Tectonics and Paleomagnetic Rotation Pattern of Yunnan (24°N-25°N, China): Gaoligong Fault Shear Versus Megablock Drift

Abstract

A wealth of paleomagnetic data from Yunnan (China) showed in the past a predominant post-Cretaceous clockwise (CW) rotation pattern, mostly explained invoking huge (hundreds of kilometer wide) blocks, laterally escaping (and/or rotating) due to India-Asia collision, separated by major strike-slip shear zones. Here we report on the paleomagnetism of the outcrops close to the Gaoligong dextral shear zone. Fifty paleomagnetic sites (503 samples) were sampled at variable distances (up to ~25 km) from mylonites exposed along the fault. Eighteen Jurassic-Cretaceous red bed sites yield systematic CW rotations with respect to Eurasia that peak at maximum (176°) close to the fault, and progressively decrease moving eastward, up to be virtually annulled ~20 km east of mylonite contact. West of the fault, 15 Pliocene-Holocene sites from the Tengchong volcanic field do not rotate. Thus, our data show that Gaoligong shear zone activity yielded significant CW rotations that were likely coeval to the main Oligo-Miocene episodes of dextral fault shear. The Gaoligong zone rotation pattern conforms to a quasi-continuous crust kinematic model and shows blocks of ≤1 km size close to the fault that enlarge moving eastward. Rotation values and width of the rotated-deformed zone translate to a 230–290 km Gaoligong shear zone dextral offset. Our work shows that fault shear plays a significant role for Indochina CW rotation occurrence. However, significant rotations at distances >30 km from main faults were also documented, so that additional tectonics—whose relative relevance has not been elucidated yet—must contribute as well to the rotation pattern.