Early Cretaceous Paleomagnetic Results From Simao Of Indochina Block And Its Tectonic Implications

Simao area is located in the northern part of the Indochina block, and in the flank zone of the Indian and Eurasian collision, where is the good area for the research to intracontinental deformation during continental collision. In this paper, paleomagnetic study was carried out on the Early Cretaceous clastic rocks at Simao area, aiming to explore the intracontinental deformation mechanism and processes, and to improve the understanding of structural deformation taken place in the southern margin of the Eurasia continent.At Simao area the lower Cretaceous Mangang formation are terrigenous clastic rocks, mainly red mudstone, siltstone and sandstone. A total of439oriented core samples were collected at Zhenyuan, Pu’er and Jiangcheng. Based on the studies of rock magnetism(k-T, IRM and triaxial IRM), have shown that goethite, magnetite and hematite are magnetic carrier minerals, and hematite is the main in samples. Thermal demagnetization results show that most of the samples exhibited two components, and a small amount of samples for the single-component and three-component.The high temperature components are all normal polarity, consistent with magnetization acquired during Cretaceous Long Normal Superchron. High temperature components from Zhenyun and Pu’er section can pass the fold test at95%confident level. Because of monoclinic strata in Jiangcheng section, combined the remanent direction with those from previous two sections also pass the fold test, these results idicate that the high temperature components are the primary remanence. The characteristic remanent magtization (ChRM) directions obtained from Zhenyuan, Pu’er and Jiangcheng area Ds=52.4°, Is=45.5°, κ=77.9, a95=6.3°, corresponding to a paleopole at42.9°N,175.9°E with A95=6.4°; Ds=46.2°,Is=46.6°, κ=50.9, a95=5.6°, corresponding to a paleopole at48.2°N,174.2°E with A95=5.8°; and Ds=8.6°, Is=42.2°, κ=117.1, a95=4.0°, corresponding to a paleopole at:81.9°N,176.8°E with A95=3.9°, respectively.The elongation/inclination (E/I) correction model was applied to evaluate the inclination error from the Pu’er and Jiangcheng sections.The results of the correction model indicate that inclination shallowing was negligible within the uncertainty in Jiangcheng, however there’s the illusion of inclination shallowing in Pu’er, as the declinations are scattered, resulting in the illusion of elongation in declination and the corrected inclination is apparent too large. The difference of declinations between all the sample sections at Simao area suggests that differential rotation has been taken place within the block, the deformation process is not only controlled by boundary faults, is also closely related with activities of the internal small faults. Taken a little shallow inclination in Cretaceous red beds in the South China block into consideration, the Simao area has been subjected to south displacement by570±310km with respect to the South China block since Cretaceous time, presumably close to the estimation of structural geology evidences.