Evolution of the Thermal and Dehydration State of Sediments Entering the North Sumatra Subduction Zone

Abstract Shallow slip on the plate‐boundary fault during the 2004 Mw 9.2 Aceh‐Andaman Earthquake, offshore North Sumatra, has been linked to thick incoming sediments on the oceanic plate with advanced diagenetic and sediment property changes at the depths of plate boundary fault development. We couple age control, physical, and thermal property measurements from International Ocean Discovery Program (IODP) drilling with multichannel seismic reflection data to reconstruct the thermal structure and evolution of the incoming sediment column, up to the point of accretion/subduction. Lithospheric thermal rejuvenation around 58 Ma is required to explain anomalously high heat flux at IODP Site U1480, and heat flux within the trench is suppressed by very high sediment accumulation rates during the development of a thick trench wedge. Accumulation of up to ∼4.5 km of thick Nicobar Fan and trench wedge sediments produces temperatures >150°C within the basal sediments where the décollement develops, resulting in total pre‐subduction diagenetic dehydration of basal sediments. The smectite‐illite transformation within these basal sediments produces sufficient fluid to explain a polarity reversal on a pre‐décollement reflector. We suggest that the boundary between basal‐pelagic and siliciclastic‐fan sediments has trapped fluid over the last ∼1 Myr, as a result of primary lithological properties, diagenetic fluid release, and cementation, controlling décollement formation at this weakened level. Pre‐subduction dehydration of large portions of the accreted sediment column strengthens the décollement beneath the prism, extending co‐seismic velocity‐weakening behavior close to the trench, which may occur at other subduction zones with similar sediment compositions, thicknesses, and/or temperatures.