Central uplift formation in very large impact structures: evidence from the Vredefort

The 80-km-wide Vredefort dome is a deeply eroded remnant of the central uplift of the Vredefort Impact Structure. It exposes upturned Archean to Paleo-proterozoic supracrustal strata and a core of Archean crystalline gneisses that have previously been interpreted as an on-edge profile of the crust and upper mantle. Our structural analysis of the gneisses in the core of the dome indicates that substantial impact-related rotation is restricted to a narrow zone close to the contact with the supracrustal strata and that the rocks responded in an essentially ductile way during doming (no megabreccia development). Furthermore, the strong orientation asymmetry of the supracrustal sequence, which is overturned in the NW sector and moderately steeply dipping in the SE sector, is attributed to symmetric rotation of an originally NW-dipping sequence, rather than asymmetric impact or post-impact tilting. Ductile deformation recorded in the basement complex is essentially Archean and, except for the pseudotachylitic breccias, no other impact related deformation has been identified. The mechanism of fabric rotation and its relationship with the pseudotachylites are discussed. Background: It is now widely accepted that large impact events can promote intense modification of deep levels of the Earth's crust. This modification is attributed to gravitational collapse of the unstable bowl-shaped transient crater cavity formed immediately following the impact. The rock movement is believed to occur hydrodynamically, analogous to central jets that rise out of a collapsing cavity in water. This " hydrodynamic collapse " is based on the fact that the strong and quasi-instantaneous ground motions triggered by the collision considerably affect the target rock rheology [1]. The abrupt change in rock property, generally referred to as acoustic fluidization, is attributed to the extreme pressure variations and/or thermal softening caused by the passage of shock waves through the target rocks during the compression and decompression stages [1]. As the effects of " rock fluidization " have yet to be constrained by geological observations, structural effects related to the formation of central uplifts remain one of the most problematic areas in impact cratering studies. This is partly due to lack of exposure of deep levels of most impact structures. Well-documented deep levels of crater basement are, however, extensively exposed in the Vre-defort dome, the eroded remnant of the central uplift of the 2.02 Ga Vredefort Impact Structure [2]. The dome comprises an ~80-km-wide roughly circular structural feature with a collar of up to overturned supracrustal strata and …