Joints in the Mesozoic sediments around the Bristol Channel Basin

Analysis has been carried out at four locations on the edges of the Bristol Channel Basin to illustrate the later phases of deformation of a sedimentary basin, and to illustrate the control on joint patterns of subtle changes in the stress system. The characteristics of the joints are described and influences on joints are determined, including the roles of faults, folds and beds. There is a low coefficient of correlation between joint spacing and bed thickness, except in very thin limestone beds, which have a high density of joints. The lengths and spacings of earlier joint phases are usually greater than those of later phases. Later joints normally abut against earlier joints.The joints abut the latest faults but are not displaced by them, so the joints post-date the main Alpine contraction. The joints formed in five main phases during reduction of the Alpine stresses. Phase 1 joints are sub-parallel to the regional compression direction (160–180°). Phase 2 joints are perturbed by faults, often curving towards points of stress concentrations along the faults. Phase 3 joints are sub-parallel to the earlier E–W-striking fold axes. Phase 4 joints are cross-joints, and phase 5 joints form polygonal patterns within joint-bound blocks. Phases 2 and 3 do not occur in the absence of faults and folds, and correspond with a reduction in horizontal compression and an increase in the importance of local factors. Phases 4 and 5 occur at all locations.     

Joint development during fluctuation of the regional stress field in southern Israel

Four trends of joint sets (WNW–ESE, NW–SE, NNW–SSE and NE–SW) are found in upper Turonian carbonate rocks within the Neqarot syncline of south-central Israel. The two most predominant sets strike parallel to the trend of maximum compressive stress directions (S_H) associated with the plate-related Syrian Arc stress field (SAS; WNW–ESE) active during the Cretaceous to present and the perturbed regional stress field (NNW–SSE) related to stress accumulation on the Dead Sea Transform during the Miocene to the present. Eighty-two percent of the beds in this study contain joints parallel with the latter trend, whereas 42% contain joints parallel to the former trend. All beds with layer thickness to spacing ratio (FSR)>1.5 have NNW–SSE joint sets compatible with the Dead Sea Transform stress field (DSS), whereas all joints sets that are not compatible with the DSS stress field fall beneath this value for FSR. Considering lithology, joints in five of six chalky limestone beds and all marly limestone beds are compatible with the DSS, whereas joints compatible with the SAS do not develop in these marly and chalky limestone beds. In the study area, the joint sets lack a consistent formation sequence where more than one set is found in a single bed. We use these observations to conclude that all studied joints are Miocene or younger, that the regional stress field from the Miocene to the present fluctuated, between DSS and SAS states, and that the higher FSRs correspond to a greater amount of joint-normal strain in response to the DSS.     

Meso-Cenozoic tectonic evolution of the Dangyang Basin,north-central Yangtze craton,central China

Based on detailed structural data and available tectonic chronological data from the Dangyang Basin, the authors propose that the north-central Yangtze craton experienced three stages of tectonic evolution since Late Triassic time. In the Late Triassic to Early Jurassic (T₃–J₁), due to the Indosinian orogeny, nearly N–S compression and shortening occurred, which initiated the Dangyang Basin as a foreland basin of the Qinling–Dabie orogen. During the Late Jurassic–Early Cretaceous (J₃–K₁) period, the Yanshanian intracontinental orogeny caused contemporaneous NE–SW and NW–SE shortening, which resulted in intense folding of the foreland basin; contraction formed a brush structure diverging in a SE direction and strongly converging in a NW direction around the Huangling anticline. In the Late Cretaceous to Palaeogene, the Yuan'an and Hanshui grabens were separated from other parts of the Dangyang Basin due to post-orogenic ENE–WSW extension. Finally, at the end of the Palaeogene, ENE–WSW shortening led to inversion and deformation of the grabens.     

Anisotropy of magnetic susceptibility (AMS) studies of Campanian–Maastrichtian sediments of Ariyalur Group,Cauvery Basin,Tamil Nadu,India: An appraisal to Paleocurrent directions

Oriented samples of sediments from Ariyalur Group, Cauvery Basin, south India, were studied for low field anisotropy of magnetic susceptibility (AMS) measurements to unravel the magnetic fabrics and paleocurrent directions. The results of AMS parameters of the sediments indicate primary depositional fabrics for Sillakkudi, Ottakovil and Kallamedu sandstone formations and secondary fabric for Kallankurichchi limestone formation. The obtained low degree of anisotropy (P _j ), oblate shape AMS ellipsoid and distribution of maximum (K ₁) and minimum (K ₃) susceptibility axes on equal area projection confirm the primary sedimentary fabric for Sillakkudi, Ottakovil and Kallamedu Formations. In the case of ferruginous, lower arenaceous, Gryphaea limestone and upper arenaceous limestone beds of Kallankurichchi Formation have recorded more than one fabric. The observed AMS parameters like shape factor (T) (prolate to oblate), q value and random distribution of minimum (K ₃) and maximum (K ₁) susceptibility axes are supported for secondary fabrics in Kallankurichchi Formation as a result of post-depositional processes. Based on petrographic studies, it can be established that K ₁ AMS axis of biotite mineral could represent the flow direction. The established paleocurrent direction for Sillakkudi is NW–SE direction while Ottakovil and Kallamedu Formations recorded NE–SW direction. Overall the paleoflow directions observed for Ariyalur Group is NE–SW to NW–SE.     

Scaling relationships of joint and vein arrays from The Burren,Co. Clare,Ireland

We present a study of the systematics of veins and joints in Carboniferous limestones of The Burren, Ireland. Scaling relationships were established for fracture arrays mapped from low elevation aerial photographs that image fractures on numerous limestone pavements for areas up to ca 1 km². The veins and joints occur in the same sequence, but have contrasting scaling properties. The veins strike north-south and cut many beds to form vertically persistent, non-stratabound arrays. They are strongly clustered and have scale invariant geometric properties. Vein geometries suggest they grew sub-critically under relatively high differential stresses, during north-south directed Variscan compression. The joints form stratabound arrays, with regular spacings that scale with bed thickness. They show greater strike variation than the veins and have lognormal length distributions. The joints formed during uplift, under low-differential stress conditions. The contrasting scaling properties of the joints and veins are attributed to different overburden stresses at the time of formation. The veins formed at greater depths than the joints, in conditions that favoured fracture propagation across mechanical discontinuities, resulting in the development of non-stratabound scaling properties.     

Relative fracture velocities based on fundamental characteristics of joint‐surface morphology

Two different joint surface morphologies, plumes and rib marks, characterize joint surfaces, but the mechanical conditions that lead to the formation of either of the morphologies are not understood well. We studied two orthogonal joint sets that cut the same Santonian chalk beds in the Judea Desert, Israel. Joints of the J₁ set are systematic, relatively long, characterized by almost exclusively by plumes and predate the shorter, non‐systematic joints of the J₂ set that are characterized by rib marks. Joints of the J₁ set formed at high stress during deformation of the Syrian Arc folding in the Late Senonian. Joints of the J₂ set formed at lower stress that occurred because of stress relaxation after the formation of the J₁ joints. A mechanical analysis indicates that the J₁ joints propagated at subcritical velocities several orders of magnitude faster than the J₂ joints. Based on previously published data of laboratory tests, the plumes and the rib marks are semi‐quantitatively placed on the subcritical part of the fracture velocity vs. stress‐intensity factor diagram.     

Present-day stress tensors along the southern Caribbean plate boundary zone from inversion of focal mechanism solutions: A successful trial

This paper presents a compilation of 16 present-day stress tensors along the southern Caribbean plate boundary zone (PBZ), and particularly in western and along northern Venezuela. As a trial, these new stress tensors along PBZ have been calculated from inversion of 125 focal mechanism solutions (FMS) by applying the Angelier & Mechler's dihedral method, which were originally gathered by the first author and published in 2005. These new tensors are compared to those 59 tensors inverted from fault-slip data measured only in Plio-Quaternary sedimentary rocks, compiled in Audemard et al. (2005), which were originally calculated by several researchers through the inversion methods developed by Angelier and Mechler or Etchecopar et al.The two sets of stress tensors, one derived from geological data and the other one from seismological data, compare very well throughout the PBZ in terms of both stress orientation and shape of the stress tensor. This region is characterized by a compressive strike-slip (transpressional senso lato), occasionally compressional, regime from the southern Mérida Andes on the southwest to the gulf of Paria in the east. Significant changes in direction of the maximum horizontal stress (σ_H = σ₁) can be established along it though. The σ₁ direction varies progressively from nearly east-west in the southern Andes (SW Venezuela) to between NW-SE and NNW-SSE in northwestern Venezuela; this direction remaining constant across northern Venezuela, from Colombia to Trinidad. In addition, the σ_V defined by inversion of focal mechanisms or by the shape of the stress ellipsoid derived from the Etchecopar et al.'s method better characterize whether the stress regime is transpressional or compressional, or even very rarely trantensional at local scale.The orientation and space variation of this regional stress field in western Venezuela results from the addition of the two major neighbouring interplate maximum horizontal stress orientations (σ_H): roughly east-west trending stress across the Nazca-South America type-B subduction along the pacific coast of Colombia and NNW-SSE oriented one across the southern Caribbean PBZ. Meanwhile, northern Venezuela, although dextral strike-slip (SS) is the dominant process, NW-SE to NNW-SSE compression is also taking place, which are both also supported by recent GPS results.     

Structural pattern and strain history of a superposed fold system in the Precambrian of Central Rajasthan,India. II. Strain history

Analysis of shapes of folds, together with other structures such as axial plane foliation boudinage, mullions and cross joints, show that the F₁ folds in the ‘main Raialo syncline’ were formed by buckling, and were subsequently modified by flattening normal to the axial planes and lengthening along the axis. The apparent buckle shortening of the F₁ folds generally ranges between 70 and 80%. The folds were formed by simple shear (giving place to pure shear at certain stages) in an almost north-south direction on subhorizontal beds. Progressive deformation in the later stage of F₁ folding resulted in gentle upright folding of F₁ axial planes on F₁′ axes slightly oblique to F₁. The F₂ folds, whose average shortening ranges from 20 to 30%, were also formed by buckling caused by horizontal compression in a nearly northwest-southeast direction. This folding was preceded and followed in some instances by homogeneous strain, as deduced fro mthe shapes of the F₂ folds and the nature of variation of the F₁ lineations. The F₃ conjugate structures developed when the maximum compressive strain was vertical and the intermediate compressive strain northwest-southeast, almost normal to the subvertical F₂ axial planes. The increase in the amplitude of the F₂ folds in the last phase of F₂ folding in certain zones resulted in an excess of vertical load, which dissipated with the formation of the F₃ structures. In the last stage of movement (F₄) the maximum compressive strain became horizontal along the strike of F₂ axial planes, whereas the minimum compressive strain was normal to them. The F₄ structures, therefore, point to a longitudinal shortening with reference to large scale F₂ folding.     

Talklüfte im Zentralen Aaregranit der Schöllenen-Schlucht (Kanton Uri, Schweiz)

The Schöllenen Gorge in the Reuss Valley of the Central Swiss Alps (Figs. 1 and 2) is a famous tourist attraction and ideal location for the study of the properties and formation mechanisms of uplift and post-uplift unloading joints. The gorge is situated in the southern part of the Central Aar Granite, a granitic batholith which intruded about 300 million years ago. The magmatic fabric of this batholith (Fig. 4) has only been locally modified during Alpine tectonic and metamorphic overprinting, mainly in the vicinity of ductile-brittle shear zones. The up to 600 m deep gorge provides an ideal opportunity to study the complex fracture systems of the batholith, and tunnels of the Göschenen hydropower system allow the study of the fracture patterns below ground surface. Outcrop, tunnel and remote mapping of fractures in the study area lead to the recognition of two probably syntectonic (Oligocene-Miocene) joint sets (S and Q joints) and three generations of uplift and post-uplift joints (unloading joints). The frequent S joints run nearly parallel to the Alpine schistosity, i.e. striking approximately E–W and dipping steeply to the south (Figs. 5 and 7). The less frequent Q joints dip steeply to SW; the angle between the two joint sets ranges between 60 and 80 degrees. The first generation of uplift joints (called L- joints) is subhorizontal and probably related to Alpine extensional veins filled with fissure quartz (Zerrklüfte). These veins formed during the late Alpine (Miocene) uplift of the Aar Granite (Mullis 1996). A first generation of post-uplift joints (T₁ joints) strikes parallel to the valley axes and dip with 30–45 degrees towards the valley bottom. This set probably formed during an earlier stage of glacial valley erosion in the Pleistocene (Figs. 9–11). The youngest generation of post-uplift joints (T₂ joints) is orientated parallel to the present ground surface of the Schöllenen Gorge and to erosional surfaces with glacial striations (Figs. 9–11 and 21). The frequency and size of these joints seems to decrease with depth below the ground surface. In one tunnel, post-uplift joints could be observed within a horizontal and vertical distance from the ground surface of 150 and 80 meters. Post-uplift joints only form in granites with a primary fabric that has not been intensively overprinted by brittle or ductile Alpine tectonic deformations. Fractographic investigations, i.e. investigations of crack propagation markers on joint surfaces, confirm this relative age of the fracture sets and give valuable insights into the formation mechanisms of post-uplift joints. Post-uplift joints show intense and 5–10 meter long plumose markings and only rarely arrest lines (Figs. 18a and 20). It can be shown that sets of post-uplift joints join at pre-existing (uplift and syntectonic) fractures to form large (50–100 m sized) curved exfoliation structures (Fig. 19). The growth direction of the post-uplift joints is mainly in subhorizontal directions (Figs. 19 and 20). Fractographic markings, spatial and depth distributions as well as the relative size of post-uplift fractures are explained within the mechanical framework of uniaxial and biaxial compression tests on intact granite samples and samples with artificial flaws. Most of these experiments have been carried out in the framework of studies related to brittle failure (spalling and rockbursting) around deep mining drifts and tunnels in hard rock’s (e.g. Hoek & Bieniawski 1965, Read et al. 1998, Eberhardt et al. 1999). As suggested already by Holzhausen & Johnson (1979), post-uplift fractures form as extension fractures in a compressive stress field with small confining stress. Laboratory tests carried out on artificial Griffith cracks suggest that the macroscopic fracture size is mainly controlled by the ratio of the smallest to the largest principal stress (σ₃/σ₁), the so-called spalling limit. In steep slopes this ratio should increase with depth below ground surface (Fig. 24c), leading to smaller exfoliation fractures with increasing depth. The spatial occurrence of post-uplift fractures along the surface topography is a function of the deviatoric stress level (Fig. 24a) and/or the development of local tensile stresses (Fig. 24d). Preliminary numerical simulations of these failure criteria in a multistage glacial erosion model (Fig. 23) allow some of the observed patterns of post-uplift fracture distributions to be reproduced. post-uplift joints in steep glacial valleys play an important role in valley erosion and in connection with the risk of rock falls, the safety of traffic corridors, and the inflow of water to near-surface tunnels and hydropower caverns. The depth dependant sizes, frequencies and hydraulic conductivities of these fractures can be directly related to the occurrence and magnitudes of the corresponding hazards.     

斜向逆冲断层相关褶皱的正演模型与实例分析

斜向逆冲作用在自然界普遍存在,研究斜向逆冲断层相关褶皱的构造几何学特征,识别断层相关褶皱是否存在斜向逆冲有重要意义。文章采用Trishear 4.5、Gocad以及Trishear3D软件构建一系列不同滑移量的断层转折褶皱和断层传播褶皱的二维正演剖面,通过连接一系列不同排列方式的二维剖面建立了三种不同逆冲滑移方向的断层转折褶皱和断层传播褶皱的假三维模型,通过不同假三维模型的比较分析来探讨斜向逆冲断层相关褶皱的构造几何学特征。研究发现,斜向逆冲断层相关褶皱区别于正向逆冲断层相关褶皱的特征主要有两点：① 正向逆冲断层相关褶皱层面等高线图上的最高点与后翼等高线中点的连线以及水平切面上的核心点与后翼中点的连线方向均与断层走向垂直,而斜向逆冲断层相关褶皱的最高点以及核心点与后翼中点的连线方向均与断层走向斜交,并且最高点与后翼等高线中点的连线方向或者核心点与后翼中点的连线方向均与逆冲滑移方向一致;② 在褶皱平行断层走向纵剖面上,正向逆冲断层相关褶皱各个层面最高点的连线是直立的,而斜向逆冲断层相关褶皱各个层面最高点的连线发生倾斜。通过这两个特征可以判别褶皱是否存在斜向逆冲以及逆冲的方向。将模型分析结果运用到四川盆地西南部三维地震勘探资料所覆盖的邛西背斜和大兴西背斜的实例中。研究结果表明,两个背斜均存在右旋斜向逆冲,逆冲方向与各自断层走向的夹角均为70°左右,邛西背斜和大兴西背斜的逆冲方向分别是NE79°和NE77°左右,这与龙门山南段晚上新世以来的主应力方向以及反演的汶川地震最大主应力方向一致。     

Mineralogy, Mössbauer spectroscopy and electrical conductivity of heterosite (Fe3+, Mn3+)PO4

Mineralogical analysis, electrical conductivity and thermopower are reported for monocrystalline heterosite (Fe³⁺, Mn³⁺)PO₄ with the orthorhombic olivine-type structure. The ⁵⁷Fe Mössbauer spectrum could be adequately described using two Fe³⁺ doublets. By impedance spectroscopy (20 Hz–1 MHz) the electrical DC conductivity σ_DC and AC conductivity σ_AC were determined parallel (∥) and perpendicular to the [001] direction (space group Pnma) in the range ～160–440 K. The graph log σ_DC?1/T shows a slightly bent curve in both directions with activation energies of E _A ～0.30 and ～0.15 eV in the high and low temperature ranges, respectively. The reduced E _A is associated with electronic conduction; σ_DC ∥ [001] follows Mott’s T ^1/4 variable range hopping law at lower temperatures with hopping between localized levels. The values of σ_AC are increased relative to σ_DC at high frequencies and low temperatures, obeying Jonscher’s universal dynamic response law; for σ_AC ∥ [001], the variation with temperature of the frequency exponent is in fair agreement with the model of small polaron hopping. The absolute thermopower Θ is negative and low between ～295 and ～440 K, Θ does hardly vary with temperatures in both directions; the temperature independency of Θ ∥ [001] is consistent with the small polaron hopping model.     

Paleostress reconstructions of Jabal Hafit structures,Southeast of Al Ain City,United Arab Emirates (UAE)

This paper presents the first paleostress results obtained from displacement and fracture systems within the Lower Eocene sediments at Jabal Hafit, Abu Dhabi Emirate, UAE. Detailed investigation of Paleogene structures at Jabal Hafit reveal the existence of both extensional structures (normal faults) and compressional structures (strike-slip and reverse faults). Structural analysis and paleostress reconstructions show that the Paleogene kinematic history is characterized by the succession of four paleostress stages. Orientation of principal stresses was found from fault-slip data using an improved right-dihedra method, followed by rotational optimisation (TENSOR program).The paleostress results confirm four transtensional tectonic stages (T1–T4) which affected the study area. The first tectonic stage (T1) is characterized by S_Hmax NW–SE σ₂-orientation. This stage produced NW–SE striking joints (tension veins) and E–W to ENE–WSW striking dextral strike-slip faults. The proposed age of this stage is Early Eocene. The second stage (T2) had S_Hmax N–S σ₂-orientation. N–S striking joints and NNE–SSW striking sinistral strike-slip faults, E–W striking reverse faults and N–S striking normal faults were created during this stage. The T2 stage is interpreted to be post-Early Eocene in age. The third stage (T3) is characterized by S_Hmax E–W σ₂-orientation. This stage reactivated the E–W reverse faults as sinistral strike-slip faults and created E–W striking joints and NE–SW reverse faults. The proposed age for T3 is post-Middle Eocene. During the T3 (S_Hmax E–W σ₂-orientation) stage the NNW-plunging Hafit anticline was formed. The last tectonic stage that affected the study area (T4) is characterized by S_Hmax NE–SW σ₂-orientation. During this stage, the ENE–WSW faults were reactivated as sinistral strike-slip and reverse faults. NE–SW oriented joints were also created during the T4 (S_Hmax NE–SW σ₂-orientation) stage. The interpreted age of this stage is post-Middle Miocene time but younger than T3 (S_Hmax E–W σ₂-orientation) stage.     

Scaling the 3-D Mohr circle and quantification of paleostress during fluid pressure fluctuation – Application to understand gold mineralization in quartz veins of Gadag (southern India)

In this study, orientations of 157 quartz veins occurring in metabasalts of the Gadag region (Dharwar craton, southern India) are used to plot the 3-D Mohr stress circle, which provides information about relative stress/fluid pressure (P_f) conditions, as well as stress state during P_f fluctuation. To scale the 3-D Mohr circle, vein orientation data are integrated with (a) available estimates from fluid inclusions of highest recorded P_f (390 MPa) and lowest recorded P_f (50 MPa) and (b) intrinsic rupture criterion that empirically quantify rock properties. Based on the scaled 3-D Mohr circle, the absolute magnitudes of the three principal stresses are quantified for high and low P_f. Of 157 veins investigated here, 14 veins are identified as having favourable orientation for dilation at high as well as low P_f. These 14 veins have a mean strike of 150°, which is similar to the orientation of the gold-bearing quartz lodes reported in the region. The effective normal stress (σ′_n = σ_n−P_f) prevalent during dilation of fracture/fabric anisotropy with 150° strike is calculated to be −11.5 MPa at high P_f, and −1.0 MPa at low P_f. Thus, it is interpreted that in the Gadag region, a change in σ′_n of 10.5 MPa prevailed during P_f fluctuation and associated separation of gold from the fluid.     

Faulting and fracturing in the Jurassic carbonate ramp folds of southern Rif ridges (Northern Morocco)

In the South Rifian ridges (SRR), the dominated structures correspond to the faulted anticline characteristic of a foreland orogeny context, front of the Rif Alpine belt. These anticlines correspond to thrust propagation folds. Geometric model of these structures shows that the normal faults have controlled the Mesozoic sedimentation during extensive episodes and participated in determining areas of thrusting during Miocene compressional phases. However, the normal fault strike which is relative to the direction of the shortening determined the geometry of diverse folds whether into the frontal ramps, lateral, or oblique. In the meantime, the systematic fracturing study in the Jurassic limestone beds, in different parts of the folds with axes oriented E-W, NW-SE, and NE-SW, permits to propose a relative fracturing chronology and tries investigating the relationship between folding and fracturing. The three main fracture families, oblique, transversal, and axial, appear simultaneously during the amplification of the fold. The simple shear in the limb contributes the latest to the folding reactivation and the density of the intensification of these microfractures. Likewise, given the important downslope fold limb dip of the ramp propagation folds, theoretically the shear intensity is more important, and micro-fractures are more important in the downslope fold limb, thus the uphill one.     

The timing and direction of metamorphic fluid flow in Vermont

A suite of metabasite dykes, emplaced within the Albee Formation, east-central Vermont, preserves evidence of interaction with a CO₂-bearing hydrous fluid during Acadian metamorphism. Coupled advective and diffusive cross-layer fluxes of CO₂ are recorded by forwards progress of the hydration-carbonation reaction: 3 amphibole + 2 epidote + 8 H₂O + 10 CO₂ = 3 chlorite + 10 calcite + 21 quartz Advection, diffusion and mineral reaction rates are modelled from reaction progress data through application of an analytical solution to the 1-D mass conservation equation for linear reaction kinetics (Lichtner 1988; Lasaga and Rye 1993; Skelton et al. in press). Dimensionless Peclet (Pe) and Damköhler (N_D) numbers which describe the ratios of advection to diffusion and reaction rate to advection are thus resolved, from which time-integrated fluid fluxes are calculated. Small Pe (<10), large N_D (>10) and small time-integrated fluid fluxes (<5 m³/m²) are obtained, predicting that cross-layer fluxes of CO₂ are largely diffusive. It follows that, within the Albee schists, fluid flow must have been layer parallel. Where fluid flow is layer parallel, it is both interesting and informative to consider the geometry of fluid flow at a fold hinge. For layer-parallel fluid flow to be maintained, the flow direction must reverse which is difficult to justify with regards to the driving force. It is perhaps more reasonable that fluid should “escape” through the fold hinge (cf. Skelton et al. 1995), either via close-spaced microveins (cf. Cole and Graham 1994) or macro-scale axial-planar quartz veins (Ferry 1992, 1994). At present, the Albee schists are ～ vertical and strike ～ N–S. This reflects: (1) E–W recumbent folding of strata (early Acadian “nappe-stage” deformation; D₁); (2) N–S arching of strata (late Acadian “dome-stage” deformation; D₂). In the Strafford Quadrangle, east-central Vermont, curious “Coarse Garnet Schists” occur in the axial regions of D₁ fold closures. On the basis of this spatial association, crystal morphologies, reaction textures and chemical zoning profiles, it is postulated that the Coarse Garnet Schists developed in response to structurally focused fluid escape. That fluid flow was syn-D₁ is implicit to this argument. As such, layer-parallel fluid flow must have been ～ horizontal. Although this conclusion is in close agreement with that of Ferry (1994), up-temperature fluid flow is not implicated.     

库车坳陷脆性构造序列及其对构造古应力的指示

在对野外脆性构造(主要是节理和断层) 大量观测的基础上, 根据它们与应力的关系, 讨论了库车坳陷白垩纪末期以来的古构造应力时空变化.结果表明, 在库车坳陷脆性构造中, 早期隆升作用形成的主要发育在中生界的NEE-SWW向系统节理被晚期同构造期的在中生界与上第三系均发育的NNW-SSE向和NW-SE向节理切割并改造, 这是对区域上构造应力场在进入新近纪时从弱伸展变化到强烈挤压这一过程的响应.基于断层滑动分析的古应力反演结果显示, 此时盆山边界处以近N-S向伸展应力状态占主导, 而坳陷内部则表现为近N-S向和NW-SE向挤压应力状态.说明在进入新近纪后, 最大主应力(σ₁) 方向从垂向变成水平, 应力场发生了转变.此后的天山快速垂向隆升是库车坳陷北缘和内部应力状态存在差异的原因.      

Exploring the influence of loading geometry on the plastic flow properties of geological materials: Results from combined torsion + axial compression tests on calcite rocks

For technical reasons, virtually all plastic deformation experiments on geological materials have been performed in either pure shear or simple shear. These special case loading geometries are rather restrictive for those seeking insight into how microstructure evolves under the more general loading geometries that occur during natural deformation. Moreover, they are insufficient to establish how plastic flow properties might vary with the 3rd invariant of the deviatoric stress tensor (J₃) which describes the stress configuration, and so applications that use those flow properties (e.g. glaciological and geodynamical modelling) may be correspondingly compromised. We describe an inexpensive and relatively straightforward modification to the widely used Paterson rock deformation apparatus that allows torsion experiments to be performed under simultaneously applied axial loads. We illustrate the performance of this modification with the results of combined stress experiments performed on Carrara marble and Solnhofen limestone at 500°–600 °C and confining pressures of 300 MPa. The flow stresses are best described by the Drucker yield function which includes J₃-dependence. However, that J₃-dependence is small. Hence for these initially approximately isotropic calcite rocks, flow stresses are adequately described by the J₃-independent von Mises yield criterion that is widely used in deformation modelling. Loading geometry does, however, have a profound influence on the type and rate of development of crystallographic preferred orientation, and hence of mechanical anisotropy. The apparatus modification extends the range of loading geometries that can be used to investigate microstructural evolution, as well as providing greater scope for determining the shape of the yield surface in plastically anisotropic materials.     

Regional stress alignments in the Kutai Basin,East Kalimantan,Indonesia: a contribution from a borehole breakout study

Borehole breakout data from 134 wells located within the Kutai Basin region, East Kalimantan were analyzed to determine the present day regional horizontal stress alignments. The data were extracted from various types of dipmeter logs. The study reveals that the majority of the data give a coherent picture of breakout orientation. The mean azimuths for the entire-unweighted, ellipticity-weighted and magnitude-weighted data sets are preferentially aligned in the regional-mean direction of 48.9° N or 128.9° N. Most of the data have a low dispersion value (So) and the ranking of reliability in Zoback’s classification is ‘A’. There is no significant azimuth variation with depth. These relatively consistent alignments of breakout azimuths indicate that the maximum regional stress direction in the study area is NW–SE. This regional-mean of breakout azimuths deviates from the axis of the anticlinorium trends and from the strike of the thrust–fault patterns in the region. It is believed that these structural patterns are influenced by reactivation of weak zones related to sediment loading (structural inversion).     

Paleocurrent analysis of sedimentary crossbed data with graphic output using three integrated computer programs

Basin analysis for paleocurrent directions commonly makes use of hundreds of cross-bedding dip and strike measurements taken on structurally tilted sedimentary beds which may be parts of plunging folds. For regional interpretations the desired forms of data are vector means and rose diagrams of dip directions after reduction to the assumed horizontal plane of deposition. Three FORTRAN computer programs have been integrated to perform the data reduction from raw field measurements to graphic plotted rose diagrams. Program PLUNGR rotates the raw field data back to the original horizontal plane of deposition. Output from PLUNGR is a deck of punched cards suitable for input into the second and third programs, with provision for various regroupings of data. VECMEN computes vector mean and other statistics on data groups defined by inserted control cards. Program ROSE, with a variety of options, plots a rose diagram of dip direction azimuths on a CalComp plotter.     

Stress fields and fault reactivation angles of the 2000 western Tottori aftershocks and the 2001 northern Hyogo swarm in southwest Japan

We estimated the stress fields of the aftershocks of the 2000 western Tottori earthquake (M_w 6.6) and the northern Hyogo swarm (max M_w 5.2) by a stress tensor inversion of moment tensor solutions reported from the National Research Institute for Earth Science and Disaster Prevention (Japan). The maximum principal stress direction of the western Tottori sequence was estimated as N107°E with a strike–slip regime. In the northern Hyogo swarm, the orientations of the principal stress directions could not be well constrained by the observed data, but after examining the detailed characteristics of the solution, we obtained a most probable solution of N113°E for the σ₁ direction. These solutions are consistent with the maximum horizontal directions roughly estimated from the strike directions of large earthquakes occurring geographically between these two seismic activities. We measured the angle between each fault–slip direction and maximum principal stress direction to investigate the frictional properties of earthquakes. The distribution of the angles was forward modeled to estimate the coefficient of friction and the stress ratio, assuming uniformly distributed fault orientations. For the western Tottori sequence, a homogeneous stress field with a coefficient of friction less than 0.4 was estimated. A high stress level was also suggested because very little change occurred in the stress field during the mainshock. For the northern Hyogo sequence, the coefficient of friction was estimated to be between 0.5 and 1.0.