East Scotia Ridge unsecured

Earthquake data indicate that the subducting plate

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: cting beneath the Sandwich plate at a rate of 70–85 km/Myr (Pelayo & Wiens, 1989). Earthquake data indicate that the subducting plate dips at 45–55° to the west, probably steepening slightly in the southern part of the subduction zone (Brett, 1977). The age of the subducting plate is 28–35 Ma below the southern part of the arc, and 50–60 Ma below the northern part of the arc (Sclater et al., 1976; Barker & Lawver, 1988). The sedimentary cover on the subducting plate consists of some 200 m of siliceous sediments in the south, and some 400 m of calcareous and siliceous sediments in the north (Barker, 1995). More than 95% of this sediment input to the trench is subducted, and NUMBER 8 AUGUST 2002 there is no significant sediment accretion at the trench (Vanneste & Larter, 2002). The South Sandwich arc consists of 11 main islands forming a distinctly curved island arc, 500 km in length. Most of the islands have abundant evidence of recent volcanic activity, and all are entirely volcanic in origin. The volcanoes belong to the tholeiitic and (relatively rare) calc-alkaline series, and the arc is regarded as a classic example of the primitive stages of island arc development (Baker, 1968, 1990; Pearce et al., 1995). All the arc magmas are depleted in high field strength elements (HFSE) such as Ti, Zr, Hf, Nb and Ta, and heavy rare earth elements (HREE), relative to normal MORB, and are therefore thought to be derived from mantle wedge material that had experienced melt extraction before the magma generation events (Hawkesworth et al., 1977; Pearce et al., 1995). All the magmas are enriched (relative to MORB) in highly incompatible fluid-mobile trace elements, such as Pb, U, Ba and Rb, derived from the subducting plate. Pearce et al. (1995) suggested that variations in incompatible trace element abundances were a result of dynamic melting processes within the sub-arc mantle and that the Nd and Sr isotope covariations of the island arc sample suite indicate the involvement of a subduction component derived from the subducted sediment and altered oceanic crust. We argue below that the sediment-derived component has locally influenced the back-arc magmatism. Previous geochemical studies Previous investigations of lavas from the East Scotia Ridge were confined to eight dredge sites (D20–D24, D56, D57, D60) sampled by R.R.S. Shackleton in 1974 and 1981, as well as volcanic rocks from the axis and the lateral flanks of segment E2 and the northern part of E3, sampled by R.R.S. James Clark Ross in 1996 (Fig. 2). Dredge sites were located on the northern tip of segment E3 (D20), east of the southern tip of E2 (D21), west of segment E5 (D22), close to the northern tip of segment E9 (D24), in the centre of segment E9 (D23), and east of the northernmost segment E1 (D56). Apart from the recently sampled segment E2 and the northern part of E3, only the dredge stations D20 and D23 are located in the neo-volcanic axis of the East Scotia Ridge. The petrographic characteristics, major and trace element geochemistry, volatile content, and Sr, Nd, Pb, O and C isotope...
View Full Document

This document was uploaded on 02/01/2014.

Ask a homework question - tutors are online