A structural analysis of a metamorphic fold–thrust belt, northeast Gagnon terrane, Grenville Province

Abstract

Northeast Gagnon terrane is located within the Parautochthonous Belt of the Grenville Orogen, near the projected intersection of the front zones of the Grenville and New Quebec orogens. The area consists principally of supracrustal units of the Early Proterozoic Knob Lake Group, and a newly recognized unit, the Equus Lake formation. Both are intruded by the Middle Proterozoic Shabogamo gabbro. Structural elements in the rocks record evidence of a polyorogenic history that is attributed to both the ca. 1800 Ma Hudsonian and the ca. 1000 Ma Grenvillian orogenies. This paper is concerned with the latter.Grenvillian deformation resulted in the formation of a relatively deep-level fold–thrust belt. Three thrust sheets can be defined on the basis of basal thrusts, variations in morphology and orientation of structural elements, and internal thrust sheet geometry. The polydeformational style of the area, rotation of fold axes into subparallelism with the tectonic transport direction, and internal imbrication lead to a complex internal thrust sheet geometry. Thrusting has produced and inverted the metamorphic gradient, with lower greenschist facies in the basal thrust sheet and upper greenschist facies in the upper thrust sheet.Documentation of the northeastern margin of Gagnon terrane as a north- to northwest-directed metamorphic fold–thrust belt corroborates similar interpretations for Gagnon terrane from elsewhere along the Grenville Front and is in accord with the models of the Grenville Province as a collisional orogen. Furthermore, it is suggested that northeast Gagnon terrane is an exhumed, internal, ductile part of a fold–thrust belt.