High-resolution multidisciplinary studies of fault zone architecture: Insights into deformation histories, fault mechanics, fluid circulation, weathering and…

Understanding how the interplay between tectonics, climate, and surface processes reflects the Earth’s endo- and exogenic dynamic behaviour, inevitably requires studying the nucleation, growth and development of faults. Faults shape the plumbing system of the Earth’s crust, promoting mass and heat transfer and steering fluid migration, storage and mineralizations. They control landscape evolution and impact society because, although they only occupy a small volume of the crust, they govern its modes of deformation by localizing earthquake slip, thus being sources of seismic hazard. To improve our understanding of faulting and produce time-constrained models firmly based on physical and chemical constraints, a deep knowledge of the structural, mechanical, hydrogeological and petrophysical properties of faults is thus required.

Long-lived, multiply reactivated faults can be architecturally complex, with every new deformation episode adding to this complexity by forming new brittle structural facies, altering the bulk and local permeability and steering the rheology of the deforming rock volume. This complicates the interpretation of the brittle archive of fault zones, which impacts on the interpretation of the local and regional deformation history. It also impacts on the seismogenic style associated with faulting (with coseismic rupturing and aseismic creep variably occurring in time and space), on modes of fluid ingress and circulation and formation of geofluid reservoirs. Recent studies have documented that this complexity is the norm rather than the exception and that it may result from deformation histories lasting many millions of years. The outcrops we study, therefore, only represent snapshots of this long history and rushed interpretations of their complexity and/or its downplaying may be grossly misleading.

To better understand the architecturally complex geometry and evolution in time and space of mature fault zones, the methodological approach to- and the first results from an ongoing study of the Carboneras Fault (CF) in the Betic Cordilleras of Spain are discussed. The CF is a NE-SW striking, 100 km long, upper crustal sinistral strike-slip fault that is described as accommodating c. 40 km offset with still ongoing distributed seismicity. It exhibits a complex architecture defined by strands of phyllosilicate-rich fault gouge enveloping domains of variably reworked host rock. Up to 14 brittle structural facies have been identified at four key outcrops. Structural analysis, X-ray diffraction and isotopic analysis of fault rocks have been systematically carried out. Sampling of each facies made it possible to define their mineralogical composition, the maximum temperature they were subjected to during faulting, their isotopic signature and the deformation mechanisms responsible for their formation. In-situ outcrop air-permeametry helped constrain the present-day permeability and its heterogeneity at the scale of the fault zone. K-Ar illite dating of eight gouge samples shows that faulting has been ongoing for >20 Myrs, and provides a comprehensive timeline for deformation localization down to the microscopic scale. Results from this high-resolution approach offer a comprehensive work protocol to untangle the spatiotemporal evolution of long-lived mature fault zones elsewhere.