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

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 na- ture 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 NICARAGUA \ MAR 8 mm/yr ß ee NAN 21mm/yr/ SOAM :GA.EAPAGOS COLOMBIA 16 mm/yr ?:i•::•:•:.•:',•,:,•:,:•:•':':.i',:::?:::....'...... 80 ø '•' - 75ø ............... .. 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 rectangle shows 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 N -o CARlB. 1.0.-0 km 9 o .91 mm/yr • domain -85 ø 84 ø -83 Qua ternary ß...-....• Quaternaryvolcanic arc ::';,'•-•'• Neogene intrusive rocks • sediments volcanicNeogene arc ;i!'...".'71::oceanicCretaceous basalts - Paleogene& sediments sedimentsNeogene i i Figure2. Tectonicmap of CostaRica showing the on-landgeologic structure relative to the offshore bathymetry(contours

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