Discussion on a tectonic model for the emplacement of the Main Donegal Granite, NW Ireland

Abstract

Mr M. J. Arthur writes: Hutton (1982) suggested that the Main Donegal Granite (MDG) was accommodated in a NE–SW zone of tension ('elongate hole'), contemporaneously with NE–SW sinistral shearing around that zone about 400 Ma ago (end Silurian–early Devonian times). Calculations from strain studies suggested that the sinistral displacement was about 20 km at its maximum, 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 established pull-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 from the 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 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 and 5 with an average of about 3 (Aydin & Nur 1982). Emplacement into an ‘elongate hole’ contemporaneously with shearing around it, is precisely a characteristic expected of a pull-apart. Although pull-aparts are perhaps best known for their occupation by sedimentary deposits they also often contain contemporaneous igneous