Large lateral ramps in the Eocene Valkyr shear zone: extensional ductile faulting controlled by plutonism in southern British Columbia

A 2.5-km high, S-facing lateral ramp links northern and southern segments of the ductile Eocene Valkyr extensional shear zone which defines the upper and outer boundary of the Valhalla core complex in southern British Columbia, Canada. The evolution of the Valkyr shear zone and of the lateral ramp is strongly influenced by the setting and geometry of Middle Jurassic tonalitic plutons and of Paleocene-Eocene leucogranite, aplite and pegmatite Jurassic plutons were emplaced in the region of the Valhalla complex as a series of interconnected, flatbottomed laccoliths with the basal surfaces of adjacent bodies at the same structural level; however, on a large scale the southern portion of the laccolithic complex is structurally 2.5 km lower than the northern portion. The lower surface of the laccolithic complex formed the upper limit of injection of gently-dipping Paleocene-Eocene leucogranite sheets. As a consequence the upper margin of leucogranite is 2.5 km structurally lower in the south than in the north. the Eocene, extensional shear zones propagated along the interface between the upper plate stiffened by the cool, strong Middle Jurassic tonalite, and the lower plate made weak and ductile by voluminous hot PaleoceneEocene leucogranite sheets. The northern and southern segments of the Valkyr shear zone propagated at two different structural levels, and a lateral ramp formed in the intervening zone. The ramp was localized at a rheological change across a S-dipping boundary between hot ductile Paleocene-Eocene leucogranite and an adjacent strong Middle Jurassic pluton. m displacement of 15–20 km occurs in the central portion of the northern segment of the Valkyr shear zone. The southern segment has only 1–3 km of displacement. This has left the pre-Eocene geological relationships of the southern Valhalla complex and its surrounding rocks nearly intact, and it can be demonstrated that the Valkyr shear zone formed with a gentle original dip.