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Restoration of basins driven by extension and salt tectonics: Example from the Cotiella Basin in the central Pyrenees
Journal of Structural Geology 69 (2014) 147e162
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Restoration of basins driven by extension and salt tectonics: Example from the Cotiella Basin in the central Pyrenees ~ oz b, Jesús García Senz c Berta Lopez-Mir a, b, *, Josep Anton Mun a
CASP, West Building, 181A Huntingdon Road, Cambridge CB3 0DH, United Kingdom ria de Pedralbes, 08028 Barcelona, Spain Institut de Recerca Geomodels, Facultat de Geologia, Universitat de Barcelona, Zona universita c gico y Minero de Espan ~ a (IGME), La Calera 1, 28760 Tres Cantos, Madrid, Spain Instituto Geolo b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2014 Received in revised form 12 September 2014 Accepted 28 September 2014 Available online 18 October 2014
The structure of the Cotiella Basin consists of four well differentiated salt-floored sub-basins which contain more than 6 km of halokinetic growth-strata and have been interpreted to be formed by gravitydriven extensional faults and diapirism during the middle Coniacian e early Santonian. The subsequent Pyrenean orogeny (late Santonian to Miocene) inverted the extensional basins and squeezed the diapirs leaving little trace of the original salt volume, although both the extensional and the salt-related features of the Cotiella Basin were preserved. This paper provides new insights into the evolution of the Cotiella Basin by establishing a correlation between sub-basins by means of structural restorations. The workflow followed and the assumptions made during the restoration of the inversion and especially of the extensional structures are outlined in detail. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Sequential restorations Salt tectonics Passive diapirism Pyrenees Cotiella basin
1. Introduction The first application of structural restoration concepts was provided by Chamberlin (1910) to determine the depth of the detachment level underlying concentric folds. However, it was not until the late 60's that the construction bed-length balanced cross-sections of the Canadian Rockies (Bally et al., 1966; Dahlstrom, 1969, 1970) demonstrated the importance of balancing to predict the geometry of the structures at depth and to validate structural interpretations. Structural balancing studies continued, particularly in contractional regions (e.g. Hossack, 1979; Boyer and Elliot, 1982), resulting in the development of kinematic structural models such as the fault-bend and fault-propagation folds (e.g. Suppe, 1983; Suppe and Medwedeff, 1990). In extensional settings, the balancing of crosssections was also applied to interpret seismic images (e.g. Gibbs, 1983) and to predict the geometry and kinematics of extensional faults (e.g. White et al., 1986; Williams and Vann, 1987; Groshong, 1990). Further research on structural balancing focused on the application of numerical (e.g. Erslev, 1991) and geomechanical
models (e.g. Maerten, 2007) to simulate and understand the evolution of geological structures (i.e. Groshong et al., 2012). In salt tectonics settings the restoration of cross-sections is also crucial to formulate models of structural evolution, but it can be more problematic. First, the constraints to restore salt-related structures are not simple because the mobility of the salt can generate uncertainties as a result of lateral salt expulsion and related vertical rotations. Second, the existing algorithms cannot be applied straightforwardly as the fundamental assumptions such as plane-strain deformation and area conservation are often invalid for cross-sections containing salt (i.e. Rowan and Ratiff, 2012). This paper presents sequential restorations of the partially inverted Cotiella salt basin in the central Pyrenees. Sequential restorations are used to validate, constrain and improve models of structural evolution. Besides the establishment of a model for the evolution of the Cotiella Basin, the workflow followed and the assumptions made during the restoration process are outlined in detail.
2. Geological setting 2.1. The central Pyrenees * Corresponding author. Present address: CASP, West Building, 181A Huntingdon Road, Cambridge, CB3 0DH, United Kingdom. Tel.: þ44 1223768290. E-mail addresses: [email protected] (B. Lopez-Mir), jamunoz@ub. ~ oz), [email protected] (J. García Senz). edu (J. Anton Mun http://dx.doi.org/10.1016/j.jsg.2014.09.022 0191-8141/© 2014 Elsevier Ltd. All rights reserved.
The Pyrenees is a Late Cretaceous to Miocene doubly-vergent collisional orogen which was developed from the inversion of previous Mesozoic extensional basins (ECORS-Pyrenees Team,
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~ oz, 2002). The location of the map below is depicted in red. b) Geological map of Fig. 1. a) Structural sketch showing the main structural units of the Pyrenees (modified from Mun pez-Mir, 2013), except for the footwall of the the Cotiella thrust sheet, indicating the main structural features and the location of Figs. 2 and 3b. This map results from our data (Lo Cotiella 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 and Ramírez, 1997). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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~ oz, 1992; Teixell, 1998). In its south-central portions, the 1988; Mun Alpine structure consists of a southwards-directed imbricate stack guret, of thrust sheets of Mesozoic and Paleogene cover rocks (Se ~ oz, 2002). From top (north) to 1972; Garrido-Megías, 1973; Mun bottom (south), these thrust sheets are: Cotiella (emplaced during ~ a Montan ~ esa the late Santonian - late Maastrichtian), Montsec-Pen (emplaced during the Early Eocene) and Gavarnie (emplaced during the Middle Eocene to Early Oligocene). The Cotiella thrust sheet developed from the positive inversion of a previous Upper Cretaceous extensional basin and was subsequently transported piggy-back at least 18 km southwards by the ~ a Montan ~ esa thrust (García-Senz, 2002; younger Montsec e Pen McClay et al., 2004). Synchronously and afterwards, it was tilted southwards and uplifted by more than 3 km by underthrusting of the younger Gavarnie and other basement-involved thrust sheets.
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As a result, the thick Upper Cretaceous succession of the Cotiella extensional 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 four seismic-scale middle Coniacian to early Santonian sub-basins: ~ a, Mediodía, and Seira (Fig. 1b). The Cotiella subCotiella, Armen basin is the largest one and consists of a broad anticline characterized by growth-strata showing a thickness expansion of up to 6 km towards a partially inverted extensional fault (Fig. 2a). To the ~ a, Mediodía and Seira subnorth, there are the smaller Armen basins, which are filled by sedimentary wedges fanning as much
Fig. 2. a) Structural cross-section across the central portions of the Cotiella thrust sheet. b) Structural cross-section across the eastern portions of the Cotiella thrust sheet. c) ~ a ridge. See Fig. 1 and cross-section 2a for location. Structural features and stratigraphic units are labeled according to the legend of Fig. 1b. Interpreted photograph of the Armen
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as 3.3 km towards extensional listric faults (Fig. 2). The synkinematic sediments consist of middle Coniacian to lower Santo~ os Fornian carbonates and calcarenites (Aguasalenz and Macin mations, respectivelly) which overlie a pre-kinematic succession made up of upper Albian to lower Coniacian post-rift carbonates guret, 1972; detached on Upper Triassic salts (Souquet, 1967; Se McClay et al., 2004). The equivalent Coniacian to lower Santonian sedimentary succession in the Cotiella fault is only 300 m thick (García-Senz, 2002; McClay et al., 2004), as happens in most of the Pyrenees. The Cotiella Basin has been interpreted to be developed by postrift gravity-driven extension above relict Upper Triassic salts in the Bay of Biscay-Pyrenean paleorift margin of the Atlantic Ocean (McClay et al., 2004). However, particular structural and sedimentary features depart from the classical models of extensional listric pez-Mir, 2013). These features and an alternative fault systems (Lo model of evolution for the Cotiella Basin are summarized below. Our model refines the previous gravity-driven extension model by the incorporation of passive diapirs isolating the sub-basins (Fig. 3) and questions the existence of a hard-link connection between subbasins as proposed by of McClay et al. (2004). One of the most peculiar salt-related features can be observed in ~ a fault (Fig. 2c), where shallow-water the hangingwall of the Armen ~ os Formation are accumulated next to the calcarenites of the Macin fault (in the theoretically structural low of the rollover). This unexpected water-depth distribution of the sediments is not adequately explained by the previously interpreted scenario of
extensional gravity-driven faults (García-Senz, 2002; McClay et al., 2004). It can be alternatively interpreted to have been developed as salt diapir-related sandstone depocenters similar to the ones described by Aschoff and Giles (2005) in La Popa basin in Mexico. The accumulation of calcarenites in this structural position, in fact, ~ a fault being supis consistent with the hangingwall of the Armen ported by salt, preventing the development of a shale-dominated ~ a sub-basin (to the basin. Additionally, at the base of the Armen north), an exceptionally large Upper Triassic salt outcrop (Fig. 1b) shows an abrupt lithological contact between the top of the salt and the overlying sediments, supporting the presence of a diapir during ~ os calcarenite Formation, as shown in the deposition of the Macin the model of Fig. 3aeI. ~ os FormaIn the Seira sub-basins, the calcarenites of the Macin 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 the ~ os Formation contain clasts sourced from the Upper Triassic Macin salt layer, which supports the model of emergent salt diapirs controlling their deposition. Regarding the largest Cotiella sub-basin, two main depocenters have been interpreted: the older and western Esera depocenter; and the younger and eastern Las Neis depocenter (Fig. 3b). These are separated by the transverse Esera transfer fault. The Aguasalenz_Fo unconformity covers both the transfer fault and the calcarenitic deposits of the Cotiella sub-basin, as can be observed in map-view
Fig. 3. a) Sketched geological model showing the main stages of evolution of the Cotiella Basin from the late middle Coniacian to the late Santonian. I) Passive diapirs are coeval with gravity-driven extension above salt; II) Extension continues and diapirs are sealed; III) Tectonic inversion and squeezing of the diapirs. b) Conceptual sketch showing the along-strike geometry of the Cotiella sub-basin at the end of the early Santonian (stage II of model above), depicting the relative chronology between the Esera and Las Neis depocenters as well as the AS_Fo unconformity and the Esera fault. The location of the cross-section is shown in Fig. 1. The intersections with Fig. 2a, b and with the model above are pez-Mir (2013). depicted. Both diagrams are modified from.Lo
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(Fig. 1b). Assuming that the deposition of the calcarenites was synchronous with the rising of diapirs, the Aguasalenz_Fo unconformity is interpreted to overlie the diapirs and related northern sub-basins (Fig. 3a-II). This unconformity crops out very nicely in the Raymond ~ a fault, where the stratd’Espouy peak, in the footwall of the Armen igraphic succession displays a spectacular fanning of more than 90 (Fig. 2a). Such a fanning is difficult to create by gravity-driven extension, but is better explained by the combination of salt tectonics with extension (although it could be partly accentuated by shortening at high angles to bedding during tectonic inversion). The migration of the thickest sedimentary interval towards the west in the Cotiella sub-basin observed in Fig. 3b is also interpreted to be related to salt inflation related to the salt displaced by the sinking depocenters. In the late Santonian, as a result of Pyrenean contraction, salt was expulsed out to the surface and then dissolved, leaving little trace of the original salt volume. Thrust-welds (which coincide with the faults observed today) were potentially created by squeezing of previous salt walls and the different sub-basins ended up attached to one another (Fig. 3a-III). Thanks to salt expulsion, the internal ~ a, Mediodía and Seira sub-basins was scarcely structure of the Armen
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deformed and the current geometry is probably similar to the original one. By contrast, the geometry of the Cotiella Basin was more severely ~ a fault for instance, strata were modified. In the footwall of the Armen steepened and overturned, resulting in the development of complex structures such as the Reduno downward-facing anticline (Fig. 2a). In areas more distant to the diapirs, such as the westernmost portions of the Cotiella Basin, tectonic inversion caused the development of folds such as the Vaquerizal anticline (Fig. 2a). 3. Restoration of the contractional structures To understand the evolution of the extensional salt-related structures, sequential restorations of the extensional faults were constructed. Before this could be done, the effects of shortening had to be removed. The contractional structures have only been partially restored in order to decipher the geometry of the Cotiella Basin before its positive inversion. Two main operations have been ~ a Montan ~ esa, Gavarnie considered: (1) the restoration of the Pen and basement-involved Axial Zone thrust sheets (Figs. (4) and (2)) the restoration of the late Santonian inversion (Figs. 5 and 6).
~ a Montan ~ esa thrust sheets in the Armen ~ a (a) and Seira (b) crossFig. 4. Workflow to restore the basement-involved thrust sheets of the Axial Zone, and the Gavarnie and Pen sections at late Santonian times, once the Cotiella basins were inverted. Four steps of restoration are depicted: I) Present day geometry; II) Geometry after removing the Axial ~ a-Montan ~ esa Zone thrust sheets; III) Geometry after removing the detachment fold of the Gavarnie thrust sheet; IV) Geometry after removing a low-angle footwall ramp of the Pen thrust sheet. The pin line, the template beds and the datum used in each step of restoration are indicated. The colors of 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).
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~ a cross-section. Four steps of restoration are depicted. a) Geometry of the Cotiella basin in the Fig. 5. Workflow to restore the overturned strata of the Reduno structure in the Armen late 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 uppermost stratigraphic 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 of ~ a and Mediodía sub-basins to the position before tectonic inversion. Two solutions are the strata considering line-length and bed-shape constant. c) Translate the Cotiella, Armen proposed 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. ~ a and Mediodía sub-basins, and interpretation of the basement geometry. Two solutions are proposed depending on the timing d) Interpretation of the eroded section in the Armen of primary welding and sealing of the passive diapirs (I and II).
~ a Montan ~ esa, Gavarnie and basement-involved thrust 3.1. Pen sheets ~ a Montan ~ esa, Gavarnie and basementThe restoration of the Pen involved thrust sheets aims to remove the folding and tilting which was acquired during their emplacement. The total transport towards the south has not been considered. This restoration has been carried out in a very simple manner because the internal structure of the Cotiella Basin was scarcely deformed. The flexural slip unfolding algorithm (Davison, 1986) that is implemented in the
Move™ software has been used, assuming longitudinal contraction during basin inversion. This method uses a pin perpendicular to, and a slip-system parallel to the template bed to control the unfolding of the beds. It maintains bed thickness variations, and both the template bed and slip-system are unfolded. Three operations have been necessary (Fig. 4): I) The tilting acquired during the emplacement of the Axial Zone basement-involved thrust sheets has been removed using the uppermost layer of the Axial Zone thrust sheets
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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 the Cotiella 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 Cotiella and 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 colors of 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 parallel to the Gavarnie thrust) as the template bed. The pin line has been inserted in the southern edge of the cross-sections as a ~a line perpendicular to the beds in the case of the Armen cross-section (Fig. 4aeI) and as the axial trace of the southernmost syncline in the case of the Seira cross-section (Fig. 4beI). The line-lengths of the rest of the stratigraphic horizons have been checked quantitatively. II) The (not very pronounced) detachment anticline observed in the Gavarnie thrust sheet has subsequently been restored. In ~ a cross-section (Fig. 4a-II), this fold has been the Armen removed using the top of the Early Eocene succession as the template bed. In the case of the Seira cross-section (Fig. 4bII), the top of the pre-extensional Upper Cretaceous succession has been used as the template. In both cases, the pin line has been inserted in the southern edge of the cross-sections,
~ a Montan ~ esa folds as a line perpendicular to the beds. The Pen observed in the southern parts of the cross-section have not been removed because their restoration does not alter the geometry of the Cotiella Basin. The geometrical changes between steps II to III are not very pronounced as the angle between the template bed and the datum is very small. The most noticeable change can be appreciated in Fig. 4a-III where the layers at the base of the Gavarnie thrust sheet are completely flat-laying. ~ a-Montan ~ esa thrust observed at III) The footwall ramp of the Pen the northern portions of Fig. 4a-III and 4b-III (which generates a slight tilting towards the north in the Cotiella Basin) ~ a Montan ~ esa thrust as the has been restored using the Pen template bed. The pin line has been inserted in the southern edge of the cross-sections, as a line perpendicular to the ~ a-Montan ~ esa thrust beds. As the footwall ramp of the Pen
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cuts the layers of its footwall at a very low angle, the changes between steps III to IV in Fig. 4 are very gentle. The most noticeable change is that the bottom of the Cotiella basin has become flat in the northern portions of the restored crosssections in step IV. The latter shows how the geometry of the Cotiella Basin looked like immediately before tectonic inversion at the late Santonian (Fig. 4a-IV and 4b-IV). 3.2. Late Santonian inversion The primary objective of restoring the late Santonian inversion is to discover what the geometry of Cotiella Basin looked like at the end of the extensional salt tectonics event (Figs. 5 and 6). This operation needs to be performed more carefully than the preceding one because it can introduce erroneous geometries, especially in the Cotiella sub-basin where the layers were more severely deformed during shortening. As a result, this step has been performed in a semi-manual manner, using the existing algorithms when possible and manually modifying the results when necessary. The assumptions made are explained as follows. Each sub-basin has been treated separately and four operations have been considered (Figs. 5 and 6): 1) Assuming that the overturned attitude of strata was acquired during tectonic inversion, the first operation comprised rotating the layers of the Cotiella Basin until all of them present a normal stratigraphic polarity. This operation has been approached in a manual manner, considering that bed-areas and bed-lengths remain approximately constant over time, as explained in Fig. 5b. Slight differences in the length or the area of the beds have been accepted because the strata could have thinned by layer-normal shortening during contractional deformation as some of the beds of the Cotiella Basin were steeply dipping before contraction started. 2) Once the overturned attitude of strata has been removed, the sub-basins have been translated to allow the salt reaching the surface as deduced by the sedimentological features above described (Fig. 5c). This assumes that during the extensional salt tectonics event the sub-basins were separated by salt walls and diapirs, as suggested by facies distribution and lithological characteristics, the spaces between sub-basins have been considered to be filled with salt and the diapirs have been depicted as narrow as possible. 3) The eroded parts of the section and the basement geometry have been interpreted manually (Figs. 5d and 6d). The basement geometry is inspired by cross-sections constructed in other areas of the Pyrenees, such as the Parentis basin (e.g. Ferrer et al., 2012), where the basement geometry is interpreted from the ECORS-Bay of Biscay and MARCONI-3 seismic profiles. The interpreted basement geometry consists of tilted blocks which dip southwards or are near-horizontal and are separated by northward-dipping planar extensional faults. These faults are decoupled from the sedimentary cover by a thick package of Upper Triassic evaporites and probably influenced the thickness changes of the Upper Triassic. This inferred geometry helps to explain the differences in thickness between the southern (thinner) and the northern (thicker) sub-basins and, additionally, it is consistent with the sequential restoration presented for the ~ a and Seira cross-sections (in the following pages). Armen 3.3. Limitations and assumptions ~ a cross-section (Fig. 2a), the first operation In the Armen (removing the overturned attitude of the layers) was problematic
(Fig. 5b). To keep the bed-length of the Reduno downward-facing anticline invariable due to the high cut-off angles of the beds with the faults; slight changes in the thickness of the beds have been accepted assuming that layers could have thinned by layernormal shortening during contractional deformation. The main ~ a fault if it problem lies in the change in the geometry of the Armen was in contact with the sediments in its footwall. This problem, however, supports our observations of thrust-weld characteristic of the presently observed fault and the possible existence of a diapir ~ a sub-basins. between the Cotiella and Armen The second operation (translating the sub-basins and filling the gaps with salt) has also been problematic in the Reduno area (Fig. 5c). Considering that the lowermost layers of the Cotiella subbasins show evidence of diapirism (such as the outcrop of Upper Triassic in the Raymond d’Espouy peak, Fig. 2c), the Cotiella and ~ a sub-basins were potentially separated by a diapir in the Armen early stages of development. If we then merely translate the ~ a sub-basin northward to allow the salt to reach the surface, Armen the resulting diapir geometry shows a landward tilted geometry, which is unlikely in extensional settings. A geometrical solution for this problem is to project the Lavasar shortcut (observed westwards, Fig. 1) above the topography of the section, in order to allow the beds to be longer and, as a result, the northern wall of the diapir to be vertical. This interpretation leads to two possible solutions (Fig. 5). The first (Fig. 5ceI) interprets a long-lived passive diapir which persisted during most of the extensional salt tectonics event. The second (Fig. 5c-II) considers that sediments covered the passive ~ os Fordiapir after the deposition of the calcarenites of the Macin mation. The second solution has been chosen to perform the sequential restorations because, as will be explained later, it better helps to explain the development of the Reduno downwards-facing anticline (Fig. 2). The contractional structures in the Seira cross-section were not so difficult to restore, as the amount of overturning is not as pro~ a cross-section (Fig. 6). To simplify the nounced as in the Armen restoration operations, the Chía klippe (Fig. 1) has been considered in continuity with the Cotiella Basin, although this relationship is actually unknown.
4. Methodology to restore the extensional structures 4.1. Workflow The workflow used to perform sequential restorations is grounded in the methods proposed by Schultz-Ela (1992) and by Rowan (1993) to restore ductile structures and salt structures in extensional terrains, respectively. These methods are more adequate 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 basement in order to keep the salt area constant. In this work, the Schultz-Ela (1992) method has been carried out separately for each sub-basin and steps have been performed in the order that we explain below (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 algorithm have been softened manually. Performing the decompaction at the beginning of the restoration (instead of doing it at the end, as suggested by Schultz-Ela, 1992) costs less computational time
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consequently the different sub-basins could have experienced different strain histories during their development. Second, the existing methods to choose the shear angle in extensional settings (i.e.: White et al., 1986) cannot be applied to restore the extension in the Cotiella Basin as the current geometry was deformed during shortening. 5) The sub-basins have been translated and gathered (Fig. 7f) assuming that diapirs widen during extension and narrow during shortening (although their height can decrease during extension). 6) The top of the basement has finally been moved (Fig. 7g), considering that the volume of the salt is constant or decreases over time (although this assumption may not be fully correct because salt can migrate laterally). Additionally, the top of the restored sections can be adapted to the interpreted bathymetry, using the Vertical Shear Unfolding to better depict how the water-depths were likely to be.
4.2. Limitations This methodology presents a limitation when steep or vertical layers exist because the simple or vertical shear algorithms do not give satisfactory results (Fig. 8a). The main problem lies in the shear angle. Applying a vertical shear algorithm to shallow-dipping layers considers that shear is perpendicular to the beds. Contrastingly, if the same shear vector is applied to steep layers, the shear becomes almost parallel to the layers and the algorithm is invalid. To solve this problem, the restoration of steep layers has been approached in a semi-manual manner, rotating and unfolding the beds individually (Fig. 8b). In some cases, manual modifications have been necessary to keep bed-areas and bed-geometries constant. To unfold the layers, vertical or simple shear algorithms have been used, and the shear angle was chosen by trial and error, as explained before. 5. Discussion of the results ~ a cross-section 5.1. Armen
Fig. 7. Method proposed by Schultz-Ela (1992) to perform sequential restorations in extensional terrains which has inspired the methodology presented in this paper. Six main steps of restoration have been considered. a) Original geometry. b) Geometry after stripping a layer. c) Decompaction. d) Rotation applied to the layers after unfolding. 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 the salt area constant. Figure modified from Schultz-Ela (1992).
because the Move™ software is much more stable in the following steps. 3) Afterwards, a rigid body rotation (Fig. 7d) has been performed semi-manually, to make the uppermost layers as parallel to a sub-horizontal datum as possible. 4) Rotation has been followed by unfolding of the layers (Fig. 7e). Hangingwall extension and salt deflation on listric faults produce strong bed rotation and diffuse deformation which is best modeled as block translation and collapse by inclined shear. A line with a slight tilting basinwards has been selected as the datum. The shear angle has been chosen by trial and error, by finding the angle that provides the least variations in the area and shape of the layers. The chosen angles vary between 40 and 70 , and consider both antithetic and synthetic shear. We have accepted great amount of variation for two main reasons. First, the Cotiella Basin was developed in a salt tectonics setting and
~ a cross-section has been The sequential restoration of the Armen carried out in the cross-section of Fig. 5d-II. This restoration has a significant limitation: the chronostratigraphy of the Cotiella Basin is not well known and the correlations between sub-basins are not well constrained. The strata of the Cotiella sub-basins are only ~ a and Mediodía sub-basins correlated with those of the Armen based on the main stratigraphic features to show that the interpreted geometries are possible, but their relationships through time are not fully constrained. Seven stages have been considered (Fig. 9): 1) Stage I depicts the development of post-rift carbonate platforms above salt. The slight thickness changes observed in those carbonate platforms result from the analysis of our field data and are indicative of an incipient salt inflation at early stages of development. 2) Stage II reveals the formation of gravity-driven extensional faults above salt, leading to the formation of rollovers. 3) Stage III considers that, after the deposition of the first layer of syn-extensional sediments (unit 2 in Fig. 9), the salt rollers in the footwall of the early listric faults pierced the topographic surface to become passive diapirs. The rising of the salt was controlled by the differential subsidence of sub-basins. Units 3 and 4 in Fig. 9 are thought to be equivalent to the Esera depocenter in the Cotiella sub-basin.
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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 layers when the existing algorithms are inadequate: first the uppermost bed has been unfolded to a datum tilted basinwards and, afterwards, the underlying beds have been consecutively unfolded 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 filled up the deepest part of the Cotiella sub-basin. The base of unit 5, accordingly, depicts the Aguasalenz_Fo unconformity described above. The end of passive diapirism was probably related to the formation of primary salt welds. 6) Stages VI and VII depict ongoing extension and the filling of the Cotiella sub-basin mostly by units 5 and 6. These units are equivalent to Las Neis depocenter which, as interpreted above, post-dates the passive diapirs.
This interpretation helps to explain the development of the ~ a fault or related structures developed in the footwall of the Armen Lavasar 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 a space problem to restore the contractional structure. This problem can be minimized if we consider that the Reduno structure was developed from a prior fold, which was cut, rotated and probably transported during shortening. This fold resembles the core of the prior antiformal geometry depicted in the restored cross-section. The piece needed to restore the structure is probably found in the
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~ a, Mediodía and Cotiella sub-basins. Seven main stages of evolution are represented: I) Formation of early normal Fig. 9. Sequential restoration showing the evolution of the Armen faults 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) Ongoing passive diapirism and extension; V) Diapirs are covered but extension continues; VI) Ongoing extension; VII) End of extension. This restoration has been done following the methodology exposed in this paper. Stratigraphic units are numbered and colored according to their relative age (colors do not depict lithologies). The bathymetric relief is slightly exaggerated. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Baranetas shortcut, a thrust system anomalously cropping out in the footwall of the Mediodía fault (Fig. 10d). The Punta Lierga fold system has been interpreted as the footwall ~ a inverted fault or of the Lavasar thrust syncline of the Armen (represented by the Piedra Blanca slice in Fig. 10c). However, the
fact that the Punta Lierga syncline is located in the same structural position as the layers covering the inferred diapir suggests that the Punta Lierga syncline could have been developed due to the differential folding of the layers. As such, the thickness differences associated with the diapir could have caused the folds to be
~ a and Mediodía sub-basins at the end of the extensional event (see Fig. 8). b) Sketch explaining the development of the Reduno Fig. 10. a) Geometry of the Cotiella, Armen downwards-facing anticline. c) Sketch explaining the development of the Punta Lierga syncline. d) Interpreted photograph of the Cotiella sub-basins showing the structural relationships between the Reduno anticline, the Baranetas shortcut and the Piedra Blanca slice (which is potentially the southeastern continuation of the Lavasar shortcut). e) ~ a sub-basins showing the structural relationships between the Punta Lierga syncline, the Aguasalenz_Fo unconformity and the Interpreted photograph of the Cotiella and Armen ~ a fault. The Raimond d’Espouy peak is located to simplify correlation between photographs. Armen
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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 early rollovers 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 falling diapir; 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 has been done following the methodology exposed in this paper. Stratigraphic units are numbered and colored according to their relative age (colors do not depict lithologies). The bathymetric relief is slightly exaggerated.
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preferentially developed in the strata covering the diapirs rather than in the strata flanking them (because the overburden is thinner and, consequently, easier to fold). This explains strong lateral variations of this fold system as well as the huge amount of fanning observed in the Raimond d’Espouy area (Fig. 10e). 5.2. Seira cross-section The sequential restoration of the Seira cross-section has been carried out in the cross-section of Fig. 6d. As in the case of the ~ a cross-section, the lack of a stratigraphic correlation beArmen 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 Northern Seira and Cotiella sub-basins (Fig. 11), combined with the salt expelled from the Esera depocenter, the development of the Southern Seira sub-basin simultaneously to the Cotiella and Northern Seira ones is not possible. Consequently, the Southern Seira sub-basin is thought to be younger than the Cotiella and Northern Seira sub-basins, as supported by the absence of upper Albian-middle Coniacian sediments at the base of the succession. The sequential restoration presented in Fig. 11 depicts the evolution of the Cotiella, Northern Seira and Southern Seira sub-basins from 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 salt inflation at early stages of development. 2) Stage II depicts the formation of early gravity-driven extensional faults and rollover anticlines. In this case, the footwall of the
early Seira fault shows evidence of diapirism with the development of calcarenitic lenses (Fig. 1), indicating that the salt roller in the footwall of the Seira fault was already rising. 3) Stage III shows that the above-described salt roller finally pierced the topographic surface to become a passive diapir, which controlled the development of the Cotiella and Northern Seira sub-basins. These sub-basins persisted during the deposition of units 3 and 4, and were possibly synchronous to the neighboring Esera depocenter. 4) At stage IV, when the bottom of these sub-basins created saltwelds, the diapir stopped rising. The formation of a primary salt-weld at the base of the Northern Seira sub-basin is consistent with the steps depicted in the basement. Additionally, the edges of the basement blocks could help to clarify some base ment exposures that are anomalously cropping out in the Esera Valley and in Las Neis Valley (Fig. 1) along the Cotiella thrust, by incorporating these interpreted basement edges into the thrust system as horses during tectonic inversion (Fig. 11-VII). 5) Stages V, VI and VII depict the development of the Southern Seira sub-basin as a result of the falling of the previous diapir. This interpretation implies that the calcarenites observed at the base of the Southern Seira sub-basin are not time-equivalent to those in the Cotiella and Northern Seira sub-basins, although their facies are very similar (Fig. 2b). The fact that the preextensional carbonates are missing at the bottom of the Southern Seira sub-basin supports the view that the development of this sub-basin was the result of a falling diapir (the same diapir that initially bounded the Cotiella and Northern Seira sub-basins). Accordingly, the development of the Southern Seira sub-basin potentially postdated the main stage of development of the Esera depocenter and consequently it was synchronous to
Fig. 12. Summary diagrams showing the evolution of the Cotiella Basin in cross-sectional and in map views. a Summary diagrams depicting the onset of passive diapirism in the Esera depocenter (I) and Las Neis depocenter areas (II). b Summary diagrams depicting the end of passive diapiris, which became a falling diapir in the Esera area (I) whereas they were covered in Las Neis area (II). Both stages roughly coincide with stages III and IV of Fig. 3, respectivelly. c Map view of stage III. d Map view of Stage IV. The Cotiella extensional fault depicted in the maps coincides with the Cotiella thrust in Fig. 1b. The location of the restored cross-sections is depicted in the maps. Dashed lines and gray colors depict buried faults and/or salt structures. Blue lines depict the structure contours of a horizon. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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the development of the western Las Neis depocenter and to the ~ a and Mediodía diapirs. covering of the Armen 6. Conclusions This paper presents sequential restorations of an extensional salt basin that was slightly deformed during subsequent tectonic inversion. The contractional structures have been restored by flexural slip, considering longitudinal deformation during basin inversion; whereas the extensional salt-related features have been restored by vertical or simple shear, considering that bed-areas and bed-shape remain constant. Manual modifications have been necessary in most of the steps to avoid problems derived from overturned or steep dip attitudes. Although using a semi-manual methodology has been awkward and time-consuming, the obtained results provide new insight to better constrain the structural evolution of this complex area and support the interpretation that the Cotiella Basin was developed by a combination of gravitydriven extension and passive diapirism. The most relevant conclusions regarding the evolution of the Cotiella Basin achieved with the sequential restorations are summarized below (Fig. 12): - The development of the Esera depocenter (Fig. 12aeI) syn~ a, Mediodía and Seira chronously with the rising of the Armen passive diapirs is corroborated by sequential restorations (Fig. 12a-II). - The Southern Seira sub-basin was potentially formed as a result of the falling of the Seira diapir which was probably triggered by the formation of a primary salt-weld at the base of the Northern Seira sub-basin (Fig. 12beI). - The formation of primary salt-welds at the base of the Esera depocenter (Fig. 12beI) potentially controlled salt expulsion westwards, the migration of the thickest sedimentary interval in the Cotiella sub-basin and the formation of Las Neis depocenter ~ a and Mediodía synchronously with the burial of the Armen diapirs (Fig. 12b-II). ~ a and - The fact that the Seira diapir fell whereas the Armen Mediodía diapirs were covered (Fig. 12c and d) indicates that the amount of salt was probably greater in the Seira diapir than in ~ a and Mediodía diapirs. This could be related to the the Armen proximity of the Seira diapir to the Esera depocenter and to the feeding of salt from Esera to Seira areas. - The formation of primary salt-welds explains the basement pieces found at the base of the Cotiella thrust sheet, which were probably incorporated into the thrust system during tectonic inversion. - These results, additionally, constrain a stratigraphic correlation between the carbonates of the Cotiella sub-basin with the ones ~ a, Mediodía and Seira sub-basins, and in the northern Armen suggest that the Southern and Northern Seira sub-basins were not synchronous. Acknowledgments The research presented has been made possible by funding from Statoil. The explained methodology was settled with the help of Martin Jackson and Mike Hudec. This work has also benefited from discussions with Kate Giles and Mark Rowan. The authors also wish to thank Clare Bond and Ian Davison for the helpful reviews. The authors acknowledge support from the n (Proyecto INTECTOSAL, Ministerio de Ciencia e Innovacio CGL2010-21968-C02-01) and the Generalitat de Catalunya (Grup lisi de Conques 2009SGR1198). de Recerca de Geodin amica i Ana pez-Mir was funded by the predoctoral grant Research by B. Lo
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n del Profesorado Universitario from the Ministerio de Formacio n, Cultura y Deporte. Midland Valley is thanked for Educacio providing the Move™ software for the construction and restoration of cross-sections. This is Cambridge Earth Sciences contribution esc.3088. References Aschoff, J.L., Giles, K.A., 2005. Salt diapir-influenced, shallow marine sediment dispersal patterns: insights from outcrop analogs. Am. Assoc. Petrol. Geol. Bull. 89 (4), 1e23. Bally, A.W., Gordy, P.L., Stewart, G.A., 1966. Structure, seismic data, and orogenic evolution of southern Canadian Rocky Mountains. Bull. Can. Petrol. Geol. 14, 337e381. Boyer, S.E., Elliot, D., 1982. Thrust systems. Bulllet. Am. Assoc. Petrol. Geol. 66, 1196e1230. Chamberlin, R.T., 1910. The Appalachian folds of central Pennsylvania. J. Geol. 18, 228e251. Dahlstrom, C.D.A., 1969. Balanced cross sections. Can. J. Earth Sci. 6, 743e757. Dahlstrom, C.D.A., 1970. Balanced cross sections. Can. J. Earth Sci. 18, 332e406. Davison, I., 1986. Listric normal fault profiles: calculation using bed-length balance and fault displacement. J. Struct. Geol. 8, 209e210. Dula, W.F., 1991. Geometric models of listric normal faults and rollover folds. Am. Assoc. Petrol. Geol. Bull. 75, 1609e1625. ECORS-Pyrenees Team, 1988. ECORS deep reflexion seismic survey across the Pyrenees. Nature 331, 508e577. Erslev, E., 1991. Trishear fault-propagation folding. Geology 19, 617e620. Ferrer, O., Jackson, M.P.A., Roca, E., Rubinat, M., 2012. Evolution of salt structures during extension and inversion of the Offshore Parentis Basin (Eastern Bay of Biscay). In: Alsop, G.I., Archer, S.G., Hartley, A.J., Grant, N.T., Hodgkinson, R. (Eds.), Salt Tectonics, Sediments and Prospectivity. Geological Society of London, pp. 359e377. Special Publications 363. cico inferior en los Pirineos García-Senz, J., 2002. Cuencas extensivas del Creta n y subsecuente inversio n. Ph.D. thesis. University of centrales, formacio Barcelona. gico de Espan ~ a 1:50.000 (2ª serie). García-Senz, J., Ramírez, J.I., 1997. Mapa geolo gico y Minero de Espan ~ a, Madrid. Hoja 213, Pont de Suert. Instituto Geolo gico y relacio n entre tecto nica y sedGarrido-Megías, A., 1973. Estudio geolo n del Secundario y Terciario de la vertiente meridional pirenaica en su imentacio rida). Ph.D. thesis. University of Granada, zona central (provincias de Huesca y Le pp. 1e395. Gibbs, A.D., 1983. Balanced cross-section construction from seismic sections in areas of extensional tectonics. J. Struct. Geol. 5, 153e160. Giles, K.A., Lawton, T.F., 2002. Halokinetic sequence stratigraphy adjacent to the El Papalote diapir, Northeastern Mexico. Am. Assoc. Petrol. Geol. Bull. 86 (5), 823e840. Groshong Jr., R.H., 1990. Unique determination of normal fault shape from hangingwall bed geometry in detached half grabens. Eclogae Geol. Helvetiae 83, 455e471. Groshong Jr., R.H., Bond, C.E., Gibbs, A., Ratcliff, R., Wiltschko, D., 2012. Preface: structural balancing at the start of the 21st century: 100 years since Chamberlin. J. Struct. Geol. 41, 1e5. Hossack, J.R., 1979. The use of balanced cross sections in the calculation of orogenic contraction: a review. J. Geol. Soc. Lond. 136, 705e711. pez-Mir, B., 2013. Extensional Salt Tectonics in the Cotiella Post-rift Basin (SouthLo central Pyrenees): 3D Structure and Evolution. Disseration. University of Barcelona. Maerten, L., 2007. Geomechanics to solve structure related issues in petroleum reservoirs. AAPG Eur. Reg. Newsl. 2, 2e3. ~ oz, J.A., García-Senz, J., 2004. Extensional salt tectonics in a McClay, K., Mun contractional orogen: a newly identified tectonic event in the Spanish Pyrenees. Geology 32 (9), 737e740. ~ oz, J.A., 1992. Evolution of a continental collision belt: ECORS-Pyrenees crustal Mun balanced cross-section. In: McClay, K.R. (Ed.), Thrust Tectonics. Chapman and Hall, pp. 235e246. ~ oz, J.A., 2002. Alpine tectonics I, the pyrenees. In: Gibbons, W., Moreno, T. Mun (Eds.), The Geology of Spain. Geological Society London, pp. 370e385. gico de Espan ~ a 1:50.000 Ríos-Aragües, L.M., Lanaja, J.M., Frutos, E., 1982. Mapa geolo gico y Minero de Espan ~ a, Madrid. (2ª serie). Hoja 179, Bielsa. Instituto Geolo gico de Espan ~ a 1:50.000 (2ª serie). Robador, A., Zamorano, M., 1999. Mapa geolo gico y Minero de Espan ~ a, Madrid. Hoja 212, Campo. Instituto Geolo Rowan, M.G., 1993. A systematic technique for the sequential restoration of salt structures. Tectonophysics 228 (3e4), 331e348. Rowan, M.G., Ratiff, R.A., 2012. Cross-section restoration of salt-related deformation: best practices and potential pitfalls. J. Struct. Geol. 41, 24e37. Schultz-Ela, D.D., 1992. Restoration of cross-sections to constrain deformation processes of extensional terranes. Mar. Petrol. Geol. 9, 372e388. guret, M., 1972. Etude ries de colle es de la partie Se tectonique des nappes et se ne es. In: Se rie Ge eologie Structuralle 2. Pubcentrale du versant sud des Pyre lications USTELA, Montpellier, pp. 1e155. superieur sud-pyre ne enne en Catalogne, Aragon et Souquet, P., 1967. Le Cretace Navarre. Ph.D. thesis. University of Toulouse.
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Suppe, J., 1983. Geometry and kinematics of fault-bend folding. Am. J. Sci. 283, 684e721. Suppe, J., Medwedeff, D.A., 1990. Geometry and kinematics of fault-propagation folding. Eclogae Geol. Helvetiae 83, 409e454. Teixell, A., 1998. Crustal and orogenic material budget in the west central Pyrenees. Tectonics 17 (3), 395e406.
White, N.J., Jackson, J.A., McKenzie, D.P., 1986. The relationship between the geometry of normal faults and that of sedimentary layers in their hangingwalls. J. Struct. Geol. 8, 897e910. Williams, G., Vann, I., 1987. The geometry of listric normal faults and deformation in their hanging walls. J. Struct. Geol. 9, 789e795.
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Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg
Restoration of basins driven by extension and salt tectonics: Example from the Cotiella Basin in the central Pyrenees ~ oz b, Jesús García Senz c Berta Lopez-Mir a, b, *, Josep Anton Mun a
CASP, West Building, 181A Huntingdon Road, Cambridge CB3 0DH, United Kingdom ria de Pedralbes, 08028 Barcelona, Spain Institut de Recerca Geomodels, Facultat de Geologia, Universitat de Barcelona, Zona universita c gico y Minero de Espan ~ a (IGME), La Calera 1, 28760 Tres Cantos, Madrid, Spain Instituto Geolo b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2014 Received in revised form 12 September 2014 Accepted 28 September 2014 Available online 18 October 2014
The structure of the Cotiella Basin consists of four well differentiated salt-floored sub-basins which contain more than 6 km of halokinetic growth-strata and have been interpreted to be formed by gravitydriven extensional faults and diapirism during the middle Coniacian e early Santonian. The subsequent Pyrenean orogeny (late Santonian to Miocene) inverted the extensional basins and squeezed the diapirs leaving little trace of the original salt volume, although both the extensional and the salt-related features of the Cotiella Basin were preserved. This paper provides new insights into the evolution of the Cotiella Basin by establishing a correlation between sub-basins by means of structural restorations. The workflow followed and the assumptions made during the restoration of the inversion and especially of the extensional structures are outlined in detail. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Sequential restorations Salt tectonics Passive diapirism Pyrenees Cotiella basin
1. Introduction The first application of structural restoration concepts was provided by Chamberlin (1910) to determine the depth of the detachment level underlying concentric folds. However, it was not until the late 60's that the construction bed-length balanced cross-sections of the Canadian Rockies (Bally et al., 1966; Dahlstrom, 1969, 1970) demonstrated the importance of balancing to predict the geometry of the structures at depth and to validate structural interpretations. Structural balancing studies continued, particularly in contractional regions (e.g. Hossack, 1979; Boyer and Elliot, 1982), resulting in the development of kinematic structural models such as the fault-bend and fault-propagation folds (e.g. Suppe, 1983; Suppe and Medwedeff, 1990). In extensional settings, the balancing of crosssections was also applied to interpret seismic images (e.g. Gibbs, 1983) and to predict the geometry and kinematics of extensional faults (e.g. White et al., 1986; Williams and Vann, 1987; Groshong, 1990). Further research on structural balancing focused on the application of numerical (e.g. Erslev, 1991) and geomechanical
models (e.g. Maerten, 2007) to simulate and understand the evolution of geological structures (i.e. Groshong et al., 2012). In salt tectonics settings the restoration of cross-sections is also crucial to formulate models of structural evolution, but it can be more problematic. First, the constraints to restore salt-related structures are not simple because the mobility of the salt can generate uncertainties as a result of lateral salt expulsion and related vertical rotations. Second, the existing algorithms cannot be applied straightforwardly as the fundamental assumptions such as plane-strain deformation and area conservation are often invalid for cross-sections containing salt (i.e. Rowan and Ratiff, 2012). This paper presents sequential restorations of the partially inverted Cotiella salt basin in the central Pyrenees. Sequential restorations are used to validate, constrain and improve models of structural evolution. Besides the establishment of a model for the evolution of the Cotiella Basin, the workflow followed and the assumptions made during the restoration process are outlined in detail.
2. Geological setting 2.1. The central Pyrenees * Corresponding author. Present address: CASP, West Building, 181A Huntingdon Road, Cambridge, CB3 0DH, United Kingdom. Tel.: þ44 1223768290. E-mail addresses: [email protected] (B. Lopez-Mir), jamunoz@ub. ~ oz), [email protected] (J. García Senz). edu (J. Anton Mun http://dx.doi.org/10.1016/j.jsg.2014.09.022 0191-8141/© 2014 Elsevier Ltd. All rights reserved.
The Pyrenees is a Late Cretaceous to Miocene doubly-vergent collisional orogen which was developed from the inversion of previous Mesozoic extensional basins (ECORS-Pyrenees Team,
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~ oz, 2002). The location of the map below is depicted in red. b) Geological map of Fig. 1. a) Structural sketch showing the main structural units of the Pyrenees (modified from Mun pez-Mir, 2013), except for the footwall of the the Cotiella thrust sheet, indicating the main structural features and the location of Figs. 2 and 3b. This map results from our data (Lo Cotiella 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 and Ramírez, 1997). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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~ oz, 1992; Teixell, 1998). In its south-central portions, the 1988; Mun Alpine structure consists of a southwards-directed imbricate stack guret, of thrust sheets of Mesozoic and Paleogene cover rocks (Se ~ oz, 2002). From top (north) to 1972; Garrido-Megías, 1973; Mun bottom (south), these thrust sheets are: Cotiella (emplaced during ~ a Montan ~ esa the late Santonian - late Maastrichtian), Montsec-Pen (emplaced during the Early Eocene) and Gavarnie (emplaced during the Middle Eocene to Early Oligocene). The Cotiella thrust sheet developed from the positive inversion of a previous Upper Cretaceous extensional basin and was subsequently transported piggy-back at least 18 km southwards by the ~ a Montan ~ esa thrust (García-Senz, 2002; younger Montsec e Pen McClay et al., 2004). Synchronously and afterwards, it was tilted southwards and uplifted by more than 3 km by underthrusting of the younger Gavarnie and other basement-involved thrust sheets.
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As a result, the thick Upper Cretaceous succession of the Cotiella extensional 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 four seismic-scale middle Coniacian to early Santonian sub-basins: ~ a, Mediodía, and Seira (Fig. 1b). The Cotiella subCotiella, Armen basin is the largest one and consists of a broad anticline characterized by growth-strata showing a thickness expansion of up to 6 km towards a partially inverted extensional fault (Fig. 2a). To the ~ a, Mediodía and Seira subnorth, there are the smaller Armen basins, which are filled by sedimentary wedges fanning as much
Fig. 2. a) Structural cross-section across the central portions of the Cotiella thrust sheet. b) Structural cross-section across the eastern portions of the Cotiella thrust sheet. c) ~ a ridge. See Fig. 1 and cross-section 2a for location. Structural features and stratigraphic units are labeled according to the legend of Fig. 1b. Interpreted photograph of the Armen
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as 3.3 km towards extensional listric faults (Fig. 2). The synkinematic sediments consist of middle Coniacian to lower Santo~ os Fornian carbonates and calcarenites (Aguasalenz and Macin mations, respectivelly) which overlie a pre-kinematic succession made up of upper Albian to lower Coniacian post-rift carbonates guret, 1972; detached on Upper Triassic salts (Souquet, 1967; Se McClay et al., 2004). The equivalent Coniacian to lower Santonian sedimentary succession in the Cotiella fault is only 300 m thick (García-Senz, 2002; McClay et al., 2004), as happens in most of the Pyrenees. The Cotiella Basin has been interpreted to be developed by postrift gravity-driven extension above relict Upper Triassic salts in the Bay of Biscay-Pyrenean paleorift margin of the Atlantic Ocean (McClay et al., 2004). However, particular structural and sedimentary features depart from the classical models of extensional listric pez-Mir, 2013). These features and an alternative fault systems (Lo model of evolution for the Cotiella Basin are summarized below. Our model refines the previous gravity-driven extension model by the incorporation of passive diapirs isolating the sub-basins (Fig. 3) and questions the existence of a hard-link connection between subbasins as proposed by of McClay et al. (2004). One of the most peculiar salt-related features can be observed in ~ a fault (Fig. 2c), where shallow-water the hangingwall of the Armen ~ os Formation are accumulated next to the calcarenites of the Macin fault (in the theoretically structural low of the rollover). This unexpected water-depth distribution of the sediments is not adequately explained by the previously interpreted scenario of
extensional gravity-driven faults (García-Senz, 2002; McClay et al., 2004). It can be alternatively interpreted to have been developed as salt diapir-related sandstone depocenters similar to the ones described by Aschoff and Giles (2005) in La Popa basin in Mexico. The accumulation of calcarenites in this structural position, in fact, ~ a fault being supis consistent with the hangingwall of the Armen ported by salt, preventing the development of a shale-dominated ~ a sub-basin (to the basin. Additionally, at the base of the Armen north), an exceptionally large Upper Triassic salt outcrop (Fig. 1b) shows an abrupt lithological contact between the top of the salt and the overlying sediments, supporting the presence of a diapir during ~ os calcarenite Formation, as shown in the deposition of the Macin the model of Fig. 3aeI. ~ os FormaIn the Seira sub-basins, the calcarenites of the Macin 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 the ~ os Formation contain clasts sourced from the Upper Triassic Macin salt layer, which supports the model of emergent salt diapirs controlling their deposition. Regarding the largest Cotiella sub-basin, two main depocenters have been interpreted: the older and western Esera depocenter; and the younger and eastern Las Neis depocenter (Fig. 3b). These are separated by the transverse Esera transfer fault. The Aguasalenz_Fo unconformity covers both the transfer fault and the calcarenitic deposits of the Cotiella sub-basin, as can be observed in map-view
Fig. 3. a) Sketched geological model showing the main stages of evolution of the Cotiella Basin from the late middle Coniacian to the late Santonian. I) Passive diapirs are coeval with gravity-driven extension above salt; II) Extension continues and diapirs are sealed; III) Tectonic inversion and squeezing of the diapirs. b) Conceptual sketch showing the along-strike geometry of the Cotiella sub-basin at the end of the early Santonian (stage II of model above), depicting the relative chronology between the Esera and Las Neis depocenters as well as the AS_Fo unconformity and the Esera fault. The location of the cross-section is shown in Fig. 1. The intersections with Fig. 2a, b and with the model above are pez-Mir (2013). depicted. Both diagrams are modified from.Lo
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(Fig. 1b). Assuming that the deposition of the calcarenites was synchronous with the rising of diapirs, the Aguasalenz_Fo unconformity is interpreted to overlie the diapirs and related northern sub-basins (Fig. 3a-II). This unconformity crops out very nicely in the Raymond ~ a fault, where the stratd’Espouy peak, in the footwall of the Armen igraphic succession displays a spectacular fanning of more than 90 (Fig. 2a). Such a fanning is difficult to create by gravity-driven extension, but is better explained by the combination of salt tectonics with extension (although it could be partly accentuated by shortening at high angles to bedding during tectonic inversion). The migration of the thickest sedimentary interval towards the west in the Cotiella sub-basin observed in Fig. 3b is also interpreted to be related to salt inflation related to the salt displaced by the sinking depocenters. In the late Santonian, as a result of Pyrenean contraction, salt was expulsed out to the surface and then dissolved, leaving little trace of the original salt volume. Thrust-welds (which coincide with the faults observed today) were potentially created by squeezing of previous salt walls and the different sub-basins ended up attached to one another (Fig. 3a-III). Thanks to salt expulsion, the internal ~ a, Mediodía and Seira sub-basins was scarcely structure of the Armen
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deformed and the current geometry is probably similar to the original one. By contrast, the geometry of the Cotiella Basin was more severely ~ a fault for instance, strata were modified. In the footwall of the Armen steepened and overturned, resulting in the development of complex structures such as the Reduno downward-facing anticline (Fig. 2a). In areas more distant to the diapirs, such as the westernmost portions of the Cotiella Basin, tectonic inversion caused the development of folds such as the Vaquerizal anticline (Fig. 2a). 3. Restoration of the contractional structures To understand the evolution of the extensional salt-related structures, sequential restorations of the extensional faults were constructed. Before this could be done, the effects of shortening had to be removed. The contractional structures have only been partially restored in order to decipher the geometry of the Cotiella Basin before its positive inversion. Two main operations have been ~ a Montan ~ esa, Gavarnie considered: (1) the restoration of the Pen and basement-involved Axial Zone thrust sheets (Figs. (4) and (2)) the restoration of the late Santonian inversion (Figs. 5 and 6).
~ a Montan ~ esa thrust sheets in the Armen ~ a (a) and Seira (b) crossFig. 4. Workflow to restore the basement-involved thrust sheets of the Axial Zone, and the Gavarnie and Pen sections at late Santonian times, once the Cotiella basins were inverted. Four steps of restoration are depicted: I) Present day geometry; II) Geometry after removing the Axial ~ a-Montan ~ esa Zone thrust sheets; III) Geometry after removing the detachment fold of the Gavarnie thrust sheet; IV) Geometry after removing a low-angle footwall ramp of the Pen thrust sheet. The pin line, the template beds and the datum used in each step of restoration are indicated. The colors of 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).
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~ a cross-section. Four steps of restoration are depicted. a) Geometry of the Cotiella basin in the Fig. 5. Workflow to restore the overturned strata of the Reduno structure in the Armen late 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 uppermost stratigraphic 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 of ~ a and Mediodía sub-basins to the position before tectonic inversion. Two solutions are the strata considering line-length and bed-shape constant. c) Translate the Cotiella, Armen proposed 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. ~ a and Mediodía sub-basins, and interpretation of the basement geometry. Two solutions are proposed depending on the timing d) Interpretation of the eroded section in the Armen of primary welding and sealing of the passive diapirs (I and II).
~ a Montan ~ esa, Gavarnie and basement-involved thrust 3.1. Pen sheets ~ a Montan ~ esa, Gavarnie and basementThe restoration of the Pen involved thrust sheets aims to remove the folding and tilting which was acquired during their emplacement. The total transport towards the south has not been considered. This restoration has been carried out in a very simple manner because the internal structure of the Cotiella Basin was scarcely deformed. The flexural slip unfolding algorithm (Davison, 1986) that is implemented in the
Move™ software has been used, assuming longitudinal contraction during basin inversion. This method uses a pin perpendicular to, and a slip-system parallel to the template bed to control the unfolding of the beds. It maintains bed thickness variations, and both the template bed and slip-system are unfolded. Three operations have been necessary (Fig. 4): I) The tilting acquired during the emplacement of the Axial Zone basement-involved thrust sheets has been removed using the uppermost layer of the Axial Zone thrust sheets
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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 the Cotiella 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 Cotiella and 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 colors of 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 parallel to the Gavarnie thrust) as the template bed. The pin line has been inserted in the southern edge of the cross-sections as a ~a line perpendicular to the beds in the case of the Armen cross-section (Fig. 4aeI) and as the axial trace of the southernmost syncline in the case of the Seira cross-section (Fig. 4beI). The line-lengths of the rest of the stratigraphic horizons have been checked quantitatively. II) The (not very pronounced) detachment anticline observed in the Gavarnie thrust sheet has subsequently been restored. In ~ a cross-section (Fig. 4a-II), this fold has been the Armen removed using the top of the Early Eocene succession as the template bed. In the case of the Seira cross-section (Fig. 4bII), the top of the pre-extensional Upper Cretaceous succession has been used as the template. In both cases, the pin line has been inserted in the southern edge of the cross-sections,
~ a Montan ~ esa folds as a line perpendicular to the beds. The Pen observed in the southern parts of the cross-section have not been removed because their restoration does not alter the geometry of the Cotiella Basin. The geometrical changes between steps II to III are not very pronounced as the angle between the template bed and the datum is very small. The most noticeable change can be appreciated in Fig. 4a-III where the layers at the base of the Gavarnie thrust sheet are completely flat-laying. ~ a-Montan ~ esa thrust observed at III) The footwall ramp of the Pen the northern portions of Fig. 4a-III and 4b-III (which generates a slight tilting towards the north in the Cotiella Basin) ~ a Montan ~ esa thrust as the has been restored using the Pen template bed. The pin line has been inserted in the southern edge of the cross-sections, as a line perpendicular to the ~ a-Montan ~ esa thrust beds. As the footwall ramp of the Pen
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cuts the layers of its footwall at a very low angle, the changes between steps III to IV in Fig. 4 are very gentle. The most noticeable change is that the bottom of the Cotiella basin has become flat in the northern portions of the restored crosssections in step IV. The latter shows how the geometry of the Cotiella Basin looked like immediately before tectonic inversion at the late Santonian (Fig. 4a-IV and 4b-IV). 3.2. Late Santonian inversion The primary objective of restoring the late Santonian inversion is to discover what the geometry of Cotiella Basin looked like at the end of the extensional salt tectonics event (Figs. 5 and 6). This operation needs to be performed more carefully than the preceding one because it can introduce erroneous geometries, especially in the Cotiella sub-basin where the layers were more severely deformed during shortening. As a result, this step has been performed in a semi-manual manner, using the existing algorithms when possible and manually modifying the results when necessary. The assumptions made are explained as follows. Each sub-basin has been treated separately and four operations have been considered (Figs. 5 and 6): 1) Assuming that the overturned attitude of strata was acquired during tectonic inversion, the first operation comprised rotating the layers of the Cotiella Basin until all of them present a normal stratigraphic polarity. This operation has been approached in a manual manner, considering that bed-areas and bed-lengths remain approximately constant over time, as explained in Fig. 5b. Slight differences in the length or the area of the beds have been accepted because the strata could have thinned by layer-normal shortening during contractional deformation as some of the beds of the Cotiella Basin were steeply dipping before contraction started. 2) Once the overturned attitude of strata has been removed, the sub-basins have been translated to allow the salt reaching the surface as deduced by the sedimentological features above described (Fig. 5c). This assumes that during the extensional salt tectonics event the sub-basins were separated by salt walls and diapirs, as suggested by facies distribution and lithological characteristics, the spaces between sub-basins have been considered to be filled with salt and the diapirs have been depicted as narrow as possible. 3) The eroded parts of the section and the basement geometry have been interpreted manually (Figs. 5d and 6d). The basement geometry is inspired by cross-sections constructed in other areas of the Pyrenees, such as the Parentis basin (e.g. Ferrer et al., 2012), where the basement geometry is interpreted from the ECORS-Bay of Biscay and MARCONI-3 seismic profiles. The interpreted basement geometry consists of tilted blocks which dip southwards or are near-horizontal and are separated by northward-dipping planar extensional faults. These faults are decoupled from the sedimentary cover by a thick package of Upper Triassic evaporites and probably influenced the thickness changes of the Upper Triassic. This inferred geometry helps to explain the differences in thickness between the southern (thinner) and the northern (thicker) sub-basins and, additionally, it is consistent with the sequential restoration presented for the ~ a and Seira cross-sections (in the following pages). Armen 3.3. Limitations and assumptions ~ a cross-section (Fig. 2a), the first operation In the Armen (removing the overturned attitude of the layers) was problematic
(Fig. 5b). To keep the bed-length of the Reduno downward-facing anticline invariable due to the high cut-off angles of the beds with the faults; slight changes in the thickness of the beds have been accepted assuming that layers could have thinned by layernormal shortening during contractional deformation. The main ~ a fault if it problem lies in the change in the geometry of the Armen was in contact with the sediments in its footwall. This problem, however, supports our observations of thrust-weld characteristic of the presently observed fault and the possible existence of a diapir ~ a sub-basins. between the Cotiella and Armen The second operation (translating the sub-basins and filling the gaps with salt) has also been problematic in the Reduno area (Fig. 5c). Considering that the lowermost layers of the Cotiella subbasins show evidence of diapirism (such as the outcrop of Upper Triassic in the Raymond d’Espouy peak, Fig. 2c), the Cotiella and ~ a sub-basins were potentially separated by a diapir in the Armen early stages of development. If we then merely translate the ~ a sub-basin northward to allow the salt to reach the surface, Armen the resulting diapir geometry shows a landward tilted geometry, which is unlikely in extensional settings. A geometrical solution for this problem is to project the Lavasar shortcut (observed westwards, Fig. 1) above the topography of the section, in order to allow the beds to be longer and, as a result, the northern wall of the diapir to be vertical. This interpretation leads to two possible solutions (Fig. 5). The first (Fig. 5ceI) interprets a long-lived passive diapir which persisted during most of the extensional salt tectonics event. The second (Fig. 5c-II) considers that sediments covered the passive ~ os Fordiapir after the deposition of the calcarenites of the Macin mation. The second solution has been chosen to perform the sequential restorations because, as will be explained later, it better helps to explain the development of the Reduno downwards-facing anticline (Fig. 2). The contractional structures in the Seira cross-section were not so difficult to restore, as the amount of overturning is not as pro~ a cross-section (Fig. 6). To simplify the nounced as in the Armen restoration operations, the Chía klippe (Fig. 1) has been considered in continuity with the Cotiella Basin, although this relationship is actually unknown.
4. Methodology to restore the extensional structures 4.1. Workflow The workflow used to perform sequential restorations is grounded in the methods proposed by Schultz-Ela (1992) and by Rowan (1993) to restore ductile structures and salt structures in extensional terrains, respectively. These methods are more adequate 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 basement in order to keep the salt area constant. In this work, the Schultz-Ela (1992) method has been carried out separately for each sub-basin and steps have been performed in the order that we explain below (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 algorithm have been softened manually. Performing the decompaction at the beginning of the restoration (instead of doing it at the end, as suggested by Schultz-Ela, 1992) costs less computational time
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consequently the different sub-basins could have experienced different strain histories during their development. Second, the existing methods to choose the shear angle in extensional settings (i.e.: White et al., 1986) cannot be applied to restore the extension in the Cotiella Basin as the current geometry was deformed during shortening. 5) The sub-basins have been translated and gathered (Fig. 7f) assuming that diapirs widen during extension and narrow during shortening (although their height can decrease during extension). 6) The top of the basement has finally been moved (Fig. 7g), considering that the volume of the salt is constant or decreases over time (although this assumption may not be fully correct because salt can migrate laterally). Additionally, the top of the restored sections can be adapted to the interpreted bathymetry, using the Vertical Shear Unfolding to better depict how the water-depths were likely to be.
4.2. Limitations This methodology presents a limitation when steep or vertical layers exist because the simple or vertical shear algorithms do not give satisfactory results (Fig. 8a). The main problem lies in the shear angle. Applying a vertical shear algorithm to shallow-dipping layers considers that shear is perpendicular to the beds. Contrastingly, if the same shear vector is applied to steep layers, the shear becomes almost parallel to the layers and the algorithm is invalid. To solve this problem, the restoration of steep layers has been approached in a semi-manual manner, rotating and unfolding the beds individually (Fig. 8b). In some cases, manual modifications have been necessary to keep bed-areas and bed-geometries constant. To unfold the layers, vertical or simple shear algorithms have been used, and the shear angle was chosen by trial and error, as explained before. 5. Discussion of the results ~ a cross-section 5.1. Armen
Fig. 7. Method proposed by Schultz-Ela (1992) to perform sequential restorations in extensional terrains which has inspired the methodology presented in this paper. Six main steps of restoration have been considered. a) Original geometry. b) Geometry after stripping a layer. c) Decompaction. d) Rotation applied to the layers after unfolding. 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 the salt area constant. Figure modified from Schultz-Ela (1992).
because the Move™ software is much more stable in the following steps. 3) Afterwards, a rigid body rotation (Fig. 7d) has been performed semi-manually, to make the uppermost layers as parallel to a sub-horizontal datum as possible. 4) Rotation has been followed by unfolding of the layers (Fig. 7e). Hangingwall extension and salt deflation on listric faults produce strong bed rotation and diffuse deformation which is best modeled as block translation and collapse by inclined shear. A line with a slight tilting basinwards has been selected as the datum. The shear angle has been chosen by trial and error, by finding the angle that provides the least variations in the area and shape of the layers. The chosen angles vary between 40 and 70 , and consider both antithetic and synthetic shear. We have accepted great amount of variation for two main reasons. First, the Cotiella Basin was developed in a salt tectonics setting and
~ a cross-section has been The sequential restoration of the Armen carried out in the cross-section of Fig. 5d-II. This restoration has a significant limitation: the chronostratigraphy of the Cotiella Basin is not well known and the correlations between sub-basins are not well constrained. The strata of the Cotiella sub-basins are only ~ a and Mediodía sub-basins correlated with those of the Armen based on the main stratigraphic features to show that the interpreted geometries are possible, but their relationships through time are not fully constrained. Seven stages have been considered (Fig. 9): 1) Stage I depicts the development of post-rift carbonate platforms above salt. The slight thickness changes observed in those carbonate platforms result from the analysis of our field data and are indicative of an incipient salt inflation at early stages of development. 2) Stage II reveals the formation of gravity-driven extensional faults above salt, leading to the formation of rollovers. 3) Stage III considers that, after the deposition of the first layer of syn-extensional sediments (unit 2 in Fig. 9), the salt rollers in the footwall of the early listric faults pierced the topographic surface to become passive diapirs. The rising of the salt was controlled by the differential subsidence of sub-basins. Units 3 and 4 in Fig. 9 are thought to be equivalent to the Esera depocenter in the Cotiella sub-basin.
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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 layers when the existing algorithms are inadequate: first the uppermost bed has been unfolded to a datum tilted basinwards and, afterwards, the underlying beds have been consecutively unfolded 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 filled up the deepest part of the Cotiella sub-basin. The base of unit 5, accordingly, depicts the Aguasalenz_Fo unconformity described above. The end of passive diapirism was probably related to the formation of primary salt welds. 6) Stages VI and VII depict ongoing extension and the filling of the Cotiella sub-basin mostly by units 5 and 6. These units are equivalent to Las Neis depocenter which, as interpreted above, post-dates the passive diapirs.
This interpretation helps to explain the development of the ~ a fault or related structures developed in the footwall of the Armen Lavasar 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 a space problem to restore the contractional structure. This problem can be minimized if we consider that the Reduno structure was developed from a prior fold, which was cut, rotated and probably transported during shortening. This fold resembles the core of the prior antiformal geometry depicted in the restored cross-section. The piece needed to restore the structure is probably found in the
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~ a, Mediodía and Cotiella sub-basins. Seven main stages of evolution are represented: I) Formation of early normal Fig. 9. Sequential restoration showing the evolution of the Armen faults 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) Ongoing passive diapirism and extension; V) Diapirs are covered but extension continues; VI) Ongoing extension; VII) End of extension. This restoration has been done following the methodology exposed in this paper. Stratigraphic units are numbered and colored according to their relative age (colors do not depict lithologies). The bathymetric relief is slightly exaggerated. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Baranetas shortcut, a thrust system anomalously cropping out in the footwall of the Mediodía fault (Fig. 10d). The Punta Lierga fold system has been interpreted as the footwall ~ a inverted fault or of the Lavasar thrust syncline of the Armen (represented by the Piedra Blanca slice in Fig. 10c). However, the
fact that the Punta Lierga syncline is located in the same structural position as the layers covering the inferred diapir suggests that the Punta Lierga syncline could have been developed due to the differential folding of the layers. As such, the thickness differences associated with the diapir could have caused the folds to be
~ a and Mediodía sub-basins at the end of the extensional event (see Fig. 8). b) Sketch explaining the development of the Reduno Fig. 10. a) Geometry of the Cotiella, Armen downwards-facing anticline. c) Sketch explaining the development of the Punta Lierga syncline. d) Interpreted photograph of the Cotiella sub-basins showing the structural relationships between the Reduno anticline, the Baranetas shortcut and the Piedra Blanca slice (which is potentially the southeastern continuation of the Lavasar shortcut). e) ~ a sub-basins showing the structural relationships between the Punta Lierga syncline, the Aguasalenz_Fo unconformity and the Interpreted photograph of the Cotiella and Armen ~ a fault. The Raimond d’Espouy peak is located to simplify correlation between photographs. Armen
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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 early rollovers 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 falling diapir; 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 has been done following the methodology exposed in this paper. Stratigraphic units are numbered and colored according to their relative age (colors do not depict lithologies). The bathymetric relief is slightly exaggerated.
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preferentially developed in the strata covering the diapirs rather than in the strata flanking them (because the overburden is thinner and, consequently, easier to fold). This explains strong lateral variations of this fold system as well as the huge amount of fanning observed in the Raimond d’Espouy area (Fig. 10e). 5.2. Seira cross-section The sequential restoration of the Seira cross-section has been carried out in the cross-section of Fig. 6d. As in the case of the ~ a cross-section, the lack of a stratigraphic correlation beArmen 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 Northern Seira and Cotiella sub-basins (Fig. 11), combined with the salt expelled from the Esera depocenter, the development of the Southern Seira sub-basin simultaneously to the Cotiella and Northern Seira ones is not possible. Consequently, the Southern Seira sub-basin is thought to be younger than the Cotiella and Northern Seira sub-basins, as supported by the absence of upper Albian-middle Coniacian sediments at the base of the succession. The sequential restoration presented in Fig. 11 depicts the evolution of the Cotiella, Northern Seira and Southern Seira sub-basins from 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 salt inflation at early stages of development. 2) Stage II depicts the formation of early gravity-driven extensional faults and rollover anticlines. In this case, the footwall of the
early Seira fault shows evidence of diapirism with the development of calcarenitic lenses (Fig. 1), indicating that the salt roller in the footwall of the Seira fault was already rising. 3) Stage III shows that the above-described salt roller finally pierced the topographic surface to become a passive diapir, which controlled the development of the Cotiella and Northern Seira sub-basins. These sub-basins persisted during the deposition of units 3 and 4, and were possibly synchronous to the neighboring Esera depocenter. 4) At stage IV, when the bottom of these sub-basins created saltwelds, the diapir stopped rising. The formation of a primary salt-weld at the base of the Northern Seira sub-basin is consistent with the steps depicted in the basement. Additionally, the edges of the basement blocks could help to clarify some base ment exposures that are anomalously cropping out in the Esera Valley and in Las Neis Valley (Fig. 1) along the Cotiella thrust, by incorporating these interpreted basement edges into the thrust system as horses during tectonic inversion (Fig. 11-VII). 5) Stages V, VI and VII depict the development of the Southern Seira sub-basin as a result of the falling of the previous diapir. This interpretation implies that the calcarenites observed at the base of the Southern Seira sub-basin are not time-equivalent to those in the Cotiella and Northern Seira sub-basins, although their facies are very similar (Fig. 2b). The fact that the preextensional carbonates are missing at the bottom of the Southern Seira sub-basin supports the view that the development of this sub-basin was the result of a falling diapir (the same diapir that initially bounded the Cotiella and Northern Seira sub-basins). Accordingly, the development of the Southern Seira sub-basin potentially postdated the main stage of development of the Esera depocenter and consequently it was synchronous to
Fig. 12. Summary diagrams showing the evolution of the Cotiella Basin in cross-sectional and in map views. a Summary diagrams depicting the onset of passive diapirism in the Esera depocenter (I) and Las Neis depocenter areas (II). b Summary diagrams depicting the end of passive diapiris, which became a falling diapir in the Esera area (I) whereas they were covered in Las Neis area (II). Both stages roughly coincide with stages III and IV of Fig. 3, respectivelly. c Map view of stage III. d Map view of Stage IV. The Cotiella extensional fault depicted in the maps coincides with the Cotiella thrust in Fig. 1b. The location of the restored cross-sections is depicted in the maps. Dashed lines and gray colors depict buried faults and/or salt structures. Blue lines depict the structure contours of a horizon. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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the development of the western Las Neis depocenter and to the ~ a and Mediodía diapirs. covering of the Armen 6. Conclusions This paper presents sequential restorations of an extensional salt basin that was slightly deformed during subsequent tectonic inversion. The contractional structures have been restored by flexural slip, considering longitudinal deformation during basin inversion; whereas the extensional salt-related features have been restored by vertical or simple shear, considering that bed-areas and bed-shape remain constant. Manual modifications have been necessary in most of the steps to avoid problems derived from overturned or steep dip attitudes. Although using a semi-manual methodology has been awkward and time-consuming, the obtained results provide new insight to better constrain the structural evolution of this complex area and support the interpretation that the Cotiella Basin was developed by a combination of gravitydriven extension and passive diapirism. The most relevant conclusions regarding the evolution of the Cotiella Basin achieved with the sequential restorations are summarized below (Fig. 12): - The development of the Esera depocenter (Fig. 12aeI) syn~ a, Mediodía and Seira chronously with the rising of the Armen passive diapirs is corroborated by sequential restorations (Fig. 12a-II). - The Southern Seira sub-basin was potentially formed as a result of the falling of the Seira diapir which was probably triggered by the formation of a primary salt-weld at the base of the Northern Seira sub-basin (Fig. 12beI). - The formation of primary salt-welds at the base of the Esera depocenter (Fig. 12beI) potentially controlled salt expulsion westwards, the migration of the thickest sedimentary interval in the Cotiella sub-basin and the formation of Las Neis depocenter ~ a and Mediodía synchronously with the burial of the Armen diapirs (Fig. 12b-II). ~ a and - The fact that the Seira diapir fell whereas the Armen Mediodía diapirs were covered (Fig. 12c and d) indicates that the amount of salt was probably greater in the Seira diapir than in ~ a and Mediodía diapirs. This could be related to the the Armen proximity of the Seira diapir to the Esera depocenter and to the feeding of salt from Esera to Seira areas. - The formation of primary salt-welds explains the basement pieces found at the base of the Cotiella thrust sheet, which were probably incorporated into the thrust system during tectonic inversion. - These results, additionally, constrain a stratigraphic correlation between the carbonates of the Cotiella sub-basin with the ones ~ a, Mediodía and Seira sub-basins, and in the northern Armen suggest that the Southern and Northern Seira sub-basins were not synchronous. Acknowledgments The research presented has been made possible by funding from Statoil. The explained methodology was settled with the help of Martin Jackson and Mike Hudec. This work has also benefited from discussions with Kate Giles and Mark Rowan. The authors also wish to thank Clare Bond and Ian Davison for the helpful reviews. The authors acknowledge support from the n (Proyecto INTECTOSAL, Ministerio de Ciencia e Innovacio CGL2010-21968-C02-01) and the Generalitat de Catalunya (Grup lisi de Conques 2009SGR1198). de Recerca de Geodin amica i Ana pez-Mir was funded by the predoctoral grant Research by B. Lo
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