Magnetization and tectonic evolution of ultraslow-spreading rate lower oceanic crust, Atlantis Bank, SW Indian Ridge (IODP Expedition 360)

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Generation of ocean crust by seafloor spreading at mid-ocean ridges is responsible for forming two-thirds of the Earth's surface. The lower crust is the key to understanding this process. It consists of the rock gabbro, a slowly-cooled, coarse-grained igneous rock formed by crystallization of magma derived from melting of the mantle at mid-ocean ridges. The lower crust therefore acts as a critical layer, receiving magma from below, crystallizing it to form gabbros and passing remaining melt upwards to form the upper crust. However, we have surprisingly few samples of lower crustal gabbros on which to test different hypotheses for seafloor spreading. These have come from a limited number of deep drill holes in the world's oceans, and the volume of all samples combined would fit inside a 2.5 m box! In addition to sampling, an important tool in determining the age of oceanic crust is the interpretation of subtle variations in the strength of the Earth's magnetic field produced by the magnetization of oceanic crustal rocks on the seafloor. These "marine magnetic anomalies" provide a global tape recording of seafloor spreading in the form of positive and negative variations in field strength that are distributed symmetrically on either side of mid-ocean ridges as a result of past reversals of the direction of the field during spreading. However, to date we have never sampled across one of these important reversal boundaries in the lower crust, and retrieving such samples would provide a major step forward in understanding of the magnetic signal in the oceans.Significant new samples of the lower crust can only be obtained by scientific oceanic drilling. International Ocean Discovery Program (IODP) Expedition 360 "SW Indian Ridge Lower Crust and Moho" will use the drillship JOIDES Resolution to recover core samples from a new 1.3 km deep hole into lower oceanic crust at Atlantis Bank, where these rocks have been brought near to the seafloor by movement on a major fault system that cuts the crust. By describing and analyzing core samples of gabbros from this special tectonic setting, the expedition will address important questions such as: how the igneous lower crust forms (or accretes); how magma migrates, crystallizes and evolves (changes composition) during accretion; and how tectonic deformation (faulting) of the crust contributes to seafloor spreading by taking up stretching of the oceanic plates. Importantly, Atlantis Bank also provides a unique opportunity to understand the source of marine magnetic anomalies as a reversal boundary is believed to lie a few hundred metres beneath the seafloor at this location.This project will analyse the magnetic properties of the lower crustal rocks recovered at Atlantis Bank. The gabbros acquired a magnetization in the direction of the Earth's field shortly after they crystallized. The direction of the magnetization trapped in the rocks can be used as a marker to determine whether the rocks have been tilted during faulting, by comparing with the known present day direction of the Earth's field at the site. However, a major problem with this approach is that IODP samples are free to spin in the core barrel during drilling, and are not oriented relative to present day north. To overcome this, we will examine oriented geophysical "pictures" of the inside of the borehole wall and match up geological structures of known orientation in these images with corresponding structures on the core samples to reorient core pieces. This will allow us to restore the magnetic data obtained from the cores to a true geographic reference framework and hence to use their magnetization directions to determine the extent to which tectonic tilting has contributed to the structural development of the massif. We will also look at how the magnetization of the gabbros changes as the reversal boundary in the hole is approached and crossed, shedding new light on the nature of source of marine magnetic anomalies.