Physical properties of southern Alaska margin sediments in the context of global convergent margins

Abstract:

At convergent margins, the deformation response due to external forcing by sedimentation, tectonic stress, and volume changes during chemical reactions is closely interrelated with the ability of excess pore pressure to dissipate. These excess pore pressures in turn can affect plate boundary fault location and strength, rates of sediment accretion or subduction, the taper angle of material on the overriding plate, and may also play a role in the generation of earthquakes and propagation of seismic slip. Offshore southern Alaska, rapid sedimentation and glacial loading are interpreted to have influenced the location of thrust faulting by rapid transport of sediment offshore, where previously active faults were deactivated by increased normal stresses as a result of sediment loading. The response of the wedge to external forcing is linked to permeability of the wedge sediments, as well as those in the underriding plate. We determined permeability of sediments from the glacial sediment dominated accretionary wedge, sampled at Sites U1420 and U1421 on the Yakutat Block, and sediments from the Surveyor Fan that overlies the Pacific Plate and are inputs to the Aleutian Trench, sampled at Sites U1417 and U1418. We found that the Surveyor Fan sediments have porosity-permeability relationships that are comparable to sediments from other reference sites worldwide. However, the sediments in the wedge have somewhat higher permeability, much larger grain sizes, and are much less compressible compared with other wedge sediments. This suggests that the physical properties that control overpressure generation and dissipation in the input sediments to the Aleutian Trench are comparable to other subduction zones, but that the accretionary wedge on the Yakutat Block is uniquely strong and well-drained due to the predominance of glacigenic sediments.