Download The Balearic Basin in the West-Mediterranean

The Balearic Basin in the West-Mediterranean:
A Back-arc Basin or a Foreland-Foredeep Basin?
Paolo Esestime*, Hannah Kearns and Phillip Hargreaves (Spectrum Geo Ltd)
Summary
Introduction
In this paper we question the origin of the Balearic
Basin, univocally described in literature as a back-arc
basin, which opened during the Oligocene-Miocene
rift. Our interpretation highlighted the presence of
an extensive Mesozoic basin, re-shaped during the
Paleogene compression and uplift. The deepest units
show polyphasic folds and thrusts, characterized
by different structural styles, active mostly during
the Paleogene. In the Early Miocene (BurdigalianSerravallian), the gradual decrease in tectonic
activity developed the final bathymetric setting of
this basin.
We propose to reclassify the Balearic Basin as forelandforedeep basin, deformed between the frontal thrusts
of the Betic Cordillera and the Pyrenees. A preliminary
regional kinematic model has been presented as an
alternative to the back-arc model, rearranging the
evolution of this basin with the regional tectonic of
the circum-Mediterranean orogens.
The West Mediterranean is classified as an episutural
basin related to the circum-Mediterranenan orogens
(Bally and Snelson, 1980), that resulted from the
convergence of Africa and Europe, the subduction
of the western Tethys Ocean, and the collision of
continental margins (Stampfli et al. 2002, Golonka
2014) (Figure 1). The Balearic Basin, within the West
Mediterranean, has previously been described as
a back-arc extensional basin, related to eastward
trench rollback and slab retreat during the MiocenePliocene subduction in the Italian Apennine Arc
(Gueguen et al. 1998). Resultant crustal stretching
caused a Late Oligocene-Early Miocene rift,
separation of the Sardinia-Corsica block from the
European Margin, and oceanic crust formation. This
tectonic model takes into account the 30-35˚ anticlockwise paleo-magnetic rotation of the SardiniaCorsica continental block during the AquitanianBurdigalian (Speranza et al. 2002, Gattacceca et al.
2007), assuming a rotation pole in the northern apex
of the block near the Italian coast.
Figure 1 Schematic sketch showing the onshore and offshore regional structural trends reconstructed in this
work. The white lines represent the seismic grid.
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In this work, we reconstruct the offshore structural
setting at the Oligocene-Miocene transition, focusing
on the relationship between the rifting described in
the back-arc model, and compressional structures
active in the Pyrenees during the Eocene-Oligocene
and the Betic Cordillera during the OligoceneMiocene (Azañon et al. 2002, Capote et al. 2002,
Verges et al. 2002).
Method
We interpreted approximately 20 000 km of 2D
marine seismic data, acquired from the 1970s to
2012 in several regional grids (Figure 1); all lines were
reprocessed in 2011 to 2013. The data are distributed
around the Balearic Archipelago and along the
Spanish and French offshore. More than thirty years
of exploration provide good well control, allowing
calibration of the seismic data with time-depth
curves and checkshots. The interpretation focused
on the distinctive Oligocene-Miocene sequence
boundary observable across the Balearic Basin.
This event is calibrated by wells and separates an
underlying highly deformed, seismic unit consisting
of discontinuous reflectors, from the overlying almost
undeformed unit characterized by continuous
reflectors.
Figure 2 The analogy between the onshore crustal section (A), and the interpreted offshore seismic (B and
B’) shows the possible continuation of the Pyrenees in the deep offshore of the Gulf of Lion. Lines of sections
are shown in Figure 1.
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Results
Two different structural domains exist within the
Balearic Basin: the Gulf of Lion and the Valencia
Trough.
The Gulf of Lion well data offer a sparse calibration of
the pre-Tertiary substratum, marked by an erosional
surface at the top, which allows correlation of the
Late Oligocene-Early Miocene unconformity from the
nearshore (Figure 2A) to deep water areas (Figure
2B). Along the coast between Spain and France, the
erosion reaches Paleozoic strata, where the offshore
continuation of the Pyrenees exists (Figure 1). In the
deep water, the substratum descends along an
escarpment where wells have intersected Mesozoic
sediments (Figure 2B and B´).
In the Valencia Trough, Mesozoic sediments can be
calibrated in most of the wells, and consist mainly of
shallow water carbonates. The seismic data indicate
this sequence is up to 5000m thick and deformed in
kilometre-scale detachment folds, detaching either
at the base of the Mesozoic or within Paleozoic
strata. The shortening is Paleogene-Early Miocene
in age, coeval with the Betic Cordillera, which
shares the east-west trend. The deformation is more
pronounced towards the Balearic Archipelago.
Along the Spanish shoreline, carbonates form an
undulated monocline and thrust fault propagation
folds. The structures are incised by canyons several
hundred metres deep, and intense karstification of
sediments suggests subaerial exposure during the
Paleogene. Neogene shallow water carbonates and
clastic deposits lie unconformably on the deformed
substratum, indicating a relatively quiescent tectonic
phase occurred from the Miocene. No evidence of
Early Miocene rifting was seen. Listric normal faults
Figure 3 Indentation model showing the tectonic evolution of the Balearic Basin during the Tertiary and the
rotation of Sardinia-Corsica microcontinent. (a) and (b) represent continental crust and margins; (c) is thin
continental crust or oceanic crust related to the Mesozoic Tethys Ocean.
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or diapiric structures are locally active during the
Late Miocene-Pliocene, detached or rooted at the
base of the Mesozoic, suggesting the presence of
mobile evaporites analogous to the Keuper Fm. in
the nearshore.
Structures resulting from several phases of
compression can be seen within the Paleozoic to
Oligocene substratum on the seismic data, typical
of pelagic accretionary wedges. The age of the
aforementioned unconformity is thought to be Late
Oligocene-Early Miocene, coeval with the end of the
Pyrenean orogeny (Figure 2). The level of shortening
between the onshore and offshore Pyrenees cannot
be compared, but several thousand metres of
difference in morphology can be explained with
variable crustal thicknesses in the orogens. This
interpretation implies a thin continental or oceanic
crust, present in the deep water sector since the
Mesozoic, which became part of the Pyrenees and
its foreland.
Conclusion
The structural setting and evolution described is
incompatible with extension required by a back-arc
model. An alternative kinematic model to the backarc has been based on a process of indentation
(Sokoutis et al. 2000) of the Sardinia-Corsica block
during the Africa-Europe convergence (Figure 3).
This model presents the Balearic Basin as a remnant
of the western Tethys, and the Sardinia-Corsica
block as an isolated micro-continent in place since
the Mesozoic. The Early Miocene rotation has been
re-arranged using a rotation pole located offshore
Barcelona. The block is pushed northwards during
the formation of the Northern Apennines, forming
an indenter within the Piemont-Liguria Ocean.
Approximately 300 km of northward migration has
been inferred, compatible with the 30-35˚ anticlockwise rotation and palinspastic reconstruction in
the Northern Apennines (Cerrina Feroni et al. 2004,
and reference therein). The abrupt onset of diapirism
in the Messinian evaporites may reflect deformation
in underlying units caused by a transcurrenttranspressive lineament in the deep water Gulf of
Lion.
In this model, the Balearic Basin remains trapped in
the external sectors of the Betic Cordillera and the
Pyrenees as a relict foreland-foredeep basin of these
orogens.
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