Crustal structure beneath southern Africa and its implications for the formation and evolution of the Kaapvaal and Zimbabwe cratons,

Abstract. The formation of Archean crust appears to involve processes unique to early earth history. Initial results from receiver function analysis of crustal structure beneath 81 broadband stations deployed across southern Africa reveal significant differences in the nature of the crust and the crust-mantle boundary between Archean and post-Archean geologic terranes. With the notable exception of the collisional 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 by a strong velocity contrast across a relatively sharp Moho. This crustal structure contrasts markedly with that beneath post-Archean terranes and beneath Archean regions affected by large-scale Proterozoic events (the Bushveld complex and the Okwa/Magondi belts), where the crust tends to be relatively thick (∼ 45-50 km) and the Moho is complex

Aeromagnetic, Gravity, and Differential Interferometric Synthetic Aperture Radar Analyses Reveal the Causative Fault of the 3 April 2017 M\u3csub\u3ew\u3c/sub\u3e 6.5 Moiyabana, Botswana, Earthquake

On 3 April 2017, a Mw 6.5 earthquake struck Moiyabana, Botswana, nucleating at \u3e20 km focal depth within the Paleoproterozoic Limpopo-Shashe orogenic belt separating the Archean Zimbabwe and Kaapvaal Cratons. We investigate the lithospheric structures associated with this earthquake using high-resolution aeromagnetic and gravity data integrated with Differential Interferometric Synthetic Aperture Radar (DInSAR) analysis. Here we present the first results that provide insights into the tectonic framework of the earthquake. The ruptured fault trace delineated by DInSAR aligns with a distinct NW striking and NE dipping magnetic lineament within the Precambrian basement. The fault plane solution and numerical modeling indicate that the cause of the earthquake was 1.8 m displacement along a NW striking and NE dipping normal fault, rupturing at 21-24 km depth. We suggest that this seismic event was due to extensional reactivation of a crustal-scale Precambrian thrust splay within the Limpopo-Shashe orogenic belt

Crustal structure of the Arabian Plate: New constraints from the analysis of teleseismic receiver functions

An edited version of this paper was published by Elsevier Science. Copyright 2005, Elsevier Science. See also: http://dx.doi.org/10.1016/j.epsl.2004.12.020; http://atlas.geo.cornell.edu/SaudiArabia/publications/Al-Damegh%202005.htmReceiver functions for numerous teleseismic earthquakes recorded at 23 broadband and mid-band stations in Saudi Arabia and Jordan were analyzed to map crustal thickness within and around the Arabian plate. We used spectral division as well as time domain deconvolution to compute the individual receiver functions and receiver function stacks. The receiver functions were then stacked using the slant stacking approach to estimate Moho depths and Vp/Vs for each station. The errors in the slant stacking were estimated using a bootstrap re-sampling technique. We also employed a grid search waveform modeling technique to estimate the crustal velocity structure for seven stations. A jackknife re-sampling approach was used to estimate errors in the grid search results for three stations. In addition to our results, we have also included published receiver function results from two temporary networks in the Arabian shield and Oman as well as three permanent GSN stations in the region. The average crustal thickness of the late Proterozoic Arabian shield is 39 km. The crust thins to about 23 km along the Red Sea coast and to about 25 km along the margin of the Gulf of Aqaba. In the northern part of the Arabian platform, the crust varies from 33 to 37 km thick. However, the crust is thicker (41?53 km) in the southeastern part of the platform. There is a dramatic change in crustal thickness between the topographic escarpment of the Arabian shield and the shorelines of the Red Sea. We compared our results in the Arabian shield to nine other Proterozoic and Archean shields that include reasonably well determined Moho depths, mostly based on receiver functions. The average crustal thickness for all shields is 39 km, while the average for Proterozoic shields is 40 km, and the average for Archean shields is 38 km. We found the crustal thickness of Proterozoic shields to vary between 33 and 44 km, while Archean shields vary between 32 and 47 km. Overall, we do not observe a significant difference between Proterozoic and Archean crustal thickness. We observed a dramatic change in crustal thickness along the Red Sea margin that occurs over a very short distance. We projected our results over a cross-section extending from the Red Sea ridge to the shield escarpment and contrasted it with a typical Atlantic margin. The transition from oceanic to continental crust of the Red Sea margin occurs over a distance of about 250 km, while the transition along a typical portion of the western Atlantic margin occurs at a distance of about 450 km. This important new observation highlights the abruptness of the breakup of Arabia. We argue that a preexisting zone of weakness coupled with anomalously hot upper mantle could have initiated and expedited the breakup

Area selection for diamonds using magnetotellurics : examples from southern Africa

Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Lithos 112 (2009): 83-92, doi:10.1016/j.lithos.2009.06.011.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 675 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 Terrain. 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 Vs [m/s] = 0.045*log(resistivity [ohm.m]).We especially thank our academic funding sponsors; the Continental Dynamics programme of the U.S. National Science Foundation, the South African Department of Science and Technology, and Science Foundation Ireland

A high resolution seismic reflection study of the carboniferous Bay St. George subbasin, western Newfoundland

A seismic reflection profile from onshore Bay St. George Subbasin in western Newfoundland was reprocessed and reinterpreted to determine the structure and extent of the Carboniferous rocks. The main emphasis of reprocessing was placed on velocity analyses and dip move-out (DMO). The quality of the data was improved significantly by the reprocessing. A few features which had not been discovered before became evident. An example of this is an unconformity at a depth of 3.0 km to 5.0 km. -- The basin has the configuration of a half graben dipping to the east. The maximum thickness of the Carboniferous sediments is about 5 km in individual depocentres. The basin appears to be bounded downwards by unconformity "U", which separates the Carboniferous rocks from either Lower Palaeozoic rocks or Precambrian rocks. -- The fault system is very complex. A few faults correspond to the surface geology. The pattern of faults suggest that the basin was opened by strike slip movements and later deformed by compressional forces

Crustal structure of the southern margin of the African continent: Results from geophysical experiments

A number of geophysical onshore and offshore experiments were carried out along a profile across the southern margin of the African Plate in the framework of the Inkaba yeAfrica project. Refraction seismic experiments show that Moho depth decreases rapidly from over 40 km inland to around 30 km at the present coast before gently thinning out toward the Agulhas-Falkland Fracture Zone, which marks the transition zone between the continental and oceanic crust. In the region of the abruptly decreasing Moho depth, in the vicinity of the boundary between the Namaqua-Natal Mobile Belt and the Cape Fold Belt, lower crustal P-wave velocities up to 7.4 km/s are observed. This is interpreted as metabasic lithologies of Precambrian age in the Namaqua-Natal Mobile Belt, or mafic intrusions added to the base of the crust by younger magmatism. The velocity model for the upper crust has excellent resolution and is consistent with the known geological record. A joint interpretation of the velocity model with an electrical conductivity model, obtained from magnetotelluric studies, makes it possible to correlate a high-velocity anomaly north of the center of the Beattie magnetic anomaly with a highly resistive bod

Seismic Structure of the Tectosphere beneath Southern Africa

The Southern Africa Seismic Experiment was designed to image the deep structure of cratonic roots. It is part of a multidisciplinary research project by Carnegie Institution, MIT, southern African academic institutions and industry collaborators to study the structure, composition, and evolution of the cratons and adjacent mobile belts of southern Africa. An array of fifty five portable broadband REFTEK/STS-2 seismic stations was deployed in April/May 1997 along a NNE-SSW transect about 1800 km long by 600 km wide in southern Africa. Approximately half the station were redeployed to new sites in April/May, 1998. A total of eighty two stations have been occupied in a swath extending from the Cape Fold Belt and Namaqua-Natal mobile belt in South Africa, through the Kaapvaal craton, across the Limpopo Belt, and into the cratons of Zimbabwe and Botswana. P-wave and S-wave delay times have been analyzed to obtain tomographic images of mantle structure beneath the array to depths in excess of 1000 km. Receiver function and surface wave inversions have been used to constrain crustal thickness and mean velocity across the array. Results based on the first several months of data show that mantle velocities beneath the craton are significantly higher than those of the adjacent Proterozoic belts, with shear velocity constrasts disproportionately large. Mantle velocities vary across the craton itself at about the 1 percent level. Highest mantle velocities are found in the southern part of the Kaapvaal craton and in the region of the Zimbabwe craton of NE Botswana and SW Zimbabwe. Clear evidence of cratonic root structures is seen to extend to at least 200--250 km depth, perhaps substantially more, with the most prominent root structures generally underlying diamondiferous regions. Preliminary results indicate a prominent low velocity feature in the mantle below about 700 km in the general region beneath the Bushveld province. The Bushveld province within the Kaapvaal craton is marked by relatively lower velocities in the mantle root. While the velocity contrast with adjacent craton is comparatively small, it does correlate also with slightly greater crustal delay times and with a zone of null SKS splitting (Gao et al., this session). The seismic results coupled with evidence of younger Re/Os model ages of mantle nodules suggests partial cratonic disruption during formation of the Bushveld