Plate convergence usually commences with intraoceanic

1
Plate convergence usually commences with
intra-oceanic Subduction, Andean margins
commonly start after ophiolite obduction and
subduction flip.

• CONVERGENT PLATE MARGINS
• Intra-oceanic (ensimatic) subduction
• Andean margins
• 3) Continent - continent collision zones

1)
2)
3)
REMEMBER, IN 3-D A CONVERGENT MARGIN MAY HAVE
DIFFERENT MATURITY ALONG STRIKE!
2
legend and estimates of plate-tectonic forces

• Fsp - Slab-pull()
• Frp - Ridge-push()
• Fsu - Suctional force ()
• For - Orogenic spreading()
• Fdf - Mantle drag-force ( or -)
• Fsr - Subduction ressistance (-)
• Fcd - Extra continental-drag(-)
• Ftr - Transform resistance (-)

Frp g e (?m ?w) (L/3 e/2) 21012 Nm-1 Can
also be expressed as a function of age Frp
g?m?T?t 1 (?m/(?m-?w)) 2?T/? 1.19x10-3 t
(Unit MPa)
g - gravity 9,8 ms-2 e elevation of spreading
ridge above cold plate 3,3 km (e- is a
function of the age t) ?m mantle density,
3,2 g cm-3 ?w water density L lithosphere
thickness 85km T - temperature(1200C),
?-thermal diffusivity ??????m?s??, ? -
coefficient of thermal expansion ? 310-5 K-1
3
Estimate of slab-pull force Fsp pr. unit length
subduction zone (see Fowler Solid Earth, Chap
7, for details)
?2z
8g? ?m T1L2Re
?2d
Fsp(z)
exp(-
) - exp(-
)
ca 2x1013Nm-1
?4
2ReL
2ReL

• z depth (d z give Fsp 0)
• ? coefficient of thermal expansion
• T1 mantle temp,
• dL thickness of the upper mantle
• L Lithosphere (plate) thickness
• Re Thermal Reynolds number

Re (?mcpvL)/2k Thermal Reynolds number
k - conductivity cp - spesific heat k -
kinematic viscosity v - subd. velocity
4
Plate velocity as a function of subduction
margin
5
Reactions and phase transitions affecting the
forces in subduction zones

• In addition to the thermal contraction and
  density change will the forces of the subducting
  litosphere be affected by
• Gabbro to eclogite transition ()
• Olivin-spinel transiton ()
• Spinel to oxides (perovskitt and periklas) (-)

6
Modeled density structure of subducted MORB,
Hacker et al. 2003
7
Temperature variation across a subduction zone

• Notice the localization of the olivin-spinel and
  spinel-oxide transitions.
• Use the next fig to explain the phenomena

8
Phase diagrams for the transititions for olivin
to spinel and spinel to post-spinel (oxides)
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THE ANATOMY OF A SUBDUCTION COMPLEX
Fore-arc basin
Active volcanic arc
Remnant-arcs from arc-splitting
Outer non-volcanic island
Back-arc basin/spreading
alternating compression amd tension
Tension
Compression
Trench
sea level
High geotherm
Low geotherm
Accreationary prism
High geotherm
Please notice that Benioff zones frequently have
an irregular shape in 3-D (ex. Banda Arc). 80
of all seismic energy is released in Benioff
zones. The low geotherm in subductions zones
makes them the prime example of high P - low T
regional metamorphic complexes. The high geotherm
in the arc-region gives contemporaneous high-T
low P regional metamorphism, together these two
regions give rise to a feature known asPaired
Metamorphic Belts
10
Accreationary Prism, Example from
Scotland. Age Late Ordovician to Late Silurian
ca 450-420 Ma
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PAIRED METAMORPHIC BELTS
12
Ophiolites normally originate here!
Blueshists normally originate here!
alternating compression amd tension
Tension
Compression
Trench
sea level
High geotherm
Low geotherm
High geotherm
Seismic quality factor (Q) The ability to
transmitt seismic energy without loosing the
energy. Low Q in high-T regions. Seismic quiet
zones---NB potential build-up to very large
quakes! Arc-splitting - tensional regime above
subductions zones. Subduction roll-back. High
heat-flow in the supra-subductions zone regime
give rise to relatively low shallow sealevel
above the back-arc basins. Most ophiolite
complexes have their origin is a supra-subduction
environment
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3 - D MORPHOLOGY
NB! NOTICE INTRA-SLAB EARTHQUAKES
SEISMICITY
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26/12-2004, Mag 9 earthquake of Sunda arc -
Andaman sea
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Link fault plane solution
Link displacement magnitude
Link earthquake information in general
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The amount of energy radiated by an earthquake is
a measure of the potential for damage to
man-made structures. Theoretically, its
computation requires summing the energy flux
over a broad suite of frequencies generated by an
earthquake as it ruptures a fault. Because of
instrumental limitations, most estimates of
energy have historically relied on the empirical
relationship developed by Beno Gutenberg and
Charles Richter log10E 11.8 1.5MS where
energy, E, is expressed in ergs.
The drawback of this method is that MS is
computed from an bandwidth between approximately
18 to 22s. It is now known that the energy
radiated by an earthquake is concentrated over a
different bandwidth and at higher frequencies.
With the worldwide deployment of modern
digitally recording seismograph with broad
bandwidth response, computerized methods are now
able to make accurate and explicit estimates of
energy on a routine basis for all major
earthquakes. A magnitude based on energy
radiated by an earthquake, Me, can now be
defined, Me 2/3 log10E - 2.9. For every
increase in magnitude by 1 unit, the associated
seismic energy increases by about 32 times.
Although Mw and Me are both magnitudes, they
describe different physical properites of the
earthquake. Mw, computed from low-frequency
seismic data, is a measure of the area ruptured
by an earthquake. Me, computed from high
frequency seismic data, is a measure of seismic
potential for damage. Consequently, Mw MW 2/3
log10(MO) - 10.7 Mw µ(area)(displacement) and
Me often do not have the same numerical value.
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Frictional heating on faults may result in
melting of any rock-coposition
19
Stress-measurements from grain-size and/or
dislocation density (4 to 5 x1013m-2) in olivine
associated with pseudotachylytes in peridotite
indicate that peridotites (mantle rocks) may
sustain extreme differential stress ?1-?3 3-600
MPa. Assuming a fault with a modest
displacement of d 1m, and a differential
stress of 300 MPa the release of energy
according to equation (1) Wf Q E? where
Q heat and E? seismic energy is Wf
d ?n d (?1-?3)/2 1m(300MPa)/2 1.5 x 108 J
m-2 or 47 kWhm-2. The seismic energy (E?) is
commonly estimated to be lt 5 of Wf on a strong
fault, ie. less than 2.3 kWh m-2 is radiated as
seismic waves, the remaining energy (Q) turns to
heat and surface energy (difficult to measure)
along the fault. The process is adiabatic since
the fault movement occurs in seconds and no heat
is lost by conduction (thermal diffusivity ?
1.5 mm2s-1). Taking the heat capacity of
lherzolite, Cp 1150 J kg-1 oC-1 and a heat of
fusion (Fo) H 8.6 x106 Jkg-1 the thermal
energy (equation 4) required to melt one kg of
peridotite (4) Q Cp(?T) H 1150Jkg-1oC-1
(1200oC) 8.6 x106 Jkg-1 2.7 x 105 Jkg-1. On
a fault with D 1m, 60 kg lherzolite may melt
pr m2 of the fault plane, corresponding to an
approximately 2 cm thick layer of ultramafic
pseudotachylyte.