Southern East Pacific Rise Research Articles

Near‐axis subsidence rates, hydrothermal circulation, and thermal structure of mid‐ocean ridge crests

We systematically investigated near‐axis subsidence on the ridge flanks of intermediate and fast spreading mid‐ocean ridges using bathymetric data from well‐surveyed portions of the Southeast Indian Ridge (spreading at 72–76 mm/yr), the northern East Pacific Rise (91–96 mm/yr), and the southern East Pacific Rise (144 mm/yr). In all three regions, the mean subsidence rate of young (<1–1.4 Ma) seafloor is less than 220 m/m.y.1/2. The distribution of individual estimates shows a distinct peak at 180–220 m/m.y.1/2 with few profiles having subsidence rates greater than 275 m/m.y.1/2. The observed subsidence rates are significantly lower than both the worldwide average (∼350 m/m.y.1/2) and subsidence rates observed for older lithosphere at the same ridge segments. Intense hydrothermal circulation at the ridge axis can result in low subsidence rates on the adjacent ridge flanks provided the vigorous flow is confined to the immediate vicinity (<∼5 km) of the axis. According to our model, the extremely vigorous hydrothermal circulation ceases off‐axis, and conductive heat flow becomes a primary mechanism of vertical heat transport on the ridge flanks. The very low geothermal gradient within the cooled portion of the uppermost lithosphere retards conductive cooling, and the cooled area needs to be heated from below before a geothermal gradient can be established which permits significant heat to be conducted out of the lithosphere. As a result, subsidence in very young (∼0.1–1 Ma) lithosphere is suppressed. A simple one‐dimensional thermal model with a Nusselt number parameterization was used to estimate the effect of hydrothermal circulation. An upper layer with a high Nusselt number and the half‐space that it overlies are initially at a temperature of 1300°C and the surface is maintained at 0°C. After 0.1 m.y. of cooling (about 3.5 to 7 km from the spreading axis), the Nusselt number of the top layer is set to 1 so that normal conduction is simulated in the cooled layer. We used an explicit finite difference method to solve for the temporal changes in temperature with depth. This model produces subsidence rates in the range that we observe for Nu in the range of 15–30. Isotherms resulting from the modeling imply rapid lithospheric thickening very near the axis, which is incompatible with most current models for the formation of the bathymetric axial high observed at fast spreading mid‐ocean ridges.

Faulting patterns near 19°30′S on the East Pacific Rise: Fault formation and growth at a superfast spreading center

Structural mapping from high‐resolution sonar and bathymetric data collected near 19°30′S on the southern East Pacific Rise (EPR) reveals multiple packets of small, closely spaced outward‐dipping faults that form antithetic to larger inward‐dipping faults. The mean spacing and height of inward‐dipping scarps are ∼1.6 and ∼1.9 times greater than those of outward‐dipping faults, and the median spacing and height of inward‐dipping scarps are ∼2.0 and ∼1.7 times greater, respectively. Accordingly, inward‐dipping faults are less abundant (35–45%) but accommodate more (50–55%) of the roughly ∼4.0% strain. Although previous studies have shown that EPR fault populations exhibit exponential size‐frequency distributions, one‐dimensional throw and spacing data from the 19°30′S region are not described well by this model. The first major (>40 m scarp height) fault is observed within 1–4 km of the ridge crest and invariably dips inward. Further off‐axis, a transition from greater inward‐dipping to greater outward‐dipping fault abundance constrains the minimum half width of the active plate boundary zone to be ∼5–15 km. Inspection of additional data sets from 16° and 18°30′S indicates a pattern of large inward‐dipping faults with associated hanging wall deformation similar to that observed in the 19°30′S region. This pattern is distinct from the horst and graben morphology observed along the fast spreading northern EPR. The pervasive hanging wall deformation observed on the flanks of the southern EPR suggests master fault geometries that are more sharply curved or kinked at depth, relative to their abyssal hill equivalents on the northern EPR. Boundary element modeling indicates the pattern and geometry of faulting observed along the southern EPR is consistent with fault nucleation above a shallow, sill‐like magma chamber with internal overpressure. Since the influence of the sill on fault growth is strongly dependent on the depth of the sill, the different patterns of faulting observed along the northern and southern EPR may reflect the small difference in axial magma chamber depth between these two regions, with a slightly shallower chamber beneath the southern EPR.

Hydrothermal plumes along segments of contrasting magmatic influence, 15°20′–18°30′N, East Pacific Rise: Influence of axial faulting

Vertical profiles of light backscattering and temperature recorded on 133 rock cores and dredge hauls between the Orozco and Rivera transform faults on the East Pacific Rise (EPR) (15°20′–18°30′N) provide an opportunity to compare the hydrothermal environment of three adjacent but distinctly different segments that span the maximum range of axial cross section at a relatively constant spreading rate. Contrary to predictions based on data from other Pacific ridges, hydrothermal plumes over the inflated 16°N segment were less extensive and weaker than along the narrower, rifted 17°N segment. Remarkably, the 17°N segment has a plume incidence equal to the mean of superfast spreading segments from the southern EPR. The data suggest that the local permeability environment in this region controls the expression of hydrothermal activity in the water column. The 16°N segment, which has little or no indication of faulting, may have its hydrothermal activity presently suppressed by widespread volcanic flows that act as an impermeable cap over much of the segment. Activity on the 17°N segment may be tectonically enhanced, with hydrothermal fluids circulating through deep faults to a cracking front. Within each segment, intense hydrothermal plumes characteristic of focused discharge seem associated with clearly rifted areas, while weaker water column signals characteristic of diffuse discharge are associated with unrifted portions of the ridge axis that appear dominated by magmatism. Previous studies at intermediate‐ to‐superfast spreading ridges have emphasized a positive correlation between local magmatic budget and hydrothermal activity. Our data suggest, however, that even at fast rates local tectonics can control the extent and nature of hydrothermal activity, as documented for several sites on the slow‐spreading Mid‐Atlantic Ridge. Despite the segment‐scale incongruity between hydrothermal activity and magmatic budget, the fraction of total ridge length between 15°20′ and 18°30′N overlain by plumes (0.39) follows the existing global correlation between plume incidence and spreading rate.

Crustal helium in deep Pacific waters

We have analyzed deep helium isotope data from World Ocean Circulation Experiment section P17 along 135°W in the Pacific Ocean. Combining these data with neon data, we have derived the isotopic ratio (Rt) of the nonatmospheric part of helium (terrigenic helium). The calculated Rt values are significantly lower than helium isotope ratios from mid‐ocean ridge (MOR) sources (RMOR ≈ 8Ra; Ra: = (3He/4He)air), which are generally presumed to dominate the isotope characteristics of helium of nonatmospheric origin. For the depths most influenced by the southern East Pacific Rise (EPR) helium plume the isotopic ratio of terrigenic helium is ∼7.4 Ra. However, both the northern EPR plume and the Juan de Fuca Ridge (JdFR) plume show significantly lower Rt values of ∼6.8 Ra. We find minimum Rt values of ∼5.6 Ra at about 20°N to 30°N from 2000 m depth to the ocean floor. We conclude that in addition to the EPR and JdFR helium sources (having MOR isotope characteristics) there is a flux of radiogenic 4He out of deep‐sea sediments and the oceanic crust which causes a significant deviation of Rt toward values lower than RMOR. Using our calculated terrigenic 4He concentrations and isotope ratios Rt and the helium isotope ratios of the two end‐members (MOR helium and crustal helium), we derive concentrations and ocean bottom fluxes of crustal 4He (4Hecrust). Maximum concentrations of 4Hecrust (up to ∼30 pmol kg−1) are found at about 20°N to 30°N between 2000 m and 4000 m along the P17 section. The mean crustal 4He flux is obtained as 1.0 (±0.4) 109 atoms m−2Earth s−1.

Geochemical heterogeneity within mid-ocean ridge lava flows: insights into eruption, emplacement and global variations in magma generation

Compositional heterogeneity in mid-ocean ridge (MOR) lava flows is a powerful yet presently under-utilized volcanological and petrological tracer. Here, it is demonstrated that variations in pre- and syn-eruptive magmatic conditions throughout the global ridge system can be constrained with intra-flow compositional heterogeneity among 10 discrete MOR flows. Geographical distribution of chemical heterogeneity within flows is also used along with mapped physical features to help decipher the range of conditions that apply to seafloor eruptions (i.e. inferred vent locations and whether there were single or multiple eruptive episodes). Although low-pressure equilibrium fractional crystallization can account for much of the observed intra-flow compositional heterogeneity, some cases require multiple parent magmas and/or more complex crystallization conditions. Globally, the extent of within-flow compositional heterogeneity is well correlated (positively) with estimated erupted volume for flows from the northern East Pacific Rise (EPR), and the Mid Atlantic, Juan de Fuca and Gorda Ridges; however, some lavas from the superfast spreading southern EPR fall below this trend. Compositional heterogeneity is also inversely correlated with spreading rate. The more homogeneous compositions of lavas from faster spreading ridges likely reflect the relative thermal stability and longevity of sub-ridge crustal magma bodies, and possibly higher eruption frequencies. By contrast, greater compositional heterogeneity in lavas at slower spreading rates probably results from low thermal stability of the crust (due to diminished magma supply and greater hydrothermal cooling). Finally, the within-flow compositional variations observed here imply that caution must be exercised when interpreting MOR basalt data on samples where individual flows have not been mapped because chemical variations between lava samples may not necessarily record the history of spatially and temporally distinct eruptions.

Expansion of the geographic distribution of a novel lineage of epsilon-Proteobacteria to a hydrothermal vent site on the Southern East Pacific Rise.

The diversity associated with a microbial mat sample collected from a deep-sea hydrothermal vent on the Southern East Pacific Rise was determined using a molecular phylogenetic approach based on the comparison of sequences from the small subunit ribosomal RNA gene (16S rDNA). The DNA was extracted from the sample and the 16S rDNA was amplified by PCR. Sixteen different phylotypes were identified by restriction fragment length polymorphism analysis; four phylotypes were later identified as putative chimeras. Analysis of the 16S rDNA sequences placed all the phylotypes within the Proteobacteria. The majority of the sequences (98%) were most closely related to a new clade of epsilon-Proteobacteria that were initially identified from an in situ growth chamber deployed on a deep-sea hydrothermal vent on the Mid-Atlantic Ridge in 1995. The similarity between phylotypes identified from Atlantic and Pacific deep-sea hydrothermal vent sites indicates that this new clade of Proteobacteria may be endemic to and widely distributed among deep-sea hydrothermal vents.

Expansion of the geographic distribution of a novel lineage of ϵ-Proteobacteria to a hydrothermal vent site on the Southern East Pacific Rise

The diversity associated with a microbial mat sample collected from a deep-sea hydrothermal vent on the Southern East Pacific Rise was determined using a molecular phylogenetic approach based on the comparison of sequences from the small subunit ribosomal RNA gene (16S rDNA). The DNA was extracted from the sample and the 16S rDNA was amplified by PCR. Sixteen different phylotypes were identified by restriction fragment length polymorphism analysis; four phylotypes were later identified as putative chimeras. Analysis of the 16S rDNA sequences placed all the phylotypes within the Proteobacteria. The majority of the sequences (98%) were most closely related to a new clade of ϵ-Proteobacteria that were initially identified from an in situ growth chamber deployed on a deep-sea hydrothermal vent on the Mid-Atlantic Ridge in 1995. The similarity between phylotypes identified from Atlantic and Pacific deep-sea hydrothermal vent sites indicates that this new clade of Proteobacteria may be endemic to and widely distributed among deep-sea hydrothermal vents.

A detailed comparison of repeated bathymetric surveys along a 300‐km‐long section of the southern East Pacific Rise

Little is known about the frequency‐magnitude relationship for volcano‐tectonic events over long sections of fast spreading ridges. A technique that can both detect and characterize the size, shape, and approximate time of a volcanic or tectonic event is to survey an area of seafloor on multiple occasions using multibeam sonar systems. We have performed a detailed comparison of repeated bathymetric surveys of a 300‐km‐long section of the southern East Pacific Rise with the objective of detecting and characterizing any changes in seafloor morphology associated with eruptions and perhaps tectonic deformation. Repeat bathymetry data were collected as part of three cruises that surveyed the axial high between 16°S and 18°40′S latitude in late 1995, in late 1996, and in mid‐1997. During the latter two surveys the way points of the 1995 survey were repeated with the same survey vessel, thus closely approximating the acquisition geometry of the first survey. We have developed new methods for removing systematic errors and suppressing random noise in the bathymetry data (which reduce errors between surveys to a root‐mean‐square value of <2 m) and for selecting regions of the seafloor that surpass threshold values in depth difference and lateral area from survey to survey. As the ability to resolve a depth anomaly is a function of both its thickness and lateral area, anomalies of small lateral area require a larger depth anomaly than larger area features to be statistically significant. For this 1.5‐year survey period, no significant seafloor changes in the bathymetric data were detected with areas ≤0.2 km2 and depth differences >8 m or with areas >0.2 km2 and depth differences >4 m. We estimate that any seafloor depth changes that occurred between the survey times must have dimensions less than these values or they would have been detected. For comparison, lava flows detected along the Juan de Fuca and Gorda ridges using similar techniques are ∼10–75 m in thickness and have estimated total areas of 0.52–2.8 km2.

Geomagnetic intensity variations over the past 780 kyr obtained from near-seafloor magnetic anomalies.

Knowledge of past variations in the intensity of the Earth's magnetic field provides an important constraint on models of the geodynamo. A record of absolute palaeointensity for the past 50 kyr has been compiled from archaeomagnetic and volcanic materials, and relative palaeointensities over the past 800 kyr have been obtained from sedimentary sequences. But a long-term record of geomagnetic intensity should also be carried by the thermoremanence of the oceanic crust Here we show that near-seafloor magnetic anomalies recorded over the southern East Pacific Rise are well correlated with independent estimates of geomagnetic intensity during the past 780 kyr. Moreover, the pattern of absolute palaeointensity of seafloor glass samples from the same area agrees with the well-documented dipole intensity pattern for the past 50 kyr. A comparison of palaeointensities derived from seafloor glass samples with global intensity variations thus allows us to estimate the ages of surficial lava flows in this region. The record of geomagnetic intensity preserved in the oceanic crust should provide a higher-time-resolution record of crustal accretion processes at mid-ocean ridges than has previously been obtainable.

Basaltic lava domes, lava lakes, and volcanic segmentation on the southern East Pacific Rise

Meter‐scale DSL‐120 sonar mapping and coregistered Argo II photographic observations reveal changes in eruptive style that closely follow the third‐order structural segmentation of the ridge axis on the southern East Pacific Rise, 17°11′–18°37′S. Near segment ends we observe abundant basaltic lava domes which average 20 m in height and 200 m in basal diameter and have pillow lava as the dominant lava morphology. The ubiquity of pillow lava suggests low effusion rate eruptions. The abundance of lava domes suggests that the fissure eruptions were of sufficient duration to focus and produce a line of volcanic edifices. Near segment centers we observe fewer but larger lava domes, voluminous drained and collapsed lava lakes, and smooth lobate and sheet lava flows with very little pillow lava. The abundance of sheet flows suggests that high effusion rate eruptions are common. Fewer lava domes and large lava lakes suggest that fissure eruptions do not focus to point sources. This pattern was observed on eight third‐order ridge segments suggesting that a fundamental volcanic segmentation of the ridge occurs on this scale. The third‐order segment boundaries also correlate with local maxima in the seismic axial magma chamber reflector depth throughout the study area and decreased across‐axis width of the region of seismic layer 2A thickening along the one segment where sufficient cross‐axis seismic lines exist. The geochemically defined magmatic segment boundaries in the study area match the locations of our volcanic segment boundaries, although rock sampling density is not adequate to constrain the variation across all the third‐order volcanic segments that we identify. These observations suggest that variation in the processes of crustal accretion along axis occurs at a length scale of tens of kilometers on superfast spreading (>140 km/Myr full rate) mid‐ocean ridges.

Oxyanions in metalliferous sediments: tracers for paleoseawater metal concentrations?

The concentration of carbon, Fe, Mn, S, P, V, Cr, As, Mo, Cd, U, and Re were determined in sediment core Y-71-7-53P near an active hydrothermal location on the southern East Pacific Rise crest (10°S). The goal was to determine the usefulness of hydrothermal iron oxides as carriers of elemental tracers of seawater concentrations. The sediments are 70% carbonate and about 1.3% detrital material, with the remainder having a hydrothermal origin. Rhenium concentrations are at detrital levels suggesting little anaerobic diagenesis beyond manganese remobilization; however, U, Cd, and Mo concentrations all indicate alteration after deposition. Oxyanion/Fe ratios of P, V, and As in surface sediments are lower than the ratios in plume particles and covary with Fe content downcore. The probable explanation for these variations is variable efficiency of incorporation of these metals during iron oxide formation or early diagenetic processes after deposition. The P/Fe ratio in the core sediment is about half that in the plume particles. Among the oxyanion ratios, only the V/As ratio in the sediments is relatively constant at the present plume particle ratio during the past 60–70 kyr. Changes in sediments older than this could reflect variability in the incorporation efficiency or changes in V/As ratio in seawater. This study demonstrates the difficulties of using hydrothermal sediment element enrichments as tracers of past seawater chemistry changes. In this core it appears that all oxyanion ratios change for reasons other than seawater metal concentration changes, with the possible exception of the V/As ratio.

Near-axis seamount distribution and its relationship with the segmentation of the East Pacific Rise and northern Pacific–Antarctic Ridge, 17°N–56°S

We used satellite-derived gravity anomaly maps and bathymetry data to analyze the distribution of off-axis seamounts on young crust on the flanks of the East Pacific Rise (EPR) and northern Pacific–Antarctic Ridge (PAR), from 17°N to 56°S latitude. We observed large-scale variations in the distribution of the volcanoes which we attribute to variations of the regional magmatic budget of the underlying mantle. Two distinct settings for off-axis volcanism were observed at the scale of second-order segments. The first one favors volcano growth near first- or second-order discontinuities. The second type occurs preferentially near the centers of second-order segments. We infer that the distribution of seamounts is controlled both by the availability of melt under the flanks of the ridge and by the vulnerability of the young lithosphere. Near the northern EPR and in some places near the PAR, the off-axis volcanism results mostly from fracturing of the crust near axial discontinuities, probably due to the thermomechanical stresses that develop in the cooling lithosphere. In the southern EPR, especially in the 14–19°S area, the young lithosphere is probably much more vulnerable to off-axis volcanism. The seamount distribution seems to reflect variations in the off-axis melt production at the scale of second-order segments (∼150 km). We infer that the difference in volcanic setting (close to segment ends or close to segment centers) between the northern and southern EPR results from the difference in spreading rate and, consequently, the variation in rheology of the young lithosphere.

Modelle der magmatischen und hydrothermalen Entwicklung am schnell divergierenden südlichen Ostpazifischen Rücken

Modelle der magmatischen und hydrothermalen Entwicklung am schnell divergierenden südlichen Ostpazifischen Rücken

Geochemistry of lavas from the Garrett Transform Fault: insights into mantle heterogeneity beneath the eastern Pacific

Young intra-transform lavas erupted as a result of extension within the Garrett Transform Fault on the southern East Pacific Rise, are more porphyritic, less evolved, have lower concentrations of incompatible trace elements, and lower ratios of more incompatible to less incompatible elements (e.g. low K/Ti and La/Sm) compared to lavas from the adjacent East Pacific Rise ridge axis. Sr, Nd and Pb isotope compositions overlap with the depleted end of the field for Pacific mid-ocean ridge basalts, but extend to lower 87Sr/ 86Sr (0.702137), 206Pb/ 204Pb (17.462), 207Pb/ 204Pb (15.331), 208Pb/ 204Pb (36.831), and higher 143Nd/ 144Nd (0.513345) than any lavas previously reported from the Pacific. Peridotites from the Garrett Transform have Nd isotope compositions within the range of the intra-transform lavas. The unusual major and trace element compositions of the Garrett lavas appear to be characteristic of other intra-transform lavas from elsewhere in the Pacific. The chemical and isotopic features of the Garrett lavas can be explained by remelting, beneath the transform, a two-component upper mantle which was depleted in incompatible element-enriched heterogeneities during melting beneath the East Pacific Rise ridge axis (within the past 1 Ma). Our data place new constraints on the trace element and isotope composition of the depleted mantle component that contributes to magmatism in the Pacific, and show that this component is heterogeneous, both on the scale of a single transform fault, and on the scale of an ocean basin.

Asymmetric Electrical Structure in the Mantle Beneath the East Pacific Rise at 17 degrees S.

The magnetotelluric component of the Mantle Electromagnetic and Tomography (MELT) Experiment measured the electrical resistivity structure of the mantle beneath the fast-spreading southern East Pacific Rise (EPR). The data reveal an asymmetric resistivity structure, with lower resistivity to the west of the ridge. The uppermost 100 kilometers of mantle immediately to the east of the ridge is consistent with a dry olivine resistivity structure indicating a mantle depleted of melt and volatiles. Mantle resistivities to the west of the ridge are consistent with a low-melt fraction (about 1 to 2 percent interconnected melt) distributed over a broad region and extending to depths of about 150 kilometers. The asymmetry in resistivity structure may be the result of asymmetric spreading rates and a westward migration of the ridge axis and suggests distinct styles of melt formation and delivery in the mantle beneath the two plates.

Fresnel zone: A pitfall in seismic imaging of mid‐ocean ridge magma lenses

We present estimates of the Fresnel zone diameter for the axial magma chamber lens along the southern and northern East Pacific Rise and compare them to the magma lens widths derived from migrated seismic sections. The mean values of that diameter are 996m and 1052m and the mean values of the magma lens width are 680m and 1056m respectively. Most individual estimates of the magma lens width fall below the Fresnel zone diameter. Similar results are presented for three other mid‐ocean ridges. The registered signal is therefore a mix of responses from the magma sill and the encasing rock and lateral resolution is not achieved. We conclude that data analysis procedures that assume a laterally homogeneous Earth, cannot provide reliable estimates of the magma lens physical properties, including whether it is liquid or not.

Anisotropy in the oceanic lithosphere from the study of local intraplate earthquakes on the west flank of the southern East Pacific Rise: Shear wave splitting and waveform modeling

Shear wave splitting is observed on ocean bottom seismometer records from local, intraplate microearthquakes on the west flank of the East Pacific Rise at 18°S. Split times reach a maximum of about 0.2 s. For most of the incoming waves with long mantle paths (∼20 km), the polarization direction of the fast arrival is subparallel to the spreading direction, which we attribute to anisotropy caused by the strain‐induced preferential orientation of olivine. In contrast, for some records with short paths or shallower sources or propagation along the spreading direction, the fast direction is nearly parallel to the ridge axis. These polarizations are probably caused by seismic anisotropy from aligned cracks in the uppermost crust. In addition, for some events, the apparent splitting is frequency dependent. To explore the pattern of shear wave splitting that would be expected for nonvertical paths in an oceanic lithosphere with two distinct anisotropic layers, we generate synthetic seismograms for a variety of source depths and mechanisms. We employ a multidomain, pseudospectral method to simulate the elastic wave fields from point sources in an inhomogeneous, anisotropic medium. Splitting parameters measured from synthetic S waves demonstrate that the apparent fast direction is not always parallel to the symmetry axes and that, in some cases, fast directions at higher frequencies will be more characteristic of the shallower crustal anisotropy while fast directions at lower frequencies are dominated by the mantle portion of the path. Most of the observed characteristics of splitting can be reproduced if there is approximately 8% S wave anisotropy in the mantle and an average of about 6% S wave anisotropy in the upper crustal, seismic layer 2.

Seismic evidence for a hydrothermal layer above the solid roof of the axial magma chamber at the southern East Pacific Rise

A full-waveform inversion of two-ship, wide-aperture, seismic reflection data from a ridge-crest seismic line at the southern East Pacific Rise indicates that the axial magma chamber here is about 50 m thick, is embedded within a solid roof, and has a solid floor. The 50--60-m-thick roof is overlain by a 150--200-m-thick low-velocity zone that may correspond to a fracture zone that hosts the hydrothermal circulation, and the roof itself may be the transition zone separating the magma chamber from circulating fluids. Furthermore, enhanced hydrothermal activity at the sea floor seems to be associated with a fresh supply of magma in the crust from the mantle. The presence of the solid floor indicates that at least the upper gabbros of the oceanic lower crust are formed by cooling and crystallization of melt in magma chambers.

Distribution of isolated volcanoes on the flanks of the East Pacific Rise, 15.3°S–20°S

Volcanic constructions, not associated with seamount (or volcano) chains, are abundant on the flanks of the East Pacific Rise (EPR) but are rare along the axial high. The distribution of isolated volcanoes, based on multibeam bathymetric maps, is approximately symmetric about the EPR axis. This symmetry contrasts with the asymmetries in the distribution of volcano chains (more abundant on the west flank), the seafloor subsidence rates (slower on the west flank) and the distribution of plate‐motion‐parallel gravity lineaments (more prominent on the west flank). Most of the isolated volcanoes complete their growth within ∼14 km of the axis on crust younger than 0.2 Ma, while seamount chain volcanoes continue to be active on older crust. Volcanic edifices within 6 km of the ridge axis are primarily found adjacent to axial discontinuities, suggesting a more sporadic magma supply and stronger lithosphere able to support volcanic constructions near axial discontinuities. The volume of isolated near‐axis volcanoes correlates with ridge axis cross‐sectional area, suggesting a link between the magma budget of the ridge and the eruption of near‐axis volcanoes. Within the study area, off‐axis volcanic edifices cover at least 6 % of the seafloor and contribute more than 0.2% to the volume of the crust. The inferred width of the zone where isolated volcanoes initially form increases with spreading rate for the Mid‐Atlantic Ridge (<4 km), northern EPR (<20 km), and southern EPR(<28 km), so that isolated volcanoes form primarily on lithosphere younger than 0.2 Ma (<4–6 km brittle thickness), independent of spreading rate. This suggests some form of lithospheric control on the eruption of isolated off‐axis volcanoes due to brittle thickness, increased normal stresses across cracks impeding dike injection, or thermal stresses within the newly forming lithosphere.

Magnetic telechemistry and magmatic segmentation on the Southern East Pacific Rise

Results from axial dredges and a profile inversion of magnetic anomaly data along the axis of the East Pacific Rise (EPR) at 13–23°S provide an estimate of the average degree of fractionation for the extrusive layer at this ultrafast-spreading (∼145 mm/yr full rate) ridge. We find a high correlation ( R=0.81) between dredge mean FeO * (total iron as FeO) and natural remanence for 34 axial dredges with multiple samples having coincident geochemical and magnetic data. We attribute this good correlation to detailed sampling spanning the full range of cooling-related magnetization changes within a flow and to the young age (0–6 ka) of these axial samples, which effectively minimizes time-dependent magnetization changes due to geomagnetic intensity or alteration. A composite axial magnetic anomaly profile shows large amplitude (up to 400 nT) fluctuations with wavelengths of 50–200 km, which theoretical considerations suggest can reliably be related to the magnetization directly beneath the ship. For much of the southern EPR, seismic data provide independent limits on the axial thickness (259 ± 55 m) and the pattern of off-axis thickening of the extrusive magnetic source layer. These data also provide evidence for an axial magma lens that effectively eliminates anomaly contributions from deeper magnetic sources. Inversion of the axial magnetic anomaly data utilizing these geophysical constraints yields a magnetization solution which, through use of the regression relating FeO * and natural remanence, may be related to the average degree of differentiation of the extrusive source layer. The magnetic data reveal a pattern of magmatic segmentation that closely parallels the tectonic segmentation of the ridge, suggesting that magma supply may be an important control on the average degree of differentiation of the extrusive layer.