Abstract

Report of a meeting of the Tectonic Studies Group held at the University of Cambridge on 21 May 1983.

About 70 geologists and geophysicists from Britain, France, Holland and Spain met to discuss the tectonic evolution of the Pyrenees and surrounding areas. Twenty short talks and 12 posters were presented and the meeting closed with a two-hour 'round table' discussion.

Hercynian geology and Alpine overprinting Vielzeuf referred to lower crustal granulites of probable Hercynian age along the North Pyrenean Fault. Zircon age determinations for these rocks are in progress. In the Aston Massif, Vissers described the progressive folding and transposition of a steep early schistosity (suprastructure) into a relatively flat-lying attitude (infrastructure) with increasing metamorphic grade. The refolding accompanied low-pressure metamorphism and probably resulted from tectonic doming during granite emplacement. Similar low-pressure metamorphic rocks and granite petrogenesis in the Trois Seigneurs Massif were discussed in a poster by Wickham. Although comparatively few participants spoke on Hercynian orogenesis, it appears to have been characterized by unusually high heat flow and intense plutonic activity; the tectonic setting of the area at this time remains obscure.

Andrews, Austin and Colwill described high strains measured on pyrite-cube pressure-shadows in the Llavorsi syncline. These indicate up to 10 km of Hercynian S-directed thrust movement in this area. A similar reverse displacement with a dextral component on mylonites along the Merens Fault was described by McCaig and Miller. ³⁹Ar-⁴⁰Ar dating clearly shows this movement to be of Alpine age, and probably <70 Ma, although excess argon