ABSTRACT

The NE-trending Longmen Shan (LMS) thrust belt, located between the Songpan-Garze terrane and the Yangtze craton, is one of the steepest topographic margins around the Tibetan Plateau. However, the exhumation history and the fault activity of the southern segment of the LMS have remained poorly constrained, particularly in the range front, where little thermochronology ages have been reported. Herein, we report 17 apatites (U-Th)/He (AHe) ages from the southern LMS along the Qingyi River across the Dachuan–Shuangshi Fault (DSF), the Yanjing–Wulong Fault (YWF), and the Gengda–Longdong Fault (GLF). The AHe ages in the hanging-wall of the DSF are ca. 6–11 Ma, which increase to ca.19–23 Ma in the footwall of the DSF. New AHe ages reveal that exhumation rates across the DSF increased significantly from 0.05 to 0.1 km/Myr in the footwall to 0.2–0.4 km/Myr in the hanging-wall since the Late Miocene. The width of the high exhumation rate (0.4–0.6 km/Myr) belt concentrated between the DSF and the GLF, and towards DSF eastward and GLF westward, the exhumation rates decrease rapidly. The results show that continuous crustal shortening along the DSF. Our results support the upper crustal thrusting model that emphasizes crustal shortening deformation along the eastern Tibetan Plateau margin.

摘要

NE向龙门山（LMS）冲断带位于松潘-甘孜地体和扬子克拉通之间，是青藏高原周围最陡峭的地形边缘之一。然而，LMS 南段的折返历史和断层活动仍然受到很大限制，特别是在山脉前沿，那里几乎没有热年代学年龄的报道。在此，我们报告了 17 个磷灰石 (U-Th)/He (AHe) 年龄，来自 LMS 南部沿青衣河穿过大川-双石断裂 (DSF)、燕京-武隆断裂 (YWF) 和更达-龙东断裂(GLF)。DSF 挂壁中的 AHe 年龄约为。6-11 Ma，在 DSF 下盘增加到约 19-23 Ma。新的 AHe 年龄显示整个 DSF 的挖掘率从下盘的 0.05 到 0.1 公里/迈尔显着增加到 0.2-0。自晚中新世以来，上盘中的 4 km/Myr。高折返率（0.4-0.6 km/Myr）带的宽度集中在DSF和GLF之间，向东向DSF和向西GLF，折返率迅速下降。结果表明地壳沿DSF连续缩短。我们的研究结果支持了强调沿青藏高原东部边缘地壳缩短变形的上地壳逆冲模型。