ACTIVE TECTONICS AND QUATERNARY MARINE TERRACES ALONG A CONVERGENT PLATE MARGIN, PENINSULA DE NICOYA, COSTA RICA

The Península de Nicoya lies along the Costa Rican Pacific margin where the Cocos plate subducts beneath the Caribbean plate at 9-10 cm/yr. Uplifted Quaternary marine terraces along the Nicoya coast signify active deformation of the fore arc crust. The objective of this study is to examine Quaternary vertical tectonics on the Península de Nicoya by aerial photo analysis, field mapping, and surveying of marine terrace surfaces. Along the northern Nicoya coast, from Tamarindo to Nosara, the 2-4 km wide Iguanazul Surface forms a low-relief coastal platform between the modern sea cliffs and the interior mountain front. This terrace surface consists of three separate wave-cut treads with preserved paleo-shorelines at 10-15 m, 18-22 m, and 26-32 m elevation. Based on correlation with Quaternary sea level curves, the terrace inner edge elevations can be linked to Pleistocene sea level high stands between 80-330 ka (Marine oxygen isotope stages 5-9). These estimated ages suggest a net uplift rate of 0.1-0.3 m/ky. In addition, Holocene beach rock samples from Playa Negra and Playa Lagarto yield calibrated radiocarbon ages of 1610 and 760 ybp, indicating net uplift at less than 0.5 m/ky. The uplift rate suggested by radiocarbon dating of Holocene deposits is consistent with that determined by sea level curve correlation for Pleistocene terraces. Documented coseismic uplift along this coast during historic large subduction earthquakes suggests that cumulative uplift may be the result of episodic events related to the seismic cycle.