East Scotia Ridge unsecured

1998 the east scotia ridge an active back arc

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Unformatted text preview: ing of mantle (Gribble et al., 1998). The East Scotia Ridge, an active back-arc spreading centre behind the South Sandwich island arc in the South Atlantic Ocean (Fig. 1), is an interesting place to study subduction-related processes for several reasons: (1) its simple tectonic setting as part of an intra-oceanic arc, without the influence of continental crust; (2) its close proximity to the active island arc, suggesting the possibility of a strong subduction influence on the back-arc mantle source (Livermore et al., 1997); (3) the well-defined chemical signature of the associated island arc (Pearce et al., 1995) and of the sediments subducting at the South Sandwich trench (Barreiro, 1983; Ben Othman et al., ∗Corresponding author. Telephone: ++49 431 880 2085. Fax: ++49 431 880 4376. E-mail: [email protected] Oxford University Press 2002 KEY WORDS: geochemistry; petrogenesis; volcanism; back-arc; subduction INTRODUCTION JOURNAL OF PETROLOGY VOLUME 43 1989; Plank & Langmuir, 1998). Recent campaigns by the British Antarctic Survey (Livermore et al., 1999) and the University of Kiel now provide sampling of the neovolcanic zone within each segment of the East Scotia Ridge. In this paper, we present major and trace element compositions together with Sr–Nd–Pb isotope data for basalt samples dredged from the ridge axis, to identify petrogenetic processes controlling geochemical variations and to identify the components contributing to the magma source of the East Scotia Ridge lavas. TECTONIC SETTING The East Scotia Ridge is situated some 250–300 km to the west of the South Sandwich trench (Fig. 1). It was mapped for the first time in 1995 using the HAWAIIMR1 swath sonar (Livermore et al., 1995) and shown to consist of nine segments, separated by non-transform offsets. Spreading rates of 60–70 km/Myr place it in the intermediate range of spreading centres, transitional between ‘fast’ (axial high) and ‘slow’ (median valley) morphologies. Six segments, E3–E8, are characterized by faulted median valleys similar to those observed at the Mid-Atlantic Ridge, whereas segments E2 and E9 both display axial volcanic ridges (Fig. 2), and are propagating into the back-arc region (Livermore et al., 1997; Bruguier & Livermore, 2001). Segment E1 extends northward into the South Sandwich trench in the form of a trough with water depths of up to 5500 m. Side-scan images of this segment show it to be surrounded by a much more chaotic backscatter pattern compared with the rest of the East Scotia Ridge, indicating recent establishment of E1 and/or a more disordered mode of extension (Livermore et al., 1997). There is a distinct along-axis trend in axial depth, from maxima of >4000 m in segments E5 and E6, to minima of >2600 m in E2 and E9. These characteristics have provided support for models involving the shallow inflow of Atlantic mantle around the ends of the subducting slab (Livermore et al., 1997). The South American plate is subdu...
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This document was uploaded on 02/01/2014.

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