The Morin Anorthosite Mass underlies an area of 2 500 square kilometers; it consists of three tectonic units: a dome, a diapir and a nappe. The units have in common a large part of their magmatic history, but their tectonic evolution is different. Under the influence of gravity, the dome and the diapir behaved as buoyant masses, while the nappe and part of the diapir spread laterally. Texture and structure of the anorthosite are correlated with these differences in tectonic evolution. Jotunitic and mangeritic rocks envelope the dome and part of the diapir, whereas a troctolite sill occurs in the zone of weakness between the dome and the nappe. The jotunites and mangerites shared a large part of their tectonic evolution with that of the dome, but while the dome was emplaced in an advanced stage of crystallization, its envelope was largely liquid. It is probable that anorthosites, jotunites and mangerites are comagmatic, in the widest sense of the word, and it is possible that troctolite was an early differentiate of the parent magma. The structural relations between the plutonic rocks and the surrounding supracrustal rocks have many characteristics in common with basement-cover relations in polycyclic otogenic zones. However, the plutonites can be considered as a basement complex only if a rather improbable wholesale remobilization of at least the jotunites and mangerites is postulated. The main phase of penetrative deformation in the supracrustal rocks gave rise to gently inclined and recumbent folds (F2) east and northeast of the Morin Mass, and is correlated with the lateral spreading of the anorthosite nappe. This phase overprinted most older structures, but relicts of the latter (Fj) are preserved locally. A set of major open upright folds (F3) is restricted to the area adjacent to the anorthosite nappe and is interpreted as a set of secondorder compression folds related to the emplacement of the nappe. A late phase of deformation gave rise to very gentle major and minor folds (F4) that slightly deform the axial lineation of F.ä-folds. The regional metamorphism is transitional between the amphibolite and granulite facies; it is roughly contemporaneous with the main phase of deformation, thus with the lateral spreading of the anorthosite nappe. The only well-defined isograd is one that separates orthopyroxene-quartz bearing granulites on the side of the Mass from hornblende-quartz bearing gneisses away from the Mass. Late metamorphism took place in and around the anorthosite dome and produced garnet and clinopyroxene from plagioclase and iron-rich mafic minerals.

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The Bic fault is exposed along the shoreline of the St. Lawrence River, 21 km southwest of Rimouski, for 210 m at Cap à l'Orignal and for 100 m at Cap Enragé. The fault brings in contact two major thrust sheets, the Des Seigneuries and the Des Iles, Cambrian lithologies of the former overlying Ordovician rocks of the latter. In the Taconic Orogen, such contacts are normally thrust faults, but the Bic fault is a dextral strike-slip fault, striking east–west and dipping southward. A study of a narrow zone straddling the fault and an adjacent part of the Des Seigneuries thrust sheet has led to the recognition of four successive stages of deformation, all compatible with a northeast–southwest-trending strain ellipsoid. The two first stages are most important: stage 1 brought about regional folding and faulting, whereas stage 2 was characterized by the development of various structural elements (C–S fabrics, stretching lineation, Riedel shears, and kink bands) exclusive to the fault zone. We infer that (i) in the study area, the Bic fault constituted a lateral ramp along which the Des Seigneuries thrust sheet slid horizontally westward; and (ii) emplacement of Ba–Pb–Zn deposits took place slightly after this movement, probably during regional uplift of the orogen in Late Ordovician to Early Silurian time. The latter hypothesis tends to be corroborated by model lead ages of galena in two deposits.

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