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_Journal of Structural Geology 32 (2010) 1250–1256_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Stylolites in limestone: Magnitude of contractional strain accommodated and scaling relationships Antonio Benedicto a,*,1 Richard A. Schultzb a Laboratoire de tectonique, CNRS INSU UMR-7072 Tectonique, Université Paris XI, 91405 Orsay, France b Department of Geological Sciences and Engineering, University of Nevada, Reno, NV 89557, USA Article info Article history: Received 23 November 2007 Received in revised form 14 August 2008 Accepted 9 April 2009 Available online 21 May 2009 Keywords: Stylolite Growth Scaling Faulting Limestones Pressure-solution Abstract The amount of contractional strain accommodated by a set of stylolites is analyzed by comparing the amplitudes of stylolitic teeth and spikes to independent measures of thinning of the enclosing layer. The scaling relations between along-strike trace length (L) and the maximum (Dmax) and average (Davg) amplitudes of stylolitic topography are also investigated. The studied stylolites occur in a 212-mm thick limestone layer dragged into the damage zone of the Gubbio normal fault zone in central Italy. Layer thinning was assessed independently from the layer geometry, with a maximum value of w23% nearest the fault. A total of 28 mm of thinning are related to 24 stylolites whose lengths range from 6.4 to 146 mm with average amplitudes from 0.1 to 1.3 mm. The average and maximum amplitudes of stylolitic topography increase with stylolite length, with Davg ? 0.0011L^0.17, r2 ? 0.57, implying propagation to greater lengths as contractional strains increase along them. The stylolites increase in number and amplitude into the most thinned area, correlating with increasing contractional strain accommodated by the layer. The average amplitude of stylolites visible in outcrop provides a measure of the minimum magnitude of contractional strain in the rock, although other mechanisms such as grain-scale dissolution appear necessary to account for the remaining layer thinning. © 2009 Published by Elsevier Ltd. 1. Introduction and background Stylolites are distinctive and pervasive structures that result from water-assisted pressure solution in rocks such as limestones and dolomites (Rutter, 1983; Passchier and Trouw, 1996). The orientations of stylolite surfaces and the associated topography (spikes and teeth) track the direction of the local stress state (e.g. Suppe, 1985; Petit and Mattauer, 1995), making them reliable paleostress indicators (Stel and De Ruig, 1989; Koehn et al., 2007) comparable to anticracks (Fletcher and Pollard, 1981) and compaction bands (Holcomb et al., 2007). Stylolites are thought to be analogous mechanically to anticracks (Fletcher and Pollard, 1981; Rispoli, 1981a,b; Tapp and Cook, 1988) that propagate to greater lengths (Tapp and Cook, 1988; Carrio-Schaffhausen et al., 1990) as contractional strains increase along them, similar to compaction bands in porous sandstones (Mollema and Antonellini, 1996; Sternlof et al., 2005; Katsman and Aharonov, 2006). Stylolites nucleate at local heterogeneities in the rock, such as grains of differing rates of dissolution, mica flakes, or other small-scale physical or chemical perturbations. When stylolites start to grow (called micro-stylolites), peaks develop in a regime in which there is a competition between the effect of strain energy that promotes the peak development and surface energy ("capillary" forces) that limits it (Renard et al., 2004; Brouste et al., 2007; Koehn et al., 2007). The result is that stylolites develop with relatively smaller amplitudes at small sizes (producing flat stylolites at outcrop scale) than at larger sizes. Once a stylolite's length L becomes sufficiently large relative to the average grain size l of the rock, however (i.e., a cross-over length L/l > 10–100; see Renard et al., 2004; Brouste et al., 2007; Koehn et al., 2007), then amplitude increases with contractional strain. As a result, stylolites record a variable minimum value of the actual magnitude of contractional strain in the rock and a scaling relationship between stylolites topography (amplitude) and length is expected. The minimum magnitude of contractional strain accommodated by a stylolite has been difficult to assess given the dependence of the amplitude of peaks on the properties and chemical heterogeneity of the surrounding rock (Brouste et al., 2007; Koehn et al., 2007). Effectively, stylolitization is a complex self-organized system in which the passive concentration of insoluble species progressively changes both the kinetics and the geometry of the stylolites with time. For example, the amplitudes of spikes and teeth associated with water-assisted diffusive mass loss across a stylolite increase with the number and density of less soluble phases (Renard and Dysthe, 2003; Koehn et al., 2007). But on the other hand, mica concentration can enhance pressure solution and passive concentration of insoluble minerals can progressively flatten the stylolites (J.P. Gratier, personal communication). Koehn et al. (2007) show how the roughening of stylolites passes from a regime of power-law growth through a regime where the roughness potentially saturates. Ultimately an increase of contractional strain can lead to a decrease of the rate of increase of the rate of increase of the peak amplitude of the stylolites. Taking into account the previous considerations, it could be expected that a population of sufficiently large stylolites (large in order to avoid the influence of "capillary" should give a minimum value of the accommodated strain. Here, we calculate this minimum value in such a population of stylolites. We analyse well-constrained fault-related stylolites within micritic limestones in order to: (1) quantify the magnitude of the contractional strain accommodated across the stylolites by comparing the amplitudes of stylolitic teeth and spikes to independent measures of layer thinning; and (2) investigate the scaling relations between along-strike trace length (L) and both maximum (Dmax) and average (Davg) amplitudes of stylolitic topography. Calculations of the minimum accommodated strain by natural stylolites, in addition to numerical modelling, are necessary to better understand the process of dissolution and mass lost under tectonic strain. 2. Study area and block The study area is located along the Gubbio normal fault in central Italy (Fig. 1a). This fault is 22 km long, with w3200 m of displacement near its midpoint, and is seismically active (Boschi et al., 1997; Boncio et al., 2000). The fault strikes NW–SE, dips to the SW, and juxtaposes Mesozoic marine limestones in the footwall against Plio-Quaternary lacustrine sediments in the hangingwall (e.g. Menichetti and Minelli, 1991; Collettini et al., 2003). The study block is located in the western cliff face of the Cava Filippi quarry (Fig. 1b), which traverses the entire fault zone near its northwestern tip. The structure of the fault zone in this quarry is described in Bussolotto et al. (2005, 2007). The block studied (Fig. 1c) is situated in the inner domain of the fault's damage zone, approximately two meters away from the main fault that separates the non-deformed footwall from the fault damage zone (Fig. 1c). This faulted domain involves fine-grained mudstone layers of the reddish marly calcareous Scaglia rossa Fm. (Coniacian-Upper Paleocene) which are here highly fractured and tilted, but still well recognizable. The block is part of a limestone layer that was thinned during normal faulting (Fig. 1d), comparable to the deformation sequence observed by Micarelli et al. (2005) along the La Remuque fault in southern France. Layer thinning was accompanied by stylolite growth in both cases, and was enhanced in the studied block by the punching effect of a piece of the subjacent layer that remained locked on a restraining bend of the fault during displacement of the hangingwall (Fig. 1d). Bending of the layer inducing stylolite growth was also accommodated by cracking and calcite vein growth. The kinematics suggests that layer thinning was greater near the fault (right-hand side of Fig. 1d) and decreased to a negligibly small value beyond w2 m away (left-hand side of Fig. 1d). 3. Approach The studied block of 20 ? 20 cm in size (Fig. 2a) was divided into 4 areas (A, B, C and D) of increasing block thinning (Fig. 2b). Area A corresponds to the undeformed part of the layer. It only contains the tips of two stylolites (S6, S24). Although the irregular morphology of the lower and upper contacts of the layer beyond Area A suggests pressure solution related to post-depositional compaction, the lack of stylolites within the same layer, farther away from the fault, indicates the initial layer thickness before faulting and stylolite growth (after compaction). We incorporate this uncertainty concerning layer thickness into the analysis below. Area B corresponds to the flat portion of the layer where stylolites start to appear. Areas C and D correspond to the thinned and rotated portion of the layer approaching the fault, separated by a calcite vein t' Ключевые слова: increase, molnar, amplitude stylolitic, geophysical, surface, rispoli, fault zone, stylolites increase, stylolite length, damage zone, study block, scale, max, gubbio stylolites, france, gubbio fault, verge, stylolites start, studied, schultz, wa, layer, journal structural, missenard, earth, plot, structure, contractional strain, cook, stylolites visible, limestone, fault, tapp, studied block, account, scaling exponent, layer thickness, approach, suppe, contractional, deformation feature, envelope, thickness, study area, scaling relation, development, dmax, minimum magnitude, thinned, stylolitic topography, block, independent measure, cook carrio-schaffhausen, rutter, collettini, shorter stylolites, uncertainty, deformation, stylolite growth, grain, heterogeneity, tectonophysics, relationship, grain scale, sternlof, accommodated, fracture, pollard, maximum, westaway, normal fault, olson, displacement, relation, calculated, dissolution, propagation, anticracks, topography, journal structural geology, journal, location, scaling relationship, rate, le, mm, calcite vein, average amplitude, study, stylolite, pressure solution, table, solution, sense, micarelli, thinning, compaction, zone, davg, structural geology, area, teeth, sciences, result, provide, strain, maximum dmax, length amplitude, passive concentration, peak, journal geophysical, boschi, boncio, tectonics, strain accommodated, structural, morphology, faulting, length, rock, magnitude, layer thinning, minimum, sedimentary, benedicto, shape, pro?les, stylolitic, geology, scholz, pressure, italy, band, total, host rock, increasing strain, kostrov, compaction band, koehn, magnitude contractional, growth, accommodated strain, measured, doi, condition, proles, average, kostrovs formula, limestone layer, consistent, stylolites, nature, stylolitic teeth, amplitude, host, layer area, theory, population, maximum amplitude, du, renard, gubbio, normal, scaling, anticracks fletcher, suggests, domain, dependence, average displacement, chemical heterogeneity, increasing