大巴山中、新生代构造剥露过程研究：低温热年代学制约

The Daba Shan is the south part of foreland fold-thrust belt of the Qinling-Dabie Orogen, it is located along the northeastern margin of Sichuan Basin, connected to the West Qinling in the northwest and separated from the Longmen Shan by the Sichuan basin (Yangtze block) in the central, the later one is regarded as the eastern to northeastern margin of the Tibetan Plateau in the Cenozoic . The Daba Shan is featured by the southwest convex orocline, the Triassic to Cretaceous were involved and preserved very well in the fold-thrust belt; moreover, no magmatic activity was recorded since the Early Paleozoic. In addition, the Daba Shan is an intensively geologic disaster region , such as earthquake and frequent landslides; the current topography is characterized by moderate elevation (2000-3000m) but steep incised river valley. Above all imply that the Daba Shan is likely a young mountains. Thus, the Daba Shan is an idea region not only to reconstruct the postorogenic exhumation of the Qinling since the Triassic, but also to test the extent of eastward growth of the Tibetan Plateau. In this study we plan to employ new lower temperature thermochronologic dating (zircon and apatite fission-track and apatite (U-Th)/He) in combination with previous studies to constrain the formation timing of the Daba Shan orocline, the denuadation timing of Sichuan Basin, and to elucidate the relationship between the current geomorphology of the Daba Shan and growth of the northeastern Tibetan in the Late Cenozoic. Zircon and apatite fission-track dating will be performed to constrain the timing when the Daba Shan orocline were mainly formed. Apatite fission-track and (U-Th)/He thermochronology , integrating with the remote sense analysis , will be addressed to reveal the Cenozoic exhumation process, and study when and how the landscape of the Daba Shan get into the current shape, this would be critical to understand the eastward growth of the northeastern Tibetan Plateau in the Cenozoic .

大巴山作为秦岭-大别造山带南缘典型的前陆冲断褶皱带，南临四川盆地，隔川中地块与青藏高原相望，其西缘与高原东缘龙门山相距不过200km；大巴山发育独特的弧形构造，较完整保存了中生代地层及变形记录，记录了秦岭造山带主造山期后陆内构造变形过程。此外，现今大巴山区内河流流向规律，山体坡度大，河谷呈深切呈V字型，这些地貌均特征均暗示其是一个隆起相对较晚的年轻山脉，很可能与青藏高原东北缘新生代末扩展隆升相关，也是探讨高原东向隆升扩展时序及机制的绝佳场所。本研究拟利用多方法低温热年代学测试（锆石、磷灰石裂变径迹、磷灰石（U-Th）/He）对大巴山地区中、新生代的构造剥露过程进行系统定量研究，研究结果不仅可以限定大巴山弧形构造带形成扩展期次及时序，研究秦岭-大别中新生代陆内构造变形过程；更可以揭示大巴山新生代地貌形成过程，进而探讨新生代青藏高原东北缘扩展隆升对秦岭及大巴山的影响程度及作用方式。

大巴山弧形构造带作为秦岭-大别造山带前陆褶皱-冲断带，是理解和认识秦岭造山带中生代前陆扩展变形和新生代青藏高原东向扩展隆升的绝佳场所。本项目针对大巴山弧形构造带和邻区开展了锆石、磷灰石裂变径迹和（U-Th）/He低温热年代学研究，建立区域冷却剥露历史，来限定弧形构造带的形成扩展时序和探讨大巴山新生代的构造隆升与青藏高原的关系。研究结果显示秦岭造山带向南侧大巴山的扩展变形从中侏罗世末开始，北大巴山逆冲推覆构造带构造活动终止于约100-90Ma，南大巴山褶皱带变形终止于85-70Ma。此后在70-40Ma，区域性的地壳伸展断陷影响到秦岭造山带和青藏高原东缘。新生代以来，大巴山西侧龙门山地区经历了三期主要的快速剥蚀，分别为70-40Ma(剥蚀速率 0.05-0.08mm/yr)，25-15Ma(其中彭灌杂岩剥蚀速率 ≤1 mm/yr；川西盆地为 ~0.2 mm/yr)和11Ma（剥蚀速率龙门山为0.80mm，大渡河谷与岷山约为0.50mm）之后。大巴山南侧的四川盆地则记录了两期盆地范围的快速剥蚀去顶事件，应该与古长江上游水系调整相关。第一期的快速剥蚀发生在45Ma并终止于中新世早期，大于1km厚的岩石被剥蚀去顶；第二期的快速剥蚀包括了龙门山、川中盆地和南大巴山，超过2km厚的岩石被剥蚀，该期快速剥蚀与下扬子水系切穿三峡袭夺盆地内水系相关。北大巴山、西秦岭地区新生代末<15-13Ma以来经历了快速的构造剥露，与青藏高原东向扩展跨越刚性四川盆地引起的地表隆升相关；而东秦岭地区，该期构造剥蚀的影响相对较弱（剥蚀厚度不大于1.6km），显示高原隆升的前锋带尚未到达该区或者影响幅度有限。