Namibia's passive continental margin records a long history of tectonic activity since the Proterozoic. The orogenic belt produced during the collision of the Congo and Kalahari Cratons in the Early Proterozoic led to a zone of crustal weakness, which became the preferred location for tectonism during the Phanerozoic. The Pan-African Damara mobile belt forms this intraplate boundary in Namibia and its tectonostratigraphic zones are defined by ductile shear zones, where the most prominent is described as the Omaruru Lineament–Waterberg Thrust (OML–WT). The prominance of the continental margin escarpment is diminished in the area of the Central and Northern Zone of the Damara belt where the shear zones are located. This area has been targeted with a set of 66 outcrop samples over a 550-km-long, 60-km-broad coast-parallel transect from the top of the escarpment in the south across the Damara sector to the Kamanjab Inlier in the north. Apatite fission track age and length data from all samples reveal a regionally consistent cooling event. Thermal histories derived by forward modelling bracket this phase of accelerated cooling in the Late Cretaceous. Maximum palaeotemperatures immediately prior to the onset of cooling range from ca. 120 to ca. 60 °C with the maximum occurring directly south of the Omaruru Lineament. Because different palaeotemperatures indicate different burial depth at a given time, the amount of denudation can be estimated and used to constrain vertical displacements of the continental crust. We interpret this cooling pattern as the geomorphic response to reactivation of basement structures caused by a change in spreading geometry in the South Atlantic and South West Indian Oceans. 相似文献
The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous hornblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U–Pb zircon age of 540±3 Ma.
Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (Nd (540 Ma)=−6.3 to −19.8; δ18O=9.0–11.6‰) but rather low and uniform initial Sr isotope compositions (87Sr/86Srinitial=0.70794–0.70982). Two leucogranites and one aplite have higher initial 87Sr/86Sr ratios (0.70828–0.71559), but similar initial Nd (−11.9 to −15.8) and oxygen isotope values (10.5–12.9‰). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K2O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr (<0.6) ratios and are probably generated by partial melting of meta-igneous (intermediate) lower crustal sources by amphibole-dehydration melting. Most of the leucogranites display higher Rb/Sr ratios (>1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material. 相似文献
We developed generalised additive models (GAMs) to estimate standardised time-series of population abundance indices for assessment purposes and to infer ecological and behavioural information on northern Benguela hakes, Merluccius capensis and M. paradoxus, using haul-by-haul commercial trawl catch-rate data as proxies for hake densities. The modelling indicated that individual ship identifiers should be used rather than general vessel characteristics, such as vessel size. The final models explained 79% and 68% of the variability in the commercial catch rates of M. capensis and M. paradoxus, respectively. The spatial density patterns were consistent and confirmed existing knowledge about these species in the northern Benguela system. Furthermore, seasonal migration patterns were described for the first time and were found to correspond to the known spawning areas and seasons for M. capensis and M. paradoxus. Spatial density patterns were validated using the geostatistical modelling results of fisheries-independent trawl survey data. Improved understanding of the relationships between fleet dynamics and fish movement can be achieved by taking into consideration the present catch-rate model and spatial and seasonal distribution maps. We conclude that the yearly standardised CPUE time-series are problematic as proxies for total stock abundance because of spatial coverage issues. Consequently, such CPUE data should not be used for stock-size assessments and fisheries advice concerning northern Benguela hakes until this is solved. We generally recommend the exclusion of standardised CPUE time-series from stock assessments when important and changing parts of the stock distribution cannot be targeted by the fishery, such as due to closed areas or seasons. 相似文献
Herero communities in northern Namibia recently formed the Ehi-rovipuka Conservancy under a national Community-Based Natural Resource Management Programme (CBNRM) that has received international acclaim for wildlife conservation and poverty alleviation. Nearly a century ago ancestors of the Herero were ousted from Etosha National Park, contiguous to the Ehi-rovipuka Conservancy. The communities have been denied access ever since. Participatory Rural Appraisal (PRA) methods were employed to elucidate relationships of the Herero and their communal conservancy with the Etosha National Park. Memory mapping and villager interviews revealed a profound sense of lost traditional territory inside Etosha and a strong desire to return to the Park, not to harvest wildlife but to restore cultural practices and reap certain benefits from the Park. We term the separation of the communities from their traditional territory as a ‘decoupling’ of people from their local environment. We further suggest that the potential benefits that villagers identify from the Park represent mechanisms for ‘recoupling’ local social-ecological systems, requiring the literal and figurative breakdown of the Park fence. We describe a model to illustrate decoupling and recoupling mechanisms, and argue that recoupled social-ecological systems are necessary for long-term conservation of biodiversity. We further suggest a collaborative landscape model for biodiversity conservation featuring institutional linkages and integration between community-conserved areas, integrated conservation corridors for connectivity, and dynamic, mobile reserves collaboratively integrated with national parks management. This model may have applicability in Namibia and similar regions elsewhere with low population densities, high species endemism and prevailing or emerging threats to biodiversity. 相似文献
The Epupa Metamorphic Complex constitutes the southwestern margin of the Congo Craton and is exposed in a hilly to mountainous terrain of northwestern Namibia, bordering the Kunene River and extending into southern Angola. It consists predominantly of granitoid gneisses which are migmatized over large areas. This migmatization locally led to anatexis and produced crustal-melt granites such as the Otjitanda Granite. We have undertaken reconnaissance geochemical studies and single zircon U–Pb SHRIMP and Pb–Pb evaporation dating of rocks of the Epupa Complex. The granitoid gneisses, migmatites and anatectic melts are similar in composition and constitute a suite of metaluminous to peraluminous, calc-alkaline granitoids, predominantly with volcanic arc geochemical signatures. The zircon protolith ages for the orthogneisses range from 1861 ± 3 to 1758 ± 3 Ma. Anatexis in the migmatitic Epupa gneisses was dated from a melt patch at 1762 ± 4 Ma, and the anatectic Otjitanda Granite has a zircon age of 1757 ± 4 Ma. Migmatization and anatexis therefore occurred almost immediately after granitoid emplacement and date a widespread high-temperature Palaeoproterozoic event at ∼1760 Ma which has not been recorded elswhere in northern Namibia. The Nd isotopic systematics of all dated samples are surprisingly similar and suggest formation of the protolith from a source region that probably separated from the depleted mantle about 2.4–2.0 Ga ago. A major Archaean component in the source area is unlikely. 相似文献
The Orange Basin records the development of the Late Jurassic to present day volcanic-rifted passive margin of Namibia. Regional extension is recorded by a Late Jurassic to Lower Cretaceous Syn-rift Megasequence, which is separated from a Cretaceous to present day post-rift Megasequence by the Late Hauterivian (ca. 130 Ma) break-up unconformity. The Late Cretaceous Post-rift evolution of the basin is characterized by episodic gravitational collapse of the margin. Gravitational collapse is recorded as a series of shale-detached gravity slide systems, consisting of an up-dip extensional domain that is linked to a down-dip zone of contraction domain along a thin basal detachment of Turonian age. The extensional domain is characterized by basinward-dipping listric faults that sole into the basal detachment. The contractional domain consists of landward-dipping listric faults and strongly asymmetric basinward-verging thrust-related folds. Growth stratal patterns suggest that the gravitational collapse of the margin was short-lived, spanning from the Coniacian (ca. 90 Ma) to the Santonian (ca. 83 Ma). Structural restorations of the main gravity-driven system show a lack of balance between up-dip extension (24 km) and down-dip shortening (16 km). Gravity sliding in the Namibian margin is interpreted to have occurred as a series of episodic short-lived gravity sliding between the Cenomanian (ca. 100 Ma) and the Campanian (ca. 80 Ma). Gravity sliding and spreading are interpreted to be the result of episodic cratonic uplift combined with differential thermal subsidence. Sliding may have also been favoured by the presence of an efficient detachment layer in Turonian source rocks. 相似文献