Ultra-high resolution (micro-bathymetric mapping and sub-bottom profiling) imaging of an active strike-slip fault, the North Alfeo Fault, offshore Catania, Eastern Sicily (Ionian Sea, Central Mediterranean)

Abstract

The goal of the ERC funded FOCUS project is to study an active fault offshore Catania using fiber optics, sea-floor geodesy, seismological stations (onshore and OBS on the seafloor) and detailed in-situ observations using an ROV and an AUV. Here, we report on the latter. In October 2020 using the ROV Victor6000 and in January 2022 using the AUV IdefX, we performed micro-bathymetric mapping (at an altitude of 50 m above the seafloor) of a 15-km-long segment of the North Alfeo Fault, covering water depths of about 1600 m to 2300 m. A prominent lozenge-shaped transpressive ridge or “pop-up” type structure is one of the primary features of this portion of the fault zone. It forms a flat-topped plateau culminating at about 1700 m water depth. It is cross-cut by a network of N-S striking faults resembling domino blocks or books in a book-shelf.Sub-bottom profiling (using a chirp system on the AUV IdefX at an altitude of 70 m above the seafloor) crossed the transpressive ridge and imaged the network of narrowly spaced (typically 100 - 200 m spacing) N-S striking faults, which are steeply W dipping normal faults. This suggests the transpressive ridge is currently collapsing. Indeed, the eastern part of the plateau is marked by a small (600 m long from headscarp to toe) submarine landslide. The overall pattern in the northern portion of the mapped area (west of the plateau) is a series of oblique secondary faults, crossing the primary fault at an angle of about 30°. Using a very simple analog model of a thin layer of cohesive granular material above two rigid plates, with a slightly curved fault track, it was possible to produce a primary strike-slip fault directly above the cut between the two plates, and several distinct transpressional ridges (pop-ups) as well as transtensional fissures or gashes. Secondary faults form obliquely to the primary fault and are oriented at about a 30° angle clockwise from the trend of the primary fault. This pattern reproduces the large-scale features observed in the micro-bathymetry from the NW prolongation of the North Alfeo fault. A series of analog experiments using different rheologies in the sediment layer is planned in the future to test the likely detachment (nucleation) depth for the strike-slip fault in the basement.