Magnetotelluric constraints on the fluid content in the upper mantle beneath the southern Canadian Cordillera: Implications for rheology

Long‐period magnetotelluric data were collected on two parallel profiles extending across the southern Canadian Cordillera and used to derive models of the electrical resistivity of the crust and mantle from the Cascadia subduction zone to the Alberta Basin. In the fore arc, the resistivity models indicate an east dipping conductor and conductive mantle wedge, caused by the release of aqueous fluids from the Juan de Fuca plate. Low resistivities are also found beneath the volcanic arc, associated with water released from the subducting slab and mantle melts. Low resistivities in the back‐arc upper mantle at depths less than 60 km suggest a shallow asthenosphere compared to the adjacent North American craton where the lithosphere‐asthenosphere boundary is at ~200 km. The resistivity of the back‐arc upper mantle was interpreted using geotherms, laboratory studies of mineral properties and melting points to determine the type and quantity of fluids present. The low resistivities in the back‐arc upper mantle require aqueous fluids, with water content increasing from 0.005 wt % at 50 km to 0.03 wt % at 150 km depth. In addition, melt fractions of up to 1.5% are required at depths less than ~135 km to explain the observed resistivities. The presence of these quantities of aqueous fluids and partial melt will lower the viscosity of the upper mantle, as required by geodynamic models that include vigorous convection in the back arc to explain the observed heat flow.