Cenozoic right-lateral wrench tectonics in the Western Pyrenees (Spain): The Ubierna Fault System

Tectonophysics 509 (2011) 238–253

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Cenozoic right-lateral wrench tectonics in the Western Pyrenees (Spain): TheUbierna Fault System

S. Tavani ⁎, A. Quintà, P. GranadoGeomodels Institut de Recerca, Departament de Geodinàmica i Geofísica, Facultat de Geologia, Universitat de Barcelona, Spain

⁎ Corresponding author. Tel.: +34934035957.E-mail address: [email protected] (S. Tavani).

0040-1951/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.tecto.2011.06.013

a b s t r a c t

Article history:Received 6 May 2010Received in revised form 3 June 2011Accepted 10 June 2011Available online 21 June 2011

Keywords:Strike-slipPyreneesMesostructural dataInversion tectonics

A study of macro and mesostructural deformation patterns of the southern margin of the Cantabrian area(Western Pyrenees, Spain) has revealed a complex Cenozoic tectonic framework. Right-lateral tectonicsreactivated inherited WNW–ESE striking faults, which developed during Late Paleozoic and Early Triassicevents, and Late Jurassic to Early Cretaceous main rifting stage. The Ubierna Fault represents the southernboundary of the Mesozoic basin. During the Oligocene (even Eocene) to present day deformation, this faultand the Ventaniella Fault located to the south in the study area acted as right-lateral slightly transpressiveelements forming a 120 km long and 15 kmwide overstep area, here named Ubierna Fault System, where thecumulative right-lateral displacement exceeds 15 km.The Cenozoic tectonic framework of the Ubierna Fault System includes reactivation along the WNW–ESEfaults, development of negative and, mostly, positive flower structures, branch faults, strike-slip duplexes, andreleasing and restraining bends. NE–SW to ENE–WSW striking reverse faults and contractional horsetailterminations, and NNW–SSE striking normal faults and joints are produced by the WNW–ESE right-lateralstrike-slip motion. The extensional elements are well developed and deformation progression implied theirincorporation in the strike-slip system as right-lateral faults (forming part of strike-slip duplexes). Theabundance of flower structures striking WNW–ESE and paralleling the main strike-slip faults, together withthe overall uplift of the overstep area, testifies for a slight compressional component.At a regional scale, the Ubierna Fault System represents the most prominent element of a Cenozoictranspressional belt, which incorporates the western portion of the Basque-Cantabrian Basin and the AsturianMassif area. Lateral transition between this transpressive belt and the dip-slip belt located to the east, occursacross an area experiencing along strike-shortening, which developed to accommodate the eastwardextrusion of the transpressional belt.

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1. Introduction

The Basque-Cantabrian and Asturian Massif areas represent thewesternportionof the PyreneanBelt, bounded to the southby theDueroForeland Basin (Fig. 1). They shared with the other portions of theIberia–Eurasia boundary a multiphase Meso-Cenozoic deformationalhistory. A Triassic extensional stage predated themain Upper Jurassic toLower Cretaceous extensional to left-lateral transtensional phaseassociated with the opening of the North Atlantic margin and the Bayof Biscay (e.g. Le Pichon and Sibuet, 1971; Muñoz, 2002; Olivet, 1996;Roest and Srivastava, 1991). Uncertainties exist about the Mesozoicrifting stage, which was constrained by magnetic anomalies of unclearorigin. In fact, it has been recently recognised thatmany anomalies usedin previous plate tectonic reconstructions are not associated with sea-floor spreading, but were caused by serpentinisation during mantle

exhumation (Sibuet et al., 2007). During the Upper Cretaceous relativemotion of the Eurasian and Iberian plates changed from divergent toconvergent leading to the inversion of previously developed basins (e.g.Muñoz, 1992; Vergés et al., 2002). In the study area such an inversionwas oblique involving amainlyWNW–ESE striking normal fault system(e.g. Barnolas and Pujalte, 2004; García-Mondéjar, 1996; Soto et al.,2007) with an overall N–S shortening direction (e.g. Muñoz, 2002). Ingreater detail,magnetic anomalies distribution in theNorthAtlantic andin the Bay of Biscay indicates the existence of three convergence stages(e.g. Rosenbaum et al., 2002 and references therein): an UpperCretaceous left-lateral oblique convergence followed by Paleocene–Eocene right-lateral wrench tectonics, with an associated displacementof about 60 km (e.g. Roest and Srivastava, 1991) and an Oligocene toMiocene right-lateral oblique convergence. In the offshoreportionof thebelt many right-lateral WNW–ESE striking faults are interpreted asreactivated faults (e.g. Boillot and Malod, 1988; Lepvrier and Martínez-García, 1990). In the onshore portion, the Ubierna Fault System couldrepresent the south-eastern segment of an about 300 km-long right-lateral strike-slip shear zone (Ventaniella-Ubierna Faults System; e.g.

Fig. 1. Geological map of the northern Iberia margin.

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Boillot and Malod, 1988; De Vicente et al., 2011) (Fig. 1). However,amount of displacement, timing and relationships between strike-slipand dip-slip movements are still under debate and, although thepresence of right-lateral directionalmovements along the Ubierna FaultSystem is documented (e.g. Hernaiz, 1994), they are considered assubordinated features within a mostly compressional framework (e.g.Alonso et al., 1996; Espina et al., 1996).

B

Fig. 2. (A) Geological map of the study area with traces of seismic sections in Fig. 3, earthqnetwork (Instituto Geográfico Nacional, Madrid). (B) Scheme of the major fault systems andIn the following figures this scheme will be used to indicate the position of an area of interCarreras Suárez et al. (1978); Portero García et al. (1978); Colmenero et al. (1982); Ambrose ePineda Velasco and Martín Serrano (1997a); Pineda Velasco and Martín Serrano (1997b); Pine

In this work we present newmesostructural data from the UbiernaFault System and surrounding area. These data provide importantinsights on the kinematic of the corresponding fault system (e.g.Keller et al., 1995; Kim et al., 2004). However, the complex structuralassemblage observed in strike-slip fault systems (e.g. Harding, 1974;Sylvester, 1988; Woodcock and Schubert, 1994), implies that theycannot be directly upscaled (e.g. Storti et al., 2006) due to thepresence of “local” patterns associated with second order structures(i.e. branch faults, flowers structures etc.). For this reason we presentand discuss meso-scale features and sites of particular interest, whichallow to fully constrain the kinematic of this fault system.

The entire area is re-interpreted as a wide right-lateral shear zone(Fig. 2), mostly reactivating an inherited extensional fault system,which includes a large range of structures associated with wrenchtectonics: positive flowers, branch faults, restraining bends, antitheticfaults, pull-apart basins with associated diapirs, strike-slip duplexes(e.g. Aydin and Nur, 1982; Cunningham andMann, 2007; Dewey et al.,1998; Dooley and McClay, 1997; Harding, 1974; Riedel, 1929;Sanderson and Marchini, 1984; Storti et al., 2003; Sylvester, 1988;Wilcox et al., 1973; Woodcock and Fischer, 1986; among others).

2. Geological setting

The Ubierna Fault System is an WNW–ESE elongated highlydeformed overstep area between the Ubierna and the Ventaniellafaults (Fig. 2). It divides the Upper Jurassic to Lower Cretaceous

A

uakes location and field sites location. Earthquakes data are from data from the MDDposition of the markers used to compute the displacement along the Ubierna Fault PDZ.est or field site. Map in (A) is modified from: Del Olmo and Ramírez del Pozo (1972);t al. (1984); Lobato et al. (1985); Olivé Davó et al. (1990); López Olmedo et al. (1997a,b);da Velasco et al. (1997).

Fig. 3. Seismic cross-sections across the Ubierna and Ventaniella faults (see Fig. 2 for location), with scheme of extensional forced-folding and subsequent inversion (D). Data from the I.G.M.E. (INSTITUTO GEOLÓGICO YMINERO DE ESPAÑA),reflectors interpretation for sections e–g from Gallastegui (2000).

240S.Tavani

etal./

Tectonophysics509

(2011)238

–253

UCUC

CenCen

LCUC

TCen

0

1

2

3

TW

T(s

ec)

1 Km1 Km

Upper Cretaceous

Paleozoic

Upper Cretaceous

Paleozoic

PaleozoicPaleozoic

SW NEVentaniella fault

0

1

2

3

1 Km1 KmPaleozoicPaleozoic

PaleozoicPaleozoic

SW NE

0

1

2

3

1 Km1 Km

SW NE0

1

2

3

1 Km1 Km

SW NE

UCUCCenCen

UCUC

CenCen

UCUC

CenCen

E

F

HG

CenozoicCenozoic

Fig. 3 (continued).

241S. Tavani et al. / Tectonophysics 509 (2011) 238–253

Basque-Cantabrian Basin to the north, and the Cenozoic Duero Basin tothe south (e.g. García-Mondéjar, 1996). The sedimentary package ofthe area consists of five major groups: Paleozoic rocks, exposed in thewestern sector of the study area; Triassic to Middle Jurassic rocks,which predated themainUpper Jurassic to Lower Cretaceous rift eventand include sandstones, evaporites, dolostones, marls and limestones;Upper Jurassic to Lower Cretaceous syn-rift siliciclastic sediments(with few calcareous levels); Upper Cretaceous limestones andCenozoic siliciclastic sediments. In the area to the north of the UbiernaFault the thickness of the Upper Jurassic to Lower Cretaceous syn-riftsediments frequently exceeds 1000 m, being reduced to less than200 m to the south of the Ubierna Fault, where the Triassic to Jurassicportion of the multilayer has been strongly eroded and/or notdeposited (e.g. Lanaja, 1987; Rodríguez Cañas et al., 1994). Thisthickness variation relates with the fact that the Ubierna Fault hasrepresented a first-order structure during the Upper Jurassic to LowerCretaceous rift (e.g. Malagón et al., 1994). However, its origin is older,probably late Paleozoic (e.g. Pulgar et al., 1999). Seismic cross-sectionsacross the Ubierna Fault (Fig. 3A) and one of its splay (Fig. 3B), andacross aNE–SWstriking transversal element (Fig. 3C) allow to imaginethe Mesozoic architecture of the extensional system. Geometricalrelationships between basement, Triassic evaporites, pre, syn, andpost-rift rocks observed in the hangingwall are those of an extensionalforced fold involving a intermediate decollement level (e.g. Maurin

and Niviere, 2000). Extension along basement faults in the presence ofthe Triassic Keuper evaporites accumulated in their hangingwall led tothe decoupling of the Paleozoic basement from the Upper Triassic andLower Jurassic cover sequence. A diagnostic feature of such anextensional forced folding is the presence of triangularly shapedbodies of Triassic Keuper evaporites, which are observed across boththe Ubierna Fault (Fig. 3A) and a NE–SW striking transversal elements(Fig. 3C). The Cenozoic inversion of such a deeply rooted extensionalsystem led to the uplift of the above mentioned bodies of Triassicevaporites, and to the development of dome structures (Fig. 3D). It isparticularly important to note that the main evidence of Cenozoicreactivation is found across the NE–SW striking transversal element.

The area between the Ubierna and Ventaniella faults is charac-terised by abundant faults and folds. The latter are frequently cored byTriassic and Lower Jurassic rocks and their wavelength is one to twoorders of magnitude smaller than in the northern block of the UbiernaFault. Both faults with a reverse component and folds strike from NE–SW to ENE–WSW (i.e. at 45° to 60° from the Ubierna Fault) andWNW–ESE (i.e. parallel to the Ubierna Fault). The Ventaniella Faultrepresents the southern boundary of the overstep area and it isexposed only in the western portion of the study area. To the SE it iscovered by Cenozoic sediments but it is still well-recognisable inseismic cross-sections (Fig. 3E–H), where negative and positiveflowers and pop-ups with opposite vergences reveal its presence.

242 S. Tavani et al. / Tectonophysics 509 (2011) 238–253

The major features to the north of the Ubierna Fault System aresynclines and anticlines roughly striking WNW–ESE. The WSW–ENEstriking Huidrobo Anticline is oblique to this trend and has beenfolded during the growth of the WNW–ESE striking Oña Anticline(Fig. 2). The Golobar and Rumaceo faults are Triassic faults that havebeen reactivated during the Late Jurassic to Early Cretaceous riftingstage and, as discussed below, show evidences of reactivation duringthe Cenozoic (Espina et al., 2004). This is particularly evident alongthe Rumaceo Fault, whose trace is recognisable in the post-riftsediments as an area of WNW–ESE striking embryonic faultsterminating in the Cenozoic Poza de la Sal Diapir (Quintà et al., inpress; Tavani et al., 2011). Other remarkable structure is the NNE–SSW striking left-lateral Ayoluengo Fault System (Fig. 2).

A weak seismic activity is present in the area, mostly associatedwith the major faults (Fig. 2). As much more abundantly observed inthe western and central sectors of the Ventaniella Fault (e.g. López-Fernández et al., 2004), the hypocenters lie along the faults trace

Fig. 4. Fault data collected in the Ubierna Fault System (A) and corresponding patterns expeextensional fault system (D). DN is data number, C.I. Is contouring interval.

down to 16 km-depth, indicating that the major faults are steeplydipping and penetrate at least down to the lower crust. This datumcontrasts with many interpretations of the area, where it is assumedthe existence of a low dipping thrust/basal decollement that wouldoffset both Ubierna and Ventaniella Fault (e.g. Alonso and Pulgar,2004; Serrano and Martínez del Olmo, 2004).

3. Structural data

3.1. The Ubierna Fault System

Fault data have been collected along the principal displacementzone (PDZ; Tchalenko, 1970) of both Ubierna and Ventaniella faultsand, mostly, along subsidiary structures.

Faults are steeply dipping and mostly strike from WNW–ESE toNNW–SSE (Fig. 4A). Right-lateral faults represent about 60% of thedataset and their slickenlines are clustered in a broad interval striking

cted in a N120° striking right-lateral fault system (B), left-lateral fault system (C), and

Table 1Geographic location of discussed geological features, referred to the I.G.M.E. Magna50geological maps. Data accessible from the I.G.M.E. website (http://www.igme.es/internet/default.asp).

N Description Source Latitude Longitude Other

1 Folded Burdigaliansediments

IGME.Magna50.

42°32′15″N

3°24′00″W

Sheet 1682 Middle Miocene growth

strataIGME.Magna50.

SE sectors ofcross-section II

Sheet 1333 Folded Middle Miocene

sedimentsIGME.Magna50.

42°42′00″N

4°22′45″W

Sheet 1334 Eocenic unconformity IGME.

Magna50.NW cornerof the map

Sheet 1335 Displaced syncline.

Southern blockIGME.Magna50.

42°44′35″N

4°13′00″W

Sheet 1336 Displaced syncline.

Northern blockIGME.Magna50.

42°41′45″N

4°03′00″W

Sheet 1347 Displaced syncline limb.

Southern blockIGME.Magna50.

42°39′00″N

3°55′50″W

Sheet 1668 Displaced syncline limb.

Northern blockIGME.Magna50.

42°35′10″N

3°47′00″W

Sheet 167

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between N90° and N180°. Left-lateral, normal and reverse faults are lessabundant. The rotaxes (i.e. the axis lying on the fault plane andperpendicular to slickenlines) of normal faults are strongly scattered,while thoseof reverse faults strikeNW–SE,NE–SWandE–W.Slickenlinesof left-lateral faults mostly strike N–S. This pattern includes elementscoherent with the theoretical distribution expected in a N120° strikingright-lateral system (Fig. 4B) (e.g. Harding, 1974; Riedel, 1929; Sylvester,1988). Theoretical distributions expected in N120° striking left-lateral(Fig. 4C) and extensional (Fig. 4D) fault systems are also shown. Theseindicate that a small portion of themeasuredmesostructures could havedeveloped during the mesozoic rift in response to a NNE–SSW orientedstretching, while in our dataset there are no significant evidences of left-lateral movements along WNW–ESE striking elements.

In its eastern termination the Ubierna Fault System is charac-terised by a rather narrow across fault width (compared with thewestern sectors). The PDZ strikes about N110° and two anticlinesoriented about N45° (forming an angle of about 65° with the PDZ)represent the contractional horsetail termination of the fault (Fig. 2).These faulted anticlines are cored by Cretaceous sediments (locally

Fig. 5. Details of the deformation pattern in the western portion of the Ubierna Fault. A) Acroslateral faults. B) Frontal view of a near vertical bed with a tilted strike-slip assemblage includireworked as right-lateral faults (near vertical fault) with associated S–C structures.

the Triassic is involved) and almost the entiremultilayer, up toMiddleMiocene sediments (Table 1, N 1) is folded. The PDZ is rather straightfor about 10 km and steeply dipping to near vertical layers strikingabout WNW–ESE (i.e. parallel to the PDZ) are reactivated as rightlateral faults (Fig. 5A). On the other hand, along the bedding surfaceare well exposed bedding-perpendicular right-lateral faults that havebeen tilted together with layers (Fig. 5B). This indicates that foldsparalleling the PDZ have grown within a strike-slip framework, hencethey represent flower structures.

Toward the west of the eastern termination of the Ubierna Fault, itbends and attains a strike of about N140° and then turns to N110°/115°(Fig. 6A). Along this area the PDZ,whichdisplays strike-slip kinematics(Fig. 6A), hosts elongated lens of Triassic evaporites and runs parallelto a set of synclines and anticlines. Secondary structures increase inboth fault walls. The northern wall is characterised by two branchfaults that join the PDZ close to the village of Montorio (Fig. 6A). Thenorthern branch terminates in a small and tight anticline and thisstructure represents the northern boundary of a basin filled byCenozoic sediments, being the southern boundary the Ubierna Fault(e.g. Serrano and Martínez del Olmo, 2004) (Fig. 2). In the centralportion of this branch structure (Fig. 6B, D) almost all faults strike in anE–W fashion showing both reverse and right-lateral kinematics, thatsuggests that this small anticline is a positive flower. Many bedding-perpendicular faults display a progressive transition from strike-slip totranstensional kinematics. Within the same branch element thedeformation becomes less penetrative toward the east. Although thedeformation pattern displays a geometry consistentwith a tilted right-lateral system,many faultswere active also after (and possibly during)folding (Fig. 6C, E) and slickenlines describe a NW–SE striking right-lateral system which evolves to transtensional, being the mainstretching direction about N–S. These data indicate that the northernbranch fault displays transpressional kinematics that evolved totranstensional. In the Ubierna Fault southern block the thrust andfold traces are both parallel and at about 45° from the main faultdirections.

At the central part of the studied area, the PDZ strikes N115°(Fig. 7A). Map-scale strike-slip duplexes characterise this segment ofthe Ubierna Fault. The deformation pattern in their vicinity includesjoints and normal faults (oriented about N–S) and pressure solutioncleavages (striking about E–W) all of them forming an angle of about45° (in opposite ways) with the main fault direction (Fig. 7B). Manyright-lateral faults oriented parallel to joints and cleavages arepresent, probably representing reactivated mesostructures (Fig. 7B).Along the main subsidiary fault of the area, which displaces a fold axis(the right-lateral displacement is about 500 m), are found S–Cstructures indicating an almost pure right-lateral kinematics(Fig. 7A). Ortophotos provide a good picture of the deformation

s-strike view of near vertical layers (Upper Cretaceous limestones) reactivated as right-ng right-lateral faults (slightly eastward dipping) and formerly t-fracture (Riedel, 1929)

Fig. 6. Structural scheme in the Montorio area. A) Schematic map (modified from Pineda Velasco and Martín Serrano, 1997b) with insets showing the summary of mesostructuraldata at given sites. (B–C) Mesostructural data from the northern branch fault, with details of the deformation pattern (D–E). See text for details.

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pattern (Fig. 7C–E): NNW–SSE to N–S striking joints and normalfaults, which form part of the strike-slip assemblage, are reactivatedas right-lateral faults in the strike-slip duplexes area (Fig. 7C, D) andleft-lateral faults (eastern sector of Fig. 7E). In Fig. 7e it is also evidentthat antithetic left-lateral faults are oriented at about 70°–80° fromthe major right-lateral faults, regardless of the orientation of thelatter, indicating that the stress field orientation was kinematicallyimposed by the right-lateral movement (e.g. Sylvester, 1988).

The Villela Fault System is located between the Ubierna andVentaniella Faults (Fig. 2), and represents an intriguing structure. Thisfault is divided in different segments. Those oriented WNW–ESEstrike parallel to the Ubierna Fault and represent right-lateral faultswith a reverse component. The WSW–ENE striking segment repre-sents the contractional horsetail termination of the fault. The segmentstriking N150° (Fig. 8) “should” correspond to a releasing bend but,instead, displays a strong reverse component. Within this fault zonemany WNW–ESE striking map-scale faults display left-lateral kine-matics and offset tilted layers. This sector of the Villela Fault has beenpreviously interpreted as both left-lateral fault (Hernaiz, 1994) and asa rotated Upper Jurassic normal fault system (García-Mondéjar et al.,2004). Our data (Fig. 8A–C) are partially consistent with the firstinterpretation. The deformation pattern includes WNW–ESE strikingright-lateral and left-lateral faults that have suitably started todevelop before tilting, as slickenlines preferentially locate at the

intersection between faults and bedding surfaces (Fig. 8B, C). In thisarea slickenlines are frequently observed along bedding surfaces and,together with fault data, indicate a transition from a SW–NE orientedshortening to NW–SE directed right-lateral movement (Fig. 8B).Pressure solutions cleavages striking NW–SE are associated withfaults striking N–S (Fig. 8a). At the local scale NW–SE oriented flowerstructures are observed, including thrusts (accounting for a NE–SWdirected shortening) emanating from right-lateral faults (Fig. 8D). Assuggested by Hernaiz (1994) this segment of the Villela Fault hasacted as a left-lateral fault and, although they are not abundant,similarly oriented left-lateral faults are observed also at a more localscale (Fig. 9). Our data also indicate that this fault segment acted as areverse and then right-lateral element. The activity of this faultsegment is roughly constrained by middle Miocene growth strataassociated with NW–SE striking folds located to the SW (Table 1, N 2).

Toward the west the strike of the Villela Fault progressivelychanges and it joins the Ubierna Fault, which strikes N110°. UpperCretaceous rocks (See Fig. 2 for location) are affected by near verticalENE–WSW striking reverse faults having slickenlines oriented aboutNNW–SSE, and by WNW–ESE oriented right-lateral faults (Fig. 10A).Folded Oligocene to Middle Miocene sediments are largely exposed inthe stepover area between the Ubierna and Ventaniella faults (Table 1,N 3). They are coeval with the growth of many anticlines (Espina et al.,1996) and unconformably overlie Jurassic to Paleogene rocks (Table 1,

Fig. 7. Structural scheme in the central sector of the study area. A) Schematic map (modified from Pendas Fernández et al., 1994) with summary of mesostructural data.B) Mesostructural data from the strike-slip duplexes area. (C–E) Orthophotos (INSTITUTO GEOGRÁFICO NACIONAL DE ESPAÑA. 2009) showing the details of the deformationpattern. Fault senses in (E) are based on the assumptions that faults are vertical and the apparent horizontal displacement provided by slightly folded layers is the real displacement.

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N 4). The deformation pattern associated with the Ubierna Faultwithin the Paleozoic rocks becomes much more complex due to thepresence of Late-Hercynian inheritances. The majority of faults strikeNW–SE and slickenlines (that are mostly right-lateral) are clusteredalong two directions: NNW–SSE and NW–SE (Fig. 10B). Late-Hercynian dykes present in the area (e.g. Corretgé et al., 1987) arefrequently offset by WNW–ESE right-lateral faults.

To the south the sites along the eastern termination of theVentaniella Fault and in the overstep area with the Ubierna Fault arecharacterised by an almost pure right-lateral kinematic, with right-lateral faults striking WNW–ESE and NNW–SSE (Fig. 10C).

3.2. The Northern block of the Ubierna Fault System

In this section we present preliminary data from the area locatedto the north of the Ubierna Fault system. This area is characterised by a

less developed Cenozoic deformation pattern that, however, displaysa strong variability and will be exhaustively addressed in futureworks. The Mesozoic extensional pattern is well-preserved andWNW–ESE faults accounted for a NNE–SSW directed stretching(Fig. 11), as already recognised in the area (e.g. Soto et al., 2007).The major faults of this area are the Golobar and Rumaceo faults (seeFig. 2 for location). The thickness and the elevation of Triassicsediments vary across both of them, with geometries that closelyresemble those of inverted normal faults (Espina et al., 2004). Ourdata indicate that these geometries aremuchmore suitably associatedwith directional movements. The right-lateral reactivation of thesetwo faults is testified by the structural assemblage found along them(Fig. 12). It includes WSW–ENE striking pressure solution cleavagesand NNW–SSE striking normal faults and joints (Fig. 12C) and S–Cstructures (Fig. 12d). This right-lateral stage is postdated by a left-lateral one, as testified by the fact that pressure solution cleavages

Fig. 8. Structural scheme in the Villela Fault area. A) Schematic map (modified fromLópez Olmedo et al., 1997a,b) with summary of mesostructural data. B, C)Mesostructural data. D) Detail of a positive flower structure.

246 S. Tavani et al. / Tectonophysics 509 (2011) 238–253

found along the Rumaceo Fault are reactivated as left lateral faults(Fig. 12C).

To the east the Rumaceo Fault continues as a zone of WNW–ESEstriking embryonic faults affecting the post-rift Upper Cretaceoussediments (Fig. 13A, B). These structures are found to be foldedwithinthe Huidrobo box-shaped anticline (Fig. 13C), which accordingly hasdeveloped in a later stage. Few slickenlines and, above all, small scalesynthetic faults indicate right-lateral strike-slip kinematics for thesefaults. In this anticline pressure solution cleavages are near perpen-dicular to bedding and strike about WSW–ENE (Fig. 13C); and fieldevidence for right-lateral reactivation of pressure solution cleavages isfound in the structure. Joints are perpendicular to bedding and strikesNNW–SSE, i.e. perpendicular to pressure solution cleavage (Fig. 13C).

Both joint and pressure solution cleavage are oriented at 45° from theWNW–ESE striking right-lateral faults. To the east, the area includingWNW–ESE striking embryonic fault system terminates in theOligocene to Miocene Poza de la Sal Diapir. Here all the measuredelements are joints (Fig. 13d) and are clustered in two perpendicularsets striking WSW–ENE and NNW–SSE.

In the WNW–ESE striking Oña Anticline WNW–ESE striking syn-sedimentary lower Cretaceous normal faults are abundant. Right-lateral faults become rare and strike NE–SW. On the other hand,WNW–ESE striking left-lateral transpressive faults are abundant andstrike parallel to the normal fault system. Themajor fault of the area isrepresented by a 50 m-wide left-lateral transpressive fault zonewhere near vertical layer are frequently reactivated as left-lateraltranspressive faults (Fig. 14a). In the same fault zone, and in othersector of this anticline, reverse mesofaults are associated with aroughly E–W oriented compression. Close to the frontal thrust andwithin the Miocene growth strata reverse faults indicate a roughly N–S oriented shortening direction (Fig. 14b, c).

4. Discussion

4.1. Amount of displacements

In the study area the apparent right-lateral displacement along thePDZ of the Ventaniella Fault is about 5 Km. This value, however, hasbeen computed by using Paleozoic markers. Due to the multiphasedeformational history recorded by these rocks this value cannot beconsidered reliable. Along the PDZ of the Ubierna Fault a faultedsyncline cored by post-rift Upper Cretaceous sediments indicates aright-lateral displacement of about 14 Km (Fig. 2) (Table 1, N5–6;Table 2 N8). This displacement value is consistent with that providedby the other, less reliable, marker of about 13 km, represented by adisplaced limb of a syncline cored by Oligocene sediments (Fig. 2)(Table 1, N7–8; Table 2 N9). In both cases the marker is a wrenchtectonics-related structure, which implies an underestimation of thecumulative displacement along the PDZ of the Ubierna Fault.

The ratio between maximum displacement and length computedfor the Ubierna Fault and for other faults in the Ubierna Fault System isextremely consistent with published values for strike-slip faults (e.g.Cowie and Scholz, 1992; Kim and Sanderson, 2005), which confirmsthe reliability of the displacement (Fig. 15). However, this value doesnot incorporate neither the displacements along the subsidiarystructures between the Ubierna and Ventaniella faults nor thedisplacement along the Ventaniella fault. For these reasons thevalue of about 15 km has to be regarded as the minimum right-lateral displacement of the system. A compressional component ispresent in the strike-slip system, as testified by the abundance ofpositive flowers and by the fact that rocks exposed in the areabetween the Ubierna and Ventaniella faults are older than thoseexposed to the north and, above all, to the south.

The estimation of the displacement associated with both Golobarand Rumaceo faults is more complex. In the area between theHuidrobo Anticline and the Poza de la Sal Diapir, the Rumaceo Faulthas acted as a strike-slip fault before layers' tilting occurred, asevidenced by tilted strike-slip faults within the Huidrobo Anticline.Accordingly, there are no available markers for computing thedisplacement. To the west, where the Golobar and Rumaceo faultsaffect pre- and syn-rift sediments, their cumulative displacementresults from the sum of a normal Mesozoic component and a Cenozoicstrike-slip component. Accordingly, the apparent stratigraphic dis-placements along the two faults provide little information.

4.2. Timing of deformation

In the Ubierna Fault System the youngest sediments clearlyinvolved in the strike-slip system are Middle Miocenic (Table 1, N 1

Fig. 9. Frontal view of a near vertical bed with a tilted right-lateral fault system (site close to the Ventaniella Fault). A formerly t-fracture in the overstep area between two right-lateral faults is reactivated as a left-lateral fault.

247S. Tavani et al. / Tectonophysics 509 (2011) 238–253

to N 3). However, the presence of wind gaps and of an abandoned andtilted river bend in the eastern contractional horsetail termination(Table 2: N 1 to 4), displaced crests and valleys (Table 2, N 5 to 7),coupled with the weak seismic activity of major faults (Fig. 2),suggests that the strike-slip system is still active. The older syn-tectonic sediments associated with folds in the Ubierna Fault Systemarea are Oligoce in age (Espina et al., 1996). However, the Eoceneunconformity (Table 1, N 4) can be suitably associatedwith the strike-slip tectonics and, accordingly, deformation could have started at leastduring the Eocene, remaining active until the present day.

To the north of the Ubierna Fault System, Miocene growth strataare present in the Oña Anticline. Miocene and Oligocene unconfor-mities constrain the activity of the Poza de la Sal Diapir. The HuidroboAnticline is older than the Oña Anticline, as testified by the fact thatthe axis of the former is folded by the latter (Fig. 2). In turn, theHuidrobo Anticline has developed after the right-lateral reactivationof the Rumaceo Fault as evinced by tilted right-lateral faults withinthe limbs of the Huidrobo Anticline.

4.3. Structural summary

Macro and mesoscale deformation patterns in the Ubierna FaultSystem are consistent with the following evolution. WNW–ESE right-lateral strike-slipmotion along thepre-existingUbierna andVentaniellafaults is accompanied by the development along their large damagezones (e.g. Kim et al., 2003) of NNW–SSE strikingmesoscale extensionalstructures and WNW–ESE striking right-lateral faults. Deformation

A B

Fig. 10. Mesostructural data from the western portion of Ubierna Fault collected in Mesozosurrounding areas.

progression implied the vertical and along-strike propagation of themajor faults, the onset of a slightly transpressional regime and thetransition from diffuse to localised shear (e.g. Riedel, 1929; Tchalenko,1970). Map-scale positive flowers and restraining bends developedduring this stage, as testified by the fact that right-lateral strike-slipassemblages were tilted together with layers. A small clockwise block-rotation (e.g. Scotti et al., 1991) in the area around the Villela Fault canbe invoked to incorporate this left-lateral fault within the right-lateralframework (requiring further paleomagnetic studies). Block-rotation, infact, could have implied the reactivation of the N150° striking segmentof this fault (a formerly releasing bendor an inheritedMesozoic transferfault) as a left-lateral fault that then evolved to transpressive. This couldexplain, in the Villela Fault area, the coexistence of WNW–ESE strikingpre-folding right-lateral faults (developed in the very early stages ofdeformation) and left-lateral faults (developed at the beginning of blockrotation), and the presence of syn- to late-folding left-lateral faults. Weinvoked this block-rotation mechanism to include this left-lateral faultwithin the right-lateral system.However, further paleomagnetic studiesare required to confirm/discard our interpretation. Pressure solutioncleavages striking NNW–SSE (i.e. almost parallel to the Villela Fault)indicate that this fault passed through an almost pure compressionalstage. With proceeding deformation the entire system evolved: theextensional structures were incorporated in the directional system asstrike-slip duplexes; the Villelia Fault evolved from reverse to right-lateral and the Montorio bend started to behave as a transtensionalstructure. The Montorio bend was bounded to the north by the branchfault to the east of the Montorio village which, consistently with this

C

ic (A) and Paleozoic (B) rocks. C) Data collected along the Ventaniella Fault and in the

Syn-sedim

entary faults

UnconformityUnconformity

SW NESandstone

Clay

Sandstone

Pole to Bedding

Bedding surfaces intersection

Fault

Fig. 11. Lower Cretaceous syn-sedimentary WNW–ESE striking normal fault system inthe Poza de la Sal area. In this outcrop, poles to bedding surfaces are clustered along aN23° striking plane (bedding surfaces intersection strikes N293). The strike of syn-sedimentary faults ranges from E–W to WNW–ESE (average value is N98°). These twoobservations allows to infer a stretching direction striking about N10/25°.

500 m N

Triassic

Jurassic

Pole to P. sol. CleavagePole to vein

Slickenlines Left-lateralFault

Slikenlines

Right-lateral

Left-lateral

Normal

Reverse

Unknown

Pole to CPole to S

A

B

C

D

Fig. 12. Orthophoto (A), structural scheme (B) and mesostructural data (C) from theRumaceo Fault. (D) Data from the Golobar Fault.

248 S. Tavani et al. / Tectonophysics 509 (2011) 238–253

interpretation, displays a transition from a transpressional to atranstensional kinematic.

Few of the data displayed in Fig. 4 can be interpreted as associatedwith the Lower Cretaceous extension, but they represent a verylimited portion of the dataset, despite the fact that this fault systemhas played an important role during Lower Cretaceous extension (e.g.Malagón et al., 1994; Serrano and Martínez del Olmo, 2004). Theconclusion is that the Cenozoic right-lateral event erased most of themesostructural evidences of older tectonic events.

To the north of the Ubierna Fault System, the degree ofdeformation associated with the Cenozoic strike-slip stage is reduced.Right-lateral WNW–ESE directed movement is testified by thereactivation of the pre-existing Golobar and Rumaceo faults. Thelatter continues to the east, affecting post-rift sediments, as an area ofpervasive fracturing, where the strike-slip deformation patternincludes: WNW–ESE striking right-lateral faults, NNW–SSE strikingextensional structures and WSW–ENE striking pressure solutioncleavages. The Rumaceo Fault terminates in the Poza de la Sal Diapir,where the mesostructural pattern includes two perpendicular jointsets. This pattern is consistent with an extensional setting (with jointsoriented at 45° from the strike-slip faults) that we associate with asmall pull-apart basin, whose development would have triggereddiapirism, a commonly observed scenario in strike-slip systems (e.g.,Koyi et al., 2008; Talbot and Alavi, 1996).

The Huidrobo Anticline strikes parallel to pressure solutioncleavage. It forms an angle of about 45° with the right-lateral faultsystem and has developed mostly after right-lateral faults. Thedevelopment of WSW–ENE striking right-lateral faults, which in theHuidrobo Anticline reactivate the pressure solution cleavage, couldrepresent an even later stage, possibly coeval with the development ofthe easterly located Oña Anticline, where structures associated withboth N–S and E–W compression are found.

A consistent Cenozoic deformation history of the area is sum-marised in Fig. 16. It includes an early (pre-folding thus suitably pre to

Early Oligocene) embryonic right-lateral stage along inheritedMesozoic WNW–ESE striking normal faults (i.e. Ubierna, Ventaniella,Globar, and Rumaceo faults), with an associated deformation patternthat mostly includes NNW–SSE striking extensional structures(Fig. 16A). Deformation progression implied the transition to aslightly transpressive regime (Fig. 16B–C), as testified by the factthat flower structures always preserve tilted right-lateral assem-blages. The formation of flower structures, as well as the developmentof the major map-scale structures including the Villela Fault and therestraining bends in the Ubierna Fault System, the Huidrobo Anticline,the contractional horsetail termination of the Ubierna Fault and thePoza de la Sal pull-apart basin with its associated diapir, would haveoccurred during this stage. An Oligocene to Miocene age is inferred forthis stage, consistently with the age of syn-tectonic sedimentsassociated with transpressive folds. With proceeding deformation(Fig. 16D), in the area comprised between the Oña and Huidroboanticlines, formerly compressive structures striking from WSW–ENEto NE–SW were reactivated as right-lateral faults. The local strike-slipmovement passed from WNW–ESE to WSW–ENE directed. Left-

Lower Cretaceous

Cenomanian-Coniacian

Santonian-Maastrichtian

Tertiary

Huidrobo

anticline

N

5 km

Triassic-Jurassic

Poza de la sal

Poza de la salDiapirDiapir

Poles to faultDN = 108CI=3%

Poles to P.S cleavageDN = 79CI=5%

Poles to jointDN = 22CI=3%

Poles to jointDN = 377CI=3%

100 mB

Huidrobo Poza de la Sal

A

DC

Fig. 13. Eastern portion of the Golobar Fault. A) Geological map (modified from Carreras Suárez et al., 1978). B) Details of the WNW–ESE oriented right-lateral fault system withassociated N–S striking joints. Mesostructural data from the Huidrobo Anticline (C) and Poza de la Sal Diapir (D).

249S. Tavani et al. / Tectonophysics 509 (2011) 238–253

lateral reactivation of roughly E–W striking cleavages close to theRumaceo Fault, left-lateral transpressional reactivation of the Meso-zoic fault system in the Oña Anticline, and development of many N–Soriented mesoscale thrusts can be interpreted as a unique eventassociated with a roughly E–W directed shortening stage.

In this framework it is uncertain the origin of the Ayoluengo left-lateral fault system. It strikes about perpendicular to the Mesozoicextensional structures and could be interpreted as the shallow ex-pression of a pre-existing extensional transfer zone reactivated duringright-lateral tectonics as a left-lateral fault. This fault, in fact, strikesabout NE–SW and forms an angle of about 30° with the expectedorientation of an antithetic left-lateral faults.

4.4. Regional implications

The major structure studied in this work, the Ubierna Fault System,is an inherited element. The Late Jurassic to Early Cretaceousextensional architecture of this element is well imaged in the seismiccross-sections depicted in Fig. 3A–C. Although the extensionalarchitecture of the Ubierna Fault is still well recognisable, the Cenozoicright-lateral event erasedmost of themesostructural evidences of oldertectonic events. Accordingly, few can be said about its detailedMesozoic history. In particular, the existence of a left-lateral componentalong this fault during the Lower Cretaceous rift (e.g. García-Mondéjar,1996) cannot be neither discarded nor confirmed by mesostructuraldata. The Villela Fault shows evidences of an early left-lateralmovement, and could be interpreted as associated with the EarlyCretaceous left-lateral transtensive tectonics. However, we havedocumented the presence of small-scale left-lateral faults having thesame orientation and forming part of the right-lateral assemblage(Fig. 9). For these reasons our conclusions are focused only on the

Cenozoic tectonics of the area, for which presented data allow toprovide a kinematically supported scenario.

In the study area, the existence of a Cenozoic compressional stagewith a N–S directed shortening, implicitly assumed in those works thatinterpret the study area as a thrust-and-fold belt (e.g. Alonso and Pulgar,2004; Hernaiz et al., 1994; Serrano and Martínez del Olmo, 2004),contrasts with themesostructural data presented in this work. Our datademonstrate that, to the south-west of the Oña Thrust, Cenozoiccompressional structures, including thrusts and folds oriented WNW–

ESE and about WSW–ENE (i.e. both parallel and at 45° from theMesozoic extensional structures), were formed within a slightlytranspressional strike-slip framework. The only evidence supportingthe existence of a N–S directed compressional tectonics are found in theOña Anticline, where WNW–ESE striking right-lateral faults are notpresent. The absence of Cenozoic right-lateralmovements alongWNW–

ESE oriented faults to the NE of the Oña Anticline is also confirmed byother detailed mesostructural studies (Quintana et al., 2006).

The coexistence of transpressive movements along WNW–ESEstriking elements in the southern and western portions of the studyarea and reverse south-directed movements in the north-easternsectors, could explain the presence of structures associated with an E–W shortening, which are present in the Oña, Rumaceo and Golobarareas. These elements include N–S striking reverse mesofaults, left-lateral mesofault striking about WNW–ESE and right-lateral meso-faults striking about NE–SW. Many of them clearly postdate foldsgrowth or E–Wstriking pressure solution cleavages, indicating that E–W compression represents a younger event in the tectonic evolutionof the area. The NNW–SSE striking Pas anticline, one of the majorstructures of the area (Fig. 17A), can be also explicated within such anE–W compression stage. This anticline and the mesostructurespresented in this work define a narrow band here interpreted as

SWSW NENE

Bedding surface

Bedding surface

1 m1 m

Slickenlines

ReverseRight-lateralLeft-lateral

Pole to bedding

Fault number = 34Contouring interval = 7%

SW NEENWS

BB10 cm10 cm

Bedd

ing

appr

oxim

ativ

e tra

ce

C

Pole tofault

BeddingA

Fig. 14. (A) Fault detail and mesostructural data from a 50 m-wide left-lateral transpressive fault zone in the Oña Anticline. (B) Photo and (C) scheme and stereoplot of tiltedconjugated reverse faults in the Miocenic growth wedge of the Oña Anticline.

250 S. Tavani et al. / Tectonophysics 509 (2011) 238–253

forming part of an accommodation zone between the transpressiveand the dip-slip sectors. In fact, synchronous south and SE-directedmovements to the east and to the west of this accommodation zone,respectively, would have determined a roughly E–W orientedcompression at the boundary between these two sectors.

Our conclusions can be used as a template to clarify the Cenozoicframework of the Asturian Massif and the western portion of the

Table 2Geographic location of discussed geomorphological features.

N Description Latitude Longitude

1 Wind gap From 42°30′00″N 3°32′34″Wto 42°29′28″N 3°32′38″W

2 Wind gap From 42°31′00″N 3°31′46″Wto 42°29′35″N 3°31′33″W

3 Wind gap From 42°31′52″N 3°29′07″Wto 42°29′28″N 3°28′23″W

4 Abandoned and tiltedriver bend

NW corner 42°31′58″N 3°28′20″WSE corner 42°31′40″N 3°28′00″W

5 Displaced Crest 42°36′26″N 3°56′22″W6 Displaced Crest 42°36′31″N 3°54′50″W7 Displaced Crest SW block cutoff 42°40′23″N 4°08′16″W

NE block cutoff 42°40′13″N 4°08′04″W8 Displaced syncline SW block cutoff 42°44′45″N 4°13′00″W

NE block cutoff 42°41′45″N 4°03′00″W9 Displaced syncline SW block cutoff 42°39′00″N 3°55′50″W

NE block cutoff 42°35′10″N 3°47′00″W

Basque-Cantabrian Basin. In the former, the Cenozoic right-lateralreactivation of the NW–SE striking Ventaniella Fault is well con-strained, being about 3–4 km its right-lateral displacement (e.g.Alvarez-Marrón, 1995). Seismic cross-sections across the southern

Fig. 15. Maximum displacement vs length diagram.

A

B

C

D

Fig. 16. Schematic evolution of the area, see text for details.

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sector of the Asturian Massif clearly show the presence of roughly E–W striking Cenozoic faults having a reverse component (e.g.Gallastegui, 2000). Other first-order structures are the E–W strikingand south verging Llanera and Cabuérniga thrusts, two reactivatedHercynic structures (e.g. Pulgar et al., 1999), which were active also

during the Mesozoic extensional stages (e.g. García-Mondéjar et al.,1986, 1996). Many works highlighted the presence of intense post-carboniferous right-lateral wrench tectonics affecting the Paleozoicbasement to the south of the Ventaniella Fault (e.g. Heredia, 1998).Evidences for right-lateral movements along roughly E–W strikingfaults also exist in Mesozoic rocks in the north-western sectors of theBasque-Cantabrian area (e.g. López-Horgue et al., 2009; Shah et al.,2010). Two possible interpretations can be proposed, reconciling ourobservation and previous works. In the first one the Asturian Massifand thewestern portion of the Basque-Cantabrian Basin still form partof a fully contractional belt. Due to their favourable orientation withrespect to the bulk N–S oriented shortening, inherited E–W and NW–

SE striking faults were respectively reactivated as reverse and right-lateral elements. In this case, E–W striking right-lateral elementsdocumented in Paleozoic rocks must be assumed as pre-alpine and/orassociated with local conditions. The second hypothesis assumes afully transpressive Cenozoic framework, with right-lateral compo-nents along E–W striking faults interpreted as Cenozoic (Fig. 17A). Inthe latter case, the observation that the displacement along theUbierna Fault System is at least 15 km, while along the VentaniellaFault it is 3–4 km, would indicate that the right-lateral displacementof the Ubierna Fault has been westerly transferred not only to theVentaniella Fault but also to other fault systems located to the south ofthis fault. This second solution would convert the Asturian Massif andthe western portion of the Basque-Cantabrian Basin in a transpressivebelt. This hypothesis is supported by the presence a roughly N–Soriented accommodation zone where E–W shortening has occurred,relating with the eastward extrusion of the eastern portion of thetranspressive belt (Fig. 17B). This system represents a small-scaleanalogue of the lateral extrusion occurring to the north of theHimalayan Orogen (e.g. Tapponnier et al., 1982). Such a regionalinterpretation of the WNW–ESE right-lateral wrench tectonics ispreferred to other possible alternatives, including that of a strainpartitioning (e.g. Fitch, 1972), as the presence of an accommodationzone where E–W shortening has occurred better matches with alateral extrusion hypothesis. Moreover, in the hypothesis of a lateralextrusion, the described Ubierna right-lateral Fault System wouldrepresent a typical foreland structure, and therefore a sort of smallintraplate strike-slip deformation belt (e.g. Storti et al., 2003), whosepresence is confirmed by Cenozoic right-lateral movements docu-mented to the south of the Ubierna Fault System, in the Iberian Chain(e.g. De Vicente et al., 2009), i.e. in an area outside the Pyrenean Belt.

5. Conclusions

Data presented in this work indicate that the southern margin ofthe Cantabrian area is characterised by the presence of intense right-lateral tectonics that started at least in the Oligocene (or even in theEocene) and is still active. In the Ubierna Fault System WNW–ESEright-lateral strike-slip (slightly transpressive) movements alongpreviously developed fault systems led to the development of anintense deformation, which includes NNW–SSE oriented extensionalstructures, NE–SW and WSW–ENE striking restraining bends, strike-slip duplexes and flower structures paralleling the main faults. Thecumulative displacement along this fault system exceeds 15 km.

To the north the degree of deformation associated with theCenozoic strike-slip stage is reduced. Right-lateral WNW–ESE direct-ed movements are testified by the presence of NNW–SSE strikingextensional structures, right-lateral reactivation of both Golobar andRumaceo faults, and by the development of the Poza de la Sal diapir-cored pull-apart basin. The Oña Anticline represents the southernfront of the “compressive” thrust-and-fold belt. Cenozoic structurestestifying for an E–Woriented compression are interpreted as formingpart of an accommodation zone between the compressional and right-lateral transpressional sectors.

Fig. 17. (A) Geological map of the Western Pyrenees with Alpine faults senses expected in the transpressional hypothesis. Modified from Alonso et al. (2009) and Soto et al. (2008).(B) Scheme of lateral extrusion. Great arrows indicate regional shortening, small arrows indicate the direction of local movements. See text for details.

252 S. Tavani et al. / Tectonophysics 509 (2011) 238–253

Acknowledgments

Two anonymous reviewers are thanked for their very helpfulreview. Eloi Carola is thanked for useful discussion. This work wascarried out under the financial support of CIUDEN (FBG305657), andINTECTOSAL (CGL2010-21968-C02-01/BTE) projects. We thank the I.G.N. (INSTITUTO GEOGRÁFICO NACIONAL DE ESPAÑA. 2009) and theI.G.M.E. (INSTITUTO GEOLÓGICO YMINERO DE ESPAÑA) for providingorthophotos, and seismic sections and geological maps, respectively.

Appendix A. Supplementary data

Supplementary data to this article can be found online at doi:10.1016/j.tecto.2011.06.013.

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