Lithospheric structure of an Archean craton and adjacent mobile belt revealed from 2‐D and 3‐D inversion of magnetotelluric data: Example from southern Congo craton in northern Namibia

Abstract

[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.

Is the Okavango Delta the terminus of the East African Rift System? Towards a new geodynamic model: Geodetic study and geophysical review

“…Our conclusion that the OG isn't a rifting zone and hence can't be considered as an analogue of the EARS rifts is in agreement with Khoza et al (2013), McCarthy (2013 and Yu et al (2015a) who questionned the dynamics of the tectonic activity. Both latter studies proposed new deformation models based on accommodation of differential far-field plates movements, consequent to the EARS dynamics according to McCarthy (2013) or to Kalahari craton displacement relatively to the central to northern Nubian plate in the case of Yu et al (2015a).…”

“…Its southern limit was originally thought to lie in central Angola, but has subsequently been redefined further south underneath the Damara Belt in northern Botswana (Singletary et al, 2003;Begg et al, 2009;Khoza et al, 2013). Its associated lithosphere is estimated to reach a thickness of up to 250 km.…”

“…The lithosphere thickness in the area was determined by local magnetotelluric studies at about 160 km for the Damara belt, 180 km for the Ghanzi-Chobe belt, at least 250 km for the Congo craton, and 220 km for the Kalahari craton (Muller et al, 2009;Begg et al, 2009;Miensopust et al, 2011;Khoza et al, 2013). The proposed Kalahari craton lithospheric thickness is supported by kimberlite pipe studies, which show a thickness of at least 200 km south of the Makgadikgadi pans (Griffin et al, 2003).…”

“…Apart from fault orientation and morphology, proponents of this hypothesis use the first geophysical studies to infer that rifting is at play in the OG in a similar way to that observed in the EARS (Fairhead and Girdler, 1969;Scholz et al, 1976;Wilson and Dincer, 1976;Girdler, 1975;Ballard et al, 1987). Few geological data are available due to the extensive cover of Kalahari sands of varying thickness but several recent geophysical studies bring new insights on the crustal and lithospheric structure, and mantle activity of the area (e.g., Forte et al, 2010;Khoza et al, 2013;Yu et al, 2015a) that call into question the rifting hypothesis for the OG. Other deformation models have been proposed (McCarthy, 2013;Yu et al, 2015a) and are supported by new available data.…”

The electrical resistivity structure of lithosphere across the Dharwar craton nucleus and Coorg block of South Indian shield: Evidence of collision and modified and preserved lithosphere

“…MT is therefore widely used to study the subsurface structures, crustal as well as upper mantle, associated with varied geological and tectonic settings, such as in ancient and active subduction/collision zones, volcanic and seismic regions, stable cratons, and rift zones [e.g., Unsworth et al ., ; Abdul Azeez and Harinarayana , ; Evans et al ., ; Zhao et al ., ; Abdul Azeez et al ., ; Thiel and Heinson , ; Padilha et al ., ]. Various MT studies have proved their efficacy in delineating the cratonic roots and upper mantle structures that could enlighten the evolution of cratons and their stability [e.g., Wei et al ., ; Jones et al ., ; Evans et al ., ; Khoza et al ., ; Thiel and Heinson , ; Adetunji et al ., ; Spratt et al ., ; Selway et al ., ]. The present study uses both broadband and long‐period (ranging up to 10,000 s) data to constrain the lithospheric electric structure along a profile that cuts across the known oldest part (nucleus) of the Dharwar craton, i.e., the Holenarsipur belt, and the Coorg block of India.…”