Brittle structures

The cleavage, folds and faults of this category transpose,
transform and translate, respectively, all earlier struc-
tures. Unlike the previous two structure groups, brittle
structures include extensional features. A range of brittle
structures produced at various times and high structural
levels have been amalgamated into this part of the struc-
tural spectrum.

Cleavage and folds

The cleavage has an open crenulation style in phyllite but
is not visible in quartzite. Earlier muscovite and biotite
are crenulated with the formation of stylolitic-type cleav-
age selvages. Finely crystalline white mica forms part of
the selvages. Chlorite, which has randomly overgrown
earlier foliations and replaced biotite and garnet in varia-
ble amounts, is folded by, and overgrows, the crenula-
tions. The chlorite retrograde metamorphism therefore,
occurred after the late open structures of the ductile
period of shortening.

Crinkles of phyllite, kinks, and widely spaced hand-
size open folds have a more variable attitude than earlier
folds. The crinkles and kinks have a single cleavage or
joint respectively through their hinges. The kink folds
have broken hinge zones. The open folds have a spaced
crenulation cleavage.

Compressional

The strike fault through Mount Tom, Island, Cow and
Richfield mountains may be a steep reverse fault, and
therefore compressional. There is some indication that
it is a dextral strike-slip fault (Struik, 1985). The strike
faults are cut by northeast-trending extensional faults.

Extensional

Northeast- and north-trending steep faults, important
hosts of gold-bearing quartz veins, appear to offset all
other structures, and therefore are assumed to be the
youngest feature in the Snowshoe Group structural pack-
age. It is not clear whether a distinct timing preference
exists for the northeast- as opposed to the north-trending
faults. Nor is it clear that the sense of movement is con-
sistent in those groups. The more northerly-trending
faults have right-lateral strike-slip as well as dip-slip,
whereas those trending northeasterly may record more
southside-down dip-slip motion than strike-slip. In this
manner the northeasterly faults may record the exten-
sional component to a northerly-oriented strike-slip
regime. Vein filling within faults of both orientations indi-
cates an overall extension. In combination, strike-slip and
extension, the system, may be thought of as transtension-
al, related to Cretaceous and/or Early Tertiary strike-slip
and uplift of the Omineca Belt metamorphic terranes.

Veins that are planar for outcrop-sized distances and
are parallel to north-trending faults or northeast-trending
joints are considered the latest generation. Locally these

va

Figure 51. Flow-folded quartz vein in Snowshoe Group
micaceous quartzite near Mount Borland. (GSC 191042)

apparently young veins are transposed along the region-
al foliation (Fig. 51). Examples of late movement along
the foliation surfaces suggest sporadic slip on the cleav-
age surfaces throughout the deformation of the Barker-
ville Terrane.

The crinkles, kinks, small open folds, and extension-
al faults represent deformation in a high viscosity regime;
sufficient to produce brittle features.

Summary

The structures of the Snowshoe Group package are
divided into three groups; shear, ductile shortening, and
brittle shortening and extension. The groups highlight
changes in the strain conditions of protracted deforma-
tion (Fig. 52).

Shear, as indicated by mylonite (as defined by Wise
et al., 1984), bedding cleavage, and rootless recumbent
flow folds, dominates the earliest deformation. It is inter-
preted as the response to the eastward overthrusting of
the Crooked Amphibolite along the Eureka Thrust. The
thrusting is reasoned to be Late Triassic or Early Jurassic.

Ductile shortening, as denoted by flow cleavage and
folds, and thrust faults, overprints the shear fabric. Con-
ditions of metamorphism were sufficient throughout the
shear and ductile shortening to crystallize and successive-
ly recrystallize muscovite, albite and quartz. Biotite and
garnet crystallized locally and once during the period of
ductile shortening. The ductile shortening is a response
to compression, possibly representing collision between
the two land masses of North America and Quesnellia.
The collision began in the Early Jurassic, assuming the
metamorphism records the collision and the metamor-
phism is Early Jurassic (Andrew et al., 1983).

The transition from ductile to less ductile compres-
sion may mark the change from Jurassic collision to
Cretaceous transpression and strike-slip (see Gabrielse,