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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 Email alerting service click here to receive free email alerts when new articles cite this article Permission request click here to seek permission to re-use all or part of this article Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes Downloaded by University of Southern California on 23 February 2010 © 2004 Geological Society of London 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. 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