Dalradian Basin Research Articles

Geometry and emplacement of nappes in the Central Scottish Highlands

Summary A deformation history related to polyphase nappe formation is presented for the Central Scottish Highlands as a model for the Dalradian tract. The genesis of a four-fold pattern of tectonic belts recognisable throughout this tract is discussed. The fan structure of the Tummel steep belt is argued to be a deep-seated rotation zone. Movements within this zone spanned the climax of Barrovian metamorphism and were responsible for creating the major belt of inverted rocks—the Loch Tay inversion—which forms the lower limb of the Tay (fold) nappe. The early-formed Tay nappe, nucleated at a relatively high tectonic level, was translated as a result of movements within the deeper rotation zone progressively towards the SE principally by simple shear. The core of the Tay nappe is recognised within the Highland Border steep belt at Aberfoyle, but does not root within the deeper levels of the Central Highlands Dalradian. The Highland Border steep belt at Dunkeld was formed by late orogenic downbending of the previously flat-lying inverted limb of the Tay nappe probably resulting from sub-vertical movements within the basement along the ancestral line of the Highland Boundary fault. A kinematic model explains the nappe geometry by density inversion of the Dalradian succession consequent on the pre-orogenic evolution of the Dalradian (marginal) basin.

A Caledonian plate tectonic model

A plate tectonic model for the Caledonides of the British Isles with three non-parallel lower Ordovician-lower Devonian subduction systems is presented. In the north-west, the Caledonian thrust front is interpreted as the site of multiple, slow ESE.-directed subduction (Sd1) of Lewisian-bearing continental foreland plate beneath continental crust containing an ensialic Dalradian basin. South of the continental crust of the Southern Uplands zone lay a ENE.–WSW.-trending plate boundary (Sd2) where Iapetus oceanic crust was subducted to the NNW. The convergence of subducted lithosphere beneath the Moine-Dalradian plate is linked to its prolonged history of diachronous deformation, metamorphism, intrusion and uplift. The oblique subduction on Sd1 and Sd2 caused collision below the Moine-Dalradian outcrop generating sinistral strike-slip faults with downthrown south-east sides, spreading north-east with time. Sinistral displacement on the Great Glen Fault system, and of the Connemara inlier is linked to this model. A third (Ordovician-Silurian) subduction system (Sd3) is defined to the north-west of a volcanic arc from the Lake District in England, through southern Ireland. Iapetus oceanic crust was subducted to the south-east here. Evidence is presented for oblique collision between Sd2 and Sd3 from late Ordovician to early Devonian, followed by an unstable triple point migrating south-westwards across the British Isles region, and subsequent dextral strike-slip movement on the suture. Triple-point migration to the south-west is measured using diachronous ending of volcanism and plutonism on Sd 2 and Sd 3 Opposed lower Devonian collision points now appear to be dextrally offset by about 900 km. Calculated closure rates on Sd 2 and Sd 3 give minimum Arenig widths for Iapetus of 600–800 km. About 1270 km of late Ordovician-Devonian dextral displacement on the Iapetus suture could have generated 570 km of foreland convergence in the East Green­land-Scandinavian Caledonides.

The pre-Silurian rocks of Clare Island, Co. Mayo, Ireland, and the age of the metamorphism of the Dalradian Ireland

The Highland Boundary Fault is considered to extend from Stonehaven in Scotland to the major Leek fault of Clare Island in western Ireland. This conspicuous fault of the island has however been erroneously correlated with the Leannan fault of Donegal, a probable branch of the Great Glen Fault. South of the Leek fault Silurian sediments rest unconformably upon a metamorphic basement. This consists of amphi-bolite facies metasediments intruded by basic and ultrabasic rocks that have also undergone amphibolite facies metamorphism. These high grade rocks of uncertain age are considered to be part of a horst which was uplifted within the Moine Dalradian basin of western Ireland during Cambrian times. The horst was probably a southern source of detritus for the Upper Dalradian turbidites now on both sides of the Leek fault. The large uplift to the south which formed this horst conforms with and expands previous ideas on the early history of the Highland Boundary Fault. The age of the Dalradian metamorphism in western Ireland is reconsidered, and it is concluded that the case for a pre- Nitidus Zone age is unsatisfactory and that a mid-Ordovician age is possible.