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Seismological Investigation of Okavango
Rift, Botswana
Youqiang Yu
Date: 04/28/2015
Geology and Geophysics Program
Missouri University of Science and Technology
Acknowledgments
 Thank my advisor Dr. Stephen Gao and co-advisor Dr. Kelly Liu
for the patient guidance and mentorship.
 Thank my committee members, Dr. Andreas Eckert, Dr. David
Rogers and Dr. Kevin Mickus for the constructive suggestions.
 Field helps from Dr. Angela M. Reusch (Mouse), Dr. Moikwathai
Moidaki (Dax), student Keletso Kaisara and local people in
Botswana are appreciated.
 Thanks for the help from our secretaries of Patti Adams, Patti
Robertson and Paula Cochran.
 Thank the helps and suggestions from my colleagues of
geophysics group.
 This study is funded by the US national Science Foundation.
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Outline
 Location
 Tectonic Background
 Introduction to the Rifting
 Observations from MTZ
 Results from SWS
 Geodynamic Mechanism
 The Reverberation-removal Technique
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Okavango Rift Zone
Seismicity analysis (1969~1973)
(Scholz et al., 1976)
Located at the southwestern
terminal of the East African
Rift System; incipient rift
between 27~40 ka (Modisi et al.,
2000, Kinabo et al., 2008)
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Tectonic Background
Situated between the
Archean cratons;
superimposed with
Proterozoic belts.
(CC: Congo Craton, DB: Damara Belt, KC: Kaapvaal
Craton, LP: Limpopo Belt, MB: Magondi Belt, Mb:
Mozambique Belt, ORZ: Okavango Rift Zone, RP:
Rehoboth Province, ZC: Zimbabwe Craton) (Hanson,
2003; McCourt et al., 2013)
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Tectonic Background
Gondwana reconstruction at
end of Neoproterozoic, from
Hoffman (1991) and
Fitzsimons (2000).
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Introduction to the Rifting
The modes for the initiation of the rifting
(Sengor and Burke, 1978):
 Active rifting: an anomalously hot and low density region in
the underlying upper mantle (hot spot). The doming and /or
volcanism should be the first detectable events.
http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-3101/Rifting02/actpass.html
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Introduction to the Rifting
The modes for the initiation of the rifting
(Sengor and Burke, 1978):
 Passive rifting: the far field stretching. local subsidence, and
accompanying rise of the asthenosphere.
http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-3101/Rifting02/actpass.html
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Studies of MTZ
Mantle Transition Zone:
Boundary between the upper and
lower mantle. Constrained at the
depths of 410 and 660 km (here
after named d410 and d660
respectively).
Mineral physics and seismological investigations have suggested that
the d410 represents the transition from olivine to wadsleyite, and
the d660 represents the transition from ringwoodite to perovskite.
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Characters of MTZ Discontinuities
 Under normal temperature and anhydrous conditions, the
estimated Clapeyron slope of the transitions:
d410: 1.5 to 3.0 Mpa/K
d660: -4.0 to -2.0 Mpa/K
 The presence of water in the MTZ has similar effects as reducing
temperature.
 Under high temperature conditions (e.g. >=1700 °𝐶), d660 is
dominated by the transition from majorite to perovskite, which has
a positive Clapeyron slope of about 1.0 Mpa/K.
(Ringwood, 1975; Bina and Helffrich, 1994, Hirose, 2002)
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410
410
660
660
(Deuss, 2007)
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Picking of the MTZ phases
The mean MTZ values:
 d410: 398±6 km
 d660: 646±7 km
 MTZ thickness: 248±6 km
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Results
The whole study has a character
of normal MTZ thickness.
Congo Craton: almost identical
with those of the IASP91 Earth
model.
Kalahari Craton: shallower
depths of d410 and d660
(approximately 395 km and 645
km respectively).
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Discussion
 Strong correlation between the d410 and d660,
indicating velocity anomalies constrained in the
upper mantle.
 15 km uplift for the Kalahari Craton, which is equal
to a 1.2% Vp anomaly existing above d410 (Gao and
Liu, 2014), leading to a 0.6s difference consistent with
the picked time residuals.
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Discussion
 There is an estimated 100150 km difference of
lithospheric thickness
between ORZ and Kalahari
Craton (James et al., 2001; Muller et
al., 2009)
P time residuals
S time residuals
 Strong coherence between
the distribution of the relative
time residuals and those of
the MTZ discontinuities
depths
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Main Findings from MTZ Study
 Absence of thermal anomalies beneath the ORZ based
on the observed normal MTZ thickness.
 Rule out the existence of superplume within the MTZ.
 Higher velocity anomalies exist beneath the Kalahari
Craton and are attributed to the thick lithosphere relative
to the ORZ.
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Studies of SWS
(Kendall et al., 2014)
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Correction of Sensor Orientation
Before:
1. Energy left in
the transverse
component.
2. Poor matching
between the
polarized
components.
3. Bad linearity of
particle motion
patterns.
After applying the
technique of Niu
and Li (2007):
1. No energy left in
the transverse
component.
2. Matching
improved.
3. linearity of
particle motions
is enhanced.
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Station Orientations of the 22 Stations
The magnitude of the orientation ranges
from -16 to 5 degrees clock wise from the
north, with a simple mean of -3 ± 1° .
http://www.ngdc.noaa.gov/geomag-web/
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Examples of Quality A Measurements
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Spatial Distributions
of SWS Results
• A total of 223 pairs of Quality A or
B SWS measurements (73 PKS,
58 SKKS and 92 SKS).
• Dominantly NE-SW fast
orientations with an average
value of 37.5 ± 20.6° .
• The mean splitting time is 1.09±
0.34s.
• Sudden change of fast
orientations at the boundary of
Kalahari Craton.
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Relationships with Previous SWS Studies
Consistent with
previous studies
from adjacent
areas.
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Estimations of Anisotropy Depths
Conditions required before applying
the spatial coherence technique (Liu
and Gao, 2010):
1. The compilation of a high-quality
dataset of individual splitting
parameters.
2. A significant but smooth spatial
variation in the splitting parameters
signifying a single layer of
anisotropy.
3. An approximately constant
anisotropic depth beneath the area
where the technique is applied.
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Examination of Rifting Mechanisms
1. Active rifting related to the
mantle plume. Ruled out by the
lacking radial pattern of fast
orientations.
Moore and Blenkinsop, 2002
2. Edge Driven Convection. Need
SWS measurements orthogonal
to the trend of the rift.
King and Anderson, 1998
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Rifting Induced by Intra-plate
Relative Motion
 The bulk of the observed
anisotropy is caused by
the simple shear in the
direction of the APM
developed in the upper
asthenosphere.
 Deviations of the fast
orientation observed in
Area C is due to the
modulation of flow by the
topography of the bottom
of the lithosphere.
Gao et al., 2008
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Geodynamic Mechanism
 The Okavango Rift possibly follows a passive model.
 The relative movements between the Archean cratons
could rupture ancient zones of lithospheric weakness
such as the Damara belt can exert a trans-tensional force
upon the lithosphere, resulting in the initiation of
continental rifting.
 Such a model is supported by the GPS study that the
south Africa is rotating clockwise relative to the Congo
Craton (Malservisi et al., 2013).
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The Reverberation-removal Technique
Yu et al., 2015
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Publications
Papers as author & co-author

Yu, Y., S.S. Gao, M. Moidaki, C.A. Reed and K.H. Liu (2015), Seismic anisotropy beneath the incipient Okavango rift:
Implications for rifting initiation (Submitted).

Yu, Y., K.H. Liu, M. Moidaki, C.A. Reed and S.S. Gao (2015), No thermal anomalies in the mantle transition zone beneath the
incipient Okavango Rift: Evidence from stacking of receiver functions (Submitted).

Yu, Y., J. Song, K.H. Liu, S.S. Gao (2015), Determining crustal structure beneath seismic stations overlying a loose
sedimentary layer using Receiver Functions, J. Geophys. Res., 120, doi:10.1002/2014JB011610.

Reed, C.A., S.S. Gao, K.H. Liu and Y. Yu (2015), Mantle plume and mantle transition zone hydration beneath the Afar
Depression and adjacent regions (submitted).

Lemnifi, A.A., K.H. Liu, S.S. Gao, C.A. Reed, A.A. Elsheikh, Y. Yu and A.A. Elmelade (2015), Azimuthal anisotropy beneath
north central Africa from shear wave splitting analyses, Geochem. Geophy. Geosy., 16, doi:10.1002/2014GC005706.

Wu, J., Z. Zhang, F. Kong, B.B. Yang, Y. Yu, K.H. Liu and S.S. Gao (2014), Complex seismic anisotropy beneath western
Tibet and its geodynamic implications, Earth Planet. Sc. Lett., 413, 167-175, doi:10.1016/j.epsl.2015.01.002.

Mohamed, A.A., S.S. Gao, A.A. Elsheikh, K.H. Liu, Y. Yu, and R.E. Fat-Helbary (2014), Seismic imaging of mantle transition
zone discontinuities beneath the northern Red Sea and adjacent areas, Geophys. J. Int., 199, 648657, doi:10.1093/gji/ggu284.

Elsheikh, A.A., S.S. Gao, K.H. Liu, A.A. Mohamed, Y. Yu, and R.E. Fat-Helbary (2014), Seismic anisotropy and subductioninduced mantle fabrics beneath the Arabian and Nubian plates adjacent to the Red Sea, Geophys. Res. Lett., 41, 2376-2381.

Liu, K.H., A. Elsheikh, A. Lemnifi, U. Purevsuren, M. Ray, H. Refayee, B. Yang, Y. Yu, and S.S. Gao (2014), A uniform
database of teleseismic shear wave splitting measurements for the western and central United States, Geochem. Geophy.
Geosy., 15, 2075-2085, doi: 10.1002/2014GC005267.

Yang, B.B., S.S. Gao, K.H. Liu, A.A. Elsheikh, A.A. Lemnifi, H.A. Refayee, and Y. Yu (2014), Seismic anisotropy and mantle
flow beneath the northern Great Plains of North America, J. Geophys. Res., 119, 1971-1985, doi: 10.1002/2013JB010561.

Gao, S. S., K. H. Liu, C. A. Reed, Y. Yu, B. Massinque, H. Mdala, M. Moidaki, D. Mutamina, E.A. Atekwana, S. Ingate, and A.
M. Reusch (2013), Seismic arrays to study African rift initiation, Eos Trans. AGU, 94, 213-214, doi: 10.1002/2013EO240002.
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