Viscoelastic Crustal Deformation in the India-Eurasia Collision Zone: Difference in Time Dependence Between the Vertical and Horizontal Components

The present-day convergence rate between the Indian and the Eurasian plates has been estimated to be about 50 mm/yr. At the collision boundary extending along the Himalayas, about 40 % of the total convergence is consumed by the subduction of the Indian plate beneath the Eurasian plate. The present crustal movement in this region is characterized by rapid uplift along the high Himalayas and large-scale horizontal deformation in and around Tibet. The fundamental causes of these two different types of crustal movement are the same; interaction between the Indian and the Eurasian plates. So far, we have represented the plate interaction by steady increase in tangential displacement discontinuity (dislocation) across the interface that divides an elastic surface layer overlying a viscoelastic half-space into the Indian and the Eurasian plates. With this single plate interaction model, we have consistently explained the present rapid uplift of the high Himalayas and large-scale horizontal deformation in and around Tibet. The former is due to the steady slip along the ramp-shaped plate interface below High Himalayas, and the latter is due to the slip deficits of 30 mm/yr at the collision boundary relative to the surrounding subduction zones. In the present study, we introduced the viscoelasticity of the lithosphere and computed the viscoelastic response to a step slip over the plate interface to examine the time dependence of the viscoelastic crustal deformation. First, using 2-D model we computed vertical component of the viscoelastic response to a step slip over the plate interface with a realistic shape observed in Himalaya. Since the lithosphere-asthenosphere system consists of the high viscosity surface layer and the low viscosity substratum, its response to a step slip is characterized by three different phases; instantaneous elastic deformation of the total system (0 <= t <= 10 yr), rapid viscoelastic relaxation of the asthenosphere (10 ^ {2} yr <= t <= 10 ^ {6} yr), and gradual viscoelastic relaxation of the lithosphere (10 ^ {7} yr <= t). The vertical component of the viscoelastic response decreases with time due to viscoelastic relaxation of the tectonic stress supporting the surface load associated with crustal uplift. Second, we computed the horizontal components of the viscoelastic step response with a simple 3-D model. We considered a plate interface with simple geometry and introduced a slip deficit region on it. The slip deficit region corresponds to the Himalayan collision boundary. At t = 10 ^ {7} yr, the stress relaxation must be completed both in the lithosphere and the asthenosphere. However, we cannot find large difference in the pattern of surface deformation between t = 10 ^ {4} yr and 10 ^ {7} yr. This result indicates that the horizontal components of the viscoelastic response are not largely affected by stress relaxation of the lithosphere, because the gravity does not suppress horizontal deformation unlike the case of vertical deformation. From this result, we can conclude that the horizontal deformation due to steady slip on the plate interface steadily increases at a nearly constant rate up to t = 10 ^ {7} yr at least.