Red-bed sequences of known or inferred Permian and Triassic age are preserved in numerous offshore basins to the north and west of Scotland. These developed following the Late Carboniferous Variscan Orogeny, as compressive uplift was superseded by a tensional stress regime causing the fragmentation of the Variscan foldbelt and its northern foreland. Basin formation persisted through the Permian and Triassic periods, and the sediment infills reflect syn-and post-depositional fault activity. Syndepositional basins take the form of deep, fault-bounded half-grabens whose development was controlled by the extensional reactivation of earlier (Precambrian/Caledonian) thrusts and transfer faults. These basins accumulated wedge-shaped packages of sediment dipping into major basin-bounding faults. In contrast, post-depositional basins are characterized by a parallel-bedded infill which appears to reflect local preservation as a result of later faulting. These basins may be Late or post-Triassic in age. The Permo-Triassic rocks were deposited during a period of major regression, with fluviatile, lacustrine and aeolian environments predominating. Principal lithologies include sandstones, that are partly conglomeratic, intercalated with mudstones and claystones. The localized development of carbonates and evaporites indicates some marine influence. Palaeontological data are commonly sparse throughout the succession and generally cannot readily differentiate the two systems. There is limited evidence for both Early Permian and Late Triassic volcanism. The Early Permian volcanism was 'within-plate' and 'continental' in character, and was probably related to the first phase of post-Variscan continental disintegration. The Late Triassic volcanism indicates a subsequent episode of synrift igneous activity west of Britain and Ireland.

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The 6 km thick Ordovician Borrowdale Volcanic Group is readily divisible into a lower 2.2–2.7 km thick predominantly pre-caldera succession dominated by basalt, andesite and dacite sheets, and an upper succession of caldera-related ignimbrites and volcaniclastic sedimentary rocks of c. 3 km thickness. The lower Borrowdale Volcanic Group rocks, here included within a single lithostratigraphical unit, the Birker Fell Formation, affords a well-exposed section through a pre-caldera sequence. The Birker Fell Formation is dominated by andesites which comprise 60% of the stratigraphy. Thin sequences of reworked volcanic detritus are commonly interbedded with the andesites; locally there are thicker units of volcaniclastic sedimentary rocks. Interpretation of the stratigraphy and eruptive history of the formation has been aided by the recognition of some distinctive lithological units including: (1) a 200-600 m thick sequence of weakly parallel-bedded basaltic tuffs resting on the pre-volcanic basement, (2) a 200-600 m sequence of single flow unit aa basalts, (3) dacite flows and ignimbrites up to > 1100 m thick in the middle and upper parts of the formation and (4) aphyric to pyroxene-phyric, simple and compound flows of basalt, 50-400 m thick, locally present in the uppermost parts. Analysis of the facies comprising the Birker Fell Formation indicates that it was emplaced as a sub-aerial, plateau-andesite sequence, formed by the coalescence of products erupted from a number of centres or fissures in an extensional, subsiding volcano-tectonic rift zone. Volcano-tectonic faulting and eruption of thick ignimbrites locally influenced development of this field.

Synopsis Pressures and temperatures have been derived from Dalradian pelitic schists within the area from Tomintoul to the Banffshire coast, predominantly in terrain west of the Portsoy–Duchray Hill lineament and straddling the andalusite–kyanite inversion isograd. These indicate that pressures up to 8–9 kb occur at the base of the Dalradian sequence compared to 4–5 kb immediately west and east of the lineament. There is also a sharp thermal break across the lineament. Replacement of andalusitc by kyanite at Portsoy and further south to the west of the lineament implies that rocks in this area underwent a significant pressure increase. P–T path calculation using garnet zoning confirms this and indicates the increase to have been about 2 kb. Following Baker (1987) it is postulated that the Portsoy lineament represents a thrust plane and that low-P, high-T Dalradian rocks of the Buchan terrain were thrust westwards, deformation that can be related to the local D2 episode. This event postdated emplacement of the Newer Basic masses. Later D3 movements resulted in rapid differential exhumation of the Dalradian which was partly taken up by further shear movement on the Portsoy lineament steepening it to its present attitude.

Mafic dykes occur in close association with, and both cut and are cut by, the Eskdale granite in the south-western Lake District. The dykes range compositionally from magnesian basalt to andesite and are divided into two groups: (1) high-Fe-Ti rocks of tholeiitic affinity forming most of the dykes and (2) a lower-Fe-Ti group, comparable in composition to the lavas of the Borrowdale Volcanic Group. The dykes extend the range of tholeiitic magmatism in the Lakes into late Ordovician, and possibly Silurian times, and indicate that published plate tectonic models partly based on the distribution of magma types are perhaps over-simplified. The Eskdale dykes form one end of a spectrum of Lake District compositions from tholeiitic to calc-alkaline. All the magma types may have shared a common mantle source, their final composition reflecting residence times in the crust or LIL-enriched mantle.