The Lower Engadine Window: sediment deposition and accretion in relation to the plate-tectonic evolution of the Eastern Alps

Abstract

The Lower Engadine Window comprises a weakly metamorphosed, heterogeneous mixture of continental fragments and oceanic ophiolites embedded in a disrupted mass of deep-sea sediments ranging from the mid-Jurassic to the Early Eocene, mostly turbidites. Its geological features are reviewed in the light of recent research and its relationships with other Penninic outcrops in the Eastern Alps discussed. In the Falknis, Sulzfluh and Tasna zones it is thought that the entire spectrum of post-Liassic sediments was deposited to the north of the Austro-Alpine continental margin. This is based on downcurrent variations of sedimentary structures to the north or northwest in the same direction as the trend of paleocurrent indicators, and on the close petrographic resemblance of coarse parent rock materials to Austro-Alpine units. Of particular significance are granitic rocks exposed by vertical movements along fault scarps associated with the first formation of oceanic crust. These granitic rocks, together with their Permo-Mesozoic sedimentary cover, were to be eroded and slumped down from the continental margin until the Senonian. Throughout the Jurassic, the clastic sediments were shed laterally onto the abyssal plain and base of the continental slope. Pelagic sedimentation during parts of the Late Jurassic and earliest Cretaceous, including Aptychus limestones deposited close to the calcite compensation depth, suggests that maximum subsidence of the initially passive continental margin had been attained by this time period. With the onset of continental convergence in the earliest Cretaceous, underthrusting of oceanic crust gave rise to an accretionary trench system. The Cretaceous sediments were deposited in a deep-sea fan environment affected by chaotic mass movements from the inner trench wall. It is believed that the close proximity to the source area of the vast majority of Jurassic and Cretaceous deposits reflects the relative motion between the plates, which must have been largely strike-slip and directed essentially E-W during the Cretaceous. An irregular leading edge of the Adriatic plate is proposed to account for Cretaceous high-pressure metamorphic events associated with higher rates of plate consumption in adjacent regions. N-S compressional movements in the Tertiary finally resulted in rapid underthrusting and continental accretion along most of the eastern Alpine arc.