拉萨地块北缘尼玛地区晚白垩世古地磁结果及其构造意义

Late Cretaceous paleomagnetic results in the Nyima area from the northern margin of the Lhasa block and its tectonic implications

通过对青藏高原拉萨地块北缘尼玛县城附近晚白垩世红层8个采点的系统古地磁测定,获得了研究区晚白垩世可靠的古地磁数据。通过系统热退磁获得了采样剖面高温特征剩磁方向平均值为:Dg=9.9°,Ig=5.5°,κg=30.3,α95=10.2°,N=8(地理坐标下);D_s=8.2°,I_s=27.4°,κs=37.6,α_(95)=9.2°,N=8(层面坐标下);对应古地磁极为:71.2°N,241.9°E,dp/dm=5.5°/10.0°。该高温剩磁分量在地理坐标下与现代地磁场方向有显著差别,具有正、反极性,并通过了倒转检验,由此认为其很可能代表岩石形成时的原生剩磁。本次研究结果表明拉萨地块北缘在晚白垩世位于~14.5°N的古纬度位置(参考点:31.8°N,87.2°E)。对比稳定亚洲大陆参考极显示晚白垩世以来拉萨地块北缘与稳定亚洲大陆之间发生了1200±630km(10.9°±5.7°)的南北向构造缩短,但并未发生明显的相对旋转作用(0.8°±5.9°)。综合地质证据提出印度与亚洲大陆碰撞所造成的亚洲大陆内部南北向构造缩短主要集中在拉萨地块以北的区域。

A paleomagnetic study of the Upper Cretaceous red beds have been carried out near the Nyima County in the northem margin of the Lhasa block.Stepwise thermal demagnetization successfully isolated high temperature characteristic directions from the Upper Cretaceous red beds.The tilt-corrected mean direction for 8 sites is Ds/Is =8.2°/27.4° with α95 =9.2°,corresponding to a paleopole at 71.2° N,241.9° E with dp/dm =5.5°/10.0°.The mean direction of high temperature characteristic directions in geographic coordinates is significantly different from the present geomagnetic field （PGF） direction.Both normal and reversal polarities were observed in the high temperature characteristic direction and it pass reversal tests at the 95% confidence level.These indicate the primary origin for the characteristic remanence of the red beds.The results indicated that the northern margin of the Lhasa block was located at about 14.5°N during the Late Cretaceous （reference point：31.8°N,87.2°E）.Compared with the Late Cretaceous paleopole of the Asian continent,we suggest that significant crustal shortening （1200 ± 630km） and insignificant rotation （0.8° ± 5.9°） have taken place between the northern margin of the Lhasa block and the stable Asian continent since the Late Cretaceous.The N-S crustal shortening resulted from India-Asia collision was mainly distributed on the north of the Lhasa block