| The Hanzhong basin, trending in NE or NEE, formed in Late Quaternary, is located on thesouthern margin of the Qingling Mountains and end of the Qingchuan fault. It begins fromMianxian in west, extending toward east to Yangxian for100km with width nearly30km. West ofHanzhong, the Liangshan uplift continues into the basin from southwest, dividing the westernbasin into northern and southern depressions. The Hanzhong basin looks like a wedge wide inwest and narrow in east, with Quaternary sediments of thickness up to700~1000m, andremarkable difference between its east and west. As the western boundary, the Qingchuan faultenters the basin from southwest extending northeast. The Qingling Mountains stand out to thenorth. The basin is bounded by faults on its northern and southern edges, which merge in the eastwhere the basin dies out.According to the geomorphic interpretation of the image of the Hanzhong basin and fieldinvestigations, this work has identified the geomorphic types and determined different levels ofgeomorphic planes in the Hanzhong basin, of which the ages of each plane was estimated throughthe chronological study. These geomorphic planes are dominated by range-front alluvial andpluvial fans and terraces at the Hanjiang River and its branches, with ages spanning over from theearly Pleistocene to Holocene.（1）The pluvial fan in the Hanzhong basin is mainly in the southern slope of the QinlingMountains，north of Hanjiang, and different stages of alluvial fans overlapped and staggered. Afterthe reformation by the river gullies, alluvial fans have also been severely eroded. Different sizesand shapes of alluvial fans are crony-like, with nearly EW extension, covering the trailing edge ofthe Hanjiang T3terrace. The alluvial and pluvial fans are composed by the alluvial sand andgravel. The gravel is angular and sub-angular. Besides, few such fans are present on the both sidesof the Liangshan Mountains.（2）The Hanjiang outcrops T1-T5terraces in the Hanzhong basin, and the tributary ofHanjiang also developed different levels of terraces near the pass from the mountain to the basin.Geochronology of the overburden strata on the terraces and the regional stratigraphic correlation suggest that the T1terrace of Hanjiang and its tributary formed in Late Pleistocene-Holocene, theT2terrace in Late Pleistocene, T3terrace in Middle Pleistocene, and T4terrace in earlyPleistocene. As lacking proper dating methods, it is not possible to determine the age of the T5terrace, though its overlying clay bed is of Middle Pleistocene.Based on investigations of the geometry, kinematic property and the activity times of the faultsaround the Hanzhong basin, this work made an analysis of the activity characteristic of these faultsince Quaternary.（1） The north-edge fault of the Hanzhong basin, with its normal faulting and nearly EW extension,is divided into two sections according to different geometry: The western part (Mianxian-Baocheng part) cuts theT3terrace of Baohe, with15m of the vertical displacement. The active time of this section is the early LatePleistocene. The east part (Baocheng-Youshuixiang part) cuts the diluvium of the Juyuanxiang and the T3terraceof Yishuihe, which was active in middle Pleistocene.（2） The south-edge fault of the Hanzhong basin, as a normal fault stretching NE, is present onthe south bank of the Hanjiang River where the Hannan complex rock links the terrace of theHanjiang River. It cuts the T4and T5terraces as well as overlying clay beds, and was active in middlePleistocene.（3） The Qingchuan fault, a right-slip fault striking NE, lies in the southwest of the Hanzhongbasin. It enters the basin north of Mianxian, and finally intersects with the Mianlue Detailedinvestigations on its section along the Jialing River show that the Qingchuan fault offsets the T3and T4terraces and range-front slope sediments. According to the research of the geochronology ofoffset strata, this fault was active in late Pleistocene, and it exhibits high-angle normal faulting in thenorth ofYanzibian.（4） The Chaba-Linansi fault spreads on the north side of Liangshan with a NE trend. Onsatellite images, the northern section of this fault is a fairly linear feature. While in the Hanzhongbasin, it is expressed by a boundary between basement rock and sediments. High-angle normalfaulting and dipping NW characterize this fault, which becomes continuous troughs in themountainous area to southwest. It is traced by troughs at terrace T4of the Yudai river north ofHujiaba, while gradually unclear father south. By the excavation of the trench in the Yuandunzi, itwas observed that the fault dislocates the piedmont slope sediments, and should be active in latePleistocene. （5） The Liangshan south margin fault lies on the southern side of the Liangshan Mountains,striking in NEE, almost parallel to the northern section of the Chaba-Lin’an fault. In the basin thisfault exposes is rupture plane, constituting the boundary between moderate-low mountains and theQuaternary basin. In the mountainous area on the southwest side, this fault manifests ascontinuous troughs which offset gravel beds of terrace T4, range-front sliderock and overlyingclay strata of Pleistocene. Seismic exploration and drilling in the basin suggest that this fault was anormal one dipping due south being active in Late Pleistocene time. In the southwest mountainousarea, this fault dips NW, also high-angle normal faulting, just opposite to its section in the basin.The analysis of the geomorphic features in the Hanzhong basin and the investigations of thefault activity surrounding the Hanzhong basin reveal that tectonic activities differ perceptiblybetween its east and west since Quaternary.（1） The temporal difference of the fault activity on the basin’s rim. The west section of the north-edgefault of the Hanzhong basin (east of Baocheng) was active in early Late Pleistocene, while itseastern section (west of Baocheng) was active was in middle Pleistocene. The south-edge fault ofthe basin was also active in middle Pleistocene. And the three faults in the southwest of the basinwere all active in late Pleistocene.（2）The difference of the thickness of basin sediments. The Hanzhong basin is a Late Cenozoic basin,hosting dominant sediments of Quaternary, which are thin in the east and thick in the west. For instance, theQuaternary System is merely100~300m thick east to Hanzhong, while as thick as700m and900m north and southof Lingshan, respectively, both in the western part of the basin. The deposit center lies north and south of Lingshanin the western basin, where Quaternary sediments become thicker from east to west.（3） The difference of the distribution of the alluvial and pluvial fans in the Hanzhong basin.In the western basin, the alluvial and pluvial fans become older southward gradually from theQinling Mountains, and most of the late Pleistocene fans developed near the Qinling, The MiddlePleistocene fans is far away from the Qinling. Those alluvial and pluvial fans in the easternHanzhong basin are of Middle Pleistocene in age.（4）The difference of the distribution of recent small earthquakes within the basin. Smallearthquakes within the basin mainly occur around Liangshan in the west side of the basin, near the Yangsan area,showing planar distribution.Afew of small earthquakes occurred in the east part of the basin.This work has collected17apatite samples for fission-track analysis from the Yunwushan, Middle Qinling. The age-elevation plot indicates that within the elevation range600~2400m, theirages are around60Ma. The average length of closed tracks is14.06~14.80m, with standarddeviations0.09~0.12. These results suggest that the rock bodies at the Yunwushan, Middle Qinlinghad experienced a rapid cooling (exhumation) stage at60Ma.By comparison of previous work on apatite fissure-track and (U-Th) He dating in the northQinling Mountain region and North China, this thesis suggests that the uplift of the Qinling has anobvious characteristic of tilting southward. It is expressed by a large rise amplitude in the northand a small rise in the south, with the center of the uplift located on the north side of the Qinling,near the Weihe basin. This tilting uplift may be related to the extension of the North Qinling andthe activity of the north-edge fault in the Qinling.From apatite fission-track age data, the North Qinling area recorded thermal history of middleto late Cenozoic, while the South Qinling area recorded its thermal history of early Cenozoic.Thus, the thermal history of the whole Qinling area since Cenozoic may include three stages.(1)Rapid cooling (exhumation) process at~60Ma, which started from an uncertain time, and ended at~50Ma. Both the North and South Qinling have experienced this process, but it was recorded bythe South Qinling, likely because of later strong uplift in the North Qinling.(2) Relatively slowcooling (exhumation) process during48.3~9.6Ma, which was recorded by the North Qinling, butnot by the South Qinling, indicating that the relevant rock did not uplifted to the surface in theSouth Qinling.(3) Rapid cooling (exhumation) period since9.6Ma, which was recorded only bythe North Qinling.This work made a reanalysis of the samples from the Micang Shan-Hannan uplift and suggeststhat the age of the north slop of the Micang shan and the Hannan uplift are in accordance. Thenthis thesis infers the uplift of the Micang Shan has the characteristic of northward titling. A rapiduplift event of～50Ma was also recorded at the Micang area, South Qinling, which was likely theconsequence of southward under thrust of a fold zone that resulted in the northward tilting uplift.Because of the southward titling uplift in the Middle-North Qinling and the northward titlinguplift in the South Qinlig, a valley formed in the Hanhong area, then the Hanjiang began todevelop in this valley. At the early stage, the Hanjiang possesses the broad valley by the planationsurface of Hanjiang on the Hannan complex body, and it extended from the south slope of theQinling to the North slope of the Micang Shan. Due to the slow uplift of the Micang Shan, the Hanjiang migrated gradually to the north area. Due to the development of the Hanzhong basin, thefaults began to be active as normal faulting, and led to rifting in the Hanzhong area. The HanjiangRiver shifted gradually to the inside of the Hanzhong basin. The continue uplift of the Qinling inQuaternary caused the development of large scale pluvial fans in the Hanzhong basin. Thesouthward tilting landform made the Hanjiang River flow along the south of the Hanzhong basin.Since the neotectonic movement, with rapid uplift of the eastern margin of the Tibetan plateau,the Longmenshan rose in12~5Ma and south edge of the Qingling uplifted in9~4Ma, respectively.These tectonic motions affected the Motianling block, and the junction area between Dabashanand Qingling. Right strike-slip and left slip strike-extension dominate the Qingchuan fault andMianlue fault, respectively, serving as the dynamic setting of the Hanzhong basin. Because ofstrike-slip motion of the Qingchuan fault and Mianlue fault, compression and uplift occurred inthe Motianling block west of the triple junction, while extension and subsidence took place in thesouth including the Hanzhong basin. Consequently, normal faults appeared on northern andsouthern edges of the basin. From Pliocene to early Pleistocene, with rapid rise of the MingshanMountains in west, the northeastern Longmenshan fault zone become less active, the Mianluefault changed into dominant left-slip strike with compression, and the Hanzhong basin stopped todevelop. By middle and late Quaternary, due to differential uplift of the Qingling in north andDabashan in south, the Hanzhong basin continued to receive sediments, resulting in Quaternarystrata of varied epochs. When activity on the faults within the basin continued at different rates,the sedimentary center of the Hanzhong basin shifted to west, and the current tectonic pattern ofthe basin was shaped up.According to the research of the Quaternary tectonic activity characteristics of the Hanzhongbasin, the investigate of the development mechanism, the reveals of the Cenozoic cooling historyin the Qinling area, the tectonic evolution of the Hanzhong basin is summarized as follows:（1）At the early Cenozoic（～50Ma）, due to the southward titling uplift in the Middle-NorthQinling and the northward titling uplift in the South Qinling, a valley landform formed in theHanhong area, then the Hanjiang River began to develop in this valley.（2）At the late Cenozoic（9～4Ma）, influenced by the eastward expansion of the Tibetanplateau, the Qingchuan fault was active as dextral strike-slip fault and the Mianlue fault active as asinistral strike-slip fault, respectively; the Hanzhong basin was situated at the triple junction of the intersection of the two faults. The sltrike-slip of the two faults formed a squeezed rising land areain the west of the Motianling block and tensile subsidence zone in the south of the Hanzhongbasin. The Hanzhong basin began to develop in the triple-junction. The basin-control faults beganto develop and be active as normal faults.At the Pliocene, as the Minshan in the west rose rapidly, the Longmenshan fault lose theposition as the boundary of the active bock. The Mianlue fault converted into sinistral strike-slipwith compression. The Hanzhong basin missed the original dynamic setting, and transformed intothe slow development stage. The outline of the Hanzhong basin bas been formed before thePliocene.At this stage, the faults around the Hanzhong basin remained active, leading to continualincrease of the north-south span of the Hanzhong basin. The Hanzhong basin received steadilysediments with its deposit center at the center of the basin. Since the Quaternary, affected by theuplift of the Qinling, the Hanzhong basin gradually developed alluvial fans at the piedmont of theQinling, then the landform in the basin tilted southward. Since the late Pleistocene, the boundaryfaults of the Hanzhong basin stopped activities gradually, meanwhile the three faults in thesouthwest of the basin were still active. Due to activity of the three normal strike faults, the spanof the west part increased and the sedimentary center shifted to the west of the Hanzhong basin.Finally, the basin had a geometry of wedge as seen at present. Entering the Holocene, as the faultsaround the basin stopped activities, the development of the Hanzhong basin wass mainlyinfluenced by the uplift of the Qinling Mountains. Since the Holocene, the Qinling Mountainsentered to the rapid uplift stage, and the landform tilted more to the south.It seems that the current active boundary of the Bayan Hara block has shifted to themiddle-southern section of the Longmenshan fault zone, Mingshan uplift, and East Kunlun faultzone, but not including the northeastern section of the Longmenshan fault zone. The possiblemechanism for this change is that the Mingshan uplift received most of the compression from theeastward motion of the Bayan Hara block, resulting in thrust and uplift of Mingshan, while thenortheastern section of the Longmenshan fault zone was little affected. A chronological studysuggests that the Mingshan Mountains began to rise rapidly at5~3Ma, consistent with theestimation of late Pliocene to early Pleistocene.As the eastward motion of the Bayan Hara block is hampered by the South China block, thrust deformation occurred along the middle-southern Longmenshan fault zone and Mingjiang Riverfault zone in early and middle late Cenozoic time. Rapid uplift and cooling happened in11~5Maon the Longmenshan and in and around Mingshan in5~3Ma, indicative of a gradually youngertrend of cooling age from the Longmenshan near the Sichuan basin toward the northwest. Sincethe Pliocene, the Mingshan area has been in an uplift state, capturing the driving force for thrust ofthe northeastern Longmenshan fault zone. In other words, the deformation of the Mingshan upliftabsorbed most of the eastward motion of the Bayan Hara block, and made the Qingchuan,Chaba-Linansi, and Liangshan south edge faults less active since the Quaternary. Thus few majorearthquakes have been documented there. As these three faults have been active to some extentsince the late Pleistocene, they are still capable of generating moderate earthquakes, though areunlikely to spawn great events. |
PDF Full Text Request