DYNAMICS OF THE PACIFIC NORTHWEST LITHOSPHERE AND ASTHENOSPHERE

Seismic imaging resolves structure beneath the Pacific Northwest (PNW) that is interpreted as: Ÿan high-velocity piece of accreted (~50 Ma) Farallon lithosphere that descends from being exposed along much of the coast ranges (i.e., Siletzia) into the lower crust of SE Washington and then turning down and extending to depths of ~600 km as a 'curtain' beneath central Idaho; Ÿa stubby Juan de Fuca slab, extending to <250 km in most places, and only to ~150 km beneath much of Oregon; Ÿand very slow 'fingers' of partially molten mantle wedge with an especially anomalous slow volume beneath central Oregon. Evacuation of asthenosphere entrained with the descending Juan de Fuca slab presents an interesting problem. Typical corner-flow supply is prevented by the east-blowing mantle wind. A stagnation line between mantle drawn west into the mantle wedge and that flowing east in the far field may exist near the Farallon curtain. Other sources of supply to the mantle wedge include: mantle flowing north beneath California, entrained with the Pacific Plate; flow from below the Juan de Fuca slab, perhaps through the Oregon gap; toroidal flow around the southern edge of the slab; and up-flow beneath the backarc. Anisotropy studies suggest the importance of toroidal flow.

PNW lithosphere stress is dominated by N-S compression, a result of the Sierra Nevada driving the PNW into British Columbia; this push and oblique subduction are responsible for PNW clockwise rotation. Modeled tractions on the Cascadia mega-thrust suggest shear stress of ~40 MPa over much of the locked mega-thrust (i.e., much more shear stress than the typical earthquake stress drop of 1-10 MPa). Southern Cascadia is relatively tensional, with margin-normal compression being much lower than typical ridge push, whereas northern Cascadia is compressional. Hence the southern Cascadia mega-thrust is more weakly coupled than the northern mega-thrust. Southern Cascadia slab rollback and extension of the Cascade graben and Basin-and-Range are enabled by the weak coupling, in conjunction with high gravitational potential energy of the southern Oregon arc and back-arc.