Preprints
https://doi.org/10.5194/egusphere-2022-229
https://doi.org/10.5194/egusphere-2022-229
 
13 May 2022
13 May 2022
Status: this preprint is open for discussion.

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

Giulio Viola1, Giovanni Musumeci2,3, Francesco Mazzarini3, Lorenzo Tavazzani4, Espen Torgersen5, Roelant van der Lelij5, and Luca Aldega6 Giulio Viola et al.
  • 1Department of Biological, Geological and Environmental Sciences, Università di Bologna, Italy
  • 2Dipartimento di Scienze della Terra, Università di Pisa, Italy
  • 3Istituto Nazionale di Geofisica e Vulcanologia, Pisa, Italy
  • 4Institute of Geochemistry and Petrology, ETH Zürich, CH-8092 Zürich, Switzerland
  • 5Geological Survey of Norway, Trondheim, Norway
  • 6Dipartimento di Scienze della Terra, Sapienza Università di Roma, Roma, Italy

Abstract. We studied the Zuccale Fault on Elba Island, 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), that is, volumes of deformed rock characterized by a given fault rock type, texture, color, 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 BSF’s requires tight constraints on their age of formation during the ZF life span 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 BSF’s. Two top-to-the E foliated gouge and talc phyllonite samples document faulting in the Aquitanian (c. 22 Ma), constraining E-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 < c. 5 Ma. The new structural and geochronological results reveal an unexpectedly long faulting history spanning a ca. 20 Ma long 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 environmental conditions during this prolonged history. We propose that the ZF started as an Aquitanian thrust which then became selectively reactivated by early Pliocene out-of-sequence thrusting during the progressive structuring of the Northern Apennines 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 Apennines 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 architecturally very complex. Their unraveling, 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 in space.

Giulio Viola et al.

Status: open (until 24 Jun 2022)

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Giulio Viola et al.

Giulio Viola et al.

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Short summary
A structural-geochronological approach helps to unravel the Zuccale Fault architecture. By mapping its internal structure and dating some of its fault rocks we constrained a deformation history lasting 20 Myr starting c. 22 Ma ago. Such long activity is recorded by now tightly juxtaposed Brittle Structural Facies, that is, different types of fault rocks. Our results have also implications on the regional evolution of the Northern Apennines, of which the Zuccale Fault is an important structure.