Large-scale over-thrusting and regional extension are both known to have been operative within the Shuswap metamorphic terrane of the southern Omineca belt. The Monashee core complex of the northern Shuswap terrane is interpreted to be a crustal-scale duplex that was developed during late Mesozoic shortening and thickening of the crust. Uplift of the Monashee complex, caused by basement duplexing, is estimated to have been in excess of 25 km. Late Cretaceous overthrusting of high-grade metamorphic and related plutonic rocks, during later stages of uplift and basement duplexing, led to weakening of the crust and to gravitationally driven extension of a regional crustal welt (Shuswap culmination). Denudation and spreading of this uplifted metamorphic terrane are attributed to displacements across listric-normal shear zones and faults localized along the outward-dipping flanks of the Shuswap culmination. Total extension across the Shuswap culmination at the northern end of the Monashee complex is estimated to be no greater than 30%. This model of denudation and gravity-induced spreading is extrapolated southward into the Valhalla and Okanagan regions, where extension may be far more significant.

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