Two phases of shortening across northeastern Tibet as a result of a
strong Qaidam lithosphere during the Cenozoic India-Asia collision
Abstract
Current geologic observations support two contrasting views on the
evolution of the northeastern Tibet north of the Qaidam basin: (1)
crustal shortening started at the onset of the India-Asia collision
about 50-60 Ma, and (2) crustal shortening did not start until after
20-15 Ma. To reconcile these two seemingly contradicting views supported
both by observations, we perform a series of 2-D thermo-mechanical
simulations with the goal of assessing the role of the lithospheric
structure and strength of the Qaidam basin in controlling the Tibet
deformation history. Our simulations yield three end-member scenarios
for the first-order Tibetan lithospheric deformaiton: (1)
mantle-lithosphere de-lamination in central Tibet accompanied by a
deformation-free northern Tibet margin; (2) northward motion and
deformation of southern Tibet accompanied by the southward subduction of
the Asian lithosphere below northern Tibet, and (3) northward motion and
deformation of southern Tibet accompanied by a deformation-free northern
Tibet margin. Due to strong crust-mantle coupling of the Qiadam
lithosphere, we suggest that the Tibetan lithospheric deformation is
accorded with the third scenario. Our model results also show that the
pre-existing weaknesses in northeastern Tibet is activated shortly after
the onset of the India-Asia collision. This deformation field stays
stable until after the removal of the mantle lithosphere in central
Tibet, during which a second wave of northward propagating shortening
sweeps across northern Tibet north of the Qaidam basin. This result is
consistent with the existing data and reconcile the two end-member views
on the tectonic history of the northern Tibet.