Formation And Evolution Of Strike-Slip Faults In The Northern Slope Of Tazhong Uplift,Tarim Basin

The northern slope of Tazhong uplift located in the middle part of the Tarim Basin is one of the key areas in the Tarim Basin with great exploration potentials.The Paleozoic strike-slip fault systems developed in this area have received growing attentions because their detailed characteristics are closely linked to the migration and accumulation of hydrocarbon resources.However,due to the lack of high quality 3D seismic data and the identification of vertical fault throw of the strike-slip faults are limited by using 2D seismic data,detailed characterization of the strike-slip fault systems haven't been carried out previously.Acquisition of high-quality 3D seismic reflection data of the study area allowed us to better constrain and understand the geometry,kinematics and formation mechanisms of the strike-slip faults.In this paper,shuntuo and shunan seismic volumes are used.Based on the detailed interpretations of the 3D seismic data,five groups of major faults(faults S1,Sn1,Sn2,Sn3 and Sn4)are are identified and characterized.These faults are named by drilling wells,e.g.,Well #S1,#Sn1.The following specific conclusion can be drawn:1.Based on the interpretations of the 3D seismic data,five strike-slip fault zones cutting through the Cambrian-Middle Devonian units are described to have distinct geometric and structural features in deeper and shallower layers.On the seismic cross sections,the deeper faults rooted in Cambrian-the Upper Ordovician layers appear as lower positive flower structures and the shallower ones restricted to the Late Ordovician-Middle Devonian units are characterized by upper negative flower structures.In map view,the lower faults show a linear geometry orienting NNE and only developed within the principal displacement zone.The upper faults are NW trending and display right-stepping en-echelon arrangements.In three dimensions,the strike-slip fault zones display a complex geometric feature that consists of a lower positive flower structure and an upper negative flower structure.2.By analyzing the change in the reverse separation of the interface and counting the fault growth index,the lower strike-slip faults may have been formed no early than the Late Ordovician and the upper en-echelon normal faults were developed during the Middle Silurian-Middle Devonian.Furthermore,the range of the angle between the strike of each en-echelon normal fault and their principal displacement zone is 40°~60°,with an average about 50°,indicating that these en-echelon normal faults are tensional structures,so-called T fractures which formed during progressive sinistral displacement above a NNE-striking basement fault.3.By counting the reverse separation of the T74 horizon along the deeper faults,,activity intensity on the southern deeper strike-slip faults is stronger than that on the northern ones.According to the comparison between the reverse separation of the T74 horizon along the deeper faults and the fault throw of the T62 horizon along the shallower faults,activity intensity of the Fault zone S1 in the different layers has similar performance that the activity intensity of the southern faults is larger than that of the northern ones.4.The horizontal displacement determined from the time slices of faults S1,Sn1 and Sn4 are 743(±150)m,1545(±150)m and 1780(±150)m,respectively.Such a variation indicates that the amount of horizontal strike-slip displacement have slipped with different magnitudes.This indicates that the amount of horizontal strike-slip displacements is likely to have a tendency to decrease from the south to the north.5.Combined with tectonic background analyses on the structures surrounding Tarim Basin,the strike-slip faults are believed to be formed as a result of two-stage evolution.In the Late Ordovician,the Tazhong Uplift experienced a strong compression from Ancient Kunlun ocean subducting in NE direction,which resulted in the formation of NNE-trending strike-slip faults.During the Middle Silurian-Middle Devonian,intense folding orogeny of the Arkin tectonic domain caused the reactivation of the lower strike-slip faults and made the formation of the en-echelon normal faults.6.The Late Ordovician décollement layer developed in the Tazhong area and the peripheral tectonic events of the Tarim Basin have been thought to be two main factors for the formatting and evolution of the strike-slip fault zones in the the northern slope of Tazhong uplift.They controlled the multi-level and multi-stage different deformations of the strike-slip faults,respectively.