CRUSTAL VELOCITY STRUCTURE OF NORTHWEST HIMALAYA USING SURFACE WAVE DISPERSION

Abstract

We present a precise 1D crustal and upper mantle velocity structure of the Northwest Himalaya, derived from the combined inversion of Rayleigh and Love wave group velocity dispersion reaching depths of up to 220 km, achieved through the application of Genetic Algorithm. Furthermore, we present a comprehensive 3D isotropic shear wave velocity model of the Indo-Gangetic plain, investigating to depths of up to 100 km, by using Rayleigh wave tomography. The Data are obtained from ~130 Indian and neighboring network broadband seismic stations. We use the dispersion dataset measurement of Rayleigh and Love wave from period 4sec to 100 sec. The study region is partitioned into three clusters of wave paths traversing the Upper Indus Basin, categorized by their epicentral locations. This division allows for a comprehensive investigation into the geological characteristics beneath the western, central, and eastern segments of the Upper Indus Basin (UIB). The analysis reveals a gradual increase in crustal thickness from west to east (~61.8 km, shear wave velocity ~4.6 km/s). The Lithosphere-Asthenosphere Boundary (LAB) is identified at 160 km depth (velocity decrease ~1.6%). A sedimentary cover of ~4 km is observed, with an assumed felsic crust similar to the southern Pamir region, possibly resulting from mafic lower crust loss via lithospheric delamination or gravitational instability. We have presented a 3D shear wave velocity model of the northern Indian-Eurasian collision zone (70°E – 80°E) using Rayleigh group velocity tomography, extending down to 80 km depth. Our study incorporated approximately 8,000 new Rayleigh wave dispersion measurements from 121 seismic broadband stations across regional networks, totaling approximately 20,000 dispersions. The methodology involved a two-step surface wave tomography process followed by regionalization with an 80 km Gaussian correlation length and depth inversion. Our regionalized 2D tomography maps effectively capture well-known geological features. At shorter periods (below 20 seconds), slower velocities observed in the Indo-Gangetic Plain, Indus River valley, and the northern Tarim Basin suggest a thicker basement. Conversely, longer periods reveal distinct velocity anomalies in the Himalayan and Tibetan Plateau regions, indicating thicker crust.