About 80
million years ago, in the late Cretaceous, the Iberian microplate was very
different from the present configuration. Simplifying a little bit: the
western part of past Iberia was above sea level whereas the eastern part was
partially under the sea. Iberia location was between the African plate and
Euroasiatic plate. Around 200 km far from the Eurasian plate, and 300 km far
from the African.
In the north, there was an oceanic ridge that pushes
counterclockwise to the peninsula, in what is known as the opening of the Bay
of Biscay. At the same time, we had an eastward movement caused by the Atlantic
ridge. As if that were not enough, the African plate and the Euroasiatic began
to converge, causing a tectonic contraction in the Iberian microplate. As a
result, two converging boundaries generated, one in the south, in the Betics
and the other in the north, which produced the Pyrenees and the Cantabrian
mountains. But why were mountains generated in the interior?
Some scientist suggest a complete bulge of the whole
peninsula which generated hight topography in the middle of Iberia. With both
subductions in the north and south and surface processes, we would have the
actual configuration of mountain belts. Other scientist suggest that tectonic
compression generated a folding of the entire lithosphere. However, the shortening caused by the
movements in the different layers of the crust can't be explained by these two
theories, so it has been suggested several detachment levels that transmit
efforts from the edges towards the interior, raising intraplate mountain
chains.
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| Figure 1. |
Through numerical models, we try to understand the
mechanisms involved in the generation of the relief of the peninsula as well as
the structure below the surface. Considering it is not possible to know exactly
the past configuration of the Iberian Peninsula, we have made several models in
order to explain the shortening differences of the crystal layers, as well as
the overall geostructure.
We start with an initial mechanical model for a profile that
goes from the bay of Biscay, through the Cantabrian range, through the basin of
the Duero reaching the Central system. We also include a detachment level in
the middle crust and two initial faults in the central system that come from
tectonic movements before 80 million years. Afterwards, we introduce the
temperature and rheologies for each zone of the model in order to reproduce the
different shortening in the crust.
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| Figure 2. |
Some studies suggest that the shortening of the upper crust
is between 70-97 km in the northern part and the shortening of the lower crust
of about 120 km, starting in Cretaceous times untill ~15My ago. That implies
small lower crust subduction under the Cantabrian area. Moreover, the central
system is estimated to have a maximum of 22 km of shortening at some points, in
the last 40 My (dashed squares in Figure 5). Likewise, there is no record of shortening in the Duero basin.
This fact does not imply that shortening has not been produced in the area,
evidence could have been erased through time. Apart from the rheologies, a
plasticity criterion is included for when there are high strain rates that
generate high levels of stress where shear zones or fractures occur.
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| Figure 3. Topography, moho and LAB profiles for most representative models. SRTM15 is real topography of transect AB from figure 1. Grey dashed line is a previous study based in density changes from Carballo et al., that also include seismic data. |
Looking into the results of the mechanical models, the one
that best explains the different shortening is the M3 model, which has as a
peculiarity a weak zone in the north that goes down to the upper mantle
lithosphere. That generates the subduction, along with the compression tectonic
movement. Model M2 has good results of shortening but the plastic criterion was
easier than the M3 model and it does not have that weak zone going into the
upper mantle. So there is no subduction, just thickening. Also, the structure
of the material was not adequate in order to compare with the real
geostructure.
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| Figure 4 |
Although shortenings coincide for M3, we can observe in
figure 3 that the topography, the discontinuity of the MOHO and the LAB are
very high than previous studies suggest.
When we introduce temperature gradients for the model
(figure 1). The relation of density and viscosity in the model change. Now is
not vertical nor linear profiles. Density changes due to the different
temperature of the lithospheres that compounds the model, in this case, one is
continental and the other is young oceanic. And viscosity changes due to
temperature, depth, pressure and strain rate in what is call power-law
creep. Using these temperature profiles
and different rheologies, strain rate accommodates in a distinct way than in mechanical
models. As we can see in figure 4, those modifications change the geodynamic evolution of the model. And
also shortenings are modified. But the geostructure is much more similar to the
real one.
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| Figure 5 |
Remarks
Weak zones, either inherited or new, localise strain rate,
increasing stress in the surroundings. Although, topograhy pattern is preserved
in all models, geostructure changed quite a lot, and including temperature fix
better with real one as strain rate is being localised in a different parts and
in distinct way. Crustal subduction happens only when a mid crustal detachment
level is predefined down to the lithosperic mantel. As we can see, plastic
criteria also plays important role in the distribution of the weaknesses.
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