Geometric analysis and scaling relations of deformation bands in porous sandstone

Displacement, length and linkage of deformation bands have been studied in Jurassic sandstones in southeastern Utah. Isolated deformation bands with lengths (L) that span more than three orders of magnitude show similar displacement (D) profiles with more or less centrally located maxima and gently increasing gradient toward the tips. Soft- and hard-linked examples exhibit steeper displacement gradients near overlap zones and immature hard links, similar to previously described fault populations. The deformation band population shows power-law length and displacement distributions, but with lower exponents than commonly observed for populations of larger faults or small faults with distinct slip surfaces. Similarly, the D_max-L relationship of the deformation bands shows a well-defined exponent of ca 0.5, whereas the general disagreement for other fault populations is whether the exponent is 1 or 1.5. We suggest that this important difference in scaling law between deformation bands and other faults has to do with the lack of well-developed slip surfaces in deformation bands. During growth, deformation bands link to form zones of densely spaced bands, and a slip surface is eventually formed (when 100 m < L < 1 km). The growth and scaling relationship for the resulting populations of faults (slip surfaces) is expected to be similar to ‘ordinary’ fault populations. A change in the D_max-L scaling relationship at the point when zones of deformation bands develop slip surfaces is expected to be a general feature in porous sandstones where faults with slip surfaces develop from deformation bands. Down-scaling of ordinary fault populations into the size domain of deformation bands in porous sandstones is therefore potentially dangerous.     

Spatial variations in damage zone width along strike-slip faults: An example from active faults in southwest Japan

Field investigations reveal spatial variations in fault zone width along strike-slip active faults of the Arima–Takatsuki Tectonic Line (ATTL) and the Rokko–Awaji Fault Zone (RAFZ) of southwest Japan, which together form a left-stepping geometric pattern. The fault zones are composed of damage zones dominated by fractured host rocks, non-foliated and foliated cataclasites, and a fault core zone that consists of cataclastic rocks including fault gouge and fault breccia. The fault damage zones of the ATTL are characterized by subsidiary faults and fractures that are asymmetrically developed on each side of the main fault. The width of the damage zone varies along faults developed within granitic rocks of the ATTL and RAFZ, from ∼50 to ∼1000 m. In contrast, the width of the damage zone within rhyolitic tuff on the northwestern side of the ATTL varies from ∼30 to ∼100 m. The fault core zone is generally concentrated in a narrow zone of ∼0.5–∼5 m in width, consisting mainly of pulverized cataclastic rocks that lack the primary cohesion of the host rocks, including a narrow zone of fault gouge (<0.5 m) and fault-breccia zones either side of the fault. The present results indicate that spatial variations in the width of damage zone and the asymmetric distribution of damage zones across the studied strike-slip faults are mainly caused by local concentrations in compressive stress within an overstep area between left-stepping strike-slip faults of the ATTL and RAFZ. The findings demonstrate that fault zone structures and the spatial distribution in the width of damage zone are strongly affected by the geometric patterns of strike-slip faults.     

Fault damage zone origin of the Teufelsmauer, Subhercynian Cretaceous Basin, Germany

Recognition of fractures as porosity-reducing deformation bands pervading all sandstone segments of the Teufelsmauer, Subhercynian Creatceous Basin, Germany, motivates a study relating the observed macroscopic and microscopic deformation to the damage zone of the nearby Harz border fault. Deformation bands, confirmed and documented by several porosity-reducing micro-mechanisms, such as cataclasis, particulate flow, pressure solution and a heavy quartz cementation, were mapped and analyzed in terms of the kinematics and deformation intensities expressed by them. Deformation band kinematics are uniform throughout the entire basin and consistent with the large-scale tectonic structures of the area. A strain intensity study highlights two narrow but long zones of deformation bands, sub-parallel to the Harz border fault. Deformation band kinematics, strain intensity, as well as micro-mechanisms are all consistent with a continuous but internally diverse deformation band damage zone of the entire Teufelsmauer structure along the Harz border fault, bringing new insights into the tectonic evolution and the origin of the heavy quartz cementation of the sandstones in the Subhercynian Cretaceous Basin.     

Damage zone and slip-surface evolution over μm to km scales in high-porosity Navajo sandstone, Utah

Detailed mapping of throw variations and deformation along two km-scale normal faults in the high-porosity Navajo sandstone, Utah, has been used to investigate fault growth in this lithology. The faults consist of one or more through-going, striated, slip-surfaces, accommodating the greater part of the offset surrounded by a damage zone consisting of deformation band clusters and short, unconnected slip-surfaces. In contrast to previous models for deformation in this lithology, we find that the nucleation of slip-surfaces begins where measurable throw is negligible and deformation bands are forming and increasing in number. The microstructure and porosity of deformation bands and slip surfaces are distinct and independent of the amount of offset that they accommodate, i.e. they represent different and yet contemporaneous deformation mechanisms. The point where measurable throw begins to accumulate (the fault tip) is marked by the first through-going connected slip-surface. Increase in throw towards the centre of the fault results in a three-dimensional strain field, producing orthorhombic structural geometries within the damage zone. We find that the total width of the damage zone increases as offset is accumulated. For these faults, the damage zone width is approximately 2.5 times the total fault throw.     

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.     

Structure and flow properties of syn-rift border faults: The interplay between fault damage and fault-related chemical alteration (Dombjerg Fault,Wollaston Forland,NE Greenland)

Structurally controlled, syn-rift, clastic depocentres are of economic interest as hydrocarbon reservoirs; understanding the structure of their bounding faults is of great relevance, e.g. in the assessment of fault-controlled hydrocarbon retention potential. Here we investigate the structure of the Dombjerg Fault Zone (Wollaston Forland, NE Greenland), a syn-rift border fault that juxtaposes syn-rift deep-water hanging-wall clastics against a footwall of crystalline basement. A series of discrete fault strands characterize the central fault zone, where discrete slip surfaces, fault rock assemblages and extreme fracturing are common. A chemical alteration zone (CAZ) of fault-related calcite cementation envelops the fault and places strong controls on the style of deformation, particularly in the hanging-wall. The hanging-wall damage zone includes faults, joints, veins and, outside the CAZ, disaggregation deformation bands. Footwall deformation includes faults, joints and veins. Our observations suggest that the CAZ formed during early-stage fault slip and imparted a mechanical control on later fault-related deformation. This study thus gives new insights to the structure of an exposed basin-bounding fault and highlights a spatiotemporal interplay between fault damage and chemical alteration, the latter of which is often underreported in fault studies. To better elucidate the structure, evolution and flow properties of faults (outcrop or subsurface), both fault damage and fault-related chemical alteration must be considered.     

碳酸盐岩断裂相分类特征——以新疆塔里木盆地柯坪露头为例

断裂相的概念为断裂带的内部结构研究提供了新的思路与建模方法,通过塔里木盆地柯坪露头断裂带的分析,碳酸盐岩断裂相特征有别于碎屑岩.柯坪露头碳酸盐岩断裂带不连续构造以滑动面、裂缝带和变形带发育为特征.根据形态识别出平直截切型、弯曲起伏型、渐变条带型三种类型滑动面.破碎带中裂缝带发育,裂缝充填少,是良好输导通道;断层核部存在...     

Permeability upscaling of fault zones in the Aztec Sandstone,Valley of Fire State Park,Nevada, with a focus on slip surfaces and slip bands

The effect of open and filled slip surfaces on the upscaled permeability of two fault zones with 6 and 14 m strike-slip in an eolian Aztec Sandstone, Nevada, USA is evaluated. Each fault zone is composed of several fault components: a fault core, bounded by filled through-going slip surfaces referred to as slip bands, and a surrounding damage zone that contains joints and deformation bands. Slip band geometry, composition, and petrophysical properties are characterized. Measurements and modeling show that slip band permeabilities can vary over 12 orders of magnitude depending on the degree of fill within the slip bands. The slip bands along with other fault zone components are represented in finite volume numerical calculations and the impact of various slip-band representations on upscaled fault zone permeability is tested. The results show 2 orders of magnitude variation in upscaled fault zone permeability in the fault-normal direction and a factor of 2 variation in the fault-parallel direction. The numerical results presented here are compared to the earlier numerical results in which structured Cartesian grids were used for the numerical simulations, and are in qualitative agreement with earlier calculations but use about a factor of 250–400 fewer numerical cells.     

Relations between shallow cataclastic faulting and cementation in porous sandstones: First insight from a groundwater environmental context

The interplay between fault zone cataclasis and cementation is important since both processes can drastically reduce the permeability of faults in porous sandstones. Yet the prediction of fault cementation in high-porosity sandstone reservoirs remains elusive. Nevertheless, this process has rarely been investigated in shallowly buried faults (<2 km; T°<80 °C) where its sealing capacity could be acquired early in the geological history of a reservoir. In this paper, the macro- and microscopic analysis of a fault zone in the porous Cenomanian quartz arenite sands of Provence (France) shows that silica diagenesis occurs in the most intensely-deformed cataclastic parts of the fault zone. This fault zone shows 19–48% of its total thickness occupied by low-porosity quartz-cemented cataclastic shear bands whose porosities range from 0 – ca. 5%. The analysis of the weathering profile around the fault zone reveals the presence of groundwater silcretes in the form of tabular, tightly silicified concretions cross-cut by the fault. Detailed transmitted light, cold-cathodoluminescence and scanning electron microscopy analyses of the silica cements (from the fault and the silcrete) reveal that all the silica cements originate from groundwater diagenetic processes. This study therefore shows that silica cementation can occur specifically in fault zones and as groundwater silcrete in the shallow context of a groundwater system, generated at the vicinity of an erosional unconformity.     

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.     

Analysis of faulting in porous sandstones

Faults in porous sandstones occur in three forms: deformation bands about 1-mm thick and tens of m long and across which offsets are a few mm; zones of deformation bands constituted of many closely spaced deformation bands across which offsets are a few cm or dm; and slip surfaces, that is, distinct surfaces within zones of deformation bands across which offsets are a few m to a few tens of m. Deformation bands represent highly localized deformation; analogous localization within a field of homogeneous deformation is theoretically possible in inelastic materials with certain ranges of constitutive parameters. Crushing and consolidation of sandstone within a band cause the material there to become stiffer than the surrounding porous sandstone. A zone of deformation bands behaves mesoscopically much as a stiff inclusion in a soft matrix. According to the constitutive model assumed to investigate the formation of deformation bands, an instability can develop, and strain increments within the zone of deformation bands can become boundlessly large when the far-field stresses reach critical values. This instability is here associated with the formation of slip surfaces.     

Polygonal deformation bands

We report for the first time the occurrence of polygonal faults in sandstone, which is compelling given that layer-bound polygonal fault systems have been observed so far only in fine-grained sediments such as clay and chalk. The polygonal faults are shear deformation bands that developed under shallow burial conditions via strain hardening in dm-wide zones. The edges of the polygons are 1–5 m long. The shear deformation bands are organized as conjugate faults along each edge of the polygon and form characteristic horst-like structures. The individual deformation bands have slip magnitudes ranging from a few mm to 1.5 cm; the cumulative average slip magnitude in a zone is up to 10 cm. The deformation bands heaves, in aggregate form, accommodate a small isotropic horizontal extension (strain <0.005). The individual shear deformation bands show abutting T-junctions, veering, curving, and merging where they mechanically interact. Crosscutting relationships are rare. The interactions of the deformation bands are similar to those of mode I opening fractures. The documented fault networks have important implications for evaluating the geometry of km-scale polygonal fault systems in the subsurface, top seal integrity, as well as constraining paleo-tectonic stress regimes.     

碳酸盐岩断层破碎带构造成岩作用——以塔中Ⅰ号断裂带为例

多旋回叠合盆地断层具有异常复杂的构造变形与成岩演变,跨学科结合的构造成岩作用研究为断层破碎带演变机制与流体-岩石作用分析提供了新思路。结合塔中Ⅰ号断裂带构造解析和地化资料分析,探讨古老碳酸盐岩断层破碎带构造成岩作用的特殊性。结果表明,塔中Ⅰ号断裂带上奥陶统台缘带碳酸盐岩断层破碎带发育多类、多期构造成岩作用,明显不同于围岩。断层破碎带宽度高达2~4km,裂缝类型多样、纵横向变化大、开启程度较高。沿断裂带压实作用较弱且有差异,发现有碳酸盐岩压实变形带发育,并有后期裂缝与溶蚀作用形成的局部高渗透带。断层破碎带观测到多期压溶低角度缝合线与高角度构造缝合线,部分具有渗透性。多达4~5期不同类型胶结作用沿裂缝带发育,大气淡水胶结较多,胶结程度相对较弱。准同生期大气淡水溶蚀与局部风化壳岩溶主要沿断层破碎带发育,而且埋藏溶蚀作用较强,发育大型缝洞体与溶蚀孔洞。构造成岩作用揭示塔中Ⅰ号断裂带东段上奥陶统碳酸盐岩断层破碎带经历断裂萌芽期-形成期-定型期-复活扩张期-局部开启期等5期演变,不同演化期构造成岩作用有明显差异。构造成岩作用的多期差异活动是造成古老碳酸盐岩断层破碎带复杂性的重要因素。     

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.     

Smoothing and re-roughening processes: The geometric evolution of a single fault zone

The geometry of a fault zone exerts a major control on earthquake rupture processes and source parameters. Observations previously compiled from multiple faults suggest that fault surface shape evolves with displacement, but the specific processes driving the evolution of fault geometry within a single fault zone are not well understood. Here, we characterize the deformation history and geometry of an extraordinarily well-exposed fault using maps of cross-sectional exposures constructed with the Structure from Motion photogrammetric method. The La Quinta Fault, located in southern California, experienced at least three phases of deformation. Multiple layers of ultracataclasite formed during the most recent phase. Crosscutting relations between the layers define the evolution of the structures and demonstrate that new layers formed successively during the deformation history. Wear processes such as grain plucking from one layer into a younger layer and truncation of asperities at layer edges indicate that the layers were slip zones and the contacts between them slip surfaces. Slip surfaces that were not reactivated or modified after they were abandoned exhibit self-affine geometry, preserving the fault roughness from different stages of faulting. Roughness varies little between surfaces, except the last slip zone to form in the fault, which is the smoothest. This layer contains a distinct mineral assemblage, indicating that the composition of the fault rock exerts a control on roughness. In contrast, the similar roughness of the older slip zones, which have comparable mineralogy but clearly crosscut one another, suggests that as the fault matured the roughness of the active slip surface stayed approximately constant. Wear processes affected these layers, so for roughness to stay constant the roughening and smoothing effects of fault slip must have been approximately balanced. These observations suggest fault surface evolution occurs by nucleation of new surfaces and wear by competing smoothing and re-roughening processes.     

Microstructures in the Cretaceous Bima Sandstone,Upper Benue Trough,N.E. Nigeria: Implication for hydrocarbon migration

Faulting related to movements along major fault zones in the Upper Benue Trough during Albian times, with evidence of deformation in the Cretaceous Bima Sandstone are common especially around the Kaltungo, Gombe, Zambuk and Teli lineaments. Conjugate extensional systems of deformation bands show increased siliceous cementation of the sandstones adjacent to these lineaments. During the Late Cretaceous compressional event, the deformation bands and faults in the Upper Benue Trough were reactivated, resulting into dilational opening of fractures believed to have acted as fluid conduits and/or barriers. These deformation bands which decrease in density away from the major faults are characterized with increasing porosity and permeability in the host sandstone abruptly away from the tectonic barrier. It is proposed here that the master faults of the Benue Trough, linking it with the Anambra Basin and the Niger Delta probably served as conduits for the migration of hydrocarbons into the Cretaceous reservoirs of the Upper Benue Trough and by extension into the Niger Delta.     

Finite-element 3D modeling of stress patterns around a dipping fault

Stresses in a block around a dipping fracture simulating a damage zone of a fault are reconstructed by finite-element modeling. A fracture corresponding to a fault of different lengths, with its plane dipping at different angles, is assumed to follow a lithological interface and to experience either compression or shear. The stress associated with the destruction shows an asymmetrical pattern with different distances from the highest stress sites to the fault plane in the hanging and foot walls. As the dip angle decreases,the high-stress zone becomes wider in the hanging wall but its width changes negligibly in the foot wall.The length of the simulated fault and the deformation type affect only the magnitude of maximum stress,which remains asymmetrical relative to the fault plane. The Lh/Lfratio, where Lhand Lfare the widths of high-stress zones in the hanging and foot walls of the fault, respectively, is inversely proportional to the fault plane dip. The arithmetic mean of this ratio over different fault lengths in fractures subject to compression changes from 0.29 at a dip of 80°to 1.67 at 30°. In the case of shift displacement, ratios are increasing to 1.2 and 2.94, respectively.Usually they consider vertical fault planes and symmetry in a damage zone of faults. Following that assumption may cause errors in reconstructions of stress and fault patterns in areas of complex structural setting. According geological data, we know the structures are different and asymmetric in hanging and foot walls of fault. Thus, it is important to quantify zones of that asymmetry. The modeling results have to be taken into account in studies of natural faults, especially for practical applications in seismic risk mapping, engineering geology, hydrogeology, and tectonics.     

塔里木盆地走滑带碳酸盐岩断裂相特征及其与油气关系

通过露头与井下资料的综合分析,塔里木盆地奥陶系碳酸盐岩走滑断裂带断裂相具有多样性,根据内部构造发育程度可以分为断层核发育、断层核欠发育两类。露头走滑带断层核部以裂缝带、透镜体、滑动面等断裂相发育为特征,断裂边缘的破碎带发育裂缝带、变形带。裂缝带主要分布在断层核附近50m的破碎带内,裂缝多开启,渗流性好。断裂核部透镜体发育,在破碎带也有分布,破碎角砾组合的透镜体多致密。滑动面具有平直截切型、渐变条带型等两种类型,多为开启的半充填活动面。变形带多为方解石与碎裂岩充填,破碎带局部部位裂缝与溶蚀作用较发育。利用地震剖面、构造图、相干图等资料可以判识塔里木盆地内部奥陶系碳酸盐岩走滑断裂相的特征及其发育程度,沿走滑断裂带走向上断裂相具有分段性与差异性,根据渗流性可以定性区分高渗透相、致密相区。沿断裂带高渗透相区是碳酸盐岩缝洞体储层发育的有利部位。断裂相的横向变化造成油气分布的区段性,形成高渗透相输导模式、致密相遮挡模式等两类成藏模式。走滑断裂带碳酸盐岩断裂相的特征及其控藏作用对油气勘探开发储层建模具有重要意义。