Geodynamics and seismicity of the Pamir-Himalayas region

Abstract

Deep seismic sounding studies carried out in 1974–79 allowed an important peculiarity of the deep structure of the Pamir-Himalayas region to be established: the thickness of the Earth's crust is almost twice as large here as on the stable plates (65–75 and 35–37 km, respectively). The absence of any evidence for doubling of crustal thickness provides grounds for rejecting the hypothesis of subduction of the rigid Hindustan plate under the geosynclinal folded constructions of the Punjab syntaxis of the Himalayas. The steep inclination of all major faults, dissecting the Earth's crust and often dislocating the M surface, is also counter to this hypothesis. Several faults reflect the dynamics and conditions of formation of deep layers of the lithosphere. For example, the structural seam of the Indus, which has an almost sheer tilt and which penetrates to subcrustal depths, is a channel along which ophiolite associations of crystalline rocks were squeezed from the mantle. The Fore Himalayan and Major Himalayan faults are the boundaries between different structural facial zones. The band of greatest thickness of crust extends within the zone of greatest thickness of the asthenospheric layer; a deep minimum in the Bouguer anomalies (−550 mGal) corresponds to this zone, as does also a depression on the surface of the geoid. Seismicity of the lithosphere of the Pamir-Himalayas region is caused by geodynamic processes manifested in the higher lithospheric layers by block displacements of the Earth's crust (mostly uplifts), and in the lower parts by shifts of the steeply inclined mantle blocks (the Pamir-Hindukush seismic focal zone).