Fabrizio Marra

The history of the Monti Sabatini and Alban Hills volcanoes : groundwork for assessing volcanic-tectonic hazards for Rome

Abstract The volcanic rocks of Rome provide chronostratigraphic constraint that is of use to geologists, archaeologists, and paleontologists. Identifying the areal distribution of these rocks, their mode of emplacement and a detailed chronology of the eruptive events also, is crucial for assessing local natural hazards in a volcanic-tectonic context, and for recognizing whether the volcanoes may erupt again in the future. We have identified and dated many of the volumetrically significant eruptive units from the Monti Sabatini and Alban Hills (Colli Albani) and propose a formalized volcanic stratigraphy for Rome. It is probable that seismic events triggered some eruptions, as several ignimbrites from the two volcanoes have statistically indistinguishable ages. Following a 200+xa0kyr period of dormancy that was coincident with a pulse of regional uplift, the Alban Hills appears to have begun recently (circa 45xa0ka), a new eruptive cycle. Therefore, accurate assessment of volcanic-tectonic hazards, and estimation of possible future volcanic activity, is necessary for the safety of the five million people living near these volcanoes.

Sea-level variability over five glacial cycles

Research on global ice-volume changes during Pleistocene glacial cycles is hindered by a lack of detailed sea-level records for time intervals older than the last interglacial. Here we present the first robustly dated, continuous and highly resolved records of Red Sea sea level and rates of sea-level change over the last 500,000 years, based on tight synchronization to an Asian monsoon record. We observe maximum natural (pre-anthropogenic forcing) sea-level rise rates below 2u2009m per century following periods with up to twice present-day ice volumes, and substantially higher rise rates for greater ice volumes. We also find that maximum sea-level rise rates were attained within 2u2009kyr of the onset of deglaciations, for 85% of such events. Finally, multivariate regressions of orbital parameters, sea-level and monsoon records suggest that major meltwater pulses account for millennial-scale variability and insolation-lagged responses in Asian monsoon records.

CORRELATION OF FLUVIODELTAIC AGGRADATIONAL SECTIONS WITH GLACIAL CLIMATE HISTORY : A REVISION OF THE PLEISTOCENE STRATIGRAPHY OF ROME

Volcanic horizons interbedded with fluviodeltaic clastic sediments of the Tiber and Fosso Galeria River valleys near Rome provide a unique opportunity to date five middle Pleistocene fining-upward aggradational sections. 40 Ar/ 39 Ar ages from the interbedded volcanic beds illustrate that aggradation occurred on an approximate 100 k.y. cycle and was initiated by sea-level rise associated with glacial melting. The age constraint additionally shows that aggradation occurs rapidly in response to sea-level change, and thus ages from volcanic strata within the aggradational sections provide high-resolution constraint on the timing of relative sea-level rise. The established chronology is consistent with other glacial proxies and their (radioisotopically or orbitally based) time scales for the timing of glacial terminations VI and V, but is significantly older than most records for the timing of termination III. Ultimately, the age of each termination should be determined by radioisotopic means, and to that end the work here shows that fluviodeltaic sedimentology provides valuable information for middle Pleistocene time-scale calibration.

Recurrence of volcanic activity along the Roman Comagmatic Province (Tyrrhenian margin of Italy) and its tectonic significance

[1]xa0The Roman Comagmatic Province (RCP) developed along the NW-SE striking extensional Tyrrhenian margin of Italy since middle Pleistocene and includes ultrapotassic small eruptive centers and large volcanic districts. By means of a review and statistical analysis of a large data set of radiometric ages we demonstrate that in the RCP, major eruptive cycles occurred at different volcanic districts almost simultaneously and with an almost constant recurrence time. Our analysis started with the Alban Hills Volcanic District, where a detailed 40Ar/39Ar chronology of the eruptive cycles has been recently achieved, and then extended to include a database of 264 radiometric ages of the RCP volcanics published since 1980. Statistical treatment of the data set shows a marked non-Poissonian behavior of volcanic activity through the whole magmatic province, with climax of each major eruptive cycle occurring simultaneously at the different districts approximately every 48 kyr. The concomitant eruption of many volcanic districts spreading over a 250-km-long chain implies a control mechanism acting at a regional scale. After discussing the possible relationship between tectonics, sea level changes, and volcanic activity we hypothesize that constant rates of slab retreat during middle Pleistocene could have been a key factor controlling the rate of magma upwelling and the major cycles of recharge of the magma chambers. As a working hypothesis, we propose that the duration of the cycles may be related to (1) progressive weakening of the crust due to magma underplating or (2) cyclic injection of convective plumes into the mantle wedge.

Pulsed uplift estimated from terrace elevations in the coast of Rome: evidence for a new phase of volcanic activity?

Abstract The most recent phase of activity from the Roman volcanoes, since 45 ka, has been interpreted to be the waning stage of the older explosive activity that peaked between 550 and 250 ka. Yet substantial regional uplift from approximately 250 ka suggests that magma chambers beneath the area are growing rather than shrinking. We have estimated the uplift history of the region using radioisotopically and paleomagnetically constrained coastal terrace elevation data that allow us to correlate these terraces with marine oxygen isotopic stages 1–22. Elevations have been corrected for glacio-eustatic sea-level change. Our study shows that over the last million years, regional uplift of coastal terraces occurred in pulses that preceded volcanic activity at roughly 800 ka (in the Monti Sabatini) and 600 ka (in the Alban Hills). The recent pulse of uplift since 250 ka occurred primarily during a period of relative volcanic dormancy that lasted from 250 to 45 ka. We hypothesize that the most recent volcanic activity since 45 ka may reflect the completion of magma chamber recharge, suggesting the start of a new volcanic epoch. It is also possible that the local tectonic stress field changed recently from one dominated by transpression to one dominated by extension, allowing magma to more easily reach the surface via normal faults.

Strike-slip faulting and block rotation: a possible triggering mechanism for lava flows in the Alban Hills?

Abstract It is suggested in this paper that two distinct tectonic triggers, tapping different depths of a magma reservoir, may be the cause of nearly contemporaneous eruptions of lava flows and ignimbrites in the Alban Hills. The area between the Alban Hills and Rome underwent alternating regional extensional and strike-slip faulting during the Middle Pleistocene. The geometries of the main structural dislocations in Quaternary strata, combined with a statistical analysis of drainage network trends, show a structural pattern that is consistent with local strain partitioning in transpressive zones at strike-slip fault bends, superimposed on regional extension. Based on this analysis, it is suggested that local clockwise block rotation between parallel N–S strike-slip faults might have generated local crustal decompression, allowing relatively volatile-free magma to rise from deep reservoirs beneath the Alban Hills, triggering peripheral fissure lava flows. In contrast, the main ignimbrite eruptions appear to tap shallow, volatile-rich magma reservoirs and are controlled by extensional processes.

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

Abstract 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

Large amplification of ground motion at rock sites within a fault zone in Nocera Umbra (central Italy)

During the two mainshocks of September 26, 1997 inthe Umbria-Marche border a strong-motion accelerographrecorded peak ground accelerations as large as 0.6 g,approximately, in the town of Nocera Umbra, atdistances of 10 to 15 km from the epicentres. Thisvalue is significantly larger than expected on thebasis of the usual regressions with magnitude anddistance. A broad-band amplification up to a factor of10 was consistently estimated in previous papers,using both weak and strong motion data recorded at theaccelerograph site during local moderate earthquakes.To study the cause of this amplification we deployedsix seismologic stations across the tectonic contactbetween the Ceno-Mesozoic limestone and the Mesozoicmarly sandstone where the accelerograph is installed.Seismograms of 21 shallow aftershocks in the magnituderange from 2.2 to 4.0 and a subcrustal Mw = 5.3event are analysed. Regardless of epicentre location,waveforms show a large complexity in an approximately200 m wide band adjacent to the tectonic contact. Thisis interpreted as the effect of trapped waves in thehighly fractured, lower velocity materials within thefault zone.

Paleomagnetism and geochronology of early Middle Pleistocene depositional sequences near Rome: comparison with the deep-sea δ18O record

Abstract A sequence-stratigraphic analysis was carried out on clastic coastal deposits (Ponte Galeria Formation – PGF) of the Tiber River delta, cropping out to the southwest of Rome, in order to correlate these sequences to Pleistocene eustasy. Tectonic subsidence affected sedimentation, but is distinguished from the glacio-eustatic signal, showing that the PGF consists of four depositional sequences that correlate to two fourth-order cycles. A paleomagnetic investigation of clay horizons deposited before and after the Brunhes–Matuyama reversal, combined with 40 Ar/ 39 Ar ages from a tuff horizon within the PGF, constrain the timing of the relative sea-level changes and allow us to tie it to the astronomically calibrated deep-sea δ 18 O record. The Brunhes–Matuyama boundary in the PGF postdates a set of highstand systems tract sediments, suggesting that the reversal occurs late in δ 18 O stage 19. This position is consistent with the recent study of deep-sea cores by Bassinot et al. [Earth Planet. Sci. Lett. 126 (1994) 91–108]. The assumption of a common continental stage marking the start of the Middle Pleistocene ( δ 18 O stage 22) in the Mediterranean region is not supported by the Tiber River sections which, in our interpretation, begin at stage 20.

Long-duration asynchronous ground motions in the Colfiorito plain, central Italy, observed on a two-dimensional dense array

[1]xa0In winter 1998 a small-aperture (∼200 m), four-station array was operating in the middle of the Colfiorito plain. Waveforms of seven small magnitude (2.1 < M < 3.7) local earthquakes are analyzed in this study. The peculiarity of the array seismograms is a ∼2-min long duration of the horizontal ground motion. The predominant frequency in the plain is around 1 Hz. In this frequency band, earthquake-induced ground shaking is synchronous for a few seconds during direct S waves; after this time window, wave trains show an increasingly chaotic behavior within the array. The energy variation of the horizontal ground shaking between the array and a nearby rock outcrop exceeds a factor of 500 at 1 Hz. In order to help understanding of observations and construct models for future numerical simulations, the buried structure of the Colfiorito plain has been investigated through seismic refraction profiles and geoelectric measurements. A three-dimensional reconstruction of the basin structure reveals an extremely complex bedrock topography, which is probably a recurrent feature for intermontane basins in tectonically active regions. Apparent velocities and backazimuths in the frequency band 0.5–1.5 Hz indicate that a 180-m deep depression NW of the array is responsible for the generation of edge-diffracted surface (Love) waves. Their arrival to the array breaks the synchronism of vertically reverberating waves which predominate in the first seconds of the S-wave window.