Oceanic Transform Faults are one of manifestations of the three major plate boundaries and a key tectonic feature of oceanic crust. They are broadly considered to accommodate strike-slip displacement along simple vertical faults and to be largely without magmatic addition. We present the first observations from broadband 3D seismic of buried, Cretaceous-aged transform faults in the Gulf of Guinea with complex internal architectures including crustal scale detachments and rotated packages of volcanics within oceanic crust. In the study area, several Oceanic Fracture Zones (OFZ) are described from Top Crust to Moho. OFZ scarps are observed to connect at depth with zones of low angle reflectivity which dip into the OFZ and perpendicular to the spreading orientation. At depth they detach onto the Moho, necking the adjacent crust in the manner of extensional shear zones. Thickly stacked and tilted reflectors, interpreted as extrusive lava flows, are common above the shear zones and infill up to 75% of the crustal thickness. The entire OFZ stratigraphy is overlain and sealed by late-stage lavas that are continuous from the abyssal hills of the trailing spreading ridge. These insights demonstrate complexity previously only predicted in numerical simulations. We propose a model with extension at a high angle to the spreading orientation along a low angle shear zone that acts as a conduit for decompression related melt and volcanism. We conclude that oceanic transforms are non-conservative and not simple strike slip fault zones, contradicting the conventional view.

Key Points

Oceanic transforms faults are interpreted from 3D Pre-Stack Depth Migrated Seismic with a fidelity unavailable with standard methods
For the first time, we document intra-transform detachments and lava growth packages along 180 km high-resolution seismic profiles and in 3D
Inconsistent with a simple strike slip model this supports numerical model outcomes of oblique extension and two stage crustal accretion

Plain Language Summary

Oceanic transform faults (OTFs) are one of three types of plate boundaries and an integral component of the seafloor fabric which is used to reconstruct the evolution of oceanic basins. They laterally offset active seafloor spreading ridges and record the geometrical configuration of past spreading ridges as the “train tracks of plate tectonics”—oceanic fracture zones. OTFs are regarded as “conservative”—simple vertical fault planes where no crustal material was added or destroyed, yet observational support for the 3D crustal-scale architecture of these faults has been lacking. Here, we present observations from a proprietary, high-resolution industry 3D seismic data set acquired in the Gulf of Guinea. We document, for the first time, the 3D architecture of these faults in Cretaceous-aged oceanic crust (105–115 Million years old), now buried underneath more than 3 km of sediments and formed shortly after South America-Africa breakup at center of the former Gondwana supercontinent. We show that oceanic transform faults have a complex 3D architecture, formed by a two-stage process of extension/deformation and infilling through lava flows. Our work highlights that this type of plate boundary can no longer be regarded as simple conservative and supports similar conclusions from recent geodynamic models.

1 Introduction

In this study we describe observations of Oceanic Fracture Zones from Albian-Aptian-aged oceanic crust (∼105–115 Ma, ICS-GTS2020 timescale) located approximately 80 km east of the São Tomé and Príncipe islands (STP) in the eastern Gulf of Guinea (Figure 1a). Oceanic fracture zones (OFZs) and Oceanic Transform Faults (OTFs) were recognized from very early on as key tectonic element of oceanic crust, providing important insights on seafloor spreading patterns across nearly all spreading regimes (slow to fast), the plate kinematic evolution of oceanic basins, and relative plate motions (e.g., Gerya, 2012; Le Pichon, 1968; Wilson, 1965 and references therein; Seton et al., 2020). OFZs are the morphological remnants of OTFs which accommodated opposing spreading directions between two offset ridge segments. Extending for tens to thousands of kilometers across ocean basins, they juxtapose segments of oceanic crust with differing ages (Figure 2). Here an 8,000 km² industry-acquired broadband 3D seismic survey from 2017 forms the basis for this study (Figure 1) in which we mapped the oceanic crust architecture beneath 3–5 km of sediment. Seismic reflector geometries within the OFZs can be dated as being of transform fault origin and illustrate a broad deformational zone containing dipping reflectivity. These provide unique insights into large-scale transform processes in young oceanic crust outboard of a continental margin. Similarities are observed with numerical models of OTF inside-corner thinning and dipping plate boundaries, as well as a second stage of extrusive magmatism infilling partially the transform valley. We therefore discuss these observations within the context of a new model for the evolution of transform faults.

Details are in the caption following the image — **Figure 1**
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Location map and plate kinematic reconstruction of the study area. (a) Location map of the study area off Central West Africa. Illuminated grid indicates bathymetry (Tozer et al., 2019) with superimposed isochrons of oceanic crust age (Seton et al., 2020) in color and annotated ages in 5 Myr intervals. Hydrocarbon exploration concessions are shown as thin white polygons, outline of 3D seismic survey as hachured green polygon. The interpreted landward limits of oceanic crust (LaLOC) is shown as dashed blue lines (light blue: Heine et al. (2013); dark blue for North Gabon to Equatorial Guinea segment: this study). Oceanic fracture zones from Global Seafloor and Magnetic Lineations project (Matthews et al., 2011; Seton et al., 2014) as thick dark red lines, mapped fracture zones in study area as thick orange lines. Eq. Guinea: Equatorial Guinea. (b) Reconstruction of the study area at 104 Ma in Africa-fixed reference frame with 3D seismic survey outline (“3D”) as green hachured polygon. Flowlines derived from plate model (Muller et al., 2019) as thick blue lines with colored points spaced in 5 Myr intervals from 140 to 105 Ma indicate relative plate motions, white vectors indicate South American plate motions (∼40 mm/a, translating to a half-spreading rate of ∼20 mm/a). Thick black line indicates modeled seafloor spreading ridge being located west of the study area in the mid-Albian. Both maps in Mercator project and same scale.

2 Oceanic Transform Fault Observations

OTF studies have primarily focused on currently active zones of oceanic spreading using a variety of geological and geophysical methods. These include in-situ observations of OTF's in marine settings through submersible dives (Mamaloukas-Frangoulis et al., 1991; Pastouret & Cyamex Scientific Team, 1981) as well as outcropping active transform faults (Gudmundsson, 1995; Khodayar et al., 2020) and remnants of oceanic crust in ophiolites (e.g., Fagereng & MacLeod, 2019), multibeam bathymetry (Embley & Wilson, 1992; Fornari et al., 1989; Searle, 1986), active source (Gasperini et al., 2017; Marjanović et al., 2020; Van Avendonk et al., 1998), seismicity and passive seismic studies (Kuna et al., 2019; Wolfson-Schwehr & Boettcher, 2019; Wolfson-Schwehr et al., 2014), as well as potential field data-based studies (Gregg et al., 2007; Wessel et al., 2015). The magmatic evolution of OTZ/OFZ features has also been directly sampled through dredges and submersibles (Dick et al., 2008). The initiation of seafloor spreading and the nucleation of OFZs/OTFs adjacent to passive continental margins is less well understood due to thick sedimentary overburdens typically overlying old oceanic crust which impedes direct sampling and good imaging with seismic and potential field methods.

Large scale side scan sonar data sets in particular have allowed for morphological description of OTFs which offset bounding spreading segments over distances of ∼10–100 km and by >1 Myr in crustal age (e.g., Fowler, 2012; Gallo et al., 1986; Sclater et al., 2005; Smoot, 1989; van Wijk & Blackman, 2005; Wolfson-Schwehr & Boettcher, 2019 and references therein). These plate boundaries are characterized by a bathymetric deep and demonstrate complex deformational features resulting from changes to the local stress field (Maia, 2019). Very thin crust (2–3 km) is often, but not always recorded at slow-spreading OTF zones with large offsets (White et al., 1984). Seismic velocities within OTFs also tend to be anomalously low compared to typical oceanic crust (Roland et al., 2012; Van Avendonk et al., 2001). White et al. (1984) note that velocities typical of Layer 3, the most consistently observed seismic structure in oceanic crust, is not often observed from OFZs.

Direct sampling through submersible dives showed that in general OTFs contain abundant basaltic material (Francheteau et al., 1976). It is relatively common to observe topographically elevated flanks adjacent to, and running parallel to OTF's (e.g., Gregg et al., 2007). In such highs in the slow rate spreading Atlantic, submersible based observation describes exposed scarps, terraces and talus cones which contain a mix of mantle and lower crust gabbroic and ultramafic rocks (Francheteau et al., 1976) such that the typical Layer 2A basalts are thin or absent (Francheteau et al., 1976; White et al., 1984).

Spreading ridges can also be laterally discontinuous without significant discrete OTF's and with a variety of complex deformational morphology including overlapping axial ridges and oblique jogs in the axial zone (Carbotte et al., 2016; van Wijk and Blackman, 2005). Where ridge to ridge offsets are typically below 30 km these features are termed non-transform offsets.

Based on the predominance of strike-slip earthquake focal mechanisms and from controlled source seismic studies, OTFs were historically viewed as conservative plate boundaries, consisting of a simple vertically oriented fault plane in which no crustal material is either being added or removed (Eittreim & Ewing, 1975; Grevemeyer et al., 2021; Kuna et al., 2019; Reston, 2009). Several lines of evidence however point toward a more complex deformational zone, in which the fraction of plate motion accommodated by aseismic deformation mechanisms is as high as 85% (Boettcher & Jordan, 2004) and where extension is localized particularly toward the inside corner of the ridge-transform intersection (hereafter referred to as the “inside corner,” Figure 2). Low velocity zones observed along mid-Atlantic ridge OTFs suggest fractured basaltic rocks under transpressive conditions which can account for the low Poisson's ratio calculated from seismic refraction data (White et al., 1984). Roland et al. (2012) note the uneven distribution of earthquakes along OTFs. Using seismic tomography, they demonstrate that the Ghofar and Quebrada OTFs on the East Pacific Rise contain a low velocity zone, which is interpreted to indicate increased (and fluid filled) porosity in gabbros and highly altered mantle peridotites. There, earthquake focal mechanisms point toward a strong lithological control on the mechanism of fault slip at OTFs through, for example, fracturing, increased porosity, and presence of fluids at seismogenic depths (Roland et al., 2012).

While OTFs have been commonly regarded as conservative, so-called “leaky transforms” in which extrusive volcanism and transtension occur along a significant proportion of its length were recognized early on from, both, continental as well as oceanic settings (e.g., Favela & Anderson, 1999; Garfunkel, 1986; MacDonald et al., 1979). Such “leakiness” occurs when the rotation pole moves in a way that generates intra-transform extension due to a significant change in spreading kinematics (Pockalny et al., 1997) or where opposing ridge segments propagate beyond the active transform to generate a rotating microplate (Bird & Naar, 1994). At the Siqueiros transform zone in the eastern Pacific, leaky ridge-transform fault zones (RTFZ, Figure 1) are partitioned into several minor transform fault offsets by small intra-transform spreading centers (Fornari et al., 1989; Hays, 2004). These ITSC's are extensional features and can be equivalent to strike-slip related pull-apart basins in continental crust (Embley & Wilson, 1992). The development of ITSC's within the Blanco OTF has been interpreted to result from transtension which prevents the development of a single through going OTF (Embley & Wilson, 1992). Recent sampling of basaltic picritic lavas within these ITSC's indicates crustal accretion occurring within the transform zone (Perfit et al., 1996). Magmatic additions within OTFs, through the pooling of lavas, or from dykes which cross the transform from adjacent spreading ridges, have been proposed by Gregg et al. (2007) to account for transform-parallel negative gravity anomalies adjacent to the main deformation zone. At the Wilkes Fracture zone on the East Pacific rise, oblique ridges and rhombus-shaped pull-apart basins are observed (Kureth & Rea, 1981), evidencing transtensional shear between the walls of the transform zone (Goodliffe et al., 1997).

Mid-oceanic ridge-transform fault processes have been explored by means of analog (e.g., Oldenburg & Brune, 1972) and numerical modeling (e.g., Fujita & Sleep, 1978) since the 1970s, although as stated by Gerya (2012), in a much less systematic way compared to their observational counterparts. For the purpose of this paper, we focus on two numerical studies which address the orientation of the fracture zone/plate boundary geometry in depth. A finite element model of slow-spreading second-order ridge discontinuities (van Wijk & Blackman, 2005) generates a similar plate boundary geometry which is transient and inherently unstable in contrast to transform fault plate boundaries which are stable until acted upon by external factors. In a plastic shear-based numerical model, Grevemeyer et al. (2021) generates oblique shear and horizontal extension within an OTF, that increases with depth and focuses on the inside corner of the ridge transform intersection. The resulting plate boundary is curved and evolves from vertical at the transform center to a shallower antithetic dip at the inside corners. From bathymetric shallowing as the OTF transitions to OFZ, Grevemeyer et al. (2021) also proposed a second stage of crustal accretion as the inside corner crust is displaced along the transform and juxtaposed against the opposing spreading ridge where the volcanism of this ridge partially fills the transform valley. These numerical model studies question the validity of a simple conservative OTF nature and modify their current understanding as strike slip fault dominated zones. The deformation styles and plate boundary geometries predicted by these numerical models have so far not been corroborated by geological case studies.

3 Study Area

The study area is located in the territorial waters of São Tomé and Príncipe (STP) in the Equatorial Atlantic, approximately 150 km west of the African coastline. Based marine magnetic anomaly patterns and plate kinematic models, it is inferred that the oceanic crust in the study area was formed during the Cretaceous Magnetic Quiet Zone (CMQZ, Granot et al., 2012; Seton et al., 2020) making precise dating through magnetic anomaly patterns impossible. Based on plate kinematic modeling, the onset of oceanic spreading in the northern segment of the South Atlantic is dated as late Barremian to early Aptian age (Heine et al., 2013), and the crust within the study area is likely to be of mid-Albian (ca. 110–105 Ma) age (Figure 1, Seton et al., 2020). The direct conjugate spreading system to our area of interest in São Tomé and Príncipe crust lies off the Brazilian Pernambuco plateau (Figure 1b). The seabed within the study area gently dips toward the west with an average water depth of 2.7 km, and at least 3 km of sediments bury the top of the oceanic crust (Figure 1).

The continental margin inboard of the oceanic crust has been described as an oblique shear margin as expressed by very steep continental slopes and a strongly laterally offset landward limit of oceanic crust (LaLOC, sensu Heine et al., 2013; Turner et al., 2003; Wilson et al., 2003). A series of OFZs terminate close to the LaLOC in the study area including the Ascension fracture zone in the south (Figures 1 and 3a).

The naming of other OFZs from the Ascension OFZ toward the North variously comprises the Bata, Campo and Kribi OFZ but is inconsistent across previous studies (Dailly, 2000; Lawrence et al., 2017; Meyers et al., 1998). The data resolution in this study also allows for the recognition of several new minor discontinuities which confuse the previous nomenclature. In this study, we apply the name Ascension OFZ as per the General Bathymetric Chart of the Oceans (https://www.gebco.net/), and Bata OFZ to the most northerly OFZ in the data set (Figures 3a and 3b) where it terminates close to the town of Bata in Equatorial Guinea. The main OFZ between these two is here termed “Central fracture zone.” This OFZ has previously been called both the Bata OFZ (Meyer et al., 1998) and the Ascension OFZ (Dailly, 2000; Lawrence et al., 2017).

The OFZs were preferentially tectonically inverted during a Santonian age compressional event seen regionally across west Africa and resulting from variation in the late Cretaceous pole of rotation (Guiraud & Bosworth, 1997; Lawrence et al., 2017; Turner et al., 2003).

4 Data Sets and Methods

The 3D broadband seismic survey used here was acquired in 2017 by CGG, on behalf of Kosmos Energy as part of their hydrocarbon exploration commitments in São Tomé and Príncipe. Fourteen streamers were used with a slanted cable increasing in depth from 10 to 30 m with length along the profile, a total cable length of 7,950 m and a streamer separation of 100 m. Overall 114 sail lines were completed in a NNE orientation with a 25 m shot spacing giving an 81 fold of data. A dual airgun source with a total volume of 4,360 cubic inches provided the acoustic energy with a source depth of 7 m. For processing purposes a Sound Velocity Profiler provided water velocity throughout the water column. The data used in this study comprise a Pre-Stack Depth-Migrated (PSDM) volume. A starting velocity model, generated by a standard Pre-Stack Time Migration workflow, was updated by joint tomographic inversion to produce an anisotropic Kirchhoff PSDM migration volume to a record length to 10 s, giving a maximum depth in the range of 11–12 km below a datum of mean sea level. A radon demultiple and denoise cleaned the gathers before stacking on a regularized 25 m bin spacing. We provide an example of a flattened gather in Figure A1. A Top Oceanic Crust (“Top Crust”) dominant frequency of ∼16 Hz gives a maximum possible vertical seismic resolution of ∼70 m. To highlight deep crustal geometries below the sediment pile a low pass bandpass filter (4.2 to 9–24 to 36 Hz) and an amplitude normalization filter (AVC) was applied to the 3D seismic data set.

A maximum vertical velocity model resolution of 100 ms at 5 km depth gives a velocity model sample point roughly every 600 m. An extraction from the final PSDM velocity model is provided in Figure A1 at the same location as the accompanying gather. The PSDM process aims for flat gathers, migration of reflections back to the correct location, and to provide for the ability of the stack to improve the signal-to-noise ratio. The velocity model shows some velocity gradient changes within the crust, but at 3–5 km burial depth it is incapable of resolving key velocity breaks in the manner of controlled source studies on currently active ridge-transform systems (Gasperini et al., 2017; Marjanović et al., 2020). Despite therefore not being processed specifically for the oceanic crust, with the resolution at this depth a smooth velocity model is fit for purpose in the aims of this study.

The polarity of the data as presented here is “SEG Normal” such that a peak (positive amplitude value) is an acoustically hard boundary and colored blue. Conversely, a trough (negative value) is an acoustically soft boundary and colored red.

Seismic horizons were mapped in 3D on the PSDM seismic volume and in the two-way-time (TWT) domain (using the PSDM interval velocity model to convert to TWT). This allowed for the depth conversion of seismic horizons using a range of alternative velocity models which were subsequently not used in this study. A combination of manual picking and autotracking was used to pick the horizons. The resulting interpretations were gridded using a minimum curvature method to allow for area calculations and attribute extraction. To assess the evolution of the OFZs and abyssal hills fabric, a number of measurements were extracted from the gridded events along profiles that were mapped using polylines. Window-based Root Mean Square (RMS) Amplitude extractions are included to highlight crustal architectures and thickness maps are calculated between mapped events for volumetric calculations.

The Gravity data (Figure 3b) is GETECH Pty's Multi-Sat 2020 satellite altimetry-based data compilation spliced with ship-borne data using grid-based data merging algorithms (Getech Group, 2020).

The ship-borne gravity data was acquired in parallel with the 3D PSDM seismic survey. The data set has undergone a conventional marine gravity processing that included Eötvös, Meter drift, base station, Free air, and 3D Bouguer corrections followed by line-oriented low-pass filtering and tie-line leveling to produce final Bouguer-corrected gravity anomaly. The 3D Bouguer correction has included calculation of terrain effects up to 100 km away from the survey area and assumed a sediment density of 2.2 and water density of 1.03 g/cc.

The MultiSat 2020 gravity has been derived from satellite altimetry measurements and had Free Air, Bouguer and terrain corrections applied. Bouguer and terrain corrections assumed rock density of 2.67 onshore and 2.2 g/cc offshore (with a sediment-water density contrast of 1.17 g/cc).

Finally, a sixth order Butterworth band-pass filter has been applied to the merged Bouguer gravity anomaly grid to enhance spatial wavelengths in the range of 10–100 km.

5 Seismic Observations

In contrast to studies of modern oceanic crust and active OTFs, the crustal architecture described here is overlain by a varying sedimentary thickness between 2,350 and 4,410 m across the entire study area. Local to OFZs the Top Crust and the overlying sediments can be tectonically inverted. This is dated to a regional Santonian compressional event as defined by Lawrence et al. (2017) (Figure 3). For the purposes of this study the Santonian inversion is not discussed. Where appropriate the seismic sections are flattened on a continuous sedimentary reflector above the Top Crust (e.g., Figure 7) to remove the Santonian inversion effect. The Santonian inversion post-dates the oceanic crustal age in the study area by ∼25 Myr and is believed to not impact the relationships between the crustal architectures described here.

5.1 Seismic Stratigraphy of the Oceanic Crust in the Study Area

The oceanic crust has a variable seismic stratigraphy broadly consisting of a tripartite subdivision. The Top Crust is clearly defined as a low frequency, high amplitude peak (positive impedance contrast). The horizon has been mapped across the complete survey, ranging between depths of 4.9 and 7 km below sealevel (Figure 3). It overlies a unit of horizontal to sub-horizontal high amplitude reflectors which are continuous over 2–5 km in length and have a mean thickness of 600 m based on an isochore extraction (Figure 5a). Based on amplitude variations, we infer a mix of acoustically softer and harder lithologies are present. This unit also shows stronger amplitudes around the OFZ valleys where it thickens to >4,000 m (“Unit 2,” Figure 3). Reflectors display draping and onlapping to the margins of the OFZ. The base of this unit is not a single reflector but variable and undulatory.

Below the reflective unit an acoustically transparent unit can be mapped down to the base of the crust (“Unit 3,” Figure 3). In places, the lower half of this section consists of numerous strong and steeply inclined high amplitude reflectors which predominantly dip toward the bounding OFZ. These inclined reflectors are limited to the western part of the central crust segment (Figure 5a). The top of the inclined reflectors is defined by a series of isolated, extremely strong, sub-horizontal, and acoustically hard reflectors with subtle dips toward the OFZ (Figure 3).

In places, a strong horizontal to sub-horizontal reflector, or series of anastomosed reflectors are observed close to the seismic record length (Figure 3). Their depths shows a good spatial correlation to Bouguer-corrected gravity anomalies, with shallow depths corresponding to more positive anomalies (Figure 4b and Figure A2b). We interpret this reflector as the oceanic Moho, indicating crustal thicknesses ranging between 4.2 and 6.7 km (Figure 5b). These crustal thicknesses match well with Moho observations from long offset 2D seismic in this area as presented by Lawrence et al. (2017).

Due to the limitations of the PSDM velocity model at this burial depth we define the crustal stratigraphy by its clearly differentiated seismic facies rather than the typical layers (e.g., Layer 2A, 2B, and 3) which are defined by velocity and velocity gradient (Christeson et al., 2019). The upper reflective unit (our Unit 2) and is interpreted to contain Layer 2A (lavas) plus sediments. The acoustically transparent middle unit (our Unit 3) is likely to consist of both Layer 2B and Layer 3 (lavas, transition to sheeted dyke complex, and gabbros). Steeply inclined reflectivity, as present in the base of Unit 3, has been interpreted in many seismic studies of slow to fast spreading oceanic crust, including Alaska (Bécel et al., 2015), the South China sea (Ding et al., 2018), the NW Pacific (Reston et al., 1999) and from the Enderby basin of the Indian Ocean (Sauter et al., 2021). The typical lack of offset in the Moho and Top Crust points away from brittle faulting and toward a magmatic or lithological origin. Interpreted mechanisms range from magmatic faults (dyke intruded faults) near the spreading ridge (Qin & Buck, 2008; Sauter et al., 2021) to a ductile-deforming shear zone coupled with stress-driven melt segregation. Mylonitization or crystal alignment can also lead to seismic anisotropy and seismic reflection generation (Bécel et al., 2015).

5.2 Structuration of the Top Oceanic Crust

The Top Oceanic Crust reflector shows relief in excess of 2,000 m and contains two clear structural fabrics: a spreading-induced abyssal hills topography, and NE-SW trending OFZ troughs (Figure 4b).

The abyssal hills fabric strikes Northwest to Southeast as a densely spaced series of ridges and troughs with a wavelength of typically 3–6 km and a relief up to 300 m (Figure 4b). The rugged relief of the Top Crust implies a strong axial ridge topography and a relatively slow spreading rate (Small, 1998). Spreading velocities interpolated from plate models agree on a full spreading rate of ∼40 mm/yr (Heine et al., 2013; Pérez-Díaz & Eagles, 2017). The second fabric consists of three deep troughs striking NE-SW coincident with the oceanic fracture zones (Figure 4b) against which the abyssal hills crests can be seen to terminate. From South to North they are the “Ascension OFZ,” the “Central OFZ,” and the “Bata OFZ.” Another minor OFZ is interpreted solely from seismic observations below the Top Crust (Section 5.3.2) and is situated immediately north of the Central OFZ. It is not well covered by the seismic data, and continues out of the survey area to the south-west. It is termed here the Northern OFZ (Figure 3).

Across the Ascension and Central OFZ, the Top Crust relief shows an abrupt deepening toward the northwest, corresponding to strong lateral changes in the gravity data (Figure 4a). The deepening is expressed as an increase in the dip of <60°NW, forming a steep scarp which is onlapped by sedimentary reflectors (Figure 3). On seismic time slices, a mean OFZ azimuth of ∼N60°E is observed (Figure 6), although the seismic expression is not simple but consists of several sinuous segments from 25 to >50 km in length with an azimuth ranging within 30° of this mean. In addition, the morphology of the Top Crust shows the OFZ scarps to be irregularly indented by the ridges and valleys along their lengths (Figure 4b).

An isochore of the onlapping sedimentary package was made using the Top Crust and a regionally continuous reflector which continuously drapes the Top Crust (Figure A1). The maps indicate that OFZs were localized topographic lows. The maximum vertical offset mapped across the Central OFZ scarp, as measured from the thickness of onlapping reflectors, is 1,280 m. The onlapping sedimentary thickness decreases to ∼100 m in the northeast, reflecting a loss of vertical offset where its topographic expression and gravity expression diminish and terminate (Figures 4a, 4b, and 5b). These observations suggest that the Central OFZ becomes a crustal scale feature only within the study area and not directly at the LaLOC which is located 20–30 km northeast beyond the data limits (Figure 4a). This fracture zone can therefore be associated with the early stages in ridge splitting and oceanic fracture zone formation.

The widths of the Central OFZ and Ascension OFZ both commonly range from ∼4 to ∼9 km based on the sedimentary onlap and lack of abyssal hills (Figures 4a and 5b). Along the Central OFZ, the fracture zone width locally indents up to 11 km into the central crust segment with an absence of abyssal hills ridges (Figure 4b). This locale is morphologically smooth on the Top Crust map and is interpreted as a nodal basin (Grevemeyer et al., 2021).

A right lateral offset is expected to have been present over these OFZ when they were actively offsetting the Albian-aged spreading ridge based on the present-day LaLOC geometry and plate kinematic models (Figure 2). To constrain the paleo offset of ridge segments across the OFZ, attempts were made to map downlaps of sedimentary packages onto the oceanic crust across the OFZ (e.g., age equivalent crust). While not fully conclusive, offsets in the order of 30–50 km are deduced from the seismic data. This degree of offset agrees with the geometry of the LaLOC as interpreted by various authors and would be reasonable given the close proximity of the study area to the LaLOC.

5.3 Oceanic Fracture Zone Internal Character

Mapping the base of Unit 2 shows that its thickness varies considerably. The base of Unit 2 deepens abruptly when transitioning into the OFZ, with the thickness increasing from ∼600 to >4,200 m (Figure 5b). Within the OFZ, Unit 2 is the dominant crustal unit, making up <85% of the crustal thickness within the Central OFZ and Ascension OFZ. Observations on time slice data indicate that the OFZs can reach ∼15 km in width at depth (Figure 4b), <150% wider than their Top Crust expression. In addition, we observe local Moho elevations around the Central, North and Bata OFZs (Figure 3) where crust thickness decreases to <4 km (Figure 5a).

Below Top Crust, Unit 2 consists of high amplitude reflectors with similar character to the abyssal hills, which consistently dip into the OFZ and display a growth geometry (Figure 7). At their downdip end, reflectors rapidly dim and onlap against the transparent seismic facies of Unit 3 at the margin of the OFZ. The onlap surface is commonly subvertical but can be low angle and dominantly progradational (Figure 7a). This dipping package is termed “Transform Normal Dipping Reflectivity” (TNDR).

At the base of the TNDR package in both the Central OFZ and Ascension OFZ are distinct reflectors which form the lower boundary. They are continuous from the Top Crust to the locally elevated Moho (Figures 7a and 7b) and are continuous along the OFZ lengths. It is a distinct, but genetically related feature and is identified as “TNDR-Base” (TNDR-B). These features are separately described below.

5.3.1 Transform Normal Dipping Reflectivity Sequences (TNDR)

The TNDR package displays significant variability. Their overall direction of dip is variable but always broadly perpendicular to the OFZ trend (Figures 7a and 7b).

Within the Ascension OFZ, the TNDR package dips and thickens toward the northwest where it downlaps on to the TNDR-B reflectors in the manner of syntectonic growth against a listric fault (Figure 7b). The deepest TNDR reflectors of the Ascension OFZ dip >8° with dips generally decreasing to sub-horizontal with shallowing.

Within the Central OFZ, the TNDR dip changes along strike (Figure 7a). In the southwest, the TNDR reflectors dip in the same direction as the TNDR-B reflector. In the northeast, the dip of the TNDR reflectors is opposing to the TNDR-B.

In 3D, the TNDR dip is however oblique to the OFZ margins as seen in seismic strike lines parallel to the Central OFZ (Figure 9). Below Top Crust the reflectors demonstrate offlapping toward the southwestern direction of spreading, terminating at depth within the Unit 3. In places the offlapping trajectory deepens and terminates, to become overlain by a thinner and younger set which continues the southwestward trend (Figure 9, boxes B). The continuity of individual reflectors within the TNDR package can reach 20 km along strike and bears resemblance to magmatic flow units in seaward-dipping reflector geometries (SDRs) well-known from magma-rich passive margins (e.g., Geoffroy, 2005 and references therein).

The shallowest point of each TNDR reflector commonly overlies the OFZ margin and on to adjacent crust. Mapping individual reflectors demonstrates the presence of onlap onto older reflectors, as well as offlap within the growth packages (Figures 7a and 7b). This is again in the manner of a sedimentary sequence within a growth fault with extensional deformation controlling local accommodation trends.

Overall, the TNDR geometries can be subdivided into distinct packages based on onlap/offlap relationships as well as the presence of discordant boundaries denoting marked dip changes. The bottom 1–2 reflectors form a package which dips relatively steeply and is spatially restricted, onlapping to the TNDR-B reflector, as well as to Unit 3 packages of mid-crustal level on the opposing margin (Figure 7 labeled package A). Above this, a thicker and wider package of TNDR sequences occurs, terminating by onlap or abruptly onto the upper Unit 3 reflectors, and much wider than the Top Crust expression of the OFZ would suggest (Figure 7 package B). This package contains most of the dip divergence and growth evidencing vertical aggradation during tectonic tilting. Onlap to the mid crust would tend also to suggest an exposed and dipping surface across which the TNDR pinchout advanced as it aggraded. Shallower packages contain geometries varying from seismically transparent (Figure 7b) to restricted and local onlapping to deeper tilted reflectors (Figure 7a).

The shallowest TNDR packages are always overlain and sealed by the subhorizontal reflectivity of Unit 2 which is continuous from abyssal hills adjacent to the OFZ. Seismic time slices at this level show J-shaped ridge terminations in which the orientation changes from southeast to southwest, verging toward the direction of spreading (Figure 6b).

On seismic time slices between 6,710 and 7,500 ms TWT (Figure 6a), a series of circular features can be mapped within the TNDR packages. They are acoustically hard, up to 1.5 km in diameter and up to 300 m thick. While typically isolated, they can be laterally amalgamated with neighboring examples. In seismic section they are subtle and difficult to identify but can be associated with reflectors displaying local undulations and short wavelength relief (Figure 6a). On amplitude extractions the circular features stand out as relatively bright. Similar features can also be mapped on the higher elevated margins of the OFZ and on the abyssal hills fabric (Figure 6a).

Onlap and offlap within the TNDR sequences as well as on adjacent crust point toward depositional, infilling geometries. The genetic link between the TNDR packages and the overlying abyssal hills facies with their J-shaped geometries, indicates a similar extrusive flow origin for both, as calibrated by ODP wells and outcropping ophiolite examples (Fowler, 2012; Larssen et al., 2009). Based on these observations we interpret the TNDR to represent a thick series of extrusive lava flows being deposited within spatially confined accommodation within the OFZ (Figures 6 and 7). The rotation and subsidence of the reflectors is likely caused by dominating extensional processes during TNDR deposition. The variability of the TNDR sequence thickness along strike of the OFZ examples (Figure 5a) demonstrates that the magmatic budget of material extruded into the OFZ is highly time-variable. In addition, the semi-circular features are interpreted as likely constructional volcanic edifices and their associated intra-crustal feeder systems. Their occurrence throughout the full depth of Unit 2 would indicate the focused eruption of lavas from isolated point sources within the OFZs and potentially on the more elevated margins of the OFZ. The dipping TNDR reflectivity shows similarities to SDR geometries observed along passive margins where plate separation is accommodated by magmatic dilation (e.g., Buck, 2017), however, the relationship to the magmatic plumbing system and clearly mappable structural features related to dilation (i.e., dykes, faults) remain enigmatic.

The late stage sealing of the TNDR package by Unit 2 with J-shaped planform geometries, constrains the relative timing of the TNDR package to be during accretion of the oceanic crust and proximal to the spreading ridge. We therefore interpret the TNDR sequences as an architecture related to magmatic and extensional transform processes rather than crustal processes acting upon a fully formed OFZ. The subdivision of TNDR sequences into coherent geometrical units also indicates common evolutionary phases, from early and localized subsidence, to accelerating syndepositional extension, and onlap to crust which was likely to be contemporaneously and actively accreting.

5.3.2 Transform Normal Dipping Reflectivity-Base (TNDR-B)

The basal reflectors of the TNDR package in the North, Central, and Ascension OFZs are a separate package of dipping reflectors which is termed the TNDR-B (Figures 7a and 7b).

Striking parallel to the margins of the OFZ, the TNDR-B connects the TNDR package base on one margin, to the Moho on the opposite margin. In planform view using time slices, the TNDR-B surfaces have a narrow, focused, and locally curvilinear geometry (Figure 8d). The Moho join can occur up to 5 km beyond the edge of the Top Crust OFZ margin, and below the abyssal hills fabric (Figure 7a). Along its shallower sections, the TNDR-B expression is broadly conformable to the base of the overlying TNDR package. At depth its expression is that of a narrow zone of anastomosing reflectors with some cross-cutting relationships (Figures 7a and 7b). Orthogonal to the OFZ trend, the TNDR-B dips steeply <60° at shallow levels, decreasing to become conformable with the Moho at depth (Figure 7). An average dip of 33° is calculated for the OFZ examples in the study area (Figure 10). In cross section, the base of Unit 2, and in places the Moho are vertically downthrown across the TNDR-B surface, with the downthrown side accommodating the thick TNDR sequence above the TNDR-B reflectors (Figures 3 and 7).

Five TNDR-B surfaces are identified within the three OFZs that have seismic coverage (North, Central, and Ascension OFZ). Each was mapped in detail and are sequentially numbered from 1 to 5 (Figure 8a). They have continuous lengths up to 75 km within an OFZ, although only two examples within the Central OFZ are mapped for their entirety within the data limits. At the along-strike termination of TNDR-B 3 we interpret a hard-link with TNDR-B 4 which has antithetic dip, analogous to antithetic faults from extensional fault systems (Figures 9a and 10a).

RMS amplitude extractions were calculated for each TNDR-B surface using a symmetric window of 150 ms TWT about the horizon. Figure 8c shows the example of TNDR-B 3 with the deepest portion showing a high amplitude response due to its intersection with Moho reflectivity. In shallower areas, high amplitudes can be seen where the thickest parts of the TNDR are located above the dipping TNDR-B reflector. Thin ribbons of high amplitudes can also be identified on TNDR-B 3 and 4 (Figure 8c), occurring at mid crustal levels where they plunge steeply across the TNDR-B structure contours. Furthermore, with shallowing the plunge of the high amplitudes often decreases by 30°–90°, becoming increasingly parallel to the TNDR-B strike (Figure 8c).

The dip profile of the TNDR-B surfaces have a geometry comparable to extensional detachment faults mapped in oceanic crust which show increasing dip with depth, detachment onto the Moho, and with a shallow detachment on to Top Crust (Reston & Ranero, 2011). The entire crustal thickness is also detached across the TNDR-B, with Unit 3 seismic facies being thinned and necked against it (Figure 6a). Low velocity zones are identified in many OTFs (Roland et al., 2012; Van Avendonk et al., 2001), and in a 2D active source study of the Marathon OFZ in mature (60–75 Ma) equatorial Atlantic oceanic crust. Davy et al. (2020) attributes the low velocity zone to a model of decreasing melt supply toward the ridge segment ends and consequent thinning of Layer 3. It is also possible that such low velocity zones and the absence of velocities typical of Layer 3 (White et al., 1984) are analogous to active crustal thinning at a TNDR-B type boundary.

Additionally, there is a clear genetic link between the TNDR-B features and the overlying and tectonically tilted extrusive flows (TNDR). Given the accommodation space-creation and the rotation of the TNDR flows in a growth type pattern, we consider a syn-depositional extensional detachment fault or shear zone as a potential interpretation. Low angle detachments are described from oceanic ridges and the inside corners of ridge-transform intersections in slow and intermediate rate spreading regimes (Blackman et al., 2008; Reston & Ranero, 2011; Sandiford et al., 2021; Smith et al., 2014). They exhume a footwall massif of peridotites, gabbros, and serpentinized lower crust rocks (Andreani et al., 2014; Pressling et al., 2012) and detach onto the top of exhumed crust. Their dip typically increases until subvertical at depth and is oriented toward the spreading axis and not the OTF unlike the TNDR-B examples (a 90° strike difference). We also do not observe a prominent footwall massif. Nevertheless, the TNDR flows commonly dip away from the footwall, a geometry that requires relative uplift of the footwall compared to the hanging wall.

In combination with accommodation space creation and a necked lower crust, the TNDR-B surfaces are interpreted to represent a broad shear zone accommodating extensional displacement at a high angle to the OTFs, below the base of the TNDR package. Their dip and strike rugosity at the kilometer scale would prohibit the TNDR-B surface from accruing significant strike slip displacement as might be expected at OTFs. The anastomosed reflectivity also suggests a zone of several tens to hundreds of meters in width with acoustic (lithological) variability along its length rather than a discrete fault plane accommodating extension. A dilational shear zone containing discrete and variable lenses of melt along its length or generating crustal alignment might satisfy these criteria (e.g., Bécel et al., 2015; Qin & Buck, 2008; Sauter et al., 2021). It is possible that the generation of reflections within the TNDR-B package is similar to lower crustal dipping reflectors along spreading ridges which relate to high temperature and syn-magmatic ductile faults (Sauter et al., 2021). The genetic link between the lava flows (TNDR), the TNDR-B surface, and potential volcanic edifices in their proximity would support the interpretation that the TNDR-B can act as melt pathways feeding sheet-like lava flows within the OTF valley. The plunging bright seismic amplitudes on the TNDR-B surfaces may indicate the position of localized melt paths from base lithosphere to upper crustal levels. However, such clear genetic link between these features supporting the tectono-magmatic evolution remains enigmatic at present.

6 Discussion

We present the first detailed structural observations of OFZs and their surroundings from an 8,000 km² region of Albian-aged oceanic crust using 3D PSDM seismic data. The interpretation of dipping reflectivity within the OFZ indicate extensional processes within OTFs as well as accretionary magmatism, and shares similarities with the results of OTF numerical models which we discuss here. To clarify terminology within the discussion, we refer to the spreading ridge which generated the crust as the “leading ridge,” and the opposing ridge that the crust is later juxtaposed against as the “trailing ridge.”

6.1 An Observation-Based Model of Extensional and Non-Conservative Transform Processes

The presence of abyssal hills facies overlying the OFZ internal architectures clearly date the TNDR to be prior to the end of ridge based volcanic processes. The mapped lengths of TNDR-B 3 and 4 are also over double the estimated ridge-ridge offset of ∼30–50 km for the Central OFZ, suggesting that this deformation style is not transient, but a continuous process with a duration in excess of 5 Myr (assuming a half spreading rate of 20 mm/yr over the 100 km length of TNDR-B 3 and 4).

In contrast to studies of active OTFs and Ridge Transform Fault Zones (RTFZs), the observations here are limited to in situ crust. On the African Atlantic margin, only crust generated on the eastern and trailing flank of the Cretaceous ridge system is preserved (Figures 2 and 11a). We therefore observe only the southern margin of the transform (Figure 11a), which in the final OFZ comprises the inside corner of the leading ridge, juxtaposed against the northern outside corner of the trailing ridge (Figure 11a). The inside corner from the northern margin of the OTF is preserved southeast of the Pernambuco Plateau/northern Sergipe basin of Brazil (Figure 1b). Assuming symmetrical OTF processes across the paleo-transform, it is reasonable to assume conjugate TNDR packages are preserved on the South American side. This opens up the intriguing possibility that the transform architectures contained two conjugate TNDR-B surfaces overlain by opposing TNDR packages, with the strike slip plate boundary separating them.

Figures 11a–11e illustrate a proposed ridge-transform kinematic model for generating these architectures assuming a right-lateral ridge-ridge offset. It is based on the observations described in our study combined with numerical models from Grevemeyer et al. (2021). It is therefore a departure from the classic assumption of a conservative, simple vertical strike slip fault accommodating opposing crustal spreading directions across the transform fault. The numerical model of Grevemeyer et al. (2021) generated a vertical strike slip fault in the initial conditions and evolved to produced inside corner extension proximal to the ridge intersections. The dip of the plate boundary decreases away from both opposing ridge segments until vertical in the transform fault center. The model overview in Figure 11a introduces the terminology used and incorporates both the plate boundary geometry and Beta stretching factor (β) contours produced by Grevemeyer et al. (2021). Our mapped TNDR-B surfaces approximate this plate boundary geometry, shallowly dipping toward the south as would be expected given the preservation of only the southern plate margin with a right-lateral ridge offset. Only the TNDR-B3 surface dips north but is hard linked with the south dipping TNDR-B4 surface. This could indicate an evolution from a dextral ridge-ridge offset (TNDR-B4), to a sinistral ridge-ridge offset (TNDR-B3), with the transition zone between them (where ridge-ridge offset was effectively zero) representing the hard linked section.

The narrow and deep zone at Top Crust incorporating the transform valley is termed the transform deformation zone (TDZ) following the nomenclature of Grevemeyer et al. (2021) (Figure 11a). The steep scarp bounding the deformation zone at Top Crust might be termed the transform fault as observed in modern studies (Maia, 2019). The 3D nature of the seismic data however shows that below Top Crust the TDZ is far wider, prior to being overlain and hidden by Unit 2 volcanics. The deposition of Unit 2 over the northern margin of the OFZ, including J-shaped ridge terminations, must come from the trailing ridge, as proposed by Grevemeyer et al. (2021) to explain a shallowing of bathymetry from TFZ to OFZ in several modern examples. These observations corroborate the 2-stage accretion of crust proposed in that study. Grevemeyer et al. (2021) also demonstrated J-shaped abyssal hills geometries will verge toward the leading ridge as seen in this study, supporting the right-lateral ridge offset predicted in plate models.

The presence of Unit 2 covering the TNDR packages on the southern side of the TDZ must come from the leading ridge prior to, or during displacement of the inside corner crust along the transform fault (Figure 11b). Answering how far from the leading ridge can Layer 2A (equivalent to our Unit 2) be deposited, will provide a constraint on the timing and location of the extension and volcanism of the TNDR package given it is contemporaneous with Unit 2 deposition.

Seher et al. (2010) documents a relatively constant thickness of Layer 2A from axial valley to off-axis at the Lucky strike segment of the Mid Atlantic Ridge. Christeson et al. (2010) observe similar off-axis thickness trends with a mean Layer 2A thickness of 485 ± 135 m at intermediate spreading rate crust adjacent to the Blanco Transform Fault along the Juan de Fuca ridge. Estep et al. (2019), in a review of Layer 2A thicknesses from both slow and intermediate spreading rates in the south Atlantic, observe a mean thickness of 780 m in crust from an intermediate spreading regime, and a mean of 690 m in crust from a slow spreading regime without rapid off-axis thickening of Layer 2A. These values are in reasonable agreement with the thicknesses of Unit 2 observed away from the OFZs (Figure 5a) and suggest the thickness of Layer 2A is built primarily within the ridge axial valley. Applying these observations, extension and thinning of the inside corner occurred extremely early, perhaps even below the leading ridge flank. Weekly et al. (2014) in contrast documents significant off-axis volcanism and thickening of Layer 2A laterally over several kilometers from the intermediate spreading rate Juan de Fuca ridge based on seismic tomography calibrated to seafloor observations. It is entirely possible therefore that while the extension is limited to the proximal inside corner as predicted by Grevemeyer et al. (2021), the TNDR package is generated over several kilometers of the transform while still being overlain by off-axis ridge originating lavas. As the TNDR package is displaced along the OTF it would also be partially overlain by a younger TNDR package being developed closer to the leading ridge and at the new (younger) inside corner. A continuous process of thinning and TNDR deposition would be preserved as offlapping packages as is seen along the length of the Central OFZ (Figure 9).

Figure 11c proposes that crustal thinning occurs primarily through shearing of the inside corner lower crust to zero thickness across the TNDR-B surface where it abuts the strike slip shearing and displacement of the transform fault. Vertical deformation features are extremely difficult to image on reflection seismic data, particularly at the significant depths in this study, and we are unable to recognize coherent reflectivity in this area which might indicate strike slip fault surfaces.

Low seismic velocity zones observed within OTFs are typically interpreted to result from fracturing and higher porosities associated with strike slip deformation (Van Avendonk et al., 1998). White et al. (1984) describe this phenomenon as the absence of seismic velocities which are typical of Layer 3 (e.g., mean global velocity range of 6.53–7.12 km/s) with low vertical gradients (Christeson et al., 2019). In addition, the lack of Layer 3 velocities could result in this model from the tectonic removal of high velocity crustal material as the crust is necked. Away from the necking zone and toward the normal oceanic crust, TNDR flows progressively onlap to the transparent seismic Unit 3, suggesting that Unit 3 crustal accretion (Layer 2B and 3) in the ridge segment was contemporaneous with thinning and lava deposition within the RTFZ.

Figure 11e proposes that once the inside corner crust is displaced adjacent to the trailing ridge, a second phase of lava deposition occurs within the TDZ. Fornari et al. (1989) suggest that at the fast spreading Sequeiros transform fault, the “overshooting ridge tips” that result in J-shaped ridge terminations may be indicative of a continuous magma chamber beneath the entire length of the adjacent ridge segment, feeding dykes directly across the transform fault. Gregg et al. (2007) in a study of 19 globally picked OTFs similarly conclude that particularly at intermediate and fast spreading crust, overshooting ridge tips across the transform may generate an increase in crustal thickness, explaining an increasingly negative gravity anomaly (e.g., mass deficit) with increasing spreading rate. At slower spreading rates then, more positive gravity anomalies at OTFs would tie with thinner crust, reflecting less magmatic ridge segment ends abutting the OTF, and restricted across-transform dyke propagation (Gregg et al., 2007). If the model outlined here is an analog for OTFs associated with slow spreading crust, then the more positive gravity anomalies described by Gregg et al. (2007) could result from inside corner necking of high velocity and density crust to be replaced with lower density lava flows, subsequently deformed and fractured.

The Moho response to this process is hard to ascertain due to the limitations of the seismic record length, however an elevated Moho at the OFZs could be a response to the crustal thinning and melt extraction to feed the extrusive flows.

6.2 Comparison to Active Oceanic Transform Faults

Extension within Ridge-Transform fault inside corners has been observed in a variety of configurations, with best documented examples being low angle detachment faults dipping toward the ridge axis on slow and ultraslow spreading ridges (Reston & Ranero, 2011). They have significantly different architectures to the TNDR-B surfaces outlined here however, dipping toward the ridge and not the transform, and are involved in the extension and exhumation of lower crust and mantle peridotites from beneath the ridge axis. Their inside corner morphology is also rough and high relief where the corrugated slip surface modifies an exhumed oceanic core complex. No high relief domal morphologies have been observed in this study, and the TNDR-B shear zone clearly dips perpendicular to these detachments. If a rough Top Crust morphology is indicative of such spreading aligned detachments then the inside corner extension detailed in this study would seem to be incompatible with such areas.

Nodal basins in contrast have a smooth and anomalously deep topography on the inside corner of slow spreading crust (Lagabrielle et al., 1992). They represent a significant topographic anomaly considering their young age and proximity to the spreading ridge (Lagabrielle et al., 1992) and are thought to be magmatically starved (Grevemeyer et al., 2021). Direct observation from submersible dives along the Vema ridge and Kane OFZ indicate some magmatic activity however, including active volcanic vents (Mamaloukas-Frangoulis et al., 1991; Mével et al., 1991). The Vema ridge constructional terrain includes a prominent volcanic ridge with basaltic pillows and sheet lava flows (Mamaloukas-Frangoulis et al., 1991) while at the Kane OFZ, deep crustal rocks are also present in an exhumed sequence capped by pillow lavas, interpreted to result from low angle stretching of heterogeneous lithosphere (Mével et al., 1991). We suggest that inside-corner extension along a TNDR-B surface as outlined here and in numerical modeling studies are a likely scenario for the formation of such topographic lows with extrusive volcanics. This process could mark the initiation of a TNDR package. We speculate that the limited spatial extent and discontinuous nature of nodal basins could indicate a transitory phenomenon related to a lower magmatic budget, preserving the tectonic stretching and subsidence characteristics. Return to a higher magmatic flux may partially fill the nodal accommodation resulting in the typical transform valley and TNDR style architectures.

To test whether the architectures described in this study are commonplace in more mature spreading settings across a range of spreading and kinematic scenarios requires the acquisition of densely spaced 2D or 3D seismic data across OFZs. We consider it possible that a similar process of transform oblique extension and non-conservative volcanism is commonplace at slow to intermediate spreading ridges where oceanic detachment faulting and core complexes are not present and where the magmatic budget is high enough. The J-shaped ridges and relatively uniform thicknesses of the oceanic crust across the study area would suggest a ready supply of melt. The breakup location in the center of former Gondwana (Heine et al., 2013) could also have affected the early seafloor spreading phases due to supercontinent insulation (Van Avendonk et al., 2017; Whittaker et al., 2008). Observations of excess magmatic activity (such as SDR wedges) along the conjugate North Gabon-Sergipe Alagoas margins and seamounts/SDRs on the oceanic crust outboard of these margin segments (Rosendahl et al., 2005) would support higher ambient mantle temperatures and hence allow for higher magmatic budgets during the early spreading phase.

Furthermore, Grevemeyer et al. (2021) document evidence for extension and late-stage volcanic sealing of the transform valley in 41 RTFZ examples from the Pacific, Atlantic and Indian Oceans, suggesting a widespread applicability of this concept. The São Tomé and Príncipe crust in the study area formed within ∼10–20 Myr after breakup in close proximity to the West African LaLOC. It has developed the TNDR-B surfaces extremely soon after ridge splitting. As such perhaps a key difference to slow spreading crust with core complex development is the relative instability of the melt supply and ridge processes in this study, and potentially the breakup position at the center of a supercontinent.

7 Conclusions

A high-quality 3D PSDM seismic survey allows for the mapping and characterization of complex dipping reflectivity within a series of oceanic fracture zones dissecting Cretaceous aged crust offshore São Tomé and Príncipe in West Africa.

The study area includes Oceanic Fracture Zones that, both, continue across, and terminate within the survey area close to the LaLOC. This suggests that spreading patterns and some ridge-transform offsets in the early oceanic crust were not inherited from continental margin offsets, but instead seem to be largely evolving as a function of minor changes in Africa-South America plate kinematics. Further, the breakup location, in the center of former Gondwana, lets us to speculate that anomalous sublithospheric thermal conditions could have affected the early seafloor spreading phases due to supercontinent insulation (Van Avendonk et al., 2017; Whittaker et al., 2008). Observations of excess magmatic activity (such as SDR wedges) along the conjugate North Gabon-Sergipe Alagoas margins and seamounts/SDR Type 3 on the oceanic crust outboard of these margin segments support higher ambient mantle temperatures.
Within all the OFZs are a series of stacked divergent reflectors dipping perpendicular and into the OFZ are identified. This TNDR is interpreted as a series of extrusive volcanic flows and sediments deposited at the ridge-transform inside corner contemporaneously with crustal accretion and displays OFZ strike-parallel aggradational patterns suggesting progressive offlapping and younging toward the spreading ridge.
The inside corner crust was thinned and necked against a basal (TNDR-B) surface interpreted as a broad dilational shear zone accommodating crustal extension. The Top crust underwent subsidence and rotation while the TNDR volcanic flows were being deposited into the transform topography.
The TNDR packages are overlain by Unit 2 seismic facies (Layer 2A plus sediments) constraining the timing of the extension as a transform process. As stretched crust is displaced past the trailing ridge segment a second phase of Unit 2 volcanic activity seals the TNDR package documenting 2-stage crustal accretion.
Numerical models (Grevemeyer et al., 2021; Van Wijk and Blackman, 2005) predict inside corner extension and plate boundaries which are vertical in the transform center while dipping away from the transform at the inside corners. We interpret the mapped TNDR-B surfaces to correspond to this predicted plate geometry. We propose that the shearing transform zone was bound on both sides by antithetic TNDR-B surfaces and the ridge-transform intersections were kinematically dynamic. A kinematic model is proposed which predicts a conjugate set of TNDR packages and antithetic TNDR-B surfaces which evolve from inner corner extension to a second magmatic phase as the inside corner is juxtaposed against the trailing ridge.

We consider our observations to have broad implications for oceanic ridge transform and slow spreading models as well as for modeling the nucleation of OFZs. In particular, the presence of topographically deep nodal basins, low seismic velocity zones within OTZs, as well as negative gravity anomalies along OFZs could result from the extension and necking of dense and seismically fast crust within the inside corner (Layer 3). Our observations support the outcomes of numerical models for active OTF systems, contradicting the view of conservative, vertical strike slip transform faults.

Acknowledgments

We would like to thank Shell International Exploration and Production and our joint venture partners Galp São Tomé and Príncipe Unipessoal, and the regulator, the Agência Nacional do Petróleo de São Tomé e Príncipe, for granting permission to publish this work. This paper would not have been possible without the exceptional subsurface data. We owe special thanks to as the Agência Nacional do Petróleo de São Tomé e Príncipe, regulator and data owner, who have kindly granted us permission to use the 3D seismic data for this paper. We gratefully acknowledge Getech Group plc (https://getech.com/) for allowing us to use their Multisat 2020 regional gravity data set. Figure 2 was created using the open source Generic Mapping Tools software (Wessel et al., 2019). Milena Marjanović and an anonymous reviewer are kindly acknowledged for constructive feedback and challenges which helped to significantly improve the manuscript. Editor Claudio Facenna is thanked for the editorial handling of the manuscript. Our families have supported extra work hours spent on this manuscript for which we'd like to express our deep gratitude.

Appendix A

Figures A1 and A2.

Open Research

Data Availability Statement

The 3D seismic data is a proprietary survey acquired by CGG in 2017 for the Oil and Gas exploration companies Kosmos Energy, Galp and Shell Global. Requests for data access can be made through Shell Global via the corresponding author. Interpretations shown in the study, including seismic events of the oceanic crust and transforms can also be requested through Shell Global via the corresponding author. The regional gravity data is a proprietary compilation of global satellite altimetry and on/offshore survey data from GETECH Group plc. (U.K), any requests or data should be addressed to GETECH (https://www.getech.com).

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Volume23, Issue11

November 2022

e2022GC010351

A New Model for the Evolution of Oceanic Transform Faults Based on 3D Broadband Seismic Observations From São Tomé and Príncipe in the Eastern Gulf of Guinea

Abstract

Key Points

Plain Language Summary

1 Introduction

2 Oceanic Transform Fault Observations

3 Study Area

4 Data Sets and Methods

5 Seismic Observations

5.1 Seismic Stratigraphy of the Oceanic Crust in the Study Area

5.2 Structuration of the Top Oceanic Crust

5.3 Oceanic Fracture Zone Internal Character

5.3.1 Transform Normal Dipping Reflectivity Sequences (TNDR)

5.3.2 Transform Normal Dipping Reflectivity-Base (TNDR-B)

6 Discussion

6.1 An Observation-Based Model of Extensional and Non-Conservative Transform Processes

6.2 Comparison to Active Oceanic Transform Faults

7 Conclusions

Acknowledgments

Appendix A

Open Research

Data Availability Statement

References

Figures

References

Information

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A New Model for the Evolution of Oceanic Transform Faults Based on 3D Broadband Seismic Observations From São Tomé and Príncipe in the Eastern Gulf of Guinea

Abstract

Key Points

Plain Language Summary

1 Introduction

2 Oceanic Transform Fault Observations

3 Study Area

4 Data Sets and Methods

5 Seismic Observations

5.1 Seismic Stratigraphy of the Oceanic Crust in the Study Area

5.2 Structuration of the Top Oceanic Crust

5.3 Oceanic Fracture Zone Internal Character

5.3.1 Transform Normal Dipping Reflectivity Sequences (TNDR)

5.3.2 Transform Normal Dipping Reflectivity-Base (TNDR-B)

6 Discussion

6.1 An Observation-Based Model of Extensional and Non-Conservative Transform Processes

6.2 Comparison to Active Oceanic Transform Faults

7 Conclusions

Acknowledgments

Appendix A

Open Research

Data Availability Statement

References

Figures

References

Related

Information

RESOURCES

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