Rainer Brandner

Permian–Triassic boundary magnetostratigraphy from the Southern Alps (Italy)

Palaeomagnetic investigations of Permian–Triassic boundary sections in the Dolomites provided a magnetostratigraphy for the uppermost part of the Permian Bellerophon Formation to the Lower Triassic Werfen Formation. Magnetite was the dominant magnetic component in most of the samples, while the presence of hematite was characteristic for the Tesero horizon, which is regarded as the immediate Permian–Triassic boundary layer. The palaeomagnetic results are consistent with earlier studies of Permian and Triassic sediments and volcanites in the Southern Alps. The mean characteristic remanence directions indicate deposition and remanence acquisition at a shallow northerly palaeolatitude and a counterclockwise rotation. A zone of reversed polarity occurs at the top of the Permian sequence and a polarity change from reverse to normal polarity shortly above the base of the Werfen Formation. The onset of a tectonically controlled sedimentary cycle marks the beginning of the basal Triassic normal polarity zone, which corresponds to most of Griesbachian time. The record of geomagnetic polarity across the Permian–Triassic boundary confirms the apparent continuity as found by sedimentologic biostratigraphic investigations which emphasise a gradual lithic transition with no evidence for a time gap at the boundary.

Microfacies of carbonate slope boulders: Indicator of the source area (Middle triassic: Mahlknecht cliff, western Dolomites)

SummaryPaleontological and microfacies criteria of limestone boulders occurring within megablocks deposited on clinogonal slopes provide indications of the source area of Middle Triassic allochthonous carbonates exposed in the Mahlknecht cliff, Seiser Alm, Dolomites. Microfacies, biotic composition, a high percentage of low-growing binding and baffling communities (sponges, algae, microproblematica) as well as the remarkable coincidence with distributional patterns observed on other Ladinian paleoslopes indicate a source area characterized by common bioconstructions, which is situated on a subtidal upper and middle foreslope and not at the platform/slope-margin or on the carbonate platforms.RiassuntoI megablocchi calcarei del Trias medio affioranti vicino al Rif. Molignon (Alpe di Siusi, Dolomiti) sono costituiti di clasti carbonatici risedimentati. Il loro studio ha permesso di formulare un’ipotesi sull’area di alimentazione dei clasti stessi.I dati paleontologici, l’analisi e la distribuzione delle microfacies, l’alta percentuale di organismi ‘binding’ e ‘baffling’ a basso tenore di crescita) spugne, alghe e microproblematici) indicano una sorgente di alimentazione caratterizzata da biocostruzioni simili a quelle osservate in altre paleoscarpate ladiniche. In particolare tale sorgente doveva essere ubicata in ambiente subtidale nella parte media ed alta della scarpata e non al margine piattaforma/scarpata o sulla piattaforma stessa.

87Sr/86Sr stratigraphy from the Early Triassic of Zal, Iran: Linking temperature to weathering rates and the tempo of ecosystem recovery

Recovery from the Late Permian mass extinction was slowed by continued environmental perturbations during the Early Triassic. Rapid fluctuations of the Early Triassic marine carbonate carbon isotope record indicate instability in the global carbon cycle, and recent δ 18 O apatite studies link elevated temperatures to the prolonged biotic recovery. High temperatures potentially caused enhanced continental weathering that was detrimental to marine ecosystems, but linking weathering rates to temperature has proven difficult. One proxy for weathering is the 87 Sr/ 86 Sr of marine carbonate; we present here an 87 Sr/ 86 Sr record from an upper Permian–lower Triassic succession near Zal, Iran, that is coupled to a δ 13 C carbonate record. An increase in the rate of 87 Sr/ 86 Sr rise from the Dienerian to the Smithian may be linked to elevated continental weathering rates caused by warming during the Smithian.

Structure, Mineralogy and Geomechanical Properties of Shear Zones of Deep-Seated Rockslides in Metamorphic Rocks (Tyrol, Austria)

Deep-seated rockslides, which are characterised by slow to extremely slow rates of movement, frequently occur in foliated metamorphic rock masses (schists, phyllites, paragneiss series). Many case studies indicate that slope displacement is predominantly localised at basal and internal shear zones. Thus, the deformation and stability behaviour of rockslides is influenced primarily by the properties of these soil-like shear zones. In this study, new findings concerning the structure, mineralogical composition and geomechanical characteristics (residual friction angle, grain size distribution) of the shear zones of deep-seated rockslides are presented. The characteristics of these shear zones are shown by case studies in paragneissic rock masses of the polymetamorphic Austroalpine Ötztal–Stubai crystalline complex in Tyrol, Austria. Differences between the laboratory scale and the in situ scale are discussed, as well as the evolution of the shear zones. Within the framework of this study, structural investigations of the shear zones were performed from surface and subsurface surveys and core logs, as well as mineralogical laboratory analyses, grain size analyses and ring shear tests. The shear zones are characterised by a complex fabric of lensoid-shaped layers of clayey-silty fault gouges embedded in sandy-gravelly fault breccias and block-in-matrix structures. The mineralogical analyses indicated high amounts of phyllosilicates, such as mica and chlorite. Swelling clay minerals were observed in small amounts in very few instances. The ring shear tests of the rockslide fault gouge samples, performed under various normal stress conditions, resulted in residual friction angles in a wide range between 19° and 28°, reached after rather short displacements.

Process-based investigations and monitoring of deep-seated landslides

Through the consolidation of alpine settlement areas there have been an increasing number of incidents in recent years related to the activity of landslides in Northern Tyrol (Austria). This has led to humans, buildings, and communication and transportation routes being increasingly threatened. In 1999 a rockfall event in Huben (Otztal, Austria) destroyed a wood mill and cut the main power supply for the inner Otztal. In the same year increased deformation rates at the Eiblschrofen (Schwaz, Austria) induced reoccurring rockfall events. In early summer 2003, parts of the deep-seated Steinlehnen rockslide system (Gries i. Sellrain, Austria) were reactivated, causing an acceleration of a sliding slab (Henzinger 2005). Secondary events in the form of increased rockfall activity were the direct consequence of these slope movements and demanded temporary evacuations and roadblocks as immediate measure. In order to protect the road and settlement area permanently a safety dam was built. After the floods in Tyrol in August 2005, parts of the complex Zintlwald landslide system (Strengen, Austria) accelerated. This was triggered on the one hand by increased water infiltration of the slope and on the other hand by intense fluvial erosion of the slope foot. As a consequence important supra-regional infrastructure such as sections of the Arlberg national road were destroyed. In addition, the possibility was given that a rapid landslide could dam the river Rosanna. Considering that a collapse of this dam would entail a sudden flood event downstream, a monitoring and warning system has been installed.