# Along-strike segmentation of the northeastern Brazilian margins controlled by geodynamics and lithospheric inheritance

Mikael Evain, Philippe Schnürle, Afonso Loureiro, Flora Gallais, Joao Marcelo Pinheiro, Alexandra Afilhado, Frauke Klingelhoefer, Nuno Dias, José Eduardo Soares, Reinhardt Fuck, et al.

► To cite this version:

Mikael Evain, Philippe Schnürle, Afonso Loureiro, Flora Gallais, Joao Marcelo Pinheiro, et al.  
Along-strike segmentation of the northeastern Brazilian margins controlled by geodynamics and  
lithospheric inheritance. Journal of South American Earth Sciences, 2023, 128, 104479 (20p.).  
10.1016/j.jsames.2023.104479. hal-04204118

HAL Id: hal-04204118

https://hal.science/hal-04204118v1

Submitted on 16 Sep 2025

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<figure><img src="image_1.png" /></figure>
Along-strike segmentation of the northeastern Brazilian margins controlled by geodynamics and lithospheric inheritance

Mikael Evain<fnref n="1" />, Philippe Schnürle<fnref n="1" />, Afonso Loureiro<fnref n="2" />,<fnref n="3" />,<fnref n="4" />, Flora Gallais<fnref n="1" />, Joao Pinheiro<fnref n="1" />, Alexandra Afilhado<fnref n="2" />,<fnref n="3" />, Frauke Klingelhoefer<fnref n="1" />, Nuno Dias<fnref n="2" />,<fnref n="3" />, José Eduardo Soares<fnref n="5" />, Reinhardt Fuck<fnref n="5" />, J.A Cupertino<fnref n="6" />, Adriano Viana<fnref n="6" />, Daniel Aslanian<fnref n="1" />, and Maryline Moulin<fnref n="1" />

**Abstract**

Passive continental margins depict a wide range of crustal architectures. Such observation mainly come from wide-angle seismic that has proven to be a valuable tool to constrain margins' onshore and offshore crustal structures. Yet, it is still unclear what are the main drivers of such diversity and whether some more general processes do also control their formation. Here, by analyzing coincident marine seismic reflection and refraction data acquired parallel to the coast of northeast Brazil we shed a unique view on the along-strike segmentation of its passive margins. Our two P-wave velocity models add up to 10 other wide-angle velocity models build in the scope of the SALSA (Sergipe ALagoas Seismic Acquisition) project to provide a coherent picture of the crustal and upper mantle architecture of the offshore domains of the Jequitinhonha, Almada, Camamu, Jacuípe, Sergipe and Alagoas margins. In the Camamu-Jacuípe segment, both crustal necking and the continent to ocean transition were shown to be sharp. Along strike, our models show extremely thin crust above a thick anomalous velocity layer and a relatively low density mantle. This architecture is interpreted as made of collapsed continental blocks above either highly intruded lower continental crust and/or serpentinized mantle that is locally exhumed. It greatly differs from the crustal structure imaged to the south, within the Jequitinhonha and Almada segments, where our models support the existence of a vast and homogeneous domain of exhumed lower continental crust. It also considerably contrasts with the offshore architecture of the Sergipe-Alagoas segment to the north. There, the necking and transition to normal oceanic crust was shown to be narrow but gradual and dominated by middle to lower continental crust. On our along-strike models, it is characterized by a thicker but also highly intruded continental crust. Crustal architecture evolves brutally at the frontiers of these segments which coincides precisely with the Camamu Triple Junction and the offshore prolongation of the Vaza Barris fault zone. The former is one of the major geodynamic features in the development of the Central Segment of the South-Atlantic Ocean while the later marks the suture between the São-Francisco Craton and the northeastern Brazilian Orogenic Belt. Therefore, we clearly evidence the first-order control of both geodynamics and lithospheric inheritance on the along-strike segmentation of the northeastern Brazilian passive margins. Our results emphasize the crucial role of plate kinematics and lithospheric strength in shaping their architectures and the exhumation of middle/lower continental crust as a more general process of their formation.

<footnote n="1">¹Geo-Ocean, Univ Brest, CNRS, Ifremer, UMR6538, F-29280 Plouzane, France</footnote>

<footnote n="2">²Instituto Politécnico de Lisboa, Instituto Superior de Engenharia de Lisboa, Lisboa, Portugal</footnote>

<footnote n="3">³Universidade de Lisboa, Faculdade de Ciências, Instituto Dom Luiz, Lisboa, Portugal</footnote>

<footnote n="4">⁴Agência Regional para o Desenvolvimento da Investigação, Tecnologia e Inovação, Funchal, Portugal</footnote>

<footnote n="5">⁵Instituto de Geociências - Lablithos, Universidade de Brasília, Campus Darcy Ribeiro, 70910-900, Brasília, Brasil</footnote>

<footnote n="6">⁶Petrobras, Cenpes Research Center, Rio de Janeiro, Brazil</footnote>
# 1-Introduction

The wide range of architectures, crustal nature and evolution observed along passive continental margins question the existence of a general crustal thinning process in their formation. Such diversity may be linked to the tectonic heritage, the lithospheric thermal structure and/or a peculiar geodynamic context. Seismic imaging of margins, from the continent to the ocean, but also their along-strike segmentation, is crucial as it is the most valuable tool to provide accurate crustal structure that can help in understanding the influence of each of these parameters.

Northeast Brazil is particularly suitable to study rift's and continental margins' segmentation. The region presents both onshore aborted rifts and fully developed continental margins that result from the opening of the Central Segment of the South-Atlantic Ocean in Lower Cretaceous (Chang et al., 1992; Matos, 1992; Moulin et al., 2010) (Figure 1a-b). The diversity of architectures and strong segmentation of the Recôncavo-Tucano-Jatobá (RTJ) aborted rifts system as well as the onshore portions of the Sergipe-Alagoas and Pernambuco marginal basins were intensely studied. Most studies have concluded that it is controlled by lithospheric inheritance (Matos, 1992; Milani & Davison, 1988; Vasconcelos et al., 2019). Offshore, well defined positive and negative anomalies on gravimetric and magnetic maps are observed both off the coast of Brazil and along its conjugate margins of West-Africa (Figure 1b-c). They are also suspected to reflect variations in margins' architecture, especially the width and magmatic content of their continent-ocean transitional crust (Blaich et al., 2008, 2010; Mohriak et al., 1998, 2000). These changes appear to be spatially correlated with major onshore-offshore transfer systems. A link between continental and oceanic lithospheric discontinuities was also further evidenced recently (Vasconcelos et al., 2019). Taken all together, past studies in the region show strong evidence that lithospheric inheritance, including its structural discontinuities and strength, exerted a strong control on the formation and segmentation of both internal rifts and margins' offshore distal domains (Blaich et al., 2010, 2008; Matos et al., 2021; Watts & Stewart, 1998).

Until recently, however, constraints on the deep offshore crustal structure of northeastern Brazilian passive margins has been limited to gravity modeling and interpretation of sparse seismic data (Blaich et al., 2008, 2010; Mohriak et al., 1998, 2000). Deep penetration seismic was thus required to finally provide a complete onshore-offshore picture of the crustal structure and fully assess the parameters controlling rifts' architecture and their segmentation. The SALSA (Sergipe ALagoas Seismic Acquisition) experiment conducted in 2014 onboard the French RV L'Atalante fulfilled this goal with an exceptional seismic dataset imaging the offshore part of each segment. Among the twelve coincident reflection and wide-angle seismic profiles acquired (Figure 1a), more than 1000 km were also acquired in a direction parallel to the coast in order to precisely map and characterize the along strike structural segmentation. This study presents the P-wave velocity modeling of two of these profiles, complementing previously published models (Loureiro et al., 2018; Pinheiro et al., 2018) and results from companion papers presented in this special issue (Aslanian et al., this issue; Loureiro et al., this issue). Our study reveals unambiguously how crustal architecture changes brutally along NE Brazilian margins. By localizing precisely this segmentation it shows how the geodynamic context and lithospheric inheritance were the main driver of these changes.

# 2-Geological background

The northeastern Brazilian margins result from the opening of the Central Segment of the South- Atlantic Ocean (Chang et al., 1992; Matos, 1992, 1999; Milani & Davison, 1988; Moulin et al., 2010). Rifting initiated in Neocomian along a north-south trend marked by the RTJ onshore rift
system and the Jequitinhonha, Almada, Camamu, Jacuípe, Sergipe and Alagoas basins (Figure 1a). During Aptian times, rifting aborted in the RTJ and break-up occurred between northeast Brazilian margins and Gabon west African margins (Chaboureau et al., 2013; Magnavita & Cupertino, 1988; Milani & Davison, 1988). These extensional domains defined the Tucano Microplate which rotated counterclockwise with respect to the South-American and African plates during this period (Milani & Davison, 1988; Szatmari & Milani, 1999). This geodynamic was strongly controlled by the reactivation of the Precambrian Pernambuco shear zone to the north and the Camamu triple-junction (CTJ) at the southern tip of the micro-plate, offshore Salvador (Mohriak et al., 2000).

In the region, Cretaceous extension disrupted both cratonic cores and orogenic belts that were amalgamated during Brasiliano/Pan African orogenies (Almeida et al., 2000; Brito Neves et al., 2014). To the south of our study area, the Brazilian basement is composed of Archaean and Palaeoproterozoic metamorphic rocks (Itabuna-Salvador-Curaçá Belt or Itabunas Belt) that belong to the southern São Francisco Craton (SFC) (Barbosa & Sabaté, 2004; Mascarenhas et al., 1984). At the time of amalgamation the SFC was separated from the Congo Craton by a long band of the Neoproterozoic belt referred as the Mantiqueira system (Matos et al., 2021). To the north, the RTJ rift system partly cuts through the SFC, with partial outcrops of Archean basement (Aporá and Salvador-Jacuípe horsts) bounding the Recôncavo basin (Figure 1a). But the RTJ further extends northward through the Proterozoic metamorphic rocks of the Sergipano Fold Belt (SFB) and the Mesoproterozoic to Neoproterozoic granite gneisses of the Pernambuco-Alagoas Massif (PAM). These two units compose the southern Borborema Province, a large Pan-African-Brasiliano orogenic assemblage at the NE tip of Brazil (Santos et al., 1984; van Schmus et al., 2008). Together with its conjugate in West-Africa, the Benin-Nigeria Province, they define an heterogeneous ensemble remarkable by the large continental shear zones they host. Such inheritance is though to have strongly influence the architecture of Brazilian margins (Matos et al., 2021).

Numerous studies have indeed highlighted the control and reactivation of pre-existing basement fabrics and lithospheric discontinuities on NE Brazilian rifts localization, structure and geometry. To the south of our studied area, the Camamu-Almada-Jequitinhonha basins have NNE-SSW basement fabrics directly inherited from deformation trends of the SFC Itabunas Belt (Barbosa & Sabaté, 2004; Ferreira et al., 2015; Gordon et al., 2013). The triple junction within the Camamu segment also coincides with a point where two perpendicular trends of foliations exist within the SFC granulite belt (Blaich et al., 2008; Mascarenhas et al., 1984; Milani & Davison, 1988). Northward, the RTJ rift geometry was controlled by a network of NE-trending normal fault that followed most of the foliations within the northern SFC basement. The rift system also exhibits a segmentation clearly controlled by NW-trending transfer faults reactivated from Precambrian transcurrent shear zones (Chang et al., 1992; Matos, 1992; Milani & Davison, 1988). The main depocenters are bounded by such features. Among them, the NE Itaporanga Transfer Zone or Vaza-Barris Fault Zone (VBFZ, Figure 1a), which localizes the transition between the SFC and SFB, marks a flip in geometry between the Central and South Tucano basins (Milani & Davison, 1988). Further north, in the PAM, the Jatobá basin developed along the already mentioned NE trending Pernambuco lineament, which acted as a barrier to the northward propagation of the RTJ rift system (Milani & Davison, 1988; Neves & Mariano, 1999; Szatmari & Milani, 1999). Finally, recent investigations of this network of Precambrian inherited structures showed how they were reactivated as normal faults controlling the rift architecture of the Sergipe-Alagoas basins, at least for its subaerial and near shore parts (Vasconcelos et al., 2019). Extending their analysis of potential field and seismic data offshore, Vasconcelos et al. (2019) further showed how they directly influenced the localization of transfer and fracture zones during break-up and early seafloor spreading.
Overall, these studies have shown that the geodynamic of rifting was purely extensional south of Camamu triple-junction while it was oblique in the RTJ rifts and with an important strike-slip component along the JSA segments (Mohriak et al., 2000). The influence of tectonic heritage on the architecture and segmentation of the deepest and distal parts of these margins is, however, less understood mainly due to the lack of deep offshore seismic data capable of revealing their distal crustal structure. By acquiring coincident reflection and wide-angle seismic profiles parallel to the coast, the SALSA experiment provided a complete picture of the crustal architecture along-strike the NE Brazilian margins.

## 3-Seismic dataset

### 3.1- Acquisition

The SALSA project was a joint initiative between the Department of Marine Geosciences at IFREMER (Institut Français de Recherche pour l'Exploitation de la MER, France), the 'Geoscience Ocean' Laboratory at IUEM (Institut Universitaire et Européen de la Mer, France), the 'Faculdade de Ciências' at the University of Lisbon (IDL, Portugal), the University of Brasília (UnB, Brazil) and PETROBRAS (Brazil). The acquisition geometry was employed by our group at numerous occasion in the past and has proven to be a valuable strategy to image the whole crustal velocity structure of passive continental margins (e.g. Afilhado et al., 2015; Evain et al., 2015; Biari et al., 2015; Moulin et al., 2020 Aslanian et al., 2021).

This contribution presents two deep seismic profiles that run broadly parallel to the NE Brazilian (Figure 1a). Profiles SL05 and SL06 are respectively 420 km and 460 km long profiles.

Respectively thirty and thirty-three 4-components Ocean Bottom Seismometers (OBS) of Ifremer's MicrOBS pool (Auffret et al., 2004) were deployed along these two lines at an interval of 7 nautical miles. The OBS recorded together with a 4.5 km, 360 channel towed marine streamer, 2539 (SL05) and 2620 (SL06) shots from a 6550 in³ air-gun array source. Combined together SL05 and SL06 strike through the Almada, Camamu, Jacuípe and Sergipe-Alagoas margin's segments. The two profiles crosses at the location where the VBFZ extends offshore. Finally the northeast extremity of SL05 runs over the Ascension Fracture Zone (AFZ). SL05 and SL06 are perpendicular to the other profiles of the SALSA experiment that targeted specific segments : SL01 and SL02 were acquired in the Sergipe-Alagoas margin (Pinheiro et al., 2018), SL03 and SL04 over the AFZ (P. Schnurle, pers. com), SL07 and SL08 in the Jacuípe margin (Aslanian et al., this issue) SL09 strike through the Camamu and distal Almada margins (Loureiro et al., this issue) and finally, SL10, SL11 and SL12 were dedicated to the Jequitinhoha-Almada margins (Loureiro et al., 2018). SL05 and SL06 are also almost coincident with a portion of industrial ION-GXT900 multi-channel seismic profile (Figure 1a) from which a line-drawing was made and will be compared with our results (section 5.1).

### 3.2- Multi-channel seismic data

SL05 and SL06 MCS data were processed with the Ifremer's SolidQC and CGG-Veritas' Geocluster softwares (Figure 2). The processing sequence was composed of: geometry, editing and water column mute, wide bandpass filtering (2-8-64-92 Hz), wave-equation multiple attenuation, shot-gather predictive deconvolution, time variant band-pass filter, random multiple attenuation. Velocity picking and normal move-out correction preceded CMP stack and a post-stack FK time migration.

### 3.3-Wide-angle seismic data
A pre-processing sequence is applied on seismic data once downloaded from the OBS. This includes internal clock-drift correction, SEG-Y file generation based on navigation and shots files and estimation of OBS position on the seafloor from travel-time picks of water-wave direct arrivals. Band-pass filtering (1-6-48-64 Hz) or upward and downward traveling waves separation are eventually applied in order to facilitate the analysis OBS gathers. By combining hydrophone and vertical seismometer OBS components a spiking predictive deconvolution can indeed be applied to the upward traveling record using the downward traveling wave as signature (Schneider & Backus, 1964). This is followed by spherical divergence correction to compensate amplitude decay of the records and traces scaling with a gain proportional to the offset in order to enhance the refracted events.

# 4-Velocity structure modeling

## 4.1-Strategy

We aim at building a P-wave velocity model for each profile that explain travel times of reflected and refracted phases interpreted on OBS data and coherent with the crustal structure that is observed on the MCS profiles.

An initial model is first built along the great circle that best fit the MCS navigation. OBS positions are then projected along this circle and the zero distance is chosen at the projected position of the southernmost instrument of each profile. Depths to the seafloor are taken from the acquired bathymetric data which are adjusted in case of large discrepancies with depth at OBS relocated position.

For seismic phase identification and travel time picking, we used the interactive plotting ZPLOT package (available at http://terra.rice.edu/department/faculty/zelt/rayinvr.html). We worked by increasing reduction time of record sections, in order to better identify every seismic phases and determine its apparent velocity.

Picked travel times of these phases are modeled using an iterative procedure of two-dimensional forward ray-tracing followed by a damped least-squares travel-time inversion using the RAYINVR software (Zelt & Smith, 1992; Zelt, 1999). The top-to-bottom modeling strategy adopted consists of fitting reflected and refracted phases in the sedimentary layers first then adjusting velocity and depth of crustal and mantle layers. Sedimentary, basement, and deeper interfaces if possible, are further constrained by MCS data by converting the velocity model to two-way travel time, superimposing the model to the MCS profile in order to ensure the best consistency of layers with the major horizons observed on the MCS. Phase amplitude modeling is further used to qualitatively check the coherence of velocity gradients and velocity contrasts at each interfaces of the model (See supplementary material S1).

Some deep mantle reflectivity are observed on OBS records suggesting a number of highly reflective interfaces. Part of this signal, consistent over the entire studied area, helped in the modeling of extra layer in the mantle. The remaining travel times correspond to very deep wide-angle reflected phases and were explained using floating reflectors within the mantle.

Finally it should be mentioned that it greatly improves confidence in the analysis of OBS sections and in the progress made during the modeling to be confronted to the analyses made by other modelers on OBS section collected along crossing profiles. SL05 and SL06 crosses respectively four and five profiles from the SALSA experiment and those have been processed by a different person. By regularly checking 1D velocity-depth structure modeled at these crossing points we carefully monitor everyone progress and ensured consistency in our analysis and in the modeling of the entire SALSA dataset.
## 4.2 MCS data analysis

The analysis on MCS data reveal that seismic facies and structural characteristics strongly evolve along the SL06 (Figure 2a) and SL05 (Figure 2b) profiles, both within the sediments and the basement. To the SW extremity of SL06, continuous but undulating sedimentary and basement horizons are dipping over the Almada margin. The sedimentary packages is about 2 s twt thick with evidence of a transparent body that possible results from salt deposit. In Camamu margin it consists of a homogeneous and strongly stratified sedimentary package about 3 s twt thick. While reflectors from upper sedimentary layers can be discontinuous, either cut by palaeo-valleys or possibly hidden by the presence of fluids, deep horizons are very clear, with strong amplitude and strike through the entire basin. The sedimentary package thins drastically as both the SL06 (200 to 360 km model distance) and the SW extremity of SL05 (-20 to 50 km model distance in figure 2b) enter the Jacuípe basin. Sediments are 1.5-2.0 s twt thick and resemble the upper sediments of the Camamu basin with wavy or short lived reflectors although the seafloor is cut by numerous valleys. At depth, the Jacuípe basin is marked by a continuous and prominent horizon at about 5 s twt that either covers basement highs or caps a 1 s twt thick layer of chaotic and incoherent reflections. At the NE extremity of SL06 and from 50 km to 250 km model distance on SL05 the Sergipe-Alagoas basin is imaged bounded by the VBFZ and AFZ. It is characterized by a 2.5 s twt thick and well stratified sedimentary package. However, the seafloor is rough, cut by numerous deep valleys, and internal horizons show strong wavy patterns. The last 150 km of SL05 evidence a 1 s twt thick but restricted volcano-sedimentary basin that sits between basement highs and volcanic edifices of the AFZ.
Regarding the top of the acoustic basement, on both SL05 and SL06 it is generally not defined by a clear reflector but rather by a contrast in seismic facies from well defined horizons to a more chaotic, short-lived reflections or a transparent facies. In places sedimentary horizons show clear onlaps on basement topographic highs. In the Camamu basin, the basement surface shows strong topography marked by numerous highs and lows, about 10 km wide and half a second high. A major rise (1 s twt) occurs approximately at the transition with the Jacuípe basin (200-240 model distance on SL06) which is also characterized by highs and lows but of much longer wavelength. Finally the basement surface in the Sergipe-Alagoas segment is less clear but appears quite flat around 6s twt. Both SL06 and SL05 show a crystalline crust with a transparent facies. Sparse and limited internal reflections mostly seen within the Camamu and the Sergipe-Alagoas segment contrast with the strong and continuous reflective band that strikes from the Jacuípe basin and continues northeastward (starting at ~200 km model distance on SL06 – figure 2a). This set of reflectors appears segmented in places (e.g. 270 and 320 km on SL06; 120 and 240 km on SL05 – figure 2b) and constantly deepens from 6.5 s. to a maximum of 10 s twt in the Sergipe-Alagoas basin. Deep reflectivity is also evident on ION-GXT-900 profile that is further discussed in section 5.1.

## 4.3 Wide-angle seismic analysis and modeling

Here we proceed with an overview of refracted and wide-angle reflected phases identified on OBS records and how those were used to progressively build our velocity models. We make references to selected key OBS sections. For clarity two representative sections were selected for each profiles based on their location. SL06OBS27 (Figure 3) and SL06OBS13 (Figure 4) are located respectively at the transition of the Almada-Camamu and Jacuípe-Sergipe basins. SL05OBS25 (Figure 5) and SL05OBS08 (Figure 6) are located to the south and north of the Sergipe-Alagoas segment, the first above the VBFZ and the second just to the SW of the AFZ. All the remaining OBS records are provided in supplementary materials (Fig S1 for SL05OBS and S2 for SL06OBS). They are all accompanied by a synthetic section, travel times picks and fit, ray-tracing through the final model and coincident portion of MCS profile.

The main sedimentary sequences and interfaces observed along the MCS profiles SL05 and SL06 are consistent with the main sets of refracted and reflected sedimentary phases visible on OBS sections. This allows us to model six and seven sedimentary layers (S1 to S7) for SL05 and SL06
respectively (Figure 2). Refracted phases (Ps1 to Ps7) consist mostly of secondary arrivals with associated reflected phases visible within the water-cone of OBS sections. Layer S1 is poorly constrained but has apparent velocity lower than 2.0 km/s in some places and allows for the time delay to fit Ps2 arrivals times. Refracted sedimentary phases coming out of the water-cone can be grouped in three main sets (e.g. SL06OBS32, SL06OBS27-Figure 3). Ps2 to Ps4 compose the first group with apparent velocities that do not exceed 3.0-3.5 km/s (e.g. SL05OBS25-Figure 5), then Ps5 and Ps6 show arrivals with apparent velocity between 3.5 and 4.5 km/s (e.g. SL05OBS25-Figure 5), and finally Ps7 has an apparent velocity around 5.0 km/s (e.g. SL06OBS27-Figure 3) and can eventually be characterized by first arrivals within offsets of 10 to 15 km outside the water-cone (e.g. SL05OBS22). Ps2P to Ps4P are usually well-defined reflected phases within the water-cone and Ps6P or Ps7P are on most of OBS records the latest reflected arrivals seen within the water-cone. Velocity characteristics for each sedimentary layer are usually quite homogeneous with only mild internal variations within a particular basin. At the locations of major basement highs that mark the transitions between these basins, however, a number of layers are absent and some interfaces pinch out to take this into account. If seven sedimentary phases have been modeled for the sedimentary package in the Camamu basin, six have been identified for the Sergipe-Alagoas basins while only five characterize the Jacuípe basin.

Wide-angle data further constrain the acoustic basement, crystalline crust and upper mantle velocity structure along SL05 and SL06. For each profiles, we modeled one basement layer (B), two crustal layers (G1-G2) and two layers for the mantle (M1 an M2). Pb refracted phase is either visible as primary or secondary arrivals following the trend of Ps7 and up to about 30 km offsets. Both refracted phases have very close apparent velocities around 5.0 km/s and velocity inversion are suggested in places by the presence of shadow zones on OBS sections (e.g. SL05OBS14). PbP, if it is observed, has very faint amplitude compared to Pg1P. The latter is indeed a clear wide-angle reflected phase marking an upper crustal interface that invisible on MCS profiles. The interface is associated with a major velocity jump since both Pg1 and Pg2 emerge on section with apparent velocities higher than at 6.0 km/s. In the Camamu basin there is a progressive increase from 6.0 km/s to 7.5 km/s in the apparent velocity of these two refracted phases toward the northeast.

Moreover, reflective phases (Pg1P and Pg2P) become less evident while Pn1 refracted phase in the upper mantle becomes increasingly strong over long offsets (compare SL06OBS27-Figure 3 with SL06OBS17). Note that, at the transition with the Jacuípe basin, the Pn1 phase emerges straight after the Pg1P with an apparent velocity slightly lower than 8 km/s (SL06OBS16 and SL06OBS17). In the Jacuípe basin itself OBS records are dominated by long offsets Pg1 refracted phase (no Pg2 are required) with apparent velocity of 6.5 km/s on average, strong pre-critical PmP reflection and a Pn with an apparent velocity of 8.0 km/s (e.g. SL06OBS13-Figure 4, SL06OBS01). On some records, strong amplitude arrivals at offsets up to 250 km suggest the presence of a very deep reflector within the mantle along SL06 (e.g. SL06OBS07). In the Sergipe-Alagoas basin, the internal structure and geometry of the basement is well constrained with clear reflection and refraction from layer G1 and G2. Except at the basin's borders where they are strongly affected by the basement's topography (e.g. SL05OBS25-Figure 5) Pg1 and Pg2 suggest again a strong velocity jump with apparent velocity exceeding 6 km/s and combine both primary and secondary arrivals over a wide range of offsets. Their apparent velocities increase to 7.0 km/s in the central part of the basin (e.g. SL05OBS22, SL05OBS14). Those decrease again in the AFZ area where they never not exceed 6.75 km/s (e.g. SL05OBS08-Figure 6, SL05OBS02). Strong amplitude and long offsets pre-critical reflected phases (Pg1P, Pg2P, PmP) reveal a well structured basement and a clear Moho. Similarly, along SL05, many OBS sections show Pn1 phases of apparent velocities close to 8.0 km/s that are associated with strong amplitude and long offsets reflected phases within the mantle (e.g. SL05OBS14).
### 4.4-Final models evaluation

SL05 and SL06 final velocity models are shown respectively in figure 7. A total of 50787 and 47950 arrival times respectively were picked and assigned to a particular reflected or refracted phases. Travel-time uncertainty was manually set for each phase ranging from 25 ms for Pw increasing with lower signal to noise ratio up to 150 ms for Pn1. The two final models have acceptable statistics as they explain respectively 92% (46766) and 90% (43048) of the travel times picked with a global RMS travel-time of 90 ms and 77 ms. Given the uncertainties assigned for each phase a normalized $\chi^2$ of 0.758 for SL05 and 0.662 for SL06 are obtained, only meaning uncertainties have been slightly overestimated. Travel-time RMS and $\chi^2$ values for each phase are synthesized for SL05 in Tables 1a and for SL06 in Table 1b.

Besides travel times fit, the spacing of interface depth nodes as well as velocity nodes is key to model lateral variations of the velocity structure with sufficient resolution, but without introducing complexity not required by the data. In figures 8 and 9 we present four parameters to assess the quality of SL05 and SL06 final models respectively. Top panel (a) represents the node parameterization of the final velocity models where the interface between layers that are constrained by reflected phases are highlighted in blue. All velocity and depth nodes in the sediments, basement and mantle have been evaluated, although those defining depth and velocity of sedimentary layers benefited from additional constraints from the MCS data. Ray hit count (b); spreading factors (c) and nodes resolution (d) are also provided to evaluate models' quality. In general, model parameters are either well resolved in the basement or of acceptable resolution in sediments and smearing is limited. The quality slightly degrades toward the edges of the model and where strong basement topography exists, but still provides confidence to the modeled values. Most interfaces in the model also show reflective segments except the top basement where the velocity contrast with the overlying sediment is very limited which prevent any significant impedance contrasts. Hit-counts decrease with depth but also at model's edges and basin boundaries due to geological complexities while smearing shows a reversed pattern. As a consequence, our final velocity models are well resolved for sediments, basement and mantle within the Camamu and Sergipe-Alagoas basins. Resolution decreases but remains within acceptable range for the Jacuípe and AFZ areas which show more topographic complexities. Mantle deepest layers shows good resolution since very few parameters define them and no major lateral variations are required. Finally, supplementary figure S3 and S4 summarizes 1-D velocity-depth profiles at each crossing points between SL05 and SL06 and other SALSA profiles. Overall, interface depths and layer velocities from the top sediments to the basement (highlighted in light blue) are very similar giving good confidence in the final modeled structures. The main input is to emphasize discrepancies in upper mantle velocities modeled on SL05 and SL06 versus anomalously high velocity zones suggested on other profiles. Deep, long-offsets refracted phases recorded by SL05 and SL06 OBS are best explained as Pn implying a sharp velocity step at the Moho and normal mantle velocities around 8 km/s. However, SL09 model suggests slightly lower than normal mantle velocities below the Moho (Loureiro et al., this issue) while anomalously high velocities (varying between 7.0 and 7.7 km/s) layers have been modeled along SL01/SL02 (Pinheiro et al., 2018), SL03/SL04 (Schnurle, pers. com.) and SL07/SL08 (Aslanian et al., this issue). In section 5.2 below we will show that gravity modeling requires lateral density variations highlighting changes in the nature of layer M1 along profiles SL05 and SL06.

## 5-Final models assessment

### 5.1-Pre-stack depth migration and comparison with ION-GXT line drawings

Pre-stack depth migration of the MCS data and residual move-out analysis allow by combining near-vertical and wide-angle seismic data sets to produce a seismic section of MCS data in depth
but also verify the accuracy of velocities in final models. The processing sequence was carried out using the Seismic Unix geophysical package (SU) (Cohen & Stockwell, 2019; Stockwell, 1999) and consists of two steps: 1) ray tracing and seismic data depth migration. The velocity model is used to compute travel-time tables regularly spaced at 200 m along the profile by paraxial ray tracing on a 50 × 24 m spaced grid, then travel-times in shadow zones are compensated by solving the eikonal equation; 2) common offset Kirchhoff depth migration is performed and migrated traces are outputted as common image gathers (CIG) binned at 25 m with 45 offset-classes between 227 and 4627 m spacing at 150 m.

We present SL05 and SL06 PSDM sections (upper profiles in figure 10 A and B) and compare them with line drawings from their coincident industrial MCS profile ION-GXT900 (lower profiles in figure 10, see also Figure 1 for localisation). We also present receiver gathers of residual move-out of MCS data in supplementary materials for both SL05 (figure S5) and SL06 (figure S6) profiles. On those later figures, major reflective events appear flat which confirm good constraints on the velocity structure within the sedimentary basins. The PSDM allow to finely image reflective events with their true geometry. All three basins in our study area show similar sedimentary structure that can be subdivided between an upper and lower package. Upper sediments are finely stratified and made of medium amplitude discontinuous reflectors while deeper reflectors have a more continuous and stronger amplitude character. This is also what is observed from our line drawings of industrial the ION-GXT profile where coherent reflectors concentrate essentially in the deepest parts of the sedimentary basins (fine yellow lines in figure 10 7). The internal basement structure is almost entirely transparent on PSDM sections. Only sparse and shot-lived strong reflections are visible which are difficult to correlate. Similarly, the ION-GXT profile reveals some but only sparse internal reflections within the basement, although they are more numerous within the Camamu basement. There is no clear evidence for a Moho horizon beneath this basin and the deepest reflectors appear to shallow from 17 km to 12 km deep from SW to NE. On the other hand, at the bottom of the Jacuípe, Sergipe-Alagoas and AFZ crust a set of strong amplitude, compact and continuous horizons is present. These reflectors are difficult to observe on our PSDM sections (figure 10) but are consistent with SL05 and SL06 MCS time sections (Figure 2). They are coherent with the Moho interface modeled from wide-angle seismic although they do not perfectly fit with the deep reflectivity of our line drawing. In the Camamu basin, there is a good match between the modeled Moho interface and the bottom of basement reflectivity. There is a good fit in the Sergipe-Alagoas basins and below the AFZ. In the Jacuípe basin, between 220 and 330 km, our modeled Moho is a few kilometers above the set of strong reflectors. Differences may come from the ION-GXT profile running closer to the coast while rapid variation of the crustal structure occurs across continental margin. Such discrepancies also remind our observations in the Santos basin further south the coast of Brazil (Evain et al., 2015) or in the Equatorial Brazilian margin (Aslanian et al., 2021) and may derive from the estimation of crustal velocities that are made from two different seismic approaches.

## 5.2-Gravity modeling

We performed gravity modeling to evaluate the consistency of our final velocity models with ship measured gravity anomaly and satellite derived gravity grid (Sandwell et al., 2014). A 2-D model consisting of homogeneous density blocks was constructed from SL05 and SL06 final models (figure 11) by converting P-wave velocities to densities according to (Ludwig et al., 1970) empirical function. The model was extended down to 100 km where isostatic compensation may be reached, and 500 km laterally on each sides to avoid edge effects. Resulting densities range from 1800 kg/m³ to 2500 kg/m³ in the sedimentary units, 2600 kg/m³ to 2900 kg/m³ in the basement and crust and 3300 kg/m³ in the mantle layer. However, applying such simple conversion leads to large discrepancies between calculated and observed gravity anomalies. Based on velocity structures and densities modeled along crossing SALSA profiles we adapted densities in the deepest crustal layer
G2 and both densities and the thickness of the upper mantle M1 as shown in figure 11. Increasing densities to a maximum 3000-3100 kg/m³ for layer G2 in the Camamu and Sergipe-Alagoas basins and lowering densities below 3200 kg/m³ in layer M1 while fixing its depth at 23 km lead to an acceptable fit. Differences are still present between calculated and observed gravity anomalies but given the highly non-cylindrical structures crossed by SL05/SL06 profiles, we deemed our fit acceptable if it is within the range of anomaly curves for profiles that are within 30 km apart. Moreover calculated lithospheric load anomalies appear well balanced and within the acceptable range (less than 200 Bar) for a crust in isostatic equilibrium (Whitmarsh et al., 1996). Gravity modeling made for SL05 and SL06 imply that our modeling of the mantle structure solely based on seismic travel-times overestimated velocities along most of layer M1 and were not able to properly constrain changes in its nature or its transitions with anomalously high velocity body at the bottom of the crust. Quantitatively investigating such issue is out of scope of this article, however, we can think of at least two main causes. A first one could be related to out of plane propagation of deep refracted waves that tend to be guided through the highest velocity structures that is normal mantle that is slightly to the east of our profiles given the sharp changes in crustal structure of these margins. A second possibility could be related to the geodynamic processes that shaped these margins (see discussion below) with a phase strike-slip plate motion that may have align mantle fabrics in a direction parallel to the coast leading to seismic anisotropy with a fast axis of propagation close to the orientation of SL05/SL06 profiles.

## 6-Crustal structure evolution along SL05/SL06

SL05 and SL06 final velocity models (figure 7), their density structure (figure 11) as well as PSDM sections (figure 10) present the crustal and upper mantle structure beneath the Camamu, Jacuípe and Sergipe-Alagoas basins to a depth of about 20-25 km although several strong amplitude wide-angle reflections originating from deeper mantle interfaces have been identified on several OBS records. Below, the along-strike variations in crustal architecture observed along SL05 and SL06 profiles is summarized.

### 6.1-Sedimentary structure

The overall sedimentary architecture evolves greatly along SL05 and SL06 combined. They suggest a 3 to 4 km thick package over the Almada slope and Camamu deep basin with well organized and stratified sedimentary units and mild lateral variations. This package shows a homogeneous velocity gradient within layers S1 to S6 that have increasing velocities from 1.8 km/s to about 4 km/s. Only the deepest layer (S7) that cover the basement differs from this trend. The continuous and strong amplitude reflectors observed at the top of the layer marks a major velocity jump of about 1 km/s to values above 5 km/s. Furthermore the layer is strongly variable in thickness due to the basement strong topography.

In Jacuípe the sedimentary infill is thinner because SL05 and SL06 runs slightly landward respectively toward the SW and the NE. Sediments do not exceed 3 km thick and the number of layers is reduced to five. This package can be subdivided in an upper and lower sedimentary sequences. The former connects directly with the Camamu upper sedimentary layers S1 to S4. It is about 1.5 km thick, has velocities lower than 3.5 km/s and shows a similar stratification with little lateral variations. By contrast, the lower package shows strong lateral variability both in seismic velocity and facies. Its top interface is marked by a strong amplitude reflector that coincides with a clear velocity jump which oscillates between 0.5 to 1.5 km/s. At the bottom of the package, however, velocities are quite similar than at the bottom of the Camamu basin and around 4.7-4.8 km/s.

A thick sedimentary package covers the deep offshore Sergipe-Alagoas basin consisting of a 2-4 km thick upper sequence (layers S1 to S5) and a 1 km thick lower sequence (S6 layer). Although the
upper sequence appears well stratified with clear parallel horizons, lateral velocity variations are observed within almost every layers. There is a progressive increase in velocity towards the center of the basin where a maximum of 4.8 km/s is reached. Beneath, the lower sequence has velocities oscillating around 5 km/s and thickens on the edge of the basin due to basement topography. It also shows clear internal stratification and a peak in velocity in its central part.

The northeastern extremity of SL05 images an enclosed sedimentary basin in between two major basement highs of the AFZ. Volcanic material clearly cut trough the sediments to form seamounts on the seafloor. Both reflectivity and velocities suggest a package with similar trends than in the adjacent Sergipe-Alagoas basin. Velocities progressively increase to reach around 5 km/s over the basement. The velocity gradient is thus greater here since the sedimentary thickness is reduced to only 3 km thick.

## 6.2-Basement heterogeneous nature

Although top basement velocities are quite stable around 5.0-5.5 km/s all along SL05 and SL06, its depth is highly variable from one basin to another (Figure 7, 10 and 11). It is deep in the Camamu basin at 6.5-7.5 km, rises abruptly up to 4.4 km depth at the transition with the Jacuípe basin to stabilize around 6 km within the basin itself. Crossing the VBFZ, it deepens abruptly to 6.5-7 km deep in the Sergipe-Alagoas basin to slightly rise again at 6 km deep within the AFZ. On SL05 and SL06 models the top basement corresponds to the top of the B layer which compares to the U layers defined on SL09 (Loureiro et al., this issue) for Camamu and SL07/SL08 for Jacuípe (Aslanian et al., this issue). Both studies point out the oceanward evolution of its facies to emphasize its highly heterogeneous, non-unique nature and interpret the layer as a volcano-sedimentary sequence.

Although SL05 and SL06 run in a perpendicular direction compared to all other SALSA profiles, seismic data also reveal clear changes in facies that oscillate between transparent and reflective pockets (see Section 3.1 and figure 2). In Camamu, on SL06, small scale transparent highs that alternate with more reflective patterns may corresponds to relics of continental blocks that gravity-slided at the toe of the slope as previously observed in the Gulf of Biscay (Thinon et al., 2003) and the Galicia Bank-Southern Iberia Abyssal Plain strike-slip margin segment (Clark et al., 2007). In Jacuípe and Sergipe-Alagoas the basement is characterized by small scale seaward dipping reflectors (SDRs) and volcanic plugs (Pinheiro et al., 2018). Overall, basement velocities remain almost constant all along the two profiles with no major jump with respect to the deepest sedimentary layer modeled above. Instead localized velocity inversions are modeled. These observations add up to what has been described by companion articles and confirm the heterogeneous volcano-sedimentary nature of the B layer along SL05 and SL06.

## 6.3-Crustal and upper mantle architecture

Along SL06, the two layers that are present below the basement form together a 7 km thick crustal unit beneath the Almada basin reducing to a 4 km thick unit in the Camamu basin. Besides a drastic increase in lower crustal velocities from 7.0 to 7.6 km/s towards the NE, the crust progressively thins until it completely vanishes at 210-220 km distance (Figure 7 and 10). From Almada to this point, the Moho depth decreases from 15 km deep to only 8 km. However, given velocity models along SL07/SL08 crossing profiles (Aslanian et al., this issue) and modeled density structures along SL06 (Figure 11) an important deep structural change may occur approximately at the crossing of SL06 and SL09. From this point toward the NE, crustal thickness and mantle velocities have probably been overestimated due to out of plane wave propagation and/or anisotropy. Instead very thin crust (only a few kilometers as suggested along SL07/SL08) may overlay an anomalously high velocity body.

The Moho deepens again up to 13 km with increasing crustal thickness in Jacuípe. Compared to Camamu, the Jacuípe crust consists of a single layer with a maximum lower crustal velocity of only 6.8 km/s. Below, instead of normal mantle velocities which again were most probably
overestimated along SL06, SL07/SL08 crossing profiles and SL06 density model suggest the continuation of the anomalously high velocity layer found in Camamu.

SL05 velocity model and companion models along SL01/SL02 (Pinheiro et al., 2018) and SL04/SL05 (Schnurle et al., pers. com.) reveal an intermediate crustal architecture in the Sergipe-Alagoas basin and AFZ area starting from the offshore prolongation of the VBFZ toward the NE. The Moho has a concave shape beneath the Sergipe-Alagoas basin which reach 22 km at its center. It then varies between 15-18 km beneath the AFZ. The Sergipe-Alagoas crust is 14 km thick at most and made of 2 layers: a 6.5-6.8 km/s upper layer and a 7.0-7.3 km/s lower layer. Within the AFZ, the two crustal units are 8-9 km thick but velocities are comprised between 5.5 and 7 km/s top to bottom. Anomalously high velocity zones between the crust and mantle are again suggested beneath the Sergipe-Alagoas and the AFZ based on our velocity and density models as well as crossing models (Pinheiro et al., 2018; Schnurle et al., pers. com.).

# 7-Discussion

## 7.1-Along-strike segmentation of northeastern Brazilian margins

The SALSA experiment was specifically designed to shed new lights on the deep crustal architecture of northeastern Brazilian offshore margins, with SL05 and SL06 profiles allowing to characterize precisely their along-strike variations and segmentation. Previous studies already discussed this segmentation. Based on MCS data and gravity modeling, the Jacuípe and Sergipe segments were interpreted as narrow volcanic margins whereas the Alagoas segment to the north and the Camamu-Almada segments to the south qualified as wide non-volcanic margins (Blaich et al., 2008, 2010; Mohriak et al., 1998, 2000). Recently, the Camamu-Almada and conjugate Gabon margins were also described as a non-volcanic rifted system with a transitional zone on the Brazilian side depicting successively in an oceanward direction: thinned continental crust, exhumed mantle, and proto-oceanic crust (Romito & Mann, 2022).

Previously published velocity models build in the scope of the SALSA project also gave insights into the segmentation of northeastern Brazilian margins, with a large domain of exhumed lower continental crust interpreted in the Jequitihonha offshore segment (Loureiro et al., 2018) as opposed to an abrupt transition between continental and oceanic crust in the Sergipe-Alagoas margin (Pinheiro et al., 2018). Integrating all available velocity models from the SALSA experiment further refine this picture by providing a continuous and unique vision of the crustal architectures of northeastern Brazilian margins.

To the south of the study area, the SW extremity of SL06 crosses SL10 where exhumed continental crust where previously interpreted (Loureiro et al., 2018). Adding up SL06 velocity-depth profiles (Figure 12) until its crossing with SL09 to the compilations previously made by (Loureiro et al., 2018) clearly show similarities in structure and velocities between Almada and Jequitinhonha basins imaged along SL06, SL09, SL10, SL11 and SL12 (Figure 12c). Loureiro et al. (2018) showed how continental crust thins progressively oceanward in the Jequitinhonha basin and is exhumed in its distal domain by analogy with the Santos basin (Evain et al., 2015). Loureiro et al. (this issue) extend their analysis and comparison of velocity-depth profiles to show that those crustal characteristics also exist beneath the Almada basin. We therefore interpret the Jequitinhonha-Almada distal margin as a vast domain of exhumed lower continental crust. In this segment, from the perspective of the SALSA dataset and the area it covers, there are neither evidence of exhumed mantle nor oceanic crust. Loureiro et al. (this issue) only mention the plausible presence of proto-oceanic crust at the south-westernmost tip of SL09.
The central part of SL09 also images the southern portion of the Camamu basin. According to Loureiro et al. (this issue), it consists of a thinner upper crust and a thicker anomalously high velocity lower crust than in Almada. Based on SL06 models, this crustal architecture is consistent with the architecture of the transitional domain of both Camamu and Jacuípe margins observed on SL06 models but also along SL07/SL08 profiles (Aslanian et al., this issue). Locally, SL06 reveals that the upper crustal layer can be absent with the volcano-sedimentary basement just underlain by the anomalous velocity layer. The later shows a strong northeastward increase in velocities that become intermediate between typical lower crustal and upper mantle velocities, the highest value (7.7 km/s) is found at the crossing with SL07. Combining velocity-depth profiles of SL06, SL07, SL08 and SL09 for the Camamu and Jacuípe basins (figure 12c) we show that they are similar to profiles in the Para-Maranhão-Barreirinhas margins in northern Brazil and in the aborted rift interpreted at the center of the Santos basin (Aslanian et al., this issue; Aslanian et al., 2021; Evain et al., 2015). We therefore suggest that exhumed and/or intruded continental crust lies above a layer of either highly intruded continental crust and/or serpentinized mantle. The trend of velocity profiles clearly diverges from typical signature of an oceanic crust which is, however, imaged just a few kilometers oceanward according to SL07 and SL08 models (Aslanian et al., this issue). The Camamu-Jacuípe margins' narrow architecture, the nature of their transitional domains and the abrupt transition towards oceanic crust greatly diverge from the architecture just described for the Jequitinhonha-Almada margins and imply that the Camamu Triple Junction marks a first major segmentation in NE Brazilian margins.

The Sergipe-Alagoas margins structure is also well imaged by the SALSA dataset. SL05 and SL06 profiles cross each other near the offshore prolongation of the VBFZ. On our models it coincides with a large change in sedimentary and crustal architecture between the Jacuípe and Sergipe-Alagoas basins. SL05 velocity model complement previously published models by Pinheiro et al. (2018) providing an along-strike view of the seaward extremity of the necking zone where the crust is considerably thicker (up to 16 km) than in the Jacuípe basin. The most striking observation is the thick and anomalously high velocity (from 6.5-7.3 top to bottom) crust of the Sergipe-Alagoas basins which is underlain by normal mantle velocities (Figure 7). Such architecture is different beneath the AFZ where the crust is thinner, with lower velocities. However, an anomalous velocity layer is also suggested by crossing SL03/SL04 profiles (P. Schnurle pers. com.) and our density model suggest a low density model (Figure 11). Compiled velocity-depth profiles (figure 13) in the Sergipe-Alagoas basin and in the AFZ area highlight the low gradient of the crust which is intermediate between normal continental and oceanic crust as shown by Pinheiro et al. (2018). Such characteristics is again close to what was observed in the Santos basin (Evain et al., 2015) and suggest the presence of thinned, probably exhumed, continental crust modified by intrusions of mafic materials. Compared to Camamu-Jacuípe, the transition toward oceanic crust is also steep within the Sergipe-Alagoas and the AFZ segments (Pinheiro et al., 2018; Schnurle et al., pers com.) but the width of the necking zone is larger and the continental crust thinned more gradually with lower inputs of mafic materials. Beside the geodynamic segmentation imposed by the CTJ, the SALSA dataset therefore confirm the structural segmentation of the offshore NE Brazilian margins by the VBFZ and the AFZ. These results are summarized on map in Figure 14.

## 7.2-Role of lithospheric inheritance on segmentation

The SALSA velocity models allow to make the necessary connection between the onshore and offshore rift system that characterizes NE Brazil. With this detailed view on variations in margins architecture we are able to confirm the VBFZ not only segmented the onshore rifts (Blaich et al., 2008; Milani & Davison, 1988; Mohriak et al., 1998) but also the offshore part of the system between Jacuípe and Sergipe-Alagoas basins. This inherited discontinuity had therefore a profound influence both temporally and spatially from the early onset of rifting within the RTJ to the final
stage of rifting and formation of the distal margin. With the connection between structural inheritance and the AFZ previously shown (Vasconcelos et al., 2019), the segmentation observed between Sergipe-Alagoas and the AFZ margins, most notably in the structure of the mantle, further shows this influence continued as far in time and space as the onset of new oceanic crust, acting on its segmentation and the localization of its main structural features.

How Brazilian inheritance controlled margins' architecture remains to be tested. The VBFZ localizes the main contact between the old Archean and Paleoproterozoique terrains of the SFC and younger Pre-Cambrian belts of the Borborema Province. These two units have different lithospheric compositions and rheological history, their overall strength were probably very different at the onset of extension (Watts & Stewart, 1998). Although the geodynamic of rifting was similar along the Camamu-Jacuípe and Sergipe-Alagoas segments (see next section), this difference in strength

means they likely accommodated extension differently thus explaining distinct final crustal architecture. Finally it is likely that the lithosphere of these two segments had different thermal state by looking at the trend of the Bahia Seamounts perfectly constrained it the prolongation of the Sergipe-Alagoas segment. This is further attested by the difference in upper mantle structure

observed along the SL05 profile, a contrast that appears to be deeply rooted up to the asthenospheric mantle according to a recent teleseismic tomographic model (Simões Neto et al., 2019).

## 7.3-Geodynamic control on segmentation

Another important insight from the SALSA dataset is to point toward the crucial role of regional geodynamic in margins' segmentation and architecture. From the vast area of exhumed continental crust evidenced in the Jequitinhonha-Almada segments to the steep thinning and sharp transition toward oceanic crust observed in the Camamu-Jacuípe-Sergipe-Alagoas segments our models are consistent with a geodynamic framework where purely divergent extension occurred to the south of the CTJ while transtension shaped the margins to the north. Integrating these results and building on the kinematic model of Moulin et al. (2010) and the stratigraphic synthesis of Chaboureau et al. (2013) the figure 15 presents the detailed evolution of regional plate motions relative to the Tucano micro-plate (considered as fixed). This point of view allows for a better understanding of the timing and movement on either side of the micro-plate. We suggest three distinct phases:

-Phase 1: From the Berriasian-Vallanginian boundary (141-139 Ma) to the Barremian-Aptian (125 Ma) boundary, rifting occurred on each side of the Tucano plate forming the RTJ basins and initiating the Jacuípe-Sergipe-Alagoas margins. These rifts are produced by two different movements: Westwards for the São Francisco plate escaping westward and the African plate moving towards the ENE. Combined together the differential created a small space at the location of the CTJ which formed what we identify as a 'Boutonnière' (Figure 14). Note that the movement of the São Francisco plate decreased strongly at the Barremian-Aptian boundary (almost no more movement will occur with the Tucano micro-plate).

-Phase 2: From the Barremian-Aptian (125 Ma) boundary to the Aptian-Albian boundary (112 Ma) a major kinematic change occurred. As the São Francisco plate is almost fixed to the Tucano plate, the CTJ ceased. The African plate movement also changes in direction to a NE direction. Overall this implies strike-slip movement between Camamu-Jacuípe-Sergipe-Alagoas and North Gabon margins. This kinematic change is probably related to African intraplate deformation, which also ended around the Aptian-Albian boundary (Guiraud & Maurin, 1992).

-Phase 3 is posterior to the Aptian-Albian boundary (112 Ma) and represents the beginning of plate separation in the Equatorial Atlantic ocean, formation of the first oceanic crust in the Central Segment of the South Atlantic and the end of the African intraplate deformation. The African Plate movement comes back to the previous ENE direction, parallel to the Ascension Fracture Zone.
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The CTJ appears as a crucial marker not only of the segmentation of NE Brazilian margins but also of the South-Atlantic margins in general. It is remarkable that salt deposits along the eastern Brazilian margins stop exactly at the CTJ with a strong asymmetry with west African margins where salt is observed as north as the AFZ (Chaboureau et al., 2013). It is the exact inverse that took place at the southern end of the Central Segment where salt deposited further south in the Santos basins while the Kwanza margin, its conjugate on the African side, remained depleted. As previously suggested, the Tucano micro-plate and its margins may represent, as the Santos-São Paulo system (Aslanian et al., 2009; Evain et al., 2015; Klingelhoefer et al., 2014; Moulin et al., 2013), buffer zones that accommodated the diachronous development of the main segments of the South-Atlantic Ocean (Moulin et al., 2010). The peculiar and more complex geodynamic of these buffer zones must therefore be carefully studied in order to understand the main parameters controlling their margins architectures.

## 8-Conclusion

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The SALSA experiment shows that the complex structure of a margin cannot be emphasized by a single seismic profile that would follow the main direction of extension. Instead, denser meshing is required, including long crossing profiles parallel to the coast. Although difficult to interpret because of the highly non-cylindrical character of the passive margins and because they are perpendicular to the general thinning process, they provide crucial information on margin's segmentation. The interpretation of SALSA along-strike profiles, fed by results of crossing dip ones, completes the onshore-offshore picture of the northeastern Brazilian rift system to better identify parameters controlling its architecture and segmentation. At the frontiers of its segments, the crustal structure changes brutally and coincides precisely with the Camamu Triple Junction, the Vaza Barris fault zone between the São-Francisco Craton and the northeastern Brazilian Orogenic Belt, and the Ascencion Fracture Zone, exhibiting a first order control by lithospheric heritage. The segmentation marked by the Vaza Barris fault zone clearly extends offshore and is attested by the presence of the Bahia seamounts on the seafloor. Besides this tectonic legacy, geodynamic appears to be another controlling parameter of the segmentation of northeast Brazilian margins with the Camamu Triple Junction as its crucial marker. It has a major impact, particularly on the peculiar buffer zone that is the Tucano micro-plate, but also on the South Atlantic margins development in general. At the regional level, it imposes a three-phases kinematic evolution north of Camamu Triple Junction with an additional strike-slip movement between the Camamu-Jacuípe-Sergipe-Alagoas and Northern Gabon margins that is coeval to the end of the African intraplate deformation (Barremian-Aptian boundary). This movement explains well the abrupt thinning, breaking of continental crust and the steep transition to oceanic crust, as well as the presence of detached continental blocks that most probably slipped at the foot of the slope. It further created some space in the Camamu segment for either highly intruded continental crust or serpentinized mantle to exhume creating its characteristic positive gravity anomaly. All these results clearly evidence the first-order control of both the geodynamic and lithospheric inheritance on the along-strike segmentation and offshore architecture of the northeastern Brazilian passive margins.

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## Contributions

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The SALSA Project was led by D. Aslanian and M. Moulin, from Ifremer, and A. Viana, from Petrobras. Processing of the high resolution seismic data was done by A. Baltzer, M. Benabdellouahed, and M. Rabineau. The hypocentral determination was done by I. Rio. The geodynamic interpretations were done by D. Aslanian and M. Moulin. Modelling of profiles SL11 and SL12 was done by A. Loureiro. Modelling of profile SL10 was done by F. Klingelhoefer. Profile SL09 was modelled by A. Afilhado and jointly interpreted by A. Loureiro. F. Gallais modelled profiles SL07 and SL08. M. Evain modelled profiles SL05 and SL06. Profiles SL01 and SL02 were modelled by J. M. Pinheiro. Modelling of profiles SL03 and SL04 and processing of the
deep-sounding reflection seismic data was done by P. Schnürle. S. Gonçalves applied RTM to
profile SL04.

**Acknowledgments:**

The authors would like to thank the Editors for handling this contribution and two anonymous
reviewers for their comments that have greatly improve the final version of this article.

The data set collected during the SALSA experiment is protected under a partnership between
IFREMER and Petrobras. Any request must be addressed to Daniel Aslanian (aslanian@ifremer.fr)
and Adriano Viana (avaina@petrobras.com.br).

The authors wish to thank the captain, crew, and MCS technical team of the French R/V L'Atalante.
Many thanks also to the offshore OBS team: J. Crozon, P. Fernagu, P. Pelleau and M. Roudaut, and
the onshore seismic team: J. Soares, R. Fuck, M. Lima, J. Pinheiro, F. Lima, M. Sobrinho, P.
Resende, R. Oliveira, N. Dias, C. Corela, J.L. Duarte, A. Loureiro and D. Alves who all made this
experiment possible. Thanks also to the remaining SALSA team who helped during the project:
Morvan, L., Mazé, J.P., Pierre, D., Roudaut Pitel, M., Rio, I., Barros Junior, P., Biari, Y., Ducatel,
C., Falcão, C., Le Piver, D., Mokeddem, Z., Rigoti, C., & Roest, W..

The authors acknowledge financial support from CAPES-COFECUB. R. Fuck acknowledges
CNPq, Brazil research fellowship (30.1065/2016-0) and INCT Estudos Tectônicos research grant
(46.5613/2014-4). A. Loureiro acknowledges funding from the Portuguese Fundação para a Ciência
e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020.

The GMT (Wessel & Smith, 1998), Seismic Unix package (Stockwell, 1999), GEOCLUSTER
(CGG-Veritas), and RAYINVR (Zelt, 1999; Zelt & Smith, 1992) softwares were used in the
preparation of this paper.
List of Tables:

<caption>Table 1: Statistics of data fit for profile SL05 (a) and SL06 (b): For every reflected or refracted phase it is specified its name and index number, the number of travel time picks, the overall travel time residual (RMS) and normalized $\chi^2$ value.</caption>

<table><caption>Table 1a: Statistics of data fit for profile SL05</caption><thead><tr><th>phase</th><th>index</th><th>npts</th><th>Trms</th><th>χ<sup>2</sup></th></tr></thead><tbody><tr><td>Pw</td><td>1</td><td>4610</td><td>0.022</td><td>0.758</td></tr><tr><td>Ps1</td><td>2</td><td>210</td><td>0.036</td><td>0.525</td></tr><tr><td>Ps2</td><td>3</td><td>293</td><td>0.035</td><td>0.497</td></tr><tr><td>Ps3</td><td>4</td><td>692</td><td>0.032</td><td>0.421</td></tr><tr><td>Ps4</td><td>5</td><td>506</td><td>0.038</td><td>0.565</td></tr><tr><td>Ps5</td><td>6</td><td>906</td><td>0.044</td><td>0.300</td></tr><tr><td>Ps6</td><td>7</td><td>171</td><td>0.050</td><td>0.389</td></tr><tr><td>Ps7</td><td>8</td><td>1519</td><td>0.052</td><td>0.415</td></tr><tr><td>Pb</td><td>9</td><td>456</td><td>0.089</td><td>0.788</td></tr><tr><td>Pg1</td><td>10</td><td>7532</td><td>0.071</td><td>0.501</td></tr><tr><td>Pg2P</td><td>11</td><td>3940</td><td>0.102</td><td>1.039</td></tr><tr><td>Pg2</td><td>12</td><td>3538</td><td>0.088</td><td>0.773</td></tr><tr><td>Ps2P</td><td>13</td><td>805</td><td>0.032</td><td>0.404</td></tr><tr><td>Ps3P</td><td>14</td><td>695</td><td>0.036</td><td>0.517</td></tr><tr><td>Ps4P</td><td>15</td><td>1542</td><td>0.034</td><td>0.468</td></tr><tr><td>Ps5P</td><td>16</td><td>2444</td><td>0.048</td><td>0.922</td></tr><tr><td>Ps6P</td><td>17</td><td>2370</td><td>0.052</td><td>0.418</td></tr><tr><td>Ps7P</td><td>18</td><td>293</td><td>0.044</td><td>0.301</td></tr><tr><td>PbP</td><td>19</td><td>1391</td><td>0.060</td><td>0.561</td></tr><tr><td>Pg1P</td><td>20</td><td>2395</td><td>0.080</td><td>0.645</td></tr><tr><td>PmP</td><td>21</td><td>6051</td><td>0.120</td><td>1.006</td></tr><tr><td>Pn</td><td>22</td><td>1696</td><td>0.129</td><td>0.735</td></tr><tr><td>Pm2P</td><td>23</td><td>2753</td><td>0.197</td><td>1.720</td></tr></tbody></table>

<table><caption>Table 1b: Statistics of data fit for profile SL06</caption><thead><tr><th>phase</th><th>index</th><th>npts</th><th>Trms</th><th>χ<sup>2</sup></th></tr></thead><tbody><tr><td>Pw</td><td>1</td><td>5343</td><td>0.025</td><td>1.019</td></tr><tr><td>Ps1</td><td>2</td><td>157</td><td>0.041</td><td>0.664</td></tr><tr><td>Ps2</td><td>3</td><td>246</td><td>0.046</td><td>0.854</td></tr><tr><td>Ps3</td><td>4</td><td>664</td><td>0.041</td><td>0.683</td></tr><tr><td>Ps4</td><td>5</td><td>849</td><td>0.051</td><td>1.022</td></tr><tr><td>Ps5</td><td>6</td><td>508</td><td>0.060</td><td>0.555</td></tr><tr><td>Ps6</td><td>7</td><td>615</td><td>0.053</td><td>0.448</td></tr><tr><td>Ps7</td><td>8</td><td>1347</td><td>0.060</td><td>0.558</td></tr><tr><td>Pb</td><td>9</td><td>1021</td><td>0.101</td><td>1.019</td></tr><tr><td>Pg1</td><td>10</td><td>4630</td><td>0.090</td><td>0.815</td></tr><tr><td>Pg2P</td><td>11</td><td>3750</td><td>0.086</td><td>0.734</td></tr><tr><td>Pg2</td><td>12</td><td>1941</td><td>0.070</td><td>0.492</td></tr><tr><td>Ps2P</td><td>13</td><td>619</td><td>0.035</td><td>0.489</td></tr><tr><td>Ps3P</td><td>14</td><td>631</td><td>0.031</td><td>0.392</td></tr><tr><td>Ps4P</td><td>15</td><td>925</td><td>0.046</td><td>0.846</td></tr></tbody></table>
<table><tr><td>Ps5P</td><td>16</td><td>2762 0.043</td><td>0.732</td></tr><tr><td>Ps6P</td><td>17</td><td>1150 0.045</td><td>0.317</td></tr><tr><td>Ps7P</td><td>18</td><td>1238 0.045</td><td>0.316</td></tr><tr><td>PbP</td><td>19</td><td>872 0.057</td><td>0.516</td></tr><tr><td>Pg1P</td><td>20</td><td>1822 0.081</td><td>0.658</td></tr><tr><td>PmP</td><td>21</td><td>1892 0.081</td><td>0.457</td></tr><tr><td>Pn1</td><td>22</td><td>6067 0.096</td><td>0.406</td></tr><tr><td>Pm2P</td><td>23</td><td>2277 0.119</td><td>0.625</td></tr><tr><td>Pn2</td><td>24</td><td>752 0.104</td><td>0.477</td></tr><tr><td>Pm3P</td><td>25</td><td>56 0.132</td><td>0.785</td></tr><tr><td>Pn2P</td><td>27</td><td>752 0.124</td><td>0.688</td></tr><tr><td>Pn2P</td><td>29</td><td>162 0.150</td><td>1.001</td></tr></table>
List of figures

Figure 1: The SALSA experiment focuses on the northeastern Brazilian margins. a) Topographic map derived from satellite data (Tozer et al., 2019) highlighting locations of the SL05 and SL06 seismic profiles, respect to others SALSA profiles, multi-channels seismic profile ION-GXT-900 and main geological features (Gordon et al., 2017; Milani & Davison, 1988; Ussami et al., 1986): onshore and offshore basins: Je: Jequitinhonha, Al: Almada, Ca: Camamu, Ja: Jacuípe, Se: Sergipe, Ag: Alagoas, Pe: Pernambuco, Re: Reconcavo, Tu: Tucano, Jt: Jatoba; main tectonic features: AFZ: Ascencion Fracture Zone, PSZ: Pernambuco Shear Zone, VBFZ: Vaza-Barris Fault Zone; main geological domains: SFC: São-Francisco Craton, SFB: Sergipano Fold Belt, PAM: Pernambuco-Alagoas Massif. b) Gravity map derived from satellite data (Sandwell et al., 2021) of the northern Central Segment of the Atlantic Ocean highlighting main fracture zones and anomaly patches along the two conjugate margins (black boxes). c-d) Magnetic maps (Meyer et al., 2017) at the two conjugate margins with references to the main basins and cratonic cores (CC: Congo Craton).

Figure 2: MCS profiles SL06 (A) and SL05 (B). Interfaces from respective velocity models are overlain in colored lines and their identification provided in the legend. Intersections with other SALSA profiles are indicated by red line. OBS location are indicated by white triangles.

Figure 3: Interpreted seismic phases for SL06OBS27. Alongside each OBS sections (a), synthetic section (b) superimposed by predicted phase travel times (c), picks colored by phase and their travel time fit (thin black line; d), and ray-tracing (e) are all calculated based on the final model. f) zoom on coincident portion of MCS profile as in figure 2. Similar figures for each OBS are provided in Supplementary material S1 and S2.

Figure 4: Same as figure 3 for SL06OBS13

Figure 5: Same as figure 3 for SL05OBS25

Figure 6: Same as figure 3 for SL06OBS08

Figure 7: Final P-wave velocity models along SL05 (A) and SL06 (B) profiles with mask applied outside areas covered by seismic rays. Text and arrows in black indicate constraints brought by crossing profiles and confirmed by gravity modeling (see text and figure 7). OBS locations are indicated by black inverted triangles. Thick blue lines indicate parts of interfaces constrained by wide-angle reflections.

Figure 8: Evaluation of SL05 velocity model. a) Model parameters including interface depth nodes (squares), top and bottom layer velocity nodes (red circles); interfaces where reflections have been observed on OBS data are highlighted in blue. b) Hit-count for velocity (colored) and depth nodes (squares). c) Smearing from Spread-Point Function (SPF) for velocity (colored) and depth nodes (squares). d) Resolution of velocity (colored) and depth nodes (squares). Zones that were not imaged are blanked

Figure 9: Same as figure 8 for SL06 velocity model.

Figure 10: Pre-stack depth migrated section of MCS profiles SL06(a) and SL05(b). Interfaces from respective velocity models are overlain in colored lines and their identification provided in the legend. Intersections with other SALSA profiles are indicated by red lines. Below each MCS profiles is a line drawing of nearly coincident (see map in Figure 1) MCS seismic profile ION-
GXT900. Red line is the seafloor, yellow and blue segments represent reflectivity within the sedimentary basins and basement respectively.

Figure 11: Gravity modeling along SL05 and SL06 profiles. a,d) Density models up to a depth of 40 km overlain by interfaces from wide-angle modeling. b,e) Free-air gravity anomaly observed (Pavlis et al., 2012) along profile (red dotted) and 10, 20 and 30 km laterally (yellow lines), measured during the SALSA experiment (blue line) and calculated (green line). c,e) Calculated load anomaly.

Figure 12: Velocity-depth (VZ) profiles analysis along SL06. a) VZ profiles were extracted every 10 km and color coded according to margin's segmentation. b) VZ profiles extracted below seafloor, plotted all together in black and separated/color coded by segments for comparison and highlight crustal structure changes. c) VZ profiles for the crust, excluding the volcano-sedimentary basement layer. VZ plotted all together in black and separated/color coded by segment for comparison with VZ from other continental margins: $SL06 crustal VZ profiles for the Almada segments are compiled with those of SL09, SL10, SL11 and SL12 for the Jequitinhonha-Almada basins to show their similarities. They are also similar to averaged velocity-depth profiles in the Santos basin (Evain et al., 2015) but diverge from typical crustal structure of oceanic crust in the Atlantic (White et al., 1992). Similarly, SL06 VZ profiles for the Camamu and Jacuípe segments are compiled with those of SL07, SL08 and SL09 profiles than cross these same margin's segments. Note how these VZ profiles diverge again from the typical crustal structure of oceanic crust in the Atlantic (White et al., 1992) but show good consistency with VZ profiles from the Barreirinhas/Ceara margin (Aslanian et al., 2021) where an anomalous velocity zone (AVZ) is also present and interpreted as exhumed continental crust.

Figure 13: Same as figure 12 for SL05 profile. a) VZ profiles were extracted every 10 km and color coded according to margin's segmentation. b) VZ profiles extracted below seafloor, plotted all together in black and separated/color coded by segments for comparison and highlight crustal structure changes. c) SL05 crustal VZ profiles are compiled with those of SL01 and SL02 for the Sergipe-Alagoas basin. They are compared to oceanic crust in the Atlantic (White et al., 1992) and averaged velocity-depth profiles in the Santos basin (Evain et al., 2015). A similar comparison is presented for VZ profiles within the AFZ area where an anomalous velocity zone (AVZ) is also drawn as it is suggested by crossing SL03/SL04 profiles (P. Schnurle pers. com.) and SL05 density model from gravity modeling (Figure 11).

Figure 14: Proposed segmentation of northeastern Brazilian margins based on a synthesis of results from the SALSA project (D. Aslanian et al., this issue; Loureiro et al., 2018, this issue; Pinheiro et al., 2018); P. Schnurle pers. com. and this article). Crustal nature interpretations from wide-angle models are showed with coloured areas and the necking zone limited by two thick dashed black lines. Crustal thickness is illustrated in color scale along each SALSA profiles. Onland, crustal thickness was taken from (Assumpção et al., 2013; Döring et al., 2022) and pictured with same color scale filling squared boxes.

Figure 15: Geodynamic evolution of the northern Central Segment of the South Atlantic Ocean. Each color represents the position of the plates at a specific time, between the fit (130 Ma) and the Aptian/Albian boundary (112 Ma). The positions of the South American and African plates, restored following the parameters given by (Moulin et al., 2010), highlight the movements relative to the Tucano micro-plate (fixed plate represented with a grey area), also indicated by colored arrows.
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Crustal thickness
from previous studies

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△ Land Seismic Station

○ OBS and MicrOBS

Profile

Geology

Fracture Zone

Fault

Cretaceous Basin

+ + Craton

Crustal thickness compilation
(From SALSA data)

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Evolution from 141-139 Ma (Berriasian/Valanginian) to 112 Ma (Aptian/Albian boundary)

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