Cardiff University, United Kingdom

The region between 12 degrees N and 16 degrees N on the Mid-Atlantic is characterized by numerous of active and inactive extensional core complexes exposing ultramafic and gabbroic rocks. The abundant outcrop of residual ultramafic and gabbroic intrusions embedded in ultramafic country rocks outcropping on the seafloor on both sides of the rift valley suggests a thin overall crustal component, low melt supply and complex unroofing of extensional core complexes. Asymmetric rifting is prevalent and not restricted to inner corners of ridge segment segment boundaries. In addition to Leg 209 drilling in the region, we have extended sampling programs from previously surveyed northern regions from 14 degrees to 16 degrees N (Cannat and Casey, 1995, Fujiwara et al., 2000) to four newly discovered core complexes (Smith et al., 2006) and adjacent ridge segments between 14 degrees and 12 degrees 30'N on a cruise of the research vessel James Cook in March-April 2007. The distribution of ultramafic and gabbroic lithologies sampled in the entire region, the newly discovered core complexes, the ridge and core complex morphotectonic expression along the ridge segments, and gravity signature and resultant crustal thickness estimates indicate that the mafic crustal section is either thin or non-existent in the region. The rheological boundary between the lithosphere and low viscosity asthenosphere is likely to be significantly below the mafic-ultramafic or serpentinized/unserpentinized seismic crust-mantle boundary. This indicates that any mafic and ultramafic boundary is strongly coupled throughout the extension along the ridge axis. Geochemical results indicate low melt supply and sampling by drilling and dredging indicate zones of significant strain softening and strain localization throughout the upper lithosphere during both high temperature and low temperature deformation in ultramafic rocks.