active orogeny of the south-central andes studied with gps geodesy

Revista de la Asociación Geológica Argentina 61 (4): 555-566 (2006) 555

INTRODUCTION

It has long been recognized that greatmountain belts are built, in large part, bycontractional processes (Argand 1924,Schiller 1912). In the Andes, for example,much of the crustal thickening due to the

most recent phase of Neogene orogenesisis attributed to tectonic shortening of theSouth American plate (Allmendinger et al.1997, Isacks 1988, Kley and Monaldi 1998,Ramos 1988, Ramos et al. 1996). Althoughthere is some question as to how much con-traction has occurred in the internal por-

tions of the mountain belt (Cristallini andRamos 2000, McQuarrie 2002, Ramos et al.1996) and the relative contribution of thin-or thick-skinned structures to crustal shor-tening is still uncertain (Kley and Monaldi1998), it is clear that during the last 10 Myrat least, the contractional belts along the

ACTIVE OROGENY OF THE SOUTH-CENTRAL ANDES STUDIED WITH GPS GEODESY

Eric KENDRICK1, Benjamin A. BROOKS2*, Michael BEVIS1, Robert SMALLEY Jr.3, Eduardo LAURIA4, Mario ARAUJO5

and Hector PARRA6

1 Department of Geological Sciences, Ohio State University, 125 South Oval 275 Mendenhall Laboratory, Columbus, OH 43210,USA, Email: [email protected] School of Ocean and Earth Science and Technology, University of Hawai'I, 1680 East-West Rd, Honolulu, HI 96822, USA,Email: [email protected] *corresponding author 3 Center for Earthquake Research and Information, University of Memphis, 3876 Central Ave Ste 1, Memphis, TN, 38152-3050,USA, Email: [email protected] Geográfico Militar Argentina, Cabildo 301, Buenos Aires, Argentina. Email: [email protected] Nacional de Prevencion Sismica, Roger Balet Norte 47. 5400 San Juan, Argentina, Email: [email protected] Instituto Geográfico Militar Chile, Dieciocho No 369, Santiago, Chile. Email: [email protected]

RESUMEN: Orogenia activa de los Andes centro-australes estudiada mediante geodesia de GPS.Se presentan mediciones GPS del campo de velocidad en los Andes centro-australes entre el extremo norte de Santa Cruz y la faja plegada deMalargüe. Se modela el campo de velocidad intersísmico como la combinación de una señal elástico/efímera asociada con el anclaje del límite prin-cipal de placas, y una componente constante, no reversible de desplazamiento asociada con una convergencia localizada en el retroarco y crecimien-to del cinturón montañoso andino. Se encuentra que esta segunda componente, por ejemplo en el desplazamiento actual relativo al cratón del ante-arco y los altos Andes, puede ser modelada muy bien como una rotación horaria constante de la microplaca andina alrededor de un polo de Eulerlocalizado en el sur de Argentina. Cerca de Malargüe, esta microplaca (o bloque) se mueve subparalelamente al rumbo del orógeno, transportandomaterial hacia la inflexión de los Andes Centrales. Más al norte, en el sector austral de los Andes Centrales, el movimiento de este bloque es casiperpendicular al rumbo de la cadena montañosa. Se sugiere que las tasas de deformación permanente en el retroarco varían entre un máximo de~ 6-7 mm/año en el Subandino Boliviano y menos de ~ 3 mm/año en la Precordillera Argentina y en la faja plegada de Malargüe. Es probableque la deformación activa más importante esté ocurriendo en una delgada faja (~ 50 km de ancho) asociada con el límite del retroarco (usualmen-te definido como el frente orogénico). En esta etapa, es imposible distinguir si estructuras específicas del retroarco está acumulando deformación.

Palabras clave: Campo de velocidades corticales, velocidad intersísmica, tasas de deformación, geodesia GPS, Andes Centrales.

ABSTRACT:We present GPS measurements of the crustal velocity field in the southern Central Andes between the Santa Cruz corner and the Malargüefold and thrust belt, and model this interseismic velocity field as the combination of an ephemeral, elastic signal associated with locking ofthe main plate boundary, and a steady and non-reversing component of displacement associated with localized backarc convergence andgrowth of the mountain belt. We find that this second component, i.e. the ongoing and permanent displacement of the forearc and the highAndes relative to the craton, can be modeled very well as a steady clockwise rotation of an Andean microplate about a pole located in sou-thern Argentina. Near the Malargüe Basin, this microplate (or block) is moving nearly parallel to the strike of the orogen, transporting mate-rial towards the bend in the central Andes. Farther north, in the southern limb of the Central Andes, the motion of this same crustal blockis directed nearly perpendicular to the strike of the mountain belt. Our results suggest that permanent deformation rates in the backarc rangefrom a maximum of ~ 6-7 mm/yr in the Bolivian Subandes to less than ~ 3 mm/yr in the Argentine Precordillera and Malargue fold andthrust belt. It is likely that most active backarc deformation is accruing in a narrow zone (~ 50 km wide) associated with the backarc boun-dary (usually defined as the thrust front) though at this stage it is impossible to distinguish whether specific backarc structures are activelyaccruing strain.

Keywords: Crustal velocity field, interseismic velocity, deformation rates, GPS geodesy, Central Andes.

0004-4822/02 $00.00 + $00.50 C 2006 Revista de la Asociación Geológica Argentina

E. KENDRICK, B. A . BROOKS, M. BEVIS, R . SMALLEY JR , E . LAURIA, M. ARAUJO AND H. PARRA556

eastern margin of the Andes (e.g.Baby et al.1997, e.g. Zapata and Allmendinger 1996b),referred to as either the foreland or the bac-karc, have accommodated most of theshortening that has occurred between thePacific coast and the stable interior of theSouth American plate. Localized backarcshortening continues today as evidenced bythe concentration of shallow crustal seismi-city (Smalley and Isacks 1990)(Fig. 1a) andthe fact that the easternmost backarc con-tractional structures are usually the oneswith evidence for most recent geologicalmovements (Brooks et al. 2000, Chiaramonteet al. 2000, e.g. Costa et al. 2000, Siame et al.2002, Zapata and Allmendinger 1996a).However fundamental questions regardingthe kinematics and geodynamics of Andeanorogenesis still remain. For instance, what isthe distribution of anelastic deformationacross the mountains and in the backarc?At any given time, is deformation uniformor is it focused on specific structures thatmay be seismically active? How does short-

term deformation contribute to the long-term growth of the mountains and how dolong-, intermediate-, and short-term ratescompare to one another? This is also aquestion of substantial practical importan-ce, particularly for seismic hazard assess-ments in the backarc, which has been thesite of large and/or damaging earthquakes(Castano 1995, Funning et al. 2005, Ka-dinsky-Cade et al. 1985).Over the past decade, the growth and matu-ration of large regional GPS geodetic net-works, such as CAP (Central Andes Pro-ject) (Bevis et al. 2001, Bevis et al. 1999,Brooks et al. 2003, Kendrick et al. 2001,Kendrick et al. 2003, Kendrick et al. 1999,Smalley et al. 2003), SAGA (South AmericanGeodyamic Activities) (Angermann et al.1999, Khazaradze and Klotz 2003, Klotz etal. 1999, 2001), and to a lesser degreeSNAPP (South America - Nazca Plate mo-tion Project) (Norabuena et al. 1998, No-rabuena et al. 1999), has illuminated the acti-ve deformation of the Andes from the

Santa Cruz bend in Bolivia to the southerntip of South America. Coastal GPS stationsnear the Nazca-South America plate boun-dary move towards the stable craton at ratesof 30 - 40 mm/yr while stations in the bac-karc move towards the craton at a third ofthis rate, or even more slowly (Fig. 1a). Theinterpretation of the GPS measurements isgreatly complicated by the fact that theinterseismic velocity field is (in very largepart) the superposition of two distinctmodes of deformation: (1) ephemeral, elas-tic earthquake-cycle deformation caused bylocking at the Nazca-South America plateboundary, and (2) ongoing accumulation ofpermanent deformation including back-arccontractional processes. Most geophysicistsagree that the elastic component of defor-mation dominates the present day crustalvelocity field within the forearc, but in thebackarc the permanent component of de-formation accounts for more than half ofthe observed velocity field (Bevis et al. 2001,Brooks et al. 2003, Liu et al. 2000).

Figure 1: a) GPS velocity fieldsuperimposed on Andean topo-graphy. Yellow dots representstation locations; black lines,velocity vectors; yellow ellipses,95% confidence ellipses; reddots, shallow crustal seismicity(< 30km ) from the catalog ofEngdahl (Engdahl et al. 1998).The map (and those following)is presented in a transverse mer-cator projection with an originchosen at 30°S, 70°W. b)Residual velocity field when thedisplacement field from a fullylocked Nazca boundary is remo-ved from the data in A. Whitetriangles are the dislocation ele-ments used to approximate theplate boundary locking displace-ment field with the upper (-10km) and lower (-50 km) bounda-ries constrained by earthquakecatalogs (Cahill and Isacks1992).

Active orogeny of the south-central Andes studied with GPS geodesy 557

Whereas the SNAPP group has inferredthat the main plate boundary is only par-tially locked during the interseismic phaseof the earthquake deformation cycle (No-rabuena et al. 1998), both the CAP and theSAGA teams have concluded independen-tly that the main plate boundary is fully loc-ked, which is hardly surprising given theextremely high rate of seismic energy relea-se along this plate interface (Bevis et al.2001, Brooks et al. 2003, Khazaradze andKlotz 2003).Different models have been proposed forthe ephemeral and non-ephemeral (or 'per-manent') modes of deformation. Liu et al.(2000) modeled the SNAPP data from thecentral Andes with a viscoelastic rheologydriven by different velocity boundary con-ditions (32-35 mm/yr and 12 mm/yr) atdifferent time-scales (< 102 and >106 years)and additional forces associated with topo-graphy. In this model the instantaneouscrustal velocity field, which declines linearlywith distance from the trench, ma-nifestselastic strain accumulation which is even-tually released in earthquakes. In contrast,the long-term behaviour of the sys-tem isthat of a viscous fluid. This long termdeformation field involves arc-normal con-traction of the submarine forearc re-gion,arc-normal extension of the Altipla-no, andarc-normal contraction of the foreland (orbackarc) fold and thrust belt. According toLiu et al.'s (2000) model the geological orpermanent stretching rate in the Altiplanois comparable in magnitude to the rate offoreland shortening, a finding that manygeologists would find hard to accept. Wesee two additional difficulties. First, Liu etal. (2000) compared their model velocitieswith the geodetic velocity solution ofNorabuena et al. (1998) which was shownby Kendrick et al. (2001) to have a large~7.5 mm/yr eastward velocity bias. Second,shortening of the foreland in Liu et al.'smodel is achieved by bulk or pervasiveshortening of the entire plate. But there is alarge body of geological evidence indicatingthat shortening within the foreland is domi-nated by shearing localized on a regionaldetachment surface or decollement thatunderlies a thrust belt or wedge. Naturally,no model in which a plate is acting as a fluidis capable of reproducing a discontinous

subsurface velocity field of the kind asso-ciated with a decollement. Liu et al's (2000)mathematical model is sophisticated, but itignores and is inconsistent with the establis-hed tectonic framework of the forelandfold and thrust belt.Lamb (2000) recognized that this heteroge-neity of deformation in a vertical sectionthrough the lithosphere violated the condi-tion of homogeneity assumed when appl-ying a thin viscous sheet model (Englandand McKenzie 1983) of deformation to theNeogene-modern velocity field in theBolivian Andes. He suggested that verticaland horizontal lithospheric viscosity varia-tions could be invoked to better explain thelate Miocene-recent velocity field. Additio-nally, in forming his Late Miocene-recentvelocity field for the Bolivian Andes from acombination of geologic and space geode-tic data, Lamb (2000) acknowledged boththe velocity bias in the Norabuena et al.(1998) data as well as the plate boundarylocking contribution to the geodetically de-termined velocities. His method, however,removed geodetic reference frame issues byconsidering relative velocities in a triangularnetwork of points and, at the time, thereported 1-sigma errors in the velocity datawere large enough (± 5 mm/yr) so that theelastic effect, especially in the backarc, waswithin the errors.Using more recently collected data withlonger occupation times, we have modeledthe composite velocity fields of the central(Bevis et al. 2001) and south-central Andes(Brooks et al. 2003) without invoking vis-cous behaviour in the lithosphere. We haveassumed that the interseismic velocity fieldarises through the superposition of twoquite distinct patterns of motion and defor-mation: ephemeral elastic signals driven bylocking of the main plate boundary supe-rimposed on asteady velocity field associa-ted with progressive backarc convergence(described by the kinematics of rigid micro-plates or blocks). Our earlier models citedabove, which can certainly be criticized assimplistic, were able to mimic the GPS ve-locity field with RMS misfits of a fewmm/yr or less, despite being equipped withrelatively few degrees of freedom. Our re-sults suggest that the permanent compo-nent of deformation within presently accu-

mulate in this plate boundary zone is con-sistent with a rigid Andean block overthrus-ting the South American craton at rates onthe order of ~3-6 mm/yr. These resultsbroadly agree with the analysis of theSAGA data set (Khazaradze and Klotz2003) as well as with some estimates of themost recent geological shortening rates inthe Argentine Eastern Precordillera (Zapataand Allmendinger 1996a) although, gene-rally, reported geological shortening ratesacross the entire backarc contractional beltstend to be higher (~10-12 mm/yr) (Echa-varria et al. 2003, Sheffels 1990, Zapata andAllmendinger 1996a). Our analysis impliesthat active deformation in the backarc is fo-cused on a narrow zone (< ~50 km wide)near the backarc thrust frontal zone; inother words, it does not appear that out ofsequence deformation of more hinterlandthrust sheets is currently detectable withGPS in the regions studied.In this paper, in order to test if the conclu-sions of rigid Andean block motion anddeformation accomodated in a narrow zo-ne at the thrust front are applicable acrosslarge portions of the entire Andes moun-tain chain, we extend our earlier analysesand model the geodetic velocity field fromthe Santa Cruz bend in the Central Andesto the Malargue fold and thrust belt. Themodel domain is close to 2000 km long(along strike) and over 800 km wide.

GPS DATA & NAZCABOUNDARY LOCKING

It is worthwhile here to review briefly themost pertinent aspects of GPS geodesy inthe above context of imaging permanentdeformation associated with Andean oro-genesis. The GPS data from the Andes todate comprise two main types: CGPS (con-tinuous GPS) and SGPS (survey GPS).CGPS sites operate on a continuous basis,while SGPS sites are occupied only occasio-nally (typically every 2 or 3 years) forperiods of one to several days. The CGPSsites form the reference backbone with res-pect to which we position the SGPS sitesusing a strategy called MOST (MultimodalOccupational Strategy) (Bevis et al. 1997).Because of their massive temporal avera-ging, CGPS data are more accurate than

E. KENDRICK, B. A. BROOKS, M. BEVIS, R. SMALLEY JR, E. LAURIA, M. ARAUJO AND H. PARRA558

SGPS data. Once a CGPS station has beenestablished for 4 years or more, its horizon-tal velocity estimates are accurate at 1-2mm/yr level, though the uncertainties asso-ciated with vertical velocity estimates are 2-3 times larger. Most published studies,however, cite mainly SGPS data becauseCGPS costs are so high that SGPS net-works are far denser. SGPS data quality aremore variable because of their short occu-pation times and they are more susceptibleto error sources such as equipment setup,atmospheric anomalies that may be spatiallyor temporally correlated over the timeperiods of observation, and annual defor-mation components related to hydrologicalloading effects (Bevis et al. 2005). Accordingly, in the Andes, when interpre-ting interseismic deformation (i.e. not thelarge cm-m scale coseismic motions thatattend large earthquakes, Klotz et al. 1999)

we only consider SGPS data that have threeor more occupations and a total time spangreater than 3 years. Furthermore, withSGPS data, we have to date only consideredhorizontal displacements because the errorsources in the vertical (particularly the hy-drologic ground noise) are currently toopoorly constrained and time series tooshort to establish reliable average rates. Aswe note below, for regions of high-anglebasement faulting such as the SierrasPampeanas, the vertical signal could be par-ticularly illustrative. We are not aware, ho-wever, of contemporaneous geophysicalstudies aimed at resolving short-term verti-cal motions using more traditonal techni-ques such as leveling (Kadinsky-Cade et al.1985).We installed the network of SGPS andCGPS stations presented here between1993 and 1997. SGPS measurements were

carried out from 1993 to 2001 in sessionsof at least 40 hours over consecutive days.Our field, processing, and velocity analysismethods have been described previously(Bevis et al. 1997, Bevis et al. 2001, Kendricket al. 2001). We realize a craton-fixed framenominally attached to stable South America(Fig.1, inset) with a root mean square(RMS) residual velocity of better than 1.0mm/yr. The principal characteristic of the GPSfield is its smoothness, both across- andalong-strike, yet one of the fundamentalcharacteristics of Neogene Andean backarcgeology south of the Arica bend is that it isstrongly heterogeneous in terms of shorte-ning style, inherited structures, and inferredgeodynamic controls (e.g. Kley and Monaldi1998). Generally, velocities are directed eas-twards towards the craton (Fig. 1). They arelargest (~35 mm/yr) near the coast, closer

Figure 2: a) Cartoon of the three plate model in a South America fixed reference frame. Magenta lines are the boundaries of the three modeledplates; N: Nazca; A: Andes; S: South America. Dashed magenta line is the locked portion of the subduction zone with degree of locking, L. b)Contour plot of the L2-norm error surface for the grid search in the region of the South American continent. The grid search was carried out forthe entire globe but the South American region was the region of lowest misfit. The plot is a slice through the 3-D parameter space at = 0.15º/yr.South of the white dashed line rotations are clockwise; north of the line rotations are counter-clockwise. c) Same as b with the slice at 39ºS latitu -de. d) Same as b, c, with the slice at 64ºW longitude.


to the Nazca boundary, and decline towardsthe craton. South of the Santa Cruz bend,backarc vectors tend to be rotated clockwi-se with respect to those in the forearc (Be-vis and Martel, 2001). As we noted pre-viously (Brooks et al. 2003), the velocityfield shows no obvious or abrupt change incharacter associated with the changing dipof the subducted Nazca plate (Fig.1).We estimate the ephemeral elastic displace-ments driven by locking of the plate inter-face in the subduction boundary using anelastic dislocation model. It is assumed thatthese displacements will be fully recoveredby the major plate boundary earthquakesthat frequently occur along the Chile trench(Klotz et al. 1999). This model is based onthe backslip formalism of Savage (1983)and it is implemented using triangular dislo-cations as discussed in Bevis et al. (2001)(Fig. 2). The principal source of error inthis type of modeling is that the calcula-tions are carried out in an elastic half-spacewith no earth-like horizontal boundariessuch as the base of the lithosphere andcrust. Nonetheless, close to the plate boun-dary, the half-space and other more sophis-ticated modeling techniques such as FiniteElement Models (FEM) (Williams andMcCaffrey, 2001) yield similar results; themain differences occur farther from thetrench.We assume that the plate interface is fullylocked all along the Nazca plate boundaryto a depth of 50 km, as we inferred in ourprevious studies (Bevis et al. 2001, Brooks etal. 2003), a conclusion also made byKhazaradze et al. (2003) in their analysis ofthe SAGA network. Lamb and Davis (2003)have suggested that this high degree ofcoupling, i.e. nearly complete and perfectlocking of the plate interface, has arisenbecause of the sediment-starved conditionof the trench since 20 Ma. The lower limitof ~50km for the coupling zone is broadlyagreed upon by various seismological stu-dies (Pacheco et al. 1993, Petersen et al.1996). Whether locking is constant downdip or not is a subject of some debate, butfor the purposes of our analyses it is asecond order effect and one which does notchange the fundamental nature of our con-clusions.In Figure 1b we present the residual velo-

city field obtained by removing the velocitycomponent due to full locking of the Nazcaboundary. Because the component due tolocking is an ephemeral signal that will berecovered by Chile trench earthquakes, weargue that this field describes the generalkinematics of the permanent componentof deformation captured over the ~8 yearsof GPS data. The residual field is characte-rized by ~north-directed vectors in the sou-thern Andes and east-directed vectors inthe central Andes. Peak magnitudes reach~13 mm/yr near the Chilean coast. Qua-litatively, this pattern mimics either a clock-wise horizontal rotation of material about apole in Argentina or flow of material fromthe south towards the north and east. Weclaim that this residual field is a fundamen-tal characteristic of current Andean defor-mation. Local details of the field will surelybe dependent on the choice of a modelused to describe Nazca boundary locking,but the general clockwise rotational charac-ter will remain. In order to more quantitati-vely investigate this residual field we expli-citly model it in the following section.

INVERTING THE GPSVELOCITY FIELD METHODOLOGY

As in our earlier papers, we consider thecrustal velocity field in the Andes in a frameof reference attached to the stable interiorof the South American plate, and assumethat it is composed of an elastic signal (dri-ven by locking of the main plate boundary)that eventually reverses itself in majorearthquakes (e.g.Savage 1983), plus a steadycomponent of motion due to ongoing,continuous convergence of the Andes andthe craton (achieved by creep of the deco-llement). This second, 'geological' compo-nent never self-reverses, but constitutespermanent relative displacement. We modelthe ephemeral component as an elastic per-turbation superimposed on steady plateconvergence. We model the geologicalcomponent using the kinematical theory ofplate (or microplate or block) motion on aspherical earth (Bevis et al. 2001, McCaffrey2002). We note, as an aside, that because ofsome confusion about our earlier termino-logy, we now use the word block instead of

plate motion to describe the long term ave-rage motion of the Andes relative to thecraton. The difference is semantic: there isno difference in the way we are modelingthe kinematics of a block and a plate (weuse Euler's theorem). Nonetheless, we ack-nowledge that the term plate may suggestto some readers that the Andean block dis-cussed here has been rigid for tens ofmillions of years, which is something thatwe do not claim in this context.To find the elastic component (Ue) of themodel displacement field we implement ourcalculations using triangular dislocation ele-ments in an elastic half-space (Bevis et al.2001). Triangular dislocations best reprodu-ce the smoothly curving geometry of thesubduction zone in this ~2000km longstretch of the Andes. To find the perma-nent component (Up) of the model we defi-ne an Andean block (A) located betweenthe Nazca (N) and South America (S) pla-tes. We assume that the A-S boundary cre-eps continuously at a constant rate on whatwe envision as a sub-horizontal décolle-ment rooted at deeper crustal levels bene-ath the Andean arc, though as noted byBevis et al. (2001), this subsurface geometrydoes not enter into the calculations them-selves. Up is then the surface velocity for theA block which we calculate using Euler vec-tors (ω(Cox and Hart, 1986) subject to theconstraint that:

ωNS = ωNA + ωAS (1).

When we superpose Ue and Up the velocityfield in A is the combination of elastic loa-ding and steady A-S plate convergence,whereas in S all motion is due to elastic loa-ding.The eastern boundary of the A block issubjectively chosen to correspond to ourbest knowledge of the position of the acti-ve Andean thrust front (e.g. Costa et al.2000). Moreover, the calculations do notdepend on, nor imply, vergence of specificstructures at the thrust front. Because thethrust front stretches for such a long distan-ce and well-defined Quaternary deforma-tion rates are only available from a smallnumber of sites along this distance, thegeometry of the boundary may be poorlyconstrained in sections. Where GPS station


spacing is dense we have taken care definethe boundary as precisely as possible.Where station spacing is sparse, however,the precise location of the surface tracedoes not affect the model results. Moreo-ver, the point of our analysis is not toattempt to solve for the spatial location ofthe thrust front at the current time; rather,it is to try to assess issues such as the parti-tioning of ephemeral and permanent mo-des of motion and deformation, and thelateral variations of the deformation field.We employ a simple grid search for ourinversion. By using the assumptions descri-bed in section 2 of full-locking to 50 kmdepth and by fixing the value of ωNS (61.1°N, 93.6° W, 0.57°/Myr) from our previousGPS-based analysis of Nazca-South Ame-rica convergence (Kendrick et al. 2003) wecan reduce the number of unknown para-meters in the inversion to the three parame-ters describing ωAS and it becomes compu-tationally feasible to use a grid search as theinversion approach. Grid searches are al-ways preferable to other inversion me-

thods because of their complete examina-tion of parameter space and their ability toaddress resolution and uniqueness a poste-riori.At each grid point in the 3-dimensionalparameter space, the inversion comprisesthe following steps: (1) assign ωAS and itscorresponding Up; (2) solve for ωNA inequation 1; (3) use ωNA to define theBurger's vectors (direction and magnitudeof slip) on the dislocations defining theNazca boundary; (4) from the Burger's vec-tors in (3) evaluate the forward problem tofind Ue; (5) estimate misfit using an L2norm; (6) proceed to the next grid point.

INVERSION RESULTS

The grid search finds a well-defined globalminimum with a best-fit AS pole at 39°Sand 64°W in south-central Argentina and aclockwise rotation rate of 0.15°/Myr (Fig.2a). There is also a local minimum associa-ted with counterclockwise rotation and anAS pole north of the Arica bend. Slices

through the best model position in ωAS lon-gitude/ωAS rate space (Figure 2b) as well asωAS rate/ωAS longitude space (Fig. 2c) con-firm the uniqueness of the best solution.Further, geological data demonstratingcounterclockwise rotations north of thebend and clockwise rotations south of it(e.g. Allmendinger et al. 2005, e.g. Lamb2000) give added preference to the sou-thern solution.In figure 3a we plot along with the data, thepredicted horizontal vectors from the bestmodel, which we call NAS-3. NAS-3 captu-res all of the first order features of thehorizontal velocity field including the mag-nitude of decrease with distance from thetrench as well as the more eastward-direc-ted displacements with distance from thetrench. The latter phenomenon, as descri-bed in Bevis and Martel (2001), is an effectof oblique convergence that can be explai-ned by a more rapid decay towards the bac-karc of trench parallel displacement incomparison to trench perpendicular displa-cement

Figura 3: a) GPS velocity fieldfrom NAS-3, data (black vectors),best 3-plate model (red vectors)superimposed on Andean topo-graphy. Magenta line denotes theeastern boundaries of the modeled'Andean' plate, with the hachures onthe overthrusting plate. The greencircle represents the position of theNAS-3 pole from the grid search(described in Fig. 2). b) Plot of pre-dicted permanent component ofdeformation (Up) from the NAS-3model.


Figure 3b shows just the permanent com-ponent, Up, of the NAS-3 model. In a simi-lar fashion to the residuals from the trench-locking elastic model (Fig. 1b) the predictedUp field displays clockwise rotation ofpoints about a pole indicated in the lowerright-hand corner of Figure 3b. Total pre-dicted horizontal velocity magnitudes rangefrom 1.6 to 6.7 mm/yr, while trench per-pendicular components range from 0.6mm/yr in the south to 6.3 mm/yr in thenorth (Fig. 5).

DISCUSSIONIMPLICATIONS FOR ANDEANEVOLUTION

In figure 4 we compare the latitudinal varia-tion (normalized by the maximum value) ofour modeled backarc permanent shorteningrate, Up, with Kley and Monaldi's (1998)compilation of geologic estimates of bac-karc shortening from balanced cross-sec-tions and the equivalent shortening curvebased on the crustal cross-sectional areafrom Isacks (1988). Both the equivalentshortening and the NAS-3 predicted Updecrease monotonically from a maximumnear 16ºS and both curves strongly coinci-de with one another, except for a smalldeviation in the equivalent shortening curvebetween 20º and 25º S. In apparent contrastto this, the shortening profile derived fromgeological balanced cross-sections does notfollow the same trend as the model Up orcrustal cross-section curves; rather, it decre-ases towards the south where it reaches aminimum at 25º S and then increases againnear 30º S.We find it unlikely that the agreement bet-ween the modeled Up and the equivalentshortening curve is pure coincidence. Whilethe Up curve is not sensitive to changes inAS rotation rate (changes in rotation rateswill not change the slope of the curve) it issensitive to changes in the AS pole position,particularly its latitude. AS pole latitudechanges of ~ 1 degree and longitude chan-ges of ~10 degrees will alter the predictedUp curve enough so that there is visual disa-greement with the equivalent shorteningcurve. In order for the two curves to becausally related, however, there must beminimal pre-Neogene contribution to the

cross-sectional area. In support of this, in areview of paleoelevation data from theCentral Andes, Gregory-Wodzicki (2000)concludes that the Altiplano-Puna plateauhad attained no more than a third of its3700 m current elevation by 20 Ma. Isacks (1988) argued that the growth oftopography that constitutes the modernCentral Andes was very largely achieved bycrustal shortening, and that latitudinal va-riation in the rate of shortening imposedthe curvature of this orocline and the verti-cal axis rotations preserved in the paleo-magnetic record (see also Allmendinger et

al. 2005). Practically all writers agree thatthe fundamental driver of this crustal shor-tening was Nazca - SoAm plate convergen-ce. Clearly this is not a complete explana-tion. Three additional factors are frequentlycited: (i) the average rate of subduction atthis boundary since 25 - 20 Ma has beenunusually high for a continental arc (Pardo-Casas and Molnar, 1987) (ii) major weake -ning of the leading edge of the SoAm plateby its long history of subduction (Isacks,1988), and (iii) consistently strong couplingacross the plate interface (Dewey and Bird1970). Lamb (2003) proposed that strong

Figure 4: Normalized magnitude versus latitude for: grey line, equivalent shortening curve deri-ved from crustal cross-sectional area (from Kley and Monaldi 1998; after Isacks 1988); yellowdots, predicted trench perpendicular component from the NAS-3 model; red dots, Neogeneshortening estimates from balanced cross-sections (compiled by Kley and Monaldi 1998).


interplate coupling has been controlled byclimatic conditions that have starved thetrench of sediments, thereby ensuring thatthe plate interface is very poorly lubricated.The position of the Euler pole describingNazca-South America convergence (and,accordingly, the direction of convergence atthe boundary) have changed very little(Gephart 1994, Kendrick et al. 2003, Pardo-Casas and Molnar 1987, Somoza 1998) (Fig.5a). The convergence rate, however, hassteadily declined from ~150 mm/yr betwe-en 20 and 25Ma to between 60-65 mm/yr

currently (Angermann et al. 1999, Kendricket al. 2003, Norabuena et al. 1999, Somoza1998) (Fig. 5b). While the magnitude of themaximum subduction rate has declinedwith time, at any given epoch the maximumrate of Nazca-SoAm plate convergence hasalways occurred near the middle of theCentral Andes Kendrick et al. 2003). IfAndean deformation is fundamentally dri-ven by plate convergence, it is reasonable toexpect that along-strike variation of bac-karc deformation also peaks towards thecentral portion of the Central Andes

(although climate conditions could perturbthis coincidence to a certain degree).So while the latitudinal variation of Up andthe shortening curve are very simlar oncethe two curves are suitably normalized, nei-ther of these curves looks anything like thebalanced cross-section curve (no matterhow it is normalized) since it has great de-partures from linearity (Fig. 5). We considerthe most likely explanations either: (1) theuncertainties or error bars associated withthe balanced cross-section measurementsmay be so large that the discrepancy withthe other curves is not statistically signifi-cant, or (2) there are significant contribu-tions to permanent deformation on geolo-gical timescales accommodated by proces-ses other than thrusting and folding in bac-karc contractional belts (after Kley andMonaldi 1998). We suggest that it is moreprobable that most of the discrepancy liesin the balanced cross-section reconstruc-tions for two reasons. It is very difficult todetermine formal estimates for the measu-rement errors or uncertainties associatedwith shortening estimates derived frombalanced cross-sections, even when there isa wealth of complementary geophysicaldata (e.g. Brooks et al. 2000). The techniqueof balanced cross sections often involvesdifficult-to-justify or difficult-to-quantifyassumptions such as the lack of transportin the direction perpendicular to each sec-tion, how the subsurface dip of a decolle-ment varies with depth (using thin-skinnedor thick-skinned models), or how muchshortening is underestimated when a mini-mum shortening value is reported (e.g.Woodward et al. 1989). Because Kley andMonaldi's compilation included estimatesfrom multiple authors each using distincttechniques and assumptions, actually trac-king and propagating the errors associatedwith each balanced cross-section is notreally viable, especially considering that rea-listic estimates of uniqueness and uncer-tainty were not reported in the original refe-rences. Second, the strong agreement bet-ween our modeled Up and the equivalentshortening curves (after normalization)adds support to the notion that time-inte-grated shortening within the Bolivian Sub-andes varies quite smoothly south of theSanta Cruz corner.

Figura 5: a) Euler pole positions from NUVEL-1A; the finite rotations obtained by Somoza(1998) and Pardo-Casas and Molnar (1987) (PC&M87) for 4.9-0 Ma (Somoza only) and 10.8-0Ma; and derived from GPS measurements from Kendrick et al. (2003). b) The Nazca-SoAmrotation rate versus time, after Kendrick et al. (2003). The average rotation rates implied by thevarious stage poles of Somoza (1998) and Pardo-Casas and Molnar (1987) are plotted at themidpoint of the averaging interval. The horizontal lines indicate the associated time intervals.Also shown are the NUVEL-1A rate and the GPS rate from Kendrick et al. 2003. The error barsfor the Kendrick et al. 2003 solution are too small to be easily visible at the scale of the plot.


IMPLICATIONS FOR BACKARCACTIVE TECTONICS

Because of the GPS technique's generallystated horizontal resolution of a 1-3 mm, itis tempting to try and interpret the data interms of activity in particular backarc re-gions or on particular structures. This is dif-ficult at this stage, however, because of therelative sparseness of the GPS stations andbecause of the relatively slow backarc ratesin comparison to the resolving power ofGPS measurements. When we consider thattotal backarc horizontal velocities (inclu-ding the plate-boundary ephemeral signal,i.e. Ue) are of order < 10 mm/yr, then themaximum velocities related to backarcdeformation are likely less than ~ 5 mm/yrand velocity differences (those related tobackarc strain) are of the order of less thana few mm/yr which approaches the resolu-tion of the technique itself. Of course, thesituation will improve: the longer we obser-ve using GPS, the lower the error levelsassociated with our velocity measurements.In light of this, the most important implica-tions of the NAS-3 model in terms of bac-karc active tectonics are: (1) active deforma-tion in the backarc is focused in a very na-rrow zone near the boundary between the'Andean' region and the South Ame-ricancraton and there is little permanent shorte-ning accumulating west of the Boli-vianSubandes and Precordillera; (2) backarcshortening rates increase from ~1 mm/yrin the south to ~ 6mm/yr in the north; (3)the permanent component of the relativedisplacements across the backarc zone ofdeformation implies transpressive strainingwithin the southern portion of this zone,and coaxial straining farther north; and (4)other 'active' regions, such as the SierrasPampeanas, do not exhibit any currentlydetectable signals of (horizontal) strainaccumulation.Of these implications, the first and secondessentially agree with geological and geo-physical results to within an order of mag-nitude, which we contend is the appropria-te scale of comparison at this time. Asnoted previously (Brooks et al. 2003), recor-ded backarc crustal earthquakes occur pre-dominantly in a narrow band associatedwith the eastern foothills of the Andes, and

this is also the area of the most focusedQuaternary deformation (Fig. 1a) e.g. Costaet al. 2000, Siame et al. 2002). Over geolo-gic time scales, most workers would agreethat the dominant portion of backarc de-formation has been focused at the thrustfront such as Argentina's Precordillera du-ring the Neogene (e.g Zapata and Allmen-dinger 1996b). Furthermore, although thereis a suggestion that GPS-determined per-manent shortening rates are slightly lowerthan geologically determined ones (e.g.Hindle et al. 2002) we consider the uncer-tainties in both data sets (particularly thedifficult-to-quantify geologic data sets) tobe too large to place much certainty on arate discrepancy, at least today.The predicted varying of strain conditionsin the backarc is one that can be comparedwith large data sets of fracture orientations,fault slip indicators, and detailed studies ofbackarc earthquake focal mechanisms. Forinstance, Dewey and Lamb (1992) showthat co-seismic backarc shortening direc-tions in our region of study are roughly co-axial with the plate convergence direction.However, as demonstrated by the GPS ve-locity field and discussed by us previously(Bevis et al. 2001, Bevis and Martel 2001,Brooks et al. 2003) backarc velocities arerotated clockwise by more than 10 degreesfrom the Nazca-South America convergen-ce direction. The Dewey and Lamb (1992)study had a regional focus, and in order tobest evaluate latitudinal variation in back-arcstrain accommodation more focused andup-to-date studies need to be undertaken.The fourth implication, that the SierrasPampeanas show no detectable signs ofpemanent deformation, is perhaps the mostinteresting and enigmatic. In the past 150years, several earthquakes greater than M 6have occurred across the Sierras Pampeanas(1944 Ms 7.4 San Juan, 1977 M 7.7 Caucete,1936 San Martin and 1934 M 5.5, 6.0Sampacho) (Costa, 1995, Costa et al. 2000,1999; Costa and Vita-Finzi 1996). Moreo-ver, the Sierras Pampeanas have certainlybeen the site of large finite strains sincetheir initial deformation ~ 6-10 Ma (Costaand Vita-Finzi 1996, Coughlin et al. 1998).Yet, our GPS results suggest that SierrasPampeanas strain rates are close to an orderof magnitude less than the neighboring

Precordillera to the west: across the entireSierras Pampeanas (distance of ~400 km),trench-perpendicular velocity differs by lessthan 2mm whereas the displacement gra-dient at the Mendoza Precordillera thrustfront is ~3-4 mm/yr of trench-perpendicu-lar velocity over ~20-30 km. Moreover,when considering that essentially none ofthe Sierras Pampeanas sites have velocitiesgreater than their errors when the plateboundary locking signal is removed (Fig.1b), the entire Sierras Pampeanas horizon-tal velocity field can be explained as anephemeral elastic effect from a fully-lockedNazca plate boundary. Perhaps modern dis-placements associated with the high anglefaults of the Sierras Pampeanas (Costa andVita-Finzi 1996) are largely vertical in natu-re, and thus are not manifested in the hori-zontal velocity field obtained using GPS.We suggest that understanding if theSierras Pampeanas are currently deforming,and if that deformation is truly contempo-raneous with Precordillera deformation, is afundamental question in lithospheric defor-mation that has important implications forseismic hazards assocated with some of Ar-gentina's population centers. For instance,one expects to need a very powerful vice tocompress a craton so strongly that the enti-re crust fails as in the Sierras Pampeanas,and competing mechanisms have been pro-posed: one is that a fully locked subductionboundary, such as the Nazca boundary forthe Andes, transmits end-load compressio-nal stresses across the mountain belt andinto the craton as implied by geometricmodels (e.g. DeCelles 2004); another is thattractions associated with the interface bet-ween a flat slab and the base of the overri-ding lithosphere produce the necessarycompressional stresses (Bird 1998). In theAndes, the apparent contemporaneity ofboth thin- and thick-skinned shortening, atleast over geological timescales (Jordan andAllmendinger 1986), creates a further me-chanical conundrum: would not the shorte-ning in the thin-skinned region alleviate thesignificant compression needed to form thethick-skinned ranges? This is an avenue offuture research that is best addressed bydensifying the GPS networks and by insta-lling continuous GPS stations so that verti-cal signals may be studied. Additionally,


SAR Interferometry (InSAR) (e.g. Euilladeset al. 2005, Pritchard and Simons 2002) stu-dies covering wide swaths will be invaluableto determining if the Sierras Pampeanas areactively deforming or not.

ACKNOWLEDGEMENTS

Funding provided by NSF grants EAR-9115576 and EAR-9615393. We gratefullyacknowledge the many people whose workwith us in in the field made this paper pos-sible. These include but are not limited to:A.A. Garcia, B. Knight, R. Barriga, W.Rubio, P. Gallardo, J. Baron, M. Blanco, P.Euillades, L. Euillades, V. Bonfils, G. Ca-brera, C. Gardini, J. Argento, S. Bea, J. Yu,M. Burns, and M. Patrick. Additionally wethank C. Vita-Finzi and E. Cristallini forthorough reviews which significantly im-proved the manuscript.

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Recibido: de , 2006Aceptado: de, 2006

active orogeny of the south-central andes studied with gps geodesy

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