Paleomagnetic study of Permian and Triassic basalts from the Baoshan block in western Yunnan, SW China

Abstract

The Baoshan Block is a wedge-shaped fragment that forms the northern tip of the Sibumasu Terrane in westernmost Yunnan Province, SW China. The fragment detached from Gondwana in the Early Permian before amalgamating with eastern Eurasia in the Late Triassic. It is now bounded by the Gaoligong Suture Zone to its west and the Chongshan Suture Zone to the east. The initial objective of this study was to use paleomagnetic data from two volcanic units to reconstruct the Baoshan Block’s rotation and translation history over this interval. However, based on the paleomagnetic analyses, the magnetizations of both the Lower Permian and the Upper Triassic basalts record secondary overprints. The age of remagnetization likely coincides with the regional crustal shortening caused by the India-Eurasia convergence at 30 Ma. The 10% untilting of the mean directions from twenty-nine sites yields a declination of 4.0°, inclination of 64.4°, where α_95= 5.9°, and k = 15.4. This gives a paleo-latitude of 46.5 ±2.9°N at that time. The result supports the “tectonic extrusion” model with ~2928 ± 534 km of southward translation of the Baoshan Block, induced by the India-Eurasia convergence. Aside from the tectonic modeling, the project’s other objective was to evaluate the efficacy of using one block-sampling per cooling site in paleomagnetic studies.