2 Super Continent cyclisity and Wilson cycle tectonics

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2) Super Continent cyclisity (?) and Wilson cycle
tectonics
Does the earths continental lithosphere go
thorough stages of assembly and disintegration to
produce periods when most continents are united
into one, a Supercontinent?. 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.
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Supercontinent cyclisity?
From Rodinia to Pangea and a future
supercontinent?? Does the earths continental
lithospheric plates assemble and rift apart in
longer term cycles?
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500 Ma
Caledonian orogenic cycle in brief
BALTICA a separate continent 550-425Ma
Notice that traditional Wilson-cycle
tectonics does not work to explain formation of
the Caledonides
http//www.geodynamics.no/platemotions/500-400/
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• A Wilson cycle produces geo-tectonic rock units
• characteristic of the various stages of the
  cycle.
• 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.)
• Intra-oceanic convergent margins (subduction
  complexes, island-arcs and
• back-arc complexes etc)
• 6) Ophiolite/island arc obduction
• 7) Andean margins (composite batholiths)
• 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
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• Large-scale tectonic rift types
• Atlantic-type rifts
• Back-arc rifts
• Syn-orogenic rifting and wrenching
• Post-orogenic extension
• Mantle plumes and hot spots
• Other large-scale classification
• Active rifts
• ( 1 2 5 above)
• Passive rifts
• ( 3 4 above)
• (key ref Ziegler and Cloething 2003)

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MODELS ARGUING CONTINUOUS VS. DISCONTINUOUS
STRETCHING OF CRUST AND MANTLE LITHOSPHERE
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MODELS ARGUING SYMMETRICAL VS. ASYMMETRICAL
STRETCHING OF CRUST AND MANTLE LITHOSPHERE What
are the implications for Localization of
magmatism Areas of subsidence vs. uplift
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Doming by asthenosphere upwelling/thermal
erosion of lithosphere
Doming by ponding of melts near the
crust-mantle boundary
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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
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Duration of rifting in failed rifts
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Duration of rifting in successful rifts that went
on to produce oceanic lithsphere.
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Problems with the Crust-mantle boundary P-wave
velocity (Vp) from V-7.8 to 8.08.2 km/s,
(crustal granulites and the olivine-dominated
mantlelithosphere). The continental Moho is not
always a sharp discontinuity, but often a
complex and variable transition zone that
generally ranges in thickness between lt 1 and 5
km, but can expand to 10 km.
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The commonly observed mismatch between
measured extension from fault-heave and from the
crustal configuration
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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
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Transitional crust from the distal Caledonian
margin of Baltica is preserved (obducted) within
the Seve Nappe Complex in Scandinavia