Palaeogene and later structure, Palaeogene volcanic districts of Scotland
|Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
The Palaeogene and later structures of the Inner Hebrides, the adjoining mainland, and Arran were strongly influenced by structures inherited from Mesozoic and earlier times (Chapter 4). These earlier structures include major, north-east-trending faults and lineaments dating from the Caledonian Orogeny or earlier, and large, predominantly north—south-orientated Mesozoic sedimentary basins with their bounding faults (P914123). There is evidence that at least some of these structures were reactivated during the Paleocene volcanism and it has been postulated that they were major factors in determining the sites of Paleocene central complexes (e.g. Richey, 1961) and the thick lava successions (Walker, 1979). Parts of the lava fields are extensively faulted, and localised faulting and folding occurs within and around the central complexes.
The central complexes of Skye and Rum are situated on, or close to, the north—south-trending Camasunary—Skerryvore Fault, which defines the western edge of the Inner Hebrides Basin. At Camasunary, on Skye, the fault affects Torridonian and Mesozoic strata but there is little, if any, displacement of the overlying Paleocene lavas, which are assumed to belong to the Skye Lava Group (p. 67). Beyond the Blà Bheinn area, the northern continuation of the fault is uncertain, but it has been suggested that the Loch Screapadal Fault on Raasay represents its northern continuation. This is a major, post-Mid Jurassic fault that is offset by a zone of north-west-trending sinistral strike-slip faults between Raasay and the Red Hills (Butler and Hutton, 1994). South of Skye, there is evidence of Paleocene, or post-Paleocene, movement on the continuation of the Camasunary Fault, since gently south-west-dipping flows of the Eigg Lava Formation crop out on Eigg but not on the east coast of Rum (except as faulted slivers in the central complex; Chapter 9). Still farther south, both Coll and Tiree are formed of Lewisian gneisses although down-faulted Paleocene lavas occur beneath the sea a short distance to the east of both islands (P914119; P914127).
The Great Glen Fault cuts across south-east Mull, where it has disturbed Mesozoic and earlier rocks but not the Paleocene lavas (Chapter 4). There is, however, evidence of displacements on this fault system that post-date dykes near Mull (Holgate, 1969). Basaltic dykes, of presumed Palaeogene age, intrude the Caledonian Strontian Granite and Moine rocks on the west side of Loch Linnhe. The probable south-east continuation of this swarm on Lismore appears to have been offset dextrally by the intervening Great Glen Fault, with further dextral offset by the north-east-trending Lynn of Lorn Fault, situated between Lismore and the mainland to the south-east. There has been a total of almost 30 km of post-Paleocene dextral displacement of the dyke swarm, 20 km of which was on the Lynn of Lorn Fault. From an analysis of the Neogene drainage pattern on both sides of the Great Glen, Holgate (1969) suggested that the final dextral movement was considerably later than the emplacement of the Mull Central Complex. As a consequence, the south-east continuation of the Mull Dyke Swarm also should be offset dextrally, and be represented by dykes well to the south and south-west of Oban. However, there is no indication of any offset of the pronounced linear magnetic anomalies associated with the Mull Dyke Swarm to the south-east of Mull, which head directly towards the Mull Central Complex. Furthermore, the dense dyke swarm at Oban and to the south-east includes significant numbers of felsic dykes and a few composite dykes, which are indicators of proximity to an associated central complex. Thus, the available evidence indicates that, although the southern extension of the Skye Dyke Swarm may have been displaced, little movement has occurred on the Great Glen Fault since the emplacement of the Mull Central Complex and the Mull Dyke Swarm.
Structure of the lava fields
The lava field of northern Skye is dissected by numerous faults and has been gently folded. The lavas have been broken into a large number of blocks by fairly major, continuous faults trending between north-west and north—south, and shorter faults trending approximately north-east. The fault pattern is well displayed in west-central Skye where the lava stratigraphy is known in detail (Williamson and Bell, 1994). The broad structure of the lava field is interpreted as an early, shallow north-west-trending syncline, the core of which is occupied by the more evolved lavas. This earlier folding is attributed to loading that resulted from lava accumulation. Later folds developed, along a north—south to north-east trend and were related to Oligocene and younger events; one such syncline is responsible for the preservation of Oligocene strata in the offshore Canna Basin (England, 1994).
Eigg and Canna
The flows of both the Eigg and the Canna lava formations are either flat-lying or dip at low angles. Normal faults are common. On Eigg there is evidence that a north-north-west-trending fault that affected the Mesozoic strata was reactivated after accumulation of the flows of the Eigg Lava Formation (Emeleus, 1997).
The detailed stratigraphy of the Mull Lava Field has not been determined (Table 10), and therefore only the broad outline of the structure is known. The lavas of north-west Mull, Ulva, the Ardmeanach peninsula and the eastern part of the Ross of Mull, and the thick Ben More succession are generally flat-lying or dip gently, although evolved lavas south-east of Ben More are involved in folding associated with the emplacement of the central complex (see below). Away from the central complex, the base of the lavas crops out in the south at Carsaig Bay, in the west at Gribun and in the north at intervals on the coast between Tobermory and Bloody Bay. This may indicate that the lavas form a broad north-north-west-trending syncline in north-west Mull. Several north-north-west- to north-west-trending faults occur on the island of Ulva, towards the western end of the Ardmeanach peninsula and south of Loch Scridain. On the Ross of Mull, near Ardtun, the Paleocene lava outcrop terminates at the west-north-west-trending Loch Assapol Fault, where they are thrown down against Moine metasedimentary rocks.
The lavas on Morvern are cut by north-north-west- to north-trending faults, and are faulted out in the east at the north-trending Inninmore Fault where they are thrown down against Moine psammitic gneisses. The base of the lavas drops from an altitude of about 400 m on Beinn Iadain in the east to sea level near Auliston Point in the west, a distance of about 13 km. Much of the change in altitude is the result of faulting but some is due to postdepositional tilting of the lavas. Near to the Inninmore Fault, the lavas directly overlie Cretaceous, Jurassic, Triassic and Moine rocks, indicating a measure of uplift and erosion before the lavas were erupted. Elsewhere in Morvern, the lavas generally rest on the thin Gribun (Beinn Iadain) Mudstone Formation, which in turn usually rests on the equally thin Upper Cretaceous Clach Alasdair Conglomerate Member (compare with Hancock, 2000; Mortimore et al., 2001). There is thus only slight evidence for erosion between the Late Cretaceous and the Paleocene in this district.
Structures associated with the central complexes
Country rocks adjacent to the central complexes are commonly folded and faulted as a consequence of the emplacement of the intrusive units. These features are most apparent around centres where granites or other silicic intrusions are present (Chapter 9).
The lavas and earlier rocks around the Cuillin Centre on Skye are generally flat lying and appear little disturbed, except for gentle folding in Glen Drynoch. However, there is considerable disruption of country rocks adjoining both the Western and Eastern Red Hills centres. Paleocene lavas, Mesozoic sedimentary rocks and Torridonian sandstones on the northern slopes of Glamaig and on Scalpay appear to define part of a dome over the Western Red Hills granites. In the Eastern Red Hills there is a suggestion of doming of Mesozoic and earlier sedimentary rocks over the Beinn na Caillich Granite. The Beinn an Dubhaich Granite occupies the core of the Broadford Anticline, an anticlinal structure in Lower Palaeozoic Durness Group dolostones. However, the granite cuts across, and hence postdates, the folds (Holroyd, 1994). Folding of lavas and Mesozoic rocks in the Coire Uaigneich area, west of Loch Slapin, is attributed to granite emplacement (Butler and Hutton, 1994).
The gentle regional dip of the Torridonian sandstone towards the west-north-west to north-west is greatly disturbed on the northern margin of the Rum Central Complex (P914142; P914143). Where they are in faulted contact with the Western Granite and the Northern Marginal Zone, the beds dip outwards at over 50° towards the north-west in the west, changing progressively to a north-east direction in the east. There is little folding or tilting of the strata next to the Eastern Layered Intrusion, but large masses of Torridonian sandstone at Mullach Ard, at Welshman's Rockand in parts of southern Rum appear to have slid away from the area of the central complex, with minor folding in places (p. 110). These structures formed during the early stages in the growth of the central complex, when central uplift led to doming, and major peripheral faulting initiated the Main Ring Fault. The central complex is cut by the north—south-trending Long Loch Fault. There is evidence of movement on this fault both before and after the emplacement of the central complex, and that it acted as a feeder zone during emplacement of the layered intrusions.
On the eastern margin of the Ardnamurchan Central Complex, the north-north-west-trending Loch Mudle Fault throws down Paleocene lavas and Mesozoic rocks to the east (P914145a). Apart from this and some other minor faulting, there is little sign of disturbance of the lavas and earlier rocks. At the western end of the peninsula, however, the Mesozoic sedimentary rocks generally dip away from Centre 2 and the northern edge of Centre 3, suggesting a dome-like structure. In contrast to the doming on Skye and Rum, this appears to be associated with predominantly gabbroic intrusions; however, there must have been a substantial contribution to uplift by the intrusion of the multitude of cone-sheets (e.g. LeBas, 1971).
One of the most notable structural features of the Hebridean Igneous Province is the set of concentric folds that almost encircle the Glen More and Beinn Chaisgidle centres on Mull (Bailey et al., 1924). The folds are developed in the surrounding older lava sequence and the various subjacent pre-Paleocene rocks. They include the Loch Spelve and Loch Don anticlines and the Duart Bay and Coire Mòr synclines (P914146). South of Loch Don, Dalradian metalimestones and phyllites form the core of a north-trending anticline, flanked successively by late-Silurian andesitic lavas, Lower and Middle Jurassic sandstones and mudstones, and Paleocene lavas. These major structures continue to the north and north-west, where Jurassic rocks form the core of the Craignure Anticline as far as Craignure Bay, with Moine rocks exposed in the core between Craignure Bay and Scallastle Bay. Elsewhere, the folding generally involves Paleocene basaltic lavas and, on the eastern flanks of Ben More, mugearites near the top of the lava succession. It is suggested that the folds formed in response to the early intrusive events in the central complex, but their age is not well understood. Furthermore, the area enclosed by the folds is domed and the folds may owe their origin to gravity-driven movement as the dome developed. From evidence in the Loch Don area, it has been suggested that doming and folding were initiated prior to eruption of the Paleocene lavas (Cheeney, 1962; Walker, 1975a), but Bailey (1962) considered the evidence to be inconclusive. Near Sgurr Dearg, the folds are cross-cut by, and thus predate, volcaniclastic breccias of the central complex.
There is spectacular deformation of the country rocks surrounding the North Arran Granite Pluton. Throughout the south-west Highlands, the Dalradian rocks strike fairly consistently north-east—south-west, but on Arran they strike approximately parallel to the near-circular outline of the Outer Granite over about 270° of arc (P914151). The structure is well illustrated by the outcrops of distinctive black, slaty and phyllitic metamudstones on the west and north-west margin of the pluton. To the north of the granite, between Loch Ranza and Catacol, the Dalradian rocks form a concentric synform, termed the Catacol Synform. Offshore, between Catacol and Imichar Point, Permo-Triassic sandstones occur in the core of a concentric synclinal structure affecting Dalradian and Carboniferous rocks. On the eastern side of the granite, the country rocks responded to granite emplacement by a combination of folding and faulting, with reactivation of pre-existing faults in the structurally complex area near Corrie. Locally, near Pirnmill on the west coast, there are small reverse faults inclined towards the granite, some of which cut Paleocene dykes. South and south-east of the granite, doming affects sedimentary rocks of Devonian, Carboniferous and Permian age, for example in Glen Cloy, west of Brodick. This deformation is attributed to the diapiric rise of the Outer Granite (England, 1992b). The later Central Arran Ring-complex cuts across the domed country rocks of the North Arran Granite Pluton and there is some evidence that the country rocks adjacent to the ring-complex are also domed on its south and south-east side, although indifferent exposure makes verification difficult.