Earth and Planetary Science Letters 414 (2015) 100–107 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Evidence for trench-parallel mantle flow in the northern Cascade Arc from basalt geochemistry Emily K. Mullen ∗ , Dominique Weis Department of Earth, Ocean and Atmospheric Sciences, 2020-2207 Main Mall, University of British Columbia, Vancouver, British Columbia V6T1Z4, Canada a r t i c l e i n f o a b s t r a c t Article history: Received 10 October 2014 Received in revised form 6 January 2015 Accepted 7 January 2015 Available online xxxx Editor: A. Yin Keywords: Cascade Arc Garibaldi Belt alkalic basalts isotopes trace elements trench-parallel flow Geochemical data for basalts from the Garibaldi Volcanic Belt (northern segment of the Cascade Arc) define arc-parallel gradients in trace elements and isotope ratios that extend at least 150 km into the arc from the northern margin of the subducting Juan de Fuca plate. Southerly increases in Zr/Nb, Ba/Nb, Th/La, Pb/Ce, 208 Pb/ 204 Pb and 176 Hf/ 177 Hf indicate greater mantle depletion and higher slab-derived contributions with distance from the slab edge. Temperatures and pressures of mantle melt segregation also decrease to the south. The gradients are most plausibly explained as a consequence of slab rollback- induced toroidal flow at the northern slab edge (Nootka fault zone), whereby enriched (OIB-type) NE Pacific asthenospheric mantle from beneath the slab is drawn into the mantle wedge in a trench-parallel southerly flow pattern. Melts of the enriched asthenosphere are progressively diluted to the south by melts of the slab-modified, depleted mantle wedge. Arc-parallel changes in slab thermal conditions cannot account for these gradients. Trench parallel geochemical gradients in the northern Cascade Arc are consistent with shear wave splitting data, numerical modeling, and experimental studies showing that trench-parallel mantle flow may be a common phenomenon near slab edges and slab gaps.  2015 Elsevier B.V. All rights reserved. 1. Introduction Two-dimensional numerical models of subduction zone dynam- ics predict that viscous coupling between the subducting plate and overlying mantle will result in mantle wedge flow that is parallel to the convergence vector (e.g., van Keken et al., 2002). However, seismic anisotropy data, 3-D numerical modeling, and experimen- tal studies have shown that mantle flow in subduction zones may be much more complex, especially at and near the terminations of the arcs (Kneller and van Keken, 2008; Long and Silver, 2009; Long and Wirth, 2013; MacDougall et al., 2014; Schellart, 2010, 2004). Because the compositions of primary arc magmas reflect the composition of the mantle wedge (variably modified by slab- derived components), spatial variations in the geochemistry of arc magmas provide independent evidence for circulation patterns in the mantle wedge. In this study, we present new high precision Sr–Nd–Hf–Pb iso- topic and trace element data for basalts from the Garibaldi Vol- canic Belt (GVB), the northern segment of the Cascade Arc (Fig. 1). An arc-parallel gradient in basalt alkalinity (Fig. 2) was first doc- umented by Green and Harry (1999) and attributed to an along- arc gradient in the age and temperature of the subducting plate. * Corresponding author. E-mail address: emullen@eos.ubc.ca (E.K. Mullen). However, spatial gradients in basalt geochemistry that are ori- ented parallel to the arc axis have been interpreted as evidence for trench-parallel mantle flow in Central America (Hoernle et al., 2008), Tonga (Turner and Hawkesworth, 1998), and Vanuatu (Heyworth et al., 2011). Similar gradients are also associated with mantle influx at slab edges in Kamchatka (Portnyagin et al., 2005) and Central America (Johnston and Thorkelson, 1997). The goal of this study is to use the new geochemical data on GVB basalts to evaluate trench-parallel mantle flow as an alternative hypothesis for the gradient in GVB basalt alkalinity, and to explore the tec- tonic implications. 2. The Garibaldi Volcanic Belt The ∼1250 km long Cascade Arc is related to subduction of the young (<11 Ma) Juan de Fuca oceanic plate and represents a thermally ‘hot’ end-member in the global spectrum of subduc- tion zones (Syracuse et al., 2010). A bend in the arc axis marks the boundary between the ∼375 km long Garibaldi Volcanic Belt (GVB) in the north and the High Cascades in the south (Fig. 1a). The GVB includes the major volcanic centers, from south to north, of Glacier Peak, Mt. Baker, Mt. Garibaldi, Mt. Cayley, Mt. Meager, Salal Glacier, and Bridge River Cones (Fig. 1). In the GVB, basaltic lavas comprise only a small fraction of total eruptive volumes and are periph- eral to the main volcanic edifices (Green, 2006). Basalts transition http://dx.doi.org/10.1016/j.epsl.2015.01.010 0012-821X/ 2015 Elsevier B.V. All rights reserved.