GEOLOGY, December 2007 1111 INTRODUCTION Subduction zones are key conduits by which eroded continental crust and oceanic sediments enter the mantle. Primary evidence for sedi- ment subduction is the anomalously small accretionary complex at most subduction zones (von Huene and Scholl, 1991). Crustal material may also be added to the mantle through tectonic erosion of the overriding plate, as indicated by margin truncation and forearc subsidence (von Huene and Scholl, 1991; Clift and Vannucchi, 2004). While some subducted sedi- ment may accrete at mid-crustal depths (von Huene and Scholl, 1991), there is now abundant evidence that a significant volume is carried into the mantle. Trace element and isotope signatures of volcanic arc magmas, including 10 Be and 207 Pb, require the involvement of subducted sediments (e.g., Morris et al., 1990; Plank and Langmuir, 1993; Elliott et al., 1997). At a number of subduction zones, a subducted sediment signature has also been identified in backarc magmas, primarily from Sr, Nd, Pb, and Th isotopes (e.g., Cousens et al., 1994; Ryan et al., 1995; Turner and Foden, 2001; Fretzdorff et al., 2002; Ishizuka et al., 2003). Whereas sediment subduction to depths of arc magma genesis is well documented, the fate of sediments beyond this point is poorly constrained. Most conceptual models assume that sediments are transported to depth with the subducting plate and stored or convectively dispersed in the upper mantle (e.g., Weaver, 1991). Conversely, recent numerical and ana- log modeling studies indicate that buoyant material, including sediments, may separate from the subducting plate at shallow depths (e.g., Gerya and Yuen, 2003; Boutelier et al., 2004). Knowing the fate of subducted sediments is critical for understanding the chemical evolution of Earth, as sediment subduction is a fundamental mechanism of mantle refertiliza- tion. In this study, we use numerical models to investigate the dynamics of sediments that are carried to mantle depths at a subduction zone. NUMERICAL MODEL FORMULATION The two-dimensional, plane strain, upper mantle scale numerical models address the subduction of an old (>70 Ma) oceanic plate and sediments beneath continental lithosphere (Fig. 1; see also the GSA Data Repository 1 ). Convergence between the two plates at 5 cm/yr is kinemati- cally imposed along the oceanic model boundary. Within the model domain, the dynamics are driven by the far-field boundary conditions and by buoy- ancy forces associated with thermal and compositional density variations. The coupled thermal-mechanical evolution of the system is calculated using arbitrary Lagrangian-Eulerian finite element techniques. All materials have a viscous-plastic rheology and temperature-dependent density. Frictional- plastic deformation uses a Drucker-Prager yield criterion with strain soften- ing. When the state of stress is below plastic yield, deformation is viscous with an effective viscosity based on laboratory-derived flow laws. The 2 km sediment layer is designed to deliver a constant sediment flux to the subduction zone. The model sediment is interpreted to be domi- nantly terrigenous with a small pelagic component. To avoid the complexity of computing phase transformations, sediments have a density consis- tent with felsic continental crust at ultrahigh-pressure (UHP) conditions (>2.6 GPa) and flow viscously according to a wet quartzite dislocation creep flow law (see GSA Data Repository 1 ). NUMERICAL MODEL RESULTS Figure 2 shows the evolution of the reference model (Δρ = ρ mantle – ρ sed = 550 kg/m 3 at 800 °C). Initially, sediments accrete in the trench region, but with continued convergence, sediments begin to bypass the accretionary complex. By 24 m.y., a well-developed subduction chan- nel has formed in which sediments are carried to ~100 km depth. As sedi- ments enter the mantle below the continental lithosphere, they detach from the subducting plate in a Rayleigh-Taylor-like instability, owing to their positive buoyancy relative to the surrounding mantle. The detached sedi- ments first accumulate in the mantle wedge corner. Later (>36 m.y.), the detached sediments are expelled laterally, forming a horizontal plume of material that intrudes the continental mantle lithosphere. Intrusion occurs where lithosphere strength is decreased owing to high temperatures, and thus lithosphere can be displaced by the buoyant sediments. In addition to injecting sediments into the shallow backarc, this process mechanically perturbs the lowermost continental lithosphere, causing it to delaminate and sink because it is cool and dense. Geology, December 2007; v. 35; no. 12; p. 1111–1114; doi: 10.1130/G24098A.1; 4 figures; Data Repository item 2007275. © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. 1 GSA Data Repository item 2007275, supplementary information on the numerical modeling approach, is available online at www.geosociety.org/pubs/ ft2007.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. The fate of subducted sediments: A case for backarc intrusion and underplating Claire A. Currie Christopher Beaumont Department of Oceanography, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada Ritske S. Huismans Department of Earth Science, Bergen University, Bergen N-5007, Norway ABSTRACT Subduction of oceanic and continental sediments into the mantle is fundamental to the geochemical evolution of Earth. Using thermal-mechanical models, we examine the dynam- ics of sediments that are subducted below continental lithosphere. Owing to their low density relative to the mantle, model sediments detach from the subducting plate at ~100 km depth. With ongoing subduction, a subhorizontal sediment plume develops and intrudes the conti- nental lithosphere. This occurs for a wide range of sediment densities and rheologies, suggest- ing that sediment detachment may be important for regions where the subducted sediment thickness is larger than ~350 m. In these areas, a reservoir of sediments may be found in the shallow backarc mantle. In contrast to models of sediment transport to the deep mantle, the detachment model predicts chemical and mechanical interactions between the sediments and backarc mantle lithosphere and a shallow sediment source for arc and backarc magmas. Keywords: subduction zones, numerical models, sediment transport, volcanic arcs.