Giusy Lavecchia

The Tyrrhenian-Apennines system: structural setting and seismotectogenesis

Abstract The available geological, geophysical and seismological data of the Tyrrhenian-Apennines system are briefly reviewed and synthesized in a number of sketch maps. Integrating and comparing these data, a large-scale simple-shear kinematic model is proposed for the Tyrrhenian-Apennines system that is here interpreted as an oblique asymmetric passive continental margin developed during the progressive counterclockwise rotation of the Adriatic foreland, from the Late Miocene to the present. In the frame of this working hypothesis, the thinning and stretching of the Tyrrhenian crust were accommodated by progressive eastward motion of extensional allochthonous slices, bounded by low-angle west-dipping oblique shear zones, rotating above a common detachment that penetrates into the mantle lithosphere. At any given time, the most external eastward moving slice is bounded by a transtensional shear zone at the top and by a transpressional one at the bottom. The displacement within the hanging wall of the transtensional shear zone is accommodated by high-angle normal faults and en echelon grabens; the displacement within the hanging wall of the transpressional shear zone is instead accommodated by co-axial en echelon sets of folds, thrusts and high-angle strike-slip faults. Thus, the development of the Late Miocene-Pliocene-Pleistocene contractional strain field of the Apennines of Italy might not be a consequence of complex subduction processes, but subordinate to the deformation processes that lead to the progressive opening of the Tyrrhenian Sea. The seismicity of the crustal Apennines of Italy is analyzed in the frame of the above history of progressive deformation, and a seismotectogenetic model is outlined.

Defining a model of 3D seismogenic sources for Seismic Hazard Assessment applications: The case of central Apennines (Italy)

Geology-based methods for Probabilistic Seismic Hazard Assessment (PSHA) have been developing in Italy. These methods require information on the geometric, kinematic and energetic parameters of the major seismogenic faults. In this paper, we define a model of 3D seismogenic sources in the central Apennines of Italy. Our approach is mainly structural-seismotectonic: we integrate surface geology data (trace of active faults, i.e. 2D features) with seismicity and subsurface geological–geophysical data (3D approach). A fundamental step is to fix constraints on the thickness of the seismogenic layer and deep geometry of faults: we use constraints from the depth distribution of aftershock zones and background seismicity; we also use information on the structural style of the extensional deformation at crustal scale (mainly from seismic reflection data), as well as on the strength and behaviour (brittle versus plastic) of the crust by rheological profiling. Geological observations allow us to define a segmentation model consisting of major fault structures separated by first-order (kilometric scale) structural-geometric complexities considered as likely barriers to the propagation of major earthquake ruptures. Once defined the 3D fault features and the segmentation model, the step onward is the computation of the maximum magnitude of the expected earthquake (Mmax). We compare three different estimates of Mmax: (1) from association of past earthquakes to faults; (2) from 3D fault geometry and (3) from geometrical estimate ‘corrected’ by earthquake scaling laws. By integrating all the data, we define a model of seismogenic sources (seismogenic boxes), which can be directly used for regional-scale PSHA. Preliminary applications of PSHA indicate that the 3D approach may allow to hazard scenarios more realistic than those previously proposed.

Architecture and seismotectonics of a regional low‐angle normal fault zone in central Italy

Information from surface geology, subsurface geology (boreholes, seismic reflection, and refraction profiles), and seismicity are used to depict the geometry and the possible seismogenic role of the Altotiberina Fault (AF), a low-angle normal fault in central Italy. The AF extends along the inner Umbria region, for a length of ∼70 km, with an average dip of ∼30° and an horizontal displacement up to 5 km. It emerges west of the inner border of the Tiber basin and deepens beneath the Umbria-Marche carbonate fold-and-thrust belt to a depth of 12–14 km. Close to the AF surface trace, low-angle synthetic east dipping normal faults extensively outcrop, whereas high-angle antithetic west dipping normal faults prevail farther east. Integrating geological and seismologic information, it can be stated that the AF behaves as an active extensional fault zone and represents the basal detachment of the west dipping seismogenic normal faults of the Umbria-Marche region. The AF belongs to a regional NE dipping low-angle normal fault system (Etrurian Fault System (EFS)), which extends for ∼350 km from northwestern Tuscany to southern Umbria. Early preliminary considerations suggest that the EFS may play an important role in controlling active extension and related seismicity in northern central Italy.

A structural model for active extension in Central Italy

This work presents a structural model for earthquake faulting in the Umbria-Marche Apennines (Central Italy). The model is derived by an integrated analysis of geological, geophysical and seismological data. At regional scale, the distribution and character of the seismicity appear to be mainly controlled by a low-angle east-dipping normal fault (Altotiberina fault, AF). The latter is the lower boundary of an active, continuously deforming hangingwall block moving toward NE. Moderate magnitude earthquakes (4 < M < 6), such as the Norcia 1979 (M = 5.9), the Gubbio 1984 (M = 5.2) and the Colfiorito 1997 (Mmax = 5.9), occur within the active hangingwall block and are related to the activity of major west-dipping normal faults detaching on the AF. The geometry of the deep seismogenic structures is listric (as in the case of Colfiorito) or more complex, because of local reactivation of pre-existing low-angle thrust (e.g. Gubbio) or high-angle strike-slip faults (e.g. Norcia). For all the analysed earthquakes the rupture nucleation is located at the base of the aftershock volumes, near the line of intersection between the SW-dipping normal faults and the east-dipping AF basal detachment. The progressive increase in depth of the earthquake foci from the north‐west (e.g. Gubbio, 6‐7 km) to the south‐east (e.g. Norcia, 11‐12 km) appears to be related to the eastward deepening of the basal detachment. These seismotectonic features are relevant for determining the seismogenic potential of the Apennine active faults, which depends not only on the length of the faults, but also on the depth of the detachment zone as well. Published by Elsevier Science Ltd.

A lithospheric-scale seismogenic thrust in central Italy

In this paper, the relatively deep (up to 90 km) seismicity of central Italy is considered in comparison with the upper crust seismicity at the outer front of the Apennine fold-and-thrust belt system and with the major crust-scale tectonic discontinuities outlined by deep crust seismic reflection and refraction profiles. In particular, attention is given to the presence of a Plio-Quaternary SW-dipping thrust zone cutting the entire Adriatic crust and emerging at the outer front of the Apennine thrust system, along the Po Plain-Adriatic belt. This discontinuity, detectable from the CROP 03 near-vertical seismic reflection profile, but only partially highlighted in previous studies, is here named Adriatic Thrust (AT). The geometry of the AT is compared with the distribution and kinematics of the Coastal-Adriatic upper crust seismicity, of the Marche Foothill lower crust seismicity and of the Apennine deep seismicity (base of the crust and upper mantle). A good spatial correspondence between the down-dip fault geometry and the seismicity is observed. The rheological stratification of the lithosphere controls the location of the seismic activity at different depth intervals along the fault plane (0–10, 15–25, 35–45 km). The reconstructed rheological conditions do not allow to justify in terms of brittle behavior the deepest earthquakes, located at depths from 55 to 70 km within the possible down-dip intra-mantle prosecution of the AT. These events might represent either co-seismic plastic instabilities due to high strain concentrations within the AT shear zone or seismic failure of the continental Adriatic lithosphere due to a strong increase in pore fluid pressure (infiltration from the asthenosphere of hydroxil-CO2 rich volatiles). The proposed interpretation of the AT as a lithosphere-scale seismogenic fault zone, has several implications: (1) the Po Plain-Adriatic front of the Apennine thrust system is still active; (2) the Apennine deep seismicity is not associated with a SW-dipping subduction plane; (3) seismogenic deformation in association with lithospheric failure is possible.

Is there a mantle plume below Italy

Some of the most diverse igneous rocks found on Earth occur along the length of Italy and in many of the islands in the southeastern Tyrrhenian Sea, all the result of Cenozoic magmatism. Magmas extremely rich in alkalis, particularly potassium, and many undersaturated with respect to silica, were erupted, as well as others of calc-alkalic affinity (see legend in Figure 1). Their origin has been the subject of heated debate, and there is still no general consensus about how they formed. Most attribute them to subduction-related processes (see Beccaluva et al. [2004] for a review); others consider them to be the result of within-plate magmatism [e.g., Vollmer, 1976; Lauecchia and Stoppa, 1996]. Still others consider magmatism the result of a deep, mantle upwelling within a slab window coupled with mixing between isotopically different reservoirs [Gasperini et al., 2002].

Late Pleistocene ultra-alkaline magmatic activity in the Umbria-Latium region (Italy): An overview

Abstract The “Umbria-Latium ultra-alkaline district” (ULUD), central Italy, consists of numerous, generally monogenetic igneous centres, all of which are Late Pleistocene in age. They show strong peculiarities in both volcanic behaviour and chemical characteristics. The igneous centres occur as cinder cones, lava flows, dykes, maars and diatremes along the Plio-Pleistocene graben faults and adjoining blocks. The existing subvolcanic rocks and lavas display rare mineral assemblages including melilite, leucite, kalsilite, monticellite, wollastonite, perovskite and BaSr-rich calcite. In addition, they contain very unusual Ti-rich garnet and ZrCaTi minerals, which are generally confined to carbonate-rich magmatic rocks. These rock types range from near-agpaitic melilitolite and melilitite to calcium-carbonatite. The pyroclastics commonly contain feldspars, mostly sanidine, and like the ULUD sub-volcanic rocks and lavas, commonly contain diopside, phlogopite, olivine and mantle micro-nodules. The pyroclastic rocks range from melilitite-carbonate tuffs to sanidine-bearing tuffs. In the ULUD, volcanic activity and rock types strongly resemble those generally observed in classical, continental-rift-related magmatic provinces. The affinity between the ULUD rocks and the Roman Campanian Province “High-Potassium Series” rocks helps in interpreting the magmatism developed at the Tyrrhenian eastern border as intra-continental and rift-related.

The 1984 Abruzzo earthquake (Italy): an example of seismogenic process controlled by interaction between differently oriented synkinematic faults

The evolution of the seismogenic process associated with the Ms 5.8 Sangro Valley earthquake of May 1984 (Abruzzo, central Italy) is closely controlled by the Quaternary extensional tectonic pattern of the area. This pattern is characterised by normal faults mainly NNW striking, whose length is controlled by pre-existing Mio–Pliocene N100F10j left-lateral strike-slip fault zones. These are partly re-activated as right-lateral normal-oblique faults under the Quaternary extensional regime and behave as transfer faults. Integration of re-located aftershocks, focal mechanisms and structural features are used to explain the divergence between the alignment of aftershocks (WSW–ENE) and the direction of seismogenic fault planes defined by the focal mechanisms (NNW–SSE) of the main shock and of the largest aftershock (Ms=5.3). The faults that appear to be involved in the seismogenic process are the NNW–SSE Barrea fault and the E–W M. Greco fault. There is field evidence of finite Quaternary deformation indicating that the normal Barrea fault re-activates the M. Greco fault as right-lateral transfer fault. No surface faulting was observed during the seismic sequence. The apparently incongruent divergence between aftershocks and nodal planes may be explained by interpreting the M. Greco fault as a barrier to the propagation of earthquake rupturing. The rupture would have nucleated on the Barrea fault, migrating along-strike towards NNW. The sharp variation in direction from the Barrea to the M. Greco fault segments would have represented a structural complexity sufficient to halt the rupture and subsequent concentration of post-seismic deformation as aftershocks around the line of intersection between the two fault planes. Fault complexities, similar to those observed in the Sangro Valley, are common features of the seismic zone of the Apennines. We suggest that the zones of interaction between NW–SE and NNW–SSE Plio-Quaternary faults and nearly E–W transfer faults, extending for several kilometres in the same way as M. Greco does, might act as barriers to the along-strike propagation of rupture processes during normal faulting earthquakes. This might have strong implications on seismic hazard, especially for the extent of the maximum magnitude expected on active faults during single rupture episodes. D 2002 Elsevier Science B.V. All rights reserved.

A geological model for the Colfiorito earthquakes (September-October 1997, central Italy)

In this study, surface and subsurface geologicaldata are integrated with seismological data in orderto reconstruct a structural model for theSeptember-October 1997 Colfiorito earthquakes. Theseismic sequence is mainly controlled by two majorSW-dipping normal faults outcropping in the area (M.Pennino-M. Prefoglio and M.Civitella-Preci faults).The activated faults detach, at depth, on a commoneast-dipping low-angle normal fault, the AltotiberinaFault (AF). The AF is interpreted as the base of anactive hangingwall block which is stretching towardNE. The decrease in maximum depth of the earthquakefoci from the Colfiorito area (about 8 km) to theSellano area (about 6 km), suggested by the available seismological data, could be related to the eastward-deepening geometry of the AFdetachment. The seismic fault planes, inferred fromfocal mechanisms and aftershock distributions, arecharacterised by a moderate dip (average 40°)toward SW, which appears to be independent from thepresence of pre-existing thrust planes.

The Umbria-Marche arcuate fold belt (Italy)

Abstract In this paper the structural setting of the Umbria-Marche fold belt is briefly described. The basement-cover relationships and the primary or secondary origin of the arcuate shape are also discussed. Mainly on the basis of structural data, it is suggested that the Umbria-Marche fold belt can hardly be the result of a thin-skinned tectonics under little overburden, but one or more deep-seated detachment levels must have worked (thick-skinned tectonics) during the Mio-Pliocene transpressional phase. Furthermore, an orocline origin for the arcuate shape of the fold belt, resulting from a progressive history of oblique right-lateral shearing, is suggested. During the first deformative stage (stage 1a, Late Miocene) the cover, detached from the basement, is deformed by a NW-SE trending linear fold belt, whilst the basement is deformed by oblique right-lateral shear. Then (stage 1b), the linear fold belt is progressively bent, by right-lateral dragging in correspondence with the deep-seated shear zones. During the second deformative stage (stage 2, Early Piocene), the cover deforms in connection with the basement: the northern and the central sectors of Umbria-Marche Apennines undergo a prevailing shortening accomodated by NW-SE, NNW-SSE dip-slip thrusting; the southern sector undergoes a prevailing right-lateral shearing, accommodated by NNE-SSW oblique thrusting.