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.
collections
- retrospective theses [1604]