Riesner Magali, Bollinger Laurent, Hubbard Judith, Guérin Cyrielle, Lefèvre Marthe, Vallage Amaury, Basnet Shah Chanda, Kandel Thakur Prasad, Haines Samuel, Sapkota Soma Nath
Earth Observatory of Singapore, NTU, Singapore, Singapore.
CEA, DAM, DIF, 91297, Arpajon, France.
Sci Rep. 2021 Nov 2;11(1):21521. doi: 10.1038/s41598-021-00297-4.
The largest (M8+) known earthquakes in the Himalaya have ruptured the upper locked section of the Main Himalayan Thrust zone, offsetting the ground surface along the Main Frontal Thrust at the range front. However, out-of-sequence active structures have received less attention. One of the most impressive examples of such faults is the active fault that generally follows the surface trace of the Main Boundary Thrust (MBT). This fault has generated a clear geomorphological signature of recent deformation in eastern and western Nepal, as well as further west in India. We focus on western Nepal, between the municipalities of Surkhet and Gorahi where this fault is well expressed. Although the fault system as a whole is accommodating contraction, across most of its length, this particular fault appears geomorphologically as a normal fault, indicating crustal extension in the hanging wall of the MHT. We focus this study on the reactivation of the MBT along the Surkhet-Gorahi segment of the surface trace of the newly named Reactivated Boundary Fault, which is ~ 120 km long. We first generate a high-resolution Digital Elevation Model from triplets of high-resolution Pleiades images and use this to map the fault scarp and its geomorphological lateral variation. For most of its length, normal motion slip is observed with a dip varying between 20° and 60° and a maximum cumulative vertical offset of 27 m. We then present evidence for recent normal faulting in a trench located in the village of Sukhetal. Radiocarbon dating of detrital charcoals sampled in the hanging wall of the fault, including the main colluvial wedge and overlying sedimentary layers, suggest that the last event occurred in the early sixteenth century. This period saw the devastating 1505 earthquake, which produced ~ 23 m of slip on the Main Frontal Thrust. Linked or not, the ruptures on the MFT and MBT happened within a short time period compared to the centuries of quiescence of the faults that followed. We suggest that episodic normal-sense activity of the MBT could be related to large earthquakes rupturing the MFT, given its proximity, the sense of motion, and the large distance that separates the MBT from the downdip end of the locked fault zone of the MHT fault system. We discuss these results and their implications for the frontal Himalayan thrust system.
喜马拉雅山脉已知的最大地震（震级M8+）使主喜马拉雅逆冲带的上部锁定段发生破裂，导致山脉前沿的地表沿着主前锋逆冲断层发生错动。然而，失序活动构造受到的关注较少。这类断层最引人注目的例子之一是一条大致沿着主边界逆冲断层（MBT）地表痕迹延伸的活动断层。这条断层在尼泊尔东部和西部以及印度更西部都留下了近期变形的清晰地貌特征。我们将重点放在尼泊尔西部，在苏尔凯特和戈拉希两个市镇之间，这条断层在此处表现得很明显。尽管整个断层系统在适应收缩，但在其大部分长度上，这条特定的断层在地貌上表现为正断层，表明主喜马拉雅逆冲断层（MHT）上盘存在地壳伸展。我们将这项研究聚焦于沿着新命名的复活边界断层地表痕迹的苏尔凯特 - 戈拉希段的MBT复活情况，该断层长约120公里。我们首先利用高分辨率昴宿星团影像的三联体生成了一个高分辨率数字高程模型，并以此绘制断层崖及其地貌横向变化图。在其大部分长度上，观察到正断滑动，倾角在20°至60°之间变化，最大累计垂直错距为27米。然后，我们展示了位于苏赫塔尔村的一条沟谷中近期正断层活动的证据。对在断层上盘采集的碎屑木炭进行放射性碳年代测定，包括主要的崩积楔和上覆沉积层，结果表明最后一次事件发生在16世纪早期。这一时期发生了造成巨大破坏的1505年地震，主前锋逆冲断层上产生了约23米的滑动。无论是否相关，与随后几个世纪断层的静止状态相比，主前锋逆冲断层（MFT）和MBT上的破裂都发生在短时间内。考虑到其距离、运动方向以及MBT与MHT断层系统锁定断层带下倾端之间的巨大距离，我们认为MBT的间歇性正断活动可能与使MFT发生破裂的大地震有关。我们讨论了这些结果及其对喜马拉雅山脉前沿逆冲系统的影响。
