A Study On The Late Quaternary Faulting Of The Huoshan Piedmont Fault Zone In The Central Shanxi Faulted Basin Belt

The Huoshan piedmont fault zone, lying on the west slope of the Mts.Huoshan, is animportant boundary active fault in the middle of the Shanxi faulted basin belt (also named theShanxi graben system or Shanxi rift system). It is the seismogenic fault of the1303HongdongMs8.0earthquake, the first event of M≥8.0in historical documents in China. The quantitativestudy of its geometry, tectonic geomorphology, faulting features, and paleo-earthquakes, is veryimportant to understanding the active history of the Shanxi faulted basin belt in the lateQuaternary as well as the earthquake disaster mitigation efforts in Shanxi Province.This thesis focuses on applications of field investigations, comprehensive remote sensinginterpretation, the paleo-earthquake method, AMS14C dating and DGPS survey, GIS technologyto research on the Huoshan piedomont fault zone in the late Quaternary. The purposes are:1) toreveal the intense normal faulting in the Cenozoic as the northeastern margin of the Linfen basin,of which the southern section located in Hongdong county and Huozhou city and northernsection located in Lingshi County and Jiexiu city have consistent active features, and2) showthat the fault is mainly characterized by at least four paleo-earthquake events during latePleistocene, of which, the latest event corresponding to the1303Ms8.0Hongdong earthquakebased on5combined trench analysis and35AMS14C dating ages providing constraints on thegiant-earthquake recurrence interval of about2000a,3) describe continuous distribution ofdifferent levels of fault triangles, quantitative river geomorphic parameters in the surroundingMts. Huoshan areas and fault plane occurrence and scratches collected during field work tosupport the intensive normal faulting of the fault with a dip-slip characteristics during lateQuaternary,4) analyze the main causes for horizontal displacements along and across faultdrainages combining the arid and semi-arid climate factors and loess hilly geomorphology, and5)discuss the dynamic model based on field data and previous studies. The major research resultsof this thesis are presented below.(1) Geometic characteristics of the Huoshan piedmont fault zoneThe Huoshan piedmont fault zone, a normal faulting from Pliocene as a Basin and Rangeboundary, located in the northeastern Linfen basin, can be recognized by significant terraindifferences between the mountains and basins. Based on comprehensive remote sensinginterpretation using high resolution satellite images such as IRS-P5and CBERS-02B data, aerial photographs and fieldwork investigations, this fault has a northeast trend, dipping northwest,extends for more than116km, along the west foot slope of the Mts. Huoshan. Starting from thesouth, it extends by Subu town, Guangshengsi temple, Xingwangyu village, Shimenyu Village,Baiting village in Hongdong county, continues along the Shawo village, Guandui village, Licaotown, Sanjiao town, and Liwan village in Huozhou City, through the Renyi river, along the westfoot slope of the mountain in the Lingshi county, and terminates at Longfeng town in Jiexiu city.According to the geometric distribution characteristics, the Huoshan piedmont fault zonecan be divided into2sections, the north section and the south section, each section can bedivided3segments further, that is A segment (Subu-Guangshengsi section), B segment(Guangshengsi-Guandui section), C segment (Guandui-Yangjiazhuang section) of south part, andD segment (Liwan-Nancheyao section), Esegment (Nancheyao-Huokou section), and F segment(Huokou-Longfeng section) of north part. In all of6segments, B, D, and E strike in NNE, C andF has strike in NE, A segment has sand liquefactions and giant landslides triggered by the1303history earthquake, but with little buried fault planes, B segment shows a significant linearfeature on images and aerial photos, with the characteristic of cutting all geomorphic units bothsides, and geomorphology such as fault scarps, trough valley, fault triangular ridges, which is theright place for trench work.(2) Tectonic geomorphology characteristics of the Huoshan piedmont fault zoneUsing aerial stereo image pair, DEM data, fieldwork investigations, spatial datingprocessing softwares, on the basis of previous research results, the tectonic geomorphologyfeatures along the fault zone such as terrain, geology, crossing-fault river system distribution,quantitative river geomorphic parameters, fault triangle shape and etc. have been studied andmany geomorphology maps were prepared in order to understand the interaction betweentectonic activity and geomorphology features.1. The main ridge of Mts. Huoshan strikes in north-northeast, its southern end is locatednear Subu town in Hongdong County, and north end located near Longfeng town in Jiexiu city.The Huoshan piedmont fault zone on the west slope of the mountain, constitutes the boundarybetween bedrock and unconsolidated sediments, and lies at the position of highly variablerange-front terrian gradients.2. This work identified a total17first level fault triangles and nearly100sub-fault trianglesfrom south to north. Each first level fault triangle is split by the main river basins. The differentscale fault triangles can be characterized as the newer and smaller ones nested in the older ones,those with similar forming eras have similar morphology and almost same vertex elevation andbase elevation, showing a regular distribution from south to north. The features of the trianglesalso show that intense tectonic uplift of the Mts. Huoshan and normal faulting of the fault sincePliocene. There are many loess hills nearby the fault, where surface erosion is limited by thescale of triangles, indicative of weak hydrodynamic conditions. It can provide an importantcandidate sites to combined trench excavating for paleo-earthquake identification.3. Analysis of planar forms of more than400crossing-fault rivers of different scales shows that that all river from south to north are not consistency offset (right-laterally or left-laterally)by the faulting. In other words, river channel features in the study area does not support theobvious horizontal movement of the fault. The main river channels with source near the Mts.Huoshan ridge are all not offset by lateral faulting movement. Some of them, the channelsections near fault traces run along the trough valleies. The flat distribution of the othersecondary rivers located at the different levels fault triangles is complicated by relative weakerhydrodynamic conditions, loess hill deposition, some of which can be seen as right-offset, someof them can also be seen like left-offset, and the rest has no lateral offsets. Its cause is the naturaldevelopment process of river basins of different scales, and can not serve as the evidence for thedextral movement of the Huoshan piedmont fault zone.4. This work selected48rivers around the Mts. Huoshan. These rivers are divided into fourgroups according to their tectonic settings: rivers north of, across and south of the Huoshanpiedmont fault zone on the west of the Mts. Huoshan, and rivers on the east slope of themountains. Based on the ASTER GDEM of30m resolution, this work calculated a series ofquantitative fluvial geomorphic parameters, such as the longitudinal profile style, streamlength-gradient index (SL index), standard stream length-gradient index (SL/K index), Hackprofile, hypsometric integral curves and its values (HI). There is a significant correlation betweenthese parameters and tectonic environment. To the rivers across the Huoshan piedmont fault zone,the longitudinal profiles of upstream and downstream, divided by the fault, show significantlydifferent features from each other, demonstrating obvious characteristics controlled by faultactivity. Along the rivers in other tectonic domains, the longitudinal profiles performance anapproximate exponential curve, indicative of a near-equilibrium. The HI values and Hypsometricintegral curves of all river basins indicate that all the rivers are in the prime of life or childhood,that is, Mts. Huoshan is in the state of strong uplift and weak erosion overall in Cenozoic era.5. Convex Hack profiles of all rivers also show that Mts. Huoshan is in the stage of intensetectonic uplift in the Cenozoic, while there exist significant differences among Hack profiles ofrivers in different tectonic domains reflecting different tectonic activities. The river segmentswith abnormally high SL/K values usually corresponded to the normal faulting, while randomappearance of high SL/K values on some rivers without fault-cut corresponds to the bedrock withlittle effect of weathering.(3) Paleo-earthquake events of the fault and surface rupture zone of the1303Hongdong M8.0earthquakeFrom analysis to trenches Tc01-Tc05in south section of B segment, this work obtains thefollowing knowledge:1. According to regional stratigraphic correlation, the strata formation in the edge of theLinfen basin and Taiyuan basin constituted by loess and talus accumulated since Pleistocene, anddeveloped paleosoil in loess strata, especially during the Holocene with suitable climate. Thelatest paleosoil layeres in the top loess deposit can be identified in the edge or center of thebasins, which reflect the regional deposit characteristics. 2. There are at least four paleo-earthquakes identified during late Pleistocene and Holocenetimes, every event is characterized by normal faulting of the main fault and/or accompanied bysecondary sub-faults activities on the hanging wall. In the four events, the oldestpaleo-earthquake occurred at12520~26380a B.P., the other three events occurred at5370-5808aB.P.,2650-3465a B.P., and709a B.P., respectively. The three events in late Holocene show theearthquake recurrence interval of about2000a.3. Among the three paleo-earthquake events occurred at late Holocene, i.e. E1:1303a, E2:2650-3465y B.P., and E3:5370-5808y B.P., the E2and E3events have the similar seismicintensity with the E1earthquake based on comparative analysis of the paleoseismic wedges anddip slip amounts.4. The Huoshan piedmont fault zone is dominated by dip slip, and its deformation traces bythe1303giant earthquake concentrated in the south section, such as earthquake landslides, sandliquefaction, collapses, and tension fissures etc. Based on the latest displacement and faultprofiles of the fault, the surface rupture zone of the1303Hongtong M8.0earthquake is116kmin length, approximating length of isoseimal long axis of X degree area. From the Subu inHongtong County in south section, end at the northern Longfeng town in Jiexiu city, the latestsurface rupture can be traced by latest wedge and14C dating ages in every trenches. The heightof the fault scarps is mostly between6-10m reflecting accumulated vertical displacements bymany earthquake events, with the average vertical displacement of about2m by every earthquake.The elapsed time of the history earthquake is709year to2012A.D. The surface rupture zoneproduced by the earthquake has been completely ruined by human activities and erosionprocesses, thus it is difficult to confirm and distinguish from the earlier events’ displacement. Sotrench paleoevent analysis was as the primary means to resume faulting history in this thesis.(4) Active faulting manner, slip rate and dynamic characteristics of the Huoshanpiedmont fault1. As previously mentioned, the Huoshan piedmont fault zone is dominated by dip-slip witha small amount of the horizontal component. Using24groups of data of fault scratches on freshfault planes from south to north, with the side rake angles concentrating in75°-85°, and thiswork calculated that the vertical component is3-10times than the horizontal component. Thisview is different from previous studies. In fact, statistical analysis to more than400streamsacross the fault indicates that most rivers with right-or left lateral.2. Based on vertical dislocation of key layers exposed on trench Tc01, Tc03and Tc04profiles and their forming ages by AMS14C dating, the vertical slip rate is0.76-1.49mm/a duringthe late Holocene.3. The Cenozoic faulting of the Huoshan piedomont fault zone is consistent with the patternof regional tectonic activity. The formation and development of the active zone systems aroundthe Ordos block are closely related to asthenospheric upwelling, which has been confirmed by alarge number geophysical data. Asthenospheric upwelling is a major dynamic source fordevelopment of all faulted basins and boundary fault activities. Previous geophysics also have confirmed the upwelling of deep materials beneath the Linfen basin and surrdounding, and theupwelling is also the main source of the normal faulting of the Huoshan piedmont fault zone.