Deciphering the 4-D evolution along and across the Insubric Line by thermochronology

The Lepontine dome, the Insubric Line, and the adjacent Southern European Alps are an archetypal example of indentation tectonics. The late-stage collision led to a highly asymmetric exhumation pattern with vertical displacement across the fault line in the range of 15 (±5) km. Exploring the faulting and exhumation history of the suture between European and Adriatic plates by applying multiple thermochronometers on both sides of the fault is the main goal of this project. Thermochronological results are embedded into paleostress analyses as well as the attempt to date brittle fault structures by U-Pb means.
(U-Th)/He dating on 44 apatite and 28 zircon samples was applied to densely-spaced horizontal as well as vertical transects across and adjacent to the Insubric Line. AHe ages N of the fault line prominently cluster around 8-12 Ma, pointing towards more recent tectonic activity than previously assumed. The ZHe age pattern is more dispersed with unexpectedly old ages of around 50 Ma from 10 samples along a vertical profile spanning 2000 m just N of the Insubric Line. As these ages from highly-sheared samples are older than prominent ductile backthrusting and backfolding between ca. 35 and 30 Ma, we tentatively suggest a slow-down of He mobility due to crystal imperfections acting as He traps. To verify this pattern we currently (1) carry out U-Pb apatite dating on the same samples, (2) complement this profile by apatite and zircon fission track dating and (3) add data from a second parallel profile for comparison.