central costa rica deformed belt: kinematics of diffuse...

TECTONICS, VOL. 19, NO. 3, PAGES 468-492 JIfNE 2000

Central Costa Rica deformed belt:

Kinematics of diffuse faulting across the western Panama block

Jeffrey S. Marshall 1 and Donald M. Fisher Department of Geosciences, Pennsylvania State University, University Park

Thomas W. Gardner

Department of Geosciences, Trinity University, San Antonio, Texas

Abstract. Fault kinematics, seismicity, and geodetic data across central Costa Rica reveal a diffuse fault zone, here named the Central Costa Rica Deformed Belt (CCRDB). The CCRDB defines the western margin of the Panama block and links the North Panama Deformed Belt (NPDB) along the Caribbean coast with the Middle America Trench (MAT) along the Pacific coast. The junction of the CCRDB and the MAT coin- cides with an abrupt transition from smooth to rough crust on the subducting Cocos plate (rough-smooth boundary). Shallow subduction of rough, thickened oceanic crust associated with the Cocos Ridge shifts active shortening into the volcanic arc along faults of the CCRDB. Variable fault kinematics along this zone may reflect three combined deformation mechanisms: horizontal shortening and shear from oceanic ridge indentation, basal traction from shallow subduction, and localized block uplift from subducting seamount roughness. Within the forearc (domain 1), mesoscale faults express transtension where steep NE striking regional-scale faults intersect the Pacific coast. Across the volcanic arc (domain 2), mesoscale faults exhibit mostly sinistral and dextral slip on NE and NW striking conjugate faults, respectively. Approaching the NPDB in the back arc (domain 3), transcurrent faulting is modified by transpression and crustal thickening. Fault kinematics are consistent with earthquake focal mechanisms and Global Positioning System (GPS) measurements. Radiometric age constraints confirm that faulting post- dates the late Neogene onset of shallow subduction. The ensu- ing deformation front has propagated northward into the volcanic arc to its present position along the seismically active CCRDB. Within the forearc, the effect of shallow subduction is overprinted by local uplift related to underthrusting seamounts.

1. Introduction

The Panama-northern Colombia region of Central and South America spans a complex deformation zone between four actively converging tectonic plates: Caribbean, South American, Cocos, and Nazca (Figure 1). Plate motions are partitioned

1Now at Department of Geosciences, Franklin and Marshall College, Lancaster, Pennsylvania.

Copyright 2000 by the American Geophysical Union.

Paper number 1999TC001136. 0278-7407/00/1999TC001136512.00

across a diverse array of actively evolving fault zones that span several indelSendent crustal blocks or microplates (e.g., Panama, North Andes, and Maracaibo) [Mann and Burke, 1984, Mann et al, 1990]. These crustal fragments function as broad deformation zones that accommodate the complicated kinematics of distributed plate boundary deformation.

This study examines the kinematics and the tectonic origin of faulting along the western margin of the Panama block in central Costa Rica. The Panama block consists of the southern

end of the Central American volcanic arc that has detached

from the Caribbean plate owing to collision with South America (Figure 1). Along Panama's eastern margin, convergence with South America is accommodated along diffuse transpressional faults of the East Panama Deformed Belt [Mann and Kolarsky, 1995] and by uplift of the Colombian Cordillera within the North Andes block [Kellogg and Vega, 1995]. This collision also drives oroclinal bending and northward thrusting of Panama over the Caribbean plate along the North Panama Deformed Belt [Wadge and Burke, 1983; Vergara-Muhoz, 1988; Silver et al., 1990, 1995]. To the south, oblique convergence between the northern Nazca plate and the Panama block occurs along the South Panama Deformed Belt [Mackay and Moore, 1990; de Boer et al., 1991; Westbrook et al., 1995; Moore and Sender, 1995; Kolarsky and Mann, 1995]. While these convergent zones de- lineate the eastern, northern, and southern boundaries of the independent Panama block, the kinematics of deformation along the western margin have remained poorly constrained.

Here we define the Central Costa Rica Deformed Belt

(CCRDB) as a diffuse zone of seismically active faulting across central Costa Rica that marks the western margin of the Panama block (Figures 2 and 3). In this paper, we investigate the nature of active faulting along the CCRDB by comparing mesoscale fault kinematics with patterns of regional-scale faults, historic seismicity, and Global Positioning System (GPS)-measured crustal displacements. We examine fault population data from 86 outcrops in conjunction with earthquake focal mechansims in order to evaluate the spatial variations in fault kinematics across central Costa Rica. In addition, we es-

tablish age constraints for offset Quaternary units and confirm that faulting along the CCRDB is active. Finally, we assess the kinematics and timing of faulting within the context of regional tectonics in order to explore the potential causes of this deformation.

The observations made in this study indicate that the CCRDB represent. s a deformation front that has propagated into the volcanic arc in response to the shallow subduction of

468

MARSHALL ET AL' FAULT KINEMATICS, COSTA RICA 469

thickened oceanic lithosphere associated with the Cocos Ridge and seamount domain on the Cocos plate (Figure 2). Faults of the CCRDB accommodate diffuse crustal shortening and sinistral shear across the volcanic arc, allowing for northeastward displacement of the western Panama block toward the back arc North Panama Deformed Belt.

2. Tectonic Framework

The Central American volcanic arc is generated by subduction of the Cocos plate at the Middle America Trench along the southwestern margin of the Caribbean plate. The Panama block consists of the southern portion of the arc, extending from the margin of South America in the east, to central Costa Rica in the west (Figure 1). This independent block spans the Cretaceous Chorotega and Choco oceanic basement terranes and encompasses several Cenozoic volcanic cordilleras and uplifted sedimentary basins that expose Paleogene deep marine and Neogene-Quaternary shallow marine, volcaniclastic, and fluvial sediments [Escalante, 1990]. Late Neogene colli-

sion with South America uplifted the Panama arc and created a land bridge that closed the Caribbean-Pacific seaway [Kellogg and Vega, 1995].

Until recently, little attention had been focused on the western boundary of the Panama block. Previous research ei-

ther did not discuss a western boundary [Adamek et al., 1988] or loosely associated it with such features as the Panama Fracture Zone [Bowin, 1976; Vergara-Muhoz, 1988] or NW trending faults traversing Panama [Mackay and Moore, 1990; Mann and Corrigan, 1990; de Boer et al., 1991]. However, recent investigations of regional stratigraphy [Astorga et al., 1991; $eyfried et al., 1991], tectonic geomorphology [Gardner et al., 1993; Marshall et al., 1995], fault kinematics [Marshall et al., 1993; Fisher et al., 1994], seismicity [Jacob et al., 1991; Giiendel and Pacheco, 1992; Goes et al., 1993; Fan et al., 1993], and geodetics [Lundgren et al., 1993, 1999] have recognized a diffuse region of active faulting across central Costa Rica. This broad deformation zone extends onland

from the NPDB along the Caribbean coast (Figures 2 and 3), traverses the volcanic arc and heavily populated Valle Central, and intersects the Pacific coast between Puntarenas and

Quepos to meet the MAT south of the Peninsula de Nicoya [Fisher and Gardner, 1991; Marshall et al., 1993; Fisher et al., 1994].

The location occupied by the CCRDB has long been recognized as a major segment boundary along the Middle America arc-trench system [Stoiber and Carr, 1973; Carr, 1976; Burbach et al., 1984]. This position within the overriding plate corresponds with the location of the "rough- smooth boundary" [Hey, 1977] on the subducting Cocos plate offshore (Figure 1). The rough-smooth boundary follows a

NICARAGUA

ß ee

:GA.EAPAGOS COLOMBIA

?:i•::•:•:.•:',•,:,•:,:•:•':':.i',:::?:::....' ...... 80 ø '•' - 75ø ...............

..

\ MAR 8 mm/yr

NAN 21 mm/yr/ SOAM

16 mm/yr

Figure 1. Tectonic setting of southern Central America showing the Central Costa Rica Deformed Belt (CCRDB) along the western margin of the Panama block (PAN). The CCRDB links the North Panama Deformed Belt (NPDB) with the Middle America Trench, and is located onshore of the rough-smooth boundary on the subducting Cocos plate (COCOS). Large arrows show plate motions relative to the Caribbean plate (CARIB) [DeMets et al., 1990]. Small arrows show velocities for Global Positioning System (GPS) sites (solid circles) relative to Panama (solid square) [Kellogg and Vega, 1995]. The Cocos Ridge is outlined by the 1000-m depth contour. The rectangle shows area of Figure 2. NAZCA, Nazca plate; SOAM, South American plate, MAR; Maracaibo block; NAN, North Andes block; EPDB, East Panama Deformed Belt; SPDB, South Panama Deformed Belt. Map is compiled from Lonsdale and Klitgord [1978], Mackay and Moore [1990], Silver et al. [1990], Kellogg and Vega [1995], Protti et al. [1995a], and Westbrook et al. [19951.

470 MARSHALL ET AL.: FAULT KINEMATICS, COSTA RICA

-o

CARlB.

9 o .91

mm/yr •

domain

N

km

-85 ø 84 ø -83

1.0.-0

Qua ternary ß .. .-. ...• Quaternary ::';,'•-•'• Neogene intrusive rocks • sediments volcanic arc

Neogene ;i!'...".'71:: Cretaceous - Paleogene Neogene volcanic arc oceanic basalts & sediments sediments

i i

Figure 2. Tectonic map of Costa Rica showing the on-land geologic structure relative to the offshore bathymetry (contours in meters). The CCRDB (outlined by dashed lines) marks the diffuse boundary between the Panama block and the Caribbean plate. The NW limit of the CCRDB aligns with the rough-smooth boundary (RSB) on the subducting Cocos plate. The RSB separates smooth crust to the NW from rough crust (seamount domain and Cocos Ridge) to the SE. The subducting Cocos Ridge aligns with a gap in the active volcanic arc (asterisks), with the uplifted intrusive rocks of the Cordillera de Talamanca (C Tim) and with the inverted sedimentary basins of the Terraba thrust belt (TB) and the Lim6n thrust belt (LB). The rectangle shows the area of Figure 3. MAT, Middle America Trench; C Gnc, Cordillera de Guanacaste; C Agt, Cordillera de Aguacate; C Cen, Cordillera Central; Nicoya, Peninsula de Nicoya; Osa, Peninsula de Osa.

linear morphologic trend on the subducting seafloor that separates smooth, low-relief crust to the NW from rough, high-relief crust to the SE (Figure 2) [yon Huene et al., 1995]. Rough, thickened seafloor SE of the boundary contains the aseismic Cocos Ridge and adjacent seamounts, products of relatively slow spreading and pervasive hotspot volcanism along the Galapagos Rift system [Holden and Dietz, 1972; yon Huene et al., 1995; Barckhausen et al., 1998; Meschede et al., 1998; Stavenhagen et al., 1998; Werner et al., 1999].

The Cocos Ridge, a primary trace of the Galapagos hotspot, began subducting along the MAT sometime in the late Neogene [Collins et al., 1995; Kolarsky et al., 1995; Meschede et al., 1999b]. Shallow ridge indentation dramati- cally decreased the subduction angle [Prottiet al., 1995a], diminished the mantle wedge, and extinguished arc volcanism within the Cordillera de Talamanca [de Boer et al., 1995]. Rapid uplift and horizontal shortening occur within a broad arch across the ridge axis,. extending from the forearc Peninsula

de Osa and Terraba belt, through the Talamanca arc, and into the back arc Lim6n belt (Figure 2) [Corrigan et aL, 1990; Gardner et al., 1992; Bullard, 1995; Collins et al., 1995; Kolarsky et al., 1995].

To the NW of the Cocos Ridge, along the central Costa Rican margin, the pronounced impact of ridge indentation di- minishes [Gardner et al., 1992]. Subducting seamounts, which ornament the moderately thickened crust on the NW ridge flank (Figure 2), deform the outer trench slope [yon Huene et al., 1995; Barckhausen et al., 1998] and produce differential uplift of forearc fault blocks [Marshall and Anderson, 1995; Fisher et al., 1998]. Shallowing of the subducting slab beneath central Costa Rica, combined with possible trench retreat by forearc erosion [e.g., Meschede et al., 1999a], has resulted in northeastward migration of the volcanic arc from the extinct Cordillera de Aguacate to the modern Cordillera Central (Figure 2) [Marshall, 1994; Marshall and Idleman, 1999]. In this paper, we suggest that shallow sub-

MARSHALL ET AL' FAULT KINEMATICS, COSTA RICA 471

CI Z •

• o o o


duction also drives active faulting across the volcanic arc along the CCRDB, as deformation propagates inland above the NW flank of the indenting Cocos Ridge.

3. Geology of Central Costa Rica The Central Costa Rica Deformed Belt traverses the width of

the volcanic arc, extending from the Pacific forearc region to the Caribbean back arc. Faults of the CCRDB offset a broad

range of lithologies, ranging from Cretaceous-Tertiary seafloor basement and marine sediments, to Quaternary terrestrial sediments and extrusive rocks. For this study, we summarize the regional geology by defining several general lithostrati- graphic units shown in Figures 2 and 3.

Within the central Pacific forearc the upper Cretaceous igneous basement (Nicoya Complex) is exposed along the outer forearc high of the Peninsula de Nicoya [Lundberg, 1982] and within uplifted inner forearc blocks [Fisher et al., 1998]. Flanking these forearc basement exposures are upper Cretaceous-Paleogene deep marine and slope sediments derived from the early Costa Rican arc [Lundberg, 1982]. Rocks of the inner forearc coastal piedmont include Neogene shallow marine to deltaic sediments of the Nicoya and Parrita basins [Astorga et al., 1991], Neogene-Quaternary volcanic and volcaniclastic rocks derived from the Aguacate arc [Denyet and Arias, 1991], and Quaternary fluvial and marine terrace deposits [Fisher et al., 1994].

The complex volcanic arc of central Costa Rica encompasses the active Cordillera Central, as well as the extinct Cordilleras de Aguacate and Talamanca (Figures 2 and 3). The Cordillera Central consists of a broad massif of Quaternary andesitic- dacitic stratovolcanos. To the SW of the active cordillera, heavily eroded remnants of the extinct Cordillera de Aguacate expose Neogene-Quaternary basaltic-andesite lavas and pyroclastic flows (Aguacate Group) [Denyet and Arias, 1991]. The Valle Central basin, situated between the extinct Aguacate arc and the active Cordillera Central, contains a volcanic tableland formed by the accumulation of a thick sequence of Quaternary andesitic to dacitic lavas, pyroclastic flows, and lahar deposits [Denyet and Arias, 1991].

The Cordillera de Talamanca, SE of the Valle Central, corresponds with a 175-km-wide gap in the active volcanic arc and represents the only area of southern Central America above 2000 m in elevation (Figures 2 and 3) [de Boer et al., 1995; Kolarsky et al., 1995]. These rugged mountains expose a suite of Neogene-Quaternary intrusive (principally granodi- orites) and extrusive rocks (andesites) that are correlative in age with the Aguacate arc to the NW. Rapid uplift driven by Cocos Ridge subduction has provoked extensive unroofing of the Talamanca range, exposing the intrusive core.

The intra-arc and back arc regions along the flanks of the Cordilleras de Aguacate and Talamanca expose both Paleogene deep marine and Neogene shelf sediments of the Candelaria and Lim6n basins (Figures 2 and 3) [Denyet and Arias, 1991; Astorga et al., 1991]. Rocks of the Candelaria basin were moderately deformed by homoclinal tilting during the late Neogene [Denyet and Arias, 1991], while sediments of the Lim6n basin have been extensively faulted and folded within the back arc thrust system of the NPDB [Astorga et al., 1991].

4. Regional-Scale Faults 4.1. Kinematic Domains

In order to facilitate our discussion of fault kinematics we

have divided the study area into three generalized kinematic domains (Figure 4). The forearc region (D l: domain 1) encompasses the inner forearc (Pacific coastal piedmont), as well as portions of the forearc basin (Golfo de Nicoya) and outer forearc high (Peninsula de Nicoya). The central volcanic arc region (D2: domain 2) spans the extinct Neogene to Quaternary volcanic arc (Cordilleras de Aguacate and Talamanca), the Valle Central basin, and the southwestern flank of the active arc (Cordillera Central). This domain also includes portions of the uplifted Tertiary-aged Candelaria basin along the flanks of the extinct arc. The back arc region (D3: domain 3) encompasses the Caribbean slope of both the extinct arc (Cordillera de Talamanca) and modern arc (Cordillera Central), as well as uplifted portions of the back arc basin (Lim6n basin).

Deformation along the CCRDB occurs across a diffuse array of regional-scale faults (lengths of several kilometers to tens of kilometers) that exhibit a range of orientations and styles of slip (Figures 3 and 4, and Table 1). Our discussion of fault kinematics begins with a description of these regional-scale features, based on a combination of field observations and a compilation of existing geologic map data.

4.2. Regional-Scale Faults: Forearc (Domain 1)

The CCRDB intersects the central Pacific coast between

Puntarenas and Quepos (Figures 2 and 3), southeast of the projected trend of the rough-smooth boundary on the subducting plate. Offshore seismic-reflection profiles reveal steep NE striking margin-perpendicular faults that show dip-slip offset of late Cenozoic outer forearc shelf sediments (Figure 3) [Barboza et al., 1995]. Onshore, a series of steep margin-perpendicular faults strike along the NE trending valleys of major Pacific slope rivers including the Rios Barranca, Jesfis Maria, T•trcoles, Tusubres, and Parrita [Madrigal, 1970]. These major faults (10-20 km in length) allow for differential uplift of a system of inner forearc blocks referred to as the Esparza, Orotina, Herradura, Esterillos, and Parrita blocks (Figure 4) [Fisher et al., 1994, 1998]. Uplift rates, estimated from Quaternary marine and fluvial terraces, vary sharply across block-bounding faults. These displacements are consistent with vertical offsets observed in Late Cretaceous through Quaternary deposits. The broadly distributed uplift generated by the subducting Cocos Ridge to the SE [Gardner et al., 1992] is corrugated locally by block uplift above subducting seamounts [Fisher et al., 1998]. The following paragraphs discuss the block- bounding faults from NW to SE along the central Costa Rican forearc.

At the northwestern edge of the CCRDB the Barranca fault (Figure 4, Fault 1) strikes NE along the Rio Barranca valley, separating the uplifted Esparza block from the low-lying Puntarenas coastal plain to the NW. Vertical offsets of up to 30 m for late Quaternary fluvial terraces across the Rio Barranca and up to 4 m for Holocene marine benches near the river mouth demonstrate active slip along this fault. A radio- carbon date of-•3.0 ka for wood beneath a colluvial wedge on the uplifted Holocene platform indicates a maximum late Holocene uplift rate of 1.3 m/ka for the Esparza block near the

MARSHALL ET AL.' FAULT KINEMATICS, COSTA RICA 473

'.Cent ra

?.San Jos•"

ß ...

% % %

%

% % % % %

83 ø 30'

Figure 4. Geologic map of the central study area showing regional-scale faults (Faults 1-44) and the 3 kinematic domains: forearc (D 1), volcanic arc (D2), and back arc (D3). The faults are keyed by number to Table 1. Heavy white dashed lines mark kinematic domain boundaries. Medium white dashed lines mark edges of forearc fault blocks: Ez, Esparza; O, Orotina; H, Herradura; Es, Esterillos; P, Parrita; Q, Quepos [Fisher et al., 1998]. Dashed shaded rectangles outline areas of Figures 6a-6c, and 6f. Volcanoes are as follows: VB, Volcfin Barva; VI, Volcfin Irazfi; VT, Volcfin Turrialba. See Figure 2 for geologic symbols.

Barranca fault. This rate is consistent with longer-term average uplift of the upper Barranca fluvial terrace (Qt], oxygen iso- tope stage 5e - 125 ka) at a rate of 1.0 m/ka [Fisher et al., 1998]. Remnants of the upper three terraces (Qt]_3) on opposite sides of the Barranca fault show the same magnitude of vertical offset (- 30 m). This observation suggests that slip along the fault is relatively recent, having begun after formation of Qt3 in the late Pleistocene.

The NE striking Jesfis Maria fault (Figure 4, Fault 3) separates the uplifted Esparza block from the low-lying Orotina block to the SE. This fault forms a prominent SE facing scarp along the NW bank of the Rio Jes•s Maria. Pleistocene lahar deposits (Tivives Formation: age <2 Ma, 4øAr/39Ar) [Marshall and Idleman, 1999] and Miocene shallow marine volcaniclastic sediments (Punta Carballo Formation: minimum age middle Miocene) [Madrigal, 1970] are offset across the Jes•s Maria fault with a NW-side-up separation of-•120 m [Fisher et al., 1994].

Within the interior of the Orotina block several minor NE

striking faults (Trinidad, Diablo, and Poz6n; Figure 4, Fault 5) show dip-slip offset of late Quaternary ash flows, volcaniclastic sediments, and fluvial terrace gravels of the Orotina Formation and expose Miocene sediments within isolated to-

pographic highs. A rhyodacitic welded tuff (-•400 ka, 4øAr/39Ar) [Marshall and Idleman, 1999] and underlying fluvial gravels, which outline a paleoriver course, show up to 40 m of vertical offset across these faults.

The NE striking Tfircoles fault (Figure 4, Fault 6) accommodates vertical motion between the low-lying Orotina block and the uplifted Herradura block to the SE. The Herradura block, which contains the highest topography within the forearc, exposes upper Cretaceous Nicoya Complex seafloor basalts which have been stripped of their sedimentary cover. While the Tfircoles fault forms the principal boundary between the Orotina and Herradura blocks, several subsidiary faults (e.g., Carara and Turrubares; Figure 4, Fault 7) outline minor fault blocks along this trend. Vertical offsets of up to 60 m for late Quaternary fluvial terraces along the Rio T,qrcoles demonstrate active slip within the T,qrcoles fault system.

The NE-striking Tusubres fault (Figure 4, Fault 10)jux- taposes the upper Cretaceous basement within the Herradura block against Paleogene slope sediments of the Esterillos block to the SE [Sak et al., 1997]. Rocks of the moderately uplifted Esterillos block are, in turn, offset from the low-lying Parrita block to the SE across the NE striking Parrita fault (Figure 4, Fault 12). An elevation difference of-•150 m for flu-

474 MARSHALL ET AL.' FAULT KINEMATICS, COSTA RICA

Table 1. Regional-Scale Faults of the Central Costa Rica Deformed Belt

Map Length, Slip Side Earthquakes Number Fault km* Strike Sense•' UpS' (M >3.0)•'•' References{}

1 Barranca 18 45 N SE

2 Barbudal, Jocote 7 132, 125 N SW, NE 3 Jesfis Maria 12 43 N NW

4 Cuarros 9 95-120 (N) SW 5 Trinidad, Diablo, Poz6n 7 65, 45, 45 N SE, NW, SE 1924 (7.0), 1989 (3.6) [A] 6 Ttircoles 21 45 N (SE) 7 Turrubares 6 55 N SE

8 Leona 8 45 N SE

9 Herradura system 5 40-65 (N) 9 10 Tusubres 10 35 N NW

11 Esterillos system 5 50 (N) 9 12 Parrita 15 40 N NW

13 Quepos system 5 25-175 N 9 14 Turrubaritos 15 115-140 9 9

15 Venado 5 57 (N) NW 16 Tigre 20 125-135 (R) SW 17 Tulin 10 65-70 L NW

18 Zapat6n 7 125-130 (R) NE 19 Candelaria 60 130-135 R/T SW, NE

20 Quivel 8, 122-124 R (SW) 21 Queb. Colorado 10 57-59 L (NW) 1995 (4.7) [O] 22 Delicias 17 38-42 L (NW) 23 La Mesa, Resba16n 17 125-135 R (NE) 24 Cortezal 7 50 L 9 1990 (5.0) [El, 1990 (5.7) [I] 25 Picagres 7 120-122 (R) 9 1990 (4.5) IF], 1990 (4.8) [K] 26 Jarls 33 120-130 R NE

27 La Garita 10 33 (L) NW 28 Ciruelas 10 45-55 (L) NW 29 Pacacua 10 55 (L) SE 30 Salitral, Tablazo 14 35, 45 L NW

Alumbre, Patio de Agua 12 50, 45 L NW 19107 31 Coralillo 7 145 R 9 1991 (4.7) [MM] 32 Higuito 38 128-132 R SW 1993 (3.7) [Y], 1997 (3.4) 33 Alajuela 20 85-115 T NE 1772, 1851, 18887 34 Hondura 10 12 (N/R) 9 1993 (3.2) [FF] 35 Sucio, A. Grande, Blanco 18 128, 155, 146 R NE, NE, SW 1952 (5.2) 36 Coris-Guarco 15 100-125 (R) 9 1841, 1910, 19127 37 Navarro 16 65-75 (L) 9 1951 (5.0)? 38 Patarrfi 6 140 (R) 9 39 Orosi 20 140 (R) 9 1910, 1951 (5.0)? 40 Pejibaye 13 50 L (SE) 1993 (4.4)[AB] 41 Gato 7 130 (R) 9 1993 (4.3) 42 Atirro 18 150 R 9 1987 (4.3) [AG], 1988 (4.5) [AF] 43 Tuis 20 120-130 (R) 9 44 Pacuare 20 130 (R) 9

1,7 7

1,7 1

1,5

1,7 1

1

1

1, 14

1, 14

1, 14 1

2

2

2

2

2

2

2

2,4 2

2

2,4

4,6

2,11

1,2

1,2 2

2,11

2,8

4,8

2,4

3,8,13

4,9

8,9

4,8 4

4

10

4,12 4

8, 12 1

1

* Mapped length (minimum). •' Predominant sense of displacement (parentheses indicate inferred): N, normal; T, reverse; L, left lateral; R, right lateral.

•'•' Magnitude in parentheses; letters indicate focal mechanisms in Figures 6a-6c and in Table 3. õ References are as follows: 1, this study; 2, Arias and Denyer [1991]; 3, Borgia et al. [ 1990]; 4, Ferndndez [1995];

5, Gaendel et al. [1989]; 6, Gaendel et al. [1990]; 7, Madrigal [1970]; 8, Montero [1994]; 9, Montero andAlvarado [1995]; 10, Montero and Miyarnura [1981]; 11, Montero and Morales [1984]; 12, Observatorio Volcanol6gico y Sismol6gico de Costa Rica (OVSICORI-UNA) [1993]; 13, Peraldo and Montero [1994]; 14, Sak et al. [1997].

vial terrace gravels across this fault demonstrates significant late Quaternary slip [Sak et al., 1997; Fisher et al., 1998].

In addition to the major NE striking faults along the block boundaries, several steep NW striking faults offset block

interiors. Within the Esparza block the Barbudal and Jocote faults (Figure 4, Fault 2) accommodate uplift of a minor highland. Within the Orotina block the WNW striking Cuarros fault (Figure 4, Fault 4) accommodates minor uplift

MARSHALL ET AL.: FAULT KINEMATICS, COSTA RICA 475

and seaward tilting of the coastal portion of the block. The interior of the Herradura block is cut by the NW striking Turrubaritos and Tigre faults (Figure 4, Faults 14 and 16), which show evidence of oblique dextral motion [Arias and Denyet, 1991], and may also accommodate some of the uplift of the Herradura block by reverse slip.

4.3. Regional-Scale Faults: Volcanic Arc (Domain 2)

Regional-scale faulting within domain 2 displays an overall conjugate pattern of NW striking dextral faults and NE striking sinistral faults (Figures 3 and 4). While some NE striking faults have lengths of up to 20 km, the dominant structures within this pattern are three major NW striking faults (Candelaria, Jarls, and Higuito) that display lengths of over 40 km (Figure 4, Faults 19, 26, and 32).

The NW striking Candelaria fault (Figure 4, Fault 19) extends for over 60 km along the seaward flank of the Aguacate arc, southeastward to merge with the Terraba thrust belt (Fila Costefta) north of Quepos. This major fault represents a tectonic boundary between the uplifted forearc blocks (Herradura, Esterillos, Parrita, and Quepos) to the SW and the extinct Aguacate volcanic arc to the NE (Figure 4). Field exposures of offset lithologic units and slickenlines display oblique dextral motion on the Candelaria fault [Arias and Denyet, 1991]. Significant uplift of the forearc basement within the Herradura block suggests a major component of vertical slip along the northern segment of the fault. Several NW striking subsidiary faults, including the Quivel and Zapat6n (Figure 4, Faults 18 and 20), trend subparallel to the Candelaria fault and also show evidence of oblique dextral slip [Arias and Denyet, 1991]. Sinistral NE striking faults that intersect the trace of the Candelaria fault include the

Tulin, Quebrada Colorado, and Parrita faults (Figure 4, Faults 17, 21, and 12) [Arias and Denyet, 1991; Sak et al., 1997].

The Jarls fault (Figure 4, Fault 26) strikes parallel to the Candelaria fault (Figure 4, Fault 19) for over 45 km southeastward from the Valle Central into the extinct volcanic arc, forming a tectonic boundary between the Cordilleras de Aguacate and Talamanca. The Jarls and Candelaria faults together bound the north tilted Candelaria basin homocline [Arias and Denyet, 1991]. At the southwestern edge of the Valle Central the Jarls fault cuts Quaternary welded tuffs as young as 400 ka [Marshall and Idleman, 1999]. Offsets along the trace of the Jarls fault suggest predominantly right-lateral motion, with a component of dip slip (NE side up) along a steeply dipping fault surface [Arias and Denyet, 1991]. This vertical motion may accommodate uplift and exposure of intrusive rocks within the northern Cordillera de Talamanca.

Another major NW striking fault, the Higuito fault, (Figure 4, Fault 32) runs along the southern margin of the Valle Central at the southwestern edge of the San Jos6 metropolitan area, marking the boundary between the Valle Central basin and the northern flank of the Cordillera de Talamanca. The

Higuito fault extends northwestward into the Valle Central and may continue beneath the Quaternary volcanic sequence to the southern flank of the Cordillera Central volcanoes.

Uplift of Miocene sediments along a prominent NW trending ridge within the Valle Central basin suggests SW-side-up motion along the Higuito fault. Toward the SE the Higuito fault extends out of the Valle Central for nearly 30 km into the

northern Cordillera de Talamanca, cutting Miocene- Quaternary sediments and volcanic rocks. A series of NE striking sinistral faults that intersect the Higuito fault along the northwestern edge of the Cordillera de Talamanca include the Salitral, Tablazo, Alumbre, and Patio de Agua faults (Figure 4, Fault 30). These faults offset Neogene-Quaternary units as well as NW trending folds within the Miocene sequence.

Drainage systems of the Valle Central have incised deeply into the Quaternary volcanic sequence along a network of fault-controlled canyons. The two principal rivers of the Valle Central, the Rios Virilla and Grande, join to form the Rio Grande de T/trcoles at the intersection of the NE striking La Garita fault and the NW striking Jateo fault (Figure 4, Faults 27 and 25). The La Garita fault runs for over 10 km, cutting Quaternary lavas, welded tuffs, and fluvial terraces. Offset of volcanic beds and paleosols record oblique left-lateral motion with the NW side up. Vertical separation of nearly 25 m for fluvial terraces suggests active late Quaternary slip across the La Garita fault.

Additional NE striking faults along the southern margin of the Valle Central include the Ciruelas and Pacacua faults

(Figure 4, Faults 28 and 29). The Ciruelas fault extends for at least 8 km and may continue farther to the NE into the modern arc beneath the cover of late Quaternary lavas. NW-side-up motion along the Ciruelas fault has uplifted a window of Miocene rocks along a fault-parallel ridge within the Quaternary volcanic tableland. The NW striking Jateo fault (Figure 4, Fault 25) may represent a left-stepping extension of the Jarls fault (Figure 4, Fault 26) that has been offset along the Ciruelas fault (Figure 4, Fault 28). The inferred Pacacua fault (Figure 4, Fault 29) runs for-10 km, cutting Miocene sediments and Quaternary lavas along the northwestern edge of the Cordillera de Talamanca.

An apparent exception to the overall NW-NE fault pattern within the Valle Central is the E-W trending Alajuela fault (Figure 4, Fault 33). This anomalous fault extends for over 18 km along the northern margin of the Valle Central, forming a clearly visible 100 to 200-m scarp at the foot of Po•s and Barva volcanoes. Quarry exposures along the trace of the Alajuela fault show clear offsets of young Quaternary volcanic deposits. Borgia et al. [1990] ascribe this scarp to fault propagation folding at the tip of a major north dipping gravitational thrust fault caused by spreading of the volcanic massif. While the abrupt scarp and Quaternary offsets attest to recent slip, seismic activity is virtually absent. Peraldo and Montero [1994], however, suggest possible association of this fault with damaging earthquakes in the eighteenth and nineteenth centuries.

4.4. Regional-Scale Faults: Back Arc (Domain 3)

Regional-scale faulting at the western edge of domain 3 deviates slightly from the conjugate pattern typical of domain 2. The WNW trending Coris, Guarco, and Agua Caliente faults (Figure 4, Fault 36) south of Cartago merge toward the east with the ENE striking Navarro fault (Figure 4, Fault 37) and the NW striking Orosi fault (Figure 4, Fault 39). The traces of these faults are marked by the alignment of numerous geother- mal springs, and recent activity is manifested by offsets of Quaternary fluvial terraces and Holocene soils.


Toward the east, along the upper Caribbean slope of the Talamanca arc, regional-scale faulting is dominated again by a strong NW-NE conjugate pattern of major faults that strike along deep linear valleys. Major NW striking faults include the Gato, Atirro, Tuis, and Pacuare faults (Figure 4, Faults 41- 44), and major NE trending faults include the Pejibaye (Figure 4, Fault 40), upper Pacuare, and Chirrip6 (beyond east edge of Figure 4). These faults are associated with bold topographic scarps and offset Quaternary river terraces. In addition, the Tertiary section and some Quaternary rocks in this region are affected by folding with NW trending axes, cut in some cases by minor NE verging thrust faults [Krushensky, 1972].

5. Fault Kinematics

5.1. Mesoscale Fault Analysis

While relative motions can be determined for many regional- scale faults in the field [e.g., Arias and Denyet, 1991], the precise kinematics of deformation across the CCRDB remain obscure. We therefore expand upon our understanding of regional-scale faulting by examining mesoscale fault populations. Mesoscale faults are outcrop-scale features (meter to decimeter length) that exhibit measurable fault surface orientations and kinematic indicators such as slickenlines.

Mesoscale fault data sets, combined with focal mechanisms from recent seismicity, allow us to better characterize strain across this diffuse plate boundary deformation zone.

Distributed faulting along plate boundaries is likely to reflect the kinematics of plate interactions, with slip on individual faults representing local increments of strain within a region of mesoscale cataclastic flow [e.g., Wojtal, 1989]. Populations of minor faults are unlikely to record a single common stress tensor. For this reason, we employ a kinematic- based method of fault analysis [Marrett and Allmendinger, 1990] rather than stress tensor reduction [e.g., Angelier, 1984]. The kinematic method determines the principal shortening and extension axes (P and T axes) on the basis of slip data from individual faults. The P and T axes from a population of faults are contoured to determine the distribution of strain axes

for that location. The strain, as expressed by P and T axes dis- tributions, should reflect the motion between regional blocks.

Mesoscale fault populations were measured at 86 locations across central Costa Rica in rocks ranging from Eocene to Holocene age (Table 2). In general, older rocks were avoided in order to exclude Cretaceous-early Paleogene faults generated during the early developmental phases of the MAT and its volcanic arc [e.g., Gutsky, 1988]. Kinematic data were collected from fresh fault surfaces exposed in quarries, excava- tions, road cuts, riverbanks, coastal cliffs, and wave-cut platforms (Table 2). Data include the strike and dip of individual fault planes, trend and plunge of slip lineations (slickenlines), and slip sense. The slip sense was determined on the basis of the geometry and nature of slicknelines, subsidiary fractures, and/or offset beds as outlined by Petit [1987].

Slickenline types vary between outcrops, mostly as a function of lithology. Fibrous mineral growth (mostly calcite) occurs on many fault surfaces within Tertiary sandstone and limestone units, as well as on some Neogene-Quaternary andesitic lavas and pyroclastic rocks (e.g., Aguacate Group).

These fibers commonly display steps or risers that are congruent with fault motion (stepping downward in the direction of the missing block), providing a relatively definitive indica- tion of slip sense.

Slickenlines within Quaternary lavas, pyroclastic rocks, and lahar deposits include mostly fault surface striations or grooves. Within Quaternary volcaniclastic and fluvial deposits, as well as soils, slip generally produces smooth streaks on fault surfaces. In the absence of congruent fibrous steps these slickenline types require subsidiary fractures to determine slip sense. In the majority of cases, subsidiary fractures were of "RM type" [Petit, 1987], minor, striated Riedel- style fractures intersecting the fault surface at shallow angles dipping in the direction of movement of the opposite block. These fractures are commonly concave upward toward the fault plane, intersecting the fault as crescents with horns pointing in the movement direction of the opposite block. In a few cases (usually in dense lavas), "T type" (tensile) fractures [Petit, 1987] were utilized as slip sense indicators. These features also tend to intersect the fault plane as crescents indicating the motion direction of the opposite block.

Fault data were analyzed using the method of Marrett and Allrnendinger [1990] and the computer software of Allrnendinger et al. [1994]. The resulting kinematic axes for fault populations are plotted on best fit fault plane solutions (Figures 5a-5c) . We interpret the kinematics of deformation along the CCRDB by examining the spatial distribution of the fault data on regional geologic maps in conjunction with regional-scale faults and earthquake focal mechanisms (Figures 6a-6f). The kinematics of both regional-scale and mesoscale faulting along the CCRDB show notable differences between domains 1, 2, and 3. In sections 5.2-5.4, we summarize the mesoscale fault data, earthquake seismicity, and regional fault patterns and discuss the observed kinematic variations moving from west to east across central Costa Rica.

5.2. Fault Kinematics: Forearc (Domain 1, Sites 1-27)

Within the forearc region (domain 1), mesoscale faults display considerable variability in orientation and slip direction (Figures 6a and 6d-6f). Normal and strike-slip faults, however, significantly outweigh thrust faults in number and in magnitude of slip. Fault population data from this area reflect transtension with a predominance of shallow T axes and P axes trending between steep and shallow.

Shallow T axes, combined with steep P axes, suggest a component of extension for a majority of populations. This is consistent with offsets of Neogene-Quaternary units observed along steep, regional-scale, block-bounding faults (Figure 4). Where mesoscale P and T axes are both shallow, the data show predominantly left-lateral motion on NE striking faults. This is consistent with sinistral transtension across the forearc

fault blocks.

Historically, shallow upper plate earthquakes within the forearc (domain 1) have been relatively rare in comparison to the volcanic arc and back arc (domains 2 and 3). A seismic sequence in 1989 centered on the coastal piedmont of the Orotina block (Figure 6a, Focal Mechanism A, and Table 3) showed oblique-normal slip along a linear NE trend [Giiendel et al., 1989]. This swarm occurred in the same location as the M=7.00rotina earthquake of 1924 (Table 1), which caused ex-


Table 2. Mesoscale Fault Population Data Field

Site Number Latitude Longitude Outcrop Location and Type* Age Total

and Fm-• Faults

1 03-96 9 ø 37'

2 02-96 9 ø 37'

3 01-96 9 ø 35'

4 07-94 9 ø 54'

5 05-94 10 ø 03'

6 06-94 10 ø 06'

Peninsula and Golfo de Nicoya (Forea/c Region, Domain 1) 85 ø 09' Punta Barrigona, Malpais (sp) Tm-st 8 85 ø 08' Quebrada Vanegas, Malpais (rc) Tm-st 8 85 ø 06' Cabo Blanco Reserve HQ (sp) Tpe-cb 10 84 ø 56' Playa Gigante (sc) Tpe-lp 10 84 ø 58' Punta Morales (sp/sc) Tpe-lp 5 84 ø 56' Pta Morales Hwy, Cerro Am6rica (rc) To-ma 13

7 05-90 9 ø 57'

8 15-94 9 ø 59'

9 01-90 10 o 01

10 02-90 9 ø 58'

11 11-90 9 ø 54'

12 08-94 9 ø 53'

13 41-93 9 ø 55'

14 04-94 9 ø 51'

15 43-93 9 ø 53'

16 18-94 9 ø 53'

17 09-90 9 ø 53'

18 16-94 9 ø 37'

Esparza and Orotina Blocks (Forearc Region, Domain 1) 84 ø 45' Punta Carballo (sp/sc) Tm-pc 16 84 ø 43' Finca Machuca, Gregg de Esparza (q) Tm-pc 14

' 84 ø 40' Rio Barranca, Marafional (rb) Tm-pc 9 84 ø 41' Esparza-Artieda road, Humo (rc) Tm-pc 10 84 ø 44' Punta Corralillo (sc) Tm-pc 7 84 ø 43' Playa Tivives (sc) Tm-pc 20 84 ø 41' Costanera Hwy, Rio Jesfis Maria (rc) Tm-pc 8 84 ø 41' Perion Bajamar (sc) TQ-t 9 84 ø 38' Cerro Tamarindo (q) Tm-pc 30 84 ø 36' Costanera Hwy, Quebrada Poz6n (rc) TQ-t 16 84 ø 3,5' Costanera Hwy, Cerro Coyote (rc) Tm-pc 11 84 ø 35' Costanera Hwy, Rio Tfircoles (rc) Qt 14

19 14-90

20 12-90

" 13-90

21 01-95

22 10-95

23 06-95

24 05-95

25 04-95

26 03-95

27 02-95

Herradura, Esterillos, Parrita, and Quepos Blocks (Forearc Region, Domain 1) 9 ø 43' 84 ø 40' Punta Leona (sp/sc) Tm-pc 15 9 ø 42'

9 ø 41'

9 ø 40'

9 ø 38'

9 ø 32'

9 ø 32'

9 ø 27'

9 ø 23'

9 ø 23'

84 ø 40' Punta Sucia (sp/sc) Tm-pc 27 84 ø 40' Playa Caletas (sp/sc) Tm-pc 84 ø 40' Playa Coyol, Puerto Escondido (sc) Tm-pc 27 84 ø 38' Hacienda Jac6, Playa Jac6 (q) Tm-pc 19 84 ø 26' Costanera Hwy, Bejuco (q) Qt 19 84 ø 16' Costanera Hwy, Vueltas (rc) Qt 6 84 ø 09' Costanera Hwy, Finca Managua (ex) Qt 7 84 ø 09' Punta Catedral, Manuel Antonio (sc) Te-ps 23 84 ø 08' Playa Escondida, Manuel Antonio (sc) Te-ps 6

28

29

30

31

32

33

34

35

36

37

38

39

40

41

09-94

50-93

51-93

78-93

80-93

66-93

81-93

83-93

82-93

15-90

59-93

09-95

60-93

58-93

10 ø 03'

10ø02 '

10 ø 04'

10 ø 02'

10 ø 01'

10 o 02'

10 ø 00'

9 ø 58'

9 ø 58'

9 ø 55'

9 ø 54'

9 ø 52'

9 ø 54'

9 ø 53'

Cordillera de Aguacate (Central Volcanic Arc Region, Domain 2) 84 ø 38' Finca Norita, San Jer6nimo (q) TQ-ga 15 84 ø 34' Pan-Am Hwy, F. Piedra Bianca (rc) TQ-ga 10 84 ø 30' Tajo Santiago, Magallanes (q) TQ-ga 12 84 ø 30' Balboa(ex) TQ-ga 17 84 ø 29' Calle Vargas, Berlin (q) TQ-ga 14 84 ø 24' Alto La Cima (rc) TQ-ga 13 84 ø 26' Rinc6n de San Isidro (ex) TQ-ga 12 84 ø 26' Atenas-S.Mateo Hwy, A. del Monte (rc) TQ-ga 5 84 ø 28' Atenas-S.Mateo Hwy, Desmonte (rc) TQ-ga 6 84 ø 28' Tajo Dantas (q) TQ-ga 7 84 ø 28' Tajo Cerro Rayos, Bols6n (q) TQ-ga 8 84 ø 28' Rio Turrubares bridge (rb) Tm-lc 15 84 ø 25' Pursical-Turrubares Hwy, Por6 (rc) TQ-ga 9 84 ø 23' Tajo Grifo Alto (q) TQ-ga 14

42

43

44

45

46

47

64-93

73-93

36-93

72-93

38-93

39-93

07-93

24-93

9 ø 56'

9 ø 56'

9 ø 58'

9 ø 59'

10 ø 01'

10 ø 02'

10 o 03'

10 ø 03'

Valle Central (Central Volcanic Arc Region, Domain 2) 84 ø 22' Planta Hidroe16ctrica La Garita (rc) TQ-ga 7 84 ø 23' Rio Tfircoles bridge, La Junta (rc) TQ-ga 18 84 ø 22' Tajo Rio Grande (q) Qv-aa 15 84 ø 21' Tajo Puente La Garita (q) TQ-ga 22 84 ø 22' Tajo La Pista (q) Qv-aa 28 84 ø 22' Tajo La Argentina (q) Qv-aa 84 ø 17' Tajo Prendas, Rio Prendas (q) Qv-aa 30 84 ø 17' Tajo Finca Chilamate, Rio Prendas (q) Qv-aa

P Axis

085,29

338,32

185,75

154,59

150,40

148,13

281,63 356,O6

167,47

232,08

227,35

213,52

038,13

192,24

330,63

020,76

156,66

114,64

360,81

180,32

142,75

294,81

173,77

274,72

157,86

061,86

280,82

336,40

197,17

033,37

005,35

194,16

194,34

351,00

195,11 008,07

349,04

330,62

001,31

337,52

220,37

035,39

186,11

339,24

010,08

005,49

020,63

T Axis

347,15

245,04

008,15

291,24

244,06

240,08

092,27

086,02

267,09

141,05

128,11

312,07

301,28

288,13

140,26

113,01

051,07

298,26

094,01

272,02

306,15

174,05

282,04

106,18

338,04

217,04

021,02

070,05 288,03

301,02 251,30

101,11

095,13

261,05

287,12

103,33

259,00

098,18

271,01

100,24

317,10

138,15

277,06

085,33

100,02

227,33

220,26


Table 2. (continued)

Field Age Total Site Number Latitude and Fm•- Faults Longitude Outcrop Location and Type*

Valle Central (Central Volcanic Arc Region, Domain 2) (continued) 48 02-94 9 ø 58' 84 ø 13' Tajo Lindora, Valle del Sol (q) Qv-aa 6 353,45 199,42 49 56-93 9 ø 58' 84 ø 09' Tajo Las Animas (E), La Caja (q) Qv-aa 54 357,44 156,44 " 57-93 9 ø 58' 84 ø 10' Tajo Las Animas (W), La Caja (q) Qv-aa " 01-94 9 ø 58' 84 ø 10' Tajo Rio Torres, S.Pedro de Pavas (q) Qv-aa 50 84-93 9 ø 58' 84 ø 08' Tajo Lagunilla, Rio Virilla (q) Qv-aa 16 206,62 338,20 51 53-93 9 ø 57' 84 ø 19' Rio Tizate bridge, Turr6cares (rc) Tm-lc 14 192,82 046,07 52 85-93 9 ø 55' 84 ø 19' Rio Jaris bridge, Piedras Negras (rc) Qv-aa 10 005,15 099,15 53 75-93 9 ø 56' 84 ø 17' Tajo Lara, Quebrada Ponciano (q) Qv-aa 15 178,25 286,33 54 28-93 9 ø 56' 84 ø 15' San Jos6-Co16n Hwy, Villa Co16n (rc) Tm-p 11 170,56 043,22 55 29-93 9 ø 54' 84 ø 15' Quebrada Honda bridge (rc/rb) Tm-p 38 182,81 325,07 " 02-93 9 ø 54' 84 ø 14' Co16n-Puriscal Hwy (q) Tm-p 56 03-93 9 ø 52' 84 ø 15' Co16n-Puriscal Hwy, Tajo Guayabo (q) Tm-lc 14 020,37 125,20 57 04-93 9 ø 52' 84 ø 14' Tajo Carlas, Tabarcia (q) Tm-pn 12 199,06 104,40 58 08-95 9 ø 56' 84 ø 13' San Jos6-Co16n Hwy, Rio Cararia (rc) Qv-aa 5 197,16 289,05 59 01-93 9 ø 56' 84 ø 12' Tajo Cerro Minas, Santa Ana (q) TQ-ga 28 226,34 332,22 " 11-93 9 ø 57' 84 ø 11' San Jos6-Co16n Hwy, Tajo Pozos (q) TQ-ga 60 13-93 9 ø 57' 84 ø 10' Los Laureles, Cerro Palomas (ex) Tm-p 20 176,82 308,06 " 14-93 9 ø 57' 84 ø 10' Los Laureles, Cerro Palomas (rc) Tm-p 61 09-93 9 ø 57' 84 ø 10' S.J.-Co16n Hwy, Cerro Palomas (rc) Tm-p 10 202,14 303,36 " 10-93 9 ø 57' 84 ø 10' S.J.-Co16n I4wy, Cerro Palomas (rc) Tm-p

P Axis T Axis

Cordillera de Talamanca (Central Volcanic Arc Region, Domain 2) 62 23-93 9 ø 52' 84 ø 03' Tajo Valverde, Valverde de Higuito (q) Tm-sm 10 014,01 284,19 63 30-93 9 ø 48' 84 ø 07' Aserri-Frailes Hwy, Tranquerillas (rc) Tm-pn 28 013,48 117,12 " 31-93 9 ø 48' 84 ø 07' Aserri-Frailes Hwy, Tranquerillas (rc) Tm-lc 64 20-93 9 ø 47' 84 ø 05' Aserri-Frailes Hwy, Rosario (rc) Tm-lc 11 166,43 275,19 65 32-93 9 ø 48' 84 ø 04' Rio Alumbre bridge, Guadarrama (rc) Tm-pn 16 182,14 273,01 66 33-93 9 ø 49' 84 ø 02' Pacayas-Copalchi road (rc) Tm-lc 16 356,31 087,02 67 17-93 9 ø 43' 84 ø 03' Tajo Angostura, San Antonio (q) TQ-ga 12 213,19 305,05 68 19-93 9 ø 44' 84 ø 02' Frailes-S.Pablo Hwy, Santa Cruz (rc) TQ-ga 8 013,58 137,19 69 15-93 9 ø 43' 83 ø 58' El Empalme-S.Maria Hwy, Jardin (rc) TQ-ga 7 121,60 280,29 70 18-93 9 ø 39' 84 ø 00' S.Maria-S.Marcos Hwy, Zapote (rc/ex) Tm-pn 13 354,28 258,11 71 16-93 9 ø 38' 83 ø 55' Pedregoso, Copey de Dota (rc) Tm-pn 8 055,40 149,05 72 21-93 9 ø 52' 84 ø 02' Tajos Guatuso, Patarrfi (q) Tm-sm 16 041,25 309,04 73 34-93 9 ø 51' 84 ø 00' Tajos Bermejo, Coris (q) Tm-sm 9 216,07 308,19 74 05-93 9 ø 49' 83 ø 57' Pan-Am Hwy, Tajo Guatuso, S.Isidro (q) Tm-lc 27 352,28 186,61

Vailes de Tejar, Orosi, and Reventaz6n (Back Arc Region, Domain 3) 75 55-93 9 ø 54' 83 ø 57' Pan-Am Hwy, Ochomogo (ex) Tm-lc 20 027,07 296,08 76 35-93 9 ø 53' 83 ø 57' Tajo Taras, Taras de Cartago (q) Tm-lc 18 016,03 285,26 77 17-94 9 ø 49' 83 ø 53' Tajo Barro Morado, Lourdes (q) Tm-sm 21 352,08 087,31 78 06-93 9 ø 50' 83 ø 53' Tajo Agua Caliente, Paraiso (q) Tm-pn 12 167,06 259,14 79 12-95 9 ø 50' 83 ø 53' Paraiso-Cachi'Hwy, Tajo Los Novios (q) Qv-r 6 290,04 047,82 80 11-95 9 ø 52' 83 ø 47' Cachi-Tuccurique road, Tajo Joyas (q) Tp-d 16 338,17 111,66 81 11-94 9 ø 50' 83 ø 42' Tajo Oriente, Rio Pejibaye (q) Tp-d 18 189,14 .288,33 82 10-94 9 ø 48' 83 ø 39' Tajo Esperanza, Rio Atirro (q) Tpe-t 29 014,07 283,08 83 14-94 9 ø 48' 83 ø 31' Tajo Las Quebradas, Bajo Pacuare (q) Tpe-t 12 213,06 115,53 84 03-94 9 ø 57' 83 ø 38' Rio Lajas, Torito de Turrialba (rb) Tp-sk 7 134,12 324,78 85 12-94 9 ø 58' 83 ø 34' Old Lim6n Hwy, Tajo Tres Equis (q) Tp-d 14 358,17 252,42 86 13-94 10 ø 01' 83 ø 37' Quebrada Linda, Bonilla (rc) Tom-u 11 219,03 321,75

* Outcrop types are as follows: q, quarry; rc, roadcut; ex, excavation; rb, river bank; sp, shore platform; sc, sea cliff. •' Sedimentary rock formations (Fm) include: Quaternary fluvial and marine terraces (Qt), the Neogene Punta

Carballo Fm (Tm-pc), Santa Teresa Fm (Tm-st), San Miguel Fm (Tm-sm), Perias Negras Fm (Tm-pn), and Pacacua Fm (Tm-p), and the Paleogene Masachapa Fm (To-ma), Punta Serrucho Fm (Te-ps), Cabo Blanco Fm (Tpe-cb), Las Palmas Fm (Tpe-lp), Suretka Fm (Tp-sk), Uscari Fm (Tom-u), and Tuis Fm (Tpe-t). Volcanic rock formations include: The Quaternary Avalancha Ardiente Fm (Qv-aa), and Reventado Fm (Qv-r), and the Neogene-Quaternary Tivives Fm (TQ-t), Grifo Alto Fm (TQ-ga) (Aguacate Group), Dofin Fm (Tp-d) (Aguacate Group), and La Cruz Fm (Tm-lc) (Aguacate Group).


FOREARC REGION (DOMAIN 1)

1

I 20 ß .i '••"• 2 2 ß 2 * 2 ß ß i'

2 26 . ß .

Figure 5a. Mesoscale fault population data for the forearc region (kinematic domain 1). Data for each fault population (analyzed using the method of Marterr and Allmendinger [1990]) are presented as best fit fault plane solutions (lower hemisphere, equal-area projections) defined by compressional (P) axes (solid circles show individual faults; letter P shows the average), and tensional (T) axes (open squares show individual faults; letter T shows the average). The data are keyed by number to Table 2 and to Figures 6a-6f.

tensive damage in central Costa Rica. According to eyewit- ness interviews, the 1924 event produced a 4-km-long NE trending ground rupture west of the town of Orotina [Giiendel et al., 1989]. The location and trend of both the 1924 ground rupture and the 1989 seismic swarm correspond with those of the Trinidad, Diablo, and Poz6n faults mapped in this study (Figure 4, Fault 5). The 1989 composite focal mechanism, the 1924 ground rupture, mapped Quaternary offsets, and mesoscale fault data are all consistent with transtension ac-

commodated by oblique slip mostly along NE striking margin-perpendicular faults within the inner forearc.

5.3. Fault Kinematics: Volcanic Arc (Domain 2, Sites 28-74)

In general, mesoscale fault populations within the central volcanic arc (domain 2) mimic regional-scale faults in displaying a strong pattern of conjugate NE and NW striking faults (Figures 6a and 6b). While NW striking regional-scale faults

show greater total lengths, crosscutting relationships suggest that faults of both orientations are of similar age. Mesoscale fault populations record predominantly shallow T and P axes, indicating mostly strike-slip motion, with left-lateral slip on NE striking faults and right-lateral slip on NW striking faults. Minor vertical components of slip show a slight extension on NE striking faults and shortening across NW striking faults.

Shallow (< 15 km) seismic activity is extremely common within domain 2, with a broad distribution of minor earthquakes (34'_<3.0) [Montero and Dewey, 1982; Ferndndez, 1995, 1996]. The focal mechanisms (Figures 6a and 6b, and Table 3) include composite mechanisms from diffuse minor earthquakes (1976-1981), as well as single-event mechanisms for minor to moderate earthquakes (M=3.0-5.0) from the 1990 Puriscal seismic swarm at the northwestern end of the Jarls

fault (Figure 4, Fault 26), and the 1993 Valle Central seismic swarm at the northwestern end of the Higuito fault (Figure 4,


CENTRAL VOLCANIC ARC REGION (DOMAIN 2)

34 .• 35 3

6 6 66 .,,• 6 . 6 ß ß . .

Figure 5b. Mesoscale fault population data for the volcanic arc region (kinematic domain 2). See Figure 5a for explanation.


BACK ARC REGION (DOMAIN 3)

Figure 5c. Mesoscale fault population data for the back arc region (kinematic domain 3). See Figure 5a for explanation.

Fault 32). In general, earthquake focal mechanisms within domain 2 are consistent with mesoscale faults in showing predominantly strike-slip motion on conjugate NE left-lateral and NW right-lateral faults. Normal and thrust mechanisms near major fault junctions may reflect the complex kinematics of intersecting conjugate faults [e.g., Ingles et al., 1999].

A notable exception to the conjugate fault pattern in domain 2 are mesoscale faults within thick packages of Quaternary pyroclastic rocks filling the Valle Central basin (Figure 6b and Table 2). These populations show mostly dip-slip motion on subvertical surfaces, with moderately inclined P and T axes. Anomalous fault kinematics within these weak surface

deposits may record vertical simple shear related to differential motion of underlying bedrock fault blocks. Transcurrent motion at depth may be transmitted into this overlying layer without clear throughgoing faults. Alternatively, these faults may have been generated by thermal contraction during cooling of these thick sequences of pyroclastic rocks.

Another exception to the conjugate pattern are mesoscale faults along the trace of the east trending Alajuela fault (Figure 4, Fault 33). These features show predominantly normal motion and may reflect extension within the crest of a thrust propagation anticline that forms the Alajuela fault scarp. This structure may represent gravitational slumping of the volcanic Cordillera [Borgia et al., 1990].

5.4. Fault Kinematics: Back Arc (Domain 3, Sites 75-86)

Mesoscale fault populations within the back arc (domain 3) show a transition from the steep conjugate transcurrent faults common in domain 2 toward an increase in shallower faults

displaying components of shortening (Figure 6c). Along the traces of major regional-scale faults, mesoscale fault populations show a mix of steep NW and NE striking conjugate strike-slip faults with shallow P and T axes. As in domain 2, dextral slip occurs principally on NW striking faults, and sinistral slip occurs principally on NE striking faults. An apparent exception to the predominantly conjugate fault pattern is the Coris-Guarco-Navarro fault system near Cartago, which displays orientations ranging from WNW to ENE (Figure 4, Faults 36 and 37). In the eastern portion of domain 3 (Figure 6c), fault populations recorded within ridges between major

fault valleys often include oblique thrust faults of varying orientations and dips that display steep T axes. These structures are more abundant approaching the NPDB toward the east and are presumably associated with NW trending folds in this region.

As in domain 2, shallow seismicity (< 15 km) is also common within domain 3. Focal mechanisms (Figure 6c and Table 3) include aftershocks of the 1991 Valle de la Estrella earthquake on the NPDB, as well as minor to moderate events (M=3.0-5.0) of the 1993 Turrialba seismic swarm along the Pejibaye fault (Figure 4, Fault 40). In general, focal mechanisms along regional-scale faults are consistent with mesoscale fault data (e.g., Figure 6c, Faults 81 and 82, and Focal Mechanisms AB and AF) in showing sinistral slip on NE striking faults and dextral slip on NW striking faults. As in domain 2, normal and reverse components of slip may reflect the kinematics of conjugate fault intersections. To the east of domain 3, focal mechanisms reported by Protti and Schwartz [1994] for aftershocks of the 1991 Valle de la Estrella earthquake show increased components of reverse slip as strike- slip faults of the CCRDB merge with thrusts of the NPDB near Puerto Lim6n.

6. Age of Faulting

The regional-scale faults examined in this study (Table 1) offset rocks ranging from Neogene to Quaternary in age. Several authors have suggested that the conjugate faults of the central volcanic arc developed during the Oligocene or Miocene under N-S compression generated by convergence between North and South America [Astorga et al., 1991; Arias and Denyer, 1991]. While many faults may have originated under a past tectonic regime, field observations confirm that these faults are presently active and that the mesoscale fault data reflect the modern kinematics.

Field geomorphic evidence of Quaternary activity (e.g., faulted soils, bold scarps, and offset fluvial and wave-cut terraces) is common along the CCRDB. Radiometric dating (4øAr/39Ar) of offset Quaternary volcanic units both along the Pacific coastal piedmont and within the Valle Central demonstrates pervasive faulting along the CCRDB during the last 400 ka [Marshall and Idleman, 1999]. The active nature

482 MARSHALL ET AL- FAULT KINEMATICS, COSTA RICA

10 ø 00'

B

9 ø 45'

9 ø 30'

Figure 6. (a-f) Geologic maps showing the distribution of mesoscale fault population data (large, shaded, numbered stereo projections; see Figures 5a-5c and Table 2 for complete data) and earthquake focal mechanisms (small, solid, lettered stereo projections; see Table 3 for data and references). Dashed white lines mark the boundaries of kinematic domains (D1, D2, and D3). See Figure 3 for map location and geologic symbols.

of individual faults throughout the zone is further supported by direct association with historic earthquakes and modern recorded seismicity (Table 1).

While field evidence and seismicity confirm that most regional-scale faults of the CCRDB are tectonically active, the age of outcrop-scale faults can be ambiguous. In most cases, mesoscale fault data were collected from Neogene rocks, intro- ducing the concern that some faults may reflect earlier deformation not representative of the active tectonic regime. We use

three observations to argue that our data do indeed reflect modem deformation kinematics: (1) lack of temporal variations in kinematics, (2) association with active regional-scale faults, and (3) consistency with earthquake focal mechanisms.

The first observation is that mesoscale fault kinematics

within individual domains are essentially the same within Neogene through Quaternary units regardless of unit age. If deformation kinematics have changed through time, fault populations would show either a broad distribution of P and T


:!San Jos6.-,

9 ø 30'

Figure 6. (continued)

axes or multiple clusters (bull's-eyes) of axes. On the other hand, if strain orientations have remained relatively constant through time, P and T axes should cluster about a single average trend and plunge. In this study, nearly all fault populations show relatively concentrated clusters of P axes (Figure ß 7). This suggests either that all faults originated in the Quaternary, that no active structures were measured, or that the deformation kinematics have not varied significantly throughout the late Cenozoic. Because the first two scenarios are highly unlikely, we propose that the kinematics of faulting in central Costa Rica have remained relatively constant throughout the Neogene and Quaternary.

The second observation is that many of the mesoscale fault populations were measured near active regional-scale faults. These fault populations are often dominated by faults with similar or conjugate orientations to the nearby, active regional-scale features (Figures 6a-6f). This direct association with known active faults supports the argument that the mesoscale data represent active kinematics.

The third observation is that mesoscale fault kinematics

across the study area are consistent with earthquake focal mechanisms (Figures 6a-6f), again suggesting that the mesoscale data reflect the modern deformation regime. While many faults may have originated sometime in the past, these


710 ø 00'


features have since inherited the active kinematics. Section 7

discusses the relationship of active seismicity along the CCRDB and the tectonics of shallow subduction.

7. Seismicity and GPS Data

7.1. Earthquake Cycles

Recurring cycles of heightened seismicity across the CCRDB provide compelling evidence for an active tectonic link between the Middle America Trench and the North

Panama Deformed Belt [Giiendel and Pacheco, 1992]. While large (M>7.0) thrust earthquakes are common along both the

MAT and the NPDB, the most damaging historical events within the heavily populated Valle Central have been moderate (M=5.0-6.5), shallow (<25 km), upper plate earthquakes along the CCRDB. Costa Rica's historical seismicity [Gonz•ilez, 1910; Peraldo and Montero, 1994] shows several periods of heightened earthquake activity across the volcanic arc following large subduction earthquakes [Giiendel and Pacheco, 1992; Montero and Alvarado, 1995]. These periods of triggered seismicity are interspersed with times of relative quiescence. Such cycles of seismic energy release along faults of the CCRDB may reflect the arcward transfer of convergent stress produced by shallow subduction.


'0 ;

-f

Qu iepo s. '":•':' ø :::::::::::::::::::::: 3 .'•:. :• •

--- ' ..... ::':" 'A :f•¾' .."..;•::'""':'.'•..':.

• ..... •:.....:. :.. ii:...:i. '!•::' ........ .:.

84 ø 1'5 •


Figure 7. Examples of P axis contour plots for mesoscale fault populations. Nearly all populations in the study area show relatively concentrated clusters of P axes centered about an average trend and plunge. This suggests that strain kinematics have remained relatively constant since faulting began. Numbers are keyed to Table 2 and Figures 5a-5c.


Table 3. Earthquake Focal Mechanism Data

Focal

Mechanism Date

A•'•- Jan. 21-23, 1989 B Sept. 26, 1994 C Feb. 9, 1993 D June 30, 1990

E June 30, 1990

F June 30, 1990 G June 30, 1990

H June 16, 1990

I Dec. 22, 1990

J June 9, 1990

K June 8, 1990

L May 29, 1990 M•'•' April 1980-Nov. 1981 N•'•' April 1980-Nov. 1981 O Date unknown, 1995 P Feb. 26, 1989 Q Jan. 12, 1993 R Jan. 10, 1993

S Jan. 23, 1993

T Aug. 17, 1982 U Aug. 3, 1993 V Jan. 8, 1993

W Jan. 10, 1993 X Feb. 13, 1993 Y Jan. 20, 1993

Z Nov. 3, 1992

AA Nov. 3, 1992 BB Jan. 30, 1993

CC•-•- July 1976-June 1979 DD Dec. 2, 1992

EE Nov. 21, 1992

FF Sept. 2, 1993 GG•'•' July 1976-June 1979 HH$•' April 1980-Nov. 1981 II Jan. 11, 1994

JJ•-•- April 1980-Nov. 1981 KK•-•- April 1980-Nov. 1981 LL Sept. 29, 1994 MM Aug. 9, 1991 NN•-•- July 1976-June 1979 OO Nov. 12, 1992 PP June 13, 1992

QQ April 24, 1994 RR March 19, 1993

SS•'• July 1976-June 1979 TT June 1982-Sept. 1982 UU Nov. 2, 1992 VV Nov. 18, 1992

WW July 14, 1993 XX May 7, 1993 YY Sept. 23, 1993 ZZ July 10, 1993 AB July 8, 1993 AC July 10, 1993 AD July 18, 1993 AE July 11, 1993 AF Jan. 31, 1988

AG Nov. 19, 1987

Depth, MagnitudeS- Latitude Longitude. km P axis

M =3.6 9 ø 56' 84 ø 35' 10.9 246, 58 M =4.3 9 ø 47' 84 ø 31' 28.1 089, 44 M =2.8 9 ø 49' 84 ø 22' 7.3 208, 13 M =4.5 9 ø 49' 84 ø 21' <15.0 011, 02 M =5.0 9 ø 51' 84 ø 23' 9.4 188, 04 M =4.5 9 ø 53' 84 ø 23' <15.0 247, 14 M =4.5 9 ø 54' 84 ø 21' <15.0 209, 01 M =4.5 9 ø 52' 84 ø 20' 14.1 184, 09 M =5.7 9 ø 53' 84 ø 20' 14.6 020, 00 M =4.5 9 ø 53' 84 ø 19' 7.2 008, 29 M =4.8 9 ø 52' 84 ø 20' 8.8 208, 09 M =4.7 9 ø 50' 84 ø 18' 16.1 195, 02 M <3.0 9 ø 53' 84 ø 17' <15.0 214, 51 M <3.0 9 ø 54' 84 ø 16' <15.0 194, 10 M =4.7 9 ø 43' 84 ø 14' <15.0 194, 17 Ms =4.8 9 ø 40' 84 ø 11' 38.0 115, 11 M =3.3 9 ø 37' 84 ø 07' 22.3 134, 09 M =3.6 9 ø 36' 84 ø 06' 26.9 339, 14 M =3.1 9 ø 35' 84 ø 07' 28.0 265, 75 Ms =5.5 9 ø 34' 84 ø 04' 37.0 194, 23 M =4.0 9 ø 33' 84 ø 10' 30.0 287, 28 M =3.3 10 ø 00' 84 ø 13' 5.5 000, 90

M =3.5 9 ø 59' 84 ø 12' 6.8 345, 83 M =4.1 9 ø 59' 84 ø 10' 14.1 040, 70 M =3.7 9 ø 59' 84 ø 11' 11.6 355, 30 M =4.1 9 ø 55' 84 ø 08' 6.5 360, 76 M =3.3 9 ø 56' 84 ø 08' 11.1 181, 30 M =3.4 9 ø 58' 84 ø 06' 16.2 037, 55 M <4.0 9 ø 59' 84 ø 02' <15.0 351, 30 M =4.8 9 ø 59' 84 ø 00' 17.8 166, 50 M =3.5 10 ø 02' 84 ø Off 13.2 230, 65 M =3.2 10 ø 03' 83 ø 59' 14.0 348, 62 M <4.0 9 ø 48' 84 ø 15' <15.0 011, 00

M <3.0 9 ø 50' 84 ø 08' < 15.0 200, 00 M =3.5 9 ø 49' 84 ø 09' 16.8 209, 20 M <3.0 9 ø 48' 84 ø 08' <15.0 190, 00 M <3.0 9 ø 52' 84 ø 05' <15.0 008, 02 M =3.3 9 ø 52' 84 ø 04' 6.6 030, 01 Ms =4.7 9 ø 44' 84 ø 03' 15.0 203, 00

M <4.0 9 ø 46' 83 ø 59' <15.0 216, 04 M =3.1 9 ø 45' 84 ø 01' 14.9 209, 20 M =3.9 9 ø 41' 84 ø 00' 1.4 229, 74 M =2.9 9 ø 43' 83 ø 57' 21.3 344, 58 M =3.2 9 ø 50' 83 ø 58' 15.9 294, 69

M <4.0 9 ø 52' 83 ø 58' <15.0 063, 46

M <3.2 9 ø 57' 83 ø 51' <15.0 150, 42

M =3.4 9 ø 53' 83 ø 46' 6.2 007, 52 M =2.7 9 ø 44' 83 ø 50' 7.6 087, 40 M =3.9 9 ø 43' 83 ø 49' 6.7 215, 80 M =3.7 9 ø 42' 83 ø 46' 3.8 200, 30 M =3.6 9 ø 42' 83 ø 42' 11.7 037, 52 M =3.0 9 ø 46' 83 ø 42' 14.8 033, 79 M =4.4 9 ø 48' 83 ø 42' 8<17 007, 04 M =5.3 9 ø 46' 83 ø 41' 13.2 075, 35

M =2.9 9 ø 45' 83 ø 39' 12.8 217, 33

M =3.4 9 ø 46' 83 ø 39' 15.4 359, 72 M =4.5 9 ø 46' 83 ø 38' 8<17 004, 22 M =4.3 9 ø 42 '• -- 83 ø 35' 8<17 014, 18

T axis

151, 03

352, 07

318,57

280, 28

280, 30

151,21

299, 36

293, 63

110, 40

272, 10

112, 34

105,25

072, 32 286, 10

285, 03

206, 03 243, 63

082,42

085, 15

073, 50

193, 10 310,0

120, 05

220, 20

105, 30 133, 10

338,59

207, 35

101, 30

336, 40

090, 19

062, 118

101, 00

110, 14

360, 68

100, 14

098, 12 120, 10

293, 00

125, 10

360, 68 121, 05

106, 18

098, 20

192, 31

300, 44

127,22 354, 03

305, 00

100, 17

156, 22

149, 05 098, 24

179,20

076, 50

237, 10

094, 01

105, 04

Reference*

5

2

2

6

6

6

6

2

2

6

2

2

8

8

2

1

2

2

2

1

2

2

2

2

2

2

2

2

7

2

2

2

7

8

2

8

8

2

1

7

2

2

2

2

7

3

2

2

2

2

2

2

9

11

2

2

9

9


Table 3. (continued)

Focal Depth, Mechanism Date MagnitudeS' Latitude Longitude km P axis T axis Reference*

AH May 14, 1991 M =4.2 9 ø 52' 83 ø 32' 21.0 035, 13 128, 09 1 AI April 24, 1991 Ms =6.1 9 ø 45' 83 ø 31' 23.0 009, 13 100, 04 1 AJ May 14, 1991 M =3.1 9 ø 38' 83 ø 34' 12.7 193, 20 103, 01 10 AK May 17, 1991 M =3.0 9 ø 36' 83 ø 33' 4.2 191, 09 097, 23 10 AL March 7, 1983 Ms =5.7 9 ø 35' 83 ø 40' 12.0 192, 21 283, 06 4 AM Dec. 21, 1993 M =2.8 9 ø 30' 83 ø 38' 7.4 071, 46 177, 15 2

* References are as follows: 1, Fan et al. [1993]; 2, Fernc•ndez [1995]; 3, G•endel [1985]; 4, G•endel [1986]; 5, G•endel et al. [1989]; 6, G•endel et al. [1990]; 7, Montero andDewey [1982]; 8, Montero and Morales [1984]; 9, OVSICORI-UNA [1993]; 10, Protti andSchwartz [1994]; 11, Ramirez et al. [1993].

•' M, local magnitude; Ms, surface-wave magnitude '•'• Composite focal mechanism.

After the 1950 M=7.7 Nicoya subduction earthquake on the MAT, a 5-year series of moderate events occurred across central Costa Rica [Montero and Alvarado, 1995]. Similarly, the 1983 Mw=7.5 (Ms=7.3) Golfo Dulce subduction earthquake triggered a cycle of moderate events within the Cordillera de Talamanca [Adarnek et al., 1987; Tajirna and Kikuchi, 1995]. Most recently, a period of increased seismicity began in central Costa Rica with the 1990 Mw=7.0 (Ms=6.9) Cobano subduction earthquake on the Pacific coast and sub- sided in the wake of the 1991 Mw=7.7 (Ms=7.5) Valle de la Estrella back arc thrust event on the Caribbean coast (Figure 8) [Gaendel and Pacheco, 1992]. The discussion in section 7.2 of the 1990-1993 seismic sequence provides important in- sights into the seismotectonics of the CCRDB and its link to shallow subduction along the Pacific margin and back arc convergence within the NPDB.

7.2. The 1990-1993 Seismic Sequence

In 1990, five moderate subduction zone earthquakes occurred along the length of the MAT from Nicaragua to Panama. The first and largest of these events, the Mw=7.0 (Ms=6.9) Cobano earthquake, was centered offshore of central Costa Rica, SE of the Peninsula de Nicoya, directly above the projected trend of the subducting rough-smooth boundary (Figure 8) [Protti et al., 1995b]. This shallow subduction earthquake immediately triggered a seismic swarm 60 km inland along the CCRDB within the Cordillera de Aguacate [Gaendel et al., 1990; Barquero et al., 1991; Gaendel et al., 1995]. This swarm continued for several months along a conjugate system of NE striking left-lateral and NW striking right-lateral faults (Figures 6a and 6b). The seismic sequence culminated with the 1990 Ms=5.7 Puriscal earthquake (Figure 8), which produced considerable damage in the Valle Central.

Four months later, the 1991 Mw=7.7 (Ms=7.4) Valle de la Estrella thrust earthquake ruptured the westernmost segment of the North Panama Deformed Belt (Figure 8) [Fan et al., 1993; Goes et al., 1993; Protti and Schwartz, 1994; Suc•rez et al., 1995]. Both local and far-field seismicity outlined a shallow SW dipping fault, consistent with NE thrusting of the Panama block over the Caribbean plate. Coseismic uplift suggested abrupt termination of this rupture at its NW edge at Puerto Lim6n [De Obaldia et al., 1991' Plafker and Ward, 1992; Denyer et al., 1994]. Aftershocks within the main rup-

ture zone showed mostly thrust mechanisms, however, a cluster of shallow events centered arcward of Puerto Lim6n

showed mostly oblique strike-slip along steep NE striking faults. Mapped surface ruptures in this area showed significant strike-slip offsets [Denyer et al., 1994]. These observations imply an onland extension of the NPDB and a transition from reverse motion to transcurrent faulting along the CCRDB.

Similar to the 1990 Cobano earthquake on the Pacific coast, the 1991 Valle de la Estrella earthquake triggered a seismic sequence farther inland along a diffuse array of faults through the Cordillera de Talamanca and the Valle Central. These

events lasted for several years and showed oblique strike-slip mechanisms on mostly NE and NW oriented faults [Protti and Schwartz, 1994; Gaendel et al., 1995]. In 1993 a seismic swarm, culminating with an M=5.3 event (Figure 8), occurred within the Turrialba region centered around the conjugate Pejibaye and Gato faults (Figure 4, Faults 40 and 41). Similar to previous events in this area, these earthquakes showed mostly transcurrent motion on NE and NW oriented faults.

Overall, the 1990-1993 seismic sequence ruptured a diffuse array of transcurrent faults spanning the volcanic arc between the epicenters of the Cobano and Valle de la Estrella earthquakes. This sequence of triggered events demonstrates an active tectonic link between the MAT and the NPDB along faults of the Central Costa Rica Deformed Belt. Repeated seismic cycles along the CCRDB may reflect the transfer of convergent stress produced by shallow subduction from the Pacific to the Caribbean margin.

7.3. GPS Measurements

Recent motion of the Panama block relative to adjacent plates has been constrained by the Central and South America (CASA) Global Positioning System (GPS) campaigns initi- ated in 1988 (Figure 1) [Kellogg and Vega, 1995]. Measurements between Panama (Panama City) and two sites in Colombia (Cartegena and Bogotfi) suggest ongoing collision between the Panama and North Andes blocks at rates ranging from 8 to 21 mm/yr [Kellogg and Vega, 1995]. GPS-based nu- merical modeling for regional deformation indicates that the Panama block moves northward over the Caribbean plate at increasing rates (10-20 mm/yr) toward the west along the NPDB from the margin of South America [Lundgren and Russo, 1995]. These observations are consistent with clock-


N

o 5o lOO

CARlB M 5.7 12/90

CCR[

GPS data 19•94•96

20 mrn/yr (95% ellipse)

M 7.6

4/91

mm/y r

9 ø

•M 3:/90

o

.. 7/9 3'

8-4..o

A A' B B' C C'

Figure 8. Tectonic map of the CCRDB showing a summary of mesoscale fault data from the three kinematic domains (D1, D2, and D3), focal mechanisms from the 1990-1993 earthquake sequence, GPS data for 1994-1996, Cocos plate bathymetry including the rough-smooth boundary (RSB), and the subducting slab along three margin-perpendicular cross sections. The shaded area outlines the CCRDB along the western margin of the Panama block (dashed solid line). See Figure 2 for on-land geology. The summary fault plane solutions (D1, D2, and D3) combine all mesoscale fault data for each kinematic domain (see Figures 5a-5c for complete data). Earthquake focal mechanisms (left to right) from: Protti et al. [1995b], Ferndndez [1995, 1996], Ramirez et al. [1993], and Goes et al. [1993]. The thin dashed lines show locations of the Wadati-Benioff zone cross sections depicted in boxes below the map [from Protti et al., 1995a]. The small arrows show GPS displacement vectors (scale at top right) with respect to a fixed Caribbean plate [Lundgren et al., 1999].

wise rotation of Panama, thrusting along the NPDB, and sinistral shear across central Costa Rica [Lundgren and Russo, 1995].

Results from the Costa Rica (CORI) GPS project (1994 and 1996) provide local constraints on active deformation across the CCRDB [Lundgren et al., 1999]. These results (Figure 8) indicate up to 30 mm/yr of sinistral shear between sites in northern Costa Rica on the Caribbean plate and sites within the actively deforming Panama block to the south. The GPS measurements show increasing sinistral shear moving

southward across the faults of the CCRDB. These observations

may reflect NE displacement of the western Panama block toward the back arc NPDB. Such short-term data should be

viewed with caution, however, considering their temporal proximity to the postseismic stage of the 1990-1993 earthquake cycle along the CCRDB. These velocity vectors may reflect postseismic deformation related to the 1991 Valle de la Estrella earthquake and interseismic strain along the southern MAT superimposed on secular motions of the Panama block [Lundgren et al., 1999].


8. Tectonic Interpretation The location of the Central Costa Rica Deformed Belt within

the overriding volcanic arc corresponds with the position of the subducting rough-smooth boundary on the Cocos plate offshore (Figure 8). This relationship implies a genetic link between shallow subduction of thickened oceanic crust and ac-

tive deformation across the volcanic arc. As suggested by previous authors, transcurrent faulting across central Costa Rica may reflect sinistral shear along the NW flank of the indenting Cocos Ridge [Montero, 1994; Kolarsky et al., 1995]. In the model presented by Kolarsky et al. [1995] the CCRDB would represent a cross-arc fault zone similar to those observed in other ridge subduction settings [e.g., Taylor et al., 1995]. Transpression where the CCRDB merges with the back arc NPDB would reflect horizontal shortening directly above the ridge axis. This model also implies that a similar dextral shear zone must exist SW of the subducting ridge in Panama, a ques- tion that remains open for further investigation.

In addition to the ridge indentation mechanism, we suggest that shallow subduction of thickened oceanic crust throughout the rough domain (not just limited to the Cocos Ridge) may increase basal traction on the overriding plate, resulting in distributed horizontal shortening and NE displacement of the western Panama block toward the back arc NPDB (Figure 8). Regardless of the precise mechanism, we suggest that active faulting within the CCRDB should be viewed in general terms as a deformation front that has propagated into the volcanic arc along the NW limit of shallow subduction (Figure 8).

The rough domain of the subducting plate originated through hotspot activity along the E-W oriented Galapagos Rift [Werner et al., 1999] (Figure 1). Shallow subduction at the MAT may be controlled primarily by increased buoyancy associated with hotspot thickening of the oceanic crust. This buoyant effect is maximized nearest the Cocos Ridge [Gardner et al., 1992] and may be amplified by decreased plate age toward the SE [Prottiet al., 1995a]. Because the subducting plate becomes shallower approaching the NW flank of the Cocos Ridge (Figure 8), the zone of crustal shortening extends progressively farther inland along an E-W zone across central Costa Rica. Thus, as the seamount domain and Cocos Ridge subduer northeastward at the trench, an E-W trending deformation front, forward of their leading edge, propagates northward into the overriding volcanic arc.

Conjugate strike-slip faults (NW and NE) of the CCRDB (domains 2 and 3) allow for north directed horizontal shortening along the deformation front above the NW flank of the indenting ridge (Figure 8). This zone also accommodates diffuse sinistral shear as fault-bounded blocks are displaced northeastward toward the NPDB. Transpression observed within the back arc (domain 3) reflects merging of the CCRDB with the thrust faults of the NPDB above the axis of the subducting Cocos Ridge. Within the forearc (domain 1) the effect of shallow subduction is overprinted by local deformation related to isolated seamounts. Steep margin-perpendicular normal faults may reflect vertical kinematics within forearc blocks overriding subducting seamounts. To the SE the indenting Cocos Ridge drives uplift and horizo•ltal shortening within the Terraba belt (Fila Costefia) and may accentuate basal traction on the overriding plate through increased coupling along the root of the Cordillera de Talamanca.

Overall, therefore, the kinematics of active faulting along the CCRDB may be understood as the combined result of horizontal shortening and shear due to ridge indentation [e.g., Taylor et al, 1995], crustal displacement from possible increased basal traction due to shallow subduction [e.g., Bird, 1998], and localized forearc uplift controlled by seamount subduction [e.g., Fisher et al., 1998]. While many faults within the CCRDB may have originated prior to indentation of the Cocos Ridge [e.g., Astorga et al., 1991, Arias and Denyer, 1991], they have since inherited the kinematics of distributed shortening and sinistral shear associated with shallow subduction of thickened oceanic crust.

Some have argued that the ridge indentation model is in- compatible with the idea of a microplate boundary shear zone across central Costa Rica [Montero, 1994; Fernc•ndez, 1996]. We suggest that these two ideas are not mutually exclusive if the CCRDB is viewed in general terms as the western limit of deformation within the Panama block. Even if ridge indentation plays a primary role in deformation, the CCRDB still defines a tectonic boundary between the actively deforming Panama block and the relatively stable Caribbean plate in northern Costa Rica.

Interestingly, shallow subduction of the Cocos Ridge at the MAT may, in an indirect way, contribute to the onland propagation of the NPDB in eastern Costa Rica. Indentation of the Cocos Ridge beneath southern Costa Rica drives rapid uplift of the Talamanca arc. Accelerated topographic erosion of the range has stripped off overlying extrusive rocks, exposing the intrusvie core (Figure 2). This erosion has generated a major pulse of sedimentation within the Costa Rica fan offshore of the Caribbean coast. Silver et al. [1995] argue that sediment loading of the offshore slope may have forced the thrust front of the NPDB onshore at Puerto Lim6n (Figures 2 and 3). In this manner, we suggest that subduction of the Cocos Ridge along the Pacific margin may indirectly influence the geometry of the NPDB along the Caribbean margin and hence its onshore extension along the CCRDB.

While the western Panama block converges northeastward with the NPDB in southern Costa Rica, this motion gradually rotates to northward displacement along the arcuate NPDB farther to the east in Panama. Within the East Panama

Deformed Belt the sense of conjugate faulting is reversed, with NE striking right-lateral and NW striking left-lateral faults allowing for oroclinal bending and northward thrusting of the arc into the back arc basin across the arcuate NPDB [Mann and Kolarsky, 1995]. This deformation reflects collision of the arc with South America to the east.

9. Conclusions

1. As defined here, the Central Costa Rica Deformed Belt (CCRDB) is a diffuse zone of active faulting that marks the western margin of the Panama block (Figure 8). This 70 to 100-km wide zone extends across the Costa Rican volcanic

arc, linking the North Panama Deformed Belt (NPDB) on the Caribbean coast with the Middle America Trench (MAT) on the Pacific coast.

2. The intersection of the CCRDB with the Pacific forearc

corresponds with the location of the rough-smooth boundary (RSB) on the subducting Cocos plate offshore. Shallow sub-

490 ,MARSHALL ET AL.' FAULT KINEMATICS, COSTA RICA

duction of thickened oceanic lithosphere (Cocos Ridge and seamount domain) SE of the rough-smooth boundary extends crustal shortening into the overriding volcanic arc and dis- places the western Panama block toward the back arc NPDB.

3. Active faulting along the CCRDB represents the arcward propagation of a deformation front along the NW limit of shallow subduction. Horizontal shortening at the deformation front and differential shear between northern and southern

Costa Rica are accommodated by transcurrent faulting along the CCRDB.

4. Fault kinematics along the CCRDB vary across three domains: (1) the forearc, (2) the central volcanic arc, and (3) the back arc. Where the CCRDB intersects the forearc (domain 1) between Puntarenas and Quepos, mesoscale fault populations express sinistral transtension across steep NE striking faults that accommodate differential uplift of forearc blocks. To the southeast of Quepos, shallow subduction of the Cocos Ridge produces flexural uplift and horizontal shortening within the Terraba thrust belt. Inland, within the central volcanic arc (domain 2), the CCRDB encompasses a conjugate system of NW and NE striking transcurrent faults. Mesoscale fault kinematics demonstrate primarily dextral slip with minor shortening on NW striking faults and sinistral slip with mi- nor extension on NE striking faults. In the back arc (domain 3), mesoscale faults show increased transpression and crustal thickening where conjugate regional-scale faults merge with thrust faults of the NPDB.

5. The observed kinematic variations along the CCRDB reflect the combination of three principal deformation mechanisms: (1) horizontal shortening and shear from oceanic ridge indentation, (2) increased basal traction from shallow subduction, and (3) localized forearc block uplift from subducting seamount roughness.

6. Active regional-scale faults along both the Valle Central and the central Pacific coast displace rocks dated (4øAr/39Ar)

as young as 400 ka [Marshall and Ildeman, 1999]. In addition, observed offsets of late Quaternary fluvial terraces, wave-cut platforms, and soils attest to the active nature of faulting.

7. Repeated earthquake cycles along faults of the CCRDB demonstrate an active tectonic link between shallow subduc-

tion at the MAT and back arc thrusting along the NPDB. Focal mechanisms agree with fault population data, suggesting that the observed mesoscale faults characterize the modern kinematics.

8. Global Positioning System (GPS) data are consistent with sinistral shear across the CCRDB and northeastward

convergence of southern Costa Rica with the NPDB. 9. While many faults of the CCRDB may have originated

prior to Cocos Ridge indentation, they have subsequently inherited the kinematics of deformation associated with shallow

subduction of thickened oceanic crust.

10. The ridge indentation and microplate boundary models for central Costa Rica are compatible if the CCRDB is viewed in general terms as a deformation front at the western edge of the Panama block.

Acknowledgments. We are very grateful to F. Gtiendel, M. Protti, and E. Malavassi (Observatorio Volcano16gico y Sismo16gico de Costa Rica, Universidad Nacional) and P. Denyer, W. Montero, and M. Fernfindez (Escuela Centroamericana de Geologia, Universidad de Costa Rica) for insightful discussions and for earthquake focal mechanisms. P. Lundgren (Jet Propulsion Laboratory) made significant contri- butions to this study by providing a tour of the Costa Rica (CORI) GPS network and by sharing GPS data. We also thank B. Idleman (Lehigh University) for critical radiometric analyses and R. Allmendinger (Cornell University) for the kinematics software used to analyze the fault data [/tllmendinger et al., 1994]. We also appreciate the valuable field assistance of R. Seelbach (University of California, Santa Cruz) and the logistical support of F. Rudin, L. Valverde, and L. Chavez (Instituto Geogrfifico Nacional de Costa Rica). Finally, we thank P. Mann, R. yon Huene, and Editor D. Scholl for helpful comments on this manuscript. This research was funded by NSF grant EAR-9214832.

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(Received June 11, 1999; revised December 9, 1999; accepted January 24, 2000.)

central costa rica deformed belt: kinematics of diffuse...

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