Geology Reference
In-Depth Information
collision with the Avalonia microplates
that have migrated from Gondwana.
The British Isles sector of the Cal-
edonides is about 300 km wide and
consists of nine distinct tectonic zones
separated by major fault boundaries
(Figure 12.2). These are, from north-
west to south-east: North-west Fore-
land (Laurentia),
Moine Thrust belt
,
Northern Highlands
,
Grampian
(or
Central
)
Highlands
,
Midland Valley
,
Southern Uplands
,
Lake District
,
Welsh Basin
and
Midlands Platform
, or
South-eastern Foreland (part of the East
Avalonia microplate). The Northern and
Grampian Highlands zones together
make up the central metamorphic core
of the orogenic belt. These zones were
established in Scotland and England;
however the Northern Highlands, Gram-
pian Highlands, Southern Uplands
and Lake District zones can also be
traced across into Ireland, though with
considerable differences in detail.
The Northwest Foreland
This zone consists of a mainly gneissose
basement of Early Proterozoic age (the
Lewisian complex
) comparable with a
formerly adjacent Early Proterozoic belt
in East Greenland at the south-eastern
margin of the Laurentian continent.
This Lewisian basement is unconform-
ably overlain by unmetamorphosed
continental clastic red-bed sequences,
the
Stoer group
and the
Torridonian
Supergroup
, deposited at around
1200 Ma and 1000 Ma respectively,
and by a Cambrian to Early Ordovi-
cian shallow-marine shelf sequence.
Laurentia plate
Metamorphic core zones
Great
Glen
fault
Laurentia
plate
Moine
thrust
Midland Valley terrane
Northwest
foreland
Southern Uplands
terrane
A
Lake District terrane
Grampian
Highlands
Welsh basin
The Moine Thrust belt
This zone is a classic foreland fold-
thrust belt in which the foreland
sequence is involved in several major
thrust packages, or nappes, each of
which is divided internally by smaller
thrust slices. An example of this struc-
ture is shown in Figure 12.3A, which
shows several nappes containing
foreland rocks overlain by a higher,
exotic nappe, resting on the
Moine
Thrust
, which forms the boundary
with the Northern Highlands zone. The
second nappe of Figure 12.3A is char-
acterised by an
imbricate structure
,
where the same Cambrian sequence
is repeated many times in succes-
sive thrust slices. Structures in this
zone, such as elongation lineations
and deformed fossil burrows, indi-
cate a WNW-directed shear sense.
Moine
thrust
Younger cover
Midland
Valley
Solway (Iapetus)
suture
strike-slip
fault
thrust
Lake
District
suture
LD
A
Welsh
basin
LM
Midlands
Platform
Solway (Iapetus)
suture
B
R
East Avalonia
plate
100km
Figure 12.2
Main tectonic zones of the British-Irish Caledonides. LD, Longford Down inlier; LM,
Leinster massif; A, Anglesey; R, Rosslare.
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