羌北地块中—晚侏罗世雁石坪群古地磁新结果

New paleomagnetic result of the Middle-Late Jurassic rocks from northern Qiangtang Block,west China

本文报道青藏高原羌北地区中—晚侏罗世雁石坪群古地磁新结果.对采自青海省格尔木市唐古拉山乡雁石坪剖面(33.6°N,92.1°E)11个灰岩采点(118块)和10个碎屑岩采点(99块)定向样品系统古地磁学研究表明,大部分样品的退磁曲线具有双分量特征.低温分量方向在地理坐标系下较为集中,应该为地层褶皱之后的黏滞剩磁.高温特征剩磁分量方向可分为两类:(1)索瓦组(J3s)和布曲组(J2b)灰岩,以磁铁矿为主要载磁矿物,高温特征剩磁分量(Ds=355.7°,Is=42.1°,k=58.2,α95=6°)可通过99%置信度的褶皱检验.(2)雪山组(J2x)和雀莫错组(J2q)碎屑岩,以赤铁矿、磁铁矿为主要载磁矿物,高温特征剩磁分量(Ds=3.3°,Is=28.9°,k=30.7,α95=8.9°)可通过95%置信度的倒转检验和99%置信度的褶皱检验.两组分量都应该是岩石形成时的原生剩磁信息.碎屑岩组的磁倾角比灰岩组偏低13°左右,其剩磁方向很可能存在着与压实作用相关的剩磁倾角变浅的状况.本文取灰岩组平均磁化方向作为雁石坪群的原生剩磁分量,获得羌北地区雁石坪群古磁极位置:80.0°N,295.2°E(dp/dm=7.4/4.5).古地磁结果表明,羌北—昌都地区晚石炭—晚二叠世期间位于南纬中低纬度地区,早三叠世以后开始大规模北向漂移,至中—晚侏罗世已到达24.3°N.其快速北向运动主要发生在早三叠至早侏罗世期间(3500km左右),与现今位置相比中晚侏罗世之后的北向迁移总量为900km左右.

A paleomagnetic study was carried out on the Middle to Late Jurassic sediments of the northern Qiangtang block in Qinghai-Tibet Plateau,west China.118 limestone oriented drill-core samples from 11 sites and 99 sandstone samples from 10 sites of the Yanshiping section（33.6°N,92.1°E） were collected.Detailed stepwise thermal demagnetization allowed us to isolate two components,the low-temperature component（LTC） isolated from most of samples failed the fold test and has been identified as a viscous remanent magnetization.The high-temperature component（HTC） could be divided into two groups：（1） the HTCs of the limestones from Suowa formation（J3s） and Buqu formation（J2b） were carried by magnetite,all with normal polarity,and passed the fold test at 99% confidence level.（2） While the HTCs of the sandstones from Xueshan formation（J2x） and Qoima Co formation（J2q） were carried by a combination of magnetite and/or hematite,displayed dual polarities,and passed the reversal test at 95% confidence level and fold test at 99% confidence level.Positive fold and reversal tests indicate that both HTC groups could be primary magnetization acquired in the Middle-Late Jurassic period.The overall mean direction of the first group（in limestones） is Ds=355.7°,Is=42.1°,k=58.2,α 95=6° after tilt-correction.The overall mean direction of the second group（in sandstones） is Ds=3.3°,Is=28.9°,k=30.7,α 95=8.9° after tilt-correction.By comparison with the mean dip angle of the first group from limestones,the contemporary mean dip angle of the second group from sandstones is about 13° shallower,this anomalously shallow inclination could be explained by inclination flattening during compaction.The paleomagnetic north pole calculated from 11 limestone sites is： 80.0°N,295.2°E with dp/dm=7.4/4.5.The paleomagnetic results indicate that the north Qiangtang region was at mid-low southern latitude regions in the late Carboniferous to late Permian periods,and started to move northward in the late Permian to early Triassic periods.The rapid northward displacement took place in the early Triassic to early Mesozoic period（about 3500 km）.The north Qiangtang region arrived at 24.3°N in the middle to late Jurassic,and up to 900 km of northward motion to current location took place subsequently.