西藏冈底斯山弧高海拔古地貌开始形成时间

The paleo-geomorphic features of the Tibetan plateau prior to the Indian-Asia collision have been one of the frontier issues in earth science. The formation process of the Andean-type Gangdese Mountain in southern margin of the Lhasa terrane is of great significance to understand the geomorphic evolution and climate change of the entire Tibetan plateau. The Shexing Fm. is the youngest terrestrial record preserved in the Gangdese retro-arc basin before the collision. Its deformation and exhumation process undoubtedly recorded the key information on the growth of the Tibetan plateau. The angular unconformity between the Shexing Fm. and overlying Linzizong volcanic rocks represents the onset of uplift of the Gangdese Mountain. However, no detailed study has been conducted on timing and mechanism of its deformation. In this work, we will perform detailed low-temperature thermochronology of clastic sediments in the Shexing Fm., associated with zircon U-Pb chronology, sedimentology and petrology, to reveal its time span and deformation age, through which we can constrain the age of the angular unconformity and explore the paleogeomorphology of the Gangdese Mountain. Combined with the cooling processes of the Gangdese batholith and forearc basin, we will provide new insights to understand the paleogeomorphology of the southern margin of the Asian continent before the India-Asia collision.

印度-欧亚大陆碰撞前的青藏高原古地貌特征一直是国际地球科学领域研究的热点问题，拉萨地体南缘安第斯型冈底斯山弧的形成过程对于理解整个青藏高原的地貌演化和气候变化具有重要的指示意义。冈底斯弧后盆地设兴组作为碰撞前的陆相沉积物，其变形与剥露过程无疑记录了高原生长的关键信息；其与林子宗火山岩之间的角度不整合代表了冈底斯山弧的初始隆升。然而，目前对设兴组的变形时代和机制尚未开展深入研究。本项目以设兴组碎屑沉积物低温热年代学为主要研究手段，辅以锆石U-Pb年代学、沉积学和岩石学等方法，揭示其沉积时间跨度和褶皱变形时间，限定角度不整合的时代，探讨冈底斯山弧初始隆升过程和古地貌特征；并结合冈底斯岩基和弧前盆地冷却过程，为全面认识印度-欧亚碰撞前亚洲大陆南缘的古地貌特征提供依据。

印度-欧亚大陆碰撞前的古地貌特征是国际研究热点问题，而藏南冈底斯山记录了碰撞前高原隆升的重要信息，弧后盆地的沉积记录是解决这一问题的关键。本项目执行期内按照申请书研究计划和研究内容，采用沉积学、锆石U-Pb年代学、低温热年代学和数值模拟等手段，对弧后盆地设兴组的沉积特征、沉积时代和构造变形时间等内容进行了详细研究。沉积学和物源分析表明，设兴组沉积环境为河流相，物源主要来自北部中北拉萨地体（>60%），南部冈底斯山及盆地内下白垩统提供了少量物源。锆石U-Pb年代学揭示设兴组沉积时代为97-72 Ma。低温热年代学研究结果表明，设兴组磷灰石裂变径迹（AFT）中值年龄为23.9±3.3至77±5.1 Ma，平均径迹长度为7.7±0.23至11.1±0.84 μm；最老的AFT峰值年龄与沉积年龄接近，表明AFT年龄为部分重置年龄；25颗磷灰石(U-Th)/He（AHe）年龄为9.09±0.4至18.3±0.3 Ma，13颗锆石(U-Th)/He（ZHe）年龄为31.24±0.91至80.29±2.27 Ma。低温热年代学年龄结合数值模拟揭示设兴组经历过四期冷却剥蚀事件，分别为晚白垩世（76-68 Ma）、始新世（56-40 Ma）、渐新世-早中新世（30-20 Ma）以及晚中新世（15-9 Ma）。最早的冷却事件发生在弧后盆地充填之后，记录了冈底斯弧后地区构造变形开始。综合上述研究结果，本项目取得了以下重要认识：（1）弧后盆地构造背景为拉张盆地，而非先前认为的弧后前陆盆地；（2）物源分析和碎屑锆石年龄图谱表明设兴组沉积时期冈底斯山并未发生大规模隆升，结合日喀则弧前盆地及海沟沉积物碎屑锆石年龄，结果显示设兴组沉积时期存在横传冈底斯的南向河流，该河流可能为古拉萨河；（3）低温热年代学揭示的最早期冷却事件与安第斯型的冈底斯山形成密切相关，可能与新特提斯大洋岩石圈的低角度俯冲有关，冈底斯弧后地区在76-68 Ma开始隆升，这与氧同位素记录的古高程研究结果一致。