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Variations in elastic thickness and flexure ofthe Maracaibo block
ARTICLE in JOURNAL OF SOUTH AMERICAN EARTH SCIENCES SEPTEMBER 2014
Impact Factor: 1.37 DOI: 10.1016/j.jsames.2014.09.014
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2 AUTHORS:
Mariano S. Arnaiz-Rodrguez
Central University of Venezuela
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Franck A. Audemard
Fundacin Venezolana de Investigaciones
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Available from: Mariano S. Arnaiz-Rodrguez
Retrieved on: 29 September 2015
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Variations in elastic thickness and exure of the Maracaibo block
Mariano S. Arnaiz-Rodrguez a, *, Franck Audemard b ,c
a Departamento de Geofsica, Escuela de Geologa, Minas y Geofsica, Facultad de Ingeniera, Universidad Central de Venezuela, Venezuelab Departamento de Ciencas de la Tierra, Fundacion de Investigaciones Simologicas (FUNVISIS), Caracas, Venezuelac Departamento de Geologa, Escuela de Geologa, Minas y Geofsica, Facultad de Ingeniera, Universidad Central de Venezuela, Venezuela
a r t i c l e i n f o
Article history:
Received 14 May 2014
Accepted 8 September 2014
Available online 30 September 2014
Keywords:
Maracaibo block
Lithosphereexure
Finite differences
Gravity
a b s t r a c t
We estimate the lateral variations of the elastic thickness of the Maracaibo block with a 3D numerical
approach by using centered nite differences. The calculation is based on solving the fourth-order partial
differential equation that governs the bending of a thin plate xed on its boundaries (zero displacement)
with variable thickness (or elastic thickness for this particular case). An initial plate-load model is built
and is iteratively modied to t the general basement conguration and gravity data. The nal result is
an elastic thickness map that covers the Maracaibo block and the surrounding sections of the South
American plate. It shows that the elastic thickness ranges from 30 km to 18 km with a mean value of
23.6 km and a mode of 26 km. The largest elastic thickness values are associated with the location of the
Santa Marta Mountains and the Barinas Apure Basin, while the smallest ones with the M erida Andes-
Maracaibo Basin exural system. The current basement conguration within the Maracaibo basin,
formed as a result of its geodynamic evolution, has affected the mechanical properties of the Maracaibo
block near the current Merida Andes position. The load of the Perija Range is compensated by a complex
stress tensor, and that of the Santa Marta Mountains does not have an isostatic root as it is held by a
relatively strong lithosphere.
2014 Elsevier Ltd. All rights reserved.
1. Introduction
A sedimentary basin is a depressed region in the Earth surface
that has been lled by sediments (Turcotte and Schubert, 2002). Aexural basin or foreland basin is a sedimentary one formed in
response to subsidence driven by vertical stress over the elastic
lithosphere (e.g.DeCelles and Giles, 1996; Watts, 2001). These ba-
sins are characterized by: (1) a thrust front of an adjacent orogen
(or load), which is responsible for the vertical stress that bends the
plate; (2) a sediment ll with a wedge shape in transverse section;
(3) the depocenter located contiguous to the thrust belt that gen-
erates the depression (e.g.Jordan,1995); and (4) aexural bulge, orforebulge, that marks the end of the basin and separates it from the
undeformed craton or plate (e.g. Karner and Watts, 1983). Typical
foreland basins are divided in four discrete sections: (a) the wedge-
top depozone that buries the active thrust front; (b) the foredeep
depozone formedby the subsidence driven by the load of the thrust
belt; (c) the forebulge, a region ofexural uplift which is the result
of a damped sinusoidal deformation; and (d) the backbulge depo-
zone, a broad region of shallow secondary exural subsidence
(DeCelles, 2012).
In Venezuela there are three foreland basins: the Eastern
Venezuela basin, the Barinas-Apure basin and the Maracaibo basin.
The two last basins are part of a double exural system driven by
the Merida Andes load. The Maracaibo basin (Fig. 1) is located
within an independent piece of crust known as the Maracaibo
block, while the Barinas-Apure Basin is situated in the South
American plate. These basins have been largely studied because of
their natural resources. Nonetheless little is known about the
behavior of the lithosphere in this region.Audemard and Audemard (2002) pointed out that both, the
Maracaibo basin and the Barinas-Apure basin, have different me-
chanical behavior, as the depocenter in the Maracaibo basin
dwhich have a major inuence over the study aread is at least
2 km deeper that in the Barinas-Apure basin.Chacn et al. (2005)
calculated the elastic thickness for the Barinas-Apure basin in
25 km, whereasMedina (2009)afrmed that the effective elastic
thickness variations within it ranged from 30 km near the craton
(Guayana Shield) to 10 km near the Merida Andes. Arnaiz-
Rodrguez et al. (2011) considered that the elastic thickness for
the Barinas-Apure basin was around 24 2 km, and estimated that,
* Corresponding author.
E-mail addresses: [email protected], [email protected]
(M.S. Arnaiz-Rodrguez).
Contents lists available atScienceDirect
Journal of South American Earth Sciences
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . co m / l o c a t e / j s a m e s
http://dx.doi.org/10.1016/j.jsames.2014.09.014
0895-9811/
2014 Elsevier Ltd. All rights reserved.
Journal of South American Earth Sciences 56 (2014) 251e264
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for the Maracaibo basin, it was around 16 2 km. Further,Arnaiz-
Rodrguez et al. (2011)pointed out that the formation of the Mar-
acaibo basin and the exural system in the Maracaibo block are not
only controlled by the Merida Andes, but also by the Perija Range
and the Santa Marta Mountains, recommending that a 3D approach
to study the exure of this microplate was necessary.Seismic anisotropy analysis suggested that SKKS split orienta-
tions at ~N45E was most likely to be caused by lithospheric
deformation parallel to the Bocono fault (Masy et al. (2011). Curie
Point Depth analysis reportedthat Maracaibo block was a thermally
stable continental basin, and that a Curie Point Depth anomaly was
due to the exure produced by the Merida Andes, or to the graben
systems located within the Barinas-Apure basin (Arnaiz-Rodriguez
and Orihuela, 2013). The studies previously referred assert that the
Maracaibo block and the South American lithospheres behave
differently.
The present research is part of an ongoing multidisciplinary
effort to understand the dynamics of the Merida Andes, its adjacent
basins, structures and terranes using different approaches: GIAME
project (Geociencia Integral de los Andes de Merida-Integral Geo-
science of the Merida Andes;Schmitz et al., 2013). It focuses upon
the Maracaibo block with the purpose of estimating the lateral
variations of elastic thickness of the block and its adjacent regions
by using a 3D numerical method.
2. Tectonic setting
The Maracaibo block (Fig. 1a), an independent piece of conti-
nental crust localized in northwestern Venezuela, is limited by
three fault systems (e.g., Mann and Burke, 1984; Taboada et al.,
2000; Audemard et al., 2005): Bocono and Oca-Ancon fault sys-
tems, both with dextral strike slip; and Santa Marta-Bucaramanga
with sinistral strike slip. Its formation and expulsion (in NNE di-
rection relative to South America) are related to the compression
generated by the subduction of the Carnegie Ridge and the collision
of the Panama Arc against northern South America (e.g.
Pennington, 1981; Audemard, 1993; Kellogg and Vega, 1995). The
Maracaibo block and the Bonaire block have been overriding the
Caribbean plate, creating an ESE-dipping, amagmatic at oceanic
subduction (e.g.,Kellogg and Bonini, 1982; Freymueller et al., 1993;Van der Hilst and Mann, 1994; Kellogg and Vega, 1995; Kaniuth
et al., 1999; Taboada et al., 2000; Audemard and Audemard,
2002; Mann et al., 2006; Bezada et al., 2010 ). It is worth noting
that the northern Andes block, of which the Maracaibo block is a
piece, is bound to the N and NW by a complex deformation belt,
where the Caribbean and South American plates meet (Taboada
et al., 2000). Given the geometry of the northwestern corner of
South America, the Caribbean plate has been subducting in two
stages: an older one, found in the NW and W, began in the Eocene-
Oligocene (~50 Ma ago; e.g., Kellogg and Bonini, 1982; Kellogg,
1984; Pindell and Kennan, 2009); a younger one, found in the N,
began in the Pliocene (~5 Ma ago) from the Southern Caribbean
Deformation Belt under the Bonaire block and the Maracaibo block
(Audemard, 1991; Taboada et al., 2000; Audermard and Audemard,2002; Duerto et al., 2006; Bezada et al., 2010). Several authors have
described the older stage inuence in the Maracaibo Block geo-
dynamic evolution (e.g., Kellogg and Bonini, 1982). However, the
stage that we refer in the text is the younger one, as is the closest to
the Maracaibo Block.
Within the Maracaibo block there are three important mountain
chains: the Merida Andes, the Perija Range and the Santa Marta
Mountains. The Merida Andes is an over 400 km long and 40 km
wide mountain range with a maximum elevation of 5 km, which
has no direct genetic relationship with the rest of the Andean
Range.Colletta et al. (1997)described the internal structure of the
Merida Andes as a compressional positiveower structure that has
been assumed to be either symmetrical (e.g. Gonzalez de Juana,
1952) or asymmetrical (e.g. Audemard, 1991; Audemard y
Fig.1. (a) Shaded relief topography of the study area showing the major tectonic features in northwestern Venezuela. The red box represents the modeled study area. Abbreviations
stand for: MA, Merida Andes; PR, Perija Range; SMM, Santa Marta Mountain Range; NCA, Northern Colombia Andes; MBa, Maracaibo Basin; BABa, Barinas-Apure Basin, BF, Bocono
fault; IF, Icotea fault; SMF, Santa Marta fault; O-AF, Oca-Ancon fault. Quaternary faults fromAudemard et al. (2000)(b) Structural map of the Maracaibo block. Major structures are
the same as inFig. 1a; thin gray lines represent minor faults (French and Schenk, 2004). Red dashed lines represent sediment thickness to the top of the basement in km ( Di Croce,
1995; Parnaud et al.,1995; Laske and Masters, 1997; Ceron et al., 2007). Blue shapes denote half-graben and basement troughs while orange shapes denote basement uplifts (Erlich
et al., 1999). (For interpretation of the references to colour in this gure legend, the reader is referred to the web version of this article).
M.S. Arnaiz-Rodrguez, F. Audemard / Journal of South American Earth Sciences 56 (2014) 251 e264252
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Audemard, 2002).TheMerida Andes has also been compared to the
Laramide-Rocky Mountains; i.e. a compressive basement block
uplift overthrusting toward the adjacent basins along blind thrust
faults (Kellogg and Bonini, 1982; De Toni and Kellogg, 1993;
Audemard, 1991). The Perija Range sits between Venezuela and
Colombia with a maximum elevation of 3.6 km. This mountain belt
is characterized as an ESE dipping monocline, resulting from the
reactivation of Jurassic faults during the Cenozoic (Garrity et al.,
2004; Duerto et al., 2006). The Santa Marta Mountains is a trian-
gular shaped mountain ranged located in northern Colombia,
covering an approximate area of 3830 km2 with maximum altitude
of 5.7 km. It is usually described as an isolated uplifted massif block
of Precambrian to Mesozoic rocks that were uplifted in three pulses
from the late Maastrichtian to the Late Miocene (Cardona et al.,
2008; Ceron-Abril, 2008). These three mountains play a major
role in the exure of the lithosphere of the South American Plate
and the Maracaibo Block. Arnaiz-Rodrguez et al. (2011) proposed
that the Merida Andes load did not uniquely control the exure of
the lithosphere in the Maracaibo block, and that the Perij a Range,
the Santa Marta Mountains and the Caribbean at slab impact on
the dynamic equilibrium in the region, as well as in the basement
morphology found today.
Adjacent to the northern foothills of the Merida Andes, lays theMaracaibo basin, a foreland basin resulting from the loading of the
Merida Andes and Perija Range (e.g., Audemard and Audemard,
2002; Audemard, 2003). The Maracaibo basin is a small basin
with a deep asymmetric depocentre (Fig. 1b) and the apparent
absence ofexural bulge (Mann et al., 2006). The great depth of this
basin (at least 9 km), in comparison with the Barinas-Apure basin
(at least 4.5 km) shows either that the Maracaibo block has a
different elastic thickness than the rest of the South American plate
(Audemard and Audemard, 2002; Arnaiz-Rodrguez et al., 2011) or
that the basement thrusting in the Merida Andes was asymmetric
(De Toni and Kellogg, 1993).
3. Gravimetric and isostatic setting
Gravimetric studies of the Maracaibo basin and the Merida
Andes have been carried out since the 70s, and most of them have
come to similar results in terms of the gravity and the isostasy in
the region (e.g.,Folinsbeei, 1972; Kellogg and Bonini, 1982; Escobar
and Rodrguez, 1995; Chacn et al., 2005; Arnaiz-Rodrguez et al.,
2011). In this section, we present gravimetric maps of the Mar-
acaibo block with a brief discussion on the signicant anomalies to
illustrate its isostatic state. The free air anomaly (Sandwell and
Smith, 2009) and the total Bouguer anomaly map (Arnaiz-
Rodrguez and Garzon, 2012) of the region are shown inFig. 2.
The free air anomaly of the Maracaibo block ranges from
593 mGal to 149 mGal, with mean values of 16.3 mGal (Fig. 2a).
Positive values are associated with the topography of the mountain
ranges in the area, while negative values are associated with the
adjacent foreland basins. Differences in the negative free air
anomaly values at the northern and southern foothills of the
Merida Andes show the discrepancy between the depocenter
depths of the Maracaibo basin (9 km) and the Barinas-Apure basin
(4.5 km). This difference is associated with the lateral variations of
mechanical properties between the Maracaibo block and South
America and to the asymmetric distribution of the masses (loads)
within the Merida Andes structure (Audemard and Audemard,
2002; Arnaiz-Rodrguez et al., 2011). Positive free air anomaly
values show complex distribution of loads in the area. Four
mountain ranges load the lithosphere: the Merida Andes, PerijaRange, Santa Marta Mountains and the Northern Colombian Andes.
The Merida Andes seems to be the most signicant of these, as the
depocenter for the Maracaibo basin is immediately adjacent to
them; however, the Perija Range and the Northern Colombian
Andes clearly bound the basin on its western side. The deepest
section of the Barinas-Apure basin is linked to the joint contribu-
tion of the Northern Colombian Andes and the Merida Andes
(Arnaiz-Rodrguez et al., 2011).
The Bouguer anomalies range from 265 mGal to 145 mGal,
with a mean of45 mGal (Fig. 2b). The highest values are associ-
ated with the Santa Marta Mountains and Perija Range, indicating
that they lack local isostatic compensation (e.g.Kellogg and Bonini,
1982). Other positive anomalies are likely related to upper crust
density contrast, basement uplift (Fig. 1b), or shallow basement inthe Barinas-Apure basin and the Maracaibo basin. Watts (2001)
proposes that positive values of Bouguer anomaly often indicate
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(a) (b)
Fig. 2. (a) Free air anomaly map of the studied region (Sandwell and Smith, 2009). (b) Complete Bouguer anomaly map of the studied region, reduced with 2.67 g/cm 3 Bouguer
density (BA;Arnaiz-Rodrguez and Garzon, 2012). Quaternary faults fromAudemard et al. (2000). Both contour maps are colored in the same color scale and contours are every
50 mGal. Gravimetric positive anomalies in the Santa Marta and Perija mountains indicate absence of isostatic compensation, while displacement to the northwest of the
gravimetric low that could be associated with the Merica Andes isostatic root reveals a complex regional compensation system. (For interpretation of the references to colour in this
gure legend, the reader is referred to the web version of this article).
M.S. Arnaiz-Rodrguez, F. Audemard / Journal of South American Earth Sciences 56 (2014) 251 e264 253
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buried loads (density contrasts in the subsurface), and thus, those
within the Merida Andes and the Northern Colombian Andes, can
be considered as indicative of this sort of loads. Regarding the low
Bouguer anomalies, these are associated with basin regions:
notable negative values are located over the depocenters for the
Maracaibo basin and Magdalena Basin. One of the most important
characteristics of the Bouguer anomaly map is that the gravimetric
low that characterizes an isostatically compensated mountain is
displaced over the northern foothills of the Merida Andes. The
negative gravimetric anomalies are most likely due to the sum-
mation of deep (Moho) and mid-depth (basement) effects (Arnaiz-
Rodrguez et al., 2011), and not to the position of the Merida Andes
isostatic root.
Fig. 3 exhibits the radially averaged power spectrum of the
Bouguer anomaly. From the slopes of this spectrum we estimate the
depth to three mayor sources of gravimetric anomalies: the Moho,
the upper crust-lower crust boundary and the basement, using a
horizontal prism model (e.g.Spector and Grant, 1970). The longest
wavelength is associated with interfaces between 50 and 40 km,
which limit the gravimetric interpretation to crustal depth. Filtering
all but the longest wavelength with a band-pass algorithm (Fig. 4),
we produce a regional map (Fig. 4a) and a residual map (Fig. 4b).
5
10
15
0
0.05
45 5 km (Moho depth)
17.4 2 km (Upper Crust - Lower Crust)
9.5 1 km (Basement/ Maracaibo Basins depocenter)
Bandpass filter (wavelenght = 0.0105883 radians/km)
0.15 0.2 0.250.1
-5
-10
Wavenumber(Radians/km)
log(Powe
r)
Fig. 3. Radially averaged power spectrum of the complete Bouguer anomaly -BA-showing the source depths estimated from the slopes of the curve. The largest wavelength
component is most likely associated with the Moho discontinuity, the mid wavelength component with the lower cruste
upper crust boundary and, the shortest one, with thebasement.
(mGal)
Gravity Anomaly
74 73 72 71 70
7
8
9
10
11
100 50 0 50 100
100
10
0
50
50
50
50
0
0
0
74 73 72 71 70
7
8
9
10
11
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
50
50
50
50
(a) (b)
Fig. 4. (a) Regional map from ltering the longest wavelength of the complete Bouguer anomaly map showing gravimetric anomalies due to Moho and basement variations. (b)
Residual map from ltering the longest wavelengths of the complete Bouguer anomaly map showing the gravimetric anomalies due to the density contrast in the upper crust and to
some structures shown inFig. 1b (half-graben and basement troughs). Quaternary faults from Audemard et al. (2000).
M.S. Arnaiz-Rodrguez, F. Audemard / Journal of South American Earth Sciences 56 (2014) 251 e264254
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The regional map (Fig. 4a) presents the gravimetric contribution
of the deeper structures, particularly the Moho. The Moho depth in
the area varies from 25 km in the Santa Marta Mountains to 45 km
intheMerida Andes northern foothills (Ceron et al.,2007). Niu et al.
(2007) suggested that the Moho in the region has a maximum value
of 49 km, while the mean Moho value is around 42 km. Positive
values in the regional map reect the location of thinner crust as
well as the Santa Marta Mountains, which is congruent with the
values proposed by Ceron et al. (2007). Negative values
under 50 mGals most likely correspond to the extent of the
regional exure due to the load distribution. Those zones with
anomalies below100 mGals may be related to places where the
crust and/or the sedimentary section are thicker.
The residual map (Fig. 4b) displays the gravimetric signature of
all the shallower structures such as basement uplifts, troughs and
faults. Some positive anomalies (exceptionally within the center of
the Merida Andes)are associated with some of the basement uplifts
inFig. 1b, although those within the Barinas-Apure Basin do not
seem to have a clear gravimetric response. Other signicant re-
sidual positive anomalies are associated with the Perija Range and
the Santa Marta Mountains. This couldbe related to Paleozoichigh-
density rocks and deformed basement present in those mountain
ranges. We do not consider here that some of these positiveanomalies represent small-buried loads, but it cannot be ruled out.
Some negative values can be linked to the locations of the basement
troughs (Fig. 1b), though not as clear as the anomalies produced by
uplifts. Other residual positive and negative values are related to
smaller structures and density contrasts that go beyond our aims.
4. Methodology
4.1. Mechanical background
Inexural studies, the lithosphere is usually represented as a 2-
D elastic beam that lays over a viscous medium (Watts, 2001). This
beam is then deformed by vertical stresses linked to the existence
of a vertical column of mass laying over it (thrust belts, ice caps,
sedimentary layers, etc). Assuming the absence of horizontal stress,
two models have been largely applied: the innite plate model (e.g.
Watts et al., 1985) and the broken plate model (e.g. Karner and
Watts, 1983a,b). The rst one is applied in cases where the load is
located relatively far away from a plate margin, while the second
one is applied when the loads are set near the limit of the plate. In
the rst scenario, the deformation is computed by solving the
fourth-order differential equation (Equation(1))
Dd4w
dx4
pq
0
p rmwg
q r inwg
(1)
where:
w is the deection of the beam
p Winkler foundation term
q Sedimentary load termrm Mantle density
rin Sediments density
g Gravity aceleration
D Flexural rigidity
D, in Equation(2),depends on the efcient elastic thickness (Te,
how much of the lithosphere behaves elastically), the Young
modulus (E) and the Poisson radius (y) of the beam (Watts, 2001).
D ETe3
12
1 y2 (2)
These equations have been widely used to study, withina simple
approach, the behavior of the lithosphere, assuming that D and Te
are constants. When more complex situations are presented, and it
is notpossible to assume Te as a constant, a numerical approach can
be used to compute the deection of a beam with variable me-chanical parameters (e.g., Bodine, 1981). The problem becomes
much more intricate when the exure of the lithosphere cannot be
simplied into a 2-D elastic beam.
In a 3-D scenario, an elastic plate is used to represent the lith-
osphere (rather than a beam), and the deformation is computed by
solving the fourth-order partial differential equation with variable
coefcients that governs the bending of a thin plate xed on its
boundaries and variable thickness (Equation (3); Eq (3.83) in
Ventsel and Krauthammer, 2001).
DV2V2w 2vD
vx
v
vx
V
2w 2
vD
vy
v
vy
V
2wV2D
V
2w
1 y!v2D
vx2
v2w
vy2 2
v2D
vxvy
v2w
vxvy
v2D
vy2
v2w
vx2! P
(3)
Wherewrepresents the bending of a plate, whose thickness varies
gradually (there is no abrupt variation in thickness). P represents
thesystem of transverse loads applied to the plate. D is describedby
Equation(4)
DETex;y3
12
1 y2 (4)
To perform the mechanical modeling it is necessary to solve
Equation(3).At this point the boundary conditions imposed to the
equation are: (1) the boundaries of the plate are xed
(displacement 0) and are far away (at least 100 km) from the
loads, and (2) the thickness of the plate (that represents the elasticthickness of the lithosphere) is variable but cannot vary abruptly.
Two more boundary conditions must be imposed, related to the
geologic situation at hand: (3) the plate sits over a Winkler foun-
dation that represents the mantle, and (4) the depression after the
exure is ll with sediments.Cardozo (2009)developed a code to
solve Equation (3) by using centered nite differences, and
considering the parameters and conditions previously specied. To
compute w(x,y), the distribution of loads P(x,y) and the variations of
the elastic thickness of the plate Te(x,y) are needed. The parameters
of the mantle and inll material (rm, r in) are also required.
4.2. The Maracaibo block scenario and modeling approach
Arnaiz-Rodriguez et al. (2011) pointed out from a series ofsimple 2D models that: (a) the Maracaibo block and the South
American plate cannot behave as a plate with constant elastic
thickness, (b) the Maracaibo block exure depends on the loads
distribution, and (c) elastic thickness variations must exist in the
region to explain the current basement morphology. They
concluded that is was necessary to apply 3D modeling to estimate
the elastic thickness lateral variations in the region. Considering the
mountain belt distribution (Fig. 1a), and given the fact that it is
difcult to establish the physical limit between both plates, we
chose to model the Maracaibo Block-South America interaction
region with a single continuous plate with variable elastic thickness
and xed boundaries.
Thus, we build an initial model taking into consideration the
area inFig. 1a and the main loads within it (M
erida Andes, Perij
a
M.S. Arnaiz-Rodrguez, F. Audemard / Journal of South American Earth Sciences 56 (2014) 251 e264 255
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Range, Santa Marta Mountains and Northern Colombian Andes).
These loads were initially represented from the down-sampled
topographic grid extracted from the V15 Global Topography
(Sandwell and Smith, 2009). Mechanical parameters needed for the
model (r, m E) as well as initial values of elastic thickness were
extracted from previous research; they can be seen in Fig. 5 (Chacn
et al., 20 05; Medina, 2009; Arnaiz-Rodrguez et al., 2011). Once the
initial model was built, the exure was computed using Cardozo's
(2009)code. Loads and elastic thickness were iteratively modied
in small steps so that the plate would t the general basement
conguration; and the residual topography, would t the down-
sampled topography. A owchart illustrating this process is pre-
sented inFig. 5.
Fig. 6presents some steps of the modeling process: the rst is
the exure of the initial model; the second is a middle step; the
third is the nal model. Finally, we computed the gravity anomaly
of the model to compare it to the regional component of the
observed total Bouguer anomaly using the Oasis Montaj 3D GM-SYS
module (Geosoft, 2007). The residual topography and gravimetric
anomaly of the nal model are presented inFig. 7; the resulting
elastic thickness map, inFig. 8.
5. Results
The elastic thickness within the Maracaibo block (Fig. 8) ranges
from 30 km to 18 km, with a mean value of 23.73 km and a mode of
26 km (Fig. 9). The orientation of the elastic thickness contours is
roughly N45E, similar to the Merida Andes and the Bocono fault
System. The largest elastic thickness values (higher than 26 km) are
associated with the location of the Santa Marta Mountains, and
with the deformed Guayana Shield to the southeast (Barinas Apure
Basin). The smallest values (less than 20 km) are associated with
the Merida Andes-Maracaibo basin exural system. Elastic thick-
ness minimum values appear in the northern ank of the Merida
Andes and the Bocono fault, which is congruent with the gravi-
metric data of the area, where the Bouguer anomaly minimum
(that characterizes a locally compensated mountain) is displaced tothe north (Fig. 2b).
The residual topography, i.e. the height of the topographic load
after the exure, ts the real downsampled topography with rela-
tively low deviations. The largest difference is of 172 m, which
represents 3.7% of the real topography (Fig. 7a).Fig. 6(III) presents
the exure of the mechanical model in meters. The exure of the
modeled plate is similar to the real basement conguration within
the basins. Accordingly, the model ts the morphologic data; the
error of the estimation is difcult to judge, since the modeling and
data-tting is done manually. The modeling process showed that
values in the center of the model, near the Merida Andes, were
more sensitive to the variations of elastic thickness and load size (as
would be expected) suggesting that the error in this area should be
small (1.0 km) due to the relatively good t of the plate congu-
ration to the basin's basement, and the insignicant difference
between the residual topography and the real topography. Larger
errors (2.5 km) can be expected near the edges of the region
considered for the modeling (Fig. 1).
6. Discussion
The results of the mechanical model prove that the basement
conguration within the Maracaibo basin is controlled by two
important load systems: the Merida Andes and the Perija Range-
Northern Colombian Andes. The Merida Andes is clearly the
largest load in the system as the orientation of the elastic thickness
contours is similar to the one of this mountain range. This afr-
mation is supported by gravimetric data, as previously discussed.
After having compared the elastic thickness gradients on both sides
of the Merida Andes foothills, we propose that elastic thickness
variations in the Barinas-Apure basin (from 27 to 24 km) are most
likely due to the exure caused by the Merida Andes load over the
relatively stable South American lithosphere (Arnaiz-Rodrguez
et al., 2011); while elastic thickness variations within the Mar-
acaibo basin (from 24 to 18 km) are due to lithospheric weakening
caused by different processes from the Jurassic extension (and
graben formation) to the present compression (uplift of the Merida
Andes and convergence between the Maracaibo block and South
America;Audemard and Audemard, 2002).
Regarding the Merida Andes isostatic state, it is evident from
gravimetric data that local isostasy is not the compensation
mechanism that supports this range (Kellogg and Bonini, 1982;Escobar and Rodrguez, 1995). Flexural evidence, regional gravi-
metric anomalies (Fig. 4a) and elastic thickness gradients adjacent
to the mountain (Fig. 8) suggest a regional compensation mecha-
nism (e.g.,Chacn et al., 2005; Arnaiz-Rodrguez et al., 2011). In a
regional isostasy scenario, we propose that the Maracaibo block
Initial model
Initial Loads:downs-sampled topographic grid
(Sandwell and Smith, 2009) Parameters:Mantle Density: 3.3 g/cm3Sediments Density: 2.4 g/cm3
Poissons Ratio: 0.25
Youngs Modulus: 100e9 Pascal
Initial constant Te: 25 kmMrida Andes
Perij Range
Santa Marta Mountains
Northern Colombian Andes Compute 3D Flexure of a thin plate using
centered finite differences (Cardozo, 2009)
Input model
Compare results to basement configuration
and calculate residual topography
Does the mechanical model
fit the geology?
Does the mechanical model
fit geophysical data?
Compute gravity
of the model
YES!YES!
NO
Endmodelling!
Modify
Loads
and/or Te
Fig. 5. Flowchart describing modeling approach. First, an initial model is created with assumed parameters (downsampled topography and mechanical parameters) and constant
elastic thickness (25 km). Then, the model is tested in the nite difference code (Cardozo, 2009). Flexure of the plate is compared to the basement morphology and residual
topography is compared to the real downsampled topography. The model is updated and tested until it ts the geological data. The gravimetric response of the best model is
computed and compared to the regional gravimetric anomaly; if the model roughly
ts the data, the modeling is
nished, if not the model is modi
ed again.
M.S. Arnaiz-Rodrguez, F. Audemard / Journal of South American Earth Sciences 56 (2014) 251 e264256
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may support the northern half of the Merida Andes, which is the
highest one, while South America supports the southern half. If we
consider that a low angle thrust is more efcient for overthusting
than shortening, and because shortening in the northern foothills
of the Merida Andes (~40 km) is far greater than in the southern
foothills (10e
12 km; Audemard and Audemard, 2002), then we
may expect that the overthrusting in the North to be much smaller
than the one in the South. The asymmetry implied by the short-
ening/overthrusting relation, previously proposed byColletta et al.
(1997), induces more uplift on the northern side than in the
southern side. Therefore the relatively weak lithosphere of the
Maracaibo block is holding a tall and narrow load that produces a
2000
2500
24
24
74 72 70
8
10
500
10001
000
1000
1000
1000
1500
1500
1500
2000
2000
2000
2500
74 72 70
8
10
1000
1500
1500
1500
150
0
2000
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200
2000
2000
2500
2500
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250
0
3000
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450045
00
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2000
4000
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6000
6
6000
6000
8000
8000
10000
10000
10000
12000
15 20 25 30
(km)
Elastic Thickness (Te)(km)
Loads Height
6
4
2 0 2 4 6
(I)
Topographic
LoadElastic Thickness Flexure due to Load
(II)
(III)
Fig. 6. Some steps of the exural model. From top to bottom three examples are presented: (I) is the initial model where the load is the same as the down-sample topography, Te is
25 km, the exure does not t the basement conguration. (II) is an intermediate step where the load is larger than the topography in the mountains and the same in the basins, Te
is different for SA (24e26 km) and for the MB (18e22 km), the exure has a similar shape to the basement conguration but the depth does not t. (III) is the nal model where the
topographic load is larger than the topography (dark red squares represents regions where the load is at least 3 km larger than the topography), Te gradients exist in all the area and
the exure roughly ts the depth and shape of the basement conguration. Quaternary faults from.Audemard et al. (2000).(For interpretation of the references to colour in this
gure legend, the reader is referred to the web version of this article).
M.S. Arnaiz-Rodrguez, F. Audemard / Journal of South American Earth Sciences 56 (2014) 251 e264 257
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large exure (9 km at the Merida Andes northern foothills), while
the strong South American lithosphere holds a widespread load
that causes a minor exure (4.5 km at the Merida Andes southern
foothills). This interpretation supports the asymmetry of the
mountain chain masses, rst proposedby De Cizancourt (1933); the
asymmetric orogenic oat model anticipated by Audemard and
Audemard (2002); and the unevenness of the Merida Andes load
distribution over the Maracaibo Block and South America. It is also
consistent with the 20e30 dipping blind thrust and 10 km uplift of
basement rock described byDe Toni and Kellogg (1993).Further-
more, Fig. 10 shows the relation between crustal thickness
(modeled from the regional Bouguer anomaly,Fig. 4a) and elastic
thickness. The region with thicker crust is related to the weakestlithosphere, but they are also associated to the highest sections of
the Merida Andes, as well as the deepest sections of the Maracaibo
100 0
74 72 70
8
10
(km)Residual Topography
6 4 2 0 2 4 6
Gravity Anomaly(mGal)
74 72 70
8
10
100
100
50
(a) (b)
Fig. 7. (a) Residual topography of the nal model; the largest value is 172 m, which represents 3.7% of the real topography. (b) Gravimetric anomalies due to the exure beam and
the masses of the modeled exural loads. The anomaly produced by the exure is similar to the regional gravimetric anomalies (Fig. 4a).
74 73 72 71 70
7
8
9
10
11
15 20 25 30
20
20
20
24
24
24
(km)
Elastic Thickness (Te)
Fig. 8. Contour map showing the lateral variations of the elastic thickness (Te) of the
MB. Largest values (>26 km) are associated with the undeformed shield to the SE and
the SMM to the NW. Small values (
7/25/2019 Variations in Elastic Thickness and Flexure of the Maracaibo Block.pdf
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basin, both acting as large but narrow loads over a weakened
lithosphere with small elastic thickness.
Other important loads are the Perija Range and the Northern
Colombian Andes, which limits the basin on its western margin.
The Perija Range hardly distorts the pattern of the elastic thickness
contours, which implies that it is in isostatic equilibrium. Since
there is no gravimetric evidence (Figs. 2 and 4) that shows the
existence of an isostatic root (e.g. Kellogg and Bonini, 1982), andelastic thickness values are not particularly large (Fig. 8), another
mechanism must be present for this equilibrium to exist. Generally,
when considering isostasy, other stresses beyond those produced
by a vertical load are not taken under consideration in the model.
One could think that horizontal stresses due to compression and
plate interaction, particularly related to the convergence between
the Maracaibo block, the Caribbean plate and South America, could
be enough to hold such load as the Perija Range in dynamic equi-
librium. Another possibility is that, given the fact that the Mar-
acaibo block is a small plate, the large subsidence of the lithosphere
caused by the Merida Andes could force the crustal block to tilt
towards the SE, uplifting the NW side. Since the Perija Range and
the Santa Marta Mountains are on this side, they might hold the
plate from rising, and therefore be on some state of dynamicequilibrium driven by a vertical (upward vs. downward) stress.
Most likely a blend between both cases exists, causing the Perija
Range not to have an isostatic root.
Larger elastic thickness values in the northwest, going from
26 km to 30 km, particularly near Santa Marta Mountains, would
explain why this mountain does not have sign of been isostatically
compensated (as free air and Bouguer anomalies are positive,
Fig. 2). This could be related to the convergence and coupling be-
tween the South American and the Caribbean plates. We cannot
rule out that a process similar to the one associated with the Perija
Range state could also play a signicant role in the Santa Marta
Mountains isostatic state.
The residual Bouguer anomaly map combined with the base-
ment topography can be used to differentiate the four sections of
the Maracaibo basin (Fig. 11). The wedge-top depozone is located
next to the Merida Andes. This depozone is expected to be narrow
due to the relationship between overthrusting and shortening in
the Merida Andes northern foothill. The foredeep depozone is
present further to the NW and is characterized by a steep basement
from 9 km to 4.5 km depth. The Position of the forebulge crest
(shown in a bold line in Fig. 11) is indicated by a cluster of 4.5 km
contours and by a positive residual Bouguer anomaly. The forebulgeof the basin has not been located before, and was thought to be
absent (Mann et al., 2006; Arnaiz-Rodrguez et al., 2011). Because
this particular forebulge shows no topographic expression (is
buried by 4 km of sediments), the Maracaibo basin is in an over-
lled state (DeCelles, 2012). Furthermore, the relative proximity
between the forebulge and the thrust front supports low elastic
thickness values found within the basin (
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the Icotea fault, a left-lateral strike slip fault, also distorts the
pattern of the elastic thickness contours in the direction of its
displacement. Therefore, the Icotea fault has, at least, a crustal in-
uence, as suggested by seismicity in its vicinities, with events up
to 40 km deep (Audemard and Audemard, 2002). Moreover, this
distortion has the appearance of continuing to the southeast, which
is compatible with the idea of itsconvergence with the Bocono fault
at some point (e.g.Beltran, 1994), or at least that it does not end in
the Maracaibo Lake area as proposed byCastillo and Mann (2006).
As the loads within the Maracaibo block have different ages, we
will briey discuss the time dependent exure of the lithosphere.
Based on the viscoelastic plate model (Walcott, 1970), two char-
acteristics of the load are important: the age and the width (Watts,
2001). Young loads are mostly correlated to high values ofexuralrigidity, while older ones tend to produce lower exural rigidity
values. Wide loads cause the lithosphere to approach faster to a
hydrostatic state (Airy's isostasy model) than a narrow load. The
Merida Andes, within the Maracaibo block exural system, can be
considered as a relatively young and narrow load that produces a
short wavelength and deep exure over the lithosphere. Such cir-
cumstances would suggest that the instantaneous exural rigidity
of the plate (and therefore its elastic thickness) should be less than
the standards values for continental lithosphere. In fact, there is no
simple relationship between instantaneous exural rigidity, the
elastic thickness variations and the age of a load (Watts, 2001), so
we cannot directly determine how much of the subsidence and
elastic thickness variations are produce by the Merida Andes load,
nor how much is inherited from previous process.
6.1. Flexural history of the Maracaibo Block
Based on the results of this research and on previous in-
terpretations of the geodynamic evolution of the Maracaibo Block,
its orogens and basins, we present a schematic portrayal of the
geodynamic history of the region (Fig.12), with particular emphasis
on the different exural stages deforming this microplate:
a. Originally, the Maracaibo block was part of the South American
plate. This lithosphere probably had a relatively large and con-
stant elastic thickness (>30 km), as suggested by regional ex-
ural studies about South America (e.g. Watts et al.,1995; Stewart
and Watts, 1997; Perez-Gussinye, 20 07).
b. In the Late Jurassic, rifting between North and South Americacreated the Proto-Caribbean seaway (e.g. Pindell and Barrett,
1990), as well as a passive margin along northern South Amer-
ica. This process would have reduced the elastic thickness to-
wards the divergent margin. Eventually, extension of the
lithosphere created a series of grabens and half grabens in the
South American crust (Parnaud et al., 1995); their formation
would have weakened the lithosphere and signicantly reduced
the elastic thickness near these structures, as suggested by
Audemard and Audemard (2002).
c. During the Cretaceous, sediments were deposited over the
continental platform causing subsidence in the lithosphere
(Duerto, 1998). In the late Maastrichtian the uplift of the Santa
Marta Mountains began, which might have affected the elastic
thickness in an uncertain way. During this period, thermal
3.0
1.0
2.0
1.5
3.0
3.0
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7.
5
7.5
74 73 72 71 70
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6.
0
WED
GE-TOP
DEPOZ
ONE
FORE
DEEP
DEPOZ
ONE
BACKBU
LGE
FOREBULGE
(mGal)
Gravity Anomaly
100 50 0 50 100
Fig. 11. Forebuldge position and depozones of the Maracaibo basin on top of the residual Bouguer anomaly and major structures in the study area. The forebulge is located within a
set of 4.5 km contours and associated to some positive residual anomalies within the basin.
M.S. Arnaiz-Rodrguez, F. Audemard / Journal of South American Earth Sciences 56 (2014) 251 e264260
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equilibrium of the lithosphere drove thermal subsidence in the
Maracaibo Block, similar to the process described in the Eastern
Cordillera in Colombia (e.g. Sarmiento, 2002), certainly reducing
the elastic thickness.
d. The rstexural deformation stage of the Maracaibo basin was
associated with the collision of the The Great Caribbean Arcwith northern Venezuela from the Paleocene to the early Eocene
(Lugo and Mann, 1995). Throughout this period, some portions
of
The Great Caribbean Arc
collided and overthrust the passive
margin. Shortening related to this process led to the Lara nappe
emplacement (Stephan, 1985) that caused subsidence in
northwestern Venezuela, reducing the elastic thickness in
northern South America, similar to the lithospheric weakening
produced by nappe emplacement described in the East Carpa-
thians (e.g.Artyushkov et al., 1996).
e The Oligocene represents an important orogenic stage because
of the uplift of the Colombian Andes and the Perij a Range, as
well as a second pulse of the Santa Marta Mountains uplift. The
Fig. 12. Geodynamic evolution of the MB, its orogens and basins, based on the reconstruction proposed by different authors, see text for details. The dotted red line shows the
variation of the elastic thickness (Te) through time (not at true vertical scale). MA current structure is based on Arnaiz-Rodrguez et al. (2011)andMonod et al. (2010)models. The
age of the stages described are as follow: (a) Pre-Jurassic, (b) Late Jurassic, (c) Cretaceous, (d) Paleocene, (e) Oligocene, (f) Middle Moicene, (g) Pliocene. In Fig. 12g, S stands for
shortening and Ot for overthusting. Thick arrows show the direction of stress (either compression or extension) and the small arrow marks the forebulge. (For interpretation of the
references to colour in this gure legend, the reader is referred to the web version of this article).
M.S. Arnaiz-Rodrguez, F. Audemard / Journal of South American Earth Sciences 56 (2014) 251 e264 261
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rst and second were related to the Nazca Plate subduction,
while the third to the at subduction of the Caribbean Plate
(Kellogg, 1984; Van der Hilst and Mann, 1994; Taboada et al.,
2000). Flat subduction of the Caribbean plate that started in
the NW at this period might have help support the load in the
Maracaibo block along the Perija Range and the Santa Marta
Mountains. It is worth noting that the crustal structure of the
Perija Range is not well known, therefore, in our reconstruction
we take the one proposed byAudemard and Audemard (2002).
f In the Middle Miocene, stress produced by the Panama Arc
collision with northern South America forced the inversion of a
Jurassic graben that led to the uplift of the Merida Andes (e.g.
Audemard and Audemard, 2002; Monod et al., 2010) and drove
the exural subsidence of the region, as well as the creation of
the large depocenter on the northern foothills of the Merida
Andes (Audemard, 2003). The formation of this foreland basin is
recorded by normal faults within it with an average trend of
S37E (Castillo and Mann, 2006). The large load of the Merida
Andes might have reduced the elastic thickness in the Mar-
acaibo basin and the Barinas-Apure basin through its uplift to
some extent, and formed the incipient forebulge of the Mar-
acaibo basin.
g. Ultimately, in the last 5 Ma, the compression generated by thePanama arccollision and the subduction of the Carnegie Ridge at
the Ecuador trench in northern South America produced the
escape of the Maracaibo block and the northern Andes and the
Bonaire block (Egbue and Kellogg, 2010). As both overrode the
Caribbean Plate, a south-dipping amagmatic at oceanic sub-
duction was created in the Southern Caribbean deformation belt
(e.gAudermard, 2009). The current uplift of the Merida Andes
and the Perija Range is driven by oblique convergence and
resulting transpression between South America and the Mar-
acaibo block (Audemard and Audemard, 2002) affecting the
exural system in an uncertain way.
7. Conclusions
Numerical modeling of the complex load system within the
Maracaibo block has allowed us to estimate the lateral variations of
the exural thickness in the region. Based on the elastic thickness
variations, we can draw the following conclusions:
1. The use of a 3D numerical approach is valid to roughly estimate
the variations of the elastic thickness of the continental litho-
sphere. This method is applicable as long as the boundary
conditions and limitations expressed by the equations are
respected, and satisfy the overall geodynamic setting.
2. The elastic thickness in the study area ranges from 30 km to
18 km, with a mean value of 23.7 km, a mode of 26 km. The
orientation of the elastic thickness contours is roughly N45E,
similar to the Merida Andes, indicating that this is the most
important load within the Maracaibo block. Large elastic thick-
ness values (higher than 26 km) are associated with the location
of the Santa Marta Mountains and with the deformed Guayana
Shield. The smallest values (less than 20 km) coincide with the
Merida Andes-Maracaibo basin exural system. Estimated er-
rors range from 1.0 km to 2.5 km.
3. The basement conguration within the Maracaibo basin seems
to be controlled by the Merida Andes (which is clearly the
largest load) and by the Perija Range-Northern Colombian
Andes (which limits the basin on its western margin).
4. The elastic thickness map shows that the 20 km contour has a
similar orientation than the Bocono fault system; this could
imply that the mechanical and geodynamic limit between the
Maracaibo block and South America is in some way associated
with this structure, even though it is improbable the fault dis-
places the Moho.
5. The Perija Range barely distorts the pattern of the elastic
thickness contours and lacks an isostatic root. There may be two
possible explanations for this: (a) horizontal stresses due to
compression and plate interaction hold the Perija Range load, or
(b) the large subsidence of the lithosphere caused by the Merida
Andes could force the Maracaibo Block to tilt towards the SE;
this would have caused the west side of the block to be uplifted
but the Perija Range and the Santa Marta Mountains prevent it
from rising. A mixture of both should not be discarded.
6. Contiguous 4.5 km contours and positive residual Bouguer
anomaly within the basin indicate the forebulge crest of the
Maracaibo basin. It shows no topographic expression as it is
buried by 4 km of sediments, which implies that the basin is in
an overlled state. Moreover, the distance between the fore-
bulge and the northern thrust front support low elastic thick-
ness values found within the basin.
7. The Santa Marta Mountains region has larger elastic thickness
values (from 26 km to 30 km). This could be related to the
convergence of South America and the Caribbean plate (CP) and
the coupling related to this process. The scenarios proposed for
the Perija Range might play a role on the Santa Marta Mountainsisostatic equilibrium as well. Moreover, the fact that the Santa
Marta Mountains was uplifted far from the region affected by
graben formation suggests that elastic thickness values in this
region were unaffected by pre-orogenic processes.
8. When looking at the full picture of the geodynamic evolution of
the Maracaibo block, it is clear that, even though the Merida
Andes is the most important load in the system, its orogenesis is
not the only process that produced the current elastic thickness
gradients within it. Consequently, the Te values within the
Maracaibo basin (from 24 to 18 km) are likely the response to
different stages in the Maracaibo block history. Particularly
Jurassic extension could have affected and weakened the lith-
osphere. Subsequently, the uplift and overthrusting of the
Merida Andes over the weak Maracaibo block lithosphere pro-duced the deep Maracaibo block.
Further work
As noted previously in this paper, classic exural studies take
into consideration vertical stresses related to the loads over the
lithosphere. Horizontal (either compression or extension) stresses
are not, but are often mentioned in the interpretation of the re-
sults obtained. As the Maracaibo block represents a region with a
relatively complex stress eld, compressional stress must be
considered. Further work will include modeling the geodynamic
situation with the nite element method so the horizontal stress
eld can be taken into account, as well as viscoelasticdeformation.
Acknowledgments
The authors would like to thank the Project GIAME and its team
for support during the research, Nestor Cardozo for designing al-
gorithms freely available to the geoscientic community, and
Michael Schmitz and James Kellogg for their thoughts on previous
stages of the research and their motivation to nish the work.
Finally, we would like to thank the reviewers whose notes, com-
ments, corrections, recommendations and thoughts greatly
improved the quality of the original manuscript, and once again to
James Kellogg Editor-in-Chief for his comments that helped us
improve the manuscript and interpretations.
M.S. Arnaiz-Rodrguez, F. Audemard / Journal of South American Earth Sciences 56 (2014) 251 e264262
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