New hydroplastic structures of the Eocene Rus Soft-sediment Detachment (Eastern Saudi Arabia) and their contribution to the dating of the Zagros Collision

The Eocene Lower and Middle members of Rus Formation are exposed at the King Fahd University of Petroleum and Minerals (KFUPM) campus and contain 'odd' structural features. Previously, such structures were overlooked or misinterpreted by other researchers. In this study, we interpret these structures as hydroplastic kinematic indicators in the basal part of the Middle Rus Member. Their occurrence is related to the Rus soft-sediment detachment, a major displacement zone at the boundary/interface between the Lower and Middle Rus. The structures are fist-sized vugs coupled with carrot- or comet-trail imprints (VCT structures), previously translated calcite geodes. VCT structures demonstrate NNW (345°) transport/slip and are found on flat to low-dipping surfaces characterized as Y, R, and P shears according to the Rus detachment orientation. The Andersonian transtension stress regime is indicated by palaeostress analysis, but it was not enough to activate the Rus soft-sediment detachment. The negative effective principal stress σ₃' and the exceptionally low frictional coefficient generated by fluid pressure resulted in detachment activity. Because it reveals the Arabian platform's instability in the larger area of the Dammam Dome during the Late Eocene, the soft-sediment Rus detachment can be considered a 'sensitive stress sensor' for the Zagros collision. The beginning of the Zagros collision, which was previously thought to occur during the Oligocene based on the well-known pre-Neogene unconformity, is credited with this instability.