Download Delamination, Slab Break-Off, and Slab Roll-Back

Delamination, Slab Break-Off,
and Slab Roll-Back
Jeff Bowman
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Outline
• Delamination
• Slab Break-Off
• Slab Roll-Back
–Discussion of related papers
–Discuss papers assigned
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Delamination
• Lithospheric delamination is…
– the detachment of thickened
lithospheric mantle from overlying crust
during continental collision (Bird, 1978).
– In other words, a chunk of lithosphere
peeling off and sinking into the
asthenosphere (Bowman, Today)
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Ways to Approach Delamination
• Geochemical analysis of
metamorphic and igneous provinces.
• Orogenic processes
–Rapid uplift
• Seismicity
–Earthquakes/Tomography
–Seismic refraction
–Seismic reflection (Cook et. al., 1998)
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Analogy
• Glue a piece of steel
to the bottom of a
block of wood that is
floating in water.
http://www.geology.um.maine.edu/geodynamics/analogwebsite/Projects2002/Gerbi20
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02/lith_delam.html
Taking A Step Back
• Oxburgh 1972 introduced
– Flake Tectonics and Continental Collision
• Large sheets of material (flakes) shearing off
the top of one of the colliding plates.
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The Accredited Start
• ‘Delamination’ was introduced as a
theory by Peter Bird of UCLA in 1978 and
1979.
– Detachment of thickened lithospheric
mantle from overlying crust during
continental collision (Bird, 1978).
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P. Bird - Delamination
• His concept has come to include a
broader range of processes,
including…
–Detachment of oceanic slabs (Sacks and
Secor, 1990; Davies and von
Blackenburg, 1995)
–Foundering of mafic lower crust and
upper mantle driven by phase changes
(Kay and Kay, 1993; Nelson, 1991).
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Continental Delamination and
the Colorado Plateau, Bird 1979
• Delamination cannot
begin until some
process breaks
through the mantle
lithosphere, allowing
asthenosphere to
contact the crust.
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Bird, 1979
Four possible causes
of delamination:
•Convective instability
•Rifting
•Plume erosion
•Continental collision
http://peterbird.name/publications/1979_d
elamination/0010s_4_ways_to_initiate.jpg
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Continental Delamination and
the Colorado Plateau, Bird 1979
• Instability is expected
to propagate at plate
tectonic rates, 5 cm/yr
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Bird, 1979
•The delamination
model implies a fast
rise of asthenosphere
to replace the sinking
lithosphere; 24 cm/yr
•Figure shows the
similarities between
subduction and
delamination
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Meissner, Mooney 1998
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Consequences of Delamination
Bird, 1979
• Regional Uplift
– Excess pressure brought by newly risen
asthenosphere
• Increased Heat Flow
• Reduced Seismic Velocities
• Mafic volcanism
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Kay and Mahlburg Kay, 1993
• Same idea of delamination as P. Bird
– “Lithospheric delamination is the
foundering of dense lithosphere into less
dense asthenosphere.”
• One step further:
– Cause for the density inversion
• thermal
• compositional
• phase changes
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Kay and Mahlburg Kay, 1993
• Other existing models rely on thermal
expansion to create density changes.
• However, compositional variations and
phase changes more easily generate
density differences.
– Mafic granulite facies -> eclogite facies
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Kay and Mahlburg Kay, 1993
• A prediction of delamination events are
tough
• Field observations are better for
identifying where delamination has
occurred
– Observable structures
– Thermal and magmatic events
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Delamination Magmatism:
Andes
• 2-3 m.y. ago, the Argentine Puna changed
– Thickened crust -> Crustal shortening
(Isacks, 1988)
•Mafic volcanism across region
•Topographic high compared to
the northern Puna (Isacks, 1988)
•This area outlines delamination
Kay and Mahlburg Kay, 1993
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Cross-section of Andean Crust
• Seismic studies show a gap
– Slab too hot due to asthenosphere
Kay and Mahlburg Kay,
1993
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Geochemical Classification
• Three general chemical groups based on
trace element characteristics
– Oceanic Island Basalt (OIB), Calc-Alkaline,
and Shoshonite
Kay and Mahlburg Kay, 1993
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Geochemical Classification
• Melting percentages
• OIB – highest
– Relates to seismic gap
on x-section
• CAC-ALK – intermediate
– Flanking OIB on margins
• Shoshonites – smallest
– Northeastern area
where lithosphere is
thicker
Kay and Mahlburg
Kay, 1993
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Kay and Mahlburg Kay, 1993
• Delamination events exist in few places
– Basin and Range
– Tibet
– Andes
• Delamination events may have been
more common (Archean)
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Lustrino 2005
• Paper explaining the geochemical
compositions of basaltic magmas found
• Uses a model involving delamination to
explain peculiarities
• Density increase
– Basalt -> amphibolite -> eclogite
• Sound familiar?
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Lustrino
2005
•A) Initial situation
•Cont. -Oceanic -Cont.
•B) Oceanic slab
subducting
•C) Cont. – Cont. collision
•D) Lithos. thickening
•Phase changes
•E) Density increase leads
to instability
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Top: temperature (degree centegrate)
field and some isotherms.
Middle: composition
by tracers,
blue = upper crust,
yellow = lower crust,
red = mantle;
in the mantle lithosphere
the initially vertical tracer columns
are almost not deformed
due to the high viscosity.
Bottom: viscous dissipation function
(non-dimensionallog10),
zoomed-in into the upper half of the
model.
http://www.geophysik.unifrankfurt.de/~schmelin/orogeny/orogenyneu.h
tml
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Schott and Schmeling, 1998
Collisional Delamination in New
Guinea: The Geotectonics of
Subducting Slab Breakoff
• Mark Cloos et. al., 2005
• Lithotectonic belt relationships
• Mechanical properties of the crust and
lithospheric mantle
• Basis for series of lithospheric-scale cross
sections showing collisional delamination
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Cook et. al., 1998
• Describes results from an ~725 km long ~100
km deep seismic reflection profile recorded by
LITHOPROBE
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Cook et. al., 1998
• Enlargement of data in Slave–Wopmay crustal
wedge
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Slab Break-Off
• A.K.A. – Slab Detachment
• Subducted slab becomes detached from
the surface slab
• Observed volcanism
• Regional tectonics
• First hypothesized based on (1970’s)
– Hypocentral distribution
– Tomographic images
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Davies & Blanckenburg 1995
• Model -> oceanic lith. detaches from
continental lith. during continental
collision
• Opposing buoyancy leads to extension in
the subducting slab
– Rifting occurs with strain localization
• Critical strain rates & lithosphere strength
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!! Fun Fact !!
• Comparison of mean life span
–Continental crust
• 2.2 Ga
–Oceanic crust
• 100 Ma
• Continental crust does not subduct
very well
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Davies & Blanckenburg 1995
• Buoyant continental crust vs. cold
dense oceanic crust
– Extensional forces acting at transition
region
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Sacks & Secor, 1990
Delamination in
Collisional Orogens
• Continental crust
buoyancy vs. slab
pull force
– Extensional stress
may lead to
extensionally rupture
• A: simple shear
• B: pure shear
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Davies & Blanckenburg 1995
• Strain localization
• Critical strain rates
– Kusznir and Park (1987) – weakening of 50 m.y
old lithosphere
• 5 x 10-15 s-1
– Other strain rates
• 0.12 x 10-15 s-1 – 0.37 x 10-15 s-1 Houseman &
England (1996)
• Critical strain rate may lower with depth*
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Davies & Blanckenburg 1995
• Figure 4 – Integrated strength vs. depth
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Consequences
• Magmatism
– Asthenospheric upwelling
• Exhumation of high-pressure rocks
• Orogen deformation and uplift
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Discussion
• Setting expected
– Syn-collisional magmatism
– Preceded by subduction of oceanic and
continental lithosphere
• Bird’s delamination model
– Difficult to start and stop
– Extensive crustal melt vs. localized/linear
crustal melt
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Ferrari 2004 fig 2
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Slab Roll-Back
• Subducting slab sweeps back through the
mantle like a paddle
• Hinge migrates away from the arc region
• Upper plate is pulled along and may lead to
back-arc spreading
• Causes/Effects
– Gravity
– Forces
– Asthenosphere upwelling
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Heuret & Lallemand, 2005, Plate
motions, slab dynamics and
back-arc deformation
• Where Δρ = density difference between slab
and mantle, L = slab length, A = age of slab, K
= constant
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Heuret, Lallemand 2005
• In contrast to the theoretical rule where rollback increases with age.
• Younger age subducting slabs roll-back faster
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Heuret, Lallemand 2005
• Rules out slab roll-back related to
slab pull
–As many advancing trenches as
retreating ones
–Slab age is not correlated with trench
retreat
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