Constraints on the formation of ultramafic and mafic pseudotachylytes in the Schistes Lustre complex, Corsica

Deseta, Natalie
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Intermediate-depth earthquakes occur at depths of 60 – 300 km at these depths high confining pressure inhibits brittle failure from generating earthquakes. Fault-related pseudotachylytes from Corsica are exhumed paleofaults from a high pressure, low temperature subduction zone environment, and are considered analogues of intermediate-depth earthquakes. Hence, it is important to analyse the physico-chemical processes by which these pseudotachylytes form in order to gain primary insight into the controls of their formation and how this seemingly paradoxical process takes place. Up until the recent discovery of high pressure pseudotachylytes there was no known direct method of evaluating the formation mechanisms of intermediate-depth earthquakes. High pressure pseudotachylytes found in subduction complexes are regarded as relict paleo-earthquakes. Previous research aimed at understanding the generation of these phenomena and the role of fluids on their origin has been based on seismic, experimental and numerical modelling. The principal aims of this project were to carry out detailed geochemical, petrographic and microtextural analyses of such pseudotachylytes located in the Eocene Schistes Lustres Complex, Corsica, and to determine whether the data from natural samples corroborate current models. The pseudotachylytes in this study reside in peridotitic and metagabbroic lozenges enclosed within serpentinites. Pseudotachylytes are notoriously complex and messy, with compositions that vary widely over small distances (< 1 mm). For this reason the pseudotachylytes in this study were systematically analysed from the outcropscale to the micron-scale according to their wallrock type. From these data it was observed that greenschist and blueschist facies hydrous minerals present in the peridotite and metagabbro wallrocks were entrained into pseudotachylyte fault veins. Back scatter electron (BSE) imaging shows that these hydrous minerals underwent wholesale fusion in the melt. No evidence for prograde dehydration reactions was observed in the wallrocks or in association with the pseudotachylytes. Electron microprobe analyses (EPMA) of the bulk matrix of the pseudotachylytes revealed variable H2O content, 0 – 14 wt % in peridotite-pseudotachylytes and 0 – 4 wt % in metagabbro-hosted pseudotachylytes. The principal minerals that underwent fusion are: clinopyroxene, plagioclase, glaucophane, Mg-hornblende and actinolite (metagabbro- hosted) pseudotachylyte), and olivine, orthopyroxene clinopryroxene, chlorite, serpentine and tremolite (peridotite-hosted pseudotachylyte). The bulk of H2O entering the melt remained in solution until it reached supersaturation, upon which it exsolved to form fluid-rich, vesicular veins. Cuspate and lobate rims of microlites (omphacite, clinopyroxene, olivine and orthopyroxene) along the boundaries of hydrous veins indicate that the melt was still molten when the fluids exsolved. The presence of hydrous fluids in the melt appears to have enhanced the fracturing process. Fault veins hosted by peridotite that have the greatest H2O content are the thickest, have more chaotic injection networks and exhibit more cataclastic deformation features than the anhydrous fault veins observed. With regard to the mechanism of pseudotachylyte generation, it is clear that water present in hydrous minerals or entrapped in the crystal lattices of anhydrous minerals plays a fundamental role in facilitating intermediate-depth earthquakes through hydrolytic weakening. A melt richer in hydrous fluid also has a lower viscosity, facilitating fault slip. Dissolved H2O is also a flux and may enhance further melting of the wallrock, relative to an anhydrous pseudotachylyte vein. Sheared, kinked and twinned wallrock minerals and survivor clasts associated with the pseudotachylyte fault veins indicate crystal-plastic deformation. No significant grain size reduction was observed in proximity to fault veins. The grain size of wallrock minerals at fault vein boundaries ranges from 5 – 20 mm. From this it was inferred that the mechanism of deformation is controlled by power law creep, temperature and high strain rate. The presence of metastable high temperature crystallisation products in the pseudotachylyte such as hoppers and dendrites of olivine, orthopyroxene and diopside (in peridotite) and Al-rich omphacite and Fe-rich anorthite (in metagabbro), are suggestive of a short-lived high temperature event resulting from thermal instability. These high temperature mineral assemblages are overprinted by ones indicating a return to ambient conditions (lower temperatures, but still high pressures), namely, glaucophane, albite and epidote (in metagabbro) and clinochore, fine-grained granoblastic olivine, enstatite and diopside (in peridotite). The observations from this detailed study of natural samples suggest that intermediate-depth seismicity may be generated by a thermal runaway process.
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2014.