The Montsant anticlinorium is part of the Pàndols-Cavalls-Montsant tectonic line situated in the southwestern area of the Catalan Coastal Ranges, adjacent to the contact with the Tertiary Ebro Basin. We have interpreted this Alpine structure as a triangular type I zone with two opposite faults. The centre of the anticlinorium is formed by middle Muschelkalk facies with intensively deformed gypsum layers and an intensively deformed zone with an associated framework of satin spar gypsum veins. A field structural analysis reveals that there are two sets of veins: one associated with a pre-folding stage (before the Alpine orogeny), and another one related to the development of the Montsant anticlinorium, and therefore syn-folding (and Alpine in age)

Griera, A.

Gómez Rivas, Enrique

Martínez-Rodríguez, C.

Playà i Pous, Elisabet

English

Griera, Albert

Diposit Digital de la Universitat de Barcelona

IntroductionMost of the literature dealing with gyp-sum precipitates in fractures focuses on fi-brous gypsum veins (satin spar), andproposes that fibrous fillings are cementsformed in open fracture planes (El Tabakhet al., 1998; Moragas et al., 2013; amongothers). Generally, satin spar veins are ob-served in relatively undeformed evaporiteunits and sub-parallel to bedding. However,complex polygenetic gypsum vein networksare commonly observed in thrust belts whendetachment levels are localized withinevaporites. This contribution presents astructural analysis of a series of deformedgypsum layers and veins cropping out at thecore of the Montsant anticlinorium. Ourmain objective is to understand the relation-ship between the vein network and thelocal deformation during the Alpineorogeny. Geological settingThe study area is located in the south-western of the Catalan Coastal Ranges(CCR), close to its NW contact with the Ter-tiary Ebro Basin (Fig. 1A). The main struc-tures of the CCR are NE-SW-trendingpolygenic basement faults, which acted asnormal faults during the Mesozoic. Later on,they were reactivated as sinistral-reversefaults that produced the inversion of theMesozoic rifted basins during the compres-sive Alpine orogeny (e.g., Guimerà, 1984).These basement faults controlled the depo-sition of the Tertiary cover and produced thedevelopment of a narrow band of intensivedeformation (the Pàndols-Cavalls-Montsanttectonic line, PCM in Fig. 1B; Teixell, 1988)between two relative undeformed areas(the Ebro and Mora basins) (Fig. 1B). Thistectonic line consists of a NE-SW elongateanticlinorium (Fig. 1). Fission track data in-dicate uplift and exhumation of the areaduring the Upper Cretaceous (Juez-Larréand Andriessen, 2006), and coherent withCornudella group that predate main Alpinephase (Teixell, 1998). The study area is located at the centreof this anticlinorium, along the Montsantriver transverse, between the villages of LaVillella Baixa and Cabacès (Fig. 1B). In thisGEOGACETA, 60, 201615Copyright© 2016 Sociedad Geológica de España / www.geogaceta.comABSTRACTThe Montsant anticlinorium is part of the Pàndols-Cavalls-Montsanttectonic line situated in the southwestern area of the Catalan CoastalRanges, adjacent to the contact with the Tertiary Ebro Basin. We haveinterpreted this Alpine structure as a triangular type I zone with twoopposite faults. The centre of the anticlinorium is formed by middleMuschelkalk facies with intensively deformed gypsum layers and anintensively deformed zone with an associated framework of satin spargypsum veins. A field structural analysis reveals that there are two setsof veins: one associated with a pre-folding stage (before the Alpineorogeny), and another one related to the development of the Montsantanticlinorium, and therefore syn-folding (and Alpine in age). Key-words: gypsum rocks, gypsum veins, Middle Muschelkalk, Alpineorogeny, structural analysis.RESUMENEl anticlinorio del Montsant forma parte de la línea tectónica de Pándols-Cavalls-Montsant, situada al Suroeste de la Cordillera Costera Catalana, yadyacente al contacto con la Cuenca del Ebro. Esta estructura Alpina se inter-preta como una zona triangular de Tipo I con dos flancos opuestos. El centrodel anticlinorio está formado por facies del Muschelkalk medio con capas deyeso altamente deformadas formando una red de venas de yeso de tipo satinspar. Mediante un análisis estructural de campo hemos identificado dos con-juntos de venas: uno asociado a la etapa previa al plegamiento (antes de laorogenia Alpina) y el otro asociado a la formación del anticlionorio del Montsantdurante la orogenia Alpina.Palabras clave: rocas de yeso, venas de yeso, Muschelkalk medio, orogeniaAlpina, análisis estructural.Recepción: 1 de febrero de 2016Revisión: 20 de abril de 2016Aceptación: 20 de Mayo de 2016Geogaceta, 60 (2016), 15-18 ISSN (versión impresa): 0213-683XISSN (Internet): 2173-6545The evolution of a gypsum vein network in compressive tectonicsettings: the Montsant anticlinorium case (NE Spain)La evolución de una red de venas de yeso en contextos tectónicos compresivos:El caso del anticlinorio del Montsant (NE España) Cristina Martínez Rodríguez1, Elisabet Playà Pous1, Albert Griera Artigas2 and Enrique Gómez Rivas31 Departament de Geoquímica, Petrologia i Prospecció Geològica, Facultat de Geologia, Universitat de Barcelona, Martí i Franquès s/n, 08028, Barcelona, Spain.tina@vallat.net, eplaya@ub.edu2 Departament de Geologia, Universitat Autònoma de Barcelona, 08193, Cerdanyola del V., Spain. albert.griera@gmail.com3 Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen AB24 3UE, UK. e.gomez-rivas@abdn.ac.ukarea, different units with ages ranging be-tween the Paleozoic and Tertiary crop out.A series of folds and thrusts affect the sedi-mentary cover. This study focuses on MiddleMuschelkalk (M2) evaporites.MethodsA detailed structural analysis of gypsumvein sets was performed in order to identifywhether each gypsum vein filled with fi-brous cement formed pre- or syn-tectoni-cally with folding and thrusting. For thispurpose, data were collected at three diffe-rent localities: one at the middle of thehigh-strain zone (outcrops B1 and B2; Fig.2), and two located at the two limbs of theanticlinorium, which are low-deformed re-gions (outcrops A and C, at the western andeastern fold limbs, respectively; Fig. 2). Therelative angle between the vein wall and fi-bres was measured in the field using a pro-tractor. The extensional direction wasobtained using the convic method of Dur-ney and Ramsay (1973). This method as-sumes that the extensional direction lays atthe acute bisector between the vein normaland the fibres. In the high-strain bands theanalyses were restricted to a 2D sectionusing the software FraNEP (Fracture Net-work Evaluation Program; Zeeb et al.,2013). This code allows an automatic sta-tistical analysis of 2D vein networks, andconstructs rose diagrams of wall veins andgypsum fibre orientations.Results and discussionsMiddle Muschelkalk facies are com-posed of red siltstones interbedded with se-condary gypsum layers, which comprise twodifferent lithofacies (laminated-to-nodulargypsum and gypsum breccia). Secondarygypsum is formed by hydration of anhydrite.The M2 rocks host a dense network of veinsthat range from several dm to tens of me-ters in length and from less than 5 mm tomore than 10 cm in width. These veins areparallel-to-oblique to the evaporite layersand are also hosted in the red siltstone in-terlayers. Deformation appears strongly localizedin a narrow synform band (high strain zone,Fig. 2) where intense folding of the M2 be-tween the two thrusts with opposite vergencecan be identified (i.e., facing to the forelandand the hinterland). A main thrust facing to-wards the foreland is found at the SE flank ofthe anticlinorium. The map, cross section (Fig.2, outcrops B1 and B2) and field observationsindicate that this structure corresponds to avertical thrust with an associated hanging-wall anticline and a steeply dipping back-limbon the SE. The thrust facing towards the hin-terland is observed at the NW flank of the an-ticlinorium and dips subvertically. Based onthe cross section (Fig. 2), values of displace-ment range between approx. 250 m for theforeland thrust and less than 100 m for thebackthrust. The structure corresponds to a“pop-down” or a type I triangular zone accor-ding to Couzens and Wiltschko (1996). Boththrusts develop a damage zone formed bybreccias and reddish-brown gouge. A denseand complex network of gypsum veins, aswell as large pieces of brecciated and lami-nated gypsum, are observed at the core ofthis structure. The maximum vein density isfound at the high strain zone in the hinge ofthe synform, while gypsum veins sharply dis-appear near the damage zone associatedwith both faults (Fig. 3H).Infilling formed in the following sequence(in chronological order): a) fibrous gypsum(satin spar) cements growing perpendicularto vein walls, and, b) large prismatic (or sub-euhedral) crystals that can partially or almostentirely overprint the fibrous crystals andgrow from parallel to slightly oblique to thefibres. Although all described infillings canappear in all veins, veins hosted in the lami-nated-to-nodular and brecciated host unit (AGEOGACETA, 60, 2016 C. Martínez Rodríguez, E. Playà Pous, A. Griera Artigas and E. Gómez Rivas16 Geología Estructural y Tectónica / Structural Geology and Tectonics Fig. 1.- A) Simplified geological sketch of the Catalan Coastal Ranges in contact with the Ebro forelandbasin. The Pàndols-Cavalls-Montsant tectonic line is indicated with a grey rectangle and the studyarea with a star. Modified from Playà et al. (2010). B) Geologic map of the studied área (modifiedfrom Institut Cartogràfic i Geologic de Catalunya, 2006). See colour figure on the Web.Fig. 1- A) Esquema geológico simplificado de la Cordillera Costera Catalana en contacto con la cuencadel Ebro. La línea tectónica de Pàndols-Cavalls-Montasant está indicada con un rectángulo gris y lazona de estudio con una estrella negra. Modificado de Playà et al. (2010). B) Mapa geológico de lazona estudiada (modificado a partir de Institut Cartogràfic i Geològic de Catalunya, 2006). Ver figuraen color en la Web.Fig. 2.- Cross-section of the study area. Location of the studied outcrops marked in the cross-section.Outcrops B1 and B2 correspond to the narrow synform band, while A and C correspond to the antiformfold limbs. See orientation of the cross-section in figure 1. See colour figure on the Web.Fig. 2.- Corte geológico de la zona de estudio donde se indica las localizaciones de los afloramientos es-tudiados. Los afloramientos B1 y B2 corresponden a la banda de deformación del sinforme, mientras queA y C a los flancos del antiforme. Ver orientación de la sección en la figura 1. Ver figura en color en la Web.and C outcrops) are less deformed and recrys-tallized. Prismatic recrystallization is muchmore abundant in veins located at the middleof the high strain zone (outcrops B1 and B2),where veins are interbedded in red siltstones,and not related with secondary gypsum hosts.In the low deformed zones (outcrops Aand C), veins display straight fibres orientedeither normal or inclined (about ~30º) withregard to the walls. These veins usually appearat a low angle or parallel to bedding. Thereare also few veins showing sigmoidal orcurved crystals, in which fibres are oriented ata high angle with respect to the vein wallsand are subparallel to the walls in the centre.Veins of the NW flank of the anticlinorium(outcrop A) strike NE-SW and dip moderatelyto the NW, while fibres plunge approximatelyGEOGACETA, 60, 2016The evolution of a gypsum vein network in compressive tectonic settings: the Montsant anticlinorium case (NE Spain)17Geología Estructural y Tectónica / Structural Geology and Tectonics Fig. 3.- A)–C) Outcrop A: A) Field picture, B) Stereoplot of veins and fibres orientations and C) Stereoplot of the extensional directions. D)–F) Outcrop C: D)Field picture, E) Steroplot of veins and fibres orientations and F) Stereoplot of the extensional directions. G) Stereoplot of outcrops A and C after back-tilting the data. H) Field picture of outcrop B1 with rose diagrams of gypsum veins strikes in zones (I) and (J). K) Field picture of outcrop B2 with rose di-agrams of gypsum veins strikes (L), and calculated extensional directions (M) using the FraNEP code. See colour figure on the Web.extensión calculadas. D)–F) Afloramiento C: D) Fotografía de campo, E) Proyección estereográfica de las venas y fibras y F) de las direcciones de extensión.G) Proyección estereografía de las direcciones de extensión desplegadas a la horizontal de los afloramientos A y C. H) Fotografía de campo interpretadadel afloramiento B1, con los diagramas de rosa de los vientos mostrando las orientaciones de venas para los sectores (I) y (J). K) Fotografía de campo delafloramiento B2 con los diagramas de rosas de los vientos mostrando las orientaciones de las venas y las direcciones de extensión calculadas mediantedel programa FraNEP. Ver figura en color en la Web.140/36 on average (Fig. 3C). Veins of the SEflank (outcrop C, Fig. 3D) also strike NE-SW(Fig. 3E) but dip to the SE. The average fibresorientation of these veins is 300/28 (Fig. 3F).On both fold flanks fibres are oriented at ahigh angle with regard to the vein walls, thussuggesting that they formed from mode I ortensile fractures. The extension direction of theveins on the NW flank is 310/38 (Fig. 3C) andis 142/30 for veins on the SE flank (Fig. 3F).Two main trends of vein orientations arerecognizable in the high-strain domain (out-crops B1 and B2). The first set of veins formsat low angles with bedding, which is foldedand has very similar strike to those of large-scale folds (i.e., striking NW-SE and with east-dipping axial planes). The second set iscomposed of slightly folded veins with subver-tical orientation, and crosscuts folded siltstones,gypsum layers and the first set of gypsum veins(at a low angle with bedding). The maximumfrequency of the vein walls is found subvertical,and veins dominantly dip to the NW. In addition to these two sets of veins,another important population of SE-dippingveins appears in outcrop B2 (Fig. 3L), but ismissing in outcrop B1. These SE-dippingveins are folded in a similar way than thehost rocks, but some of these veins arecrosscutting other veins and folds. While weinterpret the first two set of veins (NW dip-ping veins found in both outcrops) as pre-or syntectonic with the main deformationphase, the additional SE-dipping set foundin outcrop B2 is clearly syntectonic. Fibreorientations at the outcrop B2 are very ho-mogeneous, and plunge on average to theSE. Pre- and syntectonic fibres have a verysimilar orientation, although pretectonicveins tend to plunge less than syntectonicones. Fibres of pretectonic veins are de-formed and their orientation is very unlikelyto indicate the extension direction. The cal-culated extension directions are very varia-ble (Fig. 3M), but have three maximafrequency peaks. The maximum frequency iscoincident with the fibre orientation and in-dicates the extensional direction duringNW-SE folding. The other two peaks, at highand low dip angles, correspond to a pretec-tonic extensional direction, which is folded. In order to test the chronology of theveins (before, during or after the fold) wehave backtilted the data from low strainzones back considering initial horizontallayers (Fig. 3G). The results reveal that ro-tated data have a scattered distributionaround the vertical axis, thus suggestingthat at least a large number of veins formedbefore folding. Therefore, the formation ofmost of these veins is probably related tothe rehydration of anhydrite layers duringthe Upper Cretaceous-Palaeogene exhuma-tion (Juez-Larré and Andriessen, 2006), andbefore the main tectonic Alpine stage.Finally, not all data in lower strain areas(outcrops A and C) are in agreement withthis interpretation, as veins with sigmoidalor complex fibre patterns plot away fromthe vertical axis. Although we do not haveenough data to interpret sigmoidal veins, aplausible hypothesis is that they are recor-ding layer-parallel slip during folding andlimb rotation during the Alpine orogeny. ConclusionsA framework of gypsum veins cuts thehost rock layers, composed of red siltstonesand deformed gypsum rocks. The relative chronology of the veins hasbeen established according to the localAlpine structures (folds). The veins found atlow angle to bedding predate folding andthus the main Alpine phase (i.e., they arepre-folding). These veins are observed in allthe outcrops. The formation of high-angleveins with respect to bedding and SE-dip-ping veins observed in outcrop B2 are inter-preted here to have developed synchronouswith the formation of the Montsant anticli-norium (i.e., they are syn-folding). Although gypsum veins are preferen-tially observed at the core of the structure,and therefore could be interpreted as indi-cators of fluid flow and overpressure relatedto compressive deformation, our structuralanalysis indicates that most of these veinsare probably related to the exhumation andrehydration of anhydrite before the maintectonic Alpine stage. Such type of structuralanalysis has not been previously used in fi-brous gypsum veins.AcknowledgementsThe present work was supported by theGrup Consolidat de Recerca Geologia Sedi-mentària 2009SGR-1451. We thank theeditor Carlos L. Liesa and two anonymousreviewers for their helpful suggestions thatcontributed to improve this manuscript.ReferencesCouzens, B.A. and Wiltschko, D.V. (1996).Bulletin of Canadian Petroleum Geology44, 165-179.Durney, D.W. and Ramsay, J.G. (1973). In:Gravity and Tectonics (K.A. De Jongand J. Sholten, Eds.). Wiley, New York,67-96.El Tabakh, M., Schreiber, B.C. and Warren,J.K. (1998). Journal of SedimentaryResearch 68, 88-89.Guimerà, J. (1984). Geological Magazine121, 413-420.Institut Cartogràfic i Geologic de Catalunya(2006). Mapa Geológic Comarcal1:50000, Full Priorat.Juez-Larré, J. and Andriessen, P.A.M. (2006).Earth and Planetary Science Letters 243,159-180. Moragas, M., Martínez, C., Baqués, V., Playà,E., Travé, A., Alías, G. and Cantarero, I.(2013). Geofluids 13, 180-193.Playà, E., Travé, A., Caja, M.A., Salas, R. andMartín-Martín, J.D. (2010). Geofluids 10,314-333.Teixell, A. (1988). Revista de la SociedadGeológica de España 1, 229-238. Zeeb, C., Gomez-Rivas, E., Bons, P.D., Virgo,S. and Blum, P. (2013). Computers &Geosciences 60,11-22.GEOGACETA, 60, 2016 C. Martínez Rodríguez, E. Playà Pous, A. Griera Artigas and E. Gómez Rivas18 Geología Estructural y Tectónica / Structural Geology and Tectonics 

The evolution of a gypsum vein network in compressive tectonic settings: the Montsant anticlinorium case (NE Spain)

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The evolution of a gypsum vein network in compressive tectonic settings: the Montsant anticlinorium case (NE Spain)

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