Constraints on the depth, geometry and kinematics of blind detachment faults provided by fault-propagation folds: An example from the Mesozoic fold belt of South China

The Mesozoic multi-layered thin-skinned, fault-related fold belt in South China is characterized by propagation chevron folding and thrust faulting. We examine the geometry and depth of blind detachment faults in this belt using faults and folds that crop out along a well-defined cross-section. A combination of parameters such as backlimb dip angle (2θ), interlimb angle (γ), height of anticline (h), thickness of the referenced layer (h0), width of the anticline (AB) and thickness-change ratio of layering within the forelimb (tf/t) are used to develop new equations to estimate the geometry and depth of the detachment faults. In the cross-section, the Huangdushan chevron anticline has parameters revealing a blind fault with a ramp angle of 42° and a flattened segment at a depth of 4.9 km. Based on existing seismic data, we interpret this blind fault as a backthrust developed over an in-sequence imbricated duplex of Cambrian to Ordovician strata. A chevron anticline at Qiyueshan also reveals a backthrust fault. However, a broad anticline in Lengzhuba and a small cylindrical, sinusoidal anticline in Yupize reveal a flat-ramp-flat detachment fault with flat segments at depths of 7.4 and 3.4 km, respectively, suggesting that the Sinian strata were thickened probably by an out-of-sequence imbricated duplex. sults indicate that the flat-ramp-flat detachment fault along the bottom of the Sinian strata produced the out-of-sequence imbricated duplex and cylindrical sinusoidal folds. The in-sequence imbricated duplex produced backthrusts and related chevron anticlines. We consider that this fault-related fold belt was produced by collision between the Yangtze Block and North China Blocks.