Discussion on a Tectonic Model for the Emplacement of the Main Donegal Granite, NW Ireland
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Journal of the Geological Society, London, Vol. 144, 1987, pp. 201-203, 1 fig. Printed in Northern Ireland Discussion on a tectonic model for the emplacement of the Main Donegal Granite, NW Ireland Journal, Vol. 139, Part 5, 1982, pp. 615-631. Mr M. J. Arthur writes: Hutton (1982) suggested that the Main DonegalGranite (MDG) was accommodatedin a NE-SW zone of tension(‘elongate hole’), contem- poraneously with NE-SW sinistral shearing aroundthat zoneabout 400 Ma ago (end Silurian-early Devonian times). Calculations from strain studies suggested that the sinistral displacement was about 20 kmat itsmaximum, across the NW boundary of the MDG, but only about 5 km where the NW and SE boundaries united and continued to the SW (Hutton 1982). No figure was given for the offset across the SE boundary, though it was considered to be a major sinistral shear zone (Hutton 1982). However, Hutton (1982) proposed an unorthodox ‘crack opening model’ for the emplacement, which is adequately explained by the well establishedpull-apart model. The MDG has the rhomboidal/elliptical shape (Fig. 1) typical of a pull-apart produced at a releasing bend (cf. Crowell 1974, fig. 3),resulting fromthe intersection of the two throughgoing parts of a left-stepping sinistral wrench fault (cf. Ramsay 1980, fig. 18). The MDG is about 50 km long N and 11 km wide and thus has a length/width ratio of about 4.5, whilst the ratio for pull-aparts is commonly between 2 0U 510f and 5 with an average of about 3(Aydin & Nur 1982). km Emplacementinto an ‘elongate hole’ contemporaneously Donegal AI47 with shearing around it, is precisely a characteristic expected of a pull-apart. Although pull-aparts are perhapsbest known Fig. 1. The structural situation of the Main Donegal Granite after for their occupation by sedimentary deposits they also often Pitcher er al. (1964), Max & Barker (1978), Hutton (1982), Max er contain contemporaneous igneousrocks, andthe largest al. (1983): a pull-apart produced by left-step of a sinistral wrench pull-aparts are so profoundthat oceaniccrust is created fault. within them (e.g. Crowell 1974, 1979; Mann et al. 1983). Thus, Hutton’s (1982) account is entirely consistent with emplacement of the MDG by pull-apart, except that in that suggested to run in the Atlantic Ocean, about 50 km NW of case thetotal sinistraldisplacement across it must be the Leannan Fault, but with the latter still being a major 50 km-the overlap length of the boundaryfaults. splay from it (Fig. 1).The GGF probablyruns nearly Consideration of the position of theMDG within the 1000 km from W of Ireland to NE of the Shetland Islands, Caledonides,in particular its relationship tothe NE-SW and is paralleled in a zone several hundred kilometres wide Great Glen Fault (GGF), shows that the latter estimate is by other major faults, with similar (wrench) characteristics reasonable, and indeed conservative. andextent, such asthe Minch,Highland Boundary and Since Kennedy’s (1946) suggestion that the GGF was a Southern Upland faults. major wrench fault that showed about 105 km of sinistral Following Kennedy’s (1946) proposals, it has been offset across it in Upper Devonian to Lower Carboniferous frequently suggested that there were sinistral movements of times, therehave been continual discussions aboutthe about 100-300 km across the GGF around Devonian times, times,senses andamounts of wrenching that may have and probably additionally, or possibly alternatively, dextral occurred on it, andits correlation with faults in Ireland (e.g. wrenching of nomore than about 100 km across it in see Pitcher et al. 1964; Storetvedt 1975; Donovan et al. 1976; Mesozoic and/or Tertiary times. It has also been suggested Johnson & Frost 1977; Kent & Opdyke 1978; Max & Barker that about 2000km of sinistral offset occurredacross the 1978; Holgate 1980; van der Voo & Scotese 1981, 1982; Max GGF and/or associated faults in Carboniferous times (Kent et al. 1983; Smith & Watson 1983; Briden et al. 1984; & Opdyke 1978; van der Voo & Scotese 1981). The latter Cisowski 1984; Esang & Piper 1984; Irving & Strong 1984; suggestion appears untenable, but it is currentlyaccepted also accompanyingreferences and discussions). The thatthe GGF is a major wrench fault, with a net offset Leannan Fault, running parallel to and about5 km SE of the (probably sinistral) across it of100-200 km that mainly SE boundary fault of the MDG, was early on suggested to occurredin Devonian times (e.g. Smith & Watson 1983; be the continuation of the GGF, or at least a major splay Esang & Piper 1984). The most likely settingfor an fromit. Latterly the main splay of theGGF has been assemblage of major wrenches, such as theGGF and 201 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/1/201/4888934/gsjgs.144.1.0201.pdf by guest on 27 September 2021 202 DISCUSSION associated faults, is a transform margin (Harding & Lowell This discussion is published with the permission of the Director, 1979), like that containing the San Andreas and associated British Geological Survey (NERC) . faults. The San Andreas Fault Zone (SAFZ) has been traced forat least 1200 km,and is themajor fault zone ina transform margin that is a few hundred kilometres wide and Dr D. Hutton replies: I agree that a left-stepping pull-apart across which thetotal wrench offset is probably about is a possible, if somewhat simple, model for the 1000 km, though the offset on individual splays of the SAFZ emplacement of the MainDonegal granite in a sinistral is only about 100 km (e.g. Crowell 1979). Clearly, as noted shear zone. There are three reasons why it does not work. by Kennedy (1946), there are strongsimilarities between the First, the external parts of the shear zone, (beyond the settings of theGGF and SAFZ, and Storetvedt (1975) pluton) are not parallel (Hutton 1982, fig. 1). The SW part suggested that the GGF was a ‘. Palaeozoic equivalent to trends SSW at an angle of nearly 35” to the northeastern the San Andreas Fault. .’. NE-trending part. Such a discordance renders the opening It is noteworthy that the offsets on the GGF and SAFZ of a simple pull-apart, by synchronous movements on the were determinedfrom correlations of geological features two strands, nearly impossible. across them and from palaeomagnetic studies: strain studies Second, the displacement onthe shearzone does not made a negligible contribution torevealing the magnitude of drop to zero at the NE andSW ends of the pluton-which it offset across them, and indeed Parson (1979) reported that should do for a pull-apart. strainstudies did not indicateany wrenching along the Third, Arthur’s claim thatthere is some 30 km extra GGF, a finding universally discounted. It is clear that a displacement hidden in the shear zone is difficult to accept. wrench propagating through a rock unit may leave a strain It is all very well suggesting thatthe 20 km total record of its movement, but that once the wrench has fully displacementcalculated by me is an absolute andrather propagatedfurther movements onit (including reversed unrealistic minimum and that a much greater displacement ones)leave little strain record, so that strainstudies is taken up on a discrete fault within the shear zone, but generally provide only a (very) low limit for the amount of where is that fault? I was certainly never able to findit! offset on a major wrench. There are transcurrentslides within the aureole of the MDG Thus,the boundary faults of theMDG are situated (Hutton 1977, 1981; White & Hutton 1985) but to suggest between branches of the GGF on which sinistral offsets of thatthese haveanything morethan a few kilometres the order of 100 km probably occurred at about the time of displacement is defied by theremarkable stratigraphic emplacement of the MDG. Therefore, it is concluded that continuity of the country rocks right across the shear zone. those boundary faults were themselves probably splays of a Indeed thislast reason was why Pitcher et al. (1984) splay from the GGF, and thatanyway sinistral wrenching of considered that a total shear zone displacement of 20 km similar magnitude could have occurred across them when was too much! the MDG was formed. It is further concluded that Hutton’s Much of Arthur’s argument concerns possible relation- (1982) strain studies provide only minimum estimates for the ships between the main Donegal Granite Shear zone and the sinistral offset across the boundary faults of the MDG. The faults of theGreat Glen system andthat given possible recognition thatthe geometry of theMDG is that of a significant displacements onthe latter; something similar pull-apart shows thatthe true sinistral offset across its should be expected of theformer. I find thistype of boundary faults, and along their united continuations to NE argument very unconvincing. Whilst it is apparent that there andSW, must be theiroverlap length (50 km), in good may be a spatial and temporal connection between the two, agreement with the maximum estimate from thestrain studies specifjmg exactly what it is confounded by a lack of clear (20 km). data.Indeed despite Arthur’sinference that movements The setting of the MDG favours its identification as a weresynchronous, the available evidence, such as it is, pull-apartsince, like themajor wrench assemblages that would have the Irish Great Glen representatives (Leannan, cause them, pull-apartscommonly occur on transform Gweebara, Belshadefaults etc) cutting theMDG Shear margins, particularlyin association with theSAFZ (e.g. Zone (Pitcher & Berger 1972; Soper & Hutton 1984). Even Crowell 1974, 1979). Furthermore, such structureshave if synchroneity was the case, movements on a ‘splay of a already been supposed to exist in the area: Max et al. (1983) splay’ could be as big or as small as one wanted and I find suggested thatthe South Mayo and associated troughs the arguments about the San Andreas fault system to be (about 175 km SW of theMDG), which containLower especially irrelevant in this respect.