Oceanic transform faults offshore São Tomé and Príncipe highlighted by integrated density and magnetic modeling of the crust

The Eastern Gulf of Guinea hosts several buried Cretaceous-aged oceanic fracture zones. 3D broadband seismic data acquired offshore São Tomé and Príncipe revealed a complex crustal architecture. Mapped oceanic fracture zones show low-angle reflectors that detach onto or eventually cross through the Moho boundary, overlain by strong reflectors that are interpreted as transform process related extrusive lava flows. Here, we use a high resolution shipborne free-air gravity and total field intensity magnetic data set to reassess whether previously defined seismic models of the crust are in conformity with potential field data. The study area is located offshore São Tomé with a size of c. 150x150 km. Using the software IGMAS+, we model the gravity and magnetic properties of the crust (i.e. density and susceptibility) in 3D. Long record length seismic sections plus mapped seismic horizons, which include bathymetry, sediments, upper and lower crust, are used as constraints. While the general trend of the free-air anomaly can be explained within a range of expected crustal densities, the magnetic field anomaly reflects high residuals that are predominantly oriented parallel to the transform faults. This indicates that gravity and magnetic data cannot be explained by the same simple source geometry. Therefore, we first perform sensitivity tests to isolate the source of the residual magnetic anomaly, followed by a structural analysis along the transform faults with special emphasis to the extrusive lava flows in the crustal domain. Our final model reconciles seismic horizons and potential field data and will stimulate a discussion on the architecture and evolution of transform faults and their signatures in different data sets.