The lithospheric density structure below the western Himalayan syntaxis: tectonic implications

Abstract

The differences in the tectonic regimes of the western part of the India–Asia collision zone (western Himalayan syntaxis), consisting of the Ladakh–Karakoram, Hindu Kush and Pamir terrains, are examined through the analyses and modelling of gravity anomalies, computed from global gravity model and terrestrial data. Long-wavelength (>450 km) anomalies are related to isostatic compensation and correspond to topographic variations. The short-wavelength gravity highs are primarily due to upthrust blocks and are mostly linked to crustal seismicity. The intermediate depth-focused earthquake region of Hindu Kush–Pamir is characterized by a gravity low probably caused by crustal underthrusting. The deep density structure of the crust and upper mantle constructed from the modelling of gravity anomalies with constraints from seismological information suggests crustal underthrusting of the Indian and the Asian plates in the Hindu Kush–Pamir section unlike that in the Ladakh–Karakoram region, where underthrust Indian lithosphere underplates beneath the Asian plate. The density model supports a hypothesis of slab break-off of Indian and Eurasian plates in the Ladakh–Karakoram segment, and of the Indian plate in the Hindu Kush–Pamir region, whereas the Eurasian plate drastically underthrusts deeper (c. 200 km), causing deep seismicity in the Hindu Kush–Pamir section.