Showing papers on "Mid-ocean ridge published in 1994"

Constraints on hydrothermal heat flux through the oceanic lithosphere from global heat flow

Abstract: A significant discrepancy exists between the heat flow measured at the seafloor and the higher values predicted by thermal models of the cooling lithosphere. This discrepancy is generally interpreted as indicating that the upper oceanic crust is cooled significantly by hydrothermal circulation. The magnitude of this heat flow discrepancy is the primary datum used to estimate the volume of hydrothermal flow, and the variation in the discrepancy with lithospheric age is the primary constraint on how the hydrothermal flux is divided between near-ridge and off-ridge environments. The resulting estimates are important for investigation of both the thermal structure of the lithosphere and the chemistry of the oceans. We reevaluate the magnitude and age variation of the discrepancy using a global heat flow data set substantially larger than in earlier studies, and the GDHI (Global Depth and Heat Flow) model that better predicts the heat flow. We estimate that of the predicted global oceanic heat flux of 32 x 10(exp 12) W, 34% (11 x 10(exp 12) W) occurs by hydrothermal flow. Approximately 30% of the hydrothermal heat flux occurs in crust younger than 1 Ma, so the majority of this flux is off-ridge. These hydrothermal heat flux estimates are upper bounds, because heat flow measurements require sediment at the site and so are made preferentially at topographic lows, where heat flow may be depressed. Because the water temperature for the near-ridge flow exceeds that for the off-ridge flow, the near-ridge water flow will be even a smaller fraction of the total water flow. As a result, in estimating fluxes from geochemical data, use of the high water temperatures appropriate for the ridge axis may significantly overestimate the heat flux for an assumed water flux or underestimate the water flux for an assumed heat flux. Our data also permit improved estimates of the 'sealing' age, defined as the age where the observed heat flow approximately equals that predicted, suggesting that hydrothermal heat transfer has largely ceased. Although earlier studies suggested major differences in sealing ages for different ocean basins, we find that the sealing ages for the Atlantic, Pacific, and Indian oceans are similar and consistent with the sealing age for the entire data set, 65 +/- 10 Ma. The previous inference of a young (approximately 20 Ma) sealing age for the Pacific appears to have biased downward several previous estimates of the global hydrothermal flux. The heat flow data also provide indirect evidence for the mechanism by which the hydrothermal heat flux becomes small, which has often been ascribed to isolation of the igneous crust from seawater due to the hydraulic conductivity of the intervening sediment. We find, however, that even the least sedimented sites show the systematic increase of the ratio of observed to predicted heat flow with age, although the more sedimented sites have a younger sealing age. Moreover, the heat flow discrepancy persists at heavily sedimented sites until approximately 50 Ma. It thus appears that approximately 100-200 m of sediment is neither necessary nor sufficient to stop hydrothermal heat transfer. We therefore conclude that the age of the crust is the primary control on the fraction of heat transported by hydrothermal flow and that sediment thickness has a lesser effect. This inference is consistent with models in which hydrothermal flow decreases with age due to reduced crustal porosity and hence permeability.

A geological model for the structure of ridge segments in slow spreading ocean crust

Abstract: First-order (transform) and second-order ridge-axis discontinuities create a fundamental segmentation of the lithosphere along mid-ocean ridges, and in slow spreading crust they commonly are associated with exposure of subvolcanic crust and upper mantle. We analyzed available morphological, gravity, and rock sample data from the Atlantic Ocean to determine whether consistent structural patterns occur at these discontinuities and to constrain the processes that control the patterns. The results show that along their older, inside-corner sides, both first-and second-order discontinuities are characterized by thinned crust and/or mantle exposures as well as by irregular fault patterns and a paucity of volcanic features. Crust on young, outside-corner sides of discontinuities has more normal thickness, regular fault patterns, and common volcanic forms. These patterns are consistent with tectonic thinning of crust at inside corners by low-angle detachment faults as previously suggested for transform discontinuities by Dick et al. [1981] and Karson [1990]. Volcanic upper crust accretes in the hanging wall of the detachment, is stripped from the inside-corner footwall, and is carried to the outside comer. Gravity and morphological data suggest that detachment faulting is a relatively continuous, long-lived process in crust spreading at <25–30 mm/yr, that it rnay be intermittent at intermediate rates of 25–40 mm/yr, and that it is unlikely to occur at faster rates. Detachment surfaces are dissected by later, high-angle faults formed during crustal uplift into the rift mountains; these faults can cut through the entire crust and may be the kinds of faults imaged by seismic reflection profiling over Cretaceous North Atlantic crust. Off-axis variations in gravity anomalies indicate that slow spreading crust experiences cyclic magmatic/amagmatic extension and that a typical cycle is about 2 m.y. long. During magmatic phases the footwall of the detachment fault probably exposes lower crustal gabbros, although these rocks locally may have an unconformable volcanic carapace. During amagmatic extension the detachment may dip steeply through the crust, providing a mechanism whereby upper mantle ultramafic rocks can be exhumed very rapidly, perhaps in as little as 0.5 m.y. Together, detachment faulting and cyclic magmatic/amagmatic extension create strongly heterogeneous lithosphere both along and across isochrons in slow spreading ocean crust.

Martian plate tectonics

Abstract: The northern lowlands of Mars may have been produced by plate tectonics. Preexisting old thick highland crust was subducted, while seafloor spreading produced thin lowland crust during Late Noachian and Early Hesperian time. In the preferred reconstruction, a breakup margin extended north of Cimmeria Terra between Daedalia Planum and Isidis Planitia where the highland-lowland transition is relatively simple. South dipping subduction occurred beneath Arabia Terra and east dipping subduction beneath Thatsis Montes and Tempe Terra. Lineations associated with Gordii Dotsum are attributed to ridge-parallel structures, while Phelegra Montes and Scandia Colles are interpreted as transform-parallel structures or ridge-fault-fault triple junction tracks. Other than for these few features, there is little topographic roughness in the lowlands. Seafloor spreading, if it occurred, must have been relatively rapid. Quantitative estimates of spreading rate are obtained by considering the physics of seafloor spreading in the lower C 0.4 g) gravity of Mars, the absence of vertical scarps from age differences across fracture zones, and the smooth axial topography. To the first order, the height of vertical scarps across fracture zones does not involve gravity. Crustal thickness at a given potential temperature in the mantle source region scales inversely with gravity. Thus, the velocity of the rough-smooth transition for axial topography also scales inversely with gravity. Plate reorganizations where young crust becomes difficult to subduct are another constraint on spreading age. Possible plate reorganizations, for example, the end of spreading through Alba Patera, occur when the ridge axis is far from the trench. That is, rapid plate motions are inferred to have placed young oceanic crust far from the ridge axis. The preferred full spreading rate 900 from the plate pole is 80 mm yr -1. Plate tectonics, if it occurred, dominated the thermal and stress history of the planet. A geochemical implication is that the lower gravity of Mars allows deeper hydrothermal circulation through cracks and hence more hydration of oceanic crust so that more water is easily subducted than on the Earth. Age and structural relationships from photogeology as well as median wavelength gravity anomalies across the now dead breakup and subduction margins are the data most likely to test and modify hypotheses about Mars plate tectonics.

Melting phase relations of an anhydrous mid‐ocean ridge basalt from 3 to 20 GPa: Implications for the behavior of subducted oceanic crust in the mantle

Abstract: High pressure melting experiments on an anhydrous abyssal tholeiite collected from the Mid-Atlantic Ridge have been conducted over the pressure interval 3 to 20 GPa to explore the fate of subducted oceanic crust in the mantle. The composition of the mid-ocean ridge basalt (MORB) is almost identical to the average basaltic layer of the oceanic lithosphere. The melting phase relations of the MORB are summarized as follows: (1) the liquidus temperature is about 1425°C at 3 GPa, and it rises almost linearly to above 1900°C at 10 GPa. The slope of the liquidus curve decreases slightly above 10 GPa. Nevertheless, it still has a positive slope with increasing pressure. At 20 GPa, the liquidus temperature is about 2200°C. (2) The liquidus phase changes from clinopyroxene to garnet above 3.5 GPa. (3) The solidus temperature rises almost linearly to 2100 °C at 20 GPa; consequently, the melting interval is slightly narrower at high pressures (<140°C at 5 GPa, <100°C at 20 GPa). (4) Silica minerals (coesite, stishovite) are stable near and below the solidus. (5) At shallow mantle conditions (2∼7 GPa), the liquidus temperature of the MORB is slightly lower than the solidus of the mantle material (peridotite, KLB-1); however, the solidus temperature of the MORB nearly equals or exceeds the liquidus temperature of the mantle material at about 16 GPa. The solidus temperature of the mantle material is expected to exceed the liquidus temperature of the MORB again at higher pressures where the liquidus phase in the peridotite system is majorite and/or modified spinel rather than olivine. In the ordinary thermal structure of the present mantle, subducted MORB is difficult to melt in the absence of volatiles because its dry solidus is much higher than the estimated mantle geotherm. However, in the ancient Earth, when the mantle was supposedly much hotter than today, complete or partial melting of subducting MORB was quite probable and this melting process could have played a significant role in controlling the movements of subducted slab and the evolution of the mantle and continental crust.

Tomographic imaging of the shallow crustal structure of the East Pacific Rise at 9°30′N

Abstract: Compressional wave travel times from a seismic tomography experiment at 9°30′N on the East Pacific Rise are analyzed by a new tomographic method to determine the three-dimensional seismic velocity structure of the upper 2.5 km of oceanic crust within a 20×18 km2 area centered on the rise axis. The data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber. A careful analysis of source and receiver parameters, in conjunction with an automated method of picking P wave onsets and assigning uncertainties, constrains the prior uncertainty in the data to 5 to 20 ms. The new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models. The nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function, horizontal and vertical smoothing constraints, and prior model assumptions; all constraints applied to model perturbations are normalized to remove bias. We use the tomographic method to reject the null hypothesis that the axial seismic structure is two-dimensional. Three-dimensional models reveal a seismic structure that correlates well with cross- and along-axis variations in seafloor morphology, the location of the axial summit caldera, and the distribution of seafloor hydrothermal activity. The along-axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle-derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length. The seismic data are compatible with a 300- to 500-m-thick thermal anomaly above a midcrustal melt lens; such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years. Asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis. These anomalies may be the result of off-axis magmatism; alternatively, the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling.

Small-scale spatial and temporal variations in mid-ocean ridge crest magmatic processes

Abstract: Data from a suite of closely spaced lava flows recovered within the axial summit caldera and on the crestal plateau of the East Pacific Rise around lat 9°31′N indicate that eruptions on this fast-spreading part of the mid-ocean ridge occur throughout the crestal region and are not restricted to the axis. These eruptions contribute to a complex distribution of basalts of various ages and a significant thickening of seismic layer 2A away from the axis in our study area. Small-scale (<600 m) diversity and nonsystematic distribution of lava types may reflect rapid changes in magma chemistry that occur during crystallization and replenishment in small magma lenses, coupled with the effects of frequent low-volume eruptions both within and outside of the axial summit caldera.

Dynamic basis of volcanic spreading

Abstract: The established models of the structure and dynamics of a “typical” Hawaiian volcano lead inevitably to a paradox: finite element viscoelastic calculations, using TECTON [Melosh and Raefsky, 1980, 1981], predict that a compressional stress field should characterize their upper flanks; instead, extension is observed. The paradox is solved by postulating that volcanic evolution is determined by feedback processes related to growth and spreading of the volcanic edifice. Many examples of the process of spreading are documented in the geologic literature on volcanoes of a very broad size range and very different geologic settings. The process is similar across many orders of magnitude in size: the deformation of the substratum related to magma intrusion during lava dome building events is at the lower end of the scale; the spreading of seafloor at oceanic ridges is at the higher end of the scale. Volcanoes may evolve from small constructs into huge shield volcanoes and perhaps into ocean plates. Key elements for volcanic spreading are (1) the existence of a weak basal layer and (2) a sufficiently high mass and magma influx to drive the process. As the mass of a volcano grows, it passes through five phases (which may overlap, repeat, or be omitted): building, compressing, thrusting, intruding, and spreading. During the building phase the mass of a volcano does not significantly affect the stress field of the substratum; primitive magmas are likely to be erupted. As the mass increases, the volcano subsides and becomes characterized by a compressional stress field which inhibits any further intrusion of magma into the volcanic edifice; magma may differentiate in the crust, allowing for large caldera-forming eruptions to occur. In the next phase, thrusting begins on a decollement usually located at the foot of the edifice; while the stress field on the upper slopes of the volcano remains compressional, the stress at the base, close to the center, becomes extensional, allowing the magma to intrude and differentiate at the base of the edifice; a basal intrusive complex begins to grow; explosive eruptions may be expected. Finally, spreading of this complex creates an extensional stress field on the upper flanks of the volcano, confining compression to the lower slopes of the edifice; the extension allows primitive magma to erupt again at the summit. The process of spreading seems to have a fundamental influence on the structural and magmatic evolution of volcanoes. At every scale, this process results from coupling and feedback between the gravitational and thermal fields. Thus studying this process may improve our understanding of the relation between structural dynamics and magma evolution and of the origin and evolution of volcanic seismicity and eruptions.

The influence of regional stress and magmatic input on styles of monogenetic and polygenetic volcanism

Abstract: A new model is proposed to relate the development of monogenetic and polygenetic volcanoes to magmatic input and regional stress in the lithosphere. Output-stress diagrams relate magmatic output rate and crustal deformation rate (or strain rate). The magmatic output rates of polygenetic volcanoes and monogenetic volcano fields, including lava fields and central (axial) volcanoes on the mid-oceanic ridge systems, are estimated. Crustal deformation rates obtained from the literature are used as indicators of differential stress (Δσ). These data are normalized over time periods of 104 years, and over areas of 103 km2. The ratio of output rate to input rate, inferred from ophiolite sections, and from crustal deformation around volcanoes, is about 0.1 to 0.3. Based on the average ratio, an input-stress diagram may be obtained at some volcanoes. Polygenetic volcanoes are plotted in the region of both larger output rate and smaller Δσ. Monogenetic volcano fields are plotted on a rift trend, or in the region of a small output (≤1 km3/104 yr/103 km2). In some regions, polygenetic volcanoes and monogenetic volcanoes coexist, for example, during 0.1 m.y. periods, and within the area of 30 km × 30 km. The output-stress diagram indicates that coexistence of these volcano types results from variation in differential stress, or variation in production rate of magma in the mantle at different levels over periods of time less than 0.1 m.y. This is supported using extrusive volumes and crustal deformation data from the eastern volcanic zone, Iceland, Taupo Volcanic Zone, New Zealand, TransMexican Volcanic Belt, Mexico, and the volcanic field on and around the Izu peninsula, Japan. The observed relationship between deformation rate, magmatic output rate, and volcano type supports the crack interaction theory of magma-filled cracks. The output-stress diagram also indicates that the balance between local stress induced by magma accumulation and regional stress, and stress relaxation, govern the structure of a volcano.

Superplumes or supercontinents

Abstract: I attribute the Cretaceous pulse of Pacific oceanic crust formation to a global plate reorganization associated with the breakup of Pangea and rapid growth and reorganization of the Pacific plate. The Cretaceous was characterized by widespread rifting, continental breakup, rapid spreading, and global magmatism. Tomography shows that the upper mantle of the Pacific and Indian oceans is hot. These large low-velocity regions contain most of the world's hotspots and ridges and were the sites of extensive plateau and continental flood-basalt magmatism. The formation and rapid expansion of the Pacific and Indian ocean plates took place in these regions that are hot because they have not been cooled or displaced by cold oceanic lithosphere for more than 200 m.y. Plumes are not required to explain such mantle. The Pacific hemisphere is isolated from the supercontinent hemisphere by a band of cool mantle over which continents collect and into which subduction preferentially occurs.

Comparison of seafloor tectonic fabric at intermediate, fast, and super fast spreading ridges: Influence of spreading rate, plate motions, and ridge segmentation on fault patterns

Abstract: We have conducted a comparative study of the tectonic morphology of young seafloor using SeaMARC II side scan sonar surveys of the intermediate spreading Ecuador Rift, the fast spreading East Pacific Rise (EPR) (8°30′–10°N), and the super fast spreading EPR (18°–19°S). We find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments (i.e., with proximity to large- and short-offset discontinuities). We also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone. Most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer. For example, regions of super fast spreading are characterized by the largest numbers of short faults, the smallest average fault spacing and throw, and the highest fault density. In addition, clusters of short, closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area, presumably the result of a thinner, weaker brittle layer. Faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates. Rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates. Outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets. These differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge. Fault lengths and spacings in all areas approximate exponential distributions. The extensional strain represented by fault populations is calculated from the displacement and length distributions of faults, and strain estimates of ∼4% are obtained for each area. Assuming that fault spacing reflects fracture depth extent where faults initiate, we infer a brittle layer thickness of ∼1 km when faulting begins. Fault populations are examined for ridge segment scale variations in amagmatic extension. We see evidence for greater amagmatic extension associated with long-term reduced magma supply along the eastern third of the Ecuador Rift. Evidence for local increased brittle extension is also found within 15 km of transform faults. Discordant zones left by overlapping spreading centers (OSCs) are characterized by low fault abundances. At OSCs, discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting. Fault azimuths do appear to be useful indicators of plate motion. Within the EPR 8°30′–10°N area, fault trends record a recent change in Pacific-Cocos plate motion (3°–6° at ∼1 m.y.) consistent with magnetic anomaly and fault lineation data from elsewhere along the northern EPR. Within the Ecuador Rift, fault azimuths scatter within 3° of predicted trends and are consistent with constant spreading about one pole for the past 1.5 m.y.

Volcanic and hydrothermal processes associated with a recent phase of seafloor spreading at the northern Cleft segment: Juan de Fuca Ridge

Abstract: The northern portion of the Cleft segment, which is the southernmost segment of the Juan de Fuca Ridge, is the site of a seafloor spreading episode during the mid-1980s that was originally discovered by the occurrence of anomalous hydrothermal bursts (megaplumes) and later documented by seafloor mapping of new pillow mounds (NPM) that were erupted. Several field seasons of investigations using sidescan sonar, a deep-tow camera system, and the submersible Alvin reveal that about 30 km of the ridge crest is hydrothermally active and/or has experienced recent volcanic and tectonic activity associated with this episode. The most intense hydrothermal activity within this area and all the known high-temperature vents lie along a fissure from which a young sheet flow (YSF) erupted. Extinct chimneys located within 100–200 m on either side of the fissure system represent an older (>100 years) and probably less intense, hydrothermal regime. The bathymetry and the morphology of the YSF suggest that this eruption occurred over a 1–2 km section of the fissure system that forms its eastern boundary and that it flowed to the south. Fields of lava pillars concentrated at the margins of the YSF where lava probably formed when the lava stagnated near the edges of the flow. A comparison of sidescan data sets collected in 1982 and 1987 implies that the YSF was erupted at least 7 months prior to the NPM, consistent with analysis of bottom photographs that suggests that the eruptions of the YSF and NPM were only separated by a few years. The low hydrothermal flux over the NPM relative to the YSF suggests a rapidly cooled underlying heat source beneath the former. We propose that the NPM were erupted from a dike or dikes injected laterally to the north from a magma body lying beneath the YSF. Recent evidence of a decrease in the intensity of the overlying hydrothermal plumes suggests that the system is continuing to cool down.

Seismic constraints on shallow crustal emplacement processes at the fast spreading East Pacific Rise

Abstract: We present the results of nine on-bottom seismic refraction experiments carried out over young East Pacific Rise crust. The experiments are unusual in that both the source and receiver are located within a few meters of the seafloor, allowing high-resolution determinations of shallow crustal structure. Three experiments were located within the axial summit caldera (ASC) over “zero-age” crust. The seismic structure at these three locations is fundamentally the same, with a thin (<60 m) surficial low-velocity (<2.5 km/s) layer, a 100 to 150-m-thick transition zone with velocities increasing by ∼2.5 km/s, and a layer with velocities of ∼5 km/s at a depth beneath the seafloor of ∼130–190 m. The surficial low-velocity layer and transition zone are defined as seismic layer 2A, and the ∼5 km/s layer is defined as the top of layer 2B. Both the surficial low-velocity layer and the transition zone double in thickness within ∼1 km of the rise axis. We model layer 2A as the extrusive sequence and transition zone and the 2A/2B boundary as the top of the sheeted dikes. The primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from ∼150 m to ∼300 m within 1 km of the ASC. The thickening of the extrusive layer is interpreted to be due to lava that either overflows the ASC walls, is emplaced through eruptions outside of the ASC, or travels laterally from the ASC through conduits. The most probable cause for the thickening of the transition zone is sill emplacement outside of the ASC, either from magma that does not reach the surface in an off-axis eruption or magma that is transported laterally during the drainage process creating the ASC. We suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2A.

Arctic Ocean Gravity Field Derived From ERS-1 Satellite Altimetry.

Abstract: The derivation of a marine gravity field from satellite altimetry over permanently ice-covered regions of the Arctic Ocean provides much new geophysical information about the structure and development of the Arctic sea floor. The Arctic Ocean, because of its remote location and perpetual ice cover, remains from a tectonic point of view the most poorly understood ocean basin on Earth. A gravity field has been derived with data from the ERS-1 radar altimeter, including permanently ice-covered regions. The gravity field described here clearly delineates sections of the Arctic Basin margin along with the tips of the Lomonosov and Arctic mid-ocean ridges. Several important tectonic features of the Amerasia Basin are clearly expressed in this gravity field. These include the Mendeleev Ridge; the Northwind Ridge; details of the Chukchi Borderland; and a north-south trending, linear feature in the middle of the Canada Basin that apparently represents an extinct spreading center that "died" in the Mesozoic. Some tectonic models of the Canada Basin have proposed such a failed spreading center, but its actual existence and location were heretofore unknown.

Hydrothermal circulation, Juan de Fuca Ridge eastern flank: Factors controlling basement water composition

Abstract: Pore water has been analyzed from sediment cores taken from three areas on the eastern flank of the Juan de Fuca Ridge as part of FlankFlux 90, a study of hydrothermal circulation through mid-ocean ridge flanks. Seismic reflection and heat flow surveys (Davis et al., 1992a) indicate that the three areas differ in sediment thickness, basement topography, abundance of outcrops, basement temperature, and fraction of heat lost by advection versus conduction. Area 1 is on 0.6 Ma crust with nearly continuous basement outcrop, area 2 is on 1.3 Ma crust over the first buried ridge parallel to the present ridge axis, and area 3 is on 3.5–3.8 Ma crust over two axis-parallel buried ridges that penetrate the sediment cover in three locations. Each area includes a hydrothermal system in which seawater flows into basement, reacts with crustal basalt, and then exits basement either through the sediment or directly into the overlying water column. As constrained by concentrations of sulfate and lithium in the pore waters, at least some seawater enters basement in all three areas without reacting fully with the overlying sediment, even where no outcrops are known nearby. Speeds of up welling of pore water through the sediment have been estimated by fitting profiles of dissolved magnesium and chlorinity, which behave conservatively in these areas, to numerical time-dependent transport models. The estimated velocities range from <0.1 to 7.4 cm/yr; faster flows probably occur but were not sampled. Upwelling speed correlates positively with heat flow and basement highs and negatively with sediment thickness. The correlation with heat flow differs from area 2 to area 3 along with differences in physical properties of the turbidite sediment. We have documented pore water upwelling through sediment up to 100 m thick. We estimate that upwelling continues at decreasing speeds through sediment up to 160 m thick, corresponding to a heat flow of 0.44 W/m2 in area 2 and 0.3 W/m2 in area 3. Concentrations of magnesium and chlorinity in the altered seawater upwelling from basement are uniform within each area but differ from one area to the next. Both species remain at the bottom seawater concentration in area 1, where basement is cooled to <10°C at the base of the sediments mainly by advection. The concentration of magnesium decreases with increasing basement temperature in areas 2 and 3 to a minimum of 2.5 mmol/kg at about 90°C in area 3. The transition from largely advective to largely conductive heat loss occurs over only 20 km between areas 1 and 2 and corresponds to a dramatic change in the composition of fluid circulating through basement, as the uppermost basement is heated from <10° to 40–50°C. Chlorinity of the basement fluid increases above the present-day bottom seawater concentration in areas 2 and 3 and in nearly all other mid-ocean ridge flanks studied to date, as a result of rock hydration and the higher chlorinity of bottom seawater during the last glacial period. While chlorinity generally correlates positively with uppermost basement temperature in various ridge flank hydrothermal systems, it reaches a maximum in area 2 at only 40°C, probably because alteration there occurs at a lower water/rock ratio than elsewhere. For all mid-ocean ridge flanks studied to date, the temperature at the basement interface correlates better with the fraction of heat lost by advection versus conduction and with the average thickness of the sediment cover than with crustal age.

Nd and Sr isotope evidence linking mid-ocean-ridge basalts and abyssal peridotites

Abstract: PERIDOTITES found on the sea floor are widely believed to be residues left by mid-ocean-ridge basalt (MORE) melting. As such, their composition should provide insights into the nature of the sub-oceanic depleted mantle. But although these abyssal peridotites occur in mid-ocean ridge fracture zones1,2, there is little other evidence in support of their genetic link with MORB, and doubts about it have been raised3–5. Radiogenic isotopes should be able to provide a powerful test of the hypothesis, but previous studies3,8–10 on whole rocks have not provided unambiguous answers as they are generally altered by sea water. Here we present measurements of the Nd and Sr isotope compositions of a suite of abyssal peridotite clinopyroxenes which should have resisted alteration. Despite residual seawater contamination of Sr in the clinopyroxenes, the data demonstrate that the peridotites are from a depleted mantle source, identical to that of MORB. This provides a strong indication that abyssal peridotites are residues of MORB melting.

Transform migration and vertical tectonics at the Romanche fracture zone, equatorial Atlantic

Abstract: The Romanch½ transform offsets the Mid-Atlantic Ridge (MAR) axis by about 950 km in the equatorial Atlantic. Multibeam and high-resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the RIV Akademik Strakhov as part of the Russian-Italian Mid-Atlantic Ridge Project (PRIMAR). Morphobathymetric data show the existence on the northern side of the transform of a major 800-kin-long aseismic valley oriented 10 o to 15 o from the active valley; it disappears about 150 km from the western MAR segment. The aseismic valley marks probably the former location of the Romanche transform ("PaleoRomanche") that was active up to roughly 8-10 Ma, when the transform boundary migrated to its present position. A temporary microplate developed during the migration and reorientation of the transform. This microplate changed its sense of motion as it was transferred from the South American to the African plate. A prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side, reaching its shallowest part (shallower by over 4 km than the predicted thermal contraction depth) in a zone opposite the eastern MAR axis/transform intersection (RTI). Flat-top peaks on the summit of the transverse ridge are capped by acoustically transparent, weakly stratified, shallow water platform/lagunal/reef limestones. This limestone unit is a few hundred meters thick and overlies igneous basement. Evaluation of the seismic reflection data as well as study of samples of carbonates, ventifact basaltic pebbles and gabbroic, peridofitic and basaltic rocks recovered at different sites on the transverse ridge, suggest that (1) the summit of the transverse ridge was above sea level at and before about 5 Ma; (2) the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate; its summit was flattened by erosion at sea level during subsidence; (3) the transverse ridge is an uplifted sliver of lithosphere and not a volcanic construcfional feature; and (4) transtensional and transpressional tectonics have affected the transverse ridge. Hypotheses on the origin of the Romanche transverse ridge include (1) lateral heat conduction across the RTI; (2) shear heating; (3) lithospheric flexure due to thermal stresses in the cooling lithosphere; (4) viscoelastic deformation of the lithosphere; (5) hydration/dehydration of mantle peridotites; and (6) longitudinal flow of melt and igneous activity across the RTI. These processes cannot by themselves explain the transverse ridge, although some of them could contribute to its formation to a small extent. Vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge/transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence. Uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former (PaleoRomanche) and the younger transform boundaries, before and during the transition to the present boundary. After migration of the transform boundary to its present position, transpression was replaced by transtension and by subsidence of the transverse ridge. An aseismic axial rift valley impacting against the transform valley about 80 km west of the present RTI suggests eastward ridge jumping that probably followed transform migration. Localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements, such as those that formed an ultradeep (>7800 m) pull-apart basin along the transform valley.

Influence of the Sierra Leone mantle plume on the equatorial Mid-Atlantic Ridge : a Nd-Sr-Pb isotopic study

Abstract: We report on a Pb-Nd-Sr isotope and rare earth study of Mid-Atlantic Ridge (MAR) basalt glasses collected across the equatorial fracture zones from 7°S to 5°N (65 stations). The 1600-km-long profile reveals two mixing zones in the mantle that are isotopically distinct but cover the same range of (La/Sm)n ratios (0.3–2), with a gradational boundary between the Romanche and the Chain fracture zones. The potential mantle temperature profile inferred from Na2O content is also quite distinct. The north zone is dominated by a major, La/Sm and HIMU type Pb isotope anomaly centered at 1.7°N±300 km, which is flanked by two zones mildly radiogenic in Pb but depleted in light REE. A kinematic and evolutionary model describing the dispersion and interaction of the Sierra Leone plume with the asthenosphere and the MAR in the last 75 m.y. is proposed for this zone, which includes St. Paul and St. Peter's Rocks. In contrast, over the south zone the isotope/geochemical profiles are well correlated at all length scales and opposite in sign from the inferred potential mantle temperature profile and mean percent fusion. Broad negative gradients are observed between the Romanche and the Charcot fracture zones, superimposed by spikelike anomalies at the intersection with the eastern part of the Romanche and Chain transform faults, where cold plate edge effects prevail. The heterogeneous mantle model of Sleep [1984] and Langmuir and Bender [1984] is applicable to this zone, that is the volatile and radiogenic Pb-rich lumps are preferentially melted during mantle decompression and passively sampled. The lumps may reflect the early dispersion of the St. Helena or Ascension mantle plumes under a thick lithosphere, followed by redistribution due to intense shearing, continental lithosphere delamination, and secondary mantle convection. The presence of a depleted asthenosphere unpolluted by plumes along the 400-km-long MAR segment between the Charcot and Ascension fracture zones is also apparent in the data.

Seismic imaging of oceanic layer 2A between 9°30′N and 10°N on the East Pacific Rise from two‐ship wide‐aperture profiles

Abstract: We analyze two-ship multichannel seismic wide aperture profiles (WAP) acquired on the East Pacific Rise (EPR) between 9°30′C and 10°N in May and June 1985. For offsets between 2 and 5 km, most of the WAP common depth point (CDP) gathers exhibit a strong retrograde, reflection-like seismic arrival that corresponds to rays turned within a very strong velocity gradient zone underlying the uppermost crustal layer 2A. Stacking of this postcritical arrival for hundreds of CDP gathers along the WAP lines yields remarkably clear, and virtually continuous, images of the bottom of layer 2A for several tens of kilometers along and across the EPR. An innovative combination of this image with others of the seafloor (SF) and of the axial magma chamber (AMC) provides an unusually clear composite picture of young upper crust. We combine these images with detailed seismic velocity information, also obtained from the CDP gathers, to achieve an accurate determination of the layer 2A thickness in the area. Along the EPR axis between 9°30°N and 10°N, layer 2A shows a relatively constant thickness between 100 and 150 m (80–120 ms two-way travel time). Across the axis, however, a line near 9°30′N indicates that layer 2A thickness rapidly increases by approximately a factor of 2 within 2–4 km of the axis and remains nearly constant afterwards. Thus the final thickness of layer 2A is attained before the development of large-scale tectonic faulting, strongly suggesting that the thickening of layer 2A away from the axis is due to successive episodes of volcanic activity increasing the thickness of the uppermost crustal section from the top, rather than tectonic fracturing lowering the uppermost crustal seismic velocities and pushing the bottom of layer 2A downwards as the crust moves away from the axis.

Off-axis volcanism at the East Pacific Rise detected by uranium-series dating of basalts

Abstract: RECENT detailed surveys of the East Pacific Rise have revealed the complexity of the volcanic and magmatic processes occurring along and across fast-spreading ocean ridge crests1–7. In parallel with geological and geochemical investigations, it is now possible to investigate the temporal and spatial pattern of volcanism at ocean ridges by dating young basalts using mass spectrometric uranium-series disequilibria methods8–11. Here we use 238U-230Th and235U-231Pa ages for basalts to quantify the spatial extent of young volcanism and crustal accretion at 9°31′ N on the East Pacific Rise. Most of the ages are younger than would be expected based on off-axis distance and spreading rate. We infer from these anomalously young ages that most of the dated basalts on the crestal plateau were erupted 0.5–2 km outside the axial summit caldera, with some volcanism occurring as far as 4 km off-axis. Melts erupted outside the axial summit caldera can have crustal residence times and magmatic supply systems that differ from those of axial lavas.

Extensional transform zones and oblique spreading centers

Abstract: Extensional transform zones (ETZs) are plate boundary segments of order 100 km long that strike at angles between 15° and 45° to the extension direction. They are characterized by neovolcanic/tectonic zones comprising overlapping en echelon volcanic systems and/or faults that trend 30°–75° to the extension direction, sometimes accompanied by a Riedel shear. Below these surficial en echelon structures the deformation is aseismic and ductile, and the plate boundary is probably continuous. ETZs occur in fast and slow spreading and rifting environments and may persist in a stable configuration for several million years. ETZs link oblique spreading segments to transform faults in the Manus and probably the Lau backarc basins. The Reykjanes Peninsula and Tjornes Fracture Zone in Iceland and the Mak'Arrasou in Afar are ETZs that link subaerial to submarine spreading or rifting segments. The Brawley and Cerro-Prieto seismic zones appear to be ETZs in the Imperial and Mexicali valleys that link the San Andreas, Imperial, and Cerro-Prieto transform faults. Experimental and analytical models of transtensional deformation in brittle-ductile systems match many of the observed characteristics of ETZs and oblique spreading centers, including variably sigmoidal to straight en echelon faults that are not parallel to the extension direction. The contrasting fault patterns reflect the rheology of the models and lithosphere: they are more sigmoidal when the strain in the lower ductile layer is more focused, causing the axial faults to curve as they propagate toward parallelism with the less ductile rift margins. The angle (O) between the faults and the extension direction decreases with the angle (α) between the strike of the zone and the extension direction. ETZs occur in the range 15° ≤ α ≤ 45°, whereas oblique spreading centers have 45° < α < 90° and transform relay zones have 0° < α < 15°. Oblique fast spreading segments exhibit ridge-parallel faults and volcanic systems (O = α), presumably reflecting locally rotated stress fields, whereas at oblique slow spreading centers, O is closer to orthogonal (α < O < 90°).

Teleseismic arrivals at a mid‐ocean ridge: Effects of mantle melt and anisotropy

Abstract: Recent observational evidence of upwelling-mantle anisotropy at a slow spreading center has motivated the modeling of teleseismic arrivals at mid-ocean ridges. The models consider a variety of types of anisotropy and heterogeneity where the emphasis is to ascertain whether or not travel-times can be used to discriminate between the existence of partial melt and anisotropy. Two mechanisms for anisotropy in the upwelling asthenosphere are considered: one due to the preferential alignment of the fast axes of olivine crystals in the direction of mantle flow and the other due to the preferential alignment of cracks that feed melt towards the spreading axis. The results indicate that P-waves are most sensitive to even modest amounts of flowinduced asthenospheric anisotropy, while S-waves are most sensitive to the presence of mantle melt. Multiple S-wave arrivals are predicted for many models, most notably the ones with anisotropy due to crack-alignment where very large S-wave separations develop. The model which best fits existing data requires a higher degree of crystal-alignment anisotropy in the upwelling-asthenosphere than in the lithosphere. This effect has been predicted in studies of the evolution of crystal-alignment anisotropy in polycrystalline aggregates.

Lithospheric structure and compensation mechanisms of the Galápagos Archipelago

Abstract: Volcanic islands of the Galapagos Archipelago are the most recent subaerial expression of the Galapagos hotspot. These islands and numerous seamounts are constructed mainly upon a broad volcanic platform that overlies very young (<10 m.y.) oceanic lithosphere just south of the active Galapagos Spreading Center. The 91°W fracture zone crosses the platform and creates an estimated 5-m.y. age discontinuity in the lithosphere. Major tectonic features of the Galapagos include an unusually broad distribution of volcanic centers, pronounced structural trends such as the NW-SE Wolf-Darwin Lineament (WDL), and a steep escarpment along the western and southern margins of the archipelago. We use shipboard gravity and bathymetry data along with Geosat geoid data to explain the tectonic and structural evolution of the Galapagos region. We model the gravity anomalies using a variety of compensation models, including Airy isostasy, continuous elastic flexure of the lithosphere, and an elastic plate with embedded weaknesses, and we infer significant lithospheric strength variations across the archipelago. The outboard parts of the southern and western escarpment are flexurally supported with an effective elastic thickness of ∼12 km. This area includes the large shield volcanoes of Fenandina and Isabela Islands, where the lithosphere regionally supports these volcanic loads. The central platform is weaker, with an elastic thickness of 6 km or less, and close to Airy isostasy. The greatest depths to the Moho are located beneath eastern Isabela Island and the central platform. Thinner lithosphere in this region may account for the broad distribution of volcanoes, the extended period of eruption of the central volcanoes, and their reduced size. The transition from strong to weak lithosphere along the southern escarpment appears to be abrupt, within the resolution of our models, and can be best represented by a free end or faultlike discontinuity. Also, modeling the WDL as a lithospheric fault increases the match to the observed gravity anomalies. The primary feature is the weak central platform, whose existence is a logical convergence of several factors: the 91°W transform/age offset, the possible increase in age of the lithosphere along the southern part of the platform due to ridge jumping, and the formation of the central platform at a spreading ridge.