Indenter tectonics

Abstract: The topography of the eastern Southern Alps (ESA) reflects indenter tectonics causing crustal shortening, surface uplift, and erosional response. Fluvial drainages were perturbed by Pleistocene glaciations that locally excavated alpine valleys. The Late Miocene desiccation of the Mediterranean Sea and the uplift of the northern Molasse Basin led to significant base level changes in the far field of the ESA and the Eastern Alps (EA), respectively. Among this multitude of mechanisms, the processes that dominate the current topographic evolution of the ESA and the ESA-EA drainage divide have not been identified. We demonstrate the expected topographic effects of each mechanism in a one-dimensional model and compare them with observed channel metrics. We find that the normalized steepness index increases with uplift rate and declines from the indenter tip in the northwest to the foreland basin in the southeast. The number and amplitude of knickpoints and the distortion in longitudinal channel profiles similarly decrease toward the east. Changes in slope of χ-transformed channel profiles coincide spatially with the Valsugana-Fella fault linking crustal stacking and uplift induced by indenter tectonics with topographic evolution. Gradients in χ across the ESA-EA drainage divide imply an ongoing, north directed shift of the Danube-ESA watershed that is most likely driven by a base level rise in the northern Molasse basin. We conclude that the regional uplift pattern controls the geometry of ESA-EA channels, while base level changes in the far field control the overall architecture of the orogen by drainage divide migration.

Abstract: Monometamorphic metasediments of Paleozoic or Mesozoic age constituting Schneeberg and Radenthein Complex experienced coherent deformation and metamorphism during Late Cretaceous times. Both complexes are part of the Eoalpine high-pressure wedge that formed an intracontinental suture and occur between the polymetamorphosed Otztal–Bundschuh nappe system on top and the Texel–Millstatt Complex below. During Eoalpine orogeny Schneeberg and Radenthein Complexes were south-dipping and they experienced a common tectonometamorphic history from ca. 115 Ma onwards until unroofing of the Tauern Window in Miocene times. This evolution is subdivided into four distinct tectonometamorphic phases. Deformation stage D1 is characterized by WNW-directed shearing at high temperature conditions (550–600°C) and related to the initial exhumation of the high-pressure wedge. D2 and D3 are largely coaxial and evolved during high- to medium-temperature conditions (ca. 450 to ≥550°C). These stages are related to advanced exhumation and associated with large-scale folding of the high-pressure wedge including the Otztal-Bundschuh nappe system above and the Texel–Millstatt Complex below. For the area west of the Tauern Window, F2/F3 fold interference results in the formation of large-scale sheath-folds in the frontal part of the nappe stack (formerly called “Schlingentektonik” by previous authors). Earlier thrusts were reactivated during Late Cretaceous normal faulting at the base of the Otztal–Bundschuh nappe system and its cover. Deformation stage D4 is of Oligo-Miocene age and accounted for tilting of individual basement blocks along large-scale strike-slip shear zones. This tilting phase resulted from indentation of the Southern Alps accompanied by the formation of the Tauern Window.

Abstract: Structural studies of Lower Permian sequences exposed on wave-cut platforms within the Nambucca Block, indicate that one to two ductile and two to three brittle – ductile/brittle events are recorded in the lower grade (sub-greenschist facies) rocks; evidence for four, possibly five, ductile and at least three brittle – ductile/brittle events occurs in the higher grade (greenschist facies) rocks. Veins formed prior to the second ductile event are present in some outcrops. Further, the studies reveal a change in fold style from west-southwest-trending, open, south-southeast-verging, inclined folds (F01) at Grassy Head in the south, to east-northeast-trending, recumbent, isoclinal folds (F01; F02) at Nambucca Heads to the north, suggesting that strain increases towards the Coffs Harbour Block. A solution cleavage formed during D1 in the lower grade rocks and cleavages defined by neocrystalline white mica developed during D1 and D2 in the higher grade rocks. South- to south-southwest-directed tectonic transport and north–south shortening operated during these earlier events. Subsequently, north-northeast-trending, open, upright F23 folds and inclined, northwest-verging, northeast-trending F24 folds developed with poorly to moderately developed axial planar, crenulation cleavage (S3 and S4) formed by solution transfer processes. These folds formed heterogeneously in S2 throughout the higher grade areas. Later northeast–southwest shortening resulted in the formation of en echelon vein arrays and kink bands in both the lower and higher grade rocks. Shortening changed to east-northeast–west-southwest during later north-northeast to northeast, dextral, strike-slip faulting and then to approximately northwest–southeast during the formation of east-southeast to southeast-trending, strike-slip faults. Cessation of faulting occurred prior to the emplacement of Triassic (229 Ma) granitoids. On a regional scale, S1 trends east–west and dips moderately to the north in areas unaffected by later events. S2 has a similar trend to S1 in less-deformed areas, but is refolded about east–west axes during D3. S3 is folded about east–west axes in the highest grade, multiply deformed central part of the Nambucca Block. The deformation and regional metamorphism in the Nambucca Block is believed to be the result of indenter tectonics, whereby south-directed movement of the Coffs Harbour Block during oroclinal bending, sequentially produced the east–west-trending structures. The effects of the Coffs Harbour Block were greatest during D1 and D2.