Formation of the Cenozoic Ailao Shan mid-crustal tectonic discontinuity: Role of Oligo-Miocene stratified sub-horizontal middle to lower crustal flow in the southeastern Tibetan Plateau

Abstract

Structural, kinematic and geothermometry analyses are applied to unravel the structural evolution of the Ailao Shan Tectonic belt (ALTB) with a specific goal of understanding how crustal masses in southeastern Tibetan Plateau flowed during the Indian-Eurasian continental collision. A mid-crustal tectonic discontinuity that is characterized by a narrow ultramylonite belt separates the highly sheared high-grade metamorphic unit (HMU) from the low-grade metamorphic unit (LMU) along the ALTB. It is shown that rocks from the HMU and LMU possess deformation associations of identical structural geometries and kinematics, but at contrasting metamorphic conditions. Calculated mean kinematic vorticity numbers indicate that the ultramylonite belt was affected by dominant simple shearing, while the HMU and LMU on both sides of the belt were deformed by general shearing dominated by pure shearing. Geothermometry analyses on sheared two-mica schists from the HMU and quartz veins from the LMU reveal different deformation conditions, i.e., 610–777 °C, 0.4–0.6 GPa and 320–391 °C, respectively. There is also a sharp decrease in deformation temperature inferred from deformation microstructures and quartz c-axis fabrics, from 550–650 °C for the HMU to 350–400 °C for the LMU. These results suggest that there is an apparent temperature break of ca. 220 °C over a total structural distance of approximately 1 km between these two units at the present outcrop level. Our results show that the HMU and LMU are kinematically linked while rheologically decoupled, and were originally deformed by sub-horizontal shearing at different crustal levels in the same strain field. The ultramylonite belt between them potentially inherited a preexisting basement/cover contact along the ALTB and brought the two units into contact. We suggest that progressive stratified flow of middle to lower crustal masses during the extrusion of the Sundaland block at Oligocene to Early Miocene was responsible for the concurring high- and low-temperature fabrics at different crustal levels. Contemporaneous doming during the progressive crustal flow resulted in exhumation of the lower crustal rocks. During the process, the ultramylonite belt occurred as a tectonic discontinuity (TDC) that leads to the loss of a thick pile (ca. ∼6.3–7.8 km) of crustal masses in the middle crust.