Tectonic background and stress field change process

Fangzheng fault depression is located at the eastern end of the Central Asian orogenic belt and adjacent to the Pacific plate in the east (Fig. 1). It is the most complex area for the superposition and transformation of the Paleo-Asian Ocean and the Pacific Rim. From the Paleozoic to the early Mesozoic, the Central Asian-Mongolian Sea experienced many times of cracking and closure, and the basement formed. At the same time, the Mesozoic Late Triassic to Early Jurassic, the Pacific plate subduction began and entered the Pacific Rim structural domain and Era (^{Hilde et al., 1977}). The positive northward subduction movement of the western Pacific plate since the early Cretaceous caused the left lateral of the Tanlu fault zone. Under the mechanism of the left-lateral strike-slip pull, the Fangzheng fault depression formed, and the NW direction of the Late Cretaceous Western Pacific plate subducted. Fangzheng fault depression right-lateral strike-slip extrusion, leading to Cretaceous strata uplift and erosion.

Figure 1 Fangzheng fault depression structure location 

In the early Paleogene, the movement direction of the Pacific plate changed to NNW direction, the TanLu fault zone began to be left-lateral strike-slip, and the basin area has further expanded. In the late Paleogene, the movement direction of Pacific plate has changed to NWW direction, and the Tan-Lu fault zone was right-lateral strike-slip. The slippery feature is intensified. The post-arc expansion of the western Pacific plate since the Neogene has caused a lateral thrust force to the west, causing the Fangzheng fault depression and Tan-Lu fault zone, and even the eastern part of China to be in a near-East-west squeeze environment (^{Chen et al., 2015}). Fangzheng fault depression under the action of right-lateral strike-slip compression and torsion, along with the northern heat uplift, the northern part of the fault depression is uplifted and denuded, and the stress field presents a three-dimensional state, forming the current tectonic pattern.

Fangzheng fault depression is a series of narrow and Cenozoic fault basin distributed in the northeast direction in the Tan-lu fault zone, which stratum develops from bottom to top:Cretaceous (K), Paleogene Paleocene Xin'ancun Formation (E₁x), Eocene Dalianhe Formation (E₂d), Oligocene Baoquanling Formation (E₃b: E₃b1, E₃b₂, E₃b₃), Neogene Fujin Formation (Nf), and the Quaternary (Q) stratum, generally in the direction of the North East 40 °-50 °, the area is about 1460km².

Asymmetric difference strike-slip characteristics

The Fangzheng fault depression has mainly controlled by tectonic activities such as strike-slip and compression of the Tan-Lu fault zone and the thermal uplift caused by deep geological processes. It presents the characteristics of asymmetric strike-slip pull-apart basins under the superposition of multiple tectonic activities.

Fangzheng fault depression from south to north seismic profile shows different fault depression structures (Fig. 2). The southern structure is wast-faulting and east-overlapping (Fig. 2. A-A), the middle structure is double-faulting (Fig. 2. B-B, C-C), Northern structure is east-faulting and west-overlapping (Fig. 2. D-D). The entire fault depression structure presents an asymmetrical "warping" feature. This is entirely different from the extended fault basin. Therefore, the extended basin model was used to understand the Fangzheng fault depression is not comprehensive.

Figure 2 Fangzheng fault depression structure and seismic section. 

The fault depression has controlled by three faults (Fig. 2). FZ1 is the western strike-slip pull-apart fault boundary; FZ2 is the eastern strike-slip extrusion fault boundary; FZ3 is the middle strike-slip regulation fault. FZ1 and FZ3 control the Cretaceous basin; FZ1 and FZ2 control the Cenozoic basin. Hence, the fault depression has divided into the Western and Eastern fault depression by the FZ3 fault control. The internal fault structure of the West fault depression and the East fault depression is entirely different. The strata in the West fault depression are stable, the Cretaceous strata are developed, and the seismic data quality is good, the signal-to-noise ratio characteristic of the west fault depression is better than the east fault depression (Fig. 3a).

Figure 3 1200-2000ms signal-to-noise ratio image of the 3D seismic work area. 

Conversely, the strata continuity of the East fault depression is weak, the later transformation is strong, the Cretaceous strata are not developed, and the seismic data quality is poor (Fig. 3a). Through the above analysis, it shows the segmentation of the strike-slip characteristics from the south to the north and the difference of the strike-slip mechanism from east to west. In particular, there are different strike-slip fault structures on both sides of the FZ3 fault (Fig. 3b), which have the characteristics of asymmetric difference strike-slip fault basin.

Evolution of multi-stage strike-slip basin

Because of the asymmetric difference strike-slip control basin, since the strike-slip effect through the entire basin evolution process, it cannot have simply explained by the evolution model of the conventional fault basin, and the control and influence of the strike-slip activity on the fault depression must have considered. Since the formation of the Cretaceous, the Fangzheng fault depression has divided six evolution stages according to the different slipping periods (Fig.4). Including the evolution of the two stages slip-control basin in the Cretaceous, the evolution of the four stages slip-control basin in the Cenozoic, under the control of the six-stage strike-slip mechanism, experienced three stages of uplift and erosion, and the sedimentary subsidence center also experienced four migration process from south to north, and north to south.

Figure 4 Evolution mode of the multi-stage strike-slip basin in Fangzheng fault depression 

Cretaceous slip-controlled basin evolution

(1) Early Cretaceous strike-slip pull-apart stage

Early Cretaceous, affected by the high-angle subduction of the western Pacific plate, the left- lateral strike-slip of the Jiayi fault in the north of the Tan-Lu fault zone, the rigid blocks on both sides of the strike-slip fault were separated, and the Fangzheng fault depression began to form. An Early Cretaceous fault basin (Western fault depression, Figure 5) near the northwest-south-eastward distribution was created on the pre-Mesozoic granite or metamorphic rock basement.

Figure 5 Comparison of geological modeling between the East Depression and the West Depression 

(2) The end of early Cretaceous to late Cretaceous strike-slip extrusion stage

As the direction of the western Pacific plate changed from the north to the northwest, the northeastern part of China gradually entered the extrusion environment at the end of the Early Cretaceous, and the fold uplift suffered erosion of Fangzheng fault depression. The Cretaceous residual strata have developed in the southern part of the fault depression. The Cretaceous weathering crust revealed by the northern drilling confirmed the uplift.

Cenozoic slip-controlled basin evolution

In the early Paleogene, the movement direction of the western Pacific plate changed to the north-northwest, and the Fangzheng fault depression was again left-lateral strike-slip. At this time, the FZ2 fault began to become the eastern boundary fault of the Fangzheng fault depression, and as the fault deepened, the magma and the volcanic activity promoted the extension of the lithosphere, which caused the Fangzheng fault depression to enter the Paleogene main basin period, and formed the East fault depression (Fig. 5) between FZ2 and FZ3. The East Fault Depression and the West Fault Depression constitute a Cenozoic fault basin (Fangzheng fault depression).

(1) Paleocene-Eocene strike-slip pull-apart stage

The Fangzheng fault depression began to pull-apart in the north-south direction. Because the tensile stress direction is oblique to the northeast trend of the Fangzheng fault depression, this oblique pull-apart has characterized by a left- lateral torsion in the Fangzheng fault depression. Under the action of left- lateral torsional stress, the fault FZ1 in the northwestern margin and the fault FZ2 in the southeastern margin are strongly active, but the FZ3 activity is weak, at this time, the structural of the Fangzheng fault depression has a double fault, and the structural pattern of the sag-uplift fault depression is initially formed.

(2) At the end of the Eocene, the strike-slip extrusion stage

At the end of the Eocene, the Fangzheng fault depression extrusion in the right- lateral background, the overall depression of the fault depression, the basin ascends as a whole, the northern part of the basin rises higher, and the south is relatively low. The Xin'ancun Formation (E1x) and the Dalianhe Formation (E2d) were strongly denuded, in particular, the Dalianhe Formation (E2d) has been eroded in the northern part of the fault depression.. As a result, the remnants of the Xin'ancun Formation (E1x) are also relatively thin.

(3) Oligocene-Neogene differential strike-slip settlement stage During the Oligocene, as the western Pacific plate turned into high-angle subduction in the northwest direction, FZ3 began to move strongly, and the settlement of the west fault depression was small. The East fault depression received huge thick sediment under the control of FZ3, and the Fangzhengr fault depression began to enter the differential settlement stage. Affected by the paleo-geomorphology the Baoquanling Formation (E3b: E₃b₁, E₃b₂, E₃b₃) was overtaken from the north to the south.

(4) Post-Neogene strike-slip compression and uplift stage

The Fangzheng fault depression is affected by the regional right-lateral strike-slip compression stress, and the tectonic inversion occurs. The north and south sides of the fault depression are strongly uplifted and most eroded. The northern boundary forms a thrust-napped structure under the compressive stress, which is leading to the blurring of the north boundary fault. Eventually, the tectonic pattern of the asymmetry difference between the east fault depression and the west fault depression was formed (Fig. 5).