Segmentation of plate coupling, fate of subduction fluids, and modes of arc magmatism in Cascadia, inferred from magnetotelluric resistivity

Abstract

Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2-D and 3-D nonlinear 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 locking, and suggest fluid-and sediment-deficient conditions. In contrast, the north-central Oregon coastal structure is quite 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 forearc 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. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and nearby backarc. The low-to high-temperature mantle wedge transition lies slightly trenchward of the arc.

Mantle hydration and Cl-rich fluids in the subduction forearc

“…Atg antigorite serpentinite, CPx clinopyroxene, OPx orthopyroxene, and Ol olivine. Olive symbols olivine aggregate and dark green symbols pyroxene-rich peridotite resolution of 2D electrical conductivity images has greatly improved (Wannamaker, et al 2014;Worzewski, et al 2011) with imaging of fluid paths that can be compared with seismic tomography. Typical values of conductivity range between 10 −3 and 10 −4 and 1 S.m 1 , the lower values being defined by the background sensitivity in cold and fluid-poor regions, and the highest observed in zones where fluids and melts are abundant.…”

“…Magnetotellurics made great improvements in providing more accurate pictures of subduction zones from early studies that depicted rough limits between compartments of a few tens of kilometers to recent studies that give resolution of a few kilometers (Wannamaker, et al 2014;Worzewski, et al 2011). In the forearc region, the most prominent structures are high-conductivity zones that are located near the limit between the serpentinized mantle and the base of the overriding plate (Fig.…”

“…Further decrease in seismic velocity and variations of V P /V S ratio is expected if excess fluid pressure develops at depths (Peacock, et al 2011;Schubnel, et al 2006). If fluids are connected, this will also result in variations of electrical conductivity that can be mapped by magnetotelluric surveys in subduction zones (Baba, et al 2010;Wannamaker, et al 2014).…”

Resistivity distribution from mid-crustal conductor to near-surface across the 1200 km long Liquiñe-Ofqui Fault System, southern Chile

“…Kohl & Rybach 1996), the dynamics of electric resistivity, covering several orders of magnitudes compared to typical crustal rock, characterize mid-crustal conductors (e.g. Unsworth et al 2004;Hill et al 2009;Wannamaker et al 2014). The origin of these electric conductors has early been attributed to graphite (Frost et al 1989;Jödicke 1992).…”

“…The origin of these electric conductors has early been attributed to graphite (Frost et al 1989;Jödicke 1992). More recently, links to aqueous fluids and secondary minerals in fault zones (Korja et al 2008;Brasse et al 2009;Weckmann et al 2012), to partial melt (Unsworth et al 2005;Hill et al 2009), and to metamorphic processes (Wannamaker et al 2014;Zhang et al 2015) have been discussed. Worldwide, it is observed that these conductors have an upper depth limit near the brittle-ductile transition (Jones 1992;Jiracek 1995).…”