00923 1st pages / page 1 of 17
Influence of pre-Andean crustal structure on Cenozoic thrust belt
kinematics and shortening magnitude: Northwestern Argentina
David M. Pearson1,2*, Paul Kapp1, Peter G. DeCelles1, Peter W. Reiners1, George E. Gehrels1, Mihai N. Ducea1,3,
and Alex Pullen1
1
Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721, USA
Department of Geosciences, Idaho State University, 921 South 8th Avenue, Pocatello, Idaho 83209, USA
3
Facultatea de Geologie Geofizica, Universitatea Bucuresti, Strada N. Balcescu Nr 1, Bucuresti, Romania
2
ABSTRACT
The retroarc fold-and-thrust belt of the
Central Andes exhibits major along-strike
variations in its pre-Cenozoic tectonic configuration. These variations have been proposed to explain the considerable southward
decrease in the observed magnitude of Cenozoic shortening. Regional mapping, a cross
section, and U-Pb and (U-Th)/He age dating
of apatite and zircon presented here build
upon the preexisting geological framework
for the region. At the latitude of the regional
transect (24–25°S), results demonstrate that
the thrust belt propagated in an overall eastward direction in three distinct pulses during
Cenozoic time. Each eastward jump in the
deformation front was apparently followed
by local westward deformation migration,
likely reflecting a subcritically tapered orogenic wedge. The first eastward jump was at
ca. 40 Ma, when deformation and exhumation were restricted to the western margin
of the Eastern Cordillera and eastern margin of the Puna Plateau. At 12–10 Ma, the
thrust front jumped ~75 km toward the east
to bypass the central portion of a horst block
of the Cretaceous Salta rift system, followed
by initiation of new faults in a subsystem
that propagated toward the west into this
preexisting structural high. During Pliocene
time, deformation again migrated >100 km
eastward to a Cretaceous synrift depocenter
in the Santa Bárbara Ranges. The sporadic
foreland-ward propagation documented here
may be common in basement-involved thrust
systems where inherited weaknesses due to
previous crustal deformation are preferentially reactivated during later shortening.
The minimum estimate for the magnitude of
*E-mail: pearson@isu.edu.
shortening at this latitude is ~142 km, which
is moderate in magnitude compared to the
250–350 km of shortening accommodated in
the retroarc thrust belt of southern Bolivia
to the north. This work supports previous
hypotheses that the magnitude of shortening decreases significantly along strike away
from a maximum in southern Bolivia, largely
as a result of the distribution of pre-Cenozoic basins that are able to accommodate a
large magnitude of thin-skinned shortening.
A major implication is that variations in the
pre-orogenic upper-crustal architecture can
influence the behavior of the continental lithosphere during later orogenesis, a result that
challenges geodynamic models that neglect
upper-plate heterogeneities.
INTRODUCTION
Cordilleran-style orogens form during convergence of oceanic and continental plates and
are characterized by long belts of continental
magmatism and shortening. An active example
of such an orogenic system is in South America,
where shortening of the overriding plate results
in continued growth of the Andes. In spite of the
documentation of major along-strike variations
in the style and magnitude of Cenozoic shortening in the Andes (e.g., Allmendinger et al., 1983;
Isacks, 1988; Kley and Monaldi, 1998; Kley
et al., 1999), there is not a considerable alongstrike difference in the relative convergence
velocity of the oceanic and continental plates
nor in the age of the subducting oceanic Nazca
plate (Oncken et al., 2006). In contrast, some
of the observed spatial variations in the style
and magnitude of Cenozoic retroarc shortening
match with changes in pre-Andean stratigraphy
and structure of the South American plate (e.g.,
Mpodozis and Ramos, 1989; Allmendinger and
Gubbels, 1996; Kley et al., 1999). For example,
in northernmost Argentina and Bolivia (Fig. 1;
17–23°S), Cenozoic thin-skinned shortening
within a thick Paleozoic basin exceeds 300 km
(Fig. 2; e.g., McQuarrie, 2002). Southwest of
Salta, Argentina (Fig. 1; ~25°S), where this
thick Paleozoic basin was absent prior to Cenozoic time, steeply dipping reverse faults that are
locally inverted normal faults are suggested to
have accommodated <100 km of shortening
(Fig. 2; e.g., Allmendinger et al., 1983; Grier
et al., 1991). Despite this large along-strike
variation in shortening magnitude and structural style, a corresponding major southward
decrease in elevation and crustal thickness does
not accompany this transition in the Central
Andes (e.g., Isacks, 1988), prompting speculation that magmatic addition, tectonic underplating, and/or crustal flow may have contributed
significantly to the crustal volume south of the
thin-skinned Bolivian fold-and-thrust belt (Kley
and Monaldi, 1998; Husson and Sempere, 2003;
Gerbault et al., 2005).
Although inversion of rift faults and the distribution of pre-orogenic basins have long been
suggested to influence the style of deformation
in the Central Andes (e.g., Allmendinger et al.,
1983), only recently have workers integrated
geo-thermochronological results with structural
analysis in southern Bolivia to show that the spatial extent of the Altiplano Plateau was largely
controlled by the distribution of Mesozoic rift
faults and was established by ca. 25 Ma (Sempere et al., 2002; Elger et al., 2005; Ege et al.,
2007; Barnes et al., 2008). However, the influence of these pre-Cenozoic heterogeneities in
influencing the kinematics of the thrust belt has
not been sufficiently investigated in northwestern Argentina, despite the observation that early
Andean deformation spatially correlates with
Cretaceous rift basins (Kley and Monaldi, 2002;
Carrera et al., 2006; Hongn et al., 2007; Insel
et al., 2012). One study at ~25.75°S, utilizing
Geosphere; December 2013; v. 9; no. 6; p. 1–17; doi:10.1130/GES00923.1; 11 figures; 1 table; 2 supplemental files.
Received 1 March 2013 ♦ Revision received 30 August 2013 ♦ Accepted 31 October 2013 ♦ Published online XX Month 2013
For permission to copy, contact editing@geosociety.org
© 2013 Geological Society of America
1
00923 1st pages / page 2 of 17
Pearson et al.
20°S
A
Bo
ent Punaliv
Alti
ina
plan ia
o
Eas
tern
Cor
dille
SB
ra
ran
ges
Su b
and
64°W
es
Fig. 1B location
Chil
e
21°S
25°S
Arg
68°W
21°S
B
22°S
ia
liv
Bo
Chi
le
Arg
enti
na
23°S
Lomas
edo
del Olm
24°S
QdT
Arch
67°W
Fig. 2
0
50
100 km
Major thrust faults
Cenozoic sediment
Cenozoic igneous rocks
Mesozoic sed. rocks
Mesozoic igneous rocks
Paleozoic sed. rocks
Paleozoic granitoids
Mostly Cambrian rocks
ange
ean
amp
Lurac atao R
sp
Tran
25°S
Salta
66°W
65°W
64°W
63°W
Figure 1. Reference maps of study area showing (A) locations of tectonomorphic provinces
(inset); and (B) geological map of southern Central Andes. Major along-strike changes in
exposed rocks and structural style are apparent. Abbreviations: QdT—Quebrada del Toro;
SB—Santa Bárbara Ranges.
studies, evaluating the importance of pre-orogenic crustal architecture (e.g., Allmendinger
et al., 1983; Allmendinger and Gubbels, 1996;
Kley et al., 1999) is critical for understanding
the main factors influencing structural style
relative to other models that largely neglect preexisting upper-plate heterogeneities and instead
implicate climate (e.g., Lamb and Davis, 2003;
Strecker et al., 2007), mantle dynamics (e.g.,
Russo and Silver, 1994; Sobolev and Babeyko,
2005; Schellart et al., 2007; Husson et al.,
2012), or buoyant anomalies within the downgoing plate (e.g., Jordan et al., 1983; Isacks,
1988; Ramos, 2009). Also, in spite of the
hypothesized importance of shallow subduction
beneath the Central Andes during Miocene time
(e.g., Ramos, 2009), few workers have evaluated the spatio-temporal correlation between the
kinematic history of the thrust belt and an eastward migration of retroarc magmatism thought
to indicate shallow subduction.
This paper focuses on an E-W transect across
the Eastern Cordillera tectonomorphic province
of the Andean retroarc thrust belt of northwestern Argentina (Fig. 1). Results presented here
provide new constraints on the style, timing,
kinematics, and magnitude of shortening of the
fold-and-thrust belt at ~24.75°S latitude. These
results: (1) indicate that the northwestern Argentine thrust belt was deformed above a W-dipping
décollement that transferred slip to a system of
E-dipping back thrusts; (2) constrain the timing
of eastward deformation propagation within the
Eastern Cordillera and suggest that the Cretaceous rift architecture influenced the evolution
of the thrust belt at this latitude; and (3) increase
the estimate of the magnitude of shortening at
this latitude, but they still suggest that significantly less shortening was accommodated south
of the thin-skinned Bolivian fold-and-thrust
belt. This work complements existing work and
underscores the importance of the preexisting
tectonic framework in controlling the spatial
distribution of shortening, particularly during
the nascent stages of thrust belt development.
This, in turn, strongly influenced the evolution
of the orogenic system.
GEOLOGICAL BACKGROUND
apatite thermochronometry of Cenozoic basin
strata that spatially correlate to a Cretaceous rift
basin, suggests that a lack of influence of preexisting structures on thrust belt propagation is
reflected by a progressive eastward migration
of Cenozoic exhumation (Carrapa et al., 2011).
Likewise, stratigraphic and detrital provenance
analyses within a fault-bounded basin in the
Eastern Cordillera at ~23.25°S record progressive eastward migration of the thrust belt and
coupled foreland basin system and imply a
2
lack of influence of older structures on Cenozoic thrust belt propagation (Siks and Horton,
2011). These and similar studies are focused
upon Cenozoic strata that reflect regional deposystems and evolving sediment source areas.
In contrast, the approach taken here involves
(U-Th)/He apatite and zircon analysis of reverse
fault hanging walls that were uplifted and
exhumed during fault displacement. In addition
to resolving the potential spatial heterogeneity
of thrust belt kinematics implied by these prior
Geosphere, December 2013
Tectonomorphic provinces of the central
Andean retroarc include, from west to east (Fig.
1A; Jordan et al., 1983): (1) the Puna Plateau, a
relatively low-relief, topographically high (average elevation ~4 km) region of internal drainage, where Paleogene thrust belt structures are
mostly buried by Cenozoic sedimentary and
volcanic rocks (this province is the southern
continuation of the broad, lower-relief Altiplano
Plateau of Bolivia); (2) the Eastern Cordillera,
00923 1st pages / page 3 of 17
Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude
South Latitude
28°
collapse of a Carboniferous to Permian mountain belt (e.g., Kay et al., 1989), back-arc extension linked to subduction along the western
margin of South America (e.g., Welsink et al.,
1995), or failed rifting related to opening of
the Atlantic Ocean (e.g., Grier et al., 1991). In
northwestern Argentina, up to 5.5 km of Cretaceous sediment, the Salta Group, were deposited in concomitant rift basins (Salfity and
Marquillas, 1994; Monaldi et al., 2008). Across
much of the transect, Cretaceous strata unconformably overlie the Puncoviscana Formation,
demonstrating that thick overlying Paleozoic
strata present in southern Bolivia were absent in
northern Argentina prior to Andean orogenesis
(Salfity and Marquillas, 1994).
Frontal Frontal belt
belt
+ E Cord.
/
an
do
Or mbri ks
a
c
o
C
./ r
rot de
op gra
Ne low-
Revised estimate
from this study
~150 km
“deficit”
Mz
Foreland Basin Fill
24°
B
Carb
.
Dev
Siluonian
rian /
Cam
brian
A
Upper
Cenozoic
20°
Total
shortening
estimate
Transpampean
Arch
Sierras
Pampeanas
Basement
“Predicted”
magnitude of
shortening
Precordillera
shortening
Triassic CuyoBolsones Basin
Cenozoic Thrust Belt Evolution
32°
0
5 km
0
50
100
150
200
250
300
350
Magnitude of shortening (km)
Figure 2. Along-strike variations in stratigraphy and previous estimates of the magnitude of retroarc shortening. (A) N-S stratigraphic
section across retroarc thrust belt, modified from Allmendinger and
Gubbels (1996). Mz—Mesozoic; Carb.—Carboniferous; Ordo—
Ordovician. (B) Predicted (assuming an initially 40-km-thick crust
and local isostatic compensation; Isacks, 1988; Kley and Monaldi,
1998) versus observed (Oncken et al., 2006) magnitudes of shortening in retroarc thrust belt of southern Central Andes. A revised
estimate of 142 km of shortening accommodated by the Eastern
Cordillera at ~24.75°S is the sum of current results and existing estimates for the Puna Plateau (Coutand et al., 2001) and Santa Bárbara Ranges (Kley and Monaldi, 2002).
a high-relief, topographically high (peak elevations >6000 m), externally drained Cenozoic
thrust belt with predominantly west-vergent
structures in Argentina that transition northward
into a bivergent system in Bolivia; and (3) the
Santa Bárbara Ranges, a region near the modern
deformation front that consists of mainly eastdipping reverse faults, transitioning along strike
northward to the Subandes, a W-dipping thinskinned thrust belt in northernmost Argentina
and southern Bolivia.
Paleozoic Architecture
The Paleozoic geology of western Bolivia consisted of >10 km of sedimentary rocks deposited
in a back-arc setting during Cambrian to Carboniferous time (Fig. 2; e.g., Sempere, 1995).
In northwestern Argentina, this Paleozoic basin
was shallower and more limited in extent (Fig. 2;
e.g., Starck, 1995; Egenhoff, 2007) and formed
on the northeastern flank of a NNW-trending
Paleozoic basement high (Transpampean arch;
Figs. 1B and 2A; Tankard et al., 1995), thought
to reflect a remnant Ordovician (“Ocloyic”)
mountain belt (Mon and Salfity, 1995; Starck,
1995). Poorly constrained Late Devonian to
Mississippian orogenesis from central Argentina to Peru (“Eohercynian/Chañic” orogenesis)
eroded the original margins of the Ordovician
to Carboniferous basin (Fig. 2A; Starck, 1995).
Although Ordovician rocks are prevalent on
the Puna Plateau west of this regional transect,
Ordovician to Devonian rocks are not exposed
in the Eastern Cordillera southwest of Quebrada
del Toro (Fig. 1B), indicating that this locality
may approximate the northeastern boundary of
major Paleozoic deformation.
Mesozoic Extension
Widespread but low-magnitude Mesozoic
extension affected much of western South
America and has been variously attributed to
Geosphere, December 2013
Major crustal shortening in the Central
Andes began after the South American plate
overrode the subduction zone during opening of the South Atlantic Ocean (e.g., Coney
and Evenchick, 1994). Deformation propagation has been sporadic through time, but most
authors agree that shortening in the central
Andean thrust belt began in Late Cretaceous
to early Eocene time in northern Chile (Sempere et al., 1997; Arriagada et al., 2006; Jordan et al., 2007) and propagated in an overall
eastward direction. In northwestern Argentina,
growth strata and apatite fission-track data
reflect 40–30 Ma deformation in the eastern
Puna Plateau and western Eastern Cordillera,
followed by an enhanced period of exhumation from 20 to 15 Ma (Andriessen and
Reutter, 1994; Coutand et al., 2001; Deeken
et al., 2006; Hongn et al., 2007; Carrapa and
DeCelles, 2008; Bosio et al., 2009; Carrapa
et al., 2011). A pre–15 Ma (Reynolds et al.,
2000), possibly Eocene angular unconformity
across the Santa Bárbara Ranges (Salfity et al.,
1993) may reflect an early phase of shortening
or the eastward migration of a flexural forebulge (DeCelles et al., 2011).
In contrast to the 40–15 Ma evolution of the
thrust belt, interpretations of the 15–0 Ma deformation history in northwestern Argentina vary
significantly (Carrapa et al., 2011; Hain et al.,
2011). Some authors use regional correlations
of sedimentary rocks interpreted in the context
of a flexural foreland basin to infer a progressive eastward migration of the thrust belt (e.g.,
DeCelles et al., 2011). Others suggest that an
initially intact flexural depositional system was
“broken” in mid- to late Miocene time as basement-involved reverse faults were initiated or
reactivated away from what was once a continuous, along-strike thrust front and foreland basin
(e.g., Hain et al., 2011).
3
Neoproterozoic to Middle Cambrian Puncoviscana and La Paya
Formations: variably metamorphosed turbidites
Geosphere, December 2013
45
40 Ma
A
116 km shortening (95 km in Eastern Cordillera + 21 km in SB Ranges)
belt in Eastern Cordillera. Section is thus semi-balanced
21 Lower detachment of Kley and Monaldi (2002) cropped for simplicity and consistency with thrust
seismic data (Kley and Monaldi, 2002)
20 Regional elevation of Cretaceous Balbuena Subgroup and deeper basin structure constrained by
19 Décollement depth constrained by thickness of Mojotoro fault hanging wall
antithetic fault that loses slip toward the south beneath the Lerma Valley.
18 This structure, with an open, S-plunging anticline in its footwall, is likely a lower-displacement
17 Here, a remnant of Cretaceous strata is truncated by the Lesser fault (new name)
along-strike, but accommodated less displacement.
16 The Quijano fault (new name) to the east apparently branches with the Pascha fault (new name)
10
5
0
5
3
A
4
Cachi
fault
42
1
pЄp
Og
2
13.8 Max
13.8 Ma
09DP301
77
5
6
7
Calchaquí
Valley
Cz
08DP081
57
Toro Muerto
fault
66.25° W
62
KTb
Cz
32
La Poma
50
60
Calchaquí
fault
38 Ma*x
38 Ma*
09DP42
85
pЄp
90
pЄp
61
5.2 Ma
66° W
7.3 ± 0.3 Ma
Cz
53
t
Cz
79
Q d a de
09DP46
pЄp
8
Capillas
fault
34
ul
Ca
pil
l as
KTb
50
42
Cz
60
9
Solá
fault
Gólgota
fault
10 11
12
20
13
14
47
12.1 Ma
65
7.1 Ma
83
65.5° W
ЄOr
15
16
17
Lesser
fault
9.1 ± 0.2 Ma
9.0 ± 0.1 Ma
12.8 ± 0.2 Ma
Cz
65
ЄOr
63
5.2 Ma
09DP15
29
10DP08
8.5 Ma
53
9.7 ± 0.2 Ma
37
QdT
65.75° W
70
QT42
6.3 ± 0.1 Ma
*81
ЄOr
Zamanca 4.4 ± 0.1 Ma
fault
4.2 Ma
09DP47
79
28
16
4.7 Ma
10DP07
53
18
19
Lerma
Valley
Lerma Valley
Salta
Cz
ЄOr
9.3 Ma
20 Ma
35
Cz
pЄp
50
?
10
65.25° W
Mojotoro
fault
21 Max
52 Max
58 Max
74 Max 10.4 ± 0.2 Ma
Єg
45
66
83
Cz
Cz
20
65° W
Cz
Lavayén Valley
Lavayén Valley
General Güemes
Cz
Cz
e
Rang
ЄOr
Kley
KTb
Cz
d
an
21
KTb
02
ЄOr
i, 20
ald
Mon
p
ЄOr
ma
64.75° W
ЄOr
KTb
KTb
Cz
ЄOr
KTb
Modified from Kley and Monaldi, 2002
Cz
Figure 3. Regional map and balanced cross section, with sample locations and (U-Th)/He apatite and zircon age results. Superscript references: 1—Pearson et al. (2012); 2—Adams et al. (2008). Map and cross section in the Santa Bárbara Ranges are modified from Kley and
Monaldi (2002); notably, we excluded their interpreted deeper décollement. See text for additional information. Abbreviations: QdT—
Quebrada del Toro; SB—Santa Bárbara Ranges.
strata in Quebrada del Toro (DZ samples 08DP01 and 08DP03).
* Growth
U-Pb tuff sample 08DP04 = 9.4 Ma
*All (U-Th)/He apatite grains from this sample have very low eU
Youngest (U-Th)/He age from sample
Weighted mean (U-Th)/He apatite ages at 2σ uncertainty; xSingle grain
10DP07
21 Ma
U-Pb detrital zircon sample locality
13.8 Ma
Cenozoic exhumation >
thermo-chronometer closure depth
Neogene exhumation <
thermo-chronometer closure depth
(U-Th)/He apatite age
Cz
Og
A
47
structurally higher Cretaceous strata
14 Displacement along the Pascha fault increases southward, with erosion of a W-verging
hanging-wall anticline to the south
15 Strata dip more steeply here than in hanging wall of Solá fault, suggesting that these structures
were rotated during later displacement on the Solá fault
13 Possible 5-10° angular unconformity between Cambro-Ordovician rocks and a sliver of
indicating a southward increase in along-strike displacement
12 Cambro-Ordovican strata and the W-verging anticline seen to the north are eroded farther south,
trending folds indicate minor pre-13 Ma shortening on the Gólgota fault and ~10 Ma above Solá
fault
11 Subtle growth strata in the footwall of the W-dipping Solá pop-up fault within a series of ~N-S
24.75° S
89
(U-Th)/He zircon age
Overturned anticline axial trace
Overturned syncline axial trace
Cross section
Monocline axial trace
Syncline axial trace
Anticline axial trace
Reverse faults juxtaposed planar, ~30° E-dipping back limbs against overturned footwall
synclines. Folding and faulting here likely represent deformation in the footwall of the E-dipping
Capillas fault (new name) whose displacement increases toward the north.
9 Cenozoic sediment of possible Plio-Pleistocene age buried the E-dipping Zamanca fault (new
name) that bounds the western margin of the Zamanca Range.
10 E-dipping Zamanca fault is dominant due to consistent along-strike hanging wall geometry;
therefore W-dipping structure is pop-up
likely a truncated fault-propagation fold.
7 W-verging, locally overturned syncline in footwall of the Calchaquí fault (Hongn et al., 2007) is
8
pЄp
09DP071
40 Ma1
Cach
i fault
09DP061
Og
36
A’
55
Bedding orientation
36
25 Overturned bedding
Bedding parallel cleavage,
facing unknown
an overturned footwall syncline along its trend. Folding of the Puncoviscana Formation is required
to overturn this contact and folds are interpreted as fault-propagation folds.
6 Structural relief here constrains restored depth/dip of décollement
5 The Toro Muerto fault consists of a ~100 m wide zone of intense strain that structurally overlies
Cz
Czb
51
Geo- and Thermochronology
30
Dip of fault (triangle) and trend and
plunge of striae on fault surface
(diamond)
Strike-slip fault with relative
motion
Reverse fault; teeth on
hanging wall
Lithologic contact
Solid where well located, dashed where poorly located, dotted where buried
Geological symbols
Cambrian granitoids
Єg
pЄp
Lower Ordovician granitoids
Mainly Upper Cambrian to Ordovician sedimentary rocks, with local
Silurian to Devonian in the eastern part of the map area
ЄOr
Og
3 Pirgua Group conglomerates in the hanging wall of the Cachi fault south of the transect are up to
3 km thick adjacent to their western fault boundary; their absence to the west indicates that this is
an inverted normal fault at the western boundary of the Salta Rift (Carrera et al., 2006).
4 Décollement dip constrained by structural relief in syncline in Calchaquí Valley (point 4)
2 E-dipping structure is dominant, therefore W-dipping structure is inferred antithetic fault
1 Eroded hanging wall geometry constrained by (U-Th)/He data (Pearson et al., 2012)
Cross section notes
Mainly Cretaceous rocks of Pirgua and Balbuena Subgroups
Elevation (km)
Cenozoic
Cachi Range
fault
rto
ue
M
To ro
64
57
Quaternary basalt flows
64
R a nge
Pasch
a
Range
Lesse
r
Cz
Quijano fault
toro R ange
M ojo
KTb
72
ang
e
ea
ar
64.5° W
ЄOr
a Ra
nge
ne l
Cen
ti
e
ng
Ra
e te
KTb
Cz
64.25° W
A’
Deformed
Cz
0
A’
Restored A’
25 km
No Vertical Exaggeration
Cz
Piq
u
Czb
a f a u lt
Lampasillos Range
85
an c
ult
a fa
sa d
Z am
Me
o t a f a ul t
í fault
chaqu
Cal
ge
G ól g
ult
Pasch a f a u lt
Calchaquí Valley
R an
Lesser fault
51
ul t
C
s
illa
ap
n ca
im R
C a p i ll as f a
ang
e
Zama
o
Tor
Mojotoro fa
a
Unch
del
nton
io R
la s
da
San
A
a de
brad
Qu e
bra
m
ap
áf
e
Qu
02
53
ldi
, 20
S ol
67
on
a
43
dM
Zapla
an
Rock Units
Kley
4
are
a
The oldest unit exposed in the retroarc of
the Central Andes is the Puncoviscana Formation, which consists of variably metamorphosed
siltstone, argillite, and turbiditic sandstone,
and it constitutes the majority of outcrop in the
mapped area (Fig. 3). In the Cachi Range, the
westernmost mountain range in this transect, the
Puncoviscana Formation exhibits a gradational
contact with the higher-metamorphic-grade La
Paya Formation (Galliski, 1983). These rocks
have Neoproterozoic to Cambrian protoliths
and are correlative (Pearson et al., 2012); for
this reason and for simplicity, these rocks are
collectively referred to as the Puncoviscana
Formation. In general, the Puncoviscana Formation exhibits finer grain size toward the west,
including outcrops of chert south of La Poma
(Fig. 3). Locally, however, metamorphic recrystallization has increased grain size. In contrast,
in the eastern Lampasillos Range and Quebrada
de las Capillas to the east (Fig. 3), the Puncoviscana Formation consists of 10–30-cm-thick
fine-grained quartzites that are interbedded with
5–100-cm-thick slate beds. Closer to Quebrada
del Toro to the east, these rocks alternate on the
kilometer scale with low-grade metapelitic rocks
characterized by 3–20-m-thick siltite and finegrained quartzite interspersed with slate. In the
eastern Lampasillos Range and Quebrada de las
Capillas, primary depositional features are common, including flute casts and ripple marks (Fig.
4A), and some rocks are volcaniclastic. In the
Lesser and Mojotoro Ranges (Fig. 3), the Puncoviscana Formation mainly consists of low-grade,
fine-grained quartzite and metapelite.
Northwest of the Quebrada del Toro (Figs.
1 and 3), 526–517 Ma plutons (Hongn et al.,
2010) intruded the Puncoviscana Formation. In
the Cachi Range, 485–483 Ma granitoids (Fig.
2; Pearson et al., 2012) also intruded the Puncoviscana Formation and are among the northernmost outcrops of the Famatinian magmatic arc.
East of Quebrada del Toro (Fig. 3), the Middle
to Upper Cambrian angular unconformity
between highly deformed rocks of the Puncoviscana Formation and overlying quartzite and shale
of the Upper Cambrian Mesón Group (Adams
et al., 2011) is exposed. Lower Ordovician shale
and quartzite of the Santa Victoria Group overlie the Cambrian Mesón Group. Together, these
rocks constitute the majority of the Pascha,
Lesser, and Mojotoro Ranges (Fig. 3).
Up to 2 km of synrift Cretaceous nonmarine conglomerate and sandstone of the Pirgua
Subgroup disconformably overlie Paleozoic
rocks in the Santa Bárbara Ranges (Salfity and
Marquillas, 1994; Kley and Monaldi, 2002).
In several localities, synrift depocenters corre-
Legend
00923 1st pages / page 4 of 17
Pearson et al.
00923 1st pages / page 5 of 17
Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude
~N
~S
or
F
cordierite
porphyroblasts
isca
na
Pu
nc
Ba
0
u
n
ti o
ov
lbu
en
aS
ub
gro
p
m
a
B
A
0
0.25 m
C
D
~5 m
W
Toro Muerto fault
E
n
Puncoviscana Formatio
overturned angular
unconformity
tion
rma
o
F
ite
ora
Yac
0
E
E
W
brian
Cam
for
eg
rou
n
Gólgota fault
Pre 12.8 Ma angular
unconformity
dc
ol
l uvi
um
Qls
co
v
~15 m
~NW
fo
n
0
F ~SE
~
a
en
u
b
Ba l
p
rou
g
b
Su
~10 m
0
30 cm
Cambrian
0
~400 m
fo r
egro
und
collu
vium
rm
ab
le
12
. 8 Ma
lava flo
wa
Ba
v
rre
ss
an
ds
ton
e
Figure 4. Outcrop photos. (A) Flute casts in Puncoviscana Formation turbidites in Quebrada de las Capillas; (B) major angular unconformity (>450 m.y.) between Puncoviscana Formation and Balbuena Subgroup rocks in the Quebrada de las Capillas; (C) irregular folding
of likely early Paleozoic age within Puncoviscana Formation in Lampasillos Range; (D) overturned syncline and angular unconformity in
the footwall of the Toro Muerto fault in the eastern Cachi Range; (E) likely fault-propagation fold in Balbuena Subgroup rocks south of La
Poma; and (F) overturned syncline in footwall of Gólgota fault and >13 Ma (K-Ar age; Mazzuoli et al., 2008) angular unconformity below
Barres sandstone demonstrating subtle early growth. Qls—Quaternary landslide.
Geosphere, December 2013
5
00923 1st pages / page 6 of 17
Pearson et al.
spond to Cenozoic reverse fault hanging walls
(Kley and Monaldi, 2002). Much of the mapped
region west of the Santa Bárbara Ranges represents the Salta-Jujuy High, considered to be
a horst block in the central part of the Salta
rift (Salfity and Marquillas, 1994). However, a
minor remnant of Pirgua Subgroup is exposed
near the Pascha Range (Salfity and Monaldi,
1998), and up to 3 km of strata are exposed in
the southern portion of the Cachi Range (Fig. 3;
Carrera et al., 2006). Overlying the Pirgua Subgroup and adjacent structural highs, there is a
thinner but more regionally contiguous package
of postrift Upper Cretaceous to Lower Eocene
sandstone, limestone, and shale of the Balbuena
and Santa Bárbara Subgroups (Salfity and Marquillas, 1994). In the Quebrada de las Capillas,
the depositional contact between previously
deformed Puncoviscana Formation and overlying Balbuena Subgroup is exposed (Figs. 3 and
4B). Here, the angular discordance is 45°–90°,
and overlying sandstone contains angular clasts
of Puncoviscana Formation quartzite; minor
fault slip has also occurred along the primarily
depositional contact.
Most workers attribute the accommodation
space within which Balbuena rocks were deposited to postrift thermal relaxation and associated
subsidence, an interpretation that is corroborated by the spatial coincidence of Paleogene
depocenters with Cretaceous grabens (e.g.,
Starck, 2011). Elsewhere, workers have interpreted the Paleocene to Miocene succession as
part of an eastward-advancing flexural foreland
basin system (e.g., DeCelles et al., 2011). It is
likely that the foreland basin related to growth
of the Andes interacted in a complex way with
waning thermal subsidence following Cretaceous extension.
The Paleocene–Lower Eocene fluvial and
lacustrine deposits are overlain regionally by
Middle-Upper Eocene paleosols that transition
across strike to a disconformity in the eastern
part of the Eastern Cordillera (Salfity et al.,
1993; DeCelles et al., 2011). In turn, the paleosols are overlain by 2–6 km of Upper Eocene to
Lower Miocene upward-coarsening fluvial and
eolian deposits, preserved within the current
transect at the eastern side of Quebrada de las
Capillas (Fig. 3), and capped locally by middle
Miocene to Pliocene fluvial, lacustrine, and
alluvial-fan deposits (Hernandez et al., 1996;
Starck, 1996; Reynolds et al., 2000, 2001; Echavarria et al., 2003; DeCelles et al., 2011).
In mid- to late Miocene time, retroarc magmatism migrated well east of the magmatic
arc, with associated volcanic centers defining
the NW-trending Calama–Olacapato–El Toro
lineament that crosses the current transect near
the Quebrada del Toro (Figs. 1 and 3; e.g., All-
6
mendinger et al., 1983). Although no volcanic
centers are exposed along the current transect,
tuff, volcaniclastic, and flow deposits of intermediate composition (Mazzuoli et al., 2008)
occur as interbeds in a dominantly clastic sedimentary succession near the northern Quebrada
del Toro; the Las Burras (Hongn et al., 2010)
and Acay (Petrinovic et al., 1999) plutons likely
represent the intrusive equivalents of these
rocks and are exposed just north of the current
transect. Miocene magmatism was followed by
eruption of shoshonites of likely Pleistocene age
near the northern part and eastern margin of the
Cachi Range (Fig. 3; Kay et al., 1994; Ducea
et al., 2013).
In the higher-elevation regions along the
transect, Pleistocene glacial deposits and landslides are abundant. Dark alluvial and landslide
deposits, also of likely Pleistocene age (Trauth
et al., 2000), unconformably blanket Cenozoic
rocks and range-bounding reverse faults in
many places at range margins. Terrace deposits of likely late Pleistocene age, in turn, overlie these sediments and are locally covered by
Holocene alluvium. For simplicity, all Cenozoic
sedimentary rocks are considered as one map
unit (Fig. 3).
STRUCTURAL GEOLOGY
Work presented here was conducted along an
~130-km-long E-W transect across the Eastern
Cordillera at 24.5–25°S latitude (Figs. 1 and
3). Field work involved geological mapping
and structural analysis, coupled with sample
collection for U-Pb and (U-Th)/He analysis of
detrital and igneous zircon and apatite. Much of
the field work was accomplished by multiday
foot traverses across high-elevation mountain
ranges for which minimal published data exist.
This regional transect was then linked with a
balanced cross section across the Santa Bárbara
Ranges to the east (Kley and Monaldi, 2002) as
well as thermochronological results and a balanced cross section across the Puna Plateau to
the west (Coutand et al., 2001). This work also
builds upon regional-scale mapping across the
Salta province (1:500,000 scale; Salfity and
Monaldi, 1998) and west of Quebrada de las
Capillas (1:250,000 scale; Blasco et al., 1996),
and more detailed mapping in the northern Calchaquí Valley (Hongn et al., 2007), Quebrada
del Toro (Marrett and Allmendinger, 1990), and
northern Cachi Range (Pearson et al., 2012).
Paleozoic Structure
Within the study area, cleavage intensity and
the grade of metamorphism increase to a maximum toward the west in the Cachi Range. Here,
Geosphere, December 2013
bedding is transposed, and rootless isoclinal
folds prohibit assessment of stratigraphic facing
direction beyond the outcrop scale. Although
open to tight chevron folds are common across
the transect (Fig. 4C), rocks generally are less
tightly folded toward the east. Bedding, bedding-parallel cleavage, and primary cleavage
within slate, phyllite, and quartzite generally dip
moderately to steeply to the northwest or southeast across the transect (Figs. 3 and 5), with the
exception of within the Quebrada de las Capillas
and southern Lesser Range, where foliations dip
toward the northeast or southwest. Within the
Cachi Range, fine-grained metamorphic minerals include ~1-mm-diameter anhedral cordierite that increases in size toward the core of
the range; farther south, correlative rocks were
subjected to granulite-facies metamorphism and
anatexis (Pearson et al., 2012). Much of this
deformation and metamorphism is thought to
be Cambrian and Ordovician in age (Mon and
Hongn, 1991; Mon and Salfity, 1995). N-S–
striking ductile shear zones, also likely Ordovician in age, have also been documented in
the Cachi Range (Pearson et al., 2012). East of
the Cachi Range, rocks underwent lower-grade
peak metamorphic conditions, and cordierite
porphyroblasts are rare to absent. Although the
orientations of bedding, bedding parallel cleavages, and primary cleavages are variable in orientation, on the scale of the transect, the Puncoviscana Formation usually dips more steeply
than younger strata and is generally NE-SW
striking and moderately to steeply NW-SE dipping (Fig. 5), an observation consistent with
regional measurements of Puncoviscana Formation rocks (Piñán-Llamas and Simpson, 2006).
Cenozoic Shortening
Prominent Cenozoic faults consist of two
types: (1) N-S–striking, mainly E-dipping reverse
faults that are expressed in the modern topography, juxtaposed rocks of markedly different age, and are locally demonstrably inverted
Cretaceous normal faults; and (2) mainly NWstriking sinistral faults with minimal displacement that are discontinuous and often en echelon. These near-vertical faults have likely been
active during Holocene time.
Major N-S–striking reverse faults generally
dip 45°–60° toward the east and are characterized by up to 200-m-wide zones of intense fracturing, with rare through-going fault surfaces or
fault gouge. In the footwalls of reverse faults
where Upper Cretaceous and Paleogene rocks
are preserved, overturned synclines are common, with steeply dipping axial surfaces that are
subparallel to superjacent reverse faults (Fig.
3 and 4D). Corresponding hanging-wall anti-
00923 1st pages / page 7 of 17
Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude
Upper Cretaceous and Cenozoic
Bedding (n=148)
Folds (n=7)
Cambrian and Ordovician
Bedding (n=104)
Folds (n=10)
Poles to planes
Trends/plunges of fold axes
Cylindrical best fit fold axis
Kamb contours
Contour interval = 2σ
Significance level = 3σ
Puncoviscana Formation
Folds (n=62)
Bedding, bedding ll cleavage,
primary cleavage (n=238)
brada del Toro in the footwall of the Solá fault.
Analytical details are available in the Supplemental File.1
Detrital zircon ages are shown on relative ageprobability diagrams (Fig. 6). In accord with
Dickinson and Gehrels (2009), we constrain
maximum depositional ages using the youngest
age cluster in a sample defined by three or more
overlapping analyses. For the igneous sample,
we attempt to minimize errors resulting from
inclusion of inappropriate analytical data in
age calculations by reporting a weighted mean
age (Ludwig, 2001) of concordant and overlapping 206Pb/238U ages, with final uncertainties
that include all random and systematic errors
(Fig. 7).
(U-Th)/He Apatite
Figure 5. Stereograms showing attitudes of sedimentary and metasedimentary rocks and
structures.
clines were also observed in Cambrian–Ordovician, Cretaceous, and Cenozoic rocks and are
consistent with fault-propagation folding being
a dominant structural style in the Eastern Cordillera at this latitude (Fig. 3E). The (U-Th)/He
zircon data obtained from the Cachi Range suggest that major antiforms in the hanging walls
of reverse faults, also interpreted to be faultpropagation folds but at a more regional scale,
accommodated the formation of up to 15 km of
structural relief (Pearson et al., 2012). See notes
in Figure 3 for descriptions of individual Cenozoic structures.
meter-scale offset of beds, brittle fault fabrics
and kinematic indicators, tool marks, and NEstriking, antithetic dextral faults. In the Quebrada del Toro, portions of the Solá and Gólgota
reverse faults (Fig. 3) that are coincident with
the El Toro lineament (Salfity et al., 1976) are
locally NW striking with horizontal slickensides, demonstrating that late strike-slip faulting
exploited preexisting contractional structures
(Marrett et al., 1994). Despite the prevalence
of these faults, none are through-going at the
regional scale, and the more significant of these
faults are only characterized by tens of meters of
displacement (Acocella et al., 2011).
Cenozoic Strike-Slip Faults
METHODS
NW-SE-striking lineaments are visible in
aerial imagery across the southern Central
Andes (Allmendinger et al., 1983) and are associated with some of the main retroarc magmatic
complexes (e.g., Riller et al., 2001). In some
cases, these lineaments are pre-Cenozoic in age
(Monaldi et al., 2008) and appear to segment
Cenozoic contractional structures (Coutand
et al., 2001). At the latitude of this study, these
NW-SE–trending lineaments were locally confirmed as sinistral strike-slip faults based upon
U-Pb Zircon
U-Pb zircon geochronology by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS), following methods
described by Gehrels et al. (2008), was applied
to eight detrital samples to better constrain provenance, ages of deposition, and deformation. A
tuff was also collected for U-Pb zircon analysis
to constrain the age of growth strata in the Que-
Geosphere, December 2013
Apatite (U-Th)/He thermochronology is used
here to constrain the timing and magnitude of
rock exhumation, which we infer resulted from
rock deformation. Our results supplement earlier work in the region (e.g., Coutand et al.,
2001; Deeken et al., 2006; Carrapa et al., 2011)
by placing ages of low-temperature thermochronometers in a structural context. (U-Th)/He thermochronometry of apatite generally reflects the
time since cooling of the apatite below ~70 °C
(assuming an effective grain radius of 60 µm
and a cooling rate of 10 °C/m.y.; Farley, 2000).
Using apatite fission-track ages from vertical
transects in the Cumbres de Luracatao (Fig. 1),
Deeken et al. (2006) obtained a Miocene geothermal gradient of ~18 °C/km. Using stratigraphic exhumation depths and lack of complete closure of the apatite fission-track system,
Coutand et al. (2006) calculated a similar value
of <18 °C/km for the Angastaco Basin ~100 km
to the south. This low geothermal gradient and
a mean annual surface temperature of 10 ± 5 °C
yield a closure depth of the (U-Th)/He system
in apatite of 3–4 km; a more conservative gradient for a foreland basin (~22 °C/km; Allen and
Allen, 1990) yields closure depths of 2–3 km.
Rock samples collected for (U-Th)/He apatite thermochronometry consist of quartzites of
the Puncoviscana Formation and Santa Victoria
Group, and small Cambrian and Ordovician
granitoids (two plutons in the Cachi and Mojotoro Ranges, and one dike in the Lampasillos
Range; Fig. 3) that are exposed in the hanging
walls of major reverse faults. Forty-nine individual apatites were dated from 11 samples (eight
1
Supplemental File. PDF file of analytical details
of U-Pb (zircon) geochronologic analyses. If you are
viewing the PDF of this paper or reading it offline,
please visit http://dx.doi.org/10.1130/GES00923.S1
or the full-text article on www.gsapubs.org to view
the Supplemental File.
7
00923 1st pages / page 8 of 17
Pearson et al.
0.0014
0.0010
MDA: 524 Ma
09DP15 (n=93)
MDA: 543 Ma
10DP08 (n=87)
MDA: 536 Ma
10DP07 (n=88)
MDA: 476 Ma
09DP47 (n=47)
MDA: 522 Ma
09DP46 (n=88)
MDA: 556 Ma*
09DP42 (n=11)
200
400
600
800
Cenozoic
08DP03 (n=85)
Ordovician
MDA: 15.7*
Neoproterozoic-Cambrian
Normalized probability
0
0.0002
0.00
08DP01 (n=73)
1000 1200 1400 1800 2200 2600 3000 3400 3800
Age (Ma)
Figure 6. Normalized probability plot of detrital zircon ages for samples collected from sedimentary rocks across the Eastern Cordillera from 24°S to 25°S. MDA—Maximum Depositional Age.
metasedimentary and three igneous); five grains
were dated for seven of the samples, whereas
four grains were dated from the other four samples (Table 1).
Balanced Cross Section
We constructed a restorable, area-balanced
cross section at ~24.75°S (Fig. 3) using the software LithoTect®. The main goal was to better
constrain shortening estimates at this latitude
and appraise along-strike heterogeneity in the
magnitude of retroarc shortening in the Central
Andes. Additionally, the results better constrain
the subsurface structure within the basement-
8
involved, locally inverted thrust system. The
method here utilized forward modeling and iterative restorations. The thrust belt at this latitude
involved previously deformed, strain-hardened
rocks that were subjected to pre-Cenozoic metamorphism, behave as mechanical basement, and
commonly deform into pop-up structures. For
these reasons, hanging-wall strain during fault
displacement was modeled using inclined shear
in an orientation antithetic to faults (e.g., Groshong, 1989).
The cross section is constrained by regional
mapping and structural analysis. Geometries
of thrust sheets where Cambrian, Ordovician,
Cretaceous, and Cenozoic rocks are exposed
Geosphere, December 2013
Age = 9.4 ± 0.4 Ma
Mean = 9.4 ± 0.3 Ma
MSWD = 2.1
(2σ)
0.0018
0.0006
MDA: 10.5 Ma
08DP04
age
/ 238U
206Pb*
Map unit
0.0022
17
15
13
11
9
7
0.04
207Pb*
0.08
0.12
/ 235U
Figure 7. Concordia plot and mean U-Pb zircon age of tuff within subtle growth strata in
Quebrada del Toro. 2σ error includes internal and external errors. MSWD—mean
square of weighted deviates.
are better constrained than in the Lampasillos
Range and intermittently along the Quebrada
de las Capillas where exposed rocks are dominantly the Puncoviscana Formation (Fig. 3).
Where possible, we used along-strike constraints
(i.e., down-plunge viewing) to reconstruct the
geometry and style of deformation. Eroded
hanging walls were also drawn with the minimal displacement required to satisfy available
observations, yielding “minimum” shortening
estimates. However, along-strike preservation
of likely breached fault-propagation folds east
of Quebrada de las Capillas suggests that current
shortening estimates there are not greatly underestimated (Fig. 3). Although rocks younger than
the Puncoviscana Formation are generally not
exposed in the Cachi Range, (U-Th)/He zircon
and apatite thermochronological results constrain the geometries of eroded rocks (Pearson
et al., 2012).
Marine-influenced carbonates of the Cretaceous Balbuena Subgroup were used as a
regional reference horizon and provide a preAndean datum with which to estimate the
minimum Cenozoic structural relief. The undeformed regional elevation of these rocks is constrained by interpretations of subsurface seismic
lines at the flanks of the Santa Bárbara Ranges
published by Kley and Monaldi (2002). Folds
and tilted strata in fault hanging walls suggest
shallowing fault dips in the subsurface (e.g.,
Grier et al., 1991; Kley and Monaldi, 2002; this
study). This requires that slip accommodated
on multiple faults at shallower crustal levels
is transferred at depth to fewer structures that
accommodate greater slip. This observation,
coupled with the lack of major structural relief
of basement rocks above their regional elevation, precludes the presence of deep décollements or whole-scale crustal faulting at this latitude. Although a two-décollement model, such
00923 1st pages / page 9 of 17
Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude
TABLE 1. (U-Th)/HE AGES FOR INDIVIDUAL APATITES
4
U
Th
Sm
eU
He
Mass
Half-width
Corrected age
Sample
(ppm)
(ppm)
(ppm)
(ppm)
(nmol/g)
(μg)
(μm)
FT*
(Ma)
09DP37 (24.644486°S, 66.29634°W); Ordovician granitoid
grain
09DP37_ap1
1.1
2.3
178.6
1.6
0.1
4.4
58.8
0.8
15.0
09DP37_ap2
6.6
6.9
284.7
8.2
0.5
3.6
52.7
0.7
13.8
09DP37_ap3
0.7
1.8
67.8
1.1
0.0
5.6
69.3
0.8
7.5
09DP37_ap4
1.9
3.6
105.6
2.7
0.3
4.7
64.0
0.8
23.2
09DP44 (24.804378°S, 66.151696°W); granitoid of likely Cambrian age
grain
09DP44_ap1
0.8
3.9
35.2
1.7
0.3
5.5
62.4
0.8
37.9
09DP44_ap2
2.2
4.4
49.7
3.3
0.7
1.6
45.9
0.7
53.5
09DP44_ap3
0.7
2.5
26.4
1.3
0.3
2.4
48.4
0.7
53.1
09DP44_ap4
1.5
5.2
44.7
2.7
0.5
1.5
40.0
0.7
49.4
09DP45 (24.714586°S, 66.048758°W); Lower Cambrian Puncoviscana Formation
grain
09DP45ap1
6.0
8.3
33.1
7.9
0.2
0.7
31.8
0.6
7.9
09DP45ap2
35.0
34.6
186.2
43.2
1.2
0.5
31.0
0.6
9.0
09DP45ap3
1.4
6.8
126.2
3.0
0.1
3.3
58.3
0.8
7.3
09DP45ap4
8.3
79.7
59.2
27.0
0.4
0.5
30.8
0.6
5.2
09DP45ap5
3.4
18.3
270.4
7.7
0.2
0.7
33.9
0.6
6.6
09DP47 (24.80377°S, 65.809525°W); Lower Cambrian Puncoviscana Formation
grain
09DP47ap1
80.1
67.5
296.0
96.0
1.7
5.9
63.7
0.8
4.2
09DP47ap2
2.6
9.1
365.9
4.7
7.9
9.0
79.7
0.8
342.7
09DP47ap3
6.3
27.7
533.5
12.8
0.7
1.4
42.4
0.7
14.3
09DP47ap4
24.9
72.1
177.6
41.9
0.7
1.2
43.0
0.7
4.7
09DP47ap5
3.9
20.9
438.9
8.8
1.2
0.6
34.0
0.6
42.2
10DP07 (24.536595°S, 65.756178°W); Ordovician Santa Victoria Group
grain
10DP07_ap1
9.6
43.1
307.2
19.8
0.5
1.7
42.5
0.7
7.2
10DP07_ap2
9.1
59.0
81.2
23.0
0.5
1.0
35.3
0.6
6.2
10DP07_ap3
8.4
24.6
295.6
14.2
0.5
4.6
56.5
0.7
8.2
10DP07_ap4
12.0
91.8
224.7
33.6
0.6
1.5
49.1
0.7
4.7
10DP08 (24.552606°S, 65.671056°W); Ordovician Santa Victoria Group
grain
10DP08ap1
12.9
11.5
96.0
15.6
0.5
1.0
34.9
0.6
10.6
10DP08ap2
27.1
7.0
158.2
28.8
1.1
0.6
31.8
0.6
12.3
10DP08ap3
14.8
13.1
560.0
17.8
0.6
0.9
36.5
0.6
9.1
10DP08ap4
19.2
56.6
224.2
32.5
0.9
0.5
31.3
0.6
9.1
10DP08ap5
18.6
10.7
244.8
21.1
0.6
1.0
33.1
0.6
8.5
09DP15 (24.670446°S, 65.64119°W); Ordovician Santa Victoria Group
grain
09DP15_ap1
2.5
2.4
1.0
3.0
31.2
5.3
59.7
0.8
2102.5
09DP15_ap2
6.1
19.5
321.8
10.6
0.2
3.5
59.6
0.8
5.2
09DP15_ap3
13.5
49.7
123.5
25.1
1.5
5.7
62.0
0.8
15.0
09DP15_ap4
23.0
9.0
92.0
25.1
1.5
3.6
63.0
0.8
14.1
10DP16 (24.766301°S, 65.602217°W); Lower Cambrian Puncoviscana Formation
grain
10DP16_ap1
3.7
7.9
115.0
5.6
0.2
10.5
82.8
0.8
8.5
10DP16_ap2
3.9
16.2
817.8
7.7
0.5
3.0
49.6
0.7
13.6
10DP16_ap3
4.0
5.1
175.9
5.2
0.2
7.4
72.6
0.8
10.6
10DP16_ap4
15.2
161.6
576.3
53.1
1.6
5.2
67.7
0.8
7.1
10DP16_ap5
3.8
24.5
57.9
9.6
0.6
2.1
48.5
0.7
15.4
10DP15 (24.800167°S, 65.563095°W); Lower Cambrian Puncoviscana Formation
grain
10DP15_ap1
14.2
15.9
91.7
17.9
0.8
1.6
41.0
0.7
12.1
10DP15_ap2
3.3
12.3
71.4
6.2
0.5
1.1
39.3
0.6
24.8
10DP15_ap3
11.3
21.5
289.2
16.4
0.9
6.7
64.9
0.8
12.7
10DP15_ap4
1.3
9.3
19.3
3.5
0.4
1.3
37.9
0.6
33.0
10DP15_ap5
6.7
20.1
358.4
11.5
0.6
0.8
34.4
0.6
16.4
11DP01 (24.796091°S, 65.359293°W); Cambrian granitoid
grain
11DP01_ap1
53.1
4.6
293.3
54.2
4.0
1.5
40.0
0.7
20.8
11DP01_ap2
36.3
5.9
326.2
37.7
8.5
2.0
49.3
0.7
58.2
11DP01_ap3
29.4
4.0
318.7
30.3
7.3
17.2
91.7
0.8
52.5
11DP01_ap4
28.2
6.1
239.9
29.6
10.0
10.4
90.7
0.8
73.6
11DP01_ap5
0.9
1.8
244.9
1.3
0.3
23.3
108.1
0.9
35.2
10DP17 (24.719877°S, 65.339544°W); Lower Cambrian Puncoviscana Formation
grain
10DP17_ap1
13.3
5.7
145.4
14.7
0.5
4.1
55.8
0.7
9.3
10DP17_ap2
16.4
29.6
284.0
23.4
1.0
2.5
47.5
0.7
11.3
10DP17_ap3
0.8
10.2
36.3
3.2
0.3
6.0
68.9
0.8
18.9
10DP17_ap4
4.4
27.7
521.2
10.9
0.5
1.7
44.9
0.7
10.8
10DP17_ap5
0.2
5.3
7.2
1.5
0.2
6.0
69.8
0.8
38.0
Note: 2σ represents formal analytical error of individual runs. Gray text: Analysis rejected on the basis of very low effective uranium (eU < 5 ppm).
*Alpha-ejection correction.
Geosphere, December 2013
±2σ
(Ma)
0.6
0.5
0.6
0.8
1.3
2.0
2.9
2.6
1.0
0.4
0.5
0.5
1.0
0.1
10.9
0.6
0.2
1.8
0.3
0.5
0.3
0.2
0.5
0.6
0.5
0.5
0.4
61.1
0.3
0.4
0.5
0.3
0.5
0.4
0.2
0.6
0.4
0.9
0.3
1.2
0.7
0.6
1.7
1.5
2.1
1.3
0.3
0.3
0.7
0.5
1.6
9
00923 1st pages / page 10 of 17
Pearson et al.
as Kley and Monaldi’s (2002) for the Santa Bárbara Ranges, could provide a better explanation
for local structures and deep seismicity (up to
~25 km; Cahill et al., 1992), our work suggests
that structural relief was accommodated above a
regional, shallowly W-dipping décollement.
The back limb of the hanging-wall anticline in
the Mojotoro Range is roughly concordant with
the Mojotoro fault, providing a good constraint on
the décollement at 9 ± 1 km depth there (Fig. 3).
The shallower and deeper uncertainty limits
represent, respectively, the décollement depth if
the hanging wall were to deform by flexural slip
and the uncertainty in dip of the Mojotoro fault,
which may dip more steeply than hanging-wall
strata (Fig. 2). A deeper décollement is incompatible with thrust sheet thicknesses within the
Lesser, Pascha, and Mojotoro Ranges. Planar
back limbs of these thrust sheets also suggest a
décollement depth of ~9 km. This décollement is
comparable to Kley and Monaldi’s (2002) shallower décollement determined independently for
the Santa Bárbara Ranges to the east. Structures
west of the Quebrada del Toro generally involve
thicker thrust sheets and require a deeper décollement at ~11 km. Given that Balbuena Subgroup
rocks at lower structural levels (e.g., in the Calchaquí Valley) appear minimally deformed, their
post-Cretaceous structural relief constrains the
displaced thickness of supra-décollement rocks,
yielding a regional décollement dip of ~2° west
of the Quebrada de las Capillas (Fig. 3).
RESULTS
tion west of the Quebrada del Toro (Figs. 3
and 6; Pearson et al., 2012; this study). These
Ordovician rocks also contain a prominent
Neoproterozoic population of grains similar to
the youngest age peak from a sample of Puncoviscana Formation collected by Adams et al.
(2008; sample QT4 in Fig. 3) in Quebrada del
Toro (Fig. 5). One detrital zircon sample collected from an outcrop of turbidites mapped as
Puncoviscana Formation in the Quebrada de las
Capillas (09DP47; Fig. 3; Salfity and Monaldi,
1998) yielded several grains with Ordovician
ages. Two of these Ordovician analyses are reasonably concordant and have acceptable errors
but do not define a robust population (Dickinson and Gehrels, 2009). We tentatively maintain
that these rocks are Puncoviscana Formation but
suggest that additional age evaluation of turbidites in this region is warranted.
Two detrital zircon samples and a tuff were
collected from the Miocene Agujas conglomerate (Marrett and Strecker, 2000) exposed in the
western part of Quebrada del Toro. These strata
occur in the core of a tight ~N-S–trending syncline in the footwall of the Solá and Gólgota
faults, and their deposition reflects structural
growth (Fig. 3; DeCelles et al., 2011). Consistent
with the results of DeCelles et al. (2011), a U-Pb
zircon age of a tuff from this section of 9.4 ±
1.6 Ma (Fig. 7) indicates late Miocene deposition (time scale used throughout this paper is
that of Ogg et al., 2008). This is >5 m.y. after
major exhumation in the Cachi Range (Pearson et al., 2012) that was concurrent with the
establishment of a topographic high and internal
drainage at the modern eastern margin of the
Puna Plateau (Vandervoort et al., 1995; Coutand
et al., 2006). Upper Cambrian and Ordovician
zircons dominate detrital zircon populations of
samples collected from the Agujas Conglomerate near this tuff (Fig. 6). Although recycling
cannot be ruled out, this suggests that their
sediment source during deposition was from
the east, given that the main source of Ordovician grains on the Puna Plateau to the west was
already hydrographically isolated.
(U-Th)/He Apatite
Forty-seven individual apatite grains analyzed by (U-Th)/He thermochronometry yielded
mostly Miocene and early Pliocene ages, with
a lesser number of Paleocene to Eocene dates
(Table 1; Fig. 7). Two other grains yielded
Proterozoic and Paleozoic ages, with low effective U (eU = U + 0.235Th) and Th concentrations, making it unlikely that these anomalously
old ages result from radiation damage that
enhanced He retentivity; instead, these old ages
may have been compromised by He implantation (Spiegel et al., 2009). Multiple grains that
generally form well-defined Upper Miocene to
Lower Pliocene age clusters are considered here
to represent recent exhumation and cooling of
rock samples (Table 1; Fig. 8).
Four Eocene (U-Th)/He apatite ages from
the Lampasillos Range may represent an early
signal of Cenozoic exhumation and cooling
U-Pb Zircon
Longitude (DD)
-66.5
-66.3
-66.1
10
-65.7
-65.5
-65.3
0
Westward younging
5
exhumation
10
15
20
25
30
35
40
45
50
55
(U-Th)/He apatite grain age 60
(U-Th)/He zircon grain age
2
Supplemental Table. Excel file of U-Pb (zircon)
geochronologic analyses. If you are viewing the PDF
of this paper or reading it offline, please visit http://
dx.doi.org/10.1130/GES00923.S2 or the full-text
article on www.gsapubs.org to view the Supplemental Table.
-65.9
Age (Ma)
U-Pb analyses of detrital zircons and a U-Pb
zircon age on a tuff help to constrain the provenance, timing of sediment source emergence,
and the age of deposition of sedimentary and
low-grade metasedimentary rocks in the region.
U-Pb zircon results are available in the Supplemental Table.2 For Puncoviscana Formation
rocks, the youngest zircon populations dominate and vary slightly in age (Fig. 6), indicating a continuous supply of young zircons during deposition, and suggesting that maximum
depositional ages obtained from these rocks
likely approximate depositional ages (Fig. 6).
Cambrian zircons also dominate detrital zircon
populations from Ordovician rocks of the Santa
Victoria Group; these ages are comparable to
those obtained from the Puncoviscana Forma-
65
Figure 8. (U-Th)/He apatite (this study) and zircon (Pearson et al.,
2012) ages (±2σ) versus longitude across the transect. Grayed ages
lie outside of clusters and are considered partially reset.
Geosphere, December 2013
00923 1st pages / page 11 of 17
Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude
and are consistent with subtle Eocene growth
strata documented in the Luracatao and Calchaquí Valleys (Bosio et al., 2009; Hongn et al.,
2007). Unfortunately, the quality of these ages
is questionable, given their very low eU concentrations; these and eight other analyses with
eU concentrations of <5 ppm are not considered
in the following age evaluations. Nonetheless,
several (U-Th)/He zircon grains in the northern
Cachi Range also yielded Eocene ages (Fig. 8;
Pearson et al., 2012), which may attest to significant Eocene deformation at this time.
A (U-Th)/He apatite sample collected within
the core of the Cachi Range yielded a grain
age of 13.8 ± 0.5 Ma (Fig. 3; weighted mean
ages henceforth), supplementing ca. 15 Ma
(U-Th)/He zircon (Pearson et al., 2012) and apatite fission-track ages (Deeken et al., 2006) collected farther south in the same range. After this
time, (U-Th)/He apatite results suggest that the
location of exhumation jumped ~75 km toward
the east to the Lesser, Mojotoro, Pascha, and
Zamanca Ranges, which record an ~60 km westward-younging progression of cooling between
12.8 and 4.4 Ma toward the Quebrada de las
Capillas (Figs. 3 and 8). Across strike ~15 km
west of the Zamanca fault, samples collected
from the immediate footwall of the Mesada fault
(this study) and ~40 km along strike of there in
the hanging wall of the Tin-Tin fault (Carrapa
et al., 2011) disrupt the westward-younging
trend, yielding (U-Th)/He apatite ages of ca.
7 Ma. From a regional perspective, these results
build upon and modify preexisting results in the
region and suggest a pulse of exhumation and
deformation at or since Eocene time within the
Luracatao Valley, Cachi Range, and Calchaquí
Valley, followed by a second pulse in exhumation at 15–10 Ma that occurred mainly in the
Cachi, Lesser, and Mojotoro Ranges. Exhumation then progressed ~60 km westward from the
Mojotoro and Lesser Ranges, with additional
widespread unroofing occurring at ca. 7 Ma in
the Quebrada de las Capillas and Lampasillos
Range (Fig. 3).
Balanced Cross Section
Total Cenozoic shortening across the Eastern
Cordillera from the area-balanced cross section is 95 km (45% over an E-W distance of
211 km; Fig. 3). The magnitude of shortening is
not grossly underestimated within the Mojotoro,
Lesser, Pascha, Zamanca, and Cachi Ranges
(Fig. 3). However, this estimate is likely to be
a minimum in the Quebrada de las Capillas and
Lampasillos Ranges given the lack of hangingwall strata to constrain deformed thrust sheet
geometries. Adding 95 km of shortening to areabalanced shortening estimates accommodated
by the Santa Bárbara Ranges toward the east
(21 km; Kley and Monaldi, 2002) and a rough,
line-length balanced shortening estimate of the
Puna Plateau southwest of the current transect at
~25°S (26 km; Coutand et al., 2001) results in
a total shortening magnitude of 142 km (26%).
This total encompasses the entire retroarc thrust
belt east of the modern magmatic arc in northern
Chile. Additional Cretaceous to Paleogene shortening in northern Chile (Arriagada et al., 2006;
Jordan et al., 2007) would increase this estimate.
Although this work was focused in the Eastern Cordillera, field observations and limited
mapping in the eastern Puna Plateau also suggest that the existing estimate of shortening
accommodated there may be greatly underestimated. Neogene volcanic and sedimentary rocks
buried many structures that are likely Cenozoic
in age; where exposed, Ordovician rocks clearly
accommodated major shortening. Although
some of this shortening likely occurred during
Paleozoic time, apatite fission-track results and
subsurface seismic data from the eastern margin of the Puna Plateau demonstrate that significant Cenozoic shortening and exhumation
occurred locally (Coutand et al., 2001; Carrapa
et al., 2005). If the Puna Plateau accommodated
shortening equivalent to the Eastern Cordillera
(45%), the predicted total shortening across
the plateau is 90 km, which would increase the
retroarc estimate to 206 km. However, this is
still ~85 km less than predicted by mass balance
estimates that assume an initially 40-km-thick
crust and local isostatic compensation (Fig. 2;
Isacks, 1988; Kley and Monaldi, 1998).
Time-averaged shortening rates using existing
estimates (Coutand et al., 2001) and the balanced
cross section yield a shortening rate of 1.9 mm/yr
from 40 to 12 Ma for the entire thrust belt at
24–25°S (Fig. 9), which is probably a minimum
value given that shortening within the Puna Plateau is likely underestimated. This was followed
by shortening at a rate of 6.5 mm/yr in the Eastern
Cordillera from 12 to 4 Ma, a sharp increase that
occurred after the location of shortening jumped
~75 km eastward to the Mojotoro, Lesser, Pascha,
and Zamanca Ranges and Quebrada de las Capillas. A final episode of shortening at a rate of 5.3
mm/yr occurred within the Santa Bárbara Ranges
from 4 to 0 Ma (Kley and Monaldi, 2002). The
resultant time-averaged, long-term shortening
rate since ca. 40 Ma is 3.6 mm/yr.
DISCUSSION
Timing and Kinematics of Shortening
Coupled with previously published constraints, results presented here (Figs. 9 and 10)
corroborate earlier work suggesting an overall
Geosphere, December 2013
eastward migration of the fold-and-thrust belt
during Cenozoic time, with additional local
westward propagation into lesser-deformed
regions during times of inferred subcritical orogenic wedge taper. Cretaceous to Eocene growth
structures and exhumation in what is now the
forearc of northern Chile record early stages
of Cenozoic shortening near the latitude of this
study (Maksaev and Zentilli, 1999; Arriagada
et al., 2006; Jordan et al., 2007). In northwestern
Argentina, however, Cretaceous and Paleocene
time marks a period of thermal subsidence that
reflects waning Cretaceous rifting (e.g., Starck,
2011). By late Eocene time, contractional deformation and exhumation had begun in the eastern
Puna Plateau (Coutand et al., 2001; Carrapa and
DeCelles, 2008) and westernmost Eastern Cordillera (e.g., Deeken et al., 2006). Constraints on
the eastern limit of observed Eocene exhumation
are limited by data quality, but Eocene exhumation may be recorded by samples collected
from the Luracatao and Cachi Ranges (Figs. 1
and 3; Deeken et al., 2006; Pearson et al., 2012;
this study), which are prominent topographic
features that mark the western boundary of the
Cretaceous Salta rift. Eocene growth strata in
intervening valleys may also reflect deformation in the western Eastern Cordillera at this time
(Hongn et al., 2007; Bosio et al., 2009). Deformation and exhumation within the eastern Puna
Plateau continued during late Eocene to early
Oligocene time (Coutand et al., 2001), progressing westward into the interior of the Puna Plateau
into the late Oligocene (Carrapa et al., 2005), as
occurred in the Altiplano Plateau to the north
(Fig. 10; e.g., Elger et al., 2005). From 20 to
15 Ma, rocks in the Luracatao and Cachi Ranges
in the western Eastern Cordillera record another
period of major exhumation (≥8 km locally;
Deeken et al., 2006; Pearson et al., 2012); by this
time, the modern eastern extent of the Puna Plateau was established (Vandervoort et al., 1995).
Exhumation continued within the Cachi Range
until <13.8 Ma (this study).
At 12–10 Ma, results presented here suggest
that the deformation front shifted ~75 km eastward to the E-dipping Lesser and W-dipping
Mojotoro faults (Figs. 3, 9, and 10), which are
exposed within the eastern portion of the SaltaJujuy High of the Cretaceous rift system (Salfity
and Marquillas, 1994). The Mojotoro Range is
the northern continuation of the Metán Range,
which is also bounded by a major W-dipping
fault and spatially correlates with a >3-km-thick
Cretaceous synrift depocenter (Salfity and Marquillas, 1994). Thus, ca. 10 Ma (U-Th)/He apatite ages from the Mojotoro Range (this study)
support a proposed early phase of deformation
of the same age in the Metán Range (Cristallini et al., 1997; Hain et al., 2011). Structural
11
12
Growth strata
~40 Mag
Geosphere, December 2013
70
15.7
(U-Th)/He zircon
6
0
6
2
4
6
Elevation (km)
g
40
15
13.8
70
f
b
c
Supercritical
wedge
12.1
4.2 4.7 5
5.2
7.1
1
8.5 7
Salta
Greatest exhumation at
or since 5–4 Ma from
(U-Th)/He apatite ages
~12–8 Ma (U-Th)/He
apatite ages
Quebrada de las Quebrada
del Toro
Capillas
5.2 5.7
37
Cachi Range Calchaquí
Valley
Younging (U-Th)/He
apatite ages
21
9.3
e
Santa Bárbara Ranges
0
modified from Kley and Monaldi, 2002
Eroded
section
50 km
No Vertical Exaggeration
Major deformation <4 Mad
Pliocene to Recent (4–0 Ma)
Supercritical wedge
Active thrust faulting
Lavayén Valley
Active thrust faulting
Mid to late Miocene (12–4 Ma)
Late Eocene (~40 Ma)
Figure 9. Proposed kinematic history of the Eastern Cordillera and Santa Bárbara Ranges at 24–25°S. Reference abbreviations: a—Pearson et al. (2012); b—DeCelles et al. (2011); c—Carrapa et al. (2011); d—Kley and Monaldi (2002); e—Hain et al. (2011), Cristallini et al.
(1997); f—Hongn et al. (2007); g—Bosio et al. (2009).
Eocene
Mid to late Miocene
Pliocene to Recent
Generalized deformation
Growth strata
37
9.3
Not reset
Age constraints
(U-Th)/He apatite
Mainly Upper Cambrian to Ordovician
sedimentary rocks, with local Silurian to
Devonian toward the east
Neoproterozoic to Middle
Cambrian rocks
Cretaceous syn-rift rocks
of the Pirgua Subgroup
Cretaceous rocks of the
Balbuena Subgroup
Cenozoic
~15–13 Ma
(U-Th)/He apatite
and zircon agesa
Eocene Growth strata
Eocene
(U-Th)/He ~40 Maf
(U-Th)/He
zircon agesa
apatite ages
Elevation (km)
4X Vertical Exaggeration
Late Cretaceous – early Cenozoic
00923 1st pages / page 12 of 17
Pearson et al.
00923 1st pages / page 13 of 17
Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude
35n
Chil
e
Arge
ntin
a
10
00
15j
Subcritical
40a
4
10
0m
1
>15?g
14k
0m
00
10l
2
agation
6k
3k
10
Subcritical
10
0m
R i ft
22i
14-3 Ma prop
4?
nd
0m
40c
al
<4 Ma jump
~10 Ma jump
25°0′0″S
Subcritic
00
Salta-Jujuy
High
0m
40b
~40 Ma jump
00 m
2
0 00 m
38e
67°0′0″W
ary
m
ary
1000 m
30d
26°0′0″S
d
un
30e
24°0′0″S
29h
Ri f t b o
bo
u
l
a
ivi
00 m
Bo
23°0′0″S
40f
Age of contractional deformation
30e Datapoint 22–15 Ma 4–0 Ma
~10 Ma
0 Ma
8–4 Ma
Eocene-Oligocene deformation
Extent of
1000 m
Cretaceous rift and
1000-m isopachs
66°0′0″W
65°0′0″W
64°0′0″W
Figure 10. Location of Cenozoic deformation and exhumation in northwestern Argentina
closely correlates with Cretaceous synrift depocenters, as shown by isopachs of Pirgua Subgroup (1000 m contour interval; Sabino, 2002). Reference abbreviations: a—Hongn et al.
(2007); b—Carrapa and DeCelles (2008); c—Bosio et al. (2009); d—Andriessen and Reutter
(1994); e—Coutand et al. (2001); f—Ege et al. (2007); g—Salfity et al. (1993); h—Carrapa
et al. (2005); i—Deeken et al. (2006); j—Pearson et al. (2012); k—Carrapa et al. (2011);
l—Hain et al. (2011), Cristallini et al. (1997); m—Cahill et al. (1992); n—Insel et al. (2012).
Black arrows indicate rapid eastward jumps in the location of the thrust front, which
occurred at ca. 40 Ma, ca. 10 Ma, and <4 Ma. Gray arrows show local westward-propagating deformation that is the likely expression of a subcritically tapered orogenic wedge.
growth during deposition of the Agujas Conglomerate (10.5–9 Ma) also occurred within the
Quebrada del Toro to the west (Fig. 3; DeCelles
et al., 2011; this study), indicating that faulting
occurred over a >50 km width during this time.
After initiation of significant exhumation
above the Lesser and Mojotoro faults, deformation propagated progressively westward
into the Salta-Jujuy High, within the E-dipping
fault subsystem beneath the Lesser, Pascha, and
Zamanca Ranges. A westward migration of
deformation is supported by westward-younging (U-Th)/He apatite ages (Fig. 8) and westward shallowing of thrust sheet dips toward the
Quebrada del Toro that record rotation of previously deformed rocks in the hanging walls of
the younger, western faults. A >12.8 Ma (ande-
site K-Ar age; Mazzuoli et al., 2008) angular
unconformity below the Barres sandstone in the
footwall syncline of the Gólgota fault (Fig. 4F)
and ca. 10 Ma growth strata above the W-dipping Solá fault (Fig. 7) attest to an early phase
of contractional deformation here. However,
(U-Th)/He apatite results from hanging-wall
rocks structurally above these localities and
abundant Ordovician zircon grains likely originating from the east indicate that most exhumation did not occur until after 6–4 Ma, which
coincides with the timing of exhumation associated with the Mesada and Tin-Tin faults to the
west (Carrapa et al., 2011; this study).
Poor exposure and a lack of suitable rocks for
thermochronometry inhibit assessment of the
age of deformation in the Santa Bárbara Ranges,
Geosphere, December 2013
which are the easternmost portion of the thrust
belt at this latitude. Existing constraints suggest
that a pre–15 Ma unconformity in the Santa Bárbara Ranges may represent an earlier phase of
deformation (Salfity et al., 1993) or the passage
of a flexural forebulge (DeCelles et al., 2011),
followed by the main period of post–9 Ma
(Reynolds et al., 2000), likely Pliocene uplift
(e.g., Kley and Monaldi, 2002). This is indicative of another (>100 km) eastward jump in the
location of deformation. The structural similarity between the Santa Bárbara and Zapla Ranges
and the Mojotoro, Lesser, Pascha, and Zamanca
Ranges to the west is intriguing: Both areas
are bounded on the east by a major, W-dipping
structure, and the latter region records a westward migration of exhumation toward the lesserdeformed Quebrada del Toro, where recent
deformation has been documented (Hilley and
Strecker, 2005). Progressive rotation of faults
depicted by Kley and Monaldi (2002) also hints
at a westward migration of deformation from
the Piquete and Centinela synrift depocenters
in the Santa Bárbara Ranges westward toward
the minimally deformed Lavayén Valley, where
active seismicity is focused within the growing
Zapla Range (Cahill et al., 1992). This pattern of
local migration of fault subsystems away from
the foreland may be common during inversion
of rift systems, as preexisting bivergent faults
are reactivated, followed by new faults formed
in footwalls of inverted structures as the subcritical orogenic wedge gains taper (Fig. 11).
A sporadic eastward migration of deformation interspersed with local, in-sequence westward-migrating faulting is a scenario that differs markedly from that encountered ~100 km
to the south (Carrapa et al., 2011). There, Carrapa et al. (2011) documented a progressive
eastward migration of deformation. The discrepancy in results may reflect the influence of
Cretaceous
rift faults?
Figure 11. Cartoon showing hypothesized
reactivation of E-dipping Cretaceous normal
faults, followed by local westward migration
of shortening.
13
00923 1st pages / page 14 of 17
Pearson et al.
preexisting Salta rift architecture. To the south,
mainly E-dipping, preexisting Cretaceous faults
(Grier et al., 1991; Cristallini et al., 1997) were
progressively inverted in front of the orogenic
wedge in a roughly continuous E-W rift basin.
In contrast, at 24–25°S, a lack of inversionprone Cretaceous rift faults within the central
portion of the horst block may have promoted
an eastward jump in the location of deformation
(Figs. 10 and 11). The original configuration of
the rift also likely influenced the topographic
expression of mountains across the Eastern
Cordillera. South of the present transect, where
the rift basin is better developed, W-dipping,
antithetic pop-up structures are less common;
instead, rocks were deformed within a major
eastward-propagating back-thrust belt, with
subdued topography east of Cachi reflecting
strain accommodation by primarily E-dipping
structures. In contrast, at the latitude of the
Salta-Jujuy High, several W-dipping pop-up
structures form sharp topographic boundaries
on the eastern margins of ranges (Fig. 3).
Geodynamic Model
Some workers have suggested that the segmented, “broken” nature of the Laramide and
Sierras Pampeanas forelands reflects basement deformation that occurs during shallow
subduction (e.g., Dickinson and Snyder, 1978;
Jordan and Allmendinger, 1986). An eastward
sweep of magmatism across the Altiplano-Puna
Plateau from 25 to 15 Ma (e.g., Allmendinger
et al., 1997) has led some researchers to suggest
that southward-migrating shallow subduction
occurred beneath much of the Central Andes
during this time, presumably associated with
oblique subduction of the Juan Fernández Ridge
(Yañez et al., 2001). In the literature, shallow
subduction is generally thought to cause magmatic lulls; the conventional model suggests
that retroarc magmatism signals steepening of
the subducting slab that follows shallow subduction (e.g., Dickinson and Snyder, 1978).
However, retroarc magmatism occurred northwest of the Quebrada del Toro at ca. 15 Ma
(Hongn et al., 2010), which predates by ~5 m.y.
the interpreted location of the Juan Fernández
Ridge beneath 24–25°S (Yañez et al., 2001) and
the ~75 km eastward jump in the deformation
front by the Eastern Cordillera. Similarly, in the
southern Altiplano, enhanced retroarc deformation at 19–7 Ma followed the onset of retroarc
magmatism by up to 8 m.y. (e.g., Allmendinger
et al., 1997; Elger et al., 2005).
Timing constraints suggest that the inferred
interval of slab shallowing corresponds closely
with enhanced deformation and thrust belt propagation in Bolivia and northwestern Argentina.
14
One possibility is that lithospheric delamination could result in enhanced magmatism due
to an influx of asthenosphere, which may also
create space beneath the upper plate for a shallowing slab, in turn promoting further forelandward propagation of the thrust belt. This model
may explain some aspects of the Miocene
kinematic history of northwestern Argentina
whereby magmatism is followed by slab shallowing and an enhanced eastward propagation
of shortening.
Several workers have documented that the
modern spatial extent of the Altiplano Plateau
was already in place before 25 Ma (e.g., Horton
et al., 2001), following rapid Eocene and Oligocene advancement of the thrust front to regions
of preexisting rift depocenters (Sempere et al.,
2002; Elger et al., 2005; Oncken et al., 2006;
Ege et al., 2007). Additional constraints presented here refine the timing and kinematics of
the retroarc thrust belt in northwestern Argentina. During shallow subduction, the upper plate
accommodates increasing strain. As retroarc
thrust belts commonly involve a craton-ward
thinning wedge of sedimentary rocks, enhanced
foreland-ward propagation of the deformation
front during shallow subduction would thus
be likely to encounter older basement rocks
with less overlying strata and greater preexisting heterogeneities. Plateau formation may be
enhanced in regions of pre-orogenic foreland
heterogeneities because distal uplifts increase
orography and the formation of internally
drained basins, in turn providing a positive feedback for formation of an orogenic plateau (Sobel
et al., 2003). With continued deformation in the
Sierras Pampeanas, the Puna Plateau may grow
southward as intramontane basins accommodate
additional strain that follows initial reactivation
of preexisting heterogeneities, much like within
the Salta rift of northwestern Argentina.
Implications of Along-Strike Variations
in Shortening
A primary observation by tectonicists working in the Andes is that the maximum magnitude
of crustal shortening coincides with southern
Bolivia, with shortening decreasing significantly along strike (Fig. 2B; Isacks, 1988; Kley
and Monaldi, 1998). Hypotheses that seek to
explain the along-strike change include the orientation of the relative convergence (Gephart,
1994), mantle flow beneath the long subducting slab (Schellart et al., 2007), and variations
in the pre-orogenic stratigraphic architecture of
the overriding plate (Fig. 2A; Allmendinger and
Gubbels, 1996; Kley et al., 1999).
Results from the present study suggest that
the pre-Cenozoic structural and stratigraphic
Geosphere, December 2013
architecture strongly influenced the spatiotemporal evolution of the thrust belt (Figs. 2
and 10). There is a striking correlation between
the magnitude of shortening and distribution of
Paleozoic strata in the retroarc of the Central
Andes, suggesting that it is not mantle flow or
convergence parameters that control the ability
of the upper plate to deform, but rather the preorogenic architecture of the upper plate (Fig. 2;
Allmendinger and Gubbels, 1996; Kley et al.,
1999). Due to the apparent decreased ability of
the upper plate to accommodate a portion of the
convergence between the South American and
Nazca plates, the relative convergence along
strike at the subduction interface would be predicted to increase away from the Central Andes,
which may explain the formation of the Bolivian orocline (e.g., Isacks, 1988; Allmendinger
and Gubbels, 1996; Kley et al., 1999; Arriagada
et al., 2008).
Coupled with existing estimates for the Puna
Plateau (Coutand et al., 2001) and Santa Bárbara Ranges (Kley and Monaldi, 2002) near the
latitude of the current transect, results presented
here constrain a minimum estimate of 142 km
for the total magnitude of shortening at 24–25°S
(Figs. 2A and 3). These results also suggest that
at least in the Eastern Cordillera at this latitude,
this shortening estimate is not greatly underestimated. For comparison, the ~95 km of shortening within this domain is <50% of that accommodated in the Eastern Cordillera of Bolivia
(McQuarrie et al., 2008). If the kinematics of
shortening within the Altiplano and Eastern
Cordillera in Bolivia were largely controlled
by the distribution of Mesozoic rift basins,
then the northern and southern segments of the
thrust belt are very similar, but differ in their
magnitude of shortening. One possibility is that
the Cretaceous rift basin in Bolivia was wider
(e.g., Cominguez and Ramos, 1995) and more
favorably oriented for inversion than in northwestern Argentina (Fig. 10), which allowed for
a greater magnitude of distributed shortening
during Cenozoic time. At the latitude of northwestern Argentina, the basement-involved Santa
Bárbara Ranges also could not accommodate
the large-magnitude (>100 km), thin-skinned
shortening absorbed by the Bolivian Subandes
because such a thick, pre-orogenic Paleozoic
basin did not exist at this latitude (Fig. 2).
The shortening estimate calculated here is
greater than existing approximations in this
region (e.g., Grier et al., 1991; Coutand et al.,
2001), but it is still ~150 km less than predicted
(Fig. 2; Isacks, 1988; Kley and Monaldi, 1998).
Recent studies have suggested that local existence
of anomalously thin crust beneath the Puna Plateau (~42 km; e.g., Yuan et al., 2002) may indicate Cenozoic crustal loss. However, at ~25°S,
00923 1st pages / page 15 of 17
Influence of pre-Andean crustal structure on Cenozoic thrust belt kinematics and shortening magnitude
most geophysical studies suggest an ~55-kmthick crust across much of the Andes (Yuan et al.,
2002; Tassara et al., 2006; Wölbern et al., 2009).
The initial crustal thickness in the Central Andes
is poorly constrained. Assuming that the Andes
are underlain by a 55-km-thick crust and had
an initial crustal thickness of 35 km, ~190 km
of shortening would be required at this latitude.
This is 48 km more than the ~142 km of shortening documented here; given that shortening in
the Puna and western Cordillera may be underestimated, we suggest that crustal addition (e.g.,
by crustal flow, magmatic underplating, etc.) may
not be necessary to explain the observed crustal
thickness at this latitude.
CONCLUSIONS
Regional geological mapping, structural
analysis, and geo- and thermochronological
results indicate that the northwestern Argentine
thrust belt at 24–25°S was deformed above a
W-dipping décollement that transferred slip to
primarily E-dipping reverse faults in a major
back-thrust belt that propagated in an overall
eastward direction during Cenozoic time. Following rapid eastward propagation of the thrust
belt at ca. 40 Ma, (U-Th)/He and U-Pb age dating of apatite and zircon constrains an ~75 km
eastward propagation event at 12–10 Ma, when
the thrust front bypassed the central portion of
a horst block in the Cretaceous rift system, followed by subsequent initiation of new faults in a
subsystem that propagated toward the west into
this region. Subsequently, deformation again
migrated >100 km eastward to a Cretaceous
synrift depocenter in the Santa Bárbara Ranges,
likely followed by westward-migrating deformation to its current location in the Lavayén Valley. Approximately 100 km to the south, deformation migrated progressively toward the east
through time with no local westward migration
documented. This suggests that the architecture
of the thrust belt was strongly influenced by
the configuration of the Cretaceous rift, which
likely influenced the discontinuous nature of
deformation propagation in an overall foreland
direction since Eocene time; these results are in
accord with recent work in southern Bolivia.
A regional balanced cross section across the
Eastern Cordillera, coupled with existing shortening magnitude estimates for the Santa Bárbara
Ranges and Puna Plateau, increases the estimate
of the magnitude of shortening at this latitude to
~142 km, but it confirms that significantly less
shortening was accommodated south of the thinskinned Bolivian fold-and-thrust belt. We suggest that greater shortening than has been previously documented was probably accommodated
within the Puna Plateau and ancient retroarc
of northern Chile, and that crustal shortening
alone may explain the observed thick crust at
24–25°S. The overall along-strike decrease in
shortening magnitude is well explained by the
distribution of pre-Cenozoic basins that are able
to accommodate large-magnitude thin-skinned
shortening. Coupled with a likely correlation of
Cenozoic thrust belt kinematics with the spatial
distribution of the Cretaceous rift, this suggests
that the pre-orogenic architecture strongly influenced the style, kinematics, and magnitude of
shortening, which, in turn, influenced the geodynamic evolution of Andean orogenesis.
ACKNOWLEDGMENTS
This research was conducted as part of the Convergent Orogenic Systems Analysis (COSA) project,
in collaboration with and funded by ExxonMobil.
National Science Foundation grant EAR-0732436
supported data acquisition at the Arizona LaserChron
Center. This work benefited from discussions with
many people, including M. McGroder, F. Fuentes,
R. Waldrip, J. Kendall, G. Gray, R. Bennett, S. Lingrey,
T. Hersum, T. Becker, and R.N. Alonso. B. Ratliff
provided assistance with LithoTect® software.
F. Shazanee, C. Hollenbeck, A. Abbey, I. Nurmaya,
and M. Hearn helped with mineral separations. Constructive reviews by J. Barnes and G. Hilley improved
the manuscript.
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