The impact of rapid sediment accumulation on pore pressure development and dehydration reactions during shallow subduction in the Gulf of Alaska

Abstract

In the Gulf of Alaska region, sediment has rapidly accumulated (>1 km/my) in the trench sourced from intensified glaciation in the past $1.2 million years. This rapid sediment accumulation increases overburden and should accelerate dehydration of hydrous minerals by insulating the underlying sediment column. These processes have the potential to generate fluid overpressures in the low permeability sediments entering the subduction zone. A 1-D model was developed to simulate dehydration reaction progress and investigate excess pore pressures as sediments approach the trench and are subducted. At the deformation front, simulated temperatures increase by $308C due to the insulating effect of trench sediments. As a result, opal-A begins to react to form quartz while smectite remains mostly unreacted. Loading due to the trench sediments elevates excess pore pressures to $30% of lithostatic pressure at the deformation front; however, deformation front excess pore pressures are sensitive to assumptions about the permeability of outer wedge sediments. If the outer wedge sediments are coarse-grained and high-permeability rather than mud-dominated, excess pore pressures are lower but still have an insulating effect. During early subduction, simulated pore pressures continue to rise and reach $70% of lithostatic by 60 km landward. The 1-D modeling results suggest that the elevated pore pressures are primarily due to loading and that dehydration reactions are not a significant component of excess pore pressure generation at this margin.

Spatiotemporal Characterization of Smectite‐to‐Illite Diagenesis in the Nankai Trough Accretionary Prism Revealed by Samples From 3 km Below Seafloor

“…We simulated the spatiotemporal progress of the smectite‐to‐illite diagenesis in the Nankai Trough accretionary prism offshore Kii peninsula using time‐ and temperature‐dependent reaction kinetic laws, which was parameterized with available temperature models, pore water geochemical data, and estimates for the travel time and trajectory of the sediment through the prism. More generally, our modeling results are consistent with previous numerical modeling studies of the smectite‐to‐illite transition in other subduction zone forearcs showing that the conversion occurs in a temperature range of 60 to 150 °C (Bekins et al, ; Meridth et al, ; Saffer et al, ). The results also confirm expected differences in illitization progress along the strike of the margin based on the spatial variation of marine heat flow data (Harris et al, ).…”

Release of mineral-bound water prior to subduction tied to shallow seismogenic slip off Sumatra

“…The thickness and thermal state of sediments at the Cascadia, southern Lesser Antilles, and Makran margins (table S1), all affected by input of submarine fan sequences, suggest similarities to North Sumatra in terms of state of diagenesis and potential for shallow slip. We also note that although dehydration is modeled to peak within and not outboard of the Eastern Aleutians subduction zone (27), slip in the outermost forearc was recorded during the 1964 earthquake (28) (table S1). Many of these subduction zones have either never been sampled (e.g., Makran) or have a limited historic rupture record (e.g., Makran, southern Lesser Antilles, and Cascadia), but shallow slip during megathrust rupture may be possible.…”

“…26 CNRS, UMR6118 -Geosciences Rennes, University de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France. 27 Geologic Environmental Division, Korea Institute of Geoscience and Mineral (KIGAM), 124 Gwahak-ro, Yuseong-gu, Daejeon 34132, Korea. 28 Royal Holloway and Bedford New College, Royal Holloway University of London, Queens Building, Egham TW20 0EX, UK.…”

Effect of Buoyant Sediment Overlying Subducting Plates on Trench Geometry: 3D Viscoelastic Free Subduction Modeling

“…The sediment covering the subducting plates is characterized by the low density (i.e., 2,400–2,600 kg/m 3 ; Malatesta et al., 2013), low mechanical strength (i.e., <1 MPa; Ikari & Kopf, 2011), and high pore‐pressure (Meridth et al., 2017), which may promote the decoupling of the sediment from the subducting slab. However, geochemical (Chen et al., 2017) studies on intraplate volcanoes in East Asia have suggested that trench sediment can descend to the mantle transition zone (i.e., >410 km).…”