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

Abstract

Two thrusts occur on Ross Island: the Head of Ross Thrust and the more southerly Ross Island Thrust. These lie to the north of the Killarney-Mallow Fault (KMF), the boundary frequently interpreted as the Variscan Front. The Ross Island Thrust, exposed in outcrop and in seven borehole cores, has emplaced dark blue-grey limestones of the Courceyan Ballysteen Formation over pale grey-brown Rockfield Limestone Formation of Chadian-Holkerian age. These lithologies at Ross Island exhibit a continuum of deformation at both the micro-and macro-scale, beginning with the generation of a spaced cleavage, formed during layer parallel shortening, that was subsequently rotated into parallelism with fold axial planes. Extensional microstructures are predominant in thin section and are associated with attenuation of the fold limb. Calcite veins are also attenuated and lie parallel to a mylonitic fabric close to the thrust plane. Lithological boundaries, the prominent pressure solution cleavage and the southerly dipping limb of an asymmetrical antiform are all parallel and form a composite planar anisotropy. This has controlled the location of the ductile Ross Island Thrust, which formed during the attenuation and shearing of a common fold limb. Ductile thrusts within the limestones at Ross Island contrast with the reactivation of basin-margin extensional faults further to the south along the major KMF. The Ross Island Thrust is considered to result from deformation ahead of the major northerly propagating Variscan décollement thrust and does not necessitate a continuous décollement structure north of the KMF. Mineralization at Ross Island exhibits remobilization associated with the formation of a pressure-solution cleavage and probably pre-dates thrusting.

Detrital sources and sediment delivery to the early post-rift (Albian-Cenomanian) Newfoundland Basin east of the Grand Banks: results from ODP Leg 210

“…The Hercynian belt was a Permo-Carboniferous collisional orogen, bounded by a deformation front called the Hercynian Front or Variscan Front (e.g. Nex et al, 2003). Plate-motion vectors indicate essentially orthogonal convergence between Europe and the Grand Banks to form this segment of the Hercynian Orogen (Scotese et al, 1979, their figures 30 and 32;Capdevila and Mougenot, 1988;Ziegler, 1988).…”

“…This orogeny is essentially coeval with the Hercynian event in Europe -both record continent-continent collision between Africa/Europe and Laurentia. The northern to western limit of Hercynian/ Alleghenian thrusting crosses the southern UK and southern Ireland as the Hercynian Front (Nex et al, 2003), follows the approximate line of Mesozoic separation between Iberia and Newfoundland ( Fig. 1) and northwest Africa and Nova Scotia/New England (Ziegler, 1988), and then comes onshore in the southern Appalachians as the Allegheny Front.…”

“…In western Ireland, the Hercynian Front is on land (Nex et al, 2003), and the Hercynian highlands were the source for turbidites (Ross Formation) and thick deltaic units (Central Clare Group) in the Clare Basin (Rider, 1974;Lien et al, 2003) (2007). Those authors labeled the top-Cenomanian sample as Turonian, but it is essentially at the boundary between these two stages.…”

Elemental Contamination of Ancient Copper Mine in Killarney, Ireland

“…The peninsula of Ross Island is located on 2 different geologic thrusts-Head of Ross Thrust and Ross Island Thrust (Nex et al, 2003). The carbonate lithology of the region, along with a ductile nature, suggests that a ductile-thrust model is the most applicable model for the area (Nex et al, 2003). (Nex et al, 2003).…”

“…The carbonate lithology of the region, along with a ductile nature, suggests that a ductile-thrust model is the most applicable model for the area (Nex et al, 2003). (Nex et al, 2003).…”