EAST-DERIVED STRATA IN THE METHOW BASIN RECORD RAPID MID-CRETACEOUS UPLIFT OF THE SOUTHERN COAST MOUNTAINS BATHOLITH

The Jurassic-Cretaceous Methow basin of northern Washington State and southern British Columbia forms an overlap sequence linking several small tectonostratigraphic terranes during the Mesozoic suturing of the Intermontane and Insular superterranes. Sandstone petrography, sandstone and mudrock geochemistry, and detrital zircon U-Pb age and Hf analysis of mid-Cretaceous, east-derived Methow strata together document a remarkably uniform provenance signature that suggests proximal, abundant, and unchanging sediment sources throughout deposition. Detrital zircon signatures from the Harts Pass and Winthrop Formations of the Methow Basin are characterized by prominent age peaks at ca. 160 Ma and ca. 116 Ma, with less than 1% of grains older than Mesozoic age. Mesozoic detrital zircon have positive epsilon Hf values averaging +8 to +10, and mudrock geochemistry indicates a young, undifferentiated magmatic arc source with little involvement of continental crustal sources. The eastern belt of the Coast Mountains batholith, intruded into Stikine and related inboard terranes of the Intermontane superterrane, along with Jurassic plutons of the westernmost Okanogan Range, provide the best match to the provenance signature of east-derived sediment in the Methow basin during the mid-Cretaceous. Furthermore, the Late Cretaceous and Early Jurassic plutons of the eastern Coast Mountains batholith and western Okanogan Range must have been rapidly uplifted in order to provide the substantial thickness of sediment in the Methow basin, and they must have acted as a topographic barrier that effectively prevented sediment derived from the continental interior from reaching the basin. We suggest that this rapid uplift of a proximal eastern source occurred during late Early Cretaceous regional transtension, resulting in subsidence of the Methow trough and block uplift and tilting of terranes immediately east of the basin. Because Methow sediment sources apparently did not include the North American interior, the extent of post-depositional large-scale translation relative to the North American craton of the Methow basin and its proximal, eastern sources cannot be unequivocally determined.