Stratigraphic, paleontologic, and structural data from two Panama-Costa Rica forearc peninsulas located landward of the aseismic Cocos Ridge along the southernmost Middle America Trench document rapid Pliocene subsidence and basin infilling followed by Quaternary deformation and uplift. On the Burica Peninsula, an approximately 3,000-m-thick, Pliocene-Pleistocene clastic sedimentary sequence, characterized by fine-grained turbidite deposits and volumetrically minor coarser-grained channel-fill, slump, and debris-flow deposits, bears depth-diagnostic foraminifera that document rapid Pliocene subsidence and deposition on a south-dipping paleoslope. New paleobathymetric and age estimates from foraminifera in these rocks indicate shallowing of depositional paleodepths from 2,000+ to 1,200 m during the late Pliocene. Present-day exposure of these rocks indicates an average Quaternary uplift rate of ∼1 mm/yr. The turbidite section on the Burica Peninsula is interpreted to represent a trench-slope deposit and is correlated to a similar, marine sedimentary section on the adjacent Osa Peninsula of Costa Rica.

Structures developed within Pliocene-Pleistocene strata of the outer-forearc Burica and Osa Peninsulas record minor (<5%) subhorizontal, margin-perpendicular shortening and margin-perpendicular extension. Bedding orientations on the Osa and Burica Peninsulas generally record arcward tilting by 10°-30° of these outer-forearc blocks. On the Burica Peninsula, the Medial fault zone, a north-striking, high-angle, right-lateral strike-slip (?) fault, has truncated and reoriented the Pliocene-Pleistocene sedimentary sequence by fault-bend (forced) folding. A linear, margin-parallel fault, herein named the "Ballena-Celmira fault zone," bounds the arcward side of the Osa and Burica Peninsulas and has accommodated vertical displacements between the outer forearc (Burica-Osa area) and inner forearc (Terraba area) throughout the Tertiary. Structural and seismicity observations from the forearc region suggest that Neogene deformation of the leading edge of the arc in this area reflects the effects of isostatic uplift, and possibly increased interplate shear stresses, as a result of subduction of the Cocos Ridge beneath this part of the arc since 1 m.y. ago.

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