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Crustal block of the western Ganga Basin: A fragment of oceanic affinity?

GEOPHYSICS DIVISION DEPARTMENT OF PHYSICS UNIVERSITY OF TORONTO, TORONTO, ONTARIO, Canada, M5S 1A7

Abstract

Three-component, long-period New Delhi records of two earthquakes occurring near Multan, central Pakistan, are used to determine group velocities of multimode surface waves in the period range of 5 to 57 sec. Inversion of these data has provided insight regarding the crustal shear velocity structure in the western Ganga Basin, in the northwestern wedge of the Indian subcontinent. The shear velocity is less than 3.10 km/sec within the topmost 10 km of its 40-km-thick crust. The estimated velocity in the 10- to 20-km depth range is 3.80 ± 0.09 km/sec. In the lower half crust, the velocity is as high as 3.98 ± 0.08 km/sec. Unlike the continental shield crust in the New Delhi-Shillong region, the crustal structure in this western part of the Ganga Basin is strongly reminiscent of certain oceanic plateaus. This suggests that the crustal block underlying the sediments is not part of the stable core, or the craton of the Indian Shield. The available data do not allow the determination of fine velocity structure in the upper mantle. The average velocity in the uppermost 80 km of the mantle is 4.52 km/sec. This value appears to be characteristic of the entire northern Indian subcontinent and may be indicative of active tectonism in the mantle south of the Himalayas.

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