Earthquakes Beneath the Himalayas and Tibet: Evidence for Strong Lithospheric Mantle

Abstract

Eleven intracontinental earthquakes, with magnitudes ranging from 4.9 to 6, occurred in the mantle beneath the western Himalayan syntaxis, the western Kunlun Mountains, and southern Tibet (near Xigaze) between 1963 and 1999. High-resolution seismic waveforms show that some focal depths exceeded 100 kilometers, indicating that these earthquakes occurred in the mantle portion of the lithosphere, even though the crust has been thickened there. The occurrence of earthquakes in the mantle beneath continental regions where the subduction of oceanic lithosphere ceased tens of millions years ago indicates that the mantle lithosphere is sufficiently strong to accumulate elastic strain.

A Reinvestigation of Crustal Thickness in the Tibetan Plateau Using Absolute Gravity, GPS and GRACE Data

“…However, this continuum deformation is apparently confined to the plateau itself and is absorbed by crustal thickening thus causing vertical crustal movement, i.e., elevated topography of the Tibetan Plateau and a subsided crustal bottom at depth. The seismological tomographic evidence showing that the lithospheric crust under south Tibet has doubled to 80 km thickness (Chen and Yang 2004) and supports this hypothesis. Many of the large earthquakes, including the 2008 Wenchuan earthquake (M 8.0), were located on the Red River and are characterized by dip slip faults.…”

“…Instead, the Bouguer gradient must be used. According to an earlier study (Heiskanen and Moritz 1967), the Bouguer correction can be estimated as +1.1 ± 0.1 μgal cm -1 based on the local crustal density t = 2.7 -2.9 g cm -3 (Chen and Yang 2004). Thus the Bouguer gradient is obtained as -1.9 μgal cm -1 .…”

Geochemical variations in Miocene adakitic rocks from the western and eastern Lhasa terrane: Implications for lower crustal flow beneath the Southern Tibetan Plateau

“…Although previous geophysical observations (e.g., Nelson et al, 1996) and the results of geodynamic modeling (e.g., Beaumont et al, 2001Beaumont et al, , 2006 suggest that channel flow may occur at depths of 20-30 km beneath southern Tibet (i.e., mid-crustal levels), other recent studies suggest that channel flow may also occur in the lower crust, as indicated by widespread high-conductivity layers in the mid-lower crust (e.g., Jin et al, 2010;Wei et al, 2010), a pronounced low-velocity channel that extends from 30 to 70 km depth in the mid-lower crust (e.g., Cotte et al, 1999), and the fact that earthquakes are absent at depths of 30-65 km beneath the Lhasa terrane (e.g., Chen and Yang, 2004;Jackson 2002aJackson , 2002bJackson et al, 2004). These findings imply that mid-lower crustal material beneath Southern Tibet is ductile and therefore may take place ductile flow.…”

Crustal–lithospheric structure and continental extrusion of Tibet

“…Bottom diagram shows the restored South Tibetan Detachment profile for the Everest section, after Searle et al (2003Searle et al ( , 2006 during early Miocene leucogranite formation, and the flow pathways for its exhumation beneath the Lhotse detachment (LD) and the Qomolangma detachment (QD) branches of the South Tibetan Detachment. Chen & Yang (2004). LVZ, low-velocity zone.…”