Josep Anton Muñoz

Factors controlling the Alpine evolution of the central Pyrenees inferred from a comparison of observations and geodynamical models

Geodynamical numerical modeling has been combined with crustal structural restoration of the central Pyrenees in order to gain insight into fundamental processes that control the evolution of collisional orogens. Models are based on deformation of the crust by stresses transmitted upward from kinematic basal boundary conditions corresponding to the subduction of part of the lithosphere. The influence of inherited crustal heterogeneities, denudation, subcrustal loads, and crustal mechanical properties, consistent with well-constrained crustal partial restored cross sections of the central Pyrenees, is investigated by progressively incorporating them into model experiments. The primary result inferred from the modeling is that the asymmetry of the central Pyrenees double-wedge, seen as strain partitioning and in the morphological evolution, is a consequence of the asymmetric distribution of inherited crustal heterogeneity. The tectonic style of the central Pyrenees is the result of the inversion of the Early Cretaceous extensional fault system, during the early stages of the collision, and the reactivation of Hercynian heterogeneities during the late stages. Most of the upper crustal mass that entered the orogen during the calculated 165 km of convergence was accommodated by an increase of upper crustal cross sectional area or lost by denudation. To explain the upper crustal mass partitioning, as well as the geometry of the foreland basins and the preservation of synorogenic deposits in piggyback basins, a subduction load has to be applied to the models. Lower crust and mantle lithosphere were consumed by the mantle.

Eastern Pyrenees and related foreland basins: pre-, syn- and post-collisional crustal-scale cross-sections

Abstract A new crustal-scale cross-section through the Eastern Pyrenees shows a minimum of 125 km of total shortening across the belt. Convergence rates of 6 mm/yr (during early and middle Eocene time) between the northern domain of the Iberian plate and Europe can be evaluated from calculated shortening rates in both sides of the orogen. Two stages of orogenic growth can be determined in the Eastern Pyrenean transect. A first stage (from Early Cretaceous to middle Lutetian time) is characterized by a low topography, submarine emplacement of the thrust front, fast rates of south-directed shortening up to 5 mm/yr and widespread marine foreland deposition. This stage is also characterized by equivalent amounts of mountain erosion and detrital foreland accumulation. A second stage (middle Lutetian to late Oligocene) is marked by an increase in structural relief, subaerial emplacement, a decrease in shortening rates and widespread continental sedimentation. This leads towards a non-equilibrium condition in which mountain erosion is almost three times the foreland basin accumulation, leading to a large by-pass of sediments towards the Atlantic before the final endorrheic stage of the basin. Erosion rates based on area conservation between middle Lutetian and present day sections in a two-dimensional calculation indicate an average of 0.15 mm/yr. This rise is lower than middle Lutetian to early Miocene rock uplift rates in the Eastern Pyrenees, which account for 0.2–0.35 mm/yr, suggesting that erosion has been discontinuous through time. Inferred maximum river incision rates since the middle Miocene opening of the Ebro Basin towards the Mediterranean Sea account for less than 0.1 mm/yr.

The chronology of the Eocene tectonic and stratigraphic development of the eastern Pyrenean foreland basin, northeast Spain

Abundant and clearly exposed relationships between structures and syntectonic sedimentary rocks in the southern Pyrenees make this area particularly suitable for detailed studies of the sequential development of a collisional orogenic belt. During the Pyrenean orogeny, major shortening occurred in the eastern Pyrenees in Eocene and early Oligocene times. Until now, individual tectonic and depositional events have been only loosely defined temporally by biostratigraphic data. In order to provide a more precise chronological framework, four new magnetostratigraphic sections, spanning 8 km of Eocene strata and encompassing 18 m.y., have been developed in the eastern Pyrenees of northern Spain. The ages of nine major depositional sequences in the eastern Pyrenean foreland have been specified within the context of these chronologic data, beginning with the early Eocene transgression that commenced at 58 Ma. The timing of numerous tectonic events that occurred during the subsequent 16 m.y. can also be delineated, including the initial emplacement of the Pedraforca thrust sheet (58-54 Ma), the development of the Pedraforca breakback thrusts (47.5-40 Ma) at a mean shortening rate of 2.4 mm/yr, rotation of the Pedraforca footwall, three intervals of motion along the Vallfogona thrust (42.5-44.0, 40.5-41.5, and

Geometries of syntectonic sediments associated with single-layer detachment folds

Abstract Three kinematic models have been proposed to account for the geometry and kinematics of detachment folds involving a homogeneous competent layer detached over a ductile unit: Model 1 — variable limb dip-constant limb length, Model 2 — constant limb dip-variable limb length, and Model 3 — variable limb dip-variable limb length. Because the same fold shape can be generated by any of the above mechanisms, fold kinematics are best determined by the geometries of syntectonic sediments. In single-layer detachment folds, growth strata patterns are controlled by axial surface activity, limb rotation, limb lengthening, fold uplift rates, sedimentation rates and deformation mechanisms of the syntectonic sediments. Asymmetric kink folds have been modelled and compared with natural examples from the southern Spanish Pyrenees. Under conditions of high sedimentation rates, Model 1 folds produce characteristic fanning growth stratal wedges that initially onlap and then progressively overlap the detachment anticline, whereas at low sedimentation rates, anticlines with fanning growth wedges on both limbs are formed. Model 2 folds, under conditions of high syntectonic sedimentation rates, form anticlines with growth strata largely parallel to the pre-growth units and thinned over the fold crest. In contrast, at low sedimentation rates, Model 2 folds show offlapping growth strata onto both limbs. Model 3 growth folds, under conditions of high syntectonic sedimentation rates, form anticlines with intermediate features from Model 1 and Model 2; at low sedimentation rates, anticlines are formed with offlapping growth structures.

Three-dimensional reconstruction of geological surfaces: An example of growth strata and turbidite systems from the Ainsa basin (Pyrenees, Spain)

The external and internal geometry of four turbidite systems outcropping around the Buil syncline (Ainsa basin, Spanish Pyrenees) has been reconstructed with reservoir-scale resolution in three dimensions (3-D). The irregular geometry of the syncline and the resolution required for the reconstruction cannot be resolved with cross sections. Therefore, reconstruction has been carried out with a new methodology that applies a 3-D dip-domain geometrical model and 3-D restoration techniques to achieve reservoir-scale resolution in kilometric-scale reconstructions. This methodology is aimed at resolving 3-D geometries in folded areas and regions with variable thickness stratigraphy.The 3-D reconstruction of the Buil syncline reveals the synsedimentary growth of an intrabasinal anticline and the foreland lithospheric flexure associated with tectonic loading north of the Ainsa basin.

Quantified vertical motions and tectonic evolution of the SE Pyrenean foreland basin

Abstract Local isostatic backstripping analysis is performed across the eastern part of the Ebro foreland basin between the Pyrenees and the Catalan Coastal Ranges. The subsidence analysis is based on two well-dated field-based sections and four oil-wells aligned parallel to the tectonic transport direction of the eastern Pyrenean orogen. The marine infill of the foreland basin is separated into four, third-order, transgressive-regressive depositional cycles. The first and second depositional cycles are located in the Ripoll piggy-back basin and the third and fourth ones are located south of the syn-depositional emergent Vall-fogona thrust. Subsidence curves display a typical convex-up shape with inflection points recording the onset of rapid tectonic subsidence. Inflection points coincide roughly with the base of depositional cycles. Rates of tectonic subsidence are less than 0.1 mm a−1 in distal parts of the basin and up to 0.53 mm a−1 in proximal parts during second depositional cycle. Younger depositional cycles show maximum rates of tectonic subsidence of 0.26 mm a−1. The locus of subsidence within the basin migrated southward at a rate of c. 10 mm a−1. This flexural wave crossed the complete Ebro foreland basin in 10–11 Ma. The intraplate Catalan Coastal Ranges at the southeastern margin of the Ebro foreland basin produced an increase of tectonic subsidence rate at 41.5 Ma. Maximum rates of tectonic subsidence coincide with deep-marine infill of the basin, maximum rates of shortening and thrust front advance, and low topographic relief orogenic wedge. Transgressive-regressive depositional cycles can be controlled partly by reductions of available space within the basin during tectonic thickening of the sedimentary pile by layer parallel shortening, folding and thrusting. Although much less constrained, an approximation of post-thrusting exhumation and isostatic and tectonic uplift, as well as a first determination of possible amounts of eroded material of parts of the Ebro basin illustrate the impact of post-depositional erosion and uplift on the foreland.

Growth strata in foreland settings

Abstract The accurate analysis of growth strata has revealed their significance to unravel both fold kinematics and timing of deformation in both compressive and extensive settings. The increasing acquisition of 3-D multichannel seismic lines (with few tens of meters of resolution) in complex tectonic areas reveals complex interplay between growing structures and deposition either in marine or continental environments. Interestingly, growth strata reveal similar relationships in both compressive folds linked to thrusts at depth and folds related to propagating normal faults in extensive regimes. To completely document the complete kinematic history of an individual fault or a group of faults, it is necessary to combine studies at different scales to integrate the findings revealed by field work (meters of resolution), and multichannel seismic lines results. The margins of the Ebro Basin to the south of the Pyrenees have been the focus of abundant studies of syntectonic deposits. This is because of its special long-term evolution from a foreland to an intermontane basin and a final open basin to the Mediterranean Sea in Neogene times. The complete comprehension of growth strata in these natural laboratories is essential for our present needs in both petroleum exploration and earthquake prediction.

Partial melting of subducted continental lower crust in the Pyrenees

A magnetotelluric profile through the Central Pyrenees indicates the présence of very high conductive zones at lower crustal and upper mantle depths. High conductivity at upper mantle depths is interpreted as partial melts within subducted lower crust, whereas high conductivity at lower crustal depths is interpreted as rising magmas derived from the melting crustal slab. Such melts suggest that the continental lower crust, together with its lithospheric mantle may have been subducted into the mantle during the Pyrenean continental collision. Thus, magma generation can be related to thermal reequilibration of a subducted lower crust, a scenario that may serve as a model for understanding the late evolution of other collisional orogens.

Extensional salt tectonics in a contractional orogen: A newly identified tectonic event in the Spanish Pyrenees

A new Coniacian–early Santonian extensional salt tectonics event has been identified in the Cotiella thrust sheet of the central Spanish Pyrenees. We present a new interpretation of the Cotiella thrust system and describe exceptional outcrop examples of postrift, gravity-driven extensional growth faults. The thrust sheet contains three, kilometer-scale extensional faults with as much as 5.5 km of Coniacian–lower Santonian growth strata in their hanging walls. These postrift listric growth faults detached on the prerift Triassic evaporites on the uplifted and eroded southern flanks of Early Cretaceous rift basins. Early Alpine (late Santonian–Maastrichtian) contractional deformation reactivated and reversed the displacement on both the Early Cretaceous rift faults and some of the postrift-salt detachment faults. Hanging walls of the listric growth faults deformed into major antiforms, but the original extensional geometries are well preserved. Continued Alpine shortening incorporated these reactivated extensional fault systems into the thin-skinned thrust sheets of the southern Pyrenees. These postrift extensional structures account for significant thickness variations in the postrift succession and for along-strike changes in the styles of the thrust structures. The Late Cretaceous extensional salt tectonics is interpreted to result from gravity-induced northward sliding on the Early Cretaceous rift margin, toward the axis of the rift basin. The listric growth faults are similar to those found on the Gulf of Mexico margin, in the southern North Sea, and the South Atlantic margins.

Statistical grid-based facies reconstruction and modelling for sedimentary bodies. Alluvial-palustrine and turbiditic examples

The geological community is increasingly aware of the importance of geological heterogeneity for managing subsurface activities. In sedimentary bodies, facies distribution is an important factor constraining geological heterogeneity. Statistical grid-based sedimentary facies reconstruction and modelling methods (FRM methods) can be used to provide accurate representations (reconstructions or models) of facies distribution at a variety of scales, which can be conditioned to hard and soft data. These representations enable geological heterogeneity to be quantified; and therefore, serve as important inputs to manage projects involving subsurface activities. FRM methods are part of a general workflow comprising the construction of a surface-based framework, which is used to build the modelling grid in which these methods operate. This paper describes this workflow and provides an overview, classification, description and illustration of the most widely used FRM methods (deterministic and stochastic). Among others, two selected datasets comprising alluvial-palustrine and turbiditic deposits are used for illustration purposes. This exercise enables critical issues when using FRM methods to be highlighted and also provides some recommendations on their capabilities. For deterministic facies reconstruction, the main choice of the method to be used is between that employing a continuous or a categorical method. For stochastic facies modelling, choosing between the different techniques must be based on the scale of the problem, the type and density of available data, the objective of the model, and the conceptual depositional model to be reproduced. Realistic representations of facies distribution can be obtained if the available methods are applied appropriately.