Sedimentary fabrics in Alpine ophicalcites, South Pennine Arosa zone, Switzerland

Abstract

Many ultramafic complexes in the South Pennine nappes of eastern Switzerland are covered by serpentinite breccias, known as ophicalcites. The breccias are overlain by radiolarites or younger oceanic sediments. Their fabric and their components document a combined tectonic-sedimentary origin of the breccias. In the Davos area the breccias occur along distinct zones in the ultramafic host rock. Typically, nonfragmented peridotite/serpentinite grades rapidly through a narrow zone of host rock, cut by different generations of dikes, into complex breccias dominated by a carbonate matrix. The breccias are of polyphase origin, as shown by the different phases of fragmentation and generations of cementation and sediment infill. These are distinguished by color differences and by a bimodal grain-size distribution that also exhibits geopetal structures. Comparison with breccias from present-day oceanic fracture zones suggests that the Tethyan serpentinite breccias may have formed in a transform setting. A sinistral transform margin along the northern edge of the Apulian microplate or promontory is in line with current plate-kinematic reconstructions.