Joseph Clancy White

Semi-brittle deformation within the Alpine fault zone, New Zealand

Abstract This paper reports the results of an attempt to identify the deformation micromechanisms in the brittle-ductile transition zone of the Alpine fault, New Zealand. Characterization of sequentially developed microstructures was carried out using optical microscopy, HVEM and STEM analysis. It was found that the transition zone corresponds to a broad zone of retrogression produced initially by microcracking and fluid infiltration. The non-catastropic nature of the fracturing indicates that fluid-enhanced sub-critical cracking is a significant crustal deformation mechanism. After the initial phase of retrogression, both ductile and brittle processes coexist and cyclically alternate in response to stress cycling. The relative importance of macroscopic brittle processes may decrease with time as more ductile minerals, especially phyllosilicates, develop during continued retrogression.

Kinematics of rock flow and the interpretation of geological structures, with particular reference to shear zones

Abstract Current interpretations of structures are generally based on homogeneous and steady deformation models, despite the fact that both the heterogeneity of rocks (materially, rheologically and geometrically) and the time dependence of imposed geological conditions give rise to significant heterogeneous and non-steady flow. In concert with field observations, we emphasize that the expectation of heterogeneity and non-steadiness is the key to understanding natural deformation and that in order to carry out successful structural analysis and tectonic interpretation, it is necessary to recognize the first-order distinction between imposed boundary conditions typically used to define the tectonic regime (e.g. transcurrent, transpression) and the response recorded by rocks within the zone (structures and fabrics). Using SC fabric as an example, it is demonstrated how flow with a non-zero spinning component resulting from the rheological contrasts and/or geologically realistic time-dependent boundary displacement can drastically change the ‘ideal’ geometric and kinematic relations between the fabric and the host zone. In agreement with both theoretical analysis and field observation, it is shown that natural flow regimes range from pure shear to pure rotation, including super-simple shear. In consideration of the heterogeneity and non-steadiness of natural deformation, kinematic analysis is justifiable only within a homogeneous domain and steady period. Flow kinematics and mechanisms are interrelated in that, firstly, mechanisms provide internal constraints on kinematics, ensuring that only certain flows are possible and, secondly, flow kinematics will favour development of certain mechanisms.

Extreme ductility of feldspars from a mylonite, Parry Sound, Canada

Abstract Single feldspar crystals in mylonites, from the Grenville structural province of Canada, have accumulated extreme strains by ductile mechanisms. The samples studied are from a deep-crustal shear zone and were collected at Parry Sound, Ontario. The mylonites were derived from granite and leucogabbro, and the feldspar crystals originated in late syntectonic pegmatite dykes. Optical and transmission electron microstructures of microperthitic alkali feldspars show evidence of syntectonic and synchronous dislocation climb (leading to recovery), Si/Al ordering, and Na/K interdiffusion. Evidence for the operation of these processes is, respectively, ubiquitous subgrains, development and coarsening of tweed texture and transition of monoclinic to triclinic K-feldspar, and exsolution domain reorganization. These processes occurred concurrently with extreme straining of the host crystals by dislocation and diffusion mechanisms. The effects of ductile deformation preserved in these crystals are quite different from those usually observed in feldspars. The unusual microstructures are thought to be due to the conditions of high temperature and confining pressure which existed during deformation, in concert with a deformation-enhancing point defect chemistry, possibly associated with incorporation into the crystals of a species of ‘water’.

Transient discontinuities revisited: pseudotachylyte, plastic instability and the influence of low pore fluid pressure on deformation processes in the mid-crust

Abstract Occurrences of pseudotachylyte cyclically introduced as melts into the Outer Hebrides thrust are demonstrably synkinematic with crystal-plastic mylonites. Classical interpretations of these rock types as, respectively, products of pressure-dependent frictional melting and thermally-activated intracrystalline deformation create a paradox in that these processes are, to a large extent, mutually exclusive. Detailed microstructural and microcompositional analyses of host mylonites and primary and deformed pseudotachylyte were carried out by light and electron microscopy. The ambient shear zone environment in which pseudotachylyte formed was determined to comprise temperatures in the order of 500 ° C and stresses of 140–210 MPa, on which were imposed much higher transient stresses in response to heterogeneous, non-uniform flow. The latter conditions are sufficient for the development of plastic instabilities in feldspar-rich crust, if pore fluid pressures are sufficiently low. The latter is consistent with the absence of hydration during exhumation observed in the rocks under study. Low pore fluid pressure during thrust exhumation of deep crust enables activation of high-strength ductile processes dominated by dislocation glide which may be a prerequisite to instability. Whereas other studies have demonstrated the possible occurrence of such melt-generating instabilities, it is believed that this study provides the first example in which the calculated potential for instability formation is consistent with the deformation microstructures and estimated pressure-temperature conditions. Grain-size reduction to produce ultramylonites dominated by grainsize-sensitive flow is achieved by both deformation-induced dynamic recrystallization and crystallization of instability-generated melts.

Low-temperature recrystallization in calcite: Mechanisms and consequences

Vein-calcite–dominated fault rocks collected from several locations show evidence for intense intracrystalline plasticity and interface (twin and grain boundary) mobility, leading to dynamic recrystallization of calcite at temperatures (150–250 °C) significantly below those at which these features are commonly anticipated. These observations require a reappraisal of calcite deformation at low temperature, particularly the capability for dynamic recrystallization in the apparent absence of significant, thermally activated recovery processes. The cyclic introduction of coarse-grained calcite veins is observed to be essential for the initiation of intracrystalline deformation and associated dynamic recrystallization. The introduction of veins generates an essentially monomineralic rock of a grain size larger than the protolith. As a result, the mylonitization does not occur within a given protolith, but rather in the introduced secondary calcite. Through Hall-Petch–type grain- size–dependent dislocation interactions, stress is locally increased, and the resulting increase in dislocation densities promotes grain-boundary migration. The recognition that nominal high-temperature creep processes and associated microstructures can occur outside their expected temperature range has implications for fault rheology (strength) and fault permeability and porosity.

Silica gel formation during fault slip: Evidence from the rock record

Dynamic reduction of fault strength is a key process during earthquake rupture. Many mechanisms for causing coseismic weakening have been proposed based on theory and laboratory experiments, including silica gel lubrication. However, few have been observed in nature. Here we report on the fi rst documented occurrence of a natural silica gel coating a fault surface. The Corona Heights fault slickenside in San Francisco, California, is covered by a shiny layer of translucent silica. Microstructures in this layer show fl ow banding, armored clasts, and extreme comminution compared to adjacent cataclasites. The layer is composed of ~100 nm to 1 µm grains of quartz, hydrous crystalline silica, and amorphous silica, with 10‐100 nm inclusions of Fe oxides and ellipsoidal silica colloids. Kinematic indicators and mixing with adjacent cataclasites suggest the shiny layer was fl uid during fault slip. The layer therefore represents a relict silica gel that formed during fault motion, and which could have resulted in frictional instability. These observations confi rm that the silica gels formed in rock friction experiments do occur in natural faults and therefore that silica gel formation can act as a dynamic weakening mechanism in faults at shallow crustal conditions.

Mylonite to megabreccia: Tracking fault events within a transcurrent terrane boundary in Nova Scotia, Canada

Transcurrent terrane boundaries commonly evolve into long-lived faults that preserve little evidence for early docking events. A remarkable exception is exposed at Clarke Head along the Appalachian Meguma terrane boundary in Nova Scotia, Canada, where a Late Carboniferous fault megabreccia contains Devonian (369 Ma: U-Pb zircon) granulite-grade mylonite fractured by veins filled with Visean amphibole (ca. 335 Ma: Ar-Ar). This fractured mylonite was later mixed with Early Carboniferous sedimentary rocks during megabrecciation (ca. 315–310 Ma). These three fault events are reflected in the tectonostratigraphic record. Devonian (ca. 370–360 Ma) transpressional terrane docking ramped Meguma up against Avalonia and shed clastic detritus across the fault system. The Visean brittle deformation recorded by the amphibole veins was coeval with marine regression at surface. The late Namurian megabrecciation event similarly produced unconformity followed by renewed nonmarine clastic sedimentation. The Clarke Head megabreccia therefore preserves an episodic late Paleozoic fault history spanning some 55 m.y. during convergence between Laurentia and Gondwana and the assembly of the Pangean supercontinent.

Natural calcite c-axis fabrics: An alternate interpretation

Microstructures and crystallographic fabrics in natural calcite mylonites from Newfoundland, Canada are discussed and compared with other natural and experimental examples. In the Newfoundland rocks, porphyroclasts deformed mainly by twinning and intracrystalline slip, and the recrystallized grains deformed by dislocation creep assisted by dynamic recrystallization by grain boundary migration (T < 350°C). Porphyroclast c-axis fabrics consist of a broad point maximum oriented normal to the shear plane. Recrystallized grains in the ultramylonites have strong c-axis fabrics consisting of a single point maximum oriented either parallel or oblique to the normal to the shear plane. Experimental, simple shear, calcite c-axis fabrics have been divided by previous workers into low- (LT) and high-temperature (HT) fabrics. Twinning and intracrystalline r-slip have been identified as the dominant deformation mechanisms in low-temperature experiments whereas intracrystalline r- and f-slip dominate in high-temperature experiments. LT fabrics have a single point maximum which is oriented approximately parallel to the principal stress axis and is oblique to the normal to the shear plane. HT fabrics have three point maxima which are asymmetrical both in orientation and intensity with respect to the normal to the shear plane. The transition between LT and HT fabrics occurs at temperatures lower than 400°C in fine-grained limestones and lower than 700°C in marbles at laboratory strain rates. Natural calcite c-axis fabrics, in agreement with study, almost invariably consist of a point maximum oriented either normal or slightly oblique to the shear plane. Because their fabrics are similar to experimental LT fabrics, they have been interpreted as LT fabrics. By comparison with experimental and computer-simulated LT fabrics, the obliquity of their c-axis point maximum with respect to the shear-plane normal has been used as a shear sense indicator and as a measure of the degree of non-coaxiality of the deformation. Two lines of evidence mitigate against calcite c-axis fabrics as shear sense and strain path indicators in mylonite zones. First, the obliquity of the c-axis point maximum in natural examples is not consistent for a given sense of shear. Secondly, mylonite microstructures reported here and in the literature supports the contention that dynamic recrystallization and intracrystalline slip are responsible for the strong c-axis fabrics of most calcite mylonites. Therefore, natural calcite mylonite c-axis fabrics do not generally correlate with either LT or HT experimental fabrics. Instead, natural calcite mylonites have similar microstructures and fabrics to marbles that were deformed experimentally at very high temperatures (800–850°C) and underwent pervasive dynamic recrystallization by grain boundary migration. Consequently c-axis fabrics of calcite mylonites cannot be used as a measure of the degree of non-coaxiality of the deformation nor can they be used as shear sense indicators.

Shock-induced amorphous textures in plagioclase, Manicouagan, Quebec, Canada

The textural relationships and structural states of optically isotropic labradorite from the Manicouagan, Quebec, impact structure have been examined by light (optical) and transmission electron (TEM) microscopy. Two distinct diaplectic glasses have been recognized based on their contrasting morphology, timing and the inferred modes of formation. The earliest isotropic bands and grain-scale isotropism (maskelynite) optically exhibit a gradational,in situ transformation from crystalline plagioclase with preservation of relict textures (twins, grain boundaries). The same transformation from crystalline to amorphous structure is observed in TEM to occur heterogeneously at scales on the order of the unit cell. The progressive transformation of optical properties reflects an increase in the volume fraction and eventual coalescence of these amorphous units. This maskelynite-type diaplectic glass is interpreted to form in the solid-state directly from crystalline material during the compressional phase of the shock wave. The other isotropic material occurs in spatially discrete tensiongashes and planar deformation features (PDFs) that overprint the maskelynite-type glass. This second type of diaplectic glass (PDF-type) is developed homogeneously within a given glass band and exhibits sharp crystal-glass boundaries, in contrast to the gradational boundaries of the maskelynite-type glass. PDF-type glass is interpreted to form by melting in tensional release zones during passage of the rarefaction wave. These observations emphasize the ability of naturally shocked rocks to preserve subtle evidence of variations in the shock process from highly transient events.

The coating layer of glacial polish

Glacial polish has previously been thought to form by removal of material by glacier abrasion. Here we identify a micrometer-scale coating layer that suggests that the uppermost interface between ice and rock forms by accreting material to the abraded surface. Bent and broken crystals in a damage zone beneath the coating layer provide evidence for abrasion at the nanoscale, which generates the fragments and amorphous matrix that ultimately compose the coating layer. Flow and shear textures within the coating suggest that this composite material is smeared over the damage zone during ice sliding, forming a smooth surface. The coating can potentially change the shear resistance and erosion rates at the bed of temperate glaciers and likely explains the relative resistance of glacial polish to postglacial weathering.