The boundary between the Central-Iberian Zone and the Ossa-Morena Zone of the Hesperian Massif has long been placed either in Los Pedroches batholith or in the Badajoz-Cordoba shear zone, using different arguments. Here, based on mainly stratigraphic and paleogeographic criteria, we propose that this boundary is in fact a new zone of the Hesperian Massif, the Lusitan-Marianic Zone, and not just a simple lineament. The northern limit of this zone is at present concealed by the Variscan "Los Pedroches" batholith, and the southern one is the "Malcocinado Fault". The main distinctive geological features of this zone are: 1) The Proterozoic-Lower Cambrian basement rocks are similar to those of the Ossa-Morena Zone. This basement cannot be correlated with Precambrian rocks outcropping north-eastwards of the Los Pedroches batholith, even if the Alcudian-type sediments might have a Central-Iberian equivalent. 2) The Paleozoic sequences younger than Middle/Upper Cambrian, outcropping between the Malcocinado fault and the Pedroches batholith, display however clear Central-Iberian affinities, and have nothing in common with those of SW Ossa Morena. Excepting the Silurian, a persistent paleogeographic high existed in the south of the Lusitan-Marianic Zone that migrated transversally through time. The proposed new Lusitan-Marianic Zone shows more distinctive properties to be recognised as an independent entity, than other traditionally distinguished zones, e.g., the Cambrian Zone, West Asturian-Leonese Zone or Central-Iberian Zone, whose boundaries do not correspond to paleogeographic features as significant as those described here.

The boundary between the Ossa Morena and the South Portuguese zones is outline by the Aracena metamorphic belt. The main characteristics of this region of the Iberian Massif are divided into lithologic, structural, metamorphic, magmatic, geochemical, isotopic and experimental points of view and presented in this paper. The Aracena metamorphic belt is divided into two main domains: the oceanic domain is composed of MORB-derived metabasites (the Acebuches metabasites) and a former accretionary prism. The continental domain comprises aluminous gneisses and migmatites, calc-silicate rocks, leucocratic gneisses, marbles, amphibolites, syn-to post-tectonic noritic intrusions with high-Mg andesite (boninitic affinities) composition, as well as post tectonic acid-to-basic intrusive rocks. The Acebuches metabasites were firstly affected by a hig-temperature/low-pressure event, which currently shows an inverted metamorphic gradient, and is related to SW verging thrusting. The thermal peak related to this metamorphic event displays an age gradient in such a way that older ages have been obtained towards the west. The lower half of the Acebuches metabasite piles was later affected by a mylonitic deformation and a retrometamorphism related to the Southern Iberian Shear Zone. Four ductile deformation phases have been defined in the continental domain: CD-D₁ was related to the formation of km-scale recumbent folds. CD-D₂ could be associated with an extensional collapse or a gravity spreading, and is coeval with a high-temperature/low-pressures metamorphic event that affected the continental domain and gave place to several migmatitic complex. CD-D₃ produced symmetric upright folds and CD-D₄ generated south-verging thrusts and thrusts accommodation folds. The main characteristics of the Aracena metamorphic belt are interpreted as the result of a trench-trench-ridge triple juncion evolution. According to the proposed model, the oceanic ridge-trench interaction led to the formation of a slab-free window beneath the continental margin, which provoked the upwelling of the underlying astenosphere and a subsequent thermal rebound. This triple junction mugated along the contiental margin towards the east, which gave place to the generation of a high-temperature/low-pressure metamorphic belt in the contact between the Ossa Morena and the South Portuguese zones.

The imprint of the Cadomian orogeny in SW Iberia has been a controversial issue over the last two decades. At present, the amount of geological, geochronological and geochemical data available enable, one to envisage the most relevant features of the Cadomian orogenesis in SW Iberia. The Cadomian imprint mostly consist of a widespread, though not very voluminous, calc-alkaline, Upper Vendian magmatism, which has been related to a subduction zone. Locally, in the Peraleda del Zaucejo outcrop, low-grade metamorphism and a foliation of Late Precambrian age are preserved in Upper Proterozoic rocks. This is not the case in most of the Ossa-Morena Zone, where km-scale recumbent folds and thrusts have been described affecting both the Upper Proterozoic rocks and the overlying Palaeozoic succession and giving rise to the main penetrative foliation in both Precambrian and Palaeozoic rocks. Drawing on stratigraphic and geochronological data, we claim that the Cadomian orogenic events must have occurred in Late Proterozoic times, not entering the Cambrian period as proposed by other researchers. As for the magmatism, which shows an apparent continuity from Late Proterozoic to Early Cambrian, we postulate that lowermost Palaeozoic granitoids were generated in the context of a rifting process related to the beginning of the Variscan cycle. This Early Palaeozoic rifting is very well attrested by the steatigraphic record, which shows a transition from a quiet carbonate platform in Early Cambrian times to terrigenous sediments with tholeiitic-alkaline volcanics in Middle Cambrian times. Thus, the Early Cambrian can be argued to be a transient stage of tectonic quiescence between the end of the Cadomian orogeny and the beginning of the pre-Variscan rifting in Early-Middle Cambrian times. Within this same context of rifting, we also place the medium-to-high-grade-low-pressure metamorphism in the Valuengo and Monesterio areas. In these areas, there is a top-to-the-north mylonitic and synmetamorphic foliation which is folded by the Variscan structures and seems to be Cambrian in age, thus being interpreted as another record of the pre-Variscan rifting. The Ossa-Morena Zone may well have become an isolate continental block in Silurian times as a final result of the Early Palaeozoic rifting.

The present paper reviews the Ordovician, Silurian and Devonian sedimentary rocks of the Ossa-Morena Zone of the SW Hesperian (Iberian) Massif in Spain and Portugal. It gives detailed informations on the successions and faunas from the Early Ordovician to the Late Devonian, i.e. During the passive margin development that followed a Cambrian rifting phase proceded the Variscan orogenic events. Comparison of the sedimentary and faunal record from the Ossa Morena and Central Iberian zones during the Palaeozoic indicates that both regions were part of the North Gondwanan shelf, characterized by distal (OMZ) and proximal shelf conditions, respectively. This shows that the Badajoz-Córdoba Shear Zone corresponds only a Variscan shear zone and cannot be considered as the cryptic suture of a Palaeozoic ocean.

A protracted history of strike-slip movements in Soutwestern Iberian, throughout Carboniferous times, Westphalian and Stephanian in particular, has resulted in subparallel strips of terrain which are difficult to recompose into the original configuration. The nature and timing of the major strike-slip movements have become apparent as a result of detailed work on the early Westphalian basin of Peñarroya (Córdoba province) and, also, by interpreting the published information on upper Westphalian and upper Stephanian strata in the Valongo structure northeast of Oporto, the Stephainan C (lower Autunian) basin of Buçaco near Coimbra, the Santa Susana Basin west of Évora, and the mid-Autunian basin of Valdeviar (Sevilla province). The geological history of the Peñarroya Basin is summarised; research date are analysed, and comparison is made with the Teriary basins in California controlled by transform faulting. Published information on the Valongo, Boçaco, Santa Susana, and Valdeviar areas suggests similar histories to that of Peñarroya, with the corollary that major sinistral strike-slip operated in what is presently the Iberian Massif for most of Pennsylvanian times, perhaps some fifteen million years. The conclusion is reached that at least three major transforms existed, with additional subparallel strike-slip faults, came into being from Permian onwards. Structural interpretations of Ossa Morena and the adjacent part of the Central Iberian Zone should take cognisance of the important effects of Carbiniferous transform faulting.

The diagenetic and metamorphic grade of low temperature processes which affected the pelitic materials of the Central Sector of the Ossa-Morena Zone is determined by electron microscopy and X-ray diffraction criteria. Cadomian metamorphic processes produced at least greenschist facies conditions even in the lowest grade rocks of the Serie Negra. Post-Cadomian rifting affected the early Paleozoic sedimentary basin giving way to an extensional metamorphism characterised by a burial pattern, which represents the earliest response to depth-controlled diagenetic and very low-grade metamorphic reaction progress caused by basin subsidence. No significant metamorphic overprint was produced by Variscan deformation, as shown by the preservation of typically sedimentary minerals and early-to-late diagenetic values of KI in Devonian and Carboniferous sediments.

The state of knowledge about the Ossa-Morena Zone is variable. There are petrologic, geochemical, isotopic and geochronological studies of many bodies, but few are exhaustive studies; and the interpretation of these data is complicated by the superposition of the Cadomian and Variscan deformations. In this paper we present a summary of the published geochronological data and their interpretation. We also present a brief but systematic study of several granitic bodies of the Ossa-Morena Zone. Finally, we interpret the presented data in the context of the geological history of the region. The age range of the studied plutons is ~532-502 Ma and ~348-332 Ma. Cambrian ages comprise: the Culebrín tonalite, 532 Ma, with calcalkaline chemistry, and characteristic of mantelic origin rocks formed in arce areas; the Calera de León granite, 524 Ma, with alkaline typology, and also with a mantelic origin; the Monesterio migmatites, 511 Ma, and the granodiorite, orginated from deeper levels but intruded as the same time as the migmatites were forming; finally the Castillo granite, 502 Ma, with A-type affinity and mantelic origin too. Carboniferous ages are 332 Ma for Santa Olalla tonalite, and 348 Ma for Teuler granite, these bodies being of crustal derivation. For some authors Cambrian bodies are still related to the Cadomian cycle; for others the existence of a Lower Cambrian detritic-carboanted platform is considered the end of the Cadomian cycle, therefore these granites are connected to the Lower Paleozoic extensional event, that is considered the beginning of the Variscan cycle. Taking all this information, we propose the following; older granites as Culebrín and Calera are still related with the Cadomian cycle; both, magmatic and metamorphic events of Monesterio and Castillo are syn-kinematics with the deformation related to the Lower Paleozoic rifting. Younger ages would represent the late magmatism related to this extension. Plutonic bodies of Carboniferous age were generated after the first deformation phase of the Variscan Orogeny.

The IBERSEIS deep reflection seismic experiment has provided a crustal image of the Variscan orogen of southwest Iberia. A brief presentation of the entire seismic profile is given, and then the Ossa-Morena Zone (OMZ) and its boundaries are considered. The crust of the OMZ is shown to be divided into an upper crust, characterized by dominantly NE-dipping reflectivity, and a poorly reflective lower crust. The reflectivity of the upper crustal has good correlation with the geological cross-section constructed from surface mapping. In the seismic image, the upper crustal geological structures are seen to merge in the middle crust. Nevertheless, the OMZ middle crust is not a mere detachment level, as it shows very unusual features: it appears as a band of strong reflectivity and irregular thickness (the Iberian Reflective Body, IRB) that we interpret as a great sill-like intrusion of basin rocks. The boundaries of the OMZ are considered sutures of the orogen, and their geometrical features, as deduced from geological mapping and the seismic image, are in accordance with the transpressional character of the Variscan collision recorded in SW Iberia. The present Moho is flat, obliterating the root of the orogen.