Quaternary uplift astride the aseismic Cocos Ridge, Pacific coast, Costa Rica

Abstract

The Pacific coast of Costa Rica lies within the Central American forearc and magmatic-arc region that was created by northeastward subduction of the Cocos plate beneath the Caribbean plate at the Middle America Trench. From the Peninsula de Nicoya south-eastward toward the Peninsula de Osa and the Peninsula de Burica on the Panamanian border, the Middle America Trench loses its physiographic expression where it intersects the aseismic Cocos Ridge. Interaction between subduction of the buoyant, aseismic Cocos Ridge and the overriding Caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from Nicoya to Burica. The Pliocene and Pleistocene stratigraphic record and Holocene marine terraces and beach ridge complexes indicate that maximum rates of crustal uplift have occurred on the Peninsula de Osa, immediately landward of the aseismic Cocos Ridge. Crustal uplift rates decrease northwest toward the Peninsula de Nicoya, and to a lesser extent southwest toward the Peninsula de Burica. The late Quaternary stratigraphy on the Peninsula de Osa is subdivided into two major chronostratigraphic sequences from groupings of radiocarbon dates. Crustal uplift rates calculated from these sequences systematically decrease from 6.5 to 2.1 m/ka north-east across the peninsula. Deformation of the peninsula is modeled as uplifted and down-to-the-northeast-tilted fault blocks with an angular rotation rate of 0.03° to 0.06° per thousand years. Although less well constrained, crustal uplift rates on the Peninsula de Nicoya, 200 km to the northwest of the Peninsula de Osa, vary from <1 m/ka for Pliocene and Pleistocene sediments to 2.5 m/ka for Holocene marine terraces. In the Quepos region, 100 km to the northwest of the Peninsula de Osa, calculated uplift rates derived from incision of late Quaternary fluvial terraces range from 0.5 to 3.0 m/ka. On the Peninsula de Burica, only 60 km to the southwest of the Peninsula de Osa, calculated uplift rates range from 4.7 m/ka for a late Holocene marine terrace to 1.2 m/ka for post-late Pliocene deep-sea sediments. The variations in calculated uplift rates on the Peninsula de Osa constrain a dynamic model for subduction of the Cocos Ridge and the resulting uplift of the overriding Caribbean plate. Deflection of the Caribbean plate is modeled using various effective elastic thicknesses as the response of an elastic plate to the buoyant force of the subducted Cocos Ridge. Because the shape of the subducted end of the Cocos Ridge is unknown, two scenarios are evaluated: (1) a radially symmetric ridge with a slope similar to the slope of the flanks of the ridge and (2) a ridge where the subducted end was truncated by the Panama fracture zone. The best-fit model utilizes a truncated ridge that has been subducted during the past 0.5 m.y. ∼50 km beneath the overriding Caribbean plate, which has an effective elastic thickness of 5 km. The model predicts that the highest uplift rate should be ∼3.7 m/ka and occur on the southwest coast of the Peninsula de Osa. The rate of uplift slows considerably to the northeast and indicates that the Peninsula de Osa is tilting to the northeast, which agrees with observations in that region. The predicted uplift rate attributed to aseismic ridge subduction also decreases along the coast both north and south of the Peninsula de Osa, resulting in little uplift that can be attributed to Cocos Ridge subduction in the northwestern portions of the Peninsula de Nicoya.