Unit of Earth Sciences
Department of Geosciences | University of Fribourg
PREALPES
Created 09 May 1996; last updated: 2 April 2004
This page has been created by Jon Mosar. Jon.Mosar@unifr.ch
Introduction to the geology of the Préalpes
You wonder what and where the Préalpes are? The following information will, hopefully, help you solve this "problem". You will find many links to abstracts and brief discussion pages with pictures, maps, cross-sections and graphs on the different aspects of the Préalpes geology.
Simplified structural map of the western Alps in Europe, with location of the main parts of the allochthonous Préalpes klippen in front to the NW of the Helvetic nappes of Switzerland and France.
A large part of the research presented here has been done with the financial support of the Fonds national de la recherche scientifique Suisse (project numbers: 2.837-0.85; 2.686-0.82; 21-29011.90; 20-33450.92; .... ) at the universities of Fribourg, Lausanne andNeuchâtel. Other Swiss universities that have been extensively working in the Préalpes are Bern, Geneva and to some lesser extent Zürich. The Préalpes have been studied for more than 150 years and have witnessed the birth of many concepts of modern geology such as nappe tectonics and flysch, to name but two. Many of the research published and unpublished have been put together in a bibliographic data base containing some 910 references.
A brief introduction to the regional geology of the Préalpes may help you to a quickstart. Also the results presented in the following mainly concern the most important of the Préalpes tectonic nappes, namly the Préalpes Médianes.
To make a long story short or from tectonic plate to tectonic klippen
Unlike most of the Pennine units the Préalpes Médianes succeeded in escaping an untimely reshaping (overheating and awful deformation) in the accretionary prism of the closing alpin Tethys ocean, due to changes in the plate tectonic regime and the subsequent changes in thrust dynamics. Since then they enjoyed the unsustainable lightness of thin-skinned tectonics and have been surfing the frontal "wave" of the alpine orogeny.
The domains of the Préalpes- mainly Préalpes Médianes - geology presented here.
The Préalpes nappe stack rests on top of the subalpine flysch and the Molasse as tectonic klippen in front of the Helvetic nappes and the external crystalline basement massifs. They have been detached from their pennine homeland and transported to the N-NW to the front of the alpine mountain belt.
Structural Geology
The Préalpes exhibit the typical features of a foreland fold-and-thrust belt. Detached above a basal décollement in Triassic evaporites the Préalpes Médianes have developed features such as: imbrications in the frontal part of the arcute mountain belt; tear faults, backthrusts and en échelon relay structures. Fault related fold such as fault-bend, fault-propagation, lift-off and décollement folds are common. Internal deformation
Rocks have suffered only week deformation in the brittle domain. Pressure solution, weak rock cleavage, calcite twinning and cataclasites associated with faults are common. The frontal Préalpes Médianes plastiques underwent only week to almost no internal deformation, while the trailing Préalpes Médianes rigides suffered stronger, more "ductile" deformation. Cornieules are very special breccia-type rocks commonly found in the Préalpes and the alpine nappes in generall Metamorphism, isotopes (ages,temperatures)
The Préalpes sedimentary series have suffered very low-grade metamorphism ranging from diagenetic to epizonal conditions (100-350°C). This metamorphism is discontinuous and transported. Its development is mainly due to tectonic burial in the accretionary prism of the closing Tethyan ocean. Sedimentation - Paleo-tectonics
The sedimentary record of the Préalpes médianes, mainly made of limestones and marls, ranges from Triassic to Cretaceous - Tertiary in age. Many gaps (either erosional and/or depositional) exist, both in the Médianes Plastiques and the Médiane Rigides. Deposited on the S edge of the european platform during Triassic, the Préalpes médianes sediments are infilled, since Liassic, in a passive margin rim basin of the Briançonnais microcontinent. Tertiary flysch sediments deposited in the active margin environment of the closing Tethys ocean are the youngest sediments of the Préalpes médianes. Plate tectonics
The Préalpes sedimentation domain is located at the N edge of the alpine tethys ocean until Late Triassic. Since the Briançonnais s.l. (including the Préalpes) separated from mainland Europe with the opening of the Valais ocean, it evolved with the Iberic plate. Once separated from this plate in Late Cretaceous times, the Brianonnais s.l. forms an isolated tarrane which is finally incorporated in the accretionary prism of the closing Tethys ocean. Historical - Bibliography
Historical accounts of the evolution of the Préalpes geology and the many important geological discoveries can be found in Bailey (1935), Jeannet (1912) and Masson (1976). A rather complete list of references of published and unpublished papers and research work as well as a brief discussion of publication dates and places may help understand the historical evolution of the Préalpes Geology. The refernce list may also be a helpfull tool for people working on the Préalpes. Have also a look at some of the most prominent personalities of the Préalpes Geology: Hans Schardt, Maurice Lugeon, Alphonse Jeannet.
