Plate Boundary Geometry Research Articles

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.

宮城県沖地震のアスペリティとプレート境界面の形状に関する考察

宮城県沖地震のアスペリティとプレート境界面の形状に関する考察

Constraints on the structural development of Afar imposed by the kinematics of the major surrounding plates

Abstract The development of the Afar rift-rift-rift triple junction is analysed from the view-point of the Nubia, Arabia and Somali plate kinematics. A variety of constraints allow definition of a range of kinematic models that approximate well to the plate motions with a resolution of a couple to tens of kilometres. Rigid plate kinematics probably cannot resolve smaller motions. The size and location of the new area that opened between the major plates is inferred from plate kinematics and this provides a framework in which to assess the structural development that accommodates plate separation. The development of the Afar region was complicated by the presence of microplates — the Danakil and Aisha blocks — which results in a complex plate boundary geometry. This led to local deformation that does not directly reflect the divergence of the major plates, e.g. rotations of microplates and of minor blocks about vertical axes and strike-slip faulting. The opening of new area was accommodated by various crustal growth and accretion mechanisms, e.g. building of thick new igneous crust, normal seafloor spreading, and/or crustal stretching. Thus, plate motions by themselves do not determine the development of the plate boundaries, as this is strongly influenced by other factors such as lateral variations of the rates of magma supply (e.g. away from, and over, the Afar plume). The plate boundaries changed — e.g. the Gulf of Aden spreading centre propagated westward c. 2 Ma ago and normal seafloor spreading began along portions of the Red Sea axis since c. 5 Ma ago — while there were no resolvable changes in the plate motions. Such changes therefore signify a reorganization of the way in which plate divergence and addition of new area is accommodated: diffuse extension may give way to separation along a narrow spreading centre, or new plate boundaries may form at the expense of other boundaries that became inactive.

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.

Bending of the subducting oceanic plate and its implication for rupture propagation of large interplate earthquakes off Miyagi, Japan, in the Japan Trench subduction zone

We present the results of an onshore–offshore wide‐angle refraction and reflection experiment off Miyagi, in the central Japan Trench forearc region. There are two rupture zones of large interplate earthquakes here: the landward rupture zone and the trenchward rupture zone. To examine the influence of plate boundary geometry on the distributions of the rupture zones, we determined reflector geometries from reflections. The subducting oceanic plate increases its dip from about 5° to 13° around 143.2°E. This bending point in the oceanic plate corresponds to the eastern edge of the trenchward rupture zone. Moreover, another bending point may be present at approximately 142.3°E, which corresponds to the eastern edge of the landward rupture zone. The coincidence between bending points in the oceanic plate and the edges of rupture zones suggests that changes in the plate boundary geometry may affect rupture propagation of interplate earthquakes in the direction of plate convergence.

Regionalization of the Recent Tectonic Stress Field in China and Adjacent Regions

AbstractAccording to the stress state and force source character, the recent tectonic stress field of China and adjacent regions is classified into four classes: The first order tectonic stress district is mostly controlled by the geometry of plate boundary and the force acting on plate boundary, the stress action of this stress field shows uniformity and stability in large areas and long time. The stress action of second order tectonic stress district shows correlation in a large area. This stress field is controlled by regional block interaction. The stress regime of the third order tectonic stress district shows comparability in finite areas. This stress filed is mostly influenced by interior block interaction inside the areas. The stress character (direction, intensity, regime, etc) of the forth order stress district shows better coherence. This stress filed is mostly controlled by block and fault interaction, and some may be influenced by its crust structure and tectonic environment. Recent tectonic stress field in China and adjacent region is divided into two first order districts, four second order districts, five third order districts and twenty‐six fourth order districts.

Long-term elasticity in the continental lithosphere; modelling the Aden Ridge propagation and the Anatolian extrusion process

SUMMARY The evolution of the Gulf of Aden and the Anatolian Fault systems are modelled using the principles of elastic fracture mechanics usually applied to smaller scale cracks or faults. The lithosphere is treated as a plate, and simple boundary conditions are applied that correspond to the known plate boundary geometry and slip vectors. The models provide a simple explanation for many observed geological features. For the Gulf of Aden the model predicts why the ridge propagated from east to west from the Owen Fracture Zone towards the Afar and the overall form of its path. The smaller en echelon offsets can be explained by upward propagation from the initially created mantle dyke while the larger ones may be attributed to the propagating rupture interacting with pre-existing structures. For Anatolia the modelling suggests that the East Anatolian Fault was created before the North Anatolian Fault could form. Once both faults were formed however, activity could switch between them. The time scales over which this should take place are not known, but evidence for switching can be found in the historical seismicity. For Aden and Anatolia pre-existing structures or inhomogeneous stress fields left from earlier orogenic events have modified the processes of propagation and without an understanding of the existence of such features the propagation processes cannot be fully understood. Furthermore a propagating fault can extend into an active region where it would not have initiated. The North Anatolian Fault encountered slow but active extension when it entered the Aegean about 5 Ma and the stress field associated with the extending fault has progressively modified Aegean extension. In the central Aegean activity has been reduced while to the north-west on features such as the Gulfs of Evvia and Corinth activity has been increased. The field observation that major structures propagate and the success of simple elastic models suggest that the continental crust behaves in an elastic-brittle or elastic-plastic fashion even though laboratory tests may be interpreted to suggest viscous behaviour. There are major problems in scaling from the behaviour of small homogeneous samples to the large heterogeneous mantle and large-scale observations should be treated more seriously than extrapolations of the behaviour of laboratory experiments over many orders of magnitude in space and time. The retention of long-term elasticity and localised failure suggests a similar gross rheology for the oceanic and continental lithospheres. Even though it is incorrect to attribute differences in behaviour to the former being rigid (i.e. elastic) and the latter viscous, oceanic and continental lithosphere behave in different ways. Unlike oceanic crust, continental crust is buoyant and cannot be simply created or destroyed. The process of thickening or thinning works against gravity preventing large displacements on extensional or contractional features in the upper mantle. The equivalents of ridge or subduction systems are suppressed before they can accommodate large displacements and activity must shift elsewhere. On the other hand, strike-slip boundaries and extrusion processes are favoured.

The Central Basin Spreading Center in the Philippine Sea: Structure of an extinct spreading center and implications for marginal basin formation

Newly acquired marine geophysical data along the Central Basin Spreading Center (CBSC), the extinct spreading axis of the West Philippine Basin (WPB), display along‐axis variations of spreading style. We have described and analyzed the tectonic spreading fabric and segmentation patterns along a 1000‐km‐long section of the fossil ridge between 126°00′ and 133°30′E. “Slow‐spreading features” (deep rift valleys and nodal basins, rough abyssal hills on the ridge flanks, and mantle Bouguer anomaly lows beneath segment centers) are observed in the eastern CBSC. In contrast, “fast‐spreading” features (overlapping spreading centers, volcanic axial ridges, and smooth abyssal hill fabric) are identified along the western CBSC. We attribute the large morphological and geophysical variations along the CBSC to higher melt supply in the western region caused by high mantle temperature and/or mantle heterogeneity, which may be related to a relatively small‐scale mantle plume forming the oceanic plateaus located in the WPB. Another prominent feature of the CBSC is the development of a deep valley oblique to the spreading fabric. Reconstructions of the plate boundary geometry through time, using abyssal hill geometry as well as other measurements, provide evidence for a later stage of amagmatic extension (i.e., reactivation) along the CBSC after the formation of the main basin. This stage is dominated by tectonic deformation with minor amounts of volcanism (mainly located in eastern segments), resulting in the observed surface brittle deformation distributed within a broad zone of ductile deformation.

Predicting plate velocities with mantle circulation models

We predict plate motions from a comprehensive inversion of theoretical estimates of tectonic forces in order to evaluate the relative importance of these and the uncertainties of such models. Plate‐driving forces from the mantle are calculated using global flow models that are driven by tomography and subduction‐derived density fields. Observed and predicted plate velocities agree well for a variety of models, leading to varied conclusions about the relative importance of forces. The dominance of the subduction related density pattern in the mantle is confirmed; it appears that P wave models do not satisfactorily image all of the slab‐associated anomalies in the upper mantle. Furthermore, lower mantle structure always improves the plate motion fit with respect to models that are based on upper mantle anomalies and lithospheric thickening only. We show that the average torques from the lower mantle scale with the radial flow through the 660‐km phase transition; the amplitude of the lower mantle torques will be significant for a range of models if there is mass flux through 660 km. We also evaluate parameterized edge forces and find that the additional inclusion of such torques does not significantly improve the model fit. The main reason for the nonuniqueness of the inversions is plate boundary geometry since all plate motions are dominated by the trench‐ridge system, and plates move from ridges to trenches.

Global seismicity characteristics of subduction‐to‐strike‐slip transitions

There are at least 30 major plate boundary segments worldwide where the plate boundary changes from subduction to strike‐slip; these include six triple junctions and 24 two‐plate boundaries. This study investigates earthquake seismicity in the 24 two‐plate subduction‐to‐strike‐slip transition (SSST) regions by utilizing recently published earthquake relocations, ternary diagrams of focal mechanisms, and moment rate calculations. To facilitate cross‐regional comparisons, we categorize the geometry of SSST plate boundaries in terms of (1) their radius of curvature, (2) their sense of curvature, that is, whether they are convex or concave as viewed from the downgoing plate, and (3) their tectonic complexity, that is, the variability of crustal thickness and the segmentation of the plate boundary trace. We observe three main trends in SSST regions: (1) there is a conspicuous scarcity of strike‐slip earthquakes along plate boundary segments that plate motion models indicate are strike‐slip boundaries; (2) in these apparent strike‐slip segments, both the rate of occurrence of earthquakes of any kind and the moment release rate are low compared to adjacent subduction segments; and (3) there were few observable differences in seismicity between convex and concave boundaries. The observation that transform zones exhibit moment rate deficiencies, that is, have few large‐magnitude earthquakes in the historical record, may have important implications for seismic hazard assessment in SSST regions. In particular, is motion along these boundaries aseismic with little seismic hazard, or is motion expressed in very large magnitude, infrequent, but potentially devastating earthquakes? In at least three such regions, New Zealand, the Philippines, and the Dominican Republic, paleoseismic evidence and the historical record of seismicity suggest that very large, infrequent earthquakes do occur.

Thermal-rheological controls on deformation within oceanic transforms

Abstract Transform faults that offset mid-ocean ridge (MOR) segments accommodate plate motion through deformations that involve complex thermal and mechanical feedbacks involving both brittle and temperature-dependent ductile rheologies. Through the implementation of a 3D coupled thermal-mechanical modelling approach, we have developed a more detailed picture of the geometry of plate boundary deformation and its dependence on plate velocity and the age offset of MOR transforms. The modelling results show that cooling of near-ridge lithosphere (lateral heat transfer) has significant effects in the ductile mantle lithosphere for both the location and style of deformation. The region where strain is accommodated in the subjacent mantle lithosphere is systematically offset from the position of the overlying linear transform fault in the brittle crust. This offset causes the boundary to be oblique to plate motions along much of the transform’s length, producing extension in regions of significant obliquity modifying the location of the surface fault segments. An implication of this complex plate-boundary geometry is that in the near-ridge region, the older (cooler) lithosphere will extend beneath the ridge tip, restricting the upwelling of mantle to the MOR. The melt to generate the oceanic crust adjacent to the transform must migrate laterally from its offset source, resulting in a reduced volume and thinner crust. This near-ridge plate boundary structure also matches the pattern of core-complex extension observed at inside corners of many slow-spreading ridges. The oblique extensional structure may also explain magmatism that is observed along ‘leaky’ transforms, which could ultimately result in the generation of new ridge segments that effectively ‘split’ large transforms.

Anomalous seafloor spreading of the Southeast Indian Ridge near the Amsterdam‐St. Paul Plateau

The Amsterdam‐St. Paul Plateau is bisected by the intermediate‐rate spreading Southeast Indian Ridge, and numerous geophysical and tectonic anomalies arise from the interactions of the Amsterdam‐St. Paul hotspot and the spreading center. The plate boundary geometry on the hotspot platform evolves rapidly (on timescales <1 Myr), off‐axis volcanism is abundant, the seafloor does not deepen away from the axis, and transform faults do not have fracture zone extensions. Away from the hotspot platform the ridge‐transform geometry is typical of mid‐ocean ridges globally. In contrast, the Amsterdam‐St. Paul Plateau spreading segments are shorter, they often overlap each other significantly, and the intervening discontinuities are smaller, more ephemeral, and more migratory. Abyssal hills are smaller and less uniform on the hotspot platform than on neighboring spreading segments. From gravity and isostasy analysis the average thickness of the platform crust is ∼10 km, approximately 50% thicker than that of typical oceanic crust. Most of the isostatic compensation of the hotspot plateau occurs at the Moho or within the lower crust, and the effective elastic thickness of the plateau lithosphere is ∼1.6 km, less than half that of adjacent spreading segments. Away from the platform some transform faults contain intratransform spreading centers; on the platform the two transform faults have valleys which may be depocenters for abundant axial or off‐axis volcanism and mass wasting. Although not wellconstrained by magnetics coverage, the Amsterdam‐St. Paul hotspot appears to have been “captured” by the Southeast Indian Ridge, enhancing crustal production at the ridge since about 3.5 Ma. Prior to this time the hotspot formed a line of smaller, isolated volcanoes on older Australian plate. The underlying cause for the present‐day crustal accretion anomalies is the effect of melt generation from separate sources of mantle upwelling (due to plate spreading and the hotspot) which has a consequent effect of weakening the lithosphere.

Mechanical control of oceanic plate boundary geometry

We present a global analysis of oceanic plate boundary geometry based on the mechanics of relative plate motion at mid-ocean ridges and transform faults. If the observed geometry formed by the first-order segmentation of oceanic plate boundaries represents a state of mechanical equilibrium, we find the relative strength of spreading ridges, and their bounding transform faults to be fundamental to its organisation. A consideration of power dissipation along adjacent lengths of spreading ridge and transform fault leads to a simple relationship between spreading obliquity and relative strength. Increased spreading rate is found to decrease the strength of spreading segments relative to transform faults. Proximity to an active hotspot reduces the relative strength of spreading ridges.

Evolution of the Australian‐Antarctic discordance since Miocene time

In this study we chronicle the development of the Australian‐Antarctic discordance (AAD), the crenelated portion of the Southeast Indian Ridge between ∼120° and 128°E, since anomaly 6y time (19 Ma). We reconstruct satellite‐derived marine gravity fields and depth anomalies at selected times by first removing anomalies overlying seafloor younger than the selected age, and then rotating the remaining anomalies through improved finite rotations based on a very detailed set of magnetic anomaly identifications. Our gravity field reconstructions reveal that the overall length of the Australian‐Antarctic plate boundary within the AAD has been increasing since 19 Ma. Concomitantly, the number of propagating rifts and fracture zones in the vicinity of the discordance has increased dramatically in recent times, effectively dividing it into its present‐day configuration of five distinct spreading corridors (B1‐B5) that are offset alternately to the north and south and exhibit varying degrees of asymmetric spreading. Our bathymetric reconstructions show that the regional, arcuate‐shaped, negative depth anomaly (deeper than predicted by normal lithospheric cooling models) presently centered on the discordance began migrating westward before anomaly 5ad time (∼14.4 Ma), and that a localized depth anomaly low, which at time 5ad lay on the ridge axis in spreading corridor B5, has been split apart by subsequent seafloor spreading. The magnetic anomaly patterns suggest that the depth anomaly is not always associated with a particularly contorted plate boundary geometry. Although the plate boundary within the AAD has been getting progressively more crenelated with time, this effect shows little to no migration along the ridge axis since 19 Ma. Thus any geodynamic models of the evolution of the discordance must account for the following observations: (1) the crenelation of the plate boundary within the AAD has increased with time, (2) the center of the crenelated zone does not appear to have migrated along the ridge crest, and (3) both the depth anomaly and the isotopic boundary between Pacific and Indian mantle have been migrating westward along the ridge axis but at apparently different rates. We suggest that both along‐axis migration of the depth anomaly and isotopic boundary, as well as temporal variation in the upwelling mantle material beneath the AAD, and local tectonic effects are required in order to explain these observations.

Space-time patterns of Cenozoic arc volcanism in central Mexico: From the Sierra Madre Occidental to the Mexican Volcanic Belt

ABSTRACTA histogram of 778 isotopic ages of magmatic rocks younger than Eocene in centralMexico shows a multimodal distribution with peaks at about 30 Ma, 23 Ma, 10 Ma, and 5 Ma.The sample suite displays systematic spatial variations with age that likely reflect the pro-tracted transition from the north-northwest–trending arc of the Sierra Madre Occidental tothe east-west–trending Mexican Volcanic Belt. The reorientation of the arc is accompanied bya change in the dominant composition of the products from silicic ignimbrites and rhyolites toandesitic and basaltic lavas. The observed transition is related to the Miocene reorganization ofthe subduction system following the cessation of subduction off Baja California and the east-ward motion of the Caribbean–Farallon–North America triple junction along the southeast-ern margin of Mexico. Our data support an early–middle Miocene age for the initiation of sub-horizontal subduction in southern Mexico and confirm that the locus of arc volcanism wasprimarily controlled by the geometry of plate boundaries and the thermal structure of the sub-ducting slab.

Ridge-trench interactions and high- T -low- P metamorphism, with particular reference to the Cretaceous evolution of the Japanese Islands

Abstract A corollary of plate tectonics is that spreading ridges ultimately interact with trenches; this is a consequence of the closure phase of the Wilson cycle that eliminates ocean basins. Ridge-trench interactions generate distinctive igneous, metamorphic, structural and sedimentation effects, which commonly are diachronous parallel to the trench; effects vary with plate boundary geometry and rate of migration is controlled by relative motion vectors. Identification of such interactions in the geological record is uncommon, which suggests that geological events triggered by ridge-trench interactions are attributed to other phenomena, such as changes in plate motion or a singularity at the trench. The estimated greater length of plate boundaries in the past means that subduction of a spreading ridge system could have been more common earlier in Earth history. High- T -low- P metamorphism requires a tectonic setting that allows enhanced heat flux, transient advection of heat due to magma ascent, unusually large internally generated heat, or some combination of these features. Subduction of a spreading ridge system produces anomalously high temperatures at shallow crustal depth through the development of a slab window, which allows transfer of sub-slab asthenospheric mantle across the slab to create an enhanced heat flux beneath the overriding plate. This process is important along convergent margins that develop a subduction-accretion complex, and drives low P/T ratio metamorphism and anatexis of the complex. Heat transport occurs by rising magma near the trench, which leads to formation of plutons with characteristic contaminated MORB geochemistry. Near-trench igneous activity and low P/T ratio metamorphism are the most striking features attributable to spreading ridge subduction and slab window formation, and together are likely to be diagnostic of ridge-trench interactions. Geological features of the Cretaceous Ryoke and Abukuma high- T -low- P metamorphic belts in Japan are consistent with the expectations of ridge subduction and slab window formation. Based on modelled plate interactions in the North Pacific Ocean basin during the Cretaceous, a tectonic scenario is presented for the evolution of the Japanese Islands that involves interactions between the Farallon-Izanagi ridge and a trench along the Asian continental margin. Other low P/T ratio metamorphic belts that owe their high- T lineage to interactions between spreading ridges and trenches are the Shimanto and Hidaka belts of Japan, and the Chugach belt of Alaska, USA. The complex tectonic evolution of Palaeozoic orogenic belts such as the Appalachian orogen of eastern Canada and USA, implies that ridge-trench interactions were involved in their evolution, although terminal continental collision makes the effects of these interactions difficult to identify in the geological record. There are many older belts for which ridge subduction and slab window formation should be evaluated, including the Palaeoproterozoic evolution of the Arunta Inlier, Australia, and the late Archean evolution of the Slave Province, northern Canada.

Geophysical study of the East Pacific Rise 15°N–17°N: An unusually robust segment

Bathymetric, side‐scan sonar, magnetic and gravity data from the East Pacific Rise (EPR) between 15° and 17°N are used to establish the spreading history and examine melt delivery to an unusually robust spreading segment. The axial ridge between the Orozco transform fault (15°30′N) and the 16°20′N overlapping spreading center (OSC) has an average elevation of 2300 m which is 300 m shallower than typical EPR depths, and its cross‐sectional area is double the average value for the northern EPR. The total opening rate is 86 km/Myr, but the inflated segment appears to have spread faster to the east by more than 20% since 0.78 Ma. The orientation of magnetic isochrons and lineaments in the side‐scan sonar indicates a ∼3° counterclockwise rotation of the spreading direction since 1.8 Ma (C2) and reflects a change in the Pacific‐Cocos plate motion. The side‐scan lineaments also show that the percentage of inward facing faults (83%) and the spacing between faults (1.5 km) are consistent with the spreading rate dependence shown by Carbotte and Macdonald [1994]. However, the mean fault length (4.8 km) is 1.5 km shorter than expected for the spreading rate and suggests that extensive off‐axis volcanism has draped the faults. Gravity analysis shows that the inflated segment has a ∼12‐mGal bull's eye shaped low in residual mantle Bouguer anomaly. We offer several possible end‐member models for the anomaly, including a prism of 10% partial melt in the mantle and lower crust or a crustal thickness anomaly of 2.25 km. Kinematic modeling that is based on structure and magnetic data suggests that two large magmatic pulses occurred at approximately 0.8 Ma and 0.3 Ma and have reshaped the plate boundary geometry and inflated the segment.

Segmentation and morphotectonic variations of the Central Indian Ridge (21°10′S–22°25′S)

Bathymetric, magnetic, and petrographic data have been used to analyze the segmentation and the morphotectonic variations on the Central Indian Ridge (CIR) between 21°10′S and 22°25′S. Volcanoes, abyssal hills, and steep slopes, corresponding to fault scarps and to volcanic constructional slopes, have been obtained by image processing methods applied to Sea Beam data. In the survey area, segmentation of the CIR is similar to that of a slow spreading ridge. The plate boundary morphology is defined by a linear zone composed of an ensemble of median ridges and elongate depressions. Hourglass‐shaped valleys are bounded by second and third‐order discontinuities located at local along‐axis depth maxima (400–700 m depth variations) and marked by oblique extensional basins and en échelon jogs, respectively. Large‐throw faults are prevalent toward segment ends and suggest greater amagmatic extension in these places. Along the axial valley, the petrographic characteristics of the dredged basalts are correlated with the morphotectonic segmentation and suggest different magma reservoirs. The morphology and structure of the rift valley reflect the three‐dimensional structure of the axial lithosphere and the geometry of mantle upwelling beneath the spreading center. Abyssal hills are bounded by both inward and outward facing faults in ∼1.5 Ma crust, indicating that the formation of abyssal hills is not solely resulting from a combination of volcanic and tectonic processes within the central part of the ridge but that tectonic processes may also occur as far as 20–40 km from the axis by initiating or reactivating some faults at the outward facing slope of the abyssal hills. Reconstructions of the plate boundary geometry through time, using abyssal hills as isochrons, provide evidence to show that the nontransform discontinuity, located at 21°47′S, with an offset as small as 2–3 km, is associated with V‐shaped off‐axis traces in the form of alignments of disrupted abyssal hill lineations. These reconstructions suggest that a small ridge jump is responsible for the initiation of this discontinuity and that this latter migrates away from areas of higher melt delivery. Three‐dimensional inversion of magnetic data shows that this discontinuity has been initiated between 0.780 and 0.984 Ma and suggests postaccretionary crustal block rotations within the discordant zones.

Influence of sea-floor spreading on the global hydrothermal vent fauna

ONE remarkable discovery of recent decades is the presence of hundreds of unusual species, including fourteen new families, at hydrothermal vents. These animals, unknown from other habitats, live in extreme chemical and thermal conditions around vents on spreading centres of the mid-ocean ridges and back-arc basins. Chemosynthesis provides an in situ energy source for the thriving vent fauna. This habitat has existed through the Phanerozoic1,2 and probably since the Archaean, thus providing sites for long-term adaptation. We now test the hypothesis that animal distribution among hydrothermal vents is related to tectonic plate history3,4. The predominant migration pathway is most likely to occur along mid-ocean ridges rather than by shortest oceanic routes. Similarity analyses suggest that the distribution patterns of today's vent fauna display the strong imprint of the timing and geometry of ancient plate boundaries. Study of past ridge geometry provides a method to predict relationships among vent communities yet to be discovered.

Plate boundary reorganization at a large-offset, rapidly propagating rift

THE existence of rapidly spinning microplates along the southern East Pacific Rise has been documented by geophysical swath-mapping surveys1–6, and their evolution has been successfully described by an edge-driven kinematic model7. But the mechanism by which such microplates originate remains unknown. Proposed mechanisms1–10 have generally involved rift propagation11, possibly driven by hotspots or changes in direction of sea-floor spreading. Here we present geophysical data collected over the Earth's fastest spreading centre, the Pacific–Nazca ridge between the Easter and Juan Fernandez microplates (Fig. 1), which reveal a large-offset propagating rift presently reorganizing the plate boundary geometry. A recent episode of rapid 'duelling' propagation of the historically failing spreading centre in this system has created a 120 × 120 km overlap zone between dual active spreading centres, which may be the initial stage of formation of a new microplate.