Extension across the Indian-Arabian plate boundary: The Murray Ridge

Seismic reflection profiles from the Murray Ridge in the Gulf of Oman, nort hwest Indian Ocean, show a significant component of extension across the pr edominantly strike-slip Indian-Arabian plate boundary. The Murray Ridge lie s along the northern section of the plate boundary, where its trend becomes more easterly and thus allows a component of extension. The Dalrymple Trou gh is a 25 km wide, steep-sided half-graben, bounded by large faults with c omponents of both strike-slip and normal motion. The throw at the seabed of the main fault on the southeastern side of the half-graben reaches 1800 m. The northwest side of the trough is delineated by a series of smaller anti thetic normal faults. Wide-angle seismic, gravity and magnetic models show that the Murray Ridge and Dalrymple Trough are underlain by a crystalline c rust up to 17 km thick, which may be continental in origin. Any crustal thi nning due to extension is limited, and no new crust has been formed. We favour a plate model in which the Indian-Arabian plate boundary was init ially located further west than the Owen Fracture Zone, possibly along the Oman continental margin, and suggest that during the Oligocene-Early Miocen e Indian Ocean plate reorganization, the plate boundary moved to the site o f the present Owen Fracture Zone and that motion further west ceased. At th is time, deformation began along the Murray Ridge, with both the uplift of basement highs, and subsidence in the troughs tilting the lowest sedimentar y unit. Qalhat Seamount was formed at this time. Subsequent sediments were deposited unconformably on the tilted lower unit and then faulted to produc e the present basement topography. The normal faulting was accompanied by h anging-wall subsidence, footwall uplift, and erosion. Flat-lying recent sed iments show that the major vertical movements have ceased, although continu ing earthquakes show that some faulting is still active along the plate bou ndary.