Behavior of lithosphere and asthenosphere in slab window environments

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Interactions between the lithosphere and the asthenosphere control many aspects of Earth evolution including volcanism, earthquakes and mountain building. The lithosphere is rigid, overlies the asthenosphere and is divided into numerous tectonic plates. At some plate boundaries, one lithospheric plate slides beneath another and descends into the asthenosphere. These boundaries, called subduction zones, are characterized by violent volcanic activity along curved belts such as the Pacific Ring of Fire.***One significant gap in our understanding of subduction zones is the behavior of lithosphere and asthenosphere at slab windows, which are breaches in the subducting lithosphere caused mainly by tearing or ridge subduction. Several slab windows are currently active on Earth, particularly along the western margin of the Americas, in the southwest Pacific and the eastern Mediterranean. Volcanoes are commonly located above slab windows and their distribution and composition are proxies for the position and behavior of subducted lithosphere and ambient asthenosphere.***The proposed research will consist of graduate student-centered investigations in key locations in Canada and Turkey. The first will test a hypothesis that southern British Columbia has shifted from a slab window environment to a slab-present situation and back to a slab window environment during the past 55 m.y. In this investigation, a PhD student will study Eocene volcanic and plutonic rocks, an MSc student will examine Quaternary lavas and a BSc Honors student will study Oligocene volcanic rocks. The second will test a hypothesis that Turkey is underlain by subducted lithosphere of the African plate that fragmented in the Neogene. A PhD student will test this idea by examining Neogene to Holocene volcanoes of the Cappadocia belt. The work will be carried out in collaboration with faculty at a Turkish university. These two investigations will involve field research and laboratory analysis to define the current and past distribution and behavior of slab and asthenosphere. Sensitive boron and beryllium isotopes will be used to track geochemical contributions from the slab and subducted sediment and provide an independent assessment of the subsurface processes. In the third investigation, a PhD student will test the hypothesis that slab melting and fragmentation at slab windows has significantly affected the composition of the crust and mantle through geologic time. By compiling information on igneous rocks from slab windows worldwide, and developing a methodology to describe slab-edge behavior, my research will provide the first time-integrated view of the global effects of slab windows. Overall, the research will improve our understanding of how slab windows are affecting volcanism in western Canada and the Middle East, and have contributed to the compositional evolution of Earth since the onset of plate tectonics.**