Abandonment of the South Penninic-Austroalpine palaeosubduction zone, Central Alps, and shift from subduction erosion to accretion: constraints from Rb/Sr geochronology

We present new age data for the evolution of the Suture zone between lower plate South Penninic and upper plate Austroalpine units in the Central European Alps. Rb/Sr deformation ages for mylonitized rocks of the South Penninic palaeosubduction melange and tor deformed Austroalpine basement (Eastern Switzerland) shed light oil the pre-Alpine and Alpine deformation history along the suture, as well as on synsubduction interplate mass transfer. Rb/Sr age data define two age group,, The first group reflects pre-Alpine events within the upper plate basement, with varying degree of resetting by Subsequent Alpine overprints. The second group marks the waiting of subduction-related deformation along the South Penninic Austroalpine Suture Zone, at around 50 Ma, and termination at c. 47 Ma. Identical Rb/Sr ages for pervasively deformed Austroalpine and South Penninic lithologies point to tectonic erosion of the tipper plate during subduction. We propose that underplating or the Middle Penninic micro-continent at c. 50 Ma led to the cessation of deformation within the South Penninic melange, and shifted file zone of active deformation into the footwall. This also caused I Contemporaneous upper plate surface uplift and shutoff of sedimentation in Alpine Gosau-type forearc basins.