Ernest H. Rutter

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.

Experimental deformation of partially molten Westerly granite under fluid‐absent conditions, with implications for the extraction of granitic magmas

The mechanical behavior of partially molten Westerly granite was investigated in the temperature range 800°–1100°C, 250 MPa confining pressure, by means of constant strain rate, creep, and stress relaxation tests. The only water in the samples came from the breakdown of hydrous phases, biotite, minor chlorite and muscovite and alteration products of feldspars. Thus the amount of melt was controlled by the test temperature and ranged from 3% at 800°C to 50% at 1100°C. Over that temperature range, strength decreased from ≈500 MPa to less than 1 MPa, and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates. The comparative viscosity of the melt alone was estimated at 950° and 1000°C from the distance it could be made to penetrate into a porous sand under a known pressure gradient. Under all conditions, deformation of the matrix of solid grains was by brittle fracture only. Samples containing up to 10 vol % melt failed with the formation of a shear fault zone. At higher melt fractions, melt-filled “pores” collapsed by shear-enhanced compaction, squeezing the melt into axial cracks. Above 40 vol % melt, unfractured solid grains were carried about passively in the flowing liquid. There was no evidence of a “rheologically critical melt percentage” in this system. By analogy with the uniaxial compaction of water-saturated soils, a simple model is erected to describe a two-stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress. Shear-enhanced compaction is inferred to drive melt into a network of melt-filled veins, whereupon porous flow through the high-permeability vein network allows rapid drainage of melt to higher crustal levels.

Can the maintenance of overpressured fluids in large strike-slip fault zones explain their apparent weakness?

The nucleation of earthquakes on weak faults is still poorly understood. Favored models for weakening have invoked the presence of high-pressure fluids contained within the fault. Here, from detailed field mapping, permeability measurements, and modeling, we assess how pressurized fluids may be impounded by layers of low-permeability phyllosilicate- rich fault gouge. Constraints are made on the permeability anisotropy, fluid-flow pathways, and fluid-loss rates from a large transcurrent fault zone. It is concluded that phyllosilicate-rich fault gouges having permeabilities ranging from 10−17 to 10−21 m2 and cumulative layer thicknesses of ∼50–200 m (from field observations) need fluid fluxes at the base of the brittle part of a large transcurrent fault (assumed to extend from 0 to 15 km depth) of ∼0.1 m3·m−2·yr−1. Furthermore, permeability anisotropy must exceed three orders of magnitude for overpressures to develop. This finding implies that vertical permeability must be enhanced by relatively permeable inclusions of fractured protolith within the fault zone, enclosed by walls of low-permeability gouge. For cross-zone flow, the lateral continuity of the phyllosilicate-rich fault-gouge layers provides effective barriers to fluid flow both into and out of the fault zone. This geometry restricts the origin of fluids in large fault zones as it favors fluids derived from deep sources. However, the fluid flux over time of mantle-derived CO2 and water produced from dehydration reactions has been estimated and appears inadequate. Given the frictional characteristics of phyllosilicate-rich fault gouge, small earthquakes and fault creep are the most likely modes of failure, facilitated by the buildup of high-pressure fluids in cells, producing spatial weakening of the fault.

Palaeostress estimation using calcite twinning: experimental calibration and application to nature

Abstract By means of high-temperature deformation experiments on calcite rocks of different grain sizes, we have established the relationships between differential stress and twinning incidence, twin density and volume fraction, and the way that these relationships depend on grain size. Both twinning incidence and volume fraction increase with grain size at a constant level of differential stress. Log twin density is proportional to stress but is independent of grain size. At a given grain size, volume fraction of twinning increases non-linearly with differential stress. Twinning activity measured by twinning incidence increases linearly with stress. The behaviour of each of these parameters is independent of temperature, strain and strain rate. These relationships can be used as a basis for the estimation of palaeostress values in naturally deformed calcite rocks. This is illustrated by a study of twinning activity in limestones close to thrust faults in the Cantabrian zone of the Variscan orogenic belt in northwestern Spain. High peak differential stresses were measured, ranging from 150 to 250 MPa. Using the Turner Dynamic Analysis technique, however, it is clear that shear stresses resolved parallel to the fault planes were very low. The attainment of high stress differences when the resistance to faulting was low is attributed either to bending forces induced during the motion of the non-planar surfaces relative to each other or to the stress peak being attained prior to the localization of deformation onto the fault surfaces.

Experimental study of the influence of stress, temperature, and strain on the dynamic recrystallization of Carrara marble

Carrara marble has been deformed experimentally at temperatures ranging between 500° and 1000°C, at confining pressures of 200 and 300 MPa, and up to very large strains in extension and compression, in order to study the microstructural and mechanical property changes associated with dynamic recrystallization. Microstructural studies were made by optical microscopy on ultrathin sections, and automatic grain size measurement techniques were used. When the temperature is sufficiently high (≈ 600°C) and the stresses are high enough for deformation twinning to occur, twin boundary migration is a powerful recrystallization mechanism that does not modify the grain size. At stress levels too low to activate twinning, recrystallization occurs in two stages: the formation of nuclei by grain boundary bulging and subgrain rotation recrystallization in the grain boundary regions, followed by a second stage of grain boundary migration recrystallization to a larger grain size that eventually overprints the entire rock volume. The recrystallization process requires larger prestrains at the lower temperatures. The size to which migration-recrystallized grains grow seems to be limited by the stress and grain size dependent twinning field boundary or by the boundary between the dislocation creep and grain size sensitive flow mechanisms. Within the range of experimental observations, dynamically recrystallized grain size does not depend on strain rate or temperature. Separate empirical stress versus grain size relations are presented for rotation and migration recrystallization that may be used for palaeopiezometry. No unequivocal experimental evidence of weakening resulting from recrystallization to finer grain sizes at laboratory strain rates was found. However, extrapolation of the experimental data to low, natural strain rates suggests that weakening following recrystallization may occur in nature.

Deformation mechanisms and rheology: why marble is weaker than quartzite

When deformed together in nature, calcite rocks invariably appear weaker and more ductile than quartz rocks. This can be reconciled with experimental flow data only by taking into account grain-size-sensitive (GSS) flow in calcite rocks, which is predicted to dominate even at grain sizes on the order of 1mm at middle metamorphic grades. Using new experimental data that demonstrate the transition between intracrystalline plastic and GSS flow of quartz rocks, we predict that unnaturally small grain sizes at temperatures of 700°C or higher are required for GSS flow of quartz in nature. Thus natural flow of quartzite is expected to occur by intracrystalline plastic processes, even after recrystallization to a fine grain size.

On the influence of porosity on the low-temperature brittle—ductile transition in siliciclastic rocks

Abstract A compilation of available experimental data, coupled with new results on Tennessee sandstone, shows that the low-temperature brittle faulting to cataclastic flow transition in siliciclastic rocks takes place at progressively higher confining pressures as porosity is reduced. The collapse of initial porosity compensates for the tendency for brittle deformation to be dilatant. According to a stability criterion, this in turn favours spreading of the cataclastic deformation throughout the rock volume instead of fault localization. At sufficiently high strains, dilatation, and hence fault localization, supervenes. From microstructural observations on Oughtibridge Ganister (7% porosity), deformed under conditions of the faulting to flow transition, the mechanism of pore collapse involves crushing and sliding on shear-oriented grain boundaries, with accumulation of wear products in the pore spaces. This explains the enhancement of pore collapse by differential stress relative to purely hydrostatic compression.

Thermally-induced grain growth of calcite marbles on Naxos Island, Greece

The island of Naxos is composed of an elliptically shaped structural and thermal dome of Miocene age. Peak metamorphic temperatures within the central migmatite complex exceeded 700° C, decreasing to about 300° C at the most distant exposures on the island. Equigranular calcite marbles which outcrop together with metapelites and metabasites over the whole island show a systematic pattern of increasing grain-size towards the central migmatite complex, with a significant discontinuity in the pattern corresponding approximately with the 500° C isotherm. The microstructures and grain-size distributions in the marbles are consistent with normal grain-growth. The variation of grain-size with peak temperature attained can be explained equally well by the assumptions that (a) a maximum grainsize had developed, particularly at higher temperatures, or that (b) the grain-size had been frozen-in by a combination of cooling and coarsening, both of which combine to reduce the rate of grain-growth.The grain-size data do not impose strong constraints on the mechanism of transport of heat responsible for the metamorphism, whether by conduction or by advection, but the 500° C discontinuity indicates that the tectonothermal history of the migmatitic core and its envelope of metasediments were different.

Use of extension testing to investigate the influence of finite strain on the rheological behaviour of marble

Most mechanical testing of rocks at high temperatures is carried out under axisymmetric triaxial shortening to small (< 20%) strains. However, the most significant geological deformations involve much higher strains, especially in plastic shear zones where there has been strain localization. Highly strained rocks commonly are microstructurally modified by dynamic recrystallization, so that the sheared rock is microstructurally quite different from its protolith. High strain deformation in experiments can be induced by direct shear or through torsion, or by taking advantage of the tendency for extensional deformations to result in necking. It is important to be able to separate out the geometrical factors favouring localization from those arising from strain-dependent microstructural changes. Here, we present the results of high strain (> 1000%) axisymmetric extension tests on Carrara marble at 700 °C and 800 °C, in the form of strain-time plots for the evolution of material sections through the sample at different distances from the neck. At 800 °C, it is clear that total dynamic recrystallization by grain boundary migration is accompanied by substantial mechanical weakening, but sufficient strain could not be applied to reach the steady-state condition for the recrystallized material. The recrystallization is interpreted to produce weakening because the migration of grain boundaries sweeps grains clear of accumulating dislocation density faster than recovery by dislocation climb. In nature, such mechanical weakening would be expected to favour the stable development of a localized zone of deformation.

Synthetic seismic reflection profile through the Ivrea zone–Serie dei Laghi continental crustal section, northwestern Italy

A geologic cross section, restored to its original horizontal orientation in Permian-Triassic time, has been constructed for the middle and lower continental crustal rocks of the Ivrea-Verbano zone and the adjacent Serie dei Laghi of northwestern Italy. Seismic P-wave velocities of a representative suite of rock samples were measured to high-pressure and high-temperature conditions. A synthetic seismic reflection profile, ∼76 km long and 30 km thick, was computed to compare what can be deduced from the seismic profile with what is known in much more detail from geologic mapping. Imaged features correspond closely to those seen on many present-day profiles, and the broad features of the tectonic evolution would be correctly interpreted, but important recumbent fold structures would be missed, and relationships between intrusive bodies and their country rocks would be unclear.