Geochronologic, petrologic and kinematic constraints on the evolution of the Err-Platta boundary, part of a fossil continent-ocean suture in the Alps (eastern Switzerland)

Abstract

The boundary between the Austroalpine Err nappe and the Penninic Platta unit represents part of a Late Cretaceous continent-ocean suture preserved in the highest stockwerk of the Tertiary Alpine orogen. Subduction and nappe stacking (D1) and subsequent extensional uplift (D2) along part of this suture are dated with the K-Ar method on two generations of syn-kinematic white mica in the Err nappe and with Ar-Ar step-heating of syn- to post-kinematic riebeckitic amphiboles in the Platta nappe. The first generation of white micas associated with isoclinal folds and W-directed mylonitic thrusting along nappe contacts yields a 76 to 89 Ma age range, whereas the second generation of white micas that grew during tight folding and top-to-the-E extensional D2 shearing yields 67 to 80 Ma ages. The K-Ar white mica ages are interpreted as ages of formation at the approximately 300°C temperature of lower green-schist facies regional metamorphism in the southern part of the Err nappe. The two amphibole samples from the Platta nappe yield 69 and 73 Ma Ar-Ar ages, interpreted as syn-D2 formational ages. The Si-content of the white micas in chlorite-K-feldspar-quartz-bearing meta-radiolarites in the southern part of the Err nappe indicate a pressure decrease from D1 to D2 deformation, probably accompanied by an increase in temperature. This can be related to D2 extensional uplift accommodated along reactivated D1 nappe contacts in the footwall of the Julier-Bernina nappe. When combined with stratigraphic evidence, the radiometric ages and deformation-crystallization relations indicate clockwise P-T paths for the Err, Platta, and Margna nappes: Suturing of the Austroalpine continental and Liguro-Piemontese oceanic crust began with near surface thrusting and flysch sedimentation between 88 and 100 Ma and culminated with D1 folding and nappe stacking under high P/T greenschist facies conditions sometime during the period 80 to 88 Ma. The unroofing of this suture was a multi-stage process involving Late Cretaceous D2 extension under greenschist facies conditions, Early Tertiary D3 thrusting under lower anchizonal conditions, and mid-Tertiary D4 normal faulting above the uplifting Lepontine dome.