Orogen‐Parallel Migration of Exhumation in the Eastern Aar Massif Revealed by Low‐T Thermochronometry

Abstract New and published (U‐Th)/He data on zircon, apatite, and zircon fission track ages constrain the thermal overprint and cooling history of the eastern Aar Massif, Switzerland. The timing and pattern of cooling is in agreement with independent kinematic and age constraints from exposed shear zones. This suggests that the cooling ages mainly reflect exhumation and that long‐term exhumation‐dynamics were mainly controlled by crustal‐scale tectonic processes. Results of a statistical inverse model reveal significant diachrony in the timing of exhumation in the along‐strike direction. Maximum exhumation rates ( 1 mm/yr) were initially located in the central Aar Massif (from 22 to 10 Ma), then gradually migrated to the east between 10 Ma and present, while the central Aar Massif continued to exhume at slower rates ( 0.5 mm/yr). The diachrony in the timing of exhumation may be explained by lateral variations in the inherited thickness or the density of the accreted European crust. We attribute the increase in exhumation rates between 2 Ma and present to enhanced glacial erosion. Nevertheless, the post 2 Ma exhumation pattern reflects a continuation of noncylindrical massif “growth” in the eastward orogen‐parallel direction. This indicates that—although at slow rates—thick‐skinned and buoyancy‐driven compressional deformation, likely enhanced by the presence of easily erodible flysch units at the surface, might still be ongoing especially in the eastern Aar Massif. Noncylindrical massif‐growth is likely to also affect other External Crystalline Massifs or orogens, but may be overlooked because studies often focus on single orogen‐perpendicular transects.