The southern Coast Mountains, British Columbia: New interpretations from geological, seismic reflection, and gravity data

Abstract

The southern Coast Mountains of British Columbia are characterized by voluminous plutonic and gneissic rocks of mainly Middle Jurassic to Eocene age (the Coast Plutonic Complex), as well as metamorphic rocks, folds, and thrust and reverse faults that mostly diverge eastward and westward from an axis within the present mountains, and by more localized Eocene and younger normal faults. In the southeastern Coast Mountains, mid-Cretaceous and younger plutons intrude Bridge River, Cadwallader, and Methow terranes and overlap Middle Jurassic through Early Cretaceous marine clastic rocks of the Tyaughton–Methow basin. The combination of geological data with new or reanalyzed geophysical data originating from Lithoprobe and related studies enables revised structural interpretations to be made to 20 km depth. Five seismic profiles show very cut-up and chaotic reflectivity that probably represents slices and segments of different deformed and rearranged rock assemblages. Surface geology, seismic interpretations, physical properties, and gravity data are combined in two profiles across the Coast Mountains to generate two new 2-D density models that are interpreted in terms of the geological units. The western part of the southern Coast Mountains consists primarily of Jurassic to mid-Cretaceous plutons to depths of 20 km with slices of Wrangellia (in the west) and Early Cretaceous volcanic and sedimentary rocks (Gambier group) in the upper 10 km. The eastern part, east of the Owl Creek fault, consists of slices of Cadwallader and Bridge River terranes and Tyaughton–Methow basin strata with limited slices of plutonic rocks at depths less than 10 km. Below that, Eocene and Late Cretaceous plutons dominate for another 10 km.