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1 Crustal Thickness Mapping of the Central South Atlantic and the Geodynamic 1 2 Development of the Rio Grande Rise and Walvis Ridge 2 3 3 4 5 1,2 6 4 Michelle Cunha Graça* , Nick Kusznir ³, Natasha Santos Gomes Stanton¹ 7 8 5 ¹ Faculty of Oceanography, Rio de Janeiro State University, Rio de Janeiro, Brazil 9 10 2 11 6 Geological Survey of Brazil – CPRM, 404, Pasteur Avenue, Urca, Rio de Janeiro, Brazil 12 13 7 ³ Department of Earth & Ocean Sciences, University of Liverpool, Liverpool L69 3BX, UK 14 15 16 8 *corresponding author: e-mail address: [email protected] 17 18 9 19 20 21 10 22 23 11 Abstract 24 25 12 The origin of the Rio Grande Rise and Walvis Ridge within the central South Atlantic 26 27 28 13 and its implications for the separation of South America and Africa during the Cretaceous are 29 30 14 controversial. The recent report of the discovery from submersible sampling of continental 31 32 33 15 material of Proterozoic age on the Rio Grande Rise suggests that the existing explanation 34 35 16 involving ocean ridge – mantle plume interaction or simply excess ‘on-ridge’ magmatism for 36 37 38 17 the formation of the Rio Grande Rise and Walvis Ridge needs to be re-examined. We use 39 40 18 gravity anomaly inversion to map crustal thickness for the central South Atlantic area 41 42 19 encompassing the Rio Grande Rise, Walvis Ridge and adjacent South American and African 43 44 45 20 rifted continental margins. We show that the Rio Grande Rise consists of three distinct bodies 46 47 21 of anomalously thick crust (Western, Central and Eastern) separated by normal thickness 48 49 50 22 oceanic crust. The Central Rio Grande Rise forms a large elliptical body with maximum 51 52 23 crustal thickness of 25 km. The Walvis Ridge also has a maximum crustal thickness of 25 km 53 54 55 24 but has a narrower and more linear geometry. We use plate reconstructions to restore maps of 56 57 25 crustal thickness and magnetic anomaly to Early Cretaceous times to examine the 58 59 60 26 development of the Rio Grande Rise and Walvis Ridge. These restorations together with ages 61 62 63 64 65 2

27 of magmatic addition suggest that the Central Rio Grande Rise and Walvis Ridge formed a 1 2 28 single body between 90 and 80 Ma located on the ocean ridge plate boundary similar to 3 4 5 29 Iceland today. On the basis of crustal thickness mapping, the plate restorations and the 6 7 30 magmatic ages, we propose that the Rio Grande Rise was fragmented into its 3 parts and 8 9 10 31 separated from Walvis Ridge by at least 4 ocean ridge jumps during the opening of the South 11 12 32 Atlantic Ocean between approximately 90 and 50 Ma. Plate reconstructions of crustal 13 14 33 thickness showing rotated structural lineaments imply that the separation of Eastern Rio 15 16 17 34 Grande Rise and Walvis Ridge was highly complex involving simultaneous crustal extension 18 19 35 and magmatic addition. We propose that the continental material reported on the Rio Grande 20 21 22 36 Rise, if not drop-stones, was isolated from the main continental land-mass and transported 23 24 37 into the ocean by these ridge jumps during the Cretaceous formation of the South Atlantic. 25 26 27 38 28 29 39 Keywords: Rio Grande Rise, Walvis Ridge, South Atlantic, Gravity anomaly inversion, 30 31 40 Crustal thickness, Plate Reconstruction. 32 33 34 41 35 36 42 1. Introduction 37 38 39 43 The ocean basin of the central South Atlantic (Figure 1) is dominated by the Rio Grande 40 41 44 Rise (RGR) and Walvis Ridge (WR). Both are spatially extensive regions of anomalously 42 43 44 45 shallow bathymetry within an otherwise normal oceanic domain and are associated with Late 45 46 46 Cretaceous and Cenozoic magmatism. The commonly held view is that both the RGR and 47 48 49 47 WR were formed by the interaction of the Tristan da Cunha mantle plume with sea-floor 50 51 48 spreading during the formation of the S. Atlantic (Wilson, 1963, 1965; Dietz and Holden, 52 53 49 1970; Morgan, 1971). The Early Cretaceous formation of the Paraná-Etendeka Magmatic 54 55 56 50 provinces, the subsequent magma rich breakup of western Gondwana and initiation of sea- 57 58 59 60 61 62 63 64 65 3

51 floor spreading in the southern South Atlantic are also attributed to the Tristan da Cunha 1 2 52 mantle plume, as are the Late Cretaceous to Cenozoic Tristan-Gough Guyot chains. 3 4 5 53 The expectation therefore is that the RGR and WR are oceanic crust greatly thickened 6 7 54 by ocean ridge - mantle plume interaction or anomalous excess ocean ridge magmatism, 8 9 10 55 similar to that believed to be responsible for the formation of the thick crust under Iceland 11 12 56 today. However, recent geological sampling of the RGR indicates the presence of some 13 14 57 continental material of Proterozoic age in the RGR. Reported submersible sampling found 15 16 17 58 granites, granulites, gneisses, and pegmatites with ages between 500 and 2200 Ma 18 19 59 (Fioravanti, 2014). If those continental rocks are not drop-stones, the discovery of continental 20 21 22 60 material within the RGR suggests that the ocean ridge - mantle plume interaction hypothesis 23 24 61 for RGR and WR formation needs to be reconsidered. Continental material has been found in 25 26 27 62 other oceans significant distances from the nearest continental land-mass. Examples include 28 29 63 the discovery of Pre-Cambrian zircons in Mauritius in the Indian Ocean (Torsvik et al., 2013) 30 31 64 and geochemical evidence for continental material under SE Iceland (Torsvik et al., 2015). In 32 33 34 65 both of these cases plate tectonic re-organisations and ridge jumps resulted in continental 35 36 66 crust or subcontinental lithospheric mantle (SCLM) becoming isolated within the oceanic 37 38 39 67 domain. Such a mechanism has not been proposed previously for the formation of the RGR 40 41 68 and WR. 42 43 44 69 The RGR is located in oceanic lithosphere offshore of the Santos and Pelotas segments 45 46 70 of the Brazilian rifted margin and is separated from the São Paulo Plateau and the 47 48 49 71 Florianópolis Ridge by oceanic crust (Leyden et al., 1971). In this context, the presence of 50 51 72 continental rocks with affinities with the adjacent continental lithosphere of Africa and South 52 53 73 America raises questions about the nature, origin and kinematics of South Atlantic Ocean 54 55 56 74 formation since the breakup of Western Gondwana. 57 58 59 60 61 62 63 64 65 4

75 The aim of this paper is to present the results of new detailed crustal thickness mapping 1 2 76 using gravity inversion of the central South Atlantic area encompassing the RGR, WR and 3 4 5 77 adjacent SouthAmerican and African rifted continental margins, and to use plate 6 7 78 reconstructions to restore these crustal thickness maps to Early Cretaceous times in order to 8 9 10 79 examine the development of the RGR and WR. The observed ages of magmatic additions and 11 12 80 restored magnetic anomalies are also incorporated into our analysis. By doing so we aim to 13 14 81 provide new observations and constraints on the formation mechanisms of the RGR and WR, 15 16 17 82 and the development of the central South Atlantic. 18 19 83 20 21 22 84 2. The Rio Grande Rise (RGR) and Walvis Ridge (WR) 23 24 25 85 The Rio Grande Rise (RGR), along with its conjugate Walvis Ridge (WR), forms the 26 27 86 two most prominent bathymetric features in the South Atlantic ocean basin (O’Connor and 28 29 30 87 Duncan, 1990). The RGR extends between latitudes 28º and 34ºS and longitudes 28º and 31 32 88 40ºW. It is delimited to the north and south by the Rio Grande and 35.3 º S oceanic fractures 33 34 35 89 zones and consists of seamounts, guyots and escarpments (Alves, 1981). Gamboa and 36 37 90 Rabinowitz (1984) using geomorphology identified two distinct units: the Western and 38 39 40 91 Eastern RGR. In this paper, we name these the Central (CRGR) and Eastern RGR (ERGR) 41 42 92 units so we can also distinguish an additional distinct unit we call the Western RGR (WRGR) 43 44 45 93 (Figure 2). In this paper the WRGR is considered as a RGR unit due to its similar crustal 46 47 94 thickness, magnetic anomalies and proximity to the CRGR and ERGR The Western, Central 48 49 95 and Eastern RGR are separated by oceanic abyssal plains with few seamounts. The Central 50 51 52 96 RGR presents an elliptical bathymetric anomaly with extensive regions with bathymetry less 53 54 97 than 1000 m (700 m at the shallowest point) compared with the regional oceanic bathymetry 55 56 57 98 of greater than 4000 m. In comparison to the Central RGR, the Eastern RGR is much less well 58 59 99 known. The Eastern RGR is composed of two main segments with average bathymetries of 60 61 62 63 64 65 5

100 2000 m. The Western RGR shows a much less pronounced bathymetric anomaly with 1 2 101 shallowest values of approximately 3000 m. 3 4 5 6 102 Gamboa and Rabinowitz (1984) suggests that the RGR was first formed at the oceanic 7 8 103 spreading center near to sea level in the Santonian-Coniacian and then thermally subsided 9 10 104 accompanied by pelagic sedimentation. Afterwards, in the Middle Eocene, an alkaline 11 12 13 105 volcanic episode uplifted the Central RGR creating numerous oceanic islands resulting in 14 15 106 erosion and a sedimentation phase characterized by turbidite currents and landslides due to 16 17 18 107 the extension of the underlying crust (Barker, 1983; Gamboa and Rabinowitz, 1984). Flexural 19 20 108 isostasy analysis by Ussami et al. (2013) supports the interpretation that the RGR was 21 22 23 109 constructed during these two main magmatic events. Subsequently, the RGR province 24 25 110 returned to oceanic thermal subsidence and sedimentation (Detrick et al., 1977). 26 27 28 111 The Central RGR has a major rift-like structures aligned in a NW-SE direction which is 29 30 31 112 associated with the Cruzeiro do Sul Lineament (Mohriak et al., 2010) and is partially filled 32 33 113 with sediments. The Cruzeiro do Sul Lineament extends from the Cabo Frio High south- 34 35 36 114 eastwards into the RGR and involves both continental and oceanic lithosphere. Walvis Ridge 37 38 115 (WR) apparently does not show a zone of similar deformation (Mohriak et al., 2010). The 39 40 41 116 Cabo Frio High, which divides the two largest oil producing provinces in Brazil, the Campos 42 43 117 and Santos basins, appears to be the north-western continuation of the Cruzeiro do Sul 44 45 118 Lineament (Souza et al., 1993). Seamounts and guyots are located along this rift-structure in 46 47 48 119 the Central and Eastern RGR, suggesting that this volcanism was originated in the Middle 49 50 120 Eocene (Barker, 1983) as a result of within-plate extensional stresses that caused mantle 51 52 53 121 decompression and melting. 54 55 56 122 The WR is located in the eastern South Atlantic on the opposite side of the mid-Atlantic 57 58 123 ridge in an approximately conjugate position to the RGR (Figure 1). It forms part of the 59 60 61 62 63 64 65 6

124 Tristan-Gough age-progressive magmatic track which connects the Etendeka Magmatic 1 2 125 Province in the African continent with the Tristan da Cunha and Gough island groups in the 3 4 5 126 ocean part of the African plate. In the north-east it comprises the aseismic Walvis Ridge itself, 6 7 127 and the contiguous Guyot Province which extends in the south-west direction and which 8 9 10 128 bifurcates in the south-west forming two spatially separate subtracks (the Tristan and Gough 11 12 129 tracks) (Rohde et al., 2013). Fromm et al. (2017) modelled the P-wave velocity structure of 13 14 130 northeastern segment of WR and proposed that it consists of thickened oceanic crust 15 16 17 131 composed of basaltic layers, pillow basalts and sheeted dikes in the upper crust and gabbroic 18 19 132 rocks in the lower crust. 20 21 22 23 133 The first explanations for the origin of these conjugate RGR and WR features were 24 25 134 proposed by Wilson (1963, 1965), Dietz and Holden (1970) and Morgan (1971) who 26 27 135 suggested that they were formed during the opening of the South Atlantic as the South 28 29 30 136 American and African plates moved away from a stationary mantle “hot spot” located at the 31 32 137 seafloor spreading axis. Kumar and Gambôa (1979) also suggested a RGR-WR common 33 34 35 138 origin as resulting from an excess volcanism between 100 and 80 Ma in a segment of the 36 37 139 southern mid-Atlantic ridge delimited by fracture zones. The flexural isostatic analysis of 38 39 40 140 Ussami et al. (2013) is consistent with the results of South Atlantic palaeo-geographical 41 42 141 reconstruction to anomaly C34 (at 84 Ma) which puts RGR and WR together on the ocean 43 44 45 142 ridge. Following the generation of the Paraná-Etendeka Magmatic Province (PMP) during the 46 47 143 Early Cretaceous, it has been proposed that the Tristan da Cunha-Gough mantle plume 48 49 144 generated the RGR and WR at the ocean ridge spreading axis (Wilson, 1963; Morgan, 1971; 50 51 52 145 O’Connor and Duncan, 1990). However RGR-WR morphologies differ from the expected 53 54 146 "V" shaped symmetry about the sea-floor spreading axis mid-ocean ridge because at around 55 56 57 147 75 Ma multiple ocean ridge jumps occured in an eastward direction. This resulted in the 58 59 148 isolation of the RGR from the spreading axis while the WR continued its formation by 60 61 62 63 64 65 7

149 intraplate volcanism generating the Guyot Province. Afterwards, during the transition of the 1 2 150 WR to the Guyot Province, westward ocean ridge jumps also occurred (Rohde et al., 2013). 3 4 5 6 151 Ussami et al. (2013) found that RGR-WR basalts have isotope signatures with an EMI 7 8 152 (Enriched Mantle I) component which are very different from those of N-MORB (Normal 9 10 153 Mid-Ocean Ridge Basalt) indicating melting of a distinct mantle source without a continental 11 12 13 154 crust melting component and distinct from the present-day Tristan da Cunha alkaline rocks. It 14 15 155 is nearly identical to the PMP (Paraná Magmatic Province) tholeiites (133-132 Ma) (Figure 16 17 18 156 1). Isotopic data for the Eocene age alkaline basalts are still unknown. 19 20 21 157 Explanations of the origin of the EMI (Enriched Mantle I) component include thermal 22 23 158 erosion of the Subcontinental Lithosperic Mantle (SCLM) due to small-scale convection at 24 25 26 159 the edge of a cratonic lithosphere (King, 2000; King and Anderson, 1998), melting of 27 28 160 fragments of delaminated or detached SCLM before, during and after continental break-up 29 30 31 161 including mantle metasomatism and edge-driven convection (Peate et al., 1999; Smith and 32 33 162 Lewis, 1999; Foley, 2008) and thermal erosion at the base of a SCLM and mantle flow 34 35 36 163 transport (Class and le Roex, 2006; Meyzen et al., 2007). Detachment and/or thermal erosion 37 38 164 of an old SCLM is the most accepted model to explain the EMI geochemical and isotope 39 40 41 165 signatures of the RGR-WR (Hawkesworth et al., 1986; Peate et al., 1999; Gibson et al., 2005; 42 43 166 Class and le Roex, 2006; Meyzen et al., 2007). This led Ussami et al. (2013) to suggest that 44 45 167 RGR-WR magmatism results from melting of a heterogeneous lithospheric mantle which was 46 47 48 168 fragmented and left behind during western Gondwana break-up. To maintain the deep mantle 49 50 169 plume theory to explain South Atlantic magmatism with a EMI signature, it is necessary that 51 52 53 170 the a mantle plume evolved in space and time (Ussami et al., 2013). Hoernle et al. (2015) 54 55 171 consider a mantle plume to be the cause of the RGR-WR and present a spatial-temporal 56 57 58 172 geochemical zonation for the Tristan-Gough age-progressive magmatic track. 59 60 61 62 63 64 65 8

173 Others studies examine the South Atlantic magmatic track, mainly for the African plate 1 2 174 and WR, and suggest an origin at the core-mantle boundary (COB) deriving from the edge of 3 4 5 175 the Africa Low Shear Wave Velocity Province (Burke et al., 2008; Torsvik et al., 2010; 6 7 176 O’Connor et al., 2012; Steinberger and Torsvik, 2012). Some recent 40Ar/39Ar results explain 8 9 10 177 the spatial and temporal distribution of the magmatism in the southeast Atlantic Ocean as 11 12 178 being controlled by the interplay between deep-sourced mantle plumes and the motion and 13 14 179 structure of the African plate. This requires a stable (or coherently moving) deep mantle 15 16 17 180 source(s) to explain the parallel age-progressive magmatic trails (O’Connor et al., 2012; 18 19 181 O’Connor and Jokat, 2015). 20 21 22 23 182 (Rohde et al., 2013) also dated RGR-WR basalts, tephrites, trachytes, and 24 25 183 phonotephrites using 40Ar/39Ar method. Their research indicates an overall age progression 26 27 184 volcanism along the Tristan-Gough track. According to Rohde et al. (2013), the age 28 29 30 185 progression together with the decrease in the amount of volcanism along the Tristan-Gough 31 32 186 track, suggests a genesis above a deep mantle plume, which began with the formation of the 33 34 35 187 Paraná-Etendeka Magmatic Provinces. The plume then transitioned into a broad conduit 36 37 188 centered on the ridge axis, forming the RGR-WR, and then became unstable and bifurcated 38 39 40 189 around 60-70 Ma ago turned into pulsing "bubbles" between 35-45 Ma to form the Guyot 41 42 190 Province (Rohde et al., 2013). 43 44 45 191 Galvão and Castro (2017) used magnetic anomalies from EMAG2 data (Maus et al., 46 47 48 192 2009), gravity data and plate reconstructions to interpret fracture zones in the Central and 49 50 193 Eastern RGR and proposed that slivers of continental blocks rose during magma 51 52 53 194 emplacement. 54 55 56 195 The WR emplacement has also been explained by the existence of a major fracture zone 57 58 196 in the South Atlantic named Rio Grande Fracture Zone (RGFZ) (Figures 1 and 2). Some 59 60 61 62 63 64 65 9

197 authors assign the origin of the WR to the evolution of this fracture zone with an extensional 1 2 198 component (a failed rift arm of a triple junction) producing volcanism (Le Pichon and Fox, 3 4 5 199 1971; Fairhead and Wilson, 2005; Elliott et al., 2009) or to a combination of hotspot and 6 7 200 fracture zone mechanism (Haxel, 2005). In contrast Fromm et al. (2017) modelled the P-wave 8 9 10 201 velocity structure of the eastern segment of the WR and concluded that the RGFZ and WR 11 12 202 evolved independently from each other. 13 14 15 203 16 17 18 204 3. Data and Methodology 19 20 205 We use public domain free-air gravity anomaly, bathymetry and sediment thickness 21 22 23 206 data to determine crustal thickness for the RGR, WR and adjacent South Atlantic Ocean 24 25 207 region. The bathymetric data (Figures 1 and 2) used was taken from the ETOPO1 compilation 26 27 208 provided by the National Geophysical Data Center (NGDC) and described in Amante and 28 29 30 209 Eakins (2009). The free-air gravity anomaly data (Figure 3a) were derived from satellite radar 31 32 210 altimetry (Sandwell and Smith, 2009). Sediment thicknesses were taken from the NGDC 33 34 35 211 global marine compilation (Divins, 2003). We also compare the free-air gravity anomaly data 36 37 212 and crustal thickness distribution with regional magnetic anomaly from EMAG2 (version 3). 38 39 40 213 EMAG2 v3 (Figure 3b) is a compilation of marine, terrestrial, aerial and satellite magnetic 41 42 214 measurements with a lateral resolution of 2 arc-minutes, an anomaly altitude at sea level for 43 44 45 215 oceanic regions and 4 km for continental regions, a maximum degree or order of 300 and 46 47 216 minimum wavelength within our study area of 15 km (Meyer et al., 2017). 48 49 217 Regional crustal basement thickness has been determined using 3D gravity inversion 50 51 52 218 which is carried out in the spectral domain and incorporates a lithosphere thermal gravity 53 54 219 anomaly correction. The gravity inversion technique used is described in detail in Chappell 55 56 57 220 and Kusznir (2008) and Alvey et al. (2008). It is important to include a correction for the 58 59 221 lithosphere thermal gravity anomaly because of the elevated geotherm of oceanic and 60 61 62 63 64 65 10

222 continental margin lithosphere; failure to do so leads to a significant over-estimate of Moho 1 2 223 depth and crustal basement thickness. 3 4 5 6 224 The lithosphere thermal gravity anomaly correction requires information on the 7 8 225 lithosphere cooling time and also the magnitude of the initial lithosphere thermal pertubation. 9 10 226 For oceanic regions, the ocean isochrons of (Müller et al., 2008) were used to give cooling 11 12 13 227 ages and a lithosphere thinning factor (1-1/β) of 1 to derive the initial thermal perturbation (β 14 15 228 is the lithosphere stretching factor defined by McKenzie (1978) and is ∞ for oceanic 16 17 18 229 lithosphere). For the continental margin lithosphere, the breakup age is used for the 19 20 230 lithosphere cooling time and the lithosphere thinning factor for the initial thermal perturbation 21 22 23 231 is derived from the gravity inversion itself (see Chappell and Kusznir (2008) for further 24 25 232 explanation). 26 27 28 233 The sediment density used in the gravity inversion assumes a compaction controlled 29 30 31 234 dependence on depth corresponding to shaly-sand lithology (Sclater and Christie, 1980). We 32 33 235 use a crustal basement density of 2850 kg/m3 and a mantle density of 3300 kg/m3. 34 35 36 37 236 Crustal thickness derived from gravity inversion using public domain NGDC sediment 38 39 237 thickness is shown in Figure 4a. The input data used in the gravity inversion consists of 40 41 238 bathymetry, free-air gravity anomaly and sediment thickness. Of these, the least accurate is 42 43 44 239 sediment thickness. As a consequence we compare crustal thickness predicted by gravity 45 46 240 inversion for a profile for which we have more accurate sediment thickness derived from 47 48 49 241 seismic reflection data across the RGR with that using the NGDC global sediment thickness 50 51 242 compilation. The location of the profile is shown in Figure 4a. The seismic reflection data 52 53 54 243 along this profile crossing the RGR was acquired by the Institute for Geophysics of the 55 56 244 University of Texas at Austin (Barker, 1983; Gamboa and Rabinowitz, 1984) and a regional 57 58 245 Leplac seismic profile (L511) reported in Mohriak et al. (2010). Figure 4b&c show Moho 59 60 61 62 63 64 65 11

246 depth and crustal basement thickness derived from gravity inversion along this profile using 1 2 247 sediment thickness from the seismic reflection data and the global NGDC compilation. 3 4 5 248 Comparison shows that the gravity inversion using the global NGDC sediment thickness 6 7 249 compilation gives a similar Moho depth and crustal thickness to that derived using seismic 8 9 10 250 reflection sediment thicknesses. As a consequence we believe that the gravity inversion 11 12 251 mapping of crustal thickness using the NGDC public domain sediment thickness is 13 14 252 meaningful and useful. 15 16 17 18 253 In Figure 4b,c&d we also compare Moho depth derived from gravity inversion with 19 20 254 independent seismic reflection measurements from Constantino et al. (2017). The comparison 21 22 23 255 suggests that the gravity inversion method provides a reliable estimate of Moho depth and 24 25 256 crustal thickness which allows the regional S. Atlantic mapping of these parameters. The 26 27 257 cross-plot shown in Figure 4d has been used to calibrate the reference Moho depth used in the 28 29 30 258 gravity inversion and gives a value for this parameter of 35 km. 31 32 33 259 If a heterogeneous lithospheric mantle which was fragmented and left behind during 34 35 36 260 western Gondwana break-up (Ussami et al. 2013) was incorporated in the gravity inversion, it 37 38 261 would modify the lithosphere temperature structure used to calculate the lithophere thermal 39 40 41 262 gravity anomaly correction. If this resulted in a slightly cooler lithoshere under the RGR then 42 43 263 this would reduce the thermal correction and give a slightly thicker crust. 44 45 46 264 4. RGR, WR and South Atlantic crustal thickness distribution 47 48 49 50 265 51 52 266 Figure 5 shows the crustal thickness map of South Atlantic Ocean, including the RGR 53 54 and WR, derived from gravity inversion. It shows that while the South Atlantic generally has 55 267 56 57 268 oceanic crust of normal thickness (7 +/- 1 km), the Central and Eastern RGR and the WR 58 59 269 show much thicker crust with maximum crustal thickness of approximately 25 km (see Figure 60 61 62 63 64 65 12

270 4b&c also). Other features with significant thick crust (> 15 km) are the Western RGR, 1 2 271 Florianópolis Ridge, São Paulo Plateau and Torres High. The Torres High, a feature that runs 3 4 5 272 from the Ponta Grossa Arch (Figure 2) on the continental margin into the ocean, interconnects 6 7 273 with the Western RGR. The African continental margin has a different appearance compared 8 9 10 274 with the Brazilian continental margin. There is a direct connection between WR and the 11 12 275 continent through Namibia Ridge, and it does not have defined features with thick crust like 13 14 276 the São Paulo Plateau and Florianópolis Ridge. 15 16 17 277 Magmatic ages (Renne et al., 1996; O’Connor et al., 2012; Rohde et al., 2013; Geraldes 18 19 278 et al., 2013; Hoernle et al., 2015; O’Connor and Jokat, 2015) for the RGR, WR, Guyot 20 21 22 279 Province, Africa and South America are superimposed on the crustal thickness map shown in 23 24 280 Figure 5. The RGR has magmatic ages between 80 – 87 Ma and 46 Ma, while WR has ages 25 26 27 281 from 114 Ma in its northern segment decreasing to 1 Ma at its southern end. While RGR and 28 29 282 WR magmatic ages may correlate for the northern segment of WR, in the southwest of WR its 30 31 283 ages are much younger than the RGR magmatic ages. Figure 5 shows that WR ages decrease 32 33 34 284 southwestwards from the African continent towards the present mid-ocean ridge. 35 36 285 Three regional crustal cross-sections crossing the RGR have been constructed using 37 38 39 286 bathymetry, sediment thickness and Moho depth from gravity anomaly inversion and 40 41 287 compared with magnetic anomaly data. Profile AB (Figure 6b), which runs west to east from 42 43 44 288 the Pelotas margin oceanwards across the RGR, shows the three main units of the RGR, the 45 46 289 Western, Central and Eastern RGR, which are characterized by deep Moho, thick crust and 47 48 49 290 high amplitude magnetic anomalies. They are separated by oceanic crust of normal, or near 50 51 291 normal, thickness. Along profile AB both Western and Eastern RGR have similar Moho 52 53 292 depths, reaching 17 km. The deepest Moho is located in the Central RGR (> 25 km) and is 54 55 56 293 correlated with a very large magnetic anomaly (C34) reaching 450 nT. The rift valley 57 58 59 60 61 62 63 64 65 13

294 associated with the Cruzeiro do Sul Lineament can be seen on profile AB and is located above 1 2 295 where the the Central RGR has maximum crustal thickness. 3 4 5 6 296 Cross-section CD shown in Figure 6c runs from the Santos Basin oceanwards in a SE 7 8 297 direction and also crosses the Central RGR. The maximum Moho depth on this profile also 9 10 298 reaches 25 km on the Central RGR. The cross-section shows that Central RGR is separated 11 12 13 299 from the Florianópolis Ridge (and the São Paulo Plateau) by much thinner crust of 14 15 300 approximately 10 km thickness in the region of the Rio Grande Fracture Zone. The 16 17 18 301 Florianópolis Ridge and São Paulo Plateau have Moho depth of approximately 20 km and 15 19 20 302 km respectively. The crustal thickness map (Figure 6a) also shows that the entire Central 21 22 23 303 RGR has substantially thicker crust than the São Paulo Plateau and Florianópolis Ridge and 24 25 304 presents a significant crustal thickness anomaly. The anomalously thick crust under the 26 27 305 Central RGR predicted by gravity inversion is isostatically consistent with the bathymetry of 28 29 30 306 this region which is locally less than 1 km. In contrast the thinner crust of the São Paulo 31 32 307 Plateau has a greater bathymetry of over 2 km despite its greater sediment thickness. The 33 34 35 308 thicker crust of the Central RGR and Florianópolis Ridge correlate with high amplitude 36 37 309 magnetic anomalies. 38 39 40 41 310 Cross-section EF (Figure 6d) runs N-S and cuts the Central RGR where Moho depth 42 43 311 increases abruptly reaching more than 25 km depth. To both the north and south of the 44 45 312 Central RGR, crustal thickness decreases to as low as 5 km corresponding to normal, or 46 47 48 313 slightly thinner than normal, oceanic crust. The magnetic anomaly along the EF profile is not 49 50 314 shown because of its N-S orientation. 51 52 53 54 315 A direct correlation between thicker crust and large amplitude magnetic anomalies is 55 56 316 observed for all components of the RGR; 200 nT for Western RGR, 450 nT for Central RGR 57 58 317 and more than 200 nT for Eastern RGR. This suggests that igneous rocks from oceanic crust 59 60 61 62 63 64 65 14

318 are present in the RGR and possibly contribute to the anomalously large crustal thicknesses. 1 2 319 Shallow bathymetry with igneous rocks from oceanic crust may also generate the higher 3 4 5 320 amplitude magnetic anomalies. 6 7 321 No important magnetic anomaly indicating a large block of non-magnetic continental 8 9 10 322 crust seems to be present. A magnetic anomaly signature from continental upper crust rocks 11 12 323 (sedimentary, acid igneous and metamorphic rocks) with less magnitite content and remanent 13 14 324 magnetization compared with basic igneous and volcanic rocks has not been identified. If it 15 16 17 325 had been present this would have generated a large reversed polarity induced magnetic 18 19 326 anomaly. 20 21 22 327 However the presence of excess magmatism within the RGR does not necessarily 23 24 328 exclude interaction between continental and oceanic crust or lithospheric mantle within the 25 26 27 329 RGR, as evidenced by the rock samples of continental origin collected on the RGR. 28 29 330 30 31 331 5. Magnetic Anomalies, Crustal Thickness and Plate Reconstructions between 90 and 70 32 33 34 332 Ma 35 36 333 Maps of crustal basement thickness from gravity inversion and magnetic anomalies 37 38 39 334 (Figures 3b and 5) have been restored to ages of 90, 80 and 70 Ma using the Gplates 1.5 plate 40 41 335 reconstruction software with rotation poles, plate polygons and ocean isochrones from Seton 42 43 44 336 et al. (2012). 45 46 337 South Atlantic magnetic anomalies are characterized by two main zones with different 47 48 49 338 anomaly patterns. The Cretaceous Normal Superchron (CNS), which runs oceanwards from 50 51 339 the Brazilian and African margin until anomaly C34 (83 Ma) and represents a long period of 52 53 340 stable polarity of the geomagnetic field with small amplitude anomalies. Further oceanwards 54 55 56 341 the younger magnetic anomaly pattern from C34 to the present turns into a typical oceanic 57 58 59 60 61 62 63 64 65 15

342 striped pattern formed by normal oceanic sea-floor spreading and geomagmetic field 1 2 343 reversals. 3 4 5 344 For the South Atlantic ocean floor reconstructions, the spreading history along the 6 7 345 entire length of the South Atlantic from Anomaly 34 (83.5 Ma) onwards is relatively 8 9 10 346 uncomplicated. Most studies are in agreement and propose largely symmetrical sea-floor 11 12 347 spreading occurred after Anomaly 34 to the present day (Rabinowitz and Labrecque, 1977; 13 14 348 Nürnberg and Müller, 1991; Cande and Kent, 1992; Moulin et al., 2010; Seton et al., 2012). 15 16 17 349 However it is difficult to determine reconstructions before the Anomaly 34 during the 18 19 350 Cretaceous Normal Superchron (CNS) (Pindell et al., 1988; Nürnberg and Müller, 1991; 20 21 22 351 Müller et al., 1998;). 23 24 352 In Figure 7, the 90 Ma reconstruction only shows magnetic anomalies of the CNS 25 26 27 353 (Cretaceous Normal Superchron). According to the magmatic ages presented in Figure 5, the 28 29 354 magmatic components of the Central and Eastern RGR had not been formed by this time. By 30 31 355 80 Ma, the large negative magnetic anomaly corresponding to the C34 isochron are present 32 33 34 356 and pass through the Central RGR and WR which appear to form a single body at this time as 35 36 357 shown by the restored crustal thickness map. This C34 magnetic anomaly marks the divergent 37 38 39 358 plate boundary at 83 Ma and we therefore interpret this to indicate that the combined Central 40 41 359 RGR and WR were located at the divergent plate boundary at this time. The Central RGR has 42 43 44 360 basalt ages between 80 and 87 Ma coincident with this period. Therefore between 90 and 80 45 46 361 Ma, the restored magnetic anomaly and crustal thickness maps bring together RGR and WR 47 48 49 362 as a single body with active magmatism located above the sea-floor spreading axis. This is a 50 51 363 similar configuration to Iceland today (Torsvik et al., 2015). 52 53 364 Although the Tristan-Gough age-progressive magmatic track appears in the 80 Ma 54 55 56 365 reconstruction shown in Figure 7, its magmatic ages indicate its emergence starting about at 57 58 366 70 Ma, as shown in the plate reconstruction at this time. During the same time interval (80 - 59 60 61 62 63 64 65 16

367 70 Ma), the Eastern RGR emerges and has started to separate from WR at 70 Ma. By 70 Ma 1 2 368 the Central RGR was distinct and separated from the WR. 3 4 5 369 Figure 8a shows the crustal basement thickness map from gravity inversion restored to 6 7 370 83 Ma when Central RGR and WR were a single combined feature above an active ocean 8 9 10 371 ridge. Figure 8b shows a crustal cross-section along profile RW running E-W across the 11 12 372 Western RGR, Vema Channel, Central RGR and WR respectively. Note that at 83 Ma the 13 14 373 restoration suggest that the Eastern RGR had not yet been formed. Maximum Moho depths on 15 16 17 374 the Central RGR and WR reach 25 km. The region of thick crust under the Central RGR is 18 19 375 wider that under the WR. Crustal thickness exceeds 20 km for the Central RGR for a profile 20 21 22 376 distance of more than 500 km while for the WR the corresponding extent is 150 km. 23 24 377 It is notable that the shape of the regions of thick crust under the Central RGR and the 25 26 27 378 WR are different; thick crust under the Central RGR has an elliptical shape while that for the 28 29 379 WR is more linear. 30 31 380 WR shows continuity of crustal thickness from continent to ocean (Namibia Ridge- 32 33 34 381 Walvis Ridge-Tristan Gough track). In contrast, RGR is fragmented into three units and the 35 36 382 Central and Eastern RGR do not show connection in any obvious way with the continent. The 37 38 39 383 Western RGR may have been connected to the Torres High, Florianópolis Ridge and São 40 41 384 Paulo Plateau but is now separated. This could be indicative of multiple plate boundaries re- 42 43 44 385 organizations and ocean-ridge jumps during the early formation of the S. Atlantic. 45 46 386 Both the Central RGR and the WR show large magnetic anomalies (Figure 8b). Their 47 48 49 387 maximum crustal thickness of 25 km, their high amplitude magnetic anomalies and the plate 50 51 388 restorations all suggest a direct relationship between Central RGR and WR. 52 53 389 Profile RW (Figure 8b) also crosses the Western RGR where a maximum crustal 54 55 56 390 thickness of 12 km is reached. Thin crust of normal oceanic thickness (or less) is seen to the 57 58 59 60 61 62 63 64 65 17

391 west of the Western RGR and to the east between the Western and Central WGR (in the 1 2 392 Vema Channel). 3 4 5 393 6 7 394 6. Crustal Structure and Plate Reconstructions from 70 Ma to present day 8 9 10 395 11 12 396 Figure 9 shows plate restorations of crustal thickness from gravity inversion from 13 14 397 70 to 40 Ma at 10 Myr intervals. By 50 Ma the Eastern RGR and WR had separated; this may 15 16 17 398 have occurred by 60 Ma. The separation coincides with the transition from mantle plume - 18 19 399 plate boundary interaction and/or enhanced magmatism above the ocean spreading center to 20 21 22 400 off-ridge intraplate magmatism. The change of the Tristan da Cunha mantle plume to off- 23 24 401 ridge magmatism generates the Tristan-Gouch volcanic track in pre-existing oceanic 25 26 27 402 lithosphere which continues to the present day. This change from on-ridge to off-ridge mantle 28 29 403 plume magmatism has already beend described by O’Connor and Duncan (1990). 30 31 404 Alternatively, the enhanced magmatism could have ceased on the ocean ridge and started off- 32 33 34 405 ridge at approximately the same time. As shown in Figure 9, the separation process between 35 36 406 Eastern RGR and WR (specifically the Tristan-Gough age-progressive magmatic track) lasted 37 38 39 407 for about 20 Myr from 70 to 50 Ma. 40 41 408 Figure 10a shows crustal thickness from gravity inversion restored to 54 Ma. This 42 43 44 409 restoration shows that the fracture zones connecting Eastern RGR and WR have a curved 45 46 410 morphology showing rotations in the evolution of the separation of Eastern RGR from WR. 47 48 49 411 These rotated fracture zones do not exist to the north or south of the Eastern RGR and stop at 50 51 412 its north and south terminations, indicating that this region underwent differential deformation 52 53 413 and extension. What causes this rotational deformation may be explained by variations in 54 55 56 414 spreading rates in the mid-ocean ridge at that time but also may be indicative of more than 57 58 415 one active spreading center at this period. This period of complex extension and deformation 59 60 61 62 63 64 65 18

416 probably not only affected the Eastern RGR but also WR and may have some influence in 1 2 417 Guyot Province bifurcation. 3 4 5 418 The cross-sections (Figure 10b) along the axes of the Eastern RGR and WR show thick 6 7 419 crust reaching almost 20 km thickness. Both cross-sections show thicker crust in the north 8 9 10 420 than in the south. They also show high amplitude magnetic anomalies. In contrast the maps 11 12 421 show that the thick crust of the Eastern RGR and WR are separated by oceanic crust of 13 14 422 normal thickness. 15 16 17 423 The magnetic anomalies in the RGR (Figure 3b) show three distinct domains, 18 19 424 representing the 3 different components of the RGR (Figure 2). Near the coast, a series of NE- 20 21 22 425 SW short magnetic anomalies are observed, extending from the margin to the Central RGR, 23 24 426 gradually shifting orientation to N-S (in the Western RGR) and to NNW-SSE (in the Central 25 26 27 427 RGR) indicating rotation. This variation in magnetic anomaly orientation from NE-SW, 28 29 428 parallel to the margin, to N-S in the Western RGR, to NW-SE in the Central RGR suggests 30 31 429 rotation. In the Central RGR, the NW direction is distinct from the general anomaly 32 33 34 430 orientation and probably reflects one or more changes in the oceanic spreading rates. In 35 36 431 addition, oceanic spreading rates (Müller et al., 2008) indicate higher sea-floor spreading 37 38 39 432 velocities in the south-east portion of the Central RGR than in the north, suggesting an 40 41 433 internal deformationof this region. The Eastern RGR has a distinct magnetic anomaly set, 42 43 44 434 with a large positive anomaly without a clear orientation of magnetic lineaments. 45 46 435 47 48 49 436 7. The role of ridge jumps in Western Gondwana separation 50 51 437 52 53 438 Together with intense magmatism, intra-ocean ridge jumps seem to be an important 54 55 56 439 process in the South Atlantic evolution, especially in the formation of the RGR and WR. The 57 58 440 present day asymmetry of the South Atlantic spreading-axis has been attributed to a 59 60 61 62 63 64 65 19

441 succession of easterly and southeasterly ocean ridge migrations prior to C34 (O’Connor and 1 2 442 Duncan, 1990). 3 4 5 6 443 The first recorded eastward migration occurred to the north of the Torres High and WR 7 8 444 in early Albian-late Aptian time (Leyden et al., 1971; Ponte and Asmus, 1976; Cande and 9 10 445 Rabinowitz, 1978, 1979; Kumar and Gambôa, 1979; Rabinowitz and LaBrecque, 1979). A 11 12 13 446 major sea-floor spreading ridge jump has also been proposed for the formation of the São 14 15 447 Paulo Plateau on the SE Brazilian margin by transferring onto the South American plate a 16 17 18 448 thickened piece of oceanic crust which was originally part of the African plate north of the 19 20 449 Namibia Ridge (Pérez-Diaz and Eagles, 2014; Fromm et al., 2015). Kumar and Gambôa 21 22 23 450 (1979) identified a N-S structure, between the Western and Central RGR in the Vema 24 25 451 channel, as possibly being an extinct spreading center. Constantino et al. (2017) proposed that 26 27 452 this feature extends northwards from 34ºS into the Florianópolis Ridge and called this feature 28 29 30 453 as the Vema Aborted Ridge (VAR).Figure 11 shows crustal thickness from gravity inversion 31 32 454 restored to 50 Ma, on which we identify past sea-floor spreading centres south of the Rio 33 34 35 455 Grande Fracture Zone between the Torres High, the Western, Central and Eastern components 36 37 456 of the RGR and the WR. The arrows in Figure 11 show distinct regions of oceanic crust 38 39 40 457 generated as a consequence of the ridge jumps. They strike at different angles because of the 41 42 458 extinct spreading centers original direction, the different velocities of the spreading rates and 43 44 45 459 the rotation of the macroplates (the clockwise rotation of South American plate relative to 46 47 460 Africa) and microplates, as indicated by curved fracture zones between the CRGR and ERGR. 48 49 50 461 In the region of Figure 4, we suggest that the sea-floor spreading was initially between 51 52 53 462 the Torres High and the Western RGR (location 1 on Figure 11) before jumping to location 2 54 55 463 in the Vema Channel. The jump from ridge position 1 to position 2 in the Vema Channel 56 57 58 464 would have separated the Western RGR from whatever lay to the east of it. This may have 59 60 465 been the Namibian margin; at the time of this ridge jump the Central and Eastern RGR and 61 62 63 64 65 20

466 WR may not have existed as we know them at present. Sea floor spreading resulting from this 1 2 467 ridge jump into the Vema Channel shows a strong asymmetry of accreted oceanic crust in the 3 4 5 468 spreading corridor occupied by Vema Channel. Pérez-Diaz and Eagles (2014) propose an 6 7 469 eastward migration of sea-floor spreading of 400 km for the jump from ridge position 1 to 2 8 9 10 470 which is consistent with what we see in Figures 4 and 11. The different shapes of the Central 11 12 471 and Eastern RGR and WR may also have been the result of ridge jumps as suggested by 13 14 472 Gamboa and Rabinowitz (1984). At approximately 80 Ma the Central RGR rifted and 15 16 17 473 separated from the WR (which then included the Eastern RGR). This required an eastward 18 19 474 migration or jump of sea-floor spreading to ridge position 3. By 70 Ma this was followed by 20 21 22 475 rifting and separation of the Eastern RGR from the WR and required an additional eastward 23 24 476 jump of rifting and sea-floor spreading to ridge position 4 from which it evolved to the 25 26 27 477 present day. It is noteworthy that the African side of the S. Atlantic is much simpler than the 28 29 478 S. American side, consistent with dominantly eastward migrations and jumps of the sea-floor 30 31 479 spreading axes. 32 33 34 35 480 We propose that these successive ocean ridge jumps and plate boundary re- 36 37 481 organizations were the main mechanism that formed the 3 separate components of the RGR 38 39 40 482 and the WR. In addition we suggest that these jumps in rifting and sea-floor spreading 41 42 483 isolated fragments of continental crust or lithospheric mantle from their original continental 43 44 45 484 land-mass locations and transported them large distances into the oceanic domain. This may 46 47 485 support the reports of continental material recovered by submersibles on the Central RGR. 48 49 486 The dominantly eastward migration and jumps of rifting and sea-floor spreading may imply 50 51 52 487 that continental fragments within the central S. Atlantic are likely to have an African affinity 53 54 488 even though they are now located on the S. American plate. 55 56 57 58 59 60 61 62 63 64 65 21

489 Some important questions remain unanswered. When and where exactly did those 1 2 490 ridges jumps take place? Were they simultaneously active? What is the generic relationship 3 4 5 491 between ridge jumps, magmatism and continental crust in the oceanic realm? 6 7 8 492 9 10 11 493 8.Summary 12 13 14 494 15 16 495 We observe anomalously thick crustal thickness predicted by gravity inversion for the 17 18 19 496 Western, Central and Eastern RGR and the WR. Both the Central RGR and the WR have 20 21 497 maximum crustal thicknesses of 25 km which are substantially greater than that of 22 23 498 surrounding S. Atlantic oceanic crust which has normal oceanic crustal thickness. The RGR 24 25 26 499 also has thicker crust compared with the São Paulo Plateau and Florianópolis Ridge on the 27 28 500 adjacent Brazilian continental margin. 29 30 31 32 501 There is a direct correlation between anomalously thick crust and high positive 33 34 502 amplitude magnetic anomalies, suggesting that igneous rocks are present within the RGR and 35 36 37 503 WR, which is already proven by DSDP drilling and coring. The anomalously large crustal 38 39 504 thicknesses under the RGR and WR are most likely the result of magmatic addition but they 40 41 505 may contain some continental material as shown by submersible sampling on the RGR. 42 43 44 506 Gravity inversion alone cannot distinguish between continental and oceanic crust, however no 45 46 507 significant magnetic anomaly indicating a large block of non-magnetic continental crust 47 48 49 508 seems to be present. 50 51 52 509 The plate reconstruction of the C34 magnetic anomaly at 80 Ma shows the location of 53 54 the South Atlantic divergent plate boundaries and ocean ridge. Restoration of crustal 55 510 56 57 511 thickness from gravity inversion to 80 Ma shows that the C34 magnetic anomaly intersects 58 59 512 the RGR and WR which at that time forming a single body. The reported magmatic ages of 60 61 62 63 64 65 22

513 the RGR and WR are consistent with this conclusion. The similar crustal thickness, high 1 2 514 amplitude magnetic anomalies and the plate restorations suggest a direct relationship between 3 4 5 515 RGR and WR and, therefore, a likely common origin. While at 80 Ma the RGR and WR form 6 7 516 a single body located on the divergent plate boundary similar to Iceland today, by 70 Ma they 8 9 10 517 are separated. 11 12 518 The Western, Central and Eastern RGR and the WR were separated by a series of 13 14 519 eastward migrations and jumps of rifting and sea-floor spreading. These rift jumps may have 15 16 17 520 isolated fragments of continental crust or lithospheric mantle from their original continental 18 19 521 locations and transported them large distances into the oceanic domain of the S. Atlantic. 20 21 22 522 Existing models of the formation of the RGR and WR during S. Atlantic Ocean 23 24 523 formation invoke ocean ridge – mantle plume interaction or anomalous ‘on-ridge’ magmatism 25 26 27 524 to generate large magmatic thicknesses during sea-floor spreading. This interaction alone 28 29 525 cannot however explain the reported discovery of Proterozoic material of continental affinity 30 31 526 on the Central RGR. If those are not drop-stones, we suggest that the isolation and transport 32 33 34 527 of fragments of continental crust or lithospheric mantle into the ocean domain by a series of 35 36 528 eastward ridge jumps during the formation of the Central South Atlantic also plays an 37 38 39 529 important role in the formation of the RGR and WR. 40 41 530 42 43 44 45 531 46 47 48 532 Acknowledgements 49 50 533 We would like to thank Prof. Renata Schmitt and Andres Gordon for the helpful comments 51 52 53 534 and MM4 for providing useful discussions. 54 55 535 56 57 58 536 59 60 537 References 61 62 63 64 65 23

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713 Torsvik, T.H., Amundsen, H.E.F., Trønnes, R.G., Doubrovine, P. V., Gaina, C., Kusznir, N.J., 1 2 714 Steinberger, B., Corfu, F., Ashwal, L.D., Griffin, W.L., Werner, S.C., Jamtveit, B., 2015. 3 4 5 715 Continental crust beneath southeast Iceland. Proc. Natl. Acad. Sci. 6 7 716 https://doi.org/10.1073/pnas.1423099112 8 9 10 717 Torsvik, T.H., Burke, K., Steinberger, B., Webb, S.J., Ashwal, L.D., 2010. Diamonds 11 12 718 sampled by plumes from the core–mantle boundary. Nature 466, 352–355. 13 14 719 https://doi.org/10.1038/nature09216 15 16 17 720 Ussami, N., Chaves, C.A.M., Marques, L.S., Ernesto, M., 2013. Origin of the Rio Grande 18 19 721 Rise–Walvis Ridge reviewed integrating palaeogeographic reconstruction, isotope 20 21 22 722 geochemistry and flexural modelling. Geol. Soc. London, Spec. Publ. 23 24 723 https://doi.org/10.1144/SP369.10 25 26 27 724 Wilson, J.T., 1965. Submarine fracture zones, aseismic ridges and the International Council of 28 29 725 Scientific Unions Line: Proposed Western Margin of the East Pacific Ridge. Nature. 30 31 726 https://doi.org/10.1038/207907a0 32 33 34 727 Wilson, J.T., 1963. Evidence from islands on the spreading of ocean floors. Nature. 35 36 728 https://doi.org/10.1038/197536a0 37 38 39 729 Figure Captions 40 41 730 42 43 44 731 Figure 1: Regional bathymetric (Amante & Eakins 2009) map, scale in meters, overlain by a 45 46 732 shaded-relief display of free-air gravity anomaly, showing the location of RGR and WR and 47 48 49 733 its Tristan-Gough volcanic track (Guyot Province) in the South Atlantic Ocean (SAO), as well 50 51 734 as South Atlantic fracture zones (AFFZ: Agulhas-Falkland Fracture Zone; RGFZ: Rio Grande 52 53 735 Fracture Zone; RFZ: Romanche Fracture Zone) dividing the SAO in Central, Austral and 54 55 56 736 Falkland segments. The Equatorial segment belongs to the Equatorial Atlantic. The Rio 57 58 59 60 61 62 63 64 65 31

737 Grande Rise and Walvis Ridge are located southern of the RGFZ. Paraná-Etendeka Magmatic 1 2 738 Provinces are shown in red in the South America and African continents. 3 4 5 6 739 Figure 2: RGR focused bathymetric (Amante & Eakins 2009) map, scale in meters overlain 7 8 740 by a shaded-relief display of free-air gravity anomaly, which shows the three units of the 9 10 741 RGR (Western, Central and Eastern RGR) and surrouding geologic features: Brazilian 11 12 13 742 sedimentary basins (PB: Pelotas Basin; SB: Santos Basin; CB: Campos Basin), São Paulo 14 15 743 Plateau, Florianópolis Ridge, Torres High, Jean Charcot Seamounts,Vema Channel, Ponta 16 17 18 744 Grossa Arch and Cruzeiro do Sul Lineament. RGFZ: Rio Grande Fracture Zone. (b) 3D 19 20 745 bathymetric (Amante & Eakins 2009) map of the RGR. 21 22 23 746 Figure 3: (a) Satellite free-air gravity anomaly map, scale in mgal (Sandwell & Smith 2009), 24 25 26 747 overlain by a shaded-relief display of itself and (b) magnetic anomaly map, scale in nT 27 28 748 (Meyer et al. 2016), overlain by shaded-relied display of free-air gravity anomaly. Magmatic 29 30 31 749 ages for the RGR, WR and Africa and South America continents are superimposed (ages from 32 33 750 Renne et al. 1996; Rohde et al. 2013; O’Connor et al. 2012, 2015; Geraldes et al. 2013; 34 35 36 751 Hoernle et al. 2015) The RGR shows ages between 80 – 87 Ma and 46 Ma. 37 38 39 752 Figure 4: (a) Map of crustal basement thickness derived from gravity inversion for the RGR, 40 41 753 scale in kilometers, with the seismic reflection lines path in black lines (A-B-C-D-E) and 42 43 44 754 yellow circles indicating the location of the seismic Moho depth measurements from 45 46 755 Constantino et al. (2017). (b) Crustal cross-section with Moho depth derived from gravity 47 48 49 756 inversion using sediment thickness from seismic interpretation and (c) crustal cross-section 50 51 757 with Moho depth from gravity inversion using public domain sediment thickness. The blue 52 53 54 758 line is bathymetry, the green line is top basement, the red line is the Moho calculated from 55 56 759 gravity inversion and yellow circles are the location of the Moho from seismic interpretation 57 58 59 60 61 62 63 64 65 32

760 from Constantino et al. (2017). (d) Cross-plot comparing gravity Moho depths with Moho 1 2 761 depths from seismic reflection (from Constantino et al. 2017). 3 4 5 6 762 Figure 5: Map of crustal basement thickness derived from gravity inversion with 7 8 763 superimposed shaded relief free-air gravity anomaly for the South Atlantic Ocean showing 9 10 764 anomalous thick crust for RGR and WR. Magmatic ages are are shown (ages from Renne et 11 12 13 765 al. 1996; Rohde et al. 2013; O’Connor et al. 2012, 2015; Geraldes et al. 2013; Hoernle et al. 14 15 766 2015). The RGR shows ages between 80 – 87 Ma and 46 Ma. The conjugate WR ages are 16 17 18 767 shown in the 87 – 107 Ma segment of WR. 19 20 21 768 Figure 6: (a) Map of crustal basement thickness for the RGR with scale in kilometers. Shaded 22 23 769 relief free-air gravity is superimposed. (b-d) Three crustal-scale cross-sections with Moho 24 25 26 770 depth extracted from the results of the gravity inversion. Cross-section locations are shown in 27 28 771 (a). CNS: Cretaceous Normal Superchron. 29 30 31 32 772 Figure 7: Plate reconstruction using GPlates 1.5 of magnetic anomalies and crustal thickness 33 34 773 from gravity inversion (with superimposed shaded relief free air gravity) for the S. Atlantic 35 36 37 774 for ages of 90, 80 and 70 Ma. The reconstructions at 80 Ma show magnetic anomaly C34 on 38 39 775 the divergent plate boundary and RGR and WR as a single body located at that divergent plate 40 41 776 boundary. 42 43 44 45 777 Figure 8: (a) Plate reconstructions using Gplates 1.5 of crustal thickness from gravity 46 47 778 inversion (with superimposed shaded relief free-air gravity) at 83 Ma showing RGR and WR 48 49 50 779 forming a single body located at the divergent plate boundary. (b) Crustal cross-sections with 51 52 780 Moho from gravity inversion and magnetic anomaly for a profile restored to 83 Ma running 53 54 across Western RGR, Central RGR and WR. 55 781 56 57 58 59 60 61 62 63 64 65 33

782 Figure 9: Plate reconstruction, using Gplates 1.5, of crustal thickness from gravity inversion 1 2 783 (with superimposed shaded relief free-air gravity) for the S. Atlantic from 70 to 40 Ma at 10 3 4 5 784 Myr intervals. 6 7 8 785 Figure 10: (a) Plate reconstruction, using Gplates 1.5, of crustal thickness from gravity 9 10 786 inversion (with shaded relief free-air gravity) at 54 Ma. (b) Crustal cross-sections with Moho 11 12 13 787 from gravity inversion along the axes of the Eastern RGR and WR. Line locations are shown 14 15 788 in (a). 16 17 18 19 789 Figure 11: Plate reconstruction, using GPlates 1.5, of crustal thickness from gravity inversion 20 21 790 (with superimposed shaded relief free-air gravity) at 50 Ma. Past sea-floor spreading axes are 22 23 791 shown and indicate successive eastward migration and jumps of sea-floor spreading ridge 24 25 26 792 axes which lead to the separation of the Western, Central and Eastern RGR and WR. 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 34

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