Structural and geochronological studies of the Cona Detachment and Cona Rift: Implications for the Miocene evolution of the eastern Himalayan extensional structures

The various extensional structures developed at the shallow surface are the coupled embodiment of the deep dynamics, which have played significant roles in the Himalayan tectonic evolution. The two sets of intersecting extensional structures in the Cona area of eastern Himalayan orogen are studied, this paper presents new detailed field investigations, microstructures, quartz [c] axis CPO patterns, kinematic vorticity, deformation temperatures, zircon U-Pb, and mica ⁴⁰Ar/³⁹Ar geochronology. The results suggest that the Cona Detachment (CD) is mainly in pure shear deformation and the ductile deformation temperature ranges from 280 to 517℃. It was active between 19 and 16 Ma, and ceased at 15 Ma. However, the Cona Rift (CR) is mainly in simple shear deformation and its top-down-to-the-E ductile deformation is recorded at temperatures from 500 to 608℃. It initiated at ~16 Ma, and moved until 10 Ma. Statistically, it was found that the cessation of STDS and the initiation of NSTR seem to follow a youthful trend from west to east along the Himalayan orogen, and there is an overlap in the period of activity between them. Combined with previous studies, we speculate that this process resulted from the Indian plate tearing and the asthenosphere upwelling. Ultimately, this tectonic event leads to the mid-Miocene regime transition. The shallow surface reflects the change from N-S to E-W extensional movement in the mid-Miocene, while the fluid content, heat source, and stress field conditions are also changed in response.