Some observations on brittle-ductile toggle

Abstract

In low temperature deformation of polymineralic rocks the constituent minerals often show contrasting deformation mechanisms. In naturally deformed arkoses, feldspathic quartzites and grits under greenschist to aimandine-amphibolite facies condition, feldspar deforms by microbondinage (rigid-brittle behaviour), while quartz flows by a combination of dislocation creep, pressure solution and solution transfer. Boudin segments develop and separate in a phased sequential manner while quartz matrix flows in a ductile manner, indicating a brittle-ductile toggle during progressive deformation.Both the pressure solution and dislocation creep flows are volume-conservative. Therefore, a net volume increase during the above deformations is a necessity, unless compensated by a solution-transfer process. Hydrofracturing probably played a role in microboudinage formation as the ambient level of differential stress is estimated to be low around 45-75 MPa.To develop a synthetic flow law for the above type of deformation in arkoses, one needs to consider the significance of different rate-controlling mechanisms. As feldspar pull-aparts are syntectonically filled with quartz or metamorphic minerals crystallizing during progressive deformation, successive microboudin segmentation will depend on how fast/slow the matrix quartz moves to the open crack or the sealing takes place by transfer of appropriate solute components by pressure solution or solution transfer, the real rate-controlling process.

Flow laws in polymineralic aggregates deformed by a combination of diffusion creep and dislocation creep

“…Natural deformation under low ambient temperature as in the upper crust is often accommodated by a combination of several deformation mechanisms as demonstrated by a number of workers (e.g., Mitra 1976;Etheridge et al 1984;O'Hara 1990;Saha 1995). Diffusive mass transfer (DMT) is an important deformation mechanism (Eliot 1973;Rutter 1983;Groshong 1988;Knipe 1989).…”

“…Microstructures in naturally deformed rocks (e.g. Ross and Lewis 1989;Saha 1995) demonstrate that creep strain in nature, particularly in the upper crust is accommodated by a combination of dislocation creep (crystal plastic mechanisms, CPM), diffusive mass transfer (DMT) or dissolution processes, and 'brittle' microcracking. If the ambient conditions are favourable cracking on its own is a fast process.…”