Trench-parallel shortening in the Northern Chilean Forearc:
Tectonic and climatic implications
Richard W. Allmendinger†
Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 91125, USA
Gabriel González
Dipartamento de Ciencias Geológicas, Universidad Católica del Norte, Antofagasta, Chile
Jennifer Yu
Greg Hoke
Bryan Isacks
Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 91125, USA
ABSTRACT
INTRODUCTION
In the Central Andes, the frictional coupling
between South America and the subducting Nazca Plate occurs beneath the Coastal
Cordillera of northern Chile. One of the most
distinctive characteristics of the Coastal Cordillera is a suite of EW topographic scarps
located between 19° and 21.6°S latitude. These
scarps are associated with predominantly
south dipping reverse faults that have almost
pure dip slip and produce shortening parallel
to the plate boundary. Limited geochronology
as well as more regional studies indicate that
the scarps formed during the late Miocene
and early Pliocene, though some show activity
extending into the Quaternary. In several areas,
the scarps dammed local drainages, producing
internally drained basins that accumulated
evaporites. This relationship indicates that
the Coastal Cordillera was probably moister
during the Late Miocene and Pliocene than
it is today and also indicates that the Coastal
Cordillera has been significantly uplifted or
that the Coastal Escarpment of northern Chile
has advanced significantly eastward since the
Pliocene. The limited latitudinal extent of the
EW scarps and their location symmetrically
about the axis of topographic and WadatiBenioff zone symmetry suggest that they owe
their origin to the concave seaward shape of
the continental margin due to prior formation
of the Bolivian orocline.
Nearly orthogonal plate convergence in the
Central Andes (Fig. 1) south of the bend in
the coastline at 18.5°S latitude since the early
Oligocene has produced a suite of plate boundary-parallel tectonic provinces such the forearc
(Coastal Cordillera), magmatic arc, and back arc
(Subandean belt). In the Coastal Cordillera of
northern Chile between 19° and 21.6°S latitude
(Fig. 2), however, the most prominent morphostructural features are fault scarps orthogonal to
the plate boundary and approximately parallel
to the convergence direction. Because these features are restricted to the Coastal Cordillera (the
only part of subaerial South American crust in
direct contact with the Nazca Plate) but are also
latitudinally restricted to just part of northern
Chile, they must contain information about the
nature of three-dimensional plate coupling.
These scarps have previously been interpreted as the product of domino-style normal
faulting (e.g., Reijs and McClay, 1998), but we
show here that they are associated with moderately dipping reverse faults. With no significant
oblique slip component, these faults produce
horizontal shortening almost exactly parallel
to the trend of the Coastal Cordillera and the
plate boundary. In this paper we document the
structural geometry, kinematics, and ages of
these structures. Possible explanations for the
tectonic origin of the structures must take into
account their limited geographic distribution.
We conclude that they are fundamentally related
to the concave-seaward shape of the Bolivian
orocline, though they are probably not a product
of bending itself.
The margin-orthogonal scarps also control
the evolution of drainage systems in the Coastal
Keywords: forearc, Andes, Chile, deformation, geomorphology, tectonics.
†
E-mail: [email protected].
Cordillera. The history of river incision and
abandonment related to these features reflects
three fundamental processes, one climatic and
two tectonic: (1) a climatic transition from
semiarid to hyperarid environment after the
Miocene, (2) eastward base-level migration
due to coastal escarpment retreat, probably controlled by subduction erosion, and (3) Coastal
Cordillera uplift due probably to underplating in
or near the interplate seismic zone.
FOREARC TECTONIC SETTING
Plate Setting and Lithospheric Structure
The Nazca-South America plate boundary
between 18 and 24°S latitude has long been
considered most typical of an “Andean margin.” The subducted plate beneath northern
Chile (Fig. 1) dips ~30° eastward beneath the
continent (Cahill and Isacks, 1992). More than
a decade of intense geophysical exploration has
provided detailed images of the subducted plate
and its interface with South America (ANCORP
Working Group, 1999; Buske et al., 2002;
Götze et al., 1994; Husen et al., 2000; Husen
et al., 1999; Wigger et al., 1994; Yuan et al.,
2002; Yuan et al., 2000). The Peru-Chile Trench
is located just 70–150 km offshore (von Huene
et al., 1999). The Nazca-South America convergent plate boundary is responsible for some
of the largest earthquakes in recorded history
(Comte and Pardo, 1991; Tichelaar and Ruff,
1991). The 1995 Mw = 8.1 Antofagasta interplate earthquake, which occurred just south of
the region described here, showed that the interplate seismic zone extends from 20 to ~50 km
depth (Delouis et al., 1997; Husen et al., 2000;
Husen et al., 1999; Pritchard et al., 2002).
GSA Bulletin; January/February 2005; v. 117; no. 1/2; p. 89–104; doi: 10.1130/B25505.1; 15 figures; Data Repository item 2005022.
For permission to copy, contact [email protected]
© 2005 Geological Society of America
89
ALLMENDINGER et al.
Figure 1. Regional morphology of the Nazca-South America plate boundary between 10° and 35°S latitude. Onshore digital topography
from the USGS 30 arc second DEM; marine bathymetry from the ETOPO 5 DEM. The line labeled “Gephart Symmetry Plane” is the plane
of bilateral symmetry from Gephart (1994). Wadati-Benioff zone contour in kilometers depth from (Cahill and Isacks, 1992). Box shows the
location of Figure 2. The approximate rupture extent of large historic earthquakes (shaded ovals) is after Pritchard et al. (2002).
Geologic Setting
The Coastal Cordillera of northern Chile is
formed mainly by Jurassic-Early Cretaceous dioritic to granodioritic plutons and Jurassic volcanic rocks. These units comprise the remnants of a
Mesozoic magmatic arc formed at the birth of the
modern Andes (Coira et al., 1982; Mpodozis and
Ramos, 1990). The magmatic arc was emplaced
on an ensialic crust composed of Paleozoic sedi-
90
mentary and Precambrian metamorphic rocks.
The most important structure of the Coastal
Cordillera is the Atacama Fault System (Arabasz,
1971; Brown et al., 1993; Riquelme et al., 2003;
Scheuber and González, 1999) that extends for
more than 1000 km between the 21–26ºS latitude. Within our study area, the extreme northern
segment of the Atacama Fault System has been
reactivated during the late Cenozoic as the Salar
Grande fault (Fig. 3).
The Neogene to Quaternary sedimentary
record of the Coastal Cordillera attests to predominating arid and hyperarid climatic conditions. Several internal basins in the Coastal
Cordillera are composed of Oligocene-Miocene
alluvial deposits covered locally by Mio-Pliocene evaporite deposits (Chong et al., 1999;
Hartley and Chong, 2002; Hartley and Jolley,
1995), including the Salar Grande described in
this paper (Fig. 3).
Geological Society of America Bulletin, January/February 2005
FOREARC DEFORMATION, NORTHERN CHILE
Figure 2. Shaded relief map of northern Chile based on the new Shuttle Radar Topographic Mapping Mission 90 m DEM of northern Chile.
Boxes show the locations of the study areas detailed in subsequent figures. Box labeled “BA” is Barranco Alto. Camarones marks the Quebrada
de Camarones. Conical mountains in the eastern part of the DEM along the Chile-Bolivia international boundary are volcanoes in the Western
Cordillera, which marks the western boundary of the Altiplano. The solid and dashed white line shows the location of the topographic transect
used to calculate the magnitude of the plate boundary parallel shortening; the solid segment of the transect is shown in Figure 12.
Geological Society of America Bulletin, January/February 2005
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ALLMENDINGER et al.
Topography and Late Cenozoic Tectonics
The Bolivian Orocline has long been recognized and was one of the first major tectonic
features to be studied using digital elevation
models (DEMs) in the late 1980s. Isacks (1988)
compiled and used a DEM to evaluate the
amount of shortening and rotation along strike
in the Central Andes. He ascribed the orocline
to the fact that shortening is greatest at the bend
in the Andes at Santa Cruz, Bolivia, and diminishes to the north and south. Gephart (1994)
used Isacks’ DEM to show that the topography
of the central Andes has a well-developed bilateral symmetry (Fig. 1) with the symmetry axis
defined by a great circle whose pole coincides
with the pole of rotation between Nazca and
South America during a relatively stable period
from 36 to 20 Ma at the start of modern Andean
mountain building. Gephart’s symmetry plane
crosses the coastline of northern Chile at 20.5°S
latitude (Fig. 1), south of the bend in the coastline at Arica, Chile (18.5°S).
The Coastal Cordillera forms a 1000–1800 m
structural high (Fig. 2), bounded on its western
side by a 1000 m coastal escarpment (Paskoff,
1980). North of Iquique the escarpment extends
down to and is currently being undercut by the
ocean. In contrast, south of Iquique the escarpment is inactive and floored by 1000–4000 m
wide coastal plains that contain Late Pleistocene
marine terraces (Radtke, 1987; C. Casanova, personal commun., 2003). The most prevalent Late
Cenozoic structures in the Coastal Cordillera as
well as offshore are trench-parallel normal faults
(Niemeyer et al., 1996; von Huene and Ranero,
2003; von Huene et al., 1999). Although some
have suggested that the Coastal escarpment is
a single normal fault scarp (Armijo and Thiele,
1990), there are no outcrops of this inferred fault
and many of the exposed normal faults dip in the
opposite direction toward the Andes (Delouis et
al., 1998; González et al., 2003). The eastern
side of the Coastal Cordillera is formed by
gentle topography with an irregular mountain
front embayed by the sedimentary infill of the
Central Valley.
Our study of the forearc of northern Chile
employs a new high-resolution DEM produced
with radar interferometry (InSAR). These data
cover all of northern Chile at a 20 m resolution,
providing detailed images of Late Cenozoic fault
scarps and other features of tectonic and geomorphic interest. One of the first-order regional
Figure 3. Shaded relief map of Salar Grande and the Chuculay system of EW-striking fault scarps. Sites A–D are described in the text.
Stereographic lower hemisphere equal area projections show the available fault slip data and have been shaded as fault plane solutions (the
“T” quadrant in gray and the “P” quadrant in white). The principal axes of infinitesimal strain from fault slip analysis are labeled “1” for
extension and “3” for shortening. The data, though sparse, consistently indicate that the EW faults are reverse faults.
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observations made from these data are that the
Salar Grande segment (Fig. 3) of the Atacama
fault zone and related structures are characterized by a much more “youthful” appearance
(sharper, better defined fault scarps) than regions
farther south (Yu and Isacks, 1999).
EAST-WEST REVERSE FAULTING
Regional Distribution and Character
The prominent E- to ENE-striking fault
scarps are restricted in latitudinal extent from
just south of the Río Loa at 21.6°S to the
Quebrada de Camarones at 19°S (Fig. 2). The
scarps are as much as 450 m high and 35 km
long. The associated faults are completely
restricted to the Coastal Cordillera, becoming blind and plunging beneath the strata of
the Central Valley. The majority of the scarps
face northward, although between Iquique and
Salar Grande there are several well-developed
south facing scarps. The backsides of many
of the scarps display a gently inclined erosional surface on the Mesozoic “basement” of
the Coastal Cordillera. At Iquique, there is a
change in the orientation of the scarps: to the
south the scarps strike predominantly E or ESE,
but to the north they strike consistently ENE.
This change in scarp orientation accompanies a
change in regional trend of the Coastal Cordillera from north-south to the south or Iquique
to NNW-SSE to the north. Thus the scarps are
everywhere approximately perpendicular to the
trend of the Coastal Cordillera.
Key Field Localities
In a hyperarid environment, there is no
surface water to carry away the products of
mechanical weathering and the topography simply becomes buried in its own debris. The lack
of fault exposure and open fractures on the faces
of the scarps, combined with characteristic tilted
blocks led most previous authors to interpret the
structures as domino normal fault blocks (e.g.,
González et al., 1997; Reijs and McClay, 1998).
However, some of the structures are exposed
where they cut the Coastal Escarpment and
there are a few scattered exposures in the deep
canyons north of Pisagua. In every case, the
fault plane dip, the associated scarp morphology, and kinematic analysis of fault plane striations demonstrate reverse fault motion during
the Late Cenozoic.
Chuculay
The Chuculay system, mapped by Skarmeta
and Marinovic (1981), comprises several EW
scarps located east and south of Salar Grande
(Figs. 2 and 3). Though not the main topic of this
paper, one of the most remarkable structures in
the region is the Salar Grande fault scarp, which
produces ~10 m of vertical offset of the halite
surface of the Salar Grande (“A” in Fig. 3). To the
north of the region shown in Figure 3, the Salar
Grande fault produces several tens of meters of
horizontal displacement of drainages, suggesting that it is predominantly a strike-slip fault
(González et al., 2003). The Salar Grande fault
is not offset and is therefore younger than the
suite of EW scarps of the Chuculay system. The
main scarp at Cerro Chuculay is ~350 m high
and it and several related faults have produced
a suite of well-defined blocks tilted ~4–5°S (“B”
in Fig. 3). Two strands of the Chuculay fault
system offset a small antecedent drainage (“C”
in Fig. 3) that is no longer active. The northern
strand truncates and drops down the catchment
area (now a small closed basin) relative to the
channel, and the southern strand offsets the same
channel vertically but not horizontally, indicating no strike-slip motion on the fault.
Two outcrops in this system yielded exposures of the bounding faults. To the south along
the Coastal Escarpment, a brittle gouge zone
in Mesozoic igneous rocks dips to the south
beneath the uplifted block of the scarp (“D” in
Fig. 3). The fault is thus a reverse fault. Likewise, a small exposure of the fault plane along
the scarp located just north of Cerro Chuculay
shows reverse displacement with nearly downdip slickensides. The age(s) of these structures
is (are) not known. They produce a broad
smooth scarp in the surface of the Salar Grande,
however, suggesting at least some movement
post-dating salar halite deposition.
Barranco Alto
Although one of the smaller features in the
region, the 60 m high, south-facing scarp at
Barranco Alto (Figs. 2 and 4) is particularly
important for its well-preserved age relations.
The structure consists of two scarps: a northern,
more continuous scarp and a parallel southern
scarp that dies out ~0.5 km inland from the
Coastal Escarpment. The former scarp yielded
no outcrop but the southern one (“A” in Fig. 4)
highlights some key relationships. A small evaporite basin occupies the depression immediately
south of the southern scarp, and outcrops on the
edge of the Coastal Escarpment show that the
strata of this basin onlap the fault scarp (Fig. 5).
The reverse fault that produced the scarp also
cuts into the basin strata and produces a subsidiary 8–10 m scarp.
We were able to date a reworked tuff in the
basin via single crystal laser fusion ages on 19
feldspar crystals, predominantly plagioclase.
Eliminating two obvious xenocrysts, the 17
remaining crystals yield a statistically significant
isochron (Fig. 6)1 that accounts for the excess
argon in this sample. Based on the isochron, the
best estimate for age of the tuff is 5.62 ± 0.1 Ma
(T. Spell, personal commun., 2002). Large euhedral biotite crystals in the sample indicate that the
reworking is minor and that the age obtained is
probably close to the age of accumulation of the
surrounding strata in the basin. The tuff is particularly significant as it is part of the section that
onlaps the scarp in the upper plate of the reverse
fault and is itself offset by the fault where it cuts
the basin. Though the total separation of the basement-basin contact is ~60 m, the tuff is offset by
just 2–3 m in a thrust sense. Thus, the Barranco
Alto site shows that at least the southern of the
two EW scarps accrued most of their displacement prior to 5.6 Ma, but also experienced minor
thrust motion after 5.6 Ma (Fig. 5).
The topography of the Barranco Alto area
records a relationship between scarp evolution
and paleodrainages that we see repeated at
many sites. A linear drainage (“B” in Fig. 4)
cuts straight across the uplifted hanging wall
near the east end of the main scarp. This small
valley once provided continuous drainage from
the interior of the Coastal Cordillera toward the
sea although it is now folded so that the northwest segment slopes toward the coastal escarpment and the southeast segment slopes toward
the eastern end of the Barranco Alto evaporite
basin. Furthermore, there is no sign of the valley
to the southeast of the scarp, suggesting that the
basin covers the old paleovalley. Thus, the morphology records the following history (Fig. 7).
Sometime prior to 5.6 Ma, there was sufficient moisture in the Coastal Cordillera
and/or the Central Valley to cut a small valley
that drained from the interior to the ocean. In
the Pliocene, the Barranco Alto east-northeast
trending reverse fault began to uplift. Initially,
the stream had enough power to incise the
uplifting block, cutting the well-defined channel we see today. The continued uplifting and
folding of the hanging wall eventually defeated
the drainage and a closed evaporite basin
formed in the footwall. Strata accumulated in
the basin, onlapped the growing fault scarp
and covered the paleovalley in the footwall.
If the coastal escarpment were located where
it is today, the growth of the fault scarp would
have only diverted the drainage to run along
the scarp to the southwest until it reached the
escarpment. Instead, higher topography must
1
GSA Data Repository item 2005022, tables containing the original analytical data for the geochronology displayed in Figures 6 and 9, is available on the
Web at http://www.geosociety.org/pubs/ft2005.htm.
Requests may also be sent to [email protected].
Geological Society of America Bulletin, January/February 2005
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ALLMENDINGER et al.
Figure 4. Shaded relief map of the Barranco Alto system of ENE-striking fault scarps. Sites A–D are described in the text. Lower hemisphere
equal area projections are plotted as described in Figure 3. The box at site “A” shows the location of the detailed geologic map in Figure 5.
have once existed farther west to block the
outflow. The coastal escarpment, if it existed
at 5.6 Ma, must have been located still farther
west. This conclusion and the fact that the
paleovalley itself is truncated at the escarpment
(“B” in Fig. 4) indicate escarpment retreat since
5.6 Ma. Defeat of the channel described here
must have been a function of both uplift rate of
94
the hanging wall and diminishing water supply
(roughly correlated with precipitation) (Sobel et
al., 2003), and in fact the subsequent filling of
the evaporite basin indicates that the upstream
portion of the drainage continued to have sufficient water power, at least intermittently, to
carry medium grained sediment. There must
have been more water in the Coastal Cordillera
when the channel was cut prior to 5.6 Ma than
there is today, as there is insufficient water in
the Coastal Cordillera under current climatic
conditions to cut an incised valley, and virtually
all evaporite basins there are fossil.
Although the relationships are less clear, the
scarp south of Barranco Alto (“D” in Fig. 4) also
has a sandy, evaporite basin only in the footwall
Geological Society of America Bulletin, January/February 2005
FOREARC DEFORMATION, NORTHERN CHILE
Figure 5. (A) Simplified geologic map of the Barranco Alto area and (B, C) cross section showing the relations between faulting, depositional
onlap of the fault scarp, and the dated tuff horizon.
of the reverse fault that formed the scarp. These
detailed observations are in general accord
with the post ca. 6 Ma drying out of northern
Chile suggested by Hartley and Chong (2002),
although nothing in our data constrains whether
the onset of hyperaridity was at 6 or significantly earlier than 6 Ma.
The Barranco Alto site is interesting for
another reason: If one projects the northern fault
scarp westward across the Coastal Escarpment,
it coincides with a step in the coastal marine terrace of ~40 m (Fig. 4, “C”). Although no young
fault zone was found in the coastal exposures
(perhaps because of cover due to modern beach
and sand dune deposits), the facing of the step in
the coastal terrace is the same as that of the scarp.
The paleontological record on the coastal marine
terraces indicates that they are Pleistocene in age
(Casanova, personal commun., 2003).
Pisagua
To the north of Iquique, the EW scarps are
sparser than to the south, but are much longer and more distinct. Furthermore, the coast
north of Iquique lacks the narrow but relatively
continuous marine terrace that characterizes the
coast to the south. Instead, the coastal escarpment plunges directly into the ocean. The
Pisagua scarp (Figs. 2 and 8), ~65 km north
of Iquique, is a prominent north-facing scarp
~25 km long and 160–260 m high. The scarp
can be traced from the coastal escarpment to the
western edge of the Central Valley. The geology
of this area was mapped by Silva (1977). We
describe this structure below, from west to east.
Similar to, but better developed than at
Barranco Alto, the Pisagua scarp is associated
with a narrow uplifted marine terrace that is
not present either to the north or south along
the coast. Exposures of the fault at the escarpment show it to be an east-northeast trending
reverse fault that dips to the south (locality
“A,” Fig. 8). A west-dipping, north-striking
normal fault with ~400 m of vertical throw is
well exposed in the Quebrada Tiliviche just
north of the town of Pisagua (Fig. 8). The trace
of the Pisagua reverse fault is offset by the
normal fault at site “B,” suggesting as we have
seen elsewhere, that the latter is the younger
structure. A similar relationship between a
smaller north-striking normal fault and the
Pisagua fault scarp may be exposed at “C,” but
the throw on the normal fault is considerably
less and therefore the offset of the Pisagua fault
scarp is more open to question.
Somewhere between sites “D” and “E” in
Figure 8, the Pisagua scarp changes from a fault
scarp (to the west) to a fold scarp (to the east).
At site “E,” the fault is blind and instead, a welldeveloped tip line fold is exposed. The scarp is
much smoother and has a lower slope angle and
less relief than farther west. The valley that cuts
and incises the scarp at “E” has no catchment
area but simply disappears beneath the flat
surface of the Central Valley at site “F.” Thus,
we conclude that, like the example at Barranco
Alto, this is a paleovalley that predates and is
synchronous with the initial ENE reverse faulting but has been largely abandoned and covered
by the youngest deposits of the Central Valley.
A volcanic tuff is folded above the tip line
along with the rest of the sedimentary section
Geological Society of America Bulletin, January/February 2005
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ALLMENDINGER et al.
Figure 6. Isochron plot of the dated tuff
from Barranco Alto collected at the location
shown in Figure 5. Analysis performed by
the Nevada Isotope Geochronology Laboratory and interpreted by T. Spell (personal
commun., 2003).
Figure 7. Schematic interpretation of the evolution of geomorphic and structural features at Barranco Alto. See text for discussion.
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Figure 8. Shaded relief map of the Pisagua fault scarp. The lower resolution topography west of 70°10′W is from the Shuttle Radar Topographic Mapping Mission (SRTM) 90 m DEM for northern Chile as our 20 m DEM does not cover the Pisagua peninsula. Sites A–G are
described in the text. Lower hemisphere equal area projections are plotted as described in Figure 3.
at site “E.” The sample of this tuff collected for
isotopic dating yielded a separate of very thin,
fine-grained biotites with as much as 20%–30%
plagioclase. Because these are volcanic rocks
that cooled very rapidly, the mixture of the two
minerals should not produce a significant problem. Stepwise heating yielded a total gas age of
6.36 ± 0.03 Ma, but did not produce a plateau.
However, an isochron for steps 5–10 produced
a reliable age of 3.49 ± 0.04 Ma (T. Spell, personal commun., 2004), which we consider to be
the most reliable age for this sample (Fig. 9).
These data demonstrate that the Pisagua structure was active into the late Pliocene, further
lending credence to the suggestion above that
the uplifted Pleistocene marine terrace at the
coast is a produce of fault movement.
Just east of site “E” the Pisagua structure
bifurcates into two tipline folds, one trending
NE and the other nearly EW. The fold and
the tipline of the fault are well exposed in the
bottom of the Quebrada Tiliviche at site “G”
(Fig. 8). Here, the Tiliviche drainage, which
is active today, has been superimposed on and
downcut through the Mesozoic igneous basement in the core of the Pisagua fold. Fault
exposures show that the fault continues to be a
reverse fault; the fanning of shear planes and the
geometry of the tip line fold suggest that it may
be a trishear-like structure. East of the Quebrada
Tiliviche, the Pisagua structure has virtually no
surface expression at all.
Atajaña
The Atajaña scarp, located just south of the
Quebrada Camarones (Fig. 2), is the longest and
highest, as well as the northernmost, of all the
east to east-northeast striking scarps. North of
the Quebrada Camarones the Coastal Cordillera
decreases in elevation and trends out to sea; at
Arica, the Coastal Cordillera is completely submerged. It is possible that east to northeast-striking reverse fault scarps continue to be present
in the submerged part of the Coastal Cordillera
north of Atajaña.
Like most of the structures of the region, the
fault that produced the Atajaña scarp is not generally exposed, but outcrops in the Quebrada de
Chiza (“A” in Fig. 10) and along the coast (“B,”
Fig. 10) provide critical insight. Along the PanAmerican Highway where it descends into the
Quebrada Chiza, a nucleus of Mesozoic igneous
rocks and folded Cenozoic strata cut by minor
faults is exposed. Exposures in the bottom of the
canyon show that the Atajaña structure is a blind
thrust with an overlying fault-propagation fold.
The folded layers diminish in dip up-section and
the fold wavelength becomes much broader; the
entire structure resembles a trishear fault-propagation fold (e.g., Allmendinger, 1998). Exactly
where the tipline pierces the surface west of
site “A” is unknown. A good candidate would
be somewhere near point “C” where the scarp
becomes gentler, lower, and smoother.
Along its entire strike length, the hanging
wall of the Atajaña fault is composed of Cretaceous red beds whereas the footwall is made up
of Jurassic volcanic rocks (Anonymous, 2003).
This relationship suggests that the fault may
have originated as a Mesozoic normal fault and
was subsequently, at a much later time, reactivated as a Cenozoic reverse fault.
Geological Society of America Bulletin, January/February 2005
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Figure 9. Isochron plot from volcanic tuff
folded above the tipline of the Pisagua thrust,
from site E along the Pisagua fault scarp
(Fig. 8). See text for discussion. Analysis performed by the Nevada Isotope Geochronology Laboratory and interpreted by T. Spell
(personal commun., 2003).
Figure 10. Shaded relief map of the Atajaña scarp, highlighted by the arrows. Sites A–C are described in the text. Box shows location of
detailed topography displayed in Figure 11. Note the spatial association of the fault and the warped marine terrace fragment exposed at the
base of the Coastal Escarpment.
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As at Pisagua and Barranco Alto, an upwarped
Pleistocene coastal terrace remnant is present
where the Atajaña scarp intersects the Coastal
escarpment (Fig. 10, site “B”). A detail of the
topography (Fig. 10, inset) shows that the terrace
is not offset but is present at the same elevation
both north and south of the projected trace of the
fault. This suggests localized upwarping, though
small scarps beneath the resolution of the topography are possible.
REGIONAL SYNTHESIS
Orientation and Magnitude of North-South
Shortening
Figure 11. Summary diagram of the fault kinematics for all of the measured EW reverse
faults, displayed on an equal area, lower hemisphere projection. The data are displayed
as described in Marrett and Allmendinger (1990), with the solid circles representing “P”
axes and the open boxes “T” axes. The statistical best fit principal infinitesimal strain axes
labeled “1” (extension), “2,” and “3” (shortening) were used to derive a pseudofault plane
solution, shown gray.
Figure 12. Topographic profile constructed parallel to the Coastal Cordillera. Location of
profile is shown as dashed white line in Figure 2. Numbers on vertical lines show the vertical
relief of each fault scarp and the numbers in degrees show the angle of tilt of the mid-Tertiary erosional surface.
Though outcrops are few and exposures of
fault planes scarcer still, we were able to collect fault slip measurements on just over 20 fault
planes, all from the fault zones of the major EW
striking reverse faults. Following the P and T axis
method of Marrett and Allmendinger (1990), we
calculate the infinitesimal strain axes (the socalled P and T axes) and plot them in Figure 11.
The average shortening axis (“3” in Fig. 11) is
nearly horizontal and trends ~170° and the extension axis is nearly vertical. The subhorizontal
intermediate principal axis (“2”) indicates that,
on average, the deformation has almost no strikeslip component. The shortening azimuth is very
nearly exactly parallel to the regional trend of the
Coastal Cordillera in this part of northern Chile.
In this hyperarid region, which has experienced little erosion since the Pliocene and Late
Miocene when the faults were mostly active,
scarp height is an excellent proxy for the vertical throw on the fault. Scarp heights were determined on a topographic profile parallel to the
Coastal Cordillera and crossing all of the major
fault scarps (white line in Fig. 2). The southern
third of this profile, shown in Figure 12, shows
the asymmetry of the scarps and emphasizes not
only the height but also the tilting of the blocks.
In general, there is a rough correlation between
the degree of tilting and the scarp height (or
composite scarp height), indicating that tilting
and fault slip are related. In fact, removing the
effect of the tilting would make the average
shortening axis exactly horizontal.
Assuming that the faults associated with the
scarp dip 45°, which fits with what is known
of the regional kinematics (Fig. 11), the magnitude of the horizontal shortening will be
about the same as the vertical uplift. If so, the
EW reverse faults produce, in total, a little
more than 3 km of horizontal shortening in a
distance of ~300 km, or ~1% shortening. If the
faults average 60° dips, then the shortening is
~1.8 km across the same distance. The percent
shortening is marginally greater in the southern
Geological Society of America Bulletin, January/February 2005
99
ALLMENDINGER et al.
end of the transect than in the north, which is not
surprising given the poor development of scarps
in the region between Iquique and Pisagua.
Relationship to Coastal Cordillera Drainages
At several localities in the Coastal Cordillera
small, narrow, generally straight and shallow
valleys lie totally abandoned, with no evidence
that they have carried water in a very long time.
These valleys generally lack a catchment area
and incise the EW scarps, but where they intersect deep canyons or the Coastal Escarpment
they are left truncated and hanging, with no evidence that they even attempted to cut down to
modern base level. The small valley at Barranco
Alto has already been discussed in some detail
above. Another example occurs just south of the
Río Loa (Fig. 13). There, the southernmost outcrop of the EW fault scarps has been cut by two
small channels. They are inactive now and their
merged channel hangs more than 600 m above
the bottom of the Río Loa canyon.
In all of these cases, there was enough water
in the Coastal Cordillera when the EW scarps
began to form, so that these small drainages
could begin to incise the scarps. In many cases,
the footwall blocks of the EW scarps have
small evaporite basins that show no evidence
of ever having been present in the hanging
wall. Sufficient moisture continued in the area
even after the uplift of the EW scarps outpaced
the ability of the streams to continue downcutting. The scarps formed dams that impounded
the drainages and produced internally drained
basins. We know from our data at Barranco
Alto, as well as more regional data on the ages
of evaporite deposition in the area (Chong et al.,
1999; González et al., 1997; Hartley and Chong,
2002), that this wetter environment persisted
until at least 5.6 Ma and if the age of the Pisagua
tuff is reliable, until at least ca. 4 Ma.
The truncation of the small valleys shown in
Figure 13 indicates more than 600 m of downcutting of the Río Loa since those valleys were
active. In light of the proximity of this area to
the current coastline, this downcutting could
have been accomplished in either of two ways
(Fig. 14). One possibility is that most of the
uplift of the Coastal Cordillera is mid-Miocene
and younger. Alternatively, the Coastal Escarpment could have retreated significantly eastward
since the drainages became inactive at the beginning of the Pliocene. Again, the relations at Barranco Alto require some eastward retreat of the
escarpment but we cannot say how much.
Either possibility has important though different tectonic implications and both were
probably active simultaneously. As there is no
significant continental shelf or beveled marine
platform offshore, it is likely that any significant
Coastal Escarpment retreat must be the result of
tectonic (i.e., subduction) erosion of the forearc.
This process has been shown to have operated
along this continental margin, with a very rough
average rate of erosion and retreat of 1 km/1 Ma
(von Huene and Ranero, 2003; von Huene and
Scholl, 1991; von Huene et al., 1999). Although
flexure of the locked plate boundary during the
Figure 13. Shaded relief map of the southernmost EW fault scarp just south of the Río Loa, showing the relationship between drainage
development, EW faulting, and canyon incision. Sites A–C described in the text.
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Geological Society of America Bulletin, January/February 2005
FOREARC DEFORMATION, NORTHERN CHILE
interseismic part of the earthquake cycle can
explain transient uplift of the Coastal Cordillera,
the most popular explanation of long-term uplift
is the underplating of material to the base of the
continental crust beneath the Coastal Cordillera
(Armijo and Thiele, 1990; Delouis et al., 1998).
In this process, material eroded from the leading
edge of South America is transported downdip
to at or near the base of the interplate seismic
(~50 km) where it is reincorporated at the base
of the South American crust. The resultant
thickening of the continental crust produces
isostactic uplift.
At the present time we cannot distinguish
between the two. Further insight into the history
and mechanisms of river incision in the Coastal
Cordillera could provide our most exquisite
record yet of subduction erosion and underplating during the last 10 m.y. of geologic history.
TECTONIC INTERPRETATION
Any interpretation of the EW faults must
take into account three basic facts. (1) They
are limited to the Coastal Cordillera, the only
part of South American continental crust in
direct contact with the subducted Nazca Plate.
As such, they must in some way be related to
the coupling between the two plates. (2) Their
shortening direction parallels both the Coastal
Cordillera and the contours on the underlying
Wadati-Benioff zone (Fig. 1), further suggesting a link to plate coupling. And, (3) they are
limited in geographical extent to the region
between 19° and 21.6°S latitude. Thus, there
must be something unique about the area in
which they occur and they do not reflect general plate coupling processes.
The Shape of the Bolivian Orocline
The EW reverse faults are not spatially
related to the bend in the coastline at 18.5°S
but occur entirely to the south. However, the
symmetry plane that best defines the Bolivian
Orocline (Gephart, 1994) does not coincide with
the bend in the coastline either, but crosses the
coastline nearly 2° of latitude farther south at
20.5°S (Fig. 1), just south of the city of Iquique.
The EW faults are distributed equal distances
to the north and south of Gephart’s symmetry
plane. This striking spatial coincidence leads
us to the conclusion that the faults are related
to processes occurring on the inner arc of the
Bolivian Orocline.
There are two ways in which oroclinal bending
might influence the deformation in the Coastal
Cordillera. First, the buckling of a beam produces
beam-parallel shortening on the concave side of
the neutral surface. Thus, conceivably the Coastal
Cordillera, which is located on the concave side
of the Bolivian Orocline, has experienced minor
shortening parallel to the coast. While appealingly simple and easy to understand, we think it
unlikely that oroclinal bending is mechanically
so simple. Furthermore, recent paleomagnetic
data suggest little or no vertical axis rotation of
the forearc during the time that the EW structures
were active (e.g., Roperch et al., 1999).
Alternatively, it may be that the preexisting
curved shape of the plate boundary induces a
deformation field with a component of compression parallel to the boundary. Bevis et al. (2001)
have modeled the interseismic velocity field of an
elastically deforming forearc of a locked plate
boundary with concave curvature (Fig. 15A). In
the region overlying the zone of interplate coupling and locking, the velocity vectors converge
Figure 14. Diagram illustrating the two mechanisms by which the Río Loa could have been incised. Dark profile in each case represents the
modern setting wheras lighter profiles represent older positions of the Coastal Cordillera and Escarpment. The paleodrainage shown in the
two scenarios represents the intersection at site B (Fig. 13) of the small paleodrainge south of the Río Loa canyon with the Río Loa.
Geological Society of America Bulletin, January/February 2005
101
ALLMENDINGER et al.
strongly toward the symmetry plane. One can
calculate two-dimensional strain for any three
non-colinear points knowing their initial and final
positions (or initial positions and displacement
vectors). In the case of the model of Bevis et al.,
both principal axes of strain are negative (i.e., a
non-area constant deformation), and the shortening perpendicular to the symmetry plane is larger
than that parallel to the convergence direction.
The forearc of northern Chile is significantly
less curved than that shown in the model of
Bevis et al., and the GPS stations are much more
sparsely distributed (Fig. 15B). The triangle of
GPS stations that directly overlays the Gephart
symmetry plane (IQQE-PPST-PTCH) does, in
fact, yield two negative principal axes, indicating
shortening in all directions. However, the most
negative axis is oriented at a low angle to the
EW scarps and oblique to the convergence direction. Furthermore all of the other triangles yield
essentially EW shortening and NS extension.
Two dimensional analysis of just the coastal GPS
stations suggests that, in general, there is minor
shortening between those stations, particularly if
the three northern stations (ARIC-PSAG-VRDS)
are analyzed separately from the three southern
stations (VRDS-PTCH-PBLN). Thus, the current GPS data displays some orogen parallel
shortening in the region of the symmetry plane,
consistent with the Bevis et al. hypothesis, but the
case is less than compelling. Additionally, there
is no guarantee that the interseismic elastic strain
is converted into permanent strain in any straightforward manner.
Our very limited geochronology data suggest
that most of the shortening occurred during or
prior to the early Pliocene, though there may
be minor reactivation during the Quaternary. As
recently summarized by Kendrick et al. (2003),
Figure 15. (A) Model of the velocity vectors during interseismic deformation in the upper plate of a locked curved plate boundary (modified
from Bevis et al., 2001). The dark curved shaded area represents the locked segment of the plate boundary between 20 and 50 km depth. The
two triangles were used to calculate the infinitesimal principal strain axes shown below. The fact that both axes are negative means that both
are shortening axes. (B) GPS vectors from northern Chile (data from Bevis et al., 2001). The principal strain axes for the triangle shown,
which spans Gephart’s (1994) symmetry plane, are also both negative. Four letter codes identify individual GPS stations.
102
Geological Society of America Bulletin, January/February 2005
FOREARC DEFORMATION, NORTHERN CHILE
the convergence rate between Nazca and South
America has systematically decreased by a factor
of 2 since ca. 15 Ma. The magnitude of boundary-parallel shortening induced by preexisting
curvature is closely related to the convergence
rate. Thus, it is not surprising that the major
growth of the EW structure is pre–mid-Pliocene.
Trench-Parallel Shortening
Previous investigations have documented PlioQuaternary trench-parallel shortening farther
south in the Andean forearc at 33–37°S (Lavenu
and Cembrano, 1999). McCaffrey (McCaffrey,
1994, 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, although the large magnitude of the
errors in McCaffrey’s analyses does not make
this a very robust conclusion. This kinematic
analysis is complementary to the elastic modeling of Bevis et al. (2001) of a locked, curved
plate boundary. Indeed, the arc-parallel shortening predicted from the kinematic analysis arises
from the fact that the predicted velocity parallel
to the arc not only diminishes linearly from south
to north but actually changes sign, just as the arcparallel components of the velocity vectors in the
Bevis model would decrease linearly and change
sign. Perfect accord is not achieved, however, as
the change in sign in McCaffrey’s model occurs
at 23.5°S latitude whereas the change in sign
in the Bevis model would occur at the Gephart
symmetry axis at 20.5°S.
Of course, as an observational and kinematic
analysis, McCaffrey’s model is not required to
change sign anywhere. Nonetheless, McCaffrey
predicts arc-parallel shortening for all of northern
Chile from 30°S to 18°S latitude. As the velocity changes linearly, within error, the strain rate
should be constant over this entire length of the
forearc. As we have seen, our EW reverse faults
are much more restricted latitudinally than the
region of predicted arc-parallel shortening.
CONCLUSIONS
We have shown that small but significant
shortening parallel to the Nazca-South America
plate boundary between 19° and 21.6°S latitude
occurred during the Pliocene and may be continuing at present. The scarps produced by this
deformation are some of the most prominent
topographic features in this segment of the
Chilean Coastal Cordillera. The topographic
scarps were initially consistently incised by and
later dammed small, well-defined drainages in
the Coastal Cordillera that are no longer active.
The data from one site, Barranco Alto, suggests
that tectonic damming of one such drainage
produced an internally drained evaporite basin
at around 5.6 Ma. This implies, as others have
stated as well, that the Coastal Cordillera was
wetter at the start of the Pliocene than it is today.
Relations around the Río Loa imply either significant uplift of the Coastal Cordillera and/or
significant retreat of the Coastal escarpment
since the Pliocene. These two processes are
probably related to tectonic underplating and
subduction erosion, respectively.
The restriction of the EW scarps to the
Coastal Cordillera suggests that plate coupling
plays an important role in their formation. The
latitudinal distribution of the scarps symmetrically about Gephart’s symmetry plane implies
that processes associated with oroclinal bending
are also involved. Though not fitting our data
perfectly, both kinematic analysis of interplate
earthquakes and oblique convergence and elastic models of locked curved plate boundaries
indicate that the northern Chile forearc should
experience arc-parallel shortening.
ACKNOWLEDGMENTS
Our ideas about the Coastal Cordillera of northern
Chile have been shaped over a number of years by our
colleagues, including José Cembrano, Teresa Jordan,
Mike Bevis, Constantino Mpodozis, and Jack Loveless. We are indebted to Terry Spell and Kathleen
Zanetti of the Nevada Isotope Geochronology Laboratory for the geochronological analyses reported here.
We are grateful to reviewers George Hilley and Adrian
Hartley, as well as editors Michael Edwards and Peter
Copeland for numerous suggestions that improved
the manuscript. The geological work was supported
by the U.S. National Science Foundation under grant
EAR 0087431 (to Allmendinger and Isacks) and
Fundación Andes-Chile grant C-13755-12- (to Gabriel
González). The InSAR topographic data set presented
here was produced with support from NASA grants
NAG5-11424, NAG5-30126, and NSF grant EAR9706427 (to Isacks).
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Geological Society of America Bulletin, January/February 2005