REMOTE SENSING AND GIS ANALYSES FOR DETERMINING SIZE, MORPHOLOGY AND

LITHOLOGICAL MAPPING OF THE VREDEFORT IMPACT STRUCTURE, SOUTH AFRICA

Phillips M.E.1,2

, Townsend C.1, Bartels K.

1.

1Centre for GIS & Remote Sensing, School of Science, University of

Greenwich, Chatham Maritime, U.K. 2Planetary Science Research Group, Environmental Science Department,

University of Lancaster, Lancaster, U.K. (email: m.phillips@gre.ac.uk)

Introduction:

The overall geometry and size of the Vredefort Impact

Structure are still unresolved. Some workers have used

modelling based on cratering processes or geophysics

to show that the original diameter could have been at

least 250-300km or more [1]. Also, previous studies of

the Vredefort Structure have indicated that a number of

circular or semi-circular features, at varying radial dis-

tances, could be rings related to the original Vredefort

event [1-5].

Methods and Results: Landsat TM and ASTER

data were used for three interrelated project analyses of

the structure over the period 1999-2005, and four

fieldwork visits between 1998-2007. Landsat TM uses

6 bands of 30m spatial resolution in the visible and

infrared wavelengths, with 1 band of 120m in the ther-

mal infrared [6,7]. ASTER has superior spatial and

spectral resolution with 3 visible bands at 15m, 6 infra-

red bands at 30m and 5 thermal infrared bands at 90m

spatial resolutions [8]. These were the first detailed

studies of the Vredefort Structure by Remote Sensing

methods. Landsat TM was particularly useful for rec-

ognising features for the first time, such as radial faults

to the north of the Ring Collar (Fig. 1), arcuate drain-

age patterns and a likely ring feature at a radial dis-

tance of 125km, suggesting an overall size =>250km

[6,7].

Figure 1. Radial faults in the Foch Thrust Zone (FTZ)

Figure 1 shows the Foch Thrust Zone region north of

the Vredefort Ring Collar, with a known thrust fault in

yellow, normal faults in white, and a dyke in red.

Newly recognised radial faults from these studies [6,7]

are shown in black; the large magenta structure in the

lower central region is the Losberg Complex. Many of

these findings supported work by geophysical methods

[1]. Spectral profiling with ASTER data was applied to

selected regions where either vegetation cover was low,

or where other image processing methods highlighted

differences in lithologies, or for impact structure mor-

phologies. An area was investigated at Witkop, match-

ing the spectral library profile of gneiss (Fig. 2) [8].

Figure 2. Spectral Profile of gneiss found at Witkop.

In this comparison of observed spectra with library

spectra, characteristic absorption features were de-

tected at 0.65µm (band 2, visible green), 2.20µm (band

7, mid-infrared) and 2.35µm (band 9, mid-infrared) [8].

ASTER data was then used successfully for spectral

unmixing of specific geological regions and for deline-

ating lithologies [8]. Additionally, Principal Compo-

nents Analysis (PCA) was used to study Koedoesfon-

tein, a region of quartzites/conglomerates. Two differ-

ent geological units were observed. The upper half of

the sequence belongs to the Turffontein, the lower ex-

posures were discriminated as Johannesburg subgroups

of the Central Rand Group, important for gold deposits

in the region. Figure 3(a) shows a PCA RGB 123 im-

age of this area. At Dwarsberg, a Dolerite ridge was

mapped with PCA, where the ridge was highlighted in

strips of magenta and dark blue with a n.east-s.west

trend, as displayed in Figure 3(b) [8].

(a) (b)

Figure 3(a). PCA RGB123 image of Koedoesfontein.

(b). PCA RGB123 image of the Dwarsberg area.

Large Meteorite Impacts and Planetary Evolution IV (2008) 3061.pdf

GIS and Digital Elevation Model (DEM) mapping was

applied to the Vredefort Impact Structure for the first

time [9], resulting in more detailed mapping and rec-

ognition of other previously unseen and unreported

structures. The DEM revealed a previously unreported

raised, circular to lobate-like structure in the central

Vredefort core, with height ~30m above the local ter-

rain, and diameter ~25km [9]. Estimation of fault and

scarp morphologies were visualised in 3D, one exam-

ple of which is shown in Figure 5.

Figure 5. Perspective view of the FTZ, looking NE [9].

Fig.5 is a perspective view of the Foch Thrust Zone

(FTZ), looking along strike towards the NE, with a

Landsat TM RGB 641 [6] draped over a 30m DEM,

made from digitised contours of 12 x 1:50,000 topog-

raphic maps [9]. Vertical exaggeration is x2, area

~10km. A prominent outward-facing scarp morphology

is seen, with gentle back slopes and steep faces. This

matches the morphology of ring scarps in a multi-ring

basin [10]. The DEM was also used to extrapolate ra-

dial faults [9] which intersected at the centre of the

structure, confirming the radial orientation [8]; and was

also applied to an ‘inverted cone’ model for recon-

structing the crater’s original size, by extrapolation of

the structure at the FTZ and at the Ensels Thrust Zone.

This suggests that Vredefort is a multi-ringed structure

with at least two rings at diameters 146km and 186km

[9]. The model was applied to the third outer ring

structure giving a final original structural extent of

281km diameter, in good agreement with other results

[3,12]. Figures 6 and 7 show a profile and cross-section

covering ~45km out from the Inner Ring Collar.

Conclusion: 3D morphological modelling, com-

bined with recognition of arcuate structures, add to the

determination of size, structure and multiple rings at

Vredefort. Many of these findings [7,9,11] are in

agreement with workers using other multi-disciplinary

methods [1-3]. Multi-ring Basins on Earth are difficult

to confirm, however, [10,13] and further multi-

disciplinary studies integrated with other digital data

within a GIS still need to be done.

Figure 6. NNW-SSE Profile across the inner rings.

Figure 7. Cross-section of the NNW-SSE Profile high-

lighting the following features and ring structures:

i. Foch Thrust Fault Zone

ii. Potchefstroom Synclinorium

iii. Ensels Thrust Fault Zone

iv. Outer Limit of Circle 1 Zone

v. Inner Ring Collar (arcuate ring fault)

References: [1] Henkel, H. & Reimold, W.U.

1998. Tectonophysics, 287, 1-20. [2] Andreoli, M.A.G.

1991. Structural Map, Atomic Energy Corporation,

Pretoria. [3] Brink, M.C. et al. 1997. Tectonophysics,

270, 83-114. [4] Pilkington, M. & Grieve, R.A.F.

1992. Review of Geophysics, 30, 2, 161-181. [5] Ron-

dot, J. 1994. Earth Science Reviews, 35, 331-365. [6]

Phillips, M.E. 1999. Unpublished BSc thesis. [7] Phil-

lips, M.E. et al. 1999. Abstracts of the 62nd Meteoriti-

cal Society Meeting. [8] Bartels, K. 2005. Unpublished

BSc Thesis, University of Greenwich, UK. [9] Town-

send, C. 2001. Unpublished BSc Thesis, University of

Greenwich, UK. [10] Melosh, H.J. 1996. Impact Cra-

tering, A Geologic Process. Oxford Uni. Press. [11]

Phillips, M.E. 2001. Norwegian Geographical Mem-

bership, GIS Meeting, Bergen, Norway. [12] Grieve,

R.A.F. et al. 1995. GSA Today, Vol 5 No 10, p 194-

197. [13] Thierriault, A. M., et al.1997. Meteoritics &

Planetary Science, vol. 32, 71-77.

i ii iii iv v

NNW

SSE

NNW SSE

Large Meteorite Impacts and Planetary Evolution IV (2008) 3061.pdf