Characteristics Of Active Tectonics And The Deformation Pattern Of The Jiuxi Basin At The Western Qilian Shan

The Jiuxi basin is located in the westernmost part of the Qilian Shan and Hexi Corridor,where is the forefront of the northeastward growth of the Tibetan Plateau.Numerous active faults were developed within the basin as well as faults along the basin boundaries,all suggesting the region was characterized by intensive deformation since Late Quaternary.It can provide basic date for us to understand the strain partitioning along the Altyn Tagh fault by investigating the characteristics of active faults.At present,there are still some deficiencies in the study of the characteristics of the active faults in the Jiuxi basin,which affects the overall understanding of the seismic activity level and tectonic deformation.Based on systematic study of the active tectonic research in this thesis,I will derive the quantitative parameters of the Late Quaternary active faults.The characteristics of seismic activity in this area will be summarized.By integrating the regional structure research results,the deformation pattern of the Jiuxi basin will be proposed.I will also discuss the dynamic mechanism of its tectonic deformation.The main findings are as follows:The Yumen-Beidahe fault is a thrust fault with no strike-slip component.According to the landform and geometric characteristics,The Yumen-Beidahe fault can be divided into three segments.The geometry of the eastern segment is a straight line,where an earthquake rupture zone was preserved.The middle segment is composed of several thrust faults towards the central basin.The western segment is a blind-thrust fault zone.The average vertical slip rate is 0.58 ± 0.08 mm/a and 0.73 ± 0.09 mm/a at the eastern and middle segment,respectively.At the western segment,dating of the Baiyanghe terraces suggests the age of 8.6 ± 1.7 ka(T2),21.6 ± 5 ka(T3),55.3 ± 2.4 ka(T4).The age of high alluvial fan is 69.2 ± 2.3 ka.According to the measured deformation amount,we infer the average vertical slip rate is 0.94 ± 0.32 mm/a for the western segment.Trench results of the Yumen-Beidahe fault reveal at least three paleoearthquake events at the eastern segment since Holocene.The latest earthquake event occurred at about 1.7 ± 0.3 ka,and formed 20 km long surface rupture zone.The second earthquake event occurred between 2.74 ± 0.02 ka and 4.1 ± 0.6 ka.The third earthquake event occurred before 8.4 ± 1.0 ka.The surface fault scarp and the strata deformation from the trench indicate that the coseismic displacement of each earthquake is 0.6 ~ 1 m.The magnitude of these three earthquakes is estimated to be around 7.There are also a number of Holocene active faults in the Jiuxi basin.The Yinwashan fault is a high angle thrust fault,with a Late Pleistocene vertical slip rate of 0.08 ± 0.02 mm/a.Three paleoearhquake events were determined at 16 ka?10 ka?5 ka,and the vertical displacement of each earthquake is about 0.3~0.4 m.The Xinminpu fault is an overthrust fault with a Late Pleistocene vertical slip rate of 0.11 ± 0.02 mm/a.This fault is related to magnitude 6? earthquake in 1785.Another earthquake was recognized before 3.8 ± 0.3 ka along the Xinminpu fault.The south Xinminpu fault is a reverse fault with length of 5 km,which has not been reported before.Paleoearthquake research suggests the rupture was formed at about 1.5 ± 0.2 ka.Another earthquake occurred before 5.7 ± 0.8 ka.According to historical earthquake documents,the rupture zone may be related to the 365 Yumen earthquake.The Baiyanghe fault is a EW trending reverse fault.The crustal shortening rate is of 0.48 ± 0.03mm/a from the deformed terrace geometry.Two paleoearhquakes occurred at 8.7 ± 0.6 ka and 3.9 ± 0.5 ka,and the vertical displacement of each earthquake is about 0.6 m.According to the fault scale and the coseismic displacement,upper limit of the event magnitude in the Jiuxi basin is estimated to be about 6.8.Through the comprehensive comparison of the study on the paleoearhquake in the Jiuxi basin,it is found that the behavior of the single fault in the Jiuxi basin follows the characteristic earthquake model.The recurrence period is 3~5 ka.A cluster earthquake model is also found in the fault system of the Jiuxi basin.Three cluster cycles was determined since the Holocene,10~8 ka,5.5~3.5 ka and 2 ka.Each cluster lasts 2000 years.According to the characteristics of the recurrence and the elapsed time,we suggest that the Baiyanghe fault and the Yinwashan fault are of high seismic potential.The deformation in the Jiuxi basin is characterized by thrust and fold.The crustal shortening rate is about 2.3 mm/a estimated by fault slip rate and the dip angle of the faults in Jiuxi basin.The crustal shortening rate is about 2.3 mm/a parallel to the Altyn Tagh fault.The shortening rate is comparative to the decrease of the left lateral slip rate of the Altyn Tagh fault,which may suggest the Jiuxi basin is absorbing the eastward motion of the Altyn Tagh fault.Geomorphology,displacement distribution and fault slip rate of Yumen-Beidahe fault suggest the fault segments were gradually developed and grew from west to east.Most of the faults in the basin,however,develop from the middle,and gradually extended towards both ends.The present study finds the formation age of the Xinminpu fault and the Baiyagnhe fault(~ 0.17 Ma)are far later than the Hanxia-Dahuanggou fault(~ 9 Ma)and the Yumen-Beidahe fault(~ 8.3 Ma)at the southern margin of the basin.Its formation is also later than the Heishan fault(~ 3.8 Ma)and the Yinwashan fault(~2.8 Ma)at the northern margin of the basin.The fault deformation pattern of Jiuxi basin is piggy-back,over-step and back-thrust.The tectonic deformation of Jiuxi basin is resulted from both the left lateral of the Altyn Tagh fault and the northeastward growth of Qilian Shan.The complex geological structure and present landform pattern were developed due to the collision between Qilianshan block and Alashan block.