In metamorphic areas, paleogeothermal gradients are difficult to properly estimate, leading to great uncertainty in exhumation rate calculations based on fission track analysis. In this work, a new multidisciplinary approach based on fluid inclusion analysis, structural analysis and fission track dating has proven to be able to constrain the late stages of exhumation of metamorphic rocks. The study area is located in the Sesia-Lanzo unit of the Western European Alps, along the Hone shear zone. Kinematic analysis at different scales unraveled a polyphase postmetamorphic deformation history, constrained in the P-T space thanks to laboratory investigation of kinematically referenced fluid inclusions coupled with the analysis of the theological behavior of quartz and feldspar inside syn-kinematic veins. The resulting P-T path points to an increasing paleogeothermal gradient from 18 to 30 degrees C km(-1) after greenschist facies metamorphic conditions, and provides a reference framework for a quantitative interpretation of fission track data in terms of exhumation rates and fault throws. According to fission track data, throws accommodated by the Hone shear zone between 33 and 30 Ma are on the order of 4.5 km, and would be underestimated by similar to 25% if a "normal" steady gradient was assumed for the whole post-metamorphic history of the area. The deepest portions of this structure were exhumed thanks to the activity of the E-W fault system that accommodated a throw of similar to 4 km between 28 and 20 Ma, and a throw of similar to 0.8 km during the last 20 Myr. (c) 2006 Elsevier B.V. All rights reserved.
Univ Turin, Dipartimento Sci Terra, CNR, Inst Geosci & Earth Resources, I-10123 Turin, Italy; Inst Phys Globe, Lab Geosci Marines, F-75252 Paris 05, France; Univ Bologna, Dept Earth Sci & Environm Geol, I-40127 Bologna, Italy; Univ Insubria, Dept Phys Chem & Math, I-22100 Como, ItalyArticleEnglish