Structural style, shortening estimates and the thrust front of the Irish Variscides

Abstract

Previous work in the Irish Variscan orogen has emphasized the significance of the major folds in the overall structure. A series of three balanced sections through the orogen reveals that thrusts are an equally important element of the orogenic structure. The 'thrust front' is not a fundamental structural line delimiting different deformational zones as previously proposed. Major thrusts occur along the 'thrust front' between Killarney and Mallow, but are absent east of Mallow. This change in character of the 'thrust front' is a function of a change in the stratigraphic level of the sole thrust to the orogen and the current erosion level. The deeper level of the sole thrust produces the numerous Silurian cored inliers to the NE of Mallow. The balanced sections show that the overall bulk shortening in the orogen increases from 33% in the east, to 42% in the central areas. This increase may be related to the arcuate nature of the orogen.

Localized ductile thrusting north of the Variscan Front, Ross Island, southwest Ireland

“…To the east, the ENE-WSW-trending KMF extending from Loch Guitane eastwards places Devonian sandstones against Upper Carboniferous clastic rocks. Between Killarney and Mallow this fault is believed to displace rocks by up to 7 km (Cooper et al 1984). To the west the WNW-ESE-trending Black Lake Fault, extending from Muckross Lake to the Dingle Bay-Galtee fault zone, is estimated to displace rocks by no more than 2 km in the nearby Gap of Dunloe, 6 km west of Ross Island (Meere 1995a).…”

“…(i) extension and formation of the Munster Basin in the mid-to late-Devonian with the deposition of > 5 km of alluvial sheet flood sediments in an E-W-trending half-graben (Williams et al 1989); (ii) continued extension from latest Devonian through Namurian times, with a considerable decrease in the rate of sedimentation and the development of the South Munster Basin (Williams et al 1989;Meere 1995b); (iii) onset of Variscan convergence and inversion characterized by reactivation of original high-angle extensional faults and additional minor thrusting (Price and Todd 1988;Meere 1995a,b;Johnston et al 1996); (iv) layer parallel shortening, crustal thickening, then formation of kilometre-scale folds and low-angle reverse faults (Cooper et al 1984(Cooper et al , 1986Meere 1995a). …”

“…Models for the structural evolution of the Munster Basin have previously been dominated by two different interpretations: the thin-skinned, thrust-dominated model (Cooper et al 1984(Cooper et al , 1986) and the thick-skinned transpressional strike-slip model (Sanderson 1984). More recent interpretations have been largely based on the thin-skinned model with additional modifications involving inversion and reactivation of basin margin faults (Price and Todd 1988), the incorporation of minor sinistral or dextral transpression within a surge zone (Murphy 1990), and also the involvement of a buried granite and obstacle tectonics (Meere 1995a).…”

“…In spite of numerous papers decrying the use of the term 'Variscan Front' and its significance (e.g. Shackleton 1984;Cooper et al 1984;Ford et al 1992), its location and importance is still the subject of debate (Bresser and Walter 1999). The term was introduced by Suess (1886) to represent the termination of Variscan deformation by thrusting over a relatively undeformed foreland.…”

“…The KMF has been considered to be the boundary between two different tectonic zones, with well-developed tectonism to the south and less significant deformation to the north (Gill 1962;Cooper et al 1984). However, more recent interpretations have recognized the complex nature of the KMF, and although detailed strain analysis shows that there is a distinct change across the front in the vicinity of Killarney, minor thrusting and a local development of a tectonic fabric also occur to the north of the Variscan Front (Gill 1962;Naylor et al 1983;Shackleton 1984;Meere 1995a).…”