Socle varisque et tectoniques rhénanes dans le Fossé rhénan supérieur méridional : traitement et interprétation de la carte gravimétrique du fossé à partir du levé haute densité des MDPA

Abstract

The southern Upper Rhine Graben (Fig. 1) and its Paleozoic environment were affected by two main tectonic events: 1) Variscan plate collision during the Late Devonian and Early Carboniferous, and 2) graben formation cutting the European plate from the North Sea to the western Mediterranean during the Tertiary. The southern Upper Rhine Graben is also known for its recent seismic activity; in 1356 a strong earthquake destroyed the city of Basle and, more recently in 1980, an earthquake with a magnitude of 4.8 occurred in the region of Sierentz (Rouland et al., 1980) (Fig. 2). Bouguer anomalies mainly reflect the thickness variations of the sedimentary cover and density contrasts due to different petrographic units in the upper basement (Edel, 1982; Edel and Fluck, 1989; Rousset et al., 1993). Processing of a detailed gravimetric map, therefore, can provide information both on the basement and on the tectonism related to basin formation. A very dense data set was measured in the Rhine Graben by the Mines Domaniales de Potasse d'Alsace (MDPA) in the late 1940s. Although not yet interpreted quantitatively, these data have been exploited in order to 1) demonstrate that a high-density map can provide relevant information on fine structures in the upper crust, 2) specify the nature and structures of the Paleozoic basement beneath the sedimentary cover, and 3) identify and locate faults related to Eocene-Oligocene graben formation and post Late Miocene NW-SE compression. Two gravimetric maps were compiled; one covering the graben and based on the MDPA high-density data, and the other covering a larger area extending to the Vosges and the Black Forest graben borders and including data from various sources (Rousset, 1992; SEISMOS ref; etc.) (Figs. 2, 3, 4). In order to study density distribution in the Paleozoic basement, the gravimetric effect of the Mesozoic and Cenozoic sediments was computed and removed from the Bouguer anomalies (Lutz, 1999) (Fig. 5). For this purpose, isopach maps were established and digitized, and the effect of salt diapirs was modelled using parallelepipeds. The effect of the Moho topography (Edel et al., 1975) is only visible in the southern part of the map and is removed by filtering “the long wavelengths”. The new map (Fig. 6) shows some significant differences compared to the previous one (Fig. 4). A positive anomaly appears beneath the Dannemarie basin. The N060°-striking negative anomaly centred on the Mulhouse horst is emphasized and separates a narrow, N060°-striking positive anomaly from a larger one with an apparent E-W direction. Despite removal of the effects of the low density Cenozoic sediments, a negative anomaly persists beneath the potash basin. Interpretation of the corrected map in terms of Paleozoic structures and discontinuities is easier after computing first- and second-order vertical derivatives and downward continuations. On the vertical gradient map (Fig. 7), the negative anomalies can be assigned mainly to granite and the positive anomalies to a Devonian-Dinantian volcano-sedimentary succession that crops out in the southern Vosges basin and the Badenweiler-Lenzkirch zone (ZBL, Fig. 2) (Edel and Fluck, 1989). The discontinuities represent either faults or contacts between units with contrasting densities. By also using the magnetic properties derived from the magnetic map of Spreux (1975), i.e. reduced-to-the-pole and vertically derived, it was possible to draw a geological sketch map of the Paleozoic basement (Fig. 9). This map shows that the Late Devonian to Late Visean WNW-ESE volcano-sedimentary and plutonic units of the southern Vosges continue beneath the graben sediments towards the southern Black Forest. Most of the magnetic rocks show low to intermediate densities and are interpreted as monzonitic and dioritic bodies, comparable to those that crop out around the Ballons pluton (Edel and Lossy, 1975). The WNW-ESE-striking structures are cut and offset by NNE-SSW faults parallel to the Ste-Marie-aux-Mines and Hunsrück-Kohlschlag faults (FHK, Fig. 9), which were active as sinistral strike-slip faults in Late Visean times (Fluck et al., 1989; Edel and Fluck, 1989). The major offset occurred west of the Mulhouse horst. According to the previously available gravimetric data, the 10-km-wide wrench zone extends for about 200 km northward, as far as the Odenwald (Rousset et al., 1994; Edel and Weber, 1995). The major N030-035° basement faults were reactivated as normal faults during graben formation and strongly influenced sedimentation. This was particularly the case during the NNE-SSW Eocene compression and, to a lesser degree, during the W-E extension (Bergerat, 1985) (Fig. 11). In addition to density distribution within the basement, the Bouguer map provides information on the Tertiary sedimentary cover and on the faults active during the Late Tertiary to recent tectonism. By filtering the long wavelengths, through vertical derivation or downward continuation, the effects of faults and density variation in the uppermost crust are emphasized. After removing wavelengths of over 4 km, the residual anomalies, along with the first- and second-order vertical gradient maps, were used to plot gravimetric discontinuities representing faults and density contrasts within the uppermost crust (Fig. 10). The anomalies with the highest intensities and shortest wavelengths display predominantly N0-010° trends followed, in decreasing order, by N145-150°, N055-060° and N095-100° trends. These directions reflect the stress regime affecting the southern Upper Rhine Graben since the latest Miocene and coincide with the fault directions derived from focal mechanisms obtained over the last few decades (Rouland et al., 1980; Bonjer et al., 1984; Bonjer, 1997). Within the potash basin, the prominent negative anomalies are due to salt diapirs. A second family of anomalies with slightly longer wavelengths and trending N015-020° and N060-070° can be distinguished. Most coincide with the western and northern flanks of the Mulhouse horst, and others with the Oligocene faults observed on seismic sections. This N015-020° direction coincides with the main direction of the southern Upper Rhine Graben resulting from Oligocene WNW-SSE to W-E extension (Bergerat, 1985) (Figs. 10, 11). Owing to their strike, certain of the mapped discontinuities are faults susceptible to respond to the present stress regime. For the Sierentz earthquake in 1980, the interpretation of the gravimetric maps shows that the main quake was due to reactivation of a Variscan N030-035° strike-slip fault, while the aftershocks involved fractures reflecting the present stress regime (Fig. 11). This type of interpretation shows that a detailed gravimetric map is a useful tool for locating potential seismic faults and constraining seismic risks

Key words: Gravity survey maps, Basement, Faults, Strike-slip faults variscan orogeny, Rifting, Neotectonics, Bas-Rhin France, Swiss Rhine Graben, Upper Rhine Graben.