David I. Schofield

Timing and kinematics of Eburnean tectonics in the central Reguibat Shield, Mauritania

The Reguibat Shield of north Mauritania comprises a western Archaean terrane dominated by gneisses and granitic rocks and an eastern Eburnean terrane largely made up of Palaeoproterozoic granitic and metasedimentary rocks. These were juxtaposed during the c. 2.1–2.0 Ga Eburnean Orogeny, which formed an approximately north-trending front contiguous with the Eburnean belt of the Leo Shield in equatorial West Africa. Geological surveying in the Sfariat region of the Reguibat Shield has shown that a metamorphosed Palaeoproterozoic continental margin succession has been intruded by and intercalated with synorogenic granitoids and transported SW onto the Archaean foreland during sinistral oblique collision preserved as strongly partitioned D1 shortening and D2 strike-slip-dominated transpression within the Sfariat Belt. This was subsequently affected by retrogressive dextral transcurrent deformation and the propagation of the en echelon Imarkene and Tmeimichat fault zones. Laser ablation and conventional thermal ionization mass spectrometry U–Pb geochronology were carried out on five samples of granitic rock and migmatitic gneiss from the region, which reveal that anatexis and sinistral transpression took place between c. 2117 and 2064 Ma. Timing and kinematics of the Eburnean event in the Reguibat Shield are similar to those for the Leo Shield, which underwent SW-directed sinistral transpression at this time.

Precise dating of low-temperature deformation: Strain-fringe analysis by 40Ar-39Ar laser microprobe

Pyritized graptolites from the Welsh Basin (United Kingdom) slate belt acted as rigid bodies during cleavage formation, and epizonal white micas formed within the resulting. strain shadows, orthogonal to the principal stress orientation. Although the quantities of mica are small, they are a pure synkinematic mineral and have been dated by Ar-40-Ar-39 infrared laser microprobe as a means to dating cleavage. Four samples of strain-fringe mica from different hand samples yielded ages ranging from 394.4 +/- 3.1 to 397.8 +/- 1.8 Ma (2sigma), with a mean age of 396.1 +/- 1.4 Ma (2sigma). By focusing on minerals that are unequivocally synkinematic, this technique provides a novel solution to the problems of isotopically dating slaty cleavage. Previous studies have predominantly relied on dating whole-rock slate samples or separated illite grains by Ar-40-Ar-39 techniques; problems encountered included (1) separating the effects of isotopic contamination by detrital phases, (2) Ar-39 loss during the irradiation of illite mineral separates, and (3) thermally induced Ar-40 loss in nature from fine-grained minerals. By circumventing these problems, this new method provides the first unequivocal and high-precision age data for Acadian deformation in the well-characterized Welsh Basin slate belt. With such precision, the method may afford geologists the opportunity to track tectonic fronts across orogens and assess the rates of accretion processes in areas that are peripheral to sites of continent-continent collision.

Relationships between syn-tectonic granite fabrics and regional PTtd paths: an example from the Gander-Avalon boundary of NE Newfoundland

Abstract Syn-tectonic granitic plutons and their host rocks not only provide a potential wealth of information about the timing and nature of regional kinematics, but also record information about thermal conditions. Successive syn-tectonic plutons emplaced into the Gander Zone of NE Newfoundland preserve fabric overprinting formed during cooling to regional ambient thermal conditions. Ambient conditions have been approximated for each pluton by careful analysis of microstructures and consideration of potential cooling histories, and have illustrated regional exhumation during a tectonic reversal from Silurian-Devonian sinistral transpression to Devonian dextral transpression. Because the time period of emplacement is relatively short by comparison to orogenic events (typically by more than an order of magnitude), individual plutons may record a ‘snap-shot’ of the regional history. By combining information from a number of plutons emplaced sequentially during a period of regional orogenesis, a picture may be built up defining time-temperature-deformation (Ttd) paths. Such information may compliment regional metamorphic PT studies in helping to establish orogenic PTtd paths.

Polyphase Neoproterozoic orogenesis within the East Africa–Antarctica Orogenic Belt in central and northern Madagascar

Abstract Our recent geological survey of the basement of central and northern Madagascar allowed us to re-evaluate the evolution of this part of the East Africa–Antarctica Orogen (EAAO). Five crustal domains are recognized, characterized by distinctive lithologies and histories of sedimentation, magmatism, deformation and metamorphism, and separated by tectonic and/or unconformable contacts. Four consist largely of Archaean metamorphic rocks (Antongil, Masora and Antananarivo Cratons, Tsaratanana Complex). The fifth (Bemarivo Belt) comprises Proterozoic meta-igneous rocks. The older rocks were intruded by plutonic suites at c. 1000 Ma, 820–760 Ma, 630–595 Ma and 560–520 Ma. The evolution of the four Archaean domains and their boundaries remains contentious, with two end-member interpretations evaluated: (1) all five crustal domains are separate tectonic elements, juxtaposed along Neoproterozoic sutures and (2) the four Archaean domains are segments of an older Archaean craton, which was sutured against the Bemarivo Belt in the Neoproterozoic. Rodinia fragmented during the early Neoproterozoic with intracratonic rifts that sometimes developed into oceanic basins. Subsequent Mid-Neoproterozoic collision of smaller cratonic blocks was followed by renewed extension and magmatism. The global ‘Terminal Pan-African’ event (560–490 Ma) finally stitched together the Mid-Neoproterozoic cratons to form Gondwana.

Deep crustal and local rheological controls on the siting and reactivation of fault and shear zones, northeastern Newfoundland

The siting of a c. 25 km wide, transpressive, high-strain zone at the eastern margin of the Gander Zone of NE Newfoundland corresponds to the trace of a fundamental contact between two Gondwanan basement blocks displaced sinistrally relative to one another during Silurian orogenesis. Changes in plate motion during the Devonian led to kinematic reversal and reactivation of the shear zone and, at high structural levels, the development of a major brittle-ductile fault system. At a local scale within the Silurian ductile high-strain zone, the focus of deformation and shifts in siting of shear were closely related to magma presence. We consider that granite magmas exploited shear zones within the crust to aid ascent, and in doing so enhanced local deformation. Cessation of magma supply and/or cooling of magmas within conduits caused deformation to relocate elsewhere. NE Newfoundland hence provides an example of how fault/shear zone siting and reactivation may be controlled by regional-scale pre-existing basement configuration and more localised processes affecting rheology, notably plutonism.

Ganderia–Laurentia collision in the Caledonides of Great Britain and Ireland

During terrane convergence, an influx of clastic sediment from an upper plate onto a lower plate is an early indication of terrane juxtaposition. In the Caledonides of Great Britain and Ireland, units accreted to Laurentia during the early Palaeozoic Era include peri-Gondwanan terrane assemblages that earlier separated from West Gondwana. However, the Southern Uplands Terrane contains detrital zircon populations apparently derived entirely from Laurentia, characterized by a large, asymmetric Mesoproterozoic peak and a scarcity of zircon at 600 Ma and 2.1 Ga. In contrast, Cambrian and Ordovician rocks from the Lake District and the Leinster Massif of Ireland show abundant grains with these ages, together with a range of Mesoproterozoic zircon. These characteristics are shared with the Monian terrane of Anglesey and with Ganderia in the Appalachians, indicating probable derivation from Amazonia in West Gondwana. Silurian sandstones from the Lake District show an influx of Laurentia-derived zircon, and lack the peri-Gondwanan signal. This indicates that in the Caledonides, Ganderia was not accreted to the Laurentian margin until c. 430 Ma, in contrast to the Ordovician accretion of Ganderian fragments recorded in the Appalachians, suggesting that the configuration of the closing Iapetus Ocean varied significantly along the strike of the orogen. Supplementary material: Details of sample coordinates, analytical procedure and U/Pb detrital zircon analytical data are available at www.geolsoc.org.uk/SUP18739.

How was the Iapetus Ocean infected with subduction

Because subduction in the Iapetus Ocean began only ∼35 m.y. after the end of rifting, spontaneous foundering of mature passive margins is an unlikely subduction-initiation mechanism. Subduction is more likely to have entered the Iapetus from the boundary with the external paleo-Pacific, similar to the incursion of the Scotia, Caribbean, and Gibraltar arcs into the modern Atlantic. The subduction zone probably became sinuous, entraining fragments of the Gondwanan margin along its complex sinistral southern boundary where oblique collision caused Monian-Penobscottian deformation. Following Taconian-Grampian collision of part of the subduction system with Laurentia, remaining parts of the Iapetus were progressively infected with subduction, leading to Silurian closure.

A new, high precision U–Pb date from the oldest known rocks in southern Britain

A new high precision U–Pb zircon age of 710.8 ± 1.5 Ma for granophyric granitic rock from the Stanner Hanter Complex of the Welsh Borderland lies just within error of an older Rb–Sr isochron age. ϵNd values of −0.3 and −1.2 combined with T DM of 1394 Ma and 1468 Ma indicate that the magma incorporated an older crustal source component. The Nd data highlight differences with western Avalonia, the widely considered Late Neoproterozoic north American counterpart to southern Britain, and point toward a closer similarity with other Peri-Gondwanan terranes that incorporate older, cratonic source material.

A revised 87Sr/86Sr curve for the Silurian: Implications for global ocean chemistry and the Silurian timescale

Recent recalibration of the Silurian timescale and improved global chronostratigraphic correlation of Silurian strata significantly altered the Silurian 87Sr/86Sr curve and the temporal extent of available data. Whereas previous Silurian 87Sr/86Sr composites showed a generally monotonic increase throughout the Silurian, revisions to the Silurian timescale now require a major increase in the rate of change in 87Sr/86Sr at or near the onset of the Gorstian Age of the Ludlow Epoch. Similarly, improved chronostratigraphic correlations between Silurian outcrops on Anticosti Island, Canada, and Gotland, Sweden, indicate that the middle part of the Telychian Age, which is roughly 10%–15% of the total duration of the Silurian period, is undersampled and underrepresented in Silurian 87Sr/86Sr composites. A revised Silurian 87Sr/86Sr curve based on 241 new and published analyses confirms the significant increase in the rate of change of 87Sr/86Sr toward more radiogenic values near the base of the Ludlow Series. On the basis of these data, we propose that the rapid trend toward more radiogenic 87Sr/86Sr values is indicative of increased weathering of old sialic crust exposed during the Silurian uplift of portions of Baltica, Laurentia, and Avalonia. Importantly, however, the actual rate of change of 87Sr/86Sr will remain equivocal until the durations of Silurian epochs and ages are better constrained.

A new Early Silurian turbidite system in Central Wales: insights into eustatic and tectonic controls on deposition in the southern Welsh Basin

The newly recognized Nant Brianne turbidite system was a focus of laterally supplied coarse-grade sediment deposition that, along with the Caban–Ystrad Meurig system, punctuated late Hirnantian to early Telychian, mudstone-dominated slope apron deposition along the SE margin of the southern Welsh Basin. Geological mapping coupled with detailed biostratigraphy enable the depositional influence of sea-floor topography, an active Llyn Brianne Fault and eustatic sea-level changes to be tracked. The latter may represent ‘far field’ effects relating to the retreat and advance of contemporary Gondwanan ice sheets. Slope apron mudstone facies reveal a strong response to high order eustatic events; however, the response of contemporary, easterly sourced, coarse-grade turbidite systems was more complex, with some periods of increased sand and gravel input coinciding with times of rising global sea level. These anomalous relationships are explained by invoking a series of smaller-scale movements in marine base level. It was the interaction of these lower order events with the main eustatic cycles that appears to have been the primary control on sand and gravel input to the Welsh Basin. An early Telychian expansion of the Nant Brianne turbidite system records a marked increase in tectonically generated sediment at a time of palaeo-plate collision between Baltica/Avalonia and Laurentia before intra- and peri-basinal faulting led to its abandonment as a supply path to the basin centre.