Normal Faulting Of Central-southern Yadong-gulu Rift Since Late Cenozoic,southern Tibet

In the interior of Tibetan plateau,the active tectonics are primary marked by north trending rifts in southern Tibet since late Cenozoic.The formation and evolution of these rifts are always the key points within the earth science of Tibetan plateau due to their close connection with the uplift of Tibet and the adjustment of convergence deformation between India and Eurasia.In this study,the Yadong-Gulu rift,typical one of these rifts,is chosen the main object,and part of Cuona-Oiga rift is also listed.The methods are mainly consisted of interpretation of satellite images,field structure investigation,topographic measurements and geochronology,and may others.Together with many previous research,we studied on the geometry of north trending rifts,initial rifting age and evolution process,and fault slip rates at timescales of million,millennium,ten years.Based on these product,we preliminary discussed the formation of these rifts.By interpretation of satellite images,the north trending rifts are composed of eight major rifts.All of these rifts are connected with grabens or half-grabens roughly parallel distributing at the central Himalaya arc.And the spacing of rift valleys in west are smaller than east.Based on field investigation and interpretation,the southern Yadong-Gulu rift consists of three grabens with right-step oblique sequence that are Pagri-Duoqing Co graben,Nieru graben and Relong graben.Normal faults are developed on both sides of the Pagri-Duoqing Co graben.However,the activity of western margin normal faults is weak now which is the trace of the early activity.The boundary faults of Neru and Relong grabens are located in the eastern margin.The Nyemo graben group of central Yadong-Gulu rift is consisted of seven smaller grabens.Most of grabens are developed with late Quaternary sediments and are displaced by the normal fault forming typical fault scarps.Late Cenozoic sedimentary strata and thermal history modeling from low-temperature thermochronology vertical profiles provide new evidence for defining the initial rifting age of north trending rifts.Our results suggested that the initial rifting age could be reach to 10-16 Ma,i.e.middle Miocene,which is consistent with previous estimates.And there is a secondary accelerated extension process between Pliocene and Quaternary.According to the basin-mountain height difference and sediment thickness,the vertical slip rate of the Yadong-Gulu rift ranges from 0.2 mm/yr to 0.4 mm/yr on the whole,with the maximum not exceeding 0.5 mm/yr at the scale of one million years since its formation.Combined with remote sensing interpretation,field measurements and late Quaternary geochronology,the late Quaternary vertical slip rates of four grabens in the central-southern Yadong-Gulu rift range from 0.6 mm/yr to 2.0 mm/yr.Based on previous data,it is found that the vertical slip rates of the rift on a ten-thousand-year scale tend to decrease from both sides to the middle.Re-analysis of published GPS data shows that the extension rate of Yadong-Gulu rift is about 6 mm/yr,which is significantly higher than the slip rate geological estimated.The geological extension rate of Yadong-Gulu rift is about 1-1.5mm/yr evaluated by the fault dip.Then the whole north trending rifts could be reach to 8-15mm/yr,which is similar to the GPS observations.This suggested that the north trending rifts are the major structures that adjusted the the convergence deformation in southern Tibet at least since late Quaternary.Combining geometry and kinematics characteristics,fault activity and evolution of the rifts,the formation of north trending rifts may be the fracture and the extension of upper crust of southern Tibet resulting from the strong compression and convergence.The secondary accelerated extension of the rift may be related to plate tearing at the subduction front of the Indian lithosphere.