The change from rifting to thrusting in the Northern Calcareous Alps as recorded in Jurassic sediments

Facies analysis, fossil dating, and the study of the metamorphism in the La te Triassic to Early Cretaceous sedimentary successions in the central part of the Northern Calcareous Alps allow to reconstruct the tectonic evolutio n in the area between the South Penninic Ocean in the northwest and the Tet hys Ocean with the Hallstatt Zone in the southeast. The Triassic as well as the Early and Middle Jurassic sediments were deposited in a rifted, transt ensive continental margin setting. Around the Middle/Late Jurassic boundary two trenches in front of advancing nappes formed in sequence in the centra l part of the Northern Calcareous Alps. The southern trench (Late Callovian to Early Oxfordian) accumulated a thick succession of gravitatively redepo sited sediments derived from the sedimentary sequences of the accreted Tria ssic-Liassic Hallstatt Zone deposited on the outer shelf and the margin of the Late Triassic carbonate platform. During a previous stage these sedimen ts derived from sequences deposited on the more distal shelf (Salzberg faci es zone of Hallstatt unit, Meliaticum), and in a later stage from more prox imal parts (Zlambach facies zone of Hallstatt unit, Late Triassic reef belt ). Low temperature-high pressure metamorphism of some Hallstatt limestones before redeposition is explained by the closure of parts of the Tethys Ocea n in Middle to Late Jurassic times and associated subduction. In the northe rn trench (Late Oxfordian to Kimmeridgian) several hundred meters of sedime nt accumulated including redeposited material from a nearby topographic ris e. This rise is interpreted as an advancing nappe front as a result of the subduction process. The sedimentary sealing by Tithonian sediments, documen ted by uniform deep-water sedimentation (Oberalm Formation). gives an upper time constraint for the tectonic events. In contrast to current models, wh ich propose an extensional regime for the central and eastern Northern Calc areous Alps in the Late Jurassic,we propose a geodynamic model with a compr essional regime related to the Kimmerian orogeny.