Holford et al. Post-Caledonian exhumation of northwest Scotland
1 Multiple post-Caledonian exhumation episodes across northwest Scotland revealed by
2 apatite fission track analysis
3
4 Simon P. Holford1, Paul F. Green2, Richard R. Hillis1, John R. Underhill3, Martyn S. Stoker4
5 & Ian R. Duddy2
6
7 1Australian School of Petroleum, Centre for Tectonics, Resources and Exploration (TRaX), University of Adelaide, SA 5005, Australia
9 2Geotrack International Pty Ltd, 37 Melville Road, Brunswick West, Victoria 3055, Australia
10 3School of Geosciences, The University of Edinburgh, Grant Institute of Earth Science, The King’s Buildings, West Mains Road, Edinburgh
11 EH9 3JW, United Kingdom
12 4British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, United Kingdom
13
14 Abstract
15 The post Caledonian exhumation history of northwest Scotland is a controversial issue, with
16 some studies advocating largely continual emergence while others suggest dominantly early
17 Palaeogene plume-driven exhumation. AFTA data in samples of Precambrian basement and
18 Permian-Cretaceous sediments from onshore and onshore reveal multiple phases of post-
19 Caledonian cooling, viz: Triassic (beginning 245-225 Ma), Cretaceous (140-130 Ma; 110-90
20 Ma) and Cenozoic (65-60 Ma; 40-25 Ma; 15-10 Ma), all of which are interpreted at least in
21 part as recording exhumation. Basement and sedimentary cover rocks display similar thermal
22 histories, emphasising the regional nature of these episodes and implying that sedimentary
23 outliers represent the remnants of previously more extensive sequences. Significant
24 thicknesses of Jurassic rocks may once have covered northwest Scotland. Palaeocene
25 palaeothermal effects are most pronounced in the vicinity of igneous centres, probably
26 reflecting combined effects of heating by elevated heat flow, deeper burial and hydrothermal
27 activity. Most of the region underwent km-scale Neogene exhumation. Contrary to the
1 Holford et al. Post-Caledonian exhumation of northwest Scotland
28 common assumption of monotonic cooling and denudation histories, integration of geological
29 evidence with AFTA data defines an episodic thermal history involving repeated cycles of
30 burial and exhumation. We suggest that onshore passive margins and continental interiors
31 may also best be characterised by similar histories.
32
33
34
35
36 Abstract
37 The origin and development of the high topography bordering the Atlantic margin of NW
38 Europe has long been the subject of debate (Geike, 1901; George, 1966; Doré et al., 2002).
39 The mountains of Scotland and western Scandinavia are characterized by rugged topography
40 and elevations that exceed 1.3 km and 2.4 km respectively and largely comprise rocks that
41 were formed and/or deformed during the Caledonian Orogeny. Numerous lines of evidence
42 indicate that the present-day topography of both regions was initiated by tectonic uplift that
43 began during the Cenozoic or late Mesozoic and which also affected peripheral sedimentary
44 basins (Doré et al., 2002). These include; the transition from deposition of shallow-marine
45 carbonates in the late Cretaceous to rejuvenated clastic sedimentation in the early Palaeogene
46 (Doré et al., 2002)); the easterly tilt of Britain (Brodie & White, 1994); identification of
47 Palaeocene, Eocene and Pliocene-Pleistocene prograding and locally heavily incised shelf-
48 slope wedges of clastic sediments offshore Scotland and Norway (Stoker et al., 1993, 2010;
49 Underhill, 2001); apatite fission track, vitrinite reflectance and sonic velocity data which
50 record deeper burial of Cenozoic-Mesozoic sediments and Palaeozoic and older basement
51 rocks along the Atlantic margin prior to their uplift and erosion (Japsen et al., 2007; Hillis et
52 al., 2008); and, geomorphological evidence for elevation of low-relief erosion surfaces and
2 Holford et al. Post-Caledonian exhumation of northwest Scotland
53 landscape elements formed at or near sea level during the Mesozoic and Palaeogene and
54 subsequently uplifted (Japsen et al., 2002).
55
56 Despite this evidence for tectonically driven uplift of the Atlantic margin during the late
57 Mesozoic and Cenozoic, it has been recently proposed (Nielsen et al., 2009) that the present-
58 day topography of western Scandinavia and the Scottish Highlands represents remnant
59 topography from the Caledonian Orogeny that survived post-orogenic extensional collapse
60 and erosion. Nielsen et al. (2009) suggest that many of the observations listed above that have
61 been used to argue for Cenozoic tectonic uplift can instead be interpreted in terms of
62 protracted exhumation maintained by climatically-controlled erosion rates. This hypothesis
63 implies that the Scottish Highlands and western Scandinavia have been positive topographic
64 features for the past ~400 million years, experiencing no significant burial and receiving little
65 or no Upper Palaeozoic-Cenozoic sediments (cf. Macdonald et al., 2007).
66
67 Recent apatite fission track and (U-Th)/He data from basement samples in the Scottish
68 Highlands have been interpreted in terms of thermal histories dominated by continuous,
69 monotonic cooling and exhumation over the past ~400 Myr (Persano et al., 2007) and thus
70 support the idea of continuous, post-Caledonian emergence (cf. Watson, 1985). Such
71 monotonic cooling of basement rocks is inconsistent with the local geological record
72 however, because outliers of Devonian, Carboniferous, Permian-Triassic, Jurassic and
73 Cretaceous sediments overlying Neoproterozoic basement (e.g. Moine Supergroup,
74 Torridonian) are found along the coast of northwest Scotland (Fig. 1). In addition, thick
75 sequences of mainly Triassic-Jurassic age sediments are preserved in offshore basins of the
76 Malin-Hebrides sea area (Fyfe et al., 1993), whilst the overstep of Old Red Sandstone onto
3 Holford et al. Post-Caledonian exhumation of northwest Scotland
77 late Caledonian granites indicates that Caledonian topography had largely been worn down
78 by rapid uplift and erosion during the early Devonian (Hall, 1991).
79
80 Conversely, other workers have claimed that Scotland has experienced post-Caledonian
81 tectonically-driven uplift that was dominantly early Palaeogene in age and driven by
82 processes associated with the Iceland mantle plume, such as igneous underplating or
83 convectively-supported dynamic uplift (White & Lovell, 1997; Jones et al., 2002; Shaw
84 Champion et al., 2008). However, recent fission-track studies from the southern British Isles
85 have revealed a more complex Mesozoic-Cenozoic uplift history, with major phases of km-
86 scale uplift and exhumation occurring during the early Cretaceous and Neogene in addition to
87 the early Palaeogene (Holford et al., 2005; Hillis et al., 2008; Holford et al., 2009a, b, c).
88
89 The aim of this study is to test these different models for the post-Caledonian tectonic
90 evolution of northwest Scotland using a new apatite fission-track analysis (AFTA) database
91 comprising onshore and offshore samples from basement (i.e. Lower Palaeozoic and older)
92 and from the Permian-Cretaceous sediments deposited in overlying sedimentary basins. The
93 offshore database includes samples collected over a range of depths from exploration wells
94 located in the Sea of the Hebrides (134/5-1) and West Orkney (202/19-1) basins that provide
95 direct constraints on palaeogeothermal gradients and allow estimates of amounts of missing
96 section removed during exhumation. Thermal history solutions extracted from our data show
97 consistency between samples from basement and cover sequences and show that the
98 Mesozoic sediments deposited around northwest Scotland have been heated to temperatures
99 exceeding 100°C during burial by additional sedimentary sequences. The preserved
100 sediments are thus interpreted as remnants of deposits that were originally far more extensive,
101 raising the possibility that the Highlands were buried beneath a Mesozoic sedimentary cover.
4 Holford et al. Post-Caledonian exhumation of northwest Scotland
102 The additional sedimentary sequences were removed during a series of Cretaceous-Cenozoic
103 exhumation episodes, whose timing is constrained by both AFTA data and unconformities
104 that separate the sedimentary units. Our results indicate that interpretations involving
105 continual post-Caledonian emergence cannot be sustained, and that the post-400 Ma history
106 of northwest Scotland has been characterised by repeated cycles of sedimentation and
107 exhumation, suggesting that post-Caledonian events are largely responsible for the
108 physiography of northwest Scotland.
109
110 Post-Caledonian geological history of northwest Scotland
111 The Caledonian orogeny in northwest Scotland culminated during the Lower Devonian (Fyfe
112 et al., 1993). The onset of extensional tectonics in the Middle Devonian facilitated the
113 collapse of the Caledonian mountain belt (Stoker et al., 1993). Thick sequences of Old Red
114 Sandstone (ORS) continental sediments and lavas occur within the Midland Valley, southeast
115 of the Highland Boundary Fault (Fyfe et al., 1993) and large areas of Devonian sedimentary
116 rocks in northeast Scotland are remnants of the originally more extensive Orcadian Basin
117 (Hillier and Marshall, 1992) (Fig. 1). Limited ORS siliciclastic and volcanic outcrops of
118 Lower Devonian age around Oban and in the Firth of Lorne are interpreted to be the
119 preserved remnants of a formerly more extensive ORS cover (Fyfe et al., 1993).
120 Carboniferous rocks are of limited exposure in the Scottish Highlands with the exception of
121 ~100 m of Carboniferous sandstone preserved on Morvern (Fyfe et al., 1993), although
122 considerably thicker sequences (up to 3 km) of marine and nonmarine Carboniferous
123 sediments are preserved within the Midland Valley Basin (Underhill et al., 2008).
124
125 Permian-Triassic continental redbeds are widely distributed in the basins offshore northwest
126 Scotland (Fig. 1), reaching thicknesses of ~3 km in the North Minch Basin (Fyfe et al., 1993).
5 Holford et al. Post-Caledonian exhumation of northwest Scotland
127 Upper Triassic sediments are exposed on Skye, Ardnamurchan, Mull, Morvern, Applecross,
128 Gruinard Bay and on northeast Lewis where ~1.2 km of Permian-Triassic sandstones and
129 conglomerates rest with unconformable or faulted contact against Lewisian gneisses (Fyfe et
130 al., 1993). These basins also contain thick sequences of marine and nonmarine Jurassic
131 deposits, and there are onshore outcrops along the northwestern coast of Scotland and on
132 Skye and Raasay where the Jurassic succession reaches 450 m (Fyfe et al., 1993). Based on
133 detailed subsidence analyses of preserved Triassic-Jurassic rocks in the Inner Hebrides,
134 Morton (1987) proposed that the initial phase of crustal stretching responsible for the
135 Hebridean basin system commenced during the late Triassic (~210 Ma).
136
137 Cretaceous rocks are less widely distributed (Fig. 1), but this is considered to be due
138 Cenozoic uplift and erosion rather than non-deposition (Hallam, 1983). Upper Cretaceous
139 sediments are preserved on Morvern, including glauconitic and pure silica sandstones
140 overlain by silicified Chalk (Fyfe et al., 1993). The Cretaceous rocks on Morvern rest
141 unconformably on Triassic-Jurassic strata, implying an intervening period of erosion,
142 probably during the early Cretaceous (Hallam, 1983). There are minor outcrops of Upper
143 Cretaceous strata on Skye, Mull, Eigg and Arran but no rocks of this age have been proven
144 immediately offshore (Fyfe et al., 1993).
145
146 Widespread volcanicity accompanied continental breakup between Greenland and northwest
147 Europe in the late Palaeocene-early Eocene along the North Atlantic margins (Emeleus and
148 Bell, 2005). Most volcanicity in the British Isles was concentrated in northwest Scotland,
149 with major igneous centres at Mull, Ardnamurchan, Rum and Skye (Emeleus and Bell, 2005).
150 This igneous activity has been attributed to the proto-Iceland mantle plume, and petrological
151 data suggest that a significant proportion (up to 70%) of the melt generated around the British
6 Holford et al. Post-Caledonian exhumation of northwest Scotland
152 Isles was trapped at depth, leading to underplating of the lower crust, which in turn would
153 have resulted in surface uplift (Jones et al., 2002). However, the depth to the seismic
154 reflection Moho beneath northwest Scotland is low compared to southeast Scotland (varying
155 from 25 to 29 km beneath basins and basement) (Chadwick and Pharaoh, 1998) and is
156 inconsistent with the crust having been significantly thickened by igneous material.
157
158 Climatic deterioration during the Quaternary profoundly affected the landscape of the
159 Scottish Highlands, with decreasing temperatures permitting the growth of glaciers on
160 uplifted terrain which in turn promoted widespread erosion and deposition of glacial
161 glaciomarine sediments offshore northwest Scotland (Stoker et al., 1993).
162
163 AFTA methodology and database
164 The methodological and analytical aspects of AFTA are well established (e.g. Green et al.,
165 2002). AFTA utilizes the radiation damage trails (fission tracks) created by spontaneous
166 fission of 238U within the crystal lattice of apatite, which is a common detrital mineral in most
167 sandstone and occurs as an accessory phase in plutonic and high grade metamorphic rocks.
168 By counting the number tracks in a polished and etched grain surface and measuring the
169 uranium content, a fission track age can be calculated, which in the absence of other factors
170 would provide a direct measure of the time over which tracks have accumulated. Fission
171 track lengths form within a narrow range (~16 μm), but are progressively shortened as
172 radiation damage is repaired at a rate that increases with temperature (annealing). Shortening
173 leads to a reduction in the number of tracks that can intersect a polished surface, and above
174 110°C all damage is repaired and no tracks are preserved. A measured fission track age thus
175 represents a balance between production of tracks and loss by annealing, and must be
176 assessed in tandem with track length data.
7 Holford et al. Post-Caledonian exhumation of northwest Scotland
177 We present AFTA data from 78 samples collected from onshore and offshore northwest
178 Scotland (Fig. 1, Table 1). Our dataset includes 42 samples from onshore outcrops, 21
179 samples from British Geological Survey (BGS) offshore shallow boreholes and drill cores,
180 and 15 samples from two wells (202/19-1 in the West Orkney Basin (WOB) and 134/5-1 in
181 the Sea of Hebrides Basin (SOHB)). The samples cover a broad range of stratigraphic levels,
182 from Proterozoic (Torridonian and Moine) and Archean (Lewisian) basement to Phanerozoic
183 sedimentary cover rocks of Permian, Triassic, Jurassic and Cretaceous age. Data quality is
184 generally excellent, with the majority of samples yielding 20 or more single grain ages and
185 many samples yielding 100 track lengths (Table 1).
186
187 The majority of measured fission track ages are younger than the respective stratigraphic ages
188 (Table 1). Overall there is a consistent trend between fission track age and mean track length,
189 with most fission track ages in the range of 150-300 Ma regardless of the depositional or
190 metamorphic age of the host rock, and mean track lengths (MTLs) in the range 11-13 µm
191 (Fig. 2). Samples from Skye yield much younger fission track ages between 50 and 70 Ma,
192 with mean track lengths around ~13-14 µm. The main trend in Fig. 2 is very different from
193 the simple “boomerang trend” shown by samples that have undergone a single heating event
194 in which different samples have undergone varying degrees of resetting (Green, 1986), and
195 suggests that the region has undergone a relatively complex history involving a series of
196 resetting events.
197
198 With the exception of one sample (GC503-151), the fission track ages of all samples from
199 northwest Scotland are younger than 400 Ma and the overwhelming majority are younger
200 than 300 Ma i.e. at least 100 Myr younger than the latest stages of Caledonian Orogeny. This
201 immediately indicates that almost all basement and early Palaeozoic samples reached
8 Holford et al. Post-Caledonian exhumation of northwest Scotland
202 palaeotemperatures sufficient to produce severe age reduction (i.e. ~100°C or above) during
203 post-Caledonian events. Moreover, Proterozoic and Achaean basement samples tend to yield
204 similar fission track ages and length distributions to nearby Phanerozoic cover samples
205 (Table 1 and Fig. 2), suggesting that basement and cover sequences in northwest Scotland
206 have undergone similar thermal histories.
207
208 Thermal history interpretation of AFTA data
209 Basic principles
210 Extraction of thermal history information from AFTA data begins with construction of a
211 Default Thermal History (DTH), derived from the burial history defined by the preserved
212 sedimentary section and the present-day thermal gradient. For a sedimentary rock, this is the
213 history which would apply if the sample has never been any hotter than the present-day
214 temperature at any time since deposition. For basement samples, a similar approach can be
215 adopted using the age of the oldest overlying sedimentary units, which in this study we have
216 assumed to be Devonian if no sedimentary units are present. If the DTH can explain the
217 AFTA data, then it is not possible to extract further thermal history information. If the AFTA
218 data show a greater degree of annealing (i.e. fission track age and/or track length reduction)
219 than expected from the DTH, then the sample must have been hotter in the past and
220 information on the magnitude and timing of heating and cooling events can be extracted from
221 the data. All samples analysed in this study show a greater degree of annealing (i.e. fission
222 track age and/or length reduction) than that expected from the DTH , and have therefore been
223 much hotter in the past.
224
225 To extract thermal history information from AFTA data, we use a kinetic model that makes
226 full quantitative allowance for the effects of Cl content on fission-track annealing rates
9 Holford et al. Post-Caledonian exhumation of northwest Scotland
227 (Green et al., 2002). Because maximum palaeotemperatures are the key factor that dominates
228 AFTA data, this technique reveals little information on the thermal history prior to the onset
229 of cooling. Therefore, we do not attempt to constrain the entire thermal history of each
230 sample but focus on the key aspects of the thermal history that control the fission track age
231 and length distribution i.e. the maximum palaeotemperature of each sample, and the time at
232 which cooling from that palaeotemperature began.
233
234 By modelling expected AFTA parameters resulting from a range of possible thermal
235 histories, we use maximum likelihood theory similar to that described by Gallagher (1995) to
236 define the range of values of maximum palaeotemperature and the onset of cooling giving
237 predictions which match the measured data within 95% confidence limits. Palaeotemperature
238 estimates derived using this approach usually have an absolute uncertainty of better than
239 ±10°C. Many of the samples analysed in this study require at least two episodes of heating
240 and cooling to explain all facets of the data. AFTA can provide constraints on two episodes
241 provided the magnitude and timing of the palaeothermal maximum and the subsequent
242 (lower) peak palaeotemperature are sufficiently separated. In some cases (e.g. Turner et al.,
243 2008) it is possible to resolve three discrete episodes in data from a single sample. This is
244 most likely when the first episode involves a maximum palaeotemperature sufficient to
245 totally anneal all tracks (typically >110°C), followed by a subsequent peak around 90- 100°C
246 which leads to shortening of tracks formed after the initial cooling to a mean length of ~10
247 μm. Finally, cooling to a low temperature is followed by reheating to ~70°C, sufficient to
248 reduce track lengths formed after the second episode to ~12-13 μm.
249
250 Table 2 summarises estimates of peak palaeotemperatures reached by each sample and the
251 time at which cooling from these palaeotemperatures began.
10 Holford et al. Post-Caledonian exhumation of northwest Scotland
252 West Orkney Basin
253 Of six Permian-Lower Jurassic samples from offshore shallow boreholes in the WOB, most
254 require two discrete episodes of heating and cooling (Table 2); an early episode involving
255 cooling from palaeotemperatures typically >90°C beginning at some time during the mid-
256 Jurassic to late Cretaceous (180 to 85 Ma based on combined results from all samples; Fig.
257 3). These palaeotemperatures imply significant depths of burial for any reasonable
258 palaeogeothermal gradient. This was followed by reheating and a second cooling episode
259 (from palaeotemperatures typically between 50- 70°C; Table 2) that commenced during the
260 mid-late Cenozoic (between 30 and 15 Ma; Fig. 3). These results are interpreted in terms of
261 burial of the preserved WOB succession by late Triassic-Jurassic sediments that were
262 removed by exhumation that occurred in two stages (mid-Jurassic to late Cretaceous, and
263 mid-late Cenozoic)
264
265 Exploration well 202/19-1
266 WOB well 202/19-1 was drilled in the hangingwall of the Solan Bank High Fault (Evans,
267 1997) and encountered ~3 km of Upper Permian-Lower Triassic sediments beneath a thin
268 veneer of Quaternary deposits (Stoker et al, 1993). AFTA results indicate that all nine
269 samples reached temperatures of ~100°C or above prior to cooling that began during the late
270 Jurassic to late Cretaceous (Table 2), similar to results from WOB boreholes. The consistent
271 timing of cooling based on overlap for all samples is 110 to 90 Ma (Fig. 3). A later episode of
272 cooling beginning during the late Cretaceous-Cenozoic is also required for most samples,
273 with the consistent overlap suggesting an onset beginning during the mid-late Cenozoic (55
274 to 10 Ma).
275
11 Holford et al. Post-Caledonian exhumation of northwest Scotland
276 The variation of palaeotemperature with depth for the two separate episodes (Fig. 4b)
277 suggests that both can be satisfied by linear palaeotemperature profiles, with gradients close
278 to the present-day gradient of ~25.3°C km-1. This indicates that these palaeotemperatures
279 record heating caused by deeper burial and subsequent cooling caused by exhumation (cf.
280 Green et al., 2002). Assuming a palaeosurface temperature of 5°C and palaeogeothermal
281 gradients equivalent to the present-day gradient, early Cretaceous palaeotemperatures are
282 consistent with deeper burial by 3.3-3.9 km of additional Triassic-Lower Cretaceous section,
283 whilst the mid-late Cenozoic palaeotemperatures are consistent with an additional 1.2-1.7 km
284 of section (Fig. 4c). Higher surface temperatures would require correspondingly lower
285 amounts of additional section, while higher palaeogeothermal gradients (within the ranges
286 allowed by the constraints on each episode) would require lower amounts of additional
287 section.
288
289 Evans (1997) estimated the amount of deeper burial of the Permian-Triassic section at
290 202/19-1 by comparing Triassic shale sonic velocities against published velocity-depth
291 relationships for normally compacted shales. By comparison with the burial curves of Marie
292 (1975) and Magara (1976), Evans (1997) estimated gross exhumation of the Permian-Triassic
293 section at 1.85 km. These velocity-depth relationships overestimate the increase of velocity
294 with depth in comparison to more recent trends for Triassic shales from the British Isles (e.g.
295 Japsen 2000). The value reported by Evans (1997) may thus underestimate the amount of
296 gross exhumation by several hundred metres. The amount of exhumation indicated by sonic
297 data is lower than that indicated by early Cretaceous AFTA palaeotemperatures assuming a
298 geothermal gradient of 25.3°C km-1 (i.e. 3.3-3.9 km) but is reasonably close to the maximum
299 likelihood value of additional section (2.31 km) (Fig. 4c). The slightly lower values of
12 Holford et al. Post-Caledonian exhumation of northwest Scotland
300 additional section indicated by the sonic velocity data could also indicate elevated basal heat
301 flow during the early Cretaceous.
302
303 Based on AFTA and sonic data from 202/19-1 and AFTA data from shallow boreholes we
304 suggest that up to 2-3 km of Upper Triassic-Lower Cretaceous sediments were deposited in
305 parts of the WOB prior to early Cretaceous (110-90 Ma) exhumation. Although some
306 samples are from footwall blocks, many others (including those from 202/19-1) are from
307 hangingwalls which implies a regional cause for exhumation as opposed to localized
308 exhumation of footwalls during Cretaceous normal faulting and extension.
309
310 Hebridean region
311 We analysed 22 samples from the North Minch Basin (NMB), Outer Hebrides and outcrops
312 along the northwest coast of Scotland. Most require two palaeothermal episodes, and thermal
313 history solutions for Permian and younger samples are generally similar to those resolved for
314 basement samples (Table 2). When combined, results from this region reveal up to four
315 separate cooling episodes during the Mesozoic-Cenozoic (Fig. 3).
316
317 Several of the Archean and Proterozoic samples, notably those in the footwalls of the NMB
318 from Lewis and Harris and the northwest coast of Scotland, require Permian-Triassic cooling
319 from temperatures generally >100°C (Table 2). An onset of cooling beginning between 225
320 and 190 Ma satisfies all samples that show evidence for this episode (Fig. 3).
321
322 Most samples from both basement and cover rocks in the Hebridean region show evidence
323 for cooling from palaeotemperatures of up to ~105°C during the Jurassic to mid-Cretaceous
324 (Table 2). Precise constraints on the timing of this episode are often broad in individual
13 Holford et al. Post-Caledonian exhumation of northwest Scotland
325 samples suggesting that Jurassic-mid Cretaceous cooling could result from several unresolved
326 events, but Permian-Lower Cretaceous sedimentary samples from shallow boreholes in the
327 Hebrides Shelf (GC523-47 to 52) define consistent evidence for cooling from ~90-95°C
328 beginning during the early Cretaceous (140 to 130 Ma) (Fig. 3).
329
330 Likewise, most samples from the Hebridean region reveal evidence for elevated
331 palaeotemperatures during the late Cretaceous-Cenozoic. Timing constraints are quite broad
332 for some samples, which is again suggestive of several distinct phases of cooling that are
333 closely spaced in temperature and time. It is possible to identify discrete late Cretaceous-mid
334 Cenozoic and late Cenozoic cooling episodes in data from Permian-Triassic sample GC523-
335 11 from NMB borehole 71/11. AFTA suggests that this sample cooled from 95-100°C
336 between 75 and 30 Ma, with further cooling from 55-80°C between 15 Ma and the present-
337 day. In Table 2 we have assigned late Cretaceous-Cenozoic cooling observed in individual
338 episodes to discrete late Cretaceous-mid Cenozoic (75 to 40 Ma) and late Cenozoic (15 to 0
339 Ma) episodes, with the former more prevalent in basement samples on the flanks of the NMB
340 (e.g. GC503-184) and the latter clearly resolved in samples from the Hebrides Shelf (e.g.
341 GC523-49, 56). However, we acknowledge that other interpretations of these results are
342 possible given the broad timing constraints for some samples.
343
344 Exploration well 134/5-1
345 We analysed eight AFTA samples from exploration well 134/5-1, located in the SOHB ~50
346 km southwest of Skye (Fig. 1). The well intersected ~2340 m of Upper Triassic-Lower
347 Jurassic section intruded by numerous Palaeocene minor sills and dykes (i.e. <2 m thick). We
348 also analysed 29 vitrinite reflectance (VR) samples (predominantly from the Jurassic section),
14 Holford et al. Post-Caledonian exhumation of northwest Scotland
349 using the kinetic model of Burnham and Sweeney (1989) to convert observed Romax values
350 to palaeotemperatures (using an assumed heating rate of 1°C Ma-1).
351
352 BHT data indicate a present-day geothermal gradient of 47.5°C km-1, but the DTH calculated
353 on this basis for sample GC369-6 predicts total annealing, whereas this sample contains
354 tracks and gives a finite (though very young) fission track age (Fig. 5a). In samples GC369-3
355 and 4 a similar comment applies to apatites containing <0.2 wt% Cl. In addition, GC369-4
356 and 5 show less length reduction than expected from the DTH. These aspects of the AFTA
357 data suggest that present-day temperatures based on a gradient of 47.5°C km-1 are too high,
358 and are more consistent with a gradient of ≤43°C km-1. We have used this upper limit as the
359 basis for construction of the DTH used to extract thermal histories from AFTA, and on this
360 basis six samples show clear evidence of having been hotter in the past (Table 2).
361
362 Combining timing constraints derived from AFTA samples with maximum
363 palaeotemperatures derived from VR data (Fig. 5b), indicates that two distinct Cenozoic
364 cooling episodes have affected the preserved section (Table 2). VR-derived
365 palaeotemperatures show fluctuations over narrow depth intervals around intrusions
366 superimposed on a “background” linear profile (Fig. 5b). The rapid fluctuations clearly
367 reflect contact heating around the igneous intrusions. AFTA data in GC369-2 is also
368 interpreted to reflect contact heating, and a similar conclusion applies to the
369 palaeotemperature constraint from GC369-1, as it plots above the main background trend
370 defined by VR and other AFTA samples (Fig. 5b). AFTA data from the Triassic section
371 (>1.75 km depth) define mid-Cenozoic cooling that began between 45 and 20 Ma (Table 2).
372 Lower limits to the maximum palaeotemperature in this episode are consistent with the linear
373 background trend defined by the VR data through this section and shallower units (Fig. 5b).
15 Holford et al. Post-Caledonian exhumation of northwest Scotland
374 Similarly, the range of palaeotemperatures indicated by AFTA data in Jurassic sample
375 GC523-58 is also consistent with this trend and palaeotemperatures indicated by VR data at
376 similar depths. Therefore we interpret the “background” linear increase of palaeotemperature
377 with depth indicated by the shallower and deeper VR data as defining a mid-Cenozoic
378 palaeothermal peak (Fig. 3) Palaeotemperatures characterising this episode are consistent
379 with a range of palaeogeothermal gradients from ~30-50 °C km-1 (Fig. 5c). If the mid-
380 Cenozoic palaeogeothermal gradient was similar to the revised present-day gradient (43°C
381 km-1), these data imply deeper burial by between 0.64 to 0.91 km of additional post-Lower
382 Jurassic section, removed during mid-Cenozoic (45-20 Ma) exhumation (Fig. 5c). These
383 results confirm km-scale deeper burial of preserved Jurassic and Triassic strata, but also
384 suggest that early Palaeocene cooling in some parts of this region, often attributed solely to
385 exhumation, is dominantly a consequence of igneous activity.
386
387 Skye, Mull and Morvern
388 We analysed 31 onshore and two offshore samples collected on and around Skye, Mull and
389 the Morvern peninsula, and one additional sample collected from central Scotland. Samples
390 encompass a broad range of ages, from Mesoproterozoic to Upper Cretaceous (Fig. 3).
391 Thermal history interpretation indicates that most samples around Skye, irrespective of their
392 stratigraphic age (e.g. Moine sample GC523-6, near Glenancross and Jurassic sample
393 GC523-10, from BH71/10) were totally annealed during early Palaeogene heating, reaching
394 temperatures >110°C before cooling which began between 65 and 60 Ma (Fig. 3). Samples
395 that were totally annealed during the early Palaeogene enable resolution of a late Cenozoic
396 cooling episode, also observed in samples from Applecross that were not totally annealed in
397 the Palaeocene event. Combining timing constraints for all samples suggests that cooling
398 began between 20 and 10 Ma (Fig. 3). Results from around Mull are generally consistent with
16 Holford et al. Post-Caledonian exhumation of northwest Scotland
399 those from Skye, although the onset of Cenozoic cooling in Devonian sample GC523-77
400 from Iona and Moine sample GC523-78 from near Bunessan is slightly later (60 to 20 Ma)
401 (Fig. 3). This could represent the failure to resolve two distinct Cenozoic cooling episodes.
402
403 AFTA results from Morvern are reported in more detail in a subsequent section of this paper,
404 but generally define distinct cooling episodes beginning during the early-mid Triassic (245 to
405 225 Ma), early-mid Cenozoic (65 to 30 Ma) and late Cenozoic (20 to 10 Ma) (Table 2).
406
407 Integrating AFTA results with regional geological evidence
408 Triassic-early Jurassic cooling episodes (245-225 and 225-190 Ma)
409 Pre-Mesozoic samples across northwest Scotland provide evidence for two phases of cooling
410 beginning during the Triassic-Jurassic. Lewisian basement samples from the Outer Hebrides
411 (GC523-95, 97, 98) and Hebrides Shelf (GC523-55) reveal Triassic-early Jurassic cooling
412 from >105°C that began between 225 and 190 Ma (Figs. 3, 6, 7a). There is no strong
413 stratigraphic evidence for major Triassic-early Jurassic erosion in the Hebridean basins (Fyfe
414 et al., 1993). Morton (1987) suggested that the first phase of crustal stretching responsible for
415 the Hebridean basin system began ~210 Ma. Because these samples are located in the
416 footwalls of major Triassic-Jurassic normal faults (e.g. the Minch Fault; Roberts &
417 Holdsworth, 1999) we interpret this cooling as due to localized exhumation driven by
418 footwall uplift. Similar footwall uplift has been identified in the North Sea from seismic data
419 (Roberts & Yielding, 1991), although any palaeothermal effects there have been overprinted
420 by Jurassic burial, reflecting the greater degree of extension and post-rift subsidence.
421
422 Mesoproterozoic (Moine), Silurian (Strontian Granite) and Devonian sedimentary AFTA
423 samples from around Skye, Mull and Morvern reveal evidence for early Triassic cooling that
17 Holford et al. Post-Caledonian exhumation of northwest Scotland
424 began between 245 and 225 Ma (Figs. 3, 6, 7a). Sample RD17-41 also shows that cooling at
425 this time also occurred in central Scotland, emphasising the regional extent of this episode
426 (Fig. 7a). Whilst the timing of cooling is similar to that observed in the footwall of the Minch
427 Fault (e.g. samples GC523-95, 97, 98), the consistency of data and interpretations over much
428 of the study area (Tables 1 and 2, Fig. 3) suggests a more regional event. One possible
429 explanation for this early Triassic cooling is that it represents the final stage of post-
430 Caledonian unroofing that brought Precambrian and Lower Palaeozoic basement rocks to the
431 surface prior to their reburial beneath Mesozoic sediments.
432
433 Early-mid Cretaceous cooling episodes (140-130 and 110-90 Ma)
434 A key finding of this study is that Permian-Lower Cretaceous sediments now exposed at or
435 near surface levels around northwest Scotland have been heated to temperatures higher than
436 90-100°C which we interpret as reflecting regional Triassic-Jurassic burial. Particularly in
437 northern and central parts of the study area (e.g. the WOB), these maximum post-depositional
438 temperatures were attained during the early to mid-Cretaceous (Figs. 3, 7b, c).
439
440 Our results indicate two separate phases of early Cretaceous cooling, with the more recent
441 event apparently restricted to the WOB (including well 202/19-1) while the earlier event
442 affected a wider region, across the Hebrides and eastwards into central Scotland (Fig. 7b).
443 Our preferred interpretation is that these are distinct episodes based on consistencies in the
444 data that characterise the different events (Tables 1 and 2). Early Cretaceous cooling is less
445 well defined around Skye, Mull and Morvern (Fig. 7b), which we attribute to overprinting by
446 early Palaeogene heating related to igneous activity. Results from well 202/19-1 suggests that
447 heating to the early to mid-Cretaceous palaeotemperatures was dominantly due to deeper
18 Holford et al. Post-Caledonian exhumation of northwest Scotland
448 burial, with subsequent cooling to exhumation, and this conclusion can be extended to
449 cooling at this time recorded in WOB shallow borehole samples (Table 2; Fig. 3).
450
451 Cretaceous rocks are largely absent around northwest Scotland, but where preserved their
452 stratigraphic relations often provide evidence for early-mid Cretaceous exhumation and
453 deformation. Cenomanian sandstones at Morvern rest unconformably on Lower Jurassic and
454 older rocks, indicating intervening uplift and erosion (Fyfe et al, 1993). In north Skye, NE-
455 trending anticlines within Jurassic strata are not observed in the thin Upper Cretaceous
456 sequences that underlie Paleocene lavas (England, 1994). The axes of these folds trend
457 parallel to the major basin-bounding faults (e.g. the Camasunary-Skerryvore Fault), and
458 indicate early Cretaceous deformation in the Hebridean region (Bell & Williamson, 2002).
459 Further south, AFTA data from wells in the Irish Sea basin system and from samples onshore
460 Ireland record widespread early Cretaceous (~120-115 Ma) exhumation (Holford et al.,
461 2009a). AFTA and compaction data from the Lower Jurassic succession of the Mochras
462 borehole indicate ~2.5 km of early Cretaceous exhumation at that location (Holford et al.,
463 2005). These observations confirm the regional nature of early Cretaceous cooling and
464 exhumation, although slight differences in the timing of cooling between individual areas
465 results in some uncertainty with regards to identification of discrete episodes and their
466 correlations with events in the stratigraphic record.
467
468 Early Cenozoic (65-60 Ma)
469 Early Cenozoic exhumation of northern Scotland is thought to be a source of Palaeocene
470 sediments in the Faroe-Shetland Basin (White & Lovell, 1997; Stoker et al., 2010), and a
471 number of previous studies have suggested regional Palaeocene uplift and exhumation across
472 northern Scotland based either on thermochronology (Persano et al., 2007) or geophysical
19 Holford et al. Post-Caledonian exhumation of northwest Scotland
473 studies (Jones et al., 2002). However, our AFTA data shows that early Cenozoic cooling is
474 restricted dominantly to the vicinity of Palaeogene igneous centres (e.g. Skye, Mull) (Fig.
475 7c). Away from these intrusions, early Cenozoic palaeotemperatures decrease rapidly, whilst
476 some regions such as the WOB show no obvious requirement for early Cenozoic cooling
477 (Fig. 7b), and earlier or later cooling episodes dominate the thermal histories.
478
479 The clearest evidence for elevated early Cenozoic palaeotemperatures is provided by Jurassic
480 AFTA samples from Skye, where all but one sample show evidence for palaeotemperatures
481 >110°C prior to cooling beginning between 65 and 60 Ma (Table 2). This is consistent with
482 previous results from Skye that also reveal early Cenozoic palaeotemperatures >110°C
483 (Lewis et al., 1992). In contrast, VR data from onshore Middle Jurassic sediments in the
484 Inner Hebrides signify localized elevated palaeotemperatures around contemporaneous
485 igneous intrusions, while away from intrusive bodies VR values are much lower (Hudson and
486 Andrews, 1987). The compaction and cementation state of these sandstones suggest
487 maximum burial depths of only ~1 km (Hudson and Andrews, 1987), indicating that the early
488 Cenozoic palaeotemperatures of >110°C revealed by AFTA most likely do not reflect solely
489 deeper burial, but are more likely due to hydrothermal and/or contact heating, similar to that
490 observed in the 134/5-1 well (Fig. 5c) Thus, whilst early Cenozoic cooling recorded by
491 AFTA data in some samples may in part record regional exhumation, over much of this
492 region it is difficult to isolate this signal from thermal effects related to igneous activity. But
493 in other areas (e.g. WOB) early Cenozoic exhumation is clearly of lower magnitude than that
494 during the early Cretaceous and mid-late Cenozoic.
495
496
497
20 Holford et al. Post-Caledonian exhumation of northwest Scotland
498 Mid-Cenozoic (45-20 Ma)
499 Data from 134/5-1 and 202/19-1 reveal evidence for mid-Cenozoic cooling. Samples from
500 134/5-1 provide the most detailed definition of this episode, showing cooling beginning
501 between 45 and 20 Ma and indicating exhumation of Lower Jurassic and older rocks
502 involving removal of ~0.6-0.9 of additional section (Fig. 5). Results from 202/19-1 are
503 consistent with a similar timing, while requiring a larger amount of removed section. This
504 cooling and exhumation is contemporaneous with a regional Upper Eocene (~34±3 Ma)
505 unconformity reported from the Atlantic margin (Praeg et al., 2005) and coeval with
506 progradation of early Oligocene clastic wedges into the eastern North Sea, which is taken to
507 record the onset of Scandinavian uplift (Japsen et al., 2007). Jolivet (2007) provides evidence
508 for similarly timed (40-25 Ma) cooling onshore Scotland recorded by apatite fission track
509 data from Loch Ness and Strontian. Jolivet (2007) interpreted this data in terms of ~1.6 to 2
510 km of post mid-Cenozoic denudation.
511
512 There are at least three outliers of late Oligocene terrestrial sediments within our study area
513 (Fig. 1). Sediments reach a maximum thickness of ~1 km in the Canna Basin, and in all areas
514 are variably tilted and folded, and overlain with marked unconformity by Quaternary deposits
515 (Fyfe et al., 1993). Lithologies (freshwater mudstones, sandstones and lignites) are similar to
516 other Oligocene terrestrial deposits in western Britain (e.g. Lough Neagh, Cardigan Bay
517 basins). Compaction and palaeothermal data show that these sediments have been exhumed
518 by up to 1.5 km (Holford et al., 2005, 2009b). No such data exist for the Oligocene outliers
519 offshore Scotland, but their preservation and deformation point to complex local variations in
520 mid-Cenozoic vertical motions.
521
522
21 Holford et al. Post-Caledonian exhumation of northwest Scotland
523 Late Cenozoic (15-10 Ma)
524 Most AFTA samples reveal major late Cenozoic cooling from palaeotemperatures of ~60-
525 65°C to their present-day temperatures (Fig. 7d; Table 1). Data from most regions are
526 consistent with a regionally synchronous onset between 15 and 10 Ma (Fig. 3, 6). AFTA
527 samples from the WOB suggest an earlier (30-15 Ma) onset, although this could represent the
528 combined effects of mid and late Cenozoic cooling. In contrast to earlier uplift episodes
529 where there is often uncertainty as to the amount of subsequent reburial, we can quantify the
530 amount of reburial following late Cenozoic uplift because Quaternary sedimentary
531 thicknesses are well constrained (generally <100 m) (e.g. Fyfe et al., 1993; Stoker et al.,
532 1993). We use late Cenozoic palaeotemperatures to estimate amounts of exhumation since
533 the late Cenozoic. Gross exhumation estimates (i.e. uncorrected for reburial) have been
534 calculated using minimum, maximum and mid-point palaeotemperature estimates, assuming
535 a late Cenozoic palaeosurface temperature of 10°C and late Cenozoic palaeogeothermal
536 gradients that decrease linearly from 43°C km-1 at 134/5-1 to 25.3°C km-1 at 202/19-1 (i.e.
537 similar to the present-day gradients) (Fig. 8). Estimates based on mid-point
538 palaeotemperatures show a consistent northwards increase, with lowest values (>0.92 km) in
539 the vicinity of Skye, and highest values (<1.94 km) in the WOB (Fig. 8). Estimates from the
540 WOB are similar to late Cenozoic exhumation estimates across the southern British Isles
541 from AFTA and sonic velocity data (Holford et al., 2005, 2009a). Around major igneous
542 centres (e.g. Mull), a significant fraction of the section eroded during the late Cenozoic may
543 have comprised lavas that accumulated during the Palaeocene (cf. Walker, 1971).
544
545 Integration of AFTA data with stratigraphic constraints on the Morvern peninsula
546 Fourteen AFTA samples have been analysed from Morvern (Fig. 9), where sedimentary rocks
547 of Mesoproterozoic (Moine metasediments; samples GC523-2, RD17-43), Upper
22 Holford et al. Post-Caledonian exhumation of northwest Scotland
548 Carboniferous (GC523-72 & 74), Triassic (GC523-1 & 3), Lower Jurassic and Upper
549 Cretaceous (Cenomanian-?Santonian; GC523-4 & 5) ages crop out within an area of ~20
550 km2. In addition there is a ~460 m thick succession of the Palaeocene Mull Lava Group,
551 whilst the eastern part of the peninsula comprises Silurian Strontian granite (RD17-42, 44 to
552 48). This area provides a unique opportunity to integrate stratigraphic constraints with
553 thermal history information from AFTA to evaluate burial and uplift histories of basement
554 and cover rocks. In basement terrains without younger sedimentary cover, the only available
555 thermal history constraints for modelling of fission track data are the measured present-day
556 temperature and an assumed depositional/metamorphic/crystallisation temperature. However,
557 Moine rocks on Morvern are variably overlain by Triassic, Lower Jurassic and Upper
558 Cretaceous sediments, showing that the basement rocks now exposed across this region had
559 been exhumed to near-surface levels/temperatures at these times.
560
561 The Upper Triassic sandstones from which samples GC523-1 and 3 were collected
562 unconformably overlie Moine Supergroup metasediments, while the Silurian Strontian
563 Granite outcrops nearby (Fig. 9a). These pre-Caledonian rocks must therefore have been
564 exhumed to surface levels by the late Triassic, at which time a regional planation surface had
565 been created, now represented by the sub-Triassic unconformity. Samples of Moine and
566 Strontian Granite from Morvern consistently record cooling from >100°C that began between
567 245 and 225 Ma (Table 2). In other words, samples now just below the level of the
568 unconformity were at paleotemperatures >100°C a few Myr prior to the time at which they
569 reached that position and the overlying Upper Triassic sedimentary units were deposited.
570 Therefore we interpret the early Triassic cooling episode as representing the period of
571 exhumation that led to formation of the unconformity underlying the Upper Triassic
572 sediments (Fig. 9c).
23 Holford et al. Post-Caledonian exhumation of northwest Scotland
573
574 The Upper Triassic sandstones are conformably overlain by Lower Jurassic sandstones and
575 shales, in turn unconformably overlain by the ~20 m thick Cenomanian (possibly Turonian)
576 marginal marine Loch Aline White Sandstone Formation, indicating the presence of a
577 Cenomanian coastline (Lowden et al., 1992). The absence of Middle Jurassic to Early
578 Cretaceous section indicates a period of erosion or non-deposition during this interval. AFTA
579 data from Morvern provide no direct evidence for cooling associated with this unconformity
580 (Fig. 9c). But as Jurassic heating and burial and early Cretaceous cooling, exhumation and
581 deformation is observed in northern parts of our study area we suggest it is also likely to have
582 occurred in Morvern, but palaeotemperatures associated with this episode were subsequently
583 overprinted by higher early Cenozoic paleotemperatures (below).
584
585 Most samples of Moine and Strontian Granite, as well as the two samples of Upper Triassic
586 sandstone, record reheating to ~80-90°C or above in the early Cenozoic (Table 2). This event
587 is also recorded in samples GC523-4 and 5 from the Loch Aline Sandstone (Table 2), which
588 show that following deposition at ~95 Ma, it was heated to 90-105°C in the Early Cenozoic.
589 Over 100 m of Upper Carboniferous (Westphalian) sandstones and mudstones occur in a
590 faulted inlier on the southern coast of Morvern. While the relationship between this and
591 younger units is not clear, thermal histories of AFTA samples GC523-72 and 74 are similar
592 to the Cenomanian samples, defining early Cenozoic palaeotemperatures of ~100°C (Fig. 9c).
593
594 As noted earlier, early Cenozoic paleotemperatures across the study region are interpreted as
595 dominantly reflecting processes related to igneous activity, chiefly contact and hydrothermal
596 effects as well as locally elevated heat flow and possibly deeper burial, and the effects of
597 these competing processes cannot be resolved. But if the early Cenozoic paleotemperatures
24 Holford et al. Post-Caledonian exhumation of northwest Scotland
598 in these samples involved any degree of deeper burial, it is clear that the entire section must
599 have been reburied following deposition of the Cenomanian sandstone, to achieve these early
600 Cenozoic palaeotemperatures. Burial beneath a late Cretaceous Chalk cover seems plausible.
601 While Upper Cretaceous strata are poorly represented in this area, sediments of this age are
602 identified in Skye, Raasay and Eigg, and Chalk Group fragments of possible Campanian age
603 occur in volcanic vents in Arran (Fyfe et al., 1993). If such burial occurred, early Palaeocene
604 exhumation must have occurred to produce the predominantly subaerial landsurface onto
605 which lavas and pyroclastic material were erupted (Emeleus & Bell, 2005). Alternatively,
606 early Cenozoic palaeotemperatures could be related to hydrothermal and other effects
607 associated with volcanic activity during or subsequent to extrusion of lavas, although in this
608 case, reheating from near-surface temperatures is still required (Fig. 9c).
609
610 Late Cenozoic (20-10 Ma) paleotemperatures around 60-70°C are also recorded in almost all
611 samples from Morvern, as well as other areas in this study (although results in some samples
612 from Morvern may represent the unresolved effects of both early and late Cenozoic episodes;
613 Table 2). The presence of the volcanic sequence and the underlying sub-aerial surface shows
614 that the section must have been re-buried. The Mull Lava Group has a thickness of ~460 m
615 on Morvern, and zeolite minerals (e.g. mesolite, laumonite) within the lavas indicate this area
616 may have been buried by >1 km of additional lavas (Bell & Williamson, 2002). The depth-
617 related zeolite zones are superimposed on hydrothermal mineral zones related to the Mull
618 Central Complex (Bell & Williamson, 2002), and a zone of Carbonate minerals on western
619 Morvern is interpreted to have been deposited by circulating heated waters from the
620 Ardnamurchan Central Complex (Walker, 1971).
621
25 Holford et al. Post-Caledonian exhumation of northwest Scotland
622 Integration of AFTA data from various stratigraphic levels with geological evidence thus
623 reveal a complex history of post-Caledonian vertical motions across Morvern (Figure 9c).
624 Moine basement rocks have been repeatedly buried then exhumed to surface levels, and at
625 least three major cycles of heating and cooling have been identified. This is in stark contrast
626 to the monotonic post-Caledonian cooling histories proposed for this region by
627 thermochronological studies that have focused on basement rocks rather than younger
628 sedimentary cover (e.g. Persano et al., 2007). We suggest that such episodic histories are
629 unlikely to be restricted to this one location, and that such histories are likely to be the rule,
630 rather than the exception.
631
632 Discussion
633 New AFTA data from northwest Scotland reveal a complex history of vertical motions
634 characterized by multiple cycles of Mesozoic-Cenozoic burial and exhumation (Fig. 10). Our
635 data reveal regional cooling and uplift episodes that began during the early-mid Triassic
636 (245-225 Ma), early Cretaceous (140-120 Ma), early Cenozoic (65-60 Ma), mid Cenozoic
637 (45-20 Ma) and late Cenozoic (15-10 Ma) (Fig. 3, 6). AFTA data in Permian-Mesozoic units
638 from offshore wells allow evaluation of palaeogeothermal gradients and indicate that deeper
639 burial is the primary cause of heating, and that exhumation is the main cause of cooling in
640 these episodes, except for early Cenozoic heating which appears in large part to be related to
641 igneous activity. The Cenozoic history of uplift and erosion is consistent with the offshore
642 sedimentary record to the north and west of our study area, with multiple tectonically-
643 influenced sedimentation pulses of Palaeocene, Eocene, Oligocene, Miocene and Plio-
644 Pleistocene observed in the Rockall and Faroe-Shetland basins (Stoker et al, 2010). Our
645 results, which indicate a major role for Cenozoic tectonics in the development of the Scottish
646 landscape, are worth comparing with recent geomorphological studies of Ireland (Dewey,
26 Holford et al. Post-Caledonian exhumation of northwest Scotland
647 2000), which indicate that many topographic features may owe their origin due to faults
648 active during Palaeogene extension and Neogene compression.
649
650 Jurassic sedimentary cover over the Highlands?
651 A key aspect of our results is that many Permian-Triassic samples appear to have reached
652 temperatures of 90-100°C or above in the early Cretaceous. Samples of basement and
653 Palaeozoic sediments give comparable results, implying similar geological histories for
654 basement terrains and sedimentary basins over the past ~200 Myr. The sedimentary basins of
655 the Hebrides contain thick successions of marine Jurassic rocks (e.g. ~2.5 km in the NMB;
656 Fyfe et al., 1993), and our results imply that comparable successions were deposited where
657 Jurassic rocks are now missing, including across the northern Highlands. Sequence-
658 stratigraphic analyses of Middle-Upper Jurassic marine sediments in the Inner Moray Firth
659 and Hebridean basins reveal stratigraphic affinities that imply a marine connection (Underhill
660 and Partington, 1993). Underhill (1991) postulated that such a connection may have existed
661 along the trace of the Great Glen Fault. Roberts & Holdsworth (1999) argued that the
662 Highlands were traversed by a component of Mesozoic extension along NNE-SSW trending
663 faults, and that many structures that are traditionally held to be Caledonian may in fact be of
664 Mesozoic age. Quartz-filled fractures within Cambrian rocks west of the Moine Thrust Zone
665 provide independent evidence for Mesozoic extension (Laubach & Diaz-Tushman, 2009).
666 These fractures strike N-NE and are associated with post-Caledonian faults, leading Laubach
667 & Diaz-Tushman (2009) to attribute their formation to Mesozoic extension. Fluid inclusion
668 homogenization temperatures show these fractures formed at ~80°C, in good agreement with
669 early Mesozoic palaeotemperatures predicted by thermal modelling of AFTA samples that
670 from the northwest coast (Fig. 7b). AFTA indicate that a maximum of 2-3 km of Jurassic
671 sediments have been removed from northern parts of the Hebridean basin system (e.g. the
27 Holford et al. Post-Caledonian exhumation of northwest Scotland
672 West Orkney Basin) during post-Jurassic exhumation. Lower exhumation is likely in
673 southern areas where thicker Mesozoic sequences are preserved (e.g. in the vicinity of Skye,
674 where a thick Jurassic succession is preserved, diagenetic evidence suggests deeper burial of
675 these rocks by ~1 km (Hudson & Andrews, 1987)
676
677 Implications for low-temperature thermal histories of basement regions
678 The repeated cycles of heating and cooling revealed by our data and implied by the
679 geological record are in marked contrast to the results of a recent study that presented apatite
680 fission-track and (U-Th)/He (AHe) data from two locations in northwest Scotland; Clisham
681 (Achaean gneisses) and Sgorr Dhonuill (Silurian granites) (Persano et al., 2007). Persano et
682 al. (2007) interpreted their data as recording continuous, monotonic cooling over the past 400
683 My (i.e. consistent with continuous post-Caledonian emergence), with an early Cenozoic
684 acceleration in cooling. Their approach was based on forward modelling of fission-track and
685 AHe data to indentify thermal histories that were consistent with the observed fission track
686 and AHe ages.
687
688 Persano et al (2007) extracted thermal history constraints from their AHe data using the
689 systematics of He diffusion in apatite defined for Durango apatite by Farley (2000). Although
690 these systematics have been widely adopted, recent studies have shown that He diffusion in
691 apatite is more complicated than originally supposed. Green at al. (2006) reported systematic
692 inconsistencies between measured AHe ages and values predicted on the basis of thermal
693 histories derived from AFTA using the Farley (2000) systematics, with discrepancies
694 increasing with U content of the apatite and the fission track age. Further empirical data
695 showing similar effects (Flowers et al., 2007), combined with new experimental
696 investigations of AHe diffusion kinetics (Shuster et al., 2006) have shown that apatite
28 Holford et al. Post-Caledonian exhumation of northwest Scotland
697 becomes more retentive of He as radiation damage accumulates within the crystal lattice.
698 Consequently, apatites with high effective U (eU) concentrations and older fission track ages
699 retain He at much higher temperatures than young apatites with low eU concentrations such
700 as Durango apatite (Shuster et al., 2006). Because Persano et al. (2007) did not consider the
701 effects of radiation damage, the thermal history interpretations they derive from the AHe data
702 cannot be considered reliable. For all samples where Persano et al. (2007) report both AHe
703 and fission track ages, the latter are considerably older than 100 Ma (the youngest being a
704 value of 186±6 Ma sample in SD9), and so the interpretation of the AHe data in these
705 samples by Persano et al. (2007) using the Farley (2000) systematics in terms of an early
706 Cenozoic acceleration in cooling is unlikely to be valid (Green et al., 2006).
707
708 Even if this interpretation was accurate, it is far from straightforward to attribute early
709 Cenozoic cooling solely to regional exhumation, given the results here which suggest a link
710 to contemporaneous igneous activity (Fig. 2, 5b). Whilst it remains possible that regional
711 Palaeocene exhumation may contribute to early Cenozoic cooling, further work is needed to
712 isolate this signal from cooling resulting from the cessation of hydrothermal heating, contact
713 effects and more localised exhumation caused by intrusion of central complexes (Brown et
714 al., 2009).
715
716 Low-temperature thermochronological studies of basement rocks from onshore passive
717 margins and continental interiors commonly assume monotonic cooling (e.g. Dempster &
718 Persano, 2006). However, our results, best demonstrated by samples from the Morvern
719 peninsula (Fig. 9), indicate that episodic histories dominated by repeated cycles of heating
720 and cooling (i.e. burial and exhumation) are more realistic. The common assumption of
721 monotonic cooling histories for low temperature thermochronological studies is only possible
29 Holford et al. Post-Caledonian exhumation of northwest Scotland
722 occurs because such studies traditionally focus on basement rocks and ignore the significance
723 of any sedimentary outliers that may be present (Green and Duddy, 2007). We suggest that
724 this naive approach can lead to erroneous conclusions, and is no longer tenable.
725
726 Origin of uplift episodes
727 A synthesis of AFTA data from multiple wells and outcrops in the Irish Sea basins by
728 Holford et al. (2009a) identified regional (up to ~106 km2) exhumation-driven cooling
729 episodes beginning during the early Cretaceous and the early, mid and late-Cenozoic that can
730 be correlated with the episodes observed in northwest Scotland. These results suggest greater
731 complexity than previous studies that have suggested that exhumation across the western
732 British Isles is dominantly Palaeogene in age, driven by plume-related igneous underplating
733 (Brodie & White, 1994; White & Lovell, 1997; Jones et al., 2002; Persano et al., 2007).
734 Whilst underplating may well have contributed to the localized early Palaeogene exhumation
735 around Hebridean igneous centres, underplating and dynamic uplift cannot explain the
736 Triassic and early Cretaceous exhumation of northwest Scotland, and we consider it unlikely
737 that the mid and late Cenozoic exhumation episodes constrained by our AFTA data were
738 primarily plume-driven.
739
740 Holford et al. (2009a) noted close temporal relationships between the onset of Cretaceous-
741 Cenozoic exhumation episodes across the western British Isles and plate boundary
742 reorganizations associated with the North Atlantic oceanic ridge system, and increased
743 periods of convergence and coupling between the Eurasian and African plates associated with
744 Alpine collision. These correlations suggest that increased levels of intraplate stress
745 propagated from plate boundary interactions may explain the exhumation episodes
746 constrained by AFTA data. Present-day stress data from northwest Europe indicate a
30 Holford et al. Post-Caledonian exhumation of northwest Scotland
747 northwest-southeast orientation of maximum horizontal stress in the upper crust consistent
748 with control by plate boundary forces (Goelke & Coblentz, 1996). Cenozoic compressional
749 stress magnitudes in particular are likely to have been high (perhaps ~100 MPa) due to ridge
750 push forces enhanced by anomalously high temperature asthenosphere underlying the
751 elevated northeast Atlantic spreading axis, which is above sea level at Iceland (Bott, 1993;
752 Doré et al., 2008). High magnitudes of intraplate stress can explain the large number of
753 Cretaceous-Cenozoic compressional structures in post-Palaeozoic basins of the British Isles
754 and Atlantic Margin that contain weak sedimentary infills (Hillis et al., 2008). It may also
755 explain uplift of ‘uninverted’ basement regions due to the distributed effects of relatively low
756 strain rates (<10-16 s-1) operating over timescales of >1-10 Myr. The resultant cumulative
757 crustal shortening, although relatively minor (<5-10%) may nonetheless be sufficient to
758 generate long-wavelength uplift and exhumation. Low degrees of crustal shortening have
759 been invoked to explain late Cenozoic uplift and deformation of other intraplate regions e.g.
760 the West Siberian Basin (Allen & Davies, 2007) and the margins of the Australian continent
761 (Sandiford et al., 2004).
762
763 Conclusions
764 We have constrained post-Caledonian vertical motions around northwest Scotland using 78
765 AFTA samples from onshore outcrops, and offshore shallow boreholes and deep exploration
766 wells. Samples came from both Proterozoic-Lower Palaeozoic basement and Mesozoic cover
767 sequences. Our results indicate multiple Mesozoic-Cenozoic cooling and exhumation
768 episodes, several of which correlate with similar events recorded by AFTA data throughout
769 western Britain. Thermal histories of basement and cover samples are generally consistent,
770 and many Permian-Triassic samples cooled from temperatures of 90-100°C or above during
771 the Cretaceous. These results require deposition of considerable additional thicknesses of
31 Holford et al. Post-Caledonian exhumation of northwest Scotland
772 Triassic-Jurassic rocks in the Hebridean basins, and suggest deposition of Mesozoic
773 sediments across the Highlands. This additional cover was removed during regional early
774 Cretaceous and mid-late Cenozoic exhumation. AFTA data record early Cenozoic heating
775 and cooling, but it is difficult to isolate the contribution by regional uplift from thermal
776 effects associated with igneous activity. Combined with preserved stratigraphy which records
777 intermittent Upper Palaeozoic-Mesozoic sedimentation across northwest Scotland, our data
778 are incompatible with a significant Caledonian component in the present-day topography.
779 Instead, tectonic processes have repeatedly uplifted northwest Scotland over the past ~140
780 Myr and erosion has stripped away a considerable thickness of cover. Our results emphasize
781 the importance of integrating thermal history data from basement and overlying sedimentary
782 units when reconstructing the uplift histories of onshore passive margins and intraplate
783 regions, and also demonstrate the importance of integrating observations from onshore and
784 offshore.
785
786 Acknowledgements
787 This work forms part of ARC Discovery Project DP0879612, and represents TRaX
788 contribution #75. We thank Alan Roberts and an anonymous reviewer for their comments and
789 Tim Needham for editorial guidance. The contribution of Stoker is made with the permission
790 of the Executive Director, British Geological Survey (Natural Environment Research
791 Council).
792
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1057 Figure captions
1058 Fig. 1 Map of Scotland showing post-Ordovician geology and faults with evidence of post-Ordovician activity,
1059 and locations of samples used in this study (Fyfe et al., 1993; Stoker et al., 1993). Although Mesozoic and
1060 Cenozoic rocks are mainly restricted to the present-day offshore basins, sedimentary and volcanic outliers of
1061 Devonian, Carboniferous, Permian, Upper Triassic, Jurassic and Upper Cretaceous occur along the northwest
1062 coast. CB, Canna Basin; CF, Colonsay Fault; CL, Clisham; GGF, Great Glen Fault; HBF, Highland Boundary
1063 Fault; HF, Helmsdale Fault; HS, Hebrides Shelf; IMF, Inner Moray Firth; LMT, Little Minch Trough; M, Mull;
1064 MF, Minch Fault; NH, Northern Highlands; NMB, North Minch Basin; OH, Outer Hebrides; S, Skye; SD, Sgorr
1065 Dhonuill; SOHB, Sea of Hebrides Basin; SKF, Skerryvore Fault; STF, Strathconon Fault; WF, Wick Fault;
1066 WFB, West Flannan Basin; WLB, West Lewis Basin; WOB, West Orkney Basin; WSB, West Shetland Basin.
43 Holford et al. Post-Caledonian exhumation of northwest Scotland
1067 Fig. 2 Mean fission track ages plotted against mean track lengths for apatite fission track data used in this study,
1068 alongside previously published data from northwest Scotland (Lewis et al., 1992 (margins of Sea of Hebrides
1069 Basin); Thomson et al., 1999 (northern Highlands); Jolivet, 2007 (Loch Ness, Strontian); Persano et al., 2007
1070 (Clisham, Sgorr Dhonuill)). All errors shown at ±1σ. Most fission track ages are younger than 300 Ma,
1071 suggesting these samples reached late Palaeozoic-Mesozoic palaeotemperatures sufficient to produce severe age
1072 reduction (i.e. >100°C). Ages from Skye and Mull mostly range between 50-70 Ma, similar to the timing of
1073 igneous activity in these regions, and suggesting that related thermal effects heavily influence fission track data
1074 from these regions. A typical ‘boomerang trend’ based on AFTA data from the Lake District (Green, 1986) is
1075 shown in the background.
1076
1077 Fig. 3 Comparison of the timing information derived from all AFTA data that show evidence for higher
1078 palaeotemperatures post-deposition/intrusion/metamorphism. Synthesis of results identifies a number of discrete
1079 phases of cooling, indicated by the vertical bands. Open boxes and arrow indicate the range of stratigraphic ages
1080 for individual samples. Horizontal bands define range of timing for the onset of cooling derived from AFTA
1081 data in each sample within a 95% confidence interval. Note that for several samples from 202/19-1 the timing
1082 estimates for individual estimates show some overlap. Timing estimates have been shifted vertically where this
1083 is the case.
1084
1085 Fig. 4 AFTA results and thermal history reconstruction for exploration well 202/19-1. a) Left, fission-track
1086 ages; right, mean track lengths, both plotted against depth below kelly bushing. Black lines show increasing
1087 stratigraphic age with depth. Greyscale lines show the predicted patterns of fission-track age and mean track
1088 length from the DTH for apatites containing 0.0-0.1, 0.4-0.5, 0.9-1.0 and1.5-1.6 wt% Cl. The DTH is based on
1089 the burial history derived from the sedimentary section intersected by the well, combined with the present-day
1090 geothermal gradient of 25.3°C km-1 determined from corrected BHT values in this well. The fission-track ages
1091 decrease with depth and are much less than the values predicted from the DTH, indicating that the sampled
1092 sedimentary units have been hotter in the past. b) Palaeotemperature constraints from AFTA data plotted against
1093 depth. AFTA data define two separate phases of cooling that have affected the preserved Permian-Triassic
1094 succession that began during the early Cretaceous (110 to 90 Ma) and Cenozoic (55 to 10 Ma). c) Ranges of
1095 amounts of removed section and palaeogeothermal gradients (hyperbolic ellipsoids) required to explain the early
1096 Cretaceous and Cenozoic palaeotemperatures. The hyperbolic ellipsoids define the parameter ranges estimated
44 Holford et al. Post-Caledonian exhumation of northwest Scotland
1097 from AFTA that are consistent with the respective palaeotemperature constraints within 95% confidence limits.
1098 Black dots indicate the corresponding maximum likelihood solutions. Horizontal shaded band shows amount of
1099 removed section independently estimated from sonic velocity data (Evans, 1997).
1100
1101 Fig. 5 AFTA results and thermal history reconstruction for exploration well 134/5-1. a) Left, fission-track ages;
1102 right, mean track lengths, both plotted against depth below kelly bushing. Black lines show increasing
1103 stratigraphic age with depth. Greyscale lines show predicted patterns of fission-track age and mean track length
1104 from the DTH based on the preserved sedimentary section for apatites containing 0.0-0.1, 0.4-0.5, 0.9-1.0
1105 and1.5-1.6 wt% Cl. The DTH for this well has been determined using the revised geothermal gradient of 43°C
1106 km-1 that is with AFTA data. b) Palaeotemperature constraints from AFTA and vitrinite reflectance (VR) data
1107 plotted against depth. AFTA and VR data define two episodes of Cenozoic cooling. Samples recording early
1108 Cenozoic (~60 Ma) palaeotemperatures are closely associated with coeval igneous intrusions and define no
1109 linear trend with depth, suggesting that cooling occurred following the cessation of contact and hydrothermal
1110 heating associated with igneous activity. Samples recording mid-Cenozoic (45 to 20 Ma) palaeotemperatures
1111 define a linear profile with depth which has a similar gradient to that of the present-day temperature profile. (c)
1112 Mid-Cenozoic cooling in these samples is explained by removal of ~0.6 to 0.9 km of post-Lower Jurassic
1113 section during a denudation episode which began between 45 and 20 Ma, for a palaeogeothermal gradient equal
1114 to the revised present-day value of 43°C km-1.
1115
1116 Fig. 6 Post-400 Ma event diagram for northwest Scotland, comparing generalized stratigraphy with AFTA-
1117 constrained cooling episodes defined in this study and in the western British Isles (Holford et al., 2009a).
1118
1119 Fig. 7 Palaeotemperatures across northwest Scotland in each of the regional palaeothermal episodes defined in
1120 this study. a) Triassic (220-195 Ma in the Outer Hebrides and 245-225 Ma in Morvern and central Scotland).
1121 Lighter shading shows surface/seafloor outcrops of Permian-Triassic rocks, darker shading shows outcrops of
1122 Jurassic rocks. b) Early Cretaceous (110-90 Ma in the West Orkney Basin and 140-130 Ma in the Hebridean
1123 region). Lighter shading shows surface/seafloor outcrops of Permian-Triassic rocks, darker shading shows
1124 outcrops of Jurassic rocks. c) Early Palaeogene (65-60 Ma in Skye, Mull and Morvern, 75-40 Ma in the
1125 Hebridean region). Diagonal shading shows seafloor outcrops of Cenozoic sedimentary rocks, hatched shading
1126 shows surface/seafloor outcrops of Palaeogene igneous extrusive rocks, grey shading shows surface/seafloor
45 Holford et al. Post-Caledonian exhumation of northwest Scotland
1127 outcrops of Palaeogene intrusive rocks. d) Mid-late Cenozoic (30-15 Ma in West Orkney Basin, 15-10 Ma in
1128 Hebridean region, 40-25 Ma in 134/5-1 and 20-10 Ma in Skye, Mull and Morvern).
1129
1130 Fig. 8 Mid-late Cenozoic exhumation estimates across northwest Scotland based on AFTA data. a) Estimates
1131 based on mid-point temperature for each sample, assuming a surface temperature of 10°C and a geothermal
1132 gradient that decreases linearly from 43°C km-1 at 134/5-1 to 25.3°C km-1 at 202/19-1. b) Horizontal bars
1133 represent the range of gross exhumation for each sample location (reflecting the range of palaeotemperatures
1134 estimated from AFTA), and thickness of Quaternary deposits (i.e. post-exhumation reburial). Adding the
1135 thickness of Quaternary sediments to the gross estimates gives an estimate of net exhumation for each location.
1136 Results indicate that large areas of northwest Scotland and its offshore basins have had >0.5-1.0 km of section
1137 removed during late Cenozoic exhumation.
1138
1139 Fig. 9 AFTA data from the Morvern peninsula. a) Geological map of the Morvern peninsula. b) Thermal
1140 histories for AFTA samples from various stratigraphic levels. Results indicate exhumation of basement rocks to
1141 surface levels by the late Triassic, following which they were reheated during late-Triassic-Jurassic burial, and
1142 then cooled through several stages of Cretaceous-Cenozoic exhumation. c) Schematic post-400 Ma thermal
1143 history of the Morvern peninsula reconciling results from all AFTA samples.
1144
1145 Fig. 10 Schematic ~E-W cross-sections illustrating our preferred model for the post-Caledonian geological
1146 evolution of northwest Scotland. (a) Early Devonian post-orogenic uplift witnessed by the unroofing of the
1147 Newer Granites (Watson, 1985; Hall, 1991). (b) Devonian extensional collapse and subsidence permitting
1148 deposition of >2-3 km of Devonian and Lower Carboniferous rocks (Thomson et al., 1999). (c) Late
1149 Carboniferous deformation and uplift related to far-field Variscan compression (Underhill et al., 2008)
1150 removing Devonian and Carboniferous sediments. (d) Late Triassic-Jurassic extension and subsidence leading to
1151 formation of Hebridean basins (Fyfe et al., 1993) but also affecting pre-Caledonian basement rocks (Roberts &
1152 Holdsworth, 1999). Localised uplift in footwalls of major faults, and regional uplift of basement rocks in
1153 western and central Scotland. Maximum post-Caledonian burial depths across northwest Scotland are attained
1154 during the late Jurassic-early Cretaceous with up to 2-3 km of subsequently eroded syn and post-rift Jurassic
1155 rocks deposited across this region. (e) Early Cretaceous regional uplift and exhumation beginning ~120 Ma
1156 followed by late Cretaceous sea level rises permitting Chalk deposition (f). (g) Localised early Cenozoic uplift
46 Holford et al. Post-Caledonian exhumation of northwest Scotland
1157 (beginning ~65-60 Ma) around Palaeogene igneous intrusive centres. Considerable thicknesses of lavas
1158 accumulate in some areas (e.g. around Mull), much of which are eroded during later exhumation. (h) Regional
1159 exhumation beginning during the mid-Cenozoic (40-25 Ma) and late Cenozoic (15-0 Ma) removing up to 1.9
1160 km of overburden, followed by Pliocene-Pleistocene uplift not directly constrained by our AFTA data but
1161 witnessed by shelf-slope wedges of clastic sediment that prograde away from northwest Scotland (Stoker et al.,
1162 1993).
1163
1164 Table 1 Apatite fission track analysis sample details.
1165
1166 Table 2 Summary of palaeotemperatures analysis and Default Thermal History interpretations.
47 Holford et al Figure 1
Carboniferous cz Cenozoic sediments j c pt Jurassic sediments sediments Permian-Triassic g Palaeogene intrusives pt d Devonian extrusives sediments Carboniferous d g Palaeogene extrusives c d Devonian sediments extrusives WSB k Cretaceous sediments k, j, pt g 202/19-1 WLB 503-151 SBHF cz d pt 503-146 pt
523-24 503-147 523-47 503-148 pt g WOB g HS MF j 503-149 pt 503-150 WF 523-51 d 523-52 523-48 g 503-184 k, j, pt 523-15 IMF WFB NH cz 523-49 503-185 NMB k 58°N 523-23 OH 523-110 HF 523-98 523-13 CL 523-111 g 523-93 523-14 523-97 523-95 523-112 LMT 523-11 j 523-91 pt g 523-99 523-102 523-105 d g 523-109 S 523-106 STF j 523-55 523-108 g 523-83, 84 523-80 CB 523-88 RD17-41 cz 523-89 j pt 523-10 GGF 134/5-1 523-6,8 523-56 MF pt RD17-47,48 RD17-42 SOHB RD17-45,46 SD d pt 523-1,2,3 RD17-44 g 523-4,5 RD17-43 523-72,74 523-79 Fig. 9 523-71 HBF SKF 523-77 M d d cz 523-78 d
g 56°N 523-9
8°W pt CF 6°Wc 4°W c 2°W Holford et al Figure 2
16 West Lewis/West Orkney Basin Hebrides 15 Skye, Mull and Morvern Lewis et al. (1992) Thomson et al. (1999) 14 Jolivet (2007) Persano et al. (2007) Lake District ‘boomerang trend’ 13 Green (1986)
12
11
10
9
8 0 50 100 150 200 250 300 350 400 450 500 Fission track age (Ma) Holford et al Figure 3 Time (Ma) Time (Ma) 300 250 200 150 100 50 0 300 250 200 150 100 50 0
Permian Triassic Jurassic Cretaceous Cenozoic Permian Triassic Jurassic Cretaceous Cenozoic
West Orkney Basin 134/5-1 GC369-1 GC523-24 GC369-2 GC503-146 GC523-68 GC503-147 GC369-3 GC503-148 GC369-4 GC503-149 GC369-5 GC503-150 Degree of overlap 45-20 Ma GC503-151 Skye, Mull and Morvern 202/19-1 GC503-84 GC523-1 GC503-85 GC523-2 >800 GC503-86 GC523-3 GC503-87 GC523-4 GC503-88 GC523-5 GC503-89 GC523-6 >800 GC503-90 GC523-8 >800 GC503-91 GC523-9 416-359 GC503-92 GC523-10 416 Degree of overlap 110-90 Ma 30-15 Ma GC523-71 -385 GC523-72 345-328 Hebrides GC523-74 345-328 GC503-184 >570 GC523-77 416-359 GC503-185 GC523-78 >800 GC523-11 GC523-79 GC523-13 1100-800 GC523-83 GC523-14 GC523-84 GC523-15 GC523-88 1100-800 GC523-23 GC523-89 GC523-47 GC523-91 GC523-48 GC523-99 GC523-49 GC523-102 GC523-51 GC523-105 1100-800 GC523-52 GC523-106 GC523-55 2900-1100 GC523-108 GC523-56 1100 GC523-109 -800 GC523-80 >800 RD17-41 >800 GC523-93 2900-1100 RD17-42 420 GC523-95 2900-1100 RD17-43 >800 GC523-97 2900-1100 RD17-44 420 GC523-98 2900-1100 RD17-45 420 GC523-110 2900-1100 RD17-46 420 GC523-111 1100-800 RD17-47 420 GC523-112 1900-1100 RD17-48 420 225-190 Ma 140-130 Ma 75-40 Ma 15-10 Ma 245-225 Ma 135-100 Ma 65-60 Ma 20-10 Ma
300 250 200 150 100 50 0 300 250 200 150 100 50 0 Time (Ma) Time (Ma) Holford et al Figure 4 (a) 0 Quat 10 10
20 20 1 1 30 30
40 40
50 50 2 2
Permian-Triassic 60 60 Depth (km)
70 (°C) Temperature 70 3 3 80 80 stratigraphic age
4 0 100 200 300 0 4 8 12 16 Fission Track Age (Ma) Mean Length (µm) (b) Temperature (°C) 20 40 60 80 100 120 140 160 180 200 Sea bed 127 mKB Quat
GC503-84 0.5
GC503-85
1.0 GC503-86
1.5 GC503-87
GC503-88 GC503-89
Depth (km) 2.0 GC503-90 Upper Permian-Lower Triassic Upper Permian-Lower
GC503-91 2.5
GC503-92 3.0
TD 3189 m Corrected BHT measurement AFTA sampling horizon Maximum mid-late Cenozoic (55-10 Ma) palaeotemperature from AFTA Maximum early Cretaceous (110-90 Ma) palaeotemperature from AFTA (c)
10 Early Cretaceous maximum likelihood estimates Palaeogeothermal gradient: 39°C/km Additional section: 2.31 km 8 Cenozoic maximum likelihood estimates Palaeogeothermal gradient: 18.2°C/km Additional section: 2.67 km
6 Present-day geothermal gradient = 25.3°C/km
4 3.9 km
Additional section (km) 3.3 km
2 Triassic shale sonic velocities >1.85 km 1.7 km 1.2 km
20 40 60 80 100 Palaeogeothermal gradient° ( C/km) Holford et al Figure 5 (a) 0 10 10 20 20 30 30 1 1 40 40 stratigraphic age 50 50 Lower Jurassic 60 60 70 70 Temperature (°C) Temperature Depth (km) 2 2 80 80 90 90 Triassic
3 0 100 200 300 400 0 4 8 12 16 Fission T rack Age (Ma) Mean Length (µm) (b) T emperature (°C) 20 40 60 80 100 120 140 160 180 200 220 240 260 Sea bed 132 mKB
Mid-Cenozoic Pliensbachian 0.5 (45 to 20 Ma) palaeogeothermal gradient GC523-58 Inferred early Cenozoic palaeotemperature profile 1.0 GC369-1 Late Sinemurian
Early 1.5 Sinemurian
Depth (km) Present-day GC369-2 geothermal Hettangian gradient of Rhaetian GC523-68 ~43°C/km Palaeogene 2.0 GC369-3 (based on BHT intrusive rocks
and AFTA data) GC369-4 GC369-5 Triassic
2.5 TD 2472 m Corrected BHT measurement AFTA sampling horizon Revised present-day temperature derived from AFTA data Maximum mid-Cenozoic (45 to 20 Ma) palaeotemperature from AFTA Maximum mid-Cenozoic (45 to 20 Ma) palaeotemperature from VR Maximum early Cenozoic palaeotemperature from AFTA Maximum early Cenozoic palaeotemperature from VR (c) Mid-late Cenozoic maximum likelihood estimates 2 Palaeogeothermal gradient: 39.4°C/km Additional section: 0.94 km
1.5
Present-day geothermal gradient = 43°C/km 1 0.91 km
Additional section (km) 0.64 km 0.5
20 30 40 50 60 70 Palaeogeothermal gradient (°C/km) Holford et al Figure 6
Age Generalized AF TA-constrained (Ma) stratigraphy cooling episodes 0 Widespread 15-10 Ma 20-10 Ma 20-15 Ma 30-15 Ma Canna Basin 40-25 Ma
Widespread 75-40 Ma 65-60 Ma 65-55 Ma
Morvern, Skye, Mull, 100 Arran 110-90 Ma
Early Cretaceous 120-115 Ma
Cretaceousdeformation Cenozoic and uplift 140-130 Ma
Callovian hiatus Toarcian hiatus Widespread
200 Upper Triassic rocks overly 225-190 Ma basement in Morvern 245-225 Ma
Base poorly dated Variscan deformation and uplift 300 Morvern
Arran 134/5-1 Hebridean Region West Orkney Basin West
Mull, British Isles Western Arran Skye, Mull & Morvern 400 Devonian Carboniferous Permian Triassic Jurassic Holford et al Figure 7 a) b)
58°N 58°N
Triassic Early Cretaceous palaeotemperatures palaeotemperatures >110°C >110°C 100-110°C 100-110°C 90-100°C 90-100°C 80-90°C 80-90°C 70-80°C 70-80°C 60-70°C 60-70°C <60°C <60°C
56°N 56°N
8°W 6°W 4°W 2°W 8°W 6°W 4°W 2°W
8°W 6°W 4°W 2°W c) d)
58°N 58°N
Early Palaeogene Mid-Late Cenozoic palaeotemperatures palaeotemperatures >110°C >110°C 100-110°C 100-110°C 90-100°C 90-100°C 80-90°C 80-90°C 70-80°C 70-80°C 60-70°C 60-70°C <60°C <60°C
56°N 56°N
8°W 6°W 4°W 2°W 8°W 6°W 4°W 2°W Holford et al Figure 8 Late Cenozoic exhumation (km) b) 0 0.5 1.0 1.5 2.0 2.5 3.0
1.67 GC503-151 84 m 0.74 2.59 a) 8°W 6°W 4°W 2°W 1.92 GC503-146 23 m 1.54 2.30 1.69 GC523-47 52 m 1.45 1.93 1.94 GC503-148 8.5 m 1.29 2.59 1.94 GC503-147 28 m 1.29 2.59 1.72 GC503-149 28.5 m 0.86 2.59 1.43 GC503-150 12.5 m 0.71 2.14 1.47 GC523-51 8.5 m 1.18 1.76 1.62 GC523-48 12.1 m 1.32 1.91 1.47 GC523-52 10.5 m 1.03 1.91 1.32 GC523-49 10.6 m 0.97 1.67 1.74 GC523-23 1.85 m 1.39 2.08 1.64 GC523-11 9 m 1.28 2.00 1.15 GC523-91 0 m 0.54 1.76 1.01 GC523-99 0 m 0.54 1.49 1.01 GC523-102 0 m 0.54 1.49 1.49 GC523-105 0 m 1.22 1.76 1.64 GC523-109 0 m 1.18 2.10 1.10 58°N GC523-55 68 m 0.93 1.28 1.47 RD17-41 0 m 1.2 1.73 0.92 GC523-106 0 m 0.39 1.45 1.35 GC523-83 0 m 0.64 2.05 1.35 GC523-84 0 m 0.90 1.79 1.11 GC523-88 0 m 0.53 1.71 Late Cenozoic 1.22 GC523-89 0 0.26 2.18 exhumation 1.52 GC523-10 19 m 1.34 1.71 1.8-2.0 km 1.13 GC523-6 0 m 0.88 1.38 1.44 1.6-1.8 km RD17-42 0 m 1.2 1.69 1.55 1.4-1.6 km RD17-47 0 m 1.43 1.67 0.88 1.2-1.4 km RD17-48 0 m 0.59 1.18 1.55 1.0-1.2 km RD17-45 0 m 1.43 1.67 1.55 0.8-1.0 km RD17-46 0 m 1.43 1.67 1.56 RD17-44 0 m 1.44 1.68 1.08 RD17-43 0 m 0.72 1.44 0.88 GC523-4 0 m 0.47 1.28 1.33 GC523-5 0 m 0.93 1.74 1.22 GC523-72 0 m 0.81 1.63 1.22 GC523-74 0 m 0.47 1.98 1.34 56°N GC523-79 0 m 1.16 1.51 1.34 GC523-78 0 m 0.81 1.86 0500 Quaternary reburial (m) Holford et al Figure 9
Time (Ma) Time (Ma) 800 400 300 200 100 0 350300250 200 150 100 50 0 Paleocene lavas b) 0 0 Upper Cretaceous RD17-42,45,46,47,48 RD17-44 GC523-2 Moine (Meso- Lower Jurassic 25 proterozoic) 25 Upper Triassic Upper Carboniferous RD17-43 50 50 Strontian Granite (Silurian) Moine (Mesoproterozoic) Fault GC523-1,2 75 75 Temperature (°C) Temperature Temperature (°C) Temperature
GC523-3 100 100 GC523-72 and GC523-74 Carboniferous GC523-4 125 125 Time (Ma) Time (Ma) 450400 350 300 250 200 150 100 50 0 10080 60 40 20 0 0 0 GC523-4 and GC523-5 RD17-44 Upper Cretaceous GC523-5 Strontian 25 Granite 25 Sound of Mull (Silurian) Temperature (°C) Temperature
(Underlain by Moine) 50 50 Loch Morvern ? Alineline
GC523-4 75 75 Temperature (°C) Temperature Temperature (°C) Temperature
GC523-72 100 100 GC523-74
a) 125 125 0 km 4
Time (Ma) c) 400 350 300250 200 150 100 50 0 0 Summary of events Upper Upper Upper Palaeocene Unroofing of Strontian Granite prior Carboniferous Triassic Cretaceous basalts 1 to Carboniferous (and Devonian?) sandstone sandstone sandstone sedimentation 2 Late Carboniferous-Permian burial 25 and heating Moine and Strontian samples from 3 Morvern consistent with cooling 5 7 from >100°C between 245-225 Ma 1 10 4 Early-mid Triassic cooling and 2 8 exhumation 50 5 Triassic-Jurassic burial and heating Early Cretaceous cooling not 4 required by these data (overprinted 6 by Palaeocene heating?) but exhumation likely based on regional 11 results 75 Late Cretaceous burial and heating Temperature (°C) Temperature 6 7 (beneath eroded Chalk cover) Latest Cretaceous uplift to produce 8 subaerial landsurface onto which lavas were erupted Early Cenozoic palaeotemperatures 100 of 90°C or above due to either burial 9 9 beneath a lava cover of hydrothermal and contact heating associated with 3 igneous intrusions 10 Burial by Cenozoic sediments? 125 65- 40-25, 11 Mid-late Cenozoic cooling from Regional cooling episodes 245-225 Ma 140-130, 110-90 Ma 60 Ma 20-15 Ma ~65-70°C due to regional exhumation Holford et al Figure 10
(a) Early Devonian - uplift (b) Mid Devonian-Carboniferous - subsidence Carboniferous sediments onlapping and covering parts of the Highlands?
Rapidly unroofed Newer Granites Devonian ORS Lower Palaeozoic- e.g. Outer Isles e.g. Moine Precambrian basement Thrust Thrust Extensional reactivation of Caledonian thrusts cf. DRUM seismic profile
(c) Late Carboniferous - uplift (d) Triassic-Jurassic - basin subsidence, basement uplift Folding, inversion and erosion Uplifting footwall Early Triassic uplift of Carboniferous sediments e.g. Outer Hebrides Jurassic onshore Scotland post-rift Sea-level
Devonian ORS Triassic syn-rift Basement (including e.g. Minch Devonian ORS) Fault
(e) Early Cretaceous - uplift (f) Late Cretaceous - subsidence Upper Cretaceous Chalk deposited Regional uplift and erosion occurring Jurassic over sedimentary basins and basement in multiple stages and leading to post-rift Sea-level removal of Jurassic sediments
Triassic Cretaceous Jurassic syn-rift Triassic post-rift post-rift syn-rift
Continuing extension to the northwest
(g) Early Cenozoic - magmatism and localised uplift (h) Mid-Late Cenozoic - uplift Quaternary glaciation and rejuvenation Sills and dykes intruded Central complex Erosion and removal Fissure-fed lava fields of Chalk and lava of topography into Mesozoic sediments e.g. Skye e.g. Mull Sea-level
Reactivation of Hot fluid circulation basin bounding Uplift of basins and faults? basement
Localized uplift associated with igneous activity Sample location Uranium Fission track age P [χ2] (number Mean track S.D. (μm) (number Sample number Stratigraphic age (Ma) (depth bsl if offshore) content (ppm) (Ma ± 1σ)a of grains) length (μm) of track lengths) West Orkney Basin GC523-24 BGS rockdrill 58-06/642 (4.7 m) Permian-Triassic (299-200) 4 76.5 ± 13.7 17 (20) 12.12 ± 0.60 2.55 (18) GC503-146 BGS BH72/37 (9-21 m) Triassic (251-200) 17 239.3 ± 22.8 <1 (19) 11.90 ± 0.19 1.87 (100) GC503-147 BGS BH73/31 (28-31 m) Permian-Triassic (299-200) 9 300.4 ± 23.9 <1 (20) 10.98 ± 0.20 2.04 (100) GC503-148 BGS BH72/34 (14 m) Lower Jurassic (200-176) 12 201.5 ± 32.1 <1 (12) 11.26 ± 0.35 2.48 (51) GC503-149 BGS BH73/29 (31-36 m) Permian-Triassic (299-200) 13 281.5 ± 15.2 14 (20) 11.50 ± 0.17 1.83 (118) GC503-150 BGS BH72/25 (15-19m) Permian-Triassic (299-200) 10 295.8 ± 23.7 <1 (20) 11.88 ± 0.17 1.74 (101) GC503-151 BGS BH78/07 (125-128 m) Permian-Triassic (299-200) 10 428.6 ± 33.0 <1 (20) 11.54 ± 0.18 1.84 (109)
202/19-1 GC503-84 274-427 m Upper Permian (260-251) 22 138.6 ± 17.7 <1 (20) 12.27 ± 0.23 2.00 (75) GC503-85 610-762 m Upper Permian (260-251) 13 162.5 ± 19.3 <1 (20) 13.21 ± 0.19 1.77 (84) GC503-86 975-1128 m Upper Permian (260-251) 15 138.1 ± 17.0 <1 (20) 12.30 ± 0.28 2.11 (57) GC503-87 1372-1524 m Upper Permian (260-251) 10 82.5 ± 9.1 6 (20) 11.88 ± 0.51 1.52 (9) GC503-88 1753-1758 m Upper Permian (260-251) 28 65.8 ± 4.9 5 (20) 12.99 ± 0.16 1.23 (56) GC503-89 1810-1819 m Upper Permian (260-251) 23 107.8 ± 16.6 <1 (20) 12.81 ± 0.33 1.58 (23) GC503-90 1999-2008 m Upper Permian (260-251) 17 70.1 ± 8.5 93 (20) 11.82 ± 0.60 1.88 (10) GC503-91 2316-2469 m Upper Permian (260-251) 26 58.5 ± 7.2 <1 (20) 12.03 ± 0.31 1.60 (27) GC503-92 2743-2895 m Upper Permian (260-251) 21 57.4 ± 6.0 <1 (20) 11.08 ± 0.62 2.24 (13)
Hebrides GC503-184 Balchrick Torridonian (1000-530) 24 176.4 ± 15.7 <1 (20) 12.41 ± 0.24 2.36 (100) GC503-185 Tolm, Lewis Permian-Triassic (299-200) 20 212.1 ± 16.0 <1 (20) 12.11 ± 0.28 2.10 (58) GC523-11 BGS BH71/11 (19-25 m) Permian-Triassic (299-200) 12 119.2 ± 13.2 <1 (20) 10.88 ± 0.29 2.65 (82) GC523-13 BGS BH71/13 (14.5-15 m) Torridonian (1000-530) 15 133.2 ± 32.0 <1 (15) 10.08 ± 0.54 2.21 (17) GC523-14 BGS BH71/14 (23-27 m) Permian-Triassic (299-200) 11 72.2 ± 18.8 <1 (13) 11.05 ± 0.95 3.16 (11) GC523-15 BGS BH72/32 (28-36 m) Permian-Triassic (299-200) 8 139.5 ± 23.0 4 (13) 12.08 ± 0.52 2.50 (23) GC523-23 BGS rockdrill 58-08/94 (1.85-2.6 m) Permian-Triassic (299-200) 17 160.1 ± 15.0 <1 (19) 11.63 ± 0.26 2.68 (106) GC523-47 BGS BH78/05 (54 m) Permian-Triassic (299-200) 7 187.7 ± 17.2 13 (20) 12.68 ± 0.19 1.93 (104) GC523-48 BGS BH88/04 (11.15-12.61 m) Permian-Triassic (299-200) 11 184.3 ± 22.7 <1 (20) 12.37 ± 0.22 2.30 (106) GC523-49 BGS BH88/08 (11.5-12.1 m) Permian-Triassic (299-200) 13 219.8 ± 20.2 3 (20) 11.45 ± 0.27 2.75 (106) GC523-51 BGS BH90/08 (17.25-12 m) Upper Jurassic (161-145) 27 270.3 ± 27.5 <1 (20) 12.11 ± 0.17 1.91 (125) GC523-52 BGS BH90/09 (18-22.3 m) Lower Cretaceous (145-100) 7 220.8 ± 19.0 71 (20) 12.35 ± 0.26 1.52 (34) GC523-55 BGS BH90/14 (91 m) Lewisian (>2500) 18 238.9 ± 22.1 <1 (20) 12.89 ± 0.15 1.65 (114) GC523-56 BGS BH90/16 (75-76.5 m) Permian-Triassic (299-200) 9 215.7 ± 16.9 7 (20) 11.80 ± 0.31 2.40 (62) GC523-80 Dornie, Loch Duich Moine (1100-800) 15 227.3 ± 15.2 21 (20) 11.39 ± 0.26 2.68 (106) GC523-93 Luskentyre, Harris Lewisian (>2500) 4 70.8 ± 9.2 17 (20) 14.25 ± 0.69 1.70 (6) GC523-95 Northton, Harris Lewisian (>2500) 11 201.4 ± 11.3 22 (20) 14.40 ± 0.15 1.48 (102) GC523-97 Meavag, Harris Lewisian (>2500) 4 180.9 ± 15.3 70 (20) 12.95 ± 0.36 2.13 (35) GC523-98 Bowglass, Harris Lewisian (>2500) 6 216.7 ± 15.2 17 (20) 13.91 ± 0.12 1.26 (111) GC523-110 Achmelvich Bay Lewisian (>2500) 2 147.7 ± 26.8 75 (20) 11.79 ± 0.35 2.37 (46) GC523-111 Loch Lurgainn Torridonian (1000-530) 18 296.1 ± 33.6 <1 (7) 12.15 ± 0.25 2.20 (77) GC523-112 Gruinard Bay Lewisian (>2500) 1 234.2 ± 76.7 89 (20) 12.13 ± 0.85 1.90 (5)
134/5-1 GC523-58 579-731 m Pliensbachian (190-183) 13 256.7 ± 41.3 9 (5) 12.11 ± 0.42 0.84 (4) GC369-1 930-1082 m Sinemurian (197-190) 26 81.1 ± 37.6 <1 (5) 10.11 ± 1.88 4.20 (5) GC369-2 1524-1676 m Hettangian-Sinemurian (200-190) 16 57.7 ± 33.0 <1 (4) 14.30 ± 0.27 0.38 (2) GC523-68 1829-1966 m Triassic (251-200) 9 74.5 ± 37.5 <1 (8) 12.46 ± 0.45 1.09 (6) GC369-3 1966-2079 m Triassic (251-200) 16 19.6 ± 4.9 <1 (19) 10.30 ± 0.35 1.88 (29) GC369-4 2164-2255 m Triassic (251-200) 33 8.7 ± 2.1 <1 (22) 11.24 ± 0.47 1.04 (5) GC369-5 2255-2347 m Triassic (251-200) 32 7.6 ± 0.8 14 (20) 10.54 ± 0.79 3.06 (15)
Skye, Mull and Morvern GC523-1 Loch Aline, Morvern Triassic (251-200) 18 229.2 ± 13.6 63 (20) 10.66 ± 0.21 2.13 (102) GC523-2 Loch Aline, Morvern Moine (1100-800) 18 206.4 ± 14.3 52 (20) 10.43 ± 0.21 2.25 (110) GC523-3 Loch Aline, Morvern Triassic (251-200) 18 208.1 ± 12.3 69 (20) 10.75 ± 0.21 2.17 (107) GC523-4 Loch Aline, Morvern Cenomanian (100-94) 21 221.2 ± 14.5 79 (20) 10.99 ± 0.22 2.18 (102) GC523-5 Loch Aline, Morvern Cenomanian (100-94) 15 219.2 ± 18.2 <1 (17) 10.82 ± 0.37 2.19 (35) GC523-6 Glenancross Moine (1100-800) 8 59.5 ± 7.6 98 (20) 11.43 ± 0.27 2.70 (103) GC523-8 Glenancross Moine (1100-800) 15 177.8 ± 11.5 21 (20) 10.82 ± 0.23 2.29 (103) GC523-9 BGS BH71/09 (42-46 m) Devonian (416-359) 5 231.1 ± 23.0 23 (20) 10.87 ± 0.27 2.01 (57) GC523-10 BGS BH71/10 (20-22 m) Jurassic (200-145) 26 60.6 ± 15.7 <1 (14) 13.04 ± 0.17 1.83 (115) GC523-71 Ganavan Bay Lower-Middle Devonian (416-385) 11 228.4 ± 26.5 <1 (19) 12.08 ± 0.24 2.30 (88) GC523-72 Inninmore Bay, Morvern Visean (345-328) 13 157.0 ± 11.0 33 (20) 10.83 ± 0.42 2.80 (44) GC523-74 Inninmore Bay, Morvern Visean (345-328) 10 142.6 ± 14.5 <1 (20) 10.53 ± 0.33 3.44 (108) GC523-77 Fionnphort, Mull Devonian (416-359) 7 203.7 ± 16.9 9 (20) 11.48 ± 0.22 2.20 (101) GC523-78 Bunessan, Mull Moine (1100-800) 24 151.6 ± 9.1 28 (20) 10.12 ± 0.26 2.75 (110) GC523-79 Balnahard, Mull Triassic (251-200) 11 67.9 ± 5.5 20 (20) 12.58 ± 0.22 2.27 (102) GC523-83 Broadford Bay, Skye Lower Jurassic (200-176) 21 52.3 ± 6.0 <1 (20) 13.53 ± 0.29 1.66 (32) GC523-84 Broadford Bay, Skye Lower Jurassic (200-176) 16 54.7 ± 4.7 3 (20) 13.91 ± 0.24 1.53 (40) GC523-88 Drocaid Lusa, Skye Torridonian (1000-530) 23 61.9 ± 4.4 19 (20) 13.43 ± 0.41 2.20 (29) GC523-89 Loch Slapin, Skye Middle Jurassic (176-161) 12 71.2 ± 8.2 <1 (20) 14.39 ± 0.23 1.55 (45) GC523-91 Cairidh Ghlumaig, Skye Middle Jurassic (176-161) 13 219.2 ± 18.4 <1 (20) 12.11 ± 0.30 2.53 (72) GC523-99 Staffin, Skye Middle Jurassic (176-161) 22 66.4 ± 4.9 9 (20) 13.92 ± 0.27 0.99 (13) GC523-102 Culnacnoc, Skye Middle Jurassic (176-161) 19 68.0 ± 6.5 <1 (20) 14.14 ± 0.18 1.12 (39) GC523-105 Cuaig Torridonian (1000-530) 26 60.6 ± 15.7 <1 (14) 12.60 ± 0.32 2.44 (60) GC523-106 Applecross Lower Jurassic (200-176) 13 275.7 ± 18.9 8 (19) 10.85 ± 0.20 2.11 (108) GC523-108 Applecross Permian-Triassic (299-200) 16 179.6 ± 34.6 <1 (10) 10.19 ± 0.43 2.21 (27) GC523-109 Applecross Torridonian (1000-530) 13 270.9 ± 22.2 4 (20) 10.35 ± 0.26 2.69 (108) RD17-41 Fort Augustus Moine (1100-800) 29 179.8 ±10.6 <1 (20) 11.98 ± 0.14 1.41 (104) RD17-42 Strontian (A884) Strontian Granite (420) 37 207.9 ± 8.5 96 (20) 11.76 ± 0.13 1.43 (120) RD17-43 Coire nam Muc (A884) Moine (1100-800) 36 213.7 ± 9.1 23 (20) 11.89 ± 0.14 1.48 (109) RD17-44 Beinn Chlaonleud (A884) Strontian Granite (420) 32 187.7 ± 9.2 10 (20) 11.96 ± 0.11 1.13 (100) RD17-45 Beathrach (A884) Strontian Granite (420) 32 195.1 ± 13.0 <1 (20) 12.02 ± 0.11 1.08 (102) RD17-46 Beathrach (A884) Strontian Granite (420) 32 204.1 ± 12.3 <1 (20) 12.14 ± 0.12 1.28 (108) RD17-47 Liddesdale (A884) Strontian Granite (420) 30 202.9 ± 12.6 <1 (20) 12.28 ± 0.13 1.27 (101) RD17-48 Liddesdale (A884) Strontian Granite (420) 28 188.6 ± 11.1 <1 (20) 12.02 ± 0.14 1.40 (100)
Table 1. Apatite fission track analysis sample details aCentral age (Galbraith and Laslett, 1993) used for samples containing a significant spread in single grain ages (P[χ2]>5%), otherwise the “pooled age” is quoted. All ages were calculated using the zeta calibration approach of Hurford and Green (1983) using zeta values of 336.0±6.0 for samples GC369-1 to 5 (SRM612 dosimeter glass), 380.4±5.7 for samples GC503-84 to 92 (CN5 dosimeter glass), 354.8±6.3 for GC503-184 and 185 (SRM612), 386.9±6.9 for GC523-88 to 102 (CN5), 378.4±5.5 for GC503-146 to 151 (CN5), 392.9±7.4 for GC523-1 to 84 and GC523-105-112 (CN5), 348.8±5.2 for RD17-41 and 44-48 (SRM612) and 360.3±6.8 for RD17-42 and 43 (SRM612). All errors quoted at ±1σ. All analytical details are as described by Green et al. (2001), with the exception that the thermal neutron irradiation showed a significant flux gradient, and the appropriate value of ρD was determined by linear interpolation through the stack of grain mounts.
Predicted FTA Predicted Cooling episodes from DTH MTL from Pre-Cretaceous Cretaceous Early Cenozoic Mid-Late Cenozoic Sample (Ma); DTH (μm); Max. Max. Max. Max. number Onset of Onset of Onset of Onset of measured FTA measured palaeotemperature palaeotemperature palaeotemperature palaeotemperature cooling (Ma) cooling (Ma) cooling (Ma) cooling (Ma) (Ma) MTL (μm) (°C) (°C) (°C) (°C) West Orkney
Basin GC523-24 208-290 (76.5) 14.7 (12.12) ≥105 200-75 90-100 65-15 GC503-146 146-210 (239.3) 14.7 (11.90) 85-95 190-185 50-70 50-0 GC503-147 208-290 (300.4) 14.7 (10.98) 85-95 180-130 45-80 30-0 GC503-148 178-208 (201.5) 14.7 (11.26) 90-100 185-55 45-80 55-0 GC503-149 208-290 (281.5) 14.7 (11.50) 85-110 ≥45 35-85 55-0 GC503-150 208-290 (295.8) 14.7 (11.88) 80-90 190-45 30-70 45-0 GC503-151 208-290 (428.6) 14.7 (11.54) 80-105 ≥50 30-80 70-0
202/19-1 GC503-84 246 (138.6) 14.7 (12.27) 95-100 155-80 40-65 55-0 GC503-85 246 (162.5) 14.4 (13.21) ≥100 180-70 50-90 120-0 GC503-86 246 (138.1) 14.1 (12.30) ≥100 150-70 50-90 90-0 GC503-87 243 (82.5) 13.7 (11.88) ≥110 200-90 GC503-88 238 (65.8) 13.2 (12.99) ≥110 130-70 GC503-89 239 (107.8) 13.2 (12.81) ≥120 130-80 60-105 105-0 GC503-90 236 (70.1) 12.8 (11.82) ≥120 130-70 60-110 80-0 GC503-91 225 (58.5) 12.0 (12.03) ≥110 110-50 80-105 55-0 GC503-92 200 (57.4) 10.8 (11.08) ≥115 180-70 90-110 65-10 Overlap 110-90 Ma 30-15 Ma
Hebrides GC503-184 570 (176.4) 14.7 (12.41) 100-105 180-110 75-85 80-40 GC503-185 208-290 (212.1) 14.7 (12.11) 75-90 155-30 GC523-11 208-290 (119.2) 14.7 (10.88) 95-100 75-30 55-80 15-0 GC523-13 800-1100 (133.2) 14.7 (10.08) 115-125 270-90 90-100 80-0 GC523-14 208-290 (72.2) 14.7 (11.05) 100-105 90-10 GC523-15 208-290 (139.5) 14.7 (12.08) 105-115 200-100 80-95 90-20 GC523-23 208-290 (160.1) 14.7 (11.63) 95-105 140-75 60-85 50-0 GC523-47 208-245 (187.7) 14.7 (12.68) 90-105 205-130 55-70 70-5 GC523-48 208-245 (184.3) 14.7 (12.37) 95-105 180-95 55-75 55-0 GC523-49 208-245 (219.8) 14.7 (11.45) 95-100 155-75 45-70 30-0 GC523-51 208-290 (270.3) 14.7 (12.11) 80-95 260-100 50-70 40-0 GC523-52 208-290 (220.8) 14.7 (14.51) 80-105 270-80 45-75 70-0 GC523-55 1100-2900 (238.9) 14.7 (12.89) 95-105 270-170 50-65 50-0 GC523-56 208-245 (215.7) 14.7 (11.80) 85-95 170-70 45-65 25-0 GC523-80 800 (227.3) 14.7 (11.39) 100-105 235-120 70-80 80-10 GC523-93 1100-2900 (70.8) 14.7 (14.25) 100-110 100-40 GC523-95 1100-2900 (201.4) 14.7 (14.40) ≥120 225-175 45-70 100-55 GC523-97 1100-2900 (180.9) 14.7 (12.95) 105-110 240-140 45-75 100-0 GC523-98 1100-2900 (216.7) 14.7 (13.91) ≥105 270-190 45-65 120-20 GC523-110 1100-2900 (147.7) 14.7 (11.79) 100-110 250-70 75-90 65-5 GC523-111 800-1100 (296.1) 14.7 (12.15) 90-100 270-100 50-75 80-5 GC523-112 1100-1900 (234.2) 14.7 (12.13) 70-100 240-0 70-100 240-0 70-100 240-0 Overlap 225-190 Ma 140-130 Ma 75-40 Ma 15-10 Ma
134/5-1 GC523-58 184 (256.7) 14.0 (12.11) 45-90 195-0 GC369-1 180 (81.1) 13.5 (10.11) 95-100 125-0 GC369-2 167 (57.7) 12.7 (14.30) ≥125 85-25 GC523-68 129 (74.5) 9.9 (12.46) ≥100 245-20 GC369-3 96 (19.6) 10.3 (10.30) ≥120 50-20 GC369-4 29 (8.7) 10.0 (11.24) ≥115 65-10 GC369-5 18 (7.6) 10.6 (10.54) ≥125 45-10 Overlap 85-25 Ma 45-20 Ma
Skye, Mull
and Morvern GC523-1 208-290 (229.2) 14.7 (10.66) 85-90 70-10 GC523-2 800 (206.4) 14.7 (10.43) 100-120 370-100 80-95 70-10 GC523-3 208-290 (208.1) 14.7 (10.75) 80-90 65-10 GC523-4 90-97 (221.2) 14.7 (10.99) 90-105 90-25 30-65 30-0 GC523-5 90-97 (219.2) 14.7 (10.82) 90-110 97-30 50-85 30-0 GC523-6 800 (59.5) 14.7 (11.43) ≥100 70-50 45-65 25-0 GC523-8 800 (177.8) 14.7 (10.82) 100-105 255-100 85-95 30-10 GC523-9 363-403 (231.1) 14.7 (10.87) ≥100 400-210 85-95 95-5 GC523-10 146-208 (60.6) 14.7 (13.04) ≥125 70-50 65-80 25-10 GC523-71 386-408 (228.4) 14.7 (12.08) 110-120 285-190 80-90 90-35 GC523-72 333-350 (157.0) 14.7 (10.83) 95-100 100-25 45-80 25-0 GC523-74 333-350 (142.6) 14.7 (10.53) 100 70-20 30-95 25-0 GC523-77 363-408 (203.7) 14.7 (11.48) 95-105 305-135 80-90 60-20 GC523-78 800 (151.6) 14.7 (10.12) 95-100 60-20 45-90 20-0 GC523-79 208-245 (67.9) 14.7 (12.58) 100-105 75-35 60-75 25-5 GC523-83 178-208 (52.3) 14.7 (13.53) ≥115 65-40 35-90 40-0 GC523-84 178-208 (54.7) 14.7 (13.91) ≥120 70-50 45-80 45-10 GC523-88 800-1100 (61.9) 14.7 (13.43) ≥110 75-55 30-75 50-0 GC523-89 157-178 (71.2) 14.7(14.39) ≥120 95-60 20-95 65-0 GC523-91 157-178 (219.2) 14.7 (12.11) 85-95 135-55 30-75 80-10 GC523-99 157-178 (66.4) 14.7 (13.92) ≥105 85-50 30-65 45-0 GC523-102 157-178 (68.0) 14.7 (14.14) ≥110 80-55 30-65 45-0 GC523-105 800-1100 (60.6) 14.7 (12.60) 115-125 90-60 55-75 40-0 GC523-106 178-208 (275.7) 14.7 (10.85) 85-90 80-20 25-65 20-0 GC523-108 208-290 (179.6) 14.7 (10.19) 90-100 65-0 GC523-109 800-1100 (270.9) 14.7 (10.35) 95-100 280-50 55-90 40-0 RD17-41 800 (179.8) 14.7 (11.98) >100 245-195 80-90 175-60 55-75 30-0 RD17-42 420 (207.9) 14.7 (11.76) >100 280-225 75-95 225-25 60-80 25-0 RD17-43 800 (213.7) 14.7 (11.89) >100 280-170 75-90 175-25 40-70 25-0 RD17-44 420 (187.7) 14.7 (11.96) >100 260-205 70-85 205-30 70-80 30-0 RD17-45 420 (195.1) 14.7 (12.02) >100 270-225 70-80 30-0 RD17-46 420 (204.1) 14.7 (12.14) >100 265-225 70-80 35-0 RD17-47 420 (202.9) 14.7 (12.28) >100 270-220 70-80 45-0 RD17-48 420 (188.6) 14.7 (12.02) >100 255-200 80-85 100-25 35-65 20-0 Overlap 245-225 Ma 175-100 Ma 65-60 Ma 20-10 Ma
Table 2. Summary of palaeotemperatures analysis and Default Thermal History interpretation