Development of schistosity by dissolution–crystallization in a Californian serpentinite gouge

Abstract

To understand the behaviour and deformation mechanisms of serpentinites in the seismogenic zone we study the deformation macro-and microstructures of serpentinites along the Santa Ynez Fault in the San Andreas System. At the outcrop scale, deformation is localized in a gouge zone that shows three different structures: (1) micrometric undeformed fragments (clasts) of the previously serpentinized peridotite, (2) localized shear planes (Y and R) and (3) a penetrative schistosity (S). Observations under SEM and TEM reveal that the schistosity corresponds to serpentine fibres, parallel to each other, and whose orientation varies as they wrap around clasts. TEM micro-textures indicate that these long fibres result from continuous syntectonic growth rather than from reorientation of pre-existing fibres implying a slow transfer process that occurs at short distances. We propose a dissolution-diffusion-crystallization process for the formation of the schistosity that corresponds to a low strain-rate creeping process of deformation that can be effective in aseismic fault segments.

The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep

“…The observations from this study are in accord with previous studies on naturally deformed antigorite (e.g. Andreani et al, 2005;Auzende et al, 2006;Wassmann et al, 2011) that demonstrate the importance of dissolution-precipitation processes during ductile deformation of antigorite, especially in polyphase aggregates, where phase boundaries between antigorite and olivine act as sites of preferred dissolution. The general importance of phase boundaries as preferred sites of dissolution during dissolution-precipitation creep is evident from many natural rocks (e.g.…”

“…Hilairet et al, 2007;Chernak and Hirth, 2010) microfabrics can yield information on the deformation mechanisms, as well as the stress and strain-rate conditions during burial and exhumation (e.g. Skrotzki et al, 1990;van der Wal, 1993;Altenberger, 1995;Jin et al 1998;Zhang et al, 2000;Jung and Karato, 2001;Piepenbreier and Stöckhert, 2001;Stöckhert, 2002;Andreani et al, 2005;Auzende et al, 2006;Jung et al, 2006;Wassmann et al, 2011;Matysiak and Trepmann, 2012). The aim of this study is to use the successively overprinted and modified microfabrics of the partly serpentinized peridotite mylonites to differentiate between the potential record of independent grain-scale deformation processes and implications on the stress conditions.…”

Antigorite crystallographic preferred orientations in serpentinites from Japan

“…For example are the occurrences in the California Coast Ranges and foothills of the Sierra Nevada (e.g. Andreani et al, 2005;Hirauchi et al, 2008;Wakabayashi, 2004) and the majority of serpentinites in the Central Alps, including Val Malenco (e.g. Molli, 1996;Tricart and Lemoine, 1983) where serpentinite is massive, without significant preferred orientation.…”

Linking rock fabric to fibrous mineralisation: a basic tool for the asbestos hazard

“…2b). Circulation within the subduction channel results in the development of polyphase deformation fabrics and metamorphic mineral assemblages that modify the original texture, mineralogy, and rheology of pristine ophiolites (Scambelluri et al, 1995;Hermann et al, 2000;Andreani et al, 2005). Orogenic processes may also involve oceanic obduction, which constitutes the tectonic juxtaposition of the oceanic lithosphere onto continental margins (e.g.…”

“…5a, b) define the common example of large amount of localised ductile shearing, as they accommodate fault slip surfaces by the combined effect of kinematics and crystallization (e.g. Norrell et al, 1989;Hoogerduijn Strating and Vissers, 1994;Vissers et al, 1995;Karkanas, 1995;Hermann et al, 2000;Andreani et al, 2005). Within schistose serpentinites, syn-kinematic fibrous minerals are disposed parallel among them and define a structural preferential orientation (Fig.…”