DUCTILE DEFORMATION AND THE ORIGIN OF LAYERED GABBRO IN OPHIOLITES

Layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3-6 km deep. Although large, steady state magma chambers provide a possible explanation for the limited diversity of mid-ocean ridge basalts, their existence is not supported by geophysical observations. Sinton and Detrick (1992) recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small (1-4 km wide) magma chambers perched on large (approximately 8 km wide) zones of low-velocity material, which are interpreted to consist of partially molten crystal cumulates. In this paper, we compare the geology of the gabbroic rocks of the Samail ophiolite, which is thought to have formed by fast spreading, with results of a finite element analysis of the Sinton-Detrick model for fast spreading centers. Our results demonstrate that flow, similar to that postulated by Sleep [1975], within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the Samail gabbros. As a consequence of this deformation, (1) layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward die spreading center; (2) die gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime, let alone the magma chamber (3) feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering, but those that invade farther from the ridge will be lm deformed and retain crosscutting relationships; and (4) total deviatoric strain should increase down section toward the Moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros. Upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in dhe gabbros and help maintain the magma chamber in a compositional steady state.