John V. Smith

Shear thickening dilatancy in crystal-rich flows

Abstract Rheological models of molten rock are mostly based on behaviours in which viscosity is constant with respect to strain rate (e.g. Newtonian, Bingham plastic) or viscosity decreases with increasing strain rate (pseudo-plastic or shear thinning). Rheological models in which viscosity increases with increasing strain rate (shear thickening or dilatant) models have rarely been used, yet such behaviour is common in particulate systems. Whereas magmas with low crystal content may be appropriately modelled by the former behaviours, greater crystal content may lead to particle interactions resulting in shear thickening behaviour. The effect is illustrated using a powdered rhyolitic rock mixed with water which exhibits extreme shear thickening behaviour caused by particles moving apart as they move past each other. The expansion of the grain fabric during flow is opposed by the applied loads such that dilatancy is invariably associated with shear thickening rheology. Crystal-rich lava may exhibit shear thickening dilatancy, a proposition supported by observations of glass-rich microscopic shear zones disrupting crystal fabrics. Plutonic crystal mush may also exhibit shear thickening dilatancy, which can explain synplutonic intrusion involving brittle fracture followed by ductile disruption. In general, strain rate is an important contributor to the transition from ductile to brittle behaviour in crystal-rich igneous rocks.

Ductile−brittle transition structures in the basal shear zone of a rhyolite lava flow, eastern Australia

Abstract Structural and textural evidence indicates that the basal breccia of the Tertiary Minyon Falls Rhyolite of Tweed Volcano, eastern Australia, formed mainly by fragmentation within the basal shear zone of the lava flow. Cooling at the flow base led to lava behaviour passing through the ductile-brittle transition while subjected to progressive deformation. The intact lavas of the basal zone record intense ductile shearing in multiple folds, rotated phenocrysts, strong alignment of crystallites and micro-folding of these crystal alignments. Minor brittle structures, including faults on fold limbs and microfaults, occur in the intact lava immediately above the basal breccia. These structures represent incipient brecciation and complete brecciation has occurred by intense faulting of previously intact lava. Brittle deformation migrated upward into overlying intact lava as cooling occurred. This process is distinctly different from overriding and incorporation of surface breccias which is commonly invoked as the primary process of basal breccia formation.

Textural evidence for dilatant (shear thickening) rheology of magma at high crystal concentrations

Investigation of the rheology of magmas at high crystal concentrations by experimental means has proved problematic. An alternative approach is to study textures of igneous rocks that not only preserve evidence of the kinematics of magma flow, such as flow direction, but can also preserve evidence of rheology. Flow textures in multiply intruded trachyte dykes on Fraser Island, eastern Australia record evidence of dilatant flow during solidification. This conclusion is reached by interpretation of microscopic ductile shear zones that disrupt the groundmass of aligned feldspar laths. Detailed three-dimensional investigation demonstrates that the dihedral angle between conjugate micro-shear zones is approximately 65°. This conjugate angle is equivalent to that observed in dilatant granular materials such as sand. Dilatant behaviour is synonymous with shear thickening rheology indicating that the magma flow is time-dependent and resists high flow rates. Some of the dykes contain autobrecciation fragments that may represent localities where the ductile flow rate threshold was exceeded. Newtonian or pseudoplastic (shear thinning) rheology of crystal-poor magmas must progressively give way to shear thickening rheology during cooling and increasing crystal concentration.

Folds produced by gravity spreading of a banded rhyolite lava flow

Abstract Recumbent folds near the top of the Minyon Falls banded rhyolite flow, in northeastern New South Wales, Australia, indicate lateral radial extension of lava as it flowed from the vent under the influence of gravity. These structures occur near the vent of the flow and differ from the upright folds commonly observed near the tops of lava flows. Farther from the vent, the fold patterns indicate lateral radial shortening and shearing as the spread of lava was impeded. Interpretation of the folds leads to a kinematic model of coaxial flow near the vent grading to simple shear and sub-simple shear away from the vent.

Structures on interfaces of mingled magmas, Stewart Island, New Zealand

Abstract Strain analysis of magmatic rocks has been attempted using the shapes of enclaves of contrasting magmas and analysis of mineral fabrics. In composite plutonic rocks of the Anglem Complex, Stewart Island, New Zealand, interfaces between different magma types are deformed into fold-like patterns which have the potential to provide new data on strain in magmatic processes. The fold-forms have shorter wavelength and higher amplitude on interfaces at a high angle to layering whereas fold-forms are either absent or of low amplitude on interfaces parallel to layering. The fold-forms may be amplified/de-amplified irregularities or buckle folds, and in either case they indicate shortening perpendicular to rock layering. Whole-rock fabrics of the plagioclase–hornblende–biotite rocks, analysed by shape preferred orientation and centre-to-centre methods, record less intense alignment and strain than predicted by free rotation of particles in viscous media. The inhibition of crystal rotation at high crystal concentrations and the growth of late poorly aligned crystals present difficulties for calibrating whole-rock fabrics with the strain incurred by magmatic flow.

Geometry and kinematics of convergent conjugate vein array systems

Abstract Conjugate arrays of quartz veins in the Neranleigh-Fernvale turbidite beds at Norries Head, eastern Australia occur in configurations in which the trends of the veins, in a principal section, converge towards the acute bisector of the conjugate arrays (a convergent configuration). Such a configuration is common in vein arrays, and has been attributed to initiation of the veins as antithetic shear fractures based on geometric arguments. The morphology of the veins in this study indicates that the veins are hosted by extension fractures which formed by en echelon breakdown of faults. Previous models of extension fracture arrays emphasise that all fractures lie parallel to the bisector of conjugate arrays and that arrays develop after initiation of extension fractures and concurrent with fracture propagation. Sigmoidal vein shapes have been attributed to concurrent shear strain and fracture propagation. An alternative model is proposed for the geometry of conjugate arrays formed by en echelon breakdown of faults. The conjugate angle between parent faults is established before the en echelon extension fractures are formed. The fracture-array angle depends on the local displacement of the parent fault, thus, there is no necessity for the fractures in different arrays to be parallel. If the fracture-array angle is greater than half the conjugate angle between parent faults, a convergent configuration of fractures is produced. The kinematics of opening of veins in this study involved bending of rock bridges between fractures producing a gradation from planar to sigmoidal shapes in serial sections, without evidence of concurrent fracture propagation.

Interpretation of domainal groundmass textures in basalt lavas of the southern Lamington Volcanics, eastern Australia

The arrangement of crystals, vesicles, and glass in the matrix of a volcanic rock records information about the kinematics accompanying its formation. Simple rock textures involve homogeneous distributions of features such as aligned crystals, whereas more complex rock textures involve inhomogeneous distributions comprising subregions or domains of homogeneous textural types. In the groundmass of volcanic rocks, two types of domainal textures are defined by (1) differences in the degree of alignment or direction of alignment of crystals and (2) different concentrations of crystals. A petrographic survey of basalt lavas of the Tertiary Lamington Volcanics in northeast New South Wales, Australia, reveals the presence of both crystal alignment and crystal concentration domainal textures. Alignment domains are interpreted as shear zones overprinting homogenous aligned textures during flow. The formation of crystal concentration domains appears to involve multiple processes including (1) crystal clustering during flow, (2) complex overprinting of dilatant groundmass shear zones, (3) melt segregations due to stress, or (4) postvesiculation processes.

Three-dimensional morphology and connectivity of stylolites hyperactivated during veining

Abstract The limestone of the Palaeozoic Jack Formation in north Queensland, Australia, contains bedding-parallel stylolites that locally increase in amplitude adjacent to calcite veins. The veins formed in response to inhomogeneous strain locally superposed on the tightly folded strata. Stylolites were locally hyperactivated during vein formation and provided calcite for vein filling. Three-dimensional reconstructions of the morphology of the stylolites, based on detailed serial profiles, demonstrate that the side-faces of columnar stylolites form an interconnected fluid pathway through the rock. The connective morphology defines a drainage network comprising highly tortuous major trunks of high amplitude side-faces linked locally by minor pathways of variable amplitude. The permeability pathway anastomoses in three dimensions making it impossible to image in two dimensions alone.

Enéchelon sigmoidal vein arrays hosted by faults

Abstract The morphology of enechelon sigmoidal veins generally supports an extension fracture origin for host fractures. Nevertheless, a shear fracture origin has been proposed in some cases, mainly on geometric arguments, often without morphological support. This study describes a system of sigmoidal enechelon quartz veins in a deformed greywacke which displays morphological evidence that the veins are hosted by faults. The veins occur in the hinge zone of an upright horizontal antiform and their configuration is consistent with both faulting and vein opening occurring during folding. The formation of close-spaced vertical dextral faults transformed thick massive greywacke beds into a discontinuous multilayer structure of rock bridges between faults. As the length to thickness ratio of the multilayer structure increased, the deformation mechanism changed to one of rotation and bending of rock bridges along sinistral shear displacement zones resulting in sigmoidal openings between rock bridges. Morphological features which indicate that the host fractures are faults include: (1) continuity of host fractures well past the tips of the sigmoidal veins and shear displacement of markers by unopened host fractures, (2) pinnate extension veins emanating from host fractures particularly near terminations and (3) shear quartz fibres on host fractures. Basic kinematic analysis shows that shear displacement (with opposite shear sense to the array) must occur before sigmoidal forms develop as the curvature produces contractional jogs which are incompatible with vein opening. Recognition of these features may assist in determining the role of shear displacement on host fractures of sigmoidal enechelon vein arrays.

True and apparent geometric variability of en-échelon vein arrays

Abstract The wide range of vein-array angles reported for en-echelon vein arrays can be attributed to both true and apparent geometric effects. A survey of published figures supports a continuum of orientations rather than distinct populations related to different fracture mechanisms. The results of the survey indicate a modal veinarray angle of less than 45 ° which indicates area increase as described by a displacement vector oriented outward from the array. A stereometric analysis of the effects of exposure-plane orientation on the apparent vein-array angle is developed. Examples of vein arrays, from the Nambucca Slate Belt of eastern Australia, exposed on two differently-oriented surfaces are shown. These examples include non-profile exposures with apparent vein-array angles which are (1) less than, (2) greater than, (3) approximately equal to, and (4) of opposite shear sense to the true vein-array angle.