Geological and Geomorphic Evidence for Late Quaternary Activity of the Bianba-Luolong Fault on the Western Boundary of the Bangong-Nujiang Suture
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摘要:
边坝-洛隆断裂为班公湖-怒江缝合带东南段的西界断裂，正在规划建设中的川藏铁路穿经了该断裂.研究该断裂的晚第四纪活动性对认识缝合带的发展演化历史以及青藏高原东南缘现今构造变形的动力学机制具有重要意义，也直接关系到川藏铁路的规划建设和工程地震安全.边坝-洛隆断裂沿线记录有1642-1654年洛隆M≥7地震和1791-1804年边坝M 63/4地震，然而，由于缺少明确的地质地貌证据，关于边坝-洛隆断裂是否为全新世活动断裂至今仍存在较大争议.基于遥感影像解译和野外地质地貌调查，结合14C加速质谱（AMS）测年技术，笔者在八宿县城和洛隆县城一带获得了多个断错晚更新世-全新世地层的剖面，揭示边坝-洛隆断裂为一条全新世活动断裂，活动性质以左旋走滑为主，其最新一次活动发生在（3 310±30）a BP之后，可能对应1642-1654年洛隆M≥7地震事件.调查结果也显示，边坝-洛隆断裂晚第四纪活动的地表形迹非常不连续，其几何展布与班公湖-怒江缝合带的早期边界断裂并不完全一致，似乎暗示缝合带晚第四纪以来的活动可能正在发生新的演化.
Abstract:The Bianba-Luolong fault, which runs through the Sichuan-Tibet Railway under planning and construction, is the western boundary fault of the southeastern section of the Bangong-Nujiang suture. The Late Quaternary activity of the fault is of great significance for understanding the development and evolution history of the suture and researching the dynamics of the present tectonic deformation in SE Tibetan Plateau, and is also directly related to the planning and construction of the Sichuan-Tibet Railway and the safety of engineering earthquake. Historical records show that two large earthquakes, the 1642-1654 Luolong M ≥7 earthquake and the 1791-1804 Bianba M 63/4 earthquake, have occurred on the Bianba-Luolong fault. However, whether the fault has been seismically active in the Holocene remains a subject of debate due to poor research and lack of reliable geologic and geomorphological evidence. In this study, we focus on the Late Quaternary activity of the Bianba-Luolong fault based on interpretation of remote sensing imagery, analysis of field fault outcrops, faulted landforms and chronological measurement of 14C samples. It is suggested that the Bianba-Luolong fault is an active Holocene fault, and the main movement of the fault is sinistral strike-slip. Specifically, the latest faulting event occurred after (3 310±30) a BP, which possibly corresponds to the 1642-1654 Luolong M ≥7 earthquake. Survey results also show that the surface traces of the Bianba-Luolong fault activity is significantly discontinuous during the Late Quaternary, which is not completely consistent with the previous boundary fault of the Bangong-Nujiang suture, suggesting that the activity of suture zone may be undergoing a new evolution since the Late Quaternary.
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Key words:
- Bianba-Luolong fault /
- fault outcrops /
- Holocene activity /
- Bangong-Nujiang suture /
- Sichuan-Tibet Railway /
- seismology
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图 1 边坝‒洛隆断裂及其邻近地区活动构造简图
图a据Li et al.（2019）略有修改；AKMS. 阿尼玛卿‒昆仑‒木孜塔格缝合带；JS. 金沙江缝合带；BNS. 班公湖‒怒江缝合带；YZS. 雅鲁藏布江缝合带；ATF. 阿尔金断裂；KF. 喀喇昆仑断裂；MFT. 主前缘逆冲断裂；图b阴影区指示班公湖‒怒江缝合带（BNS）；F1. 嘉黎断裂；F2. 边坝‒洛隆断裂；F3. 羊达‒亚许断裂；F4. 怒江断裂；F5. 澜沧江断裂
Fig. 1. Active tectonic map of the Bianba-Luolong fault and its adjacent regions
图 2 八宿县城东郊一带断错地貌
a. 八宿城东一带断错地貌遥感影像，红色箭头指示断层形迹；b. 断错地貌（图a）解译图与剖面位置，红色线条指示断层形迹；c. 断层槽谷地貌与剖面位置，红色箭头指示断层槽谷；d. 断错地貌局部照片；e. 断错地貌（图d）解译图，蓝色线条指示小冲沟
Fig. 2. The offset geomorphic features on the east area of Basu County
图 3 八宿城东槽谷西端断层剖面（P1）
a. 断层剖面全景照片；b~d. 断层剖面局部照片，红色箭头指示断层面；e. 断层剖面（图a）解译图. ①风化破碎基岩（灰岩）；②坡洪积砾石层，砾石磨圆差，存在一定程度的胶结；③含砾砂土层，该层位采集1个14C混合样品（BSCN1）；④砂砾层，厚约30~50 cm，砾石砾径较小，略具磨圆，分选较好；⑤断层破碎带，带宽数厘米至十几厘米
Fig. 3. Fault section at the west end of the fault trough in the east of Basu County (P1)
图 4 八宿城东洪积扇上断层剖面（P2）
a. 断层剖面全景照片，红色箭头指示断层面；b. 断层剖面解译图. ①沟壁垮塌体；②紫红色含砂砾石层，局部层位发育斜层理（黄色虚线）；③灰白色砾石层；④含砾砂土层；⑤砾石层；⑥砂砾层；⑦砾石层；⑧草皮层
Fig. 4. Fault section at the alluvial fan in the east of Basu County (P2)
图 5 八宿城东台地处断层剖面（P3）
a. 断层剖面全景照片；b、c. 断层剖面局部照片，红色箭头指示断层面；d. 断层剖面（图a）解译图. ①风化（半风化）灰岩，颜色以灰绿色为主；②紫红色砂岩；③坡洪积砂砾石层；④薄层表土，局部发育；⑤断层带
Fig. 5. Fault section at the platform in the east of Basu County (P3)
图 6 八宿城西一带断错地貌与剖面位置
a. 八宿县至沙玛村一带断层形迹与剖面位置，底图为Google Earth影像，红色箭头指示断层形迹；b. 断错地貌照片；c. 断错地貌（图b）解译图，单边箭头示意断层运动方向
Fig. 6. The offset geomorphic features and fault section location on the west area of Basu County
图 7 沙玛村采砂场西壁断层剖面（P4）
a. 采砂场西壁断层剖面；b. 西壁断层剖面解译图，F1、F2为断层，F3~F6为推测断层（不排除滑坡的可能）；c. 西壁断层剖面近景照片，红色箭头指向断面. ①砾石层；②青灰色小砾石层；③粘土质砾石层；④小砾石层，偶含较大砾；⑤含粘土砾石层；⑥含砾砖红色粘土薄层；W构造楔
Fig. 7. Fault section on west wall of sand pit at Shama Village (P4)
图 8 沙玛村采砂场东壁断层剖面（P5）
a. 采砂场东壁断层剖面；b. 东壁断层剖面解译图，F1、F2为断层；c. 东壁断层剖面近景照片，红色箭头指向断面. ①青灰色小砾石层；②含粘土砾石层；③含砾砖红色粘土薄层
Fig. 8. Fault section on east wall of sand pit at Shama Village (P5)
图 9 夏果村断层剖面（P6）
a. 断层剖面全景照片；b、c. 断层剖面局部照片，红色箭头指示断面或定向排列砾石；d. 断层剖面（图a）解译图. ①半风化灰岩；②坡洪积含砾粘土层；③地表黑土薄层
Fig. 9. Fault section at Xiaguo Village (P6)
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