Dating Illitic Fault Rocks, and Applications to Cordilleran Normal Faults

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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.