Asymmetric extension and subsequent unroofing of the Pohorje pluton at the western margin of the Pannonian basin, Slovenia.

Abstract

The Pohorje pluton, an Early Miocene tonalitic to granodioritic intrusion close to the transition of the eastern Alps to the Pannonian Basin in northern Slovenia underwent rapid exhumation from mid-crustal levels to the surface within 3 Ma. The timing of crystallization as well as of the exhumation path are well constrained by various isotope systems. However, the mechanism by which the Pohorje Pluton was exhumed is still unknown. In this study we use structural fieldwork and the analysis of microstructures to shed light on the processes, which led to the exhumation of the Pohorje Mountains in the frame of Miocene Alps-Adria convergence and coeval Pannonian Basin extension. The new field data reveal three stages of deformation: D1 is characterized by isoclinally folded quartz veins and a penetrative foliation (S1), D2 by recumbent, tight folds, with axial planar foliation (S2) and are interpreted to represent nappe stacking and associated metamorphism within Austroalpine units surrounding the Pohorje pluton. D3 also affects the pluton and thus constrains the Early Miocene age of deformation. D3 is associated with the formation of E‐W oriented stretching lineations, and top‐to-the-East sense of shear within mylonitic zones under retrograde, lower greenschist metamorphic conditions. D3 deformation localized at former nappe contacts and led to the omission of part of the metamorphic succession. This asymmetric East‐directed shear suggests exhumation of the pluton along an east‐dipping low‐angle detachment, which also provides an explanation for the differential exhumation (higher in the east) of the pluton. Detachment faulting was coeval with N‐S shortening probably contributing to the doming of the structure as well as to folding and tilting of adjacent Lower Miocene sediments, which was amplified by later convergence during the Pliocene to Quarternary. The style of deformation is similar to other regions at the transition of the Alps to the Pannonian basins suggesting that footwall exhumation along low‐angle detachments is strongly linked to the Pannonian Basin extension.