Constraining erosion during arc-arc collision from turbidite sands in the Nankai

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Sedimentary rocks in accretionary complexes potentially record the evolution of subduction zones over long periods of geological time. Accretionary active continental margins are both major sinks of continental crust back into the mantle and locations for continental crustal formation. As a result understanding the processes that construct these margins is essential to constraining how the continental crust is built and maintained. Accretion of oceanic arcs to continental margins is a key stage in the building of continental crust, yet evidence for these events is often obscure. In this project we will determine what processes control the flux of sediment to a trench system and thus what might be deconvolved from similar deposits in older complexes. In particular, we will examine the impact that arc-arc collision has had on the erosion of an active margin system. We choose to study the Nankai Trough of SW Japan because it is the type-section accretionary continental margin and derives sediment, not only from Mesozoic accretionary complexes in Honshu, but also from the mountains formed by the collision of the Izu-Bonin Arc with the Japanese mainland. We further test whether this collision or the climate changes during the Mid-Pleistocene transition ~0.9 Ma are the primary controls on erosion of Japan. We will apply mineralogy and thermochronology methods to sediments recovered by IODP Expedition 316, on which Nicholson sailed as a shipboard scientist. The samples are dated as Pliocene-Recent (2.0-0.5 Ma) and are accretionary sands of the Nankai margin. We also propose to characterize modern river sediments draining both the Izu collision zone and the Shimanto Accretionary Complex, immediately onshore from the drill sites. Using the contrasting tectonic/magmatic histories of the Shimanto Complex compared to the Izu collision zone we will constrain their relative influences on sediment production and flux to the trench. Present models predict that the Izu-Bonin Arc collided with Japan around 1 Ma. We shall test this by analyzing 15 sands spanning the last 2.0-0.5 Ma in order to determine when material diagnostic of that event first reaches the Nankai Trough. We will use proven heavy mineral analyses that can identify specific grains known to be diagnostic of the collision zone. Single grain major element analysis of pyroxene and amphibole grains will constrain changes in provenance through time, allowing us to date changes in erosion patterns within 100 ky. In addition, LA-ICP-MS U-Pb dating of zircon grains will be used to further constrain changes in source area, because the Izu-Bonin collision zone supplies zircons younger than 40 Ma, contrasting with Mesozoic ages in those derived from the Shimanto Belt. Thermochronological data derived from apatite and zircon fission track analysis (AFT and ZFT), combined with biostratigraphic data, will provide constraints on the timing and rate of exhumation of the sediment source areas, allowing reconstructions of the evolution of the landscape during the collision of the arc with the Japanese mainland. In doing so we will better understand how the arc accretion progresses and thus how the continental crust of Asia has been constructed over long periods of geological time.