Formation of an intra-orogenic transtensional basin: the Neogene Wagrain basin in the Eastern Alps

Abstract

The Neogene (ca. Ottnangian to Lower Badenian) Wagrain basin formed within a halfgraben, close to the north-eastern edge of the exhuming and uplifting Neogene Tauern metamorphic core complex, along the ENE-trending, ca. orogen-parallel sinistral-transtensional Salzach-Enns and Mandling fault systems of the Eastern Alps. Based on a pronounced colour change from thin basal reddish, hematite-rich conglomerates to grey-brownish clastics, the basin infill records a climate change from a subtropical to a humid temperate climate. The change is used as a proxy for the dating of the basin as a similar regional climate shift has been reported, e.g., from other fault-bounded basins in the Eastern Alps and from the base Styrian basin predating there Ottnangian coal-bearing deposits. The lithofacies evolution of the Wagrain basin fill shows initial rapid subsidence and infilling by fluvial, massive, coarse-grained conglomerates and subsequent mica-rich immature sandstones deposited in a prodelta environment of a lake. The conglomerates were mainly shed from a polymetamorphic, medium-grade metamorphic basement similar to the Austroalpine Altkristallin basement complex, which is not exposed in the present surroundings of the basin. Provenance studies particularly suggest the Schladming/Bosenstein and Wolz Micaschist complexes as possible sources. The sandstones are extremely rich in mica, similar to ones found in many late-stage orogenic sandstones of other orogens, and garnet. Provenance analysis contrasts recent models for the Neogene evolution of the Eastern Alps, which assume a phyllitic source for similar conglomerates exposed further north and northeast. Fault and striae of the Wagrain basin fill allow deduce two post-depositional palaeostress tensor orientations: WNW-ESE extension and ca. E-W contraction. This data is consistent with palaeostress development along the Salzach-Enns fault, which also confines the Tauern metamorphic core complex, and shows an evolution from sinistral transtensional to sinistral transpressional to post-depositional dextral transpression.