Non-transform Discontinuities Research Articles

TOBI sidescan sonar imagery of the very slow-spreading Southwest Indian Ridge: evidence for along-axis magma distribution

New deep tow sidescan sonar data from the Southwest Indian Ridge reveal complex volcanic/tectonic interrelationships in the axial zone of this ultra-slow spreading ridge. While some constructional volcanic features resemble examples documented at the slow-spreading Mid-Atlantic Ridge, such as axial volcanic ridges, hummocky and smooth lava flows, their distribution and dimensions differ markedly. The largest axial volcanic ridges occur at segment centres, but fresh-looking volcanic constructions also occur at segment ends and in the deep basins marking the non-transform discontinuities. The orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge. Minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes. This observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes, with associated fractures opening in response to magma pressure, may control local emplacements of axial volcanic ridges at obliquely spreading ridges. This non-systematic pattern of ridge characteristics suggests an along-axis variation between focused and distributed magmatic supply, a model which is supported by our interpretation of low-amplitude mantle Bouguer anomalies calculated for the area. We propose that a change of the axial segmentation pattern, from two segments to the present-day three segments, may have introduced additional instability into the crustal accretion process.

The Southwest Indian Ridge between 49°15′E and 57°E: focused accretion and magma redistribution

Bathymetric, gravity, magnetic and backscattering strength data have been used to characterise the segmentation of an 800 km long portion of the ultraslow-spreading Southwest Indian Ridge (SWIR, full rate 14 mm/yr) between 49°15′E and 57°E. This analysis reveals that the segmentation defined by along-axis variations of depth and by occurrence of axial offsets does not systematically correspond to the segmentation determined by the along-axis variations of backscattering strength, mantle Bouguer anomaly (MBA) and amplitude of the central magnetic anomalies (CMA). At axial discontinuities with offsets larger than 15 km, thin crust and reduced volcanic production are suggested by the occurrence of MBA highs, almost non-existent CMA and 50% lower backscattering strength relative to the segment centres. By contrast, smaller non-transform discontinuities, with offsets smaller than 15 km, correspond to very weak variations or to no variation of the MBA, the CMA or the reflectivity of the seafloor, suggesting that there is little or no variation of volcanic production and crustal thickness associated with those small discontinuities. These small axial discontinuities bound low-relief bathymetric segments (500–700 m), corresponding to weak or no MBA lows (amplitude <11 mGal), and robust high-relief segments (>1000 m), corresponding to large MBA lows (amplitude >30 mGal). We suggest that the magma supply to these low-relief segments is controlled by near-surface processes such as melt migration and/or crustal plumbing from adjacent high-relief segments. Pronounced MBA lows at high-relief segments are thought to correspond to spreading cells where magma supply is focused in the mantle. These spreading cells are spaced by about 100 km along the SWIR axis. We suggest that the spacing of spreading cells on slow-spreading ridges is primarily controlled by the spreading rate with larger spacing between spreading cells on ultraslow-spreading ridges than on slow-spreading ridges.

Comparisons of gravity anomalies at pseudofaults, fracture zones, and nontransform discontinuities from fast to slow spreading areas

Published mechanisms for rift tip propagation at spreading centers include extensional deformation and an initial period of slow spreading. We investigate whether the gravity signal and inferred crustal structure at pseudofaults formed in medium to superfast spreading environments resemble the gravity signal at fracture zones or nontransform discontinuities formed in slow spreading environments. We find that altimetry‐based gravity anomalies on the Mathematician, Bauer, Easter, Juan Fernandez, and northern Chile Ridge pseudofaults, located in 75–150 mm/yr (full rate) seafloor spreading environments, are similar in amplitude and form to Atlantic fracture zones with 20–30 mm/yr spreading rates. A 5–15 mGal positive mantle Bouguer anomaly is observed on the pseudofault bounding the eastern Juan Fernandez microplate, comparable to those at some similar age‐offset nontransform discontinuities in slow spreading environments. Our results suggest that the deeps associated with active propagating rift tips result from both a dynamic mantle component and anomalous crust, the latter of which remains frozen at pseudofaults. We predict that any pseudofaults with age offsets more than ∼1 m.y. and not coincident with hotspot volcanism will be associated with thin (and possibly unusually dense) crust, even in superfast seafloor spreading environments.

Relationship of the Central Indian Ridge segmentation with the evolution of the Rodrigues Triple Junction for the past 8 Myr

Located near 25°33′S, 70°00′E, the Rodrigues Triple Junction is the joining point of the intermediate‐spreading Southeast Indian and Central Indian Ridges with the ultraslow spreading Southwest Indian Ridge. Bathymetric data and magnetic anomalies are used to analyze the relationship between the evolution of the Central Indian Ridge segmentation and the evolution of the Rodrigues Triple Junction for the past 8 Myr. The Central Indian Ridge domain exhibits a complex morphotectonic pattern dominated by ridge‐normal and oblique bathymetric lows interpreted as the off‐axis traces of axial discontinuities. The short‐lived nontransform discontinuities as well as the segments that lengthen or shorten along the ridge axis reveal that the Central Indian Ridge segmentation is unstable near the Rodrigues Triple Junction. The combined study of the Central Indian Ridge and Southeast Indian Ridge domains shows that the triple junction evolves between two modes: a continuous mode where the Central Indian Ridge and Southeast Indian Ridge axes are joined and a discontinuous mode where the two ridge axes are offset. Owing to spreading asymmetry, and differences in axis direction or in lengthening rates of the Central Indian and Southeast Indian ridges, the continuous mode is unstable and evolves rapidly (&lt;2 Myr) into a discontinuous mode. This last one is more stable and can evolve into a continuous mode only through the formation of a new Central Indian Ridge segment, which takes place facing the northern Southeast Indian Ridge segment. The evolution of the Rodrigues Triple Junction configuration and the evolution of the Central Indian Ridge segmentation are thus closely related.

The last spreading episode of the West Philippine Basin revisited

In 1996, bathymetric data and backscattering imagery were gathered along a 20 km‐wide NW‐SE transect across the West Philippine Basin (WPB), between Taiwan and the Palau‐Kyushu Ridge. These data reveal fine‐scaled structures of the fossil spreading axis from which we infer episodes of oblique deformation and diminished magmatic supply resulting from cessation of spreading. An original NE‐SW seafloor fabric is observed in the basin northeast of the Benham volcanic plateau. Since this trend is oblique to the more common E‐W and NW‐SE seafloor fabrics known in the WPB, it could be the result of overlap of two segments of the spreading axis. Several rhomboidal structures exist in the vicinity of the rift valley, suggesting that dextral shear occurred along the central basin rift axis during the last spreading phase. The E‐W spreading segments associated with N‐S nontransform discontinuities that occur between 127°E and 132°E were probably cross‐cut during a final, largely amagmatic, extensional phase to produce a N130° ‐trending deep rift valley.

Distributions of hydrothermal activity along the Mid-Atlantic Ridge: interplay of magmatic and tectonic controls

Hydrothermal activity has been investigated along three different sections of the slow-spreading Mid-Atlantic Ridge (MAR): 11°20′–30°N, 36–38°N and the Reykjanes Ridge, 57°45′–63°06′N. When considered in total, the incidence of venting along these three sections of the MAR compares well with the predictions of a model in which frequency of venting is linearly related to ridge-crest spreading-rate. At the scale of individual study areas, however, departure from the model is observed by up to an order of magnitude. Venting is anomalously rare along the Reykjanes Ridge but anomalously abundant along the MAR 36–38°N. Whilst such variability may be within the error of the linear spreading-rate model, we note that the interplay between magmatic and tectonic processes also differs between the three study areas. In the case of the Reykjanes Ridge we propose that the low incidence of venting reported may reflect a limitation of the sampling/investigative strategy because the style of venting which predominates may not give rise to conventional black-smoker hydrothermal plumes. Along the oblique and broadly segmented MAR 36–38°N, we propose that vigorous hydrothermal venting in broad segment-end non-transform discontinuities may be focussed along deeply penetrating active faults with the requisite heat supply being supported through some combination of along-axis magmatic intrusions and thermal release associated with the serpentinisation of crustal peridotites.

Variations of backscatter strength along the super slow-spreading Southwest Indian Ridge between 57°E and 70°E

Simrad EM12 backscatter strength data of the Southwest Indian Ridge (SWIR), between 57°E and 70°E, are used to reveal the along-axis segmentation of this super slow-spreading ridge. The backscatter properties of different geologic domains, like bathymetric highs and oblique basins within the rift valley, are characterized using 66 small test sites. We show that backscatter strength is higher on bathymetric swells, corresponding to segment centres, and lower in deep oblique basins corresponding to axial non-transform discontinuities and fracture zones. This contrast between segment centres and discontinuities is produced by both a thicker sediment cover and less frequent volcanic eruptions at segment ends. Using the model of Mitchell (1993), sediments have been estimated to be 2 to 5 m thicker in these areas than at segment centres. The distribution of the seamounts within the rift valley is controlling the long-wavelength variations of the mean backscatter strength calculated along the axis. Lower densities of seamounts and thicker sediments are producing lower and heterogeneous reflectivity levels in the deepest part of the axial valley floor between 61°45′E and 63°45′E. We propose that cooler mantle temperatures inducing construction of fewer volcanoes occur beneath this part of the ridge. The mean backscatter strength along the SWIR axis decreases dramatically toward the Rodrigues Triple Junction suggesting that volcanic production is reduced between 68°20′E and 69°20′E and that the transition from amagmatic tectonic deformation at the triple junction to new seafloor spreading occurs between 69°20′E and 70°E.

Along-axis magmatic oscillations and exposure of ultramafic rocks in a second-order segment of the Mid-Atlantic Ridge (33°43'N to 34°07'N)

New in situ observations on the Mid-Atlantic Ridge demonstrate that the along- and across-axis volcano-tectonic variability within second-order segments is larger than commonly acknowledged. In our study area, the segment center, far from being the place with present-day maximum volcanic activity, is dominated by extensive sedimentary cover, fissuring, and faulting. Furthermore, the area of most recent magmatic activity is located away from the segment center in a region of greater depth and thinner crust. Segment-tip magmatic oscillations are suggested by the distribution of rock types at both segment ends. Serpentinized peridotites and associated dolerites are exposed at the massifs located at the intersection with nontransform discontinuities (NTDs), whereas only basaltic rocks crop out on the nodal basin floors. We suggest that the combination of low magmatic budget and extension taking place at the NTDs during a segment retreat favors the uplift and exposure of ultramafic massifs.

A Survey of the Southwest Indian Ridge Axis Between Atlantis II Fracture Zone and the Indian Ocean Triple Junction: Regional Setting and Large Scale Segmentation

The study of very low-spreading ridges has become essential to ourunderstanding of the mid-oceanic ridge processes. The Southwest Indian Ridge(SWIR) , a major plate boundary of the world oceans, separating Africa fromAntarctica for more than 100 Ma, has such an ultra slow-spreadingrate. Its other characteristic is the fast lengthening of its axis at bothBouvet and Rodrigues triple junctions. A survey was carried out in thespring of 1993 to complete a multibeam bathymetric coverage of the axisbetween Atlantis II Fracture Zone (57° E) and the Rodrigues triplejunction (70° E). After a review of what is known about the geometry,structure and evolution of the SWIR, we present an analysis of the newalong-axis bathymetric data together with previously acquiredacross-axis profiles. Only three transform faults, represented byAtlantis II FZ, Novara FZ, and Melville FZ, offset this more than 1000 kmlong section of the SWIR, showing that the offsets are more generallyaccommodated by ridge obliquity and non-transform discontinuities. From comparison of the axial geometry, bathymetry, mantle Bouguer anomaly and central magnetic anomaly, three large sections (east of Melville FZ, between Melville FZ and about 65°30′ E, and from there to the Rodrigues triple junction) can be distinguished. The central member, east of Melville FZ, does not resemble any other known mid-oceanic ridge section: the classical signs of the accretion (mantle Bouguer anomaly, central magnetic anomaly) are only observed over three very narrow and shallow axis sections. We also apply image processing techniques to the satellite gravity anomaly map of Smith and Sandwell (1995) to determine the off-axis characteristics of the Southwest Indian Ridge domain, more especially the location of the triple junction and discontinuities traces. We conclude that the large-scale segmentation of the axis has been inherited from the evolution of the Rodrigues triple junction.

Structure and Stability of Non-Transform Discontinuities on the Mid-Atlantic Ridge between 24° N and 30° N

Observations of the median valley within the 24–30° N area ofthe Mid-Atlantic Ridge (MAR), using the IOSDL high resolutionside-scan sonar instrument TOBI, image four separate areas of themedian valley, containing part or all of nine spreading segments, and fivenon-transform discontinuities between spreading segments (NTDs).These high resolution side scan images were interpreted in parallel withmultibeam bathymetry (Purdy et al., 1990), giving a greater degree ofstructural precision than is possible with the multibeam data alone. Threedistinct types of NTD were identified, corresponding in part to typespreviously identified from the multibeam bathymetric survey of the area.Type 1 NTDs are termed septal offsets, and are marked by a topographic ridgeseparating the two spreading segments. The offset between the spreadingsegments ranges from 9 to 14 km. These can be further subdivided into Type1A in which the septa run parallel to the overall trend of the MAR and Type1B in which the septa lie at a high angle to the bulk ridge trend. Type 1ANTDs are characterised by overlap of the neovolcanic zones of the segmentson each side, and strong offaxis traces, while Type 1B NTDs show no overlapof neovolcanic zones, and weak offaxis traces. Type 2 NTDs arebrittle/ductile extensional shear zones, marked by oblique extensionalfractures, and associated with rotation of tectonic and volcanic structuresaway from the overall trend of the MAR. Type 3 NTDs are associated withoffsets of less than 5 km, and show no sign of any accommodating structure.In this type of NTD, the offset zone is covered with undeformed volcanics.The type of NTD developed at any locality along the ridge axis appears todepend on the amount of segment offset and segment overlap, the overalltrend of the mid-ocean ridge, the width of the zone of discontinuity, themedian valley offset and the longevity of the offset. These factorsinfluence the mechanical properties of the lithosphere across thediscontinuity, and ultimately the tectonic style of the NTD that can besupported. Thus brittle/ductile extensional shear zones are long-livedstructures favoured by large segment offsets, and small or negative segmentoverlaps. Septa can be short or long lived, and are associated with largesegment offsets. Segment overlaps vary from negative (an along axis gap) tozero, for Type 1B septal offsets, or positive to zero for Type 1A septaloffsets. Non-tectonised NTDs are generally short lived structures,characterised by small segment offsets and zero or positive overlaps.

Dynamics of the mid-ocean ridge plate boundary: Recent observations and theory

The global mid-ocean ridge system is one of the most active plate boundaries on the earth and understanding the dynamic processes at this plate boundary is one of the most important problems in geodynamics. In this paper I present recent results of several aspects of mid-ocean ridge studies concerning the dynamics of oceanic lithosphere at these diverging plate boundaries. I show that the observed rift valley to no-rift valley transition (globally due to the increase of spreading rate or locally due to the crustal thickness variations and/or thermal anomalies) can be explained by the strong temperature dependence of the power law rheology of the oceanic lithosphere, and most importantly, by the difference in the rheological behavior of the oceanic crust from the underlying mantle. The effect of this weaker lower crust on ridge dynamics is mainly influenced by spreading rate and crustal thickness variations. The accumulated strain pattern from a recently developed lens model, based on recent seismic observations, was proposed as an appealing mechanism for the observed gabbro layering sequence in the Oman Ophiolite. It is now known that the mid-ocean ridges at all spreading rates are offset into individual spreading segments by both transform and nontransform discontinuities. The tectonics of ridge segmentation are also spreading-rate dependent: the slow-spreading Mid-Atlantic Ridge is characterized by distinct “bulls-eye” shaped gravity lows, suggesting large along-axis variations in melt production and crustal thickness, whereas the fast-spreading East-Pacific Rise is associated with much smaller along-axis variations. These spreading-rate dependent changes have been attributed to a fundamental differences in ridge segmentation mechanisms and mantle upwelling at mid-ocean ridges: the mantle upwelling may be intrinsically plume-like (3-D) beneath a slow-spreading ridge but more sheet-like (2-D) beneath a fast-spreading ridge.

The Southeast Indian Ridge between 127° and 132°40′E: contrasts in segmentation characteristics and implications for crustal accretion

Abstract The Australian-Antarctic Discordance is an anomalously deep section of the Southeast Indian Ridge which overlies a colder than normal region of the upper mantle. The Southeast Indian Ridge exhibits large contrasts in its geophysical and geochemical characteristics across the eastern boundary of the Discordance. We present new geophysical data collected along the Southeast Indian Ridge between 127°00′E and 132°40′E which define the segmentation characteristics of this portion of the Indo-Australian-Antarctic plate boundary. The Southeast Indian Ridge within our survey area can be broken into three first-order segments bounded by one transform fault and two propagating rifts. The transform fault, located at the west end of the study area, forms the eastern boundary of the Australian-Antarctic Discordance. The morphology of the spreading axis is that of an axial high akin to that found along the East Pacific Rise. The non-transform discontinuities found along the axis of accretion are also similar to those encountered at fast-spreading centres. These characteristics are in marked contrast with those of the spreading axis within the Australian-Antarctic Discordance, which resembles a slow-spreading centre despite its intermediate spreading rate ( c. 74 mm a −1 ). We suggest that the geophysical and geochemical transitions that occur along the Southeast Indian Ridge near the eastern boundary of the Australian-Antarctic Discordance may be ascribed to varying mantle temperature and melt production rate along the spreading centre at constant spreading rate. The two propagating rifts, located near 127°45′E and 131°E, are actively migrating toward the Discordance down the regional, along-axis gradient in bathymetry at the rates of 43 and 46 mm a −1 , respectively. Propagating rifts play a dominant role in the reorganization of the spreading centre east of the Australian-Antarctic Discordance. Satellite gravity data reveal a number of V-shaped, west-pointing structures on the flanks of the Southeast Indian Ridge. These structures appear to have been spawned near the George V ridge-transform intersection at 139°E. Four of these features display a clear association with active propagating rifts. Some of the remaining features may be due to the migration of melting anomalies entrained by asthenospheric flow along the axis of the Southeast Indian Ridge.

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.

Ocean crust formation processes at very slow spreading centers: A model for the Mohns Ridge, near 72°N, based on magnetic, gravity, and seismic data

The Mohns Ridge, in the Norwegian Greenland Sea, is one of the slowest spreading centers of the mid‐ocean ridge system (8 mm/yr half rate). Sea Beam data acquired with R/V Jean Charcot near 72°N show that its rift valley floor is characterized by en échelon volcanic ridges, oriented obliquely relatively to the average strike of the ridge axis. These ridges are regularly spaced along the axis, about every 40 km, and are separated by nontransform discontinuities. Sharp positive magnetic anomalies, centered over the topographic highs, suggest that they are eruptive centers, considered as the surficial expression of active spreading cells. Over the rift valley, Bouguer anomalies obtained by subtracting the predicted effects due to seafloor topography from the measured free‐air gravity field are consistent with a low density body within the lower crust having its upper surface lying at about 2 km below the sea surface. This body, if it exists, probably corresponds to the zone of low viscosity that can be inferred from the model of Chen and Morgan (1990b), which predicts the existence of a decoupling region, between the upper crust and the asthenophere below. Its width varies rapidly along‐strike, from less than about 5 km to more than 15 km. In plan view, it has a pinch and swell form, which defines a series of spreading cells, the center of one cell being where the Bouguer anomaly is widest. Short wavelength (less than 10 to 20 km) along‐strike variations, such as Bouguer anomaly lows centered on the topographic highs, reflect local effects associated with the presence of the eruptive centers. Seismic tomography data from a 20×10 km active oblique volcanic ridge near 72°22′N tend to indicate that the links between the main, low‐velocity body at depth, and the magma injections centers which lie within the rift valley inner floor are probably complex.

Three‐dimensional gravity study of the Mid‐Atlantic Ridge: Evolution of the segmentation between 28° and 29°N during the last 10 m.y.

During the high‐resolution survey SARA (Segmentation Ancienne de la Ride Atlantique), Sea Beam bathymetry, magnetic, gravity, and seismic reflection data were collected on the flanks of the Mid‐Atlantic Ridge, between 28° and 29°N. This survey was designed to provide off‐axis information (up to approximately 10 m.y.) and to complement a detailed on‐axis survey carried out in 1988 and 1989 (Sempéré et al., 1990) between the Atlantis and Kane fracture zones. A previous gravity study had revealed the existence of “bulls‐eye” shaped gravity lows centered on the axial segments and gravity highs centered on the non‐transform discontinuities (Lin et al., 1990). We carried out a three‐dimensional calculation of the mantle Bouguer anomaly in order to investigate if the axial pattern of circular anomaly lows can be followed on the flanks of the spreading center. The off‐axis gravity anomalies are characterized by anomaly lows over the centers of the segments and anomaly highs over the discontinuities. After correcting for the gravity effect of lithospheric cooling away from the ridge, the segmentation configuration determined from gravity data appears to be very similar to that deduced from bathymetry. Off‐axis bathymetry is characterized by southward pointing, V‐shaped basins which indicate that the traces of the nontransform discontinuities do not follow plate motion flow lines and that each ridge segment advances and retreats continuously. However, the gravity trace of the discontinuities is always slightly offset with respect to the bathymetric lows: northward on the western flank and southward on the eastern flank. The sense of offset between the gravity and bathymetric traces appears to be related to the right‐stepping nature of the axial discontinuities.

Segmentation and morphotectonic variations along a slow-spreading center: The Mid-Atlantic Ridge (24�00? N? 30�40? N)

Analysis of Sea Beam bathymetry along the Mid-Atlantic Ridge between 24°00′ N and 30°40′ N reveals the nature and scale of the segmentation of this slow-spreading center. Except for the Atlantis Transform, there are no transform offsets along this 800-km-long portion of the plate boundary. Instead, the Mid-Atlantic Ridge is offset at intervals of 10–100 km by nontransform discontinuities, usually located at local depth maxima along the rift valley. At these discontinuities, the horizontal shear between offset ridge segments is not accommodated by a narrow, sustained transform-zone. Non-transform discontinuities along the MAR can be classified according to their morphology, which is partly controlled by the distance between the offset neovolcanic zones, and their spatial and temporal stability. Some of the non-transform discontinuities are associated with off-axis basins which integrate spatially to form discordant zones on the flanks of the spreading center. These basins may be the fossil equivalents of the terminal lows which flank the neovolcanic zone at the ends of each segment. The off-axis traces, which do not lie along small circles about the pole of opening of the two plates, reflect the migration of the discontinuities along the spreading center.

Morphology and tectonics of the Australian-Antarctic Discordance between 123� E and 128� E

The Australian-Antarctic Discordance (AAD) is an anomalously deep and rugged zone of the Southeast Indian Ridge (SEIR) between 120° E and 128° E. The AAD contains the boundary between the Indian Ocean and Pacific Ocean isotopic provinces. We have analyzed SeaMarc II bathymetric and sidescan sonar data along the SEIR between 123° E and 128° E. The spreading center in the AAD, previously known to be divided into several transform-bounded sections, is further segmented by nontransform discontinuities which separate distinct spreading cells. Near the transform which bounds the AAD to the east, there is a marked change in the morphology of the spreading center, as well as in virtually every measured geochemical parameter. The spreading axis within the Discordance lies in a prominent rift valley similar to that observed along the Mid-Atlantic Ridge, although the full spreading rate within the AAD is somewhat faster than that of slow-spreading centers (~ 74 mm a−1 vs. 0–40 mm a−1). The AAD rift valleys show a marked contrast with the axial high that characterizes the SEIR east of the AAD. This change in axial morphology is coincident with a large (~ 1 km) deepening of the spreading axis. The segmentation characteristics of the AAD are analogous to those of the slow-spreading Mid-Atlantic Ridge, as opposed to the SEIR east of the AAD, which exhibits segmentation characteristics typical of fast-spreading centers. Thus, the spreading center within and east of the AAD contains much of the range of global variability in accretionary processes, yet it is a region free from spreading rate variations and the volumetric and chemical influences of hotspots. We suggest that the axial morphology and segmentation characteristics of the AAD spreading centers are the result of the presence of cooler than normal mantle. The presence of a cool mantle and the subsequent diminution of magma supply at a constant spreading rate may engender the creation of anomalously thick brittle lithosphere within the AAD, a condition which favor, the creation of an axial rift valley and of thin oceanic crust, in agreement with petrologic studies. The morphologies of transform and non-transform discontinuities within the Discordance also possess characteristics consistent with the creation of anomalously thick lithosphere in the region. The upper mantle viscosity structure which results from lower mantle temperatures and melt production rates may account for the similarity in segmentation characteristics between the AAD and slow-spreading centers. The section of the AAD which overlies the isotopic boundary is associated with chaotic seafloor which may be caused by an erratic pattern of magmatism and/or complex deformation associated with mantle convergence. Finally, the pattern of abyssal hill terrain within a portion of the AAD supports previous models for the formation of abyssal hills at intermediate- and slow-spreading ridges, and provides insights into how asymmetric spreading is achieved in this region.

Evidence from gravity data for focusedmagmatic accretionalong the Mid-Atlantic Ridge

Gravity data from the Mid-Atlantic Ridge between latitudes 27°50′N and 30°40′N show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis. Large positive gravity anomalies equivalent to reductions of almost 50% in crustal thickness are observed over non-transform discontinuities bounding spreading segments.