羌塘盆地磁倾角浅化校正及其在构造上的应用——中侏罗纪以来约1 000 km的南北向缩短

古地磁磁倾角是古纬度的记录者,能真实反映块体在地质时期的古纬度,对块体构造活动有着很好的揭示作用。但最近众多的研究表明,沉积物所记录的磁倾角结果,往往存在浅化现象。因此,如何精确确定古地磁磁倾角方向,是研究块体南北向构造活动的关键。本文围绕古地磁研究相对薄弱的青藏高原腹地羌塘块体,针对雁石坪地区侏罗纪沉积地层开展了精细的古地磁研究,确定了雁石坪群沉积岩地层中灰岩和碎屑岩的剩磁方向(碎屑岩雀莫错组Ds=339.0°,Is=24.4°,α95=18.3°,灰岩布曲组Ds=336.9°,Is=30.9°,α95=7.8°),通过运用"E/I"统计方法对所获得的古地磁结果进行校正,发现雁石坪地区侏罗纪沉积岩由同沉积期间或沉积后的压实作用所造成的磁倾角浅化约5.7°～8.5°。该结果表明,羌塘块体在中侏罗世雀莫错组和布曲组期间存在约278 km北向漂移。校正后的古地磁结果,与同期的相邻块体塔里木极对比,揭示出磁倾角具有额外10.9°～14.6° 的浅化,可能指示了自中侏罗世以来,羌塘块体相对塔里木块体有约1 000 km的北向漂移和缩短。

"E/I" corrected paleomagnetic inclination indicates 1 000 km N-S shortening of the Qiangtang terrane since the Middle Jurassic

As paleolatitude recorder,paleomagnetic inclination can provide constraints on the tectonic activities of studied block and play a crucial role in paleogeographic reconstructing.However,recent paleomagnetic data obtained from sedimentary rocks show a pronounced bias towards too-shallow inclinations.Therefore,how to precisely determine paleomagnetic inclination is a crucial key to understand latitudinal motion of blocks.This paper provides new detailed paleomagnetic results of the Middle-Later Jurassic sediments of the Yanshiping area in Qiangtang block,central Tibetan.The high quality paleomagnetic direction of the Quemocuo Formation is D_s=339.0°,I_s=24.4°,α₉₅=18.3°,whereas the overlying limestone layers of Buqu Formation is D_s=336.9,I_s=30.9°,α₉₅=7.8°.We then utilize an effective "Elongation/Inclination' statistic analysis method to correct the inclination bias caused by Syn- /or post-depositional compaction,which reveals 5.7°～8.5° of shallowing.The 3.7° differences of corrected inclinations between the above two adjacent formations indicate that the Qiangtang block has had～278 km of northward adrift during the period.In addition,when comparing the corrected inclinations with the Tarim block of the same period,it reveals another 10.9°～14.6° inclination shallowing.It possibly indicates that there has been approximate of 1 000 km convergences between the Qiangtang and Tarim blocks since the Middle Jurassic.