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1
Flat vs. Normal Subduction Zones: A Comparison Based on 3D 1
Regional Traveltime Tomography & Petrological Modeling of 2
Central Chile & Western Argentina (29°-35°S) 3
4
M. Marot(1), T. Monfret(1), M. Gerbault(1), G. Nolet(1), G. Ranalli(2), M. Pardo(3) 5
(1) Géoazur, UNSA, IRD, CNRS, OCA, (UMR 7329), 250 Albert Einstein, 06560 Valbonne, France 6 (2) Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON, K1S 5B, Canada 7 (3) Departamento de Geofísica, Universidad de Chile, Blanco Encalada 2002, Santiago, Chile 8
9
Abstract 10
Beneath central Chile and western Argentina, the subducting oceanic Nazca lithosphere 11
drastically changes geometry from dipping at an angle of 27-35° to horizontal, along the inferred 12
subduction path of the Juan Fernandez seamount Ridge (JFR). The aim of our study is to assess the 13
differences in the seismic properties of the overriding lithosphere in these two regions, in order to 14
better understand its deep structure and the links between its surface deformations and the geometry of 15
the slab. In comparison with previous studies, we show the most complete 3D regional seismic 16
tomography images for this region, whereby we use (1) a larger seismic dataset compiled from several 17
short-term seismic catalogs, (2) a denser seismic array enabling us to better resolve the subduction 18
zone from the trench to the backarc and into the upper ~ 30 km of the slab, and (3) a starting 1D 19
background velocity model specifically calculated for this region and refined over the years. We assess 20
and discuss our seismic tomography results with: (i) supporting novel regional seismic attenuation 21
models, and (ii) predicted rock types calculated using the Hacker and Abers (2004) mineral/rock 22
database, and based on a computed thermo-mechanical model of the pressure and temperature 23
conditions at depth. Our results show significant seismic differences between the flat and normal 24
subduction zones: As expected, the lower geotherm of the flat slab region is reflected by faster seismic 25
velocities and increased seismic activity within the slab and overriding lithosphere. We find evidence 26
that the JFR slab dehydrates at the eastern tip of the flat slab segment, indicating that the flat slab 27
segment retains some fluids. The forearc region above the flat slab area is subject to high Vs and very 28
low Vp/Vs ratios (1.69) at two different depth intervals, uncorrelated with typical rock compositions, 29
increased density or reduced temperature; however, possibly linked with the aftershock effects of the 30
Mw 7.1 1997 Punitaqui earthquake, the slab geometry at depth, and/or seismic anisotropy. At the 31
surface, the Andean crust is strongly reduced in seismic velocities along the La Ramada-Aconcagua 32
deformation belt, suggesting a structural damage impact, and the seismic variations correlate with the 33
geological terranes. The Andean lower crust is also impacted by strongly reduced seismic velocities 34
matching hydrous rocks of either mafic and non-eclogitized composition, indicating a possible past 35
delamination event, or a felsic composition, in support for the existence of the Chilenia terrane at these 36
depths. We confirm previous studies that suggest the Cuyania terrane, in the backarc region, is mafic 37
and contains an eclogitized lower crust below 50 km depth. We also suspect major Andean basement 38
detachment faults (or shear zones) to extend towards the plate interface and channelize slab-derived 39
fluids into the continental crust. 40
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Keywords: central Chile; flat subduction; seismic tomography; rock composition; thermo-mechanical 43
modeling; eclogite crust; dehydration/hydration 44
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1. Introduction 45
Along the South American-Nazca convergence zone, spatial correlations exist between 46
subducting oceanic features (ridges, plateaus, fracture zones), volcanic arc gaps, backarc basement 47
uplift, and past or present occurrences of flat subduction (Skinner and Clayton, 2013). The two most 48
famous present-day flat slabs on Earth are the Peruvian and central Chilean flat slabs, both impacting 49
the Nazca Plate along the South American margin. Nevertheless, the cause for their formation is still 50
subject of debate. 51
The central Chilean “Pampean” flat slab (~ 29°-32.5°S) is perhaps the best documented so far, 52
since the region is advantaged by a high micro-seismic activity, good geological exposure levels, 53
advanced structural evolution, and has been impacting the region for a relatively long time (15-18 Ma; 54
Kay and Mpodozis, 2002; Ramos et al. 2002). It is associated with the subduction of relatively young 55
(35-40 Ma) and hydrated lithosphere (Kopp et al., 2004) and of the Juan Fernandez hotspot seamount 56
Ridge (JFR) beneath the thick continental South American lithosphere. Both the region’s high 57
seismicity and the slab’s rapid geometrical transition (from plunging 30° to 0°), render this location 58
ideal to study the seismic properties and anomalies associated to this flat subduction. 59
In normal subduction circumstances, conventional slab dehydration processes take place due to 60
high geothermal gradients at depth, resulting in arc volcanism, seismicity, and weakening of the 61
continental crust. However, in the case of flat subduction, the asthenosphere is expelled during slab 62
flattening, significantly cooling the subduction system (Kay and Mpodozis, 2002; Ramos et al., 2002; 63
Grevemeyer et al., 2003). In the case of the central Chilean flat slab, it is associated with increases in: 64
(i) plate coupling forces, (ii) compressional stresses farther inland, (iii) topography, (iv) seismicity in 65
the backarc, forearc, plate interface and slab (including a Double Seismic Zone, Marot et al. 2013), 66
and (v) cessation of arc volcanism (Kay and Abbruzzi, 1996; Kay and Mpodozis, 2002; Ramos et al., 67
2002). 68
While slab properties are commonly considered the dominant parameters influencing changes in 69
slab geometry, there is a growing consensus that the upper plate properties also play a non-negligible 70
role in provoking flat subductions (e.g. van Hunen, 2001; 2004; Gerbault et al., 2009; Manea et al., 71
2012). 72
Whereas the processes and timing of slab shallowing are well constrained from the collection of 73
evidences of backarc basement uplifts and eastward volcanic arc migration (Ramos et al., 2002; 74
Alvarado et al., 2007), the factors that triggered flat subduction and maintained it for at least 6 Myr 75
(Kay et al., 1991; Kay and Abbruzzi, 1996), as well as its relationship with the deep composition and 76
deformation of the overriding lithosphere, are ill-constrained and multiple paradoxes exist, such as the 77
obvious influence of the JFR, however, deemed too small to have created by itself the flat slab’s 78
imposing dimension (~ 200-250 km long). 79
For this reason, our study images the deep seismic structure of the continental lithosphere in 80
order to better understand its interactions with the flat slab, its impacts, and what causes and maintains 81
the flat slab. To do so, we performed 3D regional seismic tomography of P- and S-wave residual 82
travel-times using local earthquakes recorded by four temporary seismic campaigns (OVA99, 83
CHARGE, CHARSME, CHASE) to compare the normal (27-30°) subduction region south of 33.5°S 84
(taken to represent conventional subduction conditions) with the flat subduction zone, between 31°S 85
and 32°S. We then compared our absolute seismic velocities with those predicted for subduction-type 86
rocks, using the Hacker and Abers (2004) worksheet, at appropriate pressure and temperature (P-T) 87
conditions obtained from thermo-mechanical modeling. We also analyzed our results with those of 88
Wagner et al. (2005), who performed a similar local experiment for this region, using only the 89
CHARGE database. As an improvement to their work, our study incorporates a much larger 90
earthquake catalog (3770 events) representing different time periods and resulting in greater ray 91
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coverage, increased resolution, and a more complete temporal view of the seismic properties of the 92
lithospheres. Furthermore, our results include the continental crust and the upper portion of the slab 93
lithosphere, which could not be interpreted in their models due to lack of data and poor resolution. We 94
also supplemented unpublished seismic attenuation models for the region, calculated by Deshayes 95
(2008), using the OVA99 and CHARSME catalogs, to reinforce our interpretations of the nature of 96
our observed velocity perturbations. Our work offers a finer-scale, broader and more complete image 97
of the region’s subduction system. 98
We report seismic differences between the normal and flat slab region, which confirm previous 99
interpretations of (1) a generally cold, dry, and Mg-rich continental mantle, and (2) an eclogitized 100
lower crust in the Cuyania terrane. However, our results also advocate localized mantle hydration, 101
reflecting flat slab dehydration processes, as well as the absence of an eclogitized Andean arc crustal 102
root. The flat slab segment exhibits fast seismic properties which we can only explain with dense 103
eclogite or peridotite for the crust and mantle, respectively, contradicting the flat slab buoyancy 104
effects. We also notice a correlation between geological terrane boundaries at the surface and seismic 105
velocity variations. Regions of localized slow velocity and high-Vp/Vs anomalies are consistent with 106
major basement shear zone domains, previously interpreted to reach the slab interface. Finally, the 107
forearc and Punitaqui aftershock regions, located above the flat slab, show strong increases in Vs and 108
very low Vp/Vs ratios, which we interpret as possibly related to one another,. 109
110
2. Tectonic and Geological Settings 111
The Nazca Plate subducts beneath central Chile and western Argentina (29°-35°S) at a current 112
convergence rate of 6.7 ± 0.2 cm/a in the N78°E direction (Fig. 1) (Kendrick et al., 2003). The region 113
between 30°S and the subducting JFR path (~ 32.5°S at the trench) is defined as the flat slab region 114
(Fig. 1C). The flat slab segment occurs at 100-120 km depth and underplates the continental 115
lithosphere for 200-300 km eastwards, before resuming its descent at 68°W, with a 30°-dip angle (Fig. 116
1D) (e.g. Barazangi and Isacks, 1976; Cahill and Isacks, 1992; Anderson et al., 2007). It is bounded to 117
the north by a gentle (27.5°-29.5°S) along-strike transition back towards a normal dip of 30°, whereas 118
to the south, the transition is more abrupt (32.5°-33.5°S) and occurs over only ~ 100 km width (see 119
slab isocontours in Fig. 1A) (Cahill and Isacks, 1992; Araujo and Suarez, 1994; Giambiagi and 120
Ramos, 2002). The nature of the slab deformation in the southern transition zone is unclear, however, 121
is more often interpreted as a sharp bend rather than a tear (Wagner et al., 2005; Pesicek et al., 2012). 122
About 25 Ma, the Farallon Plate broke-up into the smaller Cocos and Nazca Plates, and around 123
18-15 Ma, the Nazca Plate began shallowing in central Chile from ~ 45° to 30° dip angle (Ramos et 124
al., 2002). Around 12-10 Ma, the southward migrating JFR incepted the central Chilean trench, 125
coinciding with increased slab shallowing and the cessation of arc volcanism 9 Ma (Kay and 126
Mpodozis, 2002). The JFR is currently subducting along the trench at 33°S (Fig. 1A) since 11 Ma 127
(Yañez et al., 2002), and the present flat slab geometry (Fig. 1C) is believed to have been achieved 128
since 6 Ma (Kay et al., 1991; Kay and Abbruzzi, 1996). 129
Since the onset of slab shallowing below the South American plate, eastward volcanic arc 130
migration and increasing compressional stresses have been linked with strong tectonic uplift and 131
shortening ( up to ~ 130 to 150 km, in comparison to less than < 100 km further south, Allmendinger 132
et al., 1997; Kley and Monaldi, 1998; Alvarado et al., 2007). The overriding plate is thus now 133
described by five different morpho-tectono-structural-geological provinces (from west to east) (Fig. 134
1B): the Coastal Cordillera (forearc), Principal and Frontal Cordilleras (present-day active Andean 135
belt), the Precordillera (foothill) and the Sierra Pampeanas (backarc). 136
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Significant changes occur between the flat and normal subduction zones, in association with the 137
change in slab geometry. To mention only a few changes with respect to the flat slab region (Fig. 1): 138
(i) the strike of the trench rotates from N20°E to N5°E, (ii) the geothermal gradient increases 139
(Miranda, 2001), (iii) the slab becomes younger (Yáñez and Cembrano, 2000), (iv) the arc magmatism 140
resumes, (v) the Central Depression valley re-appears, (vi) the backarc basement no longer uplifts, and 141
the Precordillera and Sierra Pampeanas disappear (Ramos, 2009), (vii) the topography decreases from 142
an average elevation of 4500 m (max. elevation is 6700 m, Mount Aconcagua, at ~ 70°W/32.5°S) to < 143
2000 m at 38°S, (viii) the crustal thickness decreases from 70 to 35 km (Tassara et al., 2006; Alvarado 144
et al., 2007), (ix) the total lithospheric thickness also decreases from 80-100 km (Tassara et al., 2006) 145
to 60 km south of 36°S (compared to > 140 km in Peru and > 160 km in the eastern Brazilian shield, 146
the thickest in South America), and finally, (x) the crustal seismic rates decrease in the slab and 147
backarc regions, however increase along the active volcanic arc. 148
Three major episodes of terrane accretion have occurred against the western proto-Gondwana 149
margin (Rio de la Plata craton) over the past 600 Ma, and have strongly influenced the structure and 150
tectonic evolution of central Chile (Ramos et al., 1986; Ramos et al., 2002; Alvarado and Ramos, 151
2011). These terranes have various provenances, compositions and ages, and comprise of (Fig. 1B): (i) 152
the para-autochthonous Pampia terrane, amalgamated at ~ 530-515 Ma, (ii) the allochthonous Cuyania 153
terrane, accreted at ~ 460 Ma, and (iii) the allochthonous Chilenia terrane (its existence is still 154
debated), added at ~ 420-315 Ma. The Chilenia terrane is believed to compose the Andean basement 155
and part of the forearc (Ramos, 2004). At the surface, suture zones, characterized by narrow ophiolitic 156
belts now representing major shear zones, separate each terrane and control the neo-tectonic style of 157
deformation of the region (Ramos et al., 2002) by influencing the thick-skinned basement uplift of the 158
Sierra Pampeanas region up to ~ 800 km away from the trench (Fig. 1B) (Ramos et al., 2002; 159
Alvarado et al., 2009). However, these terrane boundaries have never been mapped at depth, and the 160
composition of the Andean basement remains enigmatic (Chilenia vs. Cuyania). Such consequent 161
backarc basement uplift over a distance of several hundreds of kilometers, is equally observed above 162
the modern Peruvian flat slab (James and Snoke, 1994) and within the Laramide Orogeny in western 163
USA (DeCelles, 2004), where an ancient flat slab episode of the Farallon Plate is believed to have 164
shaped this uplifted region. Hence, the central Chilean flat subduction is considered and studied as a 165
modern analog of the Laramide Orogeny. 166
167
3. Seismic Data 168
3.1 Seismic Catalog 169
Our seismic tomography inversion is based on passive local earthquakes recorded by four short-170
term seismic campaigns (Fig. 1A): 171
OVA99 campaign was deployed in the region of Ovalle (30°-32°S, 72°-70°W) during the period of 172
mid-November 1999 to mid-January 2000. Thirty-seven short-period three-component receivers, 173
about 30 km apart, recorded continuously the local micro-seismicity (0.5 < Ml < 5.5) with a 174
sampling rate of 125 pts/s. 175
CHARGE (Chile-Argentina Geophysical Experiment, an American project of the University of 176
Arizona; Fromm et al. 2004; Wagner et al. 2005) continuously recorded seismicity from December 177
2000 to May 2002, and consisted of 22 broadband seismometers (10 STS-2, 10 Guralp CMG-178
3ESPs and 2 Guralp-40T) deployed mainly along two profiles at 30°S and 36 °S, and some in 179
between. Sampling rate was 40 pts/s. 180
CHARSME (CHile ARgentina Seismological Measurement Experiment) was carried out from mid-181
November 2002 to March to analyze the regional seismo-tectonic features and map the Nazca slab 182
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geometry. It comprised 29 portable broadband three-component stations (27 CMG-40T and 2 183
CMG-3T Guralp) that continuously recorded seismicity with a sampling rate of 125 pts/s. 184
CHASE (CHile-Argentina Seismic Experiment) recorded the seismicity from mid-November 2005 185
to March 2006 included, and was composed of 14 broadband and 12 short-period seismometers. It 186
focused around the area of Santiago city (33.5°-34°S and 70°-71°S) with the main goal of locating 187
shallow seismicity along fault systems and quantifying the seismic hazard of the capital region, and 188
slab events were also recorded. 189
Events recorded by 15 permanent seismic stations from the Chilean Seismological Service of the 190
University of Chile were added, to increase receptor coverage and improve hypocenter 191
determination near coastal areas. 192
OVA99, CHARSME, and CHASE seismic networks were installed and maintained by a 193
collaborative group of experts from the GéoAzur laboratory in France, the IRD, and the Geophysical 194
Department of the University of Chile, in Santiago. 195
196
3.2 1D Background Model and Event Selection 197
All events were consistently located, using the Hypoinverse program (Klein, 2000) within a 198
‘minimum’ average 1D velocity model of least-square fit that best describes the region, constructed 199
using passive and active sources from the region, and the Velest program (Kissling et al., 1994) was 200
used for depths greater than 20 km. The chosen average velocity model represents a 17 layer model 201
with average Vp/Vs ratio of 1.76, extrapolated to 215 km depth for the inversion (Fig. 2). It is used by 202
the Chilean Seismological Service of the University of Chile. In comparison with the IASP-91 global 203
average velocity model, which is used in Wagner et al. (2005)’s seismic tomography study, our model 204
produces generally faster seismic velocities and a deeper average continental Moho, which values we 205
will detail below. 206
To ensure stability and reliability of our tomography results, we selected only the highest 207
quality events from our total event catalog, with the following selection criteria for P- and S-waves, 208
respectively: (a) maximum error of manual picking of ± 0.25 s and ± 0.4 s, (b) maximum pick quality 209
index of 2 and 3 (0: excellent, 4: discarded), (c) maximum hypocenter uncertainty of 5 km in all 210
directions, (d) maximum RMS misfit of < 0.6 s (Ri2/N, where Ri is the time residual at the ith 211
station), and (e) minimum 8 and 4 station observations. Finally, we retain a total of 3770 events for the 212
inversion process, including 55128 and 54889 P- and S-wave arrival times, respectively. 213
214
4. Seismic Tomography: Method 215
4.1 3D Seismic Tomography Inversion 216
We apply the tomography code TLR3 (Latorre et al., 2004; Monteillet et al., 2005) to obtain 3D 217
P- and S-wave velocity models. Source-receiver ray tracing is calculated using the finite-difference 218
algorithm developed by Podvin and Lecomte (1991), within a fine grid model of mesh size 2x2x2 km 219
to enable ray-path smoothness. Our inversion model is discretized into a coarser regular grid of mesh 220
size 40x40x10 km, representing dimensions of 960x840 x220 km, in which body-wave traveltimes are 221
calculated, for computational efficiency purposes. The least-square 3D velocity structure is 222
progressively retrieved through a set of iterations that solve for small velocity perturbations, in order 223
to linearize the problem and reduce computational time constraints caused by the large dataset. 224
Hypocenters are simultaneously relocated at each step. The starting iteration involves the initial 1D 225
velocity model (Fig. 2) and its relocated seismicity. Values for damping and weight ratio (Cp/Cs) of P- 226
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and S-wave traveltimes were adjusted separately by fixing one parameter at a time and allowing the 227
other to vary, in order to obtain those that minimize the RMS misfit solution. The parameters that led 228
to best results were a damping value of 0.7 and a Cp/Cs value of 0.5. Mesh spacing and final velocity 229
model robustness were examined using the checkerboard and spike tests. The well resolved regions of 230
the model have a spatial resolution representative of the mesh spacing used, and represent areas where 231
the seismicity is densest. 232
4.2 Final Solution Model Quality Assessment 233
A posteriori, we assessed the resolution quality of our final model using the ray density 234
distribution and the harmonic checkerboard (Fig. 3) and spike sensitivity tests (Fig. 4). The latter two 235
examine the possible artifacts introduced in the model space during the inversion process, by assessing 236
the restoration of amplitude, shape and extent of synthetic velocity perturbations. Our checkerboard 237
tests considered three different mesh spacing, 80x80x20, 40x40x10 and 30x30x10 km, with the latter 238
two offering the best resolution solutions. We finally retained a mesh spacing of 40x40x10 km, 239
because 80x80x20 km and 30x30x10 km offer too coarse or too detailed and complex resolutions, 240
respectively. We then constructed a synthetic spike test model that reflects the main seismic 241
characteristics (perturbation sign, location and geometry) obtained in our final velocity model. As 242
shown by the checkerboard and spike tests, the regions closest to the subduction interface, where ray 243
densities are highest, are best resolved. The overriding lithosphere is well resolved above the flat and 244
normal slabs, with a good recovery of both amplitudes and geometry of the synthetic input model. 245
Also, the upper part of the oceanic lithosphere, assumed to be represented by the thick seismogenic 246
zone (~ 20-30 km inside the slab) is well recovered throughout the region. On the other hand, the 247
backarc region over the flat slab (the Cuyania terrane) is less well resolved, with some horizontal 248
smearing and diminished amplitudes, and above the normal slab, it lacks ray paths and is even more 249
poorly resolved. 250
251
5. Seismic Tomography: Results 252
5.1 The Forearc Crust 253
Between 5 and 25 km depth, the forearc crust (Coastal Cordillera) is described throughout the 254
region with fast seismic velocities and low Vp/Vs ratios, limited to the east by the western Andean La 255
Ramada-Aconcagua fold-and-thrust belt, forming the boundary between the Coastal and Principal 256
Cordilleras (Fig. 5A). Between 0 and 10 km depth and at latitudes < 32°S (in the flat slab region), Vs 257
is highest (3.6-3.7 km/s) and results in very low Vp/Vs ratios of 1.69-1.73, peaking in the Punitaqui 258
region (Fig. 5A and 6B). South of 32°S (above the subducting JFR and in the normal slab region), Vs 259
is lower (3.4-3.5 km/s) and Vp/Vs are higher (1.74-1.76). Vp is constant throughout the region (6.2 260
km/s). Directly below 10 km depth, is a 10 km thin area of reduced Vp and Vs, producing higher 261
Vp/Vs ratios (1.78) (Fig. 6A and B), and connecting the plate interface with the Principal Cordillera. 262
Two other areas of distinct reduced seismic velocities and high Vp/Vs ratios occur near the 263
surface, above the normal slab, with unknown origins: (1) northeast of Valparaiso city (70°W/33°S), 264
down to ~ 25 km depth, and which is associated with a cluster of deep seismicity (Fig. 5A); and (2) 265
directly beneath Rancagua city (~ 70.5°W/34°S), down to only ~ 10 km depth, and which displays a 266
Vp/Vs ratio that is by far the highest in the region (1.86). These high Vp/Vs ratios could suggest 267
important fluid (O’Connell and Budiansky, 1974) or (but less likely) melt (Watanabe, 1993) 268
concentrations in the rock, with a possible thermal influence inherited from the relatively recent 269
Andean magmatism (4-9 Ma, i.e. responsible for the famous El Teniente and Rio Blanco-Los Bronces 270
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porphyry copper deposits) currently active volcanism in the Eastern Principal Cordillera (Farías et al., 271
2010). In both cases, it may be of socials interest to re-evaluate the local seismic risks at these 272
localities, given these new observations. 273
5.2 The Main Andean Crust 274
The entire Andean crust, and particularly the Principal Cordillera, is characterized by 275
significantly reduced seismic velocities and moderately high Vp/Vs ratios (~ 1.78-1.79) (Fig. 5A). 276
Beneath the active and ancient volcanic arcs, down to 25-30 km depth, the Principal Cordillera 277
experiences the slowest seismic velocities together with moderately high Vp/Vs ratios (1.78-1.80) 278
(Fig. 5B). The orientation (NNW-trending) and dimension (~ 100 km wide) of this anomaly mimic 279
those of the La Ramada-Aconcagua deformation belt. Beneath and ~ 50 km west of the active volcanic 280
arc (i.e. in the Principal Cordillera and > 33°S), seismic velocities are more reduced and associated 281
with intense and deep seismicity (Fig. 5A and B), related to the arc volcanism. Alternatively, the 282
Frontal Cordillera shows less reduced seismic velocities, with the exception of the region between 283
33°S and 34°S, bounded to the east by the Cuyania-Chilenia suture zone, where Vp and Vs are as 284
reduced as in the Principal Cordillera and the Vp/Vs ratios are locally higher (Fig. 5A), suggesting a 285
similar impact factor, such as structural control, at least within shallow depths. At deeper crustal 286
depths (> 30 km), this slow velocity anomaly, originating at the surface in the Principal Cordillera, 287
propagates downward and eastward, into the Andean crustal root (beneath the Frontal Cordillera) and 288
into the sublithospheric continental mantle to the slab interface beneath Cuyania (68.5°W) (Fig. 6A 289
and B). The (seismic) continental Moho (Fromm et al., 2004) is well defined by the isocontour lines 290
Vp ~ 8.0 km/s and Vs ~ 4.5 km/s (Fig. 6). 291
5.3 The Backarc Crust 292
Our resolution of the backarc region is limited but is best within the flat slab region (< 32.5°S) 293
(Fig. 3 and 4), representing the Cuyania terrane (Precordillera and western Sierra Pampeanas), due to 294
the higher ray coverage produced by the flat slab’s seismicity. The Cuyania lithospheric column, down 295
to 75-80 km depth, is described by slightly higher Vp and Vs values and lower Vp/Vs ratios (1.75-296
1.76) (Fig. 5, 6A and B) than the environing lithosphere. Inconsistently, previous studies, based on 1D 297
velocity forward modeling, characterized it rather with high Vp/Vs values (> 1.80) (Gilbert et al., 298
2006; Alvarado et al., 2007). Finally, as mentioned in Section 4.2, our resolution of the backarc above 299
the normal slab (> 33°S) is inadequate for discussion. 300
5.4 The Continental Mantle 301
Throughout the region, between 50 and 60 km depth, the continental mantle exhibits reduced 302
Vp and Vs with higher Vp/Vs ratios (1.77-1.78) than the surrounding mantle (Fig. 6A-C). Above the 303
flat slab segment (> 70 km depth), the continental mantle is characterized by relatively high Vp (8.0 to 304
8.5 km/s), high Vs (4.5 to 4.8 km/s), and relatively low Vp/Vs ratios (1.75-1.77), with localized 305
patches of higher Vp/Vs values (1.79) (Fig. 6A and B). Beneath the Frontal Cordillera, the mantle is 306
reduced in seismic velocities, a feature which appears to extend deeper eastward to the flat slab’s 307
interface at 68.5°W, directly beneath the Cuyania terrane. This 10-20 km thin area of slow velocities, 308
located beneath the Cuyania terrane, is located above the eastern tip of the flat slab segment, before re-309
subduction occurs, and represents locally increased Vp/Vs ratios (1.78) (Fig. 6D), possibly indicating 310
hydrous mantle conditions. 311
The only marked difference in the seismic velocities of the continental mantle between the flat 312
and normal regions is observed between 35 and 50 km depth. Here, at latitudes < 32°S (in the flat slab 313
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region), Vs increases (4.0-4.4 km/s) substantially compared to above the normal slab (3.9-4.1 km/s), 314
and Vp remains constant (6.9-7.7 km/s) everywhere, resulting in very low Vp/Vs ratios (1.70-1.74) 315
(Fig. 5B), analogous to the shallow forearc directly above it. 316
5.5 The Slab 317
In the flat slab region (< 32.5°S), the slab down to ~ 50 km depth is highly seismogenic, 318
strongly reduced in seismic velocities and contains high Vp/Vs ratios (1.80-1.82), suggesting the 319
presence of fluids (Fig. 6A and B). Between 50 and 75 km depth, both the Vp/Vs ratios and the DSZ’s 320
upper plane seismic rate decrease (Fig. 6A). At 31°S, 71°W and 50-80 km depth, a local anomaly of 321
decreased Vp (8.0-8.1 km/s), strongly increased Vs (4.5-4.7 km/s) and very low Vp/Vs ratios (1.70-322
1.73), defines the 1997 Punitaqui (Mw 7.1) aftershock region (shown by the blue star in Fig. 6B). Its 323
peak low Vp/Vs value is located in the interplanar region of the DSZ. Along the flat slab segment (> 324
100 km depth), the western part of the slab, between 69°W and 70.5°W, is expressed with higher Vp 325
(8.5-8.6 km/s) and Vs (4.8-4.9 km/s) than the slightly slower eastern part (8.4-8.5 and 4.7-4.8 km/s, 326
respectively), a difference which is particularly pronounced along the JFR axis (Fig. 6A and B). 327
Nevertheless, the Vp/Vs ratios remain constant and relatively low (1.75-1.77) throughout the flat slab 328
segment. 329
In comparison, the normal slab (> 33°S) down to 50 km depth is less seismogenic and less 330
pronounced in slow seismic velocities, with slightly lower Vp/Vs ratios (1.76-1.78) (Fig. 6C). 331
Between 50 and 75 km depth, the seismic activity increases and Vp decreases, resulting in even lower 332
Vp/Vs ratios (1.74-1.76). Below 125-150 km depth, the seismic activity becomes rare. 333
334
6. Petrological Analysis: Method 335
6.1 Thermo-Mechanical Estimation of P-T Conditions 336
In order to correlate our obtained seismic velocities with those predicted for rock types likely to 337
constitute the lithospheres, we need to estimate the appropriate pressure (P) and temperature (T) 338
conditions at depth. A conventional approach consists in defining kinematical models of subduction 339
that calculate a geothermal gradient equilibrated with the velocity of subduction (Springer, 1999; 340
Gutscher et al., 2000) and assume a pressure simply equal to the lithostatic pressure. Here instead, we 341
chose to estimate the P-T field along two vertical cross-sections representing the central Chilean flat 342
(31.5°S) and normal (33.5°S) subduction, with a thermo-mechanical method that can account for some 343
tectonic overpressure as well as shear heating along the plate interface. The plane-strain finite 344
differences code Parovoz (Poliakov and Podladchikov, 1992) was used, which is based on the FLAC 345
method (Fast Lagrangian Analysis of Continuum, Cundall and Board, 1988). This code has been used 346
in a variety of geodynamic contexts (e.g. Gerbault et al. 2009, and references therein), and resolves the 347
equations of motion and heat transfer explicitly in a time-marching scheme, with self-consistent 348
elasto-visco-plastic T-dependent rheologies (details in Gerbault et al. 2009). 349
The modeled domain is first sub-divided into several rheological units describing the different 350
portions of the continental and oceanic lithospheres at present-day. The geometries of our two starting 351
modeled profiles are constructed based on our catalog hypocenter distribution at depth, and 352
complemented by seismic reflection and wide-angle data for depths less than 10 km (Flueh et al., 353
1998). The initial geotherms are evaluated according to the method proposed by Burov and Diament 354
(1996) assuming: i) effective thermal ages of the oceanic and continental lithospheres of 35 Ma and 355
200 Ma, respectively (similar to estimates by Tassara et al. 2006), ii) radiogenic heating in the 35 km 356
thick reference continental crust (8.10-10 W/kg with an exponential decay length set to 10 km), and iii) 357
9
best fitting surface heat flow data (Hamza and Muñoz, 1996). The thickness of the lithosphere (LAB) 358
is determined by the 1350°C isotherm, set at 100 km and 150 km depths above the flat and normal 359
slabs, respectively (according to values determined by Tassara et al. 2006). Plate convergence is 360
applied at 7.5 cm/yr, as proposed by Somoza and Ghidella (2005) and O'Neill (2005). Other thermo-361
mechanical parameters can be found in Gerbault et al. (2009), as it is not the aim of the present paper 362
to discuss them. 363
Our aim here is not to reproduce the progressive formation (or not) of a flat subduction, for 364
which mechanical causes are still subject of debate (for a recent discussion, see e.g. Cerpa et al., 365
submitted). Instead, we determine thermo-mechanical parameters for which the present day geometry 366
is stable over several millions of years, a timing a priori consistent with the geological record (e.g. 367
Haschke et al. 2006) and appropriate to generate the observed seismic velocities. Therefore, the 368
computational time is chosen to span ~ 5 Ma, during which the geometrical setup evolves relatively 369
little (yet external changes in kinematical and thermal initial conditions can be considered negligible at 370
this time-scale as well as internal phase changes, e.g. see discussion in Gerbault et al., 2009). As a 371
result, two synthetic P-T models are obtained, for the normal and flat dipping slab sections, shown in 372
Fig. 7. These P-T conditions can now be used as input data for the petrological analysis, with a rather 373
permissive range of uncertainty, as detailed below. 374
6.2 Petrological Modeling 375
For each cell and for several discrete sites in our 2D representative cross-sections of the central 376
Chilean flat (31.5°S) and normal (33.5°S) subduction zones (Fig. 8), we analyzed which rock types 377
matched best our calculated seismic velocities for (i) the forearc crust, (ii) the continental lower crust, 378
(iii) the continental mantle, and (iii) the resolved upper slab regions. We based our analysis on 379
experimentally measured seismic isotropic properties of mafic and ultramafic rock compositions, 380
representative of subduction zones (Hacker et al., 2003). 381
The Hacker and Abers (2004) rock and mineral database provides compositional details for 382
many common subduction rock types, including 25 MORB-type rocks, 19 hydrous peridotites (10 383
harzburgites, 9 lherzolites) and 21 anhydrous peridotites (10 lherzolites, 7 harzburgites, 1 dunite, 384
wherlite, olivine clinopyroxenite, and pyrolite). We tested for all rock types and accounted for a 385
permissive uncertainty range of ± 0.1 km/s in our Vp and Vs, ± 100°C in T and ± 0.5 GPa in P. We did 386
not compare Vp/Vs ratios, since small variations within the uncertainty constraint of either Vp or Vs, 387
or both, resulted in substantial misleading differences in the Vp/Vs values. Hence, we only 388
acknowledged absolute velocities. 389
The aim of our exercise is not to find specific rock compositions that best match our seismic 390
wave field at depth, but instead to observe whether trends exists, such as recurring rock facies (e.g. 391
eclogites vs. blueschists) or in the case of mantle rocks, hydrated vs. non-hydrated, garnet vs. 392
plagioclase peridotites. In order to do so, we calculated the total average volume percentage of water 393
and other minerals (vol%) contained in the rock solutions describing each cell (∑ (𝑉𝑜𝑙%)𝑖𝑁𝑖
𝑁, where i is 394
each rock solution and N is the total number of rock solutions per cell). In such a statistical analysis, 395
the reliability of our results is quantified by the number of rock solutions in each cell, shown in Fig. 396
8B, which in turn is dependent on the total number of rocks present in our database, considered to be 397
fairly high. 398
We have not used more complete or self-consistent mineral databases, such as Perplex 399
(Connolly, 2005) and Theriak (Capitano and Petrakakis, 2010), since these methods generally require 400
that detailed rock and mineral compositions be known (obtained from field sampling and laboratory 401
analyses), which is not our case here. Furthermore, the resolutions associated to our thermo-402
mechanical and tomographic data are too large to justify the use of more complex and precise 403
10
methods, which furthermore, are best applied to crustal domains than to mantle domains. The Hacker 404
and Abers (2004) database, in turn, has been quite well developed for mantle domains, which is our 405
principal interest in this study. 406
Figure 8B shows that the continental and oceanic mantle domains have a large set of rock 407
solutions matching our seismic velocities. Because there is little seismic distinction between 408
lherzolites and harzburgites of the same type (e.g. garnet) at similar P-T conditions, the number of 409
rock solutions for the mantle domain are much high compared to the continent crustal domain, which 410
contains a larger panoply of rock compositions, describing a larger range of seismic velocities for 411
which our velocities could match only fewer. Nevertheless, our results are considered reliable for both 412
domains, since our seismic velocities in each cell generally fitted rocks with similar compositions in 413
major elements (e.g. garnet peridotites) or trends (e.g. hydrous vs. anhydrous), whereby in the latter 414
case we calculated total averages of volume percentages (e.g. average vol% H2O). 415
416
7. Discussion and Interpretation 417
7.1 The Forearc Crust 418
The forearc crust in the flat slab region (< 32°S) displays several unusual seismic characteristics 419
which are difficultly explained, and possibly linked to one another and/or with the flat slab geometry. 420
Here, the elevated Vs and low Vp/Vs ratios, with respect to the normal slab region, suggest the 421
presence of denser and/or colder rocks, with negligible water content. Indeed, a compositional change 422
was proposed by Sallarès and Ranero (2005), who also noted similar results of faster Vs (> 4.0 km/s) 423
north of 33°S than to the south (Vs ~ 3.5 km/s), possibly attributed to the higher degree of plutonism 424
and metamorphism in the forearc basement rocks (Ramos et al., 1986). 425
Comparing our seismic velocities (Vp: 6.2 km/s, Vs: 3.6 km/s) with those predicted for typical 426
mafic rocks at 5 km depth using the Hacker and Abers (2004) database, we find that normal mafic 427
rocks should have much faster seismic velocities at such depths (average ∆Vp: +1.25 km/s, ∆Vs: 428
+0.66 km/s), suggesting either abnormal/deformed mafic rocks or prevailing felsic compositions. We 429
could not test for typical felsic rock compositions using the Hacker and Abers (2004) rock database 430
since these are not included however, we could examine the influence of increasing the silica content 431
on the calculated seismic velocities. We observe that increasing the silica content of a rock decreases 432
Vp significantly more than Vs, rather than increasing the Vs. Furthermore, we calculated seismic 433
velocities for an average granite composition (74.5% SiO2, 14% Al2O3, 9.5% Na2O and K2O, 2% 434
oxides (Fe, Mn, Mg, Ca) at 5 km depth (P: 0.5 GPa, T: 100°C) using PerpleX (Connolly, 2005) and 435
found much slower seismic velocities (Vp: 5.9 km/s, Vs: 2.5 km/s) than our observed values, in 436
particular fir Vs. Since our Vp is constant throughout the region, we consider that our Vs must be 437
abnormally high in the flat slab region, contradicting the effects of increased silica content. 438
If a compositional change poorly justifies the seismic differences between the northern and 439
southern part of the forearc crust, a compositional change between the forearc (Coastal Cordillera) and 440
the Andean crust (Principal Cordillera) seems to more easily explain its eastern boundary at or near the 441
western La Ramada-Aconcagua fold-and-thrust-belt (Principal Cordillera), since such compositional 442
contrasts are often associated with major shear zones (Twiss and Moores, 1992). This correlation was 443
reported in northern Chile, between 15°S and 23°S, where a sharp density contrast exists between the 444
forearc and the Altiplano-Puna crusts (Tassara, 2005), associated with high gravity, high resistivity 445
and low P-wave attenuation values, and interpreted as cold and rigid material acting as an indenter into 446
the weaker Altiplano-Puna lithosphere (Tassara et al., 2006; Gerbault et al., 2009). In comparison with 447
our case study, the forearc crust in the flat slab region also reflects high gravity anomalies (Andrès 448
Tassara, personal communication), but does not promote any seismic attenuation pattern, as shown in 449
11
Fig. 9A (Deshayes, 2008). Nevertheless, a compositional change associated to increases in density 450
should induce simultaneous increases in Vp and Vs, not simply Vs (as it seems to be our case). In 451
addition, a decrease in rock temperature, reflecting the flat slab’s lower geotherm, can also be 452
discarded since it should produce an equal effect as increased density. To resume, normal isotropic 453
mafic rock compositions, higher rock density, and lower rock temperatures, do not convincingly 454
explain the shallow (< 10 km depth) forearc seismic anomaly. 455
Deeper, between 35 and 50 km depth, an identical anomaly of high-Vs and very-low-Vp/Vs also 456
locates in the forearc crust at similar latitudes (< 32°S) as its shallower counterpart, suggesting a 457
relationship between one another. Understanding this deeper forearc anomaly might shed light onto 458
the shallower anomaly. 459
The deeper forearc anomaly lies within the uncertainty range of the poorly defined seismic 460
Moho (Fromm et al., 2004), and hence, it could represent crust or mantle material. Our petrological 461
analysis shows that mafic rocks, rather than mantle rocks, match far better the seismic velocities at this 462
depth range and at appropriate P-T conditions (n°8 in Fig. 10A); whereas, mantle rocks explain better 463
the deeper seismic velocities (n°9 in Fig. 10A), indicating the lower limit of this anomaly probably 464
represents the boundary between the crust and mantle, which is supported by the sudden decrease in 465
the slab’s seismic activity (Fig. 6A and B) and the change to seismic creep at this depth (50 km) 466
reported by Gardi et al. (2006) based on Coulomb stress transfer analyses of major interface events 467
that occurred in the region. However, no rocks match the very low Vp/Vs values of the shallow and 468
deep forearc crust (1.69-1.73). 469
A study by Ramachandran and Hyndman (2012) reported even lower Vp/Vs values (1.65) for 470
the forearc lower crust in the Cascadia subduction zone, located between 25 and 35 km depth, equally 471
associated with elevated Vs (4.1-4.2 km/s), however also by decreased Vp (6.6-6.8 km/s), unlike us. 472
Comparing absolute seismic velocities, our Vs is similar, however our Vp is significantly higher (7.0-473
7.1 km/s). Takei (2002) indicated that adding only 5% of silica to a rock decreases significantly its 474
Vp/Vs ratio, and Ramachandran and Hyndman (2012) explained their seismic anomaly with 20% 475
added silica, derived from consequent amounts of subducted eroded continental material precipitated 476
from slab fluids. Nevertheless, as mentioned earlier, increasing the silica content of a rock decreases 477
Vp substantially more than Vs, contradicting our observations. 478
In addition, a link with the intraslab Punitaqui earthquake aftershock region (1997, Mw 7.1), 479
located at 31°S and 70 km depth (Fig. 6B) cannot be excluded, considering that they all reflect 480
identical seismic behaviors. Another correlation can also be made with the flat slab geometry at depth. 481
To resume, within the flat slab region, two very-low-Vp/Vs anomalies occur inside the forearc 482
crust and another in the slab, for which we can find no reasonable explanation for their seismic 483
behaviors nor a link with the flat slab geometry underneath. Simple mechanisms, such as normal mafic 484
and ultramafic rock compositions, increased silica concentrations, decreased temperature or increased 485
rock density, are discarded. However, one might consider seismic anisotropy effects resulting from 486
convergence deformation, since MacDougall et al. (2012) reported strong seismic anisotropy in the 487
mantle wedge below the forearc and above the flat slab. However, this interpretation is beyond the 488
reach of our data. 489
Finally, we superposed the basement detachment fault geometry of Farías et al. (2010)’s 490
tectonic model for the Andes at 34°S (La Ramada-Aconcagua fold-and-thrust belt) onto our 491
tomography models (Fig. 6) and noticed a good correlation between the fault’s westward extension 492
and the high-Vp/Vs region separating both very-low-Vp/Vs anomalies, located at 30 km depth in the 493
forearc crust (Fig. 6B). Thus, we support Farías et al. (2010)’s proposition that a crustal shear zone 494
crosses the forearc at mid-crustal depths, reaching out for the subduction interface and possibly 495
channelizing slab fluids into the continental crust, where high-Vp/Vs ratios dominate. 496
12
7.2 The Main Andean Crust 497
The NS-trending slow velocity and high-Vp/Vs anomaly affects the upper 30 km of the 498
Principal Cordillera crust and mimics well the location, geometry and dimension of the La Ramada-499
Aconcagua fold-and-thrust belt (Fig. 1B, 6A and B) (~ 100 km wide and ~ 20-30 km deep; Ramos et 500
al., 2002; Giambiagi et al., 2003; Farías et al., 2010). Therefore, this slow anomaly is interpreted as the 501
consequence of structural damage, in addition to the regular presence of meta-sedimentary rocks and 502
fluids (reflected by the higher Vp/Vs ratios). In section 7.1, we interpreted the basement detachment 503
fault of the La Ramada-Aconcagua belt reaching the subduction interface across the forearc’s mid-504
crust. 505
The Frontal Cordillera is less affected by such strong velocity reductions, indicating stronger 506
and less fractured crust (Ramos et al., 2002), in agreement with its lower seismic rate. The only 507
exception lies in the Cordon del Plata, west of Mendoza city (33°-34°S), displaying similar significant 508
decreases in seismic velocities as in the Principal Cordillera, down to 35 km depth (Fig. 5A). Although 509
this region lies outside the limits of the Aconcagua fold-and-thrust belt at this latitude, it is also 510
interpreted to experience structural damage (Alvarado et al., 2009), in addition to being at proximity to 511
the active volcanic arc, thus creating a higher geothermal gradient that could explain slow seismic 512
velocities. 513
At depth, little is known about the composition and origin of the Andean basement rocks, as 514
there is little evidence left for the existence of the Chilenia terrane, obliterated by abundant old 515
intrusions and covered by volcanism and sedimentation. However, the continental affinity of the oldest 516
rocks found in the Frontal Cordillera (Caminos et al., 1982) supports its existence. Also, there is no 517
reconnaissance for the location of the Chilenia-Cuyania suture zone at depth (Ramos et al., 1986), 518
which would answer the questions about the Andean basement’s composition. Our results show that 519
the lower Andean crust is strongly reduced in seismic velocities, with high Vp/Vs ratios (1.77-1.79), 520
relative to the Andean mid- and Cuyania crusts (Fig. 6A and B). Our seismic velocities for the Andean 521
root, between 50 and 60 km depth, can be explained by lawsonite amphibole eclogite (57% H2O), as 522
well as jadeite epidote blueschist (74% H2O) and garnet granulite (0% H2O) assemblages. And 523
between 60 and 70 km depth (within the uncertainty range for the seismic Moho), the seismic 524
velocities match either hydrated eclogites (zoisite and zoisite-amphibole eclogites, 32 and 10% H2O, 525
respectively) or may represent mantle rocks hydrated with an average of 10-30% H2O (n°10, Fig. 526
10A). Therefore, the higher Vp/Vs ratios and the mafic rock solutions for the Andean lower crust 527
suggest it is likely hydrated and principally non-eclogitized (at least until 60 depths), contrary to our 528
expectations, since the crustal root extends well into the eclogite stability field, normally starting 529
around 50 km depth (e.g. Hacker et al., 2003; Hacker and Abers 2004). 530
In general, the absence of an eclogitized crustal root can be explained by the following: (1) the 531
rock composition is too felsic to transform to eclogite. This is highly likely, since felsic rocks have 532
slower seismic velocities than mafic rocks and the Andean crust is suspected to encompass the 533
continental Chilenia terrane; (2) the lower crust temperature is too low for eclogite transformation to 534
occur efficiently, even in the presence of catalyzing fluids (Artemieva and Meissner, 2012); and (3) 535
crustal delamination may have removed the previously dense eclogitized lower crust, due to overcome 536
gravitional instability (DeCelles et al., 2009). Crustal detachment was proposed by Kay and Gordillo 537
(1994) to explain the deep and old crustal signatures in the Pocho lava rocks above the re-subducting 538
slab (65°W, 32°S). 539
Therefore, the Andean slow velocity lower crust can be explained by a felsic or a mafic non-540
eclogitized crustal root, with the likely presence of fluids. In the case of a felsic root, this would attest 541
for the presence of Chilenia and provide insight to the location of the suture zone and distribution of 542
the Chilenia and Cuyania terranes at depth. 543
13
7.3 The Backarc Crust 544
The Grenvillian Cuyania terrane (Precordillera and western Sierra Pampeanas) stands out as a 545
fast seismic velocity anomaly down to 80 km depth, with relatively low Vp/Vs ratios (Fig 6A-B). 546
Consistent with fast seismic velocities, Cuyania is also mapped as a high gravity anomaly, overprinted 547
by a low geothermal gradient, until 37°S (Tassara et al., 2006). These observations reflect its 548
mafic/ultramafic, and cold and old character (Ramos et al., 1986; 2002; Ramos, 2004). However, we 549
can only observe its high Vp until ~ 33°S and high Vs until ~ 32°S, rather than 37°S (Fig. 5A). The 550
peak of intensity is situated more-or-less beneath San Juan city, which is directly above the inferred 551
subducting JFR track. 552
Multiple seismic and gravity studies measured a crustal thickness of 60 km beneath the 553
Precordillera and 50-55 km beneath the western Sierra Pampeanas (Regnier et al., 1994; Fromm et al., 554
2004; Alvarado et al., 2005; 2007; Gilbert et al., 2006, Calkins et al., 2006; Corona, 2007; Alvarado et 555
al., 2009; Castro de Machuca et al., 2012). These all proposed the existence of a dense eclogitized 556
lower crust to explain seismic velocity jumps and their unexpected high gravity values relative to their 557
average low elevations (~ 2 and 1 km, respectively) (Miranda, 2001; Alvarado et al., 2005; 2007; 558
Gilbert et al., 2006). Indeed, our obtained high seismic velocities for the Cuyania lower crust (around 559
68.75°W) confirm the presence of eclogites below 50 km depth, matching amphibole (26% H2O) and 560
zoisite (10% H2O) eclogites (~ 3.50 g/cm3). Between 40 and 50 km depth, lawsonite amphibole 561
eclogite (57% H2O) could occur, amongst jadeite epidote blueschist (74% H2O) and garnet granulite 562
(0% H2O). Since eclogite is not seismically distinguishable from mantle rocks, we are unable to 563
provide information about the maximum depth extent of this eclogite layer. 564
The presence of eclogite supports a mafic composition, indicating that Cuyania (and not 565
Chilenia) forms the backarc basement, constraining further the suture zone location between the 566
terranes. 567
7.4 The Continental Mantle 568
Whether the flat slab retains or releases fluids is key information to better constrain the reasons 569
for its flat geometry and the fate of deformation in the overriding lithosphere, since fluids decrease the 570
bulk rock density, increase buoyancy, and reduce rock strength (e.g. Reynard et al., 2013). 571
Above both the flat and normal slab regions, and between 50 and 60 km depth, our results show 572
that the continental mantle’s seismic velocities have a 70-80% chance of matching serpentinized 573
mantle rocks (i.e. 6 out of 9, and 5 out of 7 rocks, respectively, contain hydrous phases) with an 574
average of 30 % hydration, represented by the minerals amphibole (avg. 15%), talc (avg. < 5%), 575
chlorite and Å-phase (avg. < 10%). This is in agreement with the locally higher Vp/Vs ratios in the 576
mantle at this depth range, and the reduced seismic activity in the Wadati-Benioff zone of the adjacent 577
slab (Fig. 6A-B). The mantle in the flat slab region, down to 80 km depth, exhibits a pronounced low 578
P-wave attenuation anomaly (Fig. 9A-B) (Deshayes, 2008). However, the effects of saturated versus 579
under-saturated fluid conditions on seismic attenuation behaviors are complex and difficult to assess 580
(Toksöz et al., 1979). 581
The very small variations in the seismic velocities of the mantle above both normal and flat 582
slabs (Fig. 10A-B) imply similar degrees of mantle hydration; yet, the mantle above the normal slab 583
strongly attenuates P- and S-waves (Fig. 9C) (Deshayes, 2008) and appears able to produce sufficient 584
partial melts to feed the overlying volcanoes. Hence, we wonder what is the role of temperature in 585
causing both the flat and normal slabs to dehydrate similarly, and its influence in triggering the mantle 586
wedge’s partial melting. We also question whether the mantle wedge adjacent to the normal slab is 587
14
unusually cold and dry, or if instead the mantle above the flat slab is abnormally hydrated, given the 588
lower temperature circumstances and our expectation of differences in seismic velocities. 589
The erupted lavas of the Tupungatito, San José and Maipo volcanoes, situated between 33.5°S 590
and 35°S, reflect higher degrees of fractionation and crustal contamination than the rest of the 591
Southern Volcanic Zone (SVZ), due to longer magma ascent and residence times in the crust (Michael 592
Dungan, personal communication). This can be explained by (1) smaller magma production, relative 593
to the rest of the SVZ, reflecting poor slab dehydration processes or low mantle temperatures, or by 594
(2) high crustal compressional stresses associated to the flat subduction and consequent impeding 595
magma ascent. In both cases, it appears that the flat slab conditions to the north may affect the normal 596
slab conditions directly to its south over a few hundred kilometers, at least between ~ 32.5°-34.5°S. 597
Based on geological evidences, Folguera and Ramos (2011) proposed that current Pampean flat 598
subduction once extended further south until 38°S at 17 Ma, then its southern segment progressively 599
steepened to its present angle of subduction (30°) at 5 Ma, resulting in the SVZ. This hypothesis 600
explain the similar seismic structure of the mantle wedges above both normal and flat slabs in the 601
region, and also renders our assumed “normal” subduction conditions at 33.5°S closer to a transitional 602
state, with closer to normal conditions expected to be found further south along the SVZ (> 38°S). 603
Therefore, although the tectonics near the surface changes rapidly as a response to the dramatic shift in 604
the slab geometry at depth, on the other hand, the continental mantle appears more homogeneous 605
(seismically) and less abruptly affected by these sudden changes, perhaps owing to an ambient low 606
viscosity flow that homogenizes the mantle properties. 607
The continental mantle above the flat slab segment contains faster Vp (8.0 to 8.4 km/s) and Vs 608
(4.5 to 4.8 km/s) values, and Vp/Vs ratios mainly around 1.75-1.77, in agreement with previous 609
interpretations for a cold and dry mantle (n°12, 14, 22 and 23 in Fig. 10A). We find that mostly Opx- 610
and Mg-rich garnet peridotites (lherzolites and harzburgites) match these seismic velocities, a 611
conclusion also settled by Wagner et al. (2005; 2006; 2008), based on another processing method of 612
the CHARGE seismic data. The slower continental mantle directly below the Frontal Cordillera (n°13) 613
is significantly attenuated in P-waves (Fig. 9A) (Deshayes, 2008) and, along with the mantle directly 614
beneath the Cuyania crust (n°21), matches partially hydrated (avg. 10-20 vol% H2O) mantle rocks. 615
However, the proximity of this material to the ill-defined Moho could also indicate that it represents 616
lower crust material instead (hydrous eclogites, see section 7.3). 617
We present here additional evidence that the mantle is probably locally hydrated directly above 618
the flat slab’s eastern extremity, between 68°W and 69°W, shown by Vp and Vs reductions and 619
increased Vp/Vs ratios over a small volume of 100x100x20 km3 (Fig. 5C, 6A-B and D). This is 620
supported by other geophysical investigations which highlighted this region as strongly reduced in Vp 621
(Bianchi et al., 2013), highly attenuated in S-waves (Fig. 9A) (Deshayes, 2008) and significantly 622
conductive (Booker et al., 2004; Orozco et al., 2013), all partisan of fluids effects. Also, its 623
geographical position coincides with the subducting JFR track (Fig. 5C, 6A-B and D), indicating that 624
the oceanic lithosphere along the seamount ridge may dehydrate into the overlying continental mantle. 625
On the other hand, though Wagner et al. (2005) also found strong Vp reductions (< 7.4 km/s) 626
associated to this area, their Vs remains high (4.6 km/s), resulting in very low Vp/Vs values (~ 1.65). 627
In comparison, our model indicates a similar Vs value, but a much higher Vp (8.2 km/s) and Vp/Vs 628
ratio (1.78) (Fig. 6D). By analyzing mantle rock seismic properties over a range of possible P-T 629
conditions appropriate for here, we find no correspondence at all with Wagner et al. (2005)’s seismic 630
velocities (n°23w in Fig. 10A), suggesting that their Vp might be too low because of their use of a 631
global background model. As an interpretation of our own seismic velocities, only dry garnet 632
peridotites (n°23) can explain our results (Fig. 10A), creating a paradox for mantle hydration. 633
To remedy this paradox of too fast seismic velocities, we tested the influence on our results of a 634
slower background 1D velocity model for mantle depths (Fig. 11A). The resulting effects on the final 635
15
velocities at the flat slab’s depth are a reduction in Vp of up to 0.1 km/s, and in Vs of up to 0.3 km/s. 636
The region exhibiting mantle hydration signatures (n°23) is now described by a Vp of 8.0-8.1 km/s 637
and a Vs of 4.5-4.6 km/s (Fig. 11B), corresponding to dry or slightly serpentinized peridotites (avg. 638
10-20 vol% H2O) (Fig. 12A). 639
Therefore, we hypothesize that the flat slab segment slowly dehydrates along the track of the 640
JFR prior to its re-subduction into the deeper mantle, as a consequence of its proximity to the hot 641
asthenosphere directly east. We expect those released fluids to be stored in the adjacent continental 642
mantle where the slow velocity anomaly can be observed (n°23), and possibly to migrate upwards into 643
the Andean lower crust via crustal shear zones that cross-cut the mantle, explaining the locally higher 644
Vp/Vs ratios and the seismic attenuations. Crustal faults are believed to penetrate into the continental 645
mantle (e.g. Fromm et al., 2004; Gilbert et al., 2006), despite the region’s low geothermal gradient and 646
probably higher rock rigidity. 647
7.5 The Oceanic Lithosphere 648
The vertical resolution of our model (10 km) limits us to studying the oceanic mantle rather than 649
the oceanic crust, considering that the crust there has a normal thickness of less than 10 km, as 650
indicated by the seismic surveys offshore (~ 8 km thick along the JFR; Kopp et al. 2004). 651
In the flat slab region, the oceanic lithosphere is characterized by slow seismic velocities and 652
high Vp/Vs ratios (1.80) down to about 60-70 km depth, below which the Vp/Vs ratio diminishes to 653
1.75-1.77 (Fig. 6A-B). At 25 km depth, we match these velocities with fully serpentinized mantle 654
rocks (n°1), and with 60-70% (n°24), 50-60% (n°25) and 0-20% serpentinization at 40 km, 50 km and 655
70 km depths, respectively (Fig. 10A). Similar results are obtained for the normal slab region, with a 656
slab appearing to be 50-60% serpentinized at 50 km depth (n°9-10), and hardly (n° 17-18) or totally 657
dry (n° 14-13) below 70 km depth (Fig. 10B). Ray coverage is poor above 50 km depth, but tends to 658
assimilate to highly serpentinized rocks. Furthermore, the plate interface at 25 km depth in the flat slab 659
region is likely a zone of strong fluid concentration (n°1 in Fig. 10A), shown by the high Vp/Vs ratios 660
(1.82) and decreased Vs (Fig. 6A-B), coherent with the effects of fluid-filled pore compaction and 661
primary rock dehydration reactions causing expulsion of slab fluids at these depths (Kirby et al., 1996; 662
Peacock, 2001; Hacker et al., 2003). 663
Although the slight velocity variations (~ 0.05 km/s) between the normal and flat slab regions 664
may result in no difference in the predicted rock compositions, they nevertheless bring non-negligible 665
discrepancies in the Vp/Vs ratios. Therefore, the higher Vp/Vs ratios in the flat slab region attest in 666
favor of higher plate hydration along the subducting JFR. Indeed, there is intense lithospheric 667
hydration offshore, observed by seismic and gravity investigations and identified as the Challenger 668
Fracture zone which the JFR is part of (Yáñez et al., 2002; Grevemeyer et al., 2003; 2005; Kopp et al., 669
2004; Ranero et al., 2005; Clouard et al., 2007; Contreras-Reyes et al., 2010), whereby intense 670
fracturing and faulting allow particularly deep seawater penetration to occur, at least down to 20 km 671
depth (Clouard et al., 2007). 672
Along the flat slab segment, Gans et al. (2011) used receiver function analyses to locate the 673
slab’s surface and Moho, and advocate for a thicker oceanic crust (~ 15 km) than measured offshore (~ 674
8 km). They also interpreted a vertical offset along an inherited fault (shown in Fig. 6A-B), and found 675
that the distribution of this overthickened crust spans a larger area (~ 300 km) than the flat slab’s 676
width estimated by the dense seismicity and the JFR width offshore (both 100 km; shown by the white 677
line and band in Fig. 5; Kopp et al. 2004). In the case of a 15 km-thick oceanic crust, our maximum 678
vertical resolution of 10 km should enable us to detect the oceanic crust, which should be noticeable as 679
a slow velocity anomaly especially if it is non-eclogitized. Non- or partially-eclogitized oceanic crust 680
is suggested as a possible factor inducing flat subduction in this region (Martinod et al., 2005; 2010; 681
16
Gans et al., 2011), since basaltic crust is on average 0.5 g/cm3 lighter than eclogite (3.5 g/cm3). Its 682
effects would evermore be enhanced in the case of an overthickened crust (Gans et al., 2011). 683
At flat slab P-T conditions (400-800°C at 2-4 GPa, van Hunen et al., 2002), normal average 684
basaltic crust has Vp values between 7.2 and 7.6 km/s (Hacker et al., 2003). Not only do our results 685
not show a slow velocity anomaly along the flat slab segment possibly indicating non-eclogitized 686
conditions or enabling us to confirm Gans et al. (2011)’s observation, but our fast seismic velocities 687
(Vp ~ 8.5-8.6 km/s, Vs ~ 4.8-4.9 km/s) also correspond to only dense and anhydrous eclogites (3.5-3.6 688
g/cm3). When we test our results using a slower background velocity model (Fig. 11A), the resulting 689
smaller seismic velocities (Fig. 11B) now match hydrous amphibole eclogite (20% H2O); however, 690
the paradox for a buoyant slab is maintained, since its bulk rock density is as high as for dry eclogites 691
(3.5 g/cm3), representing a positive density anomaly of 0.2-0.3 g/cm3 relative to normal mantle (3.3 692
g/cm3). In the case where our seismic velocities, from both background models, rather illuminate 693
oceanic mantle (due to the lack of model resolution for the oceanic crust), the corresponding rock 694
types are dry and dense peridotites (n°15-17, 27 in Fig. 10A and 12A). 695
To resume, the flat slab segment is composed of dry oceanic mantle with either (1) a very dense 696
(3.5-3.6 g/cm3), thick (> 10 km) and fully eclogitized oceanic crust, or (2) a normally thick oceanic 697
crust (7-8 km) which we cannot observe in our model due to lack of resolution. This observation 698
challenges the concept of buoyancy, posing a paradox of how can flat subduction occur when the 699
lithospheric density is so high? Then one should seek for another mechanism to explain flat 700
subduction (e.g. see alternative mechanisms in Skinner and Clayton, 2013; Cerpa et al., submitted). In 701
addition, a fully transformed eclogitic crust should no longer be seismically active, contradicting the 702
very high seismic activity defining the flat slab geometry (Fig. 1C) and the subducting JFR path. 703
Furthermore, in agreement with other studies (see Section 7.4), our results show that the JFR may be 704
dehydrating at the eastern tip of the flat slab, indicating that fluids are retained in the slab until this 705
point and that eclogite transformation is incomplete. This interpretation is backed by the high mantle 706
conductivity values adjacent to the re-subducting slab (Booker et al., 2004) and by the recent (1.9 Ma, 707
Kay and Abbruzzi, 1996) volcanism above it, in San Luis located in the Sierras Cordoba province 708
(65°W). This hypothesis of a partially eclogitized and hydrous oceanic crust with thickness less than 709
10 km, explains well the flat slab’s high seismic activity and positive buoyancy. 710
711
Conclusions 712
In conclusion, our results show that there are significant seismic differences between the 713
continental lithospheres above the flat and normal slabs, in central Chile and western Argentina: 714
The forearc crust above the flat slab (< 32°S) exhibits high Vs and very low Vp/Vs ratios at two 715
distinct depth ranges, 0-10 and 35-50 km. No normal rock type was found to correlate with these 716
seismic velocities. However, we suggest possible links with transient short time-scale variations 717
similar to the Punitaqui aftershock region that portrays identical seismic behaviors, the flat slab 718
geometry, and/or seismic anisotropy. 719
The Andean crust throughout the region is characterized by significantly slow seismic velocities 720
and relatively high Vp/Vs ratios, especially at depth ranges of 0-30 km and 40-70 km. The shallow 721
anomaly is explained by intense structural damage correlated with the La Ramada-Aconcagua fold-722
and-thrust belt. And the deeper crustal root anomaly is interpreted as a fluid-bearing non-723
eclogitized mafic crust possibly indicating a past delamination event (Kay and Gordillo, 1994), or a 724
felsic crust, coinciding with a deep extent of the Chilenia terrane. 725
The backarc crust is described with fast seismic velocities outlining the Cuyania terrane, which are 726
coherent with eclogitic compositions below 50 km depth, supporting many previous interpretations. 727
17
We interpret localized regions of higher Vp/Vs ratios in the forearc crust and mantle wedge as 728
hydrated zones representing basement detachment shear zones reaching out for the subduction 729
interface and channelizing slab-derived fluids into the Andean crust. 730
The continental mantle above the flat slab portion is expressed by fast seismic velocities and 731
relatively low Vp/Vs ratios, probably enhanced by the region’s low geotherm, and matching dry 732
Mg-rich peridotite seismic properties at such depths, as previously suggested. This coincides well 733
with the more intense deformation style at the surface, and the cessation of arc volcanism at the 734
surface. 735
We show evidence that the subducting Juan Fernandez Ridge locally dehydrates at the eastern tip 736
of the flat slab (68.5°W/ 31.5°S), before re-subduction occurs, suggesting that some fluids are 737
retained within the flat slab, explaining part of its buoyancy. In contradiction with a previous study, 738
we interpret the oceanic crust along the flat slab segment as thinner than our model’s vertical 739
resolution of 10 km, since our seismicity velocities here are high and can only reflect very dense 740
eclogites. An eclogitic oceanic crust strongly disrupts the role of the slab’s buoyancy in 741
maintaining flat subduction, thus calling for an alternative causative mechanism (which is not our 742
aim to discuss here). In addition to advocating for non-overthickened flat slab oceanic crust, we 743
propose that it is instead non- or only very partially-eclogitized, which is consistent with its 744
ongoing dehydration process along the JFR, the observed dense seismic activity, as well as further 745
east, the higher mantle conductivity values (Booker et al., 2004) and recent volcanism (Kay and 746
Gordillo, 1994). 747
Nevertheless, there appears to be little seismic variations in the mantle wedges above both flat 748
and normally dipping slabs, describing similar levels and depths of hydration (until 70 km depth), and 749
indicating that this similitude is caused by the influence of, and proximity with, the flat slab impacting 750
on the region directly to its south. 751
752
Acknowledgements 753
Local data were obtained thanks to projects FONDECYT 1020972-1050758 and IRD- 754
GéoAzur. Marianne Marot and Guust Nolet are supported by the GlobalSeis project (ERC 226837). 755
Giorgio Ranalli’s participation in this project is supported by a grant by NSERC (Natural Sciences and 756
Engineering Research Council of Canada). We are very grateful to all the members who have actively 757
discussed and debated our subject with us, bringing us new insights and challenges to our topic. These 758
include, Andres Tassara, Eric Ferre, Lara Wagner, Stephan Rondenay and Manuele Faccenda. We also 759
would like to thank Perrine Deshayes who shared with us her seismic attenuation tomography results 760
for this region, which she performed during her PhD thesis project. 761
762
763
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Figure Captions 1
Figure 1: Tectono-seismo-structural-geological context of central Chile and western Argentina. (A) 2
Seismological context of the seismic networks (inverted triangles) and its recorded seismicity (small 3
circles). Shown are: active volcanoes (red triangles), main cities (white circles, capital city Santiago 4
with a star), slab contours from Anderson et al. (2007), the political border between Chile and 5
Argentina (white line), the inferred Juan Fernandez Ridge subduction path and width (semi-6
transparent white line and band, respectively). B) Tectono-structural-geological context, showing the 7
accreted terranes, major suture zones (thick black lines), geological provinces and their uplifted 8
outcrops (dotted lines), the approximate location of the La Ramada and Aconcagua fold-and-thrust 9
belts (lines with triangles), and main cities (white circles and circled labeled in (A). (C) and (D) The 10
recorded seismicity along two vertical E-W cross-sections along the flat and normal subduction 11
regions, respectively, positioned in (A). 12
Figure 2 : 1D velocity models determined for the region of Central Chile and used as initial model for 13
our tomography study. 14
Figure 3: Checkerboard test results for our final velocity models of P- and S-waves in: (A) plan view 15
at 5, 35 and 95 km depths; (B) and (C) vertical E-W cross-sections along the flat slab at 31.5°S and the 16
normal slab at 33.5°S; and (D) vertical N-S cross-section along the backarc region at 68.5°W. The 17
initial synthetic velocity perturbations for P- and S-waves are 500 and 300 m/s, respectively. The 18
horizontal and vertical node spacing distances are chosen to be 40 and 10 km, respectively. Grey 19
shaded cells in vertical cross-sections reflect areas of poor ray density, determined by fixing a certain 20
threshold value for the total ray-length crossed in each cell. Also shown are the topography and 21
geological and tectonic terrane boundaries (thick dotted and solid lines, respectively), the slab 22
geometry (isocontours in thin dotted lines, from Anderson et al. 2007)the location of the active 23
volcanoes (red triangles) and of the seismic stations used in this study (inverted blue triangles), the 24
inferred slab interface based on our relocated seismicity (dark gray line) and the expected location of 25
the subducting Juan Fernandez Ridge material (thick brow line segment), the continental Moho 26
calculated from seismic (Fromm et al., 2004) and gravity studies (Tassara et al., 2006). 27
Figure 4: Spike test results for our final velocity models of P- and S-waves, shown in: (A) plan view at 28
depth 5, 35 and 95 km depth; (B) and (C) Vertical E-W cross-sections through the flat slab at 31.5°S 29
and normal slab at 33.5°S; and (D) Vertical N-S cross-section along the backarc region at 68.5°W. The 30
initial velocity perturbations imposed on our final velocity inversion model are shown by blue 31
(positive) and red (negative) outlined boxes of amplitude ±500 m/s. We notice that the region closest 32
to the subduction front (i.e. the forearc) is well resolved with almost not distortions, however, the 33
backarc region is less well constrained by the ray coverage, leading to some horizontal smearing, 34
mostly in the continental mantle below Cuyania. 35
Figure 5: Plan view of our final 3D velocity model perturbations for P- and S-waves, relative to our 36
background model, and deduced Vp/Vs ratios, in at depths (A) 5 km, (B) 35 km, and (C) 95 km, cross-37
cutting the shallow forearc, deeper forearc and continental mantle seismic anomalies (details in text). 38
Contour lines are the absolute P- and S-wave velocities. Red dots are the seismicity at these depth 39
slices. Small empty triangle at the center indicates the location of the Aconcagua highest peak of the 40
region (6500 m). Figure legend same as Figure 3. 41
Figure 6: Vertical cross-sections of our final 3D velocity model perturbations for P- and S-waves, 42
relative to our background model, and deduced Vp/Vs ratios, along latitudes (A) 31.5°S, (B) 31°S, and 43
(C) 33.5°S, describing the flat slab segment where the JFR subducts, the Punitaqui aftershock region; 44
and the normal slab region, respectively; and along the longitude (D) 68.5°W, describing the Cuyania 45
lithosphere and its possible hydrated continental mantle. Shown are: absolute seismic velocities 46
(contour lines); the selected seismicity for our tomography along each profile(red dots, see Section 3.2 47
for details); the tectonic models for the Andes (from Farías et al. 2010) and for the backarc region 48
(from Ramos et al. 2002) (black lines) and our inferred extensions of major basement detachment 49
faults (purely hypothetical and based on our tomography results, dotted line); and another model for 50
the flat slab’s interface (dark red dotted line), oceanic Moho (pink dotted line) and speculative vertical 51
offset (black line) based on receiver function analysis by Gans et al. (2011). Figure legend same as 52
Figure 3. 53
Figure 7: Our final synthetic P-T models computed to represent the central Chilean (A) flat and (B) 54
normal subduction zones, and used for our petrological analysis. See Section 6.1 for details on the 55
computation method. Variations in temperature (T) and pressure (P) are delineated by isocontour lines. 56
The uncertainty of T and P are of the order of ±100°C and ±0.5 GPa, respectively. Figure legend same 57
as Figure 3. 58
Figure 8: Areas sampled for our petrological analysis, whereby our absolute seismic velocities 59
obtained by tomography are compared with predicted rock seismic velocities computed from the 60
Hacker and Abers (2004) worksheet. (A) Discrete and precise locations of interest. Numbers to make 61
reference to Figure 10. (B) Within in cell (40 and 10 km in length and width respectively). Numbers 62
and color code refer to the number of rock solutions found matching our seismic velocities observed in 63
each cell. The left and right columns represent the flat and normal subduction zones at 31.5°S and 64
33.5°S, respectively. Figure legend same as Figure 3. 65
Figure 9: Novel seismic attenuation models for P- and S-waves calculated by Deshayes (2008) for the 66
region using events from the OVA99 and CHARM campaigns, along the (A) flat slab at 31.5°S, (B) 67
transition zone at 32.5°S, and (C) normal slab at 33.5°S. Note that the model is limited at 72°W. The 68
color scale describes a Q-value variation from 400 to 1600, representing strongly attenuating and non-69
attenuating regions, respectively. 70
Figure 10: Petrological analysis. Comparison of our seismic tomography velocities with predicted 71
rock seismic behaviors (Vp vs. Vs) for the continental and oceanic mantle, at varying pressures (1-3 72
GPa), temperatures (400-600°C) and fluid content (0-100 vol% H2O) along (A) the flat and (B) 73
normal slabs, using the Hacker and Abers (2004) worksheet and listed ultramafic rock compositions. 74
Our seismic tomography velocities are based on the initial velocity model shown in Figure 2 and 75
sampled at locations shown in Figure 8A. We have calculated the predicted rock seismic properties at 76
varying pressures and temperatures, to account for the uncertainty range in our petrological model, 77
shown in Figure 7. Each rock type (Harzburgites, Lherzolites and others) is represented by a 78
geometrical shape and color coded according to its volume proportion of hydrous minerals (also 79
representing its percent hydration). Increasing the rock’s temperature and fluid content, decreases its 80
seismic velocities, and increasing its pressure, increases its seismic velocities. Rocks with the highest 81
seismic velocities are rich in magnesium (Mg) rather than iron (Fe). The continental mantle above the 82
flat slab segment is described with a Mg-rich and dry composition. 83
Figure 11: A comparison of our seismic tomography results when using (A) a slower background 84
velocity model (grey line) at depths of the continental mantle, with respect to the faster one initially 85
used (black line) and shown in Figure 2. The new seismic tomography results obtained using this 86
slower background model are shown along the (B) flat and (C) normal subduction zones, and describe 87
the same velocity trends as those obtained using the faster model in Figure 2, with however only a 88
slight reduction in Vp and Vs of up to 0.1 and 0.3 km/s, respectively, at flat slab depths, sufficient to 89
describe the continental and oceanic mantle with seismic velocities now closer to hydrous conditions, 90
as shown in Figure 12. 91
Figure 12: New comparison between predicted rock seismic behaviors (Vp vs. Vs) using the Hacker 92
and Abers (2004) worksheet for the continental and oceanic mantle and our seismic tomography 93
velocities obtained using a slower background velocity model shown in figure 11A. Numbers refer to 94
the locations where our seismic velocities were sampled, shown in Figure 8A. The previously 95
described as dry continental mantle above the flat slab segment is now described as lying along the 96
borderline between dry and slight hydrated. 97
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