The Mid-atlantic Ridge (31°S–34°30′S): Temporal and spatial variations of accretionary processes

Abstract

The ridge located between 31 ~ S and 34~ is spreading at a rate of 35 mm yr-1, a transitional velocity between the very slow (-<20 mm yr -1) opening rates of the North Atlantic and Southwest Indian Oceans, and the intermediate rates (60 mm yr-~) of the northern limb of the East Pacific Rise, and the Galapagos and Juan de Fuca Ridges. A synthesis of multinarrow beam, magnetics and gravity data document that in this area the ridge represents a dynamically evolving system. Here the ridge is partitioned into an ensemble of six distinct segments of variable lengths (12 to 100 km) by two transform faults (first-order discontinuities) and three small offset ( < 30 km) discontinuities (second-order discontinuities) that behave non-rigidly creating complex and heterogeneous morphotectonic patterns that are not parallel to flow lines. The offset magnitudes of both the first and second-order discontinuities change in response to differential asymmetric spreading. In addition, along the fossil trace of second-order discontinuities, the lengths of abyssal hills located to either side of a discordant zone are observed to lengthen and shorten creating a saw-toothed pattern. Although the spreading rate remains the same along the length of the ridge studied, the morphology of the spreading segments varies from a deep median valley with characteristics analogous to the rift segments of the North Atlantic to a gently rifted axial bulge that is indistinguishable from the shape and relief of the intermediate rate spreading centers of the East Pacific Rise (i.e., 21 ~ N). Like other carefully surveyed ridge segments at slow and fast rates of accretion, the along-axis profiles of each ridge segment are distinctly convex upwards, and exhibit along-strike changes in relief of 500 m to 1500 between the shallowest portion of the segment (approximate center) and the segment ends. Such spatial variations create marked along-axis changes in the morphology and relief of each segment. A relatively low mantle Bouguer anomaly is known to be associated with the ridge segment characterized by a gently rifted axial bulge and is interpreted to indicate the presence of focused mantle upwelling (Kuo and Forsyth, 1988). Moreover, the terrain at the ends of each segment are known to be highly magnetized compared to the centers of each segment (Carbotte et al, 1990). Taken together, these data clearly establish that these profound Marine Geophysical Researches 13: 1-20, 1991. 9 1991 Kluwer Academic Publishers. Printed in the Netherlands. spatial variations in ridge segment properties between adjoining segments, and along and across each segment, indicate that the upper mantle processes responsible for the formation of this contrasting architecture are not solely related to passive upwelling of the asthenosphere beneath the ridge axis. Rather, there must be differences in the thermal and mechanical structure of the crust and upper mantle between and along the ridge segments to explain these spatial variations in axial topography, crustal structure and magn...

The geological record of oceanic crustal accretion and tectonism at slow-spreading ridges

“…The plate boundary at slow-spreading ridges is characterized by a 1-to 3-km deep and 15-to 30-km wide rift valley, bounded on either side by rugged topography of the rift mountains (e.g., Needham and Francheteau, 1974;Macdonald and Luyendyk, 1977;Macdonald, 1986;Karson, 1990;Fox et al, 1991). A typical cross-section of a rift valley exhibits an inner floor -7 km wide where crust is relatively undeformed except by small 43 fissures and where zero-age crust is accreted at the ridge axis along a neovolcanic ridge.…”

“…Fundamental structural and lithological changes occur along the length of individual segments (e.g., Macdonald and Luyendyk, 1977;Carbotte eta!., 1991 ;Grindlay at al., 1991;Blackman and Forsyth, 1991).…”

“…Based on along-axis data, it has been hypothesized that segmentation is controlled by the dynamics of mantle upwelling, by melt distribution, and by mechanical extension (e.g., Whitehead et al, 1984;Macdonald et al, 1991;Mutter and Karson, 1992).…”

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

“…Such maps can reveal elements of tectonic and volcanic terrains as small as a few hundred meters laterally and 10-20 m vertically (Renard and Allenou, 1979). Seabeam coverage of the ridge axis has proved to be a high-performance tool to analyze the plate boundary morphology along long sections of mid-ocean ridges (e.g., Macdonald et al, 1984;Purdy et al, 1990;Fox et al, 1991;Semp6r6 et al, 1993). On small-scale maps, linear, dense bathymetry contours, often limiting terrain elements on one side only, are interpreted as fault scarps, whereas dense contours, but with less linearity, and often symmetrically surrounding terrain elements are inferred to be volcanic ridges (e.g., Kappel and Ryan, 1986;Fox et al, 1991).…”

“…Fast and slow spreading ridges do not differ as much in their first-order discontinuities, the fracture zones (Fox and Gallo, 1984), as in their second-order discontinuities. Between the major fracture zones, fast-spreading ridge axes are most often offset along overlapping spreading centers (e.g., Lonsdale, 1983;Macdonald and Fox, 1983), whereas slow-spreading ridge axes are offset along non-transform discontinuities such as en 6chelon jogs or oblique basins (e.g., Purdy et al, 1990;Semp6r6 et al, 1990;Fox et al, 1991;Grindlay et al, 1991). Until now, however, only the fast and slow end-members of the range of spreading rates have been analyzed with a morphotectonic approach.…”