Table of Content

20 December 2017, Volume 39 Issue 6

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A REVIEW OF LOW-TEMPERATURE THERMOCHRONOLOGY ON BEDROCK AND DETRITUS FROM RIVERS AROUND THE TIBETAN PLATEAU

LIN Xu, LIU Jing, PENG Bao-fa, LI Chang-an, WU Quan-yuan

2017, 39(6):  1091-1110.  DOI: 10.3969/j.issn.0253-4967.2017.06.001

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The large river systems are the major transfer of continental masses to the ocean and basin, playing significant roles in global geochemical cycles. The Tibetan plateau is the birthplace of many huge rivers flowing through eastern and southern Asia, in which the fluvial deposits kept not only closely relate to the geological evolution information from the source areas, but also record the river itself building process. The low-temperature thermochronology method of detrital minerals (zircon and apatite, etc.) can be used to constrain the river's source areas, establishing its source-sink system. It can also combine regional tectonic deformation analysis to determine the potential source region of the river and the formation time of the plateau geomorphology, which is a focused issue in recent years. In this study, we have summarized the research results from the large rivers in the Tibetan plateau in recent years, suggesting that the low-temperature thermochronology analysis of the detrital minerals should be focused on the river's key locations in the upstream, midstream and downstream, respectively, combining the small tributaries analysis which can give a more detailed thermal evolution history in the whole drainage basin. On the conditions of the bedrock, it is shown that in the same river's different place we should use the same low-temperature thermochronology, while in different river's places we should take several low-temperature thermochronology methods (apatite and zircon, etc.)at a same position, so we get a complete time series related to the river incision. Combining the valley bedrock and detrital river minerals with the low-temperature thermalchronology on the Tibetan plateau, together with the chronology, structure analysis and other sedimentary studies, we can obtain detailed structures and river's evolution processes.

A BRIEF INTRODUCTION TO THE NEW METHOD FOR RIVER PROFILE ANALYSIS: Integral Approach

WANG Yi-zhou, ZHANG Hui-ping, ZHENG De-wen, YU Jing-xing, LI Chao-peng, XIAO Lin

2017, 39(6):  1111-1126.  DOI: 10.3969/j.issn.0253-4967.2017.06.002

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The topography and geomorphology of active orogens result from the interaction of tectonics and climate. In most orogens, a fluvial channel is most sensitive to the coupling between tectonics, lithology, and climate. Meanwhile, the related signals have been recorded by both the drainage geometry and channel longitudinal profile. Thus, how to extract tectonic information from fluvial channels has been a focused issue in geologic and geomorphologic studies.
The well known stream-power river incision model bridges the gap between tectonic uplift, river incision and channel profile change, making it possible to retrieve rock uplift pattern from river profiles. In this model, the river incision rate depends on the rock erodibility, contributing drainage area and river gradient. The steady-state form of the river incision model predicts a power-law scaling between the drainage area and channel gradient. Via a linear regression to the log-transformed slope-area data, the slope and intercept are channel concavity and steepness indices, respectively. The concavity relates to lithology, climatic setting and incision process while the channel steepness can be used to map the spatial pattern of rock uplift. For its simple calculation process, the slope-area analysis has been widely used in the study of tectonic geomorphology during past decades.
However, to calculate river slope, the coarse channel elevation data must be smoothed, re-sampled, and differentiated without any reasonable smooth window or rigid mathematical fundamentals. One may lose important information and derive stream-power parameters with high uncertainties. In this paper, we introduce the integral approach, a procedure that has been widely used in the latest four years and demonstrated to be a better method for river profile analysis than the traditional slope-area analysis. Via the integration to the steady-state form of the stream-power river incision equation, the river longitudinal profile can be converted into a straight line of which the independent variable is the integral quantity χ with the unit of distance and the dependent variable is the relative channel elevation. We can calculate the linear correlation coefficient between elevation and χ based on a series of concavity values and find the best linear fit to be the reasonable channel concavity index. The slope of the linear fit to the χ value and elevation is simply related to the ratio of the uplift rate to the erodibility.
Without calculating channel slope, the integral approach makes up for the drawback of the slope-area analysis. Meanwhile, via the integral approach, a steady-state river profile can be expressed as a continuous function, which can provide theoretical principle for some geomorphic parameters (e.g., slope-length index, hypsometric integral). In addition, we can determine the drainage network migration direction using this method. Therefore, the integral approach can be used as a better method for tectonogeomorphic research.

SYSTEMATIC OFFSET OF BEDROCK CHANNELS ALONG ACTIVE STRIKE-SLIP FAULTS ON THE EASTERN TIBETAN PLATEAU

YAN Bing, JIA Dong

2017, 39(6):  1127-1142.  DOI: 10.3969/j.issn.0253-4967.2017.06.003

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Offset river is one of the characteristic landforms along active strike-slip fault. Whereas because of various factors such as natural meander, river capture, etc, difficulties exist while interpreting slip motion and offset amount using landforms of offset rivers. In this study, we introduced the systematic offset of bedrock channels as a method to analyze offset rivers along strike-slip fault. Systematic offset of bedrock channels is the result of coupling between tectonic process and surface process. It also describes the phenomenon of synchronous accumulation both of the offset amount and the upstream length because of head-ward erosion. Based on the interpretation, measuring and statistics of the offset river landforms, it is found that systematic offset of bedrock channels have developed along the Ganzi-Yushu, Xianshuihe and eastern Kunlun fault zones on the eastern Tibetan plateau. There is a linear relationship between the upstream length (L), measured from the headwater to the fault, and the offset amount (D):D=a·L. This study provides useful implications to the role of strike-slip faults during the geomorphic evolution of the eastern Tibetan plateau.

MEASUREMENT PROCEDURE OF SINGLE-GRAIN APATITE(U-Th)/He DATING AND ITS VALIDATION BY DURANGO APATITE STANDARD

WANG Ying, ZHENG De-wen, WU Ying, LI You-juan, WANG Yi-zhou

2017, 39(6):  1143-1157.  DOI: 10.3969/j.issn.0253-4967.2017.06.004

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(U-Th)/He isotopic dating has been developed very quickly in recent years, due to the recognition that the thermal history of rock at low temperature can be effective revealed by such dating method. In particular, He closure temperature in apatite (40~80℃) is very low, so apatite (U-Th)/He ages can reflect the thermal history information of the low-temperature stage, and have a good application prospect in the field of low-temperature thermal chronology. However, because of many influence factors and complicated measurement procedures, the development of apatite He dating in China remains in its early stage. In this study, a measurement procedure was established at the (U-Th)/He dating laboratory of Institute of Geology, China Earthquake Administration. We measured the daughter isotopic helium by diode laser heating four batches of a total seventy-five grains of Durango apatite in an Alphachron helium mass spectrometry system. Then the apatite grains were dissolved to precisely measure the concentration of parent nuclides (U, Th)using the solution isotope dilution method through an automatic sampling ICP-MS (Agilent 7900). Results show that the Th/U values of Durango apatite grains were in the range of 17.23 to 23.60, while all the 75ages were in the range of 28.61 to 34.51Ma with an average of (31.71±1.55)Ma (1σ), which are consistent with the international calibrated ages.

RESPONSES OF THE S-A DOUBLE-LOG GRAPH, CONCAVITY INDEX AND STEEPNESS INDEX OF CHANNELS TO THE TECTO-NIC MOVEMENT OF THE HUOSHAN PIEDMONT FAULT

BI Li-si, HE Hong-lin, XU Yue-ren, WEI Zhan-yu, SHI Feng, SUN Hao-yue

2017, 39(6):  1158-1172.  DOI: 10.3969/j.issn.0253-4967.2017.06.005

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The Huoshan piedmont fault is a small watershed region in Shanxi Province. We utilized the high-resolution DEM data and the stream-power incision model which describes the relationship between the tectonic uplift and fluvial incision to analyze the S-A double-log graph, concavity index (θ)and steepness index (logk_s) of the 64 channels across this fault and discuss their responses to the tectonic movement of the fault. The results show that (1)the S-A double-log graphs all exhibit an obvious convex form, which is the direct expression of the response to the situation that the bedrock uplift rate is higher than the fluvial incision rate. (2)All of the concavity index (θ)values of 64 channels are lower than 0.35 with an average value of 0.223, much lower than the empirical value (0.49)of the rivers in steady state. These low values are the quantitative reflections of the channels' slightly concave profiles. Meanwhile they imply that these channels across the fault are very young. There is no enough time for them to adjust the profiles through the fluvial incision to the steady state because of the fault's frequent and strong tectonic movements. (3)The steepness index values of the channels located in the Laoyeding Mt. are highest, while they are lower in the northern and southern mountains. Moreover, the steepness index values of the channels in the northern mountains, on average, are higher than those of the channels in the southern mountains. To a certain extent, this distribution of the steepness index corresponds to the difference in the uplift rates of the Huoshan piedmont fault. It means that the uplift rate of the middle fault segment in the Laoyeding Mt. is highest, and the uplift rate of the northern segment is higher than that of the southern segment.

A BRIEF REVIEW OF SEVERAL MODELS OF TOPOGRAPHIC EVOLUTION

YANG Rong

2017, 39(6):  1173-1184.  DOI: 10.3969/j.issn.0253-4967.2017.06.006

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With steady development of mathematical-physical models and computer technology, numerous methods of topographic simulation have emerged during the past decades. A major challenge in the modeling is how to accurately and efficiently describe processes of surface erosion at different spatial scales. This review focuses on the physical processes controlling surface erosion, including river erosion and hillslope erosion. Four popular models of topographic simulation (CASCADE, CHILD, FastScape and DAC models)and their applications are presented. Although these models have become more sophisticated in recent years, there are still some issues unsolved regarding the basics of the physical erosion processes. For example, some factors have not been taken into account, such as the impacts of changes in grain size and sediment budget during transportation on river erosion and the measurements of the rock erodibilities for various lithologies. Moreover, there is no topographic index that can be used to evaluate the modeling results. Therefore, it would be helpful to combine the models of topographic simulation with other numerical models, e.g. the low-temperature thermochronometric data modeling, to provide better constraints on the terrain modeling.

APPLICATION OF UAVLS TO RAPID GEOLOGICAL SURVEYS

SHAO Yan-xiu, ZHANG Bo, ZOU Xiao-bo, WANG Ai-guo, ZHANG Fan-yu, YUAN Dao-yang, LIU Xing-wang, HE Wen-gui

2017, 39(6):  1185-1197.  DOI: 10.3969/j.issn.0253-4967.2017.06.007

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Three-dimensional scanning with LiDAR has been widely used in geological surveys. The LiDAR with high accuracy is promoting geoscience quantification. And it will be much more convenient, efficient and useful when combining it with the Unmanned Aerial Vehicle (UAV). This study focuses on UAV-based Laser Scanning (UAVLS)geological field mapping, taking two examples to present advantages of the UAVLS in contrast with other mapping methods. For its usage in active fault mapping, we scanned the Nanpo village site on the Zhangxian segment of the West Qinling north-edge fault. It effectively removed the effects of buildings and vegetation, and uncovered the fault trace. We measured vertical offset of 1.3m on the terrace T₁ at the Zhang river. Moreover, we also scanned landslide features at the geological hazard observatory of Lanzhou University in the loess area. The scanning data can help understand how micro-topography affects activation of loess landslides. The UAVLS is time saving in the field, only spending about half an hour to scan each site. The amount of average points per meter is about 600, which can offer topography data with resolution of centimeter. The results of this study show that the UAVLS is expected to become a common, efficient and economic mapping tool.

USING STEREO-PAIR AND DIFFERENTIAL GPS TO REVEAL SURFACE DEFORMATION CHARACTERISTICS OF THE MINLE-YONGCHANG FAULT

ZOU Xiao-bo, YUAN Dao-yang, SHAO Yan-xiu, LIU Xing-wang, ZHANG Bo, YANG Hai-bo

2017, 39(6):  1198-1212.  DOI: 10.3969/j.issn.0253-4967.2017.06.008

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Qilian Shan-Hexi Corridor is located in the northeastern margin of the Tibetan plateau, which hosts many active strike-slip and thrust faults as well as folds. Previous study on this area was mostly concerned with large faults at the boundary of the corridor, while rare work on active tectonics in the interior of the corridor. On 25 October 2003, the Minle-Shandan M_S6.1 earthquake occurred in this area, which is related with the Minle-Yongchang fault hidden beneath the south piedmont of the Dahuangshan Mountains. As there is no obvious rupture trace on the surface, the quantitative study of this fault has never been reported so far.
In order to obtain quantitative parameters of this active structure efficiently, the software of ERDAS was used to generate pointscloud data from SPOT6 stereo-pair. Two-meter resolution DEM imagery from point cloud has the line accuracy of height about 1m. Three swath profiles were extracted from the DEM data, which show that high geomorphic surfaces are all uplifted and folded. By differential GPS measurement, the vertical uplift of the thrust-related fold is estimated to be about 2.0m on the T₂, and the strike of the fold deformation is nearly 311°, which is close to the result of the fault parameter determined by aftershocks, and also in agreement with the focal mechanism solutions. Furthermore, the location of fold axial zone is consistent with the actual investigation data. These indicate that there is obvious tectonic deformation in the west part of the Minle-Yongchang fault. It supports the view that this fault is the seismogenic structure of the 2003 Minle-Shandan earthquake. Stereo-pair is of high importance in active tectonics research, which can provide significant guidance for field geologic investigations and determining the location of tectonic deformation, according to this research.

THRUST OF THE SOUTHERN LONGMENSHAN FAULT IN THE LATE QUATERNARY REVEALED BY RIVER LANDFORMS

LI Wei, ZHANG Shi-min, JIANG Da-wei, GAO Yu

2017, 39(6):  1213-1236.  DOI: 10.3969/j.issn.0253-4967.2017.06.009

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The Longmenshan fault zone is divided into three sections from south to north in the geometric structure. The middle and northern segments are mainly composed of three thrust faults, where the deformation of foreland is weak. The geometric structure of the southern segment is more complex, which is composed of six fault branches, where the foreland tectonic deformation is very strong. The Wenchuan M_S8.0 earthquake occurred in the middle of the Longmenshan in 2008, activating the bifurcation of two branches, the Yingxiu-Beichuan and the Guixian-Jiangyou faults. In 2013, the Lushan M_S7.0 earthquake occurred in the southern Longmenshan, whose seismogenic structure was considered to be a blind fault. After the Lushan earthquake, the seismic hazard in the southern Longmenshan has been widely concerned.
At present, the studies on active tectonics in the southern Longmenshan are limited to the Dachuan-Shuangshi and the Yanjing-Wulong faults. The Qingyi River, which flows across the southern Longmenshan, facilitates to study fault slip by the deformation of river terraces. Based on satellite imagery and high-resolution DEM analysis, we measured the fluvial terraces along the Qingyi river in detail. During the measurement, the Sichuan network GPS system (SCGNSS)was employed to achieve a precision of centimeter grade. Besides, the optical luminescence dating (OSL)method was employed to date the terraces' ages. And the late Quaternary activities of the six branch faults in the southern Longmen Shan were further analyzed.
The Gengda-Longdong, Yanjing-Wulong and the Xiao Guanzi faults (west branch of the Dachuan-Shuangshi fault)all show thrust slip and displaced the terrace T₂. Their average vertical slip rates in the late Quaternary are 0.21-0.30mm/a, 0.12-0.21mm/a and 0.10-0.12mm/a, respectively. Since the Late Quaternary, vertical slip of the east branch of the Dachuan-Shuangshi fault was not obvious, and the arc-like Jintang tectonic belt was not active. Crustal shortening rate of the southern Longmenshan thrust fault zone in the late Quaternary is 0.48-0.77mm/a, which equals about half of the middle segment of the Longmenshan. Based on the previous study on the tectonic deformation of the foreland, we consider that the foreland fold belt in the southern Longmenshan area has absorbed more than half of the crustal shortening. The three major branch faults in the southern Longmenshan are active in the late Quaternary, which have risk of major earthquakes.

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

LIU Rui, LI An, ZHANG Shi-min, CHEN Zhi-dan, GUO Chang-hui

2017, 39(6):  1237-1255.  DOI: 10.3969/j.issn.0253-4967.2017.06.010

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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.

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

LEI Jing-hao, LI You-li, HU Xiu, XIN Wei-lin, XIONG Jian-guo, ZHONG Yue-zhi

2017, 39(6):  1256-1266.  DOI: 10.3969/j.issn.0253-4967.2017.06.011

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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.

GEOMETRIC CHARACTERISTICS OF CO-SEISMIC SURFACE RUPTURES AND ACTIVE FAULTS

HAO Hai-jian, HE Hong-lin, WEI Zhan-yu, SHI Feng

2017, 39(6):  1267-1282.  DOI: 10.3969/j.issn.0253-4967.2017.06.012

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Fault traces contain abundant information associated with the fracture process and mechanism, so an accurate and quantitative description of their geometric characteristics is of great significance to perceiving the generation and development of faults. We collected 52 co-seismic surface ruptures and 300 active fault traces from across the world to analyse their geometric characteristics by the method of power spectrum density. Our results show that (1)the average power spectrum density has a distinct three-segment charateristic in the frequency domain. In the low frequency domain it represents the geometric characteristics of the boundary of tectonic block. In the medium frequency domain, the power spectrum density reflects the processes of lateral growth and connection of secondary faults, and the turn point on the 100 meters scale represents the effective resampling length, below which the power spectrum density characteristics are meaningless. (2)In the middle and high frequency domains, the power spectrum density curves of co-seismic surface ruptures show that there are obvious differences in roughness among three fault types, i.e. reverse > normal > strike-slip, which indicates that the geometric characteristics of co-seismic surface ruptures are controlled by the fault types. (3)Compared with co-seismic surface ruptures, active fault traces have much lower power spectrum density, indicating the roughness of active fault traces becomes lower with increasing numbers of rupturing events and the lengths of active history, i.e., the fault roughness is inversly proportional to its maturity.

INFLUENCE OF TECTONICS AND CLIMATE ON THE EVOLU-TION OF FLUVIAL TERRACES: A CASE STUDY OF THE HONGSHUIBA AND MAYING RIVERS IN THE NORTHERN MARGIN OF THE QILIAN MOUNTAINS

TIAN Qing-ying, ZHENG Wen-jun, ZHANG Dong-li, ZHANG Yi-peng, XU Bin-bin, HUANG Liu-ting

2017, 39(6):  1283-1296.  DOI: 10.3969/j.issn.0253-4967.2017.06.013

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In tectonically active regions, geomorphic features such as fluvial terraces can be interpreted as the consequence of tectonic and climatic forcing. However, deciphering and distinguishing tectonic impacts and climate changes remain a challenge. In this study, we examine the terraces along the Hongshuiba river and Maying river, which flow across the Fudongmiao-Hongyazi fault in the northern margin of the Qilian Mountains. Our purpose is to analyze the relative roles of tectonics and climate in shaping orogenic topography in this area. 8~9 levels of river terraces were identified through field observations, interpretation of satellite images and using DEMs. According to relative heights and ages of T₅ of the Hongshuiba river and T₆ of the Maying river, the incision rates are calculated to be (10.2±2.0)mm/a and (12.2±2.8)mm/a, respectively. Furthermore, the thrust rate along the Fodongmiao-hongyazi fault was determined based on offset terraces and OSL dating, which are ten times less than river incision rates approximately. Comparing the uplift rate and incision rate in the northern margin of the Qilian Mountains and adjacent areas, we inferred that climate change is the most plausible controlling factor in the evolution of the river terraces, while tectonics plays a minor role in this process.

GEOLOGICAL AND GEOMORPHIC EVIDENCE FOR DEXTRAL STRIKE SLIP OF THE HELAN SHAN WEST-PIEDMONT FAULT AND ITS TECTONIC IMPLICATIONS

LEI Qi-yun, ZHANG Pei-zhen, ZHENG Wen-jun, DU Peng, WANG Wei-tao, YU Jing-xing, XIE Xiao-feng

2017, 39(6):  1297-1315.  DOI: 10.3969/j.issn.0253-4967.2017.06.014

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The horizontal movement of the Helan Shan west-piedmont fault is important to determination of the present-day boundary between the Alashan and North China blocks as well as to the exploration of the extent of the northeastward expansion of the Tibetan plateau. Field geological surveys found that this fault cuts the west wing of the Neogene anticline, which right-laterally offset the geological boundary between Ganhegou and Qingshuiying Formations with displacement over 800m. The secondary tensional joints (fissures)intersected with the main faults developed on the Quaternary flood high platform near the fault, of which the acute angles indicate its dextral strike slip. The normal faults developed at the southern end of the Helan Shan west-piedmont fault show that the west wall of this fault moves northward, and the tensional adjustment zone formed at the end of the strike slip fault, which reflects that the horizontal movement of the main fault is dextral strike slip. The dextral dislocation occurred in the gully across the fault during different periods. Therefore, the Helan Shan west-piedmont fault is a dextral strike slip fault rather than a sinistral strike slip fault as previous work suggested. The relationship between the faulting and deformation of Cenozoic strata demonstrates that there were two stages of tectonic deformation near the Helan Shan west-piedmont fault since the late Cenozoic, namely early folding and late faulting. These two tectonic deformations are the result of the northeastward thrust on the Alashan block by the Tibet Plateau. The influence range of Tibetan plateau expansion has arrived in the Helan Shan west-piedmont area in the late Pliocene leading to the dextral strike slip of this fault as well as formation of the current boundary between the Alashan and North China blocks, which is also the youngest front of the Tibetan plateau.

RIVER GEOMORPHIC PARAMETERS OF THE HUASHAN PIEDMONT AND THEIR TECTONIC IMPLICATIONS

XU Wei, LIU Zhi-cheng, YUAN Zhao-de, GAO Zhan-wu, YANG Yuan-yuan

2017, 39(6):  1316-1335.  DOI: 10.3969/j.issn.0253-4967.2017.06.015

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Based on DEM data and ArcGIS software, we extract the geomorphic parameters of drainage basins and rivers that flow through the Huashan piedmont, which include stream length-gradient index (SL), stream-power incision model normalized channel steepness index (k_sn), hypsometric integral (HI), valley floor width to valley height ratio (Vf)and mountain front sinuosity (Smf). Study shows that all parameter indexes have obviously different distributions roughly bounded by Huaxian and Huayin. In the Huaxian to Huayin section, the stream length-gradient index has extremely high abnormal values near the fault, the values of river mean SL, mean k_sn, HI, Vf and Smf are concentrated in 500~700, 120~140, 0.5~0.6, 0~0.1 and 1.0~1.1, respectively. Between Lantian and Huaxian and between Huayin and Lingbao, the parameter indexes distributional characteristics are largely the same, with the values in 300~500, 100~120, 0.4~0.5, 0.2~0.6 and 1.2~1.5, respectively. Comprehensive analysis suggests that tectonic activity is the primary factor responsible for these differences. We divide each geomorphic parameter into three classes (strong, medium, and low)and calculate the relative active tectonics (Iat)of the Huashan piedmont. The results show that the Iat values in Huaxian to Huayin section are in 1.0~1.5, those at other places are in 1.5~3.0, indicating that the tectonic activity from Huaxian to Huayin is most intense, while that of other places are relatively weak. Field geological investigations show that the Huashan piedmont fault can be divided into Lantian to Huaxian section, Huaxian to Huayin section and Huayin to Lingbao section. In Huaxian to Huayin section the fault has been active several times since Holocene indicative of strongest activity, while in Lantian to Huaxian section and Huayin to Lingbao section the fault was active only in the late Pleistocene and its activity was weaker as a whole. Tectonic activity of the Huashan piedmont derived from river geomorphic parameters is consistent with field geological investigations, indicating that geomorphic parameters of rivers can be used to characterize activity of faults on a regional scale.