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

脆性断裂和断层是地表变形最常见的表现形式。著名的例子包括著名的圣安德烈斯断层（美国加利福尼亚州）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年的旧金山地震；1999年的伊兹米特地震），因此可能对生活在受影响地区的人们造成毁灭性的影响。除了沿板块边界的这种集中变形外，脆性变形也可能更分散地发生在大陆碰撞带，如山带（造山带）或伸展环境中，导致一系列较小的脆性断层和相关褶皱。了解这样的构造是揭示一个地区时空演变的关键，因为如果仔细分析，它们会存储形成山脉链或地堑结构的力（应力）的记录。除了学术上的兴趣外，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、隧道开挖、建筑工程或矿产勘探（采矿）。脆性断裂通常通过密封、圈闭或管道控制油气分布，因此对定向石油勘探非常重要。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、全长57公里的瑞士圣哥达基线隧道工程就是最近一个备受瞩目的例子（http://news.bbc.co.uk/1/hi/world/europe/6471241）。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，这需要被理解，以便随着时间的推移建立一个一致的地质模型，无论是在一个小区域还是整个山脉的尺度上。到目前为止，直接约束脆性断裂的时间演化是非常困难的，几乎是不可能的。主要原因是低温，只有少数矿物同步生长（如石英、方解石）。此外，这些矿物通常被认为无法用普通的地质年代学方法确定年代。这正是这项提议的目的所在：它旨在开发和应用一种新技术来直接确定脆性断层的年代，这将为构造地质学和构造学提供一种新工具。最终，它将允许直接确定脆性断层的年代，从而提供以前无法获得的脆性断层的年龄，到目前为止，这只能间接地得到约束。这种新方法基于方解石纤维，这种纤维通常与脆性断层有关。这种方解石纤维可能含有相对高浓度的铀(U)，但铅（Pb）含量较低，因此适用于U/Pb定年。所附的“支持案例”中提供的初步数据表明，这种方法确实提供了与独立地质证据相一致的脆性断层年龄。我们现在想进一步将这种方法应用到奥地利东阿尔卑斯山的关键地区，那里记录了多次脆性变形事件，沉积岩的年代相对较好。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，不仅单个断层事件的年龄将受到限制，甚至它们的持续时间也可能是触手可及的，因为由于脆性断层面上方解石纤维的纤维生长，断层事件的开始和结束可能是可确定的。

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.