Abstract:
The thrust and fold belt of south Junggar basin, north Tianshan, is still activing since Cenozoic time. Geological survey, the interpretation of seismic data and well data indicate that the Huomatu anticlines and the Huomatu thrust fault in the forelimbs of the anticlines in the thrust and fold belt of south Junggar basin, which is extended to under the first row of the Qigu anticlines, is propagating toward north under the tectonic compression. The seismic data show that the Huomatu intact thrust sheet is almost not deformation, implies very weak detachments relative to internal sheet strength, i.e. weak-fault/strong-thrust sheet. Well data in the Huomatu anticline's belt show that overpressures are developed in the Paleocene Anjihai formation mudstones and Ziniquanzi formation mudstones, which are the main detachment surfaces of the Huomatu thrust fault in which tectonic over-pressures exist in this layer. The development of overpressured formation is not closely related to the depth, but depends on the thrust developed in the formation (mudstones or shales). Well test data indicate that the pore-fluid pressure coefficients of the hanging wall are separated from that of the footwall by the thrust fault. The hypotheses on which the Huomatu has been given as an elastic deformation, no deformation of intact thrust sheet, and the tectonic stresses is applied at the rear of the thrust sheet, the mechanical models of the Huomatu thrust sheet are founded around the following three points: ①The nature of the forces which cause the displacement (forces of tectonic origin, and applied by rear compression.), ②The elastical behaviour of rocks at the time of the thrusting, ③The geometric form of the overthrusted unit (rectangular dimension plus on prism of triangular). Based on our interpreted seismic data of the Huomatuo thrust sheet, two-dimensional, simplified mechanica models of hanging wall accommodation above undeformed footwalls in ramp-flat thrust models are set up. The model depends on the pore-fluid pressure coefficients, internal friction coefficient of the thrust fault and ramp angle. These include the possibility of a detachment fault in a weak basal layer with overpressures, a common feature of the Huomatuo thrust sheets and an externally applied, subhorizontal compression. In particular we vary the dimensionless ratio of shear strength to gravity stress to model hanging wall accommodation styles in different fluid overpressures ratio. In all models, we require that the flat-ramp-flat footwall provides a surface of low frictional resistance. At high ratios of shear strength to gravity stress the hanging wall blocks translate forward without bending and unbending to the form of the rigid footwall. Both without fluid pressure and with fluid pressure in the detachment thrust fault, arithmetic expressions are set up for the tectonic stress to gravity stress ratio with relation to coefficient of sliding friction in the fault, and ramp angles as well as horizontal length of the flat sector of the hanging wall to height of the thrust sheet ratio. We have found that the main factors governing the tectonic stress to gravity stress ratio is frictional resistance along the fault plane and the pore-fluid pressure coefficient (fluid overpressure), and ramp angles as well as horizontal length of the flat sector of the hanging wall to height of the thrust sheet ratio. The tectonic stress to gravity stress ratio with fluid pressure, compared with that without fluid pressure, decreases with the increase of the pore-fluid pressure coefficients along the thrust fault. When the coefficient of sliding friction in the thrust fault with fluid pressures increases, the tectonic stress to gravity stress ratio rises, because the pore-fluid pressure coefficient is a fixed value. But when the pore-fluid pressure coefficient in the thrust fault with fluid pressures increases, the tectonic stress to gravity stress ratio decreases, because the coefficient of sliding friction in the thrust fault with fluid pressures is a fixed value. The physical analog modeling of the Huomatu thrust fault and thrust sheet in this area, sand box experiment with approaching real geological setting was designed to simulate the evolution of the thrust fault with fluid pressures and without fluid pressures in the south Junggar thrust-fold belt. Loose dry quartz sands were used to construct the overlying sediments and growth strata. Silicone putty on basement represents the detachment layer with fluid pressures. The experimental results show that the whole thrust fault deformations without fluid pressures by the rear compression. The thrust sheet is relatively strong compared with weak thrust fault with fluid pressures, i.e. weak-fault/strong-sheet. A likely locus of the deformed thrust sheet strength is at fault bends, where the bulk rock of the thrust sheets must continually deform, and at the rear end of the thrust sheet as shown in this experimental model. Shear magnitudes and displacement vectors are computed by result of the sectional simulation experiment for the Huomatu thrust sheet, in which show that they are concentrated in the flat, where the concentrated maximum values of shear magnitudes are found by punches of multi-points which may reduce or enhance the fluid flow along the fault planes.
The thrust and fold belt of south Junggar basin, north Tianshan, is still activing since Cenozoic time. Geological survey, the interpretation of seismic data and well data indicate that the Huomatu anticlines and the Huomatu thrust fault in the forelimbs of the anticlines in the thrust and fold belt of south Junggar basin, which is extended to under the first row of the Qigu anticlines, is propagating toward north under the tectonic compression. The seismic data show that the Huomatu intact thrust sheet is almost not deformation, implies very weak detachments relative to internal sheet strength, i.e. weak-fault/strong-thrust sheet. Well data in the Huomatu anticline's belt show that overpressures are developed in the Paleocene Anjihai formation mudstones and Ziniquanzi formation mudstones, which are the main detachment surfaces of the Huomatu thrust fault in which tectonic over-pressures exist in this layer. The development of overpressured formation is not closely related to the depth, but depends on the thrust developed in the formation (mudstones or shales). Well test data indicate that the pore-fluid pressure coefficients of the hanging wall are separated from that of the footwall by the thrust fault. The hypotheses on which the Huomatu has been given as an elastic deformation, no deformation of intact thrust sheet, and the tectonic stresses is applied at the rear of the thrust sheet, the mechanical models of the Huomatu thrust sheet are founded around the following three points: ①The nature of the forces which cause the displacement (forces of tectonic origin, and applied by rear compression.), ②The elastical behaviour of rocks at the time of the thrusting, ③The geometric form of the overthrusted unit (rectangular dimension plus on prism of triangular). Based on our interpreted seismic data of the Huomatuo thrust sheet, two-dimensional, simplified mechanica models of hanging wall accommodation above undeformed footwalls in ramp-flat thrust models are set up. The model depends on the pore-fluid pressure coefficients, internal friction coefficient of the thrust fault and ramp angle. These include the possibility of a detachment fault in a weak basal layer with overpressures, a common feature of the Huomatuo thrust sheets and an externally applied, subhorizontal compression. In particular we vary the dimensionless ratio of shear strength to gravity stress to model hanging wall accommodation styles in different fluid overpressures ratio. In all models, we require that the flat-ramp-flat footwall provides a surface of low frictional resistance. At high ratios of shear strength to gravity stress the hanging wall blocks translate forward without bending and unbending to the form of the rigid footwall. Both without fluid pressure and with fluid pressure in the detachment thrust fault, arithmetic expressions are set up for the tectonic stress to gravity stress ratio with relation to coefficient of sliding friction in the fault, and ramp angles as well as horizontal length of the flat sector of the hanging wall to height of the thrust sheet ratio. We have found that the main factors governing the tectonic stress to gravity stress ratio is frictional resistance along the fault plane and the pore-fluid pressure coefficient (fluid overpressure), and ramp angles as well as horizontal length of the flat sector of the hanging wall to height of the thrust sheet ratio. The tectonic stress to gravity stress ratio with fluid pressure, compared with that without fluid pressure, decreases with the increase of the pore-fluid pressure coefficients along the thrust fault. When the coefficient of sliding friction in the thrust fault with fluid pressures increases, the tectonic stress to gravity stress ratio rises, because the pore-fluid pressure coefficient is a fixed value. But when the pore-fluid pressure coefficient in the thrust fault with fluid pressures increases, the tectonic stress to gravity stress ratio decreases, because the coefficient of sliding friction in the thrust fault with fluid pressures is a fixed value. The physical analog modeling of the Huomatu thrust fault and thrust sheet in this area, sand box experiment with approaching real geological setting was designed to simulate the evolution of the thrust fault with fluid pressures and without fluid pressures in the south Junggar thrust-fold belt. Loose dry quartz sands were used to construct the overlying sediments and growth strata. Silicone putty on basement represents the detachment layer with fluid pressures. The experimental results show that the whole thrust fault deformations without fluid pressures by the rear compression. The thrust sheet is relatively strong compared with weak thrust fault with fluid pressures, i.e. weak-fault/strong-sheet. A likely locus of the deformed thrust sheet strength is at fault bends, where the bulk rock of the thrust sheets must continually deform, and at the rear end of the thrust sheet as shown in this experimental model. Shear magnitudes and displacement vectors are computed by result of the sectional simulation experiment for the Huomatu thrust sheet, in which show that they are concentrated in the flat, where the concentrated maximum values of shear magnitudes are found by punches of multi-points which may reduce or enhance the fluid flow along the fault planes.
