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The Karoo Basin of South Africa covers an area of 700 000 km2 and has been identified as a possible shale gas reserve. Any evaluation of the shale gas potential of the basin must consider the widespread dolerite dykes and sills. These intrusions were emplaced into the Karoo Supergroup and are well dated at around 183 Ma. Their intrusion triggered the explosive releases of gas in the basin, marked on surface by breccia pipes and hydrothermal vents. This outpouring of gas has been proposed as a significant contributor to global climate change. Research into the three-dimensional interconnected structure of these dolerite sills and dykes and their interaction with the hydrocarbon rich layers in the lower part of the Karoo Supergroup has been limited to localized observations of outcrop, magnetic data, legacy seismic data (from the 1970s) and well core. Here we present an interpreted 65 km long higher-resolution 2D seismic reflection profile across the Karoo Basin, approximately 100 km southeast of Trompsburg. These data were collected in the 1990s and at the time deeper structures along the line interpreted. In this study we focus on the top 0.6 to 2 seconds TWT of the data. The seismic line images the interconnected and cross cutting nature of the dolerite dykes and sills along the profile. We also report possible evidence of a gas escape structure (approximately 2.5 km in diameter at surface) emerging near the edge of a dolerite sill in close proximity to the Whitehill Formation, which is the main target for shale gas exploration. This suggests that gas vents in the eastern Karoo Basin close to Lesotho are due to the release of gas from the carbonaceous shales of the Ecca Group. This is similar to breccia pipes mapped on surface in the western part of the Karoo Basin. This seismic section highlights why dolerite sills and dykes must be considered when evaluating the shale gas potential of the Karoo Basin. We propose that better characterization of the Karoo Basin subsurface by seismic and magnetic studies is necessary prior to any efforts to calculate shale gas reserves.

We apply a Spherical Cap Harmonic Analysis technique on CHAMP satellite data recorded over southern Africa between 2007.0 and 2009.0 epochs, and develop a Southern African Lithospheric Magnetic Model (SALMM) at satellite altitude. The comparative evaluation of the SALMM with the global model MF7 shows a good agreement in the Y and Z field components that are not much contaminated by external field contributions. We use the Z lithospheric field map to confirm the prominent long-wavelength anomalies over the southern African region and its surrounding ocean areas, discussing the underlying geological and tectonic structures of the identified crustal anomalies.

A full three-dimensional (3D) potential field model of the central and southern Bushveld Complex reveals information about the Complex in areas obscured by younger geological cover. Previously, two-dimensional gravity models and a few magnetic models limited to certain sections of the Bushveld Complex have been used to propose geometries for the Rustenburg Layered Suite, especially in the western and eastern lobes. These models were often used to support different emplacement models. Although these models provided valuable information, two-and-a-half-dimensional (2.5D) potential field modelling is not well suited to modelling complex 3D geology. Also, in most cases, only the magnetic or gravity data were modelled, but jointly modelling both data sets better constrains the results, as was shown recently for a 3D model of the northern lobe. Joint 3D modelling of regional gravity and magnetic data combined with published crustal thickness models derived from broadband seismic tomography studies and constrained by density and susceptibility data, geologic mapping, boreholes and seismic reflection data were used to create a 3D model of the central and southeastern sections of the Bushveld Complex, as well as the southern part of the northern lobe. The model shows a complex geometry with thick continuous Rustenburg Layered Suite S in most of the western and southeastern lobes, but less continuous Rustenburg Layered Suite in the eastern lobe. Large domes or thick granites and granophyre in the latter interrupt the continuity of the Rustenburg Layered Suite and the western and eastern lobes are strictly speaking only partially connected in places. However, they are not separate intrusions, but one disconnected by pre-existing and synmagmatic updoming. Three possible feeders were modelled in the northern, western, and south-eastern lobes.

CHAMP satellite and relatively densely spaced ground-based data measured over southern Africa between 2005 and 2010 are combined in a new regional, harmonic spline based, core field model. This new SACFM-2 model is compared to the regional SARM model, which is based only on satellite data, and the global CHAOS-6 model. The results agree well in the vertical (Z) component, with somewhat larger differences in the horizontal components. The Z component and total intensity F are used to investigate the evolution of the South Atlantic Anomaly in this region. The computed maps of main field of the Z component and total intensity F show a steady decrease in the field over the years during the study period, indicating the evolution of the South Atlantic Anomaly over southern Africa and suggesting an increase in the area of this feature.

The time at which plate tectonics were initiated on Earth is unclear. Geochemical analysis of diamonds suggests that recycled oxidized material could have been introduced to the mantle via subduction zones more than 3 billion years ago.

The AfricaArray International Geophysics Field School is a three-week program designed to introduce the complete workflow of a geophysical project. In the first week, participants design a geophysical survey of relevance to a mine, water-resource investigation, or an archaeology study. The design of the course includes considerations of cost and safety and is presented in a competitive manner. The weeklong data-collection phase ensures that participants are familiar with operation of all equipment, including gravity, magnetics, DGPS, resistivity, EM31, EM34, and reflection and refraction seismology. Data are processed in the evening, and safety issues are reviewed the following morning. The final week is spent on campus, interpreting and integrating data to form a coherent model of the subsurface. The participants are selected from three cohorts: University of Witwatersrand geophysics students, SEG Foundation and UNESCO-sponsored African participants, and U. S.-sponsored undergraduates attending as part of a Research Experience for Undergraduates (REU). Instructors include graduate students, resulting in a two-tier training system, as they learn about lecturing, logistics, and field preparation for weeks in advance. The field school has developed a wide-ranging network of geophysical talent throughout Africa, which is starting to seed new field schools.

Geophysical models image the 3D geometry of the mafic portion of the Bushveld Complex north of the Thabazimbi-Murchison Lineament (TML), critical for understanding the origin of the world's largest layered mafic intrusion and platinum group element deposits. The combination of the gravity and magnetic data with recent seismic, MT, borehole and rock property measurements powerfully constrains the models. The intrusion north of the TML is generally shallowly buried (generally 12,000 m, averaging ∼4000 m. A feeder, suggested by a large modeled thickness (>10,000 m) and funnel shape, for Lower Zone magmas could have originated near the intersection of NS and NE trending TML faults under Mokopane. The TML has been thought to be the feeder zone for the entire Bushveld Complex but the identification of local feeders and/or dikes in the TML in the models is complicated by uncertainties on the syn- and post-Bushveld deformation history. However, modeled moderately thick high density material near the intersection of faults within the central and western TML may represent feeders for parts of the Bushveld Complex if deformation was minimal. The correspondence of flat, high resistivity and density regions reflect the sill-like geometry of the Bushveld Complex without evidence for feeders north of Mokopane. Magnetotelluric models indicate that the Transvaal sedimentary basin underlies much of the Bushveld Complex north of the TML, further than previously thought and important because the degree of reaction and assimilation of the Transvaal rocks with the mafic magmas resulted in a variety of mineralization zones.

Here we present a comprehensive depth and thickness map of the main Karoo and Cape Basins using borehole and reflection seismic data. The depth to the Whitehill Formation, which is the focus of current shale gas interest within the Karoo, is also mapped. Change: The deepest part of the basin is in the south, along the northern boundary of the Cape Fold Belt (~4000 m in the southwest Karoo and ~5000 m in the southeast; ~5500 to 6000 m sediment thickness). The Whitehill Formation along this boundary reaches a depth of ~3000 m in the southwest and ~4000 m in the southeast. Limited borehole data in the southeastern Karoo show a broad deepening of the basin here compared to the southwestern Karoo. In the southeast near East London faulting has resulted in deepening of the basin close to the coast, with the Whitehill Formation deepening to over ~5000 km. Seismic and borehole data show that the Cape Supergroup pinches out below the Karoo Basin around Beaufort West and Graaff-Reinet in the southern Karoo (32.6°S for the Bokkeveld and 32.4°S for the Table Mountain Group). The Cape Supergroup reaches thicknesses of around 4 km in the south. The gravity effect of these sediments does not account for the Cape Isostatic Anomaly (CIA) in the southern part of the Karoo Basin near Willowmore and Steytlerville, i.e., an ~45 mGal Bouguer gravity low. A refraction seismic profile over the anomaly shows this region is associated with a large volume of low velocity/density shallow sediments (4.5 m/s2, 2500 kg/m3), as well as a low velocity/density anomaly associated with a normal fault and the Klein Winterhoek Thrust Fault (5.5 m/s2, 2650 kg/m3). These low density shallow sediments are explained by uplift of Karoo and Cape sediments of ~2 km or greater that is evident on Soekor reflection seismic data. This deformation has brought lower density shales (1800 to 2650 kg/m3) of the Ecca Group closer to the surface. These shallower features along with a deeper lower crust in this region (6.5 m/s2, 2900 kg/m3) are interpreted to account for the CIA.

The Bellevue drillcore intersects ~3 km of Main and Upper Zone cumulates in the Northern Limb of the Bushveld Complex. Main Zone cumulates are predominately gabbronorites, with localized layers of pyroxenite and anorthosite. Some previous workers, using bulk rock major, trace and isotopic compositions, have suggested that the Main Zone crystallized predominantly from a single pulse of magma. However, density measurements throughout the Bellevue drillcore reveal intervals that show up-section increases in bulk rock density, which are difficult to explain by crystallization from a single batch of magma. Wavelet analysis of the density data suggests that these intervals occur on length-scales of ~40 to ~170 m, thus defining a scale of layering not previously described in the Bushveld Complex. Upward increases in density in the Main Zone correspond to upward increases in modal pyroxene, producing intervals that grade from a basal anorthosite (with 5% pyroxene) to gabbronorite (with 30–40% pyroxene). We examined the textures and mineral compositions of a ~40 m thick interval showing upwardly increasing density to establish how this type of layering formed. Plagioclase generally forms euhedral laths, while orthopyroxene is interstitial in texture and commonly envelops finer-grained and embayed plagioclase grains. Minor interstitial clinopyroxene was the final phase to crystallize from the magma. Plagioclase compositions show negligible change up-section (average An62), with local reverse zoning at the rims of cumulus laths (average increase of 2 mol%). In contrast, interstitial orthopyroxene compositions become more primitive up-section, from Mg# 57 to Mg# 63. Clinopyroxene similarly shows an up-section increase in Mg#. Pyroxene compositions record the primary magmatic signature of the melt at the time of crystallization and are not an artefact of the trapped liquid shift effect. Combined, the textures and decoupled mineral compositions indicate that the upward density increase is produced by the downward infiltration of noritic magma into a previously emplaced plagioclase-rich crystal mush. Fresh noritic magma soaked down into the crystallizing anorthositic mush, partially dissolving plagioclase laths and assimilating Fe-enriched pore melt. The presence of multiple cycles showing upward increases in density in the Bellevue drillcore suggests that downward magma infiltration occurred episodically during crystallization of the Main Zone.

The source of the Beattie Magnetic Anomaly (BMA) still remains unclear, with several competing hypotheses. Here we add a piece to the puzzle by investigating available potential field data over the anomaly. Filtered magnetic data show the BMA as part of a group of linear magnetic anomalies. As the linear anomaly north of the BMA is associated with exposed supracrustals, migmatites and shear zones within the Natal thrust terranes we assume a similar source for the BMA. This source geometry, constrained by seismic and MT data, fits potential field data over the BMA and other magnetic linear anomalies in the south-central and south-western Karoo. In these models the bodies deepen from ~ 5 km towards the south, with horizontal extents of ~ 20–60 km and vertical extents of ~ 10–15 km. Densities range from 2800 to 2940 kg/m 3 and magnetic susceptibilities from 10 to 100 × 10 − 3 SI. These magnetic susceptibilities are higher than field values from supracrustal rocks (10–60 × 10 − 3 SI) but could be due to the fact that no remanent magnetisation was included in the model. The lithologies associated with the different linear anomalies vary as is evident from varying anomaly amplitudes. The strong signal of the BMA is linked to high magnetic susceptibility granulite facies supracrustals (~ 10–50 × 10 − 3 SI) as seen in the Antarctic, where the mobile belt continued during Gondwana times.

Recent interest in the main Karoo Basin of South Africa has been sparked by the possibility of extensive shale gas resources. Historical reflection seismic, petroleum exploration wells and regional magnetic data are used to better understand the distribution and geometry of dolerite intrusions within the basin that could have impacted the shale reservoir. The lowest concentration of dolerites are found in a region stretching from the southwest to the southeastern part of the basin around the town of Graaff-Reinet. These intrusions are confined to the Beaufort Group, ~1000 m shallower than the shale reservoir. In the southeastern Karoo around Queenstown, 5 to 30 km wide saucer-shaped sills extend down to ~800 m, with dips of between 2° and 8°. Further south, dolerite sheets around Somerset-East extend for over 150 km at dips of between 3° and 13°, and are imaged down to ~5 km. These dips appear to increase closer to the Cape Fold Belt in the south, although there is no correlation between the southern edge of these dolerites (i.e., the dolerite line) and the dip of sediments due to folding. Magnetic data are useful shale gas exploration to detect shallower (

As an additional opportunistic component to the southern African magnetotelluric experiment, natural-source audio-magnetotelluric (AMT) data were acquired during phase IV to investigate the local-scale electric conductivity subsurface structure in the Otjiwarongo and Katima Mulilo regions (Namibia) as an aid in locating the installation points for high-voltage direct current earth electrodes. The study showed that the shallow subsurface of areas containing one measurement site in the Otjiwarongo region and three sites in the Katima Mulilo region have appropriate high conductivities for the optimal placement of the earth electrodes. Both of the AMT surveys are situated close to the edge of the orogenic Neo-Proterozoic Damara mobile belt (DMB). Previous studies all suggest the existence of a highly conductive midcrustal zone, which correlates well with the spatial location of the DMB. Two-dimensional inverse modeling of the Otjiwarongo AMT data confirms the existence of the high-conductive zone at midcrustal depths (10–15 km). The high conductivity of the DMB is explained by the presence of interconnected graphite in the marble units present. The Katima Mulilo inversion results are characterized by a conductive upper crustal layer that does not form part of the DMB conductive belt. It was deduced that at the uppermost subsurface (maximum [Formula: see text]), Kalahari sediments are responsible for the high conductivity observed, whereas at greater depth (up to 6 km), its cause remains enigmatic, albeit the hypothesis of ironstone or graphite being present and causing the observed conductive upper crust.

Earth’s residual geoid is dominated by a degree-2 mode, with elevated regions above large low shear-wave velocity provinces on the core–mantle boundary beneath Africa and the Pacific. The edges of these deep mantle bodies, when projected radially to the Earth’s surface, correlate with the reconstructed positions of large igneous provinces and kimberlites since Pangea formed about 320 million years ago. Using this surface-to-core–mantle boundary correlation to locate continents in longitude and a novel iterative approach for defining a paleomagnetic reference frame corrected for true polar wander, we have developed a model for absolute plate motion back to earliest Paleozoic time (540 Ma). For the Paleozoic, we have identified six phases of slow, oscillatory true polar wander during which the Earth’s axis of minimum moment of inertia was similar to that of Mesozoic times. The rates of Paleozoic true polar wander (

Window-based Euler deconvolution is commonly applied to magnetic and sometimes to gravity interpretation problems. For the deconvolution to be geologically meaningful, care must be taken to choose parameters properly. The following proposed process design rules are based partly on mathematical analysis and partly on experience. 1. The interpretation problem must be expressible in terms of simple structures with integer Structural Index (SI) and appropriate to the expected geology and geophysical source. 2. The field must be sampled adequately, with no significant aliasing. 3. The grid interval must fit the data and the problem, neither meaninglessly overgridded nor so sparsely gridded as to misrepresent relevant detail. 4. The required gradient data (measured or calculated) must be valid,with sufficiently low noise, adequate representation of necessary wavelengths and no edge-related ringing. 5. The deconvolution window size must be at least twice the original data spacing (line spacing or observed grid spacing) and more than half the desired depth of investigation. 6. The ubiquitous sprays of spurious solutions must be reduced or eliminated by judicious use of clustering and reliability criteria, or else recognized and ignored during interpretation. 7. The process should be carried out using Cartesian coordinates if the software is a Cartesian implementation of the Euler deconvolution algorithm (most accessible implementations are Cartesian). If these rules are not adhered to, the process is likely to yield grossly misleading results. An example from southern Africa demonstrates the effects of poor parameter choices.

Gravity models reveal the 3-D extent of the mafic component of the Bushveld Complex, critical for understanding the origin of the world’s largest layered mafic intrusion and largest source of platinum-group elements (PGEs). New density information, broadband seismic data, borehole data and geological discoveries have improved the constraints on the gravity modelling. Furthermore, all of the models published up to now have been done in two or 2.5 dimensions which is not well suited to modelling the complex geometry of the Bushveld intrusion. Constrained three dimensional modelling takes into account effects of variations in geometry and geophysical properties of lithologies, providing better fits to the shape and amplitude of calculated fields. Gravity data reveal subsurface density contrasts to great depths and the significant density contrast between the mafic rocks of the Bushveld Complex and the surrounding granites and sediments, as well as contrasts across the crust–mantle boundary, make gravity modelling ideal for constraining the 3D geometry of the Bushveld Complex. The aim of this paper is to demonstrate the effect of the new constraints and use of full three dimensional modelling on gravity models of the Bushveld intrusion. We remodel previously published models using full three dimensional potential field modelling software to test the existing conceptual models in an equally conceptual way. Including the measured thicker crust underneath the Bushveld Complex necessitates the presence of dense material in the central area between the eastern and western lobes. The simplest way to achieve this is to model the Rustenburg Layered Suite as a single connected intrusion. This is similar to the first geometries suggested for the Bushveld Complex. In addition to these findings, variations in the lower crust and mantle densities also contribute to models of this scale and have to be considered.

Aeromagnetic data clearly delineate the mafic rocks of the economically significant Bushveld Igneous Complex. This is mainly due to the abundance of magnetite in the Upper Zone of the Rustenburg Layered Suite of the Bushveld, but strongly remanently magnetised rocks in the Main Zone also contribute significantly in places. In addition to delineating the extent of the magnetic rocks in the complex, the magnetic anomalies also provide information about the dip and depth of these units. The presence of varying degrees of remanent magnetisation in most of the magnetic lithologies of the Rustenburg Layered Suite complicates the interpretation of the data. The combination of available regional and high resolution airborne magnetic data with published palaeomagnetic data reveals characteristic magnetic signatures associated with the different magnetic lithologies in the Rustenburg Layered Suite. As expected, the ferrogabbros of the Upper Zone cause the highest amplitude magnetic anomalies, but in places subtle features within the Main Zone can also be detected. A marker with strong remanent magnetisation located in the Main Zone close to the contact with the Upper Zone is responsible for very high amplitude negative anomalies in the southern parts of both the eastern and western lobes of the Bushveld Complex. Prominent anomalies are not necessarily related to a specific lithology, but can result from the interaction between anomalies caused by differently magnetised bodies. The magnetic data provided substantial information at different levels of detail, ranging from contacts between zones, and layering within zones, to magnetite pipes dykes and faults that can have an impact on mine planning. Finally, simple modelling of the magnetic data supports the concept of continuous mafic rocks between the western and eastern lobes.

[1] Archean cratons, and the stitching Proterozoic orogenic belts on their flanks, form an integral part of the Southern Africa tectonic landscape. Of these, virtually nothing is known of the position and thickness of the southern boundary of the composite Congo craton and the Neoproterozoic Pan-African orogenic belt due to thick sedimentary cover. We present the first lithospheric-scale geophysical study of that cryptic boundary and define its geometry at depth. Our results are derived from two-dimensional (2-D) and three-dimensional (3-D) inversion of magnetotelluric data acquired along four semiparallel profiles crossing the Kalahari craton across the Damara-Ghanzi-Chobe belts (DGC) and extending into the Congo craton. Two-dimensional and three-dimensional electrical resistivity models show significant lateral variation in the crust and upper mantle across strike from the younger DGC orogen to the older adjacent cratons. We find Damara belt lithosphere to be more conductive and significantly thinner than that of the adjacent Congo craton. The Congo craton is characterized by very thick (to depths of 250 km) and resistive (i.e., cold) lithosphere. Resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during Pan-African magmatism. Graphite-bearing calcite marbles and sulfides are widespread in the Damara belt and account for the high crustal conductivity in the Central Zone. The resistivity models provide new constraints on the southern extent of the greater Congo craton and suggest that the current boundary drawn on geological maps needs revision and that the craton should be extended further south.

Igneous rocks record the direction of the Earth’s magnetic field as they cool through their Curie temperature. The mafic magmas of the 8-km-thick Bushveld Complex of South Africa took 65 k.y. to be emplaced, 180 k.y. to solidify (to 900 °C), and a further 500 k.y. for the entire intrusion to cool below 580 °C, the Curie temperature of magnetite. Once solid, the cooling of this intrusion occurred mainly from the top downward, with slower cooling through its floor. As a result, the upper rocks cooled through their Curie temperature before those at the base; the portion 6 km below the upper contact was the last to reach the Curie temperature. Thus, the intrusion records a mainly top-down sequence of three paleomagnetic reversals starting with N (normal direction). The last two are also recorded from the base of the mafic sequence upward as it cooled through 580 °C later than the top. The lateral variations in thickness of the Bushveld Complex are important in this interpretation, because thinner sections cooled more quickly. Hence, reversals do not always correlate with stratigraphy. Specific reversals provide a cooling marker horizon that may crosscut the stratigraphic layering. The interpretation of the order and number of paleomagnetic reversals presented here differs from previous interpretations that envisage the oldest paleomagnetic directions to be recorded sequentially from the base upward, and has implications for the interpretation of paleomagnetic results from all thick intrusions, mafic and felsic.

a b s t r a c t Despite many years of work, a convincing evolutionary model for the Limpopo belt and its geometri- cal relation to the surrounding cratons is still elusive. This is partly due to the complex nature of the crust and upper mantle structure, the significance of anatectic events and multiple high-grade meta- morphic overprints. We use deep probing magnetotelluric data acquired along three profiles crossing the Kaapvaal craton and the Limpopo belt to investigate the crust and upper mantle lithospheric struc- ture between these two tectonic blocks. The 20-30 km wide composite Sunnyside-Palala-Tshipise-Shear Zone is imaged in depth for the first time as a sub-vertical conductive structure that marks a fundamental tectonic divide interpreted here to represent a collisional suture between the Kaapvaal and Zimbabwe cratons. The upper crust in the Kaapvaal craton and the South Marginal Zone comprises resistive granit- oids and granite-greenstone lithologies. Integrating the magnetotelluric, seismic and metamorphic data, we propose a new tectonic model that involves the collision of the Kaapvaal and Zimbabwe cratons ca. 2.6 Ga, resulting in high-grade granulite Limpopo lithologies. This evolutionary path does not require a separate terrane status for each of the Limpopo zones, as has been previously suggested.

The structure of the crust in the environs of the Bushveld Complex has been investigated by jointly inverting high-frequency teleseismic receiver functions and 2–60s period Rayleigh wave group velocities for 16 broadband seismic stations located across the Bushveld Complex. Group velocities for 2–15s periods were obtained from surface wave tomography using local and regional events, while group velocities for 20–60s periods were taken from a published model. 1-D shear wave velocity models obtained for each station show the presence of thickened crust in the center of the Bushveld Complex and a region at the base of the crust where shear wave velocities exceed 4.0km/s. The shear wave velocity models also suggest that velocities in some upper crustal layers may be as high as 3.7–3.8km/s, consistent with the presence of mafic lithologies. These results favor a continuous-sheet model for the Bushveld Complex in which the outcropping mafic layers of the western and eastern limbs are continuous at depth beneath the center of the complex. However, detailed modeling of receiver functions at one station within the center of the complex indicates that the mafic layering may be locally disrupted due to thermal diapirism triggered by the emplacement of the Bushveld Complex or thermal and tectonic reactivation at a later time.

SUMMARY We discuss a recent publication by Tedla et al., which presents a crustal thickness map of Africa determined by Euler deconvolution of gravity. In our comment, we first outline the limitations of the data set used by the authors and the deleterious effects of working with free air gravity. Next, we discuss the grievous parameter choices (grid interval, Euler window size, structural index) and incorrect assumptions made by the authors and their serious effects on the results. Finally, we demonstrate with two examples why their results are not helpful in understanding crustal thickness and consequently the tectonic history. This is not specific to Africa, but also true for the application to other parts of the Earth.

A number of circular negative magnetic anomalies (up to 8 km across) exist within the area encompassed by the western Bushveld Complex (150 km by 100 km) on the Kaapvaal craton in South Africa. They are covered by up to 700 m of sedimentary rocks of the Karoo Supergroup, which could not produce these anomalies. Exploration boreholes into one of these magnetic anomalies revealed a hidden volcanic complex, called the Elandskraal Volcano. One of these boreholes intersected an olivine–magnetite–apatite clinopyroxenite body, which we studied. Ages on apatite–clinopyroxene pairs using Sm–Nd dating techniques yield a poorly constrained age of 1207 ± 200 Ma, because there is little variation in Sm–Nd between all the samples. This age correlates with the Pilanesberg Alkaline Province that spans the time period from 1430 to 1200 Ma, but almost all age determinations from this suite give very large errors. Three other clinopyroxenite bodies closely related to the Pilanesberg Alkaline Province have been reported, but no geochemical data have been presented. Our mineral and whole-rock geochemical data permit an interpretation of the genesis of these clinopyroxenite bodies. The basaltic lavas of the Elandskraal Volcano are extremely unusual in having very high TiO 2 (over 7 wt%), high Fe 2 O 3 (16–21 wt%) and high incompatible element contents. Modelling the crystallization sequence using MELTS of the more magnesian lava compositions yields olivine, clinopyroxene, magnetite and apatite as liquidus phases within a temperature interval of less than 30 °C, which matches that observed in the clinopyroxenite body. In our samples the Mg/(Mg + Fe) value for the mafic minerals and incompatible trace elements abundances in clinopyroxene are consistent with crystallization from these unusual basaltic compositions. Contrasts with other clinopyroxenite bodies in the Phalaborwa Complex and the Bushveld Complex are documented in terms of rock associations and mineral compositions (both major and trace elements).

SUMMARY Hotspot-ridge interactions lead to the dynamic evolution of divergent plate boundaries, including propagating and overlapping ridge segments. In southern Iceland, the Eastern Volcanic Zone (EVZ) formed approximately 2–3 Ma ago during the last eastward ridge jump from the Western Volcanic Zone (WVZ), and is propagating to the southwest into the Tertiary lithosphere of the Eastern Volcanic Flank Zone. North America–Eurasia relative plate motion is partitionedbetweentheEasternandWVZs.Weutilizenewterrestrial(dry-tilt)andspace(GPS and InSAR) geodetic data to investigate the nature of plate spreading and magma–tectonic interaction at the southern terminus of this propagating ridge system. We present a new GPS derived horizontal velocity field covering the period 1994–2006, new InSAR analyses for the periods 1993–2000 and 2003–2007, and models of plate spreading across this region. The velocity field indicates horizontal surface deformation consistent with plate spreading across and the propagation of the EVZ. The dry-tilt and InSAR data show transient deformation signals associated with magmatic processes. The velocity field is corrected for these transient deformation sources in order to investigate the nature of secular plate motion. Our model results indicate a decrease in spreading rate from northeast (15 mm yr −1 ) to southwest (9 mm –

Over the past 30 years, the water available in boreholes at the Dayspring Children's Village has slowly diminished to the point where the school's viability is threatened. In this same time period, a large stand of eucalyptus and yellow wattle trees has become well established. These trees are known to consume large quantities of water and we are investigating the effect of these trees on the local groundwater hydrology. Many types of geophysical and hydrological data—including gravity, magnetic, electrical resistivity, electromagnetic (EM) and seismic—are being acquired to understand the regional geology, seasonal changes and ultimately the impact of the trees on the local hydrology. Electrical resistivity data collected at the end of the dry season and at the end of the rainy season are used to define seasonal changes of a near-surface aquifer. By quantifying the effect of these trees on the hydrology, predictive recommendations on the benefits of tree removal can be made to government.

Palaeomagnetic data from the well-dated 2060.6 + 0.5 Ma Phalaborwa Complex in South Africa (Kaapvaal Craton) are of excellent quality. High unblocking components are carried by magnetite and single polarity remanence directions (mean declination 5.08, inclination 57.38, a95 1⁄4 5.28) yield a palaeomagnetic pole (latitude 27.78N, longitude 35.88E, A951⁄4 6.68) that overlaps with existing poles from the near coeval 2054.4 + 1.3 Ma Bushveld Complex. The Phalaborwa and Bushveld complex poles, along with poles from the well-dated Vredefort impact (2023 + 4 Ma) and Post-Waterberg Dolerites (1874.6 + 3.9 Ma), define the most reliable poles for the Kaapvaal Craton during this time interval (c. 2060–1875 Ma) and witness low rates of Mid-Palaeoproterozoic apparent polar wander. Poorly dated NE–NNE-trending dyke swarms that intrude the Phalaborwa and Bushveld complexes both yield dual-polarity remanence components that share a common mean at the 95% confidence level. Primary palaeomagnetic poles (Phalaborwa dykes pole latitude 7.68, longitude 12.18, A95 1⁄4 11.88; Bushveld dykes pole latitude 12.68, longitude 24.18, A95 1⁄4 10.88) suggest that they are of the same age as the Post-Waterberg dolerites (c. 1875 Ma). They could also be as old as the Phalaborwa and Bushveld Complexes, however; high-precision geochronology is required to resolve this issue and to enlarge the number of Palaeoproterozoic key poles for the Kaapvaal Craton. Pangea, the youngest and best-documented supercontinent on Earth, formed at the end of the Palaeozoic era. In the deep past, however, Neoproterozoic (Rodinia), Palaeoproterozoic (Columbia) and Archaean (Vaalbara) supercontinents/continents have been postulated (e.g. Hoffman 1991; Dalziel 1992; Torsvik et al. 1996; Cheney 1996; Zegers et al. 1998; Rogers & Santosh 2002; Zhao et al. 2002; Meert & Torsvik 2003; Torsvik 2003; Li et al. 2008; de Kock et al. 2009; Evans 2009). However, the timing for assembly/dispersal and continental geometry of these older supercontinents/continents differs in almost every paper. The Archaean-aged Vaalbara continent, hypothetically composed of the Kaapvaal (South Africa) and Pilbara (West Australia) cratons, is no exception and we show three different constellations (Fig. 1a) that can be seen in the literature. Vaalbara may have existed between 2.8 and 2.1 Ga (Cheney 1996; Zegers et al. 1998; Strik et al. 2003; de Kock et al. 2009) but Zhao et al. (2002) argue that Vaalbara was also intact at 2.0–1.8 Ga and became the site of two major orogenic events. The first event is that of the Limpopo mobile belt (2.0–1.9 Ga), which formed as a result of the collision between the Kaapvaal and Zimbabwe cratons (Zeh et al. 2009). The second orogenic event resulted in the formation of the Capricorn Orogen, in which the Pilbara and Yilgarn cratons were sutured together. These orogens, together with many other 2.1–1.8 Ga orogens (Zhao et al. 2002), were argued to be the result of a world-wide amalgamation of cratonic blocks to form the Columbia supercontinent (Fig. 1b). In order to test the hypothesis of the Columbia supercontinent, high-precision geochronological and palaeomagnetic data from many Palaeoproterozoic cratons are urgently required. In this account we report new palaeomagnetic data from the c. 2.06 Ga Phalaborwa Complex and dykes cutting both the Phalaborwa and Bushveld (c. 2.05 Ga) complexes in South Africa, and summarize the most From: Van Hinsbergen, D. J. J., Buiter, S. J. H., Torsvik, T. H., Gaina, C. & Webb, S. J. (eds) The Formation and Evolution of Africa: A Synopsis of 3.8 Ga of Earth History. Geological Society, London, Special Publications, 357, 9–26. DOI: 10.1144/SP357.2 0305-8719/11/$15.00 # The Geological Society of London 2011. reliable palaeomagnetic poles (c. 2.06–1.85 Ga) from the Kaapvaal/Kalahari Craton. There are no reliable 2.0 Ga poles from the Pilbara Craton (or Australia at large), and hence proposed connections between the Kaapvaal and Pilbara cratons (Vaalbara continent) remain unproven at this time. A comprehensive global analysis and aspects of the Columbia supercontinent will be dealt with elsewhere. Phalaborwa and Bushveld The Phalaborwa Complex (Harmer 2000) outcrops in an elongated kidney shape in the Limpopo Province in northern South Africa (Fig. 2). The Complex was intruded into Archaean basement gneisses at 2060.6 + 0.5 Ma (U–Pb baddeleyite; Reischmann 1995) in several alkaline cycles, which emplaced a suite of rocks ranging from ultramafic to peralkaline in character. The first phase, the intrusion of a massive ultramafic pipe-like body, consists mainly of pyroxenites. This was followed by an alkaline phase in which the surrounding syenite plug-like bodies were emplaced. At a later stage, foskorite (olivine þ magnetite þ apatite þ phlogopite rock) and banded carbonatite of the central pegmatoid body were emplaced. The final phase saw the intrusive injection of the transgressive carbonatite (PMC 1976). The Phalaborwa Complex is nearly coeval with the Bushveld Igneous Complex (2054.4 + 1.3 Ma; Scoates & Friedman 2008), the world’s largest layered intrusion. Regional aeromagnetic data reveal that northern South Africa is riddled with dykes (Fig. 3). Unfortunately, very little is known about the age of those occurring in the Phalaborwa and Bushveld Pilbara Plbara

DOUWE J. J. VAN HINSBERGEN1,2*, SUSANNE J. H. BUITER1,2,3, TROND H. TORSVIK1,2,3,4, CARMEN GAINA1,2,3 & SUSAN J. WEBB4 Physics of Geological Processes, University of Oslo, Sem Saelands vei 24, NO-0316 Oslo, Norway Center for Advanced Study, Norwegian Academy of Science and Letters, Drammensveien 78, 0271 Oslo, Norway Centre for Geodynamics, Geological Survey of Norway (NGU), Leiv Eirikssons vei 39, 7491 Trondheim, Norway School of Geosciences, University of the Witwatersrand, WITS 2050 Johannesburg, South Africa

The three-dimensional shapes of mafic layered intrusions have to be inferred from surface outcrops, in some cases aided by drilling and/or geophysical data. However, geophysical models are often equivocal. For the 2.06 Ga Bushveld Complex of South Africa, early geological models proposed a shape of a single, gently inward dipping lopolith. Subsequent resistivity and gravity data were interpreted to suggest that the eastern and western limbs were discrete, dipping wedge-shaped intrusions separated by ~150 km. A more recent gravity model that takes crustal flexure into account allows continuity and the reversal to the original model. Distinguishing between these possibilities is difficult from surface-based studies because the central regions of the Complex are obscured by large volumes of younger granites and sedimentary/volcanic cover rocks. Here, we describe xenoliths from the Cretaceous Palmietgat kimberlite pipe, located mid-way between the exposed western and eastern lobes of the Complex. They are chromite-bearing feldspathic pyroxenites considered equivalent to those of the typical outcropping Critical Zone of the Bushveld Complex. This result provides strong support for a regionally interconnected Bushveld Complex, implying its emplacement as a single sill-like body. Confirming the continuity of the Bushveld Complex greatly expands exploration opportunities and implies that other layered mafic intrusions could have similar geometry.

A 1400 km-long, 2-D magnetotelluric (MT) profile, consisting of 69 sites at 20 km intervals, across the western part of the Archaean Kaapvaal Craton, the Proterozoic Rehoboth Terrane and the Late Proterozoic/Early Phanerozoic Ghanzi-Chobe/Damara Belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern African lithosphere. The lithospheric structures of the Rehoboth Terrane and Ghanzi-Chobe/Damara Belt have not been imaged previously by geophysical methods. Temperature is the primary control on the resistivity of mantle minerals, and the MT derived lithospheric thicknesses therefore provide a very reasonable proxy for the “thermal” thickness of the lithosphere (i.e., the thickness defined by the intersection of a conductive geotherm with the mantle adiabat), allowing approximate present-day geotherms to be calculated. The work indicates the following present-day average lithospheric thicknesses, to a precision of about ± 20 km, for each of the terranes traversed (inferred geotherms in brackets): Eastern Kimberley Block of the Kaapvaal Craton 220 km (41 mW m− 2), Western Kimberley Block 190 km (44 mW m− 2), Rehoboth Terrane 180 km (45 mW m− 2) and Ghanzi-Chobe/Damara Belt 160 km (48 mW m− 2). A clear relationship between the electrical resistivity structure of the lithosphere and the tectonic stabilisation-age of the terrane is evident. Good agreement between the inferred present-day lithospheric geotherms and surface heat flow measurements indicate the latter are strongly controlled by variations in lithospheric thickness. A significant difference in lithospheric thickness is observed between the Eastern and Western Kimberley blocks, and is consistent with previous seismic tomography images of the Kaapvaal Craton. The present-day lithospheric thickness, and reduced depth extent into the diamond stability field, accounts for the absence of diamondiferous kimberlites in the Gibeon and Gordonia kimberlite fields in the Rehoboth Terrane. Previously published mantle xenolith P–T arrays from the Gibeon, Gordonia and Kimberley fields, however, suggest that the Rehoboth Terrane had equilibrated to a cooler conductive palaeo-geotherm (40–42 mW m− 2 ), very similar to that of Eastern Kimberley Block of the Kaapvaal Craton, at some time prior to the Mesozoic eruption of the kimberlites. The timing and nature of both the thermal equilibration of the Rehoboth Terrane, and the subsequent lithospheric heating/thinning event required to account for its present-day lithospheric structure, are not well constrained. A model consisting of the penetration of heat transporting magmas into the lithosphere, with associated chemical refertilisation, at an early stage of Mesozoic thermalism appears to be the most plausible model at present to account for both the present-day lithospheric structure of the Rehoboth Terrane and an earlier, cooler palaeo-geotherm. Some problems, however, remain unresolved in terms of the isostatic response of the model. Based on a compilation of xenocryst Cr/Ca-in-pyrope barometry observations, the extent of depleted mantle in the Rehoboth Terrane is found to be significantly reduced with respect to the Eastern Kimberley Block: 117 km versus 138–167 km. It appears most likely that the depletion depth in both terranes, at least in the vicinity of kimberlite eruption, is explained by refertilisation of the lower lithospheric mantle.

SUMMARY The Kaapvaal Craton (South Africa) was the host of several major magmatic events around 2000 Ma, including the Bushveld Complex, the world's largest known layered mafic intrusion (∼0.5–1 × 106 km3). The Bushveld Complex has been the subject of numerous palaeomagnetic studies, which yielded a large spread in palaeomagnetic pole positions for the different Zones, and interpreted to indicate that the Bushveld Complex was emplaced and cooled over a time span of ∼50 million years. New palaeomagnetic data collected from 100 sites (996 drill-cores) from all Zones of the Rustenburg Layered Suite of the Bushveld Complex, yield exceptional palaeomagnetic results with high unblocking (HB) components carried by magnetite. Comparable palaeomagnetic poles from all Zones (mean pole: latitude = 19.2°N, longitude = 030.8°E, A95 = 5.8°) [correction made after online publication 22 September 2009: the mean pole values have been corrected] eliminates the previously noted large spread in poles, and this observation concurs with precise age data that constrain the time period of emplacement of the Bushveld Complex to a few million years at around 2054 Ma. Bedding-corrected HB components from all zones produced better directional groupings, which together with at least seven reversals, strongly points to a primary magnetic signature. This imply that cooling of the Bushveld Complex below the blocking-temperature of magnetite (

Southern Africa, particularly the Kaapvaal Craton, is one of the world's best natural laboratories for studying the lithospheric mantle given the wealth of xenolith and seismic data that exist for it. The Southern African Magnetotelluric Experiment (SAMTEX) was launched to complement these databases and provide further constraints on physical parameters and conditions by obtaining information about electrical conductivity variations laterally and with depth. Initially it was planned to acquire magnetotelluric data on profiles spatially coincident with the Kaapvaal Seismic Experiment, however with the addition of seven more partners to the original four through the course of the experiment, SAMTEX was enlarged from two to four phases of acquisition, and extended to cover much of Botswana and Namibia. The complete SAMTEX dataset now comprises MT data from over 730 distinct locations in an area of over one million square kilometres, making SAMTEX the largest regional-scale MT experiment conducted to date. Preliminary images of electrical resistivity and electrical resistivity anisotropy at 100 km and 200 km, constructed through approximate one-dimensional methods, map resistive regions spatially correlated with the Kaapvaal, Zimbabwe and Angola Cratons, and more conductive regions spatially associated with the neighbouring mobile belts and the Rehoboth Terrane. Known diamondiferous kimberlites occur primarily on the boundaries between the resistive or isotropic regions and conductive or anisotropic regions. Comparisons between the resistivity image maps and seismic velocities from models constructed through surface wave and body wave tomography show spatial correlations between high velocity regions that are resistive, and low velocity regions that are conductive. In particular, the electrical resistivity of the sub-continental lithospheric mantle of the Kaapvaal Craton is determined by its bulk parameters, so is controlled by a bulk matrix property, namely temperature, and to a lesser degree by iron content and composition, and is not controlled by contributions from interconnected conducting minor phases, such as graphite, sulphides, iron oxides, hydrous minerals, etc. This makes quantitative correlations between velocity and resistivity valid, and a robust regression between the two gives an approximate relationship of V s [m/s] = 0.045 * log(resistivity [ohm m]) + 4.5.

Geoscientists from across Africa, the United States, and Europe are building a pan-African initiative, AfricaArray, to strengthen geoscience education and research programs in Africa and to create an Africa-to-Africa academic support system. Universities, government agencies, and exploration companies are benefiting from the establishment of AfricaArray training programs coupled to a network of geophysical observatories spanning the continent. The observatory network provides support to the training programs and fosters community building by catalyzing collaborative projects and encouraging data sharing.

Seismic data provide a unique opportunity to understand the subsurface structure of dolerite sills that have intruded the Karoo Basin at multiple levels. These sills have intruded close to the Whitehill Formation at several locations within the basin, and may impact shale gas exploration of this horizon. In the southeastern Karoo Basin, around Queenstown, seismic data reveal the presence of 5–30 km wide saucer-shaped sills with vertical thickness of up to ~270 m each, and dips of between 2° and 8° at shallow stratigraphic levels. Farther south, dolerite sheets near Somerset-East are imaged down to a depth of ~5 km and extend for over 150 km, with dips of between 3° and 13°. These dips increase closer to the adjacent Cape Fold Belt in the south. Around Lesotho, at the highest stratigraphic levels, intrusions are dominated by dykes and fluid vents that are evident on seismic data as regions of diffused reflectivity. The lowest concentration of dolerite is found in a region in the south-central part of the basin around the town of Graaff-Reinet. Here intrusions are confined to the Beaufort Group, ~ 1000 m above the shale reservoir. This change in dolerite distribution will influence the location of shale gas exploration.

Peninsular India was assembled into a continental block c. 3 million km2 in area as a result of collisions throughout the length of a 4000 km long S-shaped mountain belt that was first recognized from the continuity of strike of highly deformed Proterozoic granulites and gneisses. More recently the recognition of a variety of tectonic indicators, including occurrences of ophiolitic slivers, Andean-margin type rocks, a collisional rift and a foreland basin, as well as many structural and isotopic age studies have helped to clarify the history of this Great Indian Proterozoic Fold Belt. We here complement those studies by considering the occurrence of deformed alkaline rocks and carbonatites (DARCs) in the Great Indian Proterozoic Fold Belt. One aim of this study is to test the recently published idea that DARCs result from the deformation of alkaline rocks and carbonatites (ARCs) originally intruded into intra-continental rifts and preserved on rifted continental margins. The suggestion is that ARCs from those margins are transformed into DARCs during continental, or arc–continental, collisions. If that idea is valid, DARCs lie on rifted continental margins and on coincident younger suture zones; they occur in places where ancient oceans have both opened and closed. Locating sutures within mountain belts has often proved difficult and has sometimes been controversial. If the new idea is valid, DARC distributions may help to reduce controversy. This paper concentrates on the Eastern Ghats Mobile Belt of Andhra Pradesh and Orissa, where alkaline rock occurrences are best known. Less complete information from Kerala, Tamil Nadu, Karnataka, West Bengal, Bihar and Rajasthan has enabled us to define a line of 47 unevenly distributed DARCs with individual outcrop lengths of between 30 m and 30 km that extends along the full 4000 km length of the Great Indian Proterozoic Fold Belt. Ocean opening along the rifted margins of the Archaean cratons of Peninsular India may have begun by c. 2.0 Ga and convergent plate margin phenomena have left records within the Great Indian Proterozoic Fold Belt and on the neighbouring cratons starting at c. 1.8 Ga. Final continental collisions were over by 0.55 Ga, perhaps having been completed at c. 0.75 Ga or at c. 1 Ga. Opening of an ocean at the Himalayan margin of India by c. 0.55 Ga removed an unknown length of the Great Indian Proterozoic Fold Belt. In the southernmost part of the Indian peninsula, a line of DARCs, interpreted here as marking a Great Indian Proterozoic Fold Belt suture, can be traced within the Southern Granulite Terrain almost to the Achankovil-Tenmala shear zone, which is interpreted as a strike-slip fault that also formed at c. 0.55 Ga.

We present a large database of geophysical, petrological and mineralogical measurements for the ~3000 m Bellevue borehole through the entire Upper Zone (UZ) and about half of the Main Zone (MZ) of the Northern Lobe of the Bushveld Complex, South Africa. Magnetic susceptibilty readings were taken every 2 cm (n = 109,360) and densities were measured on average every 1.7 m (n = 2252). Petrographic data and microprobe analyses (n = 14,160) were obtained for plagioclase, mafic silicates, Fe-Ti oxides, amphibole and biotite in 502 samples throughout the entire sequence of layered mafic cumulates. The Bellevue UZ, as marked by the first appearance of cumulus magnetite, is ~1190 m thick (corrected for mean dip of 17.5°), which is less than UZ thicknesses in the Eastern and Western Lobes. A prominent 4 m thick pyroxenite horizon occurs ~390 m below the UZ-MZ boundary, but we show on the basis of mineralogy that this horizon cannot be correlated with the well known Pyroxenite Marker (PM) of the Eastern and Western Bushveld Complex. If the PM is indeed absent in the Northern Lobe, then a substantial portion, perhaps 500 m of the uppermost MZ may be missing; possible causes include non-deposition ( e.g. due to syn-magmatic upwarping or diapirism) or removal (e.g. due to emplacement of UZ magmas). The Bellevue drillcore penetrated only about half of the MZ (total dip-corrected thickness ~1270 m), and the lowermost ~200 m contains unusual olivine-bearing (or troctolitic) horizons that are atypical of MZ rocks elsewhere in the Bushveld Complex. These troctolites have mineral compositions as primitive as those of the upper Critical Zone (CZ), suggesting that they might represent a sliver of CZ rocks dismembered by intrusion of MZ magmas. Alternatively, they may represent an intrusive sill of syn- or post-Bushveld age, or merely a mineralogically unusual horizon in otherwise typical MZ lithologies. Mineral compositions show broad normal fractionation upwards, with plagioclase An78→21, opx (and inverted pigeonite) En 80→26, cpx Mg 86→27, olivine Fo 78→74 (in the lowermost troctolitic horizon) and Fo 59→06 (in the UZ olivine ferrodiorites). There are, however, numerous prominent reversals and discontinuities in mineral compositions, some of which are likely related to magma additions. The extensive dataset of mineral compositions allows the estimation of a new fractionation trend for the Bushveld Complex. On an En-An diagram, the Bushveld trend is shifted toward more An-rich plagioclase at equivalent Mg# of coexisting pyroxenes relative to those for Kiglapait or Skaergaard. This is attributed to the relative paucity in Bushveld of augite, which has a high fractionating power for Ca/Na in evolving liquids. Magnetic susceptibility data clearly reveal the presence of the UZ-MZ boundary. MZ cumulates have susceptibilty values

SUMMARY We report palaeomagnetic and 40Ar/39Ar age data for dykes that intrude the ∼2 Ga eastern Bushveld Igneous Complex (BIC). The dykes were previously assumed to be of Karroo age (Jurassic ∼ 180 Ma) based on their NE–SW orientation. Palaeomagnetic data (pole position 8.7°N, 22°E; dp/dm= 18/20.6°), however, clearly demonstrate that these dykes are Precambrian in age, either ∼1.9 Ga and close to the Early Proterozoic Bushveld age, or 1649 ± 10 Ma based on 40Ar/39Ar plagioclase laser fusion ages from one of the dykes. Both normal and reverse polarity dykes are identified, and a positive reversal test together with a semi-conclusive contact test attests to a primary magnetization. If the 40Ar/39Ar age represents a primary cooling age then palaeomagnetic poles from South Africa (Kalahari) at ∼1.9 to ∼2 Ga and ∼1650 Ma are virtually identical, and suggest an apparent polar wander loop; alternatively, the Kalahari Craton drifted from high southerly (>50°) to high northerly latitudes (or vice versa) during this interval. Conversely, if we assign a ∼1.9 Ga age for dyke emplacement as suggested from a comparison with Kalahari palaeomagnetic poles (e.g. Waterberg–Soutpansberg pole), the ∼1650 Ma 40Ar/39Ar age must relate to a thermal disturbance that did not erase the primary magnetic signature.

SUMMARY High coercivity but low unblocking (LB) temperature components (∼80–100 °C) demonstrate that goethite is the principal carrier of the natural remanent magnetization (NRM) in Jurassic and Cretaceous limestone from the Jaisalmer basin (Rajasthan, NW India). Goethite-bearing components (declination = 359.9°, inclination = 46.0°, α95= 4.0°, N = 10 sites) plot close to the present day field direction. High unblocking (HB) components (declination = 172.0°, inclination =−45.6°, α95= 9.1°, N = 7 sites) are carried by haematite, mostly with reverse polarity directions, but undoubtedly not primary. This remagnetization event, mostly within a period of reverse magnetic polarity, occurred after 35 Ma and before 780 000 yr (the last known reversal). Only palaeomagnetic data from a single Cretaceous site (declination = 302.0°, inclination =−47.2°, α95= 14.0°, N = 10 samples) has potentially preserved a primary magnetization (carried by normal polarity magnetite); if correct, this site substantiates a mid-southerly palaeolatitude for India during mid-Aptian times (∼117 Ma).

Recent gravity modelling of the Bushveld Complex indicates that the western and eastern limbs of the Bushveld Complex are connected at depth. The model predicts a downwarp in the Moho beneath the Bushveld Complex, ensuring observed Airy isostatic balance is achieved. By constraining a new Bouguer gravity model with published Vibroseis results, crustal thicknesses determined using the receiver function method, and seismic velocity modelling of the crust from the Southern African Seismic Experiment; we demonstrate that the connected model of the Bushveld Complex is consistent with all available data. Crustal thicknesses determined from receiver functions indicate that the depth to the Moho thickens from a value of ~35 to ~40km in the southern Kaapvaal craton to ~50km beneath the central region of the Bushveld Complex. This seismologically determined Moho varies significantly from that calculated from Airy isostatic balance based solely on topography as a load in this region. The corresponding crustal velocity model, determined from inverting the receiver function results for Bushveld Complex stations, also indicates a thick crust and delimits a ~6km thick high velocity zone in the upper 10km of crust attributed to the presence of the Bushveld Complex. Comparison of the seismic crustal model with drill core data on the mafic rocks of the Bushveld Complex suggests a correspondence between high seismic velocities and high densities in the upper crust. Both the gravity model and the seismological results imply a density contrast of about 0.30mg/m3 at the crust/mantle boundary beneath the Bushveld Complex. We also find that the Moho transition beneath the Bushveld Complex is significantly broader than that beneath the rest of the Kaapvaal craton, outside of the Limpopo Belt. By constraining the modelling of the gravity data with these seismological results, outcropping geology and published Vibroseis profiles, we show that the dense mafic units of the western and eastern Bushveld Complex can be interpreted as having originally been emplaced as a connected sheet (or series of connected sheets), which has subsequently been deformed and faulted. The seismological results of the Kaapvaal project support the interpretation of a connected Bushveld Complex.

Summary Geophysical data have been instrumental in the exploration and exploitation of the Bushveld Complex, South Africa. Surprising new economic discoveries are still being made, for example in the Northern Lobe, and major advances in exploration and analytical tools promise many more. We are still in the first-order characterization phase of understanding large igneous provinces and there is much to be learned on a wide range of topics. Little work has been published on linking borehole geophysical measurements with extensive geophysical data sets.

Analysis of susceptibility and density data from a borehole in the Bushveld igneous complex in South Africa was performed using statistical tools such as power spectra, wavelet analysis, and windowed histogram plots. This analysis has revealed significant patterns and cycles that correlate both with rock type and subtle layering within and between units. These results can now be used to test models of layering processes.

The formation of Archean crust appears to in- volve processes unique to early earth history. Initial results from receiver function analysis of crustal structure beneath 81 broadband stations deployed across southern Africa re- vealsignicantdierences inthenatureof thecrustandthe crust-mantle boundary between Archean and post-Archean geologic terranes. With the notable exception of the colli- sional Limpopo belt, where the crust is thick and the Moho complex, the crust beneath undisturbed Archean craton is typically thin ( 35{40 km), unlayered, and characterized byastrongvelocitycontrastacrossarelativelysharpMoho. Thiscrustalstructurecontrastsmarkedlywiththatbeneath post-ArcheanterranesandbeneathArcheanregionsaected bylarge-scaleProterozoicevents(theBushveldcomplexand the Okwa/Magondi belts), where the crust tends to be rel- atively thick (45{50 km) and the Moho is complex.

The mafic layered rocks of the Bushveld Complex are 6–8 km thick and crop out over an area of 65,000 km 2 . Previous interpretations of the Bouguer gravity anomalies suggested that the intrusion consisted of two totally separate bodies. However, the mafic sequences in these arcuate western and eastern limbs are remarkably similar, with at least six petrologically distinctive layers and sequences being recognisable in both limbs. Such similarity of sequences in two totally discrete bodies 200–300 km apart is petrologically implausible, and it is suggested that they formed within a single lopolithic intrusion. All previous Bouguer gravity models failed to consider the isostatic response of the crust to emplacement of this huge mass of mafic magma. Isostatic adjustment as a result of this intrusion would have caused the base of the crust to be depressed by as much as 6 km. With this revised whole crustal model, it becomes possible to construct a gravity model, consistent with observed data, which includes a 6 km-thick sequence of mafic rocks connecting the western and eastern limbs of the Bushveld Complex. The exact depth at which the mafic rocks of the Bushveld Complex lie in the centre of the structure cannot be constrained by the gravity data. Such a first-order model is an approximation, because there have been subsequent deformation and structural readjustments in the crust, some of them probably related to the emplacement of the Bushveld Complex. Specifically, the observed geometry of the rocks around the Crocodile River, Dennilton, Marble Hall and Malope Domes suggests that major upwarping of the crust occurred on a variety of scales, triggered by emplacement of the Bushveld Complex.