Central Andean Transect Nazca Plate to Chaco Plains, Southwestern Pacific Ocean, Northern Chile, and Northern Argentina

34 Pages·2.334 MB·English

GLOBAL GEOSCIENCE TRANSECT 6
CENTRAL ANDEAN
TRANSECT, NAZCA PLATE TO
CHACO PLAINS
SOUTHWESTERN PACIFIC
OCEAN, NORTHERN CHILE
AND NORTHERN ARGENTINA
~American
Geophysical Union
Publication No. 192 ofthe International Lithosphere Program
ISBN 0-87590-783-0
Copyright 1991 American Geophysical Union. Figures, tables and
short excerpts may be reprinted in scientific books and journals if
the source is properly cited; all other rights reserved.
Printed in the United States ofAmerica.
American Geophysical Union
2000 Florida Avenue, N.W.
Washington, D.C. 20009
Foreword
The Global Geoscience Transects Project (GGT) is surface-bound field geologist, can attest to the excite
an.ambitious international effort that draws together ment of working with geophysicists from around·the
geoscientists in a variety ofdisciplines to produce the world to consider the nature and evolution ofdeeper
best possible portrayal ofthe composition and struc partsofthecrustandtoattempttointegratethesedata
ture ofthe Earth's crust. Since its inception in 1985, with surface features.
GGT has encouraged geoscientists in all countries of GGT compilation also plays a significant role in the
the world to compile cross sections ofthe Earth up to education ofgeoscientists. The problems being inves
tigated are global and interdisciplinary in scope, and
a few thousands ofkilometers in length and drawn to
international cooperation between geologists, geo
the base of the crust using all available geological,
physicists, geochemists, and geodesists is required to
geophysical, and geochemical information. Transects
resolve them. Transects cannot be compiled without
are drawn to common scales and formats so that the
such interdisciplinary research.
Earth's crust in different parts of the world can be
Theguidelinesdevelopedforthetransectshadtobe
directly compared.
flexible enough to accommodate the variations indata
GGT was conceived by the Inter-Union Commis
available in different parts of the world. Some areas
sion on the Lithosphere (ICL), a "child" of the
haveverycomplete,freelyavailabledatabases.;others
International Union of Geological Sciences and the
have·at bestonly gravity oraeromagnetic maps as the
International Union of Geodesy and Geophysics
geophysical databasefrom which to constructcrustal
(IUGG), attheIASPEIconferenceinTokyoinAugust
cross sections.
1985. Project coordinators James W. H. Monger and
Thefirst publicationsoftheGGTProject, atransect
Hans-JurgenGotzeorganizedamultidisciplinary,mul
chartandaccompanyingbooklet, areajointactivityof
tinational committee that coordinated the global proj
ICL and AGU. The seven transects from China (2),
ect. Meeting at the IUGG.XIX General Assembly in
South America (2), Australia (2), and Syria (1), were
Vancouver in August 1987, the committee developed
drafted and h~nd colored by the £ompilers. Transects
preliminary guidelines for transect compilations, fol
combine the available data into interpretive sections
lowingsuggestionsfrom·transect compilers. Proposals
drawn to the base ofthe Earth's crust. The product is
fortransectswere solicitedfrom ICLnationalcommit
alarge display-approximately 1meterby 2meters
tees,. government geoscience agencies, universities,
and scientists. A total of 140proposals were received withthebottomlinecrosssectionasaverticaltectonic
from Mrica (41), South America (30), China (18), map ofthe crust. The booklet includes an analysis of
USSR(12), USA(12), Europe(12), AustraliaandNew the transect by the compilers and references to data
Zealand (11), India (3), and Southeast Asia (1). sources.
TheGGTProjecthasgeneratedaspecialintellectual The GGT Project is now embarking on a new
excitementamongparticipantsbyencouraginggeosci direction that will make this wealth ofdataeven more
entists to understand the evolution and physical be usefultothe scientificcommunity. ICLandAGUplan
havior ofthe lithosphere by bridging the gap between to publish future transects in electronic format (dis
surface geology and deepergeophysics. Monger, as a kettes, CD-ROM) thatcanbemanipulatedusing small
"low-end" computers. AGGTDigitizationGroupwas wide network of scientists cooperating on transects
established in 1988 to prepare guidelines for digitizing would be able to work interactively with these files,
transects; the guidelines and sample will be published adding. data to them, analyzing, reconfiguring, and
by AGU. interpreting them.
This endeavor breaks new ground and goes a long Digitizationofgeosciencematerialwillnotonlyease
way toward a goal of the ICL of closely linking the task of publishing trartsects and encourage the
geological, geochemical, and geophysical data bases exchange of data bases, it promises to allow quanti
and breaking down present barriers between different tativecomparisonsofelementsofdifferentpartsofthe
Earthsciencedisciplines. Withthedigitizedtransects, Earth's crust. The use ofdigitized transect material is
geologists,forexample, couldmanipulategravitydata the perfect training ground for the new breed: geosci
entists!
totestthe validityoftheirstructuralmodels. Aworld-
James H. Monger
Geological Survey ofCanada
100 West Pender Street
Vancouver V6B lR8
Canada
Hans-Jurgen Gotze
Institut fur Geologie, Geophysik und Geoinformatik
Freie Universitat Berlin
Malteserstrasse 47 100
D-I0oo Berlin 45 Germany
CENTRAL ANDEAN TRANSECT, NAZCA PLATE TO CHACO PLAINS,
SOUTHWESTERN PACIFIC OCEAN, NORTHERN CHILE
AND NORTHERN ARGENTINA
EDITORS
R.OMARINI
Salta,Argentina
H.-J. GOTZE
Berlin,Germany
2 GLOBALGEOSCIENCElRANSECT6
CONTENTS
LISTOFAUTHORS ANDAFFILIAnONS 3
PREFACE 4
GEOLOGICALDEVELOPMENTANDSTRUCTURES 5
R.Omarini, K.ReutterandT.Bogdanic
THECRUSTALSTRUCTUREALONGTHECENTRALANDEANTRANSECT 14
DERIVEDFROMSEISMICREFRACTIONINVESTIGAnONS
PJ.Wigger,M. Araneda,P.Giese,W.-D.Heinsohn,
P.Rower,M.SchmitzandI. Viramonte
THEGRAVITYDATABASEOFTHETRANSECTCOMPILATION 21
H.-J.Gotze,B.Lahmeyer,S.Schmidt,S.Strunk
M. Araneda,G.Chong and1.Viramonte
ACROSS-SECTIONOFELECTRICALRESISTIVITYSTRUCTURE 25
ALONGTHECENTRALANDEANTRANSECT
G. Schwarz,V.RathandD.Kruger
THEDIGITIZATIONOFTRANSECTMATERIAL 27
S.Schmidt,J.Palmer,H.-I.Gotze,G.Goltz,
M. Alvers,Th.Karnbrock,Ch.KolaxandB.Heber
APPENDIXI: CONTENTOFTRANSECTDATAFILES 31
CENlRALANDEANTRANSECT 3
LISTOFAUTHORSANDAFFILIATIONS
UniversidadNacionaldeSalta:
R.OmariniandI.Viramonte,DepartamentodeGeologIa,UniversidadNacionaldeSalta,BuenosAires 144,4400Salta,
Argentina.
UniversidaddelNorte:
T.BogdanicandG.Chong,DepartamentodeGeologIa,UniversidaddelNorte, Antofagasta,Chile.
UniversidaddeSantiago:
M. Araneda,DepartamentodeGeologIayGeoflsica,UniversidaddeSantiago,BlancoEncalada2085,Casilla2777,Santiago
deChile,Chile.
FreieUniversitatBerlin:
P.Giese,W.-D. Heinsohn,P.Rower,M.Schmitz,andP.J.Wigger,InstitutfUrGeologie,GeophysikundGeoinformatik,FR
Geophysik, FUBerlin,Rheinbabenallee49,D-l000Berlin33,Germany.
M. Alvers, H.-I.Gotze, G.Goltz,B. Heber,Th. Karnbrock, Ch. Kolax, D.KrUger,I.Palmer,V. Rath, S. Schmidt,G.Schwarz,
InstitutflirGeologie,GeophysikandGeoinformatik,FRGeophysik,FUBerlin,Malteserstrasse74-100,D-l000Berlin46,
Germany.
K.Reutter,InstitutflirGeologie,GeophysikandGeoinformatik,FRGeologie,FUBerlin,Altensteinstr.34a,D-I000Berlin33,
Germany.
B.Lahmeyer,Deichmannstrasse2,D-3000Hannover 1,Germany,formerly: FRGeophysik,FUBerlin.
S.Strunk,Riiterstrasse73,D-2000Hamburg-Wansbeck,Germany,formerly: FRGeophysik,FUBerlin.
4 GLOBALGEOSCIENCE1RANSECf6
PREFACE Most of the fundamental goals of the Global
Geoscience Transect Program (GGT), which was
This pamphlet accompanies the "Central Andean launched by the Inter-Union Commission on the
Transect, Nazca Plate to Chaco Plains, Southwestern Lithosphere (ICL) in 1985, correspond with those of
Pacific Ocean, Northern Chile and Northern the international "Andean group". It is obvious that
Argentina" (CAT). InthisareatheCentralAndeshave the detailed investigations carried out in the last
their greatest width, and all typical morphostructural decade at the western continental margin of South
elements are well developed between the Pacific America provide a useful data base for the
OceancoastinthewestandtheSubandeanfoothills in compilation of a geotransect. Following closely
theeast. On this transect thePre-Andean evolution is GGT's guidelines, we compiled strip maps,
documented by outcrops of Paleozoic and even cross-sections and diagrams as a contribution to the
Precambrian rocks. All phenomena related to the South American Transect Program. The group was
Andeancyclecan be studied along theentire transect. headed by both Ricardo Omarini (Salta), who was
The enormous amount ofrecent work, particularly in responsible for coordination of geological
northern Argentina and Chile as well as in southern compilations and tectonics, and H.-I. Gotze (Berlin),
Bolivia, has led to new ideas and models for the who coordinated geophysical contributions to the
development of this region in terms of plate transectcompilation and digitization of the transect.
interactions and orogenic cycles. Increasingevidence The sections of the accompanying pamphlet were
on the distribution of ancient magmatic arcs and the written by the project participants to ensure original
metamorphic and sedimentary sequences of old information, which is best evaluated by themselves.
orogenic belts suggests that a complex collage of Aside from them the transect compilation benefited
former collisions and amalgamations of lithospheric greatly from the discussions, experience, cooperation
plates formed the basement of the Central Andes. and former data acquisition ofcolleagues not directly
Most Andean tectonic and magmatic processes are involved as authors of this booklet. Their assistance
controlled and/or influenced directly by the andhelpisverymuchappreciated:
downgoing Nazca Plate. Motion, directions and
G. Franz, W. Heinrich, H. Soto, and R. Sureda
related forces vary in time and space but provide a
contributed to the compilation of geology and
common platform for gaining new insight into the
tectonics,
evolution of the Central Andes continental margin,
R. Carle and C.E. Di Persia worked out the
continental rifting and basins, Phanerozoic and
interpretationofseismic reflection lines in thearea
Proterozoic fold belts and crustal growth. New
ofwesternChaco,
geophysical, geological and geochemical data and
L. Febrercontributed tomagnetotelluric studies, A.
information help to define the nature and crustal
Introcaso and A.A. Cerrato to potential field
structure of the Central Andes both laterally and at
compilationand
depth.
B.L. Isacks made his digital topography model and
To keep this in mind, a multidisciplinary and
D. Fairhead his digital South American geography
international group of geoscientists from the
fileavailabletous.
Universidad Nacional de Salta (Argentina), Univer
sidad del Norte (Antofagasta, Chile), Universidad de The Central Andean Transect is one of the first
Santiago (Chile) and from both Freie Universitat and GGT compilations whose data base and graphics
Technische Universitat Berlin (Germany) focused entities are fully digitized. The principal objective is
their studies along a geotraverse through the Central tomakethistransectmore useful in termsofchanging
Andes. Active assistance and close cooperation of scale and projections of graphics, viewing and
many other colleagues from South American insti analyzing data by modeling procedures, editing and,
tutions and universities, particularly from Bolivia and most important, updating of different data sets. The
Argentina, were necessary to bring this project to a use of "low end" computer hardware and open file
satisfactory conclusion. Concerning the German softwareforbothdigitizationand modelingmakesthis
participation this interdisciplinary work was under compilationoneofthekey projects oftheentireGGT
way since 1982, supported as the research group program.
"MobilityofActive ContinentalMargins" by both the The editors owe a special debt of gratitude to all
German Research Society and the Berlin universities. colleagueswhocontributedto thefinal compilationof
TheprojectwasheadedbyPeterGiese(pUBerlin). theCentralAndeanTransect.
Central Andean Transect: Nazca Plate to Chaco Plains,
Global Geoscience Transects Southwestern Pacific Ocean, Northern Chile, and Northern Argentina Vol. 6
CEN1RALANDEANTRANSECT 5
GEOLOGICALDEVELOPMENT AND STRUCTURES
R.OMARINI,K.REUTTERANDT. BOGDANIC
ABSTRACT magmaticallyoverprintedduringtheMesozoic-Cenozoic
AndeanCycle. Thiscycleischaracterizedbyamoreor
The geological development of the Central Andes less continuous subduction of the East Pacific oceanic
along the transect is presented following the platesystem, however, with varyingparametersofplate
morphostructural units from the Coastal Cordillera to convergence,suchasrate,obliqueness,dipangle,etc.In
theChacoPlains. Theseunitsarean expressionofthe theareaofthetransect,four magmaticarcsystemswere
actual stress field in the active continental margin. It successively developed and changed their position
can be shown that, despite great paleogeographic stepwise: (1) a Jurassic-Early Cretaceous arc in the
variations, the conditions of an active continental Coastal Cordillera, (2) a Mid-Cretaceous arc in the
marginpersistedthroughoutthePhanerozoic. LongitudinalValley,(3)aLateCretaeeous-Paleogenearc
in the Chilean Precordillera, and (4) the
INTRODUCTION: GENERALFEATURES Miocene-Holocenearcin theWesternCordillera. Thus,
a migration of the tectonomagmatic processes towards
SincetheLateProterozoic,thegeologyofthewestern the east took place (Coirn et al., 1982; Reutter et al.,
margin of Gondwana has been controlled by its 1988),whichalso impliedthedisplacementin the same
interaction with the changing oceanic plate systems of directionoftherespectivebackarc andforearc structures
thePacificinthewest Twomajororogeniccyclesofthe and depocenters. The advancement of the tectonic
Phanerozoicaredistinguished: thePaleozoicPreandean processestowards the interiorofthe continent, opposed
Cycle and the Meso-Cenozoic Andean Cycle (Coirn et to the developmentduring thePreandean Cycle, canbe
al., 1982). Bothcyclesembraceslow tectonic evolution explainedwithadestructionofthecontinentalmarginby
aswellasrapidorogenicevents. Accordingtotheactive subductionerosion(Hilde,1983).
continental margin setting, the geology of the Central
Andes is not only characterized by sedimentation and THEMORPHOSTRUCTURALUNITS
defonnation, but also by volcanism and plutonism.
Thus, thick volcanic sequences and intrusive bodies With respect to their geomorphological structure,
ranging in sizefrom small stocks tohugebatholithsare the Central Andes of the transect area can be
widely distributed in time and space in that mountain subdivided into several morphostructural units
chain. (Reutteret al., 1988). These units are essentially due
The Preandean Cycle is characterized by the to the stress field that, at present, is exerted in the
developmentofhugedepocentersandsubsequentfolding continental margin by the convergent movement of
of the mostly thick sedimentary sequences during a the Nazca Plate and the South American Plate. The
tectonic event (orogeny). The sediments are partly general geotectonic subdivision in forearc, arc, and
intercalated with volcanic products indicating the backarc is well developed. Within the forearc, the
existenceofmagmaticarcs. Continentalaccretionseems structural high atthe trench slopebreakisrepresented
to have occurred during the Preandean Cycle as the by the Coastal Cordillera (CC), the Longitudinal
depocenters were developed successively farther west. Valley (LV) corresponds to a forearc basin, and the
Thus, the basin of the Upper Precambrian-Lower Chilean Precordillera (CP) separates as a morpho
CambrianPuncoviscanaFm. was situated in theEastern logical high the Longitudinal Valley from the
CordilleraandSubandeanRanges,thatoftheOrdovician Preandean Depression (PD), which is another forearc
sediments in the Eastern Cordillera (West) and, finally, basin close to the arc. The volcanoes ofthe Western
the basin of the EI Toeo Fm. (Breitkreuz, 1986) of Cordillera (WC) mark the present magmatic arc
Devonian to Permian age was situated in the whose western front is very regular, but in the
Longitudinal Valley and the Coastal Cordillera. While Neogene to Recent, volcanism extended irregularly
thefITst twodepocenters may havebeenbackarc basins into thePunahighplateau(puna=Eastern Cordillera,
withthearc lyingon acontinentalfragment tothewest western part, ECW), so that these two units are not
of the basins (possibly the so-called Arequipa Massiv; clearly separated. The ECW shows features of the
Coirnetal., 1982;Bahlburg,1990),nomagmaticarccan magmatic arc as well as of the backarc area.
be attributed to the Upper Paleozoic depocenter during However, the Eastern Cordillera sensu stricto (s.str.)
the Devonian. Only in the Permo-Carboniferous and (BCE), which is affected by young volcanism only in
possiblyEarlyTriassic,abroadmagmaticarcdeveloped its western part, belongs certainly to thebackarc area.
(Breitkreuz, 1990) at the eastern side of the EI Toco It is the high riding edge of the ECW block which,
basin,whosesedimentswerestronglyfolded during that along a great crustal overthrust system, is overriding
time. Syn-orogenic plutonism affected mainly the arc more eastern parts of the backarc. These include the
areas especially in the Late Ordovician and dwing the SubandeanBelt(SA),whichforms aforelandfoldand
LatePaleozoic(Dammetal., 1990). thrust belt, and the still active backarc basin of the
The Paleozoic basement rocks and structures of the ChacoPlains (CHP) situatedbetween the frontranges
Central Andes were tectonically, thermally and oftheAndesandtheBraziliancraton.
Copyright American Geophysical Union
Central Andean Transect: Nazca Plate to Chaco Plains,
Global Geoscience Transects Southwestern Pacific Ocean, Northern Chile, and Northern Argentina Vol. 6
6 GLOBALGEOSCIENCE1RANSECT6
GEOLOGICALDESCRIPTIONOFTHE LongitudinalValley(LV)
MORPHOSTRUCTURALUNITS
In the transect area, the LV is morphologically
littledevelopedasasmoothinflexionbetweenCCand
CoastalCordillera(CC)
CP, and Neogene sedimentsare represented only as a
ThegeologyoftheCCismainlydeterminedbythe discontinuous veneeroftheabove mentioned ttGravas
andesiticlavasandplutonsofa magmaticarc thatwas delaPampa." However, towards thenorth, theyoung
active during the Jurassic andEarly Cretaceous. The tectonic depression is increasingly better expressed
oldest rocks, consisting of micaschists and gneisses and, in the Pampa del Tamarugal, it is an important
andhavingagesbetween520and 560Ma,cropoutin depocenterwithsourcesfrom theCPo
the northern part of Mejillones Peninsula (punta In the LV, the lavas ofthe Jurassic LaNegra Fm.
AngamosFm.andJorginoFm.;BaezaandPichowiak, hadalateral transitiontonon-volcanic, mostly marine
1988). In the eastern part of the CC, Late Paleozoic calcareous sediments (Sierra el Cobre Fm.; Ferraris
sediments, Triassic sediments and volcaniclastics and and Di Biase, 1978; von Hillebrandt et al., 1986)
Liassic to Sinemurian marine limestones (Estratos de which are considered as the sedimentary filling of a
Rencoret; Ferraris and Di Biase, 1978; Munoz et al., backarc basin belonging to the Jurassic-Early
1989) underlie a sequence ofJurassic andesitic lavas Cretaceous arc in the CC. These limestones are
(La Negra Fm.) having a cumulative thickness of underlain by some marineUpperTriassic and clastic,
about 10km (Rossling, 1988). SouthofAntofagasta, volcaniclastic and volcanic rocks of Early Triassic,
the lava sequence is unconformably overlain by the Permian and Late Carboniferous age (Agua Dulce
coarse detritic Caleta Coloso Fm. and marine Fm.) as well as of Carboniferous and Devonian
limestones(ElWayFm.,Hauterivian-Aptian)ofEarly sandstones. Starting with the Kimmeridgian, the
Cretaceous age. Unconsolidated continental sedi marine sediments pass upwards mostly conformably
tt
ments ofmainly Mioceneage (ttGravasde laPampa ) intoa thicksequenceofLowerCretaceouscontinental
overlie discontinuously the older rocks. On clastics (Chacarilla Fm.; Bogdanic, 1990). On top of
Mejillones Peninsula they pass into the partly marine these sediments, a sequence, 2-3 Ian thick, of mostly
La Portada Fm. (Miocene) and Mejillones Fm. andesitic lavas, tuffs and intercalated sediments of
(pliocene). Mid-Cretaceous age (Empexa Fm., Bogdanic, 1990)
The La Negra magmatic arc volcanism was indicates that a magmatic arc was present in the area
contemporaneouswith theintrusionofhugebatholiths of the LV and the western parts of the PC.
as confirmed by various isotope age determinations Radiometric dates were obtained only from areas
(200-120 Ma: Diaz et aI., 1985; Damm et al., 1986; northofthetransect,thewesternslopeoftheSierrade
HerveandMarinovic, 1989). 87Srf6Srinitialratios of Moreno,byRogers (1985: Rb/Sr-isochron in volcanic
±
about 0.703 (Diaz et aI., 1985, Herve and Marinovic, rocks of 104.7 19 Ma) and Dobel (1989: 4°Ar/39Ar
1989) in the intrusive rocks exclude the possibility of involcanicdykes: 92.4±2.3Ma).
significant crustal contamination. The contact Theserockswerefolded (SubhercynianPhase?)and
relationsofthe intrusiverocksaswellas theirtextures post-tectonically intruded by granodioritic magmas at
show that in many cases the level of intrusion was approximately 80Maaccording to radiometric dates by
considerably more shallow than 10 km (Damm and Munoz et ale (1989) and Baeza and Pichowiak (1988).
Pichowiak, 1981). South of Antofagasta and in Latertectonics areevinced by an angular unconfonnity
southern Mejillones, however, a deeper crustal level between the defonned sequence with its plutonic
ofabout 12-15 km is exposed (Rossling, 1988) which intrusions and an overlying volcanic sequence of very
consists of basic intrusive rocks and metamorphic LateCretaceoustoEoceneage(Augusta-VictoriaFm.or
rocks.ofdioritic composition with migmatiticportions Chile-AlemaniaFm;Chong, 1973).
(BolfinComplex,probablyEarlyJurassic).
The most significant structure of the CC is the ChileanPrecordillera(CP)
orogen-parallel Atacama Fault Zone, which can be
traced over more than 1000 Ian from 200S (Iquique) The mountain ranges to theeastofthe Chilean LV
to 300S (La Serena). Its main branches cut through aremorphologicallyclearlyseparatedfrom theWCby
Mejillones Peninsula, where a vertical throw ofabout the PD. Due to strong shortening the basement is
12km can be postulated. According to Scheuberand upfolded in one or more huge anticlines, so that
Andriessen (1990), high to low grade mylonites give pre-Jurassic rocks are exposed. Schists and
evidenceofstrongsinistralstrike-slipmovements,and amphibolitesofpossiblyOrdovician metamorphic age
the relationship with Jurassic and Early Cretaceous cropoutto the north ofthe area (Limon Verde, Baeza
intrusives points to a corresponding age of andPichowiak, 1988). Theywereintrudedbygranitic
deformation. Themagmaticandtectonicdevelopment magmas during the Late Carboniferous and the
of this arc system is interpreted as an effect of a Permian. Granites ofthis age are exposed also along
transtensional stress regime caused by oblique the transect (pichowiak et aI., 1990). In other places
subduction (Scheuber and Reutter, 1991). Younger also Paleozoic sediments of Devonian or
neotectonic movements in the abandoned magmatic Carboniferous age crop out. For the most part these
arcactedmost!yasdipslipfaults. basementrocks ofdiffering ages are overlain byLate
Copyright American Geophysical Union