Stretching of the deep crust at the slow-spreading Southwest Indian Ridge

The 500 metres of gabbro drilled in the Southwest Indian Ocean during ODP Leg 118 allow direct observation of early, high-temperature extensional deformation in the deep crust of a slow-spreading rift valley. About 30% of the recovered gabbros have been plastically stretched at progressively decreasing temperatures (granulite- to lower amphibolite-facies metamorphic conditions), increasing water/rock ratio, and probably increasing deviatoric stresses. The early solid-state strain in granulite facies metamorphic conditions produced low-dipping foliations and low strain gradients. At this stage, mylonitic shear zones only formed in oxide-rich gabbros. Strain localization in the deepest and hottest part of the oceanic crust thus appears to have been influenced by the magmatic stratigraphy. In upper to lower amphibolite-facies metamorphic conditions, the ductile strength of the gabbro was probably higher, and deformation concentrated in mylonitic shear bands, which may have provided access to seawater-derived fluids. Strain localization at this stage was accompanied by brittle failure and boudinage, in the mylonitic foliation, of individual porphyroclasts, or of thick gabbro intervals. The influence of such brittle failure on hydrothermal circulations was critical: it allowed for extensive crust/seawater interactions at high temperatures (upper amphibolite facies conditions). In a spreading context where continuous magma supply provides heat and new crustal material to the axial region, the mechanical behaviour of the ridge axis lithosphere should be controlled by continuous ductile deformation in granulite-facies conditions. This may lead to the steady-state “necking” configuration described by Tapponnier and Francheteau (1978). Alternatively, a discontinuity of the magma supply, allowing the lower crust of the axial region to cool below granulite-facies temperatures, should cause mylonitic shear zones characteristic of amphibolite-facies deformation to extend deeper into the axial domain, eventually leading to the detachment fault configuration proposed by Karson (in press). As magma supply to slow-spreading oceanic ridges is likely to be discontinuous, necking and faulting periods should alternate through time.