Nature of the plate contact and subduction
zones diversity
Roberta De Franco a,∗, Rob Govers a, Rinus Wortel a
aDepartment of Earth Sciences, Utrecht University, Budapestlaan 4, 3508 TA
Utrecht, The Netherlands
Abstract
In recent studies we showed that the nature of the plate contact in subduction zones
is an important physical feature in both oceanic lithospheric subduction and con-
tinental collision. We investigated two fundamental states of the plate contact: one
based on a fault and the other based on a subduction channel. Using geodynamic
modeling, we determined the specific signatures of both states of the subduction con-
tact. In the present study, we combine results of our previous numerical experiments
with a re-analysis of published observations. Overall, our synthesis connects seismic
moment release with back-arc deformation and tectonic processes at the margin. It
leads us to identify four classes of subduction zones. The first two classes results
directly from our numerical experiments. In class 1, subduction zones are charac-
terized by a plate contact that is largely fault-like with an accretionary margin. In
class 2, the plate contacts are largely channel-type and have an erosive margin. Class
3, where the plate contact is entirely channel-like, consists of accretionary margins
with a high sediment supply. Subduction zones of class 4, mostly characterized by
an erosive convergent margin (northern Chili, Peru, Honshu and Kuril), are more
complicated. They can be explained by incorporating regional observations.
Key words: subduction zones; accretionary margins; erosive margins; large
Preprint submitted to Elsevier 7 April 2008
earthquakes; back-arc deformation
1 Introduction1
Earthquake intensity, back-arc state of stress, subduction velocity and sed-2
iment supply vary greatly from one subduction zone to the other. Under-3
standing the dynamics of the subduction process in view of these observed4
features represents a challenging problem of plate tectonics. In their classic5
study Uyeda & Kanamori (1979) group subduction zones into Marianas and6
Chili types. In the first case, the back-arc region shows extension, while in the7
second case compression prevails. Chilean type subduction zones are charac-8
terized by very strong thrust earthquakes, while Marianas type zones are not.9
Uyeda & Kanamori (1979) suggested that different subduction types result10
from different levels of coupling between the subducting and the overriding11
plate. The idea of Uyeda & Kanamori (1979) was confirmed by Conrad et al.12
(2004) who investigated the relation between back-arc stress and the magni-13
tude of the largest seismic events at each subduction zone and showed that14
there is a good correlation between these two observables.15
Convergent margins (i.e., the most shallow part of subduction contacts)16
can be roughly classified into accretionary margins and erosive margins (e. g.,17
Von Huene & Scholl, 1991, 1993; Clift & Vannucchi, 2004). Accretion transfers18
material from the subducting plate to the forearc wedge and plate boundary19
zone. It enlarges the accretionary prism and is accompanied by sediment sub-20
∗ Corresponding authorEmail address: [email protected], tel.: 0031 30 2535076, fax: 0031
30 2535030 (Roberta De Franco).
2
duction. The accretion rate depends on the balance between sediment influx21
and outflux through subduction and is favored at low subduction rate (Le Pi-22
chon et al., 1993). Subduction erosion is defined as the process that causes a23
net loss of material from the hanging wall. In this case the wedge volume is24
stationary or decreases with time (Lallemand et al., 1994). Lallemand et al.25
(1994) classified subduction erosion in two types, frontal and basal tectonic26
erosion. Frontal erosion takes place near the trench, and is thought to be due27
to the grabens or to ridges and seamounts. Basal tectonic erosion occurs at the28
base of the upper plate in a low friction environment, and might be due to a29
fast convergence rate that results in hydrofracturing by overpressuring. Such a30
first order classification into convergent and erosive margins neglects some of31
the observed variability of individual subduction plate contacts; a margin can32
evolve from one type into the other, or the plate contact may vary laterally.33
Accretion at the toe and erosion may occur simultaneously like, for example,34
in Japan and Peru (Von Huene & Lallemand, 1990).35
Trying to explain the dichotomy between the seismically active Chilean36
and the aseismic Marianas type subduction zones, Cloos & Shreve (1996) sug-37
gested that Chilean-type margins typically have thick trench fills and that38
the plate boundary zone thins arc-wards, while Marianas type margins have39
a thin or non-existent accretionary prism and the plate contact zone thickens40
with depth. They suggest that sea-mounts are subducted entirely to become41
seismogenic asperities in the Chilian type margin, while in the Marianas type42
just a truncated part of the sea-mount enters the subduction. This trunca-43
tion, in combination with the thick subduction channel, precludes subducted44
seamounts from becoming seismogenic asperities. Erosional margins are thus45
seismically quiescent, and accretionary margins exhibit high seismic moment46
release rates. Observational backup of this idea is good, although there exist47
3
some exceptions.48
Over the years several data sets have been systematically compiled and an-49
alyzed in order to understand the dynamics of subduction zones and to reveal50
the relation between different observables (e.g., Jarrard, 1986; Clift & Vannuc-51
chi, 2004; Lallemand et al., 2005; Sdrolias & Muller, 2006). Seismological stud-52
ies became more accurate, able to show new features at the subduction zones53
like the presence of inter-plate channel-like units of about 1-8 km thickness54
(Eberhart-Phillips & Martin, 1999; Oncken et al., 2003; Abers, 2005; Tsuru55
et al., 2002). Through inversion of arrival times from local earthquakes, these56
authors found an anomalous zone of low velocity at the interface between the57
plates. However, the arrival-time inversion can not resolve the thickness and58
the character of the layer independently. Investigation of the crustal fore-arc59
structures with wide-angle seismic data showed narrow, low velocity zones at60
the base of the fore-arc wedge, suggesting the existence of subduction chan-61
nels in Costa Rica, Makran, Nankai, Chile, Peru (Christeson et al., 1999; Kopp62
et al., 2000; Takahashi et al., 2003; Patzig et al., 2002). These data do not have63
the resolution to identify accurate subduction-channel geometries. Reflection64
seismic profiles from some of these fore-arcs give more accurate results; the65
disadvantage of these experiments is that they trace the subduction channel66
just to shallow depth (e.g., Von Huene et al., 2004; Ranero & von Huene,67
2000).68
The increasing evidence for variability of the plate contact motivated us to69
study its imprints on the response of subduction processes (De Franco et al.,70
2007). Through geodynamic numerical modeling we demonstrated that the71
overall plate contact nature has a dominant control on the state of stress of72
the overriding plate. In these models the entire plate contact is considered.73
We further investigated the role of the plate contact in the dynamic evolution74
4
of the subducting and overriding plate following the arrival of a terrane at75
the trench (De Franco et al., 2008). In the present study we shed new light76
on the geodynamics of subduction, based on the physical insights from our77
numerical experiments (De Franco et al., 2007, 2008) in combination with78
a re-analysis of published observations (Abers, 2005; Lallemand et al., 2005;79
Clift & Vannucchi, 2004). Our synthesis elucidates relations between back-arc80
state of stress, maximum seismic moment magnitude, and the nature of the81
all subduction plate contact, that affects the coupling level between overriding82
and subducting plate. Building on this, we identify four classes of subduction83
zones using observed back arc strain, nature of the convergent margin (erosive84
or accretionary) and maximum seismic moment magnitude. In doing this, we85
combine large scale features of subduction zones, following the approach of86
Uyeda & Kanamori (1979), with more regional observations as compiled by87
Cloos & Shreve (1996).88
2 Data and correlations89
The average dip angle of slabs at both shallow depths (depth range 0-12590
km) and great depths (depths greater than 125 km) is correlated with other91
characteristic parameters like the back-arc deformation and the nature of the92
plate contact. Recently, it was established that the magnitude of the low ve-93
locity anomaly at the top of the slab correlates with the average slab dip94
angle (Abers, 2005). Bodywave speeds decrease with increasing subduction an-95
gle at seven subduction zones: Aleutian (Ale), Alaska (Ala), Hokkaido-South96
Kurile (Hok-S.Kur), North Honshu (N. Hon), Marianas (Mar), Nicaragua97
(Nic), North Kurile-Kamchatka (N. Kur-Kam). Waveguide thicknesses vary98
5
between 2-8 km with large uncertainties. The velocity anomaly is as large as99
14 %, indicating that the waves within this layer propagate much more slowly100
than in the surrounding mantle. Between different subduction zones, the ve-101
locity anomaly varies by a factor of 2-3 down to 150 km depth.102
The low velocities may be explained by either metastable gabbro, that rep-103
resents an essential part of the oceanic crust, or low temperature hydrated104
mafic rocks that represent subduction channel-type rocks. This last hypoth-105
esis is plausible because slabs seem to dewater constantly during subduction106
(Gerya et al., 2002; Rupke et al., 2004). Quantitative experimental studies on107
rheology show that plastic deformation of upper mantle minerals is signifi-108
cantly intensified by the presence of water (e.g., Karato et al., 1986; Mei &109
Kohlstedt, 2000), suggesting that at higher water fugacity conditions weaken-110
ing effects may be quite pronounced. In this way it is possible to relate the low111
velocity to the rheology of the material. Numerical modeling experiments sup-112
port also a correlation between viscosity reduction in the subduction channel113
and increasing slab dip angle (Fig. 1 and Manea & Gurnis, 2007). Note how-114
ever, that the thickness of the subduction channel trades off with the seismic115
velocity anomaly: a thicker subduction channel with a relatively low velocity116
anomaly is analogous to a thinner channel with a higher velocity anomaly.117
Obviously we do not exclude that other factors than the plate contact may118
influence the slab dip angle. Other studies showed that trench motion, slab119
strength and evolution of the slab in time have a control on the deep slab dip120
(e.g., Cizkova et al., 2002; Billen & Hirth, 2007; Goes et al., 2008).121
In addition to the correlation between nature of the plate contact and dip122
angle, a strong correlation between back-arc deformation and subduction dip123
angle has been inferred (Lallemand et al., 2005). Back-arc spreading is ob-124
served for deep dip angles larger than 51◦, whereas back-arc shortening occurs125
6
for deep dip angles smaller than 31◦.126
When taken together, these correlations suggest that there is also a correla-127
tion between back-arc strain state and the rheological properties of the plate128
contact (see Fig. 1a and see also Chemenda et al. (2000)). In order to quan-129
tify this relation we calculate the correlation coefficient (Fig. 2) between the130
nature of the plate contact and the back-arc strain (Abers, 2005). The plate131
contact nature is parameterized by the velocity anomaly dlnVp = Vp−V0
V0, where132
Vp is the P wave velocity and V0 is the reference P wave velocity). Nicaragua is133
not included because the back-arc state of strain is not provided by Lallemand134
et al. (2005). The correlation coefficient for the data points shown in Figure135
2 a is R = −0.83. However, the seismic velocity anomalies have uncertainties136
(see Abers (2005)). Therefore we test the robustness of the correlation by sim-137
ply adding random noise (see the error bars) to the data points and evaluate138
the correlation coefficient for 1000 possible realizations. From the resulting139
distribution of correlation coefficients we compute the cumulative probabil-140
ity distribution (Fig. 2 b). 83 % of the 1000 correlation coefficients have a141
value lower than R = −0.8, indicating that the velocity jump is negatively142
correlated with the back arc state of strain (see Fig. 2 a). We conclude that143
the weaker the rheology of the subducting channel the more extensional the144
back-arc region.145
Our numerical experiments show a similar correlation (De Franco et al.,146
2007). In these geodynamic models, we linked the nature of the plate contact147
and the back-arc state of stress (tensional stress corresponds to extensional148
strain). The type of plate contact controls the dynamic response of the upper149
plate, drives the displacement of the overriding plate and, as a consequence, the150
stress distribution. We adopted two descriptions for the active plate contact: a151
subduction fault and a subduction channel, with the following characteristics:152
7
1) A deformable subduction fault is described via updated slippery nodes, in153
which the fault slip is locally kept parallel to the fault (Buiter et al., 2001).154
2) A subduction channel separates the subducting and the overriding plate.155
The channel width is assumed to be approximately 6 km (Shreve & Cloos,156
1986; Beaumont et al., 1999). Channel viscosity is taken to be Newtonian,157
ranging from 7·1017 Pa·s (Shreve & Cloos, 1986; England & Holland, 1979,158
red and yellow curves in Fig. 3) to 10 21 Pa·s (Renner et al., 2001; Stockhert,159
2002; Gerya et al., 2002, blue curve in Fig 3).160
In all the models the right-hand side of the subducting plate is free to move161
in horizontal direction, while the overriding plate is fixed on the left-hand side162
(see Fig. 3 bottom part). In Figure 3 we show the horizontal stress at the163
surface of five different models. A clear trend from high compressive to tensile164
back-arc stress is visible moving from a fault model (black curve) to a weak and165
wide subduction channel model (yellow curve). The first four models (black,166
grey, blue and red curves) are taken from De Franco et al. (2007) whereas the167
yellow curve represents a new model in which the thickness of the channel is168
increased (to about 20 km), while the viscosity is the same as in the model169
shown by the red curve (7·1017 Pa·s). The effect of the increased thickness is170
to obtain the back-arc tension. We conclude that increasing the thickness of171
the channel or further decreasing the viscosity in the channel will eventually172
lead to a tensile back-arc state of stress. In all these models a low viscosity173
wedge (LVW) is adopted above the subducting slab in order to reduce the174
down-warping, and as a consequence the magnitude of compressive stress, of175
the overriding plate in the arc/back-arc region (e.g., Billen & Gurnis, 2001,176
2003; Kelemen et al., 2004; Currie et al., 2004). The LVW is likely due to the177
presence of water released from the subducting slab or to melting processes178
at the base of the arc crust.179
8
Subsequently we investigate the relation between the nature of the plate180
contact and seismicity. Recently, Conrad et al. (2004) found that there is a181
positive correlation between back-arc state of strain and the maximum seis-182
mic moment release, meaning that strong earthquakes are characteristic for183
subduction zones with a compressive back-arc. This correlation, in combina-184
tion with the relation between back-arc deformation and the nature of the185
plate contact, suggests that the nature of the plate contact and the maxi-186
mum seismic moment are linked (see Fig. 1 b). In order to assess the validity187
of this relation, we analyze 7 subduction zones for which the channel veloc-188
ity anomaly is given by Abers (2005). We use the maximum seismic moment189
magnitude for each of these subduction zones between 1904 and 2007 taken190
from the USGS catalog (http://earthquake.usgs.gov/eqcenter). The resulting191
data points are shown in Figure 4a together with the best fitting line. The192
correlation coefficient is R = 0.92. Repeating the same procedure as before we193
compute the cumulative probability distribution of the correlation coefficient194
from 1000 noisy realizations. We find that 80 % of the 1000 correlation coeffi-195
cients have a value higher than R = 0.82 (Fig. 4b), indicating that the velocity196
anomaly is robustly correlated with the greatest moment release (see Fig. 4a).197
Note that these correlations display a basic trend that expose a relationship198
between two parameters. We do not expect perfect correlation because of the199
complexity of the subduction process and the small amount of data.200
In summary, compression in the back-arc region and strong earthquakes201
prevail for a high viscosity subduction channel, or for a very thin channel202
(represented by a fault) in which the body wave velocity anomaly is small.203
Lower compression or eventually tension and weak earthquakes prevail for a204
weak or wide channel with relative weak material fill, where the body wave205
9
velocity anomaly is more pronounced.206
3 A new subduction zone classification207
In a global view of subduction zones, another key discriminating physical fea-208
ture is the accretionary or erosive nature of the margin. Accretionary and209
erosive margins have characteristic shallow features which we aim to link with210
the deeper part of the plate contact. Cloos & Shreve (1996) suggested that211
in accretionary margins, characterized at shallow depth by thick trench fills,212
tall sea mounts are subducted entirely. In erosive margins, characterized at213
shallow depth by thin trench fills, only truncated sea-mounts are subducted.214
Our numerical models (De Franco et al., 2008) also shed light on the behav-215
ior of sea mounts upon subduction. A topographic feature (in the numerical216
model it is represented as a continental fragment) approaching the trench is217
entirely subducted in the presence of a wide and weak subduction channel218
(see Fig 5b and 5 d). In a fault type model, subduction of a steep topographic219
feature does not occur without previous delamination of the upper part of220
the incoming fragment (see Fig 5c and 5e and De Franco et al. (2008)). Our221
models confirm the idea of Cloos and Shreve (1996) if we equate the shallow222
subduction channel in the model to an accretionary margin, and if we inter-223
pret the shallow part of the model subduction fault as an erosive margin.224
According to Cloos & Shreve (1996), as we move deeper down along the225
plate contact, the shear zone becomes thinner in accretionary margins, while226
in erosive margins the shear zone thickens. Using the correlation between227
the back-arc state of stress and the nature of the plate contact found in the228
previous section, we infer that the back-arc state of stress is compressive in229
10
accretionary margins and tensional in erosive margins.230
To investigate the validity of such an interpretation we link the type of con-231
vergent margins and the back-arc state of strain of several subduction zones232
using the data of Clift & Vannucchi (2004) and Lallemand et al. (2005) (see233
Fig. 6a). We identify four different classes, in which the back-arc region is234
either in extension (T) or in compression (C), in combination with the margin235
being either accretionary (A) or erosive (E) (see Fig. 6). As expected, most236
of the accretionary margins are characterized by back-arc compression (class237
1-CA), while the erosive margins mostly have an extensive back-arc state of238
strain (class 2-TE). These two classes directly follow from our numerical re-239
sults and from the previously established correlation between back-arc state240
of strain and nature of the plate contact. We show cartoons of these two types241
of margins in Figure 7 a and Figure 7 b, respectively, in which the entire plate242
contact is shown.243
Not all the accretionary margins show the same behavior and there are244
some evident exceptions (see Fig. 6 a). For instance, Aegean arc, Makran and245
Barbados are characterized by back-arc extension (class 3-TA). Albeit less246
straightforwardly than for the first two classes, the results of the previous247
section and of De Franco et al. (2007, 2008) also account for the behavior248
of subduction zones of class 3-TA. Our results imply that a weak and wide249
subduction channel characterizes the entire plate contact. For the margins of250
class 3-TA, the fact that the sediment delivery rate and the material subduc-251
tion rate are higher than for the other margins (the sediment delivery rate is252
between 131 and 179 km3/yr and the subduction material rate is between 109253
and 150 km3/yr (Clift & Vannucchi, 2004)), explains the presence not only of254
an accretionary wedge at the inlet, but also of a thick sedimentary channel255
that decouples the two plates along the entire plate contact (Figure 7 c).256
11
Class 4-CE represents a more complicated case. In order to offer a possible257
explanation we need to supplement our results with additional information258
from various regional observational studies. The erosive margins of this class,259
contrary to the ones of class 2-TE, are characterized by very strong compres-260
sion (Fig. 6 a). As a consequence we would expect the deeper part of the plate261
contact to be described by a fault or, alternatively, by a channel with a small262
velocity anomaly (equivalent to a high viscosity channel). In the past, this263
kind of erosive margins has been envisaged as strongly coupled subduction264
zones where the erosion is due to high frictional abrasion. For instance, in265
the North Chile subduction zone, characterized by the downgoing plate cov-266
ered with less than 100 m of pelagic sediment, a sediment starved trench, and267
earthquakes of Mw = 8.0 was indicated as a typical example of such highly268
frictional margins. However, recent studies on North Chile have shown that269
some water is incorporated in the sediments at the plate contact (e.g., Sallares270
& Ranero, 2005). Despite sediment starvation, a frontal prism, constructed of271
debris, elevates pore pressure to reduce interplate friction. Therefore, processes272
other than high frictional abrasion are required to explain subduction erosion273
along northern Chile (e.g. hydrofracturing) and as a consequence a thick plate274
contact is expected. Therefore, the presence of strong back-arc compression275
instead of extension might be due to a change of the physical properties along276
the subduction channel: the material becoming stronger with depth due to de-277
watering, results in a strong coupling between the plates. Based on our model278
results (De Franco et al., 2007, 2008) and the correlation between back-arc279
state of strain and nature of the plate contact, we propose that the North280
Chile-type plate contact can be described by a subduction fault at shallow281
depth, followed deeper down by a subduction channel in which the viscosity282
increases with depth (Fig. 7 d).283
12
We propose another possibility that explains the anomalous behavior of284
class 4-CE. In Peru and North Japan, accretion and erosion are simultane-285
ously occurring (Von Huene & Lallemand, 1990). Accretion at the toe is active286
and erosion consumes the overriding plate generating subsidence further back.287
High friction erosional mechanisms generally proposed for these margins are288
not sufficient to explain the measured amount of eroded material (Von Huene289
& Lallemand, 1990). As a consequence, frontal erosion -due to the Nazca Ridge290
for Peru and the Daiichi Kashima seamount for Japan- and basal hydrofrac-291
turing are invoked. Subduction of a seamount or of a ridge is indicated as one292
of the reasons that can make an accretionary margin erosive. Since accretion293
is still active, part of the incoming sediments are used to create the accre-294
tionary wedge, decreasing the already low percentage of subducted sediments.295
We may speculate that in these cases the plate contact is characterized by296
a relatively wide inlet (accretionary margin) in which topographic features297
are subducted, followed at shallow depth by a thin plate contact where the298
coupling between the plates results in back-arc compression; deeper down, the299
plate contact enlarges again as a consequence of the sediment produced by300
basal erosion. Based on our numerical results (De Franco et al., 2007, 2008),301
we suggest that the Peru type of margin can best be described as changing302
with increasing depth from a subduction channel to a fault (that couples the303
plates), back to a subduction channel (Fig. 7 e).304
From the previous section, we know that strong earthquakes correlate with305
a thin or wide-and-strong plate contact and back-arc compression, whereas306
small earthquakes correlate with a wide-and-weak plate contact and back-arc307
extension. Therefore we expect to have strong earthquakes in classes 1-CA308
and 4-CE, and small earthquakes in classes 2-TE and 3-TA. In order to test309
this theory, we compile a list of the greatest seismic moment magnitudes for310
13
the considered subduction zones (Fig. 6 b). The results substantiate our pre-311
dictions: class 1-CA has 9.0 ≤ M ≤ 9.5 with the only exception of Japan ,312
class 2-TE has 7.2 ≤ M ≤ 8.1, class 3-TA has 7.1 ≤ M ≤ 8.0 and class 4-CE313
has 8.4 ≤ M ≤ 8.5.314
According to Cloos & Shreve (1996), class 1-CA is characterized by strong315
earthquakes, since the subducted seamounts become seismogenic asperities316
when they enter in contact with the upper plate. In class 2-TE the subducted317
truncated sea-mounts generate small earthquakes, since they do not touch the318
roof of the wide channel (see Fig. 7 a and b).319
In the North Chilean subduction zone (class 4-CE), Sallares & Ranero (2005)320
propose the release of elastic energy stored in the high viscosity channel as321
an explanation for the strong earthquakes. In the Peru/Japan type margin322
(class 4-CE), the strong earthquakes are possibly caused by the nucleation323
of seamounts where the plate contact becomes thinner, in a similar fashion324
than in the accretionary margins. This can even happen at very shallow depth325
resulting in shallow earthquakes that cause tsunamis. Without excluding the326
influence of large scale processes, in this study we highlight the importance327
of local features (rheological properties of the plate contact, geometry of the328
plate contact) in controlling the dynamic evolution of subduction zones.329
4 Conclusions330
We highlight the nature of the plate contact as an important physical feature331
of subduction zones by establishing correlations between the back-arc state of332
stress/strain, slab dip angle, and the maximum seismic moment. We identify333
four subduction zone classes. The first two classes (1-CA and 2-TE) directly334
14
follow from our numerical results and the established correlations in combi-335
nation with the observed nature of the convergent margin (accretionary or336
erosive). Although less straightforwardly than for the first two classes, our337
results also account for the behavior of subduction zones of class 3-TA. Class338
4-CE represents a more complicated case. The main characteristics of each339
class are listed below (denoting back-arc deformation as extensional (T) or340
compressional (C), and the nature of the margin as accretionary (A) or ero-341
sive (E)):342
Class 1-CA: The entire plate contact is well described by a wide channel at343
shallow depth and a fault at greater depth. This class contains accretionary344
margins mostly in back-arc compression with very strong earthquakes.345
Class 2-TE: The entire plate contact is a combination of a fault (at shallow346
depth) and a subduction channel (at deeper depth). This class contains erosive347
margins generally characterized by back-arc extension and small earthquakes.348
Class 3-TA: The entire plate contact is represented by a weak subduction349
channel. This class contains accretionary margins that exhibit back-arc ex-350
tension and small earthquakes. These margins are characterized by a high351
sediment subduction rate.352
Class 4-CE: We propose two explanations for the subduction zones in this353
class. One possibility is that the eroded material at the plate contact becomes354
stronger with depth, due to dewatering. In this case the plate contact is repre-355
sented by a fault at shallow depth followed by a subduction channel character-356
ized by an increasing viscosity with depth. Alternatively, in case accretion is357
active together with erosion (e.g., Peru and North Japan), we suggest that the358
plate contact can best be described as changing with increasing depth from a359
subduction channel to a fault (that couples the plates), back to a subduction360
15
channel. This class contains erosive margins that show back-arc compression361
and strong earthquakes.362
Acknowledgments363
Partial support for RDF and computational resources for this work were pro-364
vided by the Netherlands Research Center for Integrated Solid Earth Science365
(ISES). We thank an anonymous reviewer and Serge Lallemand for construc-366
tive comments on an earlier version of this manuscript.367
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Figure Captions489
Fig. 1 a) Schematic representation of the relations between back-arc strain,490
slab dip angle and nature of the plate contact. b) Schematic representation491
of the relations between back-arc state of strain, earthquake magnitude and492
nature of the plate contact.493
Fig. 2 a) Best fitting line in the least square sense between the back-arc state494
of strain (Lallemand et al.,2005) and the relative velocity variation in the plate495
contact zone (dlnVp, Abers, 2005). b) Cumulative probability distribution of496
R computed from 1000 noisy data points, showing that 83 % of the 1000497
correlation coefficients have a value less than or equal to -0.8.498
Fig. 3 Horizontal stress at the free surface of five subduction models. Com-499
pression is negative. The solid black line represents frictionless fault model,500
the grey line a fault model with friction, the blue line a high viscosity channel501
model, the red line low viscosity channel model and the yellow line a low vis-502
cosity channel with a greater thickness (see text for a detailed explanation).503
Bottom: subduction geometry used in the numerical experiments and velocity504
field from a flow model corresponding to the frictionless fault model (corre-505
sponding to the black line in the top panel). The green region represents a506
low viscosity wedge. The plate contact reaches 100 km depth for both channel507
and fault models (red line). This is in agreement with Kneller et al. (2005).508
On the basis of heat flow data, seismic attenuation, and velocity tomography,509
they conclude that the plate boundary zone needs to extend to at least 70 km510
depth.511
Fig. 4 a) Best fitting line in the least square sense between the greatest seis-512
mic moment (M) and the relative velocity variation within the plate contact513
21
(dlnVp, Abers, 2005). The dlnVp value of Hok-S.Kur-N.Hon is an average be-514
tween the value given for Hokkaido-S. Kurile and N. Honshu. b) Cumulative515
probability distribution of R computed from 1000 noisy data points, showing516
that 80 % of the 1000 correlation coefficients have a value greater than or517
equal to 0.82.518
Fig. 5 a) General representation of a topographic feature approaching a sub-519
duction zone. The dashed rectangle is shown enlarged in b and c. b) Schematic520
representation of an approaching topographic feature in a subduction channel-521
type model: subduction of the entire continental crust takes place. c) Ap-522
proaching topographic feature in a subduction fault-type: truncation of part523
of the continental crust takes place. d) Effective strain rate distribution of a524
continental margin arriving at the trench in a subduction channel-type model525
(corresponding to cartoon b). e) Effective strain rate distribution of a conti-526
nental margin arriving at the trench in a subduction fault-type (corresponding527
to cartoon c). The black dashed line show the initial position of the slab. The528
grey region represents the LVW, in which no strain rate is shown.529
Fig. 6 a) Level of back-arc strain for different types of convergent margins.530
On the horizontal axis C means compression, 0 means neutral E means exten-531
sion. The scale from strong back-arc compression to strong back-arc spreading532
is according to Lallemand et al. (2005) C3, C2, C1, 0, E1, E2, E3, with the533
exception of the Aegean arc and Makran taken from Jarrard (1986) and Mc-534
Call (1997). The vertical axis classifies the margins in accretionary and erosive535
and does not include any scale. The letters in the class nomenclature mean:536
C=Compression, T=(ex)Tension, A=Accretion, E=Erosion. b) A list of sub-537
duction zones with their maximum seismic moment magnitude.538
22
Fig. 7 Summary of our subduction zone classification (subducting oceanic539
lithosphere on the lower left, and continental margin on the upper right).540
a) Accretionary margin with back-arc compression. b) Erosive margin with541
a back-arc in extension. c) Accretionary margin with back-arc extension. d)542
Erosive margin with back-arc compression and increasing viscosity in the chan-543
nel. e) Erosive margin with back-arc compression and active accretion process.544
Note that the scale of the deeper part of the model differs from the upper part.545
23
Fig. 1.
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Fig. 2.
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Fig. 3.
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Fig. 4.
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Fig. 5.
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Fig. 6.
29
Fig. 7.
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