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Geological Society, London, Special Publications; 1984; v. 13; p. 41-53;
DOI: 10.1144/GSL.SP.1984.013.01.04
© 1984 Geological Society of London

Magma Chambers: Products and Processes

The structure of the oceanic upper mantle and lower crust as deduced from the northern section of the Oman ophiolite

J. D. Smewing, N. I. Christensen, I. D. Bartholomew & P. Browning

Earth Resources Institute, Department of Geology, University College of Swansea, Swansea SA2 8PP, UK
Department of Geological Sciences, University of Washington, Seattle, WA 98915, USA
Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

The lowermost 7 km of the Oman ophiolite consist of harzburgite, dunite and minor lherzolite of mantle origin overlain by layered gabbros and peridotites of the lower oceanic crust. Detailed structural measurements along three traverses through the ophiolite, complemented by olivine petrofabric determinations on oriented specimens from these traverses, reveal the following geometrical parameters of the spreading process:

(i) The ultramafic rocks of the mantle sequence have all suffered deformation by simple shear at temperatures exceeding 950°C beneath a spreading ridge.
(ii) The tectonite fabric which characterizes the mantle sequence passes upwards with no change in orientation or intensity into the layered gabbros and peridotites at the base of the crust.
(iii) The sense of shear and type of slip system vary at random with depth, reinforcing petrological arguments that the Oman mantle sequence results from the juxtaposition of various mantle ‘packages’ equilibrated at different depths beneath the ridge.
(iv) No unique kinematic model for the ophiolite can be formulated from the structural data of the three traverses; each traverse has a distinctive geometrical arrangement of the various structural elements relative to the reconstructed ridge directions. Dispersed mineral-layering planes indicate closing directions of magma chambers strung along the ridge.