The ongoing collision of India with Eurasia, which began about 50 million years ago, has uplifted the Himalayas and the Tibetan Plateau and nearly doubled the thickness of the region’s continental crust. How exactly this deformation has been accomplished, however, has been the topic of vigorous scientific debate.
Some researchers argue the deformation has been localized along major faults, forcing rigid crustal blocks to slide laterally and extruding large continental fragments eastward. Other scientists have proposed that the lithosphere deforms fluidly, causing the deformation to be continuously distributed across the Tibetan Plateau. Still others advocate for a channel of low-viscosity material in which deformation more easily occurs than in the brittle, overlying crust or the underlying mantle lithosphere. However, a lack of GPS and field data from the region has made it difficult to resolve this debate.
To offer fresh insights into how continents deform, Daout et al. have tapped into an 8-year archive of data for the region collected by the interferometric synthetic aperture radar (InSAR) instrument aboard the Envisat satellite. Finely tuned atmospheric corrections and the characterization of the seasonal ground movement signal associated with the freeze-thaw cycle of the permafrost active layer allowed the authors to separate tectonic surface displacements from interfering processes in a vast region of northwestern Tibet.
The map reveals a steep velocity gradient within a 30-kilometer-wide zone across the Altyn Tagh Fault, the primary structure separating the Tibetan and Tarim crustal blocks. This indicates an overall eastward motion of Tibet relative to the Tarim block. The map also revealed previously unrecognized faults in the Tarim basin and a more distributed gradient of deformation along the Late Triassic Jinsha suture zone, which separates two accreted terranes in central Tibet.
Using simple, 2-D fault models constrained by these results, as well as published GPS data, the team determined that the Altyn Tagh Fault averaged 10.5 millimeters per year of left-lateral strike-slip motion below a locking depth of 17 kilometers, whereas the Jinsha experienced 4–8 millimeters of left-lateral motion per year. Because of this uneven pattern of deformation, the authors conclude that the central Tibetan Plateau is not deforming as a fluid. Instead, the convergence appears to result in strain localized along major strike-slip faults and preexisting weaknesses within the lithosphere that are forcing the Tibetan block eastward.
This study highlights how state-of-the-art methods can be used to process the large archive of InSAR data from past constellations, enabling scientists to extend the record of surface deformation in this remote and tectonically complex region. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB015020, 2018)
—Terri Cook, Freelance Writer