Download Moho depths and Poisson`s ratios of Precambrian crust

Earth and Planetary Science Letters 355-356 (2012) 73–81
Contents lists available at SciVerse ScienceDirect
Earth and Planetary Science Letters
journal homepage: www.elsevier.com/locate/epsl
Moho depths and Poisson’s ratios of Precambrian crust in
East Africa: Evidence for similarities in Archean and Proterozoic
crustal structure
Fred Tugume a, Andrew Nyblade a,n, Jordi Julia b
a
b
Department of Geosciences, Penn State University, University Park, PA 16802, USA
~ em Geodinâmica e Geofı́sica, Universidade Federal do Rio Grande do Norte, Natal, Brazil
Departamento de Geofı́sica & Programa de Pós-Graduac- ao
a r t i c l e i n f o
abstract
Article history:
Received 11 January 2012
Received in revised form
20 August 2012
Accepted 22 August 2012
Editor: P. Shearer
We investigate the structure of Precambrian crust in East Africa by using estimates of Moho depth and
Poisson’s ratio for 37 new seismic stations in Uganda and Tanzania and for 32 previously deployed seismic
stations in Kenya and Tanzania. The dataset includes estimates of crustal structure distributed between all
of the major Archean and Proterozoic terrains obtained from modeling P-wave receiver functions. The
average crustal thickness for the Ubendian belt is 42 km and between 37 and 39 km for all of the terrains.
The average Poisson’s ratio for all of the terrains is either 0.25 or 0.26, indicating a felsic to intermediate
bulk crustal composition. A similar composition and thickness of the crust in East Africa for terrains
spanning some 4 byr of Earth history is different than in many other Precambrian regions. Our results
suggest that there may have been few changes over Earth’s history in the processes that formed the East
African crust, or else that processes have been at play to homogenize crustal structure, such as the flow of
lower crustal material in orogenic systems or through the foundering of eclogites into the mantle. The
finding that crustal structure is similar between the Archean and Proterozoic terrains indicates that crustal
structure, through its influence on lithospheric rheology and strength, has not had a first-order effect on the
location of rifting. We also find that there is little correlation between Moho depth and elevation across the
East African Plateau, indicating that variations in crustal thickness exert few, if any, influences on
topography in East Africa. This conclusion lends support to many studies arguing that mantle structure
and processes provide the primary buoyant support for the plateau.
& 2012 Elsevier B.V. All rights reserved.
Keywords:
crust
Africa
tectonics
Precambrian
1. Introduction
In this paper, we investigate the Precambrian crustal structure of
East Africa by using estimates of Moho depth and Poisson’s ratio for
37 new seismic stations in Uganda and Tanzania and for 32
previously deployed seismic stations in Kenya and Tanzania. Advancing our understanding of crustal Poisson’s ratio and Moho depths in
East Africa is important because this information provides fundamental constraints on the formation and evolution of the continental crust, most of which formed in the Precambrian (e.g., Goodwin,
1996). 94% of African crust is Precambrian, representing about 29%
of Precambrian crust globally (Goodwin, 1996). In comparison to
many other continents, little is known about the Precambrian
crustal structure of Africa, including East Africa, and consequently
African crustal structure is poorly represented in global studies of
crustal structure. The results presented in this study not only
advance our understanding of East African crustal structure, but
also help to address the bias inherent within global studies because
of a lack of data on crustal structure from Africa. In addition, the
results from this study are important for understanding the role
crustal structure may or may not have played in the development of
the East African rift valleys.
The estimates of Moho depth and Poisson’s ratio for the 37 new
stations come from modeling P-wave receiver functions using the
H–k stacking method (Zhu and Kanamori, 2000), while the results
from the previous studies were obtained by modeling P-wave
receiver functions using both the H–k method (Dugda et al., 2005)
and a slant-stack method (Last et al., 1997). The dataset, which
includes estimates of crustal structure distributed between all of the
major Precambrian terrains in East Africa, provides new insights into
crustal composition and thickness for many areas of East Africa that
have not been affected by Cenozoic rifting and volcanism.
2. Background
2.1. Geological setting
n
Corresponding author.
E-mail address: [email protected] (A. Nyblade).
0012-821X/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.epsl.2012.08.041
The Archean Tanzania Craton forms the nucleus of the Precambrian framework of eastern Africa and is surrounded by
74
F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81
that were partially derived from reworking and recycling of the
Tanzania Craton (Cahen et al., 1984; Schulter, 1997; De Waele
et al., 2008).
The Mozambique Belt is the longest terrain, extending along the
East African coast from Mozambique in the south to Egypt in the
north (Schulter, 1997). This belt is believed to represent a
Himalayan-type continental collision zone formed by multiple collisional events dated between 1200 Myr and 450 Myr (Cahen et al.,
1984; Shackleton, 1986). The northern part of the Mozambique Belt
consists of juvenile terrains and blocks of reworked Archean to
Mesoproterozoic continental crust, while the southern part (i.e.,
basement of southern Tanzania and northern Mozambique) consists
of Paleoproterozoic to Neoproterozoic gneisses metamorphosed
during the Pan-African Orogeny (Begg et al., 2009).
Superimposed on the Precambrian tectonic framework is the
Cenozoic East African rift system (EARS). The EARS is a classic
example of a continental rift, exhibiting characteristic patterns of
rifting, volcanism, and plateau uplift. The development of the rift
system has likely been influenced by the Precambrian tectonic
framework, with both the eastern and western branches forming
in the Proterozoic mobile belts surrounding the Archean Tanzania
Craton (Ebinger, 1989; Hetzel and Strecker, 1994; Tesha et al.,
1997; Nyblade and Brazier, 2002) (Fig. 1). The eastern branch of
the EARS starts at the Afar triple junction, continues southwards
through Ethiopia and Kenya, and terminates in northern Tanzania
in a wide zone of block faulting. The western branch of the EARS
extends from northern Uganda to Mozambique, running along the
borders of Uganda, the Democratic Republic of Congo, Rwanda,
Burundi, Tanzania, Malawi and Zambia.
2.2. Previous studies of crustal structure
Fig. 1. Map showing elevation, the tectonic framework of East Africa, and the
locations of temporary and permanent broadband seismic stations. The black
squares and diamonds give locations of the AfricaArray East Africa broadband
seismic experiment (AAEASE) phase I and II stations, respectively. Black triangles
and hexagons give locations of stations from the 1994–1995 Tanzania broadband
seismic experiment (TBSE) and the 2001–2002 Kenya broadband seismic experiment (KBSE), respectively. The black circles and inverted triangles represent the
permanent AfricaArray (AF) and GSN stations, respectively.
several Proterozoic mobile belts (Cahen et al., 1984) (Fig. 1).
To the east of the craton is the Neoproterozoic Mozambique Belt,
to the southeast and southwest are the Paleoproterozoic Usagaran
and Ubendian Belts, respectively, and to the northwest and north
of the craton are the Mesoproterozoic Kibaran and Rwenzori
Belts, respectively.
The northern part of the Tanzania Craton, referred to as the
Nyanzian terrain, is comprised of greenstone belts and granites.
The Dodoman terrain, located in the southern part of the craton,
consists of granodiorites, granitic gneisses, migmatites, and other
associated high-grade metamorphic rocks (Cahen et al., 1984;
Schulter, 1997; Manya and Maboko, 2003). The Rwenzori Belt is
an approximate E–W trending belt, also referred to as the
Buganda-Toro–Kibaran Belt, consisting of metasedimentary rocks
and tholeites, folded and cut by 1850 Myr granitoids and thrust
onto the northern margin of the Tanzania Craton ( Begg et al.,
2009). The Kibaran Belt forms a linear NE–SW oriented terrain of
amphibolite grade rocks formed during the Kibaran orogeny
which affected large areas of central, eastern and southern Africa
(Klerkx, 1987).
The Ubendian Belt consists of granulite and amphibolite facies
gneisses and metasedimentary rocks which formed during two
orogenic episodes, the first between 2100 and 2000 Myr and
the second around 1860 Myr, which also involved the emplacement of numerous granitoids and exhumation of granulites and
eclogites (Cahen et al., 1984; Lenoir et al., 1994; Schulter, 1997).
The Usagaran Belt consists mainly of granitoids and orthogneisses
Many previous investigations of crustal structure in East Africa
have focused on the eastern and western branches of the rift
system and by contrast much less work has been done on crustal
structure away from the rifts. Early studies used seismic refraction data and observations from teleseismic and regional earthquakes to examine crustal structure (Bonjer et al., 1970; Griffiths
et al., 1971; Long et al., 1972; Mueller and Bonjer, 1973; Bram and
Schmeling, 1975; Nolet and Mueller, 1982; Hebert and Langston,
1985), yielding estimates of Moho depths of 40–48 km beneath
unrifted Precambrian crust, and of 20–32 km under the rift
valleys. More detailed work on crustal structure in and around
the Kenya rift was undertaken by the Kenya Rift International
Seismic Project (KRISP) (Prodehl et al., 1994; Fuchs et al., 1997,
and references therein). Away from the rift, they obtained crustal
thicknesses of 34–40 km beneath the Tanzania Craton and
35–42 km beneath the Mozambique Belt, and an average Vp for
both terrains between 6.4 and 6.6 km/s.
More recent investigations of crustal structure in Tanzania
(Last et al., 1997; Julia et al., 2005), and Kenya (Dugda et al., 2005,
2009) using P-wave receiver functions and Rayleigh wave dispersion measurements, have provided additional information about
crustal structure in the mobile belts and craton. Data used in
those studies came from the Tanzania broadband seismic experiment (Nyblade et al., 1996) and the Kenya broadband seismic
experiment (Nyblade and Langston, 2002) (Fig. 1). For the
Tanzania Craton, Last et al. (1997) obtained Moho depths of
37–42 km and Poisson’s ratios of 0.24–0.26. For the Mozambique
Belt, they obtained Moho depths between 36 and 39 km and
Poisson’s ratios between 0.24 and 0.27, and for the Ubendian Belt
they obtained Moho depths between 40 and 45 km. Results
similar to those of Last et al. (1997) for Tanzania were obtained
by Julia et al. (2005), and Dugda et al. (2005, 2009) obtained Moho
depths of 37–42 km for the Mozambique Belt in Kenya, similar to
the KRISP results. For Uganda, Wolbern et al. (2010) reported
F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81
75
Moho depths of 20–28 km for the Rwenzori block, a horst within
the Albertine rift, and Moho depths of 30 km along the eastern
flank of the Albertine rift.
3. Data and methodology
3.1. Data
The data used for this study were recorded between August
2007 and June 2010 by the AfricaArray East African Broadband
Seismic Experiment (AAEASE), which consisted of portable broadband seismometers installed in Uganda and southern and western
Tanzania (Fig. 1). Each station was equipped with a broadband
seismometer (Guralp CMG-3T, Guralp CMG3-ESP, or Streckeisen
STS-2), a 24-bit Reftek data logger and a GPS (Global Positioning
System) clock. Data were recorded continuously at 40 samples
per second. During the first phase of the deployment, twenty
stations were installed in August 2007 in Uganda and northwestern Tanzania and operated until December 2008 (Fig. 1).
In the second phase of the deployment from January 2009 to June
2010, eighteen of the stations were removed from Uganda and
northwestern Tanzania and redeployed in southern Tanzania.
Station spacing was between 100 and 200 km. Data from the
permanent AfricaArray and IRIS/GSN stations in the region were
also used (Fig. 1).
3.2. Receiver functions
Receiver functions are time series containing secondary P-to-S
converted waves generated by the interaction of a teleseismic
P-wavefront with seismic discontinuities under the recording
station. The main amplitudes after the direct P wave are generally
a P-to-S conversion upon refraction across the Moho (Ps) and
multiple reverberated phases between the Moho and the freesurface (PpPs and PpSs). Analysis of the S–P travel-times, therefore, can be utilized to infer average crustal properties such as
crustal thickness and bulk Vp/Vs ratio (e.g., Zandt et al., 1995; Zhu
and Kanamori, 2000). Receiver functions are created by deconvolving the vertical component waveforms from the corresponding
radial component waveforms to isolate the receiver site effects
from other information contained in the teleseismic P wave coda
(Langston, 1979; Ligorria and Ammon, 1999). The deconvolution
effectively removes the signature of the source and instrument
response from the originally recorded waveforms keeping only
information on the structure local to the station.
For this study, P-wave receiver functions were computed for
each station using teleseismic events with magnitudes equal to or
greater than 5.5, and at epicentral distances between 301 and 901
(Fig. 2). A table of events used for computing receiver functions is
provided in the Supplementary material.
For computing receiver functions, the waveforms were windowed between 10 s before and 100 s after the leading P arrival,
de-trended, tapered, high pass filtered above 0.05 Hz to remove
the low frequency and instrument noise and decimated to 10
samples per second, after low-pass filtering below 8 Hz to avoid
aliasing. The horizontal seismograms were rotated into the greatcircle path to obtain radial and transverse components, and then
the vertical component was deconvolved from both radial and
transverse components using an iterative time domain deconvolution with 500 iterations (Ligorria and Ammon, 1999).
For each teleseismic event, receiver functions were computed
using two overlapping frequency bands ( o0.5 Hz and o1.25 Hz)
with Gaussian bandwidths of 1.0 and 2.5, respectively. Lower
frequency bands imply longer wavelengths and may amplify the
signal from the Moho in the presence of gradational crust–mantle
Fig. 2. Map showing the distribution of earthquakes used in this study. Black
circles and open circles represent earthquake locations used for the computation
of receiver functions for phase I and phase II AAEASE stations, respectively, plotted
using an equal distance projection. The large circles are plotted at distances of 30
and 90 degrees from the center of the seismic network.
boundaries, especially for the multiply reverberated phases
(Owens and Zandt 1997; Julia, 2007). The quality of the receiver
functions for one station is illustrated in Fig. 3 and additional
examples can be found in the Supplementary materials.
The receiver functions were evaluated for quality using a least
squares misfit criterion to assess the percentage of recovery of the
original radial component. The misfit criterion provides a measure of closeness of the receiver function to the ideal case, and it is
calculated by using the difference between the radial component
seismogram and the convolution of the vertical component
seismogram with the already determined radial receiver function.
Receiver functions with a fit of 85% and above were considered for
further processing. In addition, events with large amplitude
tangential receiver functions were not used, even if they passed
the 85% threshold.
3.3. H–k stacking method
The H–k stacking method was applied to the receiver functions
to obtain an estimate of crustal thickness (H) and Vp/Vs (k) ratio
under each seismic station (Zhu and Kanamori, 2000). In this
method, receiver function amplitudes are stacked along theoretical P-to-S phase moveout curves obtained after assuming an a
priori value for P-wave velocity (Vp). An objective function, s(H,k),
is constructed from the weighted sum given by
sðH,kÞ ¼
N
X
w1 r j ðt 1 Þ þ w2 r j ðt 2 Þw3 r j ðt 3 Þ
ð1Þ
j¼1
where ti are the S–P travel-times for the three main P-to-S converP
sions at the Moho (Ps, PpPs, and PpSs), wi are weights ð wi ¼ 1Þ
assigned to each conversion, rj(ti) is the receiver function amplitude
at ti for the j-th receiver function waveform, and N is the number of
receiver functions. For a simple layer-over-half space model, s(H,k)
reaches a maximum when H coincides with the layer parameters
76
F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81
necessary to observe at least two P-to-S conversions (Zhu and
Kanamori, 2000). For these stations, we estimated Moho depth
using a Poisson’s ratio of 0.25, which is an average value for most
of the Precambrian terrains in East Africa, the Moho Ps arrival
time, and Eq. (2) from Zandt et al. (1995). In addition, for stations
BUTI, KATE and KYLA, which were installed on rift sediments, the
receiver functions were too noisy to identify any Ps phases, and
therefore results for these stations are not reported.
4. Results
Fig. 3. A plot of radial and tangential receiver functions against ray parameter for
station BEND computed using a Gaussian filter of 1.0. The lines shows the
theoretical move-out curves calculated for each phase (Ps, PpPs, PsPsþ PpSs) for
a 40 km thick crust.
(Zhu and Kanamori, 2000), which are then taken as estimates of
crustal thickness and Vp/Vs ratio under the station.
In order to apply the H–k method, weights must be assigned to
each phase in Eq. (1), and an average crustal Vp must be selected.
A Vp of 6.5 km s 1, which is consistent with average crustal P wave
velocities from previous studies of Precambrian crust in East Africa
(Julia et al., 2005; Fuchs et al., 1997 and references therein), and a
weighting system of w1 ¼0.6, w2 ¼ 0.3, and w3 ¼0.1 were used.
However, at a few stations the PsPsþPpSs phase was not observed,
and the weights were re-adjusted so that w3 ¼ 0 (Tugume, 2011).
Results are illustrated for one station in Fig. 4, and for the other
stations the results are provided in the Supplementary material.
Receiver functions computed with a Gaussian filter of 1.0 were
used for all stations except SAKA, MLBA, BIHA, NJOM, MGOR,
DODT and KGMA, where a Gaussian filter of 2.5 was used. For
these stations, the Moho Ps phase and its multiples were more
clearly seen on the higher frequency receiver functions.
Uncertainties in H and k were obtained using a bootstrap
method, which involved repeating the stacking procedure 200
times with a resampled data set selected at random from the
original dataset, with replacement (Efron and Tibshirani, 1991).
Examples of the uncertainties are shown in Fig. 4, and the
uncertainty for each station using a mean crustal Vp of 6.5 km/s
for the H–k stacking is given in Table 1. In addition, for assessing
uncertainty in H and k resulting from the choice of the mean
crustal P wave velocity, the H–k stacks were recomputed using
mean crustal P wave velocities of 6.3 km s 1 and 6.7 km s 1
(Table 1). To obtain an overall uncertainty in H and k, the
uncertainty obtained from the bootstrap method was combined
with the range of H and k values obtained when using different
mean crustal P wave velocities. Combining the formal uncertainties with the range of H and k values obtained by varying the
mean crustal Vp from 6.3 to 6.7 km/s yields an overall uncertainty
in Moho depth for each station that is 3–4 km.
Results from this study, together with the results from studies by
Last et al. (1997) and Dugda et al. (2005) are summarized in Table 1
and Fig. 5. As mentioned above, the overall uncertainty in Moho
depth for each station is 3–4 km. For the Rwenzori Belt (stations
BEND, ROTI, MALE, PIGI, and SAKA), crustal thickness varies between
34 and 40 km with an average of 37 km, and Poisson’s ratio varies
between 0.24 and 0.27, with an average of 0.25.
For the Kibaran Belt (stations BKBA, MLBA, BIHA, KIBO, MKRE,
SULU, MBAR and KBLE), the average crustal thickness and Poisson’s ratio is 39 km and 0.25, respectively, but with larger
variability than in most of the other terrains. For example, a
significantly thinner crust and a higher Poisson’s ratio (0.28) are
found beneath station MBAR (33 km), while the thickest crust
within this terrain (station MLBA) is 44 km.
In the Ubendian Belt (stations UVZA, MBEY, PNDA, NAMA,
INZA, GOMA, KGMA, SUMB, PAND, LAEL, LOSS and TUND), the
thickest crust anywhere within East Africa is found at stations
LOSS (49 km), NAMA (48 km) and LAEL (47 km). On average, the
crustal thickness for this terrain is 42 km and Poisson’s ratio is
0.26. Our results for station MBEY, within the reported uncertainties, are consistent with those reported for nearby station PAND
by Last et al. (1997). For TUND station, Last et al. (1997) did not
report any results and therefore we only show values obtained in
this study for that station (Table 1).
Stations IRIN, MAFI, MAKA, NJOM, WINO, SONG, MGOR, CHIM
and MIKU are located in the Usagaran belt. The crustal thickness
in this terrain varies from 32 to 39 km and Poisson’s ratios range
from 0.24 to 0.28. The average crustal thickness is 37 km and
Poisson’s ratio is 0.26.
For the Nyanzian terrain within the Tanzania Craton, we report
new results from stations HAMA, GEIT and JNJA and combine
them with previous results for stations BASO and PUGE (Last
et al., 1997). Crustal thickness ranges from 37 km to 44 km and
Poisson’s ratio ranges from 0.25 to 0.28, with average values of
crustal thickness and Poisson’s ratio of 39 km and 0.26, respectively. For the Dodoman terrain, we combine results for station
DODT (this study) with results for stations MBWE, MITU, MTAN,
MTOR, RUNG, and SING (Last et al., 1997). Crustal thickness
ranges from 37 to 42 km and Poisson’s ratio ranges from 0.25 to
0.26, with average values of crustal thickness and Poisson’s ratios
of 39 km and 0.26, respectively.
In the Mozambique Belt, we combine results from station MAUS
with results from Last et al. (1997) and Dugda et al. (2005) for
stations HALE, KIBA, KIBE, KOMO, KOND, LONG, TARA, ANGA, KAKA,
KITU, KMBO, KR42, NAI and TALE. Crustal thickness varies between
36 and 42 km, with a mean Moho depth of 38 km. Poisson’s ratio
ranges from 0.24 to 0.27, with a mean value of 0.25.
3.4. Special cases
5. Discussion
For stations SUMB, MIKU, FOPO, CHIM and JNJA reverberations
from the Moho could not be easily identified on either lower or
higher frequency receiver functions. In order to constrain crustal
thickness and Vp/Vs ratio with the H–k stacking method it is
The main finding of this study is that crustal structure is similar
across all of the Precambrian terrains in East Africa. Within the
uncertainties of our estimates, there is no discernable difference in
F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81
77
Fig. 4. H–k stack of receiver functions for station BEND. To the left of each receiver function the top number gives the event backazimuth and the bottom number gives the
event distance in degrees. Contours map out percentage values of the objective function given in the text.
the crustal thicknesses or Poisson’s ratios between the Archean and
Proterozoic terrains, or within the Proterozoic terrains themselves.
The average crustal thickness for all but one of the terrains is
between 37 and 39 km. The exception is in the Ubendian terrain,
which has an average crustal thickness of 42 km. In all of the
terrains, the average Poisson’s ratio is similar, either 0.25 or 0.26.
This is in contrast to other areas in Africa, as well as within other
continents, where the Precambrian crustal structure on a regional
basis exhibits larger variability (i.e., Nair et al. (2006) and Kgaswane
et al. (2009) for southern Africa; Tokam et al. (2010) for west Africa;
Chevrot and van der Hilst (2000) and Kennett et al. (2012) for
Australia; Chen et al. (2010) for China; and Thompson et al. (2010)
for Canada).
Using Poisson’s ratio as a proxy for bulk crustal composition,
the average Poisson’s ratio of 0.25 to 0.26 indicates a felsic to
intermediate composition for the crust. For example, granitic
rocks have a Poisson’s ratio of 0.24, while intermediate composition rocks (e.g., diorite) have values around 0.27, and mafic rocks
(e.g., Gabbro) have values around 0.30 (Christensen, 1996; Tokav
and Vavakin, 1982). Poisson’s ratios of 0.28 or 0.29 are found at
stations SULU, MBAR, UVZA, PNDA, KGMA, MAKA, WINO and
HAMA (Table 1). However, when the uncertainties in these
estimates are considered, they are not necessarily anomalous
with respect to the average Poisson’s ratio of 0.25–0.26 (Table 1)
for the region.
Although there is much similarity in the average crustal
structure between terrains, there are a few stations where the
crustal thickness departs from the 37–39 km average. Thinner
crust is observed beneath stations BEND (35 km) and MBAR
(33 km). Studies by Dugda et al. (2005) and Wolbern et al.
(2010) obtained Moho depths of 33 and 30 km, respectively, for
station MBAR, consistent with our results. As noted by Dugda
et al. (2005), there are no obvious tectonic explanations as to why
the crust is thinner than average at station MBAR, nor is there an
obvious explanation for the thinner than average crust at station
BEND. Thicker than average crust is observed beneath stations
NAMA, LAEL, LOSS and SUMB, which are all located within the
Ubendian Belt. We suggest that the thickened crust beneath these
stations resulted from the collision of the Tanzania Craton with
the Bangweulu Block during the Ubendian Orogeny ( Lenoir et al.,
1994; Begg et al., 2009).
A summary of Moho depths and Poisson’s ratio for Precambrian terrains in East Africa as well as globally is given in Table 2.
In comparison to a global investigation of crustal structure by
Durrheim and Mooney (1991, 1994), who compiled Moho depth
estimates from 18 studies using a variety of seismic methods, we
find somewhat different results. Durrheim and Mooney (1991,
1994) concluded that average Proterozoic crustal thickness ranges
from 40 to 55 km, while Archean crustal thickness ranges from 27
to 40 km. We do not find such differences in the crustal thickness
between the Archean and Proterozoic terrains in East Africa.
In comparison, Rudnick and Fountain (1995), using results from
29 seismic refraction investigations, found no significant difference between the thickness of Archean and Proterozoic crust. Our
results concur in this respect with the findings of Rudnick and
Fountain (1995), however the mean crustal thickness for all but
one of the Precambrian terrains in East Africa (37–39 km) is
somewhat less than the mean crustal thickness of 43 and 44 km
reported by Rudnick and Fountain (1995) for Archean and
Proterozoic terrains, respectively. In the study by Zandt and
Ammon (1995) using receiver functions from 114 stations distributed across the globe, they found the crustal thickness of
shields and platforms to be between 37 and 42 km and
Poisson’s ratios for the bulk crust to be 0.27. Our results are
consistent with the findings of Zandt and Ammon (1995), and also
with the global compilation of Precambrian crust by Christensen
and Mooney (1995).
The finding that on average the Precambrian crust in all of the
terrains within East Africa is similar has important implications
for understanding processes of crustal formation and evolution, as
78
F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81
Table 1
Results from H–k stacking of receiver functions.
Tectonic region
Station name
N
rf
Lat.
(deg.)
Long.
(deg.)
Elev.
(m)
H (1)
(km)
k (1)
Poisson’s ratio
H, k (2)
(km)
H, k (3)
(km)
M
S
Rwenzori Belt
BEND
MALE
ROTI
SAKA
PIGI
Average
1
1
1
1
1
40
35
40
19
28
0.588
1.070
1.626
0.315
0.231
31.392
34.167
33.600
31.730
32.319
1351
1127
1108
1263
1252
34.5 þ / 0.6
38.0 þ / 0.5
40.2 þ / 1.3
37.0 þ / 1.6
36.7 þ / 0.7
37
1.73 þ / 0.02
1.70 þ/ 0.02
1.76 þ / 0.04
1.73 þ / 0.06
1.77 þ / 0.03
1.74
0.25 þ/ 0.01
0.24 þ/ 0.01
0.26 þ/ 0.02
0.25 þ/ 0.02
0.27 þ/ 0.01
0.25
36.4,
39.3,
41.8,
38.8,
38.2,
1.71
1.69
1.75
1.72
1.76
33.8,
36.5,
38.9,
36.5,
35.4,
1.73
1.71
1.76
1.71
1.78
A
A
A
A
A
1
1
1
1
1
Kibaran Belt
BKBA
MLBA
BIHA
KIBO
MKRE
SULU
MBAR
KBLE
Average
1
1
1
1
1
1
4
1
19
17
28
31
32
29
–
21
1.364
1.838
2.638
3.583
4.282
4.573
0.602
1.254
31.812
31.670
31.316
30.713
30.424
30.087
30.738
29.992
1274
1237
1459
1485
1175
1359
1390
1879
42.4 þ / 0.9
44.4 þ / 1.3
39.2 þ / 0.6
39.1 þ / 0.6
39.4 þ / 0.3
36.7 þ / 0.7
33.4 þ / 0.8
37.0 þ / 1.2
39
1.70 þ/ 0.02
1.70 þ/ 0.03
1.70 þ/ 0.02
1.72 þ / 0.03
1.75 þ / 0.01
1.84 þ / 0.02
1.82 þ / 0.02
1.75 þ / 0.03
1.75
0.24 þ/ 0.01
0.24 þ/ 0.01
0.24 þ/ 0.01
0.24 þ/ 0.01
0.26
0.29 þ/ 0.01
0.28 þ/ 0.01
0.26 þ/ 0.01
0.25
43.9,
46.1,
40.6,
40.5,
40.5,
38.2,
34.4,
38.6,
1.69
1.69
1.70
1.72
1.75
1.83
1.81
1.73
40.8,
42.9,
37.9,
37.7,
37.8,
36.9,
31.8,
35.9,
1.71
1.71
1.70
1.73
1.76
1.83
1.83
1.75
A
A
A
A
A
A
A
A
1
1
1
1
1
1
1
1
Ubendian Belt
UVZA
INZA
PNDA
NAMA
LAEL
LOSS
TUND
MBEY
PAND
GOMA
KGMA
SUMB
Average
1
2
1
1
1
1
1
5
2
2
1
1
16
–
26
20
11
12
7
–
–
–
19
–
5.104
5.117
6.352
7.510
8.566
8.417
9.295
9.000
8.983
4.839
4.878
7.953
30.393
30.399
31.061
31.041
32.059
33.158
32.771
33.250
33.242
29.693
29.633
31.620
992
975
1071
1559
1596
1195
1697
1331
1248
880
821
1837
39.0 þ / 0.9
42
40.9 þ / 1.3
47.5 þ / 2.1
46.7 þ / 1.1
49.2 þ / 2.0
43.8 þ / 1.9
40.1 þ / 3.6
35
44
39.6 þ / 1.7
44.6
42
1.80 þ/ 0.04
–
1.82 þ / 0.05
1.70 þ/ 0.05
1.72 þ / 0.03
1.79 þ / 0.03
1.77 þ / 0.05
1.69 þ / 0.07
1.72
1.74
1.81 þ / 0.17
–
1.76
0.28 þ/ 0.02
–
0.28 þ/ 0.02
0.24 þ/ 0.02
0.24 þ/ 0.01
0.27 þ/ 0.01
0.27 þ/ .02
0.23 þ/ 0.03
0.24
0.25
0.28 þ/ 0.06
–
0.26
40.4, 1.79
–
42.3, 1.82
48.9, 1.69
48.9, 1.71
51.1, 1.78
45.4, 1.76
–
–
–
41.0 1.8
–
37.6, 1.81
–
39.4, 1.83
45.6, 1.70
45.6, 1.72
47.5, 1.80
42.2, 1.78
–
–
–
38.1 1.82
–
A
C
A
A
A
A
A
A
B
B
A
C
1
2
1
1
1
1
1
1
2
2
1
1
Usagaran Belt
IRIN
MAFI
MAKA
NJOM
WINO
SONG
MGOR
MIKU
CHIM
Average
1
1
1
1
1
1
1
1
1
17
20
7
9
12
17
10
35.686
35.313
34.830
34.791
35.300
35.651
37.670
36.990
34.028
1561
1866
1685
1949
1511
1119
501
518
1100
39.4 þ / 1.4
38.8 þ / 1.4
39.1 þ / 2.3
32.3 þ / 1.3
39.6 þ / 2.5
37.0 þ / .8
36.1 þ / 2.4
37.2
38.1
37
1.74 þ / 0.04
1.74 þ / 0.04
1.80 þ/ 0.07
1.73 þ / 0.05
1.82 þ / 0.09
1.71 þ / 0.04
1.76 þ / 0.09
–
–
1.76
0.25 þ/ 0.02
0.25 þ/ 0.02
0.28 þ/ 0.03
0.25 þ/ 0.02
0.28 þ/ 0.03
0.24 þ/ 0.02
0.26 þ/ 0.03
–
–
0.26
41.0,
40.1,
40.8,
36.6,
41.0,
38.5,
37.7,
–
–
38.1,
37.3,
37.9,
31.2,
38.1,
35.5,
35.1,
–
–
1.74
1.74
1.79
1.73
1.83
1.72
1.76
–
7.762
8.306
8.847
9.367
9.757
10.670
6.828
7.400
8.826
A
A
A
A
A
A
A
C
C
1
1
1
1
1
1
1
1
1
Tanzania Craton (Nyanzian)
HAMA
GEIT
BASO
PUGE
JNJA
Average
1
1
2
2
1
6
13
–
–
–
3.832
2.881
4.324
4.175
0.446
32.642
32.217
35.138
33.184
33.182
1227
1281
1694
1350
1133
37.0 þ / 1.3
36.6 þ / 2.1
41 þ/ 4
37 þ/ 4
44.4
39
1.82 þ / 0.06
1.73 þ / 0.07
–
1.76
–
1.77
0.28 þ/ 0.02
0.25 þ/ 0.03
–
0.26
–
0.26
38.5, 1.80
38.7, 1.71
–
–
–
35.9, 1.81
36.0, 1.73
–
–
–
A
A
B
B
C
1
1
2
2
1
Tanzania Craton (Dodoman)
DODT
MBWE
MITU
MTAN
MTOR
RUNG
SING
Average
5
2
2
2
2
2
2
31
–
–
–
–
–
–
7.762
4.961
6.019
7.907
5.251
6.937
4.640
35.748
34.346
33.406
33.320
35.401
33.518
34.732
1114
1100
1566
1393
1100
1230
1462
40.9 þ / 0.5
37 þ/ 4
38 þ/ 4
37 þ/ 4
38 þ/ 4
42 þ/ 4
37 þ/ 4
39
1.71 þ / 0.02
1.74
1.76
1.76
1.71
1.76
1.76
1.74
0.24 þ/ 0.01
0.25
0.26
0.26
0.24
0.26
0.26
0.26
42.7, 1.70
–
–
–
–
–
–
39.6, 1.71
–
–
–
–
–
–
A
B
B
B
B
B
B
1
2
2
2
2
2
2
Mozambique Belt
MAUS
LONG
HALE
KIBA
KIBE
KOMO
KOND
TARA
ANGA
KAKA
KITU
KMBO
KR42
NAI
TALE
Average
1
2
2
2
2
2
2
2
3
3
3
4
3
4
3
31
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2.741
2.725
5.302
5.322
5.378
3.842
4.904
3.889
2.500
0.559
1.373
1.127
0.038
1.127
0.979
36.704
36.698
38.617
36.570
37.476
36.719
35.797
36.006
36.800
34.796
38.002
37.352
35.726
36.804
34.976
1334
1380
230
1500
997
1114
1419
1268
1000
1477
1129
1940
2157
1692
1821
39.1 þ / 1.0
37 þ/ 4
39 þ/ 4
36 þ/ 4
37 þ/ 4
36 þ/ 4
37 þ/ 4
37 þ/ 4
39 þ/ 3.0
37 þ/ 2.0
40 þ/ 3.9
41 þ/ 2.6
38 þ/ 4.8
42 þ/ 2.3
38 þ/ 3.2
38
1.67 þ / 0.02
1.78
–
–
1.74
1.72
1.78
1.74
1.70 þ/ 0.07
1.67 þ / 0.07
1.73 þ / 0.07
1.74 þ / 0.05
1.81 þ / 0.16
1.75 þ / 0.04
1.76 þ / 0.04
1.74
0.24 þ/ 0.01
0.27
–
–
0.25
0.24
0.27
0.25
0.24 þ/ 0.03
0.24 þ/ 0.03
0.25 þ/ 0.03
0.26 þ/ 0.02
0.26 þ/ 0.06
0.26 þ/ 0.02
0.27 þ/ 0.02
0.25
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
A
B
C
C
B
B
B
B
A
A
A
A
A
A
A
1
2
2
2
2
2
2
2
3
3
3
3
3
3
3
Western Branch
FOPO
1
–
0.663
30.282
1535
35
–
–
–
–
C
1
S¼ Source. 1¼ This study; 2¼ Last et al. (1997); 3¼ Dugda et al. (2005).
N ¼Network. 1 ¼AAEASE; 2¼ TBSE; 3 ¼KBSE; 4¼ GSN; 5¼ AF.
M ¼Methods. A ¼ H–k stack; B ¼ slant stack; C ¼Ps travel time.
rf ¼no. of receiver functions used.
H (1,2,3), k (1,2,3); ave. Vp used for H–k stack. (1) Vp ¼ 6.5 km/s; (2) Vp¼ 6.7 km/s; (3) Vp¼ 6.3 km/s.
1.73
1.73
1.80
1.72
1.81
1.70
1.74
F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81
well as the influence crustal structure may have exerted on the
location and development of the East African rift system. A similar
composition and thickness of the crust in East Africa for terrains
spanning some 4 byr of Earth history is different than in many
other regions of Precambrian crust, as noted previously. Our
results for East Africa suggest that there may have been few
changes over Earth’s history in the processes that formed the East
African crust, or else that processes have been at play to homogenize crustal structure, such as the flow of lower crustal material
in orogenic systems (e.g., Costa and Rey, 1995; Rey et al., 2001) or
79
through the foundering of eclogites into the mantle (e.g., Arndt,
1989; Jarchow and Thompson, 1989; Austrheim, 1991; Zandt
et al., 2004).
With regard to the rifting in East Africa, the fact that crustal
structure is similar between the Archean and Proterozoic terrains
indicates that variations in crustal structure, through its influence
on lithospheric rheology and strength, have not exerted terrainscale control on the location of rifting. It has long been noted that
the Cenozoic rift valleys have developed within the Proterozoic
mobile belts surrounding the Archean craton (e.g., McConnell,
1972; Ebinger, 1989; Nyblade and Brazier, 2002), and the results
of this study indicate that any major influence that the Precambrian lithosphere has had on the development of the rift system
must primarily come from variations in Archean versus Proterozoic lithospheric mantle and not crustal structure. However,
within individual terrains, some control on rift development
could have come from small ( o5 km) variations in crustal
thickness (e.g., Tesha et al., 1997).
Lastly, we use Airy isostasy to examine if variations in crustal
thicknesses have an influence on topography across the East
African plateau. Assuming a crustal underplating layer, h1,
beneath an uplifted region with elevation h2, and taking rc, ru
and rm as densities of the crust, underplating layer, and uppermost mantle layer, respectively, then h1 and h2 can be related
using the Airy relation h1 ¼h2 (rm ru)/rc. If we take h2, rc, ru and
rm as 1 km, 2670 kg/m3, 3000 kg/m3 and 3300 kg/m3, respectively, then for an uplift of 1 km about 10 km of crustal thickening
is required. Fig. 6 shows a plot of elevation against Moho depth
for all the stations in Table 1. While there is Z1 km variation in
elevation between the stations, there is no obvious trend between
elevation and Moho depth. The lack of correlation between Moho
depth and elevation in Fig. 6 indicates that variations in crustal
thickness do not have a first-order influence on topography in
East Africa, lending support to models showing mantle structure
and processes provide the primary buoyant support for the
plateau elevations (e.g., Moucha and Forte, 2011).
6. Summary
Fig. 5. Map of the study region showing major geological features, Moho depths,
and Poisson’s ratios (the first and the second numbers next to each station name)
obtained from this study, Last et al. (1997) and Dugda et al. (2005). Solid circles
show Moho depths of 35–39 km, open circles Moho depths from 40 to 44 km and
solid triangles Moho depths of 45 km or deeper. Solid squares show stations for
which no results are reported. Geological features are the same in Fig. 1.
P-wave receiver functions have been modeled using the H–k
stacking method to investigate crustal Poisson’s ratio and Moho
depth for Precambrian terrains in East Africa in areas away from
the Cenozoic East African rift system. The average crustal thickness for all but one of the terrains is between 37 and 39 km. An
exception is the Ubendian terrain, which has an average crustal
thickness of 42 km. In all of the terrains, the average Poisson’s
Table 2
Comparison of crustal structure obtained in this study with global averages.
Source
Country
Terrain
Moho depth range
(km)
Poisson’s ratio range
This paper
Uganda
Uganda–Tanzania
Tanzania
Rwenzori Belt
Kibaran Belt
Ubendian Belt
Usagaran Belt
Mozambique Belt
Tanzania Craton (Nyanzian)
Tanzania Craton (Dodoman)
35–40
33–44
34–49
36–39
36–42
37–41
37–42
0.25–0.27
0.24–0.29
0.25–0.28
0.25–0.28
0.24–0.28
0.25–0.28
0.24–0.26
Dugda et al. (2005)
Christensen and Mooney (1995)
Rudnick and Fountain (1995)
Kenya
Global
Global
Mozambique Belt
Precambrian
Archean Proterozoic
37–42
41.5 7 5.8 (s.d.)
43.0 7 6.3 (s.d.)
43.6 7 4.6 (s.d.)
0.24–0.28
Zandt and Ammon (1995)
Global
Shields Platforms
Durrheim and Mooney (1994)
Global
Archean Proterozoic
0.29 7 0.02 (s.d.)
0.27 7 0.03 (s.d.)
27–40 40–55
80
F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81
anonymous reviewers for helpful reviews. This study was funded
by the National Science Foundation (Grants OISE-0530062, EAR0440032, EAR-0824781).
Appendix A. Supporting information
Supplementary data associated with this article can be found in
the online version at http://dx.doi.org/10.1016/j.epsl.2012.08.041.
References
Fig. 6. A plot of Moho depth versus elevation for stations listed in Table 1.
ratio is either 0.25 or 0.26. The main finding of this study is that
crustal structure is similar across all of the terrains. There is no
discernable difference in the crustal thicknesses or Poisson’s
ratios between the Archean and Proterozoic terrains, or within
the Proterozoic terrains. This finding is in contrast to Precambrian
crustal structure elsewhere in Africa and in other continents,
where, on a terrain-by-terrain basis, discernable variations in
crustal structure can be found.
The finding that on average the Precambrian crust in all of
the terrains within East Africa is similar has important implications for understanding processes of crustal formation and
evolution, as well as the influence crustal structure may have
exerted on the location and development of the East African
rift system. A similar composition and thickness of the crust in
East Africa for terrains spanning some 4 byr of Earth’s history
is different than in many other Precambrian regions. Our
results suggest that there may have been few changes over
Earth’s history in the processes that formed the East African
crust, or else that processes have been at play to homogenize
crustal structure, such as the flow of lower crustal material in
orogenic systems or through the foundering of eclogites into
the mantle.
With regard to the rifting in East Africa, the finding that crustal
structure is similar between the Archean and Proterozoic terrains
indicates that crustal structure, via its influence on lithospheric
rheology and strength, has not had a first-order influence on the
location of rifting, at least not compared to the influence that
variations in Precambrian lithospheric mantle structure have had
on the development of the rift system. However, this conclusion
does not preclude the possibility that within individual terrains
some control on rift development may have come from small
( o5 km) variations in crustal thickness. Finally, the lack of
correlation between Moho depth and elevation across the East
African Plateau indicates that variations in crustal thickness do
not have a first-order influence on elevation in East Africa.
Acknowledgments
Support for field work is gratefully acknowledged from IRISPASSCAL, the Ugandan Geological Survey, the Tanzania Geological
Survey, the University of Dar es Salaam, and from many individuals
at those institutions and at Penn State University. We thank two
Arndt, N.T., 1989. An open boundary between lower continental crust and mantle:
its role in crust formation and crustal recycling. Tectonophysics 161, 201–212.
Austrheim, H., 1991. Eclogite formation and dynamics of crustal roots under
continental collision zones. Terra Nova 3, 492–499.
Begg, G.C., Griffin, W.L., Natapov, L.M., O’Reilly, Suzanne, Y., Grand, S.P., O’Neill, C.J.,
Hronsky, J.M.A., Djomani, Y.P., Swain, C.J., Deen, T., Bowden, P., 2009. The
lithospheric architecture of Africa: Seismic tomography, mantle petrology, and
tectonic evolution. Geosphere 5 (1), 23–50, http://dx.doi.org/10.1130/
GES00179.1.
Bonjer, K.P., Fuchs, K., Wohlenburg, J., 1970. Crustal structure of the East African
rift system from spectral response ratios of long period bodywaves.
Z. Geophys. 36, 287–297.
Bram, K., Schmeling, B.D., 1975. Structure of the crust and upper-mantle beneath
the western rift of East Africa, derived from investigations of near earthquakes.
In: Pilger, A., Rosler, A. (Eds.), Afar Between Continental and Oceanic Rifting.
Schweizerbart, Stuttgart, Germany, pp. 138–142.
Cahen, L., Snelling, N., Delhal, J., Vail, J., 1984. The Geochronology and Evolution of
Africa. Clarendon Press, Oxford, United Kingdom.
Chen, Y., Niu, F., Liu, R., Huang, Z., Tkalicic, H., Sun, L., Chan, W., 2010. Crustal
structure beneath China from receiver function analysis. J. Geophys. Res. 115,
B03307, http://dx.doi.org/10.1029/2009JB006386.
Chevrot, N.I., van der Hilst, R.D., 2000. The Poisson’s ratio of the Australian crust;
geological and geophysical implications. Earth Planet. Sci. Lett. 183, 121–132.
Christensen, N.I., 1996. Poisson’s ratio and crustal seismology. J. Geophys. Res. 101,
3139–3156.
Christensen, N.I., Mooney, W.D., 1995. Seismic velocity structure and composition
of the continental crust: a global view. J. Geophys. Res. 100, 9761–9788.
Costa, S., Rey, P., 1995. Lower crustal rejuvenation and growth during postthickening collapse—insights from a crustal cross-section through a Variscan
metamorphic core complex. Geology 23, 905–908.
De Waele, B., Johnson, S.P., Pisarevsky, S.A., 2008. Palaeoproterozoic to Neoproterozoic growth and evolution of the eastern Congo Craton: its role in the Rodinia
puzzle. Precambrian Res. 160, 127–141.
Dugda, M.T., Nyblade, A.A., Julia, J., Langston, C.A., Ammon, C.J., Simiyu, S., 2005.
Crustal structure in Ethiopia and Kenya from receiver function analysis:
implications for rift development in eastern Africa. J. Geophys. Res. 110,
B01303, http://dx.doi.org/10.1029/2004JB003065.
Dugda, M.T., Nyblade, A.A., Julia, J., 2009. S-wave velocity structure of the crust and
upper mantle beneath Kenya in comparison to Tanzania and Ethiopia:
implication or the formation of the East African and Ethiopian plateaus.
S. Afr. J. Geol. 112, 241–250, http://dx.doi.org/10.2113/gssajg.112.3-4.241.
Durrheim, R.J., Mooney, W.D., 1991. Archean and Proterozoic crustal evolution:
evidence from crustal seismology. Geology 19, 606–609.
Durrheim, R.J., Mooney, W.D., 1994. Evolution of the Precambrian lithosphere:
seismological and geochemical constraints. J. Geophys. Res. 99, 15359–15374.
Ebinger, C.J., 1989. Tectonic development of the western branch of the East Africa
Rift System. Bull. Seismol. Soc. Am. 101, 885–903.
Efron, B., Tibshirani, R., 1991. Statistical data analysis in computer age. Science
253, 390–395.
Fuchs, K., Altherr, B., Muller, B., Prodehl, C., 1997. Structure and dynamic processes
in the lithosphere of the Afro-Arabian rift system. Tectonophysics 278, 1–352.
Griffiths, D., King, R., Khan, M., Blundell, D., 1971. Seismic refraction line in the
Gregory Rift. Nature 229, 69–71.
Goodwin, A.M., 1996. Principles of Precambrian Geology. Academic Press, London
(pp. 327).
Hebert, L., Langston, C.A., 1985. Crustal thickness estimate at AAE (Addis-Ababa,
Ethiopia) and NAI (Nairobi, Kenya) using teleseismic P-wave conversions.
Tectonophysics 111, 299–327.
Hetzel, R., Strecker, M.R., 1994. Late Mozambique Belt structures in western Kenya
and their influence on the evolution of the Cenoizoic Kenya Rift. J. Struct. Geol.
16 (2), 189–201.
Jarchow, C.M., Thompson, G.A., 1989. The nature of the Mohorovicic Discontinuity.
Annu. Rev. Earth Planet. Sci. 17, 475–506.
Julia, J., Ammon, C.J., Nyblade, A.A., 2005. Evidence for mafic lower crust in
Tanzania, East Africa, from joint inversion of receiver functions and Rayleigh
wave dispersion velocities. Geophys. J. Int. 162, 555–569.
Julia, J., 2007. Constraining velocity and density contrasts across the crust mantle
boundary with receiver function amplitudes. Geophys. J. Int. 171, 286–301
http://dx.doi.org/10.1111/j.1365-2966.2007.03502.x.
F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81
Kennett, B.L.N., Salmon, M., Saygin, E., 2012. Ausmoho working group, 2012.
AusMoho: the variation of Moho depth in Australia. Geophys. J. Int. 187,
946–958.
Kgaswane, E.M., Nyblade, A.A., Julia, J., Dirks, P.H.G.M., Durrheim, R.J., Pasyanos,
M.E., 2009. Shear wave velocity structure of the lower crust in Southern Africa:
evidence for composition heterogeneity within the Archean and Proterozoic
terrains. J. Geophys. Res. 114, B12304, http://dx.doi.org/10.1029/
2008JB006217.
Klerkx, J., 1987. Crustal evolution of northern Kibaran Belt, eastern and central
Africa. Geodyn. Ser. 17, 217–233.
Langston, C.A., 1979. Structure under Mount Rainier, Washington, inferred from
teleseismic body waves. J. Geophys. Res. 84, 4749–4762.
Last, R.J., Nyblade, A.A., Langston, C.A., Owens, T.J., 1997. Crustal structure of the
East African Plateau from receiver functions and Rayleigh wave phase
velocities. J. Geophys. Res. 102, 24469–24483.
Lenoir, J.L., Liegeois, J.P., Theunissen, K., Klerkx, J., 1994. The palaeoproterozoic
Ubendian shear belt in Tanzania; geochronology and structure. J. Afr. Earth Sci.
19, 160–184.
Ligorria, J.P., Ammon, C.J., 1999. Iterative deconvolution and receiver function
estimation. Bull. Seismol. Soc. Am. 89, 1395–1400.
Long, R.E., Backhouse, R.W., Maguire, P.K.H., Sundarlingham, K., 1972.
The structure of East Africa using the surface wave dispersion and Durham
seismic array data. Tectonophysics 15, 165–178.
Manya, S.M., Maboko, M.A.H., 2003. Dating basaltic volcanism in the Neoarchean
Sukumaland Greenstone Belt of the Tanzania Craton using the Sm–Nd
method: implications for the geological evolution of the Tanzania Craton.
Precambrain Res. 121, 35–45.
McConnell, R.B., 1972. Geological development of the rift system of eastern Africa.
Geol. Soc. Am. Bull. 83, 2549–2572.
Moucha, R., Forte, A.M., 2011. Changes in African topography driven by mantle
convection. Nat. Geosci. 4, 707–712.
Mueller, S., Bonjer, K.P., 1973. Average structure of the crust and upper mantle in
East Africa. Tectonophysics 20, 238–253.
Nair, S.K., Gao, S.S., Liu, K.H., Silver, P.G., 2006. Sothern Africa crustal evolution and
composition: constraints from receiver function studies. J. Geophys. Res. 111,
B02304, http://dx.doi.org/10.1029/2005JB003802.
Nolet, G., Mueller, S., 1982. A model for the deep structure of the East African rift
system from the simultaneous inversion of teleseismic data. Tectonophysics
84, 151–178.
Nyblade, A.A., Brazier, R.A., 2002. Precambrian lithospheric controls on the
development of the East Africa Rift System. Geology 30 (8), 755–758.
Nyblade, A.A., Langston, C.A., 2002. Broadband seismic experiments probe the East
African rift. EOS Trans. AGU 83, 405–408.
81
Nyblade, A.A., Birt, C., Langston, C.A., Owens, T.J., Last, R., 1996. Seismic experiments reveals rifting of Craton in Tanzania. EOS Trans. AGU 77, 517–521.
Owens, T.J., Zandt, G., 1997. Implications of crustal property variations for models
of the Tibetan Plateau. Nature 387, 37–43.
Prodehl, C., Keller, G.R., Khan, M.A. (Eds.), 1994. Crustal and upper mantle
structure beneath Kenya rift. Tectonophysics 236, 483 (special issue).
Rey, P., Vanderhaeghe, O., Teyssier, C., 2001. Gravitational collapse of the
continental crust: definition, regimes and modes. Tectonophysics 342,
435–449.
Rudnick, R.L., Fountain, D.M., 1995. Nature and composition of the continental
crust the continental crust: a lower crustal perspective. Rev. Geophys. 33,
267–309.
Shackleton, R.M., 1986. Precambrian collision tectonics in Africa, collision tectonics.
Geol. Soc. Spec. Publ. 19, 329–349.
Schulter, T., 1997. Geology of East Africa. Borntraeger, Druckerei zu Altenburg
GmbH, Berlin, Stuttgart, Germany.
Tesha, A.L., Nyblade, A.A., Keller, G.R., Doser, D.I., 1997. Rift localization in suturethickened crust: evidence from Bouguer gravity anomalies in northern
Tanzania, East Africa. Tectonophysics 274 (1–4), 315–328.
Tokam, A.-P.K., Tabod, C.T., Nybalde, A.A., Julia, J., Wiens, D.A., Pasyanos, M.E.,
2010. Structure of the crust beneath Cameroon, West Africa, from joint
inversion of Rayleigh wave group velocities and receiver functions. Geophys.
J. Int. 183, 1061–1076, http://dx.doi.org/10.1111/j.1365-246X.2010.04776.x.
Tokav, A.P., Vavakin, V.V., 1982. Poisson’s ratio behavior in crystalline rocks:
application to study of the Earth’s interior. Phys. Earth Planet. Inter. 29, 24–29.
Thompson, D.A., Bastow, I.D., Helffrich, G., Kendall, J.M., Wookey, J., Snyder, D.B.,
Eaton, D.W., 2010. Precambrian crustal evolution: seismic constraints from the
Canadian Shield. Earth Planet. Sci. Lett. 297, 655–666.
Tugume, F., 2011. The Precambrian Crustal Structure of East Africa. Ph.D. Thesis.
The Pennsylvania State University, pp. 196.
Wolbern, I., Rumpker, G., Schumann, A., Muwanga, A., 2010. Crustal thinning
beneath the Rwenzori region, Albertine rift, Uganda, from receiver function
analysis. Int. J. Earth. Sci. (Geol Rundsch.) 99, 1545-1557 , http://dx.doi.org/
10.1007/s00531-009-0509-2.
Zandt, G., Ammon, C.J., 1995. Continental crust composition constrained by
measurements of crustal Poisson’s ratio. Nature 374, 152–154.
Zandt, G., Myers, S.C., Wallace, T.C., 1995. Crust and mantle structure across the
basin and range—Colorado Plateau boundary at 371N latitude and implications for Cenozoic extensions mechanism. J. Geophys. Res. 100, 10529–10548.
Zandt, G., Gilbert, H., Owens, T.J., Duceau, M., Saleeby, J., Jones, C.H., 2004. Active
foundering of a continental arc root beneath the southern Sierra Nevada in
California. Nature 43, 41–46.
Zhu, L., Kanamori, H., 2000. Moho depth variation in Southern California from
teleseimic receiver functions. J. Geophys. Res. 105, 2969–2980.