Central-northern Apennines Research Articles

Relationships between thrusts and normal faults in curved belts: New insight in the inversion tectonics of the Central-Northern Apennines (Italy)

The relationships between thrusts and normal faults represent primary constraints in the reconstruction of the modes and timing of pre-, syn- and post-orogenic deformation events in fold-and-thrust belts. Such relationships are well exposed in curved orogenic belts where the thrusts are oblique to the trend of normal faults.We study the NNE–SSW-trending Olevano-Antrodoco-Sibillini oblique thrust and its crosscutting relationships with NW–SE-trending normal faults in order to constrain the Neogene–Quaternary deformation history of the Central-Northern Apennine (Italy). The analysis of structural and geological data allowed us to reconstruct the geometric and kinematic constraints of two inversion events: 1 – During the Pliocene, positive inversion reactivated the NNE–SSW-trending pre-existing Ancona-Anzio normal fault as the Olevano-Antrodoco-Sibillini oblique thrust ramp and caused the shortcut of NW–SE-oriented normal faults; 2 – During the Quaternary, negative inversion reactivated NW–SE-trending pre-thrusting normal faults.The growth of the NW–SE Quaternary normal faults causes seismicity and is responsible of the development of wide Quaternary intramontane basins. Their distribution and the related seismicity have been controlled and compartmentalized by NNE–SSW-trending oblique thrusts. Thus, the crosscutting relationships between thrusts and normal faults are crucial in seismic hazard assessment.

A review on the driving mechanism of the Tyrrhenian–Apennines system: Implications for the present seismotectonic setting in the Central-Northern Apennines

The formation of the Tyrrhenian–Apennines system has been characterized by a complex distribution of major tectonic events, such as opening of basins, migration and distortion of orogenic belts, retreat of trench zones, stopping of consuming processes and starting of new subductions. The various aspects of each event, such as timing, location, dimension and deformation style, impose important constraints on the genetic mechanism. In this work we argue that the numerous constraints implied by all events together may be plausibly and coherently explained as effects of the convergence of the confining Africa, Arabia and Eurasia plates. Crucial evidence for discriminating among the various geodynamic hypotheses so far advanced is provided by the structural/tectonic information recently provided by CROP seismic sections in the study area. The proposed evolutionary history is illustrated by several paleogeographic maps, which provide a more detailed reconstruction than previous attempts. Particular attention is paid to describing the tectonic setting developed since the middle Pleistocene in the Apennines belt and its connection with seismic activity.

The last interglacial pedocomplexes in the litho- and morpho-stratigraphical framework of the central-northern Apennines (Central Italy)

Recently in Italy, a great deal of effort has been devoted to the mapping of Quaternary continental deposits. Several Unconformity Bounded Stratigraphic Units (UBSU) have been defined. The unconformities indicate important phases of changing dynamics linked to Quaternary climatic changes, and have been related to valley downcutting simultaneous with paleosol formation during interglacials. However, glacial periods are characterised by deposition of thick continental sequences. We suggest that the UBSU's belonging to the late Middle Pleistocene (MIS 6) and Late Pleistocene can also be defined under a pedostratigraphic point of view. Locally at the top of the UBSU attributed to MIS 6, buried or relict paleosols are preserved. The paleosols developed in calcareous fluvial and moraine gravels with minor flints. The pedocomplexes are made of three paleosols separated by erosion surfaces and/or stonelines. The older paleosol (MIS 5e) is severely truncated and is characterised by a Ck horizon overlain by strongly leached reddish Bt horizons with flinty rock fragments (with a diagnostic wavy lower boundary). In some sites, subsequent secondary precipitation of carbonates transformed this horizon into a Btk. This paleosol is buried by a younger paleosol (MIS 5c) with less leached reddish Btk horizons, quartz of aeolian origin, and very small and scarce calcareous clasts. The main pedofeatures are associated with colluvial processes, biological activity, and carbonate precipitation. This paleosol is also truncated and buried under a similar paleosol (MIS 5a). Therefore, the lower paleosol of the pedocomplex can be attributed to the Eemian, and constitutes a valid marker for the definition and mapping of the underlying and overlying sedimentary units. In other parts of the Apennines, no sedimentary units have been found that correspond to the unconformities separating different pedostratigraphical units. In order to map the different UBSU's, due to the poor lateral continuity of the preserved paleosols, morpho- and litho-stratigraphical investigations and correlation are always recommended.

Geometry, segmentation pattern and displacement variations along a major Apennine thrust zone, central Italy

Hundreds of kilometres-long, arc-shaped, continuous thrust faults commonly imply very large displacements by detachment-dominated thrusting. Therefore, their occurrence has a large impact on fold–thrust belt structure. One of the major thrust faults of the central-northern Apennines, the Umbria–Marche–Sabina Thrust Zone, is traditionally believed to represent one such feature, characterised by displacements of several tens of kilometres. However, detailed studies of this structure revealed that it is actually composed of a series of partially overlapping fault segments, rather than consisting of a single, continuous thrust. Cross-section balancing and restoration, carried out by the integration of surface geological data with available sub-surface information, points out relatively limited amounts of thrust displacement (<10 km). Displacement–distance profiles show moderate displacement gradients well compatible with those reported for coherent thrust sheets. They also suggest that individual thrust segments were originally isolated and then grew by lateral propagation, leading to overlap and variable fault interaction within relay zones. The relationship between maximum displacement and fault trace length tends to follow a power-law distribution, as it commonly occurs for fault populations. However, a better correlation could be obtained by further segmentation of the northernmost (blind) fault in an area of no seismic data, thus confirming that fault scaling relationships may be useful for pointing out possible problems with the structural interpretation of poorly constrained areas and for exploring viable alternative solutions.

Effects of uplift and tilting in the Central-Northern Apennines (Italy)

The morphotectonics of the Central-Northern Apennines reflect the effects of Pliocene–Pleistocene regional uplift and tectonics. High uplift rates and extensional tectonics prevailed in the axial sector, while lesser uplifting, eastward tilting, and gravity sliding affected the Periadriatic belt. The axial sector has a chocolate-slab pattern of blocks with intermontane depressions, bounded by normal faults and characterized by differential vertical movements. The Periadriatic belt is a NE dipping monocline, broken by transverse faults into separate blocks with different evolutionary development. The evolution of the axial sector can be correlated with that of the Periadriatic sector because the unconformities in the marine succession of the Periadriatic basin are linked to pulses of uplift in the axial chain. The amounts of coarse-grained sediments in intermontane depressions and in the Periadriatic belt are correlated with the erosion processes affecting the axial sector. This morphostructural pattern may be connected with the formation of a long-wavelength bulge associated with mantle upwelling.

Thick‐skinned tectonics in the external Apennines, Italy: New evidence from magnetic anomaly analysis

We discuss the tectonic implications of a new residual magnetic map of the Apennine belt/Adriatic‐Apulian foreland obtained by integrating ground and offshore data sets [Chiappini et al., 2000a]. Negative anomalies are documented over the Adriatic‐Apulian foreland areas, whereas the external Apennine belt is characterized by a ubiquitous low‐amplitude (&lt;30 nT), long‐wavelength positive anomaly. In the central northern Apennines, three ∼100 km wide more intense (100–200 nT) round‐shaped anomalies are superimposed to the long‐wavelength feature. Finally, in the Tyrrhenian Sea and margins, high‐intensity, short‐wavelength positive‐negative couplets coincide with magmatic outcrops or bodies at shallow depth. The low‐amplitude anomaly pattern over Italy suggests that the magnetic basement beneath the Triassic evaporites is ubiquitously incorporated in the external belt compressive fronts, implying a thick‐skinned tectonic style for the external Apennines. The new residual magnetic map resolves the inconsistency between previous aeromagnetic data [AGIP SpA. Italia, 1981], which suggested a lack of basement involvement in the Apennine belt, and recent seismic data, which imaged deep reflectors penetrating the basement. Two magnetic models along NE‐SW transects in the northern and southern Apennines suggest consistent structural styles. In the northern Apennines, positive anomalies roughly coincide with the external compressive fronts, although there are local second‐order differences between the belt front and the edges of the anomaly. Here the magnetic data show that the basement rises southwestward along the thrust fronts from 6–7 km depth in the Adriatic foreland to 2–3 km depth in the axial belt, where some exploration wells have penetrated basement. Within the belt front, basement exhumation is inferred to occur along high‐angle, low‐displacement thrust faults inverting preexisting normal faults. In the southern Apennines, a remarkable positive magnetic anomaly is parallel with and tens of kilometers southwest of the belt front. Seismic data and oil wells show that the basement surface cannot be shallower in the belt than in the foreland. Therefore the observed magnetic anomaly is produced by strongly magnetic basement beneath the belt, likely an internal crustal wedge tectonically interposed between the Apulian carbonate sequences and basement.

The dip of the foreland monocline in the Alps and Apennines

The foreland monocline dips underneath thrust belts and accretionary wedges, both in oceanic and continental subduction zones. We present new data on the dip of the monocline in the frontal part of two orogens, the Alps and the Apennines. There is an overall difference between the dip of the relative monoclines, and there is also a strong lateral variation along both arcs. In the Alps, the regional dip varies between 0° in the remote foreland, to an average of 2–3° at the front of the thrust belt below the foredeep, to about 5° beneath the external thrust-sheets within 40 km from the leading edge of the accretionary wedge. The regional dip of the monocline in the Apennines has an average of 4–5° at the front of the thrust belt below the foredeep, to about 10° beneath the external thrust-sheets within 40 km from the leading edge of the accretionary wedge. There are areas where the dip exceeds 20°. The Apennines though topographically lower than the Alps present higher monocline dips and a deeper foredeep. Moreover, there are variations in the dip of the monocline moving along the strike of the two belts: the low values coincide with Permian–Mesozoic inherited horsts, whereas the steeper values correspond to basinal areas, and they usually match the salients of the thrust belt front. Within the salients the distance between thrust ramps increases. Therefore, there are two orders of mean values of the dip of the foreland monocline, the first at the orogen scale (more than 1000 km wavelength), the second at the regional scale (100–200 km wavelength) within the single orogen. Lateral variations in the lithospheric buoyancy due to the inherited Mesozoic stretching may explain the second order variations in foreland dip, but not the first order mean values which seem to be more sensitive to the geographic polarity of the subduction rather than to the lithospheric composition which is rather similar in the Alpine and in the central-northern Apennines slabs.

Paleomagnetic constraints on the Plio–Pleistocene geodynamic evolution of the external central–northern Apennines (Italy)

We report on new paleomagnetic results obtained from 27 sites sampled in the Plio–Pleistocene sequences at the external front of the central–northern Apennines. Previous analyses of Miocene (Messinian) sediments indicated that the present shape of the northern Apenninic arc is due to the oroclinal bending of an originally straight belt oriented around N320° and that vertical axis rotations accompanied the migration of the thrust fronts toward the Adriatic foreland [F. Speranza et al., J. Geophys. Res. 102 (1997) 3153–3166]. We tried to provide new paleomagnetic constraints for the timing and rates of the oroclinal bending process during the Pliocene and the Pleistocene. The results suggest that CCW rotations observed in the northern part of the studied area are possibly younger than 3 Ma. No regional rotation is recorded in the Pliocene and Pleistocene sediments from the southern part of the study area, analogously to the Messinian sediments of the ‘Acquasanta’ domain of Speranza et al. [F. Speranza et al., J. Geophys. Res. 102 (1997) 3153–3166]. A local significant CCW rotation (23°±10°) is identified in the Early Pleistocene sediments that crop out along the Adriatic coast between Ascoli and Pescara, indicating differential motion of the thrust sheets. This rotation must be younger than 1.43 Ma.

Magnetic anisotropy of Plio–Pleistocene sediments from the Adriatic margin of the northern Apennines (Italy): implications for the time–space evolution of the stress field

The anisotropy of magnetic susceptibility (AMS) of the marine fine-grained Plio–Pleistocene sediments that crop out at the eastern (Adriatic) front of the central–northern Apennines (Italy) indicates a prevalent sedimentary-compactional magnetic fabric with variable overprint of the tectonic strain. The degree of anisotropy and the geometry of the AMS ellipsoids suggest a subdivision of the studied sediments in two distinct ages: Early–middle Pliocene and Late Pliocene–Early Pleistocene. The Early–middle Pliocene sediments show a weak but well defined magnetic lineation parallel to the main fold and thrust axes throughout the region, analogously to the Messinian sediments located in a more internal (western) position of the Apenninic chain. Since the Late Pliocene, the AMS data are not regionally coherent and indicate a reduced and locally variable tectonic influence on the magnetic fabrics. The AMS data have been integrated with the available geophysical information of the present-day stress in the region, with the aim to reconstruct the time–spatial evolution of the stress field in the Adriatic margin of the central-northern Apennines, during the past 5 Ma. The present-day stress field was evaluated using borehole breakout in deep wells (about 50 wells), focal mechanisms of crustal earthquakes (2.5< M d<5) that occurred in the last 10 years and a review of the historical seismicity. The synthesis of these data suggests significant changes in the time–space evolution of the stress field through the area in the last 5 Ma. In the northern sector the tectonic regime shows a maximum compressional axis trending NE–SW, that is constant at least since the Early Pliocene. In the southern sector the stress field has drastically changed: a compressional stress with a maximum compressional axis trending E–W was active during the Pliocene and was likely preserved until Early Pleistocene; then the stress field changed, presently showing a NE–SW minimum horizontal stress that mirrors the trend of the active extension observed in the southern Apennines. Between these two sectors a transitional area is identified at about 43°N, that we associate with a main NNE–SSW lithospheric discontinuity pointed out by the available geological and geophysical data.

Tectonics of the Umbria‐Marche‐Romagna Arc (central northern Apennines, Italy): New paleomagnetic constraints

We present the results of a paleomagnetic study carried out on 32 sites from mainly Messinian clayey sediments distributed throughout the external Umbria‐Marche‐Romagna Arc (UMRA). These data, together with published results from coeval sediments, demonstrate that this arc is an orocline in its central northern sector. Bending, not well constrained in time, was due to about 15° clockwise rotations of the central part of this arc and to counterclockwise rotations farther north. In this latter area, post‐Messinian counterclockwise rotations are of the same amplitude as those calculated for some classic Mesozoic paleomagnetic sections in northern Umbria, suggesting a Plio‐Pleistocene age for the rotations reported from the older sequences.

Petrologic binary diagrams for characterizing and comparing arenites

Point-counting according to the Gazzi-Dickinson method was performed on 136 samples of several Paleocene to Early Miocene turbidite units of the central-northern Apennines. The main components of the siliciclastic fraction are plotted on binary diagrams in which the coordinates are dimensionless ratios. The following petrographic parameters have been used: Qm/Q (monocrystalline quartz/total quartz); Q/Q + F (total quartz/total quartz + total feldspar); K/F (K-feldspar/total feldspar); Mw/M (white micas/total micas). For each pair of ratios, binary diagrams can be drawn in which each sandstone formation occupies a fairly restricted field. The simultaneous use of two or more diagrams allows two formations to be distinguished in at least one of the diagrams.

The Puglia uplift (SE Italy): An anomaly in the foreland of the Apenninic subduction due to buckling of a thick continental lithosphere

The Apenninic foreland shows two distinct structural signatures comparing the central Adriatic Sea and the Puglia region. During the Pliocene‐Pleistocene the central Adriatic underwent high subsidence rates due to the eastward rollback of the hinge of the west dipping Apenninic subduction. The Puglia region and the Bradanic foredeep are located southward along strike in the same foreland, but, in contrast with the central Adriatic, after Pliocene‐early Pleistocene subsidence they underwent uplift since the middle Pleistocene. The geometry and the kinematics of the frontal accretionary wedge and related foreland changed from that moment on between the two areas. At the front of the central northern Apennines, off scraping and subsidence continued, whereas the foredeep and foreland of the southern Apennines were buckled. Those differences are interpreted as being due to the larger subduction hinge rollback rate since middle Pleistocene of the central Adriatic lithosphere (70 km thick) with respect to the thicker Puglia (110 km). The different thicknesses of the continental crust and lithosphere were inherited from the Mesozoic rifting that disrupted the Adriatic plate. The different thicknesses appear to have controlled the variable degree of flexure of the lithosphere and its asthenospheric penetration rate. The Tremiti E–W alignment is the right‐lateral lithospheric transfer zone of those different tectonic regimes. The consequent different dip of the subduction in the two sections (steeper west of Puglia) could also explain the lower elevation of the southern Apennines, compared to their central‐northern sector.

Magnetic interpretation related to geo-magnetic provinces: the Italian case history

A comparison of the results obtained from the geophysical interpretation with the available information regarding the Italian geological situation permitted us to establish our so-called “geo-magnetic provinces”. The relationship between the magnetic characteristics and the tectonic evolution permitted the identification of each of the four following “provinces” that exist inside the global tectonic confines: 1. (1) The “Sardo-Corso” block, which represents the eastern boundary of stable Europe. This province is characterized by medium-wavelength low-amplitude anomalies that originate in an outcropping basement (Hercynian granites s.l.). 2. (2) The “Orogenic belt”, which constitutes the double Alps-Apennines system. From a magnetic standpoint, this system is characterized by short-wavelength, low-amplitude anomalies that are due to a Paleozoic outcropping and/or buried, acid, crystalline basement. These anomalies are found to overlie long-wavelength, medium-amplitude regional anomalies that are related to a deep, Apenninic basement (central-northern Apennines). 3. (3) The “Foreland area”, which occurs from Sicily (Hyblean Plateau) and Apulia northwards to Ancona and the Po Plain. This area is characterized by a crystalline basement which is typically “differentiated” from the standpoint of basicity. 4. (4) The “Rifting zone”, which is represented by the part of the Tyrrhenian Sea lying south of the “41st-Parallel discontinuity”, the Ionian Abyssal Plain and the Pantelleria and Linosa troughs in the Sicily Channel. The main characteristic of this province is its extreme basicity differentiation in the crystalline basement, which is induced by the geochemical processes connected with the deep, geodynamic events. These phenomena do not permit the correlation of the semioceanic basement of the Rifting zone with the continental basement. By means of interactive modeling, a structural-tectonic picture of the basement of the various Italian “geo-magnetic provinces” has been obtained. The main result was found to be that the Orogenic belt basement is undergoing intense tectonic compression and overlies the structural highs of the Foreland area, which mainly show tensive events.

Crystal chemistry of diagenetic zeolites in volcanoclastic deposits of Italy

Zeolites from the most important volcanoclastic deposits of Italy include: (1) phillipsite and heulandite from the cinerite of the central northern Apennines; (2) chabazite and phillipsite from the phonolitic tephritic ignimbrite with black pumices; (3) phillipsite from the “tufo lionato” of Vulcano Laziale; (4) chabazite and phillipsite from the Campanian ignimbrite; (5) phillipsite from the Neapolitan yellow tuff; and (6) chabazite and phillipsite from the pyroclastics of Monte Vulture. Compared with sedimentary phillipsites and chabazites described in the literature, the chabazites and phillipsites studied here have lower Si/Al ratios and higher K contents. These chemical peculiarities are correlated with both the K-rich vesuvitic-leucititic, latitic-phonolitic, and potassic alkali-trachytic chemistry of the ash from which they were derived and, very likely, with the character of the hydrologically open system environment in which they formed. The zeolite of the heulandite-clinoptilolite group from the cinerite of the central northern Apennines is classified as a true heulandite on the basis of its chemical composition and thermal behavior.

Fault-plane solutions and stress regime in Italy and adjacent regions

The paper is a sequel to one published earlier (Cagnetti et al., 1976) in which 81 earthquakes that occurred in Italy and adjacent regions were summarized and discussed. Here the characteristics of the positions of the principal stress axes of these solutions, of five separate seismic regions within the investigated area, have been studied. The stress model for each region has been determined, and by a comparison of these stress fields it appears that the whole region can be divided into two parts, with different stress regimes. The first one includes the Alps, the Dalmatian coast, Albania, western Greece and the southeastern Ionic margin of the Tyrrhenian arc and is characterized by a compressional regime; the direction of compression is generally perpendicular to the orographic axis of the mountain chains and to the seismic lines, and is originated and controlled by the relative movement between the plates in contact in the Mediterranean area. The latter, characterized by tensional stress regime, includes the central-northern Apennines, the Tyrrhenian basin and some narrow areas along the Dalmatian coast. This area is in a state of tension in a roughly east-west direction, and in the deep earthquake zone is affected by nearly vertical compressional stresses. These results are compared with the seismotectonics of the area and other geophysical data.