Characteristics And Petroleum Geological Significances Of Transverse Faults In The Longmenshan Fault Zone

Transverse fault is the fault that strikes obliquely or perpendicularly to the general structural trend of the region.Transverse fault is the pre-existed or associated structure with the thrust fault zone and closely related to the tectonics evolution and hydrocarbon accumulation of the fault zone.However,its classification standard and features are still not clear so far.In addition,the distribution,characteristics and genetic mechanisms of the transverse faults in the Longmenshan fault zone(LMSF),and their relationship with earthquakes and hydrocarbon accumulation is not clear.To clarify problems mentioned above,firstly,based on the analysis of tectonic settings and geometric characteristics of typical thrust belts,we conducted sandbox experiments in the compression and transpression settings to reveal the transverse faults formation process,meanwhile,took the pre-existing lateral structures and lateral varaitions of the thrust fault zone into consideration.According to the experiment results,we classified transverse faults in thrust fault zone into 4 types: the shear,transverse extensional,tear and pre-existed transverse faults:(1)The shear transverse faults strike obliquely to the structural trend of the thrust zone and can form in the thrust zone and the adjacent foreland basin both in the compression and transpression settings.Under the compression setting,X-shape conjugate shear transverse faults tend to forming in the foreland basin.While under the transcompression setting,they tend to in the thrust belt,usually strike parallel with each other with a certain interval.(2)Transverse extensional faults can also form in the thrust zone and the foreland basin,usually strike perpendicular to the trend of thrust fault zone and develope along the X-shape conjugate shear fractures with a shape of "sawtooth" across the fault zone.Under the compression setting,they tend to forming in the fault zone striking parallel with certain interval.While under the transcompression setting,they tend to in the forland basin.(3)The pre-existed lateral structures control the structural framework of the thrust fault zone.With the start of the thrust,the preexisted lateral structures rejuvenated as transverse faults concentrating most of the displacements.With the increased compression,there would be some transverse branch faults correlated to the pre-existed structures.(4)Tear faults were more likely to develope in the along-strike variant compression setting.The axial thickness and mechanic variation of the stratum can lead to the formation of some shear faults that are oblique to the thrust zone and usually extend between two main thrust faults.When the lateral difference displacement of the thrust zone is larger than the width of the fault zone,the transverse fault can cut through the fault zone perpendicularly.With the interpretation of remote sensing images,geological and geophysical data,field study,we summarized the identification features of transverse faults in the LMSF and found 9 transverse faults and 4 potential transverse faults.They strike NW,are roughly parallel with each other,and have a similar interval.From northeast to southwest,the 9 faults are the Bailongjiang,Nanba,Huya,Qingping,Xiaoyudong,Wolong-Huaiyuan,Dachuan,TianquanYingjing,Sanhe transverse faults,respectively.The 4 potential faults are Chaotian,QingchuanJiange,Tongkouhe,Baoxing-lushan transverse fault,respectively.In the geological and geophysical data,they appear as the NW-trending linear structures,the abrupt disconnected,the distorted or the big gradient belt.To the field outcrops,they are NW-trending large vertical fault planes(slickensides)with the conchoidal fractures and parallel striations,fracture zones and fold distortion zones,which indicate the NW-trending strike-slip movement.Based on the anslysis of fault geometry,field study,strata and tectonic setting in LMSF and its adjacent area,we found that: The LMSF mainly develops the types of pre-existed,shear and tear transverse faults.The Wolong-Huaiyuan,Xiaoyudong and Huya faults extend from the NW-trending Fubianhe,Miyaluo and Huya faults in the Songpan-Ganzi fold belt.The LMSF is divided into the northern,middle and southern segments by the Wolong-Huaiyuan and Huya faults.In the northern and southern segments,It mainly develop transverse extensional faults,while in the middle segment,it's the shear transverse faults.The strike slip of pre-existed and pre-formed transverses aggravate the thrust variation of the main thrust faults and they were also offset by the main faults during their strike slips.We finally setup the structural model and reappear the evolution of the LMSF.In view of the Longmenshan structural framework segmented by transverse faults,we thought transverse faults theirself or with the main faults can trigger earthquakes,they cut the main faults into segments that thrust asynchronously and where so did the earthquakes.Wenchuan and Lushan earthquakes are independent events in different segments.The evoluaton on multi-factors—structural factor,kinetic factor and stratal factor,indicated that the strong shock risk zones are located near the Bailongjiang,Huya,Xiaoyudong,WolongHuaiyuan,Dachuan transverse faults.Stress and strain calculation show that transverse faults can promote the along-strike differential evolution of the LMSF and adjacent foreland basin.The strike slip of transverse faults envetually cause the thickness and evolution variation of the source rock,reservior and trap along the foreland basin.In the LMS foreland basin,the source rock in the middle and southern segments are much thicker and more mature than those in northern.The erosion and depositon in different segments are also variant.The erosion volume in middle and southern segments is much more than that in northern.And before the early Jurassic,clastic sedimentary mainly deposit in the middle and northern segments while in the middle and southern segments after.Usually,the transverse faults control the root fan area.In addition,the thrust extends to the foreland basin and results in some NE-trending hydrogen traps formed in the middle and southern segments,but not in the northern.And several gas traps form a line along the Huya transverse fault.We summarized 5 hydrocarbon accumulation models related to the transverse faults in the foreland basin.They are fault blocks formed by transverse extrnsional fault,horsts formed by compressinal strike slip,anticlines related to thrust of transverse fault,grabens formed by extensional strike slip and transverse fracture zones,respectively.We concluded that fracture zones along the NW-trending transverse faults are potential areas for unconventional hydrocarbon traps,while the Huya,Qingping and Xiaoyudong fault zone are for conventional traps.