藏东南始新世剑川盆地西缘沉积特征及构造意义

Sedimentary geology of the western margin of the Eocene Jianchuan basin in southeast Tibetan Plateau and its tectonic implications

印度-欧亚大陆碰撞作用形成了以西藏-喜马拉雅造山带为主体的正向碰撞带和以"三江"造山带为主体的斜向碰撞带,二者之间的现代分界线为喜马拉雅东构造结。剑川盆地是目前已确定的、位于斜向碰撞带最南端的始新世盆地。通过对盆地西缘4条剖面的详细沉积学研究,发现盆地充填过程分为2个阶段。钾质-超钾质火山活动(37-34 Ma)之前的充填物源自盆地西南侧,由冲积扇扇根、扇中亚相逐渐转变为三角洲平原亚相;火山活动之后的充填物源自盆地北西侧,形成冲积扇扇根、扇中亚相。结合盆地充填期间西侧基底构造变形特点,认为剑川盆地为始新世前陆盆地。始新世期间,剑川盆地与藏东、藏北诸多始新世前陆盆地一样,可能属于正向碰撞带的一部分,其与斜向碰撞带之间的界线位于剑川盆地以南。由于印度地块持续向北迁移,挤入到欧亚大陆内部,导致斜向碰撞带逐渐向北生长。

The India-Eurasia collision led to the formation of an orthogonal collisional belt and an oblique collisional belt, which are the Tibet-Himalaya and the Sanjiang orogenic belts, respectively, and are now separated by the east Himalaya syntax. The Eocene Jianchuan basin is located in the southernmost region of the oblique collisional belt. Detailed sedimentary study of four profiles along the western margin of the Jianchuan basin revealed two stages of the basin infilling process, which were separated by the 37 ~ 34 Ma potassic volcanism. The earlier stage infillings were sourced from the southwest, and displayed upper and middle fan, and/then delta plain facies. The later stage infillings were derived from the northwest regions of the basin, which formed upper fan facies. Combined with the Eocene structures of the basin’ s basement, the authors hold that the Jianchuan basin is an Eocene foreland one. The Eocene Jianchuan foreland basin, along with its numerous equivalents in eastern and central Tibet, likely was previously located in the orthogonal collisional belt. The boundary between the orthogonal and oblique collisional belts might have been located in the south of the Jianchuan basin during the Eocene. The changes in tectonic location of the Jianchuan foreland basin indicate the successive northward growth of the oblique collisional belt due to the continuous northward indenting of the India continent into Eurasia.