The development of the North Giudicarie fault zone (Insubric line, Northern Italy)

Abstract

The kinematics of transpressive fault zones is generally characterized by a variable degree of strain partitioning. This process is enhanced by the presence of older faults, which partition the simple shear strain component more easily. The North Giudicarie line can be taken as a good example of strain partitioning because it is a left-lateral transpressive fault developed during the Oligocene–Miocene along an inherited normal fault system of late-Triassic to Cretaceous age. This paper analyzes two main points related to the kinematics of the North Giudicarie line: (1) the role of the inherited faults in the development of the North Giudicarie fault zone, and (2) the mechanisms of strain partitioning. These processes have been analyzed by structural analysis and detailed mapping of the North Giudicarie fault zone. The main tectonic contact of the North Giudicarie line is a northwest dipping reverse fault, which juxtaposes the Austroalpine basement rocks against the Southalpine cover. Oligocene foliated tonalites, together with basement- and limestone-derived mylonites, are found along the main fault plane. The brittle fault zone, up to 1 km thick, is mostly developed within the Southalpine cover rocks. This area is made up of several tectonic lenses, up to 1 km thick and several km long, bounded by anastomosing faults characterized by a normal offset. North-striking faults (e.g., the Trento-Cles line) branch from the North Giudicarie fault zone. Thrusting along the North Giudicarie line and left-lateral strike-slip along the anastomosing faults and the Trento-Cles line has been documented from shear-sense indicators observed in mylonites and small-scale fault surfaces. Stratigraphic data reveal that the Trento-Cles line reactivated pre-existing late Triassic or jounger normal faults. Therefore, it is suggested that the left-lateral slip component took place more easily along inherited faults. Deformation and metamorphic features of mylonites, and the estimated emplacement depth of the Oligocene foliated tonalites, can give a bulk estimate of the vertical displacement along the North Giudicarie line. The calculated thrust slip (∼21 km) is lower than the cumulative left-lateral slip along the whole Giudicarie fault system, as inferred from published regional data.