DIFFERENTIAL UPPER PLATE DEFORMATION IN RESPONSE TO CONTRASTS IN SUBDUCTING SEAFLOOR ROUGHNESS, NICOYA PENINSULA, COSTA RICA

The Nicoya Peninsula lies along the hanging wall of the Middle America Trench (MAT) in the outer fore arc of northern Costa Rica. The peninsula's margin-parallel Pacific coast lies only 50-60 km inboard of the MAT where the Cocos plate subducts NE beneath the Caribbean plate at 10 cm/yr. This segment of the MAT corresponds with a high-potential seismic gap with an estimated 40-50 year recurrence interval for large earthquakes. The last major earthquake centered beneath the peninsula (Mw=7.7, 1950) produced widespread damage and generated 0.5-1.0 m of coseismic uplift along the peninsula's central Pacific coast. As a result of its location directly above the seismogenic zone, the Nicoya Peninsula experiences pronounced deformation and rapid coastal uplift linked to the subduction cycle.

The subducting seafloor offshore of the Nicoya Peninsula exhibits a smooth domain of relatively thin crust in the north (East Pacific Rise origin), and a rough domain of hotspot-thickened crust in the south (Galapagos spreading center origin). Uplifted flights of Quaternary marine terraces along the Nicoya coast record variable patterns of upper plate deformation across the transition between the smooth and rough seafloor domains.

Along the northern Nicoya coast (inboard of smooth domain), the Iguanazul surface includes three late Pleistocene marine terraces with inner edge elevations of 10-12 m, 18-22 m, and 26-32 m. Age correlations with sea level high stands at 80-330 ka (OIS 5-9) indicate net uplift rates of 0.1-0.2 m/k.y. Radiocarbon ages for Holocene beach rock horizons correspond with recent net uplift at <0.5 m/k.y. In contrast, the Cobano surface at the peninsula's southern tip (inboard of rough domain) encompasses four late Pleistocene terraces ranging from 30-220 m elevation. Sea level correlations (as above) yield uplift rates of 1.0-2.0 m/k.y. Optically Stimulated Luminescence (OSL) ages for terrace deposits on the three lowest treads are consistent with sea level correlations, indicating ages of 65-120 ka.

The order of magnitude difference in Quaternary uplift rates between the Iguanazul and Cobano marine terraces reflects steep subduction of smooth seafloor beneath the northern Nicoya Peninsula, and shallow underthrusting of seamounts in the south.