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_Journal of Structural Geology 32 (2010) 1912-1922_ _Available online at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _The role of dilation and cementation in the formation of cataclasite in low temperature deformation of well-cemented quartz-rich rocks_ _Charles M. Onasch a,*, John R. Farver a, William M. Dunneb a Department of Geology, Bowling Green State University, Bowling Green, OH 43403, USA b Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA_ _Article history: Received 5 October 2009; Received in revised form 26 March 2010; Accepted 26 April 2010; Available online 5 May 2010_ _Keywords: Cataclasite Quartz Cementation Dilation Fluids Faults_ _Abstract_ _An important textural component of cataclasites in quartz-rich rocks deformed at low temperatures is a fine-grained (<20 µm) matrix composed of anhedral quartz. As cataclasites form during shearing, this component would typically be interpreted to result from extreme grain size reduction through comminution. Tabular bands of fine-grained quartz that texturally resemble cataclasite found in faulted quartz arenites and other quartz-rich, well-cemented sandstones have characteristics incompatible with a shearing origin. These include a lack of shear displacement and wall rock fragments, gradational contacts with the wall rock, and a fine-grained quartz fill that has different cathodoluminescence, water content, oxygen isotope chemistry, and in some cases, mineralogy than the wall rocks. Rather than being the result of cataclasis, the fine-grained quartz in these tabular bands was precipitated in a dilating fracture. In essence, these structures are veins._ _In fault zones where cataclasite is well developed and shearing is unequivocal, the fine-grained quartz matrix representing up to 50% of the fault rock volume has characteristics similar to those in the tabular bands. We interpret these volumes to be in many cases a result of cement precipitation rather than as a product of comminution. If correct, then the brittle deformation of well-cemented, quartz-rich rocks deformed at low temperatures involves much more dilation and cementation than previously recognized._ _© 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Cataclasite is a cohesive fault rock consisting of variable proportions of matrix and rock mineral clasts with a random fabric that is intermediate in clast-matrix ratio to a breccia and fault gouge (Sibson, 1977). The formation of cataclasite through cataclasis occurs by microfracturing, grain sliding, and rotation associated with faulting or shearing (Engelder, 1974; Blenkinsop, 2000). Grain size reduction through comminution is an integral part of cataclasis (Lloyd and Knipe, 1992; Knipe and Lloyd, 1994) while cementation may or may not be important (Blenkinsop and Rutter, 1986; Power and Tullis, 1989; Knipe, 1991)._ _The processes involved in the formation of cataclasite have been identified in a number of fault zones (e.g., Blenkinsop and Rutter, 1986; Knipe, 1991; Wu and Groshong, 1991; Lloyd and Knipe, 1992; Knipe and Lloyd, 1994). Typically, the process starts with the formation of extensional microfractures, which grow and link to form through-going shear zones. As deformation accumulates, continued fracturing progressively reduces both grain size and clast matrix ratio to produce protocataclasite, cataclasite, and ultracataclasite. In these studies, the fine grains in cataclasites are interpreted to be a product of comminution. Once created by shearing, these fine grains may remain in situ or transported elsewhere under high fluid pressures via fluidization (Ujiie et al., 2007)._ _Although brittle processes are considered to dominate in formation of cataclasite, crystal-plastic deformation may also play an important role. At lower temperatures, strain hardening through cold working can lead to brittle failure and voids created by dislocation motion provide nucleation sites for fractures (Stel, 1981; Lloyd and Knipe, 1992; Knipe and Lloyd, 1994). The low dislocation densities found in the microcrystalline matrix of some cataclasites may be due to recovery of highly strained grain fragments (Knipe and White, 1979; Knipe, 1991; Graves, 1992)._ _Evidence for diffusive mass transport is common in cataclasites (Knipe, 1991; Lloyd and Knipe, 1992; Knipe and Lloyd, 1994). The very fine grain size (<10 µm) and likely presence of aqueous fluids promote dissolution and precipitation (Fein, 2000). Precipitation of fine-grained quartz during or after faulting may also play a role by cementing cataclastic grain fragments (Power and Tullis, 1989; Blenkinsop and Rutter, 1986; Blenkinsop, 2000) or by filling “vein-like” structures (Stel, 1981)._ _C.M. Onasch et al. Journal of Structural Geology 32 (2010) 1912-1922_ _1913_ _In well-cemented, quartz-rich sandstones, tabular bands up to a few cm thick filled with microcrystalline quartz are common and have been interpreted as small faults with cataclasite (Wu and Groshong, 1991; Onasch and Dunne, 1993; O’Kane et al., 2007; Onasch et al., 2009). While these bands contain fine-grained material that resembles the fine-grained matrix in cataclasite, a number of characteristics in some bands are inconsistent with a shear-only origin. We propose that these bands result from dilation and will use several lines of evidence to demonstrate that microcrystalline quartz fill did not originate solely by comminution of the wall rock but was precipitated from an externally derived fluid. Given this proposition, we will also examine the characteristics of some fault zones with unequivocal evidence for shearing to assess whether this dilation behavior is present in such cases._ _2. Nature and occurrence of microcrystalline quartz bands_ _Samples described in this study come from Paleozoic rocks in the central Appalachian Alleghanian foreland fold and thrust belt and San Juan dome of southwest Colorado (Fig. 1). Samples from the Appalachians include the Middle Ordovician Martinsburg Formation, Upper Ordovician Bald Eagle Sandstone, Lower Silurian Tuscarora Sandstone and Rose Hill Shale, Lower Devonian Oriskany Sandstone, Middle Devonian Mahantango Formation, and Lower Mississippian Pocono Sandstone (Fig. 1a). Samples from the San Juan dome are from the Devonian McCracken Sandstone Member of the Elbert Formation (Fig. 1b)._ _Samples from the Tuscarora, Oriskany, and McCracken Sandstones are quartz arenites composed of >95% quartz detrital framework grains with quartz overgrowth cement. Most are well-sorted with grain sizes typically 100-400 µm. Intragranular porosity ranges from <2% in some Tuscarora Sandstone samples to as much as 15% in the Oriskany Sandstone. The Pocono Sandstone samples range from a coarse-grained (w400 µm grain size) quartz arenite to lithic arenite or wacke. Samples from the Martinsburg Formation, Bald Eagle Sandstone, and Mahantango Formation are fine to medium-grained (w50-250 µm grain size) lithic wackes. The Rose Hill Shale sample is a hematite-cemented quartz arenite. In all samples, illite is the dominant matrix mineral._ _In each of the sandstone samples, microcrystalline quartz bands are visible in outcrop or hand sample as light-colored seams (Fig. 2a) that range in thickness from 30 µm to a few cm and in length from 100 µm to several meters (Fig. 3). Geometrically, they vary from planar to curviplanar to highly irregular and exist individually or as multiple, subparallel, or anastomosing bands. They are most common in areas of more intense deformation, such as fault zones (Fig. 3a and c), fold hinges, and overturned fold limbs, but they are also found in subhorizontal strata, away from any significant larger structures (Fig. 3d). Their spatial association with faults can be seen by a decrease in number and thickness away from the fault (compare Fig. 3a and b). Preferred orientations or predictable geometric relationships with larger structures, such as parallel to faults (Fig. 3a), may or may not be present._ _As seen in thin section (Fig. 2b and c), the bands are filled with anhedral quartz with a grain size of 5-20 µm (Fig. 2c) regardless of the composition or grain size of the wall rock. X-Ray diffraction analysis of the band fill shows that it is α-quartz and not a less ordered polymorph, such as opal CT. As seen in TEM, individual quartz grains in the fill have low dislocation densities (Fig. 2d), which is in contrast to the high dislocation densities of wall rock grains. The contacts between the bands and wall rock as seen in polarized light are generally irregular and gradational. The similar optic orientation of grains at the margins of the band with adjacent wall rock grains indicates that they are syntaxial overgrowths (Fig. 2e). Many bands are zoned with finer-grained margins grading to a coarse-grained, sometimes vuggy core with euhedrally terminated quartz crystals up to 1 mm long (Figs. 2f and 4a). In addition to the grain size variation related to the vuggy cores, the fill can have a distinct banding defined by color and texture variations (Fig. 4b)._ _In cathodoluminescence (CL), the band fill is uniformly dark in comparison to the bright_ Ключевые слова: porous sandstone, contact, cementation, precipitation, aqueous uid, fault, gel, density, paleozoic rock, ne-grained quartz, nakashima, mcbride, dilating fracture, quartz-rich, drever, surface, detrital grain, lankreyer, cemented, shear, dilation, ha, solubility, si, condition, cataclasite, gel precursor, earth, fragment, host rock, ?ll, journal structural, volume, aqueous, temperature, evidence, laubach, cataclasites, dilation band, vearncombe, bedding, grain size, polarized light, journal, shear displacement, large number, onasch journal, sandstone, drop, nature, eichhubl, wolf, growth, quartz, wall, single volume, amorphous silica, process, wall rock, rock grain, water, grain, note, geological, lloyd, syntaxial overgrowth, tectonophysics, microcrystalline quartz, pressure, shear origin, vug, origin, cement, quartz precipitated, microcrystalline quartz band, international association, inclusion, onasch, knipe knipe, electrolyte concentration, wall-parallel, journal geophysical, microstructural analysis, composition, concentration, band, graves, scholz, cemented sandstone, feature, band arrow, case, formation, vein, ?ne, gradational, evans, fossen, fault zone, mahantango formation, silica, positive dilation, structural, deformation band, blenkinsop, john, dark luminescence, anhedral quartz, dunne okane, deformation, highly irregular, marshall, kronenberg, geology, dislocation density, vernon, tuscarora sandstone, mineral, light, thicknessedisplacement relationship, precipitated, mm, oxygen, sibson, thrust, quartz solubility, texture, london sibson, elsevier, water concentration, dilated fracture, shearing, fournier, structural geology, crystal, blum, zone, wa, microcrystalline, quartz band, oriskany sandstone, ?uid, size, sample, hydrolytic weakening, burnham, society, tuscarora, iler, woodcock, structure, silica gel, water content, difference, hull, quartz cement, american, ?ne-grained, cataclastic fault, bedding surface, rf, wall-normal displacement, chemistry, vuggy core, relationship, grained, kita, euhedral crystal, stel, reviews, matrix, geological society, mccracken sandstone, quartz fournier, displacement, university, stress, high, fyfe, fein, dislocation, knipe, characteristic, fracture, amorphous, journal structural geology, lander, geophysical, engelder, number, moore, luminescence, comminution, rock, ?ne-grained quartz, uid, oehler, tullis, rapid precipitation