Eos, Transactions, American Geophysical Union, v. 88, Fall Meeting Supplement, Abs T53A-1121, 2007.

Segmented Forearc Deformation Along the Nicoya Peninsula Seismic Gap,

Costa Rica

MARSHALL, J.S., Geological Sciences Dept, Cal Poly University, Pomona, CA, 91768

LAFROMBOISE, E.J., Geological Sciences Dept, Calif State Univ, Northridge, CA, 91330

GARDNER, Thomas W., Geosciences Dept, Trinity Univ, San Antonio, TX, 78212

PROTTI, J. Marino, OVSICORI, Universidad Nacional, Heredia, Costa Rica

The Nicoya Peninsula, Costa Rica deforms in response to rapid NE subduction of the Cocos plate at the Middle America Trench (9-10 cm/yr). This emergent outer fore arc peninsula lies 60-80 km inboard of the trench and coincides with a locked segment of the seismogenic zone. The Nicoya segment is a high-potential seismic gap, with a slip deficit of >5 m since the last major earthquake (M7.7, 1950). That event produced widespread damage and up to 1.0 m of coseismic coastal uplift. Net Quaternary deformation on the Nicoya Peninsula is recorded by emergent marine terraces at the coast, and by uplifted alluvial fill within interior valleys. Recent field mapping, surveying, and isotopic dating provide new constraints on deformation patterns and upper-plate faulting. Local uplift anomalies reveal upper plate faults that may accommodate a significant fraction of fore arc deformation (shortening and/or lateral sliver transport).

At the peninsula’s southern tip (Cabo Blanco), a prominent uplifted marine erosion surface (Cobano surface) encompasses at least three distinct Pleistocene terraces at 30-220 m elevation. Preliminary OSL dating yields terrace ages consistent with OIS 3-5 sea level high stands (30-120 ka), indicating net uplift at 1.0-2.0 m/k.y. A NW-striking thrust fault (Delicias fault) offsets the upper terrace by 40 m, thrusting Cretaceous basalt over Plio-Pleistocene marine sediments. Radiocarbon ages for adjacent Holocene terraces (Cabuya surface) indicate recent uplift at 1.5-3.5 m/k.y. On the peninsula’s south-central coastline (Puerto Carrillo to Playa Camaronal) marine terraces and related fluvial straths (Carrillo-Camaronal surface) occur at 20-40 m elevation. Correlations with dated Cobano terraces and Quaternary sea level curves suggest terrace formation between 80-215 ka (OIS 5-7) and net uplift rates of 0.2-0.3 m/k.y. Along the northern Nicoya coast (Tamarindo to Nosara), a 3 km wide wave-cut surface (Iguanazul surface) includes three treads with paleo-shorelines at 10-45 m elevation. Age correlations (as above) suggest terrace formation between 80-215 ka (OIS 5-7) and net uplift rates of 0.1-0.2 m/k.y. Radiocarbon ages for Holocene beachrock horizons are consistent with recent uplift at <0.5 m/k.y.

Within the interior Morote Valley, geomorphic anomalies indicate deformation along the NW-striking Morote fault (e.g., uplifted and incised alluvial fill, irregular drainage networks, stream piracy, and abrupt mountain facets). Uplifted Pleistocene alluvium (La Mansion surface) is incised 5-40 m by stream networks. At one site, fluvial gravels overlie a paleosol on fine-grain wetland deposits, 10 m above local base level. In some areas, the paleo-valley gradient is opposite that of modern incised streams, indicating capture and drainage reversal. The Morote fault forms a prominent NW-trending lineament oriented sub-parallel to the plate margin. A composite focal mechanism for shallow seismicity along this trend (CRSEIZE) shows dextral slip for a NW-striking nodal plane, consistent with NW fore arc sliver escape.

The observed differences in Quaternary deformation along the Nicoya Peninsula coincide with three contrasting domains of subducting seafloor offshore (EPR, CNS-1, CNS-2). Upper-plate segmentation may reflect along-strike variations in subducting plate roughness, coupling, and seismogenic zone geometry (e.g., dip angle, depth of up-dip and down-dip limits). This segmentation may have implications for understanding the rupture behavior and earthquake hazards along the Nicoya seismic gap.