ABSTRACT Tectonic models for arc-continent collision can be overly complex where, for example, diachronous sedimentation and deformation along a single plate boundary are attributed to separate tectonic events. Furthermore, continuous sedimentation in a single basin recording a diachronous collision along a plate margin makes it difficult to use classical unconformable relationships to date an orogenic phase. In this chapter, we describe the Ordovician South Mayo Trough of western Ireland, a remarkable example of such a basin. It originated in the late Cambrian–Early Ordovician as a Laurentia-facing oceanic forearc basin to the Lough Nafooey arc. This arc was split by a spreading ridge to form a trench-trench-ridge triple junction at the trench. The basin remained below sea level during Grampian/Taconic arc-continent collision and, following subduction flip, received sediment from an active continental margin. Sedimentation ended during Late Ordovician Mayoian “Andean”-style shortening, broadly coeval with a marked fall in global sea level. These major tectonic events are traced through the nature of the detritus and volcanism in this basin, which is preserved in a mega-syncline. The Grampian orogen is not recorded as a regional unconformity, but as a sudden influx of juvenile metamorphic detritus in a conformable sequence.

Abstract 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.

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).

The main conclusion of this paper is that some form of plate tectonics started at ca. 3.0 Ga or possibly as early as 3.1 Ga., and that, since then, plate tectonics has steadily become dominant over plumes as a mechanism of heat loss. Modern plate tectonics started at ca. 0.6 Ga. The volume history of the continental crust is one of fast Early Archean growth to generate a, probably, globally continuous crust, then a growth, probably exceeded or balanced, long-term, by crustal return to the mantle reservoir.

Abstract Sequence stratigraphy of third-order cycles is not determined by global eustasy but by local and regional tectonics interplaying with long-wavelength sea-level change. We can discern no mechanism that can cause the necessary short-wavelength, large-amplitude sea-level changes implicit in globally synchronous eustatic third-order cycles. Some mechanisms have the required amplitude but far too long a wavelength, some the necessary short wavelength but insufficient amplitude. Continental glaciation gives the correct amplitude/wavelength combination but did not operate for long periods of earth history. Much circular reasoning has been used to ‘establish’ eustatic third-order cycles. We believe that the magnitudes have been overestimated and that biostratigraphic correlation cannot be achieved to the necessary precision. The value of sequence stratigraphy lies not in its supposed relationship with global eustasy but as a basis for our understanding of basin dynamics. Long-term sea-level change is related to the episodic assembly, dismemberment and dispersal of supercontinents, which controls long-term sea-floor spreading rates, hydrography, climate, and the surficial geomorphic/stratigraphic evolution of the earth.

Abstract The tectonic evolution of the Cenozoic mountain ranges, fault systems and basins that comprise the roughly east-west Caribbean/ South America plate boundary zone from Colombia to Trinidad was controlled principally by highly-oblique dextral convergence between the Caribbean and South American Plates. The Caribbean Plate is pinned by the Central American and Lesser Antilles subduction zones and is stationary in a mantle reference frame whereas the South American Plate is moving westwards in that frame. The Caribbean Plate is of Pacific provenance and has, since early Cenozoic time, progressively invaded at an average rate of 20-25 mm/yr the Proto-Caribbean oceanic gap between North and South America, Thus, the Lesser Antilles are terminated southward by a dextral transpression zone that lengthened progressively throughout Cenozoic time. This zone showed strong partitioning between easi-west dextral strike slip faults, such as the Oca and El Pilar faults, and south-southeast ward-directed thrust nappes. The nappes loaded the South American craton to generate a coupled flexural foreland basin and peripheral bulge that migrated eastwards. At any time during this evolution, the zone between the thrust complex and the crest of the peripheral bulge was a zone of potential updip hydrocarbon migration, which moved eastwards in tandem with the relatively eastward migrating Caribbean Plate. Ln several cases, especially in Oligocene and younger Cenozoic times, E-W trending strike-slip and normal faults decoupled parts of the thrust load from the South American craton and allowed flexural recovery, the rapid uplift of coast ranges, and thick sedimentation in transtensional basins. All deformation in the Venezuela nappe pile pertaining to arc-continent collision between the Caribbean and South American crusts is of Cenozoic age and youngs from west to east. Our evolutionary tectonic reconstruction of the Caribbean/South American plate boundary zone is critically dependent upon a precise restoration of the geometry of northwest South America immediately before the Caribbean Plate began its relative eastward motion. This involved our determining the amounts of relative motion along the various faults and deformation zones of Colombia and Venezuela that have developed mainly since the late Oligocene. Retro-restoring motion on these faults allows a construction of the Cenozoic nappe front prior to 25 Ma and the shape of northwest South America prior to 60 Ma. Displacements include about 110 km of sinistral motion on the Santa Marta Fault Zone, up to 150 km of dextral slip in the Merida Andes zone, 25 km of shortening across the Sierra de Perijti, and at least 65 km and 90 km of dextral motion on the Oca Fault Zone in Colombia and Venezuela, respectively. On a retrodeformed paleogeographic grid which takes into account all of these restorations as well as removal of accreted tetranes, the paleogeographic development of Venezuela and Trinidad is traced through Cenozoic time, and important tectonic processes and controls on hydrocarbon accumulations are defined and discussed.

Alternative fits of the continents around the future site of the Caribbean about 200 Ma ago and alternative relative motions since then of North and South America and of Africa with respect to each other allow a wealth of information, including data tabulated here on the distribution of rift systems; early ocean floor; obducted ocean floor fragments and dated plutons to be assessed in relation to a history of Caribbean development. After an early rift phase, the Gulf of Mexico formed by divergence mainly before the Caribbean itself. Convergence on what are now the northern and southern Caribbean margins during the Cretaceous produced arc-systems and carried the present Caribbean ocean floor, which represents an oceanic plateau, out of the Pacific. Cenozoic convergence in the Lesser Antilles and Central America has been contemporary with more than 1000 km of roughly eastward motion, distributed in wide plate boundary zones, of the Caribbean with respect to both North and South America. Moderate internal deformation of the Caribbean plate is perhaps attributable to its oceanic plateau character because it behaves mechanically in a way that is intermediate between that of normal ocean floor and continent. Although numerous problems remain in Caribbean geology, a framework into which many of them can be accommodated is beginning to emerge.

The richly fossiliferous Silurian and Early Devonian rocks of the Arisaig area and the nearby related regions of Cape George and Lochaber in Antigonish County, Nova Scotia, have been known for almost one hundred and fifty years. The 5,000 ft of sediment in the Arisaig Group is the only fossiliferous section within the Province containing representatives of every stage of the Silurian. This monotonous succession of siltstones and mudstones is unique world-wide in providing an almost continuous faunal record of very shallow water conditions rather than an alternation of varying depth environments. The Eocoelia and related benthic animal communities persist from the early Llandovery through the entire Silurian and well into the early Gedinnian portion of the Lower Devonian. These rocks despite their relatively unmetamorphosed nature (never higher than the chlorite grade) have been highly folded and extensively faulted, beginning with disturbances associated with the Middle Devonian Acadian orogeny and extending into various intervals of the later Paleozoic and possibly Triassic. The complex structure is here outlined in more detail than is available in previous reports, thus giving both geologists and paleontologists a better framework with which to understand both faunal and geologic problems associated with the rocks. During the Gedinnian, the entire region became a site of nonmarine, Old Red Sandstone—type deposition, but there is no evidence for any stratigraphic break between the underlying marine and the overlying nonmarine beds. The Devonian vertebrates are similar to those of the Welsh Borderland of England. The rhyolitic and basaltic volcanics . . .

Abstract Aulacogens are long-lived deeply subsiding troughs, at times fault-bounded, that extend at high angles from geosynclines far into adjacent foreland platforms. They are normally located where the geosyncline makes a reentrant angle into the platform. Their fill is contemporaneous with, as thick as, and lithologically similar to the foreland sedimentary wedge of the geosyncline but in addition has periodically erupted alkalic basalt and fanglomerate. Although many aulacogens have suffered mild compressional deformation, tectonic movement within them is mainly vertical; large-scale horizontal translations are rare. Aulacogens are known throughout the Proterozoic and Phanerozoic, and incipient aulacogens occur at reentrants on modern continental margins. The 1700-to-2200-million-year-old Athapuscow Aulacogen of Great Slave Lake began as a deeply subsiding transverse graben during the early miogeoclinal stage of the Coronation Geosyncline. During the orogenic stage of the geosyncline, the aulacogen became a broader downwarp that received abnormally thick exogeosynclinal sediments from the orogenic belt. The aulacogen was compressed mildly, prior to a final stage involving transcurrent faulting, one-sided uplift, and continental fanglomerate sedimentation. The aulacogen is distinguished from the foreland sedimentary wedge of the geosyncline by having paleocurrents parallel rather than transverse to its structural trend, by having high-angle faults rather than low-angle thrusts, by its alkalic basalt volcanism, and by the lack of metamorphism. It is hypothesized that deep-mantle convective plumes produce three-armed radial rift systems (rrr triple junctions) in continents stationary with respect to the plumes. If only two of the arms spread to produce an ocean basin, the third remains as an abandoned rift extending into the continental interior from a reentrant on the new continental margin. For example, the Benue Trough, located in the Gulf of Guinea reentrant on the west coast of Africa, may be such an abandoned rift arm formed during the Cretaceous period at the time of initial rifting of Africa and South America. Inasmuch as new continental margins are predestined to become geosynclines, such abandoned rift arms are juvenile aulacogens. In this model, aulacogens and geosynclines have a common origin but differ in the extent of rifting.