西藏色林错及邻区古近纪沉积凹陷对地壳增厚的响应

青藏高原中部古近纪发育伦坡拉盆地、色林错盆地、尼玛盆地,组成伦坡拉-色林错-尼玛沉积凹陷,总体呈近东西走向,长超过250km,宽30~50km;凹陷中心古近系河湖相沉积地层厚度达5~6km,下部为古新统-始新统牛堡组砾岩、砂岩、泥岩、泥灰岩,上部为渐新统丁青湖组泥岩、页岩、粉砂岩夹油页岩,顶部被新近系河湖相沉积不整合覆盖。凹陷南部发育尼玛-色林错逆冲推覆构造,凹陷北侧发育赛布错-扎加藏布逆冲推覆构造,伦坡拉盆地北部发育薄皮推覆构造,伴有不同规模的褶皱变形。地壳深部不同深度发育多重逆冲推覆构造,羌塘地块南部自北向南逆冲推覆,拉萨地块北部自南向北逆冲推覆;两者对冲部位地壳厚度发生显著变化,地表形成古近纪沉积凹陷。根据深地震反射及构造解释,结合Airy均衡分析,表明不同深度逆冲推覆及对冲构造运动导致地壳缩短增厚,增厚地壳均衡隆升及密度差异对古近纪沉积凹陷及盆地演化具有重要控制作用。色林错凹陷及邻区古近纪沉积记录对青藏高原地壳增厚与隆升过程具有重要指示意义。

Response of Sedimentary Depression to Crustal Thickening in the Silin Co Basin and Its Adjacent Areas, Tibet

The Lunpola, Silin Co and Nima basins formed in the Paleogene along the Lunpola Silin Co Nima depression in central Tibetan Plateau. The sedimentary depression trending in east west is as long as 250km and as wide as 30~50km. Paleogenelacustrine, fluvial and alluvial sedimentary rocks as thick as 5~6km formed in the depression, including conglomerate, sandstone, mudstone and marl of the Paleocene Eocene Niubao Formation, and mudstone, shale and siltstone intercalated with oil shale of the Oligocene Dingqinghu Formation covered unconformable by the Neogene lacustrine deposits. The Nima Silin Co thrust formed in the southern depression, the Saibu Co ZajiaZangbu thrust formed in the northern depression and thin skinned thrust formed in the northern Lunpola Basin, accompanied by folding deformation. Multiple imbricate thrust occurred in deep crust along the depression. Southward thrust in the southern Qiangtang Block and northward thrust in the northern Lhasa Block result in spatial change of crust thickness across the Paleogene sedimentary depression.Such collisional thrust at different depthes caused shortening and thickening of crust, and differential uplift of the thickening crust coupled with density variation evidently controlled the Paleogene depression and basin evolution according to seismic reflection profiling and the Airy balance modeling. Paleogene sedimentary records for depression in the Silin Co basin and its adjacent areas are significant for better understanding crust thickening and uplift process of the central Tibetan Plateau.