Tectonic Movement And Paleoearthquakes On The Hejing Reverse Fault-fold Zone In The Northern Margin Of The Yangqi Basin During The Late Quaternary

The Tianshan Mountains orogen is the most important intracontinental orogen in Eurasian continent, and has a close relationship to the Tibetan Plateau. So it is a focused region in research of active tectonics. There are some foreland basins in the southern and northern sides of Tianshan. The thrust faults are different between foreland basins and outer areas. There are some rows of reverse fault and fold zones in those foreland basins, and high-dip faults outside the foreland basins. A lot of scholars have studied and discussed the deformation and the deformation rate in the Tianshan orogen from Ceneozic to present. They agree a conclusion that the deformation gradually weakens from south to north and from west to east. Those reseaches have important significances for understanding of orogen deformation mechanism.The Yanqi basin lies between the Tianshan Mountains. It formed in Jurassic. At present, it has a stress field characterized by dominant compression with strike-slip compoment. Some scholars studied active faults in the southern Yanqi basin, and concluded that the Huola Mountain thrust nappe expands into basin and the Kaidu river fault has a dextral shear charcter. This work consists of the geological mapping, active fault identification and the deformation meaturement on the Hejing reverse fault and fold zone. Based on these data, this work attempts research the structure mechanism, the deformation rate, the uplift stages and paleoearthquakes on the Hejing reverse fault and fold zones.This thesis has come to the following conclusions:(1) In the southern and northern margin of the Yanqi basin developed opposite thrust faults. The Hejing reverse fault and fold zone is a new structure which spreads at low dip into the basin in the northern margin of the Yanqi basin.(2) The deformation of Hejing reverse fault-fold zone started in the late of early Pleistocene and gradually developed from west to east. The Xiaermudun anticline was shortened by 1.79km and uplifted by 1.00km, respectively. The Haermodun anticline was shortened 0.88km and uplifted 0.59km, respectively. For the Hejing reverse fault-fold zone the average shortening rate and uplifting rate are 1.11mm/a and 0.62mm/a, respectively, during since the late of early Pleistocene. For its eastern section the average shortening rate and uplifting rate are 1.13mm/a and 0.76mm/a, respectively.(3) There are mainly reverse faults, back-reverse faults and moment faults in Haermodun anticline, which simultaneously the developed faults in this place.(4) The Haermodun anticline has four uplift stages from the late of Quatemary, which are (124.79±5.73~98.61±2.70ka), (98.61±2.70~55.00ka), (55.00~42.7±2.00ka), (42.7±2.00~3.39±0.04ka) and (3.39±0.04ka~now), and the tectonic uplift rates respectively are 1.01-2.39mm/a, 0.00-0.06mm/a,0.27-0.54mm/a, 1.07-1.23mm/a and 4.88mm/a. So the tectonic uplift rate is greater than that of climate river incision rate in stageⅠ,ⅣandⅤ(From old to new). It is shown that the structural uplift controlled terraces formed in the Haermodun anticline in these three stages.(5) Three M>6.5 paleoearthquakes are revealed in the trench, that occurred in. 3390±40a, 1690±40a and 150±40a, respectively. The inferred recurrence time is 1620±80a. Using the empirical formula between magnitude and displacement, considering different parameters proposed by some scholars, the seismogenic ability of the Hejing fault is estimated. There is a M7.5 paleoearthquake exposed in the trench, so the Hejing fault can generate earthquakes of M7.5.Compared with other researches, this work makes discussion on crustal deformation in the southeastern Tianshan Mountains, uplifting stages of the anticline in the foreland basin of the South Tianshan Mountains and the seismogenic ability of the Hejing fault:(1) The shortening rate of the Yanqi basin is 1.95~2.13mm/a, accounting for 67~74% of the shortening rate of the Tianshan Mountains which is 2.9mm/a by GPS in the northeast margin of the Tarim basin. So the Yanqi basin is one of the primary areas crust shortening in the southeastern Tianshan Mountains. And the major shortening comes from the active fault-fold zones in the northern and southern margins of the Yanqi basin.(2) The uplift of the Hejing anticline by stages in the southeast Tianshan Mountains and the Artux anticline by stages in the southwest Tianshan Mountains have some similarities. For example, the river incision rate, the beginning time of each stage and the tectonic uplift rate of each stage are consistent. The historical earthquakes show that the stage 5 is a relatively active period in tectonic activity. Whether terraces have the same or similar stages in the anticline of South Tianshan Mountains during the late Quaternary and can reflect the change of tectonic movements all over the South Tianshan Mountain? This question deserves continue study.