Cifelli, Francesca

Anisotropy of magnetic susceptibility (AMS) analysis has been carried out in the thermometamorphic aureole surrounding the Late Miocene Monte Capanne pluton (Elba Island, Central Italy). The identification and separation of the main carriers of the magnetic susceptibility by low-temperature and high-field AMS measurements demonstrate that a correct knowledge of the magnetic fabric is needed in order to use AMS for tectonic interpretations. Magnetic fabric data, combined with structural data from the aureole, and their comparison with data from the pluton itself, were used to constraint the mode of pluton emplacement. Results document an intimate linkage between the magmatic flow pattern and the syn-metamorphic fabrics acquired during pluton emplacement in the host rocks. The magnetic/structural fabric in the aureole rocks is dominated by flattening deformation and no systematic relationship with any regional tectonic feature is observed. These results suggest that local processes induced by magma ascent in the upper crust might have played a primary role in space generation for pluton emplacement in the Tuscan Magmatic Province, suggesting a revaluation of the modes of pluton emplacement during the post-orogenic evolution of the northern Apennine system as a whole.

The aim of this research is to provide an original contribution to the investigation of local macroseismic variations in urban areas by means of questionnaire surveys. In this paper, we propose a methodology to investigate earthquake effects in large cities. This procedure for a high-density macroseismic survey is here applied to the city of Rome (Italy) during the September 1997-April 1998 Umbria-Marche (Central Italy) seismic sequence. A sort of macroseismic network in the urban area was arranged, thanks to the co-operation of public high schools, where ad hoc macroseismic questionnaires were delivered to students. This method provided us with a large amount of macroseismic information related to the October 14, 1997 (Mw =-5.6; I0 VIII MCS; h ≈10 km) and March 26, 1998 (Mw = 5.3; I0 = VII MCS;h ≈50 km) earthquakes. In the first survey, 949 useful questionnaires were collected in 10 high schools and related to 669 observation points. For the second event, 1083 useful questionnaires were collected in 27 high schools and related to 928 (+39%) observation points. The mean data density in the urbanized sector reached 3.4 data/km2 in the first survey and rose to 4.7 (+38%) data/km2 in the second one. Such a high density was hardly achieved in previous macroseismic surveys in large cities. The sample reliability was checked considering the data distribution versus urban setting inhomogeneity and the percentage distribution of the main lithological units outcropping in the investigated area. Such reliability was also confirmed by the check of the data density distribution. All results confirm that the data sample is largely representative. Both the applications here shown proved that this method can be successfully performed in a large city.

The present-day arcuate shape of the Calabrian Arc has been accomplished during Neogene and Early Pleistocene by large and opposite vertical axis rotations along the two arms of the Arc. Clockwise (CW) rotations have been systematically registered in Sicily and Calabria, whereas counterclockwise (CCW) rotations were measured in Southern Apennines. Such opposite vertical axis rotations ceased in the uppermost part of the Lower Pleistocene (about 1 Ma ago) along almost the entire Calabrian Arc and are not observed in the present-day GPS velocity field. The end of the Calabrian Arc bending during the Quaternary marks a decrease in the efficiency of the tectonic processes related to the long-lived subduction of the Ionian slab, which caused the halting of the back-arc opening in the Southern Tyrrhenian Sea.

In this study, we report an extensive paleomagnetic study (76 sites) carried out in the Alborz Mts. (northern Iran), with the aim of reconstructing the rotation history and the origin of curvature of this orogenic chain. The analyzed deposits are the sedimentary successions of the Upper Red Formation (Miocene), Lower Red Formation (Oligocene) and Eocene clastic units. Paleomagnetic results indicate that the Alborz Mts. can be considered a secondary arc that originated as a linearmountain belt that progressively acquired its present day curvature through opposite vertical axis rotations along its strike. The curvature of the arc was entirely acquired after the middlelate Miocene,which is the age of the youngest investigated sediments (Upper Red Formation). Overall, our paleomagnetic data indicate that the Alborz Mts. can be considered an orocline. Our results define, for the first time, the rotational history of the entire Alborz curved mountain belt, and enable us to reconstruct the paleogeographic and tectonic evolution of northern Iran in the framework of Arabia-Eurasia continental deformation. The kinematics inferred by the pattern of paleomagnetic rotations is at odds with the present day kinematics of northern Iran, characterized by the westward extrusion of the South Caspian block, and by a left lateral shear between Central Iran and the central and western sectors of the Alborz Mts. By integrating paleomagnetic datawith stratigraphic, thermochronological, structural and GPS information,we propose that the initiation of South Caspian subduction and the activation of westward extrusion of South Caspian block occurred diachronously and that the initiation of the present-day kinematics of northern Iran was quite recent (Lower Pleistocene, b2 Ma).

Few studies have detailed the thermal architecture of large-volume pyroclastic density current deposits, although such work has a clear importance for understanding the dynamics of eruptions of this magnitude. Here we examine the temperature of emplacement of large-volume caldera-forming ignimbrites related to magmatic and phreatomagmatic eruptions at the Colli Albani volcano, Italy, by using thermal remanent magnetization analysis on both lithic and juvenile clasts. Results show that all the magmatic ignimbrites were deposited at high temperature, between the maximum blocking temperature of the magnetic carrier (600–630 °C) and the glass transition temperature (about 710 °C). Temperature estimations for the phreatomagmatic ignimbrite range between 200 and 400 °C, with most of the clasts emplaced between 200 and 320 °C. Because all the investigated ignimbrites, magmatic and phreatomagmatic, share similar magma composition, volume and mobility, we attribute the temperature difference to magma–water interaction, highlighting its pronounced impact on thermal dissipation, even in large-volume eruptions. The homogeneity of the deposit temperature of each ignimbrite across its areal extent, which is maintained across topographic barriers, suggests that these systems are thermodynamically isolated from the external environment for several tens of kilometers. Based on these findings, we propose that these large-volume ignimbrites are dominated by the mass flux, which forces the lateral transport of mass, momentum, and thermal energy for distances up to tens of kilometers away from the vent. We conclude that spatial variation of the emplacement temperature can be used as a proxy for determining the degree of forced-convection flow.

We report on results from anisotropy of magnetic susceptibility (AMS) analyses carried out on weakly deformed fine-grained sediments from the Northern Apennine orogenic system (Italy). We sampled 63 sites from preorogenic, synorogenic, and postorogenic sequences, which differ in age, composition, depositional environment, degrees of deformation, and tectonic regimes. The magnetic fabric is typical of weakly deformed sediments, with a magnetic foliation subparallel to the bedding plane and a magnetic lineation well defined in this plane. Northern Apennine chain deposits are characterized by strongly oblate magnetic susceptibility ellipsoids, indicating that the magnetic fabric is the result of both compaction process and tectonic load experienced by the sediments during diagenesis and orogenic events. The orientation of magnetic lineation is significantly different depending whether the studied sites underwent extensional or compressional tectonic regimes. In the Northern Apennine chain, the magnetic lineation is mostly oriented NNW-SSE, parallel to the main compressional structures. It suggests a tectonic origin of the magnetic lineation with an acquisition related to the Apennines compressional phases. In the extensional Tuscan Tyrrhenian margin, magnetic lineation is oriented ENE-WSW, almost perpendicular to the main extensional faults, which represent the main deformation elements of the area. Our results demonstrate a close relationship between the shape and orientation of magnetic fabric and the tectonic history of rocks, confirming that AMS represents a valuable tool to investigate the tectonic history of weakly deformed sedimentary rocks.

The post-collisional late Variscan magmatism of Sardinia-Corsica batholith attained a peak at about 290 Ma. In southern Sardinia, in the frontal part of the Variscan orogenic wedge, this magmatism is represented by three suites of granitoids, here defined as GS1, GS2 and GS3. GS1, GS2 and GS3 are slightly peraluminous and F-bearing granitoids; GS1 and GS3 granites show in addition a ferroan character, whereas GS2 rocks range from magnesian to ferroan, from granodiorites to leucogranites. From magnetic susceptibility data, GS1 and GS2 belong to the ilmenite series, whereas GS3 is a slightly oxidized rock-suite plotting on the ilmenite/magnetite series boundary. Each rock-suite shows distinctive characters, in terms of petrography, petrochemistry, rock associations, as well as metallogenic signature of the related fluids. The distinction among rock-suite types is made on the basis of both mafic and characteristic accessory minerals. Siderophyllitic dark mica as the only mafic phase, and accessory xenotime (Y) characterize the GS1 rocks; GS2 mineral associations include biotite ± hornblende + allanite + magnetite; GS3 rocks show an association of hastingsite + annite + allanite + magnetite. Chemical variations in the studied samples suggest different magmatic evolution of independent magmas. Pb, Sr and Nd isotopic data constrain the origin of magmas to lower crustal sources. Chemical composition of rocks and dark micas meet those of liquids experimentally obtained by low degrees of partial melting of different meta-igneous deep crustal sources, felsic for GS1 rock-types and more mafic for GS3 rock-types. GS1 intrusions show granophile-type (Sn-W-Mo) metallogenic signatures, very low magnetic susceptibility, and Nd model ages (referred to the Depleted Mantle - TDM) of 2.3 Ga, coherent with a possible derivation from an old (early Proterozoic-Neoarchean), reduced and weathered basement, tectonically buried under Variscan covers. A definite deep crustal inhomogeneity is mirrored by GS3 granites, whose compositional and isotopic features indicate a younger (Nd model age: 1.6 Ga) tonalitic amphibolite source. Overall, the peculiarities of the studied granitoids suggest further compositional differences in the deep crust between southern and northern portion of the Sardinia-Corsica Variscan transect. Late Variscan lithospheric delamination appears as the most reliable mechanism that may have determined the high thermal regime that triggered partial melting of the crust. The close field association, at 290 Ma, of tholeiitic dike swarms and ferroan granitoids, supports this inference.

Topography plays an important yet uncertain role in modulating the temporal and spatial evolution of the in- ternal structure of pyroclastic density currents (PDCs). Understanding such changes is critical to characterize PDC transport regimes and their hazard. Here we combine paleomagnetic data from PDC deposits of the 18 May 1980 Mt. St. Helens eruption with numerical outcomes to capture spatio-temporal temperature variations induced by topography. We show that emplacement temperatures along the northwest flank of the volcano are ≃ 100◦C colder than those recorded along the northeast flank in response to proximal topographic drops. We further report that such vertical drops lead to an initial transient regime where the PDC internal temperature, velocity, and concentration stratification is altered for periods of time that are proportional to the ratio between the drop height and the square root of the current thickness. The topographic control on PDC dynamics is attenuated moving away from the drops or when a stationary phase is attained. Collectively, our results highlight that topographic regions promoting the flow separation/reattachment process are associated with vigorous entrainment of ambient air in the lower portion of PDCs. Low temperature variability is observed in the absence of such topographic irregularities. Based on our findings, we propose a local sedimentation rate of ≃ 150 ± 100 mm s−1 for PDC deposits in a proximal reattachment region. This investigation demonstrates the importance of transient processes in PDC dynamics, introducing a new methodology to measure sedimentation rates, and highlighting that flow-topography feedbacks should be considered to assess hazards.

In this study, an extensive paleomagnetic sampling (70 sites) was carried out in north-eastern Iran with the aim of reconstructing the rotation history of the outer margin of the Eurasia-Arabia collision area represented by the Ala-Dagh, Binalud and Kopeh-Dagh mountain belts. We sampled the red beds units from the Lower Cretaceous Shurijeh Fm. and from the Middle-Upper Miocene Upper Red Fm (URF). Paleomagnetic results from all the sampled areas show a homogeneous amount of CW rotations measured in the above-mentioned Formations. These paleomagnetic results suggest that the oroclinal bending process that caused the curvature of Alborz mountain belt in north Iran after the Middle-Late Miocene, also extended to the Ala-Dagh, Binalud and Kopeh-Dagh mountain belts, at the north-eastern border of the Arabia-Eurasia deforming zone. Based on our paleomagnetic results and on GPS, seismological, geomorphological and structural data available in the area, a hypothesis of tectonic evolution of the northern Iran-South Caspian Basin area, from Middle-Late Miocene to Present, is here proposed. In this model, the initiation of the oroclinal bending processes in northern Iran occurred about 6–4 myr ago, related to the impinging of North Iran between the South Caspian block and the southern margin of the Turan platform, driven by the northward subduction of the South Caspian basement under the Aspheron-Balkhan Sill. As paleomagnetic results from this study show a pattern of vertical axis rotations that is inconsistent with the present-day kinematics of the northern Iranian blocks as described by seismicity and GPS data, we suggest that the tectonic processes responsible for the bending of northern Iran mountain chains are no longer active and that the westward motion of the South Caspian basin, and therefore the initiation of opposite strike-slip motion along the Ashk-Abad and Shahrud faults, occurred very recently (∼2 My ago). We therefore propose that initiation of the northward subduction of the South Caspian basin below the Apsheron-Balkhan Sill and the westward extrusion of the South Caspian block did not occur at the same time, with the former occurring between the late Miocene and the Pliocene, and the latter during the Pleistocene.

We report on an extensive paleomagnetic study (36 sites) of the Tuscan Nappe succession from the Northern Apennines Arc, aimed to reconstruct the tectonic evolution of the internal sector of this chain. We analyzed Eocene pelagic foreland ramp deposits (Scaglia Toscana Formation) and Oligocene–lower Miocene siliciclastic turbidites (Macigno and Falterona Formations). Paleomagnetic results show that the internal sector of the Northern Apennines underwent large counterclockwise (CCW) rotations with respect to the Adria-Africa foreland. A decrease in the rotation magnitude was observed from the southern to the northern sector of the arc (from 91 to 36°). This trend is opposite to that observed in the more external units of Northern Apennines and demonstrates that the oroclinal bending model, which has been proposed for the external units of the chain, is not appropriate to explain the evolution of the internal sector of the arc. On the basis of the observed paleomagnetic pattern, we propose a new tectonic model in which the Tuscan and Falterona-Cervarola units in the southern area were first rotated CCW along with the Corsica-Sardinia block during its lower Miocene rotational drifting and were later involved in the main phases of rotational emplacement and translation toward the outermost sector (Umbria domain), thus yielding the final curved shape of the Northern Apennines chain. Data from this study represent the first paleomagnetic evidence of the influence of the Corsica-Sardinia CCW rotation in the Apennines orogenic wedge deformation, in the general framework of the geodynamic evolution of the Central Mediterranean subduction system.