Architecture of the Igneous Lower Crust at Oceanic Core Complexes: constraints from IODP Hole U1309D

Slow spreading mid-ocean ridges are a ubiquitous part of the global ridge system, yet remain poorly understood. The crust produced at these ridges is fundamentally different than that produced at fast spreading ridges where the crustal architecture largely conforms to the standard Penrose type crust. At slow spread ridges, a reduced magma supply to the crust allows extensional faulting to play a much more important role in accommodating plate separation than at fast spread ridges. Oceanic core complexes (OCCs), a product of such faulting, denude lower crust to the surface via detachment faulting, and thus provide a means to study the architecture of slow spread lower crust. We report a detailed lithologic analysis of IODP Hole U1309D drilled into the Atlantis Massif OCC (30°N MAR). The abundance of sharp contacts between thin inter-layered gabbroic and ultramafic rocks throughout the core supports crustal construction via small (10-40m thick) injections of magma. Paleomagnetic remanance data allows re-orientation of the observed contacts and igneous fabrics to their original orientation at intrusion, and suggest that most contacts and fabrics were sub-vertical. These data therefore imply that construction of slow spread gabbroic lower crust at OCCs is dominated by dike-like intrusions rather than by sills. Combined U-Pb and (U-Th)/He zircon thermochronometry are used to predict a 3-4km thick zone of accretion that lies 6-7 km below seafloor, at the root of the detachment fault. Existing seismic data estimate the depth of the Moho to be 5km at Atlantis Massif constraining the width of the accretion zone in the footwall of the detachment fault to be 4km. Igneous fabrics from unfaulted gabbroic rocks provide an additional major constraint on the processes occurring within this zone of magmatic accretion. Electron backscatter diffraction (EBSD) was used to characterize the fabrics of slow-spread gabbros from the Atlantis Massif OCC, and other OCCs on the MAR (the 15° 20" N OCC and Kane OCC) and from the Southwest Indian Ridge (Atlantis Bank). Fabrics from the fast spread Oman Ophiolite, and from the Dufek, Stillwater and Rum layered mafic intrusions (LMIs) were analyzed for comparison. Plagioclase fabric strength was quantified using eigenvalues and J-indices, calculated using PFch5 careware (Mainprice, 1990). The E1:E2 ratio for {010} was used as a proxy for foliation strength, and the E1:E2 ratio for <100> as a proxy for lineation strength. Gabbroic rocks from the OCCs have a minimal foliation and a non-existent to very weak lineation. In contrast, gabbros from Oman (a proxy for fast spread crust), have stronger foliations and lineations; gabbros from LMIs show strong foliations and no lineations. The absence of strong fabrics in the OCC gabbros is consistent with their formation as thin, ephemeral dike-like bodies; the presence of a very weak lineation may be related to post intrusion deformation of the solidifying crystal mush during plate separation.