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Science 21 July 1978:
Vol. 201. no. 4352, pp. 249 - 251
DOI: 10.1126/science.201.4352.249
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Articles

Mantle Uplifted Block in the Western Indian Ocean

ENRICO BONATTI 1 and PAUL R. HAMLYN 1

1 Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964

An anomalous topographic high located close to the intersection of the Owen Fracture Zone with the Mid-Indian Ridge exposes exclusively ultramafic rocks for a thickness of more than 2 kilometers. The rocks, consisting of partly serpentinized spinel lherzolites, with minor harzburgites and dunites, display protogranular to porphyroclastic fabrics, but no cumulate textures. The chemistry of olivine, ortho-and clinopyroxene, and spinel crystals suggests that the rocks originated at a depth of at least 25 kilometers in the oceanic lithosphere and were partially reequilibrated and recrystallized during subsequent upwelling. Thus, field, textural, and mineral chemistry data indicate the presence of an uplifted block of upper mantle. The considerable vertical uplift can be explained by a two-stage process: mantle upwelling in the axial zone of plate accretion, followed by vertical tectonic uplift along the fracture zone. The rate of uplift in the fracture zone was of the order of 1 millimeter per year.

Submitted on January 9, 1978
Revised on April 25, 1978


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