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_Journal of Structural Geology 32 (2010) 1279–1290_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Evolution of anastomosing crack–seal vein networks in limestones: Insight from an exhumed high-pressure cell, Jabal Shams, Oman Mountains Marc Holland*,1 Janos L. Urai Structural Geology Tectonics Geomechanics, RWTH Aachen University, Lochnerstrasse 4-20, D-52066 Aachen, Germany Article info Article history: Received 12 January 2008 Received in revised form 12 August 2008 Accepted 9 April 2009 Available online 5 May 2009 Keywords: Zebra carbonate Crack–seal Anastomosing Veins Abstract We studied a special type of zebra carbonate in limestones of an overpressure cell exhumed from at 5 km depth, in outcrops on Jabal Shams, Oman Mountains. The rocks show anastomosing patterns of regularly spaced calcite veins in dark gray fine-grained carbonate; microscopic observations reveal these as dense bundles of much finer veinlets, typically 10–50 mm thick. The vein bundles are up to 5 mm thick, they contain multiple sub-parallel arrays of host rock fragments embedded in the coarse-grained vein calcite. We interpret these structures as the result of numerous mechanically effective crack and reseal events together with strong growth competition or crystallization from sparse nucleation sites. Cementation produced mechanically strong veins so that new fractures were localized along the vein-rock interface or within the matrix itself. We present simple conceptual models relating the mechanical strength of the vein and the morphology of the resulting vein network. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction In this study we discuss the formation of brightly colored, thin calcite veins in a dark carbonate matrix. Such a macroscopic texture is commonly described by the term ‘‘zebra-rock’’, which can be produced by a number of different processes, such as primary sedimentation, metasomatosis as well as different means of fracturing (e.g., Badoux et al., 2001; Merino et al., 2006; Nielsen et al., 1998, 2000; Swennen et al., 2003; Vandeginste et al., 2005; Weller, 1989). In the context of these different possibilities, the study of the micro-structural properties not only reveals the processes responsible for forming the texture, but also allows deriving the genetic conditions. Accessing these parameters is important in order to decide whether the presented zebra texture has the potential to be used as indicative information. Over a range of scales, the texture of our samples is made up of an extremely dense network of anastomosing veins, which we interpret to have formed by crack–seal processes (Andreani et al., 2004; Boullier and Robert, 1992; Gaviglio, 1986; Hilgers and Urai, 2005; Lee and Wiltschko, 2000; Petit et al., 1999; Ramsay, 1980; Renard et al., 2005; Wiltschko and Morse, 2001). The exceptional high density of healed fractures, and the corresponding pattern, * Corresponding author. Fax: +49 241 8092358. E-mail address: mholland@geomi.com (M. Holland). URL: http://www.ged.rwth-aachen.de 1 Present address: GeoMechanics International Inc., Emmerich Josef-Str. 5, D-55116 Mainz, Germany. 0191-8141 $ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2009.04.011 raises questions on how this special form of zebra pattern was formed. We attempt to explain the genetic conditions and present simple conceptual models that discuss the role of cementation in restoring the rock strength. 2. Description 2.1 Geological framework The samples of this study were collected on the southern flank of Jabal Shams in the Oman Mountains. The Oman Mountains are part of the Alpine–Himalayan chain and show a complex multiphase deformation (e.g., Al-Wardi, 2006; Al-Wardi and Butler, 2006; El-Shazly et al., 2001; Glennie, 2005; Hilgers et al., 2006; Holland et al., 2009; Loosveld et al., 1996; Searle, 2007) of which details like the strain portioning, timing and tectonic framework are still unclear (e.g., Breton et al., 2004; Gray et al., 2005a,b; Gray and Miller, 2000; Searle, 2007; Searle et al., 2005; Warren and Miller, 2007; Warren et al., 2003; Wilson, 2000). The area exposes carbonates and mudstones that belong to the ‘Autochthonous’ Hajar Supergroup. The rocks of this group were deposited in a predominantly passive margin environment (Glennie, 2005; Hughes Clarke, 1988; Searle, 2007). The autochthonous group was subject to a multiphase deformation related to the convergence of the Arabian with the Eurasian plate. This first led to a flexure of the lithosphere causing local uplift of the Arabian plate margin (El-Shazly et al., 2001; Filbrandt et al., 2006; Loosveld et al., 1996; Patton and O’Conner, 1988; Searle, 2007; Warbuton et al., 1990). With ongoing convergence two large nappe units were emplaced onto the Arabian plate margin. The load of the Hawasina and the Semail Ophiolite nappes (Campanian and Maastrichtian) reversed the trend and buried the Hajar Supergoup to several kilometers depth (Breton et al., 2004; Glennie, 2005; Hilgers et al., 2006; Holland et al., 2009). The nappe emplacement stopped with the subduction of the buoyant lithosphere, leading to a time of tectonic quiescence. The large wavelength folds of the Oman Mountains and the exhumation of the rocks are the result of a more recent phase that started in the Tertiary, continuing until today caused by the ongoing convergence of the Eurasian and Arabian plates (Breton et al., 2004; Glennie, 2005; Kusky et al., 2005). The metamorphic gradient in the exposed rocks decreases towards the Southwest with Anchizone conditions in the area of Jabal Shams (Breton et al., 2004). This multiphase orogenesis caused the formation of several generations of fractures and faults that are all cemented with calcite and minor quartz (Hilgers et al., 2006; Holland et al. in press, 2009). Hilgers et al. (2006) studied veins in the Jabal Akhdar anticline, established overprinting relationships, and reported stable isotope analyses of the cements. These authors concluded that the first generations of vein-filled fractures were created in a rock-buffered environment with fluid pressures close to lithostatic. Holland et al. (in press, 2009) studied veins in the SW of the Jebel Akhdar anticline, near Oman’s highest peak Jabal Shams. They showed that these first generation veins can be further divided into at least four sets of bedding-perpendicular veins. These four sets show no signs of mechanical interaction; abutting of veins is absent and Holland et al. proposed that the fractures were consistently cemented prior to the formation of new ones. These first generation bedding-perpendicular veins were overprinted by two more generations of veins. First-by second-generation veins formed in bedding-parallel shear zones. Hilgers et al. (2006) interpret these to have also formed in a rock-buffered environment with pressures close to lithostatic conditions. The isotopic signature of the calcite cement changes in the third generation of veins, that are associated with dilatant normal faults (Hilgers et al., 2006). Their isotopic signature shows evidence of meteoric fluids, suggesting that the normal faults drained the system, which is described as a high-pressure cell (Al-Wardi, 2006; Hilgers et al., 2006; Holland et al., 2009). 2.2 Outcrops The zebra textures presented in this study are found in outcrops of the Kahmah and Wasia groups exposed near Jabal Shams in the Oman Mountains. The metamorphic conditions of the area correspond to the anchizone with the onset of pressure solution and the evolution of cleavage in phyllosilicate-rich lithologies (Breton et al., 2004). The zebra veins are found in carbonate mudstones and wackstones at different stratigraphic positions. These veins are oriented normal to the bedding, and are therefore interpreted to be part of the first generation veins. The zebra textures are occasionally overprinted by other veins of this group including fractures with shear components. Although cross-cutting relationships with the other veins are evident no abutting is present. Outcrops with zebra veins are not common, the pattern of their occurrence is not yet clear. We note however, that the seven outcrops we studied are also located within a few tens of meters from the later (third generation) normal fault zones (Holland et al., 2009) (Fig. 1). Apart from the spatial context no direct signs for interaction or interconnection of faults and zebra veins were observed. In some cases the thick (up to 10 cm) veins grade laterally into zebras as the thick veins bifurcate into the sub-parallel strands of the zebra texture. The zebra vein network may either fade into intact rock or form a transition zone towards another massive first generation vein (Figs. 2 and 3A, D). When the interconnected veins are not aligned, the zebra veins in between commonly show curved trajectories. 1280 M. Holland, J.L. Urai Journal of Structural Geology 32 (2010) 1279–1290 Fig. 1. Location of the outcrops of the zebra patterns on the southern slope of Jabal Shams. Interpreted faults and sample sites are superimposed on a geological map (Ws ? Wasia Grp; Kh ? Kahmah Grp; Lined sections are nappe units; Box indicates section interpreted, WGS-84 map datum). Changed after Beurrier et al. (1986). The patches of zebra veins are typically in the order of a few centimeters to several 10s of centimeters wide. The bands of bright veins in the pattern are sub-parallel or show a braided pattern causing an irregular macroscopic appearance (Figs. 2 and 3A, B). 2.3 Samples Samples with zebra textures were collected from the outcrops for micro-structural and geochemical analyses. The samples were cut perpendicular to the veins and polished. After drying, the specimens were stained with an Alizarin and potassium ferricyanide III solution to show iron content a'_ M. Holland, J.L. Urai Journal of Structural Geology 32 (2010) 1279–1290 Ключевые слова: interpreted, holland, complete healing, sub-parallel, process, property, sealing, journal structural, veinlet, wall, scale bar, network, oman mountains, henderson, ramsay, geology, zebra vein, cement crystal, zebra pattern, hawasina, nollet, apparent, parallel, previous veinlet, event, searle, bar, material property, conceptional model, wiltschko, matrix, structural, university, gray, trajectory, cementation, crack–seal, isolated veinlets, geo?uids, fractal dimension, image, mm, scale, study, pra, form, growth, competition, close, binary image, vein matrix, veinlets, mechanical strength, london, curve, breton, localized, clear, single, mechanical property, successive fracture, arabian, isotope, hawasina window, urai journal, lead, geological, long period, petroleum, strength, formed, crack, case, description, isotopic signature, formation, material, element, polycrystal growth, wardi, ratio, swennen, oman, zebra texture, geoarabia, generation, crack seal, crackseal, renard, hilgers, akhdar, kraus, set, structural geology, newly, bundle, touch, zebra, geologica, comment, gaviglio, model, observed, fracture wall, inclusion, normal, deformation, holland urai, interpret, healing, vein vein, condition, cox, sedimentology, large, cement, interface, crackseal mechanism, pattern, arrangement, journal structural geology, cross-cutting, anastomosing pattern, vein network, germany, jabal shams, dense, anastomosing, limestone, journal, solid inclusion, fracturing, weller, seal, merino, fault, growth competition, loosveld, crystal, cowie, und, wilson, dimension, mountains, location, carbonate, special, woodcock, studied, tc, mechanical, box-counting algorithm, veins cement, united, jabal, result, evidence, fracture, stained sample, margin, single fracture, vein, tectonophysics, rockbuffered, area, texture, miller, beurrier, wa, vein bundle, ongoing convergence, ?rst, grain, rock, individual, individual veinlets, mineral, glennie, bedding-perpendicular vein, boullier, sample, glennie hilgers, evolution, urai, successive, moritz, calcite, fragment, smaller vein, analysis, shams, structural evolution, calcite grain