ABSTRACT: Tectonic Framework of the Precambrian Basement of Southwestern Ontario Revisited

R. M. Easton, Ontario Geological Survey, Sudbury, ON, Canada

Precambrian basement in SW Ontario consists of rocks of the Proterozoic Grenville orogen. Study of drill holes and geophysical data led to subdivision of the basement into several lithotectonic domains. This paper explores how this subdivision relates to the tectonic framework of the Grenville proposed by Carr et al. (Can. J. Earth Sci., 2000). Under the new framework, the exposed orogen in Ontario and New York consists of three tectonic elements: 1) pre-Grenvillian Laurentia and its margin with ~1740 and 1450 Ma continental arc plutons and supracrustal rocks; 2) Composite Arc Belt of allochthonous ~1300-1250 Ma volcanic arcs; and 3) Frontenac-Adirondack Belt of supracrustal and plutonic rocks, of uncertain affinity, that may represent part of the Composite Arc Belt or an offshore (micro)continent. SW Ontario basement consists mainly of rocks of pre-Grenvillian Laurentian and its margin, although Composite Arc Belt rocks occur to the east. The Composite Arc and Frontenac-Adirondack belts were amalgamated by ~1160 Ma, thrust over Laurentia during ~1080-1035 and ~1010-980 Ma convergence, and exhumed by <1040 Ma faulting. The relationship between the Midcontinent rift (~1140-1090 Ma) and Grenville orogenesis is unclear, although the rift likely established a thermal regime that influenced subsequent deformation. Penetrative deformation at ~1080-1035 Ma was restricted to rocks of the Laurentian margin and parts of the Composite Arc and Frontenac-Adirondack belts. Deposition of Late Neoproterozoic sediments (e.g., Lake Ontario, Ohio) may reflect Grenville exhumation. Despite a long pre-Grenvillian history, the present crustal architecture and most seismic reflectivity were acquired during 1080-980 Ma tectonism.

Search and Discovery Article #90907©2000 AAPG Eastern Section Meeting, London, Ontario, Canada