Abstract
From 55°45′E to 58°45′E and from 60°30′E to 62°00′E, the ultraslow‐spreading Southwest Indian Ridge (SWIR) consists of magmatic spreading segments separated by oblique amagmatic spreading segments, transform faults, and nontransform discontinuities. Off‐axis magnetic and multibeam bathymetric data permit investigation of the evolution of this part of the SWIR. Individual magmatic segments show varying magnitudes and directions of asymmetric spreading, which requires that the shape of the plate boundary has changed significantly over time. In particular, since 26 Ma the Atlantis II transform fault grew by 90 km to reach 199 km, while a 45‐km‐long transform fault at 56°30′E shrank to become an 11 km offset nontransform discontinuity. Conversely, an oblique amagmatic segment at the center of a first‐order spreading segment shows little change in orientation with time. These changes are consistent with the clockwise rotation of two ∼450‐km‐wide first‐order spreading segments between the Gallieni and Melville transform faults (52–60°E) to become more orthogonal to spreading. We suggest that suborthogonal first‐order spreading segments reflect a stable configuration for mid‐ocean ridges that maximizes upwelling rates in the asthenospheric mantle and results in a hotter and weaker ridge‐axis that can more easily accommodate seafloor spreading.
Highlights
Evolution of the Southwest Indian Ridge from 55°450E to 62°E: Changes in plate-boundary geometry since 26 Ma
This section of the Southwest Indian Ridge (SWIR) formed by northeastward propagation of the ridge-ridge-ridge Rodriguez Triple Junction between oceanic lithosphere created at the Central Indian (CIR) and Southeast Indian Ridges (SEIR) [Patriat and Courtillot, 1984; Patriat and Segoufin, 1988] (Figure 1), so the geometry and segmentation of the eastern SWIR cannot be attributed to the preexisting structure of a rifted continent as has been inferred for the MidAtlantic Ridge (MAR) [e.g., Klitgord and Schouten, 1986]
Variations in the magnitude and/or direction of asymmetric spreading between adjacent spreading segments accommodated these changes, which include growth of the Atlantis II Transform fault from $110 km to 199 km (Figure 9b), and shortening of a transform fault at 56°300E by 34 km to form an 11 km offset nontransform discontinuity (Figure 9a)
Summary
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