Abstract Ponding conditions of basalts erupted from fissure zones at São Miguel Island (Azores) were investigated through microthermometry of fluid inclusions, hosted in olivines and clinopyroxenes, and whole-rock and mineral chemistry. The Região dos Picos and Achada das Furnas fissure zones formed between central volcanoes and erupted geochemically similar magmas for the last 30 kyr. Hydrocarbonic fluid inclusions that survived to diffusion and re-equilibration recorded a maximum density value of 1000 kg m −3 (29.3 km) at both fissure zones, at the intersection with the feeding systems of older central volcanoes. The maximum density decreases to 875 kg m −3 (23.5 km) towards the central segment of Região dos Picos. Further trapping of low-density fluids is occasional and limited to a few samples. While the deepest event is interpreted to mark the vertical variation of the Moho at each location, all other events are only representative of short-time ponding with fluid re-equilibration. The deepening of the Moho towards the central volcanoes might be the effect of long-lasting underplating. However, the periodic stretching of the lithosphere, in response to the differential stress field acting at central volcanoes, would have been responsible for the thinning of the crust at the central segment of the fissure zone. Supplementary material: Whole-rock geochemistry with international standard used, and representative analyses of selected mineral phases from the two segments of the Região dos Picos fissure zone is available at www.geolsoc.org.uk/SUP18818

Abstract We investigate the evolution of the Iberia–Newfoundland margin from Permian post-orogenic extension to Early Cretaceous break-up. We used a Quantitative Basin Analysis approach to integrate seismic stratigraphic interpretations and drill-hole data of two representative sections across the Iberia–Newfoundland margin with kinematic models for lithospheric thinning and subsequent flexural readjustment. We model the distribution of extension and thinning, palaeobathymetry, crustal structure, and subsidence and uplift history as functions of space and time. We start our modelling following post-orogenic extension, magmatic underplating and thermal re-equilibration of the Permian lithosphere. During the Late Triassic–Early Jurassic, broadly distributed, depth-independent lithospheric extension evolved into Late Jurassic–Early Cretaceous depth-dependent thinning as crustal extension progressed from distributed to focused deformation. During this time, palaeobathymetries rapidly deepened across the margin. Modelling of the southern and northern profiles highlighted the rapid development of crustal deformation from south to north over a 5–10 myr period, which accounts for the rapid change in Tithonian–Valanginian, deep- to shallow-water sedimentary facies between the Abyssal Plain and the adjacent Galicia Bank, respectively. Late-stage deformation of both margins was characterized by brittle deformation of the remaining continental crust, which led to exhumation of subcontinental mantle and, eventually, continental break-up and seafloor spreading.

Abstract The South Atlantic presalt petroleum system has unique elements which are related to the basin bounding structural fabric, accommodation space and climatic conditions. The development of both high total organic carbon lacustrine source rocks and hypersaline/hyperal-kaline microbial carbonates requires sequestration of these basins from marine conditions. Sequestration is accomplished by an outer-high which was isostatically elevated by magmatic under-plating. This magmatic under-plating is occurs along the margin from the Santos Basin to the Espírito Santo Basin and the conjugate Kwanza Basin. Where the outer high is absent, such as in the Brazilian Pelotas Basin and Namibian basins, the presalt petroleum system fails to develop. In Gabon where under-plating and basin sequestration occurs, a Brazilian style presalt system fails to develop due to high clastic influx. Gabon lacks the microbial hypersaline carbonate reservoir. Similar Brazilian style presalt petroleum systems are likely however, to occur on other passive margins where similar structural styles create a sequestered basin. These basins should occur in regions which are transitional from classic volcanic margins to true nonvolcanic margins. Through better understanding of the Brazilian presalt geodynamic setting we can position ourselves to identify new basins with similar petroleum systems which have created the giant Brazilian presalt discoveries.

Published models for the Limpopo Complex as a whole include Neoarchean (ca. 2.65 Ga) continent-continent collision, Turkic-type terrane accretion, and plume-related gravitational redistribution within the crust. Hypotheses proposed for parts of the complex are Paleoproterozoic (ca. 2.0 Ga) dextral transpression for the Central Zone, westward emplacement of the Central Zone as a giant nappe, and gravitational redistribution scenarios. In this chapter these models and hypotheses are reviewed and tested against new data from geophysics (chiefly seismics and gravity), isotope geochemistry (mainly Sm-Nd and Lu-Hf data), geochronology, and petrology. Among the whole-complex models, the plume-related gravitational redistribution model and the Turkic-type terrane accretion model do not satisfy the constraints. The Neoarchean collision model remains as a viable working hypothesis, whereby (in contrast to published versions) the Zimbabwe Craton appears to be the overriding plate, with the Northern Marginal and Central Zones of the Limpopo Complex as its (possibly Andean-type) active margin and shelf, respectively. Of the partial models, gravitational redistribution in the context of crustal thickening is compatible with Neoarchean collision and can explain features at the Complex–Kaapvaal Craton boundary. Paleoproterozoic dextral transpression in the Central Zone can be superimposed on Neoarchean collision, provided that it does not itself entail a continent collision. The Paleoproterozoic metamorphism is characterized by near-isobaric prograde paths, which (along with combined teleseismic and gravity data) suggest magmatic underplating. This could be related to the Bushveld Complex, and may have weakened the crust, leading to the focusing of regional strain into transcurrent movement in the Central Zone.

The nature of petrologic and structural properties and processes that characterize the middle and lower continental crust is a long-standing problem in the earth sciences. During the past several decades significant progress has been made on this fundamental problem by synthesizing deep-crustal seismic-reflection imaging, laboratory-based seismic-velocity determinations, xenolith studies, and detailed geologic studies of exposed crustal cross sections. Geological, geochemical, and geophysical studies of crustal sections provide a crustal-scale context for a variety of important problems in the earth sciences. Crustal sections are widely used to evaluate crustal composition and petrogenesis, including lateral and vertical variations in rock types. Evidence from deep levels of crustal sections suggests seismic shear-wave anisotropy and seismic lamination result from widespread subhorizontal contacts, shear zones, and transposition fabrics, and in some sections from metamorphosed m- to km-thick, intraplated and/or underplated mafic magmatic sheets and plutons. Crustal sections also facilitate the evaluation of crustal rheology in natural settings from regional to outcrop scale. Magmatism, metamorphism, partial melting, and relatively small lithological differences control rheology, localize strain, and lead to markedly heterogeneous deformation over a wide range of crustal levels. Finally, crustal sections provide unique views of the architecture and deformation patterns of fault zones in the deep crust. As a guide to the growth and evolution of continental crust in the past 0.5 Ga, we summarize the salient features of some examples of crustal cross sections from Phanerozoic orogens. These crustal sections represent different tectonic settings, although the variation in magmatic arcs from intra-oceanic to continental-margin settings is a major theme in our synthesis. Another theme is the importance of attenuated crustal sections in reconstructing the hinterland of orogens that have experienced large-magnitude crustal extension after an earlier history of crustal contraction. The Phanerozoic crustal cross sections summarized in this chapter developed during a polyphase deformational and magmatic history that spanned 10–100s of Ma and resulted in overprinting of different events. Consequently, we conclude that there is no “typical” Phanerozoic continental crustal section, and the overall crustal composition varies markedly between sections. The thickness of lower crust that existed below an exposed crustal section is difficult to quantify. Only a few sections are in contact (typically faulted) with mantle rocks, and although xenoliths can provide important information about the unexposed parts of the deep crust and upper mantle, they are absent for most sections. The exhumation of relatively intact crustal cross sections and lower-crustal rocks probably requires an unusual sequence of tectonic events, and almost all of the sections evaluated in this chapter were exhumed by multiple mechanisms. Major exhumation is most commonly attributed to normal faults and extensional shear zones.

Ultrahigh-pressure (UHP) metamorphic terranes in contractional orogens reflect descent of continental crust bonded to a dense, dominantly oceanic plate to depths of 90–140 km. All recognized well-documented UHP complexes formed during Phanerozoic time. Rocks are intensely retrogressed to low-pressure assemblages, with rare relict UHP phases retained in tough, refractory host minerals. Resurrected UHP slabs consist chiefly of quartzofeldspathic rocks and serpentinites; dense mafic + ultramafic lithologies comprise <10% of exhumed masses. Associated garnet-bearing ultramafic lenses are of four general origins: type A peridotite + eclogite pods reflect premetamorphic residence in the mantle wedge; type B masses were mantle-derived ultramafic-mafic magmas that rose into the crust prior to subduction; type C tectonic lenses were present in the oceanic lithosphere prior to underflow; and type D garnet peridotites achieved their deep-seated mantle mineralogy long before—and independent of—the subduction event that produced the UHP-phase assemblages in garnet peridotite types A, B, and C. Geochronology constrains the timing of protolith, peak, and retrograde recrystallization of gneissic, ultramafic, and eclogitic rocks. Round-trip pressure-temperature ( P-T ) paths were completed in <5–10 m.y., where ascent rates approximated subduction velocities. Exhumation from profound depth involves near-adiabatic decompression through P-T fields of much lower-pressure metamorphic facies. Many complexes consist of thin, allochthonous sheets, but those in eastern China and western Norway are about 10 km thick. Ductilely deformed nappes generated in subduction zones allow heat to be conducted away as sheet-like UHP complexes rise, cooling across both upper and lower surfaces. Thicker UHP massifs also must be quenched. Ascent along the subduction channel is driven mainly by buoyancy of low-density crustal material relative to the surrounding mantle. Rapid exhumation prevents establishment of a more normal geothermal regime in the subduction zone. Lack of H 2 O impedes back reaction, whereas its presence accelerates transformation to low-P phase assemblages. Late-stage domal uplifts characterize some collisional terranes; erosion, combined with underplating, contraction, tectonic aneurysms, and/or lithospheric plate shallowing, may further elevate mid-crustal UHP terranes toward the surface.

The Hellenic orogen is a composite one, consisting of three orogenic belts: (1) the Cimmerian internal belt, created in pre–late Jurassic times as a result of the collision of northward-drifted Cimmerian continental fragments with Eurasia, (2) the Alpine orogenic belt, created in Cretaceous–Tertiary times after the Neo-Tethyan subduction beneath the Cimmeria-Eurasia plate and the collision of the Apulian microplate with this composite Cimmeria-Eurasia plate, and (3) the Mesogean orogenic belt along the External Hellenic arc, which resulted from the underplating of the Mesogea-Africa plate beneath the Alpine-Cimmeria-Eurasia plate in Miocene–Pliocene times and the exhumation of the Cretan–southern Peloponnesus tectonic windows. Tertiary Alpine collisional tectonics (imbrication and nappe stacking) formed the nappe pile of the Hellenic orogen. A late orogenic extension followed the lithospheric thickening and caused the collapse and crustal thinning with exhumation of the lower crustal units as metamorphic core complexes in Crete, the Southern Peloponnesus, and the Cyclades and a series of tectonic windows in continental Greece (i.e., the windows of Olympus, Ossa, Rizomata, and Krania) consisting of Mesozoic and Tertiary neritic carbonates of the External Hellenides. Five deformation events have been recognized in Greece: (1) the D 0 compressional event in the Eocene (ca. 45 Ma), contemporaneous with the high-pressure–low-temperature (H P -L T ) metamorphism that created the blueschist belt along Olympus, Ossa, Euboea, and the Cyclades; (2) the D 1 extensional event in the late Eocene–early Oligocene, contemporaneous with the greenschist metamorphism in the Hellenic Hinterland (Rhodope and Serbomacedonian zones) and some of the Internal Hellenides zones (Circum Rhodope, Axios-Vardar, and part of the Cyclades); (3) the D 2 compressional event in the late Oligocene–early Miocene, contemporaneous with the 25 Ma H P -L T metamorphism in Crete and the southern Peloponnesus, which produced the imbrication and nappe stacking; (4) the D 3 extensional event in the early–middle Miocene, which produced low-angle shear zones, thinning of the crust, uplift and exhumation of the H P -L T metamorphic rocks as core complexes, and tectonic windows in Crete, the southern Peloponnesus, and the Cyclades; and (5) the D 4 extensional phase, the final deformational event in Pliocene to recent times. The direction of the extension is generally north-south, producing east-west-trending normal faults and locally reactivating previous faults of other trends as strike-slip faults. In northern Greece, the direction of the active extension differs slightly from the general north-south direction in places, particularly in northwestern Macedonia, where it becomes almost northwest-southeast, as is deduced from both focal mechanisms of large and small earthquakes and geological field measurements along active faults.

Ultramafic and mafic xenoliths entrained in late Oligocene dikes of intraplate origin provide information about the composition of the lower crust and the processes operating beneath the eastern Rhodope metamorphic core complexes Biala Reka and Kesebir, in southeastern Bulgaria. The cumulates comprise a series of high- to medium-pressure rocks represented by olivine websterites, orthopyroxenites, clinopyroxenites, websterites, and gabbros. Thermobarometric studies and comparison with experimental works suggest that the cumulate sequence formed from hydrous (>3 wt%) mafic magma at pressures of 14–9 kilobars (45–30 km) and temperatures of 1200–850 °C. It is inferred that underplating of such hot, wet mafic intrusions modified the thermal and mechanical properties of the lower and middle crust, as is reflected in thermal metamorphism, associated extension, and hydrothermal activity producing low-sulfidation Au deposits. Findings of similar xenoliths in the alkaline basalts from other extended regions such as the eastern Mediterranean and the Basin and Range Province indicate that underplating of mafic magma plays an important role in core complex formation.