ABSTRACT Accessory mineral U-Pb geochronometers are crucial tools for constraining the timing of deformation in a wide range of geological settings. Despite the growing recognition that intragrain age variations within deformed minerals can spatially correlate to zones of microstructural damage, the causal mechanisms of Pb loss are not always evident. Here, we report the first U-Pb data for shock-deformed xenotime, from a detrital grain collected at the Vredefort impact structure in South Africa. Orientation mapping revealed multiple shock features, including pervasive planar deformation bands (PDBs) that accommodate up to 40° of lattice misorientation by <100>{010} slip, and also an ~50-µm-wide intragrain shear zone that contains {112} deformation twin lamellae in two orientations. Twenty-nine in situ secondary ion mass spectrometry (SIMS) U-Pb analyses from all microstructural domains yielded a well-defined discordia with upper-intercept age of 2953 ± 15 Ma (mean square of weighted deviates [MSWD] = 0.57, n = 29, 2σ), consistent with derivation from Kaapvaal craton bedrock. However, the 1754 ± 150 Ma lower concordia intercept age falls between the 2020 Ma Vredefort impact and ca. 1100 Ma Kibaran orogenesis and is not well explained by multiple Pb-loss episodes. The pattern and degree of Pb loss (discordance) correlate with increased [U] but do not correlate to microstructure (twin, PDB) or to crystallinity (band contrast) at the scale of SIMS analysis. Numerical modeling of the Pb-loss history using a concordia-discordia-comparison (CDC) test indicated that the lower concordia age is instead best explained by an alteration episode at ca. 1750 Ma, rather than a multiple Pb-loss history. In this example, the U-Pb system in deformed xenotime does not record a clear signature of impact age resetting; rather, the implied high dislocation density recorded by planar deformation bands and the presence of deformation twins facilitated subsequent Pb loss during a younger event that affected the Witwatersrand basin. Microstructural characterization of xenotime targeted for geochronology provides a new tool for recognizing evidence of deformation and can provide insight into complex age data from highly strained grains, and, as is the case in this study, elucidate previously unrecognized alteration events.

Field-based structural analyses addressing the formation of large terrestrial impact structures are rare. We present a field-based kinematic analysis and a three-dimensional (3-D) block model of prominent structures from supracrustal strata of the Vredefort Dome, the central uplift of the Vredefort impact structure. This study aims to better understand the kinematics of complex crater formation. Specifically, the configuration of prominent concentric and radial faults supports the hypothesis that the Vredefort Dome formed by centripetal rock movement followed by radial spreading of uplifted rocks. Centripetal rock movement led to the formation of radially disposed transpression zones, whereby distorted supracrustal strata are characterized by a remarkable structural continuity during central uplift formation. This continuity points to a rather strong mechanical coherence of strata throughout the process. Distortion of layers at the exposed crustal level was accomplished mostly by folding on the hundred-meter scale and is at variance with the concept of concentric normal faults accomplishing kilometer-scale slip within and toward the center of complex impact structures. Displacement magnitudes calculated for strata exposed in the Vredefort Dome indicate that the diameter of the transient cavity of the Vredefort impact structure was ~70 km at surface. All prominent structural elements of the Vredefort Dome can be explained by central uplift formation.

College geoscience departments keep archives of student research ranging from senior theses to master's and Ph.D. dissertations. In field geology, these archives often include maps, cross sections, stereographic projections, field notes and photographs, hand specimens, and thin sections. Subsequent publications may result from the thesis work, but much of this valuable legacy data is difficult to access and assess. Here we describe the conversion of a pre–digital-era thesis on the Vredefort Rim Synclinorium in South Africa from hard copy to digital format using Keyhole Markup Language (KML) to drape maps and inset photographs, and COLLADA (COLLAborative Design Activity) models to create stereographic projections, emergent cross sections, and virtual specimens. In addition to using the Google Earth terrain to fine-tune draped map locations, errors in field locations arising from pace and compass or bearing methods of geo-location that preceded the availability of Global Positioning Systems (GPS) were recognized and corrected. At 2.023 billion years in age and an estimated 300 km in original diameter, the Vredefort Dome is the world's oldest and largest known impact structure. The Vredefort region has been designated a World Heritage Site and specimen collection is prohibited. Only a few geologists are ever likely to visit the region, so geo-referenced field photography, specimens, and structural data are irreplaceable. An interpretative center is being planned for the Vredefort structure by South African authorities and our interactive Google Earth resources will be made available to the visiting public as well as those browsing over the Internet. Thus draped maps and scanned models provide an invaluable opportunity for enhanced instruction, continued research, and public outreach.