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'
/.,#
69°
I
'21'S
\.
r-~
\
I
1
i I
O, . . . .
50km
\
i
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!
-22"
\.
Tocopilla
! o
Caloma
e o
I
. _J
i I
U
1.1"
\ . ~ .... .i-~.
(z.. -23"
@ ?UPPERDEVONIAN-
~RBONIFEROI.JS
%4 - 2/'°
Ir'd--OEVON'AN'AR='E"=
?DEVONIAN/L.PERMIAN ?CARBONIFER.-~
/ o
i
LOWEllPERMIAN~ 5
8
./"
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~%r~L,,,.OEVONIAN/i .i"
-2~
l
?DEVDM~AN /
I /
CARBONIFEROUS? ( ~
( "'1
T a l ~ rP
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~::;~ DEVONIAN/ ~
\
/
/
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i \.
t
CARBONIFEROUS?
~ v////A
terrestriaEvolcanicsandsediments ? UpperCarboniferous-Triassic
~ morJnesedimentsond volcanics J:".","~:;J Devonian-Permian
.2~" Ch,
11
marlnesediments~ndvolcanics
Ordovician
f
~[
¢ ~ ?DEVONIAN/
i •
? Precambrian-Palaeozoic
\ \ \
'Caldem
I
(
\
)
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
S ',',',',,'t2,"! \ PazNN
\ 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.
