Crustal inheritance and arc magmatism: Constraints from the Washington Cascades on top-down control
Friday, 12 January 2018: 11:40
Salon Quinamavida (Hotel Quinamavida)
Paul Bedrosian, USGS, Crustal Geophysics and Geochemistry Science Center, Denver, CO, UNITED STATES, Jared Peacock, USGS, Geology, Minerals, Energy, and Geophysics Science Center, Menlo Park, CA, United States, Esteban Bowles-martinez, Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States, Adam Schultz, Oregon State University, College of Earth, Ocean and Atmospheric Sciences, Corvallis, OR, United States and Graham Hill, University of Canterbury, Gateway Antarctica, Christchurch, New Zealand
Abstract:
Arc volcanism occurs along relatively straight and narrow magmatic arcs, where dehydration reactions within the subducting slab and melts fluxed by dehydration fluids in the mantle wedge occur. The ‘bottom-up’ picture, that the location of arc volcanism reflects where fluids and melts are generated, explains first-order differences in trench-to-arc distance and is consistent with variations in slab thermal structure and geometry. The subducting slab is often implicated to explain arc segmentation, gaps in arc magmatism, and anomalous regions of forearc and backarc magmatism. Less well considered is the role of crustal structure in controlling magmatism. We argue that crustal structure provides a degree of ‘top-down’ control on the channeling, ascent, and possibly composition of arc magmas. We present evidence within the Washington Cascades based upon correlation between a three-dimensional resistivity model, potential-field data, seismicity, and Quaternary volcanism. We image a mid-Tertiary batholith, intruded within an Eocene crustal suture zone, and extending to 15-20 km depth. A lower-crustal conductivity anomaly, consistent with 2-4% basalt-andesite melt is additionally imaged beneath the batholith. These results address divergent interpretations on the origin of the Southern Washington Cascades Conductor (SWCC) and demonstrate the limitations of previous studies. We show the SWCC to be a combination of upper-crustal metasedimentary rocks and lower-crustal melt.We argue that this and neighboring plutons episodically channel crustal fluids and melt along their margins from the lower-crustal reservoir to steeply dipping zones of marine metasedimentary rock. Mount St. Helens, which sits directly atop such rocks, is interpreted to be fed by fluids and melt generated tens of kilometers to the east. Its unusual dacitic composition may also be related to the interpreted magma ascent path. Regionally, we argue the concealed suture zone is a mechanically weak zone of higher permeability than the surrounding plutons, controlling the distribution of forearc magmatism (and seismicity/deformation) since the mid-Miocene. Our results highlight that inherited crustal structure exerts control on both the location and style of arc magmatism.
