| The Zhongwei fault zone is one of the typical strike-slip fault zones on the northeastern margin of the Qinghai-Tibetan Plateau, being the boundary fault zone among the Qinghai-Tibet, Ordos and Alashan blocks. The kinematic study on the Zhongwei fault zone, therefore, is of great importance to the understanding of the kinematic features of the other strike-slip faults and the evolution of the Qinghai-Tibetan Plateau.1. Cenozoic strata and their deformation Stratigraphic sequences record authentically the development and evolution history of geologic structures. Therefore, the investigation and analysis of the spatial distribution, lithofacies variation and tectonic deformation of stratigraphic sequences can provide an insight into the tectonic evolution of the studied region. According to the lithofacies, stratigraphic contact and tectonic deformation of the stratigraphic sequences, the tectonic evolution process of the studied region, where the Zhongwei fault zone developed, can be divided into four stages: (1) In the early Eogene period, the Xiangshan block was situated in a gentle uplift and denudation state. At that time, the activity of the Zhongwei fault zone was not prominent, and the Paleocene series is lacking for the region. (2) In the Eocene epoch, faulting and differential movement among blocks became stronger. From Oligocene to Pliocene, however, the tectonic activity had become weaker, and sediments had changed from coarse-grained to fine-grained. (3) At the beginning of the Quaternary, the tectonic uplift of the Xiangshan block became strong again. The pluvial gravel beds developed along the piedmont of the Xiangshan Mountains and in the Hongguanguan area on the west end of the Zhongwei fault zone. The fluviolacustrine sediments of about 600m thickness were deposited within the Weining basin. One tectonic event that occurred in the late middle Pleistocene has caused strong deformation of the early-middle Pleistocene strata on both sides of the Zhongwei fault zone. (4) Since the late Pleistocene, the active behavior of the Zhongwei fault zone has been significantly changed from the previous compressive overthrusting to the later left-lateral strike-slipping. The tectonic uplift has become weaker during that time.The tectonic evolution in the first three stages was characterized mainly by intermittent uplifting, and the compressive tectonic stress field was nearly NS oriented. The evolution in the fourth stage was characterized mainly by EW-trending extensional movement, and the compressive tectonic stress field was NE-NEE oriented. Therefore, the tectonic movement since the late Pleistocene was of epoch-making significance.2. Geometric features of the Zhongwei fault zoneIn the early period, the activity of the Zhongwei fault zone was dominated mainly by overthrusting, resulting in a forward-propagating thrust nappe tectonic zone, consisting of a series of faults. In the late period, the fault zone was characterized by left-lateral strike-slipping, and its geometric structure was relatively simple. The left-lateral strike-slip followed, inherited or cut off the previous fault zone, so that the early geometric structures of the fault zone are considerably different from the late geometric structures. The early period refers here to the time from the late Pliocene to the middle Pleistocene, and the late period to the time from the late Pleistocene to the Holocene.In the early period, the Zhongwei fault zone appeared geometrically as a NE-convex arcuate structure, consisting of a series of faults. Shallow seismic prospecting and field investigation have revealed that the zone appears as an imbricate structure in cross sections. In the late period, the zone appeared as a NWW-trending linear structure on plane view. The fault planes are smooth and straight, dipping steeply. A pull-apart step-over was developed at Baidunzi on the western end of the fault zone, while swallow-tail-like or bird-claw-like branched faults were developed in the vicinity of Shiquan on the eastern end of the fault zone. Individual fault segments are connected by step-overs of various types. 3. Classification of fault types and its tectonic implicationThe left-lateral strike-slip movement of the Zhongwei fault zone since late Pleistocene has caused the differentiation of preexisting overthrust faults in the zone. Some early-formed faults or fault segments continued to be active, while the others have become inactive. In addition, some new faults were developed since then. The faults in the Zhongwei fault zone, therefore, can be classified into three categories: the newly-generated, the inherited and the rejected faults.The newly-generated fault refers to the fault that is developed newly during a certain tectonic movement. With regard to the Zhongwei fault zone, it refers to the one which has developed since late Pleistocene. Such a fault is the result of the left-lateral strike-slip movement of the Zhongwei fault zone, and did not exist during the early compressive overthrusting movement. The investigation on these newly generated faults may provide the following information: (1) the feature of tectonic stress field since late Pleistocene; (2) the starting time of the late tectonic movement, and (3) the displacement amount and slip rate of the fault.The inherited fault refers to the fault or fault segment that has existed before the late left-lateral strike-slip movement of the fault zone and has been still active after the movement. The prominent advantage of the inherited fault is that the fault contains a lot of tectonic information: (1) the inherited faults recorded the information of multiple tectonic movements; (2) they are the witness to the multiple tectonic movements along the Zhongwei fault zone; and (3) they are the important basis for the study of the tectonic evolution history.The rejected fault refers to the fault or fault segment that was the part of the main fault zone and behaved in the same way as the main fault zone during the early tectonic movement. After the early tectonic movement, the fault or fault segment has become inactive during the subsequent tectonic movement, indicating that it was rejected. The fault may reserve most or all of the information about the early tectonic movement, which is basically not disturbed or destroyed by the late tectonic movement. The investigation of rejected faults, therefore, may provide the following essential information about the early tectonic movement: (1) the ceasing time of the early tectonic movement; (2) the feature of the early tectonic stress field; and (3) the mode of faulting, i.e. stick-slip or creep-slip.4. Geological evidence and time of transformation of fault behaviorBased on field investigation, age dating and analysis, this work tries to search for geological evidence and the occurrence time of the transformation of the behavior of the Zhongwei fault zone from various aspects, including the tectonic deformation of stratigraphic sequences, the mode of faulting, fault striae, tectonic geomorphology and geophysical prospecting.4.1. FaultThe feature of faulting is the most direct evidence of the transformation of the behavior of the Zhongwei fault zone. The investigation of the rejected faults reveals that the Zhongwei fault zone was dominated by compressive overthrust napping in the early period, resulting in the development of a series of overthrust nappes and imbricate fault structures. The early activity of the fault zone ceased at about the late stage of the middle Pleistocene. The studies of newly-generated faults show that in the late period the Zhongwei fault zone was dominated mainly by left-lateral strike-slipping. The faults in the fault zone were steeply dipping, while the fault planes were smooth and straight, offsetting left-laterally the strata that were formed since late Pleistocene. The left-lateral strike-slip movement can be determined to initiate at about the early stage of late Pleistocene. The studies on inherited faults indicate that the Zhongwei fault zone was dominated by overthrusting in the early period, and by left-lateral strike-slipping in the late period. At the Yanwangpo site, three reverse faults that were formed in the early period offset the early-middle Pleistocene strata, and terminate below the bottom surface of the late Pleistocene strata. The other three faults that were formed in the late period offset the late Pleistocene and Holocene strata, appearing as left-lateral strike-slip faults. At the Dongdagou site, the left-lateral strike-slip fault that was formed in the late period dissects the overthrust fault that was formed in the early period.4.2. Fault striaeDuring field investigation, 39 sets of fault striae have been measured. Among them, the 20 sets were measured from newly-generated faults, 10 sets from rejected faults and 9 sets from inherited faults. The obtained data show that: (1) Horizontal or nearly horizontal fault striae are mostly developed on newly-generated faults, indicating the left-lateral strike-slipping of the Zhongwei fault zone since late Pleistocene. (2) Along-dip fault striae are developed mostly on rejected faults. All of the striae show that the Zhongwei fault zone was dominated mainly by compressive overthrusting in the early period, and the compressive tectonic stress field was nearly NS-oriented. (3) Both horizontal and along-dip fault striae are developed on inherited faults, indicating that two different modes of faulting had existed on the Zhongwei fault zone, i. e. the compressive overthrusting in the early period and the left-lateral strike-slipping in the late period.4.3. Tectonic geomorphologyA series of offset gullies, deformed pluvial fans and displaced river terraces along the Zhongwei fault zone indicate from various aspects the left-lateral strike-slipping of the fault zone. Tectonic geomorphology, such as fault scarps, piedmont benches and local uplift, reveals the behavior of the Zhongwei fault zone both in the early and late periods. The piedmont benches were formed in the late stage of the middle Pleistocene, that is about 150 ka B.P. It can be deduced, therefore, that the early overthrusting along the fault zone might have ended before 150 ka B.P. As compared with the Yellow River terraces, it can be postulated that during the period from 215 ka B.P to 124 ka B.P., the Zhongwei fault zone was situated in a stable state, and the left-lateral strike-slipping along the fault zone might initiate at about 124-100 ka B.P.5. Quaternary tectonic stress fieldAccording to the aforementioned studies, it can be concluded that the kinematics of the Zhongwei fault zone has once been transformed during the middle-late Quaternary, i.e. from the early compressive overthrusting to the late left-lateral strike-slipping. This means that the tectonic stress field has been correspondingly changed. According to the actually measured data of the tectonic deformation of strata, fault striae, structural joints, newly-generated and rejected faults, the tectonic stress field has been reconstructed by using stereographic projection method. The results show that there are two different states of tectonic stress fields. The early state occurred from the Neogene to the middle Pleistocene, during which the compressive tectonic stress field was nearly NS-oriented. The late state has initiated since late Pleistocene, during which the compressive tectonic stress field was NE-NEE oriented. The occurrence time of the transformation of the tectonic stress fields can be determined to be at the time between the late stage of the middle Pleistocene and the early stage of the late Pleistocene.6. Essential kinematic features of the Zhongwei fault zoneDuring the early period, owing to the action of the nearly NS-orienting compressive tectonic stress, the whole Zhongwei fault zone was dominated by northward overthrusting or obducting movement. As a result, the Xiangshan block was uplifted. During the late period, owing to the action of the NE-NEE-orienting compressive tectonic stress, the fault zone was dominated by left-lateral strike-slipping. As a result, a series of gullies across the fault zone were offset left-laterally. The period of transition from the early compressive overthrusting to the late left-lateral strike-slipping of the Zhongwei fault zone lasted from the late stage of the middle Pleistocene to the early stage of the late Pleistocene, being tectonically a relatively stable stage. The evolution process involves the early compressive stage→the transition stage→the late left-lateral strike-slipping stage. It not only has caused the change of the assemblages of the faults, but also shaped the corresponding tectonic landform in each stage.According to the aforementioned studies, the main kinematic features of the Zhongwei fault zone can be summarized as follows:(1) Since Quaternary, the Zhongwei fault zone has experienced three tectonic movement stages. The first stage was in the early-middle Pleistocene. During the stage, owing to the action of the nearly NS-orienting compressive stress, the movement of the fault zone was characterized by overthrusting. The second stage lasted from the end of the middle Pleistocene to the beginning of the late Pleistocene. In this stage, the regional compressive stress field was changed from nearly NS-orienting to NE-NEE-orienting, while the movement of the fault zone was transited from overthrusting to left-lateral slipping. The activity of the faults was relatively stable. The third stage lasted from late Pleistocene to Holocene. In this stage, the compressive tectonic stress field was transformed from nearly NS-orienting to NE-NEE-orienting, and correspondingly the fault movement was changed from overthrusting to left-lateral strike-slipping.(2) The early overthrusting on the Zhongwei fault zone has initiated not later than late Pliocene. On the basis of the estimation of elevation difference between the mountains and the river, it can be postulated that the Xiangshan block has been uplifted by about 544m, the thickness of the crust has increased by about 4352m, the uplift rate was about 0.18mm/a, the distance of horizontal napping was in the range of 7540-11960m, and the crustal shortening rate in NS direction or the overthrusting rate of the fault zone was about 2.51-3.99 mm/a. According to the uplifted height of the Yellow River terraces in the Heishan Gorge, it can be estimated that the average uplift rate since 1560 ka B.P. is 0.19 mm/a. In the early period, the Zhongwei fault zone was characterized mainly by overthrusting movement. Relatively large scale napping has led to crustal thickening and the uplift of the Xiangshan block. The overthrusting process was intermittent, so that multiple river terraces were formed.(3) The left-lateral strike-slipping on the Zhongwei fault zone initiated at about the time between the end of the middle Pleistocene and the beginning of the late Pleistocene. The movement initiated earlier on the middle-eastern segment (from Mengjiawan to Hongguliang), i.e. at about 150 ka B.P., and initiated later on the western end of the fault zone (from Yingpanshui to Hongguanguan), i.e. at about 120-100 ka B.P. As estimated from the piedmont benches, the amount of strike-slip is 274-794m, the strike-slip rate is about 2.28-5.29mm/a, the amount of vertical displacement is about 48.3-97.5m, and vertical slip rate is about 0.37-0.75mm/a. According to the local upwarps along the Zhongwei fault zone, it is estimated that the amount of the horizontal displacement is about 396-656m, and the strike-slip rate is 3.05-5.47 mm/a. According to the left-lateral offset of gullies, strata and stratigraphic unconformity interface, the left-lateral strike-slip rate is estimated to be 1.08-3.9 mm/a.(4) The late left-lateral strike-slipping along the Zhongwei fault zone was accompanied by the formation of the corresponding pull-apart step-overs or compressive step-overs. The left-lateral strike-slip on the southern and northern walls of the fault zone resulted in the compressive uplifting on the front and the tensional depression at the back of the blocks.7. ConclusionsOn the basis of the results of geological and geomorphologic investigations, age dating and analyses, incorporating with the available data from previous work, some new insights into the kinematics of the Zhongwei fault zone during late Cenozoic, especially the middle-late Quaternary, can be gained as follows:7.1. Three stages of tectonic evolutionThe first stage initiated in Eocene and ended in Pliocene. In this stage, all of the stratigraphic contacts between the strata of various ages were mostly conformable or parallelly unconformable. The sedimentary environment was changed from the initial alluvial-proluvial facies into the subsequent fluviolacustrine facies. The tectonic movement was transformed from the early strong uplifting to the late denudation and planation process.The second stage lasted from the beginning of the early Pleistocene to the late stage of the middle Pleistocene. In this stage, the conformity or parallel unconformity occurred between the lower Pleistocene and the middle Pleistocene series. The stratigraphy of the early Pleistocene consists mainly of alluvial-proluvial facies along the mountain foot, and the stratigraphy of the middle Pleistocene consists basically of aeolian loess and alluvial-proluvial sediments. During this stage, the tectonic deformation of stragtigraphic sequences was relatively strong. For example, the drape-like folds involving the early-middle Pleistocene strata were developed at the Huabaowan and other sites.The third stage lasted from the late Pleistocene to the Holocene. The upper Pleistocene and the Holocene series are in conformable contact, and locally in parallel unconformable contact. The stratigraphy consists mainly of aeolian loess, and alluvial-proluvial sediments and slope wash take the second place. In this stage, tectonic deformation of strata is considerably slight.7.2. Two important geologic interfacesTwo important geologic interfaces of angular unconformity were developed in the studied region. The first geologic interface was developed between the late Pliocene and the beginning of the early Pleistocene. The second geologic interface was developed between the late stage of the middle Pleistocene and the beginning of the late Pleistocene.7.3. Two stages of fault zone movementAccording to the regional compressive tectonic stress field and the behavior of the faults, the movement on the Zhongwei fault zone can be divided into two stages. The first stage lasted from Neogene to Middle Pleistocene. In this stage, the compressive tectonic stress field was nearly NS-orienting. The Zhongwei fault zone was dominated mainly by compressive overthrusting, resulting in a series of imbricate faults, overthrust nappe-fold and overthrust faults. Multiple along-dip and oblique fault striae were developed on the fault planes. Morphologically, thrust outliers were formed. This stage was also the main uplifting stage of the Xiangshan block. The second stage lasted from late Pleistocene to Holocene. The compressive tectonic stress field was NE-NEE-orienting. The Zhongwei fault zone was dominated mainly by left-lateral strike-slipping. The fault plane is smooth and straight, dipping steeply. The horizontal or nearly horizontal fault striae were developed on the fault planes. The compressive or pull-apart step-overs were formed between the fault segments. On the southern and northern sides of the fault, compressive uplifting occurred in the front and the normal faulting occurred in the back. Morphologically, the gullies across the faults were offset left-laterally. In this stage, significant change has occurred on the Zhongwei fault zone. Some previously active faults continued to be active and had become inherited faults. The others were no longer active and had become rejected faults. At the same time, some new faults were developed.7.4. One transformation stageThe Zhongwei fault zone experienced a transition stage from the early overthrust napping owing to the action of nearly NS-orienting compressive tectonic stress to the late left-lateral strike-slip faulting owing to the action of NE-NEE-orienting compressive tectonic stress. This transition stage was between the late stage of middle Pleistocene and the early stage of late Pleistocene, i.e. during about 200-100 ka B.P.
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