Late Quaternary Activity Of The Longriba Fault Zone And Its Kinematic Relations With Adjacent Faults

The ongoing collision of Eurasian and Indian plates caused the widespread uplift of theTibetan Plateau and has built the tectonic framework of the whole Eurasian area. This collisionalso has influenced significantly the climate and environment in the Asian area. The uplift of theTibetan Plateau is accompanied by the eastward motion of the plateau, leading to the greatestrelief in eastern Tibet than anywhere else on the plateau. Eastern Tibet has become an importantsite for studying the mechanism of eastward motion of the Tibetan Plateau and the uplift of easternTibet. The2008Wenchuan earthquake (Ms8.0) ruptured the Longmen Shan fault zone andstimulated numerous studies on eastern Tibet.Two end-member hypotheses are proposed to illustrate the deformation mechanism of easternTibet. A continental extrusion model predicts that the deformation is concentrated on majorboundary faults, whereas a lower crustal model suggests that the deformation is distributedthroughout the whole region of eastern Tibet, and the role of major faults played in the eastwardmotion of the Tibetan Plateau was not considered.The Longriba fault zone,~200km west of the Longmen Shan, is subparallel to the LongmenShan fault zone. Preliminary investigation showed a strong activity along the central Longribafault zone. Tectonically, the Longriba fault zone separates the Bayan Har block into the Aba andLongmen Shan subblocks. West of the fault zone, the landform is characterized by the flat surfaceof the Tibetan Plateau and strike-slip faults. East of the fault zone, it is marked by high relief ofthe Longmen Shan and thrusts. So, what about the fault geometry of the Longriba fault zone? Howabout the magnitude of its fault activity in the late Quaternary? How long is the elapsed time sincethe last event on this fault zone? Which model is suitable for ruptured behavior of largeearthquakes on this fault zone? In addition, the2008Wenchuan earthquake ruptured the twosubparallel faults with~15km apart: Yingxiu-Beichuan and Guanxian-Jiangyou faults. The twosubparallel strands in the central Longriba fault zone are separated by~20-30km, that raising thepossibility that a large earthquake could rupture both strands. However, no large historicalearthquake is known on this fault zone. Finally, as the first barrier on the way of eastwardmovement of the Tibetan Plateau, what role does the Longriba fault zone take in strain partitioningin eastern Tibet? The northern Longriba fault zone is linked with the Kunlun fault, how about thekinematic relation between them? The answers to these questions not only provide the quantitativeparameters for seismic hazard assessment on the Longriba fault zone, but help the understandingof the mechanism of eastward movement of the plateau and uplift of eastern Tibet.Based on the collection of previous data, we determined the geometry of the Longriba faultzone combined with high-resolution satellite images and field geological investigations. Using topographic survey of displaced landforms and dating of geomorphic surfaces (radiocarbon andOptically Stimulated Luminescence), Late Quaternary slip rates on this fault zone are calculated.We excavated trenches to reconstruct the paleoseismic history and then studied the rupturebehavior on this fault zone. In addition, we determined the slip rates along the Tazang fault.Finally, we discussed the role of the Longriba fault zone in strain partitioning in eastern Tibetusing geological and GPS slip rates. Based on the above studies, we draw the followingconclusions.1) The central Longriba fault zone consists of two strands (Maoergai and Longriqu faults)and has a clear faulted landforms. The Maoergai fault extends the northern bedrock slope ofMaoergai and Yanggong rivers and is characterized by shutter ridges and displaced channels,showing right-lateral strike-slip motion in late Quaternary. The Longriqu fault runs primarilyalong the slopes north of Longriqu and Queerdeng rivers and is marked by shutter ridges and faultscarps on the mountain-front alluvial fan. Displaced terraces indicate that the dextral rate along theLongriqu fault since~18ka is2.5±0.4mm/yr, but is1.4+0.4/-0.3mm/yr since~11ka. Its verticalslip rate is very small (0.1-0.2mm/yr), demonstrating that the Longriqu fault is predominantlystrike-slip. On the Maoergai fault, the dextral rate since~21ka is2.3+0.4/-0.3mm/yr, but is0.7±0.1mm/yr since~9.5ka. These results shows that the whole Longriba fault zone isdominantly right-lateral motion accompanying a very small south-verging thrust component onthe Longriqu fault. The dextral rate since the latest Quaternary is~4.8mm/yr, whereas the dextralrate in the Holocene is~2.1mm/yr. The slip rates on the central Longriba fault zone decreasesfrom~7.5mm/yr in the latest Pleistocene to~2.1mm/yr in the Holocene. The drop is probablyrelated to the slowdown of the eastward motion of the Tibetan Plateau.2) The southern Longriba fault zone is intermittently distributed and is only exposed nearGuanyinqiao Town. Its horizontal and vertical slip rates are0.3-0.5mm/yr and0.2-0.3mm/yr,respectively. The strike of the northern Longriba fault zone is nearly N-S trending and thedeformation spreads in at least two fault strands. The west strand probably undergoes a weaktectonic activity, where no faulted landform in late Quaternary is found. While the east strand iswell exposed and has a west-verging reverse component. It has a dextral slip rate of about0.8mm/yr and vertical slip rate of about0.3mm/yr.3) At the eastern end of the Kunlun fault, there are numerous faults active in late Quaternary.The western Tazang fault is dominated by left-lateral strike-slip motion with a Holocene rate of2.9±0.7mm/yr. Whereas, he eastern Tazang fault is characterized by south-west-verging reversemotion and has an eastward decrease along the fault from about1-1.5mm/yr to0.3mm/yr. At theeasternmost termination of the Tazang fault, it is marked by folded layers in the Holocene riverterrace at Tazang Town. The Tazang fault is probably the easternmost of the Kunlun fault zone andis linked with the master Kunlun fault (Maqu fault) via a pull-apart basin. The Minjiang fault ischaracterized by east-verging reverse motion with a vertical Holocene slip rate of0.37-0.53mm/yr.The Huya fault may have a relatively strong left-lateral motion and an east-verging reversecomponent with a vertical slip rate of0.3mm/yr in the Holocene. The Longmen Shan fault zone is dominated by thrusting with right-lateral component and has a late-Quaternary crustal shorteningrate of about3mm/yr. The Bailongjiang fault undergoes a minor slip rate in late Quaternary (＜1mm/yr).4) The trenches on the central Lognriba fault zone show that three events ruptured theMaoergai fault in late Quaternary and occurred at5170±80,7100±70, and8510±420cal yr BP.Based on fault length and the estimated coseismic horizontal offsets, the magnitude of theseevents are approximately Mw7.2-7.4. Four surface-rupturing events occurred on the Longriqufault at5080±90,11100±380,13000±260, and17830±530cal yr BP. The last event probablyruptured the Longriqu and Maoergai faults and has a magnitude of Mw~7.6. Prior to the last event,the two fault strands of the Longriba fault zone appear to experience an alternating activity. If wetake the Longriqu and Maoergai faults as a whole, the Longriba fault zone appears to undergo aregular recurrence with an interval of~2000years before the last event. Considering the elapsedtime of over5000years and relatively high slip rate, the central Longriba fault zone has a highpotential of a large earthquake in the future years.5) GPS rates and the long-term geological slip rates derived from displaced landformsdemonstrate that the Longriba fault zone plays an important role in strain partitioning in easternTibet. The Tibetan Plateau is moving due eastward and is resisted by the Longriba and LongmenShan fault zones in eastern Tibet and then collides with the rigid Sichuan block. However, theLongriba and Longmen Shan fault zones fault zone strike N~60oE. So, one portion of the motionwas transformed into dextral slip along the Longriba fault zone, and other portion continues tomove southeastward and collides with the rigid Sichuan block. The relatively soft Longmen Shansubblock accommodates the collision and finally leads to the uplift of the Longmen Shan andthrusting with dextral motion along the Longmen Shan fault zone. The two strands of the Longribafault zone merge at the decollement~20km deep similar to the Longriba fault zone. The deep dipof the Longriba fault zone helps them undertake dominant dextral motion, and the gentle dip ofthe Longmen Shan fault zone favors thrusting.6) The E-W-trending GPS section across the Min Shan platform and geological slip rates ofactive faults at the eastern end of the Kunlun fault reveals that a small portion of the sinistraldeformation on the master Kunlun fault is transformed into the reverse motion at the easternTazang fault, and a big portion is transferred to the N-S-striking Minjing, Huya, and northernLongriba fault zone characterized by E-S crustal shortening that causes the uplift of the Min Shan.This pattern at the eastern end of the Kunlun fault is somewhat similar to that at the easterntermination of the Altyn Tagh fault zone. The Roergai basin is probably a small-scale forelandbasin associated with the uplift of the western Min Shan.