Slow-spreading Mid-ocean Ridges Research Articles

Exhumed mantle‐forming transitional crust in the Newfoundland‐Iberia rift and associated magnetic anomalies

Transitional zones located between Iberia and North America formed during continental rifting and mostly consist of exhumed mantle. In this study we show that ages of exhumed mantle at Ocean Drilling Program (ODP) sites 1068 and 1070 in the Iberia Abyssal Plain and site 1277 in the Newfoundland Basin are similar to ages determined from magnetic lineations created by serpentinization during mantle exhumation. On the basis of paleomagnetic and geological data and a comparison with a fossil ocean‐continent transition in the Alps, we envisage a first episode of mantle serpentinization during which a strong component of magnetization was acquired followed by a second episode occurring at the contact with cold seawater, and which only affects the upper tens of meters of the exhumed mantle. The inversion of magnetic data (Euler deconvolution) shows that magnetic sources are N‐S trending horizontal cylindrical bodies located within the highly serpentinized upper crust. Therefore the serpentinization process is able to produce magnetic lineations formed in a similar way to those formed by seafloor spreading. Within transition zones, sequences of magnetic anomalies can provide information concerning the timing of the emplacement of crust, but not on its nature (oceanic versus exhumed mantle). This discovery enables us to date the exhumation of mantle rocks in transition zones and allows kinematic reconstructions of the final stages of continental rifting. During rifting the deep distal continental margins and the adjacent transitional zones in the Newfoundland‐Iberia rift system were formed by ultraslow extension from early Berriasian to late Valanginian–early Hauterivian and by slow extension from early Hauterivian to the late Aptian–early Albian boundary. Therefore transitional zones share many similarities with slow and ultraslow spreading midoceanic ridges.

Origin of high-Al N-MORB by fractional crystallization in the upper mantle beneath the Galápagos Spreading Center

The Galápagos Spreading Center (GSC) includes lavas with chemical compositions ranging from N-MORB to transitional and more enriched MORB. N-MORB dominate the region west of 95.5°W, far from the influence of the Galápagos hotspot. However, some N-MORB glass samples from the GSC have very high Al contents (> 16.0 wt.% Al 2O 3 at > 8.5 wt.% MgO), similar to high-Al N-MORB from other slow and intermediate spreading ridges and close to fracture zones elsewhere. GSC high-Al N-MORB are dominated mineralogically by uniform plagioclase compositions (An 80–82) with only 1–2% olivine (Fo 85–87), and have glass compositions with higher Al 2O 3 and lower SiO 2 than is predicted by normal MORB fractionation trends. Forward modeling using the MELTS and pMELTS algorithms constrained by crustal thickness measurements indicates that high-Al, low-Si MORB can be produced by high-pressure crystallization in the upper mantle using the same source as normal (low-Al) GSC N-MORB. Although high-Al glasses can be obtained by very low extents of partial melting of this mantle source, such melting models result in significant misfits in other major element oxides, especially SiO 2. Models involving significant evolution with up to 20% olivine and clinopyroxene crystallization at pressures of 0.3–0.4 GPa can account for the complete major and selected trace element compositions of these unusual MORB samples. We suggest that high-pressure fractionation is enhanced by conductive cooling of the upper mantle in this area of the GSC, consistent with other recent models correlating mantle crystallization with slow spreading mid-ocean ridges and fracture zones.

The rift to drift transition at non-volcanic margins: Insights from numerical modelling

“Non-volcanic” rifted margins exhibit very little evidence of synrift magmatism, even where the continental crust has been thinned to such an extent that the mantle has been exhumed across a transitional zone (up to ∼100 km wide), called the continent–ocean transition (COT). Using dynamical models of rifting, we explore how extension velocity, mantle composition and potential temperature influence the nature and extent of the COT and compare our results to observations at the West Iberia margin (WIM) and the ancient margins of the Liguria-Piemonte Ocean (LP) now exposed in the Alps. We find a first order relationship between extension velocity and the amagmatic exposure of mantle at the COT. For very slow half extension velocities, (< 6 mm/yr), mantle exhumation begins before melting. At these velocities, by the time melting starts at the rift centre, the area of exhumed mantle has moved sideways creating a COT, the width of which increases with decreasing velocities. However, at 10 mm/yr, a velocity probably appropriate for the exhumation of mantle at the WIM and LP, melting starts prior to mantle exhumation. In this case, our models show that by the time mantle exhumation starts, a ∼4.5 km column of melt has been produced, much more than the ∼2 km maximum mean melt thickness inferred at the COT of these margins. Even considering that 25% of the produced melt may be trapped in the mantle, as in slow-spreading mid-ocean ridges, still more melt is produced in the models than inferred from observations. Thus, extension velocity alone cannot explain the practical absence of synrift magmatism at the COT of the WIM and LP. We find that the formation of a wide, amagmatic COT requires that either the mantle was depleted in basaltic constituents by > 10% prior to rifting or that its potential temperature was ∼50 °C lower than normal (≤ 1250 °C).

OCCURRENCE AND SIGNIFICANCE OF SERPENTINITE-HOSTED, TALC- AND AMPHIBOLE-RICH FAULT ROCKS IN MODERN OCEANIC SETTINGS AND OPHIOLITE COMPLEXES: AN OVERVIEW

This paper reviews the occurrence and significance of talc- and amphibole-rich fault rocks developed in mafic-ultramafic sequences and evaluates their role in deformation and alteration of the oceanic lithosphere from different tectonic settings (from spreading mid-ocean ridges up to orogenic belts). Recently, talc and amphibole-rich fault rocks have been sampled and studied from detachment fault surfaces along slow and ultra-slow spreading mid-ocean ridges, and constraining the conditions of deformation and strain localization during the evolution of oceanic core complexes. These rocks are documented not only in oceanic core complexes, but also in other oceanic fracture zones where ultramafic rocks are exposed on the seafloor, while only few occurrences have been reported in ophiolite sequences. Samples recovered in situ in oceanic settings record heterogeneous deformation (crystal-plastic to cataclastic) under greenschist- facies conditions and are commonly restricted to localized shear zones (< 200 m) and are associated with intense talc-amphibole metasomatism. The presence of mechanically weak minerals, such as talc, serpentine and chlorite, may be critical to the development of such fault zones and may enhance unroofing of upper mantle peridotites and lower crustal gabbroic rocks during seafloor spreading. Talc in particular may be influential in lubricating and softening mylonitic shear zones and can lead to strain localization and focused hydrothermal circulation along such faults. The rheology of these rocks, and its evolution during dehydration reactions could play an important role also in subduction-zone processes and during the formation of ultramafic orogenic belts. Here, we review the occurrence and significance of talc- and amphibole-rich fault rocks in different tectonic settings on the seafloor and evaluate their role in deformation and alteration of the oceanic lithosphere.

Diapycnal mixing associated with an overflow in a deep submarine canyon

Abstract In order to close the global overturning circulation the buoyancy loss to the atmosphere associated with the formation of deep and bottom water at high latitudes must be balanced by buoyancy gain elsewhere. In case of water that is not in contact with the atmosphere except in its formation region the required buoyancy gain is accomplished primarily by diapycnal mixing. Recent observations suggest that a significant portion of the diapycnal buoyancy fluxes in the abyss is associated with overflows in submarine valleys or canyons, especially on the flanks of slow-spreading mid-ocean ridges. In the present study, hydrographic and velocity data from an overflow across a sill rising 1000 m above the floor of the rift valley of the Mid-Atlantic Ridge near 36°N were used to infer the associated diapycnal mixing. The cross-sill density gradients are characterized by a vertical dipole, with an upper layer of streamwise density increase overlying a lower layer where the density decreases along the flow. This is qualitatively consistent with the effects of diapycnal mixing transferring buoyancy from the upper into the lower layer. Hydrographic and velocity profiles provide evidence for strong mixing associated with the overflow across the sill: in addition to thick layers that are susceptible to shear instability, there are numerous density overturns indicating a spatially averaged diffusivity of order 10 - 2 m 2 s - 1 . Inversions of the density equation reveal that the cross-sill gradients can be accounted for either by adiabatic vertical advection or by vertical eddy diffusion, although the adiabatic solutions are considered energetically implausible because they require upwelling in the layer of cross-sill density increase. Eddy-diffusive solutions, both with and without adiabatic downwelling, are characterized by diffusivity profiles attaining local maxima of order 10 - 2 m 2 s - 1 ≈ 250 m above the sill depth. The vertical structure of the inversion-derived diffusivities is consistent with the Thorpe-scale based estimates and with observations from overflows in major ocean passages, which also are often characterized by local diffusivity maxima of order 10 - 2 m 2 s - 1 a few 100 m above sill depth. Combining the diffusivity profiles near the sill with budget-derived bulk estimates for 150 km of rift valley implies that ≈ 50 % of the mixing in the rift valley is associated with overflows, while the remainder is caused by other processes, including the breaking of tidally forced internal waves.

Palaeomagnetic constraints on continental break-up processes: observations from the Main Ethiopian Rift

Abstract We report the first palaeomagnetic results from the Main Ethiopian Rift (MER), the northernmost sector of the East African rift system. This part of the MER shows an along-axis tectono-magmatic segmentation pattern similar to that of slow-spreading mid-ocean ridges, which developed during the past 1.9 Ma. The aims of our palaeomagnetic, structural and geochronological studies are to test plate kinematic models for the right-stepping, en echelon 60–80 km-long magmatic segments. Twenty palaeomagnetic sites were sampled on either basalt or ignimbrite outcropping in the region adjacent to, and within, the &lt;1.9 Ma-old tectono-magmatic segments of Gademsa-Koka, Boset and Fentale-Dofan. Five K-Ar age determinations were made to bracket the age of units studied in the palaeomagnetic analyses. The natural remanent magnetization intensity possibly exhibited a unimodal distribution with a value of 6.6 A/m (σ = 5.6 A/m) for the basalts and a bimodal distribution with magnetization intensity of 0.69 A/m (σ = 0.55 A/m) and 0.03 A/m (σ = 0.02 A/m), statistically similar to values from previous studies in the Afar triple junction zone (e.g. Kidane et al. 1999, 2002). Progressive heating, alternating field analysis, and susceptibility vs. temperature measurements indicated unblocking temperature ranging between 300 °C−600 °C for basalts and between 500 °C−660 °C for ignimbrites, suggesting the magnetic mineralogy to be titanomagnetite and magnetite for the former and magnetite and titanohematite for the latter. Palaeomagnetic measurements using both TH and AF technique revealed quasi-single component of magnetization with viscous remanent magnetization (VRM) on a few samples. Principal component analysis and statistical averaging resulted in an overall mean palaeomagnetic direction of (D s = 2.3°, I s = 7.8°, α 95 = 7, K = 26.9, N = 17) which is statistically identical to the expected direction (D = 1.9°, I = 13.5°, α 95 = 2.5, K = 105.6, N = 32) from the Apparent Polar Wander Path reference curve for Africa at 1.5 Ma (Besse &amp; Courtillot 2003). The angular difference between the observed and expected directions above with their uncertainty is calculated to be 0.4° ± 7.5°. These results indicate that the Late Pliocene-Pleistocene rocks of the MER in the studied region do not suffer vertical axis rotation, arguing against transtensional and seafloor-spreading-transform kinematic models. We suggest that magma intrusion, rather than large offset faults, produce the right-stepping, en echelon magmatic segments of the MER, which is at the transition from continental to oceanic extension.

Strain accommodation in transitional rifts: extension by magma intrusion and faulting in Ethiopian rift magmatic segments

Abstract Active deformation within the northern part of the Main Ethiopian Rift (MER) occurs within approximately 60 km-long, 20 km-wide ‘magmatic segments’ that lie within the 80 km-wide rift valley. Geophysical data reveal that the crust beneath the &lt;1.9 Ma magmatic segments has been heavily intruded; magmatic segments accommodate strain via both magma intrusion and faulting. We undertake field and remote sensing analyses of faults and eruptive centres in the magmatic segments to estimate the relative proportion of strain accommodated by faulting and magma intrusion and the kinematics of Quaternary faults. Up to half the ≤10 km-long normal faults within the Boset-Kone and Fantale-Dofen magmatic segments have eruptive centres or extrusive lavas along their length. Comparison of the deformation field of the largest Quaternary fault and an elastic half-space dislocation model indicates a down-dip length of 10 km, coincident with the seismogenic layer thickness and the top of the seismically imaged mafic intrusions. These relations suggest that Quaternary faults are primarily driven by magma intrusion into the mid- to upper crust, which triggers faulting and dyke intrusion into the brittle upper crust. The active volcanoes of Boset, Fantale and Dofen all have elliptical shapes with their long axes in the direction N105, consistent with extension direction derived from earthquake focal mechanisms. Calderas show natural strains ranging from around 0.30 for Boset, 0.55 for Fantale, and 0.94 for Dofen. These values give extension strain rates of the order of 0.3 microstrain per year, comparable to geodetic models. Structural analyses reveal no evidence for transcurrent faults linking right-stepping magmatic segments. Instead, the tips of magmatic segments overlap, thereby accommodating strain transfer. The intimate relationship between faulting and magmatism in the northern MER is strikingly similar to that of slow-spreading mid-ocean ridges, but without the hard linkage zones of transform faults.

Deformation of oceanic lithosphere near slow-spreading ridge discontinuities

Transform and non-transform discontinuities that offset slow spreading mid-ocean ridges involve complex thermal and mechanical interactions. The truncation of the ridge axis influences the dynamics of spreading and accretion over a certain distance from the segment-end. Likewise, the spreading system is expected to influence the lithospheric plate adjacent to the ridge-end opposite of the discontinuity. Tectonic effects of the truncated ridge are noticeable in for example the contrast between seafloor topography at inside corners and outside corners, along-axis variations in rift valley depth, style of crustal accretion, and ridge segment retreat and lengthening. Along such slow-spreading discontinuities and their fossil traces, oceanic core complexes or mega-mullion structures are rather common extensional tectonic features. In an attempt to understand deformation of oceanic lithosphere near ridge offsets, the evolution of discontinuities, and conditions that may favor oceanic core complex formation, a three-dimensional thermo-mechanical model has been developed. The numerical approach allows for a more complete assessment of lithosphere deformation and associated stress fields in inside corners than was possible in previous 3-D models. The initial suite of results reported here focuses on deformation when axial properties do not vary along-strike or with time, showing the extent to which plate boundary geometry alone can influence deformation. We find that non-transform discontinuities are represented by a wide, oblique deformation zone that tends to change orientation with time to become more parallel to the ridge segments. This contrasts with predicted deformation near transform discontinuities, where initial orientation is maintained in time. The boundary between the plates is found to be vertical in the center of the offset and curved at depth in the inside corners near the ridge–transform intersection. Ridge–normal tensile stresses concentrate in line with the ridge tip, extending onto the older plate across the discontinuity, and high stress amplitudes are absent in the inside corners during the magmatic accretionary phase simulated by our models. With the tested rheology and boundary conditions, inside corner formation of oceanic core complexes is predicted to be unlikely during magmatic spreading phases. Additional modeling studies are needed for a full understanding of extensional stress release in relatively young oceanic lithosphere.

A REVIEW OF COMPOSITIONS AND TEXTURES OF ABYSSAL PERIDOTITES FROM THE NORTH ATLANTIC AND INDIAN OCEANS. PETROLOGICAL AND GEODYNAMICAL IMPLICATIONS

Abyssal peridotites equilibrated in the spinel lherzolite facies and sampled along ultra-slow, slow and intermediate spreading ridges, at transform faults and on-axis, in the Atlantic and Indian oceans, are compared for available mineralogical / geochemical data, with special emphasis on modal compositions and textures. Large mantle regions (100->1000 km) below the Atlantic and Indian ridges have been highly depleted by partial melting and melt extraction, not always in relation with known nearby hotspots. In those regions, residual harzburgites locally underwent secondary crystallization of clinopyroxene, spinel and olivine as a result of late-stage melt / rock reactions involving dissolution and/or incongruent (re)melting of residual orthopyroxene. In extreme cases, these reactions lead to the formation of secondary lherzolites and/or wehrlites closely associated with dunites. Trace element compositions of the secondary clinopyroxenes, as well of residual clinopyroxenes where some are still present, suggest that reacting melts derived from primary melts generated by partial melting of already depleted upper mantle. Residual peridotites sampled along transform faults are in the whole less depleted than on-axis peridotites. However, they are also characterized by a wide compositional range that includes highly depleted harzburgites. Thus, offaxis more fertile abyssal peridotite compositions probably not reflect a colder thermal regime of the ridge near the transforms. Instead, peridotite compositional variability along the transforms likely results from episodic magmatic activity at slow spreading mid-oceanic ridges, with temporal variations in the degree of melting. Pyroxenes dissolution textures and intergranular igneous clinopyroxenes can also be observed locally in less depleted, residual harzburgites and lherzolites although in much lower amounts than in the highly depleted harzburgites. In all peridotite samples, igneous clinopyroxenes and other minerals produced by melt precipitation and last-stage melt-rock reactions crystallized at the convective – conductive thermal regime transition, such that high temperatures are responsible for near-homogeneous compositions of all textural types of minerals. At a global scale, when regional averages of a large number of samples are considered, modal compositions and mineral chemistry (except sodium and the most incompatible trace elements) are relatively well correlated, and consistent with variable degrees of partial melting (~5-25%) of a lherzolitic source. Nevertheless, at a local scale, each subaxial mantle domain has its own compositional trend indicating variable melting parameters and/or source composition. Furthermore, orthopyroxene contents for a given olivine content allow to define a subgroup of orthopyroxene- rich harzburgites which is characteristically found in the most refractory subaxial mantle regions and whose clinopyroxenes are, in average, richer in sodium and chromium than those from orthopyroxene-poor harzburgites and lherzolites. Such differences in compositions might result from partial melting occurring at higher pressures in the more depleted regions. However, even in taking in account various late magmatic / metamorphic events which could have modified the primary mineral proportions, the present variations in the olivine / orthopyroxene ratios of all abyssal peridotites cannot be explained by any partial melting processes, at a global and regional / local scales. In the eastern region of the Southwest Indian ridge, along-axis compositional variability is as high as in transform zones. The scale of this compositional heterogeneity is correlated with the roughness and irregularity of the rift floor. In contrast with transform zone peridotites, however, mineral compositions and modal compositions do not correlate with each other and classical partial melting indicators are not internally coherent. The most refractory harzburgites clearly underwent metasomatism by fertile, alkali-rich, hydrous, asthenospheric melts, before subridge partial melting and high temperature annealing, while lherzolites appear to be well equilibrated hybrid rocks resulting from refertilization of refractory peridotites by basaltic melts mostly generated in the garnet stability field. Such subaxial mantle may represent heterogeneous upper mantle that upwelled beneath the present ridge and has been preserved because of very low degrees of melting. Alternatively, it may possible that in this very slow spreading / cold environment, thick subaxial lithosphere formed during the previous magmatic episode, was thermochemically eroded during the subsequent (present-day) magmatic event.

Dating the Growth of Oceanic Crust at a Slow-Spreading Ridge

Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas approximately 25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of approximately 5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank.

A multi-technique study of platinum group element systematic in some Ligurian ophiolitic peridotites, Italy

Fe–Ni–Cu sulfide mineralogy has been investigated along with bulk-rock and in-situ PGE analyses by ICP MS and LA-ICP-MS in eight lherzolites from the Internal (IL) and External Liguride (EL) ophiolites (Italy). The two EL lherzolites are fertile (2–4% partial melting) and slightly serpentinized while the six IL cpx-poor lherzolites have experienced 5–10% of partial melting, impregnation by instantaneous melt fractions [Geochim. Cosmochim. Acta 61 (1997) 4557] and have been highly serpentinized. The EL lherzolites show broadly chondritic PGE relative abundances with a slight to pronounced enhancement of the light PGE (Ru, Rh and Pd) relative to the heavy PGE (Os, Ir and Pt) (Ru N/Ir N=1.13; Rh N/Ir N=1.08–1.10; Pd N/Ir N=1.24–1.62; N=CI-chondrite normalized). Their magmatic sulfide modal abundances and S contents, similar to the orogenic peridotites values, are consistent with their very low degree of partial melting. The occurrence of Cu–Rh–Pd-rich pentlandite, however, demonstrates that, even for low degree of partial melting, a Cu–Ni-rich sulfide liquid can segregate, leaving the residual monosulfide solid solution (Mss) (now transformed into Cu-poor pentlandite) depleted in Rh and Pd (Rh N/Ir N and Pd N/Ir N<1). The IL cpx-poor lherzolites display a broadly flat PGE patterns from Os to Pt with a slight enhancement of Ru and Rh (Ru N/Ir N=1.05–1.38; Rh N/Ir N=1.01–1.31). Pd N/Ir N ratios range from chondritic to superchondritic (1.02–2.99) and cannot be interpreted in terms of partial melting models. Rh–Pd–Cu–Ni-rich pentlandite grains are associated with large corroded cpx crystals ascribed to exotic melt percolation by Rampone et al. [Geochim. Cosmochim. Acta 61 (1997) 4557]. It is concluded that precipitation of Cu–Ni–Rh–Pd-rich sulfides has significantly enhanced the Pd concentrations as well as the magmatic sulfide modal abundances. Such processes, previously documented in abyssal peridotites from slow-spreading mid-oceanic ridges, characterize residues from low to moderate melting degrees (5–10%) of the oceanic mantle as a whole, either from mature ocean or from short-lived oceanic basins.

Geochemistry of the Othris Ophiolite, Greece: Evidence for Refertilization?

The Othris peridotite massif, Greece, shows conflicting evidence for a mid-ocean ridge and supra-subduction zone tectonic setting with the presence of plagioclase peridotite that may represent an area of either incomplete melt extraction, or melt impregnation and accumulation. To address these problems we focus on a 3 km continuous section in the Fournos Kaïtsa area, consisting of layers of harzburgite, plagioclase harzburgite and plagioclase lherzolite with accurately known structural and petrographic control. Refractory, Cr-rich spinel compositions and light rare earth element depleted clinopyroxenes in the harzburgites are consistent with ∼15% dry partial melting. Simple batch and fractional melting models are not sufficient to explain the composition of the residual phases and a multistage model with some melting in the garnet stability field is proposed. The pyroxenes from the plagioclase peridotites have higher Ti and rare earth element contents than those from the harzburgites, but similar refractory spinel compositions in both rock types indicate that the plagioclase peridotites may be products of impregnation of harzburgites with a fractionating melt. These observations are in good agreement with previous structural studies and suggest that the moderately depleted Fournos Kaïtsa mantle section most probably originated at a slow-spreading mid-ocean ridge.

Geologic implications of seawater circulation through peridotite exposed at slow-spreading mid-ocean ridges

Peridotite denuded by tectonic extension and exposed at the seafloor adjacent to slow-spreading centers hosts hydrothermal circulation of seawater. The reaction of seawater with peridotite causes serpentinization, which generates a high-pH, strongly reducing fluid rich in methane and hydrogen, and is accompanied by as much as 40% volume expansion. Complete serpentinization of peridotite requires tectonic activity to open fluid paths sealed by volume expansion. Diffuse venting of serpentinization fluids causes lithification of calcareous ooze on the seafloor to chalk-like limestone. This may be the main mechanism of deposition of ophicarbonates common in ophiolites. The degree of induration is a function of the fluid flux through the sediment column. Calcareous ooze infiltrates faults and fractures and can be deformed following lithification. Focused venting of serpentinization fluids may lead to deposition of large chimneys composed of calcite, aragonite, and brucite, such as those in the recently documented Lost City vent field (30°N, Mid-Atlantic Ridge). Geophysical implications of serpentinization include (1) creation of magnetic anomalies due to growth of magnetite in serpentinite and (2) lowered seismic velocity. Integrated studies of geologic and geophysical effects of serpentinization may aid in a more complete understanding of the structure of oceanic lithosphere and the mechanisms that expose mantle peridotite at the seafloor.

Evidence from gabbro of the Troodos ophiolite for lateral magma transport along a slow-spreading mid-ocean ridge.

The lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid-ocean ridge segmentation and variations in crustal thickness. Direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust, but such studies can be made on ophiolites (fragments of oceanic crust accreted to a continent). In the Oman ophiolite, small-scale radial patterns of flow have been mapped along what is thought to be the relict of a fast-spreading mid-ocean ridge. Here we present evidence for broad-scale along-axis flow that has been frozen into the gabbro of the Troodos ophiolite in Cyprus (thought to be representative of a slow-spreading ridge axis). The gabbro suite of Troodos spans nearly 20 km of a segment of a fossil spreading axis, near a ridge-transform intersection. We mapped the pattern of magma flow by analysing the rocks' magnetic fabric at 20 sites widely distributed in the gabbro suite, and by examining the petrographic fabric at 9 sites. We infer an along-axis magma flow for much of the gabbro suite, which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust. Our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow-spreading Mid-Atlantic Ridge.

On the roughness of Mesozoic oceanic crust in the western North Atlantic

SUMMARY Seismic reflection profiles from Mesozoic oceanic crust around the Blake Spur Fracture Zone (BSFZ) in the western North Atlantic have been widely used in constraining tectonic models of slow-spreading mid-ocean ridges. These profiles have anomalously low basement relief compared to crust formed more recently at the Mid-Atlantic Ridge at the same spreading rate. Profiles from other regions of Mesozoic oceanic crust also have greater relief. The anomalous basement relief and slightly increased crustal thickness in the BSFZ survey area may be due to the presence of a mantle thermal anomaly close to the ridge axis at the time of crustal formation. If so, the intracrustal structures observed may be representative of an atypical tectonic regime.

The RAMESSES experiment-III. Controlled-source electromagnetic sounding of the Reykjanes Ridge at 57°45′N

SUMMARY A controlled-source electromagnetic sounding survey centred on an axial volcanic ridge (AVR) segment of the Reykjanes Ridge at 57°45aeN was performed as part of the RAMESSES experiment. Low-frequency (0.35‐11 Hz) electromagnetic signals were transmitted through the crust to an array of horizontal electric field recorders at the seafloor to ranges of 15 km from the source, which was a 100 m long horizontal electric dipole towed at heights of 50‐80 m from the seafloor. Coincident seismic and magnetotelluric studies were conducted during the rest of the RAMESSES experiment. Data were interpreted using a combination of 1-D forward modelling and inversion, and iterative forward modelling in two dimensions. On the axis of the AVR, the resistivity at the seafloor is 1 V m. There is a steep resistivity gradient in the upper few hundred metres of the crust, with the resistivity reaching approximately 10 V ma t a depth of 500 m. In order to explain the low resistivities, the upper layer of the crust must be heavily fractured and saturated with sea water. The resistivity increases with distance from the axis as the porosity decreases with increasing crustal age. The most intriguing feature in the data is the large diVerence in amplitude between fields transmitted along and across the AVR axis. A significant zone of low-resistivity material is required at approximately 2 km depth beneath the ridge crest in order to explain this diVerence. It is coincident with the low-velocity zone required by the seismic data, and has a total electrical conductance in excellent agreement with the results of the magnetotelluric study. The low-resistivity zone can be explained by the presence of a body of partially molten basalt in the crust. Taken together, these results provide the first clear evidence for a crustal magma chamber at a slow spreading mid-ocean ridge. The data constrain the melt fraction within the body to be at least 20 per cent, with a melt volume suYcient to feed crustal accretion at this segment of the ridge for of the order of 20 000 years. Since this body would freeze in the order of 1500 years, this finding lends support to the hypothesis that, at slow spreading rates, crustal accretion is a cyclic process, accompanying periodic influxes of melt from the mantle to a crustal melt reservoir.

The RAMESSES experiment-II. Evidence for accumulated melt beneath a slow spreading ridge from wide-angle refraction and multichannel reflection seismic profiles

The RAMESSES study (Reykjanes Axial Melt Experiment: Structural Synthesis from Electromagnetics and Seismics) targeted an apparently magmatically active axial volcanic ridge (AVR), centred on 57°45′N at the Reykjanes Ridge, with the aim of investigating the processes of crustal accretion at a slow spreading mid-ocean ridge. As part of this multicomponent experiment, airgun and explosive wide-angle seismic data were recorded by 10 digital ocean-bottom seismometers (OBSs) along profiles oriented both across- and along-axis. Coincident normal-incidence seismic, bathymetry and underway gravity and magnetic data were also collected. Forward modelling of the seismic and gravity data has revealed layer thicknesses, velocities and densities similar to those observed elsewhere within the oceanic crust near mid-ocean ridges. At 57°45′N, the Reykjanes Ridge has a crustal thickness of approximately 7.5 km on-axis. However, the crust is modelled to decrease in thickness slightly off-axis (i.e. with age), which implies that full crustal thickness is achieved on-axis and that it is subsequently thinned, most likely, by off-axis extension. Modelling also indicates that the AVR is underlain by a thin (∼100 m), narrow (∼4 km) melt lens some 2.5 km beneath the seafloor, which overlies a broader zone of partial melt approximately 8 km in width. Thus the results of this study provide the first clear evidence for a crustal magma chamber beneath any slow spreading ridge. The size and depth of this magma chamber (the melt lens and underlying zone of partial melt) are similar to those observed beneath fast and intermediate spreading ridges, which implies that the processes of crustal accretion are similar at all spreading rates. Hence the lack of previous observations of magma chambers beneath slow spreading ridges is probably temporally related to the periods of magmatic activity being considerably shorter and more widely spaced in time than at fast and intermediate spreading ridges.

Magmatic processes at slow spreading ridges: implications of the RAMESSES experiment at 57° 45′N on the Mid-Atlantic Ridge

SUMMARY This paper is the first in a series of three (this issue) which present the results of the RAMESSES study (Reykjanes Axial Melt Experiment: Structural Synthesis from Electromagnetics and Seismics). RAMESSES was an integrated geophysical study which was carefully targeted on a magmatically active, axial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57°45aeN. It consisted of three major components: wide-angle seismic profiles along and across the AVR, using ocean-bottom seismometers, together with coincident seismic reflection profiles; controlled-source electromagnetic sounding (CSEM); and magnetotelluric sounding (MT). Supplementary data sets included swath bathymetry, gravity and magnetics. Analyses of the major components of the experiment show clearly that the sub-axial geophysical structure is dominated by the presence and distribution of aqueous and magmatic fluids. The AVR is underlain by a significant crustal magma body, at a depth of 2.5 km below the sea surface. The magma body is characterized by low seismic velocities constrained by the wide-angle seismic data; a seismic reflection from its upper surface; and a region of anomalously low electrical resistivity constrained by the CSEM data. It includes a thin, ribbon-like melt lens at the top of the body and a much larger region containing at least 20 per cent melt in a largely crystalline mush zone, which flanks and underlies the melt lens. RAMESSES is the first experiment to provide convincing evidence of a significant magma body beneath a slow spreading ridge. The result provides strong support for a model of crustal accretion at slow spreading rates in which magma chambers similar to those at intermediate and fast spreading ridges play a key role in crustal accretion, but are short-lived rather than steady-state features. The magma body can exist for only a small proportion of a tectono-magmatic cycle, which controls crustal accretion, and has a period of at least 20 000 years. These findings have major implications for the temporal patterns of generation and migration of basaltic melt in the mantle, and of its delivery into the crust, beneath slow-spreading mid-ocean ridges.

Geometry of a mid-ocean-ridge normal fault

Normal faults play a dominant role in the morphology and internal structure of crust formed at slow-spreading mid-ocean ridges. The surface expression of these faults has been studied well, but few direct controls are available on their shape at depth. Multichannel seismic reflection profiles of old oceanic lithosphere potentially contain a wealth of structural information, but the origin of many of the features observed in these profiles remains controversial. Sedimented rifts provide a rare opportunity to study present-day tectonics of mid-ocean ridges with conventional seismic reflection profiles. We have used two such profiles and a recently developed inverse method to infer, from the deformation of sediments in the hanging wall, the geometry of a normal fault bounding the axis of the Juan de Fuca Ridge at Middle Valley. The fault is listric in cross section and can be traced about 5 km below the sea bed. Its shape is similar to whole-crust reflectors imaged in old oceanic lithosphere.

Abyssal peridotite mylonites: implications for grain-size sensitive flow and strain localization in the oceanic lithosphere

Microstructures preserved in abyssal peridotites dredged from the oceans record several different physical regimes of deformation. Fabrics associated with deformation processes at slow-spreading mid-ocean ridges form two major classes of abyssal peridotites based on detailed microstructural observations. The most abundant class are medium- to coarse-grained tectonites with microstructures that reflect deformation processes during mantle upwelling and emplacement to the base of the lithosphere. These tectonites give geothermometric temperatures of ∼755°C or higher, interpreted to represent lower temperature limits for diffusive exchange in coarse-grained abyssal peridotites during cooling. This conclusion is consistent with flow laws for olivine at these temperatures. The second class of abyssal peridotites, previously largely undescribed for the mid-ocean ridge environment, include fine-grained mylonites associated with faulting and shear zones that develop during extension and cooling of the oceanic lithosphere when the brittle-plastic transition extends into mantle rocks. These mylonites give temperatures of ∼600°C, which we suggest represent a lower temperature limit for plastic deformation. Reduced grain size in mylonites allows for diffusive exchange to continue to these low temperatures. Relict augen in the mylonitic samples preserve equilibration temperatures similar to those exhibited by the coarse-grained tectonites.Based on flow laws for olivine, we suggest that deformation in some fine-grained mylonites occurred by diffusion creep down to ∼600°C. Rheological data for olivine indicate that dislocation creep is not likely to occur at this temperature. We conclude that a reduction in grain size by cataclasis, or dynamic recrystallization, resulted in a transition in deformation mechanisms from dislocation- to diffusion-creep during uplift (and/or cooling). The observation of these fine-grained mylonites indicates that shear zones that extend into the upper mantle will be weaker than expected if deformation was accommodated by brittle processes or dislocation creep. Weak faults may promote the development of long-lived detachments in the upper mantle. This inference supports mid-ocean-ridge tectonic models that suggest that ultramafic rocks exposed at the inside corners of ridge-axis discontinuities are exhumed along long-lived detachment faults.