# Measuring plume-related exhumation of the British Isles in Early Cenozoic times

Nathan Cogné, Daniel Doepke, David Chew, Finlay Stuart, Chris Mark

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Nathan Cogné, Daniel Doepke, David Chew, Finlay Stuart, Chris Mark. Measuring plume-related exhumation of the British Isles in Early Cenozoic times. Earth and Planetary Science Letters, 2016, 456, pp.1-15. 10.1016/j.epsl.2016.09.053. hal-04287555

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University
of Glasgow

Cogné, N., Doepke, D., Chew, D., Stuart, F. M., and Mark, C. (2016)
Measuring plume-related exhumation of the British Isles in Early Cenozoic
times. *Earth and Planetary Science Letters*, 456, pp. 1-15.
(doi:10.1016/j.epsl.2016.09.053)

This is the author's final accepted version.

There may be differences between this version and the published version.
You are advised to consult the publisher's version if you wish to cite from
it.

http://eprints.gla.ac.uk/134604/

Deposited on: 31 January 2017
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Measuring plume-related exhumation of the British Isles in Early Cenozoic

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Nathan Cogné¹*, Daniel Doepke¹, David Chew¹, Finlay M. Stuart² and Chris Mark¹

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¹Department of Geology, Trinity College Dublin, College Green, Dublin 2, Ireland.

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²SUERC, Rankine Avenue, Scottish Enterprise Technology Park, East Kilbride G75

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0QF, United Kingdom.

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* Corresponding Author: cognen@tcd.ie

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Abstract

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Mantle plumes have been proposed to exert a first-order control on the morphology
of Earth's surface. However, there is little consensus on the lifespan of the
convectively supported topography. Here, we focus on the Cenozoic uplift and
exhumation history of the British Isles. While uplift in the absence of major regional
tectonic activity has long been documented, the causative mechanism is highly
controversial, and direct exhumation estimates are hindered by the near-complete
absence of onshore post-Cretaceous sediments (outside Northern Ireland) and the
truncated stratigraphic record of many offshore basins. Two main hypotheses have
been developed by previous studies: epirogenic exhumation driven by the proto-
Iceland plume, or multiple phases of Cenozoic compression driven by far-field
stresses. Here, we present a new thermochronological dataset comprising 43 apatite
fission track (AFT) and 102 (U-Th-Sm)/He (AHe) dates from the onshore British Isles.
Inverse modelling of vertical sample profiles allows us to define well-constrained
regional cooling histories. Crucially, during the Paleocene, the thermal history models
show that a rapid exhumation pulse (1-2.5 km) occurred, focussed on the Irish Sea.
26 Exhumation is greatest in the north of the Irish Sea region, and decreases in intensity
27 to the south and west. The spatial pattern of Paleocene exhumation is in agreement
28 with the extent of magmatic underplating inferred from geophysical studies, and the
29 timing of uplift and exhumation is synchronous with emplacement of the plume-
30 related British and Irish Paleogene Igneous Province (BIPIP). Prior to the Paleocene
31 exhumation pulse, the Mesozoic onshore exhumation pulse is mainly linked to the
32 uplift and erosion of the hinterland during the complex and long-lived rifting history of
33 the neighbouring offshore basins. The extent of Neogene exhumation is difficult to
34 constrain due to the poor sensitivity of the AHe and AFT systems at low
35 temperatures. We conclude that the Cenozoic topographic evolution of the British
36 Isles is the result of plume-driven uplift and exhumation, with inversion under
37 compressive stress playing a secondary role.
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39 Keywords: British Isles; exhumation; low temperature thermochronology; Early
40 Cenozoic; Iceland Plume

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42 1. Introduction

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44 It has long been proposed that mantle processes exert a first-order control on
45 the Earth's topography (e.g. Cox, 1989). The topographic response to mantle
46 upwelling can be either transient, with dynamically supported uplift and exhumation
47 driven by mantle upwelling, or permanent, if driven by magmatic underplating (White
48 and McKenzie, 1989). While the temperature anomaly associated with mantle
49 plumes can propagate through the sub-lithospheric mantle over 1,000 km radially
50 (White and McKenzie, 1989), the magnitude of induced uplift and exhumation can be
hard to discriminate from intra-plate exhumation caused by horizontally-transmitted
far-field tectonic stresses (Burov and Cloetingh, 2009, Braun, 2010).

In this contribution we focus on the controversial question of whether a long-
recognised phase of post-Cretaceous exhumation in the British Isles was generated
by upwelling mantle of the proto-Iceland plume (Davis et al., 2012), or by far-field
tectonic stresses transmitted from the Alpine collision (Hillis et al., 2008). The Iceland
plume is generally regarded as the source for Cenozoic anomalous magmatic activity
and dynamic support throughout Greenland and the North Atlantic basin (e.g., Jones
and White, 2003). This includes the British Paleogene Igneous Province (BIPIP),
emplaced between ca. 63-58 Ma (Ganerød et al., 2010), which shows a clear proto-
Iceland plume geochemical signature (Stuart et al., 2000). Thus the Irish Sea area
encompassed the zone of influence of the plume in the early Paleogene times. At this
time, important (i.e. kilometric scale) exhumation of the region took place (e.g.,
Rowley and White, 1998). However, as localised, poorly-dated inversion structures
caused by compressive stresses are also reported in the British Isles throughout the
Cenozoic era (e.g., Ziegler et al., 1995), the driving mechanism of the post-
Cretaceous exhumation of the British Isles remains controversial (e.g., Hillis et al.,
2008).

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## 2. Geological Context and Study Approach

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The post-Variscan evolution of Britain and Ireland and much of north-western
Europe is characterized by orogenic collapse and prolonged extension, primarily in
the Mesozoic times, which formed the basins of the Atlantic margin and the Irish and
North Seas. Repeated episodes of exhumation are superimposed on these tectonic
events, recorded by erosional unconformities in the Irish Sea basins, together with
onshore and offshore AFT ages, vitrinite reflectance, sediment compaction studies
and subsidence analyses (Rowley and White, 1998; Holford et al., 2005; Hillis et al.,
2008). The Mesozoic exhumation episodes are variously attributed to either uplift
induced by the dynamic support provided by mantle convection (Jones et al., 2012,
and references therein) or to transmitted stresses from pre-existing, or incipient plate
boundaries (Holford et al., 2005).

Determining the Cenozoic geological evolution of the British Isles is hindered
by the paucity of Cenozoic onshore outcrops outside of Northern Ireland and the
truncated stratigraphic record in many of the more accessible inboard basins. There
is no consensus on the extent, magnitude, or timing of the Cenozoic event(s) (e.g.
Hillis et al., 2008; Davis et al., 2012), for which two competing hypotheses have been
proposed: (i) an epirogenic mechanism linked to the proto-Iceland plume, either by
Early Paleogene plume-related magmatic underplating or dynamic support (e.g.
Brodie and White, 1994; Rowley and White, 1998; Jones et al., 2002) or (ii) basin
inversion under compressive stress from the opening of the North Atlantic and the
Alpine orogeny (e.g. Ziegler et al., 1995; Holford et al., 2005; Hillis et al., 2008).

The greatest magnitude of Cenozoic exhumation is generally reported around
the Irish Sea region (Lewis et al., 1992; Al-Kindi et al., 2003), which is the focus of
this study. The Irish Sea (including its southern extensions, the St George's Channel
and Bristol Channel), and the Celtic Sea to the south, comprise a series of fully-filled,
partially-inverted post-Variscan basins with little bathymetric expression, hosting late
Carboniferous to Cenozoic sedimentary units (Rowley and White, 1998; Naylor and
Shannon, 2009). Topographically, the Irish Sea is today merely a shallow (up to ca.
120 m) marine embayment, which is likely related to ice-stream excavation and
101 glacial palaeo-loading. The Celtic Sea basin preserves Jurassic and Cretaceous successions, and in common with the North Sea and the Western Irish offshore basins, these Mesozoic successions yield evidence for at least two episodes of exhumation during Mid-Jurassic times and at the Jurassic / Cretaceous boundary (Jones et al., 2012). However these episodes cannot be observed directly in the north part of the Irish Sea basin system, as Mesozoic sedimentary rocks of post-Early Jurassic age are generally absent (Naylor and Shannon, 2009), demonstrating that a significant portion of the post-rift sequence is missing. Cenozoic sedimentary rocks in the Irish Sea are only preserved locally in the St George's Channel Basin, where > 1km of Eocene-Oligocene sediments are preserved (Williams et al., 2005), and in the Cardigan Bay Basin where a ca. 600 m thick sequence of Oligocene-Miocene succession is reported (Naylor and Shannon, 2009). Cenozoic exhumation, including partial basin exhumation, has therefore long been recognised in the Irish Sea region (e.g. Kamerling, 1979).

115 Late Cretaceous chalk is present in the Ulster Basin in NE Ireland (Figure S1), and underwent subaerial erosion prior to burial by the Paleocene Antrim Plateau Basalts (Simms, 2009). Similar evidence for uplift and erosion in the Paleocene is present in Mull and Morven in western Scotland (Figure S1). Outside of these regions, the lack of onshore post-Variscan sedimentary rocks around the Irish Sea basin system makes it difficult to infer the Mesozoic to Cenozoic geological evolution. Several previous studies have utilised the apatite fission-track (AFT) low-temperature thermochronometer (Lewis et al., 1992; Green et al., 1997; Green et al., 2000; Allen et al., 2002; Green, 2002; Holford et al., 2005) to quantify and date exhumation. However, the thermal sensitivity of the AFT system alone (120-60°C; Donelick et al., 2005) is not well suited to detect cooling from shallow crustal depths of 1 – 3 km.
Green et al. (2000) argued that Ireland has suffered repeated cycles of exhumation and reburial during the Mid-Jurassic, Early Cretaceous, Early Cenozoic and a final exhumation event during Neogene times. Allen et al. (2002) preferred a more continuous exhumation history with reburial restricted to the Late Cretaceous period followed by rapid exhumation during the Cenozoic times. Using a combination of apatite (U-Th)/He and AFT dating of vertical profiles from the western Irish onshore, Cogné et al. (2014) showed that the Mesozoic exhumation history of western Ireland was closely linked to the rifting history of the Atlantic margin basins to the west. The Cenozoic history was poorly constrained, and an Early Cenozoic exhumation pulse was not detected. AFT studies from the eastern side of the Irish Sea suggest that exhumation of 2 to 3 km occurred during the Early Cenozoic epoch in NE England (Lewis et al., 1992, Green et al., 1997, Green, 2002). This did not, apparently, extend to North Wales (Holford et al., 2005), where the only reported exhumation event occurred during the Triassic – Early Jurassic times. On a regional scale, however, the outcrop pattern of post-Triassic sedimentary rocks in Britain strongly suggests southeast-directed tilting during the Early Cenozoic (Cope, 1994), while the provenance of heavy mineral assemblages in the Paleogene sands of the Hampshire Basin suggests a source in Scotland (Morton, 1982), implying that that NW Britain was being uplifted at that time.

The timing of onshore and offshore exhumation in the Irish Sea region thus remains controversial, and the relative importance of Jurassic to Early Cretaceous syn-rift versus Early Cenozoic to Neogene post-rift exhumation events remains unclear (Rowley and White, 1998; Holford et al., 2005, Hillis et al., 2008, Davis et al., 2012). In this study, we combine the AFT system with the apatite (U-Th-Sm)/He (AHe) technique, which is sensitive to temperatures between 110°C and 40°C
151 (Shuster and Farley, 2009) to yield better-constrained thermal history models on the
152 onshore part of Britain and Ireland. Crucially, we also employ a pseudo-vertical
153 profile sampling approach (i.e. summit-to-valley transects) that significantly
154 strengthens the models as they have to respect data constraints from multiple
155 samples, which results in significantly smaller uncertainties than modelling samples
156 independently (Gallagher et al., 2005). Combining the AFT and AHe systems with
157 inverse modelling of vertical sample profiles therefore allows us to define the cooling
158 history of the Irish Sea region by constraining small magnitude exhumation phases
159 (ca. 1 km), which conventional approaches (such as modelling of fission-track data
160 from individual samples) could fail to detect.

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162 **3. Thermochronology**

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164 **3.1 *Sampling strategy***

165 We favoured a pseudo-vertical profile sampling approach (i.e., summit-to-
166 valley sample transects) wherever permitted by lithology and topography. However in
167 some areas (mainly northeast Ireland and the Southern Uplands in Scotland) we
168 employed single samples that were modelled individually. In total seven vertical
169 profiles were sampled; four in Ireland, two in Wales and one from the Paleocene Isle
170 of Arran Northern Granite (Fig. 1). Three samples were collected from the Isle of
171 Man, including two from the same profile but with a small vertical offset. Importantly,
172 one sample from northeast Ireland was taken from a borehole with a present-day
173 depth of 1702 m below surface. In total, we obtained 43 AFT ages and 102 (U-Th-
174 Sm)/He individual ages.

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176 3.2 Fission track analyses

177 The detailed apatite fission track methodology is described in the
178 supplementary material. All samples were prepared and analysed in Trinity College
179 Dublin (TCD) using the etching protocol of Donelick et al. (2005). Uranium
180 concentration measurements for fission track dating were undertaken by LA-ICPMS
181 at TCD following the protocols described in Donelick et al. (2005) and Cogné et al.
182 (2014). Apatite U-Pb age data were also obtained from the same LA-ICPMS spot
183 ablations with data reduction procedures following Chew et al. (2014). U-Pb data
184 were only acquired for use as high temperature constraints for the modelling
185 procedure (see thermal history modelling methodology section below) and are not
186 presented here as they do not impact on the interpretations of the Mesozoic -
187 Cenozoic thermal histories.

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189 3.3 (U-Th-Sm)/He

190 A full description of the (U-Th-Sm)/He methodology is provided in the
191 supplementary material. All apatite grains were picked in TCD, and all (U-Th-Sm)/He
192 measurements were carried out at the Scottish Universities Environmental Research
193 Centre (SUERC). All procedures are described in detail in Foeken et al. (2006). The
194 minimum uncertainty on the calculated ages of the apatite unknowns is fixed at 8%
195 based on repeated (U-Th-Sm)/He measurements of the Durango apatite standard.

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197 4. Results

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199 For surface samples (excluding the Paleocene Isle of Arran Northern Granite),
200 the AFT ages range from 60.4 ± 4.0 Ma to 299.2 ± 27.6 Ma (all AFT age
uncertainties in this study are reported at the 2σ level; Table 1; Fig. 2). The mean track length (MTL) ranges from 12.5 ± 0.25 µm to 13.9 ± 0.22 µm (1 SE) with standard deviations between 1.06 µm and 1.82 µm. The kinetic parameter (Dpar; e.g., Donelick et al., 2005) is relatively small for all samples, ranging from 1.52 to 2.1 µm, similar to Durango apatite (mean Dpar measured for the same etching protocol is 1.66 µm). The Ire-10 and Isle of Arran samples exhibit Early Cenozoic AFT ages, which are explained by the present-day temperature of the Ire-10 sample (collected 1702m below the surface), and the Paleocene emplacement age of the Isle of Arran Northern Granite. Excluding these samples, only two samples yield Late Cretaceous/Early Cenozoic AFT ages, and they are located on the Isle of Man (IoM-3) and the Southern Uplands (Sct-1). The rest of the samples show a large spread in AFT age from the Permian to the Early Cretaceous.

The F<sub>T</sub>-corrected AHe ages of the surface samples range from 42.7 ± 3.4 Ma to 248.6 ± 19.9 Ma (all AHe age uncertainties are reported at the 1σ level; Table 2, Fig. 2). The borehole sample (Ire-10) yielded younger AHe ages due to its higher present-day temperature. In contrast to the AFT ages, many samples from around the Irish Sea exhibit Late Cretaceous/Early Cenozoic AHe cooling ages (Fig. 2). Some grains yield older F<sub>T</sub>-corrected ages than their corresponding AFT age. The amount of α-recoil damage within the apatite crystal lattice has a strong influence on the effective closure temperature of the AHe system (e.g. Green et al. 2006, Shuster and Farley 2009), which can be approximated by the effective uranium (eU) factor where eU = [U] + 0.24 [Th] (Green et al., 2006). In our dataset, the eU factor and AHe age are only weakly correlated, but most of the He aliquots that yield older AHe ages than the corresponding AFT age exhibit relatively high eU values. For these
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aliquots, the effective closure temperature for the AHe system could be higher than
that of the AFT system which would thus explain the old AHe ages.

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**5. Thermal history modelling methodology**

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Inverse modelling of the AHe and AFT (track length and age) data was
undertaken to extract thermal history information. We used the QTQt software of
Gallagher (2012), which employs a Bayesian trans-dimensional Markov Chain Monte
Carlo (MCMC) approach. The modelling procedure is described in detail in Gallagher
(2012).

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In this study, a general time-temperature box with a temperature range of 75 ±
75°C (which spans the upper-temperature sensitivity limit of the AFT system to the
present-day surface temperature) and a time range (which spans from the present
day back to a sample-specific starting point) is used as the parameter space. A
series of discrete time-temperature points are sampled within this time-temperature
space to construct a continuous thermal history, and the data likelihood is then
calculated for that model. To model the fission track data, we used the individual
track count data, the length and angle to the c-axis of confined tracks, the annealing
model of Ketcham et al. (2007) and the likelihood function of Gallagher (1995). The
measured AHe ages were modelled using a spherical diffusion formulation which
simulates both α-ejection and diffusion during the evolution of the thermal history
(Meesters and Dunai, 2002), combined with the radiation damage model of Flowers
et al. (2009). Additionally, the eU parameter is allowed to vary by up to 25% to take
into account uncertainties on the apatite grain-size measurements, deviation from an
ideal euhedral shape, and possible minor zonation of parent nuclides.

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The present-day surface temperature for the sample is dependent on the sample elevation, and is typically set at between 15 ± 5°C and 5 ± 5°C. For the vertical profiles, the present-day offset between the top and bottom sample is set at 8 ± 3°C/km to take into account the change of surface temperature with elevation. We also employed independent geological evidence for the starting time-temperate constraint for each model, such as either the depositional age of the sample, the timing of the last regional metamorphic event experienced by the sample, or the sample U-Pb apatite age as determined by LA-ICPMS. The number of iterations depends on the stability of the likelihood chain (see Gallagher, 2012 for details), but was always set at a minimum of 200,000 iterations for single samples and at 400,000 iterations for vertical profiles.

Expected models are presented. They are essentially a weighted-mean model, while credible intervals (the Bayesian equivalent of confidence intervals) were calculated using the ensemble of thermal history solutions. These credible intervals represent the range of the model parameters contained in the posterior distribution at the specified level of probability (i.e., 95%).

In addition to the inverse modelling, forward modelling has been undertaken for some samples to test whether we could extract more information than provided by the inverse models. All the predictions (i.e., modelled versus observed ages or track-length distributions) associated with the presented models are provided in the supplementary material.

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# 6. Synthesis of the thermal history models

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A detailed description of the thermal history models including the high-
temperature constraints, the geometry of the cooling/reheating path and the timing of
cooling/reheating pulses is provided in the Supplementary Materials, and forms the
basis of the synthesis of the thermal history models described below.

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## 6.1 Late Paleozoic and Mesozoic thermal history

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The models indicate that all the profiles in Palaeozoic rocks (i.e., excluding the
Paleocene Isle of Arran Northern Granite) underwent rapid cooling following the last
significant regional high-temperature event (the Variscan or Caledonian orogenies)
and had entered the apatite PAZ by the beginning of the Mesozoic era. The thermal
histories modelled from the single samples have larger uncertainties and do not
contradict the cooling histories from the profiles. All the Palaeozoic samples had
entered the apatite PAZ by the beginning of Mesozoic times.

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A discrete pulse of Mesozoic cooling is evident in almost all profiles, while the
thermal histories for individual samples can still accommodate such a cooling phase
although it is poorly constrained due to the larger confidence intervals on these
models. In Wales, the first recognised period of cooling occurred in North Snowdonia
(Fig. 3a) and Pembrokeshire (Fig. 3b) during Permo-Triassic times. In North Wales
this is coeval with an important rifting episode in the East Irish Sea Basin and in the
Cheshire Basin (Rowley and White, 1998). In Anglesey, the Mesozoic cooling
episode occurred later during Early to Middle Jurassic times (Fig. 3b), which is
contemporaneous with an important phase of rifting and clastic sediment deposition
in the Cardigan Bay Basin (Naylor and Shannon 2009). This history is similar to that
depicted by Holford et al. (2005) for NW Wales. In South Wales, the first cooling
episode recognised in Pembrokeshire is contemporaneous with the Permo-Triassic

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299 opening of the Bristol Channel to the south (Kamerling, 1979), and with the opening
300 of the St George's Channel Basin to the north during Triassic times (Welch and
301 Turner, 2000). Further east, the large end-Triassic cooling pulse in the Brecon
302 Beacons (Fig. 3c) is synchronous with the onset of the main phase of deposition in
303 the East Bristol Channel Basin (Kamerling, 1979).

304 In southern Ireland, the Galtee Mountains (Fig. 4a) and Mount Leinster (Fig.
305 4b) profiles show an important phase of cooling during the Early Jurassic period,
306 similar to the episode recognised by Green et al. (2000). This is compatible with a
307 thick Triassic to Early Jurassic sedimentary sequence in the North Celtic Sea Basin
308 to the south, and with a major rifting episode in the Central Irish Sea Basin to the
309 southeast (Naylor and Shannon 2009). The Slieve Bloom profile in central Ireland
310 (Fig. 4c) shows a rather limited phase of Mesozoic cooling compared to all other
311 profiles examined in this study, with a minor acceleration in cooling rate around the
312 Jurassic-Cretaceous boundary at ca. 145 Ma. This is coeval with the cooling episode
313 recognised by Cogné et al. (2014) in vertical profiles from the western Ireland
314 onshore but is of smaller magnitude. The Lugnaquilla profile (Fig. 4d) records a pulse
315 of cooling towards the end of the Early Cretaceous period at 110 Ma, similar to the
316 cooling pulse detected in the forward model from the Isle of Man profile (Fig. 5a).
317 This Early Cretaceous cooling episode has been widely detected in Central and East
318 Irish Sea basins (Holford et al., 2005) as well as onshore Ireland (Green et al., 2000).

319 The timing and spatial distribution of Mesozoic cooling in Ireland and western
320 Britain is thus complex and lasted over 150 Ma. It has been argued that two
321 Mesozoic exhumation episodes occurred in the vicinity of the British Isles during Mid-
322 Jurassic (ca. 175 Ma) and Early Cretaceous (ca. 140 Ma) times, due to the presence
323 of large swells that induced the development of dynamic topography over a large
region (e.g. Jones et al., 2012). However, our data seems to indicate that the Mesozoic cooling episodes recorded by our samples have a more local spatial extent, as they occurred at different times during the Mesozoic over a distance of only few tens to one hundred kilometres. While a convectively-supported exhumation mechanism can explain a component of the Mesozoic exhumation, localised uplift induced by the flexural response to extension over 50 to 100 km length scales, would explain how each profile records a localised and different cooling history. Therefore we link the late Permian to Jurassic cooling episodes primarily to the complex rifting history of the basins and sub-basins in the Irish Sea and Celtic Sea. Each region appears to have experienced cooling that is synchronous with the deposition of syn-rift sediments in the closest offshore basins. The Slieve Bloom profile is the furthest from the coast and is the only profile that does not show an important cooling episode during the Mesozoic era, showing that the flexural response diminishes rapidly away from the offshore basins, in accordance with the low elastic thickness of the British Isles' lithosphere (Tiley et al., 2003). Thus cooling is likely to be mainly linked to rift-related uplift and subsequent erosion of the hinterland during the extensional episodes that formed the different sub-basins of the Irish Sea basin system. However the Bayesian modelling approach employed in this study tends to favour simpler solutions (see Gallagher, 2012), and thus might not be able to discriminate between discrete episodes of cooling that are close together in time. Assuming a palaeogeothermal gradient of 25°C/km similar to the present day (Goodman et al., 2004, Downing and Gray, 1986), the total amount of Mesozoic rift-related exhumation ranged between 1.5 and 3 km in the studied profiles.

While the onshore Mesozoic exhumation pulse can be linked to rifting in the closest offshore basins for most of the profiles, the onshore Early Cretaceous
349 exhumation phase seen in the Lugnaquilla and Isle of Man profiles does not have a
350 corresponding rifting episode in the Irish Sea basin system. On the contrary, low-
351 temperature thermochronology data for the East Irish Sea Basin and Central Irish
352 Sea Basin suggest that the Early Cretaceous was associated with a phase of
353 exhumation (Holford et al., 2005), while at this time there is no recorded deposition in
354 the North St George's Channel Basin (Welch and Turner 2000) and in the Kish Bank
355 Basin (Naylor and Shannon, 2009). Therefore, the Early Cretaceous exhumation is
356 likely to be more regional in extent than the previous Mesozoic exhumation episodes
357 described above and is likely linked to swell-related dynamically support as proposed
358 by Jones et al. (2012). However the amount of uplift generated may be minor, as it is
359 not seen in other profiles in Ireland or Wales. Alternatively, Holford et al. (2005),
360 amongst others, argue that this episode may reflect uplift due to a period of
361 significant extension of the Atlantic Margin and the onset of seafloor spreading in the
362 Bay of Biscay and in the Goban Spur. However this explanation implies a very strong
363 lithosphere, in contradiction to the results of Tiley et al. (2003). Our results cannot
364 readily decipher between these two competing hypotheses.
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365

366 6.2 Cenozoic thermal history

367 A second major phase of post-Variscan cooling has occurred in the Early
368 Cenozoic period. The Isle of Arran profile in SW Scotland (Fig. 5b) shows that this
369 Paleocene intrusion achieved surface temperatures shortly after emplacement. This
370 cooling pulse is also evident in individual samples in the Southern Uplands (Fig. 6a)
371 and in northeast Ireland (Fig. 6b). The cooling phase is also detected in the Isle of
372 Man and Snowdonia profiles (Fig. 5a and 3a), and can be detected in individual
373 samples from Anglesey and Pembrokeshire in forward models (Fig. 3b). The two
374 profiles from the Leinster granite (Mount Leinster, Fig. 4b; Lugnaquilla, Fig. 4d) do
375 not show a well-defined rapid pulse of Early Cenozoic cooling, but importantly cooling
376 in both profiles initiated at ca. 60 Ma. This is significantly earlier than the onset of
377 post-Mesozoic cooling in southern Ireland (Galtee Mountains, Fig. 4a), central Ireland
378 (Slieve Bloom; Fig. 4c) and in western Ireland (Cogné et al., 2014), which typically
379 commenced in Neogene times. Therefore this episode of Early Cenozoic cooling is
380 restricted to the margins of the Irish Sea. It starts between ca. 65 and 60 Ma and
381 lasts for ca. 10Ma.

382 We stress here that it is the use of two thermochronometers coupled with a
383 vertical profile approach that allowed us to demonstrate the regional extent of this
384 cooling episode. As shown on Figure 7, the use of AFT data alone for the vertical
385 profile model (Fig. 7b) or modelling individual samples using combined AFT and AHe
386 data (Fig. 7c and 7d) would not detect portions of the cooling history in the studied
387 area. Using two thermochronometers with a vertical profile approach (Fig.7a) not only
388 allowed us to detect this cooling episode, but also to show that it progressively
389 decreases in intensity from north to south and to the west as discussed below.

390

391 6.3 Spatial distribution of Early Cenozoic cooling

392 The sampling density in this study does not allow for the construction of a
393 detailed Early Cenozoic cooling map, primarily because analysing multiple samples
394 in a single profile is significantly more time consuming, limiting the spatial density of
395 the resulting data. However, the behaviour of samples in the inverse modelling
396 process can also provide an indication of the intensity of Early Cenozoic cooling, as
397 an Early Cenozoic cooling episode has to be significant to be detected in single
398 samples given that the inversion modelling approach favours simpler solutions.
We can delimit four different areas (Fig. 8) based on the modelled behaviour of single samples and vertical profiles. (i) A significant pulse of Early Cenozoic cooling is present when it is detected by inverse modelling on single samples, and this area runs from the Southern Uplands in SW Scotland to northeast Ireland (red stars on Fig. 8). It also encompasses the prominent cooling pulse detected on the Isle of Arran, but it should be noted that we have not collected a sample from this region outside of the aureole of this Paleocene intrusion. (ii) The second region comprises west Wales and the Isle of Man (orange stars on Fig. 8), where inverse modelling of individual samples does not show the pulse of Early Cenozoic cooling (or is not well constrained as in sample Pb-1 from Pembrokeshire) but where it is detected by inverse modelling of vertical profiles (e.g. the Snowdonia profile). (iii) A third zone in southeast Ireland (yellow stars on Fig. 8) is characterized by vertical profiles that do not show a discrete, rapid pulse of Early Cenozoic cooling, but slow cooling nevertheless initiated at ca. 60 Ma. (iv) The final zone comprises the three profiles in this study (Brecon Beacons, Slieve Bloom and Galtee Mountains) and the five profiles from the western Irish onshore (Cogné et al., 2014) that do not show cooling during the Early Cenozoic (blue stars on Fig. 8).

416

## 7. Discussion

418

### 7.1 Linking Early Cenozoic cooling to exhumation

We associate the pulse of enhanced Early Cenozoic cooling described above with a pronounced phase of exhumation that has been previously detected in the Irish Sea basin system (e.g., Rowley and White, 1998; Holford et al., 2005). Assuming a paleogeothermal gradient of 25°C/km, similar to the present-day
424 (Goodman et al., 2004, Downing and Gray, 1986), Early Cenozoic exhumation of ca.
425 2 km is indicated in the three samples from northeast Ireland, and sample Sct-2 in
426 the NW Southern Uplands (Fig. 8). Early Cenozoic exhumation is most significant in
427 sample Sct-1 (ca. 3 km). This sample is also the only surface sample (excluding the
428 Paleocene Isle of Arran Northern Granite) that yields an Early Cenozoic AFT age in
429 our dataset, indicating that the Early Cenozoic thermal event is more significant in the
430 southwest part of the Southern Uplands than elsewhere in the studied area. Such
431 young AFT ages are relatively uncommon for surface samples in Britain and Ireland,
432 and they tend to be clustered around the NE Irish Sea (e.g. the Vale of Eden, Lewis
433 et al., 1992; Lake District, Green, 2002, East Irish Sea Basin, Lewis et al., 1992;
434 Green et al., 1997). Early studies around the East Irish Sea Basin argued for
435 exhumation of up to 3 km (e.g. Lewis et al., 1992), similar to the exhumation signal
436 we infer for sample Sct-1. However others (e.g. Green et al., 1997) have
437 subsequently argued that the peak temperatures recorded by such samples resulted
438 from a high geothermal gradient. The subsequent cooling history would then only be
439 partly associated with Early Cenozoic exhumation, as there is also a cooling
440 component associated with thermal relaxation from an initial high Early Cenozoic
441 heat flow. Therefore the total amount of denudation is somewhat unclear, but 2.5 to 3
442 km is within the upper limits of previously published exhumation estimates for the
443 East Irish Sea Basin (Rowley and White, 1998). On the other side of the East Irish
444 Sea Basin, the exhumation inferred for the Isle of Man is lower, at ca. 1.5 km.
445
446 The amount of Early Cenozoic exhumation for the Paleocene Isle of Arran
447 Northern Granite is difficult to estimate from low-temperature thermochronology
448 alone, as it cooled rapidly from magmatic temperatures following emplacement.
449 However, maximum exhumation cannot exceed emplacement depth estimates of 2-3
449 km (England, 1992); our data indicate cooling and exhumation was complete by the
450 Early Eocene. Further south in Wales, the amount of Early Cenozoic exhumation is
451 estimated at 1.5 to 2 km based on the cooling histories of the Snowdonia profile and
452 the Anglesey and Pembrokeshire samples. Finally in southeast Ireland, Early
453 Cenozoic exhumation cannot exceed ca. 1-1.5 km, but it is harder to estimate as the
454 Early Cenozoic cooling history is not represented by a discrete cooling pulse.

455

456 7.2 Causal mechanisms for Early Cenozoic exhumation

457 Several authors have argued for Cenozoic exhumation caused by localised
458 inversion due to far-field tectonic stress transmission (e.g., Hillis et al., 2008; Holford
459 et al., 2005), but this mechanism is unlikely to be the primary cause for the Early
460 Cenozoic exhumation pattern we document here. The onshore pattern of Early
461 Cenozoic exhumation is not spatially associated with any major faults nor is it limited
462 to belts of known compressive tectonism (Fig. 8). This is not to say that a localised
463 structural control on exhumation is not present; for example, it is required to explain
464 the close spatial proximity (ca. 15 km) of the Late Paleocene Mourne Mountains
465 granite and the Early Paleocene Antrim basalts at similar elevations. However, uplift
466 and exhumation is clearly centred around the Irish Sea, with the maximum amount of
467 exhumation in the north sector (SW Scotland and northeast Ireland) and decreasing
468 towards the south. Moreover, even though Cenozoic crustal shortening is well
469 documented in the southern British Isles, only a subset of this shortening can be
470 attributed to Early Cenozoic movements (Welch and Turner, 2000; Blundell, 2002).
471 Total shortening is an order of magnitude too small to account for the observed
472 exhumation of between 1 to 3 km (Davis et al., 2012). Additionally, if exhumation was
473 controlled by far-field compression then partitioning of shortening onto discrete, brittle
crustal structures is expected, together with the formation of structurally-controlled sediment traps hosting Early Cenozoic strata in zones where Early Cenozoic exhumation was minimal to absent. However, the oldest known Cenozoic sedimentary rocks in the Irish Sea basin system comprise the minor Eocene succession in the St George's Channel Basin (Welch and Turner, 2000). Late Cretaceous and Early Paleogene sedimentary rocks are entirely absent in the region, which requires regional-scale exhumation of the entire Irish Sea region during the Early Cenozoic period.

Therefore, although structural inversion under compressive stress may have played a secondary role in the observed pattern of Early Cenozoic exhumation, our data demonstrate that this exhumation in and around the Irish Sea region is primarily due to epeirogenic causes. The emplacement of the British-Irish Paleogene Intrusive Province (BIPIP) has been linked to the thermal effects of the proto-Iceland Plume. The BIPIP ranges in age from ca. 63 to 58 Ma (Ganerød et al., 2010) and comprises a series of extrusive and intrusive centres that run from the Outer Hebrides in NW Scotland through the Irish Sea as far south as Lundy (Fig. 8). The Early Cenozoic exhumation pulse thus coincides temporally and spatially with this intense phase of magmatic activity that shortly predates the opening of the North Atlantic at ca. 55 Ma (Rickers et al., 2013). Furthermore, Persano et al. (2007) have argued for a similar episode of Early Cenozoic uplift and exhumation associated with the proto-Iceland plume in western Scotland, while regional-scale Paleocene uplift of northwest Britain and the resultant southeast-directed tilting is in accordance with the outcrop pattern of post-Triassic sedimentary rocks in Britain (Cope, 1994, Fig. 8) and with the provenance of Early Cenozoic sedimentary rocks in the Hampshire basin (Morton, 1982). Regional-scale uplift is also in accordance with stratigraphic evidence for large
499 clastic fan bodies during Paleocene and Eocene times on the margins of the uplifted
500 area (White and Lovell, 1997; Jones et al., 2002; Jones and White, 2003).

501 Brodie and White (1994) proposed that ca. 5 km of Paleocene magmatic
502 underplating occurred at the base of the crust underlying the Irish Sea region
503 contemporaneous with the emplacement of the BIPIP, which induced sufficient uplift
504 to create regional-scale Paleocene exhumation. Using a wide-angle seismic profile
505 that crossed the Irish Sea, Al-Kindi et al. (2003) imaged an 8 km-thick high velocity
506 zone at the base of the crust underneath the eastern Irish Sea, and which they too
507 interpreted as underplated magmatic material. Tomlinson et al. (2006) refined the
508 shape of this body using seismic receiver function data and their calculated thickness
509 variations are a good match with the spatial distribution of exhumation detected in
510 this study (Fig. 8), and Davis et al. (2012) also argued for the presence of such a
511 high-velocity body beneath Scotland and extending southwards. It is also possible
512 that a component of the observed exhumation pulse was associated with transient
513 and intermittent dynamic uplift induced by the presence of a hot vertical convective
514 sheet in the upper asthenospheric mantle – the presumed source of the underplated
515 material – which would explain the pulsed record of deep-water clastic fans in the
516 Early Cenozoic offshore basins around the British Isles (White and Lovell, 1997,
517 Jones et al., 2002). Following the emplacement of the BIPIP, a second phase of
518 transient uplift that encompassed a large part of the British Isles occurred at the
519 Paleocene / Eocene boundary, and is linked to the presence of a hot horizontal layer
520 beneath the lithosphere (Jones and White, 2003). Both processes could have caused
521 a significant portion of the Paleocene exhumation documented here. Although the
522 spatial distribution of Paleocene exhumation (centred along a north-south axis under
523 the Irish Sea) tends to favour the vertical convective sheet model, the temporal
sensitivity of our low-temperature thermochronological data also allows for a more
widespread (but more subtle, at least in Ireland) phase of transient uplift as proposed
by Jones and White (2003). In any case, both mechanisms are linked to plume-
related (i.e. epirogenic) regional-scale exhumation during the Early Cenozoic.

Following the Early Cenozoic exhumation phase, the thermal history models
indicate that some parts of onshore Ireland and Britain may have undergone a final
limited phase of cooling (< ca. 30°C) during the Neogene period. A similar-magnitude
pulse of reheating often predates this cooling episode. However, the thermal history
models are not sensitive at temperatures below ca. 40°C, as these temperatures are
cooler than the lower temperature limits of both the AHe and particularly the AFT
techniques. Nonetheless, Neogene uplift and exhumation of some parts of the British
Isles driven by plume-related processes is in agreement with the proposal that the
present-day topography of the British Isles is dynamically supported (e.g. Jones et
al., 2002; Davis et al., 2012; Rickers et al., 2013), and with the reported presence of
a "finger" of anomalously hot asthenosphere propagating from the Iceland plume
towards the Irish Sea region. The latter is visible in the upper asthenospheric mantle
on seismic tomographic images at depths of ca. 120 to 160 km (Arrowsmith et al.,
2005; Rickers et al., 2013).

542

8. Conclusions

544

The use of two low-temperature thermochronometers (the AFT and AHe
systems) combined with a vertical profile sampling approach and inverse and forward
modelling, allows us to define the cooling history of the British Isles, and especially
the Irish Sea region, with hitherto unparalleled resolution. From end Permian to
549 Jurassic times, exhumation was likely linked to the uplift and erosion of the hinterland
during the complex and long-lived rifting history of the region, with each profile
550
recording a cooling phase synchronous with the major phase of rifting in the nearest
551
offshore basin. It is possible that during the Early Cretaceous period, transient
552
dynamic uplift was responsible for exhumation detected in two of our profiles,
553
although more work is needed to confirm this hypothesis. An Early Cenozoic pulse of
554
exhumation affected the onshore margins of the Irish Sea basin system, and is more
555
significant in the north (reaching ca. 2.5-3 km), decreasing in intensity towards the
556
south and west. Combined with the available geophysical and sedimentological
557
evidence, we demonstrate that the main cause of this regional-scale uplift and
558
exhumation is linked to the effects of the proto-Iceland plume. A body of magmatic
559
material emplaced into the base of the lower crust and which broadly corresponds in
560
seismic studies to the present-day limits of the BIPIP is likely to be the main causal
561
mechanism. It is also possible that transient dynamic support was responsible for a
562
component of the observed Early Cenozoic exhumation, while local reactivation
563
along discrete structures can account for local variations in the observed exhumation
564
pattern in the region. Subsequently, the late Cenozoic thermal history evolution of the
565
British Isles remains under-constrained, but a second phase of Neogene plume-
566
linked exhumation is possible.
567

The combination of AFT and AHe dating with vertical profile modelling has
568
proven key in resolving the hitherto disparate cooling histories documented in the
569
Irish Sea region during the Early Cenozoic. Persano et al. (2007) detected Early
570
Cenozoic underplating-driven uplift and denudation in western Scotland using a
571
similar approach; in both regions single-sample modelling of AFT data alone would
572
fail to detect the Early Cenozoic exhumation pulse (cf Fig. 7). Further AHe
573
thermochronology from vertical profiles on onshore Britain would more precisely constrain the region of influence of the proto-Iceland plume during the Early Cenozoic.

Our data supports the hypothesis that the Early Cenozoic exhumation history of the British Isles is linked to the presence of the Iceland Plume. The spatial distribution of the BIPIP onshore follows the same axis as the distribution of exhumation from the Outer Hebrides to Lundy Island. It is therefore likely that the presence of abnormally hot vertical sheet of mantle centred below the Irish Sea explains the relatively short wavelength of early Paleocene exhumation inferred for the British Isles; seismic tomographic data suggest a similar situation may exist today (Rickers et al., 2013).

## Acknowledgments

NC and DC thank Science Foundation Ireland who part-funded this work under Grant Number 13/RC/2092 (iCRAG Research Centre, project HC4.2PD6a). iCRAG is funded under the SFI Research Centres Programme and is co-funded under the European Regional Development Fund. DD is funded under The Earth and Natural Sciences Doctoral Studies Program, which is funded under the Program for Research in Third-Level Institutions and co-funded under the European Regional Development Fund. We thank S. Jones and an anonymous reviewer for their comments that significantly improved the manuscript. We would also like to thank S. Lebedev and colleagues in the Dublin Institute of Advanced Studies for many useful discussions.

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747
748
749 **Figure Captions**
750

751 Table 1: Fission track results.

752 ¹Lithology: Grt = Granitoid intrusion; Sst = Sandstone. Age: Camb = Cambrian; LDev = Lower Devonian; Ord = Ordovician; Pal = Paleogene; PreC = Precambrian; UDev = Upper Devonian.

755 ²Number of counted spontaneous tracks.

756 ³Sum of the individual grain ²³⁸U/⁴³Ca ratios measured by ICPMS and weighted by the counted area.

758 ⁴Counted area.

759 ⁵Constraint used for the inverse modelling: DA = Deposition age; EA = Emplacement age; UPb = apatite U-Pb age (unpublished); VM = Variscan metamorphism

761
762 Table 2: (U-Th-Sm/He) results

763
764 Figure 1: Topographic map of the studied area showing the sample locations and the localities discussed in the text. Inset: Location of the studied area on the Northwest European margin.

767
768 Figure 2: Plot of F<sub>T</sub>-corrected (U-Th-Sm)/He vs AFT ages. The grey bars show the timing of emplacement of the British-Irish Tertiary Igneous Province.
769
Figure 3: Thermal history models from Wales. The black boxes are user-specified temperature-time constraints. (a) Inverse model for the Snowdonia profile; (b) Inverse and forward models for individual samples from Anglesey and Pembrokeshire; (c) Inverse model for the Brecon Beacons profile.

Figure 4: Thermal history models of pseudo-vertical profiles from SE Ireland. The black boxes are user-specified temperature-time constraints. (a) Galtee Mountains; (b) Mount Leinster; (c) Slieve Bloom and (d) Lugnaquilla.

Figure 5: Thermal history models from the Isle of Man and Isle of Arran. The black boxes are user-specified temperature-time constraints. (a) Inverse and forward modelling of the Isle of Man profile; (b) Isle of Arran profile.

Figure 6: Thermal history models for Southern Uplands and northeast Ireland. The black boxes are user-specified temperature-time constraints. (a) Individual samples Sct-1 and Sct-2; (b) Inverse modelling of individual samples Ire-8, Ire-9 and Ire-10.

Figure 7: Comparison of thermal history models for the Snowdonia profile using thermochronological data from individual versus multiple samples. (a) Model with AFT and AHe data from three samples modelled using the vertical profile approach (as presented in Fig. 3a); (b) Model with AFT data *only* from three samples modelled with the vertical profile approach; (c) Model with AFT and AHe data for sample Cu-3 *only*; (d) Model with AFT and AHe data for sample Cu-4 *only*. The model for Cu-2
794

only is similar to the two other two individual samples (Cu-3,-4) and is presented in
supplementary material with the predictions.

795

796

797

Figure 8: Map of Britain and Ireland showing the spatial distribution of Early Cenozoic exhumation phase inferred from our low-temperature thermochronological data and its spatial relationship with the postulated region of magmatic underplating. The main zones of Early Cenozoic deformation along with outcrops of post-Triassic sedimentary rocks and the British-Irish Paleogene Igneous Province are also illustrated.

798

799

800

801

802
<figure><img src="image_3.png" /></figure>
<figure><img src="image_4.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 3</figcaption></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /><figcaption>Figure 4</figcaption></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>
<figure><img src="image_12.png" /><figcaption>Figure 5</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Figure 6</figcaption></figure>

<figure><img src="image_14.png" /></figure>
<figure><img src="image_15.png" /><figcaption>Figure 7</figcaption></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>
<figure><img src="image_19.png" /></figure>

Early Cenozoic cooling detected by:

<figure><img src="image_20.png" /></figure>

Inverse modelling on individual sample

<figure><img src="image_21.png" /></figure>

Inverse modelling on vertical profile and
Forward modelling on individual sample

<figure><img src="image_22.png" /></figure>

Limited on inverse modelling of vertical profile

<figure><img src="image_23.png" /></figure>

Not detected on inverse modelling of vertical profile
(including profiles from Cogné et al., 2014)
<table><thead><tr><th>Sample Name</th><th>Latitude<br>(°N)</th><th>Longitude<br>(°W)</th><th>Elevation</th><th>Lithology and<br>Stratigraphic age¹</th><th>Ns²</th><th>238U/43Ca³</th><th>Area (cm²)⁴</th><th>AFT age (Ma)</th><th>± 2σ (Ma)</th><th># grains</th><th>P(χ²)</th><th>MTL (μm)</th><th>SE (μm)</th><th>SD (μm)</th><th># tracks</th><th>Dpar (μm)</th><th>SE (μm)</th><th>Tt constraint⁵</th></tr></thead><tbody><tr><td colspan="18"><strong>Northeast Ireland</strong></td></tr><tr><td>Ire 8</td><td>53.716</td><td>6.329</td><td>3</td><td>Grt (Low Dev)</td><td>1657</td><td>1.49E-04</td><td>8.61E-04</td><td>112.8</td><td>7.8</td><td>20</td><td>0.07</td><td>13.4</td><td>0.22</td><td>1.47</td><td>100</td><td>2.01</td><td>0.27</td><td>VM</td></tr><tr><td>Ire 9</td><td>53.953</td><td>7.441</td><td>120</td><td>Grt (Low Dev)</td><td>1018</td><td>5.74E-05</td><td>8.01E-04</td><td>173.4</td><td>14.4</td><td>22</td><td>0.83</td><td>13.4</td><td>0.17</td><td>1.09</td><td>101</td><td>1.78</td><td>0.19</td><td>VM</td></tr><tr><td>Ire 10</td><td>53.639</td><td>6.706</td><td>-1702</td><td>Grt (Low Dev)</td><td>358</td><td>7.07E-05</td><td>4.68E-04</td><td>50.0</td><td>5.8</td><td>29</td><td>0.76</td><td>13.9</td><td>0.41</td><td>1.34</td><td>17</td><td>1.89</td><td>0.44</td><td>VM</td></tr><tr><td colspan="18"><strong>Central/South Ireland</strong></td></tr><tr><td>Bg-1</td><td>52.063</td><td>8.695</td><td>162</td><td>Sdt (Up Dev)</td><td>826</td><td>4.65E-05</td><td>4.03E-04</td><td>188.9</td><td>15.4</td><td>30</td><td>0.11</td><td>13.0</td><td>0.25</td><td>1.43</td><td>100</td><td>1.64</td><td>0.18</td><td>VM</td></tr><tr><td colspan="18"><em>Galtee Mountains Profile</em></td></tr><tr><td>Ga-1</td><td>52.366</td><td>8.182</td><td>909</td><td>Sdt (Up Dev)</td><td>997</td><td>5.35E-05</td><td>3.25E-04</td><td>199.4</td><td>15.2</td><td>28</td><td>0.05</td><td>13.5</td><td>0.25</td><td>1.42</td><td>103</td><td>1.59</td><td>0.21</td><td>VM</td></tr><tr><td>Ga-2</td><td>52.358</td><td>8.187</td><td>731</td><td>Sdt (Up Dev)</td><td>966</td><td>5.25E-05</td><td>3.60E-04</td><td>182.5</td><td>15.2</td><td>29</td><td>0.09</td><td>13.3</td><td>0.25</td><td>1.56</td><td>102</td><td>1.60</td><td>0.17</td><td>VM</td></tr><tr><td>Ga-4</td><td>52.321</td><td>8.176</td><td>183</td><td>Sdt (Up Dev)</td><td>870</td><td>4.69E-05</td><td>3.48E-04</td><td>172.5</td><td>14.4</td><td>28</td><td>0.1</td><td>13.3</td><td>0.23</td><td>1.42</td><td>115</td><td>1.61</td><td>0.18</td><td>VM</td></tr><tr><td colspan="18"><em>Slieve Bloom Profile</em></td></tr><tr><td>Sb-1</td><td>53.092</td><td>7.557</td><td>437</td><td>Sdt (Up Dev)</td><td>676</td><td>3.47E-05</td><td>4.01E-04</td><td>189.6</td><td>17.0</td><td>28</td><td>0.12</td><td>13.6</td><td>0.22</td><td>1.31</td><td>100</td><td>1.68</td><td>0.19</td><td>VM</td></tr><tr><td>Sb-3</td><td>53.138</td><td>7.528</td><td>142</td><td>Sdt (Up Dev)</td><td>550</td><td>2.75E-05</td><td>4.31E-04</td><td>195.9</td><td>17.6</td><td>29</td><td>0.13</td><td>13.3</td><td>0.21</td><td>1.42</td><td>102</td><td>1.64</td><td>0.21</td><td>VM</td></tr><tr><td colspan="18"><strong>Southeast Ireland</strong></td></tr><tr><td>LG17</td><td>53.087</td><td>6.370</td><td>517</td><td>Grt (Low Dev)</td><td>2601</td><td>1.68E-04</td><td>1.07E-03</td><td>153.6</td><td>9.6</td><td>21</td><td>0.09</td><td>13.6</td><td>0.20</td><td>1.06</td><td>103</td><td>1.55</td><td>0.15</td><td>UPb</td></tr><tr><td>LG18</td><td>53.087</td><td>6.362</td><td>553</td><td>Grt (Low Dev)</td><td>3125</td><td>2.34E-04</td><td>1.22E-03</td><td>132.9</td><td>7.2</td><td>22</td><td>0.12</td><td>13.7</td><td>0.19</td><td>1.16</td><td>102</td><td>1.55</td><td>0.18</td><td>UPb</td></tr><tr><td>LG19</td><td>53.066</td><td>6.337</td><td>389</td><td>Grt (Low Dev)</td><td>2556</td><td>2.01E-04</td><td>1.24E-03</td><td>126.5</td><td>7.0</td><td>21</td><td>0.08</td><td>13.8</td><td>0.18</td><td>1.22</td><td>100</td><td>1.52</td><td>0.21</td><td>UPb</td></tr><tr><td>LG32</td><td>52.804</td><td>6.717</td><td>122</td><td>Grt (Low Dev)</td><td>459</td><td>3.12E-05</td><td>1.16E-03</td><td>145.9</td><td>15.0</td><td>20</td><td>0.10</td><td>13.0</td><td>0.20</td><td>1.19</td><td>100</td><td>1.53</td><td>0.22</td><td>UPb</td></tr><tr><td colspan="18"><em>Lugnaquilla Profile</em></td></tr><tr><td>Lu-1</td><td>52.970</td><td>6.460</td><td>901</td><td>Grt (Low Dev)</td><td>608</td><td>3.47E-05</td><td>3.29E-04</td><td>172.3</td><td>14.6</td><td>20</td><td>0.72</td><td>13.6</td><td>0.25</td><td>1.59</td><td>100</td><td>1.62</td><td>0.15</td><td>UPb</td></tr><tr><td>Lu-2</td><td>52.973</td><td>6.450</td><td>698</td><td>Grt (Low Dev)</td><td>636</td><td>3.53E-05</td><td>2.83E-04</td><td>175.6</td><td>14.6</td><td>20</td><td>0.39</td><td>13.2</td><td>0.26</td><td>1.8</td><td>100</td><td>1.60</td><td>0.15</td><td>UPb</td></tr><tr><td>Lu-3</td><td>52.979</td><td>6.443</td><td>502</td><td>Grt (Low Dev)</td><td>662</td><td>4.02E-05</td><td>3.00E-04</td><td>159.9</td><td>17.4</td><td>20</td><td>0.26</td><td>13.5</td><td>0.25</td><td>1.64</td><td>100</td><td>1.62</td><td>0.19</td><td>UPb</td></tr><tr><td>Lu-4</td><td>52.982</td><td>6.439</td><td>354</td><td>Grt (Low Dev)</td><td>887</td><td>6.10E-05</td><td>3.09E-04</td><td>141.1</td><td>13.0</td><td>20</td><td>0.12</td><td>13.5</td><td>0.25</td><td>1.63</td><td>100</td><td>1.57</td><td>0.18</td><td>UPb</td></tr><tr><td>Lu-5</td><td>52.994</td><td>6.426</td><td>231</td><td>Grt (Low Dev)</td><td>949</td><td>6.71E-05</td><td>2.80E-04</td><td>138.6</td><td>9.6</td><td>20</td><td>0.24</td><td>13.7</td><td>0.25</td><td>1.56</td><td>100</td><td>1.63</td><td>0.16</td><td>UPb</td></tr><tr><td>Lu-6</td><td>52.967</td><td>6.384</td><td>126</td><td>Grt (Low Dev)</td><td>533</td><td>3.65E-05</td><td>3.51E-04</td><td>143.1</td><td>12.8</td><td>20</td><td>0.99</td><td>13.6</td><td>0.24</td><td>1.47</td><td>100</td><td>1.63</td><td>0.16</td><td>UPb</td></tr><tr><td colspan="18"><em>Mount Leinster Profile</em></td></tr><tr><td>MI-1</td><td>52.618</td><td>6.780</td><td>795</td><td>Grt (Low Dev)</td><td>1168</td><td>5.94E-05</td><td>2.80E-04</td><td>191.9</td><td>12.2</td><td>25</td><td>0.48</td><td>13.2</td><td>0.27</td><td>1.46</td><td>100</td><td>1.63</td><td>0.18</td><td>UPb</td></tr><tr><td>MI-2</td><td>52.618</td><td>6.787</td><td>596</td><td>Grt (Low Dev)</td><td>1109</td><td>6.56E-05</td><td>3.01E-04</td><td>184.7</td><td>11.4</td><td>25</td><td>0.21</td><td>13.0</td><td>0.27</td><td>1.45</td><td>100</td><td>1.64</td><td>0.15</td><td>UPb</td></tr><tr><td>MI-3</td><td>52.611</td><td>6.799</td><td>421</td><td>Grt (Low Dev)</td><td>1217</td><td>6.60E-05</td><td>3.16E-04</td><td>180.1</td><td>11.2</td><td>24</td><td>0.68</td><td>13.1</td><td>0.24</td><td>1.27</td><td>100</td><td>1.62</td><td>0.16</td><td>UPb</td></tr><tr><td>MI-4</td><td>52.599</td><td>6.814</td><td>159</td><td>Grt (Low Dev)</td><td>1414</td><td>9.12E-05</td><td>3.06E-04</td><td>172.4</td><td>10.4</td><td>23</td><td>0.17</td><td>12.5</td><td>0.25</td><td>1.27</td><td>100</td><td>1.65</td><td>0.17</td><td>UPb</td></tr><tr><td colspan="18"><strong>North and West Wales</strong></td></tr><tr><td>An-1</td><td>53.232</td><td>4.488</td><td>29</td><td>Grt (Pre-Camb)</td><td>171</td><td>8.94E-06</td><td>3.78E-04</td><td>187.7</td><td>29.8</td><td>26</td><td>0.99</td><td>13.2</td><td>0.18</td><td>1.82</td><td>98</td><td>1.72</td><td>0.18</td><td>UPb</td></tr><tr><td>Pb-1</td><td>51.866</td><td>5.282</td><td>5</td><td>Grt (Ord)</td><td>102</td><td>4.39E-06</td><td>4.43E-04</td><td>220.8</td><td>52.0</td><td>24</td><td>0.37</td><td>-</td><td>-</td><td>-</td><td>-</td><td>1.65</td><td>0.20</td><td>UPb</td></tr><tr><td>Pb-2</td><td>51.870</td><td>5.283</td><td>10</td><td>Sdt (Camb)</td><td>115</td><td>4.85E-06</td><td>3.27E-04</td><td>230.6</td><td>48.0</td><td>23</td><td>0.76</td><td>-</td><td>-</td><td>-</td><td>-</td><td>2.10</td><td>0.23</td><td>UPb</td></tr><tr><td colspan="18"><em>Snowdonia Profile</em></td></tr><tr><td>Cu-3</td><td>53.187</td><td>3.975</td><td>869</td><td>Grt (Ord)</td><td>379</td><td>1.33E-05</td><td>4.91E-04</td><td>280.5</td><td>30.0</td><td>29</td><td>0.53</td><td>13.4</td><td>0.22</td><td>1.35</td><td>100</td><td>1.72</td><td>0.19</td><td>UPb</td></tr><tr><td>Cu-2</td><td>53.191</td><td>4.000</td><td>701</td><td>Grt (Ord)</td><td>387</td><td>1.49E-05</td><td>6.15E-04</td><td>252.3</td><td>26.0</td><td>28</td><td>0.66</td><td>13.5</td><td>0.21</td><td>1.4</td><td>101</td><td>1.79</td><td>0.21</td><td>UPb</td></tr><tr><td>Cu-4</td><td>53.209</td><td>3.994</td><td>327</td><td>Grt (Ord)</td><td>369</td><td>1.43E-05</td><td>5.81E-04</td><td>250.7</td><td>28.0</td><td>28</td><td>0.61</td><td>13.2</td><td>0.23</td><td>1.4</td><td>100</td><td>1.70</td><td>0.19</td><td>UPb</td></tr><tr><td colspan="18"><strong>South Wales</strong></td></tr><tr><td colspan="18"><em>Brecon Beacons Profile</em></td></tr><tr><td>Br-1</td><td>51.882</td><td>3.708</td><td>801</td><td>Sdt (Up Dev)</td><td>736</td><td>3.56E-05</td><td>4.87E-04</td><td>205.8</td><td>16.2</td><td>31</td><td>0.69</td><td>13.9</td><td>0.22</td><td>1.39</td><td>100</td><td>1.85</td><td>0.27</td><td>DA</td></tr><tr><td>Br-2</td><td>51.853</td><td>3.687</td><td>505</td><td>Sdt (Up Dev)</td><td>779</td><td>3.62E-05</td><td>7.49E-04</td><td>213.5</td><td>16.6</td><td>29</td><td>0.87</td><td>13.7</td><td>0.20</td><td>1.27</td><td>102</td><td>1.73</td><td>0.23</td><td>DA</td></tr><tr><td>Br-3</td><td>51.837</td><td>3.682</td><td>251</td><td>Sdt (Up Dev)</td><td>838</td><td>4.28E-05</td><td>7.79E-04</td><td>196.9</td><td>16.0</td><td>38</td><td>0.13</td><td>13.7</td><td>0.21</td><td>1.43</td><td>100</td><td>1.83</td><td>0.23</td><td>DA</td></tr><tr><td colspan="18"><strong>West Britain</strong></td></tr><tr><td colspan="18"><em>Isle of Man</em></td></tr><tr><td>IoM 1</td><td>54.255</td><td>4.378</td><td>239</td><td>Grt (Dev)</td><td>800</td><td>6.25E-05</td><td>5.65E-04</td><td>127.6</td><td>11.0</td><td>20</td><td>0.01</td><td>12.6</td><td>0.28</td><td>1.52</td><td>101</td><td>1.54</td><td>0.24</td><td>UPb</td></tr><tr><td>IoM 2</td><td>54.256</td><td>4.365</td><td>162</td><td>Grt (Dev)</td><td>765</td><td>6.85E-05</td><td>6.58E-04</td><td>111.2</td><td>10.0</td><td>21</td><td>0.02</td><td>13.2</td><td>0.24</td><td>1.54</td><td>103</td><td>1.67</td><td>0.22</td><td>UPb</td></tr><tr><td>IoM 3</td><td>54.162</td><td>4.619</td><td>174</td><td>Grt (Dev)</td><td>376</td><td>4.33E-05</td><td>9.42E-04</td><td>85.5</td><td>9.4</td><td>20</td><td>0.31</td><td>13.7</td><td>0.21</td><td>1.38</td><td>100</td><td>1.48</td><td>0.21</td><td>UPb</td></tr><tr><td colspan="18"><em>Southern Upland</em></td></tr><tr><td>Sct 1</td><td>54.940</td><td>3.630</td><td>559</td><td>Grt (Low Dev)</td><td>1303</td><td>2.19E-04</td><td>5.67E-04</td><td>60.4</td><td>4.0</td><td>20</td><td>0.16</td><td>13.8</td><td>0.17</td><td>1.19</td><td>103</td><td>1.63</td><td>0.24</td><td>UPb</td></tr><tr><td>Sct 2</td><td>55.588</td><td>4.224</td><td>240</td><td>Grt (Low Dev)</td><td>622</td><td>2.02E-05</td><td>3.96E-04</td><td>299.2</td><td>27.6</td><td>21</td><td>0.27</td><td>12.7</td><td>0.23</td><td>1.43</td><td>104</td><td>2.03</td><td>0.38</td><td>UPb</td></tr><tr><td colspan="18"><em>Isle of Arran Profile</em></td></tr><tr><td>IoA 1</td><td>55.626</td><td>5.192</td><td>852</td><td>Grt (Pal)</td><td>49</td><td>7.12E-06</td><td>3.82E-04</td><td>70.6</td><td>18.8</td><td>18</td><td>0.99</td><td>-</td><td>-</td><td>-</td><td>-</td><td>1.35</td><td>0.26</td><td>EA</td></tr><tr><td>IoA 2</td><td>55.626</td><td>5.188</td><td>746</td><td>Grt (Pal)</td><td>118</td><td>1.79E-05</td><td>6.33E-04</td><td>61.1</td><td>13.4</td><td>19</td><td>0.11</td><td>-</td><td>-</td><td>-</td><td>-</td><td>1.37</td><td>0.21</td><td>EA</td></tr><tr><td>IoA 3</td><td>55.623</td><td>5.177</td><td>535</td><td>Grt (Pal)</td><td>59</td><td>1.37E-05</td><td>1.76E-04</td><td>43.3</td><td>15.6</td><td>7</td><td>0.93</td><td>-</td><td>-</td><td>-</td><td>-</td><td>1.36</td><td>0.15</td><td>EA</td></tr><tr><td>IoA 4</td><td>55.620</td><td>5.177</td><td>468</td><td>Grt (Pal)</td><td>106</td><td>1.67E-05</td><td>3.34E-04</td><td>55.3</td><td>15.0</td><td>15</td><td>0.99</td><td>-</td><td>-</td><td>-</td><td>-</td><td>1.44</td><td>0.19</td><td>EA</td></tr><tr><td>IoA 5</td><td>55.617</td><td>5.178</td><td>385</td><td>Grt (Pal)</td><td>54</td><td>8.04E-06</td><td>2.15E-04</td><td>59.1</td><td>18.8</td><td>10</td><td>0.97</td><td>-</td><td>-</td><td>-</td><td>-</td><td>1.38</td><td>0.25</td><td>EA</td></tr><tr><td>IoA 6</td><td>55.673</td><td>5.206</td><td>93</td><td>Grt (Pal)</td><td>112</td><td>1.70E-05</td><td>3.40E-04</td><td>64.9</td><td>16.2</td><td>13</td><td>0.99</td><td>-</td><td>-</td><td>-</td><td>-</td><td>1.41</td><td>0.23</td><td>EA</td></tr></tbody></table>