Morphology, sediments and structure of the Amirante Trench, Western Indian Ocean: implications for trench origin

Abstract

The Amirante Trench is a 600 km long and up to 5200 m deep topographic depression whose origin has remained enigmatic. The trench contains > 1 km of undisturbed, ponded turbidites and mass-transport deposits that have been periodically modified by northward flowing contour currents of the Deep Western Boundary Current (DWBC). Morphologically, the trench and adjacent Amirante Ridge resemble an arcuate oceanic arc-trench system and some investigators have suggested that this complex represents some type of subduction zone. However, our analysis of bathymetric and structural data indicate that the trench is not a single, continuous arcuate feature, but is actually a compound feature composed of three discrete, essentially linear segments of differing structural orientations and probable sea-floor spreading origins. The northern segment (4°20′–6°20′S) trends NE (∼030°) and apparently represents a fracture zone or transform-fault trend related to sea-floor spreading in the Mascarene Basin during the Late Cretaceous. The central segment (6°20′–8°40′S) trends NNW (∼350°) and appears to represent a fracture-zone lineament that is parallel to fracture zones and lineaments created by the opening of the Somali Basin during the Late Jurassic and Early Cretaceous. However, Somali Basin crust apparently does not extend eastward as far as this portion of the trench, so the origin of this segment remains uncertain. The southern segment (8°40′–10°S) trends NW (∼310°) and may represent a tensional rift related to sea-floor spreading within the Mascarene Basin during the early Tertiary. Further, our data show no convincing evidence that any portion of this feature was a subduction zone at any time during its development. The data also essentially rule out an extraterrestrial impact origin for this region at the end of the Cretaceous.