Luca Menegon

On the nucleation of non-Andersonian faults along phyllosilicate-rich mylonite belts

Abstract The weakness of fault zones is generally explained by invoking an elevated fluid pressure or the presence of extremely weak minerals in a continuous fault gouge horizon. This allows for faults to slip under an unfavourable normal to shear stress ratio, in contrast to E. M. Andersons theory of faulting. However, these mechanisms do not explain why faults should nucleate in such an orientation as to make them misoriented and non-Andersonian. Here we present a weakening mechanism, involving the mechanical anisotropy of phyllosilicate-bearing mylonite belts, which is likely to influence the nucleation of faults in addition to their subsequent activity. Considering three natural examples from the Alps (the Simplon, Brenner and Sprechenstein-Mules fault zones) and a review of laboratory tests on anisotropic rocks, we apply anisotropic slip tendency analysis and show that misoriented weak faults can nucleate along a sub-planar phyllosilicate-rich mylonitic foliation, constituting a large-scale mechanical anisotropy belt and preventing the development of Andersonian optimally oriented faults.

Semibrittle deformation and partial melting of perthitic K-feldspar: An experimental study

To investigate the relationships between deformation, cracking, and partial melting in the lower continental crust, axial compression and hydrostatic experiments were performed on K-feldspar single crystals at temperatures of 700° and 900°C and confining pressures between 0.75 and 1.5 GPa. Sample deformation was carried out at a constant strain rate of ~ 10−6 s−1. The samples deformed at 700°C show typical brittle behavior with formation of conjugate fractures and peak stresses that increase with confining pressure. Samples deformed at 900°C show formation of shear fractures, peak stresses below the Goetze criterion, and inverse confining pressure dependence of peak stress, indicating that along the fractures deformation was not dominantly friction controlled. Microstructural and chemical analyses reveal the presence of melt (<6 vol %) of inhomogeneous composition along the shear zones and chemical compositional changes of gouge fragments. In a hydrostatic experiment performed at 900°C, no melt and no compositional changes were observed. These observations indicate that deformation of K-feldspars at high pressures and temperatures is controlled by the simultaneous formation of brittle fractures and melt. The formation of melt is strongly accelerated and kinetically favored by cracking, as demonstrated by the absence of melting in the hydrostatic experiments. However, the melt along fractures does not dramatically weaken the samples, as the melt domains remain isolated during deformation. The fine-grained gouge fragments formed along the fracture systems undergo chemical homogenization. The dominant deformation mechanism in the gouge is likely to be melt-enhanced diffusion creep, which may also assist the chemical homogenization process.

Earthquakes as Precursors of Ductile Shear Zones in the Dry and Strong Lower Crust

The rheology and the conditions for viscous flow of the dry granulite facies lower crust are still poorly understood. Viscous shearing in the dry and strong lower crust commonly localizes in pseudotachylyte veins, but the deformation mechanisms responsible for the weakening and viscous shear localization in pseudotachylytes are yet to be explored. We investigated examples of pristine and mylonitized pseudotachylytes in anorthosites from Nusfjord (Lofoten, Norway). Mutual overprinting relationships indicate that pristine- and mylonitized pseudotachylytes are coeval and resulted from the cyclical interplay between brittle and viscous deformation. The stable mineral assemblage in the mylonitized pseudotachylytes consists of plagioclase, amphibole, clinopyroxene, quartz, biotite, ± garnet ± K-feldspar. Amphibole-plagioclase geothermobarometry and thermodynamic modelling indicate that pristine- and mylonitized pseudotachylytes formed at 650-750°C and 0.7-0.8 GPa. Thermodynamic modelling indicates that a limited amount of H2O infiltration (0.20-0.40 wt%) was necessary to stabilize the mineral assemblage in the mylonite. Diffusion creep is identified as the main deformation mechanisms in the mylonitized pseudotachylytes based on the lack of crystallographic preferred orientation in plagioclase, the high degree of phase mixing, and the synkinematic nucleation of amphiboles in dilatant sites. Extrapolation of flow laws to natural conditions indicates that mylonitized pseudotachylytes are up to 3 orders of magnitude weaker than anorthosites deforming by dislocation creep, thus highlighting the fundamental role of lower crustal earthquakes as agents of weakening in strong granulites.

The strain-dependent spatial evolution of garnet in a high-pressure ductile shear zone from the Western Gneiss Region (Norway): a synchrotron X-ray microtomography study

Reaction and deformation microfabrics provide key information to understand the thermodynamic and kinetic controls of tectono‐metamorphic processes, however, they are usually analysed in two dimensions, omitting important information regarding the third spatial dimension. We applied synchrotron‐based X‐ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite–garnet eclogite in four dimensions, where the 4th dimension is represented by the degree of strain. In the investigated samples, which cover a strain gradient into a shear zone from the Western Gneiss Region (Norway), we focused on the spatial transformation of garnet coronas into elongated garnet clusters with increasing strain. The microtomographic data allowed quantification of garnet volume, shape and spatial arrangement evolution with increasing strain. The microtomographic observations were combined with light microscope and backscatter electron images as well as electron microprobe (EMPA) and electron backscatter diffraction (EBSD) analysis to correlate mineral composition and orientation data with the X‐ray absorption signal of the same mineral grains. With increasing deformation, the garnet volume almost triples. In the low‐strain domain, garnet grains form a well interconnected large garnet aggregate that develops throughout the entire sample. We also observed that garnet coronas in the gabbros never completely encapsulate olivine grains. In the most highly deformed eclogites, the oblate shapes of garnet clusters reflect a deformational origin of the microfabrics. We interpret the aligned garnet aggregates to direct synkinematic fluid flow, and consequently influence the transport of dissolved chemical components. EBSD analyses reveal that garnet shows a near‐random crystal preferred orientation that testifies no evidence for crystal plasticity. There is, however evidence for minor fracturing, neo‐nucleation and overgrowth. Microprobe chemical analysis revealed that garnet compositions progressively equilibrate to eclogite facies, becoming more almandine‐rich. We interpret these observations as pointing to a mechanical disintegration of the garnet coronas during strain localization, and their rearrangement into individual garnet clusters through a combination of garnet coalescence and overgrowth while the rock was deforming.

Replacement reactions and deformation by dissolution and precipitation processes in amphibolites

The deformation of the middle to lower crust in collisional settings occurs via deformation mechanisms that vary with rock composition, fluid content, pressure and temperature. These mechanisms are responsible for the accommodation of large tectonic transport distances during nappe stacking and exhumation. Here we show that fracturing and fluid flow triggered coupled dissolution‐precipitation and dissolution‐precipitation creep processes, which were responsible for the formation of a mylonitic microstructure in amphibolites. This fabric is developed over a crustal thickness of >500 m in the Lower Seve Nappe (Scandinavian Caledonides). Amphibolites display a mylonitic foliation that wraps around albite porphyroclasts appearing dark in panchromatic cathodoluminescence. The albite porphyroclasts were dissected and fragmented by fractures preferentially developed along the (001) cleavage planes, and display lobate edges with embayments and peninsular features. Two albite/oligoclase generations, bright in cathodoluminescence, resorbed and overgrew the porphyroclasts, sealing the fractures. Electron backscattered diffraction shows that the two albite/oligoclase generations grew both pseudomorphically and topotaxially at the expense of the albite porphyroclasts, and epitaxially around them. These two albite/oligoclase generations also grew as neoblasts elongated parallel to the mylonitic foliation. The amphibole crystals experienced a similar microstructural evolution, as evidenced by corroded ferrohornblende cores surrounded by ferrotschermakite rims that preserve the same crystallographic orientation of the cores. Misorientation maps highlight how misorientations in amphibole are related to displacement along fractures perpendicular to its c‐axis. No crystal plasticity is observed in either mineral species. Plagioclase and amphibole display a crystallographic preferred orientation that is the result of topotaxial growth on parental grains and nucleation of new grains with a similar crystallographic orientation. Amphibole and plagioclase thermobarometry constrains the mylonitic foliation development to the epidote amphibolite facies (˜600°C, 0.75‐0.97 GPa). Our results demonstrate that at middle to lower crustal levels the presence of H2O‐rich fluid at grain boundaries facilitates replacement reactions by coupled dissolution‐precipitation and favours deformation by dissolution‐precipitation creep over dislocation creep in plagioclase and amphibole.