Giorgio Pennacchioni

The influence of grain boundary fluids on the microstructure of quartz-feldspar mylonites

Quartz-rich domains in pre-Alpine, water-deficient, amphibolite facies (510–580 °C, 250–450 MPa), pegmatite mylonites from Mont Mary (MM), western Alps, preserve a fine dynamically recrystallized grain size, without significant annealing, despite the high synkinematic temperatures and subsequent static greenschist facies Alpine overprint. The microstructure is dramatically different from more typical water-rich amphibolite facies mylonites, such as from the Simplon Fault Zone in the central Alps, where the recrystallized grain size is on the millimetre-scale. The difference reflects the dominant strain-induced recrystallization mechanism: (1) progressive subgrain rotation and grain boundary bulging for the dry MM examples; and (2) fast grain boundary migration for the wet Simplon examples. The grain boundary microstructure imaged with SEM is also very different, with most grain boundaries in the dry MM samples lacking porosity, whereas grain boundaries in the wet samples are decorated by a multitude of pores. Quartz grain boundaries from both wet and dry samples are locally coated by thin (100’s of nanometres), possibly amorphous, silica films. Despite the differences in microstructure, the crystallographic preferred orientations (CPOs) of quartz-rich domains from both areas are very similar. Water-deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism(s) but apparently have little influence on the intracrystalline slip systems, as reflected in the CPOs (strong c-axis Y maxima). In MM mylonites, both K-feldspar and plagioclase (An33-38) dynamically recrystallize, consistent with the inferred metamorphic conditions. Under water-deficient conditions, mid- to lower-crustal rocks can deform heterogeneously under transitional ductile-brittle conditions at high differential stress (for MM ca. 300–500 MPa, as estimated for dry Mohr–Coulomb failure) and preserve this high-stress microstructure, because of the low mobility of dry grain boundaries.

Earthquake rupture dynamics frozen in exhumed ancient faults

Most of our knowledge about co-seismic rupture propagation is derived from inversion and interpretation of strong-ground-motion seismograms, laboratory experiments on rock and rock-analogue material, or inferred from theoretical and numerical elastodynamic models. However, additional information on dynamic rupture processes can be provided by direct observation of faults exhumed at the Earths surface. Pseudotachylytes (solidified friction-induced melts) are the most certain fault-rock indicator of seismicity on ancient faults. Here we show how the asymmetry in distribution and the orientation of pseudotachylyte-filled secondary fractures around an exhumed fault can be used to reconstruct the earthquake rupture directivity, rupture velocity and fracture energy, by comparison with the theoretical dynamic stress field computed around propagating fractures. In particular, the studied natural network of pseudotachylytes is consistent with a dominant propagation direction during repeated seismic events and subsonic rupture propagation close to the Rayleigh wave velocity.

Rare earth and trace element mobility in mid-crustal shear zones: insights from the Mont Blanc Massif (Western Alps)

The behaviour of rare earth elements (REE) during fluid–rock interaction in mid-crustal shear zones has received little attention, despite their potential for mass balance calculation and isotopic tracing during deformation. In this study, several cases of large REE mobility during Alpine fluid-driven shear zone development in the pre-Alpine granitic basement of the Mont Blanc Massif are considered. On a regional scale, the undeformed granite compositions range within 5 wt% SiO2 (70.5–75.3 wt%) and magmatic chemical variations are of the order of 10–20%, ascribed to minor effects of crystal fractionation. Major and trace element mobility observed in shear zones largely exceeds these initial variations. Shear zones developed a range of mineral assemblages as a result of shearing at mid-crustal depths (at not, vert, similar0.5 GPa, 400°C). Five main shear zone assemblages involve muscovite, chlorite, epidote, actinolite and calcite, respectively, as major phases. In most cases, selective enrichments of light or heavy REE (and Y, Ta, Hf) are observed. REE mobility is unrelated to deformation style (cataclastic, mylonitic), the intensity of strain, and to the shear zones major metamorphic mineral assemblages. Instead, the changes in REE concentrations are ascribed to the alteration of pre-existing magmatic REE-bearing minerals during deformation-related fluid–rock interaction and to the syntectonic precipitation of metamorphic REE-bearing minerals (mainly monazite, bastnasite, aeschynite and tombarthite). Minor proportions (<2%) of these accessory phases, with grain sizes mostly <20 μm, account for enrichments of up to 5:1 compared to the initial granite whole-rock REE budget. The stability of the REE phases appears to be largely dependent on the altering fluid composition. REE mobility is ascribed to changes in pH and to the availability of CO32−, PO42−, and SO42−ligands in the fluid. Such processes are likely to influence the mobility of REE, Y, Hf and Ta in shear zones.

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.

Brittle precursors of plastic deformation in a granite: an example from the Mont Blanc massif (Helvetic, western Alps)

Abstract In the Mont Blanc Helvetic massif, granites record mesoscale Alpine structures, which include joints, veins, cataclastic to mylonitic shear zones and foliated granites. A detailed structural analysis indicates that brittle deformation predates plastic strain. Joints never pass through, and veins are offset by, cataclastic shear zones and mylonites. The mylonites progressively develop by plastic reactivation of cataclastic shear zones during greenschist facies metamorphic conditions. Plastic deformation is first localized in the brittle discontinuities and the fine-grained matrix of cataclasites. Then it involves the granite within brittle shear zones, and this is initially accomplished mainly by flow of reaction-softened aggregates of sericite, widely replacing the strain-supporting magmatic plagioclase. The brittle-to-plastic evolution has resulted in highly localized discontinuous plastic shear zones with high lateral continuity, and these characteristics are derived from reactivation of, and focusing along, pre-existing brittle discontinuities. In addition, mylonites may inherit high angles of intersection, and may contain granite porphyroclasts. These features may allow the inference of a precursor brittle deformation where the plastic overprint has completely erased the initial brittle fabrics.

Salt-rich aqueous fluids formed during eclogitization of metabasites in the Alpine continental crust (Austroalpine Mt. Emilius unit, Italian western Alps)

Abstract The metabasite bodies of the Mt. Emilius continental unit (western Italian Alps) underwent a stage of Alpine eclogite-facies metamorphism (1.1–1.3 GPa and 450–550°C) accompanied by polyphase deformation and recrystallization. The metabasites consist of two main rock types: (1) eclogites (omphacite+garnet+glaucophane+epidote+phengite/paragonite) preserving no relics of their precursors; (2) eclogitized granulites, i.e. rocks whose incomplete eclogitic recrystallization (clinopyroxene II+garnet II+epidote+amphibole+chlorite) allowed survival of textural and mineralogical relics of pre-Alpine granulitic assemblages (clinopyroxene I+garnet I+plagioclase+amphibole). In this latter case the pre-Alpine granulites were converted to eclogites as the result of infiltration of aqueous fluids during eclogitization. In both eclogites and eclogitized granulites hydrated high-pressure foliations are cut by eclogitic metamorphic veins. The bulk rock chemistry of the metabasites influenced the compositions of both the vein- and rock-forming clinopyroxenes and the compositional correlation between the vein- and rock-forming clinopyroxenes indicates that the syn-eclogitic fluids have re-equilibrated with the metabasite hosts. The predominant vein minerals (omphacite, epidote and garnet) contain primary high-salinity fluid inclusions. The fluids consist of two-phase (liquid+vapour) and of multiphase (liquid+vapour+salt+additional quartz) salty aqueous inclusions containing NaCl, CaCl 2 and MgCl 2 as the main chloride species. The vein inclusions show a salinity range from 17 to 45 wt.% salts in eclogites and from 20 to 50 wt.% salts in eclogitized granulites. In contrast, fluid inclusions in matrix minerals of the eclogitized granulites contain primary two-phase inclusions displaying a salinity range between 10 and 25 wt.% salts. The marked difference in fluid salinities recorded by the inclusions in the eclogitic veins and those in the partially re-equilibrated eclogitized granulites is interpreted in terms of progressive hydration during eclogitization of granulite-facies rocks, which caused an increase in the salt content of the residual inclusion fluids.

Strain-insensitive preferred orientation of porphyroclasts in Mont Mary mylonites

Abstract The shape preferred orientation (SPO) of porphyroclasts was determined in high temperature mylonites. The porphyroclasts approach rhomboidal (sillimanite) or elliptical (garnet, plagioclase, sillimanite) shapes, and exhibit aspect ratios ( R ) as high as 11. Particles with R >3 are dominantly rhomboidal. The long axis of the best-fit ellipse defines a very strong SPO inclined at 5–10° to the mylonitic foliation, in an antithetic sense with respect to the shear direction. This angle is independent of R . The inclination of the long sides of rhomboidal sillimanite increases from 10 to 20° with decreasing R . In contrast, the short sides have a constant orientation of 15 to 17° irrespective of R and are parallel to extensional crenulation cleavage. Low aspect ratio (mainly elliptical) objects show low intensity SPO close to the shear plane. The two SPOs appear strain-insensitive. In the case of R R >3, a stable position is acquired. This is not explained by any of the current theoretical and experimental models of SPO.

Analogue modelling of the influence of shape and particle/matrix interface lubrication on the rotational behaviour of rigid particles in simple shear

Abstract The rotational behaviour of a rigid particle embedded in a linear viscous matrix undergoing cylindrical simple shear (Couette) flow was studied in 2D rock-analogue experiments. The influence of particle shape (elliptical vs. monoclinic), aspect ratio and the nature of the matrix/particle interface (lubricated vs. unlubricated) was investigated. Both matrix (PDMS) and lubricant (liquid soap) were linear viscous, with a viscosity ratio of ca. 10 4 . Without lubricant, the rotational behaviour of all particles closely approximates the Jeffery theory. Lubricated monoclinic particles with the long diagonal initially parallel to the shear direction show back rotation and approach a stable position. Lubricated elliptical particles initially parallel to the shear direction also show back rotation but only transiently stabilize. Weak planar zones in the matrix adjacent to unlubricated elliptical particles do not induce backward rotation. In general for elliptical particles, rotation rate as a function of orientation depends on axial ratio and thickness of the lubricant mantle. For thick mantles (initially >10% of the volume of the particle), rotation rates are faster than Jeffery theory. For very thin mantles they are markedly slower compared with thick mantles, particularly when the long axis is nearly parallel to the shear direction. Rotation rates are never strictly zero, so true stabilization does not occur. However, for more elongate particles (axial ratio=6) rotation rates are so slow that a very strong shape preferred orientation would develop in a lubricated elliptical particle population. In experiments, the volume of lubricant is constant and the thickness adjacent to the long side of the particle progressively decreases with increasing strain. In natural examples of porphyroclast systems, the weak mantle continually develops by recrystallization and/or cataclasis of the rigid clast core and a steady state between production and thinning could be attained, potentially leading to true stabilization for particles with a high axial ratio.

Finite-element modelling of simple shear flow in Newtonian and non-Newtonian fluids around a circular rigid particle

Abstract The flow perturbation around a circular rigid particle during simple shear deformation has been investigated for both Newtonian and non-Newtonian (power-law) fluids by finite-element modelling. If the particle is rotating under the applied shear couple and no-slip occurs at the particle–fluid interface, a ‘bow-tie-shaped’ streamline pattern results for both Newtonian and power-law fluids and the shape of streamlines does not change noticeably if the stress exponent ( n ) is changed. In contrast, if the fluid is allowed to separate from the particle, a ‘double-bulge-shaped’ flow develops in the case of Newtonian fluids, and the type of streamline pattern is influenced by n . We suggest that both stair-stepping and non-stair-stepping geometries of porphyroclast tails may be produced in mylonites, depending on the degree of coherence between the porphyroclasts and the embedding matrix. A different behaviour of the fluid–particle interface may occur as the result of changing fluid rheology, owing to the contrasting stress fields developed for Newtonian and non-Newtonian fluids.

Ultrafine-grained quartz mylonites from high-grade shear zones: Evidence for strong dry middle to lower crust

Creep strength of the crust depends upon the rheology of the most common mineral, usually quartz. Recrystallized quartz grains in many high-grade shear zones from the middle to lower crust are typically large (millimeter sized), implying active grain boundary migration, but equivalents from old polymetamorphic and water-deficient basement sheared at similar crustal depths can be very small. For the latter, strong crystallographic preferred orientation (CPO) of quartz, with c axes aligned close to Y, progressive misorientation of crystals, subgrain and dislocation development, and core-mantle structures with recrystallized grains of 2‐8 m size, all point to dislocation glide dominantly on the prism a system with recrystallization by subgrain rotation. Recrystallized grain size piezometry of such quartz indicates high flow stress in fine-grained shear zones, while the synkinematic metamorphic mineral assemblage and the CPO are typical of amphibolite facies conditions. This is evidence that middle to lower crust is not inevitably weak due to its high temperature: water content also has an important influence.