Deformed and metamorphosed oceanic crust on the Mid-Atlantic ridge.

Highly deformed and metamorphosed gabbroic rocks were collected during detailed mapping of a portion of the median valley wall of the Mid-Atlantic Ridge at 24oN. The basaltic carapace has apparently been stripped away by back-tilting, mass wasting and erosion. The subvolcanic rocks are mainly metadiabases and metagabbros and are cut by numerous faults and shear zones. Rocks recovered from between the fault zones are fresh to slightly fractured and veined gabbros and minor diabases. Rocks from faults and shear zones include a wide variety of cataclasites, mylonites and amphibolites with petrographic evidence of recurrent tectonic deformation and hydrothermal alteration. Relatively low-T vein fillings occur in fractures cutting the deformation fabrics. Talus derived from volcanic and plutonic rocks locally overlies the faulted rocks. This study suggests that obducted oceanic crust from such a setting will not necessarily have a complete ophiolite stratigraphy. A relatively thin crustal section would be expected in which the volcanics would be very thin and locally absent; sheeted dykes would be highly fractured and possibly absent; much of the gabbroic section would be pervasively deformed and metamorphosed; and underlying peridotites would occur at shallow levels (<2 km) and might be subject to early serpentinization along fault zones. A variable thickness of breccias with clasts of basalt, diabase, gabbro and possibly serpentinite would cap this sequence. This type of structure probably forms due to extensive crustal extension in an area with very little magmatism during sea-floor spreading.-C.N.