Mantle fault zones beneath the Himalayan collision: Flexure of the continental lithosphere

The Himalayas and the Tibetan Plateau are the result of the continental collision between India and Eurasia. The Indian Plate underthrusts the Himalayan mountains and the southern Tibetan Plateau. Recorded seismicity at the Himalayan collision zone suggests that earthquakes occur mainly at upper crustal depths and near the crust–mantle boundary. The question of whether the near-Moho earthquakes are in the crust or in the upper mantle has been controversial, and has raised another question about the role of the mantle in the support of mountain loads and its ability to deform by brittle processes. Earthquake locations from several experiments place seismic events in the upper mantle. Using a finite element model, we establish a link between the recorded upper mantle seismicity beneath the Himalayan collision zone and flexural bending of the Indian lithosphere. Earthquake locations, focal mechanisms, and seismic imaging results from the HIMNT experiment, combined with previous constraints on the geometry and deformation of the Himalayan collision, are used to set up the finite element models of lithospheric loading. Our purpose is to infer the mechanical state of the lithosphere beneath the Himalayas and to evaluate the role of the lithospheric mantle in the support of the loads. The pattern of mantle seismicity can be explained by modeling the response of the Indian Plate to loads corresponding to the weight of the sediments of the Ganga basin, the Himalayan mountains and the southernmost Tibetan Plateau, combined with the effects of a horizontal force per unit length acting upon the lithospheric plate. We calculated the steady-state stress field in the Indian lithosphere, where the lithospheric mantle is assumed to be viscoelastic and non-Newtonian, and the asthenosphere is modeled as viscoelastic and Newtonian. Two model suites were tested, one with an elastic crust (Model Suite 1), and one with a viscoelastic crust (Model Suite 2). Both model suites provide a good fit to the observed patterns of seismicity, but Model Suite 2 is the one that best reproduces the observations. High differential stresses concentrate in the upper mantle, and predicted principal stress orientations match those inferred from focal mechanisms in the area. Our models show that beneath the Ganga basin and the southernmost Himalaya, earthquakes at near-Moho depths do not need to show extension, nor is the lower crust required to be weak, in order to infer that the uppermost mantle yields by brittle failure. Even when flexural stresses can generate the background stresses responsible for the generation of upper mantle earthquakes, Mohr–Coulomb theory suggests that additional factors such as the presence of lateral heterogeneities or the action of pore fluids are playing a fundamental role in bringing the upper mantle materials to brittle failure.