40Ar/39Ar Dating of Phyllonite in the Southern Rocky Mountain Trench and Adjacent Rocky Mountains Unravels Kinematic Links between the Omineca and Foreland Belts of the Southern Canadian Cordillera

Abstract

This study includes the first set of muscovite, biotite, feldspar, and whole-rock 40Ar/39Ar ages from phyllonite belts of the Southern Rocky Mountain Trench (SRMT) and the western Rocky Mountain fold-thrust belt in the southern Canadian Cordillera. Eleven samples from the northern segment of the SRMT indicate two Early Cretaceous (135 and 125 Ma), two mid-Cretaceous (111 and 96 Ma), and two Late Cretaceous (85 and 75–72 Ma) peaks of transpressional tectonism and define the Southern Rocky Mountain shear zone. Four samples from the adjacent strike-slip Walker Creek fault zone yielded late Valanginian (133 Ma), early Aptian (124 Ma), and Albian-Cenomanian (111–96 Ma) ages, with Late Cretaceous (85–68 Ma) overprint. The oblique compression Bear Foot thrust (72 Ma) and its footwall, the Valemount strain zone, yielded consistent, late Campanian to earliest Maastrichtian plateau ages. Poorly recrystallized phyllonite samples from the Monarch, Moose Pass, and Chatter Creek thrust faults in the Rockies yielded ages consistent with a late Valanginian (134–131 Ma) tectonic event. The new structural and geochronological data indicate apparently distinct pulses of protracted orogen-parallel Cretaceous tectonism along the western margin of the Rockies and the eastern margin of the Omineca belt and document the kinematic link between the Foreland belt and its hinterland through a wide zone of transpression. These ages are also consistent with previously reported ages of thrusts in the Rockies accompanied by significant depositional changes in the foreland basin. The Early Cretaceous transpression (~135 and ~125 Ma) propagated into the western Rockies as previously unrecognized, out-of-sequence thrusting pulses that may have triggered the development of the forebulge responsible for the vast sub-Cadomin–sub-Manville unconformity in the foreland basin.