Paleomagnetic study of Upper Jurassic rocks from the Sichuan basin: tectonic aspects for the collision between the Yangtze Block and the North China Block

摘要

A paleomagnetic study has been carried out on Late Jurassic redbeds in the Sichuan basin, at the northern part of the Yangtze Block. Upper Jurassic brownish-red sandstones and brownish-red siltstones were collected in the Penglaizhen Formation at 26 sampling sites around Jiangyang city (30.4°N, 104.5°E). Thermal demagnetization isolated a high temperature magnetization component with a maximum unblocking temperature of about 690°C. The primary nature of magnetization acquisition is ascertained through a 95% confidence level positive fold test, as well as a positive reversal test. The tilt-corrected overall mean direction of 24 sites is D = 25.9°, I = 26.7°(α_(95) = 4.7°), leading to a paleomagnetic pole situated at 61.3°N/222.7°E (A_(95) = 4.2°). This pole position agrees with two other Late Jurassic poles from the Sichuan basin, indicating that this basin has behaved as a rigid block since Late Jurassic. These three poles from the Sichuan basin therefore provide a characteristic Late Jurassic pole for the stable part of the Yangtze Block (64.3°N, 231.2°E, A_(95) = 6.7°). Comparison of the Late Jurassic pole of the Yangtze Block with the contemporaneous pole for the North China Block (NCB) identifies a significant difference, implying that the relative movement between the Yangtze Block and the NCB continued during, and probably after, the Late Jurassic. Paleomagnetic and geological evidence reveals that the Yangtze Block collided first with the NCB during the Early Permian, and continued to penetrate into it while undergoing a clockwise rotation until the Early Cretaceous.