Numerical Simulation Study On The Tectonic Deformation And Cenozoic Orogeny Of The Tianshan Orogenic Belt

The continental dynamics is currently one of the research frontiers in solid earth sciences.The core issue is to study the continental tectonic deformation and its dynamic mechanism.As a typical intracontinental orogenic belt, the Tianshan Mountains are an ideal natural laboratory for studying continental dynamics,due to the intense tectonic deformation and significant mountain uplift in the Cenozoic.The Tianshan orogenic belt is also an important metallogenic belt and a famous seismic zone,and is closely related to the formation and evolution of the petroliferous basins to its north and south sides.In recent years,a large amount of geological,geophysical and geodynamic research carried out in the Tianshan area, has enhanced the geodynamic understanding of the Tianshan orogenic belt.However, the mechanism of deformation and uplift of the Tianshan Mountains is still disputed, mainly because it is difficult to verify the proposed geodynamic models through experiments.For better understanding the orogenic dynamic mechanism of the Tianshan Mountains in the Cenozoic,the tectonic deformation and mountain uplift of the Tianshan Mountains have been studied in this paper, through numerical simulation methods.The main conclusions obtained are as follows:(1)On the basis of the Fortran programming language,a finite element program for solving two-dimensional elastic-plastic contact problems has been developed. Through several verification examples,the accuracy of the self-written program has been verified, by comparing the computational results of this program with the analytical solutions and the computational results of the ANSYS code.The effectiveness of the self-written program to solve the related problems has been examined through several application examples. By using the self-written program, the thrust structure in the border zone of the Tarim Basin and the Tianshan Mountains has been simulated.(2)Through using the contact element to simulate faults,the differences of faults slip and crustal stress in continuous and non-continuous models have been compared, and the impact of faults on the crustal stress has been investigated. The results show that:the non-continuous model can better simulate faults slip than the continuous model; the stress weaking, caused by simply simulating faults with weak zones, can be avoided in the non-continuous model; the elastic modulus of the faults belt, the angle between a fault and the regional principal stress,and crossing faults, are important factors that to affect crustal stress; the impact of Poisson’s ratio of the faults belt and the parallel faults on crustal stress is weak.(3) The contact model has been used to simulate the discontinuous movement of faults and the current crustal movement and deformation in the Tianshan area. The reasons for different crustal shortening in the different regions of the Tianshan Mountains have been investigated. The results show that:the crustal stress field in the Tianshan area is dominated by the near N-S compressive stress, resulting in near N-S crustal shortening; the crustal shortening in the Tianshan Mountains is not uniform, and the principal direction of shortening deformation is also not exactly the same in different regions:nearly N-S in the middle, changing to NNW-SSE in the west and to NE-SW in the east; as a result of the tectonic compression of the Tarim Basin against the Tianshan Mountains decreasing gradually from west to east, the crustal shortening defroamtion in the Tianshan Mountains decreases gradually from west to east.(4) In order to study the dynamic mechanism of intracontinental orogeny of the Tianshan Mountains in the Cenozoic,a three-dimensional thermo-mechanical model representing the lithosphere of the Tianshan orogenic belt and adjacent geological units has been developed. The influence of gravity-induced creep deformation on mountain uplift, the driving force causing mountain building, and the effect of lithospheric rheology on orogenic deformation, have been investigated. The results show that:the modeled topographic pattern coincides well with the actual topography, implying that the tectonic push of the Pamir and Tarim can cause significant uplift of the Tianshan Mountains;the lithosphere of the Tianshan Mountains is mechanically weaker than that of the platforms to the south and north of the Tianshan orogenic belt; the gravity-induced creep deformation has a significant influence on the process and elevation of mountain uplift. Therefore,when subjected to the tectonic compression resulting from the northward push of the Pamir and Tarim, the relatively weak lithosphere of the Tianshan Mountains would deform easily and hence absorb more shortening, resulting in lithospheric thickening and consequent mountain uplift.(5)On the basis of a three-dimensional model, which is used to represent the lithosphere of the Tianshan orogenic belt and adjacent geological units, the control of the tectonic push of the Pamir and Tarim on the intracontinental orogeny of the Tianshan Mountians has been investigated, by comparing the model deformation under the compression of the Pamir and Tarim. The results show that:the separate push of the Pamir and Tarim can only lead to the mountain uplift of the western section (west of about76°E) and middle-eastern section (east of about76°E) of the Tianshan Mountains, respectively; only subjected to the combined push of the both, the teconic deformation of entire Tianshan Mountains in the simulation is consistent with that observed actually. Therefore, the Cenozoic tectonic deformation of the Tianshan orogenic belt is the result of the combined compression of the Pamir and Tarim, both of which play a major role in the deformation and uplift of the western section (west of about76°E) and middle-eastern section(east of about76°E) of the Tianshan Mountains, respectively.