Silvio Seno

Plio-Quaternary tectonic evolution of the Northern Apennines thrust fronts (Bologna-Ferrara section, Italy): seismotectonic implications

The outermost, NE-verging fronts of the Northern Apennines (Italy) are overlain by a thick syntectonic sedimentary wedge filling up the basin beneath the Po Plain. Due to fast sedimentation rates and comparatively low tectonic rates, the fronts are generally buried. Evidence for their activity includes scattered historical and instrumental earthquakes and drainage anomalies controlled by growing buried anticlines. The largest earthquakes, up to M w 5.8, are associated with active compression, with a GPS-documented shortening rate <1 mm/a. We used geological, structural and morphotectonic data to draw a N-S-striking section between Bologna and Ferrara, aimed at analyzing whether and how the deformation is partitioned among the frontal thrusts of the Northern Apennines and identifying the potential sources of damaging earthquakes. We pointed out active anticlines based on the correspondence among drainage anomalies, historical seismicity and buried ramps. We also analyzed the evolution of the Plio-Quaternary deformation by modeling in a sandbox the geometry, kinematics and growth patterns of the thrust fronts. Our results (i) confirm that some of the main Quaternary thrusts are still active and (ii) highlight the partitioning of deformation in the overlap zones. We note that the extent and location of some of the active thrusts are compatible with the location and size of the main historical earthquakes and discuss the hypothesis that they may correspond to their causative seismogenic faults.

U–Pb zircon ages for post-Variscan volcanism in the Ligurian Alps(Northern Italy)

Abstract: Ages of Permian volcanic rocks from the Ligurian Briançonnais domain (Western Italian Alps) have been determined by laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircon. Three major volcanic units yielded zircons that were dated: calc-alkaline rhyolites (285.6 ± 2.6 Ma), andesites (with inherited cores yielded ages c. 476 Ma and older) and voluminous rhyodacites–rhyolites (272.7 ± 2.2 Ma). Following an amagmatic, sediment-starved time gap of c. 14 Ma, alkaline volcanic activity is recorded, at the top of the sequence, by K-alkaline rhyolites dated at 258.5 ± 2.8 Ma. The Ligurian segment of the Southern Variscan belt records transtensional and then extensional tectonics associated with the volcanic activity. The switch from calc-alkaline to alkaline activity corresponds to the transition from a post-orogenic to an anorogenic setting in the Southern Variscides; it may represent progressive and increasing delamination of the continental lithosphere, accompanied by partial melting of the lithospheric mantle. Supplementary material: Technical and data acquisition parameters, U–Th–Pb isotope analyses and calculated ages of zircons from samples and standard, and trace element compositions of selected zircons are available at: http://www.geolsoc.org.uk/SUP18322.

Analogue modeling of positive inversion tectonics along differently oriented pre-thrusting normal faults: An application to the Central-Northern Apennines of Italy

Inversion tectonics represent a key process in many orogens worldwide. The related mechanisms of fault reactivation and the effects of an articulated preshortening setting on thrust and fold development are still challenging questions. Modes and geometries of inversion have been the object of several analogue models. In this work, we analyzed the influence of an articulated high-angle preexisting discontinuity in the development of thrusts using sandbox modeling. The model geometry is based on the architecture of the major faults in the Central-Northern Apennines of Italy, where differently oriented Mesozoic–Cenozoic inherited extensional structures are clearly detectable and display contrasting styles of positive inversion tectonics. Quartz-sand is the analogue material adopted to model Mesozoic–Cenozoic sedimentary successions, and glass microbeads represent preexisting fault rocks. The geometry of the segmented preexisting structure is composed of two segments with the same dip (∼60°): one oblique and another orthogonal to the shortening direction. Our results show that different styles of positive inversion tectonics can coexist and that the obliquity angle between inherited structures and the shortening direction is a leading factor controlling the degree of inversion: The oblique segment of the discontinuity exhibits a complete reactivation, whereas along the orthogonal segment, shortcut is the prevalent mechanism. The oblique element, moreover, represents a cross-strike discontinuity that guides the localization and curved geometry of the thrusts, compartmentalizing the deformation. Our findings can be applied to fold-and-thrust belts characterized by the presence of cross-strike discontinuities.

Reconciling deep seismogenic and shallow active faults through analogue modelling: the case of the Messina Straits (southern Italy)

Abstract: The catastrophic 28 December 1908, Mw 7.1, Messina Straits earthquake was generated by a large, low-angle, SE-dipping, blind normal fault. A number of shallow, high-angle normal faults arranged in a graben-like fashion occur in the same area both on land and offshore, reaching the surface and in some instances affecting recent deposits. These faults are normally interpreted as active and have often been considered potentially seismogenic. We used an analogue modelling approach to simulate the evolution of a large, low-angle normal fault and investigate its relationships with the overlying secondary faults. We find that these faults represent the brittle surface expression of the long-term activity of the underlying master fault, and that all faults mapped by previous workers in the Messina Straits are compatible with sustained slip along the fault responsible for the 1908 earthquake. Our results confirm that analogue modelling provides a useful tool to investigate the evolution and the hierarchical relationships of fault systems, suggesting that this approach is effective in the investigation of complex seismogenic areas.

From finite strain data to strain history: a model for a sector of the Ligurian Alps, Italy

Bulk strain measurements were made in a Penninic sector of the Ligurian Alps. Previous detailed mapping, supported by petrographic and structural analyses, had shown that the oldest (D1), complex Alpine episode produced km-scale sheath-like folds, involving basement and cover, before the activation of a set of thrusts leading to nappe superposition. By combining all data, a kinematic model has been developed into which the subsequent deformation steps have been set. In particular, total strain constraints led to the suggestion of a combination of relatively low values of pure shear and simple shear for the genesis of the sheath-like folds. A computer simulation has proved that this suggestion is tenable. The model was then used to factor the bulk strain into increments—starting from the diagenetic compaction of the cover, up to the last ‘late D1’ phase—necessary to obtain results that compare well with the finite strain data. Some theories were also deduced on the possible attitude of the pre-Alpine schistosity in the basement rocks at the beginning of Late Variscan times.

Finite strain and deformation within the Briançonnais Castelvecchio-Cerisola nappe of the Ligurian Alps, Italy

Abstract The structural history of the Castelvecchio-Cerisola Brianconnais nappe (Ligurian Alps, Italy) was dominated by a southwestward overthrust shear that resulted in isoclinal folding and schistosity-lineation fabric development within the nappe. This deformation was due to the intracontinental shortening related to the collision of the European and Adriatic plates during Late Cretaceous-Eocene times. Finite strain, produced during the isoclinal folding and the schistosity-lineation fabric development, was determined from quartzites, conglomerates and tuffs using Frys method. Strain type and intensities are strongly dependent on rock type: other factors, like the proximity to the major thrusts or the position in relation to the first phase recumbent folds, seem to be of secondary importance. The quartzites and conglomerates display a constrictional strain or else they approximate to plane strain, while the pyroclastic rocks fall into the field of apparent flattening and are characterized by a higher intensity of deformation. During nappe emplacement the deformation was not accommodated by simple shear alone: the interpretation of strain patterns in the Castelvecchio-Cerisola has led to a model in which longitudinal strain was superposed on layer-parallel simple shear. Schistosity at a low angle to the major thrust faults, incremental extension directions deduced from pressure shadows, very frequent veins and stylolitic structures parallel to bedding support this hypothesis. The data presented here emphasize that subvertical thinning associated with thrust shear and a small extension perpendicular to the emplacement direction occurred in the Castelvecchio-Cerisola nappe.

The thrust zone of the Ligurian Penninic basal contact (Monte Fronté, Ligurian Alps, Italy)

ABSTRACT This paper presents a geological map and interpretative cross-sections, which illustrate the structure of a major fault zone of the Alps, that is, the Ligurian segment of the Penninic Basal Contact (PBC), which led to the emplacement of the orogenic wedge onto the European crust. Chaotic deposits, whose origin is debated, characterize the footwall of this complex thrust zone. The lower part of the sequence containing chaotic deposits consists of low deformed paraconglomerates and exotic megablocks embedded with turbidites. Conversely, the highly deformed upper part of the sequence englobes fragments of substratum-derived succession and is bounded by thrust planes. The nature of these chaotic deposits suggests an origin by gravitational processes related to the unstable front of the advancing wedge associated with offscraping of tectonic slices during the thrusting of allochtonous nappes along the PBC thrust zone.

Large sheath folds in the Briançonnais of the Ligurian Alps reconstructed by analysis of minor structures and stratigraphic mapping

This study presents a geometric and structural analysis of curvilinear sheath folds exposed in blueschist rocks of the Ligurian Alps. Field data are presented in a geological map of the structural synthesis with related geological sections (at the 1:10,000 scale) that illustrate the relationships and evolution of large-scale, sheath folds within metamorphic rocks. We based our analyses on the geometric parameters of more than 40 minor folds, as shape (hinge-Lm1 angle, main axial plane/S1 angle, interlimb angle and hinge curvature), asymmetry, fold hinge/stretching lineation obliquity and structural facing pattern. The summary of the whole data depicts a coherent 3D structure showing several orders of minor folds. Sense of asymmetry of minor folds and stratigraphic order has been used to reconstruct a reliable large-scale structure, and to define the sheath fold shape.

Fracture patterns evolution in sandbox fault-related anticlines

Timing and distribution of fractures related to folding is a crucial topic for migration and accumulation of hydrocarbons in structural traps. We performed sandbox analogue models to investigate the fracture patterns evolution in fault-related anticlines, e.g. fault-propagation and fault-bend fold. Analogue models allow us to: ( i ) reproduce geometry and kinematics of different kinds of folds, and ( ii ) describe and quantify fractures through time in all deformation stages. Three-dimensional digital model reconstructions facilitate the comparison of different steps in the evolution of fault-related anticlines and to highlight similarities and differences in fractures development. Quantitative parameters of fractures as orientation, number and average length have been acquired in each step of deformation using an automated procedure. The fault-related anticline models show that fractures development evolves with a cyclic and non-linear trend, depending on the geometry and kinematic of anticlines. We validate our approach by comparing experimental results with natural cases. Our methodology combined with other studies such as seismic surveys, field analyses and boreholes data may contribute to understanding the nature of fracture networks in real fault-related anticlines, identifying uncertainties and helping to reduce risks during exploration and drilling activities.