Collaborative Research: Investigating the Sedimentary Record of Differing Modes of Flat-Slab Subduction

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The paradigm of convergent margins involves an oceanic plate of uniform character that subducts at a relatively steep (30 to 45 degrees) angle. Oceanic plates are not uniform, however, and instead consist of broad (plateaus) and narrow (aseismic ridges) regions of thickened crust, as well as spreading ridges where molten mantle material comes to the surface and creates new crust. An increase in crustal thickness or heat flow associated with these features causes the plates to be more buoyant than typical oceanic plates and to subduct at shallower or nearly flat angles. Flat-slab subduction of buoyant oceanic plateaus and aseismic and spreading ridges occurs along some of the world?s major convergent margins. The response of sedimentary basins above a these zones of flat slab subduction is not well understood. This project investigates the response of the Cook Inlet basin in south central Alaska to changes in subduction angle with special focus on the transition from steep subduction to flat subduction. A significant impact of the proposed research will be to provide a better understanding of the response of the upper plate and the sedimentary record to flat slab subduction, results which inform future studies of potential flat slab localities. The project will advance desired societal outcomes through: 1) full participation of women in STEM through support of two early career female PIs and outreach to minority high school students; 2) increased public scientific literacy and public engagement with science and technology through public outreach; and 3) development of a diverse, globally competitive STEM workforce though activities at Cincinnati high school, a short course for students and professionals in Anchorage, and training of graduate and undergraduate students.The sequence, timing, and magnitude of upper plate processes related to flat-slab subduction are still poorly understood. Recognition of flat-slab subduction in the ancient geologic record is based primarily on changes in volcanic arc magmatism and less frequently on inboard migration of upper plate deformation, exhumation, and sedimentary basin evolution. However, very few studies have attempted to quantify the spatial and temporal patterns of surface uplift and erosion as it relates to subduction of a buoyant slab, and which result in changes to the types and locations of major sediment sources to adjacent basins. Here, a research team from the University of Iowa and University of Cincinnati will investigate the Late Cretaceous-Cenozoic sedimentary deposits of the Cook Inlet basin in south central Alaska, which constitute a complete sedimentary record of forearc basin strata deposited during three different modes of subduction. Normal subduction of oceanic crust in the Late Cretaceous was followed by flat-slab subduction of a spreading ridge (ca. 62-50 Ma), and later by subduction of an oceanic plateau (ca. 40-0 Ma). Two hypotheses are tested: (1) subduction of a spreading-ridge in late Paleocene?Eocene time resulted in a change in topography and a shift in sediment sources from the adjacent arc to the retroarc region; (2) flat-slab subduction of an oceanic plateau from Oligocene time to the present resulted in the creation of topography above the flat-slab region and an overall contraction of the forearc basin drainage area. The researchers investigate the sedimentary record of flat slab subduction in southern Alaska by examining Late Mesozoic and Cenozoic strata in core samples from the Cook Inlet forearc basin through traditional stratigraphic analysis combined with provenance and thermochronologic techniques of double-dating detrital zircons using uranium-lead and fission track dating, rare earth element analyses of mudstones, and sandstone petrography. The integration of these datasets enables the investigation of the patterns of regional exhumation, magmatism, and sediment transport across the area and interpretation these patterns in terms of regional tectonics and changes through time. This study will provide a foundation for new tectonic and provenance models of forearc basins that have been modified by flat-slab subduction processes.