Department of Geology, Johannes Gutenberg Universität, Becherweg 21, 55099 Mainz, Germany, E-mail: koehn@mail.uni-mainz.de
Abstract
Striped bedding-veins are veins that lie subparallel to bedding and have an internal layering or lineation at a small angle to the veins' long axis. They form during bedding-parallel slip and can be used as shear sense indicators. Solid inclusion trails produce the visible internal layering or lineation and track the opening direction of the veins. Elongate quartz crystals however can be oriented at an angle of up to 80° to the opening direction, are non-tracking, and contain almost no information on the shear sense. The striped bedding-veins can be separated into three types according to the geometry of their internal segmentation. Veins of type B opened parallel to jogs oriented at a low angle to bedding, veins of type J opened parallel to jogs oriented at a high angle to bedding and veins of type O opened orthogonal to bedding and jogs. Striped bedding-veins of types B and J contain crack-seal inclusion bands and displacement parallel inclusion trails. Striped bedding-veins of type O feature only crack-seal inclusion bands. The example of striped bedding-veins presented in this paper from the Orobic Alps of Italy belongs to type B. The lineation in the veins and the orientation of the inclusion bands and inclusion trails, as well as the orientation of steps in the vein wall, can be used to determine the sense of shear and the direction and amount of vein opening or bedding-parallel slip.
Koehn, D. and Passchier, C.W. (2000), Shear sense indicators in Striped Bedding-Veins. Journal of Structural Geology 22/8, 1141-1151.
* Institut für Geowissenschaften, Tectonophysics, Johannes Gutenberg Universität, Becherweg 21, 55099 Mainz, Germany, E-mail: koehn@mail.uni-mainz.de
°Geologie-Endogene Dynamik, RWTH Aachen, Lochnerstrasse 4-20, 52056 Aachen, Germany
Abstract
A two-dimensional computer model ("Fringe Growth") is presented that is used to simulate the incremental growth of crystal fibres in undeformed antitaxial strain fringes. The user can define the shape of a core-object (e.g. a pyrite crystal), the growth velocity and anisotropy of growing crystals, the rotation of fringes and core-object with respect to a horizontal datum and with respect to each other, and the opening velocity of fringes. Growth is simulated by movement of nodes connecting line segments that define the grain boundaries. Modelling results predict that face-controlled strain fringes will grow around smooth core-objects and strain fringes with displacement-controlled and face-controlled fibres around core-objects with rough surfaces. The surface roughness of the core-object determines if fibres in the fringes track the opening trajectory, since fibres follow asperities on the surface of the core-object. Rotation of the core-object and the fringes with respect to an external reference frame and with respect to each other influences the geometry of the fibres. Our modelling results indicate that fibre growth direction is not directly dependent on the orientation of the extensional instantaneous stretching axes or the finite maximum strain axes.
Koehn, D., Hilgers, C., Bons, P.D., Passchier, C.W., (2000) Numerical simulation of fibre growth in antitaxial strain fringes. Journal of Structural Geology, 22/9, 1311-1324.
* Institut für Geowissenschaften, Tectonophysics, Johannes Gutenberg Universität, Becherweg 21, 55099 Mainz, Germany, E-mail: koehn@mail.uni-mainz.de
°Universidad de Salamanca Departamento de Geologica, Area de Geodinamica, Plaza de la Merced s/n 37008 Salamanca, Spain; now at: Departamento de Geodinamica, Universidad de Granada, Campus de Fuentenueva s/n, 18071 GRANADA, Spain
Abstract
Antitaxial non-deforming strain fringes from Lourdes, France show complex quartz, calcite and chlorite fibre patterns that grew around pyrites in a slate during non-coaxial deformation. These fringes were modelled using a computer program "Fringe Growth 2.0" which can simulate incremental growth of crystal fibres around core-objects of variable shape. It uses object-centre paths as input, which are obtained from fibre patterns in thin-section. The numerical experiments produced fibre patterns that showed complex intergrowth of displacement-controlled-, face-controlled- and intermediate fibres similar to those in the natural examples. The direction of displacement-controlled growth is only dependent on the relative movement between core-object and fringe, so that core-object rotation with respect to the fringe influences the fibre patterns and produces characteristic asymmetric fibre curvatures. Object-centre paths should be used for kinematic analysis of strain fringes instead of single fibres since the paths represent one fringe as a whole. The lenght along the path can be interpreted in terms of finite strain and the curvature in terms of rigid body rotation of fringes with respect to an external reference frame.
Key words: fibres; strain fringe; numerical experiments; strain analysis
Koehn, D., Aerden, D.G.A.M., Bons, P.D., Passchier, C.W., Computer experiments to investigate complex fibre patterns in natural antitaxial strain fringes. In review, Journal of Metamorphic Geology
KOEHN,Daniel,Institut fuer Geowissenschaften,Johannes GutenbergUniversitaet,55099 Mainz, Germany, koehn@mail.uni-mainz.de; BONS,Paul D.,Department of Earth Sciences, Monash University, Clayton VIC 3168, Australia; PASSCHIER, Cees W., Institut fuer Geowissenschaften,Johannes Gutenberg Universitaet,55099 Mainz, Germany
Fibrous crystals are often used to determine the opening trajectory of veins and strain fringes. However, not all fibrous crystals track opening trajectories and it is uncertain, which parameters determine the tracking ability of such crystals. A study of the geometry of fibrous veins has shown, that a distinction can be made between elongated crystals and real fibers. It can be shown, that elongated crystals grow in crack-seal veins where the growth is restricted by the wallrock after each cracking event. Real fibers grow mostly in permanently open tension-veins and strain fringes, where their growth is restricted by diffusion of material towards the crystal interface. Both processes of crystal growth have been modelled using a computer program that simulates incremental growth and opening processes for isotropically and anisotropically growing crystals. Results show that isotropically growing real fibers can successfully track the incremental opening vector of a vein or a strain fringe. The tracking ability of these fibers is a direct function of the morphology of the growth-surface. In antitaxial strain fringes, the roughness of the central rigid object is the parameter controlling the direction of fiber growth. If the object-morphology is smooth, a face-controlled strain fringe will develop. In the case of a rough object-morphology, the fringe will be displacement-controlled. In veins the roughness of the wallrock determines whether the fibers are tracking or not. If the wallrock is smooth, crystals cannot be used to evaluate the opening vector. The results of the modelling have important implications for the interpretation of veins and strain fringes. Before using crystals as displacement indicators two things need to be checked: (1) the dominant process of formation, and (2) the roughness of the growth surface.
veins, strain fringes, fibers, tracking, crack-seal
Köhn, D., Bons, P.D. & Passchier, C.W. (1998) Modelling the tracking ability of fibrous crystals. GSA Abstracts w. Prog. 30(7), p. A-197.
Crystal fibres in strain fringes have been used by many geologists to reconstruct the host rock history. However the mechanism of fibre growth is not well understood. A 2D computer model is presented that can simulate the growth of crystals in undeformed antitaxial strain fringes by incremental growth steps. In the model the grains in the fringe grow towards the core object into a narrow open space that is created by the strained matrix. The form and surface morphology of the core object can be defined by the user as well as the opening and rotation of the fringes, rotation of the core object and the crystal morphology of the growing grains. Growth is simulated by small incremental movements of nodes that connect short line segments that outline the grains. Systematic runs of the program have been carried out varying the values: (1) core object form, (2) core object surface morphology, (3) opening rate compared to the growth rate, (4) core object rotation and (5) fringe rotation. The most important parameters that control the tracking capability of single fibres are the core object surface morphology and the opening rate compared to the growth rate. Face controlled and displacement controlled fringes are endmembers, with most of the fringes containing both tracking and non-tracking fibres. Purely face controlled fringes grow around round core objects with a smooth surface morphology. As soon as the core object has edges or a rough surface morphology, grain boundaries of fibres get locked on ridges and follow the movements of the core object. Fibre growth is not directly influenced by the orientation of the incremental extensional axis of strain. Careful simulation of incremental fibre growth steps of displacement controlled fibres can reveal information on the orientation and magnitude of the incremental extensional axis of strain, on the vorticity of the flow and on the rigid body rotation of the fringes and the core object.
Köhn, D., Passchier, C.W. & Bons, P.D. (1999) Simulation of fibre growth in strain fringes. STSG, Schweizerisches Tektoniktreffen, Lausanne (Switzerland), Feb.
Fibrous crystals in strain fringes are commonly used in modern kinematic analysis to determine the incremental strain history of deformed rocks. Displacement-controlled fibres are thought to track the opening trajectory of these strain fringes, representing the incremental extension direction or Z-axis path. Computer modelling shows that the roughness of the central object in the fringes is the critical parameter that controls the tracking ability of fibres. Face-controlled non-tracking fibres develop around smooth objects and displacement-controlled fibres around objects with a rough surface. Whether or not the displacement-controlled fibres are tracking the opening trajectories of the fringe depends on the rotation of the central object. In pure shear progressive deformation, the computer modelling predicts that displacement-controlled fibres are indeed tracking the opening trajectories of the strain fringe, and represent the Z-axis path if the central object is not rotating. Non-coaxial progressive deformation leads to more complex fibre geometries. For example, displacement controlled strain fringes that formed during non-coaxial progressive deformation around rough round pyrites from the Yilgarn Craton, Australia, show crystal fibres that have a pronounced stronger curvature than the fringe. Computer modelling of such fringes confirms that the observed fibre geometries can only be produced if the central object is rotating. This has important implications for the interpretation of displacement-controlled crystal fibres in strain fringes. Fibres in fringes are NOT necessarily tracking the incremental strain history of rocks: they are reflecting the movements of the central object. Only if the central object is not rotating, the displacement-controlled fibres are tracking the opening vector of the fringe that is representing the Z-axis path. If the object is rotating, the fibres are tracking both the opening of the fringe and the rotation of the central object. In this case, the effect of the rotation has to be determined and fringe geometry can only be translated in terms of a Z-axis path by advanced kinematic analysis.
Köhn, D., Passchier, C.W. & Bons, P.D. (1999) Kinematic analysis of strain fringes. EUG 10, Strasbourg (France), March.
Veins with inclusion bands and fibrous or elongate crystals are commonly used by geologists to evaluate a part of the geologic history of the hostrock. If the veins contain crack-seal inclusion bands and elongate crystals they are termed crack-seal veins and are thought to form by a process of repeated fracturing and sealing (Ramsay 1980). Banded-bedding-parallel-veins (BBP-veins) are a special kind of crack-seal veins that developed during bedding parallel slip of sedimentary layers. Solid and liquid inclusion bands in the veins are excellent shear sense indicators and can be used to determine the opening direction. Elongate quartz crystals are oriented at an angle of up to 80° to the opening direction, are non-tracking and contain almost no information on the shear sense. This has important implications for the interpretation of crack-seal veins in general. Elongate crystals are not necessarily tracking the displacement vector of crack-seal veins, the crack-seal inclusion bands themselves have to be used to evaluate the opening direction. The BBP-veins can be separated into three types according to the geometry of their inclusion bands. Veins of type B opened parallel to the bedding, veins of type J opened parallel to jogs on the bedding and veins of type O opened orthogonal to bedding and jogs. BBP-veins of types B and J contain two groups of inclusion bands, crack-seal inclusion bands (cs-bands) and wall rock-parallel inclusion bands (wp-bands) whereas BBP-veins of type O feature only cs-bands. A lineation in the veins and the orientation of the cs-bands and wp-bands as well as the orientation of steps in the wall rock can be used to determine the sense of shear and the direction of vein opening. The amount of bedding parallel slip can be measured along wp-bands in BBP-veins of type B and J and along lines connecting jogs in cs-bands in BBP-veins of type O. The lineation in the BBP-veins is defined by the wp-bands and not by the long axis of crystals. These veins should therefore not be confused with slickenfibres.
References:
Ramsay, J. (1980) The crack-seal mechanism of rock deformation, Nature, 284,135-139.
Koehn, D., Passchier, C.W. (1999) A Classification for Banded-Bedding-Parallel-Veins. Def. mech Conf., Neustadt a.d. Weinstrasse, Germany, 22-24 March.
To model complex fibrous strain fringes, the program "Fringe Growth" was developed. "Fringe Growth" can simulate incremental growth of crystal fibres around core-objects of variable shape during coaxial, non-coaxial and polyphase deformation. Examples from the Yilgarn Craton, Australia and from Lourdes, France, show complex fibrous growth of quartz, calcite and chlorite around pyrites in a matrix deformed during non-coaxial and polyphase deformation. Simulations have been carried out with "Fringe Growth" to reproduce the observed fibre and fringe geometries. The program is able to separate translation and rotation components of the relative movement between object and fringe. The simulations led to the following conclusions: 1) Even though most of the fibres in the fringes show displacement-controlled growth, there are face-controlled and intermediate fibres in the rims of the fringes, within displacement-controlled large fibre bands, and next to suture lines. Intermediate fibres contain face- and displacement-controlled segments which are hard to distinguish. Displacement- and face-controlled fringes are endmembers but most fringes contain both displacement- and face-controlled fibres. Only purely displacement-controlled fibres may be suitable for a strain analysis. 2) The simulations show that elongated objects do generally not rotate relative to their fringes, because the large contact area between them forces them to rotate at the same rate. When the shape of an object is close to spherical, they may rotate with respect to the fringes. Spherical pyrites can rotate the most. This rotation of the object with respect to the fringes influences the fibre patterns. Single fibres are in that case unsuitable for strain analysis. To sum up, opening paths of strain fringes should be used by geologists for strain analysis instead of single fibres. Single fibres only reflect the opening path of a fringe in a few very restricted cases.
strain, fibres, deformation, rotation, fringes
Koehn, D., Bons, P.D., Passchier, C.W., Aerden, D.G.A.M., (1999) Interpretation of fibre growth in strain fringes, GSA Abstracts w. Prog.
Fibres in antitaxial strain fringes that develop around rigid objects are commonly used by geologists for incremental structural analysis even though the development of complex fibre patterns is not well understood. We present a combined numerical and experimental study of the incremental development of antitaxial strain fringes and their fibre patterns. Strain fringes were created experimentally around wooden objects in a simple-shear box containing a Newtonian matrix. Liquid parafin wax was poured into dilatant sites next to the core-object and crystalized in several stages to model the development of antitaxial non-deforming strain fringes. The relative movement of the fringes with respect to the core-object was transfered into the computer program "Fringe Growth" that can simulate incremental growth of fibres. Conclusions are: 1) Core-object and fringes rotate with respect to each other producing complex fibre patterns. 2) Curved fibres are an effect of rigid body rotation of the fringes with respect to the shear-zone boundary and with respect to the core-object. 3) Fringes do not open necessarily parallel to the extensional incremental stretching axis. 4) The shape of the core-object and its initial orientation with respect to the shear-zone boundary have a major influence on fibre patterns and fringe forms. We present a method to interpret complex fibre patterns in antitaxial strain fringes based on our experimental observations.
Koehn, D., Bons, P.D., Passchier, C.W., (2000) Numerical and experimental modelling of strain fringes, Geoscience 2000, Manchester, Abstactvolume 126.