FOREARC BASINS AS RECORDERS OF ACCRETIONARY PROCESSES AND INDICATORS OF GEOHAZARDS: EXAMPLES FROM THE NANKAI TROUGH, SOUTHERN CASCADIA, AND SUMATRA

Forearc basin formation reflects interacting processes of accretion and subduction; the presence of a basin may yield insights into margin architecture and faulting, accretionary wedge strength, and the seismogenic zone. Assuming sediment supply is sufficient, forearc basins collect sedimentary strata that record changes in these processes due to tectonic events or changes to the input at the trench. We present three examples of forearc basins in accretionary margins, each of which records key tectonic events for a margin, provide timing constraints and yield geohazard implications. Offshore Kii Peninsula, Japan, an out-of-sequence thrust, the megasplay fault, may move co-seismically during great earthquakes. 3D seismic images of this fault system and the overlying Kumano Basin show that sediments are tilted landward in the hanging wall of the megasplay fault; timing of the tilting shows it to have occurred over < 300 kyr from 1.3 to 1 Ma. This tilting event represents either the initiation of or time of maximum movement along the megasplay fault system making the potentially tsunamigenic megasplay a recently formed and transient structure. The Eel River Basin offshore northern California lies just north of the Mendocino Triple Junction. Forearc basin strata record a reduction in accommodation space caused by regional uplift which started ~500 ka thought to represent the encroachment of the Triple Junction. Presence of strike-slip and transpressional deformation within the Basin suggests a transitional process where translational deformation can occur ahead of a migrating fault-fault-trench triple junction allowing the possibility for both megathrust and strike-slip earthquakes near the Triple Junction. The partly filled Aceh Basin offshore northern Sumatra is bound on its seaward flank by the West Andaman Fault; the accretionary prism forms a marginal plateau which is shallower bathymetrically than the Basin. Rupture during the 2004 megathrust earthquake may have occurred largely to the west of the forearc basin, which significantly influenced by strain partitioning-caused, strike-slip tectonics.