THE NEW FINDINGS OF SURFACE RUPTURE ZONES AND ITS SEISMOLOGICAL SIGNIFICANCE OF THE EASTERN MARGIN OF YUMUSHAN FAULT,NORTHEASTERN MARGIN OF QINGZANG PLATEAU
The Hexi Corridor in northwest China has obvious structural deformation and complex fracture image.With the development of several NW thrust fault zones accompanied by a large number of ancient earthquakes and historical seismic events,the earthquake disaster is relatively serious.The eastern margin of Yumushan fault is one of them.The fault is mainly developed in the east site of Yumushan Mountain,with the exposed fault plane striking NW330° and dipping about 41°～85° to the southwest as a whole.Previous research data show that the Eastern Margin of the Yumushan fault is an important part of the Qilian Mountain active thrust fault system in the northeast margin of the Tibet Plateau.It also constitutes the boundary structure between Hexi Corridor and Yumushan Uplift.Its late Quaternary tectonic deformation and recent activity characteristics reflect the northward extension process of the Qilian Mountains and the remote collision effect of the northward extrusion of the Indian Plate.However,there are still some controversies in the study of the latest activity age and deformation characteristics of the eastern Margin of the Yumushan fault zone,which directly affect the seismic risk assessment along the fault line and the Hexi Corridor,as well as the in-depth understanding of the active structural characteristics of the northeast margin of the Tibetan plateau.Combined with remote sensing image interpretation,paleoseismologic excavations,aerial photogrammetry of unmanned aerial vehicles and late Quaternary dating,this study carried out field investigations and newly discovered the surface rupture zone of The Eastern Margin of Yumushan Fault and its activity characteristics.The results show that The Eastern Margin of the Yumushan Fault strikes NW330° combined with obvious thrust movement,which is manifested as a fault scarp landform.That's revealing than the kinematics property of The eastern margin of Yumushan fault is dominated by thrust.The fault forms the dividing line between the Yumushan uplift and Zhangye Basin,and also the dividing line between pre-quaternary strata and Quaternary strata.The southwest side of the fault is dominated by pre-quaternary bedrock which constitutes a mountain landform.Late Quaternary sediments are exposed on the northeast side,and the Holocene strata are widely distributed around the Heihe River.The results show that there are obvious differences in the activity habits of the faults.With the Heihe River as the boundary,the fault activity difference is obvious on the south and north sides of the Heihe River.The latest surface fracture zone in the late Holocene was found along the Heiheokou segment(F1-1).And the Hongshahesegment(F1-2)showed pre-quaternary fault.It can be seen that the Miocene fine sandstone is in fault contact with the early Pleistocene glutenite and late Ordovician metamorphic andesite,and the fault gouge develops near the fault,which is gray-green and yellow-green with moderate hardness and easy to be wet when encountering water.The Heihekou segment(F1-1)starts from Daciyaohe River in the north,passes Xiaociyaokou,and reaches Heihekou in the south.The fracture zone moves towards NW330° and tends to SW,with a length of about 10km and a width of 3～10m.For river terraces,gullies,and platforms with young surface faults,the maximum height of the surface scarp is based on the DEM data generated by UAVs.The height of the T1 terrace fault scarp measured by two profile lines is(1.7±0.1)m to(3.3±0.2)m.In the excavating trenches,obvious evidence of fault activity such as traction bending of strata and directional arrangement of gravel can be seen.The strata consist of late Quaternary alluvial sand,gravel layer,loess layer,and silty layer.The optically stimulated luminescence dating results show that the latest surface rupture event occurred at(0.6±0.07)ka BP.According to the empirical formula between maximum vertical displacement(Dmax)and magnitude(M),the magnitude of the latest seismic event is estimated.The magnitude and potential seismic risk of the latest rupture event are evaluated.The results reveal that the maximum vertical displacement of the latest surface rupture event is(3.3±0.2)m.Based on the empirical relationship between magnitude and vertical displacement,it is concluded that a large earthquake rupture occurred in the eastern margin of the Yumushan fault in the late Holocene and the corresponding magnitude is estimated to be M7.5.Derived from the analysis of existing data,the fault in the eastern margin of the Yumushan fault may conform to the quasiperiodic earthquake recurrence behavior.And the recurrence interval of strong earthquakes may exceed 1600a.The time interval between the latest event revealed in this paper and its last seismic event is about 1800a,which is consistent with the time interval under the fault quasiperiodic earthquake recurrence model.The results show that the eastern margin of the Yumushan fault has intensive tectonic deformation in the late Quaternary and a large seismic background of M7 or above.The current kinematic mode of the fault is compressive shortening.Its geodynamic process may be mainly controlled by the northward extension of the Qilian Mountains and the remote collision effect of the northward extrusion of the Indian Plate.The deformation process of the fault may be in line with pre-spreading imbricate thrust deformation and the latest deformation has gradually extended from the basin-mountain boundary to the interior of Zhangye Basin,which provides new data to support the seismic risk assessment of the interior of the basin.At the same time,the latest deformation achievement of the eastern margin of the Yumushan fault has important scientific significance for improving the active tectonic image of the northeastern margin of the Qingzang plateau and discussing the kinematics model of the Qingzang plateau.
The eastern margin of Yumushan faultthe late Quaternary activity of the faultthe latest surfaceruptureseismic hazard of great earthquakes
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