Jean-Jacques Wagner

Ophiolites and volcanic activity near the western edge of the Arabian plate

Summary The geology of the Eastern Mediterranean is governed partly by the collision of the Arabian plate with the Anatolian block and partly by the Dead Sea Rift. The oceanic elements which have been formed between these two blocks are still visible as obducted fragments represented amongst others by the two ophiolitic massifs of Hatay and Baër-Bassit. The opening of the Atlantic ocean give rise, in the Eastern Mediterranean, to complex relative movements which can partly be resolved into a west to east shear and a south to north compression. The shear pre-dates the compression and took place between the Triassic and middle Cretaceous. It is responsible for the rifting and the volcanism in Syria, as well as the formation of a basin capable of producing ophiolites such as Hatay and Baër-Bassit. The compressive phase began in the middle Cretaceous and resulted in the detachment and the emplacement of the ophiolites on the Arabian continental margin. The timing of this phase can be determined from the age of the Baër-Bassit amphibolitic sole. Traces of this compressive episode are seen as late as the Neogene.

Tectonic history of the hatay ophiolites (South Turkey) and their relation with the dead sea rift

Abstract The Kizildag ophiolite massif lies in the Hatay region of S. Turkey. Some structural relationships in the area are examined and two geological sections through the ophiolite are presented. A sequence of structural events, dealing with the emplacement and post-emplacement history, is established. The significance of the events is discussed and it is suggested that the post-emplacement structural history was dominated by movements along the Dead Sea Rift.

Seismic Soil Effect in an Embanked Deep Alpine Valley: A Numerical Investigation of Two-Dimensional Resonance

This paper presents a 2D numerical study of the seismic response of a deep embanked sediment-filled valley (Rhone valley, Switzerland) to incident SH and SV waves. The 2D modeling is performed with a program developed by Pedersen et al. (1995), based on the IBEM technique. This valley exhibits variation in subsurface geometry. Therefore, three representative cross-sections are considered in order to investigate the influence of symmetric and asymmetric morphology on the seismic response. Comparison between 1D and 2D modeling shows that both resonant frequencies and amplification values differ to a large extent. Looking in the central part of the valley, the 2D predicted amplification value is about twice higher than in the 1D case, whereas the fundamental resonant frequency value is higher by a factor of 1.7. Moreover 2D modeling results show that this valley presents a twodimensional resonance due to its shape ratio and shear-wave velocity contrast. The main characteristic is the unchanged position of the fundamental resonance frequency, regardless of the surface site along the cross-section under consideration and the incidence angle. However, the asymmetric cross-section induces a specific spectral amplification pattern at the surface site and along the cross-section, which is also a function of the direction of incidence of the seismic wave. Such results show the importance of considering the morphology of a deep and narrow valley when investigating the local seismic response.

Toward Seismic Microzonation—2-D Modeling and Ambient Seismic Noise Measurements: The Case of an Embanked, Deep Alpine Valley

In the Sion area of Switzerland, part of a deep, embanked sediment-filled valley, investigations on soil site effects have been conducted using two independent methods. Two-dimensional (2-D) modeling was performed with a program based on the Indirect Boundary Element Method (Pedersen et. al. 1994). Numerical simulations allow taking into account the subsurface geometry of the valley and its peculiar characteristics, such as a variable shape ratio and a high shear-wave velocity contrast. The H/V method has been applied on ambient seismic noise measurements recorded on sites as close as possible to the 2-D modeling. This technique allows capturing the fundamental resonant frequency of the deepest sediments as well as identifying the existence of a surficial deposit. Both approaches agree on the fact that the fundamental resonant frequency of the valley is below 1 Hz. The amplification level of the predominant frequency obtained with numerical simulation is up to two times higher than the one given by the H/V ratio. These results provide the basis for further investigations in order to resolve differences.

The cover-basement contact beneath the Rawil axial depression (western Alps): true amplitude seismic processing, petrophysical properties, and modelling

Abstract Since 1986, several near-vertical seismic reflection profiles have been recorded in Switzerland in order to map the deep geologic structure of the Alps. One objective of this endeavour has been to determine the geometries of the autochthonous basement and of the external crystalline massifs, important elements for understanding the geodynamics of the Alpine orogeny. The PNR-20 seismic line W1, located in the Rawil depression of the western Swiss Alps, provides important information on this subject. It extends northward from the “Penninic front” across the Helvetic nappes to the Prealps. The crystalline massifs do not outcrop along this profile. Thus, the interpretation of “near-basement” reflections has to be constrained by down-dip projections of surface geology, “true amplitude” processing, rock physical property studies and modelling. 3-D seismic modelling has been used to evaluate the seismic response of two alternative down-dip projection models. To constrain the interpretation in the southern part of the profile, “true amplitude” processing has provided information on the strength of the reflections. Density and velocity measurements on core samples collected up-dip from the region of the seismic line have been used to evaluate reflection coefficients of typical lithologic boundaries in the region. The cover-basement contact itself is not a source of strong reflections, but strong reflections arise from within the overlaying metasedimentary cover sequence, allowing the geometry of the top of the basement to be determined on the basis of “near-basement” reflections. The front of the external crystalline massifs is shown to extend beneath the Prealps, about 6 km north of the expected position. A 2-D model whose seismic response shows reflection patterns very similar to the observed is proposed.