RELATIONS BETWEEN FAULTING MECHANISMS FLUID TYPES AT NON-VOLCANIC MID-OCEAN RIDGES

Parts of slow-spreading (2-5 cm/year) mid-ocean ridges and most of ultra-slow-spreading (<2 cm/year) ridges accrete plate with discontinuous or absent magmatic crust. Such ridge segments, referred to as non-volcanic ridges, dominantly accrete plate by tectonically denuding mantle peridotite and gabbro plutons. Bathymetry of non-volcanic sections of the Mid-Atlantic Ridge (MAR) indicate that mantle is denuded either by long-lived detachment faults or multiple faults with less individual slip. Strain localization on both fault types is influenced by fluid-rock interactions, but detachment faults also contain evidence of magma intrusion during strain localization. Detachment faulting denudes mantle along a single fault with >15 km slip that dips steeply beneath the ridge axis and bends to horizontal through a rolling hinge as the footwall is denuded. Rocks sampled from detachment faults along the MAR indicate that strain was localized at upper-amphibolite to granulite facies (650-800°C) and remained active to greenschist or sub-greenschist (<250°C) conditions. Peridotite fault rocks contain metasomatic mineral assemblages (Fe-Ti Oxides, apatite, zircon, and Al-rich amphibole) indicating alteration by fluids derived from evolved gabbroic magma. Gabbro fault rocks from the detachment at 16°N on the MAR contain numerous small (0.2-2 cm) diabase pods that were intruded during semi-brittle, amphibolite-grade deformation. Lithosphere from 14°N to 16°N on the MAR is dominated by peridotite that was denuded by multiple faults with 1-4 km slip that dip 10-20°. Rocks sampled from these faults contain deformation textures and minerals indicating that strain localization was controlled by serpentinization and greenschist grade alteration. These faults, however, lack metasomatic mineral assemblages indicating alteration by magmatic fluids. This suggests detachment faulting during non-volcanic spreading may be related to shallow magma intrusion.