New constraints of the shallow onshore Makran subduction zone structure

Abstract:

We have installed and operated broadband seismographs at 37 sites in SE Iran above the western Makran subduction zone. Of these, 21 sites lie along a ~400 km long profile approximately perpendicular to the subduction direction; the remaining sites are distributed about the profile. We use recording from the whole network to measure fundamental mode Rayleigh wave group velocity from ambient noise and regional earthquake, and have compute P-wave receiver functions from the teleseismic recordings made at sites along the profile. The receiver functions and dispersion data have been jointly inverted to ascertain a model of the shallow structure of the Makran subduction zone. The average structure in the coastal region consists of a low shear-wavespeed gradient between 0-10 km depth (~Vs 2.5-3.0 km/s) overlying a ~12 km-thick, near-uniform shear-wavespeed (~3.0 km/s) layer. Between ~22 and 30 km depth there is a strong gradient zone where Vs rapidly increases from ~3.0 to 4.35 km/s. This high gradient zone is likely altered oceanic crust. Beneath the coastal region and as far north as the JazMurian Depression the sediment – ocean-basement interface dips ~3^o northward. North of the JazMurian Depression at a depth of ~50 km the conversions from the oceanic crust disappear, likely as the gabbroic-basaltic crust transforms to eclogite and the subducting plate descends steeply into the into the upper mantle, matching up with the high velocity feature observed deeper in the upper mantle beneath southern Iran by teleseismic tomography. North of the JazMurian Depression the crust is continental with a near-uniform wavespeed ~3.5 km/s above a Moho at ~40 km depth. However, the upper mantle beneath the crust has a very low shear-wavespeed, presumably due to hydration of the material in the mantle wedge above the subducting slab.