Evolution of British Isles

Question 1

What drives the processes that formed the landscape?

Answer 1

• Long periods of net accumulation of sedimentary rock formations in basins
o Stretching + subsidence
o Long period
• Record broken by regional unconformities (old erosion surfaces) formed during old mountain building episodes
o Thickening + uplift
o Short period

Question 2

Importance of plate boundaries in terms of history?

Answer 2

Modern geological processes demonstrably linked to major horizontal displacements: PLATE TECTONICS
Convergent, divergent, transcurrent plate margins
Intraplate regions

Plate boundaries: dynamic, tectonically active, evolving topography/bathymetry: diverse, complex geology

Intraplate regions: stable, quiet, less complex geology

• We can use magnetic stripes & fixed hot spot reference frame to track plate movements back over the last 200 Myr
• See clear links between plate convergence & major geological processes, e.g. mountain building

Question 3

Paleogeographic changes in general?

Answer 3

• Continents split, stretching crust leading to subsidence, basin formation & development of oceans
• Continents collide: Africa, India & Australia sequentially converge upon Eurasia with time to generate the three most important modern mountain belts: Alps, Himalaya/Tibet, SE Asia/SW Pacific
• India shoots northward crashing causes Himalayan/Tibetan mountain range – removed nearly all modern oceanic crust

• Existence & breakup of supercontinent Pangaea
• When continents are stretched, they subside forming basins & they may ultimately split forming new oceans
• When continents converge & collide, crust thickens & is uplifted to form mountain belts (orogeny)
• Complex 3D motions, rates vary
• Key point: stretching & thickening events related to plate margin processes
BUT all oceanic lithosphere older than 200 Myr has been subducted
Much of the UK’s interesting history is much older!

Question 4

Importance of continental lithosphere

Answer 4

• The continents (cratons) are where the old rocks reside
• Continental crust is buoyant & will not subduct - a collage of sutured fragments of all ages
o Quartz and feldspatic rocks are buoyant and don’t sink into the mantle = stick around
• Record a long history ‘perpetual motion’ being repeatedly split apart, moved around & re-collided in different places, in different ways at different times

Question 5

Identifying tools of age?

Answer 5

•	Palaeolatitude:
o	Inclination of palaeomagnetic field to palaeohorizontal (e.g. bedding) in rocks
•	Distribution of fossils
o	Climatically controlled flora/fauna
o	Effects of continental separation

Question 6

UK history?

Answer 6

Early Paleozoic:
• Prior to mid-Silurian N & S British isles lay on different palaeocontinents
• N Britain: part of Laurentia
• S Britain: part of Avalonia (a microcontinent related to Gondwana) Separated by Iapetus Ocean

Mid Paleozoic:
• Iapetus closes as both Baltica and Avalonia collide with Laurentia in the Caledonian orogeny
• To the south lies the Rheic Ocean which opened (in Ordovician) after Avalonia rifted away from Gondwana
• Caledonian Orogeny

Mid-Late Paleozoic
• Further microcontinents rifted from Gondwana converge on Laurussia
• Devonian-Carboniferous cycle of rifting & basin accumulation on N margin of Rheic Ocean
• Followed by Variscan Orogeny (370-300 Ma) affecting S Britain due to collision of Gondwana closing Rheic Ocean & forming Pangaea
• Variscan Orogeny

Great British Divide:
• Post-Carboniferous: no major orogenic episodes - subsidence dominant - a uniform tectonic history ==> INTRAPLATE location, but proximal to Atlantic margin
• Pre-Permian: multiple orogenies – a complex disjointed history ==> PLATE MARGIN location
• Split – at plate margin in the late Permian – North/South

Paleogene uplift & tilting
• Post-Variscan cycle (300-50 Ma) dominated by periods of rifting, sedimentation & magmatism related to sequential opening of modern Atlantic Ocean
• Major magmatic event in Palaeogene in NW Britain (?early Iceland plume ~ 50 Ma) led to uplift, erosion & tilting (Tees–Exe Line)
o Iceland plume used to sit under the NW of the British isles
• Profound effect on surface & offshore geology of the British Isles
• North uplifted, South subsides
• Uplifting caused old rocks to be exposed and to move into the North Sea
o Increased sedimentation = oil
Tilting, West up, East down
Present day:
Also Isostatic rebound due to post-glacial unloading/loading effects