The Origin of Forearc Depressions

Forearc depressions form over continental subduction zones with young (age <30 Ma), slowly subducting (convergence rate <6 cm yr-1) oceanic slabs. Modern examples include the Strait of Georgia in Cascadia and the Sheliklof Strait in Alaska. There is no universal model describing why forearc depressions form due significant variation in style and character among subduction zones and because processes that control surface topography above subduction zones occur at depths (>15 km) that make them difficult to resolve. Here, we propose a general model describing why forearc depressions form. We compile megathrust earthquake, slow earthquake, geomagnetic anomaly, and seismic tomography data from subduction zones hosting forearc depressions, including those in Alaska, Cascadia, South Chile, and Japan. We show that forearc depressions are situated landward of megathrust earthquake epicenters and rupture zones, and most slow earthquake epicenters. We also show that forearc depressions are generally situated over geomagnetic anomaly highs and where the descending slab loses seismic contrast. From these data, we infer that forearc depressions are situated landward of the seismogenic zone, and overlie both the serpentinized forearc mantle and the region where the down-going slab undergoes eclogitization. We propose that forearc depressions form due to the confluence of two subsidence mechanisms operating in the same area: 1) thrusting and underplating over the seismogenic zone, causing downwards flexure over the serpentinized forearc mantle; and, 2) eclogitization of the subducting slab, leading to an evolving stress field and isostatic response that promotes subsidence in the overlying region of the upper plate.