六盘山断裂带构造活动特征及流域盆地地貌响应

2016-01-12 10:28刘兴旺,袁道阳,史志刚
地震工程学报 2015年1期

六盘山断裂带构造活动特征及流域盆地地貌响应

刘兴旺1,2,3, 袁道阳1, 史志刚4, 苏琦1

(1.中国地震局兰州地震研究所,甘肃 兰州730000; 2.兰州地球物理国家野外科学观测研究站,甘肃 兰州730000;

3.兰州大学西部环境教育部重点实验室,甘肃 兰州730000; 4.中国地质科学院地质研究所,北京100037)

摘要:通过航卫片解译和野外实地调查,对六盘山断裂带新活动特征开展详细研究。调查发现六盘山东麓断裂为一条全新世活动的逆左旋走滑断裂,而六盘山西麓断裂为晚更新世活动的挤压逆冲断裂,二者的构造活动控制和影响了本区的地貌发育和地震活动。同时利用SRTM数据提取六盘山东西两侧泾河和水洛河上游流域盆地水系,得到流域盆地面积-高程积分值(HI值)分布图,探讨本区活动构造和地貌的响应关系。分析结果表明,在相同的岩性条件下六盘山东侧的HI值要低于西侧,反映了活动断裂对本地区地貌演化特征的不同影响。上述地貌分析研究为认识和理解六盘山地区地貌演化以及控制因素提供了基础数据和思路。

关键词:流域盆地面积; 高程积分; 活动构造; 六盘山断裂带

收稿日期:*2014-04-24

基金项目:中国地震局地质研究所基本科研业务费专项(IGCEA1220);中国科学院战略性先导科技专项(XDB03020201)

作者简介:刘兴旺(1980-),男,助理研究员,主要研究方向为活动构造及地貌.E-mail:lxw_27@163.com

中图分类号:P315.2文献标志码:A

DOI:10.3969/j.issn.1000-0844.2015.01.0168

Tectonic Activity Characteristics of the Liupanshan Fault Zone

and Geomorphologic Response of Drainage Basin

LIU Xing-wang1,2,3, YUAN Dao-yang1, SHI Zhi-gang4, SU Qi1

(1.LanzhouInstituteofSeismology,CEA,Lanzhou,Gansu730000,China;

2.LanzhouNationalObservatoryofGeophysics,Lanzhou,Gansu730000,China;

3.KeyLaboratoryofWesternChina’sEnvironmentalSystemwiththeMinistryofEducation,LanzhouUniversity,

Lanzhou,Gansu73000,China; 4.InstituteofGeology,ChineseAcademyofGeologicalSciences,Beijing100037,China)

Abstract:Liupanshan is located at the border of the Ningxia Hui autonomous region,Gansu and Shanxi provinces.It belongs to the northern section of the north-south seismic structural zone, and according to its geological structures,it is located in the transitional region between the Ordos block of the western North China Plate and the Qilianshan orogenic belt of the northeastern Qinghai-Tibetan block.Its structures and landform development have their own specificity and sensitivity.Under the premise of the rapid uplift of the Tibetan Plateau and northeastern extension in the late Cenozoic,the Liupanshan area experienced strong deformational tectonic processes.These processes have controlled the development of regional landforms and evolution of rivers.The Liupanshan Fault zone includes the eastern and western Liupanshan piedmont faults,and based on the interpretation of aerial and satellite photos and data from field surveys,we undertook a detailed investigation of its activities.The total length of the eastern Liupanshan piedmont fault is ~90 km.The fault extends southward from Xiaokou to Majiaxinzhuang with an overall NW trend.The fault topography is very clearly defined along the fault.Analyses of its geomorphological features and of a typical fault section suggest that the eastern Liupanshan piedmont fault has characteristics of left-lateral thrust and that it moved in the late Holocene.According to the measurement of the offset terrace at Houmohe,the displacement of the T2 is about (25±3) m.The age of the T2 is about (9372±86) a based on 14C dating and therefore,we estimated the slip rate to be about (3.2±0.5) mm/a.This result was consistent with a previous study.The total length of the western Liupanshan piedmont fault is ~60 km.The fault extends southward from the Sanlidian Reservoir to the Taoshan Reservoir,then its trend changes from N-S to NNW-SSE.The fault topography is very clearly defined along the fault.Analyses of its geomorphological features and of a typical fault section suggest that the western Liupanshan piedmont fault is a high-angle reverse fault that moved in the early late-Pleistocene,but that has not dislocated since the middle late-Pleistocene.In this paper,we also discuss the geomorphologic response to the tectonic activity.Based on SRTM data,we established the Jinghe and Shuiluohe drainage basins and river systems that lie either side of Liupanshan,and we also obtained the hypsometric interval (HI) values of the drainage basins.In general,the HI values of bigger drainage basins reflect the influence of different regional neotectonic activity,while those of smaller basins reflect the influence of lithology and local tectonic activity.According to the map of HI values,there is a clear difference either side of Liupanshan;the two sides have similar lithologies,but the HI values to the east of Liupanshan are lower than those to the west.The reason for this might be connected to the different levels of activity between the eastern and western Liupanshan piedmont faults.Through the above research and geomorphological analysis,we provide basic data and interpretations to better understand the geomorphological evolution of the Liupanshan area.

Key words: area of drainage basin; hypsometric integral; active tectonics; Liupanshan fault zone

0引言

六盘山在地理位置上位于宁夏回族自治区、甘肃省和陕西省三省交界地带;在地质构造位置上处于鄂尔多斯地块、阿拉善地块和青藏高原块体的交汇复合部位,其构造、地貌发育有一定特殊性和敏感性。新生代以来由于印度板块与亚欧板块的持续碰撞汇聚,该地区在晚新生代发生了强烈的构造活动[1-3]。控制六盘山地质地貌发育和演化的断裂主要包括六盘山东麓断裂和六盘山西麓断裂,分别发育于六盘山东、西两侧,其中六盘山东麓断裂北端与左旋走滑的海原断裂相连接,南端与陇县—宝鸡断裂带[4]相连(图1)。在晚新生代青藏高原快速隆升及北东方向扩展挤压的前提下,六盘山地区遭受了强烈的构造变形,这种构造变形控制了该区的地貌发育、河流水系的演化等。

在活动构造研究方面,早期的研究多集中于六盘山东麓断裂和陇县—宝鸡断裂带的构造特征及相互关系上,初步得到了其第四纪左旋滑动速率为1~3 mm /a,垂直滑动速率为0.9 mm /a[5],而对六盘山西麓断裂的研究仅见于地震安全性评价中的踏勘性考察资料。近年来,史志刚等[6]对六盘山地区的断裂活动与大震危险性进行了较为详细的研究,获得了其新活动的较多证据,但是仍缺乏对整个六盘山地区的构造活动与地貌关系的研究。

研究表明,在构造活动区水系的发育受构造活动的影响,其相对均衡的状态由于构造活动改变而改变,河流水系不断调整以适应构造活动或气候变化而达到新的均衡状态,因此水系持续记录着地貌演化过程的构造活动信息。由于构造活动速率通常较慢,其效果需要长期积累,通过水系的地貌分析可反映出长时间尺度的构造活动特征和强度[7-8]。近年来,随着GIS技术和DEM数据的不断更新发展,国内外关于水系地貌的活动构造研究取得了重大进展[9-14]。本文以六盘山断裂带活动性为主要研究对象,通过航卫片解译以及野外实地调查,确定其新活动特征,进而以SRTM数据为基础,GIS技术为手段,分别提取六盘山两侧泾河和水洛河上游流域盆地,并获得流域盆地面积-高程积分值(HI值)分布图,根据HI值在六盘山东西两侧的不同来探讨流域盆地地貌发育特征对构造活动的响应。

a.索引图; b.六盘山地区构造地貌图. F 1海原断裂;F 2马东山断裂;F 3六盘山东麓断裂;F 4六盘山西麓断裂;F 5小关山断裂;F 6固关—县功断裂;F 7桃园—龟川寺断裂;F 8千阳—彪角断裂;F 9陇县—岐山—马召断裂;F 10西秦岭北缘断裂 图1 六盘山地区构造地貌图 Fig.1 Geological map of the Liupanshan region

1六盘山断裂带活动特征

1.1六盘山东麓断裂

六盘山东麓断裂是六盘山地区的主干活动断裂,断裂最北端与海原断裂带相接,南端与陇县-宝鸡断裂带相连。陇县—宝鸡断裂带从西向东包括桃园—龟川寺断裂(F7)、固关—县功断裂(F6)、千阳—彪角断裂(F8)、陇县—岐山—马召断裂(F9)。海原断裂带、六盘山东麓断裂带和陇县—宝鸡断裂带一起构成了青藏高原东北缘与鄂尔多斯块体西南缘之间的NW-SN-SE弧形断裂束[4](图1)。六盘山东麓断裂北端自硝口一带与海原断裂带相接,向东南经海子峡、开城、杨家岭,和尚铺、香水店、散庄子至马家新庄附近与陇县—宝鸡断裂带的固关—县功断裂相接(图1)。总体走向330°~335°,全长约90 km。北段具有左旋走滑特征,中南段以逆冲为主。六盘山东麓断裂地貌上线性特征表现较为明显,沿线多处可见断裂最新活动的剖面及地貌,本文仅选择2处典型断错地貌简述如下。

后磨河为固原县西南孙家庄西侧发育的一条较大河流,其发育并保存了三级阶地,河流阶地是反映新构造活动的重要地貌标志[15-17],其中T2、T3阶地保留较为完整,T1零星分布(图2(a))。六盘山东麓断裂的左旋走滑使得后磨河南北两岸的T2、T3阶地均发生了左旋位错。T3阶地拔河高度20 m,阶地冲积砾石层上覆盖有黄土沉积。野外利用激光测距仪对其边缘的左旋位错进行了测量,其位错值为(35±3) m。T2拔河12 m,阶地边缘左旋位错量为(25±3) m(图2(b))。T2阶地为典型的基座阶地,下部为红色第三系砂岩,其上为河流相阶地砾石层,上部覆盖黄土层。根据对砾石层之上土层底部14C样品的测试,测年结果为(9 372±86) a,据此得到该处六盘山东麓断裂的左旋走滑速率为(3.2±0.5) mm/a,与前人的结果基本吻合[5]。

图2 六盘山东麓断裂后磨河断错地貌 Fig.2 Fault landforms at Houmehe river of Liupanshan eastern piedmont fault

在泾源县南营村响龙河北岸共发育了四级阶地,其中T1阶地为基座阶地,拔河高度5 m。在T1阶地边缘发现断层剖面(图3)。断层西侧为层理清楚的灰绿色与紫红色泥岩互层的白垩纪岩层①,断层东侧依次为层理不清的淡紫红色砂砾石层②和更东的较致密淡紫红色、土黄色砂砾石层③构成,其时代为晚更新世早中期,构成T1阶地的基座。白垩纪地层逆冲于第四纪砂砾石地层②、③之上,使得层②砾石层近断层处发生明显的定向排列,隐约穿过层④,但不是很清晰。层⑤底部采集了一个14C样品,采样位置距顶面0.7 m,样品测定结果为(993± 59)a[13]。

对六盘山东麓断裂前人有较多的研究[5,18-20],结合前人的研究成果及野外实地调查,认为该断裂为一条全新世活动的逆左旋走滑断裂,断裂沿线多处可见冲沟左旋及断错最新地貌单元的证据。

①灰绿色、紫红色泥岩互层;②淡紫红色砂砾石层;③淡紫红色、土黄色砂砾石层;④青灰色砾石层;⑤表土层 图3 响龙河断层剖面(据文献[13]修改) Fig.3 Fault profile in Xianglonghe river (modified accordig to reference[13])

1.2六盘山西麓断裂

六盘山西麓断裂位于六盘山西侧(图1),是控制六盘山西侧地质地貌发育的主要断裂。断裂性质为挤压逆冲,倾向E或NE。地貌上表现为高山和中低山之间的分界。六盘山西麓断裂北起隆德县三里店水库,向南经罗家峡、桃山水库、奠安东、陈家堡东、文家峡、关地峡,最后消失于酒槽附近。断裂全长约60 km。断层两侧东高西低,东侧为六盘山山脉,西侧为陇西盆地。

六盘山西麓断裂在罗家峡水库附近断层地貌明显,水库北岸可见到紫红色白垩系砂岩层逆冲于砖红色砂岩与泥岩互层夹土黄色砂岩的第三系地层之上(图4(a))。通边村阳洼附近断层地貌明显,北东高,南西低,可见白垩系逆冲到于下第三系地层之上(图4(b))。水洛河两岸发育Ⅰ,Ⅱ级阶地,但两级阶地均未被错断,推测断裂晚更新世晚期新活动并不明显。

断裂在奠安乡范家峡水库坝前的庄浪河边出露明显的断层剖面(图5),白垩系紫红色砂岩逆冲到砖红色的上第三系细砂岩之上,断层破碎带宽约15~16 m,断层面附近层④为1.5~2 m宽的断层泥,其中紧靠断面为宽1 m的紫红色断层泥,湿软;向东为宽0.5 m的灰绿色断层泥,较为坚硬。再向东层③为宽7 m的灰绿色砂岩夹泥岩陡立带,层②为宽8 m的紫红色断层陡立带。断层面向上变缓,呈弧形,底部产状为345°/E∠60°。断层错断了上覆的庄浪河Ⅲ级阶地底部砾石层,垂直断距4.9 m。根据区域年代资料[16],泾河Ⅲ级阶地年代为晚更新世早期,由此判断,该断层的最新活动年代为晚更新世早中期。在层⑥处取OSL样品一个,样品测定确定此处断层的最晚活动年代为(76.0±9.4) ka左右。

图4 六盘山西麓断裂断错地貌 Fig.4 Fault landforms of the Liupanshan western piedmont fault

沿六盘山西麓断裂多处可见白垩系紫红色或灰白色砂岩、砾岩逆冲于第三系砂岩、泥岩之上,有多个较大的河流与冲沟切穿断层。切穿六盘山西麓断裂的冲沟多发育Ⅰ,Ⅱ级阶地,少数发育Ⅲ级阶地。考察中仅在奠安剖面发现断裂错断了庄浪河Ⅲ级阶地,但所有Ⅰ,Ⅱ级阶地均未发现断错现象。按照区域资料的对比,Ⅰ,Ⅱ级阶地均发育于晚更新世晚期以来,综合分析,断裂最新活动时代为晚更新世早期,晚更新世中晚期以来活动不明显。

①紫红色砂岩;②宽8 m的紫红色断层陡立带;③宽7 m的紫红色断层陡立带;④断层泥;⑤砖红色细砂岩;⑥T 3阶地底部砾石层,采OSL样品一个;⑦ T 3阶地上部亚砂土;⑧T 2阶地上部砾石层 图5 奠安断层剖面(据报告 ① 兰州地震工程研究院天平铁路近场地震构造环境.2009.修改) Fig.5 Fault profile in Dianan village (modified according to report ①)

2构造地貌特征

基于90 m分辨率的SRTM数据,从跨六盘山地区中(图1中AA′),以1 km×1 km为网格提取了长150 km的高程条带剖面(图6)。从高程条带剖面可以看出,受六盘山断裂带和小关山断裂的影响,在地形上形成一个宽约50 km的隆起带,包括了六盘山和崆峒山隆起及二者之间的山间盆地,其高度明显高于西侧的陇西盆地及东侧的鄂尔多斯块体。六盘山地区平均海拔约2 500 m以上,跨过六盘山断裂带,海拔迅速降低到2 000 m以下(图6),高程最大落差可达1 200余米。

图6 跨六盘山高程条带剖面 Fig.6 Elevation swath profile crossing the Liupanshan

六盘山两侧河流地貌是否对活动断裂的活动差异存在不同的响应呢?这是本文研究的重点,为了比较六盘山东西两侧河流流域盆地对活动构造的响应,文中分别选择了六盘山东麓的泾河上游流域盆地和六盘山西麓的水洛河上游流域盆地。泾河为渭河最大的支流,发源于宁夏六盘山东麓,于陕西高陵县注入渭河,全长451 km,年平均降水量550 mm。水洛河发源于六盘山西麓,注入渭河一级支流葫芦河,全长约100 km。同样基于SRTM数据,我们获得了流域盆地水系的分布(图7),本文利用流域盆地的面积-高程积分值来探讨地貌对活动构造的响应。面积-高程积分是某区域内不同海拔所占的相对面积百分比,面积高度曲线代表流域内在给定海拔高度之上(或之下)的相对面积百分比,通过其曲线的形态被广泛用来揭示地貌演化的阶段[21]。流域盆地的面积-高度曲线形态表明了其侵蚀的阶段,凸型曲线表示相对“年轻”侵蚀程度较低的区域,S型曲线表示中等程度侵蚀区域,凹形曲线表示相对“年老”且高度侵蚀的区域[21-23]。面积-高程积分值(HI值),即曲线下方所围限的面积,其值从0到1,在侵蚀程度较高的区域其值接近于0,而在侵蚀程度较低的地区其值接近于1。其积分值可以通过以下公式获得[24]:

式中:Hmax、Hmean、Hmin分别为流域盆地的最大高程、平均高程和最小高程。

一般来说,较大的流域盆地HI值能反映区域性的新构造活动差异对流域盆地地形的影响,而较小流域盆地的HI值更容易反映岩性和局部构造作用的影响[21]。六盘山山体区主要出露白垩系地层,为白垩系和尚铺组紫红色砂质泥岩、细砂岩。六盘山东侧主要出露古近系清水营组和寺口子组的砂岩及砂质泥岩,到盆地地区主要为上更新统和全新统风积黄土及冲洪积地层,其中河谷地区主要分布第四系冲洪积物(图8)。六盘山西侧古近系地层保存不完整,流域盆地内出露新近系甘肃群砂岩及泥岩,流域盆地低平地区主要分布上更新统浅黄色粉砂质黄土,沿河流地区分布全新统冲积物(图8)。对比六盘山两侧的岩性,其岩性特征较为相似。

图7 流域盆地水系 Fig.7 River system of the drainage basin

图8 流域盆地地质图 Fig.8 Geological map of the drainage basin

在本次研究中,我们利用1~3级流域盆地获得了研究区的HI值分布图(图9),从图中可以看出,在六盘山东侧,低HI值区的分布与山间断陷盆地和河谷的分布范围相吻合,表现出成片的低HI值区,反映了构造沉降和强烈侵蚀的结果。六盘山断裂以西的六盘山山体区出现高HI值区。在六盘山西侧,低HI值主要沿河谷分布,其余部分HI值均较高,普遍在0.5左右,跨过六盘山西麓断裂的山体区和盆地区HI值差异并不大。六盘山山体区集中表现为高HI值,表明其仍处于强烈侵蚀期,可能反映了其仍处于不断隆升的状态。对比六盘山两侧地层(图8),主要为第三系和第四系地层,在相同的岩性条件下六盘山东侧的HI值明显低于西侧,表明东侧的侵蚀要高于西侧,而引起六盘山东侧侵蚀作用较高的因素可能来源于六盘山东麓断裂强烈的挤压抬升作用。相反,在六盘山西侧断裂活动性较弱,使得本区抬升较弱,相应的侵蚀作用也较低,这就从构造地貌学的角度反映了断裂活动性强弱对地貌的控制作用。

图9 流域盆地HI值分布图 Fig.9 The HI values of the drainage basin

3结论与讨论

本文通过详细的航卫片解译和野外调查工作,结合前人研究资料,确定了六盘山断裂带的新活动特征。六盘山断裂带包括六盘山东麓断裂和六盘山西麓断裂,其中六盘山东麓断裂为一条全新世活动的逆左旋走滑断裂,六盘山西麓断裂为晚更新世早期活动的挤压逆冲断裂,晚更新世中晚期以来活动不明显。通过SRTM数据,获得了六盘山东西两侧受活动构造控制的泾河及水洛河上游流域盆地水系分布,通过1~3级流域盆地获得流域盆地面积-高程积分值(HI)分布图。对比发现,在相同的岩性条件下六盘山东侧HI值要小于六盘山西侧,反映了活动构造强弱对地貌的控制作用。上述地貌分析研究,为认识和理解六盘山地区地貌演化以及控制因素提供了基础数据和思路。

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