John Starkey

Microstructural and chemical transformations accompanying deformation of granite in a shear zone at Miville, Switzerland; with implications for stress corrosion cracking and superplastic flow

At Miéville, in the Aiguilles-Rouges Massif, granitic rocks of the basement are deformed into mylonites within a major subvertical shear zone. The ambient temperature during translation is estimated at 250° C±30° C from fluid inclusion filling temperatures in syntectonic microveins, from Δ18O quartzilmenite of+15%, and from mineralogical criteria. Porphyroclasts of both oligoclase and orthoclase feldspar decrease from initial diameters of 20 mm and assume elliptical shapes during progressive deformation, due to recrystallisation of the margins to ultra-fine polygonal grains which extend out from the porphyroclasts in thin trails: the final stable grain size is <5 μ. The recrystallised feldspar has a composition of the parent porphyroclast,+albite, requiring relative gains of Na and losses of K+Ca compared to the precursor, and implying short range redistribution of the components during deformation. Decrease of free energy associated with the deformation catalysed change in feldspar composition, coupled with stored strain energy in the porphyroclasts may account for recrystallisation to a stable aggregate of ultrafine grain size. The long trails imply exceptionally high ductility, which, coupled with microstructural criteria, and admixture of quartz from neighbouring pure quartz aggregates by grain boundary sliding, is interpreted in terms of superplastic flow. Estimated temperatures of T/Tm≈0.2 for the inferred superplastic deformation is lower by a factor of 2 than previously recorded for this flow michanism in silicates. The feldspar and quartz probably accomodated grain boundary sliding by intercrystalline diffusion.Biotite responds to deformation by bendgliding, kinking, and recrystallisation in mantles. The reaction of high-Ti parent grains to low-Ti biotite+Fe-muscovite+ ilmenite+chlorite is catalysed at all of these microstructural sites. Progressive deformation of the fine-grained products in the mantles is coupled with steady reaction to low-Fe muscovite+epidote+ sphene+rutile resulting in exceptionally ductile trails, as for the case of feldspar.Biotite grains have pervasive networks of nondisplacive intragranular fractures. At the fracture tips increase of the stress intensity has catalysed the reaction of high-Ti parent grains to low-Ti biotite+muscovite+ ilmenite which occupy the fractures. The fractures propagate and coalesce resulting in mechanical breakdown of the parent grains: these microstructures are believed to be examples of natural stress corrosion cracking. These features are also abundant in feldspar porphyroclasts where at fracture tips orthoclase→secondary orthoclase+albite, and oligoclase→secondary oligoclase+albite. Stress corrosion cracking may be significant in the steady state deformation of crustal rocks at low temperatures when intracrystalline plasticity is not generally dominant.Two way mass balance calculations utilising major and selected trace element data, reveal that deformation of the granite was essentially isochemical, involving average additions of <1 % H2O+CO2, at approximately constant specific gravity. The parameters Fe2+/∑Fe and δ18Owhole rock maintain relatively constant values across the shear zone, and this also implies limited participation of fluids in the deformation. Alkali elements and titanium display the largest percentage variation during progressive deformation, whereas SiO2, Al2O3, and P2O5, together with V, Ni, Cr,Y,Zr, and Nb remain relatively constant. All variations decrease at increasing states of deformation and this is interpreted in terms of mechanical mixing of chemical inhomogeneities of the granite precursor within the shear zone. Constraints imposed by variations in abundance of the relatively immobile elements imply that volume changes accompanying deformation in the shear zone were less than ±10%.

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.

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.

Segmentation of petrographic images by integrating edge detection and region growing

Abstract A novel approach to segmenting petrographic images is proposed in this paper. A series of edge operators with various sizes of masks are first defined. By considering a larger neighborhood, the effects of noise or surface irregularities on edges are reduced. Color edges in an image are obtained by combining the edge operators and a color edge detection algorithm. A seeded region-growing algorithm is then used to segment the image based on the color edge information and the distances between edge-pixels and non-edge pixels. Seed regions are created automatically. These regions grow simultaneously. After all pixels in the image are labeled, the boundaries shared by two regions are checked. If a boundary is weak enough, it is eliminated and the corresponding two regions are merged. In the ultimate segmented map, each region whose size is large enough corresponds to a mineral grain in the image. This approach has been implemented in C++ under the Linux environment. Three sets of petrographic images were used to test the method.

Orientation of collagen in the tunica adventitia of the human cerebral artery measured with polarized light and the universal stage.

Others have shown that collagen has a weakly birefringent molecular structure with its optical axis aligned with the longitudinal structural axis. We used a polarizing microscope and universal stage to measure the three-dimensional orientation of individual collagen fibers in sections of cerebral arteries. The arteries were fixed under a transmural pressure of 100 mm Hg, embedded with a separate reference frame of nerve fibers, and cut obliquely and longitudinally. Two dichroic enhancement stains were used—F3BA picro-Sirius red and Gomoris silver impregnation stain. Measurements on stained and unstained sections were reproducible to within approximately 1°. Three hundred measurements from 11 arterial sections revealed that the average direction of alignment was circumferential with a surface component of longitudinal fibers.

The geometry of kink bands in crystals — a simple model

A crystallographic explanation for the geometry of kink bands in crystals is proposed and three possible types of kinking are discussed, 1. simple kinking, 2. kinking associated with a phase change and 3. kinking associated with twin formation. The angular relationships at kink band boundaries have been calculated for a number of minerals and the results compared with observations in the literature. It is proposed that the crystallography of a mineral may be the dominant control of kink band geometry rather than external stress conditions as is generally assumed, this is probably particularly true of kink bands formed under natural conditons.

Plagioclase microfabrics in a ductile shear zone from the Jotun Nappe, Norway

Abstract Gabbroic and anorthositic rocks from the Jotun Nappe are transected by small ductile shear zones in which a high-grade paragenesis and a new foliation are formed. Most plagioclase grains show evidence of plastic deformation, and have recrystallized by subgrain rotation and ‘bulge’ processes to fine-grained mylonite. During these processes a plagioclase grain-shape fabric was destroyed, and with increasing strain a pre-existing plagioclase crystal fabric was successively modified, enhanced and finally obliterated. This could be evidence of superplastic flow. Inverse pole figure analysis of the typical plagioclase crystal fabric in the shear zone indicates that (021) ⊥ 〈a〉 could be an operative slip system, and planes close to (132) could be possible slip planes.

A search for the piezoelectric effect in quartz-bearing rocks

Piezoelectric effects measured in Australian granites and in a highly aligned quartzite from Poughquag, New York, have in no case exceeded the effects expected statistically in laboratory samples with finite numbers of discrete quartz grains. It is concluded that piezoelectric fabrics, which would be evidence of paleoelectric currents, probably do not exist.

IMAGE: modified for use on a microcomputer-based system

Abstract The previously published FORTRAN program IMAGE has been modified for use on a microcomputer. It has been translated into Turbo Pascal and an inexpensive tablet-type digitizer is used for input instead of a microdensitometer. This provides a readily affordable system for image analysis. The interactive nature of a tablet-type digitizer allows the operator to select what is to be digitized as the boundaries of a particle are traced. The parameters calculated for each particle remain the same as in the FORTRAN version but some parameters are derived in a different manner. The “best-fit” ellipse is constructed for each particle.

PhotoLin: a program to identify and analyze linear structures in aerial photographs, satellite images and maps

A computer program, PhotoLin, written for an IBM-PC-compatible microcomputer is described which detects linear features in aerial photographs, satellite images and topographic maps. The program accepts images saved to PCX files as input and applies noise correction and smoothing filters and thinning routines. The output consists of a skeleton containing the median lines of linear features which can be represented on a map. The branches of the skeleton can be broken into sections of constant length for which the mean orientations are obtained for the preparation of rose diagrams.