Lateral uniformity of India Plate strength over central and eastern Nepal

Abstract

The current understanding of the Himalayan lithosphere stems mostly from cross-sections through the range at the longitude of the Kathmandu Basin. In this paper we laterally extend the analyses of structures and rheology along the Nepal Himalayas between the Pokhara valley and the Arun river. We take advantage of available information and a new data set including gravity measurements and a receiver function profile. It appears that the geometry of the Moho inferred from seismological profiles and long-wavelength gravity anomalies does not exhibit major East–West variations within the 350-km-wide study area. Using thermomechanical modelling, we show that the northward deepening of the Moho observed along profiles perpendicular to the main thrust faults can be interpreted simply as the bending of a strong India Plate. This result suggests a gradual mechanical decoupling between the crust and the mantle, leading to a northward decrease of the effective elastic thickness of the Indian lithosphere from ∼75 km to ∼25 km beneath the Ganga Basin and the Tibetan Plateau, respectively. Our results also confirm (partially) eclogitized lower Indian crust beneath southern Tibet. At shorter wavelengths, the observed gravity profiles exhibit some small lateral variations that can be interpreted in terms of east–west variations of the thickness of subsurface geological structures such as the Ganga Basin and the Tethyan Sedimentary Sequence.