Figure 3. (a) Simplified geological map of the Kumaun Himalaya
and adjacent regions along with the seismological stations, and (b)
common conversion point (CCP) depth migrated RF image along the profile
AB. Red and blue colors indicate positive and negative impedance
contrast, respectively. A ramp in the MHT is identified at
\(\sim\) 130 km profile distance and a low velocity zone (LVZ)
is inferred beneath the Chiplakot crystalline belt. Abbreviations are:
HFT - Himalayan Frontal Thrust, MBT - Main Boundary Thrust, SAT - South
Almora Thrust, NAT - North Almora Thrust, MT/MCT-I - Munsiyari Thrust,
and VT/MCT-II - Vaikrita Thrust, RT-Ramgarh Thrust, SCT-South Chiplakot
Thrust, NCT-North Chiplakot Thrust, STD-South Tibetan Detachment, IGP -
Indo-Gangetic Plain, CCB - Chiplakot Crystalline Belt, BK-Bajnath Klippe
, and ASK - Askot Klippe. (From Hazarika et al., 2021)
Medved et al. (2022) obtained 3D models of the crust and uppermost
mantle beneath the NW Himalaya down to a depth of 120 km by local
earthquake seismic tomography using data of India Meteorological
Department (IMD) complemented by the Global International Seismological
Centre (ISC) Catalogue. Their results suggest that the Indian Plate not
only underthrusts northwards below the Himalaya but also bends westwards
as it gets closer to the Hindukush Region. A peculiar feature of the
model is a high-velocity anomaly in the Kaurik Chango Rift, interpreted
as a remnant of the oceanic crust, left after the closure of the
Indo-Tethys Ocean. In the seismically active Delhi-Haridwar Ridge, a
low-velocity upper crustal layer is possibly associated with sediments
of the Indo-Gangetic Plain and with a large number of fault structures.
Mir et al. (2021) estimated the shear wave velocity structure, together
with Moho depths for the NW Himalaya, Hindu Kush and the Pamirs at a
potential resolution of 0.5\(^{\circ}\)×0.5\(^{\circ}\) and at 1\(^{\circ }\)×1\(\)\(^{\circ }\) in the
surrounding area (Fig.4), by inverting fundamental mode Rayleigh wave
group velocities calculated from regional earthquake (Δ \(\le\) 2500 km) data,
and also from their joint inversion with teleseismic receiver functions
at 38 out of the 59 broadband stations in the region. The results
illuminate a) the deeper root zone structures of the main geomorphic
features, b) a pervasive low velocity layer (Vs \(\sim\) 3.1
km/s) at \(\sim\) 30 km depth beneath the NW Himalaya. Another
notable result is the distinctly shallower Moho beneath the Kashmir
Himalaya apparently segmented by arc-normal shear zones that cross the
rupture zones of the 1905 Kangra and the 2005 Kashmir earthquakes, in
turn, marked by the current epoch seismicity.
A high-resolution seismic image of the crust beneath the Arunachal
Himalaya is documented by Singh A., et al. (2021), using RF analysis of
data from 32 broadband seismic stations deployed in the Arunachal
Himalaya during 2010-2016, along with data from the HIMNT, SIKKIM,
Hi-CLIMB, and GANSSER networks. Their results reveal lateral variations
in the crustal structure with the Moho depth varying from 40-60 km. They
also observe a comparatively less complex crust, absence of a prominent
mid-crustal ramp, a highly deformed layer running parallel to the Main
Himalayan Thrust, and an intermittent anisotropic low velocity layer in
the middle crust.
In a recent study (Ravi Kumar et al., 2022), receiver function images of
the detachment, mid-crustal ramp and the Moho of the underthrusting
Indian plate along four profiles in the Arunachal Himalaya are
documented (Fig.5). The results reveal a clear Moho signature in the
depth range of 40 to 65 km, with the detachment mapped in the depth
range of \(\sim\) 10 to 20 km. A mid-crustal ramp can be traced
in the higher Himalaya especially along one profile. Singh A., et al.
(2021) imaged the crust beneath the Arunachal Himalaya using teleseismic
receiver functions. A mechanically weak middle crust beneath Arunachal
Himalaya, highly deformed layer parallel to MHT, and comparatively less
complex crust beneath Arunachal than Nepal and Sikkim are some important
observations that have been reported in this study.