title: A study of the tectonic and geomorphic history of Scotland using a joint apatite fission track and (U-Th-Sm)/He analysis approach
Creators: name: Amin, Awara
Publisher: name: University of Glasgow
Year: 2020

description: Scotland is part of the NE Atlantic passive margin with geological features and land- scapes that are evidence of a long and complex geological history. A prolonged period of extension, related to the rifting and breakup of Pangea, began in the Late Carbonif- erous and continued through the Mesozoic until breakup and the formation of oceanic crust occurred in the NE Atlantic in the Early Cenozoic. There is evidence that the Scottish landscape and surrounding basins experienced significant uplift in the Early Paleogene. However, whether this uplift can be attributed to tectonic processes re- lated to rifting or to the movement of the hot Iceland mantle plume past the west of Scotland and Northern Ireland at around 65 Ma, is still unresolved. It also remains unknown to what degree the Caledonian Highland mountains in northern Scotland, produced during the Caledonian Orogeny in the Late Devonian, have retained their original topography since they formed. To investigate long term rates of landscape evolution temporally and spatially across Scotland it is essential to constrain the timing and both the rate and amount of denudation during different stages following the Caledonian Orogeny to determine the more likely cause of uplift and erosion, especially in Late Cretaceous-Early Palaeogene. For that, this study employs apatite fission track and apatite (U-Th-Sm)/He thermochronology on samples collected from vertical profiles and from a regional NW- SE transect from the Outer Hebrides in the west towards the central and eastern Grampian Highland. In total 51 rock samples were collected. AFT analysis was performed for 50 of them using EDM and LA-ICP-MS recently installed at the University of Glasgow. For AHe analysis, 281 grains were analysed in total. During the analysis the single grain ap- proach was performed, and the new approach of treating broken grains introduced by Brown et al. (2013) and Beucher et al. (2013) was performed when samples were modelled using QTQt (5.7.0). In addition, the most recent algorithm of Gautheron et al. (2009) and Flowers et al. (2009) for radiation damage effect were incorporated to model AHe data, and Dpar was used to account for impact of chemical composition on fission track annealing. Along the study area from west to east AFT ages vary between 297.9±29.4 to 166±16.7 Ma, and the oldest age has been observed in the Central Highland. AHe mean ages are between 237.1±72.8 and 19.9±4.1 Ma. The oldest AHe mean ages have been recorded by areas around Cairngorms and Lochnagar, especially top of Lochnagar sample has provided the oldest AHe mean age across the study area. Generally measured mean track lengths varies between 11.11μm to 13.04μm along the study area, and the short- est mean track lengths have been recorded in the area around the Cairngorms. Length projection process increased mean track lengths and the maximum mean track lengths reached 14.21μm in the Outer Hebrides, and 13.99μm for the high elevation samples from the vertical profiles in the Grampian Highlands. Thermal history modelling, including the individual thermal models and joint thermal models for vertical profiles, suggest that the Scottish landscape has experienced several cooling pulses following the Caledonian Orogeny; Mid to Late Devonian (380-360 Ma), Late Carboniferous-Early Triassic (c.300-250 Ma), Early to Mid Cretaceous (120-100 Ma), and Late-Cretaceous-Early Paleogene pulse (c.80-60 Ma). Based on the predicted cooling pulses the Scottish landscape has been divided into three separate regions, the Outer Hebrides, east of the Grampian region, and west of the Grampian region. The first cooling pulse which is the oldest cooling pulse across the study area has only been predicted by samples collected around the Cairngorms, and Lochnagar (east of the Grampian Highlands). It is interpreted as an erosional cooling pulse initiated following the Caledonian Orogeny and is linked to isostatic uplift after orogenic crustal thicken- ning and granite intrusion during the Caledonian Orogeny. The Second cooling pulse seems to be regional as it has been predicted by almost all samples across the study area. Event though this pulse starts in some samples at the end of Carboniferous, majority of the samples have suggested Permian into Triassic. This pulse of cooling is coeval with the erosion and deposition of significant amount of Permo-Trassic sed- iments in the basins around Scotland (e.g. Minch Basin, Sea of the Hebrides Basin, Moray Firth Basin). During Late Permian-Early Triassic to Early Cretaceous Scotland has been through monotonic cooling and erosion based on the derived thermal models in this study. During this period uplift should have been less than the earlier pulse but the Scottish Highland has kept feeding the surrounding basins but at lower rates. The Early to Mid Cretaceous pulse which has been only observed from the vertical profile samples in the Grampian Highland region (e.g. Ben Starav, Aonach Mor, and Lochnagar) is synchronous with continental break up and sea floor spreading in the Central Atlantic region and associated rift propagation to the NE Atlantic region. Before the last cooling pulse starts, the east and west coast of the Scottish Highland possibly have been buried due to marine transgression and deposition of chalk which reached a peak at around Campanian (c. 80 Ma). Following that, the last cooling pulse starts in Late Cretaceous-Early Paleogene based on the new derived thermal models. This pulse is more likely to be linked to the proto Iceland mantle processes (including dynamic up- lift, thermal perturbation, and magmatic underplating) rather than far field stress from the continental break up at around 55 Ma, based on the time of cooling and spatial distribution of denudation associated with this pulse. However, it is clear that other geological factors like lithology and possibly fault reac- tivation have controlled the local denudation amount, therefore there is not a smooth transition in denudation amount from west to east, but still the interior part has been affected by less uplift and denudation in Late Cretaceous-early Palaeogene compared with the western regions. The arrival of hot material from the Iceland mantle pulse might be expected to have changed the thermal temperature regime at least in the west part of Scotland, but the calculated thermal gradients in this study shows that the thermal regime has not been perturbed in Late Cretaceous-Early Palaeogene when the cooling started. This is possibly because the arrival of hot mantle material and magmatism from the Iceland mantle plume stayed the the lower most part of the crust, as only when it intrudes at shallow crustal level, can it make a significant perturbation to the temperature regime in the upper most crust.