Central Costa Rica Deformed Belt: Kinematics of Diffuse Faulting Across the Western Panama Block

Home , Catedral (district), Cordillera de Guanacaste, Grifo Alto, Jaris, Middle America Trench, Neotectonics

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

Central Costa Rica deformed belt: Kinematics of diffuse faulting across the western Panama block

JeffreyS. Marshall1 andDonald M. Fisher Departmentof Geosciences,Pennsylvania State University, University Park

Thomas W. Gardner

Departmentof Geosciences,Trinity University,San Antonio,Texas

Abstract. Fault kinematics, seismicity, and geodetic data across a diverse array of actively evolving fault zones that acrosscentral Costa Rica reveal a diffuse fault zone, here named spanseveral indelSendent crustal blocks or microplates(e.g., the Central Costa Rica Deformed Belt (CCRDB). The CCRDB Panama, North Andes, and Maracaibo) [Mann and Burke, defines the western margin of the Panamablock and links the 1984, Mann et al, 1990]. These crustalfragments function as North Panama Deformed Belt (NPDB) along the Caribbean broad deformationzones that accommodatethe complicated coast with the Middle America Trench (MAT) along the kinematicsof distributedplate boundarydeformation. Pacific coast.The junction of the CCRDB and the MAT coin- This study examinesthe kinematicsand the tectonicorigin cides with an abrupt transition from smoothto rough crust on of faulting along the westernmargin of the Panamablock in the subducting Cocos plate (rough-smooth boundary). central Costa Rica. The Panama block consists of the southern Shallow subductionof rough, thickenedoceanic crust associ- end of the Central American volcanic arc that has detached ated with the Cocos Ridge shifts active shorteninginto the from the Caribbean plate owing to collision with South volcanic arc along faults of the CCRDB. Variable fault kine- America (Figure 1). Along Panama'seastern margin, conver- maticsalong this zone may reflect three combineddeformation gence with South America is accommodatedalong diffuse mechanisms:horizontal shortening and shear from oceanic transpressionalfaults of the East Panama Deformed Belt ridge indentation,basal traction from shallow subduction,and [Mann and Kolarsky, 1995] and by uplift of the Colombian localized block uplift from subductingseamount roughness. Cordillera within the North Andes block [Kellogg and Vega, Within the forearc (domain 1), mesoscale faults express 1995]. This collision also drives oroclinal bending and transtensionwhere steepNE striking regional-scalefaults in- northwardthrusting of Panamaover the Caribbeanplate along tersect the Pacific coast. Across the volcanic arc (domain 2), the North Panama Deformed Belt [Wadge and Burke, 1983; mesoscalefaults exhibit mostly sinistral and dextral slip on Vergara-Muhoz, 1988; Silver et al., 1990, 1995]. To the NE and NW striking conjugate faults, respectively. south,oblique convergencebetween the northernNazca plate Approachingthe NPDB in the back arc (domain 3), transcur- and the Panama block occurs along the South Panama rent faulting is modified by transpressionand crustalthicken- Deformed Belt [Mackay and Moore, 1990; de Boer et al., ing. Fault kinematics are consistent with earthquake focal 1991; Westbrook et al., 1995; Moore and Sender, 1995; mechanismsand Global Positioning System (GPS) measure- Kolarskyand Mann, 1995]. While theseconvergent zones de- ments. Radiometric age constraintsconfirm that faulting post- lineate the eastern, northern, and southern boundariesof the datesthe late Neogene onsetof shallow subduction.The ensu- independentPanama block, the kinematicsof deformation ing deformationfront has propagatednorthward into the vol- alongthe westernmargin have remained poorly constrained. canic arc to its present position along the seismically active Here we define the Central Costa Rica Deformed Belt CCRDB. Within the forearc, the effect of shallow subductionis (CCRDB) as a diffusezone of seismicallyactive faulting across overprinted by local uplift related to underthrusting centralCosta Rica that marksthe westernmargin of the Panama seamounts. block (Figures2 and 3). In this paper, we investigatethe nature of active faulting along the CCRDB by comparing mesoscale fault kinematics with patterns of regional-scale 1. Introduction faults, historic seismicity, and Global Positioning System The Panama-northernColombia region of Central and South (GPS)-measuredcrustal displacements. We examinefault pop- America spansa complex deformationzone betweenfour ac- ulation data from 86 outcropsin conjunctionwith earthquake tively convergingtectonic plates: Caribbean, South American, focal mechansimsin order to evaluatethe spatial variationsin Cocos, and Nazca (Figure 1). Plate motions are partitioned fault kinematics across central Costa Rica. In addition, we es- tablishage constraintsfor offsetQuaternary units and confirm that faulting along the CCRDB is active. Finally, we assess 1Nowat Department of Geosciences, Franklin and Marshall College, the kinematicsand timing of faulting within the contextof re- Lancaster,Pennsylvania. gionaltectonics in orderto explorethe potentialcauses of this deformation. Copyright2000 by the American Geophysical Union. The observations made in this study indicate that the Papernumber 1999TC001136. CCRDBrepresent. s a deformationfront that haspropagated 0278-7407/00/1999TC001136512.00 into the volcanicarc in responseto the shallowsubduction of

468 MARSHALLET AL' FAULT KINEMATICS,COSTA RICA 469

thickened oceanic lithosphere associated with the Cocos ther did not discussa western boundary[Adamek et al., 1988] Ridge and seamountdomain on the Cocos plate (Figure 2). or loosely associated it with such features as the Panama Faults of the CCRDB accommodatediffuse crustal shortening Fracture Zone [Bowin, 1976; Vergara-Muhoz, 1988] or NW and sinistral shear acrossthe volcanic arc, allowing for north- trending faults traversing Panama [Mackay and Moore, 1990; eastward displacement of the western Panama block toward Mann and Corrigan, 1990; de Boer et al., 1991]. However, the back arc North Panama Deformed Belt. recent investigationsof regional stratigraphy [Astorga et al., 1991; $eyfried et al., 1991], tectonic geomorphology [Gardner et al., 1993; Marshall et al., 1995], fault kinematics 2. Tectonic Framework [Marshall et al., 1993; Fisher et al., 1994], seismicity [Jacob et al., 1991; Giiendel and Pacheco, 1992; Goes et al., 1993; The Central American volcanic arc is generatedby sub- Fan et al., 1993], and geodetics[Lundgren et al., 1993, 1999] ductionof the Cocosplate at the MiddleAmerica Trench along have recognizeda diffuse region of active faulting acrosscen- the southwesternmargin of the Caribbeanplate. The Panama tral Costa Rica. This broad deformation zone extends onland block consistsof the southernportion of the arc, extending from the NPDB along the Caribbean coast (Figures 2 and 3), from the margin of SouthAmerica in the east,to centralCosta traversesthe volcanic arc and heavily populatedValle Central, Rica in the west(Figure 1). This independentblock spans the and intersects the Pacific coast between Puntarenas and CretaceousChorotega and Choco oceanic basementterranes Quepos to meet the MAT south of the Peninsulade Nicoya and encompassesseveral Cenozoic volcanic cordilleras and [Fisher and Gardner, 1991; Marshall et al., 1993; Fisher et uplifted sedimentarybasins that exposePaleogene deep ma- al., 1994]. rine and Neogene-Quaternaryshallow marine, volcaniclastic, The location occupied by the CCRDB has long been and fluvial sediments[Escalante, 1990]. Late Neogenecolli- recognized as a major segment boundary along the Middle sion with SouthAmerica upliftedthe Panamaarc and createda America arc-trench system [Stoiber and Carr, 1973; Carr, land bridgethat closedthe Caribbean-Pacificseaway [Kellogg 1976; Burbach et al., 1984]. This position within the and Vega, 1995]. overriding plate correspondswith the location of the "rough- Until recently, little attention had been focused on the smoothboundary" [Hey, 1977] on the subductingCocos plate westernboundary of the Panama block. Previousresearch ei- offshore (Figure 1). The rough-smooth boundary follows a

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Figure 1. Tectonic setting of southernCentral America showingthe Central Costa Rica Deformed Belt (CCRDB) along the western margin of the Panama block (PAN). The CCRDB links the North Panama DeformedBelt (NPDB) with the Middle America Trench,and is locatedonshore of the rough-smoothboundary on the subductingCocos plate (COCOS). Large arrowsshow plate motions relative to the Caribbeanplate (CARIB) [DeMets et al., 1990]. Small arrows show velocitiesfor Global PositioningSystem (GPS) sites (solid circles) relative to Panama(solid square)[Kellogg and Vega, 1995]. The CocosRidge is outlinedby the 1000-m depth contour. The rectangleshows area of Figure 2. NAZCA, Nazca plate; SOAM, South American plate, MAR; Maracaiboblock; NAN, North Andes block; EPDB, East PanamaDeformed Belt; SPDB, South PanamaDeformed Belt. Map is compiledfrom Lonsdale and Klitgord [1978], Mackay and Moore [1990], Silver et al. [1990], Kellogg and Vega [1995], Protti et al. [1995a], and Westbrooket al. [19951. 470 MARSHALL ET AL.: FAULT KINEMATICS, COSTA RICA

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Figure2. Tectonicmap of CostaRica showing the on-landgeologic structure relative to the offshore bathymetry(contours in meters).The CCRDB (outlined by dashedlines) marks the diffuse boundary between thePanama block and the Caribbean plate. The NW limitof theCCRDB aligns with the rough-smooth boundary(RSB) on thesubducting Cocos plate. The RSB separates smooth crust to theNW fromrough crust (seamountdomain and Cocos Ridge) to theSE. The subducting Cocos Ridge aligns with a gapin theactive volcanicarc (asterisks), with the uplifted intrusive rocks of theCordillera de Talamanca (C Tim)and with the invertedsedimentary basins of theTerraba thrust belt (TB) andthe Lim6n thrust belt (LB). Therectangle showsthe area of Figure3. MAT,Middle America Trench; C Gnc,Cordillera de Guanacaste; C Agt, Cordillera deAguacate; C Cen, Cordillera Central; Nicoya, Peninsula de Nicoya; Osa, Peninsula de Osa.

linear morphologictrend on the subductingseafloor that sepa- de Osa and Terraba belt, throughthe Talamancaarc, and into rates smooth,low-relief crust to the NW from rough, high-re- the back arc Lim6n belt (Figure 2) [Corrigan et aL, 1990; lief crustto the SE (Figure 2) [yon Huene et al., 1995]. Rough, Gardner et al., 1992; Bullard, 1995; Collins et al., 1995; thickened seafloor SE of the boundary containsthe aseismic Kolarskyet al., 1995]. Cocos Ridge and adjacent seamounts,products of relatively To the NW of the Cocos Ridge, along the central Costa slow spreading and pervasive hotspot volcanism along the Rican margin, the pronouncedimpact of ridge indentationdi- GalapagosRift system[Holden and Dietz, 1972; yon Huene et minishes [Gardner et al., 1992]. Subducting seamounts, al., 1995; Barckhausen et al., 1998; Meschede et al., 1998; which ornament the moderately thickened crust on the NW Stavenhagenet al., 1998; Werner et al., 1999]. ridge flank (Figure 2), deform the outer trench slope [yon The CocosRidge, a primary trace of the Galapagoshotspot, Huene et al., 1995; Barckhausen et al., 1998] and produce began subducting along the MAT sometime in the late differential uplift of forearc fault blocks [Marshall and Neogene [Collins et al., 1995; Kolarsky et al., 1995; Anderson, 1995; Fisher et al., 1998]. Shallowing of the sub- Meschedeet al., 1999b]. Shallow ridge indentationdramati- ducting slab beneathcentral Costa Rica, combinedwith pos- cally decreasedthe subductionangle [Prottiet al., 1995a], sible trench retreat by forearc erosion [e.g., Meschedeet al., diminishedthe mantle wedge, and extinguishedarc volcanism 1999a], has resulted in northeastwardmigration of the vol- within the Cordillera de Talamanca [de Boer et al., 1995]. canicarc from the extinctCordillera de Aguacateto the modern Rapid uplift and horizontal shorteningoccur within a broad Cordillera Central (Figure 2) [Marshall, 1994; Marshall and arch acrossthe ridge axis,.extending from the forearcPeninsula Idleman, 1999]. In this paper, we suggestthat shallow sub- MARSHALL ET AL' FAULT KINEMATICS, COSTA RICA 471

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• o o o 472 MARSHALL ET AL.: FAULT KINEMATICS, COSTA RICA

duction also drives active faulting acrossthe volcanic arc 4. Regional-Scale Faults alongthe CCRDB, as deformationpropagates inland above the 4.1. Kinematic Domains NW flank of the indentingCocos Ridge. In order to facilitate our discussion of fault kinematics we 3. Geology of Central Costa Rica have divided the study area into three generalizedkinematic domains(Figure 4). The forearcregion (D l: domain 1) encom- The Central Costa Rica Deformed Belt traverses the width of passesthe inner forearc (Pacific coastalpiedmont), as well as the volcanic arc, extendingfrom the Pacific forearc region to portionsof the forearc basin(Golfo de Nicoya) and outer fore- the Caribbean back arc. Faults of the CCRDB offset a broad arc high (Peninsula de Nicoya). The central volcanic arc re- range of lithologies,ranging from Cretaceous-Tertiaryseafloor gion (D2: domain 2) spansthe extinct Neogeneto Quaternary basementand marine sediments,to Quaternaryterrestrial sedi- volcanic arc (Cordilleras de Aguacate and Talamanca), the ments and extrusive rocks. For this study, we summarizethe Valle Central basin, and the southwestern flank of the active regional geology by defining several general lithostrati- arc (Cordillera Central). This domainalso includesportions of graphic units shown in Figures 2 and 3. the uplifted Tertiary-agedCandelaria basin along the flanks of Within the central Pacific forearc the upper Cretaceous the extinct arc. The back arc region (D3: domain 3) encom- igneous basement (Nicoya Complex) is exposed along the passesthe Caribbeanslope of both the extinct arc (Cordillera outer forearc high of the Peninsula de Nicoya [Lundberg, de Talamanca)and modernarc (Cordillera Central), as well as 1982] and within uplifted inner forearc blocks [Fisher et al., uplifted portionsof the back arc basin (Lim6n basin). 1998]. Flanking these forearc basementexposures are upper Deformationalong the CCRDB occursacross a diffusearray Cretaceous-Paleogenedeep marine and slope sedimentsde- of regional-scalefaults (lengthsof severalkilometers to tensof rived from the early Costa Rican arc [Lundberg, 1982]. Rocks kilometers) that exhibit a range of orientationsand styles of of the inner forearccoastal piedmont include Neogene shallow slip (Figures 3 and 4, and Table 1). Our discussionof fault marine to deltaic sedimentsof the Nicoya and Parrita basins kinematics begins with a description of these regional-scale [Astorga et al., 1991], Neogene-Quaternaryvolcanic and vol- features, based on a combination of field observations and a caniclasticrocks derived from the Aguacate arc [Denyet and compilationof existing geologicmap data. Arias, 1991], and Quaternary fluvial and marine terrace deposits [Fisher et al., 1994]. 4.2. Regional-ScaleFaults: Forearc (Domain 1) The complexvolcanic arc of central CostaRica encompasses The CCRDB intersects the central Pacific coast between the active Cordillera Central, as well as the extinct Cordilleras Puntarenasand Quepos (Figures 2 and 3), southeastof the de Aguacateand Talamanca(Figures 2 and 3). The Cordillera projectedtrend of the rough-smoothboundary on the subduct- Central consists of a broad massif of Quaternary andesitic- ing plate. Offshore seismic-reflectionprofiles reveal steepNE dacitic stratovolcanos. To the SW of the active cordillera, striking margin-perpendicularfaults that show dip-slip offset heavily erodedremnants of the extinct Cordillera de Aguacate of late Cenozoic outer forearc shelf sediments (Figure 3) expose Neogene-Quaternary basaltic-andesite lavas and [Barboza et al., 1995]. Onshore,a series of steepmargin-per- pyroclastic flows (Aguacate Group) [Denyet and Arias, pendicularfaults strike alongthe NE trendingvalleys of major 1991]. The Valle Central basin, situated between the extinct Pacific sloperivers includingthe Rios Barranca,Jesfis Maria, Aguacate arc and the active Cordillera Central, containsa vol- T•trcoles,Tusubres, and Parrita [Madrigal, 1970]. Thesemajor canictableland formed by the accumulationof a thick sequence faults (10-20 km in length)allow for differentialuplift of a sys- of Quaternaryandesitic to dacitic lavas, pyroclasticflows, and tem of inner forearc blocks referred to as the Esparza,Orotina, lahar deposits[Denyet and Arias, 1991]. Herradura, Esterillos, and Parrita blocks (Figure 4) [Fisher et The Cordillera de Talamanca, SE of the Valle Central, cor- al., 1994, 1998]. Uplift rates,estimated from Quaternarymarine respondswith a 175-km-wide gap in the active volcanic arc and fluvial terraces, vary sharply across block-bounding and represents the only area of southern Central America faults. These displacementsare consistentwith vertical offsets above 2000 m in elevation (Figures 2 and 3) [de Boer et al., observed in Late Cretaceous through Quaternary deposits. 1995; Kolarsky et al., 1995]. These ruggedmountains expose The broadly distributed uplift generated by the subducting a suite of Neogene-Quaternaryintrusive (principally granodi- Cocos Ridge to the SE [Gardner et al., 1992] is corrugated orites) and extrusive rocks (andesites) that are correlative in locally by block uplift above subductingseamounts [Fisher age with the Aguacate arc to the NW. Rapid uplift driven by et al., 1998]. The following paragraphsdiscuss the block- Cocos Ridge subductionhas provoked extensiveunroofing of boundingfaults from NW to SE along the centralCosta Rican the Talamancarange, exposingthe intrusivecore. forearc. The intra-arc and back arc regions along the flanks of the At the northwesternedge of the CCRDB the Barrancafault Cordilleras de Aguacate and Talamanca expose both (Figure 4, Fault 1) strikesNE along the Rio Barrancavalley, Paleogenedeep marine and Neogene shelf sedimentsof the separatingthe uplifted Esparza block from the low-lying Candelariaand Lim6n basins (Figures 2 and 3) [Denyet and Puntarenascoastal plain to the NW. Vertical offsetsof up to Arias, 1991; Astorga et al., 1991]. Rocks of the Candelaria 30 m for late Quaternary fluvial terraces across the Rio basin were moderatelydeformed by homoclinaltilting during Barrancaand up to 4 m for Holocenemarine benchesnear the the late Neogene [Denyet and Arias, 1991], while sediments river mouth demonstrateactive slip along this fault. A radio- of the Lim6n basin have been extensively faulted and folded carbondate of-•3.0 ka for wood beneatha colluvial wedge on within the back arc thrustsystem of the NPDB [Astorgaet al., the uplifted Holocene platform indicates a maximum late 1991]. Holoceneuplift rate of 1.3 m/ka for the Esparzablock near the MARSHALL ET AL.' FAULT KINEMATICS, COSTA RICA 473

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Figure 4. Geologic map of the central study area showingregional-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 numberto Table 1. Heavy white dashedlines mark kinematic domain boundaries.Medium white dashedlines mark edgesof forearc fault blocks: Ez, Esparza;O, Orotina; H, Herradura;Es, Esterillos;P, Parrita; Q, Quepos [Fisher et al., 1998]. Dashed shadedrectangles outline areasof Figures6a-6c, and 6f. Volcanoesare as follows: VB, Volcfin Barva; VI, Volcfin Irazfi; VT, Volcfin Turrialba. See Figure 2 for geologicsymbols.

Barrancafault. This rate is consistentwith longer-termaverage pographic highs. A rhyodacitic welded tuff (-•400 ka, uplift of the upper Barrancafluvial terrace(Qt], oxygen iso- 4øAr/39Ar)[Marshall and Idleman, 1999] and underlying tope stage 5e - 125 ka) at a rate of 1.0 m/ka [Fisher et al., fluvial gravels,which outline a paleorivercourse, show up to 1998]. Remnantsof the upperthree terraces(Qt]_3) on oppo- 40 m of vertical offset across these faults. site sidesof the Barrancafault showthe samemagnitude of ver- The NE striking Tfircoles fault (Figure 4, Fault 6) accom- tical offset(- 30 m). This observationsuggests that slip along modatesvertical motion between the low-lying Orotina block the fault is relatively recent,having begunafter formationof and the uplifted Herradura block to the SE. The Herradura Qt3 in the late Pleistocene. block, which contains the highest topography within the The NE striking JesfisMaria fault (Figure 4, Fault 3) sepa- forearc, exposesupper CretaceousNicoya Complex seafloor rates the uplifted Esparza block from the low-lying Orotina basaltswhich have been strippedof their sedimentarycover. block to the SE. This fault forms a prominentSE facing scarp While the Tfircolesfault forms the principalboundary between along the NW bank of the Rio Jes•s Maria. Pleistocenelahar the Orotina and Herradura blocks, several subsidiary faults deposits(Tivives Formation:age <2 Ma, 4øAr/39Ar) (e.g., Carara and Turrubares;Figure 4, Fault 7) outline minor [Marshall and Idleman, 1999] and Miocene shallow marine fault blocksalong this trend. Vertical offsetsof up to 60 m for volcaniclasticsediments (Punta Carballo Formation: minimum late Quaternaryfluvial terracesalong the Rio T,qrcolesdemon- age middle Miocene) [Madrigal, 1970] are offset acrossthe strateactive slip within the T,qrcolesfault system. Jes•s Maria fault with a NW-side-up separationof-•120 m The NE-striking Tusubres fault (Figure 4, Fault 10)jux- [Fisher et al., 1994]. taposesthe upper Cretaceousbasement within the Herradura Within the interior of the Orotina block several minor NE block against Paleogene slope sediments of the Esterillos striking faults (Trinidad, Diablo, and Poz6n; Figure 4, Fault block to the SE [Sak et al., 1997]. Rocksof the moderatelyup- 5) show dip-slip offset of late Quaternaryash flows, volcani- lifted Esterillos block are, in turn, offset from the low-lying clastic sediments,and fluvial terrace gravels of the Orotina Parrita block to the SE across the NE striking Parrita fault Formation and expose Miocene sedimentswithin isolated to- (Figure 4, Fault 12). An elevation differenceof-•150 m for flu- 474 MARSHALLET AL.' FAULT KINEMATICS,COSTA RICA

Table 1. Regional-ScaleFaults of the CentralCosta Rica DeformedBelt Map Length, Slip Side Earthquakes Number Fault km* Strike Sense•' UpS' (M >3.0)•'•' References{} 1 Barranca 18 45 N SE 1,7 2 Barbudal, Jocote 7 132, 125 N SW, NE 7 3 Jesfis Maria 12 43 N NW 1,7 4 Cuarros 9 95-120 (N) SW 1 5 Trinidad,Diablo, Poz6n 7 65, 45, 45 N SE,NW, SE 1924(7.0), 1989(3.6) [A] 1,5 6 Ttircoles 21 45 N (SE) 1,7 7 Turrubares 6 55 N SE 1 8 Leona 8 45 N SE 1 9 Herradurasystem 5 40-65 (N) 9 1 10 Tusubres 10 35 N NW 1, 14 11 Esterillossystem 5 50 (N) 9 1, 14 12 Parrita 15 40 N NW 1, 14 13 Quepossystem 5 25-175 N 9 1 14 Turrubaritos 15 115-140 9 9 2 15 Venado 5 57 (N) NW 2 16 Tigre 20 125-135 (R) SW 2 17 Tulin 10 65-70 L NW 2 18 Zapat6n 7 125-130 (R) NE 2 19 Candelaria 60 130-135 R/T SW, NE 2 20 Quivel 8, 122-124 R (SW) 2 21 Queb.Colorado 10 57-59 L (NW) 1995(4.7) [O] 2,4 22 Delicias 17 38-42 L (NW) 2 23 La Mesa, Resba16n 17 125-135 R (NE) 2 24 Cortezal 7 50 L 9 1990 (5.0) [El, 1990(5.7) [I] 2,4 25 Picagres 7 120-122 (R) 9 1990 (4.5) IF], 1990 (4.8) [K] 4,6 26 Jarls 33 120-130 R NE 2,11 27 La Garita 10 33 (L) NW 1,2 28 Ciruelas 10 45-55 (L) NW 1,2 29 Pacacua 10 55 (L) SE 2 30 Salitral, Tablazo 14 35, 45 L NW 2,11 Alumbre,Patio de Agua 12 50, 45 L NW 19107 2,8 31 Coralillo 7 145 R 9 1991 (4.7) [MM] 4,8 32 Higuito 38 128-132 R SW 1993 (3.7) [Y], 1997 (3.4) 2,4 33 Alajuela 20 85-115 T NE 1772, 1851, 18887 3,8,13 34 Hondura 10 12 (N/R) 9 1993 (3.2) [FF] 4,9 35 Sucio,A. Grande,Blanco 18 128, 155, 146 R NE, NE, SW 1952 (5.2) 8,9 36 Coris-Guarco 15 100-125 (R) 9 1841, 1910, 19127 4,8 37 Navarro 16 65-75 (L) 9 1951 (5.0)? 4 38 Patarrfi 6 140 (R) 9 4 39 Orosi 20 140 (R) 9 1910, 1951 (5.0)? 10 40 Pejibaye 13 50 L (SE) 1993 (4.4)[AB] 4,12 41 Gato 7 130 (R) 9 1993 (4.3) 4 42 Atirro 18 150 R 9 1987 (4.3) [AG], 1988(4.5) [AF] 8, 12 43 Tuis 20 120-130 (R) 9 1 44 Pacuare 20 130 (R) 9 1 * Mappedlength (minimum). •' Predominantsense of displacement(parentheses indicate inferred): N, normal;T, reverse;L, left lateral;R, right lateral. •'•' Magnitudein parentheses;letters indicate focal mechanisms in Figures6a-6c and in Table 3. õ Referencesare as follows:1, this study;2, Arias and Denyer [1991]; 3, Borgia et al. [ 1990];4, Ferndndez [1995]; 5, Gaendelet al. [1989]; 6, Gaendelet al. [1990]; 7, Madrigal [1970]; 8, Montero [1994]; 9, MonteroandAlvarado [1995]; 10, Monteroand Miyarnura [1981]; 11, Monteroand Morales [1984]; 12, ObservatorioVolcanol6gico y Sismol6gicode CostaRica (OVSICORI-UNA) [1993]; 13, Peraldo and Montero [1994]; 14, Sak et al. [1997].

vial terracegravels across this fault demonstratessignificant interiors. Within the Esparza block the Barbudal and Jocote late Quaternaryslip [Saket al., 1997;Fisher et al., 1998]. faults (Figure 4, Fault 2) accommodateuplift of a minor In additionto the majorNE strikingfaults along the block highland. Within the Orotina block the WNW striking boundaries,several steep NW striking faults offset block Cuarros fault (Figure 4, Fault 4) accommodatesminor uplift MARSHALL ET AL.: FAULT KINEMATICS, COSTA RICA 475

and seawardtilting of the coastalportion of the block. The in- northern Cordillera de Talamanca, cutting Miocene- terior of the Herradura block is cut by the NW striking Quaternarysediments and volcanic rocks. A series of NE Turrubaritosand Tigre faults (Figure 4, Faults 14 and 16), striking sinistralfaults that intersectthe Higuito fault along which show evidence of oblique dextral motion [Arias and the northwesternedge of the Cordillerade Talamancainclude Denyet, 1991], and may also accommodatesome of the uplift of the Salitral, Tablazo, Alumbre, and Patio de Agua faults the Herradurablock by reverseslip. (Figure 4, Fault 30). These faults offset Neogene-Quaternary units as well as NW trending folds within the Miocene 4.3. Regional-Scale Faults: Volcanic Arc (Domain 2) sequence. Drainagesystems of the Valle Central have inciseddeeply Regional-scalefaulting within domain 2 displaysan overall into the Quaternary volcanic sequencealong a network of conjugate pattern of NW striking dextral faults and NE fault-controlledcanyons. The two principalrivers of the Valle striking sinistral faults (Figures 3 and 4). While some NE Central, the Rios Virilla and Grande, join to form the Rio striking faults have lengths of up to 20 km, the dominant Grande de T/trcoles at the intersection of the NE striking La structureswithin this pattern are three major NW striking Garita fault and the NW striking Jateo fault (Figure 4, Faults faults (Candelaria, Jarls, and Higuito) that display lengths of 27 and 25). The La Garita fault runs for over 10 km, cutting over 40 km (Figure 4, Faults 19, 26, and 32). The NW striking Candelaria fault (Figure 4, Fault 19) ex- Quaternarylavas, welded tuffs, and fluvial terraces.Offset of volcanicbeds and paleosolsrecord oblique left-lateralmotion tendsfor over 60 km along the seawardflank of the Aguacate with the NW side up. Vertical separationof nearly 25 m for arc, southeastwardto merge with the Terraba thrust belt (Fila fluvial terracessuggests active late Quaternaryslip acrossthe Costefta) north of Quepos. This major fault representsa tec- La Garita fault. tonic boundary between the uplifted forearc blocks Additional NE striking faults along the southernmargin of (Herradura,Esterillos, Parrita, and Quepos)to the SW and the the Valle Central include the Ciruelas and Pacacua faults extinct Aguacatevolcanic arc to the NE (Figure 4). Field ex- (Figure4, Faults28 and 29). The Ciruelasfault extendsfor at posures of offset lithologic units and slickenlines display least8 km and may continuefarther to the NE into the modern oblique dextral motion on the Candelaria fault [Arias and arc beneath the cover of late Quaternary lavas. NW-side-up Denyet, 1991]. Significant uplift of the forearc basement motion along the Ciruelas fault has uplifted a window of within the Herradura block suggestsa major componentof Miocene rocks along a fault-parallel ridge within the vertical slip along the northern segmentof the fault. Several Quaternaryvolcanic tableland.The NW striking Jateo fault NW striking subsidiary faults, including the Quivel and (Figure4, Fault 25) may representa left-steppingextension of Zapat6n (Figure 4, Faults 18 and 20), trend subparallelto the the Jarlsfault (Figure4, Fault26) that hasbeen offset along the Candelaria fault and also show evidence of oblique dextral Ciruelasfault (Figure 4, Fault 28). The inferredPacacua fault slip [Arias and Denyet, 1991]. Sinistral NE striking faults that intersect the trace of the Candelaria fault include the (Figure 4, Fault 29) runs for-10 km, cuttingMiocene sediments and Quaternarylavas along the northwesternedge of Tulin, QuebradaColorado, and Parrita faults (Figure 4, Faults the Cordillera de Talamanca. 17, 21, and 12) [Arias and Denyet, 1991; Sak et al., 1997]. An apparentexception to the overall NW-NE fault pattern The Jarls fault (Figure 4, Fault 26) strikes parallel to the within the Valle Central is the E-W trending Alajuela fault Candelariafault (Figure 4, Fault 19) for over 45 km southeast- ward from the Valle Central into the extinct volcanic arc, form- (Figure4, Fault 33). This anomalousfault extendsfor over 18 km alongthe northernmargin of the Valle Central,forming a ing a tectonic boundary between the Cordilleras de Aguacate clearly visible 100 to 200-m scarp at the foot of Po•s and and Talamanca.The Jarls and Candelariafaults togetherbound Barva volcanoes. Quarry exposuresalong the trace of the the north tilted Candelaria basin homocline [Arias and Alajuelafault showclear offsets of youngQuaternary volcanic Denyet, 1991]. At the southwesternedge of the Valle Central deposits.Borgia et al. [1990] ascribethis scarpto fault prop- the Jarls fault cuts Quaternarywelded tuffs as young as 400 ka agationfolding at the tip of a major north dipping gravita- [Marshall and Idleman, 1999]. Offsets along the trace of the tional thrust fault causedby spreadingof the volcanicmassif. Jarls fault suggestpredominantly right-lateral motion, with a While the abrupt scarpand Quaternaryoffsets attest to recent componentof dip slip (NE side up) along a steeply dipping slip, seismic activity is virtually absent. Peraldo and fault surface [Arias and Denyet, 1991]. This vertical motion Montero [1994], however, suggestpossible association of may accommodateuplift and exposure of intrusive rocks within the northern Cordillera de Talamanca. this fault with damagingearthquakes in the eighteenthand nineteenth centuries. Another major NW striking fault, the Higuito fault, (Figure 4, Fault 32) runs along the southern margin of the Valle 4.4. Regional-ScaleFaults: Back Arc (Domain 3) Central at the southwesternedge of the San Jos6 metropolitan area, markingthe boundarybetween the Valle Central basin Regional-scalefaulting at the westernedge of domain3 and the northern flank of the Cordillera de Talamanca. The deviatesslightly from the conjugatepattern typical of domain Higuito fault extendsnorthwestward into the Valle Central 2. The WNW trendingCoris, Guarco, and Agua Calientefaults and may continuebeneath the Quaternaryvolcanic sequence (Figure4, Fault 36) southof Cartagomerge toward the east to the southern flank of the Cordillera Central volcanoes. with the ENE strikingNavarro fault (Figure 4, Fault 37) and Uplift of Miocenesediments along a prominentNW trending the NW strikingOrosi fault (Figure4, Fault 39). The tracesof ridge within the Valle Central basin suggestsSW-side-up thesefaults are markedby the alignmentof numerousgeother- motion along the Higuito fault. Toward the SE the Higuito mal springs,and recentactivity is manifestedby offsetsof fault extendsout of the Valle Central for nearly 30 km into the Quaternaryfluvial terracesand Holocenesoils. 476 MARSHALLET AL.: FAULT KINEMATICS,COSTA RICA

Toward the east, along the upper Caribbean slope of the Thesefibers commonly display steps or risersthat are congru- Talamancaarc, regional-scalefaulting is dominatedagain by a ent with fault motion (steppingdownward in the directionof strong NW-NE conjugate pattern of major faults that strike the missing block), providing a relatively definitive indica- along deep linear valleys. Major NW striking faults include tion of slip sense. the Gato, Atirro, Tuis, and Pacuarefaults (Figure 4, Faults 41- Slickenlineswithin Quaternary lavas, pyroclasticrocks, 44), and major NE trendingfaults includethe Pejibaye(Figure and lahar depositsinclude mostly fault surfacestriations or 4, Fault 40), upper Pacuare,and Chirrip6 (beyondeast edge of grooves. Within Quaternary volcaniclastic and fluvial de- Figure 4). These faults are associatedwith bold topographic posits, as well as soils, slip generally produces smooth scarps and offset Quaternary river terraces. In addition, the streakson fault surfaces.In the absenceof congruentfibrous Tertiary sectionand some Quaternaryrocks in this region are stepsthese slickenline types require subsidiaryfractures to affectedby folding with NW trendingaxes, cut in some cases determineslip sense.In the majority of cases,subsidiary frac- by minor NE verging thrustfaults [Krushensky,1972]. tureswere of "RM type" [Petit, 1987], minor, striatedRiedel- style fracturesintersecting the fault surfaceat shallowangles dipping in the directionof movementof the oppositeblock. 5. Fault Kinematics Thesefractures are commonlyconcave upward toward the fault plane, intersectingthe fault as crescentswith hornspointing 5.1. MesoscaleFault Analysis in the movementdirection of the oppositeblock. In a few cases While relativemotions can be determinedfor manyregional- (usually in denselavas), "T type" (tensile) fractures[Petit, scalefaults in the field [e.g., Arias and Denyet, 1991], the 1987] were utilized as slip sense indicators.These features precise kinematics of deformation acrossthe CCRDB remain also tend to intersectthe fault plane as crescentsindicating obscure.We therefore expand upon our understandingof the motiondirection of the oppositeblock. regional-scalefaulting by examiningmesoscale fault popula- Fault data were analyzedusing the methodof Marrett and tions. Mesoscalefaults are outcrop-scalefeatures (meter to Allrnendinger [1990] and the computer software of decimeterlength) that exhibit measurablefault surfaceorienta- Allrnendingeret al. [1994]. The resultingkinematic axes for tions and kinematic indicators such as slickenlines. fault populationsare plottedon best fit fault plane solutions Mesoscale fault data sets, combined with focal mechanisms (Figures 5a-5c) . We interpretthe kinematicsof deformation from recent seismicity,allow us to better characterizestrain alongthe CCRDB by examiningthe spatialdistribution of the acrossthis diffuse plate boundarydeformation zone. fault data on regional geologicmaps in conjunctionwith re- Distributed faulting along plate boundariesis likely to gional-scalefaults and earthquakefocal mechanisms(Figures reflect the kinematicsof plate interactions,with slip on indi- 6a-6f). The kinematicsof both regional-scaleand mesoscale vidual faults representinglocal incrementsof strain within a faulting along the CCRDB show notable differencesbetween region of mesoscalecataclastic flow [e.g., Wojtal, 1989]. domains 1, 2, and 3. In sections 5.2-5.4, we summarize the Populationsof minor faults are unlikely to record a single mesoscalefault data, earthquakeseismicity, and regionalfault commonstress tensor. For this reason,we employa kinematic- patterns and discuss the observed kinematic variations mov- basedmethod of fault analysis[Marrett and Allmendinger, ing from west to eastacross central Costa Rica. 1990] rather than stress tensor reduction [e.g., Angelier, 1984]. The kinematicmethod determines the principalshorten- 5.2. Fault Kinematics:Forearc (Domain 1, Sites1-27) ing and extensionaxes (P andT axes)on the basisof slip data Within the forearc region (domain 1), mesoscalefaults from individualfaults. The P and T axesfrom a populationof displayconsiderable variability in orientationand slip direc- faults are contoured to determine the distribution of strain axes tion (Figures 6a and 6d-6f). Normal and strike-slipfaults, for that location.The strain,as expressedby P and T axesdis- however, significantly outweigh thrust faults in number and tributions,should reflect the motionbetween regional blocks. in magnitudeof slip. Fault populationdata from this area re- Mesoscalefault populationswere measuredat 86 locations flect transtensionwith a predominanceof shallowT axesand P acrosscentral Costa Rica in rocks ranging from Eocene to axestrending between steep and shallow. Holocene age (Table 2). In general, older rocks were avoided Shallow T axes, combinedwith steep P axes, suggesta in order to exclude Cretaceous-early Paleogene faults componentof extensionfor a majority of populations.This is generatedduring the early developmentalphases of the MAT consistentwith offsetsof Neogene-Quaternaryunits observed and its volcanicarc [e.g., Gutsky, 1988]. Kinematicdata were alongsteep, regional-scale, block-bounding faults (Figure 4). collectedfrom freshfault surfacesexposed in quarries,excava- Where mesoscaleP and T axes are both shallow, the data show tions,road cuts,riverbanks, coastal cliffs, and wave-cutplat- predominantlyleft-lateral motion on NE striking faults. This forms(Table 2). Data includethe strikeand dip of individual is consistent with sinistral transtension across the forearc fault planes,trend and plunge of slip lineations(slickenlines), fault blocks. and slip sense.The slip sensewas determinedon the basisof Historically, shallow upper plate earthquakeswithin the the geometryand natureof slicknelines,subsidiary fractures, forearc (domain 1) have been relatively rare in comparisonto and/oroffset beds as outlinedby Petit [1987]. the volcanicarc and back arc (domains2 and 3). A seismicse- Slickenline types vary between outcrops,mostly as a quence in 1989 centered on the coastal piedmont of the functionof lithology.Fibrous mineral growth (mostly calcite) Orotina block (Figure 6a, Focal MechanismA, and Table 3) occurson many fault surfaceswithin Tertiary sandstoneand showedoblique-normal slip along a linear NE trend [Giiendel limestoneunits, as well as on someNeogene-Quaternary an- et al., 1989]. This swarm occurredin the same locationas the desiticlavas and pyroclasticrocks (e.g., AguacateGroup). M=7.00rotina earthquakeof 1924 (Table 1), which causedex- MARSHALL ET AL.: FAULT KINEMATICS, COSTA RICA 477

Table 2. MesoscaleFault PopulationData Field Age Total Site Number Latitude Longitude OutcropLocation and Type* and Fm-• Faults P Axis T Axis Peninsulaand Golfo de Nicoya (Forea/cRegion, Domain 1) 1 03-96 9 ø 37' 85ø 09' PuntaBarrigona, Malpais (sp) Tm-st 8 085,29 347,15 2 02-96 9 ø 37' 85ø 08' QuebradaVanegas, Malpais (rc) Tm-st 8 338,32 245,04 3 01-96 9 ø 35' 85ø 06' CaboBlanco Reserve HQ (sp) Tpe-cb 10 185,75 008,15 4 07-94 9 ø 54' 84ø 56' PlayaGigante (sc) Tpe-lp 10 154,59 291,24 5 05-94 10 ø 03' 84ø 58' PuntaMorales (sp/sc) Tpe-lp 5 150,40 244,06 6 06-94 10 ø 06' 84ø 56' Pta MoralesHwy, CerroAm6rica (rc) To-ma 13 148,13 240,08

Esparzaand OrotinaBlocks (Forearc Region, Domain 1) 7 05-90 9 ø 57' 84ø 45' PuntaCarballo (sp/sc) Tm-pc 16 281,63 092,27 8 15-94 9 ø 59' 84ø 43' FincaMachuca, Gregg de Esparza(q) Tm-pc 14 356,O6 086,02 9 01-90 10 o 01 ' 84ø 40' Rio Barranca,Marafional (rb) Tm-pc 9 167,47 267,09 10 02-90 9 ø 58' 84ø 41' Esparza-Artiedaroad, Humo (rc) Tm-pc 10 232,08 141,05 11 11-90 9 ø 54' 84ø 44' PuntaCorralillo (sc) Tm-pc 7 227,35 128,11 12 08-94 9 ø 53' 84ø 43' PlayaTivives (sc) Tm-pc 20 213,52 312,07 13 41-93 9 ø 55' 84ø 41' CostaneraHwy, Rio JesfisMaria (rc) Tm-pc 8 038,13 301,28 14 04-94 9 ø 51' 84ø 41' PerionBajamar (sc) TQ-t 9 192,24 288,13 15 43-93 9 ø 53' 84ø 38' CerroTamarindo (q) Tm-pc 30 330,63 140,26 16 18-94 9 ø 53' 84ø 36' CostaneraHwy, QuebradaPoz6n (rc) TQ-t 16 020,76 113,01 17 09-90 9 ø 53' 84ø 3,5' CostaneraHwy, CerroCoyote (rc) Tm-pc 11 156,66 051,07 18 16-94 9 ø 37' 84ø 35' CostaneraHwy, Rio Tfircoles(rc) Qt 14 114,64 298,26 Herradura,Esterillos, Parrita, and Quepos Blocks (Forearc Region, Domain 1) 19 14-90 9ø 43' 84ø 40' PuntaLeona (sp/sc) Tm-pc 15 360,81 094,01 20 12-90 9 ø 42' 84ø 40' PuntaSucia (sp/sc) Tm-pc 27 180,32 272,02 " 13-90 9 ø 41' 84ø 40' PlayaCaletas (sp/sc) Tm-pc 21 01-95 9 ø 40' 84ø 40' PlayaCoyol, Puerto Escondido (sc) Tm-pc 27 142,75 306,15 22 10-95 9 ø 38' 84ø 38' HaciendaJac6, Playa Jac6 (q) Tm-pc 19 294,81 174,05 23 06-95 9 ø 32' 84ø 26' CostaneraHwy, Bejuco(q) Qt 19 173,77 282,04 24 05-95 9 ø 32' 84ø 16' CostaneraHwy, Vueltas(rc) Qt 6 274,72 106,18 25 04-95 9 ø 27' 84ø 09' CostaneraHwy, FincaManagua (ex) Qt 7 157,86 338,04 26 03-95 9 ø 23' 84ø 09' PuntaCatedral, Manuel Antonio (sc) Te-ps 23 061,86 217,04 27 02-95 9 ø 23' 84ø 08' PlayaEscondida, Manuel Antonio (sc) Te-ps 6 280,82 021,02 Cordillerade Aguacate (Central Volcanic Arc Region,Domain 2) 28 09-94 10 ø 03' 84ø 38' FincaNorita, San Jer6nimo (q) TQ-ga 15 336,40 070,05 29 50-93 10ø02 ' 84ø 34' Pan-AmHwy, F. PiedraBianca (rc) TQ-ga 10 197,17 288,03 30 51-93 10 ø 04' 84ø 30' Tajo Santiago,Magallanes (q) TQ-ga 12 033,37 301,02 31 78-93 10 ø 02' 84ø 30' Balboa(ex) TQ-ga 17 005,35 251,30 32 80-93 10 ø 01' 84ø 29' CalleVargas, Berlin (q) TQ-ga 14 194,16 101,11 33 66-93 10 o 02' 84ø 24' Alto La Cima(rc) TQ-ga 13 194,34 095,13 34 81-93 10 ø 00' 84ø 26' Rinc6nde SanIsidro (ex) TQ-ga 12 351,00 261,05 35 83-93 9 ø 58' 84ø 26' Atenas-S.MateoHwy, A. del Monte(rc) TQ-ga 5 195,11 287,12 36 82-93 9 ø 58' 84ø 28' Atenas-S.MateoHwy, Desmonte(rc) TQ-ga 6 008,07 103,33 37 15-90 9 ø 55' 84ø 28' Tajo Dantas(q) TQ-ga 7 349,04 259,00 38 59-93 9 ø 54' 84ø 28' Tajo CerroRayos, Bols6n (q) TQ-ga 8 330,62 098,18 39 09-95 9 ø 52' 84ø 28' Rio Turrubaresbridge (rb) Tm-lc 15 001,31 271,01 40 60-93 9 ø 54' 84ø 25' Pursical-TurrubaresHwy, Por6(rc) TQ-ga 9 337,52 100,24 41 58-93 9 ø 53' 84ø 23' Tajo GrifoAlto (q) TQ-ga 14 220,37 317,10

ValleCentral (Central VolcanicArc Region,Domain 2) 42 64-93 9 ø 56' 84ø 22' PlantaHidroe16ctrica La Garita(rc) TQ-ga 7 035,39 138,15 43 73-93 9 ø 56' 84ø 23' Rio Tfircolesbridge, La Junta(rc) TQ-ga 18 186,11 277,06 44 36-93 9 ø 58' 84ø 22' Tajo Rio Grande(q) Qv-aa 15 339,24 085,33 45 72-93 9 ø 59' 84ø 21' Tajo PuenteLa Garita(q) TQ-ga 22 010,08 100,02 46 38-93 10 ø 01' 84ø 22' Tajo La Pista(q) Qv-aa 28 005,49 227,33 39-93 10 ø 02' 84ø 22' Tajo La Argentina(q) Qv-aa 47 07-93 10 o 03' 84ø 17' Tajo Prendas,Rio Prendas(q) Qv-aa 30 020,63 220,26 24-93 10 ø 03' 84ø 17' Tajo FincaChilamate, Rio Prendas(q) Qv-aa 478 MARSHALLET AL.: FAULT KINEMATICS,COSTA RICA

Table 2. (continued) Field Age Total Site Number Latitude Longitude OutcropLocation and Type* andFm•- Faults P Axis T Axis ValleCentral (Central Volcanic Arc Region, Domain 2) (continued) 48 02-94 9ø 58' 84ø 13' TajoLindora, Valle del Sol (q) Qv-aa 6 353,45 199,42 49 56-93 9ø 58' 84ø 09' TajoLas Animas (E), LaCaja (q) Qv-aa 54 357,44 156,44 " 57-93 9ø 58' 84ø 10' TajoLas Animas (W), La Caja(q) Qv-aa " 01-94 9ø 58' 84ø 10' TajoRio Torres, S.Pedro de Pavas (q) Qv-aa 50 84-93 9ø 58' 84ø 08' TajoLagunilla, Rio Virilla (q) Qv-aa 16 206,62 338,20 51 53-93 9ø 57' 84ø 19' RioTizate bridge, Turr6cares (rc) Tm-lc 14 192,82 046,07 52 85-93 9ø 55' 84ø 19' RioJaris bridge, Piedras Negras (rc) Qv-aa 10 005,15 099,15 53 75-93 9ø 56' 84ø 17' TajoLara, Quebrada Ponciano (q) Qv-aa 15 178,25 286,33 54 28-93 9ø 56' 84ø 15' SanJos6-Co16n Hwy, Villa Co16n (rc) Tm-p 11 170,56 043,22 55 29-93 9ø 54' 84ø 15' QuebradaHonda bridge (rc/rb) Tm-p 38 182,81 325,07 " 02-93 9ø 54' 84ø 14' Co16n-PuriscalHwy (q) Tm-p 56 03-93 9ø 52' 84ø 15' Co16n-PuriscalHwy, Tajo Guayabo (q) Tm-lc 14 020,37 125,20 57 04-93 9ø 52' 84ø 14' TajoCarlas, Tabarcia (q) Tm-pn 12 199,06 104,40 58 08-95 9ø 56' 84ø 13' SanJos6-Co16n Hwy, Rio Cararia (rc) Qv-aa 5 197,16 289,05 59 01-93 9ø 56' 84ø 12' TajoCerro Minas, Santa Ana (q) TQ-ga 28 226,34 332,22 " 11-93 9ø 57' 84ø 11' SanJos6-Co16n Hwy, Tajo Pozos (q) TQ-ga 60 13-93 9ø 57' 84ø 10' LosLaureles, Cerro Palomas (ex) Tm-p 20 176,82 308,06 " 14-93 9ø 57' 84ø 10' LosLaureles, Cerro Palomas (rc) Tm-p 61 09-93 9ø 57' 84ø 10' S.J.-Co16nHwy, Cerro Palomas (rc) Tm-p 10 202,14 303,36 " 10-93 9ø 57' 84ø 10' S.J.-Co16nI4wy, Cerro Palomas (rc) Tm-p Cordillerade Talamanca(Central Volcanic Arc Region, Domain 2) 62 23-93 9ø 52' 84ø 03' TajoValverde, Valverde de Higuito (q) Tm-sm 10 014,01 284,19 63 30-93 9ø 48' 84ø 07' Aserri-FrailesHwy, Tranquerillas (rc) Tm-pn 28 013,48 117,12 " 31-93 9ø 48' 84ø 07' Aserri-FrailesHwy, Tranquerillas (rc) Tm-lc 64 20-93 9ø 47' 84ø 05' Aserri-FrailesHwy, Rosario (rc) Tm-lc 11 166,43 275,19 65 32-93 9ø 48' 84ø 04' RioAlumbre bridge, Guadarrama (rc) Tm-pn 16 182,14 273,01 66 33-93 9ø 49' 84ø 02' Pacayas-Copalchiroad(rc) Tm-lc 16 356,31 087,02 67 17-93 9ø 43' 84ø 03' TajoAngostura, San Antonio (q) TQ-ga 12 213,19 305,05 68 19-93 9ø 44' 84ø 02' Frailes-S.PabloHwy, Santa Cruz (rc) TQ-ga 8 013,58 137,19 69 15-93 9ø 43' 83ø 58' El Empalme-S.MariaHwy, Jardin (rc) TQ-ga 7 121,60 280,29 70 18-93 9ø 39' 84ø 00' S.Maria-S.MarcosHwy, Zapote (rc/ex) Tm-pn 13 354,28 258,11 71 16-93 9ø 38' 83ø 55' Pedregoso,Copey de Dota (rc) Tm-pn 8 055,40 149,05 72 21-93 9ø 52' 84ø 02' TajosGuatuso, Patarrfi (q) Tm-sm 16 041,25 309,04 73 34-93 9ø 51' 84ø 00' TajosBermejo, Coris (q) Tm-sm 9 216,07 308,19 74 05-93 9ø 49' 83ø 57' Pan-AmHwy, Tajo Guatuso, S.Isidro (q) Tm-lc 27 352,28 186,61 Vailesde Tejar,Orosi, and Reventaz6n (Back Arc Region,Domain 3) 75 55-93 9ø 54' 83ø 57' Pan-AmHwy, Ochomogo (ex) Tm-lc 20 027,07 296,08 76 35-93 9ø 53' 83ø 57' TajoTaras, Taras de Cartago (q) Tm-lc 18 016,03 285,26 77 17-94 9ø 49' 83ø 53' TajoBarro Morado, Lourdes (q) Tm-sm 21 352,08 087,31 78 06-93 9ø 50' 83ø 53' TajoAgua Caliente, Paraiso (q) Tm-pn 12 167,06 259,14 79 12-95 9ø 50' 83ø 53' Paraiso-Cachi'Hwy,Tajo Los Novios (q) Qv-r 6 290,04 047,82 80 11-95 9ø 52' 83ø 47' Cachi-Tuccuriqueroad, Tajo Joyas (q) Tp-d 16 338,17 111,66 81 11-94 9ø 50' 83ø 42' TajoOriente, Rio Pejibaye (q) Tp-d 18 189,14 .288,33 82 10-94 9ø 48' 83ø 39' TajoEsperanza, Rio Atirro (q) Tpe-t 29 014,07 283,08 83 14-94 9ø 48' 83ø 31' TajoLas Quebradas, Bajo Pacuare (q) Tpe-t 12 213,06 115,53 84 03-94 9ø 57' 83ø 38' RioLajas, Torito de Turrialba (rb) Tp-sk 7 134,12 324,78 85 12-94 9ø 58' 83ø 34' OldLim6n Hwy, Tajo Tres Equis (q) Tp-d 14 358,17 252,42 86 13-94 10ø 01' 83ø 37' QuebradaLinda, Bonilla (rc) Tom-u 11 219,03 321,75 * Outcroptypes are as follows: q, quarry;rc, roadcut; ex, excavation; rb, river bank; sp, shore platform; sc, sea cliff. •' Sedimentaryrock formations (Fm) include: Quaternary fluvial and marine terraces (Qt), the Neogene Punta CarballoFm (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), LasPalmas 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 TivivesFm (TQ-t), Grifo Alto Fm (TQ-ga) (Aguacate Group), Dofin Fm (Tp-d) (Aguacate Group), and La CruzFm (Tm-lc) (AguacateGroup). MARSHALLET AL.: FAULT KINEMATICS,COSTA RICA 479

FOREARC REGION (DOMAIN 1)

ß I 20i' ß'••"•.i 2 2 ß 2 * 2 ß

2 26 . ß .

Figure 5a. Mesoscalefault populationdata for the forearcregion (kinematic domain 1). Data for eachfault population(analyzed using the methodof Marterr and Allmendinger[1990]) are presentedas bestfit fault plane solutions(lower hemisphere,equal-area projections) defined by compressional(P) axes(solid circles show individualfaults; letter P showsthe average),and tensional(T) axes (open squaresshow individual faults;letter T showsthe average).The dataare keyedby numberto Table 2 andto Figures6a-6f.

tensive damage in central Costa Rica. According to eyewit- show greatertotal lengths,crosscutting relationships suggest ness interviews, the 1924 event produced a 4-km-long NE that faults of both orientationsare of similar age. Mesoscale trending groundrupture west of the town of Orotina [Giiendel fault populationsrecord predominantlyshallow T and P axes, et al., 1989]. The location and trend of both the 1924 ground indicating mostly strike-slip motion, with left-lateral slip on rupture and the 1989 seismic swarm correspondwith those of NE striking faults and right-lateralslip on NW striking faults. the Trinidad, Diablo, and Poz6n faults mapped in this study Minor vertical componentsof slip show a slight extensionon (Figure 4, Fault 5). The 1989 compositefocal mechanism,the NE striking faults and shorteningacross NW striking faults. 1924 ground rupture, mapped Quaternary offsets, and Shallow (< 15 km) seismic activity is extremely common mesoscale fault data are all consistent with transtension ac- within domain 2, with a broad distribution of minor earth- commodatedby oblique slip mostly along NE striking mar- quakes (34'_<3.0)[Montero and Dewey, 1982; Ferndndez, gin-perpendicularfaults within the inner forearc. 1995, 1996]. The focal mechanisms(Figures 6a and 6b, and Table 3) include compositemechanisms from diffuse minor 5.3. Fault Kinematics: Volcanic Arc (Domain 2, Sites 28-74) earthquakes(1976-1981), as well as single-eventmechanisms In general, mesoscalefault populationswithin the central for minor to moderateearthquakes (M=3.0-5.0) from the 1990 volcanic arc (domain 2) mimic regional-scalefaults in display- Puriscal seismic swarm at the northwestern end of the Jarls ing a strong pattern of conjugateNE and NW striking faults fault (Figure 4, Fault 26), and the 1993 Valle Central seismic (Figures 6a and 6b). While NW striking regional-scalefaults swarm at the northwesternend of the Higuito fault (Figure 4, 480 MARSHALL ET AL.: FAULT KINEMATICS, COSTA RICA

CENTRAL VOLCANIC ARC REGION (DOMAIN 2)

34 .• 35 3

6 6 66.,,•ß ß . . 6. 6

Figure 5b. Mesoscalefault populationdata for the volcanicarc region (kinematic domain 2). SeeFigure 5a for explanation. MARSHALLET AL.: FAULT KINEMATICS,COSTA RICA 481

BACK ARC REGION (DOMAIN 3)

Figure5c. Mesoscalefault population data for theback arc region (kinematic domain 3). SeeFigure 5a for explanation.

Fault 32). In general, earthquakefocal mechanismswithin do- fault valleys often includeoblique thrust faults of varying ori- main 2 are consistentwith mesoscalefaults in showing pre- entationsand dips that display steep T axes. These structures dominantly strike-slip motion on conjugate NE left-lateral are more abundantapproaching the NPDB towardthe eastand and NW right-lateral faults. Normal and thrust mechanisms are presumablyassociated with NW trendingfolds in this re- nearmajor faultjunctions may reflectthe complexkinematics of gion. intersectingconjugate faults [e.g., Ingles et al., 1999]. As in domain 2, shallow seismicity (< 15 km) is also A notableexception to the conjugatefault patternin domain commonwithin domain 3. Focal mechanisms(Figure 6c and 2 are mesoscalefaults within thick packages of Quaternary Table 3) includeaftershocks of the 1991 Valle de la Estrella pyroclastic rocks filling the Valle Central basin (Figure 6b earthquakeon the NPDB, as well as minor to moderateevents and Table 2). These populationsshow mostly dip-slip motion (M=3.0-5.0) of the 1993 Turrialba seismicswarm along the on subverticalsurfaces, with moderatelyinclined P and T axes. Pejibaye fault (Figure 4, Fault 40). In general,focal mecha- Anomalous fault kinematics within these weak surface nisms along regional-scale faults are consistent with depositsmay recordvertical simple shearrelated to differential mesoscalefault data (e.g., Figure 6c, Faults 81 and 82, and motionof underlyingbedrock fault blocks.Transcurrent Focal MechanismsAB and AF) in showing sinistral slip on motion at depth may be transmittedinto this overlying layer NE strikingfaults and dextral slip on NW strikingfaults. As without clear throughgoingfaults. Alternatively, these faults in domain2, normaland reversecomponents of slip may reflect may have been generated by thermal contraction during the kinematicsof conjugatefault intersections.To the east of cooling of thesethick sequencesof pyroclasticrocks. domain 3, focal mechanismsreported by Protti and Schwartz Another exception to the conjugate pattern are mesoscale [1994] for aftershocksof the 1991 Valle de la Estrellaearth- faults along the trace of the easttrending Alajuela fault (Figure quake show increasedcomponents of reverseslip as strike- 4, Fault 33). These features show predominantlynormal mo- slip faultsof the CCRDB mergewith thrustsof the NPDB near tion and may reflect extension within the crest of a thrust Puerto Lim6n. propagationanticline that forms the Alajuela fault scarp.This structuremay representgravitational slumping of the volcanic 6. Age of Faulting Cordillera [Borgia et al., 1990]. The regional-scalefaults examinedin this study (Table 1) 5.4. Fault Kinematics: Back Arc (Domain 3, Sites 75-86) offset rocks ranging from Neogene to Quaternary in age. Severalauthors have suggestedthat the conjugatefaults of the Mesoscalefault populationswithin the back arc (domain 3) central volcanic arc developed during the Oligocene or show a transition from the steep conjugatetranscurrent faults Miocene under N-S compressiongenerated by convergence common in domain 2 toward an increase in shallower faults between North and South America [Astorga et al., 1991; displaying componentsof shortening(Figure 6c). Along the Arias and Denyer, 1991]. While many faults may have origi- traces of major regional-scale faults, mesoscale fault natedunder a pasttectonic regime, field observationsconfirm populationsshow a mix of steepNW and NE striking conju- that thesefaults are presentlyactive and that the mesoscale gate strike-slipfaults with shallowP and T axes. As in domain fault data reflect the modern kinematics. 2, dextral slip occurs principally on NW striking faults, and Field geomorphicevidence of Quaternaryactivity (e.g., sinistral slip occurs principally on NE striking faults. An ap- faulted soils,bold scarps,and offset fluvial and wave-cutter- parent exceptionto the predominantlyconjugate fault pattern races) is common along the CCRDB. Radiometric dating is the Coris-Guarco-Navarrofault systemnear Cartago, which (4øAr/39Ar)of offsetQuaternary volcanic units both along the displays orientationsranging from WNW to ENE (Figure 4, Pacific coastal piedmont and within the Valle Central Faults 36 and 37). In the easternportion of domain 3 (Figure demonstratespervasive faulting along the CCRDB duringthe 6c), fault populationsrecorded within ridges between major last 400 ka [Marshall and Idleman, 1999]. The active nature 482 MARSHALL ET AL- FAULT KINEMATICS, COSTA RICA

10ø 00'

9ø 45'

9 ø 30'

Figure 6. (a-f) Geologicmaps showingthe distributionof mesoscalefault populationdata (large, shaded, numberedstereo projections; see Figures5a-5c and Table 2 for completedata) and earthquakefocal mechanisms(small, solid, letteredstereo projections; see Table 3 for data and references).Dashed white lines mark the boundariesof kinematicdomains (D1, D2, and D3). See Figure3 for map locationand geologicsymbols.

of individual faults throughoutthe zone is further supported three observationsto argue that our data do indeed reflect by direct associationwith historic earthquakesand modern modemdeformation kinematics: (1) lack of temporalvariations recordedseismicity (Table 1). in kinematics,(2) associationwith active regional-scalefaults, While field evidence and seismicity confirm that most re- and (3) consistencywith earthquakefocal mechanisms. gional-scalefaults of the CCRDB are tectonically active, the The first observation is that mesoscale fault kinematics age of outcrop-scalefaults can be ambiguous.In most cases, within individual domains are essentially the same within mesoscalefault data were collectedfrom Neogenerocks, intro- Neogenethrough Quaternary units regardlessof unit age. If ducingthe concernthat somefaults may reflect earlier deforma- deformationkinematics have changedthrough time, fault pop- tion not representativeof the active tectonic regime. We use ulations would show either a broad distribution of P and T MARSHALLET AL.: FAULTKINEMATICS, COSTA RICA 483

:!San Jos6.-,

9 ø 30'

Figure 6. (continued)

axes or multiple clusters (bull's-eyes) of axes. On the other The secondobservation is that many of the mesoscalefault hand, if strain orientationshave remained relatively constant populationswere measurednear activeregional-scale faults. throughtime, P and T axes shouldcluster about a singleaver- Thesefault populationsare often dominatedby faults with age trend and plunge. In this study, nearly all fault popula- similar or conjugateorientations to the nearby, active re- tions show relatively concentratedclusters of P axes (Figure gional-scalefeatures (Figures 6a-6f). This directassociation ß7). This suggests either that all faults originated in the with known active faults supportsthe argumentthat the Quaternary, that no active structureswere measured,or that mesoscaledata representactive kinematics. the deformation kinematics have not varied significantly The third observation is that mesoscale fault kinematics throughoutthe late Cenozoic. Becausethe first two scenarios acrossthe study area are consistentwith earthquakefocal are highly unlikely, we proposethat the kinematicsof faulting mechanisms(Figures 6a-6f), again suggestingthat the in central Costa Rica have remained relatively constant mesoscaledata reflect the modern deformationregime. While throughoutthe Neogene and Quaternary. manyfaults may haveoriginated sometime in the past,these 484 MARSHALL ET AL.' FAULT KINEMATICS, COSTA RICA

710ø 00'

Figure 6. (continued)

features have since inherited the active kinematics. Section 7 MAT and the NPDB, the most damaging historical events discussesthe relationship of active seismicity along the within the heavily populatedValle Central have been moder- CCRDB and the tectonics of shallow subduction. ate (M=5.0-6.5), shallow(<25 km), upper plate earthquakes along the CCRDB. Costa Rica's historical seismicity 7. Seismicity and GPS Data [Gonz•ilez,1910; Peraldo and Montero, 1994] showsseveral periodsof heightenedearthquake activity across the volcanic 7.1. Earthquake Cycles arc following large subductionearthquakes [Giiendel and Recurring cycles of heightened seismicity across the Pacheco, 1992; Montero and Alvarado, 1995]. These periods CCRDB provide compelling evidence for an active tectonic of triggeredseismicity are interspersedwith timesof relative link between the Middle America Trench and the North quiescence.Such cycles of seismicenergy release along faults PanamaDeformed Belt [Giiendel and Pacheco, 1992]. While of the CCRDB may reflectthe arcwardtransfer of convergent large (M>7.0) thrustearthquakes are commonalong both the stressproduced by shallow subduction. MARSHALL ET AL.' FAULT KINEMATICS, COSTARICA 485

'0 ;

-f

Quiepo s.

'":•':'ø ::::::::::::::::::::::3 .'•:.:• • --- ' .....::':"'A :f•¾' .."..;•::'""':'.'•..':.

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

84ø 1'5•

Figure 6. (continued)

Figure 7. Examplesof P axis contourplots for mesoscalefault populations.Nearly all populationsin the studyarea show relatively concentrated clusters of P axescentered about an averagetrend and plunge.This suggeststhat strainkinematics have remained relatively constant since faulting began. Numbers are keyedto Table 2 and Figures5a-5c. 486 MARSHALL ET AL.: FAULT KINEMATICS, COSTA RICA

Table 3. EarthquakeFocal Mechanism Data

Focal Depth, Mechanism Date MagnitudeS- Latitude Longitude. km P axis T axis Reference*

A•'•- Jan.21-23, 1989 M =3.6 9ø 56' 84ø 35' 10.9 246, 58 151, 03 5 B Sept.26, 1994 M =4.3 9ø 47' 84ø 31' 28.1 089, 44 352, 07 2 C Feb. 9, 1993 M =2.8 9ø 49' 84ø 22' 7.3 208, 13 318,57 2 D June 30, 1990 M =4.5 9ø 49' 84ø 21' <15.0 011, 02 280, 28 6 E June 30, 1990 M =5.0 9ø 51' 84ø 23' 9.4 188, 04 280, 30 6 F June30, 1990 M =4.5 9ø 53' 84ø 23' <15.0 247, 14 151,21 6 G June 30, 1990 M =4.5 9ø 54' 84ø 21' <15.0 209, 01 299, 36 6 H June 16, 1990 M =4.5 9ø 52' 84ø 20' 14.1 184, 09 293, 63 2 I Dec. 22, 1990 M =5.7 9ø 53' 84ø 20' 14.6 020, 00 110, 40 2 J June 9, 1990 M =4.5 9ø 53' 84ø 19' 7.2 008, 29 272, 10 6 K June 8, 1990 M =4.8 9ø 52' 84ø 20' 8.8 208, 09 112, 34 2 L May 29, 1990 M =4.7 9ø 50' 84ø 18' 16.1 195, 02 105,25 2 M•'•' April 1980-Nov. 1981 M <3.0 9ø 53' 84ø 17' <15.0 214, 51 072, 32 8 N•'•' April 1980-Nov. 1981 M <3.0 9ø 54' 84ø 16' <15.0 194, 10 286, 10 8 O Date unknown, 1995 M =4.7 9 ø 43' 84ø 14' <15.0 194, 17 285, 03 2 P Feb. 26, 1989 Ms =4.8 9ø 40' 84ø 11' 38.0 115, 11 206, 03 1 Q Jan. 12, 1993 M =3.3 9ø 37' 84ø 07' 22.3 134, 09 243, 63 2 R Jan. 10, 1993 M =3.6 9ø 36' 84ø 06' 26.9 339, 14 082,42 2 S Jan. 23, 1993 M =3.1 9ø 35' 84ø 07' 28.0 265, 75 085, 15 2 T Aug. 17, 1982 Ms =5.5 9ø 34' 84ø 04' 37.0 194, 23 073, 50 1 U Aug. 3, 1993 M =4.0 9ø 33' 84ø 10' 30.0 287, 28 193, 10 2 V Jan. 8, 1993 M =3.3 10ø 00' 84ø 13' 5.5 000, 90 310,0 2 W Jan. 10, 1993 M =3.5 9ø 59' 84ø 12' 6.8 345, 83 120, 05 2 X Feb. 13, 1993 M =4.1 9ø 59' 84ø 10' 14.1 040, 70 220, 20 2 Y Jan. 20, 1993 M =3.7 9ø 59' 84ø 11' 11.6 355, 30 105, 30 2 Z Nov. 3, 1992 M =4.1 9ø 55' 84ø 08' 6.5 360, 76 133, 10 2 AA Nov. 3, 1992 M =3.3 9ø 56' 84ø 08' 11.1 181, 30 338,59 2 BB Jan. 30, 1993 M =3.4 9ø 58' 84ø 06' 16.2 037, 55 207, 35 2 CC•-•- July 1976-June1979 M <4.0 9ø 59' 84ø 02' <15.0 351, 30 101, 30 7 DD Dec. 2, 1992 M =4.8 9ø 59' 84ø 00' 17.8 166, 50 336, 40 2 EE Nov. 21, 1992 M =3.5 10ø 02' 84ø Off 13.2 230, 65 090, 19 2 FF Sept.2, 1993 M =3.2 10ø 03' 83ø 59' 14.0 348, 62 062, 118 2 GG•'•' July 1976-June1979 M <4.0 9ø 48' 84ø 15' <15.0 011, 00 101, 00 7 HH$•' April 1980-Nov. 1981 M <3.0 9ø 50' 84ø 08' < 15.0 200, 00 110, 14 8 II Jan. 11, 1994 M =3.5 9ø 49' 84ø 09' 16.8 209, 20 360, 68 2 JJ•-•- April 1980-Nov. 1981 M <3.0 9ø 48' 84ø 08' <15.0 190, 00 100, 14 8 KK•-•- April 1980-Nov. 1981 M <3.0 9ø 52' 84ø 05' <15.0 008, 02 098, 12 8 LL Sept.29, 1994 M =3.3 9ø 52' 84ø 04' 6.6 030, 01 120, 10 2 MM Aug. 9, 1991 Ms =4.7 9 ø 44' 84ø 03' 15.0 203, 00 293, 00 1 NN•-•- July 1976-June1979 M <4.0 9ø 46' 83ø 59' <15.0 216, 04 125, 10 7 OO Nov. 12, 1992 M =3.1 9ø 45' 84ø 01' 14.9 209, 20 360, 68 2 PP June 13, 1992 M =3.9 9ø 41' 84ø 00' 1.4 229, 74 121, 05 2 QQ April 24, 1994 M =2.9 9ø 43' 83ø 57' 21.3 344, 58 106, 18 2 RR March 19, 1993 M =3.2 9 ø 50' 83ø 58' 15.9 294, 69 098, 20 2 SS•'• July 1976-June1979 M <4.0 9 ø 52' 83ø 58' <15.0 063, 46 192, 31 7 TT June1982-Sept. 1982 M <3.2 9ø 57' 83ø 51' <15.0 150, 42 300, 44 3 UU Nov. 2, 1992 M =3.4 9ø 53' 83ø 46' 6.2 007, 52 127,22 2 VV Nov. 18, 1992 M =2.7 9ø 44' 83ø 50' 7.6 087, 40 354, 03 2 WW July 14, 1993 M =3.9 9ø 43' 83ø 49' 6.7 215, 80 305, 00 2 XX May 7, 1993 M =3.7 9ø 42' 83ø 46' 3.8 200, 30 100, 17 2 YY Sept.23, 1993 M =3.6 9ø 42' 83ø 42' 11.7 037, 52 156, 22 2 ZZ July 10, 1993 M =3.0 9ø 46' 83ø 42' 14.8 033, 79 149, 05 2 AB July 8, 1993 M =4.4 9ø 48' 83ø 42' 8<17 007, 04 098, 24 9 AC July 10, 1993 M =5.3 9 ø 46' 83ø 41' 13.2 075, 35 179,20 11 AD July 18, 1993 M =2.9 9 ø 45' 83ø 39' 12.8 217, 33 076, 50 2 AE July 11, 1993 M =3.4 9ø 46' 83ø 39' 15.4 359, 72 237, 10 2 AF Jan. 31, 1988 M =4.5 9ø 46' 83ø 38' 8<17 004, 22 094, 01 9 AG Nov. 19, 1987 M =4.3 9ø 42 '• -- 83ø 35' 8<17 014, 18 105, 04 9 MARSHALL ET AL.' FAULT KINEMATICS, COSTA RICA 487

Table 3. (continued) Focal Depth, Mechanism Date MagnitudeS' Latitude Longitude km P axis T axis Reference* AH May 14, 1991 M =4.2 9ø 52' 83ø 32' 21.0 035, 13 128, 09 1 AI April 24, 1991 Ms =6.1 9ø 45' 83ø 31' 23.0 009, 13 100, 04 1 AJ May 14, 1991 M =3.1 9ø 38' 83ø 34' 12.7 193, 20 103, 01 10 AK May 17, 1991 M =3.0 9ø 36' 83ø 33' 4.2 191, 09 097, 23 10 AL March 7, 1983 Ms =5.7 9ø 35' 83ø 40' 12.0 192, 21 283, 06 4 AM Dec. 21, 1993 M =2.8 9ø 30' 83ø 38' 7.4 071, 46 177, 15 2 * Referencesare asfollows: 1, Fan et al. [1993]; 2, Fernc•ndez[1995]; 3, G•endel [1985]; 4, G•endel [1986]; 5, G•endelet al. [1989];6, G•endelet al. [1990];7, MonteroandDewey [1982]; 8, Monteroand Morales [1984]; 9, OVSICORI-UNA [1993]; 10, Protti andSchwartz [1994]; 11, Ramirezet al. [1993]. •' M, local magnitude;Ms, surface-wavemagnitude '•'• Compositefocal mechanism.

After the 1950 M=7.7 Nicoya subductionearthquake on the ture zone showedmostly thrust mechanisms,however, a clus- MAT, a 5-year series of moderate events occurred across ter of shallow events centered arcward of Puerto Lim6n central Costa Rica [Montero and Alvarado, 1995]. Similarly, showed mostly oblique strike-slip along steep NE striking the 1983 Mw=7.5 (Ms=7.3) Golfo Dulce subductionearth- faults. Mapped surfaceruptures in this area showedsignificant quake triggered a cycle of moderate events within the strike-slip offsets [Denyer et al., 1994]. These observations Cordillera de Talamanca [Adarneket al., 1987; Tajirna and imply an onland extensionof the NPDB and a transitionfrom Kikuchi, 1995]. Most recently,a period of increasedseismicity reversemotion to transcurrentfaulting along the CCRDB. beganin centralCosta Rica with the 1990 Mw=7.0 (Ms=6.9) Similar to the 1990 Cobano earthquakeon the Pacific coast, Cobano subductionearthquake on the Pacific coast and sub- the 1991 Valle de la Estrella earthquaketriggered a seismic sided in the wake of the 1991 Mw=7.7 (Ms=7.5) Valle de la sequencefarther inland along a diffuse array of faults through Estrellaback arc thrustevent on the Caribbeancoast (Figure the Cordillera de Talamanca and the Valle Central. These 8) [Gaendel and Pacheco, 1992]. The discussionin section eventslasted for severalyears and showedoblique strike-slip 7.2 of the 1990-1993 seismicsequence provides important in- mechanismson mostly NE and NW orientedfaults [Protti and sights into the seismotectonicsof the CCRDB and its link to Schwartz, 1994; Gaendel et al., 1995]. In 1993 a seismic shallow subductionalong the Pacific margin and back arc swarm, culminatingwith an M=5.3 event (Figure 8), occurred convergencewithin the NPDB. within the Turrialba region centered around the conjugate Pejibaye and Gato faults (Figure 4, Faults 40 and 41). Similar 7.2. The 1990-1993 Seismic Sequence to previous events in this area, these earthquakes showed mostly transcurrentmotion on NE and NW orientedfaults. In 1990, five moderate subductionzone earthquakesoc- Overall, the 1990-1993 seismic sequenceruptured a diffuse curredalong the lengthof the MAT from Nicaraguato Panama. The first and largest of these events,the Mw=7.0 (Ms=6.9) array of transcurrentfaults spanningthe volcanic arc between Cobano earthquake, was centered offshore of central Costa the epicenters of the Cobano and Valle de la Estrella Rica, SE of the Peninsulade Nicoya, directly above the pro- earthquakes.This sequenceof triggered events demonstrates jected trend of the subducting rough-smooth boundary an active tectoniclink betweenthe MAT and the NPDB along (Figure 8) [Protti et al., 1995b]. This shallow subduction faults of the Central Costa Rica Deformed Belt. Repeatedseis- mic cyclesalong the CCRDB may reflect the transferof conver- earthquakeimmediately triggered a seismic swarm 60 km inland along the CCRDB within the Cordillera de Aguacate gent stressproduced by shallow subductionfrom the Pacific to the Caribbeanmargin. [Gaendel et al., 1990; Barquero et al., 1991; Gaendel et al.,

1995]. This swarm continuedfor severalmonths along a con- 7.3. GPS Measurements jugate system of NE striking left-lateral and NW striking right-lateralfaults (Figures 6a and 6b). The seismicsequence Recent motion of the Panama block relative to adjacent culminatedwith the 1990 Ms=5.7 Puriscal earthquake(Figure plateshas been constrainedby the Central and SouthAmerica 8), which producedconsiderable damage in the Valle Central. (CASA) Global Positioning System (GPS) campaigns initi- Four months later, the 1991 Mw=7.7 (Ms=7.4) Valle de la ated in 1988 (Figure 1) [Kellogg and Vega, 1995]. Estrella thrust earthquakeruptured the westernmostsegment Measurementsbetween Panama (Panama City) and two sites in of the North Panama Deformed Belt (Figure 8) [Fan et al., Colombia (Cartegena and Bogotfi) suggestongoing collision 1993; Goes et al., 1993; Protti and Schwartz, 1994; Suc•rez betweenthe Panama and North Andes blocks at rates ranging et al., 1995]. Both local and far-field seismicity outlined a from 8 to 21 mm/yr [Kellogg and Vega, 1995]. GPS-basednu- shallow SW dipping fault, consistentwith NE thrusting of merical modeling for regional deformation indicatesthat the the Panama block over the Caribbeanplate. Coseismicuplift Panama block moves northward over the Caribbean plate at suggestedabrupt termination of this ruptureat its NW edge at increasing rates (10-20 mm/yr) toward the west along the Puerto Lim6n [De Obaldia et al., 1991' Plafker and Ward, NPDB from the margin of South America [Lundgren and 1992; Denyer et al., 1994]. Aftershockswithin the main rup- Russo, 1995]. These observationsare consistentwith clock- 488 MARSHALL ET AL.' FAULT KINEMATICS, COSTA RICA

N GPS data 19•94•96

20 mrn/yr CARlB (95% ellipse) o 5o lOO M 5.7 M 7.6 12/90 4/91 CCR[

mm/y r

9 ø .. 7/9 3' •M 3:/90

8-4..o

A A' B B' C C'

Figure 8. Tectonicmap of the CCRDB showinga summaryof mesoscalefault data from the three kinematicdomains(D1, D2, andD3), focal mechanisms from the 1990-1993 earthquake sequence, GPS data for 1994-1996, Cocos plate bathymetry including the rough-smoothboundary (RSB), and the subductingslab along three margin-perpendicularcross sections. The shadedarea outlinesthe CCRDB along the westernmargin of the Panamablock (dashedsolid line). See Figure 2 for on-landgeology. The summaryfault plane solutions(D1, D2, and D3) combineall mesoscalefault data for eachkinematic domain (see Figures5a-5c for completedata). Earthquakefocal mechanisms(left to right) from: Protti et al. [1995b], Ferndndez [1995, 1996], Ramirez et al. [1993], and Goes et al. [1993]. The thin dashedlines show locationsof the Wadati-Benioff zone crosssec- tions depictedin boxes below the map [from Protti et al., 1995a]. The small arrows show GPS displacement vectors(scale at top right) with respectto a fixed Caribbeanplate [Lundgrenet al., 1999].

wise rotation of Panama, thrusting along the NPDB, and southward across the faults of the CCRDB. These observations sinistral shear across central Costa Rica [Lundgren and may reflectNE displacementof the westernPanama block to- Russo, 1995]. ward the back arc NPDB. Such short-term data should be Resultsfrom the Costa Rica (CORI) GPS project (1994 and viewed with caution, however, considering their temporal 1996) provide local constraintson active deformation across proximity to the postseismicstage of the 1990-1993 earth- the CCRDB [Lundgren et al., 1999]. These results(Figure 8) quakecycle alongthe CCRDB. Thesevelocity vectorsmay re- indicate up to 30 mm/yr of sinistral shear between sites in flect postseismicdeformation related to the 1991 Valle de la northern Costa Rica on the Caribbean plate and sites within Estrella earthquakeand interseismicstrain along the southern the actively deforming Panama block to the south. The GPS MAT superimposedon secular motions of the Panama block measurements show increasing sinistral shear moving [Lundgrenet al., 1999]. MARSHALL ET AL.: FAULT KINEMATICS, COSTA RICA 489

8. Tectonic Interpretation Overall, therefore,the kinematicsof active faulting along the CCRDB may be understoodas the combinedresult of hori- The location of the Central Costa Rica Deformed Belt within zontal shortening and shear due to ridge indentation [e.g., the overriding volcanic arc correspondswith the position of Taylor et al, 1995], crustal displacementfrom possible in- the subductingrough-smooth boundary on the Cocos plate creasedbasal traction due to shallow subduction[e.g., Bird, offshore(Figure 8). This relationshipimplies a geneticlink be- 1998], and localized forearc uplift controlled by seamount tween shallow subduction of thickened oceanic crust and ac- subduction [e.g., Fisher et al., 1998]. While many faults tive deformationacross the volcanic arc. As suggestedby pre- within the CCRDB may have originated prior to indentation vious authors,transcurrent faulting acrosscentral Costa Rica of the Cocos Ridge [e.g., Astorga et al., 1991, Arias and may reflect sinistralshear along the NW flank of the indenting Denyer, 1991], they have sinceinherited the kinematicsof dis- Cocos Ridge [Montero, 1994; Kolarsky et al., 1995]. In the tributed shorteningand sinistral shear associatedwith shal- model presentedby Kolarskyet al. [1995] the CCRDB would low subduction of thickened oceanic crust. representa cross-arcfault zone similar to thoseobserved in Some have argued that the ridge indentationmodel is in- other ridge subductionsettings [e.g., Taylor et al., 1995]. compatiblewith the idea of a microplateboundary shear zone Transpressionwhere the CCRDB mergeswith the back arc acrosscentral Costa Rica [Montero, 1994; Fernc•ndez, 1996]. NPDB would reflect horizontal shorteningdirectly above the We suggestthat these two ideas are not mutually exclusive if ridge axis. This model also impliesthat a similar dextral shear the CCRDB is viewed in generalterms as the westernlimit of zone must exist SW of the subductingridge in Panama,a ques- deformation within the Panama block. Even if ridge indentation that remainsopen for further investigation. tion plays a primary role in deformation,the CCRDB still de- In additionto the ridge indentationmechanism, we suggest fines a tectonic boundary between the actively deforming that shallow subductionof thickened oceanic crust through- Panama block and the relatively stable Caribbean plate in out the rough domain (not just limited to the Cocos Ridge) northern Costa Rica. may increasebasal traction on the overridingplate, resulting Interestingly,shallow subductionof the CocosRidge at the in distributedhorizontal shorteningand NE displacementof MAT may, in an indirect way, contribute to the onland the westernPanama block toward the back arc NPDB (Figure propagationof the NPDB in easternCosta Rica. Indentationof 8). Regardlessof the precisemechanism, we suggestthat active the Cocos Ridge beneath southern Costa Rica drives rapid faulting within the CCRDB shouldbe viewed in generalterms uplift of the Talamancaarc. Acceleratedtopographic erosion of as a deformationfront that has propagatedinto the volcanicarc the range has strippedoff overlying extrusiverocks, exposing along the NW limit of shallow subduction(Figure 8). the intrusviecore (Figure 2). This erosionhas generateda ma- The rough domain of the subducting plate originated jor pulseof sedimentationwithin the CostaRica fan offshoreof through hotspotactivity along the E-W oriented Galapagos the Caribbean coast. Silver et al. [1995] argue that sediment Rift [Werner et al., 1999] (Figure 1). Shallow subductionat loadingof the offshoreslope may have forcedthe thrustfront of the MAT may be controlledprimarily by increasedbuoyancy the NPDB onshoreat Puerto Lim6n (Figures 2 and 3). In this associatedwith hotspotthickening of the oceaniccrust. This manner,we suggestthat subductionof the CocosRidge along buoyanteffect is maximizednearest the CocosRidge [Gardner the Pacific margin may indirectlyinfluence the geometryof the et al., 1992] and may be amplified by decreasedplate age to- NPDB along the Caribbeanmargin and hence its onshoreex- ward the SE [Prottiet al., 1995a]. Because the subducting tension along the CCRDB. plate becomesshallower approachingthe NW flank of the While the western Panama block convergesnortheastward CocosRidge (Figure 8), the zone of crustalshortening extends with the NPDB in southernCosta Rica, this motion gradually progressivelyfarther inland along an E-W zone acrosscentral rotates to northward displacement along the arcuate NPDB Costa Rica. Thus, as the seamountdomain and Cocos Ridge farther to the east in Panama. Within the East Panama subduernortheastward at the trench, an E-W trending defor- Deformed Belt the senseof conjugatefaulting is reversed,with mation front, forward of their leading edge, propagatesnorth- NE striking right-lateral and NW striking left-lateral faults al- ward into the overriding volcanic arc. lowing for oroclinal bending and northward thrusting of the Conjugatestrike-slip faults (NW and NE) of the CCRDB arc into the back arc basin acrossthe arcuate NPDB [Mann (domains2 and 3) allow for north directedhorizontal shorten- and Kolarsky, 1995]. This deformationreflects collision of the ing alongthe deformationfront abovethe NW flank of the in- arc with South America to the east. dentingridge (Figure 8). This zone also accommodatesdiffuse sinistral shear as fault-bounded blocks are displaced northeastward toward the NPDB. Transpressionobserved within 9. Conclusions the back arc (domain3) reflectsmerging of the CCRDB with the thrust faults of the NPDB above the axis of the subducting 1. As defined here, the Central Costa Rica Deformed Belt CocosRidge. Within the forearc(domain 1) the effectof shal- (CCRDB) is a diffuse zone of active faulting that marks the low subductionis overprintedby local deformationrelated to western margin of the Panama block (Figure 8). This 70 to isolatedseamounts. Steep margin-perpendicular normal faults 100-km wide zone extends across the Costa Rican volcanic may reflectvertical kinematics within forearcblocks overrid- arc, linking the North PanamaDeformed Belt (NPDB) on the ing subductingseamounts. To the SE the indentingCocos Caribbean coast with the Middle America Trench (MAT) on Ridge drives uplift and horizo•ltal shorteningwithin the the Pacific coast. Terrababelt (Fila Costefia)and may accentuatebasal traction 2. The intersection of the CCRDB with the Pacific forearc on the overridingplate throughincreased coupling along the correspondswith the location of the rough-smoothboundary root of the Cordillera de Talamanca. (RSB) on the subductingCocos plate offshore. Shallow sub- 490 ,MARSHALLET AL.' FAULT KINEMATICS, COSTA RICA

duction of thickened oceanic lithosphere (Cocos Ridge and as young as 400 ka [Marshall and Ildeman, 1999]. In seamountdomain) SE of the rough-smoothboundary extends addition, observedoffsets of late Quaternary fluvial terraces, crustal shorteninginto the overriding volcanic arc and dis- wave-cut platforms, and soils attest to the active nature of placesthe westernPanama block toward the back arc NPDB. faulting. 3. Active faulting along the CCRDB representsthe arcward 7. Repeatedearthquake cycles along faults of the CCRDB propagationof a deformation front along the NW limit of demonstrate an active tectonic link between shallow subduc- shallow subduction.Horizontal shorteningat the deformation tion at the MAT and back arc thrusting along the NPDB. front and differential shear between northern and southern Focal mechanismsagree with fault populationdata, suggest- Costa Rica are accommodatedby transcurrentfaulting along ing that the observedmesoscale faults characterizethe modern the CCRDB. kinematics. 4. Fault kinematics along the CCRDB vary acrossthree 8. Global Positioning System (GPS) data are consistent domains:(1) the forearc, (2) the central volcanic arc, and (3) with sinistral shear across the CCRDB and northeastward the back arc. Where the CCRDB intersectsthe forearc(domain convergenceof southernCosta Rica with the NPDB. 1) between Puntarenasand Quepos, mesoscalefault popula- 9. While many faults of the CCRDB may have originated tions express sinistral transtensionacross steep NE striking prior to Cocos Ridge indentation,they have subsequentlyin- faults that accommodatedifferential uplift of forearc blocks.To herited the kinematics of deformation associated with shallow the southeast of Quepos, shallow subduction of the Cocos subduction of thickened oceanic crust. Ridge produces flexural uplift and horizontal shortening 10. The ridge indentationand microplateboundary models within the Terraba thrust belt. Inland, within the central vol- for central Costa Rica are compatibleif the CCRDB is viewed canic arc (domain 2), the CCRDB encompassesa conjugate in generalterms as a deformationfront at the westernedge of systemof NW and NE striking transcurrentfaults. Mesoscale the Panama block. fault kinematicsdemonstrate primarily dextral slip with minor shorteningon NW striking faults and sinistral slip with mi- Acknowledgments. We are very grateful to F. Gtiendel,M. Protti, and E. Malavassi(Observatorio Volcano16gico y Sismo16gicode Costa nor extensionon NE striking faults. In the back arc (domain Rica, Universidad Nacional) and P. Denyer, W. Montero, and M. 3), mesoscalefaults show increasedtranspression and crustal Fernfindez(Escuela Centroamericanade Geologia, Universidad de thickening where conjugate regional-scalefaults merge with CostaRica) for insightfuldiscussions and for earthquakefocal mecha- thrust faults of the NPDB. nisms.P. Lundgren(Jet PropulsionLaboratory) made significant contri- 5. The observedkinematic variations along the CCRDB butionsto this studyby providinga tour of the CostaRica (CORI) GPS networkand by sharingGPS data. We alsothank B. Idleman(Lehigh reflect the combinationof three principal deformationmecha- University) for critical radiometric analysesand R. Allmendinger nisms:(1) horizontalshortening and shearfrom oceanicridge (Cornell University) for the kinematicssoftware used to analyzethe indentation,(2) increasedbasal traction from shallow subduc- fault data [/tllmendingeret al., 1994]. We also appreciatethe valuable tion, and (3) localized forearc block uplift from subducting field assistanceof R. Seelbach(University of California,Santa Cruz) and the logistical supportof F. Rudin, L. Valverde, and L. Chavez seamountroughness. (Instituto GeogrfificoNacional de Costa Rica). Finally, we thank P. 6. Active regional-scalefaults along both the Valle Central Mann, R. yon Huene, and Editor D. Schollfor helpful commentson this andthe centralPacific coast displace rocks dated (4øAr/39Ar) manuscript.This researchwas fundedby NSF grantEAR-9214832.

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