Crustal structure and early Tertiary extensional tectonics of the Omineca belt at 51°N latitude, southern Canadian Cordillera

Abstract

A crustal cross section through the Omineca belt at the latitude of the Trans-Canada Highway has been drawn to satisfy available surface geological information and Lithoprobe seismic data from this part of the Cordilleran hinterland. Palinspastic restoration of Tertiary normal-sense shear zones leads to the conclusion that the Omineca belt at latitude 51°N was extended in the Eocene by approximately 45 km, 20–25% of the width of the belt. It is shown that the Okanagan–Eagle River fault, which defines the western margin of the Shuswap metamorphic core complex, is likely to have accommodated approximately 30 km of displacement. Restoration of this fault and of 15 km displacement on the Columbia River fault (eastern margin of the Shuswap complex) juxtaposes upper-crustal rocks with similar stratigraphic, structural, and metamorphic characteristics and indicates that the crust was over 50 km thick prior to Eocene extension. Comparison of the crustal geometry in the present and restored sections suggests that extensional strain was partitioned such that the upper crust was most highly attenuated above the central Shuswap complex, whereas the lower crust was most greatly stretched beneath the Intermontane and western Omineca belts.