班公湖-怒江缝合带东段晚白垩世红层古地磁研究及其构造意义

Paleomagnetic study of the Late Cretaceous red beds in the eastern segement of the Bangong-Nujiang suture zone and its tectonic implications

班公湖-怒江缝合带古地磁研究对约束印度与亚洲大陆碰撞所导致亚洲大陆内部构造缩短量具有重要意义。本文对班公湖-怒江缝合带东段晚白垩世红层开展了详细的古地磁学研究。本研究共获得了30个采点共约298块/个定向古地磁样品。通过系统热退磁分析,共获得25个采点的有效特征剩磁方向。这些特征剩磁结果同时通过了褶皱检验和倒转检验,表明岩石可能记录了原生剩磁方向。25个采点的特征剩磁方向平均值为Ds=355.7°、Is=28.5°、κs=41.6、α95=4.5°,相应的古地磁极位置为72.8°N、288.0°E、dp/dm=2.7°/4.9°。等温剩磁磁化率各向异性分析显示所获得的古地磁方向不存在明显的磁倾角浅化现象。本次研究结果限定了班公湖-怒江缝合带东段晚白垩世的古纬度为15.2±3.7°N(参考点:31.9°N/93.8°E)。通过对比拉萨、羌塘地块古地磁数据和亚洲大陆古地磁参考极表明:(1)晚白垩世以来班公湖-怒江缝合带与拉萨地块南缘之间并不存在明显的构造缩短;(2)晚白垩世以来班公湖-怒江缝合带(94°E经度方向上)与亚洲大陆刚性块体之间发生了约1150±330km的南北向构造缩短。

Paleomagnetic study for the Bangong-Nujiang suture zone is critical to understand the intra-Asia crustal shortening since the India-Asia collision.In this study,we carried out a paleomagnetic investigation of the Late Cretaceous red beds in the eastern segement of the Bangong-Nujiang suture zone.All the characteristic remanence directions yield both positive fold and reversal tests,likely indicating their primary magnetizations.The site-mean direction of the 25 sampling sites was Ds=355.7°,Is=28.5°,κs=41.6,α95=4.5°,corresponding to a paleomagnetic pole at 72.8°N,288.0°E with dp/dm=2.7°/4.9°.Moreover,anisotropy of isothermal remanent magnetization analysis implies that the obtained Late Cretaceous paleomagnetic directions have little inclination shallowing bias issue in the studied section.The paleomagnetic results imply that the Bangong-Nujiang suture zone was located at 15.2±3.7°N during the Late Cretaceous(reference site:31.9°N/93.8°E).Combined with available reliable paleomagnetic results from the Lhasa and Qiangtang blocks,and with Apparent Polar Wander Path(APWP)of the stable Asia,we suggested that:(1)there was insignificant crustal shortening between the Bangong-Nujiang suture zone and the southern margin of the Lhasa block since the Late Cretaceous;(2)there existed^1150±330 km N-S crustal shortening between the Bangong-Nujiang suture zone(at the longitude of^94°E)and the stable Asia since the Late Cretaceous.