Seafloor Spreading Centers Research Articles

Fore-arc metasomatism by hybrid slab fluids during subduction initiation: Sr–Mg–Ca isotopes of rodingite, western Yarlung Zangbo suture zone

Subduction zone metasomatism is critical for Earth's material exchanges, yet the details of slab dehydration, particularly deserpentinization beneath fore-arcs, remain poorly understood. Here, we present Sr–Mg–Ca isotopic data for Purang rodingites from the western Yarlung Zangbo suture zone (YZSZ), a remnant of the Neo-Tethys Ocean that was subducted during the collision between the Indian and Eurasian Plates. The rodingites, occurring as centimeter- to meter-sized veins and blocks in serpentinized harburgite, are dominated by amphibolite- to greenschist-facies minerals like tremolite, magnesiohornblende, and chlorite. Their cumulate textures and rare earth element patterns resemble troctolite or gabbronorite, presumably formed beneath a seafloor spreading center. The rodingites are enriched in large ion lithophile elements and depleted in high field strength elements, with higher Sr/Nb and Ba/La and lower Nb/U and Ce/Pb ratios than mid-ocean ridge basalts (MORB) and rodingites formed through seawater or serpentinizing fluid alteration of MORB-like protoliths. They also exhibit higher initial 87Sr/86Sr ratios (0.7067–0.7075) and elevated δ26Mg values (−0.22 ± 0.07 ‰ to −0.13 ± 0.02 ‰) compared to unaltered oceanic basalts, while their δ44/40Ca values (0.72 ± 0.02 ‰ to 0.87 ± 0.03 ‰) remain similar to MORB. These signatures point to a metasomatic process beyond seafloor alteration, suggesting modification of a MOR-derived protolith in the fore-arc mantle at slab depths of <40 km, driven by Sr- and Mg-rich fluids from clay-rich sediments and serpentinitized mantle. The measured Sr, Mg and Ca isotope compositions can be reproduced by mixing a MORB-like protolith with hybrid fluids derived from varying proportions of sedimentary clays and serpentinitized peridotite. Combined with previous studies on YZSZ metamorphic soles, we propose that this fore-arc metasomatism occurred during the early Cretaceous, concurrent with the incipient subduction of the Neo-Tethys oceanic rocks. These results highlight the significance of deserpentinization at shallow fore-arc mantle settings during subduction initiation, suggesting that slab dehydration is more complex than previously recognized.

Volatiles and Redox Along the East African Rift

AbstractThe upper mantle under the Afar Depression in the East African Rift displays some of the slowest seismic wave speeds observed globally. Despite the extreme nature of the geophysical anomaly, lavas that erupted along the East African Rift record modest thermal anomalies. We present measurements of major elements, H2O, S, and CO2, and Fe3+/ΣFe and S6+/ΣS in submarine glasses from the Gulf of Aden seafloor spreading center and olivine‐, plagioclase‐, and pyroxene‐hosted melt inclusions from Erta Ale volcano in the Afar Depression. We combine these measurements with literature data to place constraints on the temperature, H2O, and fO2 of the mantle sources of these lavas as well as the initial and final pressures of melting. The Afar mantle plume is C/FOZO/PHEM in isotopic composition, and we suggest that this mantle component is damp, with 852 ± 167 ppm H2O, not elevated in fO2 compared to the depleted MORB mantle, and has temperatures of ∼1401–1458°C. This is similar in fO2 and H2O to the estimates of C/FOZO/PHEM in other locations. Using the moderate H2O contents of the mantle together with the moderate thermal anomaly, we find that melting begins at around 93 km depth and ceases at around 63 km depth under the Afar Depression and at around 37 km depth under the Gulf of Aden, and that ∼1%–29% partial melts of the mantle can be generated under these conditions. We speculate that the presence of melt, and not elevated temperatures or high H2O contents, are the cause for the prominent geophysical anomaly observed in this region.

On-land manifestation of along-strike transitioning of the Red Sea from continental rifting to sea floor spreading

This study uses Landsat Thematic Mapper (TM), Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM), and aeromagnetic data to examine possible reactivation of the Ediacaran, NE-trending Ad Damm Fault Zone (AFZ) and the Fatima Fault Zone (FFZ) in the western Arabian Shield. Reactivation is by latest Paleogene to Neogene and Quaternary structures of the Red Sea. The AFZ is the tectonic boundary between the Tonian Jeddah terrane in the northwest and the Asir terrane to the southeast of the Arabian Shield. The AFZ and FFZ coincide with possible onshore continuation of a transform fault that separates the sea floor spreading segment of the Red Sea in the south from its transitional segment to the north. It has been suggested that the AFZ is a candidate for the formation of a future onshore transform fault that would result in an eastward jump of the Red Sea sea floor spreading center onto the Arabian Shield. The future spreading center will follow the latest Neogene to Quaternary volcanic rocks (Harrat) that are exposed on the eastern side of the Red Sea. The Landsat TM image and the SRTM DEM show that the AFZ dissects the Red Sea topographic escarpment, and both the AFZ and FFZ control paleo-drainage system (wadis). The aeromagnetic data show that there is a ∼100 km wide zone between the AFZ and FFZ dominated by NE-trending short wavelength magnetic anomalies, possibly related to regional tectonic fabric. Additionally, the aeromagnetic data show that both fault zones dissect NW-trending short wavelength magnetic anomalies, interpreted as latest Paleogene to earliest Neogene dike swarms associated with the opening of the Red Sea. This dissection also includes possible Red Sea related narrow marginal grabens and horsts that were formed onshore and offshore parallel to the Red Sea shoreline. Red Sea-related structure might have reactivated the AFZ and FFZ.

Strain localization instabilities and the genesis of multiple axes of seafloor spreading in the Carmen basin, southern Gulf of California

We present new insight into the tectonic evolution of the Carmen basin (CB) in the southern Gulf of California (GC) from high-resolution bathymetry and two-dimensional seismic reflection data. Our goal is to document the seafloor morphology and sub-surface acoustic characteristics to understand the structure and crustal lithology across the CB. We identify three sub-basins with distinct geometries and evolutionary histories, with basement structures displaying a strong affinity with highly reflective, sigmoidal-shaped layers and the emplacement of high-amplitude tabular material underlying sediments with varying stratigraphic thicknesses. From the extent of new oceanic crust accreted along the CB, we estimate the age of the basin using a seafloor spreading rate of 52 mm/year, which is the average velocity of the relative plate motion between the Baja California microplate and the North American plate, as documented by previous authors. The southern and central sub-basins of the CB are mainly abandoned, while the northern one is currently the locus of seafloor spreading. This is evidenced by the juxtaposition of oceanic crust younger than ~ 1.9 Ma against older oceanic crust correlating in age with the adjacent Guaymas and Farallon basins to the northeast and southwest, respectively. We propose that mantle upwelling beneath the CB is a northward continuation of the East Pacific Rise, resulting in a fast-evolving system with sharp variations in strain localization within the seafloor spreading centers of the CB.Graphical

Strain localization instability in seafloor spreading centers of the Carmen Basin, southern Gulf of California

This study provides new insights into the geological evolution of the Carmen Basin (CB) in the southern Gulf of California (GC). Using high-resolution bathymetry and seismic reflection profiles, we establish a plate-kinematic framework for this oblique-divergent rift system. By analyzing the crustal composition, we investigate the growth of the bounding transform faults and their role in accommodating transtensional shearing. We propose that the mantle upwelling beneath the CB is a northward extension of the East Pacific Rise. The CB consists of three sub-basins with distinct geometries, morphologies, and evolutionary histories. While the southern and central sub-basins are mostly abandoned, the northern sub-basin is currently experiencing seafloor spreading. This is supported by the presence of younger oceanic crust, approximately less than 1.5 Ma, juxtaposed with older oceanic crust that aligns in age with the nearby Guaymas and Farallon basins to the north and south, respectively. Our findings also indicate favorable geological conditions in the CB for the development of hydrothermal systems similar to those observed in the neighboring Guaymas and Pescadero basins in the southern GC.

An appraisal using magnetic data of the continent-to-ocean transition structure west of Iberia

SUMMARYAbout half of the rifted margins purportedly formed by extension with minor magmatism. The conceptual models of those magma-poor systems are greatly influenced by the continent-to-ocean transition structure of the archetypal magma-poor West Iberia Margin. In the past, interpretation of magnetic data of West Iberia has been used to constrain conceptual rifting models, including the structure of the transition from the exhumed mantle domain to the oceanic crust formed at a spreading centre. However, uncertainties on geophysical data were generally not considered leading to overdetailed interpretations. We use synthetic magnetic modelling to show that magnetic data acquired at sea level cannot resolve subhorizontal lithological layering in deep-water continental margins. We then present new magnetic modelling guided by a refined velocity model of the wide-angle seismic IAM-9 profile that shows that the magnetic J-anomaly correlates with oceanic crust that abuts exhumed mantle across a vertical boundary. This well-constrained observation supports that seafloor spreading initiated abruptly, terminating mantle exhumation. Conventional wisdom dictates that the sudden efficient melt extraction relates to a mechanical threshold during lithospheric thinning and concomitant asthenospheric upwelling under which melt can migrate toward the surface. However, our results support that mantle melting creating oceanic crust was probably not driven by gradual lithospheric thinning and asthenospheric upwelling, but by seafloor spreading centre propagation that cut across the lithosphere, creating the abrupt structure.

Numerical geochemical modelling of basalt-water interaction under subcritical conditions

A comprehensive hydrothermal-geochemical model was built in TOUGHREACT™ to match a detailed laboratory simulation of the interaction between tholeiitic basalt from the Elvarpahraun flow of the Svartsengi volcanic system in the Reykjanes Peninsula, Iceland and distilled de-oxygenated water. The experiment was done under subcritical conditions (350 °C and 490 bar), conditions associated with seafloor spreading centres and mid-ocean ridges.This modelling study was used to develop methodologies for building reactive transport models in TOUGHREACT™ with a particular focus on the parameters for reaction thermodynamics and reaction kinetics at high temperature and pressure conditions. The validation and update of geochemical assumptions are also integral to the methodology. Equilibrium constants for mineral and aqueous reactions were estimated using SUPCRTBL by Zimmer et al. (2016). Mineral dissolution rates were primarily adopted from Palandri and Kharaka (2004), while precipitation rates were estimated following the work of Horiuti (1957). Justifications for the modelling decisions made are presented in detail.Based on the model, precipitation rates are generally higher than expected rates from the estimated kinetic parameters and mineral surface area estimates, while there is agreement in terms of dissolution rates. Mineral solid solutions dissolved stoichiometrically, except bytownite, whose albite component was preferentially dissolved. The volume of basalt glass in the sample was higher than experimental estimates, based on the silica concentration trends. Sulfide inclusions in the glass also significantly affected the observed sulfate and sulfide trends while the glass dissolved. The potassium concentration trends in the effluents indicate the presence of K-feldspar. Lastly, based on the model results, the bulk flow rates were observed to affect the overall reaction rates.

Hyperextension and polyphase rifting: Impact on inversion tectonics and stratigraphic architecture of the North West Shelf, Australia

Restoration of interpreted 2D deep seismic lines across Australia's North West Shelf indicates that rifting style changed significantly during multiple phases of extension and continental breakup. Early phase rifting during the Neo-Proterozoic and Early Paleozoic, characterised by low-angle detachment faults, crustal-scale necking and hyperextension of the crust (Metamorphic Core Complex Mode), was localised over pre-existing Proterozoic orogenic belts formed adjacent to major cratonic blocks. Subsequent extensional events in the Late Paleozoic and Mesozoic reactivated these low-angle rift fault systems. However, a series of narrow rift basins (Narrow Rift Mode), bound by high-angle normal faults that cut into the upper mantle, also began forming due to changes in the rheological architecture of the lithosphere following previous extension, crustal thinning and post-rift cooling. Wide rift mode extension, wherein extensional deformation was distributed over broad areas, may have occurred prior to, or in conjunction with, the development of more localised narrow rifts (dual mode rifting) in areas such as the Northern Carnarvon and Browse basins. Extensional deformation progressively localised into the narrow rift basins, which in some cases, matured into seafloor spreading centres during breakup, typically outboard of the major low-angle detachment fault systems. The resulting general structural architecture evolution is consistent with several other hyperextended margins from around the world.The rift events were punctuated by periods of thermal sag, often in conjunction with pulses of compressional deformation associated with plate-scale tectonic events. Structural restoration of 2D regional seismic lines shows a marked difference in the way the different polyphase rift basin elements responded to shortening. Low-angle detachment faults were relatively well oriented for reactivation under compression, leading to the development of inversion anticlines and associated convergent unconformities towards the inboard basin margin. In contrast, high-angle normal fault systems associated with the pre-existing narrow rift basins were poorly oriented for reactivation during compression and tend to be characterised by minor wrench/flower structures, generally with relatively limited uplift and erosion on the basin margins. Regional in-plane compressional stresses also worked in combination with vertical loading due to thermal decay to downwarp the lithosphere, forming broad rapidly subsiding synclinal basins above axes of the pre-existing rift basins. The evolving structural architecture developed during the compressional events had significant impacts on the associated sequence stratigraphic architecture.

Thermal History of Lithosphere Formed Beneath Fast Spreading Ridges: Constraints From the Mantle Transition Zone of the East Pacific Rise at Hess Deep and Oman Drilling Project, Wadi Zeeb, Samail Ophiolite

AbstractWe investigate the cooling histories of peridotites and gabbros from localities that expose oceanic lithosphere formed beneath two fast seafloor spreading centers: Hess Deep as recovered from IODP Expedition 345 and ODP Leg 147, and the Oman Ophiolite as sampled by the Oman Drilling Project, ICDP Expedition 5057 (OmanDP). At these locations, relict crust‐mantle transition zones are directly sampled, enabling characterization of the thermal history of the crust‐mantle transition, and by inference, the depth extent of hydrothermal circulation beneath spreading centers. We measured major and trace element abundances in crustal gabbros and mantle peridotites from Hess Deep and OmanDP, and applied major and trace element‐based thermometers. Geospeedometric interpretation of the temperatures suggests similar cooling histories at both locations; cooling rates ranged from 0.02 to 2.6 °C/y from peak temperatures up to 1,350°C. The rates are consistent on either side of the paleo‐Moho (i.e., in the crust and mantle). Models for conductive cooling of the lower oceanic crust predict rates more than two orders of magnitude slower at the crust‐mantle transition zone, while thermal models that invoke deep and efficient hydrothermal circulation throughout the entire crustal section predict rates consistent with our observations. We infer that hydrothermal cooling extended to or near the petrologic Moho beneath the East Pacific Rise and the OmanDP paleo‐spreading center, consistent with the Sheeted Sills model for crustal accretion. Comparison with previously published rates recalculated using the methods we employed suggests the oceanic lower crust is cooled hydrothermally in some places and by conduction at others.

Diffuse spreading, a newly recognized mode of crustal accretion in the southern Mariana Trough backarc basin

Abstract South of the latitude of Guam, the Mariana Trough exhibits both trench-parallel and trench-normal extension. In this study, we examined the locus of trench-normal extension separating the Philippine Sea plate from the broadly deforming Mariana platelet. Along this boundary, we identified three distinct modes of extension and described their distinguishing characteristics using deep- and shallow-towed side-scan sonar and ship multibeam data along with regional geophysical, geochemical, and seismicity data. In the west, the Southwest Mariana Rift is an active tectonic rift exhibiting abundant strong earthquakes up to mb 6.7 and limited evidence of volcanism. In the east, the Malaguana-Gadao Ridge is a seafloor spreading center producing few and weak earthquakes less than mb 5. Between these zones, there is an ~20–40-km-wide and ~120-km-long area of high acoustic backscatter characterized by closely spaced volcano-tectonic ridges and small volcanic cones with distributed intermediate-strength seismicity up to mb 5.7. Fresh-looking volcanic rocks with high water contents and strong arc chemical affinities have been recovered from the high-backscatter zone. We interpret this morphologically and geophysically distinct zone as undergoing diffuse spreading, a distributed form of magmatic crustal accretion where new crust forms within a broad zone tens of kilometers across rather than along a narrow spreading axis. Diffuse spreading appears to be a rheological threshold effect enabled by slow opening rates and a high slab-fluid flux that facilitate the formation of a broad zone of weak hydrous lithosphere, within which new crust is accreted. Our findings describe a poorly understood process in plate tectonics, and observations of similar terrains in other backarc basins suggest that this process is not unique to the Mariana Trough.

Genesis of Manganese Deposits in the Ali Khanzai Block of the Zhob Ophiolite, Pakistan: Inferences from Geochemistry and Mineralogy

The Zhob ophiolite comprises the Naweoba, Omzha and Ali Khanzai blocks, which are surrounded by the sediments of the Alozai Group and Loralai Formation. The Ali Khanzai Block contains metamorphic, ultramafic, gabbroic, volcanic and volcaniclastic rocks with associated chert. The Zhob manganese deposits found in the Ali Khanzai Block, occur in banded, lenticular and massive forms within red to brown coloured metachert. Braunite and pyrolusite are the main constituent manganese-bearing minerals with minor hausmannite, hematite and barite while quartz is the major gangue mineral with some carbonate minerals. Geochemical evidence from the major oxides indicates that the manganese mineralization and associated metachert at Zhob were formed by hydrothermal activity with little contribution from contemporaneous volcanic materials and this is confirmed by high Fe/Mn and low Co/Zn ratios and trace element patterns. These deposits formed along with seafloor spreading centres and were later obducted as part of Ali Khanzai Block of Zhob ophiolite.

Ultra-small microorganisms in the polyextreme conditions of the Dallol volcano, Northern Afar, Ethiopia

The Dallol geothermal area in the northern part of the Danakil Depression (up to 124–155 meter below sea level) is deemed one of the most extreme environments on Earth. The area is notable for being part of the Afar Depression, an incipient seafloor-spreading center located at the triple junction, between Nubian, Somali and Arabian plates, and for hosting environments at the very edge of natural physical-chemical extremities. The northern part of the Danakil Depression is dominated by the Assale salt plain (an accumulation of marine evaporite deposits) and hosts the Dallol volcano. Here, the interaction between the evaporitic deposit and the volcanisms have created the unique Dallol hot springs, which are highly acidic (pH ~ 0) and saline (saturation) with maximum temperatures ranging between 90 and 109 °C. Here we report for the first time evidence of life existing with these hot springs using a combination of morphological and molecular analyses. Ultra-small structures are shown to be entombed within mineral deposits, which are identified as members of the Order Nanohaloarchaea. The results from this study suggest the microorganisms can survive, and potential live, within this extreme environment, which has implications for understanding the limits of habitability on Earth and on (early) Mars.

Extensive young silicic volcanism produces large deep submarine lava flows in the NE Lau Basin

New field observations reveal that extensive (up to ~ 402 km2) aphyric, glassy dacite lavas were erupted at multiple sites in the recent past in the NE Lau basin, located about 200 km southwest of Samoa. This discovery of volumetrically significant and widespread submarine dacite lava flows extends the domain for siliceous effusive volcanism into the deep seafloor. Although several lava flow fields were discovered on the flank of a large silicic seamount, Niuatahi, two of the largest lava fields and several smaller ones (“northern lava flow fields”) were found well north of the seamount. The most distal portion of the northernmost of these fields is 60 km north of the center of Niuatahi caldera. We estimate that lava flow lengths from probable eruptive vents to the distal ends of flows range from a few km to more than 10 km. Camera tows on the shallower, near-vent areas show complex lava morphology that includes anastomosing tube-like pillow flows and ropey surfaces, endogenous domes and/or ridges, some with “crease-like” extrusion ridges, and inflated lobes with extrusion structures. A 2 × 1.5 km, 30-m deep depression could be an eruption center for one of the lava flow fields. The Lau lava flow fields appear to have erupted at presumptive high effusion rates and possibly reduced viscosity induced by presumptive high magmatic water content and/or a high eruption temperature, consistent with both erupted composition (~ 66% SiO2) and glassy low crystallinity groundmass textures. The large areal extent (236 km2) and relatively small range of compositional variation (σ = 0.60 for wt% Si02%) within the northern lava flow fields imply the existence of large, eruptible batches of differentiated melt in the upper mantle or lower crust of the NE Lau basin. At this site, the volcanism could be controlled by deep crustal fractures caused by the long-term extension in this rear-arc region. Submarine dacite flows exhibiting similar morphology have been described in ancient sequences from the Archaean through the Miocene and in small batches on present-day seafloor spreading centers. This study shows that extensive siliceous lavas can erupt on the modern seafloor under the right conditions.

Diffuse Extension of the Southern Mariana Margin

AbstractExtension within the southern Mariana margin occurs both normal to and parallel to the trench. Trench‐normal extension takes place along focused and broad backarc spreading axes forming crust that is passively accreted to the rigid Philippine Sea plate flank to the northwest. To the southeast, trench‐parallel extension has split apart the Eocene‐Miocene forearc terrain accreting new crust diffusely over a 150–200 km wide zone forming a pervasive volcano‐tectonic fabric oriented at high angles to the trench and the backarc spreading center. Earthquake seismicity indicates active extension over this forearc region and basement samples date young although waning volcanic activity. Such diffuse formation of new oceanic crust and lithosphere is unusual; in most oceanic settings extension rapidly focuses to narrow plate boundary zones—a defining feature of plate tectonics. Diffuse crustal accretion has been inferred to occur during subduction zone infancy, however. We hypothesize that in a near‐trench extensional setting, the continual addition of water from the subducting slab creates a weak overriding hydrous lithosphere that deforms broadly. This process counteracts mantle dehydration and strengthening proposed to occur at mid‐ocean ridges that may help to focus deformation and melt delivery to narrow plate boundary zones. The observations from the southern Mariana margin suggest that where lithosphere is weakened by high water content narrow seafloor spreading centers cannot form. These conditions likely prevail during subduction zone infancy, explaining the diffuse contemporaneous volcanism inferred in this setting.

New constraints on the age and style of continental breakup in the South Atlantic from magnetic anomaly data

We present new constraints on the opening of the South Atlantic Ocean from a joint interpretation of marine magnetic anomaly grids and forward modelling of conjugate profiles. We use 45,000 km of recently collected commercial ship track data combined with 561,000 km of publically available data. The new data cover the critical ocean–continental transition zones and allow us to identify and downgrade some poorly navigated older ship tracks relied upon in earlier compilations. Within the final grids the mean cross-over error is 14 nT computed from 8,227 ship track intersections. The forward modelling used uniformly magnetised bodies whose shapes were constrained from coincident deep-seismic reflection data. We find the oldest magnetic anomalies to date from M10r (134.2 Ma, late Valanginian) north of the Falkland-Agulhas Fracture Zone and M3 (129.3 Ma, Barremian) south of the Rio Grande Fracture Zone. Hence, assuming the GPTS used is correct, continental breakup was contemporaneous with the Parana and Etendeka continental flood basalts. Many of the landward linear anomalies overlap seismically mapped Seaward Dipping Reflectors (SDRs). We interpret this to mean that a significant portion of the SDRs overlay crust formed by subaerial seafloor spreading. Here crustal accretion is envisaged to be similar to that at mid-ocean ridges, but sheet lava flows (that later form the SDRs) rather than pillow basalts form the extrusive component. Segmentation of the linear anomalies generated implies that this stage of continental breakup is organised and parallels the seafloor spreading centre that follows. Our results call into question the common assumption that at volcanic continental margins the first linear magnetic anomalies represent the start of conventional (submarine) oceanic crustal generation.

High-resolution seismic constraints on flow dynamics in the oceanic asthenosphere

Convective flow in the mantle and the motions of tectonic plates produce deformation of Earth's interior, and the rock fabric produced by this deformation can be discerned using the anisotropy of the seismic wave speed. This deformation is commonly inferred close to lithospheric boundaries beneath the ocean in the uppermost mantle, including near seafloor-spreading centres as new plates are formed via corner flow, and within a weak asthenosphere that lubricates large-scale plate-driven flow and accommodates smaller scale convection. Seismic models of oceanic upper mantle differ as to the relative importance of these deformation processes: seafloor spreading fabric is very strong just beneath the crust-mantle boundary (the Mohorovičić discontinuity, or Moho) at relatively local scales, but at the global and ocean-basin scales, oceanic lithosphere typically appears weakly anisotropic when compared to the asthenosphere. Here we use Rayleigh waves, recorded across an ocean-bottom seismograph array in the central Pacific Ocean (the NoMelt Experiment), to provide unique localized constraints on seismic anisotropy within the oceanic lithosphere-asthenosphere system in the middle of a plate. We find that azimuthal anisotropy is strongest within the high-seismic-velocity lid, with the fast direction coincident with seafloor spreading. A minimum in the magnitude of azimuthal anisotropy occurs within the middle of the seismic low-velocity zone, and then increases with depth below the weakest portion of the asthenosphere. At no depth does the fast direction correlate with the apparent plate motion. Our results suggest that the highest strain deformation in the shallow oceanic mantle occurs during corner flow at the ridge axis, and via pressure-driven or buoyancy-driven flow within the asthenosphere. Shear associated with motion of the plate over the underlying asthenosphere, if present, is weak compared to these other processes.

The role of pre-break-up heat flow on the thermal history of a transform margin

Abstract The paper focuses on the Romanche transform margin on the African side of the Equatorial Atlantic, and draws from thermomechanical numerical modelling and subsequent GIS-based thermal history grid processing. Our modelling indicates that the early post-break-up thermal history of the transform margin is controlled by the cooling patterns of the adjacent pull-apart terrains, the pre-rift heat-flow regime, laterally passing seafloor spreading centres, and cooling of the newly accreted oceanic crust in two corridors located in front of and behind the spreading centres. The pre-rift thermal regime controls the background heat flow on top of which the thermal transients develop. If it is cold, the transient anomalies are very distinct on this background. If it is warm, the transient anomalies tend to blend in with background heat flow much better. The most prominent anomalies are related to thinning in the pull-apart terrains located between transforms. They are up to three times wider than anomalies related to the laterally passing spreading centre, the anomaly of which becomes no wider than 20–25 km. Together with oceanic crust accreted in corridors in front of and behind the passing centre, its heat transfer into the transform margin tends to slow down the cooling of the syn-break-up anomalies developed in the pull-apart terrains by rifting.

Crust and upper mantle structure associated with extension in the Woodlark Rift, Papua New Guinea from Rayleigh‐wave tomography

AbstractThe Woodlark seafloor spreading center is propagating westward into the Australian plate near the D'Entrecasteaux Islands (DI), Papua New Guinea, generating an active transition zone from continental rifting to seafloor spreading. From March 2010 to July 2011, we deployed 31 on‐shore and 8 offshore broadband seismic stations around the DI region, to explore the dynamic processes of the lithosphere extension and the exhumation of the high‐pressure terranes exposed on those islands. We measure the multiband (10–60 s) Rayleigh‐wave phase velocities from both ambient noise and earthquake signals. These measurements are then inverted for a three‐dimensional shear‐velocity model for the crust and upper mantle. The results indicate that the lithosphere extension is localized near the rift axis beneath the DI, with a shear‐velocity structure in the upper mantle that is similar to mid‐ocean ridges. Beneath the Kiribisi Basin west of DI, an ultraslow shear‐velocity anomaly (∼4.0 km/s) is observed at shallow mantle depth (30–60 km), which can be interpreted either by the presence of excess partial melt due to slow melt extraction, or by the existence of felsic crustal material buried to mantle depth and not yet exhumed.

The contribution of the Precambrian continental lithosphere to global H2 production.

Microbial ecosystems can be sustained by hydrogen gas (H2)-producing water-rock interactions in the Earth's subsurface and at deep ocean vents. Current estimates of global H2 production from the marine lithosphere by water-rock reactions (hydration) are in the range of 10(11) moles per year. Recent explorations of saline fracture waters in the Precambrian continental subsurface have identified environments as rich in H2 as hydrothermal vents and seafloor-spreading centres and have suggested a link between dissolved H2 and the radiolytic dissociation of water. However, extrapolation of a regional H2 flux based on the deep gold mines of the Witwatersrand basin in South Africa yields a contribution of the Precambrian lithosphere to global H2 production that was thought to be negligible (0.009 × 10(11) moles per year). Here we present a global compilation of published and new H2 concentration data obtained from Precambrian rocks and find that the H2 production potential of the Precambrian continental lithosphere has been underestimated. We suggest that this can be explained by a lack of consideration of additional H2-producing reactions, such as serpentinization, and the absence of appropriate scaling of H2 measurements from these environments to account for the fact that Precambrian crust represents over 70 per cent of global continental crust surface area. If H2 production via both radiolysis and hydration reactions is taken into account, our estimate of H2 production rates from the Precambrian continental lithosphere of 0.36-2.27 × 10(11) moles per year is comparable to estimates from marine systems.

Magmatism and deformation during continental breakup

Since the 1960s, plate tectonic theory has provided a fundamental framework that explains how the configuration of the continents and ocean basins changes through time. Within this scheme, heat is lost from the Earth as continents split apart and new oceans form, a process clearly visible from the conjugate margins of continents ruptured in the past that now flank many of the globe’s mid-ocean ridges. Here I discuss observations and models of rifting processes in the Afar depression of Ethiopia, where the tectonic and volcanic processes responsible for splitting continents is still ongoing. Understanding the interaction between magma intrusion and mechanical extension is fundamental in trying to understand how continents break apart. The break-up of continents and subsequent formation of ocean basins is a fundamental component of plate tectonics that has shaped the geological record and distribution of natural resources (e.g. oil and gas) along continental margins such as the edges of the Atlantic Ocean. Continental rifts are initially characterized by relatively broad zones of mechanical extension in which faulting, ductile stretching and heating of the tectonic plate accommodates strain and defines the primary architecture of the rift zone during and after rifting (McKenzie 1978). Ultimately, however, the locus of strain must shift towards a narrowing zone that becomes the new seafloor spreading centre. It is here that magma formed from decompression melting of the mantle intrudes and creates new ocean floor (Delaney et al. 1998). Despite the importance of continental break-up in plate tectonic theory, it remains unclear how and when the transition from mechanical to magmatic extension of the plate occurs (Ebinger 2005). It also remains ambiguous how important the thermal, chemical and physical structure of the mantle are in aiding the melting of the passively upwelling mantle, and in so doing supplying magma that is intruded into the tectonic plate and/or erupted to the surface (Shillington et al. 2009).