A survey of the Indian ocean triple junction trace within the Antarctic plate. Implications for the junction evolution since 15 Ma.

The junction between oceanic crust generated, within the Antarctic plate, at the Southeast Indian Ridge and the Southwest Indian Ridge has been studied using a SEABEAM swathe bathymetry mapping system and other geophysical techniques between the Indian Ocean Triple Junction (approximately 25°S, 70° E), and a point some 500 km to the southwest (at 28°25 S, 66°35 E). The morphotectonic boundary which marks this trace of the ridge-ridge-ridge triple junction is complex and varies with age. Recent theories proposing a cyclicity of volcanic and tectonic processes at this mode of triple junctions appear to be supported by a series of regularly spaced, en echelon escarpments facing the slowly spreading (0.6 to 0.8 cm a^-1, half rate) Southwest Indian Ridge axis. The en echelon escarpments intersect at approximately right angles with the regularly spaced oceanic spreading fabric formed on the Antarctic plate at the Southeast Indian Ridge and together locally flank uplifted northward-pointing corner sections of ocean floor. The origins for the localised elevations are unclear, but may relate to intermittent and/or alternating rifting and volcanic episodes. Variations of degree of asymmetry and/or obliquity in spreading on the Central Indian Ridge and the Southwest Indian Ridge are suggested to explain detailed structural changes along the triple junction trace. It is suggested that discontinuities of the trace may be related to an intermittent development of new spreading centres beneath the most easterly part of the Southwest Indian Ridge, coupled with a more continuous process beneath the faster spreading Central Indian Ridge (2 to 2.5 cm a^-1) and the Southeast Indian Ridge (2.5 to 3 cm a^-1). A detailed history of triple junction evolution may be thus inferred from basic morphological and structural mapping along the three triple junction traces.     

Propagation of the Southwest Indian Ridge at the Rodrigues Triple Junction

The analysis of multibeam bathymetric data of the Southwest Indian Ridge(SWIR) domain between the triple junction traces from 68° E to theRodrigues Triple Junction (RTJ; 70° E) reveals the evolution of thisridge since magnetic anomaly 4 (8 Ma). Image processing has been used toshow that the horizontal component of strain due to a network of normal stepfaults increases dramatically between 69°30 E and the RTJ. Thisarea close to the RTJ is characterized by a deep graben at the foot of thetriple junction trace on the African plate and by a narrow fault-boundedridge that joins an offset of the trace on the Antarctic plate. In thatarea, spreading is primarily amagmatic and dominated by tectonic extensionprocesses. To the west of 69°30 E, some lobate bathymetricfeatures atop of a large topographic high suggest volcanic constructions.Between 68°10 E and 69°25 E the southern flank of theSWIR domain is wider than the northern one and is characterized by a series of 7 en echelon bathymetric highs similar in size,shape and orientation to the one centred at 69°30E near the present-day triple junction. Their en echelon organization along the triple junction trace on the Antarctic plate and the typical lack of conjugated parts on the northern flank show that these bathymetric highs have been shifted to the south by successive northward relocalisations of the SWIR rifting zone. This evolution results in the asymmetric spreading of the SWIR in the survey area. The off-axis bathymetric highs connect to the offsets of the triple junction trace on the Antarctic plate when the Southeast Indian Ridges lightly lengthenstoward the northwest and the triple junction is relocated to the north. We propose that the SWIR lengthens toward the northeast with two propagation modes: 1) a continuous and progressive propagation with distributed deformation in preexisting crust of the Central Indian Ridge, 2) a discontinuous propagation with focusing of the deformation in a rift zone when the triple junction migrates rapidly to the north. The modes of propagation of the SWIR are related to different localisation and distribution of strain which are in turn controlled by changes of the triple junction configurations due to propagation, recession or a symmetric spreading on the Central and Southeast Indian Ridges.     

Structural analysis of the segmentation of the Central Indian Ridge between 20°30′S and 25°30′S (Rodriguez Triple Junction)

The morphological characteristics of the segmentation of the Central Indian Ridge (CIR) from the Indian Ocean Triple Junction (25°30S) to the Egeria Transform Fault system (20°30S) are analyzed. The compilation of Sea Beam data from R/VSonne cruises SO43 and SO52, and R/VCharcot cruises Rodriguez 1 and 2 provides an almost continuous bathymetric coverage of a 450-km-long section of the ridge axis. The bathymetric data are combined with a GLORIA side-scan sonar swath to visualize the fabric of the ridge and complement the coverage in some areas. This section of the CIR has a full spreading rate of about 50 mm yr^–1, increasing slightly from north to south. The morphology of the CIR is generally similar to that of a slow-spreading center, despite an intermediate spreading rate at these latitudes. The axis is marked by an axial valley 5–35 km wide and 500–1800 m deep, sometimes exhibiting a 100–600 m-high neovolcanic ridge. It is offset by only one 40km offset transform fault (at 22°40S), and by nine second-order discontinuities, with offsets varying from 4 to 21 km, separating segments 28 to 85 km long. The bathymetry analysis and an empirical orthogonal function analysis performed on across-axis profiles reveal morphologic variations in the axis and the second-order discontinuities. The ridge axis deepens and the relief across the axial valley increases from north to south. The discontinuities observed south of 22°S all have morphologies similar to those of the slow-spreading Mid-Atlantic Ridge. North of 22°S, two discontinuities have map geometries that have not been observed previously on slow-spreading ridges. The axial valleys overlap, and their tips curve toward the adjacent segment. The overlap distance is 2 to 4 times greater than the offset. Based on these characteristics, these discontinuities resemble overlapping spreading centers (OSCs) described on the fast-spreading EPR. The evolution of one such discontinuity appears to decapitate a nearby segment, as observed for the evolution of some OSCs on the EPR. These morphological variations of the CIR axis may be explained by an increase in the crustal thickness in the north of the study area relative to the Triple Junction area. Variations in crustal thickness could be related to a broad bathymetric anomaly centered at 19°S, 65°E, which probably reflects the effect of the nearby Réunion hotspot, or an anomaly in the composition of the mantle beneath the ridge near 19°S. Other explanations for the morphological variations include the termination of the CIR at the Rodriguez Triple Junction or the kinematic evolution of the triple junction and its resultant lengthening of the CIR. These latter effects are more likely to account for the axial morphology near the Triple Junction than for the long-wavelength morphological variation.     

An evolving ridge system around the Indian Ocean triple junction

A 2°×2° map of spreading centres and fracture zones surrounding the Indian Ocean RRR triple junction, at 25.5°S, 70°E, is described from a data set of GLORIA side-scan sonar images, bathymetry, magnetic and gravity anomalies. The GLORIA images show a pervasive fabric due to linear abyssal hills oriented parallel to the two medium-spreading ridges (the Central Indian Ridge (CIR) and Southeast Indian Ridge (SEIR)). A cuvature of the fabric occurs along fracture zones, which are also located by lows in the bathymetry and gravity data and by offsets between magnetic anomalies. The magnetic anomalies also record periods of asymmetric spreading marking the development of the fracture zones, including the birth, at anomaly 2A, of a short fracture zone 50 km north of the triple junction on the CIR, and its death near the time of the Jaramillo anomaly. In some localities, a fine-scale fabric corresponds to a coarser fabric on the opposite flank of the CIR, possibly indicating a persistent asymmetry in the faulting at the median valley walls if the fabric has a tectonic and not a volcanic origin. A plate velocity analysis of the triple junction shows that both the CIR and Southwest Indian Ridge (SWIR) are propagating obliquely; the CIR appears to form an oblique trend by segmenting into a series of almost normally-oriented segments separated by short-offset fracture zones. For the last 4 m.y., the abyssal hill lineations indicate that the CIR segment immediately north of the triple junction has been spreading with an average 10° obliquity. The present small 5 km offset of the centres of the CIR and SEIR median valleys (Munschy and Schlich, 1989) is shown to be the result of this obliquity and a 30% spreading asymmetry between anomaly 2 and the Jaramillo on the CIR segment immediately north of the triple junction.     

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.     

Mantle Bouguer Anomaly Along an Ultra Slow-Spreading Ridge: Implications for Accretionary Processes and Comparison with Results from Central Mid-Atlantic Ridge

A three-dimensional analysis of gravity andbathymetry data has been achieved along the Southwest Indian Ridge (SWIR)between the Rodriguez Triple Junction (RTJ) and the Atlantis II transform,in order to define the morphological and geophysical expression ofsecond-order segmentation along an ultra slow-spreading ridge(spreading rate of 8 mm/yr), and to compare it with awell-studied section along a slow-spreading ridge (spreadingrate of 12.5 mm/yr): the Mid-Atlantic Ridge (MAR) between 28°and 31°30 N.Between the Atlantis II transform and theRTJ, the SWIR axis exhibits a deep axial valley with an 30°oblique trend relative to the north–south spreading direction. Onlythree transform faults offset the axis, so the obliquity has to beaccommodated by the second-order segmentation. Alongslow-spreading ridges such as the MAR, second-order segmentshave been defined as linear features perpendicular to the spreadingdirection, with a shallow axial valley floor at the segment midpoint,deepening to the segment ends, and are associated with Mantle BouguerAnomaly (MBA) lows. Along the SWIR, our gravity study reveals the presenceof circular MBA lows, but they are spaced further apart than expected. Thesegravity lows are systematically centred over narrow bathymetric highs, andinterpreted as the centres of spreading cells. However, along some obliquesections of the axis, the valley floor displays small topographicundulations, which can be interpreted as small accretionary segments frommorphological analysis, but as large discontinuity domains from thegeophysical data. Therefore, both bathymetry and MBA variations have to beused to define the second-order segmentation of an ultraslow-spreading ridge. This segmentation appears to be characterisedby short segments and large oblique discontinuity domains. Analysis of alongaxis bathymetric and gravimetric profiles exhibits three different sectionsthat can be related to the thermal structure of the lithosphere beneath theSWIR axis.The comparison between characteristics of segmentationalong the SWIR and the MAR reveals two major differences: first, the poorcorrelation between MBA and bathymetry variations and second, the largerspacing and amplitude of MBA lows along the SWIR compared to the MAR. Theseobservations seem to be correlated with the spreading rate and the thermalstructure of the ridge. Therefore, the gravity signature of the segmentationand thus the accretionary processes appear to be very different: there areno distinct MBA lows on fast-spreading ridges, adjacent ones on slowspreading ridges and finally separate ones on ultra slow-spreadingridges. The main result of this study is to point out that 2nd ordersegmentation of an ultra slow-spreading ridge is characterised bywide discontinuity domains with very short accretionary segments, suggestingvery focused mantle upwelling, with a limited magma supply through a cold,thick lithosphere. We also emphasise the stronger influence of themechanical lithosphere on accretionary processes along an ultra slow-spreading ridge.     

Basinal seamounts and seamount chains of the Central Indian Ocean: Probable near-axis origin from a fast-spreading Ridge

Hydrosweep mapping of crust in the Central Indian Ocean Basin reveals abundant volcanoes ocurring both as isolated seamounts and linear seamount chains parallel to flow lines. Their shapes, sizes and overall style of occurrence are indistinguishable from near-axis seamounts in the Pacific. Evidence from seamount morphology, distributions and petrography of dredged samples suggests that they were generated near the fast-spreading Southeast Indian Ridge at 50–60 Ma. If so, this style of near-axis seamount generation may be a result of fast-spreading rate rather than a peculiarity of the present Pacific spreading ridges. In fact, the results of several recent studies, taken together, suggest that the style of axis/near-axis seamount volcanism varies systematically as a function of spreading rate.     

GLORIA imagery of sea floor structures in the northern North Fiji Basin

GLORIA side-scan imagery from the northern North Fiji Basin reveals modern and relict sea-floor fabric. The South Pandora Ridge is marked by steep escarpments and small rift basins, but no recent volcanism. The northern and eastern limbs of the 16°58S, 173°55E triple junction are marked by rift grabens flanked by steep escarpments, but little recent volcanism is apparent there. At present, there is no well-organized spreading system in the northern North Fiji Basin; extension and shearing are occurring within narrowly confined areas. It is uncertain how these areas relate to one another and fit into the regional tectonic framework.     

Morphology and tectonics of the Galapagos Triple Junction

We describe the results of GLORIA and SEABEAM surveys, supplemented by other marine geophysical data, of the Galapagos Triple Junction where the Pacific, Cocos and Nazca plates meet. The data allowed detailed topographic and tectonic maps of the area to be produced. We located each spreading axis with a precision of about 1 km. All three plate boundaries change character as the triple junction is approached to take on morphologies typical of slower spreading axes: the fast-spreading East Pacific Rise develops the morphology of a medium-spreading rise, and the medium-spreading Cocos-Nazca Rise takes on the appearance of a slow-spreading ridge. The axis of the East Pacific Rise was found to be completely continuous throughout the survey area, where it runs along the 102°05 W meridian. The Cocos-Nazca axis, however, fails to meet it, leaving a 20-km-wide band of apparently normal East Pacific Rise crust between its tip and the East Pacific Rise axis. As a consequence there must be considerable intra-plate deformation within the Cocos and Nazca plates. A further 40 km of the Cocos-Nazca axis is characterised by oblique faulting that we interpret to be a sign of rifting of pre-existing East Pacific Rise crust. We infer that true sea-floor spreading on the Cocos-Nazca axis does not begin until 60 km east of the East Pacific Rise axis. Other areas of similar oblique faulting occur on the Pacific plate west of the triple junction and along the rough-smooth boundaries of the Galapagos Gore. We present a model involving intermittent rifting, rift propagation, and sea-floor spreading, to explain these observations.     

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.     

Ferrobasalts from the Central Indian Ocean Basin

 We report the occurrence of ferrobasalts recovered from the Central Indian Ocean Basin crust generated at the Southeast Indian Ridge during a phase of moderate to fast spreading accretion (∼110–190 mm/yr, full rate).The rocks are rich in plagioclase, FeO^* (13–19%), and TiO₂ (2.27–2.76%), poor in olivine and MgO (3.44–6.20%), and associated with topographic highs and increased amplitude magnetic anomalies corresponding to chrons A25 and A24. We suggest that secon dary eruptions from ancient N-MORB magma, which may have been trapped at a shallow depth in a horizon of neutral buoyancy, could have produced the ferrobasalts. Received: 27 January 1998 / Revision received: 25 May 1998     

Petrology of seamounts in the Central Indian Ocean Basin: Evidence for near-axis origin

Previous studies on the distribution and morphology of ancient seamount chains (> 50 Ma) in the Central Indian Ocean basin (CIOB) indicated their generation from the fast spreading Southeast Indian Ridge. Here we describe the petrology of some of these seamounts. Fresh glass veneers of pillow basalts from these seamounts were analyzed by electron microprobe. The rocks show a low content of TiO₂ and FeO and moderate Mg# suggesting slow eruption of a minimally fractionated N-MORB magma. In terms of chemistry and morphology, CIOB seamounts are indistinguishable from seamounts across slow spreading MAR and fast spreading EPR.     

Regional shape and tectonics of the equatorial East Pacific Rise

The geography of the East Pacific Rise (EPR) between 10°N and 6°S, redetermined by new surface ship surveys, is characterized by long spreading axes orthogonal to infrequent transform faults. Near 2°10N the EPR is intersected by the Cocos-Nazca spreading center at the Galapagos triple junction. The present pattern was established 27-5.5 m.y.b.p. by a complex sequence of rise-crest jumps and reorientations from a section of the Pacific-Farallon plate boundary. Transverse profiles of the rise flanks can be matched by thermal contraction curves for aging lithosphere, except between the triple junction and 4°S, where the east flank is anomalously shallow and almost horizontal. Most sections of spreading axis have the 10–30 km wide, 100–400 m high, axial ridge that is characteristic of fast spreading centers. However, within 60 km of the triple junction the rise crest structure is atypical, with an axial rift valley and elevated rift mountains, despite a spreading rate of 140 mm/yr. With the exception of this atypical section, the bathymetric profile along the spreading axis is remarkably even, with continuous, gentle slopes for hundreds of kilometers between major transform faults, where step-like offsets in axial depths occur. Most of the observations can be accommodated by a model in which the long spreading axes are underlain by continuous crustal magma chambers that allow easy longitudinal flow of magma, and whose size controls the style and dimensions of EPR crestal topography.Contribution of the Scripps Institution of Oceanography, new series.     

Gravity surveys over the Reykjanes Ridge and between Iceland and the Faeroe Islands

A regional survey of the southern Reykjanes Ridge (52°N to 57°N) shows an irregular topography: a rift valley which is only partly recognizable as such, with varying azimuth and some fracturezone-like interruptions. The survey also comprised gravity and magnetic measurements.The course of the axis as well as the perpendicular fractures show up well in the free air anomalies as relative troughs within an area of positive free air gravity (Figure 5). There is no indication of density variations within the topographic masses.The anomaly pattern of total magnetic intensity indicates the exact position of the rift axis and a bifurcation at about 55°N. From the parallel magnetic anomalies south of 55°N (Figure 2) a spreading rate can be deduced of 1.10 cm/yr perpendicular to the rift axis (Figure 3). This spreading rate is at the same time the plate movement involved.A survey of the Iceland-Faeroe Ridge with a 3–5 miles grid shows large gravity and magnetic anomalies over a smooth topography, indicating large pockets of light material, probably of volcanic origin. These areas have normal magnetization. Positive gravity anomalies forming a ring structure along the 200 m isobath are characterized by reversed magnetization.The dissimilarity in morphology, seismicity and inner structure between the two ridges that intersect in Iceland suggest that there is no relation between the two phenomena.Paper presented at the meeting of the International Gravity Commission, Paris, on September 8, 1970.     

Location and segmentation of the Cocos-Nazca spreading centre west of 95° W

This paper describes GLORIA sidescan sonar data from a single swath along the Cocos-Nazca Spreading Centre between the 95.5° W propagating rift and the Pacific-Cocos-Nazca triple junction. Almost the whole of the plate boundary was imaged. Five medium sized offsets of the spreading centre, ranging from 10 to 25 km, were seen. Of these, at least one (at 99° W) is a previously unknown propagating rift, propagating westwards away from the Galapagos hotspot at about 40 mm a^-1. Two other offsets have some, but not all, of the characteristics of propagating rifts, and may be poorly developed (possibly duelling) propagating rifts or migrating overlapping spreading centres. In each case the apparent propagation rate is between one and two times the half spreading rate. The average length of ridge segments in this region is 70 km, but lengths range from 12 to 135 km. The longest segments are those immediately behind actively propagating ridge offsets. The overall plan shape of the ridge axis is roughly sinusoidal, with a wavelength of 400–500 km and an amplitude of ±20 km. This nonlinear shape has arisen since the spreading centre was created, and may reflect an instability in the mantle plumes that control ridge segmentation.     

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.     

The role of the Spitsbergen shear zone in determining morphology,segmentation and evolution of the Knipovich Ridge

In 1989–1990 the SeaMARC II side-looking sonar and swath bathymetric system imaged more than 80 000 km² of the seafloor in the Norwegian-Greenland Sea and southern Arctic Ocean. One of our main goals was to investigate the morphotectonic evolution of the ultra-slow spreading Knipovich Ridge from its oblique (115° ) intersection with the Mohns Ridge in the south to its boundary with the Molloy Transform Fault in the north, and to determine whether or not the ancient Spitsbergen Shear Zone continued to play any involvement in the rise axis evolution and segmentation. Structural evidence for ongoing northward rift propagation of the Mohns Ridge into the ancient Spitsbergen Shear Zone (forming the Knipovich Ridge in the process) includes ancient deactivated and migrated transforms, subtle V-shaped-oriented flank faults which have their apex at the present day Molloy Transform, and rift related faults that extend north of the present Molloy Transform Fault. The Knipovich Ridge is segmented into distinct elongate basins; the bathymetric inverse of the very-slow spreading Reykjanes Ridge to the south. Three major fault directions are detected: the N-S oriented rift walls, the highly oblique en-echelon faults, which reside in the rift valley, and the structures, defining the orientation of many of the axial highs, which are oblique to both the rift walls and the faults in the axial rift valley. The segmentation of this slow spreading center is dominated by quasi stationary, focused magma centers creating (axial highs) located between long oblique rift basins. Present day segment discontinuities on the Knipovich Ridge are aligned along highly oblique, probably strike-slip faults, which could have been created in response to rotating shear couples within zones of transtension across the multiple faults of the Spitsbergen Shear Zone. Fault interaction between major strike slip shears may have lead to the formation of en-echelon pull apart basins. The curved stress trajectories create arcuate faults and subsiding elongate basins while focusing most of the volcanism through the boundary faults. As a result, the Knipovich Ridge is characterized by Underlapping magma centers, with long oblique rifts. This style of basin-dominated segmentation probably evolved in a simple shear detachment fault environment which led to the extreme morphotectonic and geophysical asymmetries across the rise axis. The influence of the Spitsbergen Shear Zone on the evolution of the Knipovich Ridge is the primary reason that the segment discontinuities are predominantly volcanic. Fault orientation data suggest that different extension directions along the Knipovich Ridge and Mohns Ridge (280° vs. 330°, respectively) cause the crust on the western side of the intersection of these two ridges to buckle and uplift via compression as is evidenced by the uplifted western wall province and the large 60 mGal free air gravity anomalies in this area. In addition, the structural data suggest that the northwards propagation of the spreading center is ongoing and that a `normal' pure shear spreading regime has not evolved along this ridge. This revised version was published online in November 2006 with corrections to the Cover Date.     

Ferrobasalts from the Central Indian Ocean Basin

We report the occurrence of ferrobasalts recovered from the Central Indian Ocean Basin crust generated at the Southeast Indian Ridge during a phase of moderate to fast spreading accretion (～110–190?mm/yr, full rate).The rocks are rich in plagioclase, FeO* (13–19%), and TiO₂ (2.27–2.76%), poor in olivine and MgO (3.44–6.20%), and associated with topographic highs and increased amplitude magnetic anomalies corresponding to chrons A25 and A24. We suggest that secon dary eruptions from ancient N-MORB magma, which may have been trapped at a shallow depth in a horizon of neutral buoyancy, could have produced the ferrobasalts.     

Geophysical investigations over a segment of the Central Indian Ridge, Indian Ocean

 Swath bathymetric, gravity, and magnetic studies were carried out over a 55 km long segment of the Central Indian Ridge. The ridge is characterized by 12 to 15 km wide rift valley bounded by steep walls and prominent volcanic constructional ridges on either side of the central rift valley. A transform fault at 7°45′S displaces the ridge axis. A mantle Bouguer anomaly low of −14 mGals and shallowing of rift valley over the middle of the ridge segment indicate along axis crustal thickness variations. A poorly developed neovolcanic zone on the inner rift valley floor indicate dominance of tectonic extension. The off-axis volcanic ridgs suggest enhanced magmatic activity during the recent past. Received: 24 May 1996 / Rivision received: 13 January 1997     

印度洋亚丁-欧文-卡尔斯伯格脊三联点邻近洋脊的玄武岩地球化学特征及其地幔源区性质

本文研究了亚丁-欧文-卡尔斯伯格脊(AOC)三联点邻近洋脊的玄武岩样品在主量、微量元素和Pb-Sr-Nd同位素特征上的差异和联系并分析其原因。结果表明,AOC附近卡尔斯伯格脊和希巴洋脊的玄武岩均为正常型洋脊玄武岩(N-MORB),起源自亏损地幔,其中卡尔斯伯格脊的样品较希巴洋脊样品更亏损;欧文洋脊玄武岩样品为洋岛玄武岩(OIB)特征,其地幔源区可能有残余陆块物质的混染;亚丁洋脊玄武岩样品类型包括N-MORB、E-MORB和可能的大陆玄武岩,与洋壳形成过程中大陆岩石圈物质的贡献程度有关。除了卡尔斯伯格脊外,阿法热点对各洋脊的岩浆均有一定程度的影响。