Reconciling the Geology of the Emilia Apennines and Tuscany across the Livorno-Sillaro Lineament, northern Apennines, Italy.

摘要

Surface expression of lithospheric faults may vary greatly as they can develop a wide range of geomorphic/topographic features and various kinds of superficial geological/structural mismatchings. The “Livorno-Sillaro Lineament” (Nirta et alii, 2007; Pascucci et alii, 2007; Bettelli et alii, 2012) is one of the most important transverse lineaments of the Northern Apennine orogen. The lithospheric-scale role of this structure has been recognized long time ago by various authors on the base of different geophysical, geological and geomorphic data, although its origin is still not well defined. Also the exact surface characters of this structure are still not well-defined, we think because they are mainly based on old and out-of-date geological data.udWe present a review of the more recent stratigraphical and structural data related to the geology across the “Sillaro Lineament”, SL, the northeasternmost segment of the “Livorno-Sillaro Lineament”. Based on a re-examination and reinterpretation of the existing information about the regional geology of the Northern Apennines we conclude that the supposed mismatching of the Ligurian/Subligurian Units on the two sides of this lineament is mainly due to a lack of knowledge and to an inadequate correlation between corresponding units. Nevertheless, we recognize that this structure (along with the Secchia transverse lineament) greatly influenced the growth and the evolution of the oceanic accretionary prism/Ligurian/Subligurian thrust-nappe from the late Eocene to the late Serravallian, and also later on. In particular, we point out that at least the easternmost segment of this structure not only played an important role on the differential growth of the Ligurian/Subligurian accretionary prism-thrust nappe, but that it was responsible for the different amount of translation of the Ligurian Units on both side of the lineament.udOur conclusions and interpretations include:ud1) the Sillano/Mt Morello succession, typically cropping out SE of the SL in eastern Tuscany, represents the source rocks of the Ligurian blocks forming the Sestola-Vidiciatico tectonic unit and similar units (e.g., Coscogno-Montepastore tectonic unit: Remitti et alii, 2013) cropping out NW of the SL and along the SL itself;ud2) the External Ligurian unit variously named as Samoggia/Val Sillaro/Val Marecchia Varicoloured Shales, AVS, and the overlying lower to middle Eocene turbidites (e.g., Savigno Fm) cropping out in the Emilia Apennines - i.e., NW of the SL – represents a lateral and more internal equivalent of the Sillano/Mt Morello succession. The AVS were extensively present also SE of the SL, as testified by the large klippen in the Romagna Apennines (Savio and Marecchia valleys) and many small klippens in the Umbria area (Umbertide-Gubbio area); ud3) along and SE of the SL the AVS form the stratigraphic base of the Mt Morello Fm. Therefore, also this unit is present on both sides of the SL;ud4) the pre-middle Eocene Subligurian Units cropping out NW of the SL (Argille e Calcari di Canetolo Fm and Calcari del Groppo del Vescovo Fm) do not correspond to the so called Subligurian Units cropping out SE of the SL (i.e., in Tuscany). The latter are the result of the sedimentation in a particular paleogeographic domain, transitional to the Tuscan domain, absent or not preserved NW of the SL. This seems to represent the only real difference in the geology of the Ligurian/Subligurian thrust nappes NW and SE of the SL.udAll the available data show that until the late Serravallian the thrust front of the Ligurian nappe was located in the same position across the SL. However, starting from the early-late Tortonian a differential translation of the Ligurian nappe NW of the SL took place, progressively reaching the present day position. With the exception of the Marecchia area, in the Romagna and Umbria Apennines (SE of the SL), instead, the thrust front of the Ligurian nappe remained more or less in the same position it reached in the late Serravallian. This implies that in the Northern Apennines the transverse SL played also an important role in the different amount of translation of the Ligurian thrust-nappe.udududREFERENCESudBettelli, G., Panini, F., Fioroni, C., Nirta, G., Remitti, F., Vannucchi, P. & Carlini, M. (2012), Revisiting the Geology of the “Sillaro Line”, Northern Aprnnines, Italy. Rendiconti Online Società Geologica Italiana, 22, 14-17.ududNirta, G., Principi, G. & Vannucchi, P. (2007), The Ligurian Units of Western Tuscany (Northern Apennines): insight on the influence of pre-existing weakness zones during ocean closure. Geodinamica Acta, 20/1-2, 71-97, doi:10.3166/ga.20.71-97ududPascucci, V., Martini, I.P., Sagri, M. & Sandrelli, F. (2007), Effects of transversal structural lineaments on the Neogene-Quaternary basins of Tuscany (inner Northern Apennines, Italy). In: G. Nichols, E. Williams & C. Paola (Eds.), Sedimentary Processes, Environments and Basins: a Tribute to Peter Friend (pp.155-182). Special Pubblication no. 38 of the International Association of Sedimentologists.ududRemitti, F., Balestrieri, M.L., Vannucchi, P. & Bettelli, G. (2013), Early exhumation of underthrust units near the toe of an ancient erosive subduction zone: A case study from the Northern Apennines of Italy. Geological Society of America Bullettin, 125, 1820-1832, ISSN: 0016-7606.