ARTICLES
Quaternary tectonic response to intensified
glacial erosion in an orogenic wedge
AARON L. BERGER1 *, SEAN P. S. GULICK2 , JAMES A. SPOTILA1 *, PHAEDRA UPTON3 , JOHN M. JAEGER4 ,
JAMES B. CHAPMAN5 , LINDSAY A. WORTHINGTON2 , TERRY L. PAVLIS5 , KENNETH D. RIDGWAY6 ,
BRYCE A. WILLEMS7 AND RYAN J. MCALEER1
Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, USA
Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, J. J. Pickle Research Campus, Austin, Texas 78758, USA
3
Department of Geological Sciences, 5790 Edward T. Bryand Global Sciences Center, University of Maine, Orono, Maine 04469, USA
4
Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA
5
Department of Geological Sciences, U. Texas El Paso, El Paso, Texas 79968, USA
6
Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907, USA
7
Department of Geology and Environmental Geosciences, Northern Illinois University, 312 Davis Hall, Normal Road, DeKalb, Illinois 60115, USA
* e-mail: [email protected]; [email protected]
1
2
Published online: 26 October 2008; doi:10.1038/ngeo334
Active orogens are thought to behave as internally deforming critical-taper wedges that are in rough long-term equilibrium with
tectonic influx and erosional outflux. Spatial and temporal variations in climate are therefore hypothesized to have a significant
influence on denudation, topography and deformation of orogens, thereby affecting wedge taper. However, the impact of the
most severe transition in Northern Hemisphere climate during the Cenozoic era—the onset of glaciation—has hitherto not been
empirically documented. Here we analyse the spatial patterns of denudation and deformation, and their temporal variations, in
the heavily glaciated St Elias orogen in southern Alaska. Low-temperature thermochronometry, thermokinematic modelling and
offshore seismic reflection and borehole data suggest that the global-scale intensification of glaciation in the middle Pleistocene epoch
enhanced glacier growth and caused ice streams to advance to the edge of the continental shelf. This led to focused denudation across
the subaerial reaches of the orogen and burial of the actively deforming wedge toe by the eroded sediment. We propose that this
climatically driven mass redistribution forced a structural reorganization of the orogen to maintain critical taper. Our empirical
results thus support decades of numerical model predictions of orogenesis and provide compelling field evidence for the significant
impact of climate change on tectonics.
The kinematic evolution of real-world orogenic systems during
Cenozoic cooling is an ideal but underused phenomenological
venue for quantifying the interplay between plate tectonics and
global climate1–4 . Links between these interacting systems are
commonly explored using numerical models of critical Coulomb
wedges5–9 . Within critical wedges, linked deformation and erosion
maintain mean orogen surface gradients10,11 , such that orogen
form is coupled to climatically influenced denudation pattern
and intensity5–7 . Within an orogen dominated by processes of
fluvial erosion, an increase in erosional intensity is predicted to
accelerate rock uplift and decrease orogen width and relief8,9 . As
glacial erosion can be more effective than fluvial erosion12,13 , a
climate shift towards glacially dominated denudation should have
an observable and profound impact on orogenesis. Numerical
simulations of glaciated critical wedges predict that Cenozoic alpine
glaciation should have intensified and redistributed erosion to
follow patterns of ice flux, thereby reducing mean topography,
contracting orogen width and forcing widespread structural
reorganization4,14 . Empirical studies have lagged behind models,
however, and examples of these responses have not previously been
linked to global cooling in a real-world orogen.
THE ST ELIAS OROGEN
The St Elias orogen is a product of the ongoing oblique
collision of the Yakutat terrane into the North American plate
since middle Miocene time15–18 . Deformation is currently focused
on the windward flank of the orogen within a thin-skinned
fold-and-thrust belt that accommodates ∼3 cm yr−1 of shortening
(Fig. 1)15,17,19,20 . This thrust belt may be approximated as a critical
Coulomb wedge, as it lies above a subducting slab and is
composed of poorly indurated, offscraped Cenozoic sedimentary
units with a tapered mean topographic profile15 . The orogen
has rugged coastal topography that receives heavy orographic
precipitation of ∼3–6 m yr−1 , which decreases to <0.6 m yr−1 on
the leeward flank21,22 . Half of the orogen is currently covered
by erosive, temperate glaciers that extend to sea level and
can move hundreds of metres per year (Fig. 1). During late
Pleistocene glacial maxima, regional ice coverage was nearly
complete and many glaciers reached the shelf edge through
now-submerged sea valleys (Fig. 1)21 . The history of global cooling
and regional glacial expansion is constrained by oxygen isotopes
and syn-orogenic strata within the Gulf of Alaska, respectively
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ARTICLES
Tran
0
km
50
sitio
n fa
ult
Pacific plate
144° Surveyor fan
8
Climate
and
tectonics
value
C
B
Glacial
887
log
Rare ice-rated
debris (IRD) Abundant IRD
~
C 100
My °
r –1
10
N
140°
142°
Figure 1 Thermochronometry and tectonics of the St Elias orogen. Red dots
show the location of 59 AHe ages22,32,33 . Windward flank ages range from 0.44 to
2 Myr, with the youngest ages (0.44–1 Myr) corresponding with the area shaded red.
Leeward flank ages (north of the green hatched line) range from 6 to 30 Myr. The belt
of most rapid exhumation (shaded red) correlates roughly with the position of the
‘ELA front’22 (shaded blue). The relief map (0–5,059 m elevation) includes offshore
bathymetry (100 m contour interval), major post mid-Miocene structures15,17,33 , the
modern distribution of glaciers (white) and deglaciated and ice-proximal areas
covered by alluvium (yellow). The Chugach St Elias fault (CSEF) is the Yakutat–North
American plate suture. Seismic profiles from Fig. 5 are shown in red.
Low-temperature apatite (U–Th)/He thermochronometry (AHe)
constrains the exhumation pattern in the St Elias orogen22,32,33 .
Long-term, time-averaged bedrock cooling rates indicate that
the most rapid exhumation since Mid-Quaternary time has
been focused within a narrow east–west band on the windward
flank of the orogen that correlates with the ‘ELA front’, or
the zone between the intersection of mean orogen topography
and Quaternary glacial maxima and minima equilibrium line
200 North
South
= AHe ( = extrapolated 15–100 km)
= AFT
= ZHe
= Maximum elevation
= Mean elevation
= Minimum elevation
150
100
50
0
3
4
Glacial interval C
3
Earlier cooling
2
1
ELA
front
Topography
5
0
6
Prevailing winds
2
4
Precipitation
1
120
100
80
A’
2
ELA zone
BF
0
60
Distance (km)
40
20
0
A
0
Precipitation (mm yr –1)
ONSHORE LOW-TEMPERATURE BEDROCK COOLING HISTORY
Figure 2 History of glaciation, bedrock cooling and sedimentation. a, Bedrock
time–temperature paths for the St Elias orogen. Lines encompass probable cooling
paths and are constrained by older AHe, AFT and ZHe ages that do not fall on the
plot. b, Globally distributed marine records of benthic oxygen isotopes (a proxy of
global ice volume and temperature) with specific values in black26 , range of values
shaded and average in red25 . c, History of syn-orogenic sediment accumulation in
the Gulf of Alaska (mean sedimentation rate (cm kyr−1 ) multiplied by mean sediment
dry bulk density (g cm−3 )) from Ocean Drilling Program site 887 (ref. 23). d, Log of
sedimentary facies observed in Ocean Drilling Program site 887. e, Synthesis of
climate events in the orogen including glacial intervals A, B and C and the
mid-Pliocene warm interval (MPW).
Exhumation (mm yr –1)
(Fig. 2)23–26 . Synchronous with expansion of Northern Hemisphere
continental ice sheets, the concentration of ice-rafted sediments
indicates that moderate glaciation was initiated by ∼5.5 Myr ago
(glacial interval A), but waned during the mid-Pliocene warm
interval (∼4.2–3.0 Myr). Glaciation returned as part of Northern
Hemisphere cooling from ∼3 to 2.5 Myr (glacial interval B),
and, based partly on our new data, significantly intensified in
the St Elias orogen in mid-Pleistocene time (deemed glacial
interval C, which continues to the present). We suggest this
intensification was due to a regional glacial response to a global
change from 40 to 100 Kyr glacial–interglacial climate cycles,
which occurred at ∼0.7–1 Myr (refs 27,28). The onset of glacial
intervals B and C corresponded to increases in detrital flux into
the Gulf of Alaska, including a doubling of terrigenous sediment
flux since ∼1 Myr (Fig. 2)23,24 . Because regional modifications
of Pacific–North America plate motion are not known to have
occurred in Pliocene–Pleistocene time15,17,18,29–31 , these sediment
pulses suggest that glaciation increased erosion within the St Elias
orogen. In particular, we contend that the onset of glacial interval
C had a critical role in the evolution of the orogen.
794
9
59°
0
e
A MPW
Mild glacial
Pre-glacial
Yakutat-driven orogenesis
7
4.5
d
Siliceous ooze
Yakutat
terrane
3.5
Sedimentation
(mg cm–2 kyr –1)
2,000
4,000
A verage δ 18O
Bering
trough
6
–10
0m
–200 m
–200 m
5
0 2.5
ormationa
l backstop
)
4 – 7° C
Myr –1
2
50
A2
GO
~3 cm yr –1
shortening
4
40
flank (def
B
Age (Myr)
ne
3
80
δ18O (‰)
Colder, more ice
Leeward
m
Pa
Malaspina
glacier
Cooling rate (°C Myr –1)
p lo
na
Zo
5
50
A2
GO
A
2
Elevation (km)
ng
504
A2
GO
60°
C SEF
r
120
= AHe
= AFT
= ZHe
1
Be
ri
cie
gla
160
0
Bagley fault
CSEF
Yakutat
terrane
Yakutat
terrane
Fig. 1
Pacific plate
Gulf of Alaska
c
b
St Elias orogen
Bedrock temperature (°C)
Ya
ward kutat terra
n
flank
(orog e
enic w
C My – 1
r
edge)
61°
a
North America
North
American
plate
Wind
B’
A
me leutia
gat n
hru
st
A’
~10°
142°
144°
Figure 3 Variation in bedrock cooling rate, topography and climate. Cooling
rates were calculated for AHe, AFT and ZHe data assuming linear cooling between
respective thermochronometers. Time-averaged exhumation rates (±25%) are
based on geothermal gradients accounting for thermal advection. Since the onset of
glacial interval C, cooling rates have been rapid only along the windward flank of the
orogen near the ELA front. Cooling rates before 3.5 Myr on the windward flank and
throughout Yakutat-driven orogenesis on the leeward flank were roughly steady and
considerably slower. Topography (10-km-wide swath), mean precipitation22 , the
range of ELA positions21 (modern versus glacial maxima) and the Bagley fault (BF)
are shown below. Data follow transect AA’, from Fig. 1.
altitudes22 (ELA) (Figs 1 and 3). This belt of focused and rapid
exhumation cuts across the structural trend of the orogen and is
more concentrated than orographic precipitation, suggesting that
denudation is controlled by maximum Quaternary ice flux, which
should occur at mean ELA (refs 4,14,34) (Fig. 3). Exhumation is
an order of magnitude slower on the leeward flank, which has
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ARTICLES
a
2
1
0ºC
0
k tr
North American plate
deformational backstop
200
3
aje
cto
10
Subducting basement
400
120 km
30
15
0
60 km
40 km
100 km
80 km
= Actual cooling ages
keyed to their respective
model trajectory
1 Myr
Onset
15
0 km
20 km
4
2
0
0
Tectonic influx (TI)
120 km
200
40 km
= Trajectory path 1
= Trajectory path 2
= Trajectory path 3
1.5 Myr
Onset
ZHE
60 km
20 km
2 Myr
Onset
ZHE
ZHE
150
AFT
AFT
0
HeFTy
50
0
1
AFT
AHe
AHe = 0.89 Myr
AFT = 2.24 Myr
ZHe = 8.88 Myr
2
3
Time (Myr)
4
0
1
2
3
Time (Myr)
AHe
AHe = 0.71 Myr
AFT = 1.19 Myr
ZHe = 5.10 Myr
4
HeFTy
100
HeFTy
Temperature (ºc)
80 km
Exhumation (E)
250
c
100 km
E (mm yr –1)
TI (mm yr –1)
140 km
b
5
Yakutate terrane
ries
Depth (km)
Roc
0
1
AHe
AHe = 0.46 Myr
AF T = 0.79 Myr
ZHe = 4.15 Myr
2
3
Time (Myr)
4
5
Figure 4 Two-dimensional thermokinematic model. a, Two-dimensional thermal model of the orogenic wedge after 1 Myr of accelerated exhumation, using techniques
and parameters discussed in the text. b, Component velocity profiles for a at the surface. c, Temperature–time paths for three trajectories shown in a. The solid lines and
dashed lines were calculated from the 2D and 1D model, respectively. Bands represent approximate closure temperatures for the AHe, AFT and ZHe mineral systems. Dots
represent the actual observed ages from the Yakutat terrane colour coded for position relative to each modelled trajectory. Predicted ages calculated using HeFTy (ref. 41) are
shown for trajectory path 1 on the lower right. Model parameters based on transect BB’ from Fig. 1.
been interpreted to be a deformational backstop that is isolated
from the deforming wedge by an active backthrust (Bagley fault)
that makes the orogen doubly vergent33 (Figs 1 and 3). Higher
temperature apatite fission-track (AFT) and zircon (U–Th)/He
(ZHe) thermochronometry33,35 , however, indicate that this pattern
has been in place only during the past 0.5–2 Myr (Fig. 3). During
this time frame, bedrock cooling rates across the windward flank of
the orogen increased markedly, but have remained slow and steady
on the leeward flank (Figs 2 and 3). The acceleration of bedrock
cooling in the wedge is believed to have corresponded to an increase
in average denudation rate from ∼0.3 to 3 mm yr−1 (±25%), with
peak erosion rates upwards of 4 mm yr−1 (±25%) proximal to the
ELA front. To maintain critical Coulomb form, this pronounced
acceleration in exhumation between the windward and leeward
flanks required motion along the backthrust to increase (Fig. 3).
To further constrain the timing of accelerated erosion and the
thermal evolution of the orogen, we used the finite-difference
package FLAC3D (Finite Lagrangian Analysis of Continua)36,37
to solve the advective–diffusive heat equation for constrained
tectonic15,17,19,20,33 , thermal38,39 and surficial22,33,40 boundary
conditions. Details of the model are provided in Supplementary
Information, Figs S1–S3, Video S1. The thermokinematic model
uses rock trajectory paths based on a two-sided wedge5,7 and an
initial steady-state thermal regime developed under a uniform
exhumation rate of 0.3 mm yr−1 . Using these parameters, the
model simulates an increase to spatially variable exhumation of
3–4 mm yr−1 and predicts the resulting thermal histories of rock
samples now exposed at the surface (Fig. 4). A one-dimensional
(1D) model simulating vertical rock trajectory paths was used
to constrain the pre-acceleration cooling history, owing to
uncertainties in rock trajectories earlier in the history of the orogen.
We ran the model three times, simulating an onset of accelerated
erosion at 1, 1.5, and 2 Myr. Results indicate that erosion-induced
heat advection would increase the near-surface (0–4 km depth)
geothermal gradient from 25 ◦ C km−1 to a maximum of 44 ◦ C km−1
after 1 Myr. Thermal steady state with a near-surface geothermal
gradient of 50 ◦ C km−1 would only be reached after 3 Myr. Using
the predicted thermal histories for each model, we calculated AHe,
AFT and ZHe cooling ages using the program HeFTy (ref. 41)
(Fig. 4, Supplementary Information, Note S1). Although each
model run predicts AHe cooling ages that are comparable to the
ages measured on the windward flank of the orogen (0.5–2.0 Myr)
(refs 22,33), the 2.0 and 1.5 Myr model runs predict AFT and
ZHe cooling ages that are significantly younger than observed
(Fig. 4). Likewise, the cooling ages produced when the model
is run to steady-state conditions are also inconsistent with the
thermal history of the orogen (see Supplementary Information,
Fig. S3)42 . Only the 1.0 Myr model run results in AFT and ZHe
ages that are comparable to actual measured ages (4–6 Myr and
10–13 Myr, respectively)33,35 , suggesting that the acceleration in
erosion occurred in mid-Pleistocene time. The exact timing of
the acceleration, however, varies in detail depending on the
imposed boundary conditions and modelled rock trajectories. For
example, a model with a simple vertical trajectory yields a slightly
younger, although still mid-Pleistocene, constraint on the onset
of accelerated onshore erosion (see Supplementary Information,
Fig. S4). The mid-Pleistocene timing of accelerated denudation
is therefore a robust result and is consistent with the offshore
record of sediment flux (Fig. 2). These two independent temporal
constraints, taken together, signify that an orogen-scale increase in
erosion occurred roughly coeval with intensification of glaciation
at the onset of glacial interval C.
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ARTICLES
a
GOA2502
Angular unconformity
Chaotic glacial deposits (mid-Pleistocene)
0.5
1.0
2 km
~400 m
Two-way travel time (s)
0
GOA2505
Channels and chaotic glacial deposits
Pleistocene
1.5
2.0
Ref. 45 yellow/orange
transition
Pliocene
NNW
SSE
GOA2505
0
GOA2504
GO A 2504 Initial sea valley
morphology
0.5
1.0
GOA2502
2 km
~200 m
Two-way travel time (s)
b
Angular
unconformity
WSW
Exxon 50
Angular unconformity
ENE
WSW
ENE
Figure 5 Seismic reflection profiles. a, Bering trough seismic profile (GOA2505). The orange line is interpreted to be the Pliocene–Pleistocene boundary based on
correlation with Exxon well 50 (crossed by GOA2502) and biostratigraphy43 . Dark coloured lines on the continental shelf represent glacial erosion surfaces and their
correlative conformities. The oldest of these (black) occurred in mid-Pleistocene time and is thought to coincide with the 0.7–1 Myr middle Pleistocene transition27,28 .
Evidence of glacial erosion and deposition is limited to <25 km from the current coast below the black horizon, but is present out to the shelf edge above the black horizon.
b, Cross-line seismic profiles GOA2504 and GOA2502 show the formation of a morphological sea valley and erosional resurfacing outside the Bering trough.
OFFSHORE RECORD OF GLACIATION AND DEFORMATION
Offshore seismic and borehole data indicate that the 1 Myr increase
in denudation across the subaerial deforming wedge and the coeval
increase in offshore sedimentation corresponded to an orogenwide change in glaciation. High-resolution seismic images acquired
along the Bering trough and across the seaward deformation front
(the Pamplona zone) constrain sedimentation and ice coverage
on the continental shelf (Fig. 5). An irregular low-angle angular
unconformity occurs near the shelf edge. On the basis of correlation
with biostratigraphic studies on industry wells43 and assuming early
Pleistocene sediment accumulation rates for the shelf typical of late
Neogene time, this unconformity is thought to have formed during
the mid-Pleistocene and marks the onset of what we refer to as
glacial interval C. We interpret this unconformity to represent the
first ice advance that extended beyond the present inner continental
shelf (<25 km from the modern coast) and subsequently crossed
the entire submarine extent of the deforming wedge to the shelf
edge. Two further seismic profiles show that this unconformity
occurs outside the modern sea valley and thus, was produced by
a regional erosional event (Fig. 5). These profiles also show that
the morphological sea valley did not form until after this regional
erosional event, implying that glacial advances to the shelf edge
corresponded with a transition in glacier morphology from smaller,
more dispersed termini throughout the inner shelf to focused ice
streams that carried greater ice discharge.
The mid-Pleistocene unconformity also represents a transition
in deformation style across the submarine extent of the orogenic
wedge. Before the advance of ice streams to the edge of the
continental shelf, the Pamplona zone experienced long-wavelength
folding and related faulting (Fig. 5). Growth strata between 1.5
and ∼0.5 s indicate that these structures were buried during their
latest growth, but ultimately shut-down in mid-Pleistocene time
796
as erosion bevelled the tops of the folds. Since this shelf-wide
erosional resurfacing, ∼400 and ∼200 m of undeformed sediment
has accumulated adjacent to and in the Bering trough, respectively.
Although minor shortening still occurs on the most seaward
structure of the Pamplona zone outside the Bering trough44 , it is
probably too small to significantly affect wedge taper. The lack of
significant active shortening across the toe of the wedge is unusual,
given the expectation that the frontal thrusts of a critical wedge
should accommodate the greatest fraction of convergence10,11 .
These results imply that the cessation of shortening across the
offshore extent of the wedge was coeval with, and causally related
to, both augmented onshore denudation and lowering of the
submarine critical taper by subsequent sedimentary burial. We
suggest both are a product of increased glacier coverage at the
onset of glacial interval C and represent examples of climate-forced
tectonic change.
QUATERNARY EVOLUTION OF THE ST ELIAS OROGEN
On the basis of the above synthesis of onshore and offshore
observations, we propose a kinematic model for the Quaternary
evolution of the St Elias orogen and examine it in the context of
change to a more intense glacial climate. Before the mid-Pleistocene
glacial expansion (glacial interval C), denudation rates across the
upper, subaerial portion of the orogenic wedge were relatively slow
(∼35 km2 Myr−1 unit flux) and deformation extended from the
backthrust to the offshore, forearc limit of the Pamplona zone
(Fig. 6a). Since the onset of glacial interval C, however, denudation
rates across the subaerial wedge have increased (∼190 km2 Myr−1
unit flux) and become focused at the ELA front. This change
required accelerated motion along both the backthrust and a
vertically stacked array of hinterland forethrusts (Fig. 6b). As the
wedge contracts owing to backthrust motion, these forethrusts
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2
1
0
~35 km2 Myr –1
Yakutat terrane
VE = 1:1
undifferentiated
160 km
b
140 km
Pamplona zone
Yakutat terrane basement
120 km
100 km
80 km
60 km
40 km
20 km
B’ North
South B
Pamplona Zone
140 km
~3 cm yr –1 shortening
syn-glacial strata
North American Plate
deformational backstop
Yakutat terrane
120 km
100 km
80 km
60 km
c
40 km
ks
top
Offshore
deposition
Ba
c
Ba
c
ks
top
Glacial erosion
20 km
0
4
8
12
0 km
Depth (km)
Hinterland for ethr ust s
VE = 1:1
4
3
2
1
0
Exhumation
(km Myr –1)
Prevailing winds
3–6 m precip. yr –1
~190 km2 Myr –1
<0.6 m precip. yr –1
160 km
0
4
8
12
0 km
Depth (km)
North American plate
deformational backstop
Exhumation
(km Myr –1)
a
Figure 6 Proposed model of climate-related influences on orogen kinematics. a, Structural model before glacial interval C, drawn along line BB’. Exhumational flux and
orogen architecture based on thermochronometry22,33 and geologic data15,17,33 (Fig. 1). b, Post glacial interval C structural model and exhumational flux. c, Interpretative
model of the effect of glacial erosion and deposition on the orogenic wedge. Before glacial interval C (left), the wedge is wider and has greater relief. After the intensification
of glaciation (right), deformation has been concentrated at the ELA front and the wedge has narrowed owing to the accumulation of shortening. Active faults are shown in red.
For a–c, straight dashed lines depict structures with only minor amounts of slip and wavy dashed lines depict inactive structures. VE: vertical exaggeration.
are themselves progressively truncated and incorporated into
the hanging wall of the backthrust, probably explaining the
Pliocene–Pleistocene shift in deformation away from the North
American–Yakutat terrane suture (Chugach St Elias fault)33 . Coeval
with these onshore changes, the offshore wedge was buried by
sediment and became inactive. On the basis of mass balance, only
∼20% of the sediment eroded from the subaerial wedge since
1 Myr (Fig. 6a) currently covers the inactive Bering trough segment
of the offshore wedge; the remainder was probably transported
farther offshore to the Surveyor fan and more distal portions of
the Gulf of Alaska23 . Periodic glacial advances may erode a fraction
of this cover and redeposit it on the slope and fan, but the eroded
material is replaced by renewed rapid deposition during interglacial
periods40 . This kinematic evolution fits with the classic model of
critical Coulomb wedges, in which perturbations in denudation
lead to structural reorganization and out-of-sequence thrusting to
maintain critical taper8–11 . In this case, Quaternary climate change
seems to have focused glacial denudation across the subaerial
wedge resulting in increased rock uplift through accelerated
thrust motion, while coeval glaciogenic deposition lowered the
submarine critical taper and stabilized offshore structures through
load-induced normal stress (Fig. 6c).
A consequence of the shutting-off of the offshore Pamplona
zone is that the inactive wedge has been progressively consumed
by the active onshore wedge at ∼3 cm yr−1 , such that the wedge has
narrowed by ∼20% since mid-Pleistocene time (Fig. 6). As long as
aggressive erosion continues unabated, the St Elias orogen should
continue to contract owing to the accumulation of shortening, until
the inactive deformation front merges with the active deformation
front and establishes a new equilibrium form. At the onset of
glacial interval C, however, the zone of active deformation within
the wedge narrowed by a much greater extent (∼50%) (Fig. 6),
suggesting that effective deformation within orogens may be
confined to narrow zones within a larger wedge complex and may
fluctuate depending on climatic conditions. This redistribution of
deformation is consistent with model results that indicate active
wedges should contract in response to an increase in fluvial
erosion8,9 or a transition to or intensification of glacial climate4,14 .
EFFECT OF GLACIATION ON OROGENESIS
The unique perspective offered by combining these distinct
data sets provides an empirical view of how an orogen evolves
kinematically in response to a climate shift towards intense glacial
conditions. The observations imply that Quaternary deformation
within the St Elias orogenic wedge has been directly influenced by
glacial erosion. The trigger for the reorganization of the orogenic
wedge was not the onset of Northern Hemisphere glacial conditions
at ∼3–2.5 Myr, but rather the intensification of glacier coverage at
∼1 Myr and, perhaps, a shift in glacier morpho-dynamics towards
more erosionally effective ice streams. This last process may have
been in part facilitated by the formation and development of
fjords and the resulting change in glacial morpho-dynamics45 .
The response of a critical wedge to climatic perturbations may
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© 2008 Macmillan Publishers Limited. All rights reserved.
797
ARTICLES
therefore function on the basis of thresholds, such that enhanced
glacial erosion and glaciogenic deposition may have pushed the
orogen to a tipping point, beyond which a structural reorganization
and narrowing of the entire wedge were required. An implication
of these results is that the onset and acceleration of alpine
glaciation in late Cenozoic time should have modified denudation
and deformation within numerous mountain belts worldwide,
although probably on a more subtle level than the St Elias orogen,
which is tectonically very active and has been subjected to one of the
most extreme temperate glacial climates on Earth. This hypothesis
is consistent with climate as the driver of observed changes in
exhumation rates, sedimentation rates and relief within many
orogenic systems over the past few million years1,2 . Where present,
glaciation may thus have a significant role in the internal processes
of mountain building, empirically supporting the paradigm that
orogenic architecture, kinematics and evolution may be heavily
influenced by external climatic processes.
ANALYTICAL METHODS
Thermochronometry data and methods used for this work are
presented in detail in supporting publications22,33 , although the
primary results, the pattern and history of exhumation, are
shown here (Figs 1–3). AHe, AFT and ZHe thermochronometers
have typical closure temperatures of ∼70, 110 and 180 ◦ C,
respectively. Closure temperatures vary in detail, depending on
grain size and sample cooling rate, and were estimated using
standard procedures and mineral parameters. For the background
geothermal gradient in the St Elias orogen (25 ◦ C km−1 ), which
is constrained by vitrinite reflectance data, industry wells and
onshore thermochronometry33,35,38,39 , typical closure temperatures
of these mineral systems reflect the history of exhumation from
∼2–7 km depth22,33 . As shown by numerical modelling results, rates
of exhumation on the windward flank of the orogen are sufficiently
high that advection of heat towards the Earth’s surface would
result in an elevated geothermal gradient. To account for heat
advection, AHe-based exhumation rates from the windward flank
of the orogen were calculated assuming an elevated geothermal
gradient of up to 45 ◦ C km−1 . The ±25% error of calculated
exhumation rates is based on cumulative analytical uncertainties
in laboratory measurements (±10%) and likely uncertainties in
geothermal gradient.
Two-dimensional thermal profiles were calculated using the 3D
finite-difference code FLAC3D (ref. 36,37) in which we modified the
thermal solution method to solve the advective–diffusion equation
for an imposed velocity field:
k
∂T
∂T
∂T
∂T
=
(∇ 2 T ) − u
+v
+w
− A (x,y,z)
∂t
ρC p(x,y,z)
∂x
∂y
∂z
in which k is thermal conductivity, C p is heat capacity, ρ is
density, u, v, w is velocity in an externally fixed Cartesian reference
frame and A is radioactive heat production. The model domain is
140 km long, 1 km wide and 15 km deep, representing the backstop
of the North American plate and the uppermost part of the
subducting Yakutat terrane (consisting of a thick sediment package
overlying the subducting Yakutat terrane basement). The upper
surface ranges in elevation from sea level to 2,500 m with smoothed
topography. Model parameters include a fixed surface temperature
of 0 ◦ C (ref. 21). The base of the model is given a fixed temperature
of 375 ◦ C, determined from an average geothermal gradient of
25 ◦ C km−1 (refs 38,39). Constrained by the architecture of the
orogen15,17,18,33 , modelled rock trajectories are based on a standard
two-sided wedge5,7 , which consists of conjugate thrusts along which
798
most of the deformation is taken up. Relative motion between
the North American plate and the Yakutat terrane was set at
30 mm yr−1 (refs 19,20), which is accommodated by the two thrust
faults and by exhumation. Imposed exhumation rates are based
on thermochronometry and subsidiary offshore sedimentation
data22,33,40 . The entire model domain has a thermal conductivity
of 2.5 W m−1 K−1 and uniform radiogenic heat production of
0.4 µW m−3 .
The marine seismic data presented here were collected in
2004 aboard the R/V Maurice Ewing. We acquired ∼1,800 km of
high-resolution seismic-reflection profiles along the Gulf of Alaska
continental margin using dual 45/45 cubic inch generator–injector
airguns with better than 5 m vertical resolution31 . Processing
included trace regularization, normal move-out correction,
bandpass filtering, muting, frequency–wavenumber filtering,
stacking, water-bottom muting and finite-difference migration.
Glacial erosion surfaces were identified in the data based on
stratal truncations and associated chaotic reflectivity indicative
of diamicton formed in ice-proximal/marginal settings46 .
Approximate chronology was established by correlating sequences
beneath the angular unconformity near the shelf edge in the
2004 data with strata in nearby industry wells that were dated
biostratigraphically43 (Fig. 5). In particular, the last observation of
the Pliocene planktonic foraminifera Neogloboquadrina asanoi at
∼2,250 m below the sea floor in Exxon well 50 places the seismically
imaged strata pursuant to this article in the Pleistocene and defines
the angular unconformity as mid-Pleistocene.
Received 3 March 2008; accepted 19 September 2008; published 26 October 2008.
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Supplementary Information accompanies the paper at www.nature.com/naturegeoscience.
Acknowledgements
We thank collaborators in studying the St Elias orogen and other colleagues for many helpful
discussions and ideas, including E. Berger, R. Law, J. Buscher and particularly all STEEP (St. Elias
Erosion-Tectonics Project) participants. Reviews by S. Brocklehurst and E. Kirby contributed
significantly to the manuscript. Support was provided by the National Science Foundation (NSF-EAR
0409224, NSF-EAR 0408584, NSF-EAR 0735402, NSF-ODP 0351620).
Author contributions
All authors contributed to the interpretations and hypotheses presented. Writing was done by A.L.B.,
with contributions by J.A.S. and S.P.S.G. Low-temperature thermochronometry was carried out by
A.L.B. and J.A.S. ArcGIS analyses were carried out by A.L.B. Thermokinematic modelling was carried
out by P.U., with contributions by A.L.B. HeFTy age calculations were done by A.L.B. Figures were
created by A.L.B (Figs 1–3 and 6), S.P.S.G. (Fig. 5) and P.U. (Fig. 4). Structural analysis and models
were done by A.L.B., J.B.C. and T.L.P. Seismic reflection gathering, processing and interpretation were
carried out by S.P.S.G., L.A.W., J.M.J. and B.A.W. Project planning was done by T.L.P., J.A.S.
and S.P.S.G.
Author information
Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions.
Correspondence and requests for materials should be addressed to A.L.B. or J.A.S.
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799
NEWS & VIEWS
GEOMORPHOLOGY
A glacial driver of tectonics
The interactions between climate and tectonics in active mountain ranges are complex and
important. Field and geophysical data from the St Elias Range of Alaska show that glacial
erosion can influence the dynamics of the lithosphere in such settings.
Simon H. Brocklehurst
is in the School of Earth, Atmospheric and
Environmental Sciences, University of Manchester,
Manchester M13 9PL, UK.
e-mail: [email protected]
raditionally, the movement of
tectonic plates is assumed to build
mountains, whereas rivers and glaciers
erode them away. Indeed, most of the
large mountain ranges lie along current
or former boundaries of tectonic plates.
Insights from numerical modelling have
recently challenged the idea that erosion
merely carves picturesque landscapes
from the edifices built by tectonics1. But
field-based evidence of a more dynamic
role for erosion in mountain building
has proved elusive. On page 793 of this
issue, Berger and co-authors2 synthesize
insights obtained from low-temperature
thermochronometry (radioisotope
determination of near-surface exhumation
histories) and thermo-kinematic modelling,
as well as offshore seismic reflection and
borehole data, to demonstrate that during
the middle Pleistocene epoch, the advance
of some Alaskan glaciers to the edge of the
continental shelf changed the structural
evolution of the St Elias orogen (Fig. 1).
The massive volumes of sediments
pouring from tectonically active mountain
ranges attest to the efficiency of erosion in
these settings3. In particular, the volume
of material accumulating in the fjords
of Alaska demonstrates that, in certain
settings, glaciers can erode even more
efficiently than the large rivers traversing
the Himalayas4. But whether rock can be
removed sufficiently rapidly and in sufficient
volumes to influence lithospheric dynamics
is unclear.
Numerical modelling results suggest
that erosion does elicit a response from the
lithosphere. For example, the unloading
due to erosion either by the large rivers
draining the Tibetan Plateau around each
end of the Himalayan mountain chain5, or
the rivers along the monsoon-drenched
Himalayan front6, could cause ductile
lower crustal material to be drawn towards
732
JAMES SPOTILA
T
Figure 1 Influence of climate change on the St Elias orogen. Glaciers such as the Russell Glacier in the
St Elias Range, Alaska, may have a more substantial role in the dynamics of the orogen than merely creating
picturesque scenery.
the Earth’s surface, thereby affecting the
thermal structure and dynamics of this
prominent mountain range. Precipitation,
driven by prevailing winds, tends to fall on
the upwind side of mountainous regions
and could control the distribution of
deformation within an orogen7. Changes
in the rates of river erosion or the initiation
of glacial erosion, both driven by climate
change, could control the width of glaciated
mountain ranges8,9.
The glacial cycles of the Northern
Hemisphere began in earnest about
3 million years ago, but changed their
characteristics in the middle Pleistocene
epoch. Marine oxygen isotope records of
global ice volume indicate that before about
1 million years ago, glacial–interglacial
cycles had a period of about 40,000 years,
were relatively symmetrical and had
moderate temperature oscillations
(Fig. 2). By contrast, the most recent
glacial–interglacial cycles have been
more extreme and asymmetrical, with a
period of about 100,000 years that consists
roughly of an 80,000-year build-up of ice,
10,000 years of glacial maximum and a
quick phase of deglaciation. This change
is likely to have allowed larger glaciers to
develop, but its implications for glacial
erosion and mountain building have yet to
be fully explored.
Berger and colleagues2 document a
substantial advance of Alaskan glaciers
around 1 million years ago, when smaller
glaciers were replaced by large, highly
erosive ice streams. And they interpret
low-temperature thermochronological data
as a coincident change in exhumation rates,
just in the zone of the St Elias Range where
glacial erosion would have been most
efficient. They suggest that focused glacial
erosion forced a tectonic response from
the orogen.
The model for the evolution of the
St Elias orogen during the Quaternary
period proposed by Berger and colleagues is
consistent with the predictions of the ‘critical
Coulomb wedge’ theory8–10, in which the
wedge-shaped cross-section of a mountain
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© 2008 Macmillan Publishers Limited. All rights reserved.
NEWS & VIEWS
Age Palaeomagnetic
(Myr) polarity zone
Oxygen isotopes
δ18O (‰)
Cold
5
0
Warm
4.5
4
3.5
1
2
5
6
7
0.2
8
Brunhes
0.4
9
10
11
12
13
14
0.6
15
16
17
1
0.78
0.99
1.07
19
20
21
22
Jamarillo event
0.8
Matuyama
18
1.2
25
30
31
34
Matuyama
36
1.4
37
47
52
55
2
1.77
1.95
Matuyama
1.8
Olduvai event
1.6
2.2
2.4
68
75
81
82
91
96
98
100
2.60
63
103
104
Gauss
2.6
62
Figure 2 Glacial cycles. Marine oxygen isotope records indicate a change in the period of glacial cycles, from
about 40,000-year cycles earlier than about 1 million years ago to the current 100,000-year cycles. This change
may have prompted a substantial advance of Alaskan glaciers, and consequently a reorganization of the tectonics
of the St Elias Range2. δ18O = [(18O/16O)sample/(18O/16O)standard] – 1, where the standard is Surface Mean Ocean Water
(SMOW). Reproduced with permission from ref. 11.
range is thought to be loosely analogous to
a pile of sand that is pushed by a bulldozer.
The width and surface gradient of the
range are then determined by the frictional
behaviour of the material that makes up the
range, the rate of addition of material due to
convergence, the rate of removal of material
due to erosion, and the gradient of the fault
that is treated as the base of the range. For
example, an increase in erosion rate causes a
Coulomb wedge mountain range to narrow
in order to maintain the surface gradient,
while maximum elevations are reduced8,9.
Despite its simplicity, the Coulomb wedge
model has proved remarkably effective.
Berger and colleagues find that the
intensification of glaciation during the
middle Pleistocene epoch and concomitant
increase in erosion rates caused a reduction
in the width and maximum elevations
of the St Elias wedge. Their offshore
seismic data reveal that enhanced glacial
erosion of the subaerial part of the wedge
coincided with a cessation of shortening
and deformation of the submarine toe of
the St Elias wedge. This suggests that the
high influx of sediments resulting from
increased onshore denudation buried the
submarine toe of the wedge and reduced
the critical taper, forcing the wedge to
adjust structurally. In other words, a change
in climate led to a change in tectonics.
The study by Berger et al.2 not only
provides empirical evidence that glacial
erosion has an important role in the
evolution of active mountain ranges, but
also highlights that the interaction between
climate change and tectonic response may
be more subtle than anticipated: it was
not the onset of Northern Hemisphere
glaciation that caused a tectonic response,
but the intensification of glacial coverage
around 1 million years ago. This should
encourage more detailed study of glacial
history and tectonic response in other
mountain ranges around the world, so that
we can fully grasp the relationship between
climate change and mountain building.
It should be borne in mind that the
interpretation of thermochronological
data is heavily dependent on the thermal
model of the orogen. This is often
difficult to constrain for the present, let
alone when accounting for past changes
in the thermal structure of an active
orogen. Measurements of erosion rates
from either sediment yields or various
cosmogenic isotope methods are also
difficult to extrapolate to the lifetime of an
orogen. Detailed, reliable estimates of the
topographic history of mountain ranges are
highly desirable in this context, but tend to
be elusive.
Nevertheless, Berger and colleagues2
have demonstrated that taken together,
a diverse set of field data can provide a
compelling case for a substantial change in
the dynamics of an active mountain range in
response to a change in climate and erosion.
References
Koons, P. O. Annu. Rev. Earth Planet. Sci. 23, 375–408 (1995).
Berger, A. L. et al. Nature Geosci. 1, 793–799 (2008).
Molnar, P. & England, P. Nature 346, 29–34 (1990).
Hallet, B., Hunter, I. & Bogen, J. Global Planet. Change
12, 213–235 (1996).
5. Zeitler, P. K. et al. GSA Today 11, 4–9 (2001).
6. Beaumont, C., Jamieson, R. A., Nguyen, M. H. & Lee, B.
Nature 414, 738–742 (2001).
7. Willett, S. D. J. Geophys. Res. 104, 28957–28982 (1999).
8. Whipple, K. X. & Meade, B. J. J. Geophys. Res.
109, F01011 (2004).
9. Tomkin, J. H. & Roe, G. H. Earth Planet. Sci. Lett.
262, 385–397 (2007).
10. Dahlen, F. A., Suppe, J. & Davis, D. J. Geophys. Res.
89, 10087–10101 (1984).
11. Berendsen, H. J. A. De Vorming Van Het Land, Inleiding in
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2.
3.
4.
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© 2008 Macmillan Publishers Limited. All rights reserved.
733
BACKSTORY
Flying high
Aaron Berger and colleagues leapt out of helicopters in the snow and fog in their quest to
understand the effects of glacial erosion on mountain formation.
What was the objective of the work?
We wanted to investigate the interplay
between climate, plate tectonics and
mountain evolution in a natural system.
Climate–tectonic interaction has been an
area of intense research and debate over
the past few decades. Under the St Elias
Erosion and Tectonics Project (STEEP),
we set out to examine the impact of the
onset of glacial conditions on erosion and
the activity of major faults within Alaska’s
St Elias mountain range. With the results
of our research, we were able to record
what we believe to be a first-rate example
of climate–tectonic interactions, which
has previously only been observed in
model simulations.
Why did you choose this particular
location for fieldwork?
The St Elias region has long been
recognized as the most heavily glaciated
active mountain belt on Earth. In the
mid-1990s it was becoming apparent that
temperate glaciers were, at least locally, the
most intense erosional agents on the planet.
At the same time, the scientific community
was beginning to look seriously at the
complex interplay between erosion and
tectonics, and it quickly became apparent
that St Elias was a premier site to examine
these interactions.
How long did it take to plan the work?
In one sense it took about 30 years, because
the area presents some of the greatest
logistical challenges on Earth, and were
it not for the experience of the principal
investigators we could n ever have done
the project. More specifically though,
planning for the STEEP project began
in earnest in the late 1990s, when the
National Science Foundation
Margins workshop
serendipitously
brought our
research group
together, and
helped lay the
groundwork
for the
present
study. Once
the project was
802
Helicopter towering over a cliff face in the St Elias mountain range in Alaska.
underway, each field season took about six
months to plan, in order to accomplish the
serious multitasking involved.
Did you encounter any difficulties?
The weather was the biggest obstacle
encountered throughout the field
component of the research. The prevailing
winds from the Gulf of Alaska, coupled
with the highest coastal relief on Earth
and the large ice fields in the mountains,
resulted in intense year-round rain, snow
and freezing fog. For long stretches of
time, all operations were forced to come
to a halt until the weather cleared. Owing
to these adverse conditions, it became
evident that to achieve our goals we had
to work as a united team, and constantly
re-evaluate our scientific priorities,
goals and objectives. Being stuck inside
a tent, however, turned out to be an
amazing opportunity to brainstorm and
share ideas.
Did you have any encounters with
dangerous animals?
Brown and black bears were common
throughout the field area. On several
helicopter flights we spotted around
twenty bears. Even more were seen
along the coast during the berry and
salmon season. Because the topography
is steep, and bears prefer flat terrain,
‘bear highways’ unfortunately tended to
coincide with suitable landing sites. We
managed to avoid serious accidents, but
the scientific equipment did not fare so
well; it’s amazing how efficiently a brown
bear can damage a piece of fortified
scientific equipment. What’s more, it
seemed that the bears truly delighted in
their accomplishments.
Was anything particularly memorable
about the expedition?
I will never forget flying in a helicopter
through some of the most rugged and
glaciated landscapes on Earth. The
experience gave me a completely new
understanding of geological processes
and rates of geological change. During
the project I also had the distinction of
working with extremely competent pilots,
whom I quickly realized I had to entrust
with my life. When I needed a sample
from the most treacherous mountain
around, the pilots would find a way, even
if that meant that I had to climb out of a
perfectly good helicopter hovering over
a knife ridge in the fog. Without the
expert capabilities of the helicopter pilots,
it is safe to say that the entire project,
and indeed my life, would have been
jeopardized on numerous occasions.
This is the Backstory to the work by
Aaron L. Berger and colleagues, published
on page 793 of this issue.
nature geoscience | VOL 1 | NOVEMBER 2008 | www.nature.com/naturegeoscience
© 2008 Macmillan Publishers Limited. All rights reserved.