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Scientific Challenges:
Continental rifting and other margins
Don Forsyth
MARGINS Workshop, February 15, 2010
1. What is the relationship of mantle
upwelling and melt production to
rift initiation and segmentation?
Or, What the heck are those “red” blobs in the
vicinity of rifts in seismic tomography
images of the upper mantle?
Beutel, et al. 2010
Afar triangle overlies
major hotspot
Typical magmatic and
rift segment scale 4080 km
Bastow et al.,2005
Velocity anomalies at
75 depth are
displaced from the
shallow magmatic
segments
Benoit et al., 2006
Major low velocity centers
spaced 250-300 km apart
Shear velocity
anomaly at centered
at a depth of ~ 60 km
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Velocities slow along
plate boundary (red)
with 3 (4?) centers of
particularly low
velocity 200-250 km
apart.
Low velocity
anomalies are
displaced somewhat
from spreading or
rifting centers.
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% Average Shear Velocity Anomaly 40-90 km
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Profile of anomalous shear velocities along Gulf of California. We interpret low
velocity centers as indicating higher melt concentrations, which may induce
convective upwelling in underlying asthenosphere. Low absolute shear
velocities extend to 150-175 km.
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Low velocity anomalies near Marianas backarc
spreading centers spaced about 250 km apart
with maximum anomalies at depth of ~ 50-60
km.
Pyle et al., JGR submitted, 2009
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Magde and
Sparks, 1997
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Melt Porosity
Dynamic upwelling due to melt depletion buoyancy, melt retention and
temperature anomalies predicted at 150-400 km wavelengths
Does the 200-300 km spacing of shallow-upper-mantle,
low-seismic-velocity anomalies represent dynamic
upwelling of the asthenosphere on this length scale?
If so, how does the magma produced in the upwelling
zones migrate to the shallow rift segments that are on a
significantly smaller length scale?
Or does the displacement of the anomalies from the rift
centers simply indicate that melt is produced in a
relatively broad region, but extracted more efficiently in
the vicinity of the rifts?
2. How does the subduction of a spreading center affect
the tectonic and volcanic evolution of the plate
boundary?
How does the cessation of spreading just before ridge
subduction and the persistence of slab fragments
affect the plate boundary?
Where does the subducting slab detach from the
unsubducted plate fragment?
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Atwater and Stock, 1998
Nicholson, et al. 1994
Current view.
Farallon subduction left
unsubducted fragment(s)
behind - the Monterrey
microplate offshore
California.
Nicholson et al., 1994
Magnetic anomalies
from Wilson, et al.,2005
after Lonsdale, 1991 and
Severinghaus & Atwater, 1989
Monterey microplate
Spreading ceased ~ 19 Ma
Guadalupe microplate
Spreading ceased ~ 12.5 Ma
Magdalena microplate
Burkett &Billen, 2009
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Post-subduction volcanism
From Negrete-Aranda and
Canon-Tapia (2008)
Post-subduction
volcanism in Baja
California
Negrete-Aranda &
Canon-Tapia, 2008
Infer presence of
slab, melting of
oceanic crust and
release of fluids
from fossil slab,
with resurgence of
volcanism in last 4
Ma. Slab extends
to 150+ km.
High-Mg Andesites
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Horizontal slice at 100 km
shows high shear
velocities corresponding
to slab fragment still
attached to unsubducted
microplate remanents
Spreading ceased at
~ 12.5 Ma
High-Mg andesites are
post-subduction
volcanism thought by
many to involve partial
melting of subducted
oceanic crust
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Wang, et al. V41B-2190
Colorado
Plateau
Shear velocity
at 100 km
depth
Isabella or
Central Valley
anomaly
At 70 to 90 km, low velocity region coincides with region of Quaternary volcanism
(black dots). Dashed line indicates extent of high-K volcanism at ~ 3.5 Ma. Solid line
indicates total extent of volcanism at that time (after Manley et al., Geology, 2000).
Note more circular shape of the Great Valley anomaly.
Drip may be associated
with delamination of the
lower crust and
lithospheric mantle
from beneath the
eastern Sierra Nevada
Alternative Suggestion:
Instead of oceanic
lithosphere detaching,
opening a slab window,
followed by lithospheric
instability and
delamination, the
Isabella anomaly is
simply a piece of stalled
oceanic lithosphere still
attached to the
unsubducted Monterey
microplate.
Schmandt and
Humphreys, 2010
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% P wave
anomaly
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W
36°N
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E
Latest P-wave tomography model across Isabella anomaly,
Schmandt and Humphreys, AGU 2009
Gap in post 20 Ma
volcanic activity due
to absence of slab
window?
Requires coast
ranges to translate
along with Monterey
microplate and
Pacific plate.
Wilson et al., 2005
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Brocher, et al., 1999
Oceanic crust has been traced close to the San Andreas fault with
seismic reflection and refraction. What happens at the San
Andreas and further inland is unknown.
Could the San Andreas be the location where the Pacific/Farallon
lithosphere begins to plunge downward, decoupling the Pacific
plate from North America?
Berkeley Quake Map
1970-2003
A major question:
Could the subducted
slab fragment
translate 100s of km
along with the Pacific
plate/Monterey
microplate and still
remain intact and
attached?
Benz, et al., JGR 1992
Furlong and Schwartz, 2004
Conclusions
There are remnants of the subducting Farallon slab beneath Baja
California extending to depths of ~150 km that are still attached
to the unsubducted Guadalupe and Magdalena microplates
The Isabella or Great Valley or Central Valley anomaly in
southern California is probably a slab remnant still attached to
the Monterrey unsubducted microplate rather than sinking
lithosphere related to delamination of the lithosphere beneath the
Sierra Nevada
These attached fragments of subducted oceanic lithosphere
translate laterally for hundreds of km along with the microplates
that are now part of the Pacific plate