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Abstract

Active subduction channels cannot be directly investigated due to their inaccessibility for drilling or surface examinations. Only geophysical methods, numerical modeling or sandbox simulations throw light on the shallow parts of currently active subduction channels. Hence, we additionally use direct investigations of an exposed ancient plate boundary zone to understand their internal geometry and processes operating. Here we present the anatomy of the deeper part of a plate boundary in the temperature range between 150°C and 350°C represented by a mélange zone in the Swiss Alps. A subduction channel focus deformation within subduction zones by transporting oceanic and continental material down into the mantle and by return-flow of low-viscosity material back to the surface. The subduction channel can be best defined by gradients in the flow velocity of the deforming material in respect to the upper and the lower plate. The fossil, formerly south to southeast dipping subduction channel in the Swiss Alps is sandwiched between the overlying Austroalpine nappes (African plate) and the footwall Penninic/Helvetic nappes (European plate), a setting assembled during Alpine convergent plate motion. Due to large scale tilting of this zone, a continuous outcrop today allows the identification of changes downdip along the former plate boundary zone. In this study we aim to quantify the evolution of different features (e.g. aspect ratio of clasts, localized deformation zones, mineralized vein systems) towards the south of the working area, where formerly deeper parts of the subduction channel are accessible. Therefore, we studied 8 profiles crossing the Penninic-Austroalpine boundary. In order to relocate the plate boundary zone features to their former position we projected the investigated profiles into a composite synthetic section perpendicular to the strike of the former subduction zone. This restoration is based on the NFP-20-East seismic traverse (trending N-S) covering the main geological and tectonical units in the working area. We used the software 2DMove (Midland Valley) for restoration. The former strike of the subduction zone is constraint by the orientation of the metamorphic isogrades associated with subduction and accretion of the Penninic domain, which trends ENE-WSW. In addition, the present day general trend of the external Gosau basins that represent forearc basins developed on the Austroalpine nappe stack, show the same trend. In consequence, we assume the down dip direction of the former plate interface to be SSE, and we projected the geological units and profile positions from the N-S section (NFP-20-East) into the new section rotated 20° from the original one. The first step in restoration was the removal of the offset along the Engadine line using the operations “fault parallel flow” and “join beds”. Afterwards, “line length unfold” was applied to the folded South Penninic domain, which resulted in two flat lying horizons. Overprint of the upper plate basement by Alpine deformation increases towards the south, and the shaly matrix of the subduction channel exhibits equally increasing metamorphic grade. The matrix contains clasts of different size comprising upper plate material, as well as slivers of the oceanic lower plate and its sediments. The clast size varies from a few cm to more than hundreds of meters. With increasing metamorphism and deformation both upper plate basement and metasedimentary clasts are strongly mylonitized along their rims. Mylonitic shear zones cut into the clasts and enforce their disintegration. Pseudotachylytes as evidence for unstable slip have been found at a limited depth range (app. 3-6 kbar, <350°C) directly at the base of the upper plate. Metasedimentary clasts and metabasics reveal abundant mineralized veins with partly blocky mineralization. Additionally, a vast number of foliation-parallel mineralized veins invade the matrix of the fossil subduction channel. The relationship of these mineralized fractures to seismic faulting has yet to be evaluated. They mirror dehydration processes during prograde metamorphism within the subduction zone. Rb/Sr isotope signatures of 8 lower plate carbonatic samples provide clear indications for the presence of fluids with elevated 87Sr/86Sr ratios in the subduction channel. This suggests that dehydration of continent-derived sediments was the main fluid source in contrast to the often assumed high influence of fluids derived by the dehydration of the downgoing oceanic crust, at least for the sampled depth range. The integration of the study’s results with new insights from synthetic geophysical, numerical and analogue modeling will offer the chance for a detailed identification of processes within ancient and active subduction channels.