Variations in temperature gradients identify active faults in the Oregon accretionary prism

A multi-channel seismic survey (MCS) yields high resolution images of geologic structures within the Oregon accretionary prism. In addition to revealing the internal geometry of the prism a prominent bottom simulating reflector (BSR) provides indirect estimates of temperature gradients. The BSR marks the phase transition, within the pore space of the sediments, of small amounts of solid gas hydrate above the reflector to free gas below. Although the absolute value of temperature gradients depends on assumptions of gas hydrate stability conditions, velocity functions, and seafloor temperature, relative differences reflecting regional trends and local anomalies are unaffected by the assumptions involved in calculation of temperature gradient from the position of the BSR. Broad-scale landward decreases in temperature gradient reflect the increasing depth to the oceanic crust across the accretionary prism. Correlation of anomalously high temperature gradients with two out-of-sequence thrusts indicates that these structures are currently acting as conduits for warm fluids.