Download on April 6, 2015 gsabulletin.gsapubs.org Downloaded from

Downloaded from gsabulletin.gsapubs.org on April 6, 2015
W. D. CARTER U. S. Geological Survey, Washington, D. C.
LUIS AGUIRRE LE B. Institute de Investigations Geologicas, Santiago, Chile
Structural Geology of Aconcagua Province
and Its Relationship to
the Central Valley Graben, Chile
Abstract: Aconcagua Province is herein divided
into three major structural provinces which, for the
sake of simplicity, are named the Coastal Cordillera,
Central Valley graben, and Andean Cordillera
structural provinces to correspond to the three
geomorphic provinces recognized farther south.
The coastal structural province includes the
Coastal Cordillera which is underlain mainly by
layered sedimentary and effusive rocks that strike
north and dip homoclinally to the east, range from
Triassic to Late Cretaceous in age, and are intruded
by Cretaceous granodioritic and dioritic rocks.
Igneous and metamorphic rocks largely of Paleozoic
age comprise the western coastal margin, Along the
eastern edge of the province is the Los Angeles fault
zone, a wide, poorly defined band of semiparallel,
arcuate faults which show downward displacement
to the east and which appear to have resulted mainly
from intrusion and uplift on the west. The Coastal
Cordillera, therefore, may be considered a large
horst with an intrusive granodiorite core.
The Central Valley graben is bounded on the
west by the Los Angeles fault zone and on the east
by the Pocuro fault zone. Between these two fault
zones is an area 20-30 km wide in which volcanic
rocks of Late Cretaceous age are flat lying to gently
folded and block faulted. In places, pipelike stocks
of andesitic to dioritic igneous rock intrude the area.
The Pocuro fault zone is a prominent lineament
that marks the eastern limit of the Central Valley at
Santiago and has been traced northward for 150 km.
It may, however, have a mappable length of more
than 1200 km. Vertical displacement downward to
the west has been measured near Los Andes to be at
least 2000 m. Consequently, this fault zone may
rank among the major faults of the world.
The Andean structural province is subdivided
into the Las Ollas and Juncal subprovinces. The
Las Ollas is a mountainous front range that lies east
of the Pocuro fault and extends eastward for about
25 km. Upper Cretaceous volcanic strata are gently
warped into broad, open, north-trending folds, with
local sharp flexures and faulted mainly by normal
faults. The eastern part of the subprovince is cut by
a narrow belt of Tertiary plutonic rocks, some of
which are associated with porphyry copper deposits. The Juncal structural subprovince extends
eastward into Argentina. It is typified by close,
overturned folding, vertical bedding, and thrust
faulting. In general, structural deformation increases in intensity from west to east, with imbricate
overthrusting to the east in Argentina.
Present evidence indicates that major graben
formation began during early Tertiary (preMiocene) time as a result of tensional stress developed by strain release of earlier compressional
forces that folded the Andes. Post-Miocene uplift
of the central Andean region renewed tensional
stress and opened deep-seated, north-striking
fractures and reinitiated volcanism. Most of the
active volcanos of Chile may be aligned along such
fractures.
CONTENTS
Introduction
Location
Purpose and previous work
Acknowledgments
Geomorphology
Sedimentary and igneous rocks
Structural geology
General statement
Coastal Cordillera structural province
Los Angeles fault zone
Central Valley graben
Pocuro fault zone
. . . .
652
652
652
653
654
654
656
656
657
658
658
659
Andean Cordillera structural province . . . .
660
Geologic history
660
Conclusions
662
Postscript: Chilean earthquake of March 28, 1965 662
References cited
663
Figure
1. Index map showing location of Aconcagua
Province and vicinity, Chile
652
2. Correlation chart of stratigraphic nomenclature 655
3. Map showing major structural divisions of Aconcagua Province, Chile
657
Geological Society of America Bulletin, v. 76, p. 651-664, 3 figs., 3 pis., June 1965
651
652
CARTER AND AGUIRRE LE B.-ACONCAGUA PROVINCE, CHILE
Facing
Plate
Generalized geologic map and cross section of
Aconcagua Province and vicinity, showing
structural relationship to the Central Valley graben, Chile
...........
651
INTRODUCTION
Location
Chile extends along the western flank of the
Andes Mountains for a distance of more than
4000 km. The country is divided from north to
south into three geographic and distinctly
different climatic areas: (1) a northern, extremely arid desert region; (2) a central region,
with a pleasant Mediterranean climate that
attracts most of Chile's population, and (3) a
southern lake and fjord region with abundant
rainfall.
Aconcagua Province lies between lat. 32°00'
and 33°10'S. and long. 70°00'and71°40'W.in
the central region at the northern end of the
Central Valley (Fig. 1). It covers an area of
approximately 13,000 sq km. The southern
boundary of the province is about 40 km north
of Santiago and is marked at the Cuesta de
Chacabuco by a range of hills that limits the
northern end of Chile's Central Valley.
The province includes the southernmost of
the transverse valleys and mountain ranges that
lie between the Copiapo River on the north
and the Cuesta de Chacabuco on the south.
These narrow valleys and sharp ridges extend
from the drainage divide of the Andes to the
Pacific Ocean. The major drainages include the
fertile farming valleys of the Aconcagua, Ligua,
and Petorca rivers. Of these, only the Aconcagua River has its origin at the drainage divide
that marks the Chile-Argentina frontier. The
Ligua and Petorca rivers extend into the high
Andes but are robbed of the major part of the
frontier watershed by the Choapa River and
its tributaries which flow northwestward
through Coquimbo Province on the north, and
by the Putaendo River on the south, a tributary to the Aconcagua River. The Aconcagua
Valley and the province are bounded on the
south by a rugged mountain range that includes
the Cuesta de Chacabuco (1320 m above sea
level). From this narrow pass, one can look into
Chile's vast Central Valley to the south and the
Aconcagua transverse valley to the north.
Purpose and Previous Wor\
Chile's Central Valley is a long, narrow de-
2. Structural and geomorphic features of Coastal
Cordillera and Central Valley, Chile .
3. Structural and geomorphic features of the
High Andes, Chile
662
663
pression that separates the Andes Mountains
from the Coastal Cordillera and contains most
of her population and agricultural wealth. A
few geologists have written about this prominent geologic feature, but none until now has
mapped at scales adequate for determining the
mechanics of its structural development. This
200 KILOMETERS
Figure 1. Index map showing location of Aconcagua Province and vicinity, Chile
paper, a compilation of recent geologic mapping
and interpretation of aerial photographs in
Aconcagua Province and adjacent areas of
north-central Chile, intends to show that the
Central Valley is a graben of considerable
magnitude. It is hoped that this report, although preliminary and limited in scope, will
stimulate further work on this significant
structural feature. Several areas, suggested for
future study, may provide valuable additional
information.
Domeyko (1903, p. 133-134) described the
Central Valley as being more than 900 km
long, extending from Chacabuco (40 km north
of Santiago) south to the Gulf of Ancud. He
also noted that the average width of the valley
is about 50 km and that the land surface becomes progressively lower to the south. Santiago, for example, is about 500 m above sea
level, whereas the valley plain just north of
Puerto Montt is less than 100 m.
INTRODUCTION
653
Felsch (unpub. data, 1936) and later Briig- series of volcanic debris of probable Quaternary
gen (1950) described, in general terms, the age.
structural components of the valley, mainly on
Only recently, however, with the work of
the basis of geomorphic evidence in the vicinity Thomas (1958), Aguirre Le B. (1960), Klohn
of Santiago. Both described the valley as being (1960), and Carter and Aliste (unpub. data) has
bounded by two lines of north-striking faults the stratigraphy of central Chile been clearly
that separate the Coastal Cordillera from the enough defined and field mapping been done at
high Andes. Bruggen (p. 79) mentioned that a scale at which stratigraphic and structural
one fault extends along the base of Cerro San relationships could be measured and interpreted
Ramon, immediately east of Santiago. The with reasonable accuracy.
fault is ". . . marked by large triangular rock
About 50 per cent of the geology within
facets that face the valley. These facets are cut the area of this report has been mapped by
by youthful streams with hanging valleys and reconnaissance methods and published at scales
which have deposited large piedmont deposits of 1:150,000 (Thomas, 1958) and 1:100,000
of fanglomerate along the valley margin." He (Aguirre Le B., 1960); an additional 25 per cent
pointed out that many springs, some of which has been completed on similar scales in undischarge warm waters, are aligned on or near published reports. Of this, about 20 per cent
the eastern fault. He also described, in con- was mapped between 1958 and 1962 in greater
siderable detail, a second fault that he con- detail at a scale of 1:50,000 by Carter and Aliste
sidered to have formed the western margin of in unpublished reports on the Melon, La Ligua,
the valley, and postulated several cross faults Nilhue, and San Lorenzo quadrangles of
to explain the presence of the blocklike island Aconcagua Province. Although much work
hills of bedrock that project through the allu- still remains to be done in this area, the data
vial plain and are scattered throughout the collected to date provide background for future
valley. Bruggen (p. 80) even suggested that the studies.
major graben structure may extend as far north
This report, therefore, synthesizes the geoas Arica, the northernmost city of Chile.
logic data obtained from three separate mapping
Gerth (1955, p. 223-230) agreed that the projects by Thomas, Aguirre Le B., and Carter
Central Valley was a rift valley and suggested and Aliste in Aconcagua Province. It places
that faulting was closely related to the present emphasis on the structural fabric of the area.
volcanic belts of the Andean Cordillera. One These data help confirm Briiggen's (1950)
block diagram (No. 7) included a geologic hypothesis that the Central Valley of Chile is a
section along lat. 37° S. (near Chilian) through rift valley of tectonic origin. They also indicate
Chile and Argentina in which Gerth showed that the structural development was more comtrough faulting of the Central Valley south of plex than simple doming or stretching of the
the area herein described.
earth's crust as implied by most of the classic
Munoz Cristi (1956) described the threefold examples of and experiments in graben tecphysiographic division of the central region and tonics. This report also shows that the imporreviewed the geology of Chile as it was known tant geologic structures can be traced northat that time. The reader should compare the ward even though the more obvious geomorphic
geologic section accompanying the Munoz features are not expressed. Finally, it indicates
Cristi report with that of this report (PI. 1) to several key areas where additional geologic
be aware of the differences in accumulated in- studies would contribute useful information.
formation and interpretation as a result of more
ACKNOWLEDGMENTS
recent mapping.
The geologic field investigations on which
Lomnitz (1959) made gravity profiles of the
Central Valley near Chilian (370 km south of this report is based were undertaken jointly by
Santiago) and found that the alluvial fill of the the Institute de Investigaciones Geologicas of
valley was thickest (2000 m) within a few Chile and the U. S. Geological Survey, under
kilometers of its western edge. From this data the auspices of the Agency for International
he deduced that the western edge is limited by a Development, U. S. Department of State.
The authors wish to take this opportunity to
fault with displacement of considerable magnitude. Here the west edge of the valley is acknowledge the helpful comments and data
marked by a large pluton of diorite. Lomnitz provided by Herbert Thomas, Nelson Aliste,
did not search for the eastern limit of the valley, and Carlos Ruiz of the Institute de Investigawhich in the Chilian area is covered by a thick ciones Geologicas and constructive criticism of
654
CARTER AND AGUIRRE LE B.—ACONCAGUA PROVINCE, CHILE
this report by Beatrice Levi and Jose Corvalan
of the Institute and Kenneth Segerstrom,
George Ericksen, and Robert Dingman of the
U. S. Geological Survey.
GEOMORPHOLOGY
The region of central and southern Chile
from Chacabuco (40 km north of Santiago; Fig.
1) south to Puerto Montt is divided naturally
into three geomorphic zones: A western zone
called the Coastal Cordillera that is marked by
relatively low, rounded hills, valley terraces,
and an old erosion surface dissected by a rejuvenated drainage system that indicates late
Tertiary to Recent uplift. The western zone
extends from the Pacific Coast inland for about
50-60 km and is bounded on the east by a
Central Valley zone that forms a long (900 km),
narrow (20-50 km) plain covered by alluvium
and drained by braided and meandering streams.
The Andean Cordillera forms the east edge
of the Central Valley and extends into neighboring Argentina. A drainage divide marks the
frontier of the two countries. The Andean
geomorphic zone is marked by sharp glaciated
peaks, volcanic cones, and a youthful drainage
pattern. The geomorphology of the western
half of the Andes suggests, in many places, that
the area was once a gently inclined peneplain,
the surface of which ranged between 2200 and
3000 m above sea level. Rounded, peaks more
than 4000 m in altitude, with smooth slopes
are scattered throughout the zone and may have
formed mesa-like promontories above the
peneplain. In the eastern part of the Andes
these "mesetas" have been glaciated, and the
smooth slopes are cut off sharply by U-shaped
valleys and bowl-shaped cirques. The average
elevation of peaks along the frontier is about
5000 m above sea level. Mt. Aconcagua (7021
m above sea level), in nearby Argentina, towers
above them like a monadnock.
North of the Central Valley in Aconcagua
Province, the three geomorphic divisions
largely disappear, and in places it appears that
the Andes extend westward nearly to the sea,
for some peaks of the Coastal Cordillera (e.g.,
Cerro Tabaco, Cerro Chache) reach elevations
of nearly 2500 m, and are connected to the high
Andes by rugged transverse ridges. Farther
north central valleys and coastal ranges reappear and, in places, are extensive.
SEDIMENTARY AND IGNEOUS ROCKS
Aconcagua Province is underlain by thick
sequences of effusive igneous rocks interstrati-
fied with marine and continental sediments
ranging in age from Triassic to Miocene (PL 1).
Rocks of marine origin are restricted primarily
to the Jurassic and Early Cretaceous and crop
out in the western slope of the Coastal Cordillera and in the high Andes along the frontier
with Argentina. Great thicknesses of volcanic
rock, largely of andesitic composition, are
interstratified with continental sedimentary
rocks of Late Cretaceous age and crop out in the
intervening area that forms a zone about 95 km
wide. The stratigraphy of Aconcagua Province
has been described by Thomas (1958) and
somewhat modified by Carter and others (1961)
in the Coastal Cordillera and by Aguirre Le B.
(1960) in the Andes. This is summarized in the
accompanying stratigraphic chart, Figure 2.
The oldest rocks of the area crop out at
isolated localities near the coast and consist of
contorted, highly metamorphosed mica schists,
gneisses, and phyllites that are considered to be
Paleozoic or older in age. They are unconformably overlain by less folded and weakly
metamorphosed red metavolcanic and metasedimentary rocks of the Triassic La Ligua
Formation (Thomas, 1958). The composition
of the La Ligua Formation and its relationship
to the underlying rocks are so similar to the
Triassic rocks east of Cerro Aconcagua in
Argentina (Harrington, 1956), that they are
considered to be correlative. A Triassic age for
the La Ligua Formation is further supported by
the presence of the Quebrada del Pobre Formation of Early Jurassic age, which unconformably overlies it on the east.
The Quebrada del Pobre Formation consists
mainly of dark-gray shale and limestone with
abundant marine fauna and thin, but distinctive, quartz-sandstone and conglomerate
layers. The strata, less deformed than the underlying rocks, dip gently to moderately eastward and are conformably overlain by the
Ajial and Melon Formations, also of Jurassic
age.
The Ajial Formation is composed mainly of
lavas, breccias, and indurated tuffs interbedded
with thin lenses of marine siltstone and limestone containing fossils of middle Bajocian age
(Thomas, 1958, p. 33). The contact with the
overlying Melon Formation is conformable
and may be gradational or, in part, interfingering.
The Melon Formation is divided into two
members: the Nogales Member, composed
mainly of marine limestone, and the Horqueta
Member, composed almost entirely of volcanic
SEDIMENTARY AND IGNEOUS ROCKS
breccia. Fauna of the Nogales Member indicate
that it was deposited during the Bajocian Stage
of the Jurassic Period. Although fossils are
absent in the Horqueta Member, it is also
considered to be of Middle Jurassic age because
Lagunilla Formation to the Late Jurassic
Period, largely because of its lithologic similarities to rocks of that age in Argentina. He
divided the Lagunilla Formation into three
parts: a lower member consisting of reddish-
COASTAL CORDILLERA
Carter, Perez,
and Aliste (1961)
Thomas (1958)
V
CC
CORDILLERA
Aguirre (1960)
A lluvium
1—
o
ANDEAN
Recent
7"
CC
§z
655
Terrace gravels
Terrace gravels
Horcon
Horcon
Pleistocene
*
Pliocene
CC
Miocene
p
CT
OMgocene
1—
Eocene
Paleocene
Lo Valle
Farellones
Las Chilcas
Las Chilcas
Abanico
Veta Negra
Cerro Morado
Veta Negra
Cristo Redentor
Lo Valle
to
o
o
a
a
D
1—
CC
o
d
" i""l'/K
h.
Lo Prado
O
*
o
Lo Prado
Pachacama
Patagua
H'3'"*
a
D
0
Jft
£
D
(/)
San Jose
"""i"'"" '<"!J>
^s*'
M
0
trt
UJ
HiatuS
Lagunilla
^*-
^
•a
Pachacama
E
Melon
Melon
S;
Ajial
Ajial
°
Queb. del Pobre
Queb. del Pobre
La Ligua
La Ligua
E
i-
Figure 2. Correlation chart of stratigraphic nomenclature used in the Coastal and Andean cordilleras of Aconcagua Province, central Chile
it underlies the Patagua Limestone which
Thomas (1958) included as a member of the
Melon Formation. However, later discovery
of Early Cretaceous fauna in the Patagua permitted removing it from the Melon Formation
and assigning it formational status (Carter and
others, 1961).
Late Jurassic strata are apparently absent in
the Coastal Cordillera, and a hiatus represented
by unconformity marks the contact with Early
Cretaceous rocks (Carter, 1963). In the Andes,
however, Aguirre Le B. (1960) assigned the
brown conglomerate, sandstone, and gray
dolomitic limestone; a middle member consisting of gypsum as much as 100 m thick; and
an upper member of sandstone, andesite, and
red siltstone. Correlation of the middle gypsum
member with the Auquilcoian anhydrite of
Groeber (Harrington, 1956, p. 144) is supported
mainly by its lithologic similarity and stratigraphic position below the Early Cretaceous
San Jose Formation.
The most distinctive and widespread rock
units, for correlative purposes, are fossiliferous
656
CARTER AND AGUIRRE LE E.—ACONCAGUA PROVINCE, CHILE
marine limestones of Early Cretaceous age: the
Patagua and Lo Prado Formations (Thomas,
1958; Carter and others, 1961) of the Coastal
Cordillera and the San Jose Formation of the
Andes (Aguirre Le B., 1960). Certain other
distinctive units, such as conglomerate and
fresh-water limestone with algal remains,
locally help distinguish the Las Chilcas and
Abanico Formations from other volcanic strata.
Such units, however, are lenticular and discontinuous, and in their absence, it is often
difficult to determine stratigraphic position.
The Lo Valle and Farellones Formations consist mainly of volcanic tuffs, breccias, and flows
of andesitic and basaltic composition. Soft,
multicolored tuffs and pink, welded rhyolitic
tuffs help facilitate local correlation and structural interpretation in places between the two
major Lower Cretaceous limestone belts.
Unconformities mark the base of the La
Ligua Formation of Triassic age, the Quebrada
del Pobre Formation, a marine limestone of
Early Jurassic age (Thomas, 1958), and the
Patagua Formation of Early Cretaceous age
(Carter, 1963), indicating that the Coastal
Cordilleran region, at least, was an unstable
area subjected to periods of uplift and erosion
at the end of the Paleozoic Era, at the end of
the Triassic Period, and in Late Jurassic time.
East of the unstable coastal area is the major
part of the Andean geosyncline, a narrow,
elongate trough whose axis probably lay at
about the center of what herein is referred to as
the Andean Cordillera structural province. The
floor of the trough probably sank continually
but more so each time the coast rose. Interlayered sedimentary rocks of continental and
marine origin bordering the trough indicate
that it was locally cut off from the Pacific
Ocean by periodic uplift of the Coastal Cordillera. Final separation of the Andean geosyncline from the Pacific came at the end of the
Early Cretaceous Period and was followed by
volcanism, erosion, and deposition of sedimentary and volcanic rocks of continental origin
that compose the Veta Negra, Las Chilcas,
Abanico, and Farellones Formations. Angular
unconformities mark the base of the two latter
formations (Aguirre Le B., 1960), and intraformational conglomerates mark disconformities within them.
The youngest sedimentary rocks of the area
are those of the Horcon Formation of Miocene
age (Thomas, 1958), flat-lying strata consisting
of poorly consolidated conglomerate, sandstone,
and shell beds that crop out 80-90 m above sea
level along the present coast of Chile. The
presence of these strata indicates that final
uplift of the Coastal Cordillera must have
taken place during late Tertiary or possibly
early Quaternary time. Although the Horcon
Formation has not been found in the Central
Valley near Santiago, strata of similar composition and identical age are known to crop out at
the southern end of the valley. This indicates
that there, at least, formation of the Central
Valley graben was pre-Miocene.
Granitic rocks, largely of Cretaceous age, are
exposed throughout the area. These can be
divided into two elongate belts. The largest
masses, composed mainly of granite, granodiorite, and diorite, crop out in the western
belt of the Coastal Cordillera cutting strata of
Triassic and younger age. Those intrusive
masses shown as not cutting Cretaceous strata
are herein considered to be of pre-Cretaceous
age. Intrusive rocks along the coast are in contact with metamorphic rocks and are considered
to be Paleozoic in age. Smaller plutons compose a narrow belt along the axis of the Andes,
intrude stratified rocks of Late Cretaceous and
Tertiary (?) age, and range in composition from
granodiorite to quartz monzonite. Some of these
masses are porphyritic and associated with the
so-called porphyry copper deposits. Age determinations by zircon and potassium-argon
methods suggest that Tertiary rocks of granitic
composition are largely Miocene and Oligocene
in age, whereas the prophyry types are Paleocene (Levi and others, 1963). The porphyritic
intrusives may have been the source of rhyolitic
sheets that comprise the upper part of the
Farellones Formation.
STRUCTURAL GEOLOGY
General Statement
Aconcagua Province is divided into three
structural provinces: the Coastal Cordillera,
the Central Valley graben, and the Andean
Cordillera (Fig. 3). These structural provinces
join and are named after the three geomorphic
provinces that have been described to the
south. Bounding the Central Valley are two
major fault zones herein referred to as the Los
Angeles and Pocuro fault zones. These zones
form the margins of a down-thrown block and
prove that the Central Valley is a graben.
Differences between the faults suggest that
graben development may have been somewhat
different from the classic grabens of the world
and from those developed in the experiments of
657
STRUCTURAL GEOLOGY
In general, the strata strike northward and
dip homoclinally eastward between 20° and 45°.
Near the margins of the younger intrusive
masses, moderate folding and faulting locally
disrupt the homocline. Stratified rocks also
occur as roof pendants and xenoliths within the
margins of intrusive masses. The roof pendants
Hans Cloos (De Sitter, 1956) and others. Each
of the structural provinces will be discussed in
order from west to east.
Coastal Cordillera Structural Province
The Coastal Cordillera structural province
occupies an area roughly 55 km wide that is
1
32°00'
•^
\.
1
J
J
^"
,^-
r"
{
^s,^
^-k
Petorca
S I
"*• \
£ N
• Sobrante
0^
\
1
i
\
,.
Laiiigua •
I
»Co. Chache
^
2333m
x^
2
0
"3
c
^—' \
<
•&
CORDILLE RA
STRUCTU ?AL
£>
PROVINC E
\
*
\
Las Ollas
m
\
r
\
Putaendo
\
Subprovince
Juncal
San Felipe
i
Subprovince \
^^
\
^•^
\ Los Andes
33'OC'
^
\
ANDEA
§
1
/\-X
/
^
^rf
1
K
^
GRABEN
\
\
"^
CENTRAL
c
•Cabildo
T
^
»
\1
STRUCTURAL PROVINCE
\
L
N
COASTAL CORDILLERA
VALLEY
0
/
£•
/
0 /V
1
*Portillo ^
1 Cuestade
Chacabuco\
_
0
100
200 KM
^~^ \
^
y
\^' i
).
\
Figure 3. Map showing major structural divisions of Aconcagua Province, Chile. See Figure 1 for
location.
bounded on the west by the Pacific Ocean and
on the east by the Los Angeles fault zone (Fig.
3). The area is underlain by volcanic and sedimentary rocks which have been intruded by
large, irregular masses of plutonic rocks mainly
of dioritic composition. In general, the oldest
stratified rocks lie along the coast, and progressively younger beds crop out to the east.
These strata range from Paleozoic or Triassic(P)
to Late Cretaceous in age. Most of the plutonic rocks are considered to be of Jurassic and
Cretaceous age. Along the coast some of the
intrusive rocks may be as old as Paleozoic
(Ruiz F. and others, 1960; 1961).
are generally located along the axis of the
younger intrusive belt. Xenoliths, on the other
hand, are small, generally rounded, and found
in large, pocketlike concentrations in border
zones near the contact with the country rock.
Most are composed of dark-gray, porphyritic
andesite that is characteristic of the Lo Prado
and Veta Negra Formations that bound the
intrusive; movement of the xenoliths, therefore, was probably not very great.
Although faults and fractures are abundant
in the area, most are high-angle, minor structural features that can be traced from a few
hundred meters to a few kilometers. The domi-
658
CARTER AND AGUIRRE LE B.—ACONCAGUA PROVINCE, CHILE
nant faults bear northward, but many strike
northeastward or northwestward. Several moderately dipping (45°) normal faults border and
dip away from the western edge of the eastern
intrusive mass. Two are adjacent to and show
downward displacement from two small
cupolas of granodiorite, and one at the Palqui
mine clearly shows that intrusion terminated
at the fault plane without penetrating it.
Lamprophyre dikes, however, extend from the
intrusive and cut the fault plane and the copper-bearing ore body above it. Displacements
are generally small, ranging from a few centimeters to a few tens of meters. The dominant
joints strike northeast and cut both the intrusive masses and surrounding country rock.
Faults are known to bound the Coastal
Cordillera on the east and probably also bound
it on the west. A submarine shelf extends approximately 22 km west of the Chilean coast,
where it abruptly steepens into a narrow,
elongate trench that parallels the coast. Soundings of the Chilean trench show depths ranging
from 4000 to more than 6000 m. Such a trench
must have been developed by erogenic processes, and structural features similar to those
described in central Chile are most likely
present. The Coastal Cordillera, therefore, is
probably a horst intruded by granitic rocks and
flanked by normal faults that separate it from
two grabens, a deep ocean trench on the west
and the Central Valley on the east.
Los Angeles Fault Zone
The Los Angeles fault zone marks the eastern
margin of the Coastal Cordillera and is herein
named by Carter after the Quebrada Los
Angeles (PI. 1) in central Aconcagua Province,
where the fault zone and its structural relationships were first recognized and are clearly
demonstrated. The fault zone is an irregular
and poorly defined belt a few kilometers wide,
in which the dominant structural features are
high-angle, semiparallel, rectilinear, arcuate
faults, which in most places show displacements
downward to the east. In places, the arc of the
curved faults is concave to the west, and radii
of the arcs point toward intrusive cupolas of the
Coastal Cordillera. Rotational movement is
indicated along some of these faults, for in uplifted areas they may be displaced a few
meters, whereas in collapsed areas they may
be vertically displaced as much as 500 m. A
notable example is found in the fault which
cuts Cerro Negro, east of the Cerro Negro
mine (PI. 2, fig. 1). In some places, the cumula-
tive displacement of several parallel faults may
be as much as 1000 m. Many of the major faults
split into several divergent faults or "splays."
These faults are numerous, of relatively short
extent and little displacement, and lie near the
borders of the intrusive masses. They apparently originated as tension fractures above
and along the margins of intrusive masses as
magmatic injection took place. Although individual faults probably do not extend to
great depths, their combined cumulative displacement may, in places, be large. Between
the major faults and cupolas of the intrusive
masses are minor, moderately dipping cross
faults, surrounding small blocks of gently tilted
strata that together form a complex of small
horsts and grabens. This is well displayed in the
vicinity of the Cerro Negro mines in Quebrada
Pitipeumo (PI. 1), where fresh-water limestones
and copper-bearing tuffaceous strata serve as
distinctive marker beds on which measurements are based. Northeast-trending faults
intersect northwest-trending cross faults that
are clearly of gravitational origin. They border
gently tilted fault blocks whose strata strike
northwest and dip gently south, in sharp contrast to the north-striking, east-dipping, homoclinal structure of the Coastal Cordillera elsewhere.
Central Valley Graben
The Central Valley graben is named after
Chile's Central Valley, a clearly defined geomorphic feature that extends from the Santiago
area into the Gulf of Ancud, 900 km to the
south. The structural geology of the southern
end of the Central Valley has been briefly
described by St. Amand (1961). Near the
southern boundary of Aconcagua Province the
geomorphic features of the Central Valley die
out, but its structural components have been
traced northward from the Santiago region into
the transverse mountain ranges and to the north
boundary of the province. The graben is
bounded on the west by the Los Angeles fault
zone and on the east by the Pocuro fault. Together these delimit a north-trending elongate
area about 20 km wide. In the Los Andes-San
Felipe basin, due north of the Central Valley,
strata of the youngest Cretaceous volcanic
formations (Lo Valle Formation of Thomas,
1958; Farellones Formation of Aguirre Le B.,
1960) crop out and are flat lying to gently dipping and moderately folded. Fault blocks
within the graben dip moderately to strongly
to the east. In places the strata are intruded by
STRUCTURAL GEOLOGY
small stocks of andesite, diorite, and basalt of
Tertiary(P) age (Aguirre Le B., 1960) (PL 2,
fig. 2). Faults of short extent and small displacement are numerous and diverse in
orientation. The majority, however, are gravity
faults oriented parallel or subparallel to the
major north-south tectonic fabric of the country.
Pocuro Fault Zone
The Pocuro fault forms the east side of the
Central Valley graben and separates it from
the Andean Cordillera structural province. It is
herein named for the Estero Pocuro, southeast
of Los Andes, where Aguirre Le B. (1960) first
obtained evidence for the fault. At Estero
Pocuro, bedrock is cut by a wide, well-defined
gouge zone in which mylonite shows microcataclastic texture with bent and broken
crystals of plagioclase feldspar. The trace of the
fault zone indicates that it is vertical or dips
steeply to the west.
In the Santiago region of the Central Valley
this prominent lineament is represented by the
west slope of Cerro San Ramon and adjacent
hills east of Santiago, which, in places, form a
steep escarpment; broad piedmont fans extend
westward from its base. The escarpment disappears north of Santiago near Arrayan and Lo
Curro, but the fault zone continues northward
and in marked by differences in the dip of
strata on either side of the fault, light-colored
gouge zones, and alignment or sharp bends in
the courses of subsequent stream valleys (PI. 3,
fig. 1).
Where well exposed, the fault zone is several
meters to tens of meters wide and is marked by
a light-colored alteration zone composed of
pulverized rock or gouge. In the area between
Chacabuco and the Sobrante fork of the Petorca
River (a distance of 85 km to the north), the
light-colored fault zone is accentuated by a
marked color change on either side of the fault,
which represents a difference in lithology and
formation. Cropping out on the west side,
within the graben, are pale-reddish-brown to
yellowish-brown and pale-green volcanic rocks
equivalent to Thomas' (1958) Lo Valle Formation, and Aguirre Le B.'s (1960) Guanaco or
middle member of the Farellones Formation.
On the east side, the outcrops are darker and are
composed dominantly of reddish-brown continental sedimentary rocks interbedded with
volcanic strata that are included in the older
Las Chilcas Formation (Thomas, 1958) or the
equivalent Abanico Formation of Aguirre Le
659
B. (1960). On the basis of stratigraphic correlation, Aguirre Le B. (1960) estimated that
vertical displacement along the fault is downward to the west on the order of 2000 m near
the city of Los Andes. The cross section,
Plate 1, suggests that it may be as much as
8000 m in some places. To the north in the
Sobrante area, field evidence of faulting is
nearly the same, and similar amounts of offset
are suspected. Vertical displacement is probably
different from place to place, individual blocks
within the graben being displaced more or less
than adjacent blocks.
The longitudinal extent of the Pocuro fault
zone is only partly known. As shown in this
paper, it is known to extend from Santiago
northward 150 km to the northern boundary
of Aconcagua Province. It undoubtedly continues northward and may join a major fault
mapped by Pedro Dedios (unpub. data)
near Vicuna, Coquimbo Province, or it may
arc gently westward and enter the Pacific
Ocean near the Bay of Tongoy, approximately
370 km north of Santiago. To the south, Klohn
(1960) shows this or a similar fault that extends
at least to the southern boundary of O'Higgins
Province, 210 km south of Santiago. It could
well extend to the Gulf of Ancud, 900 km south
of Santiago. If these are one and the same fault,
the known length ranges from 150 to 270 km,
and the possible length may be as much as
1270 km, making it comparable with other
major faults of the world. The fault plane,
therefore, may well extend downward through
the mantle.
The possibility exists that lateral movement
took place along the Pocuro fault, for this
fault has many of the characteristics common to
faults, such as the San Andreas of California,
that are known to have had strike-slip movement. St. Amand (1958, p. 404) listed the
following features as characteristic of strike-slip
faulting:
(1) Consistent straightness, or smooth and
gradual curvature of the strike of the fault;
(2) The occurrence of an alluvial-filled
trough along the fault, or development of a
graben marked by normal faulting on both
sides of the fault;
(3) The presence of branch faults; and
(4) Geomorphic features such as shutter
ridges and offset streams.
More convincing evidence for lateral faulting
is, of course, measurable strike-slip offset of
intrusive masses, formations, and other distinctive rock units that have been cut by
660
CARTER AND AGUIRRE LE B.—ACONCAGUA PROVINCE, CHILE
rectilineal faults. Even more convincing are the
effects that active strike-slip faults have on
man-made structures.
Although features 1, 2 and 3 have been recognized, no further evidence of lateral movement
has yet been found in the Pocuro fault zone by
present mapping. Present evidence of great
vertical displacement is outstanding and overshadows any suggestion of horizontal movement.
Major displacement is known to have taken
place during the Tertiary Period, but the
Pocuro fault zone may be the surface expression
of a zone of weakness in basement rock which
has been active since Paleozoic time and instrumental in the crustal downwarping that
formed the Andean geosyncline. This is suggested by its proximity and parallelism to the
axis of the geosyncline and differences in
stratigraphic position of unconformities of the
Jurassic Period found in the Coastal and Andean
Cordilleras.
Andean Cordillera Structural Province
The Andean Cordillera structural province is
limited to that area which lies east of the Pocuro
fault and extends eastward into Argentina. In
central Chile this province can be roughly
subdivided into two parallel structural subprovinces, herein designated the Las Ollas
subprovince on the west and the Juncal subprovince on the east.
LAS OLLAS SUBPROVINCE: The Las Ollas
subprovince is a narrow zone which extends
from the Pocuro fault eastward for about 35
km and constitutes the western front of the
Andean Cordillera. Topographically the mountains of this subprovince are somewhat lower
than those found in the high Andes of the
frontier or in the Juncal subprovince, to the
east.
The geologic structures are simple, consisting
mainly of broad, open folds, sharp flexures of
short lateral extent, and gravity faults. The
major structural axes strike to the north, and
the majority of the faults are subparallel to the
Pocuro fault, with normal displacement downward to the west.
The stratified rocks exposed in the Las Ollas
subprovince are Late Cretaceous in age and
have been intruded by irregular masses of
plutonic rocks. The majority of these plutons
are found in a long, narrow belt, 10-20 km
wide, in the center of the subprovince. Around
these intrusive masses the strata are either
locally upturned or flat lying and strongly
faulted for short distances. It is within this
intrusive belt that two of Chile's "porphyry"
copper deposits are located (Rio Blanco and
Disputada de Las Condes). Projection of the
belt both north and south indicates that similar,
better-known deposits are in the same belt
(e.g., El Teniente, Porterillos, El Salvador,
Chuquicamata) (Ruiz F. and Ericksen, 1962).
Age determinations of zircon (lead-alpha
method) contained in two samples of granodiorite from the Los Chacayes batholith, exposed south of the Aconcagua River, indicate
that these rocks are Tertiary in age, (Levi and
others, 1963). Samples listed as Chile 16 and
17 by Levi and others gave dates of 30 and 50
million years ( ± 2 0 m.y.), respectively, suggesting that the rocks were intruded during or
prior to the Miocene Epoch.
JUNCAL SUBPROVINCE: The Juncal subprovince is marked by high, jagged peaks of 4000 m
and more, vertical cliffs, glacial cirques, snowfields and U-shaped, moraine-filled, and erraticstudded valleys. These join to form a drainage
divide that marks the frontier with Argentina.
Structural deformation is more complex than
to the west and is characterized by both moderate and tight folds, some of which are asymmetrical and overturned. In general, the fold
axes bear to the north, indicating that compressional stress was oriented east-west. Upthrusting appears to be an important factor in
the frontier zone, for high-angle to vertically
dipping beds of Jurassic and Lower Cretaceous
strata have been mapped in eastern Aconcagua
Province by Aguirre Le B. (1960) and by
Klohn (1960) in frontier areas to the south.
Imbricate thrust faulting has been described by
Harrington (1956) in the area east of Mt. Aconcagua in Argentina. There Jurassic limestone
strata have been thrust eastward against a solid
plutonic mass of Paleozoic age capped by thin
layers of Triassic sediments. This relationship
suggests that compressive stress came mainly
from the west. An equal opposite stress was
offered by the stable plutonic mass and resulted
in upthrusting, overthrusting, and tight folding
in the Juncal subprovince.
GEOLOGIC HISTORY
The historical record of geologic development in central Chile is incomplete. The
presence of marine limestones of Early Jurassic
age in the Quebrada del Pobre Formation
(Thomas, 1958) indicates that the Andean
geosyncline was locally initiated near the beginning of the Mesozoic Era. Four cycles of
marine invasion, retreat, and volcanism of the
GEOLOGIC HISTORY
Jurassic and Early Cretaceous are recorded by
the stratigraphic sequence as it is known today.
Major uplift and erosion apparently took place
in the Coastal Cordillera during Late Jurassic
time, for Upper Jurassic strata are entirely
absent. To the east, in the area now occupied by
the Andes, a thick sequence of limestone,
sandstone, conglomerate, and gypsum, principally of terrigenous origin (Aguirre Le B.,
1960, p. 16) was deposited during the Late
Jurassic. The Cretaceous period brought marine
invasion and deposition of fossil-bearing sediments throughout the area.
At the end of Early Cretaceous time the
southern end of the elongate Andean geosyncline was cut off from the rest of the Pacific
basin by mild, gradual epeirogenic uplift
accompanied by volcanism. The geosyncline
became a large catchment basin for volcanic
rocks, which may have been expelled as sheet
flows from fault zones along its margins or along
its axis. During this period a thick (3000-m)
series of andesite porphyry lava flows (Veta
Negra Formation) was followed by a thick
series of effusive tuffs and flow-breccias (Cerro
Morado Formation).
Deepening of the basin, with resultant tilting
of existing strata and uplift of the Coastal
Cordillera, is indicated by conglomerates in the
Las Chilcas and Abanico Formations of Late
Cretaceous age, which record a new erosional
cycle and, in general, dip more gently to the
east than do the older strata. Angular unconformity is not always clearly demonstrated,
however, for the basal contact is not everywhere clearly marked. This is largely the result
of the fact that the upper part of the Cerro
Morado Formation is composed of poorly
bedded, blocky tuffaceous flow breccias that
interfinger with and are overlain by cobble
and boulder conglomerates which are composed
of the same tuffaceous rock, and which form
the base of the Las Chilcas and Abanico Formations. Thick lenses of such conglomerate
wedge out a few kilometers from where thicknesses of 500-1000 m of conglomerate can be
measured. The upper part of the Las Chilcas
and Abanico Formations is composed largely
of conglomerate, sandstone, siltstone, and
fresh-water limestone, all interlayered with
volcanic rocks, largely lavas and tuffs, indicating periodic volcanism and quiescence with
erosion.
Renewed and more intensive volcanic
activity at the close of the Cretaceous Period
resulted in deposition of the Lo Valle and
661
Farellones Formations, the youngest of the
folded volcanic rocks. This renewed activity
apparently heralded the forthcoming Andean
orogeny that folded and faulted the strata as
they are seen today.
Orogenic processes that formed the Andes
Mountains and the Coastal Cordillera of central
Chile appear to have begun during late Early
Cretaceous time and extended into Quaternary
time. Both compressional and tensional stresses
were significant in structural development.
Uplift, caused either by epeirogeny, intrusion, or both, along the Coastal Cordillera,
developed strong vertical stress as well as
horizontal eastward stress by lateral expansion.
The uplifted stratified rocks tilted eastward in
the coastal region. An equal, opposite horizontal
stress was created by the stable landmass to the
east in central Argentina. Thus, an east-west
oriented compressional force was developed
that resulted in folding of the intervening
rocks. North-trending overturned folds, upthrusts, and overthrusts to the east along the
frontier and in Argentina suggest that the
major horizontal stress came from the west.
Compression stopped at the end of the intrusive period. With this release, tensional
forces developed along the eastern margin of
the intruded area, forming a zone of weakness
that resulted in the Pocuro fault zone, the
Central Valley graben, and the collapse structures of the Los Angeles fault zones. The faulting is presently believed to have begun during
the early Tertiary (Paleogene), for Miocene
marine strata crop out on the east coast of the
Island of Chiloe and at the southern end of the
Central Valley. Although no such direct evidence has been found in the Central Valley
near Santiago, two lines of evidence tend to
show agreement with this hypothesis. First,
lead-alpha age determinations of the intrusive
rocks in the Las Ollas subprovince of the Andes
indicate that intrusion took place prior to or
during Miocene time. In addition, Darwin
(1845) mentioned and later Thomas (1958)
described the Horcon Formation of Miocene
age which crops out to the west along the coast
north of Concon and has a measurable thickness
of 80-90 m. This marine formation indicates
post-Miocene uplift of the Coastal Cordillera
of nearly 100 m above the present sea level.
Whether or not volcanic activity was related to formation of the Central Valley graben
is still not clear. In central Chile, Tertiary to
Recent volcanic evidence is mainly concentrated along the frontier with Argentina.
662
CARTER AND AGUIRRE LE B.—ACONCAGUA PROVINCE, CHILE
The only volcanoes included in the area of this
study are Tupungato and Tupungatito, which
lie 70 km east of Santiago. The latter still emits
fumarolic vapor from its crater. These volcanoes are the northernmost of an extensive
chain that parallels the longitudinal valley of
Chile on the east. Within the Central Valley
graben near Chacabuco are Alto del Huechiin
(PI. 2, fig. 2) and several other pluglike masses
of andesitic composition that are believed to
represent volcanic necks (Thomas, 1958, p. 74).
The Huechiin and neighboring pipes intrude
strata of Cretaceous and Tertiary (?) age and
are, therefore, younger. Their presence suggests
that faults and fractures in the Central Valley
are deep seated and once connected to a magma
chamber below the earth's crust. Some volcanoes of the high Andes, like Cerro Tupungato
(PI. 3, fig. 2), are glaciated and still have icefilled cirques near their crests; and lava flows
extending from them are interstratified with
morainal material in U-shaped valleys. Others,
like Volcan Tupungatito, are clearly younger,
for they are superposed on the older volcanoes
and mask the underlying glaciated terrain.
Near the southern part of the Central Valley,
active volcanoes are probably aligned along a
fault that forms the eastern limit of the valley
(Klohn, 1954, written communication).
Gerth (1955) suggested close volcanic relationship for all the rift zones of the Andean
Cordillera. The authors are inclined to agree
with Gerth, for the present evidence indicates
that the Central Valley graben and the volcanic
belt of the frontier are closely related in geologic time and undoubtedly formed under
similar stress conditions. Both appear to have
formed after compression and folding, probably
as a result of strain release at the end of intrusion and uplift along the Coastal Cordillera.
The tensional forces thus developed were apparently deep seated and caused the formation
of faults, such as the Pocuro and those along
the frontier that extended through the earth's
crust into the molten magma chamber below.
The Central Valley dropped and major eruption took place both within the Central Valley
graben and along the Juncal subprovince where
the effects of compressional stress had been the
greatest.
valley northward through Aconcagua Province,
even though their geomorphic expression disappears. (3) The longitudinal valley is a rift herein
called the Central Valley graben. It is a major
structure bounded by the Pocuro fault and the
Andes on the east and the Los Angeles fault
zone and Coastal Cordillera on the west. (4)
The Pocuro fault is a prominent lineament along
which several thousands of meters of vertical
displacement is marked. It probably extends to
great depth. (5) The Los Angeles fault zone,
on the other hand, comprises a band of many
normal faults of short linear extent and relatively minor displacement. They are closely
associated with masses of intrusive igneous rock
on the west and are the result of uplift by intrusion. Such faults appear to be local and
shallow. (6) Blocks within the rift are mostly
tilted to the east, suggesting that rotation,
owing to differential movement, was greater on
the east than on the west margin of the graben.
Processes that formed the Central Valley
graben and the linear chain of volcanoes in the
Andean region appear to have been similar in
origin and probably occurred at about the
same time. Both appear to have formed by
tensional forces generated by strain release
following uplift by intrusion along the coast and
compression and folding of the Andes. The
presence of Miocene strata in the southern end
of the Central Valley, however, indicates that
there, at least, graben formation was preMiocene and that gentle, late Tertiary to
Recent uplift was the latest significant tectonic
movement. Most of Chile's volcanoes appear
to be younger than Miocene, but a few may be
older. Best available evidence indicates that
many of them were formed prior to the
Pleistocene Epoch. Several are younger and
intermittently active at the present time.
Further work along the Pocuro fault zone
is needed to determine its longitudinal extent
and its sense and magnitude of movement. Such
work should include studies of its location,
geomorphology, and structure in Coquimbo
Province, north of the mapped area. In addition, detailed stratigraphic studies of the
Abanico and Farellones Formations should be
extended from the Aconcagua River to the
north.
CONCLUSIONS
It has been shown that: (1) The tripartite
geomorphic division of central Chile is controlled by structural features. (2) These structural components extend from the longitudinal
POSTSCRIPT: CHILEAN
EARTHQUAKE OF MARCH 28, 1965
The area described in this report encompasses
most of the area damaged by the Chilean earthquake of March 28, 1965. On this date, at 16
Figure 1. A north-trending f a u l t of the Los Angeles fault zone passes through the valley
separating the h i l l s of Cerro Negro (mine workings, upper left) from Cerro El Penon
(flat-topped peak). Mew is east. Note that the beds dip south at the mine but are nearly
horizontal east of the f a u l t .
Figure 2. Pluglike intrusive mass of Cerro Alto de Huechun (altitude, 1352 m) rises 600 m
above the floor of the Central Valley near Chacabuco. View is east toward the Andes.
Trace of Pocuro fault coincides approximately with snow line at about 3000 m; s k y l i n e is
about 5000 m above sea level.
STRUCTURAL AND GEOMORPHIC FEATURES OF THE COASTAL
CORDILLERA AND CENTRAL VALLEY, C H I L E
CARTER AND A G U I R R F . LE B, PLATE 2
Geological Society of America B u l l e t i n , volume 76
Figure 1. Aerial view looking north along the tract of the Pocuro f a u l t near the
headwaters of the Colina River. Note that the strata west of the f a u l t (I 1 ') dip
eastward, whereas those to the east dip westward. Minor stream valleys are
aligned along the structure. Crosscutting stream gorge at center has exposed
f a u l t gouije.
Figure 2. View looking east at Mt. Tupungato (extinct volcano, elevation 6500 m)
shows glaciated volcanic terrain p a r t i a l l y masked by recent lava flows (toreground) from Volcan Tupungatito ( f u m a r o l i c volcano) to r i g h t (south) of scene
shown. Flows rest on glacial till and a l l u v i u m .
STRUCTURAL A N I ) GFOMORPHIC FKATURKS OF THE
H I G H A.NDKS, C H I L K
CARTF.R A N I ) AGl'IRRF. I.H B., PI.ATF 3
Geological Society of America B u l l e t i n , v o l u m e 76
POSTSCRIPT: CHILEAN EARTHQUAKE OF MARCH 28, 1965
hours, 33 minutes, and 14.6 seconds (GMT),
the village and railroad station at Llai Llai,
Valparaiso Province, shook violently and were
almost totally destroyed. A large tailings pond
serving the El Soldado copper mine and mill at
the town of El Cobre collapsed, and a sea of mud
swept over the town, completely destroying it
and killing about 360 residents. Extensive
property damage was reported over an area
having a radius of at least 120 km and including
both Santiago and Valparaiso, the largest
population centers of Chile. On the basis of
severity of damage, Llai Llai was initially considered to be the epicenter of the major shock,
The U. S. Coast and Geodetic Survey report
of March 30, 1965, entitled "Preliminary
Determination of Epicenters," placed the
epicenter at long. 32.4° S., lat. 71.2° W., at La
Ligua, about 50 km northwest of Llai Llai. The
663
focus of the quake was estimated to be at a
depth of 61 km, and the energy released at the
focus was between 6.4 and 7.25 in magnitude
on the Richter scale.
The damaged areas were mainly in the Coastal Cordillera structural province and Central
Valley graben of this report. As previously
noted both areas are highly faulted, but the
faults are generally considered to be of relalively shallow depth. Movement on the Pocuro
fault is considered to be the most likely source
of the energy released. Downward projection
to the west of this steeply dipping fault in
section A-A' indicates that the fault plane
should be about 50-70 km below the approximate epicenter at Catemu and Llai Llai. This
range is in general agreement with the estimated depth of focus of 61 km.
REFERENCES CITED
Aguirre Le B., Luis, 1960, Geologia de los Andes de Chile central, Provincia de Aconcagua: Chile, Inst.
Inv. Geol., Bol. no. 9, 70 p.
Briiggen, Juan, 1950, Fundamentos de la geologia de Chile: Santiago, Inst. Geog. Militar, 374 p.
Carter, W. D., 1963, Unconformity marking the Jurassic-Cretaceous boundary in the La Ligua area,
Aconcagua Province, Chile: U. S. Geol. Survey Prof. Paper 450-E, Art. 196, p. E61-E63
Carter, W. D., Perez, E., and Aliste, N., 1961, Definition and age of Patagua Formation, Province of
Aconcagua, Chile: Am. Assoc. Petroleum Geologists Bull, v. 45, p. 1892-1896
Darwin, Charles, 1845, Viaje naturalista alrededor del mundo: Libreria el Ateneo, editorial, Buenos
Aires (Translated into Spanish by J. Hubert, 1951), 297 p.
De Sitter, L. U., 1956, Structural geology: New York, McGraw-Hill Book Co., 552 p.
Domeyko, Ignacio, 1903, Jeologia. Tomo quinto: Santiago, Imprenta Cervantes, 457 p.
Gerth, Heinrich, 1955, Der Geologische Bau der sudamerikanischen Kordillere: Berlin-Nikolassee,
Gebriider Borntraeger, p. 1-253
Harrington, H. J., 1956, Argentina, p. 129-165 in Jenks, William F., Editor, Handbook of South American
geology: Geol. Soc. America Memoir 65, 378 p.
Klohn, Carlos, 1956, Estado actual del estudio geologicode la "Formacion Porfiritica": Minerales, Ano 11,
no. 55, p. 49-60
1960, Geologia de la Cordillera de los Andes de Chile central, provincias de Santiago, O'Higgins,
Colchagua y Curico: Chile, Inst. Inv. Geol., Bol. no. 8, 95 p.
Levi, B., Mehech, S., and Munizaga, F., 1963, Edades radiometricas y petrografia de granitos chilenos:
Chile, Inst. Inv. Geol., Bol. no. 12, 41 p.
Lomnitz, Cinna, 1959, Investigaciones gravimetricas en la region de Chilian: Chile, Inst. Inv. Geol., Bol.
no. 4, 19 p.
Mufioz Cristi, J., 1956, Chile, p. 187-214 in Jenks, William F., Editor, Handbook of South American
geology: Geol. Soc. America Memoir 65, 378 p.
Ruiz, F., Carlos, and Ericksen, G. E., 1962, Metallogenetic provinces of Chile, S. A.: Econ. Geology, v.
57, p. 91-106
Ruiz, Carlos, Aguirre,Luis, Corvalan, Jose, Rose, H. J., Jr., Segerstrom,Kenneth, and Stern, T. W., 1961,
Ages of batholithic intrusions of northern and central Chile: Geol. Soc. America Bull., v. 72, no. 10,
p. 1551-1560
Ruiz F., Carlos, Segerstrom, Kenneth, Aguirre Le B., Luis, Corvalan, Jose, Rose, H. J., Jr., and Stern,
T. W., 1960, Edades plomo-alfa y marco estratigrafico de granitos Chilenos: Chile, Inst. Inv. Geol.,
Bol. no. 7, 26 p.
664
CARTER AND AGUIRRE LE B.—ACONCAGUA PROVINCE, CHILE
St. Amand, Pierre, 1958, Circum Pacific orogeny, p. 403-411 in Hodgson, J. H., Editor, The mechanics of
faulting—a symposium: Canada, Dominion Observatory, v. 20, no. 2
1961, Los terremotos de Mayo, Chile, 1960: U. S. Naval Ordnance Test Sta., China Lake, Calif.,
Michelson Labs., Tech. Art. 14, 39 p.
Thomas, Herbert, 1958, Geologia de la Cordillera de la Costa entre el valle de La Ligua y la cuesta de
Barriga: Chile, Inst. Inv. Geol., Bol. no. 2, 86 p.
MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 23, 1964
PUBLICATION AUTHORIZED BY THE DIRECTOR, U. S. GEOLOGICAL SURVEY AND THE DIRECTOR, INSTITUTO
DE INVESTIGACIONES GEOLOCICAS
Downloaded from gsabulletin.gsapubs.org on April 6, 2015
Geological Society of America Bulletin
Structural Geology of Aconcagua Province and Its Relationship to the Central Valley Graben, Chile
W. D CARTER and LUIS AGUIRRE LE B
Geological Society of America Bulletin 1965;76, no. 6;651-664
doi: 10.1130/0016-7606(1965)76[651:SGOAPA]2.0.CO;2
Email alerting services
click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article
Subscribe
click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin
Permission request
click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA
Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table,
and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post
the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists
worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society.
Notes
Copyright © 1965;, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S.
government employees within the scope of their employment.