Quaternary Uplift and Seismic Cycle Deformation, Penı´Nsula De Nicoya, Costa Rica

Quaternary uplift and seismic cycle deformation, Penı´nsula de Nicoya, Costa Rica

Jeffrey S. Marshall* Department of Earth Sciences and Institute of Tectonics, University of California, Robert S. Anderson } Santa Cruz, California 95064

ABSTRACT arcward tilting of the peninsula at an an- ing Cocos plate (Gardner et al., 1987; Protti, .(gular rotation rate of between 0.01؇ and 1991 Differing rates and styles of Quaternary 0.02؇/k.y. Considerable attention has been focused deformation along the Costa Rican fore arc Oral histories describing the M 7.7 on the rapid fore-arc uplift caused by the reflect segmentation of the trench corre- Nicoya subduction earthquake of 5 October subduction of the buoyant Cocos Ridge be- sponding with three contrasting domains of 1950 provide evidence of seismic cycle de- neath the Penıńsula de Osa (Gardner et al., subducting sea floor offshore. Rapid upward formation along the peninsula’s southwest- 1992; Wells et al., 1988; Corrigan et al., 1990). flexure of the southern fore-arc segment re- ern coast between Puerto Carrillo and However, relatively little is known about the sults from the subduction of the buoyant Nosara. Interviews with 48 residents show rates and mechanisms of fore-arc deforma- Cocos Ridge, whereas moderate deforma- that coseismic uplift of at least1maffected tion produced by the normal subduction of tion along the central fore-arc segment re- this coastline, and that a significant frac- denser and smoother sea floor beneath the flects the subduction of buoyant seamounts. tion of this uplift has subsequently been re- Penıńsula de Nicoya. Because the buoyant In contrast, the northern Costa Rican fore versed during four decades of gradual sub- influence of the Cocos Ridge becomes min- arc deforms in response to the subduction sidence. The coseismic deformation pattern, imal near the Penıńsula de Nicoya (Gardner of relatively dense sea floor devoid of ma- estimated from a uniform slip dislocation et al., 1992), additional mechanisms must be jor bathymetric anomalies. Quaternary geo- model for the 1950 earthquake, is consistent invoked to explain the Quaternary uplift ob- morphic evidence and earthquake oral his- with both geomorphic and oral history ev- served within the northern Costa Rican fore tories from the Penıńsula de Nicoya, within idence. These observations suggest that arc (Marshall, 1991). the northern Costa Rican fore arc, demon- seismic cycle deformation functions as an Our goal is to examine Quaternary verti- strate arcward tilting of the fore-arc crust, important mechanism of vertical tecto- cal tectonism on the Penıńsula de Nicoya with discrete uplift events occurring during nism within the Costa Rican fore arc. Arc- and to explore the role of seismic cycle de- large subduction earthquakes. ward rotation and doming of the Penıń- formation as a mechanism of net uplift Uplift rates calculated from the late Hol- sula de Nicoya during the Quaternary may within the Costa Rican fore arc. First, we ocene Cabuya terrace, along the peninsula’s reflect repeated cycles of sudden coseis- introduce the tectonic setting, stratigraphy, trench-perpendicular southeastern coast, mic deformation followed by gradual post- and structure of the Penıńsula de Nicoya. decrease systematically toward the arc, seismic and/or interseismic crustal move- We then discuss the Quaternary uplift his- from 4.5 m/k.y. at Cabuya to 1.7 m/k.y. at ment. tory and document the uplift rates of marine Montezuma, 8 km to the northeast. An up- terraces along the peninsula’s southeastern lifted carbonate beachrock horizon (radio- INTRODUCTION shoreline. Finally, we apply oral history in- carbon age: 4500–5200 yr B.P.), correlated terviews and dislocation modeling to inves- between Cabuya and Montezuma, is tilted The character of the lithosphere subduct- tigate seismic cycle deformation associated -0.1؇ downward toward the arc. Although ing along convergent margins largely deter- with the M 7.7 Nicoya subduction earth Quaternary uplift is evident as far arcward mines the rates and geometry of tectonism quake of 1950. as Tambor, 10 km northeast of Montezuma, within the arc and fore-arc regions of the The Penıńsula de Nicoya represents an uplifted terraces are absent between Tam- overriding plate (Cross and Pilger, 1982; excellent location for this type of study for bor and the Golfo de Nicoya. A submerged Jarrard, 1986). An excellent example of this several reasons. First, the tectonic frame- archaeological site (radiocarbon age: 2500 relationship can be observed along the Pa- work and Cenozoic history of the Costa yr B.P.), located along the Golfo de Nicoya cific coast of Costa Rica, where the struc- Rican Pacific margin are reasonably well un- coast 30 km northeast of Montezuma, dem- turally complex southern Cocos plate sub- derstood. Second, the geometry of the Pe- onstrates late Holocene subsidence of 0.5 ducts beneath the Caribbean plate and the nıńsula de Nicoya, with shorelines oriented m/k.y. These data indicate net late Holocene Panama microplate along the Middle Amer- both parallel and perpendicular to the Mid- ica Trench (Fig. 1). Within Ͻ350 km along dle America Trench, is ideal for coastal neo- the strike of the trench, stretching from the tectonic analysis. Third, the proximity of the Penıńsula de Nicoya to the similarly ori- *Present address: Geosciences Department, Penıńsula de Nicoya to the Penıńsula de Pennsylvania State University, University Park, Osa, pronounced changes occur in the ba- ented Penıńsula de Osa allows comparison Pennsylvania 16802. thymetry, age, and buoyancy of the subduct- of fore-arc deformation driven by the sub-

GSA Bulletin; April 1995; v. 107; no. 4; p. 463–473; 7 ﬁgures; 1 table.

463 MARSHALL AND ANDERSON

Variations in the Subducting Cocos Plate

Three distinct domains of subducting sea floor have been recognized along the Costa Rican Pacific margin based on contrasts in age, buoyancy, and bathymetry within the Cocos plate (Protti, 1991). The sea-floor thrusting beneath the Penıńsula de Nicoya, along the northern Costa Rican fore arc, consists of typical, relatively dense and smooth oceanic lithosphere created at the East Pacific Rise during the late Oligocene (Hey, 1977; Klitgord and Mammerickx, 1982). In contrast, the sea floor subducting just southeast of the Penıńsula de Nicoya, beneath the central Costa Rican fore arc, is characterized by buoyant seamounts created at the Cocos-Nazca boundary during the early Miocene (Hey, 1977; Lonsdale and Klitgord, 1978). Farther to the southeast, the sea floor subducting beneath the Penıń- sula de Osa, within the southern Costa Rican fore arc, consists of the buoyant Cocos Ridge generated along the Cocos- Nazca boundary during the middle Miocene (Hey, 1977; Lonsdale and Klitgord, 1978). These pronounced contrasts within the subducting Cocos plate produce significant variations in Wadati-Benioff zone geometry Figure 1. Tectonic setting of Costa Rica (modified from Protti, 1991). The Cocos, Ca- (Guëndel, 1986), seismic potential (Protti, ribbean, and Panama plates (COCOS, CARIB, and PAN) are outlined by bounding fault 1991), arc volcanism (Malavassi, 1991), zones: ENFZ, East Nicoya Fracture Zone; WCPB, Western Caribbean-Panama Boundary; trench morphology (von Huene et al., 1995), NPDB, North Panama Deformed Belt; SPDB, South Panama Deformed Belt; PFZ, Panama and fore-arc deformation (Gardner et al., Fracture Zone; and the Middle America Trench. Convergence rates, shown with direction 1992; Wells et al., 1988). arrows, are from DeMets et al. (1990). The three Costa Rican fore-arc segments are bounded by dashed lines and indicated by letters within circles: N, northern; C, central; and Variations in Fore-Arc Deformation S, southern. Triangles represent Quaternary volcanoes. Bathymetric contours shown in meters. Major Pacific Ocean peninsulas, Nicoya and Osa, are shown. Since about 1 Ma, subduction of the buoyant Cocos Ridge beneath the Penıńsula de duction of two highly contrasting lithos- the Cocos plate subducts northeastward be- Osa has resulted in rapid upward deflection pheric domains. Fourth, the Penıńsula de neath the Caribbean plate and the Panama of the overriding crust within the southern Nicoya lies along a strongly coupled seg- microplate (Fig. 1). Recent studies of re- Costa Rican fore arc (Corrigan et al., 1990; ment of the Middle America Trench that is gional geology (Astorga et al., 1991), seis- Gardner et al., 1992). Uplift rates deter- subject to repeated large subduction earth- micity (Guëndel and Pacheco, 1992; Goes mined from late Quaternary stratigraphy on quakes (M Ն 7.0) and has been designated et al., 1993; Fan et al., 1993), and fault kin- the Penıńsula de Osa range between 6.5 a high-probability seismic gap (Guëndel, ematics (Marshall et al., 1993; Fisher et al., m/k.y. and 2.1 m/k.y., decreasing systemati- 1986; Nishenko, 1989). The recognition of 1994) within the overriding plates suggest cally in an arcward direction across the pe- seismic cycle deformation within the Nicoya that the Caribbean-Panama boundary tra- ninsula (Pinter, 1988). The distribution of gap has important implications for our un- verses central Costa Rica and intersects the these uplift rates in relation to local faulting derstanding of both the processes of fore- Pacific coast just southeast of the Penıńsula implies that the Penıńsula de Osa is com- arc deformation, as well as the repeat times de Nicoya. This boundary separates the prised of arcward tilting blocks with angular of potentially damaging earthquakes along northern Costa Rican fore arc, which be- rotation rates varying between 0.03Њ and the Costa Rican Pacific coast. longs to the Caribbean plate, from the cen- 0.06Њ/k.y. (Gardner et al., 1992). tral and southern Costa Rican fore-arc Upward flexure of the overriding crust di- TECTONIC FRAMEWORK regions, which belong to the Panama micro- minishes with trench-parallel distance from plate. Although this upper-plate boundary the axis of the subducting Cocos Ridge The Costa Rican Fore Arc significantly affects tectonism landward of (Corrigan et al., 1990; Gardner et al., 1992). the fore arc, deformation within the fore arc Although the ridge probably exerts some The Costa Rican fore arc extends along itself is controlled principally by the charac- tectonic influence on the central Costa the southern Middle America Trench where ter of the sea floor subducting offshore. Rican fore arc, geomorphic studies indicate

464 Geological Society of America Bulletin, April 1995 UPLIFT AND DEFORMATION, PENI´NSULA DE NICOYA, COSTA RICA

ented fore-arc peninsula and basin directly astride the subducting Cocos Ridge. The basement of the Penıńsula de Nicoya consists of the Jurassic to Cretaceous Nicoya Complex, an intensely deformed oceanic sequence composed chiefly of pillow basalts, mafic intrusive rocks, and pelagic sediments (de Boer, 1979; Kuijpers, 1980; Lundberg, 1982; Baumgartner et al., 1984). Along the margins of the peninsula, a sequence of Up- per Cretaceous to Quaternary marine sediments unconformably overlies the Nicoya Complex (Lundberg, 1982). An up-section decrease in depositional depth and structural complexity shows progressive uplift and deformation of the Penıńsula de Nicoya since the beginning of subduction in the Late Cretaceous (Lundberg, 1982). The overall structure of the Penıńsula de Nicoya consists of a broad anticlinal dome that crests along the peninsula’s central mountains and flattens toward its northwestern and southeastern coastlines (Dengo, 1962; Kuijpers, 1980). Trench-parallel normal faults of Miocene age or younger may reflect extension resulting from arching along the Nicoya dome axis (Kuijpers, 1980). Geomorphic analyses of drainage basins and remnant erosional sur- faces within the central mountains demon- strate continued doming of the peninsula Figure 2. Map of the Penıńsula de Nicoya and adjacent Golfo de Nicoya–Rıó Tempisque during the Quaternary (Hare and Gardner, basin. Mountains, major rivers, and principal towns are indicated. Dark shading repre- 1985). The Golfo de Nicoya–Rıó Tempisque sents elevations above 200 m. basin most likely represents a synform ge- netically related to the Nicoya dome (Kuij- pers, 1980). This antiform-synform pair is an that this fore-arc segment deforms princi- Nicoya: 2 m/k.y. at Montezuma on the emergent portion of the fore-arc structural pally in response to the subduction of the southeastern coastline (Battistini and Ber- high and adjacent basin that extend along buoyant seamounts directly offshore (Wells goeing, 1983; Mora, 1985), and 1.6 m/k.y. at the entire southern Middle America et al., 1988). Uplift rates determined from Tamarindo near the peninsula’s northwest- Trench, between Costa Rica and southern late Quaternary fluvial terraces along the ern tip (Gardner et al., 1992). Mexico (Lundberg, 1982). central Costa Rican coast range between 0.5 and 3.0 m/k.y. (Drake, 1989). QUATERNARY UPLIFT HISTORY The flexural uplift model of Gardner et al. STRATIGRAPHY AND STRUCTURE (1992) shows that the Cocos Ridge exerts The Cobano and Cabuya Terraces minimal tectonic influence within the north- The Penıńsula de Nicoya (Fig. 2) covers ern Costa Rican fore arc. The Penıńsula de 2400 km2 of the northern Costa Rican fore Abundant geomorphic evidence from the Nicoya therefore deforms principally in re- arc and lies parallel to both the Middle Penıńsula de Nicoya shows continued verti- sponse to subduction of relatively dense sea America Trench and the northern Costa cal tectonism in the northern Costa Rican floor that is devoid of major bathymetric Rican volcanic arc (Cordillera de Guane- fore arc during the Quaternary. Uplifted anomalies. Although a number of authors caste). The peninsula consists of a rugged marine strandlines record recent emergence have examined Quaternary tectonism on the linear mountain chain, reaching altitudes along the peninsula’s seaward-facing coast- peninsula (Alt et al., 1980; Fischer, 1980; above 900 m, surrounded by a narrow lines, while submerged strandlines show re- Madrigal and Rojas, 1980; Bergoeing et al., coastal piedmont. Between the peninsula cent subsidence along much of the peninsu- 1983; Battistini and Bergoeing, 1983; Hare and the volcanic arc is an elongate fore-arc la’s landward-facing Golfo de Nicoya coast and Gardner, 1985; Mora, 1985), the rates basin containing the Golfo de Nicoya and (Alt et al., 1980; Marshall, 1991). This jux- and mechanisms of deformation are poorly the Rıó Tempisque. This fore-arc morphol- taposition of seaward uplift and landward constrained. Previous to this study, Quater- ogy is analogous to that along the southern subsidence suggests arcward rotation or tilt- nary uplift rates have been reported for only Costa Rican coast, where the Penıńsula de ing of the peninsula, in a manner similar to two sites (Fig. 2) on the Penıńsula de Osa and Golfo Dulce form a similarly ori- that reported for the Penıńsula de Osa.

Geological Society of America Bulletin, April 1995 465 MARSHALL AND ANDERSON

Cabuya terrace is cut into the basalts of the Nicoya Complex and the interbedded sand- stones and mudstones of the Cretaceous to Eocene Cabo Blanco Formation (Lundberg, 1982; Mora, 1985). This surface is blanketed beneath a 1- to 3-m-thick prograding beach ridge sequence composed of unconsolidated well-sorted sands and gravels containing abundant bivalves, gastropods, and corals (Calvo, 1983; Chinchilla, 1983; Anderson et al., 1989). Radiocarbon dating of these fossils (as discussed below) indicates a minimum late Holocene age for the Cabuya terrace of ca. 5.2 ka (Marshall et al., 1990; Marshall, 1991).

Uplift Rate Calculations

Nine topographic proﬁles were surveyed across the Cabuya terrace, perpendicular to the modern shoreline, and datable samples were collected at selected locations (Fig. 3). Minimum uplift rates were determined for sample localities using four basic parameters: the modern elevation of the sample, the original elevation of the sample at deposition, the paleo–sea level at the time of deposition, and the calibrated radiocarbon age of the sample. Uplift rates were calculated using the following expression:

Uplift Rate (m/ka) ϭ {[Modern Elevation(m)]Ϫ [Depositional Elevation (m)]ϩ [Paleo-Sea Level (m)]}/Age (ka)

The parameters and the calculated uplift Figure 3. Map of the late Holocene Cabuya marine terrace (shaded area) and the ad- rates for each sample are listed along with jacent late Pleistocene Cobano terrace near the towns of Montezuma and Cabuya (solid margins of uncertainty in Table 1. These val- squares). Topographic contours shown in meters. Streams appear as shaded lines. Low tide ues were determined as follows: limit shown by dotted line. Sample locations indicated by numbers (keyed to Table 1 and (1) Modern elevation. Present sample ele- Figs. 4 and 5). Circles represent shell samples, and squares represent beachrock. MAT, vations were determined by hand level sur- Middle America Trench. vey relative to the elevation of highest high tide, visible as the highest recent debris accumulation along the modern shoreline. Two prominent uplifted Quaternary ma- to late Pleistocene, led to emergence of These measurements are considered accu- rine terraces occur along the southeastern the Montezuma Formation and erosion of rate to the nearest 0.2 m. Tidal ranges were shore of the Penı´nsula de Nicoya between the Cobano surface during the maximum determined based on the tide data of Cabo Blanco and Tambor (Fig. 3). The up- late Pleistocene eustatic sea-level high- Borbon (1989). per surface, referred to as the Cobano ter- stand (isotope stage 5e) at 125 ka (Mora, (2) Depositional elevation. The deposi- race, consists of a deeply incised coastal 1985). tional elevations for shell samples within un- mesa, averaging 180 m in elevation, lying The lower surface, which we name the consolidated beach sediments were deter- between the interior mountains and the Cabuya terrace, forms the focus of this mined by comparison with similar facies abandoned Cobano sea-cliff (Hare and study. This terrace consists of a relatively within the modern beach environment. The Gardner, 1985; Mora, 1985). This terrace narrow (Ͻ1 km), low-lying (Ͻ20 m eleva- margins of uncertainty reﬂect the approxi- is cut into the Montezuma Formation, a tion) wave-cut platform located along the mate elevation ranges for the appropriate Miocene to Pleistocene transgressive shal- seaward edge of the Cobano terrace, lying facies. Beachrock samples were assigned an low marine sandstone (Lundberg, 1982; between the abandoned Cobano sea cliff original depositional elevation equivalent to Mora, 1985). Uplift, beginning in the mid- and the modern shoreline (Fig. 3). The high neap tide (Rodman and Snead, 1982).

466 Geological Society of America Bulletin, April 1995 UPLIFT AND DEFORMATION, PENI´NSULA DE NICOYA, COSTA RICA

TABLE 1. RADIOCARBON AGES, ELEVATIONS, AND UPLIFT RATES FOR 11 MARINE SHELL AND BEACHROCK crease in Holocene eustatic sea-level rise. SAMPLES COLLECTED ALONG THE CABUYA TERRACE The average uplift rates calculated from

Sample Measured Calibrated Modern Depositional Estimated Calculated these three deposits range between 1.7 number radiocarbon age radiocarbon elevation elevation paleo–mean uplift rate m/k.y. and 4.0 m/k.y. Although there is con- (yr B.P.) (yr B.P.) (m above MSL) (m above MSL) sea level (m/k.y.) (Beta Analytic Inc.) (m below MSL) siderable variation in these uplift rates (a point we address later), they are all less than Shell samples the rate of eustatic sea-level rise prior to 6.5 1 400 Ϯ 60 490 Ϯ 60 3.7 Ϯ 0.2 1.5 Ϯ 0.5 0 4.5 Ϯ 2.2 2 3800 Ϯ 60 4190 Ϯ 100 14.0 Ϯ 0.2 0. Ϯ 2.0 2.0 Ϯ 1.0 3.9 Ϯ 0.8 ka and greater than the rate of eustatic sea- 3 2260 Ϯ 70 2330 Ϯ 70 9.8 Ϯ 0.2 0 Ϯ 2.0 0.5 Ϯ 1.0 4.4 Ϯ 1.6 4 4100 Ϯ 100 4690 Ϯ 220 16.5 Ϯ 0.2 1.5 Ϯ 0.5 2.5 Ϯ 1.0 3.7 Ϯ 0.6 level rise since 5.5 ka. 5 1530 Ϯ 60 1410 Ϯ 60 5.8 Ϯ 0.2 0 Ϯ 2.0 0 Ϯ 1.0 4.1 Ϯ 2.5 Assuming that the average uplift rates of 6 780 Ϯ 90 700 Ϯ 90 4.0 Ϯ 0.2 1.5 Ϯ 0.5 0 3.6 Ϯ 1.6 the beachrock sites have remained relatively Beachrock samples constant during the late Holocene, it can be 7 4390 Ϯ 80 4935 Ϯ 125 17.4 Ϯ 0.2 1.0 Ϯ 0.5 3.5 Ϯ 1.0 4.0 Ϯ 0.5 8 4470 Ϯ 80 5150 Ϯ 180 15.1 Ϯ 0.2 1.0 Ϯ 0.5 3.5 Ϯ 1.0 3.4 Ϯ 0.4 argued that the sudden decrease in the rate 9 Undated (4852 Ϯ 140) 9.0 Ϯ 0.2 1.0 Ϯ 0.5 3.5 Ϯ 1.0 (2.4 Ϯ 0.4) 10 3980 Ϯ 80 4470 Ϯ 110 6.3 Ϯ 0.2 1.0 Ϯ 0.5 2.5 Ϯ 1.0 1.7 Ϯ 0.4 of eustatic sea-level rise between 6.5 and 5.5 11 Undated (4852 Ϯ 140) 4.3 Ϯ 0.2 1.0 Ϯ 0.5 3.5 Ϯ 1.0 (1.4 Ϯ 0.4) ka initiated a local marine regression and relative emergence of the Cabuya terrace. Note: Values and margins of uncertainty were determined as described in text. For undated beachrock samples (9 and 11), values This regression led to progressive abandon- shown in parentheses represent estimates based on the mean age of the three dated beachrock samples (7, 8, and 10). ment of uplifting shorelines and deposition of the prograding beach ridge sequence that covers the Cabuya terrace. The Cobano cliff, (3) Paleo–sea level. Eustatic sea level at the Cabuya terrace. Calibrated radiocarbon between the Cobano and Cabuya terraces the time of sample deposition is estimated ages for this horizon range between 4400 yr (Fig. 3), therefore, is an abandoned sea cliff using the sea-level curve of Pinter and Gard- B.P. and 5200 yr B.P. Paleo-eustatic sea- that marks the active shoreline at the begin- ner (1989). This allows comparison of the level data show that the rapid postglacial ning of late Holocene emergence. uplift rates reported in this study with the rise in early Holocene eustatic sea level be- uplift rates determined for the Penı´nsula de gan to slow dramatically near 6.5 ka (Fair- Arcward Tilting Osa by Gardner et al. (1992). These sea- banks, 1989; Chappell and Polach, 1991). level estimates have an uncertainty of Ϯ1.0 m. Between 6.5 ka and 5.5 ka the rate of eu- A plot showing the uplift rates for eleven (4) Sample age. Radiocarbon ages were static sea-level rise decreased from Ͼ5 sample localities versus trench-perpendicu- determined by dating either thick-shelled m/k.y. to Ͻ1.5 m/k.y. The ages of the three lar distance along the Cabuya terrace gastropod fossils or unweathered samples of dated beachrock samples correspond to a (Fig. 4) reveals a systematic arcward de- carbonate beachrock. The reported ages are period closely following this signiﬁcant decrease in average uplift rates from a maxi- calibrated radiocarbon ages based on the radiocarbon age calibration curves of Stuiver and Pearson (1986), Pearson and Stuiver (1986), and Pearson et al. (1986).

Late Holocene Emergence

The radiocarbon ages of nine samples collected from the Cabuya terrace systematically increase in age with increasing distance and elevation away from the modern shoreline (Table 1 and Fig. 3). These ages span the late Holocene, ranging from 500 yr B.P. near the shoreline to just over 5000 yr B.P. near the base of the Cobano cliff. Because eustatic sea level has been rising or steady throughout the entire Holocene (Fairbanks, 1989; Chappell and Polach, 1991), the Cabuya terrace deposits provide evidence for progressive late Holocene emergence along this section of coastline and require rates of crustal uplift that are greater than the rate of late Holocene eustatic sea-level rise. Figure 4. Calculated uplift rates with corresponding error bars for 11 samples along the A carbonate beachrock horizon, corre- Cabuya terrace plotted perpendicular to the Middle America Trench. Sample numbers with lated for over 8 km along the landward edge each data point correspond with those in Table 1 and Figure 3. Solid symbols represent of the beach ridge sequence (Fig. 3), is radiocarbon dated samples. Open symbols represent undated beachrock samples assigned among the oldest of the shore deposits on an estimated age based on the mean age of the three dated beachrock samples.

Geological Society of America Bulletin, April 1995 467 MARSHALL AND ANDERSON

it becomes exposed at the extreme low tide line. Based on tidal data for the Golfo de Nicoya (Borbon, 1989), we estimate that the modern elevation of the site is 1.75 m below mean sea level. Excavated soils indicate that the site was originally established along the margins of a coastal mangrove with an elevation just above mean sea level. An artifact from La Regla yielded a radiocarbon age of 2472 Ϯ 70 yr B.P. (Guerrero et al., 1991). At 2.5 ka, mean sea level is estimated at 0.5 m lower than modern sea level (Pinter and Gardner, 1989). Therefore, total subsidence at La Regla since 2.5 ka is roughly 1.25 m, having occurred at an average rate of ϳ0.5 m/k.y. If, as a rough approximation, the entire Penıńsula de Nicoya is modeled as a single Figure 5. Elevations of beachrock samples along the Cabuya terrace plotted perpendic- arcward rotating block, then the rates of ver- ular to the Middle America Trench. Sample numbers with each data point correspond with tical tectonism along the seaward and land- those in Table 1 and Figure 3. The best fit line shows an arcward tilt of 0.1؇ for the ward coastlines can be used to estimate the 4400–5200 radiocarbon yr B.P. beachrock strandline. angular velocity of arcward rotation. A comparison of the subsidence rate at La Regla with the uplift rate of a sample of similar age mum of 4.5 m/k.y. at Isla Cabuya to Ͻ1.7 zuma Formation (Madrigal and Rojas, 1980; at Cabuya (sample no. 4, uplift rate: 4.4 m/k.y. at Punta Colorada, 8 km to the north- Mora, 1985). Assuming a mid-Pleistocene m/k.y.) indicates an approximate minimum east (Fig. 3). This trend suggests either sig- age for these beds (Sprechmann et al., angular velocity of arcward rotation for the nificant local fault displacement, arcward 1979), this dip suggests long-term arcward entire peninsula of ϳ0.01Њ/k.y. for the late tilting of the Cabuya terrace, or some com- tilting at an average angular velocity of Holocene. bination of both. Ͻ0.01Њ/k.y. However, if this tilting is related It is likely, however, that a single-block Detailed mapping reveals that the Pleis- strictly to the latest phase of uplift, which model for the entire peninsula is an over- tocene strata of the Montezuma Formation began sometime between the mid- and late simplification. Although Quaternary fault- are affected by only very minor faulting Pleistocene (Mora, 1985), the average rate ing is minimal within the Cobano and (Lundberg, 1982; Gursky, 1988). This indi- of arcward rotation of these strata could po- Cabuya terraces (Lundberg, 1982; Gursky, cates that the displacements associated with tentially be as high as that calculated for the 1988), their southwestern margin is deline- any late Holocene faulting, along this late Holocene deposits (0.02Њ/k.y.). ated by a significant trench-parallel normal stretch of coastline, can be considered neg- Arcward projection of the decreasing fault (Fig. 3). This feature suggests that the ligible with respect to the observed magni- trend in uplift rates on the Cabuya terrace Cabuya and Cobano terraces may occupy an tude of late Holocene uplift. The arcward predicts a change from uplift to subsidence individual fault-bounded block situated decrease in uplift rates therefore suggests 6 km northeast of Montezuma in the vicinity within the larger structural framework of the late Holocene tilting of the Cabuya terrace of Tambor (Fig. 2). This location roughly peninsula’s entire southeastern coastline. between Isla Cabuya and Punta Colorada. corresponds with the northeastern margin Similar trench-parallel normal faults, found The spatial distribution of these uplift rates of the Cobano and Cabuya terraces. While throughout the peninsula (Kuijpers, 1980), indicates that arcward tilting has occurred at beach ridges and minor terraces indicate may represent boundaries of discrete fault an average angular velocity of 0.02Њ/k.y. Quaternary uplift as far northeast as Tam- blocks that rotate semi-independently at dif- Additional evidence for tilting of the bor (Battistini and Bergoeing, 1983), up- ferent angular velocities. Gardner et al. Cabuya terrace is provided by the 4400–5200 lifted terraces are absent along the coast be- (1992) proposed this type of model for de- radiocarbon yr B.P. beachrock horizon, cor- tween Tambor and the Golfo de Nicoya. formation of the Penıńsula de Osa. Further related along the landward edge of the beach Whereas the Cabuya terrace experienced detailed analysis of Quaternary faulting and ridge sequence. A plot showing the elevations net uplift during the late Holocene, net sub- vertical tectonism is required before such a of beachrock sites versus trench-perpendicu- sidence affected the peninsula’s Golfo de model can be applied conclusively to the lar distance along the coastline (Fig. 5) shows Nicoya coast 30 km to the northeast. Ar- Penıńsula de Nicoya. a systematic decrease in elevation toward the chaeological evidence from a submerged northeast. This trend corresponds to an arc- prehistoric burial ground along the Golfo de Tectonic Implications ward tilt of 0.1Њ and an average angular ve- Nicoya coast (Guerrero et al., 1991) allows locity of rotation of 0.02Њ/k.y. for the late for the estimation of a subsidence rate for Late Quaternary arcward tilting along the Holocene. the late Holocene. The La Regla burial site, Penıńsula de Nicoya’s southeastern coast- Arcward tilting of the Cabuya terrace is located offshore of the towns of Jicaral and line is consistent with regional geomorphic consistent with the gentle northeastward dip Lepanto (Fig. 2), is accessible only during evidence documenting emergence along the (2Њ to 5Њ) of the upper strata of the Monte- the two lowest spring tides of the year when peninsula’s Pacific coastlines and submer-

468 Geological Society of America Bulletin, April 1995 UPLIFT AND DEFORMATION, PENIŃSULA DE NICOYA, COSTA RICA gence along the peninsula’s Golfo de Nicoya Seismic reflection imaging of the trench order to explore the possibility of seismic coast. As discussed earlier, several authors slope offshore of the Penıńsula de Nicoya cycle deformation within the northern Costa have modeled the long-term deformational indicates that underplating may play a sig- Rican fore arc, we focused our investigation structure of the Penıńsula de Nicoya as a nificant role in accretionary prism develop- on the M 7.7 Nicoya earthquake of 1950. large anticline or dome (Dengo, 1962; Kuij- ment and long-term passive uplift along the pers, 1980; Lundberg, 1982; Hare and Gard- northern Costa Rican fore arc (Silver et al., The 1950 Earthquake: Oral History Study ner, 1985). The evidence presented in this 1985; Shipley and Moore, 1986; Shipley study suggests that this dome structure has et al., 1992). Additionally, several authors Although records from this event show been rotating in an arcward direction during have suggested that recent uplift on the Pen- that significant damage occurred in Costa the Quaternary at an average angular veloc- ńsulaı de Nicoya may have been accom- Rica’s capital city of San Jose´ and in the ity ranging between 0.01Њ and 0.02Њ/k.y. plished during discrete coseismic events as- Pacific port of Puntarenas (Louderback, These rotation rates for the Penıńsula de sociated with repeated earthquakes along 1951; Miyamura, 1980), there exists no sys- Nicoya are consistent with those of Gardner this highly coupled segment of the Middle tematic documentation of earthquake ef- et al. (1992), who reported Quaternary arc- America Trench (Fischer, 1980; Battistini fects on the Penıńsula de Nicoya. In similar ward tilting of fault-bounded blocks on the and Bergoeing, 1983; Anderson et al., 1989; cases, where rigid field data is lacking, other Penıńsula de Osa at angular velocities Marshall et al., 1990; Marshall, 1991). These researchers have effectively used anecdotal ranging between 0.03Њ and 0.06Њ/k.y. The two processes, operating independently or evidence to document the geologic effects of higher rates of uplift and rotation observed concurrently, both represent likely mecha- past earthquakes (e.g., Lyell, 1849; Dutton, on the Penıńsula de Osa can be attributed to nisms of ‘‘steady state’’ vertical tectonism 1889; Plafker, 1969; Allen, 1974; Taylor the upward flexure caused by subduction of produced by the subduction of relatively et al., 1980, 1987; Guëndel et al., 1989). the buoyant Cocos Ridge directly beneath dense and bathymetrically smooth litho- On the basis of these examples, we inter- the peninsula. Because the Cocos Ridge ex- sphere beneath the northern Costa Rican viewed 48 residents of the central Pacific erts minimal tectonic influence on the Pe- fore arc. coast of the Penıńsula de Nicoya who expe- nıńsula de Nicoya (Gardner et al., 1992), ad- rienced the 1950 earthquake. Interviews ditional mechanisms of uplift must be SEISMIC CYCLE DEFORMATION were conducted along a 25 km stretch of invoked to explain the deformation rates coastline between the towns of Puerto Car- documented in this study for the peninsula’s Penıńsula de Nicoya Earthquakes rillo and Nosara (Fig. 2), located within the southeastern shore. 1950 rupture zone (Fig. 6). As in the coseis- The proximity of this coastline to the Popular legends and historical documents mic uplift studies of Plafker (1969) and of northwestern margin of the central Costa from the Penıńsula de Nicoya describe the Taylor et al. (1980, 1987), we focused spe- Rican fore arc (Fig. 1), which deforms in recurrence of severe earthquakes spanning cifically on coastal residents who were inti- response to buoyant seamount subduction at least the past three centuries (Gonza´lez- mately familiar with the local tidal levels and (Wells et al., 1988), leads to speculation that Vı´quez, 1910; Pittier, 1959; Marshall, 1991). coastal landmarks at the time of the these rates may be anomalously high and are During the present century, seismographic earthquake. not representative of the deformation af- networks have recorded at least five large Interpretation of these interviews natu- fecting the peninsula as a whole. The over- subduction earthquakes (M Ն 7.0) in the rally involves considerable uncertainty re- steepened trench slope offshore of Cabo vicinity of the Penıńsula de Nicoya (Fig. 6; garding the accuracy of elderly people re- Blanco may have developed due to localized Guëndel, 1986; Protti, 1991, 1994). Two of calling an event that occurred nearly 40 yr rapid uplift related to subduction of the these events (5 October 1950, M ϭ 7.7, and previously. However, rural Nicoya society buoyant Fisher seamount immediately to 23 August 1978, Ms ϭ 7.0) were centered boasts a strong oral tradition, and many of the southeast of the peninsula (von Huene directly beneath the peninsula and ruptured these people, as farmers and fishermen, are et al., in press). A flexural uplift model, sim- the northern Costa Rican segment of the keen observers of their surroundings. It is ilar to that developed for the Cocos Ridge Middle America Trench. clear from the interviews that the 1950 (Gardner et al., 1992), may be applicable for With a relatively rapid convergence rate earthquake represents one of the most spec- the subducting Fisher seamount and, hence, of 9.1 cm/yr along this segment of the trench tacular events in the lives of older peninsula the vertical deformation observed along the (DeMets et al., 1990), it is unlikely that the residents. Many of those interviewed were southeastern coastline of the Penıńsula de Ms 7.0 1978 earthquake released all of the able to recall minute details of the experi- Nicoya. strain accumulated since the much larger M ence, and in most cases, uncertainty is On the other hand, oblique slip along 7.7 1950 event (Guëndel, 1986). Nishenko greatly reduced by corroboration between northeast-trending faults of the East Nicoya (1989) gives a 93% probability ofaM7.4 several different interviews. Fracture Zone (Fig. 1), which form part of earthquake on the Penıńsula de Nicoya be- the Caribbean-Panama boundary (Marshall fore the year 2009, listing it as fourth among Relative Sea-Level Changes et al., 1993; Fisher et al., 1994), may effec- the top seismic gaps in the circum-Pacific tively insulate the Penıńsula de Nicoya from region. The interviews indicate that ground shak- vertical deformation related to seamount Substantial vertical tectonism associated ing intensities throughout the Penıńsula de subduction to the southeast. In either case, with large subduction earthquakes has been Nicoya reached at least VII on the Modified the deformation affecting the majority of the reported in a variety of fore-arc settings Mercalli Scale (Wood and Neumann, 1931) Penıńsula de Nicoya should principally re- around the world (e.g., Plafker, 1972; Mat- and probably ranged as high as IX or X in flect subduction of the relatively dense and suda et al., 1978; Thatcher, 1984; Taylor alluvium and along beaches. Ground crack- smooth sea floor directly offshore. et al., 1980, 1987; Carver et al., 1994). In ing, landslides, liquefaction, and building

Geological Society of America Bulletin, April 1995 469 MARSHALL AND ANDERSON

In another example, coastal residents along the bays of Sa´mara and Puerto Car- Figure 6. Locations of rillo estimated that the high tide level after five recent large subduc- the earthquake was near present-day low tion earthquakes (M Ն tide. Currently, tidal levels in these bays are 7.0) centered near the reportedly similar or somewhat higher than Penıńsula de Nicoya. they were prior to the earthquake, following Stars represent epicen- a progressive rise in relative sea level during ters. Shaded area within the past four decades. These observations solid line is aftershock suggest a coseismic drop in relative sea level, area for the 1950 event. followed by a gradual recovery, of a magni- Dashed lines enclose af- tude similar to the average difference be- tershock areas for the tween high and low tide, or ϳ2 m (Borbon, 1978 and 1990 events. 1989). Data from Guëndel (1986) and Protti (1991, Tectonic Implications 1994). The observations of relative sea-level changes associated with the 1950 earthquake suggest the occurrence of coseismic uplift ranging from 1 to 2 m along the coast damage were widespread (Marshall, 1991). However, after the earthquake it was possi- between Nosara and Puerto Carrillo All of the interview participants who lived or ble to walk around this rocky point at low (Fig. 2). Although observable uplift may worked in 1950 along the central Nicoya tide without entering the water. These ob- have occurred as far south as Puerto Coyote, coast between Nosara and Puerto Carrillo servations suggest a coseismic drop in rela- residents of Cabuya, on the peninsula’s mentioned a significant drop in sea level as- tive sea level at this site of at least the height southeastern coast, recall no change in rel- sociated with the earthquake. Residents of of a person’s waist, or roughly 1 m. ative sea level. The interviews also suggest Sa´mara remember scouring the newly ex- According to both of the fishermen, rel- that a significant fraction of the coseismic posed sea floor just after the earthquake col- ative sea level at El Raspa Nalgas has been uplift has been reversed during a subsequent lecting an abundance of fish left stranded by rising gradually since 1950 but still remains long-term period of gradual postseismic the sudden retreat of the sea. Many people lower than it was prior to the earthquake. and/or interseismic subsidence that may explained that for many years after the One of these men indicated that the water at continue through the present. earthquake it was possible to walk to sites this site is now knee deep at low tide. This These apparent vertical tectonic displace- that previously had been inaccessible along suggests a recovery of relative sea level at ments most likely reflect a single episode in the rocky cliffs typical of that stretch of this site of at least 0.5 m since the earthquake. a recurring seismic cycle of crustal deforma- coastline. Nearly all of the coastal residents also indicated that relative sea level has been gradually rising along the central Nicoya coast since the 1950 earthquake. Reportedly, the sea has slowly reclaimed many of the sites that were exposed by its sudden coseismic retreat. Several suggested that this apparent sea-level rise occurred fairly rapidly during the first several years after the earthquake but since then has slowed noticeably. The approximate magnitude of relative sea-level changes associated with the 1950 earthquake can be estimated based on observations at well-known coastal landmarks. For example, when asked about specific sites where the sea-level changes were obvious, two fishermen from Nosara both independently referred to a rocky headland known locally as El Raspa Nalgas (‘‘The Butt Scratcher’’). Prior to the 1950 earthquake, this site represented a barrier to foot pas- Figure 7. a. Contour map of coseismic vertical deformation estimated from a uniform- sage around Punta Nosara. Even during the slip-planar dislocation model for the 1950 M 7.7 Nicoya subduction earthquake. Uplift and lowest tides of the year, deep water at this subsidence shown in meters. Dashed lines show locations of cross sections shown in Fig- site prohibited walking along the shoreline. ure 7b. Model parameters based on seismicity data from Guëndel (1986).

470 Geological Society of America Bulletin, April 1995 UPLIFT AND DEFORMATION, PENIŃSULA DE NICOYA, COSTA RICA tion within the northern Costa Rican fore eters were estimated using the seismicity net Quaternary deformation, expressed by arc related to underthrusting of the subduct- data of Guëndel (1986) and the relation- geomorphology and structure, and the rep- ing Cocos plate beneath the Penıńsula de ships between surface wave magnitude, seis- etition of the seismic deformation cycle Nicoya. The initial coseismic uplift resulted mic moment, mean slip, and rupture area (e.g., Plafker, 1972; Matsuda et al., 1978; from elastic rebound of the fore-arc crust discussed by Purcaru and Berckhemer Taylor et al., 1980, 1987; Thatcher, 1984; produced by sudden failure along the thrust (1978) and Scholz (1990). Atwater, 1987). The coseismic deformation interface (e.g., Plafker, 1972; Thatcher, Although the modeled deformation pat- pattern estimated by the dislocation model 1984). The subsequent period of progressive tern (Figs. 7a and 7b) only represents a gen- bears a striking resemblance to the dome subsidence represents gradual modification eral approximation, it is consistent with the pattern proposed for the long-term defor- of the coseismic displacements by the com- distribution and magnitude of coseismic up- mation of the Penıńsula de Nicoya. The dis- bined effects of postseismic downdip rup- lift indicated by the oral history interviews. location model, however, places maximum ture propagation (e.g., Brown et al., 1977; The model shows that a 1950 type earth- uplift along the peninsula’s seaward coast- Barrientos et al., 1992), postseismic vis- quake within the northern Costa Rican fore line, whereas structural and geomorphic coelastic readjustments within the astheno- arc is capable of producing coseismic uplift data indicate maximum net uplift along the sphere (e.g., Nur and Mavko, 1974; Thatcher on the order of 1.0 m along the central Pa- crest of the peninsula’s interior mountain and Rundle, 1984), and/or interseismic elas- cific coast of the Penıńsula de Nicoya and range (Dengo, 1962; Kuijpers, 1980; Hare tic strain accumulation within the fore-arc subsidence on the order of 0.3 m along the and Gardner, 1985). crust (e.g., Plafker, 1972; Thatcher, 1984). axis of the Golfo de Nicoya–Rıó Tempisque If long-term doming of the peninsula is depression. The magnitude of this deforma- related to seismic cycle deformation, this Dislocation Model tion decreases toward the northwest and discrepancy can be reconciled by the post- southeast, forming an elongate dome pat- seismic redistribution of vertical displace- To develop an estimate of the coseismic tern centered along the Pacific coast of the ments by either downdip fault creep (e.g., deformation pattern resulting from the 1950 peninsula flanked by an elongate depression Brown et al., 1977; Barrientos et al., 1992) or earthquake, we applied the uniform slip pla- centered along the Golfo de Nicoya. viscoelastic flow within the asthenosphere nar dislocation modeling techniques of In other fore-arc regions, researchers (e.g., Nur and Mavko, 1974; Thatcher and Mansinha and Smylie (1971). Model param- have demonstrated the connection between Rundle, 1984). Either of these processes

Figure 7. b. Cross sections A–D through the estimated coseimic deformation pattern (Fig. 7a). Approximate Paciﬁc coast locations are indicated.

Geological Society of America Bulletin, April 1995 471 MARSHALL AND ANDERSON could migrate the focus of maximum uplift coast. Late Holocene uplift rates, calculated portant contributions from seismic cycle de- landward from the trench and produce grad- from beach ridge deposits on the Cabuya formation and accretionary processes such ual subsidence in the region of maximum marine terrace, decrease systematically as underplating and subduction erosion. coseismic uplift. These readjustments, cou- toward the northeast from 4.5 m/k.y. at pled with interseismic strain accumulation, Cabuya to Ͻ1.7 m/k.y. at Montezuma. ACKNOWLEDGMENTS may explain the anecdotal observations of a Whereas Quaternary uplift is evident along coseismic drop in sea level followed by sev- the southeast coast as far arcward as Tam- This investigation was initiated based on eral decades of gradual sea-level recovery. bor, uplifted terraces are absent between the suggestions of Karen McNally of the Net Quaternary vertical tectonism in the Tambor and the Golfo de Nicoya. Archae- University of California, Santa Cruz, vicinity of both the Penıńsula de Nicoya and ological evidence from the Golfo de Nicoya Charles Richter Seismological Laboratory. the Penıńsula de Osa is characterized by coast indicates late Holocene subsidence at Research was conducted in cooperation pronounced uplift along a trench-parallel a rate of 0.5 m/k.y. These data suggest net with the Observatorio Volcanolo´gico y Sis- arcward tilting peninsula and subsidence late Holocene arcward tilting of the Penıń- molo´gico de Costa Rica, Universidad Na- along a gulf or fluvial lowland located land- sula de Nicoya at an average angular rota- cional (OVSICORI-UNA). We gratefully ward of the peninsula. The striking resem- tion rate ranging between 0.01Њ and 0.02Њ/ acknowledge their entire staff for logistical blance between this pattern and the elastic k.y. Arcward rotation at this rate is consistent support and for ideas incorporated into this coseismic deformation pattern commonly with the gentle northeastward tilt of both a paper. Special thanks is owed to Jorge attributed to subduction earthquakes leads late Holocene beachrock horizon on the Brenes M. for his insightful assistance both to speculation that the repetition of seismic Cabuya terrace and the adjacent Pleistocene in field work and in conducting oral history cycle deformation may operate as an impor- strata of the Montezuma Formation. Arc- interviews. We are especially indebted to the tant mechanism of net vertical tectonism ward tilting of the Penıńsula de Nicoya may gracious people of the Penıńsula de Nicoya within both the northern and southern be accomplished by the rotation of a system for allowing access onto their lands and into Costa Rican fore-arc regions. Both of these of semi-independent fault-bounded blocks. their lives. regions are known sources of large subduc- Oral histories describing the M 7.7 Nicoya We would also like to thank Gary Griggs, tion earthquakes (M Ն 7.0) with estimated subduction earthquake of 5 October 1950 Nicholas Pinter, and William Lettis for help- recurrence intervals of ϳ40 yr (Guëndel, provide evidence of seismic cycle deforma- ful reviews of earlier versions of this manu- 1986). Repetition of seismic cycle deformation along the southwestern coast of the script. Funding was provided by a University tion at this frequency potentially could pro- Penıńsula de Nicoya. Interviews with 48 res- of California, Santa Cruz Seed Fund grant duce a significant component of net vertical idents indicate that coseismic uplift of at to Robert Anderson and Karen McNally. tectonism. least1maffected the coastline between Radiocarbon dating was conducted by Beta Puerto Carrillo and Nosara, and that a sig- Analytic Inc. 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