Unravelling multiple thermotectonic events accommodated by the Highland Boundary Fault: insights from K–Ar dating

Multiple reactivation episodes and long slip histories lead to complex fault structures, whose unravelling remains challenging in the absence of absolute time constraints. We apply K–Ar isotopic dating of illitic fault rocks, coupled with X-ray diffraction and microstructural analyses to constrain, for the first time, timing of illite-producing brittle fault movements accommodated by the Highland Boundary Fault (HBF), Scotland. Illite Age Analysis (IAA) plots indicate multiple fault reactivation events on the HBF. IAA plots for the red foliated chaotic fault-breccia conform to a 'normal' IAA pattern with younger ages recorded for 'authigenic' 1M illite (306-276 and 300-272 Ma) and older dates for 'detrital' 2M₁ illite (554-502 and 471-427 Ma). Conversely, IAA plots for the superimposed blue fault-gouge reverse the 'normal' trend, with older 'authigenic' 1M illite (348-314 and 276-25 Ma) and younger ‘detrital’ 2M₁ illite (258-234 and 250-226 Ma) ages. We propose this results from heterogeneous shearing and strain localisation. In localised pods of the blue fault-gouge, strain accelerated clay mineral growth, increased crystallite-size, and facilitated polytypic transformation from 1M to 2M₁ illite via defect migration. Elsewhere in the blue fault-gouge, and in the red foliated chaotic fault-gouge, low strain regimes allowed the 1M polytype to remain unaltered.