EVOLUTION OF THE CASCADIA FOREARC, OREGON AND WASHINGTON (Invited Presentation)

The Cascadia convergent margin of the NW U.S. and SW Canada exhibits classic elements of a subduction zone with its active megathrust and volcanic arc. Its physiography, with a Coast Range separating offshore forearc basins from an interior seaway and lowland, is less common and more typical of young, warm slab subduction. The margin has evolved in response to several major tectonic events. The Mesozoic (Mz) emplacement of Wrangellia and the Klamath terranes set the stage for accretion of Siletzia, a 56-49 m.y.-old oceanic plateau and island chain that filled the Columbia embayment and was sutured to N. America by about 50 Ma. The collision deformed the continent, contributed to oroclinal bending of the Mz orogen, rotated Siletzia clockwise, and produced large, lithic, micaceous, arkosic clastic wedges into the forearc. A new subduction zone (Cascadia) established itself outboard (west) of Siletzia, where an accretionary complex developed, part of which is exposed in the uplifted core of the Olympic Mountains. Continued underplating of accreted sediments may be contributing to the uplift of the Olympic and Klamath segments of the forearc. Forearc basaltic and alkalic magmatism of the Tillamook episode commenced ca. 42 Ma during a period of margin-parallel extension and continued rotation, which may reflect interaction of the margin with a spreading ridge and/or hotspot. In the arc, an ignimbrite flare-up peaked ca. 35 Ma and suggests extension propagated E. into the arc. The flare-up dumped pulses of tuffaceous mud and volcaniclastics into forearc subbasins, interlayered with arkosic micaceous sand. Extension followed in the back arc with the Miocene eruption of the Columbia River flood basalts. Some far-travelled flows crossed a deformed arc and forearc, reaching the ocean as lava deltas and invasive flows over a 350-km-wide front between Newport, OR and Aberdeen, WA ca. 16.5-12 Ma. Subsequent deformation uplifted the Coast Range and depressed the CRB below sea level in the Willamette Valley and 1.3 km below sea level on the continental shelf. Neogene oblique subduction of gradually younger, more buoyant Juan de Fuca plate may have contributed to Coast Range uplift and driven the change to northward migration of the forearc, with regional clockwise rotation and right-lateral faulting that continues today.