Description:

_Journal of Structural Geology 32 (2010) 1030-1045_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Conjugate-shear folding: A model for the relationships between foliations, folds and shear zones Domingo G.A.M. Aerden a,*, Mohammad Sayab b, Mohamed L. Bouybaouen c a Instituto Andaluz de Ciencias de la Tierra (CSIC), Dpto. de Geodinámica, Universidad de Granada, C Fuentenueva s/n, 18002 Granada, Spain b National Centre of Excellence in Geology, University of Peshawar, Pakistan c Université Mohamed V, Dpt. des Sciences de la Terre, Rabat, Morocco Article history: Received 10 December 2009; Received in revised form 31 May 2010; Accepted 18 June 2010; Available online 26 June 2010 Keywords: Folding mechanism Crenulation cleavage Cleavage fanning Porphyroblast rotation Conjugate-shear zones Abstract Microstructural mapping of whole thin sections cut from two samples of micaschist containing cm-scale folds plus garnet porphyroblasts has provided new insight in the relationships between folding, shearing and foliation development. The garnets exhibit coherent inclusion-trail patterns that place important constraints on the kinematic development of both samples, which are shown to be representative of coaxial versus non-coaxial deformation in rocks containing a pre-existing schistosity. A comparison of crenulations-cleavages geometries in both samples and a review of the geometry of natural and experimental multilayer folds leads to the conclusion that folding involves conjugate shearing at different scales. At microscopic scales, crenulation cleavages nucleate as conjugate-kink or shear instabilities and develop further as a function of the macroscopic partitioning of deformation. In fold-hinge domains, bulk-coaxial deformation results in equal development of conjugate crenulations that progressively coalescence into symmetrical crenulation patterns so that, macroscopically, parallelism is achieved between foliation, fold-axial planes and long axes of strain ellipses. Fold-limb domains represent a system of conjugate-shear zones where single sets of crenulation instabilities with synthetic shearing component preferentially develop producing oblique relationships between the aforementioned elements. Cleavage fanning is inferred as a direct consequence of this conjugate-shear origin of folds. The model implies that crenulation cleavages and S-C fabrics in shear zones form by analogous processes, in both cases involving a component of shearing along foliation planes. The development of conjugate sets of foliation planes surrounding porphyroblasts during early, relatively coaxial stages of deformation explains continued “gyrostatic” behaviour during more advanced non-coaxial stages, as indicated by consistently oriented inclusion trails in the studied samples. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Tectonic foliations defined by preferred shape orientations of mineral grains are commonly classified as “continuous” foliations and contrasted with “spaced” foliations, which exhibit a distinct microstructural differentiation between discrete cleavage planes (septae) and microlithons (e.g., Passchier and Trouw, 2005). Spaced foliations are typical of polymineralic rocks and include the well-known morphological types of slaty-cleavage, crenulation cleavage and shear-band cleavage. A range of microstructural processes have been recognized as playing a role during cleavage differentiation, including micro-kinking or -folding, dissolution and reprecipitation, recrystallization, intra-crystalline deformation, mimetic * Corresponding author. Tel.: +34 958 242825; fax: +34 958 248527. E-mail address: aerden@ugr.es (D.G.A.M. Aerden). 0191-8141 $ e see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.06.010 growth and shear localization (see Passchier and Trouw, 2005 for a review). However, the precise relationships between these processes are not well understood as counts for the macroscopic relationships between foliations and strain patterns in folds and shear zones. A particularly long-standing question concerns the mechanical significance of spaced foliations, either as planes of dissolution and volume loss only, or as micro-shear zones (e.g., Siddans, 1972; Gray and Durney, 1979; Hobbs et al., 1982; Williams, 1990; Stewart, 1997). In this paper, we address these questions in the light of detailed microstructural mapping of whole thin sections that were cut from two samples of fine-grained micaschist containing cm-scale folds. The kinematic evolution of both samples is uniquely constrained by numerous garnet porphyroblasts, whose inclusion trails exhibit coherent orientation patterns exhibiting particular relationships with the host folds. Symmetrical versus asymmetrical relationships demonstrate bulk-coaxial versus bulk non-coaxial deformation in D.G.A.M. Aerden et al. Journal of Structural Geology 32 (2010) 1030-1045 the samples, respectively, which was in both cases superposed on pre-existing penetrative schistosity. A comparison of crenulation-cleavage patterns developed in both samples and a review of the geometry and kinematics of experimental or natural multilayer folds leads us to propose a new folding mechanism called “conjugate-shear folding”. The model reduces the difference between shear zones and folds to a matter of scale, and has major implications for the mechanical significance of spaced cleavages, the origin of cleavage fanning, porphyroblast rotation and the distinction between genuine and pseudo-S-C fabrics as discussed. 2. Sample -A- 2.1. General description The first studied sample (sample -A- further) is a fine-grained graphitic micaschist from the Rif mountains in North Morocco. It was collected from a coastal outcrop west of the town of Ceuta in the Palaeozoic Beni-Mzala unit of the Alpujarride-Sebtide complex (Betic-Rif orogen; Fig. 1a). Bouybaouene et al. (1995) determined peak metamorphic conditions of about 12 kbar and 500 x14C for this unit. The single thin section studied from this sample was cut perpendicular to cm-scale fold-and crenulation axes (Fig. 2a, b) and exhibits a principle anticline-syncline pair outlined by fine bedding laminae. It contains a total of 65 garnet porphyroblasts with diameters in the range of 0.75-1 mm and relatively even distribution across the fold pair, plus a dozen tiny chloritoid porphyroblasts (diameters < 0.5 mm) that cluster in a restricted area. All porphyroblasts contain straight to weakly crenulated inclusion trails composed of fine-grained graphite, opaque minerals and quartz (Fig. 3a, b). The trails are continuous with the principle matrix schistosity (S1), which appears strongly overprinted by a well-developed crenulation cleavage (S2) genetically related to the main folds. Two coarse-grained quartz-calcite veins sharply cut S2, do not show signs of internal deformation and therefore probably post-date folding. 2.2. Inclusion-trail data Accurate line tracings of all inclusion trails in sample -A- on high-magnification photomicrographs revealed their alignment along weakly undulating surfaces that follow the much tighter fold pattern in the matrix (Figs. 2b and 4). A similar geometric relationship was reported earlier by Visser and Mancktelow (1992) in a garnet-bearing, decimetre-scale fold in the Swiss Alps. These authors interpreted that the garnets predated folding but during folding experienced differential rotations in opposite fold-limb domains due to a combination of flexural-flow and homogeneous flattening. This model appeared to account for an alleged relationship between inclusion-trail orientation and porphyroblast ellipticity. However, Forde and Bell (1993) and Bell and Forde (1995) contested this relationship and proposed an alternative interpretation. They argued that the porphyroblasts nucleated early during folding and did not experience subsequent rotations relative to the axial plane of the fold (cf. Bell, 1985), so that the garnets would have effectively “fixed” an early fold-development stage. Which of these interpretations (if either) applies to sample -A-? A first point relevant to this question is that many of the inclusion trails in sample -A- are weakly crenulated, thereby demonstrating that porphyroblasts nucleated early during folding (Figs. 2b and 3a, b). Moreover, measurements for the axial planes of included crenulations (incipient S2) do not reveal a significant statistical difference between adjacent fold-limb domains, contrary to what is shown by the inclusion trails themselves (S1; Fig. 4aec). Although data are admittedly scarce in two of the fold-limb domains, these points tend to support the “fixed-fold” hypothesis. On the other hand, the relatively large spread of crenulation orientations in porphyroblasts is suggestive of differentially rotated porphyroblasts within single fold-limb domain. A possible explanation of this paradox is proposed later in this paper. 2.3. Cleavage fanning Accurate tracing of S2 cleavage septae (Fig. 2b) revealed that the changes in orientation of this foliation giving rise to cleavage fanning are not gradual, but produced discontinuously across a number of “unconformities” that separate different S2-dip domains. Indeed, two sub-sets of S2 septae can be distinguished dipping in opposite directions with respect to the axial plane of the folds and showing opposite offsets of intersected bedding laminae and S1 (Fig. 5). In principle, such offsets do not demonstrate shearing along foliation planes as they can, alternatively, be interpreted as geometric effects of dissolution and volume loss concentrated in crenulation-limb domains. However, in sample -A-, Ключевые слова: anastomosing, septae, axial, alternative interpretation, relative, johnson, developed, rigid object, fold-limb domain, press, sayab, crenulation cleavage, shear, early, scale, metamorphic, garnet porphyroblasts, matrix crenulations, sample-a-, siddans, original angle, aerden journal structural, wa, layer, alvarez fowler, multilayer, journal structural, folding mechanism, porphyroblast, vein, kinematic interpretation, shear zone, inclusion-trail, coloured blue, fabric, foliation, non-coaxial, progressive, long-limb domain, foliation development, sample, bell, progressive coalescence, vernon, respective fabric, rocks, single set, porphyroblast non-rotation, volume loss, parallel, case superposed, shear instability, selective dissolution, angle, fold, shearing, cleavage fanning, dextral, electronic supplement, case involving, -b-, geogaceta, mechanical signicance, model, development, conceptual model, pattern, mineral lineation, garnet, fanning, shearing component, individual microlithons, conjugate set, deformation, folding, plane, rotation, loss, preferred orientation, tectonophysics, bouybaouene, cleavage, -a-, relationship, s-c fabric, yeh, sample-b-, geometry, pre-existing schistosity, shear-band cleavage, origin, partitioning, axial plane, non-rotation, dissolution, conjugate, jung, sigmoidal geometry, interpretation, experimental folding, porphyroblast rotation, journal structural geology, journal, ax, carreras, multilayer fold, shortening, fold-limb, dextral displacement, aerden journal structural geology, original, bulk-shortening direction, inclusion trail, volume, sample -b-, data, trail, signicance, crenulation, stewart, spaced foliation, zone, shearing parallel, deformation partitioning, structural geology, area, aerden journal, evins, strain, mineral, williams, conjugate-shear folding, conjugate-shear, spaced, tectonics, structural, geological, single, metamorphic rock, strain ellipse, discussion, rock, timms, low-strain, hudleston, geology, non-coaxial deformation, hayward, inclusion-trail orientation, inclusion, set, cosgrove, lineation, variable width, oriented, cleavage plane, ne-grained micaschist, foliation plane, matrix, component, cm-scale fold, unit, quartz, dextral septae, symmetrical crenulations, original orientation, ramsay, aerden, cleavage refraction, nature, crenulations, passchier, converging sinistral, quartz vein, bulk, porphyroblasts, experimental, sample -a-, cleavage septae, vein segment, explanation, coaxial, domain, analysis, orientation, microstructural