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

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 similar to 4.5 km of thick Nicobar Fan and trench wedge sediments produces temperatures >150 degrees C within the basal sediments where the decollement 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-decollement reflector. We suggest that the boundary between basal-pelagic and siliciclastic-fan sediments has trapped fluid over the last similar to 1 Myr, as a result of primary lithological properties, diagenetic fluid release, and cementation, controlling decollement formation at this weakened level. Pre-subduction dehydration of large portions of the accreted sediment column strengthens the decollement 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.

Automated summary

The study reveals that shallow slip on the plate-boundary fault during the 2004 Aceh-Andaman Earthquake was related to thick incoming sediments on the oceanic plate, with the accumulation of thick Nicobar Fan and trench wedge sediments leading to high heat flux within the basal sediments. This pre-subduction dehydration of large portions of the accreted sediment column strengthens the decollement beneath the prism, suggesting potential co-seismic velocity-weakening behavior close to the trench in similar subduction zones.