R. Vegas

SEISMIC CRUSTAL STRUCTURE IN THE SOUTHWEST OF THE IBERIAN PENINSULA AND THE GULF OF CADIZ

Abstract The crust under the southwestern Iberian Peninsula and the Gulf of Cadiz has been sampled by 1200 km of deep seismic refraction/wide-angle reflection profiles, together with many seismic reflection lines and bore-holes. Wide-angle seismic data were collected during the last three decades. Commercial multichannel data provide a detailed image of the uppermost crust, improving the confidence about the models of the deeper structures. P-wave velocities within the thick column of sediments in the Gulf of Cadiz range from 2.0 to 3.8 km/s, while the Algarve and the Sines areas have higher velocities of 4.3 to 4.8 km/s. The top of the Palaeozoic basement rises to the northwest, outcropping in the South Portuguese zone of the Iberian Massif, and is characterized by P-wave velocities of 5.7–5.9 km/s. High velocities of 6.4 km/s have been found at shallow depths of 7 to 10 km in the South Portuguese zone, that could be related to the mafic and ultramafic rocks in the Beja-Acebuches zone. Lower crustal velocities are in the range of 6.7–6.9 km/s. The crustal thickness shows important lateral changes from 29 km beneath the Guadalquivir Basin/Iberian Massif contact to 35 km in the southeastern part of the South Portuguese zone. From the interpretation of these seismic data, a geodynamic model of the evolution of the crust in Southwestern Iberia and the Gulf of Cadiz is proposed. The Guadalquivir Basin, and its continuation at sea, the Gulf of Cadiz, is a flexure area of the crust that could be related to the overloading due to the overthrusting of the Alboran Domain over the Iberian plate.

Widespread Neogene remagnetization in Jurassic limestones of the South-Iberian palaeomargin (Western Betics, Gibraltar Arc)

Abstract Palaeomagnetic investigations in Upper Jurassic carbonate rocks from the Western Subbetics (Betic Cordillera, Southern Spain) testify to a widespread Neogene remagnetization. Progressive thermal and alternating field demagnetization analyses reveal the presence of two stable components of the natural remanent magnetization (NRM). The NRM is dominated by a pervasive Neogene overprint, which always shows normal polarity and maximum unblocking temperatures of 450°C throughout the whole region studied. The primary Jurassic component, however, can also be isolated in many outcrops. This component has low intensity values and maximum unblocking temperatures of about 550°C, showing both normal and reversed polarities. Palaeomagnetic and rock magnetic analyses indicate that both primary Jurassic and secondary Neogene components are carried by magnetite. The incremental fold test performed in all sampling localities demonstrates that the remagnetization occurred during some stage of the Neogene deformation of the Subbetics, being pre-, syn- or post-folding in the various folds studied.

Plate tectonics and volcanism in the gibraltar arc

Abstract Late Tertiary and Quaternary volcanism of southeastern Spain can be fitted in a platetectonics model, taking into account the post-Paleozoic evolution of the stable and semimobile Iberian areas and the new orogenic belts bordering the Mediterranean between Africa and the Iberian Peninsula. The occurrence and distribution of calc-alkaline and potassic volcanism suggest an oceanic crust sinking downwards from the Iberian plate. This active margin is causally related to the convergence and collision of Iberia and Africa during Late Cretaceous—Early Miocene time span. A pre-collision distensive phase is inferred from the stratigraphie and tectonic record between the Triassic and Late Cretaceous, while since the Late Miocene another distensive phase is related to the actualistic features.

Model of distributed deformation, block rotations and crustal thickening for the formation of the Spanish Central System

Abstract The Spanish Central System constitutes an important morphostructural intraplate feature of Iberia that resulted from a concentration of crustal strain derived from stresses applied on the boundaries of the Betics and Pyrenees. We propose a tectonic model in which the deformation of this zone was produced by a mid-late Mesozoic shear zone, a type of secondary plate boundary, that absorbed part of the Iberia-Eurasia relative motion. The deformation within this E-W shear zone was compensated by means of block rotations that resulted from the reorganization of multiple and previous faults and fractures. The subsequent N-S Africa-Eurasia convergence was partly accommodated within this intraplate zone of weakness. This caused a concentration of strain that was responsible for the homogeneous thickening of the lower crust, and for the blocking of block rotations and forced uplifting of the upper crust.

The Gulf of Valencia continental shelf: Extensional tectonics in Neogene and Quaternary sediments

Abstract Seismic reflection profiles obtained with mud-penetrator (3.5 kHz), Sparker (3000 J), Uniboom (300 J) and side-scan sonar (100 kHz), enabled us to map recent morphostructural features (extensional structures) in Neogene and Quaternary sediments of the Gulf of Valencia shelf. These structures are dominated by a group of normal faults trending subparallel to the coast. The faults produced a series of steps in the Neogene-Quaternary depositional series. These sedimentary formations are very thick throughout the area, decreasing southward in the direction of the Betic system. This distensive regime is still active today producing structures which affect the morphology of the shelf.

Palaeomagnetic results from Upper Miocene and Pliocene rocks from the Internal Zone of the eastern Betic Cordilleras (southern Spain)

Palaeomagnetic and rock-magnetic studies were carried out on samples from thirteen volcanic and nine sedimentary nsites of Late Miocene to Pliocene age from the Internal Zone of the eastern Betic Cordilleras. After comparing npalaeomagnetic results with the expected Pliocene/Miocene direction, rotated and unrotated areas can be recognized, nrotations being clockwise and counter-clockwise. Some rotations are of great magnitude. Unlike deformation in the nExternal Betics, Late Miocene to Present block rotations in the Internal Betic Zone are non-systematic, and related to the nmovement of faults as local responses to the Late Miocene-Present regional stress field, due to the N140 convergence of nAfrica and the Iberian Peninsula.

Wrench (transcurrent) fault System of the southwestern Iberian Peninsula, paleogeographic and morphostructural implications

ZusammenfassungEine Kartierung des südwestlichen Hesperischen Massivs (Südwestspanien) hat zwei SW-NE und NNW-SSE gerichtete Hauptsysteme von sich kreuzenden Blattverschiebungen ergeben. Auf Grund der Ausmae und des Zusammenhanges mit mächtigem Grundgebirge können sie als Störungen angesehen werden, die die gesamte Kruste angehen. Die Richtungen der Systeme liegen in einem Schema spät-herzynischer Blattverschiebungen, die während der Ausräumung der Iberischen Halbinsel am Ende der herzynischen orogenen Phasen entstanden. Anhand dieser groen Bruchlinien ist es möglich, a) die Gebiete mesozoischer Sedimentation und prä-alpiner Dehnung, b) die Reliefs der mesozoischen Bedeckung im Innern des Hesperischen Massivs, c) die Reliefs und die tektonischen Becken im Innern der Meseta und d) die mögliche Entwicklung der Iberischen Tafel in bezug auf die Bucht von Biskaya und die Gebiete alpiner Kompression des südlichen Randes, der Betischen Ketten, zueinander in Beziehung zu setzen.AbstractMapping of the Southwestern Hesperian Massif (South Western Spain) has permitted the definition of two principal systems of wrench faults in conjugate directions, SW-NE and NNW-SSE. By their dimensions and their association with deep basic rocks, they may be considered as accidents which interest the entirety of the crust. The directions of these systems are enclosed in a scheme of late-Hercynian wrench faults produced in the dismantling of the Iberian Peninsula at the end of the orogenic Hercynian phases. According to these large lines of fracture it is possible to relate a) the areas of Mesozoic sedimentation, pre-Alpine distension, b) the reliefs of the Mesozoic covering of the interior of the Hesperic Massif, c) the reliefs and the tectonic basins of the interior of the Meseta and d) the possible evolution of the Iberian Plate with relation to the Bay of Biscay and the areas of Alpine compression of the meridional edge, Betic Chains.ResumenUna cartografía del SW del Macizo Hesperico (SW de España) ha permitido definir dos sistemas principales de decrochements de direcciones conjugadas SW-NE y NNW-SSE. Por sus dimensiones y su asociación a rocas básicas profundas, se pueden considerar como accidentes que interesan a la totalidad de la corteza. Las direcciones de estos sistemas encajan en un esquema de decrochements tardihercínicos producidos en el desmantelamiento de la Península Ibérica al final de las fases orogénicas hercínicas. SegÚn estas grandes lineas de fractura es posible relacionar a) las áreas de sedimentación mesozoica, distensión prealpina, b) los relieves de la cobertera mesozoica del interior del Macizo Hesperico, c) los relieves y las fosas tectónicas del interior de la Meseta y d) la posible evolución de la Placa Ibérica frente a la apertura del Golfo de Vizcaya y las áreas de compresión alpina del borde meridional, Cadenas Béticas.кРАткОЕ сОДЕРжАНИЕпОлЕВАь сЩЕМкА УгО-жА пАДНОИ ЧАстИ гЕспЕРИ ИскОгО МАссИВА, УгОжАпАДНАь ИспАНИь, УстАНОВИлА Д ВЕ ОсНОВНых сИстЕМы пЕРЕкРЕЩИВАУЩИхсь с ДВИгОВ В НАпРАВлЕНИьх SW-NE И NNW-SSW. пО Их ОБЩЕМУ И Их сВьжИ с М ОЩНыМ ФУНДАМЕНтОМ Их МОжНО РАссМАтРИВАть, кАк НА РУшЕНИь, жАхВАтыВАУЩИЕ И кОРУ. пРОстИРАНИЕ ЁтИ х сИстЕМ УклАДыВАЕтс ь В схЕМУ пОжДНЕгЕРцИНскОгО сДВИгА, ИМЕВшЕгО МЕст О ВО ВРЕМь пРЕОБРАжОВ ЕНИь ИБЕРИИскОгО пОлУОст РОВА В кОНцЕ гЕРцИНскОгО О РОгЕНА. НА ОсНОВАНИИ Ё тИх БОльшИх стРУктУРНых ЁлЕМЕНт ОВ МОжНО УстАНОВИть: А) ОБ лАстИ МЕжОжОИскИх ОсАДкОНАкОплЕНИИ И Д ОАльпИИскОгО РАстьжЕНИь; Б) РЕльЕФ М ЕжОжОИскОгО пОкРОВА ВНУтРИ гЕспЕРИДскОгО МАссИВА; В) РЕльЕФ И тЕк тОНИЧЕскИЕ БАссЕИНы ВНУтРИ МЕжЕты; г) ВОжМОжНОЕ РАжВИтИЕ ИБЕРИИскОИ плАтФОРМы пО ОтНОшЕН ИУ к БИскАИскОМУ жАлИВУ И ОБлАстИ АльпИИскОг О сжАтИь УжНОгО кРАь Б ЕтскОгО хРЕБтА.

Continental-Oceanic Crustal Transition in the Bransfield Trough and the South Scotia Ridge (Antarctica); Preliminary Results

A total of 1.500 km of multichannel seismic reflection profiles recorded aboard B/O Hesperides using a seven gun array and a 96 channel streamer during the austral summer of 1991-1992 were used to study four structural provinces on the Antarctic and Scotia plates. These provinces which differ in structural style and crustal character include:

Problems and discussion on Precambrian series of the Hesperic Massif (Western Iberian Peninsula)

ZusammenfassungAnhand von bereits bestehender Literatur und noch unveröffentlichten Forschungsergebnissen der Verfasser sollen die verschiedenen präkambrischen Serien des Hesperischen Massivs beschrieben, ihre Beziehung zueinander erörtert und die durch sie auftretenden Probleme behandelt werden. Alle diese Serien bilden Sattelkerne in den herzynischen Strukturen.Da wir bis heute nur über unvollständige Kenntnisse dieser Formationen verfügen und aufgrund ihres Vorkommens an weit voneinander entfernt liegenden Orten, treten bei dem Versuch, ihre Stratigraphie festzulegen und eine Wechselbeziehung zwischen den verschiedenen Serien herzustellen, eine Reihe von Problemen auf.Betrachtet man diese Probleme als Gesamtkomplex, ist es möglich, eine vorläufige Gesamt-Stratigraphie sowie eine wahrscheinliche Wechselbeziehung zwischen den verschiedenen Einheiten aufzuzeigen.Alle Serien gehören in das jüngere Präkambrium, das in drei Teile aufgegliedert werden kann: der unterste Teil wird aus basischen und ultrabasischen Komplexen gebildet, der mittlere Teil setzt sich aus bedeutenden sauren Vulkanismen und daraus hervorgegangenem Gestein zusammen, während der oberste Teil vor allem aus pelitischen Schiefern und Grauwacken besteht.AbstractIn this paper, different Precambrian series of Hesperic Massif have been described, using some data taken from different bibliography and unpublished data by the authors. The relation among them have been discussed and also their problems stated.These Precambrian series are forming the anticlinorium nuclei of hercynian structure. The incomplete understanding of these series, as well as their occurrence in distant and isolated outcrops, give place to doubtful interpretations when traying to establish the stratigraphical sequence and he correlation among the different series.The discussion implied from the global study of these problems permit us, provisionally, to establish a general stratigraphy and a possible correlation among different units.All these series constitue an Upper Precambrian; within it three groups have been differentiated: lower group consisting of basic and ultrabasic complexes; intermediate group with a very important acid volcanism and derivated rocks, and finally upper group where pelitic schists and greywackes prevail.RésuméPar des données bibliographiques et inedites les auteurs décrivent les differentes séries precambriènnes du Massif Hésperique, en discutant leurs rapports et en établissant leurs problèmes.Toutes ces séries se situent aux coeurs des anticlinaux dans les structures hercyniennes; leur connaissance insuffisante, aussi bien que leurs affleurements distants et isolés, causent des problèmes dinterprétation dés que lon tente détablir une stratigraphie générale et une corrélation entre les différentes séries.La discussion générale de ces problèmes nous a permis de définir une stratigraphie densemble et la possibilité dune corrélation des différentes unités.Toutes ces séries appartiennent au Précambrien Supérieur, qui comprend trois ensembles: Un ensemble inférieur formé de complexes basiques et ultrabasiques, un autre intermédiaire avec volcanites acides et roches dérivées et, finalement, un ensemble supérieur pélitique-grauvackeux.ResumenSe describen con datos bibliográficos e inéditos de los autores las distintas series precámbricas del Macizo Hespérico, discutiendo las relaciones entre ellas y planteando su problemática.Estos precámbricos forman núcleos anticlinoriales de las estructuras hercínicas. El incompleto conocimiento que hasta hoy se tiene de estos terrenos y su afloramiento en zonas distantes y aisladas, da lugar a problemas de interpretación, al intentar establecer una estratigrafía y una correlación entre las diferentes series.La discusión que se plantea al estudiar estos problemas globalmente, permite establecer provisionalmente una estratigrafía de conjunto y la posible correlación entre las diferentes unidades.Todas estas series constituyen un Precámbrico Superior en el que se han podido diferenciar 3 conjuntos:Uno inferior constituido por complejos básicos y ultrabásicos; otro medio con importantes volcanismos ácidos y rocas derivadas; y otro superior formado por esquistos pelíticos y grauvacas predominantemente.Краткое содержаниеНа основании литерат урных источников и ещ е неопубликованных ре зультатов своих исследований а вторы рассматривают различные докембрийские свиты Гесперидского масси ва.Они обсуждают взаимо связь этих свит и возникающ ие при этом проблемы. Все эти свиты представля ют собой ядра антиклиналий герцинских структур.Из-за скудности сведе ний об этих формациях, установленных на бол ьших расстояниях друг от д руга, при попытке пров ести корреляцию между раз личны ми свитами, возникает ряд проблем.Рассматривая эти про блемы, как целое, можно начертить пока еще временную общую стратиграфию и возможное взаимоотн ошение между различными струк турными единицами. Весь комплекс относи тся к более позднему д окембрию, который можно под разделить на три свит ы: нижняя состоит из ос новных и ультраосновных поро д; средняя образована к ислыми породами вулк анического происхождения и по родами, возникшими из этих последних, а верх няя представлена грауваккоqaргиллитов ым составом.