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The geological record of Paleozoic strata in Northern Chile (21°-27°S) comprises ... The following is an interpretation of the north Chilean Paleozoic evolution.
THE PALEOZOICEVOLUTION OF NORTHERNCHILE: GEOTECTONIC IMPLICATIONS

Chr. Breitkreuz, H. Bahlburg & W. Zeil I n s t i t u t f u r Geologie und Pal~ontologie, Technische U n i v e r s i t ~ t B ~ r l i n , West Germany

Abstract

The geological

record of

Paleozoic s t r a t a

in Northern Chile

(21°-27°S)

comprises

Ordovician and Devonian to Permian sediments and volcanics. An evaluation of geotect o n i c concepts concerning i t s formation is given. The few r e s u l t s

available from the scarce Ordovician outcrops are ambiguous with

respect to geotectonic i m p l i c a t i o n s . The r e l a t i o n to the Ordovician series in B o l i via and Argentina is s t i l l

unclear.

We presume that i n t r a c o n t i n e n t a l processes led to the formation of the Devonian to Permian

series

control

of

rather

basin

than

processes

subsidence,

collision

of

Chilenia with

Devonian.

It

might

related

deposition

Gondwana south of

be possible

that

to

a subduction

regime.

and closure was possibly 29°S which started

the c o l l i s i o n

linked in

induced a dextral

Tectonic to

the

the Middle strike-slip

cycle in the north Chilean area as a kind of escape movement according to the reverse indenter model of EISBACHER (1985). We consider the north Chilean Late Carboniferous-Triassic volcanic series

in the

Pre- and High C o r d i l l e r a to have been formed in an i n t r a c o n t i n e n t a l tensional regime as i t

is presumed f o r the formation of the Mitu Group in the Eastern C o r d i l l e r a of

Peru and B o l i v i a .

Introduction

Knowledge of

Paleozoic volcanosedimentary development has increased s i g n i f i c a n t l y

since GARCIA's (1967) c l a s s i c survey of north Chilean geology. A wealth of newly discovered outcrops of Paleozoic s t r a t a have been reported, e x i s t i n g ones have been reinterpreted

with the help of modern methods (see e.g.

DAVIDSON et a l .

1981a,b,

1985, NIEMEYER et a l . 1985). Our research group has contributed detailed studies and surveys (see references).

Lecture Notes in Earth Sciences, Vol. 17 H, Bahlburg, Ch. Breitkreuz, P. Giese (Eds.), The Southern Central Andes © Springcr-Verlag Berlin Heidelberg 1988

88 Simultaneous to these studies, geotectonic concepts have been developed on a large scale for the Paleozoic of the Central and Southern Andes (HERVE et a l . 1982, COIRA et a l . and others.)

1981, BELL

1982, FORSYTHE 1982, DALZIEL & FOSYTHE 1985, RAMOS et a l . 1986

Here, models of

the north

Chilean Paleozoic development

have been

frequently obtained by transfering implicationG based on the geology of the Southern Andes. The following is an i n t e r p r e t a t i o n of the north Chilean Paleozoic evolution from a north Chilean viewpoint.

Geological features We begin with a short resum~ of the north Chilean Paleozoic

- detailed descriptions

and a summary of r e s u l t s are given in BREITKREUZ (1986), BAHLBURG (1987a) and BAHLBURG, BREITKREUZ & ZEIL (1987). Para- and orthometamorphic rocks of probable and certain Precambrian age occur in the north Chilean Precordillera and in the M e j i l lones Peninsula near Antofagasta

(Fig.

1).

Detailed examinations have j u s t

been

started (PACCl et a l . 1980, ZEIL 1983, BAEZA 1984, DAMM et a l . 1986, see also BAEZA & PICHOWIAK t h i s v o l . ) . Contemporaneous equivalents of the Argentinian Precambrian/ Cambrian Puncoviscana Fm. and the subsequent Cambrian Meson Group (see ACENOLAZA et a l . and KUMPA & SANCHEZ t h i s v o l . ) have not been documented in Northern Chile.

Early Ordovician marine strata occur in the north Chilean Puna (Aguada de la Perdiz Fm. and adjacent outcrops), in the northern Sierra de Almeida (CISL), and in the Precordillera (Argomedo Beds) (Fig. 1). The Aguada de la Perdiz Fm. (GARCIA et al. 1962) is composed of at least 2700 m thick, mainly s i l i c i c volcaniclastic rocks and sand-/siltstones that display

a low diversity middle Arenigian graptolite fauna

(ERDTMANN in BREITKREUZ1986). Probably Early Ordovician t h o l e i i t i c pillow lavas and associated hypabyssal stocks are reported in a hemipelagic-turbiditic facies from the 'Complejo Igneo y Sedimentario del Cord6n de Lila (= CISL; NIEMEYER 1984, et al. 1985, DAMMet al. 1986). Silicic volcaniclastic rocks also occur. The depositional features of the volcanic rocks indicate f a i r l y shallow water formation (BREITKREUZ 1986).

Fig. 1: Distribution of pre-Mesozoic strata in Northern Chile (21°-27°S). The numbers indicate the formations and localities mentioned in the text: 1) Quebrada Arcas, 2) El Toco, 3) Sierra del Tigre and Cerros de Cuevitas Fms. (Salar de Navidad), 4) Cerro Palestina, 5) Cerro 1584, 6) Aguada de la Perdiz, 7) Cordon de Lila ~I southern Sierra de Almeida, 9) Sierra de Argomedo, 10) Estratos (including CISL),1 Las T6rtolas, 12) Chinches Fm. Cerro del Medio,

89 71°W

1

70'

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'21'S

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Tocopilla

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(z.. -23"

@ ?UPPERDEVONIAN-

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CARBONIFEROUS?

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~ morJnesedimentsond volcanics J:".","~:;J Devonian-Permian

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f

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¢ ~ ?DEVONIAN/

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'Caldem

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(

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)

90 Furthermore, an at least 1200 m thick series of siliceous, fine- to coarse-grained clastic rocks outcrops in the northern Sierra de Argomedo (Argomedo Beds, BREITKREUZ 1985). I t contains a few volcanic intercalations, the volcanigenic clast content in the series is r e l a t i v e l y high. An Early Ordovician cruziana (SEILACHER in BREITKREUZ 1986) was discovered in the upper part of the exposed series. A l i t h o l o g i c a l l y similar series, not yet stratigraphically classified, has been reported in the area of C. Palestina (Fig. 1; BREITKREUZ 1986). During an orogenic phase at the Ordovician/Silurian transition ('Fase Ocl6yica', SALFITY et al.

1975) the series were folded to a varying degree. This probably

caused an u p l i f t of the western part of the Central Andes which would explain the absence of Silurian deposits in Northern Chile. During the Devonian-Carboniferous, a marine basin with two facies developed in Northern Chile: 1. A series of shelf sand- and claystones with a minimum thickness of 2700 m is exposed in a chain of outcrops at the western margin of the High Cordillera

(S.

Almeida, Fig. i , see also DAVIDSON et a l . 1981a). I t can probably be correlated with sand-/claystone series occurring to the south and the north of the S. Almeida ( i . e. Estratos Cerro del Medio, NARANJO & PUIG 1985; Chinches Fm., BELL 1985; Quebrada Arcas Beds, BREITKREUZ 1986). A limnic-brackish facies is assumed for the Chinches Fm. and the Arcas Beds. Deposition of the S. Almeida shelf series commenced in the Early Devonian on ocl6yic basement. Above t h i s , marine Middle, Late Devonian and Early Carboniferous have been confirmed by f o s s i l s

from a concordant series characterized by crossbedded

stones and c l a y - / s i l t s t o n e i n t e r c a l a t i o n s Towards the top of the series t e r r e s t r i a l

(ISAACSON et a l .

sand-

1985, BREITKREUZ 1986).

deposits occur. They are overlain with

erosional unconformity by Late Carboniferous-Permian v o l c a n i c l a s t i c rocks. 2. In the Coastal Cordillera and in some areas to the east, a thick (minimum thicknesses 2,3-3,6 km) flysch series occurs that is in parts intensely folded and the base of which is unknown (El Toco Fm., HARRINGTON 1961; Sierra del Tigre Fm., NIEMEYER et al. 1985;

Las T6rtolas Fm., BELL 1982; C. 1584 and C. Palestina areas,

Fig. 1). Some beds in the El Toce Fm. yielded Late Devonian plant fossils (SCHWEITZER in BREITKREUZ & BAHLBURG 1985), whereas near Ch~aral Early Carboniferous conodonts were found in a limestone turbidite close to the base of the Las T6rtolas Fm. (OLIVIERI in BAHLBURG 1987a). The El Toco Fm. consists of thick-bedded and coarsegrained turbidites; the Sierra del Tigre and Las T6rtolas Fm., on the other hand, are composed of thin-bedded and fine-grained turbidites and hemipelagic c l a y - / s i l t stones. Deposition took place longitudinally, mainly towards the south and subordinately towards the north. We presume the El Toco Fm., developed during the Devonian/?Early Carboniferous, was folded during the Early Carboniferous and posttecto-

91 nically

intruded during the middle Carboniferous

(320 Ma, SKARMETA & MARINOVlC

1981). As previously mentioned, sedimentation s t i l l took place during the Early Carboniferous near Cha~aral, where posttectonic intrusions, in parts with S Type a f f i n i t y , did not occur until the very ,late Carboniferous (DAMM & PICHOWIAK 1981, BERG et al. 1983). The southwardly progressing folding led to a shallowing of the marine basin, as confirmed by a transition of the lithofacies from turbidites to mud flows to spicul i t e sandstones to shallow-water limestones. There are no indications of an Early Carboniferous angular unconformity as postulated by DAVIDSON et al. Cerro 1584 area (see also NIEMEYER et al.

(1981b) for the

1985). The fossiliferous shallow-water

limestones (C. 1584-, C. Palestina areas, Cuevitas Fm., Fig. i) with Early Permian brachiopods (HOOVER in BREITKREUZ 1986) can probably be correlated to the Copacabana Fm. that is widely exposed in the Central Andes (Fig. 2, see also BARTH 1972). Some basic lavas and acidic pyroclastic and epiclastic rocks are intercalated in this platform series. These Early Permian volcanic rocks can be seen in connection with parts of the thick Late Carboniferous-Triassic volcanic series which occur in the Chilean Pre- and High Cordillera (Fig. 2, RAMIREZ & GARDEWEG 1982, DAVIDSON et al.

1985). The l a t t e r ,

predominantly acidic, volcanic rocks, referred to in Argentina as Choiyoi Fm. (Fig. 2, see also ZEIL 1981), are associated with limnic-brackish epiclastic rocks. The epiclastic rocks have u n t i l now been confined to the Carboniferous-Permian (RAMIREZ & GARDEWEG 1982, OSORIO & RIVANO 1985). Thus t h e i r synchronicity with the Early Permian limestones, as assumed in Fig. 2, is speculative.

Geotectonic implications The scarcity of pre-Devonian outcrops in Northern Chile and the lack of data allows only vague geotectonic conclusions concerning the Early Paleozoic evolution. The relation of the Early Ordovician, predominantly volcanic Aguada de la Perdiz Fm. to the magmatic rocks in the Argentinian Eastern Puna ('Faja Eruptiva') is outlined by BAHLBURG et al. (this volume). The geochemical characteristics of the t h o l e i i t i c magmatic rocks in the CISL, documented by DAMMet al.

(1986), indicate the following: The shallow extrusion depth

and the occurrence of acidic volcaniclastic rocks render an interpretation of CISL as a remnant of a normal ocean floor improbable. However, one cannot exclude the p o s s i b i l i t y of CISL having been formed in an ocean island setting. The geochemical data also allows one to presume a subduction-related formation: either as magmatism during subduction i n i t i a t i o n or in a back- or forearc r i f t

setting. CISL might

92

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\ 500 km I •

Anto~gasta

r,.,..

.--.j

/

~:/.l

.^+,

mW'-,Ir, ~

. 0 . ~ . . ^~.

volcanic intercalation volcanic series eolian deposits

~

lasticsand evapor ites

~

brackishdeposits

Santiago

shallow marine limestones after:

HELWIG 1972a,b LIMARINO* SPALLETTI 1986 MEGARD 1978 BREITKREUZ 1986

Fig. 2: Facies distribution Permi an,

J

in the Central and Southern Andes during the Lower

93 possibly be considered

as the northern continuation

of the chain of Ordovician

o p h i o l i t e outcrops as described by DALZIEL & FORSYTHE (1985) and RAMOS et a l . (1986) from the Argentinian Precordillera and Cordillera Frontal. They interpreted i t as a relict

suture of

the Middle Devonian c o l l i s i o n

of

Chilenia with

Gondwana (see

below). However, closure and folding of the CISL basin must already have taken place during the ?Late Ordovician because the ClSL is p o s t t e c t o n i c a l l y intruded by plutons at the Ordovician/Silurian t r a n s i t i o n (MPODOZIS et a l . 1983, NIEMEYER et a l . 1985). The existence of

a Late Paleozoic subduction

Southern Chile is seen as confirmed

(see e.g.

zone in

the

HERVE et a l .

area of

Central

and

1974, 1982, FORSYTHE

1982). Some authors postulate a Late Paleozoic active continental margin also for the north of Chile: a) The flysch series of the Coastal Cordillera which in parts display clear SW- to W-vergent folding are seen as forearc deposits (HERVE et a l . 1981). b) The dismembered formation on the coast near Cha~aral is interpreted as a remnant of the accretion prism ('Cha~aral m~lange', BELL 1982, 1984, 1987). c) The basic and ultrabasic volcanic rocks that also occur in the v i c i n i t y of Cha~aral are presumed to have developed in an ocean island s e t t i n g , implying that the t u r b i d i t e s were deposited on oceanic crust

(BELL 1984). d) Extensive Late Carboniferous to

Triassic magmatism caused the formation of the thick calcalkaline volcanic series and associated high level intrusions in the Chilean Pre- and High C o r d i l l e r a . This is presumed with reservation by some authors as to have been formed in a subductionrelated

setting

(COIRA et

al.

1982, DAVIDSON et

al.

1985, HERVE et

al.

1985,

NIEMEYER et a l . 1985). We would l i k e to submit the following concerning points a to d: a) The petrographic and geochemical composition of the Coastal Cordilleran t u r b i d i t i c rocks corresponds to that of a c o l l i s i o n a l orogenic source and not to that of an arc system (BAHLBURG 1987a,b). Sedimentation and folding of the flysch series had allready been completed by the time magmatism started in the Pre- and High Cordillera

(so f a r

oldest ignimbrite

age: 290 Ma, DAVIDSON et a l , 1985). This is c e r t a i n l y true of the EL Toco Fm. that was p o s t t e c t o n i c a l l y

intruded

as e a r l y

as during the middle Carboniferous.

The

series near Cha{aral Were also folded during the Carboniferous as they were posttectonically

intruded

in the very Late Carboniferous and Permian (DAMM & PICHOWIAK

1981, BERG et a l .

1983, PANKHURST & BROOK 1987). This time schedule renders any

i n t e r p r e t a t i o n of the flysch series as a forearc deposit of the assumed magmatic arc in the Pre- and High C o r d i l l e r a impossible. b) BELL (1982, 1984, 1987) reports impressive phenomena from the 'Cha~aral M~lange' which are also c h a r a c t e r i s t i c of subduction-related m~langes. However, because of the paucity of exotic blocks t h i s complex should c o r r e c t l y be c l a s s i f i e d as a

94 dismembered formation (sensu RAYMOND 1984)(BAHLBURG 1987a). The basic volcanic rocks also occur as concordant lava flows in the less tectonized t u r b i d i t e s to the east o~ the 'Cha~aral M61ange' and should thus be considered as authochthoneous material of the dismembered formation. The dismembered formations in the Cha~aral v i c i n i t y and also

in the Sierra del Tigre Fm. (BREITKREUZ 1986) could have been formed in an

intracontinental upthrust zone. NE-dipping upthrusts also occur in the El Toco Fm. (HARRINGTON 1961, BREITKREUZ & BAHLBURG 1985). No Late Paleozoic HP/LT-metamorphic rocks have yet been reported from Northern Chile indicating, as is the case in Central and Southern Chile, the existence of a subduct i o n zone. c) Geochemically, the alkaline and t h o l e i i t i c

basalts and ultrabasic volcanic rocks

near Chanaral display d e f i n i t e WPB-affinity. REE characteristics point to an i n t r a continental setting f o r the extrusion of the submarine lavas (PICHOWIAK et a l . 1987) as do the constraints given by regional geology (see DALMAYRAC et a l .

1980, MILLER

1984). d) The available geological and geochemical data is not s u f f i c i e n t to rule out the p o s s i b i l i t y of the Late Carboniferous-Triassic magmatic rocks having formed in a magmatic arc.

Nevertheless,

the percentage of

component of volcanic arc sequences,

andesitic

volcanic

rocks,

typical

is very low in the north Chilean series. The

few intermediate and basic volcanic intercalations to be found display geochemical c h a r a c t e r i s t i c s of continental r i f t rift

regime is assumed to f i t

affinity

(PICHOWIAK et a l .

1987). Continental

the formation of the Late Permian-Triassic Mitu Group

volcanic rocks (and associated c a l c a l k a l i n e plutonic

bodies) of the Peruvian and

Bolivian Eastern Cordillera (NOBLE et a l . 1978, KONTAK et a l . 1985). Radiometric dating of the plutons around the l a t i t u d e of Cha~aral confirms simultaneous i n t r u s i v e a c t i v i t y

in the Coastal Cordillera and High Cordillera during the

Permian (PANKHURST & BROOK 1987). I f the High Cordilleran magmatism r e a l l y did develop

in

the

postulated

magmatic arc,

the

intrusives of

the Coastal

Cordillera,

characterized by r e l a t i v e l y high crustal contamination (BERG & BAUMANN 1985, PANKHURST & BROOK 1987), must be seen as having formed in the forearc area. The i n t r u sion of large acidic magmas of S type a f f i n i t y

in the forearc area of an active

continental margin is impossible as the nescessary crustal thickness is not given. This is also the cas~ assuming a Devonian-Early Carboniferous f l a t - a n g l e subduction without magmatic arc a c t i v i t y to have existed in Northern Chile. Supporters of the subduction model f a i l to o f f e r any explanations for the subsequent s h i f t of the 2000 km long 'magmatic arc' from the High Cordillera at least 100 km towards the west to the Coastal C o r d i l l e r a at the beginning of the Jurassic.

95 Furthermore, the subduction model ignores the f o l l o w i n g geometrical problem: In the Late Paleozoic, Gondwana extended towards the west beyond the present continental margin

(DALMAYRAC et

al.

1980, MILLER 1984, KATO 1985). This extension must have

persisted even during the Jurassic, when a magmatic arc in the Coastal C o r d i l l e r a was active (BUCHELT & TELLEZ and BAEZA & PICHOWIAK, t h i s v o l . ) ,

as i t s forearc must

have had an extension of between I00 and 200 km (DICKINSON & SEELY 1979).

Under

these circumstances, the Late Carboniferous-Triassic magmatism of the High C o r d i l lera

would

have

b e e n located

at

a distance

of

200-300

km from

the

trench.

Subduction-related formation is thus h i g h l y improbable. In the previous paragraphs we emphasized the many inconsistencies of a simple subduction model f o r the north Chilean Late Paleozoic. We are conscious of the hypothetical

character of our ideas concerning the geotectonic development in Northern

Chile during the Devonian-Permian (Fig.

3 and 4):

part of the Central Andes probably constituted

During the S i l u r i a n the western

an elevated area, produced by the

ocl~yic orogeny. The sea did not transgress as far as the western Puna u n t i l Early Devonian (ACENOLAZA et a l .

1972, NIEMEYER et a l .

the

1985). Subsidence during the

Devonian-Early Carboniferous must have been continuous as more than 2700 m of mainly intertidal

to shallow subtidal

sediments were deposited

in the Sierra de Almeida

area during t h i s time. A deep basin formed contemporaneously in the Coastal C o r d i l lera in which thick t u r b i d i t i c

series were deposited.

We presume that the western

border of the f l y s c h trough was formed by a s t r u c t u r a l high (BAHLBURG 1987a, BAHLBURG et a l .

1987), a possible southern extension of the Peruvian Arequipa Massif

(see DALMAYRAC et al.

1980, GODOY 1983, DALZIEL & FORSYTHE 1985). Recent crustal

seismic and p e t r o l o g i c a l explanation f o r lower

crustal

investigations

in the Coastal

the disappearance of t h i s rocks

R~SSLING, B e r l i n )

(e.g.

granulites)

structural

outcrop

at

and below 20 km depth a v e l o c i t y

comm. P. WIGGER, B e r l i n ) . R~SSLING and WIGGER is

One possible

C o r d i l l e r a might o f f e r

high: the

surface

(oral.

comm. R.

inversion was detected

explanation of

an

South of Antofagasta, (oral

these phenomena given by

subduction of a continental fragment ( i n c l u d i n g l i g h t high

level crust) under the Coastal Cordilleran block during Andean convergence. This led to u p l i f t

and erosion of parts of the suprastructure giving way to the exposure of

lower crustal material. The Middle Devonian c o l l i s i o n

of the Chilenia terrane with the Argentinian Proto-

P r e c o r d i l l e r a (RAMOS et a l . 1986) could possibly have induced a response in the continental

area to the north of the c o l l i s i o n zone in form of a dextral s t r i k e - s l i p

cycle (sensu MITCHELL & READING 1986) and analogous to EISBACHER's (1985) indenter model. The f i r s t , intracontinental

rifting

transtensional and to

the

stage of the s t r i k e - s l i p

initiation

of

flysch

cycle

sedimentation

reverse led to in

the

Coastal C o r d i l l e r a during the Late Devonian at the l a t e s t . Sedimentation previous to the f l y s c h deposition must have taken place but i s t not documented. The Early

96

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70°W !

ima

PERU

~

m Lo Paz

.f(. BOLIVIA E

20os !

L.