Séminaire ISTeP - Giulio Viola

Séminaire ISTeP -  Giulio Viola

Jeudi 11 janvier 2024, à 13h, en salle Fourcade, (Tour 46-56, 5e étage. Campus Pierre et Marie Curie)

Giulio Viola (Université de Bologne, Italie - Dipartimento di Scienze Biologiche, Geologiche e Ambientali)

High-resolution multidisciplinary studies of fault zone architectures: A futile exercise or a necessary insight into regional deformation histories, fault mechanics and seismogenesis?

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 rheological/mechanical characteristics of the deforming rock volume. This remarkably complicates the interpretation of the brittle archive of fault zones, which may impact on the interpretation of the local and regional deformation history. It may also impact on the overall seismogenic style associated with faulting, with phases of coseismic rupturing and aseismic creep variably occurring in time and space. Recent studies of crustal-scale fault zones 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. Outcrops, therefore, only represent snapshots of this long history and rushed interpretations of their complexity and/or its downplaying may have negative consequences on conceptual models of deformation localization, fault mechanics and seismotectonics. 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 is 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. It belongs to the Africa–Iberia diffuse plate boundary and tectonic activity along it spans the mid Miocene -Present time interval, with still ongoing distributed seismicity. In its archetypal outcrops, the CF exhibits a complex architecture defined by strands of phyllosilicate-rich fault gouge enveloping domains of variably reworked host rock. We studied four key outcrops, aiming at elucidating the fine details of its spatial and temporal evolution to derive constraints upon its seismogenic style and mechanical evolution. Up to 14 brittle structural facies have been identified and characterized by a multidisciplinary approach including structural analysis, X-ray diffraction and isotopic analysis of fault rocks. 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 associated with different brittle structural facies are key to constrain, in an unprecedented fashion, the temporal dimension of faulting and provide a comprehensive timeline for deformation localization from the regional down to the microscopic scale. Results from this high-resolution approach are key toward a realistic analysis of the faulting history of the CF, and offer a comprehensive work protocol to untangle the spatiotemporal evolution of the mechanics and seismogenesis of long-lived mature fault zones elsewhere.