Evolution of British Isles Flashcards
What drives the processes that formed the landscape?
• 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
Importance of plate boundaries in terms of history?
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
Paleogeographic changes in general?
- 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!
Importance of continental lithosphere
• 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
Identifying tools of age?
• 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
UK history?
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