Ramps and flats of mass-transport deposits (MTDs) as markers of seafloor strain on the flanks of rising diapirs (Espírito Santo Basin, SE Brazil)

Abstract

Ramps and flats reflect variations in the morphology of basal shear surfaces in mass-transport deposits (MTDs), which are often mapped without considering their potential as strain markers. In this work ramps and flats are mapped on the margins of salt diapirs, using a high-quality 3D seismic volume from SE Brazil (Espirito Santo) to investigate how the morphology of mass-transport deposits (MTDs) relates to diapir growth and near-seafloor strain. In parallel, aspect ratios of MTDs are used to estimate their source areas and possible causal mechanisms. Our results show diapir-related MTDs to have length/width ratios ≤ 3, in a region where sediment failure was triggered by faulting and seafloor tilting due to halokinesis. Sections of MTDs that were triggered and later uplifted on the flanks of growing diapirs are termed ‘drag zones’. Ramps within this drag zones are shown as local changes in gradient at the basal shear surfaces, and are linked to promontories and older fault scarps. The results in this paper are important because they show that drag zones are elongated in a NW–SE direction, parallel to the dominant trend of basal ramps and promontories. Basal ramps and promontories in these drag zones constitute markers for seafloor strain around growing salt diapirs, with variations in the internal character of MTDs occurring across these same ramps and promontories. As a result, a spectrum of seismic and depositional facies can occur in specific quadrants of drag zones. In the study area are identified debrites (DBs), slightly deformed blocks (SDBs), coherent or unrotated blocks (CUBs), and rafted blocks (RBs).