How Old Can an Impact Crater Get? 

Impact craters are assumed to have been formed in the earliest days of planet Earth but few very old craters are known. THIS ARTICLE uses the Vredefort Dome in South Africa as a means of determining what to look for when finding old, deeply eroded, impact craters.

The Dome is very large (300km across) and very old (2 billion years). The researchers drilled across and around the dome to see if they could spot the effects a large astrobleme can be predicted to make. They could - but only just. There is thought to have been about 7 to 10 km of erosion. 10km would remove all evidence. 

The Vredefort Dome is recognised because of its shape - the central peaks, especially - and the results of shock metamorphism. Another kilometre of erosion would remove these also, leaving no trace of the impact.

The basis of the article is THIS ACADEMIC PAPER.

Gravity map of the Kaapvaal Craton. The Vredefort structure does not have a prominent gravity high or low compared to the surrounding craton. Data from the South African Council for Geoscience. Craton boundary from Hasterok et al. (2022).