Abstract During large-scale convergence of oceanic and arc-type lithospheric fragments towards a cratonic promontory along western North America from Middle Jurassic through Paleogene time, non-subductable crust of the approaching Pacific realm was deflected dextrally northward or sinistrally southward from this ‘reverse indenter’ in the California-Nevada region. Paleontologic and paleomagnetic data suggest oblique dextral displacements on the order of 1.500 to 2,000 km for the accreted terranes in the western Cordillera of Canada. These dextral displacements were first concentrated along closing sutures (from Middle Jurassic to Early Cretaceous time); later they were also taken up by pericollisional fault zones, which propagated into the western parts of the Cordilleran thrust belt and involved the Coast Plutonic Complex (mid-Cretaceous to Paleocene). A-subduction in the thrust belt and inferred B-subduction west of the Coast Plutonic Complex were thus accompanied by dextral displacements within the Omineca and Coast fault arrays respectively, imparling northwest-directed stretching fabrics onto ductile metamorphic or igneous rocks, and discrete fault strands on high-level crustal rocks. The convergent strike-slip fault motions in the Canadian Cordillera created mainly sedimentary source areas rather than subsiding basins. Pericollisional basins that did receive clastic materials from zones of oblique convergence were (1) marginal basins in the process of closing, (2) relict or tectonically overloaded depressions on accreting terranes, (3) foreland basins created by thrust propagation in the miogeoclinal succession, and (4) small pull-apart or restraining bend depressions near high-angle strike-slip faults. Basins in the accreted terrene complexes west of the Cordilleran thrust belt received most of their detrital material from exposed volcanic, plutonic, and oceanic sedimentary rocks; the predominantly turbiditic basin fill suffered repeated deformation, high sustained heat flow, and intrusive activity. The foreland basin to the east of the thrust belt, on the other hand, received most of its detrital input from carbonate and quartz-rich clastic rocks of the miogeocline and metamorphosed equivalents; the predominantly shallow-water clastic deposits of the foreland basin experienced considerably less deformation and thermal alteration than the varied sedimentary assemblages of the accreted belt.

Abstract Recent models for the tectonic evolution of the Canadian Cordillera can be tested by relating uplift of the orogenic core zones to the depositional record of the foreland and successor basins, but this requires a comprehensive understanding of paleodrainage. The Canadian Cordillera is made up of two orogens: the Columbian Orogen on the east and the Pacific Orogen on the west. Most of the late-orogenic molasse deposits are related to the Columbian Orogen (Omineca Crystalline Belt and Rocky Mountain Belt). In the Columbian Orogen three structural elements placed controlling restraints on late-orogenic drainage: uplifts, re-entrants and salients, and longitudinal intramontane fault zones. The uplift boundary can be defined as a zone of great vertical displacement of pre-orogenic sedimentary and volcanic rocks and probable involvement of terrane intruded or metamorphosed in Precambrian time. The uplift boundary separated the aggradational molasse basins from the erosional domain of the drainage system. Most of the clastic sediment derived from the uplifted core zone was transported by rivers flowing in longitudinal, intramontane valleys. These rivers merged near re-entrants of the uplift boundary and discharged their load into elongate molasse basins. During growth of folds and thrust faults within the orogen, valleys near the structural re-entrants constituted the shortest dispersal paths between the rising core zones and the subsiding foreland and successor basins. The area in front of the regional re-entrants (Crowsnest, Peace, Liard, and Peel on the east side; Chukachida and Thompson on the west side) therefore display the best developed molasse deposits in foreland basins to the east and successor basins to the west. From the structural salients only short, though locally vigorous, streams issued directly into the late-orogenic basins. The molasse of the Columbian foreland basin displays two upward coarsening megacycles: an uppermost Jurassic through Lower Cretaceous cycle (Kootenay-Blairmore Assemblage), and an Upper Cretaceous through OlLgocene cycle (Belly River-Paskapoo Assemblage). Regional drainage during deposition of the first cycle was directed to the north, whereas during the second cycle streams flowed predominantly to the southeast. Straight drainage lines connecting the re-entrants with the depositional basins are probably valid concepts for the earliest stages of uplift only. Progressive growth of folds and thrust faults in the Rocky Mountain Belt, and faulting near intramontane valleys, produced curved and even U-shaped river systems, which merged near re-entrants and effected thorough mixing of compositionally diverse sediment loads. The molasse of the successor basins reflects progressive unroofing of the crystalline core zone. The two cycles of the Columbian foreland are similar to molasse sequences in the Alps and seem to reflect two phases of isostatic uplift related to repeated intervals of tectonic crustal thickening.

Abstract The evolution of the Canadian Cordilleran eugeosyncline can be understood in terms of five rock sequences or assemblages. Three of these, Laberge, Bowser, and Sustut: and Georgia Assemblages, fulfill the requirements of epieugeosynclines (or successor basins) as defined by Marshall Kay very closely. The Laberge Assemblage (Lower and Middle Jurassic) was derived from an island complex having a granitoid core and was probably deposited within a mobile, marginal oceanic basin. The Bowser Assemblage (Upper Jurassic and Lower Cretaceous) was derived from a mobile orogenic core made up of rocks as old as Precambrian and as young as the Laberge Assemblage. The Bowser sediments were probably deposited within a marginal basin underlain b y crust transitional in character between oceanic and continental. The Sustul: and Georgia Assemblages (Upper Cretaceous and Paleogene) are characterized by extensive continental clastics that were deposited in structurally controlled basins underlain by incompletely cratonized continental crust. Intracrustal spreading of sialic material is invoked to explain the progressive continentalization during the evolution of the successor basins. The transition from predominantly marine deposition (Bowser Assemblage) to predominantly nonmarine deposition (Sustut Assemblage), seems to coincide with a time of peripheral cooling of metamorphic complexes and the emplacement of large discordant plutons. The transition from nonmarine deposition to regional erosion coincides with shallow-level plutonism, explosive volcanism, and uplift of the high-grade metamorphic core zones. It is suggested that the rate of these transitions is directly related to the magnitude of shortening of sialic crust and to the magnitude of the isostatic anomaly resulting therefrom.