Evidence from the Vredefort Granophyre Dikes points to crustal relaxation following basin-size impact cratering

Abstract

The timescale of the modification stage of basin-sized impact structures is not well understood. Owing to ca. 10 km of erosion since its formation, the Vredefort impact structure, South Africa, is an ideal testing ground for deciphering post-impact modification. Here, we present geophysical and geochemical evidence from the Vredefort Granophyre Dikes, which were derived from the -now eroded -Vredefort impact melt sheet. The dikes have been studied mostly in terms of their composition, while the timing and duration of their emplacement remain controversial. We examined the modern depth extent of five dikes, with three from the inner crystalline core of the central uplift, and two from the boundary between the core and the supracrustal collar of the central uplift, using two-dimensional electrical resistivity tomography. We found that the core dikes terminate near the present erosion surface (i.e., <5 m depth). In contrast, the dikes at the core-collar boundary extend to a depth ≥ 9 m. These observations suggest that the core dikes are exposed near their lowermost terminus. In addition, we obtained bulk geochemical composition of the dikes, finding that the andesitic composition phase is present in the core-collar dikes that is not found in the core dikes. The presence of this phase indicates the episodic emplacement of impact melt into subvertical crater floor fractures.We conclude that the dike formation was protracted and occurred over a time span of at least 10 4 years. The sequential formation of the Vredefort Granophyre Dikes points to horizontal extension of crust below the impact melt sheet above a kinematic velocity discontinuity, a crustal instability resulting from the dynamic collapse of the transient cavity.

Genesis of the mafic impact melt rock in the northwest sector of the Vredefort Dome, South Africa

“…This separation agrees well with Wannek's (2015) distinction between Felsic and Mafic Granophyre compositions, and with the data of Huber et al. (2020, 2022). The major element data (Figure 7a) indicate negative trends for CaO, MgO, Fe 2 O 3 , and Al 2 O 3 (slightly scattered) with SiO 2 .…”

“…Huber et al. (2022)—like Wannek (2015)—extended the occurrence of Mafic Granophyre into the Eldorado property southwest of Kopjeskraal. Thus, we can now summarize that Mafic Granophyre occurs widely along one specific dike of Vredefort Granophyre only—the dike that extends in the northwest of the Vredefort Dome from the Kopjeskraal, via the Eldorado and Kommandonek properties, to Rensburgdrif (dikes 7 and 8 of Therriault et al., 1996).…”

“…Lieger (2011), Lieger and Riller (2012), and, most recently, Huber et al. (2020, 2022) used major element data for these two Granophyre varieties to discuss that these two impact melt rock phases were generated in accordance with a model for the Offset Dikes at the Sudbury impact structure. These dikes are considered to comprise two different phases that were emplaced from the differentiating impact melt sheet (Sudbury Igneous Complex), where they originated from different depths.…”

“…In continuation of their 2020 work, Huber et al. (2022) provided a quite sizable table of chemical analyses for Vredefort Granophyre, with many analyses duplicated from the earlier work. The geochemical discussion is also remarkably similar.…”

“…These authors, and more recently Huber et al. (2020, 2022), proposed that two successive intrusions of melt from a differentiating impact melt sheet or generated by assimilation of a mafic lithology at the base of the melt sheet, above the original impact crater floor, had generated a composite felsic/mafic Granophyre dike. The idea was taken from the Offset Dikes of the Sudbury impact structure, which are widely considered to have been produced by multiple intrusions of impact melt (e.g., Hecht et al., 2008; Lightfoot, 2016).…”