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21 Pages·2012·2.05 MB·English

by  Ingo Wolbern

Originally published as:

Wölbern, I., Rümpker, G., Link, K., Sodoudi, F. (2012): Melt infiltration of the lower
lithosphere beneath the Tanzania craton and the Albertine rift inferred from S receiver
functions. - Geochemistry Geophysics Geosystems (G3), 13, Q0AK08

DOI: 10.1029/2012GC004167
Article
Volume13,Number1
4August2012
Q0AK08,doi:10.1029/2012GC004167
ISSN:1525-2027
Melt infiltration of the lower lithosphere beneath the Tanzania
craton and the Albertine rift inferred from S receiver functions
Ingo Wölbern and Georg Rümpker
Instituteof Geosciences, Goethe University Frankfurt, Altenhöferallee 1, DE-60438Frankfurt,
Germany([email protected]; [email protected])
Klemens Link
Instituteof Geosciences, University of Mainz, Becherweg 21,DE-55099 Mainz, Germany
([email protected])
Forough Sodoudi
HelmholtzCentre Potsdam, GFZ GermanResearch Centre for Geosciences, Telegrafenberg,
DE-14473Potsdam, Germany([email protected])
AlsoatFreie Universität Berlin,Institut fürGeologische Wissenschaften, Malteserstr. 74–100,
DE-12249Berlin, Germany
[1] Thetransitionbetweenthelithosphereandtheasthenosphereissubjecttonumerouscontemporarystud-
ies as its nature is still poorly understood. The thickest lithosphere is associated with old cratons and plat-
forms and it has been shown that seismic investigations may fail to image the lithosphere-asthenosphere
boundary in these areas. Instead, several recent studies have proposed a mid-lithospheric discontinuity of
unknown origin existing under several cratons. In this study we investigate the Tanzania craton in East
Africa which is enclosed by the eastern and western branches of the East African Rift System. We present
evidence from S receiver functions for two consecutive discontinuities at depths of 50–100 km and 140–
200 km, which correspond to significant S wave velocity reductions under the Tanzania craton and the
AlbertandEdwardriftsegments.Bycomparisonwithsyntheticwaveformsweshowthatthelowerdiscon-
tinuity coincides with the LAB exhibiting velocity reductions of 6–9%. The shallower interface reveals a
velocitydropthatvariesfrom12%beneaththecratonto24%belowtheAlbert-Edwardrift.Itisinterpreted
as an infiltration front marking the upper boundary of altered lithosphere due to ascending asthenospheric
melts. This is corroborated by computing S velocity variations based on xenolith samples which exhibit a
densesystemofcrystallized veinsactingaspathwaysoftheinfiltrating melt.Mineralassemblagesinthese
veinsarerichinphlogopiteandpyroxenitewhichcanexplainthereducedshearwavevelocities.Meltinfil-
trationrepresentsasuitablemechanismtoformamid-lithosphericdiscontinuitywithincratoniclithosphere
that is underlain by anomalously hot mantle.
Components: 10,700words,11figures,1table.
Keywords: EastAfricanRift;Tanzaniacraton;lithosphere;mantlemetasomatism.
Index Terms: 3621 Mineralogy and Petrology: Mantle processes (1038); 7218 Seismology: Lithosphere (1236); 8103
Tectonophysics:Continentalcratons.
Received27March2012;Revised20June2012;Accepted20June2012;Published4August2012.
©2012.AmericanGeophysicalUnion.AllRightsReserved. 1of20
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Wölbern, I., G. Rümpker, K. Link, and F. Sodoudi (2012), Melt infiltration of the lower lithosphere beneath the Tanzania
craton and the Albertine rift inferred from S receiver functions, Geochem. Geophys. Geosyst., 13, Q0AK08, doi:10.1029/
2012GC004167.
Theme: TheLithosphere-Asthenosphere Boundary
1. Introduction interface is too shallow to be regarded as the LAB
but the petrological origin of this additional dis-
[2] The lithosphere is commonly described as the continuity and its assumed distinctness from the
rigid lid of the earth’s mantle. It is characterized by LABremainuncertain.Severalprocesseshavebeen
highseismicvelocitieswithrespecttotheunderlying hypothesized:Amid-lithosphericinterfacehasbeen
asthenosphere,whichismechanicallymuchweaker. observed by, e.g., Savage and Silver [2008] at
However,neitherthecauseofthicknessvariationsof 150 km depth beneath the northeastern part of the
thisboundarylayernorthenatureofthetransitionto Kalahari craton using S receiver functions that was
theasthenospherearewellunderstood[e.g.,Rychert interpreted to mark a change in composition.
etal.,2010].Geophysicalstudiesshowthatthelith- Accordingly, this boundary may reflect refertilized
ospheric thickness varies from (cid:1)80 km under depletedlithospherebybasalticmeltrichinironand
oceanicregionsto>200kmbeneathcratons,where volatiles possibly related to the eruption of the
the lithosphere is postulated to be particularly cold Karoo flood-basalt and associated magma intru-
and stable [e.g., Lebedev and Van Der Hilst, 2008; sions. A mid-lithospheric velocity reduction was
Rychert and Shearer, 2009; Romanowicz, 2009]. also detected underneath central and western Aus-
In oceanic regions, the lithosphere-asthenosphere tralia at a depth range between 69 km and 85 km
boundary (LAB) appears as a pronounced feature combining results from P and S receiver functions.
at depths of 50–90 km [e.g., Kawakatsu et al., Itwasassumedtorepresenteitherachangeingrain
2009]. Beneath continents it is observed at an size or fabric, vertical variations in mantle compo-
averagedepthofabout110km.TheLABhasbeen sition or low-velocity melt accumulation [Ford
reported as a sharp boundary in areas where the et al., 2010]. Abt et al. [2010] have utilized P and
lithosphere is relatively young [Ford et al., 2010; S receiver functions as well and compared their
Rychert et al., 2005; Lekic et al., 2011] with a results with velocity profiles derived from surface-
sharpness of the velocity reductionthatmaynotbe waveanalysis.Theyreportedasimilardiscontinuity
explainedbyincreasingtemperaturealone.Eithera at depth between 59 km and 113 km beneath the
small amount of partial melt or the presence of cratoniccoreandcertainphanerozoicregionsofthe
volatiles such as water is required [Fischer et al., United States. The authors also proposed alteration
2010; Karato and Jung, 1998]. The underlying of the cratonic lithosphere by melt or the lower
asthenosphere extends to depths of about 210 km boundaryofanancient,highlydepletedlayerofthe
with its bottom defined by the Lehmann disconti- lithosphere to explain the observed interface. A
nuity[e.g.,Thyboand Perchuć,1997]. possible change in anisotropic properties was sug-
gested by Rychert and Shearer [2009] who have
[3] Underthickplatformsandcratonsthelithosphere performed a global receiver function study of the
may reach depths of more than 200 km. These thick
LAB also including velocity profiles derived from
cratonic keels exhibit melt-depleted compositions
surface-waves. S receiver functions have further
[e.g.,MillerandEaton,2010;YuanandRomanowicz,
revealed the existence of a mid-lithospheric dis-
2010] while the asthenosphere is poorly developed
continuitybeneaththeCanadianShield[Millerand
or even absent. Seismic investigations often fail
Eaton, 2010]. From its depth variations a hypo-
toimagethelithosphere-asthenosphereboundaryin thetical phase transition or a specific depth or tem-
such regions [e.g., Rychert and Shearer, 2009; perature has been excluded as the origin of this
Eaton et al., 2009]. Here, its seismic invisibility interface. Instead, the authors suggest that it is
pointstoaslowgradualvelocitydecreaseprobably causedbyremnantslabsaccretedduring theTrans-
due to a purely thermal transition [Abt et al., 2010; Hudson orogeny. In the same region Yuan and
Ford et al., 2010; Savage and Silver, 2008]. Some Romanowicz [2010] have detected changes in azi-
recentreceiver-functionstudieshaverevealedamid- muthal anisotropy coinciding with the common
lithospheric discontinuity with a negative imped- depths of mid-lithospheric interfaces. Here, this
ance contrast existing between 60 and 150 km horizonrevealsstrongtopographywhilethedeeper
under several cratons [Fischer et al., 2010]. This LABisratherflat.Theysuggestthatthelowerlayer
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Figure1. TopographicmapofEastAfrica(DEM:USGSTOPO30).Blacklinesmarksegmentsofthemajorfaults
relatedtotheeasternandwesternbranchesoftheEARSandtheconnectingAswatransformfault(afterMilesietal.
[2004]).TheoutlineoftheTanzaniacratonisgiveninredcoloraccordingtoFoleyetal.[2012].Yellowspotsdepict
volcanic fields alongthenorthern partof thewesternbranch (T:Toro-Ankole, V:Virunga, S-K: South Kivu).
reflects younger, less depleted lithosphere that was Africancontinent(Figure1).Itcomprisesthegener-
probablyaccreted atalater stageofformation. ally north–south trending eastern and western bran-
ches each stretching across distances of >2000 km
[4] In our study we explore the seismic properties [Chorowicz, 2005]. These branches encircle the
and thickness of the lithosphere beneath the Tanza-
Archean Tanzania cratonwhichdefines theVictoria
nia craton and the adjacent western branch of the
micro-plate [Calais et al., 2006]. Disregarding its
EastAfricanRiftSystem(EARS).Ouraimistomap
northwestern edge, where the northern tip of the
the lithosphere-asthenosphere boundary using the S
western branch hits Archaean cratonic lithosphere
receiver function technique and to compare the cra-
[Linketal.,2010],itisjuxtaposedtomobilebeltsof
tonic mantle lid of East Africa with the lithosphere
Proterozoicage[DeWaeleetal.,2008;Nybladeand
affectedbyrifting.Weinvestigatetheoccurrenceof
Brazier,2002].RiftinginEastAfricaprobablyorig-
the mid-lithospheric discontinuity beneath the Tan-
inated in the Afar region when a mantle plume
zaniacratonandpresentsyntheticreceiverfunctions
impinged on the lithosphere about 30 Ma ago [e.g.,
in order to develop a velocity model that is capable
Ebingeretal.,1989;EbingerandSleep,1998].From
of explaining our data. The results are finally com-
the Afar, the rifting process propagated southward
paredtoavelocitymodelcomputedfrombulkmin-
until it reached the Tanzania craton. The opening
eral composition ofthe upper mantleasconstrained
of the western branch may have been triggered by
from >500 xenolith samples. Combining seismo-
stress transmission across the rigid craton [Nyblade
logical and mineralogical information we develop a
and Brazier, 2002]. A recent tomographic study,
modelthatisabletoexplaintheseobservations.
instead, indicates that two distinct mantle plumes
may be located beneath East Africa, one being
2. The Tanzania Craton and the EARS
located beneath the Afar region and a smaller one
beneaththeeasternbranchoftheEARS,eastofLake
[5] The East-African rift system (EARS) is a major Victoria [Montelli et al., 2006]. This observation
tectonic structure traversing the eastern part of the
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agrees with results from a receiver-function study for the northernmost tip of the western branch [Link
that indicates a depression of the 410-km disconti- etal.,2010].
nuity beneath northernTanzania.Suchoffsetproba-
blyresultsfromapositivetemperatureanomalydue 3. S Receiver Functions
toplume materialspreadingout westwardunderthe
cratonic lithosphere [Nyblade et al., 2000]. This
3.1. Data and Processing
interpretationisfurthersupportedbytherathershal-
low cratonic root. In contrast, a recent study of the [7] We apply the S receiver function technique to
study the thickness of the lithosphere. This method
volcanism in the Afar Rift has revealed that plume
utilizes S-to-P converted phases (Sp) that are
influencenowadaysonlyplaysaminorroleandthat
generated at seismic discontinuities such as the
the plume signature has faded in the past [Rychert
etal.,2012]. LAB. S receiver functions have first been intro-
duced as a way to isolate Sp phases by Farra and
[6] Among the African cratons and platforms the Vinnik [2000]. Since then the method has been
Tanzania craton is reported to exhibit the least improved and is nowadays used as a standard tool
developed cratonic keel. Previous estimates of its
especially to map the LAB [e.g., Kumar et al.,
lithospheric thickness have been obtained from
2005; Yuan et al., 2006; Sodoudi et al., 2009].
different studies and vary by some tens of kilo-
When an incident shear wave passes through a
meters: Weeraratne et al. [2003] have derived a
seismic discontinuity part of its energy transforms
thickness of about 170 km from Rayleigh wave
intoacompressionalPwave.ThisS-to-Pconverted
tomography, while Ritsema et al. [1998] and
phase travels at higher velocity than the parent S
Ebinger and Sleep [1998] proposed a minimum wave and, therefore, arrives as a precursor at a
extension of the mantle lid down to 200 km specific station. For this reason, S receiver func-
from P wave tomographic inversion. Fishwick
tions are free of multiple phases in contrast to P
[2010] determined a lithospheric thickness of 150–
receiver functions. The difference in arrival times
160 km from surface wave tomography beneath
serves as a measure for the depth of the converting
Tanzania compared to about 200 km beneath the
interface. P, S and S fractions of the wavefield
V H
Kalahari craton and >225 km beneath the Congo
are separated by rotating the recorded Z, N, E
craton, respectively. Surface waves have also been
components intoalocal ray-coordinatesystem. We
used by Sebai et al. [2006]. According to their first use the theoretical back azimuth for rotation
tomographic study the Tanzania craton extends into the Z, R, T system. In a second step we rotate
down to 180 km depth while the Congo, West-
by incident angle in order to obtain L,Q,T compo-
Africa and Kalahari cratons reach about 300 km.
nents. Incident angles are selected by minimizing
Finally, Begg et al. [2009] found a rather shallow
the S wave energy and simultaneously maximizing
lithosphere-asthenosphere boundary at 150 km
the amplitude of the converted signal on the L
depth beneath Tanzania compared to the other cra-
componentwhichalmostexclusivelycontainscom-
tons throughout Africa. The depth to the LAB has
pressionalenergyafterrotation.Thesourcefunction
also been determined beneath the eastern branch of and path effects are removed from the signal by
theEARS fromtravel-time tomography withrather deconvolving the waveform of the direct S wave
low resolution at a depth between 35 and 65 km
fromtherotatedLcomponent(Pwave)reducingitto
west of Lake Victoria where the lithosphere is
a spike-like impulse. Depending on epicentral dis-
underlain by a pronounced low-velocity layer
tance and source depth S arrivals may overlap with
[Green et al., 1991]. In contrast, a minimum thick-
other strong phases such as SKS, SKKS or ScS. In
ness of 75 km has been derived for the same area
ordertoavoidinterferencethetimewindowusedfor
from basalt chemistry [le Roux et al., 2001]. In a
thedeconvolutionprocessiscustomized.Finally,the
recent study combining receiver functions and time axis is reversed to make the obtained traces
Rayleigh wave group velocities a lithospheric comparabletocustomaryPreceiverfunctions.
thickness of 70–80 km has been found beneath the
Ethiopian Plateau [Dugda et al., 2007]. For the [8] We compute S receiver functions from S and
western branch, so far, no comparable geophysical SKS arrivals that were recorded during a tempo-
study has been carried out. Geochemical data, how- rary seismological experiment carried out between
ever,suggestlateralvariationsofmantlecomposition April 2006 and October 2007. During this period a
and lithospheric thickness [Furman and Graham, seismic network was operated in the Rwenzori
1999;Chakrabartietal.,2009].Petrologicalstudies region, western Uganda, within the scope of the
indicate a thickened pre-rift lithosphere (>100 km) RiftLink research group [Wölbern et al., 2010a].
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Figure 2. Distribution of events used in this study. The circles indicate epicentral distances of 60, 80, 100 and
120degrees.ThegreenandorangedotsmarkepicenterscontributingwithSandSKSphases,respectively.Theblue
asterisk inthecenter ofthemap showsthelocation of ournetwork.
Our temporary network consisted of 23 mobile at depth of 200 km, 140 km and 60 km, respec-
short-period and broadband stations set up directly tively. The vertical resolution is in the order of
east of the Congo border. Additional data from the 8–12 km.
permanent station MBAR have been integrated in
our analysis. The distribution of the stations used [9] To enhance the signal-to-noise ratio for the
converted phases it is necessary to stack data from
in this study is shown in Figures 5g and 5h.
several stations and events. Before stacking a
Inclusion of both, S and SKS arrivals considerably
moveout correction is applied to account for vary-
increases the data coverage due to the different
ingarrival timeswhicharedependent onepicentral
rangesofincidenceangles.Weselectedteleseismic
distance. In the light of the tectonic setting, juxta-
events of magnitude ≥5.7 which occurred at epi-
central distances between 60 and 85(cid:3) and 80–115(cid:3) posing cratonic and rifted lithosphere, strong depth
variations of the discontinuities in the lithospheric
for S and SKS phases, respectively. In total, we
mantle must be considered. Consequently, traces
obtained195receiverfunctions,147fromSphases
arrivingfromdiversebackazimuthsareexpectedto
and 48 from SKS phases, respectively. Figure 2
sampleaspecificinterfaceatdifferentdepthsdueto
displays the distribution of the related epicenters.
its topography. This is expressed in varying arrival
Data were examined with different filter ranges to
times in the receiver functions as illustrated in
better distinguish between coherent arrivals gen-
Figure 3 by means of data recorded at a single sta-
erated at seismic discontinuities and incoherent
tion. If these variations are not considered, the con-
noise effects. Receiver functions used for stacking
vertedsignalwillnotstackcoherentlyandmayeven
(Figures5a–5d)wereanalyzedusingaButterworth
becanceledout.Thiseffecthascommonlybeendis-
low-pass filter with a corner frequency of 0.25 Hz.
regarded in previous receiver function studies as
The lateral resolution of our data can be estimated
topographic variations are usually expected to be
from the l/4-Fresnel-zone of the S-to-P converted
small, however, it cannot be neglected in our study
waves [Brühl et al., 1996]. We determine a diam-
due to the strong topography of the lithosphere
eter of about 116 km at 200 km depth, 97 km at
beneathEastAfrica.Thusitisessentialtoaccurately
140 km depth and roughly 65 km at 60 km depth.
estimate the position of the piercing points for the
Modeling (Figures 6–9) was performed at a lower
discontinuity in question. For this purpose we deter-
corner frequency of 0.14 Hz in order to focus on
minearrivaltimesfromindividualtracesandcalculate
large scale variations. This increases the diameter
thecorrespondingconversiondepth.Thiscalculation
of the Fresnel zone to 155 km, 130 km and 88 km
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Figure3. Dataexamplefromasinglestationillustratingtheeffectofvaryingarrivaltimesduetotopographyofan
interfacesampledatdifferentbackazimuths.(a)Thisfiguredisplayssinglereceiverfunctionsarrivingfromnorthern
to eastern directions while (b) traces in this figure cover western directions. (c) This figure merges all back azimuth
ranges. Stacked traces are shown on top of each section and clearly reveal different arrival times and corresponding
depths. The stacking procedure in our final analysis is therefore based on regional bins, as shown in Figure 5. Data
inthis example arefiltered with a bandpass2 s–20 s.
isbasedonahybridvelocitymodel(Figure4)derived whichissmallcomparedtothewidthoftheFresnel
fromregionalearthquakes[Bram,1975]fortheupper zone and thus negligible. However, an improved
125 km and on the IASP91 model [Kennett and resolution of the lithospheric structure might be
Engdahl, 1991] for depths below 125 km. We use achieved in future times when better velocity
the resulting model to compute approximate coordi- modelsbecomeavailable.Totestfortherobustness
natesofthepiercingpointsforeachconvertedphase. of the observed phases we have applied the boot-
Finally, individual receiver functions are stacked strapmethod[Pressetal.,1992]:arandom20%of
according to regional bins (Figure 5). Binning was the traces in each bin are replaced by other traces
performed separately for different discontinuities to randomly selected from the same bin so that the
accountforthecorrectpositionoftherelatedpiercing originalnumberoftracesisobtained.Inthisway20
points. subsets are randomly created for each bin that are
used to calculate the standard deviation for each
[10] Although the usage of a 1-D velocity model sum trace.2s intervals aredisplayed foreach stack
may seem problematic in the context of the strong
in Figures 5a–5d to illustrate the reliability of the
depth variations the obtained accuracy of the
sum traces obtained from the original data set in
piercing-point distribution is still acceptable: tests
each bin.
withdifferent1-Dvelocitymodelshaveshownthat
the coordinates of the related piercing points devi-
3.2. Observations
atebyonlya fewkilometers for themajority of the
raypaths. Deviations increase with distance from [11] We focus on the lithosphere-asthenosphere
the station network and also with depth, but, even boundarywhichinvolvesdecreasingvelocitieswith
for a conversion depth of 200 km the offsets for depth and should appear as a signal of negative
different velocity models do not exceed 20 km amplitude.ReceiverfunctionsinFigure5showtwo
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[12] In the resulting stacks (Figures 5a and 5c) the
positive signal generated from the Moho disconti-
nuity occurs rather uniformly at about 4 s which
translates to a conversion depth of (cid:1)33 km. In the
bins B-D this phase appears slightly earlier point-
ing to Moho depths <30 km beneath the Albert-
Edward rift graben. This observation agrees well
with depths previously derived from P receiver
functions [Wölbern et al., 2010a]. The negative
phase (labeled “MIF”) reveals strong variations in
delaytime:Theearliestarrivalsarefoundintherift
valley(binsB-D)withdelaytimesaround6s.This
corresponds to 54 km depth according to the ref-
erence model introduced in Figure 4. Beneath the
western rift shoulder (bin A) the discontinuity
deepens rapidly over short distance. Here, the sig-
nal occurs at 9.3 s delay time which translates to a
depthofabout86km.Incontrast,arrivaltimeseast
of the rift valley (bins E-H) range between 6.7 and
7.6 s corresponding to depths of 60–70 km. The
discontinuity further deepens toward the Tanzania
craton, where it exhibits strong variations in depth.
The delay time reaches a maximum of 11.1 s
corresponding to 104 km (bin O). Resulting depths
Figure 4. Velocity model used to determine conver- forallbinsaresummarizedinFigure5e.
sion depths from arrival times in the individual traces.
The upper 125 km are identical to the UVI-N74 model [13] Stacking isrepeatedinatimewindowfrom10
[Bram, 1975]. Below, velocities of IASP91 earth refer- to 25 s to account for the correct piercing points
ence model have been assumed [Kennett and Engdahl, related to the deeper discontinuity (Figures 5b and
1991]. 5d). Resulting depths are illustrated in Figure 5f.
The signal (labeled “LAB”) is less pronounced but
consecutive negative signals. The first occurs at
shows comparable depth variations as the shal-
delay times ranging from 6 to 11 s (Figures 5a and
lower structure.Theinterfaceisshallowestbeneath
5c) roughly corresponding to depths of 50 to
the Rwenzori horst in the Albert-Edward rift seg-
100 km. A second negative phase is found at delay
mentwherearrivaltimesbetween14.6sand16.7s
times between 14 and 22 s (Figures 5b and 5d) are determined (bins A′–C′). This corresponds to
corresponding to depths of 140–200 km. As
depthsroughlybetween138and160km.TheLAB
describedabove,stackingisdoneseparatelyforboth
deepens toward the Tanzania craton and adjacent
discontinuities to account for the correct position of
mobile belts to depths ranging between 180 to
the piercing points. We start our analysis with the 210km(binsD′–H′)calculatedfromdelaytimesof
first 15 s relative to the onset of the parent S wave. 18.5–21.8 s. Near the rift southeast of Lake Kivu
Coordinates of the individual piercing points are (binI′),wealsofindadeepLABatadepthofabout
calculated with respect to the first negative arrival
205 km. It should be mentioned that a weak posi-
and data from all stations are stacked in confined
tive signal occurs at variable delay times between
regionalbins(Figures5eand5g).
Figure5. Stackeddata,depthsvariationsandrelatedpiercingpointsforthetwoconsecutiveuppermantledisconti-
nuities. (a, b) These figures show the stacked receiver functions separated in two different time windows in order to
ensurethecorrectpiercingpointdistributionforbothoftheinterfaces.Datahavebeenfilteredwitha4slowpassBut-
terworth filter. Thin lines denote 2s standard deviation intervals as determined from bootstrapping (see text for
details). (c, d) These figures show the same traces including interpretations of the observed discontinuities (dotted
lines). (e, f) The depth variations of both upper mantle discontinuities are displayed in color-code, respectively. The
dots show the average depth in the related bin. Labels correspond to the stacked traces on top. (g, h) These figures
showthedistributionsofindividualpiercingpoints(crosses)andtheoutlinesofthedefinedbins.Thedotsinlightblue
denote the locations of contributing stations. The thin dotted lines in Figures 5e to 5h indicate the outline of the
Tanzania craton as shown in Figure 1.
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the two negative arrivals. The amplitude is slightly 3.3. Modeling
higherinthebinsbeneaththeriftbutitalsoappears
beneath the craton. This positive signal points to [14] We apply forward modeling of S receiver
functions based on the ray theory code of
increasing velocities with depths and could mark
Frederiksen and Bostock [2000] to constrain the
the bottom of a low velocity layer.
velocity distribution of the lithosphere. Synthetics
Figure 5
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Figure6. Simplereferencemodelsandsyntheticreceiverfunctions(7slowpassfilter)demonstratingtheeffectsof
lithospheric discontinuities. The thick line represents cratonic lithosphere including Moho and LAB. The first pro-
ducesapositivephaseatabout4s.TheLABgeneratesanegativephaseatapproximately20s.Thisphaseisnotpres-
entinthe secondmodel (thinline)which comprises only theMohodiscontinuity.
are computed by iteratively fitting the modeled velocity layer constitutingthe lower partof the lith-
waveform to the observed data utilizing a least osphere (LVL 1) and a further velocity drop at the
squares approach. This procedure has been con- transition to the asthenosphere (LVL 2). While the
ducted successfully in previous studies [e.g., velocity of all layers is kept fixed for each of
Sodoudi et al., 2009]. In order to focus on large- themodels,thethicknesshasbeenvariedinintervals
scale variations of the lithospheric structure we of 10 km (Figure 7). On the basis of the synthetic
performmodelingatalowerfrequencyrangeusing trace that reveals the best match the model has fur-
a7slowpassfilter.Thestartingmodelconsistsofa ther been refined in order to optimize the fit to the
crust with a gradual velocity increase with depth observeddata.
and a simple lithosphere of constant velocity based
on the UVI-N74 model [Bram, 1975]. The crustal [15] In the cratonic lithosphere a velocity reduction
of 11% has been assumed for LVL 1 and the LAB
thickness has been adopted from P receiver func-
is expressed in a reduction of 6%. The synthetic
tions[Wölbernetal.,2010a].Forreferencewefirst
waveforms are compared in Figure 7a (top right).
compute synthetic receiver functions representing
The optimized model (bottom) comprises a thick-
ordinary lithosphere (Figure 6). In this model the
ness of50kmofLVL1with anupperboundary at
Moho is represented by a strong positive signal. A
depth of 78 km. Here, the velocity reduction has
supplementary negative signal originates if a low-
been adjusted to 10% in order to improve the fit to
velocitylayer,suchastheasthenosphere,isincluded.
the observed trace. The LAB is located at 195 km
Toobtaininformationabouttheaverageshearwave
depthwithavelocitydecreaseof5%.Thesynthetic
velocity as a function of depth we stack data from
receiver functions, however, reveal a significant
two different regions: The first only incorporates
positive signal between 10 and 15 s which evolves
traces sampling the assumed LAB in the northern
from the lower boundary of LVL 1. The local
partof the Tanzania craton(corresponding to bin F′
maximum occurs in all synthetics regardless of the
inFigures5fand5h)whilethesecondstackcontains
layer thickness. As this positive signal is observed
only traces sampling the discontinuity beneath the
withmuchloweramplitudeinthedataweconclude
Albert and Edward rift segments (corresponding to
thatagradualtransitionhastobetakenintoaccount
binA′andB′inFigures5fand5h).Incomparisonto
at the bottom of LVL 1. Figure 7b displays the
thereferencemodel,theobservedwaveformsclearly
results for the Albert-Edward rift segment where a
exhibitanadditionalnegativephasethatdemandsfor
velocitydecreaseof19%isassumedforLVL1due
a velocity reduction within the lithosphere. The
to the strong amplitude of the negative signal,
depths of both velocity drops differ significantly
whereas a velocity reduction of 7% has been adop-
betweentheriftandthecraton.Forbothregionswe
tedattheLAB.Velocitieshavenotbeenchangedin
test a series of models incorporating a simple low-
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