North‐west propagation of the Grenville orogen: Grenvillian structure and metamorphism near Key Harbour, Georgian Bay, Ontario, Canada*

Abstract

Abstract We report structural and metamorphic data from a c. 25‐km transect across the eastern Grenville Front Tectonic Zone (GFTZ) to the Britt domain at the northern end of Georgian Bay near Key Harbour, Ontario. Constrasting Grenvillian structural and fabric elements characterize the eastern GFTZ, northern Britt domain and a narrow Transition Zone between them. Moderately to steeply dipping foliations with strong down‐dip lineations in all three divisions appear to be associated with NW‐directed thrusting. In the Transition Zone and northern Britt domain, early S = L fabrics with steep lineations are overprinted by younger structures (S > L) with shallow, SE‐SSE‐plunging lineations in which sparse, dominantly (but not exclusively) normal‐sense kinematic indicators are recognized. Pressure and temperature estimates from Grenvillian metamorphic assemblages in metadi‐abase indicate that conditions of P ± 12 kbar and T c 800° C were achieved before or during the thrust‐related deformation, with P‐T‐t paths that indicate near‐isothermal decompression to P c. 4 kbar and T c. 700° C. Correlation of fabric elements with points on the P‐T‐t paths suggests that exhumation occurred during two stages, the first associated with thrusting (≥1035 Ma) and the second with extension and thrusting (pre‐1003 Ma). The GFTZ contains steeply to moderately dipping, thrust‐related fabrics and lacks shallow, extensional structures; the latest episode of thrusting in the GFTZ is inferred to have taken place at 990‐980 Ma. The data are interpreted in terms of a tectonic model involving two stages of propagation of the Grenville orogen towards its foreland (≥1035 Ma and ≥980 Ma), with an intervening period of extension, although the tectonic regime probably remained compressional on the scale of the orogen.