Evidence for post-early Eocene tectonic activity in southeastern Ireland

Abstract

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract -The role played by Cenozoic deformation in denudation and landscape development in Ireland has historically been difficult to assess because of the lack of widespread pre-glacial Cenozoic deposits onshore. Here we combine analysis of apatite fission-track data and geomorphic observations to place constraints on the timing, kinematics and magnitude of onshore deformation in southeastern Ireland. Relationships between apatite fission-track central age and elevation for samples from the Wicklow and Blackstairs Mountains and Tullow Lowland suggest that these rocks record an exhumed apatite partial annealing zone, which after cooling was dismembered by differential vertical displacements of up to several hundred metres. We use inverted models of sample thermal history to show that samples across the region experienced very similar thermal histories up to and including a cooling event in late Paleocene or early Eocene time. This effectively rules out strongly spatially heterogeneous denudation, and implies that differential rock uplift occurred in post-early Eocene time.The central age-elevation relationships define at least three spatial domains with internally consistent apatite fission-track data, separated by known faults or topographic escarpments. Geomorphic analysis of these structures shows that patterns of catchment incision and sinuosity, as well as the presence of antecedent drainage, are best explained by differential vertical displacements at or near the domain boundaries. The kinematics and magnitudes of these displacements are consistent with those implied by the apatite fission-track results, and are compatible with other examples of known Cenozoic deformation from Ireland and the adjacent continental margin.

The late tertiary landscapes of western Ireland

“…The regional variability in the degree of denudation and tectonic activity is very apparent (Simms, 2000). In parallel with the use of newer techniques to determine denudation rates there has also been a reawakening of interest in the classic geomorphology of drainage patterns and their implications following on from the work of Davies (1970) and Mitchell (1980) for example, the work of Cunningham et al, (2003) in south-east Ireland, the use by Dewey (2000) of graded profiles and knick points associated with faults in western rivers and the review by Phillips (2001) of possible Tertiary drainage patterns.…”

Evidence for Cenozoic tectonic deformation in SE Ireland and near offshore

“…Plutons of the Leinster Granite are continuous beneath the low-relief Tullow Lowland and are capped with isolated roof pendants of Ordovician schist [Charlesworth, 1963;Brück and O'Connor, 1980] at both high and low elevations. Cunningham et al [2003] used apatite fission track data and geomorphological analysis to infer that the Tullow Lowland has been downthrown to the south by about 700 m in post-early Eocene time. Therefore the differences in elevation across SE Ireland cannot be due simply to differential erosion [Farrington, 1927;Davies, 1966].…”

“…The first led to reactivation of the Caledonian -Variscan Blackrock Fault Figure 9. Summary shaded relief image of NW Europe showing Cenozoic structures in Ireland and Britain, extensional graben in the Alpine foreland, and compression on the Faeroe-Rockall Plateau [Coward et al, 1989;Boldreel and Andersen, 1998;Cunningham et al, 2003;Parnell et al, 1988]. Also shown are generalized areas of Neogene uplift and subsidence [after Japsen and Chalmers, 2000].…”

“…[56] In SE Ireland, exhumation began post-early Eocene [Cunningham et al, 2003] as part of a regional cooling event that affected the Irish landmass at this time [Allen et al, 2002]. Cunningham et al [2003] showed that the major rivers of SE Ireland are out of equilibrium with the present topography in a manner that is consistent with the locations, magnitudes, and vertical displacements on faults, that were probably reactivated syn-or post-early Eocene.…”

“…During the Oligocene, dextral transtension of NW trending faults in NE Ireland and the Irish Sea (e.g., the Newry -Codling Faults, Figure 1) resulted in the creation of the Lough Neagh-Ballymoney pull-apart basins [George, 1967;Parnell et al, 1988;Naylor, 1992;Geoffroy et al, 1996;Meighan et al, 1999]. The general picture that emerges from the onshore stratigraphic record and recent apatite fission track studies [Green et al, 2000;Allen et al, 2002;Cunningham et al, 2003] is of a broad patchwork of repeated episodes of exhumation, mostly restricted to the high areas around the margins of the island, with a low-relief erosion surface occupying much of central Ireland (Figure 1). …”