-
lithosphere. Using a regression equation obtained from a global
distribution, data from TGA were integrated with those
1. Introduction
The African continent is made up of 4 main
Earth and Planetary Science Lettersobtained by other methods
(gravitytopography coherence and thermo-mechanical analysis)
providing a spatial coverage
sufficient to establish regional Te patterns for South America
and Africa. A comparison and association between the Te
distributions for both continents indicates that for the African
plate, the effective elastic thickness map clearly shows a
remarkable dichotomy of the Neoproterozoic rocks and reworked
older rocks. But for the case of South American plate that
is moving faster than the African plate, lower Te values are
observed only for areas where extensive tectonics with intense
volcanism has acted, suggesting that a colder mantle underlies
this continental plate, while a hotter asthenosphere is
observed
beneath the African plate. This is in part attributed to its
relatively slow motion which prevented dissipating the earlier
developed high temperature.
D 2004 Elsevier B.V. All rights reserved.
Keywords: effective elastic thickness; tidal gravity anomaly;
Gondwana; asthenosphere thermal stateLithosphere mechanical
behavior inferred from tidal gravity
anomalies: a comparison of Africa and South America
Marta S.M. Mantovania,*, Wladimir Shukowskya,
Silvio R.C. de Freitasb, Benjamim B. Brito Nevesc
aIAG-USP, Rua do Matao, 1226-, 05508-090-Sao Paulo SP,
BrazilbUFPr-CPGCG Centro Politecnico, 81531-990-Curitiba PR,
Brazil
cIG-USP, Rua do Lago, 562, 05508-080 - Sao Paulo SP, Brazil
Received 19 May 2004; received in revised form 5 November 2004;
accepted 7 December 2004
Available online 12 January 2005
Editor: S. King
Abstract
Earlier studies have shown that the amplitude difference of the
M2 gravity tidal component (TGA) between the measured
and calculated response for a viscoelastic Earth is
significantly correlated to the effective elastic thickness (Te) of
the0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights
reserved.
doi:10.1016/j.epsl.2004.12.007
* Correspon
30915034.
E-mail address: [email protected] (M.S.M. Mantovani).230 (2005)
397412
www.elsevier.com/locate/epslest Africa, Congo,cratons of
pre-Pan-African ageWding author. Tel.: +55 11 30914755; fax: +55
11Kaapvaal [1] and Tanzania [2]which have remained
stable for long periods of geological time. These
-
anetacratonic blocks are flanked by younger fold belts of
Neoproterozoic (Pan-African Cycle) and of Phaner-
ozoic development. Successive periods of rifting
occurred after these structures have stabilized [3].
This wide range of geological structures is ideal to
study the behavior of the rigidity parameter with
time.
Recent studies on the lithosphere evolution are
focusing their attention on the flexural rigidity
parameter (D), or alternatively on the effective elastic
thickness (Te), as indicative of the mechanism of
isostatic compensation beneath the plate. These
parameters are controlled by geological time scale
relaxation and, therefore, are strongly dependent on
the temperature and deformation rate [4]. This
implies that the lithosphere is stronger for slower
deformation rates and weaker when exposed to
higher temperature or, in other words, Te varies
laterally as a function of the thermal gradient and of
its derivative with time.
The stress pattern of a plate can be regarded as a
bpredictor componentQ of its evolutionary thermalstate, since
brittle failure under tension occurs at lower
stress difference than under compression [5]. The
brittle to ductile transition of rocks is a function of
temperature, and so is dependent on the local geo-
therm [6]. It is also a function of the composition, as
most of the strength of the lithosphere lies in its upper
part, which can be broadly approximated by the
rheological behavior of dry olivine [7]. The occur-
rence of thermal insulation under large continental
masses [8] and the presence of mantle plumes [9]
increase the mantle temperature, leading to a change
in the evolutionary development of the stress pattern.
In time, this will affect the lateral Te distribution of the
plate. Therefore, Te can provide information on the
thermal state and on the mechanical properties of the
lithosphere through time.
The combination of Te estimates derived from tidal
gravity anomalies and from isostatic response model-
ing was successfully applied to construct a continental
scale Te map for the South American Plate [10,11].
This method is here applied to the continental part of
the African plate. The resultant Te patterns for both
plates are then compared, and the similarities related
to the elastic parameter of the main tectonic units of
M.S.M. Mantovani et al. / Earth and Pl398each continent are
analyzed in the light of the Western
Gondwana breakup process.2. Methodology
2.1. Te from isostatic and thermo-mechanical analysis
In the isostatic analysis, Te is determined by
comparing the observed amplitude of the Bouguer
anomaly with its expected value for a locally
compensated topography. Forsyth [12] introduced
the coherence function taking into account the statisti-
cally independent subsurface loading, and making use
of the relationship between the Bouguer gravity and
topography as a function of the wavelength. The wave
number at the transition between coherent and
incoherent topography and gravity is a measure of
flexural rigidity, and hence of lithosphere effective
elastic thickness Te.
Admittance analysis [13], which considers com-
pensation only for surface loading, underestimates Te
when compared to the coherence analysis [12] which
places loads both on the surface and at the Moho.
Thermo-mechanical analysis takes into account addi-
tional parameters such as temperature, pressure,
mineralogy, xenoliths composition, age, and geometry
of geological structures, and may underestimate Te
where intraplate tectonic stresses exist. In addition,
where there is little power in the topography then any
spectral estimation technique will be biased to strong
values [14]. However, a recent study applied to
southern Africa shows that this bias can be taken into
account using a wavelet transform approach to the
analysis of gravity and topography data [15].
Assuming the lithosphere as an isotropic elastic
plate, Te data from coherence analyses are available,
for South America and Africa, over large areas (e.g.,
[1624]) or along extended profiles (e.g., [2530]).
Evaluations of Te that consider the lithosphere as a
broken elastic plate (e.g., [31]), and from thermo-
mechanical analysis (e.g., [32]) are also available.
In general, the resultant topography of an exten-
sional tectonic regime (e.g.: rifted areas, continental
sedimentary basins) and for large cratonic areas
allows one to perform 2D regional gravity surveys,
while in subduction areas and collisional tectonic
zones Te is preferentially obtained along profiles.
The spatial distribution of Te estimates determined
by isostatic response models or by thermo-mechanical
ry Science Letters 230 (2005) 397412analysis is not uniform over
continental areas. This is
so because in many situations it is not possible to
-
The strong linear dependence between Te and TGA
creates an alternative for estimating the effective
elastic thickness where no large gravity surveys exis
or for small tectonic units for which the coherence
method is not applicable, as pointed out in [17]. When
Eq. (1) is used to estimate Te from existing TGA data
the uncertainty of the estimate is given by
rTe 7:39 7:72 TGA
cos2/ 11:22 TGA
2 228:3r2TGAcos4/
s
2The regression model (Eq. (1)) is shown by the solid
line in Fig. 1. The dashed lines delimit the F1standard
deviation interval for predicted Te values
according to Eq. (2).
The regression analysis was based on worldwide
selected data, both from the point of view of tida
anetary Science Letters 230 (2005) 397412 399perform large
gravity surveys or a set of long gravity
profiles such that the study area contains a single
tectonic unit, which is a premise for the method, as
pointed out in [12] and [17]. Alternatively, Te can be
estimated from tidal gravity anomaly (TGA), as
shown in [11].
2.2. Te from tidal gravity anomaly correlation
The periodic gravity variation with tides, driven by
mass redistribution, is a characteristic of the dynamic
response of the Earth to an external stimulation. The
attraction of the Moon and Sun causes most of the
Earths observed external tide potential, a super-
position of various frequency components of the
oscillation modes. These are known as long period,
diurnal, semi-diurnal, and ter-diurnal waves, accord-
ing to their oscillation period. The oscillations occur
in different environments, interacting with the atmos-
phere (atmosphere tides), with the oceans (ocean
tides), and with the different layers of the solid Earth:
lithosphere, mantle, and core (Earth tides). The
oceanic tides exert an additional gravitational effect
on the external solid layer of the Earth causing a
flexure of the lithosphere boundary, and a tilt of the
vertical component of gravity due both to the flexure
and to the modified mass distribution. These effects
are known as ocean loading. Among the various tidal
components, the semi-diurnal lunar wave (M2) can be
determined more precisely than other components
from observational data, and is thus preferred for the
present purpose over other components.
The tidal gravity anomaly (TGA) is a useful
measure of the discrepancy between the observed
and modeled tidal gravity, which carries information
on the Earths internal structure. It is defined as the in-
phase component of the vector difference between the
observed tidal gravity corrected for the ocean loading
and the tidal gravity model for a viscoelastic radially
symmetric Earth structure.
The high linear correlation (r=0.88) between thetidal gravity
anomalies and Te estimates by the
isostatic response method available at 36 locations
worldwide allowed for the establishment of a linear
regression model [11]:
TGA
M.S.M. Mantovani et al. / Earth and PlTe 70:58F2:72 15:11F3:35
cos2/
1quality data and reliability of the corresponding
coherence Te estimates [11].
This new tool is applied to the 50 tidal gravity
stations presently available on the South American
plate, and to 34 available for the African plate. The
corresponding Te estimates are presented in Tables 1
and 2.
Data from isostatic response analysis and from
tidal gravity anomaly are merged to compose a larger
Te data set and to compare their distribution in the two
Fig. 1. Correlation of M2 tidal gravity anomaly and the
lithosphere
effective elastic thickness. Solid line is the regression model
(Eq.(1)). Dotted lines delimit F1 standard deviation for predicted
Tevalues, according to Eq. (2) (after [11]).t
,
,
l
-
Table 2
Te from tidal gravity for South America
Id Lat Lon TGA Te Std
6911 14.73 61.15 0.44 74 66975 10.65 61.40 0.03 66 67118 9.94
84.05 0.20 69 67119 10.71 85.23 1.23 43 77201 10.51 66.93 0.69 78
67202 10.55 71.50 1.59 37 77203 8.81 70.86 1.08 46 77250 4.65 74.10
0.16 63 67251 10.39 75.54 0.89 81 67252 12.62 81.69 0.55 55 67253
7.90 72.49 0.72 78 67254 3.45 76.56 0.13 68 67281 5.17 52.68 0.03
65 67303 23.56 46.73 0.38 57 77305 25.45 49.24 0.32 58 77306 29.67
53.82 1.51 100 87307 20.46 54.62 0.08 67 67308 20.77 42.87 1.18 89
77309 15.61 56.13 0.22 61 67310 16.62 49.26 0.68 78 67311 6.53
37.14 0.37 72 67312 22.12 51.41 0.14 68 67313 5.06 42.77 1.15 45
77314 22.40 43.65 0.53 54 77315 3.17 59.83 0.47 74 67316 1.50 48.50
1.31 88 77317 12.97 38.48 0.56 76 67319 23.10 46.96 0.14 68 67408
16.43 71.57 0.41 73 67409 11.99 76.84 0.21 69 67410 6.76 79.84 0.68
77 67411 3.73 73.24 1.02 83 67412 14.82 74.94 0.47 74 67450 0.19
78.50 0.80 79 67451 2.18 79.87 0.21 69 67500 16.49 68.13 0.53 75
67505 17.77 63.19 0.38 73 67506 17.79 63.16 0.38 73 67601 20.29
70.04 0.90 83 77805 34.57 58.41 1.31 99 87810 37.32 59.08 0.20 71
77812 31.67 63.89 0.04 64 77813 27.47 58.78 0.84 47 77814 31.55
68.68 0.96 89 87815 24.73 65.49 0.34 73 77816 41.12 71.42 0.85 92
87817 54.82 68.33 0.52 38 117818 45.83 67.48 0.62 43 97819 36.40
64.20 0.66 83 87895 31.67 55.93 0.95 88 8Id is the station number;
Lat and Lon are the coordinates of the
station; TGA is the M2 tidal gravity anomaly (micro Gal); Te is
the
Effective Elastic Thickness (km) by Eq. (1); Std is the
standard
deviation (km) by Eq. (2).
3801 26.0 28.0 0.58 78 73118 22.8 5.5 0.61 78 73030 2.6 36.9
0.81 51 63421 1.5 30.2 1.02 47 6
3031 3.3 38.6 0.21 62 63410 4.4 18.6 1.48 91 73399 18.8 7.3 1.07
86 73401 12.7 8.0 0.28 71 63010 15.6 32.5 0.13 68 63415 4.2 15.1
0.63 76 63505 15.1 39.3 1.94 102 83325 12.4 1.5 1.97 102 83807 33.9
18.9 0.50 78 73210 26.3 12.8 1.88 24 93005 24.1 32.6 0.51 55 7
3400 13.5 2.1 1.38 91 73312 12.6 12.2 0.37 72 63000 29.9 31.3
0.18 61 7
3321 6.2 5.0 0.14 68 63040 6.8 39.2 0.73 78 63020 2.0 45.4 1.13
46 7
3090 29.2 13.5 0.00 65 73112 36.8 3.0 0.60 82 83300 18.1 16.0
0.73 80 73311 14.4 17.0 0.56 76 63500 26.0 32.6 1.79 27 93601 18.9
47.6 0.18 62 6Id is the station number; Lat and Lon are the
coordinates of the
station; TGA is the M2 tidal gravity anomaly (micro Gal); Te is
the
Effective Elastic Thickness (km) by Eq. (1); Std is the
standard
deviation (km) by Eq. (2).
continents, positioned for 10 My after break-up, at
~130 Ma (Fig. 2).
Contouring was accomplished using the Kriging
interpolation method [33], applying an exponential
variogram model with range of 500 km based on the
spatial autocorrelation analysis of Te [11]. The
Kriging method by itself provides the uncertainty of
the interpolated Te values, as presented in Fig. 3.
The complementary comparison between the two
continental lithosphere fractions, the South American
and African, is indeed effective to strengthen the
anetary Science Letters 230 (2005) 397412Table 1
Te from tidal gravity for Africa
Id Lat Lon TGA Te Std
3014 9.0 38.7 1.21 87 73451 2.6 29.8 0.58 76 63102 33.5 5.1 0.10
68 73806 32.4 20.8 0.07 64 73420 2.1 28.6 0.59 76 63901 22.3 17.1
1.57 98 83495 17.8 31.1 0.07 64 6
M.S.M. Mantovani et al. / Earth and Pl400
-
Fig. 2. Effective elastic thickness of South America and Africa
obtained by the Krigging interpolation method. Data from isostatic
response
analysis and from tidal gravity anomaly are merged to compose a
larger Te data set and to compare their distribution in the two
continents,
positioned for 10 Ma after break-up. Distribution of data and
estimated uncertainty of the interpolated Te values are shown in
Fig. 3.
M.S.M. Mantovani et al. / Earth and Planetary Science Letters
230 (2005) 397412 401
-
anetaM.S.M. Mantovani et al. / Earth and Pl402above assumptions
and to emphasize the completely
different thermal history of these two segments,
partially, when were still connected (during the
Paleozoic), and even more after the continental drift.
The African portion underwent a more complex
history with more thermal events (slow-moving plate)
compared to the South American plate history.
Clearly, some areas have not remained stable since
break-up, and this reconstruction provides a means of
comparing post 130 Ma history.
3. Tectonic elements of the continental plates
To start any discussion related with the history of
any crustal segment one should begin from the
thermal history of that domain. To understand the
effective elastic thickness of South America and
Africa, one needs to understand the tectonic nature
of the different tectonic domains or units, their related
geophysical spatial pattern, the litho-structural com-
position, and its thermal history.
Fig. 3. Location of Te data and standard dery Science Letters
230 (2005) 397412Discussions of individual studies below intend
to
show the available coherence analysis data, their
relative variation and consistency with each tectonic
province.
4. An outline of African tectonics
As it was done for South America [34], it is
possible to record and emphasize, primarily, the
dichotomy of Syn-Pan-African cratons (stable rem-
nants of Earths early continental lithosphere), and
their circumscribing mobile belts. To Northeast and
North (Mauritanides, Atlas) and SW (Cape Fold Belt)
of this general context, narrow Permo-Triassic mobile
belts are present (Fig. 4).
In fact, a critical analysis suggests that the Congo
craton is the site of the highest effective elastic
thickness (over 60 kmFig. 4) [15]. Furthermore, the
easternmost portion of the Damara Pan-African Belt
[35] and Gariep/Saldania are clearly incorporated to
these maximum domains. Therefore, it is not excluded
viation of the interpolated Te values.
-
anetaM.S.M. Mantovani et al. / Earth and Plthat the small
thickness of the supra-crustal meta-
morphic covers (bschist beltsQ) of these mobile beltsand/or
their allocthonous character (over imposing the
cratonic basement) provided for these high observed
Te signatures.
Another high effective elastic thickness (74 km)
portion is recorded in the southern part of the
West Africa craton, more precisely the LeoMan
Shield (mostly in Senegal) where Archean nuclei
and Paleoproterozoic rocks prevail. Additional little
Fig. 4. Schematic representation of the main tectonicry Science
Letters 230 (2005) 397412 403spots of high Te (to NE and NW)
should
correspond to fractions of the Paleoproterozoic
basement within the Pan-African mobile belts not
reworked (between 600 and 500 Ma), in central
Hoggar and South Nubia (KordofanKhartoum).
The last bluish set of high Te corresponds to the
southernmost portion of the Mozambique belt, the
so-called bMozambique ProvinceQ, a poorly knownarea, where older
(pre-Neoproterozoic) rock units
occur.
units in Africa (modified after [3] and [36]).
-
The other African areas show the regional
distribution of the Pan-African belts (Neoprotero-
zoicCambrian), were superposed by several igneous
events (ring complexes and volcanic centers [36]
from the Upper Phanerozoic to the present. The
intraplate magmatism was less intense in the interior
of the cratonic nuclei and prevailingly present in
many extensional activities of the Pan-African belt
domains [3].
Over 600 ring complexes are known through
Africa and Arabia [36], of several ages, since
Neoproterozoic/Pan-African times (about 250 igneous
zones). During the Paleozoic (approximately 95
igneous zones) and especially during Mesozoic times
(over 230 igneous zones) these intraplate igneous
activities have continued, reaching the Cenozoic. It is
possible to say that over 80% of these activities
occurred within the Pan-African domains, many of
them taking advantage of previous tectonic disconti-
nuities formed during that cycle. The bpurpleQ zones(lower
effective elastic thickness) clearly follow these
Phanerozoic magmatic zones (Fig. 5).
Additionally, continental rifting has been an
important tectonic process since Late Paleozoic times
eir rel
M.S.M. Mantovani et al. / Earth and Planetary Science Letters
230 (2005) 397412404Fig. 5. Distribution of ring complexes
(Proterozoic to Tertiary) and thdomains are distinguished from the
areas with tectono-thermal activities re
are the main igneous complexes (modified after [3639]).ationship
to the Pan-African tectonic structure of Africa. The cratoniclated
to the Pan-African belts (Based on the scheme of Fig. 4). Dots
-
(D=1.410 N m): much of this area is covered withextrusive
volcanics from the rift and volcanic cones
aneta(including Mt. Kenya, Elgon, and Kilimanjaro) which
act as surface loads on the lithosphere. The intrusive
volcanic rock assemblages were classified as high-
density loads (e.g. dykes within the Gregory rift) and
low-density loads (e.g. dykes of the Kavirondo rift)
and the topography of the dome was attributed
primarily to surface volcanic loading, while a low
density (hot region or partial molten mantle) was
associated to deep subsurface loads. Ebinger et al.
[17] analyzed 4 tectonic features or provinces: the
Afar Plateau (Te=2149 km; 3 sub-regions), the East
African Plateau (Te=2764 km; 10 sub-regions),in Africa as a
whole, and it has generated numerous
fault-bounded basins, according to a series of dis-
continuous and short-lived tectonic events. Seven
major events of tectonism have been identified from
the Permian up to Recent [3]. These extensional
events are reflecting (at the Earths surface) the
occurrence of deep-seated heat sources.
In this context it is important to consider the East
African rift system, where the modern (Cenozoic)
extensional structures (and associated volcanism) lie
within the Mozambique Belt (also called EAO=East
African Orogen) [37].
5. Analysis of results
5.1. Extensional tectonic regime structures
As emphasized above, the lower value of effective
elastic thickness (Teb30 km) in Fig. 2 correspond toPan-African
basement zones where Permo-Triassic,
CretaceousEocene, and OligoceneRecent rifts
evolved (ex. East Africa rifts, Niger/Benue Through
Delta, etc.) and Phanerozoic volcanism (ex. Afar,
Cameroon Line, etc.) in different stages. Here, the
comparison of our map in Fig. 2 with Figs. 4 and 5 is
very interesting for the focusing of our discussions.
For the region of the Eastern African System of
Rifts, Te determinations are mainly given by many
authors focusing different objectives [9,15,17,24,
28,3941]. Inversion of the coherence for the Kenyan
rift yielded a best fitting elastic thickness of 25 km23
M.S.M. Mantovani et al. / Earth and Plstable cratons (Te=6480
km; 3 sub-regions), and
magmatic areas (without rifts Te=43 km; 1 sub-region). The
apparent thinning of the elastic plate
beneath the uplifted East African System of Rifts and
Afar Plateau validate the presence of a heat source
beneath the uplifted regions. In this particular area of
overcompensation, a continental breakup is under
progress, with an incipient process of ocean formation
by an extensional regime of over 6000 km in length
[17,28]. This feature is typical of a so-called weak
lithosphere. The extensional process is a result of
convective forces in the underlying asthenosphere
because, due to the low heat conductivity of the
continental block and to its nearly stability (slow
motion), the African Plate favored the accumulation
of heat along time by insulation process inducing
convection currents [8], as well as from deep plumes
as for e.g. those rising from the coremantle boundary
(CMB) beneath southern Africa that may connect to
the hot zones in the upper mantle beneath the EAR
system (e.g. [4246]).
Poudjom Djomani et al. [19] investigated the
relationship between different tectonic structures in
the West African region, as Cretaceous rifts (Benue),
Tertiary domal uplift (Adamawa volcanic uplift),
TertiaryRecent volcanoes (Cameroon Volcanic Line
or CVL), Tertiary sedimentary basins (Chad basins),
and cratonic region (Archean reworked Congolese
craton). The mentioned structures may be seen in
Fig. 4.
By the use of the coherence function analysis,
these authors obtained the minima lithosphere
strength (Te=1420 km) beneath the active rifted
and volcanic areas (Benue, CVL, and Adamawa) and
the maxima (Te ~40 km) corresponding to the
reworked unit in Congo. In the mentioned work of
Poudjom Djomani et al. (1995), only the northernmost
part of the Congo area has been analyzed.
5.2. Sedimentary basins
5.2.1. Africa
Although kinematic models of lithosphere exten-
sion account satisfactorily for the structure and
evolution of many sedimentary basins, there is little
agreement about the main aspects of the dynamical
problem [7]. Large, long-lived, and extensive con-
tinental sedimentary basins are generally associated
ry Science Letters 230 (2005) 397412 405to large parts of
cratons and three of these basins
(Chad, Iullmedden, and Congo) in Africa are
-
anetadescribed by [18]. The three basins contain mega-
sequences initiated in Late Jurassic to Early Creta-
ceous. The Chad basin, in particular, is located above
Early Cretaceous rifts which are connected at depth
to the Atlantic margin via the Benue Trough [3].
Coherence studies indicate that the lithosphere
underlying the Congo basin has a Te value in excess
of 100 km, whereas the Chad and Iullmedden overlie
substantially weaker lithosphere (Te=2030 km). The
contrasting value compared with that given in [19] is
related to the size of the analyzed area and a relative
low topography relief, for which spectral analysis
cannot be applied. A Te=84 km value for the Chad
basin reported for a coherence analysis of super-
imposed adjacent areas is rejected by these authors
due to the inclusion of mirrored wavebands which
may have caused an artificial value. Newman and
White [7] concluded that the lithosphere underlying
the Congo basin is strong, whereas the Chad and
Iullmedden basins as well as the respectively
adjacent Hoggar and Darfur Domes overlie substan-
tially weaker lithosphere, suggesting that this vast
area has been previously weakened [47]. These areas
belong to an ancient basement that was reworked
during the Pan-African orogenic cycle; many rifts
and plutonicvolcanic centers are present in these
areas (Fig. 5).
Analyzing the isostatic anomalies, Newman and
White [7] suggested that uplifts may be related to
convective upwelling in the asthenosphere involving
lateral density contrasts; this implies the existence of
thermal anomalies in the mantle underneath.
5.2.2. South America
The larger intracratonic sedimentary basins of
South America were analyzed by [22], Te=1266
km, and by [23], Te=2458 km. The Parnaiba basin
covers an area of approximately 600000 km2 of the
western part of Northeast Brazil; its maximum thick-
ness comprises about 3500 m of Silurian to Creta-
ceous sediments, intruded by magmatic rocks of
Permo-Triassic to Juro-Cretaceous age. The lower
Te values for the Parnaiba basin correspond to the
smaller areas used for coherence inversion; looking to
the shape of coherence plots, the Te=58+4/6 kmvalue is preferred
[23].
M.S.M. Mantovani et al. / Earth and Pl406The Parana basin,
located in central-south-eastern
South America covers an area of about 1,700,000km2 and is filled
by Ordovician to Cretaceous
sediments and Cretaceous volcanic rocks. Vidotti
[22] calculated the effective elastic thickness of this
lithosphere sector using the coherence analysis
technique [12]. As result of her analysis, [22]
concluded that this area is underlain by a bstrongQ(rather
rigid) lithosphere with bweakerQ (less rigid)areas within. However,
from Figs. 5.3 and 5.6 in
[22], the clear correlation observed between the size
of each analyzed sub-area and Te, supports [17]
argument for the underestimation of Te (when
analyzed areas are too small); therefore her max-
imum Te value obtained for the largest analyzed area
(Te=66+6/4 km) is the best estimate for the Paranabasin. The
evolution of the geometry of the Chaco
foreland basin (Bolivia) using seismic reflection,
gravity, and well log data was examined by [30]; the
best fit between their computational results and
experimental data was obtained for an elastic thick-
ness value ranging from 29 to 31 km.
A large volume of continental flood basalts (CFB)
erupted prior to the Gondwana break-up in the
ParanaChaco basin (308S to 108S). Eruptionsoccurred for about 10
My, with a peak of intense
activity between 133 and 130 Ma. The large amount
of erupted lava is attributed to the presence of a plume
[4850], and therefore associated with a hot astheno-
sphere that partially melted the lithosphere to produce
the CFB. TGA (Te=4768 km) values are in agree-
ment with those calculated by other techniques (such
as the coherence analysis).
The Amazonas basin is an elongated NE
structure that splits the Amazonian craton into two
large pieces. Although this basin was gravity
surveyed for petroleum exploration, its shape favors
a coherence analysis technique only along profiles.
Nunn and Aires [51] modeled 4 profiles crossing
the Medio Amazonas Basin obtaining a Te=1520
km up to a maximum value of 40 km; although
their flexure model considers the basin to be
completely filled by sediments of density 2.55 g/
cm3, gravity records indicate intrusions or partial
replacement of lower crust by mantle material.
Taking into account the inadequate assumption
(which ignores the intruded density material), they
concluded that Te was lower than expected and
ry Science Letters 230 (2005) 397412suggested that more complex
rheological models
should be applied.
-
aneta5.3. Large cratonic areas
5.3.1. Africa
Flexural rigidity in cratonic areas was investigated
by [17,21,24]. To estimate the effective elastic thick-
ness of the continental lithosphere these authors used
the coherence technique [12].
A). For the major tectonic provinces of South
Africa (Kaapvaal, to the south, Limpopo belt at the
center, and Rhodesia/Zimbabwe to the north, which
together form the Kalahari craton, Fig. 4), values of
Te=72 km for the Archean Kaapvaal craton and
Te=38 (East) km to 48 (West) km for the Mesoproter-
ozoic NamaquaNatal mobile belt were obtained by
[21]. Stark et al. [15] obtained similar values of Te
using the wavelet transform mapping method.
Doucore et al. [21] considered each tectonic
province as an independent coherent domain on the
basis of topographic features and isostatic response.
From geological and geophysical considerations, they
suggested that the contrast in flexural rigidity of the
Kaapvaal and Namaqua-Natal provinces can be
attributed to combined effects of compositional and
thickness differences of the lithosphere and to the
present asthenosphere heat flow variation. In [24] a
value of Te=64 km for the Kalahari craton was
obtained: a number between the two independent
domains of [21].
B). For the Congo craton a value of Te=101 km
was calculated [24] in agreement with the conclusions
for the Congo basin [7]. For Tanzania craton which is
underlain by hot asthenosphere [4246,52] Te=64F5km. Unlike the
other cratons, there is considerable
power in topography here.
As early mentioned in [17], it was shown that Te
estimates using the coherence technique must fulfill
the assumptions imposed by this method, one of
which is to analyze a large enough area covering the
structure.
5.3.2. South America
A). For South America, applying the coherence
method to a large regional gravity survey that covers
Uruguay and the southern portion of Rio Grande do
Sul State in Brazil, values for the Rio de La Plata
craton (RLPC) of Te=95 Km are reported [20]. For
M.S.M. Mantovani et al. / Earth and Pllatitudes between 358 and
258 and longitudes(658, 508), the RLPC is clearly depicted by
anintense high (Te=88100 km). RbSr geochronology
from the RLPC of Uruguay was described by [53,54].
Ages measured in granitoids from western Uruguay
(Piedra Alta terrane) range from 1900 to 2200 Ma.
Low Sr initial ratios (N0.7022) are a commoncharacteristic of
these rocks. These results confirm
the Paleoproterozoic ages obtained for different
lithologies [53]. This Early Precambrian age of this
lithospheric sector suggests that it has not been
significantly reworked (during late Proterozoic
cycles), and therefore it is cold and rigid in agreement
with the measured bhighQ TGA.B). A study using the coherence
technique along
profiles [25], evaluated TeN85 km for the westernGuyana shield
and for the southwestern Central
Brazilian shield, that are parts of exposures of the
Amazonian craton. Ussami and Molina [55] obtained
TeN85 km for the eastern margin of the Amazoniancraton using the
model of a lithosphere bbroken plateQand assuming as load for their
model, the Araguaia
belt.
Only two tidal stations are available for the
Amazonian craton (AC), which limits the resolution
of its boundary outline. Estimated values (Te=7488
km) are similar to those of San Francisco craton
(SFC). The oldest ages of the Amazon craton are
reported within the Carajas area, ranging from 3.1 to
2.5 Ga [56]. Its geochronological pattern decreases in
age from NE to SW, and at least five provinces that
behaved as stable platforms at the end of Meso-
Proterozoic are identified [56]. From the two available
tidal stations it is possible to devise the mentioned
NESW trend, although additional TGA stations in
the area are needed to clarify this feature. Due to the
lack of TGA stations, the AC tectonic boundary is not
clearly imaged.
C). Coherence determinations of Te for the San
Francisco craton (SFC) are not available in the
literature, up to now. Age provinces of the SFC
(3.452.0 Ga) are consistent with an Archean and
Early Proterozoic evolution for the continental crust.
The major tectonomagmatic events of SFC occurred
between 2.1 and 2.0 Ga, at the late stage of its
evolution and consolidation. Teixeira [57] identified
some episodes south of SFC that occurred at 2.82.7
Ga ago.
ry Science Letters 230 (2005) 397412 407This is in agreement
with an old and cold
lithosphere, which justifies the observed Te (76 to
-
aneta89 km). Regardless of its older age, relative to the
Rio
de La Plata Craton, the SFC has a slightly lower
rigidity. This could be explained by accepted average
lithosphere thickness: thinner for Archean provinces
compared with Proterozoic provinces [58]. Clearly
associated to SFC, the Congo craton is distinguished
in the west side of the African continent.
5.4. Collisional tectonic structures
5.4.1. Africa
For the Cape Fold Belt, a Permo-Triassic colli-
sional orogen, the southernmost structure of South
Africa, [24] obtained a value of Te=18 km. No Te
records were reported for the Mauritanides structure
of NW Africa.
For the African plate, the effective elastic thickness
map clearly shows the dichotomy of the pre-Pan-
African (older than Late Mesoproterozoic) and the
Pan-African (younger than 900 Ma) regions/mobile
belts (Neoproterozic rocks and older rocks reworked
in that period). But in the case of South America, less
than 5% is in red color while at least 95% are from
green to blue (70NTe/kmN100).
5.5. Subduction tectonic structures
5.5.1. South America
In the South American continent, subduction
structures are associated to the Andean Cordillera,
and Te determinations were obtained along several
profiles. Whitman [26] used the seismically con-
strained shape of the Moho in NW Argentina and
compared it to the gravity data to obtain the flexural
rigidity of the foreland lithosphere (10211022 N m);
he concluded that the corresponding Te (612 km)
was a factor 2 to 4 less than that estimated for the
Bolivian Altiplano [59]. Fan et al. [31] constrained
their study to the Peruvian Andes, and obtained a Te
varying from 25 to 55 km. Their model was based on
the flexural analysis of [59,60]. Watts et al. [27]
presented eight profiles between latitudes 10 and28, in
correspondence to the Nazca Plate subduc-tion. In this segment, Te
contour lines increase from
25 km near the shore to 100 km where the Brazilian
Shield outcrops; between Central Andes and the fold
M.S.M. Mantovani et al. / Earth and Pl408thrust belt, Te varies
from 50 to 75 km. Stewart and
Watts [25] analyzed 58 profiles between latitudes108N and 358S.
They used the bbroken plate modelQ,and divided the area into
northern and southern
Andes. Results are presented along each profile as
well as a Te contour map of western South America:
Te ranges from 25 km near the shore to N85 km in thecentral
Bolivian Range. Along the Andes, the highest
values are located between latitudes 08 and 108.Contours cover
part of the western portion of
continental shields where Te extrapolates 85 km.
Along most of the Andean cordillera (for latitudes
08 to 458), estimates of Te (6989 km) belong to thehigher group.
This is not in agreement with estimates
of radiometric age, but does fit with the lithospheric
thickness. In fact, a high Te, which is associated with
the rigidity parameter D, is consistent with the
existence of deep seismicity.
Hypocenters of deep earthquakes in the South
American Cordillera indicate that the down going slab
may be divided into discrete segments. The segments
beneath northern and southern Peru and beneath
central Chile have shallow dips (about 108; [61].Although
slightly displaced, probably due to gridding
effects, these shallow segments are perceptible in Fig.
2 (a relatively lower Te).
The westward elongation of a high (7593 km),
confirmed by two tidal stations, reaches the southern
Andean Cordillera at latitude 428 to 338. Part ofthis westward
extent covers the Chilenia terrane
described by [62]. In the basement of the Central
Andean Chain, Chilenia, Cuyania, and Pie de Palo are
small Proterozoic exotic terranes that have probably
been sutured to Gondwana during the Paleozoic [63].
The Chilenian basement shows a PbPb age of
1069F36 Ma, and records a complex Precambrianhistory [63]. This
may explain the higher intensity of
the blue color within the Cordillera structure.
A different interpretation is given to the volcanic
province of the Patagonia microplate (Southern
South-America; centered at 708W, 458S). Theobserved geometry for
the Patagonia low (Te=3843
km) coincides with that estimated by seismic aniso-
tropy of surface and body waves [64]. The evolution
of the Andes in this area began in the Middle to
Upper Jurassic with extrusion of voluminous acid
tuffs and lavas [65]. The origin of these volcanic
rocks seems to be related to crustal extension and
ry Science Letters 230 (2005) 397412anatexis predating the
opening of the Atlantic and the
Magellan marginal basin.
-
underlain plate is supported by the seismic records,
which delineate the subduction path, and the
anetavariation in the dip (declivity of the plate) is
reflected in the color nuances (the color intensity
in correspondence of the Nazca plate differs from the
northern and southern segments).
For Patagonia, there is a quite active tectono-
thermal history since the Neoproterozoic III and
especially during all the Paleozoic and Mesozoic
somehow associated to the evolution of SouthernNeogene volcanic
activity, mainly in the western
and central Patagonian plateau, is attributed to the
interaction of upwelling sub-slab asthenosphere flow-
ing around the trailing edge of the descending Nazca
Plate [66]. Compositional distribution of these lavas
and the modeled anomalous mantle potential temper-
ature are explained by these authors as a bweakplumeQ beneath
the slab window [66].
6. Discussions and conclusions
For the African plate, the effective elastic thickness
map clearly shows the dichotomy of the pre-Pan-
African and the Pan-African mobile belts (Neoproter-
ozic rocks and older rocks reworked in that period).
But in the case of South America, less than 5%
corresponds to Teb70, while at least 95% are withinthe interval
70NTe/km N100.
Although Watts and Burov [14] do not relate the
seismogenic Ts layer to Te, they ponder that brittle
and ductile deformation fields are roughly equally
involved in the support of loads; and because in the
continents there may be more than one brittle-ductile
transition (BDT), the elastic portion of the lithosphere
is more complex than for the oceanic plates. Accord-
ing to Kuznir and Karner [67] Te correspond to the
strong portion of the lithosphere which can be thicker
than 100 km for complex continental sectors and up to
50 km for the oceanic lithosphere with more than one
BDT.
The high values of Te observed for the Andean
range can be clearly related to a BDT effect, the
juxtaposition of a continental plate underlain by a
cold and rigid oceanic plate of the subduction
process. In spite of the depth, the rigidity of the
M.S.M. Mantovani et al. / Earth and PlAndes and to the formation
of the Austral Atlantic
Ocean. This intense volcano-plutonic activity corre-sponds to
the lower values (reddish) of the effective
elastic thickness. The low Te value and its closest
relationship with Ts relate this sector to its thermal age
[65].
In correspondence to the Amazonian craton, to
the San Francisco craton (the nucleus and its
extension under the marginal belts) and to the
assemblage of the Luis Alves, Rio de La Plata,
and partially Pampia cratons (since their limits are
not distinguishable in this analysis scale and method-
ology) the highest effective elastic thickness
(between 70 and 100 km) with some local attenu-
ation is observed.
Intermediate values are observed for the Guyana
Shield (north of the Amazon craton), probably due to
the partial connection of this area with the tectono-
thermal activity due to its interaction with the
Caribbean plate.
Comparing the color intensity of the South
America cratonic blocks with those in the African
plate, we may attribute a shallower root to the western
cratons of Gondwana compared to its central and
eastern cratons.
The Mesozoic magmatism is present and exposed
(Takutu graben and associated volcano plutonism),
but apparently is not considerable to explain the Te
observed values (between 60 and 70 km).
The Brasiliano age domains (Neo-Proterozoic), as
Tocantins, Borborema (including the portion covered
by the Parnaiba basin) and Mantiqueira, show
distinctive and coherent values (50 to 70 km; from
yellow to light red). To the south (Dom Feliciano
belt) and to the north (Espirito Santo granulitic belt)
are exceptional among the Brasiliano areas because
their Te values are not low (the observed colors
match those from the adjacent cratons). The explan-
ation of scarce presence of supra-crustals (predom-
inance of Neo-Proterozoic high-grade rocks) is yet
speculative, due to the inappropriate distribution of
measured Te.
It is convenient here to remark that the Phanerozoic
history of the Brasiliano dated domains were sub-
mitted only to a few, shallow and of little significance
events of magmatism. This fact distinguishes the
Brasiliano Domains from the above-discussed Neo-
Proterozoic domains in Africa (Pan-African ages), and
ry Science Letters 230 (2005) 397412 409that were benefited by a
rigorous magmatic and
Phanerozoic extensive history.
-
the Phanerozoic. While the Pan-African domains are
persistently red-colored (Teb60 km and even b20
anetakm), the correlated Brasiliano domains show Te
values of 50 to 70 km. This confirms a history of
plate domains, plate, and asthenospheric conditions
completely different between the two continents.
Since the Paleozoic (when the two continents were
assembled and formed Gondwana), and especially
during the Pangea history (Permo-Triassic) and during
the continental break-up (Upper Triassic to the
present), these differences were already present,
supporting the existence of a contrasting astheno-
sphere beneath the two blocks. In other words, a
colder asthenosphere beneath the bfast-movingQ SouthAmerican
plate compared to the hotter correspondent,
developed under the African plate due to its slow
motion which preserved the earlier developed high
temperature. This is in agreement with Nyblade and
Robinson [69] that attributed the mantle beneath the
African superswell to heat insulation by the super-
continent Pangea in the Late Paleozoic and Early
Mesozoic, providing possible explanation for why
deep mantle beneath the African superswell may have
elevated temperatures.
Acknowledgments
The authors are grateful to Dr. Scott King, Cindy
Ebinger, and another unknown reviewer for com-
ments and corrections that highly improved the
original manuscript. ORB and ICET provided access
to the tidal gravity database. Figures, cited as
extracted from other authors were redrawn by A.
Rugenski. CNPq, FAPESP, and CAPES financially
supported this research, through exchange coopera-The low Te
value observed for the Chaco Plain is
connected to the ParanaChaco sedimentary basin
structure [68] and is interpreted as being the site of the
thermal anomaly that produced the intense volcanism
just prior to the continental split [50].
Thus, the pre-drift between Africa and South
America is in general good (Fig. 2) showing excellent
location for the cratonic domains and the Pan-African/
Brasiliano mobile belts. For the last, it is also
observed the significant difference of behavior during
M.S.M. Mantovani et al. / Earth and Pl410tion among
institutions, travel expenses, and gradu-
ate scholarship.References
[1] D.E. James, M.J. Fouch, S. VanDecar, S. van der Lee,
Kaapvaal seismic group, Tectospheric structure beneath
south-
ern Africa, Geophys. Res. Lett. 28 (2001) 24852488.
[2] C.J. Ebinger, Y. Poudjom Djomani, E. Mbede, A. Foster,
J.B.
Dawson, Rifting Archean lithosphere: the EyasiManyara
Natron rifts, East Africa, J. Geol. Soc. (Lond.) 154 (1997)
947960.
[3] J.J. Lambiase, The framework of African rifting during
the
Phanerozoic, J. Afr. Earth Sci. 8 (2/3/4) (1989) 183190.
[4] G. Ranalli, Rheology of the Earth: Deformation and Flow
Processes in Geophysics and Geodynamics, Allen & Unwin,
London, 1987, p. 365.
[5] M.K. McNutt, Implications of regional gravity state of
stress in
the Earths crust and upper mantle, J. Geophys. Res. 85 (B11)
(1980) 63776396.
[6] C.J. Hawkesworth, S. Kelley, S.P. Turner, A. LeRoex, B.
Storey, Mantle processes during Gondwana break-up and
dispersal, J. Afr. Earth Sci. 28 (1) (1999) 239261.
[7] R. Newman, N. White, Rheology of continental litho-
sphere inferred from sedimentary basins, Nature 385
(1997) 621624.
[8] D.L. Anderson, Hotspots, polar wander, Mesozoic
convection
and the geoid, Nature 297 (1982) 391393.
[9] W.J. Morgan, Hotspot tracks and the opening of the
Atlantic
and Indian Oceans, in: C. Emiliani (Ed.), The Sea, vol. 7,
Wiley, New York, 1981, pp. 443487.
[10] M.S.M. Mantovani, W. Shukowsky, S.R.C. de Freitas,
Tectonic pattern of South America inferred from tidal
gravity anomalies, Phys. Earth Planet. Inter. 114 (1999)
9198.
[11] W. Shukowsky, M.S.M. Mantovani, Spatial variability of
tidal
gravity anomalies and its correlation with the effective
elastic
thickness of the lithosphere, Phys. Earth Planet. Inter. 114
(1999) 8190.
[12] D.W. Forsyth, Subsurface loading and estimates of the
flexural
rigidity of continental lithosphere, J. Geophys. Res. 90
(1985)
1262312632.
[13] M.K. McNutt, Compensation of oceanic topography: an
application of the response function technique to the
surveyor
area, J. Geophys. Res. 84 (1979) 75897598.
[14] A.B. Watts, E.B. Burov, Lithospheric strength and its
relation-
ship to the elastic and seismogenic layer thickness, Earth
Planet. Sci. Lett. 213 (2003) 113131.
[15] C.P. Stark, J. Stewart, C.J. Ebinger, Wavelet transform
mapping o effective elastic thickness and plate loading:
validation, using synthetic data and application to the
study
of Southern African tectonics, J. Geophys. Res. 108 (B12)
(2003) 25582577.
[16] T.D. Bechtel, D.W. Forsyth, V.L. Sharpton, R.A.F.
Grieve,
Variations in effective elastic thickness of the North
American
lithosphere, Nature 343 (1990) 636638.
[17] C.J. Ebinger, T.D. Bechtel, D.W. Forsyth, C.O. Bowin,
Effective elastic plate thickness beneath the East African
ry Science Letters 230 (2005) 397412and Afar plateaus and
dynamic compensation of the uplifts,
J. Geophys. Res. 94 (B3) (1989) 28832901.
-
M.S.M. Mantovani et al. / Earth and Planetary Science Letters
230 (2005) 397412 411[18] R.W. Hartley, P.A. Allen, Interior
cratonic basin of Africa:
relation to continental break-up and role of mantle
convection,
Basin Res. 6 (1994) 95113.
[19] Y.H. Poudjom Djomani, J.M. Nnange, M. Diament, C.J.
Ebinger, J.D. Fairhead, Effective elastic thickness and
crustal thickness variations in west central Africa inferred
from gravity data, J. Geophys. Res. 100 (B11) (1995)
22.04722.070.
[20] M.S.M. Mantovani, W. Shukowsky, S.E. Hallinan, Analise
da
espessura elastica efetiva no segmento litosferico Rio de
LaPlata-Dom Feliciano, An. Acad. Bras. Cienc. 67 (2) (1995)
200220.
[21] C.M. Doucoure, M.J. de Wit, M.F. Mushyandebvu,
Effective
elastic thickness of the continental lithosphere in South
Africa,
J. Geophys. Res. 101 (B5) (1996) 11.29111.303.
[22] R.M. Vidotti, Lithospheric structure beneath the Parana
and
Parnaiba basins, Brazil, from regional gravity analyses,
PhD,
The University of Leeds, School of Earth Sciences, UK (1998)
96 pp.
[23] M.A. de Souza, Regional Gravity Modeling and
Geo-history
of the Parnaba Basin (NE Brazil), PhD thesis, Univ. of
Newcastle upon Thyne, (1996) 126 pp.
[24] R. Hartley, A.B. Watts, J.D. Fairhead, Isotasy of Africa,
Earth
Planet. Sci. Lett. 137 (1996) 118.
[25] J. Stewart, A.B. Watts, Gravity anomalies and spatial
variations of flexural rigidity at mountain ranges, J.
Geophys.
Res. 102 (B3) (1997) 53275352.
[26] D. Whitman, Moho geometry beneath the eastern margin of
the Andes, northwest Argentina and its implications to the
effective elastic thickness of the Andean foreland, J.
Geophys.
Res. 99 (B8) (1994) 15.27715.287.
[27] A.B. Watts, S.H. Lamb, J.D. Fairhead, J.F. Dewey,
Litho-
spheric flexure and bending of the Central Andes, Earth
Planet. Sci. Lett. 134 (1995) 921.
[28] C.J. Ebinger, N.J. Hayward, Soft plates and hot spots:
views
from Afar, J. Geophys. Res. 101 (B10) (1996) 21.85921.876.
[29] N.M. Upcott, R.K. Mukasa, C.J. Ebinger, Along-axis
segmen-
tation and isostasy in the Western rift, East Africa, J.
Geophys.
Res. 101 (B2) (1996) 32473268.
[30] L. Coudert, M. Frappa, C. Viguier, P. Arias, Tectonic
subsidence and crustal flexure in the Neogene Chaco basin
of Bolivia, Tectonophysics 243 (1995) 277292.
[31] G. Fan, T.C. Wallace, S.L. Beck, C.G. Chase, Gravity
anomaly
and flexural model: constraints on the structure beneath the
Peruvian Andes, Tectonophysics 255 (1996) 99109.
[32] L.L. Lavier, M.S. Steckler, The effect of sedimentary cover
on
the flexural strength of the continental lithosphere, Nature
476
(1997) 476479.
[33] G. Davis, Statistics and Data Analysis in Geology, 2nd
edition,
John Wiley & Sons, NY, 1986, p. 646.
[34] M.S.M. Mantovani, W. Shukowsky, S.R.C. de Freitas,
Tidal
gravity anomalies as a tool o measure rheological properties
of
the continental lithosphere: application to the South
American
Plate, J. South Am. Earth Sci. 14 (2001) 114.
[35] C. Hartnady, P. Joubert, C. Stowe, Proterozoic crustal
evolution in Southwestern Africa, Episodes 8 (4)
(1985)236240.[36] J.R. Vail, Ring complexes and related rocks in
Africa, J. Afr.
Earth Sci. 8 (1) (1989) 1940.
[37] A.M. Goodwin, Precambrian Geology. The Dynamic Evolu-
tion of the Continental Crust, Academic Press, London,
1991, p. 666.
[38] P.G. Eriksson, The 2.72.0 Ga volcano-sedimentary record
of
Africa, India and Australia: evidence for global and local
changes in sea level and continental freeboard, Precambrian
Res. 97 (1999) 269302.
[39] C. Petit, C.J. Ebinger, Flexure and mechanical behavior
of
cratonic lithosphere: gravity models of East African and
Baikal rifts, J. Geophys. Res. 105 (B8) (2000) 19.15119.162.
[40] C.J. Ebinger, G.D. Karner, J.K. Weissel, Mechanical
strength of extended continental lithosphere: constraints
from the western rift system, East Africa, Tectonics 10
(1991) 12391256.
[41] N.M. Upcott, Structural segmentation of continental rifts
as
seen from ship and land gravity data: examples from East
Africa, PhD thesis, (1994) 248 pp., University of Leeds,
England.
[42] J. Ritsema, H.J. vanHeijst, J.H. Woodhouse, Global
transition
zone tomography, J. Geophys. Res. 109 (B2) (2004)
101029101047.
[43] J. Ritsema, H.J. vanHeijst, New seismic model of the
upper
mantle beneath Africa, Geology 25 (1) (2000) 6366.
[44] R. Montelli, G. Nolet, F.A. Dahlen, G. Masters, E.R.
Engdahl,
S.H. Hung, Finite frequency tomography reveals a variety of
plumes in the mantle, Science 303 (2004) 338343.
[45] E. Debayle, B.L.N. Kenneth, Anisotropy in the
Australasian
upper mantle from Love and Rayleigh waveform inversion,
Earth Planet. Sci. Lett. 184 (2000) 339351.
[46] J. Ritsema, J. van Heijst, J.H. Woodhouse, Complex
shear
wave velocity structure imaged beneath Africa and Iceland,
Science 286 (1999) 19251928.
[47] A.E. Ibrahim, C.J. Ebinger, J.D. Fairhead, Lithospheric
extension northwest of the central African shear zone in
Sudan from potential field studies, Tectonophysics 255
(1996)
7997.
[48] C.J. Hawkesworth, M.S.M. Mantovani, P.N. Taylor, Z.
Palacz,
Coupled crustmantle systems: evidence from the Parana of
South Brazil, Nature 322 (1986) 356359.
[49] D.W. Peate, M.S.M. Mantovani, C.J. Hawkesworth, Geo-
chemical stratigraphy of the Parana CFB: borehole evidence,
Rev. Bras. Geocienc. 18 (2) (1988) 212221.
[50] S. Turner, M. Regelous, S. Kelley, C.J. Hawkesworth,
M.S.M.
Mantovani, Magmatism and continental break-up in the South
Atlantic: high precision 40Ar39Ar geochronology, Earth
Planet. Sci. Lett. 1221 (1994) 333348.
[51] A.J. Nunn, J.R. Aires, Gravity anomalies an lexure of
the
lithosphere at the Middle Amazon Basin, Brazil, J. Geophys.
Res. 93 (B1) (1988) 415428.
[52] D.S. Weeraratne, D.W. Forsyth, K.M. Fisher, Evidence for
an
upper mantle plume beneath the Tanzanian craton from
Rayleigh wave tomography, J. Geophys. Res. 108 (B9)
(2003) 24272446.
[53] C. Cingolani, R. Varela, L. Dalla Salda, J. Bossi, N.
Campal,L. Ferrando, D. Pineyro, A. Schipilov, RbSr
geochronology
-
from the Rio de La Plata Craton of Uruguay, South American
Symposium on Isotope Geolog, June 1518, Campos do
Jordao, SP, 1997, pp. 775 (Extended abstract).
[54] L.A. Hartmann, J.A.D. Leite, N.J. McNaughton, J.O.S.
Santos,
Deepest exposed crust of BrazilSHRIMP establishes three
events, Geology 27 (1999) 947950.
[55] N. Ussami, E.C. Molina, Flexural modeling of the Neo-
proterozoic Araguaia belt, Central Brazil, J. South Am.
Earth
Sci. 12 (1999) 8798.
[56] C.C.G. Tassinari, K.M. Mellito, L.V. Rodrigues, The
geo-
chronological map of the Amazonian craton in Brazil, South
American Symposium on Isotope Geolog, June 1518,
Campos do Jordao, SP, 1997, pp. 326329 (Extended
Abstracts).
[57] W. Teixeira, Evolucao tectonotermal proterozoica do
craton
de Sao Francisco, com base em interpretacoes geocronolog-
icas, KAr, Simposio sobre o Craton de Sao Francisco
Evolucao Tectonica e Metalogenetica, Salvador, BA, vol. 2,
1993, pp. 1820.
[58] K.D. Nelson, A unified view of craton evolution motivated
by
recent deep seismic reflection and refraction result,
Geophys.
J. Int. 105 (1991) 2535.
[62] V.A. Ramos, G.I. Vujovich, Alternativas de la evolucion
del
borde occidental de Ameica del Sur durante el Proterozoico,
Rev. Bras. Geocienc. 23 (3) (1993) 194200.
[63] V.A. Ramos, M.A.S. Basei, Gondwana, Perigondwanan, and
exotic terranes, South American Symposium on Isotope
Geology, 1518 June, 1997, pp. 250252.
[64] V. Babuska, J. Plomerova, Seismic anisotropy and large
scale
fabric of the continental mantle lithosphere, IUGG99,
Birmingham, 2630 July 1999 (Abstracts B144, JSS44/
13-JSS44/B4).
[65] F. Herve, E. Godou, M. Parada, V. Ramos, C. Rapela, C.
Mpdozis, J. Davidson, A general review on the Chilean
Argentine Andes, with emphasis on their early history, in:
J.W.H. Monger, J. Francheteau (Eds.), Circum-pacific Oro-
genic Belts and Evolution of the Pacific Ocean Basin,
Gedynamic Series, vol. 18, 1987, pp. 97113 ILP contribution
n. 0132.
[66] C. Lomnitz, A statistical argument for the existence of
a
discontinuity in some subduction zones, J. Geophys. Res. 78
(1973) 25152612.
[67] N. Kuznir, G. Karner, Dependence on flexural rigidity
of
continental lithosphere on rheology and temperature, Naure
316 (1985) 138142.
[68] P.V. Zalan, S. Wolff, M.A.M. Astolfi, I.S. Vieira,
J.C.J.
Conceicao, V.T. Appi, E.V.S. Neto, J.R. Cerqueira, A.
M.S.M. Mantovani et al. / Earth and Planetary Science Letters
230 (2005) 397412412flexure of the Brazilian Shield beneath the
Bolivian Andes,
Earth Planet. Sci. Lett. 75 (1985) 8192.
[60] H. Lyon-Cahen, P. Molnar, Constraints on the structure of
the
Himalaya from an analysis of gravity anomalies and a
flexural model of the lithosphere, J. Geophys. Res. 88
(1983)
81718191.
[61] M. Barazangi, B.L. Isaacks, Spatial distribution of
earthquakes
and subduction of he Nazca Plate beneath South America,
Geology 4 (1976) 686692.Marques, The Parana basin, Brazil, in:
M.W. Leighton, D.R.
Kolata, D.F. Olts, J.J. Eidel (Eds.), Interior Cratonic
Basins,
AAPG Mem., vol. 51, 1990, pp. 681708.
[69] A.A. Nyblade, S.W. Robinson, The African superswell,
Geophys. Res. Lett. 21 (9) (1994) 765768.[59] H. Lyon-Cahen, P.
Molnar, G. Suarez, Gravity anomaly and
Lithosphere mechanical behavior inferred from tidal gravity
anomalies: a comparison of Africa and South
AmericaIntroductionMethodologyTe from isostatic and
thermo-mechanical analysisTe from tidal gravity anomaly
correlation
Tectonic elements of the continental platesAn outline of African
tectonicsAnalysis of resultsExtensional tectonic regime
structuresSedimentary basinsAfricaSouth America
Large cratonic areasAfricaSouth America
Collisional tectonic structuresAfrica
Subduction tectonic structuresSouth America
Discussions and conclusionsAcknowledgmentsReferences