Late Quaternary Activity Of The Gayring Co Fault And Its Implication For Dynamics Of The Southern-central Tibetan Plateau

Uplift and deformation mechanisms of the Tibet plateau are one of the issues receiving muchattention. In the1970s, the researchers determined the basic tectonic framework for the Tibetanplateau through interpretations of remote sensing images (Landsat satellite) of the United States.Afterwards, large-scale studies were launched in the past30years. With a great number of deepgeophysical exploration, researchers are trying to unravel the evolution of the collision betweenthe Indian plate and Eurasian plate, and have proposed corresponding kinetic models: continuousand discontinuous deformation models. Both models concentrate on the relationship betweendeformation and big faults in the Tibetan plateau. The discontinuous deformation model thinksthat most deformation is absorbed by big faults and block escape. While the continuousdeformation model suggests that deformation is similar to the plastic deformation which isuniform, so the role of big faults is relatively less noticeable. By the end of the last century, withthe increasing wealth of GPS data, people can more precisely measure the current plateau activetrajectories through the analysis of the velocity field. A different point of view is also put forwardto prove the correctness of these two models. However, reliable information from geologicalobservations is still an important touchstone for the two models. Due to adverse environmentaland climate conditions and other reasons, to obtain detailed information on fault activity is stillvery difficult. Consequently, most of the credibility of the existing data are confined to the edge ofthe plateau, while relevant information of the internal plateau remains relatively scarce.The Gyaring Co fault is chosen as the main study object in this thesis. It is a segment of theseries of faults in middle of the Karakoram-Jiali fault zone (KJFZ). The KJFZ is a large-scale faultzone in southern-central Tibet which is generally considered to be the southern boundary of theplateau with a total length about2000km. The formation, structure, activity and rate of KJFZ arecrucial to understanding the uplift, lateral extrusion and extension of the Tibetan plateau. TheKarakoram fault and Jiali fault, which are on either end of the KJFZ, are relatively independent, sothe formation, structure and activity patterns are clear with the research of Quaternary. Althoughsome research has been made in recent years in the central part of the KJFZ, a systematic researchon the fault is still lacking. The Gyaring Co fault is the largest branch in the middle KJFZ with thebest expression. In last five years, based on the collection of previous data, this work carried outfield surveys and topography measurements in the region around the fault three times. Besides,trenches were excavated to search for evidence for fault activity by tectonic geomorphology. Total time of field work is more than80days. Then this work made analysis to geomorphologic featuresof the area by the high-resolution images and DEM data. The primary conclusions of this thesisare presented blow.1) The geometric distribution and activity segments of the Gyaring Co fault. This fault iscomposed by a group of en echelon north-west striking segments. The total length is nearly250km. The measured targets include the dislocations in the alluvial fans, alluvial terraces, gulliesand ridges. The study suggests that this fault is dextral in nature and has a tensional normalcomponent, and the features of segments of the fault are different. According to the differences instructure and activity, it is cut off by the north-south Shenzha-Dingjie rift and NE strike Wuru Cofault. The fault can be divided into three segments. The northwest segment is between theBangonghu-Nujiang suture zone (BNSZ) and Zhangnai Co, where it is cut off by the Wuru Cofault at the southeast end. This segment is overall a broad valley without obvious fault trace,especially no obvious strike-slip found, instead a normal fault with declining north wall and risingsouth wall. The middle segment is between Zhangnai Co and Gyaring Co, which is the main partof the fault, about140km long, as wide zone of a number of lakes. Its fault trace is the mostobvious. Its dextral slip is dominant, though tensional normal activity is also strong. The southeastsegment of the fault starts from the south bank of the Gyaring Co, where the landscape is a narrowvalley filled quickly with sediments along the fault. Due to the eastward expansion of theShenzha-Dingjie Rift, activity of the fault is not strong overall. According to thesegeomorphologic features，this work speculates that this fault has a vertical dextral slip fault atdepth，and becomes a negative flower structure in the shallow subsurface because the tension.2）Activity rates of the middle segment. This work determined single seismic dislocations andmaximum magnitude of the fault. By collecting the age data of lacustrine terraces and morainesavailable, combined with the ages of the samples that this work collected, the Quaternary agegauge is established for the research area to roughly framing ages of various geomorphic units.This work measured the dislocation of lacustrine terraces, scarps and other landforms in the fieldor on the high-resolution remote sense image. The horizontal and vertical displacements caused bythe latest seismic activity are4.0m and0.6m, respectively. The slip rates in the direction of strikeand dipping are4-5mm/a and0.6mm/a respectively. This work estimates that the fault is capableto generate the maximum Mw7.5earthquake based on relationship between displacement andmagnitude.3）Determination of recurrence interval of the middle segment of the Gayring Co fault. Basedon analysis of excavated trenches and cleaned natural profiles, as well as the ages of seismicstratigraphic and colluvial wedge sediments, as well as the single seismic event dislocation andslip rate, the recurrence interval is estimated to be about1000a. And the single vertical dislocation of the segment is about0.6m.4）Activity rate and extension amount of the NS trending extensional faults in theShenzha-Dingjie rift system and its relationship with the Gayring Co fault. ETM remote sensingimages and30m resolution ASTER DEM data are processed in ArcGIS, and the northern segmentof the Shenzha-Dingjie Graben three-dimensional digital elevation model is built. Combined withfield investigations, the overall of landscape characteristics of the graben are analyzed. On thebase of the main fault nature and activities of the faults observed in the field, as well as thespreading of the fault in the rift from remote sensing interpretation, it is suggested that the fault inthe half-graben is composed by an eastward dipping major listric fault and a series of syntheticsub-faults and antithetic sub-faults. Based on the half-graben system model, a simple book obliquemodel is used to calculate the depth of the detachment surface and the magnitude of extension inthe graben system. The results show that the depth of the detachment is between9.2-20km,basically consistent with the focal depths in the region, which proves that earthquakesconcentrate in the vicinity of the detachment surface. Finally, the graben formation mechanism isdiscussed. It is suggested that the stress of upper crust is decoupled with lower crust’s due toIndian plate’s compression. The upper crust moves to south and east under gravity, thus formingthe east-dipping half-graben.5）Tectonic dynamic model for the southern-central Tibet plateau on both sides of BNSZsuture. Based on analysis30m resolution Arster DEM, there is significant negative topographywithin the range of the ellipse with a long axis1000km and a short-axis300km beside the BNSZ.In this range, there concentrate a large number of big lakes, implying significant north-southtension in this region. Besides, geophysical study confirmed the suture may be the channel forthermal material as evident in high heat flow, and there may be a plastic layer at the depth ofmid-lower crust in this area. So this work speculates that this tension-shear of the conjugated faultsystem in the upper crust of the south-central plateau presently. And this is not only the extensionin east-west direction with Qiangtang terrance’s eastward movement, but also a significantnorth-south extension along the BNSZ. Besides, the slip rate of the Gyaring Co fault is relativelysmall, far from that as the boundary fault. Instead, it only accommodates the east-west extension,while less related with the eastward escape of the Tibetan plateau. Its driving force is probablyassociated with thermal rise from the deep subsurface.