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