USING STRUCTURAL THERMOCHRONOLOGY TO RECONSTRUCT BACKFOLDING IN THE SWISS ALPS

A new interpretational method takes advantage of geometric relationships among zircon fission track ages and allows direct dating of the formation and propagation of the retroshear zone of the Swiss Alps. The retroshear zone is defined by a 20 km wide, south-vergent “backfold", formed as part of the doubly-vergent structure of the orogen. Our "structural thermochronologic" analysis relies on the concept of the isochrone, which is a surface in the earth that everywhere has the same cooling age. Isochrone surfaces are formed as rocks pass through the closure isotherm. They tend to have very gentle to flat initial dips, and they can be used, like bedded stratigraphy, to track deformation. The age gradient across uniformly dipping isochrones provides a measure of exhumation rate in datasets where typical age-elevation relationships are scattered. In the Alps, isochrone-derived exhumation rates increase more than fourfold at 16 Ma, from 0.25 km/Ma to ~ 1.0 km/Ma. Isochrones dip uniformly above and below the increase, at about 10 degrees toward the southeast. The change in exhumation rate corresponds generally to the Insubric line. Where exhumation rates are high, to the north of the Insubric line, cooling ages are Miocene, while exhumation rates to the south are low and cooling ages are Cretaceous to Triassic. We use a kinematic folding model to show that the change in exhumation rates and the uniform southward dips are both a result of steady southward propagation of the Alpine backfold.