Presentation on theme: "2) Super Continent cyclisity (?) and Wilson cycle tectonics Does the earth’s continental lithosphere go through stages of assembly and disintegration to."— Presentation transcript:
2) Super Continent cyclisity (?) and Wilson cycle tectonics Does the earth’s continental lithosphere go through stages of assembly and disintegration to produce periods when most continents are united into one, a Supercontinent?. The Wilson Cycle: The Wilson Cycle: named after J. Tuzo Wilson, one of the founding fathers of plate-tectonics and discoverer of transform faults. Wilson used his reference background, in the North Atlantic realm and the Appalachian - Caledonian orogenic belts on both sides of the Atlantic ocean to formulate a hypothesis saying that the building of mountain belts have a close relationship to the opening and closure of oceans with oceanic lithosphere. Hence he introduced the term ”the Proto-Atlantic” as a name for the postulated ocean that according to the model once opened and closed to produce the Appalachians and the Caledonides Traditional Wilson cycle model: Orthogonal opening and closure like on the previous slide, two-dimensional models. Modified Wilson cycle model: Wilson-cycle type tectonics with a modern approach;---one ocean opening--- another closing, cf. The Caledonian Wilson cycle or the Indian ocean opening -- eastern Tethyan closing.
Supercontinent cyclisity? From Rodinia to Pangea and a future supercontinent?? Does the earth´s continental lithospheric plates assemble and rift apart in longer term cycles?
http://www.geodynamics.no/platemotions/500-400/ BALTICA a separate continent ≈ 550-425Ma 500 Ma 460 Ma 440 Ma 420 Ma 400 Ma Caledonian orogenic cycle in brief 460 Ma 440 Ma Notice that traditional Wilson-cycle tectonics does not work to explain formation of the Caledonides
A Wilson cycle produces geo-tectonic rock units characteristic of the various stages of the cycle. 1)Continental rift (rift sediments and magmatic products) 2)Volcanic or non-volcanic passive margins (rift margin with thinned continental crust and associated sedimentary and volcanic products 3)Ocean continent transitional crust (highly stretched crust and dyke intruded crust) 4)Oceanic crust w/exotic elements (continental crust fragments, ocean islands hot-spots, transform complexes etc.) 5)Intra-oceanic convergent margins (subduction complexes, island-arcs and back-arc complexes etc) 6)Ophiolite/island arc obduction 7)Andean margins (composite batholiths) 8)Continent - continent collision Some important geotectonic rock units cannot be directly related to stages in Wilson cycles. Most prominent are the Large Igneous Provinces (LIPS). Also other features f.example Impact structures
Large-scale tectonic rift types: 1)Atlantic-type rifts 2)Back-arc rifts 3)Syn-orogenic rifting and wrenching 4)Post-orogenic extension 5)Mantle plumes and hot spots Other large-scale classification: 1)Active rifts ( ≈ 1; 2; 5 above) 2)Passive rifts ( ≈ 3 & 4 above) (key ref: Ziegler and Cloething 2003)
MODELS ARGUING CONTINUOUS VS. DISCONTINUOUS STRETCHING OF CRUST AND MANTLE LITHOSPHERE
MODELS ARGUING SYMMETRICAL VS. ASYMMETRICAL STRETCHING OF CRUST AND MANTLE LITHOSPHERE What are the implications for: Localization of magmatism Areas of subsidence vs. uplift
Doming by ponding of melts near the crust-mantle boundary Doming by asthenosphere upwelling/thermal erosion of lithosphere
Mathematically calculated passive margin formation with continental breakup. The crust was broken when it was thinner than a critical thickness (here 5 km) and oceanic crust was created applying a spreading velocity of 0.1 cm/year. The mathematical model is based on kinematic thinning including processes such as temperature advection and diffusion, lithospheric flexure and sediment compaction. A) Mathematically calculated temperature field for a sedimentary basin formed by extension. B) Plot of temperature versus depth. C) The corresponding crustal section providing the thickness of the upper and lower crust. From Schmalholtz et al, PGP
Duration of rifting in successful rifts that went on to produce oceanic lithsphere.
Problems with the Crust-mantle boundary: P-wave velocity (Vp) from V-7.8 to 8.0–8.2 km/s, (crustal granulites and the olivine-dominated mantle–lithosphere). The continental Moho is not always a sharp discontinuity, but often a complex and variable transition zone that generally ranges in thickness between < 1 and 5 km, but can expand to 10 km.
The commonly observed mismatch between measured extension from fault-heave and from the crustal configuration
Ocean continent transitional crust on the Norwegian Sea Atlantic margin (highly stretched and dyke and sill intruded crust) Notice the crustal extension/subsidence vs. lack of faults to accommodate the extension. We can study transitional crust with abundant sheeted dykes within the Seve Nappe Complex in the Scandinavian Caledonides
Transitional crust from the distal Caledonian margin of Baltica is preserved (obducted) within the Seve Nappe Complex in Scandinavia
Depth of oceans: (z), coeff. of thermal expansion= 3 x 10 -5 K -1 w- water depth - density or mantle(m) (3.2) water (w) t- time T- temperature, T 1 =1280 o C, T s =0 o C - thermal diffusivity 2) Column A at compensation 1) Density as function of temperature 3) Column B at compensation 4) (2), (3) and isostasy, see Stüwe p157 5) (4) first term after =, finds derivative with respect to z and wrights into integral and it gets form which says that water depth depends on the density sturcture as a function of depth 6) Inserting (1) into (5) where T(z) is the unknown (determined from heat conduction equation see Stüwe p 96) 7) Inserting heat conduction equation in (6), which simplifies to: 8) and to : 9) After taking constanst out of the integrall. If we introduce the constant n in (10) 10) We can take all the constants out to the integral and get: 11) Integral of the errorfunction is not know the 0 and z but is know for integration with Limit infinity, it is: 12) Substutuing this integral into (11) we get an expression for the water depth : 13) Which after inserting standard values for all the constanst give : 14) The water depth in oceans is proportional to the square root of the age and 5.91 times 10 -5