Laurent Arbaret

Experimental observations on the effect of interface slip on rotation and stabilisation of rigid particles in simple shear and a comparison with natural mylonites

For axial ratios R>not, vert, similar3, porphyroclasts from three mylonites all show a very strong SPO with the long axis of the best fit ellipse at an antithetic angle of 5-10° to the shear direction. This is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear. The cause of this divergence is investigated in a series of high simple shear strain (γ>15) analogue experiments, performed in a ring-shear machine (couette flow) using a linear viscous matrix (PDMS). The rotational behaviour of elongate (R=not, vert, similar5) rigid particles with elliptical and rhomboidal shapes, comparable with the natural examples, is modelled for both coherent and slipping particle-matrix interfaces. Interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction, but not stabilisation. Interface slip does result in stabilisation of the rhomboidal particle, with the long diagonal oriented at a small antithetic angle to the shear direction. For monoclinic particles, mirror image shapes (referred to here as Types 1 and 2) show different rotational behaviour. For the Type 1 particle (with a shape comparable to σ porphyroclast systems and mica fish), the long side rotates asymptotically into parallelism with the shear direction. Natural examples of Type 1 particles, such as hornblende and olivine porphyroclasts measured from the Finero mylonites (Southern Alps), show a very strong preferred orientation (for R>not, vert, similar3), with the long side parallel or at a small (<5°) antithetic angle to the mylonitic foliation. For Type 2 particles, the short side stabilises close to the shear direction, or at a small synthetic angle, as also observed for sillimanite porphyroclasts from the Mont Mary mylonites (Western Alps). In this natural case, stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation. The analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles. In natural examples, decoupling from the matrix may be affected by extensional crenulation cleavage or C-planes in S-C fabrics.

Effect of shape and orientation on rigid particle rotation and matrix deformation in simple shear flow

The rotation behaviour of rigid particles from a wide range of symmetry classes was analysed in a series of three-dimensional analogue experiments, in which the particles were embedded in a transparent linear–viscous matrix deformed to large shear strains. Comparison of the measured trajectories with theoretical periodic paths for rigid ellipsoids shows that quadratic, slightly orthorhombic and monoclinic particles have trajectory paths and rotation rates closest to theory. Orthorhombic and monoclinic particles, with a minimum aspect ratio >1.5, develop non-periodic trajectories. The results establish that the simplified theoretical model for ellipsoidal forms is a good approximation for a wide range of natural particle shapes. The relationship between the rotation of a rigid particle and deformation of the adjacent matrix was also determined in a series of experiments by placing the particle at different depths relative to an imprinted grid. The results are well illustrated by the surface passing through the centre of the particle and initially parallel to the shear plane. Two deformed regions in the matrix are recognised. In the first region, the matrix rotates with the particle, together forming an approximately ellipsoidal object. The second region is dominated by folds, of limited extent along the rotation axis and greatest extent in a direction that rotates asymptotically toward the shear direction. The folds rotate more slowly than the principal direction of maximum finite stretch.

Analogue and numerical modeling of shape fabrics: application to strain and flow determination in magmas

We summarise numerical and analogue models of shape fabrics, and discuss their applicability to the shape preferred orientation of crystals in magmas. Analyses of flow direction and finite strain recorded during the emplacement of partially crystallised magmas often employ the analytical and numerical solutions of the Jefferys model, which describe the movement of non-interacting ellipsoidal particles immersed in a Newtonian fluid. Crystallising magmas however are considered as dynamic fluid systems in which particles nucleate and grow. Crystallisation during magma deformation leads to mechanical interactions between crystals whose shape distribution is not necessarily homogeneous and constant during emplacement deformation. Experiments carried out in both monoparticle and multiparticle systems show that shape fabrics begin to develop early in the deformation history and evolve according to the theoretical models for low-strain regimes. At large strains and increasing crystal content, the heterogeneous size distribution of natural crystals and contact interactions tend to generate steady-state fabrics with a lineation closely parallel to the direction of the magmatic flow. This effect has been observed in all 3D experiments with particles of similar size and for strain regimes of high vorticity. On the other hand, studies of feldspar megacrysts sub-fabrics in porphyritic granites suggest that these ones record a significant part of the strain history. Thus, fabric ellipsoid for megacrysts evolves closer to the strain ellipsoid than for smaller markers. This behaviour results from the fact that the matrix forms of the melt and smaller crystals behave like a continuous medium relative to the megacrysts. Consequently, in absence these markers, and because the fabric intensities of smaller particles such as biotite are stable and lower than predicted by the theory, finite strain remains indeterminate. In that case, strain quantification and geometry of the flow requires the addition of external constrains based on other structural approaches.

Microstructures and rheology of hydrous synthetic magmatic suspensions deformed in torsion at high pressure

The relationship between magma rheology and characteristic magmatic microstructures was investigated by performing high-temperature high-pressure deformation experiments on hydrous synthetic magmatic suspensions in the range of 0% to 76% solid fraction (alumina grains). Torsion experiments were conducted at 300 MPa confining pressure, temperatures ranging from 475°C to 1000°C and shear strain rates ranging from 2.0 × 10 −5 to 2.1 × 10 −3 s −1 up to total strains of 21.3. Flow is Newtonian for a solid fraction of s = 0.0–0.16 with a log dynamic viscosity η = 10.3 Pa s (T = 500°C). A deviation from Newtonian behavior is observed for s > 0.16 with an increase in apparent viscosity of about 1 order of magnitude between s = 0.16 and 0.54. The shape fabric of the solid phase is characterized by a unimodal orientation that is almost stable and nearly parallel to the shear direction. Both shape fabric and deviation from Newtonian behavior originate from the increase in the number of particle clusters in the suspension. The apparent viscosity increases by 1.5 orders of magnitude between s = 0.54 and 0.65, and extrapolation of the data suggests a very sharp increase in apparent viscosity for s ≥ 0.65. At T ≥ 550°C and s = 0.54 the solid phase forms an almost entirely connected network composed of two alternating orientation domains. At T ≤ 550°C and s = 0.65, intragranular fracturing and tensile fractures result from high local stresses at contact points between neighboring particles. The resulting bulk extensional fabric is almost parallel to the shortening direction.

Porosity redistribution enhanced by strain localization in crystal-rich magmas

Magma degassing, characterized by changes in permeability and porosity distribution, has a crucial control on the style of eruption. During ascent, magma might develop large porosities and crystallize while it is subjected to shear. Shear, in turn, enhances complex fabrics that result from the reorganization of the different phases (crystals, gas, melt). Such fabrics have not yet been evaluated experimentally on a three-phase system. We performed torsion experiments on a synthetic crystal-rich hydrous magma at subsolidus conditions with 11 vol% porosity to establish a link between strain partitioning and porosity redistribution. Crystals induce non-Newtonian deformation, resulting in localization of the shear strain. Three-dimensional microtomography and two-dimensional scanning electron microscope imaging show gas accumulation in local microstructures caused by shear-induced crystal fabric. Our data show that strain localization is a mechanism that could enable magma degassing at very low vesicularity.

Rigid Particles in Simple Shear Flow: Is Their Preferred Orientation Periodic or Steady-State ?

The theory of the rotation of isolated rigid particles within a linearly viscous fluid deforming in progressive simple shear is often invoked in models of Shape Preferred Orientations (SPO) of crystals in igneous rocks. A classical result of the theoretical model is that the SPO should rotate and pulsate with increasing strain, with a periodicity equal to that of the rotation of an individual particle. However, the initial theoretical model makes a large number of assumptions, many of which are unlikely to be satisfied by actual crystalline suspensions in magmatic melts. The purpose of this note is to review three of the reasons why periodicity of rigid particle SPO may not really be expected in igneous rocks: (i) SPO in igneous rocks are generally defined by suspensions of crystals that are concentrated enough to allow mechanical interactions between the crystals; (ii) the porphyroblast/matrix interface may not always be coherent; and (iii) the aspect ratios of the crystals defining the SPO are unlikely to be unique and constant, as assumed in the model. The last point is discussed on the basis of some 2D simple calculations of the development of SPO defined by different types of heterogeneous populations of particles. The combined effects of these deviations from the standard model point to two fundamental conclusions: (i) there is no simple relationship between fabric and finite strain, and (ii) the SPO in magmatic rocks may be considered as good markers of the flow, whatever the significance of the inferred flow pattern in terms of geodynamics and/or emplacement processes.

On the conditions of magma mixing and its bearing on andesite production in the crust.

Mixing between magmas is thought to affect a variety of processes, from the growth of continental crust to the triggering of volcanic eruptions, but its thermophysical viability remains unclear. Here, by using high-pressure mixing experiments and thermal calculations, we show that hybridization during single-intrusive events requires injection of high proportions of the replenishing magma during short periods, producing magmas with 55-58 wt% SiO2 when the mafic end-member is basaltic. High strain rates and gas-rich conditions may produce more felsic hybrids. The incremental growth of crustal reservoirs limits the production of hybrids to the waning stage of pluton assembly and to small portions of it. Large-scale mixing appears to be more efficient at lower crustal conditions, but requires higher proportions of mafic melt, producing more mafic hybrids than in shallow reservoirs. Altogether, our results show that hybrid arc magmas correspond to periods of enhanced magma production at depth.

Rheology and microstructure of experimentally deformed plagioclase suspensions

We present the result of the first deformation experiments at high-temperatures and high-pressures on synthetic magmatic suspensions of strongly anisometric particles. The results highlight the interplay between the rheological response and the development of microstructures and they demonstrate the critical importance of the shape of crystals on the mechanical behaviour of magmas. Plagioclase suspensions with two crystal fractions (0.38 and 0.52) were deformed both in compression and in torsion in a Paterson apparatus. With increasing crystal fraction, the rheological behaviour of the magmatic suspension evolves from nearly steady-state flow to shear weakening, this change being correlated with a microstructural evolution from a pervasive strain to a strain partitioning fabric. Magmatic suspensions of plagioclase have viscosities approximately five orders of magnitude higher than suspensions of equivalent crystallinities made of isometric particles such as quartz.

Shear strain localization from the upper mantle to the middle crust of the Kohistan Arc (Pakistan)

Abstract Shear structures from mantle to middle crust levels of the Kohistan palaeo-island arc, in Pakistan, are described. Pre-Himalayan ductile shear zones show a wide variety in size and shape, and developed from gabbro subsolidus to amphibolite facies conditions. Their lithological context and geological history give insights into mechanisms that initiate shear strain localization, factors that control stabilization of deformation in shear zones and flow properties at the mantle-crust transition. Shear strain localization began within compositional gradients. Gabbros were more prone to localization into anastomosing patterns than diorites and granites, which show more homogeneous strain. Shear strain localization during cooling led to less numerous but longer and thicker shear zones. Viscous heating within shear zones resulted in melt production and segregation in deformation structures, and seems to have taken part in the plutonic history of the arc. Using Kohistan as an example, we suggest that the plutonic, lower crust of arcs is strongly affected by subhorizontal, synmagmatic shear zones, probably consistent with the bulk flow direction of the subduction zone. These features can obviously be preserved in collision orogens and may be mistaken for structures documenting the continental collision.

Interactions between plutonism and detachments during Metamorphic Core Complex formation, Serifos Island (Cyclades, Greece)

In order to better understand the interactions between plutonic activity and strain localization during metamorphic core complexes formation the Miocene granodioritic pluton of Serifos (Cyclades, Greece) is studied. This pluton (11.6-9.5 Ma) intruded the Cycladic Blueschists during thinning of the Aegean domain along a system of low-angle normal faults belonging to the south-dipping West Cycladic Detachment System (WCDS). Based on structural fieldwork, together with microstructural observations and anisotropy of magnetic susceptibility (AMS), we recognize a continuum of deformation from magmatic to brittle conditions within the magmatic body. This succession of deformation events is kinematically compatible with the development of the WCDS. The architecture of the pluton shows a marked asymmetry resulting from its interaction with the detachments. We propose a tectonic scenario for the emplacement of Serifos pluton and its subsequent cooling during the Aegean extension. (1) A first stage corresponds to the metamorphic core complex initiation and associated southwestward shearing along the Meghalo Livadhi detachment. (2) In a second stage the Serifos pluton has intruded the dome at shallow crustal level, piercing through the ductile/brittle Meghalo Livadhi detachment. Southwest-directed extensional deformation was contemporaneously transferred upward in the crust along the more localized Kavos Kiklopas detachment. (3) A third stage was marked by syn-magmatic extensional deformation and strain localization at the contact between the pluton and the host rocks resulting in narrow shear zones nucleation which (4) continued to develop after the pluton solidification.