Present‐day motion along the Owen Fracture Zone and Dalrymple Trough in the Arabian Sea

All plate motion data available for describing the present-day motion between the Arabian and Indian plates are analyzed to understand the present-day tectonic role of the presumed plate boundary: the Owen fracture zone and Dalrymple trough in the northern Arabian Sea. All prior plate motion models are shown to be inconsistent with some of the data along the presumed boundary. In particular, published global plate motion models predict some convergence across the Owen fracture zone and Dalrymple trough. Convergence along the Owen fracture zone is contradicted by earthquake focal mechanisms, which show right-lateral strike slip. Convergence along the Dalrymple trough is contradicted by observed normal faulting in the trough, and by normal and strike-slip focal mechanisms for earthquakes occurring along or near the trough. The sense and rate of motion is further constrained by spreading rates along the Sheba Ridge (northwest of the Owen fracture zone) and along the Carlsberg Ridge (southeast of the Owen fracture zone). To estimate the present-day motion, we determine 43 3-m.y.-average spreading rates from Sheba Ridge, Carlsberg Ridge, and Central Indian Ridge magnetic profiles and have combined them with eight transform azimuths and 15 earthquake slip vectors along these ridges and the Arabia-India boundary. We determine an Arabia-India Euler vector that predicts 2 mm/yr of right-lateral strike-slip motion along the Owen fracture zone, with a 95% confidence interval of 0–7 mm/yr, which excludes the faster rates of slips predicted by several prior models. The direction of motion is 35°–50° counterclockwise from that predicted by published global plate motion models. If we omit the data along the Arabia-India boundary, the motion differs insignificantly from zero, suggesting that magnetic profiles from the Carlsberg and Sheba ridges give spreading rates too imprecise to detect the slow Arabia-India motion. If the Owen fracture zone and Dalrymple trough are regarded as a plate boundary, it is the slowest slipping plate boundary known on Earth, with motion about 100 times slower than the fastest slipping faults along the East Pacific Rise.

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