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

摘要: [1] In the NW North American Cordillera, Cascades core region of Coast Plutonic Complex underwent Late Cretaceous (>96 Ma to locally 73 Ma) SW-NE contraction and crustal thickening followed by dextral transpression (∼73 55 Ma), then transtension ( 3 mm/yr) local thrusting in regions undergoing thickening. In central part (Chelan block), >40 km exhumation occurred between 91 45 Ma, about half which during early (driven thrusting) top-to-north, arc-oblique shear reactivation a midcrustal thrust, Dinkelman decollement. The footwall this thrust consists Swakane Biotite Gneiss, Cretaceous, metaclastic assemblage with recorded pressures 10–12 kbar, no arc-related magmatism, structures dominated pervasive top-to-north shearing. hanging wall Napeequa Complex, an oceanic 6–12 voluminous complex indicating top-to-WSW shearing, younger widespread folding. Napeequa, shearing started melt-present conditions at ≥6 kbar. Top-to-north both continued (∼1.6 cooling greenschist facies slip became increasingly localized, eventually resulting formation pseudotachylite on discrete surfaces. We suggest that extension was driven along-arc heterogeneity displacements/erosion, initially underplating continental sediments, later transtension.