Identification of ancient impact structures: Low-angle faults and related geological features of crater basements

Abstract

Ancient impact craters are commonly deeply eroded, metamorphosed and/or deformed by later tectonics. The identification of such impact structures using microstructural or mineralogical criteria are very difficult to apply under such conditions. It is proposed that fault patterns in the crater basement can be used diagnostically in eroded structures. On the basis of field observations in the Rochechouart crater, France, and numerical computations, we suggest the existence of low-angle faults dipping gently towards the crater centre. In craters with final diameters of 20-30 kin, low-angle normal faults might have a lateral extent of approximately 5 km and predominantly occur at a distance of approximately 3-8 km away from the crater centre. Our numerical model suggests that they formed at a depth of 0.5 to 4 km below the original crater surface. Lowangle normal faulting occurs during crater modification when the steep rim of the transient crater cavity starts to collapse. Gravity-driven sliding occurs in acoustically-fluidized rocks with low cohesion and reduced friction coefficients. It is suggested that the degree of fluidization changes with depth and distance from the impact centre. Faulting along gently dipping shear zones may be the result of an impact-induced theological stratification of the crater floor and a passive rotation of the shear zones due to the uplift of the central peak. Rapid and unconstrained single slip events locally lead to frictional melting and the formation of pseudotachylites. Field observations are presented that strongly support the idea of block oscillation during crater modification. 280 Kenkmann et al.

Chicxulub central crater structure: Initial results from physical property measurements and combined velocity and gravity modeling

“…Note that, in the final crater, material that started within the transient crater rim ends up beneath the peak ring. unique insight into the deep basement of the crater (Kenkmann et al 2000;Fig. 3a).…”

“…3. a) The Rochechouart crater in France (after Kenkmann et al 2000). The faults indicate the kinematics of crater collapse with subvertical collapse near the crater rim, inward sub-horizontal movement in material beneath the annular trough, and stratigraphic uplift moving upward and outward within the central crater.…”

Insights into the morphology, geometry, and post-impact erosion of the Araguainha peak-ring structure, central Brazil

“…Advanced numerical and experimental modeling and recent geophysical and fi eld-based mapping have demonstrated unequivocally that large meteorite impacts are violent catastrophic events that lead to substantial lateral and vertical movement of subsurface target rocks (e.g., Melosh, 1989;Melosh and Ivanov, 1999;Kenkmann et al, 2000;Kenkmann, 2002;Collins et al, 2002;Wünnemann and Ivanov, 2003;Lana et al, 2003aLana et al, , 2003bLana et al, , 2006aLana et al, , 2006bCollins and Wünnemann, 2005;Osinski and Spray, 2005).…”

Observations and interpretations at Vredefort, Sudbury, and Chicxulub: Towards an empirical model of terrestrial impact basin formation

“…The final kinematic indicators of one such model are shown in Fig. 14 (Kenkmann et al 2000). They indicate that the parautochthonous target rocks undergo sub-vertical collapse in the transient cavity rim area, inward sub-horizontal movement beneath the annular trough and upward and outward movement in the central structural uplift.…”

“…Some of the faults may be activated more than once. Taken from Kenkmann et al (2000). the center of the Vredefort structure (taken as the center of the Vredefort Dome, 4 km north of Inlandsee), when the location of particular shock metamorphic features from Vredefort were scaled to coincide with the relative location of the equivalent shock-metamorphic features at Sudbury.…”