Lateral middle-lower crustal flow in response to continental collision: New insights from the metamorphic complexes in the southeastern Tibetan Plateau

Abstract

There have long been controversies over the extrusion mechanism of the Sundaland block in the southeastern Tibetan Plateau in response to the tectonic convergence between the Indian and Eurasian plates. In this study, we focus on macro- and micro- structural and kinematic analysis, timing of shearing, and thermal history reconstruction of several typical metamorphic complexes in the southeastern Tibetan Plateau in order to understand the tectonic processes driving the deformation of the complexes and extrusion of the block. It is shown that Cenozoic dome structures are widely distributed in the area. The dome cores comprise high-grade metamorphic rocks with high-temperature deformation fabrics (HMU), while rocks in the mantles are characterized by low-grade metamorphism and low-temperature deformation (LMU). Foliation and lineation in the rocks of different units of the domes are generally parallel to the stratigraphic horizons, implying the dominant role of bedding-parallel shearing in all the domes. Shear discontinuity contacts exist as high strain zones between the HMUs and LMUs that the sheared complexes show kinematically coupled but rheologically decoupled two-layer structures. Kinematic indicators from all the domes show consistent LMU-to-the SE shearing. Thermochronological data reveal that the initiation of doming was coeval with ductile shearing of the middle to lower crustal rocks, but was diachronous at different places. Most of the domes possess two-stage cooling histories. We conclude that the ductile shearing, formation, and exhumation of the dome structures were ascribed to middle and lower crustal flow that accommodated a large amount of deformation during the extrusion of Sundaland block.