Magnetic signature of large exhumed mantle domains of the Southwest Indian Ridge – results from a deep-tow geophysical survey over 0 to 11 Ma old seafloor

摘要

We investigate the magnetic signature of ultramafic seafloor in theeastern part of the Southwest Indian Ridge (SWIR). There, detachmentfaulting, continuous over 11 Myr, exhumed large areas of mantle-derivedrocks. These exhumed mantle domains occur in the form of a smooth roundedtopography with broad ridges locally covered by a thin highly discontinuousvolcanic carapace. We present high-resolution data combining deep-towmagnetics, side-scan sonar images and dredged samples collected within twoexhumed mantle domains between 62° E and 65° E. We showthat, despite an ultra-slow spreading rate, volcanic areas within robustmagmatic segments are characterized by well-defined seafloor spreadinganomalies. By contrast, the exhumed mantle domains, including a few thinvolcanic patches, reveal a weak and highly variable magnetic pattern. Theanalysis of the magnetic properties of the dredged samples and carefulcomparison between the nature of the seafloor, the deep-tow magneticanomalies and the seafloor equivalent magnetization suggest that theserpentinized peridotites do not carry a sufficiently stable remanentmagnetization to produce seafloor spreading magnetic anomalies in exhumedmantle domains.