Abstract

The fundamental and higher modes of surface waves generated by 31 earthquakes and recorded at seismographic stations along the western margins of India and Pakistan (Trivandrum, Kodaikanal, Goa, Bombay, Poona, New Delhi, Nillore, and Quetta) are used to estimate the crustal structure beneath the Arabian Fan sediments. The sedimentary thickness is determined from the observed higher mode data. The observed dispersion data suggest an increase in crustal thickness northward, from an approximately 16 km crustal thickness at the southern tip of India (Trivandrum) to an approximately 28 km crustal thickness at the regions of 20°N and above latitude, with an overlying 6 km sedimentary thickness. This gradual increase in crustal thickness in the northward direction and the attaining of quasi-continental oceanic (transition from continent to ocean) structure beneath the Arabian Fan sediments suggest that the Mohorovičić discontinuity may have resulted from a change in crystal structure due to increase of pressure and not a phase change. The same material exists beneath the Moho, and it does not represent the boundary between two different materials. The transition has given rise to crustal thickening in the northward direction. Another possible explanation is that the increase in hydrostatic pressure due to the load exerted by a large sedimentary column together with horizontal pressure caused by the collision of Indian and Eurasian plates has given rise to an increase in temperature near the Moho. Because of the thermal blanketing effect of this large sedimentary column, an inferred rise in temperature may have either changed the upper mantle into material with crustal-like velocity or may have given rise to metamorphism of earlier existing sedimentary rocks. An inferred high temperature near the Moho depth beneath the Arabian Fan sediments is in close agreement with the high attenuating zone at the shallow depth (30 to 45 km) as determined by Singh (1987).

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