The largely amagmatic western branch of the East African Rift System (EARS) has been noted for unusually deep earthquakes (≥25 km), long normal faults (∼100 km) and wide grabens (∼50 km) (e.g., Craig et al., 2011;Ebinger et al., 1993;J. Jackson & Blenkinsop, 1997). The largest known normal-faulting continental earthquake, 1910 Ruwka M7.4 earthquake, occurred along the 180 km long Kanda Fault (Vittori et al., 1997) in this region. These observations have been used to hypothesize that the lithosphere here is particularly cold and strong, leading to a large elastic thickness, which controls other parameters such as

No abstract

SUMMARY This study explores the geomorphological expression and geological context of a normal fault scarp in a stable continental region (SCR) which we interpret as having failed in large (Mw >7) earthquakes. Records of such large normal faulting events in an SCR (or even in more rapidly deforming regions) are extremely rare, and so understanding this feature is of international interest. The scarp is exceptionally well-preserved due to the extensive calcrete/silcrete cementation. In areas where this cementation is reduced or absent the scarp is more diffuse, as expected for a feature formed by one or more paleoearthquakes. The exceptional preservation aids comparison with data sets based on scarps which have formed more recently. Our analysis is based on a high-resolution digital elevation model of the Hebron Fault scarp in southern Namibia using pan-sharpened Worldview-3 satellite stereophotos (0.31 m resolution). We make scarp height measurements at 160 locations providing improved estimates of the average displacement (5.9 m), maximum displacement (10.1 m), and the minimum fault length (45 km). No consistent evidence of lateral displacements in water courses or alluvial fan margins were found implying predominantly normal displacement. A newly described section in the northwest has en-echelon scarps consistent with a component of strike-slip motion that may be explained by its difference in strike from the central section. Most channels crossing the fault show a single knick-point. The displacement varies smoothly as it crosses a number of different generations of alluvial fan surfaces. No evidence of a multiscarp or a composite scarp were observed. We have therefore found no evidence for a mutiple-event origin for the scarp, although, this lack of evidence does not conclusively demonstrate a single-event origin. Published regressions, based on the limited data available for SCRs, suggest that the mean expected average displacement ($\bar{D}_{\rm av}$) for a faults of this length is 1.2–3.1 m implying that the scarp is likely to have formed in 2–5 events with an expected Mw = ∼7.1 though displacements in individual events may exceed these average values. Comparison with the regional geology and aeromagnetic data sets suggests that the fault reactivates a Mesoproterozoic ductile structure, the Nam Shear Zone, and that the location, orientation and segmentation of the scarp is controlled by the alignment of pre-existing structurally weak zones with the present-day stress regime. The fault has undergone repeated brittle reactivation, accumulating ∼110 m of vertical offset since the deposition of the Ediacaran-to-Cambrian Nama Group. This is less than expected from global compilations of total displacement and fault length data, suggesting that the fault rapidly attained its current length by recruiting an existing weak zone and is expected to accumulate displacement at a relatively constant length in the future.

The Uitenhage Group represents the earliest deposits that filled Mesozoic rift basins in the southern Cape of South Africa during the fragmentation of the supercontinent Gondwana. The sedimentology of the Enon Formation records the development of alluvial systems that drained the region since the onset of Gondwanan rifting, and therefore plays an important role in our understanding of early landscape evolution along the southern African continental margin. The mostly coarse conglomeratic unit was deposited continuously in actively subsiding, but separated, rift basins. As a result, the deposits are diachronous between basins and display highly varied thicknesses of up to well over 2000 m.

The Jurassic – Cretaceous Kirkwood Formation forms part of the Uitenhage Group, the earliest deposits to fill Mesozoic rift basins that developed in what is now the southern Cape of South Africa during the breakup of Gondwana. The Kirkwood Formation is not only palaeontologically extremely important, having yielded diverse assemblages of vertebrate, invertebrate and plant fossil taxa, but also contains suitable source rocks for hydrocarbon systems offshore of the southern Cape of South Africa. The Kirkwood Formation comprises chronostratigraphic markers in the form of radiometrically dateable volcaniclastic deposits and age-diagnostic invertebrate fossils, which may provide robust dates for the depositional history of the Uitenhage Group. The Kirkwood, including the Bethelsdorp, Colchester and Swartkops Members, is 2210 m at its thickest and comprises sandstones and mudstones deposited in fluvial and lacustrine depositional settings from the Tithonian to Valanginian. These microfossil-based ages may only apply to the unit in its type area in the Algoa Basin and not to its lithostratigraphic correlatives in the other Mesozoic rift basins, which have highly variable ages ranging from Middle Jurassic to Early Cretaceous.