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Significance This paper estimates natural hydrogen production by various geological processes and its ultimate fate in young oceanic crust near the Mid-Ocean Ridge (MOR). Hydrogen is an important source of energy for microbes living beneath the seafloor. Knowing how much is available to support these biological communities is therefore key to understanding the size and significance of this biome, one of the potential first environments for life on Earth and a potential environment for life on other planets.

Major transform fault zones link extensional segments of the North American – Eurasian plate boundary as it transects the Iceland Hotspot. Changes in plate boundary geometry, involving ridge jumps, rift propagation, and related transform fault zone migration, have occurred as the boundary has moved relative to the hotspot. Reconfiguration of transform fault zones occurred at about 6 Ma in northern Iceland and began about 3 Ma in southern Iceland. These systems show a range of different types of transform fault zones, ranging from diffuse, oblique rift zones to narrower, well-defined, transform faults oriented parallel to current plate motions. Crustal deformation structures correlate with the inferred duration and magnitude of strike-slip displacements. Collectively, the different expressions of transform zones may represent different stages of development in an evolutionary sequence that may be relevant for understanding the tectonic history of plate boundaries in Iceland as well as the structure of transform fault zones on more typical parts of the mid-ocean ridge system.

The tectonic window at Pito Deep, in the southern Pacific Ocean, permits study of the formative processes of uppermost East Pacific Rise (EPR) gabbroic ocean crust. Here we present a detailed microstructural and crystallographic study of 17 gabbroic samples from the uppermost ∼800 m of plutonic crust exposed in the Pito Deep Rift. We integrate two- and three-dimensional measurements of crystal size, shape, spatial distribution and orientation, with petrographic observations and geochemical data to constrain the formation of fast spread gabbroic ocean crust. The shallowest samples, collected < 55 metres below the sheeted dikes (mbsd), have evolved bulk-rock compositions, elongate plagioclase crystals, a clear plagioclase shape- and crystallographic-preferred orientation, and preserve only minor amounts of intracrystalline strain. The characteristics of these rocks and their proximity to the sheeted dike complex, suggests they formed by crystallization at the lateral tip of an axial melt lens that solidified as it moved off axis. Underlying samples from 96–724 mbsd, record more primitive bulk-rock compositions, less elongate plagioclase crystals and exhibit increasing strength of both plagioclase shape- and crystallographic-preferred orientation with depth below the sheeted dikes. These samples host plagioclase crystals that show increasing intracrystalline strain with depth, suggesting magmatic to hypersolidus submagmatic flow within the mush zone beneath the axial melt lens. These observations, together with inclined-to-steeply dipping mineral layering preserved below ∼180 mbsd, are interpreted to record the downward transport of crystal-rich magma originating at the bottom of the melt lens through the uppermost kilometre of the mush zone at the EPR. The location of initial crystallization along the floor of the axial melt lens determines the magmatic processes that affect the crystal-rich magma en route to solidification as lower ocean crust.

Ferrovolcanism, yet to be directly observed, is the most exotic and poorly understood predicted manifestation of planetary volcanism. Large-scale experiments carried out at the Syracuse Lava Project offer insight into the emplacement dynamics of metallic flows as well as coeval metallic and silicate flows. Here, we find that, under the same environmental conditions, higher-density/lower-viscosity metallic lava moves ten times faster than lower-density/higher-viscosity silicate lava. The overall morphology of the silicate flow is not significantly affected by the co-emplacement of a metallic flow. Rather, the metallic flow is largely decoupled from the silicate flow, occurring mainly in braided channels underneath the silicate flow and as low-relief breakouts from the silicate flow front. Turbulent interactions at the metallic-silicate flow interface result in mingling of the two liquids, preserved as erosional surfaces and sharp contacts. The results have important implications for the interpretation of possible ferrovolcanic landscapes across our solar system., Ferrovolcanism is a hypothetical form of planetary volcanism in which the erupted lava is metallic in composition. Here we show that ferrovolcanic lava is denser and less viscous than silicate lava, resulting in fast-moving, thin, braided flows.

Free hydrogen (H

Unlike most of the Mid-Atlantic Ridge, the North America/Eurasia plate boundary in Iceland lies above sea level where magmatic and tectonic processes can be directly investigated in subaerial exposures. Accordingly, geologic processes in Iceland have long been recognized as possible analogs for seafloor spreading in the submerged parts of the mid-ocean ridge system. Combining existing and new data from across Iceland provides an integrated view of this active, mostly subaerial plate boundary. The broad Iceland plate boundary zone includes segmented rift zones linked by transform fault zones. Rift propagation and transform fault migration away from the Iceland hotspot rearrange the plate boundary configuration resulting in widespread deformation of older crust and reactivation of spreading-related structures. Rift propagation results in block rotations that are accommodated by widespread, rift-parallel, strike-slip faulting. The geometry and kinematics of faulting in Iceland may have implications for spreading processes elsewhere on the mid-ocean ridge system where rift propagation and transform migration occur.

Magmatic rift systems are composed of discrete spreading segments defined by morphologic, structural, and volcanic features that vary systematically along strike. In Iceland, structural features mapped in the glaciated and exhumed Miocene age upper crust correlate with analogous features in the seismically and volcanically active neovolcanic zone. Integrating information from both the active rift zones and ancient crust provides a three-dimensional perspective of crustal structure and the volcanic and tectonic processes that construct crust along spreading segments. Crustal exposures in the Skagi region of northern Iceland reveal significant along-strike variations in geologic structure. The upper crust at exhumed magmatic centers (segment centers) is characterized by a variety of intrusive rocks, high-temperature hydrothermal alteration, and geologic evidence for kilometer-scale subsidence. In contrast, the upper crust along segment limbs, which extend along strike from magmatic centers, is characterized by thick sections of gently dipping lava flows, cut by varying proportions of subvertical dikes. This structure implies relatively minor upper crustal subsidence and lateral dike intrusion. The differing modes of subsidence beneath segment centers and segment limbs require along-axis mass redistribution in the underlying upper, middle, and lower crust during crustal construction. This along-axis material transport is accomplished through lateral dike intrusion in the upper crust and by along-axis flow of magmatic to high-temperature solid-state gabbroic material in the middle and lower crust. These processes, inferred from outcrop evidence in Skagi, are consistent with processes inferred to be important during active rifting in Iceland and at analogous magmatic oceanic and continental rifts.

A 2003 high-resolution magnetic survey conducted by the Autonomous Underwater Vehicle ABE over the low-temperature, ultramafic-hosted hydrothermal field Lost City reveals a weak positive magnetic anomaly. This observation is in direct contrast to recent observations of strong positive magnetic anomalies documented over the high-temperature ultramafic-hosted hydrothermal vents fields Rainbow and Ashadze, which indicates that temperature may control the production of magnetization at these sites. The Lost City survey provides a unique opportunity to study a field that is, to date, one of a kind, and is an end member of ultramafic-hosted hydrothermal systems. Our results highlight the key contribution of temperature on magnetite production resulting from serpentinization reactions. Whereas high temperature promotes significant production and partitioning of iron into magnetite, low temperature favors iron partitioning into various alteration phases, resulting in a magnetite-poor rock. Moreover, the distribution of magnetic anomalies confirms results of a previous geological survey indicating the progressive migration of hydrothermal activity upslope. These discoveries contribute to the results of 25 yrs of magnetic exploration of a wide range of hydrothermal sites, from low- to high-temperature and from basalt- to ultramafic-hosted, and thereby validate using high-resolution magnetics as a crucial parameter for locating and characterizing hydrothermal sites hosting unique chemosynthetic-based ecosystems and potentially mineral-rich deposits.

Bubbles are known to have a dramatic effect on lava rheology, but their impact on lava flow run out distance has not been extensively investigated. To quantify bubble effects on basaltic lava flow emplacement, we conducted a series of large-scale two-phase (melt+bubbles) experiments at the Syracuse University Lava Project facility. In this study, nine basaltic lava flows with low to intermediate vesicle contents (approximately 3-24 vol%) were emplaced over gently sloping (4°-9°) dry sand surfaces. Their velocities were measured by particle image velocimetry, and their bulk viscosities were calculated using Jeffreys' equation. Comparison with melt-only viscosities (calculated through the GRD model) allowed the isolation of the effect of bubbles on the viscosity of the experimental flows. We find that within the investigated range of vesicularities, bubbles increase the relative viscosity of the experimental flows by up to an order of magnitude. The increase appears to be steeper for sheet flows than for lobate and toey flows. Our results, which quantitatively assess the effect of bubbles on the rheology of basaltic lava flows, will provide a basis for the refinement of models for multi-phase flows that are fundamental to accurately forecasting the emplacement of lava flows.

One of the World's premier field geologists, Kristjan Saemundsson led immense geological mapping programs and authored or co-authored nearly all geological maps of Iceland during the past half century, including the first modern bedrock and tectonic maps of the whole country. These monumental achievements collectively yield the most inclusive view of an extensional plate boundary anywhere on Earth. When Kristjan began his work in 1961, the relation of Iceland to sea-floor spreading was not clear, and plate tectonics had not yet been invented. Kristjan resolved key obstacles by demonstrating that the active rifting zones in Iceland had shifted over time and were linked by complex transforms to the mid-ocean spreading ridge, thus making the concept of sea-floor spreading in Iceland acceptable to those previously skeptical. Further, his insights and vast geological and tectonic knowledge on both high- and low-temperature geothermal areas in Iceland yielded a major increase in knowledge of geothermal systems, and probably no one has contributed more than he to Icelandic energy development. Kristjan's legacy is comprised by his numerous superb maps on a variety of scales, the high quality papers he produced, the impactful ideas generated that were internationally diffused, and the generations of colleagues and younger people he inspired, mentored, or otherwise positively influenced with his knowledge and generous attitude.

Persistent, coordinated plate separation and dike intrusion generate sheeted dike complexes in oceanic crust at mid-ocean ridge spreading centers and other magmatic rifts. Although sheeted dike complexes were first described in ophiolite complexes, investigations of dikes, dike intrusion events and sheeted dike complexes in the oceanic crust have provided new constraints on how sheeted dike complexes form and their significance for the accretion of oceanic crust at spreading centers. Despite the general appearance of a monotonous array of side-by-side intrusions, details of sheeted dike complexes hold important keys to understanding the fundamentals of the tectonics, magma plumbing networks and hydrothermal/biological systems at mid-ocean ridges. In situ investigations of sheeted dikes and related upper crustal units in seafloor exposures provide fundamental observations that have implications for deformation during spreading, the reconstruction of ophiolite complexes, and the restoration of ophiolite structures to spreading center reference frames.

Viscoelastic strain in lava flows is commonly expressed as gravity-driven buckling of the lava crust. This surface folding process creates the well-known ropy pāhoehoe texture of basaltic lavas and the ogives and surface ridges of more compositionally evolved lava flows. Previous work has shown that surface fold wavelengths are proportional to the viscosity contrast between the lava crust and core, and to the thickness of the crust. Thus, fold analysis can be an important tool for understanding lava flow rheology. We analyze fold wavelength patterns of solidified natural lava flows from the Myvatn lava fields (Iceland), Piton de La Fournaise (La Reunion), and experimental lava flows from the Syracuse University Lava Project. In each case, lava flows exhibited two dominant wavelengths, consistent with multiple generations of coaxial folding. The ratio of the two dominant wavelengths for basalt (Iceland, La Reunion) is ~ 5:1 whereas the wavelength ratio for basaltic andesite (Syracuse) is ~ 3:1, suggesting a compositional control on deformation, as proposed by previous studies. Video analysis of incrementally folded Syracuse lava crusts reveals significant non-coaxial strain, which violates the assumptions of plane strain used in crustal buckling models. These results show that interpreting lava rheology from finite strain requires careful consideration of complex three-dimensional strain fields. Despite these complexities, the correlation between fold wavelength ratios and lava flow composition persists and may provide important insight into flow characterization.

Dike intrusion is a fundamental process during upper oceanic crustal accretion at fast- to superfast-spreading ridges. Based on the distribution of magma along fast-spreading centers inferred from marine geophysical data, models predict systematic steep flow at magmatically robust segment centers and shallow magma flow toward distal segment ends. Anisotropy of magnetic susceptibility (AMS) fabrics from 48 fully-oriented block samples of dikes from upper oceanic crust exposed at Hess Deep Rift and Pito Deep Rift reveal a wide range of magma flow directions that are not consistent with such simple magma supply models. The AMS is interpreted to arise from distribution anisotropy of titanomagnetite crystals based on weak shape-preferred orientation of opaque oxide and plagioclase crystals generally parallel to AMS maximum eigenvectors. Most dike samples show normal AMS fabrics with maximum eigenvector directions ranging from subvertical to subhorizontal. The distributions of inferred magma flow lineations from maximum eigenvectors show no preferred flow pattern, even after structural correction. We use a Kolmogorov–Smirnov test (KS-test) to show that the distribution of bootstrapped flow lineation rakes from Pito Deep are not statistically distinct from Hess Deep, and neither are distinguishable from Oman and Troodos Ophiolite AMS data. Magma flow directions in sheeted dikes from these two seafloor escarpments also do not correlate with available geochemistry in any systematic way as previously predicted. These results indicate distinct compositional sources feed melt that is injected into dikes at fast- to superfast-spreading ridges with no preference for subhorizontal or subvertical magma flow. Collectively, results imply ephemeral melt lenses at different along-axis locations within the continuous axial magma chamber and either direct injection or intermingling of melt from other deeper ridge-centered or off-axis sources.

Quantitative measurements of interactions between lava and ice/snow are critical for improving our knowledge of glaciovolcanic hazards and our ability to use glaciovolcanic deposits for paleoclimate reconstructions. However, such measurements are rare because the eruptions tend to be dangerous and not easily accessible. To address these difficulties, we conducted a series of pilot experiments designed to allow close observation, measurements, and textural documentation of interactions between basaltic melt and ice. Here we report the results of the first experiments, which comprised controlled pours of as much as 300 kg of basaltic melt on top of ice. Our experiments provide new insights on (1) estimates for rates of heat transfer through boundary layers and for ice melting; (2) controls on rates of lava advance over ice/snow; (3) formation of lava bubbles (i.e., Limu o Pele) by steam from vaporization of underlying ice or water; and (4) the role of within-ice discontinuities to facilitate lava migration beneath and within ice. The results of our experiments confirm field observations about the rates at which lava can melt snow/ice, the efficacy with which a boundary layer can slow melting rates, and morphologies and textures indicative of direct lava-ice interaction. They also demonstrate that ingestion of external water by lava can create surface bubbles (i.e., Limu) and large gas cavities. We propose that boundary layer steam can slow heat transfer from lava to ice, and present evidence for rapid isotopic exchange between water vapor and melt. We also suggest new criteria for identifying ice-contact features in terrestrial and martian lava flows.

Many caldera systems worldwide are interpreted to form as a result of catastrophic subsidence into space vacated by eruption. Some basaltic calderas, however, both associated with modern active volcanoes and in ancient, deeply eroded terranes, lack evidence for catastrophic formation and thus may have formed incrementally. The Vatnsdalur structural basin in northern Iceland is one such caldera-like volcanic depression. The Vatnsdalur structural basin is an elliptical depression with dimensions of ∼6 km × ∼3 km and a structural depth of ∼1.5 km. It occurs within the gently west-dipping, Tertiary basaltic lava sequences of the Skagi region, northern Iceland. Tilted basaltic to rhyolitic lava flows define a large-scale, rim monocline surrounding the basin. Exposures that form the southern rim of the Vatnsdalur structural basin reveal details of the pre-, syn-, and postsubsidence volcanic and tectonic processes during basin evolution. In these exposures, the dips of lava flows steepen continuously from horizontal to as much as 60°W, toward the center of the basin. Basaltic lava flows and mass-flow breccia units were deposited unconformably over the dipping lava flows and thicken substantially downdip, toward the basin center. The dip of bedding and flow contacts in these overlying units decreases up section, gradually becoming conformable with the regional lava-flow stratigraphy. Brittle folding of the lava flows within the rim monocline occurred by a combination of slip on minor faults and syntectonic dike intrusion. Based on structural evidence and comparisons to worldwide caldera systems, we conclude that the Vatnsdalur structural basin formed as a result of either (1) loading of the subsurface by dense, intrusive material or (2) shallow crustal magma redistribution accommodating many small subsidence events. In the context of crustal construction in Iceland, the Vatnsdalur structural basin and adjacent areas demonstrate significant along-strike variability in the mechanism(s) of crustal construction that occurs during subaerial seafloor spreading.

Summary. In this paper we present laboratory measurements of compress- ional and shear wave velocities of a diverse suite of gabbroic rocks collected from the walls of the Mid-Cayman Spreading Centre with DSRV Alvin. The degree of deformation and alteration affecting these gabbros is quite variable, and we believe that they are typical of plutonic rocks emplaced at shallow levels (upper portion of seismic layer 3 and shallower) of the oceanic crust. The compositional and textural variations are reflected in the wide range of laboratory velocities which span most of the range of seismic velocities reported for oceanic and ophiolite rock samples including basalts, gabbros, ultramafics, and their altered derivatives. Based upon the laboratory velocities and the geological setting of the Mid-Cayman gabbros, it is argued that no unique lithology, except anhydrous peridotite, can be unequivocally identified in the oceanic lithosphere from seismic velocity data alone. Furthermore, these data allow for the possibility of considerable lithologic heterogeneity within portions of the oceanic crust at the scale of a few centimetres to a few hundred metres. Such heterogeneities would go unrecog- nized because seismic refraction studies mask these variations resulting in a picture of apparent uniformity.

Inward-dipping (cone) sheet swarms and an associated central volcano are well-exposed in the deeply-eroded Tertiary crust of Vatnsdalur, Skagi Peninsula region, northern Iceland. Spatially registered orientations of 389 mafic sheets, mapped in three distinct sheet swarms define both the overall shape and magmatic source of each swarm. The Vatnsdalur sheet swarms consist of planar inward-dipping sheets that collectively define a conical shape rather than a bowl- or trumpet-shape as have been found in swarms in other locations. In the best exposed swarm, three-dimensional projection of mafic sheets into the subsurface defines two distinct foci, which are interpreted as the magmatic sources of two temporally distinct sub-swarms. These results help to establish the influence of inclined sheet intrusion on crustal accretion at central volcanoes. The geometry of the swarm constrains the thickness of material that was added to the crust during sheet intrusion. When combined with estimates of surface relief, we calculate that 2.2 to 4.1 km of subsidence were required beneath the central volcano in order to accommodate the intrusion of the sheet swarm. Similar processes of crustal thickening and subsidence likely occur in a wide variety of both continental rift and mid-ocean ridge systems where magmatic activity is focused at central volcanoes.

A suite of samples collected from the uppermost part of the plutonic section of the oceanic crust formed at the southern East Pacific Rise and exposed at the Pito Deep has been examined. These rocks were sampled in situ by ROV and lie beneath a complete upper crustal section providing geological context. This is only the second area (after the Hess Deep) in which a substantial depth into the plutonic complex formed at the East Pacific Rise has been sampled in situ and reveals significant spatial heterogeneity in the plutonic complex. In contrast to the uppermost plutonic rocks at Hess Deep, the rocks studied here are generally primitive with olivine forsterite contents mainly between 85 and 88 and including many troctolites. The melt that the majority of the samples crystallized from was aggregated normal mid-ocean ridge basalt (MORB). Despite this high Mg# clinopyroxene is common despite model predictions that clinopyroxene should not reach the liquidus early during low-pressure crystallization of MORB. Stochastic modeling of melt crystallisation at various levels in the crust suggests that it is unlikely that a significant melt mass crystallized in the deeper crust (for example in sills) because this would lead to more evolved shallow level plutonic rocks. Similar to the upper plutonic section at Hess Deep, and in the Oman ophiolite, many samples show a steeply dipping, axis-parallel, magmatic fabric. This suggests that vertical magmatic flow is an important process in the upper part of the seismic low velocity zone beneath fast-spreading ridges. We suggest that both temporal and spatial (along-axis) variability in the magmatic and hydrothermal systems can explain the differences observed between the Hess Deep and Pito Deep plutonics.

The deep oceans and global seafloor are truly Earth's last frontier. They remain largely unexplored, yet are critical to our survival on this planet. This magnificent, full-color volume transports you to bizarre landscapes hosting exotic life forms that rival the most imaginative science fiction. Starting with a historical summary of seafloor exploration and the developing technologies used to study this extreme environment, it then describes the distinctive geologic components of the Earth's ocean floor and the unusual biological communities found along the mid-ocean ridges. This is an indispensable reference for researchers, teachers, and students of marine science, and a visually stunning resource that will enlighten and intrigue oceanographers and enthusiasts alike. A suite of online resources, including photographs and video clips, combine with the book to provide fascinating insights into the hidden world of seafloor geology and biology using the latest deep-sea imaging and geological concepts.

The faulted walls of the Hess Deep Rift in the equatorial Pacific provide one of the few significant tectonic windows into the oceanic crust created at a fast-spreading mid-ocean ridge. The 5000-m-deep rift graben exposes a slightly dismembered section of oceanic crust created at the East Pacific Rise ∼1 Ma. The upper portion of the North Wall of the rift features a laterally extensive exposure of upper crustal rock units similar to those found in many ophiolite complexes. For tens of kilometers along the rift wall, basaltic lavas (with sparse dikes) grade down into a spectacular sheeted dike complex that is underlain locally by a heterogeneous assemblage of gabbroic rocks. The geometry of lava flows, dikes and fault zones that developed during magmatic accretion provide constraints on the cross-axis (temporal) evolution of the upper crust in this area. Outcrop-scale digital imagery from submersible vehicles provides a well-constrained geological framework for samples collected from these upper crustal rock units, and the geochemistry of lava and dike samples provides additional temporal constraints on crustal accretion and the role of along-axis magma transport during accretion. Integrated results of geological and geochemical investigations suggest a four-dimensional model for upper crustal construction whereby tectonic and magmatic processes act in concert to construct the upper oceanic crust within a few kilometers (few tens of thousands of years) of a fast-spreading ridge axis. In this model, a sub-axial melt lens, where it exists, is recharged from below with distinct parental melts, and limited along-axis mixing preserves geochemical segmentation of the melt lens. Dikes emanate from distinct portions of the melt lens and transport magmas vertically, as well as significant distances laterally along-axis, erupting lavas in the special case where magmas ascend through dikes to higher levels in the crust and intersect the seafloor. Differences in the phenocryst contents and compositions of lavas and dikes suggests that density filtering of buoyant magmas, among other factors including magma overpressure and orientations of local stresses, plays a role in the eruption of magma on the surface. During spreading and magmatic construction, subaxial subsidence accommodates thickening of the basaltic lava unit. Sub-axial subsidence, faulting, and block rotation result in an upper crust with inward-dipping (toward the axis) lava flows and outward-dipping (away from the axis) dikes. Substantial differences in upper crustal geology may result from segment-scale variations in which vertical magma delivery dominates in areas of robust magma supply (likely over sites of recently recharged melt lenses at segment centers) but lateral magma transport dominates in areas of relatively lower magma supply (possibly toward segment ends).

The serpentinite-hosted Lost City hydrothermal field is a remarkable submarine ecosystem in which geological, chemical, and biological processes are intimately interlinked. Reactions between seawater and upper mantle peridotite produce methane- and hydrogen-rich fluids, with temperatures ranging from

Quaternary mafic lavas from Lake Turkana (northern Kenya) provide information on processes operating beneath the East African Rift in an area of anomalous lithospheric and crustal thinning. Inferred depths of melting beneath Turkana (15---20km) are shallower than those recorded elsewhere along the rift, consistent with the anomalously thin crustal section. The mafic lavas have elevated incompatible trace element contents when compared with midocean ridge basalts, requiring an enrichment event in the source region. Basalts with low Sr isotopic ratios ( � 0� 7030) have high 143 Nd/ 144 Nd (40� 5129) and 206 Pb/ 204 Pb values ( � 19� 4) and incompatible trace element abundances that indicate derivation from a sub-lithospheric mantle source region. Quaternary mafic rocks with 10---15 wt % MgO record contributions from a mantle plume that is isotopically similar to the deep mantle source region for global hotspots. These Turkana basalts have isotope and incompatible trace element ratios that overlap with those of Quaternary mafic lavas from the Red Sea, the western Gulf of Aden, and northern Kenya, interpreted as being derived from mixtures of plume and ambient mantle sources. The Turkana data imply a common and long-lived mantle plume composition beneath both the Ethiopia and Kenya domes. This scenario is supported by tomographic results indicating a discontinuous thermal and chemical anomaly that originates in the deep mantle beneath southern Africa, and is also consistent with the seismically determined shallow mantle structure beneath East Africa.

Strontium, carbon, and oxygen isotope data and radiocarbon ages document at least 30,000 years of hydrothermal activity driven by serpentinization reactions at Lost City. Serpentinization beneath this off-axis field is estimated to occur at a minimum rate of 1.2 × 10 – 4 cubic kilometers per year. The access of seawater to relatively cool, fresh peridotite, coupled with faulting, volumetric expansion, and mass wasting processes, are crucial to sustain such systems. The amount of heat produced by serpentinization of peridotite massifs, typical of slow and ultraslow spreading environments, has the potential to drive Lost City–type systems for hundreds of thousands, possibly millions, of years.

Blueschists from the Bou Azzer inlier provide compelling evidence for Late Proterozoic subduction in the Anti-Atlas Mountains of Morocco. High-pressure/low-temperature metabasites containing blue amphibole minerals crossite and magnesioriebeckite record pressures in excess of 5 kbar. Together with regional relationships, the geologic setting of the blueschists constrains the polarity of Pan African subduction in this region, which occurred from ∼750 to 600 Ma. Blueschist facies rocks crop out in a heterogeneous assemblage of variably deformed and metamorphosed tectonic slices of ophiolitic fragments enclosed in a schistose serpentinite matrix. The melange belt containing the blueschist facies rocks is intruded by a number of diorite plutons, one of which has yielded a U/Pb radiometric age of 650 Ma. Together with Transaharan Belt to the southeast, the Anti-Atlas suture zone exposed within the Bou Azzer inlier contains among the oldest known blueschist-bearing, ophiolitic melanges in the world.

▪ Abstract Geological investigations of major fault scarps (“tectonic windows”) and DSDP/ODP Drill Holes provide direct views of the uppermost oceanic crust generated at fast- to intermediate-rate spreading centers. These areas reveal a consistent upper crustal structural geometry with basaltic lava flows defining a pattern of downward increasing (“inward”) dip toward the spreading center at which they formed and dikes in the lavas and underlying sheeted dike complex showing a similar degree of “outward” dip. Widespread fracturing, faulting, and hydrothermal metamorphism accompanied magmatic construction. These geological relationships can be interpreted in terms of dramatic, asymmetrical, subaxial subsidence of upper crustal rock units that diminishes across the very narrow (few kilometers wide) zone of lava accumulation and dike intrusion at the ridge axis. This type of crustal structure is in accord with some existing models of spreading but augments these idealized views with more realistic geological complexity.

The oceanic core complex comprising Atlantis Massif was formed within the past 1.5–2 Myr at the intersection of the Mid-Atlantic Ridge, 30° N, and the Atlantis Transform Fault. The corrugated, striated central dome prominently displays morphologic and geophysical characteristics representative of an ultramafic core complex exposed via long-lived detachment faulting. Sparse volcanic features on the massif's central dome indicate that minor volcanics have penetrated the inferred footwall, which geophysical data indicates is composed predominantly of variably serpentinized peridotite. In contrast, the hanging wall to the east of the central dome is comprised of volcanic rock. The southern part of the massif has experienced the greatest uplift, shoaling to less than 700 m below sea level, and the coarsely striated surface there extends eastward to the top of the median valley wall. Steep landslide embayments along the south face of the massif expose cross sections through the core complex. Almost all of the submersible and dredge samples from this area are deformed, altered peridotite and lesser gabbro. Intense serpentinization within the south wall has likely contributed to the uplift of the southern ridge and promoted the development of the Lost City Hydrothermal Field near the summit. Differences in the distribution with depth of brittle deformation observed in microstructural analyses of outcrop samples suggest that low-temperature strain, such as would be associated with a major detachment fault, is concentrated within several tens of meters of the domal surface. However, submersible and camera imagery show that deformation is widespread along the southern face of the massif, indicating that a series of faults, rather than a single detachment, accommodated the uplift and evolution of this oceanic core complex.

Tectonic exposures of a volcanic sequence and sheeted dike complex over a 4-km-wide region at Hess Deep (equatorial Pacific) reveal significant spatial heterogeneity (10–103 m) in the extent and nature of hydrothermal alteration in young, fast spreading East Pacific Rise crust. The volcanic sequence is fairly uniformly altered, with only minor oxidation and alteration to clay minerals. Sheeted dikes in the eastern part of the field area are highly fractured with narrow intervals of intact dikes that dip up to 60°. Their alteration characteristics show a simple depth trend such that with increasing depth the dominant secondary mafic mineral changes from chlorite to amphibole, clinopyroxene replacement increases ( 40%), whole rock δ18O values decreases (4.4–5.5‰ to 3.5–4.5‰), and calculated peak metamorphic temperatures increase (∼250°C to 450°–700°C). Within the deepest dikes, localized zones up to 400-m-wide are chlorite-rich and have low-δ18O (2.9–4.1‰) and low peak metamorphic temperatures (∼345°C). These alteration patterns likely formed within broad recharge zones whereby the low-δ18O zones developed in the regions with the highest fluid flux. In the west, massive, slightly rotated sheeted dikes near the volcanic-sheeted dike transition are δ18O and Cu depleted and display higher peak temperatures (≥345°C) than elsewhere in the shallow dikes. These characteristics are consistent with formation within a high temperature, hydrothermal discharge zone. We propose that the spreading history of a fast spreading ridge segment can create significant spatial heterogeneity in fluid flow and alteration patterns within sheeted dike complexes, similar to those preserved in many ophiolites.

Evidence is growing that hydrothermal venting occurs not only along mid-ocean ridges but also on old regions of the oceanic crust away from spreading centres. Here we report the discovery of an extensive hydrothermal field at 30 degrees N near the eastern intersection of the Mid-Atlantic Ridge and the Atlantis fracture zone. The vent field--named 'Lost City'--is distinctly different from all other known sea-floor hydrothermal fields in that it is located on 1.5-Myr-old crust, nearly 15 km from the spreading axis, and may be driven by the heat of exothermic serpentinization reactions between sea water and mantle rocks. It is located on a dome-like massif and is dominated by steep-sided carbonate chimneys, rather than the sulphide structures typical of 'black smoker' hydrothermal fields. We found that vent fluids are relatively cool (40-75 degrees C) and alkaline (pH 9.0-9.8), supporting dense microbial communities that include anaerobic thermophiles. Because the geological characteristics of the Atlantis massif are similar to numerous areas of old crust along the Mid-Atlantic, Indian and Arctic ridges, these results indicate that a much larger portion of the oceanic crust may support hydrothermal activity and microbial life than previously thought.

Major- and trace-element analyses of over 50 well-located samples from the third segment south of the Kane Transform Fault are reported. These can be grouped into four age groups based on structural relations and paleomagnetic data. Although no spatial relations are discerned and most trace-element concentrations vary with the extent of fractionation, trends of decreasing Rb and Cs and increasing Ba and Th over the past million years are revealed. The compositions of these samples are similar to basaltic rocks recovered from around the Kane Transform Fault and allow assessment of geochemical variability along a series of geologically diverse ridge segments both north of the transform and from greater than 150 km to the south. Most of the variation can be explained by polybaric fractionation with local contributions from enriched sources. Remarkably, the extreme variations in crustal thickness, segment morphology, depth of the median valley, and even the presence of the Kane Transform Fault are not associated with compositionally distinct parental basaltic liquids. A comparison with other relatively isotopically homogeneous ridge segments reveals that parental basalts from near the Kane Transform Fault are not uniquely homogeneous. This comparison suggests that crustal morphology and spreading rate do not play a dominant role in the amount of compositional diversity in parental basalts generated at mid-ocean ridges.

Reconstruction of latest Neoproterozoic Gondwana hinges on the interpretation of the subduction and collision kinematics of Pan-African orogenic belts that rim the West African craton. The Anti-Atlas suture zone of southern Morocco has presented an enigma in this reconstruction as the inferred subduction zone polarity and age of suturing appear to be incongruous with better known West African orogens to the west (Mauretanian, Bassaride and Rokelide) and Transaharan orogens to the east (Ougarta, Tuareg, Gourma and Dahomeyan). Contrary to previous interpretations, new data from the Anti-Atlas indicate a history of late Neoproterozoic (∼750–600) north-dipping subduction culminating in the (∼600 Ma) collision of the Saghro magmatic arc to the north with the north-facing rifted margin of the West African craton. Thus, the Anti–Atlas suture links a ∼6000-km long chain of Pan-African suture zones that essentially encircle the West African craton. The suture zones demarcate the former position of subduction zones that in all cases dipped away from the West African craton. The Anti-Atlas suture links the western and eastern segments of the Pan-African orogenic belts associated with the amalgamation of Western Gondwana.

Mass spectrometric measurements of 230 Th– 226 Ra, 235 U– 231 Pa and 238 U– 230 Th disequilibria are used to determine eruption ages for four mid-ocean ridge basalts from the median valley of the Mid-Atlantic Ridge south of the Kane Fracture Zone (MARK area). Three samples were collected across-axis on the Axial Volcanic Ridge (i.e. the Neovolcanic Ridge) near the Snake Pit hydrothermal mound, and one sample was collected near the crest of Serocki Volcano ∼50 km south of Snake Pit. ( 226 Ra)/( 230 Th) and ( 230 Th)/( 238 U) activity ratios are low and uniform for all four samples, while ( 231 Pa)/( 235 U) activity ratios are elevated and somewhat more variable. Age constraints suggest that these lavas, from the most robust volcanic edifices in the MARK area, are 10 000–20 000 yr old. The age data are used to evaluate the efficacy of commonly used age estimate scales based on qualitative indicators (e.g. sediment cover, glass quality) and to begin to quantify the temporal and spatial dependence of volcanic, tectonic and hydrothermal processes at slow-spreading oceanic ridges.

Large volumes of pseudotachylyte (an intrusive, fault-related rock interpreted to form by a combination of cataclasis and melting) occur in Tertiary normal faults and accommodation zones along ∼400 km of the East Greenland volcanic rifted margin. Analysis of representative pseudotachylyte samples reveals a wide range of mesoscopic and microscopic textures, mineralogies, and chemistries in the aphanitic pseudotachylyte matrix. Three distinct types of pseudotachylyte (referred to as angular, rounded and glassy) are identified based on these characteristics. Angular pseudotachylyte (found primarily in dike-like reservoir zones) is characterized by angular grains visible on all scales, with micron-scale fragments of mica and amphibole. Its matrix is enriched in Fe2O3, MgO, and TiO2 relative to the host rock, with minor increases in CaO, K2O, and small decreases in Na2O. Rounded pseudotachylyte is found in reservoir zones, injection veins (pseudotachylyte-filled extension fractures), and fault veins (small faults with pseudotachylyte along their surfaces). It is characterized by smooth-surfaced, compacted grains on microscopic scales, and encloses rounded, interpenetrative lithic clasts on outcrop scale. Its matrix is enriched in Fe2O3, MgO, TiO2, and Al2O3 relative to the host rock, with minor depletion in Na2O and K2O. Glassy pseudotachylyte is found primarily along fault surfaces. Its matrix is characterized by isotropic, conchoidally fractured material containing microscopic, strain-free amphibole phenocrysts, and is enriched in TiO2, Al2O3, K2O, Fe2O3, MgO, CaO, and Na2O relative to the host rock. These observations suggest that angular pseudotachylyte was produced by cataclasis, with enrichment in metallic oxides resulting from preferential crushing of mechanically weak amphibole and mica minerals found in the gneissic host rock. Cataclasis and concomitant frictional heating resulted in the textural and chemical modification of angular pseudotachylyte by sintering or melting, producing rounded and glassy pseudotachylyte, respectively. Compositional and textural observations constrain the temperatures reached during frictional heating (700–900°C) which in turn delimit the amount of frictional heat imparted to the pseudotachylytes during slip. Our results suggest that the East Greenland pseudotachylytes formed during small seismic events along faults at shallow crustal levels. Consistent relative ages and widespread occurrence of pseudotachylyte-bearing faults in East Greenland suggest that widespread microseismicity accompanied the early development of this volcanic rifted margin.