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Geological Society, London, Special Publications
Growth and demise of continental arcs and orogenic plateaux in
the North American Cordillera: from Baja to British Columbia
Donna L. Whitney, Scott R. Paterson, Keegan L. Schmidt, Allen F. Glazner and Christopher F.
Kopf
Geological Society, London, Special Publications 2004; v. 227; p. 167-175
doi:10.1144/GSL.SP.2004.227.01.09
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Growth and demise of continental arcs and orogenic plateaux in the
North American Cordillera: from Baja to British Columbia
D O N N A U W H I T N E Y 1, S C O T T R. P A T E R S O N 2, K E E G A N L. S C H M I D T 2'4,
A L L E N F. G L A Z N E R 3 & C H R I S T O P H E R F. KOPF 3
1Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA
(e-mail: [email protected])
2Earth Sciences, USC, Los Angeles, California 90089, USA
3Geological Sciences, UNC-Chapel Hill, North Carolina 27599, USA
4present address: Natural Sciences, Lewis-Clark State College, Lewiston, Idaho 83501, USA
Abstract: In the North American Cordillera, crustal thickening, magmatism and flow of deep
crust created an orogenic plateau, or series of related plateaux, in the Late Mesozoic-Early
Cenozoic. From west to east, the plateaux extended from the continental arcs to the inboard
crystalline belts of the Ornineca-Sevier belt. From north to south, the plateaux ranged
from British Columbia/SE Alaska to Baja California, Mexico. Although a vast region of
western North America was characterized by thickened crust (60-70 km), unroofing of
deep crust from > 30 km was largely confined to the edges of the plateaux: the arcs and the
eastern margins. Comparison of the unroofing histories of the Cordilleran arcs reveals that
they differed dramatically from each other in the amount and style, but not timing, of exhumation. The northern Cordilleran arc and northern interior (Omineca) belt were exhumed from
deep mid-crustal levels, with regional-scale Eocene extension accompanied by magmatism.
In contrast, the central (Sierra Nevada) and southern (Peninsular Ranges) arcs were unroofed
to much shallower levels (typically <15 km), primarily by erosion and local deformation.
North to south differences in exhumation style and magnitude in the Cordilleran arcs may
reflect differences in the degree of coupling between the subducting plate and the thickened
continental lithosphere in the north v. south. In the northern Cordillera, relationships
between Pacific-region plate activity and Tertiary continental extension/magmatism and
deep exhumation suggest continued geodynamic coupling between subducting plates and orogenic crust following crustal thickening and plateau formation. In contrast, the central and
southern Cordilleran arcs do not contain evidence for mechanical links with the subducting
plate after the Late Cretaceous.
The subduction of oceanic lithosphere beneath
continents has a profound effect on continental
lithosphere by driving crustal thickening and
magmatism, and is a precursor of collision
tectonics. Of interest is how deformation of the
continental lithosphere and the mechanical
relationship of the continent to the subducting
plate evolve through time following maximum
crustal thickening and subduction-related magmatism~ In particular, what is the geodynamic
relationship between plate tectonic factors (e.g.
velocities and trajectories of oceanic plates) and
processes internal to continental orogenic crust
(e.g. gravitational instability of heated, thickened
crust)? Previous studies have proposed varying
degrees of coupling between subducting plates
and thickened continental lithosphere, including
recent work emphasizing a large degree of decoupiing (e.g. Vanderhaeghe & Teyssier 2001).
The origin and fate of orogenic plateaux may
be related to these issues of coupling/decoupling
of subducting plates and thickened continental
lithosphere. Orogenic plateaux are a first-order
expression of convergent plate boundaries involving continental lithosphere (e.g. present-day
Tibet and the Andean Altiplano-Puna). Plateau
construction may involve lithosphere-scale
shortening of both crust and upper mantle, or
may involve decoupling of the crust from the
mantle, with shortening taken up by the rheologically weak deep crust. The mechanisms and rates
of construction and demise of orogenic plateaux
provide information about the thermo-mechanical behaviour of continental lithosphere from the mantle to the surface - during and
after plate convergence. In this chapter, we
examine the exhumed margins of a palaeoplateau for evidence of the degree of coupling
From: GrtOCOTT,J., MCCAFFREY,K. J. W., TAYLOR,G. & TIKOFF,B. (eds) 2004. VerticalCouplingand
Decoupling in the Lithosphere.Geological Society, London, Special Publications, 227, 167-175.
0305-8719/04/$15 9 The Geological Society of London 2004.
D.L. WHITNEY ET AL.
168
between the subducting oceanic plate and the
thickened continental lithosphere.
A large plateau likely existed in the interior
(Sevier belt, Fig. 1) of the North American Cordillera in the Late Cretaceous-Early Tertiary,
analogous to orogenic plateaux in active orogens,
as has been proposed by many authors (e.g.
Coney & Harms 1984; England & Thompson
1986; Molnar & Lyon-Caen 1988; Wernicke
et al. 1996; Wolfe et al. 1998; Dilek & Moores
1999). In this chapter, we extend this idea and
propose that the plateau regions included the
arcs in the northern, central and southern Cordillera, as well as the inboard Sevier and Omineca
belts (Figs 1 & 2a). To understand the evolution
of these plateau regions, we investigate the record
of crustal thickening and unroofing at the western
margin of the plateaux: the continental arcs.
Cordilleran crustal thickening
and unroofing
COAST~ I I ! . ~ . , ~
MOUNTAINS i . - ~ ; ~ :
O \~
i {.~
BritishColumbia~~' '~.::~.A
Washington(USA)~
~
~~."-t~"'?.-~
Fig. 3B " . . . . .
!t1-:: ,
OCEAN ] S I E R R A
. :-!.) :-!.". r
.-~.-:.--
California~[ ~:l....~..Fia.
4\1
PENINSULAP, ~IL'-'~" :'-"-': ": v:'.
,, m.'~...'.l,%/.e'.
RANGES \ li.'-~i-.'v.~, v.-.'v,
Baja,Mexico\ ~ i . ~ : : :'.:'.'.~ :" :" ::
Fig.~ 5 "~ : :
Fig. 1. Generalized map of the North American
Cordillera showing the location of the batholith belts (grey
shading) and the inboard orogenic zone (Omineca-Sevier
belt). C, Cascades. Black shading shows location of
metamorphic core complexes. Stippled region shows
location of former thickened crust (>60 kin). Stars show
general location of rocks recording >_9 kbar (arc/batholith
belts only; data not shown for Omineca-Sevier belt). The
western margin of the plateau was approximately at the
western margin of the arcs, but was located within the PRB
at the boundary between western mad eastern zones (see
Fig. 5).
Large tracts of the western continental margin
from Baja, Mexico, to British Columbia and SE
Alaska are dominated by rocks formed during
Mesozoic-Early Cenozoic magmatism, metamorphism and deformation. The continental
arcs, now unroofed batholith belts, include the
Coast Mountains-Cascades (CMC), the Sierra
Nevada (SN) and the Peninsular Ranges batholith (PRB). A parallel belt of crystalline rocks
occurs inboard of the arcs (Omineca belt in the
north; Sevier belt in the south) (Fig. 1). The batholith belts developed as Andean-style arcs generated by east-dipping subduction.
The exhumed continental arcs in the North
American Cordillera are broadly similar in that
they were generated in part by east-dipping,
Mesozoic-Early Cenozoic subduction of oceanic plates (Table 1). The timing, style (erosion
v. tectonic denudation) and magnitude of unroofing/exhumation of the different batholith
belts, however, varied from north to south, with
the most dramatic evidence for deep exhumation
of arc metamorphic rocks and high-pressure plutonic rocks in the northern Cordillera (Whitney
et al. 1999; Valley et al. 2003) (Table 1, Fig. 2).
It is important to note that the exposure of formerly deep rocks at the surface may indicate
the former presence of thickened crust, but the
absence of these rocks at the surface does not
necessarily indicate thinner crust.
Although the Tertiary tectonic evolution of the
Sevier-Omineca belt, inboard of the arcs, also
varied from north to south, we focus in this
chapter on the continental arcs because they
occupied a key tectonic position for evaluating
dynamic links between the subducting plates
and the continents. In particular, if the arcs
shared similar constructional histories, the timing
and mechanisms of unroofing of the arcs can be
compared and used to evaluate the relative
roles of external v. internal processes in the evol-
CORDILLERAN ARC AND OROGENIC PLATEAUX
ution and eventual demise of orogenic plateaux.
In the following sections, we summarize the
history and characteristics of each region of the
North American Cordillera (northern, central
and southern) to determine the magnitude of
crustal thickening, and timing and mechanism(s)
of unroofing.
N o r t h e r n Cordillera
The Coast Mountains-Cascades (CMC) orogen
is a > 1500 km long belt that extends from
Washington to SE Alaska (Fig. 1). The CMC
formed during mid-Cretaceous accretion of terranes and east-dipping subduction beneath the
accreted terranes. Intrusive activity occurred episodically from 110 to 45 Ma, including a major
magmatic episode in the Tertiary that produced
batholith-sized intrusions in the arcs (Paterson
& Miller 1998, and references therein; Stowell
& McClelland 2000, and references therein)
(Table 1) and inboard Omineca belt (Fig. 1).
The Cascade Range, at the southern end of the
Coast Mountains, is offset from the rest of the
belt by strike-slip faults (Fig. 1). Differences in
the large-scale structure and exhumation history
of the Cascades compared to the rest of the
Coast belt may be due to the position of the Cascades at the 'end' of the orogen. For example, the
overall antiformal structure of the Cascades may
represent buckling at an orogenic corner (syntaxis), analogous to the western and eastern
ends of the Himalayan orogen.
Despite some structural differences, the Cascades and Coast Mountains both contain moderately high-pressure rocks (Figs 1 & 3), including
meta-supracrustal rocks buried to depths of 3040 km. Some regions along the margins of the
crystalline core record low-P metamorphism, but
moderately high-pressure rocks ( ~ 9 - 1 2 kbar)
have been reported in SE Alaska, northern and
southern British Columbia, and throughout the
Cascades (Whitney et al. 1999, and references
therein) (Figs 1-3). Local regions in the Intermontane belt to the east of the arc also comprise
169
exhumed mid-crust (6-7kbar; Friedman &
Armstrong 1988).
Evidence for thickened crust in the Late
Cretaceous includes: (1) the presence of
12 kbar (40 km), upper amphibolite facies rocks
exhumed at the surface, indicating that the
crust must once have been thicker than at
present (32-35 kin; Potter et al. 1986; Cook
et al. 1992; Clowes et al. 1995); and (2) the geochemistry of calc-alkaline plutons indicates generation by partial melting of garnet-bearing mafic
crust at depths of ~ 6 0 km (DeBari et al. 1998).
The active magmatic arc therefore contained
thickened crust (60-70 km) in the Late Cretaceous (Miller & Paterson 2001).
In the CMC arc, exhumation commenced
during Late Cretaceous contraction (Miller &
Paterson 2001), and both the CMC and the
Omineca belts were thinned by Eocene extension. The Omineca belt is located between the
Intermontane belt and the palaeo-margin of
North America (Fig. 1), and consists of a series
of metamorphic core complexes that formed
during Eocene extension (Parrish et al. 1988).
The core complexes contain migmatitic domes
that record high-T metamorphism (up to
~800~
at mid-crustal pressures ( 7 - 1 0 k b a r )
(e.g. Spear & Parrish 1996; Norlander et al.
2002). Based on metamorphic data and geodynamic considerations (discussed below), we
propose that the entire region from the southern
CMC to the Omineca belt was thickened to at
least 60 km, primarily in the Late Cretaceous.
Central Cordillera
Magmatism in the ~600 km long Sierra Nevada
belt occurred between ~ 220 and 80 Ma, with the
greatest volume between 9 8 - 8 6 Ma (Bateman
1992; Coleman & Glazner 1997) (Table 1).
Emplacement depths of plutons decrease from
9 - 1 5 k m in the west to < 3 k r n in the east
(Ague & Brimhall 1988) (Fig. 4). Recorded
pressures of metamorphic rocks are typically
low (1.5-3 kbar), but are higher ( ~ 6 - 9 kbar)
at the southern end of the batholith where
Table 1. Comparison of the Coast Mountains- Cascades (CMC), Sierra Nevada (SN) and Peninsular Ranges
batholith (PRB) belts*
Arc
CMC
SN
PRB
Timing of major magnetism
(Ma)
Max. P (kbar) of exposed
rocks
Max. crustal
thickness t (kin)
Modern crustal
thickness (km)
110-45
98-86
164- 85
12
9 (but typically <5)
<6
> 60
40-45 or 60-70
> 55
32-35
33
25 -43
*See text for references.
t Inferred(seetext).
170
D.L. WHITNEY E T A L .
Fig. 2. (a) Cartoon showing the spatial relationships of
the arc, interior belt, thickened crust and the plateau,
with the modern and Late Cretaceous moho depths shown,
and the characteristic exposure levels of the Coast
Mountains-Cascades (CMC), Sierra Nevada (SN) and
Peninsular Ranges batholith (PRB). (b) Pressuretemperature diagram showing representative data for each
of the three arcs discussed in this chapter. See text for
references.
additional unroofing has occurred due to faulting
(Pickett & Saleeby 1993) (Figs 2 & 4). These
variations reflect differential unroofing, rather
than variations in Late Cretaceous crustal
thickness.
The western margin of crustal thickening in
the Sierra Nevada was the Western Metamorphic
Belt (WMB; Fig. 4), which was thickened
between 155 and l l 2 M a (Ague & Brimhall
1988; Paterson et al. 1991). West of this belt,
marine sediments were deposited from Late
Jurassic through Tertiary time, but the region to
the east comprised thickened crust (Wernicke
et al. 1996; Wolfe et al. 1998).
The presence of Cretaceous volcanic
sequences throughout the eastern Sierra and the
emplacement depths of plutons indicate that 5 10 km of crust has been removed since the Cretaceous. Ague & Brimhall (1988) estimated
that 6 km of Sierran crust had been removed by
erosion. These data and estimates indicate a
palaeo-crustal thickness of ~ 4 0 - 4 5 k m , but
Wernicke et al. (1996) noted that the presentday crust, from the eastern Sierra to the Basin
and Range, has been thinned by 50% over the
last 20 Ma. The present-day Moho is ~ 3 3 km
under the Sierra (Wernicke et al. 1996). This
implies a crustal thickness of 6 0 - 7 0 km prior
to extension, similar to the crustal thickness estimates derived from xenolith geobarometry
(Ducea & Saleeby 1996).
Southern Cordillera
The Peninsular Ranges batholith (PRB) extends
for 1600 km from southern California to the
southern tip of Baja California, Mexico (Figs 1
& 5). It developed as a magmatic arc in the Jurassic and Cretaceous ( ~ 1 6 4 - 8 5 Ma) along a
Neoproterozoic continental naargin (DePaolo
1981; Silver & Chappell 1988; Gastil et aL
1990; Schmidt & Paterson 2002). Cenozoic
cover obscures most of the PRB south of 28 ~ latitude, but the northern 700 km are well exposed.
The batholith is underlain by distinct western
(oceanic) and eastern (continental) belts, as
Fig. 3. Maps of (a) the northern and (b) the southern segments of the Coast Mountains-Cascades belt showing the
locations of metamorphic and plutonic rocks with recorded pressures of >9 kbar (stars). (a) is modified from Stowell &
Crawford (2000).
CORDILLERAN ARC AND OROGENIC PLATEAUX
Fig. 4. Map of the Sierra Nevada batholith showing the
emplacement depths of plutons and estimated
metamorphic pressures for country rocks and roof
pendants. Map modified from Bateman (1992), with data
from Ague & Brimhall (1988), Hanson et al. (1993) and
Pickett & Saleeby (1993). The thick black dashed line at
the western margin of the WMB marks the western limit of
crustal thickening. WMB, Western Metamorphic Belt.
identified by the petrology and geochemistry of
pluton,; (Silver & Chappell 1988) and prebatholithic stratigraphy (Gastil 1993). Deformation
was focused along the lithospheric boundary
between the belts during intrusion of much of
the batholith (Johnson et al. 1999; Schmidt &
Paterson 2002). Recorded metamorphic conditions of exhumed rocks change dramatically
across this zone from sub- to lower greenschist
facies and P < 2.5 kbar in the west to amphibolite facies and P ~ 4 - 6 k b a r across a broad
region in the east (e.g. Todd et al. 1988; unpublished data, this study) (Figs 2 & 5). Geophysical
studies suggest contrasting basement in these
belts, with faster seismic velocities (Magistrale &
Sanders 1995) and a relatively flat moho at
3 7 - 4 1 k m beneath the western PRB, and
the thickest crust (up to 43 krn) beneath the
transition zone. To the east, slower velocities and
eastward shallowing to ~25 km are related to
present-day rifting (Ichinose et al. 1996; Lewis
171
et al. 2000). Contrasts in crustal density and thickness between western and eastern zones of the
batholith in mid-Cretaceous time resulted in dramatic differences in the amount of exhumation
experienced by these two crustal belts.
Following > 3 0 Ma of structural thickening
and arc magmatism, a > 100 km wide region of
the eastern PRB was deeply denuded in the
Late Cretaceous. Approximately 20kin of
material was stripped from the eastern zone relative to the western zone during this time, with
denudation on the order of 1 mm a-1. Erosion,
coupled with local thrust faulting (Schmidt
2000), was by far the most important denudation
mechanism; only one Late Cretaceous extensional fault has been found (Erskine & Wenk
1985). Kimbrough et al. (2001) proposed that
the development of erosional topography in the
Late Cretaceous was related to the emplacement
of voluminous magmatic bodies.
Pre-exhumation reconstruction of the eastern
zone at ~100 Ma indicates crustal thickness of
the order of ~55 km (Schmidt 2000). This estimate is consistent with petrological evidence
for anomalously thick continental crust during
production of the volumetrically significant
~ 9 9 - 9 2 M a La Posta magmatic suite that
intruded a broad region of the eastern and transitional zones of the batholith (Kimbrough et al.
2001). An estimate of at least 55 km is also consistent with the modern crustal thickness for the
eastern/transition zone ( 3 5 - 4 0 k m ) plus the
amount estimated to have been removed by
erosion (~20 km),
Based on these data and inferences, we propose
that the eastern PRB formed the western margin of
thickened crust - and perhaps an orogenic
plateau - in the Late Cretaceous, separated from
the western zone by a sharp topographic break.
The fore-arc basins along the west coast received
voluminous but locally derived material from the
eastern PRB during Late Cretaceous-Paleocene
time. Not until the Eocene did regional fluvial
systems cut through the ancestral Peninsular
Ranges and deposit exotic, Sonoran-derived sediment in these basins (Axen et al. 2000).
The North American Cordilleran
plateau(x)
Evidence for thickened crust does not directly
translate into evidence for high-elevation/lowrelief landscapes. Indications of higher preMid-Eocene elevations in the North American
Cordillera come from palaeoaltimetry studies
and inferences about crustal dynamics. In the
northern Cordillera, Wolfe et aL (1998) used
172
D.L. WHITNEY ET AL.
Fig. 5. (a) Map of the Peninsular Ranges batholith, with (b) a more detailed map of part of the belt. ABF, Agua Blanca
fault; SSPM, Sierra San Pedro Martir. The thick dashed line marks the transition from magnetite-bearing (Mt) plutons to
ilmenite-bearing (Ilm) plutons. The black line (solid south of the ABF) is the boundary between low-P rocks with
> 100 Ma cooling ages and moderate-P rocks with <90 Ma cooling ages. This line therefore shows the western edge of
the proposed southern Cordillera plateau.
palaeoaltimetry based on fossil plant assemblages to propose that northeastern Washington
and southern British Columbia were at least
1 - 2 k m higher in the early Mid-Eocene
( ~ 5 0 Ma) than today. In the central Cordillera,
palaeobotanical evidence also suggests that parts
of the western USA were at least 2 km higher in
the Mid-Eocene than they are today (Wolfe et al.
1998). Palaeoaltimetry data, combined with tectonic evidence for magnitude of crustal thickness
and subsequent thinning (Wernicke et al. 1996;
Ducea & Saleeby 1996), indicate that the Sierra
may have been at elevations of 4 - 5 km in the
Late Cretaceous. The present-day average
height of the Sierran crest is 2.8 km.
Although there are no comparable data for
palaeo-elevations in Baja, Mexico, isostatic
models for the Peninsular Ranges suggest that
crust of the thickness ( > 5 5 km) and density of
the eastern zone could have supported average
surface elevations of > 4 km over a broad
region (Schmidt 2000). In contrast, the dense,
relatively little denuded, and presently anomalously thick western zone probably could not
have supported more than 1 km of average
surface elevation during the Cretaceous. We
therefore infer that the eastern PRB represented
a zone of high elevation in the Late Cretaceous.
Comparison with the modem Andean continental arc is useful for assessing the relationship
between crustal thickening and topography in
the North American Cordilleran arcs. In the
Andes, the ~ 4 km high, 350-400 km wide
Altiplano and Puna Plateaux were created by
thickening of lithosphere during subduction.
Crustal thickness varies, but is generally 5 0 70 km (Zandt et al. 1994). The plateau region is
bounded on the eastern side by a thrust belt, and
on the west by the arc. The Altiplano-Puna
Plateau comprises varied and complex basement
geology, as does the North American Cordillera,
and has been uplifted across a wide area due to
the thermo-mechanical effects of contraction of
hot, thermally weakened lithosphere. Regional
uplift of the plateau regions has been attributed
to an increase in plate convergence rate, which
l e d t o a decrease in the angle of subduction, and
thinning/weakening of the South American lithosphere (Allmendinger et al. 1997).
In the Cordillera, the CMC and Sierran belts
likely comprised the western margins of broad
plateau regions, similar to the relation between
the Andean arc and the Altiplano-Puna
Plateau. The PRB may have represented the
southern end of the plateau, analogous to the
narrow, southern end of the Altiplano-Puna
Plateau. It is difficult to identify the northern
end of the plateau because of lack of data and
effects of faulting in the northern Cordillera.
These observations and their similarities to inferences about the Mesozoic-Early Cenozoic North
American arcs support the idea that plateau formation commonly accompanies construction of
continental arcs.
Unroofing mechanisms
The unroofing of the North American Cordilleran
arcs and inboard regions can be described in terms
of the collapse of the plateau region (east of the
CORDILLERAN ARC AND OROGENIC PLATEAUX
arcs) and the unroofing of the arcs. The former is
characterized by metamorphic core complex
development and, in the central and southern
Cordillera, by Basin and Range style extension.
Unroofing of the arcs occurred by a combination of erosion and syn- to post-convergence
extension (transpression, then transtension). The
balance between these processes varied in the
northern v. the central/southern Cordilleran arcs.
The Omineca-Sevier belt may have collapsed
due to the presence of hot, thickened crust
(Livaccari 1991) weakened either by magmatism
(Armstrong & Ward 1991) or by partial melting
at depth (Vanderhaeghe & Teyssier 2001). The
exhumation of deep middle crust ( ~ 2 0 - 5 0 km
depth in a thickened orogen) may also have
been influenced by the presence of lowerdensity rocks at depth, due to the emplacement
of denser terranes on felsic basement, and/or to
the generation of partial melting at depth
during heating and decompression (Teyssier &
Whitney 2002). These are all internal processes
that are consequences of crustal thickening.
The arcs also consisted of hot crust of similar
thickness to the internal zone, but only the northern belt experienced major collapse and extension. The Sierra and PRB arcs may have been
comparable to the CMC in terms of crustal
thickening and magmatic history, but they did
not experience major extension, and only underwent about half the magnitude of exhumation.
This observation is incompatible with a model
of gravitational collapse driving unroofing of
the arcs, because collapse of hot, thickened
crust did not occur on a large scale in the
central/southern Cordilleran arcs. This suggests
that external mechanisms for driving exhumation
may have been important in the north but not in
the central and southern regions. External mechanisms include a change in plate motion factors
in the Eocene; for example, plate velocity, trajectory and/or number of small plates in the northern Pacific.
Gravitational collapse of thickened crust and
changes in Pacific plate motions have been proposed to account for extension in the northem
Cordillera. For example, rearrangement of Pacific
plates to more oblique convergence relative to
North America, consumption of the Kula plate
and/or a slowing of plate convergence rate in
the Eocene (Engebretsen et al. 1985; Stock &
Molnar 1988) may have caused extension
throughout the northern Cordillera. Plate boundary kinematics resulted in a N W - S E minimum
principal stress component, consistent with the
orientation of syn-extensional structures in the
metamorphic terrains of this region. A change in
plate boundary stresses (e.g. interaction of the
173
North American margin with the Pacific-KulaFarallon triple junction) has also long been proposed as a mechanism for Eocene magmatism in
the northern Cordillera (Ewing 1980). These
observations are consistent with geodynamic
coupling between the subducting plate and the
continental lithosphere, but the nature of the link
cannot be inferred from the observations.
Cordilleran crustal thickening, deep
crustal flow and unroofing
Previous North American Cordilleran studies
(e.g. Coney & Harms 1984; Dilek & Moores
1999) have used exposures of metamorphic
rocks to model palaeo-crustal thickness. In
these models, regions with exposures of highgrade metamorphic rocks (Omineca-Sevier
belt) are depicted as having thicker Late Cretaceous crust than regions with few or no exposures
of mid-crustal rocks. For example, these studies
show thinner crust ( < 4 0 k m ) in the region
between the Omineca and Cascades arc because
this belt (Intermontane) generally lacks exposures of high-grade metamorphic rocks. In
contrast, we argue that the location of exposed
deeper crustal levels is primarily a function of
unroofing mechanism(s), and that these mechanisms varied in different regions depending in
part on thermo-mechanical interaction between
oceanic and continental lithosphere. Thick crust
existed in the Late Cretaceous in regions that
today do not contain exposures of high-P rocks.
This proposal is supported by thermo-mechanical models that predict flow for deep crust on
short timescales (McKenzie et al. 2000). Flow
from the thickened regions (western arcs and
inboard zones) to intervening regions would
result in uniformly thick crust and a flat moho,
and reduce lateral variations in topography.
Lower crustal flow in response to pressure gradients - i.e. away from regions of thickened crust has been proposed to explain the thickening and
uplift of the Colorado Plateau (McQuarrie &
Chase 2000). Here we propose a similar mechanism for creating a broad region of thickened crust
that encompasses much of western North
America, from British Columbia to Baja and
including the continental arcs, in the Late Cretaceous-Early Tertiary. Such flow implies decoupling of the deep crust from the mantle
lithosphere. The subsequent regional-scale response of thickened crust to internal (continental)
and external (oceanic plate) factors indicates
variable coupling between oceanic and continental plates involved in the construction of the
North American Cordillera.
174
D.L. WHITNEY ET AL.
We thank Christian Teyssier, Brendan Murphy and
Graeme Taylor for their comments on the paper and the
ideas in it. This work was partially supported by NSF
grant EAR-9896017 to DLW.
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