Mantle wedge asymmetries and geochemical signatures along W- and E–NE-directed subduction zones Carlo Doglioni a, ⁎, Sonia Tonarini b , Fabrizio Innocenti b,c a Dipartimento di Scienze della Terra, Università La Sapienza, Rome, Italy b CNR-Istituto di Geoscienze e Georisorse, Pisa, Italy c Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy abstract article info Article history: Received 27 March 2008 Accepted 26 January 2009 Available online xxxx Keywords: Mantle wedge Subduction zone Westward drift B and Nd isotopes Subduction zone kinematics predict that, assuming a fixed lower plate, the velocity of the subduction equals the velocity of the subduction hinge (Vs = - Vh). In all subduction zones the subduction hinge migrates toward the lower plate. However, two main types of subduction zones can be distinguished: 1) those where the upper plate converges toward the lower plate slower than the subduction hinge (mostly W-directed), and 2) those in which the upper plate converges faster than the subduction hinge (generally E- or NE-directed). Along the first type, there generally is an upward flow of the asthenosphere in the hanging wall of the slab, whereas along the opposite second type, the mantle is pushed down due to the thickening of the lithosphere. The kinematics of W-directed subduction zones predict a much thicker asthenospheric mantle wedge, larger volumes and faster rates of subduction with respect to the opposite slabs. Moreover, the larger volumes of lithospheric recycling, the thicker column of fluids-rich, hotter mantle wedge, all should favour greater volumes of magmatism per unit time. The opposite, E–NE-directed subduction zones show a thinner, if any, asthenospheric mantle wedge due to a thicker upper plate and shallower slab. Along these settings, the mantle wedge, where the percolation of slab-delivered fluids generates melting, mostly involves the cooler lithospheric mantle. The subduction rate is smaller, andesites are generally dominant, and the lithosphere thickens, there appears to be a greater contribution to the growth of the continental lithosphere. Another relevant asymmetry that can be inferred is the slab-induced corner flow in the mantle along W-directed subduction zones, and an upward suction of the mantle along the opposite E- or NNE-directed slabs. The upward suction of the mantle inferred at depth along E–NE-directed subduction zones provides a mechanism for syn-subduction alkaline magmatism in the upper plate, with or without contemporaneous rifting in the backarc. Positive δ 11 B and high 143 Nd/ 144 Nd characterize W-directed subduction zones where a thicker and hotter mantle wedge is present in the hanging wall of the slab. However, this observation disappears where large amounts of crustal rocks are subducted as along the W-directed Apennines sub- duction zone. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The mantle wedge (Fig. 1) is the triangular section of the mantle confined between the top of the slab and the base of the upper plate (e.g., van Keken, 2003; Wiens et al., 2008). It is generally considered to be composed of asthenosphere, although some authors also include the entire lithospheric mantle section above the slab. The mantle wedge filters fluids released by the slab that melt the overlying mantle (Abers et al., 2006), and feed arc magmatism (Tatsumi et al., 1983; Syracuse and Abers, 2006). The mantle wedge is usually conceived as a relatively “hot” body, where the melting feeding the magmatic arc can take place (N 1200 °C?), bounded by lower temperatures at the inclined base (top of the slab) and the top (base of the lithosphere?). The transit and location of melting areas into the wedge have been identified by magnetotelluric or electrical conductivity studies (Brasse et al., 2002; Brasse, 2005). The mantle wedge is therefore a crucial area for plate tectonics, where relevant chemical transfer occurs and new material is produced and added to the crust. What happens in the mantle wedge can be inferred from seismic tomography, geochemistry of lavas and xenoliths, plus other indirect information such as gravimetric and geoelectrical studies. In the Tonga backarc basin, the mantle wedge has been seismically illuminated showing a series of well-bedded reflectors, indicating a form of stratified architecture (Zheng et al., 2007). Martinez and Taylor (2002) proposed an eastward flow in the mantle wedge to compensate for slab rollback, this flow being distorted by the corner flow associated with the subduction. These Lithos xxx (2009) xxx–xxx ⁎ Corresponding author. E-mail address: carlo.doglioni@uniroma1.it (C. Doglioni). LITHOS-01891; No of Pages 11 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.01.012 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos ARTICLE IN PRESS Please cite this article as: Doglioni, C., et al., Mantle wedge asymmetries and geochemical signatures along W- and E–NE-directed subduction zones, Lithos (2009), doi:10.1016/j.lithos.2009.01.012