Asymmetric fracture zones and sea-floor spreading

An asymmetric pattern is observed in the orientation of minor fracture zones about the axis of the Mid-Atlantic Ridge at five sites where relatively detailed studies have been made between latitudes 22°N and 51°N. The minor fracture zones intersect the axis of the Mid-Atlantic Ridge in an asymmetric V-shaped configuration. The V's point south north of the Azores triple junction (38°N latitude) and point north south of that junction.The rates and directions of sea-floor spreading are related to the asymmetric pattern of minor fracture zones at the sites studied. Half-rates of sea-floor spreading averaged between about 0 and 10 m.y. are unequal measured perpendicular to the ridge axis. The unequal half-rates of spreading are faster to the west north of the Azores triple junction and faster to the east south of that junction. The half-rates of sea-floor spreading calculated in the directions of the asymmetric minor fracture zones are equal about the ridge axis within the uncertainty of the direction determinations.A discrepancy exists between minor fracture zones that form an asymmetric V about the axis of the Mid-Atlantic Ridge, and major fracture zones that follow small circles symmetric about the ridge axis. To reconcile this discrepancy it is proposed that minor fracture zones are preferentially reoriented under the influence of a stress field related to interplate and intraplate motions. Major fracture zones remain symmetric about the Mid-Atlantic Ridge under the same stress field due to differential stability between minor and major structures in oceanic lithosphere. This interpretation is supported by the systematic variation in the orientation of minor fracture zones and the equality of sea-floor spreading half-rates observed about lithospheric plate boundaries.     

Neotectonic modelling of the western part of the Africa–Eurasia plate boundary: from the Mid-Atlantic ridge to Algeria

In this work we use the thin-shell approximation to model the neotectonics of the western part of the Africa–Eurasia plate boundary, extending from the Mid-Atlantic ridge to Tell Atlas (northern Algeria). Models assume a nonlinear rheology and include laterally variable heat flow, elevation, and crust and lithospheric mantle thickness. Including the Mid-Atlantic ridge permits us to evaluate the effects of ridge push and to analyse the influence of the North America motion on the area of the Africa–Eurasia plate boundary. Ridge push forces were included in a self-consistent manner and have been shown to exert negligible effects in the neotectonics of the Iberian Peninsula and northwestern Africa. Different models were computed with systematic variation of the fault friction coefficient. Model quality was scored by comparing predictions of anelastic strain rates, vertically integrated stresses and velocity fields to data on seismic strain rate computed from earthquake magnitude, most compressive horizontal principal stress direction, and seafloor spreading rates on the Mid-Atlantic ridge. The best model scores were obtained with fault friction coefficients as low as 0.06–0.1. The velocity boundary condition representing spreading on the Mid-Atlantic ridge is shown to produce concentrated deformation along the ridge and to have negligible effect in the interior of the plates. However, this condition is shown to be necessary to properly reproduce the observed directions of maximum horizontal compression on the Mid-Atlantic ridge. The maximum fault slip rates predicted by the model are obtained along the Mid-Atlantic ridge, Terceira ridge and Tell Atlas front. Relatively high slip rates are also obtained in the area between the Gloria fault and the Gulf of Cadiz. We infer from our modelling a significant long-term seismic hazard for the Gloria fault, and interpret the absence of seismicity on this fault as possibly due to transient elastic strain accumulation. The present study has also permitted better understanding of the geometry of the Africa–Eurasia plate boundary from the Azores triple junction to the Algerian Basin. The different deformational styles seem to be related to the different types of lithosphere, oceanic or continental, in contact at the plate boundary.     

The Mid-Atlantic Ridge between 29°N and 31°30′N in the last 10 Ma

The segmentation of the Mid-Atlantic Ridge between 29°N and 31°30′ N during the last 10 Ma was studied. Within our survey area the spreading center is segmented at a scale of 25–100 km by non-transform discontinuities and by the 70 km offset Atlantis Transform. The morphology of the spreading center differs north and south of the Atlantis Transform. The spreading axis between 30°30′N and 31°30′N consists of enéchelon volcanic ridges, located within a rift valley with a regional trend of 040°. South of the transform, the spreading center is associated with a well-defined rift valley trending 015°. Magnetic anomalies and the bathymetric traces left by non-transform discontinuities on the flanks of the Mid-Atlantic Ridge provide a record of the evolution of this slow-spreading center over the last 10 Ma. Migration of non-transform offsets was predominantly to the south, except perhaps in the last 2 Ma. The discontinuity traces and the pattern of crustal thickness variations calculated from gravity data suggest that focused mantle upwelling has been maintained for at least 10 Ma south of 30°30′ N. In contrast, north of 30°30′N, the present segmentation configuration and the mantle upwelling centers inferred from gravity data appear to have been established more recently. The orientation of the bathymetric traces suggests that the migration of non-transform offsets is not controlled by the motion of the ridge axis with respect to the mantle. The evolution of the spreading center and the pattern of segmentation is influenced by relative plate motion changes, and by local processes, perhaps related to the amount of melt delivered to spreading segments. Relative plate motion changes over the last 10 Ma in our survey area have included a decrease in spreading rate from 32 mm a⁻¹ to 24 mm a⁻¹, as well as a clockwise change in spreading direction of 13° between anomalies 5 and 4, followed by a counterclockwise change of 4° between anomaly 4 and the present. Interpretation of magnetic anomalies indicates that there are significant variations in spreading asymmetry and rate within and between segments for a given anomaly time. These differences, as well as variations in crustal thickness inferred from gravity data on the flanks of spreading segments, indicate that magmatic and tectonic activity are, in general, not coordinated between adjacent spreading segments.     

Mantle flow and melting beneath oceanic ridge–ridge–ridge triple junctions

Plate boundary geometry likely has an important influence on crustal production at mid-ocean ridges. Many studies have explored the effects of geometrical features such as transform offsets and oblique ridge segments on mantle flow and melting. This study investigates how triple junction (TJ) geometry may influence mantle dynamics. An earlier study [Georgen, J.E., Lin, J., 2002. Three-dimensional passive flow and temperature structure beneath oceanic ridge-ridge-ridge triple junctions. Earth Planet. Sci. Lett. 204, 115–132.] suggested that the effects of a ridge–ridge–ridge configuration are most pronounced under the branch with the slowest spreading rate. Thus, we create a three-dimensional, finite element, variable viscosity model that focuses on the slowest-diverging ridge of a triple junction with geometry similar to the Rodrigues TJ. This spreading axis may be considered to be analogous to the Southwest Indian Ridge. Within 100 km of the TJ, temperatures at depths within the partial melting zone and crustal thickness are predicted to increase by ~ 40 °C and 1 km, respectively. We also investigate the effects of differential motion of the TJ with respect to the underlying mantle, by imposing bottom model boundary conditions replicating (a) absolute plate motion and (b) a three-dimensional solution for plate-driven and density-driven asthenospheric flow in the African region. Neither of these basal boundary conditions significantly affects the model solutions, suggesting that the system is dominated by the divergence of the surface places. Finally, we explore how varying spreading rate magnitudes affects TJ geodynamics. When ridge divergence rates are all relatively slow (i.e., with plate kinematics similar to the Azores TJ), significant along-axis increases in mantle temperature and crustal thickness are calculated. At depths within the partial melting zone, temperatures are predicted to increase by ~ 150 °C, similar to the excess temperatures associated with mantle plumes. Likewise, crustal thickness is calculated to increase by approximately 6 km over the 200 km of ridge closest to the TJ. These results could imply that some component of the excess volcanism observed in geologic settings such as the Terceira Rift may be attributed to the effects of TJ geometry, although the important influence of features like nearby hotspots (e.g., the Azores hotspot) cannot be evaluated without additional numerical modeling.     

Azores mantle blob: Rare-earth evidence

Rare earths (RE) in basalts erupted within the rift of the Mid-Atlantic Ridge show a progressive change from light-RE enriched to depleted patterns from the Azores Platform (40°N) down to 33°30′N. South, the pattern remains light-RE depleted as along other “normal ridge” segments. A progressive increase in chemical variability of the basalts towards the Azores is also noted.The latitudinal RE profile and corresponding ΣFeO/ΣFeO + MgO variations, together, indicate that the origin of these basalts cannot be accounted for simply by considering variable extents of partial melting of a single mantle source and subsequent fractional crystallization during the ascent of the magmas. These two processes produce only second-order effects on the RE patterns. The data requires the presence of a distinct, light-RE richer, mantle source beneath the Azores Platform relative to that of south of 33°30′N and an intermediate zone where both mantle types mix. The relative contribution of the Azores mantle source to the mix appears to decrease fairly regularly southward along the ridge and becomes negligible at 33°30′N. Increasing chemical variability of the basalts towards the Azores is probably caused by correspondingly larger extent of fractional crystallization at shallow depth, and/or greater variability in the extent of partial melting, apparently subsequent to, and superimposed on the mixing of the two mantle sources.The combined morphological, geophysical and RE evidence along the profile are consistent with a model suggesting upwelling of a major blob (plume) under the Azores Plateau; and reveal the present extent of the blob's overflow and mixing with the asthenosphere depleted in large ionic lithophile trace elements. The influence of the Azores blob is geochemically detectable up to 1000 km southwestward beneath the ridge axis.     

Assessment of site-effects using acceleration time series. Application to São Sebastião volcanic crater

The Azores archipelago is located at the North America, Eurasia and Africa triple junction and shows seismicity and volcanism typical of an active plate boundary. Terceira Island has been affected by several damaging earthquakes being the 1980 January 1st event (M=7.2) the strongest in Azores during the last century.     

Geodynamic processes that are involved in the generation of the Northern Mid-Atlantic Ridge

This paper contains a comparative analysis of the theoretical parameters involved in the subsidence of spreading ridges into the asthenosphere: Reykjanes, Kolbeinsey, the Azores segment of the Mid-Atlantic Ridge, as well as the following aseismic ridges: the Ninety East Ridge, Maldives, Hawaiian-Emperor, and Louisville ridges due to the influence of a mantle plume. We conclude that the respective geodynamic processes involved in generating spreading ridges in the North Atlantic and the aseismic ocean ridges due to hotspot action are similar. The main phases in the evolution of the Iceland region are substantiated using geological and geophysical data and computer simulation. We discuss the Cenozoic tectonic evolution of the region, calculated and plotted paleogeodynamic reconstructions of the North Atlantic Ocean in the hotspot system for 60, 50, and 20 Ma.     

The present relative motion between Africa and Antarctica

Movement between the Africa and Antarctica plates is at present accomplished by sea-floor spreading on the Southwest Indian Ocean Ridge. This movement may be described in terms of an angular rotation vector. Bathymetric and magnetic observations from marine geophysical surveys near the Bouvet triple junction, at 52°S, 15°E and in the environs of the Prince Edward Islands are combined with spreading azimuths derived from earthquake fault plane solutions to define this vector. The rotation pole which describes the motion is located at 10.7°N, 40.9°W and the angular velocity is 1.44 × 10^?7 deg/yr. This pole is significantly different from some other poles obtained by global closure or vector addition. The possibility that the differences may be due to Africa being split into two plates is investigated but there would have to be convergence across the African Rift system for this possibility to be true. Closure of the vector velocity triangle around the Central Indian triple junction is checked by using the pole derived in this study and published poles and rates for the Africa/India and Antarctica/India motions to determine this triangle. The triangle is found to close when errors in the Africa/India and Antarctica/India motions are taken into account. This suggests that it is errors in the data that cause the differences between the observed and predicted poles.     

Multi-stage evolution of a sub-aerial volcanic ridge over the last 1.3 Myr: S. Jorge Island, Azores Triple Junction

New K/Ar dating and geochemical analyses have been carried out on the WNW–ESE elongated oceanic island of S. Jorge to reconstruct the volcanic evolution of a linear ridge developed close to the Azores triple junction. We show that S. Jorge sub-aerial construction encompasses the last 1.3 Myr, a time interval far much longer than previously reported. The early development of the ridge involved a sub-aerial building phase exposed in the southeast end of the island and now constrained between 1.32 ± 0.02 and 1.21 ± 0.02 Ma. Basic lavas from this older stage are alkaline and enriched in incompatible elements, reflecting partial melting of an enriched mantle source. At least three differentiation cycles from alkaline basalts to mugearites are documented within this stage. The successive episodes of magma rising, storage and evolution suggest an intermittent re-opening of the magma feeding system, possibly due to recurrent tensional or trans-tensional tectonic events. Present data show a gap in sub-aerial volcanism before a second main ongoing building phase starting at about 750 ka. Sub-aerial construction of the S. Jorge ridge migrated progressively towards the west, but involved several overlapping volcanic episodes constrained along the main WNW–ESE structural axis of the island. Mafic magmas erupted during the second phase have been also generated by partial melting of an enriched mantle source. Trace element data suggest, however, variable and lower degrees of partial melting of a shallower mantle domain, which is interpreted as an increasing control of lithospheric deformation on the genesis and extraction of primitive melts during the last 750 kyr. The multi-stage development of the S. Jorge volcanic ridge over the last 1.3 Myr has most likely been greatly influenced by regional tectonics, controlled by deformation along the diffuse boundary between the Nubian and the Eurasian plates, and the increasing effect of sea-floor spreading at the Mid-Atlantic Ridge.     

The Kane fracture zone in the Central Atlantic Ocean

The Kane fracture zone has been traced as a distinct topographic trough from the Mid-Atlantic Ridge near 24°N to the 80-m.y. B.P. isochron (magnetic anomaly 34) on either side of the ridge axis for a total of approximately 2800 km. Major changes in trend of the fracture zone occur at approximately 72 m.y. B.P. (anomaly 31 time) and approximately 53–63 m.y. B.P. (anomaly 21–25 time) which are the result of major reorientations in spreading directions in the central Atlantic Ocean.     

The accreting plate boundary Ardoukoˆba Rift (northeast Africa) and the oceanic Rift Valley

The Ardouko?ba Rift, subaerially exposed for ～12 km between the Ghoubbat-al-Kharab and Lake Asal in the French Territory of the Afars and Issas (northeast Africa), has intrinsic features and a regional setting consistent with arguments that it is the site of crustal accretion at approximately the same rate as that found along the rifted Mid-Oceanic Ridge (～2 cm/yr). The ～11 km wide Central Zone of the Ardouko?ba Rift has an internal relief of less than ～300 m and is set between step-like ridges standing up to 800 m above the deepest part of the rift. The lower inward-facing scarps of the Central Zone border a narrow Inner Floor. The Central Zone of the typically ～25–35 km wide oceanic Rift Valley can have a greater and rougher relief and has a width of ～8–16 km, but deep areas with an internal relief of <400 m have a maximum width that is about the same as that of the corresponding area in the Ardouko?ba Rift (～11 km). The width of the Inner Floor of the Ardouko?ba Rift varies from 2 to 5 km; in the oceanic Rift Valley the range is from less than 1 to ～9 km. Equivalence of tectonic and volcanic processes in the two settings has not been demonstrated; but a comparison of a segment of the Rift Valley in the FAMOUS area near 36°50′N in the Atlantic with the Ardouko?ba Rift encourages the tentative use of evidence from the latter to complement arguments about the pattern of vulcanism and scarp formation in the oceanic Rift Valley as a whole. The Inner Floor of the Rift Valley is the main site of horizontal extension without vertical displacements, of normal faulting that involves little or no accumulation of vertical offsets, and of constructional vulcanism, which may be further concentrated along narrow (～1 km wide) fissured zones. The normal faulting that disrupts and constrains more or less orderly growth of the Inner Floor may happen in such a way that the new graben that become new Inner Floors are laterally offset with respect to the middle line of the Rift Valley and to the axis of symmetry of a hypothetical block accounting for the central positive magnetic anomaly.Additional complexities may be introduced by syntectonic and post-tectonic vulcanism, and by normal fault displacement at any one time of young crust along only part of the distance between transform faults. Thus, although opening rate can always be equated in principle with total addition of new crust to the two plates, the assumption is suspect that the concept of spreading rate (rate of addition of crust to one plate or the other) can necessarily be applied precisely to the central part of the Rift Valley. In more general terms, the physical meaning of interpolated spreading rates on the time scale of magnetic anomalies is worth questioning. On evidence from the Rift Valley, the spreading rates need not reflect monotonic additions of new crust, and rocks of the same inferred age from opposite plates may not have the same composition. The problem is highlighted by the apparently symmetrical growth of the North Atlantic over long periods of time.     

The Newfoundland Basin: ocean-continent boundary and Mesozoic seafloor spreading history

The ocean-continent boundary in the Newfoundland Basin is defined as the seaward limit of a continental margin magnetic smooth zone. East of the Grand Banks this boundary is marked by a prominent NNE-trending magnetic anomaly that is correlated with the J-Anomaly (115 m.y.). South of Flemish Cap the smooth zone boundary strikes approximately 060° and is approximately 15 m.y. younger. Magnetic anomaly trends suggest two directions of motion during separation of Iberia and North America. The first phase of motion, commencing at J-Anomaly time with a spreading center strike of 015°, produced a rifted margin along the Grand Banks south of the Newfoundland Seamounts. No spreading occurred north of the seamounts during this phase, implying a counter-clockwise rotation of Iberia and no Grand Banks-Galicia Bank separation. The second phase began at about 102 m.y. with a shift of the pole of rotation to a location near Paris, producing a ridge orientation of approximately 060°. This spreading center extended north and east into the northern Newfoundland Basin and Bay of Biscay, producing a rifted margin south of Flemish Cap and opening of Biscay. This ridge geometry produced a component of extension across the Newfoundland Fracture Zone and the southeastward migration of the resultant “leaky” transform fault between 102 m.y. and the next pole shift produced the volcanic edifice of the Southeast Newfoundland Ridge. Fracture zone trends during this phase also exerted strong control on volcanism within the Newfoundland Seamount province; this activity ceased at about 97 m.y. The date at which the second phase ended is not well defined by presently available data. A RRR triple-junction existed in the northeastern Newfoundland Basin-western Biscay region for a short time prior to anomaly33/34 (80 m.y.) which marks the inception of a continuous Mid-Atlantic Ridge spreading center between the Newfoundland and Charlie Gibbs Fracture Zones.     

Pito and Orongo fracture zones: the northern and southern boundaries of the Easter microplate (southeast Pacific)

The Easter (Rapanui) microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on Earth is occurring today. Recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be critically constrained by improved relative motion and structural fabric data sets.During the January 1987 Rapanui expedition on the N/O “Jean Charcot” we conducted a Sea Beam/magnetics/ gravity survey of a portion of the microplate boundaries. The method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships. The northern terminus of the East Rift or eastern boundary of the microplate is expressed as a series of parallel NW-SE trending valleys including what appears to be, with 5890 m depth, the deepest active rift axis mapped in the Pacific today (Pito Rift).The northern end of the Pito Rift merges with an E-W to 083° narrow band of linear faults interpreted to be a transform fault between the Nazca and Easter (Rapanui) plates.The northern triple junction between the Easter (Rapanui), Nazca and Pacific plates is a RFF type with the two transform faults colinear along an approximately E-W direction.The southwestern boundary of the Easter (Rapanui) microplate is marked by a series of en-echelon offsets, outlined by depressions, which merge into an approximately E-W zone where shear must be predominant.The southern triple junction is a RRF junction with an overlapping ridge system.The structural data acquired during the survey provide strong constraints for kinematic models of the microplate. The structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate.     

Sea-floor spreading in the Bay of Biscay and its relationship to the North Atlantic

An analysis of the magnetic anomaly profiles in the Bay of Biscay provides evidence for the former existence of an E-W trending sea-floor spreading axis in Biscay. Identification of the magnetic anomalies indicates that the opening of the Bay of Biscay took place during the Cretaceous, between Barremian and Maestrichtian times, and involved the formation of a triple-ridge junction with the Mid-Atlantic Ridge between 80 and 73 m.y. ago. The asymmetric distribution of magnetic anomalies in the Bay of Biscay is confirmed. This evidence, together with a proposed Lower Cretaceous development of the Mid-Atlantic Ridge suggests that Biscay evolved as a result of a three-phase rotation of Iberia.     

Plate kinematics: The Americas,East Africa,and the rest of the world

Euler vectors (relative angular velocity vectors) have been determined for twelve major plates by global inversion of carefully selected sea-floor spreading rates, transform fault trends, and earthquake slip vectors. The rate information comes from marine magnetic anomalies less than 5 m.y. old, so the motions are valid for post-Miocene times. Plate motions in a mean hotspot frame of reference have also been determined, and statistical confidence limits for all the Euler vectors estimated. Among the consequences of the global motion model is the conclusion that fast-spreading ridges (separation rates greater than 3 cm/yr) have plate motion nearly perpendicular to the strike of the ridge and magnetic anomalies. Four more slowly separating ridges have an average obliquity of spreading of almost 20°.For several plate boundaries, results that differ from previous studies are in agreement with geological evidence. The North and South American plates converge slowly about a pole east of the Antilles and near the Mid-Atlantic Ridge. The results for Africa versus Somalia imply slow east-west extension on the East African Rift Valleys. The pole for motion of Eurasia relative to North America is located near Sakhalin, in accordance with evidence from Siberia and Sakhalin.     

Possible extensions of the Narmada-Son lineament towards Murray Ridge (Arabian Sea) and the eastern syntaxial bend of the Himalayas

Geophysical data contiguous with the Narmada-Son lineament suggests its possible extension westward into the Arabian Sea and eastward up to the Shillong Plateau. The airborne magnetic anomaly map of the north Arabian Sea delineates a linear trend of magnetic anomalies in line with the Narmada-Son lineament. This group of magnetic anomalies, spreading over 20°N to 22°N, starts near the west coast of India at 21°N, 69°30′E and extends to the Murray Ridge. The tectonic feature represented by this group of magnetic anomalies is buried by a thick layer of sediments. This westward extension of the lineament is also reflected in the average Bouguer gravity anomaly map of the Indian Ocean. Towards the east, the gravity and magnetic data delineate a subsurface linear tectonic feature which extends in line with this lineament to the eastern syntaxial bend. These various geophysical signatures further suggest the lineament to be a typical rift-like structure. The tectonic implications of the lineament, which extends from the western to the eastern margins of the Indian plate, is discussed.     

A new contribution to the geology of the Egyptian Red Sea shelf using geophysical data

The study examines the Egyptian Red Sea shelf and throws more light on the structural set-up and tectonics controlling the general framework of the area and nature of the crust. Herein, an integrated study using gravity and magnetic data with the available seismic reflection lines and wells information was carried out along the offshore area. The Bouguer and reduced-to-pole aeromagnetic maps were processed and reinterpreted in terms of rifting and plate tectonics. The qualitative interpretation shows that the offshore area is characterized by positive gravity everywhere that extremely increases towards the centre of the graben, supporting the presence of an intrusive zone below the axial/main trough. The gravity data were confirmed by the presence of high magnetic amplitudes, magnetic linearity and several dipoles concentrated along the rift axis for at least 250 km. The lineament analysis indicates widespread of the Erythrean (Red Sea) trend that was offset/cut by transform faults in the NE direction (Aqaba). The tectonic model suggests the presence of one tensional (N65°E) and two compressional (N15°W, N30°W) phases of tectonism, resulted in six cycles of deformations, classified into three left lateral (N35°E, N15°E and N–S) and three right lateral (N85°W, N45°W and N60°W). The basement relief map reveals a rough basement surface that varies in depth between 1 and 5.6 km. It outlines several offshore basins, separated from each other by ridges. The models show that the basement consists of tilted fault blocks, which vary greatly in depth and composition and slopes generally to the west. They indicate that the coastal plain is underlain by acidic basement blocks (continental crust) with no igneous activity while suggesting elevated basic materials (oceanic crust) below the rift axis. The study suggests that northern Red Sea forms an early stage of seafloor spreading or at least moved past the late stage of continental rifting.     

New constraints on the tectonic evolution of the eastern Indian Ocean

From marine magnetic anomaly studies, a fossil spreading ridge is identified beneath the Nicobar Fan in the northwestern Wharton Basin. Several north-south-trending transform faults offset this ridge left-laterally east of the 86°E transform fault. Our findings show that this ridge, which was part of the plate boundary between the Indian and Australian plates, ceased its spreading shortly after formation of magnetic anomaly 20 (～ 45.6m.y. B.P.). Since the breakup of Australia and Antarctica probably occurred sometime between 110 and 90 m.y. B.P., we suggest that the Indian, Australian, and Antarctic plates were moving relative to one another from about 90 to 45 m.y. B.P. A triple junction would have existed in the southeastern Indian Ocean during that period of time. At anomaly 19 time (～ 45m.y. B.P.), the junction became inactive, and Australia and India became a single plate. The northwest-southeast-trending Southeast Indian Ridge was formed by connecting the India-Antarctica spreading center with the Australia-Antarctica spreading center. Its activity has continued to the present time.     

Geochemistry of basalts drilled in the North Atlantic by IPOD Leg 49: Implications for mantle heterogeneity

IPOD Leg 49 recovered basalts from 9 holes at 7 sites along 3 transects across the Mid-Atlantic Ridge: 63°N (Reykjanes), 45°N and 36°N (FAMOUS area). This has provided further information on the nature of mantle heterogeneity in the North Atlantic by enabling studies to be made of the variation of basalt composition with depth and with time near critical areas (Iceland and the Azores) where deep mantle plumes are thought to exist. Over 150 samples have been analysed for up to 40 major and trace elements and the results used to place constraints on the petrogenesis of the erupted basalts and hence on the geochemical nature of their source regions.It is apparent that few of the recovered basalts have the geochemical characteristics of typical “depleted” midocean ridge basalts (MORB). An unusually wide range of basalt compositions may be erupted at a single site: the range of rare earth patterns within the short section cored at Site 413, for instance, encompasses the total variation of REE patterns previously reported from the FAMOUS area. Nevertheless it is possible to account for most of the compositional variation at a single site by partial melting processes (including dynamic melting) and fractional crystallization. Partial melting mechanisms seem to be the dominant processes relating basalt compositions, particularly at 36°N and 45°N, suggesting that long-lived sub-axial magma chambers may not be a consistent feature of the slow-spreading Mid-Atlantic Ridge.Comparisons of basalts erupted at the same ridge segment for periods of the order of 35 m.y. (now lying along the same mantle flow line) do show some significant inter-site differences in Rb/Sr, Ce/Yb,⁸⁷Sr/⁸⁶Sr, etc., which cannot be accounted for by fractionation mechanisms and which must reflect heterogeneities in the mantle source. However when hygromagmatophile (HYG) trace element levels and ratios are considered, it is the constancy or consistency of these HYG ratios which is the more remarkable, implying that the mantle source feeding a particular ridge segment was uniform with respect to these elements for periods of the order of 35 m.y. and probably since the opening of the Atlantic. Yet these HYG element ratios at 63°N are very different from those at 45°N and 36°N and significantly different from the values at 22°N and in “MORB”.The observed variations are difficult to reconcile with current concepts of mantle plumes and binary mixing models. The mantle is certainly heterogeneous, but there is not simply an “enriched” and a “depleted” source, but rather a range of sources heterogeneous on different scales for different elements — to an extent and volume depending on previous depletion/enrichment events. HYG element ratios offer the best method of defining compositionally different mantle segments since they are little modified by the fractionation processes associated with basalt generation.     

Subduction influence on magma supply at the East Scotia Ridge

Despite a spreading rate of 65–70 km Ma⁻¹, the East Scotia Ridge has, along most of its length, a form typically associated with slower rates of sea floor spreading. This may be a consequence of cooler than normal mantle upwelling, which could be a feature of back-arc spreading. At the northern end of the ridge, recently acquired sonar data show a complex, rapidly evolving pattern of extension within 100 km of the South Sandwich Trench. New ridge segments appear to be nucleating at or near the boundary between the South American and Scotia Sea plates and propagating southwards, supplanting older segments. The most prominent of these, north of 56°30′S, has been propagating at a rate of approximately 60 km Ma⁻¹ for at least 1 Ma, and displays a morphology unique on this plate boundary. A 40 km long axial high exists at the centre of this segment, forming one of the shallowest sections of the East Scotia Ridge. Beneath it, seismic reflection profiles reveal an axial magma chamber, or AMC, reflector, similar to those observed beneath the East Pacific Rise and Valu Fa Ridge. Simple calculations indicate the existence here of a narrow (<1 km wide) body of melt at a depth of approximately 3 km beneath the sea floor. From the topographic and seismic data, we deduce that a localised mantle melting anomaly lies beneath this segment. Rates of spreading in the east Scotia Sea show little variation along axis. Hence, the changes in melt supply are related to the unique tectonic setting, in which the South American plate is tearing to the east, perhaps allowing mantle flow around the end of the subducting slab. Volatiles released from the torn plate edge and entrained in the flow are a potential cause of the anomalous melting observed. A southward mantle flow may have existed beneath the axis of the East Scotia Ridge throughout its history.