ALBERT

Description: Highlights • Sediment accumulation rates in Nicobar Fan abruptly increase 9.5 Ma. • Increased sediment flux to eastern Indian Ocean and restructuring of sediment routing. • Nicobar Fan holds significant record of Indian Ocean sedimentation in late Neogene. • Shillong Plateau and Indo–Burmese wedge uplift drive sediment south in late Miocene. A holistic view of the Bengal–Nicobar Fan system requires sampling the full sedimentary section of the Nicobar Fan, which was achieved for the first time by International Ocean Discovery Program (IODP) Expedition 362 west of North Sumatra. We identified a distinct rise in sediment accumulation rate (SAR) beginning ∼9.5 Ma and reaching 250–350 m/Myr in the 9.5–2 Ma interval, which equal or far exceed rates on the Bengal Fan at similar latitudes. This marked rise in SAR and a constant Himalayan-derived provenance necessitates a major restructuring of sediment routing in the Bengal–Nicobar submarine fan. This coincides with the inversion of the Eastern Himalayan Shillong Plateau and encroachment of the west-propagating Indo–Burmese wedge, which reduced continental accommodation space and increased sediment supply directly to the fan. Our results challenge a commonly held view that changes in sediment flux seen in the Bengal–Nicobar submarine fan were caused by discrete tectonic or climatic events acting on the Himalayan–Tibetan Plateau. Instead, an interplay of tectonic and climatic processes caused the fan system to develop by punctuated changes rather than gradual progradation.

Description: Plate-boundary fault rupture during the 2004 Sumatra-Andaman subduction earthquake extended closer to the trench than expected, increasing earthquake and tsunami size. International Ocean Discovery Program Expedition 362 sampled incoming sediments offshore northern Sumatra, revealing recent release of fresh water within the deep sediments. Thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial-induced temperature increases in the past 9 million years. Complete dehydration of silicates is expected before plate subduction, contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction. Shallow slip offshore Sumatra appears driven by diagenetic strengthening of deeply buried fault-forming sediments, contrasting with weakening proposed for the shallow Tohoku-Oki 2011 rupture, but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records.

Description: Highlights • Magnetic fabric in the area of large slip during the 2011 Tohoku-Oki EQ are presented. • Magnetic fabric in the frontal prism reflects the WNW-directed plate convergence. • Those in underthrust sediments records a uniaxial strain state and underconsolidation. • The contrast indicates strain decoupling across the plate-boundary décollement. • Large co-seismic slip could occur in shallow décollement where plates are decoupled. Abstract The Integrated Ocean Drilling Program (IODP) Expedition 343, Japan Trench Fast Drilling Project (JFAST) drilled and cored across the plate-boundary décollement at Site C0019 near the Japan Trench, where large slip occurred during the 2011 Tohoku-Oki earthquake (MwMw 9.0). Anisotropy of magnetic susceptibility (AMS) data obtained from core samples show a striking change in magnetic fabric across the décollement. In the frontal prism above the décollement, the maximum AMS axes display a strong preferred orientation in the northeast–southwest direction, with the intermediate and minimum AMS axes distributed in the vertical plane. In the underthrust sediments below the décollement, the maximum and intermediate AMS axes are subhorizontal but variable in direction, and the minimum AMS axes display an approximately vertical preferred orientation. The AMS ellipsoids for all samples have an oblate component, but the AMS ellipsoids of the underthrust sediments are generally less oblate than those of the frontal prism. The magnetic fabric of the sediments in the prism are consistent with horizontal tectonic shortening nearly parallel to the plate convergence direction in the Japan Trench, while that in the underthrust sediments represents a vertical, uniaxial strain. The magnetic fabrics of the frontal prism and underthrust sediments indicate an abrupt change in strain across the décollement at shallow depths, and imply that large co-seismic slip occurred along a weak plate-boundary décollement that is mainly decoupled over the long-term.