TECTONICS, VOL. 30, TC3003, doi:10.1029/2010TC002707, 2011
Patterns and timing of exhumation and deformation in the Eastern
Cordillera of NW Argentina revealed by (U‐Th)/He
thermochronology
Barbara Carrapa,1,2 John D. Trimble,2 and Daniel F. Stockli3
Received 19 March 2010; revised 14 January 2011; accepted 27 January 2011; published 18 May 2011.
[1] The Eastern Cordillera (EC) and related ranges of Bolivia and Argentina exhibit a
wide variety of structural features, both thick and thin skinned, that make this region
a prime area to study the evolution of these two contrasting styles. Using a combination
of structural, geochronological, and thermochronological techniques, this study
investigates how and in what order the various structures of the Argentinean EC from 25 to
26°S have developed during the Cenozoic. New mapping in the Angastaco area preserves
one of the thickest Cenozoic stratigraphic sections and records a complex structural
evolution during the Neogene, characterized by inversion of Cretaceous Salta Rift
structures. Detrital zircon U‐Pb geochronology combined with stratigraphic and structural
features typical of synsedimentary deformation constrains the age of orgenic growth in the
area to ∼14 Ma. Detrital apatite (U‐Th)/He thermochronology on samples collected
across the width of the southernmost EC at this latitude document an eastward younging of
ages interpreted as the result of sequential eastward propagation of exhumation (and
inferred deformation) from ∼14 to 3 Ma at a rate of ∼8.3 mm/a. Our data, when compared
with existing data, show that the Puna Plateau of NW Argentina was exhuming and
deforming at the same time as the EC and inter‐Andean regions of Bolivia, suggesting that
the deformation front connects along strike despite of the differences in structural style.
Whereas the deformation front reached the sub‐Andes of Bolivia by ∼10 Ma, deformation
localized in the EC of NW Argentina until ∼4 Ma. Rates of propagation through the
whole region seem to be quasi‐uniform regardless of different structural styles.
Citation: Carrapa, B., J. D. Trimble, and D. F. Stockli (2011), Patterns and timing of exhumation and deformation in the Eastern
Cordillera of NW Argentina revealed by (U‐Th)/He thermochronology, Tectonics, 30, TC3003, doi:10.1029/2010TC002707.
1. Introduction
[2] The structural style of mountain belts greatly affects
foreland basin development, mountain building, exhumation
and sediment recycling. Although each orogenic system is
unique, all mountain belts are subject to the same fundamental tectonic processes of continental contraction and
lateral transport of upper crustal material. On the most
fundamental scale, the transported rocks of a fold‐and‐thrust
belt act like a wedge of sand or snow in front of a moving
bulldozer, incorporating new sediments and rocks at the
frontal tip across the basal thrust, and sometimes deforming
internally within the wedge to maintain a critical taper [Bally
et al., 1966; Dahlstrom, 1970].
[3] Thin‐skinned systems are characterized by low‐angle
thrust faults, multiple shallow décollements, ramp flat
1
Department of Geosciences, University of Arizona, Tucson, Arizona,
USA.
2
Department of Geology and Geophysics, University of Wyoming,
Laramie, Wyoming, USA.
3
Department of Geology, Kansas University, Lawrence, Kansas, USA.
geometries, duplex structures and a systematic progression
of deformation that generally propagates toward the foreland
through a thick package of sedimentary rocks with relatively
low mechanical strength [e.g., Bally et al., 1966; Davis et al.,
1983]. Conversely, thick‐skinned belts are characterized by
steeper faults in crystalline basement, deeper décollements,
and often experience more erratic, out‐of‐sequence development [e.g., Jordan, 1995]. Because of the relatively high
mechanical strength of basement rocks, faults in thick‐
skinned systems often nucleate along preexisting heterogeneities, such as fabrics, ancient shear zones, pervasive
fracture patterns, or inherited fault systems [Allmendinger
et al., 1983; Schmidt et al., 1995].
[4] The Eastern Cordillera of the central Andes involves
substantial amounts of mechanical ‘basement’, in this case
previously deformed low‐grade metasedimentary rocks [e.g.,
McQuarrie et al., 2005]. A key question addressed in this
paper is whether or not the timing of deformation in the
Eastern Cordillera (EC) of NW Argentina is similar to what
observed along strike to the north in Bolivia, where the EC is
characterized by a more thin‐skinned style of deformation
[Kley, 1999; McQuarrie, 2002].
Copyright 2011 by the American Geophysical Union.
0278‐7407/11/2010TC002707
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samples from exhumed Cretaceous and Cenozoic strata
from the Calchaquí Valley and surrounding ranges in the EC
(Figure 2) to determine how and in what order structures
have evolved. This multidisciplinary study provides information on the magnitude and timing of cooling and exhumation related to recent deformational events.
[7] A number of recent studies have documented rates and
styles of deformation in various parts of the central Andes
[Mon and Salfity, 1995; Echavarria et al., 2003; Elger et al.,
2005; Oncken et al., 2006; Ege et al., 2007; Barnes et al.,
2008; McQuarrie et al., 2008; Uba et al., 2009; Barnes
and Ehlers, 2009]. Our new data when combined with
existing data highlight the variations in the timing and rate
of propagation of deformation that exist on a regional scale
between the northern and southern segments of the EC
providing a basis for discussion on possible mechanisms
controlling deformation and erosion in the region.
2. Background
Figure 1. Shaded regional topography of the central Andes
of northern Argentina and southern Bolivia, showing tectonomorphic zones (labeled), thrust faults (barbed lines), and
volcanoes (circles) [modified after Sobel et al., 2003].
Black box shows area covered in Figure 2.
[5] The Eastern Cordillera (EC) in Argentina and Bolivia
is a large retroarc fold‐and‐thrust belt that has developed
during the Cenozoic along the eastern margin of the central
Andean Plateau from ∼13 to 26°S (Figure 1). The basement
of the EC in NW Argentina is mainly characterized by high‐
angle reverse faults cutting into Precambrian‐Cambrian
crystalline and metamorphic rocks, and locally Cretaceous
rift sedimentary rocks, with a décollement at >30 km depth
[Ramos, 2002]; whereas the basement in Bolivia is characterized by thrust faults cutting into Ordovician and Silurian
metasedimentary rocks [McQuarrie, 2002]. This along‐
strike variability makes the EC a prime laboratory to
examine the factors that control the sequence in which faults
activate within the orogenic system, the rate of exhumation
associated to deformation on those faults, and the style of
foreland basin fragmentation.
[6] This study combines detailed mapping of the Calchaquí Valley (Figure 1) with apatite (U‐Th)/He thermochronology and zircon U‐Pb geochronology of sandstone
2.1. Regional Setting
[8] Crustal thickening and exhumation in the central
Andes (15°–30°S) of South America is largely the result of
shortening that began in Chile during the Cretaceous and
propagated into Argentina and Bolivia during the Cenozoic
[McQuarrie et al., 2005; Arriagada et al., 2006; Carrapa
and DeCelles, 2008]. The eastward migration of the orogenic front has progressively uplifted and exhumed rocks
that were once part of the regional Andean retroarc foreland
basin system, forming the high topography and structures
observed in the Andes today (Figures 1 and 3). The Puna‐
Altiplano plateau forms the high‐altitude, relatively low
internal relief morphology of the central Andes (Figure 1).
With an average elevation of ∼4 km [Isacks, 1988], it is the
largest plateau in a retroarc setting. The Puna‐Altiplano
plateau is bounded to the west by the magmatic arc and
monoclinal flexure of the Western Cordillera and to the east
by the varied contractional systems of the Eastern Cordillera
(EC), Santa Barbara System, sub‐Andes, and Sierras Pampeanas (Figure 1).
[9] Foreland basin deformation in the central Andes
exhibits a variety of structural styles that can be broadly
correlated along strike to the angle of subduction of the
Nazca plate under the South American plate [Isacks, 1988;
Jordan and Allmendinger, 1986]. Thick‐skinned tectonics
deformation in the Sierras Pampeanas is associated with the
10° “flat slab” subduction under central Argentina, whereas
the thin‐skinned fold‐and‐thrust belts of the EC and sub‐
Andes are associated with the 30° “normal” subduction
under Bolivia [Cahill and Isacks, 1992]. However, this
relationship breaks down between 25° and 28°S where the
subducting plate is represented by a broad transitional flexure at an angle between 10° and 30° [Ramos, 2002]. In this
region, the angle of the subducting slab appears to have less
influence on the structural style of foreland deformation than
mechanical weaknesses in the crust related to preexisting
basement anisotropy. This study focuses on the southern EC
in this transitional interval, where structural style is controlled by a complex interplay between inherited structures,
inversion of Cretaceous rift structures and foreland basin
development [Strecker et al., 1989; Grier et al., 1991; Salfity
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Figure 2. General geologic map of the southern Eastern Cordillera in NW Argentina. Location of this
map is outlined as a black box in Figure 1. Apatite (U‐Th)/He sample locations are indicated with yellow
circles, and detrital zircon U‐Pb sample locations are indicated with blue stars. The general east and west
margins of the Brealito, Alemanía, and Metán subbasins of the Salta rift are outlined at the bottom
(modified after Carrera et al. [2006], Hongn and Seggiaro [2001], Marquillas et al. [2005], G. Vergani
and D. Starck (unpublished map. 1988), and new mapping from this study). Black box indicates area
mapped in Figure 5.
and Marquillas, 1994; Kley and Monaldi, 2002; Mon and
Salfity, 1995].
2.2. Eastern Cordillera
[10] The Eastern Cordillera bounds the eastern margin of
the plateau for over 1,500 km along strike, and it varies
greatly in topographic morphology, structural style, and
magnitude of shortening over that distance (Figure 1) with
average total shortening up to 270 km in Bolivia and 70 km
in NW Argentina [Kley et al., 1999; Mon and Drozdzewski,
1999; Elger et al., 2005; McQuarrie et al., 2005, 2008].
Both the thick‐skinned and the thin‐skinned portions of the
EC are bivergent fold‐and‐thrust belts, with the west half of
the cordillera verging westward and the east half of the
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Figure 3. Schematic regional cross section illustrating the general geology and major structures of the Eastern Cordillera in
NW Argentina at 25.5°–25.8°S modified after Grier and Dallmeyer [1990]. Location of the cross section is indicated on
Figure 2. Apatite (U‐Th)/He sample locations are indicated with circles, and detrital zircon U‐Pb sample locations are
indicated with stars. Sample LY was collected ∼50 km south of the line of this section and is displayed here in a laterally
equivalent structural location. Existence and depth of regional décollement inferred from Cristallini et al. [1997] and Grier
et al. [1991]. Regional geology interpreted from Carrera et al. [2006], Cristallini et al. [2004], Hongn and Seggiaro [2001],
Marquillas et al. [2005], G. Vergani and D. Starck (unpublished map. 1988), and new mapping in the Calchaquí Valley
from this study (Figure 5).
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cordillera verging eastward. There is also a difference in
timing, with deformation in the Bolivian EC initiating earlier than the Argentinean EC [McQuarrie et al., 2005].
[11] From ∼15 to 23°S, the Bolivian EC and sub‐Andes
are connected today as part of a single, retroarc fold‐and‐
thrust belt developed along the eastern margin of the Central
Andean Plateau (Figure 1), although the structures observed
today are a combination of pre‐Andean and Cenozoic
deformation. The fold‐and‐thrust belt evolved in places to
form separate east and west verging systems; deformation in
the EC and sub‐Andes has overall propagated eastward into
the foreland during the Cenozoic [Echavarria et al., 2003;
Uba et al., 2009; McQuarrie et al., 2005, 2008]. Deformation in the Argentinean EC from 23 to 26°S is markedly
different from Bolivia. It exhibits a complex structural style
dominated by high‐angle reverse faults involving intrusive
and metamorphic basement and Cretaceous sedimentary
rocks [Grier et al., 1991; Mon and Salfity, 1995].
[12] There is some evidence from sedimentology and
magnetostratigraphy to suggest that the range uplift in this
part of the EC may have swept generally eastward as it
did in the Bolivian EC [Carrera and Muñoz, 2008; Reynolds
et al., 2000], but these studies are based on indirect proxies,
and the sequence of deformation and associated exhumation
remains unresolved in this part of the EC. This study focuses
on the Angastaco area in the southernmost EC (Figures 1
and 2), where Cenozoic shortening has inverted older
north‐south trending normal fault‐bounded rocks of the
Cretaceous Salta Rift. In this region basement anisotropies
and abrupt changes in sedimentary thickness of Cretaceous
rift strata profoundly affect the style and geometry of recent
contractional deformation (Figure 3) [Mon and Salfity,
1995; Carrera et al., 2006]. Seismic reflection studies and
regional balanced cross sections suggest that the ranges of
the southwestern EC have been uplifted by the domino
rotation of semirigid basement blocks along spoon‐shaped
faults, with net shortening of 14–25% [Grier et al., 1991;
Cristallini et al., 1997; Mon and Salfity, 1995; Cristallini
et al., 2004; Kley and Monaldi, 2002]. Major folds run
parallel to fault trends in the southern EC, and synrift rocks
are often thrust over postrift and Neogene foreland strata
[Grier et al., 1991; Carrera et al., 2006]. These high‐angle
reverse faults often have limited displacement, creating
broad to overturned anticlines in their hanging walls and
synclines in their footwalls (Figure 5) [Grier et al., 1991;
Carrera et al., 2006]. Kinematic analysis of major and
minor structures has revealed a Mio‐Pliocene interval of
WNW‐ESE shortening that created the major structures
exposed at the surface, followed by a minor secondary stage
of Pliocene‐Quaternary WSW‐ESE shortening [Grier et al.,
1991; Marrett et al., 1994]. Kley and Monaldi [2002] note
that narrow folds with shallow detachments are rare within
this part of the EC, but major structures are much more
closely spaced than in the nearby thick‐skinned Sierras
Pampeanas to the south [Ramos, 2002]. The Argentinean
EC represents a type of “fold‐and‐thrust” belt that is transitional to the classic thin‐skinned and thick‐skinned models
[e.g., Kley et al., 1999], and therefore likely contains a
hybrid of the structural and kinematic features commonly
found in these two deformation regimes.
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2.3. Stratigraphy
[13] There are three major stratigraphic components in the
southern EC (Figure 2), including (1) the metamorphic
Precambrian‐Cambrian Puncoviscana Formation, (2) rift fill
sedimentary rocks of the Cretaceous Salta rift, and (3) a thick
Cenozoic sequence of synorogenic Andean foreland basin
strata (Figure 4) [Grier et al., 1991]. The Puncoviscana
Formation represents a thick passive margin sequence that
was deposited during the Neoproterozoic to Cambrian,
shortened and metamorphosed during the Pampean orogeny,
and later intruded by multiple granitic bodies [Jezek and
Miller, 1985; Willner and Miller, 1985; Grier et al., 1991].
The early Paleozoic metamorphism resulted in crustal
imbrication and in the development of east dipping shear
belts and axial planar cleavages that permeate the formation
today [Willner et al., 1987].
[14] The Cretaceous‐Eocene Salta group rests unconformably on the Puncoviscana Formation. The Salta Group
is over 5 km thick in places and is divided into three subgroups: the Pirgua, Balbuena and Santa Barbara subgroups
(Figure 4). The Pirgua subgroup is composed of over 3 km
of synrift sandstones, conglomerates, siltstones, and volcanics of the La Yesera, La Curtiembres, and Los Blanquitos
Formations [Marquillas et al., 2005]. The Balbuena subgroup is composed of 400–500 m of postrift sandstones,
limestones, and pelites. The Santa Barbara subgroup is
composed of early foreland basin sandstones, siltstones,
limestones, and shales [Marquillas et al., 2005]. Early
foreland basin strata (Paleogene) were deposited on a mixed
substratum within a continuous regional basin that was later
divided into smaller isolated basins by intrabasin range
uplift in the late Miocene [Jordan and Alonso, 1987; Grier
et al., 1991; Starck and Vergani, 1996; Bosio et al., 2009].
These strata are represented in the southern EC by four
formations of the Eocene‐Pliocene Payogastilla group
[Grier and Dallmeyer, 1990]. In the study area, the Eocene‐
Oligocene Quebrada de los Colorados Formation is composed of approximately 350 m of coarse sandstone and
conglomerate deposited in a braided fluvial to alluvial plain
system in a distal foredeep depocenter [Starck and Vergani,
1996]. The Miocene Angastaco Formation is composed of
∼4.2 km of proximal fluvial sandstones and conglomerates
that generally coarsen and thicken upward [Starck and
Vergani, 1996]. The Miocene‐Pliocene Palo Pintado Formation is composed of ∼2 km of cross‐stratified medium
grained sandstones and mudstones, topped by ∼450 m of
mudrocks, channel sandstones, pebble conglomerates, and
intercalated tuffs [Starck and Vergani, 1996]. The Pliocene
San Felipe Formation is a ∼650 m thick succession of
pebble to cobble conglomerates, medium‐grained sandstones, and sparse green mudrocks [Starck and Vergani,
1996]. The reported thickness of the Neogene formations
should be considered minimums because unit thicknesses
vary along strike, likely thicken in the subsurface, and are
eroded and crosscut by the Calchaquí Fault to the east
(Figure 2).
2.4. Chronology of Deformation in NW Argentina
[15] During the Cretaceous in the area that is now the
southern EC and Santa Barbara System (Figure 2), several
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intracratonic extensional basins of different orientations
radiated from a structural high in NW Argentina as part of
the Salta Rift system [Grier et al., 1991]. The southern arm
of the Salta rift is remarkably coincident with the location
of the modern southern Argentine EC, and can be divided
into several depocenters, including (from west to east) the
Brealito, Alemanía, and Metán subbasins (Figure 2)
[Marquillas et al., 2005]. These subbasins accumulated
more than 3.5 km of sediment from Neocomian to Maastrichtian time [Grier et al., 1991], and although Neogene
deformation has overprinted the intricate geometry of the
original rifts [Kley and Monaldi, 2002], extensional structures striking perpendicular to Andean stresses have been
preferentially reactivated as the major reverse fault bounded
ranges of the modern Argentinean EC during the Cenozoic
[Carrera et al., 2006; Mon and Salfity, 1995]. Deformation
within the Puna region and along its margins developed
since the Paleogene, and significant thick‐skinned deformation has been active along the margins of the Puna
province since the middle Eocene [Bosio et al., 2009;
Coutand et al., 2006; Carrapa et al., 2005; Deeken et al.,
2006; Hongn et al., 2007; Riller et al., 2001]. Paleogene
sedimentation in the southern central Andes occurred in a
foreland basin that was regionally extensive, partially segmented by inherited structural highs and geographically
complex [e.g., Carrapa et al., 2005; Carrapa and
DeCelles, 2008; Hongn et al., 2007]. Original paleodrainages within the Puna region were disrupted by local
range uplift starting at ca. 29–24 Ma [Carrapa et al.,
2005]. Apatite fission track thermochronology has documented that the regional foreland basin became successively compartmentalized by emergent topography as the
deformation front transferred eastward and into the EC
(Cumbres de Luracatao range) by 21 Ma (Figure 2)
[Deeken et al., 2006], as early as Eocene‐Oligocene time
[Coutand et al., 2001; Hongn et al., 2007].
[16] Hongn et al. [2007] use stratigraphic structural
relationships to argue for syntectonic deposition and growth
structure in the Eocene. If this is correct, deformation
migrated from the Plateau region to the EC almost instantaneously, which is a similar pattern to the one proposed for
the Altiplano in Bolivia [McQuarrie et al., 2005]. Alternatively, the EC was the site of regional foredeep deposition in
the Eocene as suggested by sedimentological evidence
described by Starck and Vergani [1996] and DeCelles et al.
[2008]. In this scenario the deformation was within the
Plateau in the Eocene and reached the EC in the Miocene
(∼21 Ma) as recorded by extensive coarse grained sedimentation in the foreland and Miocene exhumation of EC
basement rocks [Starck and Vergani, 1996; Coutand et al.,
2006; Deeken et al., 2006].
[17] Between ∼14 Ma and 4 Ma the deformation front was
in the Angastaco area and migrated into the Santa Barbara
System after ∼4 Ma. The Pucará, Angastaco, and La Viña
areas of the EC were connected in the early Cenozoic as part
Figure 4. General stratigraphy of the southern Eastern
Cordillera in the Calchaquí Valley and adjacent ranges
[Marquillas et al., 2005; Carrapa et al., submitted manuscript, 2011] with approximate stratigraphic location of the
analyzed samples.
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of a regional retroarc foreland basin that formed in response
to subsidence and sediment influx from the rising Puna and
Cumbres de Luracatao range (EC) directly to the west
(Figure 2) [Coutand et al., 2006].
[18] Features typical of growth strata, tilted unconformities,
and thermochronological data in the southern EC have suggested a west‐to‐east and north‐to‐south migration of deformation across the EC and Santa Barbara System starting from
Miocene time [Acocella et al., 2007; Carrera and Muñoz,
2008; Deeken et al., 2006]. The northern tip of the Sierra
de Quilmes range (Figure 2) (referred to as Cerro Negro
by Coutand et al. [2006]) began exhuming by 13–10 Ma,
resulting in the removal of Pirgua and early Payogastilla Group
strata from the range and deposition into the Angastaco and
Pucará areas (Figure 4) [Coutand et al., 2006]. From 14.5 Ma
to 5.5 Ma, the EC has been the primary source of sediment
for the Angastaco area, [Coutand et al., 2006]. Multiple apatite
fission track data sets support the exhumation of the Cerro
Runno and Cerro Durazno range (Figure 2) during the interval
of 13–10 Ma [Coutand et al., 2006; Deeken et al., 2006].
Paleodrainage reorganization in the Angastaco‐La Viña
areas suggest that the Angastaco basin was fully separated
from the rest of the foreland and was in an intermontane
position by ∼4 Ma (B. Carrapa et al., Cenozoic basin evolution in the Eastern Cordillera of northwestern Argentina
(25°–26°S): Regional implications for Andean orogenic
wedge development, submitted to Basin Research, 2011).
[19] As the sedimentary system was fragmented into a
series of isolated basins, each of those depocenters became
progressively deformed. Synclinal folding of the Pucará area
strata occurred during southward propagation of an adjacent
anticline (Figure 3), and was determined to be active
between 13.4 ± 0.4 Ma and 12.1 ± 0.1 Ma by U‐Pb geochronology on a welded tuff, lahar deposit, and lower
Payogastilla strata [Grier and Dallmeyer, 1990; Marrett
et al., 1994]. However, the timing of deformation and
exhumation in the Angastaco area and Lerma Valley has so
far been based on indirect proxies (i.e., provenance, undated
structures). Provenance and paleocurrent data indicate that
Cretaceous rocks of the adjacent Sierra de los Colorados and
Sierra de Leon Muerto (Figure 2) were uplifted and became
a dominant sediment source for the Angastaco area strata
by ∼4 Ma [Bywater‐Reyes et al., 2010]. Growth strata
relationships suggest that the Cumbres de Peñas Blancas
range east of the Lerma Valley (Figure 2) was uplifted and
exhumed during the past 5 Ma [Carrera and Muñoz, 2008].
After deformation swept across the southern EC during the
Miocene and Pliocene, out‐of‐sequence deformation and
minor strike‐slip movement occurred throughout the EC
during the Pleistocene and Holocene [Carrera and Muñoz,
2008; Marrett et al., 1994].
[20] Further south the Sierras Pampeanas, including areas
all along the southeastern margin of the Puna Plateau,
experienced a transition from regional foreland to compartmentalized basin at ∼6 Ma [Mortimer et al., 2007;
Carrapa et al., 2008a]. The most recent stratigraphic record
in the region is represented by the flat lying Pleistocene lake
deposits in the Calchaquí Valley near the town of Angastaco, which lie in angular unconformity atop the deformed
Payogastilla group and document the end of deformation
[Salfity et al., 2004].
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2.5. Study Area: Calchaquí Valley
[21] The Calchaquí Valley is an elongate depression
located in the southwestern EC near the eastern margin of
the Puna Plateau (Figure 2). It is subdivided into the
Angastaco and Tin‐Tin areas (or subbasins), and separates
the Alemanía and Brealito subbasins of the Cretaceous Salta
rift (Figure 2) [Deeken et al., 2006]. The valley’s linear
shape is controlled in part by the long N‐S trending Calchaquí fault on its eastern margin (Figure 2). The Cerro
Negro, Payogasta, and Tin‐Tin reverse faults in the northern
part of the Calchaquí valley expose synrift and postrift rocks
in their hanging walls, but there is no evidence to suggest
that synrift sedimentary rocks exist under the southern
part of Calchaquí valley, and in places foreland basin strata
lie directly on crystalline basement (Figure 2) [Carrera
and Muñoz, 2008; Mon and Salfity, 1995; G. Vergani and
D. Starck, unpublished map, 1988]. Because of its excellent
exposures and relative accessibility, the Angastaco area of
the Calchaquí Valley is an ideal location to study the details of intrabasin deformation that can occur during and
after fragmentation of the larger foreland basin system
(Figure 2). The Angastaco area is part of a valley that
continues ∼100 km north as the Calchaqui Valley and
south ∼130 km into the Santa Maria Valley; it is bounded
to the east by the inverted Calchaquí Fault, to the west by
the Sierras Pampeanas style Sierra de Quilmes uplift,
(Figure 2). The Angastaco area contains evidence of syntectonic sedimentation, intrabasin faulting and folding, and
is characterized in the southern part by large drape folds
and thick‐skinned basement uplifts [Carrera and Muñoz,
2008]. Many of these features are common in other parts
of the southern EC, such as the Luracatao Valley, the Tin‐
Tin and Amblayo areas, and the Lerma Valley (Figure 2)
[Vergani and Starck, 1988]. Detailed sampling of discrete
structures for thermochronology in the Angastaco area can
thus provide cooling ages, and by inference the timing of
deformation and exhumation, in a variety of Cenozoic
basins within the southern EC.
3. Deformation in the Calchaqui Valley
[22] In order to have a reliable stratigraphic and structural
frame for our sampling field mapping of geologic structures
within the Angastaco area was conducted using 1:30,000
scale stereoscopic aerial photographs and form the basis of
Figure 5. In particular, detailed mapping was conducted to
identify possible growth in the Angastaco Formation, which
records syndepositional deformation and thus the age of
strata can be used to determine the timing of deformation
within the basin.
[23] Evidence of synorogenic sedimentation within the
Angastaco area was observed in one location (Figure 6). To
identify growth, we were looking for truncation, thinning
or thickening of beds across strike. A 24° angular unconformity was found near the town of Angastaco in the lower
fluvial Angastaco Formation (Figure 6), with a well‐
developed paleosol interval along the unconformity (∼1 m
thick). Detrital U‐Pb geochronology on a sandstone sample
immediately below the unconformity constrains the maximum depositional age of these strata to be 14.1 ± 0.5 Ma
(see section 4.3). Continued investigation of the Angastaco
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Figure 5. Geologic map of the Angastaco area in the Calchaquí Valley. Location of this map is outlined
as a black box in Figure 2.
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LaserChron center [Gehrels et al., 2008]. For a complete
description of lab methods, consult the appendix to this
paper.
[26] There are several viable methods by which a maximum depositional age can be calculated from a young
population of detrital zircon ages. Youngest single grain age
has been shown to be compatible with depositional age 90%
of the time, but confident interpretation of young grain age
clusters can only be achieved with a weighted mean of
young ages that overlap within error [Dickinson and
Gehrels, 2009]. We present here mean ages of the youngest two or more grains that overlap within 1s error for
sandstone samples, and for intercalated ash samples the
mean ages were calculated using the Tuffzirc program of
Ludwig [2008].
Figure 6. Photo of the angular unconformity (24° angular
discordance) in the lower fluvial Angastaco Formation at the
northern edge of the town of Angastaco. A paleosol horizon
is well developed along the unconformity. Note person for
scale.
Formation yielded no other conclusive evidence for
growth.
[24] Eastward shallowing of bedding within the lower/
middle Angastaco Formation near the contact with the
basement is suggestive of basin‐scale growth or can alternatively be attributed to the synclinal flexure of the basin
(Figure 2). The contacts between the Angastaco/Palo Pintado and Palo Pintado/San Felipe Formations were observed
to be conformable at every observed location within the
study area. However, the intense folding and possible
imbrication of the sedimentary section could easily obscure
the shallow fanning of bedding that would provide more
conclusive evidence of large‐scale growth. The angular
unconformity in the lower Angastaco Formation represents
the earliest evidence of syndepositional deformation in the
area and is interpreted here to have formed by movement
along the high‐angle Sierra de Quilmes and Cerro Negro
faults to the west (Figure 2) right after ∼14 Ma.
4. Detrital U‐Pb Geochronology
4.1. Detrital U‐Pb Methods
[25] Detrital zircon U‐Pb analysis has proven a reliable
technique for determining provenance [Gray and Zeitler,
1997] and maximum depositional age of sedimentary
rocks [DeCelles et al., 2007; Dickinson and Gehrels, 2009].
Four detrital sandstone samples from the Cenozoic basin fill
preserved in the Angastaco and Pucará areas have been
analyzed in this study to better constrain the maximum
depositional age of the lower Angastaco and upper San
Felipe Formations in the region where ashes were not
present (Figure 2). In all samples, elongate euhedral zircon
crystals were preferentially analyzed to increase the likelihood of documenting a young population of ashfall zircons,
and therefore a statistically significant provenance analysis
cannot be made from this data set. All analyses were conducted by laser ablation multicollector inductively coupled
plasma mass spectrometry at the University of Arizona
4.2. Detrital U‐Pb Sampling
[27] In order to discern the age of the uppermost San
Felipe Formation, sample 04.14.08‐02 was collected in a
partially reworked ash near the top of the formation in a
small syncline (Figure 2). Because this syncline is crosscut
by the Calchaquí Fault immediately to the east (Figure 3),
the youngest detrital zircon age component constrains the
timing of recent movement on the fault as well. In order to
discern the age of the angular unconformity in the Angastaco Formation near the town of Angastaco previously in
this paper (Figure 6), two detrital sandstone samples were
collected. Sample 05.07.08‐B.Unc was collected one
meter below the base of the angular unconformity shown in
Figure 6, and sample 05.07.09‐A.Unc was collected one
meter above the same unconformity. In order to better define
the age of the Angastaco Formation in the Pucará area, a
boulder of andesite (sample 04.27.08‐AngB) was collected
within an alluvial fan conglomeratic interval of the lower
Angastaco Formation in the Pucará syncline (Figure 2).
4.3. Detrital U‐Pb Results
[28] Of the four detrital zircon samples analyzed (Table A1),
only samples 05.07.08‐B.Unc and 04.14.08‐02 had a young
population of ages capable of constraining the maximum
depositional age of the Angastaco and San Felipe Formations. For sample 04.14.08‐02, 40 detrital zircons were
analyzed and 34 grains yielded concordant ages (Figure 7a).
Zircon ages are distributed in four clusters, with major
peaks around 1–9 Ma, 460–590 Ma, 1.0–1.1 Ga, and one
old grain at 1.9 Ga. A young population of 14 grains yielded
ages younger than 3 Ma. The Tuffzirc routine of Isoplot
[Ludwig, 2008] was used to calculate a mean age of 2.3 ±
0.1 Ma from a coherent group of six detrital zircons, and the
youngest single grain age is 1.8 ± 0.1 Ma (Figure 7a). We
interpret this age to represent the formation and deposition
of volcanogenic zircons during the deposition of the
uppermost San Felipe Formation at ∼2.3 Ma. Because the
formation here is crosscut by a major fault, this age also
demonstrates that the Calchaquí fault has experienced some
reverse offset since ∼2.3 Ma. The Paleozoic and Precambrian ages are typical of first cycle Paleozoic granites and
detrital ages in the Puncoviscana Fm.
[29] For sample 05.07.08‐A.Unc, 115 detrital zircons
were analyzed and 102 grains yielded concordant ages
(Figure 7b). Zircon ages show a major Paleozoic age cluster
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at 460–540 Ma, with other zircons as old as 1.1 Ga.
Unfortunately, no zircons yielded Cenozoic ages capable of
constraining maximum depositional age of the Angastaco
Formation.
[30] For sample 05.07.08‐B.Unc, 100 detrital zircons
were analyzed and 86 grains yielded concordant ages
(Figure 7c). Zircon ages are distributed in four clusters,
with peaks around 14 Ma, 468–690 Ma, 0.9–1.1 Ga, and
1.8–2.1 Ga. Two detrital grains yielded Cenozoic ages that
overlap within 1s error, and give a maximum depositional
age (weighted mean) of 14.1 ± 0.5 Ma, with a youngest
single grain age of 13.7 ± 1.1 Ma. These young zircons
constrain growth in the lowermost Angastaco Formation to
have begun after ∼14.1 Ma. For sample 04.27.08‐AngB,
mineral separation yielded only a small zircon fraction. For
this reason only twenty zircons were analyzed, 14 of which
yielded concordant ages, and all zircons gave Precambrian
to Silurian ages (∼425–568 Ma) (Figure 7d).
[31] Although our decision to preferentially analyze
euhedral zircons prevents an unbias statistical analysis of
zircon populations to be conducted for provenance purposes, it is interesting to note that zircon ages from the lower
Angastaco and upper San Felipe Formations cluster around
three similar intervals of time in the early Paleozoic at
∼450–600 Ma, in the middle Proterozoic at ∼950–1150 Ma,
and in the early Proterozoic at ∼1.9 Ga (Figure 7). The 450–
600 Ma cluster is likely due to the dominance of zircon
sourcing from adjacent outcrops of the Puncoviscana Formation, and the presence of similar clustering in the lower
Angastaco and upper San Felipe Formations suggests that
there was no significant change in basement provenance
during this interval.
5. Apatite U‐Th/He Thermochronology
5.1. AHe System
[32] Apatite (U‐Th)/He thermochronometry (AHe) constrains the timing and magnitude of cooling through the
40°–80°C temperature window, a range that defines the
AHe partial retention zone (HePRZ) [Stockli et al., 2000;
Wolf et al., 1998]. This technique provides information on
shallow crustal processes, responds quickly to abrupt
changes in erosion rate, and is therefore especially useful
when investigating the recent history of range exhumation
and basin incision in tectonically active regions [e.g.,
Reiners and Brandon, 2006]. The AHe dating system is
based upon on the principle that radiogenic production of a
particles by 238U, 235U, 232Th, and 147Sm introduces 4He
into the crystal lattice of apatite at a predictable rate over
geologic time [Farley, 2002]. Because 4He is retained in
apatite at earth surface temperatures, but systematically
Figure 7. (a‐d) Probability density functions of zircon U‐
Pb analyses from four detrital samples. Blue boxes represent
the number of zircon ages falling into 50 My bins, with
probability density function overlain in red. Insets in boxes
Figures 7a and 7c display the youngest population of ages
from each sample. Inset of Figure 7a was calculated with
the Isoplot routine of Tuffzirc [Ludwig, 2008]. Inset of
Figure 7c displays the two youngest ages that overlap within
1s error. Sample locations are shown in Figure 2.
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CARRAPA ET AL.: EASTERN CORDILLERA EXHUMATION
diffuses out of apatite at shallow crustal temperatures, the
AHe system can be used as a precise low‐temperature
thermochronometer sensitive to as little as 2–3 km of
exhumation [Ehlers and Farley, 2003; Farley, 2002; Stockli
et al., 2000; Zeitler et al., 1987] and >4 km in case of low
geothermal gradients (<20°C). The AHe closure temperature
is well constrained to be 75 ± 5°C in Durango apatite [Wolf
et al., 1996; Ehlers and Farley, 2003], but can vary significantly depending on grain size, crystal morphology,
cooling rate, and presence of radiation damage [Shuster
et al., 2006; Ehlers and Farley, 2003; Wolf et al., 1996].
5.2. AHe Sampling
[33] Ten detrital sandstone and conglomerate rock samples were collected in the deepest exposed strata of five
fault‐bounded segments of the southern EC (Figure 2 and
Table 1). The fault‐bounded segments were sampled along a
roughly east‐west oriented transect in the Pucará Valley, the
Angastaco area of the Calchaquí Valley, the Sierra de los
Colorados anticline, the Rio de las Conchas, and off transect
in the Tin‐Tin area of the Calchaquí Valley (Figure 2). Two
samples were collected from fault‐bounded segments of the
EC, in multiple formations at each location where possible,
and are therefore described in the following as sample pairs.
All analyses were conducted using the ultra high vacuum
noble gas extraction and purification line and ICP‐MS at the
University of Kansas (U‐Th)/He laboratory, following
analytical procedures of Stockli et al. [2000] and Biswas
et al. [2007]. For complete AHe laboratory methods, consult the appendix to this paper.
5.2.1. Pucará Valley Sample Locations
[34] The Pucará valley consists of a large N‐S trending
syncline that contains Cretaceous through Neogene sedimentary rocks. Two samples were collected on the west
limb of the syncline, in the deepest parts of the Cenozoic
strata. Sample QCPucara1 was collected near the base of
the Quebrada de los Colorados Formation, and Sample
AngPuc06 was collected at the base of the Angastaco Formation just above the contact that separates these two formations (Figure 3).
5.2.2. Angastaco Area Sample Locations
[35] The Angastaco area is a structurally complex segment
of the Calchaquí Valley that contains Oligocene‐Pliocene
synorogenic sedimentary rocks. Two samples were collected
on the westernmost part of the valley, just south of the town
of Angastaco, in the footwall of a large reverse fault that
bounds the northeastern tip of the Sierra de Quilmes range.
Sample QC1EB080 was collected in the upper Quebrada de
los Colorados Formation, and sample Ang2EB was collected just above in the lower eolian facies of the Angastaco
Formation (Figure 2) (B. Carrapa et al., submitted manuscript, 2011).
5.2.3. Sierra de los Colorados Sample Locations
[36] The Sierra de los Colorados is a broad, west vergent
anticline developed in the hanging wall of the Calchaquí
Fault (Figure 3). The ridge is composed of Pirgua Subgroup
rift sediments, and is dissected in several places by antecedent drainage canyons. Sample 04.29.08‐01 was collected
in the core of the anticline where it is cut by a narrow
canyon, in a conglomeratic interval of the Los Blanquitos
Formation within the Pirgua Subgroup (Figure 2). Sample
04.14.08‐01 was also collected in upper Pirgua Subgroup
TC3003
conglomerate, approximately three meters east of the Calchaquí Fault in the overturned west limb of the same anticline. Because sample 04.29.08‐01 was collected in the core
of the anticline, it lies stratigraphically ∼400 m below
sample 04.14.08‐01.
5.2.4. Rio de Las Conchas Sample Locations
[37] Two samples were collected along the Rio de las
Conchas, and represent two different fault‐bounded segments of the EC. Sample LY is from the lowermost part of
the Pirgua Subgroup in the La Yesera Formation. It was
collected in the hanging wall of the southernmost tip of the
Calchaquí Fault, also known as the Zorrito Fault [Mon and
Salfity, 1995], at the south tip of the Sierra de Leon Muerto
where Cretaceous rift rocks are thrust over Cenozoic sedimentary rocks (Figure 2). Sample ALP is from the upper
Pirgua Subgroup in the Los Blanquitos Formation, and was
collected in the hanging wall of the La Viña Fault near the
town of Alemanía along the southwestern margin of the
Lerma Valley (Figure 2).
5.2.5. Tin‐Tin Sample Locations
[38] Two samples were collected in the northern Calchaquí Valley southeast of the town of Cachi. Sample
QCTin250, was collected in the Quebrada de los Colorados
Formation in the northern Calchaquí Valley in the hanging
wall of the Payogasta Fault (Figure 2). Sample 1TT18FT
was collected in the Santa Barbara Subgroup at the northeast
corner tip of Cerro Tin‐Tin in the hanging wall of the Tin‐
Tin fault (Figure 2).
5.3. AHe Results
[39] Fifty individual grain analyses from ten individual
rock samples show a scatter of raw AHe dates that systematically vary with the effective uranium concentration of
the grain (Table 1) (eU = U + 0.235Th) [Shuster et al.,
2006]. For detrital grains, the presence of a strong age‐eU
(Figure 8) relationship implies an episode of partial He loss
[Flowers et al., 2007]. Although these detrital samples all
come from different parts of the EC, geologic constraints
suggest that they were all buried to depths greater than 3 km
and subsequently exhumed as indicated by >6 km of
Cenozoic section that sits stratigraphically above the collected samples [Uliana et al., 1989; Coutand et al., 2006].
Assuming a standard geothermal gradient and standard
Durango apatite diffusion kinetics, all these grains would
have been thermally reset during burial, and therefore AHe
ages should represent cooling following basin incision or
exhumation, presumably related to deformation.
[40] However, most samples have two groups of grain
ages, including a scatter of dates older than depositional
age and a cluster of dates younger than depositional age
(Figure 8 and Table 1). AHe dates that are older than
depositional age (DA) of the formation in which they were
collected routinely correlate with high [eU], suggesting that
higher radiation damage increased He retentivity and raised
the effective closure temperature (Tec) in these grains,
allowing only partial resetting, and resulting in older AHe
dates. In contrast, the young clusters of dates in many
samples correspond to low [eU], indicating that these
grains have less radiation damage. The clustering of these
young populations suggests that they were more thermally
reset during burial. Thus, the older, partially annealed
populations can be momentarily disregarded, and the
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CARRAPA ET AL.: EASTERN CORDILLERA EXHUMATION
Table 1. Detrital Apatite (U‐Th)/He Data
4
Mass U
Th
eU Sm
He Length Width
rb Raw Date Est 1s Corr Date Est 1s Latitude
(Ma)
(Ma)
(Ma) Longitude
(mg) (ppm) (ppm) (ppm) (ppm) (nmol/g) (mm) (mm) Fta (mm) (Ma)
Pucará Valley Samples
AngPuc06, base of Angastaco
Fm., eolian sandstone,
deposited since 21.4 ±
0.8 Mac
a1
a2
a3
a4
a5
a6
QCPucara1, base of Quebrada
de Los Colorados Fm, sandstone,
deposited ∼37.6 ± 1.2 Mac
a1
a2
a3
a4
a5
2.0
2.7
11.6
5.5 16.4
0.65
136
1.5 13.8
1.5 62.9
2.7 6.6
2.0 2.4
1.2 29.3
25.2 19.7 109.5
25.8 68.9 17.6
5.4 7.8 18.2
9.7 4.7 23.4
9.1 31.4 94.1
0.61
0.64
0.70
0.65
0.61
145
121
136
125
127
1.4 30.6
9.7 32.9 37.0
0.64
1.4 5.5
1.5 16.2
1.1 19.4
1.1 8.3
11.6 8.3 59.7
20.6 21.0 54.8
25.7 25.4 55.1
17.1 12.3 19.5
0.60
0.62
0.56
0.59
86 0.65 44
8.5
0.5
13.2
0.8
39
40
49
44
36
47.1
36.2
24.6
8.6
38.3
2.8
2.2
1.5
0.5
2.3
77.0
56.4
35.3
13.3
62.7
4.6
3.4
2.1
0.8
3.8
114
79 0.64 40
31.7
1.9
49.7
3.0
156
132
150
109
67
76
60
72
37
39
33
36
22.1
20.1
26.8
15.8
1.3
1.2
1.6
0.9
37.2
32.3
47.4
26.7
2.2
1.9
2.8
1.6
72
79
100
89
69
0.61
0.64
0.70
0.65
0.61
0.60
0.62
0.56
0.59
25.794°S,
66.282°W
25.784°S,
66.291°W
Angastaco Area Samples
Ang2EB, base of Angastaco
Fm., eolian sandstone,
deposited since
19.2 ± 0.7 Mac
a2
a3
a4
a5
QC1EB080, top of Quebrada
de los Colorados Fm.,
sandstone, deposited
since 37.6 ± 1.2 Mac
a1
a2
a3
a4
a5
a6
a7
2.0 26.9
7.4 28.6 60.3
7.77
146
83 0.67 43
49.4
3.0
73.9
4.4
2.0 15.2
1.1 2.9
1.0 2.3
18.8 19.6 53.9
21.8 8.1 66.7
17.3 6.3 44.2
5.89
0.27
0.22
145
102
99
83 0.65 43
71 0.57 36
72 0.57 36
54.3
5.7
6.1
3.3
0.3
0.4
83.0
9.9
10.6
5.0
0.6
0.6
40.1 15.6 110.0
0.59
232
157 0.79 79
6.6
0.4
8.4
0.5
56.9 0.76
83.7 7.85
77.8 23.68
41.8 1.51
61.8 7.82
80.9 0.71
222
231
247
172
160
186
152
147
163
167
141
152
11.5
52.1
72.6
38.2
52.8
9.3
0.7
3.1
4.4
2.3
3.2
0.6
14.6
65.1
88.6
47.9
67.8
11.9
0.9
3.9
5.3
2.9
4.1
0.7
11.5
6.1
10.4 4.2
10.1 25.7
13.2 57.9
9.6 6.0
6.4 26.0
8.7 4.9
32.0
6.2
6.9
4.0
3.5
36.5
11.7
27.1
59.6
7.0
26.9
13.5
0.79
0.80
0.82
0.80
0.78
0.78
76
75
82
76
66
73
25.714°S,
66.175°W
25.715°S,
66.178°W
Sierra de los Colorados Samples (Alemania Subbasin)
04.29.08‐01, Pirgua Subgroup,
conglomerate, core of anticline,
deposited 128–70 Mad
a1
a2
a3
a4
04.14.08‐01, Pirgua Subgroup,
conglomerate, east of Calchaqui
Fault, deposited 128–70 Mad
a1
a2
a3
a4
a5
a6
0.7
9.4
20.4 14.3 18.6
0.24
83
65 0.54 32
3.1
0.2
5.7
0.3
0.6 11.8
0.9 7.9
0.8 20.5
44.6 22.3 56.5
26.2 14.1 97.4
63.1 35.4 63.6
0.55
0.46
0.98
70
80
92
65 0.51 30
76 0.57 35
64 0.54 32
4.5
5.7
5.1
0.3
0.3
0.3
8.7
9.9
9.4
0.5
0.6
0.6
2.2
42.8 19.6 127.2
1.98
113
99 0.66 46
17.7
1.1
26.6
1.6
3.7
61.1
23.5
3.8
12.9
5.35
0.21
0.30
0.99
0.38
176
69
134
144
98
60
56
76
88
66
45.3
1.1
3.8
16.8
11.5
2.7
0.1
0.2
1.0
0.7
76.2
2.4
6.1
24.8
21.0
4.6
0.1
0.4
1.5
1.3
9.6
1.3 20.6
0.4 17.7
1.6 8.8
2.3 9.9
0.9 2.8
21.5
32.0
14.3
10.8
5.9
41.2
163.3
18.7
14.2
33.3
12 of 30
0.59
0.47
0.62
0.68
0.55
34
27
40
45
33
25.624°S,
65.991°W
25.725°S,
65.967°W
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TC3003
CARRAPA ET AL.: EASTERN CORDILLERA EXHUMATION
Table 1. (continued)
4
Mass U
Th
eU Sm
He Length Width
rb Raw Date Est 1s Corr Date Est 1s Latitude
(Ma)
(Ma)
(Ma) Longitude
(mg) (ppm) (ppm) (ppm) (ppm) (nmol/g) (mm) (mm) Fta (mm) (Ma)
Rio de las Conchas Samples
LY, La Yesera Fm, sandstone,
hangingwall of Zorrito Fault,
deposited 128–90 Mad
a3
a4
a5
a6
ALP, Los Blanquitos Fm., sandstone,
hangingwall of La Viña Fault,
deposited 76–70 Mad
a1
a2
a3
a4
a5
a6
1.0 30.7 175.0 71.8 480.1
1.4
89
74 0.57 35
3.4
0.2
6.0
0.4
1.4 5.5 15.0 9.1 95.6
1.4 33.8 183.5 76.9 321.1
0.7 7.0 35.9 15.5 166.7
0.1
1.2
0.2
122
147
86
75 0.61 38
69 0.59 37
62 0.52 31
2.6
2.8
1.7
0.2
0.2
0.1
4.3
4.7
3.4
0.3
0.3
0.2
2.5 60.4
1.1
179
83 0.68 45
3.1
0.6
4.5
0.8
0.5
0.3
0.0
1.1
1.3
112
154
220
121
150
33
50
75
42
58
2.4
2.8
2.3
3.2
4.3
0.1
0.2
0.1
0.2
0.3
4.2
4.0
3.0
4.8
5.7
0.3
0.2
0.2
0.3
0.3
0.9
3.0
10.0
1.7
4.4
37.3
15.4
0.7
60.2
55.1
7.9 62.3 69.0
14.5
5.7
1.9
4.3
4.0
40.8
16.8
1.1
61.2
56.0
60.9
70.0
1.2
54.7
88.1
63
99
150
84
120
0.57
0.71
0.79
0.67
0.75
25.962°S,
65.751°W
25.632°S,
65.629°W
Tin‐Tin Samples
QCTin250, Quebrada de los
Colorados Fm, sandstone,
deposited since ∼37.6 ± 1.2 Mad
a1
a2
a3
a4
1TT18FT, Pirgua Subgroup,
sandstone, deposited 128–70 Mad
a1
a2
a3
a4
8.8 16.0
7.6 17.8 29.0
4.7
223
140 0.79 72
48.8
2.9
61.9
3.7
4.7 15.2
4.0 30.2
3.8 5.4
12.8 18.2 22.7
5.4 31.5 61.5
8.3 7.3 7.6
2.0
9.0
0.8
206
160
169
107 0.73 57
112 0.74 56
106 0.71 54
20.2
51.9
19.5
1.2
3.1
1.2
27.7
70.4
27.3
1.7
4.2
1.6
1.0 57.0 116.0 84.2 106.5
3.1
114
65 0.56 34
6.7
0.4
11.9
0.7
1.0 63.0 236.0 118.5 82.7
0.9 65.3 250.9 124.2 97.7
1.2 71.6 158.9 109.0 100.9
2.1
36.5
2.6
85
92
106
75 0.57 35
71 0.56 34
76 0.60 38
3.3
53.8
4.4
0.2
3.2
0.3
5.7
95.4
7.4
0.3
5.7
0.4
25.254°S,
66.120°W
25.113°S,
66.005°W
a
Ft is alpha ejection correction.
r is spherical radius.
Max depositional age based on detrital U‐Pb geochronology (B. Carrapa et al., submitted manuscript, 2011).
d
Salfity and Marquillas [1994].
b
c
young clustered populations of dates can be interpreted to
represent the most recent signal of exhumation (Figure 9).
Overall, there is a remarkable eastward younging trend
among the young populations of AHe ages. Raw AHe
dates systematically young eastward from ∼13 Ma in the
Pucará area to ∼4 Ma in the La Viña area, suggesting an
eastward younging wave of exhumation that propagated
across the southern EC during the Mio‐Pliocene at a rate
of ∼8.3 mm/yr (Figure 9).
5.3.1. Pucará Valley Sample Results
[41] Six apatites were analyzed from sample AngPuc06.
Two of the six grains were younger than the DA of ∼21.4 Ma
(B. Carrapa et al., submitted manuscript, 2011), with comparable cooling ages of 13.2 ± 0.8 and 13.3 ± 0.8 Ma (Table 1)
and a weighted mean age of 13.3 ± 1.1 Ma. Five apatites were
analyzed from sample QCPucara1. Three of the five grains
were younger than DA, and have disparate grain ages of ca.
26.7 ± 1.6 Ma, 32.3 ± 1.9 Ma, and 37.2 ± 2.2 Ma.
[42] QCPucara1 is stratigraphically lower than AngPuc06
(Figure 4), and therefore should have been heated to a
greater temperature than AngPuc06 during burial, but
QCPucara1 shows AHe dates considerably older than
AngPuc06. We interpret the two youngest AngPuc06 grain
dates to represent exhumation of the Pucará area because
they have comparable ages, show the lowest eU of any
apatites from the Pucará area, and are consistent with other
constraints on basin folding at ∼13.4–12.1 Ma [Marrett
et al., 1994]. Although we recognize that model results for
sample QCPucara1 are equivocal, the fact that this age
match well with evidence of growth in the adjacent
Angastaco area at ∼14 Ma supports our interpretation that
this part of the basin was exhuming at this time possibly as a
result of tilting and uplift of the east verging limb of the
Sierra de Quilmes hanging wall (Figure 3).
5.3.2. Angastaco Area Sample Results
[43] Four apatites were analyzed from sample Ang2EB
(Table 1). Two of the four grain ages are younger than
DA, and have similar grain ages of 9.9 ± 0.6 Ma and 10.6 ±
0.6 Ma. Seven apatites were analyzed from sample
QC1EB080. Three of the seven grain ages are younger than
DA, with dates of 8.4 ± 0.5 Ma, 11.9 ± 0.7 Ma, and 14.6 ±
0.9 Ma. Young AHe grain ages from these two samples
correlate strongly with eU, suggesting a basin history where
the lower Angastaco and upper Quebrada de los Colorados
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Figure 8. (U‐Th)/He ages versus effective U concentration for the analyzed samples. Green line represents depositional age.
Formations spent a significant amount of time within the
HePRZ. Additionally, because these two samples are stratigraphically close to one another, it can be assumed that
they underwent similar thermal histories. Whereas the oldest
group of ages (Table 1) can be explained by partial annealing of different closure temperatures depending on different compositions, the weighted mean of the youngest five
grain ages is used to approximate the timing of exhumation
at 10.4 ± 2.5 Ma.
5.3.3. Sierra de los Colorados Sample Results
[44] Four apatites were analyzed from sample 04.29.08‐
01, all of which are significantly younger than DA, with
ages between 5.7 ± 0.3 Ma and 9.4 ± 0.6 Ma. Six grains
were analyzed from sample 04.14.08‐01. Five of the six
grains give ages younger than DA, and cluster into two
distinct populations. The two youngest ages of 2.4 ± 0.1 Ma
and 6.1 ± 0.4 Ma generally correlate with the young exhumation signal of sample 04.29.08‐01, but three other grains
yield significantly older ages, which cluster around 21.0 ±
1.3 to 26.6 ± 1.6 Ma. The grain yielding an age of 2.4 ±
0.1 Ma is the smallest crystal of the entire study, with crystal
dimensions of 56 mm × 69 mm; this is likely the cause for its
anomalously young AHe date. Both of these samples were
collected within the same structural, and topographic feature
(the Sierra de los Colorados anticline), however sample
04.29.08‐01 despite being farther from the fault, with
respect to sample 04.14.08‐01, it is from a deeper stratigraphic level (Figure 2). The emplacement of a relatively
warm hanging wall over a cool footwall causes thermal
perturbations that depress isotherms and reduce lateral
thermal homogeneity, and there is some evidence that frictional heating along the fault plane can locally affect thermochronological systems [Ruppel and Hodges, 1994;
Ehlers and Farley, 2003]. This could explain the wide
spread of AHe dates in sample 04.14.08‐01. Because sample 04.29.08‐01 was strategically collected within the dee-
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Figure 9. Young populations of detrital apatite (U‐Th)/He grain ages plotted against sample longitude.
Errors are 1s. Dashed line represents linear best fit (r2 = 0.7). Gray halo represents the time range of sample good cooling paths through the HePRZ as modeled in HeFTy.
pest stratigraphic interval of the anticline, it was likely
subjected to greater temperatures, and it is not surprising
that it represents a simpler, less altered, and more thermally
reset signal of cooling and exhumation.
5.3.4. Rio de Las Conchas Sample Results
[45] Four apatites were analyzed from sample LY, all
of which are younger than DA, with dates between 3.4 ±
0.2 Ma and 6.0 ± 0.4 Ma. Six apatites were analyzed from
sample ALP, all of which are younger than DA, with ages
between 3.0 ± 0.2 Ma and 5.7 ± 0.3 Ma. The fact that grain
ages are significantly younger than DA suggest that both
samples were completely thermally reset and that all these
ages represent a signal of recent cooling.
5.3.5. Tin‐Tin Sample Results
[46] Four apatites were analyzed from sample QCTin250.
Two of the four grain ages are younger than DA, with
similar ages of 27.3 ± 1.6 Ma and 27.7 ± 1.7 Ma. Four
apatites were analyzed from sample 1TT18FT. Three of
the four grain ages are younger than DA, with ages of
5.7 ± 0.3 Ma, 7.4 ± 0.4 Ma, and 11.9 ± 0.7 Ma. The
young 6–12 Ma signal of sample 1TT18FT can thus be
interpreted to represent recent main exhumation of the
northern Calchaquí Valley.
5.4. Modeling of AHe Data
[47] Sample thermal history information is extracted employing an inverse modeling approach using a constrained Monte
Carlo simulations that takes into account the effects of grain
size, radiation damage, and cooling rate on the thermal history of each sample (HeFTy) [Ketcham, 2005; Flowers et al.,
2009]. Sandstone samples in this study show a relationship
between grain ages and [eU], making them prime candidates
for inverse modeling using the radiation damage trapping
model of Shuster et al. [2006]. Radiation damage impedes the
diffusion of 4He from the apatite crystal lattice, and therefore
diffusion coefficient (D) of an apatite varies between grains
and evolves within grains as a function of time. In a series of
laboratory diffusion experiments, Shuster et al. [2006]
empirically calculated the relationship between the diffusion coefficient (D) and the volume of radiation damage (Vrd)
within the grain, and the radiation damage trapping model
exploits this laboratory constrained age‐Vrd relationship to
greatly restrict the number of possible time‐temperature (t‐T)
paths that could reproduce the AHe dates observed in nature.
Inverse modeling of AHe grain ages predicts t‐T paths that
would produce the observed relationship between Tc and
radiation damage. Numerical inverse modeling can thus help
determine the detailed thermal history of a sample.
5.5. HeFTy Modeling Constraints
[48] Thermal models were run for multiple grain ages from
six AHe samples (Table 2). In all cases, He models were run
using the radiation damage trapping model calibration of
Shuster et al. [2006] (Do/a2) and the static alpha ejection
correction of Farley et al. [1996]. Gradual node randomization was chosen to reflect steady burial, and episodic node
randomization was used to represent exhumation accommodated by potentially erratic fault movements. For each
sample, wide model constraints based on geological plausible scenarios were used as input for deposition, burial, and
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exhumation so to allow maximum modeling freedom (see
Table 2). The model was allowed to run until 100 good fits
were found. If no good fits could be found, then the model
was allowed to run until 100 acceptable fits were found. All
samples were started at an age at least double the depositional age of the given sample; only the most recent, relevant
thermal history is shown in Figure 10. Known constraints on
deposition of the sample were input as a 20° temperature
boundary condition at the age of deposition. Constraints on
burial were input from the earliest known depositional age to
present, with minimum burial temperatures beginning at
30°–40°C. Depending on a sample’s grain age distributions,
we used one of two different values for maximum burial
temperature. For samples that had a group of grain ages older
than DA and a group younger than DA, we assumed that the
sample was never heated significantly above the HePRZ, and
imposed a maximum burial temperature of 100°C. For
samples that contained a single, well defined, group of grain
ages younger than DA, we assumed that the samples had
been heated well above the HePRZ, were thermally reset,
and therefore allowed a maximum model temperature of
200°C. Because the apatite system does not retain any He at
high temperatures, we did not find it necessary to impose
maximum temperatures above 200°C for any sample. A final
constraint on present‐day temperature was placed at 15°–
22°C for 0 Ma, which is consistent with average surface
temperatures. A complete list of modeling parameters can be
found in Table 2.
[49] HeFTy can accept simultaneous input from up to five
apatite aliquots, and because the apatite has a different Tec,
the model simulates thermal histories that are allowed by all
the grains, thereby constraining more time‐temperature
paths much more than would be possible with any single
grain age. Grain ages older than DA represent a partially
reset and complex T‐t signal and therefore they constrain
neither source exhumation nor basin exhumation, but
instead a combination of the two. Attempts were made to
model as many grains per sample as possible, but HeFTy is
often unable to find any good model fits when constrained
by three or more grains (especially when grains with ages
older than DA were added), even when allowed to simulate
>200,000 paths. For these reasons only grains younger than
DA were used for modeling purposes and for each of the six
successful models presented here, two to four representative
grains were used. We refer to Table 2 and Figure A1 for a
complete list and results of the grains modeled.
5.6. HeFTy Modeling Results
[50] All model results allow relatively wide boundaries on
timing of burial, but show strong, well‐constrained paths of
Figure 10. Time‐temperature paths of six AHe samples
modeled using the HeFTy computer program. The most
recent 20 Myr are illustrated here. Purple areas represent
halos around all “good” paths modeled, and green areas represent halos around all “acceptable” paths modeled. All
modeling was conducted using the radiation trapping model
of Shuster et al. [2006] (Do/a2) and the static alpha ejection
correction of Farley et al. [1996]. Complete model parameters can be found in Table 2. Sample locations are indicated on Figures 2, 3, and 4.
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Table 2. HeFTy Modeling Constraints
Deposition
Burial
Present
Sample
Age (Ma)
T (°C)
Age (Ma)
T (°C)
Age (Ma)
T (°C)
AngPuc06
Ang2EB
QC1EB080
04.29.08‐01
LY
ALP
18–21.4
14–19.2
19.2–37.6
70–128
90–128
70–76
20
20
20
20
20
20
0–21.4
0–19.2
0–19.2
0–128
0–128
0–76
30–100
40–100
40–100
40–200
40–200
40–200
0
0
0
0
0
0
10–25
10–25
10–25
10–25
10–25
10–25
most recent cooling and exhumation (Figure 10). Although
thermal models cannot determine the exact path of exhumation, they can greatly constrain a window of time and
temperature when the sample began cooling through the
HePRZ (40°–80°C). For all models, this window of exhumation has been interpreted, and it superimposed upon
Figure 9 as a cloud of time during which exhumation
through the HePRZ was occurring for each location. Model
AngPuc06 was conducted using grains a1 and a5, and
suggests that early cooling likely initiated by ∼14 Ma, and
that the sample had cooled through the HePRZ by ∼12 Ma
(Figure 10a). These results and presence of an age‐eU
relationship suggests that sample AngPuc06 cooled slowly,
and likely spent an extended period of time in the HePRZ.
[51] Model Ang2EB was conducted using grains a4 and
a5, and suggests that early cooling initiated ∼11 Ma, with
cooling through the HePRZ by ∼8 Ma (Figure 9). These
results and the presence of an age‐eU relationship suggests
that sample Ang2EB spent significant time in the HePRZ.
[52] Model QC1EB080 was conducted using grains a1
and a7. Although HeFTy was never able to find a “good”
path for these grains, 1500 “acceptable” paths were found
that suggest cooling began by ∼13 Ma, with cooling through
70°C by 12 Ma (Figure 10c). Modeling of samples
04.29.08‐01, LY, and ALP all suggest rapid cooling. Model
04.29.08‐01 was conducted using grains a2, a3, and a4, and
suggests that early cooling began ∼10 Ma, with cooling
through the HePRZ at ca. 6–7 Ma (Figure 10d). Model LY
was conducted using grains a4 and a5, and suggests that
early cooling began by ∼6 Ma, with final cooling through
the HePRZ at ∼2–4 Ma (Figure 10e). Model ALP was
conducted using grains a1, a2, a3, and a5, and suggests
that early cooling through the HePRZ had initiated at ∼35–
22 Ma, with final cooling through the 80°–40° T‐window at
ca. 0–4 Ma (Figures A1 and 10f).
5.7. Interpretation of AHe Ages and HeFTy Modeling
Results
[53] The detrital AHe system provides information on the
latest stage of exhumation in the uppermost crust, and by
exploiting the effects of radiation damage on multiple
crystals using HeFTy modeling, we can expand our confidence in the interpretation of recent cooling. While AHe
thermochronology cannot tell us about events that occurred
while the sample was buried beneath the HePRZ, incorporating the data with reasonable assumptions and known
geological constraints allows us to make interpretations that
greatly elucidate the recent patterns of exhumation in the
region.
[54] For the six Cenozoic samples that contain an old
population of partially to nonreset grain ages (Pucará,
Grains
Modeled
a1,
a4,
a1,
a2,
a4,
a1,
a5
a5
a7
a3, a4
a5
a2, a3, a5
Total
Paths
Acceptable
Paths
Good
Paths
2,003
36,892
308,236
120,547
34,514
27,180
168
2,871
1,500
1,381
357
237
100
100
0
100
100
100
Angastaco, and Tin‐Tin samples), it can be confidently
interpreted that the samples were not completely buried
beneath the HePRZ, and therefore young populations of AHe
dates closely represent the onset of exhumation. In the
Angastaco area, where corresponding apatite fission track
ages have not been annealed during burial [Coutand et al.,
2006], there is further confidence in interpreting young
AHe ages as reflecting the onset of exhumation of the basin
strata.
[55] For the two Cretaceous sandstone samples collected
in the Sierra de los Colorados anticline, just east of
Angastaco area, the sample collected in the deepest stratigraphic portion of anticline has been fully reset during burial
(sample 04.29.08‐01), whereas the shallower sample contains a mixture of nonreset, partially reset, and fully reset
ages (sample 04.14.08‐01) (see Table 1). Thus, the shallower
sample was likely buried to a maximum depth within the
HePRZ, while the deeper sample experienced temperatures
below the HePRZ; differences in composition have contributed to the observed scatter of ages. Because these samples are stratigraphically ∼400 m apart, sample 04.29.08‐01
was buried to a maximum depth just below the HePRZ, and
therefore should have entered the HePRZ shortly to immediately after the onset of Neogene exhumation. HeFTy
modeling suggests that this sample cooled through the
HePRZ at rate ≥0.5 mm/yr, and therefore the ∼400 m
stratigraphic offset between the two samples can be used to
infer that the maximum time between onset of exhumation
and entering the HePRZ is 0.8 Myr. This suggests that the
AHe ages of sample 04.29.08‐01 do represent the onset of
exhumation for this range, and that any variability is within
error of the AHe signal.
[56] The Cretaceous samples LY and ALP have been
completely thermally reset, and all grain ages are significantly younger than DA. While the AHe data for these
samples do not necessarily constrain the onset of exhumation for these samples, HeFTy modeling shows with confidence that they have experienced rapid cooling during the
last ∼6 Myr, which we interpret to reflect major range
exhumation caused by the onset of major active deformation
on adjacent reverse faults.
[57] The majority of the analyzed grains give ages that can
be interpreted to roughly represent the onset of basin
exhumation in the southern EC. The AHe grain ages and
HeFTy model results indicate that the sequence of exhumation in the southern EC has swept consistently eastward
during the Miocene and Pliocene (Figure 10). We interpret
the younging in cooling ages eastward as the result of erosion related to progressive eastward younging of fault activation. A linear regression placed through the combined
young grain age populations suggests that this wave of
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exhumation (and inferred deformation) propagated eastward
at a rate of ∼8.3 mm/yr (r2 = 0.7) (Figure 10). There is an
interesting recurrence of a second population of young grain
ages between 21 and 28 Ma in multiple parts of the study
area. We interpret this to represent a possible early, minor
exhumation signal between deposition of the Quebrada de
los Colorados and Angastaco Formations.
6. Discussion
6.1. Angastaco Area
[58] This study confirms that deposition of the Miocene‐
Pliocene sedimentary package in the Angastaco area was
contemporaneous with local deformation. U‐Pb dating in
this study and B. Carrapa et al. (submitted manuscript,
2011) limits the age range of Cenozoic strata in the Angastaco area to ∼37–2.3 Ma. The dominance of flexural slip
folding implies that the exposed strata experienced brittle
deformation at low temperature and confining pressure.
Based on detrital U‐Pb geochronology from strata below the
angular unconformity shown in Figure 6, synsedimentary
deformation initiated in the area by ∼14 Ma. AHe thermochronology documents significant rapid cooling along the
western margin of the Angastaco area by no later than
10 Ma, demonstrating that the western part of the basin was
exhuming coevally with active deposition in the central and
eastern parts of the basin. Thus, much of the sediment in the
Palo Pintado and San Felipe Formations may have been
recycled from the Quebrada de los Colorados and Angastaco
Formations just a few kilometers to the west. It is interesting
to note that samples 04.29.08‐01 and 04.29.08‐01 from the
Sierra de los Colorados yield cooling ages between 6 and
10 Ma. This indicates that cooling and exhumation in, at least
a portion of, this range initiated before the change in provenance and basin reorganization documented by paleocurrent
and conglomerate deposition at 4 Ma [Bywater‐Reyes et al.,
2010]. In turn, this suggests that despite being deformed
and eroded as early as ∼10 Ma, the Sierra de los Colorados
did not have enough topographic relief to shed coarse
material to the surrounding areas and that it was likely not
an orographic barrier. More likely this earlier exhumation is
related to the formation of local, low relief. This is supported
by fine‐grained Miocene deposits of the Anta and Jesus
Maria formations in the La Viña area, which constitute the
distal equivalents of the Angastaco Formation [Starck and
Anzótegui, 2001; B. Carrapa et al., submitted manuscript,
2011]. Overall the data indicate that the main river system
was still draining to the east and was not disrupted by the
regional uplift of ranges to the east of the Angastaco area,
which did not occur until ∼4 Ma when conglomerates
derived from the Sierra de los Colorados range were
deposited in the Angastaco and La Viña areas.
6.2. Recent Exhumation in the Southernmost EC
[59] AHe data show an eastward younging of cooling ages
that suggests an eastward migrating wave of exhumation
and related deformation during the Miocene‐Pliocene.
These data document middle Miocene exhumation of the
Pucará area and initiation of structural growth within the
Angastaco area at ∼14 Ma (Figure 10). The western
Angastaco area experienced significant exhumation by
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∼10 Ma, at which point deformation and exhumation
migrated eastward as the Sierra de los Colorados anticline
experienced rapid exhumation ∼9 Ma. The Pucará, Angastaco, and Sierra de los Colorados areas continued to exhume
as the Sierra de Leon Muerto and western Lerma Valley
experienced significant exhumation ∼6–3 Ma (Figure 10).
This timeline corresponds to the timing of shortening proposed by Marrett et al. (∼13–1 Ma) [1994], as well as the
eastward propagation of growth structures proposed in the
Mio‐Pliocene by Carrera and Muñoz [2008]. This pattern
of eastward younging deformation involved mainly westward verging thrust faults, and suggests that, regardless of
local structural geometry, overall orogenic strain propagated
eastward as expected during the development of a tapered
orogenic wedge.
6.3. Evolution of Orogenic Propagation Across the
Southern Central Andes
[60] Our new data set, when compared to existing data,
documents a wave of exhumation that propagated across the
southern EC of NW Argentina at 8.3 mm/yr between ∼14
and ∼3 Ma (Figure 11), which is roughly in agreement with
the average rate of strain propagation across the entire
central Andes (∼7.8 mm/yr) at the same latitude (Figure 11).
Major development of the southern part of the central Andes
began during the mid to late Cretaceous in the Salar de
Atacama basin [Arriagada et al., 2006; Mpodozis et al.,
2005]. Later, during the middle to late Eocene, the deformation front jumped eastward across the width of the Puna
region, causing a roughly synchronous onset of deformation
from ∼40 to 38 Ma in the Salar de Antofalla, Salar de
Arizaro and Salar de Pastos Grandes (and possibly La
Poma) basins (Figure 11) [Kraemer et al., 1999; Voss, 2002;
Carrapa et al., 2005; Mpodozis et al., 2005; Arriagada et al.,
2006; Jordan and Mpodozis, 2006; Hongn et al., 2007;
Carrapa and DeCelles, 2008]. Deformation was localized
within the eastern Puna/western EC of NW Argentina
between ∼39 and ∼22 Ma and swept eastward across the
eastern EC during the Miocene‐Pliocene (Figure 11). Evidence of deformation and exhumation in the Puna Plateau
at ca. 25 Ma [Carrapa et al., 2005] suggests out‐of‐sequence
deformation of the hinterland at this time.
[61] Despite similarities of deformation during the early
Cenozoic, the Oligocene and Neogene kinematic histories
in Bolivia and NW Argentina are significantly different.
Shortening in Bolivia continued to propagate eastward
through the EC and inter‐Andean zone throughout the
Oligocene [McQuarrie et al., 2005; Ege et al., 2007],
whereas the Puna experienced internal deformation and
exhumation [Carrapa et al., 2005] with very limited eastward propagation during this time interval (Figure 11).
During Miocene‐Pliocene time (∼22–3 Ma) deformation
was concentrated within the EC/Santa Barbara system of
NW Argentina (∼26°S, 66°W) and it was migrating through
the study area in sequence between ∼14 and 3 Ma. In the
sub‐Andes of NW Argentina‐S Bolivia [Echavarria et al.,
2003] deformation was migrating through the region
between ∼8 and 3 Ma and it was in the sub‐Andes of
Bolivia by ∼10 Ma [Gubbels et al., 1993].
[62] Interesting questions that arise from this comparison
are (1) why did different segments of the central Andes
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behave differently during the Neogene and (2) what are the
possible controlling mechanisms? Mass removal underneath
the plateau region [Kay et al., 1999] and subsequent
adjustment of the orogenic system has been proposed as one
of the main controlling factors on the evolution of Cordilleran type orogenic systems such as the Andes [DeCelles
et al., 2009]. In particular the abrupt eastward shifts of the
deformation front observed in Bolivia at ca. 45–40 Ma and
again at 10 Ma have been explained as a response to isostatic elevation gain following the removal of dense arc
roots beneath the orogenic hinterland [DeCelles et al., 2009;
Kay et al., 1999]. However, significant along‐strike variations in the Neogene kinematic history between the study
area and regions farther north suggest differences in orogenic processes. For example the Altiplano plateau to the
north has been explained in terms of orogenic‐scale lithospheric removal [Garzione et al., 2006]. Alternatively the
whole region has been explained by continuous shortening
and gradual uplift [Ehlers and Poulsen, 2009]. The Puna
Plateau to the south has been associated with smaller‐scale,
and perhaps diachronous, lithospheric removal [Carrapa
et al., 2009]. Thus, orogenic cyclicity has likely not operated uniformly over along‐strike distances of >500 km,
perhaps because of differences in shortening rates, arc melt
and dense residue production rates resulting in different
upper crustal to surficial structural responses. A younger
lithospheric removal event in the Puna Plateau at ∼3 Ma as
proposed by DeCelles et al. [2009] is consistent with
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migration of the deformation front into the Santa Barbara
system after 4 Ma.
7. Conclusions
[63] Although the central Andes express great variability
in timing and structural style along strike, remarkable similarities exist between the timing and rates of propagation of
deformation and exhumation in the northern (Bolivian) and
southern (Argentinean) portions of the central Andes, particularly when major structural features are followed along
strike regardless of the established tectonomorphic nomenclature. The development of the Bolivian EC was roughly
coeval with the Eocene‐Oligocene internal deformation of
the Puna region, and the development of the Argentinean
EC was coeval with the development of the sub‐Andean
system (Figure 11) [Carrera and Muñoz, 2008; Echavarria
et al., 2003; McQuarrie et al., 2005, 2008; Uba et al.,
2009]. Many of the geomorphic and structural features of
the Puna can be traced northward along strike into the
Bolivian EC (Figure 11), and because both provinces contain
copious evidence of Oligocene deformation, it appears that
the eastern part of the Puna correlates both geologically and
temporally with the Bolivian EC and inter‐Andean zone.
[64] If we temporarily overlook the geographic constrictions implied by predefined tectonomorphic provinces, and
instead focus on regions of the central Andes that correlate
in time, the propagation of deformation rates (and exhu-
Figure 11. Regional map of the central Andes illustrating the timing and rates of propagation of deformation across the
area. White circles represent timing (in Ma) of the earliest evidence for Cenozoic deformation, wedge top deposition, or
synorogenic deformation. Values along white lines represent rates at which the orogenic wedge propagated eastward. Base
map is a NASA SRTM digital elevation model. Tectonomorphic domains follow description in Figure 1 (tectonomorphic
provinces modified after Schoenbohm and Strecker [2009]). Timing of deformation and propagation rates are based on the
following: Balanced cross‐section restoration, apatite fission track, and zircon fission track thermochronology of McQuarrie
et al. [2008] (letter a). Modeling of 40Ar/39Ar and AFT ages after Gillis et al. [2006] (letter a*). Balanced cross‐section
restoration of McQuarrie [2002] and McQuarrie et al. [2005] (letter b). Loose estimates based on flexural modeling of
stratigraphic data combined with GPS convergence rates. Flexural modeling of stratigraphic data by DeCelles and Horton
[2003] suggests that the foreland flexural wave migrated eastward at a rate of 12–20 mm/yr during the Oligocene, which
combined with shortening rates of ∼3–6 mm/yr suggests an orogenic propagation rate of 6–17 mm/yr [after DeCelles and
DeCelles, 2001; DeCelles and Horton, 2003; Norabuena et al., 1998] (letter c). Seismic interpretation and balanced cross‐
section restoration of Uba et al. [2009] (letter d). Cross‐cutting relationships, growth strata, unconformities, provenance,
migration of facies, and accumulation rate history of Echavarria et al. [2003] (letter e). Middle‐to‐late Cretaceous onset of
deformation inferred from seismic interpretation, structural mapping, and sedimentology of Arriagada et al. [2006] and
Mpodozis et al. [2005] (letter f). Sedimentology, provenance analysis, paleocurrent interpretation, and apatite fission track
thermochronology of Carrapa et al. [2005], Kraemer et al. [1999], and Voss [2002] (letter g). Carrapa et al. [2009];
stratigraphy of Jordan and Mpodozis [2006] (letter h). Apatite fission track data and sedimentology stratigraphy; [after
Carrapa and DeCelles, 2008] (letter i). Apatite fission track thermochronology of Deeken et al. [2006] (letter j). Growth
strata deted in this study (letter k). Detrital (U‐Th)/He thermochronology in this study (letter l). Seismic interpretation,
apatite fission track thermochronology, and geochronology of Mortimer et al. [2007] (letter m). Sedimentology, stratigraphy, structural geology and U‐Pb geochronology of Carrapa et al. [2008a] (letter n). Apatite fission track data from
Carrapa et al. [2006] (letter o). Shaded gray lines show the correlation along strike of the deformation front independently
from predefined tectonomorphic domains. In this figure only studies that have a significant W‐E area coverage and/or use a
similar approach to ours (structural relationships/thermochronology) and are interpreted here as representing unequivocal
timing of in‐sequence deformation are included. Extra data are discussed in the text. For a review on available multidisciplinary data in the Andean Plateau we refer to Barnes and Ehlers [2009]. Deformation was localized within the eastern
Puna/western EC of NW Argentina between ca. 39 and ca. 22 Ma resulting in very slow (1.8 mm/yr) propagation rates.
Propagation rates in the sub‐Andean fold‐and‐thrust belt were similar, although slightly more erratic in places, migrating
eastward at ∼9.9 mm/yr from ca. 9 to 1.2 Ma at 22–23°S [Echavarria et al., 2003], ∼15 mm/yr from ca. 12 to 0 Ma at 21°S
[Uba et al., 2009], and 6–8 mm/yr from ca. 20 to 0 Ma at 19°–20°S [McQuarrie et al., 2005].
19 of 30
TC3003
CARRAPA ET AL.: EASTERN CORDILLERA EXHUMATION
Figure 11
20 of 30
TC3003
TC3003
CARRAPA ET AL.: EASTERN CORDILLERA EXHUMATION
mation) of the NW Argentinean EC is comparable to
propagation of deformation rates in the thin‐skinned sub‐
Andes during Mio‐Pliocene time. Evidence for this comes
from a variety of sources (Figure 11).
[65] In summary, whereas the Mio‐Pliocene rate of eastward propagation of the orogenic front varies along strike,
the propagation rate in the study area of NW Argentina
(8.3 mm/yr) falls within the same range of rates for the EC
of Bolivia (2.1–15 mm/a), and is remarkably similar to the
long‐term average rate of orogenic propagation in the central Andes (7.8 mm/yr) [McQuarrie et al., 2005]. More
importantly, similar rates of propagation of the deformation
front along the plateau margin characterized by significantly
different structural styles suggests that the rate at which the
deformation front has driven into the foreland is determined
more by the dynamics of regional eastward expansion of the
Andean orogeny than the by local structural style, and
therefore may develop independently of features that might
control local structural style (e.g., the nature of the local
basement). It also interesting to note that rates of propagation of deformation are similar along strike despite of the
significant differences in amounts of shortening between
Bolivia (>300 km) [Kley et al., 1997; McQuarrie, 2002] and
Argentina (<100 km) [Kley and Monaldi, 1998, and references therein]. This may be explained by the fact that the
Altiplano and Eastern Cordillera of Bolivia have deformed
mainly by simple shear whereas the Puna Plateau and
Eastern Cordillera of NW Argentina by pure shear
[Allmendinger and Gubbels, 1996].
[66] The differences in the timing of late Miocene‐
Pliocene deformation between the study area and regions
farther north suggest different structural and surficial
responses to lithospheric processes. For example the late
Cenozoic uplift history of the Altiplano in Bolivia has been
associated with large‐scale lithospheric removal [Garzione
et al., 2006], which has been used to explain changes in
taper angle and subsequent migration of the orogenic front
outward in the sub‐Andes at ∼8 Ma [DeCelles et al., 2009].
In NW Argentina, small‐scale lithospheric removal has been
proposed to have influenced Puna Plateau evolution [Kay
and Coira, 2009], late Cenozoic sedimentary basin evolution and paleogeography [Strecker et al., 2009; Carrapa
et al., 2008b]. However, in this last case the size, volume
and possible diachrounous removal of lithosphere has
probably prevented a syncronous response of deformation
forelandward.
Appendix A
A1.
U‐Pb Lab Methods
[67] Mineral separation was conducted at the University
of Wyoming using a jaw crusher, disk mill, wilfley table,
Frantz electromagnetic separator, and methyl iodide heavy
liquids. Detrital zircon U‐Pb analyses were conducted at the
Arizona LaserChron center using standard methods of laser
ablation multicollector inductively coupled plasma mass
spectrometry (LA‐MC‐ICP MS). U‐Pb data are presented in
Table A1. Zircons were ablated with a New Wave/Lambda
Physic DUV193 Excimer laser using a spot diameter of
35 mm, material was transported by He gas to the plasma
source of a GVI Isoprobe, and isotopes were measured
TC3003
simultaneously in static mode. Measurement of 238U, 232U,
208
Pb, and 206Pb was conducted with eight 1011 W Faraday
detectors. A 1012 W Faraday collector measured 207Pb, and
204
Pb was measured with an ion‐counting channel. Ion
yields were ∼1.0 mv per ppm. A typical analysis included
20 s of background measurement, 12 s integrated measurements as the laser fired, and 30 s to purge the instrument
and prepare for the next analysis. Unknowns were calibrated
against a Sri Lanka standard of known age after every fifth
zircon analysis to correct for apparent fractionation of Pb
isotopes and inter element fractionation of Pb/U. Corrections for common Pb were calculated using measured 204Pb
and an assumed initial Pb composition from Stacey and
Kramers [1975]. Measurement uncertainty was generally
1–2% (2s). Because of their sensitivity to young ages,
206
Pb/238U ages were used, and analyses with more than
30% discordance between 206Pb/238U and 206Pb/207Pb ages
were removed from consideration. For more information
concerning lab methods at the Arizona LaserChron center,
see Gehrels et al. [2008].
A2.
(U‐Th)/He Lab Methods
[68] Mineral separation and grain picking were conducted
at the University of Wyoming. Apatites were separated from
ten detrital samples using a jaw crusher, disk mill, wilfley
table, Frantz electromagnetic separator, and methyl iodide
heavy liquids. Using a Leica M165C stereomicroscope, each
apatite fraction was thoroughly screened for relatively
euhedral apatites that were free of cracks and inclusions and
had minimum dimensions greater than 60 mm. Unfortunately, detrital apatites are often small, broken, or abraded,
and thus pose a significant challenge during grain picking.
Although six individual grains were desired, in some samples only four acceptable grains could be found. All grains
were photographed and measured for Ft calculations using
the method of Farley et al. [1996].
[69] Apatite U‐Th/He analyses were conducted using the
ultra high vacuum noble gas extraction and purification line
and ICP‐MS at the University of Kansas. For each single‐
grain aliquot, 4He was extracted with a continuous mode
Nd‐YAG laser, spiked with 3He for isotope dilution, concentrated and purified with in a Janis cryogenic trap,
released, and the 3He/4He ratio was subsequently measured
with a Blazers Prisma QMS‐200 quadrupole mass spectrometer. After degassing, each sample was analyzed for U,
Th, Sm, and selected REE using a Fisons/VG PlasmaQuad
II Inductively Coupled Plasma Mass Spectrometer (ICP‐
MS). Samples were routinely calibrated against Durango
apatite standard to ensure laboratory accuracy.
A3.
Additional (U‐Th)/He Details
[70] Radiation damage introduces isolated structural
defects and vacancies into the apatite crystal structure that
can substantially retard the diffusion of 4He and lead to
anomalously old ages [Shuster et al., 2006; Shuster and
Farley, 2009]. Radiation damage controls AHe dates more
than any other known factor, is twice as influential as grain
size, can cause expansion of the HePRZ to 50°–115°C, and
generally correlates with [eU], the effective uranium concentration of the grain (eU = U + 0.235Th) [Shuster et al.,
2006]. Because radiation damage accumulates over geo-
21 of 30
SAMPLE 04.14.08‐02
04140802‐1
04140802‐3
04140802‐4
04140802‐5
04140802‐6
04140802‐7
04140802‐8
04140802‐9
04140802‐10
04140802‐11
04140802‐12
04140802‐13
04140802‐15
04140802‐16
04140802‐17
04140802‐18
04140802‐19
04140802‐21
04140802‐22
04140802‐23
04140802‐24
04140802‐25
04140802‐27
04140802‐28
04140802‐29
04140802‐30
04140802‐31
04140802‐33
04140802‐34
04140802‐35
04140802‐36
04140802‐38
04140802‐39
04140802‐40
SAMPLE 05.07.08‐A.Unc
050708‐AUNC‐1
050708‐AUNC‐2
050708‐AUNC‐3
050708‐AUNC‐4
050708‐AUNC‐5
050708‐AUNC‐6
050708‐AUNC‐7
050708‐AUNC‐8
050708‐AUNC‐9
050708‐AUNC‐10
050708‐AUNC‐11
050708‐AUNC‐12
Analysis
Isotope Ratios
Apparent Ages (Ma)
22 of 30
17.2735
17.7149
17.6687
17.6122
17.6579
17.8076
17.8131
17.6944
17.5969
17.4002
17.9142
16.0399
11614
6966
15192
11976
17030
15958
15190
22476
14036
60018
14664
43230
600
322
424
312
352
352
366
597
421
574
403
891
1.7
1.5
2.2
1.8
1.6
2.2
1.9
3.3
2.1
1.6
1.5
5.8
2.1
24.1
1.3
95.1
1.9
2.2
1.9
6.8
6.1
2.6
79.2
15.3
30.9
60.8
9.3
11.6
2.4
1.4
14.9
2.7
40.2
2.8
24.0
6.1
1.8
12.5
16.3
22.0
4.3
3.8
20.8
60.6
29.2
1.8
0.6104
0.6014
0.5939
0.6150
0.6102
0.5912
0.5904
0.5948
0.6041
0.6069
0.5906
0.7311
0.6082
0.0022
0.7920
0.0021
0.6636
1.6792
0.6016
0.0023
0.0026
1.5330
0.0023
0.0049
0.0015
0.0024
0.0030
0.0082
0.5868
5.2650
0.0021
1.5470
0.0014
0.5110
0.0022
0.0074
0.6831
0.0027
0.8628
0.0023
0.7019
0.6144
0.0023
0.0021
0.0018
1.8811
2.4
2.0
2.7
2.4
2.1
3.0
2.2
3.5
2.4
2.2
2.0
7.0
2.3
24.4
1.7
95.5
2.5
3.8
2.1
7.3
6.4
2.7
79.3
15.7
30.9
60.9
9.4
11.6
3.2
1.7
15.2
4.8
40.2
3.9
24.1
6.1
2.5
12.5
16.3
22.1
4.4
4.0
20.9
60.6
29.5
2.0
0.0765
0.0773
0.0761
0.0786
0.0781
0.0764
0.0763
0.0763
0.0771
0.0766
0.0767
0.0851
0.0773
0.0004
0.0959
0.0003
0.0839
0.1667
0.0777
0.0003
0.0004
0.1548
0.0005
0.0006
0.0003
0.0004
0.0004
0.0013
0.0740
0.3329
0.0003
0.1447
0.0003
0.0649
0.0004
0.0011
0.0850
0.0004
0.0891
0.0004
0.0834
0.0792
0.0004
0.0004
0.0003
0.1810
1.7
1.3
1.6
1.6
1.4
2.0
1.1
1.1
1.3
1.5
1.3
3.8
0.9
3.9
1.1
8.3
1.6
3.2
0.9
2.6
2.1
0.5
4.0
3.4
1.1
4.4
1.6
0.6
2.2
1.1
3.1
3.9
2.2
2.6
2.1
0.7
1.8
1.5
0.9
1.2
0.8
1.1
1.8
1.5
4.0
0.9
0.69
0.64
0.60
0.66
0.68
0.67
0.50
0.32
0.52
0.68
0.66
0.55
0.41
0.16
0.66
0.09
0.65
0.82
0.41
0.36
0.32
0.19
0.05
0.22
0.04
0.07
0.16
0.05
0.68
0.63
0.20
0.82
0.06
0.69
0.09
0.11
0.71
0.12
0.05
0.05
0.19
0.27
0.09
0.02
0.14
0.46
475.0
479.8
472.8
487.5
485.0
474.3
473.9
474.2
478.8
475.7
476.6
526.2
479.8
2.3
590.1
1.8
519.4
993.9
482.2
2.1
2.5
927.8
3.4
4.1
2.0
2.3
2.8
8.7
460.4
1852.6
2.2
871.3
2.2
405.4
2.3
7.4
525.8
2.7
550.0
2.4
516.2
491.2
2.3
2.6
2.1
1072.7
7.7
5.9
7.4
7.4
6.7
9.1
5.1
5.1
5.8
6.8
6.2
19.2
4.3
0.1
6.2
0.1
8.1
29.0
4.0
0.1
0.1
4.3
0.1
0.1
0.0
0.1
0.0
0.0
9.8
17.6
0.1
31.9
0.0
10.4
0.0
0.1
9.1
0.0
4.6
0.0
4.1
5.1
0.0
0.0
0.1
9.0
483.8
478.1
473.3
486.7
483.7
471.6
471.1
473.9
479.8
481.6
471.2
557.2
482.5
2.2
592.3
2.1
516.8
1000.7
478.3
2.3
2.6
943.7
2.3
5.0
1.5
2.4
3.0
8.3
468.8
1863.2
2.2
949.3
1.4
419.1
2.2
7.5
528.6
2.7
631.7
2.3
539.9
486.3
2.4
2.1
1.8
1074.5
9.4
7.6
10.2
9.2
8.2
11.2
8.4
13.3
9.2
8.3
7.7
29.8
8.8
0.5
7.5
2.0
10.2
24.5
8.1
0.2
0.2
16.4
1.8
0.8
0.5
1.5
0.3
1.0
12.1
14.9
0.3
29.5
0.6
13.2
0.5
0.5
10.4
0.3
77.0
0.5
18.5
15.4
0.5
1.3
0.5
13.1
525.7
470.1
475.9
483.0
477.2
458.5
457.8
472.6
484.9
509.6
445.3
686.0
495.2
−116.2
600.9
417.0
505.5
1015.6
459.5
277.6
103.5
981.1
−1059.5
425.5
−846.2
96.0
161.1
−106.1
510.2
1875.1
−23.9
1134.8
−1275.4
495.5
−106.4
57.1
540.9
29.7
935.8
−115.8
641.3
463.4
66.5
−491.4
−275.1
1078.2
38.4
34.0
47.6
39.8
34.7
49.0
42.8
73.9
45.7
35.1
34.0
124.3
46.1
601.0
27.3
673.7
42.1
44.8
43.1
156.1
143.2
53.4
1167.2
344.0
901.6
1583.6
218.4
287.0
51.8
24.5
363.3
54.6
1309.6
61.8
598.3
145.7
38.6
299.5
336.8
548.4
93.4
85.0
500.7
1759.7
756.6
35.2
475.0
479.8
472.8
487.5
485.0
474.3
473.9
474.2
478.8
475.7
476.6
526.2
479.8
2.3
590.1
1.8
519.4
1015.6
482.2
2.1
2.5
981.1
3.4
4.1
2.0
2.3
2.8
8.7
460.4
1875.1
2.2
1134.8
2.2
405.4
2.3
7.4
525.8
2.7
550.0
2.4
516.2
491.2
2.3
2.6
2.1
1078.2
7.7
5.9
7.4
7.4
6.7
9.1
5.1
5.1
5.8
6.8
6.2
19.2
4.3
0.1
6.2
0.1
8.1
44.8
4.0
0.1
0.1
53.4
0.1
0.1
0.0
0.1
0.0
0.0
9.8
24.5
0.1
54.6
0.0
10.4
0.0
0.1
9.1
0.0
4.6
0.0
4.1
5.1
0.0
0.0
0.1
35.2
90.4
102.1
99.4
100.9
101.6
103.4
103.5
100.3
98.7
93.3
107.0
76.7
96.9
−2.0
98.2
0.4
102.7
97.9
105.0
0.8
2.4
94.6
−0.3
1.0
−0.2
2.4
1.8
−8.2
90.2
98.8
−9.1
76.8
−0.2
81.8
−2.2
12.9
97.2
9.0
58.8
−2.1
80.5
106.0
3.5
−0.5
−0.8
99.5
CARRAPA ET AL.: EASTERN CORDILLERA EXHUMATION
2.8
12.2
12.1
3.3
6.3
7.0
15.8
11.6
12.6
11.6
4.6
47.7
18315 5.0 17.5152
1017 4.2 22.7779
54378 2.2 16.6871
411 1.2 18.1429
27078 3.5 17.4331
52539 3.4 13.6872
31068 2.5 17.8001
888 2.1 19.2971
1713 5.2 20.7960
12228 3.7 13.9223
729 5.8 32.2610
399 1.2 18.0740
525 0.6 29.9985
705 4.1 20.8624
1137 3.5 20.2923
3363 4.5 22.6853
10602 2.7 17.3961
21492 0.8 8.7188
1359 7.2 21.9338
67716 1.9 12.8987
474 6.2 34.6046
11025 4.5 17.5126
861 3.2 22.6877
5718 99.6 21.2060
16416 2.1 17.1542
1317 3.3 21.4510
7506 3.3 14.2336
915 3.7 22.7749
8193 2.8 16.3782
15750 2.4 17.7686
930 4.4 21.1231
384 0.6 26.3769
903 4.4 24.2709
45420 3.7 13.2694
227
2640
607
1068
284
216
370
3532
5073
53
1064
1057
2654
2237
2810
2660
189
69
3409
437
1228
161
2204
5541
162
2962
255
2294
257
156
2298
1312
2504
178
U 206Pb/
206Pb*/ Standard 207Pb*/ Standard 206Pb*/ Standard
Error 206Pb*/ Standard 207Pb*/ Standard 206Pb*/ Standard Best Age Standard Concentration
(ppm) 204Pb U/Th 207Pb* Error (%) 235U* Error (%) 238U Error (%) Correlation 238U* Error (Ma) 235U Error (Ma) 207Pb* Error (Ma) (Ma) Error (Ma)
(%)
Table A1. Detrital U‐Pb Data
TC3003
TC3003
050708‐AUNC‐13
050708‐AUNC‐14
050708‐AUNC‐16
050708‐AUNC‐17
050708‐AUNC‐18
050708‐AUNC‐19
050708‐AUNC‐20
050708‐AUNC‐21
050708‐AUNC‐22
050708‐AUNC‐23
050708‐AUNC‐24
050708‐AUNC‐25
050708‐AUNC‐26
050708‐AUNC‐27
050708‐AUNC‐28
050708‐AUNC‐29
050708‐AUNC‐30
050708‐AUNC‐31
050708‐AUNC‐32
050708‐AUNC‐34
050708‐AUNC‐35
050708‐AUNC‐36
050708‐AUNC‐37
050708‐AUNC‐38
050708‐AUNC‐39
050708‐AUNC‐41
050708‐AUNC‐42
050708‐AUNC‐43
050708‐AUNC‐44
050708‐AUNC‐45
050708‐AUNC‐46
050708‐AUNC‐47
050708‐AUNC‐48
050708‐AUNC‐49
050708‐AUNC‐50
050708‐AUNC‐51
050708‐AUNC‐53
050708‐AUNC‐54
050708‐AUNC‐55
050708‐AUNC‐56
050708‐AUNC‐58
050708‐AUNC‐59
050708‐AUNC‐60
050708‐AUNC‐61
050708‐AUNC‐62
050708‐AUNC‐63
050708‐AUNC‐64
050708‐AUNC‐65
Analysis
Table A1. (continued)
Isotope Ratios
Apparent Ages (Ma)
140
330
379
287
263
313
340
358
152
512
421
284
511
273
322
335
310
568
630
566
375
513
198
291
292
408
431
818
244
238
403
617
320
282
193
377
505
237
397
446
386
248
297
460
465
205
87
386
6538
6706
16902
14598
22614
23586
14650
17568
8546
22046
19122
17236
22184
16428
8660
30910
16152
26808
20292
24030
8542
8118
6292
9520
6258
13642
31526
19732
10462
19686
19976
28992
19986
8120
18154
14656
20230
9770
24606
35004
18142
13252
28910
26582
31740
27948
8706
23894
5.1
6.1
8.6
2.3
2.3
15.2
8.9
14.4
5.7
25.4
3.8
8.0
14.5
4.9
8.1
5.9
3.0
11.7
15.2
2.3
1.3
10.3
9.7
3.0
3.8
10.5
7.6
2.4
6.3
6.4
6.1
2.9
9.0
2.2
3.9
2.0
7.4
2.2
5.8
8.4
1.6
6.0
4.0
13.5
5.9
1.4
2.5
13.9
17.5819
16.4621
17.8625
17.3930
13.5773
17.5765
17.7632
17.8406
15.6312
17.6155
17.8672
17.1379
17.5288
14.8111
17.4928
13.8530
17.5750
17.5409
17.4866
17.5901
17.5430
17.4223
17.6830
17.6084
18.0625
17.7339
14.2932
17.4104
17.8770
14.8534
14.6417
17.3411
15.1012
19.6097
13.6823
17.2716
17.5961
17.7330
15.4115
15.0197
16.3304
15.7770
13.4216
17.7904
14.4374
13.4373
15.3626
16.3018
3.3
2.0
1.6
2.4
1.3
2.2
1.4
1.1
3.3
1.6
1.3
1.1
2.0
1.8
1.1
2.0
1.4
1.0
1.9
1.7
2.2
0.7
1.8
1.9
2.5
2.1
2.5
1.0
2.6
3.0
1.4
2.1
5.2
2.8
1.6
1.3
1.3
1.6
4.7
2.1
1.6
3.0
1.8
1.9
0.7
1.6
1.3
2.9
0.6253
0.6811
0.5848
0.6619
1.5816
0.6028
0.5914
0.5917
0.7964
0.5855
0.5912
0.6874
0.6137
1.0120
0.6050
1.5746
0.6047
0.6179
0.6158
0.6061
0.6337
0.6173
0.5914
0.6630
0.5831
0.5871
1.2476
0.5900
0.5788
1.0099
1.1280
0.6913
0.7951
0.2926
1.6547
0.6814
0.5898
0.5871
0.8447
0.9829
0.8495
0.7537
1.6919
0.6036
1.2154
1.7904
1.1927
0.7138
3.3
4.3
2.9
3.1
2.5
2.6
1.8
1.6
4.4
2.4
1.6
1.6
2.2
4.2
2.3
3.2
1.7
1.7
2.6
1.8
2.5
1.1
2.1
1.9
2.7
2.7
2.9
1.1
3.2
4.0
2.0
2.3
6.1
2.9
2.1
1.5
1.5
1.7
7.0
2.9
1.9
7.3
2.8
2.0
2.3
1.9
1.5
3.9
0.0797
0.0813
0.0758
0.0835
0.1557
0.0768
0.0762
0.0766
0.0903
0.0748
0.0766
0.0854
0.0780
0.1087
0.0768
0.1582
0.0771
0.0786
0.0781
0.0773
0.0806
0.0780
0.0758
0.0847
0.0764
0.0755
0.1293
0.0745
0.0750
0.1088
0.1198
0.0869
0.0871
0.0416
0.1642
0.0854
0.0753
0.0755
0.0944
0.1071
0.1006
0.0862
0.1647
0.0779
0.1273
0.1745
0.1329
0.0844
0.5
3.8
2.4
1.9
2.2
1.4
1.2
1.1
2.9
1.8
1.0
1.2
1.0
3.8
2.1
2.5
1.0
1.4
1.8
0.6
1.0
0.8
1.1
0.5
0.8
1.8
1.6
0.5
1.8
2.7
1.5
0.8
3.2
0.6
1.4
0.7
0.7
0.7
5.2
2.1
1.0
6.7
2.2
0.7
2.2
1.0
0.6
2.6
0.15
0.88
0.84
0.62
0.86
0.54
0.66
0.68
0.66
0.75
0.63
0.72
0.45
0.91
0.89
0.79
0.59
0.81
0.69
0.35
0.41
0.75
0.50
0.26
0.31
0.66
0.53
0.45
0.56
0.66
0.74
0.34
0.53
0.22
0.64
0.49
0.44
0.39
0.74
0.71
0.54
0.91
0.77
0.34
0.96
0.54
0.43
0.66
494.5
504.0
470.8
517.0
933.1
477.3
473.3
475.6
557.2
465.0
475.9
528.5
484.3
665.2
476.7
946.8
478.7
487.8
484.8
480.2
499.9
484.2
471.3
524.0
474.5
469.3
784.0
463.2
466.5
665.7
729.3
537.4
538.3
262.9
980.1
528.0
467.9
469.3
581.6
655.7
618.0
533.3
982.8
483.4
772.3
1036.8
804.3
522.3
2.4
18.2
11.0
9.6
18.9
6.4
5.5
4.9
15.4
8.0
4.7
5.8
4.7
24.1
9.6
21.8
4.8
6.4
8.3
3.0
4.9
3.9
4.8
2.5
3.8
8.2
11.4
2.3
8.1
16.8
10.1
4.0
16.7
1.6
12.3
3.7
3.0
3.1
28.8
13.0
6.0
34.2
19.6
3.2
16.3
10.0
4.8
12.8
493.1
527.4
467.6
515.8
963.0
479.0
471.7
471.9
594.8
468.0
471.7
531.2
485.9
709.9
480.4
960.2
480.2
488.5
487.2
481.1
498.4
488.2
471.7
516.5
466.5
469.0
822.3
470.9
463.7
708.8
766.8
533.6
594.1
260.6
991.4
527.6
470.8
469.0
621.7
695.1
624.4
570.4
1005.5
479.5
807.7
1042.0
797.2
547.0
13.0
17.6
10.9
12.6
15.7
9.9
6.8
5.9
19.8
8.9
6.2
6.6
8.6
21.5
9.0
19.6
6.7
6.5
9.9
7.0
9.7
4.3
8.0
7.8
10.0
10.3
16.4
4.3
11.9
20.6
10.8
9.4
27.6
6.7
13.4
6.1
5.6
6.5
32.5
14.8
8.8
32.0
17.8
7.6
13.0
12.5
8.1
16.3
486.8
630.3
451.7
510.5
1031.9
487.4
464.1
454.4
740.8
482.6
451.1
542.9
493.4
853.8
497.9
991.2
487.6
491.9
498.7
485.7
491.6
506.8
474.1
483.5
426.9
467.7
927.3
508.4
449.9
847.8
877.6
517.1
813.4
240.6
1016.4
525.9
485.0
467.8
770.6
824.6
647.5
721.1
1055.3
460.7
906.6
1052.9
777.3
651.3
72.4
43.8
35.0
53.7
25.9
48.0
30.0
25.2
70.3
34.7
28.4
24.5
43.8
36.9
23.2
39.6
31.2
21.9
40.8
38.1
49.6
16.0
40.7
41.1
56.5
45.4
50.7
22.3
58.5
63.3
28.0
46.8
109.0
65.5
32.9
28.3
29.5
35.2
99.0
43.5
34.0
63.8
35.9
41.5
14.1
32.5
27.9
61.9
494.5
504.0
470.8
517.0
1031.9
477.3
473.3
475.6
557.2
465.0
475.9
528.5
484.3
665.2
476.7
991.2
478.7
487.8
484.8
480.2
499.9
484.2
471.3
524.0
474.5
469.3
927.3
463.2
466.5
665.7
729.3
537.4
538.3
262.9
1016.4
528.0
467.9
469.3
581.6
655.7
618.0
533.3
1055.3
483.4
772.3
1052.9
804.3
522.3
2.4
18.2
11.0
9.6
25.9
6.4
5.5
4.9
15.4
8.0
4.7
5.8
4.7
24.1
9.6
39.6
4.8
6.4
8.3
3.0
4.9
3.9
4.8
2.5
3.8
8.2
50.7
2.3
8.1
16.8
10.1
4.0
16.7
1.6
32.9
3.7
3.0
3.1
28.8
13.0
6.0
34.2
35.9
3.2
16.3
32.5
4.8
12.8
101.6
80.0
104.2
101.3
90.4
97.9
102.0
104.7
75.2
96.4
105.5
97.3
98.1
77.9
95.7
95.5
98.2
99.2
97.2
98.9
101.7
95.5
99.4
108.4
111.2
100.3
84.6
91.1
103.7
78.5
83.1
103.9
66.2
109.3
96.4
100.4
96.5
100.3
75.5
79.5
95.4
74.0
93.1
104.9
85.2
98.5
103.5
80.2
U 206Pb/
206Pb*/ Standard 207Pb*/ Standard 206Pb*/ Standard
Error 206Pb*/ Standard 207Pb*/ Standard 206Pb*/ Standard Best Age Standard Concentration
(ppm) 204Pb U/Th 207Pb* Error (%) 235U* Error (%) 238U Error (%) Correlation 238U* Error (Ma) 235U Error (Ma) 207Pb* Error (Ma) (Ma) Error (Ma)
(%)
TC3003
CARRAPA ET AL.: EASTERN CORDILLERA EXHUMATION
23 of 30
TC3003
050708‐AUNC‐66
050708‐AUNC‐67
050708‐AUNC‐68
050708‐AUNC‐69
050708‐AUNC‐70
050708‐AUNC‐71
050708‐AUNC‐72
050708‐AUNC‐73
050708‐AUNC‐74
050708‐AUNC‐75
050708‐AUNC‐76
050708‐AUNC‐77
050708‐AUNC‐78
050708‐AUNC‐79
050708‐AUNC‐80
050708‐AUNC‐81
050708‐AUNC‐82
050708‐AUNC‐83
050708‐AUNC‐84
050708‐AUNC‐85
050708‐AUNC‐86
050708‐AUNC‐87
050708‐AUNC‐88
050708‐AUNC‐90
050708‐AUNC‐91
050708‐AUNC‐92
050708‐AUNC‐93
050708‐AUNC‐94
050708‐AUNC‐95
050708‐AUNC‐96
050708‐AUNC‐97
050708‐AUNC‐98
050708‐AUNC‐99
050708‐AUNC‐100
SAMPLE 05.07.08‐B.Unc
050708BUNC‐1
050708BUNC‐2
050708BUNC‐4
050708BUNC‐5
050708BUNC‐6
050708BUNC‐8
050708BUNC‐9
050708BUNC‐10
050708BUNC‐11
050708BUNC‐12
050708BUNC‐13
050708BUNC‐14
050708BUNC‐17
Analysis
Table A1. (continued)
Isotope Ratios
Apparent Ages (Ma)
7354 2.5 17.7912
13110 4.0 17.0601
15368 7.2 18.1485
16520 7.1 17.6770
12174 2.5 16.7117
7484 1.7 16.4425
32092 2.9 14.4929
16164 9.7 17.8554
8832 5.6 17.8501
26196 9.7 13.4664
10454 6.7 17.5396
14218 4.4 15.4779
18578 8.4 17.8661
7628 4.7 16.8421
4310 12.5 16.7121
14566 4.4 17.4333
9334 2.5 17.3488
8812 3.0 17.6661
14904 17.2 15.8897
13436 5.5 17.9244
3402 1.8 17.2524
13846 2.9 17.7121
17668 1.6 17.7922
13994 1.5 17.6028
14078 8.7 17.4607
10700 6.0 17.5084
16312 11.6 17.5718
5792 6.0 17.6977
11290 8.4 17.5228
17756 4.0 14.6760
3380 1.4 16.1641
4998 8.8 17.8253
9214 7.0 17.3671
10922 2.2 17.1253
76935 7.0 7.7557
23646 23.7 17.6252
13419 1.8 17.2050
25272 7.7 17.5059
15780 4.5 17.6861
18144 2.5 13.6948
18042 3.3 17.9257
25203 10.2 17.5751
14259 5.3 17.6516
39015 6.3 16.7757
9942 1.7 17.1931
23364 15.3 17.7090
68982 8.7 12.8935
245
267
449
328
239
120
272
262
223
370
277
189
376
264
348
247
239
230
248
284
147
213
242
257
230
311
272
170
255
327
491
180
194
259
280
474
285
426
259
116
386
567
249
425
115
395
511
24 of 30
5.9310
0.5964
0.7082
0.6013
0.6056
1.7010
0.5862
0.5920
0.6011
0.7394
0.6756
0.5882
1.6904
0.5876
0.6770
0.5712
0.6221
0.7943
0.8759
1.2640
0.6198
0.5943
0.9627
0.5993
0.8868
0.5832
0.6761
0.6765
0.6056
0.6853
0.6129
0.7077
0.6041
0.6418
0.6392
0.6468
0.6206
0.6725
0.5941
0.6172
0.6020
0.6239
0.9992
0.6499
0.6158
0.6326
0.6716
2.2
1.1
1.9
1.4
1.7
2.8
3.0
1.8
2.2
2.0
2.4
1.5
1.7
3.6
3.3
3.1
1.8
3.0
5.3
3.4
2.2
2.4
6.8
2.8
3.3
3.4
5.3
3.4
5.2
6.2
2.9
4.2
2.9
6.5
4.1
3.7
2.2
4.5
2.6
4.0
3.3
2.5
3.7
4.9
4.4
2.9
3.0
0.3336
0.0762
0.0884
0.0763
0.0777
0.1690
0.0762
0.0755
0.0770
0.0900
0.0842
0.0756
0.1581
0.0758
0.0838
0.0752
0.0798
0.0963
0.1044
0.1329
0.0803
0.0769
0.0940
0.0762
0.0995
0.0756
0.0826
0.0820
0.0766
0.0862
0.0785
0.0816
0.0785
0.0803
0.0821
0.0835
0.0792
0.0852
0.0754
0.0787
0.0773
0.0793
0.1064
0.0762
0.0796
0.0797
0.0834
1.6
0.8
0.5
0.5
0.8
1.5
1.7
0.6
0.5
1.4
0.7
0.5
1.5
1.1
2.0
0.9
1.3
1.8
4.7
2.3
1.1
0.7
4.4
1.7
1.1
1.2
1.4
0.6
4.7
5.4
1.0
1.2
1.0
1.1
0.5
2.1
1.5
3.5
1.1
2.5
2.1
0.7
2.1
2.3
0.9
0.8
2.2
0.73
0.77
0.27
0.37
0.51
0.52
0.56
0.32
0.23
0.71
0.27
0.32
0.87
0.29
0.61
0.28
0.68
0.62
0.89
0.67
0.51
0.29
0.65
0.60
0.34
0.35
0.26
0.17
0.89
0.87
0.33
0.27
0.34
0.16
0.12
0.57
0.66
0.78
0.43
0.62
0.64
0.28
0.58
0.46
0.20
0.28
0.73
1855.9
473.6
545.9
474.3
482.3
1006.3
473.5
469.0
477.9
555.3
521.4
469.5
946.1
471.2
518.6
467.3
494.7
592.5
640.4
804.2
497.7
477.8
579.3
473.6
611.8
469.6
511.5
508.0
475.6
533.2
487.4
505.4
487.3
497.9
508.7
516.8
491.5
526.9
468.8
488.1
479.8
491.9
651.6
473.3
493.8
494.3
516.5
25.8
3.8
2.6
2.3
3.9
13.6
7.6
2.7
2.3
7.6
3.3
2.3
13.1
4.8
10.1
3.9
6.0
10.4
28.8
17.4
5.3
3.2
24.3
7.7
6.7
5.4
6.7
2.8
21.5
27.4
4.5
5.6
4.6
5.1
2.4
10.4
7.1
17.7
5.1
11.6
9.9
3.3
13.3
10.4
4.2
3.9
11.0
1965.8
475.0
543.7
478.1
480.8
1008.9
468.4
472.2
477.9
562.0
524.1
469.7
1004.9
469.4
525.0
458.8
491.2
593.6
638.7
829.7
489.7
473.6
684.7
476.8
644.6
466.5
524.4
524.7
480.8
530.0
485.4
543.4
479.8
503.4
501.8
506.5
490.2
522.2
473.5
488.1
478.5
492.3
703.4
508.4
487.2
497.7
521.7
18.9
4.1
7.9
5.2
6.3
17.8
11.3
6.9
8.3
8.7
10.0
5.8
10.9
13.5
13.6
11.4
7.1
13.3
25.1
19.5
8.4
9.1
33.8
10.8
15.8
12.8
21.7
14.1
20.1
25.5
11.1
17.6
11.1
26.0
16.3
14.6
8.7
18.2
9.9
15.3
12.8
9.8
18.6
19.7
17.0
11.3
12.3
2083.5
481.3
534.4
496.3
473.7
1014.5
443.8
487.6
478.0
589.4
535.9
470.9
1135.6
460.6
552.9
416.3
474.8
597.7
632.8
898.7
452.6
453.2
1048.5
492.1
761.6
451.2
580.9
597.7
505.5
516.1
476.2
706.0
444.0
528.4
470.5
460.4
484.1
502.0
496.0
488.0
472.2
494.2
872.7
669.5
456.3
513.8
544.5
26.0
15.1
39.3
28.0
31.5
48.1
55.5
38.5
46.8
30.8
51.2
32.4
16.7
76.4
57.2
66.3
29.5
50.5
51.7
52.9
41.4
50.8
104.3
49.9
65.7
71.0
111.0
73.3
51.8
67.4
60.2
85.8
60.8
141.7
90.6
66.5
37.2
60.9
52.4
68.3
56.9
52.9
61.5
93.4
95.5
60.6
44.9
2083.5
473.6
545.9
474.3
482.3
1014.5
473.5
469.0
477.9
555.3
521.4
469.5
1135.6
471.2
518.6
467.3
494.7
592.5
640.4
898.7
497.7
477.8
579.3
473.6
611.8
469.6
511.5
508.0
475.6
533.2
487.4
505.4
487.3
497.9
508.7
516.8
491.5
526.9
468.8
488.1
479.8
491.9
651.6
473.3
493.8
494.3
516.5
26.0
3.8
2.6
2.3
3.9
48.1
7.6
2.7
2.3
7.6
3.3
2.3
16.7
4.8
10.1
3.9
6.0
10.4
28.8
52.9
5.3
3.2
24.3
7.7
6.7
5.4
6.7
2.8
21.5
27.4
4.5
5.6
4.6
5.1
2.4
10.4
7.1
17.7
5.1
11.6
9.9
3.3
13.3
10.4
4.2
3.9
11.0
89.1
98.4
102.2
95.6
101.8
99.2
106.7
96.2
100.0
94.2
97.3
99.7
83.3
102.3
93.8
112.3
104.2
99.1
101.2
89.5
110.0
105.4
55.2
96.2
80.3
104.1
88.1
85.0
94.1
103.3
102.3
71.6
109.8
94.2
108.1
112.2
101.5
105.0
94.5
100.0
101.6
99.5
74.7
70.7
108.2
96.2
94.9
CARRAPA ET AL.: EASTERN CORDILLERA EXHUMATION
1.5
0.7
1.8
1.3
1.4
2.4
2.5
1.7
2.1
1.4
2.3
1.5
0.8
3.4
2.6
3.0
1.3
2.3
2.4
2.6
1.9
2.3
5.2
2.3
3.1
3.2
5.1
3.4
2.4
3.1
2.7
4.0
2.7
6.5
4.1
3.0
1.7
2.8
2.4
3.1
2.6
2.4
3.0
4.4
4.3
2.8
2.1
U 206Pb/
206Pb*/ Standard 207Pb*/ Standard 206Pb*/ Standard
Error 206Pb*/ Standard 207Pb*/ Standard 206Pb*/ Standard Best Age Standard Concentration
(ppm) 204Pb U/Th 207Pb* Error (%) 235U* Error (%) 238U Error (%) Correlation 238U* Error (Ma) 235U Error (Ma) 207Pb* Error (Ma) (Ma) Error (Ma)
(%)
TC3003
TC3003
050708BUNC‐18
050708BUNC‐19
050708BUNC‐20
050708BUNC‐21
050708BUNC‐22
050708BUNC‐23
050708BUNC‐24
050708BUNC‐25
050708BUNC‐28
050708BUNC‐29
050708BUNC‐30
050708BUNC‐32
050708BUNC‐33
050708BUNC‐34
050708BUNC‐35
050708BUNC‐36
050708BUNC‐37
050708BUNC‐39
050708BUNC‐40
050708BUNC‐41
050708BUNC‐42
050708BUNC‐43
050708BUNC‐44
050708BUNC‐45
050708BUNC‐47
050708BUNC‐48
050708BUNC‐49
050708BUNC‐50
050708BUNC‐51
050708BUNC‐52
050708BUNC‐53
050708BUNC‐55
050708BUNC‐57
050708BUNC‐58
050708BUNC‐59
050708BUNC‐60
050708BUNC‐61
050708BUNC‐62
050708BUNC‐63
050708BUNC‐64
050708BUNC‐66
050708BUNC‐67
050708BUNC‐68
050708BUNC‐69
050708BUNC‐70
050708BUNC‐71
050708BUNC‐72
050708BUNC‐73
Analysis
Table A1. (continued)
Isotope Ratios
Apparent Ages (Ma)
371
247
358
265
552
461
118
140
669
277
310
352
255
276
278
219
416
148
364
235
655
433
464
307
181
249
372
199
259
365
80
200
223
283
327
398
470
134
171
366
141
347
459
173
330
273
269
516
23505
8385
42780
17937
31956
45312
9879
14154
45591
20115
54408
31059
14148
22923
12243
33819
73827
474
24504
19575
34785
33759
32130
21573
26796
18105
25245
14964
24681
20349
4134
49281
22074
28218
33927
20967
33729
11487
37545
19380
12669
16803
37221
12864
30768
16164
18831
18051
6.9
1.0
2.1
6.4
11.8
3.1
0.9
2.4
13.4
2.5
5.3
37.0
0.9
3.9
3.4
2.1
2.2
1.8
8.9
6.5
8.3
3.2
29.7
5.2
1.4
2.2
2.4
4.4
2.3
7.0
0.8
1.1
5.7
4.1
9.2
1.7
12.2
2.5
1.1
12.4
1.8
2.2
2.0
2.3
4.5
5.7
3.5
1.5
17.7383
17.0144
9.1449
17.7017
17.6818
16.5630
16.4654
17.2091
17.5422
17.3680
13.6416
16.9001
17.0166
17.4886
15.7829
13.1724
13.3536
15.7071
17.5488
15.6843
16.9981
13.9348
17.7250
17.5269
8.5878
16.9366
17.2198
15.3176
13.7677
17.7178
17.2900
8.1528
14.9694
16.4449
16.3294
17.4177
17.5233
17.1414
8.8016
17.3965
16.8880
16.9923
17.1283
16.8862
14.6481
16.3230
16.8751
17.1880
1.4
1.8
1.7
1.8
1.0
1.4
2.5
1.7
1.6
1.5
1.3
1.7
1.1
1.0
7.4
1.5
1.0
164.6
1.6
2.5
2.1
1.6
1.0
1.3
1.7
0.7
1.1
3.9
1.6
1.3
2.6
1.8
5.6
1.3
1.1
1.6
1.6
1.5
2.6
2.4
1.6
2.1
0.8
1.8
1.4
2.3
3.0
3.8
0.5933
0.6357
3.3879
0.6074
0.6035
0.8586
0.8888
0.7114
0.6118
0.6773
1.7016
0.6792
0.6978
0.6217
0.7020
1.9420
1.8865
0.0187
0.6190
0.8490
0.6867
1.3740
0.6059
0.6215
3.8727
0.7120
0.6932
0.8101
1.4802
0.6053
0.6174
6.2483
0.8104
0.8296
0.7782
0.6601
0.6253
0.6766
4.3080
0.6234
0.7615
0.7346
0.7086
0.7274
1.0640
0.7456
0.7187
0.6302
2.8
1.9
2.1
2.0
1.1
1.6
2.8
1.8
1.6
1.7
1.8
2.8
1.6
1.4
7.5
1.6
1.1
164.8
1.6
2.8
3.1
2.3
1.5
1.5
4.8
1.1
1.2
6.6
3.4
1.4
2.7
1.9
9.2
1.4
1.8
1.8
1.8
1.5
8.0
3.2
1.8
2.9
1.0
1.9
3.3
2.4
3.2
3.9
0.0763
0.0784
0.2247
0.0780
0.0774
0.1031
0.1061
0.0888
0.0778
0.0853
0.1684
0.0833
0.0861
0.0789
0.0804
0.1855
0.1827
0.0021
0.0788
0.0966
0.0847
0.1389
0.0779
0.0790
0.2412
0.0875
0.0866
0.0900
0.1478
0.0778
0.0774
0.3695
0.0880
0.0989
0.0922
0.0834
0.0795
0.0841
0.2750
0.0787
0.0933
0.0905
0.0880
0.0891
0.1130
0.0883
0.0880
0.0786
2.5
0.5
1.2
0.9
0.5
0.8
1.3
0.6
0.5
0.7
1.3
2.3
1.1
1.0
1.2
0.5
0.5
8.0
0.5
1.2
2.4
1.7
1.2
0.8
4.5
0.8
0.5
5.4
3.1
0.5
0.7
0.5
7.2
0.5
1.4
0.9
0.8
0.5
7.6
2.2
0.9
2.1
0.6
0.5
3.0
0.5
1.1
0.5
0.88
0.27
0.58
0.43
0.44
0.50
0.47
0.32
0.31
0.43
0.69
0.81
0.70
0.70
0.16
0.31
0.48
0.05
0.33
0.43
0.75
0.72
0.77
0.52
0.93
0.74
0.42
0.81
0.89
0.37
0.27
0.26
0.79
0.36
0.77
0.46
0.45
0.33
0.94
0.66
0.48
0.70
0.57
0.27
0.90
0.21
0.35
0.13
474.2
486.8
1306.7
484.1
480.6
632.8
650.3
548.3
483.2
527.8
1003.0
515.5
532.5
489.3
498.3
1097.1
1081.7
13.7
488.9
594.3
523.9
838.2
483.5
490.2
1393.0
540.5
535.3
555.5
888.6
482.8
480.7
2026.9
543.6
608.2
568.3
516.3
492.9
520.7
1566.1
488.1
574.9
558.7
543.9
550.1
690.4
545.3
543.5
487.6
11.4
2.3
14.2
4.0
2.3
4.8
8.2
3.0
2.3
3.6
11.7
11.3
5.6
4.8
5.6
5.0
5.2
1.1
2.5
6.8
11.8
13.2
5.5
3.7
56.0
4.3
2.6
28.7
25.3
2.3
3.3
8.7
37.7
2.9
7.4
4.2
3.7
2.5
105.3
10.1
4.7
11.0
3.0
2.6
19.8
2.6
5.8
2.4
473.0
499.6
1501.6
481.9
479.5
629.3
645.7
545.5
484.7
525.1
1009.1
526.3
537.5
490.9
540.0
1095.7
1076.3
18.8
489.2
624.1
530.8
877.9
481.0
490.8
1608.0
545.9
534.7
602.5
922.3
480.6
488.2
2011.2
602.7
613.4
584.4
514.7
493.1
524.7
1694.9
492.0
574.9
559.2
543.9
555.0
735.8
565.7
549.9
496.2
10.7
7.4
16.2
7.5
4.4
7.4
13.6
7.5
6.3
6.9
11.6
11.6
6.5
5.6
31.3
10.7
7.2
30.8
6.4
12.8
13.0
13.6
5.9
5.9
38.7
4.7
5.0
30.2
20.8
5.2
10.3
16.5
41.6
6.4
7.9
7.5
6.9
6.3
66.1
12.6
7.8
12.6
4.3
8.0
17.5
10.2
13.4
15.1
467.2
558.7
1788.6
471.7
474.3
617.0
629.8
533.8
491.8
513.7
1022.4
573.4
558.4
498.5
720.3
1092.9
1065.5
730.6
490.9
733.6
560.8
979.2
468.8
493.7
1902.3
568.7
532.5
783.5
1003.7
469.7
523.6
1995.2
831.6
632.5
647.7
507.4
494.1
542.5
1858.0
510.1
575.0
561.5
544.2
575.2
876.7
648.5
576.6
536.6
30.2
39.3
30.6
39.1
22.9
29.4
54.1
36.7
34.4
33.7
26.5
36.4
24.3
22.4
156.6
30.4
19.3
948.5
34.2
52.6
45.6
32.6
21.8
28.4
31.3
16.3
23.7
81.4
32.1
27.8
56.2
32.4
117.1
28.1
24.5
36.1
34.9
31.7
47.5
53.1
34.0
45.4
18.1
39.0
29.4
49.4
64.4
83.6
474.2
486.8
1788.6
484.1
480.6
632.8
650.3
548.3
483.2
527.8
1022.4
515.5
532.5
489.3
498.3
1092.9
1065.5
13.7
488.9
594.3
523.9
979.2
483.5
490.2
1902.3
540.5
535.3
555.5
1003.7
482.8
480.7
1995.2
543.6
608.2
568.3
516.3
492.9
520.7
1858.0
488.1
574.9
558.7
543.9
550.1
690.4
545.3
543.5
487.6
11.4
2.3
30.6
4.0
2.3
4.8
8.2
3.0
2.3
3.6
26.5
11.3
5.6
4.8
5.6
30.4
19.3
1.1
2.5
6.8
11.8
32.6
5.5
3.7
31.3
4.3
2.6
28.7
32.1
2.3
3.3
32.4
37.7
2.9
7.4
4.2
3.7
2.5
47.5
10.1
4.7
11.0
3.0
2.6
19.8
2.6
5.8
2.4
101.5
87.1
73.1
102.6
101.3
102.5
103.2
102.7
98.3
102.7
98.1
89.9
95.4
98.2
69.2
100.4
101.5
1.9
99.6
81.0
93.4
85.6
103.1
99.3
73.2
95.0
100.5
70.9
88.5
102.8
91.8
101.6
65.4
96.2
87.7
101.8
99.8
96.0
84.3
95.7
100.0
99.5
99.9
95.6
78.7
84.1
94.3
90.9
U 206Pb/
206Pb*/ Standard 207Pb*/ Standard 206Pb*/ Standard
Error 206Pb*/ Standard 207Pb*/ Standard 206Pb*/ Standard Best Age Standard Concentration
(ppm) 204Pb U/Th 207Pb* Error (%) 235U* Error (%) 238U Error (%) Correlation 238U* Error (Ma) 235U Error (Ma) 207Pb* Error (Ma) (Ma) Error (Ma)
(%)
TC3003
CARRAPA ET AL.: EASTERN CORDILLERA EXHUMATION
25 of 30
TC3003
050708BUNC‐74
050708BUNC‐75
050708BUNC‐76
050708BUNC‐77
050708BUNC‐78
050708BUNC‐79
050708BUNC‐80
050708BUNC‐82
050708BUNC‐83
050708BUNC‐84
050708BUNC‐85
050708BUNC‐86
050708BUNC‐87
050708BUNC‐88
050708BUNC‐89
050708BUNC‐90
050708BUNC‐91
050708BUNC‐92
050708BUNC‐93
050708BUNC‐94
050708BUNC‐95
050708BUNC‐97
050708BUNC‐98
050708BUNC‐99
050708BUNC‐100
SAMPLE 04.27.08‐AngB
042708AngB‐1
042708AngB‐2
042708AngB‐3
042708AngB‐4
042708AngB‐5
042708AngB‐7
042708AngB‐9
042708AngB‐11
042708AngB‐12
042708AngB‐13
042708AngB‐14
042708AngB‐17
042708AngB‐19
042708AngB‐20
Analysis
Table A1. (continued)
Isotope Ratios
Apparent Ages (Ma)
17214
336
18984
22521
12054
26226
16080
25293
19248
10821
23430
29439
24180
24306
17742
23694
22773
8958
11916
14928
14994
12810
13593
66840
15225
20496
5986
9066
6376
13638
4840
12774
9692
19520
5768
10998
12830
6424
8162
395
486
192
302
218
483
321
290
200
339
378
242
349
184
187
335
313
125
423
182
252
297
427
166
182
401
122
166
125
356
112
297
206
383
144
230
236
99
185
26 of 30
6.0
2.3
2.1
1.7
3.6
1.9
3.4
2.6
2.1
4.6
5.8
2.8
1.7
3.3
11.7
1.4
1.0
7.6
6.2
4.9
7.7
10.6
2.7
1.3
10.1
1.4
9.1
15.0
6.4
10.2
2.5
1.2
19.3
1.4
4.5
1.7
1.7
1.1
2.3
16.5795
17.3517
17.6271
18.0159
17.9001
18.2966
18.1668
17.7293
17.4357
17.9697
15.5875
17.9512
17.9111
18.0274
17.5322
14.8943
16.1942
17.6298
17.8553
17.5287
17.5970
16.2117
14.5466
17.4906
17.4422
15.9145
17.7149
16.6221
17.2460
17.6640
17.6597
17.0775
16.4528
17.1228
15.6316
17.3148
16.9084
8.5669
17.2265
0.6
1.6
2.4
2.1
1.1
2.5
1.8
1.6
1.1
2.3
2.0
29.3
3.2
2.6
0.8
30.9
1.5
1.8
2.5
2.8
2.5
2.9
1.2
2.0
1.0
2.4
2.6
2.4
1.8
1.5
1.9
1.8
1.1
1.8
4.8
3.8
2.4
1.3
2.1
0.7653
0.6579
0.6529
0.5722
0.5744
0.5270
0.5169
0.5927
0.6660
0.5917
0.7536
0.6192
0.6199
0.6019
0.6216
0.0205
0.9417
0.5972
0.5877
0.5967
0.5971
0.7835
1.2359
0.6102
0.6212
0.9442
0.5862
0.7210
0.6964
0.5901
0.6064
0.7071
0.7474
0.6973
0.7380
0.6203
0.6288
5.4105
0.6797
1.8
1.7
3.0
2.4
1.4
2.7
1.8
2.1
2.1
3.3
2.3
29.3
3.5
2.6
1.7
31.1
1.6
1.9
2.7
3.5
3.2
5.3
1.3
2.5
1.2
2.7
2.7
2.5
1.8
1.6
2.0
1.9
1.3
1.8
5.2
3.8
2.5
1.5
2.2
0.0920
0.0828
0.0835
0.0748
0.0746
0.0699
0.0681
0.0762
0.0842
0.0771
0.0852
0.0806
0.0805
0.0787
0.0790
0.0022
0.1106
0.0764
0.0761
0.0759
0.0762
0.0921
0.1304
0.0774
0.0786
0.1090
0.0753
0.0869
0.0871
0.0756
0.0777
0.0876
0.0892
0.0866
0.0837
0.0779
0.0771
0.3362
0.0849
1.7
0.6
1.8
1.2
0.9
0.8
0.5
1.4
1.8
2.4
1.2
0.6
1.3
0.6
1.5
3.7
0.7
0.5
0.9
2.1
2.0
4.5
0.5
1.5
0.5
1.3
0.9
0.8
0.5
0.5
0.5
0.6
0.6
0.5
2.0
0.7
0.6
0.8
0.5
0.94
0.37
0.61
0.51
0.66
0.30
0.27
0.65
0.84
0.73
0.53
0.02
0.38
0.21
0.87
0.12
0.41
0.27
0.34
0.61
0.64
0.84
0.38
0.60
0.44
0.47
0.31
0.30
0.27
0.32
0.25
0.32
0.49
0.29
0.37
0.18
0.23
0.55
0.24
567.5
512.8
516.8
464.8
463.6
435.8
424.7
473.5
521.3
478.9
527.0
499.8
499.3
488.3
490.4
14.2
676.3
474.4
472.9
471.3
473.5
568.0
790.1
480.6
487.7
666.9
468.1
537.3
538.4
469.8
482.2
541.2
550.7
535.3
518.0
483.6
478.8
1868.2
525.4
9.2
3.2
8.9
5.4
4.2
3.4
2.1
6.3
8.9
11.2
6.2
2.9
6.4
2.6
6.9
0.5
4.4
2.3
4.2
9.7
9.2
24.5
3.7
7.0
2.4
8.0
3.8
3.9
2.6
2.4
2.3
3.2
3.3
2.8
9.7
3.3
2.6
13.5
2.6
577.1
513.3
510.3
459.4
460.9
429.8
423.1
472.6
518.3
472.0
570.3
489.4
489.8
478.4
490.9
20.6
673.8
475.5
469.4
475.1
475.4
587.5
817.0
483.7
490.6
675.1
468.4
551.2
536.6
470.9
481.3
543.0
566.7
537.1
561.2
490.1
495.3
1886.5
526.6
7.9
7.0
11.9
8.8
5.3
9.3
6.4
8.1
8.5
12.5
10.1
114.4
13.4
10.0
6.5
6.3
8.1
7.1
10.1
13.2
12.1
23.8
7.4
9.7
4.5
13.3
10.2
10.6
7.6
6.1
7.6
8.1
5.5
7.7
22.5
15.0
9.7
12.9
9.0
614.9
515.8
481.1
432.7
447.0
398.1
414.0
468.3
505.1
438.4
746.7
440.7
445.6
431.2
493.0
842.1
665.5
480.7
452.6
493.4
484.9
663.1
891.1
498.2
504.4
702.7
470.1
609.4
529.2
476.5
477.0
550.7
631.5
544.9
740.7
520.4
572.3
1906.7
531.7
13.4
35.5
52.3
46.0
23.9
57.0
39.6
35.9
25.1
50.4
41.3
665.6
71.0
57.4
17.9
658.2
32.0
39.6
56.1
60.7
54.3
61.5
25.4
44.6
22.8
50.7
57.0
51.3
38.5
33.7
42.5
39.9
23.5
38.6
102.5
83.0
52.4
22.6
46.8
567.5
512.8
516.8
464.8
463.6
435.8
424.7
473.5
521.3
478.9
527.0
499.8
499.3
488.3
490.4
14.2
676.3
474.4
472.9
471.3
473.5
568.0
790.1
480.6
487.7
666.9
468.1
537.3
538.4
469.8
482.2
541.2
550.7
535.3
518.0
483.6
478.8
1906.7
525.4
9.2
3.2
8.9
5.4
4.2
3.4
2.1
6.3
8.9
11.2
6.2
2.9
6.4
2.6
6.9
0.5
4.4
2.3
4.2
9.7
9.2
24.5
3.7
7.0
2.4
8.0
3.8
3.9
2.6
2.4
2.3
3.2
3.3
2.8
9.7
3.3
2.6
22.6
2.6
92.3
99.4
107.4
107.4
103.7
109.5
102.6
101.1
103.2
109.2
70.6
113.4
112.0
113.2
99.5
1.7
101.6
98.7
104.5
95.5
97.6
85.7
88.7
96.5
96.7
94.9
99.6
88.2
101.7
98.6
101.1
98.3
87.2
98.3
69.9
92.9
83.7
98.0
98.8
U 206Pb/
206Pb*/ Standard 207Pb*/ Standard 206Pb*/ Standard
Error 206Pb*/ Standard 207Pb*/ Standard 206Pb*/ Standard Best Age Standard Concentration
(%)
(ppm) 204Pb U/Th 207Pb* Error (%) 235U* Error (%) 238U Error (%) Correlation 238U* Error (Ma) 235U Error (Ma) 207Pb* Error (Ma) (Ma) Error (Ma)
TC3003
CARRAPA ET AL.: EASTERN CORDILLERA EXHUMATION
TC3003
TC3003
CARRAPA ET AL.: EASTERN CORDILLERA EXHUMATION
TC3003
Figure A1. Complete time‐temperature paths of six AHe samples modeled using the HeFTy computer
program. Time and temperature modeling constraints are outlined in blue, with gradual (2G) and episodic
(2E) modeling parameters indicated. Purple areas represent halos.
logic time, He retentivity is recognized as an evolving
property of apatite that varies as a function of [eU], especially in slowly cooled rocks [Shuster et al., 2006; Flowers
et al., 2009]. Because slow cooling allows more time for He
diffusion, it enhances the effect of radiation damage, and
thus the presence of a large spread in AHe dates that
correlate to [eU] within a single sample is an indication that
the sample spent and extended period of time in the HePRZ
[Flowers et al., 2007]. During radiogenic decay, emitted a
particles can travel up to 20 mm through the apatite crystal
lattice, and some are ejected from crystal altogether. For
grains <200 mm in radius, this leads to significant He loss
and anomalously young ages [Farley, 2002; Ehlers and
Farley, 2003].
27 of 30
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CARRAPA ET AL.: EASTERN CORDILLERA EXHUMATION
[71] Because a ejection is largely dependent on the size
and surface to volume ratio of the crystal [Reiners and
Farley, 2001], the dimensions and morphology of an apatite grain are routinely used with the Ft correction method to
calculate the amount of 4He lost by a ejection and correct
the AHe age [Farley et al., 1996]. In Figure A1 we present
the complete time‐temperature paths of six AHe samples
(see Figure 10) modeled using HeFTy [Ketcham, 2005], the
radiation trapping model of Shuster et al. [2006] (Do/a2),
and the static alpha ejection correction of Farley et al.
[1996].
[72] Acknowledgments. This project was made possible through
generous support from NSF EAR 0710724. The KU (U‐Th)/He laboratory
acknowledges support from NSF EAR 0414817. The Arizona LaserChron
Center is supported by NSF EAR 0443387 and NSF EAR 0732436. We
thank Peter DeCelles, George Gehrels, and Lindsay Schoenbohm for scientific inputs and Chris Krugh, Roman Kislitsyn, and Jamey Stutz for technical and field support.
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B. Carrapa, Department of Geosciences, University of Arizona, 1040 E.
4th St., Tucson, AZ 85721, USA.
D. F. Stockli, Department of Geology, Kansas University, 1475 Jayhawk
Blvd., Rm. 120, Lawrence, KS 66045, USA.
J. D. Trimble, Department of Geology and Geophysics, University of
Wyoming, 1000 E. University Ave., Laramie, WY 82071, USA.
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