Andes Chile.pdf

Geological Society, London, Special Publications

Cenozoic evolution of the Central Andes in Bolivia and northern

Simon Lamb, Leonore Hoke, Lorcan Kennan and John Dewey

Geological Society, London, Special Publications

1997; v. 121; p. 237-264

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Cenozoic evolution of the Central Andes in Bolivia and northern

Chile

SIMON LAMB, LEONORE HOKE, LORCAN KENNAN & JOHN DEWEY

Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK

Abstract: The Central Andes in Bolivia and northern Chile form part of a wide and

obliquely convergent plate-boundary zone where the oceanic Nazca plate is being

subducted beneath the continental South American plate. In the latest Cretaceous and

Palaeocene, this part of the Central Andes formed a volcanic arc along what is today the

forearc region of northern Chile, with a wide zone of subsidence, as much as 400 km wide, at

or close to sea level behind the arc. In the Eocene, the central part of the behind-arc basin

was inverted to form a zone of uplift (proto-cordillera), about 100 km wide and along what is

today the western margin of the Eastern Cordillera of Bolivia. The Altiplano basin and an

early foreland basin were initiated at this time, receiving sediment from the Eocene

proto-cordillera. Subsequently, the proto-cordillera widened, as the rate of deformation

increased and deformation spread westwards into the early Altiplano basin, and also

eastwards towards the Brazilian Shield. In the Late Miocene, deformation essentially

ceased in the Altiplano and Eastern Cordillera. An intense zone of shortening was initiated

in what is today the Subandean Zone on the eastern margin of the Central Andes,

deforming the Oligo-Miocene foreland basin. Shortening in the Subandean Zone

accommodated both underthrusting of the Brazilian Shield and also bending of the entire

mountain belt about a vertical axis. It is suggested that much of the distinctive Cenozoic

tectonic evolution of this part of the Andes is related to pre-Andean strength

inhomogeneities in the South American lithosphere.

The Andes are one of the largest active

plate-boundary zones, forming a mountainous

region which extends for over 5000 km along the

western margin of South America (Fig. la) as a

result of the subduction since the Cretaceous of

the oceanic Nazca (or formerly Farallon) plate

beneath the South American plate (Dewey &

Bird 1970; Pardo-Casas & Molnar 1987). They

show marked variation in tectonic style and

evolution along their length, as well as several

major changes in trend.

The Andes are highest and widest in the

Central Andes of northern Chile and Bolivia,

where the present-day relative plate conver-

gence is roughly ENE-WSW at c. 85 mma -1

(Fig. 1, DeMets et al. 1990). Here, there is a

pronounced bend (Arica bend) in both the

structural and topographic trends, which swing

round from c. NW-SE, north of the bend, to

c. N-S further south (Fig. l a). An active

volcanic arc follows the western margin of the

high Andes, and there is a zone of Cenozoic

magmatism east of the arc, up to several

hundred kilometres wide. Also, in this part of

the Andes, there are thick continental sedimen-

tary sequences, deposited during the Cenozoic

tectonic evolution. The region is well populated

and easily accessible. For all these reasons, this

is an extremely good place to study the develop-

ment of wide zones of continental deformation.

We describe here the tectonic evolution since

the Cretaceous of the Central Andes in northern

Chile and Bolivia, based mainly on our own

extensive field work, unpublished oil company

data, and new geochronological, geochemical

and palaeomagnetic data.

Physiographic and geological provinces

The Andes between 16°S and 23°S form a wide

region up to 800km wide and reaching ele-

vations over 6000 m, bounded in the east by the

Peru-Chile trench and in the west by the

Amazon basin and Chaco plains (Fig. 1).

Topographic cross-sections (Fig. lb) show that

in detail there are several distinctive physio-

graphic provinces. These provide a convenient

way of describing the geology, because the

tectonic evolution of each province has been

generally distinct (Figs 1 & 2). Crustal-scale

cross-sections through the Bolivian Andes are

shown in Figures lc and 3.

From Burg, J.-P. & Ford, M. (eds), 1997, OrogenyThrough Time,

Geological Society Special Publication No. 121, pp.237-264.

237

238

S. LAMB ET A L.

(a)

(b)

Western Eastern

Cordillera Cordillera

F / ] Subandean

orearc/Altiplano/ Zone

85°W

80°W 75ow

70°W

65ow 60°W

10°S

Fig. 2 I~

TAnc_~ ~

F°reland (Amazon Basin)

15os --

--15°S

20os --

__20°S

Chaco Plains

25os --

--25°S

sea level

30os --

-- 30os

~ 20~..~

35os --

75°W

35os

(c)

85°W 80ow

70°W

65ow 60ow

Topography over 2000 m a.s.l.

~ 80

....

Crustal thickness greater than 70 km 10~ (kin)

Fig. 1. (a) Tectonic setting of the Central Andes on the western margin of South America where the oceanic

Nazca plate is being subducted beneath the South American plate in a roughly ENE-WSW direction at

c. 85 mm a -~ (large arrow, DeMets et al. 1990). The position of the trench and depth contours of the Benioff

zone are also shown (bold curves). The oceanic Nazca plate is oldest in the bend region of the trench with a

Palaeocene age. The region of topography over 2000 m a.s.l, and also crustal thickness greater than 70 km in

the Andes are shaded. Topographic cross-sections AA' and CC' are shown in (b). The box outline defines the

region shown in Fig. 2. (b) Topographic profiles AA' and CC' (see (a) for location) define a mountainous zone

up to 900 km wide, ranging from c. -6000 m in the trench to c. 4000 m in the Altiplano. Several physiographic

provinces (Forearc, Western Cordillera, Altiplano, Eastern Cordillera, Subandean Zone, Foreland) can be

recognized. (c) Crustal scale cross-section through the Bolivian Andes for an E-W transect at 20°S (BB 1in

(a))based on seismic data (after Beck et al. in press). Light shading defines original crustal thickness (c. 35 km)

and dark shading shows the crustal thickening.

100|

200

400

600

800

Foreland and Subandes

(Figs 2 and 3). Seismic refraction (Wigger et al.

1993) and reflection data (unpublished oil

company data) shows that the Subandes are part

of a thin-skinned fold and thrust belt accommo-

dating shortening above a basal d6collement at a

depth of 7-10 km and dipping at 2-3 ° towards the

west and southwest (Fig. 3). The Brazilian

Shield has been underthrust beneath the Suban-

dean Zone and Eastern Cordillera.

The foreland region along the eastern margin of

the Central Andes, in the Amazonian basin and

Chaco plains, is generally at an altitude of a few

hundred metres above sea level (Fig. lb). This is

underlain by the Brazilian Shield, which has

been a stable nucleus of South America since the

Proterozoic (Litherland et al. 1986), and over-

lain by up to 5km of Neogene sediments

immediately east of the Andes in a foreland

basin about 200 km wide (Fig. 3, unpublished oil

company data). The topography increases to-

wards the west in the foothills of the Andes,

referred to as the Subandean Zone, which is

about 100km wide and reaches altitudes over

1500 m, dominated by major valleys and ridges

which follow the general structural grain (Fig.

lb). The valleys follow synclines of Cenozoic

sediment, separated by faulted anticlinal ridges

of older Mesozoic and Palaeozoic sequences

Eastern Cordillera

West of the Subandean Zone, the mountains rise

progressively to altitudes over 4000m, in a

region referred to as the Eastern Cordillera.

This is up to 200 km wide and made up mainly of

thick (up to 10km) sequences of Palaeozoic

flysch-like deposits, with thin (<3 km) infolded

Cretaceous and Cenozoic sequences. The most

westerly parts of the Eastern Cordillera form a

high spine, referred to in the north as the

B 0 -"- -

CENOZOIC EVOLUTION OF CENTRAL ANDES

239

70 °

69 °

68 °

67 °

66 °

65 °

64 °

63 °

[

114 °

14'

Salars

Miocene Ignimbrites

Oligo-Recent-]

Pal-Eocene _l Sediments

Pre-Cenozoic

Major Faults

Volcanoes of the Major

Western Cordillera • Towns

15 ~

15 °

16'

16 °

Peru

17']-i

Coroco~•

...........

iiii'3~

..... ..........

Cocbabamb~

..............

"~ 17°

" ' I ........................... ~

........................

19°~~ k~. -- ~

[Tambo ~

I "~

4'19 °

20'~ ~ ~:~!i!iiiZililiJ, ~ ~, ,V"¢ . ~^^^~t

~I~i!i~`i!i!iiiiiiiii!iiiiiiiiiiii!~ii!!;ii~!iii!ii~iiii~i~i~ii~ii~i:i:!~i!~!~iii~i~iiiii!ij!i!~

[iiii!i!~~ I

"[20 °

22°~i~iii:~

~,~,~

\ q.9 ~

~--~:;~:il

/ ~v -I 22 °

Argentina

23'

23 °

200 kms

70 ° 69 ° 68 ° 67 ° 66 ° 65 ° 64 ° 63 °

Fig. 2. Map of the Central Andes in northern Chile and Bolivia (see Fig. 1), showing the distribution of

Cenozoic and pre-Cenozoic sedimentary sequences and major structures. Major physiographic features are also

shown, including the large salars in the Altiplano and Lake Titicaca. AA' and BB' show the line of a composite

geological cross-section in Fig. 3. Line CC' shows the line of a schematic cross-section in Fig. 4a. Numbers refer

to localities where various formations or rock units, mentioned in the text (see Figs 9 & 11), are defined: (1)

Potoco, Huayllamarca, Turco, Tihuanacu Formations; (2) Cordillera Real graniotoids; (3) Camargo,

Torotoro, Morachata/Viloma Formations; (4) Saila, Tupiza Formations; (5) Quimsa Cruz granites (Mina

Viloco and Mina Argentina bodies); (6) Petaca Formation; (7) Azurita Formation; (8) Totora, Huayllapucara,

Caquiaviri, Tambillo Formations; (9) Challapata ignimbrite; (10) Mondragon, Bolivar, Parotani, Nazareno

Formations; (11) Chilean precordillera ignimbrites; (12) Quebrada Honda Formation; (13) pre-Los Frailes

ignimbrites; (14) Yecua, Tariquia Formations; (15) Crucero, Pomata Formations; (16) Los Frailes,

Morococalla ignimbrites; (17) Umala Formation, Turco ignimbrite; (18) Perez ignimbrite; (19) Sucre-Tarabuco

tufts; (20) Emborozu Formation and recent sedimentation.

Cordillera Real and Quimsa Cruz (Fig. 2), rising

to over 6000 m where Palaeozoic, Triassic and

possibly Cenozoic intrusive granitoid bodies

outcrop. There is a marked change in vergence

of structures east and west of the central part of

the Eastern Cordillera. In the west, vergence is

mainly towards the west, whereas in the east

vergence is towards the east (Fig. 3).

In the bend region, at c. 17.5°S, a series of

Plio-Pleistocene basins occur within the Eastern

Cordillera. These are bounded by ESE-trending

normal faults with a sinistral strike-slip com-

240

S. LAMB ET AL.

A'B

II Eastern

II Cordillera

I IOverthrust of

I ITriassic pluton

B !

I Arc

[Pr~.rnmhrinn

Altiplano

Subandean

Zone

Foreland Basin

A 0 1o--

~llT ......

sea level

100km

Neogene sedimentary

basins/volcanics

Precambrian basement

Hypothetical zone of ductile

deformation

Major fault

] Mainly Palaeozoic sequences

Fig. 3. Composite crustal-scale geological cross-section through the Bolivian Andes (see Fig. 2), based mainly

on unpublished oil company data. The principal features are the presence of thick Neogene sedimentary basins

in both the Subandes and Foreland, and also the Altiplano region. Note the marked changed in vergence of

structures east and west of the central part of the Eastern Cordillera. Precambrian basement has been

underthrust beneath the Subandean Zone and Eastern Cordillera, and also outcrops on the western margin of

the Altiplano (Troeng et al. 1994). A zone of distributed ductile deformation may occur at depth beneath the

Altiplano.

ponent which comprise the Cochabamba Fault

System (Fig. 2, Dewey & Lamb 1992; Kennan

1994; Kennan et al. 1995).

and Chile, and consists of spaced Miocene and

Quaternary andesitic volcanoes and small vol-

canic centres which have erupted through a

poorly known sequence of Cenozoic, Cre-

taceous and older rocks (Figs 2 and 3). Volcanic

cones rise over 2000 m above the general land

surface, reaching elevations over 6000 m.

The western margin of the high Andes

comprises the Precordillera of northern Chile,

which consists mainly of Precambrian basement

rocks and Mesozoic sedimentary sequences and

Cenozoic intrusive and extrusive rocks. A major

fault system, which extends for hundreds of

kilometres along the Precordillera and further

south (Cordillera Domeyko), appears to have

accommodated both sinistral and dextral stike-

slip during the Cenozoic (Mpodozis et al. 1993;

Reutter et al. 1993). The Precordillera slopes

from c. 4000 m to an altitude of c. 1000 m, in a

region mantled by Miocene ignimbrites (Figs 2

& 4a). The foot of this ignimbrite slope is a fiat

region of younger Cenozoic deposits in the

central depression of northern Chile (Figs 2 &

4a). The Atacama fault, which extends for

hundreds of kilometres along the coastal parts of

Altiplano

The Altiplano forms a c. 200 km wide region of

subdued relief, west of the Eastern Cordillera, at

an average altitude of c. 3800 m (Figs lb & 2). It

is the second largest high plateau region, after

Tibet, on Earth, and is essentially a region of

internal drainage. Near La Paz the drainage has

locally broken through to the Amazon basin.

The vast salars of Uyuni and Coipasa, and also

Lake Poopo, are the remnants of once extensive

Pleistocene lakes (Servant & Fontes 1978). The

Altiplano has been an important locus of

sedimentation, where thick sequences of red-

beds have accumulated in the Cenozoic (Figs 2

and 3).

Western Cordillera and Forearc

The Western Cordillera is the active volcanic arc

along the international border between Bolivia

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