Active tectonics and landscape evolution inboard of subducting seafloor roughness: An investigation across the Costa Rican fore arc, Central America

Orthogonal subduction of rough oceanic lithosphere along the northwestern flank of the Cocos Ridge imprints a distinctive style of deformation on the overriding Costa Rican fore arc. Subduction of seamounts and ridges along the Costa Rican segment of the Middle American margin produces both subsidence offshore and differential surface uplift onshore. Near the trench inboard of subducting bathymetric highs, recovered drill cores, seismic reflection profiles, and bathymetric mapping are all consistent with subsidence. However, across the subaerially exposed portions of the fore arc, areas overriding subducting bathymetric highs record rapid uplift. For example, along the southwest coast of the Nicoya Peninsula, a northwest-trending outer-fore-arc high, exposures of latest Cretaceous and Cenozoic turbidite deposits are confined to regions overriding northeast-trending ridges. No uplifted Quaternary deposits are observed between these two isolated depo-centers. Across the central Pacific coast inboard of subducting, linear chains of seamounts, the coastal fore arc is dissected by steeply-dipping, northeast-striking faults. These faults bound blocks characterized by differential rates of surface uplift. Upthrown blocks are confined to portions of the fore arc that override subducting seamounts. Faulted, alluvial fill terraces exposed on two adjacent fault-bounded blocks are used to calculate late Pleistocene surface uplift rates of 0.8 ± 0.1 mm yr−1 for the block overriding the subducted seamount compared to 0.08 ± 0.1 mm yr−1 on the adjacent block that does not override seamounts. The fault separating these two adjacent blocks has a minimum separation rate of 0.7 mm yr−1.; The record of simultaneous offshore subsidence and onshore uplift as a result of seamount subduction is consistent with either out-of-sequence faulting or underplating beneath the inner fore arc. Arcward of the trench the fore arc basement is composed mostly of Cretaceous to early Tertiary Caribbean plate basement that has greater strength than the thinner lower-slope apron. Thus, rigid seamounts are capable of removing upper plate material along the lower apron. However, the thicker basement beneath the subaerial portion of the fore arc may act as a rigid backstop and shear subducting seamounts off the Cocos plate. This mechanism of tectonic smoothing of the downgoing plate would result in a thickened crustal sequence and broad uplifted regions inboard of subducting seamounts and ridges. This is consistent with observed narrow zones of subsidence across the lower slope and broader regions of late Quaternary surface uplift rates across the subaerial fore arc. (Abstract shortened by UMI.)...