Shallow depth extent of the Lesutoskraal granophyre dike in the Vredefort impact structure: Geophysical surveys and trenching data

Recent observations and geophysical studies at the Vredefort impact structure have indicated that the impact melt dikes in the central uplift of the structure have small depth extents. In this study, we performed magnetic and electrical resistivity tomography (ERT) surveys of the Lesutoskraal granophyre dike (LGD) and trenched to confirm its depth extent. The ERT survey showed that outcrops of the LGD are associated with shallow resistive zones with <3 m depth extent, but such zones do not occur where outcrops are absent. Visual observations in the trench confirmed that the dike has a small depth extent (~0.75 m) at this location. However, the magnetic survey revealed anomalies along the entire strike of the dike, even where no outcrops occur. We suggest that remagnetization of the host rock within a metamorphic contact aureole could explain the presence of magnetic anomalies in the absence of outcrops. Considering the results of the ERT survey, the observations made in the trench, and the surface distribution of outcrops of the LGD, we confirm that this dike has a small depth extent (<3 m) along its entire length and propose that outcrops represent the intersection of the dike terminus with the current erosional surface.     

Emplacement history of Granophyre dikes in the Vredefort Impact Structure,South Africa,inferred from geochemical evidence

The central Vredefort Impact Structure is characterised by impact melt rocks, known as the Vredefort Granophyre dikes, the mode of emplacement of which is not well known. Whole-rock and petrographic analyses of two dikes were conducted and compared to published geochemical data to elucidate the mode and timing of dike formation. The dikes are characterised by compositional and textural heterogeneity between, and within, individual dikes. Specifically, central dike portions are felsic and rich in wall rock fragments, whereas marginal dike phases are more mafic and fragment-poor. Collectively, this suggests that melt was derived from compositionally different parental melts and emplaced in at least two pulses. In addition, the chemical heterogeneity between fragment-rich and fragment-poor dike zones can be explained by variable assimilation of a mafic component, notably Ventersdorp basalt, at the base of the impact melt sheet, from which melt of the Granophyre dikes is derived. This scenario accounts for the mafic and fragment-poor character of melt emplaced first in the dikes and the more felsic and fragment-rich nature of melts of the following emplacement pulse, i.e., when the impact melt was less hot and thus less capable of digesting large quantities of (mafic) wall rock fragments. Differences in geometrical, textural, chemical and fragment characteristics between the Granophyre dikes and pseudotachylite bodies can be explained by the same process, i.e., impact melt drainage, but operating at different times after impact.     

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

Genesis and emplacement of Vredefort Granophyre, the impact melt rock exposed on the Vredefort Dome, the erosional remnant of the central uplift of the Vredefort impact structure, South Africa, have long been debated. This debate was recently reinvigorated by the discovery that besides the previously known felsic variety of >66 wt% SiO₂, a second, somewhat more mafic phase of <66 wt% SiO₂ occurs along a Granophyre dike on farms Kopjeskraal and Eldorado in the northwest sector of the dome. Two hypotheses have been put forward to explain the genesis and emplacement of this second phase: (1) successive injections of impact melt into extensional fractures opened in the course of central uplift formation/crater modification, with melts of distinct compositions derived from a differentiating impact melt body in the crater, and (2) generation of the more mafic phase as a product of admixture/assimilation of a mafic country rock component, either the so-called epidiorite of possible Ventersdorp Supergroup affiliation or the Dominion Group meta-lava (DGL), to Felsic Granophyre. In the latter model, contamination with mafic country rock would have occurred during downward intrusion and stoping into and below the crater floor. The so-called Mafic Granophyre has previously only ever been sampled on a single site (Farm Kopjeskraal). In this study, samples of Granophyre occurring along the southerly extension of this dike on farm Rensburgdrif, and from a second dike on the Rietkuil property further southwest were investigated by field work, and petrographic, geochemical, and isotopic analysis. The mafic phase indeed occurs in the interior of the dike at Rensburgdrif, and also on Rietkuil. New geochemical and Sr-Nd isotope data support the hypothesis that the Mafic Granophyre composition represents a mixture between Felsic Granophyre and a mafic country rock. A 20% admixture of epidiorite or DGL to Felsic Granophyre provides an excellent match for the chemical composition of the Mafic Granophyre. The Sr-Nd isotope data indicate that this admixture likely involved the epidiorite component rather than DGL. Together with earlier Sr-Nd-Os-Se isotopic data, and other geochemical data, these results further support formation of the Mafic Granophyre by local assimilation/admixture of epidiorite to Felsic Granophyre.     

Planar deformation features and impact glass in inclusions from the Vredefort Granophyre,South Africa

Abstract— The Vredefort Granophyre represents impact melt that was injected downward into fractures in the floor of the Vredefort impact structure, South Africa. This unit contains inclusions of country rock that were derived from different locations within the impact structure and are predominantly composed of quartzite, feldspathic quartzite, arkose, and granitic material with minor proportions of shale and epidiorite. Two of the least recrystallized inclusions contain quartz with single or multiple sets of planar deformation features. Quartz grains in other inclusions display a vermicular texture, which is reminiscent of checkerboard feldspar. Feldspars range from large, twinned crystals in some inclusions to fine‐grained aggregates that apparently are the product of decomposition of larger primary crystals. In rare inclusions, a mafic mineral, probably biotite or amphibole, has been transformed to very fine‐grained aggregates of secondary phases that include small euhedral crystals of Fe‐rich spinel. These data indicate that inclusions within the Vredefort Granophyre were exposed to shock pressures ranging from <5 to 8–30 GPa. Many of these inclusions contain small, rounded melt pockets composed of a groundmass of devitrified or metamorphosed glass containing microlites of a variety of minerals, including K‐feldspar, quartz, augite, low‐Ca pyroxene, and magnetite. The composition of this devitrified glass varies from inclusion to inclusion, but is generally consistent with a mixture of quartz and feldspar with minor proportions of mafic minerals. In the case of granitoid inclusions, melt pockets commonly occur at the boundaries between feldspar and quartz grains. In metasedimentary inclusions, some of these melt pockets contain remnants of partially melted feldspar grains. These melt pockets may have formed by eutectic melting caused by inclusion of these fragments in the hot (650 to 1610 °C) impact melt that crystallized to form the Vredefort Granophyre.     

A plume tectonics model for the Tharsis province, Mars

Morphological and structural data from the whole Tharsis province suggest that a number of shallow grabens radially oriented about the Tharsis bulge on Mars are underlain by dykes, which define giant radiating swarms similar to, e.g. the Mackenzie dyke swarm of the Canadian shield. Mechanisms for graben formation are proposed, and the depth, width, and height of the associated dykes are estimated. Structural mapping leads to define successive stages of dyke emplacement, and provide stress-trajectory maps that indicate a steady source of the regional stress during the whole history of the Tharsis province. A new tectonic model of Tharsis is presented, based on an analogy with dyke swarms on the Earth that form inside hot spots. This model successfully matches the following features: (1) the geometry of the South Tharsis Ridge Belt, which may have been a consequence of the compressional stress field at the boundary between the uplifted and non-uplifted areas in the upper part of the lithosphere at the onset of hot spot activity; (2) extensive lava flooding, interpreted as a consequence of the high thermal anomaly at the onset of plume (hot spot) activity; (3) wrinkle ridge geometry in the Tharsis hemisphere, the formation of which is interpreted as a consequence of buoyant subsidence of the brittle crust in response to the lava load; (4) Valles Marineris limited stretching by preliminary passive rifting, and uplift, viewed as a necessary consequence of adiabatic mantle decompression induced by stretching. The geometrical analysis of dyke swarms suggests the existence of a large, Tharsis-independent extensional state of stress during all the period of tectonic activity, in which the minimum compressive stress is roughly N---S oriented. Although magmatism must have loaded the lithosphere significantly after the plume activity ceased and be responsible for additional surface deformations, there is no requirement for further loading stress to explain surficial features. Comparison with succession of magmatic and tectonic events related to hot spots on the Earth suggests that the total time required to produce all the surface deformation observed in the Tharsis province over the last 3.8 Ga does probably not exceed 10 or 15 Ma.     

Geophysical research on the Kärdla impact structure,Hiiumaa Island,Estonia

Abstract— The 4 km wide and 500 m deep circular Kärdla impact structure in Hiiumaa Island, Estonia, of middle Ordovician age (～455 Ma), is buried under Upper Ordovician and Quaternary sediments. To constrain the geophysical models of the structure, petrophysical properties such as magnetic susceptibility, natural remanent magnetization (NRM), density, electrical conductivity, porosity and P-wave velocity were measured on samples of crystalline and sedimentary rocks collected from drill cores in different parts of the structure and the surrounding area. The results were used to interpret the central gravity anomaly of ?3 mGal and the magnetic anomaly of ?100 nT and also the surrounding weak positive anomalies revealed by high precision survey data. The unshocked granitic rocks outside the structure have a mean density of ～2630 kgm^?3. Their shocked counterparts have densities of ～2400 kgm^?3 at a depth of ～500 m, increasing up to 2550 kgm^?3 at a depth of 850 m. Porosity and electrical conductivity decrease, but P-wave velocity increases as density increases away from the impact point. Thus, the gradual changes in the physical properties of the rocks as a function of radial distance from the crater centre are consistent with an impact origin for Kärdla. As in many other impact structures, the magnetization of the shocked rocks are also clearly lower than those of unshocked target rocks. A new geophysical and geological model of the Kärdla structure is presented based on geophysical field measurements and data on gradual changes in petrophysical parameters of the shocked target and overlying rocks, together with structural data from numerous boreholes. An important feature of this model is the lack of an observable geophysical signature of the central uplift observed in drillcores.     

Formation of pseudotachylitic breccias in the central uplifts of very large impact structures: Scaling the melt formation

Abstract– The processes leading to formation of sometimes massive occurrences of pseudotachylitic breccia (PTB) in impact structures have been strongly debated for decades. Variably an origin of these pseudotachylite (friction melt)‐like breccias by (1) shearing (friction melting); (2) so‐called shock compression melting (with or without a shear component) immediately after shock propagation through the target; (3) decompression melting related to rapid uplift of crustal material due to central uplift formation; (4) combinations of these processes; or (5) intrusion of allochthonous impact melt from a coherent melt body has been advocated. Our investigations of these enigmatic breccias involve detailed multidisciplinary analysis of millimeter‐ to meter‐sized occurrences from the type location, the Vredefort Dome. This complex Archean to early Proterozoic terrane constitutes the central uplift of the originally >250 km diameter Vredefort impact structure in South Africa. Previously, results of microstructural and microchemical investigations have indicated that formation of very small veinlets involved local melting, likely during the early shock compression phase. However, for larger veins and networks it was so far not possible to isolate a specific melt‐forming mechanism. Macroscopic to microscopic evidence for friction melting is very limited, and so far chemical results have not directly supported PTB generation by intrusion of impact melt. On the other hand, evidence for filling of dilational sites with melt is abundant. Herein, we present a new approach to the mysterium of PTB formation based on volumetric melt breccia calculations. The foundation for this is the detailed analysis of a 1.5 × 3 × 0.04 m polished granite slab from a dimension‐stone quarry in the core of the Vredefort Dome. This slab contains a 37.5 dm³ breccia zone. The pure melt volume in 0.1 m³ PTB‐bearing granitic target rock outside of the several‐decimeter‐wide breccia zone in the granite slab was estimated at 5.2 dm³. This amount can be divided into 4.6 dm³ melt (88%), for which we have evidenced a limited material transport (at maximum, ≈20 cm) and 0.6 dm³ melt (12%) with, at most, grain‐scale material transport, which we consider in situ formed shock melt. The breccia zone itself contains about 10 dm³ of matrix (melt). Assuming melt exchange over 20 cm at the slab surface, between breccia zone and surrounding melt‐bearing host rock volume, the outer melt volume is calculated to contain the same amount of melt as contained by the massive breccia zone. Meso‐ and microscopic observations indicate melt transport is more prominent from larger into smaller melt occurrences. Thus, melt of the breccia zone could have provided the melt fill for all the small‐scale PTB veins in the surrounding target rock. Extrapolating this melt capacity calculation for 1 m³ PTB‐bearing host rock shows that a host rock volume of this dimension is able to take up some 52 dm³ melt. Scaling up 1000‐fold to the outcrop scale reveals that exchange between a host rock volume of 2 m radius around a 37 m³ breccia zone could involve some 10 m³ melt. These results demonstrate that large melt volumes (i.e., large breccia zones) can be derived, in principle, from local reservoirs. However, strong decompression would have to apply in order to exchange these considerable melt volumes, which would only be realistic during the decompression phase of impact cratering upon central uplift formation, or locally where compressive regimes acted during the subsequent down‐ and outward collapse of the central uplift.     

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

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

New observations on shatter cones in the Vredefort impact structure,South Africa,and evaluation of current hypotheses for shatter cone formation

Abstract— Shatter cones have been described from many meteorite impact structures and are widely regarded as a diagnostic macroscopic recognition feature for impact. However, the origin of this meso‐ to macroscopic striated fracture phenomenon has not yet been satisfactorily resolved, and the timing of shatter cone formation in the cratering process still remains enigmatic. Here, previous results from studies of shatter cones from the Vredefort impact structure and other impact structures are discussed in the light of new field observations made in the Vredefort Dome. Contrary to earlier claims, Vredefort cone fractures do not show uniform apex orientations at any given outcrop, nor do small cones show a pattern consistent with the previously postulated “master cone” concept. Simple back‐rotation of impact‐rotated strata to a horizontal pre‐impact position also does not lead to a uniform centripetal‐upward orientation of the cone apices. Striation patterns on the cone surfaces are variable, ranging from the typically diverging pattern branching off the cone apex to subparallel‐to‐parallel patterns on almost flat surfaces. Striation angles on shatter cones do not increase with distance from the center of the dome, as alleged in the literature. Instead, a range of striation angles is measured on individual shatter cones from a specific outcrop. New observations on small‐scale structures in the collar around the Vredefort Dome confirm the relationship of shatter cones with subparallel sets of curviplanar fractures (so‐called multipli‐striated joint sets, MSJS). Pervasive, meter‐scale tensile fractures cross‐cut shatter cones and appear to have formed after the closely spaced MSJ‐type fractures. The results of this study indicate that none of the existing hypotheses for the formation of shatter cones are currently able to adequately explain all characteristics of this fracturing phenomenon. Therefore, we favor a combination of aspects of different hypotheses that includes the interaction of elastic waves, as supported by numerical modeling results and which reasonably explains the variety of shatter cone shapes, the range of striation geometries and angles, and the relationship of closely spaced fracture systems with the striated surfaces. In the light of the currently available theoretical basis for the formation of shatter cones, the results of this investigation lead to the conclusion that shatter cones are tensile fractures and might have formed during shock unloading, after the passage of the shock wave through the target rocks.     

Hydrothermally enhanced magnetization at the center of the Haughton impact structure?

Haughton is a ~24 Myr old midsize (apparent diameter 23 km) complex impact structure located on Devon Island in Nunavut, Canada. The center of the structure shows a negative gravity anomaly of ?12 mGal coupled to a localized positive magnetic field anomaly of ~900 nT. A field expedition in 2013 led to the acquisition of new ground magnetic field mapping and electrical resistivity data sets, as well as the first subsurface drill cores down to 13 m depth at the top of the magnetic field anomaly. Petrography, rock magnetic, and petrophysical measurements were performed on the cores and revealed two different types of clast‐rich polymict impactites: (1) a white hydrothermally altered impact melt rock, not previously observed at Haughton, and (2) a gray impact melt rock with no macroscopic sign of alteration. In the altered core, gypsum is present in macroscopic veins and in the form of intergranular selenite associated with colored and zoned carbonate clasts. This altered core has a natural remanent magnetization (NRM) four to five times higher than materials from the other core but the same magnetic susceptibility. Their magnetization is still higher than the surrounding crater‐fill impact melt rocks. X‐ray fluorescence data indicate a similar proportion of iron‐rich phases in both cores and an enrichment in silicates within the altered core. In addition, alternating‐field demagnetization results show that one main process remagnetized the rocks. These results support the hypothesis that intense and possibly localized post‐impact hydrothermal alteration enhanced the magnetization of the clast‐rich impact melt rocks by crystallization of magnetite within the center of the Haughton impact structure. Subsequent erosion was followed by in situ concentration in the subsurface leading to large magnetic gradient on surface.     

Numerical Modeling of the Largest Terrestrial Meteorite Craters

Multi-ring impact basins have been found on the surfaces of almost all planetary bodies in the Solar system with solid crusts. The details of their formation mechanism are still unclear. We present results of our numerical modeling of the formation of the largest known terrestrial impact craters. The geological and geophysical data on these structures accumulated over many decades are used to place constraints on the parameters of available numerical models with a dual purpose: (i) to choose parameters in available mechanical models for the crustal response of planetary bodies to a large impact and (ii) to use numerical modeling to refine the possible range of original diameters and the morphology of partially eroded terrestrial craters. We present numerical modeling results for the Vredefort, Sudbury, Chicxulub, and Popigai impact craters and compare these results with available geological and geophysical information.     

Differentiation and emplacement of the Worthington Offset Dike of the Sudbury impact structure,Ontario

Abstract— The Offset Dikes of the 1.85 Ga Sudbury Igneous Complex (SIC) constitute a key topic in understanding the chemical evolution of the impact melt, its mineralization, and the interplay between melt migration and impact‐induced deformation. The origin of the melt rocks in Offset Dikes as well as mode and timing of their emplacement are still a matter of debate. Like many other offset dikes, the Worthington is composed of an early emplaced texturally rather homogeneous quartz‐diorite (QD) phase at the dike margin, and an inclusion‐ and sulfide‐rich quartz‐diorite (IQD) phase emplaced later and mostly in the centre of the dike. The chemical heterogeneity within and between QD and IQD is mainly attributed to variable assimilation of host rocks at the base of the SIC, prior to emplacement of the melt into the dike. Petrological data suggest that the parental magma of the Worthington Dike mainly developed during the pre‐liquidus temperature interval of the thermal evolution of the impact melt sheet (>1200 °C). Based on thermal models of the cooling history of the SIC, the two‐stage emplacement of the Worthington Dike occurred likely thousands to about ten thousand years after impact. Structural analysis indicates that an alignment of minerals and host rock fragments within the Worthington Dike was caused by ductile deformation under greenschist‐facies metamorphic conditions rather than flow during melt emplacement. It is concluded that the Worthington Offset Dike resulted from crater floor fracturing, possibly driven by late‐stage isostatic readjustment of crust underlying the impact structure.     

The South Range Breccia Belt of the Sudbury Impact Structure: A possible terrace collapse feature

Abstract— The South Range Breccia Belt (SRBB) is an arcuate, 45 km long zone of Sudbury Breccia in the South Range of the 1.85 Ga Sudbury Impact Structure. The belt varies in thickness between tens of meters to hundreds of meters and is composed of a polymict assemblage of Huronian Supergroup (2.49–2.20 Ga), Nipissing Diabase (2.2 Ga), and Proterozoic granitoid breccia fragments ranging in size from a few millimeters to tens of meters. The SRBB matrix is composed of a fine‐grained (～100 μm) assemblage of biotite, quartz, and ilmenite, with trace amounts of plagioclase, zircon, titanite, epidote, pyrite, chalcopyrite, pyrrhotite, and occasionally chlorite. The SRBB hosts the Frood‐Stobie, Vermilion, and Kirkwood quartz diorite offset dykes, the former being associated with one of the largest Ni‐Cu‐PGE sulphide deposits in the world. Optical petrography and whole‐rock geochemistry concur with previous studies that have suggested that the matrix of the SRBB is derived from comminution and at least partial frictional melting of the wall rock Huronian Supergroup lithologies. Rare earth element (REE) data from all sampled lithologies associated with the SRBB exhibit crustal signatures when normalized to C1 chondrite values. Additionally, REE data from the quartz diorites, disseminated sulphides in Sudbury Breccia, and a sample of an aphanitic biotite‐hornblende tonalite dyke exhibit flat slopes when compared to the mafic and felsic norites, quartz gabbro, and granophyre units of the Sudbury Igneous Complex (SIC), which suggests that these lithologies are representative of bulk SIC melt. We suggest that the SRBB was formed by high strain‐rate (>1 m/s), gravity‐driven seismogenic slip of the inner ring of the Sudbury Impact Structure during postimpact crustal readjustment (crater modification stage). Failure of the hanging wall may have facilitated the injection of bulk SIC melt into the SRBB, along with the Ni‐Cu‐PGE sulphides of the Frood‐Stobie deposit. Postimpact Penokean (1.9–1.7 Ga) tectonism, particularly northwest‐directed shearing along the South Range Shear Zone and associated thrust faulting, could account for the present subvertical orientation of the SRBB, and the apparent lack of a connection at depth with the SIC.     

Numerical modeling of impact heating and cooling of the Vredefort impact structure

Abstract— Large meteorite impacts, such as the one that created the Vredefort structure in South Africa?2 Ga ago, result in significant heating of the target. The temperatures achieved in these events have important implications for post‐impact metamorphism as well as for the development of hydrothermal systems. To investigate the post‐impact thermal evolution and the size of the Vredefort structure, we have analyzed impact‐induced shock heating in numerical simulations of terrestrial impacts by projectiles of a range of sizes thought to be appropriate for creating the Vredefort structure. When compared with the extent of estimated thermal shock metamorphism observed at different locations around Vredefort, our model results support our earlier estimates that the original crater was 120–160 km in diameter, based on comparison of predicted to observed locations of shock features. The simulations demonstrate that only limited shock heating of the target occurs outside the final crater and that the cooling time was at least 0.3 Myr but no more than 30 Myr.     

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

Abstract— Landsat TM, aerial photograph image analysis, and field mapping of Witwatersrand supergroup meta‐sedimentary strata in the collar of the Vredefort Dome reveals a highly heterogeneous internal structure involving folds, faults, fractures, and melt breccias that are interpreted as the product of shock deformation and central uplift formation during the 2.02 Ga Vredefort impact event. Broadly radially oriented symmetric and asymmetric folds with wavelengths ranging from tens of meters to kilometers and conjugate radial to oblique faults with strike‐slip displacements of, typically, tens to hundreds of meters accommodated tangential shortening of the collar of the dome that decreased from ?17% at a radius from the dome center of 21 km to <5% at a radius of 29 km. Ubiquitous shear fractures containing pseudotachylitic breccia, particularly in the metapelitic units, display local slip senses consistent with either tangential shortening or tangential extension; however, it is uncertain whether they formed at the same time as the larger faults or earlier, during the shock pulse. In addition to shatter cones, quartzite units show two fracture types—a cmspaced rhomboidal to orthogonal type that may be the product of shock‐induced deformation and later joints accomplishing tangential and radial extension. The occurrence of pseudotachylitic breccia within some of these later joints, and the presence of radial and tangential dikes of impact melt rock, confirm the impact timing of these features and are suggestive of late‐stage collapse of the central uplift.     

Origin and emplacement of Offset Dykes in the Sudbury impact structure: Constraints from Hess

Abstract— The Hess Offset is a steeply dipping dyke located 12–15 km north of the 1.85 Ga Sudbury igneous complex (SIC) within the 200–250 km diameter Sudbury impact structure. It is up to 60 m wide and strikes subconcentrically to the SIC for at least 23 km. The main phase of the dyke is granodioritic, but it conforms with what is locally referred to as Quartz Diorite: a term used for all the Offset Dykes of the Sudbury impact structure. Rare earth element data shows that the Hess Offset is genetically related to the SIC. Hess is most closely affiliated with an evolved Felsic Norite component of SIC and not bulk impact melt. This indicates that Hess was emplaced during fractionation of the impact melt sheet, rather than immediately following impact. The main Quartz Diorite phase of the dyke comprises a quartz + plagioclase + hornblende + biotite ± clinopyroxene ± orthopyroxene assemblage. Critically, the Hess Offset occupies a concentric fault system that marks the northern limit of a pseudotachylyte-rich, shatter cone-bearing annulus about the SIC. This fault system was active during the modification stage of the impact process.     

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

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

Multiphase emplacement of impact melt sheet into the footwall: Offset dykes of the Sudbury Igneous Complex,Canada

The offset dykes of the Sudbury Igneous Complex comprise two distinct main magmatic facies, a high-temperature inclusion-free quartz diorite (QD), and a subsequently intruded lower temperature, mineralized, and inclusion-rich quartz diorite (MIQD). The MIQD facies was emplaced after QD dykes had solidified. Key controlling factors of the two injection phases were (1) the development of a coherent roof, which confined the melt sheet; and (2) the periodic increase of melt and fluid pressure within the melt sheet. For the injection of QD melt, the melt pressure exceeded the normal stress acting on fracture surfaces. For the later refracturing of QD dykes and the injection of MIQD melt, the melt pressure increased further, exceeding the tensile strength of, and the normal stress acting on, QD dykes. We associate the melt pressure increase required for both injection episodes with degassing and devolatilization of cooling melt close to the roof. Within the hydraulically connected melt column, the related pressure increase was transmitted to the base of the melt sheet where QD and MIQD melt was extracted into dykes. Residual core to rim thermal gradients in the QD dykes produced tensile strength gradients, accounting for the typically central location of MIQD dykes within QD dykes.     

The Foelsche structure,Northern Territory,Australia: An impact crater of probable Neoproterozoic age

Abstract— The Foelsche structure is situated in the McArthur Basin of northern Australia (16°40′ S, 136°47′ E). It comprises a roughly circular outcrop of flat‐lying Neoproterozoic Bukalara Sandstone, overlying and partly rimmed by tangentially striking, discontinuous outcrops of dipping, fractured and brecciated Mesoproterozoic Limmen Sandstone. The outcrop expression coincides with a prominent circular aeromagnetic anomaly, which can be explained in terms of the local disruption and removal or displacement of a regional mafic igneous layer within a circular area at depth. Samples of red, lithic, pebbly sandstone from the stratigraphically lowest exposed levels of the Bukalara Sandstone within the Foelsche structure contain detrital quartz grains displaying mosaicism, planar fractures (PFs) and planar deformation features (PDFs). PFs and PDFs occur in multiple intersecting sets with orientations consistent with a shock metamorphic origin. The abundance and angular nature of the shocked grains indicates a nearby provenance. Surface expression and geophysical data are consistent with a partly buried complex impact crater of ?6 km in diameter with an obscured central uplift ?2 km in diameter. The deformed outcrops of Limmen Sandstone are interpreted as relics of the original crater rim, but the central region of the crater, from which the shocked grains were likely derived, remains buried. From the best available age constraints the Foelsche structure is most likely of Neoproterozoic age.