Strength and rheological evolution of the lower continental crust: an experimental study of the deformation behavior of feldspar and quartz at high pressure and temperature

Abstract

Models representing the strength of the lithosphere (i.e. strength-depth profiles) are mainly based on the rheological properties of the minerals quartz and feldspar that, to a large extent, are determined in laboratory-based rock deformation studies. The study of the deformation behavior of quartz and feldspar is of fundamental importance for the understanding and modeling of plate tectonics and orogenic processes in general. However, the rheology of the crust is intensely debated because field observations sometimes do not support models based on geophysical data. Experiments may resolve some of the discrepancies.

The aim of this study is to reproduce in the laboratory, by means of high pressure and high temperature deformation experiments, the microstructures and reactions observed in quartz and feldspar in nature in order to quantify and explore deformation processes in the lower continental crust. The deformation experiments were performed on natural single crystals and powders of quartz and feldspar with a Griggs-type solid medium deformation apparatus. High confining pressures (Pc= 0.75-1.5 GPa) and temperatures (700 °C-1000 °C) were chosen in order to simulate lower crustal conditions. As in many laboratory deformation experiments, the use of very high temperatures was necessary to promote deformation of quartz and feldspar at the strain rates that can be achieved using the Griggs apparatus (10-6 to 10-7 in this work). Two main crustal processes were studied: (1) The effect of deformation on the TitaniQ (titanium in quartz) geothermobarometer and; (2) the deformation behavior and rheology of melt bearing feldspar single crystals and gouges.

(1) The geothermobarometer TitaniQ is based on Si-Ti substitution in quartz, which is both pressure and temperature dependent. Because the microstructural evolution of quartz under varying pressure and temperature conditions is relatively well characterized, the correlation between quartz microstructures and Ti content represents a promising method to directly estimate the pressure and temperature conditions of deformation. However, the effect that different mechanisms of recrystallization have on Ti incorporation is not fully understood, and thus it is unknown whether the TitaniQ geothermobarometer can be applied to deformed rocks of the lower crust. Our high pressure and temperature deformation experiment on quartz single crystals (Chapter 3) demonstrate that in a fluid-present and Ti-saturated environment, the Si—Ti substitution in quartz is not likely to occur during deformation, regardless of the recrystallization mechanism involved in the deformation process. In our experiments, neither quartz grains that deformed by subgrain rotation recrystallization nor those showing grain boundary migration features incorporate equilibrium contents of Ti. These results suggest that the application of the TitaniQ geothermobarometer to deformed rocks under prograde metamorphic conditions is not as straightforward as previously thought.

(2) Previous experimental studies suggest that fluid-bearing feldspar deforms by viscous processes at temperatures > 700 °C. However, field observations indicate that under lower crustal conditions feldspar can deform by brittle processes, even at high temperature. Microstructural observations and mechanical data from high-temperature and high-pressure deformation experiments on K-feldspar single crystals (Chapter 4) demonstrate that deformation is initially accommodated by brittle fractures, which cause grain size reduction and formation of gouges. Dilatation along the fractures leads to a local decrease of the confining pressure and promotes melting in cracked regions. However, the presence of melt in the system does not significantly influence the strength of the rock as melt is present only in small fractions and, during deformation, it remains isolated without forming an interconnected network. Preliminary microstructural observations of the fine grained fault gouges formed along brittle fractures suggests the activity of dissolution precipitation creep as the dominant deformation mechanism.

In order to better quantify and study the processes occurring in the gouges, shear experiments were performed on layers of fine grained aggregates of feldspar (Chapter 5). Microstructural observations supported by mechanical data indicate that the composition of the starting material has only a minor influence on the deformation behavior of water bearing feldspar gouges. Furthermore, no systematic differences were observed in melt bearing and melt free aggregates. The presence of regions of gouge showing pervasive compositional changes, the nucleation and growth of new euhedral grains with a different chemical composition, grains with lobate and indented grain boundaries, and overgrowth structures all suggest that in wet feldspar gouges at high pressure and temperature deformation is mainly accommodated by dissolution precipitation processes.