Overlapping Spreading Centers Research Articles

Geochemical Signatures of Mafic Volcanic Rocks in Modern Oceanic Settings and Implications for Archean Mafic Magmatism

Abstract Greenstone belts are dominated by mafic volcanic rocks with geochemical characteristics that indicate a range of possible geodynamic influences. Many analogies with modern tectonic settings have been suggested. Increasing exploration of the modern oceans and comprehensive sampling of volcanic rocks from the sea floor are now providing unique opportunities to characterize different melt sources and petrogenesis that can be more closely compared to greenstone belts. In this study, we have compiled high-quality geochemical analyses of more than 2,850 unique samples of submarine mafic volcanic rocks (<60 wt % SiO2) from a wide range of settings, including mid-ocean ridges, ridge-hotspot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical variability spanning the full range of compositions of basalts found in greenstone belts. This diversity is interpreted to be due to variable crustal thickness, dry melting versus wet melting conditions, mantle mixing, and contamination. In particular, different melting conditions have been linked to mantle heterogeneity, complex mantle flow regimes, and short-lived tectonic domains, such as those associated with diffuse spreading, overlapping spreading centers, and triple junctions. These are well documented in the microplate mosaics of the Western Pacific. Systematic differences in mafic volcanic rock compositions in modern oceanic settings are revealed by a combination of principal components analysis and unsupervised hierarchical clustering of the compiled data. Mafic volcanic rocks from most arc-back arc systems have strongly depleted mantle signatures and well-known subduction-related chemistry such as large ion lithophile element (LILE) enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements (HREEs). This contrasts with mafic volcanic rocks in Archean greenstone belts, which show no, or at least weaker, subduction-related chemistry, a less depleted mantle, less wet melting, and variable crustal contamination. The differences are interpreted to be the result of the lower mantle temperatures, thinner crust, and subduction-related processes of present-day settings. However, mafic rocks that are geochemically identical to those in Archean greenstone belts occur in many modern back-arc basins, including the Lau basin, East Scotia ridge, Bransfield Strait, and Manus basin, which are characterized by fertile mantle sources, high heat flow, and complex spreading regimes typical of small-scale microplate mosaics. These types of settings are recognized as favorable for volcanogenic massive sulfide (VMS) deposits in modern and ancient greenstone belts, and therefore the particular geochemical signatures of the mafic volcanic rocks are potentially important for area selection in base metal exploration.

Generation of Evolving Plate Boundaries and Toroidal Flow From Visco‐Plastic Damage‐Rheology Mantle Convection and Continents

AbstractEarth's style of planetary heat transport is characterized by plate tectonics which requires rock strength to be reduced plastically in order to break an otherwise stagnant lithospheric lid, and for rocks to have a memory of past deformation to account for strain localization and the hysteresis implied by geological sutures. Here, we explore ∼107 Rayleigh number, visco‐plastic, 3‐D global mantle convection with damage. We show that oceanic lithosphere‐only models generate strong toroidal‐poloidal power ratios and features such as a mix of long‐wavelength tectonic motions and smaller‐scale, back‐arc tectonics driven by downwellings. Undulating divergent plate boundaries can evolve to form overlapping spreading centers and microplates, promoted and perhaps stabilized by the effects of damage with long memory. The inclusion of continental rafts enhances heat flux variability and toroidal flow, including net rotation of the lithosphere, to a level seen in plate reconstructions for the Cenozoic. Both the super‐continental cycle and local rheological descriptions affect heat transport and tectonic deformation across a range of scales, and we showcase both general tectonic dynamics and regionally applied continental breakup scenarios. Our work points toward avenues for renewed analysis of the typical, mean behavior as well as the evolution of fluctuations in geological and model plate boundary evolution scenarios.

Extension Dynamics of the Northern Fonualei Rift and Spreading Center and the Southern Mangatolu Triple Junction in the Lau Basin at 16°S

AbstractDue to the complexity of 2D magnetic anomaly maps north of 18°S and the sparsity of seismic data, the tectonic evolution of the northern Lau Basin has not yet been unraveled. We use a multi‐method approach to reconstruct the formation of the basin at ∼16°S by compiling seismic, magnetic, gravimetric and geochemical data along a 185 km‐long crustal transect. We identified a crustal zonation which preserves the level of subduction input at the time of the crust's formation. Paired with the seafloor magnetization, the crustal zonation enabled us to qualitatively approximate the dynamic spreading history of the region. Further assessment of the recent tectonic activity and the degree of tectonic overprinting visible in the crust both suggest a complex tectonic history including a dynamically moving spreading center and the reorganizing of the local magma supply. Comparing the compiled data sets has revealed substantial differences in the opening mechanisms of the two arms of the Overlapping Spreading Center (OSC) that is made up by the northernmost tip of the Fonualei Rift and Spreading Center in the east and the southernmost segment of the Mangatolu Triple Junction in the west. The observed transition from a predominantly tectonic opening mechanism at the eastern OSC arm to a magmatic opening mechanism at the western OSC arm coincides with an equally sharp transition from and strongly subduction influenced crust to a crust with virtually no subduction input. The degree of subduction input alters the geochemical composition, as well as the lithospheric stress response.

Geochemical Signatures of Felsic Volcanic Rocks in Modern Oceanic Settings and Implications for Archean Greenstone Belts

Abstract Felsic volcanic rocks are abundant in ancient greenstone belts and important host rocks for volcanogenic massive sulfide (VMS) deposits. About half of all VMS deposits are hosted by dacite or rhyolite, an association that reflects anomalous heat flow during rifting, partial melting of basaltic crust, and fractional crystallization in high-level magma chambers. For over 30 years, geochemical signatures of these rocks (e.g., F classification of Archean rhyolites) have been widely used to identify possible hosts for VMS deposits in ancient greenstone belts. However, comparisons with modern oceanic settings have been limited, owing to a lack of samples of felsic volcanic rocks from the sea floor. This is changing with increasing exploration of the oceans. In this study, we have compiled high-quality geochemical analyses of more than 2,200 unique samples of submarine felsic volcanic rocks (>60 wt % SiO2) from a wide range of settings, including mid-ocean ridges, ridge-hot-spot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical diversity spanning the full range of compositions of rhyolites found in ancient greenstone belts. This diversity is interpreted to reflect variations in crustal thickness, the presence or absence of slab-derived fluids (dry melting versus wet melting), and mantle anomalies. Highly variable melting conditions are thought to be related to short-lived microplate domains, such as those caused by diffuse spreading and multiple overlapping spreading centers. Systematic differences in the compositions of felsic volcanic rocks in the modern oceanic settings are revealed by a combination of principal components analysis, unsupervised hierarchical clustering, and supervised random forest classification of the compiled data. Dacites and rhyolites from midocean ridge settings have moderately depleted mantle signatures, whereas rocks from ridge-hot-spot intersections and ocean islands reflect enriched mantle sources. Felsic volcanic rocks from arc-back-arc systems have strongly depleted mantle signatures and well-known subduction-related chemistry (strong large ion lithophile element enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements [HREEs]). This contrasts with felsic volcanic rocks in Archean greenstone belts, which show high field strength element and HREE enrichment (so-called FIIIb-type) due to a less depleted mantle, a lack of wet melting, and variable crustal contamination. The differences between modern and ancient volcanic rocks are interpreted to reflect the lower mantle temperatures, thinner crust, and subduction-related processes in present-day settings. We suggest that the abundance of FIIIb-type felsic volcanic rocks in Archean greenstone belts is related to buoyant microplate domains with thickened oceanic crust that were better preserved on emerging Archean cratons, whereas in post-Archean tectonic settings most of these rocks are subducted.

Faulting and Magmatic Accretion Across the Overlapping Spreading Center Between Vance Segment and Axial South Rift, Juan de Fuca Ridge

AbstractPlate divergence along mid‐ocean ridges is accommodated through faulting and magmatic accretion, and, at overlapping spreading centers (OSC), is distributed across two curvilinear overlapping ridge axes. One‐meter resolution bathymetry acquired by autonomous underwater vehicles, combined with distribution and ages of lava flows, is used to: (a) analyze the spatial and temporal distribution of flows, faults, and fissures in the OSC between the distal south rift zone of Axial Seamount and the Vance Segment, (b) locate spreading axes, (c) calculate extension, and (d) determine the proportion of extension accommodated at the surface by faults and fissures versus volcanic extrusion over a period of ∼1300–1450 years. Our study reveals that in the recent history of the ridges, extension over a distance of 14 km across the Axial/Vance OSC was asymmetric in proportion and style: faults and fissures across 1–2 km of the Vance axial valley accommodated ∼3/4 of the spreading, whereas dike‐fed eruptions contributed ∼1/4 of the extension and occurred across 4 km of the south rift of Axial Seamount.

Separation of Gagua Rise from Great Benham Rise in the West Philippine Basin during the Middle Eocene

The West Philippine Basin (WPB) has started opening at ~ 58 Ma and ceased spreading at ~ 33 Ma, developing a fast spreading (~ 44 mm/yr half-spreading rate) magmatic episode between 58 and 41 Ma and the second amagmatic episode between 41 and 33 Ma. The occurrence of the first stage of spreading is closely related to the Oki-Daito mantle plume and related Benham Rise (BR) and Urdaneta Plateau (UP) activity. To the east of the Luzon–Okinawa Fracture Zone (LOFZ), BR was the most active volcanism from 48 to 41 Ma. The geomagnetic ages on both sides of the LOFZ have been determined; however, their causal relationship and evolution in the WPB remain unclear. In this study, we performed integrated analyses of multichannel seismic data and swath bathymetry data for the area to the west of the LOFZ. To the west of the LOFZ, the Gagua Rise (GR), is identified by a high residual free-air gravity anomaly, volcanic seamount chains and an overlapping spreading center. The GR is located at magnetic isochrons C20/C22 (50 to 44 Ma) and shows a thick oceanic crust of at least 12.7 km. We first propose an oceanic plateau named Great Benham Rise (GBR) which includes GR, UP and BR. We infer that the GR was a portion of the GBR since ~ 49 Ma and was separated from the GBR at ~ 41 Ma by the right-lateral LOFZ motion. Later, the relict GBR magmatism only continued in the area to the east of the LOFZ. Overall, the GBR dominates the spreading history of the WPB.

Subduction of the Izanagi-Pacific Ridge–transform intersection at the northeastern end of the Eurasian plate

Recent reconstructions of global plate motions suggest that the Izanagi-Pacific Ridge was subducted along the eastern margin of Eurasia at ca. 50 Ma. In the Hidaka magmatic zone (HMZ), which was located at the northeastern end of the Eurasian plate, three magmatic pulses occurred (46–45, 40–36, and 19–18 Ma). We report whole-rock geochemical and Sr-Nd-Pb isotopic data for 36 Ma high-Sr/Y (adakitic) rocks from the HMZ and show that these rocks formed by partial melting of oceanic crust and were emplaced as near-trench intrusions during ridge subduction. We reevaluate the nature of plutonic rocks in the HMZ and show that both the 46–45 and 40–36 Ma granitoids have essentially identical geochemical features. The distribution of plutons and magmatic cessation between 45 and 40 Ma are best explained by subduction of a ridge-transform intersection with a large offset of the ridge axis. The boundary between the Eocene granitoids corresponds to the position of a paleo–transform fault, and adakitic magmatism was caused by partial melting triggered by slab tearing at an overlapping spreading center. The paleoridge-transform configuration coincides with the locations of later large faults and a peridotite body.

Local rift and intraplate seismicity reveal shallow crustal fluid-related activity and sub-crustal faulting

Seismicity delineating the mid-ocean ridge system in the Panama Basin is mostly concentrated along transform offsets. Most intraplate areas show little-to-no seismic activity. Here we analyze passive recordings from a short-term deployment of 75 ocean-bottom seismographs (OBS) at the Costa Rica Rift (CRR) and across its southern flank along a flowline to Ocean Drilling Program (ODP) Hole 504B. The OBS array recorded 1061 events along the CRR and the Ecuador Fracture Zone, and 127 intraplate events around ODP Hole 504B. The seismic activity along the CRR occurred in clusters, with some events associated with an axial magma lens (AML) located at 3–3.5 km depth below seafloor (bsf), and four events exhibiting normal and reverse faulting focal mechanisms. These deep events are followed by the largest event cluster whose epicenters connect the AML to the seafloor, at a location where a hydrothermal plume was previously reported. During the same period, another event cluster occurs close to the seabed. This spatio-temporal pattern suggests that deeper events close to the AML, which might be related to thermal stresses or stress perturbations due to a volume change in the AML, trigger fluid-related seismicity within the shallower hydrothermal system. The easternmost extent of the seismicity along the CRR corresponds to an overlapping spreading center (OSC). At depth, the seismicity around the OSC is focused beneath one limb, but is spread over a larger area closer to the surface. The focal mechanisms calculated for OSC events show both normal and reverse motions, and might reflect complex stress or fault orientations. Intraplate seismicity around ODP Hole 504B shows both isolated and clustered events from the seafloor to ∼15–25 km bsf. Some of this seismicity is likely associated with the reactivation of east-west trending normal faults under high pore pressure and thermoelastic stresses conditions. Deep seismicity (i.e. between 15 and 20 km bsf) occurs in small, well-defined clusters close to ODP Hole 504B, at the theoretical depth of the brittle-to-plastic transition for a ∼6.9 Ma oceanic lithosphere. Our results show the importance of short-term OBS deployments for improving our understanding of mid-ocean ridge seismicity and hydrothermal processes, as well as intraplate seismicity and deformation mechanisms.

Rheological control on the segmentation of the mid-ocean ridges: Laboratory experiments with extension initially perpendicular to the axis

Mid-ocean ridges (MOR) axes are not straight, but segmented over scales of 10s to 100s of kilometers by several types of offsets including transform faults (TF), overlapping spreading centers (OSC) and non-transform, non-overlapping offsets (NTNOO). Variations in axial morphology and segmentation have been attributed to changes in magma supply, axial thermal structure (which depends on mantle temperature and spreading rate), and axial mechanical properties. To isolate the effect of each of these processes is difficult with field data alone. We therefore present a series of analogue experiments using colloidal silica dispersions as an Earth analogue. Diffusion of salt from saline solutions placed in contact with these fluids, causes formation of a skin, whose rheology evolves from viscous to elastic and brittle with increasing salinity. Applying a fixed spreading rate to this pre-formed, brittle plate results in cracks, faults, and ridge segments. Lithospheric thickness is varied independently by changing the surface water layer salinity. Experimental results depend on the axial failure parameter ΠF, the ratio of a mechanical length scale (Zm) and the axial elastic thickness (Zaxis), which depends on mantle temperature and spreading velocity. Slow-spreading fault-dominated, and fast-spreading fluid intrusion-dominated, ridges on Earth and in the laboratory are separated by the same critical value ΠFc±0.024, suggesting that the axial failure mode governs ridge geometry. Here, we examine ridge axis segmentation. Measurements of >4000 experimental ridge segments and offsets yield an average segment length Lm that is quasi-constant at all spreading velocities. Scaled to the Earth, Lm∼55 km, in agreement with the natural data. Experiments with low ΠF show offset size varying as dl=csteLmZaxis regardless of offset type, a correlation well explained by fracture mechanics. Finally, as on Earth, experimental ridge segments are separated by transform and non-transform discontinuities, and their nature and occurrence vary with ΠF. NTNOOs develop when ΠF<ΠFc, while OSCs develop when ΠF>ΠFc. In contrast, TF may form at any ΠF, but the proportion of TFs relative to OSCs or NTNOOs decreases when ΠF/ΠFc>> 1 or << 1, in agreement with natural MOR.

Precision Seismic Monitoring and Analysis at Axial Seamount Using a Real‐Time Double‐Difference System

AbstractSeven three‐component ocean bottom seismometers (OBS) of the Ocean Observatories Initiative (OOI) Cabled Array on top of Axial Seamount are continuously streaming data in real time to the Incorporated Research Institutions for Seismology (IRIS). The OBS array records earthquakes from the submarine volcano which last erupted on 24 April 2015, about 4 months after the array came online. The OBS data have proven crucial in providing insight into the volcano structure and dynamics (Wilcock et al., 2016, https://doi.org/10.1126/science.aah5563). We implemented a real‐time double‐difference (RT‐DD) monitoring system that automatically computes high‐precision (tens of meters) locations of new earthquakes. The system's underlying double‐difference base catalog includes nearly 100,000 earthquakes and was computed using kurtosis phase onset picks, cross‐correlation phase delay times, and 3‐D P and S velocity models to predict the data. The relocations reveal the fine‐scale structures of long‐lived, narrow (&lt;200 m wide), outward dipping, convex faults on the east and west walls of the caldera that appear to form a figure 8‐shaped ring fault system. These faults accommodate stresses caused by the inflation of magma prior to and deflation during eruptions. The east fault is segmented and pulled apart in east‐west direction due to its interaction with the Juan de Fuca Ridge, which at this location forms an overlapping spreading center. The RT‐DD system enables the monitoring and rapid analysis of variations in fine‐scale seismic and fault properties and has the potential to improve prediction of timing and location of the next Axial eruption expected to occur in the 2022–2023 time frame.

Magmatic and tectonic segmentation of the intermediate-spreading Costa Rica Rift—a fine balance between magma supply rate, faulting and hydrothermal circulation

SUMMARY 3-D tomographic modelling of wide-angle seismic data, recorded at the intermediate-spreading Costa Rica Rift, has revealed a P-wave seismic velocity anomaly low located beneath a small overlapping spreading centre that forms a non-transform discontinuity at the ridge axis. This low velocity zone displays a maximum velocity anomaly relative to the ‘background’ ridge axis crustal structure of ∼0.5 km s−1, has lateral dimensions of ∼10 × 5 km, and extends to depths ≥2.5 km below the seabed, placing it within layer 2 of the oceanic crust. We interpret these observations as representing increased fracturing under enhanced tectonic stress associated with the opening of the overlapping spreading centre, that results in higher upper crustal bulk porosity and permeability. Evidence for ongoing magmatic accretion at the Costa Rica Rift ridge axis takes the form of an axial magma lens beneath the western ridge segment, and observations of hydrothermal plume activity and microearthquakes support the presence of an active fluid circulation system. We propose that fracture pathways associated with the low velocity zone may provide the system through which hydrothermal fluids circulate. These fluids cause rapid cooling of the adjacent ridge axis and any magma accumulations which may be present. The Costa Rica Rift exists at a tipping point between episodic phases of magmatic and tectonically enhanced spreading. The characteristics inherited from each spreading mode have been preserved in the crustal morphology off-axis for the past 7 Myr. Using potential field data, we contextualize our seismic observations of the axial ridge structure at the whole segment scale, and find that the proposed balance between magmatic and tectonically dominated spreading processes observed off-axis may also be apparent along-axis, and that the current larger-scale magma supply system at the Costa Rica Rift may be relatively weak. Based on all available geophysical observations, we suggest a model for the inter-relationships between magmatism, faulting and fluid circulation at the Costa Rica Rift across a range of scales, which may also be influenced by large lithosphere scale structural and/or thermal heterogeneity.

Geological and geophysical signatures of the East Pacific Rise 8°–10°N

The East Pacific Rise (EPR) 8°–10°N is an archetype for fast-spreading mid-ocean ridge (MOR), and has been one of the most comprehensively studied MOR sections for over four decades. It covers all orders of ridge discontinuities: first-order transform faults, second-order overlapping spreading centers, third-order small offset tectonic discontinuities, and fourth-order discontinuities of eruptive fissure zones. This section has been selected as a multi-disciplinary integrated study site because of its abundance of hydrothermal vents where scientists are investigating the linkages between mantle flow, crustal accretion, volcanic, hydrothermal, and biological processes that result in chemical and physical exchanges of energy and matter between the Earth's mantle and oceans. In particular, the segment near 9°50′N is one of the only two sites where repeated seafloor volcanic eruptions were onsite observed along the MOR. In this paper, we investigate the geological and geophysical signatures of the EPR 8°–10°N on behalf of the fast-spreading MOR, present the seismic signatures of the hydrothermal pathways through full waveform inversion, and put forward some thoughts on the remaining questions to be addressed. The skewness of mantle upwelling, on- and off-axis magmatism, are playing a critical role in shaping the crustal architecture. The spatial distribution and physical property of the crustal melt lens are mutually related to the hydrothermal system. The mechanism of hydrosphere-lithosphere interaction beneath the MOR is worth further studying. This compilation should be useful to investigators new to the MOR topic but we also target advanced researchers in presentation and synthesis of findings to date.

Geochemical Variability Along the Northern East Pacific Rise: Coincident Source Composition and Ridge Segmentation

AbstractNew trace element abundances and isotope compositions for more than 100 mid‐ocean ridge basalts from 5.5°N to 19°N on the East Pacific Rise show step function variations in isotopic composition along the ridge axis that coincide with ridge discontinuities. Transform faults, overlapping spreading centers, and devals (deviation from axial linearity) mark the separation of individual clusters of distinct isotopic composition and trace element ratios that indicate source variations. This correlated chemical clustering and morphological segmentation indicates that source composition and segmentation can be closely related even on a fine scale. Substantial chemical variations within a segment are related to source composition. This suggests that even within segments the magma transport is mainly vertical, and there is limited along‐ridge transport, and there is little evidence for magma chambers that are well mixed along strike. Trace element concentrations show good correlations with isotopic compositions on a segment scale but less so on a regional scale. The trace element and isotopic variability along the northern East Pacific Rise can be explained by three mantle components: a depleted peridotite endmember, an enriched peridotite endmember, and a recycled gabbro‐like component. The gabbroic component has an isotopic signature indicating an ancient origin. The high‐resolution sampling indicates that within a segment the chemical variability is largely binary but that the endmembers of the binary mixing change from segment to segment. The endmembers of the binary variation within a segment are a combination of three of the endmembers.

Seismic Imaging and Physical Properties of the Endeavour Segment: Evidence that Skew Between Mantle and Crustal Magmatic Systems Governs Spreading Center Processes

AbstractWe invert Pg, PmP, and Pn traveltimes from an active‐source, multiscale tomography experiment to constrain the three‐dimensional isotropic and anisotropic P wave velocity structure of the topmost oceanic mantle and crust and crustal thickness variations beneath the entire Endeavour segment of the Juan de Fuca Ridge. The isotropic velocity structure is characterized by a semicontinuous, narrow (5‐km‐wide) crustal low‐velocity volume that tracks the sinuous ridge axis. Across the Moho, the low‐velocity volume abruptly broadens to approximately 20 km in width and displays a north‐south linear trend that connects the two overlapping spreading centers bounding the segment. From the seismic results, we estimate the thermal structure and melt distribution beneath the Endeavour segment. The thermal structure indicates that the observed skew, or lateral offset, between the crustal and mantle magmatic systems is a consequence of differences in mechanisms of heat transfer at crustal and mantle depths, with the crust and mantle dominated by advection and conduction, respectively. Melt volume estimates exhibit significant along‐axis variations that coincide with the observed skew between the mantle and crustal magmatic systems, with sites of enhanced crustal melt volumes and vigorous hydrothermal activity corresponding to regions where the mantle and crustal magmatic systems are vertically aligned. These results contradict models of ridge segmentation that predict enhanced and reduced melt supply beneath the segment center and ends, respectively. Our results instead support a model in which segment‐scale skew between the crustal and mantle magmatic systems governs magmatic and hydrothermal processes at mid‐ocean ridges.

Propagation and coalescence of en-echelon cracks under a far-field tensile stress regime: An experimental study

Experiments were carried out to observe the variation in propagation and linkage of parallel en-echelon cracks with varying orientation of the crack array and different relative position of the cracks within the array in an extensional regime. Two-layered analogue model, with a basal layer of pitch overlain by a layer of kaolin paste was used in the experiments. En-echelon cracks were pre-cut within the kaolin layer maintaining specific geometrical parameters of the cracks (e.g., length, centre spacing, separation) in such a manner that there was a weak (though not negligible) local tip-induced stress favouring curvature of adjacent crack tips towards one another. The results obtained were matched with natural pattern of linkage of veins, rift basins and spreading ridges, as described in the relevant literature. The experimental results showed that the final pattern of linkage between the cracks was a result of initial deflection of crack tip from its plane due to combined effect of local and far-field stress. When the deflection of tip from the crack plane was between \(0{^{\circ }}\) to \(45{^{\circ }}\), a ‘tip to wall’ linkage took place between adjacent cracks isolating a rhombohedral area in the interaction zone. The resultant structure could be geometrically comparable to a micro-plate-like structure isolated due to linkage of ridge segments initially forming an overlapping spreading centre (OSC). When the deflection of tip from the crack plane was greater than \(45{^{\circ }}\), a ‘tip to tip’ linkage between adjacent cracks took place resulting in a structure similar to a transform fault between spreading ridges and or rift basins. When effect of the remote stress opposed the tip induced stress, no linkage took place between the adjacent cracks, and finally the tips propagated straight along a plane perpendicular to the remote extension direction.

Dispersal of Hydrothermal Vent Larvae at East Pacific Rise 9–10°N Segment

AbstractA three‐dimensional, primitive‐equation, ocean circulation model coupled with a Lagrangian particle‐tracking algorithm is used to investigate the dispersal and settlement of planktonic larvae released from discrete hydrothermal habitats on the East Pacific Rise segment at 9–10°N. Model outputs show that mean circulation is anticyclonic around the ridge segment, which consists of a northward flow along the western flank and a southward flow along the eastern flank. Those flank jets are dispersal expressways for the along‐ridge larval transport and strongly affect its overall direction and spatial‐temporal variations. It is evident from model results that the transform faults bounding the ridge segment and off axis topography (the Lamont Seamount Chain) act as topographic barriers to larval dispersal in the along‐ridge direction. Furthermore, the presence of an overlapping spreading center and an adjacent local topographic high impedes the southward along‐ridge larval transport. The model results suggest that larval recolonization within ridge‐crest habitats is enhanced by the anticyclonic circulation around the ridge segment, and the overall recolonization rate is higher for larvae having a short precompetency period and an altitude above the bottom sufficient to avoid influence by the near‐bottom currents Surprisingly, for larvae having a long precompetency period (&gt;10 days), the prolonged travel time allowed some of those larvae to return to their natal vent clusters, which results in an unexpected increase in connectivity among natal and neighboring sites. Overall, model‐based predictions of connectivity are highly sensitive to the larval precompetency period and vertical position in the water column.

Vanadium isotope compositions of mid-ocean ridge lavas and altered oceanic crust

Vanadium isotope compositions of igneous rocks have the potential to constrain variations of physico-chemical conditions such as oxidation states during magmatism. Here, we present V isotope data for 27 fresh lavas (ranging from basaltic to dacitic compositions) from mid-ocean ridges, 31 altered basalts and gabbros from IODP site 1256 near the East Pacific Rise (EPR), and 2 back arc basin basalts (BABB). Our analyses of fresh mid-ocean ridge basalt (MORB) provide new constraints on the V isotope composition of MORBs, i.e. δ51V=−0.84±0.02‰ (2SE, n=22). In addition, the mean δ51V of MORBs from individual segments is correlated with the mean ridge depth and Na8.0 of the segment, which might reflect the effect of melting extent on V isotope fractionation during mantle melting.The mafic profile of intact altered oceanic crust (AOC) from the IODP site 1256 has δ51V ranging from −1.01 to −0.77‰, similar to that of fresh MORBs, suggesting that V isotope fractionation is limited during alteration of oceanic crust. These results also indicate the V isotopic homogeneity of the bulk oceanic crust with average δ51V of −0.85±0.02‰ (2SE, n=53), which is unaffected by ocean water and hydrothermal fluid alteration. Our results provide a guideline for application of V isotopes into studies of low and high temperature geochemical processes.The evolved lavas (basaltic andesites, andesites, and dacites) from the East Pacific Rise (EPR) show apparent shifts towards heavy δ51V values with increasing degree of differentiation, which can be explained by the crystal–liquid fractionation during crystallization with an inferred isotope fractionation factor of Δ51Vmineral-melt=−0.15×106/T2. The enrichment of 51V with increasing differentiation degree for the 9°N Overlapping Spreading Center (OSC) lavas is consistent with direction of the isotope shift observed in lavas from Anatahan Island (Northern Mariana Arc) and Hekla Volcano (Iceland), but the magnitude (0.3‰) is much smaller than that (2‰) reported in Prytulak et al. (2017). Modeling of V isotope fractionation between mineral and melt shows that variations in redox condition are important for controlling V isotope fractionation, but insufficient to explain the dramatically different Δ51Vmineral-melt between 9°N OSC lavas and Anatahan/Hekla suites. More studies are necessary for better understanding of mechanisms of V isotope fractionation during magmatism.

Geomorphology of the central Red Sea Rift: Determining spreading processes

Continental rifting and ocean basin formation is occurring today in the Red Sea, providing a possible modern analogue for the creation of mid-ocean ridges. Yet many of the seafloor features observed along the axis of the Red Sea appear anomalous compared to ancient and modern examples of mid-ocean ridges in other parts of the world, making it unclear, until recently, whether the Red Sea is truly analogous. Recent work suggests that the main morphological differences between the Red Sea Rift (RSR) and other mid-ocean ridges are due to the presence and movement of giant, submarine salt flows, which blanket large portions of the rift valley and thereby the oceanic crust. Using ship-based, high-resolution multibeam bathymetry of the central RSR between 16.5°N and 23°N we focus here on the RSR volcanic terrains not covered by salt and sediments and compare their morphologies to those observed along slow and ultra-slow spreading ridges elsewhere. Regional variations in style and intensity of volcanism can be related to variations in volcanic activity and mantle heat flow. The Red Sea oceanic seafloor shows typical features of mature (ultra)slow-spreading mid-ocean ridges, such as 2nd order discontinuities (overlapping spreading centres) and magma focussing in the segment centres (forming spreading-perpendicular volcanic ridges of thick oceanic crust). The occurrence of melt-salt interaction at locations where salt glaciers blanket the neovolcanic zone, and the absence of large detachment faults are unique features of the central RSR. These features can be related to the young character of the Red Sea and may be applicable to all young oceanic rifts, associated with plumes and/or evaporites. Thus, the RSR falls in line with (ultra)slow-spreading mid-ocean ridges globally, which makes the Red Sea a unique but highly important type example for initiation of slow rifting and seafloor spreading and one of the most interesting targets for future ocean research.

Near‐axis crustal structure and thickness of the Endeavour Segment, Juan de Fuca Ridge

AbstractA model of crustal thickness and lower crustal velocities is obtained for crustal ages of 0.1–1.2 Ma on the Endeavour Segment of the Juan de Fuca Ridge by inverting travel times of crustal paths and non‐ridge‐crossing wide‐angle Moho reflections obtained from a three‐dimensional tomographic experiment. The crust is thicker by 0.5–1 km beneath a 200 m high plateau that extends across the segment center. This feature is consistent with the influence of the proposed Heckle melt anomaly on the spreading center. The history of ridge propagation on the Cobb overlapping spreading center may also have influenced the formation of the plateau. The sharp boundaries of the plateau and crustal thickness anomaly suggest that melt transport is predominantly upward in the crust. Lower crustal velocities are lower at the ends of the segment, likely due to increased hydrothermal alteration in regions influenced by overlapping spreading centers, and possibly increased magmatic differentiation.

Morphotectonic and petrological variations along the southern Central Indian Ridge

In order to ascertain the effect of geomorphic and tectonic domains on the formation, enrichment, and ascension of the ridge axis melt, structural and petrological data from a nearly 300-km-long axial stretch along the slow-to-intermediate-spreading (40–60 mm/year) southern Central Indian Ridge (SCIR) were studied. The stretch, approximately between 20°30′S and 23°07′S, was disturbed by two major tectonic features—Egeria transform fault in the north and the Gemino Fracture Zone in the south—besides eight other discontinuities of variable dimensions. This stretch was chosen to test the petrological variations and mechanisms of magma supply in four distinct geomorphic and tectonic regimes: a Ridge-Crest-Flank-Valley (RCFV), a Neo-Volcanic Zone (NVZ), a large transform discontinuity (LTD), and an overlapping spreading centre (OSC). The major and trace element geochemistry of 44 glass and 47 whole rocks, extent and depth of melting (Na8 and Fe8, respectively), and melt pristinity (Mg#) of the magma indicate that rocks along this stretch were probably sourced from a reasonably primitive melt generated at a relatively greater depth and later got accumulated in pockets at a shallower level before eruption. Petrochemical analysis, and isotopic composition and ratios suggest that in contrast to largely N-MORB type of rocks at RCFV and OSC areas, the rocks from LTD and NVZ locations show signatures of enrichment to transitional (T) and enriched (E) basalts. A model explaining possible processes of enrichment and ascending framework of the melt at different tectonic regimes along SCIR are discussed.