Download Plateau uplift in western Canada caused by lithospheric

LETTERS
PUBLISHED ONLINE: 19 OCTOBER 2014 | DOI: 10.1038/NGEO2270
Plateau uplift in western Canada caused by
lithospheric delamination along a craton edge
Xuewei Bao*, David W. Eaton and Bernard Guest
Continental plateaux, such as the Tibetan Plateau in Asia and
the Altiplano–Puna Plateau in South America, are thought
to form partly because upwelling, hot asthenospheric mantle
replaces some of the denser, lower lithosphere1–4 , making
the region more buoyant. The spatial and temporal scales
of this process are debated, with proposed mechanisms
ranging from delamination of fragments to that of the
entire lithosphere1–4 . The Canadian Cordillera is an exhumed
ancient plateau that abuts the North American Craton5 . The
region experienced rapid uplift during the mid-to-late Eocene,
followed by voluminous magmatism6 , a transition from a
compressional to extensional tectonic regime7 and removal of
mafic lower crust8 . Here we use Rayleigh-wave tomographic
and thermochronological data to show that these features
can be explained by delamination of the entire lithosphere
beneath the Canadian Cordillera. We show that the transition
from the North American Craton to the plateau is marked
by an abrupt reduction in lithospheric thickness by more
than 150 km and that asthenosphere directly underlies the
crust beneath the plateau region. We identify a 250-km-wide
seismic anomaly about 150–250 km beneath the plateau that
we interpret as a block of intact, delaminated lithosphere. We
suggest that mantle material upwelling along the sharp craton
edge9 triggered large-scale delamination of the lithosphere
about 55 million years ago, and caused the plateau to uplift.
Orogenic plateaux are broad, high-standing, low-relief regions
that develop in mature continental mountain belts. Plateaux are
important because they affect climate2 , orogenesis3 and tectonic
plate interactions10 . Modern examples include the Tibetan Plateau
and the Altiplano, which developed in the Cenozoic by some combination of lithospheric delamination, lower crustal flow and regional
shortening1–4 . Although the rates, timing and mechanisms driving
plateau uplift remain controversial, lithospheric delamination continues to be a leading mechanism explaining their formation and
maintenance, albeit with debated temporal and spatial scales1–4 .
Fossil plateaux can provide important insights into orogenic
processes. In the Eocene, the interior of the Canadian Cordillera was
perhaps the highest-standing mountain belt on Earth11 and, in terms
of crustal architecture, is one of the best-studied recent orogens.
In this region, thickened crust that developed during orogenesis is
no longer present12 ; rather, present-day mountainous topography
reflects the isostatic response to regional thermal structure5 .
In this study, we integrate published thermochronology
results with new high-resolution teleseismic tomography. We use
fundamental-mode Rayleigh waves recorded during the period
2006–2013 at 86 broadband seismograph stations to construct a
new three-dimensional tomographic shear-velocity (Vs ) model to
depths of ∼300 km. Figure 1a shows Vs at 105 km depth extracted
from our tomographic model, highlighting an abrupt transition
from the high-velocity mantle of the North American Craton to
the low-velocity mantle of the Cordillera. The boundary between
these domains coincides with the southern Rocky Mountain Trench
(RMT), a conspicuous topographic lineament (Fig. 1b). In crosssection (Fig. 2), the craton edge seems to be subvertical, delineating
a step change in thickness of the seismological lithosphere from
>200 km beneath the craton to <50 km beneath the Cordillera.
As shown in the Supplementary Information, the geometrical
expression of this feature is virtually unchanged along the length
of the RMT; moreover, synthetic tests, coupled with phase-velocity
dispersion curves for closely separated paths on either side of
the RMT, show that the location, orientation, depth extent and
velocity contrast of the craton edge are robust elements of our
tomographic model.
The RMT also coincides with a major change in upper-mantle
composition and surface heat flux5 . The change in thermal state
of the crust is clearly expressed by truncation of aeromagnetic
anomalies (Fig. 1c), which originate from magnetized domains of
Precambrian age at depths of 20–25 km in the craton13 . Although
structural interpretations of crustal seismic data indicate that corresponding Precambrian domains extend as a vestigial wedge in the
lower crust for hundreds of kilometres west of the RMT (ref. 12),
the termination of magnetic anomalies occurs where crustal temperature surpasses the Curie limit for magnetite (585 ◦ C), consistent
with nearby xenolith data14 . Furthermore, spinel lherzolite xenoliths
show that, west of the RMT, the upper mantle is fertile in composition14 (that is, similar to Mid-Ocean Ridge Basalt, MORB), in contrast to melt-depleted compositions of nearby cratonic xenoliths15 .
Medium- to high-temperature thermochronological data record
variations in cooling rate that constrain exhumation history across
the RMT. Ar/Ar and K/Ar cooling ages from hornblende, biotite
and k-feldspar, and fission track ages from zircon and apatite
(see Supplementary Information) show that, west of the RMT,
the Cordillera experienced rapid cooling (∼10–20 ◦ C Myr−1 ) from
∼500 ◦ C to ∼100 ◦ C during the Eocene (about 56–34 Ma). In
contrast, fission track ages from zircon and apatite to the east
of the RMT, in the Late Cretaceous to Eocene Foreland Belt
that accommodated ∼180 km of shortening16 , show relatively slow
cooling (1–2 ◦ C Myr−1 ) from ∼110 ◦ C to 20 ◦ C between ∼80 Ma
and ∼25 Ma. Moreover, cooling ages obtained for the footwalls and
hanging walls of major Eocene normal faults west of the RMT
are synchronous, within analytical uncertainty17 , implying that the
Eocene cooling in the Cordillera was primarily caused by erosion
associated with large-scale plateau uplift rather than local tectonic
unroofing. The period of rapid cooling west of the RMT coincides
with the 59–52 Ma onset of regional extension and magmatism in
the Cordillera, whereas thrusting continued until ∼50 Ma in the
Foreland Belt; extension in the Foreland Belt started at 52–49 Ma
(ref. 16). This overlap of extension in the hinterland west of the
RMT, with contraction in the foreland to the east of the RMT, is
similar to what is observed around the actively extending Tibetan
Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada. *e-mail: [email protected]
830
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2270
LETTERS
b
60
Alberta
British Columbia
56
So
ut
Latitude (°)
54
he
rn
km
0
st
er
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nl
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yM
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Latitude (°)
elt
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c
−110
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52
50
Xenolith sample sites
−6
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Vs perturbation (%)
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6
−124
Seismic station sites
Morphotectonic belt boundaries
Rocky Mountain Trench trace
−120
−116
Longitude (°)
−300 −150
0
150
−112
300
Mag. (nT)
Figure 1 | Shear velocity (V s ), topography and aeromagnetic maps of the study region. a, Vs perturbation at 105 km depth. Black triangles represent
mantle xenolith sites. The line AA0 represents the location of the velocity cross-section in Fig. 2. b, Topography of our study region (white box) and
environs. Black circles show the locations of the 86 broadband seismic stations used. Red arrows show the position of the RMT; black arrow shows the
nearby station LLLB. c, Magnetic map showing conspicuous truncation of magnetic anomalies at the RMT (dashed line), which reflects a step increase
(from NE to SW) of temperature in the mid-crust, where through-going magnetized bodies are located13 .
and Altiplano plateaux4 . It is possible that isostatic uplift after
delamination beneath the Cordilleran hinterland altered the taper
of the orogen and promoted 55–49 Ma propagation of the Foreland
thrust belt into the foothills belt16 .
Previous studies in the interior of the Cordillera have documented a regionally flat Moho12 and hot upper mantle5,14 . West of
the RMT, our tomographic model shows that an upper-mantle layer,
with sufficiently low velocity to be asthenosphere, extends from the
base of the crust to a depth of ∼150 km, where it is floored by a
zone with higher velocity. The spatial extent of the low-velocity layer
corresponds approximately with an electrically conductive layer18 ,
interpreted as upwelling mantle material associated with regional
uplift and emplacement of voluminous mafic magmas6 . The velocity
increase at ∼150 km depth is required to fit our dispersion data,
but is incompatible with a velocity–depth profile for a homogeneous asthenosphere under adiabatic conditions (Supplementary
Information). This velocity increase was also detected by previous
surface-wave tomography19 and S-receiver function20 studies. The
positive conversion phase observed at ∼150 km in the S-receiver
function correlates with the top of the high-velocity structure imaged here (Fig. 2c), indicating that this velocity transition is a firstorder discontinuity rather than a typically gradual velocity increase
below the asthenosphere. Moreover, in the 150–200 km depth range
the shear-wave velocities beneath other tectonically active regions,
such as the northern Canadian Cordillera and the Basin and Range,
are significantly lower than here (Supplementary Fig. 8), further
supporting the anomalous character of this sub-asthenospheric
high-velocity region.
A number of possible mechanisms have been proposed to explain
thin lithosphere in the interior of the Canadian Cordillera as well
as similar tectonic settings, including flow-induced gravitational
instability in a back-arc setting21 , vigorous edge-driven convection9 ,
lithospheric extension and delamination22,23 . Explaining the
underlying mantle high-velocity zone, however, is problematic for
most of these scenarios. This high-velocity feature is not explained,
for example, by edge-driven convection alone9 , and it is unlikely
that it represents the Juan de Fuca slab, which has a significantly
steeper dip24 . On the other hand, lithospheric delamination
provides a satisfactory explanation for all of our observations,
including: history of regional uplift and magmatism; flat Moho25 ;
and shallow asthenosphere underlain by a high-velocity layer,
which in this scenario may be interpreted as a foundering block of
detached lithosphere.
Our preferred tectonic model is illustrated in Fig. 3. On the basis
of the timing implied by thermochronologic data, delamination
is interpreted to have commenced about 55 Ma. According to our
model, detachment of a block of lithosphere led to rapid advection
of heat to the base of the Cordilleran crust and the establishment
of a new, flat Moho. This was probably accompanied by removal
of mafic lower crust, rapid crustal uplift and extension west of the
RMT, together with contraction in the Foreland Belt to the east,
and small-scale convection in the uppermost mantle, as manifested
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831
NATURE GEOSCIENCE DOI: 10.1038/NGEO2270
LETTERS
Depth (km) or
heat flow (mW m−2)
120
a
Smooth heat flow
Curie depth
80
0
50
40
0
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Elevation (km)
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b
Cordillera
MOHO
RMT
Craton
Edge driven
convection
Asthenosphere
300
0
A
−124
A’
50
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100
JdF
Depth (km)
150
−122
−120
−118
−116
−114
3.8
4.0
Vs (km
4.2
4.4
4.6
−112
RMT
Erosion
Transport and deposition
Deposition
Sea level
Small-scale
convection
150
250
Intermontane and Coast belts
Omineca Belt
Foreland Belt
Foreland Basin
300
4.8
−124
s−1)
−122
−120
−118
−116
−114
−112
Longitude (°)
Figure 2 | Heat flow, Curie depth, topography and V s profile for the
southern Canadian Cordillera. a, Depth to the Curie isotherm and heat flow
profile5 , where dots show individual heat flow measurements and black
lines show spatial average values. b, Topography profile along AA0 in Fig. 1a.
c, Vs section along AA0 . Black line shows the Moho. The dashed white line
shows our inferred lithosphere–asthenosphere boundary beneath the
craton; dashed black line indicates the top of a high-velocity layer,
interpreted as delaminated lithosphere; red curve shows a stacked
S-receiver function from nearby station LLLB (ref. 20). JdF, Juan de
Fuca slab.
c
Present day
0
100
Cratonic lithosphere
150
200
250
by irregular occurrences of magmatism6 . The tomographic image
suggests that detached lithosphere remained essentially intact after
delamination; simple calculations show that a velocity anomaly
associated with a block of lithosphere of this size and with
composition that is distinct from the asthenosphere could persist for
more than 50 Myr (Supplementary Fig. 12). The inferred timing and
present-day position of the detached lithosphere implies an average
∼2 mm yr−1 foundering rate, although it is likely that sinking would
have occurred at a faster rate near the onset of delamination.
In our tectonic model, the present-day sharpness of the craton
edge was enhanced by the delamination process, but before this
an antecedent step-like structure probably existed at the transition
from craton to Cordillera8,12 . Small-scale thermal–chemical convection associated with this proto-step may have played a pivotal
role in the initiation of delamination. Previous studies9,21,22,26 show
that upwelling asthenospheric flow due to edge-driven convection
can destabilize previously hydrated lithosphere by refertilization
and shearing, causing a significant density increase and viscosity
decrease. Basal shearing may have resulted in stress and strain
concentration along the RMT, a deep rheological boundary, leading
to peeling off of a segment of lithospheric mantle. East of the
RMT, cratonic lithosphere has a much higher viscosity and thus is
capable of resisting modifications, preserving this lateral structure
over long timescales27 .
MOHO
50
Depth (km)
3.6
MOHO
JdF
3.4
−114
200
−112
Longitude (°)
3.2
−118
−116
Longitude (°)
Plateau
0
300
−120
∼40 Ma
100
250
−122
b
50
832
Craton edge
150
250
2
Cratonic lithosphere
Cordilleran
lithosphere
100
200
c
Depth (km)
Pre 55 Ma
Depth (km)
a
Delaminated
lithosphere
Asthenosphere
300
−124
−122
−120
−118
Longitude (°)
−116
−114
−112
VE 2:1
Figure 3 | Model for evolution of the Cordilleran back-arc orogenic plateau.
a, Before 55 Ma, continental lithospheric mantle extends westwards
beneath the imbricated crust of the Cordillera and autochthonous North
American crust. Above the Juan de Fuca (JdF) subducting slab, edge-driven
convection is influenced by a proto-step in the lithospheric keel. b, Mantle
delamination is accompanied by a transition from lithospheric contraction
to extension, incursion of asthenosphere to shallow depths, heating, uplift
and erosion of a newly formed orogenic plateau. c, Mature present-day
back-arc, with deeply incised fossil orogenic plateau. Delaminated
lithosphere has foundered to a present location above the JdF.
In summary, our new Rayleigh-wave tomography model reveals
that thick cratonic lithosphere, which underlies most of the North
American continent, is bounded to the west by a remarkably sharp
vertical edge. The craton edge also delineates the eastern limit
of a fossil orogenic plateau, manifested by a step change in the
present-day thermal regime and a significant contrast in crustal
exhumation history. In our proposed model, orogenic plateau
formation was initiated by a wholesale lithospheric delamination
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2270
event that commenced about 55 Ma, probably localized by an
antecedent proto-step and triggered by edge-driven convection9,26 .
The present location of detached lithosphere is imaged by an
apparently intact block of high-velocity material, the top of which
lies at approximately 150 km depth, implying an average foundering
rate of about 2 mm yr−1 . Our reinterpretation of the Cordilleran
interior as an exhumed orogenic plateau that formed in response
to lithospheric delamination differs from previous tectonic models,
which focus primarily on the back-arc tectonic setting4,5,21 . This
model explains diverse observations in the Canadian Cordillera,
provides a new lithospheric framework for evaluating the evolution
of the linked Cordilleran and Foreland contractional belts, and may
provide a template for understanding orogenic plateau formation
and evolution near cratonic margins elsewhere.
Methods
Rayleigh-wave data used in this study were recorded by 19 permanent stations of
the Canadian National Seismograph Network (CNSN), as well as 67 temporary
stations with 9 from the Alberta Telemetered Seismograph Network (ATSN),
19 from the Canadian Rockies and Alberta Network (CRANE) and 39 from
USArray. We selected 583 shallow earthquakes within an epicentral distance of
20◦ –120◦ and with magnitude ≥6 over the years 2006–2013 (Supplementary
Fig. 1). Only Rayleigh waves with high-quality dispersion and located within
±5◦ of the inter-station great circle paths were used in two-station
phase-velocity measurements.
We measured inter-station phase velocities using a cross-correlation
method28 . The robustness of phase-velocity curves was improved by averaging
measurements from multiple events, during which we decreased the influence of
off-great-circle propagation and scattering. In addition, only dispersion data
averaged from at least three different events and with standard deviation lower
than 2.5% were chose for tomography, which resulted in ∼500 to ∼1,400
phase-velocity measurements at periods from 18 to 240 s (Supplementary Fig. 2).
From the selected path average dispersion data, we used a linearized
two-dimensional inversion method29 to construct phase-velocity maps on a
1◦ × 1◦ grid. Resolution tests (Supplementary Fig. 3) and phase-velocity
distributions (Supplementary Fig. 4) at several representative periods are provided
in the Supplementary Information. To construct a three-dimensional model, we
inverted phase velocities at each grid node. Considering the period range used in
surface-wave inversion, our model constrains shear-velocity structure from
mid-crust to a depth of ∼300 km with a scale length of 50 km or more.
The definition of lithospheric thickness from shear-velocity structure,
especially for cratonic regions that lack any obvious low-velocity zone, is not
unique. When estimating the lithospheric thickness for the cratonic part of the
velocity profile, we adopted a previously defined proxy for the
lithosphere–asthenosphere boundary (LAB; ref. 15), at which depth shear velocity
is 0.5% higher than AK135. The stacked S-receiver function shown in Fig. 2c is
for western Pacific events and is projected to a location on the profile based on
piercing points at a depth of 150 km.
The earthquake waveform data used in this study can be obtained from
www.iris.edu and www.earthquakescanada.nrcan.gc.ca/index-eng.php.
Received 22 May 2014; accepted 15 September 2014;
published online 19 October 2014
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Acknowledgements
Seismic data were downloaded from the Incorporated Research Institution for
Seismology Data Management Center and Canadian National Data Center. This study
was funded by the Natural Sciences and Engineering Research Council of Canada.
Author contributions
X.B. performed Rayleigh-wave tomography. Thermal calculations and model
conceptualization were provided by D.W.E. Thermochronologic data and concepts were
provided by B.G. All authors contributed to discussion of the results and their
implications, as well as preparation of the manuscript.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints.
Correspondence and requests for materials should be addressed to X.B.
Competing financial interests
The authors declare no competing financial interests.
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