Abstract The Early Cambrian palaeogeographical enigma arises when tectonic reconstructions are made using palaeoclimatic v. palaeomagnetic data that result in possibly contradictory tropical, mid-latitude, and south polar locations for major continents. For example, NW Africa and Cadomia may have lain in a tropical zone (0° to ±30° latitude) based on the presence of archaeocyath reefs, minor evaporites, and carbonate platforms at c. 520 Ma ± 5 Ma or, alternatively, NW Africa and Cadomia may have lain in a south polar zone (90° to 60° south latitude) based on palaeomagnetic constraints. Greater Avalonia may have evolved independently from NW Africa if a dropstone constraint implying polar latitudes at c. 530 Ma and a palaeomagnetic constraint implying c. 50° latitude at c. 505 Ma are accommodated. We show here how counterclockwise rotation of Gondwana during the Cambrian about an interior axis may solve the enigma. Gondwanan apparent polar wander becomes consistent with tropical conditions inferred for NW Africa when adjusted to accommodate constraints placing the south pole near Peru for c. 540–520 Ma. Concurrent counterclockwise rotation of Baltica and Gondwana during the Middle Cambrian may have facilitated separation of Greater Avalonia from Baltica across dextral shear zones.

Abstract The Ordovician of North and West Africa comprises three main transgressive–regressive sequences understood as ‘second-order’ cycles of 10–15 myr duration. Tide- to wave-dominated shallow-marine clastic successions, preserving incidental bryozoan carbonates to the north, include fluvial deposits over the most proximal southern stretches of the platform. The boundary with Cambrian strata remains unclear but the latter are progressively less represented to the south in the undifferentiated ‘Cambro-Ordovician’. To the north, graptolites, brachiopods and trilobites combined with palynomorphs provide a robust biostratigraphic frame. Maximum flooding intervals occurred in the early to middle Tremadocian, middle Darriwilian and middle to late Katian. Two events interfered with an overall long-term transgressive trend. The ‘intra-Arenig’ (late Floian?) tectonic event highlighted palaeohighs coinciding with Paleoproterozoic basements. Gondwanan drainage basins were reorganized, which had an impact on sediment sourcing and distribution of detrital material (e.g. zircons) feeding the pre-Variscan Europe. The second event is the end-Ordovician glaciation. The domain supported the greatest part of the Hirnantian glaciers and may also have preserved pre-Hirnantian glacial archives. It is not until the very latest Ordovician that offshore conditions developed far inland; it is however suspected that this inundation benefited from a transient postglacial isostatic flexure.

ABSTRACT The Avalon terrane of southeastern New England is a composite terrane in which various crustal blocks may have different origins and/or tectonic histories. The northern part (west and north of Boston, Massachusetts) correlates well with Avalonian terranes in Newfoundland, Nova Scotia, and New Brunswick, Canada, based on rock types and ages, U-Pb detrital zircon signatures of metasedimentary rocks, and Sm-Nd isotope geochemistry data. In the south, fewer data exist, in part because of poorer rock exposure, and the origins and histories of the rocks are less well constrained. We conducted U-Pb laser ablation–inductively coupled plasma–mass spectrometry analysis on zircon from seven metasedimentary rock samples from multiple previously interpreted subterranes in order to constrain their origins. Two samples of Neoproterozoic Plainfield Formation quartzite from the previously interpreted Hope Valley subterrane in the southwestern part of the southeastern New England Avalon terrane and two from the Neoproterozoic Blackstone Group quartzite from the adjacent Esmond-Dedham subterrane to the east have Tonian youngest detrital zircon age populations. One sample of Cambrian North Attleboro Formation quartzite of the Esmond-Dedham subterrane yielded an Ediacaran youngest detrital zircon age population. Detrital zircon populations of all five samples include abundant Mesoproterozoic zircon and smaller Paleoproterozoic and Archean populations, and are similar to those of the northern part of the southeastern New England Avalon terrane and the Avalonian terranes in Canada. These are interpreted as having a Baltican/Amazonian affinity based primarily on published U-Pb and Lu-Hf detrital zircon data. Based on U-Pb detrital zircon data, there is no significant difference between the Hope Valley and Esmond-Dedham subterranes. Detrital zircon of two samples of the Price Neck and Newport Neck formations of the Neoproterozoic Newport Group in southern Rhode Island is characterized by large ca. 647–643 and ca. 745–733 Ma age populations and minor zircon up to ca. 3.1 Ga. This signature is most consistent with a northwest African affinity. The Newport Group may thus represent a subterrane, terrane, or other crustal block with a different origin and history than the southeastern New England Avalon terrane to the northwest. The boundary of this Newport Block may be restricted to the boundaries of the Newport Group, or it may extend as far north as Weymouth, Massachusetts, as far northwest as (but not including) the North Attleboro Formation quartzite and associated rocks in North Attleboro, Massachusetts, and as far west as Warwick, Rhode Island, where eastern exposures of the Blackstone Group quartzite exist. The Newport Block may have amalgamated with the Amazonian/Baltican part of the Avalon terrane prior to mid-Paleozoic amalgamation with Laurentia, or it may have arrived as a separate terrane after accretion of the Avalon terrane. Alternatively, it may have arrived during the formation of Pangea and been stranded after the breakup of Pangea, as has been proposed previously for rocks of the Georges Bank in offshore Massachusetts. If the latter is correct, then the boundary between the Newport Block and the southeastern New England Avalon terrane is the Pangean suture zone.

Abstract Turbidity currents transport globally significant volumes of sediment and organic carbon into the deep-sea and pose a hazard to critical infrastructure. Despite advances in technology, their powerful nature often damages expensive instruments placed in their path. These challenges mean that turbidity currents have only been measured in a few locations worldwide, in relatively shallow water depths (<<2 km). Here, we share lessons from recent field deployments about how to design the platforms on which instruments are deployed. First, we show how monitoring platforms have been affected by turbidity currents including instability, displacement, tumbling and damage. Second, we relate these issues to specifics of the platform design, such as exposure of large surface area instruments within a flow and inadequate anchoring or seafloor support. Third, we provide recommended modifications to improve design by simplifying mooring configurations, minimizing surface area and enhancing seafloor stability. Finally, we highlight novel multi-point moorings that avoid interaction between the instruments and the flow, and flow-resilient seafloor platforms with innovative engineering design features, such as feet and ballast that can be ejected. Our experience will provide guidance for future deployments, so that more detailed insights can be provided into turbidity current behaviour, in a wider range of settings.

Abstract The crustal structure of the Equatorial Atlantic conjugate margins (South America and West Africa) has been investigated using 3D gravity anomaly inversion, which allows for (1) the elevated geothermal gradient of the lithosphere following rifting and break-up and (2) magmatic addition to the crust during rifting and break-up. It is therefore particularly suitable for the analysis of rifted margins and their associated ocean basins. Maps of crustal thickness and conjugate-margin stretching, derived from gravity anomaly inversion, are used to illustrate how the Equatorial Atlantic opened as a set of stepped rift-transform segments, rather than as a simple orthogonal rifted margin. The influence of the transform faults and associated oceanic fracture zones is particularly clear when the results of the gravity anomaly inversion are combined with a shaded-relief display of the free-air gravity anomaly. A set of crustal cross-sections has been extracted from the results of the gravity inversion along both equatorial margins. These illustrate the crustal structure of both rifted-margin segments and transform-margin segments. The maps and cross-sections are used to delineate crustal type on the margins as (1) inboard, entirely continental, (2) outboard, entirely oceanic and (3) the ocean–continent transition in between where mixed continental and magmatic crust is likely to be present. For a given parameterization of melt generation the amount of magmatic addition within the ocean–continent transition is predicted by the gravity inversion. One of the strengths of the gravity-inversion technique is that these predictions can be made in the absence of any other directly acquired data. On both margins anomalously thick crust is resolved close to a number of oceanic fracture zones. On the South American margin we believe that this thick crust is probably the result of post-break-up magmatism within what was originally normal-thickness oceanic crust. On the West African margin, however, three possible origins are discussed: (1) continental crust extended oceanwards along the fracture zones; (2) oceanic crust magmatically thickened at the fracture zones; and (3) oceanic crust thickened by transpression along the fracture zones. Gravity inversion alone cannot discriminate between these possibilities. The cross-sections also show that, while ‘normal thickness’ oceanic crust ( c. 7 km) predominates regionally, local areas of thinner ( c. 5 km) and thicker ( c. 10 km) oceanic crust are also present along both margins. Finally, using maps of crustal thickness and thinning factor as input to plate reconstructions, the regional palaeogeography of the Equatorial Atlantic during and after break-up is displayed at 10 Ma increments.

Abstract Paleoproterozoic terranes of the Man-Leo Shield in the southern part of the West African craton host one of the world’s largest gold provinces with an overall endowment >10,000 metric tons (t). Although gold deposition commenced by ca. 2170 Ma, most deposits formed later, either during the inversion and metamorphism of intraorogenic sedimentary basins between ca. 2110 and 2095 Ma, or during later transcurrent deformation and associated widespread high K plutonism following docking of Archean and Paleoproterozoic domains within the craton at ca. 2095 Ma. Deposits formed between ca. 2110 and 2095 Ma include those with free gold in quartz veins and refractory gold in arsenopyrite and/or pyrite, and are associated with halos of carbonate, sericite, chlorite, and albite alteration. Most are located in bends and intersections between shear zones, minor faults, folds, and entrained blocks of relatively reactive igneous rock. Conglomerate-hosted gold deposits of the Tarkwa district formed early in the 15-m.y.-long period. Gold deposits that formed subsequently between ca. 2095 and 2060 Ma have a wider variety of styles, geologic settings, and metal assemblages. District-scale albite, carbonate, and tourmaline alteration, hydrothermal breccias, and a close relationship to high K granitoids characterize some of these deposits, whereas others are more typical orogenic gold deposits that are similar to those formed earlier during the craton evolution.