On the trace of the Iapetus suture in Ireland and Britain

Abstract

The Iapetus suture in Ireland and Britain is that line which separates Caledonian rocks of the Laurentian and Avalonian continents. The suture is cryptic: nowhere is there an exposed fault-zone containing ophiolite remnants, blue-schist melanges, or trench deposits. Instead a suture line may be traced with varying degrees of confidence through a series of faults that traverse the Iapetus suture zone, which contains two or more tectonostratigraphic terranes. Several data sets are utilized to constrain this trace. The distribution of faunal provinces caused by oceanic separation can be used most reliably to define early Ordovician terranes. Fauna1 intermingling in the mid-Ordovician reduces provinciality and the confidence of terrane identification using faunal data. The late Ordovician and particularly Silurian tectonostratigraphic histories of terranes either side of the suture suggest that amalgamation of the terranes, elimination of the Iapetus Ocean and development of the suture zone had begun by the Ashgill. Therefore the resolution of the suture trace becomes unreliable where Ordovician rocks do not crop out, because Silurian turbidite fans of Laurentian provenance may have dispersed across a significant width of Avalonian crust. Similarly, the precision of structural identification of the suture zone, by correlating the suture with a major fold-facing confrontation, may be weakened by late overthrusting. Tectonic interleaving of crustal flakes in the zone is supported by geophysical evidence which suggests that some major boundaries in the lower crust do not always coincide with their inferred projections in outcrop. Tectonic and sedimentary mixing of crustal blocks and sediments in the zone is further reflected by Nd and Pb isotopic patterns; the Rb-Sr patterns of granites record a stronger distinction between north and south of the suture. Despite these difficulties, we depict a possible suture trace in Ireland that departs significantly from the traditionally placed line by following a NE-SW-aligned trace through the Slane fault and the Navan-Tipperary lineament, and thence through a dog-leg to pass north of the Dingle Peninsula.

The genesis of gold mineralisation hosted by orogenic belts: A lead isotope investigation of Irish gold deposits

“…At present the Iapetus Suture (solid black line Fig. 1) runs approximately from the Shannon estuary in the south-west to Balbriggan on the east coast (Todd et al, 1991).…”

The stratigraphy, correlation, provenance and palaeogeography of the Skiddaw Group (Ordovician) in the English Lake District

“…The Bellewstown terrane is separated from the Grangegeeth Terrane by the Slane Fault, which Todd et al (1991) The Ordovician of the Leinster terrane shows fewer similarities with the Lake District succession than the other terranes. The succession along the northern edge of the Leinster terrane, at the Balbriggan and Herbertstown inliers, comprises unfossiliferous red and green mudrocks overlain by grey mudrocks and siltstones.…”

No Exploits back-arc basin in the Iapetus suture zone of Ireland

“…1; Phillips et al 1976;Todd et al 1991). Furthermore, because there was no Exploits back-arc basin, Silurian ocean closure in Ireland occurred along the same structure (Vaughan & Johnston 1992), unlike in the Canadian Appalachians where there are separate Ordovician and Silurian sutures.…”