Tectonic Deformation And Rheological Structure Around The Southern Tibetan Plateau Based On The GPS Observations

The tectonic model and dynamic mechanism in the Tibetan Plateau have been a focus in the geoscience.Two of the best-known end-member models are the‘Continent escape(Block deformation)'and‘Crust thickening(Continuous deformation)'.As a frontal area between the Indian and the Eurasian plates,the present-day tectonic deformation characteristics and evolution model in the southern Tibetan Plateau are the hot spot in the world.Some studies pointed out that the existing geoscience data is unable to clearly distinguish which deformation mode is best fit to the kinematic mechanism in the Tibetan Plateau,but at least we can explore whether the tectonic deformation of the plateau tends to be the‘localized'pattern owing to the Block model or the‘diffused'pattern in the Continuous model based on the existing geodetic observations.The determination for this basic deformation feature is also an important premise for us to analyze and explore the tectonic mechanism and geodynamic process in the Tibetan Plateau.On the other hand,surface deformation following the 2015Mw 7.8 earthquake occurred in the middle part of the Himalayan orogen was successfully recorded by the nearby GPS continuous stations,which provided valuable data for the study of the rheological structure and future seismic hazards in this region.Is it able to constrain the deep rheological properties in the Tibetan plateau based on the surface relaxation displacements after the earthquake?Whether the deep rheological structure is related to the unique tectonic characteristics in the plateau?An intensive study on these issues will not only deepen our understanding of the regional geodynamic mechanism,but also help us to comprehend the geological disaster background at present,and further predict the future trends,which are of great significance in the security of people's lives and properties and the social sustainable development.Therefore,this study focuses on the southern Tibetan Plateau and its adjacent area,as the frontal area between the Indian plate and Eurasia,to analyze and explore the regional tectonic deformation characteristics and the deep rheological structure based on the geodetic observations during the interseismic and postseismic periods.The details are presented by the following steps:(1)We first systematically process the GPS observations in Nepal and Bhutan,then obtain the time series and GPS velocities.Meanwhile,we also collect and organize the GPS observations in China,India and other countries.Then a dense velocity field relative to the Eurasian fixed reference is obtained by compiling the GPS velocities in this study and other reginal observations.We further develop an elastic block model in the southern plateau based on the tectonics setting,and estimate the motion of each block,the slip rates on the major faults and strain rates within the blocks.In addition,the intrablock deformation potency rate is introduced to quantitatively analyze the proportion of deformation within the blocks and near the faults.The results show that the deformation magnitude near the faults is the same as that in the blocks,indicating a‘diffused'upper-crust deformation feature in the southern plateau,and also infers that a simple pure elastic block model is insufficient to explain the overall deformation in the Tibetan Plateau.(2)After the 2015 Nepal earthquake,GPS observations were used to obtain the postseismic deformation signal,and further to analyze the response processes of afterslip and viscoelastic relaxation.The results show that in the early stage after the earthquake,the near-field deformation is mainly caused by afterslip on the fault,while in the later stage,it mainly comes from viscoelastic relaxation.However,in the far-field,the relaxation process is mainly controlled by viscoelastic mechanism during the whole postseismic stage.The near-field postseismic predicted signal will be less than the observation noise 18 years later.Therefore,the postseismic effects following this great earthquake must be further considered when analyzing the tectonic deformation in this area based on the geodetic data in the future.On the other hand,the viscoelastic relaxation after the earthquake supports the laterally heterogeneous rheological structure in the deep:the lower crust in the southern plateau is viscoelastic(the transient viscosity is 5×10¹⁷ Pa s,the steady viscosity is 5×10¹⁸ Pa s);but the Indian crust is close to an elastic layer.Meanwhile,considering the afterslip distributions after the Nepal earthquake and the historical earthquakes in the Himalaya,it has the potential to suffer a large earthquake in the southern and western area of this event in the future.(3)A viscoelastic block model in the study area is built based on the laterally heterogeneous rheological structure and GPS velocity field during the interseismic period.The results show that the viscoelastic earth model can produce a more reasonable interseismic deformation than a pure elastic model,indicating that the viscoelastic medium in the lower crust may be the physical premise for the observed‘diffused'deformation in the southern Tibetan Plateau.On the other hand,we can infer that the interseismic GPS deformation observed in the Tibetan Plateau at present partly contains the deep viscoelastic effects,which should be taken into account in the future numerical simulations.