CRETACEOUS AND PALEOGENE TECTONIC EVOLUTION OF THE CRYSTALLINE CORE OF THE NORTH CASCADES

The North Cascades crystalline core is characterized by amphibolite-facies metamorphism and 96 to 45 Ma arc plutons, and lies in an unique position at the southern termination of the > 1500 km long Cretaceous (~100-80 Ma) Coast belt thrust system and the northern end of a discontinuous belt of Jurassic ophiolitic and arc terranes. The Cascades core lies in the hinterland to the NW Cascades segment of this collisional thrust system, sitting above a footwall of the southernmost Insular superterrane. Juxtaposition of oceanic and arc terranes in the core predated 92 Ma plutons, and subsequent deformation was dominated by tight to isoclinal, recumbent folds and associated foliation, followed by upright folds. SW-vergent reverse shear zones with modest displacement also formed after terrane juxtaposition, but overall, deformation was dominated by pure shear.

The Cascades core was rapidly buried during mid-Cretaceous shortening. Barometric data and restoration of shortening structures indicate that a crustal section representing paleodepths of ~ 5-40 km is preserved in the southern part of the core. Crustal thickness exceeded 55 km in the Cretaceous and the core probably resembled the thickest segment of the modern Andes. The SW part of the core was rapidly exhumed following burial. Plutonism, metamorphism, and shortening continued in the deeper, more internal NE part of the core, partly in response to transpression that persisted until ~ 58 Ma. A major enigmatic event was the rapid underthrusting of Cretaceous sedimentary protoliths of the Swakane Gneiss to depths of > 40 km beneath the Cascades core between 73 and 68 Ma, which presumably disrupted the roots of the Cascades arc. Eocene magmatism after a transition from transpression to transtension was synchronous with isothermal decompression and major exhumation (~ 50-45 Ma) of the NE part of the core. Exhumation was probably in response to thermal weakening and plate boundary changes, and was facilitated by motion on a deep-level detachment and dextral-normal slip on high-angle faults. Ridge subduction may have occurred outboard of the arc at this time. The tectonics of the core exemplify the dynamic evolution of continental arcs in general and the large vertical and lateral displacements in the Cascades before initiation of the modern Cascades arc in the late Eocene.