Please use this identifier to cite or link to this item: http://hdl.handle.net/10174/31143

Title: Tectono-stratigraphic architecture of parautochthonous and allochthonous successions of Viana-Alvito region (SW of Ossa-Morena Zone)
Authors: Moreira, Noel
Araújo, Alexandre
Pedro, Jorge
Roseiro, José
Issue Date: Jan-2022
Publisher: Universidade da Extremadura
Citation: MOREIRA, N., ARAÚJO, A., PEDRO, J., ROSEIRO, J. (2022), Tectono-stratigraphic architecture of parautochthonous and allochthonous successions of Viana-Alvito region (SW of Ossa-Morena Zone). International meeting: Ossa Morena and beyond; a tribute to Teodoro Palacios, Universidade da Extremadura, Universidade da Extremadura, Badajoz, 27-28.
Abstract: Introduction and geodynamic context The Variscan Cycle in SW Iberia is controlled by the evolution of Rheic and other small Palaeozoic oceanic basins (Ribeiro et al., 2010). During Cambrian times, the rifting stage of northern Gondwana begins (Sánchez-Garcia et al., 2019), generating in the southernmost domains of the Iberian Terrane, i.e. Ossa Morena Zone (OMZ), a particular stratigraphic succession. The early Cambrian stratigraphy is characterized by an extensive carbonated platform deposited over a basal lenticular conglomerate associated with felsic volcanic rocks, which underlies the contact with previously deformed Neoproterozoic succession (Cadomian Cycle). Above the carbonate succession, a rift-related detrital Cambrian succession is developed. The extensional tectonics promote lithosphere thinning and the emplacement of numerous bimodal igneous suites during Cambrian rifting stage, showing discriminant geochemical features along the process time-span (Sánchez-Garcia et al., 2019). These igneous suites are either interbedded within Cambrian stratigraphic successions or generating some plutonic or sub-volcanic bodies. The stretching of the northern Gondwanan terranes culminated with the opening of the Rheic Ocean during the Cambrian-Ordovician transition (ca. 480 Ma; Pedro et al., 2013; Moreira et al., 2014), and the Ordovician to Lower Devonian stratigraphic succession shows geochemical and stratigraphic passive margin features (Roseiro et al., 2020). During Lower Devonian times, the closure of the Palaeozoic oceans tool place and, in the southern boundary of the OMZ, a complex suture zone is developed during subduction-collision processes that persisted until upper Carboniferous times. Thus, the southernmost domains of the OMZ consists in a complex structure resulting of thrusting of allochthonous and parautochthonous complexes into the inner sections of the relative autochthonous domain. This works proposes a new tectonostratigraphic arrangement for these complexes, based on data collected on the Viana do Alentejo-Alvito region. The Viana-Alvito region: reconstructing the stratigraphic succession The Viana do Alentejo-Alvito region is characterized by a highly deformed stratigraphic succession, showing distinct metamorphic regimes. Although the tectono-metamorphic imprint mask most of the stratigraphic characteristics of this succession, it is possible to recognize several lithological features that allows to reconstruct the original stratigraphic Neoproterozoic-Cambrian succession (Oliveira et al., 1991). From the bottom to the top: (1) a siliciclastic succession composed of shale-derived micaschists and metagreywackes, with subordinate metabasalts and meta-lydites (Série Negra succession); (2) a volcano-sedimentary complex dominated by felsic volcanic and sub-volcanic rocks; (3) a thick succession of marbles, often with interbedded mafic volcanic rocks; and (4) a bimodal volcano-sedimentary complex, with prevalence of mafic volcanic rocks over the siliciclastic component. Due to the high metamorphic conditions, it is not possible to constraint the biostratigraphic age of these units, and there are no significant geochronological studies in this region aiming to constraint the age of this succession. General tectono-stratigraphic architecture of the Viana-Alvito region The structure in this region has a complex and arcuate trend (N-S general trend in the most occidental sectors, tending to a NW-SE main direction in the most oriental ones), being result of poly-phase deformation. The discrepant metamorphic conditions and the structural features, allows to recognize 5 tectono-stratigraphic units. From West to East: (1) A high temperature unit describing a cartographic scale N-S antiform. On its core a felsic unit was recognized, being composed either by a sub-volcanic (plutonic?) gneiss or by volcanic lithotypes showing a pervasive S-L fabric. At both limbs, it is possible to identify a composite folded structure where it is noticed the Série Negra succession, the marble unit, including high-temperature olivine marbles, and volcano-sedimentary complexes, being possible to well-reconstruct the previous mentioned stratigraphic sequence, being considered an autochthonous sequence. (2) The follow tectono-stratigraphic unit is an imbricate complex, mostly composed of Série Negra derived micaschists with abundant mafic and felsic volcanic rocks. Mafic volcanic rocks express high pressure (HP) metamorphic paragenesis (Pedro et al., 2013). (3) Above, another imbricated tectono-stratigraphic unit, here mainly composed of Cambrian-like successions, including volcano-sedimentary and marble units. Also in this unit, the mafic volcanic rocks, even the ones within the Cambrian marble unit, show HP metamorphism paragenesis (Rosas et al., 2008; Pedro et al., 2013). (4) A unit composed of mafic rocks at greenschist facies rests above the HP units. It is not clear the nature of this unit, however, in three sector (Vila Ruiva, Oriola and Antas), an internal ophiolite architecture was identified (Pedro et al., 2013). (5) At the top of these tectono-stratigraphic units rests the Cubito-Moura Phyllonitic Complex, an imbricated complex mainly composed of highly deformed phyllonites, often with felsic and mafic volcanic rocks in greenschist metamorphic facies (Araújo et al., 2005). This unit include lenses of Silurian black schists and lydites with graptolites. All this tectono-stratigraphic units are detached by first order tangential shear zones.
URI: http://hdl.handle.net/10174/31143
Type: lecture
Appears in Collections:ICT - Comunicações - Em Congressos Científicos Internacionais

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