Table 13. The structures of the Cariboo Terrane organized into a series of deformations

(D) of surfaces (S), and folds (F).

Event Structure

Metamorphic mineral

D, Extension-cross faults striking NE and steeply dipping to S.
Extension faults striking NNE and N with steep dip and having

Oblique-slip and some strike-slip.

D; Open folds, warps, kinks and crenulations (F;). Chlorite?

(F3) Normal and reverse strike faults steeply dipping.

D, Pervasive axial surface cleavage (S,); asymmetric (W-directed) Quartz, albite, biotite,
(S>, flow to concentric folds (F,) that trend and plunge to NW; thrust chlorite muscovite

F,) faults E-dipping chloritoid,

D, Displayed in carbonate. Bedding-cleavage (S,):; isoclinal recumbent Quartz, albite?,

(S,) rootless fold (F,) thrust? muscovite

fold axes were recorded from these features to conclude
anything on their orientation with respect to younger fold-
ing. Those measurements taken indicate that hinges both
parallel and diverge to varying degrees from the average
orientation of the major folds. Layer parallel veinlets of
calcite crosscut the isoclines and are themselves folded
by later phases. Figure 8 shows this vein-isocline relation-
ship and the development of a crosscutting, and there-
fore later, parting.

Ductile shortening

Cleavage, folds and faults of this category display charac-
teristics of ductile flow.

Cleavage

The term cleavage as used here follows the definition
from Bayly et al. (1977, p. 9):

“Set of closely spaced secondary planar parallel
fabric elements which impart mechanical anisotropy
to the rock, without apparent loss of cohesion.’’

Included under this definition are such features as folia-
tion and schistosity.

The cleavage is pervasive throughout Cariboo Terrane
rocks of the map area. It strikes dominantly west-
northwest and dips moderately to steeply to the north-
northeast. Variations in orientation are displayed by the
Lambert net plots of Figure 24 whose domain divisions
are shown in Figure 25. The cleavage consists of parallel
surfaces of mica and is axial planar to outcrop and region-
al scale folds.

The cleavage is defined by metamorphic muscovite,
quartz, albite, chlorite and locally biotite, and its char-
acter is governed by rock type. The metamorphic micas
are generally parallel suggesting the peak temperature of
metamorphism coincided with the formation of the cleav-
age. Exceptions are on Anderson Ridge where biotite ran-
domly overgrows muscovite foliation surfaces, on Middle
Ridge where chloritoid overgrows muscovite foliation

38

(Fig. 26) and near Waverly Mountain where felted stilp-
nomelane overgrows the chlorite foliation of the Waverly
Formation basalt tuff. The temperature of metamor-
phism is mostly 300-400°C as defined by the colour alter-
ation index of conodonts (from CAI chart of Epstein et
al., 1977) extracted from the Black Stuart Group and
Greenberry Formation. Locally it may have been higher
as the conodonts are from stratigraphically high units.

The effect of rock type on cleavage will be discussed
for impure quartzite and siltite, quartzite, conglomerate,
limestone and pelite.

The impure quartzite and siltite of the Cariboo Group
has cleavage defined by parting along white mica and
minor chlorite foliation surfaces. The micas grow in a
preferred orientation and form a meshwork around resist-
ant quartz and feldspar grains. The result is not discrete-
ly spaced cleavage surfaces, but rather, a continuous mass
of oriented mica interrupted by the resistant coarser
grains. The parting is along the orientation direction and
the surfaces have a topography governed by several resist-
ant grains. A similar type of cleavage occurs in conglom-
erates of the Snowshoe Group, however, the clasts are
invariably flattened and stretched in the plane of mica
foliation. Thin pressure solution selvages associated with
the large clasts induce a spaced parting not evident in the
finer grained rocks. The relief of the parting surfaces is
half of the short axis of the largest grain.

Microscopically, pressure-solution features form an
integral part of the cleavage. Solution selvages composed
of opaque matter are erratically spaced, parallel to the
mica elongation, discontinuous and localized. They are
volumetrically more important in poorly sorted, coarser
grained clastics. Quartz grain shape is in part controlled
by solution.

In Yankee Belle Formation impure quartzite of the
Roundtop-Middle Ridge system preferentially weathered
cleavage is spaced at 0.5 to 2.5 cm intervals, imparting
a lenticular appearance to the impure quartzite (Fig. 27).

Quartzite of the Yanks Peak Formation displays
foliation, dissolution surfaces, and subgrain trains, but