Late Jurassic Paleomagnetic Results From Late Jurassic Sediments Of The Junggar Terrane And Its Tectonic Implication

In order to contribute to palaeogeographic position and tectonic evolution of the Junggar block since the late Jurassic,we carried out a systematic paleomagnetic researches the upper Jurassic continental red sedimentary rocks in the Xinjiang Manas river section of the southern margin of Junggar.The inclination shallowing was paid attention on the continental redbeds using the different methods,including the Eocene continental red sedimentary rocks in Simao area of Indosinian block.Detailed paleomagnetic and rock magnetic studies were carried out in upper Jurassic continental red sedimentary rocks in the Manas river section of the Junggar block.High-temperature magnetic components were isolated from most of the redbeds,and the positive reversal test and fold test show its primary nature of magnetization.The tilt-corrected site-mean direction of the Upper Jurassic from the Manas river section is Ds=13.3°,Is=47.2°,k=4.1°,?95=3.2°.Using Hodych and Buchan inclination rectification methods,the inclination was corrected to 53.5°,and the inclination was 54.1° with confidence interval[47.3°,62.6°]by the means of "E/I" statistic analysis method,which indicate a consistent results by two different methods.The "E/I" method indicates that the inclination shallowing induced by sedimentary compaction is 6.9°.Comparing with the 160 Ma reference pole of Eurosia,the expected value is still higher than observed value about 12.6°.The late Jurassic APWPs of the Eurosia is probably not appropriate reference pole for the Junggar block.Comparing with the late Jurassic reference pole of the Tarim block,the inclination is expected to 56.8° for the Junggar block,which is indifference with the value of the "E/I" method.Because the reference pole of the Tarim block is consistent with that of the Junggar block in the middle-late Jurassic,It seems that the Junggar had collided with Tarim block in 160 Ma.The Siberian block pole position is different with that of Junggar,it's very probably that late Jurassic pole of Eurosia could not represent the pole of Siberian block.Paleomagnetic inclination shallowing studies were carried out on the Eocene red beds at Simao area of the Indochina Block.High-temperature magnetic components were isolated from the Mengban and Mengla sections,the tilt corrected site-mean directions at Mengban and Mengla are Ds=118.2°,Is=22.1°,k=31.6,?95=10.9° and Ds=47.6°,Is=22.8°,k=20.2°,?95=5.9°,respectively,which are consistent with those previous published in the areas.The inclination shallowing correction method proposed by Hodych and Buchan(1994)and elongation/inclination("E/I")correction are used to quantitatively estimate the inclination bias of Eocene red-beds in the Mengban and Mengla areas.The corrected inclinations are 28.4°±4.3° and 30.7°[25.4°,35.9°]following the method of Hodych and Buchan(1994)and "E/I" correction,respectively.These results indicate that deposition and/or compaction induced inclination shallowing existed in the Paleogene red-beds in the southern part of Simao terrane.The usage of "E/I" correction is limited in the strata with weak deformation,in which the influence of plunging fold or local rotational deformation will strongly deviate the declinations.Hodych and Buchan's method is more reliable for the inclination correction of red beds,with considering the influence of the granular size.