Internal structure of the Cadillac tectonic zone southeast of Val d'Or, Abitibi greenstone belt, Quebec

Abstract

The Kirkland Lake – Larder Lake – Cadillac Break is a major fault zone of the Abitibi greenstone belt, well known for its spatially associated gold camps. A detailed structural analysis of this fault zone in the Val d'Or area has shown the presence of a 200 – 750 m wide zone of high strain, defined here as the Cadillac tectonic zone (CTZ), which includes the narrower schist zone generally regarded as the Cadillac Break. The boundaries of the CTZ, which coincide with major lithologic contacts, strike approximately east–west and dip 80° to the north.The internal structure of the CTZ is best analysed in terms of two increments of the same progressive deformation. The D1 strain increment is characterized by an east–west-striking, subvertical S1 foliation, at low angle in strike to the bound aries of the CTZ and containing subvertical elongation lineation. Primary lithological contacts have been transposed into S1 and folded by F1a folds. F1b intrafolial folds of S1 ranging from noncylindrical, subhorizontal folds to subvertical sheath folds, indicate dip-slip movements within the CTZ. The D1 strain increment records synchronous dextral transcurrent shearing, as required by the obliquity of S1 to the CTZ boundaries, and subvertical elongation, as indicated by elongation lineations and F1b folds. The D2 strain increment is chiefly characterized by the development of moderately to steeply plunging asymmetric Z-shaped F2 folds with associated S2 cleavage, recording dextral transcurrent shearing.The studied segment of the CTZ is best interpreted as a zone of dextral transpression, evolving from a zone with a significant shortening component (D1) into a zone increasingly dominated by a transcurrent shearing component (D2).