Journal of Conference Abstracts

Detailed Volcanic Geology, Geochemistry and Tectonics of the MARNOK Area (Mid-Atlantic Ridge North Of Kane)

Kate Lawson Department of Geological Sciences, University of Durham, South Road, Durham, DH1 3LE, UK

(Now at: Bachmanstraat 29B, 2596JB Den Haag, Netherlands)

R.C.Searle@durham.ac.uk

Roger Searle Department of Geological Sciences, University of Durham, South Road, Durham, DH1 3LE, UK

Julian Pearce Department of Geological Sciences, University of Durham, South Road, Durham, DH1 3LE, UK

Paul Browning Geology Department, University of Bristol, Bristol, BS8 1TR, UK

Pamela Kempton NERC Isotope Geosciences Laboratory, Keyworth, Nottinghamshire, NG12 5GG, UK

Simon Allerton Department of Geology and Geophysics, University of Edinburgh, West Mains Road,

Edinburgh EH9 3JW, UK

We present the results of a detailed study of the volcanic geology, geochemistry and tectonics of the median valley floor and walls of the Mid-Atlantic Ridge in two adjacent but contrasting spreading segments immediately north of the Kane transform fault. The study is based on a high-resolution deep-towed side-scan sonar (TOBI) survey, near-bottom photographic traverses, and geochemical analyses from 28 precisely sited sets of dredge samples. This has allowed us to assess the relative importance of small (intersegment) and large (transform) boundaries on magmatic processes at slow spreading ridges.

We find that the petrological effect of the transform (so-called 'transform fault effect') is minor compared to the manifestation of the second-order segmentation. The southern, 'narrowgate'-type segment has a poorly developed neovolcanic zone, which tapers towards its bathymetric centre where continuous faulting causes rapid dismemberment. Flat-topped seamounts are preferentially located at the ends of the segment, particularly near the non-transform offset. There is a striking variation in the degree of fractionation along the length of this segment: primitive lavas were sampled at the centre, while more fractionated basalts, showing a greater range of parental compositions, form discrete volcanic edifices at the ends. In contrast, the northern segment has a wider inner valley and syn-magmatic faults extend up to 15 km along the crest of its robust axial volcanic ridge. The along-segment trend of increasing MgO towards the bathymetric crest of the axial volcanic ridge is similar to, but less well-defined than, that for the southern segment.

There is no variation in the bulk degree of melting along the segments as determined from the major element geochemistry; hence, intra-crustal melt migration is favoured over focused mantle upwelling as the main cause of the crustal thickness variations which define second-order segmentation. Radiogenic isotopes show only very small variations with no systematic pattern emerging within or between the ridge segments. The isotopic irregularities are not related to the variations in incompatible elements, implying that the latter result from the action of dynamic melting processes rather than from long-lived source heterogeneity. A model is developed which relates the volcanology, tectonic style and the geochemical trends to the episodicity of magma supply and eruption.