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U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE
CIENCIAS DE LA TIERRA 10° CONGRESO GEOLÓGICO CHILENO 2003
THE EAST-WEST FAULT SCARPS OF NORTHERN CHILE: TECTONIC
SIGNIFICANCE & CLIMATIC CLUES
ALLMENDINGER1, R. W., GONZALEZ2, G., YU3, J., ISACKS4, B. L.
1Dept. of Earth & Atmospheric Sciences, Cornell University,
Ithaca, New York, 14853-1504 USA; [email protected] 2Dpto de
Ciencias Geológicas, Universidad Católica del Norte, Antofagasta,
Chile; [email protected] 3Dept. of Earth & Atmospheric Sciences,
Cornell University, Ithaca, New York, 14853-1504 USA;
[email protected] 4Dept. of Earth & Atmospheric Sciences,
Cornell University, Ithaca, New York, 14853-1504 USA;
[email protected] RESUMEN Este trabajo da cuenta del significado
tectónico de escarpes de falla de orientación E-W localizados en el
antearco de los Andes Centrales en el norte de Chile. Los escarpes
representan la propagación en superficie de fallas inversas de
orientación E-W. Estas fallas se ubican exclusivamente en la
Cordillera de la Costa entre los 19º y 21,5º S, donde los Andes
Centrales y la fosa de Chile-Peru muestran una marcada concavidad
hacia el este. Rechazos verticales medi-dos sobre la base del
desplazamiento de superficies de erosión de edad oligocena-miocena
varían entre 20 y 450 m. Cerca de la Costa las fallas desplazan
terrazas de abrasión de marina de edad pleistocena tardía. Análisis
cinemático de ejes de deformación incrementales evidencian
acortamiento N-S paralelo a la orientación general de la Cordillera
de la Costa. El origen de estas fallas se interpreta como resultado
de la deformación secundaria del antearco durante la fomación del
Oroclino Boliviano. La formación de las fallas habría ocurrido en
el Oligoceno-Mioceno bajo condi-ciones menos árida que las
prevalecientes en al actualidad. La deformación de la terraza
pleistocénica sugiere que las fallas ellas se encuentran
parcialmente activas. INTRODUCTION In the forearc of the Central
Andes in northern Chile, the Coastal Cordillera is the only part of
the South American continent actually in contact with the
subducting Nazca Plate. As such, the struc-tures of the forearc
ought to reflect the first order processes that operate at the
interface between two tectonic plates. Most forearcs are
characterized by structures that strike parallel to the mar-gin,
yet in the region between 19° and 21.5°S latitude, the Coastal
Cordillera contains a suite of well defined fault scarps that trend
EW to ENE, intersecting the margin at high angles. These
morphologic features are particularly well displayed on a new, 20 m
resolution digital elevation model (DEM) of northern Chile produced
using radar interferometry (InSAR) (Yu and Isacks, 1999). The key
questions concerning these scarps are: (1) what is their kinematic
significance? (2) when did they begin to form? (3) why are they
restricted to the region between the Río Loa and the Quebrada
Camarones? and, (4) what is their tectonic significance? Due to
extremely poor exposure in most of the northern Chilean Coastal
Cordillera, previous au-thors have interpreted these features on
the basis of their morphology as normal fault scarps (González et
al., 1997; Reijs and McClay, 1998). However, we show here that they
are produced by reverse fault scarps that began to uplift in the
latest Miocene and may still be active today. We relate the scarps
to north-south compression developed on the inner arc of the
Bolivian Orocline. Although, not the main purpose of our work, the
interaction of the scarps with other geomorphic
Todas las contribuciones fueron proporcionados directamente por
los autores y su contenido es de su exclusiva responsabilidad.
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features of the Coastal Cordillera record clues to the climatic
evolution of northern Chile since the late Miocene.
Regional Setting
The Coastal Cordillera of northern Chile is formed mainly by
Jurassic-Early Cre-taceous dioritic to granodioritic plutons and
Jurassic volcanic rocks. These units form the remnants of a
Mesozoic mag-matic arc formed at the birth of the mod-ern Andes
(Coira et al., 1982; Mpodozis and Ramos, 1990; Rutland, 1971). The
magmatic arc was emplaced into an en-sialic crust formed by
Paleozoic sedimen-tary rocks and Precambrian metamor-phics. The
most important structure of the Coastal Cordillera is the Atacama
Fault System (Arabasz, 1971) that extends for more than 1000 km
between 21 and 26ºS latitude. The Atacama Fault System is trench
parallel orientated and its forma-tion is related to the end stages
of the Mesozoic magmatic arc (Scheuber and González, 1999). The
Neogene to Qua-ternary sedimentary cover of the Coastal Cordillera
records predominantly arid to hyperarid climatic conditions during
deposition (Hartley and Chong, 2002; Hartley and Jolley, 1995;
Hartley et al., 2000). Several internal basins in the Coastal
Cordillera are integrated by Oli-gocene-Miocene alluvial deposits
cov-ered locally by Mio-Pliocene evaporite deposits (Chong D. et
al., 1999). The most spectacular evaporite basins are the Salar
Grande and Salar the Llamara ba-sin. More than a decade of intense
geophysi-cal study, spurred in part by the 1995 Mw=8.1 Antofagasta
earthquake, has made the northern Chilean Coastal Cor-dillera one
of the Geophysically best known forearcs on earth. Combined active
and passive source, off-shore and on-shore, seismic studies show
that the interplate seismic zone extends from about 20 to 50 km
depth (Buske et al.,
Figure 1. Simplified structure map of the Coastal Cordil-lera of
norther Chile between 18°30’ and 22°S latitude,showing the
distribution of the EW fault scarps and thelocalities described in
the text. The dashed line just southof Iquique is Gephart’s (1994)
best-fit symmetry plane.QC – Quebrada Camarones, QT-Quebrada
Tiliviche
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2002; Husen et al., 2000; Husen et al., 1999; Pritchard et al.,
2002; von Huene et al., 1999). Geo-detic studies of global
positioning system (GPS) station data document both interseismic
and co-seismic deformation (Bevis et al., 2001; Klotz et al.,
1999). Initial studies of digital elevation models (DEMs) of the
Central Andes renewed long standing interest in the nature and
origin of the Bolivian orocline (Gephart, 1994; Isacks, 1988). In
this study, we have taken advantage of the just-released Shuttle
Radar Topographic Mission 90 m DEM, as well as an unpublished 20 m
DEM produced by interferometric synthetic aperture radar (InSAR).
KINEMATICS OF THE EAST-WEST SCARPS The fault scarps that form the
focus of this study are spectacularly displayed in the 20 m DEM
from the Coastal Cordillera (e.g., Fig. 2), but outcrops of the
fault planes are very scarce. We were able to locate outcrops along
the Coastal Escarpment, in the deep canyons of the Quebrada
Tiliviche and the Quebrada Suca (Chiza), as well as in a few
man-made excavations and one natural exposure in the interior of
the Coastal Cordillera. In all cases, the fault that produced the
scarp is a moderately dipping reverse fault. The P and T axes
(Marrett and Allmendinger, 1990) Figure 2. Shaded relief image of
the InSAR 20 m DEM from the Salar Grande area showing (A) the Salar
Grande fault, (B) the Cerro Chuculay system of EW reverse faults,
and (C) the NNW-striking strike slip fault that offsets the trace
of the Chuculay fault. At (D), a small paleodrainage is incised
into two uplifted blocks but is not offset later-ally, indicating
no strike-slip component on the EW faults. from all of the field
measurements define a composite fault plane solution showing that
the aver-age fault dip is approximately 45°, and the average
shortening (“P”) axis is nearly horizontal with an azimuth of 170°
(Fig. 3). The fault plane solution is almost a perfect thrust
mechanism con-firming field relations of offset geomorphic features
showing no strike-slip component (Fig. 2,
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site D). The shortening axis is nearly ex-actly parallel to the
regional trend of the Coastal Cordillera.
In the northern part of the region, both the Pisagua and Atajaña
scarps change along strike from fault scarps to fold scarps as the
tip line of the fault plunges beneath the sur-face and become
blind. The fault-propagation folds are well exposed in the Quebrada
Tiliviche and the Quebrada Chiza. Thus, at the eastern edge of the
Coastal Cordillera, the faults die out: there is no evidence that
they cross the Central Valley or reappear in the Precordillera.
Because there is so little erosion, the scarp heights can be
used as proxies for the verti-cal throw on the faults and this, in
combi-nation with the observation of an average dip of 45° can be
used to determine the amount of horizontal shortening. In the 300
km between the Quebrada Camarones in the north and Río Loa in the
South, there has been about 3 km or 1% horizontal shortening. The
region south of Iquique has slightly more shortening than the area
to the north, although the fault scarps tend to be larger in the
north.
Figure 3. Equal area lower hemisphere projection show-ing the
“P” (solid dots) and “T” (open boxes) kinematicaxes calculated for
all of the reverse faults measured inthe area of Figure 1. The
black triangles show the aver-age kinematic axes: 1 = infinitesimal
principal extensionaxis, and 3 = principal shortening axis. Shown
also is thebest fit fault plane solution, which demonstrates that
thefaults have no significant strike-slip component and thatthe
average fault dip is 45°.
TIMING In this segment of the Coastal Cordillera, the dominant
Neogene structures are ~NS normal faults, NNW-striking strike-slip
faults, and the EW reverse faults (González et al., 2003a;
Gon-zález et al., 2003b; González et al., 1997). Cross-cutting
relations at Cerro Chuculay just east of Salar Grande (Fig. 2) show
that the EW reverse faults are cut by the strike-slip faults
(Skarmeta and Marinovic, 1981). At Pisagua, the ENE-striking
reverse fault is probably cut and offset by the north-striking
normal fault, although the relationship shown on the map of the
region is just the opposite of the interpretation given here.
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At Barranco Alto south of Iquique (Figs. 1, 4), an extraordinary
exposure of growth strata with intercalated tuffs provides
excellent absolute age control on the EW reverse faults (Fig. 4).
Strata in a small evaporite basin in the footwall of the thrust
onlap the fault scarp. Whereas the uncon-formity at the base of the
basin strata is offset by ~60 m, a tuff within the part of the
basin strata that onlap the scarp is offset across the fault by
just 2 m. single crystal laser dating of feldspars yields a
statistically valid isochron of 5.6 Ma for the tuff. Thus, the
Barranco Alto structure probably began growing in the latest
Miocene and was probably mostly finished growing by early Pliocene.
Preliminary geochronology from the fold scarp at the east end of
the Pisagua
Figure 4. Above: Shaded relief map of theInSAR 20 m DEM of the
Barranco Alto site.The line at point “A” shows the location of
thecross section to the right. The thrust fault dipsto the
north-northwest. The arrow at “B” high-lights the paleodrainage
incised into the up-thrown block that is described in the text.
Thearrow at “C” shows the step in the Pleistocenewave-cut platform
that is aligned with the Bar-ranco Alto fault scarps. Right:
simplified geo-logic cross section across the southern of thetwo
fault scarps at Barranco Alto; detail below,right shows the onlap
of the evaportie basinstrata onto the fault scarp. The star within
thebasin just below the fault shows the location ofthe volcanic
tuff dated at 5.6 Ma. The horizonbearing the tuff (shown as a
dashed line) is off-set by just 2 m, whereas the basal
unconformityis offset by 60 m.
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structure shows that the folding was synchronous with, or
younger than, a tuff yielding an Ar to-tal gas age of 6.36±0.03 Ma,
putting it in the same age range as the Barranco Alto structure.
Be-cause of geomorphic relations described below, we suggest that
most of the EW scarps formed at about the same time. However, as
described elsewhere in this volume (González et al., 2003a), the
existence of Pleistocene wave cut platforms locally uplifted along
strike of the prominent EW faults (Atajaña, Pisagua, Iquique,
Barranco Alto) suggests that continued, or reactivated, north-south
shortening is warping these surfaces. GEOMORPHIC RELATIONS AND
CLIMATIC IMPLICATIONS At several localities (from south to north:
Río Loa, Chuculay, Barranco Alto, Pisagua among other areas), the
uplifted blocks of the EW scarps are incised by small fossil
drainages that no longer carry any water. They were cut at a time
prior to the development of the current hyperarid conditions of the
Coastal Cordillera. The Barranco Alto site (Fig. 4, B) provides a
beautifully de-tailed record of how and when this happened: Prior
to reverse faulting, paleodrainage was to the NW towards the ocean
shoreline that must have been located several kilometers farther
west. When faulting began, there was initially enough water to keep
pace with uplift and the hanging wall block was incised.
Eventually, however the fault scarp dammed the drainage and created
an internally draining evaporite basin that covered the upstream
part of the drainage located in the footwall. Strata accumulated in
the basin include the dated 5.6 Ma tuff. The onset of hyperaridity
here post dates 5.6 Ma, a scenario in agreement with
interpretations based on more regional data (Chong D. et al., 1999;
Hartley and Chong, 2002). The paleodrainage is not incised into the
cur-rent Coastal Escarpment at all and the evaporite basin is
abruptly truncated at the escarpment, indicating the escarpment
retreated from west to east to its current position after 5.6 Ma.
TECTONIC INTERPRETATION OF THE NS SHORTENING Any tectonic
explanation of the north-south shortening of the northern Chilean
forearc must ac-count for two basic observations: (1) In east-west
extent, the structures are limited exclusively to the Coastal
Cordillera and do not extend across the Central Valley. Instead
some of the faults have been documented to die out at the eastern
limit of the Coastal Cordillera. Thus, the struc-tures are uniquely
related to the processes of the interplate zone. (2) In north-south
extent, the fault scarps are limited to the region between the
Quebrada Camarones (19°S) and just south of the Río Loa (21.6°S).
Although north-south shortening has been observed elsewhere in the
forearc of the Central Andes (e.g., Lavenu and Cembrano, 1999),
only in this range has the short-ening been sufficient to form
scarps up to 500 m high. McCaffrey (McCaffrey, 1996) has shown that
the kinematics of the forearc of subduction zones can be predicted
from the relation between the obliquity of plate convergence
(relative to the plate boundary) and the obliquity of interplate
earthquake slip vectors. When looked at in this way, most forearcs
experience arc-parallel extension. Northern Chile, however, is one
of only two forearcs surveyed by McCaffrey that displays
arc-parallel shortening. McCaffrey’s model is strictly
observational and kinematic; it does not explain why the shortening
occurs there. Gephart (1994) showed that the topography of the
Central Andes is remarkably symmetric with a best-fit symmetry
plane whose pole coincides with the pole of rotation between Nazca
and South America during the birth of the modern Andes in the early
Miocene. This symmetry plane does not coincide with the bend in the
coastline at Arica but crosses the shoreline nearly 2° of latitude
farther south at 20.5°S, close to the city of Iquique (Fig. 1). The
EW fault scarps are distributed
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symmetrically to the north and south of the symmetry plane,
indicating that they are related to formation or the modern shape
of the Bolivian orocline. Bevis and others (2001), have shown that
orthogonal convergence across a locked plate boundary that is
concave towards the subducting plate will produce an elastic
deformation field with a strong component of shortening parallel to
the plate boundary across the symmetry plane of the curvature. This
appears to be exactly the situation in northern Chile forearc. In
fact, the triangle of GPS stations that straddles Gephart’s
symmetry plane displays a component of north-south shortening
(Allmendinger et al., in prep.). CONCLUSIONS The east-west scarps
of the northern Chilean Coastal Cordillera are produced by pure dip
slip re-verse faults that resulted in about 3 km or 1% horizontal
shortening along an azimuth of 350° parallel to the trench, the
contours on the Wadati Benioff Zone, and the trend of the
Cordillera itself. Preliminary dating shows that the faults formed
in the latest Miocene and early Pliocene, although Pleistocene
wave-cut platforms are locally deformed suggesting more recent
reactiva-tion. The faults formed when the region of the Coastal
Cordillera was less arid than today; they dammed paleodrainages and
formed the small evaporite basins common to this part of the
Coastal Cordillera. Significant Coastal Escarpment eastward retreat
has occurred since the scarps formed and the drainages were dammed.
Subsequent onset of hyperaridity (Hartley and Chong, 2002) has
resulted in a region totally devoid of surface water today. The
region of trench parallel shortening straddles symmetrically the
symmetry plane of the Central Andes defined by Gephart (1994). Thus
we conclude that the deformation is related to the shape of the
Bolivian orocline, a concept borne out by GPS data and elastic
modeling (Bevis et al., 2001). ACKNOWLEDGMENTS We are grateful to a
large number of colleagues who have, over the years, enhanced our
under-standing of the geology of northern Chile. They include D.
Carrizo, J. Cembrano, G. Hoke, T. Jordan, A. Macci, and C.
Mpodozis. The dating of the tuffs reported here was done by Terry
Spell of the Nevada Isotope Geochronology Laboratory. This work was
funded by U.S. National Science Foundation grant EAR 0087431 to R.
Allmendinger & B. Isacks and by el Proyecto Fundación Andes
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