Evidence and mechanisms for Appalachian Basin brine migration into shallow aquifers in NE Pennsylvania,USA

Multiple geographic information system (GIS) datasets, including joint orientations from nine bedrock outcrops, inferred faults, topographic lineaments, geophysical data (e.g. regional gravity, magnetic and stress field), 290 pre-gas-drilling groundwater samples (Cl–Br data) and Appalachian Basin brine (ABB) Cl–Br data, have been integrated to assess pre-gas-drilling salinization sources throughout Susquehanna County, Pennsylvania (USA), a focus area of Marcellus Shale gas development. ABB has migrated naturally and preferentially to shallow aquifers along an inferred normal fault and certain topographic lineaments generally trending NNE–SSW, sub-parallel with the maximum regional horizontal compressive stress field (orientated NE–SW). Gravity and magnetic data provide supporting evidence for the inferred faults and for structural control of the topographic lineaments with dominant ABB shallow groundwater signatures. Significant permeability at depth, imparted by the geologic structures and their orientation to the regional stress field, likely facilitates vertical migration of ABB fluids from depth. ABB is known to currently exist within Ordovician through Devonian stratigraphic units, but likely originates from Upper Silurian strata, suggesting significant migration through geologic time, both vertically and laterally. The natural presence of ABB-impacted shallow groundwater has important implications for differentiating gas-drilling-derived brine contamination, in addition to exposing potential vertical migration pathways for gas-drilling impacts.     

Effects of fracture lineaments and in-situ rock stresses on groundwater flow in hard rocks: a case study from Sunnfjord, western Norway

Systematic field mapping of fracture lineaments observed on aerial photographs shows that almost all of these structures are positively correlated with zones of high macroscopic and mesoscopic fracture frequencies compared with the surroundings. The lineaments are subdivided into zones with different characteristics: (1) a central zone with fault rocks, high fracture frequency and connectivity but commonly with mineral sealed fractures, and (2) a damage zone divided into a proximal zone with a high fracture frequency of lineament parallel, non-mineralized and interconnected fractures, grading into a distal zone with lower fracture frequencies and which is transitional to the surrounding areas with general background fracturing. To examine the possible relations between lineament architecture and in-situ rock stress on groundwater flow, the geological fieldwork was followed up by in-situ stress measurements and test boreholes at selected sites. Geophysical well logging added valuable information about fracture distribution and fracture flow at depths. Based on the studies of in-situ stresses as well as the lineaments and associated fracture systems presented above, two working hypotheses for groundwater flow were formulated: (i) In areas with a general background fracturing and in the distal zone of lineaments, groundwater flow will mainly occur along fractures parallel with the largest in-situ rock stress, unless fractures are critically loaded or reactivated as shear fractures at angles around 30° to σ_H; (ii) In the influence area of lineaments, the largest potential for groundwater abstraction is in the proximal zone, where there is a high fracture frequency and connectivity with negligible fracture fillings. The testing of the two hypotheses does not give a clear and unequivocal answer in support of the two assumptions about groundwater flow in the study area. But most of the observed data are in agreement with the predictions from the models, and can be explained by the action of the present stress field on pre-existing fractures.     

Fracture and stress orientation from borehole image logs: A case study from Cambay basin,India

The Deccan trap basalt, laid down by multiple lava flows during upper Cretaceous to Paleocene times forms the basement of current study in Cambay basin. As such, there is great interest and value in fracture detection and evaluation of fractured basement reservoirs in the Cambay basin. The procedure for identification and evaluation of natural as well as induced fractures in basaltic basement of the Cambay basin is presented in this work. In this study formation micro-imager (FMI) and extended range micro-imager (XRMI) log data for fracture identification is used. The Deccan trap basaltic basement of the study area, comprising five wells in the Tarapur-Cambay block, has potential for holding commercial hydrocarbon due to the presence of fractures and weathered basement. Both image logs (FMI, XRMI) identify three types of fracture including open (conductive), partially open and closed (resistive) fractures, of which open and partially open fractures are important for hydrocarbon accumulation. Fracture dip ranges from 10° to 80°. Image logs have also identified washout, breakout and drilling-induced fracture zones. The strike direction of the open natural fractures for four wells varies from N60°E to N30°E whereas the strike direction of most natural fracture in the fifth well is oriented towards N20°W. The orientations of drilling-induced fractures and breakouts may be interpreted for the in-situ stress direction over the logged interval. Drilling-induced tensile fractures, identified over the depth interval of 1969–1972 m, and borehole breakouts over the interval of 1953–1955 m in one well, suggest an orientation of maximum in-situ horizontal compressive stress (S_H) lies in the north-south direction. The azimuths of open natural fractures in the same well vary from north-south to N30°E. It is expected that the direction of fluid flow will be controlled by open natural fractures and therefore would be in a direction parallel to the S_H direction, which is orthogonal to the minimum horizontal stress (S_h) direction. The orientations observed are consistent with the present day S_H direction in the study area of Cambay basin.     

Determination of in-situ stress direction from cleat orientation mapping for coal bed methane exploration in south-eastern part of Jharia coalfield, India

Accurate prediction of in-situ stress directions plays a key role in any Coal Bed Methane (CBM) exploration and exploitation project in order to estimate the production potential of the CBM reservoirs. Permeability is one of the most important factors for determination of CBM productivity. The coal seams in Jharia coalfield generally show low permeability in the range of 0.5 md to 3 md. To estimate the in-situ stress direction in the study area, an attempt has been made to undertake the cleat orientation mapping of four regional coal seams of two underground coal mines located at south-eastern part of Jharia coalfield, India. Cleat orientation mapping is critical to determine the maximum principal compressive horizontal stress (S_H) direction for CBM exploration and exploitation, which in turn controls the direction of maximum gas or water flow though coal beds. From the field study it is found that the average face and butt cleat azimuths are towards N15°W and N75°E respectively. Average permeability of the four above-mentioned major coal seams has been calculated from well logs of nine CBM wells distributing over an area of 7.5 km², adjacent to the underground mines. The cleat orientations are congruous with the regional lineament pattern and fits well with the average permeability contour map of the study area to infer the orientation of in-situ maximum horizontal stress. Goodness of fit for the exponential regressions between vertical stress and permeability for individual coal seams varies between 0.6 and 0.84. The cleat orientation is further validated from the previous fracture analysis using FMI well log in Parbatpur area located southern part of the Jharia coalfield. The major coal seams under the study area exhibit directional permeability, with the maximum permeability, oriented parallel to the direction of face cleat orientation.     

基于连续-非连续方法的裂隙破坏与相互作用研究

岩体内裂隙等非连续结构面对岩体的强度及变形等力学特性有着显著的影响,研究岩体裂隙起裂、扩展、相互作用和贯通机制,对工程岩体力学行为的表征和工程性能的评价十分重要。本文基于连续介质力学模型的离散元方法,通过考虑裂隙分布、模型加载条件及其与裂隙产状的关系,建立了一系列裂隙力学计算模型,研究了不同模型裂隙扩展演化特征和岩体破裂机制,分析了岩体裂隙扩展规律及其对岩体破坏路径和强度的影响,研究结果表明:（1）裂隙岩体模型加载条件下的破坏起裂点、最终贯通破坏特征及损伤分布受控于裂隙的产状及其与最大主压应力取向角度大小及围压大小。（2）裂隙弱面走向与最大主压应力取向斜交时,裂隙弱面在加载条件下其端部裂隙扩展、贯通破坏表现比较明显,反之,当裂隙弱面走向与最大主压应力取向一致时,裂隙弱面被动影响裂隙模型内新生裂隙的萌生、扩展和贯通模式,自身未出现新的扩展破坏。（3）裂隙数目的增多和围压的增大会显著增加模型内部剪切裂缝的数量和模型破坏后的破碎程度,模型内部的损伤区域主要围绕破裂面呈滑移线型交叉分布,非破裂面区域损伤呈条带状X型分布。（4）裂隙弱面走向与最大主压应力取向斜交时,裂隙对岩体模型强度的弱化程度高于裂隙弱面走向与最大主压应力取向一致的情况,而裂隙模型破坏后的残余强度则正好相反。     

上海地区浅部地应力测量及其构造地质意义分析

通过水压致裂法与超声波成像测井法相结合的方法,对上海地区浅部（180 m深度范围）原位地应力进行了测量。测量结果表明:最大水平主应力值在9.54~12.91 MPa之间,最小水平主应力值在5.41~6.96 MPa之间,最大水平主应力方向为北西42°-62°,优势方位为北西,地应力结构为S_H > S_h > S_v,可以反映区域构造应力场特征。依据区域内断裂空间分布特征和现今实测地应力结果综合分析认为,北东向断裂易于发生压性或压扭性逆断活动,断裂相对稳定;而北西向断裂易于发生张性或张扭性正断活动,在现今相对较高的应力水平状态下仍值得关注。测量结果可能对于揭示区域构造界线（江山-绍兴断裂）的走向有一定启示。      

Analogue modeling of overlapping spreading centers: insights into their propagation and coalescence

The propagation and segmentation of mid-ocean ridges is studied using centrifuged analogue models built with non-linear materials. The deformation of the brittle-ductile model is controlled by diapiric uprise of buoyant analogous asthenospheric material induced by a centrifugal body force. This linear upwelling laterally stretches the model mantle that, in turn, induces failure in the upper layer simulating the brittle crust. Arrays of fractures initiate within zones of high stress concentration above the diapir. Fractures propagate laterally in a direction perpendicular to the maximum tensile (the minimum principal) stress. Secondary tension cracks initiate in the vicinity of parent fracture tips. Through-going fractures that crosscut the model surface develop by short fractures propagating toward each other and coalescing in different types of patterns. The overlap, overstep and inclination of fractures developed at the initial stage of extension control their subsequent growth and coalescence. Non-overlapping sub-parallel fractures propagate along nearly straight paths and coalesce to produce a single planar fracture. If overlapping fractures are parallel, they propagate towards each other along curved paths that enclose an intervening elliptical core of intact material. Fracture curvature in this case results from crack–crack interaction and is similar to that of overlapping spreading centers (OSCs) observed along mid-ocean ridges. Overlapping non-parallel fractures tend to coalesce by one of their tips propagating sub-parallel to the spreading direction toward the other fracture. Such offsets can serve as models for the development of the orthogonal ridge-transform fault patterns common along mid-ocean ridges.     

Extraction and analysis of geological lineaments of Kolli hills,Tamil Nadu: a study using remote sensing and GIS

The aim of the present study is to investigate the lineaments of Kolli hills of Tamil Nadu State for which CARTOSAT-1 satellite’s DEM output has been made use of. The extracted lineaments were analysed using ArcGIS and Rockworks software. The total number and length of lineaments are 523 and 943.81 km, respectively. Shorter lineaments constitute about 3/4th of the total number of lineaments. The density of the lineaments varies from 0 to 7.41 km/km², and areas of very high to high density are restricted to the south central, central and north eastern parts, and these areas reflect the high degree of rock fracturing and shearing which makes these areas unsuitable for the construction of dams and reservoirs. However, these areas could be targeted for groundwater exploitation as they possess higher groundwater potential. The lineaments are oriented in diverse directions. However, those orienting in ENEWSW, NE-SW and NW-SE are predominating followed by those oriented in sub E-W and sub N-S directions. These orientations corroborate with results of previous regional studies and with orientations of prominent geological structures and features of the study area. Distinct variation in the predominant orientations of lineaments of varied sizes is observed, while the shorter ones are oriented in either NW-SE or NNW-SSE directions, the longer ones are oriented in either NE-SW or ENE-WSW. A comparative analysis of lineament datasets of the eight azimuth angles and the final lineament map underlines the need to extract lineaments from various azimuth angles to get a reliable picture about the lineaments.     

Conjugate faulting along pre-existing fractures in Bundelkhand granite, Hirapur, central India

Re-examination of the outcrop of conjugate of strike-slip faults mapped by Roday et al. (1989) near forest rest house at Hirapur reveals that the main dextral strike-slip fault that strikes N35°E and is a manifestation of the earliest NE-SW trending subhorizontal σ¹ that produced extensional reef system in the Bundelkhand massif. Although the change in the stress system though 90° rotation of the principal compressive stress σ₁ and σ₃ (with σ₂ maintaining near vertically) is correct, another point of interest is that the σ₁ for the system of faults bisects the obtuse angle between the two sets and not an acute one as required by the brittle failure criterion. The sinistral strike-slip faults were probably formed by rejuvenation of the initial dextral strike-slip faults that were generated when the maximum principal compressive stress was oriented NS. The reversal of fault displacement is seen on all scales in the Bundelkhand massif. The dextral strike-slip fault related to the late stress system was preferentially produced along pre-existing tensile fractures that were generated under NE-SW directed subhorizontal σ₁. Some of these fractures were converted into sinistral strike-slip faults under NS directed maximum principal compression acting subhorizontally.     

Effects of in-situ stress and joint on permeability of the coal bed in Linfen block,southeastern Ordos Basin,China

Effects of in-situ stress and joint on permeability of the coal bed depend on orientation relationships between in-situ stress and joint. In-situ stress orientations of Linfen block of southeastern Ordos Basin were determined by systematical measurements of the loess joints. In-situ stress magnitudes were calculated based on well logging and hydraulic fracturing data. Joint orientations of the No. 5 coal bed and density distributions of the overlying sandstone bed were investigated. The results show that the NE-oriented maximum horizontal principal stress, approximately parallel to the predominant joint orientation of the coal bed, is favorable to the openness of the predominant joints. The minimum horizontal principal stress, with its orientation perpendicular to the predominant joint orientation, tends to induce the closure of the predominant joints. The stress state of No. 5 coal bed is o_v > 0_H > O_h, which is favorable for the openness of subvertical joints and an increase in the permeability. The permeability of No. 5 coal bed increases obviously with increasing effective vertical stress and effective horizontal maximum stress and decreases with increasing effective horizontal minimum stress. Besides, the permeability of No. 5 coal bed exhibits a good exponential relationship with the density of predominant joint set. Therefore, predominant joints with the orientation of 45° mainly contribute to increases in the permeability of the No. 5 coal bed in Linfen block. Multidimensional analysis show good exponential relationships among the permeability, predominant joint density, and effective stress of the No. 5 coal bed.     

In situ stresses and natural fractures in the Northern Perth Basin,Australia

Present-day stress orientations in the Northern Perth Basin have been inferred from borehole breakouts and drilling-induced tensile fractures observed on image logs from eight wells. Stress indicators from these wells give an east – west maximum horizontal stress orientation, consistent with stress-field modelling of the Indo-Australian Plate. Previous interpretations using dipmeter logs indicated anomalous north-directed maximum horizontal stress orientations. However, higher-quality image logs indicate a consistent maximum horizontal stress orientation, perpendicular to dominant north – south and northwest – southeast fault trends in the basin. Vertical stress was calculated from density logs at 21.5 MPa at 1 km depth. Minimum horizontal stress values, estimated from leak-off tests, range from 7.4 MPa at 0.4 km to 21.0 MPa at 0.8 km depth: the greatest values are in excess of the vertical stress. The maximum horizontal stress magnitude was constrained using the relationship between the minimum and maximum horizontal stresses; it ranges from 8.7 MPa at 0.4 km to 21.3 MPa at 1 km depth. These stress magnitudes and evidence of neotectonic reverse faulting indicate a transitional reverse fault to strike-slip fault-stress regime. Two natural fracture sets were interpreted from image logs: (i) a north- to northwest-striking set; and (ii) an east-striking set. The first set is parallel to adjacent north- to northwest-striking faults in the Northern Perth Basin. Several east-striking faults are evident in seismic data, and wells adjacent to east-striking faults exhibit the second east-striking set. Hence, natural fractures are subparallel to seismically resolved faults. Fractures optimally oriented to be critically stressed in the present-day stress regime were probably the cause of fluid losses during drilling. Pre-existing north- to northwest -striking faults that dip moderately have potential for reactivation within the present-day stress regime. Faults that strike north to northwest and have subvertical dips will not reactivate. The east-striking faults and fractures are not critically stressed for reactivation in the Northern Perth Basin.     

The importance of the effective intermediate principal stress (σ′2) to fault slip patterns

In normal faulting regimes, the magnitudes and orientations of the maximum and minimum principal compressive stresses may be known with some confidence. However, the magnitude of the intermediate principal compressive stress is generally much more difficult to constrain and is often not considered to be an important factor. In this paper, we show that the slip characteristics of faults and fractures with complex or nonoptimal geometry are highly sensitive to variation or uncertainty in the ambient effective intermediate principal stress (σ′₂). Optimally oriented faults and fractures may be less sensitive to such variations or uncertainties. Slip tendency (T_s) analysis provides a basis for quantifying the effects of uncertainty in the magnitudes and orientations of all principal stresses and in any stress regime, thereby focusing efforts on the most important components of the system. We also show, for a normal faulting stress regime, that the proportion of potential surfaces experiencing high slip tendency (e.g., T_s ≥ 0.6) decreases from a maximum of about 38% where σ′₂ = σ′₃, to a minimum of approximately 14% where σ′₂ is halfway between σ′₃ and σ′₁, and increases to another high of approximately 29% where σ′₂ = σ′₁. This analysis illustrates the influence of the magnitude of σ′₂ on rock mass strength, an observation previously documented by experimental rock deformation studies. Because of the link between fault and fracture slip characteristics and transmissivity in critically stressed rock, this analysis can provide new insights into stress-controlled fault transmissivity.     

The Impact of Biased Sampling on the Estimation of the Semivariogram Within Fractured Media Containing Multiple Fracture Sets

Monte Carlo simulation was used to examine the error (statistical bias) introduced in estimating a sample semivariogram through application of oriented sampling patterns to variables which are correlated with fracture orientation. Sample semivariograms of the directional components of the water velocity were used to illustrate that oriented sampling schemes can provide biased data sets which result in error in the estimation of the semivariogram, particularly in the estimation of the sill (or variance). Three sampling patterns were used to analyze directional semivariograms of the components of the fluid velocity: sampling along lines parallel to the mean regional hydraulic gradient, sampling among lines perpendicular to the mean regional hydraulic gradient, and sampling along fracture segments. The first two sampling patterns were shown to introduce substantial error in the sills of the velocity variograms. It is argued that this error is due to the combination of unequal sampling of fractures with different orientations (i.e., sampling bias) and the systematic variation in the magnitude of the velocity components with orientation of the fracture. As a consequence, it is suggested that correction factors developed to correct fracture frequency statistics need to be extended to improve estimation of spatial moments of variables which are correlated with fracture orientation.     

Determination of stress directions from plagioclase fabrics in high grade deformed rocks (Além Paraíba shear zone, Ribeira fold belt, southeastern Brazil)

A new method to determine stress directions using the preferential orientation of plagioclase mechanical twins has been applied to high-temperature mylonitic rocks from the Além Paraíba shear zone, Ribeira fold belt, southeastern Brazil. We have measured the lattice-preferred orientation of plagioclase grains and calculated the orientation of the stress axes possible for the observed twin orientations. The maximum compressive stress direction (σ₁), determined for all studied samples, is a function of the mechanical twin orientations of a number of distinct plagioclase populations. The σ₁ direction is generally subperpendicular to the (010) plane. The statistical treatment for most of the plagioclase grains examined for each sample shows that σ₁ is almost perpendicular to the foliation plane, suggesting a significant coaxial component in the deformation process of these rocks.     

Variability in Early Amazonian Tharsis stress state based on wrinkle ridges and strike-slip faulting

We present evidence for a decrease in the magnitude of Tharsis-circumferential compressive stress during the Late Hesperian to the Middle Amazonian based on chronologic changes in the predominant style of faulting in southern Amazonis Planitia. Using high-resolution MOLA topography, we identify a population of strike-slip faults that exhibit Middle Amazonian-aged displacements of regional chrono-stratigraphic units. These strike-slip faults are adjacent to an older population of previously documented Late Hesperian-aged thrust faults (wrinkle ridges). Along-strike orientations of these thrust and strike-slip faults reveal the Tharsis-radial stress to be the area's most compressive remote principal stress and that this stress orientation and magnitude persisted throughout the Late Hesperian to the Middle Amazonian. We show that the change in the predominant style of faulting from thrust faulting to strike-slip faulting during this time requires a decrease of the Tharsis-circumferential compressive stress to a magnitude less than lithostatic load, with negligible change in stress orientation.     

断层相关褶皱概念模型中的裂缝域

断层相关褶皱对裂缝的控制作用主要体现在岩层的几何形态对裂缝方位的影响和变形对岩石破裂的控制程度。本文在断层相关褶皱概念模型的基础上提出了裂缝域,并以Suppe的平行膝折概念模型为对象,利用几何学方法对模型中区域性裂缝的裂缝域进行分区描述,并提出了断层相关褶皱的曲线概念模型中区域性裂缝的分布规律。以新疆卡普沙良河一处断层转折褶皱剖面上裂缝的实测数据为依据,对模型进行验证,发现测量结果同模型基本一致。该方法为前陆冲断带地区断层相关褶皱对裂缝控制作用的定量预测提供了一种全新的思路。     

Hydrogeologic control of shear-related fracturing in Blue Ridge basement rocks: a case study from production wells in the Marshall Metagranite,Fauquier County,Virginia (USA)

Borehole geophysical logging of four production wells completed in Precambrian metagranite in Fauquier County, Virginia (USA), was conducted to characterize stratigraphy, collect water-bearing fracture orientations and describe vertical hydraulic gradients in the vicinity of each borehole. Long-term (48–90 h) single-well pump test data were reevaluated for each well to better characterize this locally important aquifer system. Single-well aquifer test analyses indicate mid-to-late-time infinite acting radial flow conditions within the fractured rock aquifer, followed by increasing late-time contribution of stored groundwater from recharge boundaries. Later-time pump test results are believed to indicate that water-bearing fractures within the metagranite are ultimately recharged by groundwater stored within the regolith. Assumptions about the presence of a recharge boundary sourced by the regolith were tested with a simple groundwater flow model that was calibrated to observed drawdown data associated with one of the long-term pump tests. This study identifies fracturing associated with shear-related metamorphic fabrics in the wellbore and demonstrates the significance of these fractures as mechanisms for accessing groundwater. Results from this investigation indicate that shear-related metamorphic fabrics can be important structures for integrating transmissive fracture networks within the Marshall Metagranite and possibly within other Blue Ridge basement rocks possessing similar metamorphic history.

     

西准噶尔地区多源遥感信息的线性构造提取与定量分析

夹持在近东西向额尔齐斯断裂和天山走滑断裂系统之间的西准噶尔地区,经历了自晚古生代晚期以来长期复杂的陆内构造变形历史。线性构造的长度、方向以及空间分布能够反映构造变形的强度和样式,指示应力作用的方式。本文选取新疆西准噶尔地区为研究区,利用ASTER、Landsat等多源遥感数据通过彩色合成、主成分分析、波段比值和Sobel滤波等增强显示断裂构造在遥感影像上的空间分布和光谱信息,并利用Canny边缘检测与人工解译相结合的方法提取研究区内线性构造;运用地质统计学的原理和方法对提取出的线性构造进行定量分析。结果表明,研究区内依长度优选方位确定的主断裂走向为N50°～60°E,代表了区域一级构造即达拉布特断裂展布的方位;依线性构造数量优选方位确定的次级断裂走向为80°～90°(近东西向),代表了区域三级构造的方位;介于以上两者之间的线性构造,即数量与长度均适中的线性构造,代表了区域二级构造的方位。线性构造的区域分布,揭示了在南北向主压应力作用下,西准噶尔地区构造体系的组成与构造变形特征。由此说明,多源遥感信息提取的线性构造定量分析,对于区域断裂构造体系的厘定具有重要意义。     

Linking stress field deflection to basement structures in southern Ontario: Results from numerical modelling

Analysis of stress measurement data from the near-surface to crustal depths in southern Ontario show a misalignment between the direction of tectonic loading and the orientation of the major horizontal principal stress. The compressive stress field instead appears to be oriented sub-parallel to the major terrane boundaries such as the Grenville Front, the Central Metasedimentary Belt boundary zone and the Elzevir Frontenac boundary zone. This suggests that the stress field has been modified by these deep crustal scale deformation zones. In order to test this hypothesis, a geomechanical model was constructed using the three-dimensional discontinuum stress analysis code 3DEC. The model consists of a 45 km thick crust of southern Ontario in which the major crustal scale deformation zones are represented as discrete faults. Lateral velocity boundary conditions were applied to the sides of the model in the direction of tectonic loading in order to generate the horizontal compressive stress field. Modelling results show that for low strength (low friction angle and cohesion), fault slip causes the stress field to rotate toward the strike of the faults, consistent with the observed direction of misalignment with the tectonic loading direction. Observed distortions to the regional stress field may be explained by this relatively simple mechanism of slip on deep first-order structures in response to the neotectonic driving forces.     

En echelon crack-arrays in potash salt rock

Summary There are two types of fracture patterns in the yield pillars of the potash mines of Saskatchewan. The individual members of both patterns are tensile (extension) fractures that propagate parallel with the maximum principal stress trajectory (perpendicular to the minimum principal stress). The difference between the two patterns lies in the arrangement of the member fractures. In theen echelon tensile crack-array, the macroscopic fracture consists of individual tensile cracks that are slightly offset from each other. They have only a small overlap and the child crack seems to form randomly on either side of its parent. Consequently, the en echelon tensile crack-array inherits the axial orientation of its members. In contrast, the tensile cracks of anen echelon shear crack-array, have a larger overlap and their lateral displacement from each other is biased in one direction. Therefore, the crack-array is no longer axial but inclined 20–25 degrees from the maximum principal stress direction. With increasing stress, the shear crack-array often collapses, forming theenvelope orhourglass structures of the potash mines.