青海都兰热水-桃斯托河断裂的新发现及构造意义

doi:10.3969/j.issn.0253-4967.2020.01.002

摘要/Abstract

摘要：

在青海都兰最新开展的活动断裂调查工作中, 于都兰-茶卡高地南部新发现了1条长约40km、走向近NEE的左旋走滑、局部兼正断性质的全新世活动断裂——热水-桃斯托河断裂。文中通过野外地质调查与典型段落无人机航拍主要获得了以下2点认识: 1)热水-桃斯托河断裂及其全新世活动的发现尚属首次, 其长约40km, 断裂带东端存在长6km的地表破裂带; 2)断裂切过的冲沟和阶地存在左旋位错现象, 利用无人机获取高分辨率DEM影像, 并对冲沟沟谷的地形剖面进行恢复测量得到的位错量为(9.3±0.5)m、 (17.9±1.5)m和(36.8±2)m。对2级冲沟阶地位错进行恢复测量得到T₁/T₀阶地陡坎的位错量为(18.2±1.5)m, T₂/T₁阶地陡坎的位错量为(35.8±2)m, 可以看出冲沟位错量和阶地位错量的结果较为一致。据历史地震记载, 1938年4月10日在热水-桃斯托河断裂地表破裂的东端发生了M53/4地震, 1952年3月21日发生了M_S5.0地震, 可能均与该断裂的活动有关。但在都兰县县志等相关资料中均未发现有关于这2次地震的文献记录, 这可能与当时地震震中比较偏远、都兰县人口稀少且发震时间比较久远有关。东昆仑断裂和鄂拉山断裂最南端会聚形成1个向NW挤出的楔形断块, 位于楔形断块端部的都兰盆地受区域NE-SW向主压应力和2条边界断裂剪切作用的影响, 经历了挤压伴随局部拉张的复杂变形过程。这主要是由于2条控制楔形断块的边界断裂——东昆仑断裂与鄂拉山断裂之间存在NE-SW向的挤压作用, 在这样的挤压过程中, 楔形断块局部拉张的具体表现形式为两侧断裂发生走滑, 使断块向NW向挤出, 因此该区三角楔形挤出的变形机制与纯粹的刚性块体的挤出机制有所不同。在靠近鄂拉山断裂的都兰盆地东北缘形成一系列向SE会聚的羽列状逆冲断裂, 其形态上向E与鄂拉山断裂平行, 可能在深部汇入鄂拉山断裂; 靠近东昆仑断裂带的都兰盆地南缘断裂, 即热水-桃斯托河断裂受到东昆仑左旋走滑断裂和鄂拉山右旋走滑断裂共同作用的影响, 表现为左旋走滑的活动特征, 同时楔形断块向NW挤出, 使其东南端出现局部拉张, 故断层局部又表现为拉张性质。这些断裂共同吸收或转换高原东北缘的剪切应力。因此, 此次对都兰热水-桃斯托河断裂的新发现为更好地认识和理解青藏高原东北部断块内部的变形方式和机制等提供了重要的约束条件和依据。

关键词: 青藏高原, 热水-桃斯托河断裂, 左旋位错, 地表破裂, 全新世活断层

Abstract:

The 40km-long, NEE trending Reshui-Taostuo River Fault was found in the southern Dulan-Chaka highland by recent field investigation, which is a strike-slip fault with some normal component. DEM data was generated by small unmanned aerial vehicle(UAV)on key geomorphic units with resolution<0.05m. Based on the interpretation and field investigation, we get two conclusions: 1)It is the first time to define the Reshui-Taostuo River Fault, and the fault is 40km long with a 6km-long surface rupture; 2)There are left-handed dislocations in the gullies and terraces cut by the fault. On the high-resolution DEM image obtained by UAV, the offsets are(9.3±0.5)m, (17.9±1.5)m, and(36.8±2)m, measured by topographic profile recovery of gullies. The recovery measurements of two terraces present that the horizontal offset of T₁/T₀ is(18.2±1.5)m and the T₂/T₁ is (35.8±2)m, which is consistent with the offsets from gullies. According to the historical earthquake records, a M53/4 earthquake on April 10, 1938 and a M_S5.0 earthquake on March 21, 1952 occurred at the eastern end of the surface rupture, which may be related to the activity of the fault. By checking the county records of Dulan and other relevant data, we find that there are no literature records about the two earthquakes, which is possibly due to the far distance to the epicenter at that time, the scarcity of population in Dulan, or that the earthquake occurred too long ago that led to losing its records. The southernmost ends of the Eastern Kunlun Fault and the Elashan Fault converge to form a wedge-shaped extruded fault block toward the northwest. The Dulan Basin, located at the end of the wedge-shaped fault block, is affected by regional NE and SW principal compressive stress and the shear stress of the two boundary faults. The Dulan Basin experienced a complex deformation process of compression accompanying with extension. In the process of extrusion, the specific form of extension is the strike-slip faults at each side of the wedge, and there is indeed a north-east and south-west compression between the two controlling wedge-shaped fault block boundary faults, the Eastern Kunlun and Elashan Faults. The inferred mechanism of triangular wedge extrusion deformation in this area is quite different from the pure rigid extrusion model. Therefore, Dulan Basin is a wedge-shaped block sandwiched between the two large-scale strike-slip faults. Due to the compression of the northeast and southwest directions of the region, the peripheral faults of the Dulan Basin form a series of southeast converging plume thrust faults on the northeast edge of the basin near the Elashan Fault, which are parallel to the Elashan Fault in morphology and may converge with the Elashan Fault in subsurface. The southern marginal fault of the Dulan Basin(Reshui-Taostuo River Fault)near the Eastern Kunlun fault zone is jointly affected by the left-lateral strike-slip Eastern Kunlun Fault and the right-lateral strike-slip Elashan Fault, presenting a left-lateral strike-slip characteristic. Meanwhile, the wedge-shaped fault block extrudes to the northwest, causing local extension at the southeast end, and the fault shows the extensional deformation. These faults absorb or transform the shear stress in the northeastern margin of the Tibet Plateau. Therefore, our discovery of the Dulan Reshui-Taostuo River Fault provides important constraints for better understanding of the internal deformation mode and mechanism of the fault block in the northeastern Tibetan plateau.
The strike of Reshui-Taostuo River Fault is different from the southern marginal fault of the Qaidam Basin. The Qaidam south marginal burial fault is the boundary fault between the Qaidam Basin and the East Kunlun structural belt, with a total length of ~500km. The geophysical data show that Qaidam south marginal burial fault forms at the boundary between the positive gravity anomaly of the southern East Kunlun structural belt and the negative gravity anomaly gradient zone of the northern Qaidam Basin, showing as a thrust fault towards the basin. The western segment of the fault was active at late Pleistocene, and the eastern segment near Dulan County was active at early-middle Pleistocene. The Reshui-Taostuo River Fault is characterized by sinistral strike-slip with a normal component. The field evidence indicates that the latest active period of this fault was Holocene, with a total length of only 40km. Neither remote sensing image interpretation nor field investigation indicate the fault extends further westward and intersects with the Qaidam south marginal burial fault. Moreover, it shows that its strike is relatively consistent with the East Kunlun fault zone in spatial distribution and has a certain angle with the burial fault in the southern margin of Qaidam Basin. Therefore, there is no structural connection between the Reshui-Taostuo River Fault and the Qaidam south marginal burial fault.

Key words: northeastern margin of Tibetan plateau, Reshui-Taostuo River Fault, left-lateral offsets, earthquake surface rupture, Holocene active fault

中图分类号:

P315.2

李智敏, 任治坤, 刘金瑞, 哈广浩, 李正芳, 王勃, 王林建. 青海都兰热水-桃斯托河断裂的新发现及构造意义[J]. 地震地质, 2020, 42(1): 18-32.

LI Zhi-min, REN Zhi-kun, LIU Jin-rui, HA Guang-hao, LI Zheng-fang, WANG Bo, WANG Lin-jian. NEW DISCOVERY OF RESHUI-TAOSTUO RIVER FAULT IN DULAN, QINGHAI PROVINCE AND ITS IMPLICATIONS[J]. SEISMOLOGY AND GEOLOGY, 2020, 42(1): 18-32.

图/表 9

图 1 研究区区域地震构造图

Fig. 1 Seismotectonic map of the study area.

图 2 智尕日村西断层剖面

Fig. 2 Fault section at west of Zhigari village.

图 3 桃斯托河以西断层剖面灰色条带为断层断错标志层

Fig. 3 Fault section west of Taostuo River.

图 4 热水-桃斯托断裂西端地表破裂a 无人机DEM地貌图; b 无人机斜射地貌图

Fig. 4 Surface rupture at western end of Reshui-Taostuo River Fault.

图 5 沿着地表破裂发育的断塞塘

Fig. 5 Sag ponds developed along the surface rupture.

图 6 冲沟A左旋位错图及其恢复图a 原始地貌图; b 1次位错恢复图

Fig. 6 Left-lateral offset of gully A and its recovery.

图 7 冲沟B左旋位错图及其恢复图a 原始地貌图; b 1次位错恢复图; c 2次位错恢复图

Fig. 7 Left-lateral offset of gully B and its recovery.

图 8 断错地貌解译及位错恢复图a 原始地貌图; b 断错地貌解译图; c T1阶地位错恢复图; d T2阶地位错恢复图

Fig. 8 Interpretation of fault geomorphology and its recovery.

图9 研究区主要活动断层分布及其滑动速率(a)及热水-桃斯托河断裂成因机制(b)

Fig. 9 Major active faults and corresponding slip rates(a)and formation mechanism of Reshui-Taostuo River Fault(b).

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