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.

Eleven whole-round core samples from Integrated Ocean Drilling Program Expedition 341 Sites U1417 and U1418 were tested for vertical permeability. Subsamples of each core were taken and analyzed for grain size, biogenic silica content, and clay mineralogy. Measured vertical permeability varied from 1.8 × 10-18 to 1.5 × 10 -16 m 2 . With the exception of one sample from Site U1417, samples were dominated by clay-size (<4 µm) fractions with lesser silt-size (4-63 μm) and sand-size (>63 µm) fractions. Biogenic silica (SiO 2 ) content ranged from 2 to 15 wt% at Site U1417 and was consistently ~2 wt% in samples from Site U1418. Clay mineral abundance exceeded that of quartz, feldspar, and calcite in all samples. Smectite content ranged from 3 to 38 wt% in Site U1417 samples and from 0 to 3 wt% in Site U1418 samples. IntroductionOperations during Integrated Ocean Drilling Program Expedition 341 recovered cores from off the coast of the Gulf of Alaska ( Fig. F1; see the "Expedition 341 summary" chapter [Jaeger et al., 2014]). In this study, we used flow-through permeability tests to measure the permeability of five whole-round core samples from Site U1417 and six whole-round core samples from Site U1418. Subsamples from the core used for permeability testing were divided for grain size, biogenic silica, and clay mineral analyses. The uppermost sample at Site U1417 consists of distal Surveyor Fan sediments, and the other four samples from this site consist of pre-Surveyor Fan sediments. The samples from Site U1418 represent proximal Surveyor Fan sediments.The objective of the testing was to help characterize the sediments that are being carried toward the Aleutian Trench on the incoming Pacific plate (Fig. F1). The permeability of sediments entering subduction zones can greatly affect fluid pressures during shallow subduction, and this work provides some of the first permeability results from the incoming plate in this region. Analysis of amorphous silica and clay mineral content provides information on hydrous sedimentary minerals that can dehydrate during subduction or accretion and increase fluid pressures (Screaton, 2010 Methods Permeability testsThe methods for permeability testing are similar to those of previous studies (e.g., James and Screaton, 2015) and were based on American Society for Testing and Materials (ASTM) designation D5084-90 (ASTM International, 1990). Permeability tests used Trautwein Soil Testing Equipment Company's DigiFlow K, which consisted of a cell to contain the sample and provide isostatic effective stress and three pumps (Fig. F2). Deionized water was used as the fluid in the pumps, and a solution of 33 g NaCl per liter of water permeated the sample. Pressure was transmitted from the deionized water to the permeant across a rubber membrane in an interface chamber (Fig. F2).The retrieved core samples from Expedition 341 were stored in plastic core liners and sealed bags to prevent moisture loss and refrigerated at 4°C until immediately before sample preparation. All tests were c...