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Identifying the processes that control the rates, magnitudes, and longevity of outer forearc deformation is fundamental to our understanding of how mass is distributed within subduction zone systems. The margin of southern Costa Rica has been the target of numerous onland field studies, geophysical surveys, and the Costa Rica Seismogenesis Project (CRISP) drilling expedition. Despite these extensive data sets, the relative roles that subduction erosion, shortening, and seamount subduction play in shaping the behavior and evolution of the outer forearc remain debated. Here we analyze new and existing geomorphic, geologic, stratigraphic, and geochronologic data sets across the entire forearc‐arc‐backarc system near the Costa Rica Seismogenesis Project transect to test several conceptual models for outer forearc deformation in southern Costa Rica. The results from the compilation agree with a model where recent arcward retreat of the trench (movement of the trench in the direction of the arc) occurs due to underthrusting of the outer forearc beneath the inner forearc, and outer forearc deformation occurs primarily by transient rapid vertical tectonism due to the subduction of bathymetric relief. These results lead to a new conceptual model for Plio‐Quaternary outer forearc deformation in southern Costa Rica that does not require large amounts of net mass loss within the upper plate. Key Points: Trench retreat in southern Costa Rica can be explained by inner forearc shorteningOuter forearc vertical deformation occurs primarily due to the subduction of bathymetric reliefWe present a new model for Plio‐Quaternary deformation that does not require significant upper plate mass loss [ABSTRACT FROM AUTHOR]

Weathering rind thicknesses were measured on ∼ 200 basaltic clasts collected from three regionally extensive alluvial fill terraces (Qt 1, Qt 2, and Qt 3) preserved along the Pacific coast of Costa Rica. Mass balance calculations suggest that conversion of unweathered basaltic core minerals (plagioclase and augite) to authigenic minerals in the porous rind (kaolinite, allophane, gibbsite, Fe oxyhydroxides) is iso-volumetric and Ti and Zr are relatively immobile. The hierarchy of cation mobility (Ca ≈ Na > K ≈ Mg > Si > Al > Fe ≈ P) is similar to other tropical weathering profiles and is indicative of differential rates of mineral weathering (anorthite > albite ≈ hypersthene > orthoclase ≫ apatite). Alteration profiles across the cm-thick rinds document dissolution of plagioclase and augite and the growth of kaolinite, with subsequent dissolution of kaolinite and precipitation of gibbsite as weathering rinds age. The rate of weathering rind advance is evaluated using a diffusion-limited model which predicts a parabolic rate law for weathering rind thickness, rr, as a function of time, t(rr =κt), and an interface-limited model which predicts a linear rate law for weathering rind thickness as a function of time (rr = kappt). In these rate laws, κ is a diffusion parameter and kapp is an apparent rate constant. The rate of advance is best fit by the interface model.Terrace exposures are confined to the lower reaches of streams draining the Pacific slope near the coast where the stream gradient is less than ∼3 m/km, and terrace deposition is influenced by eustatic sea level fluctuations. Geomorphological evidence is consistent with terrace deposition coincident with sea level maxima when the stream gradient would be lowest. Assigning the most weathered regionally extensive terrace Qt 1 (mean rind thickness 6.9 ± 0. 6cm) to oxygen isotope stage (OIS) 7 (ca. 240 ka), and assuming that at time = 0 rind thickness = 0, it is inferred that terrace Qt 2 (rr = 2.9 ± 0.1 cm) is coincident with stage 5e (ca. 125 ka) and that Qt 3 (rr = 0.9 ± 0.1 cm) is consistent with OIS 3 (ca. 37 ka). These assignments yield a value of kapp of 8.6 × 10−13 cm s−1 (R2 = 0.99). Only this value satisfies both the existing age controls and yields ages coincident with sea level maxima. Using this value, elemental weathering release fluxes across a weathering rind from Qt 2 range from 6.0 × 10−9 mol Si m−2 s−1 to 2.5 × 10−11 mol K m−2 s−1. The rate of rind advance for the Costa Rican terraces is 2.8 × 10−7 m yr−1. Basalt rind formation rates in lower temperature settings described in the literature are also consistent with interface-controlled weathering with an apparent activation energy of about 50 kJ mol−1. Rates of rind formation in Costa Rica are an order of magnitude slower than reported for global averages of soil formation rates. [Copyright &y& Elsevier]

Presents a study which examined the kinematics and the tectonic origin of faulting along the western margin of the Panama block in Central Costa Rica. Tectonic framework; Geology of central Costa Rica; Regional-scale faults; Fault kinematics; Conclusions.

Presents kinematic data for late Cenozoic deformation in central Costa Rica that marks the western margin of the Panama micropalte. Extension of the boundary; Tectonic evolution across the boundary; Link of the North Panama deformed belt to the east with the Middle America Trench to the west; Earthquake focal mechanisms.

Evaluates the spatial distribution of forearc deformation in response to seamount subduction at the southeastern side of the Peninsula de Nicoya, Costa Rica. Age range of Holocene marine terraces; Maximum uplift rates based on radiocarbon dating; Block rotation.