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A Late Miocene to Pliocene Increase in Soft-Sediment Deformation in Cook Inlet Nonmarine Forearc Basin Strata—potential Evidence for Larger Magnitude Earthquakes Associated With Increased Sedimentation in the Alaska Trench

Description: 

Previous analyses of compiled megathrust earthquake data have revealed a statistically significant positive correlation between great events (>M_w8.0) and thicker sediment fill in the trench. This has been interpreted to reflect the important role sediment can play in muting bathymetric relief and smoothing the subduction interface. The reduction in potential asperities may allow ruptures to propagate for longer distances, increasing the total energy released. Seismological studies in southern Alaska have imaged a low velocity zone beneath the Kenai Peninsula, consistent with underplated sediment. This material in the subduction channel is likely sourced from the east, where immense volumes of glaciogenic sediment have been routed into the trench since Late Miocene time (~7 Ma), reflecting rapid uplift and exhumation of the Chugach – St. Elias orogen during collision of the Yakutat terrane.

The 1964 M_w9.2 earthquake in southern Alaska produced a variety of liquefaction features in terrestrial settings, including sedimentary dikes that fed surface sand volcanoes. Potentially analogous soft-sediment deformation is locally observed in the Lower and Middle Miocene Tyonek and Beluga formations along the lower Kenai Peninsula. The relative abundance of these features increases dramatically in the overlying latest Miocene to Pliocene Sterling Formation, ranging from complexly folded, convolute bedded sandstone to zones where disrupted laminations grade laterally into structureless units. These facies are often closely associated with vertical pipe-like features that are collectively interpreted as evidence for liquefaction during intense, prolonged seismic shaking. We suggest the notable rise in soft-sediment deformation at ~ 5–7(?) Ma may reflect an increase in the frequency of very large megathrust earthquakes promoted by greater Neogene sediment flux into the trench. If correct, our study significantly extends the possible record of large megathrust earthquakes in southern Alaska into deep geologic time and highlights the value of forearc basins as long-term archives of subduction zone dynamics.

Session: Structure and Behavior of the Alaska-Aleutian Subduction Zone - II

Type: Oral

Date: 5/3/2024

Presentation Time: 11:15 AM (local time)

Presenting Author: Marwan

Student Presenter: No

Invited Presentation: 

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