Petrostructural analysis of the Leg 109 serpentinized peridotites

Abstract

The serpentinized peridotites at Site 670 have a harzburgitic to lherzolitic protolith. They have been deformed plastically at high temperature and moderate deviatoric stress conditions typical of the mantle. They have subsequently been serpentinized in static conditions. The dip of the plastic foliation and the plunge of the lineation vary significantly downhole. This suggests either that the rocks recovered come from large shear polyhedra of serpentinized peridotite, which preserved their mantle structures, but rotated in a matrix of sheared serpentinite, or that Site 670 is set on a heap of dekametric blocks, fallen from a serpentinite cliff nearby. The possible modes of emplacment of the serpentinized peridotites on the seafloor of the median valley wall are discussed. A model involving tectonic stretching of a thin magmatic crust is favored. -Authors