A bathymetry and altimetry profile across the Southwest Indian Ridge crest at 31°S latitude

During March 1983 the M/V "Marion Dufresne" obtained a bathymetry profile along more than 1500 km of a SEASAT altimeter sub-track across the Southwest Indian Ridge crest at 31°S latitude. We have used these data to estimate the thermal and elastic thickness of the oceanic lithosphere in the vicinity of the ridge and to evaluate the performance of the SEASAT altimeter. The thermal thickness appears to be in the range 72-109 km while the elastic thickness is about 2-5 km. There is a good correlation between the geoid calculated from the bathymetry and its compensation and the observed geoid derived from SEASAT data. The correlation is strongest for wavelengths greater than 45 km and poorest for shorter wavelengths. We attribute the poor resolution at short-wavelengths to instrument and oceanographic noise in the altimeter data combined with the reduced sensitivity of the geoid to bathymetric relief at very short wavelengths

Updated Interpretation of Magnetic Anomalies and Seafloor Spreading Stages in the South China Sea : Implications for the Tertiary Tectonics of Southeast Asia

International audienceWe present the interpretation of a new set of closely spaced marine magnetic profiles that complements previous data in the northeastern and southwestern parts of the South China Sea (Nan Hai). This interpretation shows that seafloor spreading was asymmetric and confirms that it included at least one ridge jump. Discontinuities in the seafloor fabric, characterized by large differences in basement depth and roughness, appear to be related to variations in spreading rate. Between anomalies 11 and 7 (32 to 27 Ma), spreading at an intermediate, average full rate of ~50 mm/yr created relatively smooth basement, now thickly blanketed by sediments. The ridge then jumped to the south and created rough basement, now much shallower and covered with thinner sediments than in the north. This episode lasted from anomaly 6b to anomaly 5c (27 to ~16 Ma) and the average spreading rate was slower, ~35 mm/yr. After 27 Ma, spreading appears to have developed first in the eastern part of the basin and to have propagated towards the southwest in two major steps, at the time of anomalies 6b-7, and at the time of anomaly 6. Each step correlates with a variation of the ridge orientation, from nearly E-W to NE-SW, and with a variation in the spreading rate. Spreading appears to have stopped synchronously along the ridge, at about 15.5 Ma. From computed fits of magnetic isochrons we calculate 10 poles of finite rotation between the times of magnetic anomalies 11 and 5c. The poles permit reconstruction of the Oligo-Miocene movements of Southeast Asian blocks north and south of the South China Sea. Using such reconstructions, we test quantitatively a simple scenario for the opening of the sea in which seafloor spreading results from the extrusion of Indochina relative to South China, in response to the penetration of India into Asia. This alone yields between 500 and 600 km of left-lateral motion on the Red River-Ailao Shan shear zone, with crustal shortening in the San Jiang region and crustal extension in Tonkin. The offset derived from the fit of magnetic isochrons on the South China Sea floor is compatible with the offset of geological markers north and south of the Red River Zone. The first phases of extension of the continental margins of the basin are probably related to motion on the Wang Chao and Three Pagodas Faults, in addition to the Red River Fault. That Indochina rotated at least 12° relative to South China implies that large-scale "domino" models are inadequate to describe the Cenozoic tectonics of Southeast Asia. The cessation of spreading after 16 Ma appears to be roughly synchronous with the final increments of left-lateral shear and normal uplift in the Ailao Shan (18 Ma), as well as with incipient collisions between the Australian and the Eurasian plates. Hence no other causes than the activation of new fault zones within the India-Asia collision zone, north and east of the Red River Fault, and perhaps increased resistance to extrusion a long the SE edge of Sundaland, appear to be required to terminate seafloor spreading in the largest marginal basin of the western Pacific and to change the sense of motion on the largest strike-slip fault of SE Asia

A geomagnetic record over the last 3.5 million years from deep-tow magnetic anomaly profiles across the Central Indian Ridge

International audienceHigh-resolution records of the geomagnetic field intensity over the last 4 Myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short-lived low field intensity features associated with excursions or short polarity intervals. In order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events, we have collected six deep-tow (-500 m above the seafloor) and several sea surface magnetic anomaly profiles from the Central Indian Ridge across the Brunhes, Matuyama, and Gauss chrons (i.e., from the ridge axis to anomaly 2A). After removal of topography, latitude, and azimuth effects, we converted distances into time sequences using well-dated polarity reversal anomalies as tie points. We calculated the average signal to test the robustness of the short-wavelength anomalies. The resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the Central Indian Ridge, the East Pacific Rise, and the Pacific-Antarctic Ridge. Our results suggest that in addition to polarity reversals, to previously suggested geomagnetic events (subchrons or excursions) within the Brunhes and Matuyama chrons. A new small-scale magnetic anomaly, likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas. We observe a dense succession of microanomalies that are correlated excursions (Ontong Java 1 and 2, and Gilsa), is found after the Olduvai chron. The near-bottom results support the existence of three geo-magnetic features between the Gauss-Matuyama boundary and Olduvai. They also suggest three geomagnetic events during the C2A. I n subchron within the Gauss chron. This study emphasizes the potential of deep-tow magnetic surveys in detecting fluctuations in geo-magnetic field intensity and, in particular, short-lived excursions, a poorly constrained part of the geomagnetic field temporal variation spectrum

Motion between the Indian, Antarctic and African plates in the early Cenozoic

International audienceWe used a three-plate best-fit algorithm to calculate four sets of Euler rotations for motion between the India (Capricorn), Africa (Somali) and Antarctic plates for 14 time intervals in the early Cenozoic. Each set of rotations had a different combination of data constraints. The first set of rotations used a basic set of magnetic anomaly picks on the Central Indian Ridge (CIR), Southeast Indian Ridge (SEIR) and Southwest Indian Ridge (SWIR) and fracture zone constraints on the CIR and SEIR, but did not incorporate data from the Carlsberg Ridge and did not use fracture zones on the SWIR. The second set added fracture zone constraints from the region of the Bain fracture zone on the SWIR which were dated with synthetic flowlines based on the first data set. The third set of rotations used the basic constraints from the first rotation set and added data from the Carlsberg Ridge. The fourth set of rotations combined both the SWIR fracture zone constraints and the Carlsberg Ridge constraints. Data on the Indian Plate side of the Carlsberg Ridge (Arabian Basin) were rotated to the Capricorn Plate before being included in the constraints. Plate trajectories and spreading rate histories for the CIR and SWIR based on the new rotations document the major early Cenozoic changes in plate motion. On the CIR and SEIR there was a large but gradual slowdown starting around Chron 23o (51.9 Ma) and continuing until Chron 21y (45.3 Ma) followed 2 or 3 Myr later by an abrupt change in spreading azimuth which started around Chron 20o (42.8) Ma and which was completed by Chron 20y (41.5 Ma). No change in spreading rate accompanied the abrupt change in spreading direction. On the SWIR there was a continuous increase in spreading rates between Chrons 23o and 20o and large changes in azimuth around Chrons 24 and 23 and again at Chron 21. Unexpectedly, we found that the two sets of rotations constrained by the Carlsberg Ridge data diverged from the other two sets of rotations prior to anomaly 22o. When compared to rotations for the CIR that are simultaneously constrained by data from all three branches of the Indian Ocean Triple Junction, there is a progressively larger separation of anomalies on the Carlsberg Ridge, with a roughly 25 km misfit for anomaly 23o and increasing to over 100 km for anomaly 26y. These data require that there was previously unrecognized convergence somewhere in the plate circuit linking the Indian, Capricorn and Somali plates prior to Chron 22o. We quantify this motion by summing our new Capricorn–Somalia rotations with previously published rotations for Neogene India–Capricorn motion and for early Cenozoic Somali–India motion based solely on Carlsberg Ridge data. The most likely possibility is that there was motion within the Somalia Plate due to a distinct Seychelles microplate as young as Chron 22o. The sense of the misfit on the Carlsberg Ridge is consistent with roughly 100–150 km of convergence across a boundary passing through the Amirante Trench and extending north to the Carlsberg Ridge axis between anomalies 26y and 22o. Alternatively, there may have been convergence within the Indian Plate, either along the western margin of Indian or east of the CIR in the region of the current Capricorn–Indian diffuse plate boundary. Our work sharpens the dating of the two major Eocene changes in plate motion recognized in the Indian Ocean

On the stability of triple junctions and its relation to episodicity in spreading

A convenient representation of triple junctions that involve only ridges (R) and transform faults (F) is proposed: this representation combines in a simple way information from geographic and velocity spaces. The velocity triangle provides the budget of lithospheric surface change which directly results from interactions of the three plates. A discussion of the relative positions of the triple junction with respect to the velocity triangle demonstrates that, in general, there are several triple junction configurations that are compatible with a given triangle. This discussion stresses the importance of oblique and asymmetric spreading. Pairs of triple junction configurations that are of particular interest are RRR‐RRF and RRR‐FFR. When the triple junction lies outside of the triangle, at least one of the ridges will shorten, leading to yet another type of potentially unstable configuration. The present configurations of the Bouvet, Galapagos, and Indian Ocean triple junctions are reviewed. A method is proposed to reconstruct past configurations of the Indian Ocean junction at the time of anomalies 23 and 28. The main parameters that influence triple junction evolution are the lengths of transform faults, the availability of magma and related connectivity of magma chambers and the (also related) spreading velocities. This study confirms an earlier suggestion that activity at constructive plate boundaries occurs in two preferred modes: the effusive and tectonic modes, corresponding here to RRR and to RRF‐RFF configurations respectively. These modes apparently alternate in episodes of typically 1 Ma duration. The significance of this time constant and consequences for lithospheric mechanics are briefly discussed

On the stability of triple junctions and its relation to episodicity in spreading

A convenient representation of triple junctions that involve only ridges (R) and transform faults (F) is proposed: this representation combines in a simple way information from geographic and velocity spaces. The velocity triangle provides the budget of lithospheric surface change which directly results from interactions of the three plates. A discussion of the relative positions of the triple junction with respect to the velocity triangle demonstrates that, in general, there are several triple junction configurations that are compatible with a given triangle. This discussion stresses the importance of oblique and asymmetric spreading. Pairs of triple junction configurations that are of particular interest are RRR‐RRF and RRR‐FFR. When the triple junction lies outside of the triangle, at least one of the ridges will shorten, leading to yet another type of potentially unstable configuration. The present configurations of the Bouvet, Galapagos, and Indian Ocean triple junctions are reviewed. A method is proposed to reconstruct past configurations of the Indian Ocean junction at the time of anomalies 23 and 28. The main parameters that influence triple junction evolution are the lengths of transform faults, the availability of magma and related connectivity of magma chambers and the (also related) spreading velocities. This study confirms an earlier suggestion that activity at constructive plate boundaries occurs in two preferred modes: the effusive and tectonic modes, corresponding here to RRR and to RRF‐RFF configurations respectively. These modes apparently alternate in episodes of typically 1 Ma duration. The significance of this time constant and consequences for lithospheric mechanics are briefly discussed

The anticorrelated velocities of Africa and India in the Late Cretaceous and early Cenozoic

International audienceWe present a revised interpretation of magnetic anomalies and fracture zones on the Southwest Indian Ridge (SWIR; Africa-Antarctica) and the Southeast Indian Ridge (SEIR; Capricorn-Antarctica) and use them to calculate 2-plate finite rotations for anomalies 34 to 20 (84 to 43 Ma). Central Indian Ridge (CIR; Capricorn-Africa) rotations are calculated by summing the SWIR and SEIR rotations. These rotations provide a high-resolution record of changes in the motion of India and Africa at the time of the onset of the Reunion plume head. An analysis of the relative velocities of India, Africa and Antarctica leads to a refinement of previous observations that the speedup of India relative to the mantle was accompanied by a slowdown of Africa. The most rapid slowdown of Africa occurs around Chron 32Ay (71 Ma), the time when India's motion relative to Africa notably starts to accelerate. Using the most recent Geomagnetic Polarity Timescale (GTS12) we show that India's velocity relative to Africa was characterized by an acceleration from roughly 60 to 180 mm yr-1 between 71 and 66 Ma, a short pulse of superfast motion (∼180 mm yr-1) between 66 and 63 Ma, an abrupt slowdown to 120 mm yr-1 between 63 and 62 Ma, and then a long period (63 to 47 Ma) of gradual slowing, but still fast motion (∼100 mm yr-1), which ends with a rapid slowdown after Chron 21o (47 Ma). Changes in the velocities of Africa and India with respect to the mantle follow a similar pattern. The fastest motion of India relative to the mantle, ∼220 mm yr-1, occurs during Chron 29R. The SWIR rotations constrain three significant changes in the migration path of the Africa-Antarctic stage poles: following Chron 33y (73 Ma), following Chron 31y (68 Ma), and following Chron 24o (54 Ma). The change in the migration path of the SWIR stage poles following Chron 33y is coincident with the most rapid slowdown in Africa's motion. The change in the migration path after Chron 31y, although coincident with the most rapid acceleration of India's northward motion, may be related to changes in ridge push forces on the SWIR associated with the onset of extension along the Bain transform fault zone. The initial slowdown in India's motion relative to Africa between 63 and 62 Ma is more abrupt than predictions based on published plume head force models, suggesting it might have been caused by a change in plate boundary forces. The abrupt change in the migration path of the SWIR stage poles after Chron 24o is not associated with major changes in the velocities of either Africa or India and may reflect Atlantic basin plate motion changes associated with the arrival at the Earth's surface of the Iceland plume head. The abruptness of India's slowdown after Chron 21o is consistent with a collision event

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

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Relationship of the Central Indian Ridge segmentation with the evolution of the Rodrigues triple junction for the past 8 Myr.

International audienceLocated near 25ø33'S, 70ø00'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 (<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