Introduction to the geology of the Northern Apennines

Abstract

According to Aubouin (1965) the sedimentary evolution of the Northern Apennines geosyncline is divided into a geosynclinal stage proper, represented by eu- and miogeosynclinal sequences, a late geosynclinal and a postgeosynclinal stage. In the Apennines the eugeosynclinal rocks are almost entirely allochthonous. Their interpretation as autochthonous is held to be unrealistic on structural and paleogeographic grounds. The late geosynclinal stage is defined here mainly on the base of tectonic criteria: sediments deposited over folded eugeosynclinal rocks, later subjected to lateral tectonic transport in the same manner as their allochthonous substratum. Owing to the eastward progression of tectonic movements in the Northern Apennines, the tecto-sedimentary stages tend to overlap and coexist (e.g., Oligocene-Miocene miogeosynclinal flysch and late geosynclinal sediments). The eugeosynclinal stage is characterized by the presence of ophiolites and the early development of flysch. Four main groups of sequences are distinguished: ( 1) Upper Cretaceous to Eocene Helmynthoid Flysch sequences; ( 2) Jurassic to Eocene Vara Supergroup; ( 3) Upper Cretaceous to Middle Eocene Calvana Supergroup; and ( 4) Paleogene Canetolo Complex. The largely autochthonous miogeosynclinal rocks are represented by the Tuscan (Lower Triassic to Lower Miocene) and Umbrian (Carnian to Upper Miocene) sequences. The late geosynclinal sequences are Middle Eocene to Messinian in the Emilian Apennines, Lower and Middle Miocene in Tuscany and Romagna. The postgeosynclinal sediments are Upper Miocene to Pleistocene in the southwest (Tuscany-Latium), Pliocene and Pleistocene in the northeast and east (Emilia and Marche). Four major structural areas are distinguished: 1. ( 1) The Tyrrhenian area, a zone of intense crustal shortening, containing the axis of symmetry between the structure of the Apennines and that of the Western Alps and “Alpine Corsica”. 2. ( 2) Southwestern Tuscany, characterized by a fault block structure and an incomplete Tuscan sequence. 3. ( 3) The main fold range, with reverse faults, overturned folds and overthrusts, all directed eastward and northeastward. There are two major northwest-southeast lines of thrusting and overturned folding, and two major tectonic windows (Alpi Apuane and Monte Pisano), in which the Tuscan sequence is doubled. 4. ( 4) The outer foothills and Po Valley, where the asymmetric folding and reverse faulting become gradually attenuated. Metamorphism is generally of low grade (greenschist facies), and affects the lower part of the Tuscan sequence in small areas of western Tuscany (Alpi apuane, Monte Pisano, Montagnola Senese, Elba Island). Metamorphism tends to be associated with the tectonic doubling of the sequence. Theories on the tectonic interpretation of the Northern Apennines are summarized. The authors are inclined to accept features of both the “décollement” nappe model of Trevisan et al. (1965) and the orogenetic landslide model of Migliorini (1948) and Merla (1951). The emplacement of the allochthon was essentially through gravitational gliding; detachment and gliding affected also, in parts, rocks of the Tuscan sequence (Tuscan nappe of the Alpi Apuane). Uplift and differential movements (block faulting?) in the miogeosyncline from the Triassic to the Cretaceous are indicated by slumping and unconformities. Much slumping occurred during the Cretaceous in the eugeosyncline. The first recorded eastward gliding movements of nappes are Upper Cretaceous to Paleocene in the southern part of the eugeosyncline, Eocene in the northern part of it. Parts of the eugeosynclinal sequences were folded in the Lower-Middle Eocene and at the Eocene-Oligocene transition. The advancement of nappes onto the miogeosynclinal rocks, accompanied by folding, began at the Oligocene-Miocene transition. It continued, gradually moving eastward, until the Lower Pliocene. The detachment of rocks of the Tuscan sequence in the Alpi Apuane and southern Tuscany (Tuscan nappe) seemingly occurred in the Lower and Middle Miocene.