Surface roughness evolution on experimentally simulated faults François Renard a, b, * , Karen Mair b, c , Olav Gundersen b a ISTerre, University of Grenoble I, CNRS, BP 53, F-38041 Grenoble, France b Physics of Geological Processes, University of Oslo, Norway c Department of Geosciences, University of Oslo, Norway article info Article history: Received 3 October 2011 Received in revised form 17 February 2012 Accepted 20 March 2012 Available online 9 April 2012 Keywords: Faulting Fracture Frictional sliding Earthquake mechanics Heat dissipation abstract To investigate the physical processes operating in active fault zones, we conduct analogue laboratory experiments where we track the morphological and mechanical evolution of an interface during slip. Our laboratory friction experiments consist of a halite (NaCl) slider held under constant normal load that is dragged across a coarse sandpaper substrate. This set-up is a surrogate for a fault surface, where brittle and plastic deformation mechanisms operate simultaneously during sliding. Surface morphology evolution, frictional resistance and infra-red emission are recorded with cumulative slip. After experi- ments, we characterize the roughness developed on slid surfaces, to nanometer resolution, using white light interferometry. We directly observe the formation of deformation features, such as slip parallel linear striations, as well as deformation products or gouge. The striations are often associated with marginal ridges of positive relief suggesting sideways transport of gouge products in the plane of the slip surface in a snow-plough-like fashion. Deeper striations are commonly bounded by triangular brittle fractures that fragment the salt surface and efficiently generate a breccia or gouge. Experiments with an abundance of gouge at the sliding interface have reduced shear resistance compared to bare surfaces and we show that friction is reduced with cumulative slip as gouge accumulates from initially bare surfaces. The relative importance of these deformation mechanisms may influence gouge production rate, fault surface roughness evolution, as well as mechanical behavior. Finally, our experimental results are linked to Nature by comparing the experimental surfaces to an actual fault surface, whose striated morphology has been characterized to centimeter resolution using a laser scanner. It is observed that both the stress field and the energy dissipation are heterogeneous at all scales during the maturation of the interface with cumulative slip. Importantly, we show that the formation of striations on fault planes by mechanical abrasion involves transport of gouge products in the fault plane not only along the slip direction, but also perpendicular to it. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Earthquake traces appear as more or less linear trends on the Earth’s surface, however a closer examination reveals that these ruptures in fact display a rich morphological complexity. Similarly, exhumed fault planes have been observed to show striations at all scales (Brown and Scholz, 1985), that can be described using a self- similar or self-affine (i.e. fractal) approximation (Power et al., 1987; Renard et al., 2006; Sagy et al., 2007; Bistacchi et al., 2011). Such striations have been used to characterize directions of slip either right after an earthquake or to characterize several slip events on the same fault (Jackson and McKenzie, 1999; Liu-Zeng et al., 2010). They represent kinematic indicators to infer large scale tectonic processes (Petit, 1987; Marrett and Allmendinger, 1990; Mercier et al., 1992; Yin et al., 1999; Zeilinger et al., 2000) and may be used to calculate paleostress tensors (Angelier,1979; Fry, 1999). They are also found in other geological systems associated with large scale friction, for example bedrock below an ice-sheet (Eyles and Boyce, 1998) or the imprints left by submarine landslides on the sea floor (Gee et al., 2005). The complex morphology of striated fault surface records a plethora of mechanical processes, such as abrasion, damage and crack interactions through branching, all at work during faulting and rupture propagation both at seismic and low velocity. Since fault roughness contributes to the heterogeneity observed in fault properties, this parameter has been proposed to account for the variability of earthquake source parameters (Venkataraman and Kanamori, 2004; Choy and Kirby, 2004; Schmittbuhl et al., 2006), * Corresponding author. ISTerre, University of Grenoble I, CNRS, BP 53, F-38041 Grenoble, France. E-mail address: francois.renard@ujf-grenoble.fr (F. Renard). Contents lists available at SciVerse ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg 0191-8141/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2012.03.009 Journal of Structural Geology 45 (2012) 101e112