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Tectonophysics 399 (
Paleoseismic inferences from a high-resolution marine sedimentary
record in northern Chile (238S)
Gabriel Vargasa,b,c,T, Luc Ortliebc, Emmanuel Chaprond,1,
Jorge Valdesc,e, Carlos Marquardtf
aDepartamento de Geologıa, Universidad de Chile, Plaza Ercilla 803, Santiago, ChilebUMR-CNRS 5805 EPOC Universite Bordeaux I Avenue des Facultes, 33405, Talence Cedex, France
cIRD, UR055-PALEOTROPIQUE, 32 Avenue Henri Varagnat, 93143, Bondy Cedex, FrancedI.S.T.O. Universite d’Orleans, UMR CNRS 6113, BP 6759, 45067 Orleans Cedex, France
eFacultad de Recursos del Mar, Universidad de Antofagasta, Casilla 170, Antofagasta, ChilefSERNAGEOMIN, Avenida Santa Marıa 0104, Providencia, Santiago, Chile
Received 15 November 2002; received in revised form 10 November 2003; accepted 23 December 2004
Available online 12 February 2005
Abstract
The active margin of northern Chile is characterized by strong seismic events which induce gravity instability for
sedimentary sequences located along the outer forearc of the Andean range. Even though the narrow continental platform of this
area limits the accumulation of marine sediments, a detailed paleoceanographic reconstruction, using high-resolution
sedimentological and geochronological techniques, of a Holocene sedimentary sequence in Mejillones bay (238S) providesimportant tools to infer the occurrence of past great subduction earthquakes and/or associated tsunamis.
From the analyses of short cores retrieved in this shallow marine basin, we infer the occurrence of two strong seismic events
during historical times. The first one, dated between the years 1409 and 1449 AD, produced an angular unconformity and
associated lenticular coarse-grained deposits generated after local reworking of shallower material. The second one, dated
between the years 1754 and 1789 AD, caused slumping, erosive processes and local reworking of material. The analysis of
seismic profile data acquired in this bay allowed the recognition of a major slump deposit within the upper sedimentary
sequence, tentatively dated as of Middle Holocene age. This event might record the greatest earthquake (and tsunami) affecting
the basin within the last few thousand years.
We interpret that destabilization of sediments occurs most probably in response to local reactivation of faults within the
Mejillones peninsula during strong earthquakes, especially the Mejillones fault.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Paleoseismicity; Paleoceanography; Laminated sediments; Northern Chile; Seismic gap; Mejillones bay
T Corresponding author. Departamento de Geologıa, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile.
0040-1951/$ - s
doi:10.1016/j.tec
E-mail addre
jvaldes@uantof.1 Present addr
2005) 381–398
ee front matter D 2005 Elsevier B.V. All rights reserved.
to.2004.12.031
sses: gvargas@ing.uchile.cl (G. Vargas)8 Luc.Ortlieb@bondy.ird.fr (L. Ortlieb)8 chapron@erdw.ethz.ch (E. Chapron)8
cl (J. Valdes)8 cmarquar@sernageomin.cl (C. Marquardt).
ess: Geological Institute, ETH Zentrum, Zqrich, Switzerland.
G. Vargas et al. / Tectonophysics 399 (2005) 381–398382
1. Introduction
1.1. Regional geodynamic context in the outer forearc
of northern Chile: the Mejillones peninsula
The continental margin of northern Chile is
characterized by active tectonics and great historical
seismic events related to the subduction of the Nazca
plate under the South American plate. This tectonic
contact controls the geometry of the margin and the
disposition of the main geological features. One major
structure in this region is the N–S Atacama fault
system (Fig. 1), which was originated during the
Mesozoic period, but exhibits superficial manifesta-
tions of recent reactivations (Armijo and Thiele, 1990;
Niemeyer et al., 1996; Delouis et al., 1997; 1998;
Gonzalez et al., 2003). The Mejillones peninsula
constitutes an anomalous geomorphological feature
Fig. 1. Right: Regional geodynamical context of the forearc of northern C
coastal cordillera, indicating faults showing evidence of recent activity (Ar
b: central depression, c: pre-cordilleran basins and d: main cordillera showi
with a mean velocity around 8 cm y�1 in a N778E direction (Somoza, 199
of northern Chile, indicating the inferred rupture lengths (modified from
Pritchard et al., 2002).
which interrupts the N–S trending coastline of north-
ern Chile (Fig. 2). Uplifted Plio-Quaternary marine
terraces and coastal deposits affected by extensional
fault scarps disposed in N–S, SW–NE and SE–NW
directions, characterize this peninsula (Fig. 3). Both
vertical uplift and brittle deformation result from
strong tectonic activity in a dominant extensional
stress regime during the Late Cenozoic (Armijo and
Thiele, 1990; Ortlieb et al., 1996b; 1996c; Delouis et
al., 1998; Gonzalez et al., 2003).
Major seismic activity in this region has been
associated with the seismogenic interplate contact
zone between both tectonic plates (Comte and Pardo,
1991; Comte et al., 1992; Comte et al., 1994). The last
great earthquake, in July 30, 1995 (Mw=8.1, hypo-
center at 23.438S and 70.488W, 36 km depth; Monfret
et al., 1995), occurred at the southern limit of the area
considered as a seismic gap region, which encom-
hile showing the main morphostructural units. a: Coastal fringe and
mijo and Thiele, 1990; Delouis et al., 1998; Marquardt et al., 2002),
ng Quaternary volcanoes. The Nazca plate moves towards the trench
8). Left: Historical seismic events of magnitude MN7 along the coast
Comte and Pardo, 1991; Delouis et al., 1997; Comte et al., 2002;
Fig. 2. 3D view of the regional bathymetry in front of the Mejillones peninsula, based on a Numerical Model of Terrain (NMT) using Kriging-
method of interpolation. Original data of bathymetry from Zapata (2001). Subdivision of the continental margin in Upper Slope (US: 1000–
3000 m), Middle Slope (MS: 3000–6000 m) and Lower Slope (LS:N6000 m) is indicated. The US is composed by coherent crust affected by
normal faults. The MS is characterized by frequent terraces containing blocks that disintegrate, forming sediments and debris that constitutes the
LS (von Huene et al., 1999).
G. Vargas et al. / Tectonophysics 399 (2005) 381–398 383
passes the southern Peru and northern Chile (Comte
and Pardo, 1991). Important coseismic vertical and
horizontal deformation (coastal uplift up to 40 cm in
the southern Mejillones peninsula and 70 cm of
continental horizontal displacement toward the trench;
Ortlieb et al., 1996a; Ruegg et al., 1996), as well as
localized superficial manifestations along some por-
tions of the Atacama fault system (Delouis et al.,
1998; Gonzalez et al., 2003), illustrates how active
processes control the present day geomorphological
configuration of this region. Block-tectonic mecha-
nisms explain localized impact of co-seismic defor-
mation during strong earthquakes, as the limited
impact of the 1995 seismic event in the northern part
of the Mejillones peninsula, at a short distance north
of the epicenter (Monfret et al., 1995; Delouis et al.,
1997; 1998). Considering that the previous large
earthquake that struck the coast of northern Chile
occurred in 1877 (Comte and Pardo, 1991), the 1995
earthquake may be viewed as a precursor of the end of
the seismic quiescence in the southern portion of the
seismic gap region (Delouis et al., 1997; Fig. 1).
This important seismic activity is a source of
instability for sedimentary systems located in the
narrow continental platform of northern Chile (Fig. 2).
The interpretation of processes yielding structural
anomalous features affecting recent sedimentary
sequences in this area can provide relevant informa-
tion related to the occurrence of strong earthquakes in
the past. The hearth of the Atacama Desert, between
198S and 248S hosted very scarce inhabitants and no
written reports are available prior to the 18th century.
Ongoing high-resolution paleoceanographic recon-
struction from the sedimentary sequence in Mejillones
bay (238S) offers a potential for the determination of
great seismic events through the recognition and
dating of anomalous sedimentological and structural
features, as discontinuities and slump deposits.
1.2. The high-resolution sedimentary sequence in
Mejillones bay
While the continental margin of northern Chile is
characterized by a steep slope, a deep trench which
Fig. 3. Bathymetric chart of the Mejillones Bay with S–N and W–E bottom profiles and location of the sedimentary cores mentioned in this
paper. Onshore geomorphological and structural indicated features include uplifted marine terraces, Quaternary shorelines assigned to the
Marine Isotopic Stages (MIS) 5, 7 and 9, as well as normal Quaternary faults (dipping block indicated; from Ortlieb et al., 1996a,b,c and
Marquardt et al., 2002).
G. Vargas et al. / Tectonophysics 399 (2005) 381–398384
reaches 8000 m and a narrow (near absent) continental
platform (in the classical sense, b200 m water depth),
two small platforms are preserved north and south of
the Mejillones peninsula (Fig. 2). The Mejillones bay
is located at the northern extremity of this peninsula
and exhibits a maximum diameter of 15 km, with 125
m of maximum depth. The western margin of the bay,
which exhibits an abrupt morphology, coincides with
the northward projection of the Mejillones fault
(Ferraris and Di Biase, 1978; Fig. 3). Gentle slopes
are observed on the eastern and southern margins,
where the basin is limited by a coastal cliff exposing
Neogene marine sequences (Ferraris and Di Biase,
1978) and uplifted Quaternary marine terraces
(Ortlieb et al., 1996c). In the center of the basin, at
depths greater than 50 m, the morphological slopes
dip less than 2%.
The sedimentation within the basin is strongly
influenced by an important upwelling cell at Punta
Angamos (Rodrıguez et al., 1991), which drives high
levels of primary productivity within the bay (Marın
et al., 1993; Marın and Olivares, 1999). As the bay is
protected from the direct influence of the dominant
SW winds, pelagic sedimentation of biogenic rest
(mainly diatom-skeletons) and organic matter is
favoured (Vargas, 1998; Valdes, 1998; Ortlieb et al.,
2000; Valdes et al., 2000; Vargas, 2002). The
preservation of these sediments is in part due to
persistent bottom hypoxia within the bay (Escribano,
1998). The accumulation of organic-rich material of
marine algal origin at a short distance from the shore
is only possible because of the lack of alluvial input in
the basin, which results from limited alluvial pro-
cesses in this region of the coastal Atacama Desert
(Vargas et al., 2000). Therefore, northward eolian
transport of particles from the lowlands of Mejillones
Peninsula is the main factor controlling the input of
lithic material to the bay (Vargas, 2002).
Through downcore 14C and 210Pb analyses, mass
accumulation rates about 0.034–0.032 g cm�2 y�1
(sedimentation rate of 160–130 cm ky�1) have been
calculated in gravity cores from the central part of the
G. Vargas et al. / Tectonophysics 399 (2005) 381–398 385
basin (Vargas, 2002), while lower (than 100 cm ky�1)
values were inferred (from 14C downcore data) in
cores from the margins (Vargas, 2002). Given the high
values of mass accumulation rates in the central area
of the bay, reconstructions of ocean-climate variability
from decadal to millennial time-scales based on the
analysis of sediment cores are possible (Ortlieb et al.,
2000; Valdes et al., 2000; Valdes and Ortlieb, 2001;
Vargas, 2002; Vargas et al., 2004). This type of studies
obviously requires that a continuous record be
preserved and that the studied sequence be not
interrupted by anomalous sedimentary features. Pre-
vious works reported at least one angular uncon-
formity in several cores retrieved from this shallow
marine basin and, in one case, evidence for a slump
deposit (Valdes, 1998; Ortlieb et al., 2000, Vargas,
2002; Vargas et al., 2002; Vargas et al., 2004).
Considering that these features could be linked to
the occurrence of major seismic events in the
southern part of the seismic gap region of northern
Chile, their identification and dating are of major
importance, both for paleoceanographic reconstruc-
tions and for paleoseismological studies. The paper
thus focuses on the relationship that can be
established between sedimentary unconformities
within the Holocene marine sequence of Mejillones
bay and the former seismic activity in northern
Chile.
2. Materials and methods
2.1. Coring
Short cores were retrieved using a 3-in. diameter
gravity core aboard the boat Puri and the R/V
Purihaalar of the University of Antofagasta (Fig. 3).
Cores 23 and 24 were retrieved in 1993, while cores
32B, 32C and 33C were obtained in 1996. Cores
F981A and 41C were obtained in 1998, using a gravity
box-core of 20 cm width and 50 cm length, and a 3-in.
diameter gravity core, respectively. All these cores
were preserved at 48C until the moment of analyses.
2.2. Sampling and sedimentology of short cores
X-ray images were taken using conventional
medical equipment with mean parameters of 50 kV,
0.16 s and 1 m distance. Contact prints (positive
phases) of radiographs were produced and treated
with specialised software to characterize grey-level
variations between adjacent lamina. In the case of core
F981A, the X-ray image was obtained from the
SCOPIX equipment at the University of Bordeaux 1
(Migeon et al., 1999).
Samples for sedimentological and mineralogical
analyses were taken every 1 cm in the case of core
33C, 0.5 cm for cores F981A and 41C. Sampling
according the limits of individual lamina, thus with
variable thickness, was done in the case of core 32B.
The water content (expressed as weight percentage)
and the bulk dry density were measured after drying
up the sediment at 40–50 8C using a F10�4 g
precision balance. In the case of core F981A, repeated
measurements provided a final accuracy of F0.01 g/
cc for dry density values. Samples of core 33C were
washed and sieved at 500 Am, 160 Am and 63 Am for
grain-size analysis. Grain size fractions are expressed
as weight percentage.
The identification and quantification (weight per-
centages) of lithic minerals and amorphous silica were
performed following the FTIR (Fourier Transformed
Infrared Spectrometry) method used at IRD laborato-
ries in Bondy. This technique described in Bertaux et
al. (1998) permits to quantify the percentage of
mineralogical species in ground sediments with a
precision of F1%.
Thin sections every 10 cm from cores 33C, F981A
and 41C, were cut from resin-impregnated samples
after a replacement of water by acetone. A detailed
petrographic study on the structure, texture, sedimen-
tology and mineralogy of these sediments was
performed using a polarised-light microscope with
magnifications of �20, �80 and �320. The high-
resolution variability (samples 1 mm thick) of
minerals in core 33C was quantified using a magni-
fication �80. The minor precision of this quantifica-
tion method is estimated as 2r of the total variance.
2.3. Chronological models for sedimentary cores
The chronological model for core F981A was
elaborated through a comparison between detailed14C and 210Pb downcore analyses (Fig. 4). The novel
methodology and the obtained results (Vargas, 2002)
will be published elsewhere. The constant mass
Fig. 4. Comparison between chronological models derived from 14C
and 210Pb analyses on samples of core F981A. The respective
values of the calculated constant mass accumulation rates are
indicated. The maximum difference between both chronological
models is 14.6 years, at 42 cm depth. Original data and method-
ology used can be found in Vargas (2002).
G. Vargas et al. / Tectonophysics 399 (2005) 381–398386
accumulation rates calculated from 14C and 210Pb
results are 34F5 g cm�2 ky�1 and 32F2 g cm�2
ky�1, respectively. A maximum difference of
14.6F0.5 years calculated between both chronolog-
ical models at the base of this core (42 cm length)
indicates that the chronological framework for this
sequence is set with a high degree of accuracy, and
can be relied upon.
A lateral correlation between core F981A and the
topmost part of cores 41C, 32B and 33C was
performed comparing the variation of the concen-
tration in lithic grains, the X-ray and photography
images. We used this correlation to calculate the
corresponding mass accumulation rates for each core.
After the corroboration for the absence of any
anomalous feature within the respective sedimentary
sequences, chronological models were derived for
the top sequence of each core, through the extrap-
olation of the corresponding mass accumulation
rates.
Taking into account the two independent methods
used for the determination of each chronology, Vargas
(2002) calculated a mean value for the Local
Reservoir Effect: DR=262F13 years, valid for radio-
carbon ages obtained from the organic fraction formed
during the last few centuries in the Mejillones bay
area. This correction is necessary to determine precise
chronological models from sediments which include
CO2 with depleted activity of 14C, such as those
produced in upwelling areas (e.g. Southon et al.,
1995). We used this value of DR to calibrate the
conventional radiocarbon ages from cores 32C and
24, which have been reported in Valdes (1998) and
Ortlieb et al. (2000), as well as for the radiocarbon
ages at the base of core 33C. For calibrations we used
Calib4.3 (Stuiver and Reimer, 1993).
2.4. Seismic reflection profiling
High-resolution seismic reflection profiles data
were obtained during November 2002 in Mejillones
bay using an ODEC Bathy 2000 acquisition device.
This equipment operates on a CHIRP mode at a main
frequency of 12 kHz, modulated over 4 kHz.
Navigation on the R/V Purihaalar, controlled by
GPS, resulted in a dense grid of profiles. Data
processing using Bathy 2000 W software provided a
vertical resolution of ~20 cm and an acoustic
penetration up to 45 m below the sea floor. Two-
way travel times were converted in depths during the
processing using an acoustic constant velocity of 1500
m/s. Based on the preliminary analysis and interpre-
tation of this data set, it was possible to map changes
in the nature of the acoustic substratum of the sea
floor in the bay, and to recognize changes in
sedimentary facies and structural features within the
overlying sediments.
3. Results
3.1. Mineralogy and high-resolution sedimentology of
short cores
The retrieved sediments correspond to a diatoma-
ceous organic-rich mud, characterized by a dominant
grain size fraction b63 Am (Fig. 5). Mean concen-
trations values of amorphous silica around 46%, 55%,
52% and 50% in the upper 40–50 cm of cores F981A,
41C, 32B and 33C, respectively, reflect the high
abundance of skeletons of siliceous phytoplankton.
The mean content of lithic grains (mainly quartz,
albite and oligoclase) diminishes with distance from
the southern shoreline, from 10% (core F981A) to 4%
(core 33C).
X-ray image and microscopic observations from
thin sections confirm the preservation of the
original structure. The thickness of the individual
laminae varies from a few millimetres to a
centimetre (or slightly more), denoting a relatively
Fig. 5. X-ray image and high-resolution variability of minerals grains content of core 33C. A: Variability (every centimetre) of grain size
fractions (in Am). B: Variability of felsic minerals grains (quartz, albite, oligoclase) per centimetre, as determined trough the FTIR method
(F1%). C: Variability of total minerals grains per centimetre, as estimated from microscope observations (F2r of total variance). Correlation
between B and C is r=0.5 at pb0.05, which increases the confidence of these estimations based on two totally different techniques. D: Laminae
limits with a millimetre precision. E: Variability of total minerals grains, as counted through microscope observations (F2r of total variance)
every millimetre, on a 7 mm2 surface. Grey thick lines figure two levels anomalously enriched in lithic grains; the older one indicated by a black
arrow, and the younger one by a white arrow.
G. Vargas et al. / Tectonophysics 399 (2005) 381–398 387
rhythmic sedimentation pattern. Microscope obser-
vations revealed that dark laminae are systemati-
cally enriched in lithic particles and coarser grain
size fractions (mean value around 80–100 Am) with
respect to the adjacent light laminae (mean value
around 40–60 Am) (Fig. 5). Dark lamina are
interpreted to be formed during periods with
intensified SW winds, enhanced upwelling and
primary productivity processes, driving higher
fluxes of biogenic remains and organic matter
toward the sea floor, with respect to adjacent light
facies (Vargas, 2002; Vargas et al., 2004).
Two unconformities, observed in core 33C, are
associated with lenticular deposits anomalously
G. Vargas et al. / Tectonophysics 399 (2005) 381–398388
enriched in lithic grains (Fig. 5). The older one is
an angular unconformity, located at a 47.5 cm
depth, which is associated with a layer exhibiting a
maximum content in lithic grains (20%, from FTIR
measurements) and a high percentage of coarse
grains. The younger one, located at a 45.5 cm
depth, is associated to a local peak in lithic grain
abundance (4%, from FTIR measurements), as
observed through the high-resolution microscope
study (every 1 mm, Fig. 5). These features are
interpreted as manifestations of at least two excep-
tional geodynamic events within the more or less
rhythmic pelagic sedimentation prevailing in the
bay.
Fig. 6. Detailed sedimentological characterization of the thin section 5
variability of the concentration in mineral grains (Nb/A is number of mine
lenses which are anomalously enriched in coarse lithic grains. The grain si
unconformity situated at 47.5 cm depth in core 33C is shown also.
The detailed characterisation of the thin section 5,
which encompasses the sequence between 40.5 and
50.5 cm depth in core 33C, revealed detailed aspects
of the deposits associated to both unconformities (Fig.
6). Grain size distributions of lithic grains contained in
these levels, show mean values around 110–115 Am,
significantly greater than those of the rest of the
sequence. As this sandy grain size fraction is
dominant in shallower areas, at 25–50 m depth, we
interpret that these deposits result from sporadic local
reworking of material within the basin. This hypoth-
esis is supported by the similar mineralogical compo-
sition of the sediments from these two lenses with
respect to that of the rest of the sequence.
(40.5–50.5 cm depth) in core 33C, showing the high-resolution
ral grains per 7 mm2). The two discontinuity levels are associated to
ze distribution of the anomalous deposit associated with the angular
G. Vargas et al. / Tectonophysics 399 (2005) 381–398 389
Ortlieb et al. (2000) reported a slump deposit
between 46 and 57.5 cm depth as well as an angular
unconformity at 60 cm depth in core 23. A similar
angular unconformity was observed at 70 cm in core
24 (Ortlieb et al., 2000) and at 74 cm depth in core
32C (Valdes, 1998). The X-ray image of this core
suggests the presence of a slump deposit between 56
and 62 cm depth (Fig. 7). These anomalous structures,
tentatively correlated with those described in core
33C, can be dated using the available geochronolog-
ical data reported in this and previous works.
Fig. 7. Photography and X-ray images of cores showing anomalous struct
(black arrow) or a discontinuity/slump deposit (white arrow) observed with
the sedimentary sections analysed by the radiocarbon method. Convention
(see Table 1). A lateral correlation of anomalous structures between all th
possibly related to sudden variations of the Local Reservoir Effect (DR).
3.2. Dating anomalous structural features observed in
short cores
Conventional and calibrated radiocarbon data for
cores 24, 32C and 33C are compiled in Table 1. The
calibrated radiocarbon data suggest that the entire
cores 24 and 32C represent roughly the last millen-
nium and not the last 2000 years as interpreted earlier
(Valdes, 1998; Ortlieb et al., 2000). Two anomalously
old dates in core 32C (1717 AD and 256 AD, Table 1)
may represent sediments affected by anomalously
ural features corresponding to a discontinuity/angular unconformity
in the sequences. Grey rectangles indicate the length and position of
al radiocarbon data and corresponding calibrated ages are indicated
ese cores is proposed. Asterisks (*) indicate anomalously old dates
Table 1
Conventional and calibrated radiocarbon ages from cores 24, 32C and 33C
Core Section depth (cm) Lab. no. Conventional age
(years BP) F1rMean calibrated
age (AD)
2r interval for
calibrated ages (AD)
24 16–25 OBDY-1428 738F70 1874 1678–1950
24 36–45 OBDY-1427 848F150 1693 1446–1950
24 81–85 OBDY-1421 1428F50 1255 1153–1313
24 91–95 OBDY-1403 1928F110 716 534–981
32C 16–18 Beta-100117 810F80T 1717T 1623–1950
32C 71–74 Beta-110118 1144F100 1449 1303–1647
32C 80–82 Beta-100119 1225F60 1408 1302–1474
32C 82–87 Pa-1620 1225F80 1408 12841498
32C 8797 Pa-1612 2375F100T 256T 28–489
33C 49–50 Beta-107629 1400F40 1278 1196–1319
33C 65–66 Beta-107630 1620F40 1038 981–1151
Conventional ages for cores 24 and 32C were from Valdes (1998) and Ortlieb et al. (2000). Calibrated ages were computed with Calib4.3
(Stuiver and Reimer, 1993) and a DR value of 262F13 years (Vargas, 2002; Vargas et al., 2002). Asterisks (T) figure anomalously old results
(see text).
G. Vargas et al. / Tectonophysics 399 (2005) 381–398390
increased DR, resulting from the inclusion of boldQorganic carbon during periods with intensified upwell-
ing processes (Vargas, 2002; Vargas et al., 2002).
Taking into account the middle points of each section
dated in core 24, assuming a date of 1993 AD (the
collection year) for the top of the sequence, and
considering the calibrated ages shown in Table 1, we
calculated sedimentation rates values between 97 and
171 cm ky�1 (Fig. 8). These values seem reasonable
with respect to the mean sedimentation rates of
131F7 cm ky�1, calculated from 210Pb data, and
167F22 cm ky�1, from 14C data, for the upper 10 cm
Core 24: Calibrated ages andSedimentation rates
2000
1750
1500
1250
1000
750
5000 10 20 30 40 50 60 70 80 90
Depth (cm)
Dat
e (A
.D.)
171 cm/ky
111 cm/ky
19 cm/ky
97 cm/ky
Fig. 8. Sedimentation rates estimated for core 24. Bars indicate the
2r range for the calibrated ages (see Table 1). Conventional
radiocarbon data from Ortlieb et al. (2000).
and the entire core F981A, respectively (Vargas, 2002;
Vargas et al., 2002).
Radiocarbon data available from sediments located
below the angular unconformity in cores 24, 32C and
33C support the lateral correlation proposed for this
feature (Fig. 7). The calibrated radiocarbon ages
suggest that the generation of the discontinuity
occurred after 1255 AD (core24), 1278 AD (core
33C) and 1408 AD (core 32C). A single radiocarbon
result from above the discontinuity in core 32C
provides a minimal calibrated age of 1449 AD. Thus,
the event responsible for the generation of this angular
unconformity might have occurred between the years
1408 and 1449 AD (1302–1647 AD considering the
2r range).
The subsequent anomalous feature, observed in
cores 23 and 32C, is tentatively correlated with the
younger discontinuity in core 33C (Fig. 7). If
correct, this lateral correlation would imply that
several tens of centimetres of the sedimentary
sequence had probably been eroded prior to this
second event. Considering the lack of radiochrono-
logical data for the upper segment of this and other
cores, it is difficult to estimate the age of this event.
An attempt to meet this aim is proposed through
lateral correlations between cores F981A, 41C, 32B
and 33C. Between cores F981A and 32B, a detailed
statistically significant (r=0.60, pb0.05) lateral cor-
relation is proposed (Fig. 9). This correlation based
upon a comparison of the downcore content in lithic
Fig. 9. Detailed lateral correlation between cores F981A and 32B, using the comparison between the X-ray images and the computed grey level,
as well as the variability of the content in felsic minerals and carbonates. An acceptable correlation coefficient of 0.60 (at pb0.05) supports the
lateral correlation using the mineral grains-content.
G. Vargas et al. / Tectonophysics 399 (2005) 381–398 391
minerals (quartz, albite and oligoclase) supports the
confidence of this approach.
The lateral correlation of two marker horizons
representing the years 1907 and 1988 AD in core
F981A, with the upper sections of cores 41C, 32B and
33C, allowed the determination of mass accumulation
rates for each sequence (Fig. 10). A chronological
model for the section located below the angular
unconformity (47.5 cm depth) in core 33C was
deduced from two calibrated radiocarbon measure-
ments (Table 1). This chronological framework
suggests that cores F981A and 32B and the upper
section of core 33C (above the younger discontinuity)
represent roughly the last 250 years. In the case of
core 41C, the extrapolation of the upper mass
accumulation rate led to interpret that the former
encompasses roughly the last 550 years (Fig. 10).
Thus, the maximum content in felsic minerals (21%)
observed at 29.5 cm depth (6.95 g cm�2 of cumulated
mass depth) in this core, could be correlated with the
peak of felsic minerals (13%) observed at 43.4 cm
depth (6.93 g cm�2) in core 32B. Since the micro-
scopic observation of thin sections of core 41C
indicates that no other anomalous structure is pre-
served within the sequence; we correlate confidently
these anomalous levels with the youngest disconti-
nuity and local maximum of felsic minerals in core
33C (at 6.81 g cm�2). This proposed lateral correla-
tion led to estimate the date of occurrence of the
youngest exceptional event between the years 1754
and 1789 AD.
3.3. Acoustic profiles description and interpretation
Near 400 km of seismic reflection profiles were
acquired in the Mejillones bay area (Fig. 11). We
recognized two major seismic units: an acoustic
substratum and overlying well stratified unit (Fig.
12). Two distinct facies observed in the acoustic
substratum are probably related to the presence of
granitic basement in the nearby area (substratum-1;
Ferraris and Di Biase, 1978) and to Neogene platform
marine deposits commonly observed along the south-
ern and eastern coastline of the bay (substratum-2;
Ferraris and Di Biase, 1978). The overlying stratified
unit has a maximum thickness where the water depth
is comprised between 50 and 100 m and shows a
progressive thinning toward the NE. Three strong
Fig. 10. Lateral correlation between the upper section of cores F981A, 41C 32B and 33C, based upon the content variation in felsic minerals.
For cores 41C, 32B and the upper part of core 33C, indicated dates (to the left of each profile) are deduced from the extrapolation of the mass
accumulation rate calculated from the correlation of two marker levels (1907 and 1988 AD) in core F981A. Dates for the base of core 33C are
from Table 1. Black and white arrows designate the same anomalous structural features as in Figs. 5, 6 and 7.
G. Vargas et al. / Tectonophysics 399 (2005) 381–398392
amplitude reflections named A, B and C (Fig. 12)
characterize this unit and appear to be either regional
(A and C) or localized (B) reflections. As C seems to
Fig. 11. A: High-resolution bathymetry of Mejillones bay (isobaths every 1
in dashed line. Profiles P1123-04 and P1125-01 are indicated. The �120 m
shoreline during the LGM, is indicated. B: 3D view of bathymetry of the
denote an important change in acoustic facies within
the stratified unit, we suggest that the origin of this
reflection be associated with the onset of the dominant
0 m). Seismic profiles lines recently acquired in this basin are shown
isobath, which could correspond to the approximate situation of the
Mejillones bay. Isobaths every 10 m. Grey levels every 100 m.
Fig. 12. Interpretation of two seismic profiles along W–E (A) and SW–NE (B) directions, illustrating the sedimentary infill of Mejillones Bay.
The upper seismic unit is characterized by two regional strong reflections (A and C) and a locally strong and discontinuous reflection (B) that is
stratigraphically associated with a major slump (contorted and chaotic reflections in the centre of the bay; C and D). The substratum shows two
seismic facies. Substratum-1 is characterized by total absorption of energy (A), while substratum-2 is characterized by strong dips towards the
NE (B). The base of the sedimentary infill develops offshore downlaps above a strong erosion surface (reflection E) on top of the acoustic
basement (parallel reflections dipping toward the NE; substratum-2; B). The uppermost seismic unit (above reflection C) forms coastal onlaps,
develops a main depocentre in the centre of the bay and thins toward the NE where reflections are parallel to reflection E.
G. Vargas et al. / Tectonophysics 399 (2005) 381–398 393
G. Vargas et al. / Tectonophysics 399 (2005) 381–398394
pelagic sedimentation within the basin. This may have
occurred in the Middle Holocene, at the end of the last
marine transgression following the Last Glacial
Maximum (LGM). During the LGM (around 20 ka),
the global sea level was roughly 120 m below its
present day position (Lambeck et al., 2002), which
means that it was below the largest part of the present
bottom of Mejillones bay (Fig. 11).
The upper seismic unit is affected by a large
chaotic body in some areas of the bay (around 75 m of
water depth), between C and A (Fig. 12). B is locally
associated with the top of this chaotic acoustic facies.
This body occurs at water depths greater than 60 m
and 100 m in the central part and NE area of this
basin, respectively. In this last area, B is clearly
associated to the development of contorted reflections,
which include C. The occurrence of these chaotic
acoustic facies along a steeper slope of the bay
suggests soft-sediment deformation related to gravity
reworking phenomena. According a preliminary
interpretation of the Bathy 2000 data, no slide or
slump scar could be recognized and the shape of this
chaotic body does not indicate sediment displacement
over a long distance. Thus, the loss of internal
stratification in this chaotic body probably resulted
from limited slumping and development of sediment
liquefaction or fluidization (Lowe, 1975; Allen, 1982;
Mulder and Cochonat, 1996). Long coring is needed
to confirm the precise nature of this major event.
4. Discussion
4.1. Sedimentary instability and earthquakes
The analysis of short cores suggests that the
presence of anomalous sedimentological and struc-
tural features is associated to exceptional events that
produce gravity instability in the sedimentary
sequence, slumping and local reworking of material
from shallower areas within the basin. In situ
liquefaction of sediments due to cyclic loading and
development of soft-sediment deformation have been
described as related to cyclic gravity acceleration
during strong earthquakes (Sims, 1975; Seilacher,
1991; Moretti et al., 1999). A similar phenomenon has
been associated to pore-pressure fluctuations caused
by fluid escape, induced by cyclic and residual stress
of storm or tsunami waves (Lowe, 1975; Allen, 1982;
Molina et al., 1998, Rossetti, 1999). While it is
accepted that storm surges result in the deposition of
discrete sedimentary units, tsunami waves generally
result in deposition of sediment sheets over relatively
wide areas and considerable distances inland (Daw-
son, 1999). Moreover, the successive run-up and
backwash of tsunami waves can produce strong
seaward currents causing additional erosion and sedi-
ment re-deposition, depending on the nature of the
local coastal topography and bathymetry (Bondevik et
al., 1997; Shi et al., 1998; Dawson, 1999).
4.2. Chronological interpretation of unconformities
and slump deposits, and their correlation with
regional seismic events
From the analysis of historical reports and
chronicles from southern Peru and northern Chile,
Comte and Pardo (1991) inferred seismic parameters
(rupture length and magnitude) for strong historical
earthquakes (MS(I)N7, where MS(I) is proportional to
Mw) since the 16th century. Return periods of
118F44 (based on four events: 1513, 1604, 1784
and 1868) and 111F33 years (based on four events:
1543, 1615, before 1768 and in 1877) were estimated
for these two areas, respectively. For the two last great
tsunamigenic earthquakes, in 1868 in southern Peru
(epicenter at 17.708S and 71.608W) and in 1877 in
northern Chile (epicenter at 21.008S and 70.258W),
magnitudes of MS(I)=8.8 and MS(I)=8.8 and rupture
lengths of 500 km and 420 km were respectively
inferred (Fig. 1).
Considering the morphological configuration of
the north-western extremity of the Mejillones pen-
insula, which protects the basin from the direct
influence of the dominant S–SW winds and associated
storms (Fig. 3), we interpret that altogether the
anomalous sedimentological and structural features
observed in the short cores and the great slump
deposit detected in the seismic profiles, most probably
reflect the effects of strong earthquakes and/or
tsunamis in Mejillones bay. We suggest that a seismic
event occurred between the years 1408 and 1449 AD,
at a time during which no written archives existed.
Such an earthquake would have produced: (1) the
angular unconformity described in cores 24, 32C, 33C
and 23, and (2) local reworking of material yielding to
G. Vargas et al. / Tectonophysics 399 (2005) 381–398 395
lens-shaped deposits enriched in lithic and coarse
grains. This event would have occurred about 94–135
years before the 1543 earthquake reported in the
Tarapaca area (epicenter at 19–208S, rupture length
LN100 km; Comte and Pardo, 1991). Such interpre-
tation would be consistent with the estimated return
period (111 years) of great seismic episodes in the
region. The second event recorded in the sediment
cores, which would have occurred between the years
1754 and 1789 AD, might correspond to the 1768
historical event documented in northern Chile (Fig. 1).
The scarcity of historical reports on this event
explains that its epicenter has been imprecisely
located around 20.58S, with an inferred MS(I)N7.7
and a rupture length LN100 km (Comte and Pardo,
1991). We surmise that this earthquake produced
liquefaction and slumping within the bay, thus
generating the slump deposit seen in core 23 (and
core 32C?). Local reworking of material from shallow
areas within the basin and its later deposition in
deeper zones possibly produced the mineral-rich
deposit observed in cores 41C, 32B and 33C. We
hypothesize that the 1768 event was also responsible
for the loss of several tens of centimetres of
sedimentary sequence at the place where core 33C
was retrieved.
4.3. On the lack of evidence for local effects of the
major 1877 earthquake
It is puzzling that we did not observe, in the studied
sedimentary sequences, any anomalous structural
feature susceptible to be associated with the major
1877 earthquake. We did not observe either anom-
alous deposits associated with the 15–20 m high
tsunami waves generated during this seismic event,
known to have destroyed the Mejillones village. It
must be noted, though, that the epicenter of this
earthquake was located several hundred kilometres
north of Mejillones bay, so that the magnitude of this
event was lower than Mw=8 in the Mejillones area.
The same inference can be made to explain the
absence of any anomalous structure susceptible to be
associated to the earthquakes of 1543 and 1615
(epicenter at 19.58S and 70.58W; MS(I)=7.9; L=137
km; Comte and Pardo, 1991). Thus, we propose that
the lack of anomalous features related with these
historical earthquakes within the studied cores, may
reflect either one, or a combination, of the following
considerations:
1. the energy associated with the propagation of these
events was significantly reduced, or even stopped,
near the Mejillones peninsula, hypothesis which is
consistent with the role of tectonic-barrier proposed
for this peninsula (Armijo and Thiele, 1990;
Delouis et al., 1997; 1998);
2. the run-up and backwash fluxes of the tsunami
waves produced during the 1877 major earthquake,
were of reduced amplitude and did not allow that
the sediment reworked in the coastal area reach the
coring sites in the centre of the bay;
3. these anomalous features were associated to local
reactivations of faults that cut the Mejillones
peninsula, which generated gravity instability
yielding anomalous structures in limited areas
within the basin.
4.4. Possible effects of reactivation of the Mejillones
fault
The last hypothesis could be relevant to explain the
generation of the great slump deposit observed
through acoustic data in the sedimentary infill of the
bay. The Mejillones peninsula displays an E–W
extensional deformation during Plio-Quaternary times
and is affected by numerous normal faults, tension
gashes, tilted blocks and vertical motions (Okada,
1971; Armijo and Thiele, 1990; Hartley and Jolley,
1995; Niemeyer et al., 1996; Delouis et al., 1998).
The N–S Mejillones fault, a main feature located at
the north-western end of this peninsula (Fig. 3),
separates two blocks which were submitted to differ-
ent long-term uplift motions since the Early Pleisto-
cene, estimated at 0.4 m/ka (west of the fault) and 0.2
m/ka (on the eastern compartment) (Ortlieb et al.,
1996a; 1996b; 1996c). New chronological data, based
on cosmogenic isotopes measurements obtained from
alluvial fan’s surfaces affected by this fault, suggest a
recent vertical normal offset around 13F1 m during
the last 43F3 ka and 5.5F0.5 m during the last 20F3
ka (Marquardt et al., 2003). A detailed interpretation
of the acoustic reflection data is still pending, but
observations of profiles obtained on the western
margin of the basin already show an important
morphological scarp displacing 10–15 m the acoustic
G. Vargas et al. / Tectonophysics 399 (2005) 381–398396
substratum-1, which is systematically disposed along
the northward projection of the Mejillones fault (Fig.
12A). Considering the 5.5F0.5 m vertical offset
measured along this fault for the post-LGM period,
it is possible that local re-activations of this fault
during strong regional earthquakes occurred coevally
with the deposition of the Holocene sedimentary infill
within the basin. Actually, Delouis et al. (1998)
previously discussed the possible link between local
reactivations of this fault during some strong sub-
duction seismic events, in the light of cumulative
Coulomb’s stress in a regional E–W extensional
regime. Similar phenomena were envisioned along
some specific portions of the Atacama fault system
during the Antofagasta 1995 seismic event (Delouis et
al., 1998; Gonzalez et al., 2003).
Taking into account the mass accumulation rates
estimated from the analysis of short cores and
considering the Late Quaternary age inferred for the
whole upper seismic unit, we suggest that an
important seismic event occurred during or shortly
after the Middle Holocene. This event may have
generated a reactivation of the Mejillones fault and
induced gravity instability and slumping within the
Mejillones bay, as evidenced by acoustic data. The
wide extension of reflection B may reflect the effects
of backwash flows along the sandy coastal sediments
which succeeded to tsunami waves. We furthermore
surmise that this major tectonic event might have
played a role in the carving of the several-metre-high
cliffs which now border the southern and eastern
shores of Mejillones bay.
4.5. Final considerations
The study shows how a detailed chronological
analysis of a laminated marine sequence may yield
relevant information for both paleoceanographic and
paleoseismic reconstructions. The Holocene sedi-
mentary record preserved in Mejillones bay is quite
favourable to develop high-resolution chronological
models encompassing the last few centuries.
Ongoing and future works will thus be concentrated
on the detailed 3D interpretation of the acoustic data
and the recuperation of longer sediment cores in the
same bay that encompass longer and older time
periods. Such studies, which will involve the upper
reflection horizons identified in the seismic profiles,
should provide a more complete picture of the effect
of paleoseismic events during the last few millennia
on the recent sedimentary infill in this basin.
From the point of view of the impacts of both the
regional seismic and local faulting activities, we still
need to test hypotheses and work on the propagation
modelling of the energy released during seismic
events all along the rupture zones, the deformation
mechanisms proper to the Mejillones fault and the
propagation of tsunami waves within the Mejillones
bay.
Acknowledgements
Several aspects of this work was supported by
the program Paleotropique (Paleo-environnements
tropicaux et variabilite climatique), of IRD, France.
Acquisition of seismic profile data was partly
supported by the PCCI project bEstudio sısmico
de alta resolucion del relleno sedimentario holoceno
de la bahıa Mejillones del Sur: implicancias neo-
tectonicas y oceano-climaticasQ, of CONICYT,
Chile, and the PRODAC (Programa de Dinamica
de la Atmosfera y del Clima) of the University of
Chile.
The Department of Geology and Oceanography
of the University of Bordeaux I, the laboratory of
IRD at Bondy, the Department of Geology of the
University of Chile and the FAREMAR of the
University of Antofagasta, Chile, greatly helped for
the acquisition of cores, treatment of samples and
analyses, as well as for the acquisition of seismic
profile data.
We thank J.F. Saliege (LODYC, Univ. Paris-
CNRS-IRD) for radiocarbon data on several cores,
and Laurent Perderau (ISTO, Universite d’Orleans)
for his useful help during the acquisition of seismic
profiles. The preparation of the seismic survey
benefited largely from Emilio Vera’s (University of
Chile) experience in marine geophysics of the area.
Two anonymous reviewers provided useful
comments.
Jacques Bertaux (from IRD, Bondy) provided
important support for FTIR mineralogical analyses
of the short cores. Jacques Bertaux died quite
prematurely in August 2002, and we all miss him.
This paper is dedicated to his memory.
G. Vargas et al. / Tectonophysics 399 (2005) 381–398 397
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