Role and extent of detachment faulting at slow-spreading mid-ocean ridges

Two-thirds of the Earth's surface is paved by oceanic crust formed by seafloor spreading at the 60,000 km-long global mid-ocean ridge (MOR) system. As the rigid ocean plates are pulled apart, at rates varying from <10 to 160 mm/year, the Earth's mantle is drawn up from beneath, partly melting as it does so. The melt separates from the mantle and rises to the surface to form a continuous layer of 'magmatic' crust, typically about 6 km thick, made of basalt at the surface and gabbro, its slowly cooled equivalent, beneath.

However, over the past 15 years we have come to realise that, at spreading rates below about 40 mm/yr, this simple model cannot be correct. Instead, large tracts of mantle rocks may be exposed on the seafloor, with no magmatic crust being present. Plate separation on slow-spreading MORs such as the Mid-Atlantic Ridge (MAR) may instead be taken up in part on great dislocations - unusually large geological faults known as 'detachments' - on which tens of km of extension may be accommodated. Where exposed on the seafloor these faults typically form flat or gently domed surfaces on which mantle rocks and/or gabbro are exposed. These structures are known as 'oceanic core complexes' (OCCs). We think OCCs form when the magma supply dwindles and seawater is able to penetrate down a fault and access mantle rocks beneath. These rocks, called 'peridotites', are made mostly of the mineral olivine, which reacts easily with water to produce the weak minerals serpentine and talc, lubricating the fault and allowing it to continue slipping and develop into a long-lived detachment.

Very recently, several workers (including PI Reston) have proposed that detachment faulting is far more common than previously supposed, to the extent that up to half of all Atlantic seafloor may be generated by such 'tectonic' spreading. They view detachments as regionally continuous features that underlie all the seafloor on one side of the ridge axis, but only emerge at the surface in a few places, the OCCs.

But is detachment faulting really so widespread? From a detailed study of the 13N region of the MAR, Co-Is MacLeod and Searle came to the quite different, and much less extreme, view that detachments are discontinuous and restricted to individual OCCs. They are interspersed between volcanically active, magma-rich ridge segments, and triggered by localised waning of magma supply. In this model detachments are episodically 'killed' by renewed magmatism, often delivered laterally from adjoining segments.

How can we distinguish these very different hypotheses about the mechanism of seafloor spreading? The key data needed are: (1) the sub-surface geometry and extent of the detachments beneath the ridge axis, (2) the amount and detailed distribution of magmatic crust, and (3) the asymmetry of spreading rates associated with OCCs and volcanic seafloor (they should be similar in the regional and differ in the local detachment models).

We propose to obtain these data in a comprehensive seismic and seabed magnetic survey of the MAR in the 13N region, where detachment faults are active at the ridge axis today. We will use a large array of ocean-bottom seismographs (OBSs) to image 3D velocity variations related to different rock types using 'seismic tomography' - akin to medical CT scanning - and conduct a multi-channel reflection survey, which will image sub-surface discontinuities - like a simple X-ray. We will then leave the OBSs (to be recovered on a later cruise) to record the locations of natural micro-earthquakes in the region. These will show directly the 3D geometry and linkage of active faults. Finally, we will deploy the autonomous robot vehicle Autosub 6000, which will be programmed to make very detailed maps of magnetic field reversals (yielding seafloor age and spreading rate) and seafloor topography (helping structural interpretations) while we perform the seismic experiments.

The prime users of our results will be other researchers studying the structure and geodynamics of mid-ocean ridges, particularly the nature and role of detachments. Understanding the nature and geometry of oceanic detachments will also inform those studying detachments and other large-offset faults on land. Our work will be a landmark in seafloor spreading studies, contributing to a paradigm shift in this field.

By determining the distribution of peridotite, serpentinite and gabbro in detachment footwalls, and the history of their emplacement, we will benefit those studying the petrology, geochemistry and magnetisation of the ocean lithosphere, providing a fuller knowledge of its composition. Moreover, by increasing our knowledge of the distribution of peridotites in the oceanic crust we will benefit those working on the global carbon cycle, since extensive carbonation reactions accompany serpentinisation; this mechanism of carbon capture potentially has an important influence upon the global carbon cycle.

The work will also be of direct benefit to those studying ocean-continent margins (OCMs), where detachment faulting appears to be important and the embryonic spreading systems have much in common with slow-spreading MORs, including the exhumation of significant serpentinite bodies. Our deep geophysical imaging of a mid-ocean ridge will provide data and models that can be directly compared with OCMs.

The Autosub 6000 survey we propose will collect data that can locate hydrothermal plumes, some of which are likely to be from peridotite-hosted vents. Unlike basalt-hosted vents, these poorly-known systems are significantly influenced by serpentinisation reactions, which generate hydrogen, leading to highly reduced hydrothermal fluids. These exert an important control on ocean redox potential and can nourish vigorous microbial communities which may themselves catalyse serpentinisation; they also support ecosystems distinct from those at basalt-hosted vents, supplying twice the metabolic energy of these. By increasing the small number of known examples of such systems we will significantly benefit vent geologists, geochemists, biologists and microbiologists, and in particular contribute to the understanding of the sub-surface biosphere.

Our work will contribute to the general scientific culture of the nation, in a way that is accessible and interesting to the general public, as shown by the popularity of Earth-science-related TV shows. Responses to popular presentations of our previous work have shown the public to be eager for such knowledge.

In addition, we expect our work to have great benefit in enthusing and informing school students, who we believe are a vital community of beneficiaries, about Earth Science issues. Like astronomy, the work proposed here is both accessible and exciting, and we plan innovative ways to bring it to their attention. We will use our data to support projects for schools such as Science@Work and Nuffield Science Bursaries but, more ambitiously, we propose to take a school teacher to sea, to give them direct experience of a modern research expedition and to improve their knowledge and experience of environmental research. As well as taking part in the research activities on the ship, the teacher and the shipboard science party will actively engage with school students during the cruise, through the cruise blog, and by linked classroom activities ashore, and will undertake a short lecture tour following the cruise accompanied by a project researcher. We will also present our work at science festivals, Junior Cafés Scientifiques etc..

Finally, our work will play a significant part in increasing the professional skills and knowledge of the staff involved, especially the two postdoctoral workers. We will thus be contributing to the research skills and wealth generation capability of the nation.