Remagnetization In The Early Triassic Strata Over The Cathaysian Block

Palomagnetic studies have been conducted on the Lower Triassic statra in Fujian province (175 samples), Guangdong province (246 samples) and Guangxi province (283 samples). Most of the samples are remagnetized in the late periods. The Daye Formation in the Guangdong province is the only one that reveals primary magnetization.Paleomagnetic and rock magnetic studies on the Lower Triassic Xikou Formation in Fujian province and Lower Triassic Daye Formation in Guangdong province and Lower Triassic Majiaoling Formation and Nanhong Formation in Guangxi province show that most magnetic carriers are dominated by PSD domain state magnetite. Magnetic carriers of sites 13-18 from Huangben and sites 4-6 from Nasuo are dominated by PSD and MD domain state hematite.Stepwise thermal demagnetization on all the samples of Xikou Formation in Shangjing, Fujiang province shows double magnetic components. The low temperature component (0-200℃) is consistent with the present geomagnetic field and fails a fold test, which corresponds to a recent overprint. The overall mean direction of the high temperature stable components of Xikou Formation is D=11.2°, 1=43.9°,κ=73.4,α₉₅=6.5°at geographic coordinates and Ds=31.3°, Is=65.2°, ks= 11.1,α₉₅=17.4°at stratigraphic coordinates, showing that directions concentrate at geographic coordinates and directions decentralize at stratigraphic coordinates. Pole position isφ=205.2°,λ=79.9°, dp=8.1°, dm=5.1°and palomagnetic latitude is 25.7°N at geographic coordinates. To contrast with the APWP for Yangzte Block (YZB) of Yang et al (2001), this pole machs well with the late Cretaceous period pole of the APWP by Yang, so it should be a result of severe Yanshanian period remagnetization.Studies on the Lower Triassic Daye Formation show that the directions isolated from intermediate-temperature concentrate at geographic coordinates. To contrast with the APWP for YZB of Yang et al (2001), the intermediate-temperature (100℃～300℃) component (Ds=56.8°,Is=25.5°,α₉₅=8.8°) with pole position at 85. 1°N, 183.0°E, dp=12.7°, dm=8.0°is a Yanshanian viscous remanence.Studies on Lower Triassic Majiaoling Formation at Banxin, Guangxi province show that an overall mean direction of high temperature (350-700℃) stable components is Dg=358.5°, Ig=45.4°, kg=3210, a₉₅=2.2°at geographic coordinates and Ds= 324. 5°, Is= 67. 2°, ks=3100, a₉₅=2.2°at stratigraphic coordinates. Pole position of Banxin is 85°N, 91.4°E, dp=2.8°, dm=1.8°and palomagnetic latitude is 26.9°N at geographic coordinates. To contrast with the APWP for YZB of Zhu et al (1998), this pole matchs with the Quatermary period pole of the APWP, so it should be the result of the severe remagnetizations at Himalayan period. Studies on Lower Triassic Nanhong Formation at Nasuo, Guangxi province show that the overall mean direction of the high temperature (350-700℃) stable components is Dg=0.8°, Ig=37.5°, k=38.3,α₉₅=11°at geographic coordinates and Ds=42.5°, Ig=10.8°, k=1.4,α₉₅=105.5°at stratigraphic coordinates. Pole position of Nasuo is 89°N, 242.4°E, dp=12.9°, dm=7.6°and palomagnetic latitude is 21°N. Directions concentrate at geographic coordinates. To contrast with the APWP for YZB of Zhu et al (1998), this pole matches with the pole of Quatermary period at the APWP, so it should be the result of the severe remagnetizations in Himalayan period.The Mesozoic and Cenozoic severe remagnetizations are widely found on the Lower Triassic Statra over Cathaysian block. Widespread magma activity and tectonic movement in the Cathaysian Block and Rock efilorescence in warm and humidity climate at low latitude are the main cause to the remagnetizations.