The Multiple Deformation Of The Northern Margin Fault Of The Western Qinling And Its Tectonic Dynamic Significance

The northern margin fault of the West Qinling is a boundary fault between the Cenozoic basin and the West Qinling block in the northeast margin of Qinghai-Tibet Plateau.Its deformation history and geometric-kinematic characteristics could provide the constraints to the tectonic attributes of Cenozoic basins and spatial-temporal dynamics of crustal uplift and deformation of the northeast margin of the Qinghai-Tibet Plateau which could help understanding the remote tectonic response to the Indian-Eurasian collisional convergence.Based on the detailed structural analysis for the fault rock zoning,the morphology and geometric-kinematic characteristics of various structural elements,and the superposition and cutting relationship between different structural elements of the southern boundary F1 fault in the northern margin of West Qinling,the six phases of structural deformation with different properties and geometric-kinematic characteristics was identified in the F1 fault zone as follows: The first phase is characterized by NWW trending with northward dipping,ductile or ductile brittle extensional normal faulting by north-south tectonic extension;The second phase is characterized by NWW trending with north steep dip,brittle extrusion thrusting by north-south tectonic compression;The third phase is characterized by the north-south strike with west dipping thrust faulting formed by east-west compression;The fourth phase characterized by strike-slipping conjugate fault system with minor oblique thrusting in which NE trending fault is dextral and NW trending is sinistral formed by east-west compression as above;The fifth phase formed a conjugate fault system formed by north-south compression with sinistral NE trending faulting and dextral NW trending faulting,both fault planes is vertical;The sixth phase formed sinistral EW trending strike-slipping fault and NS trending dextral strike-slipping fault which constitute a conjugate fracture system with geometric-kinematic coordination formed by NE-SW compression.On the above F1 fault deformation analysis,in combination with the rifting basin sedimentary sequence of Oligocene-Miocene sedimentary basin,Pliocene alluvial-fluvial fan coarse conglomerate with molasses deposit,and different phases of faulting play different role with the controlling basin deposit or tectonic involved sedimentary strata in the northern margin of West Qinling,it is suggested that the first phase of F1 fault developed at Oligocene which controlled the deposition of Oligocene-Miocene extensional faulted basin;The second phase shortening-high angle thrusting deformation of F1 fault led to closure of Oligocene-Miocene basin and involved in compressive thrusting deformation and uplift,until the emergence of Pliocene alluvial-fluvial fan coarse conglomerate,therefore,this phase of deformation of F1 fault should start the end of Miocene epoch or early Pliocene and continued to the end of Pliocene;Although both the third and fourth phases deformation of F1 fault formed by E-W compression,the third phase faulting take on inverse faulting with SN trending,east-dip or west-dip fault plane,while the fourth phase take on strike-slipping conjugate fault system with minor oblique thrusting.Whether its dynamic mechanism is related to the short East-West compression of the upper crust caused by the eastward flow of the middle and lower crust induced by the crustal thickening and uplift in the west of the northeast margin of the Qinghai Tibet Plateau remains to be confirmed.Since this structural deformation has not been found in the Pliocene Hanjiagou conglomerate,it is suggested that the third and fourth phases deformation of F1 fault maybe developed in the early Pliocene;The fifth phase deformation with the NE and NW trending conjugate fracture systems and the sixth phase deformation with N-S and E-W conjugate fracture systems of F1 fault,both developed in the Oligocene-Miocene sedimentary strata and Pliocene coarse conglomerate strata;undoubtedly,the deformation occurred after Pliocene and before Quaternary.In addition,the direction of the maximum principal compressive stress forming two phase conjugate fracture systems are different,fifth phase with N-S direction and the sixth phase with NE-SW direction which indicate that the maximum principal compressive stress rotates clockwise by 30° from fifth phase to sixth phase.This rotation of the maximum principal compressive stress may be related to the block rotation in the expansion process of the northeast margin of the Qinghai Tibet Plateau.The fault deformation modes and historical evolution revealed by the rich and complex structural deformation traces of F1 fault provide an important structural evidences for clarifying the controversial understanding of the tectonic attributes of the Cenozoic red bed basin on the northeast margin of the Qinghai Tibet Plateau and the issues about the plateau deformation mechanism,that is,uniform thickening model or block strike-slipping extrusion mechanism along the fault.