Kinematics and geometry of active detachment faulting beneath the Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge

Newly acquired seismic refraction and microearthqualke data from the Trans-Atlantic Geotraverse (TAG) segment of the Mid-Atlantic Ridge at 26 degrees N reveal for the first time the geometry and seismic character of an active oceanic detachment fault. Hypocenters from 19,232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an similar to 15-km-long, dome-shaped fault surface that penetrates to depths > 7 km below the sealloor on a steeply dipping (similar to 70 degrees) interface. A tomographic model of compressional-wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall, which appears to roll over to a shallow dip of 20 degrees +/- 5 degrees and become aseismic at a depth of similar to 3 km. Outboard of the detachment the exhumed lithosphere is deformed by ridge-parallel, antithetical normal faulting. Our results suggest that hydrothermal fluids at the TAG field exploit the detachment fault to extract heat from a region near the crust-mantle interface over long periods of time.