Namibia is situated within a stable continental region (SCR), far away from plate boundary zones, and is therefore not expected to be at risk of significant earthquakes; the largest events in the instrumental record have a moment magnitude of 5.5.  Despite this, a paleoseismic fault scarp on the Hebron Fault has been interpreted as indicating much larger events have occurred in the past.  In this study, we demonstrate that a relatively small area of SW Namibia contains four more major neotectonic fault scarps.  These 16-80 km long structures have vertical separations between 0.7-10.2 m and could produce earthquakes of Mw 6.4 or greater.  Some of these scarps are interpreted to have formed through repeated failure of the same segment and they highlight the potential for further seismicity that far exceeds the maximum observed magnitude in the national catalogue. We identify strong structural controls on the location and orientation of these fault ruptures which reactivate N-S and NW-SE trending zones of crustal weakness.  These structures may be driven by E-W extension associated with the distribution of gravitational energy caused by the anomalously high elevation of the Namibian Escarpment.  If this explanation of the driving stresses is correct, these and similarly oriented faults represent a previously unrecognised source of continuing seismic hazard.  The discovery of these major fault scarps suggests that fault studies should be incorporated into seismic hazard analyses of stable Southern Africa as has been done in Australia and other SCR regions. Their apparent spatial clustering also merits further study.  At this point it is not clear if this clustering indicates a region of elevated strain rate (relative the surrounding SCR) or alternatively, an area of exceptional preservation due to a semi-arid climate and extensive calcrete-cemented surficial deposits.

Escarpment

Neotectonics

10.5194/egusphere-egu23-11719

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Citations (0)

A continental back-arc setting for the Namaqua belt: Evidence from the Kakamas Domain

Geoscience Frontiers (2022)

P.H. Macey Robert J. Thomas Alex Kisters Johann F.A. Diener M. Angombe

continental collision

Supercontinent

Continental Margin

Continental arc

10.1016/j.gsf.2022.101408

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Citations (13)

Exceptional Scarp Preservation in SW Namibia Reveals Geological Controls on Large Magnitude Intraplate Seismicity in Southern Africa

Tectonics (2023)

Robert A. Muir Benjamin Whitehead T. New Victoria L. Stevens P.H. Macey

Abstract Four previously unrecognized neotectonic fault scarps in southwest Namibia are described. These relatively straight, simple but segmented structures are 16–80 km long and have measured vertical separations of 0.7–10.2 m. We estimate that each is capable of producing earthquakes of M w 6.4 or greater, indicating that large earthquakes may occur despite limited cumulative displacement. There is strong evidence that some of these scarps were formed by repeated earthquakes. Comparison with aeromagnetic and geological maps reveal that the normal faults reactivate major crustal weaknesses that are orientated north‐south and northwest‐southeast and perpendicular to the local gravitational potential energy gradient. The presence of these structures in an area with a limited record of instrumental seismicity suggests that the M max of this region may be much larger than generally assumed. They highlight the necessity of incorporating information from fault studies into probabilistic seismic hazard assessments in this region, in a similar way to other stable continental regions such as Australia. The fact that such major structures have gone hitherto unrecorded suggests significant further research is needed to characterize these sources of hazard. The identification of an apparent cluster of large magnitude neotectonic earthquakes in the area may be related to the exceptional preservation potential of scarps rather than indicating an area of comparatively rapid deformation. If this interpretation is correct, then these scarps represent an important indication of the potential seismic hazard across the region, and the occurrence of infrequent large‐magnitude seismicity on similar structures should be considered throughout southwestern Africa.

10.1029/2022tc007693

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Citations (1)

Origin and evolution of the ∼1.9 Ga Richtersveld Magmatic Arc, SW Africa

Precambrian Research (2017)

P.H. Macey Robert J. Thomas H. Minnaar P. G. Gresse Christopher W. Lambert

Migmatite

Back-arc basin

Yilgarn Craton

Xenolith

10.1016/j.precamres.2017.01.013

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Citations (62)

Lithostratigraphy of the Mesoproterozoic Yas-Schuitdrift Batholith, South Africa and Namibia

South African Journal of Geology (2020)

C.A. Groenewald P.H. Macey

Abstract The granitic and leucogranitic Yas and Schuitdrift Gneisses occur together as a large ovoid pre-tectonic batholith that crosses the Orange River border between South Africa and Namibia. They occur in the central parts of the Kakamas Domain in the Namaqua Sector of the Namaqua-Natal Metamorphic Province where they intrude, and are deformed together with, slightly older (~1.21 Ga) orthogneisses and granulite-facies metapelitic gneisses. The Yas Gneiss occurs mainly on the outer perimeter and northern parts of the batholith and comprises equigranular leucogranite gneiss and biotite granite augen orthogneiss, whereas the Schuitdrift biotite-hornblende augen gneiss is located at the centre and southern parts of the batholith. The batholith is strongly deformed with penetrative Namaqua-aged gneissic fabrics defined by grain-flattening of quartz and feldspar in the equigranular leucogneisses and aligned K-feldspar megacrysts in the augen gneisses. The gneissic fabric is refolded during a large-scale folding event that results in the dome-shape of the batholith and controls the present outcrop pattern of its various components. Flexure along the margins of the batholith refoliated the gneisses into a zone of mylonitic rocks. The Yas and Schuitdrift Gneisses have similar geochemistry and classify as alkali granites and alkali leucogranites. They are felsic (mean SiO2: 74.5 wt%) and potassic (mean K2O: 5.8 wt%) but have low MgO, CaO and Na2O, reflecting their low mafic mineral and plagioclase contents. The Schuitdrift Gneiss yielded U-Pb zircon ages of 1 191 ± 7 and 1 187 ± 6 Ma.

Batholith

Porphyritic

Felsic

Leucogranite

Hornblende

10.25131/sajg.123.0029

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Citations (5)