The horizontal opening of vertical fractures during emplacement
of
pegmatite-dyke swarms is
an important mid-crustal mechanism of large-scale horizontal extension.
This
is documented in the south-western
Grenville Province, a deeply eroded part of the collisional Grenville orogen.
In the Georgian Bay
region of central Ontario, Grenville gneisses host c. 990 Ma old,
angular dykes which attest not only to
horizontal extension but also to vertical thinning. The original dilation
dykes probably varied in strike and
were statistically vertical. However, many dykes had subhorizontal or
inclined segments that were oblique
or quasi-concordant to the gneissic foliation in the host rocks. The number
of dykes exposed per 0.25 km2
varies on the scale of a few kilometres, and this is indicative of heterogeneous
late orogenic extension of
the Grenville gneisses. The apparent absence of regional gradients of peak
palaeo-pressure, at the present
erosion level, suggests that the extension was horizontal and initially
unaccompanied by vertical contraction
of the host gneisses. Subsequent buckling of the pegmatite dykes led to
gentle,
open or close folds with
vertical enveloping surfaces. The geometric effects of gentle buckling
of
pegmatite dykes can be difficult to
recognize in the field, especially where the late-stage vertical thinning
is
relatively weak. Among the geometric
indicators of buckle-shortened dykes, the characteristic deflection
(‘fanning’) of coplanar, inherited
folia in gneissic host rocks is most sensitive. Systematic changes in the
local degree of vertical shortening
are indicative of heterogeneous vertical thinning, and may be associated
with pull-apart structures at the
horizontal scale of several kilometres.