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Region II:
The Upper Rhine Graben System
(Alsace and Basel area)

1. Introduction

Geological Setting

Modern and historical Seismicity

2. Methodological approach

3. Identfying trench sites

4.Regional effects of large paleo- earthquakes

5. Upcoming investigations

Authors references

Regional references

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PALEOSIS  
1. Introduction   IPSN (Institute for Protection and Nuclear Safety)

(M. Cushing, F. Lemeille)

1.1. Geological Setting

As the Lower Rhine Graben (LGR), the Upper Rhine Graben (URG) is a branch of the Rhine graben system which belongs to the reactivated Cenozoic rift in western Europe. The interaction between the rift system and the compressive foreland of the Alps (Jura belt) and the direction of inherited tardi-hercynian structures make the tectonic regime of this domain more transtensive or transpressive than the LRG. Relief of the rift borders is still important and the Moho topography shows an important rise (up to 24 km) under the Kaiserstul region. Seismic (historical and instrumental) activity is an evidence of present day tectonic activity.

The quaternary faults and associated morphology along the flanks of the graben, and the values ranging from 0.8 mm/yr to 2 mm/yr vertical rate of deformation obtained from the comparison of levelings during the last 100 years (Fourniguet 1987, Mälzer et al., 1983, Chorowitz et al.1989) combined with the seismic activity are the most significant elements for recent and present-day crustal deformation. Looking at quaternary thickness map (fig), one could conclude to subsidence rates in a range from 0.02 mm/y to 0.2 mm/y. The main subsiding zones (Bartz, 1972) are near Manheim (350 m quaternary thickness), between Strasbourg and Offenburg (150 m) and East of Colmar (>225 m). This last zone is complicated by diapirism movements. Considering the thickness of tertiary sediments, similar values are obtained (Manheim area, 3300 since lower oligocene : 0,1 mm/y, this subsidence axis goes to the south under Strasbourg city - 1500 m). South of Strasbourg, the subsidence axis crosses the graben to border of the Vosges massif. Rates determined by geodetic measurements are one order of magnitude higher than geological ones.

Tertiary and Quaternary thickness maps indicate most favourables zones of highest rate of subsidence for recent period.

Based on the main geological structures and Cenozoic subsidence data, the Upper Rhine graben can be divided into several tectonic units:

  1. Borders faults (Cenozoic fault system) limiting Vosges massif to the West and the Schwartzwald to the East. The Saverne Area shows a large indentation of this fault system (the "champs de fracture de Saverne"). The graben is assymetric; to the north, the subsiding zone is located against the eastern fault system (Karlsruhe-Manheim area), to the south, the most subsiding zone is located against the Vosges border faults (Colmar-Thann area).
  2. The inner graben fault system with a presupposed recent activity (from geomorphological studies ; Vogt ). Important diapyrism occur in the south of this zone, certainly interfearing with active tectonic.
  3. The Mulhouse-basel transition zone (between transtensive tectonic to the north and transpressive to the south) wich is characterized by large ondulation of topography and thin quaternary deposits.
  4. The Jura (thrust) front organized in multiple E-W active quaternary folds (Meyer et al, 1994, Report IPSN-EOPGS 97, Report GSC 97) separated by N020 transfer left lateral ramps.

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Seismotectonic map of the Upper Rhine Graben System.