The Magnetic Signature of Oceanic Core Complexes

International audienceOceanic core complexes (OCC) are widespread tectonic features on slow spreading centers. They involve a steep detachment fault rooted on the neovolcanic zone that evolves to a flat, often exhumed detachment surface away from the axis. Their magnetic signature has so far been poorly defined: only few magnetic surveys exist and their interpretation is complex due to the multiple rock types, insufficient coverage and resolution and, for the best studied OCCs on the MAR 13-15°N, location-N-S trending magnetized features near the magnetic Equator generate negligible anomalies. We take advantage of two recent surveys to constrain the magnetic signature of OCCs. (1) Cruise CARLMAG of BHO Beautemps-Beaupré (2019) surveyed several large OCCs of the NE-SW trending Carlsberg Ridge and depicted clear sea-surface anomalies that unambiguously record geomagnetic reversals, pointing out that the oceanic crust created on large OCCs also bears the seafloor spreading magnetic stripes. As expected, the axial anomaly extends from the axial valley over a large part of the OCC, this large shift with respect to the axial valley confirming that most seafloor spreading is accounted by the detachment fault motion. Possible sources for the magnetic anomalies include gabbro bodies cooled in the neovolcanic zone and carried off-axis as the footwall progressively exhumes, or serpentinite formed near the axis as mantle peridotite cools below ~400°C and is altered by hydrothermal circulation. (2) During Cruise ODEMAR of R/V Pourquoi Pas? (2013), AUV Abyss achieved a detailed bathymetric and magnetic survey of the 13°20'N OCC at altitude ~70 m. Although the OCC presumably formed within the Brunhes chron, the magnetic data inverted to equivalent magnetization reveal a clear magnetic stripe that may reflect geomagnetic variations recorded by seafloor spreading. The sharp limits of this anomaly favor cooled gabbro bodies as the source for the magnetic anomalies observed at OCCs