Introduction -
Numerous geophysical studies
have been done in the Pyrenees to better understand the structure and composition
of the subsurface. Geophysical methods used in this region include
reflection/refraction seismic surveys, magnetotelluric surveys, and seismic
tomography.
Reflection/Refraction Surveys -
Three reflection/refraction
surveys have been conducted by the ECORS (Etude Continentale et Oceanique
par Reflexion et Refraction Sismique) Team across the Pyrenees. These
surveys were done to image the subsurface structures of the orogen and
provide better estimates of the amount of thickening and shortening within
the Pyrenees. The results have helped create well constrained balanced
cross-sections across the range (Munoz, 1992; Teixell, 1998).
The ECORS-Pyrenees survey was
conducted from 1985-1986. This seismic survey was 250km long stretching
through the Eastern Central Pyrenees from the Aquitaine basin in the north
to the Ebro basin in the south and was the first deep survey across an
entire orogenic belt (Fig 1). Results from this seismic survey show
several subsurface features (Fig 2). One observed feature in this
region is thicker Iberian crust than European crust. Flexural downbending
of the Iberian plate beneath the Pyrenees is also observed. Deformation
in this region is perceived to concentrate in a weak crustal domain considered
to be the early Cretaceous boundary between the plates (North Pyrenean
fault zone). The Axial Zone is seen on the profile as a fan-shaped
structure that separates the Ebro and Aquitaine foreland basins.
Possible structures are detected within the mantle beneath the European
plate which have been interpreted to be subducted Iberian lithosphere.
A ~15 km Moho step is also observed beneath the North Pyrenean fault zone
implying that this fault represents the plate boundary in this region (Choukroune
and ECORS Team, 1989; Daignieres et al, 1989; Roure et al, 1989).
Munoz (1992) used this seismic data to help constrain his balanced cross-section
(Fig 3).
Two other ECORS seismic surveys were conducted (Fig 1). ECORS-Biscay profile was done to study the near-shore geology of the Bay of Biscay and the onshore geology of the western Pyrenees and the Cantabrian mountains (Pinet et al, 1987). The ECORS-Arzacq survey was done to study the west central North Pyrenean Zone (Daignieres et al, 1994). Results from this seismic survey are shown in Figure 4. Teixell (1998) used this and other seismic data to better constrain his balanced cross section through the West Central Pyrenees (Fig 5).
Magnetotelluric Surveys -
Pous et al (1995) conducted
a magnetotelluric survey through the Central Pyrenees (Fig 6). Results
from this profile indicate zones of very high conductivity/low resistivity
present beneath the Axial and South Pyrenean Zones at lower crustal/upper
mantle depths (Fig 7). This zone of high conductivity is not detected
in the European plate. These results have been interpreted to indicate
the presence of partial melts within the Iberian crust. This implies
the subducted Iberian lithosphere is currently undergoing partial melting.
The lithosphere is undergoing partial melting ~25 Ma after the end of convergence
because of the time delay involved with thermal re-equilibration of thickened
crust. This partial melt is thought to be ascending up to middle
crustal levels beneath the Axial Zone (Fig7).
Seismic Tomography -
A seismic tomography profile
was also conducted down to depths of ~200km. The distribution of
seismicity representative for the Pyrenees orogen chosen for this study
is shown in Figure 8. Scatter seen in this distribution is most likely
associated with the varying depth to Moho beneath the Pyrenees (Souriau
and Granet, 1995). In the upper crust, a high velocity zone is detected
(Fig 9). This zone coincides with the presence of the North Pyrenean
Massifs and other lower crustal/upper mantle rocks. Low velocities
are observed on both sides of the high velocity zones. These low
velocity zones have been interpreted as sediments buried beneath these
granulites/lherzolites. The lower crust and upper mantle have been
interpreted as much less heterogeneous (Fig 10). Low velocities have
also been detected south of the North Pyrenean fault zone in an elongated
pattern between 80-100 km in depth. This low velocity anomaly has
been interpreted as nearly vertically subducted Iberian lithosphere.
This anomaly is not observed in the western part of the orogen implying
a differing amount of shortening along the range (Souriau and Granet, 1995).
Author: Christina M. Butzer
Spring 2001 Orogenic Systems Project
Last Updated: May 9, 2001