Lateral middle-lower crustal flow in Southeast Tibetan Plateau: structural analysis from the metamorphic complexes 

Abstract

The India-Eurasia collision and subsequent post-collision process since about 55Ma have an important influence on the middle and lower crust activities, formation and evolution of the crust structure in the Tibetan Plateau and its periphery. Several models have been proposed to explain the response of the lateral collision zone in Southeast Tibetan Plateau, such as rigid block extrusion, middle and lower crust channel flow, and crust thickening. There are several metamorphic complexes with Precambrian basement in the margins and interior of the Sundaland block. We have carried out structural analysis, kinematic analysis, shear and exhumation time studies on these complexes, which provide new important clues to explain the flow patterns in Southeast Tibetan Plateau during the India-Eurasia collision. Structural analysis of the metamorphic complexes reveals that dome structures are widely developed in the southeastern Tibetan Plateau. The cores of the domes are composed of high-grade metamorphic rocks that were deformed at high temperatures and partly superimposed by subsequent low temperature deformation. The mantles of the domes are composed of low-grade metamorphic rocks that were deformed at relatively low temperatures. Shear discontinuity contacts exist in most cases between the core and the mantle. Subhorizontal (bedding-parallel) shearing dominated the deformation of the crustal rocks during doming. Regional-scale A-type domes, local-scale mylonitic foliation, stretching lineation and outcrop-scale A-type folds were formed due to the subhorizontal shearing. Kinematically linked but rheologically decoupled two-layer structures are evidenced by the existence of tectonic discontinuity contacts between the cores and mantles in the domes. New geochronological dating results in combination with dating results in the literature suggest that middle to lower crustal shearing occurred from ca. 30 to 20 Ma. Most of the metamorphic complexes in the southeastern Tibetan Plateau began their exhumation accompanied by ductile shearing since 30 Ma, while the initiation of the exhumation of the complexes was diachronous at different locations, earlier along marginal high-strain zones and later in the interior of the Sundaland block. Exhumation of the complexes is characterized by the two-stage cooling histories of various characteristics, which indicates that the different complexes were sheared and exhumed separately. Formation and exhumation of the domes are related to middle and lower crustal flow, during which shearing, folding and exhumation are simultaneous for particular domes. The middle to lower crustal flow lines constitute a general convergent flow pattern. The counter flow, i.e., southward flow of the rigid upper crust and northward flow of the viscous middle-lower crust, is the most likely explanation for northwestward flow of the dome cores relative to the mantles.