Transcript and Wilson cycle tectonics

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: 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?
500 Ma
460 Ma
BALTICA a separate
continent ≈ 550-425Ma
Caledonian
orogenic cycle
in brief
440 Ma
400 Ma
http://www.geodynamics.no/platemotions/500-400/
Notice that traditional
Wilson-cycle tectonics
440
460 Ma
does not work to explain
420 Ma
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)
2)
3)
4)
5)
Atlantic-type rifts
Back-arc rifts
Syn-orogenic rifting and wrenching
Post-orogenic extension
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 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
A) Mathematically calculated temperature field for a
compaction.
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 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
T    m 1  T1  T 
Depth of oceans:
(z), coeff. of thermal expansion= 3 x 10-5K-1
z
A
2) Column A at compensation
  w gw   zgdz
w- water depth
0
- density or mantle(m) (3.2) water (w)
 B  m gw   m gz
3) Column B at compensation
t- time
z
o
T- temperature, T1=1280 C, Ts=0oC
m gz  w(  m  w )   zdz 4) (2), (3) and isostasy, see Stüwe p157
0
- thermal diffusivity
w(  m   w ) 
1) Density as function of temperature
z
     dz
m
z
0
w(  m   w ) 
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
z
   T  T(z)dz
m
1
0
w(  m   w ) 

z

z
n
 z 
dz
4t 
   (T  T )erfc
m
1
0
w
 z 
dz
 4t 
 mT1  T  (T1  Ts )erfc
0
w(  m   w ) 
6) Inserting (1) into (5) where T(z) is the unknown (determined from heat conduction
equation see Stüwe p 96)
s
 z 
 m T1  T  z
erfc
dz

m  w 0
 4t 
z
4t

erfcn dn 
9) After taking constanst out of the integrall. If we introduce the constant n in (10)
11) Integral of the errorfunction is not know the 0 and z but is know for integration with
Limit infinity, it is:
1
0
w
8) and to :
10) We can take all the constants out to the integral and get:
m T1  T  z
w  4t
 erfcndn
(m  w ) 0

7) Inserting heat conduction equation in (6), which simplifies to:
12) Substutuing this integral into (11) we get an expression for the water depth :

2 (T1  Ts ) t
(m  w )

w  5.91105 t
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