Structure of Ahmic domain and its vicinity, southwestern Central Gneiss Belt, Grenville Province of Ontario (Canada)

摘要

In the Central Gneiss Belt of Ontario (CGB), the parautochthon and a stack of four thrust sheets have been transformed into a complex system of inclined and upright folds. One large upright structure, the 50 km long Wahwashkesh-Ahmic Lakes antiform (WALA), folds the parautochthon and three thrust sheets represented by rock assemblages in the Ahmic, Parry Sound or Shawanaga domains. Despite its tight map pattern west of Magnetawan village, WALA is a gentle to open, disharmonic, noncylindrical, buckle fold that deforms the main foliation and other ductile-style mesoscopic structures in the Grenville gneisses. The main foliation is subhorizontal or moderately inclined, at most localities, but deforms steeply dipping quartz-feldspar veins, possible vestiges of gneissic layers, in the core region of the Ahmic domain. Such planar features could be due to a subhorizontal shortening strain associated with the development of 1080 Ma ductile thrusts in the CGB. The main foliation in the Ahmic domain, on the other hand, is attributable to 1020 Ma vertical thinning of the thrust stack. Transverse gradients of total strain, documented by others over distances of hundreds of meters, characterize the walls of major ductile thrusts. Our results of detailed strain-fabric mapping in metaplutonic rocks presently situated at the Ahmic-Parry Sound domain border, apparent sole-thrust segment of a synformal klippe, provide no evidence for such transverse gradients. We hypothesize that the original walls of the thrust were removed from the level of the present erosion surface, presumably by dip-slip extension faulting.