Reconciliation of two-sided thrusting, burial metamorphism, and diachronous uplift in the Cascades of Washington and British Columbia

摘要

Several recently published data sets have shed new light on many of the fundamental kinematic aspects of mid-Cretaceous orogeny in the Cascades of northwestern Washington and southwestern British Columbia. Specifically, (a) it is now clear that contractional deformation associated with this orogeny affected rocks east of the crystalline core of the range to a greater degree than had previously been suspected, and (b) geobarometric data from the crystalline core of the range now permit a quantitative reconstruction of the crustal-thickening event responsible for core metamorphism to be attempted. I propose a new kinematic model for mid-Cretaceous orogeny that reconciles these data with previously collected data sets that focused on deformation in the northwest Cascades-San Juan Islands thrust system. The model derived from this analysis suggests that the Cascades underwent a minimum of between 400 and 500 km of east-west shortening between about 115 and 85 Ma, owing to the collision of the greater Insular terrane, which probably included rocks of the Chilliwack-Nooksack terrane, with the western edge of the greater Intermontane terrane, which included the Methow basin and Bridge River-Hozameen terrane. On the basis of their relative and absolute times of uplift, I assign the deepest part of the metamorphic core, the Skagit Gneiss, to the greater Insular terrane, and the structurally higher carapace of the core, the Settler-Nason metapelites and the Cogburn Creek-Mad River metabasalt/metachert, to the westernmost Intermontane terrane. The contact between the Skagit Gneiss and the overlying assemblages must represent a fault or suture across which at least 120 km of displacement took place. This fault is also the root zone from which many of the nappes in the northwestern Cascades and San Juan Islands were derived. I concur with previous workers who suggested that the collisional event was not precipitated by the closure of a several-thousand-kilometer-wide ocean but was rather due to a short-lived margin-normal convergence event that both followed and was followed by periods during which the continental margin was highly influenced by oblique subduction and strike-slip faulting.