Multi-Polarity Magnetisations Acquired During Progressive Cooling of Young, Lower Oceanic Crust

IODP Expedition 304/305 sampled lower oceanic crust exhumed by detachment faulting in the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge at 30°N). Palaeomagnetic analyses on gabbros from Hole U1309D reveal a complex record of magnetic remanences. Essentially all samples record a high temperature reversed component of magnetisation, R1. An intermediate temperature normal polarity overprint, N1, and a lower temperature reversed overprint, R2, are also observed in some samples. The transition between these distinct components occurs over very narrow and consistent temperature intervals, providing strong evidence for a thermal origin of magnetic remanence. However, the variations in remanence are unrelated to either: (i) igneous stratigraphy, suggesting that the overprints are not produced by partial thermal resetting of earlier remanences by later intrusions during subsequent geomagnetic chrons; or (ii) the distribution of alteration in the core, arguing against overprinting by partial chemical re-magnetisation. Instead the pattern of multi- component magnetisations appears to be controlled by subtle variations in the grain size and related blocking temperature characteristics of the remanence carriers. We conclude that the magnetisation signature was acquired during protracted cooling of the section over a period encompassing chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. The R1 magnetisation direction provides constraints on the tectonic rotation of the section. However, mean inclinations of the N1/R2 components are steeper/shallower than that of the R1 component respectively. These differences cannot be explained by any plausible tectonic mechanism. Instead, they appear to relate to the biasing effects of a residual vertically-oriented drilling overprint that persists to intermediate unblocking temperatures despite an initial low temperature demagnetisation treatment. This highlights the need to carefully consider the potential effects of drilling overprints prior to tectonic interpretation of IODP inclination data, especially in geological settings where rotation angles inferred from palaeomagnetic analyses are sensitive to small changes in inclination.