Microscopic planar deformation features in quartz of the Vredefort structure: Anomalous but still suggestive of an impact origin

130

Abstract-Historically, there have been a range of diameter estimates for the large, deeply eroded Vredefort impact structure within the Witwatersrand Basin, South Africa. Here, we estimate the diameter of the transient cavity at the present level of erosion as -124-140 km, based on the spatial distribution of shock metamorphic features in the floor of the structure and downfaulted Transvaal outliers. Taking erosion into account ( 2 6 km) and scaling to original final rim diameter, an estimate of close to 300 km for the rim diameter is obtained. Independent estimates of the final rim diameter, based on an empirical relation of central uplift diameter to rim diameter, spatial distribution of pseudotachylites, and concentric large scale structural patterns, give a similar estimate of close to 300 km for the original final rim diameter. An impact structure of this size is expected to have had an original multi-ring form. At this size, the Vredefort impact structure encompasses the bulk of the Witwatersrand Basin, which appears to owe its preservation to the Vredefort impact. In addition, the Vredefort impact event may have been the thermal driver for some of the widespread hydrothermal activity in the area, which, in recent interpretations, is believed to be a component in the creation of the world-class gold deposits of the Witwatersrand Basin.

Section: The Vredefort Structurementioning

confidence: 99%

Section: The Vredefort Structurementioning

confidence: 99%

Original size of the Vredefort Structure: Implications for the geological evolution of the Witwatersrand Basin

Therriault

Grieve

Reimold

1997

130

“…1a and 1c). Evidence of the impact origin of the dome is largely restricted to rocks within a 30-35 km radius of its center and includes shatter cones (Hargraves 1961;Manton 1962Manton , 1965Albat 1988;Albat and Mayer 1989;Nicolaysen and Reimold 1999), planar microdeformation features in quartz (e.g., Lilly 1978;Fricke et al 1990;Grieve et al 1990; Leroux et al 1994) and zircon (Kamo et al 1996;Gibson et al 1997), coesite and stishovite (Martini 1978(Martini , 1991, microdeformation features, recrystallized diaplectic glass and shock melts in feldspars , impact melt breccia dikes (Reimold et al 1990;Koeberl et al 1996), and extremely voluminous pseudotachylitic breccia dikes (Dressler and Reimold 2004;Reimold and Gibson 2005). While these features have been studied in some detail, only limited investigation has been conducted of the larger-scale structures (faults and folds) in the collar of the dome by, among others, Manton (1962Manton ( , 1965 and Lilly (1978).…”

Section: Geological Settingmentioning

confidence: 99%

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

Wieland

Gibson

Reimold

2005

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.

“…In the course of investigations into the origin of the Vredefort structure, PDFs (e.g., Grieve et al, 1990) and shatter cones (e.g., Hargraves, 1961;Manton, 1965) were studied in some detail. Since the pressures at which these features form have been determined experimentally (e.g., Robertson et al, 1968;Roddy and Davis, 1977), we used hydrocode simulations to predict where the peak pressure of the shock wave generated by an impact would have achieved these levels.…”

Section: Introductionmentioning

confidence: 99%

Constraints on the size of the Vredefort impact crater from numerical modeling

Turtle

Pierazzo

1998

Abstract-The Vredefort structure in South Africa was created by a meteorite impact about two billion years ago. Since that time, the crater has been deeply eroded; so to estimate its original size, researchers have had to rely heavily upon comparison to other terrestrial impact structures. Recent estimates of the original crater diameter range from 160 km to as much as 400 km. In this study, we combined the capabilities of both hydrocode and finite-element modeling, using the former to predict where the pressure of an impactgenerated shock wave would have been high enough to form planar deformation features (PDFs) and shatter cones and the latter to follow the subsequent displacement of these shock isobars during the collapse of the crater. We established constraints on the sizes of the projectile and the transient crater (and, thus, on the size of the final crater) by comparing the observed locations of PDFs around Vredefort to the results of our simulations of impacts by projectiles of various sizes. These simulations indicate that a rocky projectile with a diameter of -10 km, impacting vertically at a velocity of 20 km/s generates shock pressures that are consistent with the distribution of PDFs around Vredefort. These projectile parameters correspond to a transient crater -80 km in diameter or a final crater -120-160 km in diameter. Allowing for uncertainties in our modeling procedures, we consider final craters 120 to 200 km in diameter to be consistent with the observed locations of PDFs at Vredefort. The shock pressure contour corresponding to the formation of shatter cones is almost horizontal near the surface, making the locations of these features less useful constraints on the crater size. However, they may provide a constraint on the amount of erosion that has occurred since the impact.