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

脆性断裂和断层是地球表面最常见的变形表现。著名的例子包括著名的圣安德烈亚斯断层（美国加州）或北安纳托利亚断层（土耳其北部），它们代表了构造板块的边界。这些断层通常是地球上最强烈地震的原因（例如，1906年的San Francico; 1999年的伊兹米特），因此可能对生活在受影响地区的人们造成破坏性影响。除了沿着板块边界的这种集中变形外，脆性变形也可以在大陆碰撞带（如造山带）或伸展环境中更分散地发生，从而导致一系列较小的脆性断层和相关褶皱。了解这些结构是揭示一个地区时空演化的关键，因为如果仔细分析，它们记录了形成山脉或地堑结构的力（应力）。除了学术上的兴趣之外，这种详细的地质分析还有切实的经济原因，其中最重要的包括石油/天然气勘探，隧道挖掘，建筑工程或矿产勘探（采矿）。脆性断层通常通过盖层、圈闭或导管控制油气分布，因此对于目标石油勘探非常重要。脆性断层会给隧道工程带来巨大的问题，目前世界上最长的隧道-瑞士圣哥达57公里基线隧道工程就是最近一个引人注目的例子（http：//news.bbc.co.uk/1/hi/world/europe/6471241.stm），其中各种大规模的脆性断层造成了巨大的额外成本和延误。在地质学中，脆性结构的空间排列表明了导致其存在的力（应力）。然而，同样重要的是断层事件的时间演化，需要了解这些事件，以便随着时间的推移开发一致的地质模型，无论是在小区域还是整个山脉的规模上。到目前为止，直接限制脆性断层的时间演化非常困难甚至几乎不可能。主要原因是低温，只有少数矿物（如石英，方解石）同步生长。此外，这些矿物通常被认为是不可能的日期与普通的地质年代学方法。这正是该提案的目的：它旨在开发和应用一种直接测定脆性断层年龄的新技术，这将为构造地质学和构造学提供一种新的工具。最终，它将允许直接测定脆性断层的年龄，并因此提供以前无法获得的脆性断层年龄，迄今为止只能间接限制。这种新方法是基于使用方解石纤维，这是通常与脆性断层。这种方解石纤维可能含有相对较高浓度的铀（U），但铅（Pb）含量较低，因此适用于U/Pb测年。在所附的“支持案例”中提供的初步数据表明，这种方法确实能够确定与独立地质证据一致的脆性断层年龄。我们现在想进一步将这种方法应用于奥地利东阿尔卑斯山的关键地区，在那里记录了多个脆性变形事件，并通过沉积岩确定了相对较好的年代。这使得这些地区成为验证新测年技术的理想测试地点。最后，不仅个别断层事件的年龄将受到限制，但即使是他们的持续时间可能是触手可及的，因为由于方解石纤维的纤维生长在脆性断层面，开始和结束的断层事件可能是可测的。

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.