THE LATE QUATERNARY ACTIVITY CHARACTERISTICS AND SLIP RATE OF BATANG FAULT IN SE TIBETAN PLATEAU

doi:10.3969/j.issn.0253-4967.2023.06.002

Abstract

Abstract:

The southeastern margin of the Tibetan plateau is one of the most intensely deformed regions in the continental crust. A series of active faults with varying lengths and mechanical properties have segmented the lithosphere into multiple active blocks, with the Sichuan-Yunnan block being one of the most tectonically active regions. Its eastern boundary is characterized by secondary fault zones such as the Xianshuihe-Anninghe-Zemuhe, Xiaojiang, and Daliangshan fault zone, forming a narrow and continuous strike-slip deformation zone with a total length exceeding 1 100km. The western boundary of the Sichuan-Yunnan Block is mainly composed of the Jinsha River and the Red River fault zone, with the Jinsha River fault zone consisting of more than 20 roughly parallel secondary faults, forming a complex fault zone with 30~200km width. Despite recent GNSS network observation revealing the current tectonic deformation rates in this region, there is still a lack of research on the deformation characteristics and rates of individual active faults. This limitation makes it difficult in the assessment and understanding of seismic hazards in the area, restricting the scientific understanding of the current deformation mode in the southeastern margin of the Tibetan plateau.

The Batang Fault, located within the Jinsha River fault zone at the western boundary of the Sichuan-Yunnan block, is a NE-trending main fault that obliquely cuts across the Jinsha River Fault, dividing later into northern and central segments. Presently, the Batang Fault is characterized by dominant right-lateral strike-slip motion. The deformation characteristics and rates of this fault since the Late Quaternary are crucial for understanding the spatial distribution of strong earthquakes and deformation patterns in the Sichuan-Yunnan block.

The Batang Fault has a total length of 115km and is a Holocene right-lateral strike-slip active fault. The fault extends along the margins of bedrock mountains on both sides of the Maqu river and Jinsha River valleys, trending NNE or NWW to SEE, with a steep dip. The fault exhibits linear distribution of topographic features such as slopes, ridges, triangular facets, and fault scarps, essentially controlling the boundaries of bedrock mountains. In view from the geomorphology, the Batang Fault appears continuous and straight without distinct segmentation, except for localized small-scale step-like features. The Batang Fault has preserved abundant Late Quaternary activity evidence in two areas, Huangcaoping village and Batang county. This study utilized unmanned aerial vehicle photogrammetry to establish sub-meter digital terrain data for Huangcaoping and Batang site, accurately measuring displaced features such as alluvial fans and gullies affected by faulting. In Huangcaoping site, the fault has cut through multiple mountain-front alluvial fans, causing varying degrees of horizontal displacement in features such as gullies and the margin of the alluvial fans. This provided a scale for quantifying fault displacement. In Huangcaoping, five large-angle gullies intersect with the fault, one of which is a large gully developed in the bedrock mountain area. The gully has a deep incision, a narrow valley, and a rapid downstream turn to the right after exiting the mountain. The left bank of the gully preserves two geomorphic surfaces, Qo(older)and Qi(younger)surface, with the fault cutting across both surfaces, forming linear steep terrain. The measured total right-lateral offset of this gully since exiting the bedrock mountain area is(46±9)m. To constrain the activity rate of the Batang Fault at this location, we used cosmogenic nuclide single clast dating to determine the exposure age of the oldest geomorphic surface, Qo, as(12.5±0.5)ka. Considering that the formation of the river predates the Qo geomorphic surface, the age-constrained slip rate of the fault at this location is considered a maximum value, estimated at(3.6±0.8)mm/a. At Batang county, the Batang Fault has preserved clear faulted topography when cutting through the Moqu alluvial fan. The southern edge of the Moqu alluvial fan has been displaced by the fault, providing a well-preserved geomorphic marker for determining the strike-slip displacement of the fault. The Batang Fault, when intersecting the steep edge of the Moqu River alluvial fan, caused an obvious right-lateral offset, determined by comparing the consistent morphology of the steep edge on both sides of the fault. The right-lateral strike-slip displacement along the southern edge of the alluvial fan is measured at (40±5)m. The cosmogenic nuclide depth profile dating was used to determine the age of the faulted alluvial fan. From a vertical profile excavated along a man-made road on the edge of the alluvial fan, four mixed samples of small pebbles were collected from bottom to top. The calculated exposure ages of the debris flow alluvial fan are (15.2+3.2/-5.4)ka (without consideration of erosion)and (16.4+3.9/-5.6)ka (with consideration of erosion). Combining the fault displacement along the southern edge of the alluvial fan and the cosmogenic nuclide depth profile ages, the slip rate of the Batang Fault at this location is estimated to be of(2.6±0.6)mm/a (without erosion)or(2.4±0.8)mm/a (considering erosion). We believe that the age results with consideration of erosion effects is closer to the true values, thus we take 2.4mm/a as the activity rate of the Batang Fault at this location. The two slip rate values of the Batang Fault obtained in the Huangcaoping and Batang county sites are similar, indicating a right-lateral strike-slip rate of 2~4mm/a since the Late Quaternary. This rate accounts for 50%~80% of the present GPS observation shear deformation across the western boundary of the Sichuan-Yunnan block, indicating that the Batang Fault is a major deformation absorption zone in the Jinsha River fault zone. However, this rate is lower than the predicted~10mm/a using block models. The discrepancy may be due to the different understanding of the deformation mode at the western boundary of the Sichuan-Yunnan Block. In the block model, block sliding mainly relies on the primary boundary fault to regulate, but the long-term and lower geological activity rate of the Batang Fault obtained in this study does not match the assumption of a higher activity rate for the boundary fault in this model. The continuous and diffuse deformation characteristics of crustal deformation in the southeastern margin of the Tibet plateau may corroborate the lower activity rate of the Batang Fault obtained in this study.

Key words: Sichaun-Yunan block, strike-slip fault, slip rate, Late Quaternary, terrace

摘要：

青藏高原东南缘是现今陆内地壳形变最为强烈的地区之一, 一系列长度和力学性质不同的活动断裂将岩石圈分割为多个活动块体。川滇块体是其中构造活动最为显著的区域之一, 其西边界由多条互相平行的断裂组成, 巴塘断裂是其中的一条主干断裂。了解该断裂晚第四纪以来的变形特征和速率对于认识川滇块体强震的空间分布及变形模式具有重要意义。巴塘断裂整体走向NNE, 全长115km, 是一条全新世右旋走滑活动断裂。该断裂基本控制了基岩山体边界, 其中坡中槽、三角面、断层陡崖等地貌沿断裂呈线性分布, 并在黄草坪及巴塘县城2处区域保存了丰富的晚第四纪活动证据。文中利用无人机摄影测量手段建立了黄草坪及巴塘县城分辨率达亚米级的数字地形数据, 并对被断错的洪积扇、冲沟等地貌标志物进行了精确测量。在黄草坪地区, 巴塘断裂活动造成的山前冲沟水平偏转量为(46±9)m, 同时在巴塘县城断裂活动使莫曲河洪积扇边缘被右旋断错(40±5)m。文中利用单个砾石宇宙成因核素法及深度剖面法分别确定了黄草坪最老一级地貌面和莫曲河洪积扇的年龄, 分别为(12.5±0.5)ka和(16.4+3.9/-5.6)ka, 据此可以得到巴塘断裂晚第四纪以来的右旋走滑速率为2~4mm/a。该速率值占现今川滇块体西边界整体剪切变形的50%~80%, 但小于利用GPS形变资料反演得到的约10mm/a的结果。这种差异表明川滇块体西边界可能呈现连续-弥散变形特征, 而巴塘断裂是西边界中的一条主要变形构造。

关键词: 川滇块体, 走滑断裂, 活动速率, 晚更新世, 河流阶地

HUANG Wei-liang, ZHANG Jia-le, XIANG Wen, YANG Qian-hao. THE LATE QUATERNARY ACTIVITY CHARACTERISTICS AND SLIP RATE OF BATANG FAULT IN SE TIBETAN PLATEAU[J]. SEISMOLOGY AND GEOLOGY, 2023, 45(6): 1265-1285.

黄伟亮, 张家乐, 项闻, 杨虔灏. 晚第四纪以来巴塘断裂的活动特征及滑动速率[J]. 地震地质, 2023, 45(6): 1265-1285.

Figures/Tables 11

Fig. 1 Tectonic geomorphology and fault distribution around the Southeastern margin of the Tibetan plateau.

Table1 Analytical results of terrestrial cosmogenic nuclide 10Be geochronology

样品编号	位置		高程 /m	样品深度 /cm	石英^a 质量^a /g	⁹Be载体质量 /mg	¹⁰Be/⁹Be^b	¹⁰Be浓度^b /10⁴atoms·g^-1	1σ总误差^c /%	年龄 /ka
	北纬 /(°)	东经 /(°)
BT10BE-1	30.09	99.15	2762	2	29.09	0.24	1.63×10^-13	9.68	8.08	4.1±0.3
BT10BE-2	30.09	99.16	2775	2	31.20	0.25	5.32×10^-13	10.71	4.74	12.5±0.5
XJTBE-1	30.01	99.12	2569	2	29.52	0.24	0.29×10^-13	1.65	17.1	0.8±0.1
T2BE-4	30.01	99.16	2590	2	29.98	0.24	8.24×10^-13	45.4	15.1	15.2+3.2/-5.4^c 16.4+3.9/-5.6^d
T2BE-3	30.01	99.16	2590	100	23.53	0.25	0.82×10^-13	6.05	10.1
T2BE-2	30.01	99.16	2590	130	30.42	0.24	0.99×10^-13	5.27	10.1
T2BE-1	30.01	99.16	2590	200	29.09	0.25	0.34×10^-13	1.93	17.4

Fig. 2 Distribution characteristics of Batang Fault, Huangcaoping-Batang County seat section.

Fig. 3 Fault characteristics of Huangcaoping section of Batang Fault.

Fig. 4 Linear characteristics and profile of Batang Fault.

Fig. 5 Geomorphic features of Batang County section of Batang Fault.

Fig. 6 Horizontal displacement features of the Batang Fault in the Piedmont of Huangcaoping.

Fig. 7 Cosmogenic nuclide sampling points of large boulders on the alluvial fan in Huangcaoping.

Fig. 8 Fault features on the alluvial fan of Moqu river.

Fig. 9 Age results of the cosmogenic depth profile in the Moqu river alluvial fan.

Fig. 10 The GPS horizontal velocity field in the Jinshajiang fault zone and surrounding areas(Modify by XU Xiao-xue et al., 2020).

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