NSF MARGINS Program, Central America Focus-Site Workshop, Heredia, Costa Rica, 2007.

 

 

UPPER PLATE FAULTING AND UPLIFT ALONG THE NICOYA PENINSULA

SEISMIC GAP, NORTHERN COSTA RICA FORE ARC

 

MARSHALL, Jeffrey S., Geological Sciences Dept., Cal Poly Pomona Univ., Pomona, CA, 91768, USA

LAFROMBOISE, Eli J., Geological Sciences Dept., California State Univ., Northridge, CA, 91330, USA

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

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 directly above 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 uplift at the coast. Net Quaternary deformation on the Nicoya Peninsula is recorded by emergent marine terraces along 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).

 

Recent fieldwork along the Morote Valley, within the Nicoya Peninsula’s interior, reveals geomorphic evidence of active deformation along the NW-striking Morote fault (e.g., uplifted and incised alluvial fill, irregular drainage networks, active stream piracy, and abrupt mountain front facets). Uplifted Pleistocene alluvium with a deep red soil horizon (La Mansion surface) is incised 5-40 m by local stream networks. At one site, uplifted fluvial gravels overlie a gray paleosol formed on fine-grain wetland deposits, 10 m above local base level. In some areas, the surface gradient of the paleo-valley floor 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 subduction zone. Preliminary seismicity data (OVSICORI) indicate recent earthquake activity along this trend. A composite focal mechanism shows dextral slip for the NW-striking nodal plane (Hansen et al, 2006), consistent with NW escape of a forearc sliver. The Morote fault underlies several large towns and may represent a significant seismic hazard. Ongoing field studies aim to establish better constraints on fault kinematics and deformation rates along this trend.

 

At the Nicoya 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 Pleistocene terrace deposits. Radiocarbon ages for adjacent Holocene terraces (Cabuya surface) indicate recent uplift at 1.5-3.5 m/k.y.

 

Recent mapping and surveying, along the peninsula’s southern coastline (Puerto Carrillo to Playa Camaronal) reveals a set of marine terraces (Camaronal surface) and associated fluvial straths (Río Ora) at 20-35 m elevation. Preliminary correlations with dated Cobano terraces, and with a global sea level curve, 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. Apparent terrace offset across the Río Andamojo (10 m, SE side up) suggests Quaternary slip along a local fault.

 

Regional variations in net Quaternary uplift and upper-plate fault kinematics on the Nicoya Peninsula may be controlled by such factors as convergence rate and obliquity, contrasts in subducting plate roughness, and characteristics of the underlying seismogenic zone (e.g., percent locking, asperity geometry, dip angle, and depth of up-dip and down-dip limits).