Opposed shear senses inferred from neotectonic mesofracture systems in the North Anatolian fault zone

The 1200-km long North Anatolian fault zone is a right-lateral, intracontinental transform boundary which was initiated in the Late Neogene. Sediments of Pliocene to Holocene age in basins between Cerkes and Erbaa, within the convex-northwards arc of the fault zone, are deformed by syn-sedimentary and post-depositional mesoscopic faults and joints. The mesofractures, which strike obliquely to the fault zone, include reverse faults, normal faults, normal shear joints, conjugate vertical joints and strike-slip faults. Each type of structure occurs in two geometrical groups, one comprises four systems of fractures, the other is made up of five systems. The directions of secondary compression and/or extension inferred from the first group of mesofractures, which are restricted to sediments of Pliocene to Early Pleistocene age, are interpreted as being related to left-lateral shear along the North Anatolian fault zone. The directions of compression and/or extension inferred from the second group of mesofractures, which cut sediments of Pliocene to late Holocene age, were generated during right-lateral shear.The presence of the second group of mesofractures is understandable because they are related to the shear sense which operates at the present-day, but those interpreted as being related to left-lateral shear are more puzzling: their development implies one or more reversals of the dominant sense of displacement. Several tentative models to explain such reversals are proposed, including regional and local influences, the latter related to mechanical constraints and/or the effects of other fault systems.     

Neotectonics of the Pontides: implications for ‘incompatible’ structures along the North Anatolian fault

Data for the post-Serravallian, ‘neotectonic’ evolution of the Pontides in northern Turkey indicate predominant ENE-WSW shortening with complementary NNW-SSE extension. We present a new fault plane solution for the Bartin earthquake (3 September 1968) and compare its mechanism with the movement picture of other neotectonic faults in the Pontides and northern Greece together with that of the Thessaloniki earthquake (20 May 1978). The general strain pattern exhibited by these structures agrees remarkably well with that inferred from early Tortonian-early Pleistocene structures reported from within the North Anatolian fault zone, which have been interpreted as indicating a possible reversal of the sense of movement along the North Anatolian transform fault. Here, we argue that such ‘incompatible’ structures may be related to the overall E-W shortening of Anatolia and the southern parts of the Black Sea resulting from the sideways continental escape from around the African and the Arabian promontories, rather than to hypothetical reversal of motion along the North Anatolian fault, for which there is no evidence other than the above-mentioned ‘incompatible’ structures. This new model also has important implications for seismicity and earthquake risk in regions contained within the southern part of the Black Sea plate.     

The North Anatolian fault and complexities of continental escape

Before the Plio-Pleistocene, the proto North Anatolian fault zone was occupied by two separate faults: a WNW-striking right-lateral eastern segment which extended to the Black Sea, and a WSW-striking left-lateral western segment. During the Plio-Pleistocene most of the right-lateral displacement on the eastern fault was transferred from the Black Sea extension to the western fault, converting the latter to a right-lateral structure, and giving rise to the modern North Anatolian fault zone. This model explains the evidence first reported by Hancock & Barka for an apparent Plio-Pleistocene reversal of displacement along the western part of the fault. The model may also account for the Plio-Pleistocene change in regional stress in southwestern Anatolia.     

Elastic and inelastic triggering of earthquakes in the North Anatolian Fault zone

Deformation models used to explain the triggering mechanism often assume pure elastic behaviour for the crust and upper mantle. In reality however, the mantle and possibly the lower crust behave viscoelastically, particularly over longer time scales. Consequently, the stress field of an earthquake is in general time-dependent. In addition, if the elastic stress increase were enough to trigger a later earthquake, this triggered event should occur instantaneously and not many years after the triggering event. Hence, it is appropriate to include inelastic behaviour when analysing stress transfer and earthquake interaction.In this work, we analyse a sequence of 10 magnitude M_s > 6.5 events along the North Anatolian Fault between 1939 and 1999 to study the evolution of the regional Coulomb stress field. We investigate the triggering of these events by stress transfer, taking viscoelastic relaxation into account. We evaluate the contribution of elastic stress changes, of post-seismic viscoelastic relaxation in the lower crust and mantle, and of steady tectonic loading to the total Coulomb stress field. We analyse the evolution of stress in the region under study, as well as on the rupture surfaces of the considered events and their epicentres. We study the state of the Coulomb stress field before the 1999 İzmit and Düzce earthquakes, as well as in the Marmara Sea region.In general, the Coulomb stress failure criterion offers a plausible explanation for the location of these events. However, we show that using a purely elastic model disregards an important part of the actual stress increase/decrease. In several cases, post-seismic relaxation effects are important and greater in magnitude than the stress changes due to steady tectonic loading. Consequently, viscoelastic relaxation should be considered in any study dealing with Coulomb stress changes.According to our study, and assuming that an important part of the rupture surface must be stressed for an earthquake to occur, the most likely value for the viscosity of the lower crust or mantle in this region is 5 · 10¹⁷–10¹⁸ Pa · s. Our results cannot rule out the possibility of other time-dependent processes involved in the triggering of the 1999 Düzce event. However, the stress increase due to viscoelastic relaxation brought 22% of the 1999 Düzce rupture area over the threshold value of Δσ_c ≥ 0.01 MPa (0.1 bar), and took the whole surface closer to failure by an average of 0.2 MPa. Finally, we argue that the Marmara Sea region is currently being loaded with positive Coulomb stresses at a much faster rate than would arise exclusively from steady tectonic loading on the North Anatolian Fault.     

东至断裂带构造变形特征及其构造演化

东至断裂带是皖西南一条重要的北北东向断裂带。详细的构造解析表明,该断裂带主要经过3期构造变形,分别是发生在晚侏罗世末—早白垩世初的左行平移断层、早白垩世期间的伸展构造和晚白垩世—新生代的右行平移断层。通过断层擦痕矢量反演和断层叠加改造关系分析,认为东至断裂带及其两侧多期构造变形对应的区域应力场分别为近南北向挤压、北北西—南南东向挤压、北西—南东向伸展和近东西向挤压应力场。东至断裂带的形成和演化与郯庐断裂带相似,主要与华南与华北板块俯冲碰撞、伊泽奈崎板块和古太平洋板块向欧亚板块俯冲碰撞与弧后扩张、及印度板块向北碰撞后产生向东的构造挤出等多构造体制共同作用有关。     

Geochemistry of post-collisional mafic lavas from the North Anatolian Fault zone, Northwestern Turkey

Extensive magmatic activity developed at the northwestern part of the Anatolian block and produced basaltic lavas that are situated along and between the two segments of the North Anatolian Fault zone. This region is a composite tectonic unit formed by collision of continental fragments after consumption of Neotethyan ocean floor during the late Cretaceous. Northwestern Anatolian basalts and evolved lavas exhibit both tholeiitic and calc-alkaline characteristics. Mafic lavas are moderately enriched in LILE (except depleted part of Yuvacık and İznik samples) and depleted in HFSE (but not Zr, Hf) relative to primitive mantle values, suggesting derivation from a MORB-like mantle source that is unexpected in this subduction environment. Sr and Nd isotopes are close to the mantle array and vary beyond analytical error (⁸⁷Sr/⁸⁶Sr 0.70404–0.70546, ¹⁴³Nd/¹⁴⁴Nd 0.51270–0.51289). These geochemical features may result from two possible processes: (1) melting of a MORB-like mantle source that was modified by subduction-released fluids and melts or (2) modification of mafic liquids derived from a dominantly MORB-like source by crustal or lithospheric mantle material. Geochemical characteristics of the lavas (e.g., Ba/Rb, Rb/Sr, Ba/Zr, ⁸⁷Sr/⁸⁶Sr, Sr/P) vary systematically along the fault zone from east to west, consistent with a decrease in the degree of melting from east to west or a change in the nature of the source composition itself. Thus, the difference in incompatible elements and Sr–Nd isotopic ratios seems to result from small-scale mantle heterogeneity in a post-collisional tectonic environment.     

Neotectonic and volcanic characteristics of the Karasu fault zone (Anatolia,Turkey): The transition zone between the Dead Sea transform and the East Anatolian fault zone

Abstract

The Karasu Rift (Antakya province, SE Turkey) has developed between east-dipping, NNE-striking faults of the Karasu fault zone, which define the western margin of the rift and westdipping, N-S to N20°-30°E-striking faults of Dead Sea Transform fault zone (DST) in the central part and eastern margin of the rift. The strand of the Karasu fault zone that bounds the basin from west forms a linkage zone between the DST and the East Anatolian fault zone (EAFZ). The greater vertical offset on the western margin faults relative to the eastern ones indicates asymmetrical evolution of the rift as implied by the higher escarpments and accumulation of extensive, thick alluvial fans on the western margins of the rift. The thickness of the Quaternary sedimentary fill is more than 465 m, with clastic sediments intercalated with basaltic lavas. The Quaternary alkali basaltic volcanism accompanied fluvial to lacustrine sedimentation between 1.57 ± 0.08 and 0.05 ± 0.03 Ma. The faults are left-lateral oblique-slip faults as indicated by left-stepping faulting patterns, slip-lineation data and left-laterally offset lava flows and stream channels along the Karasu fault zone. At Hacilar village, an offset lava flow, dated to 0.08 ± 0.06 Ma, indicates a rate of leftlateral oblique slip of approximately 4.1 mm?year^?1. Overall, the Karasu Rift is an asymmetrical transtensional basin, which has developed between seismically active splays of the left-lateral DST and the left-lateral oblique-slip Karasu fault zone during the neotectonic period. © 2001 Éditions scientifiques et médicales Elsevier SAS     

Neotectonic and volcanic characteristics of the Karasu fault zone (Anatolia,Turkey): The transition zone between the Dead Sea transform and the East Anatolian fault zone

The Karasu Rift (Antakya province, SE Turkey) has developed between east-dipping, NNE-striking faults of the Karasu fault zone, which define the western margin of the rift and west-dipping, N–S to N20°–30°E-striking faults of Dead Sea Transform fault zone (DST) in the central part and eastern margin of the rift. The strand of the Karasu fault zone that bounds the basin from west forms a linkage zone between the DST and the East Anatolian fault zone (EAFZ). The greater vertical offset on the western margin faults relative to the eastern ones indicates asymmetrical evolution of the rift as implied by the higher escarpments and accumulation of extensive, thick alluvial fans on the western margins of the rift. The thickness of the Quaternary sedimentary fill is more than 465 m, with clastic sediments intercalated with basaltic lavas. The Quaternary alkali basaltic volcanism accompanied fluvial to lacustrine sedimentation between 1.57 ± 0.08 and 0.05 ± 0.03 Ma. The faults are left-lateral oblique-slip faults as indicated by left-stepping faulting patterns, slip-lineation data and left-laterally offset lava flows and stream channels along the Karasu fault zone. At Hacılar village, an offset lava flow, dated to 0.08 ± 0.06 Ma, indicates a rate of left-lateral oblique slip of approximately 4.1 mm·year^–1. Overall, the Karasu Rift is an asymmetrical transtensional basin, which has developed between seismically active splays of the left-lateral DST and the left-lateral oblique-slip Karasu fault zone during the neotectonic period.     

郯庐断裂带昌邑北段隐伏活动断层探测

正活动断层是大陆地震的主要载体。活动断层有的出露地表,有的则隐伏于地下。隐伏于地下的断层称为隐伏断层或盲断层。大地震和强震不只是发生在突破地表的活动断层上,还可以发生在隐伏活动断层上。如华北地区1668年郯城Ms 8.5、1679年三河—平谷Ms 8.0和1976年唐山Ms 7.8等大地震都是未知隐伏活动断层产生的,造成了惨重的人员伤亡和巨大的经济损失。     

Stress modeling to determine the through-going active fault geometry of the Western North Anatolian Fault,Turkey

The North Anatolian Fault (NAF) is a 1200 km long dextral strike-slip fault which is part of an east-west trending dextral shear zone (NAF system) between the Anatolian and Eurasian plates. The North Anatolian shear zone widens to the west, complicating potential earthquake rupture paths and highlighting the importance of understanding the geometry of active fault systems. In the central portion of the NAF system, just west of the town of Bolu, the NAF bifurcates into the northern and southern strands, which converge, then diverge to border the Marmara Sea. At their convergence east of the Marmara Sea, these two faults are linked through the Mudurnu Valley. The westward continuation of these two fault traces is marked by further complexities in potential active fault geometry, particularly in the Marmara Sea for the northern strand, and towards the Biga Peninsula for the southern strand. Potential active fault geometries for both strands of the NAF are evaluated by comparing stress models of various fault geometries in these regions to a record of focal mechanisms and inferred paleostress from a lineament analysis. For the Marmara region, the best-fit active fault geometry consists of the northern and southern bounding faults of the Marmara basin, as the model representing this geometry better replicated primary stress orientations seen in focal mechanism data and stress field interpretations. In the Biga Peninsula region, the active geometry of the southern strand has the southern fault merging with the northern fault through a linking fault in a narrow topographic valley. This geometry was selected over the other two as it best replicated the maximum horizontal stresses determined from focal mechanism data and a lineament analysis.     

Analysis of the North Anatolian Shear Zone in Central Pontides (northern Turkey): Insight for geometries and kinematics of deformation structures in a transpressional zone

The western part of the North Anatolian Shear Zone at the southern boundary of the Central Pontides in Turkey, was investigated in the Kurşunlu-Araç area by means of a geological-structural field study. In this area the North Anatolian Shear Zone results in a transpressional deformation zone that extends between two master faults striking parallel to the main shear direction. The main systems of structures identified in the deformation zone appear to be oriented parallel to the directions predicted by Riedel theoretical model. Nevertheless, the strain partitioning is more complicated than predicted by theory. The structural analysis suggests a polyphase deformation characterized by a steady component of transcurrence associated with alternance of compression and extension. Along each of theoretical directions the combination of double verging structures can be observed, with folds and thrust surfaces root into high-angle shear zones, according to flower-type geometries. The discrepancies of directions, kinematics and geometries from theoretical models are due to transpressive and/or transtensive nature of the deformation. According to the observed outcropping structures, we propose a conceptual model for the North Anatolian Shear Zone, interpreting it as a crustal-scale positive flower structure.     

豫西马超营断裂带的构造演化及其与金等成矿的关系

马超营断裂带是豫西华北地台南缘一条重要的控矿构造带 ,具有复杂的构造演化史 ,至少发生过 5个旋回、7个世代的构造事件。其中海西—印支期的伸展滑脱构造体制和燕山期的逆冲推覆构造体制 ,对该构造带的发展演化和金、多金属成矿起着重要的作用。     

Closing-up structures, alternatives to pull-apart basins; the effect of bends in the North Anatolian Fault, Turkey

Fault blocks passing bends or stepovers in a fault zone must adapt their margins to the uneven fault trace. Two cases of adaption are distinguished for extensional bends or stepovers (transtension): (1) The fault margins close up behind a single bend ('knickpoint') of a strike-slip fault and a 'closing-up structure' (new term) arises or (2) fault-block margins are extended behind a releasing bend (double bend) or stepover parallel to the displacement and a pull-apart basin originates. The dosing up described here is accomplished by acute-angled synthetic strike-slip faults that dissect the straight fault in front of a knickpoint to form a zig-zag block boundary behind it. Crustal extension is also involved in the closing-up structure, but in a different way from typical pull-apart basins.
The closing-up structure illustrated was developed behind an extensional knickpoint in the North Anatolian Fault west of Lake Abant, NW Turkey, where the process of closing up continues to this day. The kinematic model of this closing-up structure is supported by displacements and ruptures observed during the 1967 Mudurnu valley earthquake and the 1957 Abant earthquake.     

余姚—丽水断裂带嵊州地区燕山期主要构造特征

余姚—丽水断裂带是浙东南地区活动时间长、延伸远、发育比较宽的一条NE—NNE展布的断裂构造带,在浙江嵊州地区上火山岩系磨石山群和下火山岩系永康群中构造形迹表现十分明显。余姚—丽水断裂带由一系列NE—NNE向控制区内白垩纪盆地形成与发展的正断层,以及NE—NNE走向、自北西向南东逆冲的叠瓦状断层和轴迹呈NE—NNE向的褶皱组成。通过对其构造活动特征及控制新老地层的时序关系研究,结合区域构造活动规律和时空演化关系等综合分析认为: 正断层形成时间较早,控制白垩纪盆地的形成和发展,与早白垩世岩石圈伸展减薄形成的拉张作用密切相关; 叠瓦状逆冲断层及斜歪褶皱、紧闭同斜褶皱等褶、断构造组合形成于晚白垩世之后,其动力学机制可能与古太平洋构造域向太平洋构造域的转换效应有关。研究成果为深入探讨浙东地区燕山期构造演化提供了新的素材和资料。     

阿尔金断裂带东段地区深浅部构造综合分析

阿尔金断裂带东段地区的地质构造特征及其动力学机制一直是地学工作者关注的焦点。近年来小震资料越来越多应用到活动断裂空间展布、深浅构造分析及动力学机制研究领域。本文应用双差定位法获得研究区域2008～2017年间6013次地震事件的精确定位数据,通过多条小震深度剖面清晰刻画出断裂系统的空间展布形态。综合石油地震剖面、人工地震宽角反射/折射剖面、人工地震深反射剖面,充分利用小震精确定位信息以及浅表活动构造研究成果,建立研究区断裂系统的深浅部构造模型。研究区莫霍面由北往南逐渐加深,存在三处断错,呈阶梯状展布,地壳内存在一条厚约10km的低速层,在该层以上为地震多发区,断裂系统总体呈"Y"字型,上部为一系列叠瓦状逆冲断裂,造成祁连山的隆升,向下并入一条主干断层。最后探讨了青藏高原东北缘地区构造运动的动力学机制,亚洲板块俯冲至祁连山前,上地壳以逆冲推覆构造模式造成上地壳增厚现象,而中下地壳主要为亚洲岩石圈地幔下插,上地幔的拖曳作用下发生流动引起地壳增厚,上下地壳整体增厚。     

正断层的阶区构造及生长机制：以狼山山前断层带为例

正断层带在生长过程中内部发育有阶区构造,阶区在正断层的相互作用、连接过程中起重要控制作用,同时阶区还影响地表径流和沉积盆地的发展、流体的运移和圈闭的形成。位于内蒙古河套断陷西缘的狼山山前断裂是晚新生代以来持续活动的大型正断层系统,断层带内部发育有不同类型的阶区构造。识别出了两种类型阶区的连接方式,一种是两条平行断层之间的斜坡从"软连接"到"硬连接"的演化过程;另一种是楔状阶区通过一条断层向另一条断层扩展的方式连接产生。基岩中的先存构造要素控制并影响山前正断层的展布方位及阶区的形态:基底内部NNE向糜棱面理控制山前断层带的走向,早期向SE倾斜的逆冲断层面被正断层局部利用。沿断层倾向方向,山前正断层逐渐向盆地方向扩展,最新活动的断层位于盆地边缘甚至盆地内部;沿断层走向方向,狼山山前正断层逐渐向南西侧扩展。     

滇西北程海断裂带主要古地震滑坡及其分布特征的构造解释

位于红河断裂带西北端,滇西北断陷带东侧的程海断裂带第四纪活动显著,沿断裂盆山地貌与高山峡谷地貌发育,地质灾害频发.综合利用目视解译与野外调查,对程海断裂带沿线滑坡调查发现,沿程海断裂带共发育各类滑坡940余个,含巨型滑坡61个、大型滑坡125个、中型滑坡316个、小型滑坡438个.这其中有32个巨型滑坡、61个大型滑坡...     

大别造山带与郯庐断裂带衔接区构造特征分析

大别造山带东部与郯庐断裂带南段的衔接处发育有介于二者之间的转换构造,这些转换构造对于揭示二者之间的转换关系起着非常重要的作用。本文对郯庐断裂带桐城段及其邻区发育的转换构造的构造样式和年代学特征进行了分析,并总结得出该处共出露两种不同尺度的转换构造,分别为较大规模北东向偏转的片麻岩构造带和被北东向糜棱岩带非透入性牵引转换的片麻岩带。发生构造转换的片麻岩、同构造脉体及北东向糜棱岩分别给出锆石U-Pb年龄：812～719 Ma,803～629 Ma、242～208 Ma和176～127 Ma以及735～650 Ma、288～216 Ma。结合前人年代学分析,本文认为片麻岩构造的较大规模北东向偏转活动为郯庐断裂带的初期形成活动,受大别造山活动控制;糜棱岩带的剪切活动的发生始于早侏罗世,主要受太平洋板块向欧亚板块俯冲活动的控制。     

2008年汶川地震断裂带北东段南坝地区岩石特征与内部结构

龙门山断裂带中的映秀-北川断裂带是2008年汶川地震的主发震断裂,它具有斜冲断裂的运动学特征（逆冲兼右旋走滑）,其中南西段映秀断裂带以逆冲为主,而北东段北川断裂带以右旋走滑为主。断裂带的物质组成和内部结构的研究,有利于深入认识断裂活动性质、构造变形行为和地震发震机制。本文以出露于映秀-北川断裂带北东段南坝地区的断裂岩为研究对象,通过野外地质调查、显微构造观察、XRD和μXRF等多种分析手段,探讨映秀-北川断裂带北东段的岩石组成和内部结构。研究表明发育于寒武纪粉砂质板岩中的断裂带宽度约30m,断层中心发育厚20~40cm的黑色断裂物质,2008年汶川地震的同震位移沿黑色物质层中厚约10mm滑动带滑动。断层NW侧和SE侧表现出近于对称的结构特征,两侧的角砾岩宽~4m和~3m,破碎带宽~10m和~12m。黑色断裂物质中石英含量30%~50%,长石含量18%~25%,黏土矿物总含量30%~36%,主要由伊利石、伊蒙混层和绿泥石组成,少有蒙脱石。XRD结果显示黑色断裂物质具有非晶质成分特征,SEM观测结果可见熔融结构特征,显微结构与μXRF结果呈现后期流体作用明显,其表明黑色断裂物质主要由古地震滑动形成的假玄武玻璃蚀变而成,并发育不同蚀变程度的多期假玄武玻璃。上述研究揭示出断裂摩擦熔融是汶川地震断裂带北东段南坝地区的重要动态弱化机制,其内部结构和岩石特征与南西段映秀断裂带具有明显的差异。