U-PB GEOCHRONOLOGY OF THE NORTH CASCADES—FROM MATTINSON TO THE PRESENT

James Mattinson’s pioneering work in the North Cascades used U-Pb geochronology to deconvolve its complex history. Previous field studies suggested a complicated tectonic framework, including the possibility of Precambrian basement. Mattinson demonstrated the presence of Precambrian zircons (Yellow Aster Complex) and Lower Paleozoic (Turtleback Complex), Triassic (Dumbell-Marblemount), Jurrasic-Cretaceous (Chelan Complex) and Cretaceous plutons. Almost forty years later, we continue to unravel this complex history with an integrated field-based approach and the application of Mattinson’s chemical-abrasion technique.

The Northern Cascade Mountains represent the southernmost part of the Coast plutonic complex that extends for >1500 km from Alaska to Washington, interpreted to be related to a mid-Cretaceous continental arc. The North Cascades is divided into the Wenatchee and Chelan blocks by the post-metamorphic, high-angle Entiat fault. The two blocks preserve different magmatic and metamorphic histories; the Wenatchee block records arc magmatism from ca. 96 to 88 Ma while the Chelan block records magmatism as young as ca. 45 Ma and was affected by Eocene orogen-parallel stretching and exhumation. Contractional deformation accompanied by intrusion of granitoid plutons thickened the crust to >55 km by ca. 90 Ma. Arc magmatism from ca 90–70 Ma is characterized by sheeted plutonic complexes intruded over 1–3 Ma. In the Eocene, the orogen is marked by metamorphism, magmatism, and mid-Eocene exhumation of the deep crust (Swakane Gneiss and the Skagit Gneiss Complex (SGC)), coincident with subsidence and rapid deposition in nearby non-marine basins. Swakane paragneiss protoliths were deposited ca. 71 Ma, intruded by 68 Ma pegmatites and exhumed from 11–12 kilobars. The SGC comprises 76–45 Ma tonalitic orthogneiss with metamorphism, melt generation and crystallization occurring from ~71–47 Ma terminating with nearly isothermal decompression from 10 to <5 kilobars. South of the core, sediments were deposited in the ca. 51 Ma Swauk basin, folded, and intruded by 47 Ma Teanaway basalt dikes. We have documented the temporal coincidence between: basin formation, rapid deposition, partial melting, ductile flow, and upright folding of the SGC and Swauk Formation.