A PRELIMINARY STUDY ABOUT SLIP RATE OF MIDDLE SEGMENT OF THE NORTHERN QILIAN THRUST FAULT ZONE SINCE LATE QUATERNARY

The Fodongmiao-Hongyazi Fault belongs to the forward thrust fault of the middle segment of northern Qilian Shan overthrust fault zone, and it is also the boundary between the Qilian Shan and Jiudong Basin. Accurately-constrained fault slip rate is crucial for understanding the present-day tectonic deformation mechanism and regional seismic hazard in Tibet plateau. In this paper, we focus on the Shiyangjuan site in the western section of the fault and the Fenglehe site in the middle part of the fault. Combining geomorphic mapping, topographic surveys of the deformed terrace surfaces, optically stimulated luminescence (OSL) dating, terrestrial cosmogenic nuclide dating and radiocarbon (¹⁴C) dating methods, we obtained the average vertical slip rate and shortening rate of the fault, which are ~1.1mm/a and 0.9~1.3mm/a, respectively. In addition, decadal GPS velocity profile across the Qilian Shan and Jiudong Basin shows a basin shortening rate of~1.4mm/a, which is consistent with geological shortening rates. Blind fault or other structural deformation in the Jiudong Basin may accommodate part of crustal shortening. Overall crustal shortening rate of the Jiudong Basin accounts for about 1/5 of shortening rate of the Qilian Shan. The seismic activity of the forward thrust zone of Tibetan plateau propagating northeastward is still high.     

焉耆盆地北缘和静逆断裂-褶皱带第四纪变形

焉耆盆地是塔里木盆地东北缘天山山间的重要坳陷区,盆地北缘发育的和静逆断裂-褶皱带是一条现今活动强烈的逆断裂-褶皱带,对其第四纪以来缩短量和隆升量的计算有利于分析该区域的构造活动情况,对缩短速率和隆升速率的估计可以与天山造山带其他区域的活动速率进行横向对比,从而反映出焉耆盆地在天山晚新生代构造变形的作用。在深部资料不足的情况下,对背斜形态完整、构造样式简单的和静逆断裂-褶皱带,利用地表可获得的地层和断层产状,通过恢复褶皱几何形态,计算褶皱的缩短量、隆升量和断层滑动量,得到逆断裂-褶皱带早更新世晚期(1.8Ma)、中更新世(780ka)和晚更新世中期(80ka)以来的缩短量分别为1.79km、0.88km和26m,初步估计的缩短速率分别为0.99mm/a、1.13mm/a和0.33mm/a。显示和静逆断裂-褶皱带自开始形成以来构造活动强度并不一致。与地壳形变观测结果对比,作为南天山东段最主要的坳陷区,焉耆盆地吸收了这一区域(86°～88°E)的大部分地壳缩短,且主要表现为盆地北缘新生逆断裂-褶皱带的强烈变形。     

GEOMORPHIC FEATURES AND LATE QUATERNARY SLIP RATE OF THE SOUTHERN ZONGWULONG SHAN FAULT

With the continuous collision of the India and Eurasia plate in Cenozoic, the Qilian Shan began to uplift strongly from 12Ma to 10Ma. Nowadays, Qilian Shan is still uplifting and expanding. In the northern part of Qilian Shan, tectonic activity extends to Hexi Corridor Basin, and has affected Alashan area. In the southern part of Qilian Shan, tectonic activity extends to Qaidam Basin, forming a series of thrust faults in the northern margin of Qaidam Basin and a series of fold deformations in the basin. The southern Zongwulong Shan Fault is located in the northeastern margin of Qaidam Basin, it is the boundary thrust fault between the southern margin of Qilian Shan and Qaidam Basin. GPS studies show that the total crustal shortening rate across the Qilian Shan is 5~8mm/a, which absorbs 20% of the convergence rate of the Indian-Eurasian plate. Concerning how the strain is distributed on individual fault in the Qilian Shan, previous studies mainly focused on the northern margin of the Qilian Shan and the Hexi Corridor Basin, while the study on the southern margin of the Qilian Shan was relatively weak. Therefore, the study of late Quaternary activity of southern Zongwulong Shan Fault in southern margin of Qilian Shan is of great significance to understand the strain distribution pattern in Qilian Shan and the propagation of the fault to the interior of Qaidam Basin. At the same time, because of the strong tectonic activity, the northern margin of Qaidam Basin is also a seismic-prone area. Determining the fault slip rate is also helpful to better understand the movement behaviors of faults and seismic risk assessment.Through remote sensing image interpretation and field geological survey, combined with GPS topographic profiling, cosmogenic nuclides and optically stimulated luminescence dating, we carried out a detailed study at Baijingtu site and Xujixiang site on the southern Zongwulong Shan Fault. The results show that the southern Zongwulong Shan Fault is a Holocene reverse fault, which faulted a series of piedmont alluvial fans and formed a series of fault scarps.The 43ka, 20ka and 11ka ages of the alluvial fan surfaces in this area can be well compared with the ages of terraces and alluvial fan surfaces in the northeastern margin of Tibetan Plateau, and its formation is mainly controlled by climatic factors. Based on the vertical dislocations of the alluvial fans in different periods in Baijingtu and Xujixiang areas, the average vertical slip rate of the southern Zongwulong Shan Fault since late Quaternary is(0.41±0.05)mm/a, and the average horizontal shortening rate is 0.47~0.80mm/a, accounting for about 10% of the crustal shortening in Qilian Shan. These results are helpful to further understand the strain distribution model in Qilian Shan and the tectonic deformation mechanism in the northern margin of Qaidam Basin. The deformation mechanism of the northern Qaidam Basin fault zone, which is composed of the southern Zongwulong Shan Fault, is rather complicated, and it is not a simple piggy-back thrusting style. These faults jointly control the tectonic activity characteristics of the northern Qaidam Basin.     

焉耆盆地北缘和静逆断裂-褶皱带中晚第四纪变形速率

焉耆盆地为南天山内部的一个山间盆地,盆地北缘发育1排第四纪新生褶皱带,即和静逆断裂-褶皱带。中晚第四纪以来,由于和静逆断裂-褶皱带的持续活动使得在褶皱生长过程中形成的多期洪积地貌面发生反向掀斜变形。利用高精度差分GPS,对褶皱带中部哈尔莫敦背斜区内的多期变形地貌面的地形形态进行了测绘,判定背斜的生长主要以翼旋转为主。利用背斜北翼不同地貌面的反向掀斜角度,分别计算了不同期次地貌面的隆升和缩短变形量。结合原地宇宙成因核素深度剖面法和光释光测年法,对背斜区内的F4,F3b,F2洪积台地面和T1阶地面的形成年龄进行了测定,发现背斜在距今约550ka、428.3+57.6-47.2ka和354.3+34.2-34.8ka不同时段的平均隆升速率从0.31±0.24mm/a下降至0.15±0.02mm/a,同时背斜北翼的翼旋转速度也呈逐渐减小的趋势。但背斜自起始变形开始,缩短速率却大致保持恒定为约0.3mm/a。而这一恒定的缩短速率与现今横跨和静逆断裂-褶皱带所观测的GPS速率具有很好的一致性,说明在天山内部的哈尔莫敦背斜区,短尺度的GPS速率可以代表长尺度的地壳应变速率,同时反映出山体内部一系列断层和褶皱构造在吸收和调节整体变形量时也起到一定的作用。     

焉耆盆地活动构造的遥感图像特征

焉耆盆地是一个位于天山东南部的山间盆地 ,在遥感图像上 ,可以看到盆地西面的活动构造在地表留下的痕迹 :清楚的线形构造 ,被错开的水系、冲沟等。经分析认为 ,这些活动构造主要由北边的活动褶皱和南边的右行走滑活动断裂组成。野外的实际调查与图像分析的结果十分吻合 ,表明遥感图像在研究活动构造方面是一个十分有力和快速的工具。活动断层的最大水平位移为 2 10m ,最大垂直位移为 2 0m ,主要以走滑作用为主。天山地区活动构造的发育表明印度板块向欧亚板块的碰撞仍在持续     

LATE QUATERNARY CRUSTAL SHORTENING RATE OF THE BEILUNTAI FAULT IN SOUTHERN TIAN SHAN,XINJIANG

The Beiluntai Fault is a Holocene active fault. It is the boundary between southern Tian Shan and Tarim Basin. Since the late Quaternary, steady activities of the Beiluntai fault have resulted in offsets, folds, and uplift of pluvial terraces. We used the high-resolution RTK topographic surveys to reveal that the fault scarp morphology on the Akeaiken(Ak) segment and Zhuanchang(Zc) segment of the Beiluntai fault. We found that the crustal shortening of Ak and Zc segments are dominated by thrusting and folding-uplift, respectively. We employed th optically stimulated luminescence(OSL) dating method to develop a new chronology for the different pluvial terraces, indicating that they formed at 49.14~58.51, 27±3, 13.72~14.64, 7.13±0.88, (3.32±0.43) ka, respectively. These data permitted to estimate the crustal shortening rate(about 2.4mm/a) remains largely constant on the Ak segment, while the crustal shortening rate of Zc segment was 1.43~1.81mm/a since the Fan4 pluvial terraces was abandoned. Compared with the Ak segment, the crustal shortening rate of the Zc segment declined obviously. This shows that the NS-trending crustal shortening rate of the Beituntai fault decreased gradually from west to east. A comprehensive comparison of the reverse fault-fold belt system in the front of southern Tian Shan also indicates that the crustal shortening rate drops from west to east.     

GEOMETRY AND DEFORMATION TRANSFORMATION OF THE XIMALIN FAULT

The Ximalin fault is the northwest section of the Ximalin-Shuiquan fault, which is part of the north-edge fault zone of the Yanghe Basin, located in the conjunction of the Zhangjiakou-Bohai fault zone and Shanxi fault-depression basin, and its structural geometry and deformation characteristics can facilitate the research on the interaction of the two tectonic belts. In this paper, data of geological surveys and geophysical exploration are used to study this fault exhaustively, concerning its geometry, structural features and activity as well as its relationship with adjacent faults and rule in the deformation transform of the north-edge fault zone of the Yanghe Basin. The results show that the Ximalin Fault is a strike-slip feature with thrust component. Its vertical slip rates are 0.17mm/a and 0.25~0.38mm/a, and the horizontal slip rate is 0.58~0.67mm/a and 0.50mm/a during the late Middle Pleistocene and Holocene, respectively. It is formed alternately by the NW-trending main faults and secondary NE-trending faults, of which the former is characterized by high-angle reverse with sinistral strike-slip, and the latter shows normal faulting. The two sets of structures have specific structural geometry relations, and the motion manners and deformation characteristics match each other. During the active process of the north-edge fault of the Yanghe Basin, the NW trending Ximalin fault played a role similar to a transform fault in deformation change and stress transfer, and its sinistral strike slip activity accommodated the NE trending normal faulting at the both ends.     

FAULTED LANDFORM AND SLIP RATE OF THE JINGHE SECTION OF THE BOLOKENU-AQIKEKUDUKE FAULT SINCE THE LATE PLEISTOCENE

The Bolokonu-Aqikekuduke fault zone(Bo-A Fault)is the plate convergence boundary between the middle and the northern Tianshan. Bo-A Fault is an inherited right-lateral strike-slip active fault and obliquely cuts the Tianshan Mountains to the northwest. Accurately constrained fault activity and slip rate is crucial for understanding the tectonic deformation mechanism, strain rate distribution and regional seismic hazard. Based on the interpretation of satellite remote sensing images and topographic surveys, this paper divides the alluvial fans in the southeast of Jinghe River into four phases, Fan1, Fan2, Fan3 and Fan4 by geomorphological elevation, water density, depth of cut, etc. This paper interprets gullies and terrace scarps by high-resolution LiDAR topographic data. Right-laterally offset gullies, fault scarps and terrace scarps are distributed in Fan1, Fan2b and Fan3. We have identified a total of 30 right-laterally offset gullies and terrace scarps. Minimum right-lateral displacement is about 6m and the maximum right-lateral displacements are(414±10)m, (91±5)m and(39±1)m on Fan2b, Fan3a and Fan3b. The landform scarp dividing Fan2b and Fan3a is offset right-laterally by (212±11)m. Combining the work done by the predecessors in the northern foothills of the Tianshan Mountains with Guliya ice core climate curve, this paper concludes that the undercut age of alluvial fan are 56~64ka, 35~41ka, 10~14ka in the Tianshan Mountains. The slip rate of Bo-A Fault since the formation of the Fan2b, Fan3a and Fan3b of the alluvial-proluvial fan is 3.3~3.7mm/a, 2.2~2.6mm/a and 2.7~3.9mm/a. The right-lateral strike-slip rate since the late Pleistocene is obtained to be 3.1±0.3mm/a based on high-resolution LiDAR topographic data and Monte Carlo analysis.     

THE QUANTATIVE STUDY ON ACTIVITY OF DENGDENGSHAN-CHIJIACIWO FAULTS SINCE LATE QUATERNARY

The Dengdengshan and Chijiaciwo faults situate in the northeast flank of Kuantanshan uplift at the eastern terminal of Altyn Tagh fault zone, striking northwest as a whole and extending 19 kilometers and 6.5 kilometers for the Dengdengshan and Chijiaciwo Fault, respectively. Based on satellite image interpretation, trenching, faulted geomorphology surveying and samples dating etc., we researched the new active characteristics of the faults. Three-levels of geomorphic surfaces, i.e. the erosion rock platform, terrace I and terrace Ⅱ, could be found in the northeast side of Kuantanshan Mountain. The Dengdengshan Fault dislocated all geomorphic surfaces except terrace I, and the general height of scarp is about 1.5 meters, with the maximum reaching 2.6 meters. Three paleoseismic events are determined since late Pleistocene through trenching, and the total displacement of three events is about 2.7 meters, the average vertical dislocation of each event changed from 0.5 to 1.2 meters. By collecting age samples and dating, the event Ⅰ occurred about 5ka BP, event Ⅱ occurred about 20ka BP, and event Ⅲ occurred about 35ka BP. The recurrence interval is about 15ka BP; and the vertical slip rate since the late Pleistocene is about 0.04mm/a. The Chijiaciwo Fault, however, dislocated all three geomorphic surfaces, and the general scarp height is about 2.0 meters with the maximum up to 4.0 meters. Three paleoseismic events are determined since late Pleistocene through trenching, and the total displacement of three events is about 3.25 meters, the average vertical dislocation of each event changed from 0.75 to 1.5 meters, and the vertical slip rate since the late Pleistocene is about 0.06mm/a. Although the age constraint of paleoearthquakes on Chijiaciwo Fault is not as good as that of Dengdengshan Fault, the latest event on Chijiaciwo Fault is later than Dengdengshan Fault's. Furthermore, we infer that the recurrence interval of Chijiaciwo Fault is 15ka BP, which is close to that of Dengdengshan Fault. The latest event on Chijiaciwo Fault is later than the Dengdengshan Fault's, and the vertical displacement and the slip rate of a single event in late Quaternary are both larger than that of Dengdengshan Fault. Additionally, a 5-kilometer-long discontinuity segment exists between these two faults and is covered by Quaternary alluvial sand gravel. All these indicate that the activity of the Chijiaciwo Fault and Dengdengshan Fault has obvious segmentation feature. The size of Chijiaciwo Fault and Dengdengshan Fault are small, and the vertical slip rate of 0.04~0.06mm/a is far smaller than that of Qilianshan Fault and the NW-striking faults in Jiuxi Basin. All these indeicate that the tectonic deformation of this region is mainly concentrated on Hexi Corrider and the interior of Tibet Plateau, while the activties of Chijiaciwo and Dengdengshan faults are characterized by slow slip rate, long recurrence interval(more than 10ka)and slow tectonic deformation.     

THE LATE QUATERNARY ACTIVITY CHARACTERISTICS OF THE STRIKE-SLIP FAULTS IN THE TIANSHAN OROGENIC BELT: A CASE STUDY OF THE KAIDUHE FAULT

As the most active intracontinental orogenic belt in the world, the Tianshan orogenic belt has complex and diverse internal structural deformation patterns, and among them, the particularly striking is the linear straight U-type valley landscapes which cut inside the mountains by multiple NW-SE and ENE-WSW strike-slip faults. Many of the modern strong earthquakes in Tianshan orogenic belt are closely related to these strike-slip faults. Therefore, it is important to elaborate the activity characteristics of these faults to understand the deformation process inside the Tianshan Mountains belt. This paper focuses on one of the NW-SE right-lateral strike-slip fault (the Kaiduhe Fault), which lies inside the southeastern Tianshan. Typical offset landforms and scarp lineaments on the western segment of the Kaiduhe Fault can be used to study the activity characteristics and strike-slip rate. In particular, the fault cuts through the late Quaternary alluvial fans and a series of river gullies were right-laterally faulted, producing dextral offsets ranging from 3 to 248m. A digital elevation model (DEM)with resolution of 0.25m was established by using multi-angle photogrammetry technique to stripe about 12km linear tectonic landforms along the Kaiduhe Fault. Geological and geomorphic mapping in DEM with 22 high-resolution dextral offset measurements reveals that the dextral offsets can be divide into four groups of 3.5m, 7.0m, 11.8m and 14.5m. It is presumed from the approximately uniformly-spaced offsets that the coseismic offset was 3~4m. In addition, the exposure age of an older alluvial fan surface was about 235.7ka by in situ ¹⁰Be terrestrial cosmogenic nuclide method. Combining the exposure ages and the maximum dextral offset of 248m, we found that the strike-slip rate of the Kaiduhe Fault is about 1mm/a. It is found by this study that the Kaiduhe Fault plays an important role in regulating SN compression deformation within Tianshan Mountains, and it should also be the main stress-strain accumulation area which has the risk of occurrence of strong earthquake.     

THE LATE QUATERNARY ACTIVITY FEATURES AND SLIP RATE OF THE YANGGAO-TIANZHEN FAULT

The Shanxi Graben System is one of the intracontinental graben systems developed around the Ordos Block in North China since the Cenozoic, and it provides a unique natural laboratory for studying the long-term tectonic history of active intracontinental normal faults in an extensional environment. Comparing with the dense strong earthquakes in its central part, no strong earthquakes with magnitudes over 7 have been recorded historically in the Jin-Ji-Meng Basin-and-Range Province of the northern Shanxi Graben System. However, this area is located at the conjunction area of several active-tectonic blocks(e.g. the Ordos, Yan Shan and North China Plain blocks), thus it has the tectonic conditions for strong earthquakes. Studying the active tectonics in the northern Shanxi Graben System will thus be of great significance to the seismic hazard assessment. Based on high-resolution remote sensing image interpretations and field investigations, combined with the UAV photogrammetry and OSL dating, we studied the late Quaternary activity and slip rate of the relatively poorly-researched Yanggao-Tianzhen Fault(YTF)in the Jin-Ji-Meng Basin-and-Range Province and got the followings: 1)The YTF extends for more than 75km from Dashagou, Fengzhen, Inner Mongolia in the west to Yiqingpo, Tianzhen, Shanxi Province in the east. In most cases, the YTF lies in the contact zone between the bedrock mountain and the sediments in the basin, but the fault grows into the basin where the fault geometry is irregular. At the vicinity of the Erdun Village, Shijiudun Village, and Yulinkou Village, the faults are not only distributed at the basin-mountain boundary, we have also found evidence of late Quaternary fault activity in the alluvial fans that is far away from the basin-mountain boundary. The overall strike of the fault is N78°E, but the strike gradually changes from ENE to NE, then to NWW from the west to the east, with dips ranging from 30° to 80°. 2)Based on field surveys of tectonic landforms and analysis of fault kinematics in outcrops, we have found that the sense of motion of the YTF changes along its strikes: the NEE and NE-striking segments are mainly normal dip-slip faults, while the left-laterally displaced gullies on the NWW segment and the occurrence characteristics of striations in the fault outcrop indicate that the NWW-striking segment is normal fault with minor sinistral strike-slip component. The sense of motion of the YTF determined by geologic and geomorphic evidences is consistent with the relationship between the regional NNW-SSE extension regime and the fault geometry. 3)By measuring and dating the displaced geologic markers and geomorphic surfaces, such as terraces and alluvial fans at three sites along the western segment of the YTF, we estimated that the fault slip rates are 0.12~0.20mm/a over the late Pleistocene. In order to compare the slip rate determined by geological method with extension rate constrained by geodetic measurement, the vertical slip rates were converted into horizontal slip rate using the dip angles of the fault planes measured in the field. At Zhuanlou Village, the T₂ terrace was vertically displaced for(2.5±0.4)m, the abandonment age of the T₂ was constrained to be(12.5±1.6)ka, so we determined a vertical slip rate of(0.2±0.04)mm/a using the deformed T₂ terrace and its OSL age. For a 50°dipping fault, it corresponds to extension rate of(0.17±0.03)mm/a. At Pingshan Village, the vertical displacement of the late Pleistocene alluvial fan is measured to be(5.38±0.83)m, the abandonment age of the alluvial fan is(29.7±2.5)ka, thus we estimated the vertical slip rate of the YTF to(0.18±0.02)mm/a. For a 65° dipping fault, it corresponds to an extension rate of(0.09±0.01)mm/a. Ultimately, the corresponding extensional rates were determined to be between 0.09mm/a and 0.17mm/a. Geological and geodetic researches have shown that the northern Shanxi Graben System are extending in NNW-SSE direction with slip rates of 1~2mm/a. Our data suggests that the YTF accounts for about 10% of the crustal extension rate in the northern Shanxi Graben System.     

西秦岭临潭-宕昌断裂第四纪最新活动特征

临潭-宕昌断裂是西秦岭造山带内一条重要的分支断裂,其最新活动特征是分析西秦岭构造变形的重要依据.临潭-宕昌断裂的新构造活动强烈,中强地震频繁,但目前对于断裂的新活动特征研究程度较低,未见有其全新世活动地质地貌证据的报道.文中基于遥感解译、宏观地貌分析研究断裂的长期活动表现和分段性;同时通过地质地貌考察、无人机摄影测量、...     

5·12汶川地震地表破裂基本参数的再论证及其构造内涵分析

地震地表破裂基本参数是反演地震破裂过程的基本约束条件和预测其他活动断层地震危险性不可缺少的物理量.以野外地表破裂带重要观测点全站仪或差分GPS仪实测数据为基础,结合高分辨率遥感资料解译、先存断层陡坎构造地貌标志的识别、以及地形测绘资料的考证等,重新论证了5·12汶川地震地表破裂带展布样式、长度、最大同震位移值等基本参数.结果表明,地震地表破裂带长度可达240 km,最大垂直位移为6.5±0.5 m,最大右旋走滑位移4.9 m,基于倾角向下变缓逆断层模型推测汶川地震在龙门山推覆构造带中段产生了最大~7 m的地壳缩短量,说明青藏高原东缘横向逆断层为将高原内部东向水平运动转换为高原隆升的转换构造,这一研究结果有助于深化认识青藏高原东缘隆升机理.     

2016年M6.2呼图壁地震发震构造及其对天山构造带隆升的启示:来自中近场钻孔应变观测的证据

天山山脉在新生代经历了强烈的构造隆升和地壳缩短作用,其周缘发生的地震活动是了解这一构造模式的窗口.对2016年呼图壁地震的发震构造有两种解读：向南倾斜的低角度逆断层和向北倾斜的高角度逆断层.中近场四台钻孔应变仪记录到了本次地震的同震响应,本文采用均质模型对IGP-CEA和USGS震源机制解进行模拟,结果显示发震断层为向北陡倾的反冲断层,15个原始方位和8个N-S、E-W方位观测值全部与预测值一致.对比天山北缘常见的低角度逆冲断层,反冲断层对构造隆升的贡献更有效,以断层倾角70°和19°计算,二者对隆升和缩短贡献比例分别为2.89：1和1：2.76.这一结果表明天山构造带内部的反冲构造同样具有单独发震的可能性,它们对天山现今的隆升高度同样起着不可忽视的作用.     

近场位移数据约束的2013年芦山地震破裂模型及其构造意义

以往的研究显示了2013年芦山MS7.0级地震发震断层的隐伏逆冲断层基本特征,但是破裂深部细节差异较大.本文以近场密集的同震形变数据约束芦山地震破裂面几何形状及滑动分布,结果显示芦山地震破裂面具有铲状结构,上部16km为43°~50°高角度断层,深部16~25km为小于27°的低角度断层,破裂深度与重定位的余震分布深度一致.破裂分布模型清楚显示上下两个断层上各有一个滑动幅度大于0.5m的峰值破裂区,最大滑动量1.5m位于13km深处.重定位的余震分布基本都落在最大滑动量等值线外部库仑应力增加的区域.芦山地震破裂面几何形状和滑动分布特征与2008年汶川MS8.0级地震映秀—北川破裂相似,支持龙门山冲断带发育大规模的近水平滑脱层,是青藏高原东缘地壳缩短增厚、龙门山挤压隆升的重要证据.     

THE LATE QUATERNARY TECTONIC DEFORMATION REVEALED BY THE TERRACES ON THE BAIYANG RIVER IN THE NORTHERN QILIAN MOUNTAINS

The Qilian Mountains, as a major orogenic belt in the northeastern margin of the Tibetan plateau, is the forefront of the expansion of the plateau to the northeast, where thrusts and folds dominate tectonic deformation. The Baiyang River starts from the inner Qilian Mountains, flowing northward across various structures, and finally into the Jiuxi Basin. This work focused on exhaustive investigations to the terraces on this river to characterize the Late Quaternary tectonic deformation in this region. The results show that (1)these river terraces on the Baiyang River are segmented, of which multiple levels developed at steep terrains and anticlines in the basin. Bounded by the Niutou Mountains, mainly 2-3 and 4-5 levels of terraces formed in the upper and lower reaches, respectively. (2)The longitudinal profiles along the river suggest a vertical motion rate of the Changma fault as (0.32±0.09)mm/a and crustal shortening rate (0.12±0.09)mm/a. There was no vertical activity since the formation of T₅ surface (13ka)on the Hanxia-Dahuanggou fault. At the terrace T₅ (9ka)on the Laojunmiao anticline, fold uplift amounts (6.55±0.5)m and shortening amounts (3.47±0.5)m, yielding uplift and shortening rates (1.23±0.81)mm/a and (0.67±0.44)mm/a, respectively. The Baiyang River anticline began to be active about 300ka with uplift and shortening rates (0.21±0.02)mm/a and (0.14±0.03)mm/a, respectively since 170ka. (3)In the Qilian Mountains, there were two different deformation characteristics in response to the expansion of the Tibetan plateau. Shear deformation dominates the inner Qilian Mountains, which is manifested as lateral extrusion of blocks. In the northern margin of Qilian Mountains and Jiuxi Basin, the deformation is dominated by compression, expressing crustal shortening and uplift, and the shortening within the basin accounts about half of the total deformation.     

海原断裂带老虎山段晚第四纪滑动速率精确厘定与讨论

如何准确测定断裂滑动速率是近年来活动构造研究的前沿与热点.随着高精度地形数据获取手段与第四纪测年方法的不断进步,位错量和地貌面年龄的精度均得到大大提高.在进行滑动速率计算时还要考虑地质过程是否合理,蒙特卡洛方法为获取更加符合地质过程的滑动速率提供了重要工具.本文以滑动速率研究程度较低的海原断裂带老虎山段为例,基于LiDAR高精度地形数据,测得T1—T4阶地面年龄分别为1~3 ka,9~11 ka,15~17 ka,40~45 ka,陡坎前缘的位错分别为7~14 m,28~36 m,59~66 m,180~190 m.综合多地点的左旋走滑位错量及不同时代的地貌面年龄数据,并考虑滑动历史,利用蒙特卡洛模拟方法,将位错-时间两个参数的不确定性定量化,限定老虎山断裂45 ka以来平均滑动速率为4.3±0.16 mm·a^-1,17ka以来的平均滑动速率为4.0±0.15 mm·a^-1,与前人研究得到的狭义海原断裂滑动速率4.5±1.0 mm·a^-1基本一致.综合整个海原断裂带滑动速率,本文结果更支持低滑动速率变化趋势,即海原断裂带整体滑动速率趋于稳定,向东至六盘山断裂,滑动速率开始降低,推测海原断裂带的左旋走滑在尾端主要为马东山—六盘山隆起所吸收.结合老虎山断裂历史地震资料和深部锁闭浅部蠕滑的动力学特征,推测老虎山断裂具备与相邻断裂一起触发强震的能力.     

山西长治断裂的基本特征与第四纪活动性研究

实地调查表明,山西长治断裂由三段组成,南段和中段为西倾正断层,北段为东倾正断层,总体走向北东25o,长达65km,控制着附近山地与第四纪山间盆地的构造演化和地貌发育;第四纪早期,断裂发生强烈的垂直差异活动,最大垂直活动幅度可达400m左右;第四纪晚期,断裂活动性明显减弱,中段仍有活动,至少发生过两期活动,第—期活动发生于距今10.88万年左右或以后、1.49万年以前,第二期活动发生于距今1.49万年以后,垂直错距0.65m。     

VERTICAL SLIP RATE OF MINLE-DAMAYING FAULT INDICATED BY SCARPS ON TERRACES OF DONGDA RIVER

The Qilianshan north-edge thrust (QNT)is located at the boundary between the northern margin of the Qilianshan mountain and Hexi Corridor, with a length over 700km. The Minle-Damaying fault (MDF), trending NWW, is part of the eastern section of the QNT, cutting through the Minle and Wuwei Basins. Hexi Corridor is a region of intense seismic activities, where many large earthquakes have been documented in history, such as the M7.5 Gaotai earthquake in 180, M8.5 Haiyuan earthquake in 1920, M8.0 Gulang earthquake in 1927 and the M7.6 Changma earthquake in 1932. While, there is no seismic record on the MDF. The Dongda River flows across the MDF from south to north. One of the tributary of the Dongda River, Xie River, has very well preserved terraces (T₆-T₁)which were offset by the MDF. On these terraces, there is clear trace of scarps, of which the height increases from terraces T₃ to T₆, indicating an accumulation of offset with time. In order to acquire the cross-section of scarps, unmanned aerial vehicle (UAV)scanning was implemented. With a digital camera mounted on, the UAV scanned an area of 0.52km² and digital elevation model (DEM)was generated with an accuracy of 0.2m vertically. The Thompson's method was utilized to conduct linear regressions on both the hanging wall and foot wall of the fault. The difference between the intercepts of the regression lines with the vertical line going through the intersection of the scarp surface on the fault surface is considered as the vertical offset. Terraces from T₆ to T₃ are very well preserved where MFD intercepts the Xie river, while T₂ and T₁ are badly eroded at the same location. Utilizing the cross-sections extracted from high resolution DEM, we estimate that the vertical offsets of T₆-T₃ are 13.26~15.67m, 9.74~10.13m, 5.86~7.35m and 5.03~5.60m, respectively, with 95%confidence interval. From the offsets of terraces, at least 4 paleo-seismic events are indentified. Terraces were dated by the AMS ¹⁴ C dating, yielding ages (cal BP)of T₆-T₂ as (16 405±210)a, (111 975±21)a, (5 697.5±210)a, (4 470.5±54.5)a and (3 137.5±77.5)a. Liner regression was performed for the relation between the ages and the offsets of terraces, resulting in the average vertical slip rate of MDF since the formation of T₆ as 0.91 average v. As the dip of MDF is about 35°, the shortening rate is estimated to be (1.3±0.13)mm/a. This study provides important parameters for the analysis of seismic activity in heavily populated Minle and Yongchang areas.     

Late Pleistocene and Holocene slip rate of the Northern Wadi Araba fault, Dead Sea Transform, Jordan

The Wadi Araba Valley is a morphotectonic depression along part of theDead Sea Transform (DST) plate boundary that separates the Arabian plateon the east from the Sinai subplate on the west. The Wadi Araba fault(WAF) is the main strike-slip faults one of between the Gulf of Aqaba and the E-Wtrending Khunayzira (Amatzayahu) fault that bounds the southern end ofthe Dead Sea. Just south of the Dead Sea, the WAF cuts across severalgenerations of alluvial fans that formed on tributaries to the Wadi Dahalafter the regression of Late Pleistocene Lake Lisan ca. 15 ka. Geomorphicand stratigraphic evidence of active faulting, including left-laterally offsetstream channels and alluvial-fan surfaces, yielded fault slip-rate data for thenorthern segment of WAF. Typical cumulative displacements of 54 m,39 m, and 22.5 m of stream channels and alluvial-fan surfaces acrossthe fault were measured from detailed geologic and topographic mapping.The 54 m offset of the oldest alluvial-fan surface (Q _f1 ) occurredafter the final lowering of Lake Lisan (16–15 ka) and before 11 ka yieldinga slip-rate range of 3.4 mm/yr to 4.9 mm/yr. Based on radiocarbonages of charcoal and landsnail shell samples from the buried Q _f2 alluvial-fan deposits exposed in trenches excavated across the fault, the39 m and 22.5 m offsets occurred after 9 ka and 5.8 ka, respectively. These data yield a slip-rate range between 3.9 mm/yr and 6.0 mm/yr.The small variability in these slip-rate estimates for different time periodssuggests that the northern Wadi Araba fault has maintained a relativelyconstant slip rate in the past 15 ka. We calculate an average slip rate of 4.7± 1.3 mm/yr since 15 ka based on the three separate displacementsand age estimates. Five separate offsets of 3 m were measured from gullybends and the offset of small fault-scarp alluvial fans. These displacementdata suggest a coseismic slip of 3 m in the last earthquake, or acumulative slip of 3 m in the past few earthquakes. A maximum slip of3 m correspond to a Mw 7 earthquake that ruptures about 49 km offault length. Using an average slip rate of 4.7 ± 1.3 mm/yr togetherwith a 3-m slip-per-event suggests a maximum earthquake recurrence intervalof this fault segment of 500 to 885 years.