Giuseppe Naso

Time and space variability of “thin-skinned” and “thick-skinned” thrust tectonics in the Apennines (Italy)

In the Apennine fold and thrust belt of Italy, «thin-skinned» (i.e. detachment-dominated) and «thick-skinned» (i.e. crustal ramp-dominated) structures coexist, but with marked differences in both time and space. The external part of the northern Apennines and the deeper and younger portions (buried Apulian carbonates) of the thrust belt in the central and southern Apennines show limited amounts of shortening (in the range of 5–14 km). These result from similar deformation styles, involving the occurrence of relatively low-displacement, thick-skinned thrust ramps. The latter represent, at least in the northern Apennines, preexisting basement structures reactivated and inverted during contractional deformation. Interposed between the northern and southern parts of the fold and thrust belt, the central Apennines appear to constitute a transitional area in which strike-slip tectonics is relevant and carbonate platform units become predominant over pelagic basin ones, whereas the overall structure of the thrust belt becomes similar to that of the southern Apennines. In the latter, a peculiar structural style is revealed by the integrated analysis of surface and subsurface data. Structurally, the upper part of the thrust belt consists of allochthonous units made of Mesozoic peritidal carbonate platform and pelagic basin successions, and of Miocene foredeep sediments. These are completely detached from their original substratum and transported onto the 6–7 km thick, foreland carbonates of the Apulian platform. Based on available seismic data, the latter appears to be involved, together with the underlying Permo-Triassic clastics and, we infer, also the basement, in relatively low-displacement, thick-skinned structures. Therefore, in the southern Apennines, a transition from thin-to thick-skinned tectonics appears to have occurred through time. Thin-skinned structures characterise the shallower — and older — part of the thrust belt made of detached units, while a thick-skinned tectonic style is dominant in the buried Apulian carbonates of most recent accretion. The present boundary between the two different, superposed portions of the thrust belt consists of a low-angle, large-displacement thrust fault penetrated by numerous oil wells. Different styles and modes of contractional deformation in the investigated sectors of the Apennines appear to result from the geometrical requirement of maintaining strain compatibility and overall displacement continuity along a highly segmented orogen characterised by variable mechanical stratigraphy and southward increasing amounts of shortening.RiassuntoNella catena a pieghe e sovrascorrimenti dell’Appennino coesistono strutture che vedono coinvolto nella deformazione il basamento (tipo «thick-skin») e strutture scollate da questo (tipo «thin-skin»). La parte esterna dell’Appennino Settentrionale e le parti più profonde e più giovani (i carbonati della piattaforma Apula sepolta) dell’Appennino Centrale e Meridionale mostrano un raccorciamento limitato (compreso tra i 5 e i 14 km). Ciò deriva da stili deformativi simili, che producono rigetti relativamente bassi per la presenza di rampe di sovrascorrimento di tipo «thick-skin». Queste ultime rappresentano, almeno nell’Appennino Settentrionale, delle preesistenti strutture di basamento riattivate e invertite durante la deformazione contrazionale. Interposta tra le porzioni settentrionali e meridionali della catena, l’Appennino Centrale costituisce un’area di transizione in cui la tettonica trascorrente risulta di rilevante importanza e le unità carbonatiche di piattaforma divengono predominanti su quelle dei bacini pelagici, mentre la struttura generale della catena a pieghe e sovrascorrimenti diventa simile a quella dell’Appennino Meridionale. In quest’ultimo, lo stile strutturale tipico è rivelato dall’analisi integrata dei dati di superfice e di sottosuolo. Nella parte strutturalmente superiore di questo settore di catena sono presenti unità alloctone costituite da piattaforme a carbonati peritidali del Mesozoico e da successioni bacinali, sopra le quali sono presenti sedimenti miocenici di avanfossa e di bacini satelliti. Tale parte superiore è completamente scollata dal substrato di origine e trasportata sulla piattaforama carbonatica dell’avampaese apulo, che raggiunge potenze di circa 6–7 km. Sulla base dei profili sismici diponibili, la piattaforma apula appare coinvolta, insieme ai sottostanti sedimenti clastici permo-triassici e quindi anche al basamento, in strutture con basso rigetto di tipo «thick-skin». Di conseguenza, nell’Appennino Meridionale, un passaggio da un regime tettonico di tipo «thin-skin» ad uno di tipo «thick-skin» sembra essere avvenuto nel tempo. Le strutture «thin-skin» sono caratteristiche della parte più superficiale (e più vecchia) della catena costituita da unità scollate, mentre lo stile «thick-skin» è dominante nei carbonati apuli sepolti di più recente accrezione. Il limite attuale tra le due diverse, sovrapposte porzioni della catena è costituito da una importante superficie di sovrascorrimento a basso angolo, penetrata da numerosi pozzi petroliferi. Gli stili geometrici e le modalità di deformazione contrazionale differenziati nello spazio e nel tempo sembrano essere il risultato di esigenze di compatibilità geometrica della deformazione e di continuità dei rigetti lungo un orogene fortemente segmentato e caratterizzato da una stratigrafia variabile nelle sue caratteristiche meccaniche e da un aumento del raccorciamento verso Meridione.

Influence of palaeogeography on thrust system geometries: an analogue modelling approach for the Abruzzi-Molise (Italy) case history

Abstract The Abruzzi–Molise sector in the Central Apennines is a part of a fold and thrust belt that has been deforming since the Late Cretaceous as a result of collision tectonics between the European and Adriatic plates. The superposition of different deformational styles highly reworked the originally complex palaeogeography of this portion of the southern Tethyan margin. Analogue modelling has been performed on thrusting mechanisms in the Abruzzi–Molise area in order to (1) reduce the number of admissible hypotheses regarding palaeogeographic setting, and (2) define thrusting mechanics. Both of these goals are crucial for hydrocarbon exploration purposes. The sandbox apparatus used to simulate the undeformed passive margin consisted of a thin compartment juxtaposed against a thick one along a linear boundary having variable geometries and mechanical stratigraphy; a rigid but mobile backstop was used to deform the stratigraphy in a Coulomb thrust wedge. Results from six experiments show that the geometric relationships between different structural units depend on the distribution of palaeogeographic domains. These domains are defined by mechanical and/or geometrical parameters, such as the orientation between the maximum compression direction and the palaeogeographic boundary, the mechanical stratigraphy and the thickness of the successions reproduced in the models. The present-day tectonic styles and Meso–Cenozoic palaeogeography of the Abruzzi–Molise area are discussed in terms of the mechanisms and structures analysed through the models.

Neogene–Quaternary evolution of the central Apennine orogenic system (Italy): a structural and palaeomagnetic approach in the Molise region

Abstract We report on new palaeomagnetic and magnetic fabric analyses of mainly Upper Miocene sedimentary sequences from the external central Apennine fold and thrust belt (Molise area), where the principal compressive structures are clearly non-coaxial. The sampling was carried out on the E–W-oriented Matese–Frosolone thrust sheet, that for its geographical position and structural setting (superposition of thrusting, strike-slip and extensional tectonics since Late Miocene to present-day) represents a key structure for the comprehension of the Neogene–Quaternary evolution of the entire Molise area. Palaeomagnetic results suggest that the Matese–Frosolone thrust sheet counterclockwise rotated at least 35° after Messinian times. These data confirm that the present-day trend variability observed in the main compressional structures in the Apennine chain can be related to rotations about vertical axes rather than to changes in the stress field orientation, at least since Late Miocene times. Magnetic fabric analyses indicate that the studied sediments were subjected to very mild deformation, suggesting that the surface emergence of the thrust front of the Matese–Frosolone unit is located farther north, far from the studied area. Well-defined magnetic lineations of tectonic origin were only observed in sites close to localised belts of strike-slip deformation.

On the Performances of Site Parameters for Soil Classification

The selection of specific elastic response spectra according to soil categories is the standard to account for site effects in engineering design and general-purpose hazard maps. Most of the international seismic codes are based on the average shear wave velocity of the upper 30 m (Vs30) to discriminate between soil categories. The works of Borcherdt and Glassmoyer (1992) and Borcherdt (1994) were the first to propose the adoption of the VS30 as a tool to discriminate soils with similar seismic response. Nevertheless, after Borcherdt (1994), some doubts arose about the capability of Vs30 in site effects estimation (among others, Steidl 2000; Park and Hashash 2004; Stewart et al. 2003; Castellaro et al. 2008; Lee and Trifunac 2010). The objectives of this study are: (i) to find soil classes with a similar response to an earthquake; (ii) to develop empirical amplification factors for 5 % damped response spectral acceleration in the period range T 0.04–4 s; (iii) to identify proxies for site classification (Vs30, stratigraphy, fundamental frequency, etc.). We propose a soil classification, which is not based on a priori subdivision, but it relies on the natural aggregation of empirical amplification function of the sites, obtained by normalizing the spectra of recorded motions by a reference (rock) spectrum from a ground motion prediction equation (GMPE, developed by Bindi et al. 2011, based on the ITACA 1.0 database). The empirical amplification functions of sites are aggregated by means of cluster analysis.