--> Pore pressure in sediments off the Sumatra accretionary margin

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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Pore pressure in sediments off the Sumatra accretionary margin

Abstract

A recent International Ocean Discovery Program (IODP) expedition, 362, sampled sediments approximately 225 km seaward of the deformation front of the Sumatran Subduction Zone (Dugan et al. 2017). This expedition was motivated by the 2004 Mw 9.2 earthquake and tsunami that struck North Sumatra and the Andaman-Nicobar Islands. This, followed by the Tohoku-Oki earthquake in 2011, showed unexpectedly shallow megathrust slip, and in the case of North Sumatra, this was focused beneath the distinctive plateau of the accretionary prism (Henstock et al. 2006, Bletery et al. 2016). The Sumatra accretionary margin is characterised by a 4 – 5 km thick incoming sequence of sediments at the deformation front with dominant input from the Bengal-Nicobar fan. It has a distinct seismic horizon that develops into a high amplitude negative polarity (HANP) seismic reflector near the subduction zone (Dean et al. 2010). This HANP horizon had been interpreted through seismic data and characterisation as likely weak, porous and subsequently overpressured, which would provide a locus for décollement initiation along parts of the margin (Dean et al. 2010). Core sampling during this expedition has shown a sequence of siliciclastic sediment interpreted as Nicobar Fan, with high sedimentation rates of > 100 m/Myr. This rapidly deposited sequence is underlain by mixed tuffaceous and pelagic sediment and intervals of intercalated pelagic and igneous material overlying oceanic crust that has slow rates of deposition averaging < 4m/Myr. In the lowermost and oldest intervals overlying the basement, the sediment is interbedded with extrusive and intrusive igneous rocks. The sediment section encompasses the Late Cretaceous to Recent deep- marine sedimentary cover on the ocean floor and range from predominately unconsolidated to partially lithified and lithified, from surface to basement. A 17m thick section of tuffaceous silty claystone in the lower most and slowly deposited sequence, exhibits anonymously high porosity relative to the overlying sediments and inferred compaction trend. A difference in clay type, sedimentation rate, proximity to basement intrusives, along with a high geothermal gradient result in a sediment package that is distinct in colour, texture, porosity and geochemical signature as compared to the overlying sediment. This sequence of clays also consist of low porosity high velocity calcareous oozes. Cementation is inferred to be pervasive, reducing porosity in partially lithified calcareous oozes whilst possibly helping to preserve porosity in the more siliceous material. This section of high porosity is associated with the HANP interval and the clays have been identified to be a smectite rich alteration product from the volcanics which, identified by Hupers et al (2017), are suggested to be associated with sediment strengthening mechanisms. Pore pressure estimates based on porosity proxies, indicate hydrostatic pressures down to the HANP related clays, with a HANP clay exhibiting a most likely case of mild over pressure of 1.12 SG. Mild overpressures are attributed primarily to disequilibrium compaction resulting from the rapid sedimentation of the ‘overburden’. This may have implications for proto decollement formation and the effective stress profile once these sediments reach the deformation front and form part of the accretionary wedge. The combination of mild overpressures with sediment strengthening and further overpressure mechanisms such as smectite-illite alteration switching on further down the deformation front may explain the unusual and extensive up-dip propagation of faults along the Sumatra subduction zone.