Dating Illitic Fault Rocks, and Applications to Cordilleran Normal Faults

浅层断层岩石中富钾矿物的放射性定年为解决长期以来关于上地壳断层运动和相关变形的争议提供了一个独特的机会。含粘土断层岩在已发掘的断裂带中很常见，通常保存着原生（碎屑）和新生（自生）层状硅酸盐。因此，对这些断层岩的分析反映了两种矿物种群的混合作用。利用x射线表征粘土组分中的粒度种群，并通过Ar/Ar法封装样品进行辐照和定年，开发了一种可靠的方法来确定富含伊利石和伊利石-蒙脱石的材料自生生长的年代，以跟踪断层的时间。本项目采用phylosilicate polytype characterization and quantification，并应用测年方法在美国西部科迪勒拉已知的正断层环境中检验现有假设。该方法将应用于岩心复杂断裂问题，以期解决构造地质领域长期存在的问题。如果该方法取得成功，可以回答变形的区域时间、陡脆和浅脆断层的同时位移以及脆塑性变形的时间间隔等问题。富钾断裂带材料的采样方案将被创建，包括粘土泥的完整表征和年代测定，这是一个令人兴奋的新研究领域，实际上尚未被探索过。美国公众知道有许多断层蜿蜒而过，其中最著名的是加州的圣安德烈亚斯断层。如果断层由于可能造成的潜在破坏而积极移动，那么这些地球特征对公众的重要性就会增加。如果这些断层穿过城市或郊区，可能会造成严重的危害。有很少的方法可以探测错误活动的历史，然而在这个建议中概述的工作承诺做到这一点：测量美国西部一些测试用例错误的历史。这种方法在原则上很简单。在断层运动期间生长的矿物被收集和纯化，然后测量它们的辐射年龄。测量的大块材料的年龄是断层运动的集体历史，样品中的不同矿物亚型可能包含额外的化学线索，有助于进一步破译集体历史。

Radiometric dating of K-rich minerals in shallow fault rocks offers a unique opportunity to resolve long-standing controversies about upper crustal fault movements and related deformation. Clay-bearing fault rocks are common in exhumed fault zones, which typically preserve both primary (detrital) and newly-grown (authigenic) phyllosilicates. Analysis of these fault rocks, therefore, reflects a mixture of the roles of both mineral populations. Using X-ray characterization of grain size populations in the clay fraction, and sample encapsulation for irradiation and dating by the Ar/Ar method, a reliable approach to dating authigenic growth of illite- and illite-smectite-rich materials to track the timing of faulting has been developed. This project uses phylosilicate polytype characterization and quantification, and applies the dating method to test existing hypotheses in well-known normal fault settings in the western US Cordillera. The method will be applied to core complex faulting problems, with a view toward solving long standing problems in this field of structural geology. Questions such as regional timing of deformation, concurrent displacement on steep and shallow brittle faults and time gap between brittle and plastic deformation can be answered if the approach is successful. A protocol for sampling of K-rich fault zone materials will be created, including the full characterization and dating of clay gouge, an exciting new area of research that is virtually unexplored.The public in the US understands that there are many faults that wind their way across the landscape, one of the most famous being the San Andreas Fault in California. The importance of such Earth features to the public increases if the faults are actively moving due to the potential damage they might cause. The faults can be serious hazards if they cross urban or suburban areas. There are few ways to probe the history of activity on faults, yet the work outlined in this proposal promises to do just that: Measure the history of some test case faults in the western US. The approach is in principle simple. Minerals that have growth during fault movements are collected and purified, and then their radiometric age is measured. The measured age of the bulk material is that of the collective history of fault movement, and the different mineral subtypes within the sample may contain additional chemical clues that help to further decipher that collective history.