Crustal Stress Field In SE Tibetan Plateau And Its Implication For The Crust And Upper Mantle Deformation

Rapid uplift of the Tibet Plateau one of the world's largest tectonics in the Cenozoic.The India-Asian collision induce extensive deformation in the plateau and drives the materials to move eastward.Because of the blocking of Sichuan Basin,the material extruded from the plateau escapes to southeast.Crustal earthquakes are closely related to the stress state in the crust.They are results of the interaction between the regional tectonics and the brittle deformation near the source under regional stress field.The study of the stress field in the SE Tibet Plateau helps us better understand the tectonic activity in this area.We applied the gCAP algorithm to determine 239 focal mechanism solutions(3.0 ? Mw?6.0)with records of dense ChinArray stations deployed in Yunnan,and then inverted 687 focal mechanisms(including 448 previous results)for the regional crustal stress field with a damped linear inversion.Then we compare the stress filed with the GPS observations and SKS splitting measurements,which provide important constraints on the tectonic evolution in SE Tibet Plateau.Earthquakes in SE Tibet Plateau mainly occured at 5-15 km depths along the major faults.Most of the earthquakes are strike-slip faults.Many thrust events occurred in Longmen Shan in the western margin of the Sichuan Basin.Some events with normal mechanisms are flound west Yunnan.In addition,the focal mechanisms near the Tengchong volcano are complex,which shows that the earthquakes here are not only controlled by major faults,but also influenced by the volcanic activity.In summary,the earthquakes in SE Tibet Plateau are mainly controlled by the strike-slip faults while some smaller(Mw-3.0)thrust and normal events are controlled by local tectonic activities.The tectonic stress in SE Tibet Plateau is dominated by strike slip,the maximum(?1)and minimum(?3)principal stress axes are sub-horizontal.The tectonic stress in the western margin of Sichuan Basin is characterized by thrust activities;?1 is sub-horizontal and ?3 has a large dip angle(60°?80°).The northwestern part of Yunnan is in extension in general;?3 is sub-horizontal and ?1 is vertical.We further calculated the horizontal stress orientations(i.e.,maximum and minimum horizontal compressive stress axes:SH and Sh,respectively)accordingly and found an abrupt change near?26°N.To the north,SH aligns NW-SE to nearly E-W while Sh aligns nearly N-S.In contrast,to the south,both SH and Sh rotate laterally and show dominantly fanning patterns.The minimum horizontal stress(i.e.,maximum strain axis)Sh rotates from NW-SE to the west of Tengchong volcano gradually to nearly E-W in center of Yunnan,and further to NW-SE in the South China block in the east.The Sh axis(consistent with maximum horizontal strain)in the crust is consistent with that inverted from GPS observations,whereas the maximum strain is normal to the GPS observations.The maximum strain is also normal to the gradients of the topography,suggesting the stress field in the crust mainly arises from the gravity potential and instability near the edge of the high plateau.SKS fast direction usually represents the direction of deformation of the mantle;the crustal strain field is consistent with the upper mantle strain field indicated by shear-wave splitting observations in Yunnan but not in other regions.Therefore,the crust and upper mantle in Yunnan are coupled and suffering vertically coherent pure-shear deformation in the lithosphere.