Geology Reference
In-Depth Information
The now-exposed width of the
zone is very narrow - from less than
a metre to a maximum of only 10 km.
However, the zone must originally
have extended for a considerable dis-
tance eastwards beneath the Moine
thrust and westwards to incorporate
the Caledonian thrusting in the base-
ment of the Outer Hebrides, where
the platform cover has been removed
by erosion, and possibly even further
west onto the continental shelf.
The thrusts seem mainly to have
propagated forwards, towards the fore-
land, such that the youngest thrusts,
involving only Cambrian sediments,
carry older Lewisian basement on
their roofs, with the oldest, the Moine
Thrust, overlying the whole package.
The Moine thrust differs from the
younger thrusts in being characterised
by a thick band of
mylonite
, indicating
derivation from considerable depth.
The thrust movements are attrib-
uted to the Early Silurian
Scandian
orogenic phase, discussed below.
The Moine thrust zone is well
exposed in the Assynt district of NW
Scotland, which lies in the
North West
Highland Geopark,
one of only eight
geoparks in the UK, and internation-
ally known as the area where Peach and
Horne and their colleagues of the British
Geological Survey first mapped and
explained the complex thrust geometry.
Much of the geology can be readily
appreciated in the scenery from the
roadside (e.g. Figure 12.3B) and some
of the classic exposures are explained
at viewpoints and at a visitor centre
at
Knockan Crag,
where a section
through the Moine Thrust is exposed.
The Northern Highlands
This zone contains a thick sequence
of Late Proterozoic (
Neoproterozoic
)
marine clastic sediments, the
Moine
Supergroup
, thought probably to be
the lateral equivalents of the Torrido-
nian sequence on the foreland. They
rest on a basement of Lewisian-like
gneisses, and are overlain in the east
by post-orogenic Devonian cover
(the
Old Red Sandstone
). The Moine
sequence was deposited between
~1000 Ma and ~870 Ma after the end of
the
Grenville orogeny
(
see
below) and
is usually regarded as resulting from the
erosion of the Grenville orogen. It has
been suggested that the Lewisian-like
basement that underlies the North-
ern Highlands may actually lie within
the Grenville belt and may thus have
been affected by Grenvillian orogeny.
The Moine Supergroup has been
intensely deformed and metamor-
phosed up to
amphibolite facies
, and
has experienced three major orogenic
events. The earliest of these is termed
the
Knoydartian
and took place
around 800 Ma. It is represented by
syn-orogenic granitic intrusions and
pegmatites and also by some meta-
morphic ages. The earliest recognis-
able foliation is associated with this
metamorphic event, but the nature and
extent of the Knoydartian structures
have been obscured by the younger
orogenic phases. Although the Moine
Supergroup and its deformation have
usually been considered to belong to the
Caledonian orogeny, a period of more
than 300 Ma separates the Knoydartian
event from the main Caledonian oro-
genic phase, and its origin is obscure.
The second phase, the
Grampian
orogeny
, is the main tectonic event to
affect the Grampian zone to the south,
but its effect in the Northern Highlands
is much less obvious owing to the effect
of the succeeding Scandian event;
Grampian structures have been defi-
nitely recognised only in the
Swordly
nappe
, in the eastern part of the zone.
The
Scandian orogeny
is the most
important tectono-thermal event to
affect the Northern Highlands and is
attributed to the collision between
Baltica and Laurentia in the Early Silu-
rian. Scandian structures include ductile
thrusts and related recumbent folds that
are overthrust towards the west-north-
west. These structures were re-folded
by more upright folds with a NE-SW to
NNE-SSW trend. The thrust-sense shear
zones include the Moine thrust itself,
the
Sgurr Beag thrust,
and the
Swordly
thrust
, which lie structurally above it,
to the east. Folding was accompanied
by amphibolite-facies metamorphism
dated at 435-420 Ma; these dates cor-
respond to the date of the movements
on the Moine thrust, at ~435-430 Ma.
The structures of the Moine schists in
central Ross-shire and Sutherland have
attracted structural geologists since
the 1950s, and many early innovative
studies of superimposed folding and
shear zones have been carried out there.
12
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99
The Grampian (Central) Highlands
This zone contains a thick marine
clastic sequence, the
Dalradian Super-
group
, consisting of sandstones, silt-
stones, mudstones and limestones
with a total thickness of ~20 km. It is
thought that this sequence was laid
down on the extended passive margin
of Laurentia in several
half-graben
(Figure 12.4.1;
see
also Figure 10.3).
The base of the sequence is of late
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