Gravity study of an Archaen crustal segment near Thunder Bay, Ontario

Abstract

A gravity survey in an area involving portions of the Quetico and Shebandowan subprovinces of the Superior structural province resulted in the establishment of 350 new gravity stations in addition to 50 previously established stations. For the purposes of this study the bedrock was divided into 4 major units including the metavolcanic rocks* metasedimentary rocks, assorted gneissic rocks and a group of intrusive igneous bodies. Based on gravity modeling of these principal units, the following subsurface structure results. The metavolcanic rocks vary in depth extent from 6 km to 12 km and typically occupy a trough-shaped structure flanked by a domical feature of granitoid gneisses to the south. To the north of the metavolcanic unit, the metasedimentary rocks form a basin-like structure of variable depth extent. The contact between these two units is modeled as dipping steeply southward. The Quetico granitoid gneisses to the north of the metasedimentary unit form a southward dipping wedge which extends to depths of 10 km in the model structure profiles. These gneisses are modeled as being underlain by a denser substratum representative of more basic gneiss and amphibolite. The basal surface of this unit is gently convex upward, and the unit as a whole thickens toward the north and dips toward the south in a tapered wedge extending to a depth of 10 km. This unit is inclined slightly from east to west, and is exposed at the present erosion surface in the eastern portion of the study area only. A number of quartz monzonite plutons have been emplaced near the northern portion of the metasedimentary unit and outcrops above the southern most edge of the dense substratum underlying the Quetico gneisses. A tectonic model put forward synthesizes the available information regarding Archean crustal evolution and the data from this study. The proposed model consists of a basin-like structure (Quetico subprovince) which received sedimentary and volcanic debris from a mature volcanoplutonic terrane to the north (Wabigoon subprovince). Additional sedimentation into the basin was provided from the less mature volcanoplutonic terrane to the south (Shebandowan subprovince). Subsequent deformation accompanied by high grade metamorphism resulted in a general upwarping of the basin, producing the structures now observed in this part of the Quetico subprovince.