Massimiliano R. Barchi

A low-angle normal fault in the Umbria region (Central Italy): a mechanical model for the related microseismicity

Abstract In the Northern Apennines, the upper crust is thinned by a set of east-dipping low-angle normal faults (LANFs): the easternmost and more recent of these LANFs is the Altotiberina Fault (ATF) located in Northern Umbria. The geometry of the ATF has been reconstructed matching surface geology with seismic reflection profiles and borehole data. The fault, whose average dip is ∼20°, borders the Upper Pliocene–Quaternary Tiber basin and has a displacement of about 8 km. The deeper portion of the ATF is located below the axial zone of the Northern Apennines where the strongest instrumental and historical seismicity is recorded; the microseismicity of the region ( M σ 1 , to evaluate the boundary conditions for the brittle activity of the ATF. The fault can be reactivated for low values of differential stress ( σ 1 − σ 3 T ∼10 MPa), and tensile fluid overpressure P f > σ 3 (e.g. λ v >0.93). In the peculiar situation of the Northern Apennines, the deep emissions of large amounts of CO 2 documented in the area can be entrapped in their ascent by structural seals (e.g. ATF) favouring localised fluid overpressures. The impossibility of sustaining P f > σ 3 for wide fault portions, counteracted by hydraulic fracturing, increased permeability under low effective stress and load weakening behaviour for normal faulting, would prevent the nucleation of moderate ruptures along the fault. The short-lived attainment of P f > σ 3 along small fault portions can account for the microseismic activity located along the ATF, which occurs on rupture surfaces in the range of 10 −1 –10 −3 km 2 .

The crustal structure of the northern apennines (Central Italy): An insight by the crop03 seismic line

In this paper, the CROP03-deep seismic reflection profile in the Northern Apennines is described and re-considered in light of new geophysical data and interpretations made available in the last five years (particularly from heat flow measurements, aeromagnetics, tomography, active stress determination and passive seismology). The crustal structure of the Northern Apennines is shown to be composed of two distinct domains. To the west is the Tyrrhenian domain and to the east is the Adriatic domain. These domains have distinctive geological and geophysical characteristics that exhibit distinct reflectivity patterns at all crustal levels. In the Tyrrhenian domain, the Upper Oligocene-Lower Miocene compressive structures are no longer recognizable, because they are dissected by subsequent extensional tectonic features. The seismic profile highlights the strong asymmetry of extensional deformation, and the upper crust is affected by a set of six major, east-dipping, low-angle normal faults. In the Adriatic domain, compressive tectonics have acted since the Middle-Miocene, and the pattern of shallow contractional structures is well preserved. The geological interpretation of the seismic data supports a thick-skinned style of deformation, where the basement is involved in the major thrust sheets. The good quality of seismic data also allows for determining the total shortening produced by the contractional structures. In the central part of the profile, at the border between the Tyrrhenian and Adriatic domains, seismic data shows the presence of an intermediate sector. The sector consists of a highly reflective window, where the refraction data indicate a local doubling of the crust for about 30 km. A scenario is presented that attempts to describe the geodynamics that drove the tectonic evolution of the Northern Apennines since the Upper Oligocene.

Outer Northern Apennines

The outer Northern Apennines (ONA) are an arc-shaped fold-and-thrust belt, with northeastward convexity and vergence, that plunges northwestward, extending through Romagna and Umbria—Marche to northern Latium. From the SW to the NE, it is situated between the inner Northern Apennines and the Po Plain—Adriatic foreland. To the S, the volcanic products of the Roman magmatic province cover it, while to the SE it is bounded by the Latium—Abruzzi carbonate platform. As for the Alps—ONA boundary the reader is referred to Castellarin (this vol., Ch. 13).

Seismic expression of active extensional faults in northern Umbria (Central Italy)

Abstract This paper deals with the geometry and kinematics of the active normal faults in northern Umbria, and their relationship with the seismicity observed in the area. In particular, we illustrate the contribution of seismic reflection data (a network of seismic profiles, NNW–SSE and WSW–ENE trending) in constraining at depth the geometry of the different active fault systems and their reciprocal spatial relationships. The main normal fault in the area is the Alto Tiberina fault, NNW trending and ENE dipping, producing a displacement of about 5 km, and generating a continental basin (Val Tiberina basin), infilled by up to 1500 m with Upper Pliocene–Quaternary deposits. The fault has a staircase trajectory, and can be traced on the seismic profiles to a depth of about 13 km. A set of WSW-dipping, antithetic faults can be recognised on the profiles, the most important of which is the Gubbio fault, bordering an extensional Quaternary basin and interpreted as an active fault based on geological, geomorphologic and seismological evidence. The epicentral distribution of the main historical earthquakes is strictly parallel to the general trend of the normal faults. The focal mechanisms of the major earthquakes show a strong similarity with the attitude of the extensional faults, mapped at the surface and recognised on the seismic profiles. These observations demonstrate the connection between seismicity in the area and the activity of the normal faults. Moreover, the distribution of the instrumental seismicity suggests the activity of the Alto Tiberina fault as the basal detachment for the extensional tectonics of the area. Finally, the action of the Alto Tiberina fault was simulated using two dimensional finite element modelling: a close correspondence between the concentration of shear stresses in the model and the distribution of the present earthquakes was obtained.

The Gubbio fault: can different methods give pictures of the same object?

Abstract Different investigation techniques (field geology and structural analysis, geomorphology, seismic reflection profiles and seismological data) have been used to develop images of the Gubbio fault, giving us the opportunity to compare different approaches and evaluate points of consensus and controversy in fault analysis and characterization. From our integrated analysis we define a well-focussed image of the geometry and kinematics of the Gubbio fault and the related Quaternary basin, while the seismogenic role of the fault is still ambiguous.

Interference between shallow and deep-seated structures in the Sicilian fold and thrust belt, Italy

Abstract: The fold and thrust belt in western Sicily is characterized by the presence and interference of shallow and deep-seated compressional structures, which were generated and developed at different structural levels. The shallow structures consist of imbricated thrusts and asymmetric folds, with a typical wavelength of 2 km, involving relatively thin deep-water units. These units are superimposed on thick platform carbonate units, along a wide and originally almost flat floor thrust. The axial trend of the folds is variable, as multi-phase folding often occurred, producing a characteristic interference pattern, reflecting continuous variations of the apparent transport direction during the emplacement (i.e. rotation of the allochthonous thrust sheets). The deep-seated structures consist of large, double-verging pop-up structures, with a typical wavelength of 5–10 km, involving thicker platform carbonate successions. The deep-seated structures are characterized by large folds, with vertical to overturned limbs, caused by high-angle, transpressive ramps that reactivated previous (i.e. Mesozoic) synsedimentary normal or transtensional faults. The floor thrust of the shallow structures was passively deformed by the subsequent growth of the underlying, younger deep-seated structures. Large clockwise rotations of the tectonic units occurred during the compressional deformation, and the amount of rotation is apparently related to the timing and amount of the tectonic transport.

The Umbria-Marche Apennines as a Double Orogen: Observations and hypotheses

The Umbria-Marche Apennines, an arc-shaped fold and thrust belt with eastward convexity and vergence, form the external part of the Northern Apennines. In the middle 1980s, the Umbria-Marche Apennines were interpreted by some as a classical thin-skinned foldthrust belt, with thrust sheets emplaced in an in-sequence, piggyback mode, from the interior to the exterior of the orogen, over a main, basal detachment in the Triassic evaporites. In the mechanics of this kind of edifice, the folds and thrusts would develop within a prism or tapered wedge, bounded at the base by an undeformed basement with a regional monocline dipping toward the hinterland, and at the top by a topographic and structural slope generally dipping toward the foreland. Other authors saw the Umbria-Marche Apennines as a more complex orogen, with basement involvement, possible tectonic inversions, and out-of-sequence thrusts. In the present paper, the geometry, structure, and tectono-sedimentary evolution of the Umbria-Marche Apennines are compared with the classical thin-skinned model. We suggest that this fold-thrust belt can be divided longitudinally into two sectors. The eastern part of the chain, comprising the high Mesozoic carbonate anticlinal ridges of the axial zone together with the Marche external folds, fits the classical model well, but the western part, comprising the Umbrian Pre-Apennines, shows striking differences. The boundary between the eastern and western parts of the Umbria-Marche Apennines is here termed the Scheggia-Foligno Line (SFL). East of this line, the Eastern Umbria-Marche Apennines show an eastward taper between the undeformed basement, dipping gently west, and an upper surface in which both topographic and structural elevation decrease toward the east. West of the Scheggia-Foligno Line, by contrast, seismic reflection profiles and subsurface data show basement involvement in the thrusting at shallow depths, while both the topographic and structural elevations are anomalously low compared to the more easterly parts of the chain. There is also a notable discontinuity in foredeep-basin evolution at the Scheggia-Foligno Line, with welldeveloped foredeep basins in the Umbrian Pre-Apennines and the external Marche belt (the Marnoso-arenacea and Marche Plio-Pleistocene, respectively), whereas only thrust-top basins developed in the axial zone, during the Tortonian-Messinian interval. Various mechanisms, not all mutually exclusive, might be invoked to explain the discontinuity at the Scheggia-Foligno Line. These possible explanations include causes of local character, linked to the sedimentary and tectonic evolution of the region, involving episodic departures from steady-state conditions, interrupting the regular growth of the accretionary wedge. However, it also possible that the Western and Eastern Umbria-Marche Apennines represent completely different orogenic systems, with different causes possibly with the former related to Corsica-Adria collision and the latter due to slab rollback of Adriatic lithosphere. In either case, this study demonstrates the complexity of evolution of the Northern Apennines, with adjacent zones showing abrupt variations in their history and style of deformation, which are difficult to incorporate in a single, unified geodynamic model.

Morphotectonics of the Upper Tiber Valley (Northern Apennines, Italy) through quantitative analysis of drainage and landforms

We present a geomorphological analysis of the recent extensional tectonics of a Quaternary continental basin in the Northern Apennines (Italy). The study area is focused on Upper Tiber Valley (UTV), a basin elongated for 70 km in NNW-SSE direction hosting the Tiber River. The area is characterized by a series of features that make it an excellent case study: (i) homogeneity of lithology (ii) active faults, and (iii) strong morphogenetic activity. In this study, 36 hydrographical basins, tributaries of Tiber River, have been analysed. A preliminary qualitative geomorphological setting was outlined pointing out that the drainage river network shows meaningful evidence of tectonic control, such as abrupt changes in stream directions, knickpoints and steepness anomalies alignments along meaningful length in adjacent basins. Besides, the tectonic control is well marked in base level changes and consequent tectonically induced downcutting. Signs of neotectonics are highlighted by structural landforms too. The entrenchment of alluvial fans, the triangular facets and the fault planes are mapped by field survey and aerial photo interpretation. In addition, a quantitative analysis was also performed. Linear, areal and volumetric indexes related to drainage basins and river networks are taken into account. The geometry of the escarpments delimiting the basin and the landforms detected along the adjacent piedmont are investigated. The ranges of values, according to the existing literature, confirm a condition of wide-ranging morphological disturbance. In the central part of the study area, while the western basins are almost in equilibrium, the eastern ones reveal clear signs of disequilibrium, this is particularly evident along the distal segment of the river network. These data, joined with the characteristics of the escarpment and piedmont junction, confirm that the neotectonic activity, in the centre and in the eastern side of the basin, is the main factor controlling the morphological system.

Middle Miocene–Early Pliocene wedge-top basins of NW Sicily (Italy): constraints for the tectonic evolution of a ‘non-conventional’ thrust belt, affected by transpression

The study of geological evolution of a multiphase orogenic belt is complex, expecially when the tectonic events are superimposed in a coaxial fashion. The Sicilian fold-and-thrust belt represents an interesting case study, as a non-coaxial superimposition of structures is recognizable, owing to large synkinematic clockwise rotations during each of two subsequent compressional events. These rotations involved also the syntectonic basins that developed in the wedge-top depozone. This study aims to constrain the tectono-depositional evolution of the NW Sicily fold-and-thrust belt and the associated wedge-top depozone between the middle Miocene and the early Pliocene. Integrated analyses of stratigraphic, sedimentological and structural field data allow us to better constrain the transition between the two tectonic events. The syntectonic basins developed during the first (late Serravallian–early Tortonian) tectonic event were relatively wide and characterized by marine sedimentation. The onset of the second (latest Tortonian) transpressional event induced localized deformation into the wedge-top depozone and the syntectonic successions were accommodated as basin fill in progressively narrower and laterally discontinuous basins, bounded by transpressional structures. The lateral correlation of the wedge-top successions suggests a latest Miocene regional palaeoslope with a present-day WSW dip, which fits well with the tectonic transport calculated for the early compressional event.

Tectono-stratigraphic evolution of a basin generated by transpression: the case of the Early Pliocene Lascari Basin (northern Sicily)

In the present paper integrated stratigraphy and structural analyses are aimed at the description of a synsedimentary transpressional tectonic event, driving the onset and development of the Early Pliocene Lascari Basin, located NW of the Madonie Mountains(northern Sicily). Our data show that the transpressive tectonics generated a morphostructural high flanked by a deep and narrow tectonic depression, bounded by steep and tectonically controlled slopes. Within this depression an Early Pliocene fining and deepening upward sedimentary succession was deposited, unconformably overlying the already deformed substrate. The succession is made up of «base-ofslope» breccias, cross-stratified calcarenites, and bathyal limestones deposited in a high-energy palaeostrait setting. The growth geometry that characterizes the stratal pattern of the deposits suggests the activity of a synsedrimentary transpressional event during the Early Pliocene time. The roughly E-W trending Lascari Syncline, where the Early Pliocene syntectonic basin is hosted, belongs to a system of north verging folds, deforming the pre-existing, originally flat thrusts, generated in Langhian-Serravallian time. This paper presents an original interpretation of the stratigraphic and structural evolution of the study sector of the Sicily Fold and Thrust Belt. Our data contribute also to both define the structural style and constrain the timing of transpressional tectonic event in northern Sicily. A brief comparison performed between the data here presented and those coming from adjacent sectors of northern Sicily suggests that the transpressional tectonics in these sectors of the chain was active at least since the latest Tortonian till at least the Late Pliocene.