No abstract

The northern Apennines of Italy are a classical site for studying fundamental issues in tectonic studies, such as ophiolite formation and emplacement, kinematics of thrust wedges, role of in-sequence and out-of-sequence thrusting, and of along strike segmentation, synorogenic versus postorogenic extension, and interplay between tectonics, erosion, and sedimentation. Accordingly, the northern Apennines have been extensively studied since more than two centuries ago. Despite the huge amount of available data with different resolution, a 3-D comprehensive regional view combining in a modern framework all available surface and subsurface information for contiguous sectors of the chain is still lacking. We performed such an attempt in the area framed between the Taro valley to the north and the northern termination of the Alpi Apuane to the south. The region includes the main morphostructural zones of the north-west Apennines from the Tyrrhenian coast (SSW of La Spezia), through the main topographic divide of the Apennines, up to the foothills range of the chain to the north. The area has been investigated through a multidisciplinary approach that integrated surface geological data, collected during the last two decades of structural and stratigraphic field works, and subsurface geological data. The construction of two regional NE-SW trending cross sections (the Levanto-Pontremoli-Parma to the north and the La Spezia-Sarzana-North Apuane-Cerreto to the south), intersected by a NW-SE trending Taro-Lunigiana-Alpi Apuane composite section, allowed us to illustrate (i) the role of out-of-sequence blind thrusting in the basement and (ii) the presence of low-angle normal faulting and its relationships with recent to active high-angle normal faulting. Both extensional and contractional systems have relevant implications for the tectonics of the northern Apennines as well as the seismotectonics of the studied region.

Classical frictional fault reactivation modelsindicate that slip along misprinted fault planes is not possible under most conditions.\ud Nevertheless, active or exhumed low-angle normal faults have been described in many settings worldwide. This discrepancy is addressed by contrasting models: (1) those proposing that low-angle normal faults result from postkinematic passive rotation of\ud former high-angle extensional faults; and (2) those proposing that specific conditions can promote slip along misoriented fault\ud planes. This paper describes the Tellaro detachment, a mid–late Miocene low-angle\ud normal fault that was responsible for ~500 m of tectonic vertical thinning in the carbonatedominated\ud Triassic to Lower Miocene succession of the Northern Apennines, Italy. By integrating structural, petrographic, isotopic, and fluid inclusion data, we show that: (1) the main kinematic activity of the Tellaro detachment occurred between ~8 and 4 km depths and peak temperature ~190 °C; (2) dilational breccias, tens of cubic meters in volume, are frequently associated with major low-angle\ud fault segments; (3) slip along misoriented planes was favored by elevated fluid pressures and low differential stress; and (4) the\ud fault system was characterized by transient permeability pulses and overpressure buildups, associated with multiple fracturing and\ud cementation events that caused the downward migration of master slip surfaces. Results presented in this study show that: (1) in a fluid-active regime, continental crustal thinning can occur for shallow values of fault dip; (2) low-angle normal faults have a great influence on fluid circulation within the upper crust; and (3) episodic permeability enhancement and destruction in detachment faults can promote overpressure buildups,\ud triggering deformation episodes

Thrust wedge evolution is typically characterized by out-of-sequence thrusting, which can occur in both submarine and subaerial conditions to maintain the balance between gravitational and tectonic forces. The Gran Sasso Massif, in Central Italy, is a high topography region where the kinematics and environmental conditions of deformation of some fault zones are still controversial, and this bears important implications for the Central Apennines orogenic wedge evolution. To obtain further constraints on fault activity in the Gran Sasso Massif, we studied the Monte Camicia and Vado di Ferruccio thrusts using structural, petrographical, and geochemical analyses. Such dataset allowed us to constrain the structural-diagenetic evolution of the studied faults, which has first-order implications in the characterization of their paleo-hydraulic properties. Our results indicate that in the Vado di Ferruccio out-of-sequence thrust, pressure solution-mediated mass transfer promoted low-permeability conditions in the fault core that led to a semiclosed fluid circulation, pore fluid overpressuring, and dolomite crystallization in submarine conditions. In contrast, the Monte Camicia out-of-sequence thrust was characterized by dominant cataclasis in subaerial conditions, which facilitated meteoric fluid infiltration. By considering both fault zones as belonging to the same thrust system at crustal scale, we interpret these differences as indicating the occurrence of multiple pulses of thrusting during the exhumation of this sector of the Central Apennines, up to Quaternary times, when extensional faulting eventually dissected the thrust stack. This caused extensional reactivation of the Monte Camicia Thrust and related alteration in vadose zone conditions, leading to porosity enhancement, dolomite dissolution, and calcitization.