Millennial-scale phased submarine retrogressive collapse of the owl slide complex

Retrogressive submarine landslide complexes are abundant on global margins, characterized by stepping head scarp bathymetric morphology. Investigation of major complexes showed they evolved through sequential up-dip migrating collapse on a single basal failure. Here, we explore the Owl slide complex, a relatively small scar on the southeastern Mediterranean margin. Sub-meter resolution seismic-reflection profiles and 5-m resolution bathymetry reveal that the Owl comprises a primary scar (∼1.7 km²), two secondary scars 500 m upslope of the primary scar, and additional scars along the lateral margins. The complex comprises six vertically-stacked chaotic mass transport deposit lobes, interbedded with intervals of continuous reflections, representing gradual sediment accumulation. Distinct basal surfaces of each lobe connect with its respective head scarp in an upward retrogressive pattern. Underlying the complex is a prominent unconformity correlating with the last glacial transgression. Four faults beneath the unconformity create stepped geometry reflected in the bathymetry. We suggest that the Owl complex was formed through a multistage retrogressive sequence of 4 to 6-failure events (∼16,000 BP- ∼ 1000 BP), with an average recurrence interval of ∼3000 years and intermittent periods of quiescence lasting thousands of years. Over-steepening of the head scarp by each event pre-conditioned subsequent failures. However, additional sediment accumulation of several meters was necessary before another failure could occur. Over time, consecutive retrogressive events show a linear decrease in volume, ultimately stabilizing the slope. The phased collapse mechanism described here forms the typical retrogressive sliding morphology but has different geohazard implications than the single basal failure mechanism.