Meteoritics & Planetary Science 40, Nr 9/10, 1537–1554 (2005) Abstract available online at http://meteoritics.org

Structural analysis of the collar of the Dome, — Significance for impact-related deformation and central uplift formation

Frank WIELAND, Roger L. GIBSON*, and Wolf Uwe REIMOLD

Impact Cratering Research Group, School of Geosciences, University of the , Private Bag 3, P.O. Wits 2050, , South Africa *Corresponding author. E-mail: [email protected] (Received 25 October 2004; revision accepted 13 July 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 that are interpreted as the product of shock deformation and central uplift formation during the 2.02 Ga Vredefort . 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 , 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 cm- spaced 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.

INTRODUCTION Araguainha, Bischoff and Prinz 1994) have provided some constraints on the deformation history accompanying impact impacts are catastrophic and extremely and central uplift formation. They suggest that, while complex deformational events marked by both extreme and lithology exerts some control on the types and geometries of rapidly changing centro-symmetric strain patterns. On the one the second-order structures observed, the main controlling hand, the creation of first-order structures, such as central factors appear to be the size (duration) of the impact event and uplifts in complex craters, is currently best reconciled by the level of subsequent exhumation of the hydrodynamic models and is consistent with the results of (Dence 2004). continuum numerical modeling (e.g., Melosh 1989; Melosh The Vredefort impact structure in South Africa is one of and Ivanov 1999). In contrast, however, these central uplifts the three largest impact structures known on (e.g., and the surrounding crater floor rocks contain second-order Grieve and Therriault 2000), having had an estimated original structures that indicate that localized strain heterogeneity and diameter of 250–300 km (Therriault et al. 1997; Henkel and pre-existing or newly formed structural discontinuities play a Reimold 1998). It is also the most deeply eroded of the three, major role in the impact-induced structural evolution at scales with independent estimates suggesting removal of 5–10 km of far smaller than can currently be resolved with the models overburden following its formation (McCarthy et al. 1990; (e.g., Wilshire et al. 1972; Milton et al. 1996; Kriens et al. Gibson et al. 1998; Henkel and Reimold 1998). Consequently, 1999). Several studies of the internal structure of shallowly the deep levels of the impact structure that are exposed eroded complex craters ranging in diameter from a few provide an unparalleled opportunity to study the subcrater kilometers (, Kriens et al. 1999; Kenkmann basement features relating to the impact. This study focuses et al. 2005) to several tens of kilometers (Sierra Madera, on the well-exposed Archean supracrustal rocks that occupy Wilshire et al. 1972; Gosses Bluff, Milton et al. 1996; an intermediate radial position within the 80 km wide central

1537 © The Meteoritical Society, 2005. Printed in USA. 1538 F. Wieland et al. uplift of the structure (the Vredefort Dome). It complements The limited outcrop and absence of large-scale layering previous structural studies of the crystalline basement core of in the basement gneisses in the core of the dome hampered the dome (Lana et al. 2003a) and the surrounding marginal efforts by Lana et al. (2003a) to establish whether it displays syncline (Simpson 1978; Brink et al. 1997). a megabreccia structure similar to that proposed for other large central uplifts (e.g., Ivanov et al. 1996). Assuming that GEOLOGICAL SETTING gneissic fabrics were uniformly oriented prior to impact, Lana et al. (2003a) did obtain reasonably consistent results when The Vredefort Dome is located some 120 km southwest back-rotating these fabrics in the outer parts of the core of Johannesburg, South Africa, in the center of the around axes parallel to the local strike of the adjacent collar economically important Witwatersrand Basin (Fig. 1). It strata. In this way, they identified six sectors around the consists of an ∼40 km wide core of Mesoarchean basement exposed portions of the dome. However, in the absence of gneisses enclosed by a 20–25 km wide collar of generally evidence for large radial faults delimiting these sectors in the vertical to overturned late Archean to core of the dome, they suggested that impact-induced supracrustal strata, and is surrounded by an ∼50 km wide differential slip and rotations within the core of the central structural trough known as the Potchefstroom Synclinorium uplift could have been accommodated along a pervasive (Figs. 1a and 1c). Evidence of the impact origin of the dome pseudotachylitic breccia vein-fracture network instead. is largely restricted to rocks within a 30–35 km radius of its The quality of exposure of the supracrustal strata in the center and includes shatter cones (Hargraves 1961; Manton collar is better than that of the gneisses in the core of the dome 1962, 1965; Albat 1988; Albat and Mayer 1989; Nicolaysen (Fig. 2) and structural interpretation of the collar is aided by and Reimold 1999), planar microdeformation features in the well-layered nature of the rocks, which provides quartz (e.g., Lilly 1978; Fricke et al. 1990; Grieve et al. 1990; numerous marker horizons. These supracrustal rocks range in Leroux et al. 1994) and zircon (Kamo et al. 1996; Gibson age from 3.07 Ga to ∼2.1 Ga (Armstrong et al. 1991) and et al. 1997), and (Martini 1978, 1991), comprise two volcanic sequences (the 3.07 Ga Dominion microdeformation features, recrystallized diaplectic glass and group and the 2.71 Ga Ventersdorp supergroup) intercalated shock melts in feldspars (Gibson and Reimold 2005), impact with two major sedimentary sequences (the 2.9–2.71 Ga melt breccia dikes (Reimold et al. 1990; Koeberl et al. 1996), Witwatersrand supergroup and the 2.6–2.1 Ga Transvaal and extremely voluminous pseudotachylitic breccia dikes supergroup). In addition to several small alkali granite (Dressler and Reimold 2004; Reimold and Gibson 2005). intrusions, mafic and ultramafic sills associated with the While these features have been studied in some detail, only extrusive 2.71 Ga Ventersdorp event (Pybus 1995; Reimold limited investigation has been conducted of the larger-scale et al. 2000) and the intrusive 2.06 Ga (Walraven et al. 1990) structures (faults and folds) in the collar of the dome by, Bushveld magmatic event occur in the collar rocks. among others, Manton (1962, 1965) and Lilly (1978). The present study focuses on the well-exposed Additional geophysical work by Antoine et al. (1990) and siliciclastic strata of the Witwatersrand supergroup that form geological interpretation by Lana et al. (2003a, 2003b) the innermost collar, at a radial distance 20–30 km from the considered the large-scale implications of doming in terms of center of the dome (Figs. 1 and 2). In comparison, the outer differential block rotations, but without specifically focusing parts of the collar, underlain by the Ventersdorp supergroup on any individual structures. and lower , are poorly exposed (Fig. 2). Regional reflection seismic profiles across the dome The Witwatersrand supergroup strata consist of a lower (e.g., Durrheim 1986; Henkel and Reimold 1998; Therriault sequence of pelitic, quartzite, and ironstone units (West Rand et al. 1996) indicate that the contact between the crystalline group, ∼4 km thick), and an upper, quartzite-conglomerate- basement and supracrustal sequence has been uplifted by a dominated sequence (Central Rand group, ∼3 km thick). The minimum of 12 km relative to the deepest part of the rim sedimentary strata and the Ventersdorp-age sills were syncline (Fig. 1c). The impact-related uplift in the central metamorphosed before the impact event (Bisschoff 1982; parts of the dome is estimated at a minimum of 20 km (Henkel Gibson and Wallmach 1995), with the maximum grade of and Reimold 1998; Lana et al. 2003a, 2003b). Based on an metamorphism increasing with stratigraphic depth from lower analysis of the Archean fabrics in the basement gneisses, greenschist facies (∼350 °C) in the Central Rand group to Lana et al. (2003a) concluded that little impact-related mid-amphibolite facies (∼600 °C) in the lower West Rand rotation occurred in rocks within a radial distance of ∼12 km group (Gibson and Wallmach 1995). Pressure-temperature from the center, whereas rocks beyond this distance typically constraints and geochronological data suggest that this underwent rotations of 90° or more. The transition between metamorphism accompanied the 2.06 Ga Bushveld these two domains appears to be relatively sharp, occurring magmatism that also appears to be linked to the formation of over a radial distance of only a few kilometers; consequently, the alkali granites (Gibson and Wallmach 1995; Gibson et al. the dome displays an overall plug-like geometry (Lana et al. 2000; Moser and Hart 1996). 2003a). Following the impact, the rocks experienced renewed Structural analysis of the collar of the Vredefort Dome, South Africa 1539

Fig. 1. a) A simplified geological map of the Witwatersrand Basin showing the main structural features (modified from Pretorius et al. 1986), the central position of the Vredefort Dome and its rim syncline, and the approximate limits of a 300 km diameter impact structure. For simplicity, the post-Witwatersrand supergroup lithologies are not shown. The dashed line indicates the approximate position of the section in (c). b) A simplified geological map of the Vredefort Dome (modified after Martini 1992) showing the Vredefort Granophyre dikes and tangential fold axial traces in the inner rim syncline. The discordant pre-impact metamorphic isograd (after Nel 1927) and the position of the core-collar contact in the southeastern sector of the dome (dashed line beneath Karoo cover) are discussed in the text. c) Schematic cross- section based on outcrop, drilling, and seismic data (modified from Friese et al. 1995). metamorphism through shock heating and impact-induced REGIONAL STRUCTURAL FRAMEWORK uplift. Temperatures attained in the Witwatersrand supergroup strata during this event decreased radially Given the large size and great age of the Vredefort impact outwards from ∼525 °C in the lowermost West Rand group to and the fact that the Witwatersrand rocks were deposited ∼300 °C in the Central Rand group, and the deep levels of some 700–1000 Myr prior to the impact, it is necessary to burial of the rocks presently exposed at surface facilitated consider the possibility that the rocks in the Vredefort Dome widespread recrystallization and new mineral growth, also record structures related to pre- and/or post-impact particularly in pelitic rocks (Gibson et al. 1998). deformation events. In terms of pre-impact tectonics, two 1540 F. Wieland et al.

are related to the Ventersdorp extension at ∼2.7 Ga and show little post-Transvaal (<2.1 Ga) reactivation. Gravity and magnetic geophysical data and limited borehole information from the southeastern sector of the dome suggests a complicated structure beneath the cover rocks (e.g., Pretorius et al. 1986; Antoine et al. 1990; Corner et al. 1990; Martini 1992). The dominant feature in this sector appears to be a NW-trending horst that displaces the core-collar contact radially outwards by ∼5 km along at least two inferred faults (Corner et al. 1990) (Fig. 1b). The pre-impact timing of this feature is indicated by the apparent lack of offset of impact-related post-shock annealing textures in borehole core samples from the area (Martini 1992). Given the results of Tinker et al.’s (2002) regional study, this is most likely a Ventersdorp age (2.7 Ga) structure, as are the large subsurface faults inferred in the Fig. 2. A landsat TM image of the Vredefort Dome showing the well- northern collar of the dome (Fig. 1c). exposed nature of the Witwatersrand supergroup rocks in the collar. In addition to these large faults, Albat (1988) proposed The alternating quartzite-metapelite succession of the West Rand 20–30° of pre-impact scissor rotation along a radial fault in group can be clearly distinguished from the more massive quartzite the western collar of the dome to explain anomalous shatter units of the Central Rand group in the northwestern sector. The oval cone-bedding relationships. In general, however, the outcrop pattern of an alkali granite intrusion in the northwestern sector WNW of is prominent, as are the two large pre-impact supracrustal sequence exposed in the northern and western faults in the northern sector (see text for details). sectors of the dome shows strong continuity of strata with minimal thickness variations along strike (Holland et al. major faulting events affecting the Witwatersrand basin have 1990). This points to conformable relationships of been identified (e.g., Myers et al. 1990; Roering et al. 1990): stratigraphic units and thus relatively minor tectonic NE-SW regional shortening that created a series of large disturbance prior to the impact event. fault-bounded blocks in Central Rand group times (2.89– In addition to the inferred large faults in the southeastern 2.71 Ga) and 2.7 Ga Ventersdorp-age rifting that produced sector of the dome, the few small outcrops of Witwatersrand broadly NE- and E-trending listric normal faults in the basin supergroup strata in this area display moderately steep radial (Tinker et al. 2002), some of which represent reactivated outward dips, in contrast to the predominantly steep Central Rand group faults. overturned inward dips in the remainder of the collar (Figs. 1b Evidence exists for at least two major pre-Transvaal and 3). Various explanations, including post-impact regional supergroup faults in the northern collar of the Vredefort Dome tilting toward the northwest (McCarthy et al. 1990), or NW- (Figs. 1b and 2). The larger of the two trends NNE-SSW and directed post-impact thrusting (Friese et al. 1995) have been displays ∼2.5 km offset of Central Rand group and proposed. Lana et al. (2003a) noted that the pre-impact Ventersdorp supergroup rocks and thickening of the latter to metamorphic isograd in the collar rocks cuts upward through the west, whereas the second fault, which lies to the west, the collar stratigraphy toward the northwest (Fig. 1b). By trends NW-SE and displays ∼1.2 km of offset (Fig. 1b). If the assuming that this isograd formed horizontally, they suggested 90° or more of impact-related rotation found in the collar of that the discordance and the slight NW-SE elongation of the the dome is removed (see below), these faults define a basement-supracrustal contact indicated a pre-metamorphic conjugate set bounding a central graben structure. (and thus pre-impact) northwestward tilt of the strata. Paleocurrent and clast size data from the Central Rand group However, while a tilt of only a few degrees can explain the rocks in the dome (Holland et al. 1990) do not support these discordant isograd, the tilt required to reconcile the dip faults being active in Witwatersrand times. The orientation, variation in the collar strata would have had to have been of timing, and magnitude and sense of slip of these faults are, the order of 30–60°, which would be implausible on a regional however, consistent with Ventersdorp-age rifting. Although scale. The most likely explanation is that the anomalous dips outcrop in the outer collar of the dome is poor, there appears in the poorly exposed southeastern sector reflect pre- or syn- to be little, if any, offset of the Transvaal supergroup strata, impact fault-block rotations, the former associated with the suggesting that impact-related reactivation of these faults significant zone of Ventersdorp faulting lying southeast of the must have been relatively minor at best. Interpreted seismic dome (Pretorius et al. 1986) (Fig. 1a) and the latter related to profiles from west and south of the dome (Pretorius et al. formation of the central uplift (Fig. 8). As shown in the next 1986; Tinker et al. 2002) similarly suggest that most of the section, bedding orientation varies considerably in localized large-scale fault structures in the central Witwatersrand basin areas throughout the collar of the dome. Structural analysis of the collar of the Vredefort Dome, South Africa 1541

Fig. 3. Orientation of bedding in the Witwatersrand supergroup in the Vredefort Dome. a) Interpretive sketch map of the dome from Antoine et al. (1990) showing their proposed polygonal geometry. b–g) Lower hemisphere equal area Schmidt net projections of contoured poles to bedding, grouped according to Antoine et al.’s (1990) proposed polygonal structure. Apart from the southeast sector and small areas within the northern and northeastern sectors, bedding is overturned. Statistical data: b) mean 36°/300°, maximum density 27.8; c) mean 30°/336°, maximum density 25.9; d) mean 24°/349°, maximum density 16.9; e) mean 36°/007°, maximum density 11.8; f) mean 36°/053°, maximum density 17.3; g) mean 60°/144°, maximum density 30.7.

Given the metamorphism of the Witwatersrand have shortened the northwestern part of the impact supergroup strata to lower greenschist- to mid-amphibolite- structure by ∼65 km. They also proposed uplift by several facies grades shortly before the impact (Gibson and Wallmach kilometers of the southeastern third of the structure along a 1995), older fault zone rocks are likely to have been partially major, NNE-trending lineament or flexure now lying beneath or completely annealed. Some evidence exists for at least two the Karoo basin, or as a result of NW-directed, 1.1 Ga, episodes of small-scale crenulation folding and foliation Namaqua-Natal thrusting (see also Friese et al. 1995). development in the metapelitic units in the collar during this However, they noted that these events did not substantially metamorphism (Gibson 1993; Gibson and Reimold 2001), affect the gross structure of the Vredefort Dome itself. and similar features have been described associated with bedding-parallel thrusting in the Witwatersrand goldfields RESULTS (Phillips and Law 1994). However, no map-scale structures appear to be related to this deformation. The emplacement of The present study involves interpretation of the large- the alkali granite plutons may account for some of the scale structure of the northern and western parts of the collar structural complexity of the Witwatersrand supergroup strata of the Vredefort Dome using 1:250,000 Landsat TM (Fig. 2), in their vicinity, with bedding striking radial to the dome in 1:25,000 stereoscopic aerial photographs, and 1:10,000 places (Fig. 2). orthophotos, together with both regional and detailed local Reconstruction of the Vredefort craterform based on structural mapping. Most data were collected from quartzite geophysical data led Henkel and Reimold (1998) to propose units of the West Rand and Central Rand groups because of that post-impact SE-directed thrusting, possibly related to the their generally excellent exposure (Fig. 2). The principal poorly constrained Mesoproterozoic Kheis orogeny, may results are presented below. 1542 F. Wieland et al.

Gross Collar Morphology than 5% (Table 1). Given the structural complexity of the collar, the impersistent outcrop in places, the variation in radial The results of our analysis of bedding orientations in the distance of individual marker units from the center (Fig. 2), Witwatersrand supergroup strata around the dome (Fig. 3) and the existence of Ventersdorp-age extensional faults, these confirm the mostly overturned nature of the strata in the values should be regarded as only crude approximations. For northeastern, northern, and western parts of the collar and instance, a 1 km variation in radial distance changes the moderate, right-way-up dips in the southeastern sector; calculated shortening by approximately 5%. however, dips in the northern and northeastern quadrants are also locally right-way-up. Within the limitations of the Folds dataset, which shows significant spread in bedding orientations within comparatively small areas (Figs. 3b–3g), Based on the regional-scale mapping and image analysis, the data do not appear to support Lana et al.’s (2003a) ductile strain in the collar rocks is heterogeneous. For contention that the angle of overturning is discernibly lower convenience, three structural subdomains have been in the southwestern and northeastern quadrants than in the identified, although, as argued below, they form a continuum: northwestern quadrant because of a pre-impact shallow 1) relatively straight segments up to several km long with or northwestward dip of the supracrustal strata. This implies that without internal m- to km-scale gentle to open symmetric the average 120° of overturning of the collar rocks (Fig. 3) is undulations; 2) open to tight km-scale symmetric folds a true reflection of the amount of impact-induced rotation. oriented radially to the dome; and 3) asymmetric km-scale Based on interpretation of regional gravity and magnetic open homoclinal folds with axial planes trending oblique to datasets, Antoine et al. (1990) suggested that the dome has a the dome. polygonal, rather than circular, shape. They divided the Much of the local variation in bedding orientation exposed section of the dome into six segments, arranged at recorded in individual sectors around the dome (Fig. 3) relates angles of 40° to 45° to each other, with possibly two more to the outcrop-scale gentle folding of bedding, although fault- segments to the south hidden beneath the Karoo Supergroup related block rotations also play a role locally. Smaller-scale cover rocks (Fig. 3a). In order to test this polygon model, we folds (meters to tens of meters) appear to be restricted to the subdivided our dataset to correspond to Antoine et al.’s strongly layered West Rand group. Large, km-scale, broadly (1990) segments (Fig. 3a). The resultant stereonets (Figs. 3b– symmetric, gentle to open folds occur in both the West Rand 3g) show considerable variation in bedding orientations and Central Rand group strata, but are more common in the stemming from both symmetric and asymmetric folding of former. At an even larger scale, the collar rocks show a the bedding on a ten to hundred m-scale (km-scale fold data cuspate-lobate pattern indicating heterogeneous strain around such as shown in Figs. 4 and 5 were excluded from the the dome. The cusps point both towards and away from the datasets) and fault-related block rotations (Fig. 8). The results center of the dome (Fig. 2) and are invariably faulted. indicate angles of 30° to 45° between the individual maxima The most intense km-scale folding occurs in the West of poles to bedding only in the northeastern and western Rand group in the northwestern sector of the collar where five sectors (Figs. 3b, 3c, 3e, and 3f); however, considerable synformal anticlines are developed in the Hospital Hill overlap exists even for these sectors, indicating that bedding Subgroup over a strike length of ∼13 km (Fig. 4). Symmetric rotation in response to doming was distributed on a smaller folds in the northern and northeastern sectors of the dome are scale than that envisaged by Antoine et al. (1990). A further generally more open than those in the northwestern sector, argument against the polygonal model is that our study failed with interlimb angles typically between 20° and 30°. All the to identify any large radial faults extending across the entire folds show synformal anticline geometries, with subvertical collar of the dome in the positions proposed by Antoine et al. axial planes and steeply inward-plunging hinges, both of (1990) (compare Figs. 2 and 3a). which trend radial to the dome. Bedding in the northwestern A first-order calculation of the amount of tangential segment displays a general southeasterly overturned dip shortening associated with the formation of the central uplift (Figs. 3c and 4). The three central folds (B to D in Fig. 4) are was made by comparing the cumulative strike length of largely symmetric with radially-striking axial planes and are three marker horizons in the Witwatersrand supergroup on the characterized by limbs of roughly equal length that strike 1:50,000 geological map of the Vredefort Dome (Bisschoff oblique to the general NE-SW bedding trend. In two of these 2000) with the calculated circumference based on radial folds, the layering on the southwestern limb is partially or distance from the center of the dome (Table 1). In the Hospital completely right-way-up (folds B and C, Figs. 4c, 4d, and 5), Hill Subgroup quartzite horizons of the West Rand group whereas the more open fold (D, Fig. 4e) displays overturned (Figs. 2, 4, and 6), the measured shortening by faulting and layering on both limbs. Fold D is less disrupted by faults than folding is approximately 17%, whereas the amount of folds B and C, which have been displaced radially outward by shortening in the upper Government Subgroup and the upper conjugate faults by 0.5–2 km (Figs. 4a and 5). Folds B and D units of the Central Rand group (Turffontein subgroup) is less have steep to vertical radially-trending axial planes and Structural analysis of the collar of the Vredefort Dome, South Africa 1543

Fig. 4. Large-scale symmetric and asymmetric folds in the Hospital Hill Subgroup in the northwestern collar of the Vredefort Dome. a) Portion of Landsat TM image of the northwestern collar of the dome (Fig. 2), showing the locations of the folds. b–f) Lower hemisphere equal area Schmidt net projections showing fold data (dots = poles to bedding, great circles = average bedding in limbs, bold great circle = axial plane). Note that in (c) the limb orientation is more complex, with both right-way-up and overturned portions on the western limb (see Fig. 5b for detail). moderate to steeply radially inward-plunging hinges, whereas between 45° and 90° to the regional trend of bedding. While fold C has a gently southwest-dipping axial plane and bedding remains overturned on the short limb of fold E tangential gently southwest-plunging hinges, (Fig. 4d). Given (Fig. 4f), bedding in fold A has a steep right-way-up the general radial trend of the other symmetric folds around orientation (Fig. 4b). Fold A is associated with a smaller, the dome, we suggest that post-folding fault-block rotation open, asymmetric, dextral-verging fold some 500 m to the may explain this particular anomaly. On a smaller scale, the northeast that suggests that it is, in fact, part of a larger fold limbs display m- to dm-scale secondary folds and both asymmetric box fold. The short limb of fold E is displaced the limbs and hinges are disrupted by curviplanar faults that along a radial fault with a sinistral slip sense (Fig. 7b), are typically poorly exposed. whereas fold A lies within a few km of a major oblique fault Quartzite units in the hinges of the more intense with a sinistral slip sense (compare Figs. 2, 4, and 6). The symmetric folds (e.g., fold B, Figs. 4 and 5) are highly hinge of fold E plunges steeply south-southeast and the fractured and progressive rotation of bedding around the foldaxial plane dips steeply to the southeast (Fig. 4f). In hinge is typically not seen (Fig. 5b). Instead, these hinge contrast, fold A displays a moderately south-southwest zones contain irregular networks of pseudotachylitic breccia plunging fold hinge and a moderately southwest-dipping fold up to 20–30 m wide (see also Dressler and Reimold 2004). In axial plane (Fig. 4b). A noteworthy aspect of the asymmetric contrast, pseudotachylitic breccias in the quartzites in the fold folds is that, despite exposure of more than 200° of arc-length limbs or in the straight segments of the collar are considerably around the dome, the major asymmetric folds all show a less voluminous and are generally bedding-parallel with only sinistral vergence. minor discordant offshoots (see below). Outcrop of the metapelitic units in the fold hinges is generally poor; Faults however, they do not appear to show similar voluminous network breccias. The quartzites of the Witwatersrand supergroup provide Folds A and E in the northwestern sector of the dome are excellent markers with which to investigate faulting in the asymmetric and both display sinistral vergence (Fig. 4). They collar of the Vredefort Dome (Figs. 6 and 7). Although the are homoclinal, with short southwestern limbs striking faults themselves seldom crop out, the exposures afforded by 1544 F. Wieland et al.

Fig. 5. Structural data from fold B in Fig. 4 and its vicinity showing the relationship between bedding, faults, and joints. The scatter of bedding data on the western limb of the fold (b and f) is a consequence of dm-scale folding and the change from right-way-up dips near the hinge of the fold to overturned dips in the south. a) A sketch map of Hospital Hill quartzite from which data was obtained. b) An annotated aerial photograph of fold, showing faults and absence of clear hinge curvature. Diagrams (c) to (f) are lower hemisphere equal area Schmidt net projections of the poles to bedding and joints (contour plots) in the continuations to the (c) SW and (d) NE of the fold, (e) northern limb of fold and (f) western limb of fold (great circles refer to average bedding orientations, Roman numerals to joint sets described in text). the significant topographic relief in the collar and analysis of Curvature of the faults is common, particularly in the western fractures in the vicinity of the fault traces suggest that most sector where the faults are longest. Based on Nel’s (1927) faults are vertical. The orientation of the faults varies from map, Lilly (1978) suggested that most of the curved faults are radial to oblique with respect to the circumference of the concave toward the core of the dome; in contrast, we detected dome (Fig. 7) and the largest faults can be traced for more than no preferential pattern. Faults in the Transvaal supergroup 5 km along strike. Among the larger faults, oblique trends rocks in the outer collar show similar oblique-radial strikes dominate in the western sector, whereas the northwestern, and apparent offsets to the inner collar faults (Simpson 1978; northern, and northeastern sectors display a preponderance of Bisschoff 2000), confirming that these structures are younger radial fault orientations (see also Lilly 1978) (Fig. 6). than the ∼2.7 Ga Ventersdorp age faults. Structural analysis of the collar of the Vredefort Dome, South Africa 1545

Table 1. Calculated shortening of the exposed sections of Witwatersrand supergroup strata in the Vredefort Dome based on the 1:10,000 geological map of Bisschoff (2000). Average distance from Exposed Calculated Stratigraphic unit crater center arc length arc length Measured length Percentage (subgroup) (km) (°) (km) (km) shortening Hospital Hill 21 195 72 84 17 Government 25 175 76 80 5 Turffontein 29 110 56 58 4

Horizontal to shallowly plunging grooves on slickenside difficult to locate in the more deeply weathered metapelitic fracture surfaces in the fault zones indicate that the faults and ironstone units of the Witwatersrand supergroup; involved principally strike-slip movement. Based on offset of however, sharp variations in dip angles of up to 40° between the quartzite units, displacement is typically of the order of stratigraphically-adjacent sedimentary units suggest that such hundreds of meters, although a few faults show displacements faults must exist. between 1 and 2 km. Most of the large radial and oblique faults display sinistral offset, whereas the smaller faults Joints and Shear Fractures display either sinistral or dextral offsets and define a conjugate pattern consistent with horizontal shortening Previous studies of fracture phenomena in the Vredefort having been oriented tangential to the dome. The exception is Dome have focused primarily on shatter cones (Hargraves in the northern sector where one of the large radial faults 1961; Manton 1962, 1965; Albat 1988; Albat and Mayer displays dextral offset; however, this has been interpreted as a 1989) and the so-called multiply striated joint sets (MSJS) Ventersdorp age fault (Figs. 1b and 2). The size of the faults in that Nicolaysen and Reimold (1999) linked to the shatter terms of both strike length and displacement appears to cones. The results of our study of shatter cones are the subject decrease radially outward, with faults in the Central Rand of another paper and will not be discussed further here, except group seldom displaying slip magnitudes of more than 100 m. to note that they assist in constraining the formation age of The faults cut the symmetric and asymmetric folds in the other fractures (Fig. 9a). West Rand group (Figs. 4–6), suggesting that they postdate Given the general ease with which joints appear to form ductile deformation; however, the similar sinistral asymmetry in upper crustal environments, the most obvious problem is of both the asymmetric folds and the majority of the faults and whether particular joint sets can be related with any certainty the close spatial association between symmetric folds and to the impact event. Apart from the distinctive shatter cones conjugate faults (e.g., Fig. 5) suggest a consistent strain field and MSJS, some fractures can be attributed to the impact- for their development. related deformation as a consequence of their association with The fault zones are marked by intense fracturing in the impact-related melts. The most obvious of these are the nine quartzites. They regularly contain pseudotachylitic breccia dikes of impact-melt rock (Vredefort Granophyre) that occur vein networks up to several meters wide and tens of meters in single or en-echelon radial and tangential fractures of up to long, but cataclastic fault breccias have not been found. several km length in the inner collar and outer core of the Together with the pseudotachylitic breccias in the hinges of dome (Therriault et al. 1996) (Fig. 1b). These dikes, which the folds, these are the most voluminous occurrences of this have widths ranging from 10 to 60 m, appear to be unaffected rock type in the collar of the dome. No surface exposures of by either the large-scale folds or the radial and oblique faults faults have been found in the West Rand group metapelitic in the West Rand group (Bisschoff 2000), consistent with rocks. their emplacement having occurred after tangential In the wallrocks adjacent to some of the larger faults, the shortening. This suggests that the latter stages of the evolution strike of bedding is progressively rotated in a sense that is of the dome, prior to final crystallization of the impact melt, consistent with normal drag folding related to the slip along were characterized by simultaneous tangential and radial the faults. In addition, however, some radial and oblique- extension. A similar conclusion of radially directed dilation striking faults separate rigid blocks of hundreds of meters to can also be drawn from the dm-thick bedding-parallel several kilometers length that show distinct differences in pseudotachylitic breccia veins that extend for tens to bedding dip of up to 30–50° (Fig. 8). This most likely hundreds of meters in the West Rand group quartzites, for represents scissor-type block rotation associated with the example, along the northern limb of fold E in Fig. 4. Like the faulting, and may, for example, account for the anomalous voluminous pseudotachylitic network breccias that occur orientation of the symmetric fold C shown in Fig. 4. preferentially within quartzites in the large fault zones and in In addition to the radial and oblique faults, several large, the hinges of the large-scale folds, the fillings of these veins subvertical, tangential faults showing west-side-down appear to have migrated from their sites of generation (see displacement occur along the core-collar contact in the Discussion section). southwestern sector of the dome (Lilly 1978). Such faults are In the metapelitic rocks of the West Rand group, 1546 F. Wieland et al.

Fig. 6. a) Outcrop distribution of Witwatersrand supergroup strata in the Vredefort Dome showing major faults (modified from Bisschoff 2000) and rose diagrams (b–e) of fault orientations compiled after the method of Donath (1962). The arrows indicate boundaries between domains identified for rose diagrams. Note the conjugate pattern in all diagrams and the dominance of oblique fault orientations in the western sector (e) and radial orientations in the northwestern (d), northern (c), and northeastern (b) sectors. Several large tangential faults occur in the southwestern sector. however, a more complex geometric relationship exists breccia-filled fractures in the core of the dome (Gibson and between fractures and pseudotachylitic breccia veins. The Reimold 2005) and the Central Rand group quartzites in the rocks are pervaded by a network of hairline, irregular to northeastern collar (Martini 1991) have been identified as curviplanar shear fractures that display mm- to cm-scale shock features because of the localization of shock offset of both bedding and pre-impact metamorphic metamorphic effects along their margins. porphyroblasts (Gibson et al. 1997) (Fig. 9b). Gibson (1996) Thin psammitic layers within the pelitic rocks locally and Gibson et al. (1997) described this phenomenon as a show an orthogonal to rhomboidal fracture pattern on bedding “spaced fracture cleavage” because of the intense, cm- to dm- surfaces involving up to three sets of closely spaced (typically scale spacing of the fractures. The fractures commonly mm- to cm-scale spacing and tens of centimeters long) contain mm-thick veinlets and pods of pseudotachylitic fractures, similar to the pattern seen in thinly bedded quartzite breccia that have chemical compositions consistent with units (Fig. 9c). In both scale and geometry, these fractures largely in situ derivation, and the veins and fractures are resemble the “shatter cleavage” described by Milton et al. overgrown by the post-impact metamorphic paragenesis (1996) in the thinly bedded units in the Gosses Bluff structure found in the host rock, which commonly obscures the (Australia) that they linked to the phenomenon. fractures in hand specimen (Gibson et al. 1997). Locally, Their lack of discernible dilation distinguishes them from the larger breccia networks are present. Precise slip directions larger-scale, more irregular joints, which clearly cut them and along the shear fractures are difficult to determine, and offsets which have a dilational origin (Fig. 9c). These joints form the of bedding locally suggest either horizontal tangential most striking large-scale fracture phenomenon in these rocks extension or shortening. The latter may indicate formation of (Fig. 10). Joint spacing and length is generally directly these fractures in conjunction with the large-scale folds and proportional to layer thickness, with the largest joints in the faults, while the former might be compatible with either the quartzites being hundreds of meters long and tens of meters shock compression stage or the final collapse of the central apart (Fig. 10a). Although Lilly (1978) suggested that the uplift (see Discussion section). Morphologically similar joints are parallel to the faults in the collar, his analysis was Structural analysis of the collar of the Vredefort Dome, South Africa 1547

Fig. 7. Aerial photograph interpretations of fault patterns in West Rand group strata in the a) northern (upper West Rand group NNE of Parys) and b) northwestern (lower West Rand group WNW of Parys) sectors of the dome, showing aspects of radial and oblique faulting and predominance of sinistral offset. based purely on strike data. We agree that fracture intensity Pseudotachylitic breccias are found both within the joints increases in the quartzites in the fault zones and that therefore and shear fractures (Figs. 9b and 9e). In the former case, the at least some of the fractures are fault-related. However, a full breccias appear to occupy purely dilational structures, an 3-D analysis of numerous sites around the dome indicates that interpretation supported by the local development of veins in the principal geometric control on these joints is bedding en-echelon tension gash geometries (Dressler and Reimold orientation, with the main sets typically being oriented either 2004; Reimold and Gibson 2005). The absence of clear-cut perpendicular or parallel to bedding (Figs. 5 and 10). generation planes for most of the breccias in the dome has Fractures parallel to bedding have been removed from all been noted by other authors (e.g., Reimold and Colliston stereoplots in Figs. 5 and 10, as they are represented by the 1994; Dressler and Reimold 2004). In general, thinner veins bedding orientation. of pseudotachylitic breccia are more likely to be displaced by Apart from the bedding-parallel fractures, the most small amounts along cross-cutting fractures (Fig. 9d). Thicker prominent set of joints in outcrop is typically subvertical and veins show less evidence of displacement by fractures. While perpendicular to bedding (labeled I in Figs. 5, 9a, and 10). some of these joints must be related to post-impact tectonic This set displays a general radial orientation with respect to events, this evidence suggests that the joint pattern is largely the dome. The exceptions are sections where the bedding has the product of the final stages of central uplift formation. The been folded or rotated by faulting and, thus, is no longer overall volume of pseudotachylitic breccia and the size of tangential to the dome (Figs. 5e and f). This set bisects the individual occurrences decrease radially outwards through the acute angle between two inclined shear fracture/joint sets (II Witwatersrand supergroup rocks, but veins up to 1–2 cm thick and III in Figs. 5e, 5f, 9a, and 10), although the two sets are still found locally in the Ventersdorp supergroup rocks. commonly show slightly different dips (e.g., Figs. 5 and 10) and may be unequally developed. Locally, these fractures DISCUSSION display mm- to cm-scale normal dip-slip displacement, consistent with tangential extension and vertical shortening Implications for Central Uplift and Cratering Mechanics (Fig. 9d). A fifth set (IV in Figs. 5 and 10) strikes parallel to bedding and displays a shallow outward radial dip. Joints of Structural analysis of the Witwatersrand supergroup this set are typically short and may be irregular. The rocks in the collar of the Vredefort Dome indicates that, rather metapelitic rocks show a similar joint pattern to the quartzites than exhibiting a comparatively simple geometry involving (Figs. 10b and 10c). The lack of prominence of set IV in the rigid polygonal segments separated by radial faults as metapelitic units might reflect the general lack of vertical suggested by Antoine et al. (1990), the collar displays a relief in the metapelitic outcrops. The amphibolite-grade highly heterogeneous internal structure involving both ductile metapelitic units in the West Rand group show considerably and brittle strain. The 2.06 Ga metamorphic and alkali granite less jointing than the intercalated quartzites, whereas the low- intrusive events provide a convenient time marker with which grade (or greenschist-facies metamorphic) metapelitic rocks to distinguish pre-impact brittle structures from those further from the center of the dome are typically more generated during the impact event. The association of the intensely jointed than the adjacent quartzites (Fig. 9a). folds, faults, and fractures in the collar with voluminous 1548 F. Wieland et al.

Fig. 8. a) Variable dip orientation of West Rand group quartzites disrupted by radial faults in the northwestern part of the collar. The quartzite package is 70 m wide. b) Lower hemisphere equal area Schmidt net projection of the poles to bedding in the different fault blocks B, C, and F. Average bedding orientations are shown as great circles. Data indicate more than 90° of rotation about a subhorizontal, broadly tangential axis. pseudotachylitic breccias—that are themselves overprinted development of brecciated hinge zones that are commonly by the impact-related thermal event (Gibson et al. 1997)— pervaded by pseudotachylitic breccias points to brittle supports their impact origins. deformation as well, which is consistent with the high strain The bulk of the large-scale structures (folds, radial, and rates typical of impact processes. Ductile behavior in these oblique faults) display geometries and kinematic indicators rocks may have been enhanced by the elevated post-shock that are consistent with formation during tangential temperatures, estimated at >500 °C in the inner parts of the shortening of the collar strata. The amounts of shortening collar but decreasing to ∼300 °C in the Central Rand group calculated in this study agree with those deduced by Manton rocks (Gibson et al. 1998). This temperature variation—the (1965) using Nel’s (1927) original map of the dome (he product of differential shock-induced heating and the pre- estimated 7% shortening in the West Rand group and no impact geotherm (Gibson et al. 1998; Gibson and Reimold shortening in the Central Rand group). They confirm the 2005)—may have played a role in the radial outward decrease radial outward decrease in the amount of shortening that was in the relative importance between folding and faulting seen also postulated by Simpson (1978), who noted no discernible in the collar. However, this pattern may equally be the result shortening in the Transvaal supergroup rocks in the outer of the radial outward decrease in the amount of shortening collar. The values obtained—17% in the lower West Rand across the collar, or the change from a mechanically group and <5% in the Central Rand group—fall well within heterogeneous sequence of quartzite and metapelitic units in the range of plastic strains predicted by numerical modeling the West Rand group, which should have favored folding, to of large central uplifts (Collins et al. 2004). the more homogeneous and more competent quartzite- The similarity in the strain field responsible for the dominated succession in the Central Rand group, which formation of the folds and faults and their spatial coincidence, might have favored faulting. together with the similar sense-of-shear of faults and The predominantly radial to radial-oblique arrangement asymmetric folds, suggests that formation of the ductile and of structures in the Vredefort Dome and the dominance of brittle features overlapped in time, although ongoing fault- subhorizontal displacements in their formation contrast with related block rotations may explain the somewhat varied the tangential to oblique-tangential arrangement of orientation of the folds (Fig. 4) and the generally lesser centripetally directed thrusts found in the central uplifts of amounts of overturning of the southwestern limbs of these smaller craters such as Gosses Bluff (Milton et al. 1996), folds. While fold formation indicates ductile strain, the Upheaval Dome (Kriens et al. 1996; Kenkmann et al. 2005), Structural analysis of the collar of the Vredefort Dome, South Africa 1549

Fig. 9. Fracture and joint phenomena in the Witwatersrand supergroup rocks in the collar of the Vredefort Dome. a) Shatter cones (center, left) in metapelitic unit cut by four sets of joints, Central Rand group in the northwestern collar. The vertical (II in text) and horizontal (IV) sets are most intense, with set II trending NNE across the image (right side) and set III only poorly developed (bottom, center). b) Pseudotachylitic breccia-bearing shear fractures showing mm- and cm-scale slip in Hospital Hill subgroup metapelite from the northern limb of fold A (Fig. 4). A weak conjugate pattern is suggested. Although only one set of fractures displays discernible slip, this is consistent with tangential extension (dextral slip sense). However, one fracture (center, left) shows opposing slip. Such complexity is typical of these fractures (length of pen 10 cm). c) Intense orthogonal fracture pattern in Hospital Hill quartzite in the northern collar (east of fold E, Fig. 4). The orthogonal fractures are cut by younger, irregular, conjugate joints (NE and NW orientations in the photo), which are also responsible for the way in which the slab has broken. d) View of vertical bedding surface showing conjugate shear fractures (sets II and III, see text) cutting thin pseudotachylitic breccia vein (horizontal, east-west), western limb of fold E (Fig. 3) (length of pen 10 cm). e) Shallowly outward-dipping pseudotachylitic breccia (pt 2) filling type IV joint cuts a bedding-parallel vein (pt 1), Hospital Hill subgroup, northeastern collar. 1550 F. Wieland et al.

Fig. 10. a) Joint pattern in Hospital Hill subgroup in the northwestern collar (between folds D and E) (Fig. 4). The ridge shows the intense joint spacing typical of the quartzites in the Vredefort Dome, with four joint sets of m- to dm-spacing. b) Lower hemisphere equal area Schmidt net projections (raw data and contour plot) of poles to joints in the quartzite in (a). Bold great circle shows the average bedding orientation. c) Lower hemisphere equal area Schmidt net projections (raw data and contour plot) of poles to joints in the metapelitic units southeast of the ridge (foreground). Contours range from 1–6%.

Haughton (Bischoff and Oskierski 1988), and walls. In addition, several large vertical faults in the (Wilshire et al. 1972). While this might reflect differing levels goldfields that strike radial to the dome also appear to have of erosion of the central uplifts, with the zone of convergence been active, or reactivated, during the impact event (Killick of the transient crater wall slumps still preserved in the 1993; Fletcher and Reimold 1989). These may represent smaller central uplifts, it is more likely that the center of the transpressional or transtensional structures akin to the ones was never marked by such a structure. This is postulated by Kenkmann and von Dalwigk (2000) for the because the inward propagation of the faults from the outer parts of complex craters. collapsing walls is not sufficiently fast to reach the center of a Collapse of the Vredefort central uplift is also indicated very large crater before the floor starts to rebound (Dence by the granophyre dikes and joints in the collar rocks that 2004). Such structural complexity probably exists, instead, in developed in a strain field involving simultaneous radial and the peak rings of such craters with the added complexity of a tangential extension. Impact-melt dike intrusion is unlikely to strong component of centrifugal thrusting driven by outward have been possible until vertical uplift had ceased, a fact collapse of the central uplift, opposing the centripetal motion borne out by the lack of any evidence of displacement of the of the walls. On this basis, it does not appear plausible that the dikes by the folds and faults. In all likelihood, it indicates that steeply radially-plunging folds in the collar of the Vredefort the central uplift collapsed to the point that it formed a Dome represent rotated radial transpression ridges, as topographic low, allowing at least some of the impact-melt to postulated in Kenkmann and von Dalwigk’s (2000) generic accumulate and be retained above it. model. Further evidence for outward collapse of the central At the present levels of exposure of the Vredefort crater uplift to form a peak ring may be provided by the increasing structure, indications of central uplift collapse are provided amounts of rotation measured from the center of the dome by the upright to slightly centrifugally verging folds in the rim outwards. Based on structural mapping of the crystalline syncline (Simpson 1978) (Fig. 1b). Further afield in the basement core of the dome, Lana et al. (2003a) concluded that goldfields that lie close to the inferred northern edge of the a central region approximately 25 km wide experienced impact structure (Fig. 1a), however, mining activity has minimal impact-related rotation, and that this is surrounded uncovered pseudotachylite-bearing listric normal oblique- by an annulus comprising the outer core of the dome, where and dip-slip faults that dip towards the dome (e.g., Killick ∼90° of rotation is needed to explain the present orientation of 1993), that could be the product of slumping of the Vredefort the gneissic pre-impact structures. In contrast, the inner collar Structural analysis of the collar of the Vredefort Dome, South Africa 1551 rocks studied here show an average of ∼120° of overturning central parts of the Charlevoix and Manicouagan central (Fig. 3). We can find no support for Lana et al.’s (2003a) uplifts where shock pressures exceeded 25 GPa, from which suggestion that this reflects only 90° of impact-related he suggested that the amount of brittle deformation in central rotation superimposed onto a moderately steeply dipping rock uplifts may scale inversely with shock pressure. sequence. Instead, we propose that this increase reflects the outward collapse of the outer parts of the central uplift in the Pseudotachylitic Breccias final stages of crater modification. The origin of the uniform sinistral sense-of-shear on Evidence in support of a syn-impact timing for faults and of folds in the collar of the dome is unknown. pseudotachylitic breccia development in the Vredefort Dome Pronounced asymmetry in the central uplifts of smaller is overwhelming (e.g., Dressler and Reimold 2004; Reimold complex impact structures such as Gosses Bluff and Upheaval and Gibson 2005), allowing the breccias to be used to Dome has been interpreted as the result of asymmetric mass constrain the impact origin of other structures. However, the displacement of rocks close to the surface caused by oblique exact mechanism(s) by which the breccias formed remain impact (Milton et al. 1996; Kenkmann et al. 2005), but the problematic. Three mechanisms have been proposed (see levels exposed in the Vredefort Dome are far deeper than review in Reimold and Gibson 2005): a) localized shock those studied in these smaller craters, making such an option melting caused by extreme fluctuations in shock pressure, b) less likely. It is possible that a slight asymmetry in the target friction melting along slip surfaces triggered during the rocks—for instance, pre-impact tilting of the supracrustal modification stage of cratering, or c) a combination of shock succession, but by a much smaller amount than proposed by and friction melting during the shock stage. Evidence exists Lana et al. (2003a)—might induce asymmetry in the impact- for localized enhancement of shock deformation against related structures. Unfortunately, nearly half of the Vredefort fractures hosting narrow melt breccia veins in both the core Dome is buried beneath younger cover rocks, which hampers (Gibson and Reimold 2005) and collar (Martini 1991) of the a further investigation of this problem. dome, but these fractures also invariably show small Numerical modeling suggests that a central uplift the size amounts of displacement. Given their small volumes, it is of the Vredefort Dome is likely to have formed within a likely that these melt breccias crystallized or quenched matter of 2–3 min (e.g., Henkel and Reimold 1998; Melosh virtually instantaneously, and thus constrain the fractures as and Ivanov 1999). During most of this time, the rocks in the syn-shock. However, the same argument cannot be made for central uplift appear to occupy radial positions further from the structures hosting more voluminous breccias, as these the center of the impact structure than their starting positions may have been able to retain a molten matrix for several (e.g., Collins et al. 2004); it is only in the latter stages of uplift minutes, if not hours (Ogilvie, personal communication that tangential shortening gains in importance. It thus seems 2004). In contrast to the narrow veinlets, which typically likely that the radial folds and radial and oblique faults show a close correspondence between the chemical developed towards the end of central uplift formation. composition of their matrix and that of their wallrocks, the Within a larger context, the presence of faults in the larger breccias show abundant evidence of mixing of melt collar but apparent lack of them in the core of the dome may from a variety of sources, as well as exotic clasts (Reimold reflect a combination of increased post-shock temperatures and Colliston 1994), indicating sufficient time for melts and toward the center of the dome (Gibson et al. 1998; Gibson and clasts to move distances of at least meters to tens of meters. Reimold 2005), different rock types (heterogeneously layered In the context of the cratering process, it is thus plausible collar rocks versus massive crystalline core), and lesser that the pseudotachylitic breccias could all have formed amounts of rotation in the core (plug-like geometry) (Lana simultaneously during the shock stage by either shock et al. 2003a). One option is that slip could have been melting or shock + friction melting, but that crystallization distributed more evenly through the rocks in the core of the straddled the subsequent stages of central uplift formation dome because of the increased intensity of pseudotachylitic and collapse, depending on the degree of superheating of the breccia development toward the center of the dome (Reimold melts, the host rock temperature, and the volume of melt and Colliston 1994; Gibson and Reimold 2005), thereby present. Equally, however, some or even most of the obviating the need for widely spaced large-magnitude faults breccias could have formed during the modification phase in (Lana et al. 2003a). Melosh (2005) rejected this possibility on response to high-strain-rate slip events. It is beyond the the grounds that the melts would have quenched almost scope of this study to explore this issue further, except that it immediately after forming, because of the large temperature is worth noting that thinner breccias in the collar of the difference with the host rocks. However, superheated shock dome are more likely to have been displaced by fractures melts such as those described by Gibson and Reimold (2005) than thicker breccias (compare Figs. 9d and 9e) and that a may have remained liquid long enough, given the extreme melt capable of surviving for even a few minutes within host-rock temperatures found in the center of the dome such a complex structural environment may now reside in a (Gibson et al. 1998; Gibson 2002). Dence (2004) noted a very different structural context from the one in which it similar lack of macroscopic fragmentation (i.e., faults) in the originated. 1552 F. Wieland et al.

Chronology of Impact-Related Deformation siliciclastic rocks of the Witwatersrand supergroup in the inner collar of the dome preserve a variety of fold, fault, The oldest mesoscopic impact-related structures seen in fracture, and melt breccia features related to the 2.02 Ga the collar rocks of the Vredefort Dome are the shatter cones impact event. The bulk of the fold and fault structures relate that, together with at least some of the pseudotachylitic to tangential shortening of the strata, which reached ∼17% in breccia (Martini 1991; Gibson and Reimold 2005), formed strata closer to the center of the dome, decreasing to <5% during the shock pulse. The orthogonal to rhomboidal further out. These figures are compatible with the predictions fracture cleavage observed in the quartzite units may form of total plastic strain during central uplift formation in large part of a structural continuum with the shatter cones and complex craters obtained by recent computer modeling MSJS. (Collins et al. 2004). Tangential shortening was followed by The next structures to form were the folds. Recent radial and tangential extension related to the collapse of the modeling by Collins et al. (2004) suggests that rocks in the central uplift that produced ubiquitous jointing and rarer central uplift follow a trajectory in which they are initially faults, and provided the opportunity for downward intrusion displaced downward and radially outward under shock of dikes from the impact melt sheet. The origin and timing of compression, followed by vertical uplift, only toward the end crystallization of pseudotachylitic breccias within the of which do they move closer to the center of the impact Vredefort impact event remains problematic and is the subject structure than the position from which they started. This of ongoing research. suggests that the folds most likely developed close to the end of central uplift formation, although their present overturned Acknowledgments–This work was partially funded through geometries may reflect subsequent outward collapse of the research grants from the National Geographic Society and the central uplift. Continuing tangential shortening was Barringer Family Fund for Meteorite Impact Research. accommodated by asymmetric to conjugate strike-slip Without the support of the many landowners in the Vredefort faulting. Pre-existing faults do not appear to have been Dome who granted access to their properties, this work would particularly reactivated. It is possible that some not have been possible. Comments by U. Riller, pseudotachylitic breccias could have formed at this stage F. Schwerdtner and an anonymous reviewer substantially through frictional melting along the faults; however, it is also improved earlier versions of this paper. This paper is the possible that sufficiently voluminous melts created during the University of the Witwatersrand Impact Cratering Research shock pulse (by shock and/or friction melting) could have Group Contribution No. 72. survived long enough to be driven into extensional sites opening within the younger structures, where they either Editorial Handling—Dr. Ulrich Riller quenched or crystallized. Given that the central uplift probably formed within only 2–3 min (e.g., Melosh and REFERENCES Ivanov 1999), strain-rates for the tangential shortening were probably of the order of at least 10−3 to 100 s−1. Albat H. M. 1988. Shatter cone/bedding interrelationship in the Possibly overlapping the latter stages of the contractional Vredefort structure: Evidence for meteorite impact? South phase, the collar rocks underwent some tangential faulting African Journal of Geology 91:106–113. Albat H. M. and Mayer J. J. 1989. 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