Abstract ‘Titanium-in-Quartz’ geothermometry suggests quartzites could yield reliable temperature estimates. We here apply four calibrations of the titanium-in-quartz geothermometer to contact-metamorphosed quartzites surrounding the Ballachulish Igneous Complex, Scotland. Two agree broadly with thermal modelling and pre-existing geothermometry; two give temperatures consistently too low. As reported in earlier studies, the technique suffers from difficulties in analysing low titanium (Ti) levels with high spatial and analytical precision. However, this study finds that the critical problem is one of Ti heterogeneity, which poses difficulties in constraining the chemical activity of Ti during quartz growth under metamorphic conditions. Scanning electron microscope-cathodoluminescence (SEM-CL) textures support an interpretation of extensive Ti disequilibrium despite the presence of rutile, indicating dynamic interplay between grain boundary diffusion, fluid/melt percolation and grain growth. The strong zonation suggests a possible geothermometer based on apparent volume diffusion of Ti-in-quartz to derive grain growth histories. Analysis of rutile–quartz interaction implies peak contact temperatures of 645±12 °C, precise but reliant on external estimates of cooling rate from thermal models. Our conclusions support caution in applying Ti-in-quartz geothermometry in aureole settings. However, rutile–quartz juxtaposition prior to heating to >600 °C defines a Ti diffusion couple, employable as a thermometer if cooling rates are constrained by other means.

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.

Abstract The Nanga Parbat Massif (NPM), Pakistan Himalaya, is an exhumed tract of Indian continental crust and represents an area of active crustal thickening and exhumation. While the most effective way to study the NPM at depth is through seismic imaging, interpretation depends upon knowledge of the seismic properties of the rocks. Gneissic, ‘mylonitic’ and cataclastic rocks emplaced at the surface were sampled as proxies for lithologies and fabrics currently accommodating deformation at depth. Mineral crystallographic preferred orientations (CPO) were measured via scanning electron microscope (SEM)/electron backscatter diffraction (EBSD), from which three-dimensional (3D) elastic constants, seismic velocities and anisotropies were predicted. Micas make the main contribution to sample anisotropy. Background gneisses have highest anisotropy (up to 10.4% shear-wave splitting, AVs) compared with samples exhibiting localized deformations (e.g. ‘mylonite’, 4.7% AVs; cataclasite, 1% AVs). Thus, mylonitic shear zones may be characterized by regions of low anisotropy compared to their wall rocks. CPO-derived sample elastic constants were used to construct seismic models of NPM tectonics, through which P-, S- and converted waves were ray-traced. Foliation orientation has dramatic effects on these waves. The seismic models suggest dominantly pure-shear tectonics for the NPM involving horizontal compression and vertical stretching, modified by localized ductile and brittle (‘simple’) shear deformations.

Abstract Since the early descriptions published by Callaway in 1884, the gently dipping mylonites exposed along the Moine Thrust at the Stack of Glencoul have drawn generations of geologists to the northern part of the Assynt district. These mylonites, derived from Cambrian quartzites (footwall) and Moine pelites and psammites (hanging wall), have figured prominently in: a) early research into the influence of crystal plastic deformation and recrystallization on microstructural and crystal fabric evolution; b) debates on the kinematic interpretation of macro- and micro-structures and crystal fabrics; and c) debates on the tectonic significance of such kinematic data. In this paper first we briefly review the historical aspects of this research and then, using both previously published and unpublished data, document the finite strain and quartz fabric development at this classic mylonite locality. A tectonic interpretation of these data, together with quantitative estimates of flow vorticities associated with mylonite formation at the Stack of Glencoul, are presented in a companion paper by Law (2010) .

Abstract The link between petrofabric (LPO) and seismic properties of an amphibolite-facies quartzo-feldspathic shear zone is explored using SEM/EBSD. The shear-zone LPO develops by a combination of slip systems and dauphine twinning, with a -maximum parallel to lineation ( X ) and c -maximum normal to mylonitic foliation ( XY ). The LPO are used to predict elastic parameters, from which the three-dimensional seismic properties of different shear-zone regions are derived. Results suggest that LPO evolution is reflected in the seismic properties but the precise impact is not simple. In general, the P-wave velocity ( V P ) minimum is parallel to the a -axis maximum; the direction of maximum shear-wave splitting ( AV S ) is parallel to mylonitic foliation; and the V P maximum and AV S minimum are parallel to the c -axis maximum. The seismic anisotropy predicted is significant and increases from shear zone wallrock to mature mylonite. The P-wave anisotropy ranges from 11 to 13%, fast and slow shear waves’ anisotropies range from 6 to 15% and the magnitude of shear-wave splitting ranges from 9 to 16%. Nevertheless, such anisotropy requires a considerable thickness of rock with this LPO before it becomes seismically visible (i.e. 100s of m for local earthquakes; 10s of m for controlled source experiments). However, reflections and mode conversions provide much better resolution, particularly across tectonic boundaries. The low symmetry and strong anisotropy due to the LPO suggest that multi-azimuth wide-angle reflection data may be useful in the determination of the deformation characteristics of deep shear zones.