3-D S-velocity structure of upper mantle and Moho depth variations in the Makran subduction zone



The Makran subduction zone in southeastern Iran and southern Pakistan is where the oceanic crust of the Arabian plate (Oman Sea) is subducting beneath Eurasia. Compared to other subduction zones in the world, the Makran subduction zone has some unusual features, including different seismicity patterns in its eastern and western parts. Also, the Quaternary volcanoes in the eastern part of Makran are located far from its foreland comparing to the western part of Makran. The very low seismicity of western Makran causes two different viewpoints about its current situation; i.e., whether the subducted plate is undergoing aseismicity or has been locked strongly.
The Partitioned Waveform Inversion (PWI) method is used here to image the S-velocity structure of the upper-mantle and Moho-depth variations of Makran subduction zone and explore the relationship of the Makran seismic structure with the seismicity and the volcanic arc in the region. For this purpose, we used the vertical components of the seismograms recorded by the National Iranian Seismic Network with high signal to noise ratio from the earthquakes with magnitudes of 5.5 to 7.7. Despite the limited number of stations around the Makran region, choosing proper earthquakes enables us to improve the azimuthal and path coverage and apply the PWI method in the region.
Our tomography data show that the Moho depth across the Makran subduction zone is increasing from the Oman seafloor and Makran forearc setting to the volcanic arc. Generally, the crust in the western Makran is thicker than its eastern part and the maximum crustal thickness in the Makran region reaches to 50±2 km below the Taftan volcano. The Moho map clearly depicts the western edge of the Makran subduction zone, where the Minab fault (representing the eastern edge of the Hormuz Straits) marks the boundary between the thick continental crust of the Arabian plate and the thin oceanic crust of the Oman Sea. Our results show clearly that the high-velocity slab of the Arabian plate subducts northwards beneath the low-velocity overriding lithosphere of Lut block in the western Makran and Helmand block in the eastern Makran. We found that the slab in the western Makran starts with a gentle dip (about 8?) and increases to about 55?, where it plunges into the asthenosphere beneath the volcanic arc. In eastern Makran, the slab is subducting with a low dip angle of about 8? and reaches approximately 20?below the volcanic arc. We found that the bending of the subducted plate occurs with relatively low dip and much farther beneath eastern Makran than in the western part which may explain the different volcanic arc offsets across the Makran subduction zone.