FOREARC DEFORMATION INFLUENCED BY SUBDUCTING PLATE MORPHOLOGY AND UPPER PLATE DISCONTINUITY, PACIFIC COAST, COSTA RICA

Along the southern Middle America margin, the Cocos plate subducts NE beneath the Caribbean plate at >9 cm/yr. Variations in Quaternary uplift along the Costa Rican Pacific coast relate primarily to morphologic differences in the down-going plate (roughness, thickness, slab dip). However, additional variations may reflect the added influence of structural and lithologic anisotropies in the overriding plate.

The E-W trending Murciélago fault zone (MFZ) on the Santa Elena Peninsula marks a major upper plate discontinuity along the northern Costa Rican forearc. This fault separates Cretaceous-Paleogene forearc sediments of the Sandino Basin to the north, from late Cretaceous ophiolitic rocks of the Nicoya terrane to the south. This major discontinuity may have significant impacts on forearc response to subduction.

On Punta Descartes, north of MFZ, elevated Holocene shore platforms, beach ridges, and stream terraces record active coastal emergence. A wavecut notch occurs on rocky headlands at 12-18 m elv. and valley-fill terraces within coastal embayments extend 2 km inland reaching 15-20 m elv. Streams draining to Bahía de Salinas are incised 3-5 m exposing uplifted shallow bay to intertidal muds overlain by beach ridge sands and fluvial gravels. Marine shells yield 14C ages indicating uplift rates of 2.0-3.5 m/ka.

On northern Nicoya Peninsula, south of MFZ, a pervasive Quaternary marine terrace (here named Iguanazul surface) extends from Tamarindo to Nosara with tread elevations of 12-30 m. This terrace may be a composite surface of overlapping wavecut platforms formed during multiple late Pleistocene sea level maxima (OIS 5-11). Dated Holocene shore deposits indicate recent uplift at <0.5 m/ka. On the southern Nicoya Peninsula, inboard of subducting seamounts, the Pleistocene Cobano surface consists of at least five distinct marine terrace treads separated by well-defined risers at 40-220 m elv. Holocene uplift rates in this area range from 3.0-6.5 m/ka.

The observed variations in Quaternary uplift patterns along the northern Costa Rican forearc may be controlled both by characteristics of the subducting seafloor (e.g., rough vs. smooth), as well as pre-existing structural discontinuities within the overriding fore arc crust (e.g., MFZ).