Implementation of a coupled plastic damage distinct lattice spring model for dynamic crack propagation in geomaterials

This paper studies dynamic crack propagation by employing the distinct lattice spring model (DLSM) and 3‐dimensional (3D) printing technique. A damage‐plasticity model was developed and implemented in a 2D DLSM. Applicability of the damage‐plasticity DLSM was verified against analytical elastic solutions and experimental results for crack propagation. As a physical analogy, dynamic fracturing tests were conducted on 3D printed specimens using the split Hopkinson pressure bar. The dynamic stress intensity factors were recorded, and crack paths were captured by a high‐speed camera. A parametric study was conducted to find the influences of the parameters on cracking behaviors, including initial and peak fracture toughness, crack speed, and crack patterns. Finally, selection of parameters for the damage‐plasticity model was determined through the comparison of numerical predictions and the experimentally observed cracking features.     

A geometric model for the formation of deformation band clusters

Arrays of closely-spaced (approximately <70 mm) sub-parallel cataclastic deformation bands are common structures in deformed, high-porosity (∼10–35%) sandstones. The distribution of strain onto many small-displacement deformation bands is thought by some to result from strain-hardening of the cataclasite within individual bands. Examination of both normal and strike-slip faults with displacements ≤7 m from southeastern Utah, USA, and the North Island of New Zealand suggests, however, that clusters of deformation bands systematically develop at fault geometric irregularities (e.g., fault bends, steps, relays, intersections and zones of normal drag). The strain-hardening model does not account for clustering of deformation bands at fault geometric irregularities or the associated widespread coalescence of bands, and is not unequivocally demonstrated by post-peak macroscopic mechanical responses in laboratory rock deformation experiments. A geometric model is proposed in which individual bands within clusters develop sequentially due to migration of incremental shear strains at fault geometric irregularities as part of a slip localisation, asperity removal and strain weakening process. The geometric model, which does not require strain hardening of the fault rock, applies for the duration of faulting and a range of rock types in the brittle upper crust.     

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.     

Frictional properties of simulated anhydrite-dolomite fault gouge and implications for seismogenic potential

The frictional properties of anhydrite-dolomite fault gouges, and the effects of CO₂ upon them, are of key importance in assessing the risks associated with CO₂ storage in reservoir formations capped by anhydrite-dolomite sequences, and in understanding seismicity occurring in such formations (such as the Italian Apennines). Therefore, we performed velocity-stepping direct-shear experiments on simulated dolomite, anhydrite and 50:50 anhydrite/dolomite gouges, at representative in-situ conditions (120 °C and σ_n^e = 25 MPa). They were conducted under vacuum, or else using water or CO₂-saturated water as pore fluid (P_f = 15 MPa). Friction coefficients varied between 0.55 and 0.7. All dry samples exhibited velocity-weakening behavior, whereas all wet samples exhibited velocity-strengthening behavior, without or with CO₂. This is consistent with trends previously reported for such gouges. A compilation of literature data shows that the transition from velocity-strengthening to velocity-weakening occurs in these materials between 80 and 120 °C when dry, and between 100 and 150 °C when wet. This implies little seismogenic potential for wet dolomite, anhydrite and mixed gouges under CO₂ storage conditions at 2–4 km depth. Seismic slip in the Italian Apennines at depths of ∼6 km and beyond may be explained by the velocity-weakening behavior expected in anhydrite and especially dolomite at temperatures above 150 °C.     

A two‐phase SPH model for debris flow propagation

This paper presents a model which can be used for fast landslides where coupling between solid and pore fluid plays a fundamental role. The proposed model is able to describe debris flows where the difference of velocities between solid grains and fluid is important. The approach is based on the mathematical model proposed by Zienkiewicz and Shiomi, which is similar to those of Pitman and Le and Pudasaini. The novelty of the present work is the numerical technique used, the smoothed particle hydrodynamics (SPH). We propose to use a double set of nodes for soil and water phases, the interaction between them being described by a suitable drag law. The paper presents both mathematical and numerical models, describing the main assumptions and their limitations. Then, the model is applied to (1) a simple case where shocks and expansion waves appear, (2) a dam break problem on a horizontal plane with a frictional soil phase, and (3) a debris flow which happened in Hong Kong. The main conclusions that can be drawn from the applications are:

1. Debris flows having 2 phases with important relative mobility present a rich structure of shocks and rarefaction waves, which has to be properly modeled. Otherwise, the model will have numerical damping or dispersion.
2. Dambreak exercises provide interesting information in simple and controlled situations. We can see how both phases move relative to each other.
3. Real debris flows can be simulated with the proposed model, obtaining reasonable results.

     

Earthquake activity related to seismic cycles in a model for a heterogeneous strike-slip fault

We investigate the evolution of seismicity within large earthquake cycles in a model of a discrete strike-slip fault in elastic solid. The model dynamics is governed by realistic boundary conditions consisting of constant velocity motion of regions around the fault, static/kinetic friction and dislocation creep along the fault, and 3D elastic stress transfer. The fault consists of brittle parts which fail during earthquakes and undergo small creep deformation between events, and aseismic creep cells which are characterized by high ongoing creep motion. This mixture of brittle and creep cells is found to generate realistic aftershock sequences which follow the modified Omori law and scale with the mainshock size. Furthermore, we find that the distribution of interevent times of the simulated earthquakes is in good agreement with observations. The temporal occurrence, however, is magnitude-dependent; in particular, the small events are clustered in time, whereas the largest earthquakes occur quasiperiodically. Averaging the seismicity before several large earthquakes, we observe an increase of activity and a broadening scaling range of magnitudes when the time of the next mainshock is approached. These results are characteristics of a critical point behavior. The presence of critical point dynamics is further supported by the evolution of the stress field in the model, which is compatible with the observation of accelerating moment release in natural fault systems.     

Evidence for Holocene activity of the Yilan-Yitong fault,northeastern section of the Tan-Lu fault zone in Northeast China

The Tan-Lu fault zone (TLFZ) is the largest of the major faults in eastern China. Many strong earthquakes have occurred on its section in North China, but no quake greater than M ⩾ 6 has been documented in history at its northeastern section, the Yilan-Yitong fault (YYF) in Northeast China. It is usually considered that this fault has been inactive since late Quaternary and incapable of generating moderate-sized quakes. This conclusion is, however, questioned by our recent work based on high-resolution satellite image interpretation and field investigation. We found a 70-km-long surface scarp near Fangzheng county in Heilongjiang province (HLJP) and a 20-km-long scarp near Shulan county in Jilin province (JLP), and both are associated with the YYF. The trenches across these two scarps reveal a ¹⁴C displacement date of 1730 ± 40 years BP at Fangzheng and of 4410 ± 30 years BP at Shulan. The dextral offsets of the Songhua River and Second Songhua River and nearly horizontal fault striations indicate that the new activity of the YYF has been dominated by dextral strike slipping with a normal component. These new data suggest that, at least for partial sections, the YYF has been active since the Holocene, implying a potential seismic hazard. However, current quake-protection standards in this region are very low due to the previous view that the YYF fault has not been active since the late Quaternary. If an M ⩾ 7 quake takes place on this fault, it will be a devastating event. Therefore, it is necessary to conduct a detailed study on the whole YYF and to reassess its future seismic risk.     

Validation of a model for impact breakage incorporating particle size effect

Investigation of a breakage probability model published by Vogel and Peukert [Vogel, L. and Peukert, W., 2004. Determination of material properties relevant to grinding by practicable labscale milling tests. Int. J. Miner. Process., 74S, 329–338.] has led to a modification of their model to describe the degree of impact breakage, t₁₀. The modified model takes a form similar to the JKMRC prior art breakage model, but with particle size and breakage properties incorporated explicitly in the model.     

A detachment fold model for fault zones in the Late Archean Abitibi greenstone belt

The Abitibi belt is one of the largest and most extensively studied Late Archean greenstone belts. The structural geology of the Abitibi belt consists of one generation of upright to slightly overturned, doubly plunging first-order folds with half-wavelengths of 20–60 km, and E–W-striking, steeply dipping fault zones that are parallel to the fold limbs. Two of the main fault zones are continuous for hundreds of kilometers. Previous tectonic models for the Abitibi belt interpret the fault zones to have formed as extensional growth faults bounding a volcanic-sedimentary basin, which were reactivated as thrusts during subsequent crustal shortening. Other models propose that the fault zones represent tectonic sutures, implying that the Abitibi belt is a collage of exotic terranes. However, distinct geological terranes have not been geologically demonstrated. We propose a new detachment fold model for the deformational history of the southern Abitibi belt, in Ontario, that explains the formation of the fault zones during the single, well-documented folding event that deformed the entire region. The internal structure of the fault zones, documented here with emphasis on the Porcupine–Destor fault zone, consists of isoclinally folded, strongly schistose, highly metamorphosed rock, cross-cut by numerous fault segments. We interpret that the upper crust (greenstones) was folded above a proposed detachment in the lower part of the volcanic stratigraphy. The fault zones would be, in essence, highly evolved detachment anticlines. Ultramafic metavolcanic rock that crops out within the fault zones would represent material from the detachment horizon that was emplaced in the cores of the detachment anticlines. The numerous segments that make up the mapped fault zones would be linked faults that formed within the isoclinal detachment anticlines to accommodate folding of the rheologically complex greenstones. The detachment fold model is compared to the results of analogue experiments designed to investigate crustal-scale folding, using viscous and frictional materials. Detachment folds are produced in the brittle upper crustal analogue on the limbs of folds formed in the ductile middle and lower crust analogues. The experimentally produced structures scale to the structures in the study area and indicate the detachment fold model for the southern Abitibi is mechanically viable.     

Discrete element model for crack propagation in brittle materials

We propose a discrete element model for brittle rupture. The material consists of a bidimensional set of closed‐packed particles in contact. We explore the isotropic elastic behavior of this regular structure to derive a rupture criterion compatible to continuum mechanics. We introduce a classical criterion of mixed mode crack propagation based on the value of the stress intensity factors, obtained by the analysis of two adjacent contacts near a crack tip. Hence, the toughness becomes a direct parameter of the model, without any calibration procedure. We verify the consistency of the formulation as well as its convergence by comparison with theoretical solutions of tensile cracks, a pre‐cracked beam, and an inclined crack under biaxial stress. Copyright © 2015 John Wiley & Sons, Ltd.     

Origin of fault domains and fault-domain boundaries (transfer zones and accommodation zones) in extensional provinces: Result of random nucleation and self-organized fault growth

In many extensional provinces, large normal faults dip in the same direction forming fault domains. Features variously named transfer faults, transfer zones, and accommodation zones (hereafter non-genetically referred to as fault-domain boundaries) separate adjacent fault domains. Experimental modeling of distributed extension provides insights on the origin, geometry, and evolution of these fault domains and fault-domain boundaries. In our scaled models, a homogeneous layer of wet clay or dry sand overlies a latex sheet that is stretched orthogonally or obliquely between two rigid sheets. Fault domains and fault-domain boundaries develop in all models in both map view and cross-section. The number, size, and arrangement of fault domains as well as the number and orientation of fault-domain boundaries are variable, even for models with identical boundary conditions. The fault-domain boundaries in our models differ profoundly from those in many published conceptual models of transfer/accommodation zones. In our models, fault-domain boundaries are broad zones of deformation (not discrete strike-slip or oblique-slip faults), their orientations are not systematically related to the extension direction, and they can form spontaneously without any prescribed pre-existing zones of weakness. We propose that fault domains develop because early-formed faults perturb the stress field, causing new nearby faults to dip in the same direction (self-organized growth). As extension continues, faults from adjacent fault domains propagate toward each another. Because opposite-dipping faults interfere with one another in the zone of overlap, the faults stop propagating. In this case, the geometry of the domain boundaries depends on the spatial arrangement of the earliest formed faults, a result of the random distribution of the largest flaws at which the faults nucleate.     

准噶尔盆地西北缘红3 井东侧断层构造几何学、运动学模型

红3井东侧断层的成因与中拐凸起的形成与演化密切相关,解析其构造几何学与运动学特征对于揭示中拐凸起自晚古生代以来的构造变形具有积极作用,并对于研究区的油气勘探具有重要意义。本文利用断层相关褶皱理论,不整合面识别及平衡剖面的方法,对红3井东侧断层进行精细的构造解析、建模与构造复原。中拐凸起发育白垩系-侏罗系、侏罗系-三叠系、三叠系-上乌尔禾组、上乌尔禾组底界及佳木河组-石炭系5个区域不整合面。研究认为,红3井东侧断层为一断层传播褶皱模型,发育生长三角,平面上分为北段、中段和南段。受海西晚期及燕山期的区域挤压作用的影响,红3井东侧断层在早二叠世末期（P₁f末期）至中二叠世晚期（P₂w）及早侏罗世晚期（J₁s）至晚侏罗世发生两期传播活动,并在中段发生分支,分支断层的断距较小。早期的断层活动在中段最强,南段其次,北段最弱。中-下二叠统被抬升,遭受大面积剥蚀,地层缩短量约为3.8 km。晚期的断层活动微弱,且只发生在中段和北段,地层缩短量约为200 m。红3井东侧断层自北东向南西方向的传播、推覆作用控制了中拐凸起的主体构造形态,整体具有“南北分段、早晚期构造叠加”的特征。     

Growth, linkage, and termination processes of a 10-km-long strike-slip fault in jointed granite: the Gemini fault zone, Sierra Nevada, California

Field-based structural analysis of an exhumed, 10-km-long strike-slip fault zone elucidates processes of growth, linkage, and termination along moderately sized strike-slip fault zones in granitic rocks. The Gemini fault zone is a 9.3-km-long, left-lateral fault system that was active at depths of 8–11 km within the transpressive Late-Cretaceous Sierran magmatic arc. The fault zone cuts four granitic plutons and is composed of three steeply dipping northeast- and southwest-striking noncoplanar segments that nucleated and grew along preexisting cooling joints. The fault core is bounded by subparallel fault planes that separate highly fractured epidote-, chlorite-, and quartz-breccias from undeformed protolith. The slip profile along the Gemini fault zone shows that the fault zone consists of three 2–3-km-long segments separated by two ‘zones’ of local slip minima. Slip is highest (131 m) on the western third of the fault zone and tapers to zero at the eastern termination. Slip vectors plunge shallowly west-southwest and show significant variability along strike and across segment boundaries. Four types of microstructures reflect compositional changes in protolith along strike and show that deformation was concentrated on narrow slip surfaces at, or below, greenschist facies conditions. Taken together, we interpret the fault zone to be a segmented, linked fault zone in which geometrical complexities of the faults and compositional variations of protolith and fault rock resulted in nonuniform slip orientations, complex fault-segment interactions, and asymmetric slip-distance profiles.     

郯庐断裂带临沂段地热水溶性氦气的分布特征及成因分析

绝大部分氦气矿藏赋存在油气矿藏中,对于地热井、温泉井中水溶性氦气的研究相对较少,郯庐断裂带（郯城—庐江断裂带）作为一条巨型走滑断裂,分布多处含氦气的地热井、温泉,而查明地热水中氦气的来源显得尤为重要。笔者等利用在郯庐断裂带临沂段地热井、温泉内氦气的样品分析结果,分析了郯庐断裂带临沂段地热井和温泉地下水中伴生气体氦含量、分布特征,探讨了地热水中氦气来源。结果显示：在郯庐断裂带临沂段地热水中首次发现8处明显氦含量高值点,以15×10-6作为氦含量的背景值,高值点明显高于背景值11. 1～1360. 0倍,主要分布在郯庐断裂带内及两侧;氦同位素显示氦气主要来源于地壳,少量来源于地幔,幔源氦含量最高为10. 75%,这不仅为郯庐断裂带深达地幔提供了证据,而且为地下热水的热源并非直接来源于地幔而是来源于深循环地温加热或放射性生热提供了依据。因此分析认为郯庐断裂带将地壳深部、基底岩体中U、Th等放射性元素衰变产生的大量气与热带到地表,运移过程中同时生成了部分氦气,从而形成了氦气资源、地热资源富集的特征。     

Fault growth and interactions in a multiphase rift fault network: Horda Platform,Norwegian North Sea

Physical models predict that multiphase rifts that experience a change in extension direction between stretching phases will typically develop non-colinear normal fault sets. Furthermore, multiphase rifts will display a greater frequency and range of styles of fault interactions than single-phase rifts. Although these physical models have yielded useful information on the evolution of fault networks in map view, the true 3D geometry of the faults and associated interactions are poorly understood. Here, we use an integrated 3D seismic reflection and borehole dataset to examine a range of fault interactions that occur in a natural multiphase fault network in the northern Horda Platform, northern North Sea. In particular we aim to: i) determine the range of styles of fault interaction that occur between non-colinear faults; ii) examine the typical geometries and throw patterns associated with each of these different styles; and iii) highlight the differences between single-phase and multiphase rift fault networks. Our study focuses on a ca. 350 km² region around the >60 km long, N–S-striking Tusse Fault, a normal fault system that was active in the Permian–Triassic and again in the Late Jurassic-to-Early Cretaceous. The Tusse Fault is one of a series of large (>1500 m throw) N–S-striking faults forming part of the northern Horda Platform fault network, which includes numerous smaller (2–10 km long), lower throw (<100 m), predominantly NW–SE-striking faults that were only active during the Late Jurassic to Early Cretaceous. We examine how the 2^nd-stage NW–SE-striking faults grew, interacted and linked with the N–S-striking Tusse Fault, documenting a range of interaction styles including mechanical and kinematic isolation, abutment, retardation and reactivated relays. Our results demonstrate that: i) isolated, and abutting interactions are the most common fault interaction styles in the northern Horda Platform; ii) pre-existing faults can act as sites of nucleation for 2^nd-stage faults or may form mechanical barriers to propagation; iii) the throw distribution on reactivated 1^st-stage faults will be modified in a predictable manner if they are intersected or influenced by 2^nd-stage faults; iv) sites of fault linkage and relay-breaching associated with the first phase of extension can act as preferential nucleation sites for 2^nd-stage faults; and v) the development of fault intersections is a dynamic process, involving the gradual transition from one style to another.     

滇西南2014年景谷中-强震群的地质构造成因——茶房-普文断裂带贯通过程的构造响应

2014年10—12月期间,云南景谷接连发生了Ms6.6、Ms5.8、Ms5.9三次中-强地震。为确定地震的地质构造成因,在地表调查的基础上,综合该区的地质构造情况、烈度与余震分布、震源机制解等资料,确定此次震群活动的宏观震中位于永平盆地东南侧山地,发震断层为地质与地貌表现不显著的NW向右旋走滑断层。此次震群活动及余震迁移过程指示,由于断层斜接部位岩桥的临时阻碍,Ms6.6地震破裂在向南东扩展过程中发生短暂停滞,突破障碍后进一步引发了Ms5.8和Ms5.9地震,这符合震源破裂沿NW向发震断裂分段破裂的行为。区域活动断裂的遥感解译结果发现,发震断层位置恰好处于NW向右旋走滑的茶房断裂与普文断裂之间,区域上属于该断裂带的不连贯部位,指示此次中-强震群活动应该是茶房-普文断裂带贯通过程的构造活动表现。结合思茅地块的历史地震资料发现,思茅地块地震活动多以小于等于6.8级为主,发震构造多为NW向断裂。指示在现今构造应力场作用下,该区NW向断裂的活动性相对NE向断裂更加显著,属于该区主要控震构造,应在今后的地震地质工作中给予更多关注。     

The nature and origin of off-fault damage surrounding strike-slip fault zones with a wide range of displacements: A field study from the Atacama fault system, northern Chile

Damage surrounding the core of faults is represented by deformation on a range of scales from microfracturing of the rock matrix to macroscopic fracture networks. The spatial distribution and geometric characterization of damage at various scales can help to predict fault growth processes, subsequent mechanics, bulk hydraulic and seismological properties of a fault zone. Within the excellently exposed Atacama fault system, northern Chile, micro- and macroscale fracture densities and orientation surrounding strike-slip faults with well-constrained displacements ranging over nearly 5 orders of magnitude (0.12 m–5000 m) have been analyzed. Faults have been studied that cut granodiorite and have been passively exhumed from 6 to 10 km depth. This allows direct comparison of the damage surrounding faults of different displacements. The faults consist of a fault core and associated damage zone. Macrofractures in the damage zone are predominantly shear fractures orientated at high angles to the faults studied. They have a reasonably well-defined exponential decrease with distance from the fault core. Microfractures are a combination of open, healed, partially healed and fluid inclusion planes (FIPs). FIPs are the earliest set of fractures and show an exponential decrease in fracture density with perpendicular distance from the fault core. Later microfractures do not show a clear relationship of microfracture density with perpendicular distance from the fault core. Damage zone widths defined by the density of FIPs scale with fault displacement but appear to reach a maximum at a few km displacement. One fault, where damage was characterized on both sides of the fault core shows no damage asymmetry. All faults appear to have a critical microfracture density at the fault core/damage zone boundary that is independent of displacement. An empirical relationship for microfracture density distribution with displacement is presented. Preferred FIP orientations have a high angle to the fault close to the fault core and become more diffuse with distance. Models that predict off-fault damage such as a migrating process zone during fault formation, wear from geometrical irregularities and dynamic rupture are all consistent with our data. We conclude it is very difficult to distinguish between them on the basis of field data alone, at least within the limits of this study.     

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.     

Microscopic calcite dendrites in cold-water tufa: implications for nucleation of micrite and cement

Dendritic calcite forms in an active cold-water tufa system in association with extracellular polymeric substances (EPS) that discontinuously coat bryophytes and cyanobacteria. Dendrites consist of 100–200 nm thick calcite fibres that form 3D lattice-like domains. In each dendrite domain, fibres have three structurally equal orientations, which correspond in disposition to radii from the centre of a calcite unit cell to the convex triple face junctions on its surface. Fibres do not form in the orientation of the c-axis. The external form of each dendrite has the shape of half of a shortened octahedron, with an upper triangular surface parallel to the substrate. Dendrite nucleation takes place on or in microbial EPS, whether microbial cells are present or not, and is probably effected by attraction of Ca²⁺ cations to negatively charged EPS, together with CO₂-degassing and concomitant pH increase of supersaturated spring water in stream splash zones. Ensuing dendrite growth is abiogenic and controlled by diffusion. Dendrite c-axes are perpendicular to the substrate, probably because the negative charge of EPS forces the orientation of Ca²⁺ and CO planes within the developing dendrite crystal to be parallel to the EPS film surface. Dendrites are eventually filled and overgrown by solid, syntaxial calcite, which gradually and completely obliterates the dendrites as more familiar calcite crystal forms develop. No trace of the dendritic nucleus remains in the rock record. Calcite crystal nucleation may take place by this mechanism in many marine and meteoric settings, given that microbial EPS is now assumed to be virtually ubiquitous in these environments. This phenomenon could contribute to the development of familiar fabrics such as marine micrite cement and fibrous calcite cement, radial ooids, peloids, ‘abiogenic’ stromatolites, sea floor precipitates, microbialites, tufa, travertine, speleothems, and some meteoric cements. It may also contribute to the substrate-normal orientation of c-axes of common cement fabrics.     

Antithetic fault linkages in a deep water fold and thrust belt

Deep water fold and thrust belts consist of both forethrusts and backthrusts that can link along strike to form continuous folds in the overburden. The interaction of faults of opposing dip are termed ‘antithetic thrust fault linkages’ and share the common feature of a switch in vergence of overlying hangingwall anticlines. Using three-dimensional seismic data, on the toe-of-slope of the Niger Delta, linkages are classified into three distinct structural styles. This preliminary classification is based on the vertical extent of faulting within a transfer zones relative to the branch line of the antithetic faults. The stratigraphic level of the lateral tip of the fault, the shape of lateral tip region of a fault plane and the stratal deformation within the transfer zones is also distinctive in each type of fault linkage. A Type 1 linkage comprises faults that overlap exclusively above the level of the branch line. A ‘pop-up’ structure forms within the transfer zone with sediments below remaining planar. The lower tip lines of faults climb stratigraphically towards the linkage zone creating asymmetric, upward-tapering lateral tip regions. In Type 2 linkages fault overlap occurs lower than the level of the branch line such that lateral fault tips are located within the footwall of the counterpart fault. Faulting is thus limited to the deeper section within the transfer zone and creates unfaulted, symmetric, bell-shaped folds in the overburden. Upper tip lines of faults lose elevation within the transfer zone creating asymmetric, downwards-tapering lateral tip regions. In Type 3 linkages both faults continue above and below the branch line within the transfer zone resulting in cross-cutting fault relationships. Horizon continuity across the folds, through the transfer zones, varies significantly with depth and with the type of fault intersection.