The internal structure of the Arosa Zone (Swiss-Austrian Alps)

Abstract

The Arosa Zone, part of the main Alpine suture zone between the Austroalpine and the Penninic realms, forms a heterogeneous unit composed of rocks of oceanic and continental origin. It exhibits melange character due to minor sedimentary mixing and local penetrative tectonic deformation during Cretaceous and Early Tertiary imbrication. Competent blocks of both Austroalpine and Penninic origin, covering up to 2.5 km2, are embedded in incompetent serpentinitic or shaly-calcareous matrix. On a mesoscale, disrupted strata occur in and adjacent to thrust and shear zones. Contrasting competence between blocks and matrix partitioned deformation into brittle and ductile processes. Extension veins and shear fractures affected the competent strata whereas the matrix developed a penetrative foliation during ductile flow and accommodated high strain. Flow was mainly non-coaxial in the matrix, and coaxial extension prevailed in the blocks. In a regional tectonic setting, we define the Arosa Zone as the tectonostratigraphic unit sandwiched between the Austroalpine and Penninic units. It forms a narrow and highly imbricated zone containing both South Penninic ophiolitic and sedimentary rocks as well as blocks and slices of Austroalpine origin.