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_Journal of Structural Geology 32 (2010) 1070-1084_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Stylolite interfaces and surrounding matrix material: Nature and role of heterogeneities in roughness and microstructural development Marcus Ebner a,*, Sandra Piazolo b, François Renard c,d, Daniel Koehna Tectonophysics, Institute of Geosciences, Johannes Gutenberg University, Becherweg 21, D-55128 Mainz, Germany b Department of Geology and Geochemistry, Stockholm University, 10691 Stockholm, Sweden c University Joseph Fourier & CNRS LGCA OSUG, BP 53, F-38041 Grenoble, France d Physics of Geological Processes, University of Oslo, Norway Article info Article history: Received 4 November 2009 Received in revised form 20 May 2010 Accepted 22 June 2010 Available online 30 June 2010 Keywords: Stylolite Pressure solution Heterogeneities Roughness EBSD Abstract Rough pressure solution interfaces, like stylolites, are one of the most evident features of localized slow deformation in rocks of the upper crust. There is a general consensus that the development of these rough structures is a result of localized, stress-enhanced dissolution of material along a fluid-filled interface, but little is known on the initiation of this roughness. The aim of this article is to reveal the role of heterogeneities initially present in the host-rock on roughness initiation. This should give insights on whether stylolite roughness is generated by a stress-induced instability or by the presence of disorder in the material (i.e., quenched noise). We use a microstructural approach based on SEM-EBSD analysis combined with orientation contrast (OC) image analysis of stylolites in limestones. We found that the stylolite roughness is induced by heterogeneities in the host rock (clay particles and detrital quartz grains in our case). In addition, close to mature stylolite interfaces matrix modifications occur, which can be attributed to compaction along the stylolite. The grain size decreases by 15-25% and a pre-existing shape-and lattice-preferred orientation (SPO, LPO) are significantly modified in the vicinity of the stylolite. The results presented here imply that localized pressure solution along stylolites is not necessarily restricted to the actual interface but influences the adjacent matrix. The heterogeneity data might serve as a quantitative basis for elaborate numerical models of localized compaction. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Stylolites are localized pressure solution patterns characterised by a multi-scale roughness that spans several orders of magnitude ranging from serrated grain contacts at the micron-scale up to decimetre-scale roughness amplitudes. The intriguing morphological characteristics of pressure solution surfaces have been the main focus of many qualitative studies (Alvarez et al., 1978; Bathurst, 1987; Bayly, 1986; Buxton and Sibley, 1981; Dunnington, 1954; Guzzetta, 1984; Heald, 1955; Park and Schot, 1968; Stockdale, 1922), which in turn lead to more quantitative approaches (Andrews and Railsback, 1997; Railsback, 1993). In the last decade many studies employed statistical tools and characterised the stylolite roughness by its fractal geometry showing that stylolites exhibit a fractal scaling over several orders of magnitude. * Corresponding author. Current address: Geological Survey of Austria, Neulinggasse 38, A-1030 Vienna, Austria. Tel.: +43 1 712 56 74 414; fax: +43 1 712 56 74 56. E-mail address: marcus.ebner@geologie.ac.at (M. Ebner). 0191-8141 $ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.06.014 (Drummond and Sexton, 1998; Karcz and Scholz, 2003). Some investigations of the 3D morphology of natural stylolites in limestones (Brouste et al., 2007; Ebner et al., 2009b; Renard et al., 2004; Schmittbuhl et al., 2004), experimentally generated microstylolites (Gratier et al., 2005) and numerical simulations (Ebner et al., 2009a; Koehn et al., 2007) demonstrated that the stylolite roughness has two self-affine scaling invariances, which are separated at a characteristic crossover-length of a millimetre scale. These scaling regimes have distinct Hurst or roughness exponents of 1.1 and 0.5 for small and large scales, respectively. Quasi-universal scaling exponents were reported for bedding-parallel stylolites from various different locations and with various lithological compositions (Brouste et al., 2007; Ebner et al., 2009b; Renard et al., 2004; Schmittbuhl et al., 2004). Railsback (1993) and Andrews and Railsback (1997) showed that the morphological appearance and/or abundance of stylolites in carbonate rocks changes with the rock-type of the host rock. In addition, the spatial density and intensity of dissolution seams were reported to correlate with the clay content in limestones and marls (Alvarez et al., 1978; Marshak and Engelder, 1985). Nevertheless the M. Ebner et al. Journal of Structural Geology 32 (2010) 1070-1084 1071 cause for the roughness formation of both bedding-parallel and tectonic (i.e., bedding perpendicular) stylolites remains unclear. Based on field observations, microstructural investigations, numerical and analytical considerations two main concepts for the roughening of stylolites prevail. In the first (termed the instability concept here), a stress-induced roughening instability develops along an initially flat solid-solid interface (Angheluta et al., 2008) or solid-liquid-solid interface (Bonnetier et al., 2009). In the second (Ebner et al., 2009a; Koehn et al., 2007; Renard et al., 2004; Schmittbuhl et al., 2004), heterogeneities act as quenched noise and are responsible for the roughening of the interface (and therefore termed the heterogeneity concept thereafter). In contrast to the instability concept, the heterogeneity concept produces a rough surface but no instability as such. The heterogeneity in the system may be present as “pinning” particles that dissolve slower (Koehn et al., 2007) and “pin” the interface, which results in a roughness. Surface energy counteracts the effects of these pinning particles as a stabilizing term keeping the stylolite flat on small length-scales. In contrast to the instability concept, the stress is a stabilizing term (Schmittbuhl et al., 2004) in the heterogeneity concept and flattens the interface on large length-scales. It is therefore important to note that both concepts differ fundamentally, since in the instability concept the applied external stress causes the roughening and produces an instability, whereas in the second concept the heterogeneities in the material represent the key ingredient for roughening, and the stress drives the dissolution but produces no instability. Models using heterogeneities as roughness origin have successfully generated synthetic surfaces in numerical simulations and analytical considerations that resemble the scaling features of natural stylolites (Brouste et al., 2007; Ebner et al., 2009a; Gratier et al., 2005; Koehn et al., 2007; Renard et al., 2004; Schmittbuhl et al., 2004). Although the importance of material disorder and composition on the stylolite morphology has been stressed in previous contributions investigating the rock hosting the stylolite (Andrews and Railsback, 1997; Railsback, 1993), so far no study quantitatively characterised the composition of the quenched disorder that initiates the distinct roughness of stylolite interfaces. This is indeed a difficult task in the field (Fig. 1) since pinning particles only rarely consist of macroscopically distinguishable rock fragments (Fig. 1b) or bioclasts, and thus do not register in the macroscopic roughness scaling. In previous microstructural studies, the matrix around stylolites was mainly investigated in terms of porosity reduction and its influences on fluid flow in sandstones (Baron and Parnell, 2007; Harris, 2006; Märt and Moen, 2007) and limestones (Carrio-Schaffhauser et al., 1990; Raynaud and Carrio-Schaffhauser, 1992). There is a general agreement among previous workers that pressure solution along stylolites provides a local source for cement around them. Low porosity haloes of up to several cm in width are reported (Baron and Parnell, 2007; Harris, 2006), but close examination under the SEM demonstrated that the porosity in the vicinity of the stylolite increases (within a few microns). This was termed the ‘process zone’ by Carrio-Schaffhauser et al. (1990), which is surrounded by a low porosity zone. Microstructural investigations of experimentally compacted sandstones (van Noort et al., 2008) and drilled sandstone samples from the North Sea (Märt and Moen, 2007), demonstrated that plastic deformation occurs around intergranular and localized pressure solution features. These authors report the occurrence of micro-cracks and Dauphiné twins from grain-to-grain contacts and show that Dauphiné twins are localized around stylolites. This demonstrates that localized pressure solution features concentrate stresses. Therefore they have to be carefully investigated to understand the dynamic evolution of stylolite formation from an initial interface with an undisturbed matrix to a mature interface, with a modified host-rock matrix, which will in turn modify the pressure solution process as such. Based on SEM and EBSD investigations we report direct evidence that multi-scale heterogeneities in limestones are an agent for the formation of stylolite roughness. In addition we describe matrix microstructures around mature stylolite interfaces and discuss their significance for localized pressure solution. 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