Early Triassic Paleomagnetism Of Cathysian Block And Its Tectonic Implications

The South China Block is mainly composed of three tectonic units: the Yangtze para-platform ( Yangtze Block ), the South China fold belt ( Huanan Block), and the southeast coast fold belt ( Dongnanyanhai Block ). The latter two are generally called: Cathysian Block. In recent years, researchers on the South China Block produced a set of paleomagnetic data and some achievements from the Cathysian Block. But generally speaking, paleomagnetic data from the Cathysian Block in the Early Triassic is very limited and covered limited aeras. Some Triassic paleomagnetic data from Cathysian Block lack of detail remanence analysis, stability test and rock magnetism investigation. Futhermore, the paleomagnetic data from Pre-Early Mesozoic show inconsistent results with different researchers due to the complexity of South China tectonic, particularly to the multiple stage alterations by universal magmatic activity and fault motion. Therefore there is constant dispute on the timing of collision between Yangtze block and Cathysian block according to paleomagnetic data.During the research program of "Early Triassic Paleomagnetism of South China" in recent years, we have investigated the Triassic paleomagnetism among large areas over Guangdong, Guangxi, Fujian and Zhejiang provinces. Among the samples obtained in the investigated areas, the character remanence magnetization can be distinguished from only the Huangben section in Guangdong province; therefore reasonable results have been obtained. It was found that the isolation of primary remanence in other sections was very difficult, because of the severely remagnetism affected by magmatic activity and faulty.Paleomagnetic and rock magnetic studies on 122 samples of 12 sites from the Early Triassic Daye Formation in the Huangben section, Guangdong province show that the magnetic carrier of the Early Triassic are dominated by titanium-bearing magnetite with small amount of hematite. Stepwise thermal demagnetization was performed and vector analysis was used to isolate magnetic components, which indicate two or three magnetic components: i.e. a low-temperature component (A), an intermediate-temperature component (B), and a high-temperature characteristic remanent magnetization (ChRM). The low-temperature (NRM～150°C) component is consistent with the present geomagnetic field direction and fails fold tests; the intermediate-temperature (100°C³00°C) component (Ds=56.8°,Is=25.5o,α₉₅=8.8o) is a Yanshanian overprint remanence; the high-temperature (350°C ～530°C) component passes generalized fold test and reversal test, which suggeste that the high-temperature component is probably primary magnetization. A tilt-corrected formation mean direction is Ds=51.1°, Is=15.3°, α₉₅=5.0°, corresponding to a pole position at 38.5°N, 212.7°E, dp=2.6, dm=5.1. This is overlap with the Early Triassic pole position of the Yangtze block at 95% confidence level, which reflects that Cathysian Block was already collided with Yangtze Block in the Early Triassic. The palaeomagnetic data reported in the Early Triassic from the Cathysian Block demonstrated that their magnetic inclinations are similar, but their declinations change obviously and distributing on a small circle in the equal-area projection, which implies the widespread local rotation existed in South China leads to the difference of the paleopole of all the quarters.