Tectono-sedimentary breccias associated with mantle windows in hyper-extended domains (Eastern Swiss Alps): field observations, geochemistry, and consequences for the subduction interface

We describe new geological observations and geochemical data on tectono-sedimentary breccias that formed in hyper-extended domains of the Tethyan rift system. Such breccias are markers of detachment faults in the magma-poor Iberia-Newfoundland and Tethyan rifted margins and document the continuous exhumation of fault surfaces on the seafloor. We focus on a rift-related detachment fault in the most distal Adriatic rifted margin exposed at the interface between the Margna and Malenco nappes in the Eastern Central Alps of SE-Switzerland. These are the first syn-rift breccias along the Jurassic rift related detachment system that document a mixture of both continental and mantle-derived clasts, indicating that hyperextension and the formation of mantle windows are intimately coupled.

3D mapping of the breccia combined with a 75m long drill core through the hanging and footwall of a detachment fault provide clear evidence that mantle exhumation and breccia formation was coeval. Downhole variations of major and trace elements, and ∂¹⁸O isotopes demonstrate m-scale extensive chemical exchange between crustal and mantle rocks.

The breccia-mantle contact is made of a m-wide fine-grained metasomatic chl±amph-bearing zone. The breccias on top of the ultramafic rocks are made of mm- to cm-sized amphibole-bearing mantle-derived clasts, continental-derived rounded to elongated cm- to meter-sized blocks, and a fine- to coarse-grained foliated matrix, dominated by chl+bt+fsp+qtz. Amphibole in amphibole-bearing clasts are Cr-Ni-rich, supporting the hypothesis of ultramafic clasts in the polymict breccia. Bulk rock major, minor and trace element data from the serpentinized peridotites to the continental-block bearing breccias show an increase in Al, Ca and elevated LREE. However, amphibole-rich clasts within the mica-rich matrix are very similar in composition to serpentinized peridotites with high Cr, Ni and low LREE, indicating that amphibole rich rocks are previous serpentinized peridotite clasts that are altered to form amphibole blackwalls, presumably related to recrystallization during Alpine metamorphism. We argue that recent findings of eclogite facies brecciation along the subduction interface might be the consequence of inherited brecciation of hyper-extended margins or oceanic core complexes.