Structural Architecture and Kinematic History of the Coast Mountains Orogen, southern British Columbia and northern Washington

Jordan W. Wang, University of Arizona
Paul Kapp, University of Arizona
George Gehrels, University of Arizona
John He, University of Arizona
Tristan White, University of Arizona
Jamie Cutts, Geological Survey of Canada

Abstract

Forearc lithosphere exhumed in subduction complexes and ophiolites are key records of convergent margin processes, providing insights into the evolving geodynamics of subduction systems from initiation to termination. However, the rock record within ancient and modern forearc systems is often incomplete due to (subduction) erosion, strike-slip modification of forearc lithosphere, and deep burial in forearc basins. As a result, the structural level of exposure in exhumed forearc regions along the same margin can be highly variable and present different snapshots of key processes including subduction accretion, forearc extension and ophiolite generation, and exhumation of high-pressure rocks. In Cordilleran orogens, collisional suture zones exhumed during regional terrane accretion and ensuing orogenic collapse provide unique views into the deeper structural levels of the forearc that can expose older records of the subduction interface from the hinterland of accretionary wedges, revealing processes and histories which may be buried in non-collisional along-strike segments of the same margin.

The Coast Mountains Orogen of southern British Columbia and northern Washington state is a collisional segment of the North American Cordillera that preserves a longer and older record of subduction convergence than the classic Jurassic-Cretaceous triad of California (i.e., The Franciscan Complex, Great Valley Group, and Sierra Nevada Batholith). Using new and compiled geochronologic ages of subduction metamorphism, accretion, and collisional metamorphism, we present a history of near-continuous east-dipping subduction convergence along the western margin of Quesnel terrane beginning in the Permian and terminating in the mid-Cretaceous during the terminal suturing of Wrangellia. At least two distinct episodes of upper plate rifting (in Permo-Triassic and Middle Jurassic time, respectively) are recorded in the form of suprasubduction zone ophiolite formation and higher-T amphibolite-facies metamorphism of subduction complex rocks. We interpret the former Permo-Triassic event to represent forearc rifting during east-dipping subduction initiation. The latter Middle Jurassic event coincides with a distinct increase in siliciclastic material to the accretionary wedge, suggesting a change in proximity of the subduction trench to the continent, and/or a change in coastal climate/erosion. We explore the protentional geodynamic links between these factors and discuss the implications of this longer record of subduction convergence for the rest of the Cordilleran margin.

Session

Cordilleran tectonics