Accretion Of Oceanic Crust Research Articles

Three‐dimensional seismic structure of the Dragon Flag oceanic core complex at the ultraslow spreading Southwest Indian Ridge (49°39′E)

The Southwest Indian Ridge (SWIR) is an ultraslow spreading end‐member of mid‐ocean ridge system. We use air gun shooting data recorded by ocean bottom seismometers (OBS) and multibeam bathymetry to obtain a detailed three‐dimensional (3‐D) P wave tomographic model centered at 49°39′E near the active hydrothermal “Dragon Flag” vent. Results are presented in the form of a 3‐D seismic traveltime inversion over the center and both ends of a ridge segment. We show that the crustal thickness, defined as the depth to the 7 km/s isovelocity contour, decreases systematically from the center (∼7.0–8.0 km) toward the segment ends (∼3.0–4.0 km). This variation is dominantly controlled by thickness changes in the lower crustal layer. We interpret this variation as due to focusing of the magmatic activity at the segment center. The across‐axis velocity model documents a strong asymmetrical structure involving oceanic detachment faulting. A locally corrugated oceanic core complex (Dragon Flag OCC) on the southern ridge flank is characterized by high shallow crustal velocities and a strong vertical velocity gradient. We infer that this OCC may be predominantly made of gabbros. We suggest that detachment faulting is a prominent process of slow spreading oceanic crust accretion even in magmatically robust ridge sections. Hydrothermal activity at the Dragon Flag vents is located next to the detachment fault termination. We infer that the detachment fault system provides a pathway for hydrothermal convection.

Continental Growth and Recycling in Convergent Orogens with Large Turbidite Fans on Oceanic Crust

Convergent plate margins where large turbidite fans with slivers of oceanic basement are accreted to continents represent important sites of continental crustal growth and recycling. Crust accreted in these settings is dominated by an upper layer of recycled crustal and arc detritus (turbidites) underlain by a layer of tectonically imbricated upper oceanic crust and/or thinned continental crust. When oceanic crust is converted to lower continental crust it represents a juvenile addition to the continental growth budget. This two-tiered accreted crust is often the same thickness as average continental crustal and is isostatically balanced near sea level. The Paleozoic Lachlan Orogen of eastern Australia is the archetypical example of a tubidite-dominated accretionary orogeny. The Neoproterozoic-Cambrian Damaran Orogen of SW Africa is similar to the Lachlan Orogen except that it was incorporated into Gondwana via a continent-continent collision. The Mesozoic Rangitatan Orogen of New Zealand illustrates the transition of convergent margin from a Lachlan-type to more typical accretionary wedge type orogen. The spatial and temporal variations in deformation, metamorphism, and magmatism across these orogens illustrate how large volumes of turbidite and their relict oceanic basement eventually become stable continental crust. The timing of deformation and metamorphism recorded in these rocks reflects the crustal thickening phase, whereas post-tectonic magmatism constrains the timing of chemical maturation and cratonization. Cratonization of continental crust is fostered because turbidites represent fertile sources for felsic magmatism. Recognition of similar orogens in the Proterozoic and Archean is important for the evaluation of crustal growth models, particularly for those based on detrital zircon age patterns, because crustal growth by accretion of upper oceanic crust or mafic underplating does not readily result in the addition of voluminous zircon-bearing magmas at the time of accretion. This crust only produces significant zircon when and if it partially melts, which may occur long after accretion.

The African Plate: A history of oceanic crust accretion and subduction since the Jurassic

We present a model for the Jurassic to Present evolution of plate boundaries and oceanic crust of the African plate based on updated interpretation of magnetic, gravity and other geological and geophysical data sets. Location of continent ocean boundaries and age and geometry of old oceanic crust (Jurassic and Cretaceous) are updated in the light of new data and models of passive margin formation. A new set of oceanic palaeo-age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and can be used as input for quantifying the plate boundary forces that contributed to the African plate palaeo-stresses and may have influenced the evolution of intracontinental sedimentary basins. As a case study, we compute a simple model of palaeo-stress for the Late Cretaceous time in order to assess how ridge push, slab pull and horizontal mantle drag might have influenced the continental African plate. We note that the changes in length of various plate boundaries (especially trenches) do not correlate well with absolute plate motion, but variations in the mean oceanic crust age seem to be reflected in acceleration or deceleration of the mean absolute plate velocity.

Transdimensional inversion of ambient seismic noise for 3D shear velocity structure of the Tasmanian crust

Ambient seismic noise tomography has proven to be a valuable tool for imaging 3D crustal shear velocity using surface waves; however, conventional two-stage inversion schemes are severely limited in their ability to properly quantify solution uncertainty and account for inhomogeneous data coverage. In response to these challenges, we developed a two-stage hierarchical, transdimensional, Bayesian scheme for inverting surface wave dispersion information for a 3D shear velocity structure and apply it to ambient seismic noise data recorded in Tasmania, southeast Australia. The key advantages of our Bayesian approach are that the number and distribution of model parameters are implicitly controlled by the data and that the standard deviation of the data noise is treated as an unknown in the inversion. Furthermore, the use of Bayesian inference — which combines prior model information and observed data to quantify the a posteriori probability distribution — means that model uncertainty information can be correctly propagated from the dispersion curves to the phase velocity maps and finally onward to the 1D shear models that are combined to form a composite 3D image. We successfully applied the new method to ambient noise dispersion data (1–12-s period) from Tasmania. The results revealed an east-dipping anomalously low shear velocity zone that extends to at least a 15-km depth and can be related to the accretion of oceanic crust onto the eastern margin of Proterozoic Tasmania during the mid-Paleozoic.

Sedimentary recycling in arc magmas: geochemical and U–Pb–Hf–O constraints on the Mesoproterozoic Suldal Arc, SW Norway

The Hardangervidda-Rogaland Block within southwest Norway is host to ~1.52 to 1.48 Ga continental building and variable reworking during the ~1.1 to 0.9 Ga Sveconorwegian orogeny. Due to the lack of geochronological and geochemical data, the timing and tectonic setting of early Mesoproterozoic magmatism has long been ambiguous. This paper presents zircon U–Pb–Hf–O isotope data combined with whole-rock geochemistry to address the age and petrogenesis of basement units within the Suldal region, located in the centre of the Hardangervidda-Rogaland Block. The basement comprises variably deformed grey gneisses and granitoids that petrologically and geochemically resemble mature volcanic arc lithologies. U–Pb ages confirm that magmatism occurred from ~1,521 to 1,485 Ma, and conspicuously lack any xenocrystic inheritance of distinctly older crust. Hafnium isotope data range from eHf(initial) +1 to +11, suggesting a rather juvenile magmatic source, but with possible involvement of late Palaeoproterozoic crust. Oxygen isotope data range from mantle-like (δ18O ~5 ‰) to elevated (~10 ‰) suggesting involvement of low-temperature altered material (e.g., supracrustal rocks) in the magma source. The Hf–O isotope array is compatible with mixing between mantle-derived material with young low-temperature altered material (oceanic crust/sediments) and older low-temperature altered material (continent-derived sediments). This, combined with a lack of xenoliths and xenocrysts, exposed older crust, AFC trends and S-type geochemistry, all point to mixing within a deep-crustal magma-generation zone. A proposed model comprises accretion of altered oceanic crust and the overlying sediments to a pre-existing continental margin, underthrusting to the magma-generation zone and remobilisation during arc magmatism. The geodynamic setting for this arc magmatism is comparable with that seen in the Phanerozoic (e.g., the Sierra Nevada and Coast Range batholiths), with compositions in the Suldal Sector reaching those of average upper continental crust. As within these younger examples, factors that drive magmatism towards the composition of the average continental crust include the addition of sedimentary material to magma source regions, and delamination of cumulate material. Underthrusting of sedimentary materials and their subsequent involvement in arc magmatism is perhaps a more widespread mechanism involved in continental growth than is currently recognised. Finally, the Suldal Arc magmatism represents a significant juvenile crustal addition to SW Fennoscandia.

Birth of an ocean in the Red Sea: Initial pangs

We obtained areal variations of crustal thickness, magnetic intensity, and degree of melting of the sub‐axial upwelling mantle at Thetis and Nereus Deeps, the two northernmost axial segments of initial oceanic crustal accretion in the Red Sea, where Arabia is separating from Africa. The initial emplacement of oceanic crust occurred at South Thetis and Central Nereus roughly ∼2.2 and ∼2 Ma, respectively, and is taking place today in the northern Thetis and southern Nereus tips. Basaltic glasses major and trace element composition suggests a rift‐to‐drift transition marked by magmatic activity with typical MORB signature, with no contamination by continental lithosphere, but with slight differences in mantle source composition and/or potential temperature between Thetis and Nereus. Eruption rate, spreading rate, magnetic intensity, crustal thickness and degree of mantle melting were highest at both Thetis and Nereus in the very initial phases of oceanic crust accretion, immediately after continental breakup, probably due to fast mantle upwelling enhanced by an initially strong horizontal thermal gradient. This is consistent with a rift model where the lower continental lithosphere has been replaced by upwelling asthenosphere before continental rupturing, implying depth‐dependent extension due to decoupling between the upper and lower lithosphere with mantle‐lithosphere‐necking breakup before crustal‐necking breakup. Independent along‐axis centers of upwelling form at the rifting stage just before oceanic crust accretion, with buoyancy‐driven convection within a hot, low viscosity asthenosphere. Each initial axial cell taps a different asthenospheric source and serves as nucleus for axial propagation of oceanic accretion, resulting in linear segments of spreading.

Magnetic recording of the Cenozoic oceanic crustal accretion and evolution of the South China Sea basin

We review and discuss some of the recent scientific findings made on magnetic data in the South China Sea (SCS). Magnetic anomalies bear extremely rich information on Mesozoic and Cenozoic tectonic evolution. 3D analytical signal amplitudes computed from magnetic anomalies reveal very precisely relict distributions of Mesozoic sedimentary sequences on the two conjugate continental margins, and they are also found very effective in depicting later-stage magmatism and tectonic transitions and zonation within the SCS oceanic crust. Through integrated analyses of magnetic, gravity and reflection seismic data, we define the continent-ocean boundary (COB) around the South China Sea continental margin, and find that the COB coincides very well with a transition zone from mostly positive to negative free-air gravity anomalies. This accurate outlining of the COB is critical for better tracing magnetic anomalies induced by the oceanic crust. The geometrically complex COB and inner magnetic zonation require the introduction of an episodic opening model, as well as a transform fault (here coined as Zhongnan Fault) between the East and Southwest Sub-basins, while within the East and Southwest Sub-basins, magnetic anomalies are rather continuous laterally, indicating nonexistence of large transform faults within these sub-basins. We enhance magnetic anomalies caused by the shallow basaltic layer via a band-pass filter, and recognize that the likely oldest magnetic anomaly near the northern continental margin is C12 according to the magnetic time scale CK95. Near the southern continental margin, magnetic anomalies are less recognizable and the anomaly C12 appears to be missing. These differences show an asymmetrical opening style with respect to the relict spreading center, and the northern part appears to have slightly faster spreading rates than to the south. The magnetic anomalies C8 (M1 and M2, ∼26 Ma) represent important magnetic boundaries within the oceanic basin, and are possibly related to changes in spreading rates and magmatic intensities. The magnetic evidence for a previously proposed ridge jump after the anomaly C7 is not clear. The age of the Southwest Sub-basin has yet to be further examined, most favorably with deep-tow magnetic surveys and ocean drilling. Our magnetic spectral study shows that the shallowest Curie points are located around the eastern part of the Southwestern Sub-basin, whereas within the East Sub-basin Curie depths are smaller to the north of the relict spreading center than to the south. This pattern of Curie depths is consistent to regional heat flow measurements and later-stage volcanic seamount distributions, and we therefore reason that Curie-depth variations are closely associated with later-stage magmatism, rather than with crustal ages. Although magnetic anomalies located around the northern continent-ocean transition zone (COT) are relatively quiet, this area is not a typical magnetic quiet zone since conceptually it differs markedly from an oceanic magnetic quiet zone. The relatively quiet magnetic anomalies are seemingly associated with a shallowing in Curie isotherm and thinning in magnetic layer, but our comprehensive observations suggest that the well-preserved thick Mesozoic sedimentary rocks are major causes for the magnetically quiet zone. The high similarities between various low-pass filtered marine and air-borne magnetic anomalies and satellite magnetic anomalies clearly confirm that deeper magnetic sources (in the lower crust and the uppermost mantle) have contributions to long-wavelength surface magnetic anomalies in the area, as already inferred from magnetically inversed Curie depths. The offshore south China magnetic anomaly (SCMA) becomes more prominent on low-pass filtered marine and air-borne magnetic anomalies and satellite magnetic anomalies, indicating very deeply-buried magnetic sources beneath it.

Initial burst of oceanic crust accretion in the Red Sea due to edge-driven mantle convection

The 500 m.y. cycle whereby continents assemble in a single supercontinent and then fragment and disperse again involves the rupturing of a continent and the birth of a new ocean, with the formation of passive plate margins. This process is well displayed today in the Red Sea, where Arabia is separating from Africa. We carried out geophysical surveys and bottom rock sampling in the two Red Sea northernmost axial segments of initial oceanic crust accretion, Thetis and Nereus. Areal variations of crustal thickness, magnetic intensity, and degree of melting of the subaxial upwelling mantle reveal an initial burst of active oceanic crust generation and rapid seafloor spreading below each cell, occurring as soon as the lid of continental lithosphere breaks. This initial pulse may be caused by edge-driven subrift mantle convection, triggered by a strong horizontal thermal gradient between the cold continental lithosphere and the hot ascending asthenosphere. The thermal gradient weakens as the oceanic rift widens; therefore the initial active pulse fades into steady, more passive crustal accretion, with slower spreading and along axis rift propagation.

Sedimentogenesis in the Amundsen Basin from geophysical data and drilling results on the Lomonosov Ridge

Studies in the Amundsen Basin have revealed six seismostratigraphic complexes (SSCs) in this region. The horizons bounding these complexes were dated by identifying the linear magnetic anomalies. The recognized SSCs are correlated with the seismostratigraphic and lithostratigraphic units of the Lomonosov Ridge. Based on these correlations, the lithological composition of SSCs in the Amundsen Basin is suggested. The formation of SSC2 is supposed to be due to the diagenetic processes associated with the transition of opal-A to opal-CT. It is found that, generally, the rate of sedimentation in the Amundsen Basin has consistently decreased since the beginning of its formation. However, in the Chattian time, the global regression resulted in a sharp increase in the rate of sedimentation in the basin. Arguments in favor of the duration of the Middle Cenozoic sedimentary hiatus on the Lomonosov Ridge reduced to 16.3 Ma are presented. It is supposed that the decrease in the intensity of oceanic crustal accretion in the Eurasian Basin, which is identified by the slowdown in the rate of its opening in the interval from 46 to 20–23 Ma might have resulted in a gradual sea level falling in the Arctic Ocean isolated from the World Ocean. This fact probably accounts for the Lomonosov Ridge having remained in subaerial conditions over the period from 36.7 to 20.4 Ma.

High-frequency ambient noise tomography of southeast Australia: New constraints on Tasmania's tectonic past

[1] The island of Tasmania, at the southeast tip of Australia, is an ideal natural laboratory for ambient noise tomography, as the surrounding oceans provide an energetic and relatively even distribution of noise sources. We extract Rayleigh wave dispersion curves from the continuous records of 104 stations with ∼15 km separation. Unlike most passive experiments of this type, which observe very little coherent noise below a 5 s period, we clearly detect energy at periods as short as 1 s, thanks largely to the close proximity of oceanic microseisms on all sides. The main structural elements of the eastern and northern Tasmanian crust are revealed by inverting the dispersion curves (between 1 and 12 s period) for both group and phase velocity maps. Of particular significance is a pronounced band of low velocity, observed across all periods, that underlies the Tamar River Valley and continues south until dissipating in southeast Tasmania. Together with evidence from combined active source and teleseismic tomography and heat flow data, we interpret this region as a diffuse zone of strong deformation associated with the mid-Paleozoic accretion of oceanic crust along the eastern margin of Proterozoic Tasmania, which has important implications for the evolution of the Tasman Orogen of eastern Australia. In the northwest, a narrower low-velocity anomaly is seen in the vicinity of the Arthur Lineament, which may be attributed to local sediments and strong deformation and folding associated with the final phases of the Tyennan Orogeny.

Tectonic evolution of the Knipovich Ridge based on the anomalous magnetic field

Calculation of the downward continuation for the anomalous magnetic field at the Knipovich Ridge showed more complicate segmentation of the spreading oceanic basement than was earlier considered. The structural pattern of the field is evidence that the area consists of no less than four segments separated by transform fracture zones with the azimuth of oceanic crust accretion about 310° and the normal position relative to the rift segments with the azimuth of 40°. The modern location of the axis of the Knipovich Ridge straightens the complicate divergent boundary between the plates in the strike-slip conditions between the spreading centers of the Mohns and Gakkel ridges. The axis is a detachment zone intersecting the oceanic basement having formed from the Late Oligocene. A new magnetoactive layer composed of magmatic products has not yet been formed in this structure.

Magnetostratigraphy and paleoenvironments in shallow-water carbonates: the Oligocene-Miocene sediments of the northern margin of the Liguro-Provençal basin (West Marseille, southeastern France)

AbstractThe present study proposes to estimate the influence of climate, eustatism and local tectonics on the sedimentation of a basin margin at the syn-rift to post-rift transition. For that, paleomagnetic measurements were performed on a marine marly-calcareous sedimentary succession ranging from Upper Oligocene to Lower Miocene and located on the northern margin of the Liguro-Provençal basin. The magnetostratigraphic record is correlated to the reference geomagnetic polarity scale [ATNTS04, Lourens et al. 2004] with the help of biostratigraphy based on calcareous nannofossils and planctonic foraminifers [Oudet et al., 2010]. The resulting age model shows that the 100 m-thick sedimentary succession covers a time span of 5 m.y. from the Late Chattian to the Early Burdigalian. Despite several exposure surfaces and a change in the sedimentation rate, no significant hiatus of sedimentation is documented. In addition, we also estimate the paleoenvironmental evolution through the sedimentary succession. Comparing the dated paleoenvironmental reconstruction with global δ18O and sea level curves [Miller et al., 2005], we show that the Carry-le-Rouet succession is an excellent paleoclimatic archive. Indeed, coral reefs developed at the glacial-interglacial stage transition marking the end of the Oligocene. In addition, the most diversified coral reefs occurred during the warmest period of the Aquitanian. During rifting, bathymetric variations recorded in the studied succession are related to local synsedimentary tectonics whereas, during oceanic crust accretion, global sea level changes influence the sedimentation. This result allows to characterise and to accurately date the break-up unconformity at 20.35 Ma.

SHRIMP Pb/U zircon ages constrain gabbroic crustal accretion at Atlantis Bank on the ultraslow-spreading Southwest Indian Ridge

Absolute ages of plutonic rocks from mid-ocean ridges provide important constraints on the scale, timing and rates of oceanic crustal accretion, yet few such rocks have been absolutely dated. We present 206Pb/ 238U SHRIMP zircon ages from two ODP Drill Holes and a surface sample from Atlantis Bank on the Southwest Indian Ridge. We report ten new sample ages from 26–1430 m in ODP Hole 735B, and one from 57 m in ODP Hole 1105A. Including a previously published age, eleven samples from Hole 735B yield 206Pb/ 238U zircon crystallization ages that are the same, within error, overlap with the estimated magnetic age and are inferred to date the main period of crustal growth, the average age of analyses is 11.99 ± 0.12 Ma. Any differences in the ages of magmatic series and/or tectonic blocks within Hole 735B are unresolvable and eight well-constrained ages vary from 11.86 ± 0.20 Ma to 12.13 ± 0.21 Ma, a range of 0.27 ± 0.29 Ma, consistent with the duration of crustal accretion observed at the Mid-Atlantic Ridge. An age of 11.87 ± 0.23 Ma from Hole 1105A is within error of ages from Hole 735B and permits previous correlations made between zones of oxide-rich gabbros in each hole. Pb/U zircon ages > 0.5 Ma younger than the magnetic age are recorded in at least three samples from Atlantis Bank, one from Hole 735B and two collected along a fault scarp to the East. These young ages may date one or more off-axis events previously suggested from thermochronologic data and support the interpretation of a complex geological history following crustal accretion at Atlantis Bank. Together with results from the surface of Atlantis Bank, dating has shown that while the majority of Pb/U SHRIMP zircon ages record the short-lived (< 0.5 Ma) phase of crustal accretion on-axis, results from several samples precede and post-date this period by > 1 Ma suggesting a complex and prolonged magmatic/tectonic history for the crust at Atlantis Bank.

Geodynamic evolution of crust accretion at the axis of the Reykjanes Ridge, Atlantic Ocean

The results of analysis of the anomalous magnetic field of the Reykjanes Ridge and the adjacent basins are presented, including a new series of detailed reconstructions for magnetic anomalies 1–6 in combination with a summary of the previous geological and geophysical investigations. We furnish evidence for three stages of evolution of the Reykjanes Ridge, each characterized by a special regime of crustal accretion related to the effect of the Iceland hotspot. The time interval of each stage and the causes of the variation in the accretion regime are considered. During the first, Eocene stage (54–40 Ma) and the third, Miocene-Holocene stage (24 Ma-present time at the northern Reykjanes Ridge north of 59° N and 17–11 Ma-present time at the southern Reykjanes Ridge south of 59° N), the spreading axis of the Reykjanes Ridge resembled the present-day configuration, without segmentation, with oblique orientation relative to the direction of ocean floor opening (at the third stage), and directed toward the hotspot. These attributes are consistent with a model that assumes asthenospheric flow from the hotspot toward the ridge axis. Decompression beneath the spreading axis facilitates this flow. Thus, the crustal accretion during the first and the third stages was markedly affected by interaction of the spreading axis with the hotspot. During the second, late Eocene-Oligocene to early Miocene stage (40–24 Ma at the northern Reykjanes Ridge and 40 to 17–11 Ma at the southern Reykjanes Ridge), the ridge axis was broken by numerous transform fracture zones and nontransform offsets into segments 30–80 km long, which were oriented orthogonal to the direction of ocean floor opening, as is typical of many slow-spreading ridges. The plate-tectonic reconstructions of the oceanic floor accommodating magnetic anomalies of the second stage testify to recurrent rearrangements of the ridge axis geometry related to changing kinematics of the adjacent plates. The obvious contrast in the mode of crustal accretion during the second stage in comparison with the first and the third stages is interpreted as evidence for the decreasing effect of the Iceland hotspot on the Reykjanes Ridge, or the complete cessation of this effect. The detailed geochronology of magnetic anomalies 1–6 (from 20 Ma to present) has allowed us to depict with a high accuracy the isochrons of the oceanic bottom spaced at 1 Ma. The variable effect of the hotspot on the accretion of oceanic crust along the axes of the Reykjanes Ridge and the Kolbeinsey and Mid-Atlantic ridges adjoining the former in the north and the south was estimated from the changing obliquity of spreading. The spreading rate tends to increase with reinforcing of the effect of the Iceland hotspot on the Reykjanes Ridge.

Backarc rifting, constructional volcanism and nascent disorganised spreading in the southern Havre Trough backarc rifts (SW Pacific)

High resolution multibeam (EM300 and SEABEAM) data of the Southern Havre Trough (SHT), combined with observations and sample collections from the submersible Shinkai6500 and deep-tow camera, are used to develop a model for the evolution and magmatism of this backarc system. The Havre Trough and the associated Kermadec Arc are the product of westward subduction at the Pacific–Australian plate boundary. Detailed studies focus on newly discovered features including a seamount (Saito Seamount) and a deep graben (Ngatoroirangi Rift, > 4000 m water depth floored with a constructional axial volcanic ridge > 5 km in length and in excess of 200 m high), both of which are characterised by pillow and lobate flows estimated at < 20,000 years old based on sediment cover, high reflectivity and thin Mn crusts on recovered glassy olivine basalts and basaltic andesites. Elongate volcanic ridges at 35°15′S and 34°30′S, and backarc seamounts (35°30′S, 178°30′E) occur at the eastern margin of the SHT. Similar seafloor morphology is observed in the central and western portions of the basin, suggesting that recent volcanism may be broadly distributed across the backarc. Mass balance modelling indicates a maximum crustal thickness of ~ 11 km to < 6 km, similar to estimates of crustal thickness in the Lau Basin to the north. Given such high crustal attenuation and extensive backarc mafic magmatism within deep SHT rifts, we propose that the SHT is in an incipient phase of distributed and “disorganised” oceanic crustal accretion in multiple, ephemeral, and short but deep (> 4000 m) spreading systems. These discontinuous spreading systems are characterised by failed rifts, rift segmentation, and propagation. Successive episodes of magmatic intrusion into thinned faulted arc basement results in defocused asymmetrical accretion. Cross-arc volcanic chains, isolated volcanoes and underlying basement plateaus are interpreted to represent a “cap” of recent extrusives. However, they may also be composed entirely of newly accreted crust and the spatially extensive basement fabric of elongated volcanic ridges may be the surface expression of pervasive dike intrusion that has thoroughly penetrated and essentially replaced the original arc crust with newly accreted intrusives.

Accretion of oceanic crust under conditions of oblique spreading

The accretion of oceanic crust under conditions of oblique spreading is considered. It is shown that deviation of the normal to the strike of mid-ocean ridge from the extension direction results in the formation of echeloned basins and ranges in the rift valley, which are separated by normal and strike-slip faults oriented at an angle to the axis of the mid-ocean ridge. The orientation of spreading ranges is determined by initial breakup and divergence of plates, whereas the within-rift structural elements are local and shallow-seated; they are formed only in the tectonically mobile rift zone. As a rule, the mid-ocean ridges with oblique spreading are not displaced along transform fracture zones, and stresses are relaxed in accommodation zones without rupture of continuity of within-rift structural elements. The structural elements related to oblique spreading can be formed in both rift and megafault zones. At the initial breakup and divergence of continental or oceanic plates with increased crust thickness, the appearance of an extension component along with shear in megafault zones gives rise to the formation of embryonic accretionary structural elements. As opening and extension increase, oblique spreading zones are formed. Various destructive and accretionary structural elements (nearly parallel extension troughs; basin and range systems oriented obliquely relative to the strike of the fault zone and the extension axis; rhomb-shaped extension basins, etc.) can coexist in different segments of the fault zone and replace one another over time. The Andrew Bain Megafault Zone in the South Atlantic started to develop as a strike-slip fault zone that separated the African and Antarctic plates. Under extension in the oceanic domain, this zone was transformed into a system of strike-slip faults divided by accretionary structures. It is suggested that the De Geer Megafault Zone in the North Atlantic, which separated Greenland and Eurasia at the initial stage of extension that followed strike-slip offset, evolved in the same way.

Thrusting in oceanic crust during continental drift offshore Niger Delta, equatorial Africa

Two‐ and three‐dimensional (3‐D) seismic reflection data acquired over oceanic crust in the deepwater west Niger Delta reveal convincing evidence for compressional tectonics during oceanic crustal spreading. Using the 3‐D seismic data set we describe numerous inclined seismic reflections that dissect the entire oceanic crust from the top of the crust to the level of the Moho that are interpreted as thrusts. Thrust propagation results in the development of associated hanging‐wall anticlines and footwall synclines. These structures are orthogonal to and clearly postdate normal faults that formed during the accretion of oceanic crust during continental drift and strike at right angles to them. The Charcot Ridge is located 140 km south of these thrusts and is a significantly larger structure. It is a triangular‐shaped uplifted region of oceanic crust measuring 80 by 150 km and is located along the NE–SW‐oriented Charcot Fracture Zone. Two interpretations are possible for the role of the fracture zone in the development of the Charcot Ridge: (1) A thin‐skinned model whereby the oceanic crust west of the fracture zone has been thrust southeastward, with detachment occurring close to the level of the Moho. The ridge forms as a result of translation and folding above a crustal‐scale ramp‐flat thrust geometry. (2) A thick‐skinned model where there is no detachment close to the Moho, with the thrust fault being much steeper, penetrating the crust and probably the mantle lithosphere. In this interpretation the structure formed owing to the compressional reactivation of the fracture zone. Approximate dating of onlapping reflections on either side of the ridge constrains the timing of its formation as between 25 and 120 Ma ago. The Charcot Ridge represents one of the largest thrust structures to be identified in a passive margin setting. Many other compressional folds with the same orientation formed to the northeast in the Benue Trough, probably during the Santonian, as a result of a change in the spreading direction during South Atlantic rifting. We speculate that the same causal mechanism applies for the formation of the Charcot Ridge.

Zircon Dating of Oceanic Crustal Accretion

Most of Earth's present-day crust formed at mid-ocean ridges. High-precision uranium-lead dating of zircons in gabbros from the Vema Fracture Zone on the Mid-Atlantic Ridge reveals that the crust there grew in a highly regular pattern characterized by shallow melt delivery. Combined with results from previous dating studies, this finding suggests that two distinct modes of crustal accretion occur along slow-spreading ridges. Individual samples record a zircon date range of 90,000 to 235,000 years, which is interpreted to reflect the time scale of zircon crystallization in oceanic plutonic rocks.

How supercontinents and superoceans affect seafloor roughness

Seafloor roughness varies considerably across the world's ocean basins and is fundamental to controlling the circulation and mixing of heat in the ocean and dissipating eddy kinetic energy. Models derived from analyses of active mid-ocean ridges suggest that ocean floor roughness depends on seafloor spreading rates, with rougher basement forming below a half-spreading rate threshold of 30-35 mm yr(-1) (refs 4, 5), as well as on the local interaction of mid-ocean ridges with mantle plumes or cold-spots. Here we present a global analysis of marine gravity-derived roughness, sediment thickness, seafloor isochrons and palaeo-spreading rates of Cretaceous to Cenozoic ridge flanks. Our analysis reveals that, after eliminating effects related to spreading rate and sediment thickness, residual roughness anomalies of 5-20 mGal remain over large swaths of ocean floor. We found that the roughness as a function of palaeo-spreading directions and isochron orientations indicates that most of the observed excess roughness is not related to spreading obliquity, as this effect is restricted to relatively rare occurrences of very high obliquity angles (>45 degrees ). Cretaceous Atlantic ocean floor, formed over mantle previously overlain by the Pangaea supercontinent, displays anomalously low roughness away from mantle plumes and is independent of spreading rates. We attribute this observation to a sub-Pangaean supercontinental mantle temperature anomaly leading to slightly thicker than normal Late Jurassic and Cretaceous Atlantic crust, reduced brittle fracturing and smoother basement relief. In contrast, ocean crust formed above Pacific superswells, probably reflecting metasomatized lithosphere underlain by mantle at only slightly elevated temperatures, is not associated with basement roughness anomalies. These results highlight a fundamental difference in the nature of large-scale mantle upwellings below supercontinents and superoceans, and their impact on oceanic crustal accretion.

Rotational deformation in the Jurassic Mesohellenic ophiolites, Greece, and its tectonic significance

The Jurassic Pindos and Vourinos ophiolites in the Western Hellenides of Greece are part of the Mesohellenic mafic-ultramafic slab underlying the Eocene–Miocene sedimentary basin (Mesohellenic Trough). The tectonic incorporation of this oceanic slab into the western edge of the Pelagonian subcontinent occurred via trench – passive margin collision in the late Jurassic. Much of the tectonic architecture of these ophiolites, particularly Vourinos, was acquired during progressive inhomogeneous deformation associated with the initial displacement of the Jurassic oceanic crust from its original igneous environment of formation, and its subsequent tectonic emplacement eastward onto the Pelagonian margin. The heterogeneous deformation in the mantle and crustal sequences of the Pindos–Vourinos ophiolites occurred in ductile, ductile-brittle, and brittle fields synchronously, as the Jurassic oceanic lithosphere was translated eastward; it also resulted in differential horizontal rotations within the displaced oceanic slab. Areas retaining high-temperature (diapiric) mantle fabric appear to have been “passively” translated by lower temperature ductile shearing, commonly along mylonite zones. Individual dunite bodies in the harzburgite tectonites indicate movement distances of at least kilometer scale. Pervasively mylonitic domains within the upper mantle peridotites suggest elongation on the order of five to ten times in the direction of ophiolite emplacement. Seafloor-spreading related, high-temperature mantle structures within the Pindos and Vourinos ophiolites are sub-parallel. Imprinted ductile kinematic indicators and ductile shear zones pervasive to the mantle and lower crustal sections in both ophiolites are also sub-parallel, consistent with the direction of tectonic vergence, and appear traceable across the sedimentary overburden of the Mesohellenic Trough. These geometric relations imply that the Pindos–Vourinos ophiolites retain their relative orientations from the time of cessation of ductile deformation within the ophiolitic slab. More than 90° anticlockwise horizontal rotation of the Vourinos ophiolite seems to have taken place during progressive simple shear deformation associated with the eastward emplacement of the Jurassic oceanic lithosphere. This kind of rotational deformation postdating the igneous accretion of ancient oceanic crust may be common in many ophiolites and should be carefully distinguished from tectonic extension-related vertical rotational deformation developed during their seafloor spreading evolution.