Electrical resistivity imaging of the inter-plate coupling transition at the Hikurangi subduction margin, New Zealand

Abstract

Inter-plate coupling on the Hikurangi subduction margin along the east coast of New Zealand's North Island changes from weakly coupled in the north to locked in the south. 3-D inverse modeling of magnetotelluric (MT) data across the coupling transition shows that the electrical resistivity structure is correlated with the areal strain rate, which provides a measure of plate coupling. The correlation between strain rate and resistivity is seen parallel to the strike and dip directions of the subduction. In the region where the strain is extensional, the upper plate is more conductive than in the south where contraction is occurring, and the plates are locked. The increased mid-crustal resistivity in the south can be interpreted to be a consequence of reduced fluid interconnectivity due to contraction or reduced fluid content due to decreased upward transport from sources beneath the subduction interface. The increased mid-crustal conductivity in the extensional area is interpreted to be a consequence of fluid released by subducted sediment that are also the cause of the extension. Seismicity in the upper 5 km of the subducting plate shows similar changes, with a greater concentration of near-interface seismicity beneath the conductive (extensional) region. We interpret the concentration of seismicity below the subduction interface in the extensional region to show fluid sourced from dehydration reactions within the subducting plate or from the de-watering of subducted sediments rising into the upper plate and decreasing the resistivity of the middle crust. Decreased near-interface seismicity beneath the contractional region can be interpreted to be a consequence of decreased upper plate permeability preventing upward fluid escape or reduced lower plate fluid availability.