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The evolution of the southern Cordilleran foreland thrust and fold belt and the kinematics of Cordilleran orogenesis Raymond A. Price, Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6 [email protected] The Cordilleran foreland thrust and fold belt records the convergence between the North America craton and the subduction zones along its western margin. NE-dipping subduction of oceanic lithosphere has been underway along the western side of North America since the Late Triassic; and a strip of oceanic lithosphere ~ 13,000 km wide has been consumed there in the last 150 Ma (Engebretson et al., 1992). The Cordilleran foreland thrust and fold belt formed in the region behind this NE-dipping, SW-verging oceanic subduction system, as a SW-dipping, NE-verging, accretionary wedge. It consists of supracrustal rocks that have been scraped off the under-riding North American lithosphere and accreted to the over-riding tectonic collage of oceanic magmatic-arc terranes and oceanic accretionary-complex terranes that make up most of the rest of the Canadian Cordillera (Price, 1994). It began to form in the late Early Jurassic, at ~187-185 Ma (Murphy et al., 1985) as North America began converging with the NE-dipping oceanic subduction zone that descended into the mantle under the Quesnellia oceanic magmatic arc. The ensuing collapse of the intervening Slide Mountain marginal oceanic basin, which was situated outboard of the North American continental shelf, beyond the pericratonic oceanic rocks of Kootenay terrane, involved NE-verging thrusting and folding. It culminated in the obduction of Slide Mountain terrane (the deeper-water, ocean-floor, supracrustal rocks of Slide Mountain basin) over Kootenay terrane and the outboard part of the Cordilleran miogeocline. The crustal-scale kinematics and geodynamics of the obduction process are enigmatic; but the results are clear. Slide Mountain terrane overlaps North American rocks in the Cariboo Mountains; and metamorphic geobarometry and geochronometry from various localities along this zone in southeastern British Columbia (Archibald et al., 1983; Colpron et al., 1996) indicate that Lower Paleozoic shallow-water deposits of the outboard part of the Cordilleran miogeocline were transported Southwestward to depths of about 20 km during the NE-verging obduction of Slide Mountain terrane. The mantle and lower crustal lithosphere of the Slide Mountain basin were delaminated from the overlying supracrustal rocks that comprise Slide Mountain terrane, and have disappeared into the deeper mantle, presumably by becoming entrained in the downward flow of the NE-dipping subduction system. Subsequently, continuing convergence between North America and Quesnellia was accommodated by northeastward tectonic wedging of Quesnellia and Kootenay terrane between the outboard part of the miogeocline and its underlying crystalline basement. This NE-tapering tectonic wedge was bounded below by a NE-verging thrust zone, which eventually evolved into the basal décollement of the foreland thrust and fold belt, and above by a SW-verging thrust zone. SW-verging thrusts, folds, and fold nappes developed above the advancing wedge, as the overlying miogeoclinal supracrustal rocks were tectonically delaminated from their underlying crystalline basement, horizontally compressed, and uplifted progressively toward the NE by the advancing tectonic wedge (Price, 1986; Colpron et al., 1998). At the same time, the outboard part of the North American lithosphere was being wedged under the former Quesnellia magmatic arc, delaminating Quesnellia from its mantle and lower crustal roots, which also have disappeared into the deeper mantle, presumably by becoming entrained in the downward flow of the NE-dipping subduction system. By Late Jurassic time (Oxfordian-Kimmeridgian), crustal thickening due to the advance of the tectonic wedge and the associated SW-verging thrusting and folding above it had generated sufficient gravitational (topographic) potential to drive the propagation of the “near-basement” basal décollement beyond the tip of the wedge. This produced a change in style of deformation to NEverging imbricate listric thrust faulting in a northeastward expanding critical-taper accretionary wedge (Colpron et al., 1998). There was recurrent tectonic wedging and SW-verging thrusting and folding locally within the evolving critical-taper accretionary wedge, notably in the foreland basin deposits at the tip of the wedge (the “triangle zone”), and along the west flank of the Porcupine Southern foreland and thrust belt and kinematics of Cordilleran orognesis Creek anticlinorial fan structure in the western Rockies; but the dominant structural style from Late Jurassic to Late Paleocene time was NE-verging imbricate thrusting. The supracrustal rocks of the miogeocline, and subsequently of the cratonic platform, were scraped off the under-riding North American lithosphere and accreted to the previously deformed outboard part of the miogeocline, which had become consolidated with Slide Mountain terrane, Quesnellia, and other components of Intermontane Superterrane. The growing and shifting gravitational load imposed on the North American lithosphere by the expanding thrust and fold belt produced a migrating isostatic flexure -the foreland basin -- within which clastic detritus eroded from the evolving thrust and fold belt was trapped. The growth of the thrust and fold belt, and of the foreland basin, ended in the Late Paleocene with the onset of an episode of east-west crustal extension that was linked northwestward to right-lateral displacement on the Tintina-Northern Rocky Mountain Trench fault system, and Southwestward to right-lateral displacement on the Yalakom-Ross Lake and Fraser River-Straight Creek fault systems. In the Omenica belt the extension involved the crystalline basement; rocks from mid-crustal depths, including the basal décollement beneath the thrust and fold belt, and the underlying Paleoproterozoic crystalline basement, were exhumed (Parrish et al., 1988; Doughty et al., 1998; Doughty and Price, 1999). In the Rocky Mountains and eastern Purcell Mountains preexisting thrust faults and parts of the basal décollement were reactivated as normal faults. The horizontal convergence between North America and the subducting oceanic lithosphere along its margin has varied in both rate and direction, and has generally been oblique to the trend of the Cordillera (Engebretson et al., 1985). The convergence across the foreland thrust and fold belt also has varied in both rate and direction, and has generally been oblique to the trend of the belt. The total amount of shortening across the Southern Canadian Rockies decreases from >250 km, at about Latitude 530 N, to <20 km, 750 km to the south at Latitude 460 30’ N (Price and Sears, 2000). Late Cretaceous and Paleocene displacements in this part of the accretionary wedge were linked to the northwestward rotational translation of Intermontane terrane and Insular terrane relative to North America, and underwent a 30 0 rotation about an Euler pole in northwestern Montana. The total shortening also decreases northwestward to about 75 km at Latitude 560 N. This decrease is associated with a northwestward transformation of Late Cretaceous-Paleocene thrusting into right-hand displacement on the Tintina-Northern Rocky Mountain trench intra-continental transform fault system terranes (Price and Carmichael, 1986), which also was kinematically linked to the northwestward rotational translation of Intermontane and Insular terranes (Price 1994). The kinematic history of the thrust and fold belt evidently is complex, but it can be analyzed using sequential palinspastic maps. Palinspastic reconstructions of thrust and fold belts are hampered by uncertainties about directions of thrust displacements and how they varied in space and through time. The popular simplifying assumption that displacements were perpendicular to the regional strike of the faults and folds is unwarranted. “Balanced” sections that are perpendicular to the regional tectonic strike, but oblique to fault displacement lead to unreliable palinspastic reconstructions. Reliable palinspastic reconstructions can be generated using balanced hanging wall and footwall maps, which show the distributions of rock units and structures on opposite sides of the same fault. Displacement generally varies significantly from place to place along individual faults. These variations have to be accommodated explicitly on balanced fault maps as displacement gradients, or as linkages between the fault and other faults that branch from it or are truncated by it. Each of the pair of balanced maps must be a logical counterpart of the other, and each commonly can be used as a template for the other. Footwall ramps and flats must match those on the hanging wall. Intersections of linear segments of ramps provide piercing points that uniquely define the direction and amount of displacement on the faults. These concepts can be shown to apply to faults of varying sizes from the smallest faults to major regional faults. The same principles can be adapted to the analysis of listric normal faults and the extensional detachments associated with metamorphic core complexes. About 80 per cent of the tectonic shortening across the southern Canadian Rockies is associated with displacements on a relatively small number of very large faults (e.g. Lewis thrust: maximum Southern foreland and thrust belt and kinematics of Cordilleran orognesis displacement ≈ 100 km); the remaining 20 per cent is distributed among a very large number of small faults and folds. Thus, reliable regional palinspastic reconstructions can be produced by considering balanced maps of only the small number of very large faults. The most important of the fault maps is the footwall map of the regional décollement that defines the base of the thrust and fold belt, and from which the other major faults branch. The basal décollement separates weak, layered, anisotropic supracrustal rocks above from strong, massive Paleoproterozoic rocks below; but toward the front of the belt it rises, across thrust ramps, into one or more higher, laterally extensive detachments that occur within Devonian, Mississippian, Jurassic and Upper Cretaceous strata. The largest and most conspicuous ramps in the thrust and fold belt formed along structures in the crystalline basement that are associated with the margins of former sedimentary basins. These structures, which are of crustal dimensions (3 to >10 km of throw), include the northeastern margin of the Mesoproterozoic Belt-Purcell basin, the boundary between the Cordilleran miogeocline and the cratonic platform, and the boundary between the Cambro-Ordovician miogeoclinal carbonate platform and the adjacent shale basin. During thrusting, these basins were tectonically inverted: hanging wall ramps became crustal-scale fault-bend anticlinoria (e.g. Purcell anticlinorium); footwall ramps became crustal-scale fault-bend monoclines (e.g. Vernon monocline). These relationships provide the framework for a 3-D regional palinspastic reconstruction of the thrust and fold belt. Isotope geochronometry of Jurassic to mid-Cretaceous granitic plutons that cut faults in the internal parts of the accretionary wedge (reviewed in Colpron et al., 1988 and Price and Sears, 2000) provides minimum ages for displacements on some of the thrusts; maximum ages for the final displacements on some are provided by the ages of stratigraphic units that are truncated along the footwalls of the thrusts. The stratigraphic record in the adjacent foreland basin provides additional information about times of thrusting. The superimposed Early and Middle Eocene and younger extension and righthand strike slip is dated by isotope geochronometry of cross-cutting and truncated magmatic rocks (Carr, 1992), and by cooling ages of rapidly quenched metamorphic core complexes (Doughty and Price, 1999), as well as by the ages of syn-extensional sediments. On the basis of these considerations, four contrasting episodes can be distinguished in the Late Jurassic to Eocene kinematic evolution of the foreland thrust and fold belt: (1.) left-hand transpression (155 - 105 Ma), which involved oblique southeastward convergence of Intermontane terrane and the North American craton, compressed the outer part of the miogeocline while it was still situated outboard of the craton; (2.) a hiatus (105 - 85 Ma), during which there was little, if any convergence between Intermontane terrane and the North American craton, was marked by widespread, cross-cutting, “post-tectonic” granitic plutons that “stitched” many of the preceding thrust faults; (3.) right-hand transpression (85 - 58 Ma), which produced most of the Rocky Mountain foreland thrust and fold belt, involved large-scale (>150 km) clockwise rotation and NE-verging thrusting in the southern Canadian Rockies, much of which was transformed northward into a large right-hand strike slip on the Tintina-northern Rocky Mountain trench fault system; (4.) right-hand transtension (58 - 42 Ma), which large-scale east-west horizontal extension and involved the exhumation of metamorphic core complexes in the south, was transformed northward into right-hand strike slip on the Tintina-northern Rocky Mountain trench fault system. Subsequently (42 - 0 Ma) the locus of deformation shifted to the continental margin, although intermittent, basin-and-range type extensional deformation in the interior of the accretionary wedge continued into the Neogene. References Archibald, D.A., Glover, J.K., Price, R.A., Farrar, E. and Carmichael, D.M. 1983. Geochronology and tectonic implications of magmatism and metamorphism in the southern Kootenay arc and neighbouring regions, southeastern British Columbia. Part 1: Jurassic to mid-Cretaceous. Canadian Journal of Earth Sciences, v. 20, p. 1891-1913. Southern foreland and thrust belt and kinematics of Cordilleran orognesis Carr, S. 1992. Tectonic setting and U-Pb geochronology of the early Tertiary Ladybird leucogranite suite, Thor-Odin - Pinnacles area, southern Omineca belt, British Columbia. Tectonics, v. 11, p. 258278. Colpron, M., Price, R.A., Archibald, D.A., and Carmichael, D.M. 1996. Middle Jurassic exhumation along the western flank of the Selkirk fan structure: Thermobarometric and thermochronometric constraints from the Illecillewaet synclinorium, southern British Columbia. Geological Society of America Bulletin, v. 108, 1372-1392. Colpron, M., Warren, M.J., and Price, R.A., 1998. Selkirk fan structure, southeastern Canadian Cordillera: Tectonic wedging against an inherited basement ramp. Geological Society of America Bulletin, v. 110, 8, p. 1060-1074. Doughty, P.T., Price, R.A., and Parrish, R.R. 1998. Geology and U-Pb geochronology of Archean basement and Proterozoic cover in the Priest River complex, northwestern United States, and their implications for Cordilleran structure and Precambrian continent reconstruction. Canadian Journal of Earth Sciences, v. 35, 1, p. 39-54. Doughty, P.T. and Price, R.A. 1999. Tectonic evolution of the Priest River complex, northern Idaho and Washington: A reappraisal of the Newport fault with new insights on metamorphic core complex formation, Tectonics, v. 18, 3, p. 375-393. Engebretson, D.C., Cox, A., and Gordon, R. 1985. Relative motions between oceanic plates and continental plates in the Pacific basin. Geological Society of America, Special Paper 208, 59p. Engebretson, D.C., Kelly, K.P., Cashman, H.J., and Richards, M.A. 1992. 180 million years of subduction. GSA Today, v.2, p. 93-96. Murphy, D.C., Parrish, R.R., Klepacki, D.W., McMillan, W., Struik, L.C., and Gabites, J. 1995. New geochronological constraints on Jurassic deformation of the western edge of North America, southeastern Canadian Cordillera. in Miller, D.M. and Busby, C., (eds.), Jurassic magmatism and tectonics of the North American Cordillera; Geological Society of America, Special Paper 299, p. 159-171. Parrish, R.R., Carr, S.D., and Parkinson, D.L. 1988, Eocene extensional tectonics and geochronology of the southern Omineca belt, British Columbia and Washington. Tectonics, v. 7, p. 181-212. Price, R.A. 1986. The southeastern Canadian Cordillera: thrust faulting, tectonic wedging, and delamination of the lithosphere. Journal of Structural Geology, v. 8, p. 239-254. Price, R.A. 1994. Cordilleran tectonics and the evolution of the Western Canada sedimentary basin. in Mossop, G.D. and Shetsen, I. Geological Atlas of Western Canada. Calgary, Canadian Society of Petroleum Geologists/Alberta Research Council, p. 13-24. Price, R.A. and Carmichael, D.M. 1986. Geometric test for Late Cretaceous-Paleogene intracontinental transform faulting in the Canadian Cordillera. Geology, v. 14, p. 468-471. Price, R.A. and Sears, J.W. 2000 (in press). A preliminary palinspastic map of the Mesoproterozoic Belt/Purcell Supergroup, Canada and U.S.A.: Implications for the tectonic setting and structural evolution of the Purcell anticlinorium and the Sullivan deposit. in Lydon, J.W., Slack, J.F., Höy, T. and Knapp, M. (eds.). The Geological Environment of the Sullivan Deposit, British Columbia. Geological Association of Canada, Mineral Deposits Division, MDD Special Volume No. 1. Southern foreland and thrust belt and kinematics of Cordilleran orognesis Biographical Note: Ray Price is a graduate of the University of Manitoba (B.Sc. Hons., 1955) and Princeton University (Ph.D., 1958), a former employee of the Geological Survey of Canada,, and a former Professor of Geological Sciences and Geological Engineering at Queen’s University, He mapped various parts of the southern Canadian Rockies for the Geological Survey of Canada; and, with his graduate students, he has been involved in tectonic studies in the southern Canadian Rockies, and in the Purcell, and Selkirk Mountains of southern Canada and the northern U.S.A. He is currently Professor Emeritus at Queen’s University, and is engaged in palinspastic reconstruction and tectonic analysis of the southeastern Canadian Cordillera.