Hubert Whitechurch

Periodicity in the accretion process on the Southeast Indian Ridge at 27°40′S

Abstract Image processing of a Seabeam bathymetric grid on the Southeast Indian Ridge (SEIR) at 27°40′S has been used to analyse the morphotectonics of the ridge flanks. Steep topographic slopes indicating inward-facing or outward-facing faults and those representing tilted blocks were automatically separated. An autocorrelation process allows an estimation of the periodicity of the fault spacing. The outward-facing faults first appear at 16–18 km from the axial valley, are more common on bathymetric highs, and correspond to tilted block zones in adjacent deeper areas. The tilting of the blocks is ~ 4°. The transformation of the rift valley into the smooth undulating relief of the flanks is accomplished by these outward-facing faults and tilted blocks. Inward-facing faults appear either as large faults with major throws or as a series of smaller faults with minor throws. The minor faults are mainly localized on bathymetric highs and are densely organized. The major faults, continuous and symmetrically spaced on each side of the axis zone, are organized with a periodicity of ~ 8.2 km. These faults seem to be old inner walls which have been moved outwards periodically during the spreading process. They alternate with positive magnetic lineaments reflecting old neovolcanic zones, linked to intense magmatic periods. Considering a mean half-spreading rate of 31 km/m.y. during the past million years, this would mean the development of a major fault each ~ 0.26 m.y. We propose a model for the evolution of the SEIR at 27°40′S controlled by a periodic succession of magmatic and tectonic episodes: 1. (1) faulting and volcanism are concentrated in the axial zone above the melt reservoir where the crust is weakened (magmatic period) — outside this zone the external relief is frozen-in, supported by the finite strength of the plate; 2. (2) the axial valley is enlarged, fills up with lavas and bulges due to thermal effects; 3. (3) at the end of the magmatic period, this bulged zone collapses by thermal subsidence along lines of weakness such as the faults forming the inner walls; 4. (4) when the melt reservoir is empty (tectonic period), the processes are no longer concentrated and active faulting extends over tens of kilometres from the axis — the lithosphere is thinned, a new axial valley is created, the flanks are uplifted, and outward-facing faults and block tilting are induced.

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

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