Abstract

Jean-Pierre Brun writes: Strachan et al. 1989, Strachan & Roach (1990), Treloar & Strachan (1990) as well as Strachan et al. 1990 give an interpretation of Cadomian deformation in Northern Brittany in terms of a single event of strike-slip tectonics at 540 Ma and have proposed a model of ‘terrane amalgamation’ which I find in strong contradiction with field and geochronological data (Balé & Brun 1983Balé & Brun 1989; Brun & Balé 1990). Our analysis takes into account regional-scale evidence (geological maps, strain patterns and geophysical data) and all available and reliable geochronological data. Our resulting interpretation stresses that Cadomian deformation started around 590–580 Ma ago, combining thrusting and sinistral wrenching shear zones, and ended at 540 Ma with the uplift of the St Malo migmatite. I would like to discuss three main points that, for clarity, are dealt with under the following headings.

Deformation timing. Balé and myself consider that the Coët-mieux-Fort la Latte intrusion, dated at 593 Ma (Vidal et al. 1974), is synkinematic and provides an age estimate for early Cadomian deformation in the St Brieuc area. A similar situation is observed in Normandy, where the Coutance diorite gives an age of 584 Ma (Guerrot & Peucat 1990). Strachan & Roach (1990, p. 144) write: ‘We do not believe that parallelism of an igneous fabric with a regional cleavage can alone demonstrate synchroneity of intrusion and regional deformation’. This pluton is strongly elongated with an aspect ratio of 1:4 for the visible part, and presents a well-defined L-S

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