Plate Tectonic Setting For SouthWest England
Introduction
Two major orogenic episodes have affected the rocks of the Southwest British Isles, the Palaeozoic Variscan and the Tertiary Alpine Orogenies. The earlier event, the Variscan is the primary event responsible for the evolution of Cornwall, whereas the Alpine Orogeny is reflected in the gentle folding and thrust faulting of the Mesozoic sediments of Dorset. Other earlier minor folding events also affect some of these Mesozoic sediments. These are primarily as a result of the earth movements relating to continental break-up in the North Atlantic region during the mid-Cretaceous.
A field geologist is able to unravel these separate events with relative ease.
Ask yourselves the following questions:
What is the age of the deformed rocks? Obviously the deformation post-dates the deposition!
What are the structural trends? Patterns of folding and faulting are directly related to the earth movements that created them. Measure the trends of fold axes or faults using the sight on your compass.
So how can we differentiate Variscan structures from Alpine structures? How did they arise? And where do mountains come from anyway?
Orogenesis - mountain building
An orogeny or orogen is simply geojargon for a mountain building event. Mountains have a tendency to arise out of the collision of two plates - either as convergence of two continental plates, i.e. India slamming into Asia to form the Himalayas, or as the subduction of an oceanic plate under a continental plate forming Cordilleras like the Rockies and Andes along the western margins of North and South America.In the case of the Variscan and Alpine Orogenies, we are dealing with continent - continent Himalayan-type collision. The structures associated with these events are compressional. We would expect to see folds, reverse faults and thrusts.
Thrusts are a special type of low-angle reverse fault typically associated with collision orogens. They are usually large-scale features, extending for many tens of kilometres along their strike, and capable of moving slivers of rock great distances. As a rule of thumb, fold axes and thrust fronts will lie perpendicular to the direction of plate convergence. For example India moved N to collide with Asia, and the collision zone (the suture), the fold axes and the thrust fronts trend E -W.
The Variscan Orogeny
The Variscan Orogeny was the European part of a major continent - continent collision which also formed the Appalachians of eastern North America. Cornwall lies on the northern edge of this now eroded and mostly buried mountain chain.The Variscan Orogeny occurred as a result of the northward convergence and collision of the continent of Gondwana (comprising South America, Africa, India, Australia and Antarctica) with the stable continent of Laurasia (comprising North America, Europe and Asia). On collision, the supercontinent Pangaea was formed, being a single, huge landmass in the ocean Panthalassa.
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Cornwall, or the Cornubian Massif, represents part of a trough into which sediments, eroded from the forming mountains, were accumulated. This trough, called the North Variscan Foredeep would once have extended along the whole of the strike of the belt from Poland to western Ireland. Only small fragments are now exposed.
During the final stages of continental collision, the sediments were folded and a large granite batholith was intruded into this trough, and cupolas of this granite are seen as the Dartmoor, Bodmin Moor, St. Austell, Carnmenellis, Land's End and Scilly Isles intrusions.
The Cornubian Massif also contains a feature highly typical of continent - continent collision, The Lizard Complex. This is a remnant of the old ocean that once existed between Gondwana and Laurasia (sometimes called the Rheic Ocean). Its emplacement and age are still not fully understood, but it was undoubtedly pushed North from the Variscan Suture Zone (which probably lay somewhere within what is now the Bay of Biscay) along a large-scale, major crustal thrust.