RESEARCH LETTER10.1002/2014GC005234

Kinematics of the western Caribbean: Collision of the CocosRidge and upper plate deformationDaisuke Kobayashi1,2, Peter LaFemina1, Halld�or Geirsson1, Eric Chichaco3, Antonio A. Abrego4,Hector Mora5, and Eduardo Camacho3

1Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania, USA, 2Now at Departmentof Geological Sciences, University of Idaho, Moscow, Idaho, USA, 3Instituto de Geociencias, Universidad de Panam�a, Pan-ama City, Panama, 4Engineering Division, Panama Canal Authority, West Corozal, Panama City, Panama, 5Servicio Geo-l�ogico Colombiano, Bogot�a D. C., Colombia

Abstract Subduction of the Cocos plate and collision of the Cocos Ridge have profound effects on thekinematics of the western Caribbean, including crustal shortening, segmentation of the overriding plate,and tectonic escape of the Central American fore arc (CAFA). Tectonic models of the Panama Region (PR)have ranged from a rigid block to a deforming plate boundary zone. Recent expansion of GPS networks inPanama, Costa Rica, and Colombia makes it possible to constrain the kinematics of the PR. We present animproved kinematic block model for the western Caribbean, using this improved GPS network to test a suiteof tectonic models describing the kinematics of this region. The best fit model predicts an Euler vector forthe counterclockwise rotation of the CAFA relative to the Caribbean plate at 89.10�W, 7.74�N, 1.193� Ma21,which is expressed as northwest-directed relative block rates of 11.3 6 1.0–16.5 6 1.1 mm a21 from north-ern Costa Rica to Guatemala. This model also predicts high coupling along the Nicoya and Osa segments ofthe Middle American subduction zone. Our models demonstrate that the PR acts as a single tectonic block,the Panama block, with a predicted Euler vector of 107.65�W, 26.50�N, 0.133� Ma21. This rotation manifestsas northeast migration of the Panama block at rates of 6.9 6 4.0–7.8 6 4.8 mm a21 from southern Costa Ricato eastern Panama. We interpret this motion as tectonic escape from Cocos Ridge collision, redirected bycollision with the North Andes block, which migrates to the northwest at 12.2 6 1.2 mm a21.

1. Introduction

Subduction and collision of bathymetric highs can have profound effects on the kinematics and geomor-phic development of overriding plates, affecting crustal accretion, transport of fore-arc terranes, and seismicand volcanic hazards. In the New Hebrides subduction zone, the D’Entrecasteaux Ridge, located on the sub-ducting Australian plate, collides with the West North Fiji Basin, resulting in back-arc thrusting [Calmantet al., 2003], arc fragmentation [Taylor et al., 1995], and rapid arc rotation relative to the Australian plate[Wallace et al., 2005, 2009]. The Carnegie Ridge, an aseismic ridge formed above the Galapagos hotspot andlocated on the Nazca plate, collides into and underthrusts the North Andes block along the Colombia-Ecuador trench. Carnegie Ridge collision is assumed to contribute to northeastward motion of the NorthAndes block, possibly by tectonic escape [e.g., Gutscher et al., 1999; Trenkamp et al., 2002; Egbue and Kellogg,2010]. Ridge subduction and collision results in strong mechanical coupling on the plate boundary [Vogtet al., 1976; Cloos, 1993; Scholz and Small, 1997] and horizontal forces [van Benthem and Govers, 2010] thatare accommodated by deformation within the overriding plate.

Along the western Caribbean plate boundary, the Cocos plate (CO) subducts along the Middle AmericaTrench (MAT) at rates of 67 mm a21 in Guatemala to 80 mm a21 in southern Costa Rica [DeMets et al., 2010],resulting in MW> 7 earthquakes and a Holocene volcanic arc stretching from central Costa Rica to Guate-mala (Figure 1). In southern Central America, collision of the Cocos Ridge, a 2 km high aseismic ridge stand-ing on >20 km thick oceanic crust [Walther, 2003], is the largest geodynamic force acting on the Caribbeanplate (CA) [van Benthem and Govers, 2010], driving crustal shortening directly inboard [Fisher et al., 2004],and tectonic escape of the Central American fore arc (CAFA) northwest of the ridge axis [LaFemina et al.,2009]. Cocos Ridge collision has resulted in extensive upper-plate deformation and tectonic modification ofthe western Caribbean from the Pliocene to present [MacMillan et al., 2004; LaFemina et al., 2009]. Geodeticstudies indicate interseismic elastic strain accumulation along the MAT and northwestward motion of the

Key Points:� Collision of the Cocos Ridge is

responsible for the upper platekinematics� The Panama Region acts as a single

tectonic block, migrating northeast� Interseismic coupling is high along

the Nicoya and Osa segments of thetrench

Supporting Information:� ReadMe� kobayashietal_supp

Correspondence to:D. Kobayashi,daisuke@vandals.uidaho.edu

Citation:Kobayashi, D., P. LaFemina, H.Geirsson, E. Chichaco, A. A. Abrego, H.Mora, and E. Camacho (2014),Kinematics of the western Caribbean:Collision of the Cocos Ridge and upperplate deformation, Geochem. Geophys.Geosyst., 15, 1671–1683, doi:10.1002/2014GC005234.

Received 8 JAN 2014

Accepted 15 APR 2014

Accepted article online 19 APR 2014

Published online 29 MAY 2014

KOBAYASHI ET AL. VC 2014. American Geophysical Union. All Rights Reserved. 1671

Geochemistry, Geophysics, Geosystems

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CAFA relative to the CA at 8–16 mm a21 from northern Costa Rica to Guatemala [Correa-Mora et al., 2009;LaFemina et al., 2009, and references therein]. Shallow, destructive Mw� 6.5 strike-slip earthquakes alongthe Central American volcanic arc accommodate CAFA-CA relative motion [La Femina et al., 2002; Cortiet al., 2005]. The Central Costa Rica Deformed Belt (CCRDB; Figure 1), a seismically active zone of distributeddeformation, has been proposed to mark the southeastern margin of the CAFA [Marshall et al., 2000; Lewiset al., 2008].

Directly inboard of the Cocos Ridge, marine deposits record Quaternary uplift of the outer fore arc at rates of6.5 mm a21 and 2.1–7.7 mm a21 on the Osa and Burica Peninsulas, respectively [Sak et al., 2009; Morell et al.,2011]. The inner fore arc Fila Coste~na Thrust Belt (Figure 1), an imbricate thrust system running subparallel tothe MAT, has a minimum cumulative displacement of 36 km since the middle Pliocene [Fisher et al., 2004;Sitchler et al., 2007]. The Cordillera de Talamanca, a �3800 m high extinct volcanic arc, coincides with thewidth of the Cocos Ridge (Figure 1), which has been implicated with back-arc migration of the volcanic arc incentral Costa Rica at <2 Ma [Marshall et al., 2003]. Crustal shortening is also indicated by back-arc thrustingand Mw> 7 earthquakes along the North Panama Deformed Belt (NPDB), a seismically active fold and thrustbelt [Adamek et al., 1988; Silver et al., 1990; Camacho et al., 2010] and incipient subduction zone [Camachoet al., 2010], running from central Costa Rica to the Maracaibo subduction zone, Colombia (Figure 1).

The Panama Region (PR) lies north and east of Cocos Ridge and is bounded by the Cocos (CO), Nazca (NZ),and CA plates, and the CAFA, Choco (CH), and North Andes (ND) blocks (Figure 1). Adamek et al. [1988]

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Figure 1. GPS-derived velocity field (black vectors) in a Caribbean-fixed reference frame. Velocity uncertainty ellipses are removed forclarity; see supporting information Figure S1 and Tables S1 and S2 for velocity uncertainties. Note vector scale in legend. Azimuth and rela-tive rate in mm a21 of the Cocos and Nazca plates relative to the Caribbean plate, open arrows. Holocene volcanoes, red triangles. Plateboundaries, heavy black lines. National boundaries, thin black lines. CAFA, Central American fore arc; NPDB, North Panama Deformed Belt;SPDB, South Panama Deformed Belt; UF, Unguia Fault Zone; AUFZ, Atrato-Uraba Fault Zone; Maracaibo SZ, Maracaibo Subduction Zone;BFZ, Balboa Fracture Zone; CFZ, Coiba Fracture Zone. Inset: geologic features in Costa Rica, including; CCRDB, Central Costa Rica DeformedBelt; FC, Fila Coste~na Thrust Belt; CT, Cordillera de Talamanca; NP, Nicoya Peninsula; OP, Osa Peninsula; BP, Burica Peninsula. Black box indi-cates an area of data used for profile shown in Figure 4. Topographic and bathymetric data from Amante and Eakins [2009].

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suggested that a diffuse seismic zone marked the eastern boundary of the PR. Nazca-CA relative motion ishighly oblique to the margin and is dominated by oblique convergence along the western segment of thesystem (i.e., where the Panama, Balboa, and Coiba fracture zones converge on the margin; Figure 1) andleft-lateral shear in the central and eastern segments, along the South Panama Deformed Belt (SPDB) at 37mm a21 [DeMets et al., 2010] (Figure 1). Along the Colombia Trench, the Nazca plate subducts obliquely(�20�) [Trenkamp et al., 2002] under the Choco block from 7�N to 3�N latitude and the North Andes blocksouth of �3�N (Figure 1). The NZ-ND convergence rate is 52 mm a21 at 2�N in the MORVEL56 plate model[Argus et al., 2011]. Caribbean-PR convergence is accommodated along the NPDB.

The Panama Region has been described by several tectonic models based on geologic, seismic, and geo-detic data. Silver et al. [1990] suggest internal deformation of the isthmus along strike-slip faults, whileothers suggest either the motion of a discrete tectonic block called the Panama block (PB) [e.g., Adameket al., 1988, Vergara Mu~noz, 1988], or interaction of smaller tectonic blocks [Mann and Corrigan, 1990; Tren-kamp et al., 2002; Rockwell et al., 2010]. The effect of Cocos Ridge collision on the kinematics of the PanamaRegion, itself colliding with northwestern South America since Miocene time [e.g., Kellogg and Vega, 1995;Farris et al., 2011], has not been studied or documented. We present a new GPS-derived surface velocityfield with unprecedented coverage in Central America (i.e., the western Caribbean) and northwestern SouthAmerica, and use these data to test these tectonic models. We invert the three-dimensional GPS velocitiesand earthquake slip vectors for Mw> 6 plate and block boundary earthquakes in a three-dimensional kine-matic block model [e.g., McCaffrey, 2002] to simultaneously solve for the Euler vectors that best describeGPS site velocities and the magnitude and pattern of interseismic coupling on block bounding faults. Weconstruct and test various model geometries to find the best fit CAFA-PR boundary, to improve estimates ofCAFA motion, to reveal the tectonic nature (i.e., one block versus multiple blocks) of the PR, and to detailcoupling patterns along the Nicoya and Osa segments of the MAT.

2. GPS Velocity Field and Earthquake Slip Vector Data

We present a new 154 site, GPS-derived interseismic surface velocity field in Central America and northwest-ern South America based on measurements spanning the period 1993–2012 (Figure 1; Table S1). We utilizednew GPS data collected at sites, where data existed from previous studies [e.g., Lundgren et al., 1993, 1999;Trenkamp et al., 2002; Norabuena et al., 2004; Turner et al., 2007; LaFemina et al., 2009; Mora et al., 2011], andfrom episodic and continuous GPS sites installed as part of this study. These GPS data sets were processedtogether with GIPSY-OASIS II version 5.1 in precise point positioning mode, using standard clock and satel-lite ephemeris data from the Jet Propulsion Laboratory [Zumberge et al., 1997] for consistent analysis. Dailypositions were estimated in the International Terrestrial Reference Frame (ITRF) 2005 [Altamimi et al., 2007]and time series generated. The weighted root-mean squared error (WRMS) of a linear fit to the time serieswas calculated to determine velocity uncertainties, following Geirsson et al. [2006]. The resultant velocitieswere transformed to a stable Caribbean reference frame using the ITRF05-Caribbean Euler vector fromMORVEL [DeMets et al., 2010] (Table S2). We removed episodic sites with time series shorter than 3 yearsand continuous sites shorter than 1 year. We also removed sites on volcanoes that may be influenced byvolcanic deformation. In addition to our data set, we used velocity data and their uncertainties from pub-lished velocity fields for El Salvador and Honduras [Correa-Mora et al., 2009] and Guatemala [Lyon-Caenet al., 2006]. These additional velocity data were transformed and rotated to a stable Caribbean referenceframe for consistency with our analysis (Table S2). The velocity data of Lyon-Caen et al. [2006] were firsttransformed into ITRF05.

The horizontal velocity field in a CA reference frame [DeMets et al., 2010] (Table S2) indicates the majortectonic signals for the western CA (Figures 1 and S1): (1) the MAT-CAFA system is dominated by marginparallel motion from central Costa Rica to Guatemala at rates up to 19 mm a21; (2) convergence-parallelrates on the Nicoya and Osa Peninsulas range between 12–27 mm a21 and 24–37 mm a21, respectively;(3) site velocities in central Costa Rica and western Panama indicate a fanning pattern of motion awayfrom the Cocos Ridge axis; (4) northeast-directed motion in central Panama at rates of 6–13 mm a21; and(5) sites located in northwestern South America are moving northwest at �17 mm a21. The vertical veloc-ity field demonstrates: (1) outer fore-arc subsidence along the Nicoya and Osa Peninsulas at rates of up to24 mm a21 and 11 mm a21, respectively and (2) subsidence in the Managua graben (Figure S1b).

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Earthquake focal mechanism solu-tions and slip vectors of interplateearthquakes indicate the geometryof and direction of relative platemotions on plate boundary faults.We used centroid moment tensorsolutions for MW> 6 earthquakeswith depths less than 60 km fromthe Global CMT Catalog and for thetime period 1976–2011 [Dziewonskiet al., 1981] (Figure 2; Table S3). Weselected events and nodal planesconsistent with the location andnature of known plate and blockboundaries to obtain slip vectordata that further constrained blockkinematics in our model inversion.

3. Kinematic BlockModeling

We use kinematic block modelingand invert the three-dimensional

velocity field and earthquake slip vectors (Tables S1–S3), to investigate the kinematics of the western Carib-bean, including the estimation of Euler vectors for proposed tectonic blocks, and the magnitude and spatialvariability in coupling on block bounding faults. We use the modeling code DEFNODE and the methods ofMcCaffrey [2002]. The modeled block boundaries are discrete boundaries defined using the results of pub-lished geologic mapping, seismic tomography, seismic reflection, and refraction profiles, and earthquakerelocation studies. In DEFNODE, the block boundaries are approximated by a three-dimensional meshdefined by surface nodes and the downdip geometry of the fault systems (see discussion of model resolu-tion below).

We test nine block model geometries (Models 1–9) to investigate the different tectonic and kinematic mod-els of the western Caribbean [e.g., Adamek et al., 1988; Silver et al., 1990; Trenkamp et al., 2002; McCaffrey,2002; Turner et al., 2007; LaFemina et al., 2009; Rockwell et al., 2010] (Figure 3) and compare long-term geo-logic and decadal geodetic estimates of plate boundary deformation. Models 1–5 investigate the locationand nature of the CAFA-PR boundary (Figures 3a–3d), while Models 6–9 test different models specific to thePR, whereby the isthmus is transversed by proposed active or mapped faults (Figures 3e–3h). Models 4 and5 (Figure 3d) have a discrete CCRDB block; the CCRDB is modeled as a rigid block in Model 4 and as adeforming zone in Model 5. Model 9 (Figure 3h) reproduces boundaries within the PR proposed by Rockwellet al. [2010] on the basis of paleoseismic fault studies in the Panama Canal Zone.

The spatial limitation of land-based geodetic networks makes it difficult to constrain the pattern and magni-tude of interseismic elastic deformation (i.e., coupling) near a subduction zone trench. Several paradigmshave been proposed for modeling interseismic coupling on subduction zone plate interfaces and specifi-cally the seismogenic zone. One model suggests that the shallow interface from the trench to the updiplimit of seismogenesis is a region of free slip [e.g., Bevis and Martel, 2001]. However, McCaffrey et al. [2000]and McCaffrey [2002] suggest that the estimated strain rate at these depths would be anomalously high ifthe shallow interface is not fully coupled. Wang et al. [2003] predict an exponentially decaying downdiptrend in coupling at depths deeper than the updip limit of the seismogenic zone, based on thermal modelsof the Cascadia margin [Hyndman and Wang, 1993].

The proximity of the Nicoya (�50 km) and Osa (�20 km) Peninsulas to the Middle America Trench allows usto test different models for the magnitude and spatial variability of interseismic coupling at the updip limitof the seismogenic zone. Coupling (u) here is the ratio of slip deficit to the total relative plate motion. InDEFNODE, one can impose various coupling constraints, three of which we test along the MAT interface in

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Figure 2. Earthquake focal mechanisms for Central America and western Colombiafor the time period 1976–2011. We selected MW> 6 events shallower than 60 km andconsistent with the location and nature of known plate and block boundaries. Focalmechanisms are scaled by magnitude and color-coded by depth range (see legend).Data are from the Global CMT project [Dziewonski et al., 1981].

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Figure 3. Block boundaries for Models 1–9. Thick gray lines indicate boundaries of the Central American fore arc (CAFA) and the Panama Region (PR). Thin gray lines are other blockboundaries. Thick black lines indicate boundaries that segment the PR. The Central Costa Rica Deformed Belt (CCRDB) is indicated by the gray shaded area. CO, Cocos plate; CA, Carib-bean plate; NZ, Nazca plate; CH, Choco block; ND, North Andes block; UF, Unguia Fault Zone; PANC, Panama-Caribbean block. See text for model descriptions.

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this study. The first pattern imposes no constraints and independently determines u for each node (here-after Type 0 or no constraint). This option allows for the estimation of detailed coupling patterns in high-resolution areas, namely the Nicoya and Osa segments. We tested our GPS network resolution and foundthat the network can resolve coupling patches up to 30 km (along strike) by �50 km (downdip) and up to�20 km offshore on the Osa segment, and patches up to 30 km (along strike) by �40 km (downdip) up to30 km offshore on the Nicoya segment of the MAT (Figure S2). The second constraint type imposes a lineardecrease in coupling downdip (hereafter Type 1 or linear decrease), which forces u at a node to be lessthan or equal to the node directly updip of it. In our modeling using the Type 1 constraint, each fault planeis fully coupled at the surface nodes (e.g., at the Middle America Trench), and u decreases to zero at orbefore the bottom-most node. This modeling constraint is assigned for all block-bounding faults (e.g., theCAFA-CA boundary fault or the NPDB) for which coupling is estimated in all model runs, except for the MAT(see above). The last model constraint tested is a constraint where a downdip decrease in u betweendepths Z1 and Z2 (Z1< Z2) is prescribed by an exponential function (hereafter Type 2 or exponentialdecrease) as proposed by Wang et al. [2003]. Full coupling is assigned from the surface to a depth Z1 withno coupling below Z2 in our models; Z1 and Z2 are estimated in the inversion.

For each model, we test the three coupling constraints on the MAT and solve for the Euler vectors of themodeled blocks and the magnitude and spatial variability of interseismic coupling on block boundaries,minimizing the misfit between the model and data (i.e., the three-dimensional GPS velocity field and earth-quake slip vectors). Values of the chi-square statistic are compared to determine a model of better fit (TableS4). We perform an F test [Stein and Gordon, 1984] to determine if the better fit is statistically significant.The probability of a significant improvement, not due to mere chance, is given with the F test statistics andthe F probability density function (Table S5).

4. Results and Discussion

We tested nine models to find the best fit CAFA-PR boundary to improve estimates of CAFA motion, toreveal the tectonic nature of the PR, and to detail coupling patterns along the Nicoya and Osa segments ofthe MAT. The resultant Euler vectors (Table 1) and estimated magnitudes and patterns of coupling improveour understanding of the kinematics of the western Caribbean. Below we describe the results from our bestfit models and how they reflect on the kinematics of the western Caribbean.

4.1. Modeling ResultsAll of the models tested consistently predict: (1) counterclockwise rotation of the CAFA relative to the CAabout an Euler pole located �500–600 km to its south (Figures 4a and S3–S28), and (2) moderate (�60%)coupling to a depth of at least 30 km along the Nicoya and Osa segments of the MAT (Figures 5a and S3–S28). All the models that have a nonsegmented PR (Models 2–5; Figures 3a–3d) predict counterclockwise

Table 1. Estimated Euler Vectors Relative to the Caribbean Plate

Block Longitude Latitude x (� Ma21) Azimutha emaxa emin

a

CAFAThis study, the best fit model 289.10 7.74 1.193 6 0.122 12.73 0.68 0.33Alvarado et al. [2010] 291.2 4.5LaFemina et al. [2009] 290.15 3.90 0.469Turner et al. [2007] 288.4 8.9 1.957McCaffrey [2002] 285 10.5 0.04

PRThis study, the best fit model 2107.65 26.50 0.133 6 0.265 118.9 32.32 3.05Argus et al. [2011] 2171.75 20.24 0.100Bird [2003] 2172 0 0.10

NDThis study, the best fit model 258.77 31.10 20.233 6 0.163 212.95 30.78 1.30

Argus et al. [2011] 270.82 41.3 20.198Bird [2003]b 270.74 41.04 20.197

CH 295.91 265.11 0.405 6 0.832 22.01 101.07 19.27

aAzimuth, emax, and emin refer to the azimuth of the major axis of the error ellipse and its maximum and minimum uncertainties indegrees.

bCalculated using the South America-Caribbean angular velocity by DeMets et al. [2010].

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rotation of the PB relative to the CA (Figures 4a and S6–S16). Segmented PR models (Models 6–9; Figures3e–3h) do not significantly improve model fit (Tables S4 and S5). For all the block configurations, modelswith the Type 2 coupling constraint (i.e., exponential decrease) have significantly poorer model fits com-pared to those with Type 0 or 1 constraint (see Table S4).

The best fit model is Model 2 (Figure 3b) with Type 1 coupling constraint, whose reduced v2 is 8.94 (TableS4). This model indicates that the western Caribbean can be defined as two tectonic blocks, the CAFA andPR (hereafter named the Panama block (PB); Figure 4a). The boundary between the CAFA and PB was foundto be the western limit of the CCRDB, with a relative rate of 15 6 4.2 mm a21 at an azimuth ofN73.5W 6 8.3� (Figure 4b). This indicates extension between CAFA and PB that is accommodated within theCCRDB by shallow, low-magnitude normal and strike-slip earthquakes indicating northwest-southeastextension [Marshall et al., 2000; Lewis et al., 2008].

The second and third best fit models have a comparable model fit. The second best fit model is Model 9(Figure 3h) with Type 1 coupling constraint (Model 9-1; v2 5 9.53; Figure S27; Table S4), in which the Pan-ama isthmus is segmented following Rockwell et al. [2010]. Slip rates along the faults transecting the PB inModel 9-1 are not well constrained geodetically due to the sparse data set. The third best fit model isModel 2 (Figure 3b) with Type 0 coupling constraint (v2 5 9.74; Figure S6; Table S4). This model has thesame block boundaries as the best fit model, but no coupling constraints are imposed on the MAT. Theresults for the third best fit model are in good agreement with the best fit model.

4.2. Kinematics of the Central American Fore ArcIn our best fit model, the CAFA rotation is defined by an Euler pole located at 89.10�W, 7.74�N and at a rateof 1.193 6 0.122� Ma21 (Figure 4a). Table 1 shows a comparison of CAFA Euler vectors with other studiesusing GPS data. The differences in the Euler vectors possibly result from inversion method and an extent ofthe CAFA in each model. McCaffrey [2002], LaFemina et al. [2009], and this study adopt a simultaneous inver-sion for Euler vector and elastic strain accumulation, while the others solve only for the Euler vector. Thisstudy has the longest modeled CAFA extending from northern Costa Rica to Guatemala.

The Euler vector of the CAFA in our best fit model is expressed as modeled fore-arc rates of 11.3 6 1.0–16.5 6 1.1 mm a21 from northern Costa Rica to Guatemala, respectively (Figure 4a; Table S6). For compari-son, our second and third best models result in fore-arc rates of 12.1–16.9 mm a21 and 10.6–16.5 mm a21,respectively. These results constrain the magnitude of dextral shear strain accommodated by shallowdestructive earthquakes along the CA-CAFA boundary [La Femina et al., 2002; Corti et al., 2005] and there-fore the potential seismic moment of future earthquakes along the Central American volcanic arc.

4.3. Kinematics of the Panama BlockThe Euler vector for motion of the PB relative to the CA has a pole located at 107.65�W, 26.50�N, and a rota-tion rate of 0.133 6 0.265� Ma21 (counterclockwise; Table 1). This Euler vector is significantly different fromthose estimated in other studies based mainly on geologic and seismologic observations (Table 1) [Bird,2003; Argus et al., 2011]. The rotation about this pole is expressed as northeast motion at rates of �6.9 mma21 to �7.8 mm a21 (Figure 4a; Table S6). Although the predicted Euler vector for the PB is poorly con-strained (Table 1), the modeled horizontal velocities clearly capture the overall trend of the observed veloc-ity field, constraining relative plate and block motions and indicating negligible internal deformation of thePB (Figures 4a and S3). The northeast motion of the PB results in oblique convergence with the CA alongthe NPDB, convergence with the Choco and North Andes blocks, and sinistral shear and extension alongthe SPDB.

Panama block-CA convergence in central Panama is 7.5 6 4.4 mm a21 at an azimuth of N39E 6 26� (Figure4a; Table S6). A Wadati-Benioff zone within the subducting Caribbean plate has been hypothesized andtraced to �80 km depth (assuming a dipping slab of �170 km length) along the eastern segment of theNPDB [Camacho et al., 2010]. If the Wadati-Benioff zone indicates the leading edge of the subducting CA,and we assume a constant convergence rate, onset of CA subduction occurred at least 22–23 Ma. This indi-cates that underthrusting of the CA predates Cocos Ridge collision, hypothesized at <3 Ma [MacMillan et al.,2004], but is synchronous with Panama’s collision with South America starting at 23–25 Ma [Farris et al.,2011]. In the second best fit model (Model 9-1), the central and eastern segments of the NPDB form thenorthern boundary of an elongated block, the PANC block (Figure 3h). The convergence rate (14.0 6 78.8

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mm a21) and azimuth (N35E 6 47�) between the block and the CA are poorly constrained (Figure S27).Nevertheless, this rate would result in onset of CA subduction at �12 Ma.

We tested proposed tectonic models of the PR that have suggested segmentation of the isthmus. Our Mod-els 6–9 (Figures 3e–3h and S17–S28) provide some constraints on several of the major faults proposed forthe region (e.g., the Sona-Azuero and Panama Canal fault systems); however, our estimated rates and azi-muths are poorly constrained, and the sense of slip for most faults is the reverse of that found in geologicstudies. Further expansion of the geodetic network, with a focus on investigating the major regional faults,may improve our understanding of the rates across these faults.

4.4. Earthquake Cycle DeformationOur new estimates of coupling on the CO-CAFA plate boundary have implications for interseismic elasticstrain accumulation and therefore earthquake hazards. Along the Nicoya segment of the MAT, we estimatea CO-CAFA relative rate of 72.0 6 1.0 mm a21 at an azimuth of N28.1E 6 0.7� (Figure 4b; Table S6) and pre-dict an �2500 km2 zone of moderate (40–60%) coupling, likely reflecting a smaller region of 100% coupling,located beneath central Nicoya Peninsula (Figure 5a). The location of interseismic coupling is colocatedwith the location and rupture area of historical earthquakes (Figure 5a). Earthquakes in 1900 (MS 7.1)[Pacheco and Sykes, 1992], 1950 (MS 7.7) [Abe, 1979], 1978 (MW 6.9) [Protti et al., 2001], and 2012 (MW 7.6)[Protti et al., 2013; Yue et al., 2013] were located in this region of high coupling (Figure 5a). Assuming thatthe full convergence rate was accumulated as elastic strain, �3.6 m of slip deficit accumulated prior to the1950 event (Figure 5b). If the 1950 event fully released this slip deficit, then this segment accumulated �2.0m of slip deficit by the 1978 event. Protti et al. [2001] estimate only 15% of potential slip for the 1978 eventwas released, which left �1.7 m of slip deficit (Figure 5b). Since 1978, �2.4 m of slip deficit has accumulatedfor a total of �4.1 m. Preliminary analysis of the 5 September 2012 Nicoya earthquake indicates a maximumof 3.0 m slip [Protti et al., 2013; Yue et al., 2013] (Figure 5b), but with considerable postseismic deformation[Geirsson et al., 2013].

CAFA CA

CO

NZ

NDPB

NA

CH

2˚ N

4˚ N

6˚ N

8˚ N

10˚ N

12˚ N

14˚ N

16˚ N

94˚ W 92˚ W 90˚ W 88˚ W 86˚ W 84˚ W 82˚ W 80˚ W 78˚ W 76˚ W 74˚ W

a

20 mm a relative to:

Block Rate

CA

PR

CAFA

Site Velocity

20 mm a relative to CA

Data

Model

0 200

km

CAFA

b

CA

CO

NZ

PB

CAFA

ND 11

7

15

75

73

59

3013

12

N

10 mm a

80

Figure 4. (a) Observed (black) and modeled (blue) velocities relative to CA in the best fit model. Note vector scale in legend. Arrows indi-cate azimuth and rate of block rate relative to the CA (pink), PB (green), and CAFA (white). Relative block rates and the Euler pole for theCAFA (red star) are shown with 1r uncertainty ellipses. Block boundaries are shown as thick gray lines. (b) Inset: velocity triangle diagram.Relative velocities of blocks adjacent to each other are given in mm a21. For the CAFA and PB motions, velocities in northern Costa Ricaand central Panama, respectively, are used.

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Along the Osa segment of the MAT, we estimate a CO-PB convergence rate of 73.1 6 3.8 mm a21 at an azi-muth of N22.3E 6 2.5� (Figure 4b; Table S6) and predict a broad zone of �50–75% coupling in this region(Figure 5a), which likely reflects smoothing of fully coupled patches and is consistent in area with couplingon the underthrusting Cocos Ridge axis. Large (M> 7) earthquakes have occurred in 1904, 1941, and 1983on the Osa segment, where the Cocos Ridge collides and underthrusts the PB (Figure 5a). The 1941 MS7.5and 1983 MW7.4 earthquakes were located in the same epicentral area northeast of Osa Peninsula [Adameket al., 1987] (Figure 5a). If CO-PB convergence is accommodated only as elastic strain, �3.1 m of slip deficitaccumulated between the 1941 and 1983 events (Figure 5c). Adamek et al. [1987] estimated 0.6 m of slip forthe 1983 event, indicating a remaining �2.5 m of slip deficit. Approximately 2.2 m of slip deficit has accruedsince 1983 for a total of �4.7 m, a larger slip deficit than that accumulated when the 1983 event occurred(Figure 5c). This implies either that there is the potential for a larger magnitude earthquake on the segmentor that a portion of accumulated slip deficit was released as afterslip following the 1983 event.

Another possibility is that the estimated slip for the 1983 earthquake is an accurate representation of theamount of slip deficit prior to the 1983 event. In this case, only 14 mm a21 of slip deficit accumulatedbetween 1941 and 1983, and the remaining �59 mm a21 was either released as afterslip or accommodatedby plastic deformation, such as crustal shortening, or a combination of both. The Fila Coste~na Thrust Belthas accommodated up to 40 mm a21 of cumulative slip since middle Pliocene time [Fisher et al., 2004; Sitch-ler et al., 2007]. The four sites along the Pacific coast of the Osa and Burica Peninsulas have large modelresiduals showing a fanning-out pattern from the Cocos Ridge axis (Figure S29). These residuals may indi-cate nonmodeled plastic deformation within the Fila Coste~na Thrust Belt, creeping on faults of this system,

Middle America TrenchOP

NP

BP

1904

1950

1900

1978

72

73

CCRDBNPDB

60

19831941

60

1991

CO

CAFA

PB

CO-CAFACO-CAFA

CO-PBCO-PB

20122012

1822

0 50 100

km

0.0 0.5 1.0

Phi60 mm a

a

7˚ N

8˚ N

9˚ N

10˚ N

11˚ N

86˚ W 85˚ W 84˚ W 83˚ W 82˚ W

b c

1900

Ms 7.12012

Mw 7.6

1950

Ms 7.7

1978

Mw 6.9

3.6

2.0

4.1

Slip

de

cit

(m)

Year and event

72 mm

a

2012

3.1

Slip

de

cit

(m)

Year and event

1941

Ms 7.5

1983

Mw 7.4

73 mm

a

2.5

4.6

0.6 14 mm a

60

Figure 5. (a) Estimated interseismic coupling (phi) along the Nicoya and Osa segments of the MAT as predicted by the best fit model withno coupling constraints. Open arrows show velocity of the Cocos plate relative to the overriding CAFA and PB blocks. Gray, white, andpink dots indicate epicenters of historical earthquakes along the Nicoya and Osa segments of the MAT and western NPDB [Abe, 1979; Ada-mek et al., 1987; Protti et al., 2001; Pacheco and Sykes, 1992; National Earthquake Information Center, 2012]. Dashed loops, rupture area forselected events. Color of dashed loops is the same as the corresponding epicenter. Inset: Estimated slip deficit on the (b) Nicoya and (c)Osa segments of the MAT.

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or nonmodeled geodynamic effectsof Cocos Ridge collision [e.g., LaFe-mina et al., 2009; van Benthem andGovers, 2010]. Although our best fitmodel does not require elasticstrain accumulation across the FilaCoste~na, our data are not inconsis-tent with the expected pattern ofdeformation across this system(Figure 6). If the full residual slipdeficit (59 mm a21) of the CO-PBmotion is accommodated by plasticdeformation across the Fila Coste~naand inboard of the Cocos Ridge,then the minimum slip deficitaccrued on the Osa segment sincethe 1983 earthquake is 0.4 m, 67%of the slip released during the 1983event (Figure 5c). Assuming a peri-

odic and slip predictable earthquake model, and strain partitioning across the plate boundary thrust andFila Coste~na Thrust Belt, this indicates that the next M> 7 earthquake may not occur until �2026 on theOsa segment.

Panama block-CA convergence along the western NPDB is 6.9 6 4.0 mm a21 at an azimuth of N40E 6 26�

(Figure 4a; Table S6), constraining the magnitude of back-arc thrusting. Large-magnitude earthquakes thathave occurred on this segment accommodate this relative motion. The 1822 Ms7.6 and 1991 Mw7.7 ElLimon earthquakes ruptured the same segments [Su�arez et al., 1995, and references therein], suggesting a�169 year recurrence interval. However, our estimated PB-CA convergence rate indicates that the 1822 MS

7.6 earthquake did not fully release the accumulated slip deficit. If 100% of convergence is accommodatedon the NPDB, and if earthquakes occur here periodically, then the recurrence interval for large 1991-type

0 100 200

0

20

40

OP FC CTSW NE

0.0 1.0

Phi

0

60

De

pth

(k

m)

CO CAPB

Rat

e (

mm

a)

20

40

Distance from trench (km)

MA

T

NP

DB

Figure 6. Velocity profile and generalized cross section for southern Costa Rica. Reddots show plate-motion parallel component of observed site velocity with 1r errorbars. Best fit model profile, black line. Model with the Fila Coste~na (FC) fault included,dashed line. Vertical dashed lines indicate the location of the MAT and the NPDB. Geo-logic cross section modified from Fisher et al. [2004]. Estimated coupling pattern (phi)for the CO-PB interface is shown. Location of modeled d�ecollement of FC and the PB-CA interface (i.e., NPDB) are in red.

0 1002000 200

km

Block Rate20 mm a relative to CA

CAFACA

CO

NZND

NA

CH

Cocos R

idge A

xis

2˚ N

4˚ N

6˚ N

8˚ N

10˚ N

12˚ N

14˚ N

16˚ N

94˚ W 92˚ W 90˚ W 88˚ W 86˚ W 84˚ W 82˚ W 80˚ W 78˚ W 76˚ W

NZCH

CAFAN

Figure 7. Proposed driving mechanism for motion of the Central America fore arc (CAFA) and Panama block (PB). Red vectors are relativeblocks rates from the best fit model. Motion of the Cocos Ridge and the ND and CH blocks, open white arrows. The CAFA and PB rotationssuggested by the best fit model, orange arrows. Theoretical rotation of the PB due to the Cocos Ridge collision, dashed blue arrow. Axis ofCocos Ridge, dashed blue line.

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earthquakes would be 220–830 years. In the second best fit model (Model 9-1), the CCRDB block convergeswith the CA at a rate of 15.8 6 9.1 mm a21 at an azimuth of N32E 6 38� along the western segment of theNPDB. This rate translates into a recurrence interval of 100–360 years for a 1991-type earthquake.

4.5. Escape Tectonics in the Western CaribbeanThe motion of the PB and CAFA away from the Cocos Ridge is indicative of tectonic escape [e.g., LaFeminaet al., 2009]. However, the eastern end of the region does not rotate clockwise toward the obliquely con-verging NZ as predicted for a rigid indenter [Molnar and Tapponnier, 1975]; rather, the motion is counter-clockwise toward the CA (Figure 7). Our best fit model predicts convergence of the ND block relative to theCA at 12.2 6 1.2 mm a21 at an azimuth of N56.4W 6 5.2� (Figure 4a; Table S6). The motion of the CH blockrelative to the CA is predicted to be 43.2 6 61.6 mm a21 at N81.9W 6 16.5� although it is poorly constrained(i.e., there is only one GPS site on this block; Figure 4a; Table S6). In our models, the ND-CA and ND-CH rela-tive motion is accommodated on the Atrato-Uraba fault zone (Figure 1). Here we propose that the currentmotion of the PB is driven by tectonic escape from Cocos Ridge collision with a rotation symmetrical to theCAFA; however, the CH and ND blocks drive the PB to the northwest inhibiting clockwise rotation of the PB,resulting in the northeast migration of the block and its continued collision with northwestern South Amer-ica (Figure 7).

5. Conclusions

Our kinematic block modeling demonstrates a major effect of Cocos Ridge collision on deformation anddevelopment of the western Caribbean. Northwest motion of the CAFA at 11–17 mm a21 is accommodatedby shallow destructive earthquakes along the Central America volcanic arc. Earthquakes within the CCRDBaccommodate relative extension between the CAFA and PB estimated here to be �15 mm a21. Ridge colli-sion causes uplift in the outer fore arc and shortening across the Fila Coste~na inboard of the Cocos Ridge,potentially at 50% of the CO-PB convergence rate. A pressing implication of our results is the rapid accumu-lation of interseismic strain along the Osa segment of the Middle America Trench for the next large, 1983-type earthquake in southern Costa Rica. Northeast motion of the Panama block is interpreted as tectonicescape from Cocos Ridge collision, redirected by northwestward forcing by and collision with the Chocoand North Andes blocks.

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