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| Ossa Morena Zone | |
|---|---|
| Name | Ossa Morena Zone |
| Country | Spain; Portugal |
| Region | Andalusia; Extremadura; Alentejo |
Ossa Morena Zone is a tectonostratigraphic unit in the Iberian Massif spanning southern Portugal and southwestern Spain. It forms part of the Variscan orogen and lies between the Central Iberian Zone and the South Portuguese Zone, hosting Precambrian to Paleozoic successions that record continental rifting, oceanic episodes, and collision-related deformation. The unit is a focus for studies linking Iberian geology to Appalachian, Armorican, and Moroccan terranes.
The stratigraphic architecture integrates Neoproterozoic to Carboniferous sequences with major contacts to the Central Iberian Zone, South Portuguese Zone, and the Betic Cordillera. The basal successions include Neoproterozoic metasediments correlated with the Ediacaran and Avalonia-affiliated terranes, overlain by Cambrian to Ordovician siliciclastics and carbonates comparable to units in the Armorican Massif and Moldanubian Zone. Silurian to Devonian continental red beds and marine turbidites record links to the Rheic Ocean and episodes contemporaneous with the Caledonian orogeny and the early phases of the Variscan orogeny. The stratigraphy preserves syn- to post-orogenic Carboniferous molasse and volcanosedimentary sequences similar to those in the Bohemian Massif and Massif Central.
Tectonic interpretations place the zone as an allochthonous or parautochthonous sliver within the Variscan belt, bounded by major shear zones such as the Ossa-Morena Shear Zone-type structures and linked to the West European Rift System evolution. Deformation fabrics include regional high-strain domains with mylonites, tight to isoclinal folds, and upright to recumbent nappes comparable to structures documented in the Cantabrian Zone and Pyrenees. The collision between Laurussia-derived blocks and Gondwana-derived terranes, with microcontinents like Armorica and fragments related to Avalonia, drove progressive northward subduction, obduction, and strike-slip reactivation along transcurrent faults analogous to the Variscan suture segments mapped in the Rhenohercynian Zone.
Lithologies span low- to medium-grade metasedimentary successions including schists, phyllites, quartzites, marbles, and calc-schists, together with metavolcanic rocks such as basaltic-andesites and rhyolites akin to Ordovician volcanic suites in the Ardennes and Sierra Morena. Intrusive granitoids and microgranites were emplaced during late-Variscan magmatism, comparable to plutons in the Galicia-Trás-os-Montes Zone and the Iberian Chain. Metallogeny includes polymetallic sulfide mineralization (Pb-Zn-Cu-Ag) and orogenic gold mineralization related to hydrothermal systems, with styles similar to deposits in the Iberian Pyrite Belt and the Rhenish Massif.
Fossil assemblages in carbonate and siliciclastic horizons yield trilobites, brachiopods, and graptolites that constrain Cambrian to Silurian biostratigraphy, with correlations to faunas from the Armorican Massif, Avalonian terranes, and the Bohemian Basin. Devonian assemblages include bivalves and plant remains enabling correlation to the Old Red Sandstone facies. Microfossils and palynological records assist in constraining Carboniferous marine transgressions synchronous with bioevents recorded in the Cantabrian Zone and Pennsylvanian sequences across western Europe.
Isotopic dating (U-Pb zircon, Ar-Ar mica, Rb-Sr whole-rock) records Neoproterozoic protolith ages, Ordovician to Devonian magmatic episodes, and Carboniferous thermal events contemporaneous with Variscan orogenesis observed in the Massif Central and Bohemian Massif. Metamorphic grades range from greenschist to amphibolite facies with contact and regional metamorphism, and prograde–retrograde paths imply burial and exhumation linked to nappe stacking and strike-slip exhumation as seen in the Alps and the Apennines. Thermochronology constrains cooling through the closure temperatures of biotite and muscovite, comparable to cooling histories of the Rhenohercynian Zone.
The zone hosts base-metal occurrences and orogenic gold prospects that have driven exploration and mining episodes paralleling activity in the Iberian Pyrite Belt and historical mining districts such as Aljustrel and Almaden. Industrial minerals include quarried marbles and skarn-related skarn-hosted mineralization akin to deposits exploited in the Sierra de Guadarrama and Sierra Morena. Structural controls on mineralization often follow regional shear zones analogous to ore controls in the Westerly-Directed Shear System of western Europe.
Scientific investigation accelerated during the 19th and 20th centuries with regional mapping by national surveys such as the Instituto Geológico y Minero de España and the Serviço Geológico de Portugal, and with contributions from workers associated with universities in Lisbon, Seville, Coimbra, and Madrid. Modern studies integrate field mapping, geochronology, geochemistry, and geophysical surveys drawing on methodologies developed in comparative studies of the Appalachians, Armorican Massif, and Variscan Belt research communities.