Search

The results of a study of the Paleoproterozoic Richtersveld Subprovince/Magmatic Arc (RMA) in southern Namibia are integrated with previous work in adjacent NW South Africa to produce a new, unified model for the evolution of the RMA. The RMA was variably affected by the ∼1 Ga Namaqua. In the west, low-grade metamorphic rocks (Vioolsdrif Domain) record a limited Namaqua imprint whereas in the east, the equivalent rocks were converted to amphibolite-facies gneisses (Pella Domain). Tectonic slivers of RMA rocks are also found in an imbricate thrust zone that forms the base of the over-riding Namaqua-age Kakamas Domain further east. The volcanic and equivalent plutonic rocks (Orange River Group (ORG) and Vioolsdrif Suite respectively) of the RMA form a coeval calc-alkaline magma series with characteristic island-arc geochemical signatures. The entire magmatic history took place between 1910 and 1865 Ma, contrary to the much longer time-spans previously suggested. A large inclusion of migmatite (Bankwasser Migmatite) gave Archaean detrital zircon ages. It is interpreted as a possible fragment of Archaean Kaapvaal Craton basement that was incorporated into the RMA prior to ∼1885 Ma. ∼2 Ga detrital zircon ages in the ORG and ∼2 Ga magmatic zircon inheritance and xenoliths entrained in the Vioolsdrif Suite reveal a hitherto unrecognised pre-RMA history. Recent work has shown that a pre-RMA arc (Sperrgebiet Arc) had developed to the west at ∼2020 Ma in an upfaulted block within the Neoproterozoic Gariep Orogen. Consequently, a two-arc model is proposed for the early evolution of western Namaqualand with the early Sperrgebiet Arc evolving at ∼2020 Ma which, possibly by processes of subduction migration, was cannibalised by the RMA at ∼1885 Ma. Our data also suggests that certain quartzite sequences, traditionally placed within the ORG, were deposited after cessation of RMA activity and are thus unrelated to it.

The Friersdale Charnockite is a late-Mesoproterozoic (1078 ± 10 Ma), fine- to medium-grained to porphyritic orthopyroxene-bearing monzogranite intrusion with characteristic opalescent quartz that outcrops in the Keimoes-Upington region of the Northern Cape, South Africa. The weakly to undeformed, northwest-southeast trending granitoid body represents the most extensive member of the Keimoes Suite and intruded across the tectonic boundary between the granulite facies Kakamas and Areachap Terranes of the Namaqua-Natal Metamorphic Province during the final stages of the main Namaqua orogeny. The Friersdale Charnockite is a metaluminous, ferrous and potassic monzogranite with major, trace, REE and isotope chemistries suggesting a mixed crustal source with significant input from both Paleoproterozoic- and Mesoproterozoic-aged source regions.

The Aurus Mountains in SW Namibia form a tectonic inlier (Sperrgebiet Domain) within the Neoproterozoic Gariep belt. We document the geological history from field mapping, whole-rock geochemical, metamorphic and U-Pb zircon isotope data. The calc-alkaline granodioritic orthogneisses of the Wasserkuppe Suite, dated at ∼2020 Ma, are the predominant, oldest rocks, which are interpreted to form part of a ∼2 Ga “Sperrgebiet arc”; the tectono-magmatic equivalent of the ∼250 km wide Richtersveld arc to the southeast, but ∼100 Ma older. ∼2 Ga detritus is common throughout the western Namaqua belt, but its source was hitherto unknown. The suite intruded synchronously with amphibolite facies metamorphism and deformation, considered an early phase of the Palaeoproterozoic Orange River Orogeny. The Wasserkuppe Suite is intruded by alkali granite and microgranodiorite dated at ∼1885 Ma, interpreted as equivalents of the Vioolsdrif Suite (Richtersveld arc). The Sperrgebiet Domain is thus seen as an early phase of a process of Palaeoproterozoic island-arc development at the margins of the Archaean Kaapvaal Craton/Rehoboth Province. It was cannibalised by the later Richtersveld arc which dominated the palaeogeography of this part of southern Africa 100 m.y. later, and which forms the crustal infrastructure of much of Western Namaqualand. The Palaeoproterozoic arc rocks are overlain by a clastic metasedimentary sequence; the Aurus Schist. Detrital zircon geochronology indicates that these rocks were predominantly derived from both arcs, with minor Archean and younger ∼1800 Ma detritus, of unknown source(s). Bodies of latest Mesoproterozoic (∼1000 Ma) Warmbad Granite and pegmatite are mapped throughout the area and represent the only Namaquan rocks definitively identified. Evidence for Namaquan metamorphic or tectonic imprints are lacking, apart from a garnet-clinopyroxene metabasic inclusion in a quartz-feldspar leucogneiss which gave a typical “Namaquan” P-T estimate of ∼700–750 °C at 6–7 kbar. The leucogneisses, distributed throughout the area, were dated at ∼840 Ma and are interpreted as metamorphosed felsic igneous rocks of the Richtersveld Suite, formed during early rifting of the proto-Gariep basin. The Sperrgebiet rocks are variably deformed by the Pan-African Gariep Orogeny that produced regionally-pervasive fabrics, predominantly partitioned into discrete high-strain shear zones in a NW-trending sinistral transpressive tectonic regime.

Off-fault dynamic tensile cracks form behind an earthquake rupture front with distinct orientation and spacing. These cracks explode the wall rock and create breccias, which we hypothesize will preserve a unique fingerprint of dynamic rupture. Identification of these characteristic breccias may enable a new tool for identifying paleoseismic slip surfaces in the rock record. Using previous experimental and theoretical predictions, we develop a field-based model of dynamic dilational breccia formation. Experimental studies find that secondary tensile fracture networks comprise closely spaced fractures at angles of 70–90° from a slip surface, as well as fractures that branch at angles of ∼ 30 ° from a primary mode I fracture. The Pofadder Shear Zone, in Namibia and South Africa, preserves breccias formed in the brittle–ductile transition zone displaying fracture patterns consistent with those described above. Fracture spacing is approximately two orders of magnitude less than predicted by quasi-static models. Breccias are clast-supported, monomict and can display an abrupt transition from fracture network crackle breccia to mosaic breccia textures. Brecciation occurs by the intersection of off-fault dynamic fractures and wall rock fabric; this is in contrast to previous models of fluid pressure gradient-driven failure “implosion breccias”. This mechanism tends to form many similar sized clasts with particle size distributions that may not display self-similarity; where self-similarity is observed the distributions have relatively low D-values of 1.47 ± 0.37 , similar to other studies of dynamic processes. We measure slip distances at dilational breccia stepovers, estimating earthquake magnitudes between M w 2.8–5.8 and associated rupture lengths of 0.023–3.3 km. The small calculated rupture dimensions, in combination with our geologic observations, suggest that some earthquakes nucleated within the quartz-plastic transitional zone and potentially record deep seismic slip.