Masahiko Yokoyama

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

Abstract 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° (α95=4.7°), leading to a paleomagnetic pole situated at 61.3°N/222.7°E (A95=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, A95=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.

Miocene clockwise rotation of southwest Japan and formation of curvature of the Median Tectonic Line: Paleomagnetic implications

The origin of the curvature of the Median Tectonic Line (MTL) in southwest Japan is still controversial. In order to investigate its formation, we carried out a paleomagnetic study in the Nohi area, north of the curved part of the MTL in the eastern part of southwest Japan. More than 420 paleomagnetic samples were collected at 35 sites from Cretaceous to Paleogene welded tuffs in the Kasagatake Rhyolites, Oamamiyama Group, and Nohi Rhyolite. Characteristic directions with high unblocking temperature component above 560°C were isolated from 32 sites. Paleomagnetic directions of the Kasagatake Rhyolites and Oamamiyama Group (55–66 Ma) have a clockwise deflected declination (D = 51.7°, I = 52.6°, α95 = 8.6°) which is consistent with the characteristic Paleogene paleomagnetic direction of the central part of southwest Japan. A similarly deflected declination is also observed in the peripheral part of the Nohi Rhyolite region, far away from the Atera fault system. Paleomagnetic data indicate that the Nohi area experienced a clockwise rotation through more than 45° with respect to the Asian continent as a part of southwest Japan but that the amount of rotation of the Nohi area is 23° smaller than that of the central part of southwest Japan. A new model is developed to explain the tectonic rotation. At about 15 Ma during the latest stage of the clockwise rotation of southwest Japan, the eastern end moved southward by more than 60 km along a left-lateral strike-slip fault which comprised the present north-south segment of the MTL and the Akaishi Tectonic Line. We conclude that the curvature of the MTL is a composite of the 15 Ma strike-slip fault system and a small bend of the MTL associated with the small differential rotation of the Nohi area with respect to the central part of southwest Japan.