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 Penı´nsula 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- Penı´nsula 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 Penı´nsula 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 Penı´nsula 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 Penı´nsula 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 Penı´n- 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 Penı´nsula 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 Penı´nsula 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 nı´nsula 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 ,350 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 Penı´nsula de Nicoya to the similarly ori- *Present address: Geosciences Department, Penı´nsula de Nicoya to the Penı´nsula de Pennsylvania State University, University Park, Osa, pronounced changes occur in the ba- ented Penı´nsula 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 Penı´nsula 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 Penı´nsula 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 Penı´n- 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 sub- ducting Cocos plate produce significant vari- ations in Wadati-Benioff zone geometry Figure 1. Tectonic setting of Costa Rica (modified from Protti, 1991). The Cocos, Ca- (Gu¨endel, 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 buoy- ant Cocos Ridge beneath the Penı´nsula de duction of two highly contrasting lithos- the Cocos plate subducts northeastward be- Osa has resulted in rapid upward deflection pheric domains. Fourth, the Penı´nsula 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 (Gu¨endel and Pacheco, 1992; Goes mined from late Quaternary stratigraphy on quakes (M $ 7.0) and has been designated et al., 1993; Fan et al., 1993), and fault kin- the Penı´nsula de Osa range between 6.5 a high-probability seismic gap (Gu¨endel, 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 Penı´nsula implies that the Penı´nsula 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 0.038 and the Costa Rican Pacific coast. longs to the Caribbean plate, from the cen- 0.068/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.