Pietro Paolo Pierantoni

Quaternary faults and seismicity in the Umbro-Marchean Apennines (Central Italy): evidence from the 1997 Colfiorito earthquake

Abstract Analyses of structural and geomorphological data combined with remote sensing interpretation confirm previous knowledge on the existence of an extensional Quaternary tectonic regime in the Colfiorito area (Umbro-Marchean Central Apennines). This is characterized by a maximum principal axis of finite strain oriented approx. NE–SW, which is the result of a progressive deformation process due to pure and radial extension. Surface geological data, the crustal tectonic setting (reconstructed using a CROP 03 seismic reflection profile), and seismological data relative to the autumn 1997 Colfiorito earthquake sequence constrain the following seismotectonic model. We interpret the seismogenic SW-dipping low-angle normal fault pictured by seismic data as an inverted thrust ramp located in the basement at depth between 5 and 10 km. The surface projection of this seismogenic structure defines a crustal box within which high-angle normal faults are responsible for the deformation of the uppermost crust. The regional patterns of pre-existing basement thrusts therefore control the seismotectonic zoning of the area that cannot be directly related to the high-angle normal fault systems which cut through different crustal boxes; the latter system records, in fact, re-shear along pre-existing normal faults. Moreover, Quaternary slip-rates relative to high-angle normal faults in the Central Apennines are closely related to seismic hazard within each crustal box.

Stratigraphic and structural features of the Sibillini Mountains (Umbria-Marche Apennines, Italy)

In this paper we illustrate the stratigraphic and structural features of the Sibillini Mountains on the basis of a 1:40.000 geological map. Following the “Foglio 132 Norcia” (Geological Map of Italy at 1:100.000 scale; [Scarsella, 1941][1]), this new geological map is the first cartographic document that covers the whole area of the Sibillini Mts. This area is key for understanding the geological evolution of the external zones of the Apennine orogen, mostly owing to the pronounced structural elevation of the Apennine ridge at the Sibillini Mts. area. This allowed us to: a ) carry out stratigraphic and structural analyses for the Umbria-Marche sedimentary cover to the oldest units; b ) analyze the paleotectonic setting of this sector of the Afro-Adriatic continental margin and the behavior of pre-existing structures during the subsequent deformation events; c ) investigate in depth the major Apennine thrust front (“Sibillini Mountains Thrust”) exposed in several sites (Fiastrone, Ambro, Tenna and Tronto valley) and its relationship with the units of the adjacent Messinian foredeep. Inherited (pre-thrusting) structures played an important role on the tectonic evolution of the study area. In addition to those of Jurassic age, associated to thinning of the Adriatic continental margin (the most frequent), there are also some Cretaceous-Eocene extensional faults and others probably related to Miocene foreland deformation. These extensional faults show very limited, or none, reversal of slip during contractional neogenic deformation. Usually they were tilted and, subsequently, displaced by thrust faults; locally they were deformed by buttressing processes. The paleotectonic setting also influenced the development of minor folds, as is evident in the Mt. Bove anticlinorium. Shortening-related structures are mainly represented by asymmetric, northeast-verging thrust-related anticlines involving Mesozoic-Tertiary sedimentary successions. Among these structures, the most important and well-known is the Sibillini Mts. Thrust, which bounds the Apennine mountain front, separating it from the Marche-Abruzzi foothills. This major fault affects the eastern, vertical to overturned, limb of the arcuate shaped Mt. Fiegni-Mt. Vettore anticline, which shows several complications, mostly due to the occurrence of minor thrust splays forming isolated lens-shaped tectonic slices up to several kilometers-long. The geological cross sections show that in the southernmost part of the Sibillini Mts. Thrust the displacement is partly buried and forms a detachment located at the base of the Laga Fm. Much of the Neogene compressional structures has been dissected by NNW-SSE trending Quaternary normal and oblique-slip faults, some of which reactivated older extensional structures. These faults do not seem to crosscut the Sibillini Mts. Thrust. Some of them show fresh fault scarps in the substrate and/or affect recent continental deposits and are considered responsible for the intense seismic activity of the area. [1]: #ref-141

Stratigraphic framework of the late Miocene to Pliocene Pisco Formation at Cerro Colorado (Ica Desert, Peru)

This paper describes a ∼200 m-thick section of the Pisco Formation exposed at Cerro Colorado, an important fossiliferous site in the Ica desert. In order to properly place the fauna in its correct relative position, this study establishes the stratigraphic framework within which the different fossil-bearing intervals of this site can be compared and may prove invaluable in future high-resolution studies on the faunal change. Most of the Pisco Formation deposits exposed at Cerro Colorado consist of gently dipping fine-grained sandstones, diatomaceous siltstones and diatomites with minor ash layers and dolomites deposited within nearshore and offshore settings. To facilitate detailed stratigraphic correlations within the Pisco strata for a 30 km2 area, eight marker beds have been defined and large-scale (1:10,000 scale) geological mapping conducted to determine fault positions, styles and offsets. The geological map shows that there are two important angular unconformities in the study area. The first one is the interformational basal unconformity of the Pisco Formation against folded, faulted, and planated Oligo-Miocene rocks of the Chilcatay Formation. The second is a low-angle intraformational erosional discontinuity of up to 4° angular discordance that allows the subdivision of the Pisco stratigraphy exposed in the study area into two informal allomembers. Dating of the exposed succession by diatom biostratigraphy suggests that the age of the lower allomember is late Miocene, whereas the upper allomember is late Miocene or younger.

Stratigraphic framework of the late Miocene Pisco Formation at Cerro Los Quesos (Ica Desert, Peru)

The enormous concentration of marine vertebrates documented within the Pisco Formation is unique for Peru and South America and places this unit among the prime fossil Lagerstätten for Miocene to Pliocene marine mammals worldwide. In order to provide a robust stratigraphic framework for the fossil-bearing locality of Cerro Los Quesos, this study presents a 1:10,000 scale geological map covering an area of about 21 km2, a detailed measured section spanning 290 m of strata, and a refined chronostratigraphy for the studied succession well constrained by diatom biostratigraphy and high-resolution 40Ar/39Ar isotopic dating of three interbedded ash layers. Within the apparently monotonous, diatomite-dominated sedimentary section, the Pisco Formation has been subdivided into six local members, with stratigraphic control over the different outcrops facilitated by the establishment of a detailed marker bed stratigraphy based on 15 readily distinguishable sediment layers of different nature.

Chapter 2 The Pleistocene extension of the Campania Plain in the framework of the southern Tyrrhenian tectonic evolution: morphotectonic analysis, kinematic model and implications for volcanism

Abstract The Tyrrhenian margin of the Apennine chain (TMAC) experienced widespread extensional tectonics characterized by volcanism and the formation of several marine and intermontane troughs and basins in Pleistocene times. The Campania Plain is part of this extensional system, which encompasses an area from southern Tuscany to the northern margin of Calabria. Extensional tectonics affecting these continental areas is likely to be related with the final stages of the opening of the southern Tyrrhenian Sea, which developed since Middle Tortonian times. This work presents a quantitative kinematic model explaining the relationships between extension in the Tyrrhenian Sea, basin formation in the TMAC, migration of the Apenninic arcs and geotectonic setting of the volcanism. A synthesis of the volcanic, structural and geophysical data available in the literature, coupled with a detailed morphotectonic analysis of the study areas were used in computer-aided reconstruction techniques based on interactive modelling of rigid block rotations to realistically assemble in a unique kinematic framework the first-order structures that are observed in the Apennines area and in the Tyrrhenian basin. Once established, the extension directions in the various sectors of the Apennine chain, by comparing the results of the morpho-structural analysis with data collected from the abundant geological literature, we identified two distinct kinematic elements characterizing the Apennine chain that, from Plio-Pleistocene times, moved independently with respect to the Eurasian reference plate: the Northern Apennines Arc (NAA) and the Southern Apennines Arc (SAA). On the basis of the first-order geological and goephysical constraints, as well as on trial and error experiments, we identified two distinct rotation stages for the Apennine chain. During the first stage, from 3.5 to 0.78 Ma, the NAA and the SAA migrated independently. In the second stage, from 0.78 Ma to the present, the NAA stopped migrating, while the SAA continued migrating towards SE. Thus, N-S extension in the Campania Plain is the result of the relative motion of the NAA with respect to the SAA during the first stage only, whereas the present-day NW-SE extension in this area, which is characterized by intense volcanism (e.g. Ignimbrites, Somma-Vesuvio, Ischia, Campi Flegrei), is related to the migration towards the SE of the SAA with respect to the NAA. The simplifying assumption of rigidity of the two arcs does not substantially affect the model presented, which only aims at describing the process of extension and associated magmatic activity in the TMAC. Furthermore, the model presented above could not take into account many aspects of the complex tectonic evolution of the TMCA. Nevertheless, it realistically assembles in a unique kinematic framework the first-order structures that are observed in the Apennine area and in the Tyrrhenian basin.

SEQUENCE STRATIGRAPHY AND PALEONTOLOGY OF THE UPPER MIOCENE PISCO FORMATION ALONG THE WESTERN SIDE OF THE LOWER ICA VALLEY (ICA DESERT, PERU)

The sequence stratigraphic framework and a summary of the fossil fauna of the upper Miocene portion of the Pisco Formation exposed along the western side of the Ica River (southern Peru) is presented through a new geological map encompassing an area of about 200 km 2 and detailed chronostratigraphic analyses. Extensive field mapping and sedimentological study of outcrop sections have shown that the Pisco Formation is a cyclical sediment unit composed of at least three fining-upward, unconformity-bounded depositional sequences, designated P0, P1, and P2 from oldest to youngest. In the study area, these sequences progressively onlap a composite basal unconformity from southwest to northeast. Integration of biostratigraphic and tephrochronologic age determinations constrains the ages of the three Pisco sequences within the study area. Based on the age of surrounding sediments, a conservative estimate of the age of P0 suggests deposition of these strata between 17.99 ± 0.10 Ma and 9.00 ± 0.02 Ma, whereas diatom biostratigraphy and calculated 40 Ar/ 39 Ar ages converge to indicate that strata of the P1 sequence were deposited sometime between 9.5 Ma and 8.9 Ma and that those of the P2 sequence are younger than 8.5 Ma and older than 6.71 ± 0.02 Ma. Our survey for both vertebrate and macro-invertebrate remains in the three sequences confirms the outstanding paleontological value of the Pisco Formation and contributes to depict regional faunal shifts in the fossil assemblage.

Facies analysis, stratigraphy and marine vertebrate assemblage of the lower Miocene Chilcatay Formation at Ullujaya (Pisco basin, Peru)

ABSTRACT This paper is the first integrated account of the sedimentology, stratigraphy, and vertebrate paleontology for the marine strata of the Chilcatay Formation exposed at Ullujaya, Pisco basin (southern Peru). An allostratigraphic framework for the investigated strata was established using geological mapping (1:4000 scale) and conventional sedimentary facies analysis and resulted in recognition of two unconformity-bounded allomembers (designated Ct1 and Ct2 in ascending order). The chronostratigraphic framework is well constrained by integration of micropaleontological data and isotope geochronology and indicates deposition during the early Miocene. The marine vertebrate fossil assemblage is largely dominated by cetaceans (odontocetes), whereas isolated teeth and spines indicate a well-diversified elasmobranch assemblage. Our field surveys, conducted to evaluate the paleontological sensitivity of the investigated strata, indicate that vertebrate remains only came from a rather restricted stratigraphic interval of the Ct1 allomember and reveal the high potential for these sediments to yield abundant and scientifically significant fossil assemblages.

Geological map of the Miocene-Pleistocene successions of the Mejillones Peninsula, Northern Chile

The Mejillones Peninsula (northern Chile) has undergone a complex tectono-sedimentary history from the Miocene onward and is dissected by a network of normal faults that generated three asymmetric half-grabens located in a forearc position with respect to the Chile subduction system. A new 1:50,000-scale geological map of the peninsula, covering an area of ca 800 km2 and based on a detailed facies analysis and field mapping (1:20,000 scale) in conjunction with a comprehensive interpretation of aerial photographs (1:33,000 scale), gives new and updated information about the nature of marine sedimentation and deformation during the Miocene to Pleistocene and permits the geological history of the peninsula during this time period to be reconstructed. An important aspect of organising the Miocene to Pleistocene stratigraphy has been the recognition of three regional allostratigraphic units in the hangingwall basin-fill sediments (the Miocene Caleta Herradura, Pliocene La Portada, and Pleistocene Mejillones alloformations) and two major morphostratigraphic units (PlioMMU and PleiMMU) including extensive flights of Pliocene and Pleistocene marine terraces carved on crestal and flanking areas of adjacent footwall blocks.

Intraformational unconformities as a record of late Miocene eustatic falls of sea level in the Pisco Formation (southern Peru)

ABSTRACT Field mapping and sedimentological study of outcrop sections exposed along the Ica River valley permitted the establishment of a regional allostratigraphic framework for the upper Miocene portion of the Pisco Formation. The stratigraphy of the studied interval is illustrated using a new 1:20,000-scale geological map which reveals that this formation is a cyclical sedimentary succession composed of three fining-upward allomembers. The bounding surfaces defining each allomember are transgressively modified subaerial unconformities. They converge and merge landward into a single composite surface representing the time-transgressive lower boundary of the Pisco Formation. Accordingly, the extent of the stratigraphic gap associated with the basal unconformity varies significantly throughout the basin and increases toward the basin margins. The timing of allomember-bounding surfaces coincides with that of major oxygen-isotope maxima in the deep-sea oxygen isotopic record and matches the ages of eustatic sequence boundaries identified elsewhere, indicating glacio-eustatic falls due to the growth of Antarctica ice sheets as a viable mechanism for their development.

A space-time journey through the composite Conero Geosite (Marche, Italy): a tool for teaching Earth Sciences at school

This work is part of a PhD project devoted to promoting the study of Earth Sciences at schools by presenting significant geosites in the Marche Region. The Mount Conero geosite, with several outcrops and geological trails, offers teachers prime locations for showing the most interesting moments of the geological evolution of the region, from the Cretaceous to the Pleistocene, and can serve as a springboard for introducing a variety of geoscience topics related to the major themes of Earth sciences courses at schools. These outcrops also meet certain requirements for school field trips, in terms of logistics, appeal, availability of data, and they can be modulated for students of different ages. Teachers can avail themselves of general, specific, and interdisciplinary teaching material at different school levels (http://d7.unicam.it/teachingearthsciences/) for the field trips and class activities.