J. geol. Soc. , Vol. 137, 1980, pp. 99-100. Printed in Northern Ireland.

Discussion on the Permo-Carboniferous dyke-swarm of northern and its bearingon dextral displacement on the Great Glen Fault

Journal, Vol. 136, Part 1,1979, pp. 3-11

DR N. HOLGATEwrites: Speight & Mitchellpresent The suggestion that, for some reason unspecified, 'the what they regard as evidence to disprove the Tertiary plane of the Great Glen Fault acted as a barrier to the dextral shift on the Great Glen Fault first recognized continuedsoutherly extension of the Skyeswarm' by Holgate (1969). However, they agree with him in (Speight et al. 1979, p. 9) does not seem to be substan- recognizing a 7-8 km dextral shift on themain track of tiated. the fault between and Lismore, but contest itsTertiary dating. They find noevidence of lateral DRS SPEIGHT& MITCHELLreply: It is theauthors' displacement of Tertiarydykes at the Great Glen conviction that neither the Great Glen Fault, nor the Faultand question the existence of its major splay Firth of Lorne Fault, have suffered any major dextral passing SE of Lismore. dislocationsince theemplacement of theTertiary The writer agrees with the authors that the Tertiary dyke-swarms of NW . It was never their in- Skyedyke-swarm intersects a Permo-Carboniferous tention to wholly preclude possible minor movements swarm in the Morvern area. He also accepts their field of such a nature and age in the vicinity of Lismore. If criteria (op. cit., p. 5) for the recognition of Tertiary indeed Dr Holgate's (1969, p. 113) interpretation of a dykes within thearea. But their statement (op. cit., Tertiary age for a few dykes of SE Lismore and the p. 9) that Tertiary dykes are sparse on Lismore-11 Appin district which are dextrally displaced is correct, dykes (which they arbitrarily assign to the Mull swarm) thenthis can do no morethan indicate that such in the SW end of the island-is scarcelyconvincing, movements took place, although such evidence alone since theirtraverses, on Lismore andon the Appin cannot constitute proof of a major displacement of the coast (op. cit., p. 8; Fig. 3) exclude theseveral order of 20 km between Lismore and Appin. localitiescited by Holgate (op. cit., p. 113) at which Theauthors apologizefor inadvertently placing Tertiary dykes aredisplaced dextrally by NE-SW frac- perhaps a disproportionate emphasis on evidence re- tures marginal to the Firth of Lorne fault zone which lating to lack of displacement of the Mullswarm separates Lismore from Appin. Towards the NE end gainedfrom a study of theAeromagnetic Map of of Lismore, such fractures become increasingly numer- Great Britain(Bullerwell 1968).The deduction that ous as the Firth of Lorne shore is approached. boththe Skye and Mull Tertiaryregional linear The Firth of Lorne fault diverges through some 10" swarmshave suffered no displacement of the nature in an anticlockwise sense from the Great Glen Fault at proposed by Dr Holgate (1969) arises primarily out of a point to the NE of Lismore. Since this splay shows interpretation of the mapsillustrating percentage dextral shift dislocating dykes of the Tertiary swarm, crustalstretch (dilation) for the Mullswarm (Sloan displacement in the same sense must have occurred on 1971),Jura swarm (Knaap1973), and Skyeswarm the main fault and probably also on its original align- (Speight 1972). On theshores of SE Mull between ment NW of Lismore. Can Speight & Mitchell cite a Rubhana Faoilinn andGrass Point (about 18 km locality, adjacent to this line, at which a north-easterly transect) the overall dilation of the Tertiary dykes is fracturecauses dextral displacement of aPermo- c. 4.5-5'/0. Decrease southeastwards toan overall value Carboniferous dyke, and is itself seen to be intersected of 3.5-4%dilation, over about the same length of by an unfaulted Tertiary dyke? traverse from Luing to Loch Feochan, is entirely com- Theauthors refer to the Aeromagnetic Map of patible with acorresponding rate of decrease in the Great Britain, Sheet 10 (Bullerwell 1968) as indicating Skyeregional linear swarm betweenStrathaird and the absence of offset of the Tertiary Skye swarm at the northern Sleat. Moreover, the axis of the Mull swarm Great Glen Fault. In view of the 2 km nominal separa- shows not the slightest deviation in its north-westerly tion of the E-W traverses which form the basis of the course between Croggan and the mainland. However, map, the appearance on it of evidence for such offset the percentage crustal stretch of the dykes of SE Mull could hardly be expected, given the high topographic contrasts markedly with the overall dilation of 0.5% in contrasts of the area concerned. Jura, between Corpach Bay and the NE extremity of The writerhas discussed (Holgate 1969, pp. 110, this island. This data alone indicates the impracticality 111) mechanisms which might result in the absence of of correlating the dykes of SE Mull with those of Jura, dykes of theTertiary Skyealignment in Appin.He andhence points to the absence of post-Tertiary discards the notion of dip-slip, with SE downthrow, on dyke-swarm displacement, shifting the dykes of Jura the Great Glen Fault for whatseem adequate reasons. from a previous alignment with the dykes of SE Mull.

0016-7649/80/0100-0099 $02.00 @ 1980 The Geological Society

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/1/99/4886557/gsjgs.137.1.0099.pdf by guest on 30 September 2021 100 Discussion Theoverall dilation along the length of theNW forthe Tertiary dykes on the NW shores of Loch shore of Lismore(incorporating both Permo- Linnhe. Carboniferous and Tertiary dykes) of somewhat less The diminution in intensity of the dykes, whether of than 1% is again hardly comparable with the crustal Tertiary or Permo-Carboniferousages, between the stretch on the seaboardof Argyll. Even supposing that opposingshores of LochLinnhe is irrefutable.The all of thedykes outcropping on the NW shores of authors made no attempt to offer substantial evidence LochLinnhe were of the same age, then an overall thatan 0.5 km dip-slipdisplacement onthe Great dilation of 2.5-3% is obtained for the transect from Glen Fault took place after the emplacement of the Camas Gorm to Camas Chil-Mhalieu. The markedly Tertiary swarms. Certainly, such a displacement would low dilation of dykes in Lismore, sandwiched prior to haveonly a limited effect on dilationvalues of the Tertiarydyke emplacement, according to Holgate’s Skye Tertiary dyke-swarm on either side of the fault hypothesis, between the NW shores of plane. It was, however, in this very context that the and the Lorne coast (in which latter locality the NW- barriermechanism was tentativelysuggested as a trending dykes recorded are undoubtedly Tertiary in possibleadditional solution to an otherwiseirksome age), places the island in rather an incongruous posi- problem.By no means do the authors attribute the tion. There are decreases in percentage dilation and decrease in intensity and dilation of thePermo- intensity of dykesbetween NW Loch Linnhe and Carboniferous swarm to a mere 0.5 km SE downthrow Lismore,but these are largely attributableto de- on the Great Glen Fault. On the contrary, they sug- creasesin the Permo-Carboniferous swarm. The au- gest that pre- Tertiary dyke-swarmnormal displace- thors recognize anoverall stretch of c. 2% forthe ments of greatermagnitude may also have taken Permo-Carboniferous dykes and not much over 0.5% place.

References

BULLERWELL,W. 1968. Aeromagnetic map of Great Britain. swarm. Thesis, Ph. D., Univ. London (unpubl.). Sheet10. Geol. Suro. U.K. SPEIGHT,J. M. 1972. The form and structureof the Tertiary HOLGATE,N. 1969. Palaeozoic andTertiary transcurrent dyke-swarms of Skye and . Thesis, Ph.D., movements on the Great Glen Fault.Scott. J. Geol. 5, Univ. London (unpubl.).

97-139. ~ & MITCHELL,J. G. 1979. The Permo-Carboniferous KNAAP,R. J. 1973. The form and structure of the Islay, Jura dyke-swarm in northern Argyll and its bearing on dex- and ArranTertiary basic dyke swarms. Thesis,Ph.D., tral displacement on the Great Glen Fault. J. geol. Soc. Univ. London (unpubl.). London, 136,3-11. SLOAN,T. 1971.The structureof the Mull Tertiary dyke-

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