Overview of Results from the Greenstone ... - Geology Ontario

1999) in their REE patterns and other trace element signatures, and provide the basis for refining the mapscale
distribution of these formations and, hence, definition of the earliest folding in D2. These data also
lead to inference of the tectonic setting, involving plate–plume interference.
Early stratigraphic excision (or non-deposition) resulted in an incomplete Tisdale assemblage
sequence below Porcupine assemblage sediments. To the north of a septum of Porcupine assemblage
sediments, Tisdale assemblage basalts lack the upper (geochemically defined) Gold Centre and Vipond
formations. Below the Porcupine syncline sediment, there is a contact truncating early folds (Buffam
1948), and this unconformity cuts down the Tisdale assemblage section to the east. These facts are
interpreted to indicate an angular unconformity between Tisdale and Porcupine assemblages, and that it
represents an extensional or normal faulting event that excised some Tisdale assemblage stratigraphy, and
possibly all of the Blake River assemblage if it was deposited here. These contacts are folded by F2 folds.
A D2 foliation has been systematically mapped in Tisdale assemblage rocks that is not identified in
rocks above the Timiskaming unconformity. Folds are defined by geochemical-stratigraphic patterns of
basalt formations and foliation trends with overprinting relationships. Folds associated with D2 strike
east-west, and are refolded by F3 in S-asymmetrical folds (see Figure 26). They are interpreted as
representing hanging-wall anticlines in 3 or 4 stacked south-over-north thrust panels. The Porcupine
assemblage is affected by this deformation, since it is involved in these same folds. The earliest element
of the PDDZ is C-S structures indicating south-over-north thrusting along a south-dipping zone, which is
probably the root zone to the stacked thrusts. A post-Porcupine age for thrusting coincides with age data
from the Duparquet Basin, Quebec (Mueller et al. 1996).
The F3 folds and S3 foliation are strongly developed in Tisdale assemblage rocks, and consist of a
series of en échelon folds with curved axial surfaces along the PDDZ. The F3 fold axes do not affect the
trace of the PDDZ, and do not extend across it to its southern side. In contrast to the Tisdale assemblage,
F3 folding is quite minor, yet S3 foliation in Timiskaming rocks is strong. The Timiskaming assemblage
occupies a syncline or half-graben with an faulted southern margin (the early formed Dome fault)
adjacent to a counterclockwise deviation of the PDDZ from its east-northeast trend. These facts are
interpreted to indicate formation of F3 and S3 during left lateral strike-slip movement along the shear
zone, and the syntectonic opening of the Timiskaming half-graben in a dilatational jog, followed by
foliation and folding, only the late stages of which affected the sediments, an interpretation which shares
analogies to that of Bleeker (1995).
Folding in D4 created the syncline that preserves the Timiskaming assemblage rocks. Foliation S4
crenulates S3 and is axial planar to F4 folds that commonly have Z asymmetry. Minor structures suggest
right lateral strike-slip along the PDDZ (Bleeker 1995). As part of this event, shortening strain was
partitioned on the local scale around the Timiskaming unconformity into relatively small-scale reverse
faults. The last significant event, D5, consists of an intense constrictional strain seen in Tisdale pillows,
clasts in Krist formation volcaniclastic rocks and cobbles in Timiskaming assemblage conglomerates.
Deformation event D4–D5 represents the final stage in transpressional deformation along the PDDZ.
Reversal of strike-slip direction along the shear zone converted an initially dilatational jog into a
compressional jog, accounting for constriction adjacent to the jog in the shear zone trend. The larger scale
significance of the shift from left lateral to right lateral strike-slip is unclear from this study at the scale of
the gold camp. One of either the D3 or D4 episodes may have been overall coaxial shortening with
differing strike-slip during different orientation intervals, and the other episode may have been more
strictly non-coaxial strain with a constant sense of strike-slip, as along the Larder Lake–Cadillac
deformation zone (LLCDZ)(Wilkinson, Cruden, and Krogh 1999). The solution to this question lies in
further structural analysis along the trace of the PDDZ.
68