Driving mechanisms for >40 km of exhumation during contraction and extension in a continental arc, Cascades core, Washington

Abstract

In the NW North American Cordillera, the Cascades core region of the Coast Plutonic Complex underwent Late Cretaceous (>96 Ma to locally 73 Ma) SW‐NE contraction and crustal thickening followed by dextral transpression (∼73 to 55 Ma), then transtension (<55 Ma). Exhumation occurred during all three phases. During contraction, slow exhumation (∼0.6 mm/yr) occurred along the margins of the core, driven by isostatic rebound and erosion, and faster exhumation (>3 mm/yr) by local thrusting in regions undergoing crustal thickening. In the central part of the core (Chelan block), >40 km of exhumation occurred between 91 and 45 Ma, about half of which occurred during early contraction (driven by thrusting) and half during top‐to‐north, arc‐oblique shear during reactivation of a midcrustal Cretaceous thrust, the Dinkelman decollement. The footwall of this thrust consists of the Swakane Biotite Gneiss, a Cretaceous, metaclastic assemblage with recorded pressures of 10–12 kbar, no arc‐related magmatism, and structures dominated by pervasive top‐to‐north shearing. The hanging wall consists of the Napeequa Complex, an oceanic assemblage with recorded pressures of 6–12 kbar, voluminous arc‐related magmatism, and complex structures indicating early top‐to‐WSW shearing, younger top‐to‐north shearing, and widespread folding. In the Napeequa, top‐to‐north shearing started by 73 Ma during melt‐present conditions at pressures ≥6 kbar. Top‐to‐north shearing in both hanging wall and footwall continued during exhumation (∼1.6 mm/yr) and cooling to greenschist facies conditions during which slip became increasingly localized, eventually resulting in formation of pseudotachylite on discrete slip surfaces. We suggest that arc‐oblique extension was driven by along‐arc heterogeneity in displacements/erosion, initially during transpression and underplating of continental sediments, and later during transtension.