Long wavelength satellite gravity and geoid anomalies over Himalaya, and Tibet: Lithospheric structures and seismotectonics of deep focus earthquakes of Hindu Kush – Pamir and Burmese arc

Abstract Free air anomaly map of Himalaya, Tibet and adjoining regions derived from global models including satellite observations reflect most of the tectonic elements of this region. Fold and thrust belts and suture zones are characterized by small wave length free air high anomalies representing shallow high density mafic/ultramafic rocks including ophiolitic melange. On the other hand, medium to large wavelength negative gravity anomalies which are related to crustal thickening due to isostatic compensation are better visualized in the Bouguer anomaly map. This emphasizes the importance of both free air and Bouguer anomaly maps of complex orogenic belts. Spectral analysis of the Bouguer and the geoid anomaly maps and their low pass filtered maps and modeling along a N–S profile (90°E) constrained from various seismic studies suggest low density rocks (3250 kg/m3) in the lithospheric mantle between depths of ∼130–320 km under Tibet. The low pass filtered geoid anomaly shows a prominent geoid low in the same depth range oriented NE–SW, in the direction of motion of the Indian plate that is near perpendicular to the prominent tectonics features. It coincides with the NE–SW oriented seismic anisotropy in the central part of the Himalayan collision zone. Low density rocks in the lithospheric mantle suggest under thrusted Indian and Asian lithospheres in this section that constitutes the thick lithosphere under Tibet. The Moho in general lies at a depth of 50–75 km under Tibet with deepest being under South Tibet. The Low density rocks in the upper mantle make this region buoyant that explains large scale uplifts. The prominent negative free air, Bouguer and geoid anomalies are observed over the Hindu Kush – Pamir Seismic Zone between the Kohistan arc towards the south and Pamir towards the north suggesting predominance of low density rocks approximately at a depth of ∼120 km in the lithospheric mantle. Gravity anomalies and its modeling along a profile (73°E) constrained from the hypocenters of deep focus earthquakes in this region and results from receiver function analysis in adjoining regions suggest following characteristics of the subduction model in this section: (i) the subducting plates from two sides, viz. the Indian (Kohistan arc) and the Asian plates show relatively high angle subductions compared to the east of it. (ii) The subducting plates are affected by the strike slip Chamman – Panjshir faults towards the west and Karakoram strike slip fault towards the east. (iii) The subducting plates from the two sides interact with each other in the lithospheric mantle. (iv) The subducted rocks in this section are generally comprised of high density mafic/ultramafic rocks of the Kohistan arc and Tien Shan. Combination of these factors would induce fast subduction and slab break off that usually facilitate deep focus earthquakes, as observed in case of Mariana type subductions. Similar is the case along the Burmese arc that is the only other section characterized by deep focus earthquakes in the long Himalayan collision zone (∼5000 km). Modeling of the Bouguer anomaly along 23.5°N in this case also shows relatively high angle subduction of the Indian plate which is affected by the Sagaing strike slip fault towards the east that would facilitate slab break off and generation of deep focus earthquakes as described above. Gravity modeling across Tibet, Hindu Kush and Burmese arc suggest the presence of high density rocks (∼3100 kg/m3) in the lower crust and low density rocks (∼3250 kg/m3) in the upper mantle that may be attributed to partial eclogitization and serpentinization of rocks, respectively in these sections that are typical of several orogenic belts. Such conditions are expected to facilitate delamination and detachment.

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