The major tectonic units of the Neoarchean Volgo-Uralia continent, which is ~600,000 km 2 in area, are contrastingly expressed in regional gravity and magnetic maps. Interpretations of seismic images of the crust along the TATSEIS geotraverse in combination with 3D density and magnetic crust models provide insights into the volumetric representation of tectonic structures of various ranks. Granulite-gneiss crust of Volgo-Uralia is characterized by elevated thickness (~60 km and locally up to 65–70 km). The deep structure of Volgo-Uralia assumes that the entire crustal section, including the lower crust, is composed of high-density granulite metamorphic facies rocks. Specific structural units called ovoids play the main role in the structure of this continent. The ovoids are bowl-shaped crustal blocks, round or oval in plan view, 300–600 km across, and with the base reaching a level of crust-mantle interface at ~60 km. Ovoids are bounded by conic surfaces of reverse (thrust)-faults, along which their outer parts are thrust over the framework. The Tokmov, Buzuluk, Verkhnekamsk, Krasnoufimsk, and Orenburg ovoids, which generally are not in contact with one another, are dominated by mafic granulites, gabbroic rocks, gabbroanorthosites, and ultramafic rocks. A significant contribution of deep-seated intrusive rocks suggests that metamorphism developed in the lower and middle crust at high PT parameters that exceed the maximum estimates (940–950 °C, 9.5 kbar) recorded in samples of borehole cores. The interovoidal space is occupied by elongated oval synforms up to 200–400 km long. This space is considered to be an interovoidal domain, which includes three relatively narrow, compressed synforms (Yelabuga-Bondyug, Kilmez, Chusovaya) and four oval synforms (Srednevyatka, Verkhnevyatka, North-Tatar, Almetevsk). The Tokmov ovoid is framed in the southeast by the Tuma and Penza belts. Synforms are filled with metasedimentary granulites and mafic metaigneous rocks. The protoliths were formed over the time interval from 3.4–3.2 to 3.1–3.0 Ga. The internal zoning of the Volgo-Uralia crust is related to numerous local centers within ovoids and interovoidal region. At least two high-temperature metamorphic events were followed by periods of retrogression: 2.74–2.70 and 2.62–2.59 Ga. The areal and especially high-temperature character of tectonothermal processes during formation of the Neoarchean crust of the Volgo-Uralia Craton, and distinct geometrization of space with recognition of several concentric domains, finds a universal explanation in ascent of multiple plumes.

The Bryansk-Kursk-Voronezh intracontinental collisional orogen was formed by the juvenile Middle Paleoproterozoic assemblages in combination with Early Paleoproterozoic complexes and areas of reworked Archean crust. The crucial events fall in the time interval 2.1–2.0 Ga. The orogen includes a sequence of second-order, near-meridional orogens, which, in turn, consist of tectonic belts. From the west eastward, these are the Krivoi Rog–Bryansk orogen, the orogen of the Kursk magnetic anomaly (KMA), and the East Voronezh orogen. The latter may be regarded as an axial (central) structural element, which marks a transient rupture of the continental crust. The north to south extent of the orogen along the strike of the second-order orogens reaches 900 km long and ca. 900 km in width. The Krivoi Rog–Bryansk orogen consists of two groups of tectonic belts. Its western region is composed of tectonic nappes pertaining to the Kulazhino gneiss and the Bryansk granulite-gneiss belts. The eastern region, including the Krupetsk-Znamensk belt and the Meshchevsk system of tectonic sheets, with significant role of intensely deformed banded iron formations (BIF) of the Kursk and Oskol groups, is also the western imbricated margin of the Archean Kursk craton. The volcanic-sedimentary BIF belts play the main role in the structure of the KMA orogen. The second important component is composed of subvolcanic and plutonic mafic-ultramafic, felsic, and alkaline rock complexes. Granite-gneiss domes are related to the final stage of the orogen evolution. The East Voronezh orogen consists of the Lipetsk-Losevka volcanic-plutonic belt and the Vorontsovka imbricated thrust belt, separated by the Losevka-Mamon suture. The evolution of the Bryansk-Kursk-Voronezh orogen as a whole includes: onset of rifting within the Archean Kursk craton (2.59–2.53 Ga); terrigenous and chemical sedimentation including BIF and volcanic activity (2.5–2.10 Ga), periodically accompanied by emplacement of intrusions (2.6–2.5 Ga and 2.1–2.05 Ga); metamorphism under granulite-facies conditions in the Kulazhino and Bryansk belts at 2.13 Ga and in the Vorontsovka belt at 2.10 Ga; intrusive magmatism in the Vorontsovka belt generally synchronous with high-temperature metamorphism; suprasubduction magmatism and formation of the Lipetsk-Losevka volcanic-plutonic belt 2.10–2.05 Ga ago; the collision-related reverse and thrust faulting and folding in the BIF belts accompanied by westward and south-westward overturning; rheomorphism and formation of granite-gneiss and granite-migmatite domes deforming the fold-nappe assemblages in the BIF belts of KMA; and the tectonothermal activity of postcollisional and anorogenic stages (2.07–1.9 Ga) expressed by emplacement of alkaline ultramafic and gabbro-syenite intrusions in the Lipetsk-Losevka belt and its hinterland. The most active phase of tectonic evolution spanned the time interval 2.1–2.0 Ga, which was no less than 100 Ma long and coincided with the active stage of extension, sedimentation, and high-temperature metamorphism in the northern part of the East European craton.

The Late Paleoproterozoic Lapland–Mid-Russia–South Baltia orogen surrounds the Karelian craton as a wide arc, separating it from Volgo-Uralia and Sarmatia. The orogen extends for more than 3000 km; its width in the northern and central segments is 400–700 km and increases to 1000 km in the southwest. The Lapland sector of the orogen is characterized by specific spatial distribution of tectonic belts composed of low-grade metavolcanic-metasedimentary rocks and granulite-gneiss complexes. The former are localized along the orogen boundaries; in turn, the axial zone is mainly formed by alternation of low-angle tectonic sheets varying in thickness from a few to 20–25 km. The sheets are composed of Paleoproterozoic granulite-gneiss complexes and Archean granite-greenstone and amphibolite-gneiss assemblages. The geological history of the orogen is subdivided into four stages. (1) The Early Paleoproterozoic magmatism (2.53–2.41 Ga, locally up to 2.32 Ga) corresponds to the initial stage in evolution of the superplume, which induced rifting of the Archean continent. This stage includes: the early volcanism (ca. 2.5 Ga), the layered peridotite-gabbronorite intrusions (2.53–2.42 Ga), bimodal volcanism (2.45–2.42 Ga), emplacement of the minor mafic intrusions that were later transformed into drusites (2.46–2.43 Ga), formation of the Pyrshin-Kolvitsa gabbro-anorthosite complex (2.51–2.42 Ga), and intrusions of charnockites and K-rich granites (2.50–2.43 Ga) and granitoids (2.50–2.41, up to 2.37–2.36 Ga). The Pyrshin-Kolvitsa gabbro-anorthosites underwent granulite-facies metamorphism along with mafic protolith of the Lapland and Kolvitsa-Umba granulite complexes. The domain of initial magmatism in the Kola-Karelia region is a NW-trending band, 1000–1100 km in extent and 300–450 km in width. The area of manifestation of various modes of Early Paleoproterozoic magmatic activity can be regarded as a large igneous province. (2) The initial magmatism was followed by a long-term (2.3–2.1 Ga) stage of quiescent tectonics. Sedimentation in the Karelian Province initially occurred in the vast lacustrine-alluvial plain, then in the nearshore marine and continental evaporite basin, and by the end of this stage in a shallow-water marine basin characterized by deposition of evaporite carbonate and sulfate sediments and growth of numerous stromatolite reefs. The accumulation of salt has been documented by deep drilling in the Onega depression. The basement and sedimentary sequence were cut through by a NW-trending dike swarm almost synchronously with sedimentation. In the southwestern part of the basin within Karelia, separate lava fields were formed, whereas in Kola Peninsula, volcanic activity was much more intense, and sedimentation had been suppressed. (3) The resumption of tectonic activity by the onset of the Late Paleoproterozoic is recorded in a vigorous pulse of magmatic activity (2.11–1.92, locally up to 1.88 Ga), which was somewhat similar to the Early Paleoproterozoic initial magmatism. It developed within the same NW-trending band as the Early Paleoproterozoic initial magmatism. As in the previous case, the area of the Late Paleoproterozoic magmatism corresponds to the definition of a large igneous province. It involves: (i) mafic volcanic rocks inherent to continental and oceanic rifts (including the Jormua ophiolite complex) in combination with a bimodal rhyolite-picrite association close in geochemistry to ocean-island basalt and subvolcanic minor gabbro and wehrlite intrusive bodies (2.11–1.92 Ga); (ii) volcanic-sedimentary and mafic-ultramafic subvolcanic complexes of the Onega depression (ca. 1.98 Ga); (iii) Kimozero kimberlites (ca. 2.0–1.8 Ga); (iv) Jaurijok complex of gabbro-anorthosite intrusions (from 2.0–1.95 to 1.88 Ga); and (v) alkali intrusions (from 1.97–1.95 to 1.88 Ga) and granitoid plutons (ca. 1.95 Ga). All gabbro-anorthosite bodies of the Jaurijok complex are localized at the base of nappe-thrust ensemble of the Lapland granulite belt, i.e., in exactly the same position as the Early Paleoproterozoic Pyrshin-Kolvitsa complex. Appearance of the suprasubduction magmatism in volcanic-sedimentary belts almost at the same time (1.93–1.86 Ga) marked a change from the extensional regime to compression in the Paleoproterozoic history of the East European craton for the first time in Paleoproterozoic history. (4) The subsequent stage, with predominance of collisional and postcollisional processes (1.87–1.70 Ga), resulted in eventual formation of the intracontinental collisional orogen. In the history of the Lapland and Kolvitsa-Umba granulite-gneiss belts, an emplacement of the Pyrshin-Kolvitsa gabbro-anorthosite and granulite-facies metamorphism M0 were broadly coeval with Early Paleoproterozoic plume-influenced rifting of the Archean supercontinent ca. 2.51–2.44 Ga. This period was followed by deposition of volcaniclastic protoliths of the lower part of the Lapland granulite complex, which probably occurred within the intracratonic extensional basin. The next episode of plume-related extension, which induced emplacement of the Jaurijok gabbro-anorthosite bodies, lasted from 1.97 to 1.89 Ga. High-grade metamorphism M1 (ca. 1.95 Ga) and then pervasive granulite-facies metamorphic events M2 (1.95–1.91 Ga) and M3 (1.91–1.89 Ga) followed at the end of this episode. At the same time, in the area of low-grade volcanic-sedimentary belts, opening and subsequent closure of local intracontinental oceans occurred due to rapid subduction and/or obduction of the oceanic lithosphere. The Mid-Russia sector (basement of the East European platform—territory of the Moscow syneclise) is the central part of the southern branch of the Lapland– Mid-Russia–South Baltia orogen surrounding the Karelian craton in the south. The Totma belt extends along the northern boundary, separating the Paleoproterozoic orogen from the Karelian craton, while the southern boundary, which separates the Paleoproterozoic orogen from the Volgo-Uralia continent and Sarmatia, is marked by the Aprelevka and Kazhim belts. The Kashino, Zubtsovsk-Diakonovo, Dmitrov-Galich, Moscow, Lezhsk-Grivino, and Oparino granulite-gneiss thrust-nappe belts are crucial in the upper-crustal structure of the Mid-Russia sector. They are combined with Archean or Paleoproterozoic gneiss-amphibolite-migmatite complexes in the Bologoevo, Tver, Bukalovo, and Ivanovo-Sharya belts. As a whole, the Mid-Russia sector of the Paleoproterozoic collisional orogen is a giant synform, 300–350 km wide and 1400 km long. The synform is composed of tectonic sheets, 6–8 km thick, alternating in vertical and lateral directions and formed by the Paleoproterozoic and Archean granulite-gneiss and migmatite-amphibole-gneiss complexes. It is bordered by tectonic sheets of volcanic-sedimentary belts conformably plunging southeast. The South Baltia sector of the orogen (basement of the East European platform—Southern Baltic region and Belarus) is composed of a series of arcuate (crescentic) belts consisting of metamorphic rocks ranging in grade from greenschist to epidote-amphibolite or from high amphibolite to granulite facies. The sector, which extends for 1200 km toward the northeast, is up to 800 km wide. The arcuate outlines of belts in the South Baltia sector are convex to the east. The internal structural elements of belts and their boundaries are generally characterized by westward centroclinal plunges. In the west, the South Baltia sector is cut off by the Transeuropean suture (Teisseyre-Tornquist Line), which separates it from the Hercynides of East Europe. The eastern part of the South Baltia Sector contains the Staraya Russa–South Finland granulite-gneiss belt, Ilmenozero migmatite-amphibolite-gneiss belt, and Vitebsk and Toropets granulite-gneiss allochthons. The northern segment of the South Baltia sector dominates the area and is composed of alternating arcuate (crescentic) belts: the Belarus-Baltic granulite-gneiss belt, Latvia–East Lithuania migmatite-amphibolite-gneiss belt, and West Lithuania granulite-gneiss belt at the western boundary of the East European craton. In the southern segment of the South Baltia sector, the sequence of the belt from west to east includes the Central Belarus migmatite-amphibolite-gneiss belt and the southern part of the Vitebsk granulite-gneiss allochthon. The South Baltia sector is, to a greater extent, similar in composition and structure to the other sectors of the Lapland–Mid-Russia–South Baltia intracontinental orogen. This implies that granulite-gneiss belts of the South Baltia sector most likely are underlain by Archean or Early Paleoproterozoic crust.

The earliest events in the geological history of the Early Precambrian crust of the East European craton (3.5–2.93 Ga) resulted in the emergence of spatially separate and genetically independent areas of continental crust (continental embryos), the dimensions of which rarely exceeded a few hundred kilometers across. The period between 3.05 Ga and ca. 2.75 Ga was a time of mainly plate-tectonic development: origin, evolution, and accretion of ancient island-arc systems, and collision of microcontinents. The Vedlozero-Segozero and Sumozero-Kenozero systems of greenstone belts, Tipasjärvi-Kuhmo-Suomussalmi and Central Belomorian greenstone belts in Karelia, and the Kolmozero-Voronya greenstone belt in the Kola Peninsula are interpreted as accretionary systems transformed in collisional orogens. The Belomorian eclogite province is structurally linked with the Central Belomorian greenstone (suture) belt. The Kovdozero granite-greenstone terrane is formed by granitoids and gneisses hosting metasediments and metavolcanics of several greenstone belts, which belonged to the Parandovo-Tiksheozero island arc. The amalgamation of the continental domains that made up the bulk of the Archean crust in the growing East European composite craton took from 2.82 to 2.66 Ga, but the main events had terminated by 2.75 Ga. During the period from 2.79–2.55 Ga, specific areas of intracontinental thermal and tectonic activity (hot regions) developed in the inner portions of the recently formed continent: Karelian-Belomorian and Kola areas in the eastern Fennoscandian Shield and extensive Volgo-Uralia granulite-gneiss area in the eastern part of the East European craton. These processes marked a principally new evolutionary episode in the Early Precambrian history of the East European craton. Widespread high-temperature magmatic and metamorphic processes and the development of synformal structures and linear sedimentary basins testify to an anorogenic extensional environment and a significant influx of heat to the crust, i.e., a significant event of mantle-plume type. During the Paleoproterozoic (2.53–1.87 Ga), a number of intracontinental collisional orogens were produced within the East European craton. The largest of them are the Lapland–Mid-Russia–South Baltia intracontinental orogen and the Svecofennian accretionary orogen. The Lapland–Mid-Russia–South Baltia orogen surrounds the Karelian craton as a wide arc, separating it from Volgo-Uralia and Sarmatia. The orogen extends for more than 3000 km; its width in the northern and central segments is 400–700 km and increases to 1000 km in the southwest. The Lapland sector of the orogen is characterized by spatial distribution of tectonic belts composed of low-grade metavolcanic-metasedimentary rocks and belts of high-grade metamorphic rocks, including granulite-gneiss complexes. The former are localized along the orogen boundaries; in turn, the axial zone of the orogen is mainly formed by alternation of low-angle tectonic sheets varying in thickness from a few to 20–25 km: sheets composed of Paleoproterozoic granulite-gneiss complexes with a predominance of metamorphosed juvenile intrusive and volcanic bodies and sedimentary rocks alternate with the sheets of Archean granite-greenstone and amphibolite-gneiss complexes. The Paleoproterozoic evolution of the Kola-Karelia continent and, accordingly, the Lapland–Mid-Russia–South Baltia orogen, is subdivided into four episodes: (1) ca. 2.53–2.3 Ga: failed rifting of the Archean continent; (2) 2.3–2.1 Ga: quiescent within-plate activity and diffuse rifting that can be regarded as “failed attempts” to break the supercontinent; (3) 2.1–1.95 Ga: rifting of the Kola-Karelia continent; and (4) 1.95–1.87 Ga: origin of the intercontinental collisional orogens. The Paleoproterozoic pulse of tectonic activity, which transformed the Neoarchean Kola-Karelia continent, continued for more than 600 m.y. Globally speaking, Early Paleoproterozoic magmatic and thermal activities were largely constrained within the ancient continent that then included North America and most of the eastern European continent, including the Fennoscandian Shield (Lauroscandia). Analysis of the extensive data leads us to distinguish a new type of tectonic structure: the intracontinental oval orogen formed in the inner portions of continents under the effect of large mantle plumes. It is an oval-shaped tectonic ensemble of regional rank with diameters from 600–1000 to 2500–3000 km, of which at least a part is characterized by concentric structure and metamorphic zonation or which contains bowl-shaped crustal structures. Intracontinental orogens contain granulite-gneiss complexes, derivatives of juvenile (though crust-contaminated) mafic magmas (gabbro-anorthosites and layered mafic-ultramafics), intrusions of “dry” high- temperature, within-plate–type granites, enderbites, and charnockites, and low-grade sedimentary-volcanic belts. The oval or oval-concentric structure excludes the possibility that intracontinental orogens originated as a result of processes at convergent plate boundaries. Their size and morphology and the evidence of a vast influx of mantle heat make intracontinental orogens comparable to oceanic plateaus and large igneous provinces on the continents. The fundamental changes in Earth's geological evolution that occurred at the Mesoarchean-Neoarchean boundary (ca. 2.75 Ga) can be related to the transition from Archean “microplate tectonics” to Paleoproterozoic “supercontinent tectonics” (or “microocean tectonics,” with regard to the limited size of the Red Sea–type oceans that were formed within the partly fractured supercontinent). The origin of Earth's first supercontinent, a landmass covering much of Earth's surface, by 2.80–2.76 Ga, should have played an extremely important role in restyling the system of convection cells in the underlying mantle. The style of tectonic processes and the geodynamic environment of plate tectonics in the Neoarchean–Paleoproterozoic differ from those in both the Archean and the Phanerozoic: the Archean tectonics of multiple “miniplates” was much more similar to Phanerozoic plate tectonics than to the Neoarchean–Paleoproterozoic “tectonics of supercontinents.”

Abstract We use the Bay of Biscay and Western Pyrenees as a natural laboratory to develop and apply an approach to characterize and identify distinctive rifted margin domains in offshore and onshore settings. The Bay of Biscay and Western Pyrenees offer access to seismically imaged, drilled and exposed parts of one and the same hyperextended rift system. Offshore, we use gravity inversion and flexural backstripping techniques combined with seismic interpretation to provide estimates of accommodation space, crustal thickness and lithosphere thinning. Onshore, we focus on key outcrops of the former rift domain to describe the nature of sediment and basement rocks, and of their interface. This qualitative and quantitative characterization provides diagnostic elements for the identification of five distinct structural domains at magma-poor rifted margins and their fossil analogues (proximal, necking, hyperthinned, exhumed mantle and oceanic domains). This new approach can be used to reconcile offshore and onshore observations, and to aid interpretation when only local observations are available. Onshore remnants can be placed in an offshore rifted-margin context, enabling the prediction of first-order crustal architecture. For the interpretation of offshore seismic reflection sections, geological insights into rift structures and basement nature can be suggested based on onshore analogies. Supplementary material: Sensitivity of backstripping results to flexural rigidity is available at http://www.geolsoc.org.uk/SUP18778 .

Review of the major post–Middle Ordovician lithotectonic elements of the Appalachian orogen indicates that the middle to late Paleozoic geologic evolution of the Appalachian margin was less uniform than that of the early Paleozoic. Evolutionary divergence between the northern and southern segments of the orogen started in the Late Ordovician to Silurian with staggered accretion of the first peri-Gondwanan elements to reach the Laurentia margin, Carolinia in the south and Ganderia in the north. Divergence was amplified during the Silurian, specifically with respect to the nature of the Laurentian margin and the history of accretion. During this time frame, the northern margin was convergent, whereas the amagmatic southern margin may well have been a transform boundary. In terms of accretion, the Late Silurian–Early Devonian docking of Avalonia was restricted to the northern segment, whereas the southern Appalachians appear to have been largely quiescent during this interval. The evolutionary paths of the two segments of the margin converge on a common history in the Late Devonian during the Famennian event; we suggest that this tectonism was related to the initial marginwide interaction of Laurentia with the peri-Gondwanan blocks of Meguma and Suwanee, providing a uniform tectonic template for margin evolution. The Laurentian-Gondwanan collision is marked by second-order divergences in history. Specifically, during the Carboniferous, the southern segment records a larger component of shortening than the northern Appalachians.

Terranes of the Klamath Mountains, California-Oregon, are primarily the products of transpressive-transtensional margin processes that span the mid-Devonian to Middle Jurassic time interval. One of the more enigmatic units of this orogen is the early Mesozoic, north-south–trending, medially situated North Fork terrane. Based on the integration of new and previously published data, ophiolitic rocks in the North Fork terrane are interpreted as the basement to superjacent mafic metavolcanic units and associated metasedimentary strata. We propose that these volcanic and sedimentary rocks accumulated in a forearc position relative to the eastern Klamath fringing arc. New mapping in the southern Klamath Mountains reveals that northwest-southeast–striking structures in the southern North Fork terrane are regionally consistent with those in the rest of the belt, ophiolitic rocks are variably deformed (one expression being a serpentinite-matrix mélange), and the terrane is dominated by mafic volcanic rocks and fine-grained clastic metasedimentary rocks interbedded with chert. Bulk-rock geochemical data for metasedimentary rocks indicate that chert and shale possess a continental component. Detrital zircon U-Pb ages for three sandstone samples (one from the southern and two from the central North Fork) show that (1) sedimentary rocks of both terrane segments had a similar source of Mesozoic zircons, (2) sedimentary rocks of the central North Fork terrane have ages consistent with a source in the nearby eastern Klamath Mountains, and (3) sediment of the southern North Fork was derived primarily from an early Mesozoic source. We conclude that the North Fork terrane represents tectonically disrupted remnants of a forearc basin and its ophiolitic basement that were located seaward of the early to middle Mesozoic fringing arc of the eastern Klamath terrane and landward from a coeval oceanic trench. Slightly coarser-grained sedimentary rocks are preserved in the central part of the terrane, whereas rocks in the south lack continent-derived material coarser than mud.

Sediment provenances and magmatic events of Late Neoproterozoic (Ediacaran) and Cambro-Ordovician rock complexes from the Saxo-Thuringian zone are constrained by new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating of detrital zircons from five sandstones and magmatic zircons from an ignimbrite and one tuffite. These geochronological results in combination with the analysis of the plate-tectonic setting constrained from field observations, sedimentological and geochemical data, and trends of the basin development are used to reconstruct Cadomian orogenic processes during the Late Neoproterozoic and the earliest Cambrian. A continuum between Cadomian orogenesis and the opening of the Rheic Ocean in the Cambro-Ordovician is supported by the data set. In our model, the early stage of the Cadomian evolution is characterized by a Cordilleran-type continental magmatic arc, which was established at the periphery of the West African craton between ca. 650 and 600 Ma. Subsequently, at ca. 590–560 Ma, a back-arc basin was formed behind the Cadomian magmatic arc. The back-arc basin was closed between ca. 545 and 540 Ma, leading to the development of a short-lived Cadomian retroarc basin. Subsequently, a mid-oceanic ridge was subducted underneath the Cadomian orogen. Slab break-off of the subducted oceanic plate resulted in increased heat flow, leading to voluminous magmatic and anatectic events that culminated at ca. 540 Ma. Oblique incision of the oceanic ridge into the continent caused the formation of rift basins during the Lower to Middle Cambrian. This process continued from the Middle to Upper Cambrian, finally caused the opening of the Rheic Ocean in the Lower Ordovician.

The innermost domain of the autochthon of the Iberian Massif (Central Iberian Zone) consists of Upper Proterozoic–Lower Cambrian metasedimentary rocks and Early Paleozoic augen gneisses. The former were intruded by mafic magmas as small bodies that later became amphibolites under Variscan metamorphism, gabbro-diabasic textures being sometimes well preserved. Three main groups of amphibolites can be established: (A) the light rare earth element (LREE)-depleted group, characterized by an extremely low rare earth element (REE) fractionation factor [(La/Lu) CN = 0.27–0.34] and low Ti content; (B) the flat REE pattern group, characterized by a small REE fractionation factor [(La/Lu) CN small REE fractionation factor [(La/Lu) CN = 0.95–1.25]; and (C) the LREE-enriched group, characterized by a strong fractionation factor [(La/Lu) CN = 3.53–15.04] and high Ti content. Regular major element variations for the depleted amphibolites point to a low-pressure fractional crystallization as the major process, the strongly depleted amphibolites considered to be parent magmas, although crustal contamination accounts for their broad Nd and Sr isotope ranges as well as trace element variations. Part of the enriched amphibolite samples have an ocean island basalt–like signature and plot within the mantle array, suggesting a mantle mixed source. Other samples show decoupling of Nd and Sr isotopic systems as a response to a probable mixing process involving fluid-rock interaction during subsolidus evolution, which is supported by their high δ 18 O values. Geochemical characterization of amphibolite groups is consistent with an extensional within-plate tectonic setting, whereas processes involved may account for a similar mantle-crust interaction that occurred throughout the entire Central Iberian Zone and probably all over the northern margin of Gondwana during late Neoproterozoic–Early Paleozoic times.

The Acatlán Complex of southern México comprises metasedimentary and metaigneous rocks that represent the vestige of a Paleozoic ocean. Juxtaposed against granulite-facies gneisses of Mesoproterozoic (ca. 1 Ga) age, the complex has previously been related to the Iapetus Ocean and interpreted to preserve a tectonostratigraphic record linked to that of the Appalachian orogen: (1) Cambro-Ordovician deposition of a trench or forearc sequence (the Petlalcingo Group: the Magdalena, Chazumba, and Cosoltepec Formations) and an oceanic assemblage (the Piaxtla Group), (2) polyphase Late Ordovician–Early Silurian deformation (the Acatecan orogeny) during which the Piaxtla Group underwent eclogite-facies metamorphism synchronous with megacrystic granitoid emplacement, (3) deposition of the arc-related Tecomate Formation and intrusion of megacrystic granitoid plutons during the Devonian, and (4) deformation under greenschist-facies conditions during the Late Devonian Mixtecan orogeny. However, recent structural, geochronological, and geochemical studies have shown that (1) the Cosoltepec Formation is bracketed between ca. 455 Ma and the latest Devonian and may be part of a continental rise prism with slivers of oceanic basalt; (2) the Magdalena and Chazumba Units represent a clastic wedge assemblage of Permo-Triassic age; (3) the eclogitic metamorphism is locally Mississippian in age; (4) the Tecomate Formation is an arc complex of latest Pennsylvanian–Middle Permian age; (5) the megacrystic granitoid rocks span the Ordovician and have a calc-alkaline geochemistry, whereas accompanying mafic units have mixed continental arc–tholeiitic affinities and are locally as young as the earliest Silurian; (6) the greenschist-facies tectonothermal event occurred in the Permo-Triassic; and (7) the complex records a Jurassic tectonothermal event that resulted in local high-grade metamorphism and migmatization. This revised geological history precludes any linkage to Iapetus, but is consistent with that of the Rheic and paleo-Pacific Oceans and is interpreted to record (1) development of a rift or passive margin on the southern flank of the Rheic Ocean in the Cambro-Ordovician, (2) formation of either an arc or an extensional regime along the formerly active northern margin of Gondwana throughout the Ordovician, (3) ocean closure documented by subduction-related eclogite-facies metamorphism and exhumation during the Late Devonian–Mississippian, (4) Permo-Triassic convergent tectonics on the paleo-Pacific margin of Pangea, and (5) overriding of a Jurassic plume.

Convergent margin tectonic settings involving accretion of large turbidite fans represent important sites of growth and regeneration of continental crust. The newly accreted continental crust consists of an upper crustal layer of recycled crustal detritus (turbidites) underlain by a lower crustal layer of tectonically imbricated oceanic crust, and/or rifted and thinned continental crust, along with underplated magmatic materials; the new lower crust represents additions to continental crustal volume differentiated from the mantle. This two-tiered crust is of average continental crustal thickness and is isostatically balanced near sea level, resulting in remarkable stability. The Paleozoic Tasman orogen of eastern Australia is the archetypal example of this style of orogeny, representing continental growth rates of cubic kilometers per year of material that does not return to the mantle by oceanic plate-tectonic recycling. The Neoproterozoic Pan-African Damara orogen of SW Africa is a similar orogen, whereas the Mesozoic Rangitatan orogen or Rakaia wedge of New Zealand illustrates the transition of the convergent margin from a Lachlan-type to more recognizable “ring of fire”-type orogen. These orogens illustrate continental growth from the shortening of deep marine successions and their oceanic crustal basement involving subduction-accretion. The spatial and temporal variations in deformation, metamorphism, and magmatism across these orogens illustrate how large volumes of monotonous turbidites and their relict oceanic basement eventually become stable continental crust. The timing of deformation and metamorphism reflect the crustal thickening phase, whereas the posttectonic granitoids and surficial volcanic deposits give the timing of cratonization. The turbidites represent fertile sources for crustal melting and are the main sources for the S-type granites.

Geological analysis of geophysical data obtained by the interpretation of crustal profiles indicates that in the central Mediterranean region the following structural domains can be distinguished: the foreland domains, an orogenic belt, and the Corsica-Sardinia block. The foreland domains are represented by two continental blocks, the Apulian block to the north and the Pelagian block to the south, belonging respectively to the Adria and to the Africa plates, separated by the oceanic crust of the Ionian Sea. The orogenic belt is located between two oceanic crusts: the old Ionian crust, at the present time subducting beneath the Calabrian arc, and the new crust of the Tyrrhenian Sea. The orogenic belt is represented by a multilayer allochthonous edifice composed of the Calabride chain, which tectonically overlies the so-called Apenninic-Maghrebian chain, which in turn is overthrust onto the upper Miocene and Pliocene top levels of a deep-seated thrust system that originated from the deformation of the innermost carbonates of the Apulian and Pelagian blocks (the external thrust system). The Calabride chain is composed of crystalline nappes originating, since the Eo-Oligocene, from the delaminated margin of the Europe plate. The Apenninic-Maghrebian chain tectonic units derive from the orogenic transport during Oligo-Miocene times of sedimentary sequences deposited in paleogeographic domains located between the Europe and the Afro-Adria plates. These units are composed of meso-Cenozoic shallow-water carbonate successions detached from a continental type of crustal sector named here the Panormide-Apenninic block. This is now recognizable by means of seismic lines shot in the Tyrrhenian off-shore of the southern Apennines and northern Sicily. The meso-Cenozoic basinal units that constitute the Apenninic-Maghrebian chain can be distinguished into two main groups of sequences, originally located on oceanic crusts separated by the Panormide-Apenninic crust: the external ones (Ionides) related to an original basin belonging to part of the Ionian paleo-basin involved in the orogenesis (Lagonegro, Imerese, Sicanian, and Monte Judica units) and the internal ones ascribed to the Alpine Tethys (Liguride-Sicilide units). The previously described allochthonous edifice is characterized by thin-skinned tectonics and represents a roof thrust system resting on the external thrust system, which derived from thick-skinned tectonics that produced folds and reverse faults with relatively moderate horizontal displacements. The external thrust system developed from late Miocene times, contemporaneously with the opening of the Tyrrhenian basin, and is named the Apulian thrust system in southern Italy and the Pelagian-Sicilian thrust belt in Sicily. The crustal sections of the CROP project (Deep Seismic Exploration in the Central Mediterranean and Italy) allow us to distinguish the thickness and distribution of the crusts in this area of the Mediterranean Sea, and they confirm that the foreland continental blocks, the Apulian and the Pelagian blocks, are separated by the Ionian oceanic crust. Both the foreland blocks extend below the orogenic belt, reaching the Tyrrhenian margins, with a gradual thinning and a transition to a Paleo-Ionian slab, probably not active at present time, from which the Ionides detached and overrode the external thrust system. The seismogeological data indicate the presence of a continental block, original basement of the Panormide-Apenninic platforms, that took part in the closure of the sectors of the Paleo-Ionian Sea interposed between the Panormide-Apenninic crust and the Pelagian and Apulian blocks. At the present time, it is colliding with the foreland blocks. Thus, this has been identified as collisional crust. The geologic evidence of this collisional stage is manifested in the northwest-southeast-oriented South Tyrrhenian system, which is characterized by dextral faults affecting both off-shore and on-shore areas of Sicily. A mirrorlike sinistral fault system occurs in the southern Apennines. Interpretative seismic reprocessing has permitted clear seismic imaging of the subducted Ionian slab. The distribution of the earthquakes transversally and longitudinally indicates that the slab is narrowing in a vertical direction; thus, at present, the active slab is limited to a short segment between northeastern Sicily (the Vulcano line) and southern Calabria (the Catanzaro line). To the west and to the northeast of these lines, a collisional setting can be recognized. The geological and geophysical data and the volcanological characteristics of the study area permit us to restore the paleogeographic and paleotectonic setting and have allowed us to recognize three orogenic stages: the Eo-Alpine, which originated during Cretaceous–Eocene times but is less evident in the study area; the Balearic stage (late Oligocene–early Miocene), in which the Corsica-Sardinia block collided with the Adria-Africa margins with thrusting of the Alpine Tethydes over Panormide units; and the Tyrrhenian stage (middle Miocene to present), when the onset of the Tyrrhenian back-arc basin occurred and the collisional crust followed the Ionian slab retreat, closing the interposed basin with tectonic transport of the Ionian ocean cover (the Ionides) over the foreland blocks.