Present-day Vertical Deformation In Southern Tibet Constrained By GPS Measurements

As the Indo-Asia collision continued since Cenozoic,the Tibetan Plateau rise above the surface of the earth with an average altitude of more than 4000 m.The Himalayan orogenic belt stretches 2500 km across the southern margin of the plateau.This is undoubtedly the most remarkable geological event of the Cenozoic continental active tectonics.The Tibetan Plateau is a natural proving ground for geological evolution and geophysical research with strong collision and orogeny.Inland orogeny represented by the uplift of the Tibetan Plateau is one of the core issues of lithospheric dynamics.The study of the uplift of the Tibetan Plateau involves the deduction of historical plateau elevation and the determination of the uplift rate of each period.The historical elevations are calculated based on the oxygen isotope analysis of the Tertiary lacustrine sediments in the plateau,the uplift rates are determined based on the low temperature records of the metamorphic rocks exposed since the Miocene or the Quaternary alluvial basin data in the piedmont,and traditional geodetic methods such as precise leveling are applied.Especially since the end of the last century,space geodesy technologies represented by GPS and In SAR have provided unprecedented observation capabilities for the study of crustal movement.However,for many years,the data related to the uplift of the Tibetan Plateau are still lacking in completeness and systematization.Compared with other methods,GPS has the advantages of unified datum and stable precision,but there are still large inconsistencies among existing results limited by insufficient data and GPS vertical precision,.With the increase of GPS continuous observation on the southern margin of the Tibetan Plateau,high-precision measurement of the vertical deformation field has gradually become an attainable goal.We collect the observation data of about 100 c GPS stations on the southern margin of the Tibetan Plateau from 1995 to 2020,then analyze and obtain the regional optimal noise model.The present-day vertical crustal movement velocity field on the southern margin of the Tibetan Plateau is constrained with its precision better than 1 mm/a.Using the latest models and data,we quantitatively discuss the vertical non-tectonic deformation caused by surface water variation,glacial isostatic adjustment,and geocentric motion.Vertical deformation distribution of the Indo-Asia collision front zone is clearly presented by combining GPS,Himalayan precision leveling and In SAR data.The kinematic characteristics and tectonic significance of the Indian subcontinent,the Himalayas and the Tibetan plateau are discussed respectively.The main conclusions are as follows:(1)The GPS vertical motions with rate uncertainties of < 1 mm/a is characterized by subsidence of 1-2 mm/a in the Ganges plain and Subhimalaya,uplift of 1-6 mm/a in the Himalaya and subsidence of 1-2 mm/a in the interior of the Tibetan Plateau.(2)The non-tectonic influence of the existing GPS velocity is about 1 mm/a,and the uncertainty is about 0.3?0.4 mm/a.Using the latest GRACE time-varying gravity field data,the isostatic uplift rate in the Himalayas and southern Tibet is estimated to be 0.2?0.4 mm/a.According to the existing global glacier models constrained by geodetic measurements,the postglacial rebound in Asian continent is about 0.3?0.5mm/a.According to the global geodetic survey results,the vertical motion caused by the geocentric movement is 0.3?0.5 mm/a.(3)The pattern of vertical deformation illustrated by the GPS motions attests to the descending Indian plate beneath the Himalaya and southern Tibet.The Himalaya is keeping growth and the southern Tibet may be collapsing instead due to ongoing extension east-westerly.