The Role of Mantle Residuum in Subduction-Driven Circulation: Implications for the Cascades System

This paper investigates the role of upper mantle heterogeneity in subduction systems. We utilize a kinematic laboratory subduction model that drives multiple styles of three-dimensional, time dependent mantle circulation influenced by the residuum left behind after major melt production and extraction events. Our models take into account the four-dimensional aspects of mantle-residuum interaction and highlight the importance of melt-induced mantle chemical heterogeneity in subduction systems. The results show that the presence of chemical heterogeneity in the mantle wedge impacts plate-driven mantle flows, thereby influencing mass and energy transport in the wedge. This research has specific implications for the Cascadia subduction zone of the Pacific Northwest U.S. where the ~20 Ma Columbia River/Steens flood basalt (CSFB) event and subsequent volcanic activity, High Lava Plains (HLP) and Snake River Plain-Yellowstone (SRP), is a topic of ongoing debate. In this paper we investigate explanations for HLP/SRP volcanism by comparing time dependence and material transport of residuum in the wedge to the seismic and volcanic patterns seen in the Cascades. The Yellowstone hotspot is commonly thought to be the result of a mantle plume, or thermally buoyant lower mantle upwelling. Our results indicate that a non-plume explanation, in which plate-driven upper mantle flow interacts with a time evolving CSFB residuum, may enable mantle upwelling and shallow melting beneath the HLP-SRP volcanic tracks. While our model successfully explains the westward progressing HLP track and the concurrence of melt generation beneath the HLP/SRP, it falls short in explaining the eastward progressing SRP- Yellowstone melting. We report on non-plume results that indicate that a non-plume explanation, in which trench-normal volcanic tracks form naturally from deformation of residuum from a CSFB-type event. Melt generating patterns show that our models readily explain bimodal melting beneath the HLP and SRP regions and the westward progression of melt beneath the HLP. However, replicating the age-progressive melting patterns under the SRP remains a challenge for these experiments.