Morphology and geology of the continental shelf and upper slope of southern Central Chile (33°S–43°S)

Abstract

The continental shelf and slope of southern
Central Chile have been subject to a number of international
as well as Chilean research campaigns over the last
30 years. This work summarizes the geologic setting of the
southern Central Chilean Continental shelf (33�S–43�S)
using recently published geophysical, seismological, sedimentological
and bio-geochemical data. Additionally,
unpublished data such as reflection seismic profiles, swath
bathymetry and observations on biota that allow further
insights into the evolution of this continental platform are
integrated. The outcome is an overview of the current
knowledge about the geology of the southern Central Chilean
shelf and upper slope. We observe both patches of
reduced as well as high recent sedimentation on the shelf
and upper slope, due to local redistribution of fluvial input,
mainly governed by bottom currents and submarine canyons
and highly productive upwelling zones. Shelf basins show
highly variable thickness of Oligocene-Quaternary sedimentary
units that are dissected by the marine continuations
of upper plate faults known from land. Seismic velocity
studies indicate that a paleo-accretionary complex that is
sandwiched between the present, relatively small active
accretionary prism and the continental crust forms the bulk
of the continental margin of southern Central Chile.

Full text

ORIGINAL PAPER
Morphology and geology of the continental shelf and upper slope
of southern Central Chile (33
°
S–43
°
S)
David Vo
¨
lker
•
Jacob Geersen
•
Eduardo Contreras-Reyes
•
Javier Sellanes
•
Silvio Pantoja
•
Wolfgang Rabbel
•
Martin Thorwart
•
Christian Reichert
•
Martin Block
•
Wilhelm Reimer Weinrebe
Received: 14 October 2011 / Accepted: 21 May 2012
Ó Springer-Verlag 2012
Abstract The continental shelf and slope of southern
Central Chile have been subject to a number of interna-
tional as well as Chilean research campaigns over the last
30 years. This work summarizes the geologic setting of the
southern Central Chilean Continental shelf (33°S–43°S)
using recently published geophysical, seismological, sedi-
mentological and bio-geochemical data. Additionally,
unpublished data such as reflection seismic profiles, swath
bathymetry and observations on biota that allow further
insights into the evolution of this continental platform are
integrated. The outcome is an overview of the current
knowledge about the geology of the southern Central Chil-
ean shelf and upper slope. We observe both patches of
reduced as well as high recent sedimentation on the shelf
and upper slope, due to local redistribution of fluvial input,
mainly governed by bottom currents and submarine canyons
and highly productive upwelling zones. Shelf basins show
highly variable thickness of Oligocene-Quaternary sedi-
mentary units that are dissected by the marine continuations
of upper plate faults known from land. Seismic velocity
studies indicate that a paleo-accretionary complex that is
sandwiched between the present, relatively small active
accretionary prism and the continental crust forms the bulk
of the continental margin of southern Central Chile.
Keywords Southern Central Chile  Bathymetry 
Shelf sedimentation  Shelf basins  Submarine faults 
Seismicity  Fluid seepage
Introduction
The southern Central Chilean continental margin has been
subject to geological and geophysical research for the last
D. Vo
¨
lker (&)  J. Geersen  W. R. Weinrebe
Collaborative Research Center (SFB) 574 at the GEOMAR,
Helmholtz Centre for Ocean Research Kiel,
Wischhofstrasse 1-3, 24148 Kiel, Germany
e-mail: dvoelker@geomar.de
J. Geersen
e-mail: jgeersen@geomar.de
W. R. Weinrebe
e-mail: wweinrebe@geomar.de
E. Contreras-Reyes
Departamento de Geofı
´
sica, Universidad de Chile,
Santiago, Chile
e-mail: econtreras@dgf.uchile.cl
J. Sellanes
Departamento de Biologı
´
a Marina,
Universidad Cato
´
lica del Norte, Coquimbo, Chile
e-mail: sellanes@ucn.cl
S. Pantoja
Departamento de Oceanografı
´
a y Centro de Investigacio
´
n
Oceanogra
´
fica en el Pacı
´
fico Sur-Oriental, Universidad
de Concepcio
´
n, Concepcio
´
n, Chile
e-mail: spantoja@udec.cl
W. Rabbel  M. Thorwart
Institute of Geosciences, Collaborative Research Center (SFB)
574 at the University of Kiel, Kiel, Germany
e-mail: wrabbel@geophysik.uni-kiel.de
M. Thorwart
e-mail: thorwart@geophysik.uni-kiel.de
C. Reichert  M. Block
BGR Hannover, Hannover, Germany
e-mail: Christian.Reichert@bgr.de
M. Block
e-mail: Martin.Block@bgr.de
123
Int J Earth Sci (Geol Rundsch)
DOI 10.1007/s00531-012-0795-y

decades, resulting in a variety of different data sets from
international and Chilean research cruises (Table 1). Some
of the data are published in international peer-reviewed
journals, but a number of data sets, and in particular those
collected over recent years, remain yet unpublished. In
addition, a few review articles and book chapters combine
and summarize aspects of the conducted research, and
especially the book ‘‘The Andes-Active Subduction Orog-
eny’’ (Oncken et al. 2006) provides an important overview.
However, no review article summarizes the geologic setting
of the continental shelf and uppermost slope of southern
Central Chile. Here, we tie together the current knowledge
about the geology of that part of the Chilean Shelf that has
the best data coverage (between 33°Sand43°S) by
reviewing previously published data sets and expand this
knowledge by adding previously unpublished seismic
reflection, bathymetric, seismological and biological data as
well as data sets that were published in cruise reports and
doctoral theses. The new data include (1) today’s most
comprehensive bathymetric data which we produced from
swath bathymetric data of eight scientific cruises (Weinrebe
et al. 2011) merged with a bathymetric data set of Zapata
(2001); (2) unpublished sediment-echosounder data from the
continental slope; (3) an unpublished seismic reflection line
that runs N–S from 36.75°Sto40°S (SO161-25). The
information that is assembled in this work covers the region
with a highly variable density. While we provide seismic
insights into some forearc basins (Arauco and Valdivia
Basin, Fig. 1), we lack similar data of neighbouring basins.
In total, there is still a lack of data for the comprehensive
understanding of the continental shelf, such as of bathy-
metric mapping campaigns of the shelf, margin-parallel
seismic reflection profile studies, drilling transects and bot-
tom current measurements.
Geologic and tectonic framework of the southern
Central Chilean continental margin
The convergent continental margin of southern Central
Chile between 33°S and 43°S is characterized by a trench
that is filled by up to 2.5 km of sediment. This sediment-
filled part of the Peru–Chile Trench is limited by two
elevated topographic features of the oceanic crust, the Juan
Ferna
´
ndez Ridge that enters the subduction zone at 32°S
and the Chile Ridge that subducts at the Chile Triple
Junction at around 46°S (Fig. 1). The main structural ele-
ments across the marine part of the continental margin are
the 40–80 km wide Peru–Chile Trench Basin, a young (late
Pliocene–Pleistocene) frontal accretionary prism at the
lower slope that is 5–40 km wide (Bangs and Cande 1997;
Contreras-Reyes et al. 2010; Geersen et al. 2011a), a rel-
atively smooth upper continental slope with sedimentary
slope basins, thrust ridges, deeply incised submarine
canyon systems and mass-wasting features and a conti-
nental shelf that is dissected by submarine canyons and
partly shaped by mass-wasting features (Fig. 1; detailed
maps Figures 2–6).
The tectonic framework of the southern Central Chilean
continental margin is controlled by the subduction of the
oceanic Nazca Plate underneath the South American Plate
(Fig. 1). The Nazca Plate subducts obliquely with an angle
of 80.1° at a rate of 66 mm/a (Angermann et al. 1999). The
subduction rate has varied in the past and decreased
*40 % over the last 20 Ma (Somoza 1998; Oncken et al.
2006). The volume of the 5–40 km wide accretionary
prism is not compatible with a continuous history of
accretion over time periods of tens of millions of years,
which implies episodic phases of tectonic accretion, non-
accretion and erosion (Bangs and Cande 1997
). Melnick
and Echtler (2006a) argued that during Pliocene the margin
shifted from erosive to accretionary mode in response to an
increase in trench sedimentation rate, linked to fast denu-
dation of the Andes and a coeval decrease of the subduc-
tion rate. Similarly, Kukowski and Oncken (2006)
suggested that the southern Central Chile subduction zone
has been in accretion mode since the Pliocene, following
on a period of subduction erosion that started at least in the
middle Miocene. The subduction process impacts on the
evolution of the shelf and upper slope of southern Central
Chile in a number of ways:
1. The subduction of submarine ridges, seamounts and
thickened crust has led to mass removal and local
subsidence of the marine forearc, for example at 33°S
where the subduction of the Juan Fernandez Ridge is
taking place. This process created accommodation
space for marine forearc basins on the upper conti-
nental slope that form important depocentres for
sediments close to the continental shelf (von Huene
et al. 1997; Laursen et al. 2002).
2. Basal accretion of underthrust trench sediments has
been made responsible for focused and localized uplift
of coast and shelf segments in particular off Arauco
Peninsula (Lohrmann et al. 2006). Here, *1.5 km of
uplift during Middle Pliocene has been reported
(Melnick and Echtler 2006a; Melnick et al. 2006).
3. Oblique subduction of the Nazca Plate is responsible
for the development of a forearc sliver, the Chiloe
´
Microplate, that extends from the Chile Triple Junction
at *46°S to the Arauco Peninsula at *38°S (Melnick
et al. 2009). The Chiloe
´
Microplate is decoupled from
the stable South American Plate along the Liquin
˜
e-
Ofqui Fault Zone (LOFZ, Fig. 1), a prominent margin-
parallel fault system that has been active in a
transpressional dextral motion since the Pliocene
(Cembrano et al. 2000; Rosenau 2004; Rosenau et al.
Int J Earth Sci (Geol Rundsch)
123

Table 1 Meta-information on cruises of German vessels SONNE and METEOR as well as raw bathymetric data sets are stored at the Bundesamt fu
¨
r Seeschiffahrt und Hydrographie (BSH) at
http://www.bsh.de
Inventory of scientific cruises and geophysical data offshore southern Central Chile
Ruise/ship expedition Method Data Latitudes Partly published in Cruise reports or chief scientist
RV SONNE SO101-102 Swath bathymetry Along track 21°S–44°S von Huene et al. (1997), this study Hebbeln and Wefer (1995)
Sediment echosounder Along track
Reflection seismics 37 profiles Flueh et al. (1998)
Laursen et al. (2002)
Laursen and Normark (2003)
Multicores 42 cores Lamy et al. (1998, 2001)
Hebbeln et al. (2000)
Gravity cores 63 cores Lamy et al. (1999)
RV SONNE SO103 Swath bathymetry Along track 31.5°S–34°S This study Flueh (1995)
Sediment echosounder Along track
RV SONNE SO104 Swath bathymetry Along track 20°S–32°S Laursen and Normark (2002), this study R. von Huene
Sediment echosounder Along track
Reflection seismics 14 profiles von Huene et al. (1997)
Laursen et al. (2002)
Laursen and Normark (2002)
Ranero et al. (2006)
RV SONNE SO156 Swath bathymetry Along track 22°S–44°S This study Hebbeln (2001)
Sediment echosounder Along track
Multicores 163 cores Mun
˜
oz et al. (2004)
Stuut et al. (2007)
Gravity cores 91 cores
RV SONNE SO161 Swath bathymetry Along track 23°S–38°S This study Reichert (2002)
Flueh et al. (2002)
Wiedicke-Hombach (2002)
Sediment echosounder Along track This study
Reflection seismics 35 profiles Rauch (2005)
Ranero et al. (2006)
Vo
¨
lker et al. (2006)
Contreras-Reyes et al. (2008)
Rodrigo et al. (2009)
Geersen et al. (2011b)
Gravity cores 29 cores Vo
¨
lker et al. (2008)
Blumberg et al. (2008)
Int J Earth Sci (Geol Rundsch)
123

Table 1 continued
Inventory of scientific cruises and geophysical data offshore southern Central Chile
Ruise/ship expedition Method Data Latitudes Partly published in Cruise reports or chief scientist
RV SONNE SO181 Swath bathymetry Along track 31°S–47°S This study Flueh and Grevemeyer (2005)
Sediment echosounder Along track
Reflection seismics 4 profiles Contreras-Reyes et al. (2008, 2010)
Scherwath et al. (2009)
Gravity cores 16 cores Heberer et al. (2010)
RV SONNE SO210 Swath bathymetry Along track 33°S–38.5°SVo
¨
lker et al. (2012)
Vo
¨
lker et al. (2011), this study
Linke (2011)
Sediment echosounder Along track This study
Gravity cores 14 cores This study
RRS James Cook JC23 Swath bathymetry Along track 33°S–38°SVo
¨
lker et al. (2009)
Geersen et al. (2011a), this study
Flueh and Bialas (2008)
Gravity cores 13 cores Vo
¨
lker et al. (2009)
Sidescan sonar 4 profiles Klaucke et al. (2012)
Wide-angle seismic 4 profiles Moscoso et al. (2011)
R/V CONRAD 2901 Reflection seismics 6 profiles 32°S–40°S Bangs and Cande (1997)
Dı
´
az-Naveas (1999)
Contreras-Reyes et al. (2008)
Scherwath et al. (2009)
Geersen et al.(2011a)
R/V VIDAL GORMAZ 1994 (Thioplaca) Multicores 5 cores Fossing et al. (1995)
Lamy (1998)
Hebbeln et al. (2000)
R/V VIDAL GORMAZ VG02 Swath bathymetry Along track 31°S–34°S This study J. Dı
´
az-Naveas
Reflection seismics 18 profiles Contardo et al. (2008)
R/V VIDAL GORMAZ VG06 Swath bathymetry Along track 32.5°S–37°SJ.Dı
´
az-Naveas
Reflection seismics Contardo et al. (2008)
RV METEOR M67 Swath bathymetry Along track 33°S–37°S This study Weinrebe and Schenk (2006)
Sediment echosounder Along track
Gravity cores 8 gravity cores
RV MELVILLE MV1004 Swath bathymetry Along track 34°S–38°S C. D. Chadwell
ODP leg 202 Drill cores ODP sites 1233, 1234, 1235 35°S–40°S Blumberg et al. (2008) Mix et al. (2003)
Metadata on cruises of US research vessels are stored at the Geological Data Center of the Scripps Institute of Oceanography: http://gdc.ucsd.edu/
Int J Earth Sci (Geol Rundsch)
123

2006; Thomson 2002). The collision of the Chiloe
´
Microplate that moves northward at a present rate of
6.5 mm/a (Wang et al. 2007) with the South American
Plate in the region of the Arauco Peninsula is a likely
cause for the existence of a number of SE-NW trending
upper plate faults that are mapped in the terrestrial forearc
(Melnick and Echtler 2006b; Melnick et al. 2009).
Active shortening across such faults caused the orogen-
esis of the Nahuelbuta Coastal Range (Melnick et al.
2009). An indirect consequence of the differential uplift is
the shift and reorganization of river networks on land and
their respective submarine continuations (canyon sys-
tems) that cut deeply into the shelf and continental slope
(Rehak et al. 2008).
4. The Chilean subduction zone produces powerful
megathrust earthquakes of Mw [ 8 almost in decadal
intervals (e.g. Bilek 2010), and the historic record
shows that these recur in spatially defined seismo-
tectonic segments of the forearc (Lomnitz 2004). In the
study area, the Mw 9.5 Great Chile Earthquake of 22
May 1960 and the Mw 8.8 Maule Earthquake of 27
February 2010 stand out as the largest and sixth largest
ever instrumentally recorded earthquakes in the world.
The earthquakes have historical recurrence times of
100–200 years per segment and cause coseismic
horizontal motions of some 10 m and vertical motions
of some metres of the coastal areas and the shelf
(Cifuentes 1989; Barrientos and Ward 1990; Cisternas
et al. 2005; Moreno et al. 2009; Farı
´
as et al. 2010).
Megathrust earthquakes were historically and recently
associated with tsunamis that devastated coastal areas
and deposited specific tsunami deposits in estuaries
(Cisternas et al. 2005; Vargas et al. 2011).
Climate
The denudation rate of the Andes shows a distinct climatic
component related to Hadley cell-driven precipitation
regimes (Montgomery et al. 2001). The central part of the
range (15°S–33°S) is in the subtropical belt of deserts,
where there is little precipitation on either side of the range.
To the south, Westerlies bring abundant moisture that
precipitates at the western slopes of the Andes, resulting in
a significant increase in the mean annual precipitation rate
from \0.5 m/y at around 30°S to 2–3 m/y south of 38°S
(Hoffmann 1975) and a mean annual river run-off of
0.25–0.5 mm/y (Fekete et al. 2000). In the vicinity of
the Chile Triple Junction (46°S), apatite fission track ages
from the western flank of the Andes imply that 3–4 km
of denudation occurred in this region since *17 Ma
(Haschke et al.
2006).
Oceanographic features
The poleward Gunther Undercurrent (or Poleward Under-
current) at 0.2–0.5 km water depth flows close enough to
the edge of the shelf to induce coast-parallel southward
Fig. 1 Overview map of southern Central Chile with main tectonic
units. Boxes indicate the positions of Figures 2–6. The lighter shaded
area corresponds to the outline of the Chiloe
´
Microplate (Melnick
et al. 2009). The position of shelf basins (stippled lines) is according
to Melnick and Echtler (2006b). Yellow triangles are Quaternary
volcanoes of the Southern Volcanic Zone (Siebert and Simkin 2002).
LOFZ Liquin
˜
e-Ofqui Fault Zone, LF Lanalhue Fault, JFR Juan
Ferna
´
ndez Ridge, CTJ Chile triple junction, NB Navidad Basin, CB
Chanco Basin, IB Itata Basin, AB Arauco Basin, VB Valdivia Basin,
PB Pucatrihue Basin, ChB Chiloe
´
Basin
Int J Earth Sci (Geol Rundsch)
123

sediment transport. Current velocities from 0.1 to 0.5 m/s
at depths of 100–300 m were measured (Huyer et al. 1991;
Pizarro et al. 2002). Shaffer et al. (1995, 1997, 1999)
reported a mean value of 0.128 m/s and a maximum value
of 0.689 m/s over a period of 6 years.
The coasts of Valparaı
´
so and Concepcio
´
n are well
known zones of intense coastal upwelling (e.g. Djurfeldt
1989; Figueroa and Moffat 2000). These conditions lead to
extremely high biogenic productivity (Daneri et al. 2000;
Atkinson et al. 2002) and carbon fixation resulting in
annual production rates of [200 g C/m
2
(Berger et al.
1987) which has a pronounced impact on the slope sedi-
mentation (Hebbeln et al. 2000). South of 38°S, prevailing
onshore winds of the Westerlies generally prevent coastal
upwelling (Strub et al. 1998), but nonetheless areas of high
primary productivity exist south of 40°S. Hebbeln et al.
(2000) propose either advection of the Antarctic Circum-
polar Current and/or river input as nutrient sources that
sustain this effect. Tidal currents can have a strong effect
on local deposition as they are very strong at the outlets
of estuaries and in particular at the outlet of the Gulf of
Ancud, the Chacao Channel (*4 m/s, Ca
´
ceres et al. 2003,
Fig. 6).
Sediment input
The main source area for sediments deposited at the
southern Central Chile continental margin is the western
flank of the Andean Cordillera. Sediments are brought to
the Pacific Ocean mainly by river systems (Lamy et al.
1998, 1999) that emerge from the Andean Cordillera, cross
the Central Valley of Chile and the Coastal Cordillera and
partly continue in submarine canyon systems. A fraction of
the clastic material eroded from the Andes forms the fill of
the Central Valley, another fraction is deposited in sedi-
mentary basins of the submarine forearc and the open slope
or is temporarily stored in the submarine canyons (Raitzsch
et al. 2007). A second source of sediments is the Coastal
Cordillera that reaches elevations of 2,200 m in the Val-
paraı
´
so region (33°S) and almost 1,600 m in the Cordillera
de Nahuelbuta (* 37.5°S, Fig. 4). Strong precipitation and
high river discharge transport huge amounts of terrigenous
matter to the ocean between 35° and 39°S (suspended
particles = 600–2,500 ton per month; http://www.dga.cl;
Mun
˜
oz et al. 2004).
The Southern Volcanic Zone (SVZ) of the Andes (33°S–
46°S) is associated with the Nazca Plate subduction
(Lo
´
pez-Escobar et al. 1993). The SVZ includes at least 60
historically and potentially active volcanic edifices in Chile
and Argentina, three giant silicic caldera systems (Maipo,
Calabozos and Caviahue
´
) and numerous minor eruptive
centres (Siebert and Simkin 2002; Stern 2004; Stern et al.
2007). Explosive volcanism has led to the deposition of ash
fallout deposits in prehistoric and historic eruptions (e.g.
Hildreth et al. 1984; Haberle and Lumley 1998; Naranjo
and Stern 1998, 2004; Hildreth and Drake 1992; Sruoga
et al. 2005), and the tephra layers are widespread over
Chile and Argentina. A fraction of this volcanic ash has
been deposited offshore (ODP leg 202 sites 1233,
1234,1235, Mix et al. 2003;Vo
¨
lker et al. 2006, 2009; Linke
2011).
The third major source of sediment particles is the
biogenic production related to the coastal upwelling zones.
Biogenic constituents vary in abundance and consist pri-
marily of nannofossils and diatoms with less abundant
silicoflagellates and foraminifers in general (Mix et al.
2003). Variations in the abundance and species composi-
tion of foraminifera are related to water productivity and
regional variations of upwelling (Hebbeln et al. 2000).
Patches of authigenic carbonates are observed at the sea-
floor in areas where methane seepage is reported (Linke
2011). The microbial process of anaerobic oxidation of
methane is, however, mainly restricted to the Oxygen
Minimum Zone below *800 m water depth and is related
to the high productivity areas of coastal upwelling (Treude
et al. 2005).
Morphology of the continental shelf and upper slope
The morphological information described here and pre-
sented in Figs. 2, 3, 4, 5, 6 is the result of joining swath
bathymetry data that were recorded on 12 cruises of
research vessels SONNE, METEOR, VIDAL GORMAZ
and JAMES COOK between 1995 and December 2010
(Table 1) and that mainly cover the continental slope with
a gridded Chilean data set of the shelf morphology (Zapata
2001). The swath bathymetry data, in total more than 8,000
data files comprising about 1.1 billion soundings, were
recorded with different swath bathymetry systems, but
mostly with the Kongsberg EM-120 system. We processed
the raw data using the MB-Systems software (Caress et al.
1996). Processing steps comprised the check of navigation
data, interpolation of missing navigation values, calcula-
tion of water depth and positions of the footprints of the
beams by ray tracing through the water column and
removal of artefacts and erroneous data points. Processed
data of the bathymetric systems were then combined into
digital elevation models (DEMs) with a grid point density
of 200 m. Then, we merged the grids with a clipped ver-
sion of the gridded bathymetry data of Zapata (2001). The
Zapata data set has a lower grid point density of about
800 m and lacks many details that show up in the raw
bathymetry data, but as those seldom cover the shelf we
use the Zapata data set as background information. We
clipped the Zapata data to the water depth range of
Int J Earth Sci (Geol Rundsch)
123

0–400 m and included them to the grid calculation with a
low weighting factor for the splines calculation of new grid
points. The resulting grid is satisfactory for most of the
shelf and slope areas. However, in some places, the inter-
polation between the high-density raw and low-density
gridded data produces artefacts which we had to remove
manually. Finally, the grids were combined with the land
topography data of the Shuttle Radar Topography Mission
(SRTM; Farr 2007).
Between 33°S and 43°S, the continental shelf is rela-
tively narrow with a mean width of 30–40 km (Figs. 2, 3,
4, 5, 6). The width of the shelf from the coast to the shelf
break at * 200 m water depth is narrower offshore prom-
inent promontories as well as where the Coastal Cordillera
is close to the coast such as offshore Valparaı
´
so (33°S)
and Pichilemu (34.5°S) (width 20 km), while coastal
embayments such as the Golfo de Arauco (37°S) form
regions of a wider continental platform (width 40 km). The
maximum width is offshore Arauco Peninsula at Mocha
Island and offshore Chiloe
´
Island (*60 km).
At 41.8°S, the N–S trending coastline is interrupted by
the Canal de Chacao that separates Chiloe
´
Island from the
mainland (Fig. 6). As the continental forearc subsides to
the south, the Coastal Cordillera continues as the backbone
of Chiloe
´
Island, whereas the southward continuation of the
Chilean Central Valley is drowned to form the shallow
(\250 m) Golfo Corcovado and Golfo de Ancud between
the mainland and Chiloe
´
Island. This 45-km-wide (E–W)
and 90-km-long (N–S) gulf is a semi-enclosed marine
forearc basin in the back of Chiloe
´
Island that is protected
Fig. 2 Bathymetric map of the
Chilean shelf and upper slope
from 33°Sto35°S. Blue circles
denote epicentres of aftershocks
of the Feb 27, 2010, Mw 8.8
Maule earthquake (Servicio
Sismolo
´
gico de Chile,
ssn.dgf.uchile.cl, time window
of 27.02.2010–13.04.2010). The
Pichilemu seismic sequence of
the 11.03.2010 (Farı
´
as et al.
2011) is highlighted by yellow
fill. Square symbols indicate
sediment samples described by
Hebbeln et al. (2000) and Lamy
et al. (1998). Stippled black
lines correspond to outlines of
shelf basins. Bathymetric
information is composed of a
data set of a number of RV
SONNE cruises, RRS JAMES
COOK cruise JC23 and a
shallow-water data set compiled
by Zapata (2001). Land
topography was extracted from
the SRTM data set (Farr et al.
2007), absence of bathymetric
information is indicated as grey
areas. Slope canyon names are
in yellow boxes
Int J Earth Sci (Geol Rundsch)
123

from the direct Pacific swell and unique for Chile in this
respect. With the subsidence of the Central Valley to below
sea level, the coastline south of 42°S directly touches the
Central Cordillera of the Andes and forms a fjord coast.
The shelf edge lies at a water depth range of 150–300 m.
It is well defined where the shelf is wider such as offshore
the Golfo de Arauco (37°S, Fig. 4), west of Mocha Island
(38.2°S Fig. 4) and offshore Chiloe
´
Island. Offshore
Arauco Peninsula, the shelf edge is at some places indented
by headscarps of ancient giant slope failures (Geersen et al.
2011b).
The upper continental slope shows a relatively smooth
morphology and is inclined at low angles (2–4°) to a water
depth of 2,000 m. Below 2,000 m water depth, the slope
morphology is less regular with steep slope segments (up to
30°) alternating with roughly trench-parallel belts of less
steep and even landward verging seafloor. This irregular
morphology is caused by the continuous deformation of the
*4 Ma young accretionary prism that forms the lower
continental slope.
Submarine canyon systems
A number of submarine canyons dissect the continental
slope and partly cut into the shelf to connect directly to
feeding river systems. We use the nomenclature of Rodrigo
(2010). From north to south, the submarine canyons are (1)
San Antonio Canyon, connected to the mouth of Rı
´
o Maipo
(Hagen et al. 1996; Laursen and Normark 2002, Fig. 2); (2)
Rapel Canyon; (3) Mataquito Canyon between 34°S and
34.7°S that is possibly linked to Rı
´
o Mataquillo/Mataquito
(Fig. 2); (4) Maule Canyon, connected to Rı
´
o Maule
(Fig. 3); (5) Itata Canyon, connected to Rı
´
o Itata (Fig. 3);
(6) the prominent Bı
´
oBı
´
o Canyon (with its major conflu-
ence Santa Maria Canyon) that cuts deep into the shelf and
forms a direct continuation of the BioBı
´
o River (Fig. 3);
(7) Lleulleu Canyon (or Paleo-Pellahuen Canyon) directly
north of Mocha Island once formed the marine continua-
tion of Pellahuen River before the latter was deflected due
to uplift of Arauco Peninsula according to Rehak et al.
(2008, Fig. 4); (8) Imperial Canyon is a canyon that is
Fig. 3 Bathymetric map of the
Chilean shelf and upper slope
from 35°Sto37°S (Maule
Province). Black lines indicate
the position of seismic profiles
of the SPOC project shot on RV
SONNE cruise SO161 (Reichert
2002). Pink lines indicate
PARASOUND sediment-
echosounder profiles that are
referred to in the text. Circles
denote epicentres of main and
aftershocks of the Maule
earthquake (Servicio
Sismolo
´
gico de Chile,
ssn.dgf.uchile.cl, time window
of 27.02.2010–13.04.2010 blue,
main shock: blue filled) and of
Bohm et al. 2002 (magenta).
Red diamonds indicate ODP leg
202 drill sites. Square symbols
indicate sediment samples
described by Hebbeln et al.
(2000) and Lamy et al. (1998).
CMSA concepcio
´
n methane
seepage area. Yellow stars show
observations of biocommunities
related to gas seepage (Sellanes
et al. 2004; Sellanes and
Krylova 2005). Stippled black
lines correspond to outlines of
shelf basins. Absence of
bathymetric information is
indicated as grey areas
Int J Earth Sci (Geol Rundsch)
123

supposed to incise the shelf directly south of Mocha Island
but is unresolved in our data set as we lack precise shelf
bathymetry and as the slope is deformed by a giant slope
failure (Geersen et al. 2011b); (9) Tolten Canyon might be
related to the river systems of Tolten and Imperial, but we
lack precise bathymetric data of the shelf area to test the
relationship (Fig. 4); (10 and 11) Lingue Canyon and
Callecalle Canyon, two closely spaced submarine canyon
systems lie offshore Valdivia: the northern one seems to be
related to the exit of Rı
´
o Valdivia, whereas the southern one
might rather be connected to Rı
´
o Bueno (Fig. 5). Both
canyons were confusingly referred to as Rı
´
o Bueno, Cal-
leCalle or Tolten Canyons (Thornburg et al. 1990;Vo
¨
lker
et al. 2006; Raitzsch et al. 2007; Rehak et al. 2008); (12)
Chaihuin Canyon south of Valdivia is not resolved in our
data; (13) Chacao Canyon in continuation of the Chacao
Channel (Fig. 6) and (14) Cucao Canyon offshore Chiloe
´
Island (Fig. 6). As a number of the canyons cut deeply into
the shelf, they should form effective traps for sediment that
is transported coast-parallel on the shelf by bottom currents.
The role that canyon systems play in supplying sediment
to the trench is evidenced by the submarine fan systems
that exist at the exits of the larger canyons (Valdivia
Canyon, Tolten Canyon, BioBio Canyon). These sub-
marine fans are asymmetrical and their northern (down-
slope) morphology contrasts to their southern (upslope)
morphology as they have compositional structures (fan
lobes) to the south and lag deposits and erosional structures
Fig. 4 Bathymetric map of the
Chilean shelf and upper slope
from 37°Sto39°S (Arauco
Peninsula). Black lines indicate
the position of seismic profiles
of the SPOC project shot on RV
SONNE cruise SO161 (Reichert
2002). Pink lines indicate
PARASOUND sediment-
echosounder profiles that are
referred to in the text. Yellow
stippled lines denote headwall
and sidewalls of giant slope
failures of Geersen et al.
(2011b). Circles denote
epicentres (blue: main and
aftershocks of the Maule
earthquake, Servicio
Sismolo
´
gico de
Chile, ssn.dgf.uchile.cl/,
magenta: Bohm et al. 2002, red:
main and aftershocks of the
1960 earthquake, Engdahl and
Villasen
˜
or 2002 in the time
window of 21.05–25.05.1960,
green: Haberland et al. 2006,
black: Dzierma et al. 2012).
Yellow stars show observations
of gas seepage at Mocha Island
(Jessen et al. 2011). The wide-
angle seismic profile of
Contreras-Reyes et al. (2008)is
depicted as green line. Red
stippled lines show tectonic
faults mapped on land between
36°S and 42°S by Melnick and
Echtler (2006b). CdN Cordillera
de Nahuelbuta. Stippled black
lines correspond to outlines of
shelf basins. Absence of
bathymetric information is
indicated as grey areas
Int J Earth Sci (Geol Rundsch)
123

(furrows) to the north (Thornburg et al. 1990;Vo
¨
lker et al.
2006, 2008). Within the Peru–Chile Trench, a submarine
channel of 3–5 km width and up to 200 m depth (the Chile
Axial Channel) that is inclined northwards cuts into the
flat-lying trench sediments (Vo
¨
lker et al. 2006). At its
southern end, the channel appears to be in direct continu-
ation of the Chacao Canyon from where it continues
northward over more than 1,000 km to the Juan Ferna
´
ndez
Ridge and possibly beyond. The distributary channels of
most sedimentary fans in the trench at the exits of sub-
marine canyons connect directly to this axial channel, in a
way that it appears to form a natural northward pathway for
sediments that exit from the submarine canyons. At the exit
of the San Antonio Canyon, sediments have ponded behind
an accretionary ridge and were deposited as overbank
deposits to the south of a distributary channel that breaches
the ridge (Laursen and Normark 2002).
Forearc Basins and upper plate fault zones
Between 33°S and 43°S, a number of marine shelf forearc
basins are known from seismic and petroleum exploratory
well investigations of the Chilean state oil company
Empresa Nacional del Petro
´
leo (ENAP) (Mordojovic 1981;
Gonza
´
lez 1989, Fig. 1). Two of these basins (Arauco and
Fig. 5 Bathymetric map of the
Chilean shelf and upper slope
from 39°Sto41°S (Valdivia).
Black lines indicate the position
of seismic profiles of the SPOC
project shot on RV SONNE
cruise SO161 (Reichert 2002).
Circles denote epicentres (blue:
main and aftershocks of the
Maule earthquake, Servicio
Sismolo
´
gico de Chile,
ssn.dgf.uchile.cl/, magenta:
Bohm et al. 2002, black:
Dzierma et al. 2012) Red
stippled lines show tectonic
faults mapped on land between
36°S and 42°S by Melnick and
Echtler (2006b). A yellow
diamond refers to an exploration
well, referenced in the text.
Stippled black lines correspond
to outlines of shelf basins.
Absence of bathymetric
information is indicated as grey
areas
Int J Earth Sci (Geol Rundsch)
123

Valdivia Basin) were covered by seismic sections of RV
SONNE cruise SO161 (Reichert 2002) as part of the pro-
ject ‘‘Subduction Processes off Chile’’ (SPOC) in 2001. We
show a 375-km-long deep reflection seismic line (SO161-
25, Fig. 7) that roughly follows the trend of the coast line
(Figs. 3, 4, 5) in combination with the stratigraphic infor-
mation from Mordojovic (1981) and Gonza
´
lez (1989)to
document the structure of the two forearc basins. We fur-
ther trace the positions of upper plate faults in the marine
forearc and investigate their impact on basin structure.
From North to South, shelf forearc basins are Navidad,
Chanco, Itata, Arauco, Valdivia, Pucatrihue and Chiloe
´
basins (Fig. 1). The basins have their respective depocen-
tres at the continental shelf, taper towards coast and trench
and are separated by basement highs. The creation of the
basins between 34 and 45°S is related to subsidence of the
present shelf and sectors of the slope by [1.5 km between
10.9 and 3.6 Ma (Melnick et al. 2009).
South of 38°S, the boundaries between the individual
forearc basins correlate with inferred marine continuation of
prominent upper plate faults (Figs. 4, 5, 6)thathavebeen
described by Melnick and Echtler (2006b)andMelnicketal.
(2009). Upper plate faults that strike in SE–NW direction,
oblique to the convergence direction of the Nazca Plate, are
Fig. 6 Bathymetric map of the
Chilean shelf and upper slope
from 41°Sto43°S (Chiloe
´
Island). Circles denote
epicentres (black: Dzierma et al.
2012, dark blue: Lange et al.
2007). Red stippled lines show
tectonic faults mapped on land
between 36°S and 42°Sby
Melnick and Echtler (2006b).
Stippled black lines correspond
to outlines of shelf basins.
Absence of bathymetric
information is indicated as grey
areas
Int J Earth Sci (Geol Rundsch)
123

Fig. 7 Seismic reflection profile SO161-25 across Arauco and Valdivia Basins (Figs. 3, 4, 5) from 36.75°S to 38.33°S(upper panel) and 38.33°Sto40°S(lower panel) showing major
tectonic structure and sedimentary units of the basins
Int J Earth Sci (Geol Rundsch)
123

likely the result of the northward motion of the Chiloe
´
Microplate and its collision with the South American Plate
in the region of the Arauco Peninsula (37.5°S). Some further
upper plate faults are located within individual basins.
The seismic section SO161-25 (Fig. 7) was acquired
with a recording time of 14 s two-way travel time (TWT).
It runs along the shelf in North–South direction from
36.75°Sto40°S along the axes of the Arauco Basin, the
Valdivia Basin and the northernmost area of the Pucatrihue
Basin (Figs. 3, 4, 5). As only a very small part of the entire
Pucatrihue Basin is imaged by the seismic line, the basin is
not discussed in detail.
Arauco Basin
The Arauco Basin extends over an area of 8,000 km
2
from
the latitude of Concepcio
´
n in the North to the Mocha Island
in the South (Fig. 1). In seismic line SO161-25 (Fig. 7), it is
displayed from the start of the section to the common mid-
point (CMP) 28500 at -73.693/-38.718. Around CMP
10600 (-73.559/-36.799), the V-shaped BioBio Canyon
cuts about 900 m into the shelf. Between CMPs 11900 and
15100 (-73.685/-36.905 to -73.804/-37.234), the sea-
floor shows two depressions of 7 and 15 km width and up to
650 m depth, separated by a bathymetric high (at around
CMP 13700, -73.790/-37.077). The depressions represent
indentations that belong to the upslope part of a giant sub-
marine slope failure that removed more than 350 km
3
of
slope sediment, continental framework rock and compacted
accretionary wedge material. The slope failure affected the
full width of the continental slope and shifted the shelf
break further landwards (Geersen et al. 2011b).
The Arauco Basin can be subdivided into a northern,
central and southern part based on the seismic reflection
pattern. In the northern part (start of the seismic section to
CMP 18500 at -73.833/-37.608), only the upper 0.5 s
TWT of the subsurface shows distinct seismic reflectors that
likely represent Pliocene sediments (all information about
stratigraphic units after Mordojovic 1974 and Gonza
´
lez
1989). Towards greater depth, reflections appear chaotic
and disturbed. In this part of the seismic section, it is
impossible to locate the Palaeozoic metamorphic basement.
In the central part of the Arauco Basin (CMP 18500–25000,
-73.833/-37.608 to -73.678/-38.328), distinct reflectors
are visible down to a depth of about 1 s TWT. Seismic
reflectors that again likely represent Pliocene sediments are
heavily folded and form two basin structures. At the flanks
of the basins, reflections are truncated by the seafloor
unconformably. The southern part of the Arauco Basin
between CMPs 25000 and 28500 (-73.678/-38.328 to
-73.693/-38.718) shows a distinctly different seismic
reflection pattern. Here, some high-amplitude reflectors
that likely represent Palaeozoic metamorphic basement are
observed between depth of 0.5–0.75 s TWT. A zone of
lower reflectivity on top of that unit may correspond to
Oligocene–Miocene sediments, whereas the shallowest
subsurface is formed of Pliocene to Quaternary clastic
sediments. The latter unit is represented by high-amplitude
reflectors. In this area, the continental shelf is exceptionally
wide (up to 60 km) with Mocha Island exposed up to 390 m
above sea level. The proximity of the Palaeozoic meta-
morphic basement to the seafloor may be caused by high
uplift rates in this area.
In the onshore area of the Arauco Basin, a series of
prominent SE-NW striking upper plate faults are described
that appear to continue into the marine forearc (all infor-
mation about position of continental faults from Melnick
and Echtler 2006b and Melnick et al. 2009) (Figs. 4, 5, 6).
Among those faults, the Morgu
¨
illa Fault is inferred to
intersect the seismic line SO161-25 around CMP 16000
(-73.827/-37.331). In this area, seismic reflectors in the
shallow subsurface are discontinuous, whereas towards
greater depth, no reflectors or seismic units are observed
that could indicate the position and the dip of the Morgu
¨
illa
Fault. Offsets in the seismic reflections in the shallow
subsurface indicate that the Morgu
¨
illa fault has been active
in the Pleistocene and Pliocene. Also, the Morgu
¨
illa fault
seems to develop into a flower structure in shallow depth as
is indicated by the presence of repeated small offsets.
Further to the south, two unnamed faults that branch into
a single fault zone in the continental forearc are inferred to
border the central part of the Arauco Basin. This area
is characterized by the folded basin structure between
CMPs 18500 and 25000 (-73.833/-37.608 to -
73.678/
-38.328). At the northern end of the basin structure, sev-
eral small (0.1–0.3 s TWT) offsets are observed down to a
depth of about 1 s TWT indicating ongoing deformation
and faulting since the Pleistocene. At the inferred position
of the southern branch of the unnamed fault between CMPs
24500 and 25000 (around -73.684/-38.300), the seismic
signature changes significantly. Here, the basin structure is
replaced by the high-amplitude reflections (at a depth of
0.5–0.75 s TWT) that likely represent the Palaeozoic
metamorphic basement in the southern part of the Arauco
Basin. Seismic reflections are truncated at various positions
in the subsurface. However, the shallowest reflectors (in the
upper 0.2 s TWT of the sub-seafloor) show no prominent
offsets, indicating that this fault has been inactive over the
Pleistocene.
The southern end of the Arauco Basin is located around
38.5°S at the Mocha-Villarica Fault Zone (MVFZ). Here,
the Palaeozoic metamorphic basement appears to be sud-
denly truncated. Moreover, several small offsets are
observed in the shallow sedimentary sequence indicating
that the MVFZ possibly develops into a flower structure at
shallow depth and that activity has not ceased.
Int J Earth Sci (Geol Rundsch)
123

Valdivia Basin
The Valdivia Basin lies between Mocha Island in the North
and CalleCalle Canyon in the South and, like the Arauco
Basin, extends over an area of about 8,000 km
2
. In seismic
section SO161-25 (Fig. 7), it is present from around CMP
28500 to CMP 39000 (-73.693/-38.718 to -73.816/
-39.877). At around CMP 30750 (-73.736/-38.969), the
Tolten/Imperial Canyon cuts about 250 m into the shelf
sediments. Towards the south, the basin is bordered by the
CalleCalle Canyon that offsets the seafloor more than
500 m. In the southern part of the basin, high-amplitude
reflections between 1.5 and 2.0 s TWT depths likely indi-
cate the top of the Palaeozoic metamorphic basement. The
seismic signature of the continental basement is similar to
the one described for the southernmost part of the Arauco
Basin. Between CMP 28500 (-73.693/-38.718) and
35000 (-73.789/-39.443), the reflections of the Palaeo-
zoic metamorphic basement are lacking. Reflectors in the
upper part of the basin do not show offsets at CMP 35000,
which would indicate the presence of a fault zone, but
rather seem to fade away. Therefore, we speculate that
the cause of the absence of basement between CMP 35000
(-73.789/-39.443) and the southern end of the Arauco
Basin is that here the profile is located further offshore and
thus runs west of the seaward edge of the continental
basement. Layered seismic reflections that indicate the
presence of a thick sedimentary sequence, likely Pliocene
and Miocene sediments, are observed in the upper
0.5–1.5 s TWT of the subsurface over the entire Valdivia
Basin. This is in contrast to the Arauco Basin where the
sedimentary sequence is much thinner and the basement is
met at much shallower depth.
In the Valdivia Basin, two prominent, but unnamed
faults are known (Fig. 5) that could intersect the seismic
line SO161-25. At the position of the northern of these
unnamed faults around CMP 34000 (-73.797/-39.332),
no prominent offsets are observed in the seismic line,
indicating that faulting is not active here. At the position of
the southern fault around CMP 36000 (-73.760/-39.553),
the shallow sediments as well as the continental basement
are persistently offset up to 0.1 s TWT, indicating that this
fault develops in a series of parallel faults or a flower
structure at shallow depth. Faulting at this location seems
to be active, as offsets are found directly below the seafloor
reflection. Due to the lack of resolution in greater depth, it
is unclear how this fault evolves towards depth.
Seismo-tectonics of the marine forearc
The continental shelf of Southern Central Chile is tectoni-
cally deformed at different scales by different mechanisms
the common origin of which is the (oblique) plate conver-
gence and subduction of the down-going plate. Here, we
combine information on epicentres from a number of local
seismological networks to provide a more uniform picture.
Data are from Dzierma et al. (2012), Lange et al. (2007),
Haberland et al. (2006) and Bohm et al. (2002). We added
epicentres of the main shock and aftershocks of the 1960
earthquake (Engdahl and Villasen
˜
or 2002) as well as of the
27 February of 2010, Mw 8.8 Maule earthquake (Servicio
Sismolo
´
gico de Chile, ssn.dgf.uchile.cl/). The corresponding
intra-crustal seismicity is partly diffuse, partly localized in
clusters (Figs. 2, 3, 4,
5, 6). Basically, three types of seis-
micity distributions can be observed in the marine forearc:
1. The trench-normal component of plate convergence
leads to compressional deformation of shelf sediments
and continental basement. The megathrust, as a whole,
is clearly visible in the seismicity distribution. Inside
the overriding forearc, however, the compression leads
to a rather diffuse band of seismicity in which
individual faults are difficult to identify. The frequency
of these forearc events is much higher north than south
of 36°S. Focal planes indicate mainly thrusting in the
forearc on planes subparallel to the trench (e.g.
Barrientos 2007).
2. The northward motion of the Chiloe
´
Microplate (or
Chiloe
´
Sliver, Melnick et al. 2009) with respect to the
stable Andean foreland to the east along the arc-
parallel LOFZ decreases northwards from 46°Sto
38°S (Rosenau et al. 2006; Wang et al. 2007). This
velocity gradient may have been partly accommodated
by internal deformation of the Chiloe
´
fore-arc sliver,
consistent with contractional and transpressional fault
zones in the Arauco Region that strike oblique to the
margin, such as the Lanalhue Fault and MVFZ
(Melnick and Echtler 2006a, b; Rosenau et al. 2006;
Melnick et al. 2009). At some of these faults zones
clusters of seismicity are observed that locate down to
lower crust and uppermost mantle levels (Dzierma
et al. 2012). Some clusters located offshore indicate
that the marine forearc is getting sheared in NW–SE
direction in addition to the overall background com-
pression. They show the offshore extrapolations of the
MVFZ south of Mocha Island (Haberland et al. 2006;
Dzierma et al. 2012) and of a nameless fault NW of
Valdivia close to the slip maximum of the 1960
Valdivia earthquake (Dzierma et al. 2012). The
seismicity cluster along the offshore extrapolation of
the MVFZ may have been persistent between 2004 and
2009 because it was observed by both Haberland et al.
(2006) and Dzierma et al. (2012).
3. Faulting of the down-going plate seems to continue
from the outer rise—where it is related to plate
Int J Earth Sci (Geol Rundsch)
123

bending—until beneath the forearc where it is related
to the megathrust process. This is evident from fault
displacements cutting through the entire overriding
plate down into the plate interface as imaged by
seismic reflections (Sick et al. 2006). Linear seismicity
clusters locating beneath the forearc near the plate
interface and in the down-going plate indicate that this
faulting is an ongoing process (Dzierma et al. 2012).
Two of these clusters were found beneath the Valdivia
Basin NW of Tolten and W of Mocha Island.
Paleo-accretionary complex beneath the continental
shelf
Information on the structure of the continental crust that
lies beneath the continental shelf and upper slope, under-
neath and seaward of the sedimentary basins has been
obtained by seismic investigations over the last *20 years
(e.g. Bangs and Cande 1997; Contreras-Reyes et al. 2008,
2010; Scherwath et al. 2009; Moscoso et al. 2011). Here,
we summarize recent observations on this issue.
Figure 8 shows a typical cross-section of the marine
forearc off southern Central Chile. The frontal accretionary
prism is 5–40 km wide (Contreras-Reyes et al. 2010) and
abuts the truncated continental basement (inner prism) that
extends seaward from beneath the shelf (Bangs and Cande
1997). This inner prism, presumably of Jurassic age, rep-
resents a paleo-accretionary prism. This paleo-accretionary
wedge in turn abuts the Paleozoic continental metamorphic
basement that is exposed on land in the Coastal Cordillera.
The Coastal Cordillera south of 34°S is mainly built by two
units, the Western and Eastern Series, that constitute coeval
parts of a Late Palaeozoic paired metamorphic belt domi-
nated by siliciclastic metasediments (Willner 2005). The
Western Series represents a paleo-accretionary prism
and dominantly consists of HP/LT metasediments with
subordinate metabasite intercalations’’ (see Willner 2005),
whereas the Eastern series is a belt of less deformed low-
pressure/high-temperature metasediments that represent the
retro-wedge (Willner et al. 2005; Glodny et al. 2006).
The transition between the present accretionary prism
and the sandwiched paleo-accretionary prism is visible in
seismic refraction data (Contreras-Reyes et al. 2008;
Scherwath et al. 2009) and as a morphological transition
between rough lower slope and more smooth upper slope
morphology (Geersen et al. 2011a). The landward backstop
of the paleo-accretionary prism against continental meta-
morphic basement (paleo-backstop) is manifest as velocity
gradient suggesting a change in rock type (Contreras-Reyes
et al. 2008; Scherwath et al. 2009) as well as by intraplate
seismicity (Haberland et al. 2006, 2009; Lange et al. 2007;
Dzierma et al. 2012).
Exploratory wells of the Chilean state oil company ENAP
are located landward of the paleobackstop, so they do not
help in determining the composition and age of the paleo-
accretionary complex. The degree of consolidation and
lithification of the paleo-accretionary complex is higher than
that of the frontal accretionary prism but lower than that of
the Palaeozoic continental framework. The remarkably high
lateral velocity gradient from 5.5 to [6.0 km/s implies an
abrupt change in rock type (Contreras-Reyes et al. 2008),
Fig. 8 Typical cross-section of the southern Central Chile conver-
gent margin (after Contreras-Reyes et al. 2010). The frontal
accretionary prism is typically 5–40 km wide and abuts against the
present backstop, formed by the inner prism that is 50–80 km wide.
The inner prism is likely composed of more than one rock unit.
Seismic refraction (Contreras-Reyes et al. 2008; Scherwath et al.
2009) and seismological evidence (Haberland et al. 2006; Lange et al.
2007) show the presence of a paleo-backstop structure that separates a
paleo-accretionary prism complex from the onshore exposed Paleo-
zoic continental metamorphic basement (Herve
´
et al. 1988; Glodny
et al. 2006)
Int J Earth Sci (Geol Rundsch)
123

and hence alternation between accretion and erosional
phases. The size of the paleo-accretionary complex could
have been much larger at the end of the accretion phase,
when the complex was formed. Thereafter, an integral part
of the accretionary complex was tectonically eroded
(Kukowski and Oncken 2006). At present, the width of
50 km of the paleo-accretionary complex represents the
remaining material left after the last erosional phase, which
took place in the Miocene according to Melnick and Echtler
(2006a) and Encinas et al. (2008). Assuming alternation
Fig. 9 Sample PARASOUND sediment-echosounder profiles from
the shelf and upper slope of southern Central Chile, each representing
seismic facies types. a Outcrop of lithified sediments at the shelf
edge; b angular unconformity of tilted lithified strata against thin
cover of young sediments; c subparallel strata, incised by parallel
grooves; d small landslide at the shelf edge. Location of PARA-
SOUND profiles a and b is indicated in Fig. 3, of profiles c and d in
Fig. 4
Int J Earth Sci (Geol Rundsch)
123

between accretion and erosion phases and based on the age
of the oldest shelf sediments (late Cretaceous), the estimated
age for the paleo-accretionary complex is Jurassic (Contre-
ras-Reyes et al. 2008).
Present shelf and slope sedimentation
The young sediment cover of the shelf and slope was
sampled over the last 20 years by coring campaigns along
depth transects (Table 1). ODP Leg 202 Sites 1233, 1234
and 1235 were drilled in slope basins of the upper and
middle slope in water depths of 838, 1,015 and 489 m,
respectively (Figs. 3, 5). Sediment-echosounder data were
obtained along cruise tracks of RV SONNE cruise SO161
and SO210 on the continental slope. These latter data are
presented here for the first time to image sedimentary
structures on the shelf.
The ODP coring had the goal to obtain an undisturbed
millennium-scale sediment core record of paleoclimatic
changes. Consequently, small slope basins with thick sed-
iment fill, where turbidites were expected to be channelled
away by surrounding canyons, were selected for drilling
(Mix et al. 2003). In this setting, thick and rapidly accu-
mulating hemipelagic sequences were cored, which are
characterized by extremely high bulk sedimentation rates
of 90 cm/ka (site 1234), 70 cm/ka (site 1235) and
[100 cm/ka (site 1233) over the cored intervals. The
lithology is described as homogeneous silty clay and clay
with varying, but generally low biogenic content and few
thin silt and volcanic ash layers. The low biogenic com-
ponent in spite of persistent highly productive upwelling
cells in the Concepcio
´
n area (sites 1234 and 1235) was
explained by dilution due to the overwhelmingly high
fluvial input of siliciclastic material (Mix et al. 2003).
Surface sediment samples (grab samples, gravity cores
and multicorer samples) of shelf and upper slope were
described by Lamy et al. (1998, 1999, 2001), Hebbeln et al.
(2000), Mun
˜
oz et al. (2004), Raitzsch et al. (2007) and
Stuut et al. (2007). Sediment composition of the described
samples is dominated by terrigenous input which generally
increases to the south in relation to the climatically
controlled southward increase in denudation rates of the
hinterland. Offshore mid-latitude Chile (33°S) samples
provide a record of temporal variations in the terrigenous
sediment supply that reflect changes in weathering condi-
tions related to shifts of the latitudinal position of the
Southern Westerlies (Lamy et al. 1999, 2001). Lamy et al.
(1998) showed that regional variations in silt size and bulk
mineralogy of terrigenous silts are governed by the source-
rock composition of the different geological terranes and
the relative source-rock contribution of the Coastal Cor-
dillera and the Andes as controlled by the river networks.
These trends are also reflected in the bulk chemistry (Stuut
et al. 2007).
The biogenic sediment input shows a close relation to the
environmental conditions in the Peru–Chile Current, as the
accumulation rate of organic carbon in the sediments fits
well with the present-day productivity patterns that are
related to cells of coastal upwelling known from satellite
data (Hebbeln et al. 2000). The carbonate content along the
slope varies mainly between 0 and 20 % (Hebbeln et al.
2000). For the continental slope offshore Concepcio
´
n,
sedimentation rates were determined over the past *100
years for two sites at 1,294 and 2,065 m water depth
(Mun
˜
oz et al. 2004). The very high values of 180 ± 20 cm/ka
were explained by the vicinity of the BioBı
´
oCanyon.
Seismic reflection data (Contardo et al. 2008; Geersen
et al. 2011b), bathymetric data (Vo
¨
lker et al. 2011) and the
PARASOUND sediment-echosounder data shown here
demonstrate that mass-wasting is a common effect on the
slope that affects many of the slope basins and that
focusing of sedimentation leading to extreme sedimenta-
tion rates in sheltered slope basins is contrasted by win-
nowing and sediment starved zones on the shelf.
PARASOUND sediment echo-sounder data of the shelf
break and uppermost slope around 36°S show a thin sedi-
ment cover that unconformably overlies deformed and til-
ted older strata (Fig. 9a, b). Locally, the older strata pinch
out to form hard ground basement highs lacking a young
sedimentary cover. Further south around 38°S, the rela-
tively thin young sediment cover is affected by bottom
current erosion as can be seen from v-shaped incisions
(Fig. 9c). Deformation of the topmost sediment cover due
to mass wasting is common (Fig. 9d). The overall
impression of the sediment-echosounder data is of a bot-
tom-current-dominated high energetic depositional regime
where young sediments fill sheltered basins and pockets
while elevated areas are practically swept free from young
sediments and/or subject to bottom current erosion. Prob-
ably a large fraction of the shelf sedimentation is exported
to shelf basins or eventually funnelled to the Peru–Chile
Trench via submarine canyons. On the shelf, offshore
Punta Lugurne (*36°S, Fig. 3) on the other hand, undis-
turbed and well-stratified sediments of 50-ms two-way-
travel time were observed on RV SONNE cruise SO210
(Linke 2011).
Gas and fluid seepage
The presence of solid gas hydrates is indicated by the
observation of a bottom simulating reflector (BSR) in
seismic reflection data, while the seepage of gas-charged
fluids at the seafloor is manifest by the occurrence of
chemosynthetic bio-communities as well as by the acoustic
Int J Earth Sci (Geol Rundsch)
123

detection of gas bubbles in the water column (acoustic
flares in sediment-echosounder data). Here, we report on
the present-day knowledge on the distribution of seepage-
related fauna. At other convergent continental margins,
such as of Central America and New Zealand, active
seepage of (methane-rich) fluids is a common phenome-
non, often located within a trench-parallel belt of the
middle continental slope. In those places, a variety of
active seeps appear to be long-standing structures, related
to faults that connect the plate interface with the seafloor
and form conduits (e.g. Sahling et al. 2008; Barnes et al.
2010). The presence of fluid seepage can therefore bear
information on the hydraulic properties of the forearc, its
tectonic situation and internal structure.
Seismic profiles show bottom simulating reflectors
(BSR), commonly associated with the occurrence of gas
hydrates on the continental slope of southern Central Chile
below a water depth of 650 m (Brown et al. 1996;Dı
´
az-
Naveas 1999; Grevemeyer et al. 2003; Morales 2003;
Rodrigo et al. 2009). At some places, the BSR intercepts
the seafloor below the shelf break (Rodrigo et al. 2009).
Typical members of chemosynthetic bio-communities
indicative for methane seepage (e.g. clams of the family
Vesicomyidae and tubeworms of the genus Lamellibrachia)
are known by now from many places at the Chile margin.
The first indication of seep communities was the descrip-
tion of Calyptogena australis, from the vicinities of Mocha
Island (*38°S) at 1,400 m water depth (Stuardo and
Valdovinos 1988, Fig. 4). This first report was followed by
others from offshore Concepcio
´
n Bay (Sellanes and Kryl-
ova 2005; Oliver and Sellanes 2005; Sellanes et al. 2008;
Quiroga and Sellanes 2009), a region that was termed the
Concepcio
´
n methane seep area (CMSA, Sellanes et al.
2004, Fig. 3). Three other bathyal seep sites were discov-
ered recently, located off the Limari River (at *30°S) at
1,000 m depth, off El Quisco (*33°S) at 350 m depth, and
the most recent one, off the Taitao Peninsula (*46°S) at
600 m depth (J. Sellanes, unpublished data), all of them
indicated by the presence of typical seep communities.
Fluid seepage-related features of the seafloor (patches of
high acoustic backscatter, possibly representing authigenic
carbonates) as well as acoustic anomalies in the water
column (gas flares) were detected on the upper to middle
slope in 1,500 m water depth (Flueh and Bialas 2008)
offshore Concepcio
´
n Bay (CMSA, Fig. 3). While authi-
genic carbonates that probably formed as the result of
methane-rich fluid expulsion form extensive pavements in
two distinct areas of the middle slope (north of BioBio
Canyon and around Itata Canyon), acoustic anomalies in
the water were described rarely (Klaucke et al. 2012).
According to Klaucke et al. (2012), the apparent misfit
between indications of present fluid seepage and the size of
authigenic carbonate patches and chemoherms indicates
that fluid venting must have been more intense over some
period of the past.
Chloride content of the pore waters of gravity cores
from the continental slope is meaningful for the detection
of subduction-related diagenetic processes, as a number of
these processes consume (alteration of volcanic ash to
smectite, Martin et al. 1995) or release (transformation of
smectite to illite, Kastner et al. 1991) fresh water when
trench fill is being subducted along with the down-going
plate (e.g. Hensen et al. 2004). Offshore Central Chile, pore
water geochemical measurements from gravity cores
obtained on RV SONNE cruise SO210 (Linke 2011) were
conducted by Scholz et al. (2012). They show that pore
fluids in cores of the accretionary prism show a higher
chlorinity than seawater and relate this finding to the
sequestration of water through formation of hydrous min-
erals (alteration of volcanic ash to smectite). In contrast,
cores from the upper slope have a lower chlorinity than
seawater which most likely is due to clay mineral dehy-
dration, for example the alteration of smectite to illite.
Thermal constraints from heat flow modelling let Scholz
et al. (2012) suggest that these low-salinity fluids are
generated in the upper plate, whereas the dehydration of
underthrust sediments must take place further seaward.
The occurrence of gas seeps is also known at many
intertidal and shallow subtidal places at the W side of
Mocha Island off southern Central Chile (*38°S, Fig. 4).
At this locality, two possible sources have been ascribed for
it: (a) subsurface thermogenic hydrocarbon accumulations
that are trapped within the Cretaceous rock sequence
(Comisio
´
n Nacional de Energı
´
a Chile, 2002; Sa
´
nchez 2004)
and (b) coal-bed methane, coming from coal-bearing sedi-
ments of the Trihueco Formation in the Arauco Basin in the
continental shelf (Mordojovic 1981). Recent measurements
indicate that emanations contain 70 % methane, and the
estimated methane fluxes emitted directly to the atmosphere
amount to 815 ta
-1
when considering the five subtidal and
intertidal seeps detected at the Island (Jessen et al. 2011).
The C stable isotope compositions of methane from the
intertidal seeps averaged at -43.8 ± 0.4 % (with respect to
PeeDee Belemnite) and are suggestive of a substantial
fraction derived from thermogenic sources. While stable
carbon isotopic compositions of marine benthic organisms
indicate a dominant photosynthesis-based food web, d
13
C
of some hard-substrate invertebrates were in the range
-36.8 to -48.8 %, suggesting assimilation of methane-
derived carbon by some selected taxa (Jessen et al. 2011).
Summary and conclusion
Our compilation of older and recently published data,
cruise reports, scientific theses and previously unpublished
Int J Earth Sci (Geol Rundsch)
123

geological and geophysical data on the shelf and upper
slope of Central Chile allows drawing the following
conclusions:
1. The presence of fourteen deeply incised submarine
canyon systems, their extension onto the shelf, as well
as their direct connection to river systems on land
impacts severely on shelf sedimentation, as the bulk of
fluvial transported sediment is likely funnelled down-
slope instead of being stored on the shelf. High
sedimentation rates are observed on shelf and upper
slope in spite of this deprivation of fluvial transported
material due to local zones of constant or seasonal
upwelling and due to the sheer amount of fluvial input.
Sediment-echosounder profiles show both patches of
undisturbed and well-stratified sediments on the shelf
and eroded sedimentary structures closer to the shelf
edge. The local lack of young sedimentary cover is
attributed to the Gunther Current that flows vigorously
poleward close to the shelf edge.
2. The sedimentary basins that underlie the shelf
platform and the upper continental slope consist of
Oligocene to Quaternary infill in structural basins of
the Paleozoic metamorphic basement. The thickness
of the individual units varies significantly both within
Arauco and Valdivia Basin as well as from basin to
basin. Reflectors of the sedimentary fill of those
basins are offset by six fault zones that form the
continuations of large crustal fault systems known on
land such as the Morgu
¨
illa Fault and the Mocha-
Villarica Fault Zone. The continuity of these SE-NW
trending crustal faults into the submarine forearc has
been postulated but never clearly documented before.
Five of the six fault systems appear to have been
active over the evolution of the basin from late
Cretaceous to the present, while for one fault system
activity appears to have ceased in Pleistocene. Some
of these faults zones—notably the MVFZ (Haberland
et al. 2006, offshore Arauco peninsula; Dzierma et al.
2012, cluster G), a nameless fault NW of Valdivia
(Dzierma et al. 2012, cluster F) and side branches
near the LOFZ (Dzierma et al. 2012, cluster H)—
show clusters of seismicity that extend downwards
into the lower crust, Moho and uppermost mantle.
The clusters indicate that the marine forearc is getting
sheared in NW–SE direction in addition to the overall
background compression, a tectonic regime that is
due to the transpressional docking of the Chiloe
´
Microplate to the South American Plate.
3. The structure of the upper continental margin beneath
the shelf basins is marked by a prominent transition
from a former accretionary prism that was tectonically
eroded in parts to the newly forming accretionary
prism. This transition marks the shift from a phase of
tectonic erosion to the present phase of accretion that
is supposed to have occurred about 4 Ma ago. The
transition is further expressed in the surface morphol-
ogy as a shift from smooth, gently sloping upper
continental slope to a more complex and steeper slope
below 2,000 m water depth.
4. Seepage of gas-charged fluids is observed both indi-
rectly and directly, clustered to a few locations on the
shelf and upper slope. This seepage appears to be of
shallow origin and not related to the release of fluids
from the subducting Nazca Plate by deeply connecting
conduits as was observed, for example off Costa Rica
(Sahling et al. 2008).
Acknowledgments Part of the research off Central Chile was fun-
ded by the Fondo Nacional de Desarrollo Cientı
´
fico y Tecnolo
´
gico
(FONDECYT) grants 1100166 (CMSA) and grant 1080623 (Mocha
Island), to Javier Sellanes and Silvio Pantoja, respectively. The pro-
ject SPOC (Subduction Processes off Chile) including the RV
SONNE cruise SO161 was funded by the German Federal Ministry of
Education and Research (BMBF) grant no. 03G0161A. We gratefully
acknowledge the help of the Dpto. de Geofı
´
sica de la Subdireccio
´
n
Nacional de Geologı
´
a de SERNAGEOMIN (Servicio Nacional de
Geologı
´
a y Minerı
´
a de Chile) that provided us with a bathymetric grid
of the shelf areas of Chile. Some figures were created with The
Generic Mapping Tools (GMT). We are grateful for helpful com-
ments and thorough reviews by Juan Dı
´
az-Naveas and an anonymous
second reviewer. This publication is contribution no. 226 of the
Sonderforschungsbereich 574 ‘‘Volatiles and Fluids in Subduction
Zones’’ at Kiel University.
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