ABSTRACT

The Cotiella Upper Cretaceous Basins in the central southernPyrenees host an outstandingly exposed extensional fault system andrelated folds which developed by the extensional gravitational col-lapse of the post-rift carbonate platform above relic Upper Triassicsalt at the southern margin of the Bay of Biscay-Pyrenean rift system.Among the Cotiella extensional structures, the Armeña rolloverinvolves more than 3 km of middle Coniacian to lower Santonianwedge-shaped syn-kinematic sediments which are overlying upperAlbian to lower Coniacian pre-kinematic strata detached above anUpper Triassic salt layer. Although the rollover was partially invertedduring the Pyrenean orogeny, its first order extensional architectureand the associated syn-extensional fracture pattern are well preserved.

Both the hanging-wall and foot-wall of the NE-dipping Armeñafault exposes two sets of bed-perpendicular joints which orientedabout parallel (longitudinal) and perpendicular (transverse) to thefault, respectively. Spatial distribution and reactivation pattern oflongitudinal joints indicate their early to syn-folding origin, wherewidespread longitudinal jointing occurred firstly as part of theregional extensional framework, affecting the entire rock volumewith a rather constant density. Later, laterally decreasing fault-propagation rate caused the development of a hanging-wall syncline,preferentially developed at the lateral termination of the rollover.There, folding of strata above the slowly propagating fault’s tip wasensured by development of further longitudinal joints that underwentshearing during the later stage of fault-propagation.

KEY WORDS: Extensional rollover, fractures, fault-propagationfolding, Pyrenees.

INTRODUCTION

Rollover anticlines commonly occur in extensionalframeworks involving salt detachments (e.g. DAVISON,1986; WHITE et alii, 1986; WILLIAMS & VANN, 1987). Theyconsist of folded pre-kinematic layers overlain by syn-kinematic strata thickening towards the master fault.Such a geometry can develop by fault-bend and fault-propagation-folding above listric extensional faults splay-ing from the salt detachment (e.g. HAMBLING, 1965;GIBBS, 1983; ELLIS & MCCLAY, 1988; MCCLAY, 1990;DULA, 1991; XIAO & SUPPE, 1992; COSGROVE & AMEEN,1999), with the fault and fold geometries eventually beingprogressively modified by salt mobilization (e.g. SOTO et

alii, 2007; BRUN & MAUDIT, 2008; HUDEC & JACKSON,2011). Among others, two key factors impose the fault-fold relationships of the rollover: (1) the deformationmechanism ensuring folding of pre- and syn-kinematiclayers and (2) the propagation history of the fault. Con-cerning the first factor, two end-member deformingmechanisms are commonly invoked to model folding oflayers during hanging-wall downthrow: flexural-folding(e.g. DONATH & PARKER, 1964; DAVISON, 1986) andoblique/vertical shearing (e.g. GIBBS, 1983; WHITE et alii,1986). In the first case, the bedding surface represents astrong mechanical discontinuity and folding is favored bybed-parallel slip, with negligible internal deformation oflayers. In the second case, which is by far the most com-monly invoked in extensional frameworks, the beddingsurface does not represent an important mechanicaldiscontinuity. Indeed, layers’ bending is accommodatedby shearing along directions oriented oblique or at highangles to bedding, which implies creation of newanisotropies and, thus, fracturing during folding of strata.Recognizing the operating deformation mechanism andthe shear direction is thus of primary importance to con-strain structural restorations (e.g. DULA, 1991; WHITE,1992; WITHJACK & PETERSON, 1993; YAMADA & MCCLAY,2002). Equally, the fold geometry is severely controlled bythe fault’s propagation history, and the upward propaga-tion of the fault’s tip, in both pre- and syn-kinematicstrata, strongly impacts the geometry of the rollover.Fault-propagation, in fact, causes the development of asynthetic monocline (i.e. a monocline dipping in the samedirection as the fault) in the unfaulted layers overlyingthe upward propagating fault’s tip (e.g. FERRILL et alii,2012), and consequently the development of a hanging-wall syncline. The slower the fault propagation rate, thelarger the synthetic monocline is (fig. 1). Fault-propaga-tion folding requires further internal deformation withinthe monocline (e.g. HARDY & MCCLAY, 1999), whichaccordingly is expected to hold a diagnostic fracture pat-tern, witnessing for such a fault-propagation stage, evenwhen the monocline is breached and folded.

In agreement with the above described features, inte-gration of macro and meso-structural observations canimprove our knowledge about fault-fold relationships inextensional rollovers. The aim of this paper is to reportand discuss syn-folding fracture data from the well-exposed Armeña rollover of the Cotiella Basin in thesouth-central Pyrenees. Mesostructural data collected inthe central portion of the rollover and at its lateral termi-nation are described and discussed, with the twofold pur-poses of deriving the kinematics of the Armeña Fault and

(*) Dipartimento di Scienze della Terra, dell’Ambiente e delleRisorse, Università di Napoli Federico II. Largo San Marcellino, 10 -80138 Napoli, Italy. +39 081 2538155. stefano.tavani@unina.it

(**) Geomodels, Departament de Geodínamica i Geofísica,Facultat de Geologia, Universitat de Barcelona, Barcelona, Spain.

(***) CASP, Department of Earth Sciences, University of Cam-bridge, Cambridge, UK.

Extensional fold-related fracturing in the Armeña rollover(Cotiella Massif, Southern Pyrenees)

STEFANO TAVANI (*), BERTA LÓPEZ-MIR (**) (***) & JOSEP A. MUÑOZ (**)

Ital. J. Geosci. (Boll. Soc. Geol. It.), Vol. 134, No. 1 (2015), pp. 00-00, 15 figs. (doi: 10.3301/IJG.2014.17)© Società Geologica Italiana, Roma 2015

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comparing the fracture pattern variability with the 3Darchitecture of the rollover.

GEOLOGICAL SETTING

The Pyrenees are a Late Cretaceous to Miocene doubly-vergent collisional orogen associated with the subductionof the Iberian Plate below the Eurasian Plate (CHOU -KROUNE & ECORS TEAM, 1989; ROURE et alii, 1989;MUÑOZ, 1992; TEIXELL, 1998; PEDREIRA et alii, 2003),which developed from the inversion of previous Mesozoicextensional basins (BOND & MCCLAY, 1995; MUÑOZ etalii, 1994; MUÑOZ, 2002; TAVANI et alii, 2013) (fig. 2). Thesouth-central Pyrenees consists of three southwardsdirected thrust sheets of Mesozoic and Paleogene coverrocks (SEGURET, 1972; GARRIDO-MEGÍAS, 1973; MUÑOZ etalii, 1994; MUÑOZ, 2002). According to Muñoz et alii(2013), from top (north) to bottom (south) these thrustsheets are: the Cotiella-Bóixols (emplaced during LateSantonian to Late Maastrichtian), the Montsec-Peña

Montañesa (emplaced during Early Eocene) and theGavarnie-Sierras Marginales (emplaced during MiddleEocene to Lower Oligocene).

The Cotiella thrust sheet is a major structural unit ofthe south-central Pyrenees (SEGURET, 1972) developedfrom the positive inversion of the Cotiella Upper Creta-ceous basin, which has been interpreted by GARCIA-SENZ(2002) and MCCLAY et alii (2004) to be developed by theextensional gravitational collapse of the post-rift carbonatelayers above relic Upper Triassic salt at the southern mar-gin of the Bay of Biscay-Pyrenean rift system (fig. 3).LÓPEZ-MIR (2013) interprets the internal structure of theCotiella Basin is dominated by at least four seismic-scalemiddle Coniacian to early Santonian extensional sub-basins: Cotiella sensu strictu, Armeña, Mediodía, andSeira. The Cotiella sensu strictu sub-basin is the biggestone and consists of a broad anticline characterized by a bigfan of growth-strata. Its northern boundary is disruptedby the smaller Armeña, Mediodía and Seira sub-basins,which are dominated by aesthetically appealing rolloversdeveloping above shallow-dipping listric faults (figs. 3b, 4).

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Fig. 2 - Structural sketch of the Pyrenees showing the main structural units and foreland basins of the south-central Pyrenees as well as the locationof the study area. Dashed lines depict buried thrusts according to MUÑOZ et alii (2013). Modified from MUÑOZ (1992) and MUÑOZ et alii (2000).

Fig. 1 - Scheme of fault-propagation fold asso-ciated with a listric extensional fault, showinga synthetically dipping monocline formingabove the slowly upwardly-propagating fault.

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These sub-basins involve an up to 5 km thick successionof syn-kinematic middle Coniacian to lower Santoniancalcarenites, overlying a pre-kinematic succession made

of upper Albian to lower Coniacian limestones, which inturn are on top of an Upper Triassic salt layer (SOUQUET,1967, SÉGURET, 1972; GARRIDO & RÍOS, 1972). Salt has

EXTENSIONAL FOLD-RELATED FRACTURING IN THE ARMEÑA ROLLOVER 3

Fig. 3 - Synthetic geological map (a) and cross-section (b) of the study area. The mains structural features are labelled and the location of thecross-section is depicted in the map. The map results from our data and from data from previous works (SOUQUET, 1967; GARRIDO-MEJÍAS,1973; RÍOS et alii, 1982, 1997; ROBADOR & ZAMORANO, 1999; GARCÍA-SENZ & RAMÍREZ, 1997; FERNÁNDEZ, 2004).

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played a major role during extension, with salt rollers(e.g. BRUN & MAUDIT, 2009) having formed during exten-sion at the lower termination of faults, resulting in thesynchronous hanging-wall downthrow and folding offoot-wall strata, as evidenced by the occurrence ofsteeply-dipping pre- and syn-kinematic layers in the foot-wall of major faults (fig. 3b).

Among the four sub-basins of the Cotiella Basin, theArmeña one is the best preserved as far as the outcropquality, level of erosion and negligible internal contrac-tional deformation are concerned. It is bounded by theNE-dipping Armeña Fault, which currently presents anormal character (with younger on older relationships),with a downslope extension of at least of 2.3 km in itscentral portion. In its hanging-wall, the uppermostexposed layers of the syn-kinematic sequence are shallow-dipping, while pre-kinematic strata have a steeply-dippingto near vertical attitude (fig. 4a-c). The foot-wall ischaracterized by a downward facing anticline whosecomplex geometry includes near-vertical to strongly over-turned layers, resulting from the syn-extensional saltrollers development and later syn-inversion buttressing,with associated lateral salt expulsion. Upper Triassic saltis interpreted to have been laterally expulsed along thefault during tectonic inversion, and only small (i.e. havinga cross-sectional area of few m2) smears of salt are pre-served today, which are sandwiched between the syn-kinematic rocks of the hanging-wall and pre-kinematicrocks of the foot-wall (fig. 4c), except in the large UpperTriassic salt outcrop observed on the north of the Armeñavalley (figs. 3 and 4c), belonging to the pre-kinematicpackage of the hanging-wall of the Armeña Fault.

DATA

The Lavasar and Armeña valleys are oriented roughlyperpendicular to the Armeña Fault (fig. 3a), and allow forboth hanging-wall and foot-wall strata to be exposedalong two natural cross-sections. The Armeña valley repre -sents the central portion of the Armeña rollover as thewestern continuation of the Armeña sub-basin is inter-preted to be be currently eroded, whereas the Lavasar val-ley represents the north-western termination of therollover. This interpretation is supported by extensionaloffsets of only few hundreds of meters in the Lavasar val-ley, contrasting with at least 2.3 km donwnslope exten-sion computed in the Armeña valley (LÓPEZ-MIR, 2013).

THE ARMEÑA VALLEY

The Armeña valley is the remnant of a glacial cirque,where the topographic surface at the base of the cirqueroughly coincides with the Armeña fault surface. As aconsequence, the uppermost portion of the foot-wall isexposed over such a large area (fig. 4d). The cirque isbounded to the NW by a near vertical cliff (fig. 4a-c),where hanging-wall strata are outstandingly exposed.Despite the large amount of reverse displacement of theCotiella thrust sheet (fig. 4c), Pyrenean inversion stagehas implied only rigid tilting of layers (about 15°), withnegligible distortion of the hanging-wall geometries(fig. 4c), which are characterized by syn-kinematic stratathickening and bending toward the fault. In this area, dueto the above mentioned features, the accessible outcrops,

both in the hanging-wall and in the foot-wall, are locatedin the vicinity of the Armeña Fault surface that, on theother hand, is exposed only in few sites, as it is generallycovered by debris.

Pre-kinematic strata in the foot-wall are characterizedby the widespread occurrence of meso-scale deformationstructures, including joints and abundant veins that areroughly clustered in two sets striking about parallel (lon-gitudinal) and perpendicular (transverse) to the ArmeñaFault, respectively (fig. 5a). In the strongly overturnedlayers of the foot-wall, faults mostly display an exten-sional kinematics, with few strike-slip faults also occur-ring. Faults mostly strike NW-SE and their rotationalaxes (i.e. the slip-normal directions) are clustered along aNW-SE direction too (fig. 5b), such a behaviours indicatinga dip-slip kinematics (e.g. TAVANI et alii, 2011). At severalplaces, the strong karstification affecting the foot-wall hasprevented the recognition of the local bedding surfaceand, accordingly, data in the foot-wall can be displayedonly in their present orientation. However, in those siteswhere the bedding surface is recognizable, longitudinaland transverse joints are constantly at about 90º withbedding, while extensional faults have cutoff anglesmostly ranging between 50º and 80º (fig. 5c).

Surprisingly, despite the large extensional displace-ment, the fault zone does not exhibit a well developedfault core, as neither cataclasite nor fault breccias appearin the few outcrops where the contact between the hanging-wall and the foot-wall is exposed. Kinematic data col-lected along the fault plane exposed in these sites indicatea top-to-NE extensional movement (fig. 5d), with onlyone exception, where S-C structures and slickenlines indi-cate a top-to-SSW reverse reactivation (fig. 5e).

In syn-kinematic strata of the hanging-wall, veins arealmost absent and mesostructures include joints andextensional faults. Joints are perpendicular to beddingand cluster in two main sets, striking parallel and perpen-dicular to the master fault, respectively (fig. 6a). Exten-sional faults are mostly longitudinal, with a dominantSW-dipping set, being the antithetic system of theArmeña Fault, and have sub-horizontal rotational axesoriented NW-SE (fig. 6b). Both joints and faults displayrather constant angular relationships with bedding(fig. 6c-d), indicating their early-folding development. Aspreviously remarked, only the first few meters of thehanging-wall are available for data collection, so that it isnot possible to quantitatively estimate the variability ofthe fracture pattern attributes at different distances fromthe master fault. However, as later documented, it is con-stantly “qualitatively” observed that fracture frequencyslightly and progressively increases toward the fault,being evidenced by joints cutting trough the beds in thevicinity of the fault, which instead progressively becomestratabound away from the fault.

THE LAVASAR VALLEY

The distribution of outcrops in the other valley,located at the NW termination of the rollover and ori-ented perpendicular to the strike of the Armeña Fault, isdifferent (fig. 3a). This valley exposes, in an accessiblearea, the same 20m thick syn-kinematic package for atleast 500m, providing a natural section allowing to inves-tigate the across-strike variability of the deformation pat-tern in the same mechanical unit (fig. 7), thus ensuring

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the absence of a lithological bias in the fracture patternvariability documented in the Lavasar valley. A total of383 data have been collected in 33 sites along this 500m-

long transect, where the 30º NE-dipping Armeña faultdivides a small overturned foot-wall’s block of pre-growthstrata from the hanging-wall. The latter includes two dip

EXTENSIONAL FOLD-RELATED FRACTURING IN THE ARMEÑA ROLLOVER 5

Fig. 4 - Photograph (a) and interpretation (b) of the Armeña rollover exposure in the Armeña valley (looking to the north); (c) Photo of therollover looking to the NW, with Triassic evaporites indicated; (d) Photo of the foot-wall, looking to SE, displaying steeply dipping layersbecoming near vertical and then overturned from SW to NE.

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domains, where syn-kinematic layers dip about 20° towardNE and SW, respectively.

The overturned foot-wall block is characterized bylow-angle NE-dipping synthetic faults and by a system of

WSW-ENE to NE-SW striking faults (figs. 8a, 9). Jointsare at high angle with respect to the poorly defined bed-ding surfaces, and they strike NE-SW. Once bedding dipis removed, transverse joints roughly maintain their ori-

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Fig. 5 - Stereoplot of joints and veins (a), and faults (b) collected in pre-kinematic strata of the foot-wall in the Armeña valley. DN = datanumber; (c) Photograph showing bedding perpendicular longitudinal joints and extensional faults in the overturned layer of the foot-wall ofthe Armeña valley; (d) Stereoplot of fault and slickenlines data collected along the master fault in the Armeña valley, with panoramic photoand detail showing the slipping plane of the fault; (e) Stereoplot of the fault plane data, slickenlines and S-planes, collected in the uniqueoutcrop where the Armeña fault shows a reverse kinematics. Photographs showing the S-structures (two of them are indicated by red dashedlines) and detail showing reverse slickenlines.

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entation (i.e. about NE-SW), while many faults becomeN-S striking and W-dipping (fig. 9). However, a rotationof about 150º has occurred in this foot-wall’s block, sothat large uncertainties are associated with the clustersrecognizable in the unrotated plot. Indeed, the transverseset is not bedding-perpendicular, like observed in theother valley and in the hanging-wall of the Lavasar valley.In this manner, the N-S striking “rotated” faults in thefoot-wall could be, in our opinion, pre- to early-foldingantithetic extensional faults, which prior to layers’ tiltinghad a NW-SE oriented strike, as abundantly observed inthe foot-wall of the master fault in the Armeña valley.

Pressure solution cleavages striking about NW-SEaffect the foot-wall of the Armeña Fault. No additionalevidence of inversion has been observed along this seg-ment of the fault, suggesting that development of pres-sure solution cleavage was related with buttressing effectof the master fault, which remained locked during theinversion stage. In fact, the extensional kinematics of thefault is still recognizable by unaltered mesostructures,although the Armeña Fault in the Lavasar valley displaysan extremely narrow fault zone. This is characterized bythe absence of cataclastic rocks (even coarsely grained) andby the presence of a 0.5 to 5 cm thick S-C zone (fig. 8b).Intersection between the few measurable S-planes andthe fault plane provides the slip-perpendicular direction,documenting a top-to-NW extension direction (fig. 9),which is coherent with the previously described kine-matic data collected in the Armeña valley.

Data collected in the hanging-wall block exposed inthe Lavasar area include bedding surfaces, faults andjoints (fig. 10). Poles to bedding are clustered in two sets,corresponding to the average orientation of the two dipdomains of the hanging-wall (fig. 10c). Tensorial analysisof bedding data provides the best fit great circle, being itspole (N130°, 10°) oriented parallel to the hinge line of thefold. In agreement, the computed orientation of the axialsurface dividing the two panels is N130°, 88° (strike, dip).Only 8 slickenlines have been found in this area. How-ever, both displaced markers and structural assemblagesindicate an extensional kinematics for the measuredfaults, which is in agreement with more abundant obser-vations from the Armeña valley.

Faults are characterized by four clusters (fig. 10). Afirst major one includes NW-SE striking extensional faultsdipping about 70-80° toward SW, and oriented perpendicu-lar to bedding (fig. 11a). Other two sets strike NW-SE anddip about 50-60° in opposite directions, forming a conju-gate extensional system. Faults of these two sets are lessclustered in the rotated analysis. The less developed faultset include NE-SW striking faults that are both near verti-cal and near perpendicular to bedding. Joints are arrangedin two orthogonal sets, which are bedding-perpendicularand strike NE-SW and NW-SE, respectively. These twojoint sets occur together and they mostly describe a ladderpattern (e.g. GROSS, 1993; RIVES et alii, 1994), with theNW-SE set being the systematic joint set. However, atplaces reverse abutting relationships have been found.

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Fig. 6 - Stereoplot of joints and veins (a) and faults (b) collected in syn-kinematic strata of the hanging-wall in the Armeña valley. Data aredisplayed in their present orientation and after bedding dip removal. Photograph (c) and interpretation (d) of deformation structures in thehanging-wall of the Armeña valley, showing tilted conjugate extensional faults.

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The fracture pattern in the two hanging-wall dipdomains is characterized by a marked difference that isalready noticeable in the panoramic view of fig. 7c, wherethe different “texture” of the poorly and highly fractureddomains is recognizable. At the outcrop scale, thisvariabili ty is also evident, with longitudinal joins being

mostly stratabound and not-stratabound (joints cutacross several strata) in the NE and SW domains, respec-tively. This is quantitatively evidenced by the analysis ofthe spacing of not-stratabound longitudinal (NW-SEstriking) joints, which drastically increases across theaxial surface dividing the two dip-domains (fig. 12).

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Fig. 7 - (a) Panoramic view of the Lavasar valley; (b) Highlight of the studied cliff; (c) Location of field measurement sites; (d) Structuralscheme of the cliff. Notice the different “texture” of the highly (hanging-wall panel 1) and poorly (hanging-wall panel 2) fractured domains.

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Another remarkable difference between the two domains,is the distribution of longitudinal extensional faults ori-ented perpendicular to bedding, which are mostly locatedin the south-western domain (fig. 12).

DISCUSSION

Deformation structures observed in the Armeñarollover, both in pre- and syn-kinematic strata, and both inthe foot-wall and hanging-wall, are oriented parallel andperpendicular to the Armeña fault. These mesostructuresinclude joints near perpendicular to bedding and strikingNE-SW and NW-SE, respectively. Faults have an exten-sional kinematics and strike parallel to the two joint sets.The longitudinal NW-SE striking fault set is the mostabundant one, with the antithetic SW-dipping sub-setbeing dominant. In addition, in the foot-wall veins areabundant, while they are almost absent in the hanging-

wall. Poor evidence of inversion has been found, beingrepresented by pressure solution cleavage developed in thevicinity of the main fault and, only in an outcrop in theArmeña valley, few reverse slickenlines and S-C structures.

Despite their similar attributes, extensional macro-and meso-structures in the Armeña and Lavasar valleysshow important differences, which are summarized infig. 13. Syn-kinematic layers in the Armaña valley pro-gressively bend toward the fault. Only a narrow synclineis present there, being its internal limb represented by a10 to 20 m-wide panel located close to the fault, wherelayers’ dip slightly decreases. Conversely, in the Lavasarvalley such a syncline is much more developed, with theoccurrence of a 100 to 200 m-wide synthetic panel madeof strata dipping in the same direction of the fault. Inboth valleys the fracture frequency increases toward thefault, which is qualitatively and quantitatively observed inthe Armeña and Lavasar areas, respectively. However,while in the Armeña area the increase of fracture fre-

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Fig. 8 - (a) Cross-sectional view of the Armeña Fault principal displacement zone (PDZ), showing the overturned foot-wall with associatedfaults; (b) Detail of the PDZ, showing the narrow (< 2 cm) slipping zone of the fault. Notice the absence of a fault core.

Fig. 9 - Mesostructural data collected in the foot-wall of the Armeña Fault in the Lavasar valley.

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quency is progressive, in the Lavasar area it is sharp andoccurs across a very narrow area. Another important dif-ference between the deformation pattern observed in the

two valleys concerns the longitudinal extensional faultset. These faults display rather constant cutoff angles inthe Armeña area, and in both panels of the Lavasar area

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Fig. 11 - Detail of longitudinal joints and extensional faults (oriented perpendicular to bedding) in the highly (a) and poorly (b) fractured panelsof the Lavasar valley, respectively. Notice that joints are not-stratabound in the highly fractured panel, while they are mostly stratabound inthe poorly fractured one. Additionally, in the highly fractured panel, longitudinal joints display evidence of shearing, in the photo beingrepresented by connecting tails.

Fig. 10 - Mesostructural data collected in the hanging-wall of the Armeña fault in the Lavasar valley, shown in their present orientation andafter restoring bedding dip to the horizontal.

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some conjugate sets are recognized when restoring bed-ding to horizontal, suggesting early-folding longitudinalnormal faulting in the entire rollover. However, in thesouth-western, NE-dipping and highly fractured panel ofthe Lavasar valley, another important longitudinal exten-sional fault set is present, which includes SW-dippingnormal faults oriented perpendicular to bedding and par-

allel to the longitudinal joint set (figs. 11a, 12). This faultset has resulted from shearing along previously developedlongitudinal joints, and does not occur in the other struc-tural positions of the rollover.

The synthetically dipping panel occurring in theLavasar area is here interpreted as related with theupward propagation of the Armeña fault. During exten-

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Fig. 12 - Across-strike variation of the deformation pattern in the hanging-wall of the Armeña Fault in the Lavasar valley.

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sional fault-propagation, width and dip of the syntheticmonocline increases with decreasing the propagation vsslip ratio of the fault (fig. 1) (e.g. HARDY & MCCLAY,1999). In agreement with the extensional faults growthmodels (e.g. WALSH & WATTERSON, 1988; CARTWRIGTH etalii, 1995; COWIE & SHIPTON, 1998; KIM & SANDERSON,2005), we observe that the Armeña fault had its largest

displacement in its central portion, which is exposedalong the Armeña valley. There we infer a high propaga-tion to slip ratio that has prevented the development of alarge synthetic monocline. On the contrary, this mono-cline certainly developed at the north-western termina-tion of the fault, in the Lavasar area, where a lowerpropa gation to slip ratio has implied hanging-wall down-

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Fig. 13 - Photographs and scheme showing, the main cross-sectional macro and mesostructural differences between the Armeña and Lavasarvalley. In the former, strata progressively bend toward the fault, and only a small synthetically dipping panel is occurs close to the fault. Therethe deformation pattern includes not-stratabound fractures. Away from the fault, fractures progressively become stratabound, as shown inthe two insets. In the Lavasar valley, the synthetically dipping panel is larger, and the transition from the highly and poorly fractured areasoccurs across a narrow area.

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throw to be accommodated by folding of layers overlyingthe slowly propagating fault tip. The observation that thetwo deformation domains individuated in the Lavasarvalley coincide with the two dip domains, does not allowinterpreting the highly fractured domain of the Lavasarvalley as the damage zone (e.g. COWIE & SHIPTON, 1998;KIM et alii, 2004) of the Armeña Fault, because a moreprogressive transition between the high- and the low-frac-tured areas would be expected, alike in the Armeña valley.Alternatively, correspondence between structural positionand fracture pattern, i.e. deformation and dip domains, inthe Lavasar valley supports a folding-related fracturingscenario, as the correspondence between deformationpattern and structural position within folds is long timeconsidered an evidence of syn-folding fracturing (e.g.SANDERSON, 1982; SRTIVASTAVA & ENGELDER, 1990;HARRIS & VAN DER PLUIJM, 1998; TAVANI et alii, 2012).The widespread presence of two mutually perpendicularjoint sets oriented at high angle to bedding, which isobserved in both pre- and syn-kinematic strata of bothhanging-wall and foot-wall, and in all the structural posi-tions, is a typical scenario in extensional frameworks (e.g.RAMSAY & HUBER, 1983; GROSS, 1993; ENGELDER &GROSS, 1993; RIVES et alii, 1994; BAI et alii, 2002; TAVANIet alii, 2011). The parallelism between the Armeña Faultand the NW-SE striking joint set indicates a NE-SW ori-ented regional stretching, which is confirmed by pre-sented kinematic indicators along both meso-faults andthe Armeña master fault. Local NW-SE oriented stretch-ing responsible for the development of NE-SW strikingtransverse joints was suitably induced firstly, i.e. whenstrata were almost sub horizontal, by failure along NW-SE striking longitudinal joints (e.g. GROSS, 1993; BAI etalii, 2002) and later also by along-strike stretching alongthe laterally terminating Armeña Fault (e.g. DESTRO,1995; TAVANI & MUÑOZ, 2012) (fig. 14). The presence ofthese two joint sets in both deformation domains of theLavasar valley, and their perpendicularity with the bed-ding surface, indicate an early-folding origin for part ofthese joints. As longitudinal and transverse joints affectsyn-kinematic strata, in fact, it must be deduced that atleast part of the Armeña Fault was already formed duringinitiation of bedding-perpendicular longitudinal andtransverse jointing. Accordingly, we infer that these twojoint sets have started to develop during the propagationof the Armeña Fault, when layers were sub-horizontal, orextremely shallow-dipping and not yet faulted (fig. 15a).Some longitudinal conjugate extensional faults have alsodeveloped during this early layer-parallel stretching stage.The evidence that frequency of the longitudinal jointsincreases in the SW domain synthetic panel of theLavasar valley, where these joints have been reactivatedas shear surfaces, indicates the presence of a subsequentdeformation stage. In this later step, at the lateral termi-nation of the master fault, extensional fault-propagationfolding associated with the upward propagation of thelistric Armeña fault induced the development of a the syn-thetically NE-dipping panel, flanking to the SW the mainSW-dipping limb of the rollover anticline (fig. 15b).Layer-parallel stretching is commonly assumed in thisfault-propagation folding scenario (e.g. WITHJACK et alii,1990; COSGROVE & AMEEM, 1999; JACKSON et alii, 2006)that, according to our data, operated mainly in the NE-dipping synthetic panel, causing increase of longitudinaljoints frequency. Layer parallel stretching, however,

requires occurrence of layer-parallel slip (WITHJACK etalii, 1990; JACKSON et alii, 2006) of which there is no evi-dence in the Armeña rollover. Limited layer-parallel slip,together with a possible reorganization of the stress fielddue to the presence of inherited bedding perpendicularlongitudinal joints (e.g. BERGBAUER & POLLARD, 2004),could have operated together to keep the stretching direc-tion oriented parallel to bedding during the early stages offault-propagation folding in the Lavasar valley (fig. 15b).Faulting and folding progression caused dip increase inthe NE-dipping synthetic panel and, as a consequence,the two above mentioned processes were not longer ableto keep the parallelism between stretching direction andbeds. This caused the rotation of the minimum stress,which suitably re-acquired a sub-horizontal attitude. As aresult, longitudinal conjugate extensional faults weredeveloped. Contextually and more importantly, previouslydeveloped longitudinal joints, which must be consideredprecursory structure (CRIDER & PEACOCK, 2004; BLENK-

EXTENSIONAL FOLD-RELATED FRACTURING IN THE ARMEÑA ROLLOVER 13

Fig. 14 - Deformation mechanisms allowing the development oftransverse extensional structures. The stress release mechanismassumes that an opening longitudinal joint acts as a traction-freesurface. There, the stress component paralleling the regional stretchingdirection becomes 0. An area of stress reduction surrounds the joint,where the magnitude of the regional σ3 is reduced. In this area, ifthe regional σ2 is negative, an inversion between the regional σ3 andσ2 can occur, with the consequent development of transverse joints.The along strike stretching mechanism assumes that differentialdisplacements along the fault, causes line-length variation in thehangingwall, which imposes a local tensile stress oriented perpendicularto the regional σ3. Eventually, the same mechanism can operate inthe footwall undergoing differential uplift.

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14 S. TAVANI ET ALII

Fig. 15 - 3D scheme of the deformation pattern in the Armeña rollover: a) Early development of bedding-perpendicular longitudinal jointsand conjugate extensional faults before folding in response to the remote stress field. During this stage, development of transverse joints isinduced by failure along longitudinal joints (e.g. GROSS, 1993; BAI et alii, 2002); b) In the subsequent stage, increase of the longitudinal jointsfrequency in the highly-fractured panel of the Lavasar area is due to layer-parallel stretching associated with fault-propagation folding.Along-strike stretching imposed by the laterally decreasing displacement causes coeval further transverse jointing (e.g. DESTRO, 1995; TAVANI& MUÑOZ, 2012). In the central portion of the fault, exposed in the Armeña valley, rapid propagation of the fault implied the development ofa small highly fractured synthetic panel; c) Folding and faulting progression in the Lavasar area causes further layers’ tilting. Inherited joints arerotated together with layers and attain a favourable attitude for their shear reactivation. In the central portion of the fault, such a deformationstage did not occurred, as layers were passively translated along the fault.

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INSOP, 2008), progressively attained a favorable dip fortheir reactivation, and further folding was ensured bybedding perpendicular shearing along them (fig. 15c).These mechanisms of extra jointing and shearing alongjoints, did not operate in the Armeña valley (representingthe central portion of the extensional rollover) where theinferred high propagation to slip ratio prevented thedevelopment of a large synthetic panel and, accordingly,inhibited the development of extra longitudinal jointingand their subsequent shearing.

Finally, a consideration must be done about the reac-tivation of the Armeña fault during Pyrenean orogeny.Preservation of S-C extensional structures along the slip-ping zone exposed in the Lavasar valley, as well theabsence of mesostructures indicating a reverse reactiva-tion of the fault (with the exception of those illustrated infig. 5e), point out that no significant reverse slip hasoccurred along the Armeña fault during Pyrenean conver-gence. This implying that the shortening must have beenalmost entirely accommodated by foot-wall’s layers re-folding, which must be assumed to explain the stronglyoverturned attitudes in the foot-wall. The occurrence ofonly few isolated small bodies of Triassic evaporites alongthe fault, in fact, rules out an important syn-inversionup-dip salt expulsion along the Armeña fault, whichwould have left many more evidence and, above all,would have implied the destruction of thin mesostruc-tures like those illustrated in fig. 8b. In conclusion,absence of both remarkable reverse displacement andsignificant along-dip salt expulsion, indicate that the onlymechanism that can be invoked to explain the stronglyoverturned attitude of layers in the footw-wall of theArmeña fault, is a foot-wall buttressing accompanied bylateral, along-strike, salt expulsion.

CONCLUSIONS

The hanging-wall of the Armena Fault in its centralarea forms an outstanding rollover, with strata bendingtoward the master fault and the mesostructural assem-blage being dominated by early folding structures. Theseinclude two sets of bedding-perpendicular joints strikingparallel and perpendicular to the master fault, and faultsmostly oriented parallel to the Armena Fault. These struc-tures have developed when strata were almost sub-hori-zontal and have been later progressively tilted togetherwith layers. At its lateral termination, the rollover is char-acterized by the occurrence of two deformation domains:a synthetic NE-dipping and highly fractured domain anda north-eastern, SW-dipping, poorly fractured one. Longi-tudinal joint frequency increases in the synthetic panel,where several longitudinal joints have been reactivated asextensional faults during folding. We have interpretedsuch a framework as related with extensional fault-propa-gation folding, where extra jointing and subsequentshearing occurred to ensure folding in the syntheticallydipping panel overlying the slowly propagating fault tip.

ACKNOWLEDGEMENTS

Antonio Teixell and Ruth Soto are warmly thanked for reviewingthe manuscript. This work was carried within the framework ofINTECTOSAL (CGL2010-21968-C02- 01/BTE) projects and the “Grupde Recerca de Geodinàmica i Anàlisi de Conques” (2009SGR-1198).

The cross-section of fig. 3 has been elaborated using Move (MidlandValley). Pablo Granado and Fabrizio Balsamo are warmly thankedfor constructive and useful discussions in the field.

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Manuscript received 1 February 2014; accepted 26 May 2014; published online 13 january 2015; editorial responsability and handling by C. Monaco.

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