Browsing by Author "Ebinger, C.J."
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Ranganai, R.T.; Ebinger, C.J. (Elsevier Science Ltd. www.elsevier.com/locate/jappgeo, NaN, 2008)[more][less]
Abstract: Aeromagnetic (AM) and Landsat Thematic Mapper (TM) data from the south-central Zimbabwe Craton have been processed for the purpose of regional structural mapping and thereby to develop strategic models for groundwater exploration in hard-rock areas. The lineament density is greater on TM than on AM images, partly due to the resolution of the different datasets, and also because not all TM lineaments have a magnetic signature. The derived maps reveal several previously undetected lineaments corresponding to dykes, faults, shear zones and/or tectonically-related joints, striking predominantly NNE, NNW and WNW. We suggest the possible hydrogeological significance of some of these patterns as follows: the aeromagnetic data can be used to map faults and fractures of considerable depth which are likely to be open groundwater conduits at depth (typically under tension), while TM lineaments, although not necessarily open (mostly under compression), represent recharge areas. The interpreted persistent lineation and well developed fracture patterns are correlated with existing boreholes and indicate a spatial relationship between regional structures and high borehole yields (N3 m3/h). This relationship is combined with other lithological and hydrogeological information to identify potential regional groundwater sites for detailed ground investigations. These are defined as dyke margins, faults, fractures/joints or intersections of any combination of these structures. Priority should be given to coincident AM/TM lineaments (e.g., NNW and NNE fractures) and continuous structures with large catchment areas (e.g., NNE and WNW faults). The late Archaean (2.6 Ga) granites are considered the most favourable unit because of their associated long and deep brittle fractures between numerous bornhardts (inselbergs) and kopjes. Several small-scale TM lineaments also form important local sources of groundwater for hand-dug wells. Based on measured rock susceptibilities from the area, we present a model of the typical magnetic responses from the possible groundwater exploration targets. The developed magnetic model could be applicable to similar terrains in other Archaean Cratons. URI: http://hdl.handle.net/10311/341 Files in this item: 1
Ranaganai_JAG_2008.pdf (3.168Mb) -
Ranganai, R.T.; Whaler, K.A.; Ebinger, C.J. (Elsevier Science Ltd. www.elsevier.com/locate/jafrearsci, NaN, 2008)[more][less]
Abstract: The granite-greenstone terrain of south-central Zimbabwe, encompassing the Belingwe (Mberengwa) greenstone belt and sections of the Great Dyke, provides important constraints on models for the evolution of the Zimbabwe craton and the Archaean crust in general. In this paper we enhance and model existing and recently acquired gravity data from the region and correlate the anomalies and their derivatives with the known basement geology to evaluate models for greenstone belt development. We also study the spatial gneiss-granite- greenstone association in general, and the geologic implications of models of the anomaly patterns in particular. Although the Belingwe greenstone belt has been mapped, its subsurface geometry is poorly known. Similarly, the Great Dyke is well studied, but no systematic study of the extent and cross-cutting relations of other mafic dykes in the Archaean crust has been undertaken. The regional gravity field shows no evidence for crustal thickness variations in the area and the gravity anomalies can be explained by lateral density variations of the supracrustal rocks. Prominent gravity highs are observed over the high density (≤ 3000 kg/m3) volcanosedimentary piles (greenstone belts) and ultramafic complexes. Well-defined elongate, sub-oval/elliptical gravity lows are associated with intrusive granitic plutons. The granite-greenstone contacts are marked by steep gravity gradients of up to 5 mGal/km that imply steeply dipping or near-vertical contacts for the anomalous bodies. This is tested and confirmed by 2½D modelling of gravity profiles across the Belingwe and Fort Rixon greenstone belts, constrained by measured densities and observed geological data. The modelling also indicates that these belts, and possibly all the belts in the study area (based on comparable densities and anomaly amplitudes), have limited depth extents in the range of 3–5 km. This is comparable to thicknesses obtained elsewhere from deep seismic reflection data and geoelectrical studies, but mapped stratigraphic thicknesses give a maximum depth extent of about 9.5 km. Present studies and previous work support the idea that the volcanics were extruded within rift zones and laid on older granitic crust, followed by subsidence and rapid deposition of sediments that were sourced from the adjacent basement terrains. The volcano-sedimentary sequences were subsequently deformed by intruding younger plutons and affected by late-stage strike-slip activity producing cross-cutting structures. URI: http://hdl.handle.net/10311/276 Files in this item: 1
Ranganai_JAES_2008.pdf (4.603Mb)
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