rough crust subduction, forearc kinematics, and quaternary...

992

ABSTRACT

Orthogonal subduction of bathymetrically rough oceanic lithosphere along the north-western fl ank of the Cocos Ridge imprints a distinctive style of deformation on the over-riding Costa Rican forearc. We divide the Costa Rican forearc into three 100–160-km-long deformational domains based on the bathymetric roughness and thickness of the Cocos plate entering the Middle American Trench, the dip of the subducting plate, the variation in surface uplift rates of late Quaternary coastal deposits, and the orien-tations and types of faults deforming Paleo-gene and Neogene sedimentary rocks. In the ~100-km-long Nicoya domain, coastal deposits show localized surface uplift and arcward tilting above the downdip projec-tions of the fossil trace of the Cocos-Nazca-Panama (CO-NZ-PA) triple junction and the Fisher seamount and ridge. In the ~120-km-long central Pacifi c forearc domain between the Nicoya Peninsula and Quepos, shallower (~60°) subduction of seamounts and plateaus is accompanied by trench-perpendicular late Quaternary normal faults. Steeply dipping, northeast-striking, margin-perpendicular faults accommodate differential uplift asso-ciated with seamount subduction. Uplift and faulting differ between the segments of the forearc facing subducting seamounts and ridges. Inner forearc uplift along the

seamount-dominated segment is greatest inboard of the largest furrows across the lower slope. Localized uplift and arcward tilting of coastal deposits is present adja-cent to subducting seamounts. In contrast, inboard of the underthrusting aseismic Cocos Ridge, along the ~160-km-long Fila Costeña domain between Quepos and the Burica Peninsula, mesoscale fault popula-tions record active shortening related to the ~100-km-long Fila Costeña fold-and-thrust belt. The observed patterns of fault-ing and permanent uplift are best explained by crustal thickening. The uplifted terraces provide a fi rst-order estimate of permanent strain along the forearc in Costa Rica. The permanent strain recorded by uplift of these Quaternary surfaces exceeds the predicted rebound of stored elastic strain released during subduction-zone earthquakes.

INTRODUCTION

Convergent margins can experience either accretion or subduction erosion, and rates depend upon such factors as volume of sedi-ment input, subduction angle, basal friction, and seafl oor roughness (see, e.g., Cloos and Shreve, 1988a, 1988b; von Huene and Lallemand, 1990; Stern, 1991; von Huene and Scholl, 1991; Lal-lemand et al., 1992, 1994; Bangs and Cande, 1997; Ranero and von Huene, 2000; Clift and Vannucchi, 2004; von Huene et al., 2004). Con-vergent margins dominated by subduction ero-sion, such as those along Peru, Costa Rica, Gua-

temala, northern Chile, Japan, New Britain, and Tonga, are the most prevalent type of margin and are commonly characterized by rapid con-vergence and limited sediment input (von Huene and Scholl, 1991; Clift and Vannucchi, 2004). The subduction of seamounts and ridges at the Nankai (Bangs et al., 2006), northern Chile (von Huene and Ranero, 2003), Peruvian (von Huene and Lallemand, 1990), Solomon Islands (Mann et al., 1998; Taylor et al., 2005), Middle Ameri-can (from Guatemala to Costa Rica) (Gardner et al., 1992, 2001; Fisher et al., 1998; Bilek et al., 2003; Vannucchi et al., 2004), and Tonga (Ballance et al., 1989; Scholz and Small, 1997) trenches results in tectonic deformation of the forearc and infl uences interplate coupling. The Cocos plate subducts beneath both the Carib-bean plate and Panama microplate at the Middle American Trench off the western coast of Costa Rica. The forearc response to subduction of rough lithosphere varies as a function of distance from the trench. The distribution of faulting and uplift in the subaerial portion of the Costa Rican forearc mimics the distribution of bathymetric features on the subducting Cocos plate outboard of the Middle American Trench (e.g., Gardner et al., 1992, 2001; Fisher et al., 1998; Marshall et al., 2000; Sak et al., 2004a).

The Costa Rican segment of the Middle American Trench is a suitable location for inves-tigating the relationship between the ongoing subduction of bathymetric features and upper-plate deformation because of the diversity in the surface morphology of the downgoing plate. The morphology of the Cocos plate offshore of

For permission to copy, contact [email protected]© 2009 Geological Society of America

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. FisherDepartment of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA

Thomas W. GardnerDepartment of Geosciences, Trinity University, San Antonio, Texas 78212, USA

Jeffrey S. MarshallDepartment of Geological Sciences, Cal Poly Pomona University, Pomona, California 91768, USA

Peter C. LaFeminaDepartment of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA

†E-mail: [email protected]

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

Forearc deformation along the Costa Rican segment of the Middle American Trench

Geological Society of America Bulletin, July/August 2009 993

Costa Rica changes abruptly from a few iso-lated seamounts and narrow ridges mantled by a ≤500-m-thick cover of sediment offshore of the Nicoya Peninsula to a rough surface where ~40% of the ocean fl oor is covered by linear arrays of seamounts ranging in size from 1 to 2.5 km high and 10 to 20 km wide (Ranero and von Huene, 2000) and plateaus between the Nicoya and Osa Peninsulas. Further to the southeast, the Osa Peninsula overrides the aseis-mic Cocos Ridge, an area of thickened oceanic lithosphere and elevated seafl oor (von Huene et al., 2000; Walther, 2003) that is interpreted as a trace of the Galapagos hotspot (Barckhausen et al., 2001).

In this paper, we evaluate the relationship between forearc deformation and the morphol-ogy and geometry of the subducting plate along the Middle American Trench in Costa Rica. We present a regional compilation of mesoscale fault populations and age dates collected from previ-ously published studies along the Costa Rican segment of the Middle American Trench (Gard-ner et al., 1992, 2001; Marshall and Anderson, 1995; Fisher et al., 1998, 2004; Marshall et al., 2000; Sak et al., 2004a) in addition to new data from the Nicoya Peninsula and the central Pacifi c coast. Fault data are used to recognize regional-scale trends in forearc kinematics. Radiocarbon ages of marine terraces are evaluated using the recent IntCal04 calibration (Reimer et al., 2004) and recent sea-level curves (Fleming et al., 1998; Lambeck and Chappell, 2001) to constrain Qua-ternary surface uplift rates in a common refer-ence frame. The spatial distribution of calculated uplift rates along and across the Costa Rican forearc are then combined with the fault kine-matic data to constrain plausible mechanisms resulting in permanent uplift within the forearc.

REGIONAL SETTING

The Central American isthmus occupies a complex deformational zone that responds to the interaction of four tectonic plates (Carib-bean, Cocos, Nazca, and South American) and the Panama microplate (Fig. 1A). Deformation across the forearc in southern Central America is due to the rapid subduction of the Cocos plate beneath the Caribbean plate and Panama block (Corrigan et al., 1990; Gardner et al., 1992; Kolarsky et al., 1995; Marshall et al., 2000; Fisher et al., 1998, 2004). Along strike of the Middle American Trench, the character (i.e., dip, roughness, crustal age, and thickness) of the subducting Cocos crust changes (Figs. 1A and 1C) (Protti et al., 1995a; von Huene et al., 2000; Barckhausen et al. 2001; Walther, 2003). Across the Costa Rican segment of the Middle American Trench, the relative convergence rate

increases southward from ~8.5 cm yr–1 across the Nicoya segment to 9.1 cm yr–1 across the Osa segment (Dixon, 1993; DeMets, 2001). South of Quepos (Figs. 1C and 2), up to 4 cm yr–1 of Cocos-Caribbean convergence is accommodated permanently within the Fila Costeña fold-and-thrust belt (Sitchler et al., 2007). Cocos-Carib-bean convergence becomes oblique (10°–15°) at the southern end of the Nicoya Peninsula, resulting in ~8 mm yr–1 of northwest-directed forearc sliver transport (McCaffrey, 2002; Nor-abuena et al., 2004).

Across the Costa Rican segment of the Mid-dle American Trench, from the Nicoya Penin-sula in the northwest to the Osa Peninsula in the southeast (Fig. 1C), a distance of ~300 km, the forearc response to ongoing subduction varies as a function of the lateral changes in dip of the subducting slab, age, and thickness of the crust entering the trench, seafl oor roughness, and distance from the trench (e.g., Corrigan et al., 1990; Gardner et al., 1992, 2001; Fisher et al., 1998; Walther, 2003). High-resolution bathy-metric mapping of the subducting Cocos plate offshore of Costa Rica reveals a rough surface morphology characterized by ridges, plateaus, and seamounts (Fig. 1C). Surface morphology of the lower slope arcward of the Middle Ameri-can Trench refl ects the roughness of the incom-ing Cocos plate outboard of the trench. Recov-ered drill cores (Kimura et al., 1997; Vannucchi et al., 2001, 2003), seismic-refl ection profi les (i.e., Hinz et al., 1996), and bathymetric map-ping (von Huene et al., 1995, 2000) all indicate subsidence by subduction erosion. Damage to the portions of submarine forearc facing sub-ducting bathymetric highs is interpreted as the morphologic signature of subduction erosion (Ranero and von Huene, 2000).

Where bathymetric highs enter the trench, the trench axis is defl ected arcward (Fig. 1C). Offshore of the southern half of the Nicoya Peninsula (between Punta Guiones and Cabo Blanco), an ~75-km-wide swath of relatively smooth crust is bound at either side by elongate northeast-trending ridges rising >700 m above the adjacent seafl oor (von Huene et al., 2000). The fossil trace of the Cocos-Nazca-Panama tri-ple junction is marked by a ridge that intersects the Middle American Trench offshore of Punta Guiones (TJ in Fig. 1C). This narrow (~2 km wide), discontinuous, asymmetric ridge has an abrupt southeast fl ank that rises ~700 m above relatively smooth oceanic crust to the south-east and a gently sloping northwest fl ank (von Huene et al., 2000; Barckhausen et al., 2001). This ridge is coincident with the boundary sepa-rating crust produced at the East Pacifi c Rise to the northwest from crust produced at the Cocos-Nazca spreading center to the southeast, and

it is interpreted as the fossil trace of the initial opening of the Cocos-Nazca spreading center (Fig. 1A) (von Huene et al., 2000; Barckhausen et al., 2001). To the north of this ridge, seafl oor magnetic spreading anomalies trend 120°, sub-parallel to the Middle American Trench, whereas south of the ridge, the seafl oor magnetic spread-ing anomalies are oriented 030° (Barckhausen et al., 2001). Northwest of the fossil trace of the Cocos-Nazca-Panama triple junction trace (TJ), the crust is ~24 m.y. old at the Middle American Trench (Fig. 1A) (Barckhausen et al., 2001). From the ridge, crustal age decreases to the southeast from ~23 Ma to 21.5 Ma across an ~75-km-wide swath of smooth crust to the Fisher Seamount and Ridge (Barckhausen et al., 2001). The conical Fisher Seamount rises ~1.6 km above the subjacent ca. 20 Ma Cocos-Nazca spreading center–derived crust and has a basal diameter of ~16 km (Werner et al., 1999; Barckhausen et al., 2001). Southwest (outboard) of the Fisher Seamount, is the narrow (~5–7 km wide) northeast-trending, continuous Fisher Ridge, which rises ~1 km above the adjacent ocean fl oor (Fig. 1C).

The bathymetric roughness of the Cocos plate changes southeast of the Fisher Seamount and Ridge from elongate ridges to the broad Que-pos Plateau (QP, Fig. 1C), which is elongate and oblique to the Cocos-Caribbean convergence vector and to linear arrays of conical seamounts oriented at low angles to the relative convergence vector. Because the chains of subducting fea-tures are oriented at a low angle to the relative Cocos-Caribbean convergence vector, the effects of ongoing rough crust subduction are limited to narrow regions along the margin (Fisher et al., 1998). Seamounts have broad bases (~15–20 km) and rise >1.5 km above the adjacent ocean bottom (von Huene et al., 1995, 2000). The seamounts, composed of 13–14.5 Ma basalts, are younger than the 15–20 Ma subjacent oceanic crust (Wer-ner et al., 1999; Barckhausen et al., 2001). Wide-angle seismic-refraction investigations indicate that the depth to the Moho increases from ~11 km across to the Quepos Plateau to 21 km beneath the Cocos Ridge (Walther, 2003).

The underthrusting of irregular oceanic litho-sphere has a strong impact on the morphology and structure of the submarine forearc. High-resolution bathymetric swath mapping across the plate-boundary region documents the effects of rough crust subduction on the forearc bathym-etry (von Huene et al., 1995, 2000; Ranero and von Huene, 2000; Hühnerbach et al., 2005). Where bathymetric highs enter the trench, the trench axis is defl ected arcward. For example, the underthrusting fossil trace of the triple junc-tion offshore of Punta Guiones deforms the lower slope. Across the lower slope inboard of

Sak et al.

994 Geological Society of America Bulletin, July/August 2009

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Location of mesoscale fault population

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Figure 2. (A) Mesoscale fault populations, regional faults, and general geology of the Garza region. Geology is modifi ed from Baumgartner et al. (1984) and supple-mented by this study. Mesoscale fault popu-lation data (lower-hemisphere, equal-area projections) are keyed to the map by letter (Table 2). Compressional (P) axes (black cir-cles—individual faults; black square—aver-age) and tensional (T) axes (black circles–individual faults; black square—average) defi ne best-fi t fault-plane solution for each fault population (Marrett and Allmend-inger, 1990). Black dots—locations of dated samples from the Garza surface (Table 1). See Figure 1C for location. (B) Generalized geologic map of the Cabo Blanco region after Mora and Baumgartner (1985), Gardner et al. (2001). Lower-hemisphere, equal-area projections of mesoscale fault populations (from Marshall et al., 2000). See Figure 1C for location.

Sak et al.


the subducting ridge, there is a narrow (



Pliocene-Pleistocene Montezuma Formation (Hare and Gardner, 1985; Mora and Baumgart-ner, 1985; Marshall and Anderson, 1995; Gard-ner et al., 2001). This regionally extensive sur-face is mantled by a >3-m-thick red (2.5 YR) oxisol. The elevation of the Cobano surface decreases from a maximum of >200 m at the southern boundary to ~100 m north of the town of Cobano. The measured tilt of the Cobano surface is consistent with the calculated axis of rotation for the Holocene Cabuya surface (Gard-ner et al., 2001).

An uplifted, deeply weathered fl at-topped ero-sional surface cut into thinly bedded Paleogene turbidites and Nicoya Complex pillow basalts is exposed at Playa Camaronal (Figs. 1C and 2A). This locally extensive surface, referred to as the Camaronal surface, is exposed at an elevation of 15–20 m. The Camaronal surface extends for ~3 km along the coast and up to 1 km inland to the interior mountains of the Nicoya Penin-sula. The Camaronal surface is mantled by a >3-m-thick red (2.5 YR) oxisol that is similar to

soils developed on the Cobano surface ~60 km to the southeast. As in the case of the Cobano surface, a low-lying, less-weathered marine terrace is exposed below the highly weathered Camaronal surface. The lower surface, exposed at an elevation of 1.5 m, is correlative with the dated (ca. 4 ka) Garza surface. We postulate that the extensively weathered, pedogenically simi-lar Camaronal and Cobano surfaces represent remnants of a regionally extensive erosional surface cut during marine oxygen isotope stage 5e (ca. 120 ka). If the Camaronal and Cobano surfaces are isolated remnants of a regionally extensive surface, this would suggest a dra-matic increase in uplift rate over the late Qua-ternary in the vicinity of Playa Camaronal from 0.1 m k.y.–1 to 0.4 m k.y.–1 (Figs. 1C and 2A).

Across the southern half of the Nicoya Pen-insula, late Quaternary uplift rates vary along strike of the Middle American Trench and as a function of distance from the trench. Uplift rates are greatest in the portions of the forearc oppo-site where the trace of the Cocos-Nazca-Panama

triple junction enters the Middle American Trench (offshore of Garza) and where the Fisher Seamount and Ridge impinges on the trench at Cabo Blanco (Fig. 3). Both the Holocene marine terraces and the late Pleistocene Cobano surface, exposed in the vicinity of Cabo Blanco, dem-onstrate that the magnitude of late Quaternary uplift decreased as a function of distance inboard of the Middle American Trench (Marshall and Anderson, 1995; Gardner et al., 2001).

Fault Kinematics of the Nicoya Peninsula

Mesozoic and Tertiary sediment are dissected by numerous steeply dipping faults that have trace lengths of kilometers to tens of kilome-ters and two dominant orientations that defi ne a kinematic framework: (1) northwest-striking (parallel to faults seismically imaged offshore within the slope apron; McIntosh et al., 1993; shaded box in Fig. 1C), and (2) northeast-strik-ing faults (parallel to the relative convergence vector) (Figs. 2A and 4). Slip can be directly

0.1 - 1.01.1 - 2.02.1 - 3.03.1 - 4.0

4.1 - 5.0

Uplift rate (mm/ yr)

5.1 - 6.06.1 - 7.0

NCosta Rica

area of Figure 3

North

0 10 20

km

34

22

n = 71

9°N

85°W

84°W

8°NCO-CA

QQQQQQQPQPQPQPQQQ

FSFSRR

TJTJJJJT

CRCCMAMAMAAAAAAAAAAAAATTTTTTTT

MAMAMAMAAAAAAAAAAATTTTTTTTT

MAMAAAAMAAAAATTTTTTTTTTTTT QP

FSR

TJ

CRMAT

MAT

MAT

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.

Sak et al.


TA

BLE

1. R

AD

IOC

AR

BO

N S

AM

PLE

S L

IST

ED

BY

LO

CA

TIO

N A

ND

AS

SO

CIA

TE

D V

ALU

ES

US

ED

TO

CA

LCU

LAT

E U

PLI

FT

RA

TE

S.

Are

a S

ampl

e*

Latit

ude

(°N

) Lo

ngitu

de

(°W

)

14C

age

(ra

w)†

(y

r B

.P.)

Age

§ (k

a)

(T)

Mod

ern

elev

atio

n#

(m)

(Z)

Fac

ies

dept

h**

(m)

(F)

Sea

leve

l††

(m)

(S)

Ran

ge o

fup

lift r

ate§

§ (m

/k.y

.)

(R)

Gar

za

(127

237)

9°

55′

85°3

8 ′3.

48 ±

0.0

63.

892 –

3.74

52.

4 ±

0.1

0.0

(±1.

2)– 1

.5–0

.1–0

.6G

arza

(1

3125

9)

9°55

′ 85

°38′

4.14

± 0

.07

4.78

5 –4.

632

2.0

± 0.

10.

0 (±

1.2)

–2.5

0.5–

1.5

Gar

za

(131

260)

9°

55′

85°3

8′4.

46 ±

0.0

65.

337 –

5.22

02.

5 ±

0.1

0.0

(±1.

2)–3

.50.

7–1.

6C

abo

Bla

nco

(131

258)

5 9°

42′

85°1

3′

1.29

± 0

.06

1.34

3 –1.

231

0.9

1.2

(±0.

6)–0

.5–0

.4–0

.7C

abo

Bla

nco

(121

785)

5 9°

40′

85°1

1′

1.52

± 0

.06

1.47

6–1.

402

2.0

1.2

(±0.

6)– 0

.50.

4–1.

4C

abo

Bla

nco

(121

787)

5 9°

39′

85°1

1′

1.90

± 0

.06

1.95

6–1.

834

1.8

0.6

(±0.

6)–0

.80.

7–1.

5C

abo

Bla

nco

(121

784)

5 9°

39′

85°1

0′

2.59

± 0

.06

2.83

2 –2.

755

1.2

0.6

(±0.

6)–1

.20.

4–0.

9C

abo

Bla

nco

(121

783)

5 9°

39′

85°1

0′

5.07

± 0

.07

5.95

9 –5.

803

1.1

1.2

(±0.

6)–5

.50.

6–1.

2C

abo

Bla

nco

(121

775)

5 9°

38′

85°0

9′

3.79

± 0

.07

4.34

7–4.

139

5.4

0.0

–2.0

1.2–

2.3

Cab

o B

lanc

o (1

1737

6)5

9°38

′ 85

°09′

6.

94 ±

0.0

67.

889–

7.75

55.

81.

2 (±

0.6)

–10.

01.

6–2.

1C

abo

Bla

nco

(121

776)

5 9°

38′

85°0

9′

4.90

± 0

.08

5.79

1 –5.

641

5.6

1.2

(±0.

6)–5

.01.

3–2.

0C

abo

Bla

nco

(121

774)

5 9°

37′

85°0

9′

3.59

± 0

.07

4.04

2–3.

883

5.8

0.6

(±0.

6)– 2

.01.

6–2.

0C

abo

Bla

nco

(GX

2537

0)5

9°37

′ 85

°08′

6.

660

7.58

35.

50.

0– 9

.01.

6–2.

2C

abo

Bla

nco

(121

788)

5 9°

36′

85°0

8′

1.86

± 0

.07

1.92

9–1.

772

2.8

1.2

(±0.

6)–0

.80.

9–1.

7C

abo

Bla

nco

(127

435)

5 9°

36′

85°0

8′

3.12

± 0

.07

3.46

2 –3.

315

5.9

0.0

–1.5

1.8–

2.6

Cab

o B

lanc

o (0

9410

0)5

9°35

′ 85

°08′

1.

62 ±

0.0

61.

617–

1.47

14.

10.

0– 0

.52.

0–4.

0C

abo

Bla

nco

(121

791)

5 9°

35′

85°0

8′

0.97

± 0

.05

0.92

6–0.

853

3.1

0.6

(±0.

6)0.

01.

9–3.

8C

abo

Bla

nco

(121

790)

5 9°

34′

85°0

8′

1.34

± 0

.06

1.36

3–1.

288

3.5

1.2

(±0.

6)–0

.51.

5–2.

7C

abo

Bla

nco

(121

769)

5 9°

34′

85°0

7′

0.66

± 0

.06

0.72

7–0.

686

2.6

0.6

(±0.

6)0.

01.

8–3.

9C

abo

Bla

nco

(121

770)

5 9°

34′

85°0

7′

1.28

± 0

.06

1.34

3–1.

227

5.2

0.6

(±0.

6)– 0

.53.

3–4.

7C

abo

Bla

nco

(121

771)

5 9°

34′

85°0

6′

1.49

± 0

.06

1.47

2–1.

362

7.2

1.2

(±0.

6)–0

.53.

9–5.

3C

abo

Bla

nco

(032

360)

2 9°

35′

85°0

5′

0.40

± 0

.06

0.56

8 –0.

485

3.4

± 0.

21.

2 (±

0.6)

0.0

2.5–

6.2

Cab

o B

lanc

o (0

3639

7)2

9°35

′ 85

°05′

4.

39 ±

0.0

85.

110–

4.91

017

.1 ±

0.2

0.6

(±0.

6)– 3

.03.

5–4.

3C

abo

Bla

nco

(034

835)

2 9°

35′

85°0

5′

3.80

± 0

.06

4.31

6–4.

145

13.7

± 0

.20.

0 (±

1.2)

–2.0

3.1–

4.4

Cab

o B

lanc

o (0

3235

8)2

9°36

′ 85

°05′

2.

26 ±

0.0

72.

315–

2.21

59.

5 ±

0.2

0.6

(±0.

6)–1

.03.

9–4.

8C

abo

Bla

nco

(032

359)

2 9°

36′

85°0

5′

4.10

± 0

.10

4.76

8–4.

574

16.2

± 0

.21.

2 (±

0.6)

– 2.5

3.3

–4.2

Cab

o B

lanc

o (0

3639

6)2

9°36

′ 85

°05′

4.

47 ±

0.0

85.

343–

5.21

414

.8 ±

0.2

0.6

(±0.

6)– 3

.53.

0–3.

7C

abo

Bla

nco

(034

834)

2 9°

36′

85°0

5′

1.53

± 0

.06

1.47

8–1.

410

5.5

± 0.

20.

0 (±

1.2)

–0.5

3.1–

5.2

Cab

o B

lanc

o (0

3236

1)2

9°36

′ 85

°05′

0.

78 ±

0.0

90.

851 –

0.70

83.

71.

2 (±

0.6)

0.0

2.0–

4.7

Cab

o B

lanc

o (1

2177

2)5

9°37

′ 85

°05′

1.

24 ±

0.0

70.

787 –

0.69

54.

60.

6 (±

0.6)

0.0

4.2–

6.8

Cab

o B

lanc

o (1

2177

3)5

9°37

′ 85

°05′

0.

72 ±

0.0

81.

318–

1.17

92.

40.

6 (±

0.6)

–0.5

1.2–

2.5

Cab

o B

lanc

o (0

3755

8)2

9°39

′ 85

°05′

3.

98 ±

0.0

82.

315–

2.21

59.

5 ±

0.2

0.6

(±0.

6)–1

.03.

9–4.

8C

abo

Bla

nco

(121

781)

5 9°

40′

85°0

4′

2.65

± 0

.07

2.90

5–2.

786

1.3

0.0

(±1.

2)– 1

.20.

4–1.

4C

abo

Bla

nco

(121

778)

5 9°

40′

85°0

4′

2.04

± 0

.06

2.12

0–1.

983

1.7

0.0

(±1.

2)– 0

.80.

6–1.

9C

abo

Bla

nco

(121

782)

5 9°

40′

85°0

35.

76 ±

0.0

76.

701 –

6.54

36.

21.

2 (±

0.6)

–7.0

1.5–

2.1

Cab

o B

lanc

o (1

2272

4)5 *

**

9°41

′ 85

°01′

4.

07 ±

0.0

44.

672–

4.57

23.

91.

2 (±

0.6)

–2.5

0.8–

1.5

Cab

o B

lanc

o (1

2177

9)5

9°41

′ 85

°01′

2.

03 ±

0.0

72.

119 –

1.97

42.

30.

6 (±

0.6)

–0.8

0.8–

1.6

Cab

o B

lanc

o (1

2178

0)5

9°42

′ 85

°00′

2.

58 ±

0.0

82.

830–

2.74

23.

00.

6 (±

0.6)

–1.2

1.0–

1.6

Pun

ta C

arba

llo

(880

01)3

,4

9°57

′ 84

°44′

3.

05 ±

0.0

63.

412–

3.26

03.

80.

0 (±

1.2)

–1.5

1.2–

2.0

Est

erill

os

(118

143)

9°

31′

84°3

2′

1.16

± 0

.08

1.22

8 –1.

102

3.0

0.0

(±1.

2)–0

.51.

8–4.

4E

ster

illos

(1

1814

4)

9°31

′ 84

°31′

1.

01 ±

0.0

61.

034–

0.95

61.

50.

0 (±

1.2)

–0.5

0.7–

3.5

Est

erill

os

(109

632)

9°

32′

84°3

1′

1.33

± 0

.07

1.36

1–1.

276

1.5

1.2

(±0.

6)–0

.51.

8–3.

1E

ster

illos

(1

0963

3)

9°32

′ 84

°30′

1.

65 ±

0.0

61.

674 –

1.56

81.

00.

0 (±

1.2)

–0.5

0.1–

1.8

Pla

ya B

alle

na

(129

337)

7 9°

06′

83°4

1′

5.54

± 0

.07

6.45

7 –6.

345

–0.4

4 ±

0.4

0.0

(±0.

6)–6

.00.

7–1.

3N

W O

sa

(154

118)

6 ***

8°

41′

83°4

0′

44.1

1 ±

0.9

249

.11

±0.

9226

0 ±

1.2

–52

1.5–

1.7

NW

Osa

(1

4220

8)6

8°40

′ 83

°43′

27

.22

± 0

.69

32.2

2 ±

0.6

915

< –15

–82

> 3

.4N

W O

sa

(142

206)

6 ***

8°

40′

83°4

3′

34.2

9 ±

0.4

439

.29

± 0

.44

47

3.6

NW

Osa

(1

5411

4)6 *

**

8°40

′ 83

°43′

40

.69

± 0

.63

45.6

9 ±

0.6

337

0 ±

1.2

–67

2.1–

2.3

NW

Osa

(1

5057

2)6 *

**

8°40

′ 83

°43′

46

.00

± 2

.00

51.0

0 ±

2.0

038

0 ±

1.2

–65

1.9–

2.1

NW

Osa

(1

4220

7)6 *

**

8°40

′ 83

°43′

49

.33

± 1

.554

.33

± 1

.541

± 1

0 ±

1.2

–76

6.5#

#

NW

Osa

(1

5411

5)6 *

**

8°40

′ 83

°43′

37

.95

± 0

.49

42.9

5 ±

0.4

961

± 1

–9 ±

6–6

26.

5##

(C

ontin

ued)



TA

BLE

1. R

AD

IOC

AR

BO

N S

AM

PLE

S L

IST

ED

BY

LO

CA

TIO

N A

ND

AS

SO

CIA

TE

D V

ALU

ES

US

ED

TO

CA

LCU

LAT

E U

PLI

FT

RA

TE

S.(

Con

tinue

d)

Are

a S

ampl

e*

Latit

ude

(°N

) Lo

ngitu

de

(°W

)

14C

age

(ra

w)†

(y

r B

.P.)

Age

§ (k

a)

(T)

Mod

ern

elev

atio

n#

(m)

(Z)

Fac

ies

dept

h**

(m)

(F)

Sea

leve

l††

(m)

(S)

Ran

ge o

fup

lift r

ate§

§ (m

/k.y

.)

(R)

NW

Osa

(1

5411

6)6 *

**

8°40

′ 83

°43′

40

.65

± 0

.63

45.6

5 ±

0.6

364

± 1

–9 ±

6–6

76.

5##

NW

Osa

(1

5411

7)6 *

**

8°40

′ 83

°43′

34

.87

± 0

.35

39.8

7 ±

0.3

579

±1

< –15

– 83

6.5#

#

NW

Osa

(1

4220

4)6 *

**

8°38

′ 83

°44′

40

.99

± 0

.645

.99

± 0

.620

± 1

–9 ±

6–6

74.

2##

NW

Osa

(1

4220

5)6 *

**

8°38

′ 83

°44′

34

.0 ±

0.2

439

.00

± 0

.24

35 ±

1–9

± 6

–83

4.2#

#

NW

Osa

(1

5506

2)6

8°38

′ 83

°44′

42

.54

± 0

.87

47.5

4 ±

0.8

752

± 1

< –15

– 75

4.2#

#

NE

Osa

(2

4918

)1

8°33

′ 83

°25′

30

.07

± 0

.52

35.7

2 ±

0.7

450

–9 ±

6–9

03.

8 –4.

6N

E O

sa

(267

80)1

8°

32′

83°2

5′

35.2

9 ±

0.6

240

.29

± 0

.62

40–9

± 6

–90

3.1–

3.8

NE

Osa

(2

0941

)1

8°32

′ 83

°24′

29

.78

± 1

.80

34.7

8±

1.8

40–9

± 6

–92

3.6–

4.6

NE

Osa

(D

IC–3

153)

1 8°

32′

83°2

4′

0.98

± 0

.04

39.5

3 –1.

3240

–9 ±

6– 8

33.

0–3.

7N

E O

sa

(249

17)1

8°

33′

83°2

1′

6.35

± 0

.07

7.38

9–7.

299

50.

0 (±

1.2)

–91.

5–2.

3N

E O

sa

(208

41)1

8°

33′

83°2

1′

7.15

± 0

.08

8.09

8–7.

979

4.5

0.0

(±1.

2)–9

1.3–

2.1

NE

Osa

(2

0840

)1

8°32

′ 83

°22′

33

.07

± 0

.52

38.0

7 ±

0.5

27

0.0

(±1.

2)– 7

92.

0–2.

5N

E O

sa

(249

14)1

8°

31′

83°1

9′

34.8

8 ±

0.5

139

.88

± 0

.51

25–9

± 6

–91

2.8 –

3.5

NE

Osa

(2

0938

)1

8°29

′ 83

°18′

2.

02 ±

0.0

62.

100 –

1.95

24.

60.

0 (±

1.2)

–11.

7–3.

9N

E O

sa

(208

38)1

8°

29′

83°1

8′

20.1

4 ±

0.1

424

.353

–23.

983

13.4

0.0

(±1.

2)–1

385.

9–6.

6N

E O

sa

(DIC

–336

2)1

8°29

′ 83

°19′

22

.73

(+0.

94, –

1.06

)26

.23

± 1

.01

10.0

–9 ±

6–1

425.

5–6.

8N

E O

sa

(208

37)1

8°

28′

83°1

7′

22.2

1 ±

0.2

925

.710

± 0

.29

12–9

± 6

–142

5.8–

6.8

NE

Osa

(2

4919

)1

8°27

′ 83

°17′

26

.77

± 1

.13

31.7

7 ±

1.1

35

< –

15–9

4>

3.5

NE

Osa

(2

0836

)1

8°25

′ 83

°17 ′

1.

39 ±

0.0

61.

410 –

1.32

59

0.0

(±1.

2)–0

.55.

3–8.

7N

E O

sa

(DIC

–315

4)1

8°26

′ 83

°17′

0.

98 ±

0.0

40.

990 –

0.95

74.

30.

0 (±

1.2)

02.

3–6.

6

Not

e: 1—

sam

ples

from

Gar

dner

et a

l. (1

992)

; 2—

sam

ples

from

Mar

shal

l and

And

erso

n (1

995)

; 3—

sam

ples

from

Fis

her

et a

l. (1

998)

; 4—

sam

ples

from

Mar

shal

l (20

00);

5 —sa

mpl

es fr

om G

ardn

er e

t al.

(200

1); 6

—sa

mpl

es fr

om S

ak e

t al.

(200

4a);

7 —sa

mpl

e fr

om F

ishe

r et

al.

(200

4).

*B

eta

Ana

lytic

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ple

num

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41).

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Labs

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adio

isot

ope

Co.

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rect

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INT

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mer

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ount

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uatio

ns in

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osph

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cent

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ns (

Bec

k et

al.,

200

1).

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oder

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), m

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det

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s, tr

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veys

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ts w

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in a

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ies

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ples

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sign

ed to

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n se

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±6 m

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<–1

5 m

. Inh

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repo

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s is

the

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that

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imat

e ha

ve

rem

aine

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lativ

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cons

tant

ove

r th

e sa

mpl

ing

inte

rval

.

††P

aleo

–sea

leve

l (S

) at

the

time

of d

epos

ition

(T

), m

easu

red

posi

tive

upw

ard

from

mea

n se

a le

vel,

was

det

erm

ined

from

pub

lishe

d eu

stat

ic s

ea-le

vel c

urve

s (F

lem

ing

et a

l., 1

998;

Lam

beck

and

C

happ

ell,

2001

).

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plift

rat

e (R

) w

as c

alcu

late

d fr

om E

quat

ion

1 us

ing

accu

mul

ated

err

ors

liste

d he

re

R

ZF

S

T(m

/ka)

mm

m

ka=

()−

()−

()

()

. (1)

## U

plift

rat

e w

as d

eter

min

ed a

ssum

ing

the

com

plex

his

tory

of l

ocal

ized

sub

side

nce

and

uplif

t (S

ak e

t al.,

200

4a).

***A

ccel

erat

or M

ass

Spe

ctro

met

ry a

ge.

Sak et al.


determined for most regional-scale faults in the fi eld using offset marker beds in combination with fault striae. The regional kinematic frame-work as determined from the faults is supple-mented here by detailed analysis of mesoscale fault populations. Mesoscale fault populations are outcrop-scale features (tens to hundreds of meters in length) that displace measurable fault surfaces containing kinematic indicators (e.g., slickenlines). We use kinematic analysis (e.g., Marrett and Allmendinger, 1990) to interpret dense fault arrays in terms of the strain patterns recorded at the margin and interior of faulted regions. This kinematic method determines the principal shortening and extension axes (P and T axes, respectively) for a given population based on slip data from individual faults. The sense of slip was determined using the criteria of Petit (1987). Kinematic axes for individual mesoscale fault populations measured on the Nicoya Peninsula are plotted as P and T axes from best-fi t fault-plane solutions on equal-area stereonets (Table 2; Figs. 2A and 4) (Allmend-inger et al., 2004).

Mesoscale fault populations were measured at 13 exposures of Mesozoic and Tertiary marine sediment on the Nicoya Peninsula. Most of these populations are characterized by shallow T axes and steeply inclined P axes, suggesting a com-ponent of extension (Table 2). The observed pat-tern of steeply dipping, northeast-striking faults is similar to the geometry of regional-scale faults (Fig. 3A) (Baumgartner et al., 1984). Pop-ulations characterized by steeply dipping, north-west-striking nodal planes and steep P and shal-low T axes are consistent with active downslope extension along northwest-striking normal faults observed in seismic-refl ection profi les across the upper slope offshore of Punta Guiones (McIn-tosh et al., 1993), suggesting that such mesoscale fault populations (Table 2; Fig. 2A, insets c, d, and i; Fig. 4) formed under the same kinematic conditions. Some of the faults imaged across the upper slope displace the ocean bottom, indicat-

ing recent displacement (McIntosh et al., 1993). There is a set of faults exposed in the sea cliff at Punta Indio that has the same orientation and kinematics as those imaged offshore by McIn-tosh et al. (1993). The overall kinematics are further complicated by additional faults in these uplifted Paleogene strata.

Four of the ten sites (Table 2; Fig. 2A, insets b, e, g, and j; Fig. 4) measured between Punta Gui-ones and Puerto Carrillo indicate strike-slip offset across steeply dipping fault planes. Sites b and e (Fig. 2A) record dextral motion along northeast-striking faults, while site j records dextral motion along a west-northwest–striking fault (Fig. 2A). Site g is consistent with sinistral motion along a northeast-striking fault (Fig. 2A). Similarly, in the vicinity of Cabo Blanco, Marshall et al. (2000) measured three mesoscale fault popula-tions in exposures of mid- to upper Tertiary lime-stone. Two of these populations (sites k and l) are characterized by shallowly plunging P and T axes and strike-slip motion along steeply dipping northeast-striking faults (Table 2; Fig. 2B, insets k and l; Fig. 4). Steep P and shallow T axes char-acterize the mesoscale faults north-northeast of Cabo Blanco at site m (Fig. 2B; Table 2).

THE CENTRAL PACIFIC COAST

The ~120-km-wide set-back portion of the coastline between the Nicoya and Osa peninsu-las is dissected by steeply dipping faults striking at a low angle to the Cocos-Caribbean conver-gence vector (Fig. 1C). These faults separate ~20-km-wide blocks characterized by variable surface uplift rates (Fisher et al., 1998; Marshall et al., 2000). From northwest to southeast, six fault-bounded blocks are mapped: the Esparza, Orotina, Herradura, Esterillos, Parrita, and Que-pos blocks (Fig. 1C). This region corresponds to a pronounced change in the morphology of the subducting plate from elongate ridges to linear arrays of conical seamounts oriented at low angles to the relative convergence vector.

Rapidly uplifting blocks override subducting seamounts (Fisher et al., 1998).

Quaternary Uplift Rates Along the Central Pacifi c Coast

The record of late Quaternary deformation across the central Pacifi c coast region is well constrained by alluvial and marine terraces. Northeast-trending faults along the central Pacifi c coast offset Quaternary alluvial terraces and a pyroclastic fl ow yielding a 352 ± 40 ka Ar/Ar age (Marshall et al., 2003). Rapid surfi -cial weathering along the Pacifi c coast of Costa Rica facilitates differentiation of the late Qua-ternary alluvial deposits on the basis of pedo-genic characteristics (e.g., Wells et al., 1988; Drake, 1989; Bullard, 1995; Marshall, 2000; Sak et al., 2004b). Surface soil properties range from reddish brown (2.5YR–10R), ≥540-cm-thick B horizons (Marshall, 2000), with average weathering rind thicknesses of 6.9 ± 0.6 cm for basaltic andesite clasts from Quaternary terrace (Qt) 1, to brown (10YR), 200-cm-thick B hori-zons, with average weathering rind thicknesses of 0.9 ± 0.1 cm for basaltic andesite clasts from Qt 3 (Sak et al., 2004b). Terrace ages were estimated using a physiochemical model for weathering rind formation that is constrained by radiocarbon ages (Fisher et al., 1998) and Ar/Ar ages for pyroclastic fl ows (Marshall et al., 2003). This model yields ages of ca. 240 ka, ca. 120 ka, and ca. 37 ka, respectively for Qt 1, 2, and 3 (Sak et al., 2004b).

Dated alluvial terraces of known elevation were used to calculate average incision rates. Large rivers eroding through weak rocks near the coast are assumed to have suffi cient stream power to keep pace with uplift. As such, the shape of the river longitudinal profi le is a proxy for the longitudinal profi le at the time of terrace deposi-tion, and it was used to calculate average surface uplift rates for fault-bounded blocks. From north-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).



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Sak et al.


TABLE 2. MESOSCALE FAULT POPULATION DATASite* Latitude

(°N) Longitude

(°W) Outcrop location and type† Age§ Total faults P axis T axis

Nicoya Peninsula a 9°58′ 85°41′ 16 ,05371 ,41172gP )ps( adaleP atnuPb 9°55′ 85°40′ 11 ,39123 ,19203gP )ps( senoiuG atnuPc 9°54′ 85°39′ 80 ,95396 ,11191gP )ps( senoiuG atnuPd 9°54′ 85°39′ 93 ,03084 ,19151gP )ps( azraG atnuPe 9°53′ 85°36′ 30 ,50022 ,57202gP adarbeuQ ocraB ayalPf 9°53′ 85°32′ 70 ,23147 ,51051K )br( atnaraT adarbeuQg 9°53′ 32 ,00354 ,48111gP )cr( senoretsE adarbeuQ 43°58h 9°54′ 85°32′ 60 ,31387 ,58151K )cr( ywH aramaS-ayociNi 9°51′ 85°31′ 20 ,30268 ,62352gP )cs( oidnI atnuPj 9°53′ 85°29′ 32 ,03252 ,33371K )ps( ollirraC otreuPk1 9°37′ 85°09′ 51 ,74392 ,5808gN)ps( siaplaM ,anogirraB atnuPl1 9°37′ 85°08′ Quebrada Vanegas, Malpais (rc) Ng 8 338, 32 245, 04 m1 9°35′ 85°06′ 51 ,80087 ,58101gP )cr( evreseR ocnalB obaCCentral 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′ 90 ,76274 ,7619gN )br( lanonaraM ,acnarraB oiRp1 9°58′ 84°41′ Esparza-Artieda road, Humo (rc) Ng 10 232, 08 141, 05 q1 9°54′ 84°44′ 11 ,82153 ,7227gN )cs( ollilarroC atnuPr1 9°53′ 84°43′ 70 ,21325 ,31202gN )cs( seviviT ayalPs1 9°55 84°41′ Costanera Hwy, Rio Jesus Maria (rc) Ng 8 038, 13 301, 28 t1 9°51′ 84°41′ 31 ,88242 ,2919QgN )cs( ramajaB nonePu1 9°37′ 84°35′ Costanera Hwy, Rio Tarcoles (rc) Q 14 114, 64 298, 26 v1 9°43′ 84°40′ 10 ,49018 ,06351gN )cs ,ps( anoeL atnuPw1 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′ 00 .90000 ,99002gN )ps( saduJ atnuPz 9°31′ 84°31′ 00 ,69000 ,68161gN )ps( saduJ atnuPaa 9°32′ 84°26′ 25 ,07243 ,12111Q )cr( ocujeB ,ywH arenatsoCab1 9°32′ 84°16′ 81 ,60127 ,4726Q )cr( satleuV ,ywH arenatsoCac 9°33′ 84°15′ 20 ,05388 ,6919gN )cr( samaD morf htron daoRad1 9°23′ 84°09′ Punta Catedral, Manuel Antonio (sc) Ng 23 061, 86 217, 04 ae 9°23′ 84°07′ 46 ,10210 ,29291gN )cs ,br( ojnaraN oiRFila Costeña af2 9°13′ 83°50′ 96 ,23002 ,43271gN )ps( satirgeN acoRag2 9°05′ 83°41′ 67 ,65311 ,61283gN )ps ,cs( sanatneV ayalPah2 9°06′ 83°42′ 61 ,32036 ,85261gN )ps ,cs( aleuniP atnuPai2 9°07′ 83°43′ 14 ,03094 ,80241gN )ps ,cs( aenemihC atnuPaj2 9°08′ 83°44′ 96 ,09081 ,7326gN )ps( etohciP ardeiPak2 9°08′ 83°44′ 27 ,41371 ,41141gN )ps( asogerdeP ayalPal2 9°11′ 83°47′ 36 ,64352 ,44111gN )ps( oticetreuP atnuPam2 9°12′ 83°48′ Road to Escalares in Queb Diablo Basin (rc) Ng 7 273, 16 058, 71 an2 9°11′ 83°48′ 22 ,94375 ,22201gN serodartsoM atnuPao2 9°13′ 83°50′ Costanera Hwy, SE of Dominical (rc, q) Ng 12 306, 22 172, 60 ap2 9°14′ 83°51′ 28 ,02360 ,09071gN )ps ,cs( lacinimoD atnuPaq2 9°16° 83°52′ Dominical–San Isidro road (rc) Ng 24 242, 03 349, 80 ar2 9°12′ 83°45′ 96 ,25361 ,41251gN )br( noreugiH oiRas2 9°03′ 83°36′ 57 ,36211 ,73081gN )br ,cr( soiR serTat2 9°04′ 83°35′ Unnamed stream, Rio Coronado basin (rb) Ng 14 276, 04 175, 70 au2 9°04′ 83°36′ 58 ,58050 ,54211gN )br( airF adarbeuQav2 9°04′ 83°38′ 97 ,88080 ,42271gN )br( alaM atnuP oiRaw2 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′ 76 ,14022 ,93211gN )br( agutroT oiR 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.



blocks with alluvial terraces, uplift rates are vari-able: 0.7 m k.y.–1 for the past ~240 k.y. on the Esparza block (Marshall, 2000), 0.4 m k.y.–1 for the past ~240 k.y. on the Orotina block (Marshall, 2000), 1.2 m k.y.–1 for the past ~120 k.y. on the Esterillos block (Sak, 2002), 0.14 m k.y.–1 for the past ~120 k.y. on the Parrita block (Sak, 2002), 0.19 m k.y.–1 for the past ~120 k.y. on the Quepos block (Murphy, 2002), 0.11 m k.y.–1 for the past ~120 k.y. on the Rio Savegre (Murphy, 2002), and ~1.1 m k.y.–1 for the past ~120 k.y. on the Rio Terraba (Murphy, 2002).

Holocene marine terraces in coastal exposures across the seamount-dominated segment of the Middle American Trench provide constraints on late Holocene surface uplift. For example, to the west of the village of Esterillos Oeste (Fig. 5), there is an ~6-km-long, dissected, late Holo-cene wave-cut marine terrace, the Esterillos surface. The Esterillos surface, which truncates folded Miocene beds, is confi ned to elevations of

Sak et al.


including the 2004 Mw 6.4 Damas (Pacheco et

al., 2006) and the 1924 Ms = 7.0 San Casimiro

(Montero, 1999) (Fig. 4) events, suggest ongo-ing transtensional deformation across the forearc inboard of subducting seamounts.

In contrast, southeast of the Quepos block, the subaerially exposed forearc is dissected by northwest striking, margin-parallel, seaward-verging, shallowly dipping thrust faults. These faults accommodate ≥17 km of shortening across the active Fila Costeña fold-and-thrust belt (Fisher et al., 2004; Sitchler et al., 2007). Mesoscale fault populations measured across the Fila Costeña are characterized by shallow P and steep T axes (Table 2; Fig. 4). Observed map patterns and kinematic analysis of the mesoscale fault populations are consistent with top to the southwest displacement along north-west-striking thrust faults (Fisher et al., 2004).

THE OSA PENINSULA

The Osa Peninsula is an ~60-km-long high in the outer forearc directly inboard of the axis of the aseismic Cocos Ridge. Across the northwestern and southeastern coasts of the peninsula, uplifted late Quaternary marine deposits have been dated (Gardner et al., 1992; Sak et al., 2004a). The Quaternary deposits disconformably overlie beveled exposures of semilithifi ed Late Tertiary and Quaternary sediment of the Charco Azul and Armuelles Formations (collectively mapped as TQs) and the Paleogene Osa mélange (Sprech-mann, 1984; Corrigan et al., 1990; DiMarco et al., 1995; Vannucchi et al., 2006).

Quaternary Uplift Rates on the Osa Peninsula

Late Quaternary surface uplift rates across the northwestern coast of the Osa Peninsula are constrained by a suite of 14 shell and woody debris samples. The samples, collected from nine measured stratigraphic sections of late Pleistocene marine sands, record a complex history of subsidence followed by rapid (locally > 6 m k.y.–1) uplift, which has been attributed to the passage of relief along the axis of the underthrusting Cocos Ridge (Sak et al., 2004a) (Table 1; Fig. 3). Gardner et al. (1992) used 15 dated samples of shells and woody debris col-lected across the southeastern coast of the Osa Peninsula to quantify late Quaternary surface uplift rates and demonstrate that surface uplift rates over the past 50 k.y. decrease to the north-east, away from the trench. In Table 1, we used the eustatic sea-level curve of Lambeck and Chappell (2001) to recalculate the uplift rates for deposits predating the Last Glacial Maxi-mum studied by Gardner et al. (1992) (Fig. 3). The deposits younger than 10 ka used by Gard-ner et al. (1992) to constrain uplift rates were reevaluated using the Fleming et al. (1998) sea-level curve (Table 1). The revised uplift rates preserve the originally observed pattern (Gardner et al., 1992) of decreasing uplift rate as a function of distance away from the Middle American Trench, as would be expected based on increases in depth to the plate interface or with the passage of relief through the subjacent plate interface (Sak et al., 2004a).

Fault Kinematics Along the Osa Peninsula

Across the northwest coast of the Osa Penin-sula, exposures of the late Pleistocene Marenco formation are dissected by northwest-striking planar faults, with separations locally in excess of 40 m; no penetrative mesoscale faults are recognized within the Marenco formation, sug-gesting that strain across the Osa Peninsula is accommodated along the widely (kilometer scale) spaced, northwest-striking faults. Sak et al. (2004a) interpreted these subvertical faults as the surface expression of up-shear required to accommodate the arrival of rough crust along the axis of the subducting Cocos Ridge. Along the northeast coast of the Osa Peninsula, uplift rates decrease linearly to the northwest (Gard-ner et al., 1992). Superimposed on this trend of decreasing uplift rate, there is a series of poorly exposed northwest-striking subvertical faults that result in northeast-side-up separa-tion (Gardner et al., 1992). The recognition of northwest-striking subvertical faults along both the northwest and northeast coasts of the Osa Peninsula may refl ect the deformation caused by relief along the crest of the underthrusting Cocos Ridge (Sak et al., 2004a).

DISCUSSION

Late Quaternary uplift rates vary both along and across the forearc of the Middle American Trench in Costa Rica (Figs. 3 and 6). Uplift rates are greatest across the outer coasts of the Nicoya (≤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 pedo-genically 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.



Dep

th (

km)

350 300 250 200 150 100 50 0

TJ FSRQP Cocos

-2

-1

0

1

2

3

4

5

6

7

8

9

10

Upl

ift r

ate

(mm

/yr)

246

8101214

A

NicoyaSegment

SeamountSegment

OsaSegment

n = 66

H EsP

Ez

84°W85

°W9°N

Osa

Q

PlayaCamaronal

N0 20

km

Nicoya

O

RS

RT

TJ FSR

Moho

Upper CrustQP Cocos

C

D

4

2

0

6

Upl

ift r

ate

(mm

/yr)

PCPI

10°N

CO-CA

375 300

Distance (km)

Distance (km)325

Upl

ift r

ate

(mm

/yr)

Distance along coast (km)

8

7

6

5

4

3

2

1

00 5 10 15 20

1020

km

NicoyaPeninsula

CaboBlanco 0

y = 5.0 - 0.20xR2 = 0.8

E

N

NESW

8

7

6

5

4

3

2

14 6 8 10 12

Faul

t zon

e

14

Upl

ift r

ate

(mm

/yr)

Distance along coast (km)

OsaPeninsula

CM 15 k

m

0

y = 6.7 - 0.91xR2 = 0.7

y = 11.6 - 1.2xR2 = 0.7

F

N

NESW

B

Sak et al.


and of lesser magnitude (≤4.4 m k.y.–1) along the central Pacifi c coast (Figs. 3, 6A, and 6B). The pattern of along-strike variations in uplift rate matches the distribution of bathymetric highs across the subducting plate (Figs. 3 and 6C). For example, terraces along the Nicoya coast show the greatest uplift rates by Punta Guio-nes and Cabo Blanco, inboard of the subduct-ing bathymetric features related to the Cocos-Nazca-Panama triple junction trace and the Fisher Seamount and Ridge, respectively. There is a sag in the elevation of uplifted marine ter-races that lies directly inboard of the sag in the bathymetry between the fossil triple junction trace and the Fisher Seamount and Ridge on the subducting plate (Fig. 6D). The leading edges of the two outer forearc peninsulas are charac-terized by late Quaternary uplift rates locally ≥6 m k.y.–1. Low-lying, Holocene marine ter-races exposed along the northeast-trending coastline of the southern tip of Nicoya indicate that long-term uplift rates diminish away from Cabo Blanco to both the northeast (away from the trench; Fig. 6E) and the northwest (away from the Fisher Seamount and Ridge; Marshall and Anderson, 1995; Gardner et al., 2001). Late Pleistocene marine terraces exposed along the northeast-trending coastline of the southern tip of the Osa Peninsula record a similar pattern of diminishing long-term uplift rates away from the trench (Gardner et al., 1992) (Fig. 6F).

The crustal thickening in the subaerial por-tion of the forearc is in direct contrast to the net crustal thinning imaged (McIntosh et al., 1993; von Huene et al., 2000) and modeled (Dominguez et al., 1998) in the outer forearc. There are three mechanisms for thickening of the subaerially exposed portions of the forearc: underplating (accretion of subducted seamounts and/or sediment from the outer slope; i.e., Fisher et al., 1998; Bangs et al., 2006) (Figs. 7 and 8), shortening along out-of-sequence thrust faults (active faults that lie arcward of the axis of the Middle American Trench; i.e., Fisher et al., 1998, 2004) (Figs. 8A and 8B), or transmission of shortening across the rigid forearc to the back arc (Mann et al., 1998) (Fig. 7A). Although the difference between submarine subsidence and uplift within the subaerially exposed portions of the forearc might be explained by any of these mechanisms, or a combination of processes, the distribution of uplift and forearc kinemat-ics sheds insight into the dominant mechanism. Where underplating is the dominant means of crustal thickening, no shortening within the inner forearc is required to explain crustal thickening and forearc mountain building. The uplift of the inner forearc by underplating could cause sea-ward tilting of slope deposits and thinning of the slope apron toward the inner forearc. In contrast,

the development of out-of-sequence thrust faults results in shortening across the inner forearc and/or a fault truncation of the arcward margin of the slope apron. Under these circumstances, the greatest subsidence in the outer forearc occurs in the footwall of the reverse fault that trun-cates the slope apron, and slope sediment could thicken landward. Alternatively, where offshore stresses associated with rough crust subduction are transmitted across a rigid forearc to the back arc, internal shortening (within the forearc) may be accommodated by the development of large-scale folding (Fig. 7). Following Marshall et al. (2000), who defi ned the Central Costa Rica deformed belt on the basis of changes in fault kinematics from sinistral transtension across steeply dipping northeast-striking faults in the forearc domain to a system of conjugate north-west- and northeast-striking transcurrent faults across the Central Costa Rica deformed belt, we propose that this boundary represents the arcward extent of deformation related to colli-sions in the forearc (Fig. 7B). The observation in seismic-refl ection profi les that upper slope sedi-ment thickens landward (McIntosh et al., 1993; Hinz et al., 1996) is consistent with either trun-cation along an out-of-sequence thrust fault near shore that separates the uplifting inner forearc from the subsiding outer forearc or an increase in sediment thickness closer to an onland source area. Seismic lines of the last two decades do not cross the shallow offshore region where such a fault could be imaged.

The passage of bathymetric features results in narrow scarp-bounded corridors of subsi-dence across the submarine slope and sustained broader uplift and segmentation across the inner forearc. The structural history and the late Pleis-tocene landscape evolution observed inboard of subducting rough crust are consistent with the model shown in Figure 9. Circa 1 Ma, in the absence of subducting seamounts, the subma-rine margin was laterally continuous (Fig. 9A). With the onset of rough crust subduction, the margin became embayed, and arcward retreat of the trench began (Fig. 9B). Inboard of the leading edge of the Cocos Ridge, some of the convergence was transferred from the Middle American Trench to the Fila Costeña fold-and-thrust belt (Fig. 9B). Continued subduction of rough crust resulted in further arcward retreat of the Middle American Trench and scalloping of the submarine portions of the forearc in the wake of subducting seamounts (Fig. 9C). Inboard of where the Cocos Ridge intersects the Middle American Trench, shortening continued within the Fila Costeña fold-and-thrust belt with the development of additional thrust faults imbri-cating Paleogene and older strata (Fig. 9C). Ongoing subsidence and erosion of the forearc

inboard of subducting seamounts contrasts with broader regions of uplift within the inner forearc. Across the central Pacifi c coast region, margin-perpendicular faults accommodated differential rates of uplift. Areas with the greatest rates of surface uplift expose Paleogene and older rocks and are inboard of localized zones of subsidence imaged across the submarine portions of the forearc (Fig. 9D). Opposite the Cocos Ridge, continued shortening in the Fila Costeña fold-and-thrust belt resulted in development of suc-cessive active frontal thrust faults seaward of the previous frontal thrust (Fig. 9D).

Mesoscale fault populations suggest that the forearc can be subdivided into three kinematic domains that correlate to the geometry and morphology of the subducting plate (Fig. 4), as well as distance from the trench. Along the Nicoya Peninsula, where the crust currently entering the Middle American Trench is rela-tively smooth and the Benioff zone defi nes a steeply dipping slab, mesoscale fault popula-tions are consistent with margin-perpendicular extension and steeply dipping faults at high angles to the margin that allow differential dis-placement along the margin, while many of the other mesoscale fault populations measured in the vicinity of Punta Guiones (Figs. 2A and 4) are consistent with the normal faults imaged by McIntosh et al. (1993) across the upper slope, offshore of Punta Guiones. Some of these faults, which have been attributed to downslope creep of poorly consolidated sediment overlying the older prism, displace the ocean fl oor, indicating that the deformation is ongoing (McIntosh et al., 1993). Across the Osa Peninsula, like the central Nicoya Peninsula, steeply dipping, northwest-striking faults dissect late Pleistocene marine sediment. The northwest-striking subvertical faults on the Osa Peninsula are interpreted to accommodate leading-edge-up shear necessary to accommodate irregular bathymetry along the axis of the indenting Cocos Ridge (Sak et al., 2004a). Elsewhere along the margin, steeply dipping, northwest-striking faults imaged across the submarine forearc have been attributed to collapse of the surface in the wake of sub-ducting seamounts as a result of basal erosion (i.e., Hinz et al., 1996; Dominguez et al., 1998; Ranero and von Huene, 2000; Hühnerbach et al., 2005). Similarly, along the northern Puerto Rico–Virgin Islands margin, rough crust sub-duction has resulted in forearc collapse in the wake of subducting ridges, attributed to basal erosion (Grindlay et al., 2005).

Along the central Pacifi c coast, northeast-striking steeply dipping nodal planes accommo-date differential uplift associated with ongoing seamount subduction (Fig. 4). Further to the southeast, in the inner forearc and inboard of



A

OsaN. Panama

deformed beltFila Costena

Leading edge of the Cocos Ridge

Subducted remnant of the Panama fracture zone

50 km

Seamount

Outer forearc Inner forearc

(1)

(2)Outer forea

rough crust subduction, forearc kinematics, and quaternary...

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