Letter to the Editor

A PRE-DALRADIAN LINEAMENT IN CONNEMARA

SIRS—The recent account by Dunning (1972) of the 1971 Edinburgh symposium on the dating of the Metamorphic Caledonides shows that no entirely satisfactory explana­tion has yet been offered for the apparent discrepancy between the ages of the end-Dalradian orogenesis in Connemara and in Scotland; Dunning observes that the isotopic evidence indicates a difference of 10-15 m-y- I n accord with this, the palaeontological evidence suggests the event was pre-Arenigian in Connemara, but post-Arenigian— possibly post-Llanvirnian—in Scotland. The suggestion that the orogenesis was a dia-chronic event (Dewey and Pankhurst 1970) involves difficulties, in consequence of which Dunning prefers the idea that "the change in date of onset of the main orogenic phase took place across the Highland Boundary Fault".

In fact, however, the evidence available only allows this proposition to be made with respect to the east-west trending portion of the Highland Boundary Fault in Connemara. A new suggestion is now offered based upon the postulation that in the latter area a major pre-Dalradian, east-west lineament acted as a fundamental geological control over Upper Proterozoic and Lower Palaeozoic events.

This assertion is supported by the number and variety of special characteristics possessed by the Dalradian of Connemara and by the succeeding Lower Palaeozoic of the South Mayo Trough (Fig. 1); of these, the following are relevant to this inter­pretation.

1. The Connemara Schists comprise the only extensive Dalradian outcrop south of the Highland Boundary Fault.

2. The end-Dalradian orogeny imposed an approximately east-west grain on this area; deformation commenced with tight isoclinal Fx folds (Leake 1969), and not with recumbent nappes as in the Grampians (Rast 1963).

3. The early Mt metamorphism attained amphibolite-facies conditions; elsewhere it produced only greenschist-facies.

4. Basic and ultra-basic intrusions follow ¥x Mx and are involved in later amphibolite-facies metamorphism and migmatisation (Leake 1969); in Aberdeenshire the basic intrusions are synchronous with or immediately following the M3 meta­morphism of the adjoining MacDuff Slates (Pankhurst 1970; Fettes 1971).

5. The post-F4 Oughterard Granite gives a Rb/Sr whole-rock isochron age of 525 ± 35 tn.y. (Leggoef al. 1966); similarly determined values for the Aberdeen­shire gabbro are 486 ± 17 ni.y. (Pankhurst 1970; revised to 501 ± 17 m.y. by Pankhurst in Dunning 1972), and 483 ± 1 m.y. for the Donegal Main Granite which was synchronous with the final deformation (Leggo et al. 1969).

Scott. J. Geol. 8, (4}, 363-367, 1973

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364 LETTER TO THE EDITOR

6. The thick Ordovician and Silurian sequence of the South Mayo Trough is entirely younger than the end-Dalradian orogeny (Dewey et al. 1970). This is the only Lower Palaeozoic turbidite-volcanic sequence supposedly north of the Southern Uplands Fault. The trough, its axial currents and post-Silurian folds are all oriented east-west (Dewey 1963).

7. An upper limit for the end-Dalradian orogeny in Connemara is provided by the recognition of Arenigian faunas in the Ordovician of the South Mayo Trough (Dewey et al. 1970), whereas in the Grampian the younger Dalradian metasedi-ments have yielded Arenigian or Llanvirnian microfaunas (Downie et al. 1971).

Also significant is the fact that the Highland Boundary Fault and other major faults of north-west Ireland turn towards and converge upon an east-west line through Clew Bay. (Pitcher 1969).

Dewey (19690, 1971) suggested that the Caledonian orogenic belts and preceding geosynclines evolved in relation to an ancient ocean; the South Mayo Trough was

FIG. 1. Connemara Lineament in relation to Moine-Dalradian and Lower Palaeozoic outcrops of central Western Ireland. Abbreviations—C.B.F.—Clew Bay Fault: H.B.F.—Highland Boun­dary Faults; L.F.—Leek Fault; S.U.F.—Southern Uplands Fault.

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LETTER TO THE EDITOR 365

regarded as a local geosynclinal trough formed within the Grampian orogenic belt. However, Dewey's plate-tectonic model can provide the basis for another interpretation of both the trough and pre-trough events. If it is accepted that the Moine-Dalradian belt was originally continuous into North America, then it is probable that south of Connemara there is a subsurface crop with a broadly Caledonoid trend. Thus, in Western Ireland as a whole the Moine-Dalradian occupies an S-shaped belt. It is postu­lated that this reflects an original continent-ocean margin which was off-set in Connemara along a major east-west fault-zone already present in the pre-Moine/Dalradian basement. The original magnitude of this off-set cannot be determined because of later orogenic shortening; but it could also have caused an off-set in the new ocean-spreading axis where it would be accommodated by a transform fault. Thus, an east-west lineament developed comprising the original continental fault-zone and an oceanward extension as the inactive and active parts of the transform fault. Locally ocean crust and continental shelf north of the off-set would adjoin continental shelf and foreland to the south (Fig. 2).

Dewey considered that at a later stage ocean contraction was made possible when the oceanic lithosphere decoupled from the continent, and was consumed down a subduction zone now marked by the Southern Uplands Fault. The position of this fault—indeed its existence-in west-central Ireland is unknown, although Leake (1963) and Rast (1969) argued that it was diverted to the south of Connemara. It is now suggested that the Southern Uplands Fault terminated southwards against the Connemara lineament; southwards, a subduction zone opened east of the off-set Dalradian sub-crop, and this might be coincident with the structure identified by Rast as a continuation of the Southern Uplands Fault.

The Caledonian deformation was, by common consent due to overall north-west south-east stress, i.e., oblique to the lineament. Differential movements could develop

FIG. 2. Diagrammatic representation of relation between Connemara Lineament and Moine-Dalradian outcrops: (a) prior to, and (b) after deformation.

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366 LETTER TO THE EDITOR

along it between the north and south sectors of the plate if the two subduction zones accommodated different proportions of this stress. Thus, transpression would occur along the lineament if the northern sector was driven into the southern, and the adjacent portions of the continental shelf would be tightly buckled into parallel east-west folds. Moreover, the presence of the fundamental and essentially vertical crustal lineament would facilitate: (a) a high heat-flow, and (b) the early uprise of mantle material; thus, the repeated amphibolite-facies metamorphisms and the early basic and ultra-basic intrusions are accounted for.

Reversal in the relative movement across the lineament would arise, for instance, by a change in the efficacy of the subduction zones. This could terminate the local trans­pression and bring the Grampian phase to an end in Connemara although still continuing elsewhere against the subduction zones. Later in the Arenigian, actual separation of the two sectors is recorded by the formation of the South Mayo Trough as a narrow structure within the opened lineament now represented in the north and south by the Clew Bay and Lough Nafooey fault zones respectively. A trough formed in this manner as a gore would be floored by new ocean crust, and be deep enough to accommodate the 12000 m of Ordovician turbidites and volcanics. Continuing instability within this east-west framework determined the outcrop pattern of the sub-Silurian surface, the intra-Silurian faulting and sediment-facies patterns (Piper 1972). During the end-Silurian polyphase deformation it determined that the folding was again east-west with related conjugate wrench-faults (Dewey 1969b).

Arguments in favour of a pre-Arenig age for the Highland Boundary Fault have been given previously (George i960; Dewey 1961; Cobbing 1964) and the writer supposes that this structure was active during the deposition of the Upper Dalradian. Thus, during the deformation along the Connemara lineament, its orientation would have been modified to produce the convergence noted by Pitcher (1969).

In conclusion, the special circumstances attributed to the Connemara Dalradian out­crop show that: (1) its apparent position between the Highland Boundary and Southern Upland Faults does not necessarily provide any indication of the nature of the sub-Palaeozoic floor of the Midland Valley Tract in Ireland or Scotland; and (2) whilst its deformation is an event within the Grampian orogenic phase, it does not reflect the typical sequence and timing of the latter.

REFERENCES

COBBING, E. J., 1964. The Highland Boundary Fault in East Tyrone. Geol. Mag. 101, 496-501. DEWEY, J. F., 1961. A note concerning the age of the metamorphism of the Dalradian rocks of

Western Ireland. Geol. Mag., 98, 399-405. , 1963. The Lower Palaeozoic stratigraphy of central Murrisk, Co. Mayo, Ireland and the evolution of the South Mayo Trough. Q.Jl. geol. Soc. Lond., 119, 313-344. , 1969a. Evolution of the Appalachian/Caledonian orogen. Nature Lond., 222, 124-129. , 1969b. Structure and sequence in paratectonic British Caledonides. In Kay, M. (ed.), North Atlantic—geology and continental drift. Mem. Atner. Ass. Petrol. Geol. 12, 309-335.

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LETTER TO THE EDITOR 367 DEWEY, J. F., 1971. A model for the Lower Palaeozoic evolution of the southern margin of

the early Caledonides of Scotland and Ireland. Scott. J. Geol., 7, 219-240. , M c K E R R O W , W . S. and M O O R B A T H , S. 1970. The relationship between isotopic ages, uplift and sedimentation during Ordovician times in Western Ireland. Scott. J. Geol., 6,133-145. and PANKHURST, R. J. 1970. Evolution of the Scottish Highlands and their radiometric age

pattern. Trans. R. Soc. Edinb., 68, 361-389. , RICKARDS, R. B . and SKEVINGTON, D. , 1970. New light on the age of Dalradian deformation and metamorphism in Western Ireland. Norsk. Geol. Tidsskr., 50, 19-44.

DOWNIE, C , LISTER, T. R., HARRIS, A. L., and FETTES, D . J., 1971. A palynological investi­gation of the Dalradian rocks of Scodand. Rep. No. 71/9. Inst. geol. Sci. 30pp.

D U N N I N G , F. W . 1972. Dating events in the Metamorphic Caledonides: impressions of the symposium held in Edinburgh, September 1971. Scott. J. Geol., 8, 179-192.

FETTES, D . J. 1971. Relation of cleavage and metamorphism in the MacDufT Slates. Scott. J. Geol., 7, 248-253.

GEORGE, T. N . , i960. Stratigraphical Evolution of the Midland Valley. Trans, geol. Soc. Glasg., 24, 32-107.

LEAKE, B. E., 1963. The location of the Southern Uplands Fault in Central Ireland. Geol. Mag., 100, 420-423. , 1969, for 1970. The origin of the Connemara migmatites of the Cashell district, Connemara, Ireland. Q.Jlgeol. Soc. Lond. 125, 219-75.

LEGGO, P. J., COMPSTON, W . and LEAKE, B . E., 1966. The Geochronology of the Connemara granites and its bearing on the antiquity of the Dalradian Series. Q. Jl geol. Soc. Lond., 122, 91-118. , TANNER, P. W . G. and LEAKE, B. E., 1969. Isochron study of Donegal Granite and certain Dalradian rocks of Britain. In Kay, M. (ed.), North Atlantic—geology and continental drift. Mem. Assoc. Petrol. Geol., 12, 354-362.

PANKHURST, R. J., 1970. The geochronology of the basic igneous complexes. Scott. J. Geol., 6, 83-107.

PIPER, D . J. W. , 1972. Sedimentary environments and palaeogeography of the late Llandovery and earliest Wenlock of North Connemara, Ireland. Jl geol. Soc. Lond. 128, 33-52.

PITCHER, W . S., 1969. North-east trending faults of Scotland and Ireland, and chronology of dis­placements. In Kay, M. (ed.), North Atlantic—geology and continental drift. Mem. Amer. Assoc. Petrol. Geol., 12, 724-733.

RAST, N . , 1963. Structure and metamorphism of the Dalradian rocks of Scotland. In Johnson, M. R. W . and Stewart, F. H . (eds.) The British Caledonides. Oliver and Boyd, Edinburgh, 123-142. , 1969. Orogenic belts and their parts. In Kent, P. E. et al. (eds.). Time and Place in Orogeny. London, 197-213.

J. W. BAKER Department of Geology University College, Cardiff

MS received 22nd August 1972 Revised MS received 8th September 1972

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