西藏地區喀喇崑崙-嘉黎斷裂帶東段活動性探討

Abstract

喀拉崑崙－嘉黎斷裂帶是西藏唯一大型右移走滑斷層系統，在青藏高原向東逃脫模型中扮演很重要的角色。本研究工作利用各種衛星影像，與適合第四紀相關的定年技術，進行喀拉崑崙-嘉黎斷層東段活動構造與地形學的研究。本研究工作的主要成果有二：第一，斷裂帶中段的格仁錯斷層相當活躍，依據光螢光定年結果推估，在五萬年內的滑移速率約在14.6 ± 3.1 mm/yr。而且，斷層似乎以每次約3公尺的特徵地震行為活動，依據地形證據顯示此現象至少重複發生七次以上，配合斷層速率計算格仁錯斷層大約每兩百年左右就會引發規模7.2-7.4的大地震。第二，同一系統的嘉黎斷層卻似乎不具活動性，依據定年的結果推估主斷層線的部份已有兩萬年未曾有活動的紀錄。反而是原先認為從屬於嘉黎斷層的分支斷層－那曲斷層在地形上有許多活動跡象。格仁錯斷層、那曲斷層與發生1951年規模八地震的崩錯斷層具有相近的地形特徵與斷層活動性，其所組成雁行排列右移斷層系統為目前西藏中部最活躍的構造單元，並獨立於原本的喀拉崑崙－嘉黎斷層之外。

The Karakoram-Jiali fault zone (KJFZ) is the only major dextral fault system in Tibet, and plays an important role in the eastward extrusion of the Tibetan Plateau. Armijo et al. (1989) first recognized the KJFZ and proposed it as the structural boundary along which eastward movement of the Qiangtang terrane relative to the Lhasa terrane. This study focus on the activity of the eastern KJFZ, including with the Jiali fault (JF) and the Gyaring Co fault (GCF). Using high resolution satellite images, I was able to remap those two faults. Several geochronological methods are used, including radiocarbon, in-situ cosmogenic exposure, and optically stimulated luminescence (OSL). The GCF, one of the en echelon faults of the KJFZ, has been reported as a dextral fault, striking N50°-60°W. According to our OSL ages, a slip rate of the GCF can be estimated as 14.6 ± 3.1 mm/yr since ca. 50ka. This study also focuses on a section of the western segment of the GCF, where the slip has been recognized to have occurred 3.0 ± 1.6 m more than 7 times. This ~3 m slip implies Mw 7.2-7.4 earthquakes recurring to the western segment in every 200 yrs, and may reach about Mw 7.7 if both segments break at the same time. Along JF, the main Jiali fault has become less active since ca. 20ka. The field investigation shows there may have been only one small possible event during the Holocene, but none in the Late Pleistocene. The Nagqu fault (NF), previously mapped as the horsetail branch of the JF, shows many evidences to indicate that it is an active fault. The slip rate of the NF is 13~26 mm/yr, as fast as the GCF. The GCF and the NF are similar to another en echelon minor fault, named the Beng Co fault (BCF), which was located on the 1951 M8 event. Such a high slip rate suggests that the central en echelon minor faults of the original KJFZ are kinematically different from the Karakoram and Jiali faults. Turning to inactive along the main Jiali fault may be attributed to the orientation of the Jiali fault, which is parallel to the extensional axis and has no differential stress acting across it. On the other hand, the en echelon faults play essential roles in accommodating the stress in a certain tectonic system.