The Origin of Forearc Depressions

Forearc depressions form over continental subduction zones with young, slowly subducting slabs and thick trench fills. They are bound seaward by a coast range and landward by a volcanic arc such that subsidence in forearc depressions occurs between orogens and in areas characterized by plate convergence. We propose a model for forearc depression formation based on geophysical and seismic data from four circum-Pacific subduction zones. Coast range crests coincide with >100 mGal gravity anomalies, which are attributed to underplated material and indicate that underplating drives coast range uplift. Coast range crests are situated near the down-dip termini of megathrust earthquake rupture zones, showing that coast ranges overlie where subduction interface sliding behaviour transitions from frictional to semi-frictional. This transition causes subduction interface shear stress to begin decreasing with depth and triggers underplating as shear stress becomes insufficient to drag buoyant material deeper. Forearc depressions are situated landward of inter-plate seismic phenomena, indicating they overlie the hydrated forearc mantle. Forearc depressions form as counter-flexural basins over the hydrated forearc mantle; in this position the upper plate crust is not supported by the flexurally rigid slab and can bend downwards. Forearc depressions do not form over old slabs because old slabs do not exceed the temperature threshold for semi-frictional sliding prior to intersecting the mantle wedge corner. Fast convergence rates and thin trench fills promote subduction erosion along the subduction interface, thereby prohibiting the formation of coast ranges, and by extension, forearc depressions.