Late Quaternary Behavior Of The Active Tectonic System In The Urmuqi Transform Region Of The North Tianshan

The North Tienshan piedmont tectonics has obvious E-W variations near Urumqi.The characteristics of the tectonic deformation on the surface are complicated here.Normal faults, reverse faults, and strike-slip faults are co-existing. This researchattempts to minimize the uncertainties in understanding of the late Quaternaryactivities of the tectonics, the relationship among the structures, and deformationmechanism of this area.The main work and approaches are follows:(1) Construction of the time scale of the geomorphic evolution in the study areaThe Quaternary geomorphic surfaces developed well in the study area.Topographic analysis by 3S techniques and field surveys were performed toinvestigate the geomorphic surface. Based on that, regional geomorphic surfaces weredivided and dated. Coupling among the tectonics, climate, and earth surface processwas analyzed. Then the time scales of the geomorphic evolution in the study areawere established.(2) Research on active faults and paleoearthquakes at several typical sitesIn order to minimize the uncertainties of the study, a landform-oriented mappingtechnique at 1:50,000 scale for investigating the activity of faults and a fewapproaches for identifying paleo-events recorded by the pressure ridge scarps weredeveloped. These approaches and time scale of the geomorphic evolution were usedto study the active faults and paleoearthquakes at several typical sites.(3) Analysis of relationship among the major active structuresIn the view of the tectonic system and tectonic process, spatial and temporalrelationships of the main active tectonics are analyzed. Then the mechanism oftransform of the tectonic styles of the north Tienshan piedmont thrus system arediscussed.(4) study of the late Quaternary behavior of the main structures in the Urmuqitransform regionBased on the fault activities and paleoearthquake research results at eachsurveyed sites and the temporal-spatial relationship among main active structures inthe study area, late Quaternary behaviors of the active tectonic system in the Urmuqi transform region were analyzed. Besides, two suggestions were given to the divisionof the seismic tectonic units and to prediction of the future seismic risk.The main results are as follows:(1) Five regional geomorphic surfaces are determined to define time scales of thegeomorphic evolution.Regional geomorphic surface V was deposited at about 730~550ka B.P.Intermittent uplifts occurred between 550ka B.P. when an interglacial period started,and 200ka B.P. during which multiple high pedestal-terraces were developed.Regional geomorphic surface IV was deposited at about 200~130ka B.P. It'suplifted during the last interglacial period, which is from 130ka B.P. to 74ka B.P. Anancient soil layer is widespread on the surface IV.Regional geomorphic surface III is covered with loess. The first period of theloess deposition is about 74~59ka B.P.. Part of the loess was calcified on the surfaceduring 59 ~ 55ka B.P. The second period of the loess deposition is 55~ 42ka B.P.Regional geomorphic surface II started to form at about 42~23ka B.P., when theearly regional geomorphic surfaces III and IV were cut and eroded. Tributaries, suchas Wangjiagou, formed. The eroded channels and alluvial fans of the tributaries builtthe embryonic form of regional geomorphic surfaces II. During 23~15ka, the regionalgeomorphic surfaces II may aggraded partly by lateral erosion. At 15~12ka B.P.,regional geomorphic surfaces II became widespread terraces or the Gobi plain. Itssurface began to develop incomplete calcium soil. The surface loess turned into soil.Regional geomorphic surfaces I was formed after the regional geomorphicsurfaces II was exposed in a large range. After the young Dryas event, it went into theHolocene. During 12~8.5 ka B.P., the accumulation of the main rivers was dominatedby lateral erosion. From 8.5~4ka B.P., it was the most suitable climate in Holocene forriver incision. A widespread river terrace formed under the present Gobi plain.(2) The E-W transform of the North Tienshan piedmont thrust system isaccommodated by the Urumqi transform zone, where thrusts are domainant with backthrusts, sinistral strike-slip faults, and normal faults coexisting. A few back faults havepossibly exprienced inversion since the late Pleisstocene. The deformation mechanismof the tectonic system in the Urumqi transform region can be explained using themodel of two-direction oblique compression.(3) The surface active tectonics in the Urmuqi transform region can be classifiedinto several tectonic series as: the first, the southern marginal fault of the Chaiwopu basin and the Yamalike fault; the second, the Xishan fault group (including the Xishan,Sidaocha NW, Wangjiagou, Jiujiawang fault groups) and the Wanyaogou fault; thethird, Bagang-Shihua fault; and the fourth, the Ganquanpu fault. Among them, theXishan fault group-Wanyaogou fault and Ganquanpu fault are the main active faultsin the Holocene. The former have had occurred 4 paleo- events since late Pleistocene:Event I (ca.59.7~45.5 ka B.P.), normal faulting along the Jiujiawang fault group;Event II (ca.38.8~37.9 ka B.P.), thrusting and folding along three faults north of theWangjiagou fault group; Event III (ca.27.5~23.2 ka B.P.), thrusting and folding alongthe Wangyaogou fault; and Event IV (ca.12 ~ 10.5 ka B.P.), thrusting and foldingalong three faults north of the Wangjiagou fault group, normal faulting along theJiujiawang fault group. The average earthquake recurrence interval is about 13ka.Surface ruptures may occur in the future. The latest two events along the Ganquanpufault are as follows: Event I (after ca. 51.1 ~ 50.3ka B.P., before 26.9ka B.P., morelikely close to the later one), this event may correspond to the event III of theWanyaogou fault; Event II (ca. 14 ~ 11 ka B.P.), probably a surface disturbance eventcaused by the Bukang-Jimusaer fault in the east.In addition, this dissertation reports detailed parameters of late Quaternaryactivities at all of the surveyed sites along the faults, and gives two suggestions ondivision of the seismic tectonic units and prediction of the future seismic risk.