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_Journal of Structural Geology 32 (2010) 1158-1169_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Micromechanics of brittle faulting and cataclastic flow in Tavel limestone Veronika Vajdova*, Wei Zhu, Tzu-Mo Natalie Chen, Teng-fong Wong Department of Geosciences, Stony Brook University, Stony Brook, NY 11794-2100, USA Article info Article history: Received 23 June 2009; Received in revised form 29 June 2010; Accepted 26 July 2010; Available online 3 August 2010 Keywords: Micromechanics Limestone Pore collapse Shear failure Microstructure Experimental Abstract Previous studies reveal that while compact carbonate rocks display exclusively dilatancy under compressive deformation, compaction may be observed in their more porous counterparts. In their compactive behavior the porous carbonate rocks are more akin to e.g., sandstone. Whereas the micromechanics of brittle faulting and cataclastic flow in sandstone has been studied extensively, little is known about these processes in a porous limestone. To investigate both failure modes we deformed samples of Tavel limestone with porosity 10-14% to various stages of deformation in conventional triaxial configuration at confining pressures corresponding to brittle faulting and cataclastic flow and described the microstructures associated with the damage evolution using optical and electron microscopy. In this porous micritic limestone, cataclasis is the dominant mechanism of deformation. The microcracks initiate as pore-emanated and while all pores contribute to microcrack initiation, it is the large pores that drive crack propagation and coalescence leading to failure. In brittle faulting, dilatancy arises from microcracks growing parallel to maximum principal stress, with their coalescence leading to shear localization. In cataclastic flow, microcracks do not have preferred orientation. Interplay between pore collapse and formation of new microcracks determines the compactive versus dilatant character of the cataclastic flow. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Brittle-ductile transition in rocks is associated with a broad spectrum of deformation mechanisms and failure modes that depend on extrinsic variables such as confining and pore pressures, temperature, strain rate, and pore fluid chemistry and intrinsic variables such as modal composition, grain size, and porosity (Paterson & Wong, 2005). In field settings this leads to complex deformation phenomena that cannot easily be reproduced in laboratory. Laboratory investigations under controlled conditions can provide useful insights into how some of these failure modes develop and what the underlying mechanisms are. Laboratory studies have underscored that the failure mode of a rock is intimately related to the porosity and its changes in response to an applied stress. On one hand, dilatancy is universally observed as a precursor to the inception of shear localization in the brittle faulting regime (Brace, 1978). On the other hand, plastic flow (associated with crystal plasticity and diffusive mass transfer) does not involve any volumetric change (Paterson & Wong, 2005). In the transitional regime of cataclastic flow (associated with), * Corresponding author: Currently at: NOV Downhole, 500 Conroe Park West, Conroe, TX 77303, USA. Fax: +1 631 632 8240. E-mail address: veronika@gmail.com (V. Vajdova). 0191-8141 $ e see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.07.007 homogeneously distributed microcracking), the scenario is more complicated since the pore space may dilate or compact in response to an applied stress field. The transition from brittle faulting to plastic flow in compact carbonate rocks has been studied in detail, partly because it can readily be investigated in the laboratory without the use of elevated temperatures. Limestones and marbles, even the ones with very low porosity like the Carrara marble, undergo the brittle to plastic transition at room temperature for confining pressures accessible in the laboratory (Heard, 1960; Rutter, 1974). This is possibly because calcite requires relatively low shear stresses to initiate mechanical twinning and dislocation slip even at room temperature (Turner et al., 1954; Griggs et al., 1960). In Carrara marble, Fredrich et al. (1989) observed appreciable dilatancy while the compact marble undergoes cataclastic flow. In contrast, inelastic compaction was observed in association with cataclastic flow in limestone and chalk with porosities ranging from 3 to 45% (Baud et al., 2000; Vajdova et al., 2004). Whereas the micromechanics of compaction and cataclastic flow in a porous siliciclastic rock such as sandstone have been studied extensively, not much is known about these processes in a porous limestone. The primary objective of this study is to characterize the damage evolution in deformed and failed samples of a porous micritic limestone, so as to gain insights into the micromechanics of brittle faulting and compactive cataclastic flow. V. Vajdova et al. Journal of Structural Geology 32 (2010) 1158-1169 1159 To investigate these two failure modes, two suites of triaxially compressed samples of Tavel limestone (with initial porosities in the range of 10-14%) were acquired at confining pressures of 30 and 150 MPa, respectively. To contrast with the failure under confinement, we conducted uniaxial compressive experiments on two samples. In addition we obtained a hydrostatically compacted sample for reference. In parallel with this study, we also conducted a similar investigation of two allochemical limestones of higher porosities, which we intend to report in another publication. Microstructural observations described in these two studies motivated the development of a micromechanical model that treats a rock as a dual porosity medium, containing micropores and macropores. This model has been presented by Zhu et al. (2010). Here we use their findings to analyze the role of pores in Tavel limestone during both failure modes, brittle faulting and cataclastic flow. 2. Experimental procedure 2.1. Sample material and preparation Tavel limestone is beige micritic limestone quarried in Tavel, France. Our blocks are considered to be similar to that studied previously by Vinck? et al. (1998). The composition is dominated by calcite but a small amount of quartz (<10%) may be present. Tavel limestone is relatively homogeneous, with a small number of sparry grains (w1-1% of rock volume) embedded in a microcrystalline matrix with average grain size of w5 mm. The sparry grains are usually elongate with an average equivalent diameter of w25 mm. The elongate axes of the sparry grains seem not to have any preferred orientations. In total we used ten deformed samples of Tavel limestone for microstructural observations. Two of these samples were previously deformed by Vajdova et al. (2004), and six additional samples from the same block were deformed for the present study. The initial porosity of these eight samples ranged between 9.5 and 11.4%, with an average value of 10.6%. We also deformed two samples (tu1 and th240) from a new block that has a higher porosity of w13.5% (see Table 1 for list of samples). The porosity was calculated from the density of a dried sample assuming a solid composition of 100% calcite. Cylindrical samples were cored perpendicular to the sedimentary bedding and ground to diameter of 18.4 mm and length of 38.1 mm. After it had been dried in vacuum at 80°C for 48 h, each sample was first jacketed in a thin copper foil (of thickness 0.05 mm), and then polyolefin (heat-shrinkable) tubings were used to separate the rock from the confining medium (kerosene). Except for the two uniaxial compression tests (in which the axial strain was monitored solely by an externally mounted displacement transducer), two TML electric resistance strain gages (type FLA 10-11) were attached in orthogonal directions to the copper jacket to measure axial and transverse strains. 2.2. Mechanical deformation The jacketed samples were deformed in the conventional triaxial configuration at room temperature using the same procedure as Vajdova et al. (2004). The triaxial compression experiments were performed at confining pressures of 30 and 150 MPa. Two samples were also deformed without any confinement. The confining pressure was measured with accuracy of 0.1 MPa, and during triaxial loading it was held constant to within 1%. The axial load was measured with an external load cell with an accuracy of 1 kN. To calculate the axial stress from the recorded axial load, the effect of bulging in a deformed sample was accounted for by evaluating the relative increase in cross-sectional area from the transverse strain. The displacement was measured outside the pressure vessel with a displacement transducer (DCDT) mounted between the moving piston and the fixed upper platen. With the knowledge of the stiffness of the loading frame (2.38 × 10^8 N m), the axial displacement of the sample was obtained by subtracting the displacement of the loading frame from the apparent displacement recorded by the DCDT. The axial displacement was servo-controlled at a fixed rate (corresponding to a nominal strain rate of 1.3 × 10^-5 s^-1). The load, displacement, and strain gage signals were acquired by a 14-bit A/D converter at a sampling rate of 0.5 s^-1 with resolutions of 0.3 MPa, 1 mm, and 10^-5, respectively. Uncertainty in strain was estimated to be 2 × 10^-4 (when calculated from the DCDT signal) and 10^-5 (when measured directly by the strain gages). The volumetric strain was calculated using the relation ev = ejj + 2et, where ejj and et are the axial and transverse strains, respectively. This formula neglects second-order contributions of strains to the volume change that may be appreciable at relatively large strain. While the transverse strains were generally small, the axial strains in some experiments exceeded 3%. 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