Transcript Blackman.RCL06 - RCL

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Upper mantle flow associated with plate motion
and melt-enhanced upwelling
Influences of ridge segmentation & asymmetric
opening
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Development of mantle seismic anisotropy
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Modeling flow-induced anisotropy
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Ideas for further work in Gulf of California
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Plate-driven mantle flow
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Upwelling and decompression melting of peridotite
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Buoyancy-enhanced upwelling if melt is retained
within crystal matrix
Spreading rate (and/or regional viscosity) control on
nature of flow and, therefore, the rate/structure of
new crust produced
Motion of
plates
induces flow
in upper
mantle
Decompression
melting
Along-strike variation in melt production
and/or migration
>> stronger flow velocity gradients
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Evidence of 3-D upwelling and/or melt supply?
Variation in lithospheric structure
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Cooling with age & changes in plate thickness at ridge offsets
Central vs end-of-segment crustal structure?
Evolution in time
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Changes in rift/ridge-transform plate boundary geometry
Response to change in plate motion?
Reflection of changes in melt supply? (entrained
heterogeneities)
>>> degree of coupling between lithosphere and
asthenosphere…
Van Wijk & Blackman, Tectonophysics, in press (basic results similar to earlier work by us/others)
Top velocity (10 mm/yr, half rate), initial plate boundary position prescribed
T-dependent viscosity, coupled deformation (finite element, Tekton revised
for 3-D) & temperature (finite difference) calculation
Axial zone is weak
Visco-elastic, power
law rheology with
separate upper
crust, lower crust, &
mantle properties
Melt production
follows McKenzie &
Bickle 1988
Axial zone ~2
orders of magnitude
weaker than
surrounding
lithosphere
kilometers
Non-transform offset
Map
View
top
top
55
km
top
transform
offset
(free slip)
Vertical
Profile
Depth (km)
mm/yr
km
12 km W of axis
km
45 km W of axis
Transform offset
kilometers
Map View,
20 km depth
Melt buoyancy enhanced upwelling
Small component of along-strike flow near segment end
Map View
Vertical
Profiles
along
spreading
axis
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Mantle minerals have inherently anisotropic elastic
structure
Flow-induced alignment of crystal orientations
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Poly-crystalline aggregates undergo deformation
Possible contribution of retained/migrating melt
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Distribution of melt >> anisotropic signature
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Influence on bulk seismic velocity
Effective elastic constants & modeling surface
seismic signature
Development of mineral texture along flowlines
Strain-induced alignment of
mineral grains
Distribution of grain orientations
Peridotite is olivine (black poles)
+ pyroxene (green poles), ~70:30
ol1341
Compute Effective Elastic
Constants
Voigt average over all the
individual grain contibutions
Oriented single-crystal EC
projected onto global frame to
get x,y,z contribution
en1341
ol1322
en1322
ol1341
Compute Effective Elastic
Constants
Voigt average over all the
individual grain contibutions
Oriented single-crystal EC
projected onto global frame to
get x,y,z contribution
en1341
Fast direction
Magnitude anisotropy
ol1322
en1322
Buoyancy Enhanced Upwelling
predicted body-wave
anomaly due solely
to presence of melt
relative travel time delay (secs)
Passive Flow Model
symbols show
different models
of melt 'inclusion'
geometry
(Blackman &
Kendall, 1997)
Passive vs. Buoyancy-Enhanced Upwelling
slow,
symmetric
spreading
Flowlines &
finite
strain
P-wave
anisotropy
(degree &
fast-axis
direction)
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East Pacific Rise
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MELT Experiment 17°S
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Suite of models & comparison with OBS data
Western US
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Mantle Wedge flow behind subducting plate
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CSEDI Project with Thorsten Becker & Vera Schulte-Pelkum
With/without backarc shearing
Proposal for Gulf of California
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Collaboration with Frank Vernon & Graham Kent, Harold Magistrale,
& Gary Pavlis
Shear Wave Splitting determined
along the MELT OBS array
(Wolfe et al., Science 1998)
Reference EPR
17°S Flow Model
Migrates 32 mm/yr
Constant asthenospheric
viscosity
S Splitting at Vertical Incidence
Flow
across
600 km
depth
is not
allowed
Flowlines &
finite strain
P-wave
anisotropy
S-wave
Splitting
at nearvertical
incidence
Incidence + 20°
Incidence - 20°
Temperature
Anomaly
associated with
Pacific Superswell
influences flow
(Toomey et al., EPSL 2002)
Predictions for seismic
anisotropy & heterogeneity
match MELT data better
than the other models
tested
Subduction Zone
Anisotropy
Direction of fast-seismic
propagation has been
determined to parallel the
trench in several cases
Broadscale mantle flow?
Backarc shearing (Hall et
al., 2000)?
Effect of water on fastaxis orientation during
texturing (Jung & Karato,
2001)?
2-D Corner Flow
P-wave
anisotropy
max
S- wave
Splitting
Vertical
S-wave
Splitting
Add Along-trench Flow
P-wave
anisotropy
max
S- wave
Splitting
Vertical
S-wave
Splitting
Mantle flow and
predicted anisotropy
in the Lau Basin
Conder et al., GRL 2002
Backarc spreading flow and
melting in addition to
subduction-induced mantle
wedge flow
Western US
Finite Strain Ellipses
Becker et al, EPSL submitted 2005
Color-coded for depth
Surface velocity condition (white arrows), global
seismic tomography proxy for density, radially varying
viscosity, Kaminsky & Ribe method for LPO
Profile view of finite strain
Predicted Fast S Polarization Direction (black bar)
Comparison of predicted (black bars) &
observed (white bars) SKS splitting
Western US
initial results
Gray wedge indicates variation in prediction due to
method of synthetic calculation (single layer
approximates 375 km deep region vs. multiple,
variable layers)
Overall fit is reasonable
for central/southern
area. NW and Basin &
Range are not matched
well.
Local structure & effects
on flow (not included in
low resolution global
model) preclude match in
complex areas (S Great
Valley)
Relation to complete
crustal transect?
Anomalously hot?
Depth extent of
upwelling?
Southern (oceanic
spreading) vs.
Northern (rifting)
Gulf structure?
Buoyancy vs platedriven flow?
Gaherty, Collins,
Rebollar surface wave
study designed to
address several
aspects
Continue to pursue
additional work (SIO,
SDSU, CICESE, Indiana)
Seismometer
Deployments
NARS
~5 yr, 18 broadband
Collins et al. OBS
~15 mo, 18 broadband
Proposed Array
60 PASSCAL
22 OBS
All broadband
15-18 months
Seismometer
Deployments
NARS
~5 yr, 18 broadband
Collins et al. OBS
~15 mo, 18 broadband
Previously Proposed
Array
60 PASSCAL
22 OBS
All broadband
15-18 months
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Architecture of the crust across the rift system
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Teleseismic and local event tomography
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Mapping of Moho across onshore & offshore parts of system
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Local EQ source parameters
Strength and deformation of the lower crust
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Imaging of forward scattered P-S conversions
Upper mantle thermal structure and flow
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seismic velocity structure & attenuation (scale of ~10 km)
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Shape of seismic discontinuities (upward or downward deflection?)
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Seismic anisotropy (splitting, polarization direction of fast S wave)
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Linked models of mantle flow, lithospheric deformation, development of
textural (+/- melt) anisotropy, and seismic anisotropy
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Converted phase imaging for detecting possible foundered slab
Coordinate with findings of other Gulf of CA studies
(Poppeliers & Pavlis, 2002)
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Architecture of the crust across the rift system
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Teleseismic and local event tomography
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Mapping of Moho across onshore & offshore parts of system
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Local EQ source parameters
Strength and deformation of the lower crust
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3.
Imaging of forward scattered P-S conversions
Upper mantle thermal structure and flow
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seismic velocity structure & attenuation (scale of ~10 km)
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Shape of seismic discontinuities (upward or downward deflection?)
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Seismic anisotropy (splitting, polarization direction of fast S wave)
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Linked models of mantle flow, lithospheric deformation, development of
textural (+/- melt) anisotropy, and seismic anisotropy
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Converted phase imaging for detecting possible foundered slab
Coordinate with findings of other Gulf of CA studies
Jones et al., 1994
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Mantle structure & flow are likely to vary both laterally and with depth on scale
of several km
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Along-strike changes in GoC rifting
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Influence of transform offsets
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Large scale flow at depth
Combination of research approaches needed to fully address problems
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Separate effects of T, melt, textural anisotropy
Series of inverse studies and selected forward modeling tests to assess possible
contributions of various structure and implied geodynamic consequences
Opportunity to assess both crust & mantle structure to infer processes of
rifting as a full system
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Combine body wave, surface wave and active source seismics
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Current data will provide resolution of mantle on several 10 ‘s km scale
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Denser onshore/OBS array would improve resolution to several km
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Avoid ‘bias’ due to possible superpostion of segmentation and longer wavelength rifting
signatures
Recognize cross-axis structure which could be key to understanding rifting processes
Key Constraints Needed
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Surface velocity
• Plate boundary geometry, temporal evolution
Cooling rates of crustal rocks
Caution about
• Guide vertical flow predictions
possible ‘bias’
due to limited
Information on melting
areas (core
• Degree/depth of melting
complexes)
emphasized in
• Composition
many studies …
Seismic/EM measurements
• All phases (P, S, Surface Waves)
• As many backazimuths & angles of incidence as possible
Savage & Sheehan,
2000
Pole Figures illustrate development of texture
Texture predicted depends on assumptions but
fundamental result is often similar between
methods
Wenk & Tomé, JGR 1999, model recrystallization
via strain-controlled nucleation & growth of new
grains
Sub-vertical shear imparts strong texture in
upwelling zone; diffusion occurs in corner;
subhorizontal shear generates plate-spreading
signature
P-wave anisotropy along flowline:
different models
Comparison of Texture
& Finite Strain
Anisotropy Prediction
P-wave fast axis orientation for
slow-spreading, passive model