J. M. Azañón

Mode of extensional tectonics in the southeastern Betics (SE Spain): Implications for the tectonic evolution of the peri‐Alborán orogenic system

The Gibraltar arc, which closes the westernmost part of the Mediterranean basin, is a Miocene A-type subduction arc formed by the continental collision of various pre-Miocene terranes in the major zone of collision between the Iberian and African cratons. The hanging-wall block, known as the Alboran domain, has undergone more than 300 km migration from a more easterly position, where it was the continuation of the Alpine Cretaceous-Paleogene orogen. Contemporaneous with thin-skinned thrusting in the footwall, the Alboran domain underwent two episodes of nearly orthogonal extension in which extensional systems developed with directions of extension varying from a NNW-SSE system, orthogonal to the belt axis, in the late Burdigalian-Langhian to a WSW directed orogen-parallel one in the Serravallian. The superposition of these two systems resulted in a chocolate tablet megastructure. This extensional pattern is not satisfactorily explained in previously proposed models for the evolution of the arc. Orthogonal extension is plausible in a process of the gravitational collapse of an overthickened crust; nevertheless, orogen-parallel extension is more difficult to explain in this context. We advocate that the WSW directed low-angle normal faults formed during large-scale extension in connection with important westward arc migration. The driving force of extension in a general context of convergence is controversial and varies between a convective removal model and a delamination model. Constraints on both the timing and the kinematics of extension, as presented in this paper, seem to support the contribution of both mechanisms. Convective removal may have started the process, but continued N-S convergence could have resulted in westward tectonic escape and asymmetric lateral inflow of asthenospheric material accompanying lithospheric delamination.

Exhumation during a continental collision inferred from the tectonometamorphic evolution of the Alpujarride Complex in the central Betics (Alboran Domain, SE Spain)

The tectonometamorphic evolution of the Alpujarride Complex within the internal zones of the Betics reveals a complex structural and metamorphic succession of continental collision, synmetamorphic exhumation, nappe forming in the final stage of exhumation, and rifting. A complex tectonic evolution is deduced from the superposition of structures observed in several Alpujarride units of the central Betics together with their metamorphic record. The following sequence of events is suggested: (1)A first stacking event (D1) is deduced from the presence of pre-S2 high-pressure-low-temperature metamorphic assemblages, (2) The synmetamorphic exhumation is inferred from the almost isothermal decompression pressure-temperature paths during which the S2 main foliation developed; the condensation of the mineral zones perpendicularly to the S2 foliation suggests large-scale vertical shortening during event D2, (3) A new stacking event (D3) is inferred from post-metamorphic thrusts and nappes, which are probably associated with kilometer-scale recumbent folds affecting the S2 main foliation, (4) A thinning event (D4), revealed by extensional fault systems, is associated with crustal spreading on the concave side of the Gibraltar Arc, which resulted in the opening of the Alboran Basin from the early Miocene (Burdigalian), (5) From late Tortonian to Pliocene a continuous N-S to NW-SE compression took place (D5). Stratigraphic, paleontological, and geochronological data suggest that event D3 could be lower Miocene in age (Aquitanian); consequently, event D1 and D2 would be pre-Miocene.

Stress analysis of NW Jordan: New episode of tectonic rejuvenation related to the Dead Sea transform fault

We performed a stress analysis based on fault-slip data for two of the main geological structures in NW Jordan; the Amman-Hallabat (AHF) and Shueib (SHF) faults. Both structures formed in Cretaceous times under E-W to ESE-WNW maximum compressive stress and have been considered inactive in Neogene and Quaternary times. We have collected data from 14 field stations in the area between the Dead Sea and the cities of Amman and Al Salt. This dataset comprises about 306 fault-slip data that include fault orientations and kinematics, striations, and joints. With the aid of the software T-TECTO 3.0, we have obtained the main stress axes for each station. Our results suggest that most of the structures are coherent with the present-day stress pattern associated with the Dead Sea fault system. In most of the field stations, new striations clearly overprint the older ones. Present-day stress field in the region has horizontal to sub-horizontal maximum and minimum compressive axes (σ1 and σ3), striking NNW-SSE and ENE-SWS, respectively, and a vertical intermediate stress axis (σ2). These new findings suggest a rejuvenation of the AHF and the SHF in the Quaternary in the context of the Dead Sea transform fault tectonic activity.

Distinguishing lateral folds in thrust systems: examples from Corbières (SW France) and Betic Cordillera (SE Spain): Discussion

INTRODUCTION SOME FIELD DATA AND DISCUSSION OF THE THRUST MODEL FOR THE BETICS In their paper on lateral folds in thrust systems belonging to arcuate mountain belts, Frizon de Lamotte & GuCzou (1995) discuss field criteria and kinematic analysis useful in distinguishing whether the folds in the lateral branch of an orocline are truly lateral or, rather, a frontal overprint. The authors illustrate their discussion with examples from two erogenic regions, the Betic Cordillera (southeastern Spain) and the Corbilres thrust belt (northeastern Pyrenees, France). We focus the present discussion on the Betic Cordillera in order to show that the structural evolution these authors propose-a tectonic imbrication resulting in nappe stack culminations-is in contradiction with (a) field data and (b) most of the bibliography on this subject. Therefore, although the Betic Cordillera offers one of the best examples of extensional processes in Mediterranean back-arc basins (Horvath & Berkhemer 1982), for both onshore (Garcia-Duefias et al. 1992) and offshore studies (Comas et al. 1994), it should not be used to illustrate classical thrust structures and distinguish folds related with thrust systems. Kinematic vectors in the eastern Betics: are they associated with thrust or extensionalfaults?