Surface Deformation and Lower Crustal Flow in Eastern Tibet

Abstract

Field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern Tibetan plateau since about 4 million years ago. Instead, central eastern Tibet has been nearly stationary relative to southeastern China, southeastern Tibet has rotated clockwise without major crustal shortening, and the crust along portions of the eastern plateau margin has been extended. Modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle. This model also predicts east-west extension on the high plateau without convective removal of Tibetan lithosphere and without eastward movement of the crust east of the plateau.

Crustal–lithospheric structure and continental extrusion of Tibet

“…The lower crust flow model for eastern Tibet (Royden et al 1997;Clark & Royden 2000) has no geological evidence from the upper crust to support it, although undoubtedly the temperature and rheology of the lower crust should allow it to flow. Unlike the Himalaya, the Long Men Shan range along the eastern margin of Tibet shows no horizontal extrusion of middle or deep crustal metamorphic rocks, there are no equivalents of the Main Central Thrust or South Tibetan Detachment detachments, and there is no flexural foreland basin to the east in Sichuan.…”

“…5. Lower crustal flow model for eastern Tibet, after Royden et al (1997) and Clark & Royden (2000), showing flow lines around the Eastern Himalayan syntaxis and also around the stable Sichuan basin.…”

“…Recent surface-wave tomography studies imply a highvelocity lithospheric mantle beneath the whole plateau (except the far north under the Kun Lun) to a depth of 225-250 km (Priestley & McKenzie 2006;Priestley et al 2008), arguing strongly against any lithospheric delamination beneath Tibet. Thin viscous sheet models also ignore depth-dependent behaviour and do not allow for lateral shear or detachments between upper and lower crust or between lower crust and mantle (Royden et al 1997).…”

“…Based mainly on GPS observations and using a 3D viscous model, Royden et al (1997Royden et al ( , 2008 proposed a model for eastern Tibet based on lower crustal flow around the Eastern Himalayan syntaxis and also around the rigid Sichuan basin (Fig. 5).…”

Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from 40Ar/39Ar and (U‐Th)/He thermochronology

“…The development of this high topography presents something of an enigma in that shortening of the upper crust during the Cenozoic appears to be rather limited [Burchfiel et al, 1995]. This observation, coupled with the absence of a Cenozoic foredeep in the Sichuan Basin and slow geodetic rates of shortening between the plateau and the Sichuan Basin [e.g., Chen et al, 2000], led Royden et al [1997] to suggest that the development of the plateau in eastern Tibet was the consequence of thickening and flow within a weak lower crust. Thus, in addition to addressing the regional question of when the eastern plateau developed, estimates of the magnitude and distribution of exhumation across this margin will place first-order constraints on the degree of upper crustal deformation in this region.…”