EUCOR-URGENT
1. Neotectonics and Seismic Hazard
1.5. Characterization of Pliocene-Quaternary landforms, sedimentological evolution, active faulting and
geomorphology
Sub-project coordinators:
Dr. T. Winter, BRGM
Prof. A. Wetzel, University of Basel
A major feature in the Plio-Pleistocene evolution of the Rhine Graben is the deviation of its principal
drainage system, from its Pliocene course across Burgundy into the Bresse Graben towards the Rhine
Graben proper (Villinger, 1998). The "Sundgau Schotter", of probably Pliocene age, which were deposited
by this palaeo-Rhine, are now located some 500 m above the Quaternary alluvial plain of the URG. The
cause of this fundamental change in the drainage pattern is unknown and could involve a number of
mechanisms such as tectonic uplift of the Burgundy sill (thermal or compressional lithospheric flexure)
and/or a lowering of the erosional base level of the URG due to breaching of the Rhenish Shield.
1.5a Surface geological and geomorphologic studies
Regional geomorphologic studies on terrace systems (geometry, inclination and age of terraces) and
rivers (valley/slope-distance analysis; stream gradient), based on air photography, satellite images and a
review of old maps, as well as isopach maps of the Plio-Quaternary graben fill (Simer and Théobald,
1970; European Program INTERREG, 1995), supported by sedimentological studies (transport direction,
depositional environment, analysis of grain size changes, distribution of wetlands), may be able to identify
zones of tectonic deformation and resulting changes in erosional base-level that had occurred during PlioPleistocene times. In this respect, accurate dating of the different terrace systems by biostratigraphic
and/or cosmogenic isotope methods is all important (see e.g. Grüger, 1996).
Geomorphologic analyses could also contribute towards discriminating between seismogenic and notseismogenic faults. Large Digital Elevation Models (DEM), coupled with satellite images, aerial photos
and surface geological surveys, could allow to identify and characterize active fault segments (kinematics,
surficial segmentation, slip-rates etc.). Such geomorphologic studies are presently being carried out by
the Institut de Physique du Globe (Strasbourg) in areas south of Mulhouse. Similar studies on terraces,
tracking the uplift of the Rhenish Shield, are in progress at the University of Bonn (H. Wuestefeld). Field
work addressing the neotectonics of the Jura chains near Ferrette may yield information on movements
related to the 1356 Basel earthquake. A major project on landform subsidence and uplift has been
initiated in 1997 in the Netherlands under the guidance of the Netherlands Research School of
Sedimentary Geology (Netherlands Environmental Earth System Dynamics Initiative [NEESDI]).
In order to assess the evidence for, as well as the pattern and the rate of Plio-Pleistocene deformation of
the Rhine Graben area, the morphological and sedimentological evolution of its drainage systems must
be assessed in an effort to evaluate the relative effects of endogenic (tectonic) and exogenic forcing (e.g.
climate) processes. This necessitates a geomorphostructural analysis of the landscape and a sequential
analysis of Plio-Pleistocene sediments.
Such studies will involve surface geological surveys, analysis of available borehole data, mapping of
isochronous surfaces (e.g. alluvial terraces), establishment of digital elevation models, acquisition of
geophysical data (radar, electric, reflection-seismic, see 1.5 below)), determination of the isotopic
signature of successive generations of deposits and their absolute age dating with established methods
and methods under development (C14, U/Th, Ar/Ar, U234/U238, cosmogenic isotopes [Be10], ERS,
optical simulation luminescence, thermoluminescence), diffusion models applied to recent erosive and/or
seismic scarps, and data available on climatic changes, both at a regional (palynology, isotopic signature)
and a global scale (d18O).
The pattern of active tectonic movements and their relation with active structures (geometry, kinematics),
as well as the establishment of deformation rates over various time spans, as derived from
geomorphologic and sedimentological studies, will be compared with regional precision levelling and
geodetic data sets, data on the present stress regime and with tectonic maps of the area (e.g. van den
Berg, 1994). The ultimate aim is to assess Plio-Quaternary deformation patterns in terms of a) the
geological structure of the area, and b) endogenic forcing mechanisms.
GéoFrance3D and the BRGM are able to make available satellite images covering the entire Rhine
Graben area (SOULTZ). The Universities of Strasbourg, Freiburg i.Br. and Basel are prepared to
contribute to this sub-project. For areas north of Karlsruhe, potential contributors have still to be identified.
The European Program INTERREG II will be approached for support.
The BRGM, the Universities of Strasbourg, Freiburg i.Br. and Basel and the Geologische Landesamt
Hessen and LGRB Baden-Württemberg participate in this sub-project.
1.5b Subsurface evidence for recent deformation and palaeoseismisity
The recently published compilation map, giving the thickness of Quaternary sediments in the Rhine
Graben (European Program INTERREG, 1995), is based on well data only and shows rapid lateral
thickness changes that are difficult to explain (incised fluvial channels and/or Quaternary faults). The
BRGM Région Alsace reported from shallow high-resolution reflection-seismic data evidence of faulting
extending into and through Quaternary deposits.
An on-going search for active faults in the URG is conducted under the EU project ENV4-CT97-0578
"Evaluation of the potential for large earthquakes in regions of present day low seismic activity in Europe".
The Swiss Earthquake Service cooperates in this project.
Recent seismic and/or a-seismic deformations cover a time scale in the range of 0 to 1.000.000 years. 3D
characterization of active deformation within the Rhine Graben requires the identification of markers,
recording tectonic movements over a time window of at least some 100.000 years. The spatial extension
of such markers, and their qualification as isochrones, should permit to assess
1.
the distribution of shallow deformation
2.
the geometry and kinematics of active structures at depth
3.
the dynamic segmentation of fault systems.
In view of this, consideration will be given to record, as a pilot project, a combined Sparker and Watergun
survey (both harmless to fish) on navigable canals and on the Rhine in an effort to determine the potential
role of young fault activity in the observed thickness variations of Quaternary deposits (for examples see
De Batist and Versteeg, 1999 ). If successful, additional land high-resolution shallow reflection-seismic
surveys may be considered for neotectonically active areas. In areas of proven Quaternary fault activity,
3D Georadar and electromagnetic (EM31/41) surveys may be of help to characterize the response of
fluvial systems to differential subsidence. Results of a recent 3D Georadar survey recorded by the
Kantonsgeologie Basel along the river Wiese are very encouraging.
Such a pilot project could possibly be carried out in conjunction with the European Program INTERREG
II. The BRGM sees as a first priority in geophysical investigations of selected area, such as across
regional and active faults having a morphological expression, presenting potential a seismic hazard
zones.
In addition, attention will be paid to indications of palaeo-seismic activity in an effort to evaluate the
recurrence time of strong earthquakes. To this end, the ETH-Zürich project "Earthquake Scenarios for
Switzerland" has recently recovered core material from the lakes Seewen (located 14 km S of Basel in
the Plateau Jura) and Bergsee (located 2 km N of Säckingen). This material is currently being analyzed.
So far, three correlative horizons of sediment disturbance are recognized that may be attributed to
palaeo-earthquake activity. In this context, an inventory of all natural lakes in the Vosges and Black
Forest area should be made and each lake evaluated in terms of its age and suitability for follow-up
drilling.
The ETH-Zürich, BRGM, the IPG University of Strasbourg, Kantonsgeologie Basel Stadt, the Universities
of Freiburg i.Br. and Basel and the LGBR Baden-Württemberg participate in this project.