Rise Axis Research Articles

Distribution of magma beneath the East Pacific Rise between the Clipperton Transform and the 9°17′N Deval from forward modeling of common depth point data

We have reprocessed seven cross‐axis common depth point (CDP) profiles between the Clipperton transform and the 9°17′N Deval (deviation in axial linearity) on the East Pacific Rise (EPR) to understand the relationship between axial magma chamber (AMC) width and seafloor morphology. Forward modeling of cross‐axis CDP profiles suggests a segmented AMC in which significant variations in width occur across minor rise axis discontinuities (e.g., Devals). The modeled rise segment widths bounded by the 9°53′N‐9°35′N Devals, the 9°35′N‐9°17′N Devals, and south of the 9°17′N Deval were < 0.7 km, 1.0–1.2 km, and 4.15 km, respectively. Transition in AMC width across these discontinuities is unclear due to the sparseness of cross‐axis line spacing; however, a simple association of Devals with decreased magma supply is doubtful: the minimum (250 m) and maximum (4150 m) AMC widths are found near the 9°35′N and 9°17′N Devals, respectively. The reprocessing of CDP profiles included repicking stacking velocities to ensure a proper stack of the AMC reflector and its associated diffractions, imaging postcritical reflections from the base of layer 2A, finite difference time migration, ray theoretical depth migration, and travel time modeling of AMC diffraction patterns. Constraints on AMC width were derived from forward modeling techniques based on analytic raytracing. Velocity models were constructed from SeaBeam bathymetry and modified expanding spread profile (ESP) velocity‐depth functions. ESP velocity models were altered to compensate for off‐axis thickening of layer 2A as imaged in the CDP reflection data. Continuous two‐dimensional velocity models constructed from modified ESP velocity‐depth functions and SeaBeam bathymetry should account for ray bending at the seafloor/basalt interface and any lateral velocity gradients induced by a thickening layer 2A. Stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration. Reflector positions were input into our forward modeling scheme to produce a zero‐offset travel time response of our migrated solution. The travel time response was then overlain on the stacked section to ensure an adequate match, especially to diffractions generated at the AMC edge. Forward modeling of AMC diffraction patterns reveals that original AMC width estimates were too large. The under‐migration of the AMC reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual CMP gathers, thus resulting in improperly collapsed diffraction hyperbolae. Ship wandering relative to the AMC edge can account for variations in AMC reflector amplitude and dropout on the along‐axis CDP line. The continuity of the AMC appears unbroken across several ridge axis discontinuities between the Clipperton transform and the 9°17′N Deval which suggests an AMC whose along‐axis dimension exceeds 75 km. Reflectivity modeling of CMP gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush. Such a thin layer of melt might inhibit along‐axis mixing of magmas and thereby account for variations in magma composition along the rise crest. This melt lens model for the AMC would also produce strong diffraction patterns as imaged in the CDP data.

Variation in cross‐sectional area of the axial ridge along the East Pacific Rise: Evidence for the magmatic budget of a fast spreading center

Along the fast and ultrafast spreading East Pacific Rise, the cross‐sectional area of the axial ridge varies significantly over length scales similar to its morphologic segmentation. Using an automated method, we measure the ridge area (volume per kilometer along axis) where there is complete bathymetric coverage. Along 3500 km of the northern and southern East Pacific Rise, our two study areas encompass numerous transform faults, large overlapping spreading centers, small overlapping spreading centers, and smaller discontinuities (first‐, second‐, third‐, and fourth‐order discontinuities). The cross‐sectional area variation mimics the undulation of the ridge crest depth; local area maxima occur toward the middle of segments, and the axial area decreases by 40% or more at first‐ and second‐order discontinuities. Third‐order discontinuities are generally marked by smaller disruptions in the ridge area, and fourth‐order offsets do not systematically correspond with features of the cross‐sectional area profile. The correlation between shallower ridges and larger ridge areas breaks down at some locations because axial cross‐sectional area represents a longer term average of the ridge's magmatic state than axial depth. A correlation between large ridge areas and negative residual gravity anomalies indicates that inflated ridges are underlain by low‐density crust and mantle. Also, a correlation between larger area and higher MgO content of axial basalts suggests that inflated areas generally erupt hotter magmas which are presumably supplied more rapidly to the neovolcanic zone. The cross‐sectional area of the axial ridge appears to correlate with the width of the axis‐centered low‐velocity zone in the crust. These observations, as well as the absence of large, relict axial ridges off‐axis, indicate that the axial ridge originates from buoyancy due to thermal expansion and the presence of melt in the crust and mantle within about 10 km of the rise axis. Portions of the northern East Pacific Rise underlain by a magma chamber reflector generally occur where the ridge cross‐sectional area is greatest; this supports the connection between processes which inflate the axial ridge and those which heat the crust and upper mantle and produce melt. Thus, while the axial high on fast spreading centers resembles a constructional volcano in cross section, it is more like a long, narrow balloon whose cross‐sectional area is a sensitive indicator of magma supply. Using this relationship, we predict with 75% confidence that at least 45% of the unsurveyed northern East Pacific Rise (18°N to 13°N and 9°N to 5°N) and at least 60% of the southern East Pacific Rise (4°S to 23°S) is underlain by a magma chamber reflector.

The role of density in the accumulation of basaltic melts at mid‐ocean ridges

It is commonly assumed that magma ponds at a level of neutral buoyancy in the shallow crust where melt densities are equal to the bulk density of the surrounding crust. At the East Pacific Rise this neutral buoyancy level lies only 100–400 m below the sea floor, significantly shallower than the depths (> 1–2 km) of the magma bodies imaged in multichannel reflection data, suggesting that other factors must control the collection of melt in these reservoirs. The apparent inverse relationship between magma chamber depth and spreading rate at intermediate and fast spreading ridges suggests that the thermal structure of the rise axis, not the buoyancy of melt, is the primary factor that controls the depth at which melt ponds in crustal magma chambers beneath mid‐ocean ridges.

The renavigation of Sea Beam bathymetric data between 9� N and 10� N on the East Pacific Rise

We present a gridded Sea Beam bathymetric map of a 5100 km2 area between 9° and 10° N on the East Pacific Rise (included as a color separate accompanying this issue). The raw bathymetric data are renavigated using a technique for calculating smooth adjustments to navigation that incorporates absolute constraints from satellite fixes and acoustically-located explosive shots, and relative constraints from the misfit of bathymetric data at ship track crossovers. We describe a back-projection technique for gridding the bathymetric data that incorporates an approximation for the power distribution within a narrow-beam echo sounding system and accounts for the variable uncertainties associated with multi-beam data. The nodal separation of the resulting map is ~ 80 m in both latitude and longitude, and the sampling of grid points within a 60 × 85 km2 region is in excess of 99%. A formal analysis of variance is applied to the gridded bathymetric data. For each grid point, the difference between the variance of data from within a track versus data from between tracks provides an upper bound on the magnitude of bathymetric misfits arising from navigational errors. The renavigation results in an 88% reduction in this quantity. We also examine the effects of renavigation on the misfit of magnetic and gravity data at crossovers and compare our results with other bathymetric surveys. A striking feature of the final bathymetric map is the sinuous regional shape of the rise axis. In plan view, the local trend of morphology sometimes varies by up to 15° and the distances separating changes in morphological trend are about 10–20 km. In cross section the slopes of the rise flanks are notably asymmetric and show some correlation with the offset of the axial magmatic system as detected by seismic methods.

Seismic Structure of the Southern East Pacific Rise

Seismic data from the ultrafast-spreading (150 to 162 millimeters per year) southern East Pacific Rise show that the rise axis is underlain by a thin (less than 200 meters thick) extrusive volcanic layer (seismic layer 2A) that thickens rapidly off axis. Also beneath the rise axis is a narrow (less than 1 kilometer wide) melt sill that is in some places less than 1000 meters below the sea floor. The small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge-crest thermal models. However, the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate. These observations indicate that the paradigm of ridge crest magma chambers as small, sill-like, midcrustal bodies is applicable to a wide range of intermediate- and fast-spreading ridges.

The East Pacific Rise and its flanks 8?18� N: History of segmentation, propagation and spreading direction based on SeaMARC II and Sea Beam studies

SeaMARC II and Sea Beam bathymetric data are combined to create a chart of the East Pacific Rise (EPR) from 8°N to 18°N reaching at least 1 Ma onto the rise flanks in most places. Based on these data as well as SeaMARC II side scan sonar mosaics we offer the following observations and conclusions. The EPR is segmented by ridge axis discontinuities such that the average segment lengths in the area are 360 km for first-order segments, 140 km for second-order segments, 52 km for third-order segments, and 13 km for fourth-order segments. All three first-order discontinuities are transform faults. Where the rise axis is a bathymetric high, second-order discontinuities are overlapping spreading centers (OSCs), usually with a distinctive 3:1 overlap to offset ratio. The off-axis discordant zones created by the OSCs are V-shaped in plan view indicating along axis migration at rates of 40–100 mm yr−1. The discordant zones consist of discrete abandoned ridge tips and overlap basins within a broad wake of anomalously deep bathymetry and high crustal magnetization. The discordant zones indicate that OSCs have commenced at different times and have migrated in different directions. This rules out any linkage between OSCs and a hot spot reference frame. The spacing of abandoned ridges indicates a recurrence interval for ridge abandonment of 20,000–200,000 yrs for OSCs with an average interval of approximately 100,000 yrs. Where the rise axis is a bathymetric low, the only second-order discontinuity mapped is a right-stepping jog in the axial rift valley. The discordant zone consists of a V-shaped wake of elongated deeps and interlocking ridges, similar to the wakes of second-order discontinuities on slow-spreading ridges. At the second-order segment level, long segments tend to lengthen at the expense of neighboring shorter segments. This can be understood if segments can be approximated by cracks, because the propagation force at a crack tip is directly proportional to crack length.

The Seismic Attenuation Structure of a Fast-Spreading Mid-Ocean Ridge

The two-dimensional P-wave attenuation structure of the axial crust of the East Pacific Rise was obtained from an inversion of waveform spectra collected during an active-source seismic tomography experiment. The structure shows that attenuation near the surface is high everywhere but decreases markedly within 1 to 3 kilometers of the rise axis. The near-axis variation is attributed to the thickening of the surface basalt layer and possibly to in situ changes in porosity related to hydrothermal circulation. High attenuation is also observed beneath the rise axis at depths ranging from about 2 kilometers (less than 1 kilometer beneath the axial magma lens) to the base of the crust. The levels of attenuation in this deeper region require at most only a small fraction of partial melt.

Microearthquakes on and near the East Pacific Rise, 9°–10°N

Records from a seismic network deployed as part of an active tomography experiment at 9°30′N on the East Pacific Rise (EPR) provide an opportunity to characterize local microearthquake activity over an 8‐day period. With the exception of the region around the 9°03′ overlapping spreading center (OSC), no events were located on the rise axis. One microearthquake, located from P and S‐wave arrival times, lies adjacent to the network, 18 km to the west of the rise axis. Five more events, located from P‐wave and T‐phase data, are to the south of the network. Three cluster around the western arm of the 9°03′ OSC, while two are located 25–30 km to the east of the OSC. The seismic moments of the microearthquakes range from 5 × 1019 to 4 × 1020 dyn cm, several orders of magnitude larger than those reported in other studies of shallow events along the axis of the EPR. These results suggest that overlapping spreading centers may be the loci of substantial microearthquake activity and that at this location the normal faults that form on the young flanks of the EPR are active off‐axis to distances of at least 20 km.

Tomographic image of the axial low‐velocity zone at 12°50′N on the East Pacific Rise

The 1982 MAGMA seismic refraction experiment yielded a large set of accurate P wave travel times corrected for bathymetry and anisotropy which sample the structure of the East Pacific Rise (EPR) at 12°50′N. The arrivals were recorded using ocean bottom seismographs at three sites: on axis, and at 7 km and 16 km east of the axis. We invert 2320 travel times for the twodimensional crustal seismic velocity structure across the EPR axis, assuming that the velocity structure is invariant along strike. The travel times provide strong evidence for compressional velocity anisotropy in the upper crust corresponding to ∼10% faster velocities for propagation parallel to the axis than perpendicular to the axis; the travel tunes used for the tomography are corrected for the effects of this azimuthal anisotropy. Our preferred model contains only the structure clearly required by the data (structure which is stable under excessive smoothing) and achieves a variance reduction of 81% relative to the laterally homogeneous starting model. We resolve a substantial zone beneath the rise axis in which the velocity is reduced by 0.4 to 0.7 km/s; this low‐velocity zone (LVZ) is about 7 km wide and extends from a depth of about 1.5–2.0 km down to Moho at a depth of 5.5 km. The LVZ is slightly asymmetric, extending 1 km further to the east than to the west of the axis. In the shallow (<1.0 km depth) crust, a pattern of velocity variations is imaged in which velocities are high at the spreading axis, decrease between 3 km and 7 km east of the axis, and then increase again between 10 and 15 km east of the axis. We investigate the resolution of the inversion using an impulse response method; the LVZ and off‐axis upper crustal variations are well resolved. In addition, the travel time data indicate that an axial high‐velocity anomaly less than 2 km wide exists in the upper crust but is not resolved by the inversion. The small velocity reductions of the LVZ are consistent with hot rock containing only a small quantity of melt. These results, combined with the multichannel seismic reflection lines and expanded spread profiles from the northern EPR, suggest that the zone of high melt fraction under the spreading center is confined to a narrow, thin lens capping a broad zone of hot plutonic rocks. The upper crustal velocity reduction within 1 km of the axis reflects near‐axial thickening of the extrusive layer and the later reduction probably reflects porosity increases due to near‐axial tectonism; the upper crustal velocity increase beyond 15 km off axis is attributed to porosity decreases associated with hydrothermal alteration.

Petrology and magma chamber processes at the East Pacific Rise ∼ 9°30′N

We present new major and trace element data for a set of closely spaced (< 1.8 km) dredges along a well‐studied portion of the East Pacific Rise (EPR) axis near 90°30′N (9°17′N to 90°50′N). With the exception of enriched mid‐ocean ridge basalt (E‐MORB) at 9°35′N, the lavas are all normal mid‐ocean ridge basalt (N‐MORB) with a limited range of MgO (8.40–6.22 wt %). Major element and trace element data favor derivation of the melts from a single parental composition by low pressure crystallization of olivine, plagioclase and clinopyroxene in the ratio of 16:62:22. This model is consistent with liquid line of descent models but inconsistent with petrographic observations in that most of the N‐MORB lavas have only plagioclase phenocrysts. We ascribe this to gravitational crystal settling of mafic phases and flotation of plagioclase, supported by crystal size distribution data and density relations. Most likely this occurred in the axial magma chamber (AMC) that underlies the EPR in this area. The chemistry of axial lavas varies along axis and correlates roughly with elevation of the axis and depth to the AMC. We interpret these correlations as favoring an AMC that is chemically zoned along‐axis, with Fe‐rich melts at its distal ends. This favors a central injection of MgO‐rich melt with lateral along‐axis shallow transport. The height of eruptions along axis is apparently controlled by magma density such that least dense MgO‐rich melts build local volcanic constructs of the highest elevation. The E‐MORB lavas are older than the N‐MORB and probably erupted at a time when the AMC was absent or was much smaller in size than presently. E‐MORB could have originated by deep melting processes or very shallow contamination of N‐MORB. Its presence in the 9°30′N area supports the notion that magma chambers are not truly steady state. Instead they probably come and go on a time scale of 3000–6000 years. A single parental magma seems to supply melts to the AMC along the entire 60 km segment of the EPR, suggesting that central supply injection sites are widely spaced. Based on our data, there is no evidence for petrologic segmentation corresponding to 4th‐order tectonic segmentation in this area; however, it may be present below the resolution of our sampling.

Kinematic framework of the Cocos‐Pacific Plate Boundary from 13°N to the Orozco TRANSFORM FAULT: RESULTS FROM AN EXTENSIVE MAGNETIC AND SEAMARC II SURVEY

During the summer of 1987, magnetic anomaly data were collected by surface ship as part of an extensive SeaMARC II investigation of the East Pacific Rise (EPR) from 13°N to the Orozco transform. The survey extended to either side of the rise axis onto seafloor at least 1.8 million years (m.y.) in age, enabling the recent evolution of the structural and kinematic framework of the plate boundary to be studied in detail. North of 13°50′N there has been a major perturbation in the evolution of the plate boundary. Swaths of lineaments that trend oblique to EPR‐parallel topography form a north pointing, V‐shaped discordant zone on the Pacific and Cocos plates that is broadly symmetric about the EPR axis. On the Pacific plate a zone of discordant morphology 130 km long and between 6 and 14 km wide with a structural grain that is highly oblique to the present‐day spreading direction is observed on seafloor 0.9–1.8 m.y. in age. A similar but more subtle feature of the same age is also present on the Cocos plate. These zones of discordant lineaments can be correlated with changes in the magnetic lineation pattern. On the Pacific plate the disturbed zone lies between anomalies J and 2, creating greater than normal distance between the anomalies. On the Cocos plate the disturbed zone is characterized by a distinct, high‐amplitude, northwestward trending magnetic anomaly. The observed structural grain and the changes in the magnetic anomaly patterns associated with the disturbed zones are very similar to those observed at propagating ridges. Based on the magnetic anomalies, a propagation rate of 10.8 cm/yr in a N10°W direction is estimated for the past 1.8 m.y. A detailed examination of the structures developed within the disturbed zone on the Pacifc plate indicates that the rift propagation in this area can best be explained by the model of Wilson (1990) which involves cyclic rift failure with inward curvature of both rift tips. Plate adjustment to the propagation event is ongoing. There is a pronounced change in the morphology of the rise axis with distance from the propagation event. Close to and for ∼100 km behind the propagator tip, the EPR crest is not well developed and is characterized by a series of low (relief of 100–200 m) ridges and troughs with a poorly defined neovolcanic zone. Further away (>100 km) from the propagator tip the rise crest is a single, linear, horst‐shaped ridge with a well‐developed narrow axial graben at the axis of the ridge. Regional morphologic and magnetic data suggest that this cycle of rift propagation may have begun approximately 2.5 million years before present (m.y.B.P.) near the eastern ridge‐transform intersection of the paleo‐O'Gorman transform and is continuing today at the Orozco transform.

Fossil clues to paleoecology of deep-sea hydrothermal vent fauna: summary of recent findings

An abundant and unusual fauna is associated with deep-sea hydrothermal vents on the crest of the global mid-ocean ridge system. Precipitation of metal sulfides, silica, iron-oxyhydroxides and other minerals at these vents encrusts and replaces the remains of dead organisms to form fossils that may be preserved in the geologic record. For example, molds and casts of tubiculous polychaete and vestimentiferan worms are abundantly preserved in Recent hot spring deposits on the mid-ocean ridge crest, and similar worm molds also have been found in Tethys ophiolite sulfide mineral deposits of mid-Cretaceous age. The aragonitic and calcareous shells and tests of other vent species are also good candidates for fossilization by encrustation and replacement of their carbonate constituents with hydrothermal minerals.Recent and ongoing studies of the ecology and mineralization of vent organisms at active hot spring sites on the mid-ocean ridge in the eastern Pacific provide knowledge needed for gleaning paleoecological information from fossiliferous marine hydrothermal deposits in the geologic record. At modern vents, intact larval shells (prodissoconchs or protoconchs) are present on the surfaces of many juvenile molluscs. Preservation of these larval shells by hydrothermal mineralization may provide a powerful paleoecological tool for interpretation of the life history strategies of many sessile invertebrates associated with ancient submarine hydrothermal vents. Bacterial mats that grow on the outer surfaces of sulfide mineral structures have been preserved by silica deposition in inactive deposits on the East Pacific Rise (EPR) axis at 9°27'N. Studies of the ecology and growth history of this bacterial species at active vents are in progress.Preservation of ridge crest vent fauna within volcanic rock units can occur when lava erupts on the ridge crest near hydrothermal vent communities. Along the axis of the EPR at 9°50.6'N, vent mussels and vestimentiferan worms were found partially buried by lava flows and volcanic collapse rubble. Animals trapped beneath eruption-associated rubble may be coated or replaced by hydrothermal minerals precipitating from fluids circulating in the cooling rocks. In addition, worm-tube molds were created at the EPR 9°50.6'N site where lava quenched around living vestimentiferans (analogous to the formation of tree-molds in Hawaiian lava flows). These lava molds contained pyritized remnants of the chitinous tubes of the vestimentiferans.

Upper crustal resistivity structure of the East Pacific Rise near 13°N

An active source electromagnetic (EM) sounding has been conducted on the axis of the East Pacific Rise (EPR) at 13° 10′N. 1D inversion and modelling techniques, seeking resistivity as a function of depth, have been applied to 8 Hz amplitude data collected along the ridge crest. Resistivity is seen to increase monotonically between 50 m and 1 km below the seafloor, increasing from ∼1Ωm to around 90Ωm. We observe no intrinsic difference in upper crustal resistivity structure between the rise axis and 100,000 year old crust. Inferred surface porosities of 20% are larger than those recorded in 5.9 my old crust in DSDP hole 504B. Our data do not require, and lack sufficient information for, the reliable inclusion of a conductive termination to the model below 1.2 km.

Magmatic processes at superfast spreading mid‐ocean ridges: Glass compositional variations along the East Pacific Rise 13°–23°S

Major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading (150 mm/yr) East Pacific Rise (EPR), 13°–23°S, and near‐ridge seamounts comprise 212 chemical groups. We interpret these groups to represent the average composition of individual lava flows or groups of closely related flows. Groups slightly enriched in K2O (T‐MORB) are distributed variably along the axis, in contrast to the Galapagos Spreading Center where T‐MORB are extremely rare. This result is consistent with the interpretation that T‐MORB magmas arise from low‐melting temperature, K‐rich heterogeneities in the subaxial EPR mantle. The Galapagos Spreading Center, which is migrating to the west in an absolute reference frame, is underlain by mantle previously processed and depleted in the T‐MORB component during melting events giving rise to earlier EPR magmas. Excluding T‐MORB, there are nearly monotonic, twofold increases in K/Ti and K/P of axial lavas from 23°S to 13°S. From 22°S to 17°S these gradients correlate with isotopic ratios, but north of 17°S there is a reversal of isotopic gradients, indicating (recent?) decoupling of the isotopic and minor element ratios in the subaxial mantle. A strong, southward increase in degree of differentiation for approximately 200 km north of the large offset at 20.7°S correlates with a gradient in bathymetry, consistent with previous interpretations that this offset is propagating to the south. Samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive, evolved fractionation signatures of rift propagation, suggesting that propagating rift tips have been abandoned preferentially to failing rift tips. Glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset, and possibly also south of 22°S; the latter region may be affected by deformation accompanying northward growth of the Easter Microplate. Near‐ridge seamounts on the Pacific Plate between 18°–19°S comprise two distinct populations: those aligned approximately parallel to the spreading direction are extremely variable in major element composition, but consistently enriched in Sr relative to nearby axial lavas; smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and Sr relative to axial lavas. The degree of differentiation of axial lavas between 18°–19°S can be related to the structural development of the rift axis and/or vigor of hydrothermal activity of individual segments. Glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area. Primary magmatic segmentation mainly reflects mantle source variations, the boundaries of which correlate with the largest physical offsets in the rise axis between the Easter Microplate and Garrett Transform Zone. A secondary magmatic segmentation, defined by the along‐axis continuity of similar parental magma compositions or liquid lines of descent, occurs with a length scale varying from 11 to 185 km, with an average of 69 ±57 (1σ) km. The boundaries of these segments mainly occur at overlapping spreading centers. All first‐, second‐ and third‐order physical offsets correspond to secondary magmatic segment boundaries, but some secondary magmatic segment boundaries also occur at small, fourth‐order ridge axis discontinuities. The secondary magmatic segments define the length scale of mantle melting variations, mainly variations in extent of melting, but not the scale of melt extraction processes that feed the axis. This scale must be smaller than that of the secondary magmatic segments and probably corresponds to the length scale of fourth‐order physical discontinuities along axis. There is a good positive correlation of average secondary magmatic segment length with spreading rate for four well‐sampled areas varying from 20 to 150 mm/yr. Secondary magmatic segments also become more variable in axial length with increasing spreading rate. The average lengths of secondary magmatic segments are smaller than those predicted by gravitational instability considerations at all spreading rates. Superposed on the axial magmatic segmentation are variations reflecting subaxial magmatic temperature, defined by extent of magmatic differentiation, which bears little systematic relation to physical or other kinds of magmatic segmentation. At 13°–23°S, the length scale of this variation is 217±60 (1σ) km, approximately corresponding to the wavelength of “rolls” in the gravity field observed off‐axis. Taken together, the various kinds and scales of magmatic variations observed for this superfast spreading ridge suggest that regional temperature of the upwelling asthenosphere, magma supply to the axis, and crustal magmatic temperature reflect independent, regionally decoupled processes.

The three-dimensional seismic velocity structure of the East Pacific Rise near latitude 9° 30′ N

Three-dimensional images of crustal seismic structure beneath the East Pacific Rise show pronounced axial heterogeneity over distances of a few kilometres. A linear high-velocity anomaly, approximately 1–2 km in width and restricted to the uppermost 1 km of the crust, is centred on the rise axis. An axial low-velocity anomaly at depths of 1–3 km varies in amplitude along the axis, consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis. This apparent thermal segmentation along axis is consistent with injection of mantle-derived melt midway along a locally linear, 12-km-long segment of the rise.

The structure of 0‐ to 0.2‐m.y.‐old oceanic crust at 9°N on the East Pacific Rise from expanded spread profiles

We analyze four expanded spread profiles acquired at distances of 0, 2.1, 3.1, and 10 km (0–0.2 m.y.) from the axis of the East Pacific Rise between 9° and 10°N. Velocity‐depth models for these profiles have been obtained by travel time inversion in the τ‐p domain, and by x‐t forward modeling using the WKBJ and the reflectivity methods. We observe refracted arrivals that allow us to determine directly the uppermost crustal velocity structure (layer 2A). At the seafloor we find very low Vp and VS/Vp values around 2.2 km/s and ≤ 0.43. In the topmost 100–200 m of the crust, Vp remains low (≤ 2.5 km/s) then rapidly increases to 5 km/s at ∼500 m below the seafloor. High attenuation values (Qp < 100) are suggested in the topmost ∼500 m of the crust. The layer 2–3 transition probably occurs within the dike unit, a few hundred meters above the dike‐gabbro transition. This transition may mark the maximum depth of penetration by a cracking front and associated hydrothermal circulation in the axial region above the axial magma chamber (AMC). The on‐axis profile shows arrivals that correspond to the bright AMC event seen in reflection lines within 2 km of the rise axis. The top of the AMC lies 1.6 km below the seafloor and consists of molten material where Vp ≈ 3 km/s and VS = 0. Immediately above the AMC, there is a zone of large negative velocity gradients where, on the average, Vp decreases from ∼6.3 to 3 km/s over a depth of approximately 250 m. Associated with the AMC there is a low velocity zone (LVZ) that extends to a distance no greater than 10 km away from the rise axis. At the top of the LVZ, sharp velocity contrasts are confined to within 2 km of the rise axis and are associated with molten material or material with a high percentage of melt which would be concentrated only in a thin zone at the apex of the LVZ, in the axial region where the AMC event is seen in reflection lines. Away from the axis, the transition to the LVZ is smoother, the top of the LVZ is deeper, and the LVZ is less pronounced. The bottom of the LVZ is probably located near the bottom of the crust and above the Moho. Moho arrivals are observed in the profiles at zero and at 10 km from the rise axis. Rather than a single discontinuity, these arrivals indicate an approximately 1‐km‐thick Moho transition zone.

Single plume model for asynchronous formation of the Lamont Seamounts and adjacent Eeast Pacific Rise terrains

Sonar reflectance is used to quantify the relative ages of young volcanic terrains on a seamount chain and a nearby spreading center in the north equatorial Pacific. The reflectance is a measure both of the burial of lava by pelagic sedimentation and of the microrelief of the lava. The extent of sediment cover and the lava type distribution are calibrated with thousands of bottom photographs. The relative ages indicate that the locus of volcanic eruptions has migrated toward the East Pacific Rise axis. Zero‐age activity lies just to the west of the East Pacific Rise south of the Clipperton Transform and is probably the cause of an anomalously shallow neovolcanic zone. The evidence fits a scenario in which eruptive centers appear, evolve, and become extinct in succession, with little or no overlap in time. The eruptive centers are fed in sequence through conduits from buoyant, tightly focused magma plumes. The near identical heights of many of the centers at the time of their latest eruptions suggest that the seamounts tap the same mantle depth range. Broad summit plateaus with calderas are formed if the plume volume is such that the hydraulic head is reached and magma is trapped within the volcano in ephemeral magma chambers or conduits. If the plume volume is small and the hydraulic head is not reached, the seamount remains small with only a small crater. The plumes ascend asynchronously from a common region of nucleation in the asthenosphere. Plumes that follow in close succession build a composite seamount with a string of summit calderas. Plumes that rise less often result in a line of discrete circular volcanoes.

A two‐ and three‐dimensional analysis of gravity anomalies associated with the East Pacific Rise at 9°N and 13°N

We have used a unique data set collected during the 1985 East Pacific Rise multichannel seismic experiment to reevaluate the constraints that gravity data can place on the crustal structure of the East Pacific Rise (EPR). The close spacing of track lines within the two main survey areas (8°45′N‐9°55′N and 12°20′‐13°30′N) allowed us to perform three‐dimensional analyses of high‐quality gravity data (+/−1 mGal uncertainity) obtained with the Bell Aerospace BGM‐3 gravity meter. our gravity modeling was enhanced by the availability of high‐resolution Sea Beam bathymetric data and by independent structural constraints provided by seismic reflection and refraction data. To model the crustal and upper mantle density structure at the ridge axis, we first calculated the gravity anomalies due to the density contrasts at the water/crust and crust/mantle boundaries and the changes in density caused by the cooling of the lithosphere with age and subtracted these predictable signals from the observed free‐air anomaly. The residual anomalies were then used to place constraints on the magnitude and distribution of anomalous mass at the EPR. Our results show that over 90% of the power in the observed free‐air anomaly can be modeled by these predictable components of the gravity signal. The amplitude and wavelength of the small residual anomaly can be modeled by a broad region (∼20 km wide), centered on the rise axis, of slightly lower‐than‐normal crustal and/or upper mantle densities (−0.03 Mg m−3). This region of anomalous mass and density variations in the underlying mantle provide the principle isostatic support for the axial topographic high. The anomalies are consistent with, but do not require, the presence of a low‐density, crustal magma chamber at the rise axis. If a largely molten magma chamber exists, then the gravity data require that it be a narrow, volumetrieally small body. There are first‐order differences in the distribution of anomalous mass between the two survey areas with the 9°N region generally being characterized by lower densities in the axial region. Along‐axis variations in anomalous mass within each of the survey areas correlate with first‐order changes in the depth and cross‐sectional shape of the EPR axial high. We attribute these differences in axial morphology and the amplitude of the associated gravity anomalies to along‐strike changes in the size of the crustal magma chamber, the width of the surrounding zone of cooling crustal rocks, and densities in the uppermost mantle beneath the rise axis.

Axial and off‐axial heterogeneity of basaltic rocks from the East Pacific Rise at 12°35′N–12°51′N and 11°26′N–11°30′N

Surface ship operations and 40 submersible dives have provided a large amount of field observations and rock samples from two segments of the EPR at 12°35′N–12°51′N and at 11°26′N–11°30′N and from four adjacent seamounts centered at less than 18 km from the rise axis. The basaltic samples show a great variety of morphological features and a diversity in the proportions of early formed mineral phases and chemical composition. There is no correlation between lava flow morphology, geological settings, and compositions. Based on mineral and compatible element composition, these basalts were classified into three types. The least evolved olivine‐basalts and highly phyric plagioclase‐basalts (type A) have high Fo87–89, high Mg # (66–70), and high Ni (>100) contents. The most evolved plagioclase‐olivine basalts with or without pyroxene (type B and C) have relatively low Fo78–86, low Mg # (55–65), and low Ni (60–100 ppm) contents. Simple crystal fractionation (3–18%) accounts for some of the observed compositional range. However, based on their incompatible element contents, the above basalts are divided into three groups (the various types A to C are found in each group): depleted basalts with low K/Ti (0.04–0.15) ratios, low Zr (<100 ppm) and low Nb (<0.4 ppm) contents, undepleted basalts with the highest K/Ti (0.25–0.46), Zr (150–170 ppm), and Nb (8–16 ppm) contents and the transitional basalts having compositions between these two extremes. These three basaltic groups occur within limited portions of the axial graben and its limbs in areas of less than 10 km in length and less than 3–4 km in width near 12°43′N–12°51′N on the EPR. Similar volcanic diversity in a more limited area (<3 km2) is observed on off‐axial seamounts and other constructional features centered up to 6 and 18 km from the rise axis. Depleted and undepleted melts are believed to have been formed during sequential batch melting of a composite mantle. The transitional basalts probably result from a mixing of depleted and undepleted liquids which have undergone variable degrees of crystal fractionation. Two distinct mantle sources are distinguishable by their incompatible element contents and their susceptibility to melting; we suggest that a lherzolitic mantle, with minerals having a variable behavior under partial melting, constitutes this type of composite mantle. Indeed, clinopyroxene, enriched in incompatible elements, will be consumed at an earlier stage of melting than the olivine plus orthopyroxene. Eruption of compositionally distinct magma batches in close proximity on axial and off‐axial structures implies periods of quiescence between successive magmatic stages and cycles.

Strontium, neodymium and lead isotope constraints on near-ridge seamount production beneath the South Atlantic

STUDIES1–7 of seamounts near the East Pacific Rise (EPR) have shown that, although most seamount lavas are petrographically and chemically identical to mid-ocean-ridge basalt, they are chemically and isotopically more diverse than those erupted on the rise axis. They are also generally more primitive (higher MgO content) and, in some cases, more depleted in incompatible elements than the axial basalts. This indicates that although near-ridge sea-mounts and the EPR share a common mantle source, there must be significant differences in their magma-supply processes. Sea-mounts are probably built by small batches of melt that rise rapidly to the surface, preserving evidence of heterogeneity in the mantle source region. By contrast, the processes of melting and melt segregation, storage and extrusion along the fast-spreading (9–13 cm yr–1) EPR are more efficient in mixing and homogenizing basalt compositions1–7. Here we show that the relationship of strontium, neodymium and lead isotope data between seamounts in the South Atlantic and the nearby axis of the slow-spreading Mid-Atlantic Ridge (MAR) is similar to that seen in the Pacific. This indicates that the processes leading to formation of near-ridge seamounts are similar at a wide range of spreading rates. Differences in the specific isotope signatures of lavas from near-ridge seamounts and axes of the EPR and MAR reflect regional differences in the upper-mantle source of mid-ocean-ridge basalts.