Underplating in the Himalaya-Tibet Collision Zone Revealed by the Hi-CLIMB Experiment

Abstract

We studied the formation of the Himalayan mountain range and the Tibetan Plateau by investigating their lithospheric structure. Using an 800-kilometer-long, densely spaced seismic array, we have constructed an image of the crust and upper mantle beneath the Himalayas and the southern Tibetan Plateau. The image reveals in a continuous fashion the Main Himalayan thrust fault as it extends from a shallow depth under Nepal to the mid-crust under southern Tibet. Indian crust can be traced to 31 degrees N. The crust/mantle interface beneath Tibet is anisotropic, indicating shearing during its formation. The dipping mantle fabric suggests that the Indian mantle is subducting in a diffuse fashion along several evolving subparallel structures.

Petrogenesis of the Early Eocene adakitic rocks in the Napuri area, southern Lhasa: Partial melting of thickened lower crust during slab break-off and implications for crustal thickening in southern Tibet

“…50) gabbros and basalts are characterized by depleted Sr-Nd-Hf isotopic compositions (e.g., Chu et al, 2006;Dong et al, 2008;Ji et al, 2009;Mo et al, 2007Mo et al, , 2008Wen et al, 2008aWen et al, , 2008b, indicating that southern Lhasa has a juvenile middle-lower crust that was not a suitable source for the Napuri adakitic rocks. Conversely, there is abundant geophysical data that indicates that the Indian continental lithosphere has subducted beneath the Qiangtang block (e.g., Nábělek et al, 2009;Owens and Zandt, 1997;Schulte-Pelkum et al, 2005) and the Indian crust can be traced to 31°N (Nábělek et al, 2009). Thus, we propose that the enriched isotopic component of Cenozoic crust-derived adakitic rocks results from a significant involvement of the Indian continental crust.…”

The 2015 April 25 Gorkha (Nepal) earthquake and its aftershocks: implications for lateral heterogeneity on the Main Himalayan Thrust

“…This line is referred to as the 'locking line' and separates the frictionally locked-to-aseismically creeping regions on the MHT, through a downdip zone of tapered slip (Stevens & Avouac 2015). The MHT, beneath Nepal Himalaya, has been imaged using a variety of seismological techniques and is known to have a ramp-flat-ramp geometry, from surface to depth (Hirn et al 1984;Monsalve et al 2006;Priestley et al 2008;Nábělek et al 2009;Acton et al 2011) (Fig. 9).…”

“…1). The Hi-CLIMB experiment (Nábělek et al 2009) spanned the central Nepal Himalaya and southern Tibet and provided a 1-D velocity structure immediately west of the epicentre of the main-shock (profile A-A in Fig. 1).…”

“…8) within ±50 km on either side of the profile, projected onto the profile. These profiles are chosen exactly along the Hi-CLIMB (Nábělek et al 2009) (A-A and HIMNT (Monsalve et al 2006) (B-B seismological experiment profiles, and the geometry of the MHT and Moho has been digitized from these studies. Location of MFT, MBT and MCT, taken from Lave & Avouac (2001), has been projected onto the profile.…”

“…The STF is defined using a series of overlapping isosceles triangles of fixed half-width duration. For the source region we chose a velocity model based on previous studies (Monsalve et al 2006;Nábělek et al 2009). The observed and synthetic waveforms are aligned by precisely reading the P-wave arrival time from the broad-band record.…”