Structural characterization and K–Ar illite dating of reactivated, complex and heterogeneous fault zones: lessons from the Zuccale Fault, Northern Apennines

Abstract. We studied the Zuccale Fault (ZF) on Elba, part of Northern Apennines, to unravel complex deformation history that is responsible for remarkable architectural complexity fault. The ZF characterized by a patchwork at least six distinct, now tightly juxtaposed brittle structural facies (BSF), i.e. volumes deformed rock given fault type, texture, colour, composition, and age formation. rocks vary from massive cataclasite foliated ultracataclasite, clay-rich gouge highly sheared talc phyllonite. Understanding current spatial juxtaposition these BSFs requires tight constraints their formation during lifespan integrate geometries characteristics over time dimension faulting. present new K–Ar dates obtained three samples two different BSFs. Two top-to-the-east phyllonite document faulting in Aquitanian (ca. 22 Ma), constraining east-vergent shearing along already earliest Miocene. A third sample constrains later exclusively brittle, flat-lying principal slip surface < ca. 5 Ma. geochronological results reveal an unexpectedly long spanning 20 Myr interval framework evolution Apennines. architecture heterogeneous as it formed very times under conditions this prolonged history. propose started thrust then became selectively reactivated early Pliocene out-of-sequence thrusting progressive structuring Apennine wedge. These require critical analysis existing geodynamic models call alternative scenarios continuous convergence between late Oligocene with major intervening phase extension middle Miocene allowing isostatic re-equilibration Extension again still active innermost portion In general terms, long-lived, mature faults can be architecturally complex. Their unravelling, including understanding dynamic mechanical properties, multidisciplinary approach combining detailed analyses dating events recorded internal architecture, which phenomenal archive through space.