Transcript Document

S-wave velocity structure
beneath the Kaapvaal Craton
from surface-wave inversions
compared with
estimates from mantle xenoliths
Angela Marie Larson
M.S. defense
16 July 2004
16 July 2004
Acknowledgments
I would like to thank my advisor, Arthur Snoke, as well as my committee
members, David James and Martin Chapman
Thanks also go to the VT Geosciences Dept for their support as well as the
Aubrey E. Orange Award
I think an eternal debt is owed to Connie and Richard for always making
things go a little more smoothly
Thanks to all of my friends and associates in the dept
A special thank you goes to Brian, Laura O, and my animal family for your
support (I know I can be hard to deal with…)
16 July 2004
This presentation compares two methods for calculating
the S-wave velocity structure of the Kaapvaal Craton
Why? The purpose behind the study
Where and Who? Study area and previous work
How? Xenoliths, surface waves, inversions
Oh my!
What? Conclusions
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The purpose of this research is to:
Calculate the S-wave velocity structure of the
Kaapvaal Craton using surface waves
Compare xenolith analysis and surface-wave
analysis for the upper-mantle velocity structure
Look for the existence of a low-velocity zone
beneath the Archean craton using synthetics
This slide is for Maddy!
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The region contains almost 1 Ga of Archean history
and has remained largely unaltered since its formation
Political boundaries
Geologic provinces
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The Southern African Seismic Experiment collected
seismic data in a dense (1º spacing) array
There were 79 stations deployed for a minimum of 1 year
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The Kaapvaal Project researchers also performed
geochemical/petrologic studies on local xenoliths
Approximately 100 on-craton samples with ages ~90 Ma were
analyzed geothermobarometrically
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Kimberlite pipes are for more than just diamonds; some
can be analyzed for their records of mantle properties
http://www.amnh.org/exhibitions/diamonds/
The procedure of analyzing mantle xenoliths for upper mantle
seismic structure has not been utilized much…yet
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The Kaapvaal xenoliths provide a snapshot of seismic
velocities and density for the craton at ~90 Ma
The xenoliths studied were emplaced in the southern half of the
craton
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The xenolith results indicate a slight decreasing trend in
S-wave velocities for the applicable depth range
The xenoliths reflect seismic velocities and density for a depth
range of 50 to 180 km.
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The surface-wave data is selected by choosing wellrecorded earthquakes
The earthquake must be large enough to record in the array
with a reasonable signal-to-noise ratio
Favorable earthquakes had epicenters with great-circle paths
passing through the NE and SW quadrants of the array
These earthquakes must cross continent-ocean boundaries
and mid-ocean ridges (if applicable) at near-normal incidence
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Five events were used for study
There were four events near the coast of central Chile and one in
Iran. Their great-circle paths to the center of the array are shown
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Frequency-time analysis helps with the selection of a
range of useful periods and frequencies
A range for this event would be 3.40 - 4.05 km/s and about 25 16 July 2004
180 seconds
Station pairs have their own selection criteria
At least 200 km between stations
No more than 3º difference in backazimuths
Well-recorded at both stations
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NO Bushveld in the interstation path
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The Bushveld Complex appears to affect the velocity
structures of the Kaapvaal Craton
To work with “pure” craton, I excluded all station pairs with
interstation paths within the Bushveld
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I found 16 paths from the five events using the selection
criteria
Several events use the same station pair and some events use
collinear paths
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“The velocity with which an observable, individual wave
or wave crest is propagated through a medium”
The phase velocities are calculated for many (or all) of the periods
over a range to make a dispersion curve
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A dispersion curve was calculated for each of the 16
different event/paths
The solid line on the left is the dispersion curve calculated from
the xenolith-based model
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The 16 sets were combined into a single composite
curve and compared with the xenolith-based curve
The curve fits the data very well, and this is a pre-inversion result
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The Neighbourhood Algorithm (NA) is used to find an
ensemble of models that fit a chosen misfit criterion
The NA is a non-linear direct search method. The search results
are not limited to a single maximum
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The misfit criterion was essentially a standard deviation
with the potential for a penalty to be assessed
The penalty is designed to cause physically-unlikely models to be
thrown out
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There were eight non-independent parameters arranged
over the depth range for the model
All solution models were constrained to match the reference
model values for velocities greater then 400 km
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After 10,000 model iterations, 1,788 models fit within the
misfit cutoff of 0.015
This selected misfit allowed for enough potential models that their
dispersion curves visually filled the space of the error bars 16 July 2004
Between the “best” model and an “average” model, I
chose the average model
The best model had the smallest overall misfit, but it is a
physically less likely model
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To test for uniqueness, I compared the xenolith-based
results with those from different reference models
These global models are not plausible for continental shields. The
dispersion curves produced from the unaltered models don’t fit
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After inversions performed by the NA, models are found
that have reasonable fits to the original seismic data
The gentle reverse “s”-curves are a result of the fewer parameters
of NA and from using reference models that were so unlikely
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The results are that the xenolith-derived results match
the seismic-derived S-wave velocity model
The region of most interest is from 50 to 180 km depth
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I ran synthetic tests to check for evidence of a lowvelocity zone (LVZ)
The SYNTH model was made by perturbing the XNLTH model; a
LVZ was put in starting at 180 km depth.
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An NA inversion run produced acceptable models that fit
the SYNTH generated dispersion velocities
There is no LVZ starting at 180 km depth --- the models are
significantly different
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In summary
The xenolith analysis and surface-wave analysis agree
for the upper-mantle velocity structure
This means that the geotherm of the Kaapvaal Craton is
almost the same today as it was 90 Ma
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And
There is no LVZ starting at 180 km depth or less
beneath the Kaapvaal craton
Therefore, either there is a significant tectonic root and
the LVZ is deeper than 180 km or low velocities and low
viscosities are not closely correlated
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Conclusions:
The xenolith analysis and surface-wave analysis agree
for the upper-mantle velocity structure
and
Mischief managed!
There is no LVZ starting at 180 km depth or less
beneath the Kaapvaal craton
Questions?
16 July 2004