9:00am - 9:15am
The Drosendorf Unit in the Austrian part of the Bohemian Massif: Does it host the oldest rock fragments of Variscan Europe?
1Department of Chemistry and Physics of Materials, University of Salzburg, Austria; 2NAWI Graz Geocenter – Institute of Earth Sciences, University of Graz, Austria; 3Department of Geography and Geology, University of Salzburg, Austria
Remnants of pre-Cadomian rocks are scarce in the Variscides including the Bohemian Massif. In the latter, numerous Variscan metamorphosed Mesoproterozoic and Early Neoproterozoic sediments and granitoids are contained in the Drosendorf Unit (DU) in Lower Austria. Here, we present U-Pb zircon ages for two orthogneisses from this unit showing even older magmatic formation ages of 2.10 Ga and 2.05 Ga. These granitoid gneisses with volcanic-arc and within-plate characteristics belong to the oldest rocks known from the Central European Variscides: the Gaberkirche Gneiss (~2.1 Ga), occurring as a relatively small, ~0.3 km² orthogneiss body near Drosendorf, and the Schallaburg Gneiss (~2.05 Ga) located in the south-eastern outskirts of the Bohemian Massif near Melk, where it forms two small bodies with a total area of ~2.5 km².
Although tectonically incorporated into the Moldanubian Zone (Armorica) during the Variscan orogeny, the DU likely represents a part of the Brunovistulian Terrane (BT), which lay north of the Rheic Ocean before the Variscan collisional events. Rare Palaeoproterozoic remnants have also been identified in other parts of the BT in the Velké-Vrbno Dome and the Rzeszotary Horst, the latter being interpreted as a tectonic splinter from north of the Tornquist Line. However, the Meso- to Neoproterozoic rocks of the DU typically show a detrital and inherited Palaeoproterozoic zircon signal, and may thus have been originally associated with a Palaeoproterozoic basement. This could be an important new aspect for future palaeogeographic interpretations.
9:15am - 9:30am
The Saxothuringian “Wrench-and-Thrust Zone” – the connecting link between the Peri-Gondwana shelf and the Variscan orogen.
Technische Universität Bergakademie Freiberg, Germany
The Saxothuringian Zone of the Central European Variscides preserves the sedimentary record of the post-Cadomian shelf and an Early Carboniferous synorogenic basin. Thus, this area reflects the transition from a passive continental margin setting to an active plate boundary zone. Particularly the record of the so called “Wrench-and-Thrust Zone” (WTZ) can be regarded as the connecting link between the Peri-Gondwana shelf and the Variscan orogen. The WTZ separates the complex metamorphic stack of the Erzgebirge-Fichtelgebirge Zone to the SE and the Paleozoic lithologies of the Schwarzburg Antiform to the NW. Compared with the adjacent Schwarzburg area, the WTZ differs in two essential points: i) It contains the record of a Late Devonian phase of bimodal magmatism, and ii) it experienced Early Carboniferous stacking that was partially related to a greenschist facies metamorphic overprint. Based on detailed studies such as structural mapping and 3D-modeling we propose the tectono-sedimentary evolution of the WTZ as follows. Late Devonian strike-slip faulting dissected the inner shelf of the W-African promontory of the Gondwana plate, culminating in localized and short-lived magmatism. Continued sedimentation on the segmented shelf is indicated by facies variations and prevailed until the onset of synorogenic sedimentation in the Tournaisean. Due to ongoing Gondwana – Laurussia plate convergence, first collisional tectonics, (D1) occurred in the Middle Viséan and led to SW-directed nappe stacking and the juxtaposition of low grade and non-metamorphic lithologies. The evolved synorogenic basin has been overfilled c. 10 Myrs after the D1 deformation. Late orogenic (N)NW-(S)SE-directed transpression finally overprinted the entire area.
9:30am - 9:45am
The Paleozoic supercontinent cycle and regional tectonics
1TU Bergakademie Freiberg; 2GFZ Potsdam, Germany; 3University of Calgary, Canada
Ancient plate boundary processes define the first order architecture of consolidated continental crust. Therefore, regional tectonic features allow for the reconstruction of plate tectonic processes. Here we explain the Paleozoic tectonics of various orogens of Europe and both Americas in terms of the Pannotia – Pangea supercontinent cycle. Early Paleozoic separation of Gondwana and Siberia from the eastern and western edges of North America, respectively, is compensated by convergent tectonics at plate boundaries surrounding the East-European Craton, eventually leading to the Scandian orogeny of the Caledonides and the initial formation of the Uralides. The complex opening scenario transformed passive continental margins into active ones and culminated in the Ordovician Taconian and Famatinian accretionary orogenies at the Peri-Laurentian margin and at the South American edge of Gondwana, respectively. The final assembly of western Pangea is characterized by the prolonged and diachronous closure of the Rheic Ocean (~400-270 Ma). Continental collision started within the Variscan - Acadian segment of the Gondwana – Laurussia plate boundary zone. Subsequent zipper-style suturing affected the Gondwanan Mauretanides and the conjugate Laurentian margin from north to south. In the Appalachians, previously accreted island arc terranes were affected by Alleghanian thrusting. The Ouachita – Marathon – Sonora fold-and-thrust belts of southern Laurentia evolved from the transformation of a vast continental shelf area into a collision zone. Slab pull as major plate driving force is sufficient to explain the entire Pannotia – Western Pangea supercontinent cycle for the proposed scenario.
10:00am - 10:15am
Does regional geology help to assess earthquake hazard in continental interiors?
1Geoscience Center, University of Göttingen, Germany; 2Unit ‚Engineering Seismology‘, Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany
Earthquake hazard assessment is crucial for different planning tasks, including the search for a German nuclear waste repository. Germany is located in an intraplate setting with a low level of seismicity and the seismically active faults are incompletely known. To solve this problem, seismotectonic regions (SR) of assumed uniform seismicity can be defined and used as a basis to define seismic area sources to be used in seismic hazard analyses. We have elaborated a new concept for a transparent implementation of geological data. Our basic assumption is that the intensity of past geologic deformation controls the propensity of an area for renewed fault slip and earthquakes. Based on a compilation of published geological maps we analyzed the post-Variscan (<300 Ma) evolution of Germany´s fault network and created maps of geologic deformation intensity for six time slices. The time slice maps were superimposed to give a map of total deformation intensity. Regions of similar total geologic deformation intensity define SR. Comparison of these geology-based SR with recent seismicity (1000 years) shows good correlation in Cenozoic rifts (Lower Rhine, Upper Rhine, Eger grabens) and fair correlation of sparser seismicity in areas of strong and repeated Mesozoic deformation (particularly the “Mesozoic inversion belt” of central Germany). However, the prominent earthquake clusters of Brabant and the Swabian Jura occur in “stable” areas of little past deformation. We conclude that regional geology is a valuable source of information for seismotectonic regionalizations but should initially be analyzed separately from recent seismicity to avoid circular reasoning.
10:15am - 10:30am
Analysis of continent-scale geological maps
Ludwig-Maximilians-University of Munich, Germany
Geological maps are important products of geological work that display results of generations of field geologists’ work. Most original geological maps are generated and utilized at local scales. At regional scales, geological maps have gained in practical significance ever since William Smith’s 1815 geological map of England exemplified the powerful nature of mapping and correlating strata beyond local scales. However, by comparison, geological maps compiled at continental-scales appear to be of limited use outside of geological circles. Often, they are oversized which inhibits their practical use, so they decorate our geoscience hallways and lecture halls for their beautiful colors and their general esthetic appearance. Few outsiders can even read these maps. Their special color-coding, the multiple non-diverging color schemes and their complex legends further inhibit non-geologists from being able to recognize the enormous knowledge stored in these maps. I present an analysis of continent-scale geological maps by visualizing time not represented by the rock record (hiatus) and by examining the dimensions of hiatal surfaces at interregional scales. The maps yield great variability in dimensions and space-time patterns of hiatal surfaces, a behavior which is to be expected in light of interregional-scale processes induced by both, the plate and the plume mode of mantle convection. However, to test models of mantle convection rigorously, the temporal resolution of continent-scale maps must be increased to stages level, i.e., the scale at which tectonic processes take place.