Shuswap terrane of British Columbia: A Mesozoic “core complex”

The Shuswap terrane of the northwest Cordillera owes its distinctive characteristics to deformation and high-grade regional metamorphism that occurred primarily during the Mesozoic. Within the terrane, the Monashee Complex, the overlying Selkirk allochthon, and the intervening Monashee decollement record strain and metamorphism that occurred in Middle to Late Jurassic time, during and subsequent to the accretion of a western allochthonous terrane. The Selkirk allochthon moved eastward across the Monashee Complex after emplacement of the western allochthonous terrane but before late Mesozoic to early Cenozoic telescoping of the Rocky Mountain Belt. In this latter event, the Monashee Complex and overlying allochthonous slices were transported eastward relative to the North American craton on a sole fault that developed during listric thrusting of the Rocky Mountain foreland. Synmetamorphic to late-metamorphic elongation lineations and associated fabrics of the mylonitic rocks of the Monashee decollement zone were generated by shear strain during the Middle Jurassic emplacement of the Selkirk allochthon and are not related to any upper crustal extension that occurred in the Tertiary. Uplift, normal faulting, brittle reactivation of the mylonitic decollement zone, widespread resetting of K-Ar and Rb-Sr mineral dates, and arching of the terrane are events that culminated in the Eocene. *Present address: Geotex Consultants Limited, 1200-100 West Pender Street, Vancouver, British Columbia V6B 1R8, Canada