藏南冈底斯白垩纪弧后盆地的地壳变形及初始高原的形成

摘要

在藏南“南冈底斯岩浆弧”的北侧,发育一个白垩纪碳酸盐-碎屑岩组成的弧后盆地,盆地基底为早侏罗纪火山岩,其上被大面积晚白垩世-古新世林子宗群火山岩(62 ～ 45Ma)角度不整合覆盖,以及65 ～ 40Ma花岗岩基的侵位.南冈底斯弧后盆地的主要地壳变形表现为:在自北向南剪切应变下,以早侏罗世火山岩与晚侏罗-白垩世沉积岩之间的冈底斯滑脱带(GD)为主要构造底面,与上部白垩纪地层的强烈褶皱和铲式构造一起组成的“滑脱-褶皱”构造样式.研究表明,弧后盆地的滑脱-褶皱构造是90 ～ 62Ma期间与新特提斯大洋岩石圈板片俯冲有关的弧后盆地地壳缩短、加厚和造山作用的表征.大面积存在的冈底斯林子宗火山岩与其下部地层的角度不整合是一种“火山披盖式”的不整合,说明冈底斯弧后盆地经历伸展到地壳缩短变形、造山隆升和剥蚀夷平的演化过程,标志洋-陆俯冲到陆-陆碰撞的转换.提出南冈底斯初始高原在晚白垩世俯冲条件下开始形成的新认识.%A Cretaceous carbonatite-clasolite-bearing back-arc basin,with a basement of Jurassic volcanic rocks,developed in the south of ‘ Gangdese magmatic arc’,South Tibet.The basin was featured by a large-scale angular unconformity,overlaid by Linzizong Formation of uppermost Cretaceous-lower Tertiary volcanic-bearing strata,and extensive emplacement of 65 ～40Ma granitic plutons.The upper crust deformation in South Gangdese could be mainly expressed as the Gangdese Décollement (GD) of "dfcollement-fold" structural style,which consists of 1) a main structural plane between Jurassic basement and overlying sediments with a top-to-south shearing sense and 2) overlying Cretaceous sedimentary units with tight folds and listric faults.We suggest the ‘ Gangdese Décollement’,attributed to the coeval crustal shortening,crustal thickening and orogenesis,was formed due to the northward subduction of Neo-Tethys oceanic lithosphere during 90 ～ 69Ma.The angular unconformity between Linzizong Formation and underlying strata shows as a ‘ volcanic drape-style unconformity’ type,recording the transition from ocean-continent subduction to continental collision,accompanied by regional crustal extension,shortening,orogen uplifting,then denudation and planation.Hence,we propose that an Andes-type South Gangdese Proto-Plateau triggered by northward subduction of Neo-Tethys oceanic lithosphere beneath the southern margin of Asia during the Late Cretaceous,then the whole regional planation occurred before Middle Eocene.