Paolo Scandone

Segmentation and configuration of subducted lithosphere in Italy: An important control on thrust-belt and foredeep-basin evolution

The Apennine foredeep-basin system, located on continental crust, consists of four roughly parallel basin segments, each containing a flexural outer rise and an inner trough that deepens continuously toward the thrust belt. Flexural modeling of subsidence data across the basin indicates that this foredeep basin results mainly from deep subsurface loads acting on the subducted slab from within the subduction zone rather than from the topographic load of the Apennine Mountains. The geometry of the four basin segments observed within the Apennine foredeep is thus inferred to reflect subduction of segmented lithosphere, where each lithospheric segment corresponds to a segment of the foredeep basin. Beneath the basin and in the foreland the lithosphere is continuous: segmentation occurs only at depth beneath the thrust belt. This pattern olf segmentation of the subducted lithosphere at depth is reflected in the large-scale geometry of the outer part of the Apennine thrust belt and appears to control it. Similarly, the pattern of segmentation also appears to control the position of the eastern limit of back-arc type extension in western Italy. These relations strongly suggest a genetic relation between subduction-zone processes, foredeep-basin geometry, and thrust-belt evolution, in which the configuration of the subducted lithosphere largely controls the large-scale surface deformation observed in the Apennine system.

Construction of a Seismotectonic Model: The Case of Italy

Abstract—Procedures for constructing a seismotectonic model of Italy, designed to be used as a basis for hazard assessment, are described. The seismotectonic analysis has essentially been based on a GIS-aided cross-correlation of three data sets concerning:¶— the 3-D structural model of Italy and surrounding areas;¶— the space distribution of historical and present seismicity; ¶— the kinematic model of the Central Mediterranean region, referred to the last 6 Ma and including the available information on the present-day plate motion and stress field.¶The seismicity pattern in the study area is controlled by a quite complex geodynamic framework which includes:¶— continent–continent convergence (Alps and Dinarides) with development of a neutral arc bordering the plate margins;¶— plate divergence across margins characterized by passive slab sinking (Northern Apennines and Calabrian Arc), with development of backarc basins (Northern Tyrrhenian Sea and Southern Tyrrhenian Sea) flanked by forelandward migrating thrust belt-foredeep systems;¶— plate divergence across a margin previously characterized by lithosphere sinking and afterwards discharged from the subducted slab (Southern Apennines), with development of quite peculiar rift processes within the inactive thrust belt;¶— transpression (Northern Sicily) due to the combined effect of plate convergence (Africa-Europe) and high-rate flexure-hinge retreat of an intervening plate (Adria microplate) with high angles between the respective slip vectors;¶— intraplate strain partition and fault activity (mainly combined strike-slip and thrust motions), possibly in correspondence of inverted structures.¶The results of the seismotectonic analysis are synthesized in a zonation of Italy in which every delimited zone corresponds to the surface projection of a kinematically-homogeneous segment of a seismogenic fault system. In Cornell-type hazard evaluations every polygon should be considered as a homogeneous source-zone, seat of randomly-distributed earthquakes. A homogeneous mechanical behaviour of an entire zone and a random earthquake-distribution within a single source zone obviously represent oversimplified assumptions since every zone includes one or more master-fault segments responsible for the greatest events in the area and several second-order associated faults responsible for the background minor seismicity. Therefore, major faults and background seismicity should be treated separately. Nevertheless, the oversimplified assumption of homogeneous seismic zones was the price the authors consciously paid to produce, in a reasonably short time, a homogeneous product relative to the entire national territory, suitable for earthquake hazard evaluation and for decisions regarding risk mitigatiton.

Tyrrhenian basin and Apennines. Kinematic evolution and related dynamic constraints

The post-Oligocene kinematic evolution of the central Mediterranean region is synthetically described. The proposed reconstruction is based on a comparative study of tectonically- controlled sedimentary sequences and of coeval tectonic features in areas experiencing stretching (Algero-Provencal Basin, Tyrrhenian Basin) and shortening (Apennines). The derived step by step palinspastic reconstruction at Langhian times states a number of constraints for the geodynamic processes that affected the area during Neogene and Quaternary times. Both continental (often thinned) and oceanic lithosphere were involved in subduction processes. The former prevailed in the northern sector, while both oceanic and continental lithosphere were present in the southern area. The long duration of the subduction processes in the southern sectors may be related to the existence of large portions of sinking oceanic lithosphere which dragged down intervening portions of continental lithosphere.

Seismic exploration in complex terrains : A processing experience in the Southern Apennines

We discuss a data‐processing sequence adopted to reprocess a seismic line that crosses the Italian southern Apennines from the Tyrrhenian Sea to the Adriatic margin and investigate both the overthrust and foreland areas. We first determine the main causes of the very low S/N ratio in the field data and then propose a processing sequence aimed at exploiting the signal content, also making use of a priori geological knowledge of this area. Our work indicates a combination of causes for the very low quality of the seismic data. These include length of the spread (about 20 km) that is unfavorable because of the rapid variation in the near‐surface geology, tectonic complexity, crooked‐line acquisition, and the rough topography associated with outcropping rocks characterized by highly variable velocities. Based on the outcome of this data analysis, we present a processing sequence driven by knowledge of the regional tectonic setting and by knowledge of the shallow subsurface geology. The main effort is in remov...

Ligurian-derived olistostrome in the Pseudomacigno Formation of the Stazzema Zone (Alpi Apuane, Italy). Geological implications at regional scale

In the Stazzema Zone (southern part of the Alpi Apuane tectonic window) an olistostrome of kilometric extent (here informally called Ricavo olistostrome) incorporating blocks and slides of Lower Cretaceous limestones referable to the “Argille a Palombini” Formation of the Internal Ligurian Units forms a well-defined lens-shaped body in the upper portion of the Pseudomacigno Formation. The Pseudomacigno Formation is an Oligo-Miocene siliciclastic foredeep deposit making up the upper portion the Alpi Apuane metamorphic sequence. In spite of the greenschist metamorphic imprint of the sequence, the exotic limestones included in the olistostrome have locally escaped recrystallization so that their primary depositional texture and microfossil content are still recognizable. The microfacies of these limestones are represented by mudstones/wackestones with abundant calcitized radiolarians and sponge spicules associated with rare calpionellids and some planktonic forams indicative of the Valanginian. The occurrence of exotic materials derived from the Internal Ligurian Units in the Pseudomacigno Formation is consistent with the current model of forward migration of the thrust belt-foredeep system in the Northern Apennines according to which the tectonic transport of the Ligurian/Subligurian Nappes was accompanied by the emplacement of olistostromes in the flysch deposits. The presence of an olistostrome derived from the Ligurian Nappes in the Pseudomacigno Formation contributes in a roundabout way to highlight some contradictions existing between the 27 Ma age of metamorphism of the Alpi Apuane Unit obtained from radiometric measurements, widely accepted in the literature, and the regional paleogeographic model currently adopted by the Apennine geologists. The mere presence of Ligurian-derived materials in the Pseudomacigno Formation, in fact, establishes new constraints that make a 27 Ma age of the metamorphic peak incompatible with the paleogeographic reconstruction of the Northern Apennines that relocates the domain of the metamorphic Tuscan Units east of the original domain of the Tuscan Nappe. Following this restoration, ages of metamorphism not older than 13–14 Ma would be expected.