Sedimentary Evolution and Seismogenic Slip at the Sumatra Subduction Zone

Abstract:

IODP Expedition 362 drilled Sites U1480 and U1481 to characterize lithology, sedimentation history, diagenesis, and physical, chemical, and thermal properties of the input section to the Sumatra subduction zone to improve our understanding of the 2004 ~Mw 9.2 Sumatra-Andaman earthquake and tsunami that killed more than 250,000 people. The project also provided data to assess controls of the unusual ~150 km wide forearc plateau and improve our knowledge on the Bengal-Nicobar fan system. The uppermost stratigraphic section, lithologic Units I-II and part of the Nicobar Fan system, are mud rich with interbedded sand beds, span the Quaternary to late Miocene (9-10 Ma), and had Pliocene-Miocene sedimentation rates that exceeded 200 m/Myr. Unit III has slower sedimentation, is devoid of sands, and is separated from Unit IV by a 41 Myr hiatus. Units IV and V are composed of lava flows, intrusions and limestone. Unit VI sampled basaltic oceanic crust. Provenance analysis of the high sedimentation rate sections documents a Himalayan source and the highest sedimentation rates from 9 to 2 Ma exceed those in the Bengal fan. This high sedimentation pattern in the Nicobar fan suggests massive, rapid changes in the Bengal-Nicobar fan system dominated by tectonic and climate forcing. The high depositional rates of the fan and the trench wedge also provide optimal conditions for diagenetic dehydration and strengthening and development of a decollement system at depth within the sediments that facilitated shallow slip, such, as in 2004. Sites U1480 and U1481 both show increased amorphous silica and pore fluid freshening within Unit III. Fluid production and thermal modeling during continued burial of this layer suggests temperatures of >150°C at the base of sediments at the deformation front (>4.5 km depth) and highest fluid production occurs seaward of this location, providing conditions favorable for unstable sliding. This early completion of dehydration is controlled by the rapid Miocene to present sedimentation rates and thermal structure. Combining these results enhances our knowledge on sediment delivery to the Bengal-Nicobar fan system, explains processes that led to a great earthquake, and provides comparison for other great earthquakes such as Tohoku-Oki that have slow sedimentation histories and weakening behavior.