Paleomagnetic Data Bearing on Tectonic Rotations in Northern Indochina South of the Ailao Shan Shear Zone

During middle Cenozoic time Indochina moved SE relative to South China and motion has been interpreted to be a more or less rigid translation coupled with small magnitude clockwise rotation of Indochina during India-Eurasia collision. The northern boundary of Indochina is the Red River fault, which was active as the left-lateral Ailao Shan shear zone, southwest of which is the Lanping-Simao fold/thrust belt, defined by Mesozoic and Cenozoic redbeds folded in the middle Cenozoic. Structures in the Lanping-Simao tectonic belt show strongly sigmoidal fold and fault traces and a similar sinuous boundary with terranes, of a more marine arc affinity, to the west. Interpretation of these structures suggests these rocks have undergone significant regional and internal CW rotations of more than 60^o. Our study involves Upper Jurassic to lower Tertiary redbeds across and along the fold/thrust belt to quantify rotations in the Lanping-Simao area. The growing data set, based on a collection of some 9000 samples, including recently collected ones in northwest Vietnam, is interpreted to support variable(up to 90^o) but consistently clockwise rotations. Inclination data imply a southward translation of this terrane, but the magnitude and statistical significance are unknown. Northeast of the Ailao Shan shear zone, crust was also affected by modest vertical axis rotation that appears to diminish in magnitude northeastward, as shown in previous work and current studies. The northern part of Indochina was complexly deformed during SE motion in a style incompatible with simple left-lateral translation and large-scale block rotation. Structures in rocks west of the Lanping-Simao unit show similar trends suggesting most of northern Indochina has been subject to complex internal deformations around a developing eastern Himalayan syntaxis.