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¹⁰Be imaging of sediment accretion and subduction along the northeast Japan and Costa Rica convergent margins

¹ Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, USA

Sediment accretion and subduction at convergent margins play an important role in the nature of hazardous interplate seismicity (the seismogenic zone) and the subduction recycling of volatiles and continentally derived materials to the Earth's mantle. Identifying and quantifying sediment accretion, essential for a complete mass balance across the margin, can be difficult. Seismic images do not define the processes by which a prism was built, and cored sediments may show disturbed magnetostratigraphy and sparse biostratigraphy. This contribution reports the first use of cosmogenic ¹⁰Be depth profiles to define the origin and structural evolution of forearc sedimentary prisms. Biostratigraphy and ¹⁰Be model ages generally are in good agreement for sediments drilled at Deep Sea Drilling Project Site 434 in the Japan forearc, and support an origin by imbricate thrusting for the upper section. Forearc sediments from Ocean Drilling Program Site 1040 in Costa Rica lack good fossil or paleomagnetic age control above the decollement. Low and homogeneous ¹⁰Be concentrations show that the prism sediments are older than 3–4 Ma, and that the prism is either a paleoaccretionary prism or it formed largely from slump deposits of apron sediments. Low ¹⁰Be in Costa Rican lavas and the absence of frontal accretion imply deeper sediment underplating or subduction erosion.

Key Words: cosmogenic radionuclides • ¹⁰Be • convergent margin • geochemistry • sediment dynamics • margin tectonics

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