A new method for dating brittle deformation: U-Pb dating of carbonate fibres

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Brittle fractures and faults are the most common expression of deformation at the Earth's surface. Well known examples include the famous San-Andreas-Fault (California, USA) or North-Anatolian-Fault (N Turkey), which represent tectonic plate boundaries. Such faults are often responsible for the most powerful earthquakes on Earth (e.g. San Franscico, 1906; Izmit, 1999), and as such may have devastating effects for people living in the areas affected. Apart from such concentrated deformation along plate-boundaries, brittle deformation can also occur more dispersed in continental collision zones such as mountain belts (orogens) or extensional settings, resulting in an array of smaller brittle faults and associated folds. Understanding such structures holds the key for unravelling the spatial and temporal evolution of a region because - if analyzed carefully - they store a record of the forces (stresses) that formed the mountain chain or graben structure. Besides this being of academic interest, there are tangible economic reasons for such detailed geological analyses, the most important of which include oil/gas exploration, tunnelling, construction work or mineral exploration (mining). Brittle faults typically control oil/gas distribution via seals, traps or conduits and as such are very important for targeted petroleum exploration. Brittle faults can cause enormous problems for tunnelling projects, with the current 57 km Gotthard base tunnel project in Switzerland, the world's longest tunnel, being a recent high-profile example (http://news.bbc.co.uk/1/hi/world/europe/6471241.stm), where various large-scale brittle faults caused enormous additional costs and delays. In geology, the spatial arrangement of brittle structures indicates the forces (stresses) that led to their existence. However, equally important is the TEMPORAL evolution of faulting events that need to be understood in order to develop a consistent geological model over time, be it on the scale of a small area or an entire mountain range. Constraining the temporal evolution of brittle faulting directly has been very difficult to almost impossible so far. The main reasons are the low temperatures involved where only a few minerals grow synkinematically (e.g. quartz, calcite). Moreover, these minerals are normally considered impossible to date with common geochronological methods. This is exactly where this proposal is coming in: It aims to develop and apply a new technique for the direct dating of brittle faulting, which would yield a new tool for structural geology and tectonics. Ultimately, it would allow dating of brittle faults directly and as such provide the previously unavailable ages of brittle faulting, which so far can only be constrained indirectly. This new method is based on using calcite fibres, which are commonly associated with brittle faults. Such calcite fibres may contain relatively high concentrations of uranium (U) but low levels of lead (Pb) and as such are applicable to U/Pb dating. Preliminary data presented in the attached 'Case for Support' suggest that this method indeed furnishes ages of brittle faulting consistent with independent geological evidence. We want to take this now further and apply this method to key areas of the Austrian Eastern Alps, where multiple brittle deformation events are recorded and relatively well-dated via sedimentary rocks. This makes these areas ideal test sites for validating the new dating technique. Finally, not only the ages of individual faulting events will be constrained but even their duration may be within reach, since due to the fibrous growth of calcite fibres on brittle fault planes, start and end of faulting episodes may be datable.