the late cenozoic geodynamic evolution of the central segment of the andean subduction zone
TRANSCRIPT
305
ISSN 0016-8521, Geotectonics, 2009, Vol. 43, No. 4, pp. 305–323. © Pleiades Publishing, Inc., 2009.Original Russian Text © T.V. Romanyuk, 2009, published in Geotektonika, 2009, No. 4, pp. 63–83.
INTRODUCTION
The Andean subduction zone, as a convergentboundary between the oceanic Nazca Plate and theSouth American continent, is characterized by manyalmost extreme geological and geophysical parameters.The most important features are as follows.
(1) The fold–thrust belt of the Andean mountainchains extends along the entire western margin ofSouth America and is characterized by recent volcanicactivity at its western margin (Fig. 1b). The Altiplano–Puna Plateau, at elevations of ~4 km masl, is situated in thecentral segment of this continental margin (10–
30°
S).The height difference of the topography in the centralsegment of the Andean subduction zone attains 13 km:the depth of the oceanic trench is ~7 km, whereas theheight of the volcanoes in the Andes is ~6 km.
(2) GPS experiments [58, 63, 69, 76] have shownthat the oceanic Nazca Plate moves eastward with a
velocity of 7–8 cm/yr, slightly deviating to the north.The South American Plate moves approximately to thenorthwest with a velocity of 1–2 cm/yr (http://sideshow.jpl.nasa.gov/mbh/series.html). Thus, the rate of west-ern drift is about 0.5–1.0 cm/yr. Both movements takentogether ensure a rate of convergence in the Andeansubduction zone of about 9 cm/yr (one of the world’shighest rates of subduction).
(3) The Benioff seismofocal zone beneath the cen-tral Andes is traced down to a depth 670 km (Fig. 2a);earthquakes with magnitudes reaching 9.5 have beenrecorded here (Fig. 1a).
(4) The Andean mountain system is characterizedby extremely great crustal thickness (up to 70–75 km[104, 107]). In the Altiplano–Puna Plateau, the anom-aly of geoid is estimated at +60 m, while the Bougueranomaly reaches –400 mGal [41, 64], characterizingone of the greatest lithospheric density anomalies onthe Earth.
The Late Cenozoic Geodynamic Evolution of the Central Segment of the Andean Subduction Zone
T. V. Romanyuk
Institute of Physics of the Earth, Russian Academy of Sciences, Bol’shaya Gruzinskaya ul. 10, Moscow, 123995 Russiae-mail: [email protected]
Received March 26, 2007
Abstract
—The presented model of the Late Cenozoic geodynamic evolution of the central Andes and the com-plex tectonic, geological, and geophysical model of the Earth’s crust and upper mantle along the CentralAndean Transect, which crosses the Andean subduction zone along 21
°
S, are based on the integration of volu-minous and diverse data. The onset of the recent evolution of the central Andes is dated at the late Oligocene(27 Ma ago), when the local fluid-induced rheological attenuation of the continental lithosphere occurred farback of the subduction zone. Tectonic deformation started to develop in thick-skinned style above the attenu-ated domain in the upper mantle and then in the Earth’s crust, creating the bivergent system of the present-dayEastern Cordillera. The destruction of the continental lithosphere is correlated with ore mineralization in theBolivian tin belt, which presumably started at 16
°
S and spread to the north and to the south. Approximately19 Ma ago, the gently dipping Subandean Thrust Fault was formed beneath the Eastern Cordillera, along whichthe South American Platform began to thrust under the Andes with rapid thickening of the crust in the easternAndean Orogen owing to its doubling. The style of deformation in the upper crust above the Subandean ThrustFault changed from thick- to thin-skinned, and the deformation front migrated to the east inland, forming theSubandean system of folds and thrust faults verging largely eastward. The thickening of the crust was accom-panied by flows at the lower and/or middle crustal levels, delamination, and collapse of fragments of the lowercrust and lithospheric mantle beneath the Eastern Cordillera and Altiplano–Puna Plateau. As the thickness ofthe middle and lower crustal layers reached a critical thickness about 10 Ma ago, the viscoplastic flow in themeridional direction became more intense. Extension of the upper brittle crust was realized mainly in glidingand rotation of blocks along a rhombic fault system. Some blocks sank, creating sedimentary basins. The rateof southward migration estimated from the age of these basins is 26 km/Ma. Tectonic deformation was accom-panied by diverse magmatic activity (ignimbrite complexes, basaltic flows, shoshonitic volcanism, etc.) withinthe tract from the Western Cordillera to the western edge of the Eastern Cordillera 27–5 Ma ago with a peak at7 Ma; after this, it began to recede westward; by 5 Ma ago, the magmatic activity reached only the western partof the Altiplano–Puna Plateau, and it has been concentrated in the volcanic arc of the Western Cordillera duringthe last 2 Ma.
DOI:
10.1134/S0016852109040050
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The Central Andes are separated from the coastalregions by the Precordillera Fault Zone in the west andfrom the Chaco Plain by the Subandean Thrust-FaultZone in the east. In contrast to the South American Plat-form, which has remained stable since the Proterozoic[85, 99], the Andes were characterized by periods ofsedimentation, orogeny, rifting, and magmatic activity,at least from the Proterozoic [27, 93]. The onset ofrecent tectonomagmatic reactivation in the centralAndes, which is continuing now, is dated at the late Oli-
gocene [94]. High velocities of vertical and horizontalmovements, volcanism, and seismicity are noted atpresent.
The central Andes consist of five tectonic and oro-graphic provinces that extend parallel to the Pacificcoast and are subdivided into several subprovinces(Figs. 1, 2b): (i) the andesitic volcanic belt of the West-ern Cordillera, (ii) the high-elevated Altiplano–PunaPlateau, (iii) the fold–thrust complexes of the EasternCordillera, (iv) the IntraAndean, and (v) Subandean
75
°
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70
°
65
°
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°
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°
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mm/yr
Mw = 9.5; 1960
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Mw = 8.0; 19951
km
2
km
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km
77mm/yr (NUVEL-1A)GPS NZ-SA
South America
PACIFIC OCEAN
(a)
S
Main axis of Neogene magmatism
Boundary between hinterland and fold–nappe belt
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AltiplanoPlateau
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°
Central segment ofthe fold
-
nappe belt
Thin-skinneddeformation offoreland
Thick-skinneddeformation offoreland
PunaPlateau
(b) 40
°
S
Fig. 1.
The modern geodynamics of the Andean subduction zone. Panel (a): the integrated results of GPS observations at the westernmargin of the South American continent. Arrows indicate movements of stations relative to the rigid core of the South Americancontinent. To the north of
22°
S, the data from Cornell University, obtained over two years of observations(http://www.earth.nwu.edu/research/snapp.html) are presented. The arrow designated as GPS NZ-SA shows the motion of theNazca Plate toward South America, deduced from measurements at the stations located on the islands pertaining to the Nazca Plate.The arrow designated NUVEL-1A ~77 mm/yr shows the motion velocity of the Nasca Plate calculated from the NUVEL-1A model.To the south of
~22°
S, the data from [63], obtained over 1994–1996, are presented. The motion velocity of the Nazca Plate is esti-mated at 65 mm/yr. The hatched area covers the source zones of the two strongest earthquakes in the region; earthquake mechanismsare shown as a projection on the lower hemisphere. The schematic topography of the Cordilleras is shown in gray color. Panel (b):a generalized scheme of tectonic elements at the western margin of South America, after [60].
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Peru Bolivia
La PazThe Central Andean-
Argentina
Santiago
3.7 kmFig. 2b
(a)
Earthquakehypocenters, km
0–6060 –100100–200200–400400–700
(b)
PACIFICOCEAN
SouthAmerica
The Central
Oceanic trench Western
Cordillera
PunaPlateau
EasternCordillera and
Subandes
AltiplanoPlateau
70 57
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4050
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35 67 66 58
585967 51
5845425636
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°
Á.‰. 60
°
1
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7
Transect
Andean Transect
Fig. 2.
State-of-the-art seismic profiling and tectonic zoning of the central Andes. Panel (a): seismicity of the central Andean sub-duction zone. Earthquakes with magnitudes >5 and the line of the Central Andean Transect [1] are shown. The darkest domain inthe continuous-tone sketch of the of South American topography embraces elevations higher than 3.7 km. Panel (b): the main tec-tonic provinces (Western Cordillera, Altiplano and Puna plateaus, Eastern Cordillera, and Subandes) and the state-of-the-art seismicprofiling, after [12]. Numerals are depths of the Moho discontinuity. (
1
) DSS lines 1984–1989 on land; (
2
) DDS lines on land andpilot PISCO 1994 passive seismic experiments; (
3
) marine and on-land DSS lines and CINCA 1995 passive seismic experiments;(
4
) ANCORP-96 reflected-wave line and passive seismic experiments 1996; (
5
) PUNA 1997 passive seismic experiments;(
6
) BANJO-SEDA 1997 passive seismic experiments; (
7
) APVC 1999 passive seismic experiments.
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domains, which are characterized by slightly deformedbasement and thin-skinned tectonics of sedimentarycover.
The central Andes, as one of the most extremeregions of the Earth, attract special attention of geolo-gists and geophysicits and are studied actively. In par-ticular, important international projects have beenimplemented here in the most recent decades: PISCO1994, LITHOSCOPE 1994, BANJO-SEDA 1995,CINCA 1995, ANCORP 1996, PUNA 1997, APVC1999, ISSA 2000, ECCO 2001, etc. These projects haveenhanced the knowledge of this region, especially itscentral sector (Fig. 2b) to a quite new level. For exam-ple, the seismic structure of the continental crust hasbeen examined by deep seismic sounding [79, 91, 105],seismic reflection methods [7–9, 109], converted waves[21, 108], seismic tomography with P-waves [12, 43,47, 92], and with both P- and S-waves [32, 75, 97].Detailed gravity measurements [41, 64] and magneto-telluric sounding [22, 34, 82] have been carried out.Integrating works on the thermal regime [48, 50, 95],geology and tectonics [6, 10, 16, 17, 19, 40, 44, 47, 49,51, 54, 55, 59–62, 65, 66, 68, 70, 72, 77, 90, 93, 94,101–103], petrology, and geochemistry [13, 31, 36, 56, 57]have been published. As a result, the central segment ofthe Andean subduction zone has become one of the beststudied regions of the Earth with respect to its deepstructure. The Central Andean Transect along
~21°
S,the best studied geotraverse, has been comprehensivelyexamined by a complex of methods.
The model of the Late Cenozoic geodynamic evolutionof the central Andes and the complex tectonic, geological,and geophysical model of the Central Andean Transectpresented in this paper are based on an extensive body ofcomprehensive information and demonstrate the struc-tural features of the crust and upper mantle in the centralsegment of the Andean subduction zone.
THE LATE CENOZOIC TECTONICS OF THE CENTRAL ANDES
As early as in the 1980s, it became clear that the lateOligocene is a special moment in the evolution of theAndean subduction zone [54, 94]. Approximately27 Ma ago, the Nazca and Cocos plates separated fromthe Farallon Plate and began their own movement withsharply increasing rate of convergence between theNazca and South American plates [78]. Further, duringthe Neogene and Quaternary, the geodynamic settingsalong the Pacific margin of South America underwentsubstantial rearrangement, including change of the sub-duction angle of the Nazca Plate in different segments;enhanced lateral shortening of the central Andes; acti-vation of magmatic activity (Fig. 3), etc.
At least three stages are recognized in the tectonicevolution of the central Andes (Fig. 4). At the first stage(27–19 Ma ago), thrusting and faulting of both westernand eastern vergence occurred in the Eastern Cordil-
lera, and the Altiplano–Puma Plateau began slowly ris-ing. During the second and the third stages, the forma-tion of westward-verging folds practically ceased, andthe eastern front of deformation started to migrateinland; 10 Ma ago it reached the present-day IntraAn-dean domain and 5 Ma ago, the Subandean domain.The third stage, which began about 10 Ma ago, is char-acterized by an abrupt increase in the intensity of ign-imbrite magmatism at the Altiplano–Puma Plateau(Fig. 3) and by uplift of the entire orogen continuinguntil now. No less than 2 km of uplift of the centralAndes pertain to the last 10 Ma [10, 35, 37, 44, 54].
Post-Oligocene tectonic deformations were accom-panied by magmatic activity, which at the initialmoment of activation covered the territory from thepresent-day Western Cordillera to the western bound-ary of the Eastern Cordillera. In particular, one of theworld’s largest Frailes ignimbrite complexes, dated at7 Ma, was located at the boundary between Altiplanoand the Eastern Cordillera. Beginning from 5 Ma ago,the magmatic activity receded to the west; it is currentlylocalized only in the Western Cordillera [13, 56, 57].
High-resolution seismic tomography [12, 19, 43, 92]and geochemical data [13, 56, 57] testify to possibledelamination of the continental lithosphere beneath theAltiplano–Puna Plateau in the central Andes. Togetherwith GPS evidence for slow migration of the Andesinland to the South American Platform, this implies theactive role of the continental lithosphere in the LateCenozoic activation of the central Andes.
THE CENTRAL ANDEAN TRANSECT: A COMPLEX MODEL
Initial Data
The composite tectonic, geological, and geophysi-cal model of the Central Andean Transect is shown inFig. 5. The structure of the continental crust is deducedfrom the DDS section [105]. The details of the structurein the coastal zone, in particular the thickness of low-velocity rocks on the shelf and the oceanic Moho dis-continuity, are given after [79]. The velocities in themantle wedge are shown according to [12, 43, 91]. TheBenioff zone down to a depth of 670 m (Fig. 2a)beneath the margin of the South American continent iscontoured from the hypocenters of strong earthquakes[23, 104] determined from the global seismic network.The surface of the subducted Nazca Plate near the Cen-tral Andean Transect within a depth interval down to50 km is mapped from the high-resolution determina-tions of hypocenters of weak earthquakes recorded bya local network of stations (Fig. 2b); within the depthinterval of 50–100 km it is mapped from processing ofreflected wave records [8, 9]; and below 100 km it ismapped from converted waves [21, 108]. The zones of ele-vated electric conductivity beneath the PrecordilleranFault Zone and the Western Cordillera are contoured fromthe results of magnetotelluric sounding [22, 34]. The
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THE LATE CENOZOIC GEODYNAMIC EVOLUTION OF THE CENTRAL 309
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°
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°
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Oxaya
Chair Kkollubasaltic
28–17 Ma
Bolivia
Rondal
Chile Argentina
Basalticfields
flows
Shoshonites
Queva
Aquilar
Aqua Dulce
La Coipa
Segerstrom
Altosde Pica
(a)
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°
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°
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°
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°
68° 67° 66° 65° 64°
S
17–12 Ma
(b)
20°
21°
22°
23°
24°
25°
26°
70° W 69° 68° 67° 66° 65° 64°
S
12–3 Ma
(c)
20°
21°
22°
23°
24°
25°
26°
70° W 69° 68° 67° 66° 65° 64°
S
3–0 Ma
(d)
CerroRico
Tasna
ChocayaMorokho
Bonete
Smalldomes >13
Queva
AguaEscondida
Galan
ValleAncho
19°
Morococola 7
Frailes 7
Pastos Grandes 3.6–8
Panizos 7
Guacha 4.8 Vilama Cerito 7–8.5Corarzull 6–7Rachaite>7
Pairique 11La Pacana 4–5.5
Queva 7–10V. AntofallaGroup 7–11
Coplapo
basaltic <7Galan4–6
flows
7–11 Laguna VerdeGroup 3–4
19°
The main present-day
volcanic zone
Shoshonites
AltiplanoPlateau
PunaPlateau
Alkalibasalts F
ol
d-
na
pp
eb
el
t
of
Ea
st e
r nC
or d
i ll e
r aShoshonites
Alkalibasalts
Vol
cani
c ar
c of
Wes
tern
Co
rdil
lera
Solo
ChaoTuzgle
Cerro Gale 2.2–2.7
Fig. 3. The largest dated magmatic fields (inscriptions) and centers (symbols), after [57]. Calderas filled with ignimbrites are des-ignated by circles and stratovolcanoes by squares. The size of the symbols is approximately proportional to the size of the magmaticsystems; age, Ma is shown near the names. The open circles in panel (c) indicate events younger than 7 Ma; the filled circles indicateevents older than 7 Ma. APVC is the Altiplano–Puna Volcanic Complex (Panizos–Palrique).
structure of the uppermost crust in the central Andes cor-responds to the integral geological section [73].
The morphology and depth of the PrecoldilleranFault are shown in the style of [109] for 33° S. Regions
of strong seismic reflectors, visible as bright spots inrecord-section presentations [9], were revealed beneaththe Western Cordillera and at the base of the Uni–Ken-wani and San Vicente folds; the Quebrada Blanca spot
agoago
agoago
Kari Kari
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00.01
0
50
A ~40–19 Ma
SA Platform
ç2O?
0
50
km
km
~19 Ma
~19–10 MaåÄí
åÄF 10–5 Ma
0
50
0
50 km
WesternCordillera
Altiplano-Puna Plateau
EasternCordillera
IntraAndean Domain
Subandean Domain
SUT 5–0 Ma0
50 km
100
Flow perpendicular tofigure plane
200 400 600
0.1
1
10Paleoflora
11 Ma
Paleoflora17.5 Ma Uplifted
peneplanesurfaces 11 Ma Inversion
of uplift rateimplying
homogeneouscrustal
shortening
B
a
b
Uplift
mm/yr
ForelandAltiplanoEastern Cordillera Subandes
MohoR i g i d p l a t eV i s c o u s f l o w
Moho Continental lithosphere
50 km
H2O?Suggested
fluidimpact
attenuatingcontinentallithosphere
Domain ofmagmatic activity
Rate ofuplift
Magmauplift
Normal and thrust faults
‡ bLower
crust of SA Platforma) mafic and
b) eclogitized or undergone other
high-pressuretransformations
Fragments of delaminated
lower crust andlithospheric
mantle
Western Cordillera
Altiplano–Puna Plateau
EasternCordillera
IntraAndeanDomain
SubandeanDomain
km
I
I
II
III
III
rate,
GEOTECTONICS Vol. 43 No. 4 2009
THE LATE CENOZOIC GEODYNAMIC EVOLUTION OF THE CENTRAL 311
(QBBS) is the brightest. It is important that the Mohodiscontinuity is traced along the DSS line only frag-mentarily and is lost in some segments. Another appre-ciable feature is the considerable discrepancy in theposition of the Moho discontinuity determined fromrefracted and converted waves.
The slope and position of the Subandean ThrustFault in the upper crust were estimated from the resultsof a seismic survey [7]. Both the seismic reflectionmethod [4] and magnetotelluric sounding [82] testify tothe existence of a specific weakened zone at the exten-sion of the Subandean Thrust Fault, which plungesbeneath the region of thin-skinned tectonics in the Sub-andes. This zone is described in [70] as the bottom of afold complex.
The crustal structure in the zone transitional fromthe Subandes to the Chaco Plain is poorly studied byseismic survey and therefore is approximated by arbi-trary horizontal layers. The topography of the sedimen-tary basin bottom close to the Subandean Thrust Zonewas estimated from a density model [87].
The oceanic crust of the subducted Nazca Plate, 30–40 Ma in age, is approximated by basaltic layer 2C andgabbroic layer 3. The seismic velocities and densities inboth layers increase with depth. It is suggested thatlayer 2 pinches out by a depth of 20 km because thepores in the basalt are closed at a high pressure so thatthe density of the basalt approaches the density of thegabbro [24]. No data are available on the depth of theasthenosphere under the Nazca Plate. The thickness ofthis plate is accepted in correspondence to its age andtemperature [25].
Petrological Estimates for Some Deep Elementsof the Model
Four different P–T paths are recognized along theCentral Andean Transect: (1) the path of the oceaniclithosphere, about 40 Ma in age; (2) the path of layer 3(gabbroic layer) of the oceanic crust, pertaining to thesubducted Nazca Plate; (3) the path beneath the West-ern Cordillera and Altiplano Plateau; and (4) the path ofthe Precambrian South American continental platform(Fig. 6a). These paths are plotted on a diagram with thestability fields of mineral assemblages in the metabasicrocks (Fig. 6b); the lines of dry and wet solidus of peri-dotite, quartz–coesite transition, etc., are added. Thisplot makes it possible to estimate the state and densityof the matter at a depth. It should be noted that the fieldsof metamorphic facies of basic rocks are not applicable to
the upper and middle continental crust (paths 3 and 4),which is mainly sialic in composition.
Path 1: the oceanic lithosphere. Because data onthe seismic structure of the lithosphere of the NazcaPlate are not available, the calculated temperature dis-tribution [95] was used for its subdivision into layers.The stationary path (1) for the oceanic lithosphere,40 Ma in age, predicts a temperature of 700°ë at thebottom of the apparently elastic part of the plate at adepth of 50 km. The wet melting of peridotite at a depthof 70 km occurs at a temperature of 900°ë, while drymelting in the asthenosphere at a depth of 100 kmrequires 1300–1400°ë.
Path 2: the subducted oceanic crust (gabbroiclayer). According to [33], the Andean subduction zoneis cold. In the oceanic crust of the Nazca Plate, the gab-bro–eclogite transition is possible at a depth of 120 km.This estimate is consistent with the results of the studyof the Andean subduction zone using converted waves[108], which indicates that the oceanic crust is tracedhere to a depth of 120 km.
Path 3: the Western Cordillera and Altiplano.The temperature of the middle and lower crust beneaththe Altiplano Plateau remains uncertain; only the twoextreme variants are shown in Fig. 6b by dashed lines.The high-temperature path calculated in [95] corre-sponds to the stationary model. In this case, the temper-ature at the Moho discontinuity is estimated at~1200°ë. The low-temperature path corresponds to themiddle and upper crust, created over the last 27 Ma,mainly at the expense of the supply of cold material ofthe South American Platform. If this was the case, thetemperature at a depth of 70 km (Moho discontinuity)should be not higher than 700°ë and the P–T condi-tions at depth of 70 km (point T shown in Fig. 6b withaccount of 4-km height of mountains) should corre-spond to quartz–coesite transition. Complex seismicsurveying shows that the middle and lower crustbeneath the Altiplano is characterized by low andmedium velocities with the Poisson ratio varying fromlow to normal values. The crust is felsic or intermediatein composition [97] with a temperature of about 800°Cat the base. This estimate is consistent with a low(<10%) magmatic addition to the crust and the largelytectonic nature of its thickening. The temperature of thelower crust and the upper mantle fits the conditions ofgranulite-facies metamorphism (Fig. 6b) and wet par-tial melting of felsic rocks.
Fig. 4. The Late Cenozoic tectonic evolution of the Central Andes: (A) Tectonic horizontal shortening and vertical thickening of thecrust along the Central Andean Transect. The Roman numerals are stage numbers; see Fig. 5 for the abbreviations of faults. Ele-ments of the crust are shown by patterns and gray color; (B) suggested styles of crustal deformation explaining variable rates ofcrustal shortening and uplift in different domains of the central Andes, after [66]: (a) gray domains correspond to the rate of upliftingcalculated from the inverted rate of deformation on the assumption of homogeneous crustal thickening; over the last 10 Ma, theAltiplano and the Eastern Cordillera have experienced significant uplift with minimal horizontal shortening; (b) the South AmericanPlatform is thrust as a rigid block under the Subandes and the Eastern Cordillera; further, beneath the Altiplano, the lower crust andthe upper mantle underwent viscoplastic deformation. See Fig. 5 for other notations.
312
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5.6
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n w
ithV
p >
8.4
km
/s d
etec
ted
by
Depth, km
~ 1
cm/y
r
700°
C
900°
C
1250
°C
500°
C6.
7
8.2
6.2
5.9
8.1
100
200
seis
mic
tom
ogra
phy
GEOTECTONICS Vol. 43 No. 4 2009
THE LATE CENOZOIC GEODYNAMIC EVOLUTION OF THE CENTRAL 313
Path 4: the South American Platform. The calcu-lated temperature distribution [95] was used for subdi-vision of the continental mantle into layers. Stationarypath 4 for the continental lithosphere estimates the tem-perature at the bottom of the apparently elastic plate at700°ë (a depth of 70 km); the wet melting of peridotiteat a depth of 120 km requires 900°ë; at a depth of 200 km,the temperature reaches 1250°ë. The bottom of thecrust beneath the South American Platform is located ata depth of 40 km (temperature is estimated at ~500°ë)(Fig. 6b). For the onset of eclogitization or other high-pressure transformation of the lower crustal basicrocks, downgoing of the lower crust by 10 km or heat-ing by 100°ë is sufficient.
A Density Model
The gravity field in Bouguer reduction above thecentral Andes is characterized by a deep and flat mini-mum more than 1000 km in width (Fig. 5). Preliminarydensity models [41, 42, 45, 87] and additional calcula-tions for a model with constant densities in the crustallayers of the South American Platform and the oceaniccrust of the subducted Nazca Plate revealed a defi-ciency of almost 200 mGal and thus showed a high den-sity of the mantle beneath the Andes. Therefore, an ulti-mate density model has been constructed, with a down-ward increase in density of the layers in moving plates,taking the effect of metamorphism into account (Fig. 6b).The relationships of density versus depth in the layersselected in Fig. 6a are shown in Fig. 6c.
The abrupt changes of average density of the con-solidated crust from the Coastal Zone to the WesternCordillera and from the Subandean Zone to theIntraAndean Zone are shown in Fig. 5. The first featuredepends on the changed structure and temperatureregime of the crust, and the second one marks theboundary between the doubled and weighted crust ofthe South American Platform and the lightened uppercrust of the Andean mountain chain. The deep-seatedpositive density anomaly beneath the Altiplano isrelated to the crust–mantle transitional zone.
MANIFESTATION OF ANCIENT AND RECENT GEODYNAMIC PROCESSES IN THE PRESENT-
DAY LITHOSPHERE ALONG THE CENTRAL ANDEAN TRANSECT
The subducted Nasca oceanic plate and mantlewedge. The Nazca oceanic plate, being subductedbeneath the continental margin, undergoes dehydrationin the upper part of the plate and eclogitization of basaltand gabbro in the oceanic crust [29, 33, 39, 46, 80, 81,83, 96]. The fluid released from the slab gives rise toserpentinization of peridotite in the mantle wedge andits wet melting, leading to a decrease in seismic veloc-ities and reducing the Q-factor of the wedge [12, 92].
The upper crust of the central Andes. The uppercrust is a layer bounded from below by a high-velocitylayer (HVL) at a depth of ~20 km (Fig. 5). The uppercrustal layer is a lens that thickens to 20–25 km beneaththe mountains and pinches out near the shore and theSubandean Thrust Fault. This is a relict sedimentarybasin of the marginal continental type, formed in thePaleozoic at the present-day western periphery of theancient core of the continent of South America ornearby [17, 27]. The HVL that separates the upper crustof the Andes from the lower crust may be interpreted asa relic of the basified continental crust and/or oceaniccrust of the former marginal sea. The bottom of theHVL is treated as a paleoMoho [40].
The coastal zone. Presumably in the Late Jurassic,the volcanic island arc was thrust under the backarcsedimentary basin from the side of ocean [85], and thevolcanosedimentary crust between the coast andpresent-day volcanic arc of the Western Cordillera wasdoubled. The cold crust is characterized here byinclined seismic boundaries and high average seismicvelocity and density.
The Western Cordillera is a volcanic belt withspectacular post-Miocene volcanic activity. The fluidreleased from the subducted Nazca Plate disturbed thegeochemical equilibrium of the overlying rocks andprovoked wet melting of peridotite in the mantlewedge, producing suprasubduction magmatism.
The Altiplano Plateau is a high-standing (>3.7 km)slightly differentiated block. It is a basin filled with Ter-tiary and Quaternary sedimentary and volcanic rocks
Fig. 5. The integrated tectonic, geological, and geophysical model along the Central Andean Transect.See Fig. 2 for transect location. The relatively dark color corresponds to elevated seismic velocities and densities; the scales of tonesfor the crust and the mantle are different. Numerals are seismic velocities, km/s. The heavy solid lines in the upper crust are regionalwrench and thrust faults: PC, Precordilleran Fault Zone; WF, Western Fissure; UK, Uni–Kenwani Fault Zone; SV, San Vicente FaultZone; T, Tupiza Thrust Fault; C, Camargo Fault Zone; MAT, Main Andean Thrust Fault; MAF, Main Frontal Thrust Fault; SUT,Subandean Thrust Fault. The structure of the uppermost crust of the central Andes corresponds to the integrated geological section,after [73]. The black spot in the crust indicates the approximate position of bright reflectors (QBBS) in reflection record sections [9].HVZ is the high-velocity layer from DSS data at a depth of 20 km; its bottom is interpreted as a paleoMoho [40]. LVZ is the low-velocity zone of converted waves beneath the Altiplano Plateau interpreted as a domain of magmatic and fluid activity [108]. Thegray dots are zones of elevated electric conductivity contoured by magnetotelluric sounding [22]. The isotherms are shown after[95]; the isotherm of 900°C beneath the Altiplano is corrected in agreement with estimates in [97]. The domain marked by blackdots with the inscription Q = 250 corresponds to the reduced seismic quality, after [92]. The high-velocity (Vp > 8.4 km/s) domainin the continental lithosphere beneath the Altiplano (dark color) was detected by seismic tomography [43].
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~Depth, km
200
400
600
800
1000
1200
12345
1400
8
(a)
1
Bot
tom
of
appa
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Pressure, GPa
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T
GEOTECTONICS Vol. 43 No. 4 2009
THE LATE CENOZOIC GEODYNAMIC EVOLUTION OF THE CENTRAL 315
and located between the still higher Western and East-ern Cordilleras [68].
The middle and lower crust beneath the WesternCordillera and Altiplano Plateau. The seismic surveydemonstrates very low velocity throughout the crustand the absence of HLV beneath the Western Cordilleraand Altiplano. The seismic survey did not reveal dis-tinct boundaries within the middle and lower crust,which is devoid of layered structure beneath the West-ern Cordillera and Altiplano. Because the electric resis-tance and seismic Q-factor sharply decreased in thisblock [12], it may be suggested that the present-day orrecent magmatic and/or fluid activity was predominanthere. This process obliterated the preceding structuralgrain. The crustal and upper mantle rocks are intrudedby intrusive bodies and metamorphosed. In particular, aspecial study using converted waves [26] established alow-velocity zone (Vs < 0.5 km/s) interpreted as a sill-like magmatic body 750–810 m thick beneath theAPVC volcanic complex (see Fig. 3 for the complexlocation) at a depth of 19 km.
As concerns the bright seismic reflectors in theupper crust (QBBS is the brightest spot, see Fig. 5), ithas been shown that these phenomena are recorded inthe domains of intense tectonic deformation andlocated in the transitional zone from elastic deforma-tion and brittle failure to the viscoplastic flow. Com-monly, these are root zones of newly formed activefaults accompanied by high seismicity [89]. In domainsof intense deformation, rocks accumulate elastic energy(a future earthquake source). When such domains aretransmitted by seismic waves, the medium irradiates apart of the stored energy due to nonlinear effects; this isexpressed in the recorded sections of reflected waves asarrivals with increased amplitudes, i.e., bright spots,which mark the strained state of the rocks and probablytheir saturation with fluids or magmatic melt. However,it cannot be ruled out that both processes are interre-lated.
The Moho discontinuity beneath the WesternCordillera and Altiplano Plateau. No sharp boundarybetween the crust and the mantle is detected beneath theWestern Cordillera and Altiplano. In the records ofrefracted seismic waves, Pn arrivals are almost not dis-cernible. Beneath the Altiplano, only two short reflec-tors are noted at a depth of about 70 km [105]. Theirposition deviates rather strongly from the position ofthe receiver function Moho at a depth of 60 km [107,108]. The isostatic Moho beneath the central Andes is
estimated at 66–72 km [42]. All estimates are close tothe maximum possible thickness of the crust controlledby the depth limit of quartz stability (65–75 km)deduced from quartz–coesite transition [28]. It is mostlikely that at this depth the crust–mantle boundary is agradient zone 10–15 km thick, transitional from largelycrustal to predominantly mantle rocks.
The Eastern Cordillera, IntraAndian, and Sub-andian zones are a fold-nappe belt mainly composedof Paleozoic and Mesozoic sedimentary rocks. The old-est rocks now exposed in the Eastern Cordillera are theOrdovician metasedimentary complexes sporadicallyoverlapped by Cretaceous and Tertiary sediments. Sil-urian to Triassic rocks crop out in the IntraAndeanZone, while Carboniferous to Pliocene rocks areexposed in the Subandean Zone.
Currently, this fold–nappe belt is amagmatic; how-ever, igneous rocks related to the initial stage of Andeantectogenesis are identified in the boundary zonebetween Altiplano and the Eastern Cordillera. Thismagmatic episode is not related directly to supra-subduction magmatism, and to a great extent is a prod-uct of melting of the upper 10–15 km of the crustcomposed of specific granitic series [74]. In particu-lar, S-granites derived from metasedimentary sourceshave been identified here.
The South American Platform moves as a rigidplate beneath the Subandes. Zones of plastic and vis-cous deformations appear at a depth, and the beds arethickening due to the folding. Mainly viscous and plas-tic deformations occur beneath the Altiplano [66].
Metamorphism in the subducted continental crustgradually increases the density and seismic velocity. Inthe lower crust, high-pressure metamorphism (Fig. 6)increases the density of rocks up to the mantle values.Because of this, the refractive Moho discontinuity islost beneath the Eastern Cordillera and the structuralbottom of the subducted crust of the South AmericanPlatform does not coincide with the currently recordedrefractive seismic M surface, being located at a greaterdepth between the Altiplano and the Eastern Cordillera(Fig. 5).
The continental upper mantle. The tectonic short-ening of the crust in the central Andes implies a corre-sponding rearrangement in the continental lithosphere.As follows from the geochemical data, the lithosphereof the South American Platform is traceable westwardnot farther than to 65–66° W [3]. The seismic data and
Fig. 6. Composition and density of the lithosphere in the Central Andes: (a) schematic section along the Central Andean Transectand location of P–T paths (numerals in circles correspond to the numerals in panel (b); numerals in squares correspond to the numer-als in panel (c)); (b) P–T stability fields of mineral assemblages pertaining to most important metamorphic facies (basaltic protolith),after [80]; dry and wet peridotite solidus lines and line of quartz–coesite equilibrium, after [28]; P–T paths (numerals in circles):(1) oceanic lithosphere, (2) gabbroic layer of the subducted oceanic crust, (3) lithosphere beneath the Western Cordillera and theAltiplano Plateau, (4) continental lithosphere of the South American Platform; the horizontal dashes on the paths mark the Mohodiscontinuity; (c) density versus depth relationships obtained from inversion of gravity field in some segments of the Central AndeanTransect; each symbol on the plot corresponds to one block of the model; the patterns of the symbols match the patterns of layersin panel (a); the proved and inferred relationships are shown by solid and dashed lines, respectively.
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Shoreline
(a) 22°S
24
SubductedNazca Plate
Crustalmagma chambers
Crust–mantletransition zone
Coastal Cordillera
Cordillera
Western
Lower crustaleclogite bodies
Sinking of lithosphericfragments into the mantle Calderas
Puna Plateau
EasternSubandes
Cordillera
70° W 68 66
(b)
AtacamaUKF
1 2 Lipez
3 45
67
8
9
101112
13
AF
14/15
Arizaro
Santa Maria
IF
16
17 18
20
TTF19 Pipanaco
La Riojia
1
2
68° W 66° 64°
22°S
23°
24°
26°
27°
28°
29°
(c)18°
50 40 30 20 10 S
20°22°24°26°28°30°
Latitude
Age, Ma
26km/Ma
Corque (6–14 km)Lipez (>4 km)
Atacama (3.7 km)Antofalla (1.3 km)Hombre Muerto (5 km)
Santa Maria (2–5 kmRipanaco (0.5 km)
La Riojia (0–0.4 km)
Fig. 7. Neogene deformation of the Puna Plateau andthe southern part of the central Andes, after [86].(a) Meridional extension of the upper crust in the PunaPlateau increased 10 Ma ago by reaching the criticalthickness of the crust. Arrows indicate general direc-tions of shortening and elongation in the brittle uppercrust. Extension is realized in strike-slip offsets androtation of blocks in the rhombic fault system.Calderas filled with ignimbrites are controlled by thefaults formed in the transtensional regime. Delami-nated fragments of the lower crust and lithosphericmantle of the South American Platform sank into themantle. (b) The present-day rhombic fault system ofthe Puna Plateau. The near-meridional left-lateralfaults parallel to the general Andean trend are black;the NE-trending right-lateral faults are gray. The faultsystems make up rhombic deformation domains,including sedimentary basins (dotted). The mecha-nisms of earthquakes are shown in the projection onthe lower hemisphere; the dots correspond to the direc-tion of shortening. The main faults (letters in figure):UKF, Uni–Kenwani Fault; AF, Acazoque Fault; TTF,Tucuman Transform Fault; IF, Iconza Transform Fault.Volcanic calderas (numerals in figures; age, Ma isshown in parentheses): 1, Pastos Grandes (8.1–5.4);2, Panizos (7.9–6.7); 3, Coruto (6.2); 4, Vilama Cerito(8.9–8.5); 5, Pairique (11.2); 6, Guacha (4.1); 7, Purico(1.3); 8, La Pacana (5.8–2.4); 9. Coranzuli (6.6);10, Ramadas (8.8–8.5); 11, Aguas Calientes (10.0–10.5); 12, Negra Muerta (7.4); 13, Galan (2.2);14, Wheelright (8.8–6.1); 15, Laguna Escondida (8.8–6.1);16, Cerro Bayo (8.5); 17, Mulas Muertas (6.1); 18, SanFrancisco; 19, Farallon Negro (12.6–8.6); 20, Incapillo(2.9–1.1); (1) direction of horizontal shortening;(2) direction of maximum extension close to calderas;(c) latitudinal variations of age, Ma and depth, km(numerals in parentheses) of separate rhombic basinsin the southern portion of the central Andes. The dashedline corresponds to the average rate of propagation of thesouthward deformation front over 26 km/Ma; the basindepth decreases to the south consistently with reducedcrustal shortening across the Andes.
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THE LATE CENOZOIC GEODYNAMIC EVOLUTION OF THE CENTRAL 317
Santo DomingoSan Rafael
QuenamariSaritaCondoriquna
Palca XIFabulosa
MilluniChacaltaya
Rosario
Migration oore mineralization
with time
Plateau Altiplano
Large and superlargeore deposits(numerals are age, Ma)
APVCLarge ignimbritefields and their age, Ma
Chojila 19.5Kellhuani 21.3
VilocoCaracoles 24.9
ChicoteKami
Japo
Santa Fe–Morococola 20
Huanuni 20Llallague-
Catavi Colquechaca 22.6Cerro Rico
de Potosi 13.8Frailes
7 Caracota
Tatasi Tasna
Chorolque 16
Pulacaya 14 IscaiascaChilcobija
Chocaya 16
APVC11-7Loma Blanka 7
Pirquitas
Pun
a
El–Laco 5.3–1.6 Eastern C
ordillera
Pastos-Grandes 6.3
SerroGalle2–3
Plat
eau
Bolsa Negra
Oruno 16
Morococola
WesternCordillera
Central Andean Transect
7
Age of oremineralization
middle Mioceneearly Miocenelate OligoceneLate Triassic-Early Jurassic
16°
18°
20°
26° S
72° W 70° 68° 66°
Fig. 8. Centers of ore mineralization in the Bolivian tin belt migrating in time. Geological age is shown after [74]; the isotopic age,Ma of large deposits is after [2]; the location of large ignimbrite complexes corresponds to Fig. 3. The average rate of meridionalmigration to the south is 44 km/Ma.
in tin belt
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GEOTECTONICS Vol. 43 No. 4 2009
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geochemistry of the igneous rocks testify to the delam-ination of fragments of the continental lithospherebeneath the Altiplano–Puna Plateau. The high-velocity(Vp > 8.4 km/s) domain in the mantle beneath Altiplanois interpreted as accumulation of high-density eclogiticinclusions or other high-pressure metamorphic rocks inthe downgoing continental lithosphere of the SouthAmerican Platform.
MECHAMISM AND DRIVING FORCES OF LATE CENOZOIC TECTONIC ACTIVATION
IN THE CENTRAL ANDES
According to [60], the tectonic horizontal shorten-ing of the crust in the backarc domain of the EasternCordillera and Subandes occurs along the entire west-ern coast of the South American continent and reachesa maximum of 230 km (250–290 km by other esti-mates) in the central Andes. The intense recent and cur-rent magmatic activity in the Altiplano–Puna Plateauand the Western Cordillera hampers estimation ofdeformation values. The estimates of crustal shorteningin the coastal zones at latitude 23° S were obtained onlyrecently, and the minimum total shortening in theBolivian Andes was estimated to be no less than 500 kmover 70 Ma [71, 72]. The kinematically balanced mod-els [15, 16, 30, 71, 72, 77, 90] show that these valuesare quite sufficient to explain the existing thickness ofthe crust.
Various geodynamic scenarios have been suggestedto explain the thickened crust in the central Andes. Thesubdivision of the Late Cenozoic evolution of the cen-tral Andes into stages was likely proposed for the firsttime in [54], when it was established that at the firststage the thickening occurred due to simple shearingdistributed more or less uniformly throughout the crust[5]. The regional thrust zone (Subandean Thrust Fault)was formed during the second stage. The further thick-ening was realized mainly by doubling caused bythrusting of the South American Platform under theupper crust of the Andes along the Subandean Fault.The subsequent models detailed and specified the prin-cipal two-stage model with involvement of the conti-nental lithosphere. The model of ablative two-sidedsubduction [84, 98] and models of crustal shortening inthe subduction zone, taking the climatic features of thecentral Andes [14, 15, 67] in account, should be noted.Although these models explain the crust thickening inqualitative terms, they cannot explain many docu-mented features of this process, in particular the lateOligocene jump of the locus of deformation far back ofthe subduction zone and the existence of the relativelyundeformed block of the Altiplano–Puna Plateaubetween the volcanic arc and fold–nappe belt; thechange of deformation from thick-skinned type at theinitial stages to the thin-skinned type at the late stages,etc. The idea of thermal attenuation of the continentallithosphere far back of the subduction zone as a primor-dial cause of the Late Cenozoic tectonic activation in
the central Andes was probably stated first in [54] andthen considered in detail in [103]. In terms of thismodel, the initial compression arises in the mantleabove the domain of the thermally attenuated lithos-phere. As the crust was thickening, the deformationlocus shifted from the mantle into the crust. When thecrustal thickness reaches a maximum possible value(the �65-km crust becomes gravitationally unstable),the zone of deformation starts to migrate toward a rela-tively thin crust and the foldbelt widens (inland migra-tion of deformation front).
However, the idea of thermal attenuation of the con-tinental lithosphere is poorly consistent with the lowmagmatic activity in the Eastern Cordillera in the lateOligocene. If the initial attenuation of the continentallithosphere was thermal, deformation should be moreintense in the Altiplano, where the Late Cenozoic mag-matism is intense and the present-day heat flow is high;the plateau, however, has remained little deformed untilnow. At the same time, the location of domains of thick-skinned deformation is in agreement with the geometryof the subducted slab. Tectonic reconstruction showsthat the Nazca Plate beneath the central Andes flattenedin the Oligocene and remained flat into the Miocene(Fig. 3a) just in the domains of tectonic activation. Inthe post-Miocene time, the flattened subduction zonemigrated southward and after that the zone of thick-skinned deformation shifted in the meridional direc-tion. At present, this zone embraced a wide region backof the subduction zone at 30–35° S (Fig. 1b).
The poor correlation of the localization of tectonicdeformation with the manifestation of magmatism andthe good correlation with the geometry of the sub-ducted slab show that abrupt local mechanical attenua-tion of the continental lithosphere originally was not ofthe thermal nature. The fluid effect upon the continentallithosphere seems to be the most probable. Either thelow-angle subducted Nazca Plate itself [46] or the man-tle regions disturbed by this plate might be sources offluid. For example, the lower layer of the upper mantle(400–670 km), where reservoirs of water-saturatedrocks are expected [20, 100], could be such regions.The impact of fluid on orogenic processes is suggestedin many provinces [11, 52].
The tectonic flows in the middle and lower crust [38,53, 106], along with delamination of the lower crustand the continental lithosphere [37, 56], were otherimportant processes in the evolution of the centralAndes. Indeed, all seismic studies show that a high-velocity layer (Vp = 7.0 km/s), presumably mafic incomposition, is located at the bottom of the crustbeneath the Eastern Cordillera. However, the lowvelocities of seismic waves beneath the Altiplano–Punaadmit only an insignificant contribution of mafic rocksto the crust, consistent with the idea of eclogitization ofthe lower crust in the course of underthrusting of theSouth American Platform. The eclogitic layer, detachedfrom the overlying rocks, sank into the mantle together
GEOTECTONICS Vol. 43 No. 4 2009
THE LATE CENOZOIC GEODYNAMIC EVOLUTION OF THE CENTRAL 319
with fragments of the underlying continental lithos-phere and therefore did not participate in the formationof the middle and lower crust beneath the Altiplano andPuna (Figs. 4, 7a). The collapse of eclogite into themantle most likely was caused by the gravitationalinstability of a heavy layer consisting of eclogitizedlower crust and the continental lithospheric mantle andlying on the relatively light lithospheric layer. This sce-nario is difficult to prove by direct arguments, but con-trasting variations of the Moho depth from 65 to 80 km(Fig. 2) and Pn seismic velocity immediately under theMoho discontinuity [19, 107] beneath the Altiplano–Puna Plateau and the western margin of the EasternCordillera may serve as indirect evidence.
The variation of the intensity and type of magma-tism in the volcanic arcs is commonly referred to thechange in the regime of the subduction zone. In partic-ular, the eruption of an enormous volume of ignim-brites 7–11 Ma ago is explained by variable geometryof the Benioff zone or plate kinematics, and the rotationof the upper crustal blocks in the Andean Orogenaround a vertical axis is ascribed to bending of the oro-gen (Andean Orocline) [54]. However, as has beenshown in [86], the collapse calderas filled with ignim-brites are genetically related to the faults that broke thePuna Plateau and the southern part of the Altiplano intorhombic segments (domains) partly occupied by sedi-mentary basins (Fig. 7b). The rhombic basins started toform in the Eocene and propagated to the south at a rateof ~26 km/Ma (Fig. 7c). Thus, at the initial stages, thehorizontal shortening across the strike of the orogenwas compensated by thickening of the crust. By reach-ing the maximum possible thickness at the onset of thethird stage, the meridional tectonic flows in the middleand lower crust beneath the Altiplano–Puna Plateaubecame more intense and brought about longitudinalextension of the orogen. Fault zones that developed inthe transtensional regime served as conduits for theascent of viscous silicic melts from the deep crust. Thedetailed 3D kinematic model of the central Andean seg-ment of the subduction zone, which takes into accountboth lateral and longitudinal movement of materail[106], predicts rotation of upper crustal blocks by 6–25°, in qualitative agreement with paleomagnetic andother data [18, 88].
The deposits of the Bolivian tin belt are related toplutonic and subvolcanic rocks, including S-granites,localized at the western margin of the Eastern Cordil-lera and in the Altiplano–Puna Plateau (Fig. 8). TheCenozoic deposits become younger to the south, con-sistent with meridional migration of the front of tec-tonic reworking of the middle and lower crust and theunderlying lithospheric mantle.
CONCLUSIONS
Summing up the available data, the following prin-cipal events should be pointed out in the Late Cenozoicgeodynamic evolution of the central Andes.
(1) In the late Eocene, the local rheological attenua-tion of the continental lithosphere, presumably causedby fluid impact, occurred in the present-day centralAndes far back of the subduction zone. The lateral com-pression of the subduction zone led to deformation ofthe lithosphere above the weakened domain. The locusof deformation thus migrated upward from the mantleinto the crust.
(2) Induced by tectonic pulse from a depth, tectonicdeformation in the Oligocene developed in thick-skinned style, creating the symmetric bivergent systemof the Eastern Cordillera. The deformation of the crustresulted in its thickening. At the initial stage, this pro-cess was not accompanied by fast uplift of crustalblocks and formation of high-mountain topography.Most likely, high-density lower crustal rocks held thecrust in the submerged state. Beneath the Altiplano, thecrust was thickened and heated due to magma ascent,which ensured the slow isostatic emergence of thisblock.
(3) Approximately 19 Ma ago, the gently dippingSubandean Thrust-Fault Zone arose beneath the East-ern Cordillera, and the lithosphere of the South Ameri-can Craton started to thrust under the Andes along thiszone. Over a short time, the thickness of the crustalmost doubled, giving rise to the isostatic uplift of theEastern Cordillera. The upper crust above the Suban-dean Thrust Fault experienced thin-skinned deforma-tion with the formation of east-verging folds andnappes. The deformation front rapidly migrated east-ward inland to the continent
(4) Approximately 10 Ma ago, the crust attained thegreatest possible thickness. Viscoplastic tectonic flowsin the middle and lower crust started to develop alongthe orogen axis, and a rhombic fault system was formedin the upper crust. The relative elongation in the uppercrust was realized in the rotation of blocks around thevertical axis and displacements along faults. Faulting inthe transtensional regime created conduits for magma,which erupted in the form of giant ignimbrite fields.
(5) Fragments of the lower crust and subcrustallithospheric mantle beneath the Eastern Cordillera andAltiplano–Puna Plateau were detached from the overly-ing rocks and sank into the mantle. This process hascontinued until now.
(6) The character of the Late Cenozoic magmatismof the Altiplano–Puna Plateau is determined largely bythermomechanical transformation of the lithosphere ofthe Andean Orogen and is only indirectly related to thekinematics of the subducted Nazca Plate.
ACKNOWLEDGMENTS
This study was partially supported by the RussianFoundation for Basic Research, project nos. 04-05-65092 and 07-05-00109.
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