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

Segmented Fore Arc Deformation Along the Nicoya Peninsula Seismic Gap, Costa Rica

Jeffrey S. Marshall1, Eli J. LaFromboise2, Thomas W. Gardner3, and Marino Protti4

1Geological Sciences Department, Cal Poly Pomona University, Pomona CA, 91768, USA 2Geological Sciences Department, California State University, Northridge CA, 91330, USA 3Geosciences Department, Trinity University, San Antonio TX, 78212, USA 4OVSICORI, Universidad Nacional, Heredia 3000, Costa Rica

Abstract 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. Optically Stimulated Luminescence (OSL) dating yields terrace ages consistent with OIS 3-5 sea level high stands (30-120 ka), indicating 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 late Pleistocene marine sediments. Radiocarbon ages for adjacent Holocene terraces (Cabuya surface) indicate recent uplift at 1.0-6.5 m/k.y. On the peninsula’s south-central coastline (Puerto Carrillo to Playa Camaronal) marine fluvial terraces (Carrillo-Camaronal surface) occur at 20- 40 m elevation. Correlations with dated Cobano terraces and Quaternary sea level curves suggest terrace formation during the OIS 5a-e high stands (80-120 ka) and 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 two treads with paleo-shorelines at 10-45 m elevation. Age correlations (as above) suggest terrace formation during the OIS 5a-e high stands (80-120 ka) and 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.

Introduction The Nicoya Peninsula, Costa Rica provides an optimal setting for the study of upper plate deformation along the Middle America convergent margin. Located only 60-80 km inboard of the Middle America Trench (Figs. 1 and 2), this outer fore arc peninsula sits directly above the seismogenic zone within a high potential seismic gap (Fig. 3). The last major earthquake centered beneath the Nicoya peninsula occurred on 5 October 1950 with a moment magnitude of M=7.7 (Protti et al., 2001). This event produced widespread damage and generated 1.0-1.5 m of coseismic uplift along the peninsula's central coast (Marshall and Anderson, 1995). The net pattern of upper plate deformation related to large subduction earthquakes is recorded by emergent Quaternary marine terraces along the peninsula’s coast and uplifted alluvial fill terraces within interior valleys (Hare and Gardner, 1985; Marshall and Anderson, 1995; Gardner et al., 2001; Marshall et al., 2003 and 2005). Here, we present new results from ongoing field studies of marine and fluvial terraces on the Nicoya Peninsula.

Tectonic and Geologic Setting Costa Rica is part of the Central American volcanic arc (Figs.1 and 2), formed by NE subduction of the Cocos plate beneath the Caribbean plate and Panama block at the Middle America Trench (MAT). Plate convergence offshore of Costa Rica is nearly orthogonal and occurs at 9-10 cm/yr. The subducting Cocos plate consists of seafloor produced along both the East Pacific Rise and the Cocos-Nazca spreading center (Fig. 1; Hey, 1977). Hotspot volcanism at the Galapagos Islands generates a rough domain of thickened seafloor that includes the Cocos Ridge and adjacent seamounts. Two major segment boundaries on the subducting Cocos plate intersect the MAT offshore of the Nicoya Peninsula (Fig. 4; Barckhausen et al., 2001). These include an abrupt morphologic break between smooth and rough sea floor domains, and a fracture zone trace that divides crust derived at the East Pacific Rise (EPR) from that formed along the Cocos-Nazca spreading center (CNS-1 and 2). Contrasts in subducting plate thickness, roughness, and dip across these boundaries produce along-strike variations in seismicity, volcanism, and upper plate deformation (e.g., Wells et al., 1988; Gardner et al., 1992; Protti et al., 1995; Fisher et al., 1998; Marshall et al., 2000 and 2003; von Huene et al., 2000). The Nicoya Peninsula spans an emergent segment of the Costa Rican fore arc (Fig. 2), exposing Cretaceous seafloor basement (Nicoya Complex) overlain by an upward-shallowing sequence of late Cretaceous-Quaternary marine sediments (Fig. 3; Lundberg, 1982). Tectonic erosion at the trench has removed a substantial volume of the upper plate, inducing subsidence of the offshore margin wedge (Ranero and von Huene, 2000; Vannuchi et al., 2001). However, flights of Quaternary marine terraces along the Nicoya coast and uplifted alluvium within interior valleys attest to active uplift 60-80 km inboard of the trench (Hare and Gardner, 1985; Marshall, 1991; Marshall and Anderson, 1995; Gardner et al., 2001). This uplift suggests net crustal shortening beneath the Nicoya peninsula associated with repeated slip along the plate interface. The Nicoya segment of the MAT produces frequent large earthquakes interspersed by periods of low background seismicity (Protti et al., 2001). Historical large magnitude events (>M 7.0) include the earthquakes of 1826, 1853, 1900, and 1950. The 1950 earthquake (M 7.7) is thought to have ruptured the entire length of the Nicoya Peninsula (Fig. 3). In contrast, the M 6.9 event of 1978 ruptured only 20% of the Nicoya asperity, and is estimated to have released only 6% of the accumulated energy since the 1950 earthquake (Protti et al., 2001). Based on scarce background seismicity, a rapid convergence rate (9-10 cm/yr), and the relative frequency of large earthquakes (every 40-60 years), the Nicoya segment is considered a high potential seismic gap. The limits of the Nicoya seismic gap (Fig. 3) are defined by the aftershock areas of the 1990 and 1992 earthquakes centered in adjacent seismogenic segments SE and NW of the peninsula.

Purpose and Methods The goal of this study is to constrain the lateral extent and elevations of uplifted Quaternary marine and fluvial terraces along the length of the Nicoya seismic gap. At present, uplifted marine terraces and paleo-shorelines have been mapped along three segments of the Nicoya Peninsula’s coastline (Fig. 5): 1) the northern coast (Iguanazul surface), 2) central coast (Carrillo-Camaronal Surface), and 3) the peninsula’s southern tip (Cobano surface). In addition, uplifted and incised alluvial fill terraces have been mapped in the peninsula’s interior (La Mansión surface) The results of this investigation allow us to assess the relationship between Quaternary deformation patterns, morphology and thermal state of the subducting seafloor, and the geometry of the seismogenic zone. The regional extent of the Iguanazul, Carrillo-Camaronal, and Cobano marine terraces (Fig. 5) was determined through field reconnaissance and examination of aerial photographs. We then selected three representative coastal segments for more detailed study (Figs. 6a-c). In the northern and central study areas (Fig 6a-b), the Iguanazul and Carrillo-Camaronal surfaces (10- 40 m elev.) consist of one to two discrete marine terrace treads located between the shoreline and the coastal mountains. In the southern study area (Fig. 6c), the highly dissected Cobano surface (30-220 m elev.) consists of three discrete terrace treads separated by well-defined risers. In all three areas, terraces were mapped and surveyed along a series of nine transects extending inland from the coast (Figs 6a-c). The location of terrace treads, risers, outer, and inner edges were recorded on base maps and the spatial coordinates for these features (map location and elevation) were determined using a Garmen 76S GPS unit and a Sokkia barometric altimeter. Altimeter readings were corrected for barometric pressure drift using a stationary reference barometer at a coastal base station. Based on closure of survey loops, the elevation data are accurate to within +/- 2m. The survey elevations were projected onto straight profile lines using the formula [cos(θ)(northing)] + [sin(θ)(easting)], where θ is the projection line azimuth and northing and easting are GPS coordinates. Topographic profiles of the terrace sequence were then plotted along these nine projection lines (Figs. 7a-c). Terrace ages and uplift rates were initially estimated by correlation with late Cenozoic sea level high stands (Bull, 1985; Lajoie, 1986; Anderson et al., 1999). Inner edge elevations were plotted (Figs. 8a-c) in conjunction with the late Quaternary sea level curve (Imbrie et al., 1984; Chapell and Shackleton, 1986). Assuming a constant uplift rate, terrace inner edges were linked to sea level high stands using a straight line where slope = uplift rate. Multiple scenarios were examined in order to determine a best-fit correlation. Further age constraints were established for the Cobano terrace sequence using Optically Stimulated Luminescence (OSL) dating. Quartz sands were collected from terrace deposits at multiple locations (shown on Fig. 6c), and the results, combined with sea level curve correlations, allow for age extrapolation between all three sites. In addition, carbonate beachrock horizons (Fig. 9), exposed along the modern shoreline of the northern study area (Fig. 6a), were sampled for radiocarbon dating by Beta Analytic, Inc. Calibrated ages combined with surveyed elevations allow for estimation of maximum late Holocene uplift rates. Holocene uplift rates for the southern study area (Fig. 6c) were determined in earlier investigations (Marshall, 1991; Marshall and Anderson, 1995; Gardner et al, 2001). These short-term uplift rates for both areas were compared with longer-term rates estimated in this study for the late Pleistocene terraces.

Results The late Quaternary Iguanazul surface (Fig. 5) extends along on the northern coast of the Nicoya Peninsula between Tamarindo (south of Cabo Velas) and Nosara (north of Punta Guiones). This 2-4 km wide, low relief geomorphic surface (Fig. 11) encompasses at least two discrete wave-cut treads (Fig. 6a) that preserve paleo-shorelines at 10-45 m elevation (Fig. 7a). Correlation of these terraces with the late Pleistocene sea level high stands during marine oxygen isotope stages 5a-e (80-120 ka) indicate net uplift rates of 0.1-0.2 m/k.y. (Fig. 8a). Five samples of Holocene beach rock horizons along the upper swash face of Playa Negra, Playa Pochotes, and Playa Socorro (Figs. 6a and 9), yielded calibrated radiocarbon ages ranging between 760 (+130/-90) ybp and 1880 (+100/-70) ybp. These resistant horizons of carbonate- cemented beach sand generally form within the upper half of the intertidal zone (Marshall, 1991). The radiocarbon ages and modern position of these samples on the upper beach face are consistent with net Holocene uplift at < 0.5 m/k.y.. The Carrillo-Camaronal surface (Fig. 5) on the peninsula’s central coastline (Puerto Carrillo to Playa Camaronal) encompasses two marine terraces (Fig. 6b) and related alluvial fill terraces at 20-40 m elevation (Fig. 7b). Sea level curve correlations suggest terrace formation during the OIS 5a-e high stands (80-120 ka) and net uplift rates of 0.2-0.3 m/k.y. In contrast to the other two sites, the Cobano surface at the Nicoya Peninsula's southern tip (Fig. 5) includes three distinct Pleistocene terrace treads (Figs. 6c and 12-14) separated by well- defined risers at 30-220 m elevation (Fig. 7c). Radiocarbon dating (AMS) of marine shells on the lowest terrace (Qt4, location 6 km NE of Fig. 6c) yielded calibrated ages of ~48 ka, linking this terrace tread with the OIS stage 3 sea level high stand (Fig. 8c). Preliminary OSL dating of Qt3 and Qt1 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., an order of magnitude greater than the other two sites. A NW-striking thrust fault (Delicias fault) offsets Qt1 by 40 m, thrusting Cretaceous basalt over late Pleistocene marine sediments. A low-lying Holocene terrace referred to as the Cabuya surface (Marshall, 1991) runs along the seaward edge of the Pleistocene Cobano surface (Figs. 6b and 12). Holocene uplift rates determined by radiocarbon dating of uplifted beach deposits on the Cabuya surface range from 1.0-6.5 m/ka (Marshall, 1991; Marshall and Anderson, 1995; Gardner et al., 2001). As in the case of longer-term uplift rates for the Pleistocene terraces, these Holocene rates are an order of magnitude greater than those indicated by dated beachrock deposits along the northern Nicoya coast.

Discussion and Conclusions The observed differences in Quaternary deformation along the Nicoya Peninsula coincide with three contrasting domains of subducting seafloor offshore (EPR, CNS-1, CNS-2). While the northern and central coasts lie onshore of the EPR and CNS-1 "smooth domain", the peninsula’s southern tip sits inboard of subducting seamounts of the CNS-2 "rough domain" (Fig. 4). Overall, 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. Plate motion and historical seismicity data suggest recurrence intervals of 40-60 years for large Nicoya earthquakes (Protti et al., 2001). The last event in 1950 (M = 7.7) generated 1.0-1.5 m of coseismic uplift along the Nicoya Peninsula’s central coast (Marshall and Anderson, 1995). If we assume that this is permanent deformation that recurs every 50 years, then we would expect an unlikely net uplift rate of 20-30 m/ka (two orders of magnitude greater than that indicated by field data for the Iguanazul surface!). Therefore, while frequent Nicoya earthquakes may produce meter-scale coseismic uplift, a significant fraction of this is likely recovered through interseismic subsidence. The net result is gradual coastal uplift during the Quaternary at the observed rates.

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Figure 1. Digital elevation model of Central America and adjacent sea floor of the Cocos, Nazca, and Caribbean plates (IFM-GEOMAR). The Middle America Trench marks the convergent margin where the Cocos Plate subducts northeastward at 8-10 cm/yr beneath the Caribbean Plate and Central American volcanic arc. This image reveals a sharp contrast on the Cocos Plate between smooth sea floor formed at the East Pacific Rise (EPR) and rough, hotspot- thickened sea floor formed along the Cocos-Nazca spreading center (CNS). This boundary intersects the Middle America Trench offshore of Costa Rica's Nicoya Peninsula. The rectangle outlines the area of Costa Rica shown in Fig. 2.

Figure 2. Digital elevation model of southern Central America, showing NASA-SRTM topography for Nicaragua and Costa Rica linked to offshore IFM-GEOMAR multi-beam bathymetry (courtesy of C. Ranero, ICM-CSIC, Barcelona). This image reveals the relationship between the morphology of the subducting Cocos Plate and the morphotectonic structure of the overriding fore arc and volcanic arc of the Caribbean Plate and Panama block.

Figure 3. Geologic map of the Nicoya Peninsula showing boundaries of the Nicoya seismic gap (orange dashed lines), epicenters of large subduction earthquakes (red circles), and aftershock zones of the 1950, 1978, 1990, and 1992 events (red dashed lines). The limits of the 1990 and 1992 ruptures coincide with the edges of the seismic gap. The last major rupture of the Nicoya segment (1950; M=7.7) produced 0.5-1.5 m of coseismic uplift along the central Nicoya coast (Marshall and Anderson, 1995). Most of this has been recovered by intersiesmic subsidence over the past six decades.

Figure 4. Digital elevation model of Costa Rica (SRTM) linked to offshore bathymetric data (courtesy of C.J. Petersen, IFM-GEOMAR). Two segment boundaries on the subducting Cocos Plate (Barckhausen et al., 2001) intersect the margin offshore of the Nicoya Peninsula: 1) a morphologic break between smooth and rough sea floor domains (thin dashed line); and 2) a fracture zone trace (thick dashed line) that divides crust formed at the East Pacific Rise (EPR) from that formed at the Cocos-Nazca spreading center (CNS-1 and CNS-2). The southern edge of the Nicoya Peninsula coincides with the Central Costa Rica deformed belt (red dashed line), a diffuse transpressional fault zone between the Caribbean Plate and Panama block inboard of the subducting Cocos Ridge (Marshall et al., 2000). Rectangle outlines the Nicoya Peninsula as shown in Fig. 5.

Figure 5. Upper Image: Digital elevation model of the Nicoya Peninsula (NASA-SRTM) showing areas of uplifted marine and fluvial terraces within the late Pleistocene Iguanazul, Carrillo-Camaronal, Cobano, and La Mansion geomorphic surfaces (red areas); and the Holocene Garza and Cabuya surfaces (yellow areas). Rectangles outline the three coastal study areas shown in Figs. 6a-c. Lower Image: Summary diagram showing uplifted terrace elevations, age data, and uplift rates within each coastal study area. Numbers in circles indicate specific study sites for Pleistocene and Holocene terraces. Terrace ages based on sea level curve correlations and isotopic dates (OSL and 14C, as indicated). Differences in uplift rates for each study area reflect morphologic and thermal variations in the subducting Cocos plate seafloor across the EPR, CNS-1, and CNS-2 segment boundaries.

Fig. 6a

Fig. 6b

Fig. 6c

Figures 6a-c. Maps of late Pleistocene and Holocene marine terrace treads (Qt 1-3 and Qth) in each of the three study areas (see Fig. 5 for locations). Transect lines (e.g., A-A') indicate the locations of topographic profiles surveyed across the terrace sequences (Figs. 7a-c). Colored boxes show isotopic age data (OSL and 14C) from specific sites (white circles). Differences in uplift rtes for each study area reflect variations in the subducting seafloor and seismogenic zone structure across the EPR, CNS-1, and CNS-2 segment boundaries.

Fig 7a

Fig 7b

Fig 7c

Figures 7a-c. Topographic profiles surveyed across marine terrace treads along the transect lines shown in Figs. 6a-c. Sharp breaks in slope correspond with terrace inner edges (paleo- shorelines). Elevation data and map locations for survey points (black dots) were determined by differential barometric altimetry and GPS. These data were projected onto transect lines using the formula [cos(θ)(northing)] + [sin(θ)(easting)], where θ is the projection azimuth and northing and easting are GPS coordinates.

Fig 8a

Fig 8b

Fig 8c

Figures 8a-c. Sea level correlation diagrams for terrace sequences in each of the three field areas. Terrace inner edge elevations (at left) are linked to eustatic sea level high stands with colored lines that represent uplift pathways (slope = uplift rate). The preferred correlation scenario (solid lines) and an alternative scenario (dashed lines) are shown. The indicated range of uplift rates incorporates both scenarios. Sea level curve based on marine oxygen isotope records (Imbrie et al., 1984; Chappell and Shackleton, 1986).