Rheology of the continental lithosphere: Progress and new perspectives

While the surface of Tibet is undergoing pervasive pure shear, stable terranes, straddling subsurface sutures, remain in the sub-continental lithospheric mantle (SCLM), attesting to its strength. Furthermore, sub-horizontal, cohesive remnant of Indian SCLM is traced northward from the Himalayan deformation front for about 600 km, exemplifying the longevity of buoyant, strong SCLM of Archean shields. Bimodal distribution of earthquake depths, with peaks concentrating in the upper/middle crust and near the Moho, has been a longstanding evidence for strong SCLM. Recent results from the Himalayas—Tibet and along the East African rift system not only corroborate the bimodal distribution but also firmly established that large earthquakes occur below the Moho. Intriguingly, non-volcanic tremors—newly discovered mode of elastic strain release—also occur near the Moho but well below the seismogenic zone in the upper/middle crust. Considering recent field observations and laboratory experiments of viscosity contrast across the Moho, the SCLM must be strong enough to accumulate elastic strain, a prerequisite for earthquakes, over geological time. Moreover, under laboratory conditions, recent advances that link the termination of frictional instability, an analogue for earthquakes, and the onset of crystal plasticity, provided a physical basis for limiting temperatures of crustal (~ 300–400 °C) and mantle (~ 600–700 °C) earthquakes. While any single rheological model cannot possibly account for all tectonic settings (which also evolve with time), lithological contrast across the Moho is important in shaping the bimodal distribution of strength in the continental lithosphere.