Reversals in the polarity of structural facing across an early ductile thrust; the Central Donegal Slide, northwest Ireland

Abstract

The Dalradian succession of central Donegal represents a tectono-stratigraphic sequence everywhere separated into two distinct successions by the Central Donegal Slide. Each stratigraphic package consistently youngs away from this major D, structure which originated during lower greenschist facies metamorphism and corresponds with the development of locally preserved penetrative S, fabrics. The tectonic slide and associated reversal in the direction of structural facing are commonly marked by mylonites and tectonic schists, which have been subsequently folded and reactivated by kilometric-scale sheath folds and the Ballybofey Nappe.Structural correlations with northwest Donegal suggest that the Central Donegal Slide relates to the Knockateen Slide, and represents an early thrust with top to the northwest sense of shear. The Central Donegal Slide cuts up-section through the Middle Dalradian sub-basin centred on southern Donegal, emplacing younger, anoxic basinal sediments and turbidites over an older, inverted, shelf succession. The location of the Central Donegal Slide adjacent to the margin of this sub-basin suggests that the siting and form of ductile thrusts may be strongly influenced by the geometry of the pre-existing sedimentary basin.

Ductile transpression and localization of deformation along tectonic boundaries in the Caledonides of northwestern Ireland

“…Importantly, however, D 1 stratigraphic inversions resulting in reversals in the polarity of later structural facing are also observed at similar stratigraphic levels further east along the Central Donegal Slide (Fig. 1) (Alsop, 1992;Hutton & Alsop, 1997). These consistent relationships suggest that the base of the Easdale subgroup in central and southwest Donegal has localized early D 1 deformation, and this may reflect the development of a major Argyll Group basin and its vicinity to the regional strike-swing.…”

“…4), and this reversal is indicative of pre-D 2 inversion of stratigraphy in the footwall of the Glencolumbkille Slide. F 2 facing relationships across this structure are therefore considered a consequence of D 1 tectonic sliding, and are thought to be analogous to those observed further east in central Donegal (Alsop, 1992) (see Section 7).…”

“…No major F 1 folds or D 1 structures analogous to those described from central Donegal are identified (see Alsop, 1992;Hutton & Alsop, 1995. Minor F 1 folds noted by Howarth, Kilburn & Leake (1966) to the south of Skelpoonagh Bay are now considered to be F 2 in age.…”

Localization of mid-crustal thrust ramps by metadolerite sheets in the Dalradian of northwest Ireland

“…General studies of the Boulder Bed with reference to the Glencolumbkille peninsula have been described by Kilburn, Pitcher & Shackleton (1965) and Alsop & Hutton (1990), with detailed work on the Slieve League peninsula reported by Howarth (1971) and Johnston (1993). The classic work on the Dalradian of northwest Donegal, summarized in Pitcher & Berger (1972) and Hutton (1977Hutton ( , 1979Hutton ( , 1983 has recently been extended to southeast Donegal and Tyrone in a series of papers by Alsop (1991Alsop ( ,1992a and Alsop & Hutton (1990,1993a.…”

“…However, this interpretation has been called into question by Alsop (1992a). The region also encompasses a dramatic change in stratigraphical thickness from west to east associated with lateral facies variations within the Argyll Group (Pitcher & Berger, 1972).…”