Latest Cretaceous to early Tertiary dextral strike-slip faulting on the southeastern Yalakom fault system, southeastern Coast Belt, British Columbia

Abstract

The Yalakom fault system cuts the Cadwallader, Bridge River, and Methow terranes for 300 km along the northeast side of the Coast Belt in southwestern British Columbia. The fault system had major dextral slip after middle Cretaceous contractional deformation and early Late Cretaceous volcanism, which ceased by late Eocene time. At discrete times, the Yalakom fault system had major changes in the configuration of active faulting, which resulted in four stages of faulting over ≈40 m.y. The latest Cretaceous dextral slip occurred on the associated Castle Pass-Downton Creek fault zone and possibly the Yalakom fault. Strike slip recurred on the Yalakom fault during the Paleocene(?)-middle Eocene, and there is an associated series of left-stepping faults with zones of contraction and uplift in the stepovers. This associated deformation included early to middle Eocene dextral shear, and south-vergent thrusting within the Shulaps metamorphic belt on the southwest side of the Yalakom fault. The Marshall Creek fault became an important component of the system during middle Eocene time, and this led to tectonic unroofing of the Shulaps belt along normal faults localized in the right-stepping transfer zone linking the Marshall Creek and Yalakom faults. Magmatism was widespread but minor during faulting and was commonly localized along faults. Magmatism may have aided uplift of the Shulaps belt. No basinal deposition accompanied the strike-slip faulting. An estimate of 115 km dextral, strike-slip displacement on the Yalakom fault is based on offset of a unique three-part structural succession of the Methow, Cadwallader, and Bridge River terranes. About 5 km of dextral slip occurred on subsidiary faults that do not merge with the Yalakom fault; therefore, 120 km of dextral offset is estimated for the Yalakom fault system. All but ≈10 km of dextral offset occurred in the Eocene (and Paleocene?). Thus our interpretations preclude the Yalakom fault from being a major contributor to large-scale northward translation of the northwestern Cordillera (the Baja British Columbia hypothesis) interpreted from paleomagnetic data to have occurred largely before Eocene time. On the basis of our correlations across the Yalakom fault, such large faults must lie east of the Methow terrane. The Yalakom fault system probably was linked to the Hozameen and Ross Lake faults (now exposed more than 100 km to the south on the east side of the Fraser and Straight Creek faults). An internally consistent regional model of strike-slip faulting based on offset of geologic units and timing on faults suggests ≥250 km of dextral displacement between ca. 80 and 35 Ma within the southeast Coast Belt. This model resolves the apparent conflict in fault data from Washington and British Columbia and gives a template for comparison of pre-Late Cretaceous geology.