Imaging the Moho and the Main Himalayan Thrust in Western Nepal With Receiver Functions

Abstract

The crustal structure of Western Nepal is studied for the first time by performing receiver function analysis on teleseismic waveforms recorded at 16 seismic stations. The Moho geometry is imaged as it deepens from ~40‐km depth beneath the foothills and the Lesser Himalaya to ~58‐km depth beneath the Higher Himalayan range. A midcrustal low‐velocity zone is detected at ~15‐km depth along ~55‐km horizontal distance and is interpreted as the signature of fluids expelled from rocks descending in the footwall of the Main Himalayan Thrust. Our new image allows structural comparison of the Moho and of the Main Himalayan Thrust geometry along‐strike of the Himalayas and documents long‐wavelength lateral variations. The general crustal architecture observed on our images resembles that of Central Nepal; therefore, Western Nepal is also expected to be able to host large (MW > 8) megathrust earthquakes, as the 1505 CE event.

Seismic imaging of a mid-crustal low-velocity layer beneath the northern coast of the South China Sea and its tectonic implications

“…For example, the well-known mid-crustal LVL in Tibet was proposed to host channel flow, playing an important role in accommodating lithospheric deformations during extension (Nelson et al, 1996;Beaumont et al, 2001;Bao et al, 2015;Sun and Zhao, 2020), however, its geometry, connectivity and therefore importance are still debated (Hetényi et al, 2011). The shallow-angle, long-extent and low-velocity detachments observed in Himalaya (Schulte-Pelkum et al, 2005;Caldwell et al, 2013;Subedi et al, 2018), Central Andes (Yuan et al, 2000) and eastern America (Long et al, 2019;Marzen et al, 2019) indicate a fundamental deformation mode in continental collision zones. In several cases, intra-crustal LVLs result from accretionary complexes that reflect a tectonic transformation from subduction to extension (Smit et al, 2016).…”

Eastward Dipping Style of the Underthrusting Indian Lithosphere Beneath the Tethyan Himalaya Illuminated by P and S Receiver Functions

“…The depths of both the Moho and the MHT are easily concluded to exhibit significant variations along the west‐east strike and across the strike of the Himalaya. Most studies to the south of our profile reveal that the MHT begins to dip northward at depths of 16–20 km beneath the Higher Himalaya (Singer et al., 2017; Subedi et al., 2018). Taking this feature as the reference, our findings suggest the existence of a north‐south mid‐crustal ramp in the MHT, which reaches northward to 42–54 km beneath the Tethyan Himalaya with a dip angle of 16 ± 2°.…”

“…Many disputes continue about the selection of the conversion phase interpreted as the MHT in previous studies. The MHT is defined with a changing polarity due to the lithologic juxtapositions in the Garhwal Himalaya (Caldwell et al., 2013), a negative conversion in western and central Nepal (Nabelek et al., 2009; Subedi et al., 2018) and a major crustal anisotropic shear zone in eastern Nepal and Bhutan (Schulte‐Pelkum et al., 2005; Singer et al., 2017). We prefer to interpret the positive conversion at depths of 42–54 km as the MHT, which is consistent with a strong P wave velocity increase downward from seismic reflection (Zhao et al., 1993) or the bottom of a LVZ from the wide‐angle seismic experiment (Zhang & Klemperer, 2010).…”

High‐Resolution P‐T‐Time Paths Across Himalayan Faults Exposed Along the Bhagirathi Transect NW India: Implications for the Construction of the Himalayan Orogen and Ongoing Deformation

“…Balanced, deformed-state, cross-section through the eastern Himalaya at about longitude 87.3°E (Schelling & Arita, 1991) connected to a seismic reflection profile (Zhao et al, 1993). We also overlie the P-to-S receiver function migration imaging the crust after Subedi et al (2018). The Moho and the intracrustal low-velocity zone are highlighted with a black dashed line.…”