Dating Illitic Fault Rocks, and Applications to Cordilleran Normal Faults

浅层断层岩中富钾矿物的放射性测年为解决长期存在的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。 含粘土的断层岩在折返断层带中很常见，通常保存原生（碎屑）和新生长（自生）的层状硅酸盐。因此，对这些断层岩的分析反映了两种矿物群体的混合作用。使用X射线表征的粒度群体的粘土部分，和样品封装的照射和定年的Ar/Ar法，一个可靠的方法来定年自生生长的伊利石和伊利石-蒙皂石丰富的材料，以跟踪断层的时间已经开发。 该项目使用硅酸盐多型表征和量化，并应用测年方法来测试现有的假设，在美国西部科迪勒拉著名的正断层设置。 该方法将被应用于岩心复杂断层问题，以期解决这一构造地质领域长期存在的问题。 如果该方法成功，则可以回答诸如区域变形时间、陡脆性断层和浅脆性断层的同步位移以及脆性变形和塑性变形之间的时间差等问题。 将建立一个富钾断层带物质采样协议，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，几乎没有探索过。美国公众知道有许多断层蜿蜒穿过景观，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正在积极移动，那么这些地球特征对公众的重要性就会增加。 如果断层穿过城市或郊区，可能会造成严重的危害。 探测断层活动历史的方法很少，但本提案中概述的工作有望做到这一点：测量美国西部一些测试案例断层的历史。 这种方法原则上很简单。 在断层运动期间生长的矿物被收集和纯化，然后测量它们的放射性年龄。 大块物质的测量年龄是断层运动的集体历史，样品中不同的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.