Scientific technical report
Description / Table of Contents:
Modern concepts on processes of seismically active parts of converging plate interfaces are derived from lab experiments, theoretical inferences, and geophysical observations, which have either poor resolution, or are strongly dependent on insufficiently constrained assumptions. Therefore, we studied a continuous exposure of an ancient subduction channel in the depth range of its former seismogenic zone in the Central Alps of Europe. This subduction channel developed due to Late Cretaceous - Early Tertiary subduction and accretion of the South Penninic lower plate underneath the Adriatic upper plate (Austroalpine domain). Additionally, we include information from Southern Chile, where material, which formerly underwent deformation within a subduction channel, was exhumed to the surface by large scale basal accretion. There, we concentrated on the formation of mineralized vein systems. However, we mainly focused on the exhumed plate interface zone in the European Alps. During subduction of the South Penninic ocean, material from both the continental upper plate and the oceanic lower plate was progressively involved into the subduction factory and transported downwards, forming either the shaly and serpentinic matrix of the subduction mélange, or competent clasts. Rb/Sr deformation ages for mylonitized rocks of the South Penninic mélange and for deformed Austroalpine basement shed light on the pre-Alpine and Alpine deformation history along the suture, as well as on the mode of syn-subduction interplate mass transfer. According to our Rb/Sr deformation ages and our structural data, the latest increment of subduction-related deformation occurred at ~50 Ma, and is characterized by a roughly top-W direction of tectonic transport. Identical Rb/Sr ages for pervasively deformed Austroalpine and South Penninic rocks point to tectonic erosion of the upper plate during subduction. This is also evidenced by the presence of upper plate clasts in the subduction mélange, and from the syn-subduction evolution of Gosau forearc basins pointing to tectonic erosion as prevailing mass transfer mode during the time of subduction. Lack of a metamorphic contrast between the South Penninic mélange and the Austroalpine upper plate favors exhumation of the suture zone due to a combination of tectonic underplating and surface erosion. The end of sedimentation in the forearc Gosau basins is contemporaneous with basal accretion of the South Penninic mélange and the Middle Penninic units at ~50 Ma. Therefore, we hypothesize a causal link of both processes, with the change from tectonic erosion to basal accretion caused by underplating of subducted material, which is responsible for a regional uplift leading to inversion of the forearc basins. The end of subduction-related deformation is most likely caused by locking of the South Penninic paleosubduction interface due to underplating of the Middle Penninic micro-continent, so that the active subduction interface is relocated into the new Middle Penninic footwall. Pseudotachylytes along a restricted segment of the upper plate base delineated by ca. 200°C updip and ca. 300°C downdip - define the limits of the unstable slip region within the fossil seismogenic coupling zone. Our 40Ar/39Ar ages constrain the generation of pseudotachylytes during a time span between 60 Ma to 80 Ma. The heterogeneous texture of the ultra fine grained pseudotachylyte groundmass is composed of a mixture of amphibole, feldspar and biotite, as well as of incorporated rock fragments and single minerals of comparable size. Due to the temporal similarity between subduction and pseudotachylyte formation, and the fact that the pseudotachylytes occur subparallel to the main thrust where Austroalpine rocks were overthrust onto South Penninic rocks, we interpret the generation of pseudotachylytes to be related to unstable slip processes occurring along the plate interface zone during subduction.The zone of unstable slip coincides with a domain of intense formation of foliation-parallel mineralized veins with partly blocky minerals in the subduction mélange. We suggest that the mineralized veins reflect seismic failure in the mélange due to their similarity in spatial distribution and textures compared to pseudotachylytes. Mineralized veins, and brittle fractures continue into the conditionally stable region below, maybe indicating a domain of slow earthquakes and non-volcanic tremors as recently discovered for this depth range along many active convergent margins. The conditionally stable zone above the unstable slip area is devoid of mineralized veins, but displays ample evidence of fluid-assisted processes like the deeper zone: solution-precipitation creep and dehydration reactions in the mélange matrix, hydration and sealing of the base of the upper plate. Seismic rupture is possibly expressed by ubiquitous localized deformation zones. Fluids are most likely provided by dehydration during subduction of sedimentary material from different sources. This is indicated by elevated Sr isotope signatures of marine (meta-) carbonates from the South Penninic mélange, which are caused by the interaction of syn-subduction fluids with old continental crust.In summary, the exposed plate interface has experienced flow and fracturing over an extended period of time reflecting a multistage evolution, but resembles active convergent plate margins in terms of e.g. sediment input, earthquake distribution, fluid circulation, and possible slow slip events and associated tremors.
Type of Medium:
Scientific technical report / GeoForschungsZentrum Potsdam STR 08/01
Berlin, Freie Univ., Diss., 2007