Stratigraphy,structure and geochronology of ore leads in the Matsitama Schist belt of Northern Botswana

The Matsitama schist belt in northeastern Botswana comprises an area of metasediments, notably quartzites, limestones, shales and amphibolites that are bounded by granites and gneisses. The belt lies southwest of the Rhodesian cration and north of the Limpopo mobile belt.Stratigraphic, structural and lead isotopic evidence indicates that the Matsitama metasediments are equivalent to the Shashi metasediments in the Limpopo belt. There is strong evidence that the Matsitama and Shashi metasediments stratigraphically underlie volcanic rocks of the Tati belt which have been correlated with Archaean schist belts of about 2700 Ma of Rhodesia. Therefore, the Matsitama and Shashi rocks are at least as old as the schist belts of the Rhodesian craton and may represent a shallow-water facies that occurs only in the Limpopo area.There is no structural evidence that the Matsitama and Shashi metasediments were deposited unconformably on basement rocks, although the presence of gneiss, amphibolite and ironstone pebbles in a Matsitama conglomerate, as well as the presence of orthoquartzites, shows the existence of a basement source region. However, the surrounding granites intrude the Matsitama and Shashi metasediments and all underwent several deformation phases.The structural history of the Matsitama rocks can be described in terms of five phases of deformation. The main cleavage-producing deformation phase, F₂, folded the rocks into a major synform and intensely deformed them. Before this, however, the rocks had been folded and thrust so that part of the succession shows downward-facing F₂ structures and there are possibly repetitions of the stratigraphy due to imbrication. Structures of the F₃ and F₄ phases fold the main cleavage but locally are sufficiently intense to modify the shape of the finite strain ellipsoid. There is a major ductile shear zone of F₄ age, south of which F₄ folds are tight, while to the north, F₄ deformation is negligible. All of these structures can be correlated with deformation phases in the Tati schist belt to the east and in the northern part of the Limpopo mobile belt.Lead isotope evidence suggests that mineralization in the Matsitama metasediments occurred at least 2200 Ma ago, and that leads from Dihudi/Thakadu and Messina, in the centre of the Limpopo belt, underwent a two-stage history of events at 2600–2700 Ma and 2000–2100 Ma ago, agreeing with other geochronological evidence. The leads from Matsitama and Messina are isotopically distinct from leads from the Rhodesian schist belts, which show evidence of transfer to the crust some 3500 Ma ago. The absence of this 3500 Ma-old lead from the Matsitama and Messina environments may indicate different crustal conditions and possibly the absence of the Rhodesian-type early basement.     

The roles of surface heat flux and ocean heat transport convergence in determining Atlantic Ocean temperature variability

The temperature variability of the Atlantic Ocean is investigated using an eddy-permitting (1/4°) global ocean model (ORCA-025) forced with historical surface meteorological fields from 1958 to 2001. The simulation of volume-averaged temperature and the vertical structure of the zonally averaged temperature trends are compared with those from observations. In regions with a high number of observations, in particular above a depth of 500 m and between 22° N and 65° N, the model simulation and the dataset are in good agreement. The relative contribution of variability in ocean heat transport (OHT) convergence and net surface heat flux to changes in ocean heat content is investigated with a focus on three regions: the subpolar and subtropical gyres and the tropics. The surface heat flux plays a relatively minor role in year-to-year changes in the subpolar and subtropical regions, but in the tropical North Atlantic, its role is of similar significance to the ocean heat transport convergence. The strongest signal during the study period is a cooling of the subpolar gyre between 1970 and 1990, which subsequently reversed as the mid-latitude OHT convergence transitioned from an anomalously weak to an anomalously strong state. We also explore whether model OHT anomalies can be linked to surface flux anomalies through a Hovmöller analysis of the Atlantic sector. At low latitudes, increased ocean heat gain coincides with anomalously strong northward transport, whereas at mid-high latitudes, reduced ocean heat loss is associated with anomalously weak heat transport.     

Crustal lineaments and shear zones in Africa: Their relationship to plate movements

Many of the major lineaments in southern Africa are major ductile shear zones with large displacement, occurring within, though often bounding orogenic belts. An example is the boundary to the Limpopo belt in Botswana and Zimbabwe. However, some of these shear zones only record slight displacement when considered on a crustal scale; they are merely planes recording differential movement on much larger, flat to gently dipping, shear zones where the boundary to the orogenic belt is a low-angle thrust zone. These different types of shear zones are clearly shown in the Pan-African belt of Zambia where large ENE-trending lineaments have been recorded. Recent work has shown the northern group of shears to be large lateral ramps; for example, the rocks of the copper belt are part of an ENE-verging thrust package, the southern boundary of which is a major, oblique to lateral ramp. In southern Zambia shears are more analogous to major transform faults; they form as tear faults separating zones of different thrust vergence. A possible plate tectonic model is given for this part of Africa, showing the different relative plate movement vectors estimated from the geometry of the Pan-African shear zones.     

Recent changes in the North Atlantic circulation simulated with eddy-permitting and eddy-resolving ocean models

Three eddy-permitting (1/4°) versions and one eddy-resolving (1/12°) version of the OCCAM ocean model are used to simulate the World Ocean circulation since 1985. The first eddy-permitting simulation has been used extensively in previous studies, and provides a point of reference. A second, improved, eddy-permitting simulation is forced in the same manner as the eddy-resolving simulation, with a dataset based on a blend of NCEP re-analysis and satellite data. The third eddy-permitting simulation is forced with a different dataset, based on the ERA-40 re-analysis data. Inter-comparison of these simulations in the North Atlantic clarifies the relative importance of resolution and choice of forcing dataset, for simulating the mean state and recent variability of the basin-scale circulation in that region. Differences between the first and second eddy-permitting simulations additionally reveal an erroneous influence of sea ice on surface salinity, dense water formation, and the meridional overturning circulation. Simulations are further evaluated in terms of long-term mean ocean heat transport at selected latitudes (for which hydrographic estimates are available) and sea surface temperature errors (relative to observations). By these criteria, closest agreement with observations is obtained for the eddy-resolving simulation. In this simulation, there is also a weak decadal variation in mid-latitudes, with heat transport strongest, by around 0.2 PW, in the mid-1990s. In two of the eddy-permitting simulations, by contrast, heat transport weakens through the study period, by up to 0.4 PW in mid-latitudes. The most notable changes of heat transport in all simulations are linked to a weakening of the subpolar gyre, rather than changes in the meridional overturning circulation. It is concluded that recent changes in the structure of mid-latitude heat transport in the North Atlantic are more accurately represented if eddies are explicitly resolved.