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NARROW
GeoRef Subject
-
all geography including DSDP/ODP Sites and Legs
-
Africa
-
East Africa
-
Tanzania (1)
-
Zambia (1)
-
-
Kalahari Desert (1)
-
Southern Africa
-
Barberton greenstone belt (1)
-
Botswana (1)
-
Kaapvaal Craton (19)
-
Karoo Basin (5)
-
Lesotho (3)
-
Orange River (1)
-
South Africa
-
Bushveld Complex (3)
-
Cape fold belt (1)
-
Cape Province region (1)
-
Eastern Cape Province South Africa (4)
-
Free State South Africa
-
Vredefort Dome (83)
-
-
Gauteng South Africa
-
Johannesburg South Africa (2)
-
-
Klerksdorp Field (2)
-
KwaZulu-Natal South Africa (4)
-
Mpumalanga South Africa (1)
-
Northern Cape Province South Africa
-
Kimberley South Africa (2)
-
-
North-West Province South Africa
-
Ventersdorp South Africa (1)
-
-
Transvaal region (6)
-
Vaal River (1)
-
Witwatersrand (15)
-
-
-
-
Antarctica
-
Transantarctic Mountains (1)
-
-
Asia
-
Yakutia Russian Federation
-
Udachnaya Pipe (1)
-
Udachnaya Russian Federation (1)
-
-
-
Australasia
-
Australia
-
South Australia
-
Olympic Dam Deposit (1)
-
-
Western Australia
-
Pilbara (1)
-
-
-
-
Canada
-
Eastern Canada
-
Baffin Island (1)
-
Ontario
-
Sudbury Structure (1)
-
-
-
Nunavut
-
Baffin Island (1)
-
-
Western Canada
-
Manitoba (1)
-
Northwest Territories (1)
-
-
-
Commonwealth of Independent States
-
Russian Federation
-
Karelia Russian Federation (1)
-
Yakutia Russian Federation
-
Udachnaya Pipe (1)
-
Udachnaya Russian Federation (1)
-
-
-
-
Europe
-
Karelia Russian Federation (1)
-
-
South America
-
Brazil
-
Goias Brazil (1)
-
-
-
-
commodities
-
aggregate (1)
-
construction materials (1)
-
diamond deposits (3)
-
metal ores
-
gold ores (5)
-
lead-zinc deposits (1)
-
nickel ores (1)
-
-
mineral deposits, genesis (3)
-
placers (1)
-
-
elements, isotopes
-
carbon
-
C-13/C-12 (2)
-
organic carbon (1)
-
-
chemical ratios (3)
-
hydrogen
-
D/H (1)
-
-
isotope ratios (11)
-
isotopes
-
radioactive isotopes
-
Os-187/Os-186 (1)
-
U-238/Pb-206 (1)
-
-
stable isotopes
-
C-13/C-12 (2)
-
D/H (1)
-
Hf-177/Hf-176 (1)
-
O-18/O-16 (2)
-
Os-187/Os-186 (1)
-
Os-188/Os-187 (3)
-
Sr-87/Sr-86 (1)
-
U-238/Pb-206 (1)
-
-
-
metals
-
actinides
-
uranium
-
U-238/Pb-206 (1)
-
-
-
alkali metals
-
lithium (1)
-
sodium (1)
-
-
alkaline earth metals
-
calcium (2)
-
magnesium (1)
-
strontium
-
Sr-87/Sr-86 (1)
-
-
-
aluminum (2)
-
chromium (1)
-
gold (1)
-
hafnium
-
Hf-177/Hf-176 (1)
-
-
lead
-
U-238/Pb-206 (1)
-
-
platinum group
-
iridium (1)
-
osmium
-
Os-187/Os-186 (1)
-
Os-188/Os-187 (3)
-
-
palladium (1)
-
platinum (1)
-
rhodium (1)
-
ruthenium (1)
-
-
rare earths
-
europium (1)
-
neodymium (1)
-
-
titanium (2)
-
-
oxygen
-
O-18/O-16 (2)
-
-
phosphorus (1)
-
selenium (1)
-
-
fossils
-
burrows (1)
-
Chordata
-
Vertebrata
-
Pisces
-
Chondrichthyes
-
Elasmobranchii (1)
-
-
-
Tetrapoda
-
Reptilia
-
Anapsida (1)
-
Synapsida
-
Therapsida
-
Cynodontia (2)
-
Dicynodontia
-
Lystrosaurus (1)
-
-
-
-
-
-
-
-
ichnofossils (1)
-
Invertebrata
-
Protista
-
Foraminifera (1)
-
Radiolaria (1)
-
-
-
microfossils (2)
-
palynomorphs
-
acritarchs (1)
-
-
Plantae
-
Spermatophyta
-
Gymnospermae
-
Glossopteridales
-
Glossopteris (1)
-
-
-
-
-
-
geochronology methods
-
(U-Th)/He (1)
-
Ar/Ar (3)
-
paleomagnetism (4)
-
Pb/Pb (3)
-
Rb/Sr (4)
-
Re/Os (5)
-
Sm/Nd (2)
-
Th/U (1)
-
U/Pb (18)
-
U/Th/Pb (1)
-
-
geologic age
-
Cenozoic
-
Quaternary
-
Holocene (1)
-
Pleistocene
-
upper Pleistocene (1)
-
-
upper Quaternary (1)
-
-
Tertiary
-
Neogene
-
Miocene (1)
-
-
-
-
Mesozoic
-
Cretaceous
-
Upper Cretaceous (2)
-
-
Jurassic
-
Clarens Formation (2)
-
Lower Jurassic (1)
-
-
Triassic
-
Fremouw Formation (1)
-
Lower Triassic
-
Permian-Triassic boundary (3)
-
-
Upper Triassic
-
Molteno Formation (2)
-
Stormberg Series (1)
-
-
-
-
Paleozoic
-
Permian
-
Ecca Group (2)
-
Lower Permian (1)
-
Upper Permian
-
Permian-Triassic boundary (3)
-
-
Vryheid Formation (3)
-
-
upper Paleozoic
-
Dwyka Formation (2)
-
-
-
Phanerozoic (2)
-
Precambrian
-
Archean
-
Mesoarchean (3)
-
Neoarchean (10)
-
-
Central Rand Group (2)
-
Hadean (1)
-
upper Precambrian
-
Proterozoic
-
Malmani Subgroup (1)
-
Mesoproterozoic (2)
-
Neoproterozoic (1)
-
Paleoproterozoic (14)
-
Pretoria Group (1)
-
-
-
Ventersdorp Supergroup (3)
-
Witwatersrand Supergroup (10)
-
-
-
igneous rocks
-
igneous rocks
-
carbonatites (1)
-
granophyre (10)
-
kimberlite (8)
-
plutonic rocks
-
diabase (1)
-
gabbros
-
norite (2)
-
-
granites
-
aplite (1)
-
-
granodiorites (1)
-
pegmatite (4)
-
ultramafics
-
peridotites
-
harzburgite (2)
-
-
pyroxenite
-
garnet pyroxenite (1)
-
-
-
-
volcanic rocks
-
andesites (1)
-
basalts
-
flood basalts (1)
-
-
dacites (1)
-
rhyolites (1)
-
-
-
-
metamorphic rocks
-
metamorphic rocks
-
amphibolites (3)
-
eclogite (3)
-
garnetite (1)
-
gneisses (9)
-
granulites (4)
-
hornfels (1)
-
impactites
-
impact breccia
-
lunar breccia (1)
-
-
-
metaigneous rocks (1)
-
metaplutonic rocks (1)
-
metasedimentary rocks
-
metapelite (2)
-
-
migmatites (2)
-
mylonites
-
pseudotachylite (18)
-
-
quartzites (4)
-
schists
-
greenstone (3)
-
-
-
-
meteorites
-
meteorites
-
stony meteorites
-
chondrites
-
ordinary chondrites
-
L chondrites (1)
-
-
-
-
-
-
minerals
-
carbonates
-
calcite (2)
-
-
halides
-
fluorides
-
fluorite (1)
-
-
-
minerals (2)
-
native elements
-
diamond (4)
-
-
oxides
-
baddeleyite (1)
-
ilmenite (1)
-
magnetite (2)
-
-
phosphates
-
monazite (5)
-
xenotime (1)
-
-
silicates
-
chain silicates
-
amphibole group
-
orthoamphibole
-
holmquistite (1)
-
-
-
pyroxene group
-
clinopyroxene
-
spodumene (3)
-
-
orthopyroxene (1)
-
-
-
framework silicates
-
feldspar group
-
alkali feldspar
-
K-feldspar (1)
-
microcline (1)
-
-
plagioclase (2)
-
-
silica minerals
-
quartz (5)
-
stishovite (1)
-
-
-
orthosilicates
-
nesosilicates
-
garnet group
-
majorite (1)
-
pyrope (1)
-
-
olivine group
-
olivine (2)
-
ringwoodite (1)
-
-
zircon group
-
zircon (21)
-
-
-
-
sheet silicates
-
clay minerals
-
kaolinite (2)
-
-
mica group
-
biotite (3)
-
muscovite (1)
-
phlogopite (2)
-
-
-
-
-
Primary terms
-
absolute age (28)
-
Africa
-
East Africa
-
Tanzania (1)
-
Zambia (1)
-
-
Kalahari Desert (1)
-
Southern Africa
-
Barberton greenstone belt (1)
-
Botswana (1)
-
Kaapvaal Craton (19)
-
Karoo Basin (5)
-
Lesotho (3)
-
Orange River (1)
-
South Africa
-
Bushveld Complex (3)
-
Cape fold belt (1)
-
Cape Province region (1)
-
Eastern Cape Province South Africa (4)
-
Free State South Africa
-
Vredefort Dome (83)
-
-
Gauteng South Africa
-
Johannesburg South Africa (2)
-
-
Klerksdorp Field (2)
-
KwaZulu-Natal South Africa (4)
-
Mpumalanga South Africa (1)
-
Northern Cape Province South Africa
-
Kimberley South Africa (2)
-
-
North-West Province South Africa
-
Ventersdorp South Africa (1)
-
-
Transvaal region (6)
-
Vaal River (1)
-
Witwatersrand (15)
-
-
-
-
Antarctica
-
Transantarctic Mountains (1)
-
-
Asia
-
Yakutia Russian Federation
-
Udachnaya Pipe (1)
-
Udachnaya Russian Federation (1)
-
-
-
associations (1)
-
Australasia
-
Australia
-
South Australia
-
Olympic Dam Deposit (1)
-
-
Western Australia
-
Pilbara (1)
-
-
-
-
biogeography (1)
-
Canada
-
Eastern Canada
-
Baffin Island (1)
-
Ontario
-
Sudbury Structure (1)
-
-
-
Nunavut
-
Baffin Island (1)
-
-
Western Canada
-
Manitoba (1)
-
Northwest Territories (1)
-
-
-
carbon
-
C-13/C-12 (2)
-
organic carbon (1)
-
-
Cenozoic
-
Quaternary
-
Holocene (1)
-
Pleistocene
-
upper Pleistocene (1)
-
-
upper Quaternary (1)
-
-
Tertiary
-
Neogene
-
Miocene (1)
-
-
-
-
Chordata
-
Vertebrata
-
Pisces
-
Chondrichthyes
-
Elasmobranchii (1)
-
-
-
Tetrapoda
-
Reptilia
-
Anapsida (1)
-
Synapsida
-
Therapsida
-
Cynodontia (2)
-
Dicynodontia
-
Lystrosaurus (1)
-
-
-
-
-
-
-
-
clay mineralogy (2)
-
construction materials (1)
-
crust (10)
-
crystal chemistry (1)
-
crystal growth (3)
-
crystal structure (2)
-
dams (1)
-
data processing (2)
-
deformation (14)
-
diamond deposits (3)
-
Earth (1)
-
earthquakes (2)
-
Europe
-
Karelia Russian Federation (1)
-
-
faults (10)
-
folds (5)
-
foliation (1)
-
fractures (5)
-
geochemistry (6)
-
geochronology (3)
-
geomorphology (1)
-
geophysical methods (9)
-
heat flow (1)
-
hydrogen
-
D/H (1)
-
-
ichnofossils (1)
-
igneous rocks
-
carbonatites (1)
-
granophyre (10)
-
kimberlite (8)
-
plutonic rocks
-
diabase (1)
-
gabbros
-
norite (2)
-
-
granites
-
aplite (1)
-
-
granodiorites (1)
-
pegmatite (4)
-
ultramafics
-
peridotites
-
harzburgite (2)
-
-
pyroxenite
-
garnet pyroxenite (1)
-
-
-
-
volcanic rocks
-
andesites (1)
-
basalts
-
flood basalts (1)
-
-
dacites (1)
-
rhyolites (1)
-
-
-
inclusions
-
fluid inclusions (2)
-
-
intrusions (14)
-
Invertebrata
-
Protista
-
Foraminifera (1)
-
Radiolaria (1)
-
-
-
isotopes
-
radioactive isotopes
-
Os-187/Os-186 (1)
-
U-238/Pb-206 (1)
-
-
stable isotopes
-
C-13/C-12 (2)
-
D/H (1)
-
Hf-177/Hf-176 (1)
-
O-18/O-16 (2)
-
Os-187/Os-186 (1)
-
Os-188/Os-187 (3)
-
Sr-87/Sr-86 (1)
-
U-238/Pb-206 (1)
-
-
-
lava (2)
-
magmas (4)
-
mantle (9)
-
Mesozoic
-
Cretaceous
-
Upper Cretaceous (2)
-
-
Jurassic
-
Clarens Formation (2)
-
Lower Jurassic (1)
-
-
Triassic
-
Fremouw Formation (1)
-
Lower Triassic
-
Permian-Triassic boundary (3)
-
-
Upper Triassic
-
Molteno Formation (2)
-
Stormberg Series (1)
-
-
-
-
metal ores
-
gold ores (5)
-
lead-zinc deposits (1)
-
nickel ores (1)
-
-
metals
-
actinides
-
uranium
-
U-238/Pb-206 (1)
-
-
-
alkali metals
-
lithium (1)
-
sodium (1)
-
-
alkaline earth metals
-
calcium (2)
-
magnesium (1)
-
strontium
-
Sr-87/Sr-86 (1)
-
-
-
aluminum (2)
-
chromium (1)
-
gold (1)
-
hafnium
-
Hf-177/Hf-176 (1)
-
-
lead
-
U-238/Pb-206 (1)
-
-
platinum group
-
iridium (1)
-
osmium
-
Os-187/Os-186 (1)
-
Os-188/Os-187 (3)
-
-
palladium (1)
-
platinum (1)
-
rhodium (1)
-
ruthenium (1)
-
-
rare earths
-
europium (1)
-
neodymium (1)
-
-
titanium (2)
-
-
metamorphic rocks
-
amphibolites (3)
-
eclogite (3)
-
garnetite (1)
-
gneisses (9)
-
granulites (4)
-
hornfels (1)
-
impactites
-
impact breccia
-
lunar breccia (1)
-
-
-
metaigneous rocks (1)
-
metaplutonic rocks (1)
-
metasedimentary rocks
-
metapelite (2)
-
-
migmatites (2)
-
mylonites
-
pseudotachylite (18)
-
-
quartzites (4)
-
schists
-
greenstone (3)
-
-
-
metamorphism (35)
-
metasomatism (5)
-
meteorites
-
stony meteorites
-
chondrites
-
ordinary chondrites
-
L chondrites (1)
-
-
-
-
-
mineral deposits, genesis (3)
-
mineralogy (2)
-
minerals (2)
-
Moon (2)
-
orogeny (1)
-
oxygen
-
O-18/O-16 (2)
-
-
paleoclimatology (1)
-
paleoecology (1)
-
paleogeography (1)
-
paleomagnetism (4)
-
paleontology (1)
-
Paleozoic
-
Permian
-
Ecca Group (2)
-
Lower Permian (1)
-
Upper Permian
-
Permian-Triassic boundary (3)
-
-
Vryheid Formation (3)
-
-
upper Paleozoic
-
Dwyka Formation (2)
-
-
-
palynomorphs
-
acritarchs (1)
-
-
paragenesis (1)
-
petrology (5)
-
Phanerozoic (2)
-
phase equilibria (2)
-
phosphorus (1)
-
placers (1)
-
Plantae
-
Spermatophyta
-
Gymnospermae
-
Glossopteridales
-
Glossopteris (1)
-
-
-
-
-
plate tectonics (3)
-
Precambrian
-
Archean
-
Mesoarchean (3)
-
Neoarchean (10)
-
-
Central Rand Group (2)
-
Hadean (1)
-
upper Precambrian
-
Proterozoic
-
Malmani Subgroup (1)
-
Mesoproterozoic (2)
-
Neoproterozoic (1)
-
Paleoproterozoic (14)
-
Pretoria Group (1)
-
-
-
Ventersdorp Supergroup (3)
-
Witwatersrand Supergroup (10)
-
-
remote sensing (1)
-
reservoirs (1)
-
sea-level changes (2)
-
sedimentary petrology (2)
-
sedimentary rocks
-
carbonate rocks
-
dolostone (1)
-
-
chemically precipitated rocks
-
chert (1)
-
-
clastic rocks
-
arenite
-
sublitharenite (1)
-
-
argillite (2)
-
conglomerate
-
quartz-pebble conglomerate (1)
-
-
mudstone (3)
-
red beds (1)
-
sandstone (3)
-
subarkose (1)
-
tillite (1)
-
-
-
sedimentary structures
-
biogenic structures
-
bioturbation (1)
-
-
planar bedding structures
-
cross-bedding (1)
-
cross-stratification (1)
-
-
soft sediment deformation (1)
-
-
sedimentation (8)
-
sediments
-
clastic sediments
-
alluvium (1)
-
boulders (1)
-
gravel (1)
-
pebbles (1)
-
sand (2)
-
-
-
seismology (1)
-
selenium (1)
-
soils (1)
-
South America
-
Brazil
-
Goias Brazil (1)
-
-
-
stratigraphy (5)
-
structural analysis (9)
-
structural geology (2)
-
tectonics (11)
-
tunnels (1)
-
weathering (1)
-
-
rock formations
-
Beaufort Group (3)
-
Elliot Formation (4)
-
Karoo Supergroup (5)
-
-
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Free State South Africa
Uranium–lead geochronology applied to pyrope garnet with very low concentrations of uranium
Diamonds reveal subducted slab harzburgite in the lower mantle
A positive syn-fold test from the Neoarchaean Klipriviersberg Group of South Africa: Quo vadis Vaalbara?
Amphibolite facies metamorphism in lower Witwatersrand Supergroup rocks exposed in the Vredefort Dome – a Ventersdorp LIP connection
Protogenetic clinopyroxene inclusions in diamond and Nd diffusion modeling—Implications for diamond dating
ABSTRACT The Vredefort granophyre dikes have long been recognized as being derived from the now-eroded Vredefort melt sheet. One dike, in particular, the Daskop granophyre dike, is notable for a high abundance of lithic clasts derived from various stratigraphic levels. In this study, we mapped the distribution of the clasts throughout the continuously exposed section of the dike using field mapping and aerial drone photography and attempted to constrain the emplacement mechanisms of the dike. We found that the clasts are not homogeneously spread but instead are distributed between clast-rich zones, which have up to 50% by area clasts, and clast-poor zones, which have 0–10% by area clasts. We examined three models to explain this distribution: gravitational settling of clasts, thermally driven local assimilation of clasts, and mechanical sorting of clasts due to turbulent flow. Of the three models, the gravitational settling cannot be supported based on our field and geophysical data. The assimilation of clasts and turbulent flow of clasts, however, can both potentially result in inhomogeneous clast distribution. Zones of fully assimilated clasts and nonassimilated clasts can occur from spatial temperature differences of 100 °C. Mechanical sorting driven by a turbulent flow can also generate zones of inhomogeneous clast distribution. Both local assimilation and mechanical sorting due to turbulent flow likely contributed to the observed distribution of clasts.
ABSTRACT This contribution is concerned with the debated origin of the impact melt rock in the central uplift of the world’s largest confirmed impact structure—Vredefort (South Africa). New major- and trace-element abundances, including those of selected highly siderophile elements (HSEs), Re-Os isotope data, as well as the first Se isotope and Se-Te elemental systematics are presented for the felsic and mafic varieties of Vredefort impact melt rock known as “Vredefort Granophyre.” In addition to the long-recognized “normal” (i.e., felsic, >66 wt% SiO 2 ) granophyre variety, a more mafic (<66 wt% SiO 2 ) impact melt variety from Vredefort has been discussed for several years. The hypothesis that the mafic granophyre was formed from felsic granophyre through admixture (assimilation) of a mafic country rock component that then was melted and assimilated into the superheated impact melt has been pursued here by analysis of the two granophyre varieties, of the Dominion Group lava (actually meta-lava), and of epidiorite mafic country rock types. Chemical compositions, including high-precision isotope dilution–derived concentrations of selected highly siderophile elements (Re, Os, Ir, Pt, Se, Te), and Re-Os and Se isotope data support this hypothesis. A first-order estimate, based on these data, suggests that some mafic granophyre may have resulted from a significant admixture (assimilation) of epidiorite to felsic granophyre. This is in accordance with the findings of an earlier investigation using conventional isotope (Sr-Nd-Pb) data. Moreover, these outcomes are in contrast to a two-stage emplacement model for Vredefort Granophyre, whereby a mafic phase of impact melt, derived by differentiation of a crater-filling impact melt sheet, would have been emplaced into earlier-deposited felsic granophyre. Instead, all chemical and isotopic evidence so far favors formation of mafic granophyre by local assimilation of mafic country rock—most likely epidiorite—by a single intrusive impact melt phase, which is represented by the regionally homogeneous felsic granophyre.
ABSTRACT Finite deformation patterns of accessory phases can indicate the tectonic regime and deformation history of the host rocks and geological units. In this study, tectonically deformed, seismically deformed, and shocked zircon grains from a granite sample from the core of the Vredefort impact structure were analyzed in situ, using a combination of Raman spectroscopy, backscatter electron (BSE) imaging, electron backscattered diffraction (EBSD) mapping, electron probe microanalyses (EPMA), energy-dispersive X-ray spectroscopy (EDS) qualitative chemical mapping, and cathodoluminescence (CL) imaging. We aimed to reveal the effects of marginal grain-size reduction, planar deformation bands (PDBs), and shock microtwins on the crystal structure and microchemistry of zircon. Deformation patterns such as PDBs, microtwins, and subgrains did not show any significant effect on zircon crystallinity/metamictization degree or on the CL signature. However, the ratio of Raman band intensities B 1g (1008 cm –1 ) to E g (356 cm –1 ) slightly decreased within domains with low misorientation. The ratio values were affected in shocked grains, particularly in twinned domains with high misorientation. B 1g /E g ratio mapping combined with metamictization degree mapping (full width at half maximum of B 1g peak) suggest the presence of shock deformation features in zircon; however, due to the lower spatial resolution of the method, they must be used in combination with the EBSD technique. Additionally, we discovered anatase, quartz, goethite, calcite, and hematite micro-inclusions in the studied zircon grains, with quartz and anatase specifically being associated with strongly deformed domains of shocked zircon crystals.
Extreme plastic deformation and subsequent Pb loss in shocked xenotime from the Vredefort Dome, South Africa
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.
ABSTRACT Structural analysis of overturned metasedimentary strata of the lower Witwatersrand Supergroup in the inner collar of the Vredefort Dome reveals the presence of tangential folds and faults associated with the 2.02 Ga impact. The folds are distinct from previously identified subradially oriented, vertical to plunging-inclined, gentle folds that are interpreted as the products of convergent flow (constriction) during the initial stages of central uplift formation. The tangential folds comprise disharmonic, open, asymmetric, horizontal to plunging-inclined anticline-syncline pairs with centripetally dipping axial planes and right-way-up intermediate limbs. They display centripetal-down vergence (anticline radially outward of the syncline) that is consistent with steep inward-directed shear of the overturned strata. We attribute this kinematic pattern to subvertical collapse of the Vredefort central uplift during the latter stages of crater modification. The folds are cut by pseudotachylite-bearing steep to vertical tangential faults that display center-down slip of <10 m up to ~150 m. Both the tangential folds and the faults suggest that the large-scale overturning of strata related to outward collapse of the Vredefort central uplift was accompanied by a component of inward-directed collapse via layer-parallel shearing and folding, followed by faulting. Subradially oriented faults with conjugate strike separations of 1–2 km in the NNE collar of the dome suggest penecontemporaneous tangential extension of the inner collar rocks. This evidence indicates that second-order structures in the metasedimentary collar of the Vredefort Dome preserve a complex, multistage record of evolving strain associated with both initial convergent and upward flow (constriction) related to central uplift rise and later divergent and downward flow (flattening) linked to its collapse, and that centripetally directed collapse features may be important components of the structural inventory of very large central uplifts.