Ancient orogenic belts - Introducing Tectonics, Rock Structures and Mountain Belts - page 103

which is likely, therefore, to belong
to the stretched margin of Laurentia.
These ophiolites, together with the
more extensive oceanic assemblages
in Newfoundland, East Greenland and
Norway that have been thrust over
the Laurentian and Baltican forelands
respectively, may represent back-arc
basins (e.g. see Figure 12.4.2). The
present-day boundary of the Midland
Valley terrane with the Grampian High-
lands is the steeply-inclined Highland
Boundary Fault, ( see above and Figure
12.3D). The fact that the sequences
presently juxtaposed across the High-
land Boundary Fault cannot be directly
matched means that it must represent
a terrane boundary, and the ophiolite
thus represents a suture between Lau-
rentia and the Midland Valley terrane.

Figure 12.4 opposite Cartoon sections across the British Caledonides. An interpretation of the tectonic
evolution of the crust in this sector of the Caledonides from mid-Cambrian to early Devonian times;
the section is along the line A-B in Figure 12.2. 1 Mid-Cambrian (~510 Ma). Shelf sediments and the
continental-slope Dalradian sequence are deposited on stretched Laurentian basement incorporating
the late Proterozoic Moine Supergroup; a wide ocean (the Iapetus Ocean) separates Laurentia from
the Gondwana supercontinent. 2 Late Cambrian (~490 Ma). The Dalradian sequence is complete;
an oceanic volcanic arc, the Midland Valley arc (MVA), forms at a subduction zone within the Tethys
Ocean. 3 Early Ordovician (480 Ma). A piece of oceanic crust (from the back-arc basin of the MVA?)
is obducted onto the Laurentian margin, forming the ophiolite nappe and initiating NW-directed
overfolding/thrusting and metamorphism of the Dalradian in the north (the Grampian event). 4 Mid-
late Ordovician (~470 Ma). The Midland Valley arc collides with the Grampian Highlands, causing
SE-directed back-folding in the south in a continuation of the Grampian event. A volcanic arc, the
Lake District arc (LDA) forms at the margin of Gondwana, with a back-arc basin (the Welsh basin)
behind it. 5 Early-mid Silurian (~450 Ma). A piece of Gondwana (Avalonia) has broken away and
travelled towards Laurentia. The SE-directed subduction beneath the Lake District has ceased and a
new, NW-directed subduction zone formed beneath the Midland Valley as the two continental plates
converge, causing an accretionary prism to form in the Southern Uplands. 6 Late Silurian-early
Devonian (~400 Ma). The two continental plates, Laurentia and Avalonia, have collided, the Solway
(or Iapetus) suture marking the junction between them. The subduction zone has moved further NW,
and a post-tectonic granite suite, with associated volcanics, forms in the Grampian Highlands. The
Midland Valley, Southern Uplands, Lake District and Welsh Basin all experience folding and slate-
grade metamorphism. Because of the oblique nature of the collision (transpressional), sinistral strike-
slip faults form (including the Great Glen (GGF), Highland Boundary (HBF), and Southern Uplands
(SUF) faults). The Northern Highlands experience NW-directed overfolding and thrusting (e.g. the
Moine Thrust zone (MTZ) and Sgurr Beag thrust (SBT) in the Scandian event as a result of the
Baltica-Laurentia collision.

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The Southern Uplands
This zone consists of several fault-
bounded packages of steeply dipping
Ordovician to Silurian strata (Figure
12.3E). The beds in each individual
package become younger northwards,
although the more south-eastern pack-
ages contain younger material. The
individual successions in the north
contain at their base Arenig-age basalts
and cherts overlain by black shales that
seem to have acted as weak detach-
ment surfaces. These are believed to
represent thrusts that have been steep-
ened into their present attitude as a
result of the deformation as shown in
Figure 12.3E. The shales are succeeded
by sedimentary sequences dominated
by greywacke turbidites. The steeply
inclined strata trend uniformly NE-
SW and are affected by asymmetric
upright folds. The shales possess a slaty
cleavage. The zone is bounded in the

the presence of Moine sequences
with similar dates on both sides of
the fault suggests that it need not be
regarded as a major terrane boundary.

ophiolite assemblage. The ophiolites are
succeeded by a sequence of Ordovician
to mid-Silurian, mainly clastic, sedi-
ments containing clasts ranging up to
boulder size, derived from a volcanic arc
believed to lie in the central part of the
zone in the period ~490-420 Ma (Figure
12.4.2-5). The sediments are unmeta-
morphosed and gently folded. The
Midland Valley is thus believed to repre-
sent an oceanic volcanic arc terrane that
has been tectonically welded to Lauren-
tia. Fossil assemblages both here and in
the Southern Uplands have Laurentian
rather than Gondwanan affinities.
Geophysical evidence indicates that
the Ordovician cover lies on a crystalline
basement similar in properties to that
underlying the Dalradian to the north,
and in Ireland, ophiolites are seen to
be thrust over gneissose basement,

The Midland Valley
This zone is relatively narrow - only
about 80 km across - but it represents
an important component of the orogen
that is better represented elsewhere,
as will be seen. Lower Palaeozoic rocks
are only exposed in a few small inliers
near the northern and southern margins
of the zone, the Caledonian history of
which is mostly concealed by Upper
Palaeozoic cover. Nevertheless, the
inliers have been intensively studied
and have yielded vital information on
the history of the British Caledonides.
The oldest rocks are of Arenig age (low-
ermost Ordovician) and consist of an