1. The transition from linear to diffuse plate boundary in the Azores-Gibraltar region: results from a thin sheet model
We use the thin-sheet plane-stress approach (UHURU)
to study the present-day dynamic behaviour of the plate boundary
between Eurasia and Africa along the Azores-Gibraltar region. This
plate boundary, which extends from the Azores triple junction to
the Gibraltar strait, shows a tectonic regime that changes from
transtension in the West to transpression in the East, with a strike-slip
motion in its central segment. Seismological data reveal that the
western and central segments are currently marked by a linear series
of earthquakes indicating that the plate boundary is located in
a narrow zone. In contrast, the eastern segment is not so well defined
and deformation spreads over a much broader area. To apply the thin
sheet approach, we combined heat flow, elevation and crustal thickness
data to calculate the steady-state geotherm and the total strength
of the lithosphere. Several models with different fault friction
coefficients and geometries at the eastern segment of the plate
boundary were tested. Results are compared with the maximum compressive
stress directions from the World Stress Map, and the calculated
seismic strain rates and slip vectors from earthquake data. The
best fitting models are consistent with the rotation pole of Argus
et al. (1989), and show that the rheological behaviour of the plate
boundary must necessarily change from the western and central segments
to the eastern segment. The diffuse character of the plate boundary
east of the Gorringe Bank is dominated by the transition from oceanic
to continental lithosphere, the weakness of the Alboran domain,
and the convergence between the African and the Eurasian plates.
The displacement of the Alboran domain relative to the African plate
may play a major role in stress propagation through the Iberian
Peninsula and its Atlantic margin.
a, b, c) Scores obtained from models group B. The fault strength
coefficient is set to q=7 to the western segment of the plate boundary
and varies from 1 to 3 in the easternmost segment.
d, e, f) Strain rate and velocity fields obtained from the best
models in groups B1 (qE=2.0), B2 (qE=1.2), and B3 (qE=1.4) .
White thick arrows indicate the velocity boundary conditions imposed
to each model. Velocities (black thin arrows) less than 0.5 mm/a
are not plotted.
see paper for details: pdf
2. Thin-shell modeling of neotectonics in the Azores-Gibraltar region
We applied the thin-shell neotectonic modeling method
to study the neotectonics of the Africa/Eurasia plate boundary in
the Azores-Gibraltar region. Results are compared with seismic strain
rates, fault slip rates and stress orientations. The best estimate
for the fault friction coefficient is 0.1-0.15 meaning that the
plate-boundary is only about 1/4 as strong as the adjacent lithosphere.
The largest fault slip rates (>1.5 mm/yr) are obtained along
the Gloria fault (strike-slip), and the Betic (transpressive) and
Rif-Tell (compressive) thrust systems. Whereas tectonic activity
in the Atlas region is comparable to that obtained along the plate
boundary, the fault slip rates in the west Iberia fault systems
are one order of magnitude less.
Zone of study, modeled faults, boundary conditions and mesh. The
plate boundary consists of a simple fault system running from Azores
to the Gorringe Bank where it branches along the Betics and Rift-Tell
thrust fronts. Major faults in west Iberia and NW Africa have also
Mean stress azimuth error and seismic correlation for different
fault frictions. Ideally, both curves would have maxima at the
same value of fault friction, but they do not. The grey strip
indicates the best compromise values for the fault friction.
Predicted scalar strain rates and fault slip rates from the best
thin-shell model. White points are epicenters from the ISC catalog.
Most-compressive horizontal principal stress directions from the
best thin-shell model, compared to data from the World Stress Map.
Color indicates stress regime.
3. Neotectonic modelling of the western part of the Africa-Eurasia plate boundary: from the mid-Atlantic ridge to Algeria
In this work we use the thin-shell approximation to
model the neotectonics of the western part of the Africa/Eurasia
plate boundary, extending from the Mid-Atlantic ridge to Tell Atlas
(northern Algeria). Models assume a nonlinear rheology and include
laterally variable heat flow, elevation, and crust and lithospheric
mantle thickness. Including the Mid-Atlantic ridge permits us to
evaluate the effects of ridge push and to analyse the influence
of the North America motion on the area of the Africa/Eurasia plate
boundary. Ridge push forces were included in a self-consistent manner
and have been shown to exert negligible effects in the neotectonics
of the Iberian Peninsula and northwestern Africa. Different models
were computed with systematic variation of the fault friction coefficient.
Model quality was scored by comparing predictions of anelastic strain
rates, vertically-integrated stresses and velocity fields to data
on seismic strain rate computed from earthquake magnitude, most
compressive horizontal principal stress direction, and seafloor
spreading rates on the Mid-Atlantic ridge. The best model scores
were obtained with fault friction coefficients as low as 0.06-0.1.
The velocity boundary condition representing spreading on the Mid-Atlantic
ridge is shown to produce concentrated deformation along the ridge
and to have negligible effect in the interior of the plates. However,
this condition is shown to be necessary to properly reproduce the
observed directions of maximum horizontal compression on the Mid-Atlantic
ridge. The maximum fault slip rates predicted by the model are obtained
along the Mid-Atlantic ridge, Terceira ridge and Tell Atlas front.
Relatively high slip rates are also obtained in the area between
the Gloria fault and the Gulf of Cadiz. We infer from our modelling
a significant long-term seismic hazard for the Gloria fault, and
interpret the absence of seismicity on this fault as possibly due
to transient elastic strain accumulation. The present study has
also permitted better understanding of the geometry of the Africa-Eurasia
plate boundary from the Azores triple junction to the Algerian Basin.
The different deformational styles seem to be related to the different
types of lithosphere, oceanic or continental, in contact at the
Common logarithm of the smoothed strain rate from the 3_PLATES model
set with a fault friction coefficient of 0.07. For comparison with
seismic strain rate we also display earthquakes of magnitude mb
equal or greater than 4 (circles), from the CNSS seismic catalogue.
Fault slip rates and tectonic regime from the 3_PLATES model set with
a fault friction coefficient of 0.07.