Oceanic Fracture Zones Research Articles

Contribution of global potential field data to the tectonic reconstruction of the Rio Grande Rise in the South Atlantic

The application of advanced enhancement techniques for geophysical anomalies to global gravity (WGM2012) and magnetic (EMAG2) models sheds light on the complex tectonic evolution of the Rio Grande Rise (RGR) in the southern South Atlantic. Long wavelength Bouguer gravity lows indicate a thicker crust beneath of the ridge, whose nature can be related to a microcontinent or an excess of volcanism within the oceanic realm. Recently dredged continental rocks reinforce the hypothesis of a microcontinent or, at least, slivers of continental crust. However, the reserval magnetic pattern of the processed magnetic anomalies provide no evidence of aborted spreading center similar to the well-studied Jan Mayen microcontinent and the surrounding (inactive) Aegir and (active) Kolbeinsey ridges in the North Atlantic Ocean. The reversal magnetic anomalies show a series N-S trending parallel stripes roughly follow the current South American coastline and segmented by E-W oriented oceanic fracture zones (FZs). The magnetic stripes are bended eastwards at the RGR, showing a more complex magnetic pattern similar to that in the Iceland. The aborted Cruzeiro do Sul Rift (CSR) and the Jean Charcot Chain (JCC) are structures that cross the RGR and contribute to the understanding of the tectonic evolution of the South Atlantic Ocean. NW-SE oriented extensive gravity lows and bathymetric valleys, which mark the CSR, are segmented by E-W trending magnetic lineaments related to FZs. This structural configuration suggests that the extensional event, which formed the rift and the seamounts chain, was replaced by strike-slip movements along the FZs. In addition, we constructed a plate kinematic model for the evolution of the RGR based on bathymetric, free-air and Bouguer gravity and magnetic data. Our model comprises five main stages of the RGR formation and evolution between late Cretaceous and Paleocene (ca. 95 - 60 Ma), separated by published seafloor isochrones. The proposed model suggests that the RGR was built at the mid-Atlantic ridge by increased magmatism probably related to the Tristan da Cunha hotspot.

Trace-element geochemistry of transform-fault serpentinite in high-pressure subduction mélanges (eastern Cuba): implications for subduction initiation

ABSTRACTThe Sierra del Convento and La Corea mélanges (eastern Cuba) are vestiges of a Cretaceous subduction channel in the Caribbean realm. Both mélanges contain blocks of oceanic crust and serpentinite subducted to high pressure within a serpentinite matrix. The bulk composition of serpentinite indicates spinel-harzburgite and -herzolite protoliths. The samples preserve fertile protolith signatures that suggest low melting degrees. High concentration of immobile elements Zr, Th, Nb, and REE contents (from ~0.1 to ~2 CI-chondrite) point to early melt–rock interaction processes before serpentinization took place. Major- and trace-element compositions suggest an oceanic fracture-zone–transform-fault setting. A mild negative Eu anomaly in most samples indicates low-temperature fluid–rock interaction as a likely consequence of seawater infiltration during oceanic serpentinization. A second, more important, serpentinization stage is related to enrichment in U, Pb, Cs, Ba, and Sr due to the infiltration of slab-derived fluids. The mineral assemblages are mainly formed by antigorite, lizardite, and chlorite, with local minor talc, tremolite, anthophyllite, dolomite, brucite, and relict orthopyroxene. The local presence of anthophyllite and the replacements of lizardite by antigorite indicate a metamorphic evolution from the cooling of peridotite/serpentinite at the oceanic context to mild heating and compression in a subduction setting. We propose that serpentinites formed at an oceanic transform-fault setting that was the locus of subduction initiation of the Proto-Caribbean basin below the Caribbean plate during early Cretaceous times. Onset of subduction at the fracture zone allowed the preservation of abyssal transform-fault serpentinites at the upper plate, whereas limited downward drag during mature subduction placed the rocks in the subduction channel where they tectonically mixed with the upward-migrating accreted block of the subducted Proto-Caribbean oceanic crust. Hence, we suggest that relatively fertile serpentinites of high-pressure mélanges were witness to the onset of subduction at an oceanic transform-fault setting.

Active tectonics of Peru: Heterogeneous interseismic coupling along the Nazca megathrust, rigid motion of the Peruvian Sliver, and Subandean shortening accommodation

AbstractOver 100 GPS sites measured in 2008–2013 in Peru provide new insights into the present‐day crustal deformation of the 2200 km long Peruvian margin. This margin is squeezed between the eastward subduction of the oceanic Nazca Plate at the South America trench axis and the westward continental subduction of the South American Plate beneath the Eastern Cordillera and Subandean orogenic wedge. Continental active faults and GPS data reveal the rigid motion of a Peruvian Forearc Sliver that extends from the oceanic trench axis to the Western‐Eastern Cordilleras boundary and moves southeastward at 4–5 mm/yr relative to a stable South America reference frame. GPS data indicate that the Subandean shortening increases southward by 2 to 4 mm/yr. In a Peruvian Sliver reference frame, the residual GPS data indicate that the interseismic coupling along the Nazca megathrust is highly heterogeneous. Coupling in northern Peru is shallow and coincides with the site of previous moderate‐sized and shallow tsunami‐earthquakes. Deep coupling occurs in central and southern Peru, where repeated large and great megathrust earthquakes have occurred. The strong correlation between highly coupled areas and large ruptures suggests that seismic asperities are persistent features of the megathrust. Creeping segments appear at the extremities of great ruptures and where oceanic fracture zones and ridges enter the subduction zone, suggesting that these subducting structures play a major role in the seismic segmentation of the Peruvian margin. In central Peru, we estimate a recurrence time of 305 ± 40 years to reproduce the great 1746Mw~8.8 Lima‐Callao earthquake.

Macroscopic strength of oceanic lithosphere revealed by ubiquitous fracture-zone instabilities

The origin of plate tectonics is one of the most fundamental issues in earth and planetary sciences. Laboratory experiments indicate that the viscosity of silicate rocks is so strongly temperature-dependent that the entire surface of the Earth should be one immobile rigid plate. The rheology of oceanic lithosphere is, however, still poorly understood, and there exist few constraints on the temperature dependency of viscosity on the field scale. Here we report a new kind of observational constraint based on the geoid along oceanic fracture zones. We identify a large number of conspicuous small-scale geoid anomalies, which cannot be explained by the standard evolution model of oceanic lithosphere, and estimate their source density perturbations using a new Bayesian inversion method. Our results suggest that they are caused most likely by small-scale convection involving temperature perturbations of ∼300 K±100 K. Such thermal contrast requires the activation energy of mantle viscosity to be as low as 100±50 kJmol−1 in case of diffusion creep, and 225±112 kJmol−1 in case of dislocation creep, substantially reducing the thickness of the stiffest part of oceanic lithosphere. Oceanic lithosphere may thus be broken and bent much more easily than previously thought, facilitating the operation of plate tectonics.

Structural characterization of the Nigerian sector of Benin Basin using geopotential field attributes

The structural dispositions of the Nigerian sector of the Benin Basin have been investigated using attributes of geomagnetic and gravimetric fields. Aeromagnetic anomalies were reduced to the equator to improve the correspondence of the anomalies with the causative bodies. The residual, upward continued, tilt and horizontal derivatives, and pseudogravity attributes and forward models of both geomagnetic and gravimetric anomalies were computed to accentuate geological features including shallow and regional faults, fracture network, basement block pattern and depth to magnetic basement. Three generations of sinistral faults were identified. The oldest generation of faults (F1) is the NE-SW trend corresponding to the oceanic fracture zones trend. The F1 is truncated by the second generation of faults (F2) with E-W trends. The third generation of faults (F3) assumes NW-SE trend and is offset by F2. Shallow and deep regional faults and fractures envisaged to play major role in migration and entrapment of hydrocarbons and localization of mineral resources in this area were imaged. The coastline, Lagos and Lekki Lagoons surface geometry showed high degree of similarity with their underlying basement block pattern, thus implying that these surface features are structurally controlled. The basement morphology is not flat but of horst and graben architecture in which sediment thickness attained about 4 km within the graben. Hence, the graben has significant hydrocarbon potential. This study has shown the capabilities of geopotential field attributes in providing information about the structural architecture of frontier basin. Such knowledge will aid the understanding of the geology of the basin and its resources.

Water, oceanic fracture zones and the lubrication of subducting plate boundaries—insights from seismicity

We investigate the relationship between subduction processes and related seismicity for the Lesser Antilles Arc using the Gutenberg–Richter law. This power law describes the earthquake-magnitude distribution, with the gradient of the cumulative magnitude distribution being commonly known as the b-value. The Lesser Antilles Arc was chosen because of its along-strike variability in sediment subduction and the transition from subduction to strike-slip movement towards its northern and southern ends. The data are derived from the seismicity catalogues from the Seismic Research Centre of The University of the West Indies and the Observatoires Volcanologiques et Sismologiques of the Institut de Physique du Globe de Paris and consist of subcrustal events primarily from the slab interface. The b-value is found using a Kolmogorov–Smirnov test for a maximum-likelihood straight line-fitting routine. We investigate spatial variations in b-values using a grid-search with circular cells as well as an along-arc projection. Tests with different algorithms and the two independent earthquake cataloges provide confidence in the robustness of our results. We observe a strong spatial variability of the b-value that cannot be explained by the uncertainties. Rather than obtaining a simple north–south b-value distribution suggestive of the dominant control on earthquake triggering being water released from the sedimentary cover on the incoming American Plates, or a b-value distribution that correlates with on the obliquity of subduction, we obtain a series of discrete, high b-value ‘bull's-eyes’ along strike. These bull's-eyes, which indicate stress release through a higher fraction of small earthquakes, coincide with the locations of known incoming oceanic fracture zones on the American Plates. We interpret the results in terms of water being delivered to the Lesser Antilles subduction zone in the vicinity of fracture zones providing lubrication and thus changing the character of the related seismicity. Our results suggest serpentinization around mid-ocean ridge transform faults, which go on to become fracture zones on the incoming plate, plays a significant role in the delivery of water into the mantle at subduction zones.

Erratum: Corrigendum: Seismic precursors linked to highly compressible fluids at oceanic transform faults

Nature Geoscience 7, 757–761 (2014); published online 14 September 2014; corrected after print 24 December 2015. The version of this Letter originally published described hydrothermal fluids circulating within oceanic fracture zones near ridge-transform intersections as super-critical fluids. However, Daniel Broseta, Matthew Steele-MacInnis and Thomas Driesner pointed out that, even if this term is of common use in the scientific literature, this description is incorrect and potentially misleading for multi-component fluids such as seawater.

Source mechanism of the 1998 Mw 7.4 intraplate strike-slip earthquake in the West Philippine Basin revealed by Coulomb stress changes

On April 11, 2012, Mw 8.6 and Mw 8.2 earthquakes occurred off the west coast of northern Sumatra as a result of strike-slip type events within the Indo-Australia plate. It is suggested that these two earthquakes were linked to the occurrence of the 2004 great Sumatra–Andaman earthquake. Over the last few decades, several strike-slip type earthquakes have been observed within the West Philippine Basin (WPB). However, little investigation has been conducted to understand these strike-slip earthquakes and the possible geohazard potential that they may bring. In our study, to determine the rupture plane for these strike-slip events, we calculated the static Coulomb Failure Stress changes (ΔCFS) induced by the 1998 Mw 7.4 strike-slip earthquake and compared them with the distribution of aftershocks. Our results show that when the fault plane is trending in the NW–SE direction, the aftershocks that occurred in the region show a good agreement with the maximal positive ΔCFS. Consequently, instead of sliding along the pre-existing oceanic fracture zones, the 1998 strike-slip event that occurred in the WPB should have ruptured in the NW–SE direction. Moreover, we also observed a correlation between the activities of the normal faulting type earthquakes along the trench and the positive ΔCFS induced by the 1998 Mw 7.4 event, which suggests that the occurrence of intraplate strike-slip events in oceanic plates might affect the distribution of earthquakes along the nearby subduction systems.

Late Cretaceous to Cenozoic geodynamic evolution of the Atlantic margin offshore Essaouira (Morocco)

AbstractAfter Mesozoic rifting, the Atlantic margin of Morocco has recorded the consequences of the continental collision between Africa and Europe and the relative northward motion of the African plate over the Canary Island hotspot during Cenozoic times. Interpretation of recently acquired 2D seismic reflection data (MIRROR 2011 experiment) presents new insights into the Late Cretaceous to recent geodynamic evolution of this margin. Crustal uplift presumably started during the Late Cretaceous and triggered regional tilting in the deep‐water margin west of Essaouira and the formation of the Base Tertiary Unconformity (BTU). An associated hiatus in sedimentation is interpreted to have started earlier in the north (presumably in the Cenomanian at well location DSDP 416) and propagated to the south (presumably in the Coniacian at well location DSDP 415). The difference in the total duration of this hiatus is postulated to have controlled the extrusion of Late Triassic to Early Jurassic salt during the Late Cretaceous to Early Palaeocene non‐depositional period, resulting in regional differences in the preservation of salt structures: the Agadir Basin in the south of the study area is dominated by salt diapirs, whereas massive canopies characterised the Ras Tafelnay Plateau farther north and salt‐poor canopies and weld structures the northernmost offshore Essaouira and Safi Basins. Accompanied by volcanic intrusions, a presumably Early Palaeogene reactivation of previously existing basement faults is interpreted to have formed a series of deep‐water anticlines with associated gravity deformation of shallow‐seated sediments. The orientation of the fold axes is roughly perpendicular to the present day coast and the extensional fault direction; therefore, not a coast‐line parallel pattern of extensional faults, related to the rifting and break‐up of the margin, but rather a coast‐line perpendicular oceanic fracture zone might have caused the basement faults associated with the deep‐water folds. Both the volcanic intrusions and the formation of the deep‐water anticlines show a comparable age trend which gets progressively younger towards the south. A potential tempo‐spatial relationship of the BTU and the reactivation of basement faults can be explained by the relative northward motion of the African plate over the Canary Island hotspot. Regional uplift producing the BTU could have been the precursor of the approaching hotspot during the Late Cretaceous, followed during the Early Palaeogene by a locally more pronounced uplift above the hotspot centre.

Niger Delta gravity-driven deformation above the relict Chain and Charcot oceanic fracture zones, Gulf of Guinea: Insights from analogue models

The Niger Delta is a classic example of a passive margin delta that has gravitationally deformed above an overpressured shale decollement. The outboard Niger Delta clastic wedge, including the Akata Formation overpressured shale decollement, is differentially thickened across relict oceanic basement steps formed at the Chain and Charcot fracture zones. In this study, five analogue models were applied to investigate the effects of a differentially thickened overpressured shale decollement across relict stepped basement on Niger Delta gravity-driven deformation. Gravity-driven delta deformation was simulated by allowing a lobate, layered sandpack to deform by gravity above a ductile polymer. A first series of experiments had a featureless, horizontal basement whereas a second series had differentially thickened polymer above Niger Delta-like basement steps. Two syn-kinematic sedimentation patterns were also tested. Surface strains were analysed using digital image correlation and key models were reconstructed in 3D. All five model deltas spread radially outward and formed plan view arcuate delta top grabens and arcuate delta toe folds. The arcuate structures were segmented by dip-oriented radial grabens and delta toe oblique extensional tear faults, which were formed by along-strike extensional strains during spreading. Basement steps partitioned delta toe gravity spreading into dual, divergent directions. Similarities between the analogue model structures and the Niger Delta strongly suggest a history of outward radial gravity spreading at the Niger Delta. The Niger Delta western lobe has potentially spread downdip more rapidly due to a thicker or more highly overpressured underlying Akata Fm. shale detachment. Faster western lobe spreading may have produced the Niger Delta toe ‘dual lobe’ geometry, perturbed up dip Niger Delta top growth fault patterns, and implies that western lobe toe thrusts have been very active.

The Hyblean xenolith suite (Sicily): an unexpected legacy of the Ionian–Tethys realm

The extensive study of a great number of deep-seated xenoliths from Tortonian tuff-breccia pipes in the Hyblean area (Sicily) revealed the following fundamental evidence: (1) typical continental crust rocks are completely absent in the entire xenolith suite; (2) mantle ultramafics are more abundant than gabbroids; (3) sheared oxide–gabbros, closely resembling those from oceanic fracture zones, are relatively common; (4) secondary mineral assemblages, compatible with alteration processes in serpentinite-hosted hydrothermal systems, occur both in peridotites and gabbros. Among the products of this hydrothermal activity, organic compounds, having abiotic origin via Fischer–Tropsch synthesis, occur in some hydrothermally altered gabbro and ultramafic xenoliths, as well as in hydrothermal clays. Moreover, the U–Pb dating of hydrothermal zircon grains, hosted in a xenolith of metasomatized tectonic breccia, indicated an Early–Middle Triassic age of the fossil hydrothermal system. Another line of evidence for the oceanic nature of the Hyblean–Pelagian basement is the complete absence of continental crust lithologies (granites, felsic metaigneous, and metasedimentary rocks) in outcrops and in boreholes, and the oceanic affinity of the Tertiary volcanic rocks from the Hyblean Plateau and the Sicily Channel (Pantelleria and Linosa Islands), which lack of any geochemical signature for continental crust contamination. A reappraisal of existing geophysical data pointed out that serpentinites form the dominant lithologies in the lithospheric basement of the Hyblean–Pelagian area down to a mean depth of 19 km, which represents the regional Moho considered as the serpentinization front, marking the transition from serpentinites to unaltered peridotites. On these grounds, we confirm that Hyblean xenoliths contain mineralogical, compositional, and textural evidence for tectonic, magmatic, and hydrothermal processes indicating the existence of fossil oceanic core complexes, in the geotectonic framework of the Paleo–Mesozoic, ultra-slow spreading, Ionian–Tethys Ocean forming the present Ionian–Hyblean–Pelagian domain.

Serpentinization and Deserpentinization Reactions in the Upper Mantle beneath Fuerteventura Revealed by Peridotite Xenoliths with Fibrous Orthopyroxene and Mottled Olivine

Mantle xenoliths collected from Fuerteventura, one of the easternmost Canary Islands, exhibit a complex evolutionary history comprising events of depletion, serpentinization, dehydration and melt metasomatism. Each of these events left imprints on both the texture and chemistry of the xenoliths. Extensive partial melting is shown by complete lack of primary clinopyroxene, the ultra-refractory trace element composition of orthopyroxene porphyroclasts, and low heavy rare earth element contents as compared with abyssal peridotites sampled along mid-ocean ridges and oceanic fracture zones, in the xenoliths least affected by later metasomatism. In many xenoliths the original orthopyroxene porphyroclasts and some olivines are replaced by fibrous aggregates of orthopyroxene and/or large, deformed olivine porphyroclasts with mottled rims with stringy glass and fluid inclusions. Such features are very rare in ocean island xenoliths. Unusually high H2O and Cl concentrations, together with very high H2O/Ce and Cl/K ratios in interstitial glasses, suggest that the fibrous orthopyroxene formed by local serpentinization by hot seawater. The volume increase accompanying the serpentinization caused extensive fracturing of adjacent olivine porphyroclasts. The most likely scenario for local mantle invasion by hydrous fluids is along deep faults and fractures caused by tectonic movements along the continent‐ocean transition during the early phases of the opening of the Atlantic Ocean. The peridotites were later (probably during the Canary Islands magmatism) dehydrated, causing the serpentine minerals to be replaced by porous domains of fibrous orthopyroxene. Hydrous fluids released by the deserpentinization escaped into neighbouring and overlying rocks leaving trails of fluid inclusions along fractures and grain boundaries causing mottled rims and zones in olivine porphyroclasts. During the Canary Islands magmatism the upper mantle beneath Fuerteventura was also infiltrated by enriched silicate magmas that caused different degrees of Fe‐Ti-metasomatism. A higher degree of melt metasomatism in rocks with fibrous orthopyroxene and mottled olivine than in the massive harzburgites strongly suggests that the sublithospheric Canarian magmas reused serpentinized extensional faults during their rise to the surface. The strongest degree of melt metasomatism appears to have resulted in the formation of lherzolites, wehrlites, and dunites.

Erratum to: Petrology and tectonic significance of seamounts within transitional crust east of Orphan Knoll, offshore eastern Canada

The Early Cretaceous separation of Newfoundland from Iberia–Ireland is a classic example of a magma-poor continental margin with hyperextension and with widespread minor magmatism resulting in seamounts. This study defines the distribution of seamounts east of Orphan Knoll, and documents and interprets the geochemical character of the one recovered lava sample. Video imagery of lava outcrops, and the sample, were obtained by ROV from Orphan seamount, one of a linear series of small seamounts overlying transitional thinned continental crust on the seaward side of Orphan Knoll. New multibeam bathymetry and legacy seismic data show several seamounts that extend irregularly along the fault-bound NE margin of Orphan Knoll. Whole rock geochemistry shows the sample to be highly alkaline basanite or possibly tephrite. Diopside–hedenbergite, kaersutite and K-feldspar phenocrysts were analyzed by electron microprobe and scanning electron microscope, and alteration minerals including kaolinite were identified by X-ray diffraction. The highly alkaline character of the basanite is similar only to Early Cretaceous volcanic and sub-volcanic rocks erupted through thick continental crust of the Mesoproterozoic Grenville Orogeny. The location of the linear set of seamounts is related to margin-parallel faults on the seaward side of Orphan Knoll that provided a pathway for magma, although ENE-trending lineaments in individual seamounts or seamount groups suggest the influence of oceanic fracture zones. A lower gradient crest to Orphan seamount above 2,200 m suggests subaerial erosion, consistent with the presence of kaolinite as an alteration product and the absence of lava pillows at and above this depth.

Subduction of fracture zones controls mantle melting and geochemical signature above slabs.

For some volcanic arcs, the geochemistry of volcanic rocks erupting above subducted oceanic fracture zones is consistent with higher than normal fluid inputs to arc magma sources. Here we use enrichment of boron (B/Zr) in volcanic arc lavas as a proxy to evaluate relative along-strike inputs of slab-derived fluids in the Aleutian, Andean, Cascades and Trans-Mexican arcs. Significant B/Zr spikes coincide with subduction of prominent fracture zones in the relatively cool Aleutian and Andean subduction zones where fracture zone subduction locally enhances fluid introduction beneath volcanic arcs. Geodynamic models of subduction have not previously considered how fracture zones may influence the melt and fluid distribution above slabs. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations of subduction, we show that enhanced production of slab-derived fluids and mantle wedge melts concentrate in areas where fracture zones are subducted, resulting in significant along-arc variability in magma source compositions and processes.

Crustal thickening and extension induced by the Great Sumatra–Andaman earthquake of 26 December 2004: revealed by the seismic moment tensor element Mrr

In a complex geological environment, it is generally difficult to assimilate a complete understanding of the main stress regime without being distracted by various fault configurations. In this study, we examine the changes in radial seismic moment (Mrr) of earthquakes that occurred in the Sumatra area. This approach allows for simplification of the various focal mechanism solutions by compressive and extensional slip vectors. The results show that the epicenter of the 2004 mainshock is located near the area where the accumulated Mrr was highest during the inter-seismic period. Simultaneously, a pattern of negative accumulated Mrr observed at depths between 40 and 100 km suggests a down-dip extension mechanism caused by the slab pull effect, which could be strengthened by the locking procedure in shallow portions of the slab. More- over, the right-lateral strike-slip Sumatra Fault and the left- lateral oceanic fracture zones exhibited positive and neg- ative accumulated Mrr prior to and immediately after the 2004 mainshock. This transform of Mrr indicates stress release associated with the unlocking of the asperities during the earthquake. Therefore, the distinctly different Mrr patterns for the inter- and post-seismic period suggest that the occurrence of a substantial earthquake could change the intraplate stress state.

Introduction to the special issue on SFB 574 “Volatiles and fluids in subduction zones: climate feedback and trigger mechanisms for natural disasters”

combine field work on land with marine cruises probing the seafloor. During the course of the SFB 574, many different aspects of subduction zone processes were investigated. Geophysical investigations identified and quantified the input of water through hydration of the bend-faulted subducting plate as far as mantle depths. This process and water release from the subducting slab deeper in the subduction zone was investigated by numerical modeling. These studies highlighted the important role of hydration and dehydration of mantle rocks for the global water cycle. Subducted oceanic fracture zones are another conduit for water transported deep into the subduction zone. Compactional and thermal dehydration of the subducted sediment layer affects the strength of interplate coupling and the depth and lateral extension of the seismogenic zone where disastrous earthquakes are generated. The fluids, largely generated by clay mineral transformation, are expelled through the forearc by splay faults. At the seafloor, the sites of cold seeps, often associated with mud volcanoes, are populated by biota that control the carbon transfer to the ocean. The most prominent manifestations of cold seeps are authigenic carbonates that form from anaerobic oxidation of methane and which serve as archives of forearc processes. The largest cold seep emissions occur at faults generated by the subduction of volcanic seamounts. Petrological analyses of ancient, exhumed subduction zone metamorphic rocks revealed the important role of pervasive, typically channelized fluids in high-pressure metamorphic reactions of dewatering subducted igneous crust and mantle. Slab-derived fluids hydrate the mantle wedge along the slab–wedge interface and form a subduction channel in which mixing of different fluids and fluid–rock interaction causes metasomatic overprinting. The element redistributions associated with the liberation of fluids from Sonderforschungsbereiche (SFBs) are a successful funding model in use by the German Science Foundation (DFG) for over 30 years to strengthen basic research first locally at universities and later also supra-regionally by including academic institutions at different cities and states. Literally translated, SFB means “special research area” that comprises research that complements but does not duplicate research at participating institutions and departments. The English terminology used by the DFG is “Collaborative Research Centre,” which better describes the expected approach by emphasizing collaboration and interdisciplinary efforts in such a way that the overall result is better than the sum of individual results. The SFB 574 had united more than 70 scientists with expertise in structural geology, geophysics, sedimentology, geochemistry, empirical and experimental petrology, volcanology, and biology for 11 years (2001–2012). The overarching theme addressed the role of volatiles in subduction zone tectonic, hydrological, metamorphic and magmatic processes, and resulting hazards. The main areas of research were the subduction zones of Central America and southern Chile. Both extend across shorelines from deepsea trenches to arc-volcano summits and thus required to

Could a Sumatra-like megathrust earthquake occur in the south Ryukyu subduction zone?

A comparison of the geological and geophysical environments between the Himalaya-Sumatra and Taiwan-Ryukyu collision-subduction systems revealed close tectonic similarities. Both regions are characterized by strongly oblique convergent processes and dominated by similar tectonic stress regimes. In the two areas, the intersections of the oceanic fracture zones with the subduction systems are characterized by trench-parallel high free-air gravity anomaly features in the fore-arcs and the epicenters of large earthquakes were located at the boundary between the positive and negative gravity anomalies. These event distributions and high-gravity anomalies indicate a strong coupling degree of the intersection area, which was probably induced by a strong resistance of the fracture features during the subduction. Moreover, the seismicity distribution in the Ryukyu area was very similar to the pre-seismic activity pattern of the 2004 Sumatra event. That is, thrust-type earthquakes with a trench-normal P-axis occurred frequently along the oceanward side of the mainshock, whereas only a few thrust earthquakes occurred along the continentward side. Therefore, the aseismic area located west of 128°E in the western Ryukyu subduction zone could have resulted from the strong plate locking effect beneath the high gravity anomaly zone. By analogy with the tectonic environment of the Sumatra subduction zone, the occurrence of a potential Sumatra-like earthquake in the south Ryukyu arc is highly likely and the rupture will mainly propagate continentward to fulfill the region of low seismicity (approximately 125° E to 129° E; 23° N to 26.5° N), which may generate a hazardous tsunami.

Nature of crust in the central Red Sea

A transition between continental crust in the northern Red Sea and oceanic crust in the southern Red Sea coincides broadly with a southward increase in plate tectonic separation rate and with a decrease in upper mantle seismic velocity. We re-evaluate here the nature of crust in the intervening central Red Sea with the results of legacy seismic refraction experiments and recently released marine gravity anomalies derived from satellite altimeter measurements. In the refraction data, collected east of Thetis Deep, velocities of 6.6–6.9kms−1 of a deep refracting layer, which are similar to measured velocities of unaltered gabbro samples, extend outside the deep to 65km from the axis. The new version of the marine gravity field reveals trends crossing the central Red Sea. Whereas some of them connect with major lineaments in the surrounding African-Arabian shield, those around Thetis Deep die out towards the coastlines. They can be paired across the ridge and lie slightly oblique to plate motions, as is typical of oceanic fracture zones or non-transform discontinuities migrating away from hotspots. Taken together these observations support the view that an oceanic rather than extended continental crust underlies this part of the central Red Sea.The crestal mountains around the median valleys of slow-spreading ridges are typically 500–1000m lower at spreading discontinuities. Around Thetis Deep, the similar pattern in the gravity field to those of slow-spreading ridges suggests that the crestal mountains may variably block or impede flowage of evaporites towards the spreading centre, whereas the discontinuities may mark areas where flowage is unobstructed. Limited multibeam data collected in transits outside Thetis Deep show oblique fabrics as expected from these predicted movements.

Heat flow in the southern Chile forearc controlled by large-scale tectonic processes

Between 33°S and 47°S, the southern Chile forearc is affected by the subduction of the aseismic Juan Fernandez Ridge, several major oceanic fracture zones on the subducting Nazca Plate, the active Chile Ridge spreading centre, and the underthrusting Antarctic Plate. The heat flow through the forearc was estimated using the depth of the bottom simulating reflector obtained from a comprehensive database of reflection seismic profiles. On the upper and middle continental slope along the whole forearc, heat flow is about 30–60 mW m–2, a range of values common for the continental basement and overlying slope sediments. The actively deforming accretionary wedge on the lower slope, however, in places shows heat flow reaching about 90 mW m–2. This indicates that advecting pore fluids from deeper in the subduction zone may transport a substantial part of the heat there. The large size of the anomalies suggests that fluid advection and outflow at the seafloor is overall diffuse, rather than being restricted to individual fault structures or mud volcanoes and mud mounds. One large area with very high heat flow is associated with a major tectonic feature. Thus, above the subducting Chile Ridge at 46°S, values of up to 280 mW m–2 indicate that the overriding South American Plate is effectively heated by subjacent zero-age oceanic plate material.

The evolution of rifting on the volcanic margin of the Pelotas Basin and the contextualization of the Paraná–Etendeka LIP in the separation of Gondwana in the South Atlantic

The Pelotas Basin is the classical example of a volcanic passive margin displaying large wedges of seaward-dipping reflectors (SDR). The SDR fill entirely its rifts throughout the basin, characterizing the abundant syn-rift magmatism (133–113 Ma). The Paraná–Etendeka Large Igneous Province (LIP), adjacent to west, constituted the pre-rift magmatism (134–132 Ma). The interpretation of ultra-deep seismic lines showed a very different geology from the adjacent Santos, Campos and Espírito Santo Basins, which constitute examples of magma-poor passive margins. Besides displaying rifts totally filled by volcanic rocks, diverse continental crustal domains were defined in the Pelotas Basin, such as an outer domain, probably constituted by highly stretched and permeated continental igneous crust, and a highly reflective lower crust probably reflecting underplating.The analysis of rifting in this portion of the South Atlantic is based on seismic interpretation and on the distribution of regional linear magnetic anomalies. The lateral accretion of SDR to the east towards the future site of the breakup and the temporal relationship between their rift and sag geometries allows the reconstitution of the evolution of rifting in the basin. Breakup propagated from south to north in three stages (130–127.5; 127.5–125; 125–113 Ma) physically separated by oceanic fracture zones (FZ). The width of the stretched, thinned and heavily intruded continental crust also showed a three-stage increase in the same direction and at the same FZ. Consequently, the Continental-Oceanic Boundary (COB) shows three marked shifts, from west to east, from south to north, resulting into rift to margin segmentation. Rifting also propagated from west to east, in the direction of the final breakup, in each of the three segments defined. The importance of the Paraná–Etendeka LIP upon the overall history of rupturing and breakup of Western Gondwanaland seems to have been restricted in time and in space only to the Pelotas Basin.