Tectonic structure and metamorphism, Southern Uplands-Down-Longford Terrane, Northern Ireland

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Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.

T B Anderson

Tectonic structure

The accretionary prism model for the formation of the Southern Uplands-Down-Longford Terrane. (P947803)
The Southern Uplands-Down-Longford Terrane in Northern Ireland. (P947802)
Tight, upright F1 syncline showing differential thickening of beds into fold hinge. Kearney Siltstone Formation (Hawick Group), Kearney Point [J 647 513], 5.5 km east of Portaferry, Co. Down. (Hammer 33 cm long). (P947980)
The Southern Uplands-Down-Longford Terrane in Northern Ireland. (P947802)
Typical fold-cleavage relationships in the Southern Uplands-Down-Longford Terrane (7). (P947809)
Hinge area of gentle, upright F1 anticline with well developed S1 slaty cleavage refracting to form convergent fans in siltstone beds and divergent fans in the softer interlayered mudstone. Kearney Siltstone Formation (Hawick Group), Kearney Point [J 644 511], 5.5 km east of Portaferry, Co. Down. (Hammer 33 cm long). (P947981)
Typical fold-cleavage relationships in the Southern Uplands-Down-Longford Terrane (7). (P947809)
Very tight, upright F1 anticline refolded by open F2 fold pair verging and gently inclined to the SSE. Kearney Siltstone Formation (Hawick Group), Whitehouse Port [J 645 550], about 1 km south of Cloghy, Co. Down. (Hammer 33 cm long). (P947982)
Detail of very tight upright F1 fold hinge in (P947983)
Dextral kink band deforming a composite fabric of vertical bedding and slaty cleavage. Kearney Siltstone Formation (Hawick Group), exposed on horizontal wave-cut platform at eastern end of Knockinelder Bay [J 642 514], 4.5 km east of Portaferry. (Scale shows 1cm squares). (P947984)

In the coastal outcrop of Co. Down the simple but important observation is that the rocks are well bedded with little evidence of changes in bed thickness and that dips are almost everywhere steep. The coherent nature of the beds may be a consequence of high pore-water pressure during and after deposition while the lack of stratal disruption and rarity of slump folding may reflect low depositional slopes [1]. The generally steep dip of the beds is part of a fold style which relates to the process of accretion.

Folding

At outcrop or even at tract scale, the accretionary folding (P947803) is expressed in several distinct structural facies [2].

Facies 1: Where the turbidites are coarse-grained and thickly bedded the beds are typically uniformly steeply inclined or vertical and young consistently to the northwest. This regular pattern of strike and dip is only locally interrupted by sporadic fold pairs in which vergence is consistently to the southeast. The fold pair is composed of an anticline and a syncline with the trace of the anticline northwest of the trace of the syncline. The common ‘short limb’ separating the two fold traces is, with respect to the dominant northwest-younging long limbs, rotated to the southeast. Sequences of this structural facies are typified by the Ordovician Ballygrot Tract at Grey Point [J 458 834] and by the Gala tracts between the Orlock Bridge Fault and Donaghadee (P947802). Although much of the Millisle Tract (Gala 7) is also essentially of this facies, the beds are commonly overturned so that they dip moderately to the south but young down to the north.

Facies 2: In this common structural facies there are many closely spaced, generally upright and upward facing, close, tight or even isoclinal folds, associated with minor faulting and shearing, particularly in the tighter fold cores and also separating adjacent folds (P947980). Although the folds are generally gently plunging, the amount and direction of plunge commonly varies so that some gently plunging anticlines are locally periclinal. In Co. Down easterly plunges are dominant [3] but very steeply plunging folds, some of which are completely overturned and downward facing, characterise a 1km long coastal strip north of Ballywalter (P947802). Areas with a high density of fold hinges have been, somewhat misleadingly, described as flat belts [4], not because the dips are flat but because the fold envelope, or faltenspiegel, has a gentle or flat inclination. Facies 2 is best developed in the thinner bedded, fine-grained Hawick Group sandstones and siltstones, particularly in the Portaferry and Ballyquintin tracts.

Facies 3: The two preceding structural facies account for some 90% of the Lower Palaeozoic outcrop in Counties Down and Armagh. However, there are some large areas, (eg. between Ballyhalbert and Portavogie extending inland along strike to the southwest), where beds young consistently to the southeast. Even more remarkable are significant areas of Co. Armagh, for example south of Tandragee and specifically in Glasdrummond quarry [H 940 388] southwest of Markethill, where the beds are horizontal or dip gently and are inverted as if on the overturned limbs of recumbent folds. The tracts of predominantly southeasterly younging strata, which also occur in Scotland, have been interpreted as packets of northwesterly overthrusted and obducted strata within the general northwesterly underthrusting and subduction of the accretionary prism [5]. As yet there is insufficient evidence to justify any hypothesis attempting to explain the inverted flat-lying beds but it is relevant to note that the steeply inclined tract boundaries appear unaffected, continuing ENE-WSW across Co. Armagh. There are no obviously similar sheets of flat lying inverted strata recorded in the Scottish part of the terrane.

Cleavage

A well-developed slaty cleavage is present in the mudstone interbeds, locally penetrating the finer grained turbidite beds, particularly in the Hawick Group (P947809). In the Northern Belt and northern tracts of the Central Belt, the cleavage is commonly axial planar to the contemporary accretion-related folds. Where best developed (P947809), cleavage fans are alternately convergent in the fine-grained turbidites and divergent in the mudstones (P947981).

In Hawick Group lithologies south of the Cloghy Fault, the cleavage commonly demonstrates a transecting relationship, in plan view striking at up to 30º clockwise of the fold axial traces [6], [7]. Where the folds are strongly overturned the cleavage may cut across the northwest-younging limb so that bedding faces downward on cleavage in the north-western fold limbs and upward on the same cleavage in the southeast-younging limbs (P947809). The various hypotheses explaining the origin of the transecting geometry essentially involve horizontal sinistral shear or rotation contemporaneous with folding and accretion [8], [9], [10].

In several parts of the terrane the dominant, accretion-related folds and cleavage have been refolded and the later folds have associated crenulation cleavages. Locally it is therefore possible to establish deformation sequences in which the structures are assigned to D1, D2 or D3 deformation phases. Thus at least two crenulation cleavages are recognised deforming the original slaty cleavage in the Orlock Tract of the Northern Belt and upright, accretion-related folds in many Hawick Group exposures are refolded by both north- and south-verging fold pairs [11].

It is important to recognise that these deformation chronologies relate only to observations in a particular tract. There is ample evidence that deformation tracks sedimentation diachronously from northwest to southeast across the terrane as required by both the viable terrane models [12]. Thus, in the Northern Belt tracts the D1 deformation occurred before many of the Central and Southern Belt sequences had accumulated.

The later ductile deformations appear to be non-penetrative and relatively minor in scale, the fold pairs typically having short limb lengths of no more than a few metres. In the Hawick Group outcrops of southeast Co. Down, the second folds tend to be open structures, plunging gently eastward along gently inclined axial surfaces and verging southward (P947982) and (P947983). The associated crenulation cleavage is commonly seen only at the fold hinges and in the short limbs. Third folds have a very similar style, scale, fabric and morphology except that their sense of vergence is invariably northward [13].

The well-developed slaty cleavage in many Hawick Group outcrops has acted as the mechanically essential plane of anisotropy for the genesis of an orthorhombic system of kink bands [14]. The kink bands dip steeply and displace the earlier fabric both sinistrally and dextrally (P947984). Profile cross sections of fine examples occur in slate on the wave-cut platform immediately west of Knockinelder Bay on the western side of Kearney Point [J 644 511].

Faults

Many small faults, with variable orientation and displacement directions, occurred in association with the accretionary deformation, particularly in the hinge zones of the early folds. Groups of steeply inclined reverse faults effect imbrication of the strata on a small scale. A fine example of a strike-slip duplex imbricating the Birkhill Shales occurs below and some 3m north of the low cliff on the southern edge of Coalpit Bay [J 595 788].

Later brittle shear fractures include a regular and systematic set of strike-slip faults which displace fold hinges, the various elements of the ductile structural fabric and some, but not all, of the late Caledonian lamprophyre dykes. In the coastal outcrop of Co. Down, sinistral strike slip faults are twice as common as dextral and both show preferred orientations, sinistrals striking at 015º and dextrals at 125º.[9] Displacements are invariably small, typically ranging up to a few hundred metres, sinistral slips tending to be larger than dextral. On occasions the strike-slip faults displace the tract boundaries such as between Gala 6 and Gala 7 so that coarse turbidites of the Millisle Formation (Gala 7) now form the low-lying intertidal outcrops eastward directly along strike from the Moffat Shale Group at Coalpit Bay (Gala 6). There is an apparent 1km sinistral displacement of tract boundaries, now concealed by Triassic rocks in the Dundonald gap and there are similar displacements of tract boundaries in Counties Armagh and Monaghan.

The fabric of the Orlock Bridge Fault provides further evidence of the importance of sinistral strike-slip in the later Caledonian deformation of the terrane. The fault extends for some 400 km from Slieve Glah in Co. Cavan, through outcrop at Clontibret on the Co. Monaghan-Co. Armagh border, Craigantlet and Orlock and across southern Scotland to the eastern limit of the Lower Palaeozoic outcrop 11 km southwest of Dunbar. The width of the zone of fault-related deformation appears to decrease eastward but it is still over 100 m across at Orlock Bridge, 1 km southeast of Orlock Point (P947802). In the central, most intensely deformed part of the zone, protomylonite, mylonite and ultramylonite textures are developed. The deformation overprints and deforms the local accretion-related cleavage but at Orlock Bridge it is in turn intruded by a late Caledonian lamprophyre dyke which is only marginally affected. Steeply plunging, sinistrally-verging folds indicate a large left-lateral slip [15].

Metamorphism

Throughout the Southern Uplands-Down-Longford Terrane there is ample evidence of low-grade metamorphism. Rocks now exposed at surface vary from zeolite facies to prehnite-pumpellyite facies, the latter more commonly developed in volcaniclastic greywacke and metabasic volcanic rocks such as occur in north Co. Down and in Co. Cavan [16]. Illite crystallinity studies on metapelites from the turbidite sequence in southwest Scotland demonstrate regional patterns of late diagenetic to anchizonal grades that were generated by burial and underplating in an accretionary thrust stack at depths of 12 km or more [17]. Local shear-zone metamorphism also occurred.

Deformation and metamorphism in the Southern Uplands-Down-Longford Terrane thus appears to be largely related to diachronous, south-vergent accretion and thrusting which progressed northwest to southeast across the ocean or basin in which its sediments had been recently deposited. This deformation was increasingly modified and overprinted by sinistral strike-slip, expressed both in ductile, locally penetrative, shear fabrics and in systems of late brittle faults. There is remarkably little evidence of climactic, collisional tectonics which might have been expected to accompany the final closure of Iapetus.

References

  1. ↑ Anderson, T B. 2001 (for 2000). Structural interpretations of the Southern Uplands Terrane. Transactions of the Royal Society of Edinburgh: Earth Sciences, 91, 363–73.
  2. ↑ Stone, P. (editor) 1996. Geology in south-west Scotland; an excursion guide. Keyworth, Nottingham: British Geological Survey.
  3. ↑ Barnes, R P, Anderson, T B, and McCurry, J A. 1987. Along-strike variation in the stratigraphic and structural profile of the Southern Uplands Central Belt in Galloway and Down. Journal of the Geological Society, London, 146, 807–16.
  4. ↑ Craig, G Y, and Walton, E K. 1959. Sequence and structure in the Silurian Rocks of Kirkcudbrightshire. Geological Magazine, 96, 209–20.
  5. ↑ McCurry, J A, and Anderson, T B. 1989. Landward vergence in the Lower Palaeozoic Southern Uplands-Down-Longford terrane. Geology, 17, 630–33.
  6. ↑ Cameron, T D J. 1981. The history of Caledonian deformation in East Lecale, County Down. Journal of Earth Sciences: Royal Dublin Society, 4, 53–74.
  7. ↑ Anderson, T B. 1987. The onset and timing of Caledonian sinistral shear in County Down. Journal of the Geological Society, London, 144, 817–25.
  8. ↑ Cameron, T D J. 1981. The history of Caledonian deformation in East Lecale, County Down. Journal of Earth Sciences: Royal Dublin Society, 4, 53–74.
  9. ↑ 9.0 9.1 Anderson, T B. 1987. The onset and timing of Caledonian sinistral shear in County Down. Journal of the Geological Society, London, 144, 817–25.
  10. ↑ Stringer, P, and Treagus, J E. 1980. Non-axial planar S1 cleavage in the Hawick rocks of the Galloway area, Southern Uplands, Scotland. Journal of Structural Geology, 2, 317–31.
  11. ↑ Anderson, T B, and Cameron, T D J. 1979. A structural profile of Caledonian deformation in Down. In: Harris, A L, Holland, C H, and Leake, B E (eds.). The Caledonides of the British Isles, reviewed. Special Publication of the Geological Society of London, No. 8, 263–7.
  12. ↑ Barnes, R P, Lintern, B C, and Stone, P. 1989. Timing and regional implications of deformation in the Southern Uplands of Scotland. Journal of the Geological Society, London, 146, 905–8.
  13. ↑ Anderson, T B, and Cameron, T D J. 1979. A structural profile of Caledonian deformation in Down. In: Harris, A L, Holland, C H, and Leake, B E (eds.). The Caledonides of the British Isles, reviewed. Special Publication of the Geological Society of London, No. 8, 263–7.
  14. ↑ Anderson, T B. 1969. The geometry of a natural orthorhombic system of kink bands. In: Baer, A J, and Norris, D K (eds.). Proceedings, Conference on research in Tectonics (Kink bands and brittle deformation), Geological Survey of Canada Paper, 68-52, 200–28.
  15. ↑ Anderson, T B, and Oliver, G J H. 1986. The Orlock Bridge Fault: a major late Caledonian sinistral fault in the Southern Uplands terrane, British Isles. Transactions of the Royal Society of Edinburgh: Earth Sciences, 77, 203–22.
  16. ↑ Oliver, G J H, Smellie, J L, Thomas, L J, Casey, D M, Kemp, A E S, Evans, L J, Baldwin, J R, and Hepworth, B C. 1984. Early Palaeozoic metamorphic history of the Midland Valley, Southern Uplands-Longford-Down massif and the Lake District, British Isles. Transactions of the Royal Society of Edinburgh: Earth Sciences, 75, 245–58.
  17. ↑ Merriman, R J, and Roberts, B. 2001 (for 2000). Low grade metamorphism in the Scottish Southern Uplands terrane: deciphering the patterns of accretionary burial, shearing and cryptic aureoles. Transactions of the Royal Society of Edinburgh: Earth Sciences, 91, 521–37.
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