Paieoearthquakes And Their Geomorphic Response Of Segments Of The Kalpin Thrust System In The Southwestern Tianshan Since The Late Pleistocene

Previous paleoseismic research usually focuses on only one fault or one segmentof the fault to study its recurrence and rupture behavior. It seems to be based on such ahypothesis that faults or fault segments are relatively independent during their ruptureprocesses. However, increasing evidence from large earthquakes shows that there maybe possible correlation among neighboring faults or fault segments. Some scholarshave revealed such relationships for strike faults. For example, the three segments ofthe Haiyuan fault zone exhibit distinct rupture behaviors, which was explained by thesingle fracture, double fracture and whole fracture models, respectively. TheAnninghe fault possibly shows a cascading rupture behavior, on whichpaleoearthquakes were found corresponding to those major events on the adjacentZemuhe fault. On contrary, the Zemuhe fault is featured by single-direction rupturing,on which no response to the events on the Anninghe fault was identified.The similar characteristics are also present in thrust systems. For example, theBeichuan-Yingxiu fault and the Jiangyou-Guanxian fault simultaneously rupturedduring the Wenchuan Ms8.0earthquake of2008. The Longmen Shan fault zoneshows a cascading rupture behavior in its middle and the northern segments. Theresearch has proved that paleoseismic events like the Wenchuan earthquake haveoccurred in the Longmen Shan fault zone before several thousands years. Comparedwith strike faults, the rupture behavior of the thrust system is more complex, as it cansimultaneously rupture along the strike and the vertical direction. How to describe therupturing behavior of paleoearthquakes in thrust fault systems is an important issue tobe studied in detail.Many thrust faults developed in the Tian Shan Mountains in northwest China.Because the humid monsoons of the Pacific and Indian Ocean are far away, the climate is arid in this region. Plus less erosion, the Tian Shan orogenic belt is an idealarea to study active tectonics. But a lack of samples for14C dating restricts the studyof the recurrence intervals of paleoearthquakes. The key technique is an adaptivedating to determine the intervals of thrust paleoearthquakes in the Tian Shan area. Theterrestrial cosmogenic nuclide dating is a good dating method to solve this problem.The advantages of10Be nuclide dating include its suitability for sampling and the highreliability of the estimated exposure age of the alluvial fans in arid areas. In previouswork, attempt has been made to estimate the timing of boulder deposition andabandonment of the faulted fan by TCN dating and calculate the average recurrenceinterval of the surface rupture on the alluvial fan in the Owens valley. The result wasbased on two assumptions that the influence of the inherited nuclide concentrationwas negligible and that the displacements created on the surface of the alluvial fans bythe various earthquakes were similar. Because10Be depth profile dating has beenimproved in recent years, the exposure age of the alluvial fans can be determinedmore accurately and the errors of the age can be more reliably estimated by the MonteCarlo simulation method.Many researches show the earthquake usually not rupture whole fault but onesegment of the fault. If the end point of the surface rupture is regular, the fault has anobvious segmentation feature. So the segmentation is important to determine theseismic risk. The length of the paleoearthquake surface rupture zone is difficult todetermine by the trench method in the case lacking historical records. As thepaleoseismic scarp cannot be preserved well for many years, segments ofpaleoearthquake derived from scarp mapping is not very credible. Active tectonics canaffect geomorphic evolution. Therefore, paleoearthquakes could leave records of theevolution of alluvial fans near the fault. Geomorphic parameters of alluvial fans arevariable in every paleoseismic segment which has different displacements andintervals of paleoearthquake. Combined with geomorphic parameters andpaleoseismic scarps, it is possible to improve the investigations of paleoseismicsegments.This work attempted to determine the ages of the alluvial fans by10Be TCN dating, the deformation of fault scarps by GPS measurement, and the displacement ofpaleoseismic events by a geomorphic trench survey. With the combination of the dataobtained, the interval and slip rate of the large-earthquake in every segment of theKalpin system were estimated and the recurrence of paleoearthquakes in the wholeKalpin system was calculated. Besides, in terms of geomorphic parameters of alluvialfans and fault scarps, this thesis discusses segmentation of the Kalpintage fault. Theprimary research results of this work are summarized below.(1) The front two rows of front-range faults in the eastern Kalpin thrust systemruptured in the same period with roughly same total displacement of paleoearthquakes.As a whole, the paleoseismic events of this system can be explained by the model ofthe characteristic earthquakes, of which each event has3m of displacement and5kaof the recurrence interval. The Kalabuketage fault of the third row is not active sincethe Quaternary.(2) The three rows of range-front faults of the western Kalpin system are activeat present, but their activity diminishes from south to north. The west Kalpintage faultof the first row mountain is most active, while the Tuoketage fault of the third row isthe weakest in activity. As displacements of paleoseismic events differ much betweenthe faults of these rows and time constraints of intervals are not enough, it isimpossible to determine the correlation between rows of faults.(3) In the arid area that contains large deposits of coarse gravel that lack14Csamples, a paleoseismic study that combines TCN daing with a geomorphicdeformation survey and trench excavation is an alternative approach for datingtectonic activities. The accuracy of intervals of paleoseismic events is limited byintervals of two alluvial fan ages and times of paleoearthquakes.(4) Using detailed scarps mapping and alluvial fan geomorphic analysis, thiswork found segments of the fault scarp are very consistent with segments of alluvialfans. There are two obvious boundaries of segments in the Kalpintege fault: one is thePiqiang fault; the other is a protruding point of the fold mountain30km far awayfrom the Kalpin fork. The two boundaries divide the first row of the range-front faultat the Kalpintage in to three segments: the west Kalpintage segment west of the Piqiang fault, the east Kalpintage segment east of the Piqiang fault, and the Kepinfork segment in the easternmost30km. The highest scarp with cumulative height4mis near the Wujianfang in the west Kalpintge segment, where scarps are about1.5-2mhigh in newest alluvial fans and lower on two flanks. The geomorphic characteristicsof alluvial fans express highly active structures: the constant slope in longitudinalprofile of alluvial fans; the WLF value>1; the “open fans” plan-view geomorphologyand the slip rate of the fault is1.45(+1.68/-0.44) mm/year and so forth. The site of thehighest scarp with3m is near the Sanchakou in the west Kalpintge segment with1-1.5m height scarps in newest alluvial fans, also lowering toward two flanks. Thegeomorphic characteristics of alluvial fans show low activity: the concave slope inlongitudinal profile of alluvial fans; the WLF value <1; the “closed fans” plan-viewgeomorphology and the slip rate of fault is0.31(+0.21/-0.18) mm/year. No faultscarps in the Kepin fork segment was seen, and the geomorphic characteristics ofalluvial fans indicate non-active tectonics: the WLF and WLD value>1; small slopesof alluvial fans and basins; and the concave slope in longitudinal profile of alluvialfans.(5) The Mangute fault and the bifurcation of fold belt near the Sanchakou are notthe permanent boundary and can not obstruct occurrence of earthquakes in the eastKalpintage fault.(6) The cascading rupture pattern and the diversity of paleoearthquakes in theKalpin thrust system: the front two rows, east Kalpintage fault and the Saergantagefault in the eastern segment possibly have a cascading rupturing behavior with3m ofthe total displacement of paleoearthquakes, which might ruptured simultaneously. Oncontrary, the west Kalpintage fault, the Aozitage fault and the Tuoketage fault in thewestern segment show a diversity of paleoearthquake events. The different intensitiesof tectonic activity may be the reason for differences between the eastern and thewestern Kalpin thrust system. High slip rates of the faults in the west Kalpin canexplain short recurrence intervals of earthquakes, and this section can not accumulateenough energy for cascading ruptures on multiple faults. On contrary, the low sliprates of faults in the east segment may lead to accumulation of large energy in its two faults, where the rupture of one fault could induce break of the other fault, or even thetwo faults might rupture at the same time resulting in earthquakes.