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  • From outcrop scanlines to discrete fracture networks, an integrative workflow
    J. Struct. Geol. (IF 3.128) Pub Date : 2020-01-16
    Baptiste Lepillier; Pierre-Olivier Bruna; David Bruhn; Eivind Bastesen; Alexandros Daniilidis; Óscar Garcia; Anita Torabi; Walter Wheeler

    Understanding fractures and fracture networks is essential for the investigation and use of subsurface reservoirs. The aim is to predict the fractures and the fracture network when there is no direct access to subsurface images available. This article presents a universal workflow to numerically compute a discrete fracture network by combining the 1D scanline survey method, processed with the newly written SkaPy script, together with the multiple point statistic method (MPS). This workflow is applied to a potential geothermal site in Mexico called Acoculco. We use Las Minas outcrops and quarries as surface analogues for the Acoculco reservoir, as Las Minas and Acoculco are both formed by the influence of a plutonic intrusion into the Jurassic-Cretaceous carbonate sequence of the Sierra Madre Oriental in the Trans-Mexican volcanic belt (TMVB). The intrusion is associated with contact metamorphism and metasomatic phenomena, providing the basis for the mining activities at Las Minas. The results obtained using this workflow demonstrate the feasibility of the approach, which presents a solution combining the efficiency of data processing and an interpretation-driven approach to build realistic discrete fracture networks. This workflow can be used in the process of estimating the permeability of a fracture controlled reservoir, with using only scanline surveys data as input. This is essential in the process of evaluating the feasibility to develop an enhanced geothermal system.

  • Deformation and structural evolution of mantle peridotites during exhumation on transform faults: A forced transition from ductile to brittle regime
    J. Struct. Geol. (IF 3.128) Pub Date : 2020-01-13
    Leonardo Mairink Barão; Barbara Trzaskos; Rodolfo José Angulo; Maria Cristina de Souza
  • Chert nodules in pelagic limestones as paleo-stress indicators: A 3D geomechanical analysis
    J. Struct. Geol. (IF 3.128) Pub Date : 2020-01-10
    Marco Antonellini; Leonardo Del Sole; Pauline N. Mollema
  • From hot to cold - The temperature dependence on rock deformation processes: An introduction
    J. Struct. Geol. (IF 3.128) Pub Date : 2020-01-09
    E. Gomez-Rivas; R.W.H. Butler; D. Healy; G.I. Alsop

    Understanding rock deformation processes in solid Earth materials, from the crystal to the tectonic plate scale, is essential for characterising the evolution of the lithosphere and for predicting how rocks behave in the subsurface. Temperature is a key parameter that determines what rock deformation processes are active and therefore how tectonic structures form. Moreover, it controls the migration of fluids and melt in the Earth's crust that lead to phase transformations and changes in rock rheology. This special issue gathers a collection of research papers following the 21st International Conference on Deformation Mechanisms, Rheology and Tectonics, which was held in Inverness (Scotland) in April–May 2017, organised by the University of Aberdeen. These contributions provide a significant advance in the study of rock deformation and fluid/melt migration at multiple crustal levels, from deformation bands near the Earth's surface to shear zones in partially molten rocks in the lower crust. In this introductory article, we first provide an overview of how temperature controls deformation mechanisms and then introduce the collection of research papers ordered from those analysing deformation processes occurring at high temperatures to contributions reporting deformation at shallow crustal conditions.

  • Sensitivity analysis of segmentation techniques and voxel resolution on rock physical properties by X-ray imaging
    J. Struct. Geol. (IF 3.128) Pub Date : 2020-01-07
    Y.D. Shou; Z. Zhao; X.P. Zhou

    Pore and grain phases separation significantly affect the rock physical properties extracted by digital rock measurement techniques (DRMTs). Previous studies mainly focus on approaches to predict the physical properties of rocks, and less attention is paid on the effects of segmentation techniques and voxel resolution on rock physical properties. Physical properties of rocks are not still well understood using DRMTs with less experiment effort. In this work, 6 segmentation algorithms and 4 voxel resolutions combined with X-ray CT imaging are applied to the Stainton and Clashach sandstone, Indiana limestone and Estaillades carbonate samples to understand their effects on rock physical properties. Moreover, threshold-porosity relations are proposed to accurately segment the pores and grains in rocks. Results show that voxel resolution has little to no impact on the rock physical properties. However, the segmentation algorithms significantly affect these properties, in which the Ostu's method, Iterative self-organizing data (IsoData) method and Fuzzy c-mean clustering (FMC) method should be alternative methods to separate the pore and grain phases in sandstones. For carbonates, under-segmentation is still inevitable with these techniques. Rock physical properties calculated by the proposed threshold-porosity relations are closer to the experiment results than those calculated by the classical segmentation methods.

  • Faults in tight limestones and dolostones in San Vito lo Capo, Sicily, Italy: Internal architecture and petrophysical properties
    J. Struct. Geol. (IF 3.128) Pub Date : 2019-12-27
    I. Kaminskaite; Q.J. Fisher; E.A.H. Michie

    Low-porosity carbonates in San Vito lo Capo underwent two episodes of faulting: extensional faulting in the late Miocene and strike-slip faulting in the Plio-Pleistocene. Limestones and dolostones reacted differently to these faulting events. The first extensional faulting event within the limestone formed localized faults with a single fault core surrounded by a damage zone. The fault rock was either dolomitized as deformation proceeded or resealed by calcite cement after brecciation. The dolomitized fault rocks were reactivated during the following strike-slip faulting, forming polyphase breccias. On the other hand, strain was distributed in dolostones throughout a wider area during the extensional faulting forming a pulverized fault zone. The pulverized rock experienced cataclastic deformation during the following strike-slip faulting forming anastomosing networks of cataclastic shear bands. Fault cores hosted in the limestone appear to have acted as flow conduits until they were cemented or hardened due to evolving cataclasis. The cataclastic shear bands in the dolostone are likely to form baffles to flow, at least on a local scale. The fracture spacing in the damage zone also varies significantly between the lithologies. In particular, damage zones in the limestone have a 5–10 cm fracture spacing whereas fracture spacing is 0.5–3 cm in the dolostone. It is likely that the differing mechanical and chemical properties of the dolostone and limestone were responsible for creating contrasting fault zone architectures.

  • Petrography and isotopic geochemistry of bedding-parallel fibrous gypsum veins in the Neogene Qingshuiying Formation of the Ningnan Basin, North China: Insights for growth model of antitaxial fibrous veins
    J. Struct. Geol. (IF 3.128) Pub Date : 2019-12-28
    Bangsheng Zhao; Rongxi Li; Xiaoli Wu; Xiaoli Qin; Di Zhao; Ahmed Khaled; Futian Liu

    Bedding-parallel fibrous gypsum veins, which are widely developed in the Neogene Qingshuiying Formation of the Ningnan Basin, a Tertiary basin located in the northeastern margin of the Tibetan Plateau, provide valuable information on stress field, fluid origin and host rock deformation during their formation. The systematic petrographic observation and isotope analysis of both of veins and wall rocks were carried out to investigate the formation mechanism of bedding-parallel fibrous gypsum veins. The distribution of fibrous gypsum veins shows vertical heterogeneity, and mainly occurs in the middle and lower part of the Qingshuiying Formation with relatively high gypsum content. Single fibrous gypsum veins consist of a median zone and two fibrous zones. The median zone is characterized by blocky gypsum crystals and multiple host rock fragments formed by crack-seal events. The fibrous zone is composed of parallel gypsum fibres a high angle to the vein walls, indicating shear component. Vein opening is considered to be the result of fluid overpressure caused by horizontal compression in low-permeability mudstone during basin inversion. Fibrous gypsum crystals grow continuously without obvious growth competition between adjacent crystals. This indicates that growth of gypsum fibres occurs in limited space, even without fractures. The main driving force for vein dilation is force of crystallization of gypsum fibre growth, and fluid pressure also has a certain contribution. The sulfur and strontium isotopes of gypsum veins exhibit a high degree of consistency with wall rocks, which indicates the material of the fibrous gypsum veins is derived from the wall rock. A suitable growth model of bedding-parallel fibrous gypsum veins in the Ningnan Basin is established, which explains the process of vein interaction, and the formation of V-shaped or U-shaped solid inclusions.

  • 3D structure and evolution of an extensional fault network of the eastern Dampier Sub-basin, North West Shelf of Australia
    J. Struct. Geol. (IF 3.128) Pub Date : 2019-12-29
    Hongdan Deng; Ken McClay; Awad Bilal

    Insights of spatial and temporal development of fault network in 3D is crucial for understanding the process evolution of complex fault network and for evaluating the regional and local stresses control on structure development. We demonstrate a fault network on the eastern Dampier Sub-basin, North West Shelf of Australia, which consists of (1) a ENE-trending fault array that has a through-going segment at depth and a series of left-stepping fault splays at upper levels, and (2) a network of ENE- and NNE-trending intersecting faults decoupled from the basement structures. This research shows that the segmented ENE-trending fault array developed through three extensional phases in the Late Paleozoic, in the Early Jurassic, and in the Late Middle Jurassic. Fault analysis shows that the summed displacement of the segmented, en échelon faults behaves as a single fault and that the basement fault controlled the fault array in the upper section through vertical linkages– a typical coherent fault system. The NNE- and ENE-trending intersecting faults formed simultaneously in the Late Middle Jurassic; as such, they might have controlled by 3D strain field released from the Rosemary and Mermaid fault systems bounding the fault network. This implies that fault geometry derived from 3D seismic interpretation need to be treated with caution as the alignment of fault sets may not directly relate to regional, far-field stress but, in some cases, significantly modified by local stresses induced by reactivated larger faults. This study provides an analogue for the interpretation of other rift systems, where structures were controlled by competing forces of regional and local stresses and where reactivated and newly-formed structures coexist in polyphase of extensions.

  • Semi-automatic mapping of dyke and dyke-related fractures using UAV-based photogrammetric data: A case study from Sijiao Island, coastal Southeastern China
    J. Struct. Geol. (IF 3.128) Pub Date : 2020-01-03
    Hua Zhang; Ninghua Chen; Bowen Gao; Yaochen Xu; Jianyu Chen

    Mapping dyke and dyke-related fractures is important for magma propagation and emplacement mechanisms analysis. Here, we propose a semi-automatic workflow for dyke and fracture mapping using unmanned aerial vehicle (UAV)-based photogrammetric data. Our workflow integrates mean-shift and GrabCut in one cut (GCOC) algorithms to segment dykes, and harnesses the phase symmetry algorithm and a series of follow-up actions to detect fractures. Sijiao Island of Zhejiang Province (SE China) is examined as a case study to validate the processing chain. With our workflow, the generated dyke map attains more than a 97% recall rate when compared with the results of expert interpretation, and the ratio of the number of detected false fracture segments to the number of total segments is less than 18%. The dyke strike and fracture strike calculation match well with field measurements. Moreover, by combining visual interpretation and the resulting geometric information, our results manifest that dyke magma invaded along pre-existing fractures to form dykes on Sijiao Island, and the difference between dyke fractures and host rock fractures may be because of the effect of major changes of regional stress. This workflow has great potential for high-resolution terrestrial mapping.

  • The impact of inter-bed cohesion on fold-related fracture development, stillwell anticline, west Texas (USA)
    J. Struct. Geol. (IF 3.128) Pub Date : 2020-01-03
    Ben Surpless; Sarah S. Wigginton

    The interpretation of fracture networks in contractional folds is challenging due to the range of factors that control fracture formation. We use outcrop-based analysis of fractures in plan-view pavements and in a 9-bed cross-sectional exposure to evaluate the fracture system within Cretaceous limestone layers of a Laramide fold in west Texas, the Stillwell anticline. Opening-mode extension fractures (veins) at high angles to bedding dominate the fracture population, although shear fractures and faults cut bedding at low angles within the forelimb. Analysis of extension fractures reveals NW-striking axial parallel and NE-striking axial-perpendicular fracture sets interpreted to have formed during contractional folding, a third N-striking fracture set formed during subsequent Basin and Range extension, and a fourth ESE-striking fracture formed due to unloading during exhumation. Fracture fill textures suggest that many fracture apertures increased during exhumation. The relative abundances of the four fracture sets and the intensity of each set vary from bed to bed in cross section. Because beds display no significant differences in mechanical strength and there is no correlation between bed thickness and fracture intensity, we attribute this bed-to-bed variability to differences in cohesion between beds. Bed decoupling, when combined with low extensional (<0.5%) strains, helps explain low vertical persistence. We hypothesize that weak bed cohesion results in variability of fracture patterns within planar fold limbs. In fold limbs with low fracture-related strains, we propose that high angles between fracture sets increases the likelihood of high bed-parallel fracture connectivity, and low fracture vertical persistence results in low vertical connectivity.

  • Present day in-situ stress magnitude and orientation of horizontal stress components in the eastern Illizi basin, Algeria: A geomechanical modeling
    J. Struct. Geol. (IF 3.128) Pub Date : 2020-01-02
    Rafik Baouche; Souvik Sen; K. Boutaleb

    We analyzed drilling induced tensile fractures from resistivity image log data to ascertain the orientation of maximum horizontal stress (SH) from the eastern Illizi basin, Algeria. An average SH azimuth of 150°N (± 10°) has been interpreted from B-quality induced fractures, as per world stress map guidelines. The overall NW-SE orientation of SH translates to the relative plate motion of the African and Eurasian plates. Vertical stress (Sv) gradient of 1.07 PSI/ft has been derived from density log. Pore pressure estimated from sonic slowness reveals overpressure in Silurian shale, deposited in a transgressive depositional environment, whereas Devonian and Ordovician hydrocarbon reservoirs have been seen to be normally pressured. Poroelastic strain model has been employed to quantify maximum and minimum horizontal stress (Sh) magnitudes. An effective stress ratio of 0.6, interpreted from leak-off test has also been used to model Sh. Using frictional faulting theory, upper limit of SH has been quantified. SH/Sv ratio of 1.04 (1.01–1.26) has been seen in the study area. Based on the relative stress magnitudes (SH > Sv > Sh), a present day strike-slip faulting regime has been inferred in the eastern Illizi basin, Algeria. Fault reactivation potential at reservoir level has been inferred from stress polygon analysis.

  • Numerical modelling and comparison of the temporal evolution of mantle and tails surrounding rigid elliptical objects in simple shear regime under stick and slip boundary conditions
    J. Struct. Geol. (IF 3.128) Pub Date : 2019-12-20
    Kieran F. Mulchrone; Soumyajit Muhkerjee

    Structures associated with rigid inclusions are a rich source of evidence to understand the local deformation regime. The behaviour of rigid objects in modelled here as being immersed in a linear Newtonian fluid with either (i) a stick boundary condition (continuity of stress and velocity across the boundary) or (ii) a slip boundary condition (continuity of boundary normal stress and velocity across the boundary with zero shear stress at the boundary). Of particular interest are the types of structures developed in a concentric region adjacent to the object termed the mantle. A model of the displacement of points around the inclusion comprises a set of ordinary differential equations which are solved numerically. A comprehensive set of simulations for a variety of mantle sizes, object aspect ratios, initial orientations as well as different boundary conditions has been performed. A comparison between natural examples and model output indicates a level of consistency. The resulting structures differ in detail and in a broader sense. In general δ-type structures only develop when stick boundary conditions are in operation. In contrast, σ-type structures at high strain are restricted to slip boundary conditions. Slip conditions also tend to be the source of complex mantle types involving more than one generation of mantle structures or wings. Furthermore, our model indicates that using asymmetry of orientation of objects relative to the shear direction may be problematic when used alone, particularly if stick boundary conditions prevail but that together with mantle structures there is less chance of confusion.

  • Rift propagation in rotational versus orthogonal extension: Insights from 4D analogue models
    J. Struct. Geol. (IF 3.128) Pub Date : 2019-12-13
    Frank Zwaan; Guido Schreurs; Matthias Rosenau

    In rift settings, extension rates often vary along strike, due to rotation about a vertical axis or Euler pole, yet tectonic modelers traditionally apply constant along-strike deformation rates. Here we compare rift development and propagation under traditional orthogonal extension versus rotational extension conditions. The set-ups involve brittle-viscous layering and localize deformation through structural weaknesses (seeds). Our models provide first-order insights into the differences in rift development between both boundary conditions: orthogonal extension produces a rift basin with constant synchronous along-strike features, whereas rotational extension induces along-strike structural gradients, diachronous rift development causing rift propagation and the development of V-shaped basins. We observe important viscous flow associated with differential pressure gradients in rotational extension. We also describe the important effects of strain partitioning between rift axis and model boundaries, the quantifying of which is crucial to avoid incorrect model interpretations. Although our model results are first-order only, they are in good agreement with various natural examples and previous modeling studies and highlight the importance of considering the third dimension when studying tectonic systems.

  • Using quartz fabric intensity parameters to delineate strain patterns across the Himalayan Main Central thrust
    J. Struct. Geol. (IF 3.128) Pub Date : 2019-12-12
    Jesslyn K. Starnes; Sean P. Long; Stacia M. Gordon; Jingyi Zhang; Emmanuel Soignard

    Disagreements over how to define shear zones within packages of pervasively recrystallized rock impede our ability to map and correlate these structures. Here, we explore a new approach for delineating the spatial extents of shear zones using quartz petrofabric, temperature, and kinematic data collected from two transects across the Main Central thrust (MCT) in western Bhutan. We calculated cylindricity, a fabric intensity parameter that has been previously interpreted as a proxy for finite strain magnitude, for each sample. Cylindricity values (measured on a scale from 0 to 1) increase upward from 0.11 to 0.47 between 2.3 and 1.3 km below the MCT, vary between 0.55 and 0.93 between 700 m below and 200 m above the MCT, and decrease upward from 0.68 to 0.37 between 2.4 and 11.7 km above the MCT. Fabric intensity increases with proximity to the MCT and defines a ∼900 m-thick zone of elevated intensity that overlaps spatially with an interval of inverted metamorphism. Our results add to a growing number of recent studies that suggest that spatial patterns of fabric intensity, when integrated with temperature data and kinematic observations, can be used to delineate relative strain patterns across shear zones in the Himalaya and elsewhere.

  • Structural anisotropy: Using image analysis to quantify block-in-matrix fabrics
    J. Struct. Geol. (IF 3.128) Pub Date : 2019-11-30
    Alexander P. Clarke; Paola Vannucchi

    Block-in-matrix rock assemblages — such as mélanges — are structurally complex units whose fabric is primarily defined, not by planar or linear features as most rock units are, but by the orientation and aspect ratio of irregularly shaped blocks. Despite this, previous attempts to quantitatively characterize mélange fabrics have predominantly focused on measuring the foliation using traditional techniques. Here we introduce a method of characterizing block-in-matrix fabrics using image analysis of field photos to discern the aspect ratios and orientations of blocks and define the structural anisotropy of the rock unit. We also include the software to calculate structural anisotropy and the trend of the fabric from image analysis data.

  • Seismic liquefaction features in the Kashmir Karewas: Natural seismograms of the paleoearthquakes
    J. Struct. Geol. (IF 3.128) Pub Date : 2019-11-28
    Rais Ahmad Khan

    Karewas occur within the seismically active Kashmir Valley. Natural seismograms in the form of seismites exist within the Karewa sediments. Seismites were identified and characterized to know their genesis. The magnitude of the paleoearthquakes were ranging from 6.0 to 7.4 with paleointensity IO = 8.5. LSI values computed for the paleomagnitudes were ranging from 16 to 63 mm. PGA computed for the paleoearthquakes were ranging from 0.18 g to 0.77 g using cyclic stress method. PGA computed for the simulated historical and instrumental earthquakes using GMPE were of the order of 1.11 g, 0.86 g and 0.83 g. FS, VS and Dr values indicate that the sediments of the source stratum near the paleoliquefaction sites are still liquefiable and have retained their liquefaction potential since Early Pleistocene. FS values of the paleoearthquakes indicate that the geological conditions were appropriate to liquefy sediments as FS = 1 and are still susceptible to liquefaction as advocated by the FS values of the simulated historical and instrumental earthquakes. Empirical relationships that account for the age of soil deposits show significant influence on PGA values of the paleoearthquakes and no effect on FS values of the paleoearthquakes as FS = 1. However, significant influence was observed on FS values of the historical and instrumental earthquakes.

  • Monoclinic and triclinic 3D flanking structures around elliptical cracks.
    J. Struct. Geol. (IF 3.128) Pub Date : null
    Ulrike Exner,Marcin Dabrowski

    We use the Eshelby solution modified for a viscous fluid to model the evolution of three-dimensional flanking structures in monoclinic shear zones. Shearing of an elliptical crack strongly elongated perpendicular to the flow direction produces a cylindrical flanking structure which is reproducible with 2D plane strain models. In contrast, a circular or even narrow, slit-shaped crack exhibits a reduced magnitude of the velocity jump across the crack and results in smaller offset and a narrower zone of deflection than predicted with 2D-models. Even more significant deviations are observed if the crack axes are oriented at an oblique angle to the principal flow directions, where the velocity jump is oblique to the resolved shear direction and is modified during progressive deformation. The resulting triclinic geometry represents a rare example of triclinic structures developing in monoclinic flow and may be used to estimate the flow kinematics of the shear zone.

  • The structure of the Hallstatt evaporite body (Northern Calcareous Alps, Austria): A compressive diapir superposed by strike-slip shear?
    J. Struct. Geol. (IF 3.128) Pub Date : 2014-03-01
    Anja Schorn,Franz Neubauer

    Based on old detailed mining maps and own observations in the Hallstatt salt mine, we reinterpret the structure of the Hallstatt evaporite body of the Upper Permian to Lower Triassic Haselgebirge Fm. within the Northern Calcareous Alps (NCA). The Haselgebirge Fm. represents a rocksalt mylonite with abundant lenses of sulphates, mudstones and limestones. In comparison to published results of analogue modeling we interpret the present shape of the Hallstatt body as a WNW-ESE elongated compressive teardrop-like diapir. This is overprinted by NNE-SSW shortening and dominantly sinistral shearing along a W-trending shear zone. The internal structure shows steeply dipping rock units and foliation. Earlier dextral ductile shear fabrics of likely late Early Cretaceous age are preserved in sulphate rocks and are subsequently overprinted by mylonitic fabrics in rocksalt and cataclastic fabrics in other rocks. The low strength of halite results in recent subvertical shortening and a strain rate [Formula: see text] of 8 × 10-10 [s-1] is deduced from deformed subhorizontal boreholes. This value is similar to such strain rates (10-10 to 10-9 s-1) estimated by the halite grain size distribution from other salt mines in the NCA and thus indicative of sub-recent formation of the halite microfabrics.

  • Polyhalite microfabrics in an Alpine evaporite mélange: Hallstatt, Eastern Alps.
    J. Struct. Geol. (IF 3.128) Pub Date : 2013-01-01
    Anja Schorn,Franz Neubauer,Manfred Bernroider

    In the Hallstatt salt mine (Austria), polyhalite rocks occur in 0.5-1 m thick and several metre long tectonic lenses within the protocataclasite to protomylonite matrix of the Alpine Haselgebirge Fm.. Thin section analysis of Hallstatt polyhalites reveals various fabric types similar to metamorphic rocks of crust-forming minerals, e.g. quartz and feldspar. Polyhalite microfabrics from Hallstatt include: (1) polyhalite mylonites, (2) metamorphic reaction fabrics, (3) vein-filling, fibrous polyhalite and (4) cavity-filling polyhalite. The polyhalite mylonites contain a wide range of shear fabrics commonly known in mylonitic quartzo-feldspathic shear zones within the ductile crust and developed from a more coarse-grained precursor rock. The mylonites are partly overprinted by recrystallised, statically grown polyhalite grains. Metamorphic reaction fabrics of polyhalite fibres between blödite (or astrakhanite) [Na2Mg(SO4)2.4H2O] and anhydrite have also been found. According to previous reports, blödite may occur primarily as nodules or intergrown with löweite. Reaction fabrics may have formed by exsolution, (re-)crystallisation, parallel growth or replacement. This fabric type was only found in one sample in relation with the decomposition of blödite at ca. 61 °C in the presence of halite or slightly above, testifying, therefore, a late stage prograde fabric significantly younger than the main polyhalite formation.

  • A low-temperature ductile shear zone: The gypsum-dominated western extension of the brittle Fella-Sava Fault, Southern Alps.
    J. Struct. Geol. (IF 3.128) Pub Date : 2015-11-03
    Esther Maria Bartel,Franz Neubauer,Bianca Heberer,Johann Genser

    Based on structural and fabric analyses at variable scales we investigate the evaporitic gypsum-dominated Comeglians-Paularo shear zone in the Southern Alps (Friuli). It represents the lateral western termination of the brittle Fella-Sava Fault. Missing dehydration products of gypsum and the lack of annealing indicate temperatures below 100 °C during development of the shear zone. Despite of such low temperatures the shear zone clearly exhibits mylonitic flow, thus evidencing laterally coeval activity of brittle and viscous deformation. The dominant structures within the gypsum rocks of the Lower Bellerophon Formation are a steeply to gently S-dipping foliation, a subhorizontal stretching lineation and pure shear-dominated porphyroclast systems. A subordinate simple shear component with dextral displacement is indicated by scattered σ-clasts. Both meso- and microscale structures are characteristic of a subsimple shear type of deformation with components of both coaxial and non-coaxial strain. Shortening in a transpressive regime was accommodated by right-lateral displacement and internal pure shear deformation within the Comeglians-Paularo shear zone. The shear zone shows evidence for a combination of two stretching faults, where stretching occurred in the rheologically weaker gypsum member and brittle behavior in enveloping lithologies.

  • Structure and evolution of a rocksalt-mudrock-tectonite: The haselgebirge in the Northern Calcareous Alps.
    J. Struct. Geol. (IF 3.128) Pub Date : 2011-05-01
    Christoph Leitner,Franz Neubauer,János L Urai,Johannes Schoenherr

    The Northern Calcareous Alps are part of the Eastern Alps in Austria and Germany. The Mesozoic units of this fold-and-thrust belt were detached, thrusted and stacked along the evaporitic Haselgebirge Formation. Exposed in salt mines, rocksalt and mudrock form a two component tectonite: The rock type "haselgebirge" consists of 10-70 wt % halite with silt- to gravel- or block-sized components within a halite matrix, and the "kerngebirge" with >70 wt % halite. All rock types studied are fault rocks. By use of a temperature-independent subgrain size piezometer, the paleo-differential stress of halite was calculated at ca. 2.5 MPa in Altaussee and ca. 4.5 MPa in Berchtesgaden. Including data from a grain-size piezometer, temperatures were estimated at ca. 150 ± 20 °C and 110 ± 10 °C. This implies very high strain rates, which are about 10-10-10-9 s-1. During the tectonic movement, the halite deformed, recrystallized, and crystallized as veins in mudrock fractures. We interpret high overpressure of the pore fluid to have significantly contributed to fracturing of the mudrock.

  • Displacement-length scaling of brittle faults in ductile shear.
    J. Struct. Geol. (IF 3.128) Pub Date : 2011-11-01
    Bernhard Grasemann,Ulrike Exner,Cornelius Tschegg

    Within a low-grade ductile shear zone, we investigated exceptionally well exposed brittle faults, which accumulated antithetic slip and rotated into the shearing direction. The foliation planes of the mylonitic host rock intersect the faults approximately at their centre and exhibit ductile reverse drag. Three types of brittle faults can be distinguished: (i) Faults developing on pre-existing K-feldspar/mica veins that are oblique to the shear direction. These faults have triclinic flanking structures. (ii) Wing cracks opening as mode I fractures at the tips of the triclinic flanking structures, perpendicular to the shear direction. These cracks are reactivated as faults with antithetic shear, extend from the parent K-feldspar/mica veins and form a complex linked flanking structure system. (iii) Joints forming perpendicular to the shearing direction are deformed to form monoclinic flanking structures. Triclinic and monoclinic flanking structures record elliptical displacement-distance profiles with steep displacement gradients at the fault tips by ductile flow in the host rocks, resulting in reverse drag of the foliation planes. These structures record one of the greatest maximum displacement/length ratios reported from natural fault structures. These exceptionally high ratios can be explained by localized antithetic displacement along brittle slip surfaces, which did not propagate during their rotation during surrounding ductile flow.

  • The lateral boundary of a metamorphic core complex: The Moutsounas shear zone on Naxos, Cyclades, Greece.
    J. Struct. Geol. (IF 3.128) Pub Date : 2013-09-01
    Shuyun Cao,Franz Neubauer,Manfred Bernroider,Junlai Liu

    We describe the structure, microstructures, texture and paleopiezometry of quartz-rich phyllites and marbles along N-trending Moutsounas shear zone at the eastern margin of the Naxos metamorphic core complex (MCC). Fabrics consistently indicate a top-to-the-NNE non-coaxial shear and formed during the main stage of updoming and exhumation between ca. 14 and 11 Ma of the Naxos MCC. The main stage of exhumation postdates the deposition of overlying Miocene sedimentary successions and predates the overlying Upper Miocene/Pliocene conglomerates. Detailed microstructural and textural analysis reveals that the movement along the Moutsounas shear zone is associated with a retrograde greenschist to subgreenschist facies overprint of the early higher-temperature rocks. Paleopiezometry on recrystallized quartz and calcite yields differential stresses of 20-77 MPa and a strain rate of 10-15-10-13 s-1 at 350 °C for quartz and ca. 300 °C for calcite. Chlorite geothermometry of the shear zone yields two temperature regimes, 300-360 °C, and 200-250 °C. The lower temperature group is interpreted to result from late-stage hydrothermal overprint.

  • Preferential cataclastic grain size reduction of feldspar in deformation bands in poorly consolidated arkosic sands.
    J. Struct. Geol. (IF 3.128) Pub Date : 2012-10-01
    Ulrike Exner,Cornelius Tschegg

    This study presents microstructural as well as bulk and mineral chemical investigations of deformation bands in uncemented, friable arkosic sands of Miocene age (Vienna Basin, Austria). Our microstructural study indicates grain size reduction by grain flaking in deformation bands with small offsets (0.5-8 cm), and dominant intragranular fracturing and cataclasis of altered feldspar grains at larger displacements (up to 60 cm). Relative to quartz, the sericitized feldspar grains are preferably fractured and abraded, which additionally leads to an enrichment of mainly phyllosilicates by mechanical expulsion from feldspar. Both cataclasis of quartz and feldspar grains and enrichment of phyllosilicates result in grain size reduction within the deformation bands. The measured reduction in porosity of up to 20% is in some cases associated with a permeability reduction, reflected in the retention of iron-oxide rich fluids along deformation bands. These deformation bands formed at very shallow burial depths in unconsolidated sediments indicate that fault sealing may occur in the absence of chemical alteration of the deformation bands and lead to a compartmentalization of a groundwater or hydrocarbon reservoir.

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