Rough Crust Subduction, Forearc Kinematics, and Quaternary Uplift Rates, Costa Rican Segment of the Middle American Trench

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Rough crust subduction, forearc kinematics, and Quaternary uplift rates, Costa Rican segment of the Middle American Trench

Peter B. Sak† Department of Geology, Dickinson College, Carlisle, Pennsylvania 17013, USA Donald M. Fisher Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Thomas W. Gardner Department of Geosciences, Trinity University, San Antonio, Texas 78212, USA Jeffrey S. Marshall Department of Geological Sciences, Cal Poly Pomona University, Pomona, California 91768, USA Peter C. LaFemina Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA

ABSTRACT seamount-dominated segment is greatest temala, northern Chile, Japan, New Britain, and inboard of the largest furrows across the Tonga, are the most prevalent type of margin Orthogonal subduction of bathymetrically lower slope. Localized uplift and arcward and are commonly characterized by rapid con- rough oceanic lithosphere along the north- tilting of coastal deposits is present adja- vergence and limited sediment input (von Huene western fl ank of the Cocos Ridge imprints a cent to subducting seamounts. In contrast, and Scholl, 1991; Clift and Vannucchi, 2004). distinctive style of deformation on the over- inboard of the underthrusting aseismic The subduction of seamounts and ridges at the riding Costa Rican forearc. We divide the Cocos Ridge, along the ~160-km-long Fila Nankai (Bangs et al., 2006), northern Chile (von Costa Rican forearc into three 100–160-km- Costeña domain between Quepos and the Huene and Ranero, 2003), Peruvian (von Huene long deformational domains based on the Burica Peninsula, mesoscale fault popula- and Lallemand, 1990), Solomon Islands (Mann bathymetric roughness and thickness of the tions record active shortening related to et al., 1998; Taylor et al., 2005), Middle Ameri- Cocos plate entering the Middle American the ~100-km-long Fila Costeña fold-and- can (from Guatemala to Costa Rica) (Gardner Trench, the dip of the subducting plate, thrust belt. The observed patterns of fault- et al., 1992, 2001; Fisher et al., 1998; Bilek et the variation in surface uplift rates of late ing and permanent uplift are best explained al., 2003; Vannucchi et al., 2004), and Tonga Quaternary coastal deposits, and the orien- by crustal thickening. The uplifted terraces (Ballance et al., 1989; Scholz and Small, 1997) tations and types of faults deforming Paleo- provide a fi rst-order estimate of permanent trenches results in tectonic deformation of the gene and Neogene sedimentary rocks. In strain along the forearc in Costa Rica. The forearc and infl uences interplate coupling. The the ~100-km-long Nicoya domain, coastal permanent strain recorded by uplift of these Cocos plate subducts beneath both the Carib- deposits show localized surface uplift and Quaternary surfaces exceeds the predicted bean plate and Panama microplate at the Middle arcward tilting above the downdip projec- rebound of stored elastic strain released American Trench off the western coast of Costa tions of the fossil trace of the Cocos-Nazca- during subduction-zone earthquakes. Rica. The forearc response to subduction of Panama (CO-NZ-PA) triple junction and the rough lithosphere varies as a function of distance Fisher seamount and ridge. In the ~120-km- INTRODUCTION from the trench. The distribution of faulting and long central Pacifi c forearc domain between uplift in the subaerial portion of the Costa Rican the Nicoya Peninsula and Quepos, shallower Convergent margins can experience either forearc mimics the distribution of bathymetric (~60°) subduction of seamounts and plateaus accretion or subduction erosion, and rates features on the subducting Cocos plate outboard is accompanied by trench-perpendicular late depend upon such factors as volume of sedi- of the Middle American Trench (e.g., Gardner et Quaternary normal faults. Steeply dipping, ment input, subduction angle, basal friction, and al., 1992, 2001; Fisher et al., 1998; Marshall et northeast-striking, margin-perpendicular seafl oor roughness (see, e.g., Cloos and Shreve, al., 2000; Sak et al., 2004a). faults accommodate differential uplift asso- 1988a, 1988b; von Huene and Lallemand, 1990; The Costa Rican segment of the Middle ciated with seamount subduction. Uplift Stern, 1991; von Huene and Scholl, 1991; Lal- American Trench is a suitable location for inves- and faulting differ between the segments lemand et al., 1992, 1994; Bangs and Cande, tigating the relationship between the ongoing of the forearc facing subducting seamounts 1997; Ranero and von Huene, 2000; Clift and subduction of bathymetric features and upper- and ridges. Inner forearc uplift along the Vannucchi, 2004; von Huene et al., 2004). Con- plate deformation because of the diversity in vergent margins dominated by subduction ero- the surface morphology of the downgoing plate. †E-mail: [email protected] sion, such as those along Peru, Costa Rica, Gua- The morphology of the Cocos plate offshore of

GSA Bulletin; July/August 2009; v. 121; no. 7/8; p. 992–1012; doi: 10.1130/B26237.1; 10 ﬁ gures; 2 tables.

Costa Rica changes abruptly from a few iso- increases southward from ~8.5 cm yr–1 across it is interpreted as the fossil trace of the initial lated seamounts and narrow ridges mantled by the Nicoya segment to 9.1 cm yr–1 across the Osa opening of the Cocos-Nazca spreading center a ≤500-m-thick cover of sediment offshore of segment (Dixon, 1993; DeMets, 2001). South (Fig. 1A) (von Huene et al., 2000; Barckhausen the Nicoya Peninsula to a rough surface where of Quepos (Figs. 1C and 2), up to 4 cm yr–1 of et al., 2001). To the north of this ridge, seafl oor ~40% of the ocean fl oor is covered by linear Cocos-Caribbean convergence is accommodated magnetic spreading anomalies trend 120°, sub- arrays of seamounts ranging in size from 1 to permanently within the Fila Costeña fold-and- parallel to the Middle American Trench, whereas 2.5 km high and 10 to 20 km wide (Ranero thrust belt (Sitchler et al., 2007). Cocos-Carib- south of the ridge, the seafl oor magnetic spread- and von Huene, 2000) and plateaus between bean convergence becomes oblique (10°–15°) ing anomalies are oriented 030° (Barckhausen the Nicoya and Osa Peninsulas. Further to the at the southern end of the Nicoya Peninsula, et al., 2001). Northwest of the fossil trace of southeast, the Osa Peninsula overrides the aseis- resulting in ~8 mm yr–1 of northwest-directed the Cocos-Nazca-Panama triple junction trace mic Cocos Ridge, an area of thickened oceanic forearc sliver transport (McCaffrey, 2002; Nor- (TJ), the crust is ~24 m.y. old at the Middle lithosphere and elevated seafl oor (von Huene et abuena et al., 2004). American Trench (Fig. 1A) (Barckhausen et al., al., 2000; Walther, 2003) that is interpreted as a Across the Costa Rican segment of the Mid- 2001). From the ridge, crustal age decreases to trace of the Galapagos hotspot (Barckhausen et dle American Trench, from the Nicoya Penin- the southeast from ~23 Ma to 21.5 Ma across al., 2001). sula in the northwest to the Osa Peninsula in the an ~75-km-wide swath of smooth crust to the In this paper, we evaluate the relationship southeast (Fig. 1C), a distance of ~300 km, the Fisher Seamount and Ridge (Barckhausen et between forearc deformation and the morphol- forearc response to ongoing subduction varies al., 2001). The conical Fisher Seamount rises ogy and geometry of the subducting plate along as a function of the lateral changes in dip of the ~1.6 km above the subjacent ca. 20 Ma Cocos- the Middle American Trench in Costa Rica. We subducting slab, age, and thickness of the crust Nazca spreading center–derived crust and has a present a regional compilation of mesoscale fault entering the trench, seafl oor roughness, and basal diameter of ~16 km (Werner et al., 1999; populations and age dates collected from previ- distance from the trench (e.g., Corrigan et al., Barckhausen et al., 2001). Southwest (outboard) ously published studies along the Costa Rican 1990; Gardner et al., 1992, 2001; Fisher et al., of the Fisher Seamount, is the narrow (~5–7 km segment of the Middle American Trench (Gard- 1998; Walther, 2003). High-resolution bathy- wide) northeast-trending, continuous Fisher ner et al., 1992, 2001; Marshall and Anderson, metric mapping of the subducting Cocos plate Ridge, which rises ~1 km above the adjacent 1995; Fisher et al., 1998, 2004; Marshall et al., offshore of Costa Rica reveals a rough surface ocean fl oor (Fig. 1C). 2000; Sak et al., 2004a) in addition to new data morphology characterized by ridges, plateaus, The bathymetric roughness of the Cocos plate from the Nicoya Peninsula and the central Pacifi c and seamounts (Fig. 1C). Surface morphology changes southeast of the Fisher Seamount and coast. Fault data are used to recognize regional- of the lower slope arcward of the Middle Ameri- Ridge from elongate ridges to the broad Que- scale trends in forearc kinematics. Radiocarbon can Trench refl ects the roughness of the incom- pos Plateau (QP, Fig. 1C), which is elongate and ages of marine terraces are evaluated using the ing Cocos plate outboard of the trench. Recov- oblique to the Cocos-Caribbean convergence recent IntCal04 calibration (Reimer et al., 2004) ered drill cores (Kimura et al., 1997; Vannucchi vector and to linear arrays of conical seamounts and recent sea-level curves (Fleming et al., 1998; et al., 2001, 2003), seismic-refl ection profi les oriented at low angles to the relative convergence Lambeck and Chappell, 2001) to constrain Qua- (i.e., Hinz et al., 1996), and bathymetric map- vector. Because the chains of subducting fea- ternary surface uplift rates in a common refer- ping (von Huene et al., 1995, 2000) all indicate tures are oriented at a low angle to the relative ence frame. The spatial distribution of calculated subsidence by subduction erosion. Damage to Cocos-Caribbean convergence vector, the effects uplift rates along and across the Costa Rican the portions of submarine forearc facing sub- of ongoing rough crust subduction are limited to forearc are then combined with the fault kine- ducting bathymetric highs is interpreted as the narrow regions along the margin (Fisher et al., matic data to constrain plausible mechanisms morphologic signature of subduction erosion 1998). Seamounts have broad bases (~15–20 km) resulting in permanent uplift within the forearc. (Ranero and von Huene, 2000). and rise >1.5 km above the adjacent ocean bottom Where bathymetric highs enter the trench, (von Huene et al., 1995, 2000). The seamounts, REGIONAL SETTING the trench axis is defl ected arcward (Fig. 1C). composed of 13–14.5 Ma basalts, are younger Offshore of the southern half of the Nicoya than the 15–20 Ma subjacent oceanic crust (Wer- The Central American isthmus occupies a Peninsula (between Punta Guiones and Cabo ner et al., 1999; Barckhausen et al., 2001). Wide- complex deformational zone that responds to Blanco), an ~75-km-wide swath of relatively angle seismic-refraction investigations indicate the interaction of four tectonic plates (Carib- smooth crust is bound at either side by elongate that the depth to the Moho increases from ~11 km bean, Cocos, Nazca, and South American) and northeast-trending ridges rising >700 m above across to the Quepos Plateau to 21 km beneath the Panama microplate (Fig. 1A). Deformation the adjacent seafl oor (von Huene et al., 2000). the Cocos Ridge (Walther, 2003). across the forearc in southern Central America The fossil trace of the Cocos-Nazca-Panama tri- The underthrusting of irregular oceanic litho- is due to the rapid subduction of the Cocos plate ple junction is marked by a ridge that intersects sphere has a strong impact on the morphology beneath the Caribbean plate and Panama block the Middle American Trench offshore of Punta and structure of the submarine forearc. High- (Corrigan et al., 1990; Gardner et al., 1992; Guiones (TJ in Fig. 1C). This narrow (~2 km resolution bathymetric swath mapping across Kolarsky et al., 1995; Marshall et al., 2000; wide), discontinuous, asymmetric ridge has an the plate-boundary region documents the effects Fisher et al., 1998, 2004). Along strike of the abrupt southeast fl ank that rises ~700 m above of rough crust subduction on the forearc bathym- Middle American Trench, the character (i.e., relatively smooth oceanic crust to the south- etry (von Huene et al., 1995, 2000; Ranero and dip, roughness, crustal age, and thickness) of east and a gently sloping northwest fl ank (von von Huene, 2000; Hühnerbach et al., 2005). the subducting Cocos crust changes (Figs. 1A Huene et al., 2000; Barckhausen et al., 2001). Where bathymetric highs enter the trench, the and 1C) (Protti et al., 1995a; von Huene et al., This ridge is coincident with the boundary sepa- trench axis is defl ected arcward. For example, 2000; Barckhausen et al. 2001; Walther, 2003). rating crust produced at the East Pacifi c Rise to the underthrusting fossil trace of the triple junc- Across the Costa Rican segment of the Middle the northwest from crust produced at the Cocos- tion offshore of Punta Guiones deforms the American Trench, the relative convergence rate Nazca spreading center to the southeast, and lower slope. Across the lower slope inboard of

Geological Society of America Bulletin, July/August 2009 993 Sak et al.

′ , ′ c Rise– xed Caribbean plate xed Caribbean plate (DeMets, 2001); X to ed from Protti et al., 2001). Protti ed from (modifi ′ ected arcward of subducting bathymetric features. Letters in of subducting bathymetric features. ected arcward ected arcward, but the depth to the top of the Wadati-Benioff zone Wadati-Benioff but the depth to top of ected arcward, , and C-C ′ Figure 1. (A) Generalized tectonic framework of the Central American Generalized tectonic framework of the Central 1. (A) Figure underlined Plate names are plate tectonic features. isthmus with major American and in bold. CARIBBEAN—Caribbean plate; SOAM—South plate; NAZCA—Nazca COCOS—Cocos PAN—Panama NPDB—North Panama deformed belt; SPDB—South Pan- microplate; ama deformed belt; CCRDB—Central Costa Rica PPB— compiled were features Tectonic paleo–plate boundary; RJ—ridge jump. et al. (1990), Mann and Kolarsky (1990), Silver Mackay and Moore from et al. (2001). Heavy black (1995), Marshall et al. (2000), and Barckhausen to a fi plate motions relative arrows—present-day (DeMets, 2001); thin black lines with small numbers—orientation and Index et al., 2001). (B) age (Ma) of magnetic anomalies (Barckhausen of C. Dotted white line—boundary between the inner map showing area (C) Digital eleva- (to the southwest) forearc. (to the northeast) and outer and Costa Rica, merg- (MAT) Trench America tion model of the Middle von bathymetry (from ing 30 m topographic data with high-resolution contours of the depth Huene et al., 2000). Superimposed on the map are et al., 2001). Note where Protti zone (from Wadati-Benioff to the top of axis the trench Trench, American bathymetric highs intersect the Middle is defl is not defl et al., 1998). Symbols: white ovals denote fault-bound blocks (Fisher TJ—trace of the East Pacifi Seamount and Ridge; FSR—Fisher center–Caribbean triple junction; QP—Quepos Cocos–Nazca spreading arrow—present-day heavy red Trench; America Plateau; MAT—Middle to a fi plate motions relative is location of Figure 6C; highlighted black lines—contours of the depth is location of Figure et al., 2001); shaded Protti zone (from Wadati-Benioff to the top of location of imaged normal faults accommodat- gray box—approximate with McIntosh et al., 1993); circles ing margin-parallel extension (from and G show loca- letters D, E, F, spokes—active volcanoes; white circled Jaco, and Paritta Scars of Hühnerbach et Tarcoles, tions of Rio Bongo, block; H—Herradura; Es— al. (2005). Ez—Esparza block; O—Orotina Location of Esterillos block; P—Parrita Q—Quepos block. (D) A-A zone along sections Wadati-Benioff the historic earthquakes across B-B D Back arc Arc Inner forearc forearc Outer C 3700 (m) 2000 (m) 0 (m) -3000 (m) C’ Cabo Matapalo X’ N 0 80

160 240 Cocos Ridge Cocos Depth (km) Depth B’ Costa Rica Q P 91 mm/yr CO-CA 100 km Es G H O

Fig. 5

F Pacific Ocean QP Ez Jaco 5°N E B 10°N AB A’ 75°W SOAM Cabo

100 km Blanco 85° W 0 80

160 240 Fig. 2B MAT (km) Depth D CARIBBEAN PI NAZCA PC

NPDB 54 m/k.y. 40 km SPDB PANAMA

Fig. 2A FSR

CCRDB Punta

85°W

22 15

Guiones CR

18 RJ A 86° W 84° W X 85 m/k.y.

A A’ B B’ C C’ W 0160 0160 0160 0 TJ CH Distance (km) Distance (km) Distance (km) O 2

AMERICA N PPB MAT 85 mm/yr

E 0

80 9° N 8° N TRE N 11° 10° N Depth (km) 160 Depth 20 240 COCOS MIDDL 15

994 Geological Society of America Bulletin, July/August 2009 Forearc deformation along the Costa Rican segment of the Middle American Trench

a b c T d e T T T P P P P P T

28 Unit Descriptions Geologic Map of 24 Location of mesoscale Quaternary alluvium A B the Garza Region fault population 25 Bedding attitude Paleogene marine 36 29 sediment

0 2 km Cretaceous marine N sediment 9°55’N 9°55’N 15 35 B 15 33 H Cretaceous ophilotic C Garza 40 30 32 F basement Punta D G Guiones 67 36 82 E34 25 42 40 33 4514 44 32 I J40 Playa Pacific Ocean Camaronal A 85°40’W 85°30’W

f g h i j P T T P P P T T P T

85°05’W Figure 2. (A) Mesoscale fault populations, regional faults, and general geology of the Cobano Garza region. Geology is modifi ed from Baumgartner et al. (1984) and supple- Cobano Surface mented by this study. Mesoscale fault population data (lower-hemisphere, equal-area 9°40’N 9°40’N projections) are keyed to the map by letter Holocene beach (Table 2). Compressional (P) axes (black cir- Montezuma deposits cles—individual faults; black square—aver- Undifferentiated age) and tensional (T) axes (black circles– alluvial deposits individual faults; black square—average) outer boundary defi ne best-fi t fault-plane solution for each Pacific Ocean Cobano surface k fault population (Marrett and Allmend- T l Plio-Pleistocene P (Montezuma Fm.) inger, 1990). Black dots—locations of dated P sandstones and T Cabuya samples from the Garza surface (Table 1). m conglomerates See Figure 1C for location. (B) Generalized N T Miocene shelf clastics (St Teresa Fm.) geologic map of the Cabo Blanco region after P Mora and Baumgartner (1985), Gardner et D Eocene deep-water carbonates U (Punta Cuevas Fm.) al. (2001). Lower-hemisphere, equal-area 1.5 3 km Cretaceous seafloor basalt projections of mesoscale fault populations (Nicoya Complex) and Paleocene turbidites (from Marshall et al., 2000). See Figure 1C Cabo Blanco (Cabo Blanco Fm.) for location. B 85°05’W El Flor fault

Geological Society of America Bulletin, July/August 2009 995 Sak et al. the subducting ridge, there is a narrow (<5 km Along the Nicoya Peninsula, Quaternary upward from mean sea level, was determined by wide) linear depression oriented parallel to the marine terraces are observed in segments of mapping facies into paleodepositional environ- Cocos-Caribbean convergence vector extend- the forearc inboard of subducting bathymetric ments. Reconstructed depositional environments ing ~15 km arcward from the trench (Fig. 1C). features. Uplifted late Quaternary marine ter- were subsequently assigned probable water Inboard of the arcward terminus of this confi ned races exposed in the vicinity of Punta Guiones depths based upon comparisons to the modern zone of subsidence, the continental margin over- and Cabo Blanco that are inboard of the triple shoreface environment. For example, following riding the trace of the triple junction is elevated junction trace and Fisher Seamount and Ridge, the facies designations of Gardner et al. (1992) (~500 m) above the adjacent slope apron sedi- respectively, indicate localized surface uplift and Sak et al. (2004a), bimodal cross-bedded ment. Similarly, where the arcward extension rates in excess of the rate of late Quaternary sea- coarse-grained sandstones with shell lag debris of the Fisher Seamount and Ridge intersects level rise. In the following section, we present deposited on subhorizontal planation surfaces the trench, the trench axis is defl ected arcward. new constraints from mesoscale fault analysis, cut into the bedrock were assigned to the mean Inboard of where the Fisher Seamount and radiocarbon dating, and correlation of marine sea-level facies, and poorly sorted medium- to Ridge impinges upon the trench, there is a con- terraces across the southern half of the Nicoya coarse-grained massive sands with disarticulated vergence vector–parallel depression that extends Peninsula between Punta Guiones to Cabo abraded thick-walled shells were assigned to the ~20 km arcward from the trench. Blanco (Fig. 1C). These results provide insight subwave base facies. An assumption inherent Three furrows parallel the Cocos-Caribbean into forearc deformation and constrain the dis- in the reported facies depths is that tidal ranges convergence vector inboard of the seamount- tribution, timing, and rates of surface uplift both and wave climate have remained relatively con- dominated domain further to the southeast (von inboard of subducting bathymetric highs and stant over the sampling interval. Paleo–sea level, Huene et al., 1995, 2000). The largest of these inboard of smooth subducting crust. measured positive upward from mean sea level, furrows, the Tarcoles Scar, is 12 km wide, and was determined from published eustatic sea- it extends ~55 km arcward of the trench (E in Quaternary Geology of the Nicoya Peninsula level curves (Fleming et al., 1998). Holocene Fig. 1C) (Hühnerbach et al., 2005). Inboard uplift rates for the Garza surface range from –0.1 of these furrows, domal features that protrude Uplifted Holocene and late Pleistocene to 0.6 m k.y.–1 for sample 127327 and –0.7 to above the adjacent slope apron are interpreted marine terraces exposed in the vicinity of Cabo 1.6 m k.y.–1 for sample 131260 (Table 1; Fig. 3). to be located opposite the present location of Blanco and Punta Guiones are used to constrain Uplifted, laterally extensive Holocene and subducting seamounts (von Huene et al., 1995, uplift rates along the Nicoya Peninsula. The late Pleistocene marine terraces are exposed 2000; Hühnerbach et al., 2005). Analog model- Garza terrace, a wave-cut Holocene platform, is at the southeastern tip of Nicoya Peninsula ing of seamount subduction suggests that the exposed intermittently in pocket beaches along (Fig. 3B) (Marshall and Anderson, 1995; Gard- width of the scar across the lower slope is equal the coast of the Nicoya Peninsula between Playa ner et al., 2001) and are confi ned to the portions to the basal diameter of the subducting seamount Guiones and Playa Islita (Fig. 1C). Locally the of the forearc overriding the leading edge of the inboard of the scar (Dominguez et al., 1998). No Garza surface is covered by a <50-cm-thick, Fisher Seamount and Ridge (Fig. 3). Narrow further morphologic evidence of seamount sub- shell debris lag overlain by a >1-m-thick, light (<100 m) exposures of uplifted Holocene ter- duction is resolvable inboard of protruding coni- gray, fi ne-grained sandy entisol. Near Garza races extend ~20 km along the coast in either cal highs at the arcward terminus of the fault- (Fig. 3A), three samples of disarticulated, thick- direction from Cabo Blanco (Figs. 2B and 3) bounded scar. This mass wasting–induced scar walled gastropod shells collected from the con- (Marshall and Anderson, 1995; Gardner et al., in the wake of a subducting seamount persists tact with the subjacent bedrock at 2.0–2.5 m 2001). At a distance of 15–20 km along the for ~140 k.y. until sediment accretion from the above mean sea level (msl) yield radiocarbon coast, in either direction from Cabo Blanco, trench or slope restores the frontal prism geom- ages ranging from 3.745–3.892 ka to 5.220– Holocene marine terraces decrease in elevation etry (von Huene et al., 2004). 5.337 ka (Table 1). To the southeast, the eleva- from ~16 m to <1 m (Gardner et al., 2001). At tion of this platform decreases, and 15–20 km Cabo Blanco, surface uplift rates locally exceed THE NICOYA PENINSULA east of Garza at Playa Camaronal (Figs. 1C and 6 m k.y.–1 and decrease linearly as a function 2A), it is exposed no higher than the high tide of distance along the coast away from Cabo The Nicoya Peninsula is an ~100-km-long, debris line (1.2 ± 0.6 m, assuming the modern Blanco (Gardner et al., 2001) (Table 1). More northwest-trending, outer forearc anticlinal(?) tidal range of 2.4 m). When the modern sample than ~20 km northeast and northwest of Cabo high cored by the Mesozoic Nicoya Complex elevations (Z), the facies depth (F), and sea level Blanco, uplifted Holocene marine terraces are (deBoer, 1979; Kuijpers, 1980; Bourgois et al., (S) at the time of deposition (T), are known, the no longer observed above mean sea level, and 1984). The cover sequence of Late Cretaceous surface uplift rate (R) may be calculated by >50 km north of Cabo Blanco, along the north- to Eocene turbidites outcrops inboard of where eastern coast of the Nicoya Peninsula, a sub- the Cocos-Nazca-Panama triple junction trace merged 2.5 ka prehistoric burial ground (Guer- ZFS()mmm−− () () intersects the Middle American Trench in the R (m/k.y.) = . (1) rero et al., 1991) records <2 m k.y.–1 subsidence vicinity of Garza (Figs. 1C and 2A) (Baumgart- T ()ka on the Gulf of Nicoya coast (Marshall, 1991). ner et al., 1984). At the southeastern tip of the The Cobano surface of presumed late Pleisto- Nicoya Peninsula in the vicinity of Cabo Blanco The modern sample elevation, measured posi- cene age lies landward of the Holocene terraces a sequence of shallowing-upward Late Creta- tive upward from mean sea level, was determined between the interior mountains of the Nicoya ceous to Miocene marine sediment (including precisely (±0.5 m) using a Sokkia AIR-HB-1L Peninsula north of the town of Cobano and the turbidites like those in the vicinity of Garza) is handheld digital barometer. To compensate 80-m-high abandoned sea cliff around Monte- exposed inboard of where the Fisher Seamount for temporal variations in barometric pressure, zuma (Fig. 3B)(Hare and Gardner, 1985; Mora, and Ridge subducts (Figs. 1C and 2B) (Lund- measurements were recorded every 15 min at a 1985). The 400 km2, northwest-dipping (2°–3°) berg, 1982; Baumgartner et al., 1984; Mora, fi xed base station during all elevation surveys. Cobano surface is an erosional surface cut into 1985; Astorga, 1987; Winsemann, 1994). Facies depth of deposits, measured positive the subjacent, gently northwest-dipping (3°–5°)

996 Geological Society of America Bulletin, July/August 2009 Forearc deformation along the Costa Rican segment of the Middle American Trench

Uplift rate 84°W (mm/ yr) 0.1 - 1.0 4.1 - 5.0 1.1 - 2.0 5.1 - 6.0 2.1 - 3.0 6.1 - 7.0 3.1 - 4.0 n = 71 85°W

2 2 4 3

MAMATAT MATMAAT CRC

MATMAAT QQP FSRFSR TJTJ

Costa Rica N CO-CA area of 8°N 01020Figure 3 9°N North km Figure 3. Calculated uplift rates of late Pleistocene to recent marine terraces overlain on a map meshing the bathymetry of the Middle American Trench region (from Ranero et al., 2003) and topography from a 90 m digital elevation model (DEM) derived from the U.S. National Aeronautics and Space Administration’s SRTM-3 data set. Circles are colored for the mean of the calculated uplift rate (Table 1). Numbers in circles refer to the number of samples used to calculate uplift rate at sites constrained by multiple age dates. Symbols: FSR—Fisher Seamount and Ridge; TJ—trace of the East Paciﬁ c Rise–Cocos–Nazca spreading center–Caribbean triple junction; QP— Quepos Plateau; CR—Cocos Ridge; MAT—Middle America Trench.

Pliocene-Pleistocene Montezuma Formation soils developed on the Cobano surface ~60 km triple junction enters the Middle American (Hare and Gardner, 1985; Mora and Baumgart- to the southeast. As in the case of the Cobano Trench (offshore of Garza) and where the Fisher ner, 1985; Marshall and Anderson, 1995; Gard- surface, a low-lying, less-weathered marine Seamount and Ridge impinges on the trench at ner et al., 2001). This regionally extensive sur- terrace is exposed below the highly weathered Cabo Blanco (Fig. 3). Both the Holocene marine face is mantled by a >3-m-thick red (2.5 YR) Camaronal surface. The lower surface, exposed terraces and the late Pleistocene Cobano surface, oxisol. The elevation of the Cobano surface at an elevation of 1.5 m, is correlative with the exposed in the vicinity of Cabo Blanco, dem- decreases from a maximum of >200 m at the dated (ca. 4 ka) Garza surface. We postulate that onstrate that the magnitude of late Quaternary southern boundary to ~100 m north of the town the extensively weathered, pedogenically simi- uplift decreased as a function of distance inboard of Cobano. The measured tilt of the Cobano lar Camaronal and Cobano surfaces represent of the Middle American Trench (Marshall and surface is consistent with the calculated axis of remnants of a regionally extensive erosional Anderson, 1995; Gardner et al., 2001). rotation for the Holocene Cabuya surface (Gard- surface cut during marine oxygen isotope stage ner et al., 2001). 5e (ca. 120 ka). If the Camaronal and Cobano Fault Kinematics of the Nicoya Peninsula An uplifted, deeply weathered ﬂ at-topped ero- surfaces are isolated remnants of a regionally sional surface cut into thinly bedded Paleogene extensive surface, this would suggest a dra- Mesozoic and Tertiary sediment are dissected turbidites and Nicoya Complex pillow basalts is matic increase in uplift rate over the late Qua- by numerous steeply dipping faults that have exposed at Playa Camaronal (Figs. 1C and 2A). ternary in the vicinity of Playa Camaronal from trace lengths of kilometers to tens of kilome- This locally extensive surface, referred to as the 0.1 m k.y.–1 to 0.4 m k.y.–1 (Figs. 1C and 2A). ters and two dominant orientations that deﬁ ne Camaronal surface, is exposed at an elevation Across the southern half of the Nicoya Pen- a kinematic framework: (1) northwest-striking of 15–20 m. The Camaronal surface extends for insula, late Quaternary uplift rates vary along (parallel to faults seismically imaged offshore ~3 km along the coast and up to 1 km inland strike of the Middle American Trench and as a within the slope apron; McIntosh et al., 1993; to the interior mountains of the Nicoya Penin- function of distance from the trench. Uplift rates shaded box in Fig. 1C), and (2) northeast-strik- sula. The Camaronal surface is mantled by a are greatest in the portions of the forearc oppo- ing faults (parallel to the relative convergence >3-m-thick red (2.5 YR) oxisol that is similar to site where the trace of the Cocos-Nazca-Panama vector) (Figs. 2A and 4). Slip can be directly

Geological Society of America Bulletin, July/August 2009 997 Sak et al.

) §§ 0.6 0.7 4.2 ## ## ) 0.9 1.7 5.3 4.8 5.2 4.7 6.8 4.8 1.4 2.1 2.0 3.1 1.6 1.4 1.5 1.2 2.3 2.0 2.0 2.6 2.7 1.5 2.1 2.2 4.0 3.8 3.9 4.7 6.2 4.3 4.4 3.7 2.5 1.9 1.5 1.6 1.6 4.4 3.5 1.8 1.3 1.7 2.3 2.1 – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – R ( 0.1 0.4 (m/k.y.) (m/k.y.) – – Range of Continued uplift rate ( †† ) 5267 1.5 62 2.1 6.5 82836576 > 3.4 > 3.6 1.9 6.5 S 3.50.8 0.7 0.7 2.51.2 0.5 0.4 9.00.8 1.6 0.9 0.50.52.01.0 3.3 2.5 3.9 3.50.5 3.1 3.9 0.5 3.3 1.0 3.0 1.2 3.1 0.87.02.5 1.2 0.8 3.9 1.2 0.4 1.5 0.6 0.5 1.5 0.5 0.8 0.5 0.8 0.5 1.0 6.0 1.2 1.8 0.7 1.8 0.1 0.7 1.5 0.5 0.55.52.0 0.4 5.02.0 0.6 1.2 1.5 1.3 1.6 0.5 1.8 1.5 0.5 2.0 3.0 3.5 ( 10.0 1.6 (m) – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – Sea level

) 15 15 F – – ( (m) 9 ± 6 – Facies depth**

# ) Z ( (m) Modern 0.44 ±0.44 0.4(±0.6) 0.0 elevation –

§ ) 4.6325.220 0.1 2.0 ± 2.755 0.1 2.5 ± 7.755(±1.2) 0.0 (±1.2) 0.0 1.21.772 5.80.853 0.6 (±0.6) 2.81.362 1.2 (±0.6) 3.14.1452.215 1.2 (±0.6) 4.574 7.21.410 0.6 (±0.6) ± 0.2 13.7 0.708 0.2 ± 9.5 0.695 ± 0.2 16.2 2.215 1.2 (±0.6) (±1.2) 0.0 0.2 ± 5.5 0.02.786(±0.6) 0.6 (±0.6) 1.2 3.76.543 4.64.572 0.2 ± 9.5 (±1.2) 0.0 1.9742.742 1.9 1.33.2601.102 1.2 (±0.6) 6.2(±0.6) 0.6 0.956 0.6 (±0.6) 3.91.276 2.31.568 3.06.345 0.0 (±1.2) 3.8 0.0 3.0 0.0 1.2 (±0.6) 1.5 1.2 (±0.6) 1.5 0.6 (±0.6) 1.0 0.6 (±0.6) 0.0 (±1.2) 2.0 0.0 (±1.2) 4.2 0.0 (±1.2) 1.2 (±0.6) 0.0 (±1.2) 3.7451.2311.402 0.1 2.4 ± 1.8345.8034.139 0.9(±1.2) 0.0 2.05.641 1.83.883 1.1 5.43.315 1.2 (±0.6) 1.471 1.2 (±0.6) 5.6 0.6 (±0.6) 5.81.2880.686 1.2 (±0.6) 1.227 5.9 0.0 4.10.485 1.2 (±0.6) 4.910 0.6 (±0.6) 3.5 2.6 5.2 0.2 3.4 ± 5.214 0.0 ± 0.2 17.1 0.0 1.2 (±0.6) 0.6 (±0.6)(±0.6) 1.2 1.179 0.6 (±0.6) ± 0.2(±0.6) 14.8 0.6 1.983 0.0(±0.6) 0.6 0.0 2.4 1.7 1.8 2.5 0.6 (±0.6) 0.0 (±1.2) – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – T ( (ka) Age D ASSOCIATED VALUES USED TO CALCULATE UPLIFT RATES. TO CALCULATE USED VALUES D ASSOCIATED

† 6.660 7.583 5.5 0.0 (yr B.P.) (yr B.P.) C age (raw) 1.16 ± 0.081.16 1.228 4.07 ± 0.044.07 ± 0.072.03 4.672 2.119 ± 0.075.54 6.457 3.48 ± 0.063.48 3.892 1.01 ± 0.061.01 ± 0.071.33 1.034 1.361 1.65 ± 0.061.65 1.674 1.90 ± 0.061.90 1.956 1.28 ± 0.061.28 1.343 2.58 ± 0.082.58 2.830 ± 1.549.33 ± 1.5 54.33 41 ± 1 ± 1.2 0 4.46 ± 0.064.46 5.337 0.40 ± 0.060.40 ± 0.063.80 ± 0.072.26 ± 0.104.10 ± 0.084.47 ± 0.061.53 0.568 ± 0.090.78 ± 0.071.24 4.316 ± 0.080.72 2.315 ± 0.083.98 4.768 ± 0.072.65 5.343 ± 0.062.04 1.478 0.851 0.787 1.318 2.315 ± 0.063.05 2.905 2.120 3.412 0.66 ± 0.060.66 0.727 6.94 ± 0.066.94 ± 0.073.12 ± 0.061.62 7.889 ± 0.050.97 3.462 1.617 ± 0.084.39 0.926 5.110 4.14 ± 0.074.14 4.785 2.59 ± 0.062.59 ± 0.075.07 2.832 5.959 3.59 ± 0.073.59 ± 0.071.86 4.042 1.929 ± 0.061.49 1.472 3.79 ± 0.073.79 4.347 1.34 ± 0.061.34 1.363 1.29 ± 0.061.29 ± 0.061.52 1.343 ± 0.084.90 1.476 5.791 14 44.11 ± 0.9244.11 ± 49.11 0.92 26 ± 1.2 0 40.69 ± 0.6340.69 ± 0.63 45.69 37 0 ± 1.2 46.00 ± 2.0046.00 ± 2.00 51.00 38 0 ± 1.2 34.29 ± 0.4434.29 ± 0.44 39.29 47 < 27.22 ± 0.6927.22 ± 0.69 32.22 15 < 37.95 ± 0.4937.95 ± 0.49 42.95 61 ± 1

′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ (°W) (°W) 85°03 ± 0.07 5.76 6.701 84°30 85°05 85°05 85°04 83°40 83°43 84°32 85°11 85°10 85°08 83°43 83°43 85°09 85°01 85°01 83°41 83°43 84°31 84°31 85°08 85°08 85°07 85°06 83°43 85°00 85°05 85°05 85°05 85°05 85°05 85°05 85°05 85°04 84°44 85°13 85°11 85°10 85°09 85°05 85°09 85°09 85°08 85°08 85°08 85°07 85°05 Longitude Longitude 85°38

85°38

83°43 ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′

(°N) 8°40 8°41 9°31 8°40 8°40 8°40 9°38 9°41 9°41 9°42 9°06 8°40 9°55 9°31 9°32 9°32 9°55 9°35 9°35 9°36 9°36 9°36 9°36 9°36 9°37 9°37 9°39 9°40 9°40 9°40 9°57 8°40 9°42 9°40 9°39 9°39 9°39 9°38 9°38 9°37 9°37 9°36 9°36 9°35 9°35 9°34 9°34 9°34 9°34 9°35 9°55 Latitude Latitude TABLE 1. RADIOCARBON SAMPLES LISTED BY LOCATION AN LOCATION BY LISTED SAMPLES RADIOCARBON TABLE 1.

5 5 5 5 2 5 7 5 5 5 5 5 5 5 5 5 5 5 5 5 2 2 2 2 2 2 2 5 5 2 5 5 5 5 6 *** *** *** *** *** *** *** 3,4 5 6 6 6 6 6 6

Cabo Blanco Cabo Blanco Cabo Blanco (032359) Cabo Blanco (121772) Cabo Blanco (121781) (122724) (109633) Esterillos NW Osa (154115) Cabo Blanco Cabo Blanco Cabo Blanco Cabo Blanco Carballo Punta (121782) (121779) (121780) (88001) Playa Ballena NW Osa (129337) NW Osa NW Osa (154118) (150572) (142207) Cabo Blanco Cabo Blanco Cabo Blanco (121776) Cabo Blanco Cabo Blanco (121788) Cabo Blanco Cabo Blanco (121791) Cabo Blanco (121790) Cabo Blanco Cabo Blanco (121770) (121771) Cabo Blanco (036397) (034835) (032358) Cabo Blanco (034834) (037558) Cabo Blanco Cabo Blanco (036396) Cabo Blanco Cabo Blanco (121769)

Cabo Blanco Cabo Blanco (032360) Cabo Blanco Cabo Blanco Cabo Blanco (032361) (121773) (121778) Cabo Blanco Cabo Blanco (121785) NW Osa NW Osa (142206) (154114) Cabo Blanco Cabo Blanco (121783) Cabo Blanco (127435) (118143) Esterillos Garza (127237) Garza (127237) Garza (131259) Area Sample* Area Garza (131260) Garza (131260) (118144) Esterillos (109632) Esterillos NW Osa (142208) Cabo Blanco Cabo Blanco Cabo Blanco Cabo Blanco Cabo Blanco (131258) Cabo Blanco (121787) Cabo Blanco (121784) Cabo Blanco (121775) (117376) Cabo Blanco (121774) (GX25370) (094100)

998 Geological Society of America Bulletin, July/August 2009 Forearc deformation along the Costa Rican segment of the Middle American Trench

§§ ## ## ## ## ## ) 4.6 4.6 3.7 2.3 2.1 2.5 3.5 6.6 6.8 6.8 8.7 6.6 3.8 3.9 – – – – – – – – – – – – – – R ( (m/k.y.) (m/k.y.) Range of uplift rate †† ) ) 91.5 91.3 11.7 67 6.5 83677590 6.5 4.2 83 4.2 3.8 7991 3.0 2.0 2.8 8390 4.2 3.1 92 3.6 94 > 3.5 S 0.5 5.3 – 142142 5.5 5.8 – – 138 5.9 ( (m) – – – – – – – – – – – – – – – – Sea level Continued

—samples from Gardner et al. et al. Gardner from —samples 5 modern shoreface environment. shoreface environment. modern ) dal ranges and wave climate have have and wave climate dal ranges 15 15 15 F – – – ( (m) 9 ± 6 9 ± 6 9 ± 6 9 ± 6 9 ± 6 9 ± 6 9 ± 6 9 ± 6 9 ± 6 9 ± 6 – – – – – – – – – – Facies depth**

# ) Z ( (m) atic sea-level curves (Fleming et al., 1998; Lambeck and Lambeck and al., 1998; (Fleming et curves atic sea-level Modern elevation —samples from Marshall (2000); (2000); Marshall —samples from 4 B v.5.0.2 (Stuiver et al., 2005). Age calculated from 2006. Late Late 2006. from calculated Age 2005). al., et (Stuiver v.5.0.2 B pths is the assumption that ti that assumption is the pths m); high tide line 1.2 m (±0.6 m). Pleistocene samples were samples were Pleistocene (±0.6 m). 1.2 m line tide high m); ixed base station during all elevation surveys. all elevation during base station ixed C concentrations (Beck et al., 2001). 2001). al., (Beck et C concentrations 14

§ 1.32 40 ) 7.2997.979 13.423.983 (±1.2) 5 4.51.3250.957 0.0 0.0 (±1.2) 0.0 (±1.2) 9 4.3 0.0 (±1.2) 0.0 (±1.2) 0 2.3 1.952 4.6 0.0 (±1.2) – – – – – – – T ( (ka) Age C values. 13 δ samples from Fisher et al. (1998); (1998); al. Fisher et samples from —

3 † 1.06) ± 1.01 26.23 10.0 – (yr B.P.) (yr B.P.) mean sea level, was determined from published eust published from was determined sea level, mean C age (raw) 0.98 ± 0.040.98 0.990 7.15 ± 0.087.15 8.098 ± 0.061.39 1.410 34.0 ± 0.2434.0 ± 0.040.98 ± 0.24 39.00 35 ± 1 39.53 40.99 ± 0.640.99 ± 0.6 45.99 ± 0.076.35 ± 0.062.02 ± 1 20 7.389 2.100 42.54 ± 0.8742.54 ± 1.8029.78 ± 0.87 47.54 ± 0.5233.07 34.78 ± 1.8 ± 0.1420.14 52 ± 1 ± 0.52 38.07 ± 1.1326.77 40 24.353 < ± 1.13 31.77 7(±1.2) 0.0 5 < 34.87 ± 0.3534.87 ± 0.35 39.87 79 ± 1 < 34.88 ± 0.5134.88 ± 0.2922.21 ± 0.51 39.88 ± 0.29 25.710 25 12 40.65 ± 0.6340.65 ± 0.5230.07 ± 0.63 45.65 ± 0.6235.29 ± 0.74 35.72 64 ± 1 ± 0.62 40.29 50 40 14 ounting for fluctuations in atmospheric atmospheric in fluctuations for ounting

22.73 (+0.94, 22.73

′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ from mean sea level, was determined from topographic maps, transit surveys, metric tape, or using a Sokkia AIR-HB-1L handheld handheld Sokkia AIR-HB-1L a using or tape, surveys, metric maps, transit topographic from sea level, was determined from mean (°W) (°W) localized subsidence and uplift (Sak et al., 2004a). 2004a). et al., (Sak uplift and subsidence localized Longitude Longitude 5568 yr half-life and corrected with measured with measured corrected and yr half-life 5568 83°43 83°43 83°44 83°25 83°24 83°24 83°21 83°22 83°19 83°18 83°19 83°17 83°17 83°17 83°44 83°44 83°25 83°21 83°18 83°17 ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ′ ), measured positive upward from from upward positive ), measured T —samples from Marshall and Anderson (1995); Anderson —samples from (1995); and Marshall (°N) 2 8°40 8°40 8°38 8°38 8°38 8°33 8°32 8°32 8°32 8°33 8°33 8°32 8°31 8°29 8°29 8°29 8°28 8°27 8°25 8°26 Latitude Latitude

—sample from Fisher —sample from et (2004). al. 7 GX—Geochron Labs; DIC—Dicarb Radioisotope Co. Labs; Radioisotope DIC—Dicarb GX—Geochron . (1) )

), upward measured positive ), 1 1 1 Z TABLE 1. RADIOCARBON SAMPLES LISTED BY LOCATION AND ASSOCIATED VALUES USED TO CALCULATE UPLIFT RATES. TO CALCULATE USED VALUES AND ASSOCIATED LOCATION BY LISTED SAMPLES RADIOCARBON TABLE 1. (

6 1 1 1 1 1 1 1 1 1 1 1 1 ( *** *** *** *** 6 6 6 6 ), measured positive upward from mean sea level, were subsequently assigned probable water depths based upon comparisons to the comparisons to upon based water depths probable assigned subsequently were sea level, mean from upward positive measured ), 3153) 3362) 3154) − F – – – ) ) ka ( ( T ) at the time of deposition ( deposition of time the ) at − S ) mmm ( ZFS = ) a —samples from Sak et al. (2004a); —samples from (2004a); Sak et al. —samples from Gardner et al. (1992); al. et from Gardner —samples 6 1 : e (m/k Uplift rate was determined assuming the complex history of assuming the complex was determined Uplift rate Paleo–sea level ( Paleo–sea level Uplift rate (R) was calculated from Equation 1 using accumulated errors listed here here listed errors accumulated using 1 Equation from calculated was (R) Uplift rate Calendar calibrated age and one sigma calibration error. Ages were calibrated using the INTCAL04 (Reimer et al., 2004) and CALI 2004) al., et INTCAL04 (Reimer the using calibrated were Ages error. calibration sigma one and age calibrated Calendar Modern elevation of the sample ( the sample of elevation Modern R Uncorrected ages are reported using the conventional reported are ages Uncorrected § # †† §§ † ##

Chappell, 2001). Chappell, 2001). Pleistocene ages assume a 5 k.y. offset between U/Th yr and C corrected age acc age and C corrected yr U/Th between offset a 5 k.y. ages assume Pleistocene

*Beta Analytic sample number (20841). (20841). number sample Analytic *Beta ***Accelerator Mass Spectrometry age. Spectrometry age. Mass ***Accelerator Given a tidal range of 2.4 m, facies of Holocene samples were assigned to mean sea level 0.0 m (±1.2 m); swash zone 0.6 m (±0.6 m 0.6 swash zone m); m (±1.2 level 0.0 sea mean to assigned were Holocene samples of m, facies of 2.4 range tidal Given a facies de reported the in Inherent m. subwave base <–15 m); (±6 –9 m wave base (±1.2 m), above m 0.0 sea level mean assigned to digital barometer. To compensate for temporal variations in barometric pressure, measurements were recorded every 15 min at a f a at 15 min every were recorded measurements pressure, barometric in temporal variations for compensate To barometer. digital **Facies depth of deposits ( of **Facies depth Not (2001);

remained relatively constant over the sampling interval. sampling interval. the constant over remained relatively

NE Osa NE Osa NE Osa (20941) (20838) NE Osa NE Osa NE Osa (24917) (20837) NW Osa (142205) Area Sample* Area NW Osa NW Osa NW Osa NW Osa NE Osa (154116) (154117) (142204) (155062) (24918) NE Osa NE Osa (20836) NE Osa NE Osa NE Osa NE Osa NE Osa NE Osa NE Osa (26780) (DIC NE Osa NE Osa (20841) (20840) NE Osa (24914) (20938) (DIC (24919) (DIC

Geological Society of America Bulletin, July/August 2009 999 Sak et al. determined for most regional-scale faults in the ing recent displacement (McIntosh et al., 1993). Rapidly uplifting blocks override subducting fi eld using offset marker beds in combination There is a set of faults exposed in the sea cliff seamounts (Fisher et al., 1998). with fault striae. The regional kinematic frame- at Punta Indio that has the same orientation and work as determined from the faults is supple- kinematics as those imaged offshore by McIn- Quaternary Uplift Rates Along the Central mented here by detailed analysis of mesoscale tosh et al. (1993). The overall kinematics are Pacifi c Coast fault populations. Mesoscale fault populations further complicated by additional faults in these are outcrop-scale features (tens to hundreds of uplifted Paleogene strata. The record of late Quaternary deformation meters in length) that displace measurable fault Four of the ten sites (Table 2; Fig. 2A, insets b, across the central Pacifi c coast region is well surfaces containing kinematic indicators (e.g., e, g, and j; Fig. 4) measured between Punta Gui- constrained by alluvial and marine terraces. slickenlines). We use kinematic analysis (e.g., ones and Puerto Carrillo indicate strike-slip offset Northeast-trending faults along the central Marrett and Allmendinger, 1990) to interpret across steeply dipping fault planes. Sites b and e Pacifi c coast offset Quaternary alluvial terraces dense fault arrays in terms of the strain patterns (Fig. 2A) record dextral motion along northeast- and a pyroclastic fl ow yielding a 352 ± 40 ka recorded at the margin and interior of faulted striking faults, while site j records dextral motion Ar/Ar age (Marshall et al., 2003). Rapid surfi - regions. This kinematic method determines along a west-northwest–striking fault (Fig. 2A). cial weathering along the Pacifi c coast of Costa the principal shortening and extension axes (P Site g is consistent with sinistral motion along Rica facilitates differentiation of the late Qua- and T axes, respectively) for a given population a northeast-striking fault (Fig. 2A). Similarly, ternary alluvial deposits on the basis of pedo- based on slip data from individual faults. The in the vicinity of Cabo Blanco, Marshall et al. genic characteristics (e.g., Wells et al., 1988; sense of slip was determined using the criteria (2000) measured three mesoscale fault popula- Drake, 1989; Bullard, 1995; Marshall, 2000; of Petit (1987). Kinematic axes for individual tions in exposures of mid- to upper Tertiary lime- Sak et al., 2004b). Surface soil properties range mesoscale fault populations measured on the stone. Two of these populations (sites k and l) from reddish brown (2.5YR–10R), ≥540-cm- Nicoya Peninsula are plotted as P and T axes are characterized by shallowly plunging P and T thick B horizons (Marshall, 2000), with average from best-fi t fault-plane solutions on equal-area axes and strike-slip motion along steeply dipping weathering rind thicknesses of 6.9 ± 0.6 cm for stereonets (Table 2; Figs. 2A and 4) (Allmend- northeast-striking faults (Table 2; Fig. 2B, insets basaltic andesite clasts from Quaternary terrace inger et al., 2004). k and l; Fig. 4). Steep P and shallow T axes char- (Qt) 1, to brown (10YR), 200-cm-thick B hori- Mesoscale fault populations were measured acterize the mesoscale faults north-northeast of zons, with average weathering rind thicknesses at 13 exposures of Mesozoic and Tertiary marine Cabo Blanco at site m (Fig. 2B; Table 2). of 0.9 ± 0.1 cm for basaltic andesite clasts from sediment on the Nicoya Peninsula. Most of these Qt 3 (Sak et al., 2004b). Terrace ages were populations are characterized by shallow T axes THE CENTRAL PACIFIC COAST estimated using a physiochemical model for and steeply inclined P axes, suggesting a com- weathering rind formation that is constrained ponent of extension (Table 2). The observed pat- The ~120-km-wide set-back portion of the by radiocarbon ages (Fisher et al., 1998) and tern of steeply dipping, northeast-striking faults coastline between the Nicoya and Osa peninsu- Ar/Ar ages for pyroclastic fl ows (Marshall et is similar to the geometry of regional-scale las is dissected by steeply dipping faults striking al., 2003). This model yields ages of ca. 240 ka, faults (Fig. 3A) (Baumgartner et al., 1984). Pop- at a low angle to the Cocos-Caribbean conver- ca. 120 ka, and ca. 37 ka, respectively for Qt 1, ulations characterized by steeply dipping, north- gence vector (Fig. 1C). These faults separate 2, and 3 (Sak et al., 2004b). west-striking nodal planes and steep P and shal- ~20-km-wide blocks characterized by variable Dated alluvial terraces of known elevation low T axes are consistent with active downslope surface uplift rates (Fisher et al., 1998; Marshall were used to calculate average incision rates. extension along northwest-striking normal faults et al., 2000). From northwest to southeast, six Large rivers eroding through weak rocks near observed in seismic-refl ection profi les across the fault-bounded blocks are mapped: the Esparza, the coast are assumed to have suffi cient stream upper slope offshore of Punta Guiones (McIn- Orotina, Herradura, Esterillos, Parrita, and Que- power to keep pace with uplift. As such, the shape tosh et al., 1993), suggesting that such mesoscale pos blocks (Fig. 1C). This region corresponds of the river longitudinal profi le is a proxy for the fault populations (Table 2; Fig. 2A, insets c, d, to a pronounced change in the morphology of longitudinal profi le at the time of terrace deposi- and i; Fig. 4) formed under the same kinematic the subducting plate from elongate ridges to tion, and it was used to calculate average surface conditions. Some of the faults imaged across the linear arrays of conical seamounts oriented at uplift rates for fault-bounded blocks. From north- upper slope displace the ocean bottom, indicat- low angles to the relative convergence vector. west to southeast across the six fault-bounded

Figure 4. Map meshing the bathymetry of the Middle American Trench region (from Ranero et al., 2003), age of subducting seafl oor (from Barckhausen et al., 2001), and forearc geology (modifi ed from Lew, 1983; Bullard, 1995; Tournon and Alvarado, 1995; Marshall et al., 2000; Fisher et al., 2004; Sak et al., 2004a), supplemented by our mapping efforts, the distribution of mesoscale fault population data (shaded stereographic projections) from Marshall et al. (2000), Fisher et al. (2004), and this study (Table 2), and historical upper-plate earthquake focal mechanisms (larger black and white stereographic projections; historical upper-plate earthquake focal mechanisms are from Montero [1999] and Pacheco et al. [2006]). The fault-plane solutions depict the shortening and extension axes of individual mesoscale fault populations. Slickenline morphology varies predominately as a function of lithology. Fault surfaces in sandstone and limestone are typically ornamented with stepped fi brous (mostly calcite) mineral growth. The orientation of fi brous calcite steps commonly refl ects motion along the fault plane, with down-stepping fi bers in the direction of transport of the missing block (Petit, 1987). In the absence of stepped fi brous growths, fault slip was determined using grooved and striated fault surfaces in combination with Riedel-type shears (Petit, 1987). Symbols: FSR—Fisher Seamount and Ridge; TJ—trace of the East Pacifi c Rise–Cocos–Nazca spreading center–Caribbean triple junction; QP—Quepos Plateau; CR—Cocos Ridge; MAT—Middle America Trench; heavy arrow—present-day plate motions relative to a fi xed Caribbean plate (DeMets, 2001).

1000 Geological Society of America Bulletin, July/August 2009 Forearc deformation along the Costa Rican segment of the Middle American Trench N

ica 84°W CR CR Costa R CO-CA 15 Fig. 4 Fig. area of P T T T P P km

l l

P a

T q E P P T 01020 P T P T P P T T North T P P T P T T 16 P T T P A P P T MA MAT 8°N

a T

T e

l l

q E P P T P

q E P

T a

q E T 17 P QP P T 85°W P T Focal mechanism solution for historical upper-plate earthquake mechanism solution for Focal population measured Mesoscale fault in Neogene strata population measured Mesoscale fault and older strata in Paleogene T T P T P P P T T P 18 P T T T A P MA MAT T P P T P T T P P P 19 T T P P T P T T T P T P FSR T P Neogene sediment sediment Paleogene Campanian/Maastrichtian sediment Ophiolitic basement P T 21.5 Legend T P 22 P 86°W T T T P A P MA MAT T T P 9°N T P P T P T P Nicoya SegmentNicoya Coast Segment Central Pacific Fila Costeña Segment T T P TJ TJ Quaternary sediment Quaternary ignimbrites Neogene - Quaternary volcanic Neogene - Quaternary intrusives 10°N 24

Geological Society of America Bulletin, July/August 2009 1001 Sak et al.

TABLE 2. MESOSCALE FAULT POPULATION DATA Site* Latitude Longitude Outcrop location and type† Age§ Total faults P axis T axis (°N) (°W) Nicoya Peninsula a 9°58′ 85°41′ Punta Pelada (sp) Pg 27 114, 17 350, 61 b 9°55′ 85°40′ Punta Guiones (sp) Pg 30 291, 32 193, 11 c 9°54′ 85°39′ Punta Guiones (sp) Pg 19 111, 69 359, 08 d 9°54′ 85°39′ Punta Garza (sp) Pg 15 191, 48 030, 39 e 9°53′ 85°36′ Playa Barco Quebrada Pg 20 275, 22 005, 03 f 9°53′ 85°32′ Quebrada Taranta (rb) K 15 015, 74 132, 07 g 9°53′ 85°34 Quebrada Esterones (rc) Pg 11 184, 45 300, 23 h 9°54′ 85°32′ Nicoya-Samara Hwy (rc) K 15 185, 78 313, 06 i 9°51′ 85°31′ Punta Indio (sc) Pg 25 326, 86 203, 02 j 9°53′ 85°29′ Puerto Carrillo (sp) K 17 333, 25 230, 23 k1 9°37′ 85°09′ Punta Barrigona, Malpais (sp) Ng 8 085, 29 347, 15 l1 9°37′ 85°08′ Quebrada Vanegas, Malpais (rc) Ng 8 338, 32 245, 04 m1 9°35′ 85°06′ Cabo Blanco Reserve (rc) Pg 10 185, 78 008, 15 Central Pacific coast region n1 9°57′ 83°43′ Finca Machuca, Gregg de Esparza (q) Ng 14 356, 06 086, 02 o1 10°01′ 84°40′ Rio Barranca, Maranonal (rb) Ng 9 167, 47 267, 09 p1 9°58′ 84°41′ Esparza-Artieda road, Humo (rc) Ng 10 232, 08 141, 05 q1 9°54′ 84°44′ Punta Corralillo (sc) Ng 7 227, 35 128, 11 r1 9°53′ 84°43′ Playa Tivives (sc) Ng 20 213, 52 312, 07 s1 9°55 84°41′ Costanera Hwy, Rio Jesus Maria (rc) Ng 8 038, 13 301, 28 t1 9°51′ 84°41′ Penon Bajamar (sc) NgQ 9 192, 24 288, 13 u1 9°37′ 84°35′ Costanera Hwy, Rio Tarcoles (rc) Q 14 114, 64 298, 26 v1 9°43′ 84°40′ Punta Leona (sp, sc) Ng 15 360, 81 094, 01 w1 9°40′ 84°40′ Playa Coyol, Puerto Escondido(sc) Ng 27 142, 75 306, 15 x1 9°38′ 84°38′ Hacienda Jaco, Playa Jaco (q) Ng 19 294, 81 174, 05 y 9°31′ 84°31′ Punta Judas (sp) Ng 20 099, 00 009. 00 z 9°31′ 84°31′ Punta Judas (sp) Ng 16 186, 00 096, 00 aa 9°32′ 84°26′ Costanera Hwy, Bejuco (rc) Q 11 121, 34 270, 52 ab1 9°32′ 84°16′ Costanera Hwy, Vueltas (rc) Q 6 274, 72 106, 18 ac 9°33′ 84°15′ Road north from Damas (rc) Ng 9 196, 88 350, 02 ad1 9°23′ 84°09′ Punta Catedral, Manuel Antonio (sc) Ng 23 061, 86 217, 04 ae 9°23′ 84°07′ Rio Naranjo (rb, sc) Ng 19 292, 01 201, 64 Fila Costeña af2 9°13′ 83°50′ Roca Negritas (sp) Ng 17 234, 20 032, 69 ag2 9°05′ 83°41′ Playa Ventanas (sc, sp) Ng 38 216, 11 356, 76 ah2 9°06′ 83°42′ Punta Pinuela (sc, sp) Ng 16 258, 63 023, 16 ai2 9°07′ 83°43′ Punta Chimenea (sc, sp) Ng 14 208, 49 030, 41 aj2 9°08′ 83°44′ Piedra Pichote (sp) Ng 6 237, 18 090, 69 ak2 9°08′ 83°44′ Playa Pedregosa (sp) Ng 14 114, 17 314, 72 al2 9°11′ 83°47′ Punta Puertecito (sp) Ng 11 144, 25 346, 63 am2 9°12′ 83°48′ Road to Escalares in Queb Diablo Basin (rc) Ng 7 273, 16 058, 71 an2 9°11′ 83°48′ Punta Mostradores Ng 10 222, 57 349, 22 ao2 9°13′ 83°50′ Costanera Hwy, SE of Dominical (rc, q) Ng 12 306, 22 172, 60 ap2 9°14′ 83°51′ Punta Dominical (sc, sp) Ng 17 090, 06 320, 82 aq2 9°16° 83°52′ Dominical–San Isidro road (rc) Ng 24 242, 03 349, 80 ar2 9°12′ 83°45′ Rio Higueron (rb) Ng 15 214, 16 352, 69 as2 9°03′ 83°36′ Tres Rios (rc, rb) Ng 18 037, 11 263, 75 at2 9°04′ 83°35′ Unnamed stream, Rio Coronado basin (rb) Ng 14 276, 04 175, 70 au2 9°04′ 83°36′ Quebrada Fria (rb) Ng 11 245, 05 085, 85 av2 9°04′ 83°38′ Rio Punta Mala (rb) Ng 17 224, 08 088, 79 aw2 9°00′ 83°32′ Unnamed stream, Rio Baslar basin (rb) Ng 34 358, 04 118, 82 ax2 9°03′ 83°36′ Costanera Hwy, SE of Coronado (rc, rb) Ng 16 246, 15 048, 75 ay2 9°03′ 83°39′ Costanera Hwy, SE of Punta Mala (rc) Ng 8 088, 21 244, 68 az2 9°06′ 83°39′ Costanera Hwy, Tortuga Abajo (rc) Ng 16 201, 29 031, 60 ba2 9°04′ 83°38′ Rio Tortuga (rb) Ng 11 239, 22 041, 67 Note: 1—site from Marshall et al. (2000); 2—site from Fisher et al. (2004). *Sites refer to letters assigned in the text. †Outcrop types: q—quarry; rc—road cut, rb—river bank; sp—shore platform; sc—sea cliff. §Deposit ages: K—Cretaceous; Pg—Paleogene; Ng—Neogene, Q—Quaternary.

1002 Geological Society of America Bulletin, July/August 2009 Forearc deformation along the Costa Rican segment of the Middle American Trench

84 30' W 34 Description Thrust fault D U Qt Quaternary fluvial v Location of fault terrace deposits measurement site Mpj Miocene Punta Judas Fm v Knc Figure 5. Geologic map span- River Tps Tertiary pelagic sediments ning the central seamount seg- D Normal fault U Cretaceous Nicoya Complex: ment of the Costa Rican forearc Herradura block Knc Basalts and deep marine showing the geology of the Her- 34 Primary road sediments radura, Esterillos, Parrita, and 9 40'N Secondary road w 34 Quepos blocks. Mesoscale fault

239 Knc population data (lower-hemi- x Esterillos block sphere, equal-area projections) 22 Knc are keyed to the map by letter Jaco 34 (Table 2). Compressional (P) D U U Tps 239 axes (black circles—individual D Rio Parrita block Tps faults; black square—average) U Parrita D Mpj U and tensional (T) axes (black Qt D Qt Qt circles—individual faults; black aa Qt 34 square—average) deﬁ ne the Punta Judas y/z Parrita 9 30'N ab ac best-ﬁ t fault-plane solution for Esterillos 34 each fault population (Marrett Oeste Damas and Allmendinger, 1990). Black dots—locations of dated sam- Pacific Ocean ples from the Esterillos surface Scale Quepos (Table 1). See Figure 1C for N 0 5 10 km location. v w x y z aa ab ac ad D T T T U ad

T T P P P P P T P T P P T T P

blocks with alluvial terraces, uplift rates are vari- 1.568–1.674 ka (Table 1). The calculated late blocks (Figs. 1C and 4). Here, strain localization able: 0.7 m k.y.–1 for the past ~240 k.y. on the Holocene surface uplift rate ranges from 0.1–1.8 results in segmentation of the overriding plate Esparza block (Marshall, 2000), 0.4 m k.y.–1 for to 1.8–4.4 m k.y.–1 (Table 1; Fig. 3). To the north- into ~20-km-wide fault-bounded blocks that the past ~240 k.y. on the Orotina block (Marshall, west on the Esparza block, a ca. 3 ka wood sam- accommodate differential rates of surface uplift. 2000), 1.2 m k.y.–1 for the past ~120 k.y. on the ple collected from beneath a colluvial wedge on Mesoscale fault populations measured near Esterillos block (Sak, 2002), 0.14 m k.y.–1 for the an uplifted marine platform at the base of Punta block boundaries and within block interiors are past ~120 k.y. on the Parrita block (Sak, 2002), Carballo (Fig. 1C) yielded a Holocene uplift characterized by shallow P and T axes (Table 2; 0.19 m k.y.–1 for the past ~120 k.y. on the Quepos rate of 1.2–2.0 m k.y.–1 (Table 1; Fig. 3) (Fisher Fig. 4). Intrablock faults are concentrated near block (Murphy, 2002), 0.11 m k.y.–1 for the past et al., 1998). To the southeast, in the vicinity of block boundaries and typically display less ~120 k.y. on the Rio Savegre (Murphy, 2002), the mouth of the Rio Terreba, ca. 6.4 ka wood apparent displacement (~<10 m) than the block- and ~1.1 m k.y.–1 for the past ~120 k.y. on the Rio collected from beneath a beach ridge yielded a bounding faults (>100 m). The presence of a Terraba (Murphy, 2002). Holocene uplift rate of 0.7–1.3 m k.y.–1 (Table 1; steep gradient between localized zones of high Holocene marine terraces in coastal exposures Fig. 3) (Fisher et al., 2004). strain separated by relatively little brittle defor- across the seamount-dominated segment of the mation, as is observed between the Herradura Middle American Trench provide constraints on Fault Kinematics Along the Central Pacifi c and Parrita blocks (Fig. 5), may be attributed late Holocene surface uplift. For example, to the Coast to strain localization. Both observed regional- west of the village of Esterillos Oeste (Fig. 5), scale fault patterns and kinematic analysis of there is an ~6-km-long, dissected, late Holo- Along the central Pacifi c coast, the subaerially mesoscale fault populations along the margins cene wave-cut marine terrace, the Esterillos exposed forearc is dissected by steeply dipping of the fault-bounded blocks are consistent with surface. The Esterillos surface, which truncates northeast-striking faults. These faults bound transtension along steeply dipping northeast- folded Miocene beds, is confi ned to elevations blocks characterized by differential rates of sur- striking faults (Figs. 4 and 5). Along the south- of <3 m. Overlying a thin (<50 cm) horizon of face uplift (Fisher et al., 1998). Inboard of the ern tip of the Nicoya Peninsula, Marshall et al. shell debris (thick-wall gastropod shells and ~10-km-wide Tarcoles Scar (E in Fig. 1C) on the (2000) recognized transtension (Table 2; Fig. 4, coral debris), there is a light-gray, fi ne-grained slope apron, an ~60-km-wide, broadly uplifted insets k and l) opposite where Fisher Seamount sandy entisol. Radiocarbon dating of shell sam- region is observed, extending from the Orotina and Ridge enters the Middle American Trench. ples yielded ages ranging from 0.956–1.034 to block east across the Herradura and Esterillos Similarly, historical upper-plate earthquakes,

Geological Society of America Bulletin, July/August 2009 1003 Sak et al.

including the 2004 Mw 6.4 Damas (Pacheco et Quaternary Uplift Rates on the Osa Fault Kinematics Along the Osa Peninsula al., 2006) and the 1924 Ms = 7.0 San Casimiro Peninsula (Montero, 1999) (Fig. 4) events, suggest ongo- Across the northwest coast of the Osa Penin- ing transtensional deformation across the forearc Late Quaternary surface uplift rates across sula, exposures of the late Pleistocene Marenco inboard of subducting seamounts. the northwestern coast of the Osa Peninsula are formation are dissected by northwest-striking In contrast, southeast of the Quepos block, constrained by a suite of 14 shell and woody planar faults, with separations locally in excess the subaerially exposed forearc is dissected by debris samples. The samples, collected from of 40 m; no penetrative mesoscale faults are northwest striking, margin-parallel, seaward- nine measured stratigraphic sections of late recognized within the Marenco formation, sug- verging, shallowly dipping thrust faults. These Pleistocene marine sands, record a complex gesting that strain across the Osa Peninsula is faults accommodate ≥17 km of shortening history of subsidence followed by rapid (locally accommodated along the widely (kilometer across the active Fila Costeña fold-and-thrust > 6 m k.y.–1) uplift, which has been attributed scale) spaced, northwest-striking faults. Sak et belt (Fisher et al., 2004; Sitchler et al., 2007). to the passage of relief along the axis of the al. (2004a) interpreted these subvertical faults Mesoscale fault populations measured across underthrusting Cocos Ridge (Sak et al., 2004a) as the surface expression of up-shear required the Fila Costeña are characterized by shallow (Table 1; Fig. 3). Gardner et al. (1992) used 15 to accommodate the arrival of rough crust along P and steep T axes (Table 2; Fig. 4). Observed dated samples of shells and woody debris col- the axis of the subducting Cocos Ridge. Along map patterns and kinematic analysis of the lected across the southeastern coast of the Osa the northeast coast of the Osa Peninsula, uplift mesoscale fault populations are consistent with Peninsula to quantify late Quaternary surface rates decrease linearly to the northwest (Gard- top to the southwest displacement along north- uplift rates and demonstrate that surface uplift ner et al., 1992). Superimposed on this trend west-striking thrust faults (Fisher et al., 2004). rates over the past 50 k.y. decrease to the north- of decreasing uplift rate, there is a series of east, away from the trench. In Table 1, we used poorly exposed northwest-striking subvertical THE OSA PENINSULA the eustatic sea-level curve of Lambeck and faults that result in northeast-side-up separa- Chappell (2001) to recalculate the uplift rates tion (Gardner et al., 1992). The recognition of The Osa Peninsula is an ~60-km-long high in for deposits predating the Last Glacial Maxi- northwest-striking subvertical faults along both the outer forearc directly inboard of the axis of the mum studied by Gardner et al. (1992) (Fig. 3). the northwest and northeast coasts of the Osa aseismic Cocos Ridge. Across the northwestern The deposits younger than 10 ka used by Gard- Peninsula may reﬂ ect the deformation caused and southeastern coasts of the peninsula, uplifted ner et al. (1992) to constrain uplift rates were by relief along the crest of the underthrusting late Quaternary marine deposits have been dated reevaluated using the Fleming et al. (1998) Cocos Ridge (Sak et al., 2004a). (Gardner et al., 1992; Sak et al., 2004a). The sea-level curve (Table 1). The revised uplift Quaternary deposits disconformably overlie rates preserve the originally observed pattern DISCUSSION beveled exposures of semilithiﬁ ed Late Tertiary (Gardner et al., 1992) of decreasing uplift rate and Quaternary sediment of the Charco Azul and as a function of distance away from the Middle Late Quaternary uplift rates vary both along Armuelles Formations (collectively mapped as American Trench, as would be expected based and across the forearc of the Middle American TQs) and the Paleogene Osa mélange (Sprech- on increases in depth to the plate interface or Trench in Costa Rica (Figs. 3 and 6). Uplift rates mann, 1984; Corrigan et al., 1990; DiMarco et with the passage of relief through the subjacent are greatest across the outer coasts of the Nicoya al., 1995; Vannucchi et al., 2006). plate interface (Sak et al., 2004a). (≤6.8 m k.y.–1) and Osa (6.5 m k.y.–1) Peninsulas

Figure 6. (A) Distribution of surface uplift rates calculated from marine and alluvial terraces across the Costa Rican forearc plotted as a function of distance along the trench. (B) Map of the Pacifi c Coast of Costa Rica showing the locations of dated late Quaternary samples, rupture areas (enclosed by gray lines), and epicenters of historic earthquakes (white dots). (C) Crustal structure of the incoming Cocos plate from von Huene et al. (2000). Location of transect in C is shown in Figure 1. Note that portions of the forearc characterized by the greatest uplift rates are opposite bathymetric highs on the subducting Cocos plate shown in C, with the exception of the portion of the forearc opposite the Quepos Plateau (QP). The Quepos Plateau, unlike the other bathymetric features on the subducting plate, is oriented oblique to the Cocoas-Panama convergence vector (Fig. 1). (D) Detail across the south-central Nicoya Peninsula from Punta Guiones to Cabo Blanco. Open gray triangles represent uplift rates for Garza surface exposures at Playa Camaronal and at Playa Islita and a pedogenically similar exposure at Puerto Coyote, assuming these deposits are 4 ka. (E) Calculated uplift rate for dated samples of Holocene marine terraces exposed along the northeast coast of the Nicoya Peninsula plotted as a function of distance from Cabo Blanco. Linear best-fi t regression illustrates the signifi cant decrease in uplift rate away from the tip of the peninsula. (F) Calculated uplift rate for dated samples of late Quaternary deposits exposed across the northeastern Osa Peninsula. Samples are projected onto the black line (shown in the inset) and plotted as a function of distance northeast of Cabo Matapalo. Linear best-fi t regression illustrates the signifi cant decrease in uplift rate away from the tip of the peninsula. Superimposed upon this trend is displacement across a steeply dipping, northwest-striking fault. Solid inverted triangles—Gardner et al. (1992). Symbols: open triangles—this study; black inverted triangles—Gardner et al. (1992); open diamonds—Marshall and Anderson (1995); open circle—Fisher et al. (1998); black diamonds—Gardner et al. (2001); black square— Fisher et al. (2004); gray squares—Sak et al. (2004a); dashed vertical gray lines—block-bounding faults; horizontal gray bars—uplift rates of fl uvial terraces (Marshall, 2000; Murphy, 2002) and this study; Ez—Esparza block; O—Orotina block; H—Herradura block; Es— Esterillos block; P—Parrita block; Q—Quepos block; PI—Playa Islita; PC—Puerto Coyote; gray lines and large open dots—rupture areas and epicenters, respectively, for historical earthquakes (from Adamek et al., 1987; Tajima and Kikuchi, 1995; Protti et al., 2001; Bilek and Lithgow-Bertelloni, 2005); RS—Rio Savegre; RT—Rio Terraba; TJ—fossil trace of the East Pacifi c Rise–Cocos–Nazca spreading center– Caribbean triple junction; FSR—Fisher Seamount Group; QP—Quepos Plateau; Cocos—Cocos Ridge.

1004 Geological Society of America Bulletin, July/August 2009 Forearc deformation along the Costa Rican segment of the Middle American Trench

Nicoya Seamount Osa Segment Segment Segment

D 6 SW NE 8 Nicoya 4 E Peninsula 7 20 km

2 10

10 (mm/yr) 6 Uplift rate Cabo 5 Blanco 0 N 9 0 TJ FSR 4 8 3 y = 5.0 - 0.20x R2 = 0.8 7 2 375 325 300 Distance (km) (mm/yr) Uplift rate 1 6 0 0510 15 20 5 Distance along coast (km) 4

3 Uplift rate (mm/yr) Uplift rate 2 SW NE Osa 1 8 Peninsula

7 0 CM 15 km 6 0 N -1 5 y = 11.6 - 1.2x -2 A n = 66 2 4 R = 0.7 Ez 10°N RS B Q 3 zone Fault O P RT H Es Uplift rate (mm/yr) Uplift rate 2 y = 6.7 - 0.91x Nicoya R2 = 0.7 F PI PC Osa 1 Playa 468101214 Camaronal Distance along coast (km) 020 85°W 9°N CO-CA 84°W N km TJ FSR 2 QP Cocos 4 Upper Crust 6 8 10 Moho

12 Depth (km) C 14

350 300 250 200 150 100 50 0 Distance (km)

Geological Society of America Bulletin, July/August 2009 1005 Sak et al. and of lesser magnitude (≤4.4 m k.y.–1) along the the development of out-of-sequence thrust faults inboard of subducting seamounts contrasts with central Pacifi c coast (Figs. 3, 6A, and 6B). The results in shortening across the inner forearc and/ broader regions of uplift within the inner forearc. pattern of along-strike variations in uplift rate or a fault truncation of the arcward margin of Across the central Pacifi c coast region, margin- matches the distribution of bathymetric highs the slope apron. Under these circumstances, the perpendicular faults accommodated differential across the subducting plate (Figs. 3 and 6C). greatest subsidence in the outer forearc occurs rates of uplift. Areas with the greatest rates of For example, terraces along the Nicoya coast in the footwall of the reverse fault that trun- surface uplift expose Paleogene and older rocks show the greatest uplift rates by Punta Guio- cates the slope apron, and slope sediment could and are inboard of localized zones of subsidence nes and Cabo Blanco, inboard of the subduct- thicken landward. Alternatively, where offshore imaged across the submarine portions of the ing bathymetric features related to the Cocos- stresses associated with rough crust subduction forearc (Fig. 9D). Opposite the Cocos Ridge, Nazca-Panama triple junction trace and the are transmitted across a rigid forearc to the back continued shortening in the Fila Costeña fold- Fisher Seamount and Ridge, respectively. There arc, internal shortening (within the forearc) may and-thrust belt resulted in development of suc- is a sag in the elevation of uplifted marine ter- be accommodated by the development of large- cessive active frontal thrust faults seaward of the races that lies directly inboard of the sag in the scale folding (Fig. 7). Following Marshall et previous frontal thrust (Fig. 9D). bathymetry between the fossil triple junction al. (2000), who defi ned the Central Costa Rica Mesoscale fault populations suggest that the trace and the Fisher Seamount and Ridge on the deformed belt on the basis of changes in fault forearc can be subdivided into three kinematic subducting plate (Fig. 6D). The leading edges kinematics from sinistral transtension across domains that correlate to the geometry and of the two outer forearc peninsulas are charac- steeply dipping northeast-striking faults in the morphology of the subducting plate (Fig. 4), terized by late Quaternary uplift rates locally forearc domain to a system of conjugate north- as well as distance from the trench. Along the ≥6 m k.y.–1. Low-lying, Holocene marine ter- west- and northeast-striking transcurrent faults Nicoya Peninsula, where the crust currently races exposed along the northeast-trending across the Central Costa Rica deformed belt, entering the Middle American Trench is rela- coastline of the southern tip of Nicoya indicate we propose that this boundary represents the tively smooth and the Benioff zone defi nes a that long-term uplift rates diminish away from arcward extent of deformation related to colli- steeply dipping slab, mesoscale fault popula- Cabo Blanco to both the northeast (away from sions in the forearc (Fig. 7B). The observation in tions are consistent with margin-perpendicular the trench; Fig. 6E) and the northwest (away seismic-refl ection profi les that upper slope sedi- extension and steeply dipping faults at high from the Fisher Seamount and Ridge; Marshall ment thickens landward (McIntosh et al., 1993; angles to the margin that allow differential dis- and Anderson, 1995; Gardner et al., 2001). Late Hinz et al., 1996) is consistent with either trun- placement along the margin, while many of the Pleistocene marine terraces exposed along the cation along an out-of-sequence thrust fault near other mesoscale fault populations measured in northeast-trending coastline of the southern tip shore that separates the uplifting inner forearc the vicinity of Punta Guiones (Figs. 2A and 4) of the Osa Peninsula record a similar pattern of from the subsiding outer forearc or an increase are consistent with the normal faults imaged by diminishing long-term uplift rates away from in sediment thickness closer to an onland source McIntosh et al. (1993) across the upper slope, the trench (Gardner et al., 1992) (Fig. 6F). area. Seismic lines of the last two decades do not offshore of Punta Guiones. Some of these faults, The crustal thickening in the subaerial por- cross the shallow offshore region where such a which have been attributed to downslope creep tion of the forearc is in direct contrast to the fault could be imaged. of poorly consolidated sediment overlying the net crustal thinning imaged (McIntosh et al., The passage of bathymetric features results older prism, displace the ocean fl oor, indicating 1993; von Huene et al., 2000) and modeled in narrow scarp-bounded corridors of subsi- that the deformation is ongoing (McIntosh et al., (Dominguez et al., 1998) in the outer forearc. dence across the submarine slope and sustained 1993). Across the Osa Peninsula, like the central There are three mechanisms for thickening of broader uplift and segmentation across the inner Nicoya Peninsula, steeply dipping, northwest- the subaerially exposed portions of the forearc: forearc. The structural history and the late Pleis- striking faults dissect late Pleistocene marine underplating (accretion of subducted seamounts tocene landscape evolution observed inboard of sediment. The northwest-striking subvertical and/or sediment from the outer slope; i.e., subducting rough crust are consistent with the faults on the Osa Peninsula are interpreted to Fisher et al., 1998; Bangs et al., 2006) (Figs. 7 model shown in Figure 9. Circa 1 Ma, in the accommodate leading-edge-up shear necessary and 8), shortening along out-of-sequence thrust absence of subducting seamounts, the subma- to accommodate irregular bathymetry along the faults (active faults that lie arcward of the axis of rine margin was laterally continuous (Fig. 9A). axis of the indenting Cocos Ridge (Sak et al., the Middle American Trench; i.e., Fisher et al., With the onset of rough crust subduction, the 2004a). Elsewhere along the margin, steeply 1998, 2004) (Figs. 8A and 8B), or transmission margin became embayed, and arcward retreat dipping, northwest-striking faults imaged across of shortening across the rigid forearc to the back of the trench began (Fig. 9B). Inboard of the the submarine forearc have been attributed arc (Mann et al., 1998) (Fig. 7A). Although the leading edge of the Cocos Ridge, some of the to collapse of the surface in the wake of sub- difference between submarine subsidence and convergence was transferred from the Middle ducting seamounts as a result of basal erosion uplift within the subaerially exposed portions of American Trench to the Fila Costeña fold-and- (i.e., Hinz et al., 1996; Dominguez et al., 1998; the forearc might be explained by any of these thrust belt (Fig. 9B). Continued subduction of Ranero and von Huene, 2000; Hühnerbach et mechanisms, or a combination of processes, rough crust resulted in further arcward retreat of al., 2005). Similarly, along the northern Puerto the distribution of uplift and forearc kinemat- the Middle American Trench and scalloping of Rico–Virgin Islands margin, rough crust sub- ics sheds insight into the dominant mechanism. the submarine portions of the forearc in the wake duction has resulted in forearc collapse in the Where underplating is the dominant means of of subducting seamounts (Fig. 9C). Inboard of wake of subducting ridges, attributed to basal crustal thickening, no shortening within the inner where the Cocos Ridge intersects the Middle erosion (Grindlay et al., 2005). forearc is required to explain crustal thickening American Trench, shortening continued within Along the central Pacifi c coast, northeast- and forearc mountain building. The uplift of the the Fila Costeña fold-and-thrust belt with the striking steeply dipping nodal planes accommo- inner forearc by underplating could cause sea- development of additional thrust faults imbri- date differential uplift associated with ongoing ward tilting of slope deposits and thinning of the cating Paleogene and older strata (Fig. 9C). seamount subduction (Fig. 4). Further to the slope apron toward the inner forearc. In contrast, Ongoing subsidence and erosion of the forearc southeast, in the inner forearc and inboard of

1006 Geological Society of America Bulletin, July/August 2009 Forearc deformation along the Costa Rican segment of the Middle American Trench

A Cerro B MAT Turrubares Seamount CCRDB

SM 50 km 50 km

CO-CA Figure 7. (A) Simpliﬁ ed regional scale cross section from the subducting Cocos plate across the Middle American Trench to the arc showing the geometry of the subducting slab beneath across the Herradura block along B to B′ (Fig. 1C) drawn without vertical exaggeration. Here, forearc thickening results from the transmission of stress through the rigid forearc (represented by converging gray arrows). Seamounts shown on the downgoing plate (SM) have 1.5 km of relief and a 20 km footprint. It is unlikely that underplating is the dominant mechanism of crustal thickening here, given that the depth to the top of the downgoing slab beneath the Cerro Turrubares Herradura block is ~40 km. (B) Block diagram summarizing geologic structures measured across the forearc of the central Paciﬁ c coast domain. CO-CA—Cocos-Caribbean.

A Outer forearc Inner forearc Figure 8. (A) Simpliﬁ ed regional scale N. Panama Seamount Osa Fila Costena cross section from the Cocos plate across deformed belt the Middle American Trench to the north Panama deformed belt showing the geometry of the subducting slab along C to C′ Subducted remnant of the (Fig. 1C) drawn without vertical exagger- Panama fracture zone Leading edge of the ation. (B) Cartoon showing forearc thick- Cocos Ridge ening as a result of: (1) underplating, and 50 km B Inner forearc (2) the development of an out-of-sequence Outer forearc C thrust fault. In the case of underplating, (2) the seamount detaches from the top of the downgoing plate and couples with (1) the base of the forearc, and no forearc shortening is required. In contrast, where forearc thickening is the result of shortening along an out-of-sequence fault, forearc shortening is required, and the arcward margin of the perched sedimentary basin (light gray) is truncated along a fault. (C) Block diagram summarizing geologic structures measured across the forearc of the Fila Costeña domain. CO- CA—Cocos-Caribbean. CO-CA

Geological Society of America Bulletin, July/August 2009 1007 Sak et al.

ca. 1 Ma ca. 500 ka A FC B

MAT MAT

QP CR

TJ TJ QP CR FSR North CO-CA North CO-CA

ca. 250 ka FC Present C FC D

Nicoya Osa

MAT MAT

QP FSR TJ QP CR FSR CR TJ North CO-CA North CO-CA Figure 9. Tectonic evolution of the Pacifi c coast of Costa Rica for the past 1 m.y. This history could include more old seamount collisions that have left no record due to healing at ~140 k.y. time scales (von Huene et al., 2004). Symbols and units: white—Cocos plate; darkest gray polygons—seamounts on the Cocos plate outboard of the Middle American Trench (heavy black line); intermediate gray polygons— footprints of subducting seamounts beneath the submarine forearc; dashed lines—projections of subducted bathymetric features; lightest gray—shelf sediments; light gray—slope sediments; dark gray—Paleogene to recent deposits; black—Nicoya Complex and Osa mélange (undifferentiated); circles with spokes—volcanoes. (A) 1 Ma: Convergent margin with a laterally continuous lower slope, forearc basin, and volcanic arc. (B) 500 ka: Subduction of seamounts and plateaus resulting in scalloping of the trench slope and shifting of convergence across southern Costa Rica from the trench axis to the inner forearc. (C) 250 ka: Ongoing rough crust subduction resulting in trench retreat and subsidence of the upper slope, uplift, and erosion of the inner forearc resulting in exposure of the basement within the inner forearc. (D) Present: Continued subsidence and erosion of the outer forearc, uplift, and erosion of the inner forearc with arc-perpendicular faults that allow for variations in uplift rate along the central Pacifi c coast segment of the inner forearc. Exposure of basement rocks within the inner forearc is confi ned to the regions with the fastest uplift rates. CR—Cocos Ridge; MAT—Middle America Trench; CO-CA—Cocos- Caribbean; TJ—fossil trace of the East Pacifi c Rise–Cocos–Nazca spreading center–Caribbean triple junction; FSR—Fisher Seamount Group; QP—Quepos Plateau. FC—Fila Costeña.

1008 Geological Society of America Bulletin, July/August 2009 Forearc deformation along the Costa Rican segment of the Middle American Trench where the aseismic Cocos Ridge is underthrust, during the 1990 event (two subevents between mid-Miocene sediment observed along both the mesoscale faults generally record top-to-the- 18 and 24 km depth) and the aftershock pattern margins of the Orotina and Esterillos blocks southwest slip, and mesoscale fault populations led Protti et al. (1995b) to suggest that the ini- and the absence of growth-strata geometries are characterized by northwest-striking nodal tial rupture area was ≤600 km2. The location of suggest that uplift of the Herradura block post- planes and gently inclined P and steeply inclined the hypocenter in the vicinity of the Fisher Sea- dates mid-Miocene deposition. The 1.7–1.1 Ma T axes (Table 2; Fig. 4). Both mapped regional- mount and Ridge and the potential presence of a Tivives Formation lahars are defl ected to the scale and mesoscale fault populations across the subducted seamount in high-resolution seismic northwest around the Herradura high onto the Fila Costeña fold-and-thrust belt are consistent tomography suggest that this event may have Orotina Fan (Fig. 4) (Marshall, 2000; Marshall with a minimum of 17 km of shortening in the resulted from the rupture of seamount-induced et al., 2003). Thus, uplift of the Herradura high last 2–5 m.y. (Fisher et al., 2004) (Fig. 4) and asperity, as suggested by Protti et al. (1995b) postdates deposition of both the Miocene Punta a minimum of 36 km of post–middle Pliocene and Husen et al. (2002). Seismic tomography Carballo and Punta Judas Formations and pre- shortening inboard of where the axis of the reveals that individual seamounts along the trend dates emplacement of the Tivives lahars. Cocos Ridge intersects the Middle American of the Fisher Seamount and Ridge have a basal This post–mid-Miocene maximum age of Trench (Sitchler et al., 2007). footprint of ~300–400 km2 (Husen et al., 2002). initiation of rapid uplift of the coastal forearc is Surface uplift has been observed inboard of Bilek et al. (2003) suggested that the ≤600 km2 consistent with the proposed (5–6.5 Ma) onset many erosive margins, including at the Nan- rupture area calculated by Protti et al. (1995b) of accelerated subsidence offshore of the central kai (Bangs et al., 2006), northern Chilean (von may refl ect rupture of multiple seamounts. Nicoya Peninsula (Vannucchi et al., 2001, 2003) Huene and Ranero, 2003), Peruvian (i.e., von One of the most striking examples of the con- and with reported (3–0.5 Ma) estimates for the Huene and Lallemand, 1990; Bourgois et al., trast between thinning of the submarine portions onset of rough crust subduction as marked by 2007), Solomon Islands (Mann et al., 1998; Tay- of the forearc and thickening of the subaerial the Cocos Ridge (Gardner et al., 1992; Collins lor et al., 2005), Tongan (Ballance et al., 1989; forearc occurs in the inner forearc along the et al., 1995; Kolarsky et al., 1995; Gräfe et al., Scholz and Small, 1997), and Middle Ameri- central Pacifi c coast. Offshore of the Herradura 2002; Sitchler et al., 2007; Morell et al., 2008). can (from Guatemala to Costa Rica) (Gardner block, the lower slope is scarred with one of the Either tectonic smoothing of the downgoing et al., 1992, 2001; Fisher et al., 1998; Bilek largest embayments recording the passage of a plate or transmission of stress across the rigid et al., 2003; Vannucchi et al., 2004) trenches. subducting seamount (Fig. 1C) (von Huene et forearc is consistent with both the observations Along the seamount-dominated segment, inner al., 1995). In the wake of the subducting sea- that uplifted portions of the subaerially exposed forearc uplift is greatest inboard of the extensive mount, there is an ~10-km-wide, northeast- forearc are wider than the scarred portions of the Tarcoles and Jaco scarps (Fig. 1C). In contrast, trending, scarp-bounded zone of subsidence lower slope (Fig. 1C) and that mapped fl ights there is a laterally extensive thrust belt inboard extending ~50 km arcward from the trench to a of alluvial fi ll terraces inboard of the seamount of the Cocos Ridge. cuspate headwall that is inferred to refl ect frontal domain diverge upstream (i.e., Pazzaglia et al., The subaerial portion of the forearc is com- erosion (Ranero and von Huene, 2000). Inboard 1998; Marshall, 2000). Upstream divergence posed predominately of Mesozoic Caribbean of this localized region of intense subduction of correlated alluvial terraces is consistent with oceanic plateau basement, which has greater erosion is Cerro Turrubares, the highest point increasing rates of surface uplift toward the rear strength than the sediment and thinner base- within the Costa Rican forearc (Fig. 1C). Cerro of the inner forearc. However, given the dip of ment closer to the trench (Sinton et al., 1998; Turrubares is within an ~60-km-wide, broadly the subducting slab (Fig. 7A), it is unlikely that Christeson et al., 1999). This competent base- uplifted region that extends from the Orotina the crustal thickening along the central Pacifi c ment may act as a backstop, shearing subduct- block east across the Herradura and Esterillos coast region is the result of underplating. The ing seamounts off of the older subjacent oceanic blocks (Fig. 1C). The Tertiary cover is mostly top of the Wadati-Benioff zone is ~40 km deep lithosphere (Werner et al., 1999). Accumula- eroded off of the Herradura block, suggesting beneath the Herradura block (Figs. 1C and 7A). tions of pelagic sediment deposited on juvenile the block has experienced extensive subaerial Alternatively, the permanent uplift opposite oceanic crust prior to off-axis seamount genesis exhumation (Fisher et al., 1998). subducting bathymetric highs in the absence of (i.e., Watts and Tenbrink, 1989; Scheirer et al., Across the Herradura block, the rates of upper-plate shortening may be explained by the 1996) may act as detachment surfaces. In this long-term uplift are diffi cult to resolve because transmission of stresses associated with the rigid scenario, the décollement steps down, trans- very few planation surfaces are cut across the forearc (Fig. 7A). High-resolution sidescan sonar ferring mass of the subducting seamount and ubiquitous exposures of the competent pillow surveys across the Parrita and Jaco Scarps (G and eroded lower-slope sediment entrained arcward basalt. However, both the neighboring Orotina F in Fig. 1C) reveal decreasing amounts of sur- of detached seamount from the downgoing to block to the northwest and the Esterillos block face uplift and increasing fracturing and faulting the overriding plate. Thus, this mechanism of to the southeast expose Miocene strata. Along with increasing distance from the Middle Ameri- tectonic smoothing would result in a thickened the eastern edge of the Orotina block, the Ter- can Trench, attesting to the increasing rigidity of upper plate. Due to the greater fl exural rigidity, tiary Punta Carballo Formation dips steeply the arcward-thickening forearc (Hühnerbach et on the order of 1020 N m2 (Gardner et al., 1992), (>50°) westward, away from the Herradura al., 2005). At the Tarcoles Scar (E in Fig. 1C), uplift of the subaerial portions of the forearc is block (Marshall, 2000). To the northwest, away 55 km inboard of the Middle American Trench, distributed across a wider area than the base from the Herradura block boundary, the dip of no surface uplift above the subducting seamount of the subducting seamount. For example, the the Punta Carballo decreases to ~10°–15° to the is recognized (Hühnerbach et al., 2005). extensive (~400 km2) uplifted Cobano surface, west (Marshall, 2000). Similarly, on the Ester- Along the Costa Rican forearc, all three which coincides with a relatively free-slipping illos block, the Punta Judas Formation dips mechanisms of crustal thickening—underplat- (<15% locking) portion of the plate interface steeply (~50°) eastward, away from the Herra- ing (Cabo Blanco area, Nicoya Peninsula, and (Norabuena et al., 2004), is in the vicinity of the dura high in the immediate vicinity of the block Osa Peninsula), transmission of stress across

1990 Mw = 7.0 Gulf of Nicoya earthquake (Protti boundary, and decreases to ~10°–15° to the east the rigid forearc (central Paciﬁ c coast region), et al., 1995b). The complex nature of the rupture across the Esterillos block. The steeply dipping and out-of-sequence thrusting (Fila Costeña)—

Geological Society of America Bulletin, July/August 2009 1009 Sak et al.

Active volcanic arc Extinct volcanic arc (5) The distribution of late Quaternary ter- Out of sequence thrust races and best-ﬁ t P and T axes for mesoscale fault populations across the Costa Rican forearc MAT provide insights into spatial variations in late Quaternary deformation. Permanent vertical Trench-parallel uplift is driven by both out-of-sequence thrust normal faults faults and the transfer of stress resulting from s Coco rough crust subduction across the rigid forearc. ge Rid Out-of-sequence thrusting occurs along the inner forearc southeast of Quepos in the Fila Costeña fold-and-thrust belt. Elsewhere, permanent surface uplift is consistent with the arcward transmission of stresses associated with rough Figure 10. Schematic block diagram showing the three proposed crust subduction across the rigid forearc driving mechanisms (stress transmission across the rigid arc, underplat- inner forearc uplift. ing, and out-of-sequence thrusting) of crustal thickening. MAT— (6) Across the subaerial portions of the Middle America Trench. Geologic units: Cocos plate—dark Costa Rican forearc, variations in the forearc gray; forearc basin sediment—light gray; extinct Talamancas response to rough crust subduction correlate to volcanic arc—short thick stippled pattern; Nicoya Complex— variations in the morphology of the subducting long narrow stippled pattern; volcanics of the active volcanic Cocos plate. arc—intermediate gray; Paleogene sedimentary rocks—stippled intermediate gray; roots of volcanic arc—white. ACKNOWLEDGMENTS

This research was funded by National Science may occur predominately in the outer forearc >40% shortening across the Fila Costeña (Fisher Foundation grants EAR-9909699 and EAR-0337456 with additional funding from the Dickinson Col- (Fig. 10). Along the outer forearc, four mecha- et al., 2004; Sitchler et al., 2007). lege Research and Development Committee. Marino nisms of crustal thinning—collapse in the wake Protti, César Ranero, and Paola Vannucchi reviewed of subducting bathymetric features, collapse in CONCLUSIONS earlier versions of this paper. We thank Roland von the wake of subducting seamounts, arc-parallel Huene and César Ranero for providing us with GEO- extension accommodating subduction of bathy- Our analysis of the distribution of surface MAR bathymetry data offshore of Costa Rica. The uplift and fault kinematics across the Costa Observatorio Vulcanológico y Sismológico de Costa metric highs, collapse of the outer forearc as a Rica (OVSICORI-UNA) provided logistical support. result of forearc thickening by other mecha- Rican forearc permits the following conclusions This paper has benefi ted from thoughtful reviews by nisms—may occur (Fig. 10). 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