The development regularity and genetic mechanism of intracratonic faults under the control of regional tectonic background and pre-existing structures--A case study of Lunnan area in northern Tarim Basin

The Tarim Basin is a multi-stage and multi-cycle superimposed basin developed on a stable craton basement. The Lunnan area is located in the central and eastern part of the Tabei uplift in the northern Tarim Basin. It has developed some fault systems with different strikes, properties, evolution stages. Based on the tectonic interpretation of 3D seismic data, we investigated the geometry ang kinematics of faults in Lunnan area.The formation and evolution of faults in Lunnan area occurred with the help of the pre-existing faults and were influenced by the regional compression/extensional direction transformation.

In the Neoproterozoic, the Tarim Basin was in an extensional tectonic background as a whole. The Lunnan area developed two rifts in the EW and NE directions, and the boundary normal faults were used as pre-existing structures. In the early Caledonian, the near NS direction Cambrian platform margin belt in the eastern part of Lunnan area is developed as the pre-existing weak belt.In the middle of the Caledonian, with a nearly NS-trending extrusion, the near EW-trending Lunnan fault grew and developed upward on the basis of the upper high-angle pre-existing fault surface of the early rift boundary normal fault, and the high-angle thrust fault was developed. Under the pure shear deformation mechanism, an X-type conjugate strike-slip fault system composed of two groups of NNE-and NNW-trending faults was developed.In the late Caledonian-early Hercynian, with a nearly NS-trending extrusion, the rapid uplift in the central and western parts of Lunnan area leads to a large amount of erosion of Ordovician strata to form a NEE-trending lithologic weak zone. Under the action of oblique compression, the pre-existing weak zone was activated by strike-slip and formed a series of NEE-trending strike-slip faults. At the boundary of the nearly NS-trending Cambrian platform margin zone in the eastern part of the Lunnan area, a nearly NS-trending fault was formed by the activation of the pre-existing weak zone under oblique compression.In the late Hercynian, under the NWW-trending extrusion, the near EW-trending Lunnan fault, NEE-trending and near NS-trending strike-slip faults continued to active.The NE-trending Lungu 7 fault inherits the high-angle fracture surface in the upper part of the NE-trending pre-existing rift normal fault, and develops a high-angle thrust fault. The pre-existing structure is not developed in the deep layer of the near EW-trending Sangtamu fault, and a thrust fault with a gentle dip angle conforming to the Anderson model is formed under the forward extrusion.With the change of regional compressive stress direction and the transformation from carbonate strata to clastic strata, the conjugate X-type strike-slip fault gradually disappeared.In the early Indosinian period, the Tarim Basin still showed a near NS-trending compressive stress background. In the middle and late stages, it was transformed into a NW-trending extensional background. The early stage of the fault still inherited the compressional nature, and the late stage superimposed extension-strike slip activity.In the Yanshanian-Early Himalayan period, the NW-trending extensional tectonic background induces the formation of tenso-shear echelon faults in the shallow layer.