LITHOPROBE, southern Vancouver Island, British Columbia: geology

Abstract

Phase 1 of LITHOPROBE involved co-ordinated geophysical and geological studies on southern Vancouver Island conducted from 1984 to 1986. Geological studies, the focus of this report, included 1:50 000 scale mapping as well as stratigraphic, geochemical, and structural studies in a corridor centred on Port Alberni and on southernmost Vancouver Island in the vicinity of Sooke. In the Port Alberni region the geological architecture of Wrangellia is expressed by three volcanic/plutonic successions separated by shallow marine carbonate and clastic deposits, the total ranging in age from mid-Paleozoic to Early Jurassic. Post accretionary clastic sediments of Late Cretaceous and Tertiary ages overlie Wrangellia and the adjacent Pacific Rim and Crescent terranes. The Alberni region is partitioned by northwesterly trending thrust faults and the Cowichan Anticlinorium, the latter dislocated by northerly trending faults and hypabyssal intrusions. The Metchosin Igneous Complex of southernmost Vancouver Island is part of the Crescent Terrane. It consists of layered gabbro, sheeted dykes, pillow basalts and flows forming a partial ophiolite. The complex represents an emergent volcanic island formed in a marginal basin setting. It was transported northwestward by and emplaced beneath the Leech River Fault. Integrated geological and seismic reflection data indicate that the Wrangellian crust of Vancouver Island is largely granitic rocks of the Island Intrusions with pendants and enclaves of Paleozoic to Lower Jurassic volcanic and sedimentary rocks. Prominent inclined reflectors in the seismic records are identified with the surface traces of thrust faults.