Daniel J. Tatham

Deformation Mechanisms, Rheology and Tectonics

This collection of papers presents recent advances in the study of deformation mechanisms and rheology and their applications to tectonics. Many of the contributions exploit new petrofabric techniques, particularly electron backscatter diffraction, to help understand evolution of rock microstructure and mechanical properties. Papers in the first section (lattice preferred orientations and anisotropy) show a growing emphasis on the determination of elastic properties from petrofabrics, from which acoustic properties can be computed for comparison with in-situ seismic measurements. Such research will underpin geodynamic interpretation of large-scale active tectonics. Contributions in the second section (microstructures, mechanisms and rheology) study the relations between microstructural evolution during deformation and mechanical properties. Many of the papers explore how different mechanisms compete and interact to control the evolution of grain size and petrofabrics. Contributors make use of combinations of laboratory experiments, field studies and computational methods, and several relate microstructural and mechanical evolution to large-scale tectonic processes observed in nature.

Constraints on the seismic properties of the middle and lower continental crust

Abstract For the past two decades geodetic measurements have quantified surface displacement fields for the continents, illustrating a general complexity. However, the linkage of geodetically defined displacements in the continents to mantle flow and plate tectonics demands understanding of ductile deformations in the middle and lower continental crust. Advances in seismic anisotropy studies are beginning to allow such work, especially in the Himalaya and Tibet, using passive seismological experiments (e.g. teleseismic receiver functions and records from local earthquakes). Although there is general agreement that measured seismic anisotropy in the middle and lower crust reflects bulk mineral alignment (i.e. crystallographic preferred orientation, CPO), there is a need to calibrate the seismic response to deformation structures and their kinematics. Here, we take on this challenge by deducing the seismic properties of typical mid- and lower-crustal rocks that have experienced ductile deformation through quantitative measures of CPO in samples from appropriate outcrops. The effective database of CPO and hence seismic properties can be expanded by a modelling approach that utilizes ‘rock recipes’ derived from the as-measured individual mineral CPOs combined in varying modal proportions. In addition, different deformation fabrics may be diagnostic of specific deformation kinematics that can serve to constrain interpretations of seismic anisotropy data from the continental crust. Thus, the use of ‘fabric recipes’ based on subsets of individual rock fabric CPO allows the effect of different fabrics (e.g. foliations) to be investigated and interpreted from their seismic response. A key issue is the possible discrimination between continental crustal deformation models with strongly localized simple-shear (ductile fault) fabrics from more distributed (‘pure-shear’) crustal flow. The results of our combined rock and fabric-recipe modelling suggest that the seismic properties of the middle and lower crust depend on deformation state and orientation as well as composition, while reliable interpretation of seismic survey data should incorporate as many seismic properties as possible.

Mica-controlled anisotropy within mid-to-upper crustal mylonites: an EBSD study of mica fabrics in the Alpine Fault Zone, New Zealand

Abstract The lattice preferred orientation (LPO) of both muscovite and biotite were measured by electron backscatter diffraction (EBSD) and these data, together with the LPOs of the other main constituent minerals, were used to produce models of the seismic velocity anisotropy of the Alpine Fault Zone. Numerical experiments examine the effects of varying modal percentages of mica within the fault rocks. These models suggest that when the mica modal proportions approach 20% in quartzofeldspathic mylonites the intrinsic seismic anisotropy of the studied fault zone is dominated by mica, with the direction of the fastest P and S wave velocities strongly dependent on the mica LPOs. The LPOs show that micas produce three distinct patterns within mylonitic fault zones: C-fabric, S-fabric and a composite S–C fabric. The asymmetry of the LPOs can be used as kinematic indicators for the deformation within mylonites. Kinematic data from the micas matches the kinematic interpretation of quartz LPOs and field data. The modelling of velocities and velocity anisotropies from sample LPOs is consistent with geophysical data from the crust under the Southern Alps. The Alpine Fault mylonites and parallel Alpine schists have intrinsic P-wave velocity anisotropies of 12% and S-wave anisotropies of 10%.

Permeability and seismic velocity and their anisotropy across the Alpine Fault, New Zealand: An insight from laboratory measurements on core from the Deep Fault Drilling Project phase 1 (DFDP-1)

The Alpine Fault, a transpressional plate boundary between the Australian and Pacific plates, is known to rupture quasi-periodically with large magnitude earthquakes (Mw ~8). The hydraulic and elastic properties of fault zones are thought to vary over the seismic cycle, influencing the nature and style of earthquake rupture and associated processes. We present a suite of laboratory permeability and P (Vp) and S (Vs) wave velocity measurements performed on fault lithologies recovered during the first phase of the Deep Fault Drilling Project (DFDP-1), which sampled principal slip zone (PSZ) gouges, cataclasites and fractured ultramylonites, with all recovered lithologies overprinted by abundant secondary mineralization, recording enhanced fluid-rock interaction. Core material was tested in three orthogonal directions, orientated relative to the down-core axis and, when present, foliation. Measurements were conducted with pore pressure (H2O) held at 5 MPa over an effective pressure (Peff) range of 5 - 105 MPa. Permeabilities and seismic velocities decrease with proximity to the PSZ with permeabilities ranging from 10-17 to 10-21 m2 and Vp and Vs ranging from 4400 - 5900 m/s in the ultramylonites/cataclasites and 3900 - 4200 m/s at the PSZ. In comparison with intact country rock protoliths, the highly variable cataclastic structures and secondary phyllosilicates and carbonates have resulted in an overall reduction in permeability and seismic wave velocity, as well as a reduction in anisotropy within the fault core. These results concur with other similar studies on other mature, tectonic faults in their interseismic period.

Deformation mechanisms, rheology and tectonics: microstructures, mechanics and anisotropy: introduction

This special publication of the Geological Society of London presents recent advances in the study of deformation mechanisms and rheology and their application to tectonics. We have subdivided the papers into two themed sections. The inference of deformation processes, conditions and rheology at depth in active tectonic settings is of fundamental importance to a quantitative geodynamic understanding of deformation in the Earth. The papers in the section on Lattice Preferred Orientations and Anisotropy are extremely important as they underpin our ability to make such geodynamic interpretations from global seismic data. These papers reflect the growing emphasis on the determination of elastic properties from microstructures, from which acoustic properties can be computed for comparison with in situ seismic measurements. The component of the microstructure that receives most attention is the lattice preferred orientation (LPO), otherwise known as the crystallographic preferred orientation (CPO) or the texture (the term used in material science and metallurgy). The papers include new LPO measurements (made almost exclusively by the relatively new technique of electron backscatter diffraction or EBSD), exploration of the significance of these data for seismic properties of both the crust and the mantle and modelling of LPO generation. An invited contribution from Mainprice and colleagues introduces a computational toolbox to help researchers calculate anisotropic physical properties from their LPO data. Rock microstructures evolve during deformation and rock physical properties, including both elastic properties and creep rheology, evolve with the microstructures as a function of strain and time. The section on Microstructures, Mechanisms and Rheology reflects the fundamental importance of understanding microstructural evolution to our ability to estimate deformation processes and conditions from recovered samples or geophysical data and to our modelling of tectonics. An invited contribution from Austin focuses on some of the key issues from the last few decades: how different mechanisms (grain size sensitive and grain size insensitive) compete and interact to control the evolution of grain size and LPO. Many of the other papers touch on these issues and make use of combinations of laboratory experiments, field studies and computational methods to explore the controls on microstructural evolution and to relate microstructural evolution to rheology and largescale tectonic processes. It is clear from this collection of papers that resolution of the controls on microstructural evolution in rocks remains at the cutting edge of Earth sciences.

Inferences from shear zone geometry: an example from the Laxfordian shear zone at Upper Badcall, Lewisian Complex, NW Scotland

Abstract This contribution presents a Laxfordian age shear zone near Upper Badcall, NW Scotland, as an example of using field data and theory to assess the kinematics and nature of deformation in a shear zone. The deformation zone includes quartzofeldspathic background gneiss and a dolerite dyke that cuts the gneissic banding at a high angle. A detailed field description of the deformation zone, which is critically discussed in terms of pure and simple shear, is presented. Analysis of gneissic banding and mineral lineation data, together with a consideration of the outcrop pattern, shows that the deformation zone is best described in terms of a simple shear zone with varying finite stretching direction. To analyse this deformation we introduce the concept of local plane strain. Although the deformation of the zone as a whole is 3D, at each point there is a direction in which it does not change its length. This direction is perpendicular to the local shear direction and so varies in orientation across the shear zone. In a reference frame defined at a point, the deformation can thus be understood in terms of conventional simple shear. Details of strain are hence determined according to this conclusion. A stereographic method for the determination of the reorientation of lines is used to calculate shear strain. The shear strain values across the shear zone are then used to restore the sheared dolerite dyke to its undeformed geometry. The success of the restoration provides supports for the strain calculation and also the conclusion of simple shear deformation.

Microstructure Evolution During Creep and Annealing of Minerals and Rocks

In the Earth’s crust and mantle syn-tectonic (dynamic) and post-tectonic (static) recrystallization of rocks can modify grain sizes, shapes and crystallographic orientations [1]. This affects physical properties and anisotropies and is central to the interpretation of the mechanical behavior of rocks in major fault zones along plate boundaries, geological terrains in mountain belts [2] and seismic anisotropy data. Bulk measurements of lattice preferred orientations (LPOs), using texture goniometry and more recently neutron and synchrotron techniques, have been available since the middle of last century. However in the last 2 decades the advent of electron backscatter diffraction (EBSD) in the scanning electron microscope (SEM) has offered the opportunity to measure spatially resolved crystallographic misorientations between grains in the same mineral and/or between phases [3]. Particularly, EBSD in the SEM is essential to our understanding of recovery, recrystallization and grain growth during dislocation creep and annealing of minerals and rocks. Recrystallization models, such as sub-grain rotation, grain boundary bulging and migration, currently applied in the Earth sciences produce microstructures that can be tested using EBSD. Data available mainly on rocksalt, olivine and MgO [4; 5] are consistent with current recrystallization models (Fig.1a), however microstructures observed in calcite, quartz, plagioclase and orthopyroxene (all minerals that twin) cannot be accounted for by such models alone [6; 7]. These show that newly recrystallized grains are characterized by high misorientation angles between each other and to their parent grains (Fig.1b), and display near to random misorientation axis distributions. Grain boundary sliding has been proposed as a deformation mechanism that could account for these observations. More recently we have observed that the same microstructures could be explained by sub-grain rotation, grain boundary bulging and migration assisted by the presence of twin boundaries. This can result in complex, segmented boundaries that may be unstable and rapidly become general high angle boundaries (Fig.2a and b) [8]. To date this twin assisted recrystallization mechanism has been documented only in experimentally deformed then annealed rock samples, it occurs rapidly and evidence of its evolution is difficult to gather in final microstructures. In order to identify twin assisted recrystallization confidently in deformed and annealed microstructures, new EBSD data on general and special boundary geometries, misorientations, grain distortions, dynamics and kinematics of low and high angle grain boundary migration are needed. These may be achieved by performing in-situ annealing experiments in the SEM [9] on suitable geological materials, such as for example anhydrite. Nonetheless such experiments are challenging and should always be substantiated by standard rock deformation tests. A novel time-series EBSD analysis technique is currently under development. This uses a custom built SEM stage to analyze cylindrical sample surfaces accurately and sequentially, after each stress, strain, temperature or time increment achieved in a deformation apparatus. Thus time-series EBSD coupled with rock deformation experiments for the first time, will yield information on microstructure evolution at the scale of individual grains and grain boundaries. This will be the basis upon which more realistic recrystallization models can be constructed and will underpin our interpretation of synand posttectonic processes in the Earth’s crust and mantle. Microsc Microanal 15(Suppl 2), 2009 Copyright 2009 Microscopy Society of America doi: 10.1017/S1431927609096287 412