Ocean basins near the Scotia–Antarctic plate boundary: Influence of tectonics and paleoceanography on the Cenozoic deposits

The distribution of seismic units in deposits of the basins near the Antarctic–Scotia plate boundary is described based on the analysis of multichannel seismic reflection profiles. Five main seismic units are identified. The units are bounded by high-amplitude continuous reflectors, named a to d from top to bottom. The two older units are of different age and seismic facies in each basin and were generally deposited during active rifting and seafloor spreading. The three youngest units (3 to 1) exhibit, in contrast, rather similar seismic facies and can be correlated at a regional scale. The deposits are types of contourite drift that resulted from the interplay between the northeastward flow of Weddell Sea Bottom Water (WSBW) and the complex bathymetry in the northern Weddell Sea, and from the influence of the Antarctic Circumpolar Current and the WSBW in the Scotia Sea. A major paleoceanographic event was recorded by Reflector c, during the Middle Miocene, which represents the connection between the Scotia Sea and the Weddell Sea after the opening of Jane Basin. Unit 3 (tentatively dated ∼Middle to Late Miocene) shows the initial incursions of the WSBW into the Scotia Sea, which influenced a northward progradational pattern, in contrast to the underlying deposits. The age attributed to Reflector b is coincident with the end of spreading at the West Scotia Ridge (∼6.4 Ma). Unit 2 (dated ∼Late Miocene to Early Pliocene) includes abundant high-energy, sheeted deposits in the northern Weddell Sea, which may reflect a higher production of WSBW as a result of the advance of the West Antarctic ice-sheet onto the continental shelf. Reflector a represents the last major regional paleoceanographic change. The timing of this event (∼3.5–3.8 Ma) coincides with the end of spreading at the Phoenix–Antarctic Ridge, but may be also correlated with global events such as initiation of the permanent Northern Hemisphere ice-sheet and a major sea level drop. Unit 1 (dated ∼Late Pliocene to Recent) is characterized by abundant chaotic, high-energy sheeted deposits, in addition to a variety of contourites, which suggest intensified deep-water production. Units 1 and 2 show, in addition, a cyclic pattern, more abundant wavy deposits and the development of internal unconformities, all of which attest to alternating periods of increased bottom current energy.     

The Mecina Extensional System: Its relation with the post-Aquitanian piggy-back Basins and the paleostresses evolution (Betic Cordilleras,Spain)

The Alboran Domain has undergone a westsouthwestern-east-northeastern post-Aquitanian minimal extension of 104% in two stages. In the first one (Burdigalian-early Tortonian), the Mecina Extensional System was active, the Neogene Basins were piggy-back ones, and stress ellipsoids were oblate. In the second stage (Tortonian-Present), stress ellipsoids were prolate, and the original shape of the outcropping Neogene Basins was nearly the same as their present morphology. An orogenic wedge model is proposed in which the westward strain of the wedge has been produced by variations in the spreading rates of the Ligurian-Balearic and Tyrrhenian basin oceanic crusts.     

Progressive extensional shear structures in a detachment contact in the Western Sierra Nevada (Betic Cordilleras,Spain)

Abstract

The present contact caused by the superposition of the Alpujarride complex over that of the Nevado-Filabride in the western area of Sierra Nevada and Sierra de Filábres corresponds to a detachment. The deformation in the footwall associated with this contact, produced mylonitic fabrics with a significant stretching-lineation, over which brittle structures are superimposed. The deformation in the hanging wall associated with this contact is, on the other hand, essentially brittle. These deformations are subsequent to a series of syn-to post-metamorphic structures related to thrust phases.

The micro- and meso-structures indicate that the hanging wall has moved towards the west-south-west.

Other brittle structures, which began during the same extensional regime, are superimposed on the detachment and have continued to develop up to the present time. These structures were produced in an extensional regime with a non-coaxial deformation component and suggest the possibility of a tectonic evolution similar to that described for core complexes in the USA.     

Superposition of extensional detachments during the Neogene in the internal zones of the Betic cordilleras

 In the internal zones of the Betic cordilleras, extensional structures have developed from the Upper Oligocene to the present day; they are contemporaneous with compressional structures (folds and thrusts) in the external zones. From the Upper Oligocene to the Aquitanian, extension occurred in the Maláguide/Alpujárride detachment, and related structures show varying kinematics in different sectors. Younger deformations with a top-to-the-N sense of movement have affected Nevado-Filábride (ductile shear zones), Alpujárride (ductile and brittle shear zones) and Maláguide rocks (normal faults). At least from the Late Burdigalian up to the Lower Tortonian, displacements have occurred in the Alpujárride/Nevado-Filábride detachment. Deformations have been generally non-coaxial, with a top-to-the-W sense of movement. Stretching lineation trends in the Nevado-Filábride rocks curve from E to W suggesting a progressive variation of the ductile-shear-zone kinematics related to the Alpujárride/Nevado-Filábride detachment between the Aquitanian and Lower Tortonian stages. Deformations from the Lower Tortonian to the present day are normal faults, formed in extensional settings in the upper part of the crust, and folds and strike-slip faults which indicate N–S to NNW–SSE shortening directions and E–W to ENE–WSW extension directions. Received: 26 December 1995 / Accepted: 26 January 1996     

Highly ordered antigorite from Cerro del Almirez HP–HT serpentinites,SE Spain

The Cerro del Almirez ultramafic massif offers an example of high pressure and high temperature antigorite serpentinites. A sharp antigorite-out isograd separates them from Chl-harzburgites, consisting of olivine + enstatite + chlorite. Antigorite is characterized by aluminium contents as high as 4 wt.% Al₂O₃. The microstructural study shows that, prior to the transformation, antigorite is exceptionally ordered and consists of the polysome m = 17. No polysomatic defect occurs in antigorite forming most of the Cerro del Almirez serpentinites. Close to the antigorite-out isograd, limited disorder features may occur, mainly as (001) twins, reaction rims and reduction of m down to 14–15. Here, local recrystallization phenomena lead to sporadic growth of large antigorite and chlorite crystals.     

Models of magnetic and Bouguer gravity anomalies for the deep structure of the central Alboran Sea basin

 Magnetic and gravimetric data from the central Alboran Sea allow identification of two axes of crustal thinning, which were probably active during the Oligocene–Early Miocene. The western Alboran basin axis is subparallel and may be related in origin to the Gibraltar Arc. The ENE–WSW trending Alboran Channel axis is probably intruded by basic igneous rocks and may represent the western end of the Algerian–Balearic basin rift. Present-day small areas with high heat flow may well be related to volcanism and an anomalous mantle. Areas of active deformation in the Alboran Sea accommodate the present Eurasia-Africa convergence. Received: 17 May 1996 / Revision received: 19 April 1997     

Repeated palaeoseismic activity of the Ventas de Zafarraya fault (S Spain) and its relation with the 1884 Andalusian earthquake

One of the most destructive historical earthquakes (M 6.7) in Spain occurred in 1884 along the normal Ventas de Zafarraya Fault located in the Central Betic Cordilleras. Palaeoseismic and radiocarbon data presented in this study are the first to constrain the timing of the pre-1884 fault history in the last 10 ka. These data yield a recurrence interval of between 2 and 3 ka for major earthquakes, under the assumption of uniform return periods along the normal fault. The Holocene slip rate is estimated to be in the order of 0.35±0.05 mm/year, which is significantly higher than the mean slip rate of 0.17±0.03 mm/year since the Tortonian. Several of the most important deformations and secondary features, such as landslides and liquefaction, are related to strong ground motion and document the Holocene activity of the Ventas de Zafarraya Fault.