2014) the Thermal History of the Western Irish Onshore. Journal of the Geological Society, 171 (6
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Cogne, N., Chew, D., and Stuart, F. M. (2014) The thermal history of the Western Irish onshore. Journal of the Geological Society, 171 (6). pp. 779- 792. ISSN 0016-7649 Copyright © 2014 The Geological Society of London A copy can be downloaded for personal non-commercial research or study, without prior permission or charge Content must not be changed in any way or reproduced in any format or medium without the formal permission of the copyright holder(s) When referring to this work, full bibliographic details must be given http://eprints.gla.ac.uk/98110/ Deposited on: 26 November 2014 Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk 1 THE THERMAL HISTORY OF THE WESTERN IRISH ONSHORE 2 3 Nathan Cogné1*, David Chew1 and Finlay M. Stuart2 4 1Department of Geology, Trinity College Dublin, College Green, Dublin 2, Ireland. 5 2 SUERC, Rankine Avenue, Scottish Enterprise Technology Park, East Kilbride G75 6 0QF, United Kingdom. 7 * Corresponding Author: [email protected] 8 9 Abstract 10 11 We present here a low-temperature thermochronological study that combines 12 the apatite fission track and (U+Th)/He dating methods with a pseudo-vertical 13 sampling approach to generate continuous and well-constrained temperature-time 14 histories from the onshore Irish Atlantic margin. The apatite fission track and 15 (U+Th)/He ages range from the Late Jurassic to Early Cretaceous and the mean 16 track lengths are relatively short. Thermal histories derived from inverse modelling 17 shows that following post-orogenic exhumation the sample profiles cooled to ~75°C. 18 A rapid cooling event to surface temperatures occurred during the Late Jurassic to 19 Early Cretaceous and was diachronous from north to south. It was most likely 20 caused by ~2.5 km of rift-shoulder related exhumation and can be temporally linked 21 to the main stage of Mesozoic rifting in the offshore basins. A slow phase of 22 reheating during the Late Cretaceous and Early Cenozoic is attributed to the 23 deposition of a thick sedimentary sequence that resulted in ~1.5 km of burial. Our 24 data imply a final pulse of exhumation in Neogene times, probably related to 25 compression of the margin. However, it is possible that an Early Cenozoic cooling 1 26 event, compatible with our data but not seen in our inverse models, accounts for part 27 of the Cenozoic exhumation. 28 29 Supplementary material: Apatite fission track and (U+Th)/He methodologies are 30 available at www.geolsoc.org.uk/ 31 32 Historically, the almost complete absence of post-Variscan sediments outside 33 of northeastern Ireland has made it difficult to assess the Mesozoic and Cenozoic 34 evolution of onshore Ireland. In contrast, offshore Ireland and Britain make a superb 35 natural laboratory in which to evaluate the various techniques for estimating the 36 timing and magnitude of exhumation because of the wealth of data provided by the 37 hydrocarbon industry (e.g. Corcoran & Doré 2005), such as apatite fission track, 38 vitrinite reflectance and shale compaction data. Therefore the evolution of the 39 surrounding offshore basins has often been used as a proxy to infer the post- 40 Variscan history of the Irish onshore (e.g. Naylor 1992). However without well- 41 established links between the onshore exhumation history and the evolution of the 42 offshore basin fill, such methods cannot be directly used to infer the post-Variscan 43 history of the onshore portion of the Irish Atlantic margin. 44 The thermal evolution of the Irish landmass has been investigated by 45 extensive thermal maturation studies (e.g. vitrinite reflectance and conodont 46 alteration data) of the Carboniferous and Devonian rocks that cover much of onshore 47 Ireland (Clayton et al. 1989, Goodhue and Clayton 1999). These studies require 48 several kilometres of burial and exhumation to reach their present-day exposure 49 level. Because post-Carboniferous rocks on the Irish mainland do not yield high 50 maturation values, peak maturation temperatures and the majority of the subsequent 2 51 exhumation has been attributed to the Variscan orogeny and its unroofing. However 52 many of the details of the subsequent post-Variscan thermal history remained 53 unresolved. 54 The apatite fission track (AFT) technique has been used during the past two 55 decades to determine more precisely the post-Variscan thermal history of onshore 56 Ireland (e.g. McCulloch 1993, 1994; Kelley et al. 1993; Green et al. 2000; Allen et al. 57 2002), as low-temperature thermochronological techniques have the advantage of 58 yielding temporally controlled thermal history information (e.g. Gallagher et al. 1998). 59 Green et al. (2000) and McCulloch (1993) have shown that the thermal history of 60 western Ireland is characterised by cycles of exhumation (with subsequent re-burial) 61 during the Mid-Jurassic, Early Cretaceous and Early Cenozoic followed by final 62 exhumation in the Neogene. This Cenozoic thermal history is in broad agreement 63 with the pattern of Cenozoic exhumation on a larger scale (e.g. Ireland and Britain, 64 Hillis et al. 2008). Allen et al. (2002) in contrast argued for more a continuous and 65 progressive Mesozoic to Cenozoic exhumation history, with a phase of reburial 66 restricted to the Late Cretaceous that was then followed by rapid exhumation during 67 the Cenozoic. Therefore aspects of the post-Variscan exhumation history of the Irish 68 onshore remains controversial despite a fairly large database of apatite fission track 69 data. 70 Consequently in this study we also employ the apatite (U+Th)/He (AHe) 71 thermochronometer in conjunction with the AFT method with the aim of producing 72 better resolved thermal histories. Moreover we also employed a pseudo-vertical 73 profile approach (i.e. summit-to-valley sample transects) on selected targets along 74 the western Irish coast. The thermal history of each profile is characterised by 75 integrating these two thermochronometers to generate better constrained continuous 3 76 temperature-time histories than previous thermal history studies from the Irish 77 onshore. Combining data from different samples using a vertical profile approach 78 reduces the uncertainties on the final thermal history model (Gallagher et al. 2005). 79 80 Geological setting 81 82 Onshore Ireland 83 84 Most of central and mid-western Ireland is characterised by relatively flat and 85 subdued topography and is mainly composed of Late Devonian to Carboniferous 86 sedimentary rocks. In contrast, the western Irish Atlantic coastline exhibits 87 comparatively high relief with summits that range from over 750 m in Donegal in the 88 northwest to over 1000 m in Kerry in the southwest. These high elevation regions are 89 typically formed of older rocks, ranging in age from Precambrian to Devonian (Figs. 1 90 and 2). 91 Post-Carboniferous outcrops are scarce in western Ireland. In southwest 92 Ireland, the oldest is the Mid-Jurassic (~180 Ma) Cloyne clay (Fig. 1, Higgs and 93 Beese 1986). Vitrinite reflectance values of ~0.35 RO% (Clayton, 1989) show that the 94 Cloyne clay has undergone approximately 1 km of burial. The Ballydeenlea Chalk 95 (Fig. 1) has featured prominently in many studies of the post-Carboniferous evolution 96 of Ireland (e.g. Dewey 2000; Green et al. 2000; Allen et al. 2002). It is dated as 97 Campanian in age (Barr 1966), which corresponds to an age range of 83.6-72.1 Ma 98 (all stratigraphic ages quoted in this study use the timescale of Gradstein et al., 99 2012). This outcrop is of particular interest because vitrinite reflectance data (Evans 100 and Clayton 1998) demonstrates that the Ballydeenlea Chalk was buried and heated 4 101 to temperatures of 60 - 80°C (0.46 – 0.53 RO%). In northwest Ireland the aureoles of 102 Palaeocene (62-58 Ma) dyke swarms (Thompson 1985) have yielded AHe ages that 103 are indistinguishable from the intrusion ages of the dykes implying relatively low 104 amounts (<1.5 km) of Cenozoic exhumation in this region (Bennett 2006). Cenozoic 105 sediments locally infill karstified Carboniferous limestones in central Ireland. The 106 oldest examples are Oligocene in age (Fig. 1, Ballygiblin, Simms and Boutler 2000; 107 Ballymacadam, Watts, 1957) but most them are either undated or are Miocene to 108 Pliocene in age (Coxon and McCarron 2009). The preservation of such deposits 109 clearly indicates locally low amounts of Neogene denudation in central Ireland. 110 Based on topographic and fluvial studies, Dewey (2000) argued for fragmented 111 Cenozoic uplift of western Ireland associated with Eocene-Oligocene extensional 112 normal faulting and Miocene to recent compression, with little denudation of the Irish 113 Midlands during the Mesozoic and Cenozoic. 114 115 The western Irish offshore basins 116 117 On the western Irish margin, proto-Atlantic rifting is characterized by a series 118 of discrete extensional episodes during the Permo-Triassic, Middle to Late Jurassic 119 and Early Cretaceous (Naylor and Shannon 2005) that led to the formation of the 120 western Irish offshore basins (Fig. 2). The Middle to Late Jurassic rifting event is the 121 most significant (Naylor and Shannon 2009). It was diachronous, commencing in the 122 Middle Jurassic in the Slyne and Erris basins (Dancer et al. 1999; Chapman et al. 123 1999), in the Late Jurassic to Early Cretaceous in the Porcupine Basin (Moore 1992; 124 Shannon et al. 1999; Naylor and Shannon 2005) and in the Early Cretaceous in the 125 Goban Spur Basin (Masson et al. 1984; Naylor and Shannon 2005). 5 126 During the Cretaceous and Cenozoic, the evolution of the western Irish 127 offshore basins mainly reflects the effects of thermal subsidence (Shannon 1991; 128 Shannon et al.