Josep M. Casas

On folding and shear zone-development: a mesoscale structural study on the transition between two different tectonic styles

Abstract An example of coeval development of ductile shear zones and folds in schistose terrains is documented by detailed mapping and structural analysis. Folds and shear zones were formed under retrogressive conditions after the peak of the regional Hercynian metamorphism, and both affect a previous regional schistosity. A gradual change in structural style, on a regional scale, is evidenced by the disappearance in higher grade domains of late folds and the preponderance of shear zones. Geometrical analysis reveals that the bulk structural pattern is compatible with a non-coaxial strain history. Structural variation reflects a gradual change in mechanical behaviour of schists, which show a marked variation in crystallinity, at the time of deformation.

U-Pb zircon age of Ordovician magmatism in the Albera Massif(Eastern Pyrenees)

New geochronological data from the Albera Massif confirm the presence of an Early – Mid Ordovician igneous event (472 - 465Ma) recorded in the pre-Variscan rocks of the Pyrenees. This event resulted in the emplacement of a large granitic body in the lower part of the pre-Upper Ordovician metasedimentary succession and in the intrusion of a series of metric sized dykes in the middle and upper parts of it. The two types of igneous rocks were gneissified during subsequent Variscan deformation. The geochronological data confirm the occurrence of the gneiss as having derived from an Ordovician intrusive sheet, as in other Pyrenean massifs. The dykes are considered to be the subvolcanic equivalent of the intrusive sheet. The data also provide insight into the age of the metasedimentary series of the massif and enable us to correlate the dated rocks with other gneissic and subvolcanic bodies of the Variscan massifs of the Pyrenees and Iberia.

On the role of the Hercynian and Alpine thrusts in the Upper Paleozoic rocks of the Central and Eastern Pyrenees

The structure of the Pyrenean pre-Hercynian rocks involved in the “Axial Zone” antiformal stack, results from the association of Hercynian cleavage-related folds and Hercynian and Alpine thrusts. S...

Thrust tectonics and paleogene syntectonic sedimentation in the Empordà area, southeastern Pyrenees

AbstractThe structure of the southern Pyrenees, east of the Albanya fault (Emporda area), consists of several Alpine thrust sheets. From bottom upwards three main structural units can be distinguished : the Roc de Frausa, the Biure-Bac Grillera and the Figueres units. The former involves basement and Paleogene cover rocks. This unit is deformed by E-W trending kilometric-scale folds, its north dipping floor thrust represents the sole thrust in this area. The middle unit is formed by an incomplete Mesozoic succession overlain by Garumnian and Eocene sediments. Mesozoic rocks internal structure consists of an imbricate stack. The floor thrust dips to the south and climbs up section southwards. The upper unit exibits the most complete Mesozoic sequence. Its floor thrust is subhorizontal. The lower and middle units thrust in a piggy-back sequence. The upper unit was emplaced out of sequence.Lower Eocene sedimentation in the Biure-Bac Grillera unit was controlled by emergent imbricate thrusts and synchronic ex...

Petrogenesis of Ordovician Magmatism in the Pyrenees (Albera and Canigó Massifs) Determined on the Basis of Zircon Minor and Trace Element Composition

Zircon composition (U, Th, rare earth elements, and Hf) was tested as a tracer of petrogenetic processes in a set of metaigneous rocks from two pre-Ordovician massifs in the Pyrenees, Canigó and Albera. Two groups were differentiated after analyzing a number of elements in zircon: (1) Casemí gneiss and Marialles amphibolite and (2) subvolcanic metaporphyries and Cadí and Sureda orthogneisses. Casemí gneiss and Marialles amphibolite from the Canigó massif have high Th, Th/U, and Ce/Sm and low Yb/Gd and U/Ce that define linear trends in most of the plots used. The anomalous trend of the data in the Th/U-versus-Hf plot suggests mantle involvement in the origin of these rocks and the participation of fractional crystallization during their evolution. Zircon of the metaporphyries and the Cadí and Sureda orthogneisses exhibit similar characteristics despite a difference in age. Zircon has low Th, Th/U, Ce/Sm, and and high Yb/Gd and U/Ce, suggesting that this mineral grew in a melt formed by anatexis of a continental crust, with stable plagioclase. These petrogenetic data are consistent with the previously studied whole-rock geochemistry and Sr-Nd isotopes and confirm the use of zircon as a marker of petrogenetic processes in preference to a lithological tracer.

Sequences of mesostructures related to the development of Alpine thrusts in the Eastern Pyrenees

Abstract The mesostructures formed during the development of an antiformal stack in the Eastern Pyrenees are studied. The following structural evolution is proposed: a pressure-solution cleavage develops by layer-parallel shortening at initial stages of the thrusting. The cleavage is present in all units but mainly in the marls and marly-limestones. In the siliceous layers, a weak cleavage and small-scale thrust structures develop. The cleavage and the bedding are deformed during the thrusting. Different structures form in various places of the antiformal stack: where the bedding is flat-lying, southward-directed layer-parallel shears predominate; in the hangingwall ramps kink-bands, disjunctive stylolitic cleavage and layer-parallel shear opposite to the sense of thrust movement develop. The described structures constitute a kinematical model for mesostructures developed during the evolution and ramping of thrusts.

An example of three-dimensional analysis of thrust-related tectonites

Abstract A spaced pressure-solution cleavage related to Alpine thrusts in Mallorca (Balearic Isles) is deformed by several sets of shear planes and minor folds. All these structures formed synchronously, related to the thrust sheet movement. Shear plane sets show different orientations irrespective of the cleavage disposition. Slip along these shear planes shows that deformation did not result in plane strain. Minor fold axes show a wide range of orientations. Neither folds nor shear planes enable us to directly deduce the sense of thrust motion, indicating that these criteria must be used with care. Previous strain characterization is required for definite conclusions to be reached.

Inversion of a transfer system into lateral ramps: An example from the South-Central Pyrenees (Spain)

Field work in the South-Central Pyrenees suggests that omission contacts (i.e. younger over older rocks) occur at the base of the Cadí unit (Cadí thrust), and pass laterally into thrusts. This change occurs across tear faults which are present in the hangingwall of the Cadí thrust sheet and which controlled the deposition of Upper Cretaceous sediments (Adraén formation). Detailed mapping in the contact area between the Nogueres and Cadí units has shown that the actual thrust geometry in the study area is controlled by preexisting normal and transfer faults which developed in an already compressional context. Lateral ramps or tear faults develop depending on the angle between the pre-existing extensional transfer fault and the thrust transport direction.

Evidence for Devono-Carboniferous extensional tectonics in the central Pyrenees

AbstractDetailed mapping of the prc-Variscan rocks exposed in the Southern Central Pyrenees reveals the occurrence of extensional tectonic contacts overprinted by main phase and pre-main phase folds. The interpretation of the former contacts as normal faults and the reinterpretation of other contacts described in the literature lead us to suggest an extensional event during Devonian and/or early Carboniferous times, prior to the contrac-tional Variscan paroxysm.

Carboniferous normal faults in the Eastern Pyrenees: evidences and age constrains of syn-orogenic Variscan extension

In low grade rocks of the Eastern Pyrenees syn-orogenic Variscan extension is achieved by kilometric scale low-angle brittle normal faults. Evidence of these faults is generally depicted by subtractive contacts between Devonian upon Cambro-Ordovician rocks. Normal faults are cut by a Variscan granodiorite pluton and U-Pb available geochronologic data of the granodiorite, 305 Ma ± 3 [30], indicates that the age of extension can be attributed to Moscovian times. Extension postdates the main period of Variscan crustal thickening and occurs in N-S to NE-SW direction, roughly perpendicular to the trend of the main Variscan compressional structures. Such relationships point out that the onset of Variscan extension occurs after compression and prior to the granodiorite emplacement and to the deposition of post-orogenic volcanics.