青藏高原中东部早期构造隆升对古近纪盆地充填和演化的影响

Early tectonic uplift affecting sedimentary filling and evolution of Paleogene basins in the central-eastern Tibetan Plateau

青藏高原中东部分布着一系列中小型古近纪断陷盆地和走滑一拉分盆地。印度一欧亚板块碰撞已引起盆地构造、沉积和岩浆活动等地质事件的响应。古近纪断陷盆地和走滑一拉分盆地中广泛分布的巨厚粗碎屑岩充填、新特提斯海湾消亡、大规模地壳挤压褶皱冲断和高钾岩浆活动、周缘前陆盆地形成、干旱一温暖极热事件以及古近纪盆地的封闭和裂解等。详细的野外地质调查、盆地构造一沉积学、生物地层学和岩浆岩同位素年代学研究结果表明,北部玉树一囊谦地区断陷盆地发生了大规模挤压掀斜和冲断,在盆缘形成高陡地层和挤压向斜,盆地内地层发生明显的褶皱变形。盆地内部充填了巨厚层状底砾岩、紫红色陆源碎屑岩夹火山碎屑岩、碳酸盐岩和石膏层,并被晚期岩浆岩所切割。南部巴塘一丽江地区形成走滑一拉分盆地。区域地层对比、细碎屑岩内孢粉和古植物、火山碎屑岩和侵入岩的 U-Pb和^40Ar/^39Ar年代学结果表明,盆地内充填沉积物形成于始新世(56-32Ma)。古近纪紫红色细粒沉积物、碳酸盐岩和石膏层的出现表明盆地封闭期处于干旱一炎热的古气候环境。38-32 Ma是自印度一欧亚板块陆一陆碰撞以来,青藏高原中东部从转换挤压到转换伸展的过渡阶段,出现了大规模高钾火山喷发和随后的岩浆侵入,并导致青藏高原中东部古近纪盆地的封闭和裂解。北部盆地的封闭时间(约 37Ma)早于南部盆地的裂解(约 32-28Ma)。青藏高原中东部古近纪盆地的封闭和裂解主要是自约38 Ma以来,印度一欧亚板块碰撞引起的陆壳挤压、变形和缩短,及由高原早期构造隆升诱导的逆冲挤压和走滑拉分引起的。

Many medium-and small-sized Paleogene graben and strike-slip basins are distributed in the central- eastern Tibetan Plateau. The Indo-Asia plate collision has already resulted in the responses to the geological events including tectonic, deposition, and magmatism in the basins, such as the formation of hugely thick coarse clastic rock widely distributed in the Paleogene basins, the disappearance of the New Tethys Bay, the large-scale fold and thrust due to crustal compression, high-potassium magmatism, formation of the peripheral foreland basins. Field geological mapping and the studies of basin tectonic- sedimentology, biostratigraphy and isotope chronology for magmatic rocks show that the large-scale compression tilt and thrust occurred in the Yushu-Nangqian graben basins, formation of high-angle strata and expression syncline at the basin margin, and obvious development of fold deformation in the strata in the basins. The Paleogene basins are filled with hugely thick basal conglomerate, purple-red terrigenous clastic rocks interbedded volcanic clastic rocks, carbonate rocks and gypsum layers, and were cut by late magmatic rocks. Strike-slip and pull-apart basins were formed in the Batang-Lijiang area. The studies of regional stratigraphic correlation, sporo-pollen and paleoflora in fine clastic rocks, and U-Pb and 40 Ar/39 Ar dating of volcanoclastic and intrusive rocks revealed that the sediments in these basins were formed in Eocene(56 32 Ma). The appearance of the Paleogene crimson fine-grained sediments, carbonate rocks and gypsum layers indicates that the basins were in a dry-hot paleoclimate environment in their closure period. The period of 38^32 Ma is a transitional stage from transgression to the transtension in the east- central Tibet after the Indo-Asia plate collision, as shown by the occurrence of a large-scale high-K volcanic eruption followed by magmatic intrusion and the closure and cracking of the Paleogene basins in east-central Tibet. The closure ( 37 Ma) of the northern basins is earlier than the cracking of the southern basins ( 32 28 Ma). The closure and cracking of the Paleogene basins in east-central Tibet was mainly caused by the compression, deformation and shortening of the continental crust, and the thrust compression, strike-slip and pull-apart due to the early tectonic uplift of the plateau after the collision of the Indo-Asian plate at about 38Ma.