Paleomagnetic dating of tectonically influenced Plio-Quaternary fan-system deposits from the Apennines ( Italy )

The Roveto Valley is a narrow, elongated, NW-trending depression filled with continental Plio-Quaternary deposits that outcrop at different topographic elevations. A morpho-lithostratigraphic succession of the continental deposits has been defined in order to reconstruct the geological Quaternary evolution of the area. These deposits do not contain materials suitable for biostratigraphic dating; therefore, in order to determine their chronology, three different units were sampled for magnetostratigraphic investigations. Paleomagnetic results demonstrated that standard demagnetization techniques are effective in removing secondary remanence components and in isolating the characteristic remanent magnetization, allowing us to determine, for each cycle, whether it was deposited before or after the Brunhes-Matuyama geomagnetic reversal at 781 ka. Preliminary rock magnetic analyses indicated that magnetite is the main magnetic carrier and that hematite, which gives the pink colour to the matrix, is in the superparamagnetic grain-size range and thus does not retain any paleomagnetic remanence.


Introduction
The Roveto Valley area is part of a segment of the Apennine mountain range, which was intensively deformed by Neogene-Quaternary compressional and extensional tectonics [Saroli et al. 2003 and references therein, Saroli and Moro 2012].This sector of the chain was mostly formed during the Neogene, in the context of the eastward piggy-back propagation of the Apennines thrust system.Geomorphologically, the Roveto Valley is a narrow, elongated, NW-trending morphological depression, bounded by some of the most significant regional tectonic features of the whole Apennine range, such as the Simbruini -Ernici Mts.basal (leading) thrust and the Val Roveto -Atina -Caserta Line (Figure 1).

Electron microprobe and magnetostratigraphic investigations
Cycles 1, 2, and 3 of the identified morpho-lithostratigraphic succession were sampled and 4 thin sections were analyzed with an electron microprobe model Cameca SX-50 equipped with 5 WDS spectrometers and an energy dispersive spectrometer (EDS).Results are shown in Table 1.Preliminary electron microscope investigations of polished sections indicate that magnetite, ilmenite, titanomagnetite, and garnet are distributed within the pink-coloured matrix (Figure 3).The primary cement is mainly composed of calcium carbonate (Table 1, Point 9) and shows a clear difference in mineralogical composition compared to the more recent weathering cement (Table 1, Point 8).
For the paleomagnetic analysis, about 80 cores were drilled from the three cycles of the morpho-lithostratigraphic succession.An ASC 280E gasoline-powered portable drill was used with a water-cooled diamond bit, and the cores were oriented in situ using a magnetic compass.
The magnetic measurements were carried out at the paleomagnetic laboratory of the Istituto Nazionale di Geofisica e Vulcanologia in Rome.All the specimens were demagnetized in 11-20 steps and the remaining natural remanent magnetization (NRM) was measured after each step with a 4.5 cm access pass-through 2G cryogenic magnetometer housed in a Lodestar Magnetics shielded room.For each site, 50% of the samples were stepwise alternating-field (AF) demagnetized and the remaining samples were subjected to stepwise ther- mal demagnetization, from room temperature up to 650°C.
The results indicated that thermal and AF demagnetization are equally effective in removing secondary remanence components and in isolating the characteristic remanent magnetization.Preliminary magnetic coercivity and thermomagnetic analyses indicated that magnetite is the main magnetic carrier.We therefore suggest that the hematite responsible for the pink colour of the matrix is in the paramagnetic or superparamagnetic grain-size range (<28 nm as determined by Banerjee [1971]) and thus does not retain any paleomagnetic remanence.
Demagnetization data were plotted as demagnetization-intensity graphs and Zijderveld [1967] plots.NRM directions were calculated by principal component analysis [Kirschvink 1980] and plotted on equalarea projections.Selected results are shown in Figure 4.The site mean directions are quite well-defined, and are better grouped in the geographic reference frame before tilt correction.This grouping suggests that these units were deposited onto an inclined surface, along the slope of calcareous reliefs.The in-situ mean directions are close to the directions of a recent reverse geocentric axial dipole field recognized in the central Apennines.The analysis of the two sites belonging to the first and the second depositional cycles (Rava del Corvo -Case Querceto and Campoli Appennino, respectively) gives a reverse magnetic polarity.The third site, belonging to the third depositional cycle (San Marciano), has a normal polarity.We surmise that the latter unit has been deposited during the Brunhes Normal Chron, and is therefore younger than 781 ka [Gradstein et al. 2012].On the basis of the available tectono-stratigraphic constraints [Saroli et al. 2003 and references therein], we infer that the deposition of the first two units occurred during the reverse Matuyama Chron, probably between 2.581 and 0.781 Ma.

Conclusions
The integration of morpho-lithostratigraphic data and paleomagnetic measurements allowed us to date the samples of the first and second cycle to the early Pleistocene (Matuyama Reverse Chron).The samples of the third cycle, the most recent in the morpho-lithostratigraphic succession, have a normal polarity (Brunhes Normal Chron) and must therefore be younger than 781 ka.Paleomagnetic results demonstrated that standard demagnetization techniques are effective in removing secondary remanence components and in isolating the characteristic remanent magnetization in these alluvial fan deposits.These methods allowed us to determine, for each cycle, whether it was deposited before or after the Brunhes-Matuyama paleomagnetic boundary at 781 ka.Information from paleomagnetic analyses, integrated with stratigraphic, morphological and tectonic features, could in the future be used to define a more complete geological and tectonic Quaternary evolution of the Campoli Appennino sector.

Figure 4 .
Figure 4. Selected paleomagnetic results.Upper panel, a)-f ): Zijderveld plots showing demagnetization behaviour under AF and thermal treatment for samples from each of the three cycles.Best-fit PCA directions are shown as overlaid straight line segments.Lower panel: g) third-cycle NRM directions plotted on an equal-area projection; h) demagnetization-intensity plot for two third-cycle samples.

Table 1 .
Element concentrations obtained by EDS microprobe analysis of thin sections.Locations of points 6, 10, and 14 on thin sections are shown in Figure3.
Figure3.Electron micrographs of thin sections.Mineral identifications are given for selected points; see Table1for detailed composition data.