The Interplay Between Extension And Strike-slip Faulting In Western Shandong Rise-jiyang Depression Since Late Mesozoic

Based on the field survey and well logging and seismic data,this paper researched the geometric and kinematic characteristics of normal faults and strike-slip faults,the evolution of oblique faults,and the relationships between extensional faults and strike slip faults in Western Shandong Rise-Jiyang Depression by structure analysis.Moreover,data on geothermal field,lithosphere structure,geochemical characteristics of igneous rocks and Tanlu fault zone's evolution were used to analyzed the mechanism of faults and basin in different global tectonic background.Firstly,the method of geometric analysis was used to reveal the characteristics of faults in planes,in seismic sections and in outcrops.The results show that the NW-trending faults are almost parallel aligning from south to north in Western Shandong Rise,and they are SW-dip,deep,in large scale,and control the sediments of J₃-E_1-2.Besides,there are cataclasite,fault gouge,slickenside,steps in the fault zone.Radiant faults in the north of Western Shandong Rise developed in Late Mesozoic in the way of either left or right lateral strike-slip,and extended into Jiyang Depression.However,the Faults in Rise stopped to act,and the parts in depression changed to extend after Middle Eocene.Moreover,the faults in Jiyang Depression could be divided into NW,EW,NE by trending,and of course the normal,reverse,strike-slip and oblique fault by the relative movement of two wall.And they control the sediments since Late Mesozoic.In addition,the combinations of extensional faults present the parallel,the en-echelon,the ring-radiant in the plane,and the step-shaped,the graben and horst,the“Y”shaped in section.Correspondingly,the combinations of strike slip faults present the oblique,the brush,the en-echelon in the plane,and the negative flower,the half flower,“horsetail”shaped in the section.Secondly,the method of kinematic analysis was applied to study the parameters of faults'activity.The results show that the extension in Western Shandong Rise was far less than the Jiyang Depression according to the balanced sections,and the extension mostly concentrated in the middle of the sag and J₃-K₁,Es₄-Ek period.The faults'activity indicated the process of changing from NW to NE-trending after calculating the vertical slip.Vector analysis indicated that the faults were more inclined to be normal.However,the east part of Chennan fault have acted by strike-slip since Late Mesozoic,and so did the NW-trending faults in Western Shandong Rise in Early Cenozoic.The depth of faults'detachment are18-34km in Western Shandong Rise and 10-15km in Jiyang Depression after restoring the faults'plane.Besides,the paper analyzed the mechanism of forming the oblique faults,the evolution of pre-existing faults,and the sedimentary response of oblique faults.The deformation styles in forming oblique faults consist of pure shear,simple shear,general shear and stress fields transformation.Since the stress field transformed and the basin oblique rifting,the pre-existing faults would develop by the type of the abandoned,the elapsed,the migrated,the inherited according to the activity.The oblique faults have controlled the sediments and formed the sedimentary imbricates,inclined fan-folds.The strike-slip faults could associate the normal faults,and accommodate the differences produced by the faults'extension.What's more,the faults'extension and strike-slip could be inverted.This study separated the strike slip faults into 6 zones and analyzed its contributing factor.Meanwhile,the relationships between extensional fault and strike slip fault were divided into the associated,the accommodated,the inverted.The associated relationships consisted of associating faults in the left or right walls,and in the cap rock.The accommodated relationships could be divided into the oblique and the transfer faulting,and the latter could be divided further into two:strike slip fault accommodate conjugate or synthetic extensional faults.The inherited relationships could be divided into“pre-strike slip and post-extension”,“pre-extension and post-strike slip”.The lithosphere of study area extended by the way of wide rift mode in J₃-K₁,and showed the multiple layered extensional characteristics:the crust deformed by simple shear and the lithospheric mantle deformed by pure shear.Meanwhile,the geochemical characteristics of igneous rocks indicated that the study area was located in back-arc tensional regime,the lithospheric mantle was EM1 pattern under the Jiyang depression,but the lithospheric mantle was EM2-like pattern under Western Shandong Rise due to the subduction of Yangtze plate.Combining with the stress and deformation fields in the study area,the stress source of deformation were deduced to be the tension mainly generated by the relative motion within the plate,and formed the extensional-strike-slipping faults and sags.However,lithosphere of study area extended by the way of narrow rift mode in Cenozoic.,the crust and the lithospheric mantle deformed all by pure shear,but the later extended more intensively.Additionally,the geochemical characteristics of igneous rocks indicate that the basin involved into continental riftting,the asthenosphere upwelled and transformed the lithospheric mantle.Apparently,the tension produced by the upwelling of asthenosphere was identified as the crucial role in basin rifting,and formed the extensional or oblique extensional faults and sags.