SEGMENTATION OF PLATE COUPLING, FATE OF SUBDUCTION FLUIDS, AND MODES OF ARC MAGMATISM IN CASCADIA, INFERRED FROM MAGNETOTELLURIC RESISTIVITY

Five magnetotelluric profiles have been acquired across the various segments of the Cascadia subduction system and transformed using 2D and 3D non-linear inversion to yield electrical resistivity cross sections to depths of ~200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate coupling, and suggest fluid and sediment deficient conditions. In contrast, the northern-central Oregon coastal structure is conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low-resistivity fluidized zones develop at slab depths of 35-40 km starting ~100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Subarc resistivity structure also varies from north to south along the subduction zone. Under the arc in north-central Washington and southern British Columbia, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In the central Cascadia segment, extensional deformation is interpreted to segregate upper mantle melts leading to ponding and low resistivities at Moho to lower crustal levels below the arc and near backarc. Subduction input to arc magmatism is considered high under southern Cascadia, consistent with low upper mantle resistivities in the 50-100 km depth range.