Active Flexural-slip Fault Scarps And Earthquakes In The Southwest Tianshan-pamir Foreland Basin

In recent times, some moderate-large earthquakes occurred in active folds andthrusts, such as the1906Manas ML7.7,1983Coalinga M6.5,1944San Juan Ms7.4,1968Inangahua Ms7.0,1994Northridge M6.5and2013Lushan Ms7.0events,which seem not directly related with known active faults on the surface and did notform surface ruptures. Their features are that the displacement of the seismogenicfault deceases rapidly upward and tends to zero at the surface, does not cause or formonly a small surface rupture and displacement. Instead their surface deformation issometimes characterized by uplifting folds. Although such individual earthquakesmight correspond to a known surface active fault, most of them occurred under activefolds, formed by displacement of burial thrusts which are located at depth of tenskilometers beneath the folds. Stein named such earthquake as “folding earthquake”. Itis quiet a challenging issue to study and assess seismic hazards of folding earthquakesoccurred in compressive tectonic regions with active folds and burial thrusts.The flexural-slip fault is formed by bedding slip during flexural-slip folding,includes the reverse flexural-slip fault and normal flexural-slip fault. Derived fromactive folding secondary faults such as flexural-slip faults, bend-moment faults, it iseasier to identify that the fold itself. These secondary faults at the surface havecoseismic slip and record active history of seismogenic thrusts which provide aneffective way to study the seismic activity of blind thrusts.Over the past20years, researchers have found flexural-slip faults offset terracesurface and formed fault scarps in many regions with active folds. However, there stillhave many issues remaining unclear, for example, what is the exact positionflexural-slip faults developed in a fold, what kind of deformation characteristicsflexural-slip fault is present, how large is deformation in the region with flexural-slipfaults, how about the capability of producing an earthquake by a flexural-slip fault,and how to calculate flexural-slip related parameters. So far most studies aboutflexural-slip faults are still focused on simple qualitative description.This thesis selects the southwest Tianshan-Pamir foreland basin as study area.This region is characterized by strong tectonic activity in Late Quaternary and hasbeen one of areas with most frequent and intensive earthquakes in Central Asia. Itshistory has documented the1902Atush M81/4earthquake in Xinjiang, and1985Wuqia Ms7.4earthquake. It is an ideal place to study active tectonics, tectonicgeomorphology, river terrace evolution, orogenic uplifting, and basin-mountaincoupling, receiving much attention of researchers at home and around the world.Through1m resolutionized Google Earth image interpretation and fieldgeological investigations, this work discovered fold-slip faults cutting late Quaternaryriver terraces and several rows of subparallel fault scarps with same trends as the foldaxial and zonal distribution in four active folds in the southwest Tianshan-Pamirforeland basin.By large-scale structural mapping, high-resolution differential GPSmeasurements, trenching and other means, this work proposes a method of calculatingthe flexural-slip related parameters, and makes a preliminary study on the distribution rule, deformation characteristics, expressions of geomorphology of offset surface,paleoseismology, coseismic slipping models, relations with major earthquakes offlexural-slip fault scarps in the southwest Tianshan-Pamir foreland basin.This study focuses on the following aspects:1) Trough1meter resolutionized Google earth image interpretation and fieldexpeditions, this thesis completed geological mapping and initially determined strikes,lengths, and widths of flexural-slip fault scarps, and other basic parameters.2) Based on the previous work and combined flexural-slip fault bedding,development characteristics, back-tilting scarps, this thesis proposes a reliable formulato calculate flexural-slip fault related parameters, and builds a foundation to forwardlystudy flexural-slip fault scarps.3) Taking use of high-resolution differential GPS data to measure flexural-slipfault scarps, this work obtains abundant fundamental data. Through calculatingflexural-slip fault-related parameters, it attains preliminary understanding ofdeformation characteristics and distribution rules of fault scarps. Combining riverterrace exposed ages, the amounts of crustal shortening and uplift, as well as theirrates are calculated, separately.4) Three trenches were excavated across different flexural-slip fault scarps in theterrace T3at the north bank of the Kezilusu river, south limb of the Mingyaoleanticline. The3trenches proved closely spaced flexural-slip faults with independentactivities, as well as repeated activity. Finally this work acquired coseismicdisplacement values of the flexural-slip fault during the last event.5) The trench excavated across the1985Wuqia Ms7.4earthquake surfacerupture at the south limb of Mingyaole anticline demonstrated flexural-slip faultcoseismic slipped in the earthquake, ruptured ground surface and formed a northdipping reverse small scarp. This thesis also explores the possible models of faultcoseismic slip in the earthquake.Through the work above, the following conclusions are drawn:1）There found many flexural-slip fault scarps in several active fold developedregions in the southwest Tianshan-Pamir foreland basin. Flexural-slip fault mainlydeveloped in the steep beds with similar rock mechanical properties closed to activeaxial surface. Affected by the rock mechanical properties, flexural-slip fault scarpsoccur at nearly equidistant or multiple distance spacings on the same terracesurface.Overall, away from the active axial surface, flexural-slip fault relatedparameters gradually decease.2) The Mingyaole anticline is a box-shaped fold with a steep north limb andgentle south limb. Flexural-slip fault scarps mainly developed in the steep beds ofthick coarse and fine-grain sandstone with similar rock mechanical properties in arange of50to1200meter from axial surfaces b, d and i, wide90-1000m.3)For the non box-shaped folds Wulagen anticline, Bieertuokuoyi anticline,Wulagen syncline, flexural-slip faults mainly developed in steep beds from the activeaxial surface within a range of380-1270m, with width280-550m. 4) The size and trench profiles of flexural slip fault scarps on different terracesurface revealed the characteristics of multiple activities of flexural-slip faults, theolder terreace, the larger size of flexural-slip fault scarps.Since the formation of terrace T3at the north bank of the Kezilesu river, thesouth limb of the Mingyaole anticline, terrace T3has a horizontal shortening of8.3±1.4m and shortening rate of1.0±0.2mm/a，vertical uplift of9.8±1.2m and upliftrate of1.2±0.2mm/a. At the east bank of the Kalanggoulvke river, the south limbof the Mingyaole anticline, terrace T7has a horizontal shortening of4.2±0.8m andvertical uplift of6.9±0.9m. At the east bank of the Kalanggoulvke river, the northlimb of the Mingyaole anticline, terrace T7has a horizontal shortening8.3±0.7m andvertical uplift of7.9±0.7m. At the north limb of the Mingyaole anticline，east bank ofthe Kalanggoulvke river, terrace T3has a horizontal shortening of2.2±0.4m andvertical uplift of5.1±0.4m，terrace T2has a horizontal shortening of0.7±0.3m andvertical uplift of2.3±0.3m，terrace T2has a horizontal shortening of1.2±0.3m andvertical uplift of3.2±0.3m. At the south bank of the Kezilesu river, south limb ofWulagen anticline, the terraces Tk1has a horizontal shortening of2.9±0.3m andvertical uplift of3.8±0.4m; Tk3has a horizontal shortening of3.3±0.3m andvertical uplift of4.6±0.4m. At south bank of Bieertuokuoyi river, terrace T3has ahorizontal shortening of8.8±0.4m and vertical uplift of10.7±0.6m. At the northlimb of the Wulagen syncline terrace T2has a horizontal shortening2.6±0.7m andvertical uplift3.9±0.9m. Except T7at east bank of the Kalanggoulvke river, northlimb of the Mingyaole anticline, vertical uplift is larger than horizontal shortening ateach terrace, indicating most flexural-slip faults are high-dip thrust faults, with faultvertical displacement greater than the horizontal displacement.5) Two flexural-slip faults at the south limb of the Mingyaole anticline coseismicslipped during the1985Wuqia Ms7.4earthquake. There exist two possible modelsof coseismic slipping: coseismic triggering and seismogenic fault slip along theflexural-slip plane. Cumulative displacement of the earthquake scarp is2-4times thecoseismic displacement, which indicates that this area could have probablyexperienced1-3approximately M7.4earthquakes before the Wuqia earthquake.5) The flexural-slip fault is a secondary， shallow and rootless structure,accompanied with the growth of a fold. It does not have the ability to produce adevastating earthquake, but has the risk of coseismic rupturing of the ground surface,so the region with flexural-slip faults should become "avoidance zone" with width of~1000m in engineering construction in the future.6) The flexural-slip fault could become an “indicator” for studying the activehistory of blind thrusts.