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

41

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Keywords: central Chile; flat subduction; seismic tomography; rock composition; thermo-mechanical 43

modeling; eclogite crust; dehydration/hydration 44

2

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

3

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

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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

6

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

7

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

8

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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