Environmental Engineering Reference
In-Depth Information
MORPHOTECTONIC LANDSCAPES IN BRITAIN
applications
Rocks bear the signature of their formative environment and subsequent history, a theme
developed in Chapter 12, but our interest is heightened when we can fit them into a
sequence of real events on the world stage. Tectonic episodes in Britain's past,
comparable with modern plate boundary and sea-floor activity, can be detected if we
know how and where to look. Terranes from two of Britain's three Phanerozoic orogens
appear in the same satellite image (Colour Plate 2, between pp. 272 and 273) where the
older,
Caledonian
terrane of the south-east Lake District contrasts clearly with the
Variscan
terrane of the western Yorkshire Dales. They tell the story of a micro-plate,
Avalonia
, pirouetting equatorwards between the southern hemisphere continent
Gondwana
and northern hemisphere continents of
Laurussia c
. 460-310 Ma ago. During
that time, first the ancient
Iapetus
and then the
Rheic
Oceans progressively closed. The
resultant intercontinental collisions formed the global-scale Caledonian and
Variscan
cordilleran mountain systems respectively. For maximum benefit, this box should be
studied in conjunction with the satellite image, Chapter 12 and the section of Chapter 1
on Upper Wharfedale.
Central Lake District rocks of mid-late Lower Palaeozoic age (
c
. 460-400 Ma ago)
reflect the later stages and eventual closure of the Iapetus Ocean, when 'southern Britain'
lay on a fragment of the Gondwana continental plate (Avalonia), drifting north from 60°
to 30° south of the equator. A 6·5 km thick accumulation of eruptive rocks, the
Borrowdale Volcanic Group, formed in an island fore-arc environment. It was located in
a B-subduction zone on the outer continental margin of the shrinking Iapetus Ocean. The
lower 2·5 km are andesite lavas, erupted in an oceanic setting, whilst the upper 4 km are
mostly intermediate-acid volcanic ashes probably accumulated around terrestrial
calderas
(see Chapter 12).
This volcanic episode was short but intense, lasting perhaps less than 10 Ma, followed
by rapid erosion and caldera collapse as the heat (and associated thermal dome) went out
of the eruptions. Crustal subsidence coincided with global sealevel rise, as Gondwana's
south polar ice sheet melted, reflooding the fore-arc basin. Five kilometres of marine
sediments eventually accumulated there, dominated by
turbidite
muds, and were
deformed as the Iapetus Ocean finally closed
c
. 400 Ma ago, to become the younger
Bannisdale Slates and Coniston Grits of the southern Lake District. They were caught up
with the older volcanic rocks in the tremendous orogenic compression and uplift of the
Caledonian mountains and intruded, at depth, by granite batholiths. Emplaced during the
final continental collision, two particular batholiths feature in our story. Shap granite,
emplaced 7-10 km below the surface
c
. 395 Ma ago, now
outcrops
at the surface as a
result of subsequent erosion. It was part of an intrusive swarm, together with
Wensleydale granite still concealed below the Yorkshire Dales, which domed the Lake
District.
The British sector of the Iapetus suture lies north of the Lake District today, in the
southern uplands of Scotland. In geological terms, it was not long before Gondwana itself
caught up, steadily closing the Rheic Ocean south of 'Britain's' new location near the
equator
'Northern England' now found itself in a back arc basin of
Carboniferous
age
