Structural analysis of the collar of the Vredefort Dome, South Africa—Significance for impact‐related deformation and central uplift formation

Abstract

Abstract-Landsat TM, aerial photograph image analysis, and field mapping of Witwatersrand supergroup meta-sedimentary strata in the collar of the Vredefort Dome reveals a highly heterogeneous internal structure involving folds, faults, fractures, and melt breccias that are interpreted as the product of shock deformation and central uplift formation during the 2.02 Ga Vredefort impact event. Broadly radially oriented symmetric and asymmetric folds with wavelengths ranging from tens of meters to kilometers and conjugate radial to oblique faults with strike-slip displacements of, typically, tens to hundreds of meters accommodated tangential shortening of the collar of the dome that decreased from ∼17% at a radius from the dome center of 21 km to <5% at a radius of 29 km. Ubiquitous shear fractures containing pseudotachylitic breccia, particularly in the metapelitic units, display local slip senses consistent with either tangential shortening or tangential extension; however, it is uncertain whether they formed at the same time as the larger faults or earlier, during the shock pulse. In addition to shatter cones, quartzite units show two fracture types-a cmspaced rhomboidal to orthogonal type that may be the product of shock-induced deformation and later joints accomplishing tangential and radial extension. The occurrence of pseudotachylitic breccia within some of these later joints, and the presence of radial and tangential dikes of impact melt rock, confirm the impact timing of these features and are suggestive of late-stage collapse of the central uplift.

Structural evolution of the 40 km wide Araguainha impact structure, central Brazil

“…7b). These folds are similar in many aspects to radial gentle folds observed in the collar of the Vredefort Dome (Wieland et al 2005), and are probably associated with buckling and thickening of the Furnas Formation. In addition, several outcrop-scale folds (ranging from vertically plunging recumbent to shallowly NW plunging isoclinal folds; Figs.…”

“…Impact craters ranging in size from a few kilometers to several tens or hundreds of kilometers have provided important information on the final stages of the impact cratering process (Wilshire et al 1976;Roddy 1979;Shoemaker and Shoemaker 1996;Milton et al 1996;Lana et al 2003aLana et al , 2003bLana et al , 2006bWieland et al 2005;Osinski and Spray 2005;Kenkmann et al 2005). Structural analyses of small-to medium-sized complex structures (diameter between 2 and 25 km) have revealed a complex deformation history with lateral and vertical constriction of the target rocks during crater collapse (e.g., Wilshire et al 1972;Roddy 1979;Shoemaker and Shoemaker 1996;Milton et al 1996;Osinski and Spray 2005;Kenkmann et al 2005;Scherler et al 2006).…”

“…The constriction of the sediments led to formation of radially outward plunging gentle folds. Radially plunging folds have been observed in central uplifts of different sizes (Wilshire et al 1976;Milton et al 1996;Kenkmann et al 2005;Osinski and Spray 2005;Wieland et al 2005) and seem to accommodate much of the constrictional strain created by the inward movement of the target rocks during transient cavity collapse. At Araguainha, these folds might have contributed to further thickening of the strata during or shortly after thrust-related imbrication of the large-scale sheets (Fig.…”

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

“…Both the Vredefort and Sudbury structures display the classic pattern of the upturning of lithologies and the exposure of deeper stratigraphic levels in the parautochthonous rocks of the crater floor, as the center of the structure is approached (Lana et al 2003;Riller 2005;Wieland et al 2005). This is characteristic of the process of structural uplift in transient cavity modification at complex impact structures (Grieve and Therriault 2004).…”

“…The collar rocks at Vredefort indicate that uplift and rotation to near vertical to overturned attitudes, with the variation likely due to the effect of a pre-impact regional dip (Lana et al 2003;Wieland et al 2005), occurred prior to the outward-directed radial thrusting (Simpson 1978;Brink et al 1997). The former is indicative of the well-known inward and upward kinematics in the parautochthonous rocks of the crater floor observed in structural uplifts at the centers of smaller terrestrial impact structures (Grieve and Therriault 2004).…”