Downloaded from http://jgs.lyellcollection.org/ at Servicio Nacional De Geologia Y Mineria on November 11, 2014
Journal of the Geological Society
Sedimentary fill of the Chile Trench (32−46°S): volumetric distribution and
causal factors
David Völker, Jacob Geersen, Eduardo Contreras-Reyes and Christian Reichert
Journal of the Geological Society 2013, v.170; p723-736.
doi: 10.1144/jgs2012-119
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Notes
© The Geological Society of London 2014
2013
research-articleResearch ArticleXXX10.1144/jgs2012-119D. VÖLker et al.Chile Trench Fill Volume
Downloaded from http://jgs.lyellcollection.org/ at Servicio Nacional De Geologia Y Mineria on November 11, 2014
Journal of the Geological Society, London, Vol. 170, 2013, pp. 723–736. http://dx.doi.org/10.1144/jgs2012-119
Published Online First on July 24, 2013
© 2013 The Geological Society of London
Sedimentary fill of the Chile Trench (32–46°S): volumetric distribution and causal
factors
David Völker 1* , Jacob Geersen 2 , Eduardo Contreras-Reyes 3 & Christian Reichert 4
1Collaborative Research Center (SFB) 574 at the GEOMAR, Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse
1–3, 24148 Kiel, Germany
2University of Southampton, National Oceanography Centre Southampton, Waterfront Campus, European Way,
Southampton SO14 3ZH, UK
3Departamento de Geofísica, Universidad de Chile, Santiago, Chile
4BGR, Stilleweg 2, D-30655 Hannover, Germany
*Corresponding author (e-mail: [email protected])
Abstract: The Chile Trench of the convergent continental margin of Central Chile is a sediment-filled basin
that stretches over 1500 km in a north–south direction. The sediment fill reflects latitudinal variations in
climate as well as in the morphology and geology of Chile, but also of sediment transport processes to the
trench and within the trench. We try to untangle these signals by calculating the total volume and the latitudinal volume distribution of trench sediments and by relating this distribution to a number of factors that
affect this pattern. The volume calculation is based on a model geometry of the top of the subducting oceanic
plate that is buried beneath trench sediments and the sea floor as measured by swath bathymetry. We obtain
the model geometry of the subducting plate by interpolating between depth-converted seismic reflection
profiles that cross the trench. The total volume of the trench fill between 32 and 46°S is calculated to be
46000 ± 500 km3. The resulting latitudinal volume distribution is best explained by a sedimentation model
that alternates between (1) glacial phases of high sediment flux from Southern Chile combined with active
latitudinal sediment transport within the trench and (2) interglacial phases over which sediment input is
dominated by local factors.
Supplementary material: Top of the oceanic basement (TOB) grid is available as ascii raw data files (xyz)
at www.geolsoc.org.uk/SUP18664.
Convergent plate boundaries are key areas for the long-term evolution of continents as well as for the exchange of matter between
lithosphere, hydrosphere and atmosphere. Continents grow when
material is transferred from the subducting oceanic plate to the overriding continental plate by frontal and basal accretion and arc volcanism; they may shrink when material is scraped off from the
continental plate by tectonic erosion leading to forearc subsidence
and coastal retreat; or they retain their size and general shape if there
is a balance between accretion and erosion (e.g. von Huene & Scholl
1991; Stern & Scholl 2010; Stern 2011; Roberts 2012). Switching
between these modes of forearc evolution appears to be controlled
by convergence velocity and the structure and topography of the
downgoing plate (e.g. Clift & Vanucchi 2004), and the amount and
nature of sediments that are delivered to and stored in the deep-sea
trenches in front of subduction zones. The latter control is exemplified by accretionary margins where sediment-flooded trenches coincide with huge accretionary complexes, whereas sediment-starved
trenches often coincide with tectonic erosion and retreating coastlines (e.g. Ranero et al. 2006). A certain flux of sediments into the
trenches is necessary to prevent the consumption of upper plate
material by levelling out the roughness of the oceanic plate, and a
higher flux is needed to initiate the buildup of accretionary prisms.
The sediment fill of trenches at subduction zones also appears to
affect the seismological behaviour of subduction zones by modifying the coupling of the plates, if sediments subduct along with the
downgoing plate (e.g. Ruff 1989; Dean et al. 2010; Heuret et al.
2012). Finally, trench sediments, when subducted, bring clastic
material as well as water and water-transported volatile material into
the ‘subduction factory’. How much of the volatile material is
released over the marine forearc or at the volcanic front, how much
is stored and how much is lost to the mantle globally is still a matter
of debate (Jarrard 2003; Hacker 2008; van Keken et al. 2011). It is
clear, however, that this budget is crucial for the global geochemical
exchange between lithosphere, hydrosphere and atmosphere. For all
the considerations above, and in particular for budget calculations, it
is essential to have reliable information about the volumes and volumetric distribution of trench sediments.
The Chile Trench between the Juan Fernandez Ridge (32°S) in the
north and the Chile Triple Junction (46°S), where the Chile Ridge
intersects the marine forearc in the south, is a c. 1550 km long, semienclosed sedimentary basin, filled by sediments of mainly turbiditic
origin (Thornburg & Kulm 1987; Mix et al. 2003). To the east the
basin is limited by the deformation front at the toe of the continental
slope of Chile where the basin fill becomes partly incorporated into
the accretionary prism or is subducted with the Nazca Plate (Fig. 1).
To the west, the basin is less clearly limited by the outer rise of the
bulging Nazca Plate (Völker et al. 2008; Fig. 2a and b). Within the
limits of the trench, the sea floor forms a flat abyssal plain, devoid of
contour-parallel features such as contourites. The most prominent
bathymetric feature is a trench axial channel that is continuous from
44°S to 31°S (Fig. 2a and b).
In this study we quantify the total volume of the sediment that is at
present stored in the Chile Trench between the Juan Fernandez Ridge
and the Chile Triple Junction and its spatial volumetric distribution.
This is achieved by creating a surface model of the top of the oceanic
basement (TOB) and calculating the volume between this surface and
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D. VÖLker et al.
Fig. 1. (a, b) Overview maps of the study area between 32 and 46°S, with seismic profiles used for the calculation of a model of the top of the oceanic
basement (TOB). The polygon that was used to limit the volume calculation is indicated as a green outline around the dashed contour lines (light grey in
printed version). In addition, some patch areas (yellow polygons, white in printed version) were added to account for the different seaward extension of
seismic profiles. The position of the terminal moraine front of the last Quaternary (Llanquihué) glaciation according to Caldenius (1932) is indicated as a
thick blue line (black in printed version) in (b). The trench is filled by sediments of turbiditic origin south of 32°S (Thornburg & Kulm 1987). ODP Leg
202 Site 1323 is indicated by a star in (b). Isopachs of the trench fill thickness are given in 500 m contours (dashed black lines). Names and outlines of
perennial river systems on land and their corresponding submarine canyon systems are indicated. JFR, Juan Fernandez Ridge; CTJ, Chile Triple Junction.
Offshore of the Arauco Peninsula, three giant submarine landslides (NGSF, CGSF, SGSF; northern, central and southern giant slope failures) have
significantly modified the continental slope. The evacuation areas are shown as shaded regions.
the sea floor within the lateral confinements of the trench. The TOB grid
is the product of an interpolation between 30 reflection seismic profiles
that cross the trench in east–west direction. The sea-floor grid is produced from high-resolution swath bathymetric data. In the following
sections we describe the methods employed and discuss the total
derived trench fill volume and its latitudinal distribution. The spatial,
and in particular the latitudinal variations in the trench fill volume are
discussed in relation to the most important factors that influence the
sediment input along this c. 1550 km long segment of the Chilean coast.
We focus on climate (precipitation rates), sediment feeder systems (rivers, submarine canyons, submarine fans), accommodation space (elastic thickness, plate age, crustal thickness of lower plate, fracture zones),
submarine mass wasting and volcanic activity (volume distribution of
extruded magmas along the Southern Volcanic Zone of Chile). The
match or mismatch of the volume distribution with the factors that are
supposed to control the sediment input yields information about their
relative importance and the effectiveness of latitudinal sediment transport within the trench.
Tectonic and geological setting
The tectonic framework of the South Chilean forearc north of the
Chile Triple Junction is controlled by the subduction of the Nazca
Plate under the South American Plate at a present rate of 6.6 cm a−1
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Chile Trench Fill Volume
725
Fig. 2. (a) Detailed bathymetric map of the toe of the continental slope, trench and outer rise showing the intense bend faulting of the Nazca Plate. (b)
Detailed bathymetric map of the trench showing the incoming Valdivia Fracture Zone and the axial channel of the Chile Trench. (c) Perspective view of
the BioBio Canyon and northern giant slope failure (NGSF). JFR, Juan Fernández Ridge; CTJ, Chile Triple Junction.
and at a convergence azimuth of about 80° (Fig. 1; Angermann
et al. 1999). Oblique orientation of ocean plate isochrons results in
a northward increase in Nazca Plate age at the trench of about 2 Ma
per 100 km from 0 Ma at the Chile Triple Junction (46°S) to c.
35 Ma at the latitude of the Juan Fernandez Ridge (32°S) (Tebbens
et al. 1997). The most prominent morphological features on the
Nazca Plate seaward of the trench are six fracture zones that separate Nazca Plate segments of different age (the Darwin, Guamblin,
Guafo, Chiloé, Valdivia and Mocha fracture zones; Fig. 1).
The marine forearc of the study area has experienced subduction erosion in the Palaeogene. In the middle Miocene (Kukowski
& Oncken 2006) or Pliocene (Melnick & Echtler 2006) this
regime changed drastically, and since then sediment accretion has
been active and resulted in the buildup of an accretionary prism of
10–40 km width. The alternation from subduction erosion to sediment accretion may have occurred during the onset of glaciation
in the Patagonian Andes about 6 Myr ago, resulting in increased
sediment flux to the trench (e.g. Bangs & Cande 1997; Melnick &
Echtler 2006).
Primary information on sedimentation rates in the trench comes
from Ocean Drilling Program (ODP) Leg 202, Site 1232 (Mix et al.
2003). The site lies on the seaward side of the trench, c. 70 km from
the subduction front. Sediment ages were estimated from calcareous nannofossils and planktonic foraminifers. Preliminary results
gave basal ages of 0.26 Ma for 126 ± 5 m below the sea floor (mbsf),
0.46 Ma for 259 ± 5 mbsf, and 1.7 Ma for 362 mbsf.
The Chile Trench as semi-closed basin
The subducting Chile Ridge forms a topographic barrier to latitudinal sediment transport within the trench at the southern end of the
study area (Fig. 1b). At the northern end, the Juan Fernandez Ridge
forms a linear seamount chain rather than a massive ridge but narrows the trench and seems to dam turbidity currents, at least over
certain time intervals. As a result, sea-floor depth increases significantly across this barrier from south to north, indicating that significantly less sediment is filling the trench north of the Juan
Fernandez Ridge. The seaward limit of the trench is defined by the
outer rise of the bending Nazca Plate, which forms a trench-parallel
bulge above the floor of the Chile Trench. This seaward limit is
somewhat ill-defined, as the relative height (and curvature) of the
outer rise above the trench floor changes drastically from south to
north (Fig. 2a and b). In particular, south of 41°S the outer rise is
subdued and turbidity currents may transport sediment far onto the
Nazca Plate (Fig. 2b), whereas north of this latitude the outer rise
forms a pronounced obstacle to the longitudinal transport and
defines a 20–70 km wide basin (Fig. 2a). The onlap of the turbiditic
infill of the Chile Trench onto pelagic background sediments of the
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D. VÖLker et al.
trench axial submarine channel (Fig. 2a and b) that originates from
the Chacao Canyon at 41°S and is connected to the exits of large
submarine canyon systems of the continental margin (the Callecalle
Canyon, Tolten Canyon, Biobio Canyon and San Antonio Canyon;
Thornburg & Kulm 1987; Thornburg et al. 1990; Laursen &
Normark 2002; Völker et al. 2006). According to Laursen et al.
(2002), the channel is traceable across the Juan Fernandez Ridge.
Minor sediment volumes may pass this gate, but the thick sheet
turbidite deposits that form the flat sea floor are absent north of the
Juan Fernandez Ridge. According to Laursen & Normark (2002),
the San Antonio Seamount, which is now subducting beneath the
continental slope about 45 km west of the Chilean coast, has
blocked the latitudinal transport through this gate for >1 Myr.
Method of model creation
The dataset that we used to build the TOB geometry consists of 30
seismic reflection profiles that cross the Chile Trench roughly east–
west or west–east (Fig. 1). The profiles were recorded over the last
20 years on various cruises of R.V. Sonne and R.V. Conrad.
References to the various datasets are given in Table 1. We picked
the bright high-amplitude TOB reflector, which could be easily distinguished from the sedimentary infill, in each of the 30 profiles.
Sedimentary thickness was derived by converting the depth of
TOB from two-way-time (TWT) to metres using the following
relationship:
hs = Vo
Fig. 3. Latitudinal distribution of the trench fill volume. (a) Height
difference between the sea floor at the deformation front and the TOB
underneath the deformation front (subduction window); (b) trench depth
at the deformation front and TOB depth underneath the subduction
front as imaged by seismic profiles; (c) latitudinal volume distribution
of trench-fill sediments, normalized to 1 km of trench length. Purple
lines (dark grey in printed version), volumes integrated over latitudinal
segments of 1° length; orange lines (light grey in printed version),
volumes integrated over latitudinal segments of 0.5° length. The light
blue shaded area (grey in printed version) represents the range of results
for different models of the TOB.
Nazca Plate is clearly discernible in seismic reflection profiles and
sediment echosounder records (Völker et al. 2008).
The successive filling of the trench is counteracted by the removal
of trench fill that is either incorporated into the accretionary prism
(upper part of the sediment stack) or underthrust along with the
Nazca Plate (lower part of the stack) at the rate of plate convergence.
If the trench width and convergence rate are considered, a period of
0.6–1 Myr is required for sediments that are deposited at the seaward
(western) margin of the trench to meet the deformation front.
Accordingly, the oldest trench sediments that lie directly on the TOB
close to the deformation front should have an age of <1 Ma in general. Rauch (2005) dated the oldest trench sediments between 38 and
36°S to 0.64 Ma, based on the interpretation of seismic reflection
profiles. South of 41°S, where the outer rise is poorly defined and
turbidites extend far onto the Nazca Plate, older sediments are
encountered (e.g. 1.7 Ma at the base of ODP Leg 202 Site 1232).
Deposition of turbidites has led to a flat sea floor within the
trench. This plain is inclined northwards by 0.1° and the water
depth at the deformation front increases from 3200 m at 46°S to
5950 m at 32°S (Fig. 3b). In line with this gradient, there appears to
be a general south to north transport of sediment within the trench.
Such transport is indicated by a 5 km wide and up to 150 m deep
e
k ( t2 − t1 ) / 2
−1
k
where hs is sedimentary thickness, t1 is the TWT for reflections
from the sea floor, t2 is the TWT for reflections from the top of the
oceanic basement, and k is the vertical velocity gradient. Vo corresponds to the seismic velocity of seawater (1.5 km s−1). A value of
0.5 km s−1 km−1 was chosen for k, and is based on velocity–depth
models obtained from active seismic studies (Scherwath et al.
2009). The resulting profiles give the depth of the TOB for each
position along the 30 reflection seismic lines.
To obtain a sufficiently high number of raw data points for creating
the TOB grid, we programmed a routine for the interpolation of depth
points between the depth-converted profiles. This routine (1) determines the number n of equally spaced points along a profile and the
number m of interpolants between neighbouring profiles, (2) interpolates along a pair of neighbouring profiles to generate two arrays of
xyz points of equal dimension n, (3) interpolates across the profiles
between the corresponding points of these arrays by spline interpolation to generate an array of xyz points of dimension m × n and (4)
performs a loop over all of the profiles. The output is then binned into
groups and gridded to a grid of 0.005° × 0.005° (18 × 18 arc seconds).
For volume calculation, the grid is cropped to the limits of the
trench. These limits are defined at the landward side by the deformation front of the accretionary prism. This deformation front coincides
with the lower end of the toe of the continental slope in general.
However, at some places, we find trench-parallel thrust ridges seaward of the toe of the continental slope (Fig. 4b). These ridges are
related to thrust faults, indicating compressive deformation of the
trench fill, and delimit the initial integration of this material into the
accretionary prism, so the deformation front steps into the trench fill.
For simplicity we choose the most prominent break in slope, where
the generally flat trench floor meets the 5–30° steep lower slope, as a
boundary. As discussed above, to the seaward side of the trench the
limits are somewhat arbitrary, as the trench fill onlaps onto the hemipelagic sediments of the Nazca Plate as a westward thinning unit. For
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Chile Trench Fill Volume
727
Table 1. Seismic reflection profiles used to create the model geometry of the top of the downgoing oceanic basement (TOB), sorted from north to south
Profile name
First point
Last point
UTM18S
Direction
Decimal
geographical
Decimal
geographical
Longitude
(W)
Latitude (S)
x
y
Longitude
(W)
Latitude (E)
x
condor-5
72.69625
32.0768
717445
6448731
72.62832
32.07640
723860
6448634 W–E
condor-17
72.75789
32.85381
709815
6362691
72.70884
32.85481
714404
6362482 W–E
condor-24
condor-26
72.71425
72.92635
33.0336
33.11819
713467
693470
6342664
6333696
72.86775
72.69386
33.03193
33.14384
699132
715103
6343152 E–W
6330399 W–E
condor-29
72.80494
33.55469
703780
6285059
73.17607
33.58441
669263
6282431 E–W
condor-35
72.97455
33.92156
687232
6244693
73.22263
33.82925
664474
6255355 E–W
enap-1
spoc-22
scs-04
enap-2
spoc-24
spoc-47
spoc-45
spoc-44
spoc-43
enap-4
spoc-42
74.03455
74.14790
74.10504
74.59725
74.75532
74.42021
74.53006
75.00116
74.58737
74.81947
74.81947
35.91490
36.07783
36.16172
36.33180
36.76467
36.93534
37.30804
37.47242
37.75281
37.94149
37.94149
587108
576723
580496
536146
521837
551631
541644
499897
536349
536941
515862
6025059
6007083
5997743
5979173
5931205
5912143
5870851
5852718
5821530
5802799
5800661
74.42940
74.57032
74.50782
74.19651
74.34636
74.90880
74.89139
74.49039
75.08529
74.50211
74.50211
35.90570
36.02354
36.10658
36.32768
36.79851
36.88772
37.27357
37.53758
37.64601
38.00690
38.00690
551712
538714
544387
572116
558311
508126
509628
545022
492475
498969
543709
6026335
6013355
6004102
5979406
5927279
5917579
5874773
5845367
5833456
5803459
5793302
E–W
E–W
E–W
W–E
W–E
E–W
E–W
W–E
E–W
E–W
W–E
spoc-37
spoc-35
spoc-34
spoc-33
spoc-31
enap-6
spoc-29
spoc-27
spoc-26
scs-03
scs-01
scs-02
RC2901-743
RC2901-748
75.06397
74.66576
75.48819
74.82004
75.24916
74.94106
74.94542
75.47972
74.91495
75.17717
75.31500
75.65341
75.95464
74.10504
38.09039
38.37864
38.74988
38.90008
39.09994
39.24803
39.48339
39.64981
39.84999
40.69668
42.81724
44.48941
45.59027
36.16172
494390
529193
457578
515604
478455
505085
504693
458842
507275
485031
474248
448046
425536
426483
5784154
5752119
5710865
5694296
5672103
5655697
5629577
5610999
5588888
5494898
5259431
5073559
4951031
4902834
74.54696
75.06773
74.67906
75.69982
74.85108
75.36158
75.20336
74.88363
75.99982
76.18915
75.99980
75.99984
76.13467
74.11027
38.14749
38.29747
38.74992
38.90001
39.09990
39.24886
39.48327
39.64988
39.85001
40.80527
42.90532
44.56104
45.64038
36.16101
539696
507587.29
527888
439315
512877
468799
482511
509983
414065
399695
390591
395376
411774
433672
5777723
5757707
5710925
5694086
5672126
5655544
5629572
5611095
5588406
5482178
5244132
5057756
4945383
4906022
W–E
E–W
W–E
E–W
W–E
E–W
E–W
W–E
E–W
E–W
E–W
E–W
E–W
W–E
practical reasons, we defined the western limit by connecting the
positions where the trench sediments onlap onto the hemipelagic
sediments. These points define a polygon, and allow us to cut out the
relevant part of the grid. As the trench fill thins to c. 50 m at the western end of the trench (compared with up to 2700 m at the eastern
end), the exact position of the seaward trench limit matters little for
the final volume calculation. In locations where short profiles did not
extend far enough westwards onto the Nazca Plate to resolve the
onlap point of the trench sediments onto the hemipelagic cover of the
oceanic plate, we instead connected the onlap points of the neighbouring profiles to the north and south, where these were long
enough. The resulting additional ‘patch areas’ (Fig. 1) were then considered to be covered uniformly by 100 m of sediment and their volume was added to the total volume. The resulting differences in the
calculated volumes are small (up to 2%).
UTM18S
Reference
y
Flueh 1995; Laursen
et al. 2002
Flueh 1995; Laursen
et al. 2002
Flueh 1995
Flueh 1995; Laursen
et al. 2002
Flueh 1995; Laursen
et al. 2002
Flueh 1995; Laursen
et al. 2002
Geersen et al. 2011b
Rauch 2005
Geersen et al. 2011b
Geersen et al. 2011a
Geersen et al. 2011a
Völker et al. 2006
Rauch 2005; Völker
et al. 2006; Geersen
et al. 2011a
Völker et al. 2006
Geersen et al. 2011a
Geersen et al. 2011b
Völker et al. 2006
Bangs et al. 1992
Bangs et al. 1992
The upper surface of the trench fill is defined by the sea-floor
grid that we produced from bathymetric data recorded on 12 cruises
of research vessels Sonne, Meteor, Vidal Gormaz and James Cook
between 1995 and 2011. These data, in total more than 8000 data
files comprising about 1.1 billion soundings, were recorded with
different swath bathymetry systems, but mostly with the Kongsberg
EM-120 system. Processing steps comprised the check of navigation data, interpolation of missing navigation values, calculation 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. Data gaps were patched by spline interpolation with the global GINA dataset (Lindquist et al. 2004). We calculated the volume between the TOB grid and the sea-floor grid for
different combinations of interpolation numbers, bin sizes and grid
sizes; this resulted in very similar values.
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D. VÖLker et al.
Fig. 4. (a) Isopach map of the trench fill within the limits defined by the outer rise and the toe of the continental slope; isopach contours are 400 m.
(b) Seismic reflection profile across the trench at 38°S, showing the wedge-shaped geometry of the trench fill, the outer rise and onlap of trench
turbidites onto hemipelagic drapes of the Nazca Plate as well as the axial channel (Völker et al. 2006).
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Chile Trench Fill Volume
As a correction, we clipped regions where seamounts or ridges
protrude above the flat floor of the trench. In those areas, obviously the trench-fill thickness is zero. Such a situation is met
where the Valdivia and Mocha fracture zones enter the trench
and west of the Tolten Canyon exit (Fig. 1), where a small seamount towers some 100 m above the western end of the trench.
However, huge seamounts in the trench that would significantly
modify the accommodation space are not discernible in bathymetric or seismic data.
Results
Shape of the top of the oceanic basement
The sediment fill between sea floor and TOB forms a 20–80 km
wide (west to east) and 1580 km long (north to south), wedgeshaped body with a maximum thickness of up to 2700 m at the
deformation front (Fig. 4a). The wedge shape results from the general west to east increase in the depth of the TOB. The TOB depth
at the deformation front varies locally between neighbouring profiles and decreases at the extreme northern and southern ends of the
study area (Fig. 3a, stars), but generally parallels the northward
increase in depth of the sea floor that is related to the northward
increase in plate age. In particular, no prominent maxima in accommodation space formed by local troughs of the Nazca Plate are
resolved close to the deformation front. Looking in detail, however,
it can be seen that the TOB is often offset by trench-parallel escarpments of some 100–300 m that are continuous over a number of
seismic profiles. These escarpments are formed by bend faults and
correspond to the trench-parallel half-grabens that are obvious in
the bathymetric data of the Nazca Plate directly west of the trench,
in particular between 35°S and 37.5°S (Fig. 2a).
Volume and volume distribution
The total volume of the trench fill between 32°S and 46°S amounts
to 46000 ± 500 km3. The uncertainty range given here is defined by
(1) the calculated values from 12 models that use the same raw
input data (depth-converted seismic profiles) and the same cropping polygon but different interpolation, binning and gridding
intervals and (2) by using different versions of the deformation
front for cropping the grid (as explained above). The volume corresponds to a mean trench fill thickness of 840 ± 10 m, over an area
of 55000 ± 500 km2.
The vertical plane through which sediment passes from the
trench into the accretion prism or subduction channel (hereafter
referred to as the subduction gate) is defined by the deformation
front as upper limit and its downward projection onto the TOB as
lower limit. This subduction gate has an area of 2700 km2, a mean
height of 1.68 km and a maximum height of 2.7 km between 35°S
and 33.5°S (Fig. 3a and b). With the present rate of convergence, a
total sediment volume of 0.178 km3 (1.145 × 10−4 km3 per km of
trench length) passes this subduction gate per year to become either
frontally accreted or subducted.
The latitudinal volume distribution is shown in Figure 3c. The
distribution shows a northward linear increase from the southern
end of the sediment-filled trench at 46°S to an absolute maximum
of 55 km3 trench sediments per 1 km of latitudinal distance between
41°S and 40°S. From 40°S to the northern end of the study area at
the Juan Fernandez Ridge the volumes decrease, but show local
maxima. North of 40°S, the volume drops sharply to a local minimum at 39.5–39°S. Smaller maxima are reached at 38.5°S, 36°S
and 33.5°S with local minima between them. Towards the northern
end the volume decreases again.
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The reasons for this heterogeneous volume distribution are not
obvious and need to be further investigated. Apparently, there is no
direct correlation between the height of the subduction gate (Fig.
3a) and the latitudinal volume distribution, with the exception of
the northernmost and southernmost tips of the trench. In particular,
the maximum height of the subduction gate is reached close to the
northern end of the study area, where the sediment volume of the
trench is intermediate. Also, the sharp drop in trench volume
between 40°S and 39°S has no correspondence in the height of the
subduction gate. Other factors need to be considered.
Discussion
In the following section we introduce the factors that we consider
most likely to influence the sedimentary input to the trench and discuss how they are expressed in the volume distribution. Generally,
we can distinguish between factors that influence the transport of
sediments from land to trench at a given latitude (climate and precipitation, local sediment feeder systems, volcanic activity of the arc,
extent of glaciations and mass wasting) and factors that influence the
latitudinal (south to north) transport of sediment within the trench
and onto the Nazca Plate (oceanic plate properties, position of subducting oceanic fracture zones, giant submarine slope failures).
Factors for sediment transport to the trench
(external factors)
Precipitation and denudation of the hinterland. The present mean
annual precipitation, as exemplified by 23 meteorological stations
along the Chilean coast and Andean foreland, is shown in Figure
5b. The data were extracted from a website of the Department of
Geophysics of the Universidád de Chile, Santiago de Chile (http://
www.atmosfera.cl). The meteorological data were compiled from
the following sources: Normales de precipitación, temperatura
media, temperatura mínima media y temperatura máxima media,
1961–1990, Dirección Meteorológica de Chile, 1991; Perspectivas
de desarrollo de los recursos de al región Aysén del General Carlos
Ibañez del Campo: Caracterización Climática, IREN-Corfo, Publicación 26, 1979; Perspectivas de desarrollo de los recursos de la
VII Región: Climatología, IREN-Corfo, Publicación 25, 1979
(http://www.atmosfera.cl/HTML/datos/datos_02.html).
The central part of the Andean Range to the north of our study
area (33–15°S) lies within the subtropical belt of deserts. The
northern part of our study area has a Mediterranean climate with
rainfall rates below 500 mm a−1. To the south, Westerlies bring
abundant moisture, resulting in precipitation rates of >1500 mm a−1
south of 40°S and >2500 mm a−1 south of 45°S (Fig. 5b) and a mean
annual river runoff of 250–500 mm a−1 (Fekete et al. 2000) along
the western slopes of the range. Lamy et al (2001) showed that the
position of the rainfall minimum in the north and maximum in the
south along the coast and hinterland of Chile oscillated during the
Holocene in relation to latitudinal shifts of the Southern Westerlies.
The general pattern of increasing rainfall to the south, however,
remained stable. The long-term denudation rate of the Andes shows
a prominent climatic component related to these Hadley cell-driven
precipitation regimes (Montgomery et al. 2001).
The rainfall gradient shows up in the volume distribution by the
general southward increase in volume from 32°S to 40.5°S. South
of that latitude, however, the trench volume diminishes although
rainfall rates continue to increase (Fig. 5b and c).
Sediment feeder systems (rivers, canyons, fans). A number of river
systems drain the continental forearc of Chile in an east–west direction, bringing water and sediments from the western slopes of the
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D. VÖLker et al.
Fig. 5. Latitudinal distribution of the trench
fill volume in relation to plate tectonic and
climatic parameters, volcanic extrusive
activity of the Southern Volcanic Zone
and giant mass-wasting events. (a) Age
of the incoming Nazca Plate at the Chile
Trench according to Tebbens et al. (1997,
stars) and cumulative volumes of volcanic
edifices per arc segment according to
Völker et al (2011), normalized to segment
length in cubic kilometres per kilometre
arc length (red circles and continuous
line, dark grey in printed version). The arc
segments are numbered from I to VII. If
the largest described tephra volumes (Loma
Seca Tuffs) are added, the cumulative
volume for segment II increases (yellow
circle and arrow, light grey in printed
version). (b) Precipitation pattern and river
mouth positions. (c) Latitudinal volume
distribution of trench-fill sediments,
normalized to 1 km of trench length.
Purple lines (dark grey in printed version),
volumes integrated over latitudinal
segments of 1° length; orange lines (light
grey in printed version), volumes integrated
over latitudinal segments of 0.5° length;
also volumes of giant slope failures of
Geersen et al. (2011a), SGSF, southern
giant slope failure; CGSF, central great
slope failure; NGSF, northern great slope
failure. Also shown are trench positions
of subducting features of the Nazca Plate
(JFR, Juan Fernández Ridge; MFZ, Mocha
Fracture Zone; VFZ, Villarica Fracture
Zone; GFZ, Guafo Fracture Zone;CFZ,
Chiloé Fracture Zone; triangles).
Andes and the Coastal Cordillera to the Pacific Ocean (Fig. 1). The
spatial proximity of source to sink implies short sediment pathways. The river outlets are indicated in Figure 5b. Many of the
rivers connect directly to submarine canyons that guide sediments
down the continental slope. The largest canyon systems (Callecalle, Tolten, Biobio) have formed sedimentary fans in the Chile
Trench (Thornburg et al. 1990; Völker et al. 2006; Fig. 2a and b).
South of the Canal Chacao (42°S) the situation changes, as Chiloé
Island defines a gulf at its eastern side (Golfo de Ancúd, Golfo
Corcovado; Fig. 1) into which the river systems of the Andes
debouch. Thus, the clastic load of those rivers is transported to the
Chile Trench indirectly by tidal currents and through the outlets of
the gulf (Canal Chacao and Golfo Corcovado; Fig. 1b) rather than
directly by river and submarine canyon systems. Seaward of both
of these outlets, submarine canyons document that such transport
out of the gulf has been active at some time. Small river systems
drain Chiloé Island directly westward. To the south of Golfo Corcovado (44°S) the coast is characterized by deep, glacially eroded
fjords and the Chonos Archipelago, which inhibit the direct fluvial
transport of clastic sediments derived from the Andes to the trench
(Fig 1b). There is little information about the bathymetry of the
fjords between 44°S and 46°S, but owing to the common origin of
ice-flow excavation, we assume similar characteristics to those of
the fjord system south of 47°S, characterized by deep troughs of
>800 m water depth (e.g. Sievers et al. 2002). Baker Channel at
47°45’S, for example, has a maximum depth >1200 m but is separated from the open ocean by a sill of c. 200 m water depth (Paskoff
2010). Such morphology should effectively capture land-derived
sediments at times of retracted glaciers such as today.
Although the larger river–submarine canyon systems (Callecalle,
Tolten, Biobio) have built fan systems into the trench (Fig. 2a and
b), these point sources do not correspond in the volume distribution
to local maxima (Fig. 5b and c). The decrease of trench fill volume
south of 41°S might be partially due to the fact that the fjord coast
and the gulf between Chiloé Island and the mainland provide sediment traps during warm periods.
Glaciations. The extent of glaciers from the Andes into the piedmont zone of the foreland during the stages of the last Quaternary
glaciation (Llanquihué Glaciation) is well documented by glacial
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Chile Trench Fill Volume
deposits (Caldenius 1932; Rabassa & Clapperton 1990). The maximum extent thus is depicted by a moraine belt that parallels the east
coast of Chiloé Island and continues northward within the Central
Valley of Chile (Fig. 1b). A terminal position of glaciers on Chiloé
Island would bridge the gulf between Chiloé Island and the mainland (Golfo de Ancúd, Golfo Corcovado) and bring glacial detritus
more directly to the Pacific coast.
According to Mercer (1976) and Rabassa & Clapperton (1990),
the greatest Patagonian glaciation developed during the Early
Pleistocene, when glaciers terminated on the Pacific shelf south of
latitude 43°S. The extension of glaciers onto the continental shelf
would favour the discharge of immense outlet lobes directly onto
the continental slope, bypassing the shelf as a sediment trap.
Both effects should favour enhanced flux of material to the
trench in the southernmost part of the study area (south of c. 43°S)
during glaciations. We would expect such flux to be expressed as a
volume signal, with high volumes in the high latitudes. Sediments
delivered to the trench during the Early Pleistocene glaciation
should, however, mostly have been subducted or accreted by now.
In contrast to this expectation, the sediment volumes in the trench
are low in the southernmost area and increase steadily northwards to
around 41°S (Fig. 3c). Also, the width of the accretionary prism
does not increase south of 43°S, which we would expect if the
trench had received huge sediment volumes during the Pleistocene.
Volcanic activity. The solid volcanic products at volcanic arcs (volcano edifices, lava flows and tephra layers) reflect the extrusive
fraction of magma production at depth. Völker et al. (2011) provided volume estimates of 65 Mid- to Late Pleistocene to Holocene
volcanic edifices of the Chilean Southern Volcanic Zone listed in
the ‘Volcanoes of the World’ database of the Smithsonian Institution (Siebert & Simkin 2002). As these rocks are relatively easy to
erode (e.g. Heberer et al. 2010), latitudinal variations in extruded
magmas might be expressed in the volume distribution of trench
sediments. Volcanic ash falls or tephra sheets, which can be voluminous in Chile (e.g. Hildreth et al. 1984; Sruoga et al. 2005), are
only partly included in the volume curves of Völker et al. (2011),
as their volumetric determination requires detailed field mapping
and geochemical modelling. Volcanic ashes constitute 10–40% of
the total erupted volume of the arid Central Volcanic Zone of the
Andes from the Early Miocene to Holocene (Francis & Rundle
1976; Baker 1981) and may have contributed to erupted volumes in
the Southern Volcanic Zone in a similar order of magnitude.
Völker et al. (2011) observed a maximum in the volcanic extrusion in their segment V (between the Valdivia Fracture Zone and
Chiloé Fracture Zone, c. 41.1–39.7°S), mainly owing to large arc
volcanic centres (e.g. Mocho–Choshuenco, Cordón Caulle–
Puyehue, Antillanca and Osorno), and another smaller maximum in
their segment II between 35.5°S and 34.5°S (Fig. 5a, curve), here
partly owing to large volcanic centres of the back-arc (e.g. volcano
Sosneado and volcano Guanaqueros). This second maximum is
significantly increased if the two largest documented tephra units
on land (Diamante Tuff and Loma Seca Tuffs) are added (Hildreth
et al. 1984; Sruoga et al. 2005; Fig. 5a). Interestingly, the positions
of both maxima coincide rather precisely with volume peaks in the
trench; that is, with the absolute peak at around 41°S and a local
peak at around 35°S (Fig. 5a and c).
The absolute values of extruded material (normalized to 1 km of
trench length) are smaller by a factor of 1:5 to 1:7 than the corresponding trench fill volume values (with the exception of segment
VI, where this factor is c. 1:11). At first sight, these relations seem
to suggest that variations in the volcanic ‘productivity’ of the
Southern Volcanic Zone should not have a significant influence on
the trench volume distribution. However, several points should be
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kept in mind, as follows. (1) The volumes of volcanic extrusions
have been calculated for an estimated age range of 0.5 Myr, whereas
the trench sediments encompass c. 1 Myr, which would reduce the
discrepancy between the two volumes. (2) The volume distribution
curve for volcanic extrusions (Fig. 5a) accounts for only the fraction that is still present as volcanic edifice. The morphological
appearance of the majority of these volcanic edifices, however,
bears witness to massive erosion. (3) Trench sediment samples
show that volcanic lithic fragments (Lv) represent the dominant
lithic fragment type (Heberer et al. 2010). Thus, the provenance
signal of volcanic sources is very strong (Heberer et al. 2010), documenting the easy mobilization of large amounts of ash fall present
after each eruption as well as the fact that volcanic edifices on land
are quickly eroded and transported to the sea.
Factors for sediment transport within the trench
(internal factors)
Oceanic plate properties (age, thickness, fracture zones). The plate
age of the Nazca Plate increases from 0 Ma at the Chile Triple Junction to 35 Ma at the Juan Fernandez Ridge (Fig. 5a, stars). The journey of a sediment particle from the western end of the trench to the
subduction front takes less than 1 Myr (Rauch 2005; Geersen et al.
2011a), regardless of the plate age, so that these age differences
should matter little directly. However, the age does influence the
buoyancy of the downgoing plate as well as its flexure (ContrerasReyes & Osses 2010). This is reflected in the northward increase in
the depth of the TOB and the trench floor along the deformation
front (Fig. 3b). In theory, the plate age difference could account for
a significant difference in the thickness of the hemipelagic drape of
the Nazca Plate. This is, however, not observed, as the drape thickness is small (200 m) over all of the study area.
As mentioned above, the outer rise of the oceanic Nazca Plate
forms a pronounced obstacle of some 100 m height to westward
sediment transport north of 41°S (Fig. 3a). At this latitude, the
trench basin is clearly confined between the outer rise and deformation front. The situation is different south of 41°S, where the outer
rise is poorly developed, reflecting the low rigidity of the young
oceanic Nazca Plate (Contreras-Reyes & Osses 2010; Fig. 3b).
Fracture zones form linear ridge-like structures on the Nazca
Plate seaward of the trench. This is particularly true for the Valdivia
Fracture Zone, which enters the trench as a continuous, dam-like
structure (Fig. 2b) with a maximum height of 1500 m. The Mocha
Fracture Zone is less prominent as a bathymetric feature.
The lower plate properties thus could have a major impact on
sediment transport in the trench by forcing a northward transport
direction, by allowing or limiting the transport of turbidites beyond
the trench, and by damming the latitudinal transport within the
trench at those latitudes where fracture zones are subducting. In
this respect, it is noteworthy that the absolute maximum of trench
volumes lies directly south of the Valdivia Fracture Zone and the
second largest maximum lies directly south of the Mocha Fracture
Zone (Fig. 5a and c).
Mass wasting. Submarine landslides are common on the continental slope of Central Chile (Völker et al. 2009, 2012; Geersen
et al. 2011a). The majority of the landslide features are too small
(<2 km3) to noticeably affect the volume of the sedimentary
trench fill. Also, many of the mass-transport deposits rest in continental slope basins and do not reach the trench at all (Völker
et al. 2012). Three giant slope failures that could potentially
affect the trench volume are documented with minimum volumes
of 253 km3 (central giant slope failure), 388 km3 (northern giant
slope failure) and 472 km3 (southern giant slope failure) (Geersen
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D. VÖLker et al.
et al. 2011a; Fig. 2a and c). As these mass-transport deposits are
spread over a defined latitudinal distance in the trench, we can
show how they would contribute to the volume of the trench fill
by dividing the landslide volume by the affected latitudinal distance (Fig. 5c). The position of the southern giant slope failure,
the largest slope failure, matches that of the local trench-fill maximum at around 38.5°S (Fig. 5c). The central and northern giant
slope failures coincide instead with a local minimum. The direct
contribution of the two largest landslides is about 50% of the
trench fill over some tens of kilometres. Another indirect effect
has to be considered: if mass-wasting events bring such huge
amounts of material into the trench over short periods of time, we
would expect some kind of barrier effect to latitudinal transport.
In particular, the southern giant slope failure could have blocked
south to north transport within the trench.
Interplay of factors
In this section we propose two opposing end-member scenarios
that could explain many of the latitudinal variations in the volume
of the trench fill (Fig. 6). The two scenarios build on the factors
discussed above and favour either a predominance of local sediment input parameters over trench transport processes (scenario 1,
Fig. 6a) or vice versa (scenario 2, Fig. 6b). Models should be able
to answer four striking questions that arise from the volume curve,
as follows.
(1) What defines the absolute volumetric maximum at around
40–41°S?
(2) Why does the volume decrease south of 41°S, although precipitation further increases southwards?
(3) What is the reason for the second maximum at around
35–36°S and the other maxima north of 41°S?
(4) Why do the largest river systems (Tolten and Biobio) not
correspond to maxima in the volume distribution?
Finally, we propose a third model that alternates between the
end-member scenarios and matches best with the observed volume
distribution.
End member 1, local control on sediment input. This scenario postulates that the volume distribution results from a combination of
latitudinal variations of the extrusion rate of easily erodible volcanic material within the Southern Volcanic Zone and the latitudinal variations in rainfall rates and temperature. The transport from
land to the trench is characterized by a small distance and effective
drainage networks north of Chiloé Island and the sedimentary signals are stored in the trench at the respective latitude of their origin
(i.e. there is little transport within the trench).
(1) 46–42°S (Fig. 6a, 1). Volcanic extrusion rates are low owing
to relatively small and widely spaced volcanic centres (Fig.
5a). The material that is subject to weathering (which is
mainly mechanical owing to low temperatures) is old, hard
bedrock (Patagonian Batholith) rather than young volcanic
rocks (Hervé et al. 2007). In addition, the Fjord Coast, the
Chonos Archipelago and Chiloé Island represent sediment
traps that effectively capture river material (Fig. 1b). As a
result, the volume (sediment input) is low.
(2) Around 40–41°S (Fig. 6a, 2). Effective river networks (Rio
Bueno, Rio CauCau, Rio Callecalle), high rainfall rates
(Fig. 5b) and moderate temperatures (e.g. Lamy et al. 1998)
work together to erode and transport the extrusive material
of several huge arc volcanic centres, which form a peak in
volcanic extrusion in the segment V of Völker et al. (2011;
Fig. 5a). The easily erodible material is transported via
submarine canyons (e.g. Lingue and Callecalle Canyons;
Fig. 1b) that connect to river systems and is dumped close
to the canyon exits. In addition, the bulge of the outer rise
is less pronounced here than further northwards (Fig. 3a)
and turbidity flows can easily run far onto the Nazca Plate,
shifting the trench fill area further westward. As a result, we
observe a broad belt of trench sediments (Fig. 1b) and find
the maximum of trench fill here (Fig. 5c). South to north
transport in the trench is not very effective as the peaks of
the trench-fill volumes are not shifted relative to the corresponding peaks of volcanic productivity (Fig. 5a and c).
(3) 40–36.5°S (Fig. 6a, 3). A number of volcanic centres exist
in the Southern Volcanic Zone, but they are generally
smaller than at around 40–41°S (Fig. 5a). Rainfall rates are
still high but decrease northwards, and moderate temperatures favour chemical weathering. River networks exist. As
a result, the sediment input decreases in line with the rainfall (Fig. 5b and c).
(4) Around 35°S (Fig. 6a, 4). Huge volcanic centres in the forearc
but mostly in the back-arc are responsible for a second peak
in volcanic extrusion rate (Fig. 5a). Erosional detritus from
the volcanic edifices of the Argentinian back-arc would,
however, never reach the Chile Trench by fluvial transport
as they stand on the Atlantic side of the Andean drainage
divide. This clearly applies to the volcanoes Cerro Payun
Matru (36.42°S, 69.2°W), Tromen (37.14°S, 70.03°W) and
Domuyo (36.63°S, 70.42°W), which are the three largest edifices (Völker et al. 2012), but also to most of the volcanoes
that are located in Argentina. On the other hand, this segment
is noteworthy for extensive and extremely voluminous tephra
deposits (Fig. 5a). These deposits are present on both sides
of the Andean drainage divide, and they are easy to erode
and could therefore partly be finally deposited in the trench.
Although rainfall rates diminish (Fig. 5b), this signal is transported to the trench by effective river networks that continue
as submarine canyons (Rio Mataquillo–Mataquito Canyon,
Fig. 1a). As a result, we find a second, smaller peak here.
(5) 35–32°S (Fig. 6a, 5). Volcanic extrusive volumes further
diminish, as do rainfall rates (Fig. 5a and b). River network
systems connected to submarine canyons exist (e.g. Rio
Maipo–San Antonio Canyon), but bring lesser sediment. As
a result, the trench volume decreases significantly and the
trench is not completely filled.
Although this scenario matches many of the observed characteristics, there are two major problems with it. First, we would expect
a high flux of material to the trench in the southernmost part of the
study area (south of 43°S) during the glaciations when glaciers
extended onto Chiloé Island or even further onto the shelf. This
signal does not appear, which calls for massive displacement of
material. The same applies to the point sources of submarine canyon outlets, which should show up as local maxima. Second, the
existence of the prominent trench axial channel and also the systematic asymmetry of submarine fan systems (Figs 2a, b and 6)
indicate at least two phases of active south to north transport in the
past (Völker et al. 2006). If not active, the axial channel would
become completely buried within c. 300 kyr at the calculated sedimentation rates of c. 50 cm ka−1 of Mix et al. (2003).
End member 2, control by trench-parallel sediment transport. The
trench sediment volume is influenced by climatic factors, the geology of the hinterland and the transport from source to trench, but is
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Chile Trench Fill Volume
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Fig. 6. Schematic representation of the
dominating factors of the two opposed endmember models for sediment transport into
the trench and within the trench.
mainly governed by redistribution within the trench. This redistribution is primarily driven by the height difference between the young
Nazca Plate in the south and the 35 Myr old Nazca Plate in the north
(Fig. 3b), but it is also guided by obstacles within the trench. These
obstacles (fracture zones and giant slope failure deposits; Fig. 5a and
c) act as dams and block the northward transport until they are overcome by the material ponded in front of them. The relative height of
the outer rise over the trench floor is another factor that either permits
sediment transport onto the Nazca Plate or restricts the transport to
within the narrow trench (north of 41°S, Fig. 3b). In line with arguments of Melnick & Echtler (2006) there was a significant and fast
increase in sediment flux to the trench caused by the onset of glacial
denudation in the Patagonian Andes about 6 Myr ago. With the glaciation being most intense in the south it seems probable that this flux
was greatest in the far south than further north.
(1) 46–42°S (Fig. 6b, 1). Pleistocene glaciations were most
intense and the ice front was closest to the coastline or even
on the shelf (Fig. 1b). The obstacle that the fjord coast and
Chonos Archipelago form for sediment transport to the
trench in warm periods was bridged by the ice. The turbidity currents that deliver material to the trench were diverted
northwards owing to the slope gradient within the trench.
As a result, the immense flux of material to the trench is not
manifest in this trench segment, but further north.
(2) Around 40–41°S (Fig. 6b, 2). The Valdivia Fracture Zone
has formed an effective barrier to the northward transport of
sediments within the trench. Today, this barrier forms an up
to 1500 m high, uninterrupted wall that narrows the trench
considerably (Fig. 2b). Effective river networks (Figs 1 and
5b), high rainfall rates (Fig. 5b) and moderate temperatures
contribute to add the extrusive material of several huge arc
volcanic centres (Fig. 5a). The material from the south and
from local sources is ponded in front (south) of the Valdivia
Fracture Zone (Fig. 2b) until the trench is filled and further
northward transport (as well as transport far onto the Nazca
Plate) is possible. This must have happened within the last
1 Myr, as no step is observable in the trench floor gradient.
As a result, the trench volume maximum is located here.
(3) 40–38°S (Fig. 6b, 3). A number of volcanic centres exist in the
Southern Volcanic Zone, but they are generally smaller than at
around 41°S (Fig. 5a). Rainfall rates are still high but decrease
northward. The northward transport of trench sediments
across the obstacle of the Valdivia Fracture Zone has not yet
worked long enough to fill this trench segment completely.
As a result, we observe a drastic decrease in trench volume
compared with around 41°S and a steady increase towards
38.5–39°S, where the next obstacle is met (Fig. 5a and c).
(4) Around 39–38.5°S (Fig. 6b, 4). The southern giant slope
failure transferred enormous masses of slope material into
the trench at some point in time before 0.56 Ma (Geersen
et al. 2011a). The material that was once deposited all over
the trench floor is still filling much of the trench and probably formed a second barrier behind which turbidites originating from the south were ponded. The Mocha Fracture
Zone forms a less impressive morphological structure than
the Valdivia Fracture Zone but might have acted as an additional barrier. As a result, we find a second maximum in
trench fill volumes here (Fig. 3c and 5c).
(5) 38.5–32°S (Figs 5 and 6b). The trench receives the tail of
transport from the south, and transport is confined by the
steep trench walls. Local input is low.
Again, this scenario matches many of the observed aspects, but
there are two main problems. First, north of 39°S, rainfall rates and
supply from the south diminish (Fig. 5b). However, we have the two
maxima at 35.5–36°S and at 33.5–34°S. In line with the arguments
above, this would call for some kind of sediment transport barriers (or
alternative explanations). However, no fracture zones are subducted
here and submarine mass-wasting features are too small to form obstacles in the trench. Second, independent information from provenance
analysis of trench sediment samples seems to indicate that redistribution and amalgamation of sediment material along the trench axis is
negligible. These sedimentological and mineralogical investigations
were conducted by Roeser (2007) and Heberer et al. (2010) on 16
gravity core sites from within the trench and submarine fans between
36°S and 46°S. The mineralogy of the cores reflects the erosional conditions and local rock types of the same latitude. In particular, a distinct
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D. VÖLker et al.
Fig. 7. Morphology of the Chile Trench
axial channel in map view (a) and as
a PARASOUND cross-section (b). In
map view the meandering of the internal
channel within the broader outer channel
bed is clearly visible. In the profile a flat
floor and banks of the internal channel are
distinguishable. Map scale is 1:550000;
vertical exaggeration of PARASOUND
profile is 1:28.
reduction in plagioclase content from north to south, coeval with an
increase in quartz, is attributed to the replacement of rhyolitic to
andesitic lavas of the northern part by the Patagonian Batholith as the
main source rock at higher latitudes (Roeser 2007, p. 69; Heberer et al.
2010). In summary, trench sediment mineralogy resembles source
rock lithologies of the corresponding latitude closely.
Final model: alternating control. Each of the end-member models
can explain many characteristics of the volume distribution but
either fails in explaining particular points or is at odds with independent data. We therefore propose a third, intermediate model,
which alternates between the two end-member scenarios roughly in
the rhythm prescribed by glaciations and deglaciations. As it is clear
that enormous sediment masses must have been supplied to the
trench south of 43°S when glaciers extended from the Andes to the
coast, we simply need active transport to the north to explain the
little sediment at the southern end of the study area. This sediment
input ceased during the Holocene as glaciers retreated and the fjords
and gulfs formed the main depocentres. The bulk of glacially
derived material was dammed behind the major obstacle of the Valdivia Fracture Zone, which had to be levelled out before further
northward transport was possible. Local factors such as the peaks of
volcanic activity (Fig. 5a) and river networks (Fig. 5b) overprint this
pattern but were more dominant during the Holocene when the flux
of material from the south decreased as well as to the north, further
away from the southern source. These local factors could be the
main causes for local minima and maxima north of 41°S. Such a
model need not be in disagreement with the findings of Roeser
(2007) and Heberer et al. (2010), as their studies were performed on
gravity cores of 170–650 cm length, thus most probably representing the Holocene phase, when latitudinal transport had ceased.
In this view, the excavation of the axial channel took place during
the Pleistocene when there was a high sediment input from the south,
whereas after deglaciation its activity ceased. Once the glacier front
had retreated inland, less sediment reached the trench and the broad
axial channel started to fill. Some details of the axial channel morphology support this interpretation, as they indicate that the channel
bed is being filled at present, but that some transport is still occurring.
In particular, between 37°S and 39°S we observe a small channel that
meanders between banks that have formed within the wider bed of
the (relict) axial channel (Fig. 7a and b).
Conclusions
We created a model geometry of the top of the Nazca Plate in the
Chile Trench, where it is buried underneath sediments of up to
2700 m thickness. The model is created based on spline interpolations between depth-converted seismic reflection profiles that cross
the trench. The model serves to compute the total amount and the
regional distribution of trench sediment volumes. The latitudinal
Downloaded from http://jgs.lyellcollection.org/ at Servicio Nacional De Geologia Y Mineria on November 11, 2014
Chile Trench Fill Volume
volume distribution curve shows characteristics in the position and
dimension of maxima that are not easily explained by a single
cause such as the position of river systems. Instead, many of the
features can be explained by a combination of regional factors such
as latitudinal gradients in the rainfall and denudation rate, production rates of easily erodible volcanic material in the Southern
Volcanic Zone of the Andes, effectiveness of river systems and
coastal morphology, which together create regional input regimes.
Some aspects of the volume distribution curve are, however, not
consistent with such a model of regional input dominance but
rather point to effective transport processes within the trench.
These transport processes would be directed northwards in line
with the bathymetric gradient of the trench floor but also guided by
the changing morphology of the bulging Nazca Plate and by obstacles that are formed by subducting fracture zones and mass-transport deposits. Both models (local factors and trench transport) have
their merits and need not be in opposition as they form end members of a mixed system that could have switched according to the
pulse of glaciations and deglaciations. This combined model could
also solve a paradox between the existence of a well-preserved
trench axial channel witnessing northward sediment transport in
the trench on the one side and young sediment samples from the
trench that indicate a close relationship to local onshore geology on
the other.
This publication is Contribution 251 of the Sonderforschungsbereich 574
‘Volatiles and Fluids in Subduction Zones’ at Kiel University. J.G. gratefully acknowledges funding through the European Commission’s Seventh
Framework Programme FP7-PEOPLE-2011-IEF under the grant agreement
number PIEF-GA-2011-301763. We thank the two anonymous reviewers
and the subject editor G. Hampson for their efforts to improve the paper.
The processing of the bathymetric raw data was performed using the
MB-Systems software (Caress et al. 1996). The TOP grid was calculated
using the GMT software (Wessel & Smith 1998).
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Received 12 October 2012; revised typescript accepted 5 April 2013.
Scientific editing by Gary Hampson.