Deformation mechanisms in sediments entering a subduction complex to shallow seismogenic slip

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Earthquakes challenge our ability to predict, yet doing so remains keenly important for risk mitigation in modern society. With increasing awareness of the rich variety of seismic events, from massive, destructive earthquakes like the 2004 Sumatra earthquake to slow-slip quakes, the imperative to understand potential geologic controls increases. These objectives motivated an IODP Expedition to drill and sample the Bengal/Nicobar Fan sequence and sediments down to the Indian Ocean basaltic crust in order to characterize the materials that will enter the subduction zone and will contribute to an end-member earthquake. The Sumatra earthquake is of special interest because it occurred closer to the trench than expected, increasing earthquake and tsunami size. A recently published paper suggests that the shallow slip offshore Sumatra is driven by diagenetic strengthening of deeply buried fault-forming sediments associated with complete dehydration of silicates before subduction, contrasting with conventional models. To contribute to the better understanding of this atypical shallow seismogenic slip, we propose to characterize micro-fabrics and pore structures of core samples, a subset of which were frozen in LN2 on site during this expedition in order to (1) look for anomalies in microstructure that when combined with seismic and physical property data might define future fault localization and decollement horizons and (2) deformation mechanisms during compaction and small-scale faulting that will help extrapolate mechanical properties of rocks from their current seaward location into the Sumatra subduction complex. To fulfill these objectives, we will investigate progressively compacted and mildly deformed samples preserved on site in cryogenic conditions (i.e. no drying damage and preservation of pore water) and more numerous slowly dried samples, using (cryogenic) Broad Ion Beam polishing and (cryogenic) Scanning Electron Microscopy. We will compare these results with microstructures of core samples for which the stress-strain path is measured in the laboratory, to test hypotheses about how the sedimentary section will respond to additional burial and shear stress when entering the subduction complex.