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

Abstract: 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 for deformed Austroalpine basement (Eastern Switzerland) shed light on 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 groups. 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 waning 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 upper plate during subduction. We propose that underplating of the Middle Penninic micro-continent at c . 50 Ma led to the cessation of deformation within the South Penninic melange, and shifted the zone of active deformation into the footwall. This also caused a contemporaneous upper plate surface uplift and shutoff of sedimentation in Alpine Gosau-type forearc basins.