A linkage criterion for segmented normal faults

A detailed field study of 39 centimetre- to metre-scale relay ramps from two outcrops was performed to investigate the development of a linkage criterion for segmented normal faults. We analysed the displacement distribution and the geometry of fault arrays containing three types of relay ramp: open, linked, and fully breached, in order to identify which parameters are relevant to fault linkage, and to establish a linkage criterion. Each relay ramp geometry has a specific graphical field on a relay displacement–separation diagram. The field including all the linked geometries (initiation of linkage) separates open and fully breached relay ramps and is interpreted as a value of relay displacement to separation ratio for which faults link during their overlap. A ‘linkage threshold’, in each studied fault system, is defined as the best-fit linear trend of linked relays. We discuss the scaling and the variability of the linkage criterion using published datasets from a wide variety of settings and scales. The observed linkage threshold is linear, with a slope value varying less than one order of magnitude. This suggests that linking relay ramps have self-similar geometries from centimetre- to kilometre-scale and that normal fault linkage is governed by similar fault interaction across a broad range of scales. The linkage criterion, which can be an effective tool to estimate relay ramp geometry at depth or at the earth surface, could therefore be used to improve investigations in determining fluid entrapment or in the evaluation of potential surface of seismic ruptures.     

Normal fault corrugation: implications for growth and seismicity of active normal faults

Large normal faults are corrugated. Corrugations appear to form from overlapping or en échelon fault arrays by two breakthrough mechanisms: lateral propagation of curved fault-tips and linkage by connecting faults. Both mechanisms include localized fault-parallel extension and eventual abandonment of relay ramps. These breakthrough mechanisms produce distinctive hanging wall and footwall geometries indicative of fault system evolution. From such geometries, we can estimate the positions of tilted relay ramps or ramp segments and ramp internal deformation in incompletely exposed or poorly imaged fault systems. We examine the evolution of normal fault corrugations at Fish Slough (California), Yucca Mountain (Nevada), and Pleasant Valley (Nevada), in the Basin and Range province. We discuss how evolution of the Pleasant Valley and Yucca Mountain systems relates to seismicity. For example, the 1915 Pleasant Valley earthquake produced four en échelon ruptures that appeared as overlapping segments of a single immature fault at depth. At Yucca Mountain, we argue that an en échelon array, which includes the Solitario Canyon and Iron Ridge faults, should be considered a single source, such that western Yucca Mountain could experience up to a M_w 6.9 earthquake compared to M_w 6.6 estimates for the largest individual segment.     

Geometry and architecture of faults in a syn-rift normal fault array: The Nukhul half-graben, Suez rift, Egypt

The geometry and architecture of a well exposed syn-rift normal fault array in the Suez rift is examined. At pre-rift level, the Nukhul fault consists of a single zone of intense deformation up to 10 m wide, with a significant monocline in the hanging wall and much more limited folding in the footwall. At syn-rift level, the fault zone is characterised by a single discrete fault zone less than 2 m wide, with damage zone faults up to approximately 200 m into the hanging wall, and with no significant monocline developed. The evolution of the fault from a buried structure with associated fault-propagation folding, to a surface-breaking structure with associated surface faulting, has led to enhanced bedding-parallel slip at lower levels that is absent at higher levels. Strain is enhanced at breached relay ramps and bends inherited from pre-existing structures that were reactivated during rifting. Damage zone faults observed within the pre-rift show ramp-flat geometries associated with contrast in competency of the layers cut and commonly contain zones of scaly shale or clay smear. Damage zone faults within the syn-rift are commonly very straight, and may be discrete fault planes with no visible fault rock at the scale of observation, or contain relatively thin and simple zones of scaly shale or gouge. The geometric and architectural evolution of the fault array is interpreted to be the result of (i) the evolution from distributed trishear deformation during upward propagation of buried fault tips to surface faulting after faults breach the surface; (ii) differences in deformation response between lithified pre-rift units that display high competence contrasts during deformation, and unlithified syn-rift units that display low competence contrasts during deformation, and; (iii) the history of segmentation, growth and linkage of the faults that make up the fault array. This has important implications for fluid flow in fault zones.     

Structural evolution of a normal fault array in the Gambier Embayment,offshore Otway Basin,South Australia: insights from 3D seismic data

This study was undertaken to determine the structural evolution of a normal fault array using detailed kinematic analysis of normal fault tip propagation and linkage, adding to the growing pool of research on normal fault growth. In addition, we aim to provide further insight into the evolution of the offshore Otway Basin, Australia. We use three-dimensional (3D) seismic reflection data to analyse the temporal and spatial evolution of a Late Cretaceous–Cenozoic age normal fault array located in the Gambier Embayment of the offshore Otway Basin, South Australia. The seismic reflection data cover a NW–SE-oriented normal fault array consisting of six faults, which have grown from the linkage of numerous, smaller segments. This fault array overlies and has partial dip-linkage to E–W-striking, basement-involved faults that formed during the initial Tithonian–Barremian rifting event in the Otway Basin. Fault displacement analysis suggests four key stages in the post-Cenomanian growth history of the upper array: (1) nucleation of the majority of faults resulting from resumed crustal extension during the early Late Cretaceous; (2) an intra-Late Cretaceous period of general fault dormancy, with the nucleation of only one newly formed fault; (3) latest Cretaceous nucleation of another newly formed fault and further growth of all other faults; and (4) continued growth of all faults, leading to the formation of the Cenozoic Gambier Sub-basin in the Otway Basin. Our analysis also demonstrates that Late Cretaceous faults, which are located above and dip-link to basement-involved faults, display earlier nucleation and greater overall throw and length, compared with those which do not link to basement-involved faults. This is likely attributed to increased rift-related stress concentrations in cover sediments above the upper tips of basement faults. This study improves our understanding of the geological evolution of the presently under-explored Gambier Embayment, offshore Otway Basin, South Australia by documenting the segmented growth style of a Late Cretaceous normal fault array that is located over, and interacts with, a reactivated basement framework.     

青藏高原的地幔结构:地幔羽、地幔剪切带及岩石圈俯冲板片的拆沉

正断层位移长度关系是近年来研究正断层的一种常用方法 ,它将断层的研究从二维扩展到三维。研究正断层位移长度关系对于研究正断层演化、盆地演化以及油气勘探等方面具有重要意义。讨论了影响正断层位移距离剖面形态的主要因素 :断层端的脆韧性变形、围岩强度、远程应力以及断层之间的相互作用和连接等。根据正断层位移距离剖面的几何形态和断层发育阶段 ,可将断层的位移模式分为 3个类型 :(1)对称的椭圆状或钟状代表简单的单条断层 ;(2 )不对称的似椭圆状或钟状代表断层间的相互作用 ;(3)不规则的锯齿状代表由多条断层连接而成的断层。介绍了伴随正断层发育的传递斜坡、盆内高地等构造单元以及断层位移距离剖面的测量制作方法。测量时应注意恢复其剥蚀部分。     

Segment,Linkage,and Extensional Fault-Related Fold in Western Liaodong Bay Subbasin,Northeastern Bohai Sea,China

The western Liaodong (辽东) Bay subbasin displays examples of segment,linkage of extensional fault,and fault-related folds.The Liaoxi (辽西) extensional fault system consists of a series of NNE- and NE-trending segments that were linked through relay ramps.The fault hanging walls are characterized by a series of en echelon synclines with axial traces sub-parallel to the faults.The synclines are doubly plunging located on the hanging wall of normal faults,with the strata dip sub-parallel to the fault.These folds result from along-strike displacement variations of the individual fault segments,as well as from extensional fault-related folding.In the study area,the synclines are separated by transverse intra-basin highs and relay ramps that formed where segment linkage occurred.These hanging wall synclines and their relation to fault displacement variations indicate that they are formed by extensional fault-related fold.     

Field evidence for normal fault linkage and relay ramp evolution: the Kırkağaç Fault Zone,western Anatolia (Turkey)

Linking of normal faults forms at all scales as a relay ramp during growth stages and represents the most efficient way for faults to lengthen during their progressive formation. Here, I study the linking of normal faulting along the active K?rka?aç Fault Zone within the west Anatolian extensional system to reconstruct fault interaction in time and space using both field- and computer-based data. I find that (i) connecting of the relay zone/ramp occurred with two breaching faults of different generations and that (ii) the propagation was facilitated by the presence of pre-existing structures, inherited from the ?zmir-Bal?kesir transfer zone. Hence, the linkage cannot be compared directly to a simple fault growth model. Therefore, I propose a combined scenario of both hangingwall and footwall fault propagation mechanisms that explain the present-day geometry of the composite fault line. The computer-based analyses show that the approximate slip rate is 0.38 mm/year during the Quaternary, and a NE–SW-directed extension is mainly responsible for the recent faulting along the K?rka?aç Fault Zone. The proposed structural scenario also highlights the active fault termination and should be considered in future seismic hazard assessments for the region that includes densely populated settlements.     

正断层中继构造扩展演化机制及对成矿构造研究的启示

正断层的分段构造型式及其扩展演化机制的核心内容可以归结为正断层的中继构造模式。本文从正断层的断块变形、断裂扩展以及断裂力学特征等角度,总结了正断层中继构造的扩展演化机制。正断层的断块变形是指分段断层位移转换形成的中继斜坡,可分为内倾和外倾两种斜坡类型,位移曲线揭示了中继带变形的特征。正断层的断裂扩展有多种扩展方式,可分为断层内、断层间和断层系扩展,且可以形成不同型式的中继构造。正断层中继构造是在三维空间扩展的,将断层边缘的断裂力学性质和断层面的放射状扩展方向相结合,可完整描述中继构造的空间分布规律。正断层形成于伸展构造环境,是控制油气和金属矿产资源的重要构造型式,结合SEDEX型矿床同生断层和岩体侵位控制的正断层控矿研究实例,初步讨论了正断层中继构造扩展演化在SEDEX型矿床和正断层相关控矿断裂成矿构造研究中的作用。     

珠江口盆地惠州凹陷北部边界断裂复合联接和转换

边界断裂控制断陷盆地的形成和构造格局,不同边界断裂联接模式对不同类型盆地演化具有差异性.基于井控高精度3D地震资料,通过对边界断裂几何学特征描述和“四级小层”刻画,结合裂陷Ⅰ幕边界断裂不同区段的活动差异性以及与沉积中心迁移的空间匹配关系,剖析珠江口盆地惠州凹陷北部边界断裂的形成和演化.惠州凹陷北部边界断裂始新世早期分段孤立发育,逐渐以纵向和横向双向联接的模式发展.纵向联接为断层软联接和硬联接复合联接和转换,形成转换斜坡和横向背斜,控制凹(洼)陷的结构与演化,制约沉积中心及层序的迁移.横向联接表现为转换斜坡内横向断层的多阶段联接,联接过程可划分为孤立正断层、同向叠置及硬联接3个阶段,控制转换斜坡带内沉积体系的发育和展布.研究给出了一个裂陷盆地边界断裂时空演化、复合联接和转换模式的独特案例,对丰富裂陷盆地边界断裂及其与沉积层序、凹陷演化和区域动力学机制的响应关系的研究具有积极的意义和价值.      

Growth of normal faults in multilayer sequences: A 3D seismic case study from the Egersund Basin,Norwegian North Sea

We investigate the structural style and evolution of a salt-influenced, extensional fault array in the Egersund Basin (Norwegian North Sea) through analysis of 3D reflection seismic and well data. Analysis of fault geometry/morphology, throw distribution and syn-kinematic strata reveal an intricate but systematic style of displacement and growth, suggesting an evolution of (1) initial syn-sedimentary fault growth contemporaneous with salt mobilization initiated during the Late Triassic, (2) cessation of fault activity and burial of the stagnant fault tips, and (3) subsequent nucleation of new faults in the cover above contemporaneous salt re-mobilization initiated during the Late Cretaceous, with downward propagation and linkage with faults. Stage 3 was apparently largely controlled by salt mobilization in response to basin inversion, as reactivated faults are located where the underlying salt is thick, while the non-reactivated faults are found where salt is depleted. Based on the 3D-throw analyses, we conclude that a combination of basement faulting and salt (re-) mobilization is the driving mechanisms behind fault activation and reactivation. Even though the sub- and supra-salt faults are mainly geometrically decoupled through the salt, a kinematic coupling must have existed as sub-salt faults still affected nucleation and localization of the cover faults.     

Fault growth and main controlling factors in deep area of Gaoyou SagEI北大核心CSCD

Study of fault growth in deep area of the Gaoyou Sag has great significance for the understanding of the structural evolution and hydrocarbon accumulation of the Gaoyou Sag. Based on high-precision 3D seismic data, the displacement-length relationship and throw data of faults are integrated to reconstruct the growth of the boundary fault and secondary faults in the eastern deep area of the Gaoyou Sag and controlling factors of the fault growth are discussed by analyzing the tectonic stress field and numerical simulation. This study revealed that the Zhen 2 boundary fault had six segments which grew independently in the Eocene Dainan Period and subsequently linked into four segments in the Eocene Sanduo Period. The Zhen 2 fault growth included two phases: short rapid lateral lengthening initially in Dainan Period, followed by a longer phase of slip accumulation and linkage of segments in the Sanduo Period. The strike of Zhen 2 fault which is widely and gently undulate kept in step with Zhen1 extensional boundary fault that controlled the structural evolution of the Subei Basin. The structure of Zhen 2 fault tips is contributed to interaction of fault segments. Two segments overlapped and linked with its neighbor by relay zone, otherwise the segments kept away from the fault and linked near the tip of the fault which interacting the growth progress. The secondary faults are linked alternatively by segments of NEE strike and EW strike and the segments of NEE strike are developed initially in the Dainan Period. Numerical simulation demonstrated that the extensional direction in the area of secondary faults is parallel to the extensional direction of the region and barely influenced by the Zhen 2 boundary fault. The orientational change of the tectonic stress field between the Paleocene and Eocene led to the linkage of secondary faults, which means that the segments of NEE strike are developed by pre-existing fabrics of the Paleocene and the segments of EW strike linked the NEE segments later in the Eocene. ©, 2015, Science Press. All right reserved.     

Strain compatibility and fault linkage in relay zones on normal faults

Relay zones on normal faults are unlikely to have tabular geometries as depicted in idealised models. Rotation of a relay ramp between non-parallel and non-planar relay-bounding faults will inevitably lead to strain compatibility problems causing open gaps or overlaps within the relay zone. Linkage of relay-bounding faults does not evolve from a single branch point. Rather, linkage occurs at multiple points along the fault tip lines giving rise to initially discontinuous branch lines. Where linkage occurs along a discontinuous slip-aligned branch line, displacement at different levels within the relay zone is partitioned between variable amounts of ramp rotation and slip across the branch line. The linking fault propagates when strain compatibility can no longer be maintained by continuous deformation processes, such as thickening or thinning of incompetent layers within the relay ramp. Step-like changes in vertical displacement vs. distance (d − x) profiles on horizons containing apparently intact relay ramps are probably indicative of incipient breaching and can be used predict the presence of a slip-aligned branch line in the sub-surface. Despite the complexity of the strain distribution within relay zones, the total vertical displacement across the relay remains geometrically coherent at all levels.     

New insights into the structure and evolution of the Isle of Wight Monocline

High quality seismic reflection data acquired during hydrocarbon exploration activities provide evidence for the subsurface structure and evolution of one of England's most well known structures at outcrop: the Isle of Wight Monocline. It is generally seen as a major northerly verging monoclinal structure linked to the Purbeck Monocline to the west. The Isle of Wight Monocline is the result of the interplay between two inverted east-west trending, southerly dipping and overlapping down-to-the-south major syndepositional normal faults that were active during Triassic and Jurassic times: the Needles and Sandown faults. The area between the two faults tips forms an easterly-dipping relay ramp, down which sequences of all ages thicken. Both of these major normal faults were inverted during Cenozoic (Miocene: Alpine) compressional events, folding the overlying post-rift sequences of early Cretaceous to early Cenozoic (Palaeogene) age. Interpretation of the seismic reflection data suggest that previously unknown high angle, down-to-the-north reverse faults cut the northern limbs of both anticlines forming the composite monocline and are likely to come to crop in the steeply-dipping Chalk and/or the drift-covered Cenozoic sequences. Their identification marked a period of discussions and testing of the model by detailed field mapping. The existence and location of such faulting was proved through an iterative process with the result that a reverse fault zone is now mapped along the northern limb of the northern Sandown Anticline section of the Monocline. The main reverse faults on the Brighstone and Sandown anticlines result in up to circa 550 m of displacement at top Chalk level. It is thought that a series of smaller footwall short-cut faults affect the Cenozoic strata to the north of the main reverse fault, producing up-faulted sections of flatter-lying Cenozoic strata. Reverse displacements and the severity of folding on the inverted faults decreases on each fault segment in a complementary fashion in the area of the relay ramp as one fault takes up the movement at the expense of the other. The swing in strike of the Chalk in the area of shallowly dipping strata between Calbourne and Garstons is a result of deformation of the post-rift sequences across the relay ramp established between the overlapping fault tips of the Needles and Sandown faults and the interaction of the folds developed at the tips of the reverse faults.     

裂谷盆地构造控制地形-沉积体系演化研究与面临问题

裂谷盆地早期构造作用强烈，充填作用发生在由构造控制的盆地内。盆地的古地形通过物源区侵蚀和沉积物流的盆内输送而影响沉积体的类型与堆积样式。断裂的生长、传播、相互作用、连接与死亡控制了古地形的周期演化，对沉积体系的演化施加了强烈的影响或控制。伸展盆地地层发育的控制与过程的研究主要反映在断层的生长过程、裂谷盆地的侵蚀、汇水和沉积物流等方面的精细研究。此方面研究面临的问题不仅包括研究的精细程度和技术，而且包括相关分支学科的合作。     

The influence of pre-existing structure on the growth of syn-sedimentary normal faults in a deltaic setting,Niger Delta

Three dimensional seismic-reflection data from the western Niger Delta were used to investigate the segmentation and linkage of a syn-sedimentary normal fault array and to estimate the influence of a pre-existing normal fault on the geometry and growth of younger faults. The nucleation, growth and linkage of a regional (seaward-dipping) deltaic fault system were analyzed on reflectivity time-/horizon slices and vertical seismic sections. In the deep subsurface, a master fault that consists of two segments (northwestern, NW, and southeastern, SE) grew through time into a single fault by lateral tip propagation reaching a final length of about 15 km. After attaining this length, displacement along the fault system developed non-uniformly through time. The analysis of the hanging-wall sediments of the deep-seated master fault shows two different processes of vertical linkage above the NW and SE segment. The SE segment links vertically to several younger faults contemporaneously with displacement accumulation on the master fault; in contrast, fault linkage above the NW segment occurred only after an interval of master-fault inactivity connecting the deep-seated structure upwards to a single syn-sedimentary normal fault. The observed differences in fault development suggest that although multi-segment deltaic faults form single fault systems after segment linkage, individual pre-linkage characteristics can be preserved, supporting a possibly diverse upward growth and connection to younger faults in the overburden. The geological interpretations presented highlight the influence of large deep-rooted structures on the development, location and geometry of shallow deltaic faults, documenting the influence of an older structural grain on delta tectonics.     

3D seismic analysis of the structure and evolution of a salt-influenced normal fault zone: A test of competing fault growth models

In this paper we determine the structure and evolution of a normal fault system by applying qualitative and quantitative fault analysis techniques to a 3D seismic reflection dataset from the Suez Rift, Egypt. Our analysis indicates that the October Fault Zone is composed of two fault systems that are locally decoupled across a salt-bearing interval of Late Miocene (Messinian) age. The sub-salt system offsets pre-rift crystalline basement, and was active during the Late Oligocene-early Middle Miocene. It is composed of four, planar, NW–SE-striking segments that are hard- linked by N–S-striking segments, and up to 2 km of displacement occurs at top basement, suggesting that this fault system nucleated at or, more likely, below this structural level. The supra-salt system was active during the Pliocene-Holocene, and is composed of four, NW–SE-striking, listric fault segments, which are soft-linked by unbreached relay zones. Segments in the supra-salt fault system nucleated within Pliocene strata and have maximum throws of up to 482 m. Locally, the segments of the supra-salt fault system breach the Messinian salt to hard-link downwards with the underlying, sub-salt fault system, thus forming the upper part of a fault zone composed of: (i) a single, amalgamated fault system below the salt and (ii) a fault system composed of multiple soft-linked segments above the salt. Analysis of throw-distance (T-x) and throw-depth (T-z) plots for the supra-salt fault system, isopach maps of the associated growth strata and backstripping of intervening relay zones indicates that these faults rapidly established their lengths during the early stages of their slip history. The fault tips were then effectively ‘pinned’ and the faults accumulated displacement via predominantly downward propagation. We interpret that the October Fault Zone had the following evolutionary trend; (i) growth of the sub-salt fault system during the Oligocene-to-early Middle Miocene; (ii) cessation of activity on the sub-salt fault system during the Middle Miocene-to-?Early Pliocene; (iii) stretching of the sub- and supra-salt intervals during Pliocene regional extension, which resulted in mild reactivation of the sub-salt fault system and nucleation of the segmented supra-salt fault system, which at this time was geometrically decoupled from the sub-salt fault system; and (iv) Pliocene-to-Holocene growth of the supra-salt fault system by downwards vertical tip line propagation, which resulted in downward breaching of the salt and dip-linkage with the sub-salt fault system. The structure of the October Fault Zone and the rapid establishment of supra-salt fault lengths are compatible with the predictions of the coherent fault model, although we note that individual segments in the supra-salt array grew in accordance with the isolated fault model. Our study thereby indicates that both coherent and isolated fault models may be applicable to the growth of kilometre-scale, basin-bounding faults. Furthermore, we highlight the role that fault reactivation and dip-linkage in mechanically layered sequences can play in controlling the three-dimensional geometry of normal faults.     

Modelling cover deformation and decoupling during inversion,using the Mid-Polish Trough as a case study

Seismic sections across the NW part of the Polish Basin show that thrust faults developed in the sedimentary units above the Zechstein evaporite layer during basin inversion. These cover thrust faults have formed above the basement footwall. Based on the evolution of the basin, a series of scaled analogue models was carried out to study interaction between a basement fault and cover sediments during basin extension and inversion. During model extension, a set of normal faults originated in the sand cover above the basement fault area. The distribution and geometry of these faults were dependent on the thickness of a ductile layer and pre-extension sand layer, synkinematic deposition, the amount of model extension, as well as on the presence of a ductile layer between the cover and basement. Footwall cover was faulted away from the basement only in cases where a large amount of model extension and hanging-wall subsidence were not balanced by synkinematic deposition. Model inversion reactivated major cover faults located above the basement fault tip as reverse faults, whereas other extensional faults were either rotated or activated only in their upper segments, evolving into sub-horizontal thrusts. New normal or reverse faults originated in the footwall cover in models which contained a very thin pre-extension sand layer above the ductile layer. This was also the case in the highly extended and shortened model in which synkinematic hanging-wall subsidence was not balanced by sand deposition during model extension. Model results show that inversion along the basement fault results in shortening of the cover units and formation of thrust faults. This scenario happens only when the cover units are decoupled from the basement by a ductile layer. Given this, we argue that the thrusts in the sedimentary infill of the Polish Basin, which are decoupled from the basement tectonics by Zechstein evaporites, developed due to the inversion of the basement faults during the Late Cretaceous-Early Tertiary.     

Segmentation and inversion of the Hangjinqi fault zone,the northern Ordos basin (North China)

This paper deals with the segmentation and inversion of the Hangjinqi fault zone (HFZ), which is the dominant structure in the northern part of the Ordos basin in North China. HFZ was reactivated during the Late Triassic and obliquely inverted during the Middle Jurassic shortening. Subsurface geological mapping and structural analysis were carried out to determine the segmentation and kinematic history of the deformation. The HFZ was a left-stepping fault zone and was made up of three segments: the Porjianghaizi fault (PF), Wulanjilinmiao fault (WF) and Sanyanjing fault (SF), which are separated by two relay ramps. Two distinct phases can be identified in its structural evolution: (1) during the Late Triassic compressional deformation, the HFZ was characterized by shortening and thrusting to the north; and (2) During the Middle Jurassic phase the HFZ was oblique to the extensional fault trends, the reverse faults were reactivated as dextral strike-slip faults as a result of transtensional inversion. The inversion ratio of the HFZ indicates an increase in deformational degree from east to west over the whole region. The first deformation stage resulted from the N–S compression between the South China and North China plates during the Late Triassic. The second deformation stage of compression was related to the west-northwestward subduction of the paleo-Pacific plate during the Middle Jurassic. In the Jurassic deformation framework, the HFZ may be interpreted as an accommodation structure parallel to the Yanshan–Yinshan orogenic belt developed in the northern Ordos area.     

Clay-smear continuity and normal fault zone geometry – First results from excavated sandbox models

The continuity of clay-rich fault gouge has a large effect on fluid transmissibility of faults in sand–clay sequences, but clay gouge continuity and composition in 3D are not well known. We report observations of 3D clay smear continuity in water-saturated sandbox experiments where the sheared clay layers were excavated after deformation. The experiments build on existing work on the evolution of clay gouge in similar 2D experiments where interpretations were made in profile view.We used well-known model materials (“Benchmark” sand and uncemented kaolinite–sand mixtures) that were further characterized using standardized geotechnical tests and triaxial compression experiments at effective pressures corresponding to the sandbox experiments. Results show a nonlinear failure envelope of the sand, in agreement with existing models. Unconfined compression experiments with the clay show cohesion around 50 Pa and brittle behavior.A sheared, ductile clay layer embedded in sand above a 70° dipping basement fault reveals a complex, natural-looking clay gouge architecture with relay ramps, breached relays and fault lenses. The clay gouge shows clear variations in composition and thickness and becomes locally discontinuous at throw-thickness ratios above 7, in contrast to our earlier 2D observations where discontinuous clay-gouge only formed in cemented clay layers. In addition to tectonic telescoping in the relays, the thin, continuous parts of the clay gouge were transformed from an initial pure clay by mechanical mixing of sand and clay.We also discuss the applicability of these results to the evolution of normal fault zones and deformation bands in sand–clay sequences at effective pressures below the onset of cataclasis and conclude that in fault zones a higher degree of internal segmentation reduces the probability of the formation of discontinuities.     

Oblique slip and the geometry of normal-fault linkage: mechanics and a case study from the Basin and Range in Oregon

Map patterns of normal fault linkages near Summer Lake, Oregon, show a systematic relationship between échelon step-sense, oblique-slip sense, and the position of linking faults. Where the step sense is the same as the sense of oblique slip (e.g. left step and left-oblique slip), the faults are linked in the lower part of their relay ramp. Where the step-sense and slip-sense are opposite (e.g. left-step and right-oblique slip), the faults are linked in the upper part of the ramp. A boundary-element code is used to calculate the stress field around échelon normal faults during oblique slip, and the model results reveal a relationship similar to the field observations. If step sense and oblique-slip sense are the same, there is a greater potential for deformation ahead of the tip of the front fault and in the lower part of the ramp. If step sense and oblique-slip sense are opposite, there is a greater potential for deformation ahead of the tip of the rear fault and in the upper part of the ramp. The field-model comparison confirms that oblique slip modifies the mechanical interaction among fault segments and thus influences fault growth and the geometry of fault linkage.