ALBERT

Description: 〈p〉In the first application of the developing plate tectonic theory to the pre-Pangaea world 50 years ago, attempting to explain the origin of the Paleozoic Appalachian–Caledonian orogen, J. Tuzo Wilson asked the question: ‘Did the Atlantic close and then reopen?’. This question formed the basis of the concept of the Wilson cycle: ocean basins opening and closing to form a collisional mountain chain. The accordion-like motion of the continents bordering the Atlantic envisioned by Wilson in the 1960s, with proto-Appalachian Laurentia separating from Europe and Africa during the early Paleozoic in almost exactly the same position that it subsequently returned during the late Paleozoic amalgamation of Pangaea, now seems an unlikely scenario. We integrate the Paleozoic history of the continents bordering the present day basin of the North Atlantic Ocean with that of the southern continents to develop a radically revised picture of the classic Wilson cycle The concept of ocean basins opening and closing is retained, but the process we envisage also involves thousands of kilometres of mainly dextral motion parallel with the margins of the opposing Laurentia and Gondwanaland continents, as well as complex and prolonged tectonic interaction across an often narrow ocean basin, rather than the single collision suggested by Wilson.〈/p〉

Description: 〈sec〉〈st〉Extract〈/st〉〈p〉Sir Archibald Geikie (1835–1924) was a formidable and authoritarian figure who played a central part in British geology in Victorian and Edwardian times. He was a protégé of Sir Roderick Impey Murchison and became Professor of Geology at the University of Edinburgh (1871), Director of the Geological Survey of Scotland (1871) and Director-General of the Geological Survey of Great Britain (1882), a position that he held with stern, but kindly, attention to his staff until his retirement to Haslemere in 1901. He was a prolific writer of both biographies of his mentors and a huge number of books and papers on a wide variety of geological topics. His rather long-winded and self-congratulatory autobiography (Geikie 1924) was published in the year of his death. His principal hobby was as a proficient sketcher and water colourist, mostly of scenes of geological interest, many of which adorn and illustrate his published works. Geikie had a powerful influence on Victorian and Edwardian geology and was rewarded by many honours, including Fellow of the Royal Society (1864), a knighthood (1891) and the Order of Merit (1914).〈/p〉〈/sec〉〈p〉〈b〉...〈/b〉〈/p〉 〈p〉〈i〉This 250-word extract was created in the absence of an abstract.〈/i〉〈/p〉

Description: The current view regarding the timing of regionally developed penetrative tectonic fabrics in sedimentary rocks is that their development postdates lithification of those rocks. In this case, fabric development is achieved by a number of deformation mechanisms, including grain rigid body rotation, crystal-plastic deformation, and pressure solution. The latter is believed to be the primary mechanism responsible for the domainal structure of cleavage in low-grade metamorphic rocks. In this study we combine field observations with strain studies to characterize considerable (〉50%) Acadian crustal shortening in a Devonian clastic sedimentary sequence from southwest Ireland. Despite these high levels of shortening there is a marked absence of the domainal cleavage structure and intraclast deformation that are expected with this level of deformation. Fabrics in these rocks are predominantly a product of rigid body rotation and repacking of extraformational clasts during deformation of a clastic sedimentary sequence before lithification was complete.

Description: Extract Sir Archibald Geikie (1835–1924) was a formidable and authoritarian figure who played a central part in British geology in Victorian and Edwardian times. He was a protégé of Sir Roderick Impey Murchison and became Professor of Geology at the University of Edinburgh (1871), Director of the Geological Survey of Scotland (1871) and Director-General of the Geological Survey of Great Britain (1882), a position that he held with stern, but kindly, attention to his staff until his retirement to Haslemere in 1901. He was a prolific writer of both biographies of his mentors and a huge number of books and papers on a wide variety of geological topics. His rather long-winded and self-congratulatory autobiography (Geikie 1924) was published in the year of his death. His principal hobby was as a proficient sketcher and water colourist, mostly of scenes of geological interest, many of which adorn and illustrate his published works. Geikie had a powerful influence on Victorian and Edwardian geology and was rewarded by many honours, including Fellow of the Royal Society (1864), a knighthood (1891) and the Order of Merit (1914). ... This 250-word extract was created in the absence of an abstract.

Description: The early Ordovician ( c . 485 Ma) Bay of Islands Ophiolite Complex was obducted onto the Laurentian rifted margin as the fore-arc of an oceanic arc that collided with the margin during the mid-Ordovician ( c . 470 Ma). The subduction zone was nucleated on an oceanic transform–fracture zone, part of whose remnants occur as the polyphase-deformed and intruded mafic and ultramafic rocks of the Coastal Complex. The ophiolite formed as a suprasubduction-zone fore-arc ophiolite at the spatial and temporal continuation of the ridge normal to the transform–fracture zone–subduction zone system and NE of a trench–trench–ridge triple junction. A two-pyroxene garnet granulite–garnet amphibolite–epidote amphibolite mafic metamorphic sole at the base of the ophiolite was generated, roughly synchronously with the ophiolite, by the metamorphism of mid-ocean ridge basalt mafic rocks in the descending slab at about 10 kbar and quickly attached to the base of the overlying ophiolite during slab flattening, and not by subduction zone extrusion. The metamorphic sole is not a metamorphic aureole at the base of a hot obducting ophiolite. Plate slip vector triangles around the triple junction, before collision and during obduction, are constructed from the orientation of dykes in the sheeted complex and the trends of structures in the high- to lower-temperature parts of the sole and the obducted nappes of oceanic and continental margin rocks beneath the ophiolite. Linear structures in the sole amphibolites trend NNW (the subduction direction); those in the greenschist-facies, obducted, oceanic and continental margin rocks trend WSW (the obduction direction).

Description: Ordovician strata of the South Mayo Trough in western Ireland contain clastic deposits that represent materials eroded from a large and diverse continental area over a time scale that spans much of the Earth’s history. Therefore, it is a useful region to use detrital zircons to construct a continental crustal growth model. Here, we report integrated U–Pb, Lu–Hf and O isotope measurements obtained from in situ analyses of 160 zircons from the South Mayo Trough. U–Pb zircon crystallization ages define three major magmatic episodes of crustal reworking in the Archaean (Lewisian), Mesoproterozoic (Grenville), and Ordovician (Grampian). These data, together with oxygen isotope data and Hf model ages, suggest that crustal growth, recorded in the strata of the South Mayo Trough, started at c. 4 Ga and continued until 1.4 Ga, with two major growth periods at 2.3–2.1 and 2.0–1.5 Ga. We find that the crustal incubation time is decoupled from the duration of supracrustal alteration processes; some zircons with very long crustal incubation times have pristine mantle δ18O signatures suggesting minimal low-temperature surface processing in their source regions. Identifying such zircons is the key for future studies in constructing realistic net continental crustal growth models unaffected by crustal recycling.Supplementary materials: Data tables for U–Pb, Lu–Hf and oxygen isotopes for detrital zircons from South Mayo Trough, as well as plots of values for zircon standards (δ18O for R33, U–Pb ages for 91500 and R33, and 176Hf/177Hf for 91500, GJ-1, and Plešovice) and reverse concordia plots of zircon samples are available at www.geolsoc.org.uk/SUP18543.

Description: In the first application of the developing plate tectonic theory to the pre-Pangaea world 50 years ago, attempting to explain the origin of the Paleozoic Appalachian–Caledonian orogen, J. Tuzo Wilson asked the question: ‘Did the Atlantic close and then reopen?’. This question formed the basis of the concept of the Wilson cycle: ocean basins opening and closing to form a collisional mountain chain. The accordion-like motion of the continents bordering the Atlantic envisioned by Wilson in the 1960s, with proto-Appalachian Laurentia separating from Europe and Africa during the early Paleozoic in almost exactly the same position that it subsequently returned during the late Paleozoic amalgamation of Pangaea, now seems an unlikely scenario. We integrate the Paleozoic history of the continents bordering the present day basin of the North Atlantic Ocean with that of the southern continents to develop a radically revised picture of the classic Wilson cycle The concept of ocean basins opening and closing is retained, but the process we envisage also involves thousands of kilometres of mainly dextral motion parallel with the margins of the opposing Laurentia and Gondwanaland continents, as well as complex and prolonged tectonic interaction across an often narrow ocean basin, rather than the single collision suggested by Wilson.