Jesús Galindo-Zaldívar

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

AbstractThe 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 Filabres 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 simila...

40Ar/39Ar geochronology of Alpine tectonism in the Betic Cordilleras (southern Spain)

The 40Ar/39Ar dating method has been applied to metamorphic rocks of the Alpujarride and Nevado-Filabride nappes (Alboran domain, SE Spain) in a first attempt to discriminate individual phases of deformation and metamorphism. The upper Nevado-Filabride nappes experienced an early eclogitic and blueschist metamorphism for which a barroisitic amphibole indicates a minimum age of 48 Ma. An Early Miocene age is attributed to the subsequent amphibolite facies metamorphism. Deformation associated with this metamorphic evolution is of unknown direction. The Alpujarride nappes record a plurifacial metamorphic evolution with the superimposition of low-pressure assemblages upon high-pressure ones with variable P–T ratios. Phengite from a carpholite-bearing high pressure-low temperature (HP/LT) assemblage gives an age of 25 Ma interpreted to reflect the end of the high-pressure evolution. Biotite and muscovite from high-grade metamorphic rocks overprinted under low-pressure conditions yield similar closure ages of 19 Ma dating cooling after the main episode of E–NE directed ductile deformation. This deformation was followed by W–SW directed extensional events producing brittle structures in the Alpujarride nappes and ductile-brittle shearing in the Nevado-Filabride nappes. Biotite and muscovite from ductile sheared rocks in the detachment zone between the two complexes have concordant ages of 16–17 Ma related to the end of the extensional ductile deformation. Therefore, a correlation of metamorphic and tectonic events between the two nappe complexes seems possible only since the Early Miocene and later.

Active continental subduction beneath the Betic Cordillera and the Alborán Sea

P- and S-wave seismic tomography detect a low-velocity anomaly in the upper mantle beneath the Betic Cordillera and the Alboran Sea region. The anomaly is associated with the intermediate-depth seismicity (h

Stress and palaeostress in the Betic-Rif cordilleras (Miocene to the present)

Palaeostress orientation in the Betic-Rif Cordilleras can be determined for the Miocene to the Quaternary by analysis of brittle microstructures (faults, joints, striated pebbles and stylolites). In the central and northern South Iberian Domain, σ1 was subhorizontal and trended NW-SE, from the Early Miocene to the present. In the northern Alboran Domain, from the Burdigalian to the Serravallian, palaeostress ellipsoids were oblate, with σ3 subhorizontal and trending ENE-WSW. In the southern part of the South Iberian Domain and in the northern part of the Alboran Domain (Betic Cordillera) from the Tortonian to the Quaternary, palaeostress ellipsoids were variable: in the Granada region, they were prolate with σ1 subvertical; but in the Almeria region they were triaxial with σ2 subvertical and σ3 subhorizontal and trending NE-SW to E-W. In the Rif, the Alboran and African-Maghrebian domains were affected by palaeostresses with σ1 subhorizontal, varying from NE-SW in Tortonian times to N-S in the Plio-Quaternary. A study of earthquake focal mechanisms by the right-dihedra method indicates that the present-day general stress field around the Betic-Rif cordilleras is compressional, with σ1 subhorizontal and trending NW-SE. In the Betic-Rif Cordilleras, this general stress field is modified and the present-day orientations of σ1, σ2 and σ3 are nearly the same as those of the palaeostresses in the Pliocene and Quaternary. The distribution and evolution of stresses in the Betic-Rif cordilleras, from the Miocene to the present, can be correlated with the main Neogene deformations: extensional structures in the central and northern Alboran Domain and the southern part of the South Iberian Domain, and compressional structures in the South Iberian and African-Maghrebian domains and in the southern part of the Alboran Domain.

The Alpujárride-Nevado-Fibábride extensional shear zone, Betic Cordillera, SE Spain

In the Betic Cordillera, the initial thickening of the Alboran Domain produced an Alpine high pressure-low temperature (HP-LT) metamorphism in the Alpujarride and Nevado-Filabride complexes. After 19 Ma this thickening process evolved to an extensional stage with uplift rates of between 1 and 2 mm year−1. The extensional stage generated structures in a heterogeneous shear regime with a top-to-the-west sense of movement. The present-day Alpujarride-Nevado-Filabride contact is a detachment fault generated in this shearing regime. In the footwall, the first structure developed in this extensional stage is a planar-linear fabric characterized by constrictive strain, that is axial planar to kilometric recumbent tight to isoclinal folds. The planar-linear fabric is folded by inclined close to open folds with axes parallel to the stretching lineation. The planar-linear fabric and folds are deformed by a late extensional crenulation cleavage near the detachment surface. Finally, brittle deformation was generated within the same kinematic framework as that of the ductile structures. In the hanging wall, meanwhile, deformation had a brittle character producing faults and joints.

Recent and present-day stresses in the Granada Basin (Betic Cordilleras): Example of a late Miocene-present-day extensional basin in a convergent plate boundary

The diffuse convergent boundary between the Eurasian and African plates in the western Mediterranean is associated with a seismicity zone more than 300 km wide. Although the two plates are converging NW–SE, the Betic and Rif Cordilleras contain extensional structures that have been active since the Miocene. The extensional tectonics in the region, which occurred simultaneously with the uplift of the cordillera, have been analyzed in the southeastern sector of the late Miocene to recent Granada Basin, using earthquake focal mechanisms, the determination of paleostresses from the study of the orientation and kinematics of microfaults, and the study of the major structures. Both the geological surface data and the focal mechanisms indicate present-day regional conditions of NE–SW extension, with triaxial to prolate stress ellipsoids. However, the stress field is heterogeneous, with local variations in stress over time, with different stresses sometimes even acting simultaneously in adjacent areas. The most frequent changes consist of pluridirectional or NE–SW extension, favored by the prolate character of the stress ellipsoids, and NW–SE subhorizontal compression, favored by the regional tectonic setting. Strike-slip faults are scarce even though they are the most likely structure to be expected in a region with SW–NE extension and NW–SE compression. Seismicity is concentrated in the upper crust and may correspond to the activity of low- to high-angle normal faults similar to the surface faults, although they can not be correlated with them. The lower cutoff of this seismicity probably coincides with the 300°C isotherm and suggests a low thermal gradient for the region. Present-day regional stresses have σ1 vertical at the surface but in depth plunge toward the SW.

Inversion of moment tensor focal mechanisms for active stresses around the microcontinent Iberia: Tectonic implications

The Iberian microcontinent and its connected oceanic crust are affected by deformations related to the Eurasian-African plate boundary. Active stress inversions from populations of moment tensor focal mechanisms have been performed around and inside the Iberian peninsula, using a total of 213 moment tensor estimates. Main results are: 1) The tensorial solutions show better consistency and lower misfits compared to those obtained previously from first P arrival focal mechanisms. 2) Along the Eurasia- Africa western boundary, the type of active stresses progressively changes easternwards from triaxial extension to uniaxial compression along the Terceira Ridge, the Gloria Fault zone and the Gulf of Cadiz. 3) In the Betics-Alboran-Rif zone, uniaxial extension predominates with Shmax N155oE trending. 4) In N Algeria, uniaxial compression reappears. 5) The Iberian foreland is currently under strike-slip to uniaxial extension tensorial conditions.

Contourite deposits in the central Scotia Sea: the importance of the Antarctic Circumpolar Current and the Weddell Gyre flows

Abstract New swath bathymetry with multichannel and high resolution seismic profiles shows a variety of contourite drift, sediment wave morphologies, and seismic facies in the central Scotia Sea. The deposits are to be found at the confluence between the two most important bottom current flows in the southern ocean: the eastward flowing Antarctic Circumpolar Current (ACC) and the northward outflow of the Weddell Sea Deep Water (WSDW). The contourite drifts are wedge-like deposits up to 1 km thick, that exhibit aggradational reflectors along axis thinning towards the margins. The contourite drifts occur in areas of weaker flows along the margins of contourite channels and in areas protected by obstacles. The elongate-mounded drifts are best developed along the left-hand margins of channelized bottom current flows, due to the Coriolis force. A contourite fan has a main channel and two distributary channels that expand over a gentle seafloor. The proximal fan exhibits sediment waves with the distal fan incised by furrows. Sediment wave fields are well developed in areas of intensified bottom flows without channels, particularly at the confluence of the ACC and the WSDW. Small sediment waves occur where unidirectional bottom current flows predominate. Sediment waves may develop under the influence of internal waves produced by the interaction of the flows and sea-bottom relief. The stratigraphic sequence above the oceanic crust of Early to Middle Miocene age contains six seismic units separated by major reflectors. All the units were shaped under the influence of strong bottom current flows, although they exhibit distinct seismic facies changes that record the variations of the bottom current pathways over time. The age of the units was calculated based on the age of the oceanic crust and sedimentation rates of deep-sea deposits in the region. The oldest, Units VI–IV, are of Early to Middle Miocene age and developed under the influence of the ACC. They are characterized by a southward progradational pattern of the seismic units and sedimentation rates of 5–8 cm/ky. Unit III, with an estimated Middle Miocene age, evidences the first incursion of WSDW into the central Scotia Sea, when plate movement caused openings in the South Scotia Ridge and allowed the connection with the northern Weddell Sea through Jane Basin and gaps in the ridge. Unit II, estimated to be of Late Miocene to Early Pliocene age, extends over the area and is characterized by internal unconformities. A major unconformity at the base of Unit II records an important reorganization of bottom current flows that may predate the onset of grounded ice sheets on the Antarctic Peninsula shelf. Unit I, of Late Pliocene to Quaternary age, shows intensified bottom currents. The unconformity at the base of Unit I probably predates the onset of major Northern Hemisphere glaciations and the greater expansion of Antarctic ice sheets during the Late Pliocene. The extensive distribution of contourite deposits above the oceanic crust testifies to the long-term production of Antarctic Bottom Water. Cold, deep water was swept northward from the Weddell Gyre, interacting with the ACC, and possibly exerting profound influences on the global circulation system and the onset of major glaciations.

Continental fragmentation along the South Scotia Ridge transcurrent plate boundary (NE Antarctic Peninsula)

Abstract The study of the South Scotia Ridge on the basis of swath bathymetry, multichannel seismic and magnetometry profiles, obtained during the HESANT92/93 cruise and complemented with satellite gravimetry and seismicity data illustrates the tectonics of the region. The thinned continental crust fragments of the ridge are bounded by oceanic crust of the Scotia Sea to the North and Powell Basin to the South. The northern boundary represents the contact between the Scotia and Antarctic plates. This boundary is a sinistral transpressional fault with transtensional segments and moderate recent tectonic activity. Another fault located at the southern boundary appears inactive and does not reveal any features that would enable the kinematics to be determined. Both faults have associated steep scarps since they separate oceanic and continental crust types. The most significant active deformation lies in the axial depression of the ridge, within a band delineated by fault systems with WSW-ENE and SW-NE strikes. These faults develop pull-apart basins, which separate the northern and southern blocks of the ridge. The northern block is being fragmented from the Antarctic Plate by a zone of transtensive faults, and is probably a crustal element independent of the Antarctic Plate. The axial depression, which crosses the ridge slightly obliquely, is characterized by deep basins locally more than 5000 m deep and associated high seismicity. The fault geometry and earthquake focal mechanisms indicate an active sinistral transtensive regime for the fault system, although it may locally have transpressive regimes depending on the fault plane and the stress field orientations. The internal basins are characterized by an asymmetric development showing itself as depositional wedges generally thickening northward. Deposits onlap the southern margins and are affected by normal faults in the northern margins. The seismicity around the Scotia Plate shows that the present stresses are compressive along the northern boundary with the South American Plate (σ 1 SW-NE and subhorizontal) and along the western boundary with the Antarctic Plate (σ 1 WNW-ESE and subhorizontal). For the South Scotia Ridge, however,σ 1 is steeply inclined andσ 3 is subhorizontal with a NW-SE trend. The stress distribution in Bransfield Strait is similar to the ridge and the recent extension could be partially explained by the westward continuation of the active fault system of the central South Scotia Ridge. The fragmentation of continental crustal blocks, due to the tectonic activity along the transcurrent plate boundaries, is a mechanism that contributes to the deformation of the northeastern end of the Antarctic Peninsula. This area appears appropriate for the analysis of continental plate fragmentation processes.

Active faulting in the internal zones of the central Betic Cordilleras (SE, Spain)

The internal zones of the Betic Cordilleras show a present-day relief that is mainly controlled by kilo- metre-size, symmetrical or north-vergent folds which developed mostly since Middle Miocene times. The Sierra Nevada, Sierra Alhamilla, Sierra de Los Filabres, Sierra Tejeda and Sierra de Gador, among others, are roughly E-W trending high mountain ranges, corresponding to antiforms where metamorphic rocks crop out. The surrounding depressions are located in synforms, where Neogene rocks are preserved from erosion. Field evidence shows that the growth of the folds is coeval with fault development, and that at least three of them, i.e. the Padul Fault, the Zafarraya Fault, and the Balanegra Fault, may be considered to be active seismogenetic structures. The Zafarraya Fault, in particular, is thought to be responsible for the 1884 Andalucia Earthquake. The fault is located at the northern limb of the Sierra Tejeda antiform, and could be interpreted as a collapse structure developed along the external arch of the uplifted fold. The Padul and Balanegra faults are located at the southeastern border of the Granada Basin and south of the Sierra de Gador, respectively. They belong to a set of NW-SE oriented faults that are mainly normal in character and indicate NE-SW extension. The set up, since 1999, of a GPS network within and around the Granada Basin and the planed installation of a new network in the Sierra Tejeda, will give us new insights on the present-day deformation behaviour of both folds and faults in the area.