Imaging the Indian subcontinent beneath the Himalaya

Abstract

The rocks of the Indian subcontinent are last seen south of the Ganges before they plunge beneath the Himalaya and the Tibetan plateau. They are next glimpsed in seismic reflection profiles deep beneath southern Tibet, yet the surface seen there has been modified by processes within the Himalaya that have consumed parts of the upper Indian crust and converted them into Himalayan rocks. The geometry of the partly dismantled Indian plate as it passes through the Himalayan process zone has hitherto eluded imaging. Here we report seismic images both of the decollement at the base of the Himalaya and of the Moho (the boundary between crust and mantle) at the base of the Indian crust. A significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths. North of the Himalaya, the lower Indian crust is characterized by a high-velocity region consistent with the formation of eclogite, a high-density material whose presence affects the dynamics of the Tibetan plateau.

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.…”

Crustal–lithospheric structure and continental extrusion of Tibet

“…Crustal thickness beneath SW, western and southern Tibet (c. 75-90 km; Wittlinger et al 2004;Schulte-Pelkum et al 2005;Rai et al 2006) suggest that the lower Indian crust must at present be in eclogite (wet) or high-pressure granulite (dry) facies. Thermobarometry and U-Pb dating of monazites in exposures of exhumed lower crust metamorphic rocks and S-type granites in the Karakoram show that P-T conditions (650-800 8C; 10-12 kbar) have been high almost continuously since c. 65 Ma (Searle et al 2010a).…”

“…The South Tibetan Detachment has been folded around the North Himalayan gneiss domes, the northern extension of the Greater Himalayan metamorphic rocks, and is not active today. The active southern front of the Himalaya is the Main Boundary Thrust, which dips to the north and merges with the Main Himalayan Thrust as imaged on seismic profiles (Nelson et al 1996;Schulte-Pelkum et al 2005) The Indus-Yarlung suture zone is a narrow and near-vertical feature containing remnant ophiolites and Tethyan sedimentary rocks. At about 18 km depth a horizontal band of dense ophiolite has been imaged beneath the Indus-Yarlung suture zone (Makovsky et al 1999).…”

Seismic anisotropy inferred from direct <i>S</i>-wave-derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau

“…Singh et al, 2015) and crustal anisotropic effects should be accounted for in the splitting measurements obtained using direct S and SKS/SKKS phases. In the Himalayan region, highly anisotropic crust (∼ 20 %) has been reported using an inversion of receiver functions (Schulte-Pelkum et al, 2005;Singh et al, 2010), while a similar approach at a few seismic stations covering Tibet suggests approximately 4-14 % seismic anisotropy within the Tibetan crust (Sherrington et al, 2004;Ozacar and Zandt, 2004). Ozacar and Zandt (2004) have accounted for splitting of < 0.5 s over SKS split times due to the observed anisotropy of > 10 % in the crust.…”