Strike-slip overprinting of initial co-axial shortening within the toe region of a submarine landslide and a model for basal shear surface growth: a case study from the Angoche Basin, offshore Mozambique

Abstract

Submarine landslides (slides) are some of the most voluminous sediment gravity-flows on Earth and they dominate the stratigraphic record of many sedimentary basins. Their general kinematics and internal structure are relatively well-understood. However, how slides increase in volume and internally deform as they evolve, and how these processes relate, in time and space, to the growth of their basal (shear) zone, are poorly understood. We here use three high-resolution 3D seismic surveys from the Angoche Basin, offshore Mozambique to map strain within a shallowly buried, large, and thus seismically well-imaged slide (c. 530 km3). We document several key kinematic indicators, including broadly NW-trending lateral margins and longitudinal shears bounding and within the slide body, respectively, and broadly NE-trending symmetric pop-up blocks in the slide toe. Approximately 7 km downdip of the slide toe wall, thrusts and related folds also occur within otherwise undeformed slope material, with thrusts detaching downwards onto the downslope continuation of the basal shear zone underlying the slide body. Based on the style, trend and distribution of these features, and their cross-cutting relationships, we propose an emplacement model involving two distinct phases of deformation: (i) bulk shortening, parallel to the overall SE-directed emplacement direction, with contractional shear strains reaching c. 8%; and (ii) the development of broadly emplacement direction-parallel shear zones that offset the earlier-formed shortening structures. We infer that the contractional strains basinward of the slide body formed due to cryptic basinward propagation of the basal shear zone ahead of, and to accommodate updip sliding and shortening associated with, the entire slide mass. Our study demonstrates the value of using 3D seismic reflection data to reveal slide emplacement kinematics, especially the multiphase, non-coaxial nature of deformation, and the dynamics of basal shear zone growth.