Martin Casey

Ductile extrusion of the Higher Himalayan Crystalline in Bhutan: evidence from quartz microfabrics

Quartz textures measured from deformed quartz tectonites within the Lesser Himalaya and Higher Himalaya Crystalline of Bhutan show similar patterns. Orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense: the asymmetry of c-axis and a-axis patterns consistently indicates top-to-the-south shearing. The obliquity of the texture and the inferred finite strain (plane strain to moderately constrictional), suggest the strain regime had a combination of rotational and irrotational strain path. In most of the samples from the Bhutan Himalaya, the inferred deformation mechanisms suggest moderate- to high-temperature conditions of deformation that produced the observed crystallographic preferred orientation. Much higher temperature of deformation is indicated in the quartz veins from a leucogranite. The observed ductile deformation is pervasively developed in the rocks throughout the investigated area. The intensity of deformation increases only slightly in the vicinity of the Main Central Thrust. Simultaneous southward shearing within a large part of the Higher Himalaya Crystalline near and above the Main Central Thrust and normal faulting across the South Tibetan Detachment, is explained by the tectonically induced extrusion of a ductily deforming wedge. The process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes. Channel flow accounts for some observed phenomena in the Himalayan orogen such as inverted metamorphic sequences near the Main Central thrust, not related to an inversion of isotherms, and the syntectonic emplacement of leucogranites into the extruding wedge, locally leading to an inversion of isotherms due to heat advection.

Continental breakup in magmatic provinces: An Ethiopian example

Mechanical processes largely control the along-axis segmentation of continental rifts; however, asthenospheric processes strongly influence the along-axis segmentation of mid- ocean ridges. We examine the distribution of strain and magmatism in the transitional active Main Ethiopian rift. Magmatic construction, diking, and faulting during the past 1.6 m.y. have created ∼20-km-wide, ∼60-km-long magmatic segments with or without axial valleys. Magmatic segments are arranged en echelon within the ∼100-km-wide rift valley bounded by mid-Miocene border faults. Geodetic data show that magmatic segments accommodate >80% of the strain across the rift, indicating that border faults are no longer the locus of extension. Comparison with mid-ocean ridges suggests that magmatic segments, rather than detachment faults, mark the ocean-continent boundary in rifts with a ready magma supply. Magmatic margins, therefore, may contain detachments abandoned during continental breakup. The processes of localized dike intrusion with underplating would produce strips of mafic crust transitional to oceanic crust, but without coherent seafloor-spreading magnetic anomalies.

Role of shear in development of the Helvetic fold-thrust belt of Switzerland

The geometric features of the Helvetic nappes of the Alps show intense internal deformation set up predominantly by simple shear during nappe transport. The nappes are bounded by thrusts that parallel incompetent layers and ramp upward stratigraphically across competent units in the direction of tectonic transport; stratal layering is oblique to nappe boundaries. Thrust surfaces formed after ductile shear zones developed in underlying pre-Triassic gneissic basement. The broad zone of shearing narrowed down as deformation increased to form zones of intense shear and ultimately to form thrust faults. Contraction along competent layers obliquely inclined to shear surfaces produced buckle folds. The geometry of these folds was modified by further shearing as their normal limbs became aligned in the extension field of the strain ellipse for simple shear, producing normal faults.

An illustration of the advantages of a complete texture analysis described by the orientation distribution function (ODF) using quartz pole figure data

Abstract The preferred orientation of eight crystal directions of quartz was measured with an X-ray texture goniometer for selected quartz-bearing tectonites with a clear pattern of c -axis preferred orientation. The advantages of data analysis by means of calculations of the orientation distribution function and other forms of data presentation derived from the ODF are demonstrated.

Fluid-flow properties of faults in sandstone: The importance of temperature history

Sandstone rheology and deformation style are often controlled by the extent of quartz cementation, which is a function of temperature history. Coupling findings from deformation experiments with a model for quartz cementation provide valuable insights into the controls on fault permeability. Subsiding sedimentary basins often have a transitional depth zone, here referred to as the ductile-to-brittle transition, above which faults do not affect fluid flow or form barriers and below which faults will tend to form conduits. The depth of this transition is partly dependent upon geothermal gradient. In basins with a high geothermal gradient, fault-related conduits can form at shallow depths in high-porosity sandstone. If geothermal gradients are low, and fluid pressures are hydrostatic, fault-related conduits are only formed when the sandstones have subsided much deeper, where their porosity (and hence fluid content) is low. Mineralization of faults is more likely to occur in areas with high geothermal gradients because the rocks still have a high fluid content when fault-related fluid-flow conduits form. The interrelationship between rock rheology and stress conditions is sometimes a more important control on fault permeability than whether the fault is active or inactive.

The microfabric of calcite tectonites from the Helvetic Nappes (Swiss Alps)

Summary The crystallographic orientations of calcite in tectonites from the Morcles Nappe and the Glarus Overthrust have been measured by X-ray texture goniometry. The orientation data are represented by pole figures, inverse pole figures and as orientation distribution functions. The patterns of preferred orientation observed in the specimens from the Morcles Nappe are correlated with intracrystalline slip mechanisms. The patterns exhibit quasiaxial symmetry and in some cases the axis of symmetry is oblique to the symmetry of the macroscopic fabric. This obliquity is interpreted as the result of a rotational strain path and may be used to infer direction and sense of shear. Some specimens of the Lochseiten mylonite from the Glarus Thrust show no strong preferred crystallographic orientation. This suggests that grain boundary sliding was the major deformation mechanism.

TEXTURE OF SOLNHOFEN LIMESTONE DEFORMED TO HIGH STRAINS IN TORSION

Solnhofen limestone was deformed in torsion to shear strains (γ) ranging from 1 to 12, at a temperature of 750 °C, 300 MPa confining pressure and a maximum strain-rate of 10−3 s−1. These deformation conditions correspond to the intracrystalline power-law dislocation creep field close to the boundary to the grain-size-sensitive superplastic creep field. The grain-shape microstructure was observed using orientation contrast by backscattered electrons in the SEM. The grains remain sub-equant with an average grain size of around 4 μm, even to the highest strains. Lattice preferred orientation was determined using both X-ray texture goniometry and automated electron back-scatter diffraction. The c-axis preferred orientation develops from two main maxima with a weak sub-maximum, through two maxima to a single maximum perpendicular to the shear plane. The rate of increase of the intensity of the single maximum with increasing strain diminishes, and it appears that there is a tendency to a steady-state texture. The final single c-axis maximum is displaced slightly towards the shortening direction of the applied simple shear. The a-axes tend to a girdle perpendicular to the c-axis maximum. It is proposed that the partitioning of deformation between intra- and inter-crystalline mechanisms results in a pulsating strain state in the grains, contributing to the maintenance of sub-equant grains. It is argued that the lattices of constituent grains rotate continuously with no stable end orientation and that this can lead to a steady-state texture. The experimental preferred orientation compares well with that of natural calcite mylonites in the position of the c-axis maximum and the a-axis girdle.

Individual orientation measurements in quartz polycrystals: advantages and limitations for texture and petrophysical property determinations

Individual orientation determination of quartz grains by electron channelling in principle gives the complete orientation. However, in routine analysis the noise level in electron channelling patterns (ECPs) does not permit the determination of handedness of a quartz grain in a polycrystal. In practice, all quartz grains are arbitrarily indexed as right-handed. Hence, Dauphine twins can be identified, but not Brazil twins. This practice also means that only the centrosymmetric petrophysical properties can be determined from texture measurements. These include most geologically relevant properties (e.g. thermal conductivity, thermal expansion and elasticity). However, other properties (e.g. piezoelectricity) which are not centrosymmetric cannot be calculated from such texture measurements. Some texture-forming processes (e.g. dislocation glide) can also be considered to be centrosymmetric in quartz, whereas others (e.g. grain boundary migration) may not be. The method of quantitative texture analysis from individual measurements is briefly recalled. As an example, 382 grains from Tongue quartzite are used to illustrate the advantages of texture analysis from ECPs. The orientation distribution function (ODF) is calculated from ECPs and X-ray pole figures of the same sample. The agreement is found to be good between the two methods, proving that ECPs can be used for quantitative analysis. The methods used in local texture analysis and the definitions of the various misorientation distribution functions (MODFs) are given. Data collected from a traverse of a quartzo-feldspathic shear zone in Lewisian gneiss (Torridon ‘quartzite’) are used to illustrate local texture analysis. Examples from a region of shear strain of about one are given of core and mantle subgrains and Dauphine twins. Dispersion trails of the crystallographic axes within a single grain show an apparent rotation about the intermediate structural axis Y. Detailed analysis of the subgrain misorientation axes in specimen and crystallographic co-ordinates show an important scatter, implying that the subgrains resulted from local incompatibility strains rather than specimen-scale kinematics. The method of calculation of physical properties from individual orientation measurements is given for second-and fourth-order tensors. Using the texture data from Tongue quartzite we have calculated thermal conductivity, thermal expansion and seismic velocities. All these properties are extremely anisotropic in quartz. However, it is emphasized that the presence of a second phase on grain boundaries (e. g. water, graphite) may completely alter a physical property (e.g. electric) and render the values calculated from texture measurements inappropriate.

Seismic anisotropy as an indicator of reservoir quality in siliciclastic rocks

Abstract Improving the accuracy of subsurface imaging is commonly the main incentive for including the effects of anisotropy in seismic processing. However, the anisotropy itself holds valuable information about rock properties and, as such, can be viewed as a seismic attribute. Here we summarize results from an integrated project that explored the potential to use observations of seismic anisotropy to interpret lithological and fluid properties (the SAIL project). Our approach links detailed petrofabric analyses of reservoir rocks, laboratory based measurements of ultrasonic velocities in core samples, and reservoir-scale seismic observations. We present results for the Clair field, a Carboniferous–Devonian reservoir offshore Scotland, west of the Shetland Islands. The reservoir rocks are sandstones that are variable in composition and exhibit anisotropy on three length-scales: the crystal, grain and fracture scale. We have developed a methodology for assessing crystal-preferred-orientation (CPO) using a combination of electron back-scattered diffraction (EBSD), X-ray texture goniometry (XRTG) and image analysis. Modal proportions of individual minerals are measured using quantitative X-ray diffraction (QXRD). These measurements are used to calculate the intrinsic anisotropy due to CPO via Voigt-Reuss-Hill averaging of individual crystal elasticities and their orientations. The intrinsic anisotropy of the rock is controlled by the phyllosilicate content and to a lesser degree the orientation of quartz and feldspar; the latter can serve as a palaeoflow indicator. Our results show remarkable consistency in CPO throughout the reservoir and allow us to construct a mathematical model of reservoir anisotropy. A comparison of CPO-predicted velocities and those derived from laboratory measurements of ultrasonic signals allows the estimation of additional elastic compliance terms due to grain-boundary interactions. The results show that the CPO estimates are good proxies for the intrinsic anisotropy of the clean sandstones. The more micaceous rocks exhibit enhanced anisotropy due to interactions between the phyllosilicate grains. We then compare the lab-scale predictions with reservoir-scale measurements of seismic anisotropy, based on amplitude variation with offset and azimuth (AVOA) analysis and non-hyperbolic moveout. Our mathematical model provides a foundation for interpreting the reservoir-scale seismic data and improving the geological modelling of complex reservoirs. The observed increases in AVOA signal with depth can only be explained with an increase in fracturing beneath the major unit boundaries, rather than a change in intrinsic CPO properties. In general, the style and magnitude of anisotropy in the Clair field appears to be indicative of reservoir quality.

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.