Absolute and relative plate motions and hypotheses on the origin of five aseismic ridges in the Indian Ocean

Paleopositions of the African, Indian and Antarctic plates are used as an independant set of data to test various hypotheses, derived from geophysical data, on the origin, emplacement mechanism and evolution of five aseismic structures of the Western and Southern Indian Ocean (Crozet and Kerguelen plateaus, Marion Dufresne, Léna and Ob seamount chain, Madagascar and Mascarene ridges).Except for a continental fragment of very limited extent bearing the Seychelles Islands, these structures, together with the Madagascar and Mascarene ridges, are probably due to anomalous volcanic episodes at —or near— active plate boundaries, rather than to intraplate emplacement. Their creation processes are therefore mainly related to the relative motions between the Antarctic, African and Indian plates. The volcanic episodes are synchronous with major changes in the relative motions between these plates and thus substantiate the role played by frequent irregularities in the kinematic pattern of the Western and Southern Indian Ocean, such as ridge jumps, asymmetric spreading and rapid variations in spreading velocity or direction. Finally, we think that thermal anomalies, located not far from active spreading boundaries, may have played a role in the physical processes creating the aseismic ridges.     

Seismic stratigraphy and paleogeographic evolution of Fairway Basin,Northern Zealandia,Southwest Pacific: from Cretaceous Gondwana breakup to Cenozoic Tonga–Kermadec subduction

We present the first comprehensive seismic‐stratigraphic analysis of Fairway Basin, which is situated on the rifted continent of Zealandia in the Tasman Sea, southwest Pacific, between Australia and New Caledonia. The basin is 700 km long, 150 km wide, and has water depths of 500–3000 m. We describe depositional architecture and paleogeographic evolution of this basin. Basin formation was concurrent with two tectonic events: (i) Cretaceous rifting during eastern Gondwana breakup and (ii) initiation and Cenozoic evolution of Tonga–Kermadec subduction system to the east of the basin. To interpret the basin history we compiled and interpreted 2D seismic‐reflection profiles and make correlations with DSDP boreholes and the geology of New Caledonia. Five seismic‐stratigraphic units were defined. The deepest and oldest unit, FW3, folded and faulted can be correlated with volcaniclastic sediments and magmatic rocks in New Caledonia that are associated with Mesozoic Gondwana margin subduction. Alternatively, given the basin location 200–300 km west of New Caledonia and inboard of the ancient plate boundary, the unit could have formed as Gondwana intra‐continental basin with no known correlative. The overlying unit FW2b records syn‐rift deposition, probably associated with Cretaceous Gondwana breakup. Subaerial erosion supplied terrigenous sediment into the deltas in the northern part of the basin, as suggested by the truncation surfaces on the basement highs and sigmoid reflector geometries within unit FW2b respectively. Above, unit FW2a records post‐rift sedimentation and passive subsidence as the Tasman Sea opened and the Fairway Basin drifted away from Australia. Subsidence led to the flooding of the basement highs and burial of wave‐cut surfaces. Eocene compressive deformation resulted in minor folding and tilting within the Fairway Basin and was associated with the formation of many diapiric structures. The top of unit FW2 is an extensive unconformity that is associated with erosion and truncation on surrounding ridges. Above this unconformity, unit FW1b is interpreted as a turbidite system sourced from topography created during the Eocene tectonic event, which we interpret as being related to Tonga–Kermadec subduction initiation. Pelagic carbonate sedimentation is now prevalent. Unit FW1a has progressively draped the basin during Oligocene to Pleistocene subsidence. Many small volcanic cones were erupted during this final phase of subsidence, either as a delayed consequence of subduction initiation, or related to Tasmantid and Lord Howe hotspot trails. The northern Fairway Ridge remains close to sea level and its reef system continues to supply carbonate detrital sediments into the basin, most likely during sea‐level lowstands. Fairway Basin contains a nearly continuous record of tectonic and paleoclimatic events in the southwest Pacific since Cretaceous time. Its paleogeographic history is a key piece in the puzzle for understanding patterns of regional biodiversity in the southwest Pacific.     

La région axiale de la dorsale sud-ouest indienne entre 53° est et 59° est: Son evolution depuis 10 Ma

Resumé Cet article présente des données bathymétriques et magnétiques de la région axiale de la dorsale sud-ouest indienne au voisinage de la zone de fracture majeure Atlantis II. Elles proviennent pricipalement de la campagne MD34 (Marion-Dufresne, 1983).L'axe de la dorsale est défini par la vallée et l'anomalie magnétique qui lui est associée. Le rilief le long de l'axe varie localement très rapidement; A l'ouest de la zone de fracture Atlantis II, le plancher axial présente deux bombements séparés par une dépression importante (4600 m). Cette étude met en évidence la corrélation entre ces hauts bathymétriques, la forme de la vallée et la l'amplitude de l'anomalie magnétique axiale: lorsque la profondeur du plancher axial diminue, la vallée se creuse et son encaissement augmente. On observe ainsi sur les hauts bathymétriques une section d'axe très encaissée, associée à une anomalie magnétique d'amplitude plus importance.L'identification de l'anomalie 5 (10 Ma) sur chaque flanc de la dorsale sud-ouest indienne permet la reconstitution de cette isochrone qui montre clairement une évolution de la géométrie de l'axe: à l'époque de l'anomalie 5, l'axe était composé de segments perpendiculaires à la direction d'expansion, décalés par des failles transformantes, alors qu'il apparait actuellement continu et formé sur les hauts topographiques de courts segments perpendiculaires à la direction d'expansion (et dans les dépressions par des sections d'axe très obliques).La carte bathymétrique met en évidence des lignes de crêtes grossièrement Nord-Sud (007°) dont la direction diffère de la direction d'expansion (357°) déduite des reconstructions, et parallèle à la zone de fracture majeure Atlantis II. Sur les dorsales lentes, les zones de fractures mineures, n'indiqueraient donc pas la véritable direction d'expansion.

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The axial region of the Southwest Indian Ridge between 53° E and 59° E: Evolution during the last 10 Ma

An interpretation of bathymetric and magnetic data obtained aboard the R/V Marion Dufresne provides us with new information concerning the evolution of the Southwest Indian Ridge, in the region of the Atlantis II Fracture Zone (57° E), since 10 Ma. On all profiles, the ridge axis and the axial magnetic anomaly have been clearly recognized. Bathymetric data illustrate the rapid variation of depth along the axis. On the western side of the Atlantis II Fracture Zone, the along axis profile is characterized by a succession of two highs, and an important depression between them.Our data show a strong relationship between the regional axial depth, the steep-sidedness of the axial valley and the signature of the central magnetic anomaly. In particular, where the axis is deepest (4500 m), there is a wide, shallow axial valley which is oblique to the spreading direction, and a non-typical central magnetic anomaly signature. In contrast, where the regional axial depth is shallow (3500 m), the axial valley is deep, narrow, perpendicular to the spreading direction, and the central magnetic anomaly is high in amplitude. The ridge axis on the western side of the Atlantis II Fracture Zone appears to consist of short segments located on the axial highs, which are linked by oblique zones. On the eastern side, the ridge axis is continuous, and appears to be oblique to the spreading direction.Clearly lineated magnetic anomalies 3A (5 Ma) and 5 (10 Ma) have been identified and mapped. These magnetic data allow a reconstruction which shows an evolution of the axial geometry since 10 Ma. On the western side of the Atlantis II Fracture Zone, the axis at anomaly 5 time consisted of segments perpendicular to the spreading direction which were offset by transform faults. On the eastern side, the isochron A5 appears to be parallel to the present-day ridge axis. From this plate reconstruction, a spreading direction of 357° was deduced, and appears to be parallel to the Atlantis II Fracture Zone.On each flank of the Suuthwest Indian Ridge, our bathymetric data show elongated ridges, aligned in a north-south direction, which correlate with the axial topographic highs. This direction is not precisely parallel to the spreading direction deduced from plate reconstruction. The differences in these directions suggest that transverse relief on show spreading ridge flanks (which could be interpreted as indicating the location of minor fracture zones) may not be indicative of the seafloor spreading direction.

     

GLORIA image processing: The state of the art

This chapter presents a summary of the image-processing techniques being used at present in the Institute of Oceanographic Sciences Deacon Laboratory's GLORIA long-range sidescan sonar system. It begins with a brief review of the development of GLORIA, and then describes in outline the present shipboard data acquisition, recording and replay system, including simple image-processing techniques that can be used on-board ship. Next, a detailed form of the sonar equation is developed, and this is evaluated factor-by-factor, to demonstrate the effects of beam directivity, refraction and water depth on the form of intensity variation to be expected in the final image. Finally, we discuss recent developments in shore-based image-processing. These include the development of improved radiometric corrections to normalize range-dependent intensity variations, recovery of true backscattering levels and estimation of backscattering coefficients, and combination of GLORIA with other data sets into single, colour digital images. As an example of the last process we show a digital mosaic of sonar data from the Southwest Indian Ridge, coloured as a function of depth derived from Sea Beam data in the same area.     

Linking the Canary and Cape-Verde Hot-Spots, Northwest Africa

The Canary and Cape-Verde archipelagos are two groups of volcanic islands often cited as case examples of the surface expression of two distinct hot-spot plumes. However, several considerations that␣we enumerate suggest a link between the two archipelagos. Using seismic profiles we describe a continuous morphological basement ridge that exists between the two archipelagos. We then examine the stratigraphic record available from field data on Fuerteventura Island (Canary) and Maio Island (Cape-Verde) and from a few Deep Sea Drilling Project (DSDP) holes. The geological history of these volcanic islands is very similar since the formation of their oceanic basement during the Late Jurassic. They share the same and synchronious sedimentary evolution (subsidence, uplift and emersion) as well as very similar timing of volcanism and deformation. The two distinct hot-spots model does not appear adapted to account for the formation of these structures as it ignores the existence of the ridge, as well as most of the geological coincidences. By describing the coinciding geological incidents, we argue that it is misleading to treat these two regions apart.     

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.     

Segmentation of the Central Indian Ridge between 12°12′ S and the Indian Ocean Triple Junction

The present morphology and tectonic evolution of more than 1500 kilometres of the Central Indian Ridge are described and discussed following the integration of GLORIA side-scan sonographs with conventional geophysical datasets. Segmentation of the ridge occurs by a series of ridge axis discontinuities ranging in periodicity along strike from 275 km to less than 30 km. These segment boundaries we have classified into two types: first order fracture zones of offsets greater than 50 km which bound five major (mega-) segments, and smaller scale structures of a variety of offset styles and amplitudes which cut four of these segments. We refer to these as ridge-axis discontinuities. The frequent opposite sense of offset identified between the first order structures and the subordinate discontinuities between these major structures is interpreted as resulting from the adjustment to new kinematic parameters after magnetic anomaly 20. As far as our data allows us to determine, the central major segment is not subdivided by minor ridge axis discontinuities, which we suggest is a result of its proximity to the Rodriguez hotspot.     

Lower cretaceous basalt and sediments from the Kerguelen Plateau

Geological samples from the southern Kerguelen Plateau include Lower Cretaceous basalt and lava breccia, probable Lower Cretaceous conglomerate and shelf limestone, Upper Cretaceous chert with dolomite, Upper Cretaceous-Eocene ooze, and Tertiary conglomerate. Neogene sediments are only a few hundred m thick, and include foraminiferal and diatomaceous ooze, and ice-rafted debris. In conjunction with seismic reflection profiles, the samples indicate Early Cretaceous near-shore volcanism, followed by erosion, sedimentation, and subsidence through Cretaceous; arching of the plateau at the end of Cretaceous; subsidence through Paleogene; widespread emergence in mid-Tertiary; and slow subsidence through Neogene.     

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.