Rift Sequence Research Articles

A reappraisal of the Namibian Damara stratigraphy in part of the Southern Swakop Terrane and its implications to basin evolution

The basin evolution of the Panafrican belts in Namibia was controlled by major rift systems comprising the Kaoko-West Congolian and Damara Branches with their associated parallel rift zones. Rifting along the Kaoko Branch caused continental spreading, while the Damara Branch formed the terrestrial Otjiwarongo and Windhoek Rift Zones, separated by the Usakos Horst, in which the rift related coarse clastics (Etusis Formation) and associated basinal sediments (Khan Formation) were deposited. Thermal subsidence along the Otjiwarongo Rift Zone produced an oceanic basin with thinned continental crust. In the Windhoek Rift Zone, terrestrial conditions prevailed and the newly deposited rift sequences were subjected to erosion. Succeeding the primary rifting was the Chuos Formation glacial/marine event, which produced thick deposits along the northern and western margins of the Windhoek Rift Zone. After the glaciation, a shelf carbonate unit was deposited in the slow subsiding Otjiwarongo Rift Zone. In the Windhoek Rift Zone, however, northeast trending faulting along the Okahandja Lineament produced the submerged Okawakuejawa rift unit in which the Adler dolomite and Quelle calcareous turbidites were deposited. The Okawakuejawa rift unit also divided the Windhoek Rift Zone into a northern Usakos Horst and southern Uitdraai Block. Renewed tectonism and uplift produced rifts splaying from the Okahandja Lineament northward to trend parallel to the Abbabis-Welwitchia Lineament. On the Usakos Horst, basaltic volcanism was associated with the rifting, while those on the Uitdraai block was more fissure like, non fault-related extrusions. The graben floors were submerged while the scarp shoulders were still areal as water laid and arid, non-reworked successions were deposited on them respectively. Further south in the Windhoek Rift Zone, continental separation caused the cessation of the Omusema volcanism and thermal subsidence of the entire Windhoek Rift Zone with dolomite platforms being formed on the Usakos Horst and Uitdraai block terranes. Beneath the platform edges, continuing subsidence created peri-platforms and below the Usakos Horst influx of clastic material produced the Tinkas Formation comprising the alternating dolomite and pelite sequence of the Kudulecke Member and dolomite/pelite turbiditic flows and slump deposits, in incised canyons, of the Jacobs Member. On the Uitdraai block, the Flachen quartzites were deposited onto the platform dolomite, while the Lievental Member dolomitic turbidites were deposited on the slopes. Continuing subsidence caused the north and eastward transgression of the pelitic turbidites of the Fahlwater Formation, which eventually covered the entire Windhoek Rift Zone area. Rifting of a Kibaran-aged continent produced a spreading Kaoko Branch with a subsidiary splayed Damara Branch in which the Rift Zones developed at different rates. The northern Otjiwarongo Rift Zone produced a submarine basin underlain by continental crust, while the southerly Windhoek Rift Zone stayed terrestrial. In the Windhoek Rift Zone, rejuvenation along the Okahandja Lineament lead to renewed rifting ending in continental spreading and thermal subsidence forming a passive continental margin.

Reflection seismic evidence for Caledonian deformed sediments above Sveconorwegian basement in the southwestern Baltic Sea

Two complexes of orogenic structures are interpreted to the south of the Baltic Shield from seismic reflection sections in the southern Baltic Sea. Dipping reflections from the pre‐Mesozoic basement are interpreted as (1) an upper, Caledonian (450–440 Ma) complex of SSW dipping low‐angle thrusts that formed in relation to Avalonia's accretion to Baltica and (2) a lower, Sveconorwegian (1150–900 Ma) complex of west to SW dipping ductile thrusts and shear zones in the crystalline crust of Baltica. The two complexes are separated by a regional seismic marker, the O horizon, from the basal part of Baltica's lower Palaeozoic cover and, in places, also by reflections from an underlying Neoproterozoic clastic rift sequence. The complex tectonic structures were resolved from a network of commercial seismic reflection profiles to 4 s two‐way travel time. The results imply that (1) the late Proterozoic crust of southwestern Baltica extends southward, with little lateral offset, across the Sorgenfrei‐Tornquist Zone, below the southwestern Baltic Sea and the North German Basin: (2) in the study area a distinction should be made between the northernmost occurrence of Caledonian deformation structures and the Caledonian suture proper: (3) Caledonian deformation did not involve the crystalline basement in the study area, but was accommodated by the main decollement, near the top of the O horizon, and by thrusts in the overlying lower Palaeozoic sequence; and (4) with such data it is possible to add more detail and partly revise previous interpretations of the Neoproterozoic and Caledonian tectonic evolution of the southwestern part of Baltica.

Thermo-mechanical role of a Cambro-Ordovician paleorift during the Variscan collision: the NE margin of the Bohemian Massif

The Staré Město (SM) belt (NE margin of the Bohemian Massif) represents a preserved example of an intracontinental Cambro-Ordovician rift that has been shortened during the Variscan orogeny. The rifted sequence consists of gabbroic intrusions and a leptyno-amphibolite complex of Cambro-Ordovician protolith age. The latter suffered medium pressure, granulite facies, Cambro-Ordovician metamorphism associated with extension in a continental protorift. The Variscan tectonometamorphic event is manifested by convergent orogenesis with syn-convergent intrusion of a Carboniferous tonalitic sill, high temperature–medium pressure compressional deformation of gabbros and high temperature metamorphism of peridotites.Numerical thermal and rheological modelling suggests that the SM paleorift after 140Ma of cooling represented a stiff heterogeneity between an easterly situated Cadomian continent and a westerly lying Variscan orogenic root zone. Carboniferous underplating of the paleorift by tonalitic magma was probably responsible for the thermal weakening of both the upper mantle and the mafic lower crust, and the subsequent uplift of the remaining rigid part of the Cambro-Ordovician crust. All the units were later deformed in transpressional heterogeneous greenschist facies regime at a supracrustal level.

Evolution of the Congo rift basin, West Africa: an inorganic geochemical record in lacustrine shales

The inorganic geochemistry and mineralogy of synrift lacustrine shales from the Early Cretaceous Congo rift basin provide insight into source terrane, palaeoclimate and evolution of rift topography. The basin formed in a Late Proterozoic metamorphic belt as South America and Africa began to separate. Within the synrift section, a late rift sequence (Marnes Noires and Argilles Vertes Formations) can be distinguished from an underlying active rift sequence (Vandji, Sialivakou and Djeno Formations), based on diminished faulting and more uniform subsidence.Provenance did not vary significantly until deposition of the youngest part of the rift sequence. Al2O3/TiO2 and K2O/Na2O ratios, generally constant, rise sharply in upper Marnes Noires and Argilles Vertes, demonstrating decreased contribution from Proterozoic volcanic sources. Parameters including (quartz+feldspar)/total clay and SiO2/Al2O3 suggest that depositional systems reorganized during lower Djeno deposition, possibly due to rejuvenation of faulting. Grain size decreases in the late rift section; however, the parameter SiO2/Al2O3 increases. This is attributed to chemical or biogenic deposition of silica.The proportion of chemical sedimentation increases upward in the synrift section, peaking in the Marnes Noires Formation, where the concentrations of organic carbon and carbonate and the proportion of SiO2 to siliciclastic‐associated elements reach a maximum. This is interpreted as resulting from input of high dissolved chemical load in the late rift stage and is attributed to increased chemical weathering in the basin as rift topography diminishes. The slower flow of ground and surface water to the rift lakes enhanced weathering and dissolution of minerals in rocks and sediments surrounding the rift lake. The carbon isotopic composition of carbonate decreases from +6 at the base of the rift section, associated with carbon from Late Proterozoic carbonates, to 0, indicating increasing contribution of light carbon; the source of light carbon is interpreted as vegetation, which increases as rift topography degrades.

Three-dimensional subsidence analysis and gravity modelling of the continental margin offshore Namibia

SUMMARY Seismic re£ection pro¢les and gravity anomaly data have been used to determine the structure and evolution of the Namibian continental margin. In comparison to other margins, the gravity anomaly at the Namibian margin shows a number of distinctive features. It lacks an oishore gravity ‘low’, and, despite the presence of up to 9 km of sediments, the gravity ‘high’ is displaced landwards of the maximum sediment thickness. In an attempt to explain these features, the Early Cretaceous^Recent stratigraphic record is analysed using a combined 3-D backstripping and gravity modelling technique that enables constraints to be placed on the long-term mechanical properties of the lithosphere. A neutral depth of necking and a relatively high £exural rigidity (or equivalent elastic thickness, Te ,o f*25 km) can explain part of the anomaly, at least in the south of the margin. However, large residual anomalies exist that can be reduced by the presence of pre-rift low-density sediments within the zone of maximum stretching, south of 22 0 S, high-density volcanic material at the eastern pinch-out of the lower rift sequence and a lateral sediment density variation across the shelf, slope and rise. In addition, the presence of a magmatic body at the base of the crust, provided that the £exural strength of the margin is high, signi¢cantly improves the ¢t between observed and calculated gravity anomalies. Recovered stretching factors enable predictions of the geometry of the Moho to be made. Palaeobathymetry is estimated along the entire margin through time by comparing the subsidence determined from backstripping to that predicted by simple models of rifting. These estimates are within the errors of observed palaeowater depths where well control exists. The requirement of high Te rather than Te that increases with time suggests that stretched continental crust extends oishore for some distance and the continent^ocean boundary occurs in relatively deep water near magnetic anomaly M4. Reconstruction of the gravity ¢eld of the South Atlantic at 100 Ma illustrates that, in terms of their gravity signature, the South Atlantic margins are asymmetric.

Source basins for sediment-hosted stratiform Cu deposits: implications for the structure of the Zambian Copperbelt

The Central African Copperbelt in Zambia and the Democratic Republic of Congo is the world's largest sediment-hosted stratiform Cu province. The source for the Cu in sediment-hosted stratiform Cu deposits is generally believed to be thick sections of oxidised siliciclastic sediments (red beds) and volcanic rocks deposited in early rift sequences underlying or laterally adjacent to the ore-bearing sediments. The volume of red beds beneath or lateral to the Zambian portion of the copperbelt relative to the amount of known metal in the deposits is small in comparison to the volumes of source beds in other well-known districts such as the Polish Kupferschiefer and the White Pine District, USA. Previous structural studies suggest that the rocks hosting the Zambian Cu deposits may be allochthonous or para-autochthonous. The apparent absence of source sediments for the metals, combined with the known structural geology, strongly suggests that the Zambian deposits were tectonically displaced from their source rocks. Defining the present location of the source basins would constrain the amount of tectonic transport in this portion of the Lufilian Arc and would stimulate mineral exploration in new areas.

U-PB AND SM-ND GEOCHRONOLOGICAL CONSTRAINTS ON THE CRUSTAL EVOLUTION AND BASEMENT ARCHITECTURE OF CEARÁ STATE, NW BORBOREMA PROVINCE, NE BRAZIL: IMPLICATIONS FOR THE EXISTENCE OF THE PALEOPROTEROZOIC SUPERCONTINENT "ATLANTICA"

Sm-Nd whole-rock and U-Pb zircon geochronological studies of the basement gneisses of Ceará (NW Borborema Province, NE Brazil) have identified two major pulses of Paleoproterozoic crustal growth within the state; the first between ca. 2.35 to 2.30 Ga, and the second from ca. 2.19 to 2.05 Ga. The former was characterized exclusively by juvenile growth and accretion, whereas the latter involved the amalgamation of new juvenile crustal material, reworked or enriched crust, and Archean crustal fragments. It was during this second event, also known as the Transamazonian orogeny (ca. 2.2 to 2.0 Ga), that Ceará’s fundamental crustal framework was assembled. Isotopic and geologic data indicate that this assembly involved the fusion of three distinct crustal blocks; the Northwest Ceará (AKA Médio Coreaú) Domain (NCD), the Central Ceará Domain (CCD), and the Rio Grande do Norte Domain (RND). This network of crustal blocks was subsequently affected by an episode of Late Paleoproterozoic intracratonic rifting at around 1.8 Ga. The presence of similar rift sequences of this age in other parts of South America and Africa lend support to the existence of the Paleoproterozoic supercontinental mass Atlantica. Based on the available data, this supercontinent appears to have included the Paleoproterozoic (and older) basement of the Brasiliano/Pan-African provinces in Brazil and West Africa as well as the cratons adjacent to them.

U-PB AND SM-ND GEOCHRONOLOGICAL CONSTRAINTS ON THE CRUSTAL EVOLUTION AND BASEMENT ARCHITECTURE OF CEARÁ STATE, NW BORBOREMA PROVINCE, NE BRAZIL: IMPLICATIONS FOR THE EXISTENCE OF THE PALEOPROTEROZOIC SUPERCONTINENT "ATLANTICA"

Sm-Nd whole-rock and U-Pb zircon geochronological studies of the basement gneisses of Ceara (NW Borborema Province, NE Brazil) have identified two major pulses of Paleoproterozoic crustal growth within the state; the first between ca. 2.35 to 2.30 Ga, and the second from ca. 2.19 to 2.05 Ga. The former was characterized exclusively by juvenile growth and accretion, whereas the latter involved the amalgamation of new juvenile crustal material, reworked or enriched crust, and Archean crustal fragments. It was during this second event, also known as the Transamazonian orogeny (ca. 2.2 to 2.0 Ga), that Ceara's fundamental crustal framework was assembled. Isotopic and geologic data indicate that this assembly involved the fusion of three distinct crustal blocks; the Northwest Ceara (AKA Medio Coreau) Domain (NCD), the Central Ceara Domain (CCD), and the Rio Grande do Norte Domain (RND). This network of crustal blocks was subsequently affected by an episode of Late Paleoproterozoic intracratonic rifting at around 1.8 Ga. The presence of similar rift sequences of this age in other parts of South America and Africa lend support to the existence of the Paleoproterozoic supercontinental mass Atlantica. Based on the available data, this supercontinent appears to have included the Paleoproterozoic (and older) basement of the Brasiliano/Pan-African provinces in Brazil and West Africa as well as the cratons adjacent to them.

Along-Axis Segmentation and Growth History of the Rome Trough in the Central Appalachian Basin1

The Rome trough, a northeast-trending graben, is that part of the Cambrian interior rift system that extends into the central Appalachian foreland basin in eastern North America. On the basis of changes in graben polarity and rock thickness shown from exploration and production wells, seismic lines, and gravity and magnetic intensity maps, we divide the trough into the eastern Kentucky, southern West Virginia, and northern West Virginia segments. In eastern Kentucky, the master synthetic fault zone consists of several major faults on the northwestern side of the trough where the most significant thickness and facies changes occur. In southern West Virginia, however, a single master synthetic fault, called the East-Margin fault, is located on the southeastern side of the trough. Syndepositional motion along that fault controlled the concentrated deposition of both the rift and postrift sequences. The East-Margin fault continues northward into the northern West Virginia segment, apparently with less stratigraphic effect on postrift sequences, and a second major normal fault, the Interior fault, developed in the northern West Virginia segment. These three rift segments are separated by two basement structures interpreted as two accommodation zones extending approximately along the 38th parallel and Burning-Mann lineaments. Computer-aided interpretation of seismic data and subsurface geologic mapping indicate that the Rome trough experienced several major phases of deformation throughout the Paleozoic. From the Early(?)-Middle Cambrian (pre-Copper Ridge deposition), rapid extension and rifting occurred in association with the opening of the Iapetus-Theic Ocean at the continental margin. The Late Cambrian-Middle Ordovician phase (Copper Ridge to Black River deposition) was dominated by slow differential subsidence, forming a successor sag basin that may have been caused by postrift thermal contraction on the passive continental margin. Faults of the Rome trough were less active from the Late Ordovician-Pennsylvanian (post-Trenton deposition), but low-relief inversion structures began to form as the Appalachian foreland started to develop. These three major phases of deformation are speculated to be responsible for the vertical stacking of different structural styles and depositional sequences that may have affected potential reservoir facies, trapping geometry, and hydrocarbon accumulation.

Trace element and Sm–Nd systematics of volcanic and intrusive rocks from the 3 Ga Lumby Lake Greenstone belt, Superior Province: evidence for Archean plume–arc interaction

The metavolcanic Lumby Lake belt comprises mafic tholeiites intercalated with thin felsic pyroclastic units. Al-undepleted komatiites are present towards the top of the stratigraphy. Identification of Al-depleted pyroclastic komatiites associated with chemical and siliciclastic sedimentary rocks indicates the upper portion of the 3 Ga Lumby Lake stratigraphy is directly comparable to a stratigraphic sequence developed on a paleoregolith in the nearby Steep Rock greenstone belt. The lower portion of the Lumby Lake sequence therefore represents a rarely preserved association of komatiite–tholeiite and calc alkaline volcanism developed prior to rifting episodes identified in ∼3 Ga terranes of the northern Superior Province. Al-undepleted komatiites are characterised by elevated MgO (18–24 wt.%) and Ni (600–1500 ppm) contents, in conjunction with variable LREE depletion (La/Sm n =0.5–0.8). Intercalated spinifex textured komatiitic basalts possess lower MgO (10–11 wt.%) and Ni (150–180 ppm) and flat to weakly enriched LREE (La/Sm n =0.9–1.1). Pyroclastic Al-depleted komatiites (Al 2O 3/TiO 2=4–5) are strongly LREE enriched (La/Yb n =6.7–10.5) with variable HFSE anomalies. Compositionally uniform tholeiites with variably depleted to enriched LREE (La/Sm n =0.8–1.2) and minor HFSE anomalies dominate the stratigraphy of the belt. A distinct subset of tholeiites, occurring towards the centre of the belt, is characterised by low Al 2O 3/TiO 2 ratios, LREE enrichment and the absence of HFSE anomalies. Minor intermediate (SiO 2=53–64 wt.%) volcanic rocks with pronounced REE fractionation (La/Yb n =0.8–1.1), high Al 2O 3/TiO 2 and Zr/Y ratios also occur throughout the belt. Two distinct subtypes of felsic pyroclastic rocks are recognised intercalated sporadically throughout the stratigraphy. Both types display pronounced LREE enrichment (La/Sm n =3.9–6.1) but Type 1 has strongly fractionated HREE patterns (Gd/Yb n =1.5–4.6) whereas, Type 2 HREE patterns are generally flat (Gd/Yb n =1.7–1.9). Intrusive counterparts to both subtypes have been identified within the Marmion Lake batholith to the south of the greenstone belt. The felsic subtypes may be derived from similar parental magmas by variable degrees of hornblende involvement during fractionation processes. Sm/Nd isotope systematics from a range of rock compositions have yielded ε Nd values of +2 to +5, typical of the range for Archean volcanic and intrusive rocks. Komatiite–tholeiite associations within Archean terranes are interpreted as the result of plume related magmatism likely in a geodynamic setting comparable to Phanerozoic oceanic plateaux. In contrast intermediate and felsic rocks are typical of calc alkaline suites generally attributed to Archean subduction related environments. The coeval eruption of the two magma suites in the Lumby Lake belt is best interpreted as the result of the sporadic subduction of plume-modified oceanic spreading centres over 10s of m.y. and the eventual impingement of a mantle plume upon an active subduction zone. This process can also account for the young age of basement material relative to overlying rift sequences in 3 Ga terranes of the Superior Province.

Geomagnetic paleosecular variation in the Brunhes period, from the island of El Hierro (Canary Islands)

We have sampled a sequence of 69 flows in the island of El Hierro (Canary Islands). 46 of them belong to the El Golfo formation and 23 to the most recent rift formation. Radiometric dating brackets this section between 442 ka and 134 ka with six intermediate datings. All the samples were stepwise demagnetized, partly with alternative field, partly thermally with very similar results. The large majority of the samples are characterized by a single stable component of magnetization. ChRM isolated after the first steps of demagnetization are all of normal polarity. Inclinations are on average shallower than expected from a geocentered dipole field, but consistent with results from other areas at the same latitude. Mean declinations of the rift and El Golfo sequences are both easterly deviated (about 5° and 15° respectively) and significantly different suggesting that the section has undergone a progressive clockwise rotation. VGP scatter and directional scatter calculated for the rift sequence are consistent with available data from other geographical areas, and also with recent models of paleosecular variation. On the other hand, a significantly smaller directional scatter is obtained for the lower El Golfo sequence, suggesting that a period of low secular variation occurred in the Canary Islands area between approximately 440 and 260 ka.

Templates from mainland Africa and the Red Sea for interpreting the early evolution of the South Atlantic

Abstract Findings drawn from the Tertiary pre-rift, rift and post-rift sequences of Africa and the Red Sea are applied to the South Atlantic margins with a view of aiding hydrocarbon exploration. In relation to gravity and magnetic prediction of top basement, template rifts tend to follow pre-existing lines of weakness and the upper crust underlying these rifts may have geophysical characteristics which are different from those of the rift flanks. Template rifts may or may not be associated with volcanicity. In sparsely volcanic rifts, syn-rift volcanism is located on rift axis structural highs. Extensive volcanism is accompanied by regional doming and erosional loss of the pre-rift section is likely in rift sectors preceded by volcanism. Consequently, it is doubtful whether pre-rift sediments are preserved beneath the major volcanics offshore from the Parana-Etendeka areas in the South Atlantic. The Kalahari Group of southern Africa is identified as a good analogue for pre-rift reservoir objectives in the South Atlantic, and facies and thickness criteria are discussed that help discriminate pre-rift sequences from rift sequences. Where sedimentation has kept pace with rift subsidence, so that fault scarp facies have not developed, then the Okavango Delta provides a useful analogue. Parallels are drawn between major sand dykes interpreted from seismic in the deepwater deposits of Lake Tanganyika and similar units within similar facies in the Recôncavo Basin of Brazil. Direct analogue links between the Red Sea and the South Atlantic are less strong, mainly because marine incursions, and salt deposition, occurred at a later structural stage in the South Atlantic. The onset of salt deposition in the Gulf of Suez-Red Sea occurred whilst rift block faulting was still in progress, and the post-rift sag phase, though partly salt filled, rapidly turned to fully-marine deposition. The earliest part of the post-rift sag phase in the South Atlantic is represented by laterally extensive lacustrine source rocks. Salt deposition in the South Atlantic occurred only after full development of the sag phase.

Tectonostratigraphical development of the Upper Karooforeland basin: Orogenic unloading versus thermally-induced Gondwana rifting

The origins of the Karoo Basin have never been adequately explained, but twomain models have been suggested: (1) a purely foreland basin (retro-arc) model; and (2) a polyphase successor basin, beginning with extension during Upper Table Mountain Group (Silurian) times to a foreland basin in the Early Karoo (Permian) and extension in the Upper Karoo (Upper Triassic-Jurassic). Subsidence and forward modelling on the known stratigraphy of the Karoo Basin do not fit a simple foreland basin model. This is particularly true for the Upper Karoo where the stratigraphy, stacking patterns and age of the succession suggest that the foreland basin model is not viable, and that continental extension played a major role in late basin development. Evidence for extension, and when this occurred, is provided by: (1) sequence stratigraphical identification of a major Middle-Late Carnian low stand sequence boundary unconformity, related to uplift of a newly emergent source area in the southeast; and (2) the presence of volcanic detritus in Upper Karoo sediments, chemically similar to the Karoo basalts, suggesting a possible chemogenetic link between them and, by implication, with early volcanism and incipient Gondwana rifting, now well-preserved in the rift sequences of East Antarctica. This early volcanism, which began not later than the Carnian (∼230 Ma), and provenance uplift, may be related to a thermal anomaly linked to mantle plume-induced updoming, crustal extension and faulting approximately 40 Ma prior to maximum uplift preceeding Karoo Basalt eruption at 183 Ma. Thermal uplift occurred some 200 km off the present southeast coast of South Africa, just south of the Karoo Hotspot, but away from the direct influence of any known hotspot or hotspot track. This uplift, aligned parallel to the southeast coast of South Africa, at a high angle to the Cape Fold Belt, was located close to the site of later rifting and separation along the Agulhas Falkland Fracture Zone.

The palaeozoic Koonenberry Fold and Thrust Belt, Western NSW: a case study in applied gravity and magnetic modelling

Geophysical modelling combined with recent geological mapping indicates that the Koonenberry belt of far western NSW is a Mid Palaeozoic fold and thrust system.This new interpretation relies upon the different petrophysical and structural attributes of four distinct tectonostratigraphic packages.Package I comprises mixed, multiply deformed, Late Neoproterozoic-Late Cambrian rift and continental margin sequences. Package II comprises Late Cambrian-Early Ordovician mixed carbonate-siliciclastic facies, and has been subjected to two minor and one major deformation event. Package III contains fluvial-lacustrine red beds and volcanics with one major and one minor set of folds. Package IV is the Late Devonian Mulga Downs Group, a generally flat-lying fluvial cover sequence with restricted folding.Fault kinematics are constrained by analysis and modelling of geophysical data, which indicate across-strike repetitions of various sequences. High spatial frequency magnetic linear features require steep surface dips, but listric character is demanded by lower frequency magnetic anomalies that are best fitted by sub-horizontal bodies at mid-crustal depths. Detailed analysis shows many anomalies are skewed to the east near the positions of suspected faults. However, the major Koonenberry Fault is a west-dipping back-thrust. These features strongly suggest that the Koonenberry belt is a west-vergent thrust package that detaches in the mid-crust. The principle fold-thrust deformation occurred during the Silurian, prior to deposition of package III, and probably corresponds to the Benambran event in the Lachlan Fold Belt. This deformation overprints a Late Cambrian event that has deformed package I.These conclusions indicate that the Koonenberry belt is a zone of overlap between the Lachlan and Delamerian Orogens.

Gravity signature of the western Paraná basin, Brazil

Volcanism in the Paraná basin, one of the largest continental flood basalt provinces on Earth, was contemporaneous with the Early Cretaceous opening of the South Atlantic Ocean. Problematic is the lack of crustal extension associated with this flood basalt event, although three Palaeozoic–Jurassic extensional episodes have been reported in parts of the basin. Gravity measurements at 185 new stations were established to constrain the geometry and magnitude of Bouguer anomalies that characterise the poorly understood western portion of the Paraná basin, and to interpret new and existing data in light of its poly-phase extensional history. The Bouguer map of the Paraná basin is characterised by two gravity lows (up to −100 mGal) in the eastern and western portions with a relative high in its centre (around −50 mGal). The spectral characteristics of the N–S-oriented western gravity lows and flanking regions suggest density contrasts at 7.4–4 km, which we interpret as variable depth to basement beneath the Palaeozoic–Cretaceous sedimentary and Cretaceous volcanic rocks. Forward modelling (2.5 D) of two profiles across the N–S-trending western gravity low shows that the low can be explained by shallow, steeply deepening structures, bounding what we believe to be a rift basin buried beneath the Mesozoic flood basalts and sedimentary strata. Comparison with other gravity data, heat flow, contour maps of crustal extension, basement and structural maps all support our interpretation of a pre-Cretaceous rift basin. We also compare our results to Palaeozoic–Mesozoic rift basins in Brazil, the North Sea and Central Africa with similar gravity signatures and better subsurface control. These comparisons also support a 3–5 km-deep, thick rift sequence beneath the western Paraná basin sequences.

The Bashkirian “Fenestella Shales” and the Moscovian “Chaetetid Shales” of the Tethys Himalaya (South Tibet, Nepal and India)

The “Fenestella shales” are a mid-Carboniferous marker unit which has long been described from classic localities of the NW Himalaya (Kashmir, Spiti). Correlative shaly units have recently been traced in central Nepal and as far as South Tibet, where they yielded varied brachiopod assemblages indicative of Bashkirian age. A second distinct interval of black shales, characterized by the abundance of chaetetids and directly underlying the widespread Gondwanan diamictites, has been dated as Moscovian in Spiti and represents the youngest fossiliferous horizon hitherto identified in the Upper Carboniferous of the Tethys Himalaya. The “Chaetetid shales” are recognized also in Manang, whereas in South Tibet the stratigraphic framework still needs improved definition. These major fossiliferous black shale units, marking repeated transgressive events in the middle part of the Himalayan rift sequence, have not only major stratigraphic significance but also represent a fundamental landmark in palaeogeographic and palaeoclimatic reconstructions of Northern Gondwana. With the onset of continental rifting, arid tropical climates at the close of the Tournaisian were replaced by temperate humid conditions in the Visean-Serpukhovian, when diamictites were deposited in South Tibet. After this first cooling stage, the “Fenestella shales” mark a widespread transgression at the very beginning of the Late Carboniferous, coupled with reduced tectonic activity and temperate to temperate-warm climates. After renewed tectonic activity during a second cooling episode, marked by local deposition of diamictites in central Nepal, the “Chaetetid shales” represent another major transgression in the Moscovian, shortly preceding the final and most intense cooling event marked by deposition of glacio-marine diamictites in the whole Tethys Himalaya from Kashmir to South Tibet during the latest Carboniferous/earliest Permian. Two fossiliferous horizons containing very similar brachiopod faunas of early Late Carboniferous age have recently been found also in North Karakorum, at lower southern latitudes, where climatic conditions always remained temperate and there is no trace of Upper Palaeozoic glacial deposits or ice-rafted debris.

Structure, contents and setting of Pb‐Zn mineralisation in the McArthur Basin, northern Australia

Quantitative regional geophysical analysis of the McArthur Basin in northern Australia has shown that it includes sequences with a total thickness in excess of 20 km. Little of the deeper section, which includes thick volcanic and rift sequences, is exposed but the implied stratigraphy is consistent, and may be grossly correlated, with the adjacent western sequence of the Mt Isa Basin. Thick basal felsic and covering flood basalts can be inferred within the primary rift structures. Thinner cover sequences, including the predominantly siliciclastic Tawallah and dolomitic McArthur Groups, are developed in subsidiary basins controlled by underlying rift elements while the overlying Roper Group is much thicker and more widespread. Major erosive events are recognisable across the basin. Most tectonic activity has been focused in the zone known as the Batten Trough although this name is not appropriate because the region has been largely uplifted throughout its history. Other uplifted zones such as the Murphy Tectonic Zone tend to be oriented east‐southeast to east‐northeast across an original crustal grain which may have been oriented northwest‐southeast and north‐northeast‐south‐southwest. The basin may have evolved from a series of aligned convective cells which produced a disjointed rift pattern along strike. Older basin elements were not simply connected but were overtopped and the covering sequences have interlinked. No clearly defined ultimate limits have been recognised for the basin which appears to be an extension of the Mt Isa Basin to the southeast. Significant Pb‐Zn mineralisation within the basin appears to be related to regions where rift margins have been persistently uplifted. Known mineralised sites lie on the rim horsts in zones where crustal fracturing is focused, and where young granitoids, which may have influenced basin circulation, are nearby.

Late Precambrian through Early Paleozoic Stratigraphic and Tectonic Evolution of the Nanling Region, Hunan Province, South China

The Late Precambrian through Silurian tectonic evolution of east-central South China is modeled in terms of a history of rift, drift, and collision during Late Proterozoic, Sinian, and Late Ordovician-Early Silurian times, respectively. We review the regional stratigraphie development of this area, focusing particularly on north-central Hunan province, and argue from our observations and those of others that the Jiangnan, Xuefeng, and Jiuling ranges of the Nanling realm approximately demarcate the paleogeographic transition in Sinian to Ordovician times of shelf to off-shelf environments developed along a passive-type continental margin that started rifting in the pre-Sinian Late Proterozoic. The rift sequence is recorded by the Penhsi (= Banxi) Group, which rests unconformably above an older-presumed Middle to early Late Proterozoic-low-grade metamorphic basement. The Penhsi varies markedly in thickness but is everywhere characterized by nonmarine to paralic clastic facies. The Penhsi conformably to disconformably underlies the Sinian through Lower Paleozoic sequence throughout central South China, which developed along an E-facing, passive-type continental margin. This passive-type margin was destroyed by the Guangxian Orogeny. The Guangxian Orogeny was marked initially by the northwestward progradation of deep-marine turbidites of Late Ordovician age in the most off-shelf regions, progressing to earliest Silurian age on the shelf to the northwest. Folding and concomitant thrusting in the off-shelf regions, and subsequent erosion beneath the unconformably overlying nonmarine Middle Devonian strata, truncate the stratigraphie record of the orogen within the Early Silurian. Farther northwest, in regions undisturbed by the Guangxian Orogeny, Silurian foreland-basin sedimentation included the entire Lower Silurian succession, which grades rapidly upward from basinal to inter-tonguing marine and nonmarine elastics. This reflects a change from flexurally induced subsidence first outpacing local sedimentation, followed by sedimentation outstripping and then keeping pace with subsidence.

Structural segmentation and the influence of basement structure on the Namibian passive margin

Passive margins are segmented into discrete elements by segment boundaries exhibiting a variety of structural styles. The Namibian margin is bounded by a Hinge Line that is offset by segment boundaries. The most important is the Cape Cross Segment Boundary. Here, pre-existing basement lineaments and ‘Karoo’ rift elements were reactivated during late Mesozoic rifting. It marks the northern limit of the earliest of three rift sequences and relates to a temporal change in break-up and formation of the South Atlantic. A similar change occurs across other important segment boundaries but smaller examples lack this. Major segment boundaries with a temporal change in rifting and break-up exhibit a strong connection to the basement province.

Application of magnetic methods to deep basin structures

Although magnetic methods have long been used in basin studies the data are often underutilised since the formations of a relatively undeformed basin approximate sub-horizontal tabular sources except at basin edges. Horizontal sources are known to generate negligible anomalies. Very small angular deviations from tabularity, or horizontality, however, will generate very large responses at basin scale. This response is largely independent of source contrast, thickness, dip, or the orientation of either the source or the field but is very sensitive to small changes in dip differentials between upper and lower surfaces of the source.Classical rift sequence forms may account for many of the anomalies observed in basins and no presumption of basement sources is universally justified. Anomalies must be treated with basin perspective and scale and not as isolated elements.