Autochthonous Domain Research Articles

Tectonic setting and isotopic sources (Sm–Nd) of the SW Iberian Autochthon (Variscan Orogen)

Metasedimentary rock successions in Sierra Albarrana Group (SW Iberia Autochthonous Domain) were deposited during the Early Paleozoic influenced by the evolution of a peri-Gondwanan active margin. Their geochemical composition indicates dominant contributions from felsic igneous sources and an upper continental crust provenance. The high mineralogical and geochemical maturity, together with negative εNd(530), ranging from − 11.3 to − 4.5, and old Nd model ages (TDM: 1388–1897 Ma) imply reworked materials from old continental source areas with a limited juvenile contribution for the siliciclastic sedimentary rocks in Sierra Albarrana. Both, geochemical and isotope features were caused by the progressive denudation of rocks bearing the isotopic imprint from an old basement exposed along the Gondwanan margin. The paleobasin for the SW Iberia Autochthon probably occupied an outboard position across the peri-Gondwanan margin during Ediacaran–Cambrian times. In a convergent scenario, the interaction between the peri-Gondwanan trench and the external part of the continent generated tectonic uplift, giving way to exposure of crust formed during earlier stages of the Cadomian Orogeny and erosion. Sediment redistribution through Early Paleozoic times supplied recycled detrital materials, which likely filled a retro-arc basin formed after the switch from an extensional to a compressional regime in the upper plate of the Cadomian Orogen that is recorded throughout the peri-Gondwanan domain of Iberia. A subsequent extensional stage led to a progressive widening of its marginal basins, thus bringing the onset of a passive margin from the Cambro–Ordovician onwards. Nd model ages of the Sierra Albarrana Group overlap those of Early Cambrian series from the southernmost Central Iberian Zone, and are considered an indication for the paleogeographic closeness between all these sequences during Cambrian times, occupying eastern positions closer to Tuareg Shield and the Sahara Metacraton sections along the Gondwanan margin.

U–Th-total Pb ages of monazite from the Eckergneiss (Harz Mountains, Germany): evidence for Namurian to Westfalian granulite facies metamorphism at the margin of Laurussia

Four samples of garnet-bearing metapsammitic gneisses and one sample of a kinzigitic sillimanite-garnet-cordierite gneiss from the Eckergneiss Complex (Harz Mountains, Germany) were studied in detail for U–Th-total Pb dating with the electron microprobe. Monazite in all samples occurs in the matrix and show similar chemical compositional variation. A single stage monazite growth during the highest stages of metamorphism was deduced and chemical ages of all samples are between 313 ± 4 and 328 ± 6 Ma. These ages are interpreted as the time of granulite facies metamorphism in the Eckergneiss Complex. This metamorphism post-dates the formation of the main foliation, presumably developed under amphibolite facies conditions, as indicated by overprinting static grain growth and partial melt microstructures. It pre-dates the significantly younger intrusion of the Harzburg Gabbronorite and the Brocken Granite that are of post-Variscan age. Our results, in combination with P–T conditions for the peak of granulite facies metamorphism, show that the Eckergneiss Complex must have resided at a depth of at least 15 km during the Namurian to Westfalian. Later exhumation of the Eckergneiss Complex and its tectonic emplacement within the low-grade metamorphic Rhenohercynian rock units of the NW-Harz most likely occurred along the Acker-Bruchberg Thrust Zone, which marks the transition between the allochthonous and autochthonous domains of the Harz Mountains. At Early Permian time, this imbricated sequence was intruded and sealed by the post-Variscan intrusions.

Multiple Paleozoic magmatic-orogenic events in the Central Extremadura batholith (Iberian Variscan belt, Spain)

The Central Extremadura batholith located in the southeast part of the Central Iberian Zone (e.g. Iberian Autochthonous domain of the Iberian Variscan belt) was originally thought to have been generated entirely during Carboniferous igneous activity. However, some recent geochronological work has shown the existence of Ordovician plutonic rocks. The aim of this study is to re-examine the age of granitic rocks in the Central Extremadura batholith and complement this information with new field and geochemical data. This data set is used: to constrain the relative timing of plutons emplacement, as well as deformation and metamorphism preserved in the host rocks; to track deep crustal rocks and granitic magma sources; and to discuss prevailing tectonic evolutionary models for the Paleozoic evolution of the Iberian Variscan belt. We use geochemical and SHRIMP U-Pb zircon geochronology data of three granitic plutons (Ruanes, Plasenzuela and Albala) from the Central Extremadura batholith to track magmatic sources and provide a better understanding of temporal and spatial relationships between deformation and magmatism in the Iberian Variscan belt. Ruanes tonalite dated at 464 ± 2 Ma is peraluminous, magnesian and calc-alkaline, as typical of a magmatic arc setting. We report, for the first time, the occurrence of a Middle Ordovician intrusion spatially and temporally related to host deformed rocks of the Central Iberian Zone (e.g. the Iberian Autochthonous domain), which reached high-grade metamorphic conditions. Plasenzuela two-mica leucogranite is strongly peraluminous and of anatectic origin and includes a Neoproterozoic and Ordovician population of inherited zircon grains. This granite possibly derived from the partial melting of a crustal source composed of Neoproterozoic metapelites and metagreywackes (Schist-Greywacke Complex) and/or Lower Ordovician gneisses (Ollo de Sapo Formation), both including greywackes of volcano-sedimentary origin and peraluminous composition. The crystallization age of 330 ± 7 Ma obtained for the syn-kinematic Plasenzuela two-mica leucogranite constrains the functioning of D2 dextral strike-slip shear zones within the Iberian Autochthonous domain. The age of 309 ± 2 Ma obtained for the Albala cordierite-bearing monzogranite matches the age interval of the calc-alkaline magmatic suite post-dating the main Variscan D1–D3 structures in the Iberian Autochthonous domain. The new data presented in this study make it possible to recognize multiple Paleozoic magmatic-orogenic events (e.g. Caledonian, Variscan and Cimmerian) in the Central Extremadura batholith. During the Ordovician, the emplacement of intermediate magmas at shallow depths gave rise to extensive metamorphism due to heat transfer to the host rocks. The onset of this Ordovician plutonic–metamorphic complex in the Iberian Autochthonous domain is contemporaneous with the development of an active continental margin probably related to the subduction of the Iapetus–Tornquist Ocean (i.e. the Caledonian orogeny). During the Lower Carboniferous, these D2 strike-slip domains acted as lateral margins of large-scale gravitational collapses associated with the SE-direct transport of low-angle extensional shear zones (i.e. the Variscan cycle). The emplacement of Upper Carboniferous arc type granitic rocks is interpreted in the context of the amalgamation of Pangaea and the spatial proximity of Iberia relative to the Eurasian active margin in the course of Paleotethys subduction (i.e. the Cimmerian orogeny).

Magnetotelluric Imaging of the Lithosphere Across the Variscan Orogen (Iberian Autochthonous Domain, NW Iberia)

AbstractA new magnetotelluric (MT) survey comprising 17 MT soundings throughout a 30 km long N30°W transect in the Iberian autochthons domain of NW Iberia (Central Iberian Zone) is presented. The 2‐D inversion model shows the resistivity structure of the continental crust up to 10 km depth, heretofore unavailable for this region of the Variscan Orogen. The MT model reveals a wavy structure separating a conductive upper layer underlain by a resistive layer, thus picturing the two main tectonic blocks of a large‐scale D2 extensional shear zone (i.e., Pinhel shear zone). The upper layer represents a lower grade metamorphic domain that includes graphite‐rich rocks. The lower layer consists of high‐grade metamorphic rocks that experienced partial melting and are associated with granites (more resistive) emplaced during crustal thinning. The wavy structure is the result of superimposed crustal shortening responsible for the development of large‐scale D3 folds (e.g., Marofa synform), later deflected and refolded by a D4 strike‐slip shear zone (i.e., Juzbado‐Penalva do Castelo shear zone). The later contribution to the final structure of the crust is marked by the intrusion of postkinematic granitic rocks and the propagation of steeply dipping brittle fault zones. Our study demonstrates that MT imaging is a powerful tool to understand complex crustal structures of ancient orogens in order to design future prospecting surveys for mineral deposits of economic interest.

S-type granite generation and emplacement during a regional switch from extensional to contractional deformation (Central Iberian Zone, Iberian autochthonous domain, Variscan Orogeny)

Zircon grains extracted from S-type granites of the Meda-Escalhao-Penedono Massif (Central Iberian Zone, Variscan Orogen) constrain the timing of emplacement and provide information about potential magma sources. Simple and composite zircon grains from three samples of S-type granite were analyzed by LA-ICP-MS. New U–Pb data indicate that granites crystallized in the Bashkirian (318.7 ± 4.8 Ma) overlapping the proposed age range of ca. 321–317 Ma of the nearby S-type granitic rocks of the Carrazeda de Anciaes, Lamego and Ucanha-Vilar massifs. The timing of emplacement of such S-type granites seems to coincide with the waning stages of activity of a D2 extensional shear zone (i.e. Pinhel shear zone) developed in metamorphic conditions that reached partial melting and anatexis (ca. 321–317 Ma). Dykes of two-mica granites (resembling diatexite migmatite) are concordant and discordant to the compositional layering and S2 (main) foliation of the high-grade metamorphic rocks of the Pinhel shear zone. Much of the planar fabric in these dykes was formed during magmatic crystallization and subsequent solid-state deformation. Field relationships suggest contemporaneity between the ca. 319–317 Ma old magmatism of the study area and the switch from late D2 extensional deformation to early D3 contractional deformation. Inherited zircon cores are well preserved in these late D2-early D3 S-type granite plutons. U–Pb ages of inherited zircon cores range from ca. 2576 to ca. 421 Ma. The spectra of inherited cores overlap closely the range of detrital and magmatic zircon grains displayed by the Ediacaran to Silurian metasedimentary and metaigneous rocks of the Iberian autochthonous and parautochthonous domains. This is evidence of a genetic relationship between S-type granites and the host metamorphic rocks. There is no substantial evidence for the addition of mantle-derived material in the genesis of these late D2–early D3 S-type granitic rocks. The eNd arrays of heterogeneous crustal anatectic melts may be just inherited from the source, probably reflecting mixing of a range of crustal materials with different ages and primary isotopic signatures. The generation of the Bashkirian S-type granites has been dominated by continental crust recycling, rather than the addition of new material from mantle sources.

Structural and stratigraphical significance of U–Pb ages from the Mora and Saldanha volcanic complexes (NE Portugal, Iberian Variscides)

Abstract Geological mapping and structural investigations were carried out in the eastern rim of the Morais Allochthonous Complex in NE Portugal (NW Iberian Massif) along the first-order tectonic boundary between the lower part of the Galicia–Trás-os-Montes Zone (the parautochthonous Schistose Domain) and the autochthonous Central Iberian Zone. The aim was to correlate the Palaeozoic stratigraphic pile of both domains, and to establish their pre- and synorogenic Variscan evolution. An Early Palaeozoic volcanic complex forms part of the pre-orogenic record of the parautochthonous Schistose Domain. It includes two volcanic units, the oldest of which is described for the first time in this work and is named the Mora Volcanics. It is a bimodal volcanic suite with basic to acid rocks which has yielded a late Cambrian age ( c. 493 Ma). Above the Mora Volcanics, the Saldanha Volcanics show petrographic, geochemical and age ( c. 483 Ma) similarities to the Ollo de Sapo Formation, a volcano-detritic ensemble found in the autochthonous domain of NW and Central Iberia. The stratigraphic record and the geochemical signatures of both volcanic units indicate that they were originated by fast partial melting induced by extension along the northern Gondwana margin at the Cambro-Ordovician boundary, coeval with the opening of the Rheic Ocean.

The late Variscan HT/LP metamorphic event in NW and Central Iberia: relationships to crustal thickening, extension, orocline development and crustal evolution

Abstract The Variscan metamorphic evolution of the autochthonous domain of NW and Central Iberia is characterized by a Barrovian gradient followed by a high-temperature–low-pressure (HT/LP) event associated with voluminous granite magmatism. The structural, metamorphic and magmatic histories of the region are described briefly and the relations between them are explained. A coherent model for evolution of the continental crust is proposed using published radiometric ages, thermal models and seismic reflection profiles. The metamorphic evolution, including the high-temperature event, is explained by crustal thickening resulting from the Gondwana–Laurussia collision followed by a period of thermal relaxation and a long-lasting extensional stage. The fact that the highest temperatures were reached in the core of the Central Iberian arc, partly occupied by remnants of a huge allochthonous nappe stack, is discussed in relation to both the emplacement of the allochthon and subsequent oroclinal bending. The overburden provided by the allochthonous pile was decisive in triggering the high-temperature event. Orocline development mostly occurred later and had no significant effect on the metamorphic evolution, although it was important for the present localization of gneiss domes and granitoids. The possible role of the mantle in supplying additional heat to explain the HT/LP event is also discussed. It would seem that little mantle contribution was needed and there are no strong arguments for mantle delamination, although some kind of mantle–crust interaction is expected beneath the hot regions presently occupying the core of the Central Iberian arc.

Rare earth elements in coastal sediments of the northern Galician shelf: Influence of geological features

The Northern coast of Galicia, NW Iberian Peninsula, exhibits a variety of geological features: Ortegal allochthonous complex, Ollo-de-Sapo autochthonous domain and massifs of Bares, Barqueiro and San-Ciprian. In order to examine the influence of terrestrial lithologies on coastal sediments, 103 samples were collected in the Rias of Ortigueira, Barqueiro and Viveiro, their neighbouring shelf and the estuaries of Mera, Sor and Landro rivers. Aluminium, Fe, Sc, particulate inorganic and organic carbon and rare earth elements (REE) were determined in the <2mm fraction. In addition, calcite, muscovite, quartz and riebeckite minerals were identified and quantified in 33 selected samples. The distributions of riebeckite and Fe reflect the influence of Ortegal complex on the coastal areas around the Cape Ortegal. The highest concentrations of ΣREE were found in fine sediments from confined inner parts of the Rias (up to 233mgkg−1), while most of the sands contained 11–70mgkg−1. ΣREE normalised to European Shale (ES) highlights the relative abundance of lanthanides (ΣREEN>6) near Cape Ortegal and the innermost ria zones. The ratio between light and heavy REE (L/H) showed lower values (4–11) around Cape Ortegal and the shelf while higher ratios (15–23) were detected in west of the Cape Estaca-de-Bares and in the inner Viveiro Ria due to elevated contributions of La and Ce. The L/H values normalised to ES reflects the importance of HREE in the adjacent area to Ortegal Complex (LN/HN<0.8) and the LREE (LN/HN>1.4) in the inner estuaries and west Cape Estaca-de-Bares. The highest REE individual ES normalised were measured in fine-grained sediments of the Mera and Sor estuaries. Sediments from the eastern shelf of Cape Ortegal presented enhanced ratios only for HREE. These results indicate that distribution of REE in the northern Galician region is highly depending on the neighbouring lithological pattern, contrasting with the situation found in the western Galician shelf and the Bay of Biscay. Lanthanides can, thus, provide a useful tool to follow the sediment pathway in the land–sea boundary zones, denoting continental geochemical imprint or fluvial outputs accordingly to the existing hydrological and geological conditions.

North-directed Triassic nappes in Northeastern Vietnam (East Bac Bo)

A careful re-examination of the previous tectonic descriptions and the acquisition of new structural and kinematic data have been undertaken in Northeastern Vietnam. The structure of the area consists of a system of slightly metamorphosed but ductilely deformed nappes, including recumbent folds, formed during the Triassic, prior to the unconformable deposition of the Upper Triassic terrigenous sediments. These results confirm the previous interpretation of “preyunnanaises nappes”, represented by Middle-Upper Paleozoic foliated limestone resting through a flat mylonitic contact over an intermediate “Song Mien unit” ( Deprat, 1915), the latter made essentially of deformed Lower Triassic sedimentary and volcanic formations, which previously filled the Song Hien rift ( Bourret, 1922a,b). In the external part of the belt, the Triassic strata conformably overlie slightly deformed and unmetamorphosed Upper Paleozoic rocks, forming an autochthonous domain. Alternatively, the same Triassic succession can be seen resting tectonically through décollement zones, directly over the Middle Paleozoic marbles, to form a distinct but subsidiary allochthonous unit. Our data show that the Song Chay orthogneiss and its Lower Paleozoic sedimentary country rocks belong to the main nappe. They also demonstrate that this nappe was transported to the N–NE. A newly discovered ophiolitic mélange of supposed oceanic nature and Triassic age, lying along the Song Chay Fault, North of the Nui Con Voi, could hypothetically represent the rooted zone of the nappe. A Tertiary overprinting event, in particular accommodated by strike-slip movements, likely accounts for the present orocline of NE Vietnam.

Surface and subsurface signatures of gas seepage in the St. Lawrence Estuary (Canada): Significance to hydrocarbon exploration

The seabed of the St. Lawrence Estuary is characterized by many fluid releasing features. On multibeam bathymetric images these features correspond to crater-like depression (pockmarks) predominantly found on the northwestern shoulder of the ∼300 m deep Laurentian Channel, as well as on the channel floor. Aligned pockmarks, which define segments up to 12 km long, are frequent in the Laurentian Channel, whereas they are preferentially associated with submarine landslides on the northwestern shoulder of the channel. On high-resolution seismic profiles, pockmarks found in the Laurentian Channel are characterized by seismic chimneys that may be traced down to the autochthonous Paleozoic rocks (St. Lawrence platform), suggesting a thermogenic origin for the gas. On the northwest shoulder of the Laurentian Channel, the seismic signature of pockmarks does not extend downward to the reflector that corresponds to bedrock (St. Lawrence platform and/or Grenvillian province) suggesting, together with high sedimentation rate, a biogenic origin for the gas. These results are discussed in the light of the data collected in the onshore parts of the St. Lawrence platform and suggest the presence of a mature hydrocarbon source. The conclusions are supportive arguments for the hydrocarbon prospectivity of both the onshore and offshore segments of the Paleozoic autochthonous domain (St. Lawrence platform).

A OROGÊNESE BRASILIANA NO SEGMENTO CENTRAL DA FAIXA RIBEIRA, BRASIL

The structure of the central segment of the Ribeira belt is defined by three large thrust sheets (Lower, Intermediate and Upper) which override the autochthonous domain, towards the southern portion of the Sao Francisco Craton. Four lithotectonic associations are individualized within the domains: basement rocks (pre-1,8 Ga rocks); orthogneisses of unknown age; Post-1,8 Ga metasedimentary cover and Brasiliano granitoid rocks. Continental tholeiitic deformed dykes and disrupted lenses within the cover indicate extensional periods. Investigation of the tectono-metamorphic and magmatic aspects, associated with new U/Pb geocronological data support the subdivision of Brasiliano Orogeny within three main tectonic periods: a) the syn-collisional period (590-563 Ma) is represented by the main deformation (D 1+2 ), coeval with the M 1 metamorphic stage and γl -γ3 granitoid rocks. Crustal shortening took place initially through ductile thrusts and tight recumbent folds. The late stage of this collisional event is characterized by oblique convergence, with pervasive deformation throughout the belt. M 1 had its thermal peak during D 2 , and is characterised by intermediate to high pressures. The spatial distribution of the metamorphic zones defines an inverted gradient. Deformed I and S type granitoid rocks occur during this period, b) the post-collisional period (535-520 Ma) is characterized by D 3 deformation, coeval with M 2 metamorphic stage. During D 3 phase, the oblique convergence continued, but was accomodated by late to post-metamorphic peak strike slip ductile shear zones. Between these shear zones, shortening was achieved by steep folding of S 2 foliation. The Ma stage with lower pressure regime is associated with intense melting of the basement and the cover, resulting in the generation of I-type magmatism and subordinated S-type; and c) the post-tectonic period is represented by the Post-D4 γ 5 magmatism registered between 503 to 492 Ma. This magmatism may be related to melting of crustal rocks associated with mantelic contribution related with high temperature conditions and uplift just after the Brasiliano Orogeny

Structures and tectonic evolution of the external zone of Alpine Corsica

This paper presents a structural analysis of the external zone of Alpine Corsica, including the autochthonous domain and overlying external nappes (Santa Lucia and Balagne nappes). Two stages of nappe emplacement are identified occurring prior to and after the deposition of the Eocene sediments which were laid down upon first generation thrust contacts but are imbricated with their composite (continental and ophiolitic) basement by second generation thrusts. Five generations of structures with regional extent have been distinguished. However, the first generation has not been recognized within the visible part of the autochthon domain. Eoalpine first generation structures, restricted to allochthonous units, and Late Eocene to Early Oligocene second generation structures were nearly contemporaneous with the two stages of thrusting. The precise significance of E-W third generation structures is poorly understood. Broadly N-S fourth generation structures resulted from Oligocene compressive tectonics (folding and local backthrusting). Finally, fifth generation structures were generated during a Miocene extensional stage. These results are partly consistent with structural features previously reported in the southern and the northern outcrops of the Schistes lustrés, i.e. the main part of the allochthonous domain. A summary of a regional tectonic evolution is thus proposed for Alpine Corsica from Eoalpine obduction to Miocene extension.