Late Quaternary Paleoseismology And Faulting Behavior Of The Internal And Western Boundary Faults Of Northwest Sichuan Subblock

Tibetan Plateau and its its surrounding area are one of the most complex tectonic deformation and strong seismic activity region in China's mainland.Its deformation mode has always been the hot spot of geological research.The "Rigid Block" and "Continuous Deformation" modes have been widely concerned.Block movement is the main deformation feature of China's mainland,controlling the majority of strong earthquakes in China.The hierarchy and grading of Active Blocks can be seen as a geological way to couple "Rigid Block" and "Continuous Deformation" modes,the fine division of Active Blocks makes the geologic "Rigid Active Blocks" become smaller in scale.The deformation mode of the diminishing Active Block can increasingly represent the characteristic of its local deformation,and make regional deformation gradually dispersion,which is closing to the Continuous Deformation.The activity strength and frequency of block-internal faults is an important basis to judge whether the Active Block can be further graded.From the perspective of the strong earthquake distribution,about 14% of M ? 7 earthquakes and 10-40% of M6-6.9 earthquakes occured on the block-internal fault belts in China's mainland.Some block-internal faults among Qingzang Block Zone show highly activity.The recent Maqu M7.4 earthquake occured in Bayankala Active Block may also be controlled by the Gande-Maqu block-internal fault,which reminds us to strengthen the study of the block-internal faults.The southeast margin of the Tibetan Plateau is the most complex deformation area in the Tibetan Plateau nowadays,and also the area where the subblocks and block-internal faults is very developed.Northwest Sichuan-Yunnan Subblock is a active block of Level III on the southeast margin of the Tibetan Plateau.Its boundary faults,internal Litang and Yunongxi faults have been strongly active since the Holocene.Strong earthquakes above M6 like 1948 Litang M7.3 earthquake have occurred in Northwest Sichuan-Yunnan Subblock,which is an ideal place to study the block-internal faults and its regional effect.We set the Litang,Yunongxi,and Batang fault as the object of study.We have obtained the fault parameters such as geometric features,motion properties,activity strength,and fracture behavior,through image interpretation,photogrammetry,geological investigation and ancient seismic research.On this basis,We discussed the mutual response relationship and their deformation mode between the block and block-internal faults The paper mainly obtains the following results:(1)The relationship between existing earthquake surface rupture and historical earthquake is determined.The 1948 Litang M7.3 earthquake simultaneously broke up the Litang and Kanga-Dewu segments,with a total rupture length of 76 km,a maximum coseismic dislocation of 3.1m.There is a non-permanent segmented boundary between Litang and Kangga-dewu segment,both of which have independent rupture ability and have at least three cascade ruptures since the Holocene.The surface rupture in the northern margin of Maoya Dam Basin may be the result of the history earthquake in 1729.(2)The Rupture Cycle model of large earthquake recurrence behavior in Litang fault zone was established.Since Holocene,three large Rupture Cycles on the Litang fault zone show the non-specific rupture behavior of the rupture events on the Litang fault zone,and show the continuous migration characteristics along the Litang fault zone.The number of ruptures in each segment is approximately the same to maintain the uniform release of stress.The time span of the three Rupture Cycles in the Litang fault zone revealed by the ancient earthquake was rapidly reduced from about 6000 years to 1800 to 520 years,which was basically consistent with the paleo-seismic characteristics revealed by each exploration trench.It shows that the activity of Litang fault zone has been increasing since Holocene.(3)The ancient earthquake sequence of Batang Fault was obtained.Batang fault showed weak activity in the early Holocene,but the activity intensified since the0 AD,which means the Batang Fault entered a cluster stage,and the recurrence interval was shortened to about 830 years.(4)The ancient earthquake sequence of Yunongxi Fault was obtained.It is proved that the recurrence period of Late Pleistocene is longer,the activity of Holocene is accelerated and may follow the characteristic earthquake model,the recurrence interval of large earthquake is about 2000-2500 years,and the elapse time is more than 2600 years,the future earthquake risk is worthy of attention.(5)The tectonic deformation mode of Northwest Sichuan-Yunnan Subblock is discussed.The abnormal acceleration activity of the Litang fault zone may be a direct response to the acceleration activity of Xianshuihe fault and Batang fault.The uneven movement and local deformation in Northwest Sichuan-Yunnan Subblock reflect that it can be divided into lower grade blocks(grade 4),named Yajing Subblock in the northwest and Zhongdian Subblock in the southeast.The structural deformation of the southeast margin of the Tibetan Plateau is more "distributed",the continuous deformation may be the more suitable model for the deformation mode of the southeast margin of the Tibetan Plateau.