Herbert Henkel

Integrated geophysical modelling of a giant, complex impact structure: anatomy of the Vredefort Structure, South Africa

Abstract Only three very large, confirmed impact structures are known on Earth: the Chicxulub Crater (Mexico), 65 Ma, ca. 180 km wide; the Sudbury Structure (Canada), 1.85 Ga, 200 km in diameter, and the Vredefort Structure in South Africa, 2.02 Ga. While extensive data on large impact structures have been obtained by remote sensing studies of such features on other planetary bodies, only this small number of large terrestrial impact structures can provide data crucial to understanding these catastrophic impact processes on Earth. Integrated modelling of gravity and magnetic data, constrained by geological as well as refraction and reflection seismic data, accomplished the reconstruction of the Vredefort impact structure in South Africa, approximately 250 km wide. The original Vredefort impact structure covered the whole extent of the Archaean Witwatersrand Basin, distinguished by enormous gold resources, as it is structurally preserved today. In fact, it is clear that the preservation of vast volumes of economically important Witwatersrand strata is the direct result of the formation of the ring basin around the central uplift (the Vredefort Dome) of the impact structure. This study is the first attempt to create an integrated and geophysically well-constrained model of this very large, complex impact structure.

Magnetic model of the central uplift of the Vredefort impact structure, South Africa

The Vredefort structure is the largest known impact structure on earth, with an original diameter estimated at—at least—250 km. The central uplift of this structure is about 80 km wide and contains a 40- to 50-km wide core of crystalline basement surrounded by a so-called collar of supracrustal sequences. Previous investigations of the structure of this large impact crater, based on integrated geophysical modeling, have shown that the central rise structure is an upper crustal feature with a presently recognized uplift of some 12 km, decreasing to about 4 km at the depth of the Moho. This study concentrates on the structure of the central uplift region (the Vredefort Dome), using aeromagnetic data along a SSW–NNE profile across its center. Available rock magnetic data were used to constrain the magnetic modeling. On the basis of this modeling, it is concluded that a 60- to 70-km wide region in the center of the structure experienced a magnetic overprint in connection with the impact event. This overprint is related to the occurrence of high-coercive magnetite and its subsequent thermal magnetization. Three sources for this thermal event are envisaged—the potential occurrence of a now eroded impact melt complex, the impact-related rise of hot crustal material of the crystalline core, and the thermal energy deposited by the shock wave. Thermal demagnetization of rocks from the crystalline core has shown that magnetite, most likely formed by shock dissociation of Mg–Fe silicates, is the dominant carrier mineral of the remanent magnetization. The thermal overprint has also included those parts of the upturned metasedimentary collar rocks, which contained suitable magnetic carrier minerals. Its present manifestation is the generally negative magnetic anomaly centered on, and encircling, the central uplift of the impact structure. Within the crystalline core, the magnetic models indicate structures of outward collapse along low-angle thrust surfaces. The impact event has, thus, caused an increased magnetic complexity both by drastic changes of the magnetic properties and by the structural perturbation of the upper crystalline crust.

Standard diagrams of magnetic properties and density—a tool for understanding magnetic petrology

Abstract Bivariate diagrams of magnetic susceptibility, density, remanent magnetization and Q -ratio displaying the variation patterns from 7384 rock samples are used to demonstrate the natural variation fields of these properties of paramagnetic minerals, magnetite, pyrrhotite, and hematite. These minerals represent the most frequent source of magnetic anomalies and the variation of their content provides a definite link between lithology, geological processes, and anomalies in gravity and magnetic measurements. The magnetic susceptibility of ferrimagnetic pyrrhotite id determined empirically from the variation trend in a density—susceptibility diagram of pyrrhotite-containing rocks.

Geophysical Investigations of the Siljan Impact Structure — A Short Review

Siljan in southwest Sweden is the largest impact structure in western Europe, with a present topographic diameter of ca. 75 km. Recent age determinations indicate an age of 377 Ma. The bedrock geology of the region has recently been re-mapped by the Geological Survey of Sweden in the scale of 1:50 000. There is now complete coverage with airborne geophysics. New maps of the geophysical data have been prepared for this review. In connection with the Deep Gas Project, further geophysical studies were made and two drill holes were sunk to over 6 km depth in the central uplift of the structure. The Deep Gas Project produced a large number of reports and publications, which are listed in the summary report of Juhlin (1991). Some of the results are compiled and shortly summarized here. Digital elevation data are available with 50 m spatial resolution, and a gray tone map has been prepared with the regional trend removed. A profile of these data shows that the peak ring of the structure is still visible in the morphology.

Dislocation sets in northern Sweden

Abstract The interpretation of low-altitude, aeromagnetic measurements from six areas (each 50 × 50 km) in the Precambrian of northern Sweden yields distinct sets of dislocations with main directions of −35°, 0° and 55°. These dislocations appear to be of transcurrent-fault type. The displacement for the −35° direction is mainly sinistral and vertical, and for the 55° direction exclusively dextral. The 0° direction shows displacement patterns that may be interpreted as exclusively vertical. The apparent, mean, lateral displacement is 400 m, with slight changes in different directions. The dislocations occur at preferred distances with a maximum at about 30 km. On a regional scale, the two main dislocation sets are parallel to the present, active, fracture zones, as indicated by recent seismic activity and mapped, post-glacial fault movements. It is concluded that the low-altitude aeromagnetic anomalies give very clear indications on a regional scale of dislocations and their displacement.

Putative fossil life in a hydrothermal system of the Dellen impact structure, Sweden

Impact-generated hydrothermal systems are commonly proposed as good candidates for hosting primitive life on early Earth and Mars. However, evidence of fossil microbial colonization in impact-generated hydrothermal systems is rarely reported in the literature. Here we present the occurrence of putative fossil microorganisms in a hydrothermal system of the 89 Ma Dellen impact structure, Sweden. We found the putative fossilized microorganisms hosted in a fine-grained matrix of hydrothermal alteration minerals set in interlinked fractures of an impact breccia. The putative fossils appear as semi-straight to twirled filaments, with a thickness of 1-2 m, and a length between 10 and 100 m. They have an internal structure with segmentation, and branching of filaments occurs frequently. Their composition varies between an outer and an inner layer of a filament, where the inner layer is more iron rich. Our results indicate that hydrothermal systems in impact craters could potentially be capable of supporting microbial life. This could have played an important role for the evolution of life on early Earth and Mars. (Less)

The spacing calculator software-A Visual Basic program to calculate spatial properties of lineaments

A software tool is presented which calculates the spatial properties azimuth, length, spacing, and frequency of lineaments that are defined by their starting and ending co-ordinates in a two-dimensional (2-D) planar co-ordinate system. A simple graphical interface with five display windows creates a user-friendly interactive environment. All lineaments are considered in the calculations, and no secondary sampling grid is needed for the elaboration of the spatial properties. Several rule-based decisions are made to determine the nearest lineament in the spacing calculation. As a default procedure, the programme defines a window that depends on the mode value of the length distribution of the lineaments in a study area. This makes the results more consistent, compared to the manual method of spacing calculation. Histograms are provided to illustrate and elaborate the distribution of the azimuth, length and spacing. The core of the tool is the spacing calculation between neighbouring parallel lineaments, which gives direct information about the variation of block sizes in a given category of structures. The 2-D lineament frequency is calculated for the actual area that is occupied by the lineaments.

Åvike Bay — a 10 km Diameter Possible Impact Structure at the Bothnian Sea Coast of Central Sweden.

Avike Bay is a 270° degree wide near-circular, 114 m deep bay on the Swedish coast of the Bothnian Sea, northeast of Sundsvall. The structure has a diameter of about 10 km. It was classified as a probable impact structure because of its extraordinary circular topography in the overwiew of impact structures in Fennoscandia. Recent studies lend further support to this interpretation. The structure has a submarine central mound, which is elevated some 40 m above the adjacent sea floor. It has a very distinct tangential and radial on-shore fracture pattern as seen in the topographic map. Along the southwestern shore of the Bay, an enigmatic quartzite breccia of unknown age occurs as part of a larger outcrop of polymict breccia with clasts of crystalline rocks and quartzite of unknown age. In thin section, planar fractures can be observed in quartz and feldspar grains. A detailed investigation showed that in a few cases the quartz grains contained microdeformation features closely resembling PDFs.

A Deep rock laboratory in the Dellen impact crater

The Deep Rock Lab is a platform to establish a comprehensive subsurface bedrock characterization approach, by integrating site characterization techniques applied from different disciplines of geo-mechanics, geochemistry, hydrogeology, structural geology, lithology and geophysics, with consideration of the effects of coupled geological processes of importance for the understanding of groundwater renewal, continental shield deformations, engineering issues related to geological disposal of nuclear waste and CO2, and geothermal energy retrieval in crystalline rocks. The approach will focus on the physics and chemistry of crystalline rocks and groundwater with down-the-hole measurements of relevant variables, using and developing more efficient geo-scientific site investigation techniques for deep boreholes at a chosen site, and develop more advanced down-the-hole measurements and numerical modelling methods with more advanced inversion algorithms to help integrate data interpretations and object representations. The goal is to develop this platform into a long-term research facility that can be readily used by the scientific community for both subsurface fundamental and engineering-oriented research. Such a platform will be especially important for the education of PhD students for generations to come. The integrated drilling and research facility is suggested to be located at the Dellen site. This site has an impact crater with a large range of expected physical property changes with depth, complex and multiple thermal processes that have affected the bedrock, a favorable infrastructure and local supporting activities, and a large body of existing geo-scientific data.