GEOCHRONOLOGICAL CONSTRAINTS ON SYNCHRONOUS REGIONAL TRANSTENSION, BASIN FILL, METAMORPHISM, AND EXHUMATION OF HIGH PRESSURE GNEISSES IN THE NORTH CASCADES, WA

The North Cascades metamorphic core, at the southern terminus of the Coast plutonic complex, is the remnant of a ~55 km thick Cretaceous to Paleogene continental magmatic arc. Extension ca. 45-50 Ma ago exhumed an aerially extensive complex of partially migmatitic tonalitic to granitic gneisses, plutons, and metasedimentary rocks known as the Skagit Gneiss Complex (SGC). New U-Pb zircon dates from late gneissic sheets and plutonic bodies in the southern SGC demonstrate that magmatism and solid state fabric formation was ongoing as late as 49 Ma in the Cascades core. New biotite and muscovite Ar/Ar cooling ages of ca. 47 Ma from the Skagit are consistent with regional 50-45 Ma Ar/Ar and K/Ar biotite and hornblende dates. Overlapping both high grade metamorphism and magmatism is the deposition of thick sequences of non-marine sedimentary rocks in a series of fault-bounded extensional basins including the Chuckanut basin in western Washington, the Swauk basin immediately to the south of the metamorphic core, and the Chumstick basin, which occupies a graben-like structure between blocks of dominantly crystalline rocks. New ID-TIMS zircon dates from interbedded tuffs in the three basins offer insight into development of the Cascades core. A thin silica-rich volcanic layer from the lower Chuckanut contains zircons that are ca. 57 Ma, suggesting that this basin spans the Paleocene-Eocene boundary and was active during an apparent magmatic hiatus in the crystalline core. An ash flow tuff from near the top of the Swauk basin is ca. 51 Ma, indicating that basin subsidence had begun before rapid cooling of the crystalline core through Ar closure. Two ca. 48 Ma ashes from near the middle of the Chumstick section bracket ~650 m of clastic sedimentary rock and differ in age by ~0.35 Myr, consistent with rapid accumulation/subsidence coeval with the latest episode of ductile shear and plutonism in the metamorphic core and Teanaway dike intrusion to the SW. The close temporal and spatial relationship between these basins and exhumation of the SGC suggests that a common mechanism, perhaps a system of oblique normal-sense fault structures, accommodated both.