阿尔金断裂走滑作用对青藏高原东北缘山脉形成的古地磁证据

Strike-slip movement of the Altyn Tagh fault and implications for mountain formation inferred from paleomagnetic data in northeastern Tibetan Plateau

通过对青藏高原北部阿尔金断裂东缘早白垩世-第三纪红层与玄武岩38个采点的系统古地磁测定,获得了研究区早白垩世-第三纪高温特征剩磁分量。结果表明,昌马乡早白垩世红层与玄武岩剖面层面坐标下高温特征剩磁平均方向(Ds=32.8°,Is=59.4°,κs=36.2,α95=8.1°)和北大窖早白垩世玄武岩剖面层面坐标下高温特征剩磁平均方向(Ds=335.4°,Is=55.1°,κs=34,α95=9.6°)均通过了褶皱检验,可能代表岩石形成时的原生剩磁。旱峡地区早白垩世地层层面坐标下高温特征剩磁平均方向(Ds=26.1°,Is=49.5°,κs=28.6,α95=7.3°)和红柳峡早第三纪地层层面坐标下高温特征剩磁平均方向(Ds=355.4°,Is=48.3°,κs=135.8,α95=7.9°),这两组高温特征剩磁方向在地理坐标下均远离现代地磁场方向,且具有正、反双极性特征,说明其也可能代表了岩石形成时的原生剩磁方向。结合已有阿尔金断裂及周边早白垩世-第三纪古地磁结果,提出柴达木块体在新生代印度/欧亚大陆碰撞挤压下并没有发生明显的整体顺时针旋转作用,青藏高原东北地区的块体旋转作用是阿尔金断裂左旋走滑作用在青藏高原东北缘转换的重要表现形式。

To help understand the deformational history of the northeastern Tibetan Plateau, paleomagnetic samples were collected from 38 sites in the Early Cretaceous to Pliocene sedimentary and volcanic rocks distributed in four general regions adjacent to the Altyn Tagh fault at the northeastern edge of the Tibetan Platean. In the Changma area, 10 sites in the Early Cretaceous sediments and basalt flows yield a mean paleomagnetic direction （Ds = 32. 8° ,Is = 59.4°, Ks = 36. 2, α95 = 8.1°）, and 8 sites in Early Cretaceous basalt flows from the Beidayao area provide a concordant paleomagnetic direction （Ds =335.4° ,Is =55. 1° ,Ks =34,α95 =9. 6°）. The Early Cretaceous paleomagnetic directions from 15 sites in the Early Cretaceous sediments and basalt flows have a concordant mean paleomagnetic direction （Ds = 26. 1°, Is = 49.5°, Ks = 28.6, α95 = 7.3°）. 4 sites in Pliocene strata at Hanxia area yield a paleomagnetic direction （Ds = 355.4°, Is = 48.3°, Ks = 135. 8, α95 = 7. 9°）. Fold or reversal tests imply primary magnetizations. Together with previously published paleomagnetie data in the Qaidam basin and its surroundings, these new results indicate that （ 1 ） the Qaidam block has not undergone wholescale vertical axis rotation with respect to the stable Eurasia due to the penetration of India into Asia; （2） occurrence of vertical axis clockwise rotations in areas adjacent to the eastern edge of the Ahyn Tagh fault may be an important tectonic model to absorb the India-Asia collision-induced left-slip movement of the Altyn Tagh fault at the northeastern edge of the Tibetan Plateau.