Models for continental lithospheric strength are not well resolved due to a lack of direct measurement; however, numerical simulations provide means for evaluating the physicality of endmember cases. We simulate the 3‐D dynamics of continental lithosphere within the India‐Eurasia collision zone and compare results to observed deformation. Three‐dimensional lithospheric deformation is approximated with creeping flow in a layered spherical cap. We partition strength with depth according to endmember models, using laterally varying estimates of vertically averaged effective viscosity to constrain the absolute values of 3‐D viscosity in the upper crustal and mantle layers. Comparisons between dynamic solutions and observed geophysical data indicate: (1) Deformation within the subducted Indian slab is required to simulate uplift along the entire Himalayan front with 1022 Pa·s representing an upper bound for slab strength. (2) Along‐strike variations in crustal strength are necessary to match surface observations. (3) Balance of gravitational forces acting on Tibetan lithosphere with a weak lower crust are able to replicate observed vertical deformation patterns. A west‐to‐east decrease in the lateral extent of the underthrust Indian slab is required for reproducing observed surface motions in Tibet. Geodynamic simulations yield subsidence in southern Qiangtang and Southeast Asia consistent with global positioning system‐derived dilatation. We note a trade‐off between upper crustal strength and surface velocity rotation around the Eastern Himalayan syntaxis. Simulations with stronger upper crust replicate velocities in western Tibet but not the east, while those with weaker upper crust produce observed rotation in eastern Tibet but overpredict magnitudes to the west.
The deep structure of the continental collision between India and Asia and whether India’s lower crust is underplated beneath Tibet or subducted into the mantle remain controversial. It is also unknown whether the active normal faults that facilitate orogen-parallel extension of Tibetan upper crust continue into the lower crust and upper mantle. Our receiver-function images collected parallel to the India–Tibet collision zone show the 20-km-thick Indian lower crust that underplates Tibet at 88.5–92°E beneath the Yarlung-Zangbo suture is essentially absent in the vicinity of the Cona-Sangri and Pumqu-Xainza grabens, demonstrating a clear link between upper-crustal and lower-crustal thinning. Satellite gravity data that covary with the thickness of Indian lower crust are consistent with the lower crust being only ∼30% eclogitized so gravitationally stable. Deep earthquakes coincide with Moho offsets and with lateral thinning of the Indian lower crust near the bottom of the partially eclogitized Indian lower crust, suggesting the Indian lower crust is locally foundering or stoping into the mantle. Loss of Indian lower crust by these means implies gravitational instability that can result from localized rapid eclogitization enabled by dehydration reactions in weakly hydrous mafic granulites or by volatile-rich asthenospheric upwelling directly beneath the two grabens. We propose that two competing processes, plateau formation by underplating and continental loss by foundering or stoping, are simultaneously operating beneath the collision zone.
more » « less- NSF-PAR ID:
- 10193139
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 117
- Issue:
- 40
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- p. 24742-24747
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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