Lateral variations in the crustal structure of the western Himalaya and Ladakh-Kohistan arc inferred from surface wave analysis

Shear wave velocity variation in the western Himalaya and the Ladakh-Kohistan arc crust is investigated at 30-50 km horizontal resolution by inverting Rayleigh wave dispersion (from 5 to 60 s) computed from ambient noise and earthquake waveforms. We image an unreported arc perpendicular crustal-scale transverse boundary in the western Himalaya, located between the Kishtwar window and Kullu-Rampur window (76-77°E), characterized by low-velocity zones (LVZs) (Vs < 3.4 km/s) at the mid to lower crustal depths. In the northern edge, the transverse structure is marked by extreme low velocities (3.2 > Vs < 3.4 km/s) between 20 to 50 km depth beneath the Gianbul dome in the Zanskar Himalaya. The Gianbul dome is a large-scale migmatitic structure in the Zanskar Himalaya representing sub-vertical exhumation of high-grade metamorphic rocks. Although laterally discontinuous, LVZs of the transverse structure run the full width of the Himalayan arc. To the west of this boundary in the Kashmir Himalaya, the Main Himalayan Thrust (MHT) dips northward at a very low angle at 15-20 km depth to a distance of 150-200 km north of the Main Frontal Thrust (MFT). The Indian upper crust defining the MHT is mapped as a feature with Vs ~ 3.6 km/s. The average crustal velocity is ~ 3.7 km/s up to 50 km depth except near the Kishtwar region where a thick underplated lower crust (Vs > 4.0 km/s, at 20-50 km depth) is observed. To the east of the transverse boundary in the Himachal Himalaya, the MHT is characterized by crustal-scale ramps dipping gently northward (~16°) from ~10 km depth below the Lesser Himalaya to ~25 km depth below the Tethys Himalaya to ~40 km below southern Tibet. Such a gently dipping MHT is typically observed in the central Himalaya. Furthermore, our velocity model reveals a significant lateral variation in the crustal structure below the Kohistan-Ladakh arc. The Kohistan arc shows a uniform upper and middle crust (Vs ~ 3.5 km/s) underlain by a thick (~25 km) high velocity lower crust (Vs > 4.0 km/s). In contrast, the Ladakh arc crust is largely characterized by a mid-crustal LVZ (Vs < 3.4 km/s) between 15 to 40 km depth, and an absence of the thick mafic basal layer at greater depths. These results present a first-order signature of the variability in the deep structure of the underthrusting Indian crust beneath the Himalaya and Ladakh-Kohistan arc.