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 270°C and apatite around 100°C show a westward diachronism during cooling of the tectonic pile :

-In the Queyras, zircon ages range from 26.9±1.5 Ma to 21.9±1.2 Ma and apatite range from 22.3±1.6 to 9.4±1.1 Ma

-In the eclogitic ophiolite (Monviso) the ages are 19.6±0.8 Ma on zircon and 8.6±1.7 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.