University of Wyoming, United States

The Mid-Atlantic Ridge (MAR) north and south of the 15-20 Fracture Zone (FZ) produces upper oceanic lithosphere composed dominantly of mantle peridotite with gabbro intrusions. In the absence of diapirism, mantle peridotite can only be exposed on the seafloor by extensional faulting, thus much of the seafloor in this region must be exposed fault surfaces. This area contains several domal "oceanic core complexes" that have been interpreted to result from "magma-limited" or "non-volcanic" seafloor spreading, in which tectonic extension accommodates much of plate separation. However, only 3% of the seafloor near the 15-20 FZ is characterized by oceanic core complexes. A far greater area is dominated by widely-spaced, 15-40km long (parallel to the MAR), approximately 2000m vertical relief bathymetric ridges with 10-15km wide slopes that dip approximately 15 degrees . Drilling these ridges during O.D.P. Leg 209 revealed that they are composed of mantle peridotite with small (<50m wide) gabbro intrusions. These ridges are capped by long-lived detachment faults, but also contain numerous brittle and ductile faults at depth that have both steep and gentle dips. The domal core complex drilled during Leg 209 is capped by a detachment fault, but, unlike the peridotite ridges, it is composed of gabbro and is relatively undeformed at depth. This observation is consistent with other domal core complexes drilled on MORs. Should both the peridotite ridges and gabbro domes be considered "oceanic core complexes"? Deformation microtextures, amphibole-plagioclase thermometry, and Ti-in-zircon thermometry in detachment fault samples from both types of structures indicate strain localization near the ductile-to-brittle transition and continuous down-temperature deformation during denudation to sub-greenschist conditions. However, the significant geologic and morphologic differences between them suggest that each are formed under distinct conditions of magma-limited spreading, with gabbro domes likely representing periods of greater shallow magma intrusion than peridotite ridges. Thus, these features should be treated separately.