plate convergence usually commences with intra-oceanic subduction, andean margins commonly start...

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e convergence usually commences with intra-oceanic uction, Andean margins commonly start after ophioli ction and subduction flip. ONVERGENT PLATE MARGINS ONVERGENT PLATE MARGINS Intra-oceanic (ensimatic) subduction Andean margins Continent - continent collision zones 1) 2) 3) REMEMBER, IN 3-D A CONVERGENT MARGIN MAY HAVE DIFFERENT MATURITY ALONG STRIKE!

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

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

CONVERGENT PLATE MARGINSCONVERGENT PLATE MARGINS1) Intra-oceanic (ensimatic) subduction2) Andean margins3) Continent - continent collision zones

1) 2)

3)

REMEMBER, IN 3-D A CONVERGENT MARGIN MAYHAVE DIFFERENT MATURITY ALONG STRIKE!

Page 2: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

legend and estimates of 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 = ge(m – w) (L/3 +e/2) ≈ 2*1012 Nm-1

g - gravity ≈ 9,8 ms-2

e – elevation of spreading ridge above cold plate ≈ 3,3 km

m – mantle density, ≈ 3,3 g cm-3 w – water density

L – lithosphere thickness ≈ 85km

Page 3: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

Estimate of slab-pull force Fsp pr. unit length subduction zone (see Fowler: Solid Earth, Chap 7, for details)

z – depth (d = z give Fsp = 0) – coeficient of thermal expansion T1 – mantle temp,

d+L – thickness of the upper mantle L – Lithosphere (plate) thickness Re – Thermal Reynolds number

Fsp(z) = 8g m T1L2Re

4[exp(-

2ReL

2z) - exp(-

2d

2ReL)] = ca 2*1013Nm-1

Re =(mcpvL)/2k

Thermal Reynolds number

k - conductivity

cp - spesific heat

k - kinematic viscosity

v - subd. velocity

Page 4: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

Plate velocity as a function of % subduction margin

Page 5: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

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) (-)

Page 6: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

Modeled density structure of subducted MORB, Hacker et al. 2003

Page 7: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

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

Page 8: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

Phase diagrams for the transititions for olivin to spinel

and spinel to post-spinel (oxides)

Page 9: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

THE ANATOMY OF A SUBDUCTION COMPLEXTHE ANATOMY OF A SUBDUCTION COMPLEX

Trench

Accreationaryprism

Fore-arcbasin

Active volcanic

arc

Back-arcbasin/spreading

Tension

alternatingcompressionamd tension

sea level

Remnant-arcsfrom

arc-splitting

Outernon-volcanic

island

Compression

Low geotherm

Low geothermHigh geothermHigh geotherm

High geothermHigh 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 regionsgive rise to a feature known as”Paired Metamorphic Belts”

Page 10: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

Accreationary Prism, Example from Scotland.Age:Late Ordovician to Late Silurian ca 450-420 Ma

Page 11: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

PAIRED METAMORPHIC BELTS

Page 12: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

Trench

Tension

alternatingcompressionamd tension

sea level

Compression

Low geotherm

Low geothermHigh geothermHigh geotherm

High geothermHigh 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. Subductionroll-back.High heat-flow in the supra-subductions zone regime give rise torelatively low shallow sealevel above the back-arc basins. Mostophiolite complexes have their origin is a supra-subduction environment

Ophiolites normallyoriginate here!

Blueshists normallyoriginate here!

Page 13: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

SEISMICITY

3 - D MORPHOLOGY

NB! NOTICE INTRA-SLABEARTHQUAKES

Page 14: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT
Page 15: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

26/12-2004, Mag 9 earthquake of Sunda arc - Andaman sea

Page 16: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

Link: fault plane solution

Link: displacement magnitude

Link: earthquake information in general

Page 17: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

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 betweenapproximately 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] and Me often do not have the same numerical value.

Page 18: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

Frictional heating on faults may result in melting of any rock-coposition

Page 19: Plate convergence usually commences with intra-oceanic Subduction, Andean margins commonly start after ophiolite obduction and subduction flip. CONVERGENT

Stress-measurements from grain-size and 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 < 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.