Brittle tectonism in relation to the Palaeogene evolution of the Thulean/NE Atlantic domain: a study in Ulster

Abstract

The tectonic effects of the Thulean mantle plume on the opening of the North Atlantic Ocean is still poorly understood. An analysis of the brittle deformation affecting the Late Cretaceous Chalk and Lower Tertiary igneous formations cropping out in Ulster (Northern Ireland), part of the Thulean Province, leads to the recognition of two tectonic phases. Each of these phases is characterized by different stress regimes with similar trends of the horizontal maximum principal stress ocI. The first phase, syn-magmatic and dominated by NE-SW to ENE-WSW extension, occurred during the Palaeocene. It is followed by a second post-magmatic phase, characterized initially by a probably Eocene strike-slip to compressional palaeo-stress regime with oI ( = 0") trending NE-SW to NNE-SSW associated with the partial reactivation (as reverse faults) of normal faults formed during the first phase NE-SW extension. This episode is postdated by an Oligocene extension. with ( = 02) still striking NNE-SSW/NE-SW, which reactivated Eocene strike-slip faults as nearly vertical dip-slip normal faults. This Palaeogene tectonic evolution is consistent with the tectonic evolution of similar age in western Scotland and in the Faeroe Islands. In particular, the post-magmatic NE-SW compression is here related to the 'Faeroe compressive event'. which is related to the earliest stages of drift of the Greenland plate.

Evidence for post-early Eocene tectonic activity in southeastern Ireland

“…Shannon, 1991;Naylor, 1992;Tate, 1993;Brodie & White, 1995), and coarse clastic sedimentation (Stoker, 1997;Stoker, van Weering & Svaerdborg, 2001). Most of the direct onshore evidence for Cenozoic tectonic deformation in Ireland is based on the displacement of Upper Cretaceous chalk, Paleocene basalt flows and Oligocene sequences in the northeast of the country (Parnell, Shukla & † Author for correspondence: mcunningham@rocketmail.com Meighan, 1988;Naylor, 1992;Geoffroy, Bergerat & Angelier, 1996). However, little is known about the Cenozoic tectonic history outside this region due to an absence of stratigraphic resolution (e.g.…”

“…8). These basins were filled with a thick sequence of Oligocene clays and lignites (George, 1967;Fletcher, 1977;Parnell, Shukla & Meighan, 1988;Naylor, 1992), and fault activity on the Newry fault is inferred to have continued into post-Oligocene time (Jenner, 1981;Parnell, Shukla & Meighan, 1988;Geoffroy, Bergerat & Angelier, 1996;Readman, O'Reilly & Murphy, 1997). The Newry fault has been extended south to join the Codling fault, which lies ∼ 30 km offshore to the east of the Dublin Basin and the Wicklow Mountains (Fig.…”

Lough Neagh: the site of a Cenozoic pull-apart basin

“…Cessation of igneous activity was followed, in the Mid-Palaeogene (end Paleocene to Oligocene), by NNE-SSW compression resulting in NNW-trending dextral and NE-trending sinistral faulting of the lavas and structural inversion (Geoffroy et al 1996;Mitchell 2004). …”

“…1; Jenner 1981;Dunford et al 2001), but due to lack of mapped evidence of displacement (GSNI 1997) it is difficult to assess the amount of movement on the NNWtrending faults controlling the formation of the Lough Neagh pull-apart. (Kerr 1987;Millar 1990;Geoffroy et al 1996). Geoffroy et al (1996) recognise a syn-magmatic Paleocene event in which normal faulting and extensional strike-slip occurred, and assign the dextral offsetting of the Carlingford and Slieve Gullion intrusions to this event.…”

“…(Kerr 1987;Millar 1990;Geoffroy et al 1996). Geoffroy et al (1996) recognise a syn-magmatic Paleocene event in which normal faulting and extensional strike-slip occurred, and assign the dextral offsetting of the Carlingford and Slieve Gullion intrusions to this event. An Oligocene NW-SE extensional event, recognized by Geoffroy et al (1996), fits well with the proposed dextral movement on the bounding NNWtrending faults of the Lough Neagh pull-apart in the Late Oligocene (Fig.…”