Quantitative structural geology in the deep ocean using photogrammetry: Implications for the polyphased tectonic evolution of the Buteur Ridge, French Guiana

The Buteur Ridge is a 7 km-long and 6 km-wide relief on the eastern rifted margin of the Demerara Plateau, offshore French Guiana. This margin was formed during the Lower Cretaceous. In early 2023, the oceanographic cruise DIADEM used a manned deep submersible (Nautile) to sample and directly observe the eastern flank of this 3750 m-deep ridge.

This study presents a new underwater photogrammetric method developed from the Nautile video records. This method incorporates both camera and submarine movements to produce photogrammetric 3D models of the seafloor below decimetre scale. The presented underwater photogrammetric method can be used for both past and future video records, as long as both underwater device and camera movements are recorded.

The data obtained from the 3D reconstructions allow to reconstruct detailed stratigraphic and structural framework of the Buteur Ridge. The eastern flank of the Buteur Ridge is entirely composed of sedimentary layers. Structural analysis at bed scale suggests prograding structures likely associated with a N-S-directed paleocurrent. At ridge scale, structural analysis combined with seismic observations suggests that the Buteur Ridge is the result of a polyphased tectonic evolution. We highlight that Cretaceous tilted blocks, bounded by east-dipping faults, are overlapped by a post-rift unit. The syn-rift units are in contact with the east-dipping post-rift unit by a post-rift unconformity. Subsequent tilt and fault reactivation offset both the post-rift unconformity and the post-rift unit, resulting in the present relief of the Buteur Ridge.