Geographically re-oriented magmatic and metamorphic foliations from ODP Hole 735B Atlantis Bank, Southwest Indian Ridge: Magmatic intrusion and crystal-plastic overprint in the footwall of an oceanic core complex

Abstract

When core pieces are drilled they lose all geographic information, making crustal-scale structural analysis of in situ oceanic crust near impossible. In order to better understand the orientation of deformation fabrics in the footwall, and therefore the formation and evolution of the lower oceanic crust, magmatic and high-temperature metamorphic foliations were re-oriented back to the geographic reference frame over the interval 90–600 m below sea floor from Ocean Drilling Program Hole 735B at the Atlantis Bank oceanic core complex, Southwest Indian Ridge. Fractures measured on cores were correlated with borehole images of fractures to re-orient core pieces, and therefore re-orient any element within those core pieces. Simple 2-D models of extension predict that high-temperature fabrics should dip toward or away from the spreading ridge, however, only ∼30% of each foliation type follows this prediction. The lack of systematic orientation of high temperature deformation foliations seems to indicate that there is a complex 3-D orientation and distribution of magmatic and metamorphic foliations with respect to the detachment shear zone and that the strains that these foliations recorded is not compatible with simple plane strain assumptions related to ridge-perpendicular extension at slow-spreading ridges.