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_Journal of Structural Geology 33 (2011) 255-270_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _Low-temperature deformation of limestone, Isola Palmaria, northern Apennine, Italy - The role of primary textures, precursory veins and intracrystalline deformation in localization_ _Giancarlo Molli a, Joseph Clancy White b,*, Lori Kennedy c, Veronica Taini_ _a Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 56126 Pisa, Italy_ _b Department of Geology, University of New Brunswick, 2 Bailey Drive, Fredericton, NB E3B 5A3, Canada_ _c Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada_ _article info_ _Article history: Received 1 January 2010; Received in revised form 9 November 2010; Accepted 23 November 2010; Available online 8 December 2010_ _Keywords: Calcite Localization Rheology Low-temperature deformation TEM Apennine orogen_ _abstract_ _Fine-grained limestones of the Tuscan nappe from Isola Palmaria (northern Apennine, Italy) contain sporadically developed bedding-parallel shear zones, some of which exhibit well-developed calcite mylonites. The presence of shear zones that formed at temperatures less than 250°C allows the study of limestone deformation mechanisms active under low-temperature, upper-crustal conditions. Two materially distinct types of shear zones developed during the syn-nappe stage of deformation and have been characterized by integrated field, strain, microstructural (optical microscopy, cathodoluminescence, transmission electron microscopy) and EBSD fabric studies. Type A shear zones occur within decametre-thick packages accommodating heterogeneously distributed strain in very fine-grained limestone (micrite). Type B shear zones are localized within calcite veins that develop into recrystallized calc-mylonites. Both types form at the contact with dolomite, although this does not play an important role in strain localization. In both cases, low-temperature ductility occurs by extensive dislocation glide sustained by athermal network-accommodated recovery. Although the end-state microstructures of both shear zone types cannot be differentiated optically, differences do exist in the crystallographic preferred orientations (CPO) and the grain boundary structure of deformed micrite and vein calcite. Deformed micrite grain boundaries are rich in voids consistent with extensive fluid flux that can enhance material transport and independent grain displacements, while suppressing CPO development. The crystallographically defined grain boundaries in recrystallized vein calcite suggest that lower grain boundary diffusivity enables more typical dislocation-mediated CPO formation._ _© 2011 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Ductile localization within carbonate rocks is central to tectonic detachment within a host of geological regimes (Schmid, 1975; Linzer et al., 1995; Lickorish et al., 1996; Kennedy and Logan, 1997; Teixell et al., 2000; Thomas et al., 2002; Ulrich et al., 2002; Iannace and Vitale, 2004; McQuarrie, 2004; Spratt et al., 2004; Herwegh and Püffner, 2005; Behrmann and Tanner, 2006; Ebert et al., 2007a; Vitale et al., 2007). Such localization is variously characterized as a consequence of parameters that include boundary conditions, temperature, pressure, differential stress, fluid activity, grain size and other material attributes. With_ _∗ Corresponding author. Tel.: +1 506 447 3187; fax: +1 506 453 5055. E-mail addresses: gmolli@dst.unipi.it (G. Molli), clancy@unb.ca (J.C. White), lKennedy@eos.ubc.ca (L. Kennedy)._ _0191-8141 $ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.11.015_ _the objective of further elucidating the deformation mechanisms of upper-crustal carbonates, shear zones have been examined on Isola Palmaria, in the northern Apennine of Italy (Fig. 1)._ _The study examines fine-grained carbonates and veins deformed at shallow crustal depths on the order of 7-9 km and temperatures of 200-250°C. Although the shear zones are parallel to bedding, the boundary conditions induced by this mechanical anisotropy are apparently not in themselves sufficient to induce localization, as demonstrated by those rock volumes in which shear localization does not form. The question of "why localization here?" becomes equally one of "why no localization there?" The small grain size of the protolith and low temperature of deformation, coupled with the presence of fluids during deformation portend an important role for deformation mechanisms such as fracture and pressure solution or other grain-size-sensitive (GSS) processes (Rutter, 1974; Schmid, 1976; Kennedy and White, 2002 and references therein). Nevertheless,_ _256_ _G. Molli et al. Journal of Structural Geology 33 (2011) 255-270_ _Fig. 1. Simplified geological map of the inner northern Apennine (see inset) with location of the study area on Isola Palmaria. Dashed lines are traces of cross-sections shown in Fig. 2. SP e La Spezia; MS e Massa._ _well-developed shear zone fabrics and calc-mylonites indicative of plastic deformation are observed. The latter suggest that dislocation glide and dynamic recrystallization also play an important role in the deformation. Demonstration of such intracrystalline deformation would confirm scattered observations from naturally deformed limestone (e.g., Burkhard, 1990; Teixell et al., 2000; Badertscher and Burkhard, 2000; Kennedy and White, 2002) and attest the common occurrence of low-temperature crystal plasticity in calcite._ _2. Geological setting_ _The shear zones in this study occur within Upper Triassic early Liassic carbonates (Portoro beds) on Isola Palmaria at the southern termination of the La Spezia promontory, Italy (Fig. 1) within the inverted limb of the La Spezia fold (Fig. 2), part of the Tuscan nappe, a continent-derived unit of the northern Apennine nappe stack (Elter, 1960, 1975). The Portoro limestone forms part of the lowermost 100-200 m of the Tuscan nappe strata and is overlain by_ _approximately 2000 m of younger units (Ciarapica and Passeri, 1980). Portoro is characterized by a fine-grained black matrix of calcite with alternating layers of mixed calcite and dolomite plus white carbonate veins, gold-coloured dolomite and limonite-rich stylolites. Two major pre-nappe dolomitization events that influence shear zone formation are recognized within the Portoro limestone (Miller, 1988; Miller and Folk, 1994)._ _Tuscan nappe units, including the Portoro limestone, were accreted at a shallow structural level in the northern Apennine orogenic wedge beginning in early Miocene (Federici and Raggi, 1976; Bernoulli, 2001; Cerrina Feroni et al., 2002; Molli and Vaselli, 2006). Burial occurred under a sequence of thrust sheets now represented in the overlying Canetolo and Ligurian units (Fig. 1). The shear zones of interest formed during thrusting as demonstrated by the asymmetry of shear foliations within different limbs of the fold (Gianmarino and Giglia, 1990; Taini, 2003). The nappe stack was subsequently modified by low-angle normal faults and related tectonic thinning (Carmignani et al., 1995; Storti, 1995;_ _Fig. 2. Composite cross-section (from sections A, B, C of Fig. 1) across the western promontory of La Spezia. Note location of Isola Palmaria study area in the inverted limb of the kilometre-scale La Spezia fold._ _G. Molli et al. Journal of Structural Geology 33 (2011) 255-270_ _257_ _Molli and Vaselli, 2006). The La Spezia backfold and overprinting upright folds and low-angle normal faults (Fig. 2) are related to later Miocene evolution of the northern Apennine wedge (Molli, 2008)._ _Metamorphism to anchizone grade during burial and deformation has been tectono-stratigraphically constrained to a maximum depth of 7 km for the youngest nappe unit, Macigno sandstones (Reutter et al., 1978). Temperature estimates extrapolated to the burial depth of the Portoro limestone are summarized in Fig. 3._ _3. Overview and methodology_ _Bedding-parallel shear zones in the Portoro beds were first described by Carter and Mosher (1987) and were subsequently studied by Carter (1990, 1992) and Carter and Dworkin (1990). In our study, two types of bedding-parallel shear zones are distinguished by their mesoscopic characteristics (Fig. 4). At the outcrop scale, both reflect some control by the mechanical anisotropy of contrasting micritic limestone and dolomitic layers. Nevertheless, these boundary conditions do not in themselves explain localized shearing in that outwardly equivalent exposures may or may not contain shear zones. Shear zone of both types have been examined with the aim of extracting the relationships among strain, microstructure, crystallographic texture and deformation mechanisms that contribute to localization._ _Type A shear zones develop in arrays that form strata-parallel packages several metres thick. Individual shear zones 2-15 cm thick develop within the protolith micrite and are recognized by distorted bedding-perpendicular veins. Non-coaxial deformation dominates with near simple shear conditions being estimated (Carter, 1992; this study). Variation in shear strain can be calculated at different distances from the shear zone center (e.g., Ramsay, 1967) as_ _g ? cotg$a ? cotg$a0_ _(1)_ _In Eq. (1), a and a0 are, respectively, the angles between the shear zone boundary and undeformed deformed veins, assuming the vein to be a passive strain marker. Bulk shear strain (g) estimated by assuming passive vein reorientation ranges from 0.1 to 10. These are necessarily minimum values given the internal deformation of the vein; nevertheless, they provide constraints on the deformation gradient that are useful for comparing different areas. Type B bedding-parallel shear zones occur between metre-thick massive dolomites, and are singularly associated with vein-derived white calc-mylonites 5-30 cm thick. Shear_ Ключевые слова: twin, image plane, carbonate, process, evolution, carbonate mylonites, dislocation creep, evans, vein material, creep, dynamic recrystallization, type, twinning, higher stress, geological, recrystallized grain, vein, band, experimental deformation, shear zone, anisotropy, montomoli, american, la, journal, vein calcite, eds, stress, tuscan nappe, limestone, rate, dislocation glide, science, carter, journal geophysical, microstructural evolution, elter, paterson, study, boundary, equant grain, mpa, spiers, undeformed material, grain size, molli, glide, relationship, scale, deformed, intracrystalline deformation, experimental study, journal structural, twin density, dynamic, geology, size, microstructures, logan, strain, micrite grain, stress estimate, international, ?uid, tectonics, white, twinned volume, distribution, dislocation network, pole gures, rutter, ter heege, natural deformation, type shear, deformation mechanism, host micrite, pattern, grain boundary, nite strain, schmid, elsevier, nature, paleostress estimate, law, herwegh, bresser, peierls, structural, cpo, spezia, calcite vein, zattin, boundary anisotropy, kennedy, microstructural feature, glide dislocation, shear, orientation, temperature, miller, plane, differential stress, mechanical anisotropy, smallest grain, hobbs, grain, portoro limestone, host, journal structural geology, northern, progressive misorientation, area, thesis university, eds mineral, structural geology, high, burkhard, slip, gss, apennines, low-temperature, isola palmaria, szb calc-mylonite, maximum, calcite, rock, strain area, type shear zone, consistent, tectonophysics, university, nappe, inverted limb, range, tullis, observed microstructures, szb, slip band, difference doe, glide plane, bulletin, ?ne-grained, uid, wenk, ebert, burgers vector, mancktelow, deformation band, molli journal, mechanism, ha, dislocation, geophysical, deformation, shear strain, undeformed, mechanical response, ?ow, affect conclusion, structure, grain anisotropy, additional, della, recrystallized, data, deformed micrite, strain rate, localization, exhibit, turner, material, sza, eq, stress exponent, northern apennine, applied, observed, mm, portoro, control, zone, undeformed micrite, doi, image, micrite, difference, estimate, italy, esbd pattern, walker, condition, regime, nature journal, crystal plane, recrystallization, form