MARINE TERRACES, SUBDUCTION EARTHQUAKES, AND QUATERNARY UPLIFT OF THE NICOYA PENINSULA, COSTA RICA, CENTRAL AMERICA
Along the northern Nicoya coastline, a prominent Quaternary marine terrace (Iguanazul surface) extends from Tamarindo to Nosara. This terrace is a composite surface, consisting of at least three separate wave-cut treads that preserve paleo-shorelines at 10-12 m, 18-22 m, and 26-32 m elevation. Preliminary correlations with late Pleistocene sea level high stands at 80-330 ka (marine oxygen isotope stages 5-9) indicate net uplift rates of 0.1-0.3 m/ka. Holocene beach rock horizons along the upper shore face of the active beach yield calibrated radiocarbon ages of 1610 and 760 ybp (Playa Negra and Playa Lagarto). These ages are consistent with net Holocene uplift at <0.5 m/ka.
In contrast to the Iguanazul surface, the Cobano surface at the peninsula's southern tip (Cabo Blanco) consists of at least five distinct Pleistocene marine terrace treads separated by well-defined risers at 30-220 m elevation. Holocene uplift rates here range from 3.0-6.5 m/ka. While the northern Nicoya Peninsula lies onshore of the Cocos plate "smooth domain", the southern peninsula sits inboard of subducting seamounts of the "rough domain". The order-of-magnitude difference in Quaternary uplift rates between the northern and southern Nicoya Peninsula may be linked to sharp contrasts in the roughness, thickness, and dip of the subducting Cocos plate offshore.
Plate motion and seismicity data suggest recurrence intervals of <100 years for large Nicoya earthquakes. While these frequent events may produce meter-scale coseismic uplift, a significant fraction of this is likely recovered by interseismic subsidence. The net result is gradual coastal uplift at the observed rates.