E. Roca

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 Neogene structure of the eastern Iberian margin: Structural constraints on the crustal evolution of the Valencia trough (western Mediterranean)

Abstract The presence of a thinned crust beneath the Valencia trough has been recognized for a long time. However, to date no attempt to link this thinning, inferred from geophysical data, with the superficial structures has been made. A quantification of the Neogene extensional processes in the crustal evolution of the Valencia trough is presented. As a consequence of the complexity of the Betic-Balearic domain, the analysis has been done for the Catalan-Valencian domain (northwestern areas of the trough). Crustal and superficial data in this area show that Neogene extension was accommodated by a listric normal fault system detached in a deep upper crustal level (13–15 km) in the marginal zone of the domain. Quantitative analysis of the Neogene structure shows the incoherence between the upper crustal thinning inferred from the geometric analysis of the superficial structures (β factor 1.4–1.5) and from geophysical data (1.8). Although the latter value agrees with that deduced from the analysis of Neogene subsidence, this incoherence suggests that the present thinned crust would be partially inherited from the Mesozoic extension. In this way, Mesozoic subsidence analysts and the recognized Mesozoic structure show that the Mesozoic extension was almost as important as the Neogene one.

Bed-by-bed fold growth by kink-band migration: Sant llorenç de Morunys, eastern Pyrenees

Ahstract<rowth strata deposited over and against the flank of the Sant Llorenc de Morunys fold during its final stages of deformation have been mapped at high resolution as the basis for unraveling the kinematics of fold growth. We use restoration techniques based on normal balancing assumptions to decipher the detailed kinematic history of folding. The progressive restorations, as well as balanced forward modeling, show that the last few hundred meters of fold growth were dominated by kink-band migration of a sort that is typical of much faultrelated folding. The kink-band migration has produced complex anticlinal hinge-zone geometry, including segmented fold hinges linked by disconformities and unconformities, which has direct and detailed explanation in terms of fluctuations in deposition rate relative to curved-hinge kink-band migration rate. Large fluctuations in the convolution of non-steady sedimentation and deformation are demonstrated, although the absolute fluctuations in deformation and sedimentation are unknown. At a length scale of 100 m, kink-band migration with little or no deposition is interspersed with sedimentation with little or no deformation. At the length scale of 500 m, deposition ranges from 200% to 50% of uplift. 0 1997 Elsevier Science Ltd. All rights reserved.

Evolution of the central Catalan margin of the Valencia trough (western Mediterranean)

Abstract The evolution of the central Catalan margin was conditioned by major fault activity and thermal processes generated by crustal thinning. The continental margin structure is characterised by major NE-SW-striking, southeast-dipping basement faults which have given rise to a graben, half-graben and horst system. These faults acted essentially as normal faults. No major Neogene strike-slip activity has affected the continental margin, although minor episodic variations from the dominant extensional regime to strike-slip tectonics cannot be discounted. Two major structural evolutionary stages took place at the margin, the transition from one stage to the other being gradual: (1) rifting stage (Late Oligocene? to early Burdigalian) related to the opening of the Valencia trough and from which originated both the Barcelona and Valles-Penedes half-grabens; (2) thermal subsidence stage (late Burdigalian to present) generated by the subsequent crustal cooling and thinning. Fault activity during this stage was essentially concentrated in the major half-graben bounding faults. The sedimentary evolution of the central sector of the Catalan margin was controlled by the tectonic and crustal evolution as well as by the successive sea-level and paleoclimatic changes which affected the region. Drastic changes in the depositional systems took place during the transition from the rifting stage into the thermal subsidence stage. During Aquitanian-Langhian time—characterized by an overall trend of pulsating encroachment of marine conditions over the continental margin—a noticeable diversity of depositional systems (alluvial, lacustrine, coastal evaporitic, fan delta-bay-shelf, carbonate platform systems) developed. From the latest Serravallian up to the present, the depositional framework was less varied due to a general lowering of sea-level and to the final onlapping of most of older structural highs, which are now buried below the Miocene sediments. Two progradational terrigenous shelf-talus systems separated by the Messinian erosive surface developed during the latest Serravallian-Tortonian and Pliocene-Quaternary.

Cenozoic magmatism of the valencia trough (western mediterranean): Relationship between structural evolution and volcanism∗

Abstract The Cenozoic magmatism of the Valencia trough is characterized by the existence of two volcanic cycles. The first cycle (Early Miocene-Middle Miocene) comprises calc-alkaline rocks which have been recognized in the central to eastern offshore areas and in Mallorca. The second cycle (Middle Miocene to Recent) is represented mainly by poorly differentiated alkaline rocks which are predominantly distributed along the Iberian margin of the Valencia trough. In accordance with the inferred structural evolution of the Valencia trough, petrological and geochemical data, as well as volcanological evidence, we suggest that two different geodynamic environments existed during these magmatic cycles. The Early to Middle Miocene calc-alkaline volcanism developed under compressive tectonic conditions and seems to be associated with the northwestern dipping subduction of the African plate. In contrast, the Middle Miocene to Recent alkaline volcanism is associated with extensional tectonics and can be explained within the framework of a low volcanicity rift model. Three zones—Catalan, Valencian and Columbretes—each located at different parts of the rift structure, can be distinguished in the Middle Miocene to Recent volcanism. This intraplate volcanism results from partial melting, due to extension-driven decompression, with the largest degrees of melting in the centre of the Valencia trough (Columbretes volcanic zone). In the Catalan volcanic zone a progressive increase in the extension rate from the first to the last volcanic episode has occurred, favouring the interaction of two mantle sources.

Oligocene to Middle Miocene evolution of the central Catalan margin (northwestern Mediterranean)

Abstract The Catalan margin belongs to the northern sector of the margin that separates the extremely thinned continental crust of the Valencia trough from the undeformed or thickened crust of the Iberian microplate. The integration of the available onshore and offshore data (mapping, stratigraphic sections, seismic profiles and well data) enables the present structure of the central Catalan margin to be identified as a major northwest-directed, thick-skinned thrust sheet compartmentalised in a system of NE–SW oriented horsts and grabens. The geometry and depositional features of the Tertiary successions denote that this structure resulted from two successive evolutionary episodes. The first episode was compressional and gave rise to the emplacement of the Catalan major thrust sheet; the second was extensional, linked to the opening of the Valencia trough. The compressional (pre-extensional) episode is recorded by Lower to lower Upper Oligocene deposits sedimented in a piggyback basin that developed in the present-day offshore. The pre-rift signature of these sedimentary successions is clear (they show constant thickness, are cut by all the normal faults, etc.) and their facies distribution indicates a mostly southwestward spreading of the sediments. The extensional episode includes the syn-rift and post-rift stages. The syn-rift stage resulted in the present horst-and-graben structure of the Catalan margin and it is recorded by the uppermost Oligocene–lower Burdigalian (Lower Miocene) sediments restricted to the graben troughs. The post-rift stage started in the late Langhian and has continued until the present. It is characterised by the attenuation of the tectonic activity, the sediment spreading over the horsts (most of them were completely overlapped during this stage) and no sediment thickness changes near the faults. The upper Burdigalian and lower Langhian sequences record that the transition between the syn-rift and post-rift stages was not sharp. These units show intermediate features since they concealed some minor faults and overlapped horsts, but were still affected by most of the major normal faults. The onshore and offshore tectono-sedimentary and palaeogeographic evolution show that: (1) the northwestern Mediterranean basin existed during the early to middle Oligocene as an extensional basin in the Gulf of Lions and as a contractional piggyback basin in the northeastern Valencia trough; (2) the extension started in the early Oligocene in southern France and migrated progressively southwestward, affecting the northeast Valencia trough during the latest Oligocene–Aquitanian.

The Neogene evolution of the outer Carpathian flysch units (Poland, Ukraine and Romania): kinematics of a foreland/fold-and-thrust belt system

Abstract Various techniques of geometric and kinematic analyses, including cross-section balancing and forward modeling, have been applied successfully to the outer Carpathian flysch units along regional geologic profiles crossing the belt from its undeformed foreland to the Pieniny klippen belt (western Carpathians, Poland) or to the central east Carpathian allochthon (eastern Carpathians, Ukraine and Romania). The amounts of Neogene shortening as well as the mode and the location of basement consumption are discussed on the basis of balanced cross-sections compiled along the three most representative regional traverses. In addition, the complete kinematic evolution of complex Carpathian structures, such as the Fore—Dukla triangle zone (southeastern Poland), the Borislav—Pokut duplexes (Ukraine) and the Marginal Folds (Romania) are described in more detail by using a forward modeling approach. Major results concern the tectonic heritage of Laramian inverted structures (Mesozoic normal faults reactivated as reverse faults during the Late Cretaceous—Paleocene compression or transpression) in the development of the Neogene outer Carpathian edifice. Pre-Eocene deformation and erosion effectively induce discontinuities in the Mid-Cretaceous blackshale detachment horizon, and help to localize the Neogene deformations in complex triangle zones. The thrust sequence itself evolves alternatively in a piggyback or in an out-of-sequence mode. Clearly, the critical taper of the outer Carpathians is preserved by a constant balance between erosion and thrust reactivations in the inner part of the wedge, and sedimentation and tectonic accretion at its front.

Lacustrine oil-shale basins in Tertiary grabens from NE Spain (Western European rift system)

Abstract In the NE Iberian Plate, the convergent motion and collision of the European, Iberian and African plates resulted in the development of Paleogene compressional features (strike-slip systems, thrust-fold belts) and late Oligocene to mainly Neogene extensional structures which are superimposed on the former. These extensional structures are represented by horsts, half grabens and tilted blocks, often developed in connection with preexisting, inherited faults. From the late Oligocene and during the Neogene both strike-slip and extensional regimes alternated and coexisted, giving rise to a number of fault-bounded basins. The deposition of organic-rich facies, and in particular oil-shales, took place in the lacustrine complexes developed in some of these basins: Campins Basin (late Oligocene); Ribesalbes and Rubielos de Mora basins (early-middle Miocene); Libros Basin (late Miocene) and Cerdanya Basin (late Miocene). Deep lacustrine sequences ranging from 100 up to 250 m thick were deposited in all the above mentioned basins. The organic-rich sequences are characterized by thin lamination, absence of bioturbation, rare benthonic fauna (if present, it is supplied from other parts of the basin) and excellent preservation of “exotic” fossils (plant leaves, insects, amphibians). The suitable depth conditions needed for permanent stratification in the water bodies and anoxia in the deeper parts of the lakes, were due essentially to increasing subsidence outstripping deposition. Intense tectonic activity is recorded by the occurrence of olisthostromes and slumps affecting the lacustrine deposits as well as by the syntectonic structures recorded in the basin-fill successions. The late Oligocene-late Miocene paleoclimatic regimes, ranging from warm tropical to subtropical conditions, were favourable for the development of permanently stratified lakes. Moreover chemical ectogenic meromixis may have contributed in some cases to the establishment of permanent stratification. The Eastern Iberian rift system, makes up the southernmost part of the larger Western European rift system. The occurrence of lacustrine sediments ranging from the late Eocene to late Miocene, is frequent in the fault-bounded basins of this complex rift system and lacustrine oil-shale deposits have been recorded in several of these basins. Thus the Western European rift system presents an interesting model of intracontinental rifting in a foreland platform setting, where favourable conditions for organic rich deposits often took place. The high potential of this kind of tectonic settings for lacustrine oil-shale exploration must be stressed.

Analysis of the geological evolution and vertical movements in the València Trough area, western Mediterranean

Abstract The Valencia Trough, located between the Iberian Peninsula and the Balearic Islands, corresponds to the south-west prolongation of the Provencal Basin. The analysis of recently released seismic lines shows that the present day tectonics of the Valencia Trough are the result of the coeval development of (1) a system of horsts and grabens in the north-west part of the basin during the Late Oligocene(?) — Early Miocene (Catalan-Valencian domain) and (2) a thrust and fold system which lasted up to the Middle Miocene and which was followed by extensional tectonics during the Late Miocene (Betic-Balearic domain). In an attempt to characterize the evolution of the Valencia Trough, subsidence curves have been calculated for 18 wells and three regional cross-sections distributed across the basin. Backstripping of these data shows two major stages of tectonic subsidence history: (1) a first stage, coeval with basin formation (30-15 My), characterized by a rapid and strong tectonic subsidence which attains its maximum values in the central basin areas and close to the Balearic Islands; and (2) a second stage (15-0 My) in which the tectonic subsidence decreases gradually. The amount of crustal thinning estimated from a comparison of the observed and modelled tectonic subsidence ranges between 1.5 (Iberian margin areas) and 4.5 (central and south-western basin areas). These values agree with the crustal thicknesses inferred from seismic refraction data and gravimetric modelling. Nevertheless, such a degree of thinning cannot be explained by the Neogene extensional structure. The presence of thick Mesozoic deposits suggests that the present crustal thinning is partly due to previous Mesozoic extension.

Pre-Neogene evolution of the Western Carpathians: Constraints from the Bochnia-Tatra Mountains section (Polish Western Carpathians)

On the basis of field studies and subsurface data, this first balanced transect crosses most of the Carpathian foreland basin and the entire Polish Western Carpathian thrust units (Outer flysch nappes, Pieniny Klippen Belt, and Inner Carpathian Tatric thrust sheets). It shows that (1) me Outer Carpathians are made up of several flysch units whose internal structure is strongly controlled by initial thicknesses, whereas the thin flysch successions are strongly deformed by tight folds and include several imbricated thrust sheets; by contrast, the thick flysch successions are less deformed and accreted in slightly imbricated thrust sheets; and (2) the Inner Carpathians are structured in an antiformal stack of basement thrust sheets. The unfolding of this cross section shows that the post-middle Oligocene shortening in the European foreland platform and Outer Carpathians exceeds 180 km. In the substratum, most of this shortening seems to be solved by basement consumption vertically beneath the suture zone of the Pieniny Klippen Belt. Additional basement shortening took place also (1) in the Inner Carpathians, where, during the Neogene, buried basement thrust sheets were piled up to form an antiformal stack, and (2) beneath the inner zones of the Outer Carpathians. Palinspastic restoration of the cross section reflects two well-differentiated zones prior to the late Oligocene-late Miocene compression: (1) the present Inner Carpathians and the Pieniny Klippen Belt, characterized by a Late Cretaceous thrust belt unconformably overlain by an undeformed Palaeogene cover, and (2) the Outer flysch Carpathians, in which pre-upper Oligocene configuration comprised a typical foreland basin flexed toward the SSW, with a system of NW trending highs and troughs.