AN ABSTRACT OF THE THESIS OF
Sarah A. Johnston for the degree of Master of Science in Geolo
presented on June 10
2005.
Title: Geologic Structure and Exhumation Accompanying Yakutat Terrane Collision,
Southern Alaska.
Abstract approved:
Andrew J. Meigs
The climatic and tectonic framework of the St. Elias orogen makes it an excellent
place to study the interactions between tectonic processes such as deformation and
erosive processes, in particular glacial erosion. One type of link between these processes,
proposed by numerical models of orogenic development, is an effect of orographic
rainfall on the distribution of erosion, exhumation, strain, and the motion of rock through
an orogen. The St. Elias orogen is investigated with the goal of quantifying exhumation
rates, patterns, and magnitudes, while a structural interpretation of the foreland is
proposed. New apatite and zircon fission-track data are combined with existing apatite
and zircon (U-Th)/He data. Together, the apatite fission-track and (U-Th)/He data show
a trend of increasing age with distance from the coast, implying a contrast in the rate of
exhumation across terrane boundaries from the windward to leeward sides of the orogen
for at least the last 5-6 Myr. Exhumation rates based on these chronometers are as high
as -3mm/yr, but are generally modest (< 1 mm/yr), and significantly smaller than shortterm estimates based on sediment yields. Both zircon (U-Th)/He and fission-track ages
are largely unreset, suggesting that stratigraphic and/or structural burial of the deepest
parts of the active wedge did not exceed the zircon partial annealing zone (~ 10 km depth
assuming 25 C/km). This estimate is less than but similar to estimates of current wedge
thickness and stratigraphic reconstruction based on a balanced cross section. The cross
section also indicates 81 km of shortening, or about 47% shortening has been absorbed
within the fold and thrust belt of the Yakutat terrane. A present snapshot of material
within this wedge, based on the cross section, indicates approximately 64% of the cross-
sectional area is part of the actively deforming orogen, while 36% is subducting.
Together, the thermochronology and the cross section suggest that the fold and thrust belt
has absorbed very little of the total terrane convergence, which is as high as 600 km by
some estimates. If total convergence estimates are correct, then a significant amount of
material has been subducted, or the stratigraphic influx was low prior to the last 5-6 Myr.
©Copyright by Sarah A. Johnston
June 10, 2005
All Rights Reserved
Geologic Structure and Exhumation Accompanying Yakutat Terrane Collision, Southern
Alaska
by
Sarah A. Johnston
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements or the
degree of
Master of Science
Presented June 10, 2005
Commencement June 2006
Master of Science thesis of Sarah A. Johnston presented on June IQ, "'UUS.
APPROVED:
Major Professor, representing Geology
Chair of the department of Geosciences
Dean of the Graduate School
I understand that my thesis will become part of the permanent collection of Oregon State
University libraries. My signature below authorizes release of my thesis to any reader
upon request.
Sarah A. Johnston. Author
ACKNOWLEDGEMENTS
I would like to thank my advisor Andrew Meigs for his enthusiasm about this
project, his guidance and good advice throughout my time at OSU, and for at least a
handful of funny jokes. I would also like to acknowledge Jim Spotila for sharing his
mineral separates and John Garver for the use of his laboratory and assistance with
fission-track dating. Steve Reese, director of the OSU reactor, is gratefully
acknowledged for facilitating irradiation of fission-track samples. Thanks also to
committee members David Graham and Adam Kent for their helpful comments and
suggestions.
TABLE OF CONTENTS
Page
INTRODUCTION ......................................................................................1
ST. ELIAS OROGEN ................................................................................4
Tectonic setting .........................................................................4
Yakutat terrane stratigraphic influx ...........................................6
Structural overview of Yakutat terrane and St. Elias Orogen .....11
Orography and topography ........................................................ 12
Exhumation ...............................................................................12
FISSION-TRACK DATING ......................................................................16
Background ...............................................................................16
Methodology .............................................................................18
Results ......................................................................................24
Apatite fission-track ages .......................................................... 24
Zircon fission-track ages ...........................................................25
Multiple Chronometers .............................................................28
GEOLOGIC CROSS SECTION ................................................................. 30
Results ...................................................................................... 33
DISCUSSION ............................................................................................ 38
Orography and deformation in the St. Elias orogen ...................38
Exhumation rates ......................................................................40
Shortening and accretion ........................................................... 41
SUMMARY AND CONCLUSIONS ..........................................................43
REFERENCES ........................................................................................... 45
LIST OF FIGURES
Figure
Page
1. Conceptual model ................................................................................... 2
2. Regional tectonic map of southern Alaska ..............................................5
3. Geologic map of study area ....................................................................7
4. Stratigraphic section of units in Figure 3 .................................................8
5. Summary of orographic precipitation ......................................................13
6. A. Apatite (U-Th)/He age vs. distance from coast ................................... 15
B. Apatite fission-track age vs. distance from coast ............................... 15
7. Map of study area showing summary of all thermochronometry .............19
8. A. Selected cooling histories from the Yakutat terrane ............................29
B. Cooling history of sample 99CH2 from Chugach terrane ...................29
9. A. Deformed state cross section through Yakutat terrane ....................... 31
B. Restored state cross section ............................................................... 31
10. Structural vs. thermal reference frames ................................................ 35
11. Scaled right-triangles with thermal reference frame .............................. 37
12. Estimates of eroded area from fold-and-thrust belt................................39
LIST OF TABLES
Table
Page
1. Apatite fission-track data ....................................................................... 20
2. Zircon fission-track data ......................................................................... 21
3. Zircon binomial peak ages ......................................
Geologic Structure and Exhumation Accompanying Yakutat Terrane Collision, Southern
Alaska
INTRODUCTION
The suggestion that climate and erosion play key roles and may even govern
aspects of convergent orogenic development has prompted much investigation into the
link between deformation and surface processes (e.g., Brozovic et al., 1997; Molnar and
England, 1990; Willett, 1999; Zeitler et al., 2001). Numerical models of accretionary
wedge growth and continental collision are one means of exploring the relative
importance of tectonic and erosive processes (e.g., Beaumont et al., 1992; Koons, 1990;
Willett et al., 1993; Willett, 1999). These models, which build on original critical wedge
theory (e.g., Dahlen, 1990; Davis et al., 1983), tie the cross sectional shape of an orogen,
subduction along the orogen's base, internal deformation and erosion. When erosion
occurs in the model, mass is removed from the surface, critical form is perturbed, and
deformation within the orogen adjusts to replace missing material (Willett, 1999).
This type of link between erosion and deformation is of particular interest for
orogenic belts where erosion is asymmetric across the orogen due to a pronounced
gradient in orographic precipitation. In areas with a dominant track of moisture delivery
and wind direction, high topography causes incoming air to rise, which leads to
decompression, saturation, and rainfall on the windward side of the range, while the
leeward side can remain very dry (Barry, 1981). If asymmetry.in moisture delivery is a
proxy for erosional efficiency, coupling between contrasts in erosion across a range and
several features of the orogen can be explored (Figure 1). The effects of orographic
precipitation on mountain range evolution is still widely debated (Burbank et al., 2003;
Reiners et al., 2003).
For the specific case where the dominant wind direction has the same polarity as
the subduction direction (as opposed to the opposite), models predict that erosion is
focused on the same side as which the rocks enter the orogen (Figure 1). The enhanced
erosion on the windward side produces a broad region of exhumation (motion of rock
with respect to the surface) that acts to preferentially draw incoming material towards
that side to replace the eroded material (Figure 1). If erosional efficiency is high enough
to attain steady state (erosional outflux equals rates of tectonic influx), deformation in the
2
Active wedge
windward
Backstop
leeward
wind direction
V
'I
no erosion
no deformation
S..-
S
Figure 1: Coupled erosion deformation model of an endmember orogen
type in which the tectonic influx of rocks into the orogen is from the
same direction as moisture delivery, producing asymmetry in patterns of
erosion and deformation (modified from Willet et al. (1993)). Dark
gray shaded area is the region of deformed rocks within the orogen,
stippled pattern represents undeformed rocks outside of the orogen.
Light gray area at base represents subducted material that is not added to
the orogen. Curved lines with arrows indicate particle trajectories, with
dots representing equal time positions of particles. Note that the deepest
level rocks are exhumed in the core on the windward side of the active
wedge, and take the longest time to be exposed at the surface, whereas
no trajectories enter the rear of the wedge. T represents the thickness of
incoming strata that are incorporated into the orogen, V is convergence
velocity. The sketch illustrates key model predictions and also the key
variables that contribute to the development of the orogen.
leeward side of the wedge is diminished or zero (Willett, 1999) (see complete summary
of model predictions in Figure 1).
These model predictions are significant because they provide an erosional and
tectonic framework against which major features of a convergent orogen including
material influx direction, magnitude of exhumation (outflux), internal structure, and
topography and orography can be compared. Material influx is determined by the rate (v)
and duration of plate convergence, and the thickness of the incoming sedimentary section
(T) (Figure 1). Exhumation is the motion of rock towards the surface, and is a proxy for
erosional flux out of an orogen. Internal geologic structure requires constraint of the
style of deformation, and the magnitudes and temporal development of shortening and
thickening. Particle trajectories through the orogen predicted by erosion-deformation
models are apparently related to all of these variables. Thus, constraint of these
characteristics and variables within a convergent orogen allows for the significance of
climate-tectonic interactions to be obtained from an actual orogen.
In this paper the St. Elias orogen of southern Alaska is investigated from the
perspective of these variables. The orogen matches well a "wet prowedge" model, where
orographic precipitation is localized on the tectonic influx side of the wedge. Because of
attributes such as fast plate tectonic rates and extensive glacial cover, the Chugach-St.
Elias Range is considered by many to be one of the premier orogens in the world where
interactions between tectonic and climatic processes are likely to be observed (e.g.,
Gulick et al., 2004; Jaeger et al., 2001; Meigs and Sauber, 2000). To address key
questions in the St. Elias orogen, in this paper I review the tectonic and erosional
understanding of the orogen, present new thermochronometric work that constrains
exhumation, and a new interpretation of structure to produce a balanced cross section that
documents the internal structure. The principal goals are to: 1) determine the trends of
exhumation spatially across the orogen and with time, 2) understand the nature of
accretion and deformation by estimating the amount of material added to the orogen, the
amount of synconvergent shortening. These observations constrain particle trajectories
through the orogen from coupled structural and temperature history data in order to
assess the importance of erosion versus structure in the development of the orogen.
ST. ELIAS OROGEN
Tectonic setting
The St. Elias orogen is located at a complex plate boundary, and is of interest not
only to study the relationship between tectonic and climatic processes, but also oblique
convergence and terrane accretion (Bruhn et al., 2004; Pavlis et al., 2004). Along the
active margin of southern Alaska, motion between the Pacific and North American plates
switches from strike-slip along the Queen Charlotte/Fairweather fault in the southeast to
subduction along the Aleutian megathrust to the west (Figure 2). Subduction/ accretion
of the Yakutat terrane into this margin is inferred to be the process most directly
responsible for the growth of the St. Elias orogen (Bruhn et al., 2004; Plafker, 1987).
The Yakutat terrane is a composite oceanic-continental microplate that has moved northnorthwest along the transform boundaries of southeast Alaska and British Columbia from
a more southerly location. The terrane may have originated along North America in the
southeast Alaska/B.C. region and began moving north at -30 Ma when the Pacific-North
America transform plate boundary moved eastward (Plafker et al., 1994). It has also
been proposed, however, that the terrane migrated from a position as far south as
California (Bruns, 1983). Current motion of the Yakutat terrane based on GPS
measurements of plate velocity indicate the terrane is moving northwest on the order of
44 mm/yr with respect to North America (Fletcher and Freymueller, 1999; Sauber et al.,
1997). This motion is similar to Pacific plate motion, although at a slower rate and in a
more westerly direction (see plate vectors, Figure 2). The extent to which the Pacific
plate and Yakutat terrane are coupled is not clearly understood.
The backstop against which the Yakutat terrane is converging comprises a series
of terranes of varying age and origin accreted to the continent during Paleozoic through
Eocene time (Figure 2) (Plafker, 1987). Two terranes to the north, the Chugach and
Prince William terranes, directly bound the Yakutat terrane. The Chugach terrane is an
accretionary complex, of which the different assemblages accreted during the latest
Triassic to earliest Tertiary, although mostly during the late Cretaceous and early
Paleocene. It is generally accepted that the Chugach terrane formed somewhere offshore
of present day British Columbia, which would indicate it tapped the various sediment
5
Figure 3
PACIFIC
PLATE
Alaska
56"N
150°1N
Figure 2: Regional map of southern Alaska showing major tectonic elements of
the North American margin and Yakutat terrane (after Plafker, 1987; Plafker et al.
,1994). Prince William, Chugach, Alexander and Peninsular-Wrangellia are
terranes accreted during the Paleozoic-Eocene. Present day GPS velocities for
the Pacific plate and Yakutat terrane are indicated (Sauber et al., 1997; Fletcher
and Freymueller, 1999; DeMets et al., 1994). CSEF is the Chugach-St. Elias
fault, CF is the Contact fault, BRF is the Border Ranges fault, KIZ is the Kayak
Island zone, PZ is the Pamplona zone, DRZ is the Dangerous River zone, YB is
Yautat Bay, lB is Icy Bay, and CR is the Copper River. Note the location of the
Dangerous River zone relative to the Chatham Strait fault as a potential offset
marker for total Yakutat terrane displacement (see text). Geology north of the
Denali fault is not shown. Location of Figure 3 is indicated.
sources there during northward transport along transform fault systems (Plafker et al.,
1994 , and sources therein). The Prince William terrane formed in a similar fashion,
although it is younger (late Paleocene
- middle Eocene), with likely sediment sources in
Coast Mountains of British Columbia and Alaska (Kluane arc) , and a minor component
from the nearby Chugach terrane (Plafker et al., 1994, and sources therein).
Sedimentation in the Prince William terrane is coeval with a major phase of uplift in the
Coast Mountains between 62 and 48 Ma (e.g., Hollister, 1982; Plafker et al., 1994).
Subduction of a spreading center beneath the North American margin during the Eocene
produced a suite of rocks metamorphosed as high as amphibolite grade known as the
Chugach Metamorphic Complex within the Chugach and Prince William terranes
(Hudson and Plafker, 1982; Sisson et al., 1989; Sisson et al., 2003). For the rocks beyond
the Chugach metamorphic complex, the "background" metamorphism of the Chugach
and Prince William terranes are generally greenschist and laumontite/ prehnite-
pumpellyite facies, respectively, and are considered to have been fully "continentalized"
prior to Yakutat collision (Plafker et al., 1994).
Yakutat terrane stratigraphic influx
The Yakutat terrane is blanketed with a sedimentary sequence with an age range
that spans the Cenozoic. Beneath the sedimentary cover, the Yakutat terrane contains
two types of basement: continental rocks inferred to be a fragment of the Chugach
terrane, and thickened oceanic crust. The boundary between these basement types is the
Dangerous River zone (DRZ), which is inferred to be an offset equivalent of the Chatham
Strait fault isolated within the terrane after the Yakutat terrane was detached from North
America (Figure 2) (Plafker et al., 1994). This study focuses on the Yakutat terrane west
of the DRZ (Figures 2 and 3). Offshore, the Cenozoic basinal strata thicken abruptly
from east to west across the DRZ. The Cenozoic basinal strata are divided into three
formations, estimated to be 9500-10000m thick, based on both on- and offshore data
(Plafker, 1987). From oldest to youngest, these units are the Kulthieth, Poul Creek, and
Yakataga Formations, respectively. All of these units exposed onshore have little or no
evidence of metamorphism. Stratigraphic units are summarized in Figure 4, and the
{
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99CH2
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01CH449
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Major terrane-bounding thrust/shear zone
Key:
---------01 CH39
Foreland faults and fold axes, dashed where inferred
Sample number and position
Figure 3: Geologic map of study area showing locations of all thermochronology samples (Figures 6 and 9, Tables 1-3) and cross section line A-A',
shown in Figure 7. See Figure 4 for key to all stratigraphic units. Offshore
faults are labelled with the names of associated anticlines given by Bruns and
Schwab, (1983). SA is the Sullivan anticline. After Miller (1971); Plafker
(1987); and Hudson and Plafker (1982).
Surficial deposits
Includes glaciofluvial, fluvial, glacial moraine and marine shoreline
deposits
Q
YAKUTAT TERRANE
Yakataga Formation
Glacio-marine strata; 5000 m thick; 5.6 Ma- present
U_
O
N
0
C
Poul Creek Formation
Marine siltstone and sandstone; 1900 m thick; - 40-20 - 11-6? Ma
TP
(1)
0
Kulthieth Formation
Nonmarine and shallow marine sandstone and coal; -2800 m thick;
48-40 Ma. This unit may locally include basal part of Poul Creek
Fm. on map
0
_U
0
N
0
Co
a>
2
Yakutat Group
Metamorphic basement of eastern Yakutat terrane, interpreted to be
equivalents of the Late Triassic to Early Cretaceous Chugach terrane
"melange assemblage" of Plafker et al., (1994). Overprinted by
Paleogene greenschist metamorphism. Exposed mostly east of map
area.
PRINCE WILLIAM TERRANE
0
0
N
0
C
To
, I
(1)
0
0
0
N
0
Co
a)
Orca Group (undifferentiated)
Accretionary complex, Eocene in age.
Laumontite and prehnite-pumpellyite facies, local greenschist, and
amphibolite facies metamorphism within the Chugach Metamorphic Complex.
CHUGACH and OLDER TERRANES
Valdez Group
Subgreenschist-greenschist facies slate, phyllite, semischist and
mafic volcanic rocks. Metamorphosed to form amphibolite facies
Chugach Metamorphic Complex schist and gneiss zones during
Eocene ridge-trench interaction. Note that color gradation within this
unit represent high (dark green) to low (light green) metam. grade.
Undifferentiated Paleozoic and Mesozoic sedimentary and metamorphic rocks north of the Border Ranges fault.
INTRUSIVE ROCKS
Biotite granodiorite and granite plutons (lower Eocene) associated
with Chugach Metamorphic Complex
Ti
li
I
Chitina Valley Batholith
Jurassic Biotite hornblende quartz diorite, tonalite, quartz monzodiorite, and granodiorite
Figure 4. Map units for Figure 3. See text for full descriptions of Cenozoic Yakutat
terrane sediments.
surface expression of these units is shown in Figure 3. Key attributes of these units
including stratigraphic thickness, depositional age, and provenance are reviewed below.
Kulthieth Formation
The Kulthieth Fm. (Tk) is mostly a sandstone unit, partly arkosic with coal
interbeds (Miller, 1971; Plafker, 1987). It records a marine regression, with non-marine
alluvial- and delta- plain facies, shallow marine facies, whereas deeper marine delta
facies are expressed in equivalent formations such as the Tokun Fm. (Plafker, 1987).
Unit thickness is hard to determine and highly variable due to intensive deformation,
although estimates from its type locality within the Yakataga area are 2800 m or more
(Miller, 1957; Plafker, 1987; Wahrhaftig et al., 1994). This number is also consistent
with estimates attained by subtracting the better-known Poul Creek and Yakataga Fm.
thickness out of the estimated 10 km thickness of the whole offshore section. The
depositional age of the Kulthieth Fm. is not well constrained. The age of the Kulthieth
and Tokun Fms. in the Yakataga area, based on marine mollusk fauna and leaf flora, are
presented as -48-35 Ma by Plafker (1987, p. 244 Figure 4), but the various broadest
estimates for the Kulthieth Fm and equivalents range from early Eocene to early
Oligocene (~57-30 Ma based on the 1983 DNAG timescale (Palmer, 1983)).
Poul Creek Formation
The Kulthieth Fm. is conformably overlain by the Poul Creek Fm. The Poul
Creek records a marine transgression, characterized by slow deposition, with marine
siltstone and sandstones that are in part glauconitic, along with intercalated water laid
tuffs, breccia and pillow lava (Miller, 1971; Plafker, 1987). The unit is
1900 m thick in
the Yakataga area (Lagoe, 1983; Plafker, 1987). The age of the Poul Creek is given as
late Eocene- early Miocene (-40-20 Ma 1983) according to Plafker (1987). Lagoe
(1993) indicates that the Poul Creek is as young as latest Miocene based on a 5.6 Ma
K/Ar date on an upper most glauconite bed, with a regional hiatus during the mid
Miocene.
I
The sandstone compositions of both the Kulthieth and Poul Creek suggest a
complex volcanic-arc provenance with granitic and metamorphic sources (Plafker et al.,
1994). Following the Plafker model of terrane origin 600 km south of its present
position, the most likely source of the pre-orogenic strata is the Coast Mountains of
Alaska and British Columbia, including the nearby Chugach terrane. If the terrane
traveled from a position farther south, it is likely that sediments would largely be drawn
from the length of the British Columbia coast.
Yakataga Formation
Overlying the Poul Creek Fm. is the Yakataga Fm., a thick package of
synorogenic, glacial marine strata. Onshore at Yakataga Reef within the Yakataga
district (Figure 3), the contact between Poul Creek and Yakataga is conformable, and
marked by a paucity of glauconitic beds and the influx of coarser strata (Lagoe, 1983),
although-elsewhere onshore in the Robinson Mountains area, a slight angular
unconformity is recognized at the contact (Miller, 1971). As determined from well data,
the contact offshore from the study area is an unconformity that separates mid-Pliocene
glacial marine strata from deep water Oligocene Poul Creek Rocks (Zellers, 1993).
Abundant dropstones present within the Yakataga Fm. are interpreted to record
the onset of tidewater glaciation in the St. Elias (Lagoe et al., 1993) at 5-6 Ma. The
formation changes upsection from massive diamictites at the base to complex
megachannels in the middle of the formation, which suggests an increase in depositional
instability and rapid resedimentation (Zellers, 1993). The offshore Yakataga is fairly
well imaged seismically, and thickness estimates range from 4000-6000 in in various
areas, although the most common estimate is -5000 in, which is the thickness of the
section offshore from the study area (Bruns and Schwab, 1983). Sources of the Yakataga
include rocks from the Chugach and St. Elias Mountains, such as the Chugach and Prince
William terranes and the Kulthieth and Poul Creek Formations from Yakutat terrane.
I
Structural overview of Yakutat terrane and St. Elias orogen
Onshore, the Yakutat terrane is delimited by the Chugach-St. Elias (CSEF) and
Fairweather faults on the north and east, respectively, and offshore by the Kayak Island
zone (KIZ) on the west and the Transition fault on the south (Figure 2). The Chugach-St.
Elias fault is a major terrane boundary, marking the location where the Yakutat terrane is
underthrust beneath the Prince William and Chugach terrane backstop, whereas the
Fairweather fault, a right-lateral strike-slip fault and an extension of the Queen Charlotte
fault, forms the southeastern transform boundary with the Chugach terrane. The Kayak
Island zone is a series of folds and thrusts that mark the diffuse northeastern extension of
the Aleutian megathrust. The nature of the Transition fault, which is the boundary
between the Pacific plate and Yakutat terrane, is not well understood. The fault is a relic
transform fault that may have evolved into a main detachment underlying the Yakutat
terrane, accommodating subduction of the Pacific plate, or may still be a high angle
transform absorbing strike slip motion (Plafker, 1987). There is little evidence for
deformation in young strata that drape the fault (see discussion in Pavlis et al., 2004).
Important boundaries within the terrane include the Dangerous River and
Pamplona zones. The Dangerous River zone is inferred to be an old transform boundary,
possibly correlative with the Chatham straight fault (Figure 2), which separates the two
types of Yakutat basement and is coincident with an abrupt increase in sedimentary cover
from east to west. The Pamplona zone is the frontal-most region of active, northnortheast directed foreland faulting and folding, separating the undeformed portion of the
Yakutat terrane in the southeast from the deformed portion in the northwest (Figure 2)
(Plafker, 1987; Plafker et al., 1994).
Bruhn et al. (2004) divided the St. Elias orogen into three segments with similar
styles and history of deformation. The area of interest for this paper (Figures 2 and 3) is
located within the "central segment", a zone where the sense of motion along major
foreland faults and the Chugach-St. Elias fault is almost pure reverse motion. The region
west of Icy Bay is a thin-skinned fold-and-thrust belt that has deformed the Cenozoic
strata of the Yakutat terrane against the backstop of the Chugach-St. Elias fault. This
fold-and-thrust belt extends offshore to the Pamplona zone. Growth strata are present in
the Yakataga Fm. both onshore and offshore, indicating deformation during
sedimentation through the middle Pleistocene ("Horizon A" time of Bruns and Schwab,
1983), and the bathymetric character of offshore anticlines suggests active deformation as
young as the Holocene (Bruns and Schwab, 1983; Miller, 1971).
Orography and Topography
The mean elevation of the St. Elias Range is -1225 m, and increases from values
of about 900 to >2500 m from west to east, respectively (measured with distance east of
the Copper River, see Figure 9 of Meigs and Sauber, 2000). The range also includes the
two high peaks Mt. Logan (6050 m) and Mt. St. Elias (5490 m). This high topography
and proximity of the range to the coast captures a large percentage of the precipitation
moving north off the Pacific Ocean (Figure 5). The abundant rain and snowfall,
combined with a high latitude setting have generated a series of large icefields and
tidewater glaciers (note, for example extent of glacial cover in Figure 3). The heavy
precipitation on the south flank of the Chugach-St. Elias contributes to a low elevation
snowline on this side of the range (Figure 5b). Snowline serves as a rough proxy for the
equilibrium line altitude (ELA), which is the point on a glacier where accumulation is
equal to ablation and ice flux is greatest. The location of highest ice flux is also the
location of greatest glacier sliding velocity, and therefore inferred to be the location of
most intense glacial erosion. Meigs and Sauber (2000) demonstrated that in the
Chugach-St. Elias, mean topography trends with ELA.
Exhumation
Previous studies in the St. Elias orogen have quantified erosion and exhumation
over several spatial and time scales. Short-term (100 yr timescales) estimates are erosion
rates inverted from sediment yields at the mouth of southern Alaskan fjords that infer
primary glacial erosion rates in excess of 10 mm/yr and even as high as 50 mm/yr (Hallet
et al., 1996). It is likely that this type of short-term measurement records transitory high
pulses of sediment delivery associated with landscape and glacier disequilibrium during
climate changes from glacial to interglacial periods (Koppes and Hallet, 2002; Meigs and
13
a
E
4
a)
C
C
CO
-4 C
Co
52
a)
E
cN
Col
Co N
C2
E E
0
B.
4
3
ClOtIv h
snowline
2
1
0
Modified from Pewe (1975)
200 km
Figure 5.
A. Mean annual precipitation and annual temperature plotted with
distance.
B. Present and last glacial maximum snowlines plotted with topography.
Snowline is the line on a glacier surface marking >50% surface cover by
snow in the late summer and is used as a proxy for ELA (see text). Note
the dramatic reduction in precipitation north of, and the low snowline
south of the Chugach mountains. After Pewe (1975).
o
I
Sauber, 2000). Average erosion rates estimated over the Holocene (104 yr timescale)
provide a more conservative estimate of 5.1 mm/yr for the whole range (Sheaf et al.,
2003). This rate is based on the volume of sediment shed from the St. Elias into the Gulf
of Alaska based on seismic reflection data inverted to equivalent rock volume.
Longer term (106 - 10' yr timescales) estimates of exhumation are useful for
orogenic studies because they measure erosion over multiple glacial cycles, avoiding
confounding short-term effects, and producing estimates that reflect erosion through
multiple glacial advances and retreats. Exhumation is defined as the motion of rock
towards the surface, which can be accomplished by erosion, normal faulting, or ductile
thinning (Ring et al., 1999). In fold-and-thrust belts where thrusting is the dominant
tectonic style, rocks are primarily exhumed by erosion, although other factors can affect
the rock's path to the surface. Therefore, measurements of exhumation in a convergent
orogen most directly reflect erosion, although tectonic processes such as extensional
exhumation can also be important. Thermochronometers such as (U-Th)/He and fissiontrack dating record the time it takes a sample to reach the surface after passing through a
critical isotherm after which point the chronometer becomes a closed system.
Thermochronology studies in the St. Elias orogen record exhumation rates that are
substantially lower than the shorter-term estimates (O'Sullivan and Currie, 1996; Spotila
et al., 2004).
O'Sullivan and Currie (1996) used apatite fission-track dating to measure
exhumation of Mt. Logan. An age-elevation relationship from this study records three
pulses of exhumation during the late Cenozoic, with rates varying from 0.3 - 0.7 mm/yr.
Spotila et al. (2004) incorporated these data with apatite and zircon (U-Th)/He data to
measure the spatial pattern of exhumation across the range. Apatite (U-Th)/He data
suggest average rates of 0.4-1.2 mm/yr, with a maximum value of - 3 mm/yr. The
pattern that emerges from Spotila et al.'s data indicates that the most recent phase of
exhumation is greatest closer to the coast and decreases with distance northward (Figure
6a). This trend is interpreted by Spotila et al. as a correlation between the locations of the
highest erosion rates and heaviest orographic precipitation, densest glacial cover, and
lowest mean F LA
backstop
Yakutat
PW
Chugach
45
A.
40
35
ra
ro
i
5
0
20
0
40
0
80
100
120
140
40
B
35
01CH41
30
not reset
01CH55
reset
O
O
41
01 CH5
20
40
60
80
100
120
140
Distance from shore (km)
Figure 6.
A. Apatite (U-Th)/He ages (+26) plotted with distance from coast, from Spotila et al
(2004). Location of terranes are indicated, note that orographic divide is at - 60 km.
B. Apatite fission-track ages (+ Icy) from this study plotted with distance from coast.
16
(Meigs and Sauber, 2000; Pewe, 1975; Spotila et al., 2004). This locus of exhumation
occurs within the fold-and-thrust belt of the interior Yakutat terrane, south of the
orographic divide (Figures 5 and 6a).
FISSION-TRACK DATING
Background
Fission-track dating is commonly applied to apatite and zircon crystals to
determine the amount of time a sample took to transect the upper crust, as well as place
absolute age control on the events causing its exhumation. The method relies on the
damage to crystal structure cause by spontaneous fission of 238U. During fission decay of
238U,
two highly charged nuclei repel from each other, producing a linear trail of damage
(a fission track) (e.g., Fleischer et al., 1975). Thereby, the fission-track dating method is
similar to other radiometric dating systems, with a 238U "parent" and fission track
"daughter" product. At sufficiently high temperatures, fission tracks anneal as quickly as
they are produced, but when cooled below a critical temperature that is specific to the
mineral, tracks are retained and the radiometric "clock is set". This critical temperature,
or closure temperature (Ta) is ~ 110 C in apatite and - 240 C in zircon. In actuality,
"closure" occurs within the partial annealing zone (PAZ), a range of temperatures within
which a percentage of tracks anneal. In apatite, this zone is between -60 and 110 C, and
in zircon is between ~ 200 and 275 C (e.g., Brandon et al., 1998).
If a geothermal gradient for the crust is assumed (usually 20 - 30 C/km), then
closure temperature can be translated to depth, and the fission track cooling age
constrains the time a sample took to move from that depth to the surface. This
methodology is similar to that of the (U-Th)/He system, also applied to apatite and
zircon, which is based on the production of He within a crystal and the retention of He
which is temperature dependent. The critical temperatures for retention of He within
apatite and zircon are -70 C and -180 C, respectively (Reiners et al., 2002; Wolf et al.,
1996). When FT and (U-Th)/He for apatite and zircon are combined, these chronometers
constrain passage through 4 discrete isotherms within the top -10 km of the crust.
Detrital sample ages and partial resetting
All samples from the Yakutat terrane are from clastic sedimentary units that
display little to no mineralogical evidence of metamorphism. Detrital fission-track ages
are unlike other cooling ages because each grain within the sample has a potentially
unique cooling history that can be measured. For these reasons presentation and
interpretation of fission-track grain ages requires that the differences be highlighted
between unrest, fully reset, and partially reset samples. A given grain age can be older or
similar to the depositional age (unreset), or younger than the depositional age (reset) of
the stratigraphic unit (Brandon, 1992). Discerning the degree of resetting requires a good
estimate of the depositional age range for a sample and is particularly important issue for
zircon (U-Th)/He and fission-track chronometers because of the higher temperatures
required for resetting. Whereas an entire spectrum of grain ages is possible for a sample,
an age distribution can generally be broken down into component populations
characterized by grains with similar ages (Brandon, 1992). Depending on the
temperatures a detrital rock is exposed to after deposition, some or all of the grains may
be reset with respect to a given thermochronometer. If all grains are reset, the mixture of
different grain ages is erased and all grains record a new cooling event that is younger
than the sediment from which the grains were sampled. Alternatively, if only some
grains are reset, then the grain age distribution will again consist of multiple populations,
where the youngest population represents the time of resetting and older populations
variably retain source terrain ages (Garver et al., 2002).
Partial resetting such as this occurs when a group of grains within the sample are
reset by a thermal event, whereas other, more retentive grains retain their older source
terrain age. In apatite grains, fission track stability is largely a function of mineral
chemistry (see, for example, review in Brandon et al., 1998, and sources therein), where
stability of fission-tracks decreases with lower Cl contents. Fluorapatite, in particular has
the lowest track stability and accordingly, the lowest closure temperature (Tc). For
zircon, the dominant factor affecting track and thermal stability is the amount of
accumulated radiation damage (Brandon et al., 1998; Garver et al., 2005; Kasuya and
Naeser, 1988). Radiation damage results from both fission and a decay, although the
greater frequency of a events makes it the more significant process. Both processes
disrupt crystal structure, making grains more amorphous and thereby less retentive of
fission tracks and also more chemically reactive (note section on sample etch times
below). The amount of radiation damage a zircon has accumulated depends on the
concentration of U and Th and time since original cooling of the crystal (Garver et al.,
2005). Fission-tracks in less retentive grains are less stable than tracks in the more
retentive grains, and thus reset at lower temperatures.
Methodology
Laboratory methods
Fission-track ages were determined for a suite of samples that span the Yakutat
fold and thrust belt and the metamorphic and plutonic rocks of the backstop (Figures 3,
6b and 7). In this study, 15 new apatite fission-track ages and 8 new zircon fission-track
ages
were determined for the suite of samples measured by Spotila et al. (2004) for
apatite and zircon (U-Th)/He (Figure 7, Tables 1, 2, 3). Apatite and zircon separates
were prepared and analyzed following standard procedures for the external detector
method (Gleadlow, 1981; Naeser, 1976). Apatite samples were mounted in thin section
epoxy and etched in HNO3
20 seconds to reveal fission tracks. Zircon samples were
mounted in PFA Teflon and etched in a KOH:NaOH eutectic at 228 C. Two separate
grain mounts were created for each zircon sample to allow for both a short and long
etching. Different etch times are necessary for detrital suites because accumulated
radiation damage (from a decay of 238U and 232Th) affects the chemical reactivity of
zircon and varies greatly between individual grains. Because damage is largely a
function of age, long etches are used to target young, harder to etch grains, whereas short
etches preserve the older populations eradicated by the long-etch process (e.g. Bernet and
Garver, in press; Garver and Kamp, 2002; Naeser et al., 1987).
For this study the long etches were 30 hours and short etches ranged from 21-26
hours. We generally focused on short-etch mounts. After etching, apatite and zircon
grain mounts were fitted with low-U mica detectors and stacked along with age standards
9
02-19:
AHe ->
AFT -+
18.3
ZHe-+
N
ZFT --'
North America
1-47
13.3
99-2
H
81
10 km
26.0
28 . 2 (PR )
terrine
H
-
99-2%1-47"-
0
1-49
9J
1 s1,45
1-09
13.0
0 1-53:
Contact fault
sj
02,32
'-'---_
1-1- 02 28
- --1-53
02-3
-___02-12.2B,
c1cgGs`s01-38t1-36rft132
ge
1.5
13.3
3.4
4.8
Ot 36
39
Chugach - St E as fau t
'
_s6`]I
02-31:
ta
Isample s
partially reset
01-48
138
Yakutat
AK
0134
13z .
629:
13.9
01
01_55`)
1,2
3.8
01-43
1-39:
21
4.0
47.7(N8)
1-34:
1.2 (NR) 12.4 (NR)
6.3 (NR?)
54 (NR)
1.43:
01=41
141;
16.3 (N
31.5 (N
1IV
gaV
1.0
28.7
31.8
62.6 (NR)
29.9
5.7
8.0 (NR)
Figure 7: Simplified map of study area showing sample locations and summary of all apatite and zircon (U-Th)CI le an d
fission-track dates (Ma) obtained for each sample from this study and by Spotila et al. (2004). Ages are listed in order of
closure temperature (see key at upper right). Dashes indicate that no age has been determined for a particular
chronometer, and boxes with only two ages or less indicate dates were obtained from apatite systems only. All ages ar e
interpreted to be reset except where noted "NR" (not reset) and "PR" (partially reset). Zircon fission-track ages are all
youngest peak ages (Table 3). The extent of the onshore Yakutat terrane is indicated by the tan color. DRZ is the
Dangerous River zone.
Table 1: Summary of apatite fission-track data
Sample Elevation Latitude, Longitude
99-2
01-29a
01-32a
01-34a
01-36a
01-39a
01-41a
01-43a
01-45a
01-47a
01-48a
01-49a
01-53a
01-55
02-31
754
1082
442
579
883
297
274
279
1783
1494
1920
2012
1326
1204
1387
60.7203°,
60.2926°,
60.3434°,
60.2134°,
60.3340°,
60.2625°,
60.0592°,
60.1809°,
60.6572°,
60.7672°,
60.7911°,
60.6793°,
60.4181°,
60.1866°,
60.2855°,
142.5435°
142.3558°
142.4418°
142.6270°
142.6010°
143.3121°
141.9400°
141.1756°
142.4146°
142.6065°
142.6368°
142.5340°
142.5119°
141.0804°
141.7640°
N,
ps
2.72 x 105
1.00 x 105
9.29 x 104
1.66 x 105
5.84 x 104
1.26 x 105
4.77 x 105
1.07 x 105
7.09 x 105
8.02 x 105
1.63 x 105
3.53 x 105
7.90 x 104
1.21 x 105
1.42 x 105
140
12
13
63
10
20
107
23
211
160
13
'
152
17
77
66
Pi
3.67 x 106
3.13 x 106
2.60 x 106
2.29 x 106
1.77 x 106
3.95 x 106
2.65 x 106
1.56 x 106
8.64 x 106
5.23 x 106
3.80 x 106
4.78 x 106
3.34 x 106
7.71 x 105
3.43 x 106
Pd
1885
375
364
869
303
625
594
337
2572
1043
303
2059
718
492
1593
3.908 x 106
3.560 x 106
3.604 x 106
3.626 x 106
3.647 x 106
3.669 x 106
3.691 x 106
3.713 x 106
3,734 x 106
3.756 x 106
3.778 x 106
3.800 x 106
3.821 x 106
3.865 x 106
3.887 x 106
Nd
6385
5817
5888
5924
5959
5995
6030
6065
6101
6136
6172
6207
6243
6314
6349
x`
Age
-la +la
20
73.1
17
9.9
94.5
0.0
84.2
0.0
3.8
0.0
28.8
34.2
0.0
86.3
0.0
13.8
5.5
6.2
6.3
5.8
4.0
31.5
5.7
14.5
27.3
5.0
13.3
3.4
9.7
0.0
28.7
3.8
-1.3
-1.6
-1.8
-1.2
-1.9
-1.1
-3.4
-1.8
-1,2
-2.5
-1.8
-1.2
-1.0
-3.5
-0.8
n
20
18
20
20
20
16
15
20
13
20
19
29
15
+1.4
+2.0
+2.2
+1.4
+2.4
+1.3
+3.8
+2.3
+1.3
+2.7
+2.4
+13
+1.2
+4.0
+0.9
U ±2se
37.4 ±2.0
35.0 ±3.7
28.7 ±3.1
25.1 ±1.8
19.3 ±2.3
42.8 ±3.5
28.6 ±2.4
16.7 ±1.9
92.2 ±4.1
55.4 ±3.6
40.0 ±4.7
50.0 ±2.5
34.8 ±2.7
7.9 ±0.7
35.1 ±2.0
Note: Elevations are given in meters, ps is the density (cm2) of spontaneous tracks and Ns is the number of spontaneous tracks counted; pi is the density
(cm2) of induced tracks; and pd is the density (cm2) of tracks on the fluence monitor (CN1); n is the number of grains counted; and x2 is the Chi-squared
probability (%); U is uranium concentration (ppm). Fission track ages (± la) were determined using the Zeta method, and ages were calculated using the
computer program and equations in Brandon (1992), All ages with x2>5% are reported as pooled ages, otherwise, x2 are shown. For apatite, a Zeta factor of
94.61 ± 4.16 (± I se - SJ) is based on determinations from both the Fish Canyon Tuff and the Durango apatite. Glass monitors (CNI for apatite), placed at
the top and bottom of the irradiation package were used to determine the fluence gradient. All samples were counted at 1250x using a dry 100x objective (10x
oculars and 1.25x tube factor) on an Olympus BMAX 60 microscope fitted with an automated stage and a digitizing tablet.
N
O
Table 2: Summary of zircon fission-track data
Sample Elevation Latitude, Longitude
ps
N.
Pi
Ni
99-2
01-34
754
579
60.7203°, 142.5435°
60.2134°, 142.62700
4.92 x 106
5.07 x 106
885
2288
7.96 x 106
5.78 x 106
1431
01-36
01-41
01-48
01-53
883
274
1920
1326
60.3340°,
60.0592°,
60.7911°,
60.4181°,
142.6010°
141.9400°
142.6368°
142.5119°
4.57 x 106
6.65 x 106
4.73 x 106
4.80 x 106
1146
1118
1334
2039
5.43 x 106
7.05 x 106
4.25 x 106
6.15 x 106
1363
1185
1199
2615
01-55
02-31
1204
1387
60.1866°, 141.0804°
60.2855°, 141.7640°
5.22 x 106
5.10 x 106
1364
2251
7.47 x 106
6.12 x 106
1950
2703
2610
Pd
3.286 x 105
3.178 x 105
3.190 x 105
3.166 x 105
3.010 x 105
2.962 x 105
2.914 x 105
2.926 x 105
2.878 x 105
2.842 x 105
2.854 x 105
Nd
3874
3746
3760
3732
3547
3490
3433
3490
3391
3348
3362
n
x2
Age
-la +lo
U±2se
14
1.0
32
0.0
34.4
39.6
-1.7
-1.6
+1.8
+1.7
297.9 ±18.2
223.8 ±10.5
20
44.8
33.8
48.4
31.4
-2.0
-2.0
-2.4
-1.4
+2.1
+2.1
211.1 ±12.6
288.0 ±18.1
176.7 ±11.2
33
0.0
0,0
0.0
0.0
22
37
0.4
0.0
34.0
37.3
-1.5
-1.5
+1.5
+1.5
15
21
+2.6
+1.4
259.6 ±12.5
319.1 ±17.5
264.9 ±13.5
Note: Elevations are given in meters, ps is the density (cm2) of spontaneous tracks and Ns is the number of spontaneous tracks counted; pi is the density (cm2)
of induced tracks; and pd is the density (cm2) of tracks on the fluence monitor (CN5; n is the number of grains counted; x2 is the Chi-squared probability (%); and
U is uranium concentration (ppm). Fission track ages (± la) were determined using the Zeta method, and ages were calculated using the computer program and
equations in Brandon (1992). All ages with X2>5% are reported as pooled ages, otherwise, x2 ages are shown. For zircon, a Zeta factor of 348.45± 6.51 (± 1 se SJ) is based on determinations from both the Fish Canyon Tuff and the Buluk tuff. Glass monitors (CN5), placed at the top and bottom of the irradiation
package were used to determine the fluence gradient. All samples were counted at 1250x using a dry 100x objective (10x oculars and 1.25x tube factor) on an
Olympus BMAX 60 microscope fitted with an automated stage and a digitizing tablet.
22
Table 3. Zircon fission-track binomial component ages.
Sample
Unit
Nt
01-34
Yakataga
32
34.1 Ma -2.6 +2.8
01-41
Yakataga
15
35.5%
38.0 Ma -2.1 / +2.3
68.4%
02-31
Poul
Creek
Poul
Creek
37
34.4 Ma -1.9 / +2.0
53.4 Ma -3.7 / +3.9
20
58.9%
41.2 Ma-2.41+2.6
71.3%
41.1%
66.8 Ma-7.2/+8.1
28.7%
01-53
Kulthieth
33
26.4 Ma -2.0 /+2.1
33.7%
41.9 Ma -2.61+2.7
49.0%
01-55
Kulthieth
22
29.9 Ma -2.7 / +2.9
40.8 Ma -3.51 +3.8
01-48
Chugach
21
99-2a
Chugach
14
55.0%
30.7 Ma -4.2 / +4.9
15.6%
28.2 Ma -3.6 / +4.1
34.2%
45.0%
57.5 Ma -3.2 / +3.4
73.2%
39.6 Ma -3.1 / +3.4
65.8%
01-36
P1
(Ma)
P2 (Ma)
51.0 Ma -3.71 +4.0
48.0%
P3(Ma)
100.4 Ma -9.5/ +10.4
16.5%
113.5 Ma -12.2113.7
31.6%
68.4 Ma -5.91+6.4
17.2%
122.6 Ma -23.4/ 28.8
23.3%
Note: Nt = number of dated grains; Uncertainties cited at ±1 SE. Binomial component ages were determined
using the BINOMFIT peak fitting program of Brandon (1992, 1996).
23
and uranium enriched glass dosimeters (CN1,CN5) in poly tubes. These irradiation
packages were irradiated with thermal neutrons in the TRIGA reactor at Oregon State
University at fluences of 8 x 1015 neutrons/cm2 (apatite) and 2 x 1015 neutrons/cm2
(zircon). The glass dosimeters are placed at each end of the package and used to
interpolate the neutron flux at each position in the package, as flux is highest close to the
neutron source (top of package), and decreases steadily away from the source. Age
standards used to calculate zeta calibration factors (Hurford and Green, 1983) for this
study include Durango fluorapatite and Fish Canyon Tuff for apatite, and Fish Canyon
and Buluk Tuff for zircon. Zeta factors used are 94.61 ± 4.16 based on 10 analyses for
apatite and 348.48 ± 6.51 based on 9 analyses for zircon (± 1 standard error). For apatite
samples, about 20 grains per sample were counted depending on the abundance of
acceptable grains. For zircon samples between 14 and 37 grains were counted per sample
from mostly short-etch mounts.
Analysis of sample ages
The x2 test was used to determine if individual grain ages for each sample belong
to one concordant grain-age distribution (Brandon, 1992; Galbraith, 1981; Green, 1981).
Samples that pass the x2 test (P(x2) > 5%) are reported as pooled ages, whereas samples
that fail (P(x2) < 5%) are reported as x2 ages, which is essentially the pooled age for the
youngest fraction of "plausibly related" grain ages (Brandon, 1992). All zircon samples
failed the x2 test and ages reported in Table 2 are all x2 ages, but peak fitting methods
were also used to determine component populations (Table 3). Peak ages were
determined using the Binomfit program of Brandon, which deconvolves a sample's
collection of grain-ages into component Gaussian distributions, thereby preserving
information about grain-age populations older than the youngest fraction (Brandon, 1992,
1996). The youngest grain-age population is more conservatively estimated by the x2 age
than the youngest grain-age population (P1) determined by peak fitting methods.
Statistically, it is ideal if the youngest peak age (P1) and the x2 are similar.
24
Results
Of the 15 new AFT ages presented, 10 are from Yakutat terrane sedimentary units
(Kulthieth, Poul Creek, Yakataga), and 5 samples are from the Chugach terrane (Figures
3 and 4). Sample lithologies from the Yakutat terrane are all sandstones/graywackes,
whereas Chugach terrane lithologies samples include schists/phyllites, gneisses, and
granitoids. Most of the samples have both apatite (U-Th)/He and fission-track ages, and
several also have and zircon (U-Th)/He and fission-track ages.
Apatite fission-track ages
Apatite FT ages for the 15 samples fall broadly into three groups, which also
show a rough trend with sample distance from the coastline, similar to the trend identified
by Spotila et al. (2004) (Table 1, Figure 6b). The first group is represented by AFT ages
that are significantly older than the depositional age of the unit sampled and thus clearly
not reset. Only one sample, O1 CH41 from the Yakataga Fm., fits this description, with a
depositional age range of 5-6 Ma-present and an AFT age of 31.5 (-3.443.8). This
sample is also the sample closest to the coast, from the flank of the Sullivan Anticline
(Figure 3).
A second group of ages dominates the rest of the samples from the Yakutat
terrane fold-and-thrust belt (Figure 6b). These ages range from 3.4 to 6.3 Ma regardless
of stratigraphic unit and are thus reset with respect to depositional ages. A notable
exception to this pattern is sample 01 CH55 (Kulthieth Fm.) from the eastern part of the
study area in the vicinity of Mt. St. Elias, which has an AFT age of 28.7 Ma (-3.544.0).
The AFT age is younger than (or very close to) deposition, but older than samples both in
the backstop and the Yakutat terrane fold-and-thrust belt.
A third group of samples with like ages are all from the Chugach terrane in the
backstop, with an age range of 13.3-14.5 Ma (Figure 6b). However two others samples
from the backstop, 01 CH47 and 01 CH48 are significantly older (27.3 Ma), and younger
(5.0 Ma), respectively, than the 13-14.5 Ma samples.
25
Interpretation
With the exception of O1CH41, the near-coast Yakataga formation sample, all
samples have been reset with respect to apatite fission track closure at 110 C. Several
(mostly foreland detrital) samples failed the x2 test (Table 1) suggesting that temperatures
were not high enough to reset all grains. Reset AFT samples from the Yakutat terrane are
all 3.4 to 6.3 Ma, with no perceivable pattern to the distribution of ages within the terrane
(Figure 6b). Samples from the Chugach terrane, although more variable, are mostly
older, with a cluster of ages at 13-14 Ma. This distribution is similar to AHe ages of
Spotila et al. (2004),(Figure 6a). Spotila et al. suggested that synorogenic exhumation
has been greater near the coast, consistent with a coastward increase in precipitation and
lowered ELA (Meigs and Sauber, 2000). Whereas the AFT data support this contrast in
exhumation across the range, the data define age populations that do not vary in space
within the Yakutat terrane and backstop, respectively. These 2 sample groups are
distinguished by not only a contrast in climatic variables, but also a major terrane
boundary. The pattern of AHe and AFT ages suggest that over at least the last 5-6 Ma,
the Yakutat terrane, on the windward side of the range, has experienced greater
exhumation than the backstop on the leeward side of the range.
Zircon fission-track ages
All zircon fission-track data are presented in Tables 2 and 3. Six of the 8 samples
dated by zircon fission-track are from the Yakutat foreland (2 from each stratigraphic
unit), and 2 are from the Chugach terrane backstop. All samples failed the x2 test, thus
peak fitting methods were applied to determine age populations for each sample. The
youngest peak ages are most important for the purposes of this paper. Samples from the
Yakataga Fm. and Poul Creek Fm are all unreset (populations older than unit age) and
display from 2 to 3 age populations. Youngest peak ages (P1) for these two units range
between 34.1 Ma and 41.2 Ma (Table 3). This age is substantially older than the
depositional age (5.6 Ma- present) of the Yakataga Fm., indicating that the Yakataga Fm.
samples are neither reset, nor are their youngest peaks an approximation of the maximum
depositional age of the samples. This result is not surprising because the Yakataga Fm
26
samples dated by ZFT were both also dated by AHe and AFT and were unreset with
respect to these lower temperature systems. Despite the range in possible minimum Poul
Creek deposition times (as young as latest Miocene or old as Oligocene, see Figure 4) the
youngest peaks for these samples (02-31, 01-36) are older than the depositional age.
Thus the Poul Creek samples, like Yakataga, are unreset with respect to ZFT, but unlike
Yakataga, are reset with respect to AHe and/or AFT. A second age population (P2) at
51.0-66.8 Ma is recorded by samples from both the Yakataga and Poul Creek Fins. The
third population (P3) is present only in Yakataga Fm samples and is 100.4-113.5 Ma
(Table 3).
ZFT population distributions from the Kulthieth Fm. samples are more
complicated to interpret, largely because of the poor depositional age control on this unit.
Youngest peak (P1) ages for Kulthieth Fm samples are 26.4 (01-53) and 29.9 Ma (01-55).
The depositional age of the Kulthieth Fm and related Tokun Fm_ in the Yakataga area are
-48 - 35 Ma according to Plafker (1987)(Figure 4), but the various broadest estimates for
the Kulthieth Fm. and equivalents range from early Eocene to early Oligocene (-' 57 - 30
Ma on 1983 timescale (Palmer, 1983)) (Plafker, 1987; Plafker et al., 1994). Thus,
depending on the upper bound on depositional age, the P1 ages are within error of
deposition, and may be unreset like other foreland samples. In this case we would
interpret the Kulthieth samples much the same as Yakataga and Poul Creek, were Pl, P2
and P3 are a series of age populations inherited from the original source area.
A second interpretation of the Kulthieth samples is that they have been partially
reset. In this case, the P1 peaks at 26.4-29.9 Ma represent the time of cooling, above the
zircon PAZ, as recorded by only the least retentive grains. If the sample is partially reset,
the significance of P2 and P3 (~41 Ma and 68 Ma respectively) is ambiguous, because
these peaks could consist of retentive zircons that have retained their source area ages
throughout the partial resetting thermal event, or they also represent some unknown
degree of partial resetting. Complete resetting is not likely given the existence of
multiple peak ages.
The two samples from the Chugach terrane (01CH48, 99CH2) also contain peaks
of multiple ages. This is surprising because both are located within amphibolite facies
27
rocks of the Chugach metamorphic complex, that formed from thermal events
associated with ridge-trench interactions during ~54-48 Ma (e.g., Bradley et al., 2003;
Sisson et al., 1989; Sisson et al., 2003). Sisson et al. (1989) documented a maximum
metamorphic temperature range from -
400 C - 650 C across the complex from its edge
to the highest-grade gneissic core. Sample 99CH2 is a gneiss collected east of the Tana
Glacier, near Granite Creek, and is thus located within the high-grade core (schist and
gneiss zone of Hudson and Plafker (1982). Sample 01CH48, a schist-phyllite was
collected farther north, near the transition between the highest grade core and the
intermediate, or schist zone of Hudson and Plafker (1982). The intermediate zone is also
amphibolite facies though, and 01CH48 is well within Sisson et al.'s zone of minimum
temperatures of 400 C (1989).
Interpretation
Zircon fission-track ages are largely unreset for Yakutat terrane samples with the
potential exception of Kulthieth Fm. samples 01-53 and 01-55, which are interpreted as
being partially reset. Whether or not these samples have been partially reset has
important consequences for our interpretation of original sample depth. Published
vitrinite reflectance values of 0.37% to 3.41 % for Kulthieth Fm. samples indicate
maximum temperatures of -44 C to -275 C according to the conversion of Barker
(1988; Johnsson et al., 1992). These data were used to compile the Thermal Maturity
Map of Alaska, which has very extensive sample coverage within the study area of this
project (Johnsson and Howell, 1996). In the study area, the temperatures indicated for
outcrops of the Kulthieth Fm. towards the rear of the wedge, near the Chugach-St. Elias
fault are all greater than 200 C and as high as 272 C. Also, in the particular case of
sample 01 CH55, the minimum zircon FT age is similar to the sample's ZHe, and AFT
ages, suggesting that the minimum ZFT age represents a cooling age that is also recorded
by the shallower, reset chronometers, rather than a coincidental depositional age.
28
Multiple Chronometers
Multiple chronometers are useful to constrain the changes in exhumation rates
and the absolute dates associated with these changes. The combination of 4
chronometers per sample produces a time-temperature history spanning ~10 km of the
crust and constrained at 4 points. However, the fact that most samples in this study are
unreset with respect to both zircon (U-Th)/He and zircon fission-track means that for
most samples, rates can only be determined from the apatite systems (upper -4 km). The
combined data from the apatite systems are generally complimentary, with ages that
indicate roughly similar rates of cooling from the respective AHe and AFT closure
temperatures, although AHe cooling is often slightly faster.
The few samples that do have reset ZHe ages or partially reset ZFT ages are
worth highlighting however, because their longer cooling histories record changes in
exhumation rate and show a contrast between the Yakutat terrane and backstop cooling.
For example, sample 01CH53, a Kulthieth Fm. sample from the northernmost Yakutat
terrane, records relatively slow cooling between ZFT closure at 26.4 Ma and AFT closure
at 3.4 Ma, followed by faster cooling between 3.4 Ma and the present (Figure 8). This
faster cooling from -3-6 Ma to the present is typical of most Yakutat terrane samples,
such as 01CH29 (Figure 8). Sample 99CH2, from the Chugach terrane, also records a
period of slow cooling starting at 26.0 Ma (ZHe), although unlike the Yakutat terrane
samples, slow cooling continues through AFT and AHe closure to the present (Figure 8).
These histories underscore the idea that the windward, actively deforming Yakutat
terrane has been subject to faster exhumation for the last ~5-6 Ma than the leeward
backstop.
It is important to note at this point, however, that the recorded cooling rate of a
sample is governed by not only by the actual rate of the sample's motion, but also by the
trajectory of that motion (e.g., Batt and Brandon, 2002). For example, a rock could move
at relatively fast rates through an orogen, but if its motion is highly non-vertical (i.e.
parallel to isotherms), then it takes longer to traverse isotherms and thus records a slower
cooling history. For this reason, changes in cooling rate cannot be uniquely interpreted as
29
Age (Ma)
10
15
Yakutat terrane
2
5.5
4
3.4
6
8
to
ZFT (9.6 km)
0
B.
10
5
15
20
25
30
o
A 99CH2
Backstop
2
AHe (2.8 km)
8.2
---------------------------------
4
--------------Ico%9-
13.8
6
---------------------------------------
(7.2 km)
26.0
28.2
-----------------------------------------to
ZFT (9.6 km)
Figure 8: A. Plot of age versus depth for two samples from Yakutat terrane. Both
samples shown are from the Kulthieth Fm.
B. Plot of age versus depth for sample 99CH2 from the Chugach terrane (backstop).
Note phase of slow cooling between 26 Ma and -5 Ma recorded by both Yakutat
terrane samples and the backstop sample. This is followed by rapid cooling from 5
Ma - present in the Yakutat terrane samples and continued slow cooling in the backstop sample. All depths of closure assume a 25°C/km geothermal gradient.
pure changes in rate of rock motion, as they could also indicate a change in the cooling
trajectory. The rest of the foreland wedge samples that are not reset with respect to
zircon (U-Th)/He, fission-track, and in some cases even the apatite systems, are useful
data in and of themselves because they help constrain wedge geometry (maximum
thickness) relative to the surface. The partially reset ZFT samples suggest that their
maximum burial depth is very close to the zircon partial annealing zone, as they have
been heated enough to lower their age, but have not been heated enough to anneal all
tracks, as burial for sufficient time below the PAZ would do. Depending on the
assumption of geothermal gradient, but using 25 C/km, this places the deepest samples at
~10 km, which is similar to the depth of the lowest thrust sheet within the current wedge
(see cross section discussion, Figure 9a). Alternatively, these samples could have been
raised to temperatures higher than the PAZ, but for a relatively limited duration. The
implications of maximum sample depth are discussed below.
GEOLOGIC CROSS SECTION
Surface geology of the Yakataga District/Robinson Mountains area mapped by
Miller (1971) was used to construct a balanced cross section through the fold and thrust
belt that extends from offshore north to the Chugach-St. Elias fault (Figures 3 and 9).
This region of the Yakutat terrane is located within the "central segment" of Bruhn et al.
(2004), a zone where the sense of motion along major foreland faults and the Chugach-St.
Elias fault is almost pure reverse motion. The line of section through this area is nearly
parallel to Yakutat-North America convergence (Figures 2,3), and is roughly equivalent
to B-B' of Plafker (1987; 1994, Figure 3). The offshore structures incorporated onto the
leading edge of the section are based primarily on interpretations of USGS seismic
reflection survey line 406 by Bruns and Schwab (1983).
In general, the cross section has been constructed to produce a conservative
estimate of shortening, using a minimum number of major thrusts and adhering to
Miller's (1971) surface geology and dip data as well as published stratigraphic
thicknesses. Three major thrust systems are present on the section from north to south.
An upper thrust sheet composed of imbricate thrust faults in the Kulthieth Fm., is bound
Pr nce W am
A.
Yakutat terrane
C hugach terranes
A
A'
N
S
®®0
3km0
KF
3 km
HCF LS
0
-
5 km
5 km
10 km.
10 km
15 km.
H=V
I
E
N
E
O
30 km
1
40 km
15 km
1
50 km
60 km
70 km
80 km
90 km
B.
0
0
5 km
10 km
5 km
10 km
15 km
20 km
15 km
50 km
100 km
150 km
Figure 9. A: Deformed state cross section through Yakutat fold-thrust belt. SF- Sullivan fault, W RS- White River syncline
MCF- Miller Creek Fault, LS- Leeper syncline, HCF- Hope Creek fault. KF- Kusakuts fail][. Ty- Yakataga Fm.. Tp- Pout
Creek Fm.. Tk- Kulthieth Fm. Numbers 1-5 denote thrust sheets and their locations in B. Anticlines A I and A2 are named
after Bruns and Schwab ( 1993) seismic line 406. Growth strata have been imaged on these anitelines but are not depicted.
See text tier data sources and assumptions. B: Restored state section, shown at 505, of original scale. Numbers 1-5 denote
locations thrust sheets in A. Note that undeform ed sedimentary taper is controlled by 2" regional dip and the Yakataga Fm.
31
by the Hope Creek fault on its southern end to the north by the Chugach-St. Elias fault.
The next thrust sheet to the south, in the footwall of the Hope Creek fault, is marked on
the south by the Sullivan fault. A third system includes the two offshore reverse faults
and a detachment horizon at the Poul Creek/ Kulthieth Fms. boundary offshore south of
the Sullivan fault. For the purposes of restoration, it is assumed that motion on these
major faults was initiated at progressively younger time intervals from north to south.
Line-length balancing was used to restore the section. Total shortening absorbed by the
thrust belt and the original undeformed positions of rocks currently exposed at the surface
were measured from the restoration.
Several features of the section should be noted because they depart from the
mapped geology. In the hanging wall of the Hope Creek fault (Kulthieth Fm.), Miller
indicates a series of minor folds. Presumably these folds are superimposed on a broader
regional dip, although the regional dip for rocks bounded by the Hope Creek and
Kosakuts fault is unclear. Further north, between the Kosakuts fault and the Chugach-St.
Elias fault, the Kulthieth formation is also highly deformed by folding (Bruhn et al.,
2004; Miller, 1971) but Miller indicates that strata are generally north-dipping. Thus the
north-dipping panel between the Hope Creek and Kosakuts faults is based mostly on
geology to the north of it. More importantly, the presence of internal folding between the
Hope Creek fault and the backstop is not incorporated into the cross section, thus the
shortening estimated for this thrust sheet is likely a minimum. The second major
difference is the northern-most fault on the cross section (labeled with a "?", Figure 9).
This fault is not indicated where the section is located on Miller's map, but has been
drawn in accordance with the Kulthieth Fm. thickness, dip data, and is consistent with a
mapped fault located along strike to the west. A third discrepancy exists between the
section and onshore wells. Reports from these wells, which are extrapolated onto the
section from along strike, places Kulthieth Fm. at an unknown position along their depth
based on lithology. The interpretation of the Sullivan fault and the White River syncline
on the cross section do not place Kulthieth Fm. in these wells (Figure 9).
Results
A total cross sectional area of 630 km2 of Yakutat terrane sediments represents the
actively deforming orogenic wedge. This is the thrust-bounded area above the basal
decollement, south of Chugach-St. Elias fault, north of the southernmost thrust (at
anticline Al) and below the topographic surface. Below the basal decollement and
between the foreland pin on the south and the Chugach-St. Elias fault to the north, 354
km2 are being subducted (Figure 9a). Maximum wedge thickness is - 15 km, measured
from the highest topography to basal detachment. In the undeformed section, projection
of the stratigraphic section based on a 2 regional basal decollement dip implies the
section thickens from ~10 km to -17 km from south to north, respectively. This
stratigraphic taper results from the syntectonic Yakataga Fm. taper, which thickens
towards the north from an original thickness of 5 km in the south (Figure 9b). The
validity of this assumption is questionable, however, as all estimates for the Yakataga
Fm. are a maximum of 6000 m deposited over the last 5-6 Myr. The consequences of
this assumption are discussed fully in the discussion section below. The total eroded area
(which includes the thickened Yakataga Fm.) is 1158 km2 (Figure 9b). The undeformed
state section also indicates a minimum of 81 km of total shortening absorbed by the thrust
belt. The net motion of individual thrust sheets during emplacement can also be
determined. Estimates of horizontal motion ranges from a minimum of 29 km (sheet 1)
to a maximum of 90 km (sheet 5). Vertical motion ranges from 7 km (sheet 1) to 15 km
(sheets 3,4) (Figure 9).
Combined structure and thermochronometry
Thermochronometric data provide an independent constraint on the restored depth
of the thrust sheets from which they were sampled. Estimates of restored depth are
important because they constrain wedge geometry and are a means by which the
magnitude of erosion in the orogen can be determined. Used alone, thermochronometers
provide a one-dimensional estimate of erosion at each individual sample location.
However, when used to constrain the restored depth of the thrust sheets from which they
were sampled, a net eroded area for the thrust belt can be determined for which the
3
horizontal extent is constrained by structural reconstruction and the vertical extent is
constrained by thermochronology. The thermochronometric results from this project are
well suited for this approach because the combination of reset and unreset chronometers
bracket maximum burial depths. Moreover, the age control provided by
thermochronometric data can be used to determine the duration over which erosion has
occurred.
In order to combine the restored structural interpretation and thermochronometric
data, assumptions about paleotopography and geothermal gradient need to be made. The
structural and thermochronometric data sets are based on fundamentally different
reference frames (Figure 10). The thermochronometric reference frame is the thermal
structure of the crust relative to the surface, while the reference frame of the restored
structure section is the undeformed stratigraphic section. Samples restore to original
depths between 7 and 15 km in the undeformed stratigraphic section (Figure 9). These
depths are relative to a horizontal projection of sea level and include the thickened
Yakataga Fm. For thermochronology, restored depths based on the closure temperatures
(T.) of the four mineral systems is used to determine maximum sample depths prior to
exhumation (Figure 11). Because there are no independent constraints on
paleotopography, a flat surface is assumed for the sake of illustration. The assumption of
a flat surface also facilitates comparison between the paleodepth interpretation in the
thermochronometric reference frame and in the restored stratigraphic reference frame.
Geothermal gradient
It is common to assume a continental geothermal gradient between 20 C/km and
30 C/km in thermochronometric studies. In an apatite fission-track study of Mt. Logan
in the St. Elias, O'Sullivan and Currie (1996) determined two estimates of
paleogeothermal gradient for the Mt. Logan area from their age-elevation profile.
Assuming that upper and lower breaks in age versus elevation represent the exhumed
bounds of the partial annealing zone at 60 and 110 C, each break can be assigned an
elevation and the width of the PAZ can be inverted for the geothermal gradient. They
estimated a paleogeothermal gradient of 50 C/km between Eocene and Miocene cooling
Structural reference frame
Thermal reference frame
T
AHe
............
AFT
................
ZHe
...ZFT
Figure 10. A. Sketch of a simple thrust fault to illustrate net rock
motion that can be derived from geologic structure. Note that the reference frame of particle motion is relative to the hanging wall (HW) and
footwall (FW) strata with no constraint on the location of the surface.
B. Motion of a particle towards the surface as recorded by thermochronometers. In the thermal reference frame, motion of rock is at high
angles to isotherms, and provides no information on particle motion
parallel to isotherms. Isotherm shape and spacing is a function of
erosion rate and topography (e.g., Ehlers (2002); Stuwe et al. (1994);
Mancktelow and Grasemann (1997)).
events, and 36 C/km between Miocene and Pliocene cooling events. Spotila et al.
(2004) assumed a lower gradient of 30 C/km.
Current geothermal gradients for the Yakutat terrane can also be estimated from
vitrinite reflectance data from on- and offshore wells. These well data along with
extensive surface thermal maturity data were compiled by Johnsson et al. (1992) to
accompany the thermal maturity map of Alaska (Johnsson and Howell, 1996). In
accordance with the authors of these reports, the conversion T( C) = 148+104[ln(Rm)] of
Barker (1988) is used to determine maximum temperature (T) from mean vitrinite
reflectance values (Rm). The two onshore wells (White River #2, Yakutat #2) are about
-3.5 km deep and have highly variable vitrinite reflectance values along their length, in
some places even suggesting a decrease in temperature with depth. It is assumed that the
complicated nature of the vitrinite values from these wells reflects the varied burial
depths of individual stratigraphic units that may have been juxtaposed by faults or other
contacts. In contrast, the offshore Yakutat #1 well is located in the relatively undeformed
central section of the offshore Yakutat terrane. Vitrinite values from this well indicate a
clear (r2= 0.9) geothermal gradient of 23-25 C/km from between 2- 4.5 km depth. A
25 C/km gradient is inferred on the basis of these data.
Using the assumptions of no topography and a 25 C/km geothermal gradient, the
thermochronometric data indicate maximum depths of burial for the samples are
consistently shallower than their stratigraphic reconstructions (Figures 9b and 11). A
25 C/km geothermal gradient places AHe closure at 2.8 km, AFT at 4.4 km, ZHe at 7.2
km, and ZFT at 9.6 km. A higher geothermal gradient of 30 C/km shifts the closure
depths of all systems between 0.5-1.6 km shallower, thus also in disagreement with the
structure section. A lower geothermal gradient of 20 C/km shifts the closure depths
between 0.7 km to 2.4 km deeper, placing some of our samples within the depth estimates
of the structural reconstruction, but not others.
The maximum depth for any sample as determined by ZFT is ~ 10 km (8-12km)
based on the partial resetting of the farthest interior (northward) sample O1CH53 (and 01-
55 off-section). Towards the south, at the front of the onshore wedge, sample O1CH41
indicates the least burial, as it is not reset for any chronometer including AHe. This
01CH53
AHe
AFT
3.4 Ma
zre
ZFT
26 Ma?
01CH32; O1CH 36
IAHe
'AFT
ZHe
ZPT
01CH29
3.9 Ma
5.5 Ma
AHe
AFT
ZHe
?
ZFT
OICH34
63Ma
NR
01CH41
CCCCCC
AHe
AFT
ZHe
ZFT
NRS
NR
0
0
U)
5 km
O2
10 km
t,
B.
N
-TiF
0
50
n
I
Too
r
150
: Scaled right triangles from Figure 9 with overlay of AHe (2.8). AFT (4.4). Me (7.2), and ZFT (9.6) "closure
depths" in kills assuming a 25°Gkm geothermal gradient. Cooling ages for a given mineral method are marked by an age and
black square in A. Horizontal distances are from Figure 9, and vertical distances are from thermal data. Restored sample
location according to cross section are shown as circles and maximum depths derived from thermochronometers are shown as
boxes in restored section (B). Red lines denote critical isotherms associated with closure of each system. listed on the right.
Question marks indicating a lack of data lot a given chronometer.
Figure I I
15 km
38
indicates that this sample could not have been below 2.8 km (2.3-3.5 km). Combining
the structural interpretations from the cross section and depth constraints from
thermochronology measures the total exhumation experienced by the orogen (-730 km),
that in this case is less than the magnitude of erosion predicted by the structural model
alone. Furthermore, because the exhumation for the last -5 Myr is well constrained (by
reset AFT and AHe ages on several samples), the estimate of net erosion that occurred
during this time period (~480 km) is particularly robust (Figure 12). Beyond
uncertainties with the geothermal gradient, the substantial difference in the estimates
provided by these two methods may reflect the incorporation of a stratigraphic taper
within the Yakataga Fm. of the restored section. In other words, the projection of the
Yakataga Fm. to the north end of the restored section (above thrust sheets 2-5) assumes
that all thrusting occurred after the onset of Yakataga Fm. deposition at ~ 5 Ma. This
assumption is not supported by surface geology, as no Yakataga Fm. is exposed north of
the Hope Creek fault on the section or anywhere along strike (Figure 3 and 8), although it
could have been present and since been eroded. Even more significantly, sample
01 CH53, from the most northern part of the wedge (thrust sheet 5), cooled below ZFT
closure (-10 km) at 26.4 Ma, suggesting that rocks from the deepest part of the wedge
were already shallower than the projected depth of Yakataga Fm. prior to the onset of
deposition at 5 Ma. This supports the idea that the onset of shortening occurred before
Yakataga Fm. deposition.
DISCUSSSION
Orography and deformation in the St. Elias orogen
The strong gradient in orographic precipitation across the St. Elias orogen allows
for comparison with the predictions of coupled erosion-deformation models. Apatite (UTh)/He and fission-track data indicate that rates and magnitudes of exhumation have been
greater on the windward side of the orogen, within the Cenozoic sediments of the Yakutat
terrane, for at least the last 5-6 Myr. AHe data show a clear trend of increasing age with
distance from the coast, which Spotila et al. (2004) attributed to more intensive erosion
due to the extensive glacial cover and lower regional ELA towards the coast. The new
CD
Ln
eroded area based on structure = 1158 km2
?T
CCNl
0
50
100
CD
0
Lf)
eroded area from structure and thermochron = -730 k
5 km
D
10 km
L2
15 km
CCID
0
50
100
150
0
0
5 km
5
10 km
10
15 km
15
20
0
100
150
area removed from the Yakutat told-and-thrust belt based on c ross section restoration and
e
Figure 12. A. Estimate of total coded
stratigraphic taper. B. Estimat e of total eroded area from restored cross section and thermochronum etric depth constraints. C.
Estimate of a rea eroded during the last -5 Myr. based on cross-section restoration and thermochrono logy.
40
AFT data presented here demonstrate a similar trend, although rather than a continuous
trend, the AFT data constitute 2-3 populations located on different sides of the orographic
divide. It is important to note that in the study area, the orographic divide trends closely
with the major terrane boundaries between the Yakutat and Prince William/ Chugach
terranes. The AFT data do not uniquely distinguish between an orographically controlled
erosion signal and a signal related to the different tectonic and deformational history of
the terranes. It is possible that young exhumation at 3-6 Ma within the Yakutat terrane is
in response to the propagation of deformation southward from the terrane boundary at
that time, so that exhumation is younger within the active fold and thrust belt, whereas
the Chugach terrane became increasingly isolated from the locus of most active rock
uplift. Note, however, that one key prediction of the orographic erosion-deformation
model is that high erosion on the windward flanks monopolizes material influx, making
the leeward part of the wedge (Chugach terrane) less active with respect to both erosion
and deformation (Figure 1).
Exhumation rates
Regardless of potential orographic control on the locus of erosion, the
thermochronometry data provide an estimate of long-term erosion rates in a heavily
glaciated orogen. If glaciers are generally considered to erode more efficiently than
rivers (Hallet et al., 1996; Montgomery, 2002), and it is assumed that exhumation rates
derived from thermochronometers primarily reflect long-term glacial erosion, then one
might expect that exhumation rates in the Chugach-St. Elias are among the world's
highest. This supposition has been circumstantially supported by factors such as the
impressive relief in parts of the St. Elias orogen, including the world's steepest
topographic escarpment between Icy Bay and the peak of Mount St. Elias, and also by the
more robust data that indicate short term erosion rates are very high in the orogen (Hallet
et al., 1996; Sheaf et al., 2003). AHe data indicate that the highest exhumation rates
(from the foreland) are between -0.5 and 3 mm/yr, while exhumation rates from AFT are
all between -0.75 and 1.5 mm/yr. In contrast, exhumation rates based on fission-track in
regions of the Himalaya are 2-5 mm/yr, 1.5-6.0 mm/yr in Taiwan, and as high as 11
41
mm/yr in the Southern Alps of New Zealand (Batt and Brandon, 2002; Burbank et al.,
2003; Dadson et al., 2003). Considered in the context of other active and even nonglaciated orogens worldwide, these rates are not particularly high, which may suggest
that while the pattern of exhumation in the St. Elias may correlate with the most intense
glacial erosion, the rate of exhumation itself is not as clearly tied to the effects of glacial
erosion. Alternatively, the simple explanation could be offered that long-term glacial
erosion rates in orogenic settings are just not as high as short-term rates, and may be
similar or even less than long-term fluvial rates.
Shortening and accretion
The cross section, generally drawn to produce a conservative estimate of
shortening, indicates that the fold and thrust belt in the Yakataga area has undergone a
total of -81 km of margin-normal shortening, or about 47% shortening from the
undeformed to deformed state. This number is very low in the context of total Yakutat
convergence estimates of 600 km (Plafker et al., 1994). It is also low relative to the total
convergence estimate over just the last 5 Myr., which is 200 km and is attained by
extrapolation of the current plate velocity (-40 mm/yr). If the terrane has subducted 600
km of material, then only -14% of that convergence has been absorbed by offscraping
and repeating of the stratigraphic section.
A limitation to this interpretation is that the amount of deformation the Yakutat
terrane underwent en route, along the transform boundaries prior to collision and fold and
thrust development, is unconstrained. For example, the Kulthieth Fm. in the northern
part of the belt is very intensely deformed (Bruhn et al., 2004; Miller, 1971), and Miller
also notes a general difference in the structural style of the preorogenic strata north of the
Miller Creek fault compared to the area to the southeast, although this observation could
be attributed to lithologic differences alone. Another question is how much of the
convergence estimate (based on Yakutat GPS relative to stable North America) has been
absorbed by deformation or motion along structures inboard of the Yakutat terrane foldthrust-belt.
42
The Chugach-St. Elias is an active young orogen with a heavy orographic
signal and high convergence rates. Mean elevation correlates with ELA, suggesting an
erosional control of glaciers on topographic form (Meigs and Sauber, 2000). These two
characteristics suggest that glaciers may be operating at significant rates on orogenic
timescales in the St. Elias. Long-term exhumation rates, however, are not remarkably
high, even in the most intensely eroding areas. Meanwhile, structural characteristics of
the active wedge (amount of absorbed convergence; relatively small and shallow
structure) suggest that tectonic influx into the orogen via the fold and thrust belt is
relatively low. The fact that potentially 600 km (2000-3000 km2 last 5 Myr.) of
convergence can be accommodated with 3 simple thrust sheets, and thermochronometry
and structure indicate no more than 730 km2 maximum removal in the last 30 Myr (and
-480 km2 over the last 5-6 Myr.), suggests that the orogen may not be as dynamic as
speculated. An important question is whether a large amount of material has been
subducted, preventing a voluminous tectonic influx, or if some other factor, such as thin
stratigraphic cover prior to Yakataga deposition has limited tectonic influx. It appears
that in the Chugach-St. Elias, glaciers may be highly efficient, but unimpressive tectonic
inputs have not provided a rapid influx of material necessary to maintain high
exhumation rates.
43
SUMMARY AND CONCLUSIONS
1) Apatite fission-track ages for the St. Elias orogen fall generally into 3 groups: unreset
samples at the coast, reset samples in the Yakutat terrane that cooled below AFT closure
at 6-3 Ma, and reset samples from the backstop that cooled at - 13 Ma. The difference
between Yakutat terrane and backstop AFT cooling may be due to heavier precipitation
and erosion on the windward side of the orogen, or could reflect exhumation in response
to a young phase of rock uplift captured only in the Yakutat terrane.
2) All zircon fission-track samples have multiple peak ages. Only the most internal
rocks of the Yakutat terrane are potentially partially reset. Samples from the backstop
also have multiple peak ages, which is surprising given the metamorphic temperatures
these rocks were exposed to.
3) Structure within the Yakutat fold-and-thrust belt is divided along three principal thrust
sheets that are younger from north to south.
4) A minimum of -81 km of shortening has been absorbed by the fold-and-thrust belt
(47% of undeformed section).
5) The restored stratigraphic section indicates 1158 km2 of material has been eroded,
although much of this estimate includes an unreasonably thick assumed Yakataga Fm.
Unreset thermochronometers indicate less erosion, suggesting that structural estimates of
burial are likely overestimated.
6) The cross section predicts a cross sectional area of 630 km2 in the actively deforming
orogen and 354 km2 are subducted (~64% and -36%, respectively).
7) Particle trajectories through the orogen, as constrained by both structure and
thermochronology, have large horizontal components relative to the vertical, suggesting
that sample cooling may have occurred over average paths that are highly non-vertical.
8) The St. Elias orogen matches several predictions of coupled erosion-deformation
models of a "wet prowedge" system including greater exhumation on the windward side,
a locus of the most deeply exhumed rocks on the windward side of the orogen interior, a
lack of erosion and deformation on the leeward side, and relatively shallow particle
44
trajectories through the orogen. However, the data do not uniquely distinguish
between orographically vs. tectonically controlled erosion.
45
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