2) Super Continent cyclisity (?) and Wilson cycle tectonics

Does the earth’s continental lithosphere go through stages of assembly and disintegration to produce Does the earth’s continental lithosphere go through stages of assembly and disintegration to produce

periods when most continents are united into one, aperiods when most continents are united into one, a Supercontinent?.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 - Caledonianorogenic 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 accordingto 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?Supercontinent cyclisity?

From Rodinia to Pangea and a future supercontinent?? Does the earth´s continental lithospheric plates assembleand rift apart in longer term cycles?

http://www.geodynamics.no/platemotions/500-400/

BALTICA a separatecontinent ≈ 550-425Ma

500 Ma460 Ma

440 Ma

420 Ma400 Ma

Caledonian orogenic cyclein brief

460 Ma440 Ma

Notice that traditionalWilson-cycle tectonicsdoes not work to explainformation of the Caledonides

A Wilson cycle produces geo-tectonic rock unitscharacteristic 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 products3) 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 obduction7) Andean margins (composite batholiths)8) Continent - continent collision

Some important geotectonic rock units cannot be directly related to stages inWilson cycles. Most prominent are the Large Igneous Provinces (LIPS).Also other features f.example Impact structures

Large-scale tectonic rift types:1) Atlantic-type rifts2) Back-arc rifts3) Syn-orogenic rifting and wrenching4) 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 STRETCHINGOF CRUST AND MANTLE LITHOSPHERE

MODELS ARGUING SYMMETRICAL VS.ASYMMETRICAL STRETCHINGOF 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-mantleboundary

Doming by asthenosphereupwelling/thermal erosionof lithosphere

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Mathematically calculated passive margin formation withcontinental breakup. The crust was broken when it wasthinner than a critical thickness (here 5 km) and oceaniccrust was created applying a spreading velocity of0.1 cm/year. The mathematical model is based onkinematic thinning including processes such as temperatureadvection and diffusion, lithospheric flexure and sedimentcompaction. 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 failed rifts

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 measuredextension 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 10-5K-1

w- water depth- density or mantle(m) (3.2) water (w)

t- timeT- temperature, T1=1280oC, Ts=0oC

- thermal diffusivity

€

T( ) = ρm 1+α T1 −T( )( )

σ A = ρwgw + ρ z( )0

z

∫ gdz

σ B = ρmgw + ρmgz

ρmgz + w(ρm + ρw ) = ρ z( )0

z

∫ dz

w(ρm − ρw ) = ρ z( ) − ρm( )0

z

∫ dz

w(ρm − ρw ) = ρm0

z

∫ α T1 −T(z)( )dz

w(ρm − ρw ) = ρm0

z

∫ α T1 −T − (T1 −Ts)erfcz

4κt

⎛

⎝ ⎜

⎞

⎠ ⎟

⎛

⎝ ⎜

⎞

⎠ ⎟dz

w(ρm − ρw ) = ρm0

z

∫ α (T1 −Ts)erfcz

4κt

⎛

⎝ ⎜

⎞

⎠ ⎟dz

w =ρmα T1 −T( )ρm − ρw 0

z

∫ erfc z

4κt

⎛

⎝ ⎜

⎞

⎠ ⎟dz

n =z

4κt

w = 4κtρmα T1 −T( )(ρm − ρw ) 0

z

∫ erfc n( )dn

0

∞

∫ erfc n( )dn =1

π

w =2ρα (T1 −Ts)

(ρm − ρw )

κt

π

w ≈ 5.91∗10−5 t

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 formwhich 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 conductionequation 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 withLimit 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