藏南邛多江地堑的晚新生代沉积地层及对南北向裂谷形成时代的初步限定

邛多江地堑构成了藏南近南北向裂谷带最东侧的错那-沃卡裂谷中段，是由地堑西缘高角度正断层主控的半地堑式断陷盆地。详细的地质、地貌调查表明，该地堑内主要充填有晚新生代以来的多套河湖相、冰碛及冰水沉积地层。河湖相地层底部以黏土和粉砂为主，上部以砾石层为主，向上砾石砾径逐渐变大，顶部为早更新世冲积砾石层；冰碛主要发育于地堑中部山前地带，构成宽缓的冰碛台地或者终碛垄、侧碛堤。地层的测年结果表明，该区主要发育两套晚新生代河湖相地层，早期沉积时代早于5Ma，晚期为晚第四纪；而冰碛及冰水沉积的时代主要为中更新世。综合该区地质地貌、沉积和构造等分析结果表明，早期的河湖相沉积与盆地发生初始裂陷后的主边界正断层发生强烈垂直活动有关，而晚期的河湖相沉积主要形成于盆地后期萎缩过程中，成因可能与中更新世以来的冰川堰塞湖有关。由于邛多江地堑受控于西侧主边界正断层，早期沉积应晚于其初始裂陷时代。因此，进一步综合现有年龄数据资料认为，藏南近南北裂谷的初始裂陷时代应早于5~10Ma，但晚于约15Ma。

Late Cenozoic Sedimentary Strata of Qiongduojiang Graben, South Tibet:Preliminary Constraint on the Initial Rifting Age of the SN- trending Rift

The Qiongduojiang graben, which is located in the central part of the Cuona- Oiga rift in the eastern end of the near NS- trending rift in South Tibet, is a half- graben faulting basin controlled mainly by the high- angle normal fault in the western margin of the graben. Detailed geological and geomorphic survey shows that the basin is filled with several beds of fluvio- lacustrine, moraine and outwash sediments since the Late Cenozoic. The fluvio- lacustrine sequence is dominantly clay and silt stone at bottom, gravels at the upper part with the diameter increasing gradually, and early Pleistocene alluvial gravel at top. Moraine is mainly developed in the piedmont zone of the central graben, forming broad and gentle glacial terraces and lateral moraine dams. The dating results of the sediments indicate that there are two sets of fluvio- lacustrine deposited before 5 Ma and Quaternary respectively and moraine and outwash sediments formed since the middle Pleistocene. Comprehensive analysis of geology, geomorphology, sedimentary and structure features suggest that the old fluvio lacustrine sediments were related to intensive vertical faulting of the main basin boundary after initial rifting of the basin, while late fluvio- lacustrine sediments formed during the process of shrinking of the basin, and were probably related to the glacial lake since the middle Pleistocene. Because the Qiongduojiang graben was controlled by the main boundary normal fault of the western margin, the early fluvio- lacustrine deposition should be later than initial rifting time. Thus, further integration with the existing age data suggests that the initial rifting age of the South Tibet rifts should be older than 5- 10Ma and younger than ca.15Ma.