TECTONICS, VOL. 19, NO. 3, PAGES 468-492 JIfNE 2000
Central
Costa
Rica
deformed
belt:
Kinematics of diffuse faulting acrossthe western Panama block
JeffreyS. Marshall
1 andDonaldM. Fisher
Departmentof Geosciences,
PennsylvaniaStateUniversity,UniversityPark
Thomas
W. Gardner
Departmentof Geosciences,
Trinity University,SanAntonio,Texas
Abstract.
Fault kinematics, seismicity, and geodetic data
acrosscentral Costa Rica reveal a diffuse fault zone, here named
the Central Costa Rica Deformed Belt (CCRDB). The CCRDB
definesthe westernmargin of the Panamablock and links the
North Panama Deformed Belt (NPDB) along the Caribbean
coast with the Middle America Trench (MAT) along the
Pacific coast.The junction of the CCRDB and the MAT coincides with an abrupt transition from smoothto rough crust on
the subducting Cocos plate (rough-smooth boundary).
Shallow subductionof rough, thickenedoceaniccrust associated with the Cocos Ridge shifts active shorteninginto the
volcanic arc along faults of the CCRDB. Variable fault kinematicsalongthis zone may reflect three combineddeformation
mechanisms:horizontal shortening and shear from oceanic
ridge indentation,basal traction from shallow subduction,and
localized block uplift from subductingseamountroughness.
Within the forearc (domain 1), mesoscale faults express
transtensionwhere steepNE striking regional-scalefaults intersect the Pacific coast. Across the volcanic arc (domain 2),
mesoscalefaults exhibit mostly sinistral and dextral slip on
NE and NW striking conjugate faults, respectively.
Approachingthe NPDB in the back arc (domain 3), transcurrent faulting is modified by transpressionand crustalthickening. Fault kinematics are consistent with earthquake focal
mechanismsand Global Positioning System (GPS) measurements. Radiometric age constraintsconfirm that faulting postdatesthe late Neogeneonsetof shallow subduction.The ensuing deformationfront has propagatednorthwardinto the volcanic arc to its present position along the seismically active
CCRDB. Within the forearc, the effect of shallow subductionis
overprinted by local uplift related to underthrusting
seamounts.
1. Introduction
The Panama-northernColombia region of Central and South
America spansa complex deformationzone betweenfour actively convergingtectonicplates:Caribbean,SouthAmerican,
Cocos, and Nazca (Figure 1). Plate motions are partitioned
1NowatDepartment
of Geosciences,
Franklin
andMarshall
College,
Lancaster,Pennsylvania.
acrossa diverse array of actively evolving fault zones that
spanseveralindelSendent
crustalblocksor microplates
(e.g.,
Panama, North Andes, and Maracaibo) [Mann and Burke,
1984, Mann et al, 1990]. These crustalfragmentsfunctionas
broad deformationzonesthat accommodatethe complicated
kinematicsof distributedplate boundarydeformation.
This studyexaminesthe kinematicsand the tectonicorigin
of faulting along the westernmargin of the Panamablock in
central Costa Rica. The Panama block consists of the southern
end of the Central
American
volcanic
arc that has detached
from the Caribbean plate owing to collision with South
America (Figure 1). Along Panama'seasternmargin,convergence with South America is accommodatedalong diffuse
transpressionalfaults of the East Panama Deformed Belt
[Mann and Kolarsky, 1995] and by uplift of the Colombian
Cordillera within the North Andes block [Kellogg and Vega,
1995]. This collision also drives oroclinal bending and
northwardthrustingof Panamaover the Caribbeanplate along
the North Panama Deformed Belt [Wadge and Burke, 1983;
Vergara-Muhoz, 1988; Silver et al., 1990, 1995]. To the
south,oblique convergencebetweenthe northernNazca plate
and the Panama block occurs along the South Panama
Deformed Belt [Mackay and Moore, 1990; de Boer et al.,
1991; Westbrook et al., 1995; Moore and Sender, 1995;
Kolarskyand Mann, 1995]. While theseconvergent
zonesdelineate the eastern, northern, and southern boundariesof the
independentPanama block, the kinematicsof deformation
alongthe westernmarginhaveremainedpoorlyconstrained.
Here
we define
the Central
Costa Rica
Deformed
Belt
(CCRDB) as a diffusezoneof seismicallyactivefaultingacross
centralCostaRica that marksthe westernmarginof the Panama
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
faults, historic seismicity, and Global Positioning System
(GPS)-measured
crustaldisplacements.
We examinefault population data from 86 outcropsin conjunctionwith earthquake
focal mechansimsin order to evaluatethe spatialvariationsin
fault kinematics across central Costa Rica. In addition, we es-
tablishage constraints
for offsetQuaternaryunitsand confirm
that faulting along the CCRDB is active. Finally, we assess
the kinematicsand timing of faultingwithin the contextof regionaltectonicsin orderto explorethe potentialcausesof this
deformation.
Copyright
2000bytheAmerican
Geophysical
Union.
The observations made in this study indicate that the
Papernumber1999TC001136.
CCRDBrepresent.
s a deformation
frontthathaspropagated
0278-7407/00/1999TC001136512.00
into the volcanicarc in response
to the shallowsubduction
of
468
MARSHALLET AL' FAULT KINEMATICS,COSTARICA
thickened oceanic lithosphere associated with the Cocos
Ridge and seamountdomain on the Cocos plate (Figure 2).
Faults of the CCRDB accommodatediffuse crustalshortening
and sinistralshear acrossthe volcanic arc, allowing for northeastward displacementof the western Panama block toward
the back arc North Panama Deformed
2. Tectonic
Belt.
Framework
469
ther did not discussa westernboundary[Adameket al., 1988]
or loosely associated it with such features as the Panama
Fracture Zone [Bowin, 1976; Vergara-Muhoz, 1988] or NW
trending faults traversingPanama [Mackay and Moore, 1990;
Mann and Corrigan, 1990; de Boer et al., 1991]. However,
recent investigationsof regional stratigraphy [Astorga et al.,
1991; $eyfried et al., 1991], tectonic geomorphology
[Gardner et al., 1993; Marshall et al., 1995], fault kinematics
[Marshall et al., 1993; Fisher et al., 1994], seismicity [Jacob
et al., 1991; Giiendel and Pacheco, 1992; Goes et al., 1993;
The Central American volcanic arc is generatedby sub- Fan et al., 1993], and geodetics[Lundgrenet al., 1993, 1999]
ductionof the Cocosplateat the MiddleAmericaTrenchalong have recognizeda diffuse region of active faulting acrosscenthe southwestern
marginof the Caribbeanplate. The Panama tral Costa Rica. This broad deformation zone extends onland
block consistsof the southernportionof the arc, extending from the NPDB along the Caribbean coast (Figures 2 and 3),
from the margin of SouthAmerica in the east,to centralCosta
Rica in the west(Figure 1). This independent
blockspansthe
CretaceousChorotega and Choco oceanic basementterranes
and encompassesseveral Cenozoic volcanic cordilleras and
uplifted sedimentarybasinsthat exposePaleogenedeepmarine and Neogene-Quaternaryshallow marine, volcaniclastic,
and fluvial sediments[Escalante,1990]. Late Neogenecollisionwith SouthAmerica upliftedthe Panamaarc and createda
landbridgethat closedthe Caribbean-Pacific
seaway[Kellogg
and Vega, 1995].
Until recently, little attention had been focused on the
westernboundaryof the Panamablock. Previousresearchei-
traversesthe volcanic arc and heavily populatedValle Central,
and
intersects
the
Pacific
coast
between
Puntarenas
NICARAGUA
\
MAR
8 mm/yr
ß
ee
NAN
21mm/yr/ SOAM
:GA.EAPAGOS
?:i•::•:•:.•:',•,:,•:,:•:•':':.i',:::?:::....'
......
80ø
COLOMBIA
16 mm/yr
'•' - 75ø
...............
..
Figure 1. Tectonic setting of southernCentral America showingthe Central Costa Rica Deformed Belt
(CCRDB) along the western margin of the Panamablock (PAN). The CCRDB links the North Panama
DeformedBelt (NPDB) with the Middle AmericaTrench,and is locatedonshoreof the rough-smooth
boundary on the subducting
Cocosplate(COCOS).Largearrowsshowplatemotionsrelativeto 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
Americanplate, MAR; Maracaiboblock; NAN, North Andes block; EPDB, East PanamaDeformedBelt;
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.
and
Queposto meet the MAT south of the Peninsulade Nicoya
[Fisher and Gardner, 1991; Marshall et al., 1993; Fisher et
al., 1994].
The location occupied by the CCRDB has long been
recognized as a major segment boundary along the Middle
America arc-trench system [Stoiber and Carr, 1973; Carr,
1976; Burbach et al., 1984]. This position within the
overriding plate correspondswith the location of the "roughsmoothboundary"[Hey, 1977] on the subductingCocosplate
offshore (Figure 1). The rough-smooth boundary follows a
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
ß
...-.
...• volcanic
Quaternary
::';,'•-•'•
Neogene
intrusive
rocks •
arc
Neogene
arc
volcanic
;i!'...".'71::
Cretaceous
- Paleogene
oceanic
basalts
& sediments
Qua ternary
sediments
Neogene
sediments
i
i
Figure2. Tectonic
mapof CostaRicashowing
theon-land
geologic
structure
relativeto theoffshore
bathymetry
(contours
in meters).
TheCCRDB(outlined
bydashed
lines)marks
thediffuse
boundary
between
thePanama
blockandtheCaribbean
plate.TheNW limitof theCCRDBalignswiththerough-smooth
boundary(RSB)onthesubducting
Cocos
plate.TheRSBseparates
smooth
crustto theNW fromroughcrust
(seamount
domain
andCocos
Ridge)to theSE.Thesubducting
Cocos
Ridgealigns
witha gapin theactive
volcanic
arc(asterisks),
withtheuplifted
intrusive
rocksof theCordillera
deTalamanca
(C Tim)andwiththe
inverted
sedimentary
basins
of theTerraba
thrustbelt(TB) andtheLim6nthrustbelt(LB). Therectangle
shows
theareaof Figure
3. MAT,MiddleAmerica
Trench;
C Gnc,Cordillera
deGuanacaste;
C Agt,Cordillera
deAguacate;
C Cen,Cordillera
Central;
Nicoya,Peninsula
deNicoya;Osa,Peninsula
deOsa.
linear morphologictrend on the subductingseafloorthat separates smooth,low-relief crust to the NW from rough, high-relief crustto the SE (Figure 2) [yon Huene et al., 1995]. Rough,
thickened seafloor SE of the boundary containsthe aseismic
Cocos Ridge and adjacent seamounts,productsof relatively
slow spreading and pervasive hotspot volcanism along the
GalapagosRift system[Holden and Dietz, 1972; yon Huene et
al., 1995; Barckhausen et al., 1998; Meschede et al., 1998;
Stavenhagenet al., 1998; Werner et al., 1999].
The CocosRidge, a primary trace of the Galapagoshotspot,
began subducting along the MAT sometime in the late
Neogene [Collins et al., 1995; Kolarsky et al., 1995;
Meschedeet al., 1999b]. Shallow ridge indentationdramatically decreasedthe subductionangle [Prottiet al., 1995a],
diminishedthe mantle wedge, and extinguishedarc volcanism
within the Cordillera de Talamanca [de Boer et al., 1995].
Rapid uplift and horizontal shorteningoccur within a broad
arch acrossthe ridge axis,.extendingfrom the forearcPeninsula
de Osa and Terraba belt, throughthe Talamancaarc, and into
the back arc Lim6n belt (Figure 2) [Corrigan et aL, 1990;
Gardner et al., 1992; Bullard,
1995; Collins et al., 1995;
Kolarskyet al., 1995].
To the NW of the Cocos Ridge, along the central Costa
Rican margin, the pronouncedimpact of ridge indentationdiminishes [Gardner et al., 1992]. Subducting seamounts,
which ornament the moderately thickened crust on the NW
ridge flank (Figure 2), deform the outer trench slope [yon
Huene et al., 1995; Barckhausen et al., 1998] and produce
differential uplift of forearc fault blocks [Marshall and
Anderson, 1995; Fisher et al., 1998]. Shallowing of the subducting slab beneathcentral Costa Rica, combinedwith possible trenchretreat by forearc erosion[e.g., Meschedeet al.,
1999a], has resulted in northeastwardmigration of the volcanicarc from the extinctCordillerade Aguacateto the modern
Cordillera Central (Figure 2) [Marshall, 1994; Marshall and
Idleman, 1999]. In this paper, we suggestthat shallow sub-
MARSHALL ET AL' FAULT KINEMATICS,
COSTA RICA
CI
Z
•
• o oo
471
472
MARSHALL ET AL.: FAULT KINEMATICS, COSTA RICA
duction also drives active faulting acrossthe volcanic arc
alongthe CCRDB, as deformationpropagates
inlandabovethe
NW flank of the indentingCocosRidge.
4. Regional-Scale Faults
4.1. Kinematic
Domains
In order to facilitate
3. Geology of Central Costa Rica
The Central Costa Rica Deformed
Belt traverses the width of
the volcanic arc, extendingfrom the Pacific forearc region to
the Caribbean
back arc. Faults of the CCRDB
offset a broad
range of lithologies,ranging from Cretaceous-Tertiaryseafloor
basementand marine sediments,to Quaternaryterrestrialsediments and extrusive rocks. For this study, we summarizethe
regional geology by defining several general lithostratigraphicunits shown in Figures 2 and 3.
Within the central Pacific forearc the upper Cretaceous
igneous basement (Nicoya Complex) is exposed along the
outer forearc high of the Peninsula de Nicoya [Lundberg,
1982] and within uplifted inner forearc blocks [Fisher et al.,
1998]. Flanking these forearc basementexposuresare upper
Cretaceous-Paleogene
deep marine and slope sedimentsderived from the early CostaRican arc [Lundberg, 1982]. Rocks
of the inner forearccoastalpiedmontincludeNeogeneshallow
marine to deltaic sedimentsof the Nicoya and Parrita basins
[Astorga et al., 1991], Neogene-Quaternaryvolcanic and volcaniclasticrocks derived from the Aguacatearc [Denyet and
Arias, 1991], and Quaternary fluvial and marine terrace deposits [Fisher et al., 1994].
The complexvolcanicarc of central CostaRica encompasses
the active Cordillera Central, as well as the extinct Cordilleras
de Aguacateand Talamanca(Figures 2 and 3). The Cordillera
Central consistsof a broad massif of Quaternary andesiticdacitic stratovolcanos. To the SW of the active cordillera,
heavily erodedremnantsof the extinct Cordillera de Aguacate
expose Neogene-Quaternary basaltic-andesite lavas and
pyroclastic flows (Aguacate Group) [Denyet and Arias,
1991]. The Valle Central basin, situated between the extinct
Aguacatearc and the active Cordillera Central, containsa volcanictablelandformedby the accumulationof a thick sequence
of Quaternaryandesiticto dacitic lavas, pyroclasticflows, and
lahar deposits[Denyet and Arias, 1991].
The Cordillera de Talamanca, SE of the Valle Central, cor-
respondswith a 175-km-wide gap in the active volcanic arc
and represents the only area of southern Central America
above 2000 m in elevation (Figures 2 and 3) [de Boer et al.,
1995; Kolarskyet al., 1995]. These ruggedmountainsexpose
a suite of Neogene-Quaternaryintrusive (principally granodiorites) and extrusive rocks (andesites)that are correlative in
age with the Aguacate arc to the NW. Rapid uplift driven by
Cocos Ridge subductionhas provoked extensiveunroofingof
the Talamancarange, exposingthe intrusivecore.
The intra-arc and back arc regions along the flanks of the
Cordilleras de Aguacate and Talamanca expose both
Paleogenedeep marine and Neogene shelf sedimentsof the
Candelariaand Lim6n basins(Figures 2 and 3) [Denyet and
Arias, 1991; Astorga et al., 1991]. Rocks of the Candelaria
basin were moderatelydeformedby homoclinaltilting during
the late Neogene[Denyet and Arias, 1991], while sediments
of the Lim6n basin have been extensively faulted and folded
within the back arc thrustsystemof the NPDB [Astorgaet al.,
1991].
our discussion of fault kinematics
we
have divided the study area into three generalizedkinematic
domains(Figure 4). The forearcregion (D l: domain 1) encompassesthe inner forearc (Pacific coastalpiedmont),as well as
portionsof the forearcbasin(Golfo de Nicoya) and outerforearc high (Peninsula de Nicoya). The central volcanic arc region (D2: domain 2) spansthe extinct Neogeneto Quaternary
volcanic arc (Cordilleras de Aguacate and Talamanca), the
Valle Central basin, and the southwestern flank of the active
arc (CordilleraCentral). This domainalso includesportionsof
the uplifted Tertiary-agedCandelariabasinalongthe flanks of
the extinct arc. The back arc region (D3: domain 3) encompassesthe Caribbeanslope of both the extinct arc (Cordillera
de Talamanca)and modernarc (Cordillera Central), as well as
uplifted portionsof the back arc basin(Lim6n basin).
Deformationalongthe CCRDB occursacrossa diffusearray
of regional-scalefaults (lengthsof severalkilometersto tensof
kilometers) that exhibit a range of orientationsand styles of
slip (Figures 3 and 4, and Table 1). Our discussionof fault
kinematics begins with a description of these regional-scale
features, based on a combination of field observations and a
compilationof existing geologicmap data.
4.2. Regional-ScaleFaults: Forearc (Domain 1)
The
CCRDB
intersects
the central
Pacific
coast between
Puntarenasand Quepos (Figures 2 and 3), southeastof the
projectedtrend of the rough-smoothboundaryon the subducting plate. Offshore seismic-reflectionprofiles reveal steepNE
striking margin-perpendicularfaults that show dip-slip offset
of late Cenozoic outer forearc shelf sediments (Figure 3)
[Barboza et al., 1995]. Onshore,a seriesof steepmargin-perpendicularfaults strike alongthe NE trendingvalleysof major
Pacific sloperivers includingthe Rios Barranca,JesfisMaria,
T•trcoles,Tusubres,and Parrita [Madrigal, 1970]. Thesemajor
faults(10-20 km in length)allow for differentialuplift of a system of inner forearc blocksreferredto as the Esparza,Orotina,
Herradura, Esterillos, and Parrita blocks (Figure 4) [Fisher et
al., 1994, 1998]. Uplift rates,estimatedfrom Quaternarymarine
and fluvial terraces, vary sharply across block-bounding
faults. These displacementsare consistentwith vertical offsets
observed in Late Cretaceous through Quaternary deposits.
The broadly distributed uplift generatedby the subducting
Cocos Ridge to the SE [Gardner et al., 1992] is corrugated
locally by block uplift above subductingseamounts[Fisher
et al., 1998]. The following paragraphsdiscussthe blockboundingfaultsfrom NW to SE alongthe centralCostaRican
forearc.
At the northwesternedge of the CCRDB the Barrancafault
(Figure 4, Fault 1) strikesNE along the Rio Barrancavalley,
separatingthe uplifted Esparza block from the low-lying
Puntarenascoastalplain to the NW. Vertical offsetsof up to
30 m for late Quaternary fluvial terraces across the Rio
Barrancaand up to 4 m for Holocenemarinebenchesnear the
river mouth demonstrateactive slip along this fault. A radiocarbondate of-•3.0 ka for wood beneatha colluvial wedgeon
the uplifted Holocene platform indicates a maximum late
Holoceneuplift rate of 1.3 m/ka for the Esparzablocknearthe
MARSHALL ET AL.' FAULT KINEMATICS, COSTA RICA
'.Cent
473
ra
?.San Jos•"
ß
...
%
%
%
%
%
%
%
%
%
83 ø 30'
Figure 4. Geologic map of the central study area showingregional-scalefaults (Faults 1-44) and the 3 kinematic domains:forearc(D 1), volcanicarc (D2), and back arc (D3). The faults are keyed by numberto Table 1.
Heavy white dashedlines mark kinematic domainboundaries.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 shadedrectanglesoutline areasof Figures6a-6c, and 6f. Volcanoesare as follows: VB, Volcfin
Barva;VI, Volcfin Irazfi; VT, Volcfin Turrialba. SeeFigure2 for geologicsymbols.
Barrancafault. This rate is consistent
with longer-termaverage
uplift of the upperBarrancafluvial terrace(Qt], oxygenisotope stage 5e - 125 ka) at a rate of 1.0 m/ka [Fisher et al.,
1998]. Remnantsof the upperthree terraces(Qt]_3) on oppositesidesof the Barrancafault showthe samemagnitudeof ver-
tical offset(- 30 m). This observation
suggests
that slip along
the fault is relatively recent,having begunafter formationof
Qt3 in the late Pleistocene.
The NE strikingJesfisMaria fault (Figure 4, Fault 3) separates the uplifted Esparza block from the low-lying Orotina
block to the SE. This fault formsa prominentSE facing scarp
along the NW bank of the Rio Jes•s Maria. Pleistocenelahar
deposits(Tivives Formation:age <2 Ma, 4øAr/39Ar)
[Marshall and Idleman, 1999] and Miocene shallow marine
volcaniclasticsediments(Punta Carballo Formation:minimum
age middle Miocene) [Madrigal, 1970] are offset acrossthe
Jes•s Maria fault with a NW-side-up separationof-•120 m
[Fisher et al., 1994].
Within
the interior
of the Orotina
block several minor NE
striking faults (Trinidad, Diablo, and Poz6n; Figure 4, Fault
5) show dip-slip offset of late Quaternaryash flows, volcaniclastic sediments,and fluvial terrace gravels of the Orotina
Formation and expose Miocene sedimentswithin isolated to-
pographic highs. A rhyodacitic welded tuff (-•400 ka,
4øAr/39Ar)[Marshalland Idleman,1999] and underlying
fluvial gravels,which outline a paleorivercourse,showup to
40 m of vertical offset across these faults.
The NE striking Tfircoles fault (Figure 4, Fault 6) accommodatesvertical motion between the low-lying Orotina block
and the uplifted Herradura block to the SE. The Herradura
block, which contains the highest topography within the
forearc, exposesupper CretaceousNicoya Complex seafloor
basaltswhich have been strippedof their sedimentarycover.
While the Tfircolesfault forms the principalboundarybetween
the Orotina and Herradura blocks, several subsidiaryfaults
(e.g., Carara and Turrubares;Figure 4, Fault 7) outline minor
fault blocksalongthis trend.Vertical offsetsof up to 60 m for
late Quaternaryfluvial terracesalong the Rio T,qrcolesdemonstrateactive slip within the T,qrcolesfault system.
The NE-striking Tusubres fault (Figure 4, Fault 10)juxtaposesthe upper Cretaceousbasementwithin the Herradura
block against Paleogene slope sediments of the Esterillos
block to the SE [Sak et al., 1997]. Rocksof the moderatelyuplifted Esterillos block are, in turn, offset from the low-lying
Parrita block to the SE across the NE striking Parrita fault
(Figure 4, Fault 12). An elevationdifferenceof-•150 m for flu-
474
MARSHALLET AL.' FAULT KINEMATICS,COSTARICA
Table 1. Regional-ScaleFaultsof the CentralCostaRica DeformedBelt
Map
Number
Fault
Length,
km*
Strike
Slip
Sense•'
Side
UpS'
Earthquakes
(M >3.0)•'•'
References{}
1
Barranca
18
45
N
SE
2
Barbudal,Jocote
7
132, 125
N
SW, NE
7
3
Jesfis Maria
12
43
N
NW
1,7
4
5
6
Cuarros
Trinidad,Diablo,Poz6n
Ttircoles
9
7
21
95-120
65, 45, 45
45
(N)
N
N
SW
SE,NW, SE
(SE)
7
Turrubares
6
55
N
SE
1
8
Leona
8
45
N
SE
1
9
Herradurasystem
5
40-65
(N)
10
Tusubres
10
35
N
11
Esterillossystem
5
50
(N)
12
Parrita
15
40
N
13
Quepossystem
5
25-175
N
9
1
14
Turrubaritos
15
115-140
9
9
2
15
16
Venado
Tigre
5
20
57
125-135
(N)
(R)
NW
SW
2
17
Tulin
10
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)
21
22
23
24
25
Queb.Colorado
Delicias
La Mesa, Resba16n
Cortezal
Picagres
10
17
17
7
7
57-59
38-42
125-135
50
120-122
L
L
R
L
(R)
(NW)
(NW)
(NE)
9
9
26
Jarls
33
120-130
R
NE
27
28
29
La Garita
Ciruelas
Pacacua
10
10
10
33
45-55
55
(L)
(L)
(L)
NW
NW
SE
30
Salitral, Tablazo
14
35, 45
L
NW
31
32
33
34
35
36
37
38
39
40
41
42
43
44
Alumbre,Patiode Agua
Coralillo
Higuito
Alajuela
Hondura
Sucio,A. Grande,Blanco
Coris-Guarco
Navarro
Patarrfi
Orosi
Pejibaye
Gato
Atirro
Tuis
Pacuare
12
7
38
20
10
18
15
16
6
20
13
7
18
20
20
50, 45
145
128-132
85-115
12
128, 155, 146
100-125
65-75
140
140
50
130
150
120-130
130
L
R
R
T
(N/R)
R
(R)
(L)
(R)
(R)
L
(R)
R
(R)
(R)
NW
9
SW
NE
9
NE, NE, SW
9
9
9
9
(SE)
9
9
9
9
65-70
1,7
1
1924(7.0), 1989(3.6) [A]
1,5
1,7
1
9
NW
1, 14
9
1, 14
NW
1, 14
2
2
1995(4.7) [O]
2,4
2
2
1990 (5.0) [El, 1990(5.7) [I]
1990 (4.5) IF], 1990(4.8) [K]
2,4
4,6
2,11
1,2
1,2
2
19107
1991(4.7) [MM]
1993(3.7) [Y], 1997(3.4)
1772, 1851, 18887
1993 (3.2) [FF]
1952(5.2)
1841, 1910, 19127
1951 (5.0)?
2,11
2,8
4,8
2,4
3,8,13
4,9
8,9
4,8
4
4
1910, 1951(5.0)?
1993(4.4)[AB]
1993 (4.3)
1987(4.3) [AG], 1988(4.5) [AF]
10
4,12
4
8, 12
1
1
* Mappedlength(minimum).
•' Predominant
senseof displacement
(parentheses
indicateinferred):N, normal;T, reverse;L, left lateral;R, rightlateral.
•'•' Magnitudein parentheses;
lettersindicatefocalmechanisms
in Figures6a-6candin Table 3.
õ References
are asfollows:1, thisstudy;2, Arias andDenyer [1991];3, Borgiaet 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, Saket al. [1997].
vial terracegravelsacrossthis fault demonstrates
significant interiors. Within the Esparza block the Barbudal and Jocote
lateQuaternary
slip [Saket al., 1997;Fisheret al., 1998].
faults (Figure 4, Fault 2) accommodateuplift of a minor
In additionto the majorNE strikingfaultsalongthe 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
and seawardtilting of the coastalportion of the block. The interior of the Herradura block is cut by the NW striking
Turrubaritosand Tigre faults (Figure 4, Faults 14 and 16),
which show evidence of oblique dextral motion [Arias and
Denyet, 1991], and may alsoaccommodate
someof the uplift of
the Herradurablock by reverseslip.
northern Cordillera
4.3. Regional-Scale Faults: Volcanic Arc (Domain 2)
sequence.
Regional-scalefaulting within domain 2 displaysan overall
conjugate pattern of NW striking dextral faults and NE
striking sinistral faults (Figures 3 and 4). While some NE
striking faults have lengths of up to 20 km, the dominant
structureswithin this pattern are three major NW striking
faults (Candelaria, Jarls, and Higuito) that display lengths of
over 40 km (Figure 4, Faults 19, 26, and 32).
The NW striking Candelaria fault (Figure 4, Fault 19) extendsfor over 60 km along the seawardflank of the Aguacate
arc, southeastwardto merge with the Terraba thrust belt (Fila
Costefta)north of Quepos. This major fault representsa tectonic boundary between the uplifted forearc blocks
(Herradura,Esterillos,Parrita, and Quepos)to the SW and the
extinct Aguacatevolcanic arc to the NE (Figure 4). Field exposures of offset lithologic units and slickenlines display
oblique dextral motion on the Candelaria fault [Arias and
Denyet, 1991]. Significant uplift of the forearc basement
within the Herradura block suggestsa major componentof
vertical slip along the northern segmentof the fault. Several
NW striking subsidiary faults, including the Quivel and
Zapat6n (Figure 4, Faults 18 and 20), trend subparallelto the
Candelaria fault and also show evidence of oblique dextral
slip [Arias and Denyet, 1991]. Sinistral NE striking faults
that
intersect
the trace
of the Candelaria
fault
include
the
Tulin, QuebradaColorado, and Parrita faults (Figure 4, Faults
17, 21, and 12) [Arias and Denyet, 1991; Sak et al., 1997].
The Jarls fault (Figure 4, Fault 26) strikes parallel to the
Candelariafault (Figure 4, Fault 19) for over 45 km southeastward from the Valle Central into the extinct volcanic arc, form-
ing a tectonicboundarybetween the Cordillerasde Aguacate
and Talamanca.The Jarls and Candelariafaults togetherbound
the north tilted Candelaria basin homocline [Arias and
Denyet, 1991]. At the southwesternedge of the Valle Central
the Jarlsfault cuts Quaternarywelded tuffs as young as 400 ka
[Marshall and Idleman, 1999]. Offsets along the trace of the
Jarls fault suggestpredominantlyright-lateral motion, with a
componentof dip slip (NE side up) along a steeply dipping
fault surface [Arias and Denyet, 1991]. This vertical motion
may accommodateuplift and exposure of intrusive rocks
within the northern Cordillera
de Talamanca.
Another major NW striking fault, the Higuito fault, (Figure
4, Fault 32) runs along the southern margin of the Valle
Central at the southwesternedge of the San Jos6metropolitan
area, markingthe boundarybetweenthe Valle Central basin
de Talamanca,
475
cutting Miocene-
Quaternarysedimentsand volcanic rocks. A series of NE
striking sinistralfaults that intersectthe Higuito fault along
the northwesternedge of the Cordillerade Talamancainclude
the Salitral, Tablazo, Alumbre, and Patio de Agua faults
(Figure 4, Fault 30). These faults offset Neogene-Quaternary
units as well as NW trending folds within the Miocene
Drainagesystemsof the Valle Centralhave inciseddeeply
into the Quaternary volcanic sequencealong a network of
fault-controlledcanyons.The two principalrivers of the Valle
Central, the Rios Virilla and Grande, join to form the Rio
Grande de T/trcoles at the intersection of the NE striking La
Garita fault and the NW striking Jateo fault (Figure 4, Faults
27 and 25). The La Garita fault runs for over 10 km, cutting
Quaternarylavas,weldedtuffs, and fluvial terraces.Offset of
volcanicbedsand paleosolsrecordobliqueleft-lateralmotion
with the NW side up. Vertical separationof nearly 25 m for
fluvial terracessuggestsactive late Quaternaryslip acrossthe
La Garita fault.
Additional NE striking faults along the southernmargin of
the Valle
Central
include
the Ciruelas
and Pacacua faults
(Figure4, Faults28 and 29). The Ciruelasfault extendsfor at
least8 km and may continuefartherto the NE into the modern
arc beneaththe cover of late Quaternary lavas. NW-side-up
motion along the Ciruelas fault has uplifted a window of
Miocene rocks along a fault-parallel ridge within the
Quaternaryvolcanic tableland.The NW striking Jateo fault
(Figure4, Fault 25) may representa left-stepping
extensionof
theJarlsfault (Figure4, Fault26) thathasbeenoffsetalongthe
Ciruelasfault (Figure4, Fault 28). The inferredPacacuafault
(Figure4, Fault 29) runsfor-10 km, cuttingMiocenesediments and Quaternarylavas along the northwesternedge of
the Cordillera
de Talamanca.
An apparentexceptionto the overallNW-NE fault pattern
within the Valle Central is the E-W trending Alajuela fault
(Figure4, Fault 33). This anomalous
fault extendsfor over 18
km alongthe northernmarginof the Valle Central,forminga
clearly visible 100 to 200-m scarpat the foot of Po•s and
Barva volcanoes.Quarry exposuresalong the trace of the
Alajuelafault showclearoffsetsof youngQuaternary
volcanic
deposits.Borgia et al. [1990] ascribethis scarpto fault propagationfolding at the tip of a major north dippinggravitational thrustfault causedby spreadingof the volcanicmassif.
While the abruptscarpand Quaternaryoffsetsattestto recent
slip, seismic activity is virtually absent. Peraldo and
Montero [1994], however, suggestpossibleassociationof
this fault with damagingearthquakesin the eighteenthand
nineteenth
centuries.
4.4. Regional-Scale
Faults: Back Arc (Domain3)
Regional-scale
faulting at the westernedgeof domain3
deviatesslightlyfrom the conjugatepatterntypicalof domain
Higuito fault extendsnorthwestwardinto the Valle Central 2. The WNW trendingCoris,Guarco,andAgua Calientefaults
and may continuebeneaththe Quaternaryvolcanicsequence (Figure4, Fault 36) southof Cartagomergetowardthe east
to the southern flank of the Cordillera Central volcanoes.
with the ENE strikingNavarrofault (Figure4, Fault 37) and
Uplift of Miocenesedimentsalonga prominentNW trending theNW strikingOrosifault (Figure4, Fault 39). The tracesof
ridge within the Valle Central basin suggestsSW-side-up thesefaultsare markedby the alignmentof numerousgeotherby offsetsof
motion along the Higuito fault. Toward the SE the Higuito mal springs,and recentactivityis manifested
Quaternaryfluvial terracesand Holocenesoils.
fault extendsout of the Valle Central for nearly 30 km into the
and the northern flank of the Cordillera de Talamanca. The
476
MARSHALLET AL.: FAULT KINEMATICS,COSTARICA
Toward the east, along the upper Caribbean slope of the
Talamancaarc, regional-scalefaulting is dominatedagain by a
strong NW-NE conjugatepattern of major faults that strike
along deep linear valleys. Major NW striking faults include
the Gato, Atirro, Tuis, and Pacuarefaults (Figure 4, Faults 4144), and major NE trendingfaults includethe Pejibaye(Figure
4, Fault 40), upperPacuare,and Chirrip6 (beyondeastedgeof
Figure 4). These faults are associatedwith bold topographic
scarps and offset Quaternary river terraces. In addition, the
Tertiary sectionand some Quaternaryrocks in this region are
affectedby folding with NW trendingaxes, cut in some cases
by minor NE vergingthrustfaults [Krushensky,1972].
5. Fault
Kinematics
Thesefiberscommonlydisplaystepsor risersthatare congruent with fault motion (steppingdownwardin the directionof
the missingblock), providing a relatively definitive indication of slip sense.
Slickenlineswithin Quaternary lavas, pyroclasticrocks,
and lahar depositsinclude mostly fault surfacestriationsor
grooves. Within Quaternary volcaniclastic and fluvial deposits, as well as soils, slip generally producessmooth
streakson fault surfaces.In the absenceof congruentfibrous
stepsthese slickenlinetypes require subsidiaryfracturesto
determineslip sense.In the majorityof cases,subsidiaryfractureswere of "RM type" [Petit, 1987], minor, striatedRiedel-
stylefracturesintersecting
the fault surfaceat shallowangles
dippingin the directionof movementof the oppositeblock.
Thesefracturesare commonlyconcaveupwardtowardthe fault
plane, intersectingthe fault as crescentswith hornspointing
5.1. MesoscaleFault Analysis
in the movementdirectionof the oppositeblock.In a few cases
(usually
in denselavas), "T type" (tensile) fractures[Petit,
Whilerelativemotionscanbe determined
for manyregional1987]
were
utilized as slip senseindicators.These features
scalefaults in the field [e.g., Arias and Denyet, 1991], the
also tend to intersectthe fault plane as crescentsindicating
precise kinematics of deformation acrossthe CCRDB remain
the motiondirectionof the oppositeblock.
obscure.We therefore expand upon our understandingof
Fault data were analyzedusing the methodof Marrett and
regional-scale
faulting by examiningmesoscale
fault populaAllrnendinger
[1990] and the computer software of
tions. Mesoscalefaults are outcrop-scalefeatures(meter to
decimeterlength)that exhibit measurablefault surfaceorienta- Allrnendingeret al. [1994]. The resultingkinematicaxesfor
fault populationsare plottedon best fit fault planesolutions
tions and kinematic
indicators
such as slickenlines.
(Figures 5a-5c) . We interpretthe kinematicsof deformation
Mesoscale fault data sets, combined with focal mechanisms
from recent seismicity,allow us to better characterizestrain alongthe CCRDB by examiningthe spatialdistributionof the
fault data on regional geologicmaps in conjunctionwith reacrossthis diffuseplate boundarydeformationzone.
gional-scalefaults and earthquakefocal mechanisms
(Figures
Distributed faulting along plate boundariesis likely to
6a-6f). The kinematicsof both regional-scaleand mesoscale
reflect the kinematicsof plate interactions,with slip on indifaulting along the CCRDB show notable differencesbetween
vidual faults representinglocal incrementsof strain within a
domains 1, 2, and 3. In sections 5.2-5.4, we summarize the
region of mesoscalecataclasticflow [e.g., Wojtal, 1989].
fault data, earthquakeseismicity,andregionalfault
Populationsof minor faults are unlikely to record a single mesoscale
commonstresstensor.For this reason,we employa kinematic-
basedmethodof fault analysis[Marrett and Allmendinger,
1990] rather than stresstensor reduction [e.g., Angelier,
1984]. The kinematicmethoddeterminesthe principalshortening and extensionaxes(P andT axes)on the basisof slip data
from individualfaults.The P andT axesfrom a populationof
faults are contoured to determine the distribution of strain axes
for that location.The strain,as expressed
by P and T axesdistributions,shouldreflect the motionbetweenregionalblocks.
Mesoscalefault populationswere measuredat 86 locations
acrosscentral Costa Rica in rocks ranging from Eocene to
Holoceneage (Table 2). In general, older rocks were avoided
in order to exclude Cretaceous-early Paleogene faults
generatedduring the early developmentalphasesof the MAT
and its volcanicarc [e.g., Gutsky, 1988]. Kinematicdata were
collectedfrom freshfault surfacesexposedin quarries,excavations,road cuts,riverbanks,coastalcliffs, and wave-cutplatforms(Table2). Data includethe strikeand dip of individual
patterns and discussthe observed kinematic variations moving from west to eastacrosscentralCostaRica.
5.2. Fault Kinematics:Forearc (Domain 1, Sites1-27)
Within the forearc region (domain 1), mesoscalefaults
displayconsiderable
variability in orientationand slip direction (Figures 6a and 6d-6f). Normal and strike-slipfaults,
however, significantlyoutweigh thrust faults in number and
in magnitudeof slip. Fault populationdata from this area reflect transtension
with a predominance
of shallowT axesandP
axestrendingbetweensteepand shallow.
Shallow T axes, combinedwith steepP axes, suggesta
componentof extensionfor a majority of populations.This is
consistentwith offsetsof Neogene-Quaternary
unitsobserved
alongsteep,regional-scale,
block-bounding
faults(Figure4).
Where mesoscaleP and T axes are both shallow, the data show
predominantlyleft-lateral motion on NE striking faults. This
is consistent
with
sinistral
transtension
across the forearc
fault planes,trendandplungeof slip lineations
(slickenlines),
fault blocks.
and slip sense.The slip sensewas determinedon the basisof
the geometryand natureof slicknelines,subsidiaryfractures,
and/oroffsetbedsas outlinedby Petit [1987].
Slickenline types vary between outcrops,mostly as a
functionof lithology.Fibrousmineralgrowth(mostlycalcite)
occurson many fault surfaceswithin Tertiary sandstoneand
limestoneunits,as well as on someNeogene-Quaternary
andesiticlavas and pyroclasticrocks(e.g., AguacateGroup).
Historically, shallow upper plate earthquakeswithin the
forearc(domain 1) have been relatively rare in comparisonto
the volcanicarc and back arc (domains2 and 3). A seismicsequence in 1989 centered on the coastal piedmont of the
Orotina block (Figure 6a, Focal MechanismA, and Table 3)
showedoblique-normalslip along a linear NE trend [Giiendel
et al., 1989]. This swarm occurredin the same locationas the
M=7.00rotina earthquakeof 1924 (Table 1), which causedex-
MARSHALL ET AL.: FAULT KINEMATICS,
COSTA RICA
477
Table 2. MesoscaleFault PopulationData
Field
Site Number Latitude Longitude
OutcropLocationandType*
Age
Total
and Fm-• Faults P Axis
T Axis
Peninsula
andGolfodeNicoya(Forea/cRegion,Domain1)
1
03-96
9 ø 37'
2
02-96
9 ø 37'
3
01-96
9 ø 35'
4
07-94
9 ø 54'
5
05-94
10 ø 03'
6
06-94
10 ø 06'
7
05-90
9 ø 57'
8
15-94
9 ø 59'
9
01-90
10 o 01 '
10
02-90
9 ø 58'
11
11-90
9 ø 54'
12
08-94
9 ø 53'
13
41-93
9 ø 55'
14
04-94
9 ø 51'
15
43-93
9 ø 53'
16
18-94
9 ø 53'
17
09-90
9 ø 53'
18
16-94
9 ø 37'
19
14-90
20
12-90
85ø 09'
85ø 08'
85ø06'
84ø 56'
84ø 58'
84ø 56'
PuntaBarrigona,Malpais(sp)
QuebradaVanegas,Malpais(rc)
CaboBlancoReserveHQ (sp)
PlayaGigante(sc)
PuntaMorales(sp/sc)
PtaMoralesHwy, CerroAm6rica(rc)
Tm-st
Tm-st
Tpe-cb
Tpe-lp
Tpe-lp
To-ma
8
8
10
10
5
13
085,29
347,15
338,32
245,04
185,75
008,15
291,24
150,40
244,06
148,13
240,08
EsparzaandOrotinaBlocks(ForearcRegion,Domain1)
84ø 45' PuntaCarballo(sp/sc)
Tm-pc
84ø43' FincaMachuca,Greggde Esparza(q)
Tm-pc
84ø40' Rio Barranca,Marafional(rb)
Tm-pc
84ø41' Esparza-Artieda
road,Humo(rc)
Tm-pc
84ø44' PuntaCorralillo(sc)
Tm-pc
84ø43' PlayaTivives(sc)
Tm-pc
84ø41' Costanera
Hwy, Rio JesfisMaria (rc)
Tm-pc
84ø41' PerionBajamar(sc)
TQ-t
84ø 38' CerroTamarindo(q)
Tm-pc
84ø 36' Costanera
Hwy, QuebradaPoz6n(rc)
TQ-t
84ø 3,5' Costanera
Hwy, CerroCoyote(rc)
Tm-pc
84ø 35' CostaneraHwy, Rio Tfircoles(rc)
Qt
16
14
9
10
7
20
8
9
30
16
11
14
281,63
356,O6
167,47
092,27
086,02
26
03-95
27
02-95
Herradura,Esterillos,
Parrita,andQuepos
Blocks(ForearcRegion,Domain1)
9ø43'
84ø40' PuntaLeona(sp/sc)
Tm-pc 15
9 ø 42'
84ø40' PuntaSucia(sp/sc)
Tm-pc 27
9 ø 41'
84ø40' PlayaCaletas(sp/sc)
Tm-pc
9 ø 40'
84ø40' PlayaCoyol,PuertoEscondido
(sc)
Tm-pc 27
9 ø 38'
84ø 38' HaciendaJac6,PlayaJac6(q)
Tm-pc 19
9 ø 32'
84ø 26' Costanera
Hwy, Bejuco(q)
Qt
19
9 ø 32'
84ø 16' CostaneraHwy, Vueltas(rc)
Qt
6
9 ø 27'
84ø 09' Costanera
Hwy, FincaManagua(ex)
Qt
7
9 ø 23'
84ø09' PuntaCatedral,ManuelAntonio(sc)
Te-ps
23
9 ø 23'
84ø08' PlayaEscondida,
ManuelAntonio(sc)
Te-ps
6
28
09-94
10 ø 03'
29
50-93
10ø02 '
30
51-93
10 ø 04'
31
78-93
10 ø 02'
32
80-93
10 ø 01'
33
66-93
10 o 02'
34
81-93
10 ø 00'
35
83-93
9 ø 58'
36
82-93
9 ø 58'
37
15-90
9 ø 55'
38
59-93
9 ø 54'
39
09-95
9 ø 52'
40
60-93
9 ø 54'
41
58-93
9 ø 53'
42
64-93
9 ø 56'
43
73-93
9 ø 56'
"
13-90
21
01-95
22
10-95
23
06-95
24
05-95
25
04-95
44
36-93
9 ø 58'
45
72-93
9 ø 59'
46
38-93
10 ø 01'
39-93
10 ø 02'
47
07-93
10 o 03'
24-93
10 ø 03'
CordilleradeAguacate(CentralVolcanic
Arc Region,Domain2)
84ø 38' FincaNorita,SanJer6nimo
(q)
TQ-ga
84ø 34' Pan-AmHwy, F. PiedraBianca(rc)
TQ-ga
84ø 30' TajoSantiago,
Magallanes
(q)
TQ-ga
84ø 30' Balboa(ex)
TQ-ga
84ø29' CalleVargas,Berlin(q)
TQ-ga
84ø24' Alto La Cima(rc)
TQ-ga
84ø26' Rinc6ndeSanIsidro(ex)
TQ-ga
84ø26' Atenas-S.Mateo
Hwy, A. delMonte(rc)
TQ-ga
84ø28' Atenas-S.Mateo
Hwy, Desmonte
(rc)
TQ-ga
84ø28' TajoDantas(q)
TQ-ga
84ø28' Tajo CerroRayos,Bols6n(q)
TQ-ga
84ø 28' Rio Turrubaresbridge(rb)
Tm-lc
84ø25' Pursical-Turrubares
Hwy, Por6(rc)
TQ-ga
84ø23' TajoGrifoAlto (q)
TQ-ga
ValleCentral (CentralVolcanicArc Region,Domain2)
84ø22' PlantaHidroe16ctrica
La Garita(rc)
TQ-ga
84ø23' Rio Tfircolesbridge,La Junta(rc)
TQ-ga
84ø22' Tajo Rio Grande(q)
Qv-aa
84ø21' Tajo PuenteLa Garita(q)
TQ-ga
84ø 22' Tajo La Pista(q)
Qv-aa
84ø 22' Tajo La Argentina(q)
Qv-aa
84ø 17' Tajo Prendas,Rio Prendas(q)
Qv-aa
84ø 17' Tajo FincaChilamate,Rio Prendas(q)
Qv-aa
15
10
12
17
14
13
12
5
6
7
8
15
9
14
7
18
15
22
28
30
154,59
267,09
232,08
141,05
227,35
128,11
213,52
312,07
038,13
301,28
192,24
288,13
140,26
113,01
330,63
020,76
156,66
114,64
051,07
298,26
360,81
094,01
180,32
272,02
142,75
294,81
306,15
173,77
282,04
174,05
274,72
106,18
157,86
338,04
061,86
217,04
280,82
021,02
336,40
070,05
288,03
197,17
033,37
301,02
005,35
251,30
194,16
101,11
194,34
095,13
351,00
261,05
195,11
008,07
287,12
103,33
259,00
349,04
330,62
001,31
337,52
220,37
098,18
271,01
100,24
317,10
035,39
138,15
186,11
339,24
277,06
085,33
010,08
100,02
005,49
227,33
020,63
220,26
478
MARSHALLET AL.: FAULTKINEMATICS,COSTARICA
Table 2. (continued)
Field
Site Number Latitude Longitude
OutcropLocationandType*
Age Total
andFm•-Faults P
Axis
T Axis
ValleCentral(CentralVolcanic
ArcRegion,
Domain2) (continued)
48
49
"
"
50
51
52
53
54
55
"
56
57
58
59
"
60
"
61
"
02-94
56-93
57-93
01-94
84-93
53-93
85-93
75-93
28-93
29-93
02-93
03-93
04-93
08-95
01-93
11-93
13-93
14-93
09-93
10-93
9ø58'
9ø58'
9ø58'
9ø58'
9ø58'
9ø57'
9ø55'
9ø56'
9ø56'
9ø54'
9ø54'
9ø52'
9ø52'
9ø56'
9ø56'
9ø57'
9ø57'
9ø57'
9ø57'
9ø57'
62
63
"
64
65
66
67
68
69
70
71
72
73
74
23-93
30-93
31-93
20-93
32-93
33-93
17-93
19-93
15-93
18-93
16-93
21-93
34-93
05-93
9ø52'
9ø48'
9ø48'
9ø47'
9ø48'
9ø49'
9ø43'
9ø44'
9ø43'
9ø39'
9ø38'
9ø52'
9ø51'
9ø49'
84ø 13'
84ø09'
84ø 10'
84ø 10'
84ø08'
84ø 19'
84ø 19'
84ø 17'
84ø 15'
84ø 15'
84ø 14'
84ø 15'
84ø 14'
84ø 13'
84ø 12'
84ø 11'
84ø 10'
84ø 10'
84ø 10'
84ø 10'
TajoLindora,
ValledelSol(q)
TajoLasAnimas
(E),LaCaja(q)
TajoLasAnimas
(W), LaCaja(q)
TajoRioTorres,S.Pedro
dePavas(q)
TajoLagunilla,
RioVirilla(q)
RioTizatebridge,
Turr6cares
(rc)
RioJarisbridge,
Piedras
Negras
(rc)
TajoLara,Quebrada
Ponciano
(q)
SanJos6-Co16n
Hwy,VillaCo16n
(rc)
Quebrada
Hondabridge(rc/rb)
Co16n-Puriscal
Hwy(q)
Co16n-Puriscal
Hwy,TajoGuayabo
(q)
TajoCarlas,
Tabarcia
(q)
SanJos6-Co16n
Hwy,RioCararia
(rc)
TajoCerroMinas,Santa
Ana(q)
SanJos6-Co16n
Hwy,TajoPozos
(q)
LosLaureles,
CerroPalomas
(ex)
LosLaureles,
CerroPalomas
(rc)
S.J.-Co16n
Hwy,CerroPalomas
(rc)
S.J.-Co16n
I4wy,CerroPalomas
(rc)
Qv-aa 6
Qv-aa 54
Qv-aa
Qv-aa
Qv-aa 16
Tm-lc 14
Qv-aa 10
Qv-aa 15
Tm-p 11
Tm-p 38
Tm-p
Tm-lc 14
Tm-pn 12
Qv-aa 5
TQ-ga 28
TQ-ga
Tm-p 20
Tm-p
Tm-p 10
Tm-p
353,45 199,42
357,44 156,44
206,62
192,82
005,15
178,25
170,56
182,81
338,20
046,07
099,15
286,33
043,22
325,07
020,37
199,06
197,16
226,34
125,20
104,40
289,05
332,22
176,82 308,06
202,14 303,36
Cordillerade Talamanca(CentralVolcanic
ArcRegion,Domain2)
84ø03'
84ø07'
84ø07'
84ø05'
84ø04'
84ø02'
84ø03'
84ø02'
83ø58'
84ø00'
83ø55'
84ø02'
84ø00'
83ø57'
TajoValverde,
Valverde
deHiguito(q)
Aserri-Frailes
Hwy,Tranquerillas
(rc)
Aserri-Frailes
Hwy,Tranquerillas
(rc)
Aserri-Frailes
Hwy,Rosario
(rc)
RioAlumbre
bridge,
Guadarrama
(rc)
Pacayas-Copalchi
road(rc)
TajoAngostura,
SanAntonio
(q)
Frailes-S.Pablo
Hwy,Santa
Cruz(rc)
El Empalme-S.Maria
Hwy,Jardin
(rc)
S.Maria-S.Marcos
Hwy,Zapote(rc/ex)
Pedregoso,
CopeydeDota(rc)
TajosGuatuso,
Patarrfi
(q)
TajosBermejo,
Coris(q)
Pan-AmHwy,TajoGuatuso,
S.Isidro
(q)
Tm-sm
Tm-pn
Tm-lc
Tm-lc
Tm-pn
Tm-lc
TQ-ga
TQ-ga
TQ-ga
Tm-pn
Tm-pn
Tm-sm
Tm-sm
Tm-lc
10
28
014,01 284,19
013,48 117,12
11
16
16
12
8
7
13
8
16
9
27
166,43
182,14
356,31
213,19
013,58
121,60
354,28
055,40
041,25
216,07
352,28
275,19
273,01
087,02
305,05
137,19
280,29
258,11
149,05
309,04
308,19
186,61
Vailesde Tejar,Orosi,andReventaz6n(BackArcRegion,Domain3)
75 55-93
9ø54'
83ø57' Pan-AmHwy,Ochomogo
(ex)
Tm-lc 20
027,07 296,08
76 35-93
9ø53'
83ø57' TajoTaras,TarasdeCartago
(q)
Tm-lc 18 016,03 285,26
77 17-94
9ø49'
83ø53' TajoBarroMorado,
Lourdes
(q)
Tm-sm 21
352,08 087,31
78 06-93
9ø50'
83ø53' TajoAguaCaliente,
Paraiso
(q)
Tm-pn 12
167,06 259,14
79 12-95
9ø50'
83ø53' Paraiso-Cachi'Hwy,
TajoLosNovios(q)
Qv-r
6
290,04 047,82
80 11-95
9ø52'
83ø47' Cachi-Tuccurique
road,TajoJoyas
(q)
Tp-d
16
338,17 111,66
81 11-94
9ø50'
83ø42' TajoOriente,RioPejibaye
(q)
Tp-d
18
189,14 .288,33
82 10-94
9ø48'
83ø39' TajoEsperanza,
RioAtirro(q)
Tpe-t 29
014,07 283,08
83 14-94
9ø48'
83ø31' TajoLasQuebradas,
BajoPacuare
(q)
Tpe-t
12
213,06 115,53
84 03-94
9ø57'
83ø38' RioLajas,ToritodeTurrialba
(rb)
Tp-sk
7
134,12 324,78
85 12-94
9ø58'
83ø34' OldLim6nHwy,TajoTresEquis(q)
Tp-d
14
358,17 252,42
86 13-94
10ø01'
83ø37' Quebrada
Linda,Bonilla(rc)
Tom-u 11
219,03 321,75
* Outcrop
typesareasfollows:
q, quarry;
rc,roadcut;
ex,excavation;
rb,riverbank;sp,shore
platform;
sc,seacliff.
•' Sedimentary
rockformations
(Fm)include:
Quaternary
fluvialandmarine
terraces
(Qt),theNeogene
Punta
Carballo
Fm(Tm-pc),SantaTeresaFm(Tm-st),SanMiguelFm(Tm-sm),Perias
NegrasFm(Tm-pn),andPacacua
Fm(Tm-p),andthePaleogene
Masachapa
Fm(To-ma),PuntaSerrucho
Fm(Te-ps),CaboBlanco
Fm(Tpe-cb),
LasPalmas
Fm(Tpe-lp),Suretka
Fm(Tp-sk),UscariFm(Tom-u),andTuisFm(Tpe-t).Volcanic
rockformations
include:
TheQuaternary
Avalancha
Ardiente
Fm(Qv-aa),andReventado
Fm(Qv-r),andtheNeogene-Quaternary
TivivesFm(TQ-t),GrifoAltoFm(TQ-ga)(Aguacate
Group),DofinFm(Tp-d)(Aguacate
Group),andLa CruzFm
(Tm-lc) (AguacateGroup).
MARSHALLET AL.: FAULT KINEMATICS,COSTARICA
FOREARC
REGION
(DOMAIN
479
1)
1
I
20ß
'••"•
.i 2
2 ß 2 * 2
ß
i'
2
ß
26 . ß .
Figure 5a. Mesoscalefault populationdatafor the forearcregion(kinematicdomain1). Data for eachfault
population(analyzedusingthe methodof Marterr and Allmendinger[1990]) are presentedas bestfit fault
planesolutions(lower hemisphere,
equal-areaprojections)definedby compressional
(P) axes(solidcircles
show individualfaults; letter P showsthe average),and tensional(T) axes (open squaresshow individual
faults;letterT showsthe average).The dataare keyedby numberto Table 2 andto Figures6a-6f.
tensive damage in central Costa Rica. According to eyewitness interviews, the 1924 event produced a 4-km-long NE
trendinggroundrupturewest of the town of Orotina [Giiendel
et al., 1989]. The location and trend of both the 1924 ground
rupture and the 1989 seismicswarm correspondwith those of
the Trinidad, Diablo, and Poz6n faults mapped in this study
(Figure 4, Fault 5). The 1989 compositefocal mechanism,the
1924 ground rupture, mapped Quaternary offsets, and
mesoscale
fault
data are all consistent
with
transtension
ac-
commodatedby oblique slip mostly along NE striking margin-perpendicularfaults within the inner forearc.
5.3. Fault Kinematics: Volcanic Arc (Domain 2, Sites 28-74)
In general, mesoscalefault populationswithin the central
volcanic arc (domain 2) mimic regional-scalefaults in displaying a strong pattern of conjugateNE and NW striking faults
(Figures 6a and 6b). While NW striking regional-scalefaults
show greatertotal lengths,crosscuttingrelationshipssuggest
that faults of both orientationsare of similar age. Mesoscale
fault populationsrecord predominantlyshallow T andP axes,
indicating mostly strike-slip motion, with left-lateral slip on
NE strikingfaults and right-lateralslip on NW strikingfaults.
Minor vertical componentsof slip show a slight extensionon
NE strikingfaults and shorteningacrossNW strikingfaults.
Shallow (< 15 km) seismic activity is extremely common
within domain 2, with a broad distribution of minor earth-
quakes (34'_<3.0)[Montero and Dewey, 1982; Ferndndez,
1995, 1996]. The focal mechanisms(Figures 6a and 6b, and
Table 3) include compositemechanismsfrom diffuse minor
earthquakes(1976-1981), as well as single-eventmechanisms
for minor to moderateearthquakes(M=3.0-5.0) from the 1990
Puriscal
seismic
swarm
at the northwestern
end of the Jarls
fault (Figure 4, Fault 26), and the 1993 Valle Central seismic
swarm at the northwesternend of the Higuito fault (Figure 4,
480
MARSHALL ET AL.: FAULT KINEMATICS, COSTARICA
CENTRAL VOLCANIC ARC REGION (DOMAIN 2)
34
.•
6
35
6
3
66
.,,• 6.
ß ß
.
.
6
Figure 5b. Mesoscalefault populationdatafor the volcanicarcregion(kinematicdomain2). SeeFigure5a for
explanation.
MARSHALLET AL.: FAULT KINEMATICS,COSTARICA
BACK ARC REGION
(DOMAIN
481
3)
Figure5c. Mesoscale
faultpopulation
datafor thebackarcregion(kinematic
domain3). SeeFigure5a for
explanation.
fault valleysoften includeobliquethrustfaults of varyingorientationsand dips that display steepT axes. These structures
are more abundantapproaching
the NPDB towardthe eastand
are presumablyassociatedwith NW trendingfolds in this region.
As in domain 2, shallow seismicity (< 15 km) is also
commonwithin domain 3. Focal mechanisms(Figure 6c and
Table 3) includeaftershocksof the 1991 Valle de la Estrella
earthquakeon the NPDB, as well as minor to moderateevents
(M=3.0-5.0) of the 1993 Turrialba seismicswarm along the
Pejibayefault (Figure 4, Fault 40). In general,focal mechaAnomalous
fault
kinematics
within
these weak
surface
nisms along regional-scale faults are consistent with
depositsmay recordvertical simple shearrelatedto differential mesoscalefault data (e.g., Figure 6c, Faults 81 and 82, and
motionof underlyingbedrockfault blocks.Transcurrent Focal MechanismsAB and AF) in showing sinistral slip on
NE strikingfaults and dextral slip on NW strikingfaults. As
motion at depth may be transmittedinto this overlying layer
in domain2, normaland reversecomponents
of slip may reflect
without clear throughgoingfaults. Alternatively, these faults
the kinematicsof conjugatefault intersections.
To the east of
may have been generated by thermal contraction during
domain 3, focal mechanismsreportedby Protti and Schwartz
coolingof thesethick sequencesof pyroclasticrocks.
of the 1991 Valle de la EstrellaearthAnother exception to the conjugate pattern are mesoscale [1994] for aftershocks
quake show increasedcomponentsof reverseslip as strikefaults alongthe trace of the easttrendingAlajuela fault (Figure
slip faultsof the CCRDB mergewith thrustsof the NPDB near
4, Fault 33). These featuresshow predominantlynormal moPuerto Lim6n.
tion and may reflect extension within the crest of a thrust
propagationanticlinethat forms the Alajuela fault scarp.This
structuremay representgravitationalslumpingof the volcanic 6. Age of Faulting
Cordillera [Borgia et al., 1990].
The regional-scalefaults examinedin this study(Table 1)
offset
rocks ranging from Neogene to Quaternary in age.
5.4. Fault Kinematics: Back Arc (Domain 3, Sites 75-86)
Severalauthorshave suggested
that the conjugatefaultsof the
Mesoscalefault populationswithin the back arc (domain 3)
central volcanic arc developed during the Oligocene or
show a transition from the steep conjugatetranscurrentfaults
Miocene under N-S compressiongeneratedby 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 faultsmay have origitraces of major regional-scale faults, mesoscale fault
natedundera pasttectonicregime,field observations
confirm
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 occursprincipally on NW striking faults, and
Field geomorphicevidenceof Quaternaryactivity (e.g.,
sinistral slip occurs principally on NE striking faults. An apfaultedsoils,bold scarps,and offset fluvial and wave-cutterparent exceptionto the predominantlyconjugatefault pattern races) is common along the CCRDB. Radiometricdating
is the Coris-Guarco-Navarrofault systemnear Cartago, which
(4øAr/39Ar)
of offsetQuaternary
volcanic
unitsbothalongthe
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
demonstrates
pervasivefaultingalongthe CCRDB duringthe
last 400 ka [Marshall and Idleman, 1999]. The active nature
6c), fault populationsrecorded within ridges between major
Fault 32). In general,earthquakefocal mechanismswithin domain 2 are consistentwith mesoscalefaults in showing predominantly strike-slip motion on conjugate NE left-lateral
and NW right-lateral faults. Normal and thrust mechanisms
nearmajor faultjunctionsmay reflectthe complexkinematicsof
intersectingconjugatefaults [e.g., Ingles et al., 1999].
A notableexceptionto the conjugatefault patternin domain
2 are mesoscalefaults within thick packages of Quaternary
pyroclastic rocks filling the Valle Central basin (Figure 6b
and Table 2). These populationsshow mostly dip-slip motion
on subverticalsurfaces,with moderatelyinclinedP and T axes.
482
MARSHALL ET AL- FAULT KINEMATICS,
COSTA RICA
10ø 00'
B
9ø 45'
9ø 30'
Figure 6. (a-f) Geologicmapsshowingthe distributionof mesoscalefault populationdata (large, shaded,
numberedstereoprojections;see Figures5a-5c and Table 2 for completedata) and earthquakefocal mechanisms(small, solid, letteredstereoprojections;seeTable 3 for data and references).Dashedwhite lines mark
the boundariesof kinematicdomains(D1, D2, and D3). SeeFigure3 for map locationand geologicsymbols.
of individual faults throughoutthe zone is further supported
by direct associationwith historic earthquakesand modern
recordedseismicity(Table 1).
While field evidence and seismicity confirm that most regional-scalefaults of the CCRDB are tectonically active, the
age of outcrop-scalefaults can be ambiguous.In most cases,
mesoscalefault data were collectedfrom Neogenerocks,introducingthe concernthat somefaults may reflect earlier deformation not representativeof the active tectonic regime. We use
three observationsto argue that our data do indeed reflect
modemdeformationkinematics:(1) lack of temporalvariations
in kinematics,(2) associationwith active regional-scalefaults,
and (3) consistency
with earthquakefocal mechanisms.
The first observation
is that mesoscale
fault
kinematics
within individual domains are essentially the same within
NeogenethroughQuaternaryunitsregardless
of unit age. If
deformationkinematicshave changedthroughtime, fault populations would show either a broad distribution
of P and T
MARSHALLET AL.: FAULTKINEMATICS,COSTARICA
483
:!San Jos6.-,
9ø 30'
Figure 6. (continued)
axes or multiple clusters(bull's-eyes) of axes. On the other
hand, if strain orientationshave remained relatively constant
throughtime, P and T axes shouldclusterabouta singleaverage trend and plunge. In this study, nearly all fault populations show relatively concentratedclustersof P axes (Figure
ß
7). This suggests either that all faults originated in the
Quaternary, that no active structureswere measured,or that
the deformation kinematics have not varied significantly
throughoutthe late Cenozoic. Becausethe first two scenarios
are highly unlikely, we proposethat the kinematicsof faulting
in central Costa Rica have remained relatively constant
throughoutthe Neogene and Quaternary.
The secondobservationis that many of the mesoscalefault
populations
weremeasured
nearactiveregional-scale
faults.
Thesefault populationsare often dominatedby faults with
similar or conjugateorientationsto the nearby, active regional-scale
features(Figures6a-6f). This directassociation
with known active faults supportsthe argumentthat the
mesoscaledata representactivekinematics.
The third observation is that mesoscale fault kinematics
acrossthe study area are consistentwith earthquakefocal
mechanisms(Figures 6a-6f), again suggestingthat the
mesoscaledata reflect the moderndeformationregime. While
manyfaultsmay 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
discussesthe relationship of active seismicity along the
CCRDB
and the tectonics of shallow subduction.
7. Seismicity and GPS Data
7.1. Earthquake Cycles
Recurring cycles of heightened seismicity across the
CCRDB provide compelling evidence for an active tectonic
link
between
the Middle
America
Trench
and the North
PanamaDeformedBelt [Giiendel and Pacheco, 1992]. While
large (M>7.0) thrustearthquakesare commonalong both the
MAT and the NPDB, the most damaging historical events
within the heavily populatedValle Central have been moder-
ate (M=5.0-6.5), shallow(<25 km), upperplate earthquakes
along the CCRDB. Costa Rica's historical seismicity
[Gonz•ilez,1910;Peraldo and Montero, 1994] showsseveral
periodsof heightened
earthquake
activityacrossthe volcanic
arc following large subductionearthquakes[Giiendel and
Pacheco, 1992; Montero and Alvarado, 1995]. Theseperiods
of triggeredseismicityare interspersed
with timesof relative
quiescence.
Suchcyclesof seismicenergyreleasealongfaults
of the CCRDB may reflectthe arcwardtransferof convergent
stressproducedby shallow subduction.
MARSHALL ET AL.' FAULT KINEMATICS, COSTARICA
'0
;
-f
Quiepo
s.
---
•
'":•':'
ø
::::::::::::::::::::::
3 .'•:.:• •
'
.....
::':"
'A:f•¾'
.."..;•::'""':'.'•..':.
.....
•:.....:.
:..
ii:...:i.
'!•::'
........
.:.
84ø 1'5•
Figure 6. (continued)
Figure 7. Examplesof P axiscontourplotsfor mesoscale
fault populations.
Nearly all populations
in the
studyareashowrelativelyconcentrated
clustersof P axescenteredaboutan averagetrendandplunge.This
suggests
thatstrainkinematics
haveremained
relativelyconstant
sincefaultingbegan.Numbersare keyedto
Table 2 and Figures5a-5c.
485
486
MARSHALL ET AL.: FAULT KINEMATICS, COSTA RICA
Table 3. EarthquakeFocalMechanismData
Focal
Mechanism
A•'•B
C
D
E
F
G
H
I
Date
Jan.21-23, 1989
Sept.26, 1994
Feb. 9, 1993
June30, 1990
June30, 1990
June30, 1990
June30, 1990
June 16, 1990
Dec. 22, 1990
J
June 9, 1990
K
June 8, 1990
L
M•'•'
N•'•'
O
P
Q
R
S
May 29, 1990
April 1980-Nov. 1981
April 1980-Nov. 1981
Date unknown, 1995
Feb. 26, 1989
Jan. 12, 1993
Jan. 10, 1993
Jan. 23, 1993
T
Aug. 17, 1982
U
Aug. 3, 1993
V
Jan. 8, 1993
W
Jan. 10, 1993
X
Feb. 13, 1993
Y
Jan. 20, 1993
Z
Nov. 3, 1992
AA
Nov. 3, 1992
BB
Jan. 30, 1993
CC•-•- July 1976-June1979
DD
Dec. 2, 1992
EE
Nov. 21, 1992
FF
Sept.2, 1993
GG•'•' July 1976-June1979
HH$•' April 1980-Nov. 1981
II
Jan. 11, 1994
JJ•-•- April 1980-Nov. 1981
KK•-•- April 1980-Nov. 1981
LL
Sept.29, 1994
MM
Aug. 9, 1991
NN•-•- July 1976-June1979
OO
Nov. 12, 1992
PP
June 13, 1992
QQ
April 24, 1994
RR
March 19, 1993
SS•'•
TT
UU
July 1976-June1979
June1982-Sept.1982
Nov. 2, 1992
VV
Nov. 18, 1992
WW
XX
YY
ZZ
AB
AC
AD
AE
July 14, 1993
May 7, 1993
Sept.23, 1993
July 10, 1993
July 8, 1993
July 10, 1993
July 18, 1993
July 11, 1993
AF
AG
Jan. 31, 1988
Nov. 19, 1987
Depth,
MagnitudeS- Latitude Longitude. km
M =3.6
M =4.3
M =2.8
M =4.5
M =5.0
M =4.5
M =4.5
M =4.5
M =5.7
M =4.5
M =4.8
M =4.7
M <3.0
M <3.0
M =4.7
Ms =4.8
M =3.3
M =3.6
M =3.1
Ms =5.5
M =4.0
M =3.3
M =3.5
M =4.1
M =3.7
M =4.1
M =3.3
M =3.4
M <4.0
M =4.8
M =3.5
M =3.2
M <4.0
M <3.0
M =3.5
M <3.0
M <3.0
M =3.3
Ms =4.7
M <4.0
M =3.1
M =3.9
M =2.9
M =3.2
M <4.0
M <3.2
M =3.4
M =2.7
M =3.9
M =3.7
M =3.6
M =3.0
M =4.4
M =5.3
M =2.9
M =3.4
M =4.5
M =4.3
9ø 56'
9ø 47'
9ø 49'
9ø 49'
9ø 51'
9ø 53'
9ø 54'
9ø 52'
9ø 53'
9ø 53'
9ø 52'
9ø 50'
9ø 53'
9ø 54'
9ø 43'
9ø 40'
9ø 37'
9ø 36'
9ø 35'
9ø 34'
9ø 33'
10ø 00'
9ø 59'
9ø 59'
9ø 59'
9ø 55'
9ø 56'
9ø 58'
9ø 59'
9ø 59'
10ø 02'
10ø 03'
9ø 48'
9ø 50'
9ø 49'
9ø 48'
9ø 52'
9ø 52'
9ø 44'
9ø 46'
9ø 45'
9ø 41'
9ø 43'
9ø 50'
9ø 52'
9ø 57'
9ø 53'
9ø 44'
9ø 43'
9ø 42'
9ø 42'
9ø 46'
9ø 48'
9ø 46'
9ø 45'
9ø 46'
9ø 46'
9ø 42'• --
84ø 35'
84ø 31'
84ø 22'
84ø 21'
84ø 23'
84ø 23'
84ø 21'
84ø 20'
84ø 20'
84ø 19'
84ø 20'
84ø 18'
84ø 17'
84ø 16'
84ø 14'
84ø 11'
84ø 07'
84ø 06'
84ø 07'
84ø 04'
84ø 10'
84ø 13'
84ø 12'
84ø 10'
84ø 11'
84ø 08'
84ø 08'
84ø 06'
84ø 02'
84ø 00'
84ø Off
83ø 59'
84ø 15'
84ø 08'
84ø 09'
84ø 08'
84ø 05'
84ø 04'
84ø 03'
83ø 59'
84ø 01'
84ø 00'
83ø 57'
83ø 58'
83ø 58'
83ø 51'
83ø 46'
83ø 50'
83ø 49'
83ø 46'
83ø 42'
83ø 42'
83ø 42'
83ø 41'
83ø 39'
83ø 39'
83ø 38'
83ø 35'
10.9
28.1
7.3
<15.0
9.4
<15.0
<15.0
14.1
14.6
7.2
8.8
16.1
<15.0
<15.0
<15.0
38.0
22.3
26.9
28.0
37.0
30.0
5.5
6.8
14.1
11.6
6.5
11.1
16.2
<15.0
17.8
13.2
14.0
<15.0
< 15.0
16.8
<15.0
<15.0
6.6
15.0
<15.0
14.9
1.4
21.3
15.9
<15.0
<15.0
6.2
7.6
6.7
3.8
11.7
14.8
8<17
13.2
12.8
15.4
8<17
8<17
P axis
246, 58
089, 44
208, 13
011, 02
188, 04
247, 14
209, 01
184, 09
020, 00
008, 29
208, 09
195, 02
214, 51
194, 10
194, 17
115, 11
134, 09
339, 14
265, 75
194, 23
287, 28
000, 90
345, 83
040, 70
355, 30
360, 76
181, 30
037, 55
351, 30
166, 50
230, 65
348, 62
011, 00
200, 00
209, 20
190, 00
008, 02
030, 01
203, 00
216, 04
209, 20
229, 74
344, 58
294, 69
063, 46
150, 42
007, 52
087, 40
215, 80
200, 30
037, 52
033, 79
007, 04
075, 35
217, 33
359, 72
004, 22
014, 18
T
axis
Reference*
151, 03
5
352, 07
2
318,57
2
280, 28
6
280, 30
151,21
299, 36
293, 63
110, 40
272, 10
112, 34
105,25
072, 32
286, 10
285, 03
206, 03
243, 63
082,42
085, 15
073, 50
193, 10
310,0
120, 05
220, 20
105, 30
133, 10
338,59
207, 35
101, 30
336, 40
090, 19
062, 118
101, 00
110, 14
360, 68
100, 14
098, 12
120, 10
293, 00
125, 10
360, 68
121, 05
106, 18
098, 20
192, 31
300, 44
127,22
354, 03
305, 00
100, 17
156, 22
149, 05
098, 24
179,20
076, 50
237, 10
094, 01
105, 04
6
6
6
2
2
6
2
2
8
8
2
1
2
2
2
1
2
2
2
2
2
2
2
2
7
2
2
2
7
8
2
8
8
2
1
7
2
2
2
2
7
3
2
2
2
2
2
2
9
11
2
2
9
9
MARSHALL ET AL.' FAULT KINEMATICS, COSTA RICA
487
Table 3. (continued)
Focal
Mechanism
AH
AI
AJ
AK
AL
AM
Date
May 14, 1991
April 24, 1991
May 14, 1991
May 17, 1991
March 7, 1983
Dec. 21, 1993
MagnitudeS' Latitude Longitude
M =4.2
Ms =6.1
M =3.1
M =3.0
Ms =5.7
M =2.8
9ø 52'
9ø 45'
9ø 38'
9ø 36'
9ø 35'
9ø 30'
83ø 32'
83ø 31'
83ø 34'
83ø 33'
83ø 40'
83ø 38'
Depth,
km
P axis
T axis
Reference*
21.0
23.0
12.7
4.2
12.0
7.4
035, 13
009, 13
193, 20
191,09
192, 21
071, 46
128, 09
100, 04
103, 01
097, 23
283, 06
177, 15
1
1
10
10
4
2
* References
are 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, localmagnitude;Ms, surface-wavemagnitude
'•'• Compositefocal mechanism.
After the 1950 M=7.7 Nicoya subductionearthquakeon the
MAT, a 5-year series of moderate events occurred across
central CostaRica [Montero and Alvarado, 1995]. Similarly,
the 1983 Mw=7.5 (Ms=7.3) Golfo Dulce subductionearthquake triggered a cycle of moderate events within the
Cordillera de Talamanca [Adarneket al., 1987; Tajirna and
Kikuchi, 1995]. Most recently,a periodof increasedseismicity
beganin centralCostaRica with the 1990 Mw=7.0 (Ms=6.9)
Cobano subductionearthquakeon the Pacific coast and subsidedin the wake of the 1991 Mw=7.7 (Ms=7.5) Valle de la
Estrellaback arc thrustevent on the Caribbeancoast(Figure
8) [Gaendel and Pacheco, 1992]. The discussionin section
7.2 of the 1990-1993 seismicsequenceprovidesimportantinsights into the seismotectonicsof the CCRDB and its link to
shallow subductionalong the Pacific margin and back arc
convergencewithin the NPDB.
7.2. The 1990-1993 SeismicSequence
In 1990, five moderate subductionzone earthquakesoccurredalongthe lengthof the MAT from Nicaraguato Panama.
The first and largest of these events,the Mw=7.0 (Ms=6.9)
Cobano earthquake, was centered offshore of central Costa
Rica, SE of the Peninsulade Nicoya, directly abovethe projected trend of the subducting rough-smooth boundary
(Figure 8) [Protti et al., 1995b]. This shallow subduction
earthquakeimmediately triggereda seismicswarm 60 km inland along the CCRDB within the Cordillera de Aguacate
[Gaendel et al., 1990; Barquero et al., 1991; Gaendel et al.,
1995]. This swarm continuedfor severalmonthsalong a conjugate system of NE striking left-lateral and NW striking
right-lateralfaults (Figures 6a and 6b). The seismicsequence
culminatedwith the 1990 Ms=5.7 Puriscal earthquake(Figure
8), which producedconsiderabledamagein the Valle Central.
Four months later, the 1991 Mw=7.7 (Ms=7.4) Valle de la
Estrella thrust earthquakeruptured the westernmostsegment
of the North Panama Deformed Belt (Figure 8) [Fan et al.,
1993; Goes et al., 1993; Protti and Schwartz, 1994; Suc•rez
et al., 1995]. Both local and far-field seismicity outlined a
shallow SW dipping fault, consistentwith NE thrusting of
the Panamablock over the Caribbeanplate. Coseismicuplift
suggestedabruptterminationof this ruptureat its NW edge at
Puerto Lim6n [De Obaldia et al., 1991' Plafker and Ward,
1992; Denyer et al., 1994]. Aftershockswithin the main rup-
ture zone showedmostly thrust mechanisms,however, a cluster
of
shallow
events
centered
arcward
of
Puerto
Lim6n
showed mostly oblique strike-slip along steep NE striking
faults. Mapped surfacerupturesin this area showedsignificant
strike-slip offsets [Denyer et al., 1994]. These observations
imply an onlandextensionof the NPDB and a transitionfrom
reversemotion to transcurrentfaulting along the CCRDB.
Similar to the 1990 Cobanoearthquakeon the Pacific coast,
the 1991 Valle de la Estrella earthquaketriggered a seismic
sequencefarther inland along a diffuse array of faults through
the Cordillera
de Talamanca
and the Valle
Central.
These
eventslastedfor severalyears and showedoblique strike-slip
mechanismson mostlyNE and NW orientedfaults [Protti and
Schwartz, 1994; Gaendel et al., 1995]. In 1993 a seismic
swarm, culminatingwith an M=5.3 event (Figure 8), occurred
within the Turrialba region centered around the conjugate
Pejibayeand Gato faults (Figure 4, Faults 40 and 41). Similar
to previous events in this area, these earthquakes showed
mostly transcurrentmotion on NE and NW orientedfaults.
Overall, the 1990-1993 seismic sequenceruptureda diffuse
array of transcurrentfaults spanningthe volcanic arc between
the epicenters of the Cobano and Valle de la Estrella
earthquakes.This sequenceof triggered events demonstrates
an active tectoniclink betweenthe MAT and the NPDB along
faults of the Central CostaRica DeformedBelt. Repeatedseismic cyclesalongthe CCRDB may reflect the transferof convergent stressproducedby shallow subductionfrom the Pacific
to the Caribbeanmargin.
7.3. GPS Measurements
Recent motion of the Panama block relative to adjacent
plateshas been constrainedby the Central and SouthAmerica
(CASA) Global Positioning System (GPS) campaigns initiated in 1988 (Figure 1) [Kellogg and Vega, 1995].
MeasurementsbetweenPanama(PanamaCity) and two sitesin
Colombia (Cartegena and Bogotfi) suggestongoing collision
betweenthe Panamaand North Andes blocks at rates ranging
from 8 to 21 mm/yr [Kellogg and Vega, 1995]. GPS-basednumerical modeling for regional deformation indicatesthat the
Panama block moves northward over the Caribbean plate at
increasing rates (10-20 mm/yr) toward the west along the
NPDB from the margin of South America [Lundgren and
Russo, 1995]. These observationsare consistentwith clock-
488
MARSHALL ET AL.' FAULT KINEMATICS, COSTA RICA
N
GPS
CARlB
o
5o
data
20
(95%
19•94•96
mrn/yr
ellipse)
lOO
M 5.7
M
12/90
7.6
4/91
CCR[
mm/y
r
9 ø
.. 7/9 3'
•M
3:/90
o
8-4..o
A
A'
B
B'
C
C'
Figure 8. Tectonicmap of the CCRDB showinga summaryof mesoscalefault data from the threekinematicdo-
mains
(D1,D2,andD3),focalmechanisms
fromthe1990-1993
earthquake
sequence,
GPSdatafor 1994-1996,
Cocos plate bathymetry including the rough-smoothboundary (RSB), and the subductingslab along three
margin-perpendicular
crosssections.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 eachkinematicdomain(seeFigures5a-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 crosssections depictedin boxesbelow the map [from Protti et al., 1995a]. The small arrows show GPS displacement
vectors(scaleat top right) with respectto a fixed Caribbeanplate [Lundgrenet al., 1999].
wise rotation of Panama, thrusting along the NPDB, and
sinistral shear across central Costa Rica [Lundgren and
Russo, 1995].
Resultsfrom the CostaRica (CORI) GPS project(1994 and
1996) provide local constraintson active deformation across
the CCRDB [Lundgren et al., 1999]. These results(Figure 8)
indicate up to 30 mm/yr of sinistral shear between sites in
northern Costa Rica on the Caribbean plate and sites within
the actively deforming Panama block to the south. The GPS
measurements show increasing sinistral shear moving
southward across the faults of the CCRDB. These observations
may reflectNE displacement
of the westernPanamablocktoward
the back
arc NPDB.
Such short-term
data should
be
viewed with caution, however, considering their temporal
proximity to the postseismicstage of the 1990-1993 earthquakecycle alongthe CCRDB. Thesevelocityvectorsmay reflect postseismicdeformationrelated to the 1991 Valle de la
Estrella earthquakeand interseismicstrain along the southern
MAT superimposedon secular motions of the Panama block
[Lundgrenet al., 1999].
MARSHALL ET AL.: FAULT KINEMATICS, COSTA RICA
8. Tectonic Interpretation
The location of the Central Costa Rica Deformed Belt within
the overriding volcanic arc correspondswith the position of
the subductingrough-smoothboundary on the Cocos plate
offshore(Figure 8). This relationshipimpliesa geneticlink between shallow subduction
of thickened
oceanic crust and ac-
tive deformationacrossthe volcanic arc. As suggestedby previous authors,transcurrentfaulting acrosscentral Costa Rica
may reflect sinistralshearalongthe NW flank of the indenting
Cocos Ridge [Montero, 1994; Kolarsky et al., 1995]. In the
model presentedby Kolarskyet al. [1995] the CCRDB would
representa cross-arcfault zone similar to thoseobservedin
other ridge subductionsettings [e.g., Taylor et al., 1995].
Transpressionwhere the CCRDB mergeswith the back arc
NPDB would reflect horizontal shorteningdirectly above the
ridge axis. This model also impliesthat a similar dextralshear
zone must exist SW of the subductingridge in Panama,a question that remainsopen for further investigation.
In additionto the ridge indentationmechanism,we suggest
that shallow subductionof thickened oceanic crust throughout the rough domain (not just limited to the Cocos Ridge)
may increasebasal traction on the overridingplate, resulting
in distributedhorizontal shorteningand NE displacementof
the westernPanama block toward the back arc NPDB (Figure
8). Regardless
of the precisemechanism,
we suggestthat active
faultingwithin the CCRDB shouldbe viewed in generalterms
as a deformationfront that haspropagatedinto the volcanicarc
alongthe NW limit of shallow subduction(Figure 8).
The rough domain of the subducting plate originated
through hotspotactivity along the E-W oriented Galapagos
Rift [Werner et al., 1999] (Figure 1). Shallow subductionat
the MAT may be controlledprimarily by increasedbuoyancy
associatedwith hotspotthickeningof the oceaniccrust. This
buoyanteffect is maximizednearestthe CocosRidge [Gardner
et al., 1992] and may be amplified by decreasedplate age toward the SE [Prottiet al., 1995a]. Becausethe subducting
plate becomesshallower approachingthe NW flank of the
CocosRidge (Figure 8), the zone of crustalshorteningextends
progressively
farther inland along an E-W zone acrosscentral
Costa Rica. Thus, as the seamountdomain and Cocos Ridge
subduernortheastwardat the trench, an E-W trending deformation front, forward of their leading edge, propagatesnorthward into the overriding volcanic arc.
Conjugatestrike-slip faults (NW and NE) of the CCRDB
(domains2 and 3) allow for north directedhorizontalshortening alongthe deformationfront abovethe NW flank of the indentingridge (Figure 8). This zone also accommodates
diffuse
sinistral shear as fault-boundedblocks are displaced northeastward toward the NPDB. Transpressionobservedwithin
the backarc (domain3) reflectsmergingof the CCRDB with
489
Overall, therefore,the kinematicsof active faulting along the
CCRDB may be understoodas the combinedresult of horizontal shortening and shear due to ridge indentation [e.g.,
Taylor et al, 1995], crustal displacementfrom possible increasedbasal traction due to shallow subduction[e.g., Bird,
1998], and localized forearc uplift controlled by seamount
subduction [e.g., Fisher et al., 1998]. While many faults
within the CCRDB may have originated prior to indentation
of the Cocos Ridge [e.g., Astorga et al., 1991, Arias and
Denyer, 1991], they have sinceinheritedthe kinematicsof distributed shorteningand sinistral shear associatedwith shallow subduction
of thickened
oceanic
crust.
Some have argued that the ridge indentationmodel is incompatiblewith the idea of a microplateboundaryshearzone
acrosscentral Costa Rica [Montero, 1994; Fernc•ndez, 1996].
We suggestthat these two ideas are not mutually exclusive if
the CCRDB is viewed in generalterms as the westernlimit of
deformationwithin the Panama block. Even if ridge indentation plays a primary role in deformation,the CCRDB still defines a tectonic boundary between the actively deforming
Panama block and the relatively stable Caribbean plate in
northern
Costa Rica.
Interestingly,shallow subductionof the CocosRidge at the
MAT may, in an indirect way, contribute to the onland
propagationof the NPDB in easternCostaRica. Indentationof
the Cocos Ridge beneath southern Costa Rica drives rapid
uplift of the Talamancaarc. Acceleratedtopographicerosionof
the range has strippedoff overlying extrusiverocks,exposing
the intrusviecore (Figure 2). This erosionhas generateda major pulseof sedimentation
within the CostaRica fan offshoreof
the Caribbeancoast. Silver et al. [1995] argue that sediment
loadingof the offshoreslopemay haveforcedthe thrustfront of
the NPDB onshoreat Puerto Lim6n (Figures2 and 3). In this
manner,we suggestthat subductionof the CocosRidge along
the Pacific marginmay indirectlyinfluencethe geometryof the
NPDB along the Caribbeanmargin and hence its onshoreextensionalong the CCRDB.
While the western Panama block convergesnortheastward
with the NPDB in southernCosta Rica, this motion gradually
rotates to northward displacement along the arcuate NPDB
farther
to
the
east
in
Panama.
Within
the
East
Panama
Deformed Belt the senseof conjugatefaulting is reversed,with
NE striking right-lateraland NW striking left-lateral faults allowing for oroclinal bending and northward thrusting of the
arc into the back arc basin acrossthe arcuateNPDB [Mann
and Kolarsky, 1995]. This deformationreflectscollisionof the
arc with South America
9.
to the east.
Conclusions
the thrustfaults of the NPDB above the axis of the subducting
1. As defined here, the Central Costa Rica Deformed Belt
CocosRidge.Within the forearc(domain1) the effectof shallow subductionis overprintedby local deformationrelatedto
isolatedseamounts.Steepmargin-perpendicular
normalfaults
may reflectverticalkinematicswithin forearcblocksoverriding subductingseamounts.To the SE the indentingCocos
Ridge drives uplift and horizo•ltal shorteningwithin the
Terrababelt (Fila Costefia)and may accentuate
basaltraction
on the overridingplate throughincreasedcouplingalongthe
(CCRDB) is a diffuse zone of active faulting that marks the
western margin of the Panama block (Figure 8). This 70 to
root of the Cordillera de Talamanca.
100-km
wide zone extends across the Costa Rican volcanic
arc, linking the North PanamaDeformed Belt (NPDB) on the
Caribbean coast with the Middle America Trench (MAT) on
the Pacific coast.
2.
The intersection
of the CCRDB
with the Pacific
forearc
correspondswith the location of the rough-smoothboundary
(RSB) on the subductingCocos plate offshore. Shallow sub-
490
,MARSHALLET AL.' FAULT KINEMATICS, COSTA RICA
duction of thickened oceanic lithosphere (Cocos Ridge and
seamountdomain) SE of the rough-smoothboundaryextends
crustal shorteninginto the overriding volcanic arc and displacesthe westernPanamablock toward the back arc NPDB.
3. Active faulting along the CCRDB representsthe arcward
propagationof a deformation front along the NW limit of
shallow subduction.Horizontal shorteningat the deformation
as young as 400 ka [Marshall and Ildeman, 1999]. In
addition, observedoffsets of late Quaternary fluvial terraces,
wave-cut platforms, and soils attest to the active nature of
faulting.
7. Repeatedearthquakecycles along faults of the CCRDB
demonstrate
an active
tectonic
link
between
shallow
subduc-
Costa Rica are accommodatedby transcurrentfaulting along
tion at the MAT and back arc thrusting along the NPDB.
Focal mechanismsagree with fault populationdata, suggesting that the observedmesoscalefaults characterizethe modern
the CCRDB.
kinematics.
front
and differential
shear between
northern
and southern
4. Fault kinematics along the CCRDB vary acrossthree
domains:(1) the forearc, (2) the central volcanic arc, and (3)
the back arc. Where the CCRDB intersectsthe forearc(domain
1) between Puntarenasand Quepos, mesoscalefault populations express sinistral transtensionacross steep NE striking
faults that accommodatedifferentialuplift of forearcblocks.To
the southeastof Quepos, shallow subduction of the Cocos
Ridge produces flexural uplift and horizontal shortening
within the Terraba thrust belt. Inland, within the central vol-
canic arc (domain 2), the CCRDB encompassesa conjugate
systemof NW and NE striking transcurrentfaults. Mesoscale
fault kinematicsdemonstrateprimarily dextral slip with minor
shorteningon NW striking faults and sinistral slip with minor extensionon NE striking faults. In the back arc (domain
3), mesoscalefaults show increasedtranspressionand crustal
thickening where conjugateregional-scalefaults merge with
8.
with
Global Positioning System (GPS) data are consistent
sinistral
shear across the CCRDB
and northeastward
convergenceof southernCosta Rica with the NPDB.
9. While many faults of the CCRDB may have originated
prior to Cocos Ridge indentation,they have subsequentlyinherited the kinematics
subduction
of deformation
of thickened
associated with shallow
oceanic crust.
10. The ridge indentationand microplateboundarymodels
for centralCostaRica are compatibleif the CCRDB is viewed
in generaltermsas a deformationfront at the westernedge of
the Panama block.
Acknowledgments. We are very grateful to F. Gtiendel,M. Protti,
andE. Malavassi(ObservatorioVolcano16gico
y Sismo16gico
de Costa
Rica, Universidad Nacional) and P. Denyer, W. Montero, and M.
Fernfindez(Escuela Centroamericanade Geologia, Universidad de
CostaRica) for insightfuldiscussions
and for earthquakefocal mechathrust faults of the NPDB.
nisms.P. Lundgren(JetPropulsionLaboratory)madesignificantcontributionsto thisstudyby providinga tour of the CostaRica(CORI) GPS
5. The observedkinematic variations along the CCRDB
networkand by sharingGPS data.We alsothankB. Idleman(Lehigh
reflect the combinationof three principal deformationmecha- University) for critical radiometric analysesand R. Allmendinger
nisms:(1) horizontalshorteningand shearfrom oceanicridge (Cornell University)for the kinematicssoftwareusedto analyzethe
indentation,(2) increasedbasal traction from shallow subduc- fault data [/tllmendingeret al., 1994]. We alsoappreciatethe valuable
of R. Seelbach(Universityof California,SantaCruz)
tion, and (3) localized forearc block uplift from subducting field assistance
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 helpfulcommentson this
andthecentralPacificcoastdisplace
rocksdated(4øAr/39Ar) manuscript.This researchwas fundedby NSF grantEAR-9214832.
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(ReceivedJune 11, 1999;
revised December 9, 1999;
acceptedJanuary24, 2000.)