Ancient orogenic belts - Introducing Tectonics, Rock Structures and Mountain Belts - page 104

north by the Southern Uplands Fault ,
which has experienced at least 10 km
of sinistral strike-slip displacement, as
there is no direct match between the
Lower Palaeozoic sequences on each
side. This fault is one of a set of such
faults that occur throughout the belt,
and it seems that some of the steep-
ened thrusts have been re-activated in
a strike-slip sense. The total sinistral
offset across the zone is probably con-
siderable, and took place during the
early Devonian in the closing stages of
the Caledonian orogeny. The effects of
sinistral shear during this late Caledo-
nian deformation are reflected in places
by the alignment of the slaty cleavage,
which is clockwise with respect to the
fold axial planes (Figure 12.3F); this
is consistent with the effects of sinis-
tral transpression ( see Chapter 4).
The Southern Uplands zone is
bounded on its south-eastern side
by the Solway (or Iapetus ) suture ,
which is not exposed, but has been
seismically imaged as a major discon-
tinuity inclined at a moderate angle
north-westwards beneath the zone,
and which lies at a depth of around
12 km in the central part of the zone.
The basement beneath the suture is
interpreted as part of the Avalonian
plate. This zone is represented in
Ireland by the Longford Down massif .
The Southern Uplands has long
been regarded as an accretionary
prism, formed above a NW-dipping
subduction zone at the north-western
margin of the Avalonian microplate
(Figure 12.4.5) although alternative
interpretations have been suggested.

The Lake District zone
The Caledonian rocks of the English
Lake District consist of a sequence of
Ordovician arc-type volcanics, suc-
ceeded by Silurian marine deposits;
these rest on a late Precambrian
basement, which is exposed to the
south in Anglesey and NW Wales. The
south-western continuation of this
zone in Ireland is represented by the
Leinster massif . The zone is regarded
as an Ordovician volcanic arc situated
at the northern margin of the Avalo-
nian microplate (Figure 12.4.4). These
Lower Palaeozoic rocks were deformed
and subjected to slate-grade meta-
morphism in the late Silurian, at the
same time as the Southern Uplands.

Norway and Greenland to the north. The
Northern Highland and Grampian High-
land zones are seen as part of a regional
metamorphic 'core' zone thrust onto the
Laurentian continent, and which is rep-
resented also in Newfoundland and East
Greenland. Both there and in Ireland
this zone is overthrust on its south-
eastern side by Ordovician arc terranes.
South of these zones, the polarity
changes to south-east-directed units,
including the Ordovician accretionary
prisms in Newfoundland, Scotland and
Ireland, and the volcanic arc terranes
on the northern margin of Avalonia.
An important role is played by major
strike-slip faults - the Great Glen, High-
land Boundary and Southern Uplands
faults being only the more obvious. The
total sinistral strike-slip displacement
on these is unknown, but estimates
have ranged from a few hundred to over
1000 kilometres. Consequently, none of
the terranes south of the Moine thrust
can be directly linked to its neighbour,
which makes interpretation difficult.
The strike-slip faulting results from
the late Silurian collision between Lau-
rentia and Avalonia, which must have
been oblique to the plate boundary
such that the convergence was par-
titioned into orthogonal (i.e. at right
angles to the boundary) and strike-slip
components. The collision between
Laurentia and Baltica, which appears to
have been more or less contemporane-
ous with the collision with Avalonia,
seems to have been more nearly at right
angles to the Baltica plate margin. The
effect of the Baltica collision on the
British Isles - the Scandian event - is
not so obvious; only in the Northern
Highlands have major tectonic effects
been ascribed to it. It is likely that this

The Welsh Basin
The Lower Palaeozoic rocks of Wales
have been intensively studied by gen-
erations of geologists and were regarded
as an example of a eugeosyncline in the
1930s. They comprise around 10 km of
Cambrian to Silurian sediments includ-
ing a large proportion of turbidites.
Volcanics are an important constitu-
ent in the south-west, and especially
in the north-west, in Snowdonia. The
modern model is of a back-arc basin
situated on thinned Avalonian crust
behind the Lake District arc (Figure
12.4.4). The rocks of the zone are vari-
ably deformed in the late Silurian: tight
folds with associated slaty cleavage
in the north give way to more gentle
folds in the south-east (Figure 12.3G).

Regional context
Figure 12.5 is an interpretation of how
the zones of the British-Irish sector
of the Caledonides might link up
with Newfoundland to the south, and