Geology topics

Susan J. Webb

Publications and source records attributed to Susan J. Webb.

4 recordsLinked to original sources

USGS2024

Deep magmatic staging chambers for crustal layered mafic intrusions: An example from the Bushveld Complex of southern Africa

The deep mafic magmatic staging chambers of layered mafic intrusions have been conjectured but not imaged. Their existence has long been postulated from geochemical models which require multiple magma injections from staging chambers to account for their multi-scale igneous layering and variations in sources and degrees of crustal contamination. For the Bushveld Complex of southern Africa, the world’s largest layered mafic intrusion, seismic receiver functions identify a diffuse crust-mantle transition beneath the Complex that suggests high velocity lower crust and/or uppermost lithospheric mantle. Here we present 3D gravity modelling of the Bushveld Complex that includes dense material at the crust-mantle boundary, imaging for the first time, remnants of magma staging chambers. They underlie the whole Bushveld Complex and extend westwards to the Molopo Farms Complex in Botswana. Feeders to the Bushveld Complex coincide with intersections of major faults like the Thabazimbi-Murchison Lineament and Sugarbush Fault with the staging chamber. This identification of magmatic staging chambers beneath the Bushveld relates to similar geophysically imaged lower crustal features beneath the Duluth and Stillwater Complexes. Comparison of seismic and gravity data with geochemical models from other complexes aid in development of models for the magmatic architecture of layered mafic intrusions in general.

Precambrian Research↗

USGS2021

Geometry of the Bushveld Complex from 3D potential field modelling

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.

Precambrian Research↗

USGS2015

Mapping the 3-D extent of the Northern Lobe of the Bushveld layered mafic intrusion from geophysical data

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 <1500 m) with a modeled area of &sim;160 km × &sim;125 km. The modeled thicknesses are not well constrained but vary from &sim;<1000 to >12,000 m, averaging &sim;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.

Precambrian Research↗

USGS2013

Overview of the magnetic signatures of the Palaeoproterozoic Rustenburg Layered Suite, Bushveld Complex, South Africa

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.

Bushveld Complex↗