印欧大陆碰撞前亚洲大陆南缘古位置再研究:林周盆地上白垩统红层的古地磁新结果

Reconnaissance of pre-collisional paleolatitudes of the southern margin of Eurasia:New paleomagnetic results from Upper Cretaceous red beds in the Linzhou basin,Tibet

拉萨地块林周盆地白垩系红层的古地磁数据一直都有较大争议.过去认为磁倾角变浅可能是造成这些分歧的主要原因.我们在林周盆地设兴组背斜两翼进行了系统的古地磁采样,15个采样点的特征剩磁分量在倾斜校正和倾伏褶皱校正后平均方向为D=339.3°,I=22.9°(α_(95)=5.1°).特征剩磁分量在大约69%展开时获得最大集中,表明其为同褶皱重磁化;此时平均方向为D=339.1°,I=27.3°(α_(95)=4.1°),对应的古地磁极为65.4°N,327.5°E(A_(95)=3.5°),参考点29.3°N/88.5°E的古纬度为15.0°N±3.5°.薄片镜下分析显示赤铁矿为次生矿物,岩石磁组构(AMS)也表现为过渡型构造变形组构.样品的特征剩磁方向应为重磁化的结果,E/I(elongation vs inclination)校正法显示特征剩磁方向并没有发生倾角变浅.根据区域构造,重磁化时代约为72.4±1.8 Ma到64.4±0.6 Ma.综合考虑拉萨地块东西部的古地磁数据以及地震层析成像资料后我们认为,碰撞前拉萨地块大约呈NW-SE向准线性分布,并处于~10°N-15.0°N;自~70 Ma以来,拉萨地块与稳定欧亚大陆之间至少存在1200±400 km(11.1°±3.5°)的南北向构造缩短量;印度大陆与欧亚大陆的碰撞不应晚于55 Ma.

Paleomagnetic data of Cretaceous red beds in the Lhasa block remains highly controversial. Possible inclination shallowing is believed to be a critical issue responsible for such a high discrepancy in paleolatitudes deduced from the Shexing Formation （red beds）. In this study, characteristic remanent magnetizations （ChRM） have been successfully isolated from 15 sites in both limbs of the Shexing Fm anticline with mean direction of D=339.3°,I=22.9°（α95=5.1°） after tilt-correction with plunging fold axis. However, the ChRM reaches its optimal concentration at ～69% unfolding, yielding a mean direction of D=339.1°,I=27.3° （α95=4.1°） and a corresponding paleomagnetic pole at 65.4°N, 327.5°E （A95=3.5°）. Thin section analysis indicates that the hematites are secondary products, which is consistent with the structural interpretation of anisotropy of magnetic susceptibility （AMS）. The ChRM is hence considered to be a syn-folding remagnetization, which is very likely acquired between 72.4±1.8 Ma and 64.4±0.6 Ma in the light of geologic setting, when the Lhasa block was located at about 15.0°N±3.5° （reference site： 29.3°N, 88.5°E）. The E/I （elongation versus inclination） method indicates that the ChRM was not significantly affected by inclination shallowing. Taking available paleomagnetic data from the western and eastern Lhasa block as well as the seismic tomographic image into account, we believe that the Lhasa block was located at ～10°N—15°N with a quasi-linear geometry prior to the India-Asia collision. This result indicates that the collision between India and Eurasia occurred no later than ～55 Ma. Compared with the expected paleomagnetic directions from the Eurasia plate, the total latitudinal convergence was accommodated by at least 1200±400 km （11.1°±3.5°） between the Lhasa block and stable Eurasia since ～70 Ma.