Tectonothermal evolution of a foreland fold and thrust belt: the Cantabrian Zone (Iberian Variscan belt,NW Spain)

Analysis of the conodont colour alteration index and the Kübler index of illite allowed us the characterization of four types of very low‐ or low‐grade metamorphism in the Cantabrian Zone (CZ) and determination of their regional and temporal distribution. These types are: (1) an orogenic Variscan metamorphism present only in restricted areas of the western and north‐western parts of the CZ where epizonal conditions are reached; (2) a burial metamorphism that appears in the basal part of some nappes, where anchizonal conditions are sometimes achieved; the thermal peak preceded emplacement of the nappes; (3) a late‐Variscan metamorphism in the southern and south‐eastern parts of the CZ; a cleavage, cutting most of the Variscan folds, is associated with this metamorphism, which has been related to an extensional episode; (4) a contact metamorphism and hydrothermal activity associated with minor intrusive bodies. The extension continued after the Variscan deformation giving rise to hydrothermal activity during Permian times.     

Thin skinned tectonics in the Ponga region (Cantabrian Zone,NW Spain)

The Variscan thrust and associated fold structures of the Ponga Region (Cantabrian Zone, NW Spain) are interpreted in terms of thin skinned tectonics, with a dominant eastward transport direction.A 2.5 km thick sequence of Paleozoic rocks was deformed by an east vergent thrust system that includes an imbricate fan and a duplex. The thrust surfaces have a very irregular map outcrop pattern due to the existence of a set of folds (longitudinal and transverse systems). A strike-normal balanced section illustrates the geometry of the thrusts and their related folds. The minimum value of accumulate transport is about 62 km. A tear fault can be recognized in a transverse cross-section.A later out-of-sequence thrust system (e.g. Peña Ten Thrust) with dominant southward direction is superimposed upon the earlier eastward verging thrust system. These thrust reactivate the earlier lateral structures as frontal structures.

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Zusammenfassung Die Vartszische Front und damit verbundene Faltenstrukturen des Pongagebietes (Kantabrische Zone, NW Spanien), werden mit Hilfe von »thin skinned tectonic« mit dominierendem ostwärts gerichtetem Transport interpretiert. Eine 2,5 km mächtige Folge paläozoischer Gesteine wurde durch ostvergente überschiebung deformiert. Dabei wurde ein Schuppenfächer und eine Duplexstruktur ausgebildet.Das Ausbei\en der überschiebungsbahn an der Oberfläche ist duch die Existenz longitudinaler und transversaler Faltensysteme sehr unregelmä\ig. Ein Profil senkrecht zum Streichen verdeutlicht die Geometrie der überschiebungen und Faltensysteme.Die gesamte Transportweite beträgt mindestens 62 km.In einem diagonalen Profil kann eine »tear fault« festgestellt werden.Ein späteres überschiebungssystem (z. B. Peña Ten überschiebung), mit hauptsächlich Südwärts gerichtetem Transport hat das frühere Ostwärts vergierende System überprägt. Das jüngere überschiebungssystem hat die älteren Lateralstrukturen als Frontstrukturen reaktiviert.

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Resumen La estructura de la Región del Ponga (Zona Cantábrica, NO de España) es interpretada en el contexto de su situación en la zona externa de una cordillera, y consiste en un sistema de cabalgamientos que han sido emplazados predominantemente hacia el Este con un conjunto de pliegues asociados. Las superficies de cabalgamiento muestran un trazado cartográfico muy irregular debido a la existencia de un conjunto de pliegues transversales a ellos. La Ventana Tectónica del Río Monasterio representa una estructura lateral de estos mantos. El desplazamiento mínimo calculado en una sección a través de la parte central de la región es de 62 Km.Posteriormente, tiene lugar el emplazamiento hacia el Sur de un nuevo sistema de cabalgamientos que reactivan estructuras laterales del sistema previamente emplazado, pasando a constituir estas las estructuras frontales de los nuevos cabalgamientos.Estas estructuras fueron formadas durante la orogénesis varíscica y afectan a un conjunto de materiales Paleozoicos de 2.5 Km. de espesor.

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Tectono-thermal evolution in a region with thin-skinned tectonics: the western nappes in the Cantabrian Zone (Variscan belt of NW Spain)

 The palaeotemperature distribution in the transition from diagenesis to metamorphism in the western nappes of the Cantabrian Zone (Somiedo, La Sobia and Aramo Units) are analysed by conodont colour alteration index (CAI) and illite crystallinity (IC). Structural and stratigraphic control in distribution of CAI and IC values is observed. Both CAI and IC value distributions show that anchizonal conditions are reached in the lower part of the Somiedo Unit. A disruption of the thermal trend by basal thrusts is evidenced by CAI and IC values. There is an apparent discrepancy between the IC and CAI values in Carboniferous rocks of the Aramo Unit; the IC has mainly anchizonal values, whereas the CAI has diagenetic values. Discrepant IC values are explained as a feature inherited from the source area. In the Carboniferous rocks of the La Sobia Unit, both IC and CAI indicate diagenetic conditions. The anchimetamorphism predated completion of emplacement of the major nappes; it probably developed previously and/or during the early stages of motion of the units. Temperature probably decreased when the metamorphosed zones of the sheets rose along ramps and were intensely eroded. In the context of the Iberian Variscan belt, influence of tectonic factors on the metamorphism is greater in the internal parts, where the strain and cleavage are always present, than in the external parts (Cantabrian Zone), where brittle deformation and rock translation are dominant, with an increasing role of the burial on the metamorphism. Received: 11 May 1998 / Accepted: 19 January 1999     

Relationships between structural units in the Tormes gneiss dome (NW Iberian massif, Spain): geometry, structure and kinematics of contractional and extensional Variscan deformation

Situated in the inner zone of the Iberian massif, the Tormes gneiss dome is composed of two units with different lithological contents and metamorphic evolution. The upper unit consists of a thick sequence of low- to high-grade metasediments, ranging in age from Late Proterozoic to Silurian. The lower unit is a high-grade metamorphic complex composed mostly of granitic orthogneisses and minor amounts of metasediments. Four Variscan deformations are distinguished. At deep structural levels, the most prominent D1 ductile structures are recumbent anticlines with NE vergence, cored by orthogneisses, and separated by narrow synclines. These recumbent folds grade upward into less-flattened and NE-vergent steeper structures. The overall structure is that of a large-scale stacking of orthogneissic slices underlying a shortened and thickened sedimentary sequence that formed a huge orogenic wedge in this region. During the heterogeneous and ductile D2 deformation, the rheological behaviour of the orthogneisses and metasediments became similar. The vertical D2 shortening associated with a strong top-to-the-SE shearing in a large-scale subhorizontal shear zone folded the prior SW-dipping structures, developing SW-vergent folds with axes close to NW–SE L2 mineral and stretching lineations. D2 corresponds to post-collisional crustal thinning following D1 crustal thickening. The D3 and D4 late structures are much more localized and occurred under retrograde conditions, but have a significant effect on the final geometry of the metamorphic complex. This sequence of contractional and extensional deformative events permits a tectonic interpretation in the framework of the dynamic wedge theory based on the evolution in the time of the stress configuration applied to a portion of the crust.     

Phreatomagmatic volcanism during the Ordovician of the Cantabrian Mountains (NW Spain)

In the Barrios Formation of the southern Cantabrian Mountains pyroclastic rocks occur which have an Ordovician age. They form the filling of several craters and pipes, which are unconformably overlain by Ordovician respectively Silurian sediments. The pyroclastic material mainly consists of fractured quartz grains which are mixed with angular hydrothermally altered glas lapilli. The strong fragmentation of the volcanoclastic components, even in the lower parts of the pipes, is caused by phreatomagmatic eruptions under shallow marine conditions. The Barrios Formation which is disrupted by the volcanic necks, was deposited in a shallow marine environment with a very low sedimentation rate.Sills occur in the Oville-Formation below the Barrios Formation. They are partially older than the pyroclastic rocks. These older sills intruded into not yet lithified shales and sands where a high hydrostatic pressure prevented a phreatomagmatic explosion.Major and trace element analyses reveal that the sills in the Oville-Formation, volcanic bombs, subvolcanic xenoliths and a plug within the pyroclastic rocks belong to the alkali rock series. They are enriched in K, Ti, P and Zr and are similar to alkali olivine basalts and trachy-basalts.The Ordovician volcanism of the Cantabrian Mountains occured on continental crust which was subject to rifting in the Early Palaeozoic. The rifting processes are seen in connection with the separation of the Ibero-Armorican microplate from Gondwana.

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Zusammenfassung In der Barrios-Formation des südlichen Kantabrischen Gebirges treten pyroklastische Gesteine auf, die ein ordovizisches Alter haben. Sie bilden die Füllung mehrerer Krater und Schlote, die an ihrer Obergrenze diskordant von ordovizischen bzw. silurischen Sedimenten überlagert werden. Das pyroklastische Material besteht zum größten Teil aus zerbrochenen Quarzkörnern mit eingestreuten, scharfkantigen, hydrothermal veränderten Glaslapilli. Die starke Fragmentierung des vulkanoklastischen Anteiles selbst in den tieferen Bereichen der Schlote wurde durch phreatomagmatische Eruptionen unter flachmarinen Bedingungen verursacht. Der Barriosquarzit, der von den Schloten und Kratern durchschlagen wird, wurde in einem flachmarinen Bereich mit sehr niedriger Sedimentationsrate abgelagert.Lagergänge treten in der Oville-Formation im Liegenden der Barrios-Formation auf. Sie sind teilweise älter als die pyroklastischen Gesteine. Diese älteren Lagergänge intrudierten in die noch nicht verfestigten Tone und Sande, wo ein zu großer hydrostatischer Druck eine phreatomagmatische Explosion verhinderte.Haupt- und Spurenelementanalysen zeigen, daß die Lagergänge in der Oville-Formation, vulkanische Bomben, subvulkanische Xenolithe und ein Lavapfropfen innerhalb der Pyroklastika zu den Alkaligesteinen gehören. Sie sind angereichert an K, Ti, P und Zr und ähneln Alkaliolivinbasalten und Trachybasalten.Der ordovizische Vulkanismus des Kantabrischen Gebirges ereignete sich auf kontinentaler Krustenunterlage, welche im Altpaläozoikum einer Dehnung ausgesetzt war. Die Dehnungsprozesse werden im Zusammenhang mit der Abtrennung der ibero-amerikanischen Mikroplatte von Gondwana gesehen.

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Résumé Dans la formation de Barrios, dans les Monts Cantabriques méridionaux, existent des roches pyroclastiques d'âge ordovicien. Elles forment le remplissage de cratères et de cheminées surmontés en discordance par des sédiments ordoviciens ou siluriens. Pour leur plus grande part, ces matériaux pyroclastiques consistent en grains brisés de quartz mêlés de fragments hyaloclastiques anguleux altérés par des solutions hydrothermales. Les composants volcanoclastiques sont fortement fragmentés, même dans les zones les plus profondes des cheminées, phénomène causé par des explosions phréatomagmatiques dans une mer peu profonde. De fait, la formation du quartzite de Barrios, qui est percée par les cheminées, s'est déposée dans un milieu marin peu profond, à faible vitesse de sédimentation.La formation d'Orville, située sous la formation de Barrios, renferme des sills. Ces sills, en partie plus vieux que les roches pyroclastiques, se sont intrudés dans des pélites et des sables non encore lapidifiés, où la haute pression hydrostatique empêchait une explosion phréatomagmatique.Les analyses des éléments chimiques majeurs et en trace montrent que les sills, les bombes volcaniques, les xénolithes subvolcaniques et un culot de lave dans les roches pyroclastiques font partie de la séries alcaline. Ces roches sont enrichies en K, Ti, P, Zr et semblables aux basaltes alcalins à olivine et aux trachybasaltes.Le volcanisme ordovicien des Monts Cantabriques s'est manifesté dans une croûte continentale soumise à extension pendant le Paléozoque inférieur. Cette extension est mise en relation avec la séparation entre la micro-plaque ibéroarmoricaine et le continent de Gondwana.

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Rheological control on the tectonic evolution of a continental suture zone: the Variscan example from NW Iberia (Spain)

The Variscan continental suture zone exposed in NW Iberia is examined to uncover the long-lived rheological control exerted by the strata deposited over the external parts of Gondwana on its geodynamic evolution. The suture occurs within a set of allochthonous terranes whose limits were taken as domain boundaries to interpret the Variscan stacking of Paleozoic continental domains and retrodeform the resulting nappe pile. The suture zone formed due to closure of ocean basins located between Gondwana and Laurussia during the Late Paleozoic and consists of relics of oceanic and transitional crust. The suture zone exhibits a tabular to lens shape due to repeated tectonic events dominated by non-coaxial deformation (thrusts and low-angle normal faults). Thrusting and normal faulting also involved the margins of the continents bounding the suture. The structure of the continental blocks, however, is dominated by folds, particularly large nappe folds with pronounced superimposed flattening. The upper part of the basal allochthonous units comprises a rheologically incompetent domain below the suture zone. This domain is typified by the carbonaceous-rich strata, which are probably Ordovician–Silurian sediments based on U–Pb detrital zircon populations. The rheology of this layer determined the location of the first accretionary thrust that initiated the Late Devonian subduction of the Gondwana margin below the suture zone. By favoring fault development, the upper sequence of the basal allochthonous units as a whole influenced the exhumation of deep-seated continental crust, the transference of the suture zone over Gondwana, and the re-equilibration of the resulting overthickened crust.     

A geometrical and kinematical approach to the nappe structure in an arcuate fold belt: the Cantabrian nappes (Hercynian chain,NW Spain)

In northwest Spain thrust sheets occur in an arcuate fold belt. The fault style consists of an array of thrusts, merging downdip into a single décollement surface. Most of the thrust sheets were initiated as thrusts cutting across flat lying beds. Folds above the hanging-wall ramps and some minor structures indicate that the body of the nappes has been subjected to an inhomogeneous simple shear parallel to bedding (y = 1.15), with slip concentrated along bedding planes. This allows the rocks forming the nappe to remain unstrained. At the base of the nappes a thin zone of deformed rock exists. The thrust sheets die out laterally against an anticline-syncline couple, oblique to the thrust direction. A geometrical analysis shows that if anticline and syncline axes are oblique, the thrust sheet was emplaced with a rotational movement, which can be evaluated. As deformation progressed two sets of folds were formed: a circumferential set, following the arc, and a radial set. An arcuate trace of the thrust structures remains after unfolding the radial folds. With a rotational emplacement, the displacement vector for successive points has a progressively greater length, and forms a progressively lower angle with the thrust. The main thrust units are broken into several slices with rotational movements, so that each unit was curved as it was being emplaced, producing a first tightening of the arc. Later folding increased the arc curvature to its present shape. The palaeomagnetic data available support the above conclusions.     

Diagenesis of Pennsylvanian phylloid algal mounds from the southern Cantabrian Zone (Spain)

The Pennsylvanian phylloid algal mounds exposed in the Cervatina Limestone of the Cantabrian Zone (NW Spain) developed during the highstands of high-frequency shallowing-upward cycles and lack evidence of subaerial exposure at their tops. Mound core facies are composed of massive bafflestones with variable amounts of calcite cements and anchicodiacean phylloid algae with cyathiform thalli preserved in growth position. Through standard petrographic, isotopic (δ¹⁸O and δ¹³C), major and trace element (Ca, Mg, Fe, Mn, Sr) and cathodoluminescence analyses, five calcite cement phases (cement 1 (C1)–cement 5 (C5)) have been identified filling primary and secondary pores. Early marine diagenesis is represented by micritization and non-luminescent to mottled-dull luminescent high-Mg calcite fibrous marine cement (C1). A dissolution phase then occurred and created vuggy and moldic pores. Based on the absence of field or petrographical or geochemical evidence of exposure, it is inferred that dissolution occurred in near-surface undersaturated marine waters with respect to aragonite related to progressive organic matter oxidation. Secondary porosity was subsequently filled by dull-bright-dull bladed high-Mg calcite (C2), which precipitated in the early shallow burial from marine-derived pore waters. Remaining porosity was occluded by shallow-burial precipitates consisting of non-luminescent scalenohedral low-Mg calcite (C3) followed by non-ferroan dull luminescent calcite spar (C4). Latter phases of calcite spar exhibiting non- and dull luminescence (C5) are associated with burial calcite veins. Low δ¹⁸O values (around ?8‰), moderately depleted δ¹³C values (around 0.5‰) and the homogeneity of trace element contents of carbonate matrix, cements and vein-filling calcites suggest burial isotopic re-equilibration and recrystallization, probably in Early Permian times during post-thrusting orocline formation.     

Granites built by sheeting in a fault stepover (the Sanabria Massifs, Variscan Orogen, NW Spain)

The Sanabria region (Central Iberian Zone, Variscan belt of Spain) shows an asymmetric thermal dome marked by migmatites accompanied by the Ribadelago and the Sotillo plutons. These small plutons display pronounced mineralogical variations. Biotite granodiorites and tonalites prevail, and granites and gabbros are common. Both plutons are confined in the releasing stepover of a transpressional shear zone that strikes 120°E and dips 70°SW. Most of the igneous rocks form sheets parallel to the shear zone. Magnetic foliations and lineations in the igneous rocks are parallel, respectively, to the shear bands and stretching lineations observed in the shear zone. The formation of these igneous sheets at high angle to the main axis of the regional field stress is explained by a combination of the fault-valve behaviour of the shear zone with the power of melt overpressure to open and ascend through previously formed planar structures, like S- or C-planes.     

Mineral and fluid thermobarometry within the Variscan shearzone of Monteferro-El Rosal (NW Galicia, Spain): a preliminary investigation

The Monteferro El Rosal shear zone is characterized by an intense ductile deformation produced during the third phase of the Variscan orogeny. The petrology of the metasediments and the study of the fluid inclusions from synkinematic andalusite-bearing quartz veins indicate that the major Variscan phase of deformation occurred at around 2. 5–4 kbar and 565 ± 25°C.     

Aptian— Albian tectonic pattern of the Basque— Cantabrian Basin (Northern Spain)

During the Aptian and Albian thick terrigenous and carbonate successions of sediments up to 5000 m thick, including shallow water rudist limestones (Urgonian facies), were deposited in the Basque– Cantabrian Basin of northern Spain as a result of an intense rift-related subsidence. Based on criteria from surface and subsurface data, syn-sedimentary faults, folds, angular unconformities, diapirs and sub-basins are distinguished within the Urgonian successions. Faults are grouped into N– S, E– W, NW– SE and NE– SW families and most are normal and strike-slip. Folds are gentle anticlines and synclines related to major faults. The angular unconformities have small hiatuses, poor lateral continuity and they are associated with either folds or tilted blocks. Diapirs are related to the intersection of major basement faults and in at least one instance the diapir was fossilized by Late Albian times. Strong differential subsidence controlled by basement faults determined the division of the basin into many subbasins of different sizes, which acted as depocentres (e.g. Bilbao). Despite the tectonic inversion which affected the basin during the Tertiary and created thrusts in their margins and centre, the present position of the syn-sedimentary tectonic structures gives approximate clues about the broad structural style and this reveals the original model of basin extension. Features characteristic of strike-slip identified in different parts of the basin are displaced geological lines, wrench corridors, drag effects, thickness shifts, paired uplifts and basins, vaulting of ‘slabs’, decreasing displacements, horsetail and fault splays, ‘chessboard’, oroflexural bending, pull-apart geometries, in-line horst slices, and restraining/releasing bends. Sinistral strike-slip movements along major NW– SE faults are supposedly responsible for transtension, which characterized the basin particularly during the Albian. In this scenario, the main wrench movements would have concentrated along the Oiz domain (Biscay Tertiary Synclinorium) and is a situation that has more in common with the strike-slip model proposed by some workers for the western Pyrenees, than with the simple extension models proposed for the northern margin of the Bay of Biscay.     

The detection and significance of early deformation in the southern Variscan Pyrenees,Spain; implications for regional Paleozoic structural evolution

Detailed structural analysis of part of the Variscan southcentral Pyrenees revealed the occurrence of several deformation generations, of which the most important one, called the mainphase folding and striking WNW-ESE, seems to be the oldest. Directional analysis of structural elements related to mainphase folding (sedimentary bedding, mainphase cleavage, small-scale foldaxes and intersection lineations) shows, however, that sedimentary bedding must have been non-planar before mainphase deformation took place. This observation suggests that premainphase folding occurred as well, and indeed the areal distribution of intersection lineations in the studied area demonstrates the existence of two early Variscan fold systems. They are characterized by very open NNW-SSE and WSW-ENE folds and have subvertical axial planes and subhorizontal foldaxes. In strong contrast to mainphase folds, penetrative axial plane foliations did not develop during deformation. Pre-mainphase folds in varying orientations have been reported from many other areas in the central Variscan Pyrenees, but a reinterpretation of existing maps and other data shows that also in these cases two pre-mainphase deformation generations must be present, rather than just one as suggested in most previous work. Again, the interference pattern of the two fold systems as well as field evidence indicates that axial planes are steep and strike approximately N-S and E-W, but locally strong reorientation due to Alpine deformation (mainly thrusting) has taken place. The significance of pre-mainphase folding in the Variscan Pyrenees is discussed in the light of an overall dynamic/ kinematic model involving alternating convergent and divergent right-lateral oblique-slip movements along the north-eastern boundary of the Iberian (micro-)plate. The occurrence of pre-mainphase folds is related to

1. the transition from divergent to convergent obliqueslip movement (NNW-SSE folds), and
2. initial oblique convergence of the Iberian and European plates (WSW-ENE folds) prior to mainphase collision.

     

Radioactivity monitoring in the vicinity of Jawalamukhi thrust NW Himalaya,India for tectonic study

Natural Hazards - This work reports radon-thoron monitoring at two depths (60 and 90 cm) and at 82 sites around Jawalamukhi thrust of NW Himalaya, India using Solid State Nuclear Track...     

Structure and kinematics of the Louth-Eumarra Shear Zone (north-central New South Wales,Australia) and implications for the Paleozoic plate tectonic evolution of eastern Australia

The ～E–W-trending Olepoloko Fault and ～ENE-trending Louth-Eumarra Shear Zone in north-central New South Wales are approximately orthogonal to the dominant ～N–S-trending structural grain of Paleozoic eastern Australia. These structures have been interpreted to represent the boundary between the Thomson and Lachlan orogens, but their exact geometry and kinematics remain unclear owing to the scarcity of surface exposure. Using gridded aeromagnetic data and limited field mapping, we obtained new data on the tectonic history of the Louth-Eumarra Shear Zone, which seems to represent a broad zone of dextral shearing with a component of crustal thickening indicated by the recognition of kyanite growth in a mica-schist. The timing of deformation is relatively poorly constrained, but at least a component of the dextral shearing appears to be coeval or younger than the age of displaced late Silurian and Early Devonian granitoids. Additional indicators for dextral kinematics farther north, along the ～ENE-trending Culgoa Fault, suggest that the width of the zone that was subjected to dextral deformation is possibly >100 km. This raises the possibility that a large component of dextral displacement was accommodated in this region. In a broader geodynamic context, we discuss the possibility that the precursor of the Louth-Eumarra Shear Zone and Olepoloko Fault originated from segmentation between the northern and southern Tasmanides, perhaps during the Cambrian. The existence of such a discontinuity may have buttressed the process of oroclinal bending in the Silurian. The observed dextral kinematics has possibly resulted from reactivated deformation during the Tabberabberan and Alice Springs orogenies.     

Geological structure of the Ubinka River valley (Northwestern Caucasus)

A thick olistostrome strata of late Paleocene-early Eocene age is outlined in the northern flank of the Northwestern Caucasus folded structure in the Ubinka river valley, which shows the significant role of earlier Cenozoic tectonic movements in forming the alpine structure of the region. The largest part of the strata is composed of dark weakly calcareous clays, which were earlier recognized as Lower Cretaceous deposits. Olistoliths and large olistoplaques are mostly of light calcareous rocks in which microfauna of Cenomanian and Maastrichtian ages were discovered in dark clays. A poor series of foraminifers was recognized in the dark clays hosting these olistoliths; this series do not enable one to determine with certainty the age of the strata (of approximately the late Paleocene-early Eocene). Small structural forms were recognized in olistoliths and olistoplaques, which are not traced in the matrix, this indicates that a series of folded and fissured structures were formed before these olistoliths and olistoplaques appeared in the olistostrome strata.     

Extent,kinematics and tectonic origin of the Precambrian Aswa Shear Zone in eastern Africa

The Aswa Shear Zone (ASZ) is a fundamental Precambrian lithospheric structure that has been shaped by many tectonic events in eastern Africa. It separates the Saharan Metacraton in the northeast from the Northern Uganda terrane (which represents part of the Northeastern Congo block of the Congo craton) to the southwest. Nonetheless, its tectonic evolution is not fully understood. We used high-resolution airborne magnetic and radiometric data over Uganda integrated with Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) in South Sudan to assess the extent, kinematics and contribute to the understanding of the tectonic origin of the ASZ. (1) Our results showed that the ASZ extends in a NW–SE direction for ~ 550 km in Uganda and South Sudan. (2) The airborne magnetic and radiometric data revealed a much wider (~ 50 km) deformation belt than the 5-10 km of the exposed surface expression of the ASZ. The deformation belt is defined by three NW-trending sinistral strike-slip shear zones bounding structural domains with magnetic fabric showing splays of secondary shear zones and shear-related folds. These folds are tighter close to the discrete shear zones with their axial traces becoming sub-parallel to the shear zones. A similar fold pattern is observed in South Sudan from the SRTM DEM. We interpreted these folds as due to ENE–WSW contraction associated with the sinistral strike-slip movement. (3) To the northeast, the magnetic patterns and radiometric signatures suggest the presence of a series of W-verging nappes indicative of strong E–W to NE–SW contraction deformation. (4) We relate the evolution of the ASZ to E–W to NE–SW Neoproterozoic oblique collision between East and West Gondwana. The deformation related to this collision was partitioned into E–W to NE–SW contraction resulting in W-verging thrusts in the east and a sinistral strike-slip movement along the NW-trending ASZ with the strain localized at the boundary between the Saharan Metacraton and the Northern Uganda terrane.     

Structural and kinematic analysis of the Corredoiras detachment: evidence for early Variscan synconvergent extension in the Ordenes Complex, NW Spain

 An important detachment is described in the allochthonous Ordenes Complex, in the NW Iberian Massif, and its meaning is related to the kinematics of contemporaneous convergent structures. The Corredoiras Detachment (CD) separates a hangingwall unit, characterised by a medium-pressure metamorphic gradient, from a footwall high-pressure and high-temperature unit and an underlying ophiolitic unit. An associated ductile shear zone, nearly 2000 m thick, developed in the lower part of the hangingwall unit, where the Corredoiras Orthogneiss, a Lower Ordovician metagranite, was progressively transformed into augengneisses, mylonitic and ultramylonitic gneisses. The attitude of the stretching and mineral lineation in the mylonites varies due to late refolding at map scale, but the sense of movement can be estimated, being roughly top to the SE. According to crosscutting relationships, the CD developed subsequent to the thrusting of the high-pressure/high-temperature unit onto the ophiolitic unit, and prior to younger extensional detachments, upright folding and strike-slip tectonics. The geometric relationships of the CD with the previous structures in the footwall unit, the subtractive character of the metamorphic gap between its hangingwall and footwall, and the available isotopic data suggest that the CD is an early Variscan, ductile extensional detachment, the movement of which was roughly simultaneous with the onset of thrusting of the allochthonous complexes over their relative autochthon. Received: 17 November 1998 / Accepted: 4 April 1999     

Structural study of a semiductile strike-slip system in the Central Iberian Zone (Variscan Fold Belt,Spain): structural controls on gold deposits

The Villalcampo shear system is a regional dextral strike-slip fault zone that affects Late Variscan granites and their metamorphic country rocks over an area of about 150 km². The detailed geometry of this subvertical north-west—south-east shear zone is outlined. The system forms an extensional fan to the northwest and extends to the south-east as a broad extensional duplex. Particular attention is focused on the distribution of fault rocks and associated veins in its north-west splay. A structural study of the shear bands (encompassing both geometric and kinematic criteria) and a microscopic study of the fault rocks has led to the interpretation of the system as a brittle—ductile shear zone. Calculations give a shear strain value of = 1.5 and a minimum displacement of s = 3700 m. The localization of gold mineralization in mylonite-filled subvertical extensional veins is a product of the formation of the Villalcampo shear system. The subvertical faults and veins underwent a process of cyclical sealing and reopening. As such they acted as valves controlled by fluid pressure regulating fluid—rock interactions and gold deposition. Conditions favouring these processes occur near the base of the seismogenic zone in the vicinity of the frictional—quasi-plastic transition at mid-greenschist metamorphic conditions (T = 350°C and 10–15 km depth).     

老挝西北部琅勃拉邦构造带放射虫硅质岩及其构造意义

琅勃拉邦构造带内放射虫硅质岩含有放射虫Entactinia vulgaris Won, Entactinosphaera palimpola Foreman和Belowea variabilis (Ormiston et Lane),时代为早石炭世,为老挝境内首次报告。硅质岩样品具有很高的SiO2质量分数(95.29%~98.17%),大部分样品表现出相类似的稀土配分模式,部分样品配分模式图表现为上凸状,具有中稀土富集,均具有明显的Ce负异常,Ce/Ce*值为0.64~0.74,其中部分样品具有Eu的负异常,为0.58~0.68。Y/Ho比值为31.05~40.96,类似日本Sasayama中-晚二叠世的远洋硅质岩。地球化学显示其为含酸性火山碎屑非热液成因的远洋硅质岩。这些研究证实了在思茅板块和印支板块之间存在一个开阔的石炭纪时期弧后盆地。