3.2.3. Seismicity and neotectonic deformation of the Armorican Massif
There is increasing evidence that the lithosphere of the North-Alpine foreland responds to the build-up of intraplate compressional stresses by long-wavelength folding that is controlled by the strong part of the lithospheric mantle. On-going lithospheric folding in a low strain-rate regime is thought to control uplift patterns, river incision and the location of drainage divides in the area of the Armorican Massif. This Massif is characterised by a SE-NW trending belt of increased seismic activity, reflecting reactivation of the Paleozoic Armorican shear zones.
|
|
| Fig. 2. Seismicity map of Europe, illustrating present-day active intraplate deformation. Also shown are intraplate areas of Late Neogene uplift (circles with plus symbols) and subsidence (circles with minus symbols). Background elevation images are extracted from the ETOPO2 data set. Earthquake epicentres are from the NEIC data centre, and are shown as red dots. Inset map: Cenozoic rift system of Europe (after Dèzes et al., 2004). |
Repeated precision levelling surveys indicate that the western and south-western parts of Brittany, forming the NW branch of the Armorican Massif, are currently being uplifted at rates of up to 1mm/yr (Lenôtre et al., 1999). Geomorphologic studies indicate that this uplift pattern controls the magnitude of fluvial incision, the location of drainage divides and repeated river captures. Moreover, Pleistocene deposits are locally folded and faulted (Bonnet et al., 1998; Bonnet et al., 2000; Brault et al., 2001). As the spatial pattern and timing of uplift inferred from river incision cannot be explained by glacio-eustatic sea level fluctuations, the underlying vertical crustal motions must be attributed to deformation of the lithosphere under the present-day NW-SE directed compressional stress field (Müller et al., 1997). This is compatible with the occurrence of a broad belt of increased seismic activity that extends from the Massif Central to the western tip of the Armorican Massif (Fig. 2), reflecting reactivation of the Paleozoic Armorican shear zones. Although there is geological and geodetic evidence for local fault reactivation, the wave length of about 250 km for the observed broad uplift points to a lithospheric mantle control on the on-going deformation.
|
|
| Fig. 5. Velocities of crustal motion for a four-block model of Europe calculated by least-squares estimation. The velocities at permanent GPS stations are shown as black arrows, while rates at virtual points, taken 50 km on average close to the border of the blocks, are shown as white arrows. The black lines represent the generalized borders between the north-eastern, north-western and the south-western block, while the Alpine chain is taken as the border between the northeastern and the south-eastern block. White contour lines denote the national borders (after Tesauro et al., 2005). |
TOPO-EUROPE research will focus on quantifying the neotectonic deformation rates of the Armorican Massif by applying astronomical dating technologies to terrace systems, repeated precision levelling and SAR and InSAR measurements. Furthermore, geodetic data will be modelled that suggest that under the present NW-directed compressional stress field, reactivation of the Armorican shear zones is caused by the clockwise rotation of Northern France with respect to Southern France and Central Europe as a consequence of transtensional opening of the Rhine Graben system under the present NW-directed compressional stress field (Fig. 5; Cloetingh and Cornu, 2005b; Tesauro et al., 2005). It will be of special interest to assess when post-Mesozoic deformation of the Armorican Massif commenced and how it relates to the evolution of ECRIS and the inversion of the Western Approaches and Channel Basins and upwarping of the Weald-Arois Axis (Ziegler et al., 2002; Dèzes et al., 2004; Ziegler and Dèzes, 2007).