Previous Next Tübinger Geowissenschaftliche Arbeiten, Series A, Vol. 52, p. 18.
Abstracts of the 4th Workshop on Alpine Geological Studies, Tübingen 21-24 Sept. 1999

N039

Significance of syn-convergence extension in the Western Alps revealed by fission track analysis and geophysical modelling

Stéphane Schwartz* 1, Jean-Marc Lardeaux 1, Anne Paul 2, Pierre Tricart 3, Gérard Poupeau 3


 1 

Laboratoire Dynamique de la Lithosphère-CNRS/UCB & ENS Lyon, France

 2 

Laboratoire de Géophysique Interne et Téctonophysique, Grenoble, France

 3 

Laboratoire de Géodynamique des Chaînes Alpines CNRS, Grenoble, France

 * 

Correspondence:  F-69622 Villeurbanne, France (schwartz@univ-lyon1.fr)

 

A multidisciplinary study was investigated in the framework of "Geo-France 3D Alps project" on a key area corresponding of the Piemontais Schistes lustrés zone of the Cottian Alps in order to obtain a 3D representation of an active collision belt.

The combination of structural analysis, field investigation and satellite image interpretations allows to characterise the recent strain pattern which corresponds to an important syn-convergence extensional tectonics. Furthermore the new thermochronological data revealed by fission track on zircon closure temperature around 270C and apatite around 100C show a westward diachronism during cooling of the tectonic pile :

-In the Queyras, zircon ages range from 26.91.5 Ma to 21.91.2 Ma and apatite range from 22.31.6 to 9.41.1 Ma

-In the eclogitic ophiolite (Monviso) the ages are 19.60.8 Ma on zircon and 8.61.7 Ma on apatite.

This diachronism can be linked with the observed extensional tectonics. These combined results suggest that the external units were earlier exhumed than the most internal units (Monviso area) in a denudation/exhumation regime.

We present and discuss a crustal-scale cross section constrained by structural and seismic tomography study, where extensional tectonics corresponds to the accommodation of vertical extrusion related to mantle indentation. The mantle indenter acts like a rigid piston pushing crustal units towards the earth surface during convergence, inducing the progressive development of both inverse and normal faults.