The Elan Bank microcontinent was separated from East India during the Early Cretaceous break-up. The crustal architecture and rifting geometry of East India and the Elan Bank margins document that the early break-up between India and Antarctica was initiated in the eastern portions of the Cauvery and Krishna–Godavari rift zones, and in the southern portion of Elan Bank. However, the westwards break-up propagation along the Krishna–Godavari Rift Zone continued even after the break-up in the overstepping portion of the Cauvery Rift Zone. Eventually, the western propagating end of the Krishna–Godavari Rift Zone became hard-linked with the failed western portion of the Cauvery Rift Zone by the dextral Coromandel transfer fault zone. Consequently, the break-up location between India and Antarctica shifted from its initial to its final location along the northern portion of the Elan Bank formed by the western Krishna–Godavari Rift Zone. The competition between the two rift zones to capture continental break-up and asymmetric ridge propagation resulted in a ridge jump and the Elan Bank microcontinent release.

In Chapter 2, we showed that the eclogite-facies metamorphism within the Mesoarchean–Neoarchean Belomorian eclogite province can be correlated with “hot” subduction. However, was such a thermal regime specific to the Archean? Comparison of pressure-temperature-time paths and data for peak metamorphic parameters demonstrates the general similarity of the Archean and Paleoproterozoic eclogites worldwide and their association with anomalously “hot” environments. In contrast, Phanerozoic high- and ultrahigh-pressure eclogite complexes formed in connection with “warm” subduction zones. The high-temperature character of the eclogite-facies metamorphism in the Proterozoic is impossible to explain through a distinctively anomalous thermal regime, as is often suggested for the Archean mantle. The occurrence of high-temperature conditions during eclogite-facies metamorphism can be attributed to either subduction of a mid-ocean ridge (Archean, Belomorian eclogite province) or to interaction with mantle plumes (Proterozoic).

This work integrates the available geological information and geochronology data for the Cretaceous–Recent magmatism in the South Atlantic, represented by onshore and offshore magmatic events, including the oceanic islands along the transform faults and near the mid-ocean ridge. The analysis of the igneous rocks and their tectonic settings allows new insights into the evolution of the African and Brazilian continental margins during the South Atlantic opening. Following the abundant volcanism in the Early Cretaceous, the magmatic quiescence during the Aptian–Albian times is a common characteristic of almost all Brazilian and West African marginal basins. However, rocks ascribed to the Cabo Granite (104 Ma) are observed in NE Brazil. In West Africa, sparse Aptian–Albian ages are observed in a few coastal igneous centres. In the SE Brazilian margin, an east–west alkaline magmatic trend is observed from Poços de Caldas to Cabo Frio, comprising igneous intrusions dated from 87 to 64 Ma. Mafic dyke swarms trending NW also occur in the region extending from the Cabo Frio Province towards the Central Brazilian Craton. On the West African side, Early Cretaceous–Recent volcanism is observed in the Walvis Ridge (139 Ma), the St Helena Ridge (81 Ma) and the Cameroon Volcanic Line (Early Tertiary–Recent). Volcanic islands such as Ascencion (1.0–0.65 Ma), Tristão da Cunha (2.5–0.13 Ma) and the St Helena islands (12 Ma) most probably correspond to mantle plumes or hot spots presently located near the mid-Atlantic spreading centre. Within the South America platform and deep oceanic regions, the following volcanic islands are observed: the Rio Grande Rise (88–86 Ma), Abrolhos (54–44 Ma), the Vitória–Trindade Chain (no age), Trindade (2.8–1.2 Ma) and Fernando de Noronha (12–1.5 Ma). There are several volcanic features along the NW–SE-trending Cruzeiro do Sul Lineament from Cabo Frio to the Rio Grande Rise, but they have not been dated. The only known occurrence of serpentinized mantle rocks in the South Atlantic margin is associated with the Saint Peter and Saint Paul Rocks located along the São Paulo Fracture Zone. The Cameroon Volcanic Line in NW Africa is related to the magmatism that started in the Late Cretaceous and shows local manifestations up to the Present. The compilation of all available magmatic ages suggests an asymmetrical evolution between the African and South America platforms with more pre-break-up and post-break-up magmatism observed in the Brazilian margin. This is most likely to have resulted from the different geological processes operating during the South Atlantic Ocean opening, shifts in the spreading centre, and, possibly, the rising and waning of mantle plumes.Supplementary material:A complete table with radiometric dates that have been obtained by universities, government agencies and research groups is available at: www.geolsoc.org.uk/SUP18596

Along Pacific convergent margins of the Americas, high-standing relief on the subducting oceanic plate “collides” with continental slopes and subducts. Features common to many collisions are uplift of the continental margin, accelerated seafloor erosion, accelerated basal subduction erosion, a flat slab, and a lack of active volcanism. Each collision along America’s margins has exceptions to a single explanation. Subduction of an ~600 km segment of the Yakutat terrane is associated with >5000-m-high coastal mountains. The terrane may currently be adding its unsubducted mass to the continent by a seaward jump of the deformation front and could be a model for docking of terranes in the past. Cocos Ridge subduction is associated with >3000-m-high mountains, but its shallow subduction zone is not followed by a flat slab. The entry point of the Nazca and Juan Fernandez Ridges into the subduction zone has migrated southward along the South American margin and the adjacent coast without unusually high mountains. The Nazca Ridge and Juan Fernandez Ridges are not actively spreading but the Chile Rise collision is a triple junction. These collisions form barriers to trench sediment transport and separate accreting from eroding segments of the frontal prism. They also occur at the separation of a flat slab from a steeply dipping one. At a smaller scale, the subduction of seamounts and lesser ridges causes temporary surface uplift as long as they remain attached to the subducting plate. Off Costa Rica, these features remain attached beneath the continental shelf. They illustrate, at a small scale, the processes of collision.

The proposed ages for the collision of the Carnegie Ridge with the South America trench, offshore Ecuador, range from 1 to 15 Ma. In this time frame, many geological features of Ecuador are commonly linked to the subduction of the Carnegie Ridge. (1) After the ridge collided with the trench at ca. 15 Ma, the subsequent interplate coupling produced high exhumation rates of volcanic materials at ca. 9 Ma. (2) The oblique convergence of the Carnegie Ridge would have caused the northward drift of the North Andean block and the opening of the Gulf of Guayaquil. (3) During the late Miocene, the subduction of the Carnegie Ridge would have triggered a regional tectonic inversion along the forearc. (4) Along the collision front of the ridge with the trench, subduction-related erosion is occurring, and the Ecuadorian continental margin is being uplifted in the present day. (5) The chemistry of the active volcanic arc is explained as resulting from the arrival of the Carnegie Ridge into the trench. For instance, the adakitic signal, which appears at 1.5 Ma, is thought to be ridge-induced. (6) The buoyancy of the subducted Carnegie Ridge would explain the flatness of the slab beneath Ecuador. In this paper, we review the geological evolution of the Northern Andes in order to establish which of these geological events may be related to the subduction of the Carnegie Ridge. This review suggests that there is no clear deformation linked with the subduction of the Carnegie Ridge or with its landward prolongation postulated at depth.

The Sierras Pampeanas in the west-central part of Argentina are a modern analog for Laramide uplifts in the western United States. In this region, the Nazca plate is subducting beneath South America almost horizontally at about ~100 km depth before descending into the mantle. The flat-slab geometry correlates with the inland prolongation of the subducted oceanic Juan Fernández Ridge. This region of Argentina is characterized by the termination of the volcanic arc and uplift of the active basement-cored Sierras Pampeanas. The upper plate shows marked differences in seismic properties that are interpreted as variations in crustal composition in agreement with the presence of several Neoproterozoic to Paleozoic accreted terranes. In this paper, we combine the results from the CHile-ARgentina Geophysical Experiment (CHARGE) and the CHile-ARgentina Seismology Measurement Experiment (CHARSME) passive broadband arrays to better characterize the flat-slab subduction and the lithospheric structure. Stress tensor orientations indicate that the horizontal slab is in extension, whereas the upper plate backarc crust is under compression. The Cuyania terrane crust exhibits high P-wave seismic velocities (Vp ~6.4 km/s), high P- to S-wave seismic velocity ratios (Vp/Vs = 1.80–1.85), and 55–60 km crustal thickness. In addition, the Cuyania terrane has a high-density and high-seismic-velocity lower crust. In contrast, the Pampia terrane crust has a lower Vp value of 6.0 km/s, a lower Vp/Vs ratio of 1.73, and a thinner crust of ~35 km thickness. We integrate seismic and gravity studies to evaluate crustal models that can explain the unusually low elevations of the western Sierras Pampeanas. Flat-slab subduction models based on CHARGE and CHARSME seismic data and gravity observations show a good correlation with the predicted Juan Fernández Ridge path beneath South America, the deep Moho depths in the Andean backarc, and the high-density and high-seismic-velocity lower crust of the Cuyania terrane. The Cuyania terrane is also the region characterized by more frequent and larger-magnitude crustal earthquakes.

Suprasubduction-zone ophiolites have been recognized in the geologic record for over thirty years. These ophiolites are essentially intact structurally and stratigraphically, show evidence for synmagmatic extension, and contain lavas with geochemical characteristics of arc-volcanic rocks. They are now inferred to have formed by hinge retreat in the forearc of nascent or reconfigured island arcs. Emplacement of these forearc assemblages onto the leading edge of partially subducted continental margins is a normal part of their evolution. A recent paper has challenged this interpretation. The authors assert that the “ophiolite conundrum” (seafloor spreading shown by dike complexes versus arc geochemistry) can be resolved by a model called “historical contingency,” which holds that most ophiolites form at mid-ocean ridges that tap upper-mantle sources previously modified by subduction. They support this model with examples of modern mid-ocean ridges where suprasubduction zone–like compositions have been detected (e.g., ridge-trench triple junctions). The historical contingency model is flawed for several reasons: (1) the major- and trace-element compositions of magmatic rocks in suprasubduction-zone ophiolites strongly resemble rocks formed in primitive island-arc settings and exhibit distinct differences from rocks formed at mid-ocean-ridge spreading centers; (2) slab-influenced compositions reported from modern ridge-trench triple junctions and subduction reversals are subtle and/or do not compare favorably with either modern subduction zones or suprasubduction-zone ophiolites; (3) crystallization sequences, hydrous minerals, miarolitic cavities, and reaction textures in suprasubduction-zone ophiolites imply crystallization from magmas with high water activities, rather than mid-ocean-ridge systems; (4) models of whole Earth convection, subduction recycling, and ocean-island basalt isotopic compositions ignore the fact that these components represent the residue of slab melting, not the low field strength element–enriched component found in active arc-volcanic suites and suprasubduction-zone ophiolites; and (5) isotopic components indicative of mantle heterogeneities (related to subduction recycling) are observed in modern mid-ocean-ridge basalts (MORB), but, in contrast to the prediction of the historical contingency model, these basalts do not exhibit suprasubduction zone–like geochemistry. The formation of suprasubduction-zone ophiolites in the upper plate of subduction zones favors intact preservation either by obduction onto a passive continental margin, or by accretionary uplift above a subduction zone. Ophiolites characterized by lavas with MORB geochemistry are typically disrupted and found as fragments in accretionary complexes (e.g., Franciscan), in contrast to suprasubduction-zone ophiolites. This must result from the fact that oceanic crust is unlikely to be obducted for mechanical reasons, but it may be preserved where it is scraped off of the subducting slab.

The plate tectonic processes, or “plate,” model for the genesis of melting anomalies (“hotspots”) attributes them to shallow-sourced phenomena related to plate tectonics. It postulates that volcanism occurs where the lithosphere is in extension, and that the volume of melt produced is related primarily to the fertility of the source material tapped. This model is supported in general by the observation that most present-day “hotspots” erupt either on or near spreading ridges or in continental rift zones and intraplate regions observed or predicted to be extending. Ocean island basalt-like geochemistry is evidence for source fertility at productive melting anomalies. Plate tectonics involves a rich diversity of processes, and as a result, the plate model is in harmony with many characteristics of the global melting-anomaly constellation that have tended to be underemphasized. The melting anomalies that have been classified as “hotspots” and “hotspot tracks” exhibit extreme variability. This variability suggests that a “one size fits all” model to explain them, such as the classical plume model, is inappropriate, and that local context is important. Associated vertical motion may comprise pre-, peri-, or post-emplacement uplift or subsidence. The total volume erupted ranges from trivial in the case of minor seamount chains to ∼108 km3 for the proposed composite Ontong Java–Manihiki–Hikurangi plateau. Time progressions along chains may be extremely regular or absent. Several avenues of testing of the hypothesis are being explored and are stimulating an unprecedented and healthy degree of critical debate regarding the results. Determining seismologically the physical conditions beneath melting anomalies is challenging because of problems of resolution and interpretation of velocity anomalies in terms of medium properties. Petrological approaches to determining source temperature and composition are controversial and still under development. Modeling the heat budget in large igneous provinces requires knowledge of the volume and time scale of emplacement, which is often poorly known. Although ocean island basalt–type geochemistry is generally agreed to be derived from recycled near-surface materials, the specifics are still disputed. Examples are discussed from the Atlantic and Pacific oceans, which show much commonality. Each ocean hosts a single, currently forming, major tholeiitic province (Iceland and Hawaii). Both of these comprise large igneous provinces that are forming late in the sequences of associated volcanism rather than at their beginnings. Each ocean contains several melting anomalies on or near spreading ridges, both time- and non-time-progressive linear volcanic chains of various lengths, and regions of scattered volcanism several hundred kilometers broad. Many continental large igneous provinces lie on the edges of continents and clearly formed in association with continental breakup. Other volcanism is associated with extension in rift valleys, back-arc regions, or above sites of slab tearing or break-off. Specific plate models have been developed for some melting anomalies, but others still await detailed application of the theory. The subject is currently developing rapidly and poses a rich array of crucial but challenging questions that need to be addressed.

Asthenosphere plume-to-ridge flow has often been proposed to explain both the existence of geochemical anomalies at the mid-ocean ridge segments nearest an off-axis hotspot and the existence of apparent geochemical provinces within the global mid-ocean spreading system. We have constructed a thin-spherical-shell finite element model to explore the possible structure of global asthenosphere flow and to determine whether plume-fed asthenosphere flow is compatible with present-day geochemical and geophysical observations. The assumptions behind the physical flow model are described in the companion paper to this study. Despite its oversimplifications (especially the steady-state assumption), Atlantic, Indian, and Pacific mid-ocean ridge isotope geochemistry can be fit well at medium and long wavelengths by the predicted global asthenosphere flow pattern from distinct plume sources. The model suggests that the rapidly northward-moving southern margin of Australia, not the Australia-Antarctic discordance, is the convergence zone for much plume material in the southern hemisphere. It also suggests a possible link between the strike of asthenosphere flow with respect to a ridge axis and along-axis isotopic peaks.

In this article we examine whether it is viable to form an age-progressive ridge-crossing seamount chain using a nonplume mechanism. Nonthermal melt sources considered include fertile mantle blobs and subsolidus mantle while lithospheric stresses generated at the ridge and at ridge-transform intersections (RTIs) are tapped to bring the mantle to the surface. Finite element models, analog models, and an analysis of the Tristan de Cunha chain all show that ridge-crossing seamount chains may be created using these mechanisms. Essentially, as a ridge migrates or reorganizes, excess magmatism may appear to switch sides of the ridge as areas of extensional stress at the RTI migrate with the ridge.

A conceptual shift is overdue in geodynamics. Popular models that present plate tectonics as being driven by bottom-heated whole-mantle convection, with or without plumes, are based on obsolete assumptions, are contradicted by much evidence, and fail to account for observed plate interactions. Subduction-hinge rollback is the key to viable mechanisms. The Pacific spreads rapidly yet shrinks by rollback, whereas the subduction-free Atlantic widens by slow mid-ocean spreading. These and other first-order features of global tectonics cannot be explained by conventional models. The behavior of arcs and the common presence of forearc basins on the uncrumpled thin leading edges of advancing arcs and continents are among features indicating that subduction provides the primary drive for both upper and lower plates. Subduction rights the density inversion that is produced when asthenosphere is cooled to oceanic lithosphere: plate tectonics is driven by top-down cooling but is enabled by heat. Slabs sink more steeply than they dip and, if old and dense, are plated down on the 660 km discontinuity. Broadside-sinking slabs push all sublithosphere oceanic upper mantle inward, forcing rapid spreading in shrinking oceans. Down-plated slabs are overpassed by advancing arcs and plates, and thus transferred to enlarging oceans and backarc basins. Plate motions make sense in terms of this subduction drive in a global framework in which the ridge-bounded Antarctic plate is fixed: most subduction hinges roll back in that frame, plates move toward subduction zones, and ridges migrate to tap fresh asthenosphere. This self-organizing kinematic system is driven from the top. Slabs probably do not subduct into, nor do plumes rise to the upper mantle from, the sluggish deep mantle.

The transition from Hadean-style convective overturn, driven by heating from below, and Phanerozoic-style plate tectonics, driven by the sinking of cool lithospheric slabs, was a major turning point in the thermal evolution of the Earth. Prior to this transition the formation of stable, long-lived crust was rare; after this transition, the formation and amalgamation of continental crust became a central theme of tectonic processes. Evidence for Phanerozoic-style plate tectonics includes (1) the formation of oceanic crust at mid-ocean ridge spreading centers, (2) the formation of island arc volcanic and plutonic complexes, and (3) the formation of accretionary mélange complexes during the subduction of oceanic crust. Rock assemblages characteristic of oceanic crust and island arc suites may also be formed during Hadean-style convection, making the recognition of accretionary mélange complexes the most reliable indicator of Phanerozoic-style tectonics. Accretionary mélange complexes result from the juxtaposition of oceanic crust and pelagic sediments in a matrix of arc-derived constituents and may be recognized even after high-grade metamorphic events. At least three potential Archean mélange complexes have been recognized to date: the Dongwanzi ophiolite mélange of the North China craton, the Schreiber-Hemlo mélange of the Superior province, and the Farmington Canyon complex of the Wyoming province; it has also been proposed that the Isua supracrustals of the Greenland craton represent an accretionary complex, but this proposal has been disputed. Each of these terranes contains lithotectonic elements that could not have formed in a single tectonic setting but are now intimately intermixed. Oceanic components include mafic and ultramafic volcanic rocks, gabbros, harzburgite tectonites, and pelagic sediments (chert); arc components are represented by the mélange matrix, consisting of felsic gneiss with a greywacke composition. These studies of Archean crust show that recognition of accretionary mélange complexes may be a reliable indicator of Phanerozoic-style plate tectonics and that the initiation of recognizable plate-tectonic processes occurred by 3.0 Ga, which coincides with the oldest Phanerozoic-style ophiolite assemblages. This transition may have started as early as 3.8 Ga, which coincides with the end of the terminal lunar bombardment.

The sedimentary facies and seafloor topography of the Mississippi-Alabama continental shelf are a product of late Pleis-tocene-Holocene regression and transgression. This region lies between the fine-grained facies associated with Holocene Mississippi River delta deposition and the carbonate facies on the western Florida shelf. Sediments in the shallow subsurface of the inner shelf were deposited by fluvial and coastal systems that developed on the shelf during Pleistocene sea-level fluctuations. Shoreface retreat associated with Holocene transgression allowed marine and coastal processes to modify these deposits, resulting in the present sediment distribution. Recent sedimentation on the inner shelf in this region has been minor and restricted primarily to a nearshore fine-grained facies.The Southeast Banks area (SEBA), located on the Mississippi-Alabama continental shelf approximately 23 km south of Morgan Peninsula in water depths of 17 to 27 m, was selected for study based on the irregular seafloor topography and unusual bottom sediments present at Southeast Banks fishing ground, a site well known to local fisherman. Three sedimentary facies were identified in the Southeast Banks area: (1) a sand facies, (2) a shell gravel and sand facies, and (3) a mud, shell gravel and sand facies. Hardbottoms, which consist of shell debris as well as rubble and large fragments of carbonate-cemented sandstone and coquina, occur in association with the sedimentary facies at Southeast Banks.The sea floor in the SEBA is characterized by a series of northwest-trending ridges and troughs. Lengths and widths of these ridges are highly variable. Additionally, variations in sediment textures are coincident with changes in seafloor topography. Ridge morphology, orientation, and sediment textures associated with the ridges indicate that these features developed as shoreface-connected ridges through marine reworking of older sediments and topographic features. In the present environment, ridge features and bottom sediments are disturbed primarily by periodic high-energy storm events.

Aseismic ridges on underthrusting oceanic plates often trend into cusps or irregular indentations in the trace of the subduction zone. For example, the Hawaii-Emperor Ridge trends into the Kuril-Aleutian cusp, and the Marianas arc is bounded by the Marcus-Necker Ridge on the north and the Caroline Ridge on the south. The association between ridges and cusps is too common to be due to chance; it is proposed that the extra buoyancy of the plate with its aseismic ridge gives the plate greater resistance to sinking. This would inhibit back-arc extension and thereby produce a notch in the subduction zone. Island arcs may, therefore, acquire their curvature by additional constraints than the Earth’s curvature. The geology of about 15 such cusp areas is examined for evidence to test the hypothesis that cusps were caused by subducted aseismic ridges. This hypothesis applies only to cases where extensional basins lie behind the arcs. There also appear to be cases where the trace of the subduction zone has been modified not by inhibited back-arc spreading but by splintering of the overthrusting and possibly the underthrusting plate as well. Extremely high, massive aseismic ridges might induce arc polarity reversals and thereby assume the role of protocontinental nuclei. Seismicity and volcanism are examined where aseismic ridges are being subducted; there are several examples of reduced seismicity that cannot be explained by insufficient sampling time. By modifying the geometry of the subduction zone, the downgoing ridges necessarily affect seismicity. In addition, the plate containing the ridge may be thinner and hotter and more likely to deform by creep. There is no systematic increase or decrease in the number of andesite volcanoes where the ridges are subducted. However, lines of volcanoes and sometimes other kinds of geologic and seismic provinces may stop or start at the arc-ridge intersections. This is attributed to segmenting of the lithosphere into distinct tongues, each tongue acting more or less independently. Aseismic ridges would act as lines of weakness along which the downthrust slab becomes detached.