Late Permian–Middle Jurassic lithospheric thinning in Peru and Bolivia, and its bearing on...

Late Permian–Middle Jurassic lithospheric thinning in Peru and

Bolivia, and its bearing on Andean-age tectonics

Thierry Sempere a,*, Gabriel Carlier b, Pierre Soler c, Michel Fornari d, Vıctor Carlotto e,Javier Jacay f, Oscar Arispe g, Didier Neraudeau h, Jose Cardenas e, Silvia Rosas i,

Nestor Jimenez j

aIRD, Apartado Postal 18-1209, Lima 18, PerubIRD and Laboratoire de Mineralogie, Museum National d’Histoire Naturelle, 61 rue Buffon, 75005 Paris, France

cIRD and LODYC-UMR 7617 CNRS-IRD-UPMC, Institut Pierre-Simon Laplace, Universite Pierre-et-Marie-Curie, Boıte 100, 4 Place Jussieu,

75252 Paris cedex 05, FrancedIRD and Laboratoire de Geochronologie, UMR 6526 Geosciences Azur, Universite de Nice-Sophia Antipolis, 06108 Nice cedex 02, France

eDepartamento de Geologıa, Universidad Nacional de San Antonio Abad del Cusco, Cusco, PerufUniversidad Nacional Mayor de San Marcos, Apartado Postal 3973, Lima 100, Peru

gCasilla 4836, La Paz, BoliviahLaboratoire de Paleontologie, Geosciences, Universite de Rennes I, Campus de Beaulieu, Avenue du General Leclerc,

35042 Rennes cedex, FranceiSociedad Geologica del Peru, Arnaldo Marquez 2277, Lima 11, Peru

jUniversidad Mayor de San Andres, Casilla 6568, La Paz, Bolivia

Received 30 March 2000; received in revised form 10 December 2000; accepted 15 December 2000

Abstract

Integrated studies and revisions of sedimentary basins and associated magmatism in Peru and Bolivia (8–22�S) show that

this part of western Gondwana underwent rifting during the Late Permian–Middle Jurassic interval. Rifting started in central

Peru in the Late Permian and propagated southwards into Bolivia until the Liassic/Dogger, along an axis that coincides with the

present Eastern Cordillera. Southwest of this region, lithospheric thinning developed in the Early Jurassic and culminated in the

Middle Jurassic, producing considerable subsidence in the Arequipa basin of southern Peru. This � 110-Ma-long interval of

lithospheric thinning ended � 160 Ma with the onset of Malm–earliest Cretaceous partial rift inversion in the Eastern

Cordillera area. The lithospheric heterogeneities inherited from these processes are likely to have largely influenced the

distribution and features of younger compressional and/or transpressional deformations. In particular, the Altiplano plateau

corresponds to a paleotectonic domain of ‘‘normal’’ lithospheric thickness that was bounded by two elongated areas underlain

by thinned lithosphere. The high Eastern Cordillera of Peru and Bolivia results from Late Oligocene–Neogene intense inversion

of the easternmost thinned area. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Lithospheric thinning; Andean orogeny; Mesozoic; Cenozoic; Peru; Bolivia

0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0040-1951 (01 )00211 -6

* Corresponding author.

E-mail addresses: [email protected] (T. Sempere), [email protected] (G. Carlier), [email protected] (P. Soler),

[email protected] (M. Fornari), [email protected] (V. Carlotto), [email protected] (O. Arispe),

[email protected] (D. Neraudeau), [email protected] (J. Cardenas).

www.elsevier.com/locate/tecto

Tectonophysics 345 (2002) 153–181

1. Introduction

The Andes Cordillera is classically considered as a

prime example of a mountain belt built along a

continental margin by tectonic processes related to

subduction of an oceanic plate, in an apparently non-

collisional setting. This outstanding orogenic belt

shows considerable longitudinal variations (Fig. 1).

The Central Andes form the largest and most

mountainous segment of the Andes Cordillera. This

� 4000-km-long segment is itself segmented into the

northern Central Andes (5�300– � 13�S; entirely lo-

cated in Peru), Bolivian Orocline (� 13–28�S; oversouthern Peru, Bolivia, northern Chile, northwestern

Argentina), and southern Central Andes (28–37�S;over central Chile and west–central Argentina). The

mountainous area of the largest of these segments, the

Bolivian Orocline, covers � 1,300,000 km2 (we use

here the term ‘orocline’ only in a descriptive sense,

which refers to the characteristic oceanward concavity

of this segment). This paper is concerned with the

northern Central Andes, and with the Peruvian and

Bolivian parts of the Bolivian Orocline.

The geological structure of the Central Andes is

relatively poorly known. Many tectonicists and geo-

physicists have implicitly tended to consider most of

the orogen as somewhat homogeneous (e.g., Isacks,

1988; Lamb et al., 1997). Characteristically enough,

descriptions of the Central Andean tectonic structure

are generally based on the present-day geomorphic

features. A consensus exists, however, about the idea

that the Central Andean active continental margin has

undergone significant crustal thickening since the Late

Cretaceous (e.g., Jaillard and Soler, 1996).

Little is known of the causes and processes that

result in the crustal thickening of a continental

margin submitted to an ocean–continent conver-

gence. The Central Andes happen to provide geo-

scientists with an extreme case of convergence-

related continental thickening, and are thus worthy

of detailed studies. In order to constrain the actual

structure and history of the Central Andes, a precise

knowledge of the pre-orogenic history and structure

is obviously needed, because ancient structures are

likely to have played significant tectonic roles during

Andean shortening.

This paper provides a detailed summary of new and

updated data, and of often entrenched information, and

demonstrates that the study area was submitted to

lithospheric thinning during the Late Permian–Middle

Jurassic. We observe that a Late Permian–Liassic rift

system developed along what is today the high Eastern

Cordillera of Peru and Bolivia. We infer that distribu-

tion of pre-orogenic thinned areas coincide with spe-

Fig. 1. Segmentation of the Andes Cordillera as used in this paper.

Topography from USGS data.

Fig. 2. Mesozoic–Cenozoic stratigraphic synopsis of the Central Andes from � 10�S to � 20�S. Note overall decrease in marine influence

from W to E, and N to S. Abbreviations: G =Gramadal Fm, He =Huancane Fm, Hh =Hualhuani Fm, Ipa = Ipaguazu Fm, L=Labra Fm,

M=Muni Fm, Mc=Murco Fm, P +C=Puente and Cachıos Fms, S = Sipın Fm, Sc = Saracocha conglomerates. Framed numbers: 1 = Late

Permian–Triassic rifting (diachronous from N to S); 2 = downwarping of Arequipa basin; 3 = downwarping of Chicama basin; 4 = latest

Jurassic–Early Cretaceous uplifts (4 * = Lagunillas uplift). References for northern Chile are mainly from Ramırez and Huete (1981), Skarmeta

and Marinovic (1981), Bogdanic (1990), Ladino et al. (1999); for other areas, see text and Figs. 6 and 8.

T. Sempere et al. / Tectonophysics 345 (2002) 153–181154

T. Sempere et al. / Tectonophysics 345 (2002) 153–181 155

cific Andean structural traits, and we explore the idea

that pre-existent extensional and/or transtensional

structures determined many actual characteristics of

the northern Central Andes and Bolivian Orocline.

The Mesozoic–Cenozoic stratigraphic evolution

concerning the study area is summarized in three

stratigraphic transects (Fig. 2). For correspondence

between chronostratigraphic stages and absolute ages,

we use Hardenbol et al.’s (1998) chart. Our use of the

term ‘Andean’ refers to the Andes (or proto-Andes) as

a mountain belt, and to the related period of mountain

building.

2. Late Permian–Middle Jurassic rifting in the

Eastern Cordillera of Peru and Bolivia

2.1. Introduction

Late Permian–Triassic rifting diachroneously de-

veloped in the Eastern Cordillera of Peru (Megard,

1978; Laubacher, 1978; Noble et al., 1978; Dalmayrac

et al., 1980; Kontak et al., 1985; Rosas and Fontbote,

1995; Rosas et al., 1997; Jacay et al., 1999) and

extended into Bolivia in the Late Triassic–Middle

Jurassic (McBride et al., 1983; Sempere, 1995; Sem-

pere et al., 1998, 1999). The main axis of the rift

system appears to coincide with the axis of the Eastern

Cordillera in both countries (Fig. 3). In this paper we

dedicate special attention to data from the still little

known Bolivian continuation of this rift system.

Current research in southern Peru and Bolivia shows

that Mesozoic Bolivian basins were mostly connected

to Peruvian basins, and not to southern, Argentine–

Chilean basins.

Reconstruction of the rift system in map view shows

that it splits into two branches at about 19�S (Figs. 3

and 4). The southeastern, ‘‘Entre Rıos branch’’ extends

into the Chaco Subandean belt and dies out in the

Bolivia–Argentina border area. The southern, ‘‘Tupiza

branch’’ strikes (presently) N10E and apparently

extends into the Argentine Puna. In map view, this

geometry is reminiscent of the present-day Red Sea rift

system, which to the north splits into the now inactive

Suez Gulf and the active Aqaba Gulf rift and Dead Sea

wrench–fault system (Fig. 5). In the following, we

consider the ‘‘Tupiza branch’’ as the southern contin-

uation of the main rift axis.

In Peru, strata of Permian through Jurassic age are

divided into the Mitu and Pucara groups, which were

respectively deposited in continental and marine set-

tings (McLaughlin, 1924; Steinmann, 1929; Harrison,

1943, 1951; Jenks, 1951; Newell et al., 1953). The

clastic and volcanic Mitu Group is mostly known

from the Eastern Cordillera of central and southern

Peru and accumulated in subsident grabens, reflecting

Late Permian–Triassic rifting (e.g., Megard, 1978;

Dalmayrac et al., 1980; Kontak et al., 1985). North-

west of Cusco, the carbonate-dominated Late Trias-

sic–Liassic Pucara Group has a wider distribution

(Fig. 3) and was deposited during the thermal sag that

followed rifting in this region; more to the southeast,

thick fluvio-eolian sandstones were deposited during

this thermal sag period (Sempere et al., 1998, 1999,

2000a).

In the grabens produced by rifting, conformable or

deformed Late Paleozoic strata were generally pre-

served below the Mitu Group, whereas they were

eroded out from the neighbouring rift shoulders.

Intense magmatism commonly occurred at depth

beneath the floor of the grabens, and predominantly

alkaline volcanic rocks were erupted. Consistent iso-

topic ages obtained on volcanic and plutonic rocks

point to a Late Permian–Middle Jurassic age for the

rifting (see Kontak et al., 1985, 1990; Soler, 1991;

Jacay et al., 1999, for summaries).

2.2. Pre-rift strata and deformation

2.2.1. Pre-rift stratigraphic units and relationships

The commonly >500-m-thick Pennsylvanian–

Early Permian (Tarma-)Copacabana Group was

deposited prior to rifting, and forms a guide unit as

it was frequently preserved in the Mitu grabens. It is

of shallow-marine origin and consists of fossiliferous

limestones and subordinate sandstones, black shales,

and cherty limestones. To the southeast, in the Chaco

Subandean belt and lowlands of Bolivia, the Copaca-

bana carbonate platform grades into sandstones of

eolian and fluvial origin (Cangapi Formation; Sem-

pere, 1995). In some areas of the Eastern Cordillera

where the Copacabana Group was preserved in Mitu

grabens, a rapid transition with overlying black shales

is observed. In other localities of the Eastern Cordil-

lera, strata of the Copacabana Group are folded

(locally intensely, and metamorphized) and either


Fig. 3. Synopsis of the main Mesozoic geologic elements of Peru and Bolivia. The axis of the Late Permian–Middle Jurassic rift system is

defined by occurrences of the Mitu Group, coeval granitoids, and basic dyke swarms, and approximately coincides with the axis of the Eastern

Cordillera of Peru and Bolivia. Localities: A: Arequipa, C: Cochabamba, Cu: Cusco, L: Lima, P: Potosı, SC: Santa Cruz, Tu: Tupiza.


intruded by Mitu granitoids (e.g., Soler and Bon-

homme, 1987) or unconformably overlain by the Mitu

Group (e.g., Laubacher, 1978; Megard et al., 1983).

This contrasted variety of stratigraphic relationships

(Fig. 6) provides information on the tectonic frame-

work of early rifting (see below).

Fig. 4. Location map of relevant data concerning the northern part of the Bolivian orocline.


2.2.2. Restricted-marine deposits coeval of early

rifting: the Vitiacua and Chutani formations, Bolivia,

and Ene Formation, Peru

The Vitiacua Formation of southern Bolivia consists

of black shales, siliceous carbonates (mainly lime-

stones and dolomites with common chert), dark red

mudstones, and subordinate sandstones, which form an

overall shallowing-upward succession. Marine black

shales are characteristic of the lower Vitiacua Forma-

tion, whereas restricted-marine chert-rich carbonates

are especially common in the upper part of the unit

(Sempere et al., 1992). TheVitiacua Formation overlies

the fluvio-eolian sandstones of the Cangapi Formation

with a rapid transition (marking a transgression), and is

sharply overlain by the mudstone-dominated, gypsum-

bearing, continental red strata of the Ipaguazu Forma-

tion.

Palynomorphs and a coelacanth jaw bone from the

organic-rich lower part of the Vitiacua Formation

indicate a Late Permian age (Sempere et al., 1992),

whereas fossil molluscs and fish from its uppermost

part suggest that it reaches the Triassic, and possibly the

Late Triassic (Beltan et al., 1987; Suarez-Riglos and

Dalenz, 1993), although an Early or Middle Triassic

age seems more likely (Sempere et al., 1992, 1998).

Conchostracans suggestive of a Late Permian age are

recorded from the Vitiacua Formation (Palaeolimnadia

sp., Palaeolimnadiopsis cf. eichwaldi) and from the

overlying Ipaguazu Formation (Cyzicus [Lioestheria]

sp.) (Tasch, 1987); however, ages indicated by South

American conchostracans are not securely calibrated.

The stratigraphy of Bolivia shares many aspects

with the stratigraphy of Brazil (Sempere, 1995). The

Late Permian Irati Formation of the Parana basin, Bra-

zil, is quite similar to the organic-rich lower part of the

Vitiacua Formation and represents its eastern equiva-

lent (Sempere et al., 1992). The Irati Formation is

conformably overlain by units (Serra Alta, Teresina,

and Rio do Rasto formations in the south; Corumbataı

Formation in the north) that correlate with overlying

members of the Vitiacua Formation and range into the

Triassic (Franc�a et al., 1995); continental red strata

known as the Piramboia Formation unconformably

overlie this Late Permian–Early Triassic stratigraphic

set, this contact lying somewhere in theMiddle Triassic

(Franc�a et al., 1995). As the continental red strata of theIpaguazu Formation sharply overlie the Vitiacua For-

mation, and as Bolivia and Brazil both belonged to the

cratonic area of western Gondwana at that time, a

Middle Triassic age for the Vitiacua/Ipaguazu contact

is suggested. The sedimentary discontinuity at the base

of the Ipaguazu and Piramboia formations is inter-

preted to mark a large-scale destabilization of conti-

nental depositional systems and, given its age, to

distally reflect onset of rifting to the west.

Fig. 5. Comparison between the map view geometries of the Red

Sea rift system and of the Central Andean Late Permian–Middle

Jurassic rift system (now inverted and shortened). Branching of the

rift axis is observed in both cases.


In the Eastern Cordillera of Bolivia, the Vitiacua

Formation is found only at three localities, which are

located within the main rift axis: Pasto Waykho, 6 km

west of Vitichi; Quebrada Aymaraj Hueko, 6 km

southwest of Torotoro; Iglesiani, 40 km NNW of

Cochabamba (Fig. 4). It is likely that the Vitiacua

Formation was preserved there due to tectonic down-

warping during rifting. At all three localities the

Vitiacua Formation is overlain by altered basalt flows

and/or conglomerates with � 15-cm-size basalt clasts

that mark the base of the Serere Group. At the first

two localities, the chert-rich facies and chert breccias

that characterize the upper part of the Vitiacua For-

mation are observed at the top of the local succession,

suggesting that the Vitiacua Formation is stratigraph-

ically complete; furthermore, the contact between the

Vitiacua Formation and overlying red strata is nearly

transitional there, and it seems obvious that no large

time gap separated deposition of the two units. At

Iglesiani, two rich palynomorph assemblages (1:

Hamiapollenites karrooensis, Tornopollenites toreu-

tis, Lueckisporites virkkiae, Corisaccites alutas, Pro-

tohaploxypinus enigmaticus, Taeniaesporites sp. [sp.

1 Jardine, 1974], Paravittatina cincinnata, Punctatis-

porites gretensis, . . ., and numerous acritarchs includ-

ing several species of Micrhystridium; and 2:

Lueckisporites virkkiae, Corisaccites alutas, Guttula-

pollenites gondwanensis, Gondwanipollenites ovatus,

Schweitzerisporites maculatus, Weylandites sp., with

numerous Botryococcus) indicate marine to restricted-

marine environments, and Middle to Late Permian

ages (J. Doubinger, unpublished).

About 70 km southwest of the main rift axis, the

Chutani Formation crops out in the Tiquina area of

southeastern Lake Titicaca, where it overlies the

Copacabana Group (Oviedo, 1962, 1964). The Chu-

tani Formation displays facies that are similar to those

of the Vitiacua Formation, including dark shales in its

lower part, and should thus be of similar Late Per-

mian–Early? Triassic age (Sempere et al., 1998).

Fossil plants of Late Permian age occur in the upper

Chutani Formation (Iannuzzi et al., 1997; Vieira et al.,

1999a,b). Altered basalt flows are intercalated with

coastal plain deposits and stromatolitic beds at the top

of this unit. These flows are conformably overlain by

the brown-red fluvial sandstones and mudstones of the

Tiquina Formation, which correlates with the Ipa-

guazu Formation.

In Peru, the Ene Formation displays facies similar

to those in the Vitiacua and Chutani formations. In

particular, the lower part of the Ene Formation pre-

dominantly consists of organic-rich black shales of

Late Permian age (e.g., Mathalone and Montoya,

1995; Carlotto et al., 2000). At two localities near

the Mitu rift axis (Fig. 4), these black shales con-

formably overlie the Copacabana Group and grade

upward into siliceous carbonates and/or shallow-

marine to fluvial or eolian sandstones; this continuous

Fig. 6. Synopsis of Permian through Jurassic main stratigraphic, deformational, and magmatic data discussed in text concerning the Eastern

Cordillera of Peru and Bolivia, the Chaco basin of Bolivia, and the Parana basin of Brazil.


succession is in turn conformably overlain by altered

volcanic rocks and red strata (including mudstones,

sandstones and gypsum) that represent the local facies

of the Mitu Group (Carlotto et al., 2000). The Ene

Formation is widespread in the Subandean belt and

lowlands of Peru, i.e. east of the Mitu rift system

(Mathalone and Montoya, 1995).

The black shales that characterize the lower parts

of the Vitiacua, Chutani and Ene formations represent

a marine transgression of Late Permian age, which is

also represented by units in the Parana basin, Brazil,

and the Karoo basin, South Africa (Sempere et al.,

1992; Tankard et al., 1995). This widespread trans-

gression is likely to have extended over a very broad

region of western Gondwana, but was also coeval

with the early stage of the rift-related Mitu magma-

tism (Fig. 6). In southern Peru, limestones bearing

Late Permian fusulinids are locally intercalated within

red strata of the Mitu Group (Laubacher, 1978), de-

monstrating that marine ingressions occurred within

some Mitu grabens.

The Vitiacua, Chutani and Ene formations con-

formably overlie the Copacabana Group, and, given

their age, appear as restricted-marine time-equivalents

of the older Mitu syn-rift deposits, whereas they pre-

date younger Mitu syn-rift deposits as they are over-

lain by Mitu or Mitu-equivalent red strata (Fig. 6).

Given the approximate Middle Triassic age of the top

of these restricted-marine units, the Mitu rifting ap-

pears to have been twofold, with a first stage spanning

the Late Permian–Early? Triassic interval, and a

second stage beginning in the Middle Triassic. This

analysis agrees with the idea of a twofold Mitu Group

set forth by Soler (1991) on independent bases (see

below).

2.2.3. Deformation and rifting

As stressed above, the sedimentary continuity

commonly observed in Bolivia and southern Peru

between the Pennsylvanian–Early Permian Copaca-

bana Group and overlying Late Permian–Early? Tri-

assic units markedly contrasts with the pre-Mitu,

locally intense deformation observed in the Copaca-

bana Group in some areas of the Eastern Cordillera

between � 11�S and � 17�300S. This deformation,

traditionally explained by Late Permian ‘‘tardi-hercy-

nian tectonics’’ (e.g., Dalmayrac et al., 1980), is in

fact restricted to a narrow belt within the Eastern

Cordillera (Sempere, 1995); given the contemporane-

ity of Late Permian deformation and deposition, this

belt was probably discontinuous, deformation occur-

ring in specific areas while shale-dominated sedimen-

tation was quietly going on in other areas of the same

Eastern Cordillera domain.

Such relationships are strongly suggestive of a

transcurrent rift system in which transtensional seg-

ments would have been separated by transpressional

‘‘nodes’’. We favor the idea that local transpression

caused deformation of pre-Mitu strata at the onset of

continental dislocation, before general graben forma-

tion and intense magmatism developed. Coeval trans-

tension produced slow downwarping of elongated

areas, where the Copacabana Group was preserved

and deeper marine shales conformably deposited over

it, before accelerating rifting processes enhanced mag-

matism and formed true grabens.

A similar scenario, albeit later in time, could also

explain the occurrence of Late Triassic plutons show-

ing deformation that was contemporaneous with their

emplacement. In the Cordillera Real of western Boli-

via, the Zongo–Yani pluton yielded Late Triassic U–

Pb ages (Table 1); emplacement of this foliated,

peraluminous, two-mica granite was contemporane-

ous with schistosity and low-pressure metamorphism,

reflecting a high heat flow (Bard et al., 1974). In

nearby Peru, the similar, foliated and peraluminous,

two-mica Limacpampa pluton is dated to near the

Triassic–Jurassic boundary (Table 1). South of Aban-

cay, a cataclastic ‘‘quartz-diorite’’ yielded a Late Tri-

assicU–Pb age (Table 1).We suggest that emplacement

and early deformation of these intrusions might have

occurred in local transpressional settings at a later, Late

Triassic, stage of rifting.

Triassic uplift of plutons is recorded by clasts of

Mitu-age granitoids that are commonly found in

conglomerates and pyroclastites of the Mitu Group

in central Peru (Megard, 1978), suggesting a twofold

development of rifting (Soler, 1991). Such uplifts are

likely to have been caused by lithospheric deforma-

tion related to rifting.

2.3. Rift-related magmatism

2.3.1. Southern Peru

Intense magmatism was associated with the Mitu

rifting in southern Peru (Egeler and De Booy, 1961;


Table 1

Isotopic ages mentioned in text. See original references for details; bi: biotite, mu: muscovite, pl: plagioclase, hb: hornblende, wr: whole rock, gl: glass

Region Unit Method Age (Ma) Observations Source

Central Peru: Equiscocha granite K–Ar 253 ± 11 (mu), 204 ± 9 (pl) Soler, 1991

Eastern Cordillera San Ramon-type batholiths Rb–Sr, wr 246 ± 10 recalculated by Soler, 1991,

based on data by

Capdevila et al., 1977

Talhuis–Carrizal-type granitoids K–Ar, bi 245 ± 11 and 233 ± 10 intrusive into the San

Ramon-type batholiths

Soler, 1991

Southern Peru: Quillabamba granite U–Pb 257 ± 3 Lancelot et al., 1978

Eastern Cordillera Abancay ‘‘quartz–diorite’’ U–Pb 222 ± 7 cataclastic Dalmayrac et al., 1980

monzodiorite dyke (Fig. 4) K–Ar, hb 185 ± 6 this paper (Table 2)

rhyolite dyke (Fig. 4) K–Ar, bi 244 ± 6 this paper (Table 2)

Allinccapac volcanics Rb–Sr 200–180 peralkaline Kontak et al., 1985, 1990

Macusani syenite K–Ar 184.2 Stewart et al., 1974

Macusani syenite K–Ar 173.5 ± 3.1 Kontak et al., 1985

Coasa granite U–Pb 238 ± 11 Lancelot et al., 1978

Aricoma adamellite U–Pb 234 ± 9 Dalmayrac et al., 1980

Limacpampa 2-mica pluton Rb–Sr 199 ± 10 foliated, peraluminous Kontak et al., 1990

Southern Peru: Antarane granodiorite Ar–Ar � 277 Clark et al., 1990b

Altiplano Antarane granodiorite K–Ar, bi 275.2 ± 5.8 Clark et al., 1990b

Antarane granodiorite K–Ar, mu 263.6 ± 5.2 from an associated quartz vein Clark et al., 1990b

‘‘lava flows’’ K–Ar, wr 272 ± 10 type of lava was not indicated Klinck et al., 1986

Southern Peru:

Arequipa

Punta Coles gabbro-

monzotonalite super-unit

U–Pb 188.4 and 184 Mukasa, 1986

Southern Peru:

coast

basaltic flow, lower Chala Fm Ar–Ar � 177 Romeuf et al., 1993

Bolivia:

Western Cordillera

Nevado Tata Sabaya granite K–Ar 188.1 ± 4.0 and

181.6 ± 3.9

slightly cataclastic and altered Sempere et al., 1998

Bolivia: Altiplano ‘‘basalt flow’’ Serranıas de Chilla K–Ar 279.9 ± 3.3 (wr),

244.9 ± 2.9 (gl)

S. McBride, in

Kontak et al., 1985;

ages recalculated by

Kontak et al., 1990

T.Sem

pere

etal./Tecto

nophysics

345(2002)153–181

162

‘‘altered gabbro south of

Lake Titicaca’’

K–Ar 258 ± 13 Bolivian National Oil Company

unpublished data

Saavedra et al., 1986

Bolivia: Zongo–Yani 2-mica pluton U–Pb 222.2 + 7.7/� 9.1 pervasively foliated facies Farrar et al., 1990

axis of Eastern Zongo–Yani 2-mica pluton U–Pb 225.1 + 4.1/� 4.4 weakly foliated facies Farrar et al., 1990

Cordillera Cordillera Real plutons K–Ar � 212 McBride et al., 1983;

Kontak et al., 1985

Cerro Sapo alkaline complex K–Ar 97.7 ± 2.8 dated material is from a

breccia-pipe bearing kimberlitic clasts

Kennan et al., 1995;

Tawackoli et al., 1999

Cerro Grande high-K gabbroic

to syenitic intrusion

K–Ar, bi 120.0 ± 0.5 obtained on 2 biotite fractions Tawackoli et al., 1999

Cornaca basanite dyke K–Ar, wr 184 ± 4.9 Tawackoli et al., 1996, 1999

Bolivia: Tarabuco–Uyuni sill K–Ar, wr 171.4 ± 4.2 continental tholeiite Sempere, 1995

Entre Rıos branch

of rift

Entre Rıos sill (altered) K–Ar, wr 108, 104,

and younger ages

continental tholeiite; Bolivian

Gulf Oil Company unpublished data

Saavedra et al., 1986;

Sempere et al.,

1992; Lopez-Murillo and

Lopez-Pugliessi, 1995

Camiri basalt sill K–Ar, wr 233 Bolivian Gulf Oil Company

unpublished data

Saavedra et al., 1986;

Sempere et al.,

1992; Lopez-Murillo and

Lopez-Pugliessi, 1995

Rejara pluton K–Ar 141 ± 10 this Precambrian pluton is intruded

by dolerite dyke swarms

Aranıbar, 1979

NW Argentina Los Alisos alkaline ultrabasic dyke K–Ar 303 ± 10 high-K Mendez and Villar, 1979

Sierra de Rangel alkaline granites Rb–Sr, bi 146 ± 1.6 and 122 ± 1.5 Menegatti et al., 1997

T.Sem

pere

etal./Tecto

nophysics

345(2002)153–181

163

Vivier et al., 1976; Noble et al., 1978; Dalmayrac et

al., 1980; Carlier et al., 1982; Kontak, 1984; Bon-

homme et al., 1985; Kontak et al., 1985, 1990; Clark

et al., 1990a; Cenki, 1998). In a major contribution,

Kontak et al. (1985) clearly identified that the entire

Mitu-age magmatism in southern Peru was rift-

related, and recognized that ‘‘the predominantly basic

Mitu Group volcanics and the batholithic granodior-

ites and monzogranites (. . .) are most probably repre-

sentative of a continuum with a cause and effect

relationship’’. In particular, these authors underlined

that this magmatism was very similar to the one known

from the Early Permian Oslo aborted rift in Norway

(Kontak, 1984; Kontak et al., 1985).

Several types of apparently unrelated mantle-

derived magmas, including alkaline basalts and

locally thick peralkaline facies, occur among the Mitu

volcanic rocks (Kontak et al., 1985). Basic volcanics

can form up to 20% of the total Mitu volcanism and

consist of tholeiitic or alkaline spilitized basalt flows

that are generally intercalated with the Mitu Group

sedimentary rocks (Vivier et al., 1976; Kontak, 1984).

Swarms of basic dykes and sills intruding Paleozoic

and Mesozoic strata are known from the Altiplano,

Eastern Cordillera, and Subandean belt of southern

Peru (Newell, 1949; Laubacher, 1978), and are likely

to have been produced by the Triassic–Jurassic litho-

spheric thinning as in nearby Bolivia.

A Permian, early stage of magmatism is docu-

mented in the Peruvian Altiplano by dated ‘‘lava

flows’’ from 21 km northwest of Juliaca, and by the

Antarane granodiorite � 30 km WSW of the same

city (Table 1; Fig. 4).

In the Eastern Cordillera, the 2000-m-thick Early

Jurassic, Allinccapac peralkaline volcanic rocks sur-

round and overlie a large peralkaline syenite complex

that yielded Early to Middle Jurassic apparent ages

(Table 1). The syenite complex and surrounding vol-

canic rocks are probably cogenetic (Kontak et al.,

1985), suggesting that the related peralkaline magma-

tism developed during the Liassic and, possibly, early

Dogger. A predominantly mantle-derived magmatism

thus developed in this segment of the Eastern Cordil-

lera during the first half of the Jurassic (Kontak et al.,

1985).

In contrast with the mantle-derived Mitu volcanic

rocks, the Carabaya batholith plutons derive from

crustal melts, and are also similar to plutons known

in the Oslo rift (Kontak, 1984; Kontak et al., 1985,

1990). Intrusion of the main plutons occurred in the

Late Permian and Triassic (Table 1). The Carabaya

batholith is commonly cut by coeval and younger

alkaline, Ti-rich, basic dykes that display character-

istics similar to the basalts known in the Mitu Group

(Kontak et al., 1985, 1990).

In addition to the published ages on the Mitu

magmatism in southern Peru, we have obtained new

K–Ar ages of 244 ± 6 and 185 ± 6 Ma, from a rhyolite

dyke and a monzodiorite dyke, respectively (Tables 1

and 2; Fig. 4).

All these data agree with the idea that Late Per-

mian–Middle Jurassic lithospheric thinning in the

Eastern Cordillera and Altiplano of southern Peru

generated a variety of mantle-derived magmas, along

with a high heat flow that produced significant

amounts of crustal melting in the Late Permian–

Triassic (Kontak et al., 1985). In this area, lithospheric

thinning lasted over nearly 100 Ma.

2.3.2. Bolivia

Rift-related magmatism in Bolivia was dominated

by basic magmas. Plutons derived from crustal melts

are only known northwest of 17�S, i.e. in the prolon-

gation of the Eastern Cordillera of southern Peru, and

have yielded Late Triassic U–Pb and K–Ar ages

(Table 1). The basic magmatism related to the main

Table 2

Analytical data concerning the new ages presented in this paper

Sample # (lab. #) Nature Location Method and

material dated

% K Rad. Ar

(nl/g)

Atm. Ar

(%)

Age (Ma) ± 2 s

98-08-18-01 (H475.98) rhyolite dyke � 14�420S–70�430W K–Ar, bi 6.815 69.218 6 244 ± 6

M-371 (H289.CP) monzodiorite dyke � 13�320S–72�330W K–Ar, hb 0.859 6.493 21 185 ± 6

Samples were dated at the Laboratorio de Geocronologıa of SERNAGEOMIN, Santiago, Chile; see Fig. 4 for location; bi: biotite, hb:

hornblende.


rift axis displays alkaline compositions (Aldag, 1913;

Smulikowski, 1934; Soler and Sempere, 1993; Ta-

wackoli, 1999), whereas the giant sill known in the

‘‘Entre Rıos branch’’ of the rift (Sempere et al., 1998)

has a tholeiitic composition (Fig. 7; Soler and Sempere,

1993). All these basic rocks indicate ‘‘intraplate’’

mantle-derived magmatism and lithospheric thinning.

Although several Permian to Jurassic isotopic ages

have been obtained (Table 1), samples from these

commonly altered basic rocks are difficult to date

securely, and many have yielded Cretaceous and Pale-

ogene apparent ages (see Saavedra et al. (1986), Ru-

biolo (1997) and Tawackoli et al. (1999) for reviews).

2.3.2.1. Magmatism and heat flow in the main rift

axis. The Late Triassic granitoids in the Eastern

Cordillera northwest of 17�S (Cordillera Real) are

likely to have been emplaced at depth along the

Eastern Cordillera rift system, because they are similar

to those known in nearby southern Peru (McBride et

al., 1983; Kontak et al., 1985), and include peralumi-

nous plutons (Bard et al., 1974).

Southeast of 17�S, such granitoids are unknown

and the reconstructed rift axis is characterized by

elongated swarms of basic dykes and sills that intrude

mainly Paleozoic rocks (Figs. 3 and 4). When Mes-

ozoic strata occur in areas with basic dyke swarms,

Fig. 7. Plot of some Bolivian Mesozoic basaltic rocks in Cabanis and Thieblemont’s (1988) Tb–Th–Ta diagram (modified after Soler and

Sempere, 1993).


basaltic flows, sills and/or dykes are generally also

observed in the Mesozoic, strongly suggesting that the

dykes in the Paleozoic are of Mesozoic age (Stein-

mann, 1906, 1923; Kozlowski, 1934; Sempere et al.,

1998). This strip rich in basic dykes, sills and flows

coincides with the axis of the present-day Eastern

Cordillera, which was recognized by Kozlowski

(1934) in luminous words: ‘‘Tous les gisements des

roches eruptives mesozoıques que je connais se trou-

vent exclusivement dans la Cordillera Oriental et

surtout dans sa partie centrale, ou ils sont disposes

le long d’une zone parallele a la direction de la

chaıne (. . .)’’ (‘‘All the occurrences of Mesozoic

eruptive rocks that I know are exclusively found in

the Eastern Cordillera and especially in its central

part, where they are distributed along a strip parallel to

the mountain belt’’). Kozlowski (1934), after Stein-

mann (1923), correctly conjectured that the alkaline

basic rocks they collected and studied from the East-

ern Cordillera of Bolivia were probably of Triassic–

Jurassic age. An unaltered basanite dyke that intrudes

Ordovician rocks northeast of Tupiza yielded a Toar-

cian age (Table 1).

2.3.2.2. Magmatism in the ‘‘Entre Rıos Branch’’ of the

rift system. The ‘‘Tarabuco basalt’’ (from the area

located from � 30 km north to � 20 km southeast of

Tarabuco) and the ‘‘Uyuni–Incapampa basalt’’ (from

the area located � 30 to � 50 km south of Tarabuco)

intrude pre-Maastrichtian Mesozoic strata and belong

to the same � 100–150-m-thick basaltic sill, here-

after called the Tarabuco–Uyuni sill. In the Uyuni–

Incapampa syncline, this noteworthy sill clearly

intrudes a red, � 400-m-thick, fining-upward succes-

sion that has yielded trackways of primitive tetrapods

(currently under study, but suggesting a Triassic age)

and should be ascribed to the Ipaguazu ( = Sayari)

Formation (Sempere et al., 1998); the sill displays

gabbro facies in its middle part, and branches into

three to four large sills in the west. Near Uyuni del

Pilcomayo, 0.3–3-m-thick basic dykes are known

from below and above the sill. The Tarabuco–Uyuni

sill was sampled 5 km north of Tarabuco, where it

intrudes eolian sandstones, and yielded an early Mid-

dle Jurassic apparent age (Table 1).

The relationships and characteristics of the sedi-

mentary and igneous rocks in the Tarabuco–Incapa-

mpa area are nearly identical to the set formed by the

Ipaguazu Formation, ‘‘Entre Rıos basalt’’, and Tacuru

sub-Group in the southern Chaco Subandean belt. The

Entre Rıos Basalt is a � 130-m-thick sill (Padula and

Reyes, 1958; Sempere et al., 1998); it is altered in all

known outcrops and has yielded 108 Ma, 104 Ma, and

younger apparent ages (Table 1).

Samples from the Tarabuco–Uyuni and Entre Rıos

sills have a continental tholeiite composition so close

that they cannot be geochemically distinguished (Soler

and Sempere, 1993). Both intrude Mesozoic strata and

reach similar thicknesses (130–150 m). Their respec-

tive outcrop areas are presently only � 50 km apart,

and are separated by the narrow Interandean structural

belt where no rock unit younger than Permian is

known (Fig. 4). All this strongly suggests that, prior

to Andean deformation, the two basalt sills formed

one same giant sill (Sempere et al., 1998), which we

propose to name ‘‘Tarabuco–Entre Rıos sill’’. Given

the alteration of the Entre Rıos basalt, the early Dog-

ger age obtained on the Tarabuco basalt should be

considered as a better estimate of the time of emplace-

ment of this sill. Because Bolivia was part of the

Gondwana craton prior to the Cretaceous (Sempere,

1995), it is interesting that latest Liassic–earliest

Dogger giant tholeiitic sills are known in South Africa

and Antarctica (Encarnacion et al., 1996; Fleming et

al., 1997), as these regions were also located along the

southwestern edge of Gondwana and underwent co-

eval lithospheric thinning.

A feeding dyke of the Entre Rıos sill can be

observed at Abra Castellon, 8 km northwest of Entre

Rıos, and it is possible that the dykes knownwest of the

Chaco Subandean belt (see below, and Fig. 4) served as

feeding dykes of the giant sill. Present known differ-

ences in rock composition does not preclude this

hypothesis, as alkaline and tholeiitic magmatisms can

coexist during rifting. Furthermore, the early Dogger

apparent age of the giant sill agrees with the Toarcian

apparent age obtained on a dyke from the Tupiza area.

This suggests that magmatic activity, including alkaline

and tholeiitic magmas, was significant in southern

Bolivia in the late Liassic–early Dogger (190–170

Ma).

Our recent field study of the Camiri basalt (sensu

Sempere et al., 1998) has showed that it is a � 30-m-

thick sill that intrudes the Pennsylvanian–Early Per-

mian Cangapi Formation in a small area southeast of

Camiri (Fig. 4). This also altered rock yielded a


Middle Triassic apparent age (Table 1), suggesting

that basic magmatism may have been active in the

area as early as the Triassic; this age, however, needs

confirmation. No geochemical data are available on

the Camiri sill yet, but its overall aspect and relation-

ships are similar to the Tarabuco–Entre Rıos sill.

2.3.2.3. Other areas of magmatism and heat flow in

Bolivia. Apart from the giant tholeiitic sill present in

the ‘‘Entre Rıos branch’’, the Mesozoic magmatism

known in Bolivia has proved so far to be of alkaline

and/or intraplate type, inclusively in some areas that

were apparently located outside the rift system (Fig. 4;

Soler and Sempere, 1993; Tawackoli et al., 1999).

Close to the Bolivian border in northwestern Argen-

tina and west of the Chaco Subandean belt, a potassic

alkaline ultrabasic sheet intrusion (Meyer and Villar,

1984) is derived from a deep lithospheric mantle

source (Barbieri et al., 1997) and might represent

one of these manifestations (Rubiolo, 1997), in spite

of an apparent age of 303 ± 10 Ma (Table 1). North of

the border, � 55 km SSWof Tarija, the Rejara pluton,

a Neoproterozoic or Cambrian monzonite to grano-

diorite that underwent some cataclastic metamor-

phism, is intruded by dolerite dykes that also cut

overlying Ordovician strata; the K–Ar apparent age

of 141 ± 10 Ma obtained on this pluton (Table 1)

possibly reflects a Jurassic emplacement of the doler-

ite dykes and related heat flow (Rubiolo, 1997;

Sempere et al., 1998).

A basalt ‘‘flow’’ from the Serranıas de Chilla south

of Lake Titicaca yielded apparent Permian and Early

Triassic ages (Table 1). In the same region, a Permian

apparent age was obtained on an altered ‘‘gabbro’’

from an unprecised area south of Lake Titicaca (Table

1); given its apparent age, the sampled basic rock is

probably related to the Serranıas de Chilla basalt.

Permian magmatism is also known west of Juliaca

in nearby Peru (Fig. 4).

In the northwestern Beni Subandean belt, in the

Rıo Yanamanu (Serranıa de Uchupiamonas), a 8-m-

thick lava flow or sill, of ‘‘andesitic’’ aspect, occurs in

the Jurassic Beu Formation, 400 m above its base

(Ponce de Leon et al., 1972). In the same area,

numerous basic dykes and sills intrude Early Devon-

ian strata, whereas grains of basic volcanic rocks are

common in the Beu Formation sandstones (Ponce de

Leon et al., 1972).

At the northern foot of Nevado Tata Sabaya in the

Western Cordillera of Bolivia, a slightly cataclastic

and altered granite crops out below the Cenozoic

cover and possibly belongs to the Precambrian ‘‘Are-

quipa massif’’ (Rivas, 1989). This granite yielded two

Toarcian apparent ages (Table 1). Plausible hypoth-

eses are that these ages reflect a significant thermal

reset in the Toarcian, or emplacement of the pluton at

that time; both hypotheses agree with coeval exten-

sion-related magmatism and/or heat flow in the

nearby southern extension of the Arequipa basin (see

below).

Lead–zinc(–silver) ore deposits are commonly

associated with ancient rifts, and the stratabound

deposits known in the Late Triassic–Liassic Pucara

Group of central Peru are no exception (Rosas and

Fontbote, 1995). Although they occur in the Paleo-

zoic, the lead–zinc(–silver) ore deposits known in the

Eastern Cordillera of Bolivia are distributed along

both sides of the main rift axis and thus possibly

formed at depth in relation with the rift system

(Sempere et al., 1998).

2.3.3. Central Peru

Intense magmatism also occurred in central Peru

during the Mitu rifting (Megard, 1978; Dalmayrac et

al., 1980; Carlier et al., 1982; Soler, 1991). Numerous

plutons intrude the metamorphic basement exposed in

the Eastern Cordillera and were probably emplaced in

the rift ‘roots’. They consist of a variety of granites,

granodiorites and alkaline granitoids, which appa-

rently display two clusters of ages: a set of mainly

northern plutons was emplaced during the Mississip-

pian, and a larger one during the Late Permian and

Triassic (Jacay et al., 1999). This suggests that Late

Permian–Triassic rifting developed along an area

where magmatism had already been significantly

active in the Mississippian.

Soler (1991) recognized the following chronology

of pluton emplacement in the Eastern Cordillera of

central Peru (Table 1): (1) the Equiscocha granite; (2)

the San Ramon-type batholiths, which are intruded by

(3) the Talhuis–Carrizal-type granitoids; and (4) basic

dykes (including microgabbros and microdiorites) that

cut through the latter. This chronology is based in part

on the observation that theMitu Group syn-rift deposits

contain clasts derived from the erosion of San Ramon-

type granitoids, but so far no clasts derived from


Talhuis–Carrizal-type granitoids. These relationships

support the idea that rifting developed in two stages.

The early, small Equiscocha pluton is a fine-

grained hololeucocratic granite that contains abundant

muscovite and scarce small-size garnets, and is

derived from crustal melting. The large San Ramon-

type granitoids are metaluminous to slightly peralu-

minous, calc-alkaline granites that closely resemble

the Mitu granitoids from southern Peru (Soler, 1991)

and thus were probably mainly derived from crustal

melts; Sr isotopic ratios (Capdevila et al., 1977) from

the San Ramon batholith suggest that these crustal

melts partly mixed with mantle-derived magmas

(Soler, 1991). In contrast, the Talhuis–Carrizal-type

granitoids have a marked alkaline affinity and show

many characteristics of granitoids emplaced in litho-

spheric thinning settings; the late basic dykes have

alkaline compositions similar to the alkaline basalts

from southern Peru, and were comagmatic with the

Talhuis–Carrizal-type granitoids (Soler, 1991).

As in southern Peru, volcanic rocks associated with

the Mitu rift system in central Peru are predominantly

acid, and are mainly represented by dacitic to rhyolitic

pyroclastic rocks. Most of the Mitu volcanics of

central Peru have alkaline compositions and are prob-

ably comagmatic of the Talhuis–Carrizal granitoids

(Soler, 1991).

Present knowledge of the rift-related magmatic

evolution in central Peru suggests that lithospheric

thinning initially produced crustal melts (e.g., the

Equiscocha granite) that were partly contaminated at

a later stage by mantle-derived melts (San Ramon-

type batholiths), and that these mantle-derived alka-

line magmas finally dominated (Talhuis–Carrizal-

type granitoids and comagmatic basic dykes, and Mitu

volcanics).

2.4. Syn-rift deposits

2.4.1. Syn-rift basins in Peru: the Mitu Group

The Mitu Group consists of a red to purple, locally

>2000-m-thick succession of conglomerates, sand-

stones and mudstones, with local carbonates and

evaporites, that accumulated in subsident grabens

(Megard, 1978; Dalmayrac et al., 1980; Carlotto,

1998). These sedimentary rocks are commonly inter-

bedded with locally dominant volcanic and volcani-

clastic rocks, and/or intruded by subvolcanic to

plutonic rocks that do not intrude overlying units (this

is particularly clear in the Nunoa area of southern

Peru; Fig. 4). Paleoenvironments identified in the

Mitu Group include alluvial fans, fluvial depositional

systems and (playa-)lakes.

A thick coarsening-upward unit of reddish sand-

stones and conglomerates known in the Ocona–

Camana area of coastal southern Peru is traditionally

assigned to the Mitu Group, but this attribution needs

confirmation (see Soler, 1991 for a summary).

2.4.2. Syn-rift basins in Bolivia: the lower Serere

Group

The lower part of the Serere Group of Bolivia

(Sempere et al., 1998) comprises the red-coloured

Ipaguazu and Tiquina formations (because correla-

tions seem now secured, the more recent denomina-

tion ‘‘Sayari Formation’’ should be abandoned in

order to simplify the stratigraphic nomenclature).

The Ipaguazu Formation consists of red mudstones

with subordinate sandstones and local evaporites

(gypsum, rarely halite) of dominantly lacustrine ori-

gin, and crops out in the Chaco Subandean belt

(mostly in the ‘‘Entre Rıos branch’’ of the rift axis)

and in the main rift axis of the Eastern Cordillera. The

Tiquina Formation predominantly consists of interca-

lations of red sandstones and mudstones, and of

locally thick conglomeratic sandstones or basalt-clasts

conglomerates, and is of alluvial origin; it crops out in

some localities of the Eastern Cordillera (including the

main rift axis) and in the Tiquina area. The Ipaguazu

and Tiquina formations respectively represent fine-

grained and coarse-grained end-members of the Boli-

vian equivalent of the Mitu Group. Both units are

generally � 500 m thick and conformably overlain by

the thick fluvio-eolian sandstones of the Ravelo For-

mation (Andean domain) or Tacuru sub-Group (Chaco

Subandean belt). The Ipaguazu and Tiquina forma-

tions generally occur at specific localities or in restricted

areas, suggesting they were deposited in paleograbens,

whereas the overlying fluvio-eolian sandstones are

present over much broader regions.

In the rift axis, the Ipaguazu or Tiquina formations

include basalt flows in their lower part; at four Andean

localities, these flows overlie the Late Permian–Early?

Triassic Vitiacua or Chutani formations (see above).

The Ipaguazu Formation unconformably overlies the

Vitiacua Formation in the ‘‘Entre Rıos branch’’ of the


rift axis. The observed successions characteristically

include an unconformable basal unit, generally no

more than several tens of meters thick, that consists

of pale alluvial or eolian sandstones, reddish conglom-

erates and/or basalts; this basal unit rapidly grades into

a thick, brown-red to brown-purple, mudstone-domi-

nated unit of alluvial to lacustrine origin.

At Quebrada Aymaraj Hueko, 6 km southwest of

Torotoro (Fig. 4), a previously unknown >300-m-

thick clastic unit overlies the Vitiacua Formation

with a very rapid transition, and represents a local,

coarse facies of the Tiquina Formation that strongly

resembles some deposits in the Mitu Group of Peru.

The 2–6-m-thick basal conglomerate of this unit

contains abundant basaltic clasts up to 15 cm in

size. The overlying strata dominantly consists of

basalt-clasts conglomerates and red conglomeratic

sandstones, in which at least one basalt flow is

intercalated, and is intruded by a � 100-m-thick

basalt sill. Furthermore, this Tiquina Formation and

underlying strata are folded and overlain by the

Cretaceous Torotoro Formation with a clear angular

unconformity.

2.5. Post-rift deposits

2.5.1. Post-rift deposits northwest of the Bolivian

Orocline, Peru: the Pucara Group

The Mitu rifting generated a thermal sag that

progressively expanded the basin. The Pucara Group

carbonates were deposited over the Mitu Group dur-

ing the Norian–Liassic interval (Megard, 1978; Stan-

ley, 1994) and onlapped across the rift shoulders.

They reflect a transgression that initiated in the Norian

and progressed from north to south following the Mitu

rift axis (Megard, 1978; Loughman and Hallam, 1982;

Rosas et al., 1997; Sempere et al., 1998). Maximum

inundation is marked by the organic-rich shales and

marls of the < 50-m-thick, late Rhaetian–early Sine-

murian Aramachay Formation. To the east, in the

Peruvian Oriente, red alluvial and eolian strata (lower

Sarayaquillo Formation) grade westwards into the

Pucara carbonates (Megard, 1978). Basalts with an

intraplate signature commonly occur in the Pucara

Group, which hosts a number of lead–zinc(–silver)

stratabound ore deposits (Rosas and Fontbote, 1995;

Rosas et al., 1997). The Pucara Group is unknown in

the Cordillera Oriental southeast of Cusco.

2.5.2. Post-rift deposits in the Bolivian Orocline, Peru

and Bolivia: the upper Serere Group

In the Cusco–Sicuani area in southern Peru, the

Mitu Group is post-dated by fluvio-eolian sandstones

(Caycay Formation; Carlotto, 1998) that are locally

intercalated with basalt flows. Northwest of Lake

Titicaca, the fluvio-eolian sandstones of the Quilca-

punco Formation conformably overlie the Mitu Group

(where present) or the Paleozoic (outside the Mitu

grabens) (Sempere et al., 2000a). The Quilcapunco

Formation is overlain by the Sipın Formation carbo-

nates, which yielded Rhaetian–early Bajocian echinids

(see below). The red mudstones of the Muni Formation

unconformably overlie the latter, and are transitionally

overlain by the fluvio-eolian Huancane s.s. Formation

(Fig. 2; Sempere et al., 2000a). The Muni Formation

was deposited in an alluvial to coastal plain environ-

ment and contains thin marine intercalations with

fossils suggestive of a late Dogger–early Malm age

(Newell, 1949). The Sipın and Muni formations thin

out to the north and east, and the fluvio-eolian Quilca-

punco and Huancane s.s. formations become coales-

cent in these directions (Sempere et al., 2000a).

These Jurassic fluvio-eolian units from southern

Peru correlate in Bolivia with the Beu Formation of

the Beni Subandean belt, the Ravelo Formation of

Andean Bolivia, and the Tacuru sub-Group of the

Chaco Subandean belt, which all are of similar facies

and origin (Oller and Sempere, 1990; Lopez-

Pugliessi, 1995; Lopez-Murillo and Lopez-Pugliessi,

1995; Sempere, 1995; Sempere et al., 1998). These

unfossiliferous sandstones can be over 1000 m thick

and locally include basaltic flows and sills, and

conglomerates with basalt clasts. Their distribution

shows they onlapped laterally from the initial rift axis

(Sempere, 1995). Their top is an erosional surface that

is onlapped by Cretaceous strata (Puca Group).

The Tacuru sub-Group includes, in stratigraphic

order, the (San Diego-)Tapecua (fluvio-eolian sand-

stones), Castellon (fluvial sandstones and subordinate

red and green mudstones), Ichoa (eolian sandstones)

and Yantata (fluvial sandstones partly equivalent to

the Ichoa) formations (see discussion in Sempere et

al., 1998). Semionotiformes fish occur in the Castel-

lon Formation (det. M. Gayet, Universite de Lyon,

France), and are also common in the Late Triassic/

Early Jurassic lower Tacuarembo Formation of the

Parana basin in Uruguay (Sprechmann et al., 1981).


Four new species of ostracods (Bisulcocypris laci-

niata boliviana, B. truncata, B. castellonensis, B.

lubimovae) from the Castellon Formation are similar

to Jurassic taxa of the Parana basin in Brazil but were

tentatively assigned to the lowermost Cretaceous

(Wealdian) due to a traditional attribution of the unit

to the Cretaceous (Damiani-Pinto and Sanguinetti,

1987). In spite of apparent discrepancies, all paleon-

tologic ages published on the Vitiacua through Cas-

tellon stratigraphic set in the Chaco Subandean belt

belong to the Late Permian–lowermost Cretaceous

interval (Sempere et al., 1998).

Abundant eolian sandstones in Bolivia and south-

east Peru complement to the west the large Jurassic

desertic domain defined by coeval eolian units in the

Parana and Karoo basin (e.g., Franc�a et al., 1995).

Global paleoclimatic models place Bolivia and south-

east Peru within a broad desertic domain in the

Jurassic (Chandler et al., 1992).

3. Early and Middle Jurassic lithospheric thinning

in southwestern Peru

3.1. Introduction

The Arequipa basin (Fig. 4) originated by litho-

spheric thinning during the Liassic and Dogger. The

Jurassic infill of this basin is formed by a 4500–6000-

m-thick succession that provides a prime sedimentary

record of the regional geologic evolution (Jenks,

1948; Benavides, 1962; Vicente, 1981, 1989; Vicente

et al., 1982). We refer to this succession as the ‘‘Yura

Group’’, in a modified sense. As used here, the Yura

Group comprises, from bottom to top, the Chocolate

s.s., Socosani, Puente, Cachıos, and Labra formations,

because we think that this name should reflect the

entire activity of the basin in which it accumulated.

3.2. Igneous rocks

The dominantly volcanic and volcaniclastic Choc-

olate s.s. Formation (i.e., sensu Jenks, 1948) is >900–

1500 m thick (its base rarely crops out). Near its top, it

includes limestone beds that yielded Sinemurian

ammonites, and is disconformably overlain by the

late Liassic carbonates of the Socosani Formation

(Vicente, 1981).

In the coastal area, volcanic and volcaniclastic rock

units traditionally correlated with the Chocolate For-

mation (Bellido and Guevara, 1963) were paleonto-

logically and isotopically dated (Roperch and Carlier,

1992; Romeuf et al., 1993, 1995). In the Chala area, a

basaltic flow from the lower part of the Chala For-

mation yielded an early Dogger age (Table 1), dem-

onstrating that this >3000-m-thick unit represents the

southern extension of the Dogger Rıo Grande For-

mation (Ruegg, 1956; Caldas, 1978); both formations

unconformably overlie Precambrian and Late Paleo-

zoic rocks. In the Tacna area, the ‘coastal’ Chocolate

Formation is overlain by the volcano-sedimentary,

>3000-m-thick Guaneros Formation, the base of

which has yielded late Bajocian–Bathonian ammon-

ites (Romeuf et al., 1993, 1995) and apparently

records a sea level maximum. This ‘coastal’ Choco-

late formation is constituted by a >3000-m-thick

volcano-sedimentary succession intruded by Hettan-

gian to Toarcian granodiorites (Clark et al., 1990a;

Romeuf et al., 1993) and is thus likely to include

Triassic deposits.

The Rıo Grande, Chala, and Guaneros formations

show geochemical characteristics which suggest that

they accumulated in a subduction-related volcanic

arc setting (Romeuf et al., 1993, 1995). The consid-

erable thickness of these units points to high sub-

sidence rates, and rather suggests that these volcanic

and volcano-sedimentary rocks accumulated in an

extensional back-arc setting, close to the arc proper.

As the coastal formations attributed to the Choc-

olate Formation are neither chronologic nor genetic

equivalents, we suggest that these homonymous vol-

canic units should now be prudently distinguished.

Therefore, the arc setting reconstructed for the Middle

Jurassic coastal volcanic rocks should not be gener-

alized to the Late Triassic–early Liassic Chocolate s.s.

Formation of the interior Arequipa basin.

3.3. Late Triassic–early Bajocian record of incipient

lithospheric thinning

Although the Chocolate s.s. volcanic rocks remain

virtually unstudied, the association of this Late Tri-

assic–early Liassic thick volcanic unit with the over-

lying late Liassic–Bajocian Socosani shallow-marine

carbonates is strongly reminiscent of the genetic link

between the Late Permian–Triassic Mitu Group syn-


rift volcanics and the overlying Late Triassic–Liassic

Pucara Group sag shallow-marine carbonates in cen-

tral Peru. Following Vicente et al.’s (1982) sugges-

tion, we thus propose that the Arequipa basin

originated by rifting, and given the high subsidence

evidenced in the coastal area, that this rifting probably

developed in an extensional back-arc setting.

The accumulation of the thick Chocolate s.s. vol-

canic rocks reflects intense tectonic subsidence, as

do the overlying, sedimentary, Toarcian–Bathonian

Socosani and Puente formations, in which synsedi-

mentary extensional features are abundant (Vicente et

al., 1982). Toarcian emplacement of the Punta Coles

gabbro-monzotonalite super-unit (Table 1) must have

Fig. 8. Stratigraphy of the Arequipa basin (modified after Vicente, 1989). Age indications are based on fossils (Vicente, 1981, 1989).


occurred in the same framework; the area of emplace-

ment of these plutons probably underwent more in-

tense lithospheric thinning and is located southwest of

the classical outcrop area of the Yura Group, where no

synsedimentary magmatic manifestations are known.

In the northeastern extension of the Arequipa

basin, we found Diademopsis sp., an echinid genus

indicative of the Rhaetian–early Bajocian interval

(Thierry et al., 1997), in marine limestones of the

Sipın Formation (Newell, 1949). This 0–40-m-thick

unit is thus correlative of the Socosani Formation and

reflects the same regional subsidence-related trans-

gression (Fig. 2).

3.4. Downwarping of the Arequipa basin in the middle

Dogger

Lithospheric thinning culminated in the late Bajo-

cian–early Callovian (� 162–167 Ma) with consid-

erable deepening and tectonic downwarping of the

Arequipa basin. Early Bajocian shallow-marine lime-

stones are abruptly overlain by late Bajocian ‘starved

basin’ facies (Upper Socosani Formation) that are in

turn overlain by a 700-m-thick Bathonian and early

Callovian turbidite succession (Puente Formation)

(Vicente et al., 1982; Vicente, 1989; Fig. 8). The

turbidites were deposited in an elongated trough

parallel to the present Andean trend, and show

NW! SE paleocurrents.

The overlying, � 500-m-thick, Cachıos Formation

predominantly consists of organic-rich shales. Subor-

dinate sandstones are distributed in channels and in

coarsening-upward slumps and olistolites. Facies

overall indicate a submarine slope paleoenvironment

(Vicente, 1981; Vicente et al., 1982). The Cachıos

Formation has yielded early and late Callovian

ammonites (Vicente, 1989).

Callovian ammonite-bearing shales and minor

coarser sediments are widespread southwest of Lake

Titicaca (Douglas, 1920; Jenks, 1948; Newell, 1949;

Benavides, 1962; Bellido and Guevara, 1963; Portu-

gal, 1974; Vicente, 1981), which probably indicates

that the Jurassic regional maximum inundation oc-

curred during this stage. In the northeastern extension

of the Arequipa basin, fossiliferous marine beds

intercalated in the Muni Formation (Newell, 1949)

are correlative of this late Dogger highstand (Fig. 2;

Sempere et al., 2000a).

3.5. Filling-up of the Arequipa basin in the Oxfor-

dian–Kimmeridgian

The upper part of the Cachıos Formation shows

shallowing-upward facies and grades into the 300–

1500-m-thick sandstone-dominated Labra Formation

(Vicente, 1981), which was deposited in a siliciclastic

shelf to shoreface setting (Vicente et al., 1982). The

overall thickening- and coarsening-upward trend indi-

cates deltaic-like progradation. Sandstones are com-

monly interbedded with shallow-marine limestones

(Vicente, 1981). The age of the Labra Formation is

bracketed by late Callovian ammonites in the under-

lying Cachıos Formation and early Tithonian ammon-

ites in the overlying Gramadal Formation (Vicente,

1989); the Labra thus appears mostly of Oxfordian–

Kimmeridgian age (� 159–151 Ma).

Progradation of the thick Labra shallow-marine

sandstones onto relatively deep-marine shales implies

that the basin shallowed markedly in the Oxfordian–

Kimmeridgian, while remaining very subsident. Paleo-

currents and cumulative sandstone thicknesses indi-

cate that sands were being derived from the north and

northeast. This invasion by sands perceptibly de-

creased during the early Tithonian (Gramadal Forma-

tion), when transgressive shallow-marine carbonates

were commonly deposited southwest of the present-

day Western Cordillera.

4. Late Jurassic–earliest Cretaceous tectonism in

the Eastern Cordillera

The Eastern Cordillera of central Peru is tradition-

ally believed to have behaved as a structural high

(‘‘Maranon geanticline’’ or ‘‘Axial Swell’’) since the

Late Triassic (Megard, 1978, 1987; Dalmayrac et al.,

1980; Jaillard, 1994), mainly because in this area Early

Cretaceous strata onlap Precambrian and Paleozoic

rocks and are much thinner than to the west and east.

Our reconstruction of a Late Permian–Liassic rift

system along the same area implies instead that syn-

rift, and probably thermal sag, deposition must have

occurred in the Eastern Cordillera domain during this

time span—as is indeed observed at a few localities

(Rosas and Fontbote, 1995). The absence of Late

Triassic–Jurassic deposits and the Early Cretaceous

onlap must thus reflect that this area underwent uplift


and erosion before the Early Cretaceous. Uplift of this

previously rifted area suggests that some kind of gentle

rift inversion occurred in the Late Jurassic and/or ear-

liest Cretaceous.

Latest Jurassic–earliest Cretaceous erosion in the

Eastern Cordillera of central Peru is documented by

the Copuma (J. Jacay, unpublished) and Upper Sar-

ayaquillo conglomerates, which overlie Lower to

Middle Jurassic strata respectively west and east of

the ‘‘Maranon geanticline’’. The Copuma conglomer-

ates underlie the Valanginian–Aptian Goyllarisquizga

Group with an angular unconformity, and, to the west,

grade into red mudstones and sandstones. More to the

west, their lateral equivalent is likely to be the con-

glomerate-bearing Tinajones Formation, which is of

probable Berriasian age (Jaillard, 1994) and forms the

upper unit of the Chicama Group of western central

Peru (Fig. 3); older units of this group record a

considerable and abrupt downwarping of the local

basin floor during the Tithonian, as well as neighbour-

ing uplifts (Jaillard and Jacay, 1989; Enay et al.,

1996). The Goyllarisquizga Group disconformably

overlies the Chicama Group in the west, and uncon-

formably onlaps older rocks to the east (Jaillard et al.,

1997), including the Precambrian in the Eastern

Cordillera. The erosional surface at the base of the

Goyllarisquizga Group has a regional importance

(Fig. 2).

In coastal southern Peru, the post-Dogger, pre-

Tithonian, unconformity identified by Ruegg (1961)

reflects at least local tectonic motions. Invasion of the

Arequipa basin by the northeast-derived, Oxfordian–

Kimmeridgian, Labra sands suggests that they were

produced by coeval uplifts in the northeast, possibly by

gentle inversion of the Eastern Cordillera rift system.

The early Tithonian Gramadal Formation, up to 300 m

thick in the south, rapidly thins to the northeast and

grades into red silty mudstones of coastal plain origin,

as observed by us at Chivay; near this locality, this unit

abruptly overlies the Labra Formation and locally dis-

plays coarse conglomerates at its base.

In the Lagunillas area (Fig. 4), 150-m-thick, very

coarse conglomerates overlie folded Sinemurian–

Kimmeridgian strata with a marked angular uncon-

formity, and are transitionally overlain by an overall

fining-upward succession consisting of conglomeratic

sandstones, sandstones and red mudstones, overlain in

turn by the late Middle Cretaceous Ayabacas Forma-

tion (as correctly described by Newell (1949); these

observations are at odds with Portugal (1974) and

Jaillard and Santander (1992)). This demonstrates that

the Jurassic marine strata near Lagunillas were

deformed in the latest Jurassic and/or Early Creta-

ceous, and that reliefs were tectonically created in the

area at that time. Lower temperature portions of39Ar–40Ar spectra ‘‘strongly suggest’’ that the region

� 35 km north of the Lagunillas area underwent

thermal overprinting at � 130–120 Ma (Clark et al.,

1990b).

In Bolivia, east-derived conglomerates are domi-

nant in the uppermost unit of the Serere Group � 25

km west of the Jurassic rift axis, suggesting that

erosion affected an uplifted structure derived from

inversion of the rift system prior to the Early/Middle

Cretaceous onlap. The angular unconformity observed

near Torotoro between Cretaceous strata and folded,

Mitu-equivalent, volcaniclastic deposits (see above)

reflects this deformation.

Small igneous bodies emplaced after cessation of

rifting show that magmatic activity did not disappear

completely from the lithospheric heterogeneity cre-

ated by rifting (Fig. 4): the Cerro Sapo alkaline

complex includes a nepheline syenite locally enriched

in sodalite, carbonatites, and a breccia-pipe (bearing

kimberlitic clasts) of Middle Cretaceous age (Table 1);

the undated Carpacayma phonolite overlies the Copa-

cabana Group � 25 km southwest of Torotoro

(Kozlowski, 1934); farther south, a high-K, gabbroic

to syenitic, Early Cretaceous intrusion occurs at Cerro

Grande (Table 1); in the Argentine Puna, along the

same lineament, alkaline rocks are known (Rubiolo,

1997) and include Early Cretaceous alkaline granites

(Table 1).

Available data thus suggest that a number of uplifts

(and downwarps) occurred in Peru and Bolivia in the

Late Jurassic–Early Cretaceous, and especially in the

Tithonian–Berriasian interval (Jaillard, 1994; Sem-

pere et al., 1999); new data concerning uplifts of this

age in southern Peru and Bolivia will be presented in

detail elsewhere. Uplifts in Peru are post-dated by

easterly or northeasterly onlap of Valanginian to

Middle Cretaceous strata, and an erosional surface is

found at its base (Megard, 1978; Laurent, 1985;

Jaillard, 1994). In Bolivia, this erosional surface is

represented by the unconformity that separates the

Serere Group from the overlying Puca Group; depo-


sition on this surface started in the Middle Cretaceous

(Sempere, 1995).

5. Consequences of Late Permian–Jurassic

lithospheric thinning for Andean shortening

5.1. Andean inversion of the Eastern Cordillera rift

system

As underlined above, the axis of the well docu-

mented Late Permian–Jurassic rift system closely

coincides with the axis of the present-day Eastern

Cordillera of Peru and Bolivia. This coincidence

strongly suggests that the Eastern Cordillera results

from tectonic inversion of this rift system. It is logical

that many thrusts in the Eastern Cordillera originated

by compressional to transpressional reactivation of

earlier extensional or transtensional faults (Sempere,

2000). The dip and overall geometry of the Late

Permian–Jurassic rift faults determined the vergence

of many Andean-age thrusts, as west (resp. east)-

vergent thrusts are predominant west (resp. east) of

the paleo-rift axis. In the N-trending segment of the

Eastern Cordillera, south of 19�S, Andean-age tec-

tonic displacements are likely to have been initially

more transpressional (Herail et al., 1996; Tawackoli,

1999) due to the obliqueness of pre-existent structures

relative to stress (although more recent, pure compres-

sional slips have obliterated striations produced by

older motions on these faults).

Tectonic inversion and wrenching were more

intense near the axis of the main rift system, as

suggested by exposures of structurally deeper regions

of the rift. For example, in the Eastern Cordillera

northwest of 16�300S, the granitoids that had been

emplaced in the rift ‘roots’ are now exposed at the

highest altitudes. Outcrops of these ‘roots’ are gen-

erally composed of Precambrian to Early Paleozoic

metamorphic rocks (Sempere et al., 1999); along this

axis, granitoids and metamorphic rocks northwest of

16�300S are commonly intruded by basic dykes, as are

non-metamorphic Paleozoic strata southeast of

16�300S.In central Peru, poorly studied peridotite bodies

occur within Precambrian metamorphic rocks, Mis-

sissippian plutons and strata, and at the Mitu/Pucara

contact (Aumaıtre et al., 1977; Grandin and Zegarra-

Navarro, 1979; Jacay, 1996; Megard et al., 1996;

Quispesivana, 1996; Jacay et al., 1999). Given these

geologic relationships, it is possible that these peri-

dotite bodies were tectonically emplaced in the Juras-

sic in relation with major stretching and/or wrenching

of the crust, if, due to its earlier onset, rifting in this

segment reached a state significantly more advanced

than in southern areas. It is also possible, however,

that these peridotite bodies are fragments of Precam-

brian or Paleozoic mantle lithosphere that were tec-

tonically emplaced into younger rock units during rift

inversion due to particularly intense upward expulsion

of deep material. In any case, the occurrence of

peridotite bodies in this context indicates that in

central Peru rift inversion partly reworked structural

levels as deep as the mantle lithosphere.

Because rift inversions can affect different struc-

tural depths, the amount of inversion-derived uplift

can also be perceived from the distribution of Late

Paleozoic–Jurassic granitoids. Although the rift sys-

tem continues into Ecuador (Rivadeneira and Baby,

1999), the abundance of exposed granitoids typically

decreases north of 6�S, where they nearly disappear.

This suggests that in Peru shortening in the Eastern

Cordillera considerably decreases north of 6�S.Shortening and/or depth of rift inversion in the

Eastern Cordillera apparently also decrease southeast

of 17–18�S, where exposed granitoids disappear and

only basic dyke swarms crop out southwards on (Fig.

3). Lower shortening in this region is also reflected by

the fact that this segment of the Eastern Cordillera,

although of tectonic origin, is not a true, high and

narrow, cordillera but a broad highland region affected

by large-scale erosional surfaces.

5.2. Paleotectonic status of the Altiplano

The Bolivian Orocline is the second most prom-

inent mountain range on earth. Crustal thickness

locally reaches 75 km (Beck et al., 1996), a figure

comparable to the maximum crustal thickness in the

Himalayas, which contrastingly result from conti-

nental collision. In this region is found the Alti-

plano, which in size is the second high plateau on

earth after Tibet. Under a geomorphic point of view,

the Altiplano is a large endorrheic basin largely

covered (and filled) by Cenozoic sedimentary and

volcanic deposits. It is bounded by the Western


Cordillera, a high elongated area formed by clusters

of Neogene volcanoes, and by the Eastern Cordil-

lera. In contrast with Tibet, the origin of the

Altiplano remains largely obscure and is currently

a matter of vigorous debate.

Although outcrops of pre-Cenozoic rocks are rare

on the Altiplano s.s. (i.e., excluding its northern

region), it appears that this domain includes Precam-

brian rocks, a poorly known Paleozoic cover, and, in

its easternmost part, thin remnants of the Jurassic

Ravelo Formation (which thickens to the east, i.e.

toward the Triassic–Jurassic rift axis); the Altiplano

domain was only later onlapped by Late Cretaceous or

younger strata (Sempere, 1995). Because there is no

evidence for pre-Late Cretaceous erosion of would-be

Mesozoic accumulations from the Altiplano, these

relationships indicate that the Altiplano domain was

submitted to little or no subsidence during the Trias-

sic–Jurassic interval.

The Eastern Cordillera bounds the present-day

Altiplano to the east and, because it results from the

inversion of the Triassic–Jurassic rift system, the

latter must have bounded the Altiplano domain prior

to the Andean orogeny. To the west, the Altiplano

domain was bounded by the Arequipa basin, which

apparently continued southwards into northern Chile

(Munoz et al., 1988; Munoz and Charrier, 1993;

Vicente, 1989). Although more studies remain neces-

sary, Dogger rifting is recognized in the north-Chilean

Precordillera (22�S; Gunther et al., 1997), whereas

Triassic–Liassic rifting is documented in the Domey-

ko Cordillera (25–26�S; Mpodozis and Cornejo,

1997); and Early to Middle Triassic rapid uplift and

exhumation of the Limon Verde block (Franz and

Lucassen, 1997) probably reflect coeval rifting west

of this area. Triassic–Jurassic lithospheric thinning is

thus very likely to have developed west of the entire

present-day Altiplano (Figs. 3 and 4).

A major consequence of this analysis is that, before

the Andean orogeny, the non-subsident lithospheric

domain that today corresponds to the Altiplano was

bounded on both its east and west sides by areas

where the lithosphere had been thinned. The litho-

sphere below the Altiplano must thus have had

‘‘normal’’ thickness characteristics before the Andean

orogeny. We find worthy of mention that, according to

recent seismic tomography studies, the Altiplano crust

is indeed still underlain by a 65–80-km-thick mantle

lithosphere (Myers et al., 1998; Schmitz et al., 1999).

However, the current crustal thickness below the

Altiplano varies from � 55–60 km at � 16�S to

70–74 km at 20�S (Beck et al., 1996) and, because

these values considerably exceed ‘‘normal’’ crustal

thickness, the crust below the Altiplano must have

been thickened during the Andean orogeny.

However, the fact that the Altiplano underwent

little surface shortening in the Cenozoic (� 15 km,

i.e. < 10%; Rochat et al., 1999) indicates that its upper

crust behaved relatively rigidly during the orogeny.

Consistently, propagation of upper crustal shortening

apparently ‘‘jumped’’ across the Altiplano in the late

Oligocene, from an area west of the Altiplano to an

area that at least partly coincides with the Eastern

Cordillera (Sempere et al., 1990). This implies that the

Eastern Cordillera resulted from the compressional

failure of the homonymous rift system, and not from

progressive eastward propagation of deformation in

the upper crust (Sempere, 2000).

Because thicknening of the Altiplano upper crust

was weak, crustal thickening mostly affected its lower

crust. This decoupling between the evolutions of the

Altiplano upper and lower crusts (see also Yuan et al.,

2000) can have been caused by ductile flow into the

Altiplano lower crust from overthickened lower crust

beneath the Western and Eastern cordilleras (Sempere

et al., 2000b). This important issue, however, remains

a matter of debate.

5.3. Andean-age inversion of other basins

In central Peru, the main Andean thrusts appear to

coincide with paleogeographic boundaries (Janjou et

al., 1981; Mourier, 1988), suggesting that they derive

from inversion of Mesozoic structures produced by

lithospheric thinning (Jaillard, 1990). In southern

Peru, the compressional tectonics that affects the

Arequipa basin is interpreted to be of Late Creta-

ceous–Early Paleogene age (Vicente, 1989) and pos-

sibly resulted from inversion of this thinned area.

Jaillard (1990) stated that ‘‘in northern Peru, the

inversion of the crustal extensional structures and

the related shortening of the superimposed sedimen-

tary wedge can explain the whole observed crustal

thickness, whereas in southern Peru, it acted as a

minor but not negligible parameter.’’ In these areas,

however, more detailed studies seem necessary to


assess the influence of pre-existent structures on

Andean shortening.

Farther east, in the central and northern Oriente

region of Subandean Peru, Cenozoic structures appa-

rently derive from reactivation of pre-existent faults

(Laurent, 1985).

6. Conclusions

Recognition of pre-orogenic lithospheric thinning

in the Andes of Peru and Bolivia sheds light on the

structure and building history of the Andes at these

latitudes. Although the information used to recon-

struct subsident basins and regions of thinned litho-

sphere does not necessarily come from the areas that

underwent the most intense lithospheric thinning, we

underline that it shows that such processes developed

in the Central Andes during the Late Permian–Middle

Jurassic interval.

The available evidence consistently demonstrates

that the present-day Eastern Cordillera of Peru and

Bolivia underwent significant lithospheric thinning

during the Late Permian–Middle Jurassic interval.

Development of this rift system is no extraordinary

phenomenon, as coeval similar processes were com-

mon in western Gondwana (e.g., Tankard et al., 1995)

due to the contemporaneous dislocation of Pangea.

Onset of rifting seems to have been diachronous,

propagating from north to south. Isotopic ages for the

Mitu magmatism clearly tend to be older, Late Per-

mian, in the north, although Permian (280–260 Ma)

ages are known west and south of Lake Titicaca (Fig.

6; Table 1). Mitu syn-rift strata were apparently

deposited earlier in the north than in the south, where

they overlie a Late Permian–Early? Triassic partly

marine unit that was not deposited in a rift setting.

Carbonate deposition linked to thermal sag along the

Mitu rift axis (Pucara Group) progressed from north to

south, but did not penetrate southeast of Cusco

(Dalmayrac et al., 1980). Lithospheric thinning devel-

oped more to the southwest, in the Arequipa basin,

during the Liassic and Dogger. End of rifting in the

Eastern Cordillera proceeded from onset of gentle

inversion of the rift system in the Late Jurassic.

The Late Permian–Middle Jurassic episode of

lithospheric thinning developed to the east and south-

east of coastal central Peru, where further considerable

lithospheric thinning occurred in the Middle Creta-

ceous (Atherton and Webb, 1989; Atherton, 1990;

Soler, 1991). These two episodes of lithospheric

thinning are likely to have derived from evolving

asthenospheric flow patterns related to large-scale

mantle convection. The major change recorded

at� 89 Ma from stretching-dominated to shortening-

dominated conditions in the Andean lithosphere

(Sempere, 1995) must therefore reflect a major change

in the regional mantle flow pattern around that time.

This conclusion is supported by increasing evidence

that a major change in Pacific mantle dynamics

occurred at � 84 Ma (Sager and Koppers, 2000).

In the areas that had undergone lithospheric thin-

ning, it is obvious that the pre-orogenic crust re-

mained thin until the Andean orogeny developed,

which resulted in its thickening. On the other hand,

it remains unclear whether the mantle lithosphere was

partly reconstituted at the expense of cooling astheno-

sphere while heat flow waned. In any case, however,

Andean-age shortening of the mantle lithosphere must

have triggered delamination processes when its in-

creasing thickness reached a stability threshold (e.g.,

Kay et al., 1994; Carlotto et al., 1999).

We suggest that regional-scale pre-existence of a

thinned crust or lithosphere was a necessary condition

for subsequent non-collisional development of con-

siderable shortening and crustal thickening during the

Central Andean orogeny. It is a trivial principle that

thickening of a continental area is less difficult when

its crust has been previously thinned, as is illustrated

by many ancient passive continental margins that

have been considerably shortened (e.g., in the Teth-

yan belt). It is another trivial principle that the stress

required for sliding on a pre-existent fault is less

than the stress required for a new fault to form. A

consequence of this principle is that pre-existent

faults and other tectonic heterogeneities generally

influence, and may even control, the propagation of

deformation in areas submitted to shortening. In

particular, knowledge of the pre-Andean lithospheric

heterogeneities is crucial to understand why and how

the Bolivian Orocline formed. In this respect, the fact

that the Eastern Cordillera corresponds to a paleo-rift

system and the Altiplano to an unthinned paleotec-

tonic domain suggests that the regional pre-orogenic

structure was a key factor in the formation of the O-

rocline.


Acknowledgements

This study was funded by the Institut de Recherche

pour le Developpement (IRD, previously Orstom) and

conducted under an agreement with the Universidad

Nacional San Antonio Abad del Cusco (UNSAAC),

Cusco, Peru. We thank J. Doubinger, R. Iannuzzi, E.

Robert, and J.-C. Vicente for providing us with

important information. We thank G. Herail for in-

formation regarding northern Chile, and E. Jaillard for

fruitful discussions.

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