Rapid sedimentation and overpressure in shallow sediments of the Bering Trough, offshore southern Alaska

Abstract

Pore pressures in sediments at convergent margins play an important role in driving chemical fluxes and controlling deformation styles and localization. In the Bering Trough offshore Southern Alaska, extreme sedimentation rates over the last 140 kyr as a result of glacial advance/retreats on the continental shelf have resulted in elevated pore fluid pressures in slope sediments overlying the Pamplona Zone fold and thrust belt, the accretionary wedge resulting from subduction of the Yakutat microplate beneath the North American Plate. Based on laboratory experiments and downhole logs acquired at Integrated Ocean Drilling Program Site U1421, we predict that the overpressure in the slope sediments may be as high as 92% of the lithostatic stress. Results of one‐dimensional numerical modeling accounting for changes in sedimentation rate over the last 130 kyr predicted overpressures that are consistent with our estimates, suggesting that the overpressure is a direct result of the rapid sedimentation experienced on the Bering shelf and slope. Comparisons with other convergent margins indicate that such rapid sedimentation and high overpressure are anomalous in sediments overlying accretionary wedges. We hypothesize that the shallow overpressure on the Bering shelf/slope has fundamentally altered the deformation style within the Pamplona Zone by suppressing development of faults and may inhibit seismicity by focusing faulting elsewhere or causing deformation on existing faults to be aseismic. These consequences are probably long‐lived as it may take several million years for the excess pressure to dissipate.

Impact of Gas Saturation and Gas Column Height at the Base of the Gas Hydrate Stability Zone on Fracturing and Seepage at Vestnesa Ridge, West-Svalbard Margin

“…The overpressure gradient at which the median prediction for the minimum gas saturation required to open fractures reaches zero was highest for Site 4 and lowest for Site 1 and 5 (0.4 kPa/m for Site 1 and 5, 0.75 kPa/m for Site 12 and 1.35 kPa/m for Site 4). A rigorous analysis using sediment-compaction model with lab constrained sediment physical properties, sedimentation rates and ODP data will help predict realistic overpressure estimates for the VR and constrain our fracturing analysis [78,79].…”

Submarine landslide and tsunami hazards offshore southern Alaska: Seismic strengthening versus rapid sedimentation

“…Sedimentation rates offshore southern Alaska are fast enough to induce overpressure in the shallow sediments, as observed at Site U1421 [ Daigle et al ., ]. An additional difference is age of sediments at 25 mbsf.…”

“…Exceptionally high sedimentation rates occur proximal to glacially eroded regions [ Elverhoi et al ., ; Nygard et al ., ]. Very high sedimentation rates and fluid overpressure have been recently documented in the near‐seafloor sediments along the southern Alaskan margin from IODP Expedition 341 [ Jaeger et al ., ; Gulick et al ., ; Daigle et al ., ]. Globally, most high‐latitude regions are currently located in relatively stable, passive tectonic settings [ Montelli et al ., ].…”