COMBINING STRUCTURAL ANALYSIS AND DATING OF AUTHIGENIC-SYNKINEMATIC ILLITE: A FURTHER STEP TOWARD UNRAVELLING BRITTLE FAULTING IN TIME AND SPACE (Invited Presentation)

Faulting accommodates deformation in response to the complex interplay of processes such as frictional sliding, cataclasis, fluid ingress and flow as well as rheological changes within generally dilatant rock volumes. Brittle faults thus become archives of the stress state and the physical/chemical conditions at the time of both initial strain localization and subsequent reactivation(s). Accessing those archives, however, is challenging due to the commonly convoluted internal architecture of faults and the broad spectrum of coexisting fault rocks. This is because, once formed, faults remain sensitive to variations in stress field and environmental conditions and are thus prone to reactivation also in regions only affected by weak, far-field stresses. A detailed, multi-scalar structural analysis of faults and fault rocks, therefore, has to be the starting point of any study aiming at the reconstruction of the complex framework of brittle deformation, either within a single structure or at the regional scale. However, considering that currently exposed faults only represent the end result of their often protracted and heterogeneous histories, the obtained structural and mechanical results have to be integrated over the full life span of the studied fault systems.

Dating of synkinematic illite/muscovite to constrain the time-integrated evolution of faults is, therefore, the natural addition to detailed structural analyses. By means of selected studies from different orogenic and tectonic settings, it will be demonstrated how this integrated approach may lead to the high-resolution reconstruction of brittle histories and, in turn, how it allows for multiple constraints to be placed on strain localization, deformation mechanisms, fluid flow, mineral alteration and authigenesis within brittle fault zones. Multi grain-size illite dating permits the investigation in time coordinates of the subtle details of initial localization and subsequent reactivation(s). This innovative approach makes it possible to refine the understanding of fundamental geological processes such as seismogenesis and strain localization/partitioning and helps us move forward towards fully time-constrained structural models.