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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. |