Magmatic Budget Research Articles

Magma budgets and steady‐state activity of Vulcano and Stromboli

We present three models for magma budget during steady‐state volcanic activity, by which non‐erupted magma is emplaced as dykes or cumulates or crystallises in place. Using gas and thermal data we apply our models at Vulcano to calculate degassing, cooling and crystallisation of magma at a rate of 40–375 kg s−1 within a magma body with an upper surface at a ∼2 km depth. At Stromboli we calculate a steady magma supply of 300–1300 kg s−1 to shallow (<1 km) depths.

The ultrafast East Pacific Rise: instability of the plate boundary and implications for accretionary processes

The Pacific–Nazca plate boundary evolves continuously through the frequent, rapid propagation of ridge segments and through the growth or abandonment of microplates. Propagation events can initiate at overlapping spreading centres only a few kilometres wide as well as within large transform faults. This instability of the ultrafast East Pacific Rise (EPR) probably results from the presence of a hot, thin lithosphere in the axial region, coupled with a melt supply that may be temporally or spatially variable. It indicates that along–axis magma transport can be efficient at rates corresponding to propagation rates, up to 1000 mm yr -1 . To a first–order, the tectonic segmentation of the ridge correlates with along–axis variations of the axial morphology and other physical parameters suggesting a diminished magmatic budget near offsets larger than a few kilometres. A similar correlation between axial segmentation and variations in physical characteristics at the Mid–Atlantic Ridge (MAR) is commonly interpreted to indicate that mantle upwelling is focused near mid–segment at slow–spreading ridges (three dimensional). Accordingly, mantle upwelling may be focused at discrete intervals along the ultrafast EPR. However, fluctuations of the along–axis characteristics are considerably more subdued at the EPR than at the MAR. This has been interpreted to reflect smoothing of the structural variations by efficient transport within the shallow crust and upper mantle of the material brought up through focused upwelling. Alternatively, it has been argued that mantle flow is essentially uniform along–axis (two dimensional) at the faster spreading centres. It is proposed here that the actual pattern of mantle flow along the EPR combines aspects of both models. Fast spreading centres may be supplied by vertical mantle flow nearly continuously along–axis, but the intensity of this upwelling can fluctuate both temporally and spatially, hence favouring along–axis transport away from the magmatically most robust areas.

Hydrothermal methane and manganese variation in the plume over the superfast-spreading southern East Pacific Rise

Onboard analyses of dissolved CH4 and Mn were conducted on the water column along the southern East Pacific Rise between 13 °50′ and 18°40′S during the 1993 RIDGE FLUX Cruise. Many hydrothermal plumes were identified by chemical anomalies which overlie more than half of the surveyed ridge crest. The CH 4/Mn ratio in the plumes shows wide variability ranges from about 0.05 to 3.9 due to wide range of CH 4 concentration. The CH 4-rich plumes were detected in the region of 16°00′—18°40′S where vigorous magmatic budget was confirmed by previous studies. Moreover, the CH 4/Mn ratio of the plumes is highly correlated with other volatile/metal parameters such as the S/Fe ratio in plume particulate. These results strongly suggest that volatile input caused by magmatic perturbation to hydrothermal systems is responsible for the CH 4/Mn variability in the southern EPR plumes, based on analogy of the case at the EPR 9°50′N site. The detection of several volatile-rich plumes and the extensive hydrothermal flux of volatile species are attributed to frequent magmatic events in the superfast spreading southern EPR.

Along-axis magmatic oscillations and exposure of ultramafic rocks in a second-order segment of the Mid-Atlantic Ridge (33°43'N to 34°07'N)

New in situ observations on the Mid-Atlantic Ridge demonstrate that the along- and across-axis volcano-tectonic variability within second-order segments is larger than commonly acknowledged. In our study area, the segment center, far from being the place with present-day maximum volcanic activity, is dominated by extensive sedimentary cover, fissuring, and faulting. Furthermore, the area of most recent magmatic activity is located away from the segment center in a region of greater depth and thinner crust. Segment-tip magmatic oscillations are suggested by the distribution of rock types at both segment ends. Serpentinized peridotites and associated dolerites are exposed at the massifs located at the intersection with nontransform discontinuities (NTDs), whereas only basaltic rocks crop out on the nodal basin floors. We suggest that the combination of low magmatic budget and extension taking place at the NTDs during a segment retreat favors the uplift and exposure of ultramafic massifs.

Formation of large sulfide mineral deposits along fast spreading ridges. Example from off-axial deposits at 12°43′N on the East Pacific Rise

Submersible investigations with Nautile near the 13°N hydrothermal field on the EPR were made in 1992. We selected diving areas based on deep towed side scan sonar imaging. Close to a pit-crater at the top of a young seamount 2 km east of the axial ridge we discovered one of the largest (70 m high and 200 m in diameter) sulfide mounds formed at a fast spreading ridge (12 cm/yr). Despite an overlap in composition between axial and off-axial deposits the general trend reflects real differences in compositions. Off-axis deposit differs from axial deposits by the scarcity of zinc sulfides and cobalt and selenium enrichment. Sulfur isotope variation is minimal for the off-axis seamounts and suggests that sulfides precipitated from unmodifed end-member hydrothermal solutions. Axial sulfides have a broader range due to reduction of seawater sulfate at mixing fronts between hydrothermal and seawater solutions along the axial graben fault system. Lead isotopic compositions of sulfides are in two overlapping but distinct fields indicating different Pb sources and separate convective systems for the seamount and the axial deposits. Our observations suggest that the plumbing system is stable and centred on a shallow localised magma chamber under the seamount. This configuration is by far more efficient than unstable axial processes to create, during several hydrothermal episodes, major sulfide deposits near a fast spreading ridge. Off-axis seamounts are localised areas with high magmatic budget able to drive and focus hydrothermal systems at the same place for a long time. Other known occurrences of sulfides related to seamounts are at less than 11 km from the ridge axis. Thus we suggest that off-axial volcanoes close to the ridge are first order targets to discover active or inactive large deposits along fast- to medium-spreading ridges.

Anomalous opening of the Equatorial Atlantic due to an equatorial mantle thermal minimum

The geology of the Equatorial Atlantic is dominated by a broad east-west megashear belt where a cluster of large fracture zones offsets anomalously deep segments of the Mid-Atlantic Ridge (MAR). The origin and evolution of this megashear region may lie ultimately in an equatorial mantle thermal minimum. The notion of a mantle thermal minimum in the Equatorial Atlantic is supported by an equatorial minimum of zero-age topography, a maximum in mantle shear waves seismic velocity and a minimum in the degree of melting, indicated by the chemistry of MAR basalts and peridotites. This thermal minimum has probably been a stable feature since before the Cretaceous separation of Africa from South America; it caused a pre-opening equatorial continental lithosphere thicker and colder than normal. The Cretaceous Benue Trough in western Africa and the Amazon depression in South America are interpreted as morphostructural depressions created or rejuvenated by strike-slip, transpressional and transtensional tectonics ducing extension of the cold/thick equatorial lithosphere. The oceanic rift propagating northward from the South Atlantic impinged against the equatorial thicker, colder and, therefore, stronger than normal continental, lithosphere that consequently acted as a ‘locked zone’. This, and a low magmatic budget due to the cold upper mantle, caused a lower than normal rate of propagation of the oceanic rift into the equatorial belt, with diffuse deformation during mostly amagmatic extension. The thick/cold lithosphere prevented major Cretaceous igneous activity from the St. Helena plume. Eventually initial ‘weak’ isolated nuclei oceanic lithosphere were emplaced, separated by E-W continent/continent transforms. Opening occurred largely by strike-slip motion along these initial transforms. The consequences were that the Equatorial Atlantic opened prevalently along an E-W direction, in contrast to the N-S opening of the North and South Atlantic, and that sheared continental margins are particularly well developed in the Equatorial Atlantic. After further continental separation the cold equatorial mantle caused a low degree of melting (with Na-rich MORB and alkali basalt rather than normal MORB and with undepleted mantle peridotities), thin crust, depressed ridge segments and a prevalence of amagmatic extension. Similar conditions still exist today. Long transforms offsetting short ridge segments kept sea floor spreading unstable and dominated by transform tectonics, with transform migration, transpression, and transtension causing strong vertical motion, emersion and subsidence of lithospheric blocks, development of deep pull-apart basins, and preservation of relict slivers of old lithosphere (occasionally even of continental lithosphere) within younger crust. The equatorial transforms are caused ultimately by a long lived thermal minimum in the upper mantle and not vice versa; however, they then create second-order ‘rebound’ thermal effects that help maintain the thermal minimum in the upper mantle. It can be speculated that mantle thermal minima at the Earth's equator might be related to true polar wander triggered by subduction of dense masses into the mantle.

Geological indexes of hydrothermal venting

Hydrothermal venting occurs on mid‐ocean ridge axes with a diverse array of morphological, structural, and petrological characteristics. Consequently, it is not clear if certain geological environments are more conducive than others to hydrothermal activity. This paper uses complementary data sets recently collected on multiple‐tectonic‐segment scales along intermediate to superfast spreading ridge sections to systematically examine this issue. The spatial density of venting is gauged by plume incidence, the fraction of linear ridge crest overlain by hydrothermal plumes. Five geological indexes are compared with plume incidence: ridge‐axis elevation, ridge‐axis cross‐sectional area, the percentage of ridge axis underlain by an axial magma chamber (AMC) reflector, MgO wt% in basaltic glass, and spreading rate. A mean value for plume incidence and each index, where available, is calculated for each of 14 second‐ to fourth‐order tectonic segments from the Juan de Fuca Ridge (44°30′ to 48°45′N) and the northern (9° to 11°50′N) and southern (13°50′ to 18°40′S) East Pacific Rise. Plumes are most common on segments with a cross‐sectional area >3.5 km2, a net elevation >0.35 km, AMC coverage >60%, and MgO >7 wt%. Spreading rate is not deterministic on the segment scale. The data suggest two classes of ridge segments. Segments with relatively low geological index values have a uniformly low plume incidence indicative of a presently feeble magmatic budget. Those with high index values have a plume incidence ranging from low to complete coverage, apparently a consequence of a magma supply rate sufficient to produce frequent dike intrusions and highly variable hydrothermal activity. The best individual predictors of hydrothermal activity are cross‐sectional area and net elevation. Applying these indexes to the East Pacific Rise between 18°N and 33°S suggests that the unexplored segments with the highest probability of activity are those at 15°30′–16°N, 6°–9°N, 5°–6°S, 27°–28°S, and 30°30′–33°S.

The effects of spreading rate, the magma budget, and the geometry of magma emplacement on the axial heat flux at mid‐ocean ridges

We explore two‐dimensional, steady state, flow and thermal models of oceanic spreading center structure. These models include the effects of hydrothermal heat transport and crustal accretion for two basic accretion geometries: (1) a lens model where all crust below the sheeted dike layer passes through a magma lens at the base of the dike layer before subsiding and flowing to deeper levels; and (2) a dike model where crust is emplaced in a vertical slot from the surface to Moho and subsequently moves horizontally (rigidly) away from the axis of accretion. The axial heat flux resulting from these two end‐member accretion geometries is compared, and an assessment is made of the feasibility of using various observations to differentiate between these two accretion geometries. We find that the steady state axial heat flux is predominantly influenced by three factors: the spreading rate, the magmatic budget (crustal thickness) and the efficiency of hydrothermal cooling. The total steady state heat flux from the neovolcanic zone (2 km wide) increases almost linearly with the spreading rate for both the dike and lens accretion geometries. The major difference between the dike and the lens models is nonthermal: they predict different accumulated strain distributions within off‐axis crust. Crustal flow due to crustal accretion within a crustal‐height “dike” leads to little accumulated strain, while intense crustal strain results from crustal subsidence and flow below a steady state magma lens. Ophiolite and marine seismic observations of crustal layering appear to be the strongest observational tests to discriminate between these two accretion geometries; they currently favor a lens‐like model of lower crustal accretion.

Long-lived oceanic transform boundaries formed above mantle thermal minima

The causes of segmentation of mid-ocean ridges by long-lived transform boundaries are poorly understood. Large (>200 km) offsets of the Mid-Atlantic Ridge are distributed neither regularly nor randomly, but are mostly clustered in the equatorial region, where they can be traced outside the active transform to South American and African sheared continental margins. This permanent cluster of transforms is located above a long-lived upper mantle thermal minimum. Permanent transforms at 50°–55°N and perhaps at 45°–50°S are associated with mantle secondary thermal minima. A cluster of transforms disrupts the mid-ocean ridge above a mantle thermal minimum at the Australian-Antarctic discordance. Conversely, “hot” stretches of the Mid-Atlantic Ridge are devoid of major transforms. Low magmatic budget and high-strength rheology of cold, thick lithosphere above cold mantle caused the formation of weak and unstable oceanic rift segments during the opening of the equatorial Atlantic and Australian-Antarctic oceans, favoring the development of initial transform clusters and maintaining them as permanent structural and/or geochemical boundaries. Thus, some long-lived transform clusters may be caused ultimately by upper mantle thermal minima, and not vice versa; thermal anomalies created in the mantle by these transforms (“transform cold edge effect”) are second-order “rebound” effects.

A three‐dimensional gravity analysis of the East Pacific Rise from 18° to 21°30′S

Multibeam bathymetry and gravity coverage of the East Pacific Rise (EPR) between 18° and 21°30′S is used to investigate the relation between melt supply and tectonic segmentation at ultrafast spreading rates. The long‐wavelength features in the residual anomaly show a good correlation with those in the bathymetry. The highest residual anomaly values occur over the broad discordant zone of the 20°40′S overlapping spreading center (OSC), for seafloor ages of 0 Ma to at least 1.5 Ma. We interpret the deepening of the bathymetry and the increase of the residual anomaly toward that discordant zone as due to a decrease of 500 plusmn;200 m of the crustal thickness. Hence the 20°40′S OSC has been associated with a reduced magmatic budget for at least the past 1.5 m.y. and represents a persistent segmentation of the EPR. This is consistent with models in which mantle upwelling, even at the fastest spreading centers, is enhanced between large discontinuities rather than evenly distributed along axis. However, this decrease of the crustal thickness toward an axial discontinuity is several times smaller than that typically documented for slow spreading ridges, which suggests that mantle upwelling is less focused at fast spreading ridges, or that along‐axis transport of crustal material is more efficient, or both. Across the study area, the residual anomaly decreases toward the NW by 15–20 mGal. This regional gradient can be modeled with lateral temperature variations in the upper mantle of up to 60°C, increasing toward the NW. This interpretation is consistent with the numerous seamounts present to the NW and the robust magmatic budget of the ridge between 17° and 18°S, and it could also explain why the ridge segments defined by the smaller OSCs between 18° and 19°S propagate very rapidly away from the robust area. Similar patterns of ridge propagation away from the shallowest section of a ridge have been documented near the Galapagos and Easter Island hot spots. Hence these shorter ridge segments may not be associated with significant individual melt sources. Rather, they may represent a superficial segmentation due to the interaction between the EPR and a mantle heterogeneity located between 17° and 18°S.

Seismic Images of Active Magma Systems Beneath the East Pacific Rise Between 17°05′ and 17°35′S

Seismic reflection data from the East Pacific Rise between 17 degrees 05' and 17 degrees 35'S image a magma lens that varies regularly in depth and width as ridge morphology changes, confirming the notion that axial morphology can be used to infer ridge magmatic state. However, at 17 degrees 26'S, where the ridge is locally shallow and broad, the magma lens is markedly shallower and wider than predicted from regional trends. In this area, submersible dives reveal recent volcanic eruptions. These observations indicate that it is where the width and depth of the magma chamber differ from regional trends, indicating an enhanced magmatic budget, that is diagnostic of current magmatism.

Hydrothermal plumes along the East Pacific Rise, 8°40′ to 11°50′N: Plume distribution and relationship to the apparent magmatic budget

The interactions between hydrothermal circulation and large-scale geological and geophysical characteristics of the mid-ocean ridge cannot be ascertained without large-scale views of the pattern of hydrothermal venting. Such multi-ridge-segment surveys of venting are accomplished most efficiently by mapping the distribution and intensity of hydrothermal plumes. In November 1991, we mapped hydrothermal temperature ( Δθ) and light attenuation ( Δc) anomalies above the East Pacific Rise (EPR) continuously from 8°40′ to 11°50′N, a fast spreading ridge crest portion bisected by the Clipperton Transform Fault. Plume distributions show a precise correlation with the distribution of active vents where video coverage of the axial caldera is exhaustive (9°09′–54′N). Elsewhere in the study area the sketchy knowledge of vent locations gleaned from scattered camera tows predicts only poorly the large-scale hydrothermal pattern revealed by our plume studies. Plumes were most intense between 9°42′ and 9°54′N, directly over a March/April, 1991, seafloor eruption. These plumes had exceptionally high Δc Δθ ratios compared to the rest of the study area; we suggest that the phase-separated gas-rich vent fluids discharging here fertilize an abundant population of bacteria. Hydrothermal plume distributions define three categories: intense and continuous (8°48′–8°58′N, 9°29′–10°01′N and 11°05′–11°27′N), weak and discontinuous (8°58′–9°29′N) and negligible. The location of each category is virtually congruent with areas that are, respectively, magmatically robust, magmatically weak and magmatically starved, as inferred from previous measurements of axial bathymetric undulations, cross-axis inflation and magma chamber depth and continuity. This congruency implies a fine-scale spatial and temporal connection between magmatic fluctuations and hydrothermal venting. We thus speculate that, at least along this fast spreading section of the EPR, cyclic replenishment, eruption and freezing of the thin axial melt lens exerts greater control over hydrothermal venting than the more enduring zones of crystal mush and hot rock. We found intense, and continuous, plumes along 33% of the surveyed ridge crest, an observation implying that any point on the ridge is, on average, hydrothermally active one-third of the time. Combining this result with the 20% plume coverage found along the medium-rate Juan de Fuca Ridge suggests that superfast ( ∼ 150 mm/yr) spreading ridges should support vigorous venting along ∼ 50% of their length, if spreading rate and along-axis plume coverage are linearly related.

Correction to “Variation in cross‐sectional area of the axial ridge along the East Pacific Rise: Evidence for the magmatic budget of a fast spreading center” by Daniel S. Scheirer and Ken C. Macdonald

Correction to “Variation in cross‐sectional area of the axial ridge along the East Pacific Rise: Evidence for the magmatic budget of a fast spreading center” by Daniel S. Scheirer and Ken C. Macdonald

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.

Hydrothermal venting and the apparent magmatic budget of the Juan de Fuca Ridge

The fact that vigorous hydrothermal discharge occurs on ridge segments with a variety of spreading rates and morphological characteristics makes it difficult to discern the large‐scale geological controls on the distribution of hydrothermal venting along the mid‐ocean ridge. We undertook to simplify this problem by mapping the distribution of hydrothermally produced thermal and light attenuation anomalies along the crest of the Juan de Fuca Ridge, a medium‐rate spreading center with segments of markedly differing morphology. Hydrothermal anomalies were detected by towing a high‐precision conductivity‐temperature‐depth‐transmissometer (CTDT) package in a sawtooth pattern through the lowermost 1000 m of the water column above each segment. These surveys, conducted between 1985 and 1989, have produced the first quantitative and continuous visualizations of hydrothermal plumes along a multisegment portion of the mid‐ocean ridge. Hydrothermal discharge is strongest on those segments, or portions of segments, where the apparent magmatic budget is highest, as indicated by the degree of along‐axis inflation and other morphological characteristics. This correspondence implies that magma is episodically delivered in small and spatially discontinuous bodies of melt rather than by the constant resupply of a steady state magma chamber. We propose that the probability of discharge occurring at any point on the mid‐ocean ridge is proportional to the spreading rate. This probability, scaled by the observation that significant plumes presently emanate from ∼20% of the Juan de Fuca Ridge, is expected to range from ∼5% on the slowest‐spreading ridge to ∼50% on the fastest‐spreading ridge.

Crustal structure at the Blake Spur Fracture Zone from expanding spread profiles

We present results from WKBJ and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles (ESPs), along flow lines parallel to the Blake Spur fracture zone across 140 Ma Atlantic crust. These profiles provide detailed constraints on variations in crustal structure at the fracture zone. Seven profiles on either side of the fracture zone show a normal structure for old oceanic crust. A 2–3 km thick upper layer with a steep velocity gradient is underlain by 4–5 km of crust with velocities of 6.5–7.2 km/s and low gradient and a sharp transition to upper mantle velocities of 8 km/s. Several velocity discontinuities were detected within the upper 2–3 km, but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles. This structure shows little regional variation toward the fracture zone. Despite the small magnetic anomaly offset (∼12 km) and the indistinct topographic signature of the fracture zone, three ESPs within a 10–20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure, relative to normal oceanic crust. A 2–4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7.2–7.6 km/s and high Poisson's ratio (probably at least 0.29), which is consistent with 15–30% serpentinization of upper mantle peridotites. This interpretation requires the action of off‐axis hydrothermal circulation, penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km. The original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent “normal” crust, which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments.

The axial summit graben and cross-sectional shape of the East Pacific Rise as indicators of axial magma chambers and recent volcanic eruptions

The axis of the East Pacific Rise (EPR) undulates up and down hundreds of meters over distances of 30–200 km along strike, the deep areas occurring at transform faults and other ridge axis discontinuities such as overlapping spreading centers (OSCs). We have suggested that systematic variations in depth and cross-sectional shape of the rise are indicators of the changes in the local axial magmatic budget along a given ridge segment [1]. A comparison of recently collected multichannel seismic (MCS) data [2] with our Sea Beam and SeaMARC II data have allowed us to test and advance this hypothesis. Along the EPR from 9° to 13°N there is an excellent correlation between three parameters that are all directly related to the phase of a magmatic cycle along a given ridge segment: the cross-sectional shape of the rise, the presence or absence of an axial summit graben, and the presence or absence of a shallow axial magma chamber (as interpreted from MCS data). Where the axial magma chamber is present, the cross-sectional shape of the ridge is broad and an axial summit graben is recognized along the axis. In contrast, where the cross-sectional shape of the rise is narrow and triangular, an axial magma chamber is not detected and an axial summit graben is absent. These ridge axis characteristics tend to occur along deeper portions of a given ridge segment, often near ridge axis discontinuities. We suggest that these systematic variations in ridge axis morphology (cross-sectional shape) and structure (presence or absence of an axial graben) reflect spatial and temporal variations in the magmatic budget of the ridge axis. Where the magmatic budget is waxing, shallow-level magma reservoirs in the crust and underlying upper mantle swell, creating a broad axial bulge with a summit graben. Where the magmatic budget is diminished, the crustal magma chamber is small (< 2 km wide) or absent and the ridge axis is characterized by a narrow triangular edifice that lacks a clearly defined and continuous axial graben. A summit graben is missing because the underlying axial magma reservoir is not large enough to produce a significant collapse structure or a caldera. This proposed correlation of shape, structure and magmatic parameters fails along only two short portions of the ridge. In these areas there is evidence for an axial magma chamber and the rise has a broad cross-sectional shape, but there is no summit graben. Bottom photographs and submersible results, however, show that in these areas the rise crest is covered with very fresh lavas undisrupted by faulting, suggesting that the summit graben has been recently filled in by lava flows, and the development of a summit graben (or a linear caldera) by volcano-tectonic collapse has not yet occurred.

Deep crustal structure and magmatic processes: the inception of seafloor spreading in the Norwegian-Greenland Sea

Summary An extensive investigation into the deep crustal structure of the conjugate Norwegian and E Greenland margins utilizing two-ship Expanded Spread and Wide Aperture CDP Profiling has led to the identification of a unique suite of crustal structures developed during the onset of seafloor spreading. On both margins, a region of low velocity crust was found proximal to marginal escarpments. Immediately seaward of the low velocity crustal block seaward-dipping reflectors occur within thick crust that is interpreted to be wholly igneous, a product of voluminous melt production enhanced by convective partial melting processes. This earliest formed oceanic crust is actually several kilometres thicker than the low velocity crust or the adjacent continental crust. Normal oceanic crustal thicknesses are not produced until about 5 My after the initiation of spreading when convective partial melting has abated and the magma budget has reduced. The extrusive unit reduces to normal thicknesses long before the intrusive and plutonic section, indicating that the thickening or thinning of oceanic crust is accomplished primarily by variations in the latter.

Detachment faulting and amagmatic extension at mid-ocean ridges: The Josephine ophiolite as an example

Lithospheric extension at the Josephine paleo-spreading center occurred by a combination of magmatic and amagmatic processes. The amount of extension by amagmatic processes appears at least as large as that from magmatic processes. The structural processes responsible for amagmatic extension in the absence of a magma chamber appear to be normal faulting and block rotation in the brittle upper lithosphere and ductile flow in the lower lithosphere. An extensive detachment shear zone occurs beneath the fault blocks, approximately 1 km below the base of the crustal sequence. The amount of amagmatic extension can be approximated from a simple geometric model relating extension to rotations of sheeted dikes. Similarly, attenuated crustal thicknesses can be related to fault-block rotations. The results suggest that thin crustal sequences commonly observed in ophiolites and near fracture zones may result from attenuation during amagmatic extension. Furthermore, detachment faulting and block rotation may be characteristic of spreading centers where the magma budget is low.

Fine scale study of a small overlapping spreading center system at 12�54? N on the East Pacific Rise

Overlapping spreading centers (OSCs) are a type of ridge axis discontinuity found along intermediate and fast spreading centers. The ridges at these locations overlap and curve towards each other. and are separated by an elongate overlap basin. A high resolution Deep-Tow survey was conducted over the 12°54′ N OSC (offset ≈1.6 km) on the East Pacific Rise in order to study the structure of a small OSC on a fine scale. A detailed tectonic study and Deep-Tow 3-D magnetic inversion were performed on the data. Towards the tips of both limbs, the apparent age of lava flows increases, the density of exposed faults and fissures increases, and the axial graben loses definition and disappears. No active hydrothermal vents were detected in the area. These observations suggest that the magmatic budget steadily decreases along axis approaching and OSC, even where the offset is small. In contrast with OSCs which have a large offset (>5 km), the 3-D magnetic inversion solution for this OSC produced no evidence for highly magnetized areas near the tip of either spreading center.