LINKING GEOCHEMISTRY OF CENOZOIC VOLCANIC CLASTS FROM THE WRANGELL ARC, ALASKA, TO UPPER PLATE AND SUBDUCTING SLAB TECTONIC PROCESSES

The Wrangell arc (WA) is a ~26 Ma magmatic belt in south-central Alaska that lies above the edges and leading front of the Yakutat microplate, a buoyant oceanic plateau that is causing shallow subduction in the region. The WA is set in a transition zone between “normal” Aleutian subduction to the west and right-lateral strike-slip tectonics to the east, accomplished by the Denali fault system and its splays. This unique geologic setting offers a chance to study the interrelations between subduction, strike-slip motion, and slab-edge effects in a relatively young and well-exposed arc. Aside from a few detailed studies (Richter et al., 1990; Preece and Hart, 2004; Trop et al., 2012; Skulski et al., 1991, 1992), this area remains largely unexplored and understanding the detailed temporal history of magmatic/tectonic processes is the objective of this project. To understand this spatial and temporal history, we need large datasets of geochronology as well as geochemistry. The WA covers a huge area (~10,000 km²) and is rugged and remote, which makes sampling on a large-scale a difficult task. Rather than in situ bedrock sampling, we are using a novel technique of collecting volcanic clasts from rivers encircling/draining from the WA in order to sample portions of the WA that are not easily accessible for bedrock sampling. Major and trace element geochemistry of the clasts are being linked to clast ages in order to understand how magmatism and the processes controlling magmatism have evolved through time. Previous studies indicate we will likely find: “normal” subduction calc-alkaline to transitional chemistries (Preece and Hart, 2004, Trop et al., 2012); adakites related to slab edge effects (Preece and Hart, 2004); alkaline chemistries related to adiabatic melting that erupts via strike-slip faults (Skulski et al., 1992); and intra-arc extensional tholeiitic chemistries (Preece and Hart, 2004; Trop et al., 2012). By linking chemistries to their respective ages, we hope to understand how the WA magmatic and tectonic processes evolved through time and space. We then plan to merge this new dataset with existing datasets and unpublished data from our larger study of the WA to build a more comprehensive picture of magmatic/tectonic evolution.