Chapter 1. Introduction
Location and limits of the study area
The Northern Apennines (NA) of Italy are an arcuate orogenic belt, formed within the convergent boundary between the European continental crust (the Corsica-Sardinia block) and Adria (considered either as a promontory of the African Plate or as an independent microplate). The NA are comprised between the Western Alps arc to the North and the Central-Southern Apennines arc to the South (fig. 1). From a geographical point of view, the onland portion of the belt comprehends almost entirely the regions of Tuscany, Emilia-Romagna, Umbria and Marche, along with the NW portion of the Latium. The offshore part comprehends the Northern Tyrrhenian Sea (from Corsica to the Tuscan coast), which is the hinterland of the NA (or of the Tyrrhenian-Apennines system); and the Northern Adriatic Sea, which is part of the present-day foredeep.
Figure 1. Geological sketch of the Neogene-Quaternary basins of the Northern Apennines.
Hinterland Basins - AL: Albegna; AQ: Aquila; CA: Casino; CO: Colfiorito; CH: Valdichiana; CT: Casentino; EL: Valdelsa; FI: Firenze; GU: Gubbio; MU: Mugello; NO: Norcia; PA: Punta Ala; RA: Radicofani; RD: Radicondoli; RI: Rieti; SI: Siena; TE: Valtiberina; VA: Valdarno; VI: Viareggio; VO: Volterra. Foreland Basins - CM: Camerino-Matelica; LA: Laga; MA: Marnoso-Arenacea; MV: Monte Vicino; PA: Po Plain-Adriatic; US: Urbania-Serraspinosa. Major Neogene-Quaternay thrusts - tn: Tuscan Nappe thrust; mr: Main Ridge thrust; pa: Po Plain-Ariatic thrust.
The NA are commonly interpreted as the result of the convergence between the already formed Alpine orogen and the continental crust of the Adriatic promontory of the African plate (e.g. Reutter et al., 1980; Doglioni et al., 1998). In this view, the Late Miocene-Quaternary tectonic evolution of the NA, on which this paper focuses, is superimposed on previous compressional events, related to the formation of the Alpine orogen (Cretaceous-Eocene), and to the consumption of the oceanic lithosphere of the Western Thetys (Late Oligocene-Early Miocene), simultaneous to the rotation of the Corsica-Sardinia microcontinent (Alvarez, 1972; Carminati et al., this volume; Molli et al., this volume).
As the other arcs of the Apennines and of the Mediterranean region (e.g. Lister et al., 1994; Jolivet et al., 1998b), the NA incorporate tectonic units derived from the Mesozoic Thetys ocean and the adjacent continental passive margins. Top to bottom (i.e. hinterland to foreland), they are: the Liguride units (mainly oceanic), the Tuscan and the Umbria-Marche units (both deposited on the continental passive margin of Adria). The pre-orogenic successions are unconformably overlain by syn-compressional units, mainly consisting of turbidite sandstones, deposited in foreland basins (foredeep and/or thrust-top basins): the eastward younging age of these basins marks the progression of the compressional deformation from the hinterland to the foreland. Both pre-orogenic and syn-orogenic successions are involved in the compressional belt, which is successively uplifted and partially eroded. In the western part of the NA the compressional structures are disrupted by later extensional faults, bordering hinterland basins, where continental and/or shallow marine clastic successions are deposited: the age of these syn-extensional units also becomes younger from SW to NE. Neogene magmatic activity accompanies and post-dates normal faulting. Both compressional and extensional structures are segmented by transverse faults, which were active simultaneously to the main structures they are related to. The origin, role and relevance of these transversal structures is still debated (e.g. Pascucci et al., 2007).
Aim and outline of the paper
This paper is not a summary of the very complex stratigraphy and tectonics of the NA region (for a recent review of these aspects see Carmignani et al., 2001; 2004; Barchi et al., 2001). Instead, it is focussed on the peculiar characters of the recent (Neogene to present) tectonic evolution of the region, and how this evolution is reflected in both shallow and deep structures (as depicted by geomorphological, geological and geophysical surveys) and eventually in the present-day stress and strain patterns (depicted by seismological and geodetic data, respectively). The aim is to show that at least the first-order features of the NA can be framed into a relatively simple scheme, beyond many local variations and peculiarities.
These concepts are not new and some of them are remarkably old, based on the collection of extensive data-sets, mainly performed by field geologists of the last century: successively, modern geophysical surveys and up-to-date geodynamic models have offered new technical and theoretical support to these ideas.
After this introduction, Chapter 2 deals with the deep (crust and mantle) structure of the NA, as imaged by a multiplicity of geophysical data, revealing the existence of two well-distinguishable domains, namely the Tyrrhenian and the Adriatic domains.
This crustal structure is the result of a peculiar tectonic evolution, characterised by the contemporaneous activity and eastward migration of coupled compression (in the foreland) and extension (in the hinterland). Chapter 3 summarises the sedimentological, structural, morphological and magmatological evidences constraining the nature and timing of this long-lasting geological process.
Chapter 4 will go deeper into some details of the deformation style of the upper crust, illustrating how the mechanical stratigraphy of the upper crust influenced the geometry and kinematics of both compressional and extensional structures.
Chapter 5 will go back to the present-day setting, analysing the presently active stress and strain field, here regarded as a snapshot of the present-day stage of the NA evolution. This snapshot is captured through the seismicity (both historical and instrumental) and the geodetic (GPS) data.
The final chapter 6 compares the long-term tectonic evolution of the NA and their present-day setting, in order to propose a uniformitarian model of the tectonic process which built up the NA, explaining the observed space (horizontal and vertical) and time relationships between compressional and extensional deformation. Finally, alternative geodynamic scenarios are briefly illustrated, where the NA evolution can be framed.