The Superior Boundary Zone (SBZ) forms the northwestern margin of
the Archean Superior craton and constitutes a tectonic foreland
of the ca. 1.8 Ga Trans-Hudson Orogen. The Superior Boundary
Fault (SBF) separates the SBZ from the adjacent Reindeer Zone, a
collage of Paleoproterozoic juvenile intraoceanic rocks.
LITHOPROBE seismic reflection and magnetotelluric data were
acquired along two profiles crossing the SBZ in an attempt to
better constrain the deformation and crustal geometry resulting
from Trans-Hudson orogeny.
Analysis and interpretation of spatially coincident regional
seismic and magnetotelluric data acquired along the southern 200
km profile indicate:
1) the Reindeer Zone accretionary collage
forms an east-dipping, eastward-steepening, crustal-scale
tectonic stack of moderately conductive rocks near the SBZ.
2) The SBZ is characterized at shallow depths (<5-6 km) by steep
to moderately east-dipping reflectivity that is associated with
the limbs of third generation folds (F3, D3) and east-side-up
shear zones. At greater depth, the SBZ crust is highly resistive
and is contiguous to the east with resistive crust beneath the
Superior craton proper.
3) The SBF is recognized in the
subsurface as an abrupt resistivity contrast between the Reindeer
Zone and the SBZ, extending subvertically to 15 km depth.
4) Moderately conductive rocks of the Reindeer Zone extend eastward
for 30 km beneath the SBZ at depths of 15-45 km.
Seismic
reflection data from a second crossing located 100~km NE along
strike indicate a similar crustal structure with some
notable exceptions:
1) The SBF is recognized in the subsurface as
a truncation of interpreted collisional fabrics, and extends
subvertically to ~30 km depth.
2) There is no compelling
evidence for the eastward continuation of Reindeer Zone lower
crust beneath the SBZ. To explain the present day SBZ crustal
structure, we propose that the nature of the SBZ evolved over it
ca. 200 ma convergent margin history from a lower plate
collisional thrust belt setting at >1.88-1.81 Ga, through
lithospheric delamination at ~1.82-1.80 to a steep
transpressive plate boundary at 1.80-1.72 Ga.
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