Active, long-lived upper-plate splay faulting revealed by thermochronology in the Alaska subduction zone

The lack of subaerial forearc geological records in active subduction zones has hindered our understanding of the roles of upper-plate structures and their interactions with plate interface processes in accommodating forearc deformation. Forearc splay faults, a type of upper-plate structure, are of particular interest due to their high efficiency in triggering tsunamis during great earthquakes. The coastal area of the Kodiak Islands, Alaska, USA exhibits stratigraphic and geomorphologic records of Miocene to Recent vertical tectonism and Quaternary thrust faults, suggesting potential splay-fault-involved deformation over geological timescales. To better understand the mechanisms of forearc long-term strain accumulation and the roles of splay faults, we investigate the spatial and temporal pattern of recent forearc exhumation in the Kodiak accretionary prism by conducting zircon and apatite (U-Th)/He (ZHe and AHe) thermochronologic analyses and thermal history modeling. These results are supplemented by field investigations, detrital zircon geochronology analyses and offshore active fault mapping. Most of the ZHe ages record cooling through the ZHe closure temperature in the late Eocene-early Oligocene, temporally and spatially consistent with the Eocene-early Oligocene broad antiformal exhumation previously documented by zircon and apatite fission track thermochronological ages. However, the AHe ages record cooling through the AHe closure temperature from early Miocene to Pliocene and exhibit an overall trenchward younging trend, with all the Pliocene ages (3–5 Ma) in the regions closest to the trench. Our thermal history modeling and field survey suggest that the trenchward coastal area of the Kodiak Islands experienced a change from early-middle Miocene basin subsidence to recent deformation and rapid uplift from 6–7 Ma to recent, while the rest of the island experienced an early-middle Miocene decrease in the prolonged exhumation from the Eocene-Oligocene. The newly revealed long-term exhumation pattern resembles the estimated uplift patterns based on elevated marine terraces and geodetic data. The early-middle Miocene change in exhumation pattern might be caused by a change in the dominant deformation mechanism affecting the Kodiak Islands, from broad underplating along the subduction interface mainly during the Eocene-Oligocene to hanging-wall uplift due to an active crustal splay thrust fault system since the late Miocene (the Kodiak Shelf Fault). We further discuss the dip-slip rate and geometry of the Kodiak Shelf Fault system and how inherited forearc upper-plate structures and lithology may affect forearc fluid distribution and facilitate the development and persistent deformation of the Kodiak Shelf Fault system.