Previous models of the effects of larger scale impacts on early (~ 4.6 - 3.8 Ga) terrestrial crus... more Previous models of the effects of larger scale impacts on early (~ 4.6 - 3.8 Ga) terrestrial crustal evolution have relied on analogies with the moon. There are, however, some important differences. Due to the Earth's larger gravitational cross-section, it will have been subjected to more impacts of unit mass and these impacts will have a higher relative energy density than the moon. Most importantly, the relatively higher planetary gravity of the Earth will have resulted in more severe effects due to the differential scaling of crater dimensions relative to the volume of the target recording shock metamorphic effects with event size. As a consequence, impact melt volumes will exceed the volumes of target material excavated and displace by the cratering flow-field at transient cavity diameters > 400 km on Earth, compared to > 3000 km on the moon. At these sizes, the terrestrial equivalents of lunar impact basins will have formed in strengthless melt pools and would not hav...
The SIC is a layered body within the Sudbury Structure, Canada. A detailed mineralogical and petr... more The SIC is a layered body within the Sudbury Structure, Canada. A detailed mineralogical and petrological study reveals that the original SIC melt crystallized in a water-rich environment. Its unusual petrology is evidence of its derivation from an impact melt.
Hypervelocity impact involves the near instantaneous transfer of considerable energy from the imp... more Hypervelocity impact involves the near instantaneous transfer of considerable energy from the impactor to a spatially limited near-surface volume of the target body. Local geology of the target area tends to be of secondary importance, and the net result is that impacts of similar size on a given planetary body produce similar results. This is the essence of the utility of observations at impact craters, particularly terrestrial craters, in constraining impact processes. Unfortunately, there are few well-documented results from systematic contemporaneous campaigns to characterize specific terrestrial impact structures with the full spectrum of geoscientific tools available at the time. Nevertheless, observations of the terrestrial impact record have contributed substantially to fundamental properties of impact. There is a beginning of convergence and mutual testing of observations at terrestrial impact structures and the results of modeling, in particular from recent hydrocode models. The terrestrial impact record provides few constraints on models of ejecta processes beyond a confirmation of the involvement of the local substrate in ejecta lithologies and shows that Z-models are, at best, first order approximations. Observational evidence to date suggests that the formation of interior rings is an extension of the structural uplift process that occurs at smaller complex impact structures. There are, however, major observational gaps and cases, e.g., Vredefort, where current observations and hydrocode models are apparently inconsistent. It is, perhaps, time that the impact community as a whole considers documenting the existing observational and modeling knowledge gaps that are required to be filled to make the intellectual breakthroughs equivalent to those of the 1970s and 1980s, which were fueled by observations at terrestrial impact structures. Filling these knowledge gaps would likely be centered on the later stages of formation of complex and ring structures and on ejecta.
Three “new” buried impact structures have been identified in the Williston Basin in Canada. The E... more Three “new” buried impact structures have been identified in the Williston Basin in Canada. The Elbow structure (106°45'W; 50°58'N) is 8 km in diameter, circular in plan, with a ring depression and an uplifted core of fractured and brecciated target rocks [1]. The structural form is that of a complex impact structure. It has been drilled for hydrocarbons although there is no current production. Originally, the Elbow structure was thought to results from salt tectonics [2] or cryptovolcanism [1]. Sampling of drill hole material indicates the presence of quartz grains with decorated planar deformation features (PDFs; Fig.1a). The Maple Creek structure (109°20'W; 49°51'N) is barely visible at the surface, where repeated sections of the Upper Cretaceous Bearpaw Formation are very poorly exposed. A structural anomaly was noticed by Whitaker [4] and subsequent Vibroseis seismic profiles indicated structural disturbance to ~ 1 km depth and a slightly elliptical zone ~ 6 km ...
The Vredefort Dome is located near the center of the Witwatersrand Basin, about 120 km southeast ... more The Vredefort Dome is located near the center of the Witwatersrand Basin, about 120 km southeast of Johannesburg, South Africa. Its origin is enigmatic, ranging from a major impact event to endogenous processes, either igneous or tectonic. A unique melt rock, the 'Bronzite' Granophyr, occurs in the Vredefort structure as vertical ring dikes along the contact between sedimentary collar and core of Archaean granites, and as vertical dikes extending northwest-southeast and northeast-southwest in the granitic core. The granophyre rocks have an unusual composition and high content of recrystallized sedimentary inclusions compared to common intrusive igneous rocks with similar SiO2 content (61 to 70 percent by weight). The unique nature of the granophyre has been underlined in previous studies and origin hypotheses as an impact melt or as a highly contaminated intrusive mafic magma have also been discussed. We present new results obtained from a recent detailed petrographic and ge...
The Vredefort structure is located approximately 120 km southwest of Johannesburg, South Africa, ... more The Vredefort structure is located approximately 120 km southwest of Johannesburg, South Africa, and is deeply eroded. Controversies remain on the origin of this structure with the most popular hypotheses being: (1) by impact cratering about 2.0 Ga; (2) as a cryptoexplosion structure about 2.0 Ga; and (3) by purely tectonic processes starting at about 3.0 Ga and ending with the Vredefort event at 2.0 Ga. In view of recent work in which the granophyre dikes are interpreted as the erosional remants of a more extensive impact melt sheet, injected downward into the underlying country rocks, the impact origin hypothesis for Vredefort is adopted. In order to estimate the original dimensions of the Vredefort impact structure, it is assumed that the structure was initially circular, that its predeformation center corresponds to the center of the granitic core, and that the pre-Vredefort geology of the area prior to approximately 2.0 Ga ago is as suggested by Fletcher and Reimold. The spatia...
The Vredefort impact structure contains a suite of granophyric dykes, referred to as the Vredefor... more The Vredefort impact structure contains a suite of granophyric dykes, referred to as the Vredefort Granophyre, occurring within and at the edge of the Archaean basement core. New whole-rock chemical analyses, together with previous data, represent a complete suite of the Granophyre occurrences. These data show that the Vredefort Granophyre has a remarkable chemical homogeneity, within and between dykes, on a regional scale, and a unique composition ( approximately 67 wt.% SiO 2 , approximately 1 wt.% TiO 2 , approximately 13 wt.% Al 2 O 3 , approximately 7 wt.% Fe 2 O 3 , approximately 3 wt.% MgO, approximately 4 wt.% CaO, approximately 3 wt.% Na 2 O. approximately 2 wt.% K 2 O). Five volumetrically abundant regional lithologies are: Transvaal carbonate, Ventersdorp lava. Witwatersrand quartzite, Witwatersrand shale, and Outer Granite Gneiss. These lithologies are used as components in both harmonic and least-squares mixing calculations to reproduce the Vredefort Granophyre composit...
Consisting mainly of uplifted crystalline basement, the 40-50 km Vredefort Dome is in a near-cent... more Consisting mainly of uplifted crystalline basement, the 40-50 km Vredefort Dome is in a near-central position within the Witwatersrand Basin [1,2](Fig. 1). The Vredefort impact structure (27 degrees 00'S, 27 degrees 30'E) consists of the Vredefort Dome and a wide "collar" of metasediments and metavolcanic rocks. Prior to the 1970's the diameter of the Vredefort structure was estimated at 60 km, and possibly up to 200 km [3,4]. In the 1970's and 1980's, the diameter was estimated at 100-120 km, while in the early 1990's 170-180 km was preferred. In relation to the outer edge of the Dome, the radial limit of shatter cones (26-37 km), PDFs (~13 km), and downfaulted Transvaal outliers (45-55 km), yield an estimate of 192-300 km for the final crater diameter 15](Fig. 2). These spatial indicators are subject to erosional effects. At the Sudbury Structure, a 45 km thick impact melt sheet has been preserved, under which PDFs have been annealed from the targ...
Previous models of the effects of larger scale impacts on early (~ 4.6 - 3.8 Ga) terrestrial crus... more Previous models of the effects of larger scale impacts on early (~ 4.6 - 3.8 Ga) terrestrial crustal evolution have relied on analogies with the moon. There are, however, some important differences. Due to the Earth's larger gravitational cross-section, it will have been subjected to more impacts of unit mass and these impacts will have a higher relative energy density than the moon. Most importantly, the relatively higher planetary gravity of the Earth will have resulted in more severe effects due to the differential scaling of crater dimensions relative to the volume of the target recording shock metamorphic effects with event size. As a consequence, impact melt volumes will exceed the volumes of target material excavated and displace by the cratering flow-field at transient cavity diameters > 400 km on Earth, compared to > 3000 km on the moon. At these sizes, the terrestrial equivalents of lunar impact basins will have formed in strengthless melt pools and would not hav...
The SIC is a layered body within the Sudbury Structure, Canada. A detailed mineralogical and petr... more The SIC is a layered body within the Sudbury Structure, Canada. A detailed mineralogical and petrological study reveals that the original SIC melt crystallized in a water-rich environment. Its unusual petrology is evidence of its derivation from an impact melt.
Hypervelocity impact involves the near instantaneous transfer of considerable energy from the imp... more Hypervelocity impact involves the near instantaneous transfer of considerable energy from the impactor to a spatially limited near-surface volume of the target body. Local geology of the target area tends to be of secondary importance, and the net result is that impacts of similar size on a given planetary body produce similar results. This is the essence of the utility of observations at impact craters, particularly terrestrial craters, in constraining impact processes. Unfortunately, there are few well-documented results from systematic contemporaneous campaigns to characterize specific terrestrial impact structures with the full spectrum of geoscientific tools available at the time. Nevertheless, observations of the terrestrial impact record have contributed substantially to fundamental properties of impact. There is a beginning of convergence and mutual testing of observations at terrestrial impact structures and the results of modeling, in particular from recent hydrocode models. The terrestrial impact record provides few constraints on models of ejecta processes beyond a confirmation of the involvement of the local substrate in ejecta lithologies and shows that Z-models are, at best, first order approximations. Observational evidence to date suggests that the formation of interior rings is an extension of the structural uplift process that occurs at smaller complex impact structures. There are, however, major observational gaps and cases, e.g., Vredefort, where current observations and hydrocode models are apparently inconsistent. It is, perhaps, time that the impact community as a whole considers documenting the existing observational and modeling knowledge gaps that are required to be filled to make the intellectual breakthroughs equivalent to those of the 1970s and 1980s, which were fueled by observations at terrestrial impact structures. Filling these knowledge gaps would likely be centered on the later stages of formation of complex and ring structures and on ejecta.
Three “new” buried impact structures have been identified in the Williston Basin in Canada. The E... more Three “new” buried impact structures have been identified in the Williston Basin in Canada. The Elbow structure (106°45'W; 50°58'N) is 8 km in diameter, circular in plan, with a ring depression and an uplifted core of fractured and brecciated target rocks [1]. The structural form is that of a complex impact structure. It has been drilled for hydrocarbons although there is no current production. Originally, the Elbow structure was thought to results from salt tectonics [2] or cryptovolcanism [1]. Sampling of drill hole material indicates the presence of quartz grains with decorated planar deformation features (PDFs; Fig.1a). The Maple Creek structure (109°20'W; 49°51'N) is barely visible at the surface, where repeated sections of the Upper Cretaceous Bearpaw Formation are very poorly exposed. A structural anomaly was noticed by Whitaker [4] and subsequent Vibroseis seismic profiles indicated structural disturbance to ~ 1 km depth and a slightly elliptical zone ~ 6 km ...
The Vredefort Dome is located near the center of the Witwatersrand Basin, about 120 km southeast ... more The Vredefort Dome is located near the center of the Witwatersrand Basin, about 120 km southeast of Johannesburg, South Africa. Its origin is enigmatic, ranging from a major impact event to endogenous processes, either igneous or tectonic. A unique melt rock, the 'Bronzite' Granophyr, occurs in the Vredefort structure as vertical ring dikes along the contact between sedimentary collar and core of Archaean granites, and as vertical dikes extending northwest-southeast and northeast-southwest in the granitic core. The granophyre rocks have an unusual composition and high content of recrystallized sedimentary inclusions compared to common intrusive igneous rocks with similar SiO2 content (61 to 70 percent by weight). The unique nature of the granophyre has been underlined in previous studies and origin hypotheses as an impact melt or as a highly contaminated intrusive mafic magma have also been discussed. We present new results obtained from a recent detailed petrographic and ge...
The Vredefort structure is located approximately 120 km southwest of Johannesburg, South Africa, ... more The Vredefort structure is located approximately 120 km southwest of Johannesburg, South Africa, and is deeply eroded. Controversies remain on the origin of this structure with the most popular hypotheses being: (1) by impact cratering about 2.0 Ga; (2) as a cryptoexplosion structure about 2.0 Ga; and (3) by purely tectonic processes starting at about 3.0 Ga and ending with the Vredefort event at 2.0 Ga. In view of recent work in which the granophyre dikes are interpreted as the erosional remants of a more extensive impact melt sheet, injected downward into the underlying country rocks, the impact origin hypothesis for Vredefort is adopted. In order to estimate the original dimensions of the Vredefort impact structure, it is assumed that the structure was initially circular, that its predeformation center corresponds to the center of the granitic core, and that the pre-Vredefort geology of the area prior to approximately 2.0 Ga ago is as suggested by Fletcher and Reimold. The spatia...
The Vredefort impact structure contains a suite of granophyric dykes, referred to as the Vredefor... more The Vredefort impact structure contains a suite of granophyric dykes, referred to as the Vredefort Granophyre, occurring within and at the edge of the Archaean basement core. New whole-rock chemical analyses, together with previous data, represent a complete suite of the Granophyre occurrences. These data show that the Vredefort Granophyre has a remarkable chemical homogeneity, within and between dykes, on a regional scale, and a unique composition ( approximately 67 wt.% SiO 2 , approximately 1 wt.% TiO 2 , approximately 13 wt.% Al 2 O 3 , approximately 7 wt.% Fe 2 O 3 , approximately 3 wt.% MgO, approximately 4 wt.% CaO, approximately 3 wt.% Na 2 O. approximately 2 wt.% K 2 O). Five volumetrically abundant regional lithologies are: Transvaal carbonate, Ventersdorp lava. Witwatersrand quartzite, Witwatersrand shale, and Outer Granite Gneiss. These lithologies are used as components in both harmonic and least-squares mixing calculations to reproduce the Vredefort Granophyre composit...
Consisting mainly of uplifted crystalline basement, the 40-50 km Vredefort Dome is in a near-cent... more Consisting mainly of uplifted crystalline basement, the 40-50 km Vredefort Dome is in a near-central position within the Witwatersrand Basin [1,2](Fig. 1). The Vredefort impact structure (27 degrees 00'S, 27 degrees 30'E) consists of the Vredefort Dome and a wide "collar" of metasediments and metavolcanic rocks. Prior to the 1970's the diameter of the Vredefort structure was estimated at 60 km, and possibly up to 200 km [3,4]. In the 1970's and 1980's, the diameter was estimated at 100-120 km, while in the early 1990's 170-180 km was preferred. In relation to the outer edge of the Dome, the radial limit of shatter cones (26-37 km), PDFs (~13 km), and downfaulted Transvaal outliers (45-55 km), yield an estimate of 192-300 km for the final crater diameter 15](Fig. 2). These spatial indicators are subject to erosional effects. At the Sudbury Structure, a 45 km thick impact melt sheet has been preserved, under which PDFs have been annealed from the targ...
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