LAUSR

Abstract A high-resolution magnetic survey conducted over the apparently inactive ultramafic-hosted hydrothermal site Clamstone reveals a positive magnetic anomaly, in accordance with the magnetic response of other ultramafic-hosted sites known to date. Here this magnetic signature is mainly the result of in-place bulk-magnetized serpentinite underlying the site rather than any concentration of magnetite within the stockwork zone, the latter being probably negligible due to the limited fluid venting. Conversely, the nearby high-temperature active ultramafic-hosted hydrothermal site Rainbow illustrates the key contribution of hydrothermal activity in producing large amounts of magnetite in stockwork zones within ultramafics. At a larger scale, we propose from the analysis of sea-surface magnetic anomalies that the underlying Rainbow Hill has been formed as an oceanic core complex associated with the Amar Minor North segment, later dismantled by the northward propagation of the Amar Minor South segment and the coeval recession of the Amar Minor North segment. This model solves the apparent paradox of a high-temperature hydrothermal site sitting on an oceanic core complex usually associated with cold, poorly magmatic spreading environments: the core complex belongs to the cold, receding Amar Minor North segment whereas the heat fueling site Rainbow comes from the hot, propagating Amar Minor South segment. It also explains why the Rainbow Hill, although interpreted as an oceanic core complex, lacks the typical corrugations of such core complexes – the latter have been dismantled by the deformation associated with the northward propagation of the non-transform offset.