A unified interpretation of vertical movement in Himalaya and horizontal deformation in Tibet on the basis of elastic and viscoelastic dislocation theory

Abstract The present-day convergence rate between the Indian and the Eurasian plates has been estimated to be about 50 mm/year. 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 rest of about 60% is consumed by the internal deformation of 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. In this study we represent the plate interaction by steady increase in tangential displacement discontinuity (dislocation) across the interface that divides a surface layer overlying a viscoelastic half-space into the Indian and the Eurasian plates. First, given a steady slip of 20 mm/year at the plate interface with a ramp-shaped undulation below the high Himalayas, we computed the profile of surface uplift rates along a line perpendicular to the Himalayan arc. The result accords with observed free-air gravity anomalies and the intermediate- and short-term uplift rates estimated, respectively, from the present heights of river terraces and levelling data. This means that the rapid uplift of the high Himalayas is due to the steady slip along the ramp-shaped plate interface below it. Second, given the steady slips of 20 mm/year along the 2000-km-long collision boundary and 50 mm/year along the remaining portions of the India–Eurasia plate boundary, we computed the increase rates of horizontal deformation in and around Tibet. The result accords with the observed strikes and slip rates of major Quaternary active faults. This means that the horizontal deformation in and around Tibet is due to the slip deficits of 30 mm/year at the collision boundary. From these two results we can conclude that the present rapid uplift of the high Himalayas and large-scale horizontal deformation in and around Tibet are consistently explained by a single plate interaction model based on elastic and viscoelastic dislocation theory. This study sheds new light on the driving mechanism of the crustal deformation in the India–Eurasia collision zone as follows. The plate interaction on single plate interface consists of coexisting two different physical mechanisms: (1) slip along the ramp undulation (spatial changes in slip direction), (2) slip deficit at the collision boundary (spatial changes in slip rates). As a consequence, the two different types of crustal deformation in the India–Eurasia collision zone are driven by these two different physical mechanisms.