Emplacement and Denudation History of the Lewis-Eldorado-Hoadley

Home , Appekunny Formation

[American Journal of Science, Vol. 301, April/May, 2001,P.359–373] EMPLACEMENT AND DENUDATION HISTORY OF THE LEWIS-ELDORADO-HOADLEY THRUST SLAB IN THE NORTHERN MONTANA CORDILLERA, USA: IMPLICATIONS FOR STEADY-STATE OROGENIC PROCESSES JAMES W. SEARS Department of Geology, University of Montana, Missoula, Montana 59812 ABSTRACT. The northern Montana Cordilleran mountain chain provides unusual opportunities to evaluate long-term orogenic processes in a foreland thrust belt. The thick Lewis-Eldorado-Hoadley thrust slab and an imbricated thrust wedge in its footwall dominate the structure of the region. The slab is composed of the displaced Mesoproterozoic Belt-Purcell depositional basin and overlying Phanerozoic strata, tectonically inverted into the broad, deeply dissected Purcell anticlinorium. The geometric simplicity and stratigraphic integrity of the structurally strong, thick slab constrain crustal-scale cross sections. Horizontal displacement of the slab increases from 40 km in central western Montana to 140 km at the United States-Canada border. Thermochronometers from the exhumed thrust slab and its footwall coupled with erosional products in the neighboring foreland basin measure regional denudation during Maastrichtian to middle Paleocene tectonic convergence. During the 15 my period of slab emplacement (74-59 Ma), regional denudation appears to have been minimal as the footwall subsided isostatically beneath the advancing tectonic load; erosion did not breach the Paleozoic cover of the slab. Similarities in burial and denudation patterns from central Montana to southern Canada indicate that regional erosional denudation was steady-state with respect to horizontal displacement and displacement rate. The steady-state configuration of the slab was interrupted immediately upon cessation of thrusting at 59 Ma, when major uplift and denudation of the slab and its footwall began, yoked to isostatic rebound of an autochthonous mega- thrust ramp at the depositional margin of the Belt-Purcell basin. The post-thrusting rebound bent the slab into the Purcell anticlinorium, and the flexure was accompanied by extensional faulting of the slab. Although parameters of orogenic relief are limited by its ancient nature, the northern Montana Cordillera is significant to evaluation of steady-state orogenic processes because its history is clearly divided into a syn-orogenic phase with low denudation rates and an immediate post-orogenic rebound phase with high denudation rates.

introduction The Cordilleran mountain chain of northern Montana and adjacent Canada constitutes one of the world’s classic foreland fold-thrust belts (Willis, 1902; Bally, Gordy, and Stewart, 1966; Price and Mountjoy, 1970; Boyer and Elliott, 1982; Mudge and Earhart, 1983). Unique aspects of the structural geometry, a large geological data base, and emerging thermochronometry make the region ideal for analysis of long- term orogenic processes in a foreland setting. Analysis of this region augments our understanding of steady-state orogenic processes, which is largely based on young, active systems along accretionary plate boundaries (for example, western Washington, Taiwan, and New Zealand). A region of the Montana Cordillera encompassing 250 km along strike and 100 km across strike appears to have maintained a steady-state denudation profile during 15 my of tectonic convergence, despite significant along- strike variation in horizontal shortening and strain rate. In northern Montana (fig. 1), the crustal-scale Lewis-Eldorado-Hoadley (LEH) thrust slab and an imbricated thrust wedge in its footwall were emplaced during the Late Cretaceous and Early Paleocene Laramide orogeny and denuded by Tertiary erosion and extension during post-orogenic rebound (Price and Mountjoy, 1970; Sears, 1988). Erosional products derived from the distinctive internal stratigraphy of

359 360 James W. Sears—Lewis-Eldorado-Hoadley thrust slab in the northern Montana

Fig. 1. Tectonic map of northern Montana Cordillera, showing location of Rogers Pass cross section (ﬁg. 2). CDS ϭContinental Divide Syncline, MDBϭMontana Disturbed Belt

the structurally simple LEH slab provide one measure of its denudation (Price, 1966). In addition, the slab and its footwall both contain abundant Cretaceous igneous rocks including a widespread andesite sill complex that provide a preliminary thermochronometric data base for assessing Late Cretaceous-Paleocene tectonic burial and denudation (Sears and Chamberlain, 1999). Detailed fission-track studies elucidate the burial and exhumation history in Canada (Osadetz, Stockmal, and Lebel, 2000). Widespread middle Eocene and middle Miocene angular unconformities mark stages in the post-thrusting uplift and denudation of the region (Sears and Fritz, 1998; Price, 1966). structural geometry The Lewis-Eldorado-Hoadley (LEH) thrust slab comprises a massive, east-tapering allochthon, 70 to 110 km wide and as much as 30 km thick that dominates the northern Montana thrust belt from the Lewis and Clark line shear zone northeast to the Rocky Mountain front, from west-central Montana to southern Canada (figs. 1, 2). The slab is folded across the broad Purcell anticlinorium and Continental Divide syncline and is split by a number of Tertiary grabens (Harrison, 1972; Price, 1981). The interleaved Lewis, Eldorado, Hoadley, and other thrusts (Mudge and Earhart, 1983) break the leading wedge-edge of the slab. The LEH slab has 40 km of horizontal displacement along a cross section through Rogers Pass (fig. 2), and displacement increases northward to 140 km at the United States-Canada border (Bally, 1984; Price and Fermor, 1986), indicating a general clockwise rotation during slab emplacement (Price and Sears, 2001) (fig. 3). Geologic data discussed below indicate that, along its entire length, the slab was emplaced during the 15-my-time interval from 74 to 59 Ma. At Rogers Pass, the slab thus moved at an average rate of ϳ3 km/my, while at the Cordillera, USA: implications for steady-state orogenic processes 361

Fig. 2. Cross section through Rogers Pass, Montana, showing present geometry (A), geometry restored to termination of thrusting at 59 Ma (B), and to pre-thrusting at 75 Ma (C). The sections align along their right edges. Horizontal and vertical scales are equal. Geology modified from Price and Sears (2001), Mudge and Earhart (1983), and Dolberg (ms). (A) Present geometry of Rogers Pass cross section. Hatched area above erosional profile indicates rock removed by erosion and extension following cessation of thrusting at 59 Ma upon rebound of the mega-thrust ramp at the depositional margin of the Proterozoic Belt-Purcell basin (compare with B). The rebound established the form of the Purcell anticlinorium. Volcanics on the Eocene erosional surface indicate that 10 km of denudation occurred by 47 Ma. The vertically ruled area denotes the rebound of the mega ramp from its position in (B). The open stippled pattern at the base of the section shows the rebound of the crustal keel from its position in (B). Trajectories of points A, B, C, D, and E are shown on figure 4.

United States-Canada border, it moved at ϳ9 km/my. The displacement gradient permits an evaluation of whether orogenic processes such as denudation can be steady state with respect to net horizontal displacement and strain rate over this time scale. timing of leh thrust slab emplacement Displacement of the LEH slab began at ϳ74 Ma from central Montana to southern Canada. In Montana, the movement broke the structural and stratigraphic continuity of Campanian-Maastrichtian volcanogenic formations (Viele and Harris, 1965; Gwinn and Mutch, 1965) that are capped by 74-Ma ash beds (Rogers, Swisher, and Horner, 1993). The thick Elkhorn Mountains Volcanics capped the LEH slab while the correlative Two Medicine Formation and Adel Mountains Volcanics (Harlan and others, 1991) were buried in the footwall. The Two Medicine Formation is overlain by the early Maastrichtian Bearpaw and St. Marys River formations, which constitute the youngest pre-thrust stratigraphic units in the footwall of the LEH slab in Canada (Jerzykiewicz, Sweet, and McNeil, 1996). Fission-track data from within the LEH slab in Canada indicate profound cooling at ϳ75 Ma, interpreted to indicate onset of movement on the Lewis thrust (Osadetz and others, 2000; Osadetz, Stockmal, and Lebel, 2000). Catuneanu and Sweet (1999) interpret reciprocal sequence stratigraphy 362 ae .SasLwsEdrd-ode hutsa ntenrhr Montana northern the in slab thrust Sears—Lewis-Eldorado-Hoadley W. James

Fig. 2(B) Rogers Pass cross section restored to 59 Ma. Denudation products in the foreland to the east show that erosion had not breached the Paleozoic cover of the massive Lewis-Hoadley-Eldorado thrust slab. To accommodate the slab in the section, the Moho subsided from its pre-thrust configuration, forming a crustal keel that was the inverse of the Belt-Purcell basin margin mega-ramp (compare C). The subsidence brought the footwall thrust wedge to depths that permitted resetting of the biotite age of an andesite sill, K-metasomatism of bentonites, and annealing of fission tracks in apatite but not zircon. Calibration of the geothermal gradient from cratonic heat flow studies (right side of diagram) brings sample sites to proper depths within thrust wedge. Note that the form of the basal decollement matches the mega-ramp of (C), shifted eastward as the footwall subsided beneath the load of the slab. odlea S:ipiain o taysaeooei processes orogenic steady-state for implications USA: Cordillera,

Fig. 2(C) Rogers Pass cross section restored to 75 Ma. The pre-thrust configuration places the Belt-Purcell Supergroup in its depositional basin. The Paleozoic onlapped the basement across the zero edge of the Belt-Purcell Supergroup. An andesite sill complex invaded the Cretaceous shales across the future LEH slab and its future footwall. Youngest pre-thrust deposits on this cross section are 74 Ma ash beds in a volcanogenic pediment (Elkhorn Mountains volcanics and Two Medicine Formation). The modern erosional profile is shown, along with the locations of a number of stocks and sills that have yielded K-Ar ages older than 75 Ma, indicating the minimum depth to the biotite closure temperature for the LEH slab, consistent with the geothermal gradient calibrated from cratonic heat flow studies (right side B). Black dots show positions of thermochronometric data. 363 364 James W. Sears—Lewis-Eldorado-Hoadley thrust slab in the northern Montana

Fig. 3. Palinspastic map of the Montana-Canada Cordillera, showing increased displacement of the Lewis-Eldorado-Hoadley thrust slab northward from Rogers Pass to the United States-Canada border. The slab was emplaced from 74 to 59 Ma along its length and may have experienced clockwise rotation about a pole near Helena, Montana (Price and Sears, 2001). Note that the northwestern part of the Belt-Purcell basin experienced Late Jurassic to Early Cretaceous displacement on the Purcell, Moyie, and other faults, not part of the present analysis. Based on restored balanced cross sections. After Price and Sears (2001). Cordillera, USA: implications for steady-state orogenic processes 365 in the Canadian foreland to indicate that the orogenic wedge moved in two major pulses between 74 and 60 Ma. Movement of the LEH slab ended at ϳ59 Ma. In Montana, undeformed 59-Ma porphyritic dikes cut thrusts at the leading edge of the slab (Schmidt, 1978). In Canada, the youngest strata that are deformed in the kinematically linked thrust belt adjacent to the LEH slab comprise the Middle Paleocene (ϳ60 Ma) Paskapoo Formation of Alberta (Price and Mountjoy, 1970; Hamblin, 1997). thermal structure of leh slab Comparison of the thermal structure of the LEH slab and its footwall allows evaluation of their respective uplift, burial, and exhumation histories. Exposures across the Purcell anticlinorium and in the footwalls of normal faults (fig. 2A) provide surface access to the upper 25 km of the LEH slab and permit evaluation of its thermal structure. The Belt-Purcell Supergroup dominates the LEH slab. This dominantly siliclastic sequence underwent Mesoproterozoic burial metamorphism to greenschist facies (Harrison, 1972). The entire supergroup occupies the 2M-mica zone, and the deepest levels, associated with thick mafic sills, lie within the garnet-biotite-oligoclase zone and record burial metamorphic temperatures of 500° to 600°C (Cressman, 1989; Porder, Hyndman, and Copeland, 1997). Within most of the LEH slab, temperatures during thrust emplacement did not exceed those of the Proterozoic burial metamorphism of the Belt-Purcell Supergroup. For example, at 25 km depth, white mica grew in Cretaceous cleavage that is confined to thin argillite layers, but burial metamorphic porphyroblasts of biotite and chlorite rotated in the Cretaceous cleavage as rigid bodies (Sears and others, 1989). Throughout most of the exposed area of the LEH slab northeast of the Lewis and Clark line, even at the deepest levels of exposure, quartzite layers and diabase sills are characterized by brittle fabrics, and argillite and carbonate layers are at most only weakly cleaved. The resulting preservation of delicate primary bedding has permitted extensive research into the sedimentology of the Belt-Purcell Supergroup in this region (Winston, 1986). The exposed portions of most of the slab were therefore colder than the threshold temperature for plastic deformation of quartzite, approx 250° to 300°C (Suppe, 1985; Gleason and Tullis, 1995) during slab emplacement. Fine continuity of mafic sill reflectors in seismic profiles (Cook and Van der Veldon, 1995; Sheriff, 1984; Harris, ms) suggests that diabase remained in its brittle field to the base of the slab at conditions colder than approx 500° to 550°C (Mackwell, Zimmerman, and Kohlstedt, 1998). North of the Lewis and Clark line, the LEH slab generally lacks tectonic veins, suggesting a drought of syndeformational fluids. Because the rock temperatures during slab emplacement did not exceed Proterozoic burial metamorphic conditions, no metamorphic fluids were released within the slab. Thus, most of the LEH slab deformed in a dry, brittle state. Argon ages from throughout the LEH slab (fig. 2C) are consistent with the above observations and show that exposed country rock did not exceed the biotite blocking temperature of 280°ϩ/- 40°C, except near intrusive contacts. Burial biotite porphyroblasts retain a K-Ar age of 1330 Ma (Obradovich and Peterman, 1965) from a 20-km depth (west of the cross-section line). Biotite from a 23-km depth retains Ar-Ar ages Ͼ800 Ma (Porder, Hyndman, and Copeland, 1997, west of cross-section line). A number of Cretaceous granitic stocks cross the slab near the Rogers Pass section, at depths exhumed from 2 to 15 km (fig.2). These yield biotite argon ages of 76 to 100 Ma, close to their crystallization ages (Berg and Daniel, 1981; Wallace, Lidke, and Schmidt, 1990; Wallace and others, 1987). For example, the Garnet stock, intruded at 7 km depth, yielded a zircon age of 83 Ma (Richard Tosdal, United States Geological 366 James W. Sears—Lewis-Eldorado-Hoadley thrust slab in the northern Montana

Survey, written communication) and cooled through blocking temperatures for horn- blende at 82 Ma and biotite at 79 Ma (Ruppel and others, 1981). A 23-km deep mafic sill with a 1456 Ma zircon age (west of cross-section line, Sears, Chamberlain, and Buckley, 1998) yielded an amphibole age of 1380 Ma (Cressman, 1989). A widespread mafic sill near the thin leading edge of the LEH slab retained a K-Ar whole-rock age of 750 Ma (Mudge and others, 1982). In Canada, apatite and zircon fission-track data show that the LEH slab reached maximum Phanerozoic temperatures prior to motion on the Lewis thrust, interpreted to indicate burial beneath a 7-km thick Campanian-early Maastrichtian clastic wedge (Osadetz and others, 2000). Maximum temperatures achieved at the lower Belt-Purcell Supergroup (Appekunny Formation) level were Ͻ255°C at a depth of 15 km (Osadetz, Stockmal, and Lebel, 2000). The relatively cool geothermal gradient indicated for the LEH slab from the above observations is consistent with the continental geothermal gradient calculated from heat flow measurements of the Montana and Wyoming cratons (Decker and others, 1980; Wong and Chapman, 1990). The geothermal gradient from heat flow studies is indicated on the right side of the Rogers Pass cross section (fig. 2B). The cool geothermal conditions preserved the high fracture strength of the thick LEH slab, which may have approached 2 GPa near the base during thrust emplacement (Sears and others, 2000). Its high strength abetted its mechanical emplacement as a coherent slab across the large region.

thermal structure of leh footwall The Montana disturbed belt constitutes the exhumed footwall of the LEH slab. It underwent heating during tectonic burial followed by cooling during erosional denudation. Thermal conditions appear to have been consistent from the Rogers Pass area to the United States-Canada border. A pyroxene-biotite andesite sill complex intruded this region of the foreland basin at about 76 Ma (Sears and others, 1997), immediately prior to the beginning of emplacement of the LEH slab at 74 Ma. The sill complex appears to have spread radially outward from the area of the Boulder batholith, within Upper Cretaceous shale (Sears and Chamberlain, 1999). Thick sills and a dike swarm are preserved in Upper Cretaceous shales atop the LEH slab and yielded a biotite Ar-Ar age of 76 Ma (Sears and others, 1997)(fig. 2A). Upper Cretaceous sills also intrude Upper Creta- ceous shales in nine separate thrust sheets (Mudge and Earhart, 1983) in an exhumed duplex within the footwall of the LEH slab (Dolberg, 1986). A sample of one sill from the footwall yielded a biotite Ar-Ar age of 59 Ma, with a disturbed gas-release spectrum, and a sericite Ar-Ar age of 60 Ma (Sears and others, 1997). These observations suggest that this level of the footwall experienced conditions above the biotite blocking temperature of 280° ϩ/- 40°C as late as 59 Ma. Cretaceous bentonite samples from shallower structural levels in the LEH footwall, up-dip and to the east of the andesite sill, underwent potassium metasomatism to illite, indicating temperatures of 100° to 175°C (Hoffman, Hower, and Aronson, 1976). The metasomatic reactions occurred over the time interval 74 to 57 Ma, according to K-Ar dating of the illite (Hoffman, Hower, and Aronson, 1976). Hoffman, Hower, and Aronson (1976) interpreted these relationships to date tectonic burial of the bentonites in the thrust wedge because the stratigraphic burial was insufficient to achieve the thermal conditions. In Canada, temperatures in the footwall of the Lewis thrust exceeded the 110°C apatite fission track blocking temperature, but not the 230°ϩ/- 25°C zircon fission track blocking temperature during Late Cretaceous and Paleocene tectonic burial (Osadetz and others, 2000). Peak coalification of and hydrocarbon generation from Cordillera, USA: implications for steady-state orogenic processes 367 rocks below the LEH slab were achieved by tectonic burial below the cooling and eroding slab (Osadetz and others, 2000). Pre- and syn-kinematic calcite veins are abundant in the LEH footwall near Rogers Pass in rocks as young as 74 Ma and contain fluid inclusions that record temperatures of 70° to 347°C (Lerman, ms, 1999). Sericitized Maastrichtian Two Medicine Forma- tion host rock cut by the calcite veins yielded a 67 Ma Ar-Ar age (D. Archibald, written communication). A Cretaceous sill from the LEH footwall in Canada was remagnetized after 75 Ma, after some tectonic tilting, probably because of chemical alteration due to fluid flushed from beneath the advancing LEH slab (Enkin and others, 1997). Hot fluids driven from the deepest part of the restored footwall may have augmented the ambient footwall temperatures by horizontal convective heat transfer and helped drive metasomatic reactions such as chloritization and sericitization of the andesite sill and volcanics and illitization of bentonite. Similar mechanisms have been proposed for illitization of bentonite beds in the Appalachian basin (Elliott and Aronson, 1987), for maturation of coal in the Welsh basin (Gayer, Garven, and Rickard, 1998), and for fluid migration in foreland basins in general (Oliver, 1986). foreland response to thrust emplacement Denudation products of the LEH slab deposited in the neighboring foreland basin between 74 and 60 Ma were derived chiefly from Mesozoic foreland basin deposits that covered the LEH slab (Mack and Jerzykiewicz, 1989). Conspicuously absent from this stratigraphic interval in the foreland basin are grains of distinctive red argillite, siltite, and quartzite from the Belt-Purcell Supergroup. The western Canadian foreland basin exhibits reciprocal sequence stratigraphy for the 74 to 60 Ma interval, interpreted as indicating isostatic response to major orogenic pulses and intervening periods of quiescence (Catuneanu, Sweet, and Miall, 1997; Catuneanu and Sweet, 1999). During the pulses, the basin subsided under the increasing load of the orogenic belt, and during quiescence, it rose due to erosional unroofing and isostatic rebound of the orogenic belt (Price, 1973; Beaumont, Quin- lan, and Stockmal, 1993). Because the LEH slab dominates the structure and because the imbricate wedge formed in response to LEH slab emplacement, the foreland stratigraphic record may provide a proxy for movement of the LEH slab. The denudation products of the LEH slab help constrain the 59-Ma restoration of the Roger Pass section at the termination of thrusting (fig. 2B). Since all structures in the thrust wedge had formed by 59 Ma, the 59 Ma restoration (fig. 2B) can be constructed by rotating the thrust geometry of figure 2A down to appropriate depths and restoring the stratigraphic cover. Preservation of the entire Belt-Purcell Super- group and much of its Phanerozoic cover places the footwall of the slab at depths of 5 to 20 km. This places the andesite sill in the footwall duplex marginally close to the minimum biotite-closure isotherm, given the independently constrained geothermal gradient. In this configuration, the K-bentonite samples of Hoffman, Hower, and Aronson (1976) plot within the geothermal range 100° to 175°C. Furthermore, the Ͼ75 Ma argon age determinations from within the LEH slab plot above the minimum biotite closure isograd. In southern Canada, the LEH slab appears to have had a similar geologic profile at the termination of thrusting. The footwall zone of the LEH slab in Canada occupied the temperature range appropriate for resetting fission track ages of apatite but not those of zircon (Osadetz and others, 2000). The hanging wall of the Flathead fault, a large Tertiary listric-normal fault which post-dated thrusting, preserves a 4-km section of the Paleozoic through Lower Cretaceous stratigraphic cover of the Lewis plate, overlain unconformably by the Late Eocene-Oligocene Kishenehn Formation in the Flathead graben (Price, 1966). Clasts derived from the footwall indicate that Mississip- pian formations were preserved on the Lewis plate until Late Eocene (Price, 1966). A 368 James W. Sears—Lewis-Eldorado-Hoadley thrust slab in the northern Montana model based on fission-track data suggests that the erosional surface on the eastern part of the LEH slab in Canada, just prior to motion on the Flathead normal fault, was at the stratigraphic level of the Lower Cretaceous Alberta Group (Osadetz, Stockmal, and Lebel, 2000). The above observations imply that the LEH slab behaved in a similar fashion from west-central Montana to southern Canada during its emplacement, with relatively minimal erosional denudation of its 5 to 7 km-thick Mesozoic cover.

mega-thrust ramp collapse The great thickness of well-stratified rocks in the LEH slab permits crustal-scale cross-section-balancing, which elucidates the evolution of regional erosional denudation in this area. The Rogers Pass section reconstructed to pre-LEH slab emplacement (fig. 2C) shows the Belt-Purcell Supergroup restored into its basin of deposition. The northeast margin of the basin forms a distinct southwest-facing ramp that presently underlies the southwest limb of the Purcell anticlinorium (compare fig. 2A). At ϳ74 Ma, the strong, cold LEH slab detached from its basement. On the Rogers Pass transect, the leading edge of the slab was imbricated into the Hoadley, Eldorado, and Steinbach thrusts and overrode the adjacent foreland basin. The thermal and sedimen- tological constraints outlined above indicate that the basin-margin ramp subsided in front of the LEH slab, so that little structural uplift was achieved during its emplacement. Rather, the form of the collapsing mega-ramp migrated eastward with the moving slab (compare B and C of fig. 2). Along the Rogers Pass section, the LEH slab moved only 40 km, which was barely sufficient for the rocks at the base of the ramp to override the top of the ramp. Farther north, however, the slab moved an additional 100 km across the foreland basin (Bally, 1982; Price and Fermor, 1986) (fig. 3). Similarities in hanging wall denudation products and footwall burial temperatures with increasing slab displacement northward imply that the pattern inferred for the Rogers Pass section was a steady-state configuration. That is, the craton subsided continuously in front of the advancing slab, and surface denudation was limited to the upper few kilometers of the stratigraphic section. The 59-Ma reconstruction (fig. 2B) closely resembles crustal sections constructed for active orogenic belts by Lillie (1991) and Lillie and others (1994) on the basis of interpretation of gravity anomalies. The thrusting inverted the form of the Belt-Purcell mega-ramp at the base of the crust, creating a crustal keel. The deepest part of the keel lay directly beneath the original top of the mega-ramp because that area had the thickest combination of sub-thrust crust and LEH slab. Advance of the slab would broaden the keel. The flexural strength of the lithosphere propagated the subsidence 400 km eastward across the foreland basin to the hingeline (Catuneanu and Sweet, 1999). During periods of quiescence, the keel rebounded, creating an erosional surface out to the hingeline. But during an orogenic pulse, the keel again subsided, leading to renewed sedimentation in the proximal foreland basin (Price, 1973; Beaumont, Quinlan, and Stockmal, 1993).

post-thrust rebound and denudation Upon cessation of thrusting at ϳ59 Ma, the mega-thrust ramp began its final rebound. By Middle Eocene, the LEH slab was truncated by a faulted erosional surface that had at least 2 km of local relief, judging from incised bedrock valleys filled with volcanogenic rocks, most of which K-Ar date from 47 to 44 Ma (Carter, ms). The erosional surface is split by grabens that are floored by southwest-dipping listric normal faults that flatten into the decollement above the mega-ramp (fig. 2A). The post-thrust normal faults accommodated 7 km of rebound of the mega-ramp on the Rogers Pass cross-section. Cordillera, USA: implications for steady-state orogenic processes 369

Denudation above the rebounding mega-ramp had progressed to a depth of 11 km between the termination of thrusting at 59 Ma and beginning of volcanism by 47 Ma, removing most of the hatchured area on figure 2A. The deepest denudation on the Rogers Pass section occurred directly above the top of the mega-ramp in response to rebound of the deepest part of the crustal keel. The Lincoln Valley graben contains Eocene lacustrine deposits associated with volcanics (Mudge and others, 1982), indicating that the graben formed contemporaneously with sedimentation. The graben is one of a set of extensional structures that began to form after cessation of thrusting within the Cordilleran orogenic wedge (Janecke, 1994). These include the Kishenehn basin (Flathead graben) astride the Canadian border (Constenius, 1982), the Bitterroot Valley graben and adjacent Bitterroot core complex in western Montana (Foster and Fanning, 1997; Hodges and Applegate, 1993), and the Hope fault in northwestern Montana (Filipone and Yin, 1994). The Late Eocene-Oligocene Kish- enehn basin records progressive denudation of the footwall of the Flathead extensional fault, and some conglomerates appear to fill deep paleo-valleys cut into Paleozoic carbonates (Price, 1966). By Late Eocene, distinctive Belt-Purcell quartzite clasts appeared on an unconformity that truncated Paleocene foreland basin deposits on the High Plains east of the Rocky Mountains. This unconformity may represent uplift of a broad rift shoulder on the east flank of the extensional domain (Janecke, 1994). The erosion rate appears to have decreased radically after about 47 Ma, as indicated by widespread preservation of Middle Eocene volcanics above the angular unconformity in the Rogers Pass area. A coherent system of bedrock channels of Middle Miocene age incised the area of the mega-ramp (Rassmussen, 1969). The Middle Miocene bedrock channels are consistently about 70 to 80 m above the Pleistocene channels west of the modern Continental Divide. The exact parameters of the post-Middle Eocene denudation history await further study.

belt-purcell basin inversion The post-thrusting rebound inverted the form of the Belt-Purcell basin into the Purcell anticlinorium. Figure 2B shows that bedding in the LEH slab was relatively flat at the termination of thrusting. Rebound of the crustal keel restored the original pre-thrust configuration of the flattened mega-ramp and bent the overlying LEH slab into the Purcell anticlinorium. The anticlinorium is a large-scale fault-bend fold that represents the inverted form of the Belt-Purcell basin (Price, 1981). However, the present analysis indicates that the bending of the slab into the anticlinorium post- dated thrusting. Furthermore, the net horizontal extension across the LEH slab that is represented by the Tertiary grabens approximates that necessary to have bent the slab into the Purcell anticlinorium above the rebounding mega-ramp. Most of the mega- ramp rebound appears to have occurred between cessation of thrusting at 59 Ma and initiation of volcanism at 47 Ma, because, regionally, the Middle-Late Eocene volcanics unconformably overlie the deepest exposure levels of the Purcell anticlinorium.

conclusions and implications for steady-state orogenic processes The analysis of the Rogers Pass cross-section implies that the 30 km-thick LEH slab gained a few km of elevation during its displacement, while its footwall subsided greatly. The subsidence propagated 400 km eastward of the slab as recorded by the reciprocal stratigraphy of the neighboring foreland basin from 74 to 59 Ma. Denuda- tion products in the foreland basin and preserved stratigraphy on the LEH slab show that the Paleozoic cover was not breached during slab emplacement. Geological relationships indicate that the slab moved three times farther in Canada than near Rogers Pass over a common 15 my time period. This may have been due to clockwise rotation of the strong slab about a pole near Helena, Montana (Price and Sears, 2001). 370 James W. Sears—Lewis-Eldorado-Hoadley thrust slab in the northern Montana

The increased northward displacement and linear velocity of the slab has conceptual implications for steady-state orogenic processes. Because the denudation rate and thermal evolution of the footwall were similar throughout the region, the slab appears to have achieved steady-state conditions for these factors relative to its horizontal displacement. That is, the footwall appears to have subsided to a stable depth upon advance of the slab and maintained that depth to the termination of thrusting. The displaced wedge grew wider northward, but denudation did not vary signiﬁcantly as a function of width. Figure 4 summarizes the burial and uplift trajectories of various points on the Rogers Pass cross section (ﬁg. 2). Subsidence of footwall points was driven by isostatic

Fig. 4. Burial and uplift trajectories of various points on the Rogers Pass cross section. See ﬁgure 2A for location of points A, B, C, D, and E. Burial history of footwall elements begins at 74 Ma and culminates at 59 Ma. Rapid exhumation begins at 59 Ma and decreases after 47 Ma, when widespread volcanics erupt over an angular unconformity. The exhumation may have been driven by isostatic rebound upon cessation of thrusting. Cordillera, USA: implications for steady-state orogenic processes 371 loading of the wedge-shaped LEH slab as it advanced across the points. After 59 Ma, all points moved upward as the LEH slab and its footwall underwent rapid isostatic rebound accompanied by extensional and erosional denudation. After 47 Ma denudation appears to have slowed dramatically. A ﬁssion track study of the many Cretaceous stocks and sills that cross cut the LEH slab in the Rogers Pass region (Lewis, 1996) may yield further insight into the post-orogenic denudation history of the system.

References

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