新疆北部古地磁研究

PALEOMAGNETIC STUDY OF THE NORTHERN XINJIANG

通过和布克赛尔、克拉玛依、玛纳斯—乌鲁木齐地区泥盆纪到白垩纪古地磁研究,主要取得以下结果:(1)首次建立了准噶尔西北缘及南缘石炭纪—白垩纪古地磁极移曲线,由石炭纪到二叠纪的古地磁极位置基本在同一区间,说明该时期这些地区为一个统一的构造单元,而其古地磁极明显与塔里木、哈萨克斯坦、西伯利亚地块存在着差异。(2)该地区侏罗纪及白垩纪古地磁结果与塔里木地块结果一致,侏罗纪乌鲁木齐与和布克赛尔磁偏角相差30°左右,说明和布克赛尔地区相对乌鲁木齐地区逆时针旋转了30°左右,晚古生代以后曾发生过南向移动,而侏罗、白垩纪以来均向北发生了相当规模的北向运动,并发生了相对旋转,目前东、西准噶尔的构造格局可能就是由于局部相对旋转造成的。(3)中国大陆在早二叠世还不是一个联合的整体,而是以相互分离的独立块体分布于45°N—15°S的古特提斯洋中。(4)该地区二叠纪的磁偏角为165°—168°,而塔里木为218°,哈萨克斯坦为229°,说明存在35°—55°的逆时针旋转,这个旋转可能是由于西部推覆构造造成的。如果将西准噶尔超基性岩带顺时针旋转35°—55°后,东准噶尔超基性岩带、西准噶尔超基性岩带和斋桑泊—鲁布佐夫斯克超基性岩带应在同一构造带上。(5)该地区晚古生代古纬度变化不明显。

Analyses and statistics of 1,150 samples of Devonian to Cretaceous from 191 sites inHoboksar, Karamay, and Manas-Urumqi area,reveal the following main results: (1) The pa-leomagnetic pole paths of northwestern and southern margins of Junggar Block during Carboniferous to Permian were built for the first time. The paleomagnetic poles in the two areas, utterly different from those of Tarim,Kazakhstan and Siberia blocks,were distributed in the same section. It indicated that the two areas were located in the same structural unit. (2) The paleomagnetic results of Jurassic to Cretaceous from northwestern and southern margins of Junggar Block accord with those of Tarim Block. It shows that the paleomagnetic characteristics of both blocks are of low inclination and average paleolatitude of 24° N. The difference of declination (about 30°) during Jurassic between Urumqi and Hoboksar areas reveals 30 anticlockwise rotation of Hoboksar area relative to Urumqi area assuming magnetization in Urumqi is normal. Paleomagnetic results also show southward migration since late Paleozoic and substantial northward migration and relative rotation during Jurassic and Cretaceous. The modern structural configuration was probably caused by partially relative rotation. (3) China Continent was scattered in the paleo-Tethys Ocean within 45° N-15° S, as relative separated blocks,but not as a united entirety. (4) The different declination values of Permian, 164°-168° for northwestern and southern margins of Junggar Block,218° for Tarim Block and 229° for Kazakhstan Block, respectively,reveal 35°-55° anticlockwise rotation,probably caused by western nappe structures. The ultrabasic rock belt of West Junggar, as rotated anticlockwise by 35°-55°,should be located in the same structural zone with the ultrabasic rock belts of East Junggar and Gesangpo-Labozovk. (5) Northwestern and southern margins of Junggar Block have a paleolatitude of 30°-45° N (averaging 36°N) during late Paleozoic, showing the warm climates of the north,and paleolatitude of 20°-30° N (averaging 24° N) during Mesozoic, obviously southward migrated, showing humid and warm tropical climates. (6) The peculiar magnetic features (magnetic susceptibility and magnetic intensity) of Jurassic coal bed can be used to confine the fire zone in the covered area.