Lopez-Mir Et Al - 2014 - JSG

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Journal of Structural Geology 69 (2014) 147e162

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Journal of Structural Geology

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Restoration of basins driven by extension and salt tectonics: Examplefrom the Cotiella Basin in the central Pyrenees

Berta Lopez-Mir a, b, *, Josep Anton Mu~noz b, Jesús García Senz c

a CASP, West Building, 181A Huntingdon Road, Cambridge CB3 0DH, United Kingdomb Institut de Recerca Geomodels, Facultat de Geologia, Universitat de Barcelona, Zona universit�aria de Pedralbes, 08028 Barcelona, Spainc Instituto Geol�ogico y Minero de Espa~na (IGME), La Calera 1, 28760 Tres Cantos, Madrid, Spain

a r t i c l e i n f o

Article history:Received 3 June 2014Received in revised form12 September 2014Accepted 28 September 2014Available online 18 October 2014

Keywords:Sequential restorationsSalt tectonicsPassive diapirismPyreneesCotiella basin

* Corresponding author. Present address: CASP, WeRoad, Cambridge, CB3 0DH, United Kingdom. Tel.: þ4

E-mail addresses: [email protected] (J. Anton Mu~noz), [email protected] (J. García

http://dx.doi.org/10.1016/j.jsg.2014.09.0220191-8141/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The structure of the Cotiella Basin consists of four well differentiated salt-floored sub-basins whichcontain more than 6 km of halokinetic growth-strata and have been interpreted to be formed by gravity-driven extensional faults and diapirism during the middle Coniacian e early Santonian. The subsequentPyrenean orogeny (late Santonian to Miocene) inverted the extensional basins and squeezed the diapirsleaving little trace of the original salt volume, although both the extensional and the salt-related featuresof the Cotiella Basin were preserved. This paper provides new insights into the evolution of the CotiellaBasin by establishing a correlation between sub-basins by means of structural restorations. The workflowfollowed and the assumptions made during the restoration of the inversion and especially of theextensional structures are outlined in detail.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The first application of structural restoration concepts was pro-vided by Chamberlin (1910) to determine the depth of the detach-ment level underlying concentric folds. However, itwas not until thelate 60's that the construction bed-length balanced cross-sections ofthe Canadian Rockies (Bally et al., 1966; Dahlstrom, 1969, 1970)demonstrated the importance of balancing to predict the geometryof the structures at depth and to validate structural interpretations.Structural balancing studies continued, particularly in contractionalregions (e.g. Hossack, 1979; Boyer and Elliot, 1982), resulting in thedevelopment of kinematic structural models such as the fault-bendand fault-propagation folds (e.g. Suppe, 1983; Suppe andMedwedeff, 1990). In extensional settings, the balancing of cross-sections was also applied to interpret seismic images (e.g. Gibbs,1983) and to predict the geometry and kinematics of extensionalfaults (e.g. White et al., 1986; Williams and Vann, 1987; Groshong,1990). Further research on structural balancing focused on theapplication of numerical (e.g. Erslev, 1991) and geomechanical

st Building, 181A Huntingdon4 1223768290.(B. Lopez-Mir), [email protected]).

models (e.g. Maerten, 2007) to simulate and understand the evo-lution of geological structures (i.e. Groshong et al., 2012).

In salt tectonics settings the restoration of cross-sections is alsocrucial to formulate models of structural evolution, but it can bemore problematic. First, the constraints to restore salt-relatedstructures are not simple because the mobility of the salt cangenerate uncertainties as a result of lateral salt expulsion andrelated vertical rotations. Second, the existing algorithms cannot beapplied straightforwardly as the fundamental assumptions such asplane-strain deformation and area conservation are often invalidfor cross-sections containing salt (i.e. Rowan and Ratiff, 2012).

This paper presents sequential restorations of the partially inver-ted Cotiella salt basin in the central Pyrenees. Sequential restorationsare used to validate, constrain and improve models of structuralevolution. Besides the establishment of a model for the evolution ofthe Cotiella Basin, theworkflow followed and the assumptions madeduring the restoration process are outlined in detail.

2. Geological setting

2.1. The central Pyrenees

The Pyrenees is a Late Cretaceous to Miocene doubly-vergentcollisional orogen which was developed from the inversion ofprevious Mesozoic extensional basins (ECORS-Pyrenees Team,

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Fig. 1. a) Structural sketch showing the main structural units of the Pyrenees (modified from Mu~noz, 2002). The location of the map below is depicted in red. b) Geological map ofthe Cotiella thrust sheet, indicating the main structural features and the location of Figs. 2 and 3b. This map results from our data (L�opez-Mir, 2013), except for the footwall of theCotiella thrust that results from a compilation of existing studies (Souquet, 1967; Garrido-Megías, 1973; Ríos-Aragües et al., 1982; Robador and Zamorano, 1999; García-Senz andRamírez, 1997). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

B. Lopez-Mir et al. / Journal of Structural Geology 69 (2014) 147e162148

B. Lopez-Mir et al. / Journal of Structural Geology 69 (2014) 147e162 149

1988; Mu~noz, 1992; Teixell, 1998). In its south-central portions, theAlpine structure consists of a southwards-directed imbricate stackof thrust sheets of Mesozoic and Paleogene cover rocks (S�eguret,1972; Garrido-Megías, 1973; Mu~noz, 2002). From top (north) tobottom (south), these thrust sheets are: Cotiella (emplaced duringthe late Santonian - late Maastrichtian), Montsec-Pe~na Monta~nesa(emplaced during the Early Eocene) and Gavarnie (emplaced dur-ing the Middle Eocene to Early Oligocene).

The Cotiella thrust sheet developed from the positive inversionof a previous Upper Cretaceous extensional basin and was subse-quently transported piggy-back at least 18 km southwards by theyounger Montsec e Pe~na Monta~nesa thrust (García-Senz, 2002;McClay et al., 2004). Synchronously and afterwards, it was tiltedsouthwards and uplifted by more than 3 km by underthrusting ofthe younger Gavarnie and other basement-involved thrust sheets.

Fig. 2. a) Structural cross-section across the central portions of the Cotiella thrust sheet.Interpreted photograph of the Arme~na ridge. See Fig. 1 and cross-section 2a for location. St

As a result, the thick Upper Cretaceous succession of the Cotiellaextensional basin is presently juxtaposed over Upper Cretaceous,Paleocene and Lower Eocene rocks of the Cotiella thrust's footwall(Figs. 1 and 2).

2.2. The Cotiella Basin

The internal structure of the Cotiella Basin contains fourseismic-scale middle Coniacian to early Santonian sub-basins:Cotiella, Arme~na, Mediodía, and Seira (Fig. 1b). The Cotiella sub-basin is the largest one and consists of a broad anticline charac-terized by growth-strata showing a thickness expansion of up to6 km towards a partially inverted extensional fault (Fig. 2a). To thenorth, there are the smaller Arme~na, Mediodía and Seira sub-basins, which are filled by sedimentary wedges fanning as much

b) Structural cross-section across the eastern portions of the Cotiella thrust sheet. c)ructural features and stratigraphic units are labeled according to the legend of Fig. 1b.


as 3.3 km towards extensional listric faults (Fig. 2). The syn-kinematic sediments consist of middle Coniacian to lower Santo-nian carbonates and calcarenites (Aguasalenz and Maci~nos For-mations, respectivelly) which overlie a pre-kinematic successionmade up of upper Albian to lower Coniacian post-rift carbonatesdetached on Upper Triassic salts (Souquet, 1967; S�eguret, 1972;McClay et al., 2004). The equivalent Coniacian to lower Santoniansedimentary succession in the Cotiella fault is only 300 m thick(García-Senz, 2002; McClay et al., 2004), as happens in most of thePyrenees.

The Cotiella Basin has been interpreted to be developed by post-rift gravity-driven extension above relict Upper Triassic salts in theBay of Biscay-Pyrenean paleorift margin of the Atlantic Ocean(McClay et al., 2004). However, particular structural and sedimen-tary features depart from the classical models of extensional listricfault systems (L�opez-Mir, 2013). These features and an alternativemodel of evolution for the Cotiella Basin are summarized below.Our model refines the previous gravity-driven extension model bythe incorporation of passive diapirs isolating the sub-basins (Fig. 3)and questions the existence of a hard-link connection between sub-basins as proposed by of McClay et al. (2004).

One of the most peculiar salt-related features can be observed inthe hangingwall of the Arme~na fault (Fig. 2c), where shallow-watercalcarenites of the Maci~nos Formation are accumulated next to thefault (in the theoretically structural low of the rollover). This un-expected water-depth distribution of the sediments is notadequately explained by the previously interpreted scenario of

Fig. 3. a) Sketched geological model showing the main stages of evolution of the Cotiella Bwith gravity-driven extension above salt; II) Extension continues and diapirs are sealed; Ialong-strike geometry of the Cotiella sub-basin at the end of the early Santonian (stage Idepocenters as well as the AS_Fo unconformity and the �Esera fault. The location of the cross-depicted. Both diagrams are modified from.L�opez-Mir (2013).

extensional gravity-driven faults (García-Senz, 2002; McClay et al.,2004). It can be alternatively interpreted to have been developed assalt diapir-related sandstone depocenters similar to the onesdescribed by Aschoff and Giles (2005) in La Popa basin in Mexico.The accumulation of calcarenites in this structural position, in fact,is consistent with the hangingwall of the Arme~na fault being sup-ported by salt, preventing the development of a shale-dominatedbasin. Additionally, at the base of the Arme~na sub-basin (to thenorth), an exceptionally large Upper Triassic salt outcrop (Fig. 1b)shows an abrupt lithological contact between the top of the salt andthe overlying sediments, supporting the presence of a diapir duringthe deposition of the Maci~nos calcarenite Formation, as shown inthe model of Fig. 3aeI.

In the Seira sub-basins, the calcarenites of the Maci~nos Forma-tion directly overlies Upper Triassic sediments and, additionally,the footwall of the Seira fault exhibit calcarenitic lenses (Fig. 1b)similar to the carbonate lentils described by Giles and Lawton(2002) in strata flanking passive diapirs. The calcarenites of theMaci~nos Formation contain clasts sourced from the Upper Triassicsalt layer, which supports the model of emergent salt diapirs con-trolling their deposition.

Regarding the largest Cotiella sub-basin, two main depocentershave been interpreted: the older andwestern �Esera depocenter; andthe younger and eastern Las Neis depocenter (Fig. 3b). These areseparated by the transverse �Esera transfer fault. The Aguasalenz_Founconformity covers both the transfer fault and the calcarenitic de-posits of the Cotiella sub-basin, as can be observed in map-view

asin from the late middle Coniacian to the late Santonian. I) Passive diapirs are coevalII) Tectonic inversion and squeezing of the diapirs. b) Conceptual sketch showing theI of model above), depicting the relative chronology between the �Esera and Las Neissection is shown in Fig. 1. The intersections with Fig. 2a, b and with the model above are


(Fig. 1b). Assuming that the deposition of the calcarenites was syn-chronouswith the rising of diapirs, the Aguasalenz_Fo unconformityis interpreted to overlie the diapirs and related northern sub-basins(Fig. 3a-II). This unconformity crops out very nicely in the Raymondd’Espouy peak, in the footwall of the Arme~na fault, where the strat-igraphic succession displays a spectacular fanning of more than 90�

(Fig. 2a). Such a fanning is difficult to create by gravity-drivenextension, but is better explained by the combination of salt tec-tonics with extension (although it could be partly accentuated byshortening at high angles to bedding during tectonic inversion). Themigration of the thickest sedimentary interval towards the west inthe Cotiella sub-basin observed in Fig. 3b is also interpreted to berelated to salt inflation related to the salt displaced by the sinkingdepocenters. In the late Santonian, as a result of Pyrenean contrac-tion, salt was expulsed out to the surface and then dissolved, leavinglittle trace of the original salt volume. Thrust-welds (which coincidewith the faultsobserved today)werepotentiallycreatedbysqueezingof previous salt walls and the different sub-basins ended up attachedto one another (Fig. 3a-III). Thanks to salt expulsion, the internalstructure of the Arme~na,Mediodía and Seira sub-basins was scarcely

Fig. 4. Workflow to restore the basement-involved thrust sheets of the Axial Zone, and thsections at late Santonian times, once the Cotiella basins were inverted. Four steps of restZone thrust sheets; III) Geometry after removing the detachment fold of the Gavarnie thrustthrust sheet. The pin line, the template beds and the datum used in each step of restorationinterpretation of the references to colour in this figure legend, the reader is referred to the

deformedand thecurrent geometry isprobably similar to theoriginalone. Bycontrast, the geometryof theCotiella Basinwasmore severelymodified. In the footwall of theArme~na fault for instance, strataweresteepened and overturned, resulting in the development of complexstructures such as the Reduno downward-facing anticline (Fig. 2a). Inareasmoredistant to thediapirs, such as thewesternmost portions ofthe Cotiella Basin, tectonic inversion caused thedevelopmentof foldssuch as the Vaquerizal anticline (Fig. 2a).

3. Restoration of the contractional structures

To understand the evolution of the extensional salt-relatedstructures, sequential restorations of the extensional faults wereconstructed. Before this could be done, the effects of shortening hadto be removed. The contractional structures have only beenpartially restored in order to decipher the geometry of the CotiellaBasin before its positive inversion. Two main operations have beenconsidered: (1) the restoration of the Pe~na Monta~nesa, Gavarnieand basement-involved Axial Zone thrust sheets (Figs. (4) and (2))the restoration of the late Santonian inversion (Figs. 5 and 6).

e Gavarnie and Pe~na Monta~nesa thrust sheets in the Arme~na (a) and Seira (b) cross-oration are depicted: I) Present day geometry; II) Geometry after removing the Axialsheet; IV) Geometry after removing a low-angle footwall ramp of the Pe~na-Monta~nesaare indicated. The colors of the stratigraphic horizons are shown in.Figs. 1 and 2. (Forweb version of this article).

Fig. 5. Workflow to restore the overturned strata of the Reduno structure in the Arme~na cross-section. Four steps of restoration are depicted. a) Geometry of the Cotiella basin in thelate Santonian. b) Different operations to remove the overturned attitude of strata in the Cotiella sub-basin: I) calculate the thicknesses of the strata in two separate sections(generally next to the bounding faults) in order to keep the amount of thickness expansion constant in the following operations; II) interpret the geometry of the uppermoststratigraphic interval which was potentially eroded during shortening; III) restore some of the folds acquired during tectonic inversion by drawing manually the top and the base ofthe strata considering line-length and bed-shape constant. c) Translate the Cotiella, Arme~na and Mediodía sub-basins to the position before tectonic inversion. Two solutions areproposed considering that: I) the passive diapirs persisted during the entire extensional event and; II) the passive diapirs where covered by sediments before the end of extension.d) Interpretation of the eroded section in the Arme~na and Mediodía sub-basins, and interpretation of the basement geometry. Two solutions are proposed depending on the timingof primary welding and sealing of the passive diapirs (I and II).


3.1. Pe~na Monta~nesa, Gavarnie and basement-involved thrustsheets

The restoration of the Pe~naMonta~nesa, Gavarnie and basement-involved thrust sheets aims to remove the folding and tilting whichwas acquired during their emplacement. The total transport to-wards the south has not been considered. This restoration has beencarried out in a very simple manner because the internal structureof the Cotiella Basin was scarcely deformed. The flexural slipunfolding algorithm (Davison, 1986) that is implemented in the

Move™ software has been used, assuming longitudinal contractionduring basin inversion. This method uses a pin perpendicular to,and a slip-system parallel to the template bed to control theunfolding of the beds. It maintains bed thickness variations, andboth the template bed and slip-system are unfolded. Three opera-tions have been necessary (Fig. 4):

I) The tilting acquired during the emplacement of the AxialZone basement-involved thrust sheets has been removedusing the uppermost layer of the Axial Zone thrust sheets

Fig. 6. Workflow to restore the overturned strata of the Baciero area and the Seira sub-basin in the Seira cross-section. Four steps of restoration are depicted. a) Geometry of theCotiella Basin in the late Santonian. b) Interpretation of the eroded section in the Cotiella sub-basin. c) Remove the overturned layers and interpret the relation among the Cotiellaand Seira sub-basins following the same methodology than in Fig. 5b and c d) Interpretation of the eroded section in the Seira sub-basin and of the basement geometry. The colorsof the stratigraphic horizons are shown in.Figs. 1 and 2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).


(which is the top of the Lower Triassic red beds and is parallelto the Gavarnie thrust) as the template bed. The pin line hasbeen inserted in the southern edge of the cross-sections as aline perpendicular to the beds in the case of the Arme~nacross-section (Fig. 4aeI) and as the axial trace of the south-ernmost syncline in the case of the Seira cross-section(Fig. 4beI). The line-lengths of the rest of the stratigraphichorizons have been checked quantitatively.

II) The (not very pronounced) detachment anticline observed inthe Gavarnie thrust sheet has subsequently been restored. Inthe Arme~na cross-section (Fig. 4a-II), this fold has beenremoved using the top of the Early Eocene succession as thetemplate bed. In the case of the Seira cross-section (Fig. 4b-II), the top of the pre-extensional Upper Cretaceous succes-sion has been used as the template. In both cases, the pin linehas been inserted in the southern edge of the cross-sections,

as a line perpendicular to the beds. The Pe~naMonta~nesa foldsobserved in the southern parts of the cross-section have notbeen removed because their restoration does not alter thegeometry of the Cotiella Basin. The geometrical changesbetween steps II to III are not very pronounced as the anglebetween the template bed and the datum is very small. Themost noticeable change can be appreciated in Fig. 4a-IIIwhere the layers at the base of the Gavarnie thrust sheet arecompletely flat-laying.

III) The footwall ramp of the Pe~na-Monta~nesa thrust observed atthe northern portions of Fig. 4a-III and 4b-III (which gener-ates a slight tilting towards the north in the Cotiella Basin)has been restored using the Pe~na Monta~nesa thrust as thetemplate bed. The pin line has been inserted in the southernedge of the cross-sections, as a line perpendicular to thebeds. As the footwall ramp of the Pe~na-Monta~nesa thrust


cuts the layers of its footwall at a very low angle, the changesbetween steps III to IV in Fig. 4 are very gentle. The mostnoticeable change is that the bottom of the Cotiella basin hasbecome flat in the northern portions of the restored cross-sections in step IV. The latter shows how the geometry ofthe Cotiella Basin looked like immediately before tectonicinversion at the late Santonian (Fig. 4a-IV and 4b-IV).

3.2. Late Santonian inversion

The primary objective of restoring the late Santonian inversionis to discover what the geometry of Cotiella Basin looked like at theend of the extensional salt tectonics event (Figs. 5 and 6). Thisoperation needs to be performedmore carefully than the precedingone because it can introduce erroneous geometries, especially inthe Cotiella sub-basin where the layers were more severelydeformed during shortening. As a result, this step has been per-formed in a semi-manual manner, using the existing algorithmswhen possible andmanuallymodifying the results when necessary.The assumptions made are explained as follows.

Each sub-basin has been treated separately and four operationshave been considered (Figs. 5 and 6):

1) Assuming that the overturned attitude of strata was acquiredduring tectonic inversion, the first operation comprised rotatingthe layers of the Cotiella Basin until all of them present a normalstratigraphic polarity. This operation has been approached in amanual manner, considering that bed-areas and bed-lengthsremain approximately constant over time, as explained inFig. 5b. Slight differences in the length or the area of the bedshave been accepted because the strata could have thinned bylayer-normal shortening during contractional deformation assome of the beds of the Cotiella Basin were steeply dippingbefore contraction started.

2) Once the overturned attitude of strata has been removed, thesub-basins have been translated to allow the salt reaching thesurface as deduced by the sedimentological features abovedescribed (Fig. 5c). This assumes that during the extensional salttectonics event the sub-basins were separated by salt walls anddiapirs, as suggested by facies distribution and lithologicalcharacteristics, the spaces between sub-basins have beenconsidered to be filled with salt and the diapirs have beendepicted as narrow as possible.

3) The eroded parts of the section and the basement geometryhave been interpreted manually (Figs. 5d and 6d).

The basement geometry is inspired by cross-sections con-structed in other areas of the Pyrenees, such as the Parentis basin(e.g. Ferrer et al., 2012), where the basement geometry is inter-preted from the ECORS-Bay of Biscay and MARCONI-3 seismicprofiles. The interpreted basement geometry consists of tiltedblocks which dip southwards or are near-horizontal and are sepa-rated by northward-dipping planar extensional faults. These faultsare decoupled from the sedimentary cover by a thick package ofUpper Triassic evaporites and probably influenced the thicknesschanges of the Upper Triassic. This inferred geometry helps toexplain the differences in thickness between the southern (thinner)and the northern (thicker) sub-basins and, additionally, it isconsistent with the sequential restoration presented for theArme~na and Seira cross-sections (in the following pages).

3.3. Limitations and assumptions

In the Arme~na cross-section (Fig. 2a), the first operation(removing the overturned attitude of the layers) was problematic

(Fig. 5b). To keep the bed-length of the Reduno downward-facinganticline invariable due to the high cut-off angles of the bedswith the faults; slight changes in the thickness of the beds havebeen accepted assuming that layers could have thinned by layer-normal shortening during contractional deformation. The mainproblem lies in the change in the geometry of the Arme~na fault if itwas in contact with the sediments in its footwall. This problem,however, supports our observations of thrust-weld characteristic ofthe presently observed fault and the possible existence of a diapirbetween the Cotiella and Arme~na sub-basins.

The second operation (translating the sub-basins and filling thegaps with salt) has also been problematic in the Reduno area(Fig. 5c). Considering that the lowermost layers of the Cotiella sub-basins show evidence of diapirism (such as the outcrop of UpperTriassic in the Raymond d’Espouy peak, Fig. 2c), the Cotiella andArme~na sub-basins were potentially separated by a diapir in theearly stages of development. If we then merely translate theArme~na sub-basin northward to allow the salt to reach the surface,the resulting diapir geometry shows a landward tilted geometry,which is unlikely in extensional settings. A geometrical solution forthis problem is to project the Lavasar shortcut (observed west-wards, Fig. 1) above the topography of the section, in order to allowthe beds to be longer and, as a result, the northernwall of the diapirto be vertical.

This interpretation leads to two possible solutions (Fig. 5). Thefirst (Fig. 5ceI) interprets a long-lived passive diapir which per-sisted during most of the extensional salt tectonics event. Thesecond (Fig. 5c-II) considers that sediments covered the passivediapir after the deposition of the calcarenites of the Maci~nos For-mation. The second solution has been chosen to perform thesequential restorations because, as will be explained later, it betterhelps to explain the development of the Reduno downwards-facinganticline (Fig. 2).

The contractional structures in the Seira cross-section were notso difficult to restore, as the amount of overturning is not as pro-nounced as in the Arme~na cross-section (Fig. 6). To simplify therestoration operations, the Chía klippe (Fig. 1) has been consideredin continuity with the Cotiella Basin, although this relationship isactually unknown.

4. Methodology to restore the extensional structures

4.1. Workflow

The workflow used to perform sequential restorations isgrounded in the methods proposed by Schultz-Ela (1992) and byRowan (1993) to restore ductile structures and salt structures inextensional terrains, respectively. These methods are moreadequate than the classical methods to restore normal faults (e.g.Dula, 1991) when extensional structures are associated with salt.

The Schultz-Ela (1992) method consists of six basic steps: (1)strip a layer; (2) unfold the other layers by simple shear; (3) rotate;(4) translate; (5) decompact; and (6) move the top of the basementin order to keep the salt area constant. In this work, the Schultz-Ela(1992) method has been carried out separately for each sub-basinand steps have been performed in the order that we explainbelow (Fig. 7):

1) The first operation has been stripping a layer (Fig. 7b).2) Subsequently, the layers have been decompacted (Fig. 7c) and

the bumps resulting from applying the decompaction algorithmhave been softened manually. Performing the decompaction atthe beginning of the restoration (instead of doing it at the end,as suggested by Schultz-Ela, 1992) costs less computational time

Fig. 7. Method proposed by Schultz-Ela (1992) to perform sequential restorations inextensional terrains which has inspired the methodology presented in this paper. Sixmain steps of restoration have been considered. a) Original geometry. b) Geometryafter stripping a layer. c) Decompaction. d) Rotation applied to the layers afterunfolding. e) Shear vector and datum line used to unfold the layers by simple shear. f)Translation of the different fault blocks. g) Move the top of the basement to keep thesalt area constant. Figure modified from Schultz-Ela (1992).


because the Move™ software is much more stable in thefollowing steps.

3) Afterwards, a rigid body rotation (Fig. 7d) has been performedsemi-manually, to make the uppermost layers as parallel to asub-horizontal datum as possible.

4) Rotation has been followed by unfolding of the layers (Fig. 7e).Hangingwall extension and salt deflation on listric faults pro-duce strong bed rotation and diffuse deformation which is bestmodeled as block translation and collapse by inclined shear. Aline with a slight tilting basinwards has been selected as thedatum. The shear angle has been chosen by trial and error, byfinding the angle that provides the least variations in the areaand shape of the layers. The chosen angles vary between 40 and70�, and consider both antithetic and synthetic shear. We haveaccepted great amount of variation for two main reasons. First,the Cotiella Basin was developed in a salt tectonics setting and

consequently the different sub-basins could have experienceddifferent strain histories during their development. Second, theexisting methods to choose the shear angle in extensional set-tings (i.e.: White et al., 1986) cannot be applied to restore theextension in the Cotiella Basin as the current geometry wasdeformed during shortening.

5) The sub-basins have been translated and gathered (Fig. 7f)assuming that diapirs widen during extension and narrowduring shortening (although their height can decrease duringextension).

6) The top of the basement has finally been moved (Fig. 7g),considering that the volume of the salt is constant or decreasesover time (although this assumption may not be fully correctbecause salt can migrate laterally). Additionally, the top of therestored sections can be adapted to the interpreted bathymetry,using the Vertical Shear Unfolding to better depict how thewater-depths were likely to be.

4.2. Limitations

This methodology presents a limitation when steep or verticallayers exist because the simple or vertical shear algorithms do notgive satisfactory results (Fig. 8a). Themain problem lies in the shearangle. Applying a vertical shear algorithm to shallow-dipping layersconsiders that shear is perpendicular to the beds. Contrastingly, ifthe same shear vector is applied to steep layers, the shear becomesalmost parallel to the layers and the algorithm is invalid.

To solve this problem, the restoration of steep layers has beenapproached in a semi-manual manner, rotating and unfolding thebeds individually (Fig. 8b). In some cases, manual modificationshave been necessary to keep bed-areas and bed-geometries con-stant. To unfold the layers, vertical or simple shear algorithms havebeen used, and the shear angle was chosen by trial and error, asexplained before.

5. Discussion of the results

5.1. Arme~na cross-section

The sequential restoration of the Arme~na cross-section has beencarried out in the cross-section of Fig. 5d-II. This restoration has asignificant limitation: the chronostratigraphy of the Cotiella Basinis not well known and the correlations between sub-basins are notwell constrained. The strata of the Cotiella sub-basins are onlycorrelated with those of the Arme~na and Mediodía sub-basinsbased on the main stratigraphic features to show that the inter-preted geometries are possible, but their relationships throughtime are not fully constrained. Seven stages have been considered(Fig. 9):

1) Stage I depicts the development of post-rift carbonate platformsabove salt. The slight thickness changes observed in those car-bonate platforms result from the analysis of our field data andare indicative of an incipient salt inflation at early stages ofdevelopment.

2) Stage II reveals the formation of gravity-driven extensionalfaults above salt, leading to the formation of rollovers.

3) Stage III considers that, after the deposition of the first layer ofsyn-extensional sediments (unit 2 in Fig. 9), the salt rollers inthe footwall of the early listric faults pierced the topographicsurface to become passive diapirs. The rising of the salt wascontrolled by the differential subsidence of sub-basins. Units 3and 4 in Fig. 9 are thought to be equivalent to the �Esera depo-center in the Cotiella sub-basin.

Fig. 8. a) Sketch showing the problems of the unfolding vertical/shear algorithm to unfold vertical or overturned layers. b) Method used to restore vertical and overturned layerswhen the existing algorithms are inadequate: first the uppermost bed has been unfolded to a datum tilted basinwards and, afterwards, the underlying beds have been consecutivelyunfolded and rotated considering that the datum is the base of the preceding layer. 10 restoration steps (I to X) are depicted and the shear vectors used in each step are indicated.


4) Stage IV depicts ongoing passive diapirism.5) Stage V considers that the passive diapirs of stages III and IV

were covered by younger sediments, which simultaneously fil-led up the deepest part of the Cotiella sub-basin. The base of unit5, accordingly, depicts the Aguasalenz_Fo unconformitydescribed above. The end of passive diapirism was probablyrelated to the formation of primary salt welds.

6) Stages VI and VII depict ongoing extension and the filling of theCotiella sub-basin mostly by units 5 and 6. These units areequivalent to Las Neis depocenter which, as interpreted above,post-dates the passive diapirs.

This interpretation helps to explain the development of thestructures developed in the footwall of the Arme~na fault or relatedLavasar shortcut during shortening, as detailed below (Fig. 10).

The formation of the Reduno downward facing anticline (Fig. 10b)is difficult to understand because, as explained above, it generates aspace problem to restore the contractional structure. This problemcan be minimized if we consider that the Reduno structure wasdeveloped from a prior fold, which was cut, rotated and probablytransported during shortening. This fold resembles the core of theprior antiformal geometry depicted in the restored cross-section.The piece needed to restore the structure is probably found in the

Fig. 9. Sequential restoration showing the evolution of the Arme~na, Mediodía and Cotiella sub-basins. Seven main stages of evolution are represented: I) Formation of early normalfaults affecting post-rift upper Albian-middle Coniacian carbonate platforms above salt; II) Formation of early rollovers and salt rollers; III) Onset of passive diapirism; IV) Ongoingpassive diapirism and extension; V) Diapirs are covered but extension continues; VI) Ongoing extension; VII) End of extension. This restoration has been done following themethodology exposed in this paper. Stratigraphic units are numbered and colored according to their relative age (colors do not depict lithologies). The bathymetric relief is slightlyexaggerated. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).



Baranetas shortcut, a thrust system anomalously cropping out inthe footwall of the Mediodía fault (Fig. 10d).

The Punta Lierga fold system has been interpreted as the footwallsyncline of the Arme~na inverted fault or of the Lavasar thrust(represented by the Piedra Blanca slice in Fig. 10c). However, the

Fig. 10. a) Geometry of the Cotiella, Arme~na and Mediodía sub-basins at the end of thedownwards-facing anticline. c) Sketch explaining the development of the Punta Lierga synlationships between the Reduno anticline, the Baranetas shortcut and the Piedra BlancaInterpreted photograph of the Cotiella and Arme~na sub-basins showing the structural relatArme~na fault. The Raimond d’Espouy peak is located to simplify correlation between photo

fact that the Punta Lierga syncline is located in the same structuralposition as the layers covering the inferred diapir suggests that thePunta Lierga syncline could have been developed due to the dif-ferential folding of the layers. As such, the thickness differencesassociated with the diapir could have caused the folds to be

extensional event (see Fig. 8). b) Sketch explaining the development of the Redunocline. d) Interpreted photograph of the Cotiella sub-basins showing the structural re-slice (which is potentially the southeastern continuation of the Lavasar shortcut). e)ionships between the Punta Lierga syncline, the Aguasalenz_Fo unconformity and thegraphs.

Fig. 11. Sequential restorations of the Seira and Cotiella sub-basins. I Formation of early normal faults affecting post-rift carbonate platforms above salt; II Formation of earlyrollovers and salt rollers; III Onset of passive diapirism; IV Ongoing passive diapirism and extension, until salt welds are created; V Falling of the previous diapir; VI Ongoing fallingdiapir; VII End of extension and diapirism. The location of a basement piece that was incorporated into the subsequent thrust system as a horse is depicted. This restoration hasbeen done following the methodology exposed in this paper. Stratigraphic units are numbered and colored according to their relative age (colors do not depict lithologies). Thebathymetric relief is slightly exaggerated.



preferentially developed in the strata covering the diapirs ratherthan in the strata flanking them (because the overburden is thinnerand, consequently, easier to fold). This explains strong lateral var-iations of this fold system as well as the huge amount of fanningobserved in the Raimond d’Espouy area (Fig. 10e).

5.2. Seira cross-section

The sequential restoration of the Seira cross-section has beencarried out in the cross-section of Fig. 6d. As in the case of theArme~na cross-section, the lack of a stratigraphic correlation be-tween sub-basins has complicated the sequential restorations.

The structure observed today may suggest that the Cotiella,Northern Seira and Southern Seira sub-basins were synchronous.However, if we consider salt upwelling due to the sinking NorthernSeira and Cotiella sub-basins (Fig. 11), combined with the saltexpelled from the �Esera depocenter, the development of theSouthern Seira sub-basin simultaneously to the Cotiella andNorthern Seira ones is not possible. Consequently, the SouthernSeira sub-basin is thought to be younger than the Cotiella andNorthern Seira sub-basins, as supported by the absence of upperAlbian-middle Coniacian sediments at the base of the succession.

The sequential restoration presented in Fig. 11 depicts the evo-lution of the Cotiella, Northern Seira and Southern Seira sub-basinsfrom late Albian to early Santonian time.

1) Stage I is governed by post-rift carbonate platforms above salt,showing slight thickness changes related to incipient saltinflation at early stages of development.

2) Stage II depicts the formation of early gravity-driven extensionalfaults and rollover anticlines. In this case, the footwall of the

Fig. 12. Summary diagrams showing the evolution of the Cotiella Basin in cross-sectional a�Esera depocenter (I) and Las Neis depocenter areas (II). b Summary diagrams depicting thewere covered in Las Neis area (II). Both stages roughly coincide with stages III and IV of Fig.fault depicted in the maps coincides with the Cotiella thrust in Fig. 1b. The location of the resfaults and/or salt structures. Blue lines depict the structure contours of a horizon. (For interweb version of this article).

early Seira fault shows evidence of diapirism with the devel-opment of calcarenitic lenses (Fig. 1), indicating that the saltroller in the footwall of the Seira fault was already rising.

3) Stage III shows that the above-described salt roller finallypierced the topographic surface to become a passive diapir,which controlled the development of the Cotiella and NorthernSeira sub-basins. These sub-basins persisted during the depo-sition of units 3 and 4, and were possibly synchronous to theneighboring �Esera depocenter.

4) At stage IV, when the bottom of these sub-basins created salt-welds, the diapir stopped rising. The formation of a primarysalt-weld at the base of the Northern Seira sub-basin is consis-tent with the steps depicted in the basement. Additionally, theedges of the basement blocks could help to clarify some base-ment exposures that are anomalously cropping out in the �EseraValley and in Las Neis Valley (Fig. 1) along the Cotiella thrust, byincorporating these interpreted basement edges into the thrustsystem as horses during tectonic inversion (Fig. 11-VII).

5) Stages V, VI and VII depict the development of the SouthernSeira sub-basin as a result of the falling of the previous diapir.This interpretation implies that the calcarenites observed at thebase of the Southern Seira sub-basin are not time-equivalent tothose in the Cotiella and Northern Seira sub-basins, althoughtheir facies are very similar (Fig. 2b). The fact that the pre-extensional carbonates are missing at the bottom of theSouthern Seira sub-basin supports the view that the develop-ment of this sub-basinwas the result of a falling diapir (the samediapir that initially bounded the Cotiella and Northern Seirasub-basins). Accordingly, the development of the Southern Seirasub-basin potentially postdated the main stage of developmentof the �Esera depocenter and consequently it was synchronous to

nd in map views. a Summary diagrams depicting the onset of passive diapirism in theend of passive diapiris, which became a falling diapir in the �Esera area (I) whereas they3, respectivelly. c Map view of stage III. d Map view of Stage IV. The Cotiella extensionaltored cross-sections is depicted in the maps. Dashed lines and gray colors depict buriedpretation of the references to colour in this figure legend, the reader is referred to the


the development of the western Las Neis depocenter and to thecovering of the Arme~na and Mediodía diapirs.

6. Conclusions

This paper presents sequential restorations of an extensionalsalt basin that was slightly deformed during subsequent tectonicinversion. The contractional structures have been restored byflexural slip, considering longitudinal deformation during basininversion; whereas the extensional salt-related features have beenrestored by vertical or simple shear, considering that bed-areas andbed-shape remain constant. Manual modifications have beennecessary in most of the steps to avoid problems derived fromoverturned or steep dip attitudes. Although using a semi-manualmethodology has been awkward and time-consuming, the ob-tained results provide new insight to better constrain the structuralevolution of this complex area and support the interpretation thatthe Cotiella Basin was developed by a combination of gravity-driven extension and passive diapirism.

The most relevant conclusions regarding the evolution of theCotiella Basin achieved with the sequential restorations are sum-marized below (Fig. 12):

- The development of the �Esera depocenter (Fig. 12aeI) syn-chronously with the rising of the Arme~na, Mediodía and Seirapassive diapirs is corroborated by sequential restorations(Fig. 12a-II).

- The Southern Seira sub-basin was potentially formed as a resultof the falling of the Seira diapir which was probably triggered bythe formation of a primary salt-weld at the base of the NorthernSeira sub-basin (Fig. 12beI).

- The formation of primary salt-welds at the base of the �Eseradepocenter (Fig. 12beI) potentially controlled salt expulsionwestwards, themigration of the thickest sedimentary interval inthe Cotiella sub-basin and the formation of Las Neis depocentersynchronously with the burial of the Arme~na and Mediodíadiapirs (Fig. 12b-II).

- The fact that the Seira diapir fell whereas the Arme~na andMediodía diapirs were covered (Fig.12c and d) indicates that theamount of salt was probably greater in the Seira diapir than inthe Arme~na and Mediodía diapirs. This could be related to theproximity of the Seira diapir to the �Esera depocenter and to thefeeding of salt from �Esera to Seira areas.

- The formation of primary salt-welds explains the basementpieces found at the base of the Cotiella thrust sheet, which wereprobably incorporated into the thrust system during tectonicinversion.

- These results, additionally, constrain a stratigraphic correlationbetween the carbonates of the Cotiella sub-basin with the onesin the northern Arme~na, Mediodía and Seira sub-basins, andsuggest that the Southern and Northern Seira sub-basins werenot synchronous.

Acknowledgments

The research presented has been made possible by fundingfrom Statoil. The explained methodology was settled with thehelp of Martin Jackson and Mike Hudec. This work has alsobenefited from discussions with Kate Giles and Mark Rowan. Theauthors also wish to thank Clare Bond and Ian Davison for thehelpful reviews. The authors acknowledge support from theMinisterio de Ciencia e Innovaci�on (Proyecto INTECTOSAL,CGL2010-21968-C02-01) and the Generalitat de Catalunya (Grupde Recerca de Geodin�amica i An�alisi de Conques 2009SGR1198).Research by B. L�opez-Mir was funded by the predoctoral grant

Formaci�on del Profesorado Universitario from the Ministerio deEducaci�on, Cultura y Deporte. Midland Valley is thanked forproviding the Move™ software for the construction and restora-tion of cross-sections. This is Cambridge Earth Sciences contri-bution esc.3088.

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