Testing models of metamorphic sole formation and early subduction evolution in the Easton Metamorphic Suite, Northwest Cascades, Washington

Western Washington University, Bellingham WA

Investigators

Abstract

Subduction zones occur where cold and dense oceanic crust descends beneath warmer buoyant oceanic or continental crust. Active subduction zones are responsible for most of Earth’s large earthquakes and volcanic activity. Ancient subduction zones provide important information on processes operating at great depths that are not observable in active subduction zones, such as compositional changes in the crust through time, the history and correlation of past tectonic events, and the timescales of subduction zone processes. Understanding how subduction zones form and change over time is therefore critical to understanding the Earth and the processes responsible for earthquakes and volcanism. This project will test multiple hypotheses for how subduction zones form and change with time. The research will promote the progress of science by: 1. increasing representation in STEM by recruiting and involving underrepresented undergraduate and graduate students in the project, 2. improving undergraduate and graduate education through new laboratory exercises, new advanced courses, and expanded field opportunities, and 3. developing enhanced STEM expertise through undergraduate and graduate field research and collaboration in multiple laboratories in the U.S. The mechanisms by which subduction zones initiate and evolve towards steady state are not fully understood. Subduction initiation is often preserved in suprasubduction zone ophiolites and metamorphic soles, remnants of a newly subducted plate that was heated by the overlying mantle, scraped off, and accreted to the upper plate during the first ten million years of subduction. This research will address several questions related to early subduction processes, specifically: Does subduction initiate spontaneously via a gravitational instability or is subduction induced by forced convergence? Do inverted metamorphic gradients in metamorphic soles form via downward heat advection or by the progressive accretion of subducted material? Do accreted units within subduction zones cool by thermal relaxation, exhumation within the subduction channel, or underplating and accretion? These questions will be resolved through an investigation of the Easton Metamorphic Suite in the Northwest Cascades of Washington. The region contains Jurassic ophiolite sequences and a Jurassic-Cretaceous metamorphic sole formed during subduction initiation beneath western North America. Combined field mapping, structural and microstructural analysis, metamorphic petrology, and geochronology will determine: 1. the relative timing of suprasubduction zone ophiolite and metamorphic sole formation, 2. the timing and conditions of metamorphism within each unit of the metamorphic sole, and 3. the structural relationship of contacts between subducted units. The results will establish a detailed reconstruction of the subduction history that can be used to test proposed models for early subduction evolution. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.