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Images of Lithospheric Heterogeneities in the Armorican Segment of the Hercynian Range in France

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Images of Lithospheric Heterogeneities in the Armorican Segment of the Hercynian Range in France Tectonophysics 358 (2002) 121–134 www.elsevier.com/locate/tecto Images of lithospheric heterogeneities in the Armorican segment of the Hercynian Range in France S. Judenherca,*, M. Graneta, J.-P. Brunb, G. Poupinetc, J. Plomerova´ d, A. Mocquete, U. Achauera a Ecole et Observatoire des Sciences de la Terre, UMR 7516, Strasbourg, France b Ge´osciences Rennes UPR 4661, Rennes, France c Laboratoire de Ge´ophysique Interne et de Tectonique, UMR 5559, Grenoble, France d Geophysical Institute, Czech Academy of Sciences, Prague, Czech Republic e Laboratoire de Plane´tologie et de Ge´ophysique, FRE-CNRS 2129, Nantes, France Received 4 September 2000; received in revised form 4 April 2001; accepted 15 June 2002 Abstract The Armorican Massif is located in western France. It is a part of the Hercynian Range formed in several phases of SE–NW compression from Devonian to Carboniferous time (ca. 400–300 Ma). The main tectonic features are the North and South Armorican shear zones with WNW–ESE strike. In the framework of the Ge´oFrance3D program, we have installed 35 seismological stations along two NS lines crossing the main Hercynian WNW–ESE fault systems. The data set collected during the experiment allows us to compute a 3D P-velocity model and to map S-wave seismic anisotropy at depth using teleseismic shear waves splitting measurements. The South Armorican shear zone (SASZ) is characterized by a strong (4–5%) velocity contrast and a fast shear wave azimuth parallel to its strike. The North Armorican shear zone (NASZ) does not show any significant lithospheric signature. The tomographic image and anisotropy results suggest that the Armorican lithosphere consists of the juxtaposition of distinct lithospheric blocks assembled in a subduction type process predating the Hercynian Orogeny. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Seismic tomography; Seismic anisotropy; Continental collision; Hercynian lithosphere 1. Introduction such as the Himalayas or, in Europe, the Alps or the Pyrenees. The geophysical imaging of these active Formation of mountain ranges is one of the most regions often shows that not only the crust but the spectacular effects of plate tectonic. Thus, it is not sur- whole lithosphere is involved in the mountain building. prising to see many Earth science studies, in particular, This paper focuses on the Armorican Massif (north- in the field of geophysics, focusing on active structures western France), which is a preserved segment of the Hercynian Orogeny (400–250 Ma). The main question arising when considering this old and presently stable * Corresponding author. Dipartimento di Scienza Fisiche, Com- plesso Universitario Monte S. Angelo, Via Cintia, Naples 80125, region concerns the present-day structure of the litho- Italy. sphere. Does the lithosphere beneath a 250-Ma-old E-mail address: [email protected] (S. Judenherc). mountain range still keep the signature of the related 0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S0040-1951(02)00420-1 122 S. Judenherc et al. / Tectonophysics 358 (2002) 121–134 geodynamic events? Do the tectonic features, such as two major strike–slip fault zones of Carboniferous age fault systems, involve the whole lithosphere? (c 350–300 Ma). The Hercynian Range is a Paleozoic continental (1) The North Armorican Domain (NAD) is dom- collision range. It extends in western Europe from inantly constituted of late Precambrian age formations southern Spain to the Bohemian Massif. In France, strongly deformed and metamorphosed during the the main remnants include the Armorican Massif, the Cadomian Orogeny (650–540 Ma) (Cogne´ and Massif Central and the Vosges Mountains (Matte, Wright, 1980). (2) The Central Armorican Domain 1986). The Hercynian Range was built from the con- (CAD) is separated from the NAD by the North vergent motion of two continental blocks: Gondwana Armorican Shear Zone (NASZ) and is bounded to to the South and Laurasia to the North. The present-day the south by the South Armorican Shear Zone (SASZ). Armorican Massif was located on the southern border The CAD is mainly constituted of Paleozoic sediments of Laurasia and was bounded to the south by the lying unconformably on Brioverian sediments, rang- northward subduction of oceanic lithosphere (Matte, ing in age from Neoproterozoic to Cambrian (c 600 1986; Burg et al., 1987). At the beginning of Devonian Ma) (Brun et al., 1991). (3) The South Armorican ( c 400 Ma), the closing of this oceanic basin lead to Domain (SAD) mainly consists of metamorphic and the continental collision. The latest events related to the plutonic rocks that have undergone an intermediate to collision are of Permian to Triassic age (250–200 Ma). high-pressure metamorphism associated with crustal thickening during Devonian and Carboniferous (400– 1.1. Geological settings 300 Ma). The Paleozoic formations of the CAD are only af- The Armorican Massif (Fig. 1) is organized into fected by anchizonal to epizonal metamorphism indi- three E–W-trending elongated domains separated by cating very shallow burial (Le Corre, 1977). Together Fig. 1. Map of the main Hercynian features of the Armorican Massif. The shaded area in the inset represents the approximate extension of the European Variscides (simplified after Matte, 1986). The locations of the temporary stations installed during the ARMOR2 experiment (1997) and the three short-period permanent stations (SGMF, GRR, and QUIF) used for the normalization process are shown. The legend at the lower left corner indicates the instrument type (1C: vertical sensor, 3C: three-component sensor), the eigenperiod of the instrument (BB stands for broad band) and the seismological recorder type (trig.: triggered station, cont.: continuous recording). S. Judenherc et al. / Tectonophysics 358 (2002) 121–134 123 with the Brioverian sediments, the Paleozoic forma- high beneath the Armorican Massif, ranging from 8.15 tions are affected by a single event of deformation to 8.23 km/s (Sapin, 1973; Hirn, 1977). Comparable during the Carboniferous characterized by long-wave- Pn velocities were obtained by Judenherc et al. (1999) length and low-amplitude folding across the whole using data from the permanent seismic network sta- CAD associated to a vertical slaty cleavage and a nearly tions. This analysis of the anisotropic tomography also horizontal stretching lineation. This deformation pat- shows that the Pn velocity is higher when the waves tern results from dextral simple shear related to strike– propagate parallel to the NASZ in the NAD and slip displacement along the SASZ (Gapais and Le northern CAD and parallel to the SASZ in the SAD. Corre, 1980). Dating of synkinematic emplacement At greater depth, a detailed study of P arrival times and and mylonitisation of leucogranites (Vidal et al., shear wave polarization from active-source experi- 1984) along the SASZ gives an age of late Devon- ments led Hirn (1977) to propose the existence of an ian–early Carboniferous for the dextral simple shear of anisotropic layer between 60 and 100 km beneath the CAD. It is especially important to note that the basal central Brittany. In summary, one can state that the envelope of unmetamorphosed Ordovician formations knowledge of the deep structure of this region is (c 450 Ma) remains nearly horizontal across the whole limited, in particular, the deep extension of the Hercy- CAD. The above arguments indicate that the sediments nian tectonic features is still an open question. Thus, of the CAD have not been affected by thrusting and the main goal of this study is to characterize the consequently suggest that the CAD could have escaped Hercynian lithospheric seismic structure at depth and significant crustal thickening during the Hercynian to image the deep geometry of the two main Hercynian continental collision. This is in contrast to the presence shear zones. Both would put new constraints on the of eclogitic rocks immediately south of the CAD long-term evolution of this mountain range. between the two branches of the SASZ (Fig. 1). Since the end of the Hercynian Orogeny, the massif has recorded a series of discrete second-order tectonic 2. The seismological study events related to the opening of the North Atlantic during the early Cretaceous and to continental exten- Tomographic images contain information about the sion during the Oligocene. Recent tectonics is moder- mineralogy and thermal state of the study area (e.g. ate; it is characterized by diffuse seismicity with Sobolev et al., 1996). These images reveal the deep earthquake magnitudes rarely exceeding 4.0, and 0.5 geometry of the geological units observed at the sur- to 1.1 mm/year of uplift of the northwestern border of face with a wavelength (c 10 km) compatible with the the Armorican Massif as demonstrated by river inci- lithospheric structural heterogeneities. Moreover, seis- sion during the Quaternary (Bonnet et al., 1998) and mic anisotropy studies provide information on the leveling measurements (Lenoˆtre et al., 1999). Other- deformation pattern and large-scale fabric at depth wise, no major tectono-thermal events have occurred (Babusˇka and Cara, 1991; Vinnik et al., 1992; Silver, in the past 250 My. 1996). In addition, it provides better constraints on the tectonic history of the study area. 1.2. Geophysical settings In order to map the lateral seismic velocity and seismic anisotropy variations beneath the Armorican Geophysical knowledge of the upper mantle of the Massif, a temporary seismic network was operated Armorican Massif is rather poor. Several active-source from March to December 1997 (Fig. 1). The geometry seismic sounding experiments have been run in north- of the network was designed to record teleseismic P- ern and central Brittany (e.g. Bitri et al., 1996) that all and S-waves across the NASZ and SASZ. The array show a quite flat Moho (average depth of 31–32 km) consisted of two 150-km-long N–S-oriented, nearly beneath the massif and, in particular, across the North parallel profiles, with a station spacing of about 10 km Armorican shear zone.
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