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We present a new regional three-dimensional (3D) slab reconstruction of the Eastern Mediterranean Basin utilising the UU-P07 global tomography model and two earthquake data packages (GCMT and ISC) to produce 3D slab models to a depth of 2900 km. The model data are permissive of the presence of a south-eastward-propagating horizontal tear in the Aegean slab beneath the Rhodope Massif in the Balkanides extending towards the Thermaic Gulf. Alternatively: i) the local pattern of reduced amplitudes at ~ 200km depth could also reflect a different type of lithosphere; and/ or ii) tearing might have been preceded by down-dip stretching, resulting in abrupt thinning of the lithosphere in the extended zone.Further to the southeast, beneath the Peloponnese and Crete, the model data support the existence of multiple subduction-transform (or STEP) faults. The subduction–transforms have since themselves begun to founder, and to roll back towards the southeast. Even further east, beneath Cyprus, the model data appears to support the existence of yet another STEP fault, linking the slab to the east flank of the Arabia indenter. The 3D geometry of the subducted slabs demonstrates ‘lithological steps’ that formed as the lithosphere tore and bent while descending. Previous 3D reconstructions of the region’s deep lithospheric geometry confirmed the presence of fragmented segments but details on: i) the vertical extent of the descended slabs; and ii) the correlation between surface deformation structure and geometry at depth had yet to be established. In order to allow such a correlation, the 3D model was floated [or returned to the planet surface] utilizing a wire mesh with a Delaunay tessellation, using the program Pplates. This enabled area-balancing and therefore a more accurate approximation to the areal extent of the slabs prior to their subduction. The floated slab(s) can be incorporated in a 2D+time tectonic reconstruction to provide additional constraints not available using surface geology. The inferred tears correlate with surface structures such as the Strabo and Pliny trenches between the Hellenic Arc (Aegean Trench) and the Cyprian Trench near the Cyprus Arc, as well as with the seaward extent of the East Anatolian Fault separating the Cyprus Arc and the Arabian indenter. Such correlations between surface and deep lithospheric structures have four-dimensional (4D) implications for episodic closure of the West Tethys suture from its Mesozoic onset, through the tectonically active Tertiary to the present-day.

A major contribution to the crustal growth and construction of the Congo Craton was the addition and preservation of the ≤ 45 000 km2 Kunene AMCG Complex (KC), which straddles the international border between Angola and Namibia. KC magmatism encompasses dominantly juvenile anorthositic rocks (anorthosite, leuco-gabbro, -norite, -troctolite) and A-type granitoids (Red Granite Suite) of mixed crustal and juvenile signature. High-precision U-Pb dates of zircon and baddeleyite from the exposed western parts of the KC (~15 000 km2) in between 1500 and 1360 Ma indicate that both the anorthosites and Red Granites were pulsed and exceptionally long-lived. The remaining eastern portion of the KC can only be imaged using potential field geophysical methods as it is covered by a thin (≤ 300 m) cover of Cenozoic Kalahari sediments. Field mapping and recent remote sensing in the exposed part of the complex, together with airborne geophysics of the entire KC, indicate that the anorthosites were emplaced in up to 12 layered or massive batholiths, which are elliptical in a NNE-SSW or E-W direction. They are commonly separated by relatively thin and elongated KC granitoid bodies and are in tectonic or intrusive contact with Paleoproterozoic basement rocks.Regional horizontal contraction in the Angolan portion of the KC is dated by U-Pb in zircon and Ar-Ar in micas at 1400-1370 Ma. Contraction formed N-S to NE-SW-striking, cm- to km-wide, discrete, syn- to post-magmatic thrust zones mainly localised in KC granitoids. The shear zones are parallel to magmatic foliation in the granitoids and magmatically layered anorthosites. A compilation of crystallisation ages (n = 60) suggests that the regional shortening triggered the magmatism that formed ~ 60% of the exposed KC by mobilising magmas from deep crustal mush zones. In contrast, the southern part of the KC in Namibia exhibits E-W- to ENE-WSW-striking magmatic layering, gneissic foliations and shear zones formed at amphibolite to greenschist facies conditions. These are compatible with north-directed ductile to brittle thrusting over the Angolan KC. Northward thrusting post-dates KC emplacement and is broadly constrained in between 1360 and 1330 Ma by Ar-Ar dating of micas. Airborne aeromagnetic and satellite gravimetric data indicate that the southern KC is parallel to and overlies a crustal and continental-scale geophysical lineament, which is interpreted as the relic of a linear Mesoproterozoic orogenic belt extending to the Kibaran Belt of Central Africa. The orogenic activity was terminated by 1127 Ma, which is the oldest age of a suite of mafic dykes crosscutting post-KC and undeformed capping siliciclastic units. U-Pb dates of detrital zircon and Hf-in-zircon data for these siliciclastic rocks overlap with those of the KC granitoids, indicating local recycling of KC rocks between 1360 and 1127 Ma.Our results highlight that the 1500-1360 Ma period of the Congo Craton was a time of significant crustal growth in the form of voluminous Kunene Complex magmatism. The assembly of the entire KC magmatic edifice was facilitated by syn- to post-magmatic contractional deformation that juxtaposed distinct crustal domains during two newly defined Mesoproterozoic orogenic events.

We report preliminary results based on the construction of a new 3D model for the geometry of the subducted lithosphere of the Nazca Plate. This new 3D model differs from Slab2 in that it enables capture of the slab geometry in greater detail and allows the identification of previously unrecognised potential slab tears, both down-dip and along strike, as well as slab gaping as tears open. These differences in the inferred 3D structure emerge because previous models were excessively smoothed. Here we use the interpolation of line strings derived from the interpretation of individual cross-sections: a method that has the capacity to capture detail, and to accurately drape and thus derive a 3D geometry consistent with the observed hypocenter patterns, by using Delaunay triangulation alongside with 3D grid interpolation. The 3D slab morphology obtained provides insight into the interplay between subduction earthquakes at different depths. The variation in slab morphology reinforces the concept that the megathrust comprises distinct rupture segments that behave differently in terms of their overall seismotectonic behaviour. The slab morphology also links to changes in the broad geodynamics of the subduction zone, with links between shallow, intermediate, and deep seismicity that are consistent with variation in 3D slab morphology. It is also possible to explain the variation in surficial crustal tectonic processes in the context of geodynamic processes inferred to be taking place at depth in specific segments of the descending slab.Surficial structures in the form of megathrust ruptures can be seen to be both a consequence of the highly segmented nature of the overriding plate, particularly in the Peruvian Andes, as well as the deeper variation in 3D slab morphology. For example, the inferred slab tears at the northern (Puna) edge and the southern (Payenia) edge of the Pampean segment may be explained as due to significant variation of the slab morphology and are reflected in changes in the characteristics of earthquake ruptures, and different tectonic modes in the supra-subduction zone lithosphere. Lineaments that separate the segments of the frontal megathrust also coincide with inherited surface fault systems on the South American plate. Clusters of intermediate extensional earthquakes weaken the lithosphere south of the Taltal Ridge (TR) and north of the Juan Fernandez Ridge (JFR). These structures mark the edge of the magmatically inactive plate interface due to the termination of magmatism in shallow dipping slab segments associated with the JFR, or in the flat slab of the Pampean segment.

Peninsular India is a collage of Archaean cratonic domains separated by Proterozoic mobile belts. A number of cratonic basins, known as “Purana basins” in the Indian literature, formed in different parts of the Indian Peninsula during extensional tectonic events, from Paleoproterozoic through Neoproterozoic times. In this contribution, we present a diversity of new geochronological data for different units within the Kaladgi and the Bhima basins, which overlie the western and eastern Dharwar cratons, respectively. The new geochronology data are discussed in terms of depositional history and provenance of these poorly understood Proterozoic intracratonic basins. For the Kaladgi Group, a U–Pb baddeleyite age of 1,861 ± 4 Ma obtained for a dolerite dyke intruding the Yendigere Formation is used to constrain the minimum age of deposition of the lower Kaladgi Group. This result demonstrates that this part of the succession is comparable in age to the Papaghni Group of the Cuddapah Basin, heralding onset of Purana sedimentation at ~1,900 Ma. The detrital zircon populations from the clastic rocks of the Kaladgi and Bhima basins show unique and distinct age patterns indicating different source of sediments for these two basins. Palaeocurrent analysis indicates a change in provenance from south or southeast to west or northwest between the Kaladgi and Bhima clastic sedimentation. New U–Th–Pb and Rb–Sr radiometric dates of limestones and glauconite-bearing sandstones of the Bhima Group (Bhima Basin) and the Badami Group (Kaladgi Basin) indicate deposition at around 800–900 Ma, suggesting contemporaneity for the two successions. Thus, the unconformity between the Kaladgi Group and the overlying Badami Group represents a time gap of up to 1,000 Myr. These new results demonstrate the complex multistage burial and unroofing history of the Archaean Dharwar Craton throughout the Proterozoic, with important implications for exploration of metal deposits and diamonds in Peninsular India. (Less)

A new U-Pb zircon SHRIMP age of 1284 Ma from the Kanigiri granite, India, is reported to help constrain the middle to late Mesoproterozoic tectonic evolution of the Nellore schist belt (NSB). The Kanigiri granite has whole-rock chemical characteristics of A-type granites and is marked by light rare earth element enrichment, a strong negative Eu anomaly, and negative Ba, Sr, P, Ti, and Yb anomalies, indicating feldspar, apatite, and ilmenite/magnetite fractionation. Samples show Y/Nb versus Yb/Ta ratios in the range for granites associated with ocean island basalts. This two-mica granite is peraluminous and alkali-calcic to calc-alkalic, and it has high annite to phlogopite proportions (92%–98%). Strong alignment of flattened mafic microgranular enclaves in the granite, together with relatively high- to moderate-temperature crystal plastic deformation fabric in shear zones within the granite, suggest overprinting of subsolidus deformation over a relict magmatic fabric, a feature not very common in ...

New U–Pb zircon SHRIMP ages are presented and used together with published age and geochemical data and geological observations to review the Palaeoproterozoic evolution of the Angolan Shield of Central Africa. The shield comprises at least three main crustal tectonic domains: the Central Shield Zone in the east and the Central Eburnean Zone and Lubango Zone in the west. The latter extends south into Namibia as the Epupa Metamorphic Complex and further south is exposed in the Kamanjab and Grootfontein inliers. The magmatic events associated with these tectonic domains have been attributed to the formation of a system of arcs that developed along the active western and southern continental margins of the Congo Craton, with peak magmatic events at 2.0–1.96 Ga (Eburnean Event), 1.88–1.83 Ga (Kamanjab-Bangweulu Event) and 1.80–1.77 Ga (Epupa Event). Along the western continental margin, precise age data confirms the presence of Eburnean granitoids from the Huambo-Andulo region in Angola south to the Kamanjab-Grootfontein region in NW Namibia, a distance of over 1000 km. Geochronology and isotope geochemistry indicates that Neoarchaean crust was involved in the formation of these granitoids. Field relations indicate that this ~2.0 Ga granitic crust was metamorphosed and deformed during orogeny at 1967 Ma. Along the southern continental margin within the Kamanjab Inlier a second magmatic event is documented at c. 1.84 Ga, with inherited grains suggesting the involvement of older crust with a similar age to that exposed in SW Angola and in the nearby Grootfontein-Tsumkwe Inlier. This event may be related to magmatism within the basement to the Lufilian Arc and Bangweulu Block. In the Epupa Metamorphic Complex a third magmatic event is documented at c. 1.77 Ga. It is likely that additional events will be recognized with further high precision geochronology, as is observed for instance along the Andean continental margin which has been periodically active at least since the early Paleozoic.

Proterozoic basins in cratonic India, referred to as “Purana Basins”, cover about 20% of the Archean basement of the subcontinent. Although the stratigraphy of most of these basins has been well established, it is only recently that radiometric age constraints have been obtained for some of the sedimentary sequences. This study provides new data for two of the Purana Basins in southern India using SHRIMP U–Pb analysis of detrital zircons. For the Pranhita–Godavari Basin, an age limit of 709 Ma is provided for the age of deposition of the Sullavai Group. This prolongs the duration of Proterozoic sedimentation to approximately one billion years (ca. 1700–709 Ma) and establishes the Venkatpur Sandstones of Sullavai Group as the youngest Purana Basin succession identified in India so far. A major zircon provenance age peak at about 1000 Ma, and zircon ages between 800 and 750 Ma, are correlated with major tectonothermal events in the Eastern Ghats Mobile Belt (EGMB) and intrusion of granites and associated pegmatites at that time. The main provenance area of the Venkatpur Sandstones is interpreted to be the EGMB, with subordinate supply of sediments from the Eastern Dharwar craton. It is suggested that portions of the EGMB must have been an integral part of Peninsular India at the time of deposition of these sediments. For the Cuddapah Basin, new U–Pb detrital zircon age constraints are provided for the Nallamalai Group sediments (Bairenkonda Quartzites within the Nallamalai Fold Belt), the youngest population of which yielded an age of ca. 1550 Ma. This age is similar to the emplacement age of the Vinukonda Granite (1589 ± 4.4 Ma) in the Nellore Schist Belt. This suggests that the Nellore Schist Belt was a source region for the Bairenkonda Quartzites and that deposition continued until at least 1550 Ma. A high-precision Archean age (ca. 3366 Ma) for a detrital zircon grain indicates the presence of Paleoarchean components within the sediment source region. Provided that Paleoarchean rocks are as yet unknown from the largely juvenile Neoarchean Eastern Dharwar craton, there exists a possibility that the Nallamalai Group was sourced from the Ongole Domain of the EGMB.

In an attempt to better understand the tectonic evolution of the continental crust forming SW Angola, zircon grains from the principal Precambrian rock types exposed in the Lubango area have been analysed using the sensitive high-resolution ion microprobe method. U–Pb ages of 2038 ± 28 Ma and 1954 ± 6 Ma were obtained on weakly deformed granite samples from the basement below the Humpata Plateau. The Chela Group on the Humpata Plateau is a relatively undeformed Palaeoproterozoic supracrustal sequence with an eruptive age of 1798 ± 11 Ma on ignimbrite of the Humpata Formation. The age of the northern part of the Kunene Complex is constrained by zircon data from a xenolith of basement gneiss and a mangerite dyke cutting anorthosite, which give an emplacement age of 1385 ± 7 Ma. The c . 2.0 Ga granites below the Chela Group are part of a Palaeoproterozoic granitoid terrane that extends from north of Lubango in Angola into NW Namibia. This terrane is referred to as the Angolan Shield. Regionally, the Angolan Shield is interpreted to be part of a Palaeoproterozoic magmatic arc that extends NE from Angola and Namibia under Phanerozoic cover into NW Zambia. The resultant crustal terrane defined the southern margin of the developing Congo Craton at c . 2.0 ± 0.04 Ga. Supplementary materials: Tables summarizing the zircon U–Pb data are available at www.geolsoc.org.uk/SUP18577.

Kimberlites can be viewed as time capsules in a global plate tectonic framework. Their distribution illustrates clustering in time and space. Kimberlite ages span the assembly and break-up of a number of supercontinents, such as Rodinia and Gondwana. These supercontinents show time lines with (i) broad periods devoid of kimberlite magmatism corresponding to times of continent stability, and (ii) narrow kimberlite emplacement windows corresponding to times of fundamental plate reorganizations. This episodicity implies that kimberlite emplacement events are intrinsically related to particular stages in the life cycle of supercontinents. The onset of kimberlite magmatism is closely associated with thermal perturbations (thermal insulation, mantle upwelling?) beneath a stagnant or sluggish supercontinent. These perturbations may have caused uplift and the onset of continental break-up through fracture zones propagating into the supercontinent. Subsequent spreading and ocean floor development is marked by apparent cusps and jogs in plate motion paths. Resultant strain is accommodated along trans-lithospheric corridors with episodic uplift and erosion and focused kimberlite melt migration. The corridors are manifest as discontinuities in the lithosphere mantle, measured as geophysical gradients and as changes in mantle lithosphere composition. Within the crust, these corridors are expressed as (a) terrane boundaries, (b) incipient continental rifts, (c) fracture zones, or (d) major dyke swarms. Some kimberlite populations are clustered along parallel sets of corridors widely distributed across a large part of a subcontinent and repeated magmatism is seen within many of the clusters. The association of kimberlite occurrences with discontinuities may be ascribed to favorable conditions for melt production and to resultant melt focusing along high strain zones that contain fractures and faults. Such conditions may be attained during different stages in the evolution of continents: (a) supercontinent formation; (b) incipient rifting (driven by far-field stresses?) and onset of continental break-up; and (c) strain accommodation along the continental continuation of oceanic fracture zones during spreading. Type (c) may show concomitant kimberlite magmatism in separate continents after break-up.

A 1400 km-long, 2-D magnetotelluric (MT) profile, consisting of 69 sites at 20 km intervals, across the western part of the Archaean Kaapvaal Craton, the Proterozoic Rehoboth Terrane and the Late Proterozoic/Early Phanerozoic Ghanzi-Chobe/Damara Belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern African lithosphere. The lithospheric structures of the Rehoboth Terrane and Ghanzi-Chobe/Damara Belt have not been imaged previously by geophysical methods. Temperature is the primary control on the resistivity of mantle minerals, and the MT derived lithospheric thicknesses therefore provide a very reasonable proxy for the “thermal” thickness of the lithosphere (i.e., the thickness defined by the intersection of a conductive geotherm with the mantle adiabat), allowing approximate present-day geotherms to be calculated. The work indicates the following present-day average lithospheric thicknesses, to a precision of about ± 20 km, for each of the terranes traversed (inferred geotherms in brackets): Eastern Kimberley Block of the Kaapvaal Craton 220 km (41 mW m− 2), Western Kimberley Block 190 km (44 mW m− 2), Rehoboth Terrane 180 km (45 mW m− 2) and Ghanzi-Chobe/Damara Belt 160 km (48 mW m− 2). A clear relationship between the electrical resistivity structure of the lithosphere and the tectonic stabilisation-age of the terrane is evident. Good agreement between the inferred present-day lithospheric geotherms and surface heat flow measurements indicate the latter are strongly controlled by variations in lithospheric thickness. A significant difference in lithospheric thickness is observed between the Eastern and Western Kimberley blocks, and is consistent with previous seismic tomography images of the Kaapvaal Craton. The present-day lithospheric thickness, and reduced depth extent into the diamond stability field, accounts for the absence of diamondiferous kimberlites in the Gibeon and Gordonia kimberlite fields in the Rehoboth Terrane. Previously published mantle xenolith P–T arrays from the Gibeon, Gordonia and Kimberley fields, however, suggest that the Rehoboth Terrane had equilibrated to a cooler conductive palaeo-geotherm (40–42 mW m− 2 ), very similar to that of Eastern Kimberley Block of the Kaapvaal Craton, at some time prior to the Mesozoic eruption of the kimberlites. The timing and nature of both the thermal equilibration of the Rehoboth Terrane, and the subsequent lithospheric heating/thinning event required to account for its present-day lithospheric structure, are not well constrained. A model consisting of the penetration of heat transporting magmas into the lithosphere, with associated chemical refertilisation, at an early stage of Mesozoic thermalism appears to be the most plausible model at present to account for both the present-day lithospheric structure of the Rehoboth Terrane and an earlier, cooler palaeo-geotherm. Some problems, however, remain unresolved in terms of the isostatic response of the model. Based on a compilation of xenocryst Cr/Ca-in-pyrope barometry observations, the extent of depleted mantle in the Rehoboth Terrane is found to be significantly reduced with respect to the Eastern Kimberley Block: 117 km versus 138–167 km. It appears most likely that the depletion depth in both terranes, at least in the vicinity of kimberlite eruption, is explained by refertilisation of the lower lithospheric mantle.

Southern Africa, particularly the Kaapvaal Craton, is one of the world's best natural laboratories for studying the lithospheric mantle given the wealth of xenolith and seismic data that exist for it. The Southern African Magnetotelluric Experiment (SAMTEX) was launched to complement these databases and provide further constraints on physical parameters and conditions by obtaining information about electrical conductivity variations laterally and with depth. Initially it was planned to acquire magnetotelluric data on profiles spatially coincident with the Kaapvaal Seismic Experiment, however with the addition of seven more partners to the original four through the course of the experiment, SAMTEX was enlarged from two to four phases of acquisition, and extended to cover much of Botswana and Namibia. The complete SAMTEX dataset now comprises MT data from over 730 distinct locations in an area of over one million square kilometres, making SAMTEX the largest regional-scale MT experiment conducted to date. Preliminary images of electrical resistivity and electrical resistivity anisotropy at 100 km and 200 km, constructed through approximate one-dimensional methods, map resistive regions spatially correlated with the Kaapvaal, Zimbabwe and Angola Cratons, and more conductive regions spatially associated with the neighbouring mobile belts and the Rehoboth Terrane. Known diamondiferous kimberlites occur primarily on the boundaries between the resistive or isotropic regions and conductive or anisotropic regions. Comparisons between the resistivity image maps and seismic velocities from models constructed through surface wave and body wave tomography show spatial correlations between high velocity regions that are resistive, and low velocity regions that are conductive. In particular, the electrical resistivity of the sub-continental lithospheric mantle of the Kaapvaal Craton is determined by its bulk parameters, so is controlled by a bulk matrix property, namely temperature, and to a lesser degree by iron content and composition, and is not controlled by contributions from interconnected conducting minor phases, such as graphite, sulphides, iron oxides, hydrous minerals, etc. This makes quantitative correlations between velocity and resistivity valid, and a robust regression between the two gives an approximate relationship of V s [m/s] = 0.045 * log(resistivity [ohm m]) + 4.5.

We report new single zircon ages for two Archean banded gneisses from the Shangani Batholith in central Zimbabwe. The ages of ~2.77 and ~2.84 Ga do not support previous views that the area west of the current exposure of the Tokwe Segment was part of an early- to mid-Archean (>2.95 Ga) domain (Sebakwe Proto-craton). The data correspond to ~2.9 to 2.8 Ga dates for TTG granitoids of the Chingezi Suite. Members of this suite occur as intrusions within the south-central part of the Zimbabwe Craton and are associated with contemporaneous felsic volcanic rocks. On the basis of their age distribution, isotopic characteristics and outcrop pattern with respect to the Tokwe Segment, the formation of these igneous rocks is consistent with the development of a volcanic arc along the western margin of the Sebakwe proto-craton between ~2.9 to 2.8 Ga.

Regional and local structural controls on the emplacement of 1326 Southern African kimberlites and related rocks (kimberlites sensu lato, 11% of which are dated) are analysed using a framework of lineaments defined by combining geology, aeromagnetics, gravity and geomorphological data. Spatial analysis of occurrences within clusters of kimberlites less than 100km across resolves variable trends, depending on the age and position of the cluster; but on a regional scale the distribution of these clusters is statistically controlled by four lineament trends: 040°, 096°, 134° and 165°. Similar regional trends are observed as aspect lineaments that can be followed over large distances from modelling the variation in dip direction of the Southern African topography. These observations suggest that different geological parameters exert a control on the distribution of kimberlites. Local structures may include en-echelon fault arrays, Riedel, R’-, P- or T-structures within trans-continental lithosphere structures (cryptic continental corridors). Many cryptic continental corridors are collinear with fracture zones along the Atlantic and Indian continental margins of Southern Africa, and may have found their origin in events resulting from plate reorganization during the break-up of the supercontinent Gondwana. Fault resistance may have rapidly changed the stress state of the African continent causing the deep lithospheric faults to be the loci of episodic extension, allowing kimberlite fluids to ascend through the faults and cluster within near-surface structures. A progressive age variation of kimberlite magmatism in Southern Africa may be attributed to stress propagation along deep lithospheric fractures.

Late Archaean (∼2.68 Ga) clastic sedimentary rocks of the Shamvaian Group form a shear zone-bounded lithotectonic unit in the Midlands greenstone belt (MGB) of Zimbabwe. The sedimentary rocks are wedged in between mid-Archaean granitoid gneisses and late Archaean mafic and bimodal volcanic rocks. The ≥2000-m thick sedimentary sequence consists of marine turbidite deposits overlain by high-energy shelf sandstone, fluvial braid-plain pebbly sandstone and alluvial fan conglomerate. Deposition took place in a northward-deepening basin with fluvial systems draining a source dominated by granitoid gneiss and banded chert. Basin formation was associated with a west-directed thrusting event, and the basin fill was incorporated into the thrust stack soon after deposition. The Shamvaian Group is similar to modern foreland and forearc successions and possibly accumulated during collision of a continental fragment with an island arc-like complex.

Shallowing-upward carbonate cycles in the 2650 Ma Cheshire Formation, Belingwe greenstone belt (Zimbabwe), closely resemble their Proterozoic and Phanerozoic counterparts. The cycles form part of a karstified carbonate-ramp sequence that is overlain by, and grades basinward into, siliciclastic turbidites. A single section of 74 cycles (1.5 m average thickness) was studied in detail. Two basic cycle types are recognized, both with an asymmetric facies stacking pattern. One cycle type contains open marine, subtidal shale at the base. Shale is intercalated with storm-generated sandstone and grainstone beds that become more common and thicken upward, indicating progressive shallowing. Wave-rippled ooid-intraclast grainstone beds and bedsets overlie shale or form the base of the second cycle type. Grainstone formed at or above fair-weather wave base as shoreface sand sheets in an agitated, shallow subtidal setting. Microbial laminites constitute the top of both cycle types and are interpreted as peritidal deposits. In the upper part of the studied section, microbial boundstones with aragonite pseudomorphs are intercalated with, or overlie, laminites and formed in a supratidal environment. The vertical facies distribution within a cycle is indicative of rapid submergence followed by gradual shallowing of relative sea level. High-frequency eustatic sea-level changes are favored over an autocyclic mechanism and tectonically induced allocyclicity as the controlling mechanism for the cyclicity. Hierarchies of stratigraphic cyclicity occur on different scales and may be a result of the combined effects of several orders of sea-level oscillations. Cycle recurrence ratios correspond well to the Milankovitch frequencies calculated for the Late Archean, suggesting that orbital climatic forcing may have been in operation in Archean times.

Shales of the ∼2.7 Ga Zeederbergs Formation, Belingwe greenstone belt, Zimbabwe, form thin (0.2–2 m) horizons intercalated with submarine lava plain basalts. Shales of the overlying Cheshire Formation, a foreland basin sedimentary sequence, form 1–100 m thick units intercalated with shallow–water carbonates and deep-water, resedimented basalt pebble conglomerates. Zeederbergs shale is characterized by high contents of MgO and transition metals and low concentrations of K2O and LILE as compared to average Phanerozoic shale, indicative of an ultramafic to mafic source terrain. Cheshire shales have similar major and trace element contents, but MgO and transition metals are less enriched and the LILE are less depleted. Zeederbergs shales have smoothly fractionated REE patterns (LaN/YbN = 2.84–4.45) and no significant Eu anomaly (Eu/Eu* = 0.93–0.96). REE patterns are identical to those of the surrounding basaltic rocks, indicating local derivation from submarine reworking. Cheshire shales have rather flat REE patterns (LaN/YbN = 0.69–2.19) and a small, negative Eu anomaly (average Eu/Eu* = 0.85), indicative of a mafic provenance with minor contributions of felsic detritus. A systematic change in REE patterns and concentrations of transition metals and HFSE upwards in the sedimentary succession indicates erosion of progressively more LREE-depleted basalts and ultramafic volcanic rocks, followed by unroofing of granitoid crust. Weathering indices confirm the submarine nature of Zeederbergs shale, whereas Cheshire shale was derived from a source terrain subjected to intense chemical weathering.

Metre-thick horizons of ironstone lithologically similar to ‘sulphide-facies’ banded iron formation, but interpreted as silicified and sulphide-impregnated shear zones, are a common component of the greenstone stratigraphy of the Archaean Zimbabwe craton. Such tectonic ironstones separate different lithostratigraphic units commonly regarded as autochthonous rock sequences. On outcrop scale, shearing along ironstone horizons is indicated by anastomosing foliation domains, folding, boudinage and mylonitic fabrics, and, on a regional scale, by truncation of bedding and/or foliation, an anastomosing geometry of the horizons and duplication or juxtaposition of lithostratigraphic units. Tectonic ironstone formation is attributed to in situ silicification and iron (sulphide)-impregnation of rocks, commonly sediments, by mineralized fluids that penetrated the shear zones and their immediate wall rocks. The shear zones formed as a result of thin-skinned thrust tectonics that gave rise to horizontal accretion, imbrication and juxtaposition of volcanic and sedimentary rock units before deformation associated with granitoid diapirism. As a result, ‘layer-cake’ stratigraphic models of greenstone sequences containing tectonic ironstone ‘layers’ have to be treated with care, and the use of ironstone horizons as stratigraphic markers should be discouraged.

The Chirwa dome in northeast Zimbabwe is situated at the boundary between the Zimbabwe craton and Archaean gneisses of the Zambezi mobile belt. This circular granite intrusion has long been regarded as Archaean granite which was remobilized during Pan-African times and emplaced as a mantled gneiss dome into a Proterozoic metasedimentary sequence. Field mapping, structural work and new zircon dates indicate that the Chirwa dome and surrounding rocks underwent a very different history. The supracrustal sequence was deposited and moderately deformed between ~2613 and ~2601 Ma ago, as bracketed by a provenance date and an intrusive date, respectively. Shortly after deposition, the sequence was thrust westward and juxtaposed against a greenstone terrain of the Zimbabwe craton. This active thrust stack was intruded by successive syntectonic granitoids at ~2601 and ~2593 Ma. The Chirwa dome granite, ~2570 Ma old, represents the last intrusive event before cessation of deformation. Our data indicate that while the central Zimbabwe craton already behaved as a stable crustal block at ~2600 Ma, tectonism was active along its northeastern margin.

The ~2.65 Ga old Shamvaian Group sedimentary rocks occur as a folded succession in the central part of the Bindura–Shamva greenstone belt of Zimbabwe. The strata comprise distinct, shear zone-bounded tectonostratigraphic units which may be stratigraphically arranged as follows. The lower part of the succession is represented by a transgressive, fining-upward sequence of alluvial fan conglomerate, overlain by fluvial braid-plain pebbly sandstone and marine shoreface sandstone. Detritus was derived from a mid-Archaean granitoid-gneiss terrain situated to the east. Sediment supply and subsidence rate must have been high. Shallow shelf sedimentation was followed by deep-water (sub-wave base) deposition by turbidity currents, giving rise to a thick succession of fine to coarse clastic material. The turbidite deposits were locally overlain by shallow-marine sandstone and fluvial to alluvial fan conglomerate. An upward increase in the abundance of intermediate and felsic volcanic clasts suggests an increase in the proximity of a volcanic terrain, such as a volcanic arc. Deposition was followed by layer-parallel shearing during thrust belt-style tectonism. Major shear zones developed preferentially along the contact between shallow- and deep-marine facies associations. Basin initiation may have been related to extensional tectonics, possibly on rifted continental crust, whereas later stages of basin history were characterized by compression, suggesting a foreland or fore-arc basin setting. Sedimentary facies, stratigraphy, and facies distribution are remarkably similar to some late Archaean sedimentary sequences of the Superior Province in Canada.

Detailed structural data from the Midlands greenstone belt show that, in contrast to pre-existing models, the evolution of the greenstone belt involved an early episode of thin-skinned thrusting affecting separate lithological domains. These display different sedimentary and structural-metamorphic histories prior to their juxtaposition across steep, west-directed thrust zones. The domains include 3.5 Ga old gneiss, 2.67–2.88 Ga, mafic–felsic volcanic units and early-syn-tectonic, clastic sedimentary sequences. Concomitant with thrusting along domain boundaries, upright folding and ‘minor’ shear accommodated strain within domains. The early thin-skinned, and later, steeper, thrusting events can be interpreted as progressive stages in an accretionary and crustal thickening sequence, possibly associated with under-plating or low-angle subduction. The clastic sedimentary sequences contain intraformational clasts and show coarsening upward cycles below major early thrust horizons, and may have formed on alluvial fans developing in front of west-moving nappes. Strike-slip motion on the main shear zones in the Midlands greenstone belt only occurred late in the tectonic history and there is no evidence that this resulted in large displacements. A shift of σ 1 , from E–W to N–S during the late stages of Archaean deformation of the greenstone belt can be attributed to extensional collapse following E–W shortening and crustal thickening of the Zimbabwe craton.

Based on the Zimbabwe craton, it is suggested that, during the Archaean, full decoupling between a strong upper crust and a strong upper mantle across a weak detachment zone at the Moho allowed the independent development of crustal and mantle geometries in response to lithospheric shortening. This is an effective way to explain the field observations made in the Zimbabwe craton, which suggest a late-Archaean interplay between lateral accretionary processes through low angle thrust stacking and underplating and deep seated lineament zones with a possible mantle origin. The lineament zones play an important role in the localisation of mineral deposits such as base metals, gold, and possibly diamonds. Thickening of the mantle lithosphere occurred independently from the crust, through early Archaean melt segregation and/or lithospheric underplating.

In order to constrain the temporal relationship between granite (sensu lato) emplacement and metamorphism, isotope work was carried out on the minerals zircon and apatite (U–Pb), garnet (Pb–Pb) and hornblende (Ar–Ar) from wall rock samples in the Shamva area in Zimbabwe. The area, encompassing parts of the Chinamora and Murehwa batholiths and a wedge-shaped greenstone belt segment in between, is commonly quoted in the literature as an example illustrating pluton emplacement processes and deformational models for the Archean. New U–Pb dating of apatite from a boudinaged pegmatite within mafic schists in the batholith–greenstone contact zone has yielded an age of 2619 +28/–24 Ma. This age is interpreted as the best estimation of the intrusion age of this unit, depending on the assumed closure temperature, and provides an upper age limit for the syntectonic emplacement of the now gneissic granites. Pb–Pb dating of late kinematic garnets in cordierite-bearing rocks within the greenstone belt wall rocks gives an age of 2623±8 Ma. Together, this timing of relatively late, syntectonic plutonism and metamorphic mineral growth at ca. 2.62 Ga compares well with existing zircon crystallization ages for felsic volcanics (2645±4 Ma, 2643±8 Ma) and post-tectonic porphyritic monzogranites (2601±14 Ma). Ar–Ar hornblende ages for mafic schists from different areas within the greenstone belt wall rocks range between 2621 and 2498 Ma and have been interpreted to indicate mixing between metamorphic ages and cooling ages. The data support a geological model whereby volcanism and sedimentation are associated with an early phase of regional deformation at ca. 2.64 Ga, which may have started earlier and lasted longer, and evolves into the voluminous emplacement of granites (now gneissic granites) in the batholiths at approximately 2.62 Ga. Emplacement of post-tectonic tabular monzogranites takes place at ca. 2.60 Ga.

The c. 2.65 Ga old sedimentary Cheshire Formation of the Belingwe greenstone belt (BDB), central Zimbabwe, has been studied in detail for the first time to shed some light on the much debated evolution of this classical belt. The Cheshire Formation rests sharply on a mafic volcanic unit (Zeederbergs Formation) and comprises a basal, eastward-sloping carbonate ramp sequence built of shallowing-upward, metre-scale sedimentary cycles. The cycles strongly resemble Proterozoic and Phanerozoic carbonate cycles and might have formed by small-scale eustatic sea level changes. The top of the carbonate ramp is represented by a karst surface. The carbonates are overlain by and grade laterally to the east into deeper water (sub-wave base) siliciclastic facies. Conglomerate, shale and minor sandstone were deposited by high- to low-density turbidity currents and were derived from the erosion of Zeederbergs-like volcanic rocks from the east. Shortly after deposition, the Cheshire Formation and underlying volcanics were affected by a northwest-directed thrusting event. Thrusting gave rise to the deformation of semi-consolidated sediments and resulted in the juxtaposition of a thrust slice of Zeederbergs basalts onto Cheshire sediments. The stratigraphy, asymmetric facies and sediment thickness distribution, palaeogeographic constraints and evidence for an early horizontal tectonic event suggest that the Cheshire Formation formed in a foreland-type sedimentary basin.

In northern Zimbabwe, the Archaean Pfunzi Orogen comprises an east-west-trending belt of migmatitic gneisses separating the Zimbabwe Craton from the Pan-African Zambezi Belt farther north. Previously available Rb-Sr dates for the Pfunzi Orogenic Belt have been interpreted to record formation of granulites at ca 3.0 Ga and subsequent amphibolite-facies metamorphism at ca 2.6 Ga. Here the first conventional IDTIMS U-Pb zircon dates for the Pfunzi Belt are reported, from orthogneisses in the eastern part of the belt in northeastern Zimbabwe. Two intrusive masses of granitic and tonalitic gneiss, together with a granitic leucogneiss inferred to have formed by leucosome segregation during migmatisation, yield crystallisation ages of ca 2.62 Ga. These results are interpreted to date a major tectonothermal event along the northern margin of the Zimbabwe Craton, involving regional amphibolite-facies metamorphism and migmatisation, as well as emplacement of compositionally diverse granitoid plutons and retrogression of older granulites. This event is roughly coeval with orogenesis in the Limpopo Belt along the southern craton margin and with emplacement of the craton-wide ca 2.6 Ga Chilimanzi Granite Suite.

The origin of dome-and-keel structural geometries in Archean granite–greenstone terrains appears to lack any modern analogues and is still poorly understood. The formation of these geometries is investigated using structural and anisotropy of magnetic susceptibility (AMS) data for the Chinamora batholith in Zimbabwe. The roughly circular-shaped batholith is surrounded by ca. 2.72–2.64 Ga greenstones. The batholith granitoid suites have been divided on the basis of their ages and fabric relationships into four distinct units: (i) banded basement gneisses; (ii) granodioritic gneisses; (iii) equigranular granites; and (iv) central porphyritic granites. In the gneissic granites a partial girdle (N–S) of poles to the magnetic foliation is developed that has been folded around a consistent, flat lying magnetic lineation plunging at shallow angles to the E or W. In the equigranular granites, the magnetic lineation generally plunges to the NW. The magnetic foliation has a variable strike, no clear trends can be distinguished. The AMS measurements of the porphyritic granite revealed a NW–SE striking foliation and showed subhorizontal magnetic lineations. The magnetic foliation is subparallel to the macroscopic foliation. Wall rocks are moderately inclined and show radial or concentric lineations, triaxial strain ellipsoids and kinematics that demonstrate off-the-dome sliding and coeval pluton expansion. The results of the observations do not point to a single emplacement process. Neither the observed structural data nor the magnetic fabric support a model envisaging spherically ‘ballooning’. It is argued that pluton diapirism played a major part in the formation of the fabrics in the gneisses, whereas the fabrics in the porphyritic granites reflect emplacement as laccolith-like sheets.

Structural-metamorphic relationships in the 2.71–2.64 Ga Harare-Shamva greenstone belt in northern Zimbabwe demonstrate that the Archean stratigraphy of the Zimbabwe craton can not be regarded as a coherent and nonrepetitive, autochthonous succession of (ultra)mafic and felsic greenstones. The greenstone belt shows an early set of amphibolite-grade D1/M1 structures and associated synkinematic metamorphic assemblages that were formed during an episode of layer-parallel shearing at about 2.64 Ga. This resulted in sequential west directed imbricate stacking and recumbent folding of distinct tectonostratigraphic greenstone sequences and includes the development of a syntectonic sedimentary succession. D2/M2 fabrics, strain patterns, and contact aureoles overprint D1 geometries and are related to the diapiric emplacement of the Chinamora batholith. The structural data indicate that granite-gneiss terrains can be subdivided into two types: (1) structurally emplaced terrains (the Madziwa gneiss terrain) and (2) diapiric domes (the Chinamora batholith). The history of the Harare-Shamva greenstone belt indicates that the late Archean (2.7–2.6 Ga) evolution of the Zimbabwe craton must allow for accretion of diverse crustal fragments including oceanic and back arc mafic crust, volcanic arc felsic crust, continental crust, and related sedimentary sequences. The process of accretion was probably diachronous across the craton and may have involved concomitant diapirism, strike-slip faulting, magma intrusion, and sedimentation.

The Zambezi belt is a key segment of the network of Neoproterozoic/lower Paleozoic orogenic belts in southern Africa that formed during amalgamation of central Gondwana. We present new geochronological data from the easternmost Zambezi belt in northeast Zimbabwe, near the junction between that belt and the Mozambique belt. Allochthonous high-pressure granulite-facies migmatitic and mylonitic rocks at the highest exposed structural levels in this part of the Zambezi belt are tectonically juxtaposed against amphibolite-facies supracrustal rocks, which are intruded by a regionally extensive, sheet-like composite granitoid batholith having peralkaline affinities. This batholith separates the Zambezi supracrustal rocks from a zone of Archean basement that wraps the northeast margin of the Zimbabwe craton and shows variable degrees of structural and thermal overprinting. U–Pb zircon and titanite and 40 Ar/ 39 Ar hornblende geochronological data from the allochthonous granulites are interpreted to record high-grade migmatization at ca 870 to 850 Ma, with pervasive amphibolite-facies retrogression at ca 535 Ma during tectonic emplacement into mid-crustal levels. U–Pb zircon and titanite data from the peralkaline batholith indicate that it crystallized at 805.2±11.1 Ma; partial thermal resetting of the U–Pb system occurred in the same time frame as retrogression of the allochthonous granulites. Hornblende from two samples in thermally overprinted Archean basement farther south yields 40 Ar/ 39 Ar plateau ages of 507.9±2.5 and 491.3±2.1 Ma. Together these new data indicate that tectonic elements in the easternmost Zambezi belt have a protracted history, involving early, lower crustal, granulite-facies metamorphism (ca 870–850 Ma) followed by intrusion of a peralkaline granitic batholith (ca 800 Ma) into supracrustal rocks within the belt. The major tectonostratigraphic units in the easternmost Zambezi belt were juxtaposed under amphibolite-facies conditions at ca 535 Ma, followed by relatively rapid cooling through Ar closure temperatures in hornblende. The 535 Ma event reflects deformation in the Zambezi belt in the same time frame as widespread orogenesis that is recorded in other Pan-African belts in southern Africa and is related to final stages in Gondwana assembly.

The Makuti Group of northwest Zimbabwe is composed of mafic and intermediate biotite-rich gneisses interlayered with quartzofeldspathic gneisses of granitic composition, and minor sedimentary units. The gneisses have experienced a multi-staged metamorphic history, including an early high temperature-high pressure event and subsequent reworking at upper- to mid-amphibolite-facies conditions. They are positioned along the strongly deformed, southern margin of the east-west trending Zambezi Belt, and have been correlated with supracrustal gneiss units along the northern margin of the Zimbabwe Craton. The Makuti Group is characterised by an intensely developed gneissic layering and complex disharmonic folds that resulted from non-coaxial deformation involving repeated stages of transposition. The basal contact of the g roup coincides with a decrease in strain intensity, but not with a directional change of characteristic structural elements (e.g. lineations, fold axes), nor with a clear change in rock types. Pink quartzofeldspathic gneisses of granitic composition are typical for the Makuti Group, but locally intrude basement gneiss as well. The quartzofeldspathic gneisses occur as porphyritic and non-porphyritic varieties that are, invariably, intensely sheared. The age and nature of the basal contact of the Makuti Group and its relationship to the quartzofeldspathic gneisses has been investigated. Samples for single zircon PbPb dating were collected from a felsic biotite gneiss just below (2704 ± 0.3 Ma) and above (2510 ± 0.4 Ma) the lower contact of the Makuti Group at an ‘unconformity’ 2 km northwest of Vuti. Further samples were collected from pink quartzofeldspathic units at the base (737 ± 0.9 Ma), central part (764 ± 0.9 Ma; 797 ± 0.9 Ma) and top (794 ± 0.5 Ma; 854 ± 0.8 Ma) of the Makuti Group. Two samples of Kariba orthogneiss (1920 ± 0.4 and 1963 ± 0.4 Ma) underlying the Makuti Group in the northwest were also collected. In all samples, long-prismatic, colourless to brown, igneous zircon grains were selected. Dates were obtained using a stepwise single-grain evaporation technique. Although this technique only allows minimum age estimates, the dates are highly reproducible, indicating that they approximate emplacement ages. The ages conform with the field observations that the basement has been reworked in the Makuti Group and that the quartzofeldspathic units may have been emplaced as granites. It is proposed that the Makuti Group represents a crustal scale shear zone that partly reworked basement gneisses and acted as a conduit for granite emplacement. Shearing took place in an extensional setting around 800 Ma ago, and may have resulted in the exhumation of lower crustal rocks.

Detailed structural metamorphic data from the Trojan nickel mine area in the Shamva-Bindura Greenstone Belt (SBGB) show that two groups of ductile structures associated with separate amphibolite facies metamorphic assemblages can be distinguished. D 1 /M 1 structures are related to a network of anastomosing shear zones that accommodated W-directed imbricate stacking of the stratigraphy, while metamorphic conditions in the mine area reached about 500°C at pressures of 3-4 kbar. D 1 shears are locally strongly silicified and preserve fine-grained mylonitic textures. They have been wrongly identified in the past as banded iron stones or stratigraphic chert horizons. Truncations of primary layering associated with such fine-grained mylonitic quartzites are tectonic and not stratigraphic in origin, and the units can not be used as tectonic marker horizons, but instead represent glide planes across which tectonic imbrication of the stratigraphy has occurred. An important silicified shear zone of this nature occurs along the boundary of the Iron Mask and Arcturus Formations. D 2 /M 2 structures are related to doming of the Chinamora Batholith, and a contact metamorphic overprint and recrystallisation of M 1 assemblages. M 2 temperatures in the mine area reached 565°C. Such high contact metamorphic temperatures at 3 km from the contact of the batholith can only be explained if the entire Chinamore Batholith was emplaced as a relatively hot (>750°C) intrusive body in an already anomalously hot greenstone sequence. Metamorphic fluids during M 1 and M 2 where CO 2 -rich and carbonate alteration was pervasive. D 1 /M 1 and D 2 /M 2 events may have been separated by as much as 70 Ma. D 1 structures in the Trojan area can be related to a large mantled-gneiss, nappe structure represented by the Madziwa Batholith and a mantle of mafic greenstones to the N of the SBGB. The footwall thrust of this nappe occurs along the N boundary of the Shamvaian sediments in the centre of the SBGB. The imbricate stacking of the stratigraphy in the Trojan area can be interpreted as secondary structures in the footwall of the nappe. Horizontal accretion and stacking of crustal fragments during D 1 was followed by thermal perturbations that resulted in the diapiric rise of domes like the Chinamora Batholith during D 2 . After doming the terrain appears to have cooled and stabilized, with further deformation partitioned into narrow strike-slip shear zones.

The Dando-Kwanza concession is situated in the incised highlands of Bie Province in central Angola. An aerial reconnaissance survey conducted in 2005 drew attention to artisanal alluvial mining, in the vicinity of six kimberlite occurrences discovered during colonial times. Exploration commenced in 2007 and the subsequent 3-year work program led to the discovery of 39 new kimberlite occurrences. The concession is underlain by Archean gneisses of the Central Shield Zone of the Angolan Shield, which have been intruded by Paleoproterozoic granitoids at ~1.97 Ga (this study). The basement is overlain by Paleozoic red beds of the West Congolian Inkisi Group which are host to many of the kimberlites. The Lubia cluster kimberlites occur within the northern part of the concession and were emplaced at ~238 Ma, based on zircon U–Pb and mica-in-kelyphite Ar–Ar geochronology. The kimberlites are pipes, some with multiple lobes, with surface areas of up to 16.7 ha. They occur as resedimented volcaniclastic, pyroclastic, and coherent magmatic varieties and have been classified as Group 1 kimberlites. Facies types have been interpreted as crater deposits with narrow hypabyssal feeders and one calcite kimberlite as extrusive lava. Microdiamond sampling of the kimberlites returned low modeled grades and it is postulated that erosion of an enriched and eluviated ferricrete cap on many of the kimberlites may explain the prevalence of alluvial diamonds immediately downstream of the kimberlites. Low diamond grades and the low to moderate interest major element composition of garnets and chromites may be related to perturbation of the SCLM by high-T melt-related processes during Paleoproterozoic times.

Evans, Rob L.et al.--16 páginas, 6 figuras, A regional-scale magnetotelluric (MT) experiment across the southern African Kaapvaal craton and surrounding terranes, called the Southern African Magnetotelluric Experiment (SAMTEX), has revealed complex structure in the lithospheric mantle. Large variations in maximum resistivity at depths to 200–250 km relate directly to age and tectonic provenance of surface structures. Within the central portions of the Kaapvaal craton are regions of resistive lithosphere about 230 km thick, in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography, but thinner than inferred from seismic body wave tomography. The MT data are unable to discriminate between a completely dry or slightly “damp” (a few hundred parts per million of water) structure within the transitional region at the base of the lithosphere. However, the structure of the uppermost ∼150 km of lithosphere is consistent with enhanced, but still low, conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for Kaapvaal xenoliths. The electrical lithosphere around the Kimberley and Premier diamond mines is thinner than the maximum craton thickness found between Kimberley and Johannesburg/Pretoria. The mantle beneath the Bushveld Complex is highly conducting at depths around 60 km. Possible explanations for these high conductivities include graphite or sulphide and/or iron metals associated with the Bushveld magmatic event. We suggest that one of these conductive phases (most likely melt-related sulphides) could electrically connect iron-rich garnets in a garnet-rich eclogitic composition associated with a relict subduction slab., Three institutions and one company came together to initiate SAMTEX in 2002: the Dublin Institute for Advanced Studies (academia),Woods Hole Oceanographic Institution (academia), the Council for Geoscience (government), and De Beers Group Services (industry). Seven others joined SAMTEX during the four phases of acquisition. In chronological order: the University of the Witwatersrand (academia), Geological Survey of Namibia (government), Geological Survey of Botswana (government), Rio Tinto Mining and Exploration (industry), BHP Billiton (industry), Council for Scientific and Industrial Research of South Africa (government), and ABB Sweden (industry) for the Namibian Power Corporation (government). Other members of the SAMTEX team include: L. Collins, C. Hogg, C. Horan, G. Wallace, M. Miensopust (DIAS), A. D. Chave (WHOI), J. Cole, P. Cole, R. Stettler (CGS), T. Ngwisanyi, G. Tshoso (GSB), D. Hutchins, T. Katjiuongua (GSN), E. Cunion, A. Mountford, T. Aravanis (RTME), W. Pettit, D. Khoza (BHPB), H. Jelsma (De Beers), P.‐E. Share (CSIR), and J. Wasborg (ABB). We gratefully acknowledge the tremendous contribution made to this work by a large number of people involved in several phases of data acquisition across southern Africa. Many, but not all, are employed by SAMTEX consortium members. We also thank Phoenix Geophysics, the Geological Survey of Canada, and the U.S. EMSOC for providing instrumentation. In addition to the funding and logistical support provided by SAMTEX consortium members, this work is also supported by research grants from the National Science Foundation (EAR‐0309584 and EAR‐0455242 through the Continental Dynamics Program), the Department of Science and Technology, South Africa, and Science Foundation of Ireland (grant 05/RFP/ GEO001).

The Bindura-Shamva greenstone belt (northern Zimbabwe) and adjacent batholiths are regarded as one of the type areas for granite-greenstone tectonics. Structural observations and three-dimensional strain analyses have been used to propose a model for the tectonic evolution of this late Archaean greenstone belt. The data indicate that pluton emplacement triggered the deformation of the greenstone sequences and that the deformation history is single-phase progressive rather than polyphase. They do not, however, lead to an unambiguous interpretation of the emplacement mechanism. Structural features are best explained by a combination of pluton diapirism and ballooning plutonism. Criteria used as evidence for pluton-related deformation include the pattern of foliation and stretching lineation trajectories, the variation in strain type and strain intensity, fold types, sense of shear at the batholith-greenstone interface, metamorphic zoning and timing of porphyroblast growth. Specific structural and strain patterns within cleavage triple points, interdomal synclinoria (saddle culminations) and batholith margins demonstrate interference between strain fields developed around each batholith.

Metamorphic conditions within arenaceous, calcareous and argillaceous supracrustal rocks of the Magondi Mobile Belt (Zimbabwe) range from greenschist to granulite facies. Within the high-grade segment, basement gneisses of early Proterozoic age and argillaceous rocks of the Mid-Proterozoic Piriwiri Group are intruded by charnockites and enderbites. Metamorphic mineral assemblages and thermobarometric data for enderbitic granulites of Nyaodza show temperatures of 700–800°C and pressures of 5–7 kbar for the peak of granulite-facies metamorphism. Microthermometry and Raman microspectroscopy reveal that CO 2 , associated with minor N 2 , has been the dominant fluid phase during granulite-facies metamorphism. The chronology of the CO 2 inclusions and the development of microtextures and mineral assemblages in the enderbites indicates that isolated negative crystal shaped CO 2 inclusions in quartz and plagioclase porphyroclasts entrap syn-metamorphic fluids of medium-high densities (0.88–0.90 g/cm 3 ). Lower density (0.71–0.77 g/cm 3 ) CO 2 inclusions in trails and clusters within the same minerals were formed from local re-equilibration and re-entrapment of the former (near-) peak granulitic CO 2 inclusions. As in many other granulites, syn-metamorphic CO 2 is associated with intrusives emplaced near the peak of metamorphism.

The Kaapvaal Craton is one of the world’s best natural laboratories for studying the lithospheric mantle given the wealth of xenolith and seismic data that exist for it. The Southern African Magnetotelluric Experiment (SAMTEX) was launched to complement these databases and provide further constraints on physical parameters and conditions by obtaining information about electrical conductivity structures within the lithosphere. Initially, magnetotelluric data acquisition was planned on profiles spatially coincident with the Kaapvaal Seismic Experiment. However with seven more partners joining the original four through the course of the experiment, SAMTEX was enlarged from two to four phases of acquisition, and extended northwards to cover much of Botswana and Namibia. The complete SAMTEX dataset now comprises MT data from over 730 distinct locations in an area of over one million square kilometres, making SAMTEX the largest regional-scale MT experiment conducted to date. Preliminary images of electrical resistivity and electrical resistivity anisotropy at 100 km and 200 km, constructed through approximate one-dimensional methods, map resistive regions spatially correlated with the Kaapvaal, Zimbabwe and Angola Cratons, and more conductive regions spatially associated with the neighbouring mobile belts and the Rehoboth Terrane. Known diamondiferous kimberlites occur primarily on the boundaries between the resistive or isotropic regions and conductive or anisotropic regions.

Fluid and solid inclusions have been studied in selected samples from a series of spinel-bearing Crdiopside-and Al-augite-series ultramafic (harzburgites, lherzolites, and olivine-clinopyroxene-rich rocks), and gabbroic xenoliths from Hierro, Canary Islands. In these samples several generations of fluid inclusions and ultramafic-and mafic-glass inclusions may be texturally related to different stages of crystal growth. The fluid inclusions consist of pure, or almost pure, CO2. The solid inclusions in the ultramafic xenoliths comprise early inclusions of devitrified ultramafic glass, sulphide inclusions, as well as polyphase inclusions (spinel+clinopyroxene±glass±other silicates) believed to have formed from trapped basaltic melts. Vitreous basaltic glass±CO2±sulphide±silicates are common as secondary inclusions in the ultramafic xenoliths, and as primary inclusions in the gabbroic xenoliths. Microthermometry gives minimum trapping temperatures of 1110° C for the early ultramafic-and mafic-glass inclusions, and a maximum of 1260–1280° C for late inclusions of host basaltic glass. In most samples the CO2 inclusions show a wide range in homogenization temperatures (-40 to +31° C) as a result of decrepitation during ascent. The lowest homogenization temperatures of about-40° C, recorded in some of the smallest CO2 inclusions, indicate a minimum depth of origin of 35 km (12 kbar) for both the Cr-diopside-and Al-augite-series xenoliths. The gabbroic xenoliths originate from a former magma chamber at a depth of 6–12 km.

The Archean Zimbabwe Craton in northeastern Zimbabwe is bounded along its northern margin by the east-trending Zambezi belt, which has been subdivided from south to north into the Archean Migmatitic Gneiss Terrain, overlain by Proterozoic gneiss units of the Marginal Gneiss Terrain and the Allochthonous Gneiss Terrain. The transition from the Zimbabwe Craton into the Migmatitic Gneiss Terrain occurs over a 10–15-km-wide zone across which the metamorphic grade increases from 3 to 4 Kbar and 500 °C in the northern part of the craton, to 6–7 Kbar and 700 °C in the Migmatitic Gneiss Terrain. This P-T (Pressure-Temperature) gradient was established in the late Archean and reflects exhumation of the Migmatitic Gneiss Terrain as a result of D1 sinistral transpression due to oblique collision between the Zimbabwe Craton and an unknown rock mass to the north. D1 structures are absent within the Marginal and Allochthonous Gneiss Terrains. The craton margin and Migmatitic Gneiss Terrain were subsequently intruded and crosscut by the 2.58 Ga Great Dyke. Between 1.1 and 0.5 Ga renewed tectonism occurred along the craton margin, forming the Zambezi belt. Deformation fabrics linked to this are mainly preserved in the Marginal and Allochthonous Gneiss Terrains and include 0.85–1.05 Ga high-P granulite-facies fabrics (D2) extensively overprinted by D3 fabrics that formed ∼0.80 b.y. ago. D3 fabrics preserve mylonitic characteristics across the Marginal Gneiss Terrain, which is part of a 3–5-km-thick crustal-scale, shallowly north- to northeast-dipping shear zone that can be traced along the full length of the Zambezi belt north of the Zimbabwe Craton. This shear zone accommodated extension and exhumation of rocks in the Marginal Gneiss Terrain, the Migmatitic Gneiss Terrain, and the craton margin. A renewed burial and exhumation cycle affected the rocks of the Marginal Gneiss Terrain, the Migmatitic Gneiss Terrain, and the craton margin ca. 0.53 Ga, as units of the Allochthonous Gneiss Terrain overthrust the margin of the Zimbabwe Craton.

In this contribution we discuss the rheology of the Archean continental lithosphere. Decoupling of the upper crust and mantle lithosphere may be a common situation in any tectonically active region and was probably the rule rather than the exception during the Archean. This has major implications for the style of Archean tectonism.

The Shamva gold mine is hosted within the Shamva greenstone belt and is related to steeply dipping reverse-oblique crustal shear zones. Fluid infiltration resulted in widespread sulphidisation, K alteration, chloritisation, silicification and carbonatisation. Fluid inclusions from two types of quartz veins were used to estimate the composition and pressure-temperature conditions of gold mineralisation: (1) Au mineralised sulphide ± carbonate bearing veins; (2) late tensional barren veins. The vein types contain aqueous, mixed H2OCO2 (±CH4) and CO2 (±CH4) rich inclusions. Fluid inclusions in the mineralised and barren veins are similar in composition and resulted from trapping of an immiscible aqueous and CO2 fluid. The pressure-temperature conditions of Au mineralisation are constrained to 250–450°C and 1–3 kbar. The similar pressure conditions found for Au mineralisation and the peak of metamorphism implies that the retrograde pressure-temperature path followed isobaric cooling. Thermodynamic modelling of the fluid shows that: (1) XCO 2 (XCO 2 + XCH 4 ) of the metamorphic fluid ranged between 0.75 and 0.85; (2) XH2O of the Au mineralising fluid ranged between 0.85 and 0.90; and (3) metamorphic temperatures ranged between 500 and 530°C, assuming an oxygen fugacity buffered by quartz-fayalite-magnetite.

Structural-metamorphic data and mineral ages from the northern parts of the Zimbabwe craton indicate that Archean crustal formation and stabilization evolved in two stages. In the Shamva-Bindura greenstone belt, early layer-parallel shear zones began to form at 2670 Ma and accommodated imbricate stacking of oceanic and volcanic arc material between large nappe structures with felsic gneiss cores. The resultant crustal pile of anomalously hot felsic and mafic crustal slices reached isostatic and mechanical equilibrium at a thickness of 35 km. Further shortening of this pile caused strain partitioning into vertical strike-slip zones. The subsequent establishment of an equilibrium geotherm resulted in large-scale crustal melting and diapirism between 2620 and 2600 Ma. The rise of the melt and diapirs caused a second transient metamorphic imprint and much of the strain pattern regarded as typical for the Zimbabwe craton. This diapiric stage led to cooling and stabilization of the craton.