剑川-兰坪盆地古近纪沉积-构造变革及其区域构造意义

滇西新生代兰坪盆地和剑川盆地分别位于哀牢山–红河断裂带两侧,青藏高原东构造结内,其沉积过程和构造变形对青藏高原东南缘的构造演化有重要的启示意义。通过对这两个盆地古近纪沉积和构造过程的研究,我们发现兰坪盆地和剑川盆地及邻区的构造变形分为三期:始新世早期的强烈挤压变形、始新世中晚期的伸展变形、渐新世的走滑变形。始新世早期的挤压变形主要表现为兰坪地区的褶皱–冲断系统、哀牢山-红河断裂的逆冲活动和剑川盆地的宽缓褶皱。沉积方面,古新统勐野井组(E_1m)较为稳定的细粒滨湖相沉积转变为始新统宝相寺组(E_2b)较粗的具有前陆盆地性质的河流相沉积,特别是宝相寺组底部发育的一套快速堆积的磨拉石建造,可能是对始新世强烈挤压环境下的沉积响应。始新世中晚期伸展变形体现在盆地的构造环境由早期的挤压环境变为伸展环境和该时期大量富钾岩体和岩脉的侵入,沉积学上,下始新统宝相寺组的河流相转变为中始新统金丝厂组(E_2j)具有快速堆积磨拉石特征的曲流河沉积,极可能是对构造体制变革的沉积响应。渐新世的走滑变形则体现在渐新统的缺失和哀牢山–红河断裂的早期左行走滑。因此,我们认为剑川–兰坪地区在始新世中期和渐新世均发生了显著的运动学转换,这一认识也得到了始新世中期兰坪和剑川盆地物源明显变化的支持。结合青藏高原东南部始新世中晚期岩浆的活动,渐新世大型剪切带(崇山剪切带、高黎贡剪切带)的强烈走滑和保山块体的旋转,我们推测青藏高原东南缘古近纪的构造演化为古新世-始新世早期的挤压、始新世中晚期的伸展、渐新世的转换压缩。

Paleogene Sedimentary and Structural Evolution of the Jianchuan-Lanping Basins,Western Yunnan and its Regional Tectonic Implications

The Cenozoic Lanping and Jianchuan basins are located at either side of the Ailaoshan-Red River Fault Zone,western Yunnan.This area is highly deformed as it was involved in the eastern Himalayan syntaxis.The sedimentary and deformation features of the basins have significant implications for the tectonic evolution of southeast margin of the plateau.The Paleogene deposition and subsequent structural activities in these two basins illustrate three phases of deformation,including strong compression deformation in the Early Eocene,extensional deformation in the Middle Eocene and the strike-slip deformation during Oligocene.The very first compression deformation exhibits as fold-thrust system in the Lanping area,thrust movement along the Ailaoshan-Red River Fault,and broad folds in the Jianchuan Basin.Regarding the sedimentation,the Paleocene Mengyejing Formation is characteristics by fine grained rocks and shore-lacustrine facies,while the Eocene Baoxiangsi Formation occurs as fluvial deposition and has the feature of foreland basin deposition.The switch from shore-lacustrine to fluvial deposition may result from the strong compression during the Eocene.Afterwards,the basins' tectonic regime transferred from the early compression to extension,and the intrusion of high-K granites and dikes during the Middle-Late Eocene indicates an extensional background.At the same time,the deposition was transformed from the fluvial facies of Baoxiangsi Formation to meandering fluvial facies with molasse features in the Eocene Jinsichang Formation.The absence of Oligocene sediments and the onset of the sinistral strike-slip along the Ailaoshan-Red River Fault suggest strike-slip deformation during the Oligocene.Therefore,the Jianchuan-Lanping Basins experienced twice kinematics transform at the Middle Eocene and Oligocene,which is also supported by the provenance study of the basins.Integrating the studies about the Eocene magmatism in the southeast plateau,the huge Oligocene strike-slip movements along the Chongshan and Gaoligong Shear Zones,and the rotation of the Baoshan Block at the southeast margin of the Tibetan Plateau,the Paleogene tectonic evolution includes compression during the Paleocene-Early Eocene,extension during the Middle-Late Eocene,and transpression during the Oligocene.