Lecture Notes in Earth Sciences

Lecture Notes in Earth Sciences Edited by Somdev Bhattacharji, Gerald M. Friedman, Horst J. Neugebauer and Adolf Seilacher

17 H. Bahlburg Ch. Breitkreuz RGiese (Eds.)

The Southern Central Andes Contributions to Structure and Evolution of an Active Continental Margin

Springer-Verlag Berlin Heidelberg New York London Paris Tokyo

Editors Dr. Heinrich Bahlburg Priv. Doz. Dr. Christoph Breitkreuz Institut für Geologie und Paläontologie Technische Universität Berlin Ernst-Reuther-Platz 1, D-1000 Berlin 10, FRG Prof. Dr. Peter Giese Institut für Geophysikalische Wissenschaften Freie Universität Berlin Rheinbabenallee 49, D-1000 Berlin 33, FRG

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STRUCTURES AND CRUSTAL DEVELOPMENT OF THE CENTRAL ANDES BETWEEN 21° AND 25°S

K.-J. REUTTER*, P. GIESE**, H.-J. GÖTZE**, E. SCHEUBER*, K. SCHWAB***, G. SCHWARZ** & P. WIGGER** * I n s t i t u t für Geologie der Freien Universität B e r l i n A l t e n s t e i n s t r . 34 a, 1000 B e r l i n 33 * * I n s t i t u t für Geophysik der Freien Universität B e r l i n Rheinbabenallee 49, 1000 B e r l i n 33 I n s t i t u t für Geologie und Paläontologie der Technischen Universität L e i b n i z s t r . 10, D-3392 C l a u s t h a l - Z e l l e r f e l d

ABSTRACT

The t e c t o n i c s

o f the morphostructural u n i t s o f the Central Andean segment between

21°S and 25°S are reviewed and t h e i r r e l a t i o n t o the deep c r u s t a l s t r u c t u r e s , as f a r as known from geophysical

research,

i s discussed.

Special

regard

i s given t o the

superposition o f s t r u c t u r e s due t o the stepwise eastward displacement Systems subsequently d e v e l o p i n g on

the C o n t i n e n t a l

margin

during

o f f o u r arc

the Andean Cycle:

(1) L i a s - Early Cretaceous, (2) Mid-Cretaceous, (3) Latest Cretaceous - Eocene, and (4)

Miocene

activity

can

- Holocene. Within these be d i s t i n g u i s h e d :

The

arc Systems,

subduction

zone

three areas o f main and

subduction

tectonic

complex, the

magmatic arc, and the backarc r e g i o n . The subduction complexes o f the f o s s i l

stages

are not preserved and the t e c t o n i c a c t i v i t y o f the present subduction complex a f f e c t s the c o n t i n e n t only l o c a l l y along the coast. The s t r u c t u r e s o f the f o u r magmatic arcs are exposed r e s p e c t i v e l y i n the Coastal

Range, the Longitudnal V a l l e y , the Chilean

P r e c o r d i l l e r a , and i n the broad area extending from the Preandean Depression western p a r t o f the Eastern C o r d i l l e r a . Notwithstanding great s t r u c t u r a l

t o the

differences

between the i n d i v i d u a l arcs, there are common f e a t u r e s such as the close r e a t i o n s h i p between deformation and magmatism, the i n c o r p o r a t i o n o f basement i n t o f o l d and hörst s t r u c t u r e s , and conjugate reverse f a u l t Systems. In the case o f oblique subduction, longitudinal Coastal

strike

slip

faulting,

which may

be left-handed (Atacama Fault o f the

Range) or right-handed (West Fissure o f the Chilean P r e c o r d i l l e r a ) ,

follows

the magmatic a r c . In the backarc r e g i o n , east vergent f o l d and t h r u s t b e l t s developed only i n the stages 3 and 4. A l l the d i f f e r e n t arc stages gradually t o crustal

seem t o have c o n t r i b u t e d

t h i c k e n i n g as there i s a general development i n these

from ens'ialic m a r i n e c o n d i t i o n s over an e n v i r o n m e n t o f C o n t i n e n t a l

lowlands

Systems t o the

present high plateau S i t u a t i o n . Lecture Notes in Earth Sciences, Vol. 17 H. Bahlburg, Ch. Breitkreuz, P. Giese (Eds), The Southern Central Andes © Springer-Verlag Berlin Heidelberg 1988

232

X X X X X

Coast Range

Western Cordillera

Longitudinal Valley

Neogene-Holocene Volcanism

Andean foreland(Chaco)

Altiplano

Chilean Precordillera

Salt lakes (Solares)

Eastern Cordillera

B

Preandean Depression

Subandean Ranges

- JC^-H Q) with Precambrion autcrops I b) without Precambrian outcrops clEostern front of Neogene-Holocene volcan.

n 0

100

200

300 km

f j c u _ l : The morphostructural u n i t s o f the Central Andean segment under c o n s i d e r a t i o n .

233

INTRODUCTION

The

distinct

morphostructural

u n i t s of

elements of an arc geotectonic

the

Central

Andes

( f i g . 1)

are

tectonic

s e t t i n g generated at the western rim of the South

American continent caused by i t s convergence w i t h the oceanic p l a t e of the P a c i f i c . Within t h i s s e t t i n g , the Western C o r d i l l e r a represents the magmatic arc, the backarc area compriSes the A l t i p l a n o , the Eastern C o r d i l l e r a , the Subandean Ranges as well as the Chaco lowlands, Chilean

Central

w h i l e the

Valley,

the

inner slope Chilean

o f the trench, the Coastal

P r e c o r d i l l e r a , and

the

Range, the

Preandean

Depression

belong t o the f o r c a r c System ( f i g . 2 ) . Morphology, t e c t o n i c behaviour, and

internal

s t r u c t u r e s of these u n i t s are not only determined by the stresses a c t i v e i n the upper plate

as

a

consequence

Parameters such as fluids

and

levels,

the

as

e.g.

as

inhomogeneities may

plate

convergence,

but

are

c o n t r o l l e d by

lithology,

temperature d i s t r i b u t i o n ,

of

magmatic bodies w i t h i n

presence

well

In t h i s respect,

of

by

liquid

geological

history,

as

the

partial

the

physical

pressure

different

preexisting

of

crustal

structures

and

development at l e a s t during the Mesozoic and

the

c o n t r o l the younger deformation.

the geotectonic

Cainozoic, i . e . during the Andean Cycle, should be considered.

In the segment of the

Andes studied the trend of displacement of the arc system towards the i n t e r i o r of the continent i s well developed, b e t t e r than i n other parts o f the Central Andes ( f i g . 7). Thus, i t i s known t h a t the chain of the orogenic a n d e s i t i c volcanism s h i f t e d from the Coastal Position

Range, where i t was

in

the

Precordillera

Western

during

s i t u a t e d during the Early J u r a s s i c , t o i t s present

Cordillera with

Cretaceous

and

Early

intermediate Tertiary

Evidence of a s i m i l a r m i g r a t i o n o f arc plutonism 1985.

was

Together w i t h the magmatic arc the Continental

stress f i e l d s and deformations

in

the

(COIRA e t

published

by

arc system as

Chilean

a l . , 1982).

RIVANO e t a l . , a whole, i . e .

c h a r a c t e r i s t i c f o r the f o r e a r c , arc and backarc region

must have moved eastwards during t h a t time. This parts o f the Central

positions

times

is clearly

shown i n the

eastern

Andes where the backarc t e c t o n i c s , i . e . the Subandean t h r u s t

b e l t p r o g r e s s i v e l y extends i n t o the Subandean f o r e l a n d not y e t a f f e c t e d by Andean deformation

( f i g . 7 ) . This

also means t h a t the present

a f f e c t areas which some time before had

arc

and

forearc tectonics

been deformed, r e s p e c t i v e l y , i n the backarc

and arc regimes. A s t r u c t u r a l comparison of the Central Andean morphostructural

units

has t h e r e f o r e t o take t e c t o n i c superpositions i n t o account which, as a consequence, become more intense-from for

the

surface

east t o west. This consideration should not only be

s t r u c t u r e s and

s t r u c t u r e s detected by geophysical work

(WIGGER, t h i s

•agnetotelluric

issue)

surveys

morphological

features

methods. Seismological

gravity

measurements

(Schwarz, et a l , 1986)

c r u s t a l s t r u c t u r e s of the Central Andes.

but

f o r deep

s t u d i e s , seismic

(GÖTZE et are

also

a l . , this

valid

crustal

refraction issue)

and

a v a i l a b l e t o probe the deeper

234 Residual 10 Bouguer Anomaly 0

f k L _ 2 : Cross section through the Central Andes a t 21°25'S showing c r u s t a l s t r u c t u r e s according t o g e o l o g i c a l and geophysical data. The subsurface s t r u c t u r e s o f the f o l d and t h r u s t b e l t o f the Subandean Ranges are h y p o t h e t i c a l .

In t h e f o l l o w i n g , t h e surface s t r u c t u r e s and, as f a r as they are known, the deep crustal

structures

o f t h e morphostructural

units

o f t h e Central

Andean

Segment

between 21° and 25°S s h a l l be reviewed b r i e f l y i n order t o e l u c i d a t e the s t r u c t u r a l development during the Andean Cycle.

TECTONICS OF THE MORPHOSTRUCTURAL UNITS

THE ANOEAN FORELAND (CHACO)

The

Subandean

lowlands

o f t h e Chaco

are an a c t i v e

depositional

basin

whose

C o n t i n e n t a l deposits are supplied by r i v e r s t h a t d r a i n the Eastern C o r d i l l e r a and the Subandean

Ranges. The Late

Tertiary

and Pleistocene

Sediments

(Chaco Fm.) show

growing thicknesses from E t o W exceeding

2000 m near the Subandean f o o t h i l l s . They

overlie

Tertiary

either

notably

thinner

Early

and Cretaceous

S e d i m e n t s , or

a l t e r n a t i v e l y d i r e c t l y T r i a s s i c o r Carboniferous Sediments a t the top o f a more o r less

complete

and t h i c k

Palaeozoic

sedimentary

sequence. There

i s no pronounced

235

| *

* \s

|

1 Precambrian

LVZ = Low Velocity Zone

angular unconformity between the Neogene Sediments and t h e i r Substrate, but as i n the vicinity active

o f t h e Subandean Ranges, t h e underlying formations structures

( o i l production)

beneath

a flat

surface

form young and s t i l l o f recent

deposition.

I n t e r n a l unconformities should e x i s t w i t h i n the Chaco Fm. i t s e l f . In conformity w i t h t h e increasing

sedimentary thicknesses and i n c i p i e n t

structures,

the top o f the Precambrian basement i s i n c l i n e d towards the Subandean Ranges and i s s i t u a t e d a t a depth o f about 7 t o 10 km (ALLMENDINGER e t a l . , 1983) i n t h e zone o f the

foothills

(MARTINEZ e t a l . , 1973). Based on t h e view t h a t

Brazilian shield al., and

the crust

s t a r t s t o point downwards underneath the C o r d i l l e r a

of the

(LYON-CAEN e t

1987), c r u s t a l thickness i s accordingly supposed t o increase i n t h a t d i r e c t i o n may reach 35 km. Except f o r the absence o f young marine Sediments, a l l these

sedimentary and s t r u c t u r a l typical quite

foreland. normal

features

are c h a r a c t e r i s t i c

o f a Molasse basin

and a

The Bouguer g r a v i t y f i e l d o f the Chaco area i s characterized by

values o f g r a v i t y

normal g r a v i t y o f c o n s i l i d a t e d

f l u c t u a t i n g around -50 mGals. This

i s nearly t h e

s h i e l d areas.

SUBANDEAN RANGES

The

boundary

o f t h e Subandean

Ranges

appearance o f young Andean s t r u c t u r e s

with

t h e foreland

i s determined

by

the

a t t h e surface. The s t r a t i g r a p h i c column i s

very s i m i l a r t o t h a t o f the Chaco. The thicknesses o f the Lower Palaeozoic formations gradually

increase from the east (roughly 4 km) towards the Eastern C o r d i l l e r a , where

236

values of approximately 15 km are reported ( C o r d i l l e r a Real: MARTINEZ, 1978). In the same d i r e c t i o n , also the Lower T e r t i a r y and Upper Cretaceous sequences become t h i c k e r and s t r a t i g r a p h i c a l l y more complete, although t h e i r thicknesses are subject t o local v a r i a t i o n s (MINGRAMM et a l . , 1979;

SALFITY & MARQUILLAS, 1981). As a consequence of

Neogene and Quarternary u p f o l d i n g , the basin of f o r e l a n d Sedimentation was g r a d u a l l y s h i f t e d t o the east i n t o i t s present p o s i t i o n . The Subandean Ranges are s t r u c t u r a l l y characterized as a schuppen zone or a b e l t of f o l d s t r u c t u r e s w i t h broad synclines and r e l a t i v e l y narrow a n t i c l i n e s whose eastern f l a n k s are f r e q u e n t l y d i s r u p t e d by east verging u p t h r u s t s . As none of the s t r u c t u r e s comprise S e d i m e n t s o l d e r than Devonian or S i l u r i a n , the f a u l t s can a l l be expected t o bend l i s t r i c a l l y towards the west and j o i n a f l a t l y i n g main t h r u s t System at depth which descends stepwise

i n the same d i r e c t i o n

through

the u n d e r l y i n g , not folded

formations. Thus, the Subandean Ranges seem t o form a t y p i c a l

"Cordilleran fold

and

t h r u s t b e l t " (PRICE, 1981), which sheared o f f along Devonian and S i l u r i a n formations and o v e r l i e s unfolded Ordovician, Cambrian, and basement rocks. Based on a

balanced

cross section near the B o l i v i a n border (MINGRAMM e t a l . , 1979), ALLMENDINGER e t a l . (1983) c a l c u l a t e d a minimum shortening of the Subandean zone o f 60 km since the Late Miocene. The development of the Subandean zone during the Cretaceous and

Early T e r t i a r y

has

been i n t e r p r e t e d by GALLISKI & VIRAMONTE ( i n press) as a f o r e l a n d p a l e o r i f t of "low v o l c a n i c i t y type". I t corresponds

to a Continental

backarc basin. This S i t u a t i o n i s

underlined by the e x t r u s i o n o f a l k a l i - b a s a l t i c lavas during the Late Cretaceous. Neogene and Quarternary t e c t o n i c s can be considered as an i n t r a c r u s t a l

The

t h r u s t or A-

Subduction system (BALLY, 1975) which came i n t o being i n the previous backarc basin. There are only few data a v a i l a b l e about the rocks underlying the Subandean t h r u s t plane. I t i s now

supposed t h a t the Ordovician, Cambrian and

the Precambrian rocks

have not been a f f e c t e d by the young Andean compressional

t e c t o n i c s (ALLMENDINGER et

al.,

increasing thicknesses

1983;

Palaeozoic

ROEDER, 1986). and

As

a

consequence

of

younger Sediments, the top of the

the

Precambrian

had

already

of

dipped

towards the west before t h r u s t t e c t o n i c s began. The p i l i n g up of t h r u s t sheets must have increased t h i s d i s p o s i t i o n . The crust can t h e r e f o r e also be expected t o t h i c k e n from E t o W. According t o g r a v i t y models by STRUNK (1985) and GÖTZE e t a l . (1986), the The

Moho descends t o about 50 km near the boundary t o the Eastern C o r d i l l e r a . magnetotelluric p r o f i l e

(SCHWARZ et

s i g n i f i c a n t f e a t u r e s i n t h i s respect. Low

a l . , 1986,

f i g . 3)

does

not

show

any

r e s i s t i v i t i e s are found i n an upper layer

about 5 km t h i c k representing non-metamorphic Sediments ( T e r t i a r y , Mesozoic and Upper Palaeozoic).

In

the

section

stretching

from

Villamontes

to

Tarija

and

Tupiza

( B o l i v i a , f i g . 1 ) , at l e a s t i n the eastern part of the Subandean Ranges no essential

237

t e c t o n i c stacking o f Sediments seems t o be indicated by m a g n e t o t e l l u r i c sounding. In the border zone between the Subandean Ranges and r e s i s t i v i t i e s o f about 100 Ohm

m may

the Eastern C o r d i l l e r a , uniform

be i n t e r p r e t e d as a t h i c k

(2 km)

stacking o f

Palaeozoic and Mesozoic metamorphic Sediments.

EASTERN CORDILLERA, ALTIPLANO, PUNA

The eastern boundary o f the Eastern C o r d i l l e r a i s c h a r a c t e r i z e d by the appearance of s t r a t i g r a p h i c a l l y deeper l e v e l s , e.g.

Lower Palaeozoic and

exposed i n the Subandean Ranges. Precambrian

Precambrian,

than those

rocks are widely d i s t r i b u t e d

i n the

Eastern C o r d i l l e r a o f NW-Argentina; they disappear as a consequence o f a s t r u c t u r a l plunge

- that

at l e a s t

i n p a r t had

existed since pre-Cretaceous

times - towards

B o l i v i a , under Cambrian q u a r t z i t e s and t h i c k (>5 km) Ordovician sequences o f shales and

sandstones.

For

the

same reason,

the easternmost

structures

o f the

Eastern

C o r d i l l e r a i n Argentina merge along s t r i k e i n t o the westernmost p a r t o f the Subandean t h r u s t b e l t east o f T a r i j a . From the h i g h l y u p l i f t e d eastern border o f the Eastern C o r d i l l e r a , the s t r u c t u r a l

level

also gradually descends towards the A I t i p i a n o . In

B o l i v i a , the A l t i p l a n o , characterized by i t s mainly Cainozoic Sediments, i s c l e a r l y separated by f a u l t s from the mostly Palaeozoic Sediments o f the Eastern C o r d i l l e r a . No g e o l o g i c a l l y well defined l i m i t between the Puna and the Eastern C o r d i l l e r a e x i s t s in Argentina. The s t r u c t u r e s i n both areas are very s i m i l a r . Therefore the whole area is treated

together i n t h i s

paper.

As

a result

o f the possibly

fault

controlled

southeastward extension o f the young volcanism (SALFITY, 1985), the western boundary of the Puna and the A l t i p l a n o , as drawn by the present volcanic arc o f the Western C o r d i l l e r a , i s very irregulär. Both areas form an endorheic high plateau w i t h several a c t i v e intramontane d e p o s i t i o n a l basins between the two C o r d i l l e r a s . In the Puna and the A l t i p l a n o , the s t r u c t u r e o f the c r u s t underlying the sedimentary sequences o f the Andean cycle changes i n such a way t h a t mechanical

influences on the

Andean t e c t o n i c s can be expected. While the f o r e l a n d , the Subandean Ranges, and the Eastern C o r d i l l e r a o f B o l i v i a d i d not s u f f e r

important deformations p r i o r

t o the

Andean Cycle and, hence, t h e i r t h i c k mainly p e l i t i c S e d i m e n t s are supposed t o o v e r l i e a r e l a t i v e l y undisturbed basement, the S i t u a t i o n changes t o the west and

southwest

w i t h what i s c a l l e d the "Faja Eruptiva de l a Puna". Volcanic i n t e r c a l a t i o n s i n the Ordovician Sediments, g r a n i t i c

intrusions,

folding,

and

some metamorphism

indicate

the e f f e c t s o f an orogeny ascribed t o the "Oclöyic Phase" o f the Famatinian

Cycle

( O r d o v i c i a n - S i l u r i a n boundary; ACENOLAZA & TOSELLI, 1976,

1973;

1981; MENDEZ et a l . ,

PALMA et a l . , 1986; f o r more d e t a i l s see BAHLBURG et a l . , t h i s i s s u e ) . This orogeny was

.interpreted as the r e s u l t

of a c o l l i s i o n

o f a hypothetical

"Arequipa

Massif"

(DALMAYRAC et a l . , 1977; MARTINEZ, 1978) w i t h the B r a z i l i a n Shield. Precambrian

rocks

in the Chilean P r e c o r d i l l e r a and i n the western part o f the A l t i p l a n o were considered

238

as i n d i c a t i o n s o f t h i s massif (LEHMANN, 1978). During the S i l u r i a n and Devonian the "Oclöyic Orogen" formed a broad u p l i f t t h i n continental

(=Arco Puneiio), a t the western side o f which

and shallow marine Sediments, c o n t r a s t i n g w i t h the contemporaneous

f a c i e s t o the east o f the u p l i f t , have been described (DONATO & VERGANI, 1985) which represent the marginal f a c i e s o f sedimentary sequences i n Chile (BREITKREUZ, 1986). The d i f f e r e n t c o n d i t i o n o f the area t o the west o f the Puna and the A l t i p l a n o i s also underlined by the development from the Late Carboniferous t o the T r i a s s i c when a broad magmatic b e l t developed w i t h mainly acid and intermediate volcanics and shallow marine as well as c o n t i n e n t a l sedimentary i n t e r c a l a t i o n s (COIRA e t a l . , As

a consequence

o f t h e Palaeozoic

tectonic

events

and p o s s i b l e

1982). pre-Cretaceous

movements, Cretaceous and Early T e r t i a r y Sediments, as f i r s t deposits o f the Andean Cycle i n t h e area o f the Eastern C o r d i l l e r a ,

Altiplano

and Puna (Salta and Oran

Groups), unconformably o v e r l i e Palaeozoic and Precambrian

rocks. I n c o n t r a s t t o the

Subandean Ranges, these Sediments were d i s t r i b u t e d throughout the area and f i l l e d up depressions w i t h several 1000 m of mostly c o n t i n e n t a l deposits (SALFITY & MARQUILLAS, 1981; MINGRAMM e t a l . , of

1979; MARQUILLAS & SALFITY, t h i s i s s u e ) . Enormous thicknesses

more than 5000 m l o c a l l y

Special

(MARTINEZ, 1978;

SALFITY, 1985;

SCHWAB, 1985)

are a

f e a t u r e o f the A l t i p l a n o and the Puna. According t o ARANIBAR (Lecture, 4 t h

C h i l . Geol. Congr., Antofagasta, 1985), blocks w i t h reduced sedimentary thicknesses may

border on other blocks w i t h very great thicknesses, so t h a t these blocks are

assumed t o be separated from each other by f a u l t s such as the San Andreas and Coniri f a u l t s o f the northern part o f the A l t i p l a n o (MARTINEZ, 1978) which might possibly be r e a c t i v a t e d p a l a e o - f a u l t s o f the basement (SALFITY, 1985). Some marine i n t e r c a l a t i o n s (e.g.

Yacoraite Fm.) show t h a t

Sedimentation

took place

i n lowlands. A r a t h e r

tensional regime during S e d i m e n t a t i o n i s i n d i c a t e d by some b a s a l t i c flows o f Early and Late Cretaceous

age and, i n the Puna, l o c a l

basic p l u t o n i c rocks. (GALLISKI &

VIRAM0NTE, i n press). After

intense

Oligocene

compressional

("Incaic

Phase"),

intramontane l o n g i t u d i n a l ALONSO, 1987)

tectonics

a t t h e end o f t h e Eocene and during the

Neogene S e d i m e n t a t i o n

took place i n several

basins i n the Puna and the Eastern C o r d i l l e r a

under the influence o f graben-like t e c t o n i c s (SCHWAB, 1985,

separate (JORDAN & fig. 3).

I n t e r n a l unconformities, g e n e r a l l y a t t r i b u t e d t o the "Quechua and D i a g u i t a Phases", underline the synorogenic character o f Neogene S e d i m e n t a t i o n . Another Special feature of

t h e Neogene

and Quarternary

Sedimentation

i s i t s frequent a s s o c i a t i o n

volcanism. Miocene t o Holocene volcanics and subvolcanic bodies are found Altiplano,

Puna and i n the western p a r t o f the Eastern C o r d i l l e r a

1982). These eastern volcanics show backarc a f f i n i t i e s

with

i n the

(COIRA e t a l . ,

as t h e i r potassium content i s

r e l a t i v e l y high (KUSSMAUL e t a l . , 1977). As t h i s Neogene magmatism i s p a r t l y older than t h e i n s t a l l a t i o n

o f the A-Subduction

o f the Subandean Zone during

t h e Late

Miocene (ALLMENDINGER e t a l . , 1983), no causal r e l a t i o n s h i p s can be e s t a b l i s h e d .

239

f i a . 3: The subsequent stages o f the t e c t o n i c development o f the Puna, along a section a t 23°40'S l a t . i n the Salar de Cauchari area (SCHWAB, 1985, m o d i f i e d ) . ( 1 ) Formation o f backarc basins d u r i n g the Cretaceous and Early T e r t i a r y (lower two sections), ( 2 ) development o f a backarc f o l d and t h r u s t b e l t (small i n s e r t e d s e c t i o n ) , and ( 3 ) s u p e r p o s i t i o n o f conjugate reverse f a u l t i n g i n the course o f arc t e c t o n i c s (upper s e c t i o n ) .

240

The

Cretaceous

and Palaeogene

sedimentary

development

as well

as t h e r e l a t e d

magmatism c h a r a c t e r i z e the region o f A l t i p l a n o , Puna and the Eastern C o r d i l l e r a as an e n s i a l i c backarc area w i t h respect t o an arc s i t u a t e d i n the Chilean P r e c o r d i l l e r a a t t h a t time. Hence, Late Eocene and Oligocene t e c t o n i c movements a f f e c t e d the backarc where a f o l d and t h r u s t b e l t was generated. I n the Puna an eastwardly f a c i n g f o l d o f Salta group

Sediments was reconstructed by SCHWAB (1985, f i g . 3 ) . The deformation

must have occurred between the Late Eocene and Early Miocene, as the f o l d i n g i s older than Miocene block movements. From northern Peru MEGARD (1984) described a f o l d and thrust

belt

Therefore

o f the same age and e x a c t l y

i t seems q u i t e

possible t h a t

causing these eastvergent s t r u c t u r e s

within

t h e same geotectonic

A-Subduction

i n a backarc

started

regime.

position.

i n Oligocene

times,

Neogene and Quarternary

intramontane Sedimentation and t e c t o n i c s were superimposed over the previous backarc. The

structural

reverse

fault

developed

Systems

and b u i l d

SCHWAB (1985, Altiplano, uplift

s t y l e i s d i f f e r e n t t o t h a t o f the Palaeogene t e c t o n i c s as conjugate

f i g . 3 ) . The t e c t o n i c

o f t h e region and graben

continental have

vergencies

t o t h e west

as well

hörst and graben

as t o t h e east are

structures

i s the result

o f horizontal

shortening

t h i c k e n i n g and (SCHWAB, 1985;

and a thermal root (FROIDEVAUX & ISACKS, 1984). The compressive structures

and t h e connection

arc i n a broad zone and cannot

caused

described by

s t r u c t u r e s o f the Neogene deformation i n the

Puna and Eastern C o r d i l l e r a are symmetrical. The c r u s t a l

ALLMENDINGER, 1986) hörst

with

up t h e compressive

t h e east-facing

backarc

with

volcanism

characterize

be r e l a t e d t o A-Subduction which

structures

during

t h e Oligocene.

the

might The

superposition o f these d i f f e r e n t t e c t o n i c s t y l e s shows eastward m i g r a t i o n o f the arc configuration. The A-Subduction o f the Subandean zone i s contemporaneous t o t h e s t r u c t u r e s o f the A l t i p l a n o , Puna and Eastern C o r d i l l e r a . I t s sole t h r u s t (ROEDER, 1986: "Main Andean Thrust")

dips beneath the Eastern

basement a t a major

Cordillera

where

i t i s supposed t o enter t h e

ramp. Whereas i n Argentina t h e f r o n t a l

Eastern C o r d i l l e r a i s

characterized by a series o f steep upthrusts, i n B o l i v i a the huge Sama a n t i c l i n e near T a r i j a can be considered a f r o n t a l frontal

ramp a n t i c l i n e . The high c r u s t a l

s t r u c t u r e i s r e f l e c t e d by p o s i t i v e i s o s t a t i c and residual

uplift of this

Bouguer

anomalies

(GÖTZE e t a l . , 1987). According t o the estimates o f ALLMENDINGER e t a l . (1983), the Eastern C o r d i l l e r a moved a t l e a s t 60 km t o the east w i t h respect t o t h e foreland i n the

course o f Subandean t h r u s t t e c t o n i c s . ROEDER (1986) estimated 100 km o f Neogene

t e c t o n i c t r a n s p o r t f o r the Andes o f Northern B o l i v i a . As during t h a t time the Eastern C o r d i l l e r a was compressed and shortened, i t was displaced as a whole w i t h respect t o the Chaco p i a i n o r the Subandean basement. The m a g n e t o t e l l u r i c measurements near T a r i j a by SCHWARZ e t a l . , (1986) ( f i g . 4) do not reveal

this

important t h r u s t

fault,

but f u r t h e r west,

near Tupiza,

very low

r e s i s t i v i t i e s were found i n a depth o f about 10 km and deeper, which may be r e l a t e d

241

to t h a t t h r u s t and/or the young volcanic manifestations o f the western p a r t of the Eastern C o r d i l l e r a . Towards the west, beneath the A l t i p l a n o , the depth of the layer of very low r e s i s t i v i t y i n the c r u s t descends t o 20 and 40 km. These anomalies may

be

i n t e r p r e t e d as magmatic impregnations or shear zones which p o s s i b l y separate a r i g i d upper c r u s t from a more d u c t i l e lower c r u s t . I t also i s possible t h a t the main shear zone o f A-subduction passes through t h i s l a y e r . This megathrust must have caused a considerable c r u s t a l t h i c k e n i n g i n a d d i t i o n t o the internal

thickening

of the c r u s t

of the

Eastern

Cordillera,

Altiplano

and

Puna

region. The residual g r a v i t y of the A l t i p l a n o and Puna i s also mainly characterized by NE-SW o r i e n t e d highs and lows which i n d i c a t e deeper sedimentary basins and of

Palaeozoic

rocks

(e.g. the Faja Eruptiva O r i e n t a l ) .

Strong

local

belts

gradients of

g r a v i t y mark the main f a u l t s or Systems of f a u l t s i n the p i c t u r e o f g r a v i t y . ßased on gravity

mesurements

GÖTZE,

(1986)

calculated

crustal

thicknesses

beneath t h a t region from about 50 km (E) t o 70 km (W). Thus, i t may

which

increase

be supposed t h a t

two c r u s t a l elements of normal thickness override each other (ROEDER, 1986).

WESTERN CORDILLERA

The Western C o r d i l l e r a represents the Miocene-Holocene v o l c a n i c arc which c o n s i s t s of rhyolitic

ignimbrites

and

andesitic

volcanoes.

While

i t s eastern

border

i s very

irregulär due t o volcanoes, volcanic i n t r u s i v e s and i g n i m b r i t e s extending (according to SALFITY, 1985) along r e a c t i v a t e d transverse f a u l t Systems i n t o the A l t i p l a n o

and

Eastern C o r d i l l e r a , i t s western border i s easier t o d e f i n e . I t shows t h a t , w i t h i n the segment considered here, the axis of the Western C o r d i l l e r a i s not a s t r a i g h t

N-S

s t r u c t u r e but t h a t the p o r t i o n north of 23°30' trends NNW,

and

only the part t o the south of 25°S trends The p a r t s t h a t do not run N-S at

a

low

angle.

Thus,

the middle p o r t i o n SSW

N-S.

i n t e r s e c t w i t h the neighbouring morphostructural u n i t s

from

23°30'S

towards

NNW,

the

Western

Cordillera

is

successively superposed on the northern prolongations of the Salar de Atacama and the Upper Loa V a l l e y , both elements o f the Preandean Depression, and, n o r t h of 21°S, the

Chilean

Precordillera.

The

southern

extensions

of the

Salar de

on

Atacama are

i n t e r s e c t e d by the middle p o r t i o n of the Western C o r d i l l e r a i n the segment. These contrasting determined

directions by

are

intracrustal

due

to

the

fact

that

the

stresses whereas the p o s i t i o n

tectonic

structures

are

o f the v o l c a n i c arc i s

c o n t r o l l e d by the subcrustal subduction o f the Nazca Plate beneath the c r i t i c a l

depth

of about 110 km (GILL, 1981). The v o l c a n i c arc o f the Western C o r d i l l e r a came i n t o being d u r i n g the Miocene east of an e x t i n c t

Latest Cretaceous

- Eocene arc s i t u a t e d

i n the ambit

o f the

Chilean

242

pTocopiUa *> • I Cao l mo» • i r> - * C H I L E V'Antofagasta 70"

68°

—

66' .•»»t»chn Villamontes Tupizg B O L I V I A >B

-

—

60 km

,/

V \ ARGENTINA • MT-Station 200 300hm 6266" 61"

250 k m

f i g . 4: E l e c t r i c r e s i s t i v i t y p r o f i l e through northern Chile and southern B o l i v i a based on m a g n e t o t e l l u r i c data ( R e s i s t i v i t y d i s t r i b u t i o n c a l c u l a t e d from 1D models, SCHWARZ e t a l . , 1986).

243

P r e c o r d i l l e r a and, hence, i n the former backarc area. Due t o p r i o r backarc t e c t o n i c s and subsequent erosion, the Substrate o f the Miocene-Holocene v o l c a n i c s may consist o f Palaeogene and Cretaceous rocks as well as o f Palaeozoic v o l c a n i c s , Sediments and intrusives. segment

Palaeozoic

along

rocks as a Substrate p r e v a i l

t h e Argentinian-Chilean border,

i n t h e southern

thus

showing

part o f t h e

stronger u p l i f t and

erosion p r i o r t o Neogene volcanic a c t i v i t y . The

effects

o f t h e Miocene-Holocene arc t e c t o n i c s

border zone o f the Western C o r d i l l e r a than w i t h i n

can b e t t e r it.

be observed

The growing

i n the

s t r u c t u r e s were

possibly buried beneath the extruding volcanic products whose s y n t e c t o n i c nature i s also revealed by i n t e r n a l unconformities (LAHSEN, 1982). I t has been suggested

that

the volcanoes formed along f r a c t u r e zones and t h a t block f a u l t i n g occurred w i t h i n the Western C o r d i l l e r a (LAHSEN, 1982), but e v i d e n t l y there are no important normal or even graben s t r u c t u r e s . Generally, there seems t o be no fundamental between

the tectonics

Altiplano

except

o f t h e Western

f o r the Special

Cordillera

mechanical

and those

conditions

faults

difference

o f t h e neighbouring

imposed

by t h e large

q u a n t i t i e s o f intruded and extruded volcanic m a t e r i a l . The minimum o f the Bouguer g r a v i t y f i e l d o f about -450 mGals was observed i n the area o f the Western C o r d i l l e r a gravity

field

also a t t a i n s

(GÖTZE, 1986,

GÖTZE e t a l . 1987). Although t h e regional

i t s minimum we l e a r n t

from p o t e n t i a l

field

Separation

techniques t h a t the negative values mentioned are p a r t l y caused by g r a v i t y sources o f the

points

t o t h e anomalies

of electrical

c o n d u c t i v i t y and t h e i r i n t e r p r e t a t i o n . The l e v e l

upper

crust

(down

t o 5 km).

This

o f low r e s i s t i v i t y

i n the western

p a r t o f the A l t i p l a n o shallows towards W and reaches minimum values o f less than 1 Ohm m below the western p a r t o f the Western C o r d i l l e r a a t a depth o f merely 8-10 km. This

extremely

high

conductivity

may be i n t e r p r e t e d

s o l i d i f i e d magmatic i n t r u s i o n s a t t h a t l e v e l first

alternative

by not o r not completely

o r by c i r c u l a t i n g thermal waters. The

seems t o be confirmed by r e f r a c t i o n

seismics along a l i n e

from

Chuquicamata i n t o the A l t i p l a n o and the Eastern C o r d i l l e r a , as WIGGER (1986) noted a strong a t t e n n u a t i o n o f seismic waves beneath the Western C o r d i l l e r a .

PREANDEAN DEPRESSION

In the northern p a r t o f Chile considered here, an important morphologic and t e c t o n i c depression between the Western C o r d i l l e r a and the Chilean P r e c o r d i l l e r a i s developed, most spectacular expression o f what i s the basin o f the Salar de Atacama. To the south i t i s succeeded by the Salar de Punta Negra, and s t i l l

f u r t h e r south endorheic

basins along the western border of the Western C o r d i l l e r a i n d i c a t e the persistence o f t h i s morphostructural element. To the north o f the Salar de Atacama the depression o f the upper Loa V a l l e y can be regarded as a s t r u c t u r e o f the same type.

244

As mentioned above, the Salar de Atacama region was a f f e c t e d d u r i n g the Late Eocene and the Oligocene

by strong compressive movements which caused f o l d i n g and f a u l t i n g

of the P u r i l a c t i s Group ( e s s e n t i a l l y Latest Cretaceous - Eocene according t o CHARRIER & REUTTER, i n prep.), a p a r t i a l equivalent t o the Salta Group i n Argentina (MARQUILAS & SALFITY, t h i s

i s s u e ) . During

t h e Miocene and Pliocene f u r t h e r compression also

a f f e c t e d t h e Sediments which had been accumulating depression

since

t h e Late

Oligocene.

w i t h great thicknesses

The s i m i l a r i t y

i n tectonic

i n the

and sedimentary

developments between the A l t i p l a n o and the Salar de Atacama depression suggests the l a t t e r was t e c t o n i c a l l y a part o f the A l t i p l a n o u n t i l

that

t h e i n s t a l l a t i o n o f the

Western C o r d i l l e r a oblique t o the t e c t o n i c g r a i n separated both areas i n the course of the Miocene. There are, however, also d i f f e r e n c e s . The area o f the present Preandean

Depression

was covered by Jurassic marine and Lower Cretaceous c o n t i n e n t a l Sediments forming the eastern p a r t s o f an e n s i a l i c Jurassic backarc basin. These Sediments were completely eroded p r i o r lavas

t o t h e Sedimentation

i n t h e upper

proximity

t o an

part

of this

Eocene

o f the P u r i l a c t i s group

(and o l d e r )

Group. Furthermore,

andesitic

(CHARRIER & REUTTER, i n prep.) volcanic

arc s i t u a t e d

indicate

i n t h e Chilean

Precordillera. The Preandean Depression came i n t o being contemporaneously t o the Western C o r d i l l e r a . The Depression

only roughly followed p r e e x i s t i n g t e c t o n i c s t r u c t u r e s . Therefore,

i f

the s t r a i g h t north-south t r e n d i n g axis o f the P r e c o r d i l l e r a i s taken as a reference l i n e , the Salar de Punta Negra depression developed f a r t h e r west than t h e Salar de Atacama, so t h a t

i t would c u t i n t o t h e P r e c o r d i l l e r a .

Depression

lies

individual

t e c t o n i c depressions

other

by ranges.

farther

west

structurally

than

Furthermore,

t h e Upper Loa

t h e Salar de Atacama. A l l these

are not d i r e c t l y connected but separated

I t may be supposed

that

these

depressions

from each

developed

due t o

compression between t h e i s o s t a t i c a l l y u p r i s i n g blocks o f the P r e c o r d i l l e r a and the Western C o r d i l l e r a w i t h i n

a crustal

p o r t i o n weakened by magmatic processes.

This

assumption i s supported by the i n t e r p r e t a t i o n o f m a g n e t o t e l l u r i c measurements i n the upper Loa Valley by SCHWARZ e t a l . (1986: S t a t i o n ARL) who found r e s i s i t i v i t i e s o f about 2 Ohm m down t o 15 km and o f less than

1 Ohm m below 20 km ( f i g . 4 ) . The

absorption o f seismic Signals from Chuquicamata towards the east (WIGGER, 1986) also p o i n t s i n t h e same d i r e c t i o n . Residual

g r a v i t y i n t h e Preandean Depression

zone i s

c o n t r o l l e d by an enormous g r a v i t y high o f 60-100 mGals. This anomaly Covers the area between Calama i n the NW and the Argentinan Puna crossing the Salar de Atacama and the Western C o r d i l l e r a (GÖTZE e t a l . , t h i s i s s u e ) . The extension, width and s t r i k i n g of

this

hitherto

unknown

anomaly corresponds

perfectly

Occidental" proposed by PALMA e t a l . (1986) The l o c a l

with

t h e "Faja

Eruptiva

negative anomalies caused by

s a l t deposits i n t h e Salar de Atacama are completely masked by t h i s g r a v i t y high.

245

Other l o c a l

p o s i t i v e anomalies

here are r e l a t e d t o deep seated i n t r u s i o n s o f basic

magmas. These authors i n d i c a t e a c r u s t a l

thickness o f about

70 km, and almost the

same thickness was i n t e r p r e t e d by WIGGER (1986) from r e f r a c t i o n seismic data f o r the P r e c o r d i l l e r a t o the south o f Chuquicamata.

CHILEAN PRECORDILLERA

The

mountain

ranges

t o t h e east

o f t h e Chilean

Longitudinal

Valley

( S i e r r a de

Domeyko, S i e r r a de Moreno) r i s e t o heights o f about 4.000 m and are morphologically clearly

separated

structural

from the Western C o r d i l l e r a by the Preandean Depression.

From a

p o i n t o f view, the P r e c o r d i l l e r a presents a good example o f what may be

c a l l e d 'arc t e c t o n i c s ' . The pre-Jurassic development o f the P r e c o r d i l l e r a i s s i m i l a r t o t h a t o f the Preandean Depression

and t h e Western C o r d i l l e r a .

Palaeozoic

Sediments and p l u t o n i c rocks o f

d i f f e r e n t ages are mostly o v e r l a i n by Late Carboniferous t o T r i a s s i c volcanics and Sediments.

During

t h e Late

Triassic,

t h e Lias o r l o c a l l y

t h e Dogger,

a marine

transgression occurred which led t o the d e p o s i t i o n o f t h i c k carbonatic sequences. The eastward

extension o f t h e sea i s not known, as i n t h e Preandean Depression and

f a r t h e r east, erosion preceded the d e p o s i t i o n o f Cretaceous S e d i m e n t s ; t o t h e west the basin was l i m i t e d by the Jurassic volcanic arc, which was then located i n the Chilean Coastal

Range (v.HILLEBRANDT e t a l . , 1986).

The Jurassic

palaeogeographic

c o n f i g u r a t i o n i s g e n e r a l l y i n t e r p r e t e d as an e n s i a l i c backarc basin. At the approximate

time o f the Jurassic-Cretaceous boundary, marine

gradually

by t r a n s i t i o n a l

thick.

replaced

They are conformably

and c o n t i n e n t a l

or unconformably

clastics,

overlain

Sediments were

locally

several km

by v o l c a n i c formations o f

intermediate and acid composition. These volcanics i n d i c a t e t h a t a new magmatic arc was

b u i l t up w i t h i n t h e former Jurassic backarc

basin a f t e r the e x t i n c t i o n o f the

corresponding magmatic arc i n the present Coastal Range during the Early Cretaceous. From t h e southwestern

p a r t o f t h e S i e r r a de Moreno ROGERS (1985) has dated "Mid"

Cretaceous v o l c a n i c rocks (Rb/Sr-isochron: 104,7±19 Ma). P l u t o n i c rocks o f about the same age are reported by MARINOVIC & LAHSEN (1984) from t h e southern p a r t o f the S i e r r a de Moreno (K/Ar i n b i o t i t e : 103+4 Ma). The "Mid" Cretaceous volcanics u n d e r l i e w i t h angular unconformity (MUfiOZ, 1986) a younger, only s l i g h t l y

warped volcanic

sequence (Chile-Alemania Fm., CHONG (1973), o r e q u i v a l e n t s ) o f Latest Cretaceous t o Eocene age (HERVE e t a l . , 1985).

In t h e southern

Palaeogene formation has a great areal without

intervening

continental

clastic

Cretaceous rocks o f t h e P r e c o r d i l l e r a

part

this

extension and o v e r l i e s , Sediments,

latter

t h e folded

and t h e Longitudinal

essentially

unconformably and Palaeozoic t o

V a l l e y . As Palaeogene

246 volcanics can also be found i n the Preandean Depression too, the Palaeogene magmatic arc may

have had a s l i g h t l y more w e s t e r l y p o s i t i o n than the i l l

defined Cretaceous

arc, i . e. both arc Systems overlap each other. The

Precordillera

i s s t r u c t u r a l l y c h a r a c t e r i z e d as a b e l t o f strong compressional

t e c t o n i c s w i t h intense f o l d i n g and f a u l t i n g . However, i t i s not a f o l d and

thrust

b e l t o f the f o r e l a n d type as can be seen from the f a c t t h a t Palaezoic sedimentary p l u t o n i c rocks and

locally

and

also Precambrian metamorphic rocks are involved i n the

f o l d s t r u c t u r e s . These rocks appear i n the cores o f two or t h r e e a n t i c l i n e s whose limbs c o n s i s t of Mesozoic sequences. This implies t h a t broad

(10-15 km)

the s t r u c t u r e s

are

rather

although, normally, they are s t r o n g l y compressed t o such a degree

t h a t the limbs are steep or p a r t l y overturned and that the core i s upthrusted w i t h respect t o limbs. In these a n t i c l i n e s vergencies to the w and t o the E are developed, i t seems, however, t h a t the westward vergencies are s l i g h t l y more widespread. The

flanks

and

e s p e c i a l l y the cores

d a c i t i c Stocks (1985)

which can

therefore

are

frequently

i n p a r t be considered

proposed

a

model

i n which

intruded

by

granodioritic

as synkinematic. the

deformation

or

CHONG & REUTTER was

triggered

by

i n t r u s i o n s that d e s t a b i l i z e d the stressed c r u s t and allowed shear movements i n the upper r i g i d

level

o f the c r u s t w i t h respect t o the lower weak and

viscous

( f i g . 5 ) . The wavelength o f the f o l d s would imply a detachment at an o r i g i n a l of

about

8-10 km.

This

value

corresponds

to

the

conductive l a y e r beneath the Preandean Depression.

depth

I t may

of

the

level depth

present

highly

thus be concluded

that a

s i m i l a r anomaly e x i s t e d i n the P r e c o r d i l l e r a at the end o f the Eocene. Another kind o f deformation can o c c a s i o n a l l y be observed i n the steep limbs of the PrecordiHeran

anticlines.

The

Mesozoic s t r a t a

are folded

around almost

vertical

axes. The Z-array of the f o l d s suggests t h a t they were formed by d e x t r a l t r a n s c u r r e n t faults

along

the

strike

of

the

limbs. The

sense

of

shear

along

these

faults

corresponds t o an oblique northeastward subduction of the Farallon Plate during the Palaeogene (WHITMAN et a l . , 1983, somewhat o l d e r than wrenching acted The

cannot

the

, f i g . 6 ) . In the cases mentioned,

transcurrent

be e a s i l y

movement, but,

as

i n less

folding is

inclined

rocks

recognized, i t i s supposed t h a t compression and

shear

contemporaneously.

exact age o f the deformation i s not known. In some places i t i s evident t h a t

there were two t e c t o n i c events, p o s s i b l y due t o t e c t o n i c s o f the Mid Cretaceous and the l a t e s t Cretaceous - Eocene arc. Thus, i n the S i e r r a de Argomedo (southern part of the

segment)

unconformably

the

Palaeogene

volcanics o f

the

Chile-Alemania

Formation,

which

o v e r l i e Jurassi c Sediments i n the western f l a n k o f a Precordi Heran

a n t i c l i n e , were upfolded i n a steep position, t h a t i s t o say t h a t here structures of Cretaceous

(probably Late Cretaceous)

age

were r e a c t i v a t e d

d u r i n g the Palaeogene

247

X -i 'x xPQleogene x*> KßathoLithic Intrusion*" >

X

® L

L

:

x

L

L

X

x J

L

)

•Je"-*' 1

L

L

L

L

L

L

L

L

L

J urassi c L

L

L

L

L

L L L L L L L Late Pal.-Trias volcanics , L

X

: x x x x >

X

Marine L

L

x

L

L L

L

L

L

L

L

LL L L LL L L L L L L

I I I I I I I I I I I I I II

Premesozoic basement

I I I I I I I I I I I I I

f i g . 5: Hypothetical model showing the development o f a r c - r e l a t e d compressional s t r u c t u r e s i n t h e upper c r u s t o f the P r e c o r d i l l e r a : Stage A: Plate convergence produces t a n g e n t i a l stress i n t h e c r u s t . Due t o the lack o f horizons s u i t a b l e f o r detachement, no deformation occurs. Stage B: Acid melts d e s t a b i l i z e t h e l e v e l o f i n t r u s i o n and enable f o l d i n g t o occur i n the r i g i d upper c r u s t a l l e v e l . Stage C: Increasing shortening steepens t h e f l a n k s o f the basement a n t i c l i n e g i v i n g way t o f u r t h e r f o l d i n g i n t h e sedimentary cover on the limbs of t h e a n t i c l i n e . D i a p i r i t i c r i s e o f g r a n i t i c magma i s thus possible. Stage D: Further shortening r e s u l t s i n upthrusts on t h e f l a n k s which, i n turn, enhance t h e r i s i n g o f t h e core. Further shortening occurs i n t h e synclines on the f l a n k s leading t o t h e formation o f Special f o l d s and cleavage.

248

(probably

Late

Eocene and

Choja), DAMM e t a1.(1986) syntectonic, w i t h

Oligocene).

In the northern parts

o f the

segment

(Q.

dated a m o n z o d i o r i t i c i n s t r u s i o n , which can be considered

43,7±3,8 Ma.

This

age

would confirm an

Early or Middle

Eocene

t e c t o n i c event, w h i l e no e f f e c t s of a Late Cretaceous event can be observed i n t h a t region.

In

other

places,

transitional

areas

between the Longitudinal

compressional

tectonics

especially

only

during

in the

the

southern

V a l l e y and

Cretaceous.

parts

of

the

segment,

the P r e c o r d i l l e r a The

Precordillera

a f f e c t e d by Neogene mainly v e r t i c a l movements as i s demonstrated

suffered was

also

l o c a l l y by f a u l t i n g

and t i l t i n g o f the Miocene Pampa Gravels i n the v i c i n i t y o f these ranges. The P r e c o r d i l l e r a i s s i t u a t e d i n a p a r t o f the present f o r e a r c area, which i s now not very a c t i v e t e c t o n i c a l l y . volcanic

arc

during

I t s magmatic e v o l u t i o n

the

Palaeogene

and

shows t h a t

the

i t was

contemporaneous

a Continental

compressive

and

t r a n s c u r r e n t (= t r a n s p r e s s i v e ) t e c t o n i c s have t o be c l a s s i f i e d as subduction l i n k e d arc t e c t o n i c s .

The

structures

of the Palaeogene arc are p a r t l y

those of the Cretaceous arc which was

superimposed over

probably s i t u a t e d r e l a t i v e l y

nearby,

t o the

west o f the P r e c o r d i l l e r a , and both arcs came i n t o being i n the backarc area o f the Jurassic - Early Cretaceous arc System. The

interpretation

of

gravity

thickness values o f about

data

60 km.

according

to

In the residual

GÖTZE field

(1986)

leads

Chilean P r e c o r d i l l e r a i s characterized by a g r a v i t y minimum which may accumulation of r e l a t i v e l y l i g h t acid material

to

crustal

of Bouguer anomalies,

the

be due t o the

i n the magmatic arc supported by arc

t e c t o n i c s . By means o f r e f r a c t i o n seismic data along a p r o f i l e from Chuquicamata t o the

south WIGGER (1986), obtained a c r u s t a l thickness of 70 km. He detected a high

velocity

level

velocities crustal

of

might

7.3 km/s

at

correspond

thickness

should

a depth

of

only

35 km

and

suggested

these

t o a Jurassic palaeo-Moho. Accordingly, the

have

been

achieved

by

magmatic

high

present

u n d e r p l a t i n g of

basic

material which thus formed a new and t h i c k Tower c r u s t .

LONGITUDINAL VALLEY

There

are

important

differences

in

structures

and

palaeogeological

development

between the northern and the southern p a r t of the Andean segment d e a l t w i t h These

differences

northern

part,

morphologically

the

are

particularly

Pampa del

evident

Tamarugal

separating the Coastal

debris o f which accumulate

i n the

represents

Range from

Longitudinal a young

the Chilean

Valley.

tectonic

here.

In the

depression

Precordillera,

the

i n t h a t basin. Tensional t e c t o n i c s do not seem t o play a

r o l e - i n the formation o f t h a t depression; t i l t i n g

of the now

rigid

block o f the

P r e c o r d i l l e r a towards the west i s more l i k e l y t o be a kinematic motive. The Miocene and younger Sediments are not very t h i c k ; elevations of Mesozoic and Palaeozoic rocks

249 emerging from t h a t peneplain show t h a t t h i s morphostructural u n i t was

subject t o

strong t e c t o n i c s and subsequent erosion p r i o r t o the Miocene. On comparsion w i t h the southern p a r t , the u n i t can probably be said t o be of Cretaceous age. To the east o f Antofagasta, the mountains of the Coastal Range merge o r o g r a p h i c a l l y i n t o the P r e c o r d i l l e r a , i . e. a morphologically d i s t i n c t L o n g i t u d i n a l l y V a l l e y does not

exist

there.

The

Situation

changes

south

P r e c o r d i l l e r a i s separated from the Coastal occupied

of

24°30'

S,

by the Palaeogene v o l c a n i c Chile-Alemania

folded

and

faulted

transitional

area

the

Chilean

Formation (CHONG, 1973). These

basic and acid lavas, ignimbrites, tuffs and volcanic Stocks the

where

Range by a 50 km wide h i l l y peneplain

between

the

unconformably overlie

bordering morphostrustural

u n i t s , but the volcanic formation i t s e l f s u f f e r e d almost no deformation except i n the ambit o f the P r e c o r d i l l e r a . The s t r u c t u r e s of the substratum, which can be observed i n the mountaineous region east o f Antofagasta, appear t o be s i m i l a r t o those of the P r e c o r d i l l e r a , since a n t i c l i n e s w i t h cores o f Palaeozoic rocks e x i s t . Therefore the age o f the t e c t o n i c s i s o l d e r than the l a t e s t Cretaceous -

possibly

Mid-Cretacebus.

S i m i l a r l y t o the P r e c o r d i l l e r a , compression w i t h i n a d e s t a b i l i z e d c r u s t may have been the

reason for folding which, therefore, might have occurred i n the ambit of a Mid

Cretaceous magmatic arc whose l o c a t i o n , however, i s not well known. According t o the i n t e r p r e t a t i o n o f seismics (WIGGER, 1986) and

g r a v i t y data (GÖTZE,

1986), the c r u s t o f the Longitudinal Valley has a thickness o f about 50 km. Neither the the

Bouguer g r a v i t y f i e l d nor the residual g r a v i t y p o i n t t o an abnormal thickness of sedimentary

cover.

The

Longitudinal

Valley

i s even

controlled

by

positive

residual g r a v i t y anomalies which cover both the Central V a l l e y and the Coastal Range w i t h values up t o +80 mGals. Zones of low e l e c t r i c r e s i s t i v i t y

could not be detected

in

The

the m a g n e t o t e l l u r i c measurements

intense

young

tectonics

of

that

(SCHWARZ et a l . , 1986). present

forearc

region

spaceous and

coincide

with

not these

geophysical data.

COASTAL RANGE

Düring the Jurassic and Early Cretaceous the magmatic arc was s i t u a t e d i n the Coastal Range (BUCHELT & TELLEZ, t h i s i s s u e ) . I t c o n s i s t s o f a n d e s i t i c lavas, l o c a l l y more than 10 km

thick

and

o f large

plutons of mainly d i o r i t i c

composition. The

thickness of the volcanics as well as t h e i r composition showing t h o l e i i t i c i n the e a r l y stages ("early basics", PICHOWIAK e t a l . , 1988)

great

affinities

i n d i c a t e a geotectonic

s e t t i n g d i f f e r e n t t o t h a t o f the l a t e r arc-systems. The v o l c a n i c s o v e r l i e some Early Jurassic

marine

Sediments,

Triassic

and

Upper

Palaeozoic

deposits,

Palaeozoic

g r a n i t o i d s as well as rocks o f the metamorphic basement (probably Cambrian, DIAZ et al.,

1985,

DAMM et

a l . , 1986).

Marine

intercalations

i n the Jurassic volcanics

250 i n d i c a t e a d e p o s i t i o n a l environment more or less at sea l e v e l . The e x t r u s i o n o f these volcanics was thus accompanied by a considerable c r u s t a l subsidence and the i n t r u s i o n of huge d i o r i t i c b a t h o l i t e s as e a r l y as Jurassic times. A second p l u t o n i c pulse took place i n the Early Cretaceous,

when smaller plutons o f t o n a l i t i c

composition i n t r u d e d along N-S-trending Steep t o n e a r l y v e r t i c a l

to granodioritic

faults.

f a u l t s are the most c h a r a c t e r i s t i c t e c t o n i c f e a t u r e of the

Coastal Range. Some o f them are young and seem t o be s t i l l

a c t i v e . The most important

System of f a u l t s c o n s t i t u t e s the N-S

t r e n d i n g Atacama Fault Zone (AFZ) which can

traced

(19°S)

over

displacements occurred

1000 km along

from these

Iquique faults

along the El Way

can

be

to

La

very

Serena

Fault (RÖSSLING e t a l . 1986)

blocks between the main l o n g i t u d i n a l

faults

( 30°S).

important, e.

are s t r o n g l y

g.

after

a

The

be

vertical

12.5 km

throw

the Barremian.

The

i n c l i n e d i n some places,

thus c o n t r a s t i n g w i t h the i n c l i n a t i o n of the neighbouring blocks

(SCHEUBER e t a l .

1986). The

Post-Neocomian v e r t i c a l

displacements

along

the

faults

o f the

Coastal

Range

r e s u l t e d i n the phenomenon t h a t rocks t h a t formed i n a deep c r u s t a l l e v e l are exposed over

a large

(Bolfin

area.

The

rocks

Complex, RÖSSLING,

(1987)

showed t h i s

period

of wrenching

show features o f metasomatism and

1987)

as

shear deformation along

the AFZ.

well

as

ductile

partial

melting

shear deformation. SCHEUBER

belonged t o a Jurassic t o Early Cretaceous The

deformation

was

closely

related

t o the

i n t r u s i o n o f p l u t o n i c bodies o f t h a t time. P e t r o l o g i c a l data i n d i c a t e medium t o high grade

c o n d i t i o n s f o r Jurassic

shear

zones

and

low

grade

c o n d i t i o n s f o r Early

Cretaceous mylonites w i t h metamorphic pressures intermediate between low pressure and medium pressure

series

(35-70°C/km).

The

decreasing

metamorphic

grade

indicates

c r u s t a l u p l i f t i n the Coastal Range at l e a s t since the Early Cretaceous. The sense o f shear i n the d u c t i l e shear zones i s u n i f o r m l y s i n i s t r a l

and

corresponds

t o r e c o n s t r u c t i o n s of p l a t e c o n f i g u r a t i o n s and the d i r e c t i o n s of movement f o r the SEP a c i f i c (LARSON & PITMAN I I I ,

1972, ENGEBRETSON e t a l . , 1985, f i g . 6 ) . Because o f the

close r e l a t i o n between the arc magmatism o f the Coastal Range and the wrenching the AFZ,

along

t h i s f a u l t zone can be viewed as an a r c - r e l a t e d s t r u c t u r e or as a "trench-

l i n k e d s t r i k e - s l i p f a u l t " sensu W00DC0CK (1986). It

can

be presumed t h a t the Jurassic t o Early Cretaceous f a u l t s were r e a c t i v a t e d

l a t e r i n the f o r e a r c s t r e s s regime when the subsequent magmatic arcs had developed f a r t h e r t o the east. The

Neogene t o recent a c t i v i t y of the f a u l t s o f the Coastal

Range does not reveal any s t r i k e - s l i p movements and r e f l e c t s the v e r t i c a l of the s t r u c t u r a l

high at the outer r i m o f the f o r e a r c region. The

cause the huge scarp

along the coast

segment) mark the t r a n s i t i o n

(about 2000 m i n the southern

tectonics

faults,

which

p a r t o f the

t o the inner trench slope which as a consequence o f

f i g . 6: Reconstructions o f p l a t e c o n f i g u r a t i o n s and d i r e c t i o n s o f spreading. L e f t : Early Cretaceous, r i g h t : Palaeogene (Modified a f t e r LARSON & PITMAN I I I , 1972 ( l e f t ) and WHITMAN e t a l . , 1983 ( r i g h t ) ) .

252 t e c t o n i c erosion has i t s own Special s t r u c t u r e s (BOURGOIS e t a l . , 1988). The Pliocene t o recent f a u l t Systems o f the M e j i l l o n e s Peninsula may

be considered as an example

of these slope t e c t o n i c s exposed on the c o n t i n e n t . The

crustal

structure

of the Coastal

Range was

r e f r a c t i o n measurements c a r r i e d out i n 1987 running along the coast and velocity distribution

investigated

by

detailed

seismic

(WIGGER, pers. comm.). Reversed p r o f i l e s

i n an inland d i r e c t i o n also give an impression of the

of the area under study. Well expressed

first

a r r i v a l s show

very c l e a r l y t h a t i n the upper c r u s t o f the Coastal Range at an average i n a depth of between 5 and

15 km s u r p r i s i n g l y h i g h - v e l o c i t y material

km/s.

The

e x i s t s w i t h values between

6.5

and 6.8

rocks must be i n t e r p r e t e d as u p l i f t e d deeper c r u s t a l

The

r e l a t i v e g r a v i t y high observed by GÖTZE (1986) along the Coastal

levels.

Range i s i n

agreement w i t h t h i s seismic r e s u l t . The geophysical anomalies c o i n c i d e p e r f e c t l y w i t h the

area

i n which

the B o l f i n

complex mentioned above, a probable element o f the

middle or lower c r u s t (RÖSSLING, 1987) i s s i t u a t e d . This S i t u a t i o n r a i s e s the question o f the mechanism t h a t caused the u p l i f t

of the

Chilean Coastal Range w i t h respect t o the c e n t r a l u n i t s o f the Andes. I f the B o l f i n complex represents a c r u s t a l crust

i n Jurassic times,

element which

and

which

formed a p a r t of the lower ( o r middle)

i s now

situated

i n the

upper

part

of

the

C o n t i n e n t a l c r u s t o f normal t h i c k n e s s , u n d e r p l a t i n g must have occurred. F i s s i o n - t r a c k dating

of

apatites

i n a Jurassic amphibolite S

of Antofagasta

provided

an

age

estimate o f 118 + 13 Ma (ANDRIESSEN, pers. comm.). This age designates the time when the

temperature of the rock f e i l

below 100°C. Thus, a great p a r t o f the u p l i f t

and

consequently of the u n d e r p l a t i n g probably took place i n Early Cretaceous times and so t h i s phenomenon may also be r e l a t e d t o the arc t e c t o n i c s .

CONCLUSIONS

In

a

converging

plate

System,

the

greatest

amount

of

crustal

shortening i s

accommodated i n the subduction zone, i . e . at the i n t e r f a c e between the upper p l a t e and

the downgoing

plate,

and

i n the subduction

between the trench and the s t r u c t u r a l

complex

forming the

inner slope

high (DICKINSON & SEELY, 1979). At the a c t i v e

Continental margin of the Central Andes, these s t r u c t u r e s are not exposed above sea l e v e l , perhaps w i t h the exception of some blocks near the coast Peninsula).

Due

to

the

consequence o f t e c t o n i c

displacement

of

the

arc

system

erosion of the Continental border

towards

(e.g. M e j i l l o n e s the

east

(RUTLAND, 1971;

as

a

HILDE,

1983) the subduction complexes o f the e a r l y stages o f the Andean Cycle cannot have been preserved.

253 The

v i s i b l e s t r u c t u r e s of the Andes owe

still

t h e i r existence

t o Stresses which were,

are, t r a n s m i t t e d through the subduction zone i n t o the Continental

and

c r u s t of the

upper p l a t e . The t e c t o n i c m o b i l i z a t i o n of the c r u s t i s supposed t o be achieved by i t s decoupling

from the

underlying

mantle wedge by magmatic processes. The

s e t t i n g s o f the arc c o n f i g u r a t i o n s t h a t developed during t h a t t h e i r formation was

influenced not only by the i n h e r i t e d c r u s t a l c o n d i t i o n s but

also by the v a r i a b l e conditions of p l a t e motion, e.g. of

plate

motion

structural

relative

to

the

trench

development shows t h a t intense

subduction complex, but transpressive shortening

different

the Andean Cycle suggest

also

the

axis,

convergence r a t e , obliqueness

subduction

tectonic a c t i v i t y

area of the

dip not

magmatic arc,

and

others.

The

only

affected

the

where compressive

or

s t r u c t u r e s generated. During some stages i n the backarc area c r u s t a l

was

also accomodated i n f o l d and

t h r u s t b e l t s . The

backarc outside

f o l d and t h r u s t b e l t and e s p e c i a l l y the forearc between the s t r u c t u r a l

high and

the the

magmatic arc were r e l a t i v e l y stable areas or subject only t o slow v e r t i c a l movements. Amounts and v e l o c i t i e s of t e c t o n i c t r a n s p o r t i n the mobilized part of the Continental crust o f the upper p l a t e are

important, although

c e r t a i n l y much less than those i n

the subduction zone and complex.

In the backarc area, according may

t o the present S i t u a t i o n , a huge c r u s t a l t h r u s t System

be developed caused by underthrusting of the f o r e l a n d under the mobilized

o f the c e n t r a l parts of the orogen. This A-subduction (BALLY, 1975)

crust

confers a c e r t a i n

b i l a t e r a l symmetry t o the orogenic System. According t o i t s nature as a f l a t dipping shear zone, only d i p s l i p movement i s possible, hence strong c r u s t a l shortening

can

take place here. The magmatic arc also shows e f f e c t s o f c r u s t a l shortening. I t s c r u s t i s d e s t a b l i l i z e d by i n t r u s i o n s , by which the upper s t i l l

r i g i d part o f the c r u s t i s enabled t o react

by f o l d i n g and steeply dipping conjugate t h r u s t s while the lower p a r t may

be deformed

by

necessarily

more or

less

viscous

flow.

Important

low

angle

thrusts

are

not

developed and a t h i c k e n i n g of the c r u s t i s achieved by an i n t e r n a l s o r t of pure shear deformation

and

not

by

crustal

underthrusting.

The

weak c r u s t

and

the

tectonic

s t r u c t u r e s of the magmatic arc also allow the accommodation of stresses p a r a l l e l t o the

trench

Jurassic

and

axis

resulting

from

oblique

subduction

(W00DC0CK,

1986).

As

fossil

Palaeogene s t r u c t u r e s show (SCHEUBER, 1987), the magmatic arc can

a f f e c t e d by l o n g i t u d i n a l

almost v e r t i c a l l y dipping s t r i k e s l i p f a u l t s and

s t r u c t u r e s p e r t a i n i n g t o t h i s type of

be

secondary

deformation.

In the segment under c o n s i d e r a t i o n , f o u r d i f f e r e n t stages of formation o f Continental arc Systems, one developing

a f t e r the other, can be recognized f o r the time from the

Jurassic t o the present ( f i g . 7 ) . As f a r as the magmatic arc i s concerned, each stage involved

the

deformation;

formation i t was

of

a

volcanic

chain,

intrusion

followed by a period of t e c t o n i c and

of

plutonic

bodies

and

magmatic quiescence w i t h

254

• A .A • A • A • A !

I + ++ + k + + + + I ± +± +

V 7

Ii /// 8

9

— 10

f i g . 7: Schematic presentation o f the Central Andes s t r u c t u r a l e v o l u t i o n since the Jurassic (Andean Cycle). 1. T r i a s s i c volcanics and c o n t i n e n t a l deposits, 2. arc magmatism, 3. backarc magmatism, 4. marine deposits (backarc), 5. c o n t i n e n t a l backarc Sedimentation, 6. intramontane deposits, 7. compressional arc t e c t o n i c s , 8. wrenching, 9. f o l d and t h r u s t b e l t t e c t o n i c s , 10. u n c o n f i r m i t y .

255 u p l i f t and

erosion before, once again, a new

volcanic chain i n a new

magmato-tectonic stage

S i t e , t o the east o f the preceding

started with a

chain. I f the volcanic

a c t i v i t y i s regarded i n i s o l a t i o n , these stages are (1) Jurassic, (2) Mid-Cretaceous, (3)

Latest Cretaceous - Eocene, and (4) Neogene-Quarternary (see COIRA et a l . , 1982).

Not only the s i t e s but also the c o n f i g u r a t i o n s of these

arc Systems as well as the

accompanying deformations d i f f e r e d from each other (COIRA et a l . , 1982 also i n d i c a t e d differences

of the magmatic products). Nevertheless,

there seems t o be

a regulär

trend o f e v o l u t i o n i n the arc Systems and the c o n t i n e n t a l c r u s t where they developed from a marine environment t o a c o n t i n e n t a l environment and, f i n a l l y , t o high plateau conditions. The

Jurassic magmatic arc, the f i r s t

of the Andean Cycle,

was

installed within

subsiding e n s i a l i c marine basin i n the area o f the present Coastal Range. I t may

a

have

d i v i d e d the basin i n t o a marine forearc region and a l i k e w i s e marine backarc basin. These marine c o n d i t i o n s may flattening

under the load

have been a consequence of some c r u s t a l of the

f o r e a r c , but i f the subduction

volcanics. Nothing

spreading

i s known about the

and

Jurassic

System t h a t gave r i s e t o the Andean Cycle came i n t o

being a f t e r a long period (Permian and T r i a s s i c ?) without p l a t e convergence, i t i s possible t h a t oceanic c r u s t was subduction

adhered by the newly formed f o r e a r c . A backarc

A-

i n the e n s i a l i c backarc basin cannot be i n f e r e d from the s t r u c t u r e s of the

exposed marine Jurassic Sediments. On the c o n t r a r y , some volcanic extrusions a t t e s t local

tensional regimes. However, the basin was

crustal

uplift,

the

connection

t e c t o n i c s cannot a p r i o r i

of

which

be excluded.

l i m i t e d t o the east by

with

possible

Jurassic

important

compressional

Intense t e c t o n i c a c t i v i t y took place i n the

magmatic arc, where, as a consequence of oblique subduction 1972), transpression caused

a zone of

left

lateral

strike

(LARSON & PITMAN I I I , slip

motion (SCHEUBER,

1987) and compressional warping and block t i l t i n g . As r a d i o m e t r i c age determinations

i n the considered

segment are scarce, only

little

is known about a Cretaceous magmatic arc t h a t i s supposed t o have developed t o the east o f the Jurassic arc. Some c a l c - a l k a l i n e volcanic formations and p l u t o n i c rocks of

the S i e r r a de Moreno have been dated as

"Mid"

Cretaceous and

may

t h e r e f o r e be

a t t r i b u t e d t o an arc of t h a t time. There i s no evidence of a Cretaceous A-subduction in

the

backarc, but pre-Maastrichtian u p l i f t ,

leading t o the erosion

of

Jurassic

Sediments i n the area o f the present Preandean Depression and the Western C o r d i l l e r a may

be seen i n t h i s context.

Starting

i n the

latest

Cretaceous

and

during

the

Paleocene

important magmatic arc developed i n the area of the present (Cordillera

Domeyko,

Cretaceous arc. Cretaceous and

Due

Sierra t o the

de

Moreno),

m i g r a t i n g arc

somewhat

to

the

and

Eocene,

a

new

Chilean P r e c o r d i l l e r a east

t e c t o n i c s , i n the

of

western

the

supposed

part

Palaeogene lavas o v e r l i e folded Jurassic backarc Sediments and

latest rocks

256 belonging t o t h e supposed Cretaceous arc. I n t h e eastern part o f t h e new arc, the volcanics o v e r l i e o r i n t e r f i n g e r w i t h Palaeogene c o n t i n e n t a l backarc Sediments. South o f 24°S, t h e volcanics o f t h i s age extend i n t o the L o n g i t u d i n a l V a l l e y and even i n t o the Coastal

Range, which formed t h e f o r e a r c o f t h i s arc System. Consequently these

volcanics were not a f f e c t e d by the well developed arc t e c t o n i c s o f t h e P r e c o r d i l l e r a . Oblique

subduction

i n a northeastern d i r e c t i o n

( c f . WHITMAN e t a k , 1983, f i g . 6)

during the Palaeogene i s revealed by f o l d s w i t h v e r t i c a l due

t o N-S

directed

Maastrichtian throughout

marine

right

lateral

Sediments

t h e Palaeogene,

slip

i n t h e backarc

lowland

axial

displacements area

show

conditions prevailed.

plunge and Z-arrays

i n that that

magmatic a r c . there,

The mostly

probably

continental

Sediments o f the backarc were i n t e n s e l y folded during t h e Late Eocene and Oligocene (Incaic

Phase)

i n t h e region comprising

Eastern C o r d i l l e r a . thrust

belt,

t h e present

These t e c t o n i c s c e r t a i n l y

similar

t o that

described

Preandean Depression

t o the

l e d t o t h e formation o f a f o l d and

by MEGARD (1984)

from

t h e northwestern

A l t i p l a n o , although there i s no proof o f a s i g n i f i c a n t A - s u b d u c t i o n System ( f i g . 3 ) . The

present

arc regime which has taken over

since t h e Early Miocene upon folded

Palaeogene backarc rocks developed several new f e a t u r e s .

I n t h e segment studied,

volcanism extends

from the V i r t u a l magmatic arc o f the Western C o r d i l l e r a f o r more

than

t h e backarc.

200 km i n t o

There,

an important and, w i t h

shortening, e f f e c t i v e A-Subduction i s a c t i v e and, f i n a l l y ,

respect

t o crustal

the Andes are much more

upwarped than during t h e previous arc stages, which i s c e r t a i n l y a consequence o f an enormous

crustal

thickening.

Also

t h e area

o f arc t e c t o n i c s ,

c h a r a c t e r i z e d by

conjugate inverse f a u l t Systems (SCHWAB, 1985), i s probably broader than t h a t o f the preceding arcs, as i t comprises the young s t r u c t u r e s o f t h e Preandean Depression, t h e Western C o r d i l l e r a and t h e Altiplano-Puna region ( f i g . 7 ) . This c r u s t a l

shortening

w i t h i n the upper p l a t e i s probably much more e f f e c t i v e than before. A l l these e f f e c t s may be due t o a higher p l a t e convergence v e l o c i t y than before w i t h no or only s l i g h t obliquity

(PARDO CASAS & MOLNAR, 1987).

Furthermore,

f o r t h e present

configuration

some f i n a l remarks about the subduction complex a t t h e inner slope o f t h e trench can be made. According t o t h e postulated t e c t o n i c erosion o f t h e c o n t i n e n t a l border, no accretionary wedge e x i s t s , however, blocks separated by f a u l t s are t i l t e d and subside until

t h e i r remainders

are underthrust together w i t h t h e subducting slab (ARABASZ,

1971; EMAIS e t a l . , 1988; B0URG0IS e t a l . , 1988). Thus t e c t o n i c erosion i n t h e outer f o r e a r c and u p l i f t i n t h e inner f o r e a r c seem t o be interdependent. The

foregoing considerations show t h a t throughout the Andean Cycle t h e c o n t i n e n t a l

c r u s t on which t h e Andean arc Systems were i n s t a l l e d underwent intense defortnations. One o f t h e main e f f e c t s was t h e shortening o f the c o n t i n e n t a l c r u s t and, as pointed out,

i t occurred

i n three t e c t o n i c a l l y

a c t i v e p a r t s o f t h e upper p l a t e :

(1)

the

subduction complex, ( 2 ) the magmatic arc, and (3) t h e backarc. The t e c t o n i c erosion o f the c o n t i n e n t a l margin by subduction since the Jurassic may be aproximately 200 km

257

f i g - 8: Diagram i l l u s t r a t i n g c r u s t a l shortening during t h e f o u r arc stages since the Lias i n a s e c t i o n through the Central Andes from the trench t o t h e eastern border o f the Subandean Ranges. Shortening i s achieved by t e c t o n i c erosion i n t h e subduction zone, compression i n the subsequent magmatic arcs and, during the l a s t two stages, by the i n s t a l l a t i o n o f f o l d and t h r u s t b e l t s i n the backarc areas. Tectonic erosion i s considered t o amount t o 200 km corresponding t o the distance between the Jurassic and the present v o l c a n i c , a r c s . shortening during the f i r s t three stages i s estimated w i t h 10 km i n each case (about 17%) and w i t h 40 km between the eastern border o f the Chilean P r e c o r d i l l e r a and t h e center o f t h e Eastern C o r d i l l e r a (about 11%). Crustal shortening i n t e backarc f o l d and t h r u s t b e l t o f t h e p r e s e n t l y a c t i v e arc System i s c a l c u l a t e d w i t h 80 km (ALLMENDINGER, 1983), while f o r the Late Cretaceous - Eocene stage an amount o f 50 km i s p o s t u l a t e d . The r e s u l t i n g c r u s t a l thickness i s conformable w i t h t h a t obtained from geophysical research.

258 which corresponds t o the present distance between t h e Coastal

Range and the Western

C o r d i l l e r a . The t e c t o n i c erosion mainly a f f e c t e d the Continental c r u s t which, a f t e r some subduction a t the surface o f the downgoing oceanic c r u s t , may have been added t o the present

Andean c r u s t by melting and u n d e r p l a t i n g . Tectonic

magmatic arc can roughly minimum),

be estimated

and f o r t h e Latest

shortening

i n the

only f o r the Puna (SCHWAB, 1985: 14% as a

Cretaceous

- Eocene

arc s i t u a t e d

i n t h e Chilean

P r e c o r d i l l e r a (CHONG & REUTTER, 1985: 25%) Underthrusting i n t h e backarc r e l a t e d t o the Miocene - Holocene arc stage amounts t o a minimum value o f 80 km (ALLMENDINGER 1983). Based on these values

i n f i g . 8 assumptions o f shortening are made f o r the

four arc stages which sum up t o a t o t a l

shortening o f 400 km between t h e trench and

the eastern border o f the Subandean Ranges, i . e . w i t h i n a present width o f 775 km. I f an o r i g i n a l

c r u s t a l thickness o f 35 km i s assumed t h i s amount o f c r u s t a l

shortening

s u f f i c e s t o explain the present c r u s t a l thicknes o f the Central Andes. A crosscheck based on a p l a n i m e t r i c c a l c u l a t i o n o f the present c r u s t a l volume i n the same section 775 km Wide leads a f t e r an extension o f 400 km t o a medium o r i g i n a l c r u s t a l thickness of 36,7 km. These estimations demonstrate t h a t the present thickness o f the Andean c r u s t can p e r f e c t l y be explained merely by c r u s t a l shortening. The f o u r arc stages and t h e i r respective t e c t o n i c s can only roughly be r e l a t e d t o the numerous t e c t o n i c events (phases) which have been described i n t h a t region (CHARRIER, 1981,; FRUT0S, 1981). Thus, the two Quechua phases probably belong magma-tectonic episode, and

t h e two d i f f e r e n t

t o the youngest

Incaic phases t o t h e Palaeogene

episode,

the Laramien (Peruvian) and Subhercynian phases t o the Mid o r Late Cretaceous

episode.

E a r l i e r phases are less well defined. FRUT0S (1981), BUSSEL (1983), PARD0-

CASAS & MOLNAR (1987)

and other authors

phases

convergence

and high

plate

rates.

see a r e l a t i o n s h i p I f this

i s true,

between t h e t e c t o n i c t h e magmato-tectonic

episodes o f t h i s region would be independent o f , or only i n d i r e t l y l i n k e d w i t h , the changing r a t e s o f p l a t e convergence.

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