Delineation of lithosphere structure and characterization of the Moho geometry under the Himalaya–Karakoram–Tibet collision zone using surface-wave tomography

Abstract

Group velocities for a period range of 6–60 s for the fundamental mode of the Rayleigh wave passing across the Himalaya–Karakoram–Tibet orogen are used to delineate the structure of the upper lithosphere using the data from 35 broadband seismic stations. 2D tomography velocity maps of group velocities were obtained at grids of 1° separation. Redefined local dispersion curves are inverted non-linearly to obtain 1D velocity models and to construct a 3D image of the S-wave structure down to a depth of 90 km.The Moho discontinuity is correlated with c. 4.0 km s−1 S-wave velocity. The results depict a NE-dipping trend of the Moho depth from c. 40 km beneath the frontal part of the Himalaya to up to c. 70–80 km beneath the collision zone before shallowing substantially to c. 40 km beneath the Tarim Basin. The study also reveals thick deposits of sediments in the Indo-Gangetic plains and the Tarim Basin. A broad low-velocity zone at mid-crustal depth in the western Tibetan Plateau, the Karakoram region and the surface-collision part of the India–Eurasia tectonic plates is interpreted as the effect of partial melting and/or the presence of aqueous fluid. The high velocities in the southern deeper part indicate that the lower crust and uppermost mantle of the Indian Plate are dense and cold.

A review of seismological research in India during the past five years (2015-2019)

“…A similar midcrustal low velocity layer is observed beneath both the Lhasa Terrane and the Tethyan Himalayas, which is uninterrupted by the Karakoram Fault, indicating that the fault does not cut through the entire crust (shallow fault) (Gilligan et al, 2015). Both the studies (Kumar et al, 2017(Kumar et al, , 2019 indicated high velocities in the deeper part in the south. While the former interpreted the high velocities in terms of dense and cold lower crust and uppermost mantle of the Indian plate, the latter attributed it to lateral heterogeneity of the crust and strong anisotropy in the lower crust beneath the frontal Himalayan region (western Himalaya) due to development of preferably oriented folds/ faults/fractures and reorientation of crystals as a consequence of the ongoing deformation caused by the continent-continent collision.…”

“…The dipping geometry of the Indian plate is captured by a gradual increase in both the Conrad and Moho depths (Kumar et al, 2017). This was subsequently corroborated by the NE-dipping Moho inferred by Kumar et al (2019), studying the Rayleigh wave group velocities in the period range of 6-60 s across the Himalaya-Karakoram-Tibet. The Moho depth increases from ~40 km beneath the frontal part of the Himalaya to ~70-80 km beneath the collision zone and shallows down substantially beneath the Tarim basin.…”

“…However, they observed a sharp decrease (~20-30 km) in the Moho depth to the north of the Altyn Tagh fault compared to the south. Further, Kumar et al (2019) observed a broad low-velocity zone at midcrustal depths beneath the western Tibetan Plateau, the Karakoram region and the surface-collision part of the India-Eurasia tectonic plates (Indus-Yarlung Suture Zone), which is attributed to the presence of partial melting and/or aqueous fluid. A similar midcrustal low velocity layer is observed beneath both the Lhasa Terrane and the Tethyan Himalayas, which is uninterrupted by the Karakoram Fault, indicating that the fault does not cut through the entire crust (shallow fault) (Gilligan et al, 2015).…”

“…Sunil Kumar et al (2019) determined a kinematic slip distribution model for the 2011 Mw 6.9 Sikkim earthquake. They suggested a NE SW trending, steeply dipping sinistral source zone within the underthrusting Indian slab.…”

Crustal architecture and evolution of the Himalaya–Karakoram–Tibet Orogen: introduction

“…The data recorded during a period of three days before and after the earthquakes quantify the displacement of the GNSS station from its original position. Kumar et al (2019) delineate the structure of the upper lithosphere using seismic data collected from 35 broadband seismic stations located across the HKT orogen. The results suggest a NE-dipping Moho, increasing in depth from c. 40 km beneath the frontal part of the Himalaya down to c. 70-80 km beneath the collision zone, then shallowing substantially beneath the Tarim basin.…”