Radial transpression ridges: A new structural feature of complex impact craters

Abstract

The gravity-driven collapse of complex impact craters induces the mass transfer of large rock volumes. In distal parts of a crater, inward movements dominate; whereas in the centre of an impact structure, outward movements occur if the central uplift collapses. The particle trajectory field is centrosymmetric, which signifies that the conditions for plane-strain deformation are not fullfilled. Converging particle trajectories can be compensated either by a bulk thickening of inward sliding masses (folding, repetition of rock units along thrust faults, plastic flow) or by the formation of localised radial transpression ridges (RTR) at the edges of individual landslides. In these transpression ridges, material is uplifted to accommodate the converging mass flow. Different modes of uplift are possible including radial folding, lateral overthrusting, and the formation of positive flower structures. A simple geometric model is used to estimate the amount of transpression thickening and bulk thickening of inward sliding masses on the base of volumetric considerations. The existence of RTRs is confirmed by structural investigations at the Siljan impact structure, Sweden, as well as at other complex impact craters on Earth.