Journal of Applied Geophysics 49 (2002) 129–147 www.elsevier.com/locate/jappgeo

Magnetic and gravity model of the Morokweng

Herbert Henkel a,b, Wolf Uwe Reimold b,*, Christian Koeberl c

aDepartment of Geodesy and Photogrammetry, Royal Institute of Technology, SE-10044 Stockholm, Sweden bImpact Cratering Research Group, Department of Geology, School of Geosciences, University of the Witwatersrand, Jan Smuts Avenue, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa cInstitute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria Received 27 October 2000; accepted 30 October 2001

Abstract

The geophysical characteristics of the 145-Ma-old Morokweng impact structure in South Africa have been studied by modeling combined gravity and magnetic data along a 100-km cross section. Rock physical properties to guide the modeling were measured on samples from three drill cores in the central part of the structure. Based on the resultant model, the current crater diameter is estimated at 65–70 km, and a central uplift of approximately 4 km is derived. This estimated diameter is in good agreement with a value of about 70 km recently obtained from analysis of a deep borehole from just outside of the envisaged crater rim. The Morokweng impact structure has almost no gravity expression, but has a distinct, irregular magnetic anomaly of 25–35 km diameter at its center. This anomaly is surrounded by an annular region with discontinued regional anomaly features. Such magnetic patterns are typical for a number of other impact structures and have been related to impact- induced changes in magnetization properties of target rocks and the occurrence of impact melt and bodies. At Morokweng, the most likely cause for the central magnetic anomaly pattern is the presence of remnants of a central impact melt body and impact-induced variation in remagnetization of the crystalline basement rocks. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Morokweng; Impact structure; Potential fields; Gravity; Aeromagnetics; Integrated modeling

1. Introduction ture, in this area. The most prominent geophysical expression of this structure is a central, positive, irreg- Based on the recognition of near-circular aeromag- ular aeromagnetic anomaly of near-circular shape and netic patterns in the region of the Ganyesa Dome, 25–35 km width, and a surrounding annulus of 20 km North West Province (South Africa, Fig. 1), Corner width, which is comparatively anomaly-free and has an (1994), Andreoli et al. (1995) and Corner et al. (1996) outer boundary that terminates a series of magnetic initially proposed the existence of a large dyke features trending in northeast–southwest direc- impact structure, termed the Morokweng impact struc- tion. Corner et al. (1997) provided the first evidence of shock metamorphic effects in quartz from selected surface samples found in the Morokweng region. * Corresponding author. Fax: +27-11-339-1697. Koeberl et al. (1997) and Hart et al. (1997) confirmed E-mail address: [email protected] (W.U. Reimold). the presence of shocked minerals in Morokweng rocks.

0926-9851/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S0926-9851(01)00104-5 130 H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147

Fig. 1. Location of the Morokweng impact structure in Southern Africa.

They also reported the existence of a large impact melt Group (Karoo Supergroup) and post-impact Kalahari rock body, which not only has the chemical character- Group sediments in this region (Bootsman et al., istics of impact melt and includes shocked clasts, but 1999) have been interpreted by Reimold et al. contains up to 5% of a meteoritic component (Koeberl (1999) as an indication for a possible 200-km width et al., 1997, 2000, in press). Further detailed studies of of the Morokweng impact structure. And yet, these and basement rocks from drill cores from authors also reported that geological detail around the the central part of the structure have recently been Ganyesa Dome, such as subhorizontal attitude of reported by Andreoli et al. (1999) and Reimold et al. metasedimentary strata just outside of the strong (1999). aeromagnetic anomaly pattern in the Ganyesa Dome The time of impact is well constrained from single area and lack of shocked basement only 20 km to the zircon studies by Hart et al. (1997) and Koeberl et al. southeast of the Dome, could indicate a much smaller (1997) (cf. also Reimold et al., 1999 for further diameter. Andreoli et al. (1999) still favoured a dia- chronological data). Both groups independently ob- meter of > 140 km. tained an age of 145F2 Ma for the Morokweng im- The geology of the Morokweng region (Figs. 2 pact structure. and 3) is not well constrained. With the exception of What has remained controversial, however, is the limited exposures in the environs of the Ganyesa size of the Morokweng impact structure. Early pub- Dome, the whole area is covered by sands and sil- lications (e.g., Corner et al., 1997; Hart et al., 1997) cretes/calcretes of the Cenozoic Kalahari Group. Only recognized the distinct geophysical features around sporadic outcrop, mainly to the southeast of the Dome, the Ganyesa Dome of f35 km radial extent. But, of Archean granitoid basement is accessible. In addi- beyond this, both groups speculated—from regional tion, a detailed hydrogeological survey provided geo- geophysical features — that the Morokweng impact logical subsurface information from a large number of structure could have a diameter as large as 340 km. boreholes, from which a 1:250000 geological map More recently, this figure has been revised down- (Sheet Morokweng; Council for Geoscience, 1974) wards. Isopach patterns for the pre-impact Dwyka was available. H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147 131

Fig. 2. Geological map of the Morokweng structure and its nearest surroundings. A— distorted beds, B and C— annular occurrence of silcrete and downfaulted Dwyka diamictite, respectively, D — eastward bend of quartzite horizon.

This map demonstrates that the central part of the are only locally disturbed and generally have an B region contains Archean granitoids. To the east of the outward directed dip of 5–12 . Ganyesa Dome, Dwyka diamictite occurs in patches. Hart et al. (1997), Koeberl et al. (1997), Reimold et It is possible that small inliers of amphibolitic schist al. (1999) and Andreoli et al. (1999) described three occur in the central part, and earlier workers have short drill cores, which were obtained from within the assumed that they belong to the Archean Kraaipan central aeromagnetic anomaly. One of these boreholes Group (Anhaeusser and Walraven, 1997), which intersected an approximately 150-m-thick impact melt mainly occurs further to the east and contains similar body, before penetrating strongly deformed basement lithologies. To the west of the Dome, metasediments granitoids. of the Proterozoic Transvaal Supergroup and Griqua- Recently, studies of two boreholes into and at the land West Group occur, including Black Reef Quartz- margin of the Morokweng impact structure are being ite, limited exposures of which are found just north of carried out. Andreoli et al. (Andreoli, University of Morokweng township and near Melrose to the south the Witwatersrand, pers. commun., 2000) are engaged of the Dome, as well as dolomite, jaspilite and tillite, in detailed analysis of a drill core made available to and extensive exposures of ironstones. These strata them by De Beers Geoscience Centre, Johannesburg, 132 H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147

Fig. 3. Schematic stratigraphy of the rocks at the Morokweng impact structure. which was reportedly obtained some 15 km south of The present work represents an attempt to better Morokweng township, on or close to the Ganyesa constrain the diameter of the Morokweng impact Dome (Figs. 2 and 4). This borehole allegedly pene- structure and to make a contribution to the under- trated a top layer of ca. 60 m Kalahari Group sedi- standing of the structure in an upper crustal cross ments, before entering a thick sequence of polymict section, as far as this can be achieved by using fragmental (also termed lithic) impact , with existing geophysical and geological data. A qualita- abundant clasts with shock metamorphic effects tive interpretation has been made of the aeromagnetic (Andreoli, pers. commun., 2000). and gravity maps available at the scale 1:500000 and Our group was given the opportunity by Anglo- a combined gravity/magnetic model has been calcu- gold, Johannesburg, to study drill core from an lated along an almost 100-km-long profile from WSW approximately 3500-m-deep borehole (KHK-1, drill- across the center of the structure and beyond its ENE ing site indicated in Fig. 4b) sunk to the west of the margin (Fig. 2). Morokweng structure. A thick succession of Trans- vaal Supergroup metasediments overlying a sequence of volcanic rocks and, finally, granitoids, was encoun- 2. Background tered in this borehole. The rocks are essentially undeformed (Reimold and Koeberl, 1999; Reimold Impact craters are explosion structures formed as et al., 2000) and display subhorizontal attitude. Only a the result of the impact of an extraterrestrial body. The single, 10-cm-wide occurrence of was diagnostic features of an are the occur- detected. On the basis of these findings, Reimold et rence of shock metamorphic effects [such as presence al. (2000) concluded that this borehole could, at best, of impact (shock) melt, specific shock microdeforma- be located at the edge of the impact structure, but— tions, for example, planar deformation features — so- most likely — was situated on the outside of the crater called PDFs — or diaplectic glass of minerals, and rim — thus delimiting the diameter of the impact formation of high-pressure polymorphs of minerals — structure to a maximum of about 70 km. e.g., Grieve and Pilkington, 1996; French, 1998] in H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147 133 the lithologies of the target region, contamination of In the northwestern sector, the boundary between impact with extraterrestrial material, and the dolomite and ironstones is displaced to the east, which structural imprint on the Earth’s crust by a crater with implies down-faulting, as the structures are dipping a characteristic shape related to the size of the impact. gently towards the west. In this region — at a radial The present 3-dimensional geometry of an impact distance of 40 km from the center of the structure — crater can only be assessed with geophysical techni- the beds of the ironstone formation are locally dis- ques or drilling. torted. The quartzite horizon at the base of the oldest The following aspects of impact cratering are of cover rocks, in the southern part of the structure, is importance for the modeling of the resulting structures displaced slightly to the east and then bends markedly (for a more detailed account, see Melosh, 1989): The to the west—just inside the deep part of the Kalahari impact of an extraterrestrial body results in a shock beds. The easterly displacement may be the result of wave that passes through the projectile and target down-faulting. To the north, an occurrence of silcrete materials, causing a hemispherical pattern of shock aligns with the deep Kalahari annular trough at a transformations and deformations in the target. These radial distance of 35 km. To the east of the structure, different deformation zones range from vaporization, scattered occurrences of shale and diamictite of the melting and plastic deformation to brittle fracturing, Dwyka Group (lower part of the stratigraphy of the along a transect away from the explosion center. This Karoo Supergroup, ca. 180–300 Ma) are in a down- initial phase of shock transformation is followed by faulted position at a radial distance of 30–35 km. As the crater formation stage, when accelerated material the thickness of the Kalahari beds is reduced to less is ejected out of the evolving crater and injected into than 60 m in the central part of the structure, a distinct the crater basement. The resulting cavity is transient, crater topography must have existed at pre-Kalahari and for larger structures, such as the Morokweng time (cf. also Bootsman et al., 1999). crater, the excavation is accompanied by the forma- tion of a central uplift. This central uplift will, together with the transient cavity, collapse in a subsequent 4. The geophysical database stage of reversed flow of material. After this collapse, the crater has been widened to about twice its transient Aeromagnetic and gravity data for the study region diameter to the final crater diameter. The remaining were made available by Geodass Geophysical Explo- uplift is about 0.08 times the final diameter. The crater ration Services (now Fugro Airborne Surveys), Johan- will subsequently be affected by geological processes, nesburg (Fig. 4b and c). The aeromagnetic data had which may erode it or preserve it under sedimentary been obtained along a grid of 1000 m line spacing, deposits. with a flight height of 120 m. The gravity data had been collected along roads and tracks across this region, in the irregular pattern illustrated in Fig. 4a. 3. Some geological considerations The average spacing between gravity stations is ap- proximately 3.5 km. The center of the Morokweng impact structure is Six representative drill core samples of the gener- taken as the center of the 25- to 35-km-diameter ally very homogeneous impact melt rock and six drill irregular magnetic pattern noted by Corner et al. core samples of basement granitoid, covering the (1997). It is also roughly the place where the three whole variety of lithologies encountered in the base- boreholes are located (Fig. 2). ment part of the drill core, have been measured with The geological map (Council for Geoscience, respect to their magnetic susceptibility and density. 1974; a simplified version is shown in Fig. 2) pro- Four samples of melt rock and four samples of base- vides several interesting clues regarding the occur- ment granitoid were selected for demagnetization. The rence of an impact structure. The thickness of the drill holes are vertical and, thus, the inclination of the Kalahari beds exceeds 150 m in a roughly annular remanent magnetization could be defined (whereas pattern along the southern and western edge of the the declination remains undefined), together with its structure at 25–35 km radial distance from the center. intensity and coercivity. The magnetic carrier mineral 134 H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147 could be identified by measurements of the magnetic The aeromagnetic data (Fig. 5) reveal several as- susceptibility variation with temperature. These de- pects related to the , overprinted onto pre- magnetization measurements were carried out at the existing magnetic structures. Model calculations to Paleomagnetic Laboratory of the Technical University determine the most likely remanence directions were of Lulea˚. conducted, using structural information from the geo-

Fig. 4. Geophysical data provided by Geodass. (a) Distribution of gravity stations across the Morokweng study region. (b) Free air gravity anomaly map. (c) Aeromagnetic anomalies over the Morokweng region. H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147 135

Fig. 4 (continued). 136 H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147

Fig. 5. Qualitative interpretation of the aeromagnetic anomaly patterns. 1 — Western margin of relatively strongly magnetic basement rocks, 2 — eastern edge of magnetic part of ironstone formation with eastwards- and southeastwards-located positive contact anomalies, 3 — eastern edge of ultramafic intrusive with southeastwards-extending negative contact anomaly in its southern part, 4 — outer limit of central irregular anomaly pattern, 5 — outer limit of pattern for discontinued magnetic dykes and onset of magnetic basement pattern (without noticeable contact anomalies). A — eastwards displaced magnetic ironstone formation, B — central region with irregular positive anomaly pattern, C — annular region with discontinued magnetic dyke pattern (and with scattered remnants of dikes at depth). D — area with broken shallow parts of ultramafite, E — region with positive magnetic anomalies of deeper located ultramafite, lacking distinct exterior contact anomalies. Thin lines: continuous magnetic dykes. Broken lines: dislocation zones. logical map in conjunction with the Geovista (1994) in addition, more than two strike directions occur. The modeling software. Such estimates are feasible when basic information used are the sign, location, ampli- the strike and dip of a structure are known and when, tude, and orientation of undisturbed magnetic anoma- H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147 137 lies due to contact between units of different magnet- magnetic structure, the reappearance of the dike izations. pattern can be seen in the SW sector. In the NW The pre-impact structures reflected in the aero- sector, an eastward displacement of the magnetic magnetic anomaly pattern are related to: anomalies related to the ironstone formation is noted. These anomalies are characterized by large E- and SE- . the occurrences of strongly and reversed magne- oriented positive contact anomalies, which indicate B tic (i.e., with inclination of ca. 60 and declination of reversed magnetization. The S–N-trending structure B ca. À45 ) parts of the ironstone formation; is also subdivided into smaller blocks, both north and . an unknown strongly magnetic structure trending south of the central irregular anomaly. S–N for about 75 km east of the ironstone occur- The interpretation of the changes in pre-impact rences and just west of the center of the impact anomaly patterns and the impact-related effects clearly structure, with a likely remanence declination of shows a ring structure where the original magnet- B B À30 and inclination of À45 in its southern part, ization is decreased and structures are block-faulted B B and declination of about 110 and inclination of À80 and discontinuous. The outer edges of this ring are in the northern part; typically irregular in shape. The central region with an . numerous magnetic dikes from several dike irregular anomaly pattern marks the extent of impact- swarms trending WSW–ENE, W–E and S–N; and induced magnetization of basement rocks (see below, . variably magnetic parts of the Archean granitic petrophysical measurements) around the central uplift, basement (more magnetic to the SE), with induced together with potential (only locally, in a few bore- magnetization. holes, confirmed) occurrences of variably magnetic impactites (impact melt and suevite). Several of these anomaly patterns serve as excellent The gravity data reveal large anomalies related to reference structures to the impact-induced changes of the occurrence of pre-impact lithologies (Fig. 6). magnetization and structure. The impact-generated Anomalies that can be directly related to the impact anomaly pattern consists of several typical features do not occur, as the imposed density contrast is small also observed in other impact structures. At Morok- (see below) and confined to structures that are too weng, they are found east of the S–N-trending mag- small to be detected with the present spatial resolution netic structure and include (1) a central, strongly of the data. The large, regional gravity low seen at the variable anomaly pattern with small, scattered positive structure and to the east of it is related to the up- anomalies that occupy a roughly circular area of 25– doming of relatively low-density granitic rocks in the 35 km diameter, an estimated remanence with decli- Ganyesa Dome. Cover rocks surrounding the Dome B B nation À20 , and an inclination of À40 and (2) a 15- are generally associated with positive gravity anoma- to 20-km-wide annular zone around the center of the lies and, thus, denser rock types. Indirectly, the effects structure, with an irregular outer border and an outer of the impact event are seen in the S–N-trending diameter of approximately 65 km. Inside this region, structure discussed previously, which is made up of the typical magnetic dyke patterns are discontinuous dense material producing anomalies of +20 to +45 and are only seen as scattered remains with a lesser mgal amplitude, which are broken into several blocks. gradient of the anomalies, indicative of a deeper On the anomaly map, a very weak negative anomaly source. The outer border of this zone is seen where occurs over the deeper parts of the Kalahari beds and dike anomalies reappear and become distinctly con- other occurrences of relatively young, low-density tinuous. There are no obvious contact anomalies cover rocks. associated with this border, indicating a generally Petrophysical data (Fig. 7a and b) were obtained inward-dipping attitude. on drill core samples. The density was measured on A distinct change in the attitude of the S–N- six samples of basement granitoids and six samples of trending magnetic structure is observed. It has south- melt rock, which were, on petrographical and drill easterly oriented contact minima in its southern part core stratigraphic grounds, considered representative and essentially lacks this feature in its central and for these lithologies. The susceptibility was measured northern parts. To the west of the S–N-trending with a 3-cm-diameter coil on the plane, 45- to 60- 138 H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147

Fig. 6. Qualitative interpretation of the gravity anomaly patterns. 1 — Zone with weak positive anomalies, 2 — trace of weak negative anomalies, 3 — outline of weak central low area. A — positive anomalies related to ironstone formation, B — positive anomalies related to ultramafite (numbers indicate relative gravity anomaly in milligals). Thin lines: dislocation zones. mm-wide surface of split drill core samples (in total, bility, with values ranging from 8Â10À4 to 4Â10À2 8 measurements on granitoids and 39 on melt rock, SI, corresponding to a magnetite content of up to 1%. respectively). This measurement geometry may result The melt rock has an average density of 2.647, and in values that could be slightly too low. The melt rock the fractured granitic basement of 2.50 Mg mÀ3, has a rather homogeneous, low magnetic susceptibil- respectively. ity of 1.5Â10À3 SI, whereas the underlying basement The magnetic carrier mineral in both melt and rocks are highly variable in their magnetic suscepti- basement rocks was found to be magnetite, with a H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147 139 B typical Curie temperature of approximately 570 C (Fig. 8a). In the basement sample (sample 63), an initially occurring magnetic phase is oxidized at about B 250 C and is not seen in the cooling path (dotted line in Fig. 8a). The alternating field demagnetization (Fig. 8b) shows a highly coercive (single domain) remanent magnetic phase in the melt (sample 33) as well as the basement (sample 64) rock. The inclination of this B rather stable remanence is in the interval 15–24 ,on B average 18 . An analysed basement rock sample from the deepest part (>250 m depth) of borehole MWF 05 (e.g., Reimold et al., 1999) has the soft remanence B B inclination of À59 , which rapidly drops to À17 during AF demagnetization, that is, to about the same value determined for the melt rock samples. The common stable remanence inclination of melt and basement rock samples is interpreted to represent thermo-remanence, which is related to the cooling of the impact melt body and underlying basement after the impact event. The high initial inclination angle measured for one of the basement samples represents a viscose remanence close to the direction of the present geomagnetic field in the region. The inclination obtained from demagnetization of melt and basement rock samples is close to that used in the magnetic modeling. The apparent polar wander curve for Africa during the Cretaceous indicates a B paleopole in the early Cretaceous at declination 116 B B B and inclination 30 (or À64 and À30 , respectively, for the reversed polarity; Hargraves, 1989). The trend towards the late Jurassic is latitudinal with decreasing inclination. The modeling and demagnetization results seem to fit with this trend, thus implying that the B paleopole for Africa was located some 10 further south in latest Jurassic time.

5. The combined gravity and magnetic model

5.1. The procedure

Anomalies of the gravity and magnetic fields were interpolated at 500-m intervals from the free air anomaly maps and gridded magnetic anomaly data along an almost 100-km-long profile from WSW to Fig. 7. Rock physical properties (density and magnetic suscepti- bility) measured on drill core samples. Susceptibility and density of ENE across the structure. This profile azimuth was impact melt rock and its immediate basement (averages of several chosen to be representative of the impact-related susceptibility measurements per sample). magnetic anomaly pattern and to minimize the num- 140 H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147

Fig. 8. Demagnetization of samples from impact melt (sample 33) and basement rocks (samples 63, 64 and 70). (a) Variation of magnetic susceptibility with temperature. The typical magnetic carrier phase is magnetite. The cooling path is dotted. (b) Alternating field demagnetization showing stable remanence in samples 33 (melt) and 64 (basement), as opposed to the low coercive remanence in the basement sample 70. ber of intersected magnetic dikes (compare Fig. 5). specified points (usually along a selected profile). It is For the modeling, a versatile software package allow- also capable of approximating interactively the geo- ing 2.5-dimensional modeling (i.e., allowing variation logical source structures with prismatic bodies of of strike length, strike direction, and offset from the polygonal cross section and vertical end-surfaces. profile) was used (Geovista, 1994), which allows the The software accounts for the strike direction and simultaneous calculation of both magnetic and gravity strike-length of source structures. These parameters anomalies caused by source structures approximated are selected for each structure from the respective with polygonal cross sections. anomaly map. Geovista can, further, include off- The 2.5-dimensional Geovista modeling software profile source structures. Through rotation of source B allows to calculate the gravity and magnetic effects at structures through 90 , their dip with respect to the H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147 141 calculation profile can be determined. This software and Reimold (1998) for the case of the ca. 250-km- can handle source structures located within each other. diameter Vredefort Structure. Fortunately, such a With these tools, fairly complex structures can be complexity does not occur for the Morokweng struc- approximated. Structures that are smaller than the ture. spacing between individual measurements can, how- ever, not be resolved with confidence. As this spacing 5.2. The Morokweng modeling is different for aeromagnetic and gravity data (usually smaller for magnetic data), the option for inside From the gravity field, a constant value of 200 gu source structures is normally used to include small was subtracted, resulting in a zero level over the magnetic sources within larger gravity bodies. central part of the Ganyesa Dome, thereby restricting The gravity and magnetic anomalies decrease with the analysis to the uppermost crust. The following the second and third power of the distance to their parameters were implied in the modeling: profile B source structures. Therefore, the dip of prismatic end- azimuth: 70 ; start at: 7054N and 097E, end at: surfaces has only a very minor effect on the anomaly 7065N and 190E; number of interpolated points: at the calculation point, justifying their approximation 197; distance between interpolated points: 500 m; with vertical dip. Another consequence of the depend- local geomagnetic field: 30000 nT, with declination B B ence on distance is the reduction of spatial resolution of À20 and inclination of À60 ; calculated field: with depth. Consequently, only surface-near features magnetic total intensity and vertical acceleration of can be modeled in detail. The amplitudes of gravity gravity; flight elevation: 120 m; ambient density: and magnetic anomalies are essentially the result of 2.65 Mg mÀ3; and ambient susceptibility: 3Â10À4 SI. the product of volume and contrast in the respective physical property. The symmetry/shape of anomalies 5.3. Modeling results depends essentially on the strike and dip of the related source structures. The strike-length option of the The combined gravity and magnetic model is modeling software allows an improved volume esti- shown in Fig. 9a and b for a 5-km-deep section of mate, and the option of strike-direction variation the uppermost crust within the study region. The rock results in improved dip estimates. physical properties of the modeled structures are listed Against this background, the issue whether it is in Table 1. appropriate to approximate a 3-dimensional structure, The gravity anomaly along the selected profile is such as an impact structure, with 2.5-dimensional totally dominated by a high-density structure, here source structures can be discussed. The modification modeled as an ultramafic intrusive with a density of by impact of existing structures and the generation of 3.15 Mg mÀ3, located just west of the center of the new structures is — laterally — limited to the final impact structure. This structure is also strongly mag- crater diameter and — vertically — to a small fraction netic in its lower part (source structure no. 11, Table 1), of the crater diameter (usually < 0.1 of the final crater resulting in a highly magnetic anomaly with smaller diameter). The general shape of impact-modified gradients that has been uptilted to the east by the central source structures, thus, approximates a layer geome- uplift of the impact crater. To the west, parts of the older try. This implies that the roughness of their approx- cover rocks (dolomite and ironstone formation) con- imation away from calculation points (or the profile) tribute to minor positive gravity anomalies (source has no negative influence. The account of strike- structure nos. 1, 2 and 15, Table 1). To the east, an length and strike-direction allows a better geometric almost constant low-gravity field prevails over the description along the profile (i.e., the precision is central part of the Ganyesa Dome (source structure placed where it can be best utilized). no. 13). Neither the central uplift nor the ring basin of A problematic feature to model is the central uplift the impact structure produces any significant anoma- of large impact structures. It may require a complex lies in the gravity field and are, therefore, derived from ‘description’ with inside source structures, where the magnetic modeling. crustal layers of strongly different properties are The magnetic anomaly along the selected profile involved. This has been demonstrated by Henkel reflects both deep and shallow anomaly sources with 142 H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147 H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147 143

Table 1 variable magnetizations. A low-gradient, high-ampli- Rock physical properties of modelled magnetic and gravity anomaly tude anomaly is related to the lower part of the source structures ultramafic structure delineated by the gravity model- Source Magnetic Remanence q, Density Lithology B ing (source structure no. 11). The geometric config- structure susceptibility inclination [ ], (Mg mÀ3) B no. (SI) declination [ ] uration derived from the gravity data results in easterly oriented negative contact anomalies, as can 1 0.8 3.5 ironstone formation be seen in the ring part of the structure (to the north 2 0.5 3.5 ironstone and south of the profile, respectively). As this low is formation missing in the center of the structure, a counteracting 3 0.06 ambient western magnetic source structure (nos. 1, 2 and 15, Table 1) is basement needed and has been modeled as a magnetic central 4 ambient 2.45 Kalahari beds 5 0.03 3, À40, 2.50 remagnetized uplift with ambient density (i.e., not contributing to À20 basement the gravity field) (source structure no. 12). Thus, 6 0.06 2.9 amphibolite in together with the near-surface remagnetized basement W basement (source structure nos. 5, 7, 19 and 20), three magnetic 7 0.07 6, À40, 2.50 remagnetized sources contribute to the magnetic high between 40 À20 basement 8 0.0015 3, À40, 2.65 impact melt and 60 km in the center of the profile. An increased À20 susceptibility and a change in the direction of magnet- 9 0.0015 3, À40, 2.65 impact melt ization towards steeper inclination would allow the À20 magnetic basement uplift below it to be less magnetic. 10 0.025 2.85 western The general shape of the structures would, however, dolerite 11 0.3 3.15 magnetic not change significantly (as the position of the ultra- ultramafic rock mafic volume, source structure nos. 11 and 16, is 12 0.15 ambient central constrained by its gravity anomaly). The volume basement rise around the central rise and between the magnetic 13 0.05 ambient eastern western and eastern basement blocks is modeled with basement 14 0.035 2.85 eastern dolerite ambient petrophysical properties (density 2.65 Mg À3 À4 15 ambient 2.85 dolomite m and susceptibility 3Â10 SI). Therefore, it does 16 0.004 3.15 paramagnetic not contribute to the gravity anomaly, but gives rise to ultramafite a central magnetic low. 17 ambient 2.75 lower western The high-amplitude and high-gradient anomalies to basement 18 ambient 2.45 diamictite and the west (source structure nos. 1 and 2) are modeled as shale ironstone layers dipping shallowly to the west. In the 19 0.05 5, À40, 2.50 remagnetized central uplift region, a series of local, variably high, À20 basement anomalies is modeled as magnetic, irregular, and 20 0.075 6, À40, 2.50 remagnetized shallow basement layers (source structure nos. 5, 7, À20 basement ambient 0.0003 2.65 19 and 20), interrupted by melt rock occurrences of low magnetization (source structure nos. 8 and 9). The

Fig. 9. Combined gravity and magnetic model of a 100-km-long and 4.5-km-deep section across the Morokweng impact structure. The rock physical properties of the numbered source structures applied in the modeling are listed in Table l. The un-patterned region in the crater structure has background physical properties and represents in this case the region, together with the central top position sources, that is altered by the impact event. The vertical scale is 5Â the horizontal to better show the shallow thin parts of the structure. Measured anomalies are marked with single symbols and calculated anomalies with a thin line. Note: The largest differences between measured and calculated anomalies are 15 nT and 10 gu, respectively. (a) Complete profile. (b) Enlargement of the central part with magnetic anomalies related to the impact melt bodies (8 and 9), their underlying remagnetized basement (5, 7, 19 and 20), and the eastern edge of the trough in the Kalahari bed thickness (4). Dots represent measured magnetic anomalies and the difference to the calculated anomalies is related to the surrounding sources not shown here but represented in (a). (c) Geological interpretation of the combined magnetic and gravity model in terms of the regime of the central uplift complex and distribution of impact melt rock, demagnetized basement rocks, and two types of fill in the ring basin surrounding the central uplift. 144 H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147 thickness of this complex is less than 0.6 km. The uplift; in other words, the deeper and denser crustal rock physical properties obtained from the drill core layers were not involved in the uplift. samples were used for the modeling. To achieve the The uplifted basement at Morokweng is rather measured amplitude of the anomalies, a Q-value (ratio magnetic and causes a weak but consistent long- of remanent to induced magnetization) of 3–6 was wavelength magnetic high in the center of the struc- applied for the various source structures. ture. The ring basin may also contain rocks with As the direction of the impact-related remanent densities that could balance the effect of uplift of magnetization is not well constrained, the precise denser basement. However, this is not a very likely geometry of these source structures cannot be as- scenario, as seen from the gravity model of the sessed in detail. A few dolerite dikes approach the Vredefort structure, where an annular gravity low profile both at the western and eastern margins. They separates the gravity highs of the ring basin from have been modeled with their real strike direction and the high of the central uplift. This intervening low is are, thus, cut obliquely by the profile (resulting in a related to the upturned low-density basement rocks widened cross section). that occurred in the central uplift region before impact (Henkel and Reimold, 1998). Around the Ganyesa Dome (with the exception of the Morokweng struc- 6. Discussion ture), the Proterozoic cover rocks are essentially undisturbed, dipping at very shallow angles, and the The diameter of the Morokweng impact structure regional pattern of magnetic dykes is likewise intact. has previously been estimated between 70 and 340 The best estimate of the present diameter of the km (Corner et al., 1997; Hart et al., 1997; Bootsman Morokweng impact structure is obtained from the et al., 1999; Andreoli et al., 1999; Reimold et al., extent of interrupted regional magnetic patterns re- 1999; Reimold et al., 2000). The larger diameter lated to several extensive dyke swarms, and the de- includes the negative gravity anomaly of the Ganyesa creased magnetic susceptibility, presumably caused by Dome and its wider surroundings. However, impact subsequent oxidation of basement rocks involved in structures of over 100 km size would invariably be the crater wall collapse. The occurrence of such a low- characterized by a distinct central gravity high, as magnetic ring of distorted basement is seen also in deeper and denser crustal material would be consid- other impact structures that have formed in crystalline erably uplifted. This has been well illustrated, for rocks (e.g., the Dellen and Mien craters in Sweden, as example, for the Vredefort impact crater (Henkel and discussed by Henkel, 1992). The decreased magnet- Reimold, 1998). In contrast, the geophysical charac- ization is interpreted as the result of oxidation of teristics of the Ganyesa Dome are strikingly similar to ferrimagnetic minerals in the fractured basement. the basement culminations found in the northern Within the low-magnetic ring at Morokweng, remains Baltic Shield (Henkel, 1991). The dominance in the of the externally occurring magnetic dykes can be gravity field of local anomaly sources (such as the seen as isolated anomalies with subdued gradients, ultramafic body) that are unrelated to the effects of indicating a deeper location of the source structure. the impact event is a feature occurring when the The undisturbed dike anomalies have a typical strike dimensions of local geological structures are less than continuity that is lacking inside of the edge of the that of the impact structure, that is, in very large impact structure. craters. This is, for example, seen in the Siljan impact The central, variably magnetic anomaly pattern structure (Juhlin, 1991). The lack of a recognizable observed at the Morokweng crater structure is also a positive gravity anomaly related to the substantial typical feature of impact structures formed in crystal- central uplift of 4 km might be an unexpected feature, line rocks. The causes of this pattern have been but has likewise been noticed in other crater struc- discussed by Henkel (1992) and Pilkington and tures, for example, at Dellen and Siljan. It can be Grieve (1992), and include (1) occurrence of suevite explained only if the crystalline basement before the [as in the Ries crater (Pohl and Angenheister, 1969)] impact extended homogeneously (with respect to and of melt bodies [Mien and Dellen craters (Henkel, density) far deeper than the amount of impact-induced 1992)], (2) remagnetized basement structures [for H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147 145 example, Vredefort (Henkel and Reimold, 1998) and ties of the model are related to the depth of the ring Manson (Plescia, 1996)], or (3) combinations of these basin and the height of the central uplift (which both features. depend on their real petrophysical contrasts), whereas There are other examples of low-magnetization the horizontal dimension of the structure is quite well melt bodies in craters formed in crystalline rocks [for constrained. The model accounts for all observed example, Lappaja¨rvi in Finland (Kukkonen et al., anomaly patterns and, thus, represents a possible 1992)]. The formation of the magnetic effects is related solution regarding the present distribution of rock to the amount of a suitable ferrimagnetic carrier phase types in the Morokweng crater. In Fig. 9c, a possible occurring in the rock (such as magnetite, certain Morokweng crater geological scenario, based on the pyrrhotite modifications, or titanomagnetite). This car- distribution of schematic sources modeled in this rier phase must either exist before the impact or form study, is presented. during the impact, and it must have sufficient coerciv- The lack of a significant gravity high in the center ity to keep a remanent magnetization over geologic of the structure excludes a very large crater diameter time periods. The magnetization of such carrier min- (impacts over 100 km in diameter would inevitably eral phases is created when the material cools through involve uplift of deeper and denser crustal material, its Curie temperature (and represents a thermorema- which is well illustrated in the Vredefort structure). nence), and it would have a remanence direction The model presented here is consistent with a current parallel to the — then — ambient geomagnetic field. diameter of the Morokweng impact structure of 65– In the Morokweng crater, the melt body is low 70 km. magnetic and cannot produce the observed magnetic The remaining central uplift amounts to about 4 anomalies. Instead, these anomalies represent the km; the expected uplift is 0.08Âfinal crater diameter, expression of remagnetized basement. As the sur- that is, 5.2–5.6 km). The central region of the crater rounding granitic basement has fairly low magnetiza- contains scattered, local magnetic anomalies, here tion, with susceptibilities of ca. 0.03 SI (corresponding related to the occurrences of remagnetized basement. to 1% of magnetite), additional magnetic phases may This anomaly pattern is highly characteristic and has have formed. Such new phases are likely to be mainly been found in several other impact structures, like the magnetite formed by shock dissociation of hydrous Ries crater [reversed magnetized suevite, melt and ferromagnesian minerals, such as biotite or hornblende basement (Pohl and Angenheister, 1969)], Mien and (Fel’dman, 1994). Dellen (normal magnetized melt and suevite — Hen- The nature of the strongly magnetic, high-density kel, 1992), and Manson (remagnetized basement — body in the pre-impact basement is not certain. It may Plescia, 1996). The Morokweng impact occurred at represent a fragment of the Molopo Farms Complex the edge of the pre-existing Ganyesa Dome. In the of mafic to ultramafic lithologies, which are wide- model, the orientation of the cover rocks encircling spread just north and northeast of the Morokweng the dome is included. Their gentle outward dip cannot structure (Reichhardt, 1994). Alternatively, the study be a result of the impact process. of borehole KHK-1 from just west of the Morokweng This finding has significant implications for the impact structure (Reimold et al., 1999, 2000) revealed previously (Hart et al., 1997; Koeberl et al., 1997) massive occurrence of mafic volcanics of still un- discussed fact that the Morokweng impact event dated known stratigraphic association. Such material may at 145F2 Ma is contemporaneous with the Jurassic– also be a possible cause of a high-magnetic, high- Cretaceous (J/K) boundary, which is marked by a density anomaly. minor mass extinction in the biological record. In The combined geophysical modeling of the Mo- contrast to the Cretaceous–Tertiary (K/T) boundary, rokweng impact structure accounts for the observed where the stratigraphic record demonstrates a tempo- geological features such as dikes, igneous, and sedi- ral coincidence of a major mass extinction with a mentary units. It also accounts for the obvious impact- major impact event (e.g., see review in Montanari and related structural features, such as the central uplift, Koeberl, 2000), no findings of bona fide impact and for the observed rock physical properties of indicators (shock metamorphic minerals, iridium impact melt and basement. The remaining uncertain- enrichment) have been reported from the J/K boun- 146 H. Henkel et al. / Journal of Applied Geophysics 49 (2002) 129–147 dary to date (e.g., Kudielka et al., 2001). It remains to edged. Marco Andreoli is thanked for his information be discussed what the environmental effects of an on the De Beers borehole. Drs. Elo and Thybo are impact catastrophe of Morokweng (ca. 70 km diam- thanked for their constructive comments on an earlier eter) dimension, in comparison to the demonstrated version of this paper. This is University of the Wit- global catastrophe caused by the K/T Chicxulub watersrand Impact Cratering Research Group Contri- (180–200 km) impact event, may have been. bution No. 29.

7. Conclusions References

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