The Connemara Antiform and its possible Silurian history

Abstract

The Connemara Antiform has iconic status in the geology of western Ireland. With its eastto-west trend and eastward plunge and the disposition of Dalradian stratigraphy about its axis, it forms the dominant structure in the present-day landscape and is a focal feature of geological maps of the area. Its genesis has long been ascribed to D4, the last main major deformation phase of the Ordovician Grampian Orogeny that affected the Dalradian rocks of Connemara. But does the structure have such a straightforward and simple origin? This paper investigates the possible role of middle late Silurian deformation in the origin of the Connemara Antiform. Overview of the geology of Connemara Connemara was part of the south-eastern Laurentian margin Dalradian sequence that experienced a major short-lived early Ordovician orogenic episode, the Grampian Orogeny, caused by collision with, and obduction of, an arc that developed above oceanward-directed subduction (Chew 2009). The Connemara Terrane became detached from the Laurentian margin and now lies to the south of the obducted arc and the South Mayo Trough (Fig. 1). The translation of the Connemara Terrane from the Laurentian margin to its current position is indicated by an influx of coarse clastic rocks of Connemara provenance into the c. 464Ma Bunnacunneen Member of the Mweelrea Formation of the South Mayo Trough (McConnell et al. 2009). Dewey (2005) provides a succinct account of the local plate-tectonic, metamorphic and structural history of the Connemara Terrane and the South Mayo Trough during the Grampian Orogeny, and Graham et al. (1989), Leake and Tanner (1994), McConnell et al. (2002) and Long et al. (2005) provide a comprehensive overview of the detailed geology of the area. The Grampian Orogeny began with the subduction of rifted Laurentian margin sediments beneath a continent-facing oceanic arc c. 478Ma (Dewey 2005). The Connemara Dalradian sediments were buried in a thick nappe pile of D2 and D3 folds and experienced high-pressure, lowtemperature Barrovian metamorphism (Leake and Tanner 1994). Palaeomagnetic studies indicate that the D3 folds originally dipped c. 208 south (Robertson 1988). Between 474Ma and 470Ma (Friedrich et al. 1999) a suite of metagabbros and orthogneisses (the Metagabbro Orthogneiss Complex) were injected into the deforming nappe pile during D2 D3 in an east west belt that extends for more than 80km along the southern margin of the terrane (Fig. 1). The high-temperature metagabbros and orthogneisses drove sillimanite-grade, hightemperature, low-pressure Buchan metamorphism. Migmatites resulting from the metagabbro heat pulse formed at c. 6kbar but crystallised at much lower pressures of c. 3.5kbar, indicating rapid exhumation (Barber and Yardley 1985). Rapid exhumation may have been facilitated by extensional collapse. The Renvyle Bofin Slide, a lowangle post-D3 extensional detachment, has, on the Irish Journal of Earth Sciences 32 (2014), 71 78 doi: 10.3318/IJES.2014.32.71 # 2014 Royal Irish Academy