Insights into the role of tectonic extension and compression vs. subduction erosion in the tectonics of forearcs: Examples from the Japan Trench and the Middle America Trench

The forearc region remains key in understanding the dynamics of convergent plate tectonics. This study focuses on the mechanisms governing tectonic processes within the overriding plate forearc which spans from the trench to the volcanic arc at two key and relatively well studied regions: the Japan Trench and the Middle America Trench offshore SE Costa Rica. We address the questions that have arisen concerning material input into the plate boundary, whether by subduction, accretionary prism formation, or tectonic erosion. In the Japan Trench case study, while tectonic accretion occurs near the trench axis, significant forearc subsidence suggests net material removal, possibly through tectonic erosion that has transferred material to the subducting slab. Debate surrounds the mechanism driving forearc subsidence, with recent studies proposing extensional tectonism as a possible mechanism to exclude subduction erosion. However, seismic evidence challenges this hypothesis, as normal faults indicative of forearc extension are not prominent. Moreover, a quantitative mass-balance analysis fails for the forearc if extensional tectonics rather than tectonic erosion is assumed to have predominantly shaped the margin. The spatio-temporal progression of subsidence across the forearc is further explored; this indicates that peak subduction erosion has occurred beneath the lower slope. The Middle America Trench in SE Costa Rica has also been extensively studied with several drilling expeditions, with particular focus on the area where the aseismic Cocos Ridge is subducting beneath the Caribbean plate. Here the subduction of topographic relief has been traditionally viewed as a process that enhances subduction erosion. Recent studies have challenged this perspective, suggesting instead that subducting topography might lead to net accretion to the margin through various mechanisms. Ocean drilling expeditions provide valuable data on sedimentary successions and forearc tectonic evolution. These drilling data have been not always used to the best of their capacity, which has led to significant discrepancies between drilling-based inferences and seismic interpretations, in particular regarding the presence and nature of unconformities within the forearc sediments. Borehole observations strongly favor the inference that inboard the Cocos Ridge a large amount of subsidence has occurred, linked to recent subduction erosion beneath this forearc.