COASTAL UPLIFT AND STREAM PIRACY AS INDICATORS OF NET SEISMIC CYCLE DEFORMATION ALONG THE COSTA RICA PACIFIC MARGIN: PUERTO CARRILLO HEADLAND AND RíO ORA VALLEY, NICOYA PENINSULA

The Nicoya Peninsula is an emergent segment of the Costa Rican forearc, located ~60 km inboard of the Middle America Trench where the Cocos plate subducts under the Caribbean plate at ~9 cm/yr. Rapid convergence results in systematic crustal deformation and net uplift related to the subduction earthquake cycle. Historical records indicate a ~50 year recurrence interval for megathrust earthquakes (>M7.5) beneath the peninsula. The last event (M7.7, 1950) produced abrupt coseismic coastal uplift (up to 1.5 m) followed by gradual interseismic subsidence. Ongoing tidal encroachment continues to inundate the central Pacific coast (Nosara to Puerto Carrillo) as strain builds toward the next earthquake and coseismic uplift event. Along the Puerto Carrillo headland and Río Ora valley, late Quaternary marine and fluvial terraces record the long-term pattern of net uplift and landscape evolution over multiple seismic cycles. Two marine terraces at Puerto Carrillo and Playa Camaronal (~35 m and ~15 m elv) are correlated with OSL dated terraces at Cabo Blanco on the peninsula’s southern tip. These correlations indicate terrace formation from 125-80 ka (OIS 5 sea level high stand) and uplift at 0.2-0.3 m/k.y. A prominent alluvial-fill terrace (~35-40 m elv) occurs along the length of the Río Ora valley and is characterized by deeply weathered fluvial gravels and a distinctive surface lag of resistant radiolarite boulders, derived from Cretaceous Nicoya Complex oceanic basement. The Río Ora terrace diverges from the modern river near the town of Estrada and continues along a paleo-valley to Puerto Carrillo where it merges with the upper marine terrace (125 ka). Uplift of the Puerto Carrillo headland subsequently blocked the paleo-river mouth, resulting in stream piracy and capture of the lower Río Ora, which now follows an incised channel through coastal hills to Playa Camaronal. A lower strath terrace (~20 m elv) at Estrada marks the capture point and is correlated with the lower marine terrace at Playa Camaronal (80 ka). Mapping, surveying, and age correlation of late Quaternary terraces at Puerto Carrillo and Río Ora provide constraints on both the timing of river capture and the rate of net uplift associated with the seismic cycle. Our results provide new insights into the tectonic evolution and earthquake hazards of the Nicoya coastline.