Structure of the Dingle Peninsula, SW Ireland: evidence for the nature and timing of Caledonian, Acadian and Variscan tectonics

Abstract

The Palaeozoic rocks of the Dingle Peninsula provide a record of the evolution of the Caledonides, Acadides and Variscides. The succession ranges from Early Ordovician deep-water sediments, through Silurian shallow marine to non-marine sediments and volcanic rocks to an Old Red Sandstone (ORS) succession topped by Carboniferous marine shales. Comparison of structural styles in the unconformity-bounded groups, together with a detailed analysis of fault zones, allows the tectonic history to be deduced. The older rocks record Caledonian processes on the margin of Avalonia during Early Ordovician time and convergence then soft collision with Laurentia during Silurian time. The Dingle Basin was developed during the late Silurian – Early Devonian transtension in the Iapetus suture zone and was inverted in the latest Emsian Acadian orogenic episode. Post-Dingle Group ORS groups in the north of the peninsula were deposited in a syn-rift footwall block to the main Munster Basin. The Acadian transpressional and Munster Basin extensional structures were reactivated or overprinted in the Variscan deformation such that Acadian folds are transected by Variscan cleavage in both plan and vertical views. After Iapetus closure, changes in the tectonic regime are believed to be a result of adjustments in the geometry of subduction of the Rheic Ocean.

Prelithification and synlithification tectonic foliation development in a clastic sedimentary sequence: COMMENT

“…In other words, as much 88% of the total bulk strain in the Dingle Group is likely due to Variscan shortening, not Acadian. The cleavage transects the axial planes of the Dingle Group folds in both plan and section, commensurate with Variscan overprinting of preexisting and gently tightened Acadian folds and hence is not a product of Acadian transpression (Todd, 2015). (3) Strain patterns are highly dependent on bedding orientation and lithology and fabric of the sediments.…”

“…The folding and faulting of the Dingle Group clearly accomplishes more shortening (~40%) than is evidenced by Variscan-only folds (~14%) above the Acadian unconformity (Todd, 2015). However, when strain data from the entire area are considered, there is a smaller difference in the R s yz strains in mudrocks above (~2.5) and below (~2.8) the Acadian unconformity (Todd, 2015). In other words, as much 88% of the total bulk strain in the Dingle Group is likely due to Variscan shortening, not Acadian.…”

“…Structural observations indicate that Old Red Sandstone (ORS) rocks demonstrably younger than the Acadian unconformity have experienced ~14% shortening by folding and faulting and a R s yz strain of ~2.5 (or 40% shortening). The strain is associated with a cleavage that is axial planar to the folds in plan and occurs in symmetrical fans on the axial plane in cross section (Todd, 2015). In the west of the peninsula, this Variscan deformation is evidenced by steeply dipping and cleaved Late Devonian ORS on the north coast and inferred from the substantial uplift of the Acadian unconformity above regional.…”

“…That the late Silurian to Emsian Dingle Group was deformed during the Acadian is not disputed (Todd, 2015, and references therein). Rather, it is the conclusion that the single tectonic foliation is Acadian and was generated before lithification was complete is refuted by several factors, not covered by Meere et al, but which are evident from assessment of the entire area (Todd, 2015).(1) The entire Palaeozoic of the area was affected by the endCarboniferous Variscan orogenic shortening. Structural observations indicate that Old Red Sandstone (ORS) rocks demonstrably younger than the Acadian unconformity have experienced ~14% shortening by folding and faulting and a R s yz strain of ~2.5 (or 40% shortening).…”

“…That the late Silurian to Emsian Dingle Group was deformed during the Acadian is not disputed (Todd, 2015, and references therein). Rather, it is the conclusion that the single tectonic foliation is Acadian and was generated before lithification was complete is refuted by several factors, not covered by Meere et al, but which are evident from assessment of the entire area (Todd, 2015).…”