Structural evolution and exhumation of the Yulong dome: Constraints on middle crustal flow in southeastern Tibetan Plateau in response to the India-Eurasia collision

摘要

It is generally accepted that lateral rigid block extrusion of the Sundaland block in southeastern Tibetan plateau accommodated the India-Eurasia collision. Recent studies, however, revealed that channel flow of low-viscosity middle to lower crust can explain outward growth of the Tibetan plateau. Controversies continue as to which of these processes is most significant and how the crustal mass flew if happened in response to the India-Eurasia collision. In this contribution, we focus on characterizing the Cenozoic ductile deformation of the Yulong complex beyond the eastern Himalayan Syntaxis. Detailed structural, microstructural, fabrics and thermo-chronological studies demonstrate that the Yulong complex constitutes an anticlinal dome structure. Three stages of structural deformation, i.e., D-1-a pure shear-dominated deformation, D-2-a simple shear-dominated progressive deformation and D-3-a superposition deformation, contributed to the formation and exhumation of the dome structure. The main stage of exhumation of the Yulong dome occurred in Oligocene-Miocene (34-8 Ma). The core part experienced fast cooling from similar to 425 degrees C at ca. 11-12 Ma to similar to 110 degrees C at ca. 8-9 Ma with cooling rate of 105-109 degrees C/Ma. However, the limb experienced relatively slow cooling, with cooling rate of 10.8 degrees C/Ma from 33.5 Ma to 16 Ma. Such a cooling pattern (earlier, slower in the limbs and later, faster in the core) is interpreted as the result of exhumation by anticlinal folding. Structural analysis reveals that the doming was coeval with bedding-parallel shearing of the Yulong complex. We show, from the present study, that Oligo-Miocene crustal flow occurred along subhorizontal shear zones throughout the middle to lower crust in eastern and southeastern Tibetan Plateau. Top-to-the south or southeast subhorizontal shearing during doming possibly resulted from combined effects of southward gravitational sliding of rigid upper crust and northward flow of viscous lower crust relative to the middle-upper crust.