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.

A new model is proposed for the problematic preservation of an Ordovician forearc basin, which records a complete sedimentary record of arc-continent collision during the Grampian (Taconic) orogeny in the west of Ireland. The South Mayo Trough represents an arc and forearc complex developed above a subduction zone in which the slab dipped away from the Laurentian passive margin. The collision of this arc with Laurentia caused the Middle Ordovician Grampian orogeny. However, the South Mayo Trough, in the hanging wall of this collision zone, remained a site of marine sedimentation during the entire process. Early sediments show derivation from an island-arc complex, an ophiolitic backstop, and polymetamorphic trench sediments. These are conformably overlain by marine deposits derived from a more evolved arc complex and an emerging juvenile orogen. This transition is dated as being coeval with the Grampian metamorphism of the Laurentian footwall. The problem remains as to why subsidence continued in a basin on the hanging wall. It is proposed that the suppression of the expected topography is due to the nature of the Laurentian continental margin. Geophysical and geological evidence suggests that this was a volcanic margin during Neoproterozoic rifting. It is argued that the subduction of this margin caused the formation of eclogites, which reduced its buoyancy. Simple numerical models are presented which show that this is a viable mechanism for the suppression of topography during early stages of arc-continent collision and hence for the preservation of forearcs.

ABSTRACT Three Silurian basin fills, the Llandovery–Wenlock Croagh Patrick and Killary Harbour–Joyce Country successions and the Ludlow–Pridoli Louisburgh–Clare Island succession, overstep the tectonic contacts between elements of the Grampian (Taconic) accretionary history of the Caledonian-Appalachian orogeny in western Ireland. New U-Pb detrital zircon data from lower strata of these Silurian rocks provide insight into basin evolution and paleogeography. The shallow-marine Croagh Patrick succession unconformably overlies the Clew Bay Complex and the northern part of the Ordovician South Mayo Trough. Two samples have zircon populations dominated by Proterozoic grains typical of the Laurentian margin, with few younger grains. Up to 13% of the grains form a cluster at ca. 950–800 Ma, which is younger than known Grenville magmatism on the local Laurentian margin and older than known magmatism from Iapetan rifting; these may be recycled grains from Dalradian strata, derived from distal Tonian intrusions. The Killary Harbour–Joyce Country succession overlies the structural contact between the Lough Nafooey arc and the Connemara Dalradian block and records a transgressive-regressive cycle. Four samples of the Lough Mask Formation show contrasting age spectra. Two samples from east of the Maam Valley fault zone, one each from above Dalradian and Nafooey arc basement, are dominated by Proterozoic grains with ages typical of a Laurentian or Dalradian source, likely in north Mayo. One sample also includes 8% Silurian grains. Two samples from west of the fault overlie Dalradian basement and are dominated by Ordovician grains. Circa 450 Ma ages are younger than any preserved Ordovician rocks in the region and are inferred to represent poorly preserved arc fragments that are exposed in northeastern North America. Cambrian to late Neoproterozoic grains in association with young Ordovician ages suggest derivation from a peri-Gondwanan source in the late stages of Iapetus closure. The Louisburgh–Clare Island succession comprises terrestrial red beds. It unconformably overlies the Clew Bay Complex on Clare Island and is faulted against the Croagh Patrick succession on the mainland. The Strake Banded Formation yielded an age spectrum dominated by Proterozoic Laurentian as well as Ordovician–Silurian ages. Although the basin formed during strike-slip deformation along the Laurentian margin in Ireland and Scotland, sediment provenance is consistent with local Dalradian sources and contemporaneous volcanism. Our results support ideas that Ganderian continental fragments became part of Laurentia prior to the full closure of the Iapetus Ocean.