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

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surface characters of this structure are still not well-defined, we think because they are mainly based on old and out-of-date geological data. We 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. Our conclusions and interpretations include: 1) 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; 2) 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); 3) 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; 4) 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. All 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. REFERENCES Bettelli, 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. Nirta, 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.20.71-97 Pascucci, 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. Remitti, 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

New insights into the late Miocene extent of the Ligurian nappe on the Adriatic side of the Northern Apennines, SE of the Sillaro Line, Italy

On the Adriatic side of the Northern Apennines, SE of the Sillaro Line, both the original extent/areal distribution and the stratigraphic composition of the Ligurian thrust-nappe are still unknown and represent a highly debated topic. Few contraints have been proposed in recent papers. Based on the presence of “Ligurian olistostromes” within the Tortonian Marnoso arenacea Fm and their absence, instead, within younger deposits, some workers have been suggested that to SE of the Sillaro Line (i.e., in the Romagna and Umbria Apennines) the Ligurian nappe was located in a more internal position. However, other workers claimed for a Ligurian thrust-sheet up to 5 km thick, partially or completely covering the internal Marnoso arenacea Fm of the Romagna Apennines. The latter hypothesis was based either on vitrinite reflectance data and apatite fission tracks data or on the composition of the Quaternary fluvial terraced deposits cropping out along the Romagna foothills. Following this latter hypothesis, the complete disappearance of the Ligurian nappe between the Sillaro Line and the Marecchia area, was the result of a differential widespread event of tectonic uplift since the Pliocene times enhancing subaerial erosion. Here we show that a re-examination and reinterpretation of the existing knowledge about the regional geology of the Northern Apennines is essential to solve this problem. In fact, in the External Ligurian Units cropping out in the south-eastern sector of the Emilia Apennines and in the Romagna Apennines (between the Enza and the Marecchia valleys) a particular distinct unit, variously named as Val Samoggia, Val Sillaro or Val Marecchia Varicoloured Shales - AVS - have been recently distinguished from the rest of the Ligurian Units. The AVS typically consist of green, black, red and grey shales with interbedded layers of predominantly limestones, marlstones, sandstones and manganese-rich thin layers ranging in age from the Early Cretaceous and the early Eocene. The AVS are stratigraphically inchoerent and pervasively deformed at all scales of observation, showing the typical characters of dismembered and broken formations. The AVS form the stratigraphic base of different early-middle Eocene flysches of the External Ligurian Units (i.e., Sporno, Savigno and M. Morello formations) and they share the same lithological, stratigraphical and structural character independent of the overlying formation. Interestingly, the AVS crop out also either in few scattered outcrops (klippens) in eastern Tuscany (Casentino and Pieve Santo Stefano areas) and Umbria (between Umbertide and Nocera Umbra) where they have been interpreted not as klippens but as “olistostromes” within the Marnoso arenacea Fm. In both latter areas the AVS are unconformably overlain by few scattered remnants of the late Oligocene to middle Miocene epi-Ligurian Antognola, Monte Fumaiolo or S. Marino formations. These data clearly imply that in the whole Romagna and Umbria Apennines on the internal Marnoso arenacea Fm a thin thrust sheet of Ligurian rocks exclusively formed of AVS and an uncomplete overlying epi-Ligurian succession, similar to that cropping out in the Val Marecchia area, was present at least up to the middle Miocene. Therefore, it follows that the thickness of the Ligurian nappe was not the same across the Sillaro Line, and that the thrust-front of the nappe to NW and SE of this tectonic lineament clearly had a quite different evolution. These findings again show that in the Northern Apennines the Sillaro Line played also an important role inside the evolution of the Ligurian nappe. KEY WORDS: Ligurian Units, Ligurian nappe, Northern Apennines, Romagna Apennines, Sillaro Line, Umbria Apennines, Varicoloured shales

Deformation, fluid flow, and mass transfer in the forearc of convergent margins: A two-day field trip in an ancient and exhumed erosive convergent margin in the Northern Apennines

This guide provides background information and an itinerary for a two-day fi eld trip in the Northern Apennines leaving from Modena and ending at Riolunato (Modena). The proposed fi eld trip route leads through the Po Valley side of the Northern Apennines, in the Emilia region. The fi eld trip provides opportunities to examine the exposed geological features related to a phase of early-middle Miocene convergence between the European and Adria plates. In particular, outcrops have been selected that exhibit features characterizing deformation in an exhumed plate boundary shear zone, interpreted as an erosive plate boundary shear zone. The sedimentary evolution and deformation of the upper and lower plates are also highlighted

Deformation, fluid flow, and mass transfer in the forearc of convergent margins: A two-day field trip in an ancient and exhumed erosive convergent margin in the Northern ApenninesDeformation, Fluid Flow, and Mass Transfer in the Forearc of Convergent Margins: Field Guides to the Northern Apennines in Emilia and in the Apuan Alps (Italy)

This guide provides background information and an itinerary for a two-day field trip in the Northern Apennines leaving from Modena and ending at Riolunato (Modena). The proposed field trip route leads through the Po Valley side of the Northern Apennines, in the Emilia region. The field trip provides opportunities to examine the exposed geological features related to a phase of early-middle Miocene convergence between the European and Adria plates. In particular, outcrops have been selected that exhibit features characterizing deformation in an exhumed plate boundary shear zone, interpreted as an erosive plate boundary shear zone. The sedimentary evolution and deformation of the upper and lower plates are also highlighted

Deformation, fluid flow, and mass transfer in the forearc of convergent margins: A two-day field trip in an ancient and exhumed erosive convergent margin in the Northern Apennines

This guide provides background information and an itinerary for a two-day fi eld trip in the Northern Apennines leaving from Modena and ending at Riolunato (Modena). The proposed fi eld trip route leads through the Po Valley side of the Northern Apennines, in the Emilia region. The fi eld trip provides opportunities to examine the exposed geological features related to a phase of early-middle Miocene convergence between the European and Adria plates. In particular, outcrops have been selected that exhibit features characterizing deformation in an exhumed plate boundary shear zone, interpreted as an erosive plate boundary shear zone. The sedimentary evolution and deformation of the upper and lower plates are also highlighted

Tectonic and erosional exhumation processes in the western Northern Apennines of Italy: coeval compressional and extensional tectonics affecting an eroding orogenic wedge

In the Northern Apennines of Italy the Ligurian and Subligurian Units (LSU) represent far-travelled/allochthonous units which, since early Miocene to Recent, migrated towards NE, above the Tuscan-Umbrian foredeep deposits, reaching the present-day frontal ranges of the chain. In the NE-facing side of the western Northern Apennines the present-day geometry of the LSU does not reflect the shape they acquired during the late Oligocene, when the early orogenic collisional phases produced a wedge whose geometry tapered out towards the foreland area, i.e. to the E-NE. In fact, at present, geological evidences indicate that the LSU are characterized by a doubly-tapering geometry, whose thickness ranges from less than 1 km at the main ridge zone (SW) up to more than 4 km along the NE slope of the chain and then tips out again few km N of the Northern Apennines topographic front, underneath the late Messinian to Recent marine and continental deposits of the Po Plain foredeep. These data imply that since late Oligocene the LSU geometry has been deeply reshaped inside a context of overall growth of the orogenic wedge. Therefore the reshaping of the LSU is related to their post-early Miocene progressive emplacement over the foredeep units and to the late orogenic evolution of the Northern Apennines, characterized by the coeval activity of extensional and compressional tectonics. In order to put new constraints on the relationships existing between timing and modes of reshaping of the LSU and the Miocene to Recent evolution of the western Northern Apennines, we adopted a multidisciplinary approach which took into account field evidences, low temperature thermal and thermochronological data (vitrinite reflectance, clay mineral analyses, apatite fission track dating), numerical modelling of the AFT cooling ages through the use of Pecube finite element code, recently published works on the evolution of the external slope of the chain, and a new interpretation of seismic lines and boreholes data. This multidisciplinary approach allowed us to: 1) build a 3D representation of the LSU present-day geometry; 2) highlight two main stages of reshaping of the LSU, first during late Miocene (10 to 6 Ma), and subsequently during Pliocene to Recent (?) time (6 to 0 (?) Ma); 3) relate, during the first stage, the LSU thinning processes, achieved by means of low-angle extensional faults, to the tectonic denudation of the foredeep units; 4) envisage the importance of coupled tectonic (high-angle extensional tectonics) and progressively increased surface erosion processes as responsible for the denudation of the foredeep units during the second stage; 5) define estimates of the exhumation/erosion rates characterizing the investigated area during the last 10 My. These results allowed us to give new insights, temporal and spatial constraints on the morphostructural evolution of the western Northern Apennines, which has been mainly affected initially (10-6 Ma) by the interplay between deep compressional and coeval shallow extensional tectonics (< 20 and < 4 km, respectively), and subsequently by the progressively more relevant onset of surface erosion processes

Tectonic and erosional exhumation processes in the western Northern Apennines of Italy: coeval compressional and extensional tectonics affecting an eroding orogenic wedge

In the Northern Apennines of Italy the Ligurian and Subligurian Units (LSU) represent far-travelled/allochthonous units which, since early Miocene to Recent, migrated towards NE, above the Tuscan-Umbrian foredeep deposits, reaching the present-day frontal ranges of the chain. In the NE-facing side of the western Northern Apennines the present-day geometry of the LSU does not reflect the shape they acquired during the late Oligocene, when the early orogenic collisional phases produced a wedge whose geometry tapered out towards the foreland area, i.e. to the E-NE. In fact, at present, geological evidences indicate that the LSU are characterized by a doubly-tapering geometry, whose thickness ranges from less than 1 km at the main ridge zone (SW) up to more than 4 km along the NE slope of the chain and then tips out again few km N of the Northern Apennines topographic front, underneath the late Messinian to Recent marine and continental deposits of the Po Plain foredeep. These data imply that since late Oligocene the LSU geometry has been deeply reshaped inside a context of overall growth of the orogenic wedge. Therefore the reshaping of the LSU is related to their post-early Miocene progressive emplacement over the foredeep units and to the late orogenic evolution of the Northern Apennines, characterized by the coeval activity of extensional and compressional tectonics. In order to put new constraints on the relationships existing between timing and modes of reshaping of the LSU and the Miocene to Recent evolution of the western Northern Apennines, we adopted a multidisciplinary approach which took into account field evidences, low temperature thermal and thermochronological data (vitrinite reflectance, clay mineral analyses, apatite fission track dating), numerical modelling of the AFT cooling ages through the use of Pecube finite element code, recently published works on the evolution of the external slope of the chain, and a new interpretation of seismic lines and boreholes data. This multidisciplinary approach allowed us to: 1) build a 3D representation of the LSU present-day geometry; 2) highlight two main stages of reshaping of the LSU, first during late Miocene (10 to 6 Ma), and subsequently during Pliocene to Recent (?) time (6 to 0 (?) Ma); 3) relate, during the first stage, the LSU thinning processes, achieved by means of low-angle extensional faults, to the tectonic denudation of the foredeep units; 4) envisage the importance of coupled tectonic (high-angle extensional tectonics) and progressively increased surface erosion processes as responsible for the denudation of the foredeep units during the second stage; 5) define estimates of the exhumation/erosion rates characterizing the investigated area during the last 10 My. These results allowed us to give new insights, temporal and spatial constraints on the morphostructural evolution of the western Northern Apennines, which has been mainly affected initially (10-6 Ma) by the interplay between deep compressional and coeval shallow extensional tectonics (< 20 and < 4 km, respectively), and subsequently by the progressively more relevant onset of surface erosion processes