Late Cenozoic to recent transtensional deformation across the Southern part of the Gaoligong shear zone between the Indian plate and SE margin of the Tibetan plateau and its tectonic origin

Abstract

Within the southeastern part of the Tibetan plateau the Gaoligong Shan shear zone forms the boundary between the Tengchong and Baoshan blocks and is one of the main Cenozoic strike–slip fault systems of the eastern Himalayan syntaxis. The mountain ranges rise to a maximum elevation of 3623 m, with a width of 20 km and a length of 400 km. The core of the shear zone consists of a narrow belt of mylonitic rocks with right-lateral strike–slip fabrics formed during the northward movement of India relative to Eurasia in Oligocene and Miocene time. To the south the mountains decreases in elevation and end against the Wanding fault. The latter is one of a series of NE–SW striking, active left-lateral strike–slip faults within the southeastern part of the Tibetan plateau. The cause and timing for the change in elevation of the Gaoligong Shan is unknown, but based on our recent structural and fission-track geochronological work, the decrease in elevation is the result of east-west extension across the southern part of the Gaoligong shear zone inferred from the late Cenozoic and active normal faults along its eastern and western boundaries. Southward the eastern boundary fault system merges with a series of splays of the Wanding strike–slip fault, while the western boundary fault system is bent into parallelism with the Wanding fault. This geometric relationship suggests that the normal faulting along the eastern and western boundary of the Gaoligong shear zone was caused by transfer of the NE–SW left-lateral movement along the Wanding fault into E–W extension on faults to the northeast. The resultant normal faulting caused the lowering of the topography in the southern part of the shear zone. Fission track data from 11 samples from the mylonitic rocks in the footwall of the Ruili oblique slip fault, (left-lateral thrust slip) yielded ages between 8.4 and 0.9 Ma, coeval with volcanism within the Jietou and Mangbang basins. The left-lateral strike–slip movement along the Wanding and Ruili faults is interpreted to have accommodated differential clockwise rotation of the Gaoligong shear zone and its adjacent tectonic units during late Cenozoic to Recent time, during the northward movement of the Indian plate with respect to the southeastern margin of the Tibetan plateau along the right-lateral Sagaing fault.