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Discussion on the Timing and Kinematics of Cadomian Deformation (Northern Brittany)

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Discussion on the Timing and Kinematics of Cadomian Deformation (Northern Brittany) Journal of the Geological Society, London, Vol. 149, 1992, pp. 151-155. Printed in Northern Ireland Discussion on the timing and kinematics of Cadomian deformation (Northern Brittany) Journal, Vol. 147, 1989, pp. 423426 Special Publication, No. 51, 1990, pp. 133-150 Special Publication, No. 51, 1990, pp. 151-168 Jean-Pierre Brun writes: Strachan et al. (1989), Strachan & comments (e.g. ‘The St Brieuc Thrust where it is exposed at the Roach (1990), Treloar & Strachan (1990) as well as Strachan Plage de Chateau Serein is a late subvertical fault of limited et al. (1990) givean interpretation of Cadomian deformation in displacement which displays none of the deformation features Northern Brittany in terms of a single event of strike-slip tec- to be expected if this were the synmetamorphic crustal scale tonics at 540 Ma andhave proposed a model of ‘terrane amal- structure inferred by Bale & Brun’; Strachan et al. 1990, p. 81). gamation’ which I find in strong contradiction with field and We are obviously aware of the distinction between brittle and geochronological data (Bale & Brun 1983, 1989; Brun & Bale ductile structures and have never maintained that Chateau 1990). Our analysis takes into account regional-scale evidence Serein (cf. location on fig. 1, Bale & Brun 1989) might be a (geological maps, strain patterns and geophysical data) and type-locality for observing the characteristics of the St Brieuc all available and reliable geochronological data. Our result- Thrust. It is located along what we consider to be a lateral ramp ing interpretation stresses that Cadomian deformation started and not the thrust itself. The ‘deformation features to be ex- around 590-580Ma ago, combining thrusting and sinistral pected’ have been described in detail by Bale & Brun (1989) wrenching shear zones, and ended at 540 Ma with the uplift of (maps of foliation and lineation, strain measurements, shear the St Malo migmatite. I would like to discuss three main criteria, texture goniometry analyses . .). Our analyses and points that, for clarity, are dealt with under the following interpretation can be summarized as follows. The St Brieuc headings. Thrust system has an arcuate shape(fig. l), joining the NW-SE- oriented front of the NE-SW-trending lateral ramp. The whole Deformation timing. Bale and myself consider that the Coet- thrust pile displays two major thrust slices. The lower one is mieux-Fort la Latte intrusion, dated at 593 Ma (Vidal et al. composed, from base to top, of coarse-grained amphibolites 1974), is synkinematic and provides an age estimate for early derived from gabbros and diorites (Yffmiac Formation), fine- Cadomian deformation in the St Brieuc area. A similar situ- grained amphibolites with strongly deformed pillow lavas ation is observed in Normandy, where the Coutance diorite (Lanvollon Formation) and metasediments (LeguC Forma- gives an age of 584Ma (Guerrot & Peucat 1990). Strachan & tion). Several serpentine pods occur along the basal contact. Roach (1990, p. 144) write: ‘We do notbelieve that parallelism The rocks are strongly deformed and display a SW- or WSW- of an igneous fabric with a regional cleavage can alone demon- trending stretching lineation. The upper thrustslice iscomposed strate synchroneity of intrusion and regional deformation’. of only two of the previous formations which correspond, from This pluton is strongly elongated with an aspect ratio of 1 :4 base to top, to fine-grained amphibolites with moderately de- for the visible part, and presents a well-defined L-S fabric that formed pillow lavas (Roselier formation, equivalent of Lan- developed in a sub-solidus state (‘magmatic’ s.1.) parallel to vollon) and metasediments (Binic Formation, equivalent of strongly deformed basic inclusions. The pluton and its internal Legue). The rocks in this upper thrust unit are moderately to fabric are both parallel to the regional structural grain. How weakly deformed and less metamorphosed than those of the can Strachan and his colleagues explain such a structural pat- lower slice. Stretching lineations trend SW or WSW. In the tern if not in terms of a synkinematic emplacement? Despite the whole pile,shear criteria indicate a top tothe SW or WSW sense obvious deformationof this pluton, the Rb-Srsystem shows no of shear and strain intensity increase towards the base. resetting towards a younger (c. 540Ma) age, and therefore it The basal units of the St Brieuc Thrust system are brought appears that early Cadomiandeformation occurred in the into contact with the Lamballe formation (pelites, pelitic range 590-580 Ma. This accordswith recent 40Ar/39Ardating of sandstones and black cherts) which are strongly migmatized in metamorphic and magmatic amphiboles that indicates ‘a main the GuingampBelle Isle en Terre area. The paragneisses, mig- regional tectonothermal event prior to c. 570 Ma’ (Brown et al. matites and associated leucogranites present unequivocal shear 1990~). criteria indicating a top to the SW or WSW sense of shear. To the SE, the St Malo migmatite belt is separated from the St Kinematics of deformation. Bale & Brun (1989) and Brun & Brieuc units by the St Cast sinistral shear zone which runs Bale (1990) describe the kinematics of Cadomian deformation parallel to the basal lateral ramp of the St Brieuc thrust sys- in terms of thrusting and wrenching (fig. 1). Strachan and col- tem. The migmatites (dated at 549 Ma; Peucat 1986) and para- leagues concentrate their criticism on thrusting and ignore our gneisses are first thrust toward the S-SSW and then strongly analysis of wrenching. Their criticisms range from puredenial sheared along the St Cast and Cancale sinistral shear zones. (e.g. ‘There is no field evidence to support the suggestion that Finally, it is important tonote that the contour geometry of the major crustal units were amalgamated by thrusting’, Strachan frontal and lateral ramps of the St Brieuc thrust system are & Roach 1990, p. 148) to fallacious citations and misleading well marked by strong magnetic and gravimetric anomalies, 151 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/149/1/151/4891848/gsjgs.149.1.0151.pdf by guest on 26 September 2021 152 DISCUSSION and picked out by weakly dipping reflectors on the SWAT 10 I am very grateful to all my colleagues and students involved in the seismic line (Lefort & Bardy 1987; Brun & Bale 1990). study of the Cadomian of North Brittany: S. Aitomar, B. Auvray, P. My main criticisms of Strachan and colleagues are as Bale, M. Ballevre, P. Bardy, R. Charlot, J. J. Chauvel, J. Cogni, D. follows. Gapais, V. Genestier, P. Graviou, E. Hallot, C. Le Corre, J. P. Lefort, P. Maillet, H. Martin, J. J. Peucat, P. Vidal from the Rennes 1 Univer- (1) Their 540 Ma strike-slip shearzone interpretation sity and J. Chantraine, E. Egal, E. Le Goff, C. Guerrot and D. Rabu ignores regional-scale structures as they are displayed on geo- from the B.R.G.M. As a group and as individuals, all of them have logical and geophysical maps. These are first order structures contributed to some extent to the ideas that I defend here. Several of that must be considered in making a regional scale interpreta- them share my opinions and could have written a rathersimilar discus- tion. I would suggest that Strachan and his colleagues have sion. Thanks are dueto M. S. Carpenter for improvements of English spent little time mapping inland, leading them to write: ‘The style and to W. Gibbons for very constructive suggestions. poorly exposed Yffiniac metagabbros’ (Strachan et al. 1990, p. 72) or ‘Difficulties in accepting this model arise from the 10 December 1990 almost complete lack of inland exposures’ (Strachan & Roach 1990, p. 145). The 1/50.000 cooperative mapping programme undertaken by the B.R.G.M. and Rennes 1 University demon- R. A. Strachan, M. Brown, R. S. D’Lemos, R. A. Roach & P. J. strates that,even in this areaof poor exposure, it is possible to Treloar reply: The Short Paper of Strachan et al. (1989) sum- find enough outcrops to make precise maps. marizes the work of Strachan & Roach (1990), Treloar & (2) On the coastal maps of Strachan & Roach (1990) and Strachan (1990) and Brown et al. (1990a). We interpret the Treloar & Strachan (1990), only ENE-WSW-trending and Cadomian belt of the North Armorican Massif as a collage of shallow-plunging stretching lineations are portrayed. There is variably displaced terranes produced by the amalgamation at no trace of NNE-SSW-trending lineations. One exception c. 540Ma of Cadomian continental arcs and marginal basin exists for a single area on fig. of Treloar & Strachan (1990). 4 complexes by sinistral transpression along a continental mar- But this single recognized thrust is interpreted as due to mig- gin above a subduction zone. This model contrasts with that of matite diapirism during strike-slip shearing. Evidence for SW- Bale & Brun (1983,1989) and Brun & Bale (1990)who propose trending lineationshas been known for a long time (Brun 1977 that thrusting at c. 590-580 Ma resulted in the obduction of a in the St Malo migmatite belt; Bale & Brun 1983 for the St ‘back-arc basin’ (Baie de St Brieuc) southwestwards over a Brieuc thrust system). ‘continental margin’ (St Malo-Mancellia). The resultant (3) Strachan et al., whilst failing to recognize the importance crustal thickening is thought by them to have led to partial of thrust-related structures, emphasize the presence of steep melting of Brioverian rocks at depth and tothe formation of the sinistral strike-slip zones. They fail, however, to give fair St Malo migmatites and the Mancellian granites (cf. also recognition to the fact that these strike-slip structures have Graviou & Auvray 1985; Graviou et al. 1988). We thank Brun been known for some time (e.g. Brun 1979, 1981; Bale& Brun for his discussion of our papers and model, and welcome the 1989; Brun & Bale 1990; Aitomar 1982; Bale 1986).
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