Cinque Terre geology aims at describing succintly the often undervalued geological value of this celebrated stretch of coastline in Eastern Liguria.
The Ligurian coastline is possibly the most iconic in Italy. In addition to being extremely scenic and beautiful in its own right, the coast of the Cinque Terre is home to some incredibly complex and exciting geology. It is composed of the Maritime Alps in the west and the Ligurian ‘nappes’ of the Apennines in the east. Along this coast, the north-northwest convergence of the Alpine orogeny gives way to the active east-northeast convergence along the Apennines.
Something surprising about the coastline in this region is how dramatically the relief is so near the seafront – the mountains along the coast rocket out of the sea over 1,100 meters within 5 kilometers of the coastline: this is greater than any of the peaks in the Scottish highlands, well known for their significant relief.
The western promontory of La Spezia Gulf ('Golfo della Spezia') and its northward prolongation along the Ligurian Sea is an excellent area where to study the sedimentary and deformational history of the Northern Apennines, as they reach the coastline. One of the most spectacular geological cross-sections of the Apennines is visible on the cliffs along the coast with outcrops of units of the Mesozoic Tethys ocean domain and of the Mesozoic to Cenozoic basins of the Adria continental margin. These units record significant deformations from the Jurassic ocean spreading phase to the Late Cretaceous-Miocene subduction and collisional phases. The La Spezia fold is a pluri-km structure that involves the Adria margin units (Tuscan Nappe); then, the oceanic realm – the so-called Ligurian Domain – is entered with the beautiful exposures of the steep Cinque Terre sea-cliffs.
Regional Geologic Setting
The complex geologic history of the region is punctuated by several major tectonic transitions:
All three major lithologic divisions in the Cinque Terre area – the Ligurian, Sub-Ligurian, and Tuscan units – are components of the Northern Apennines fold-and-thrust belt. The rocks exposed along the Cinque Terre cliffs are highly deformed as a result of the Adria-Europe continental collision.
During the Late Miocene continental plate collision, Ligurian and Sub-Ligurian rocks deposited on the European Plate smashed into Tuscan sediments deposited on the Adria Plate. Ligurian and Sub-Ligurian rocks were deformed during the initial phases of the collision and were then over-thrusted over the Tuscan units.
Subjected to extreme pressures, internally the Tuscan units overthrusted themselves, leading to a doubled section. The lower or underthrusted Tuscan section is highly metamorphosed and is referred to as the Metamorphic Tuscan Succession; the over-riding portion of the Tuscan units is called the Tuscan Nappe.
Ligurian Units (Jurassic to Paleocene)
The rocks of the Ligurian units consist of marine sedimentary rocks and ophiolite sequences deposited in the Ligurian-Piedmont Ocean (“Oceano Ligure-Piemontese”). This ocean basin was a former piece of oceanic crust associated with the Tethys Ocean. Fragments of Piedmont-Ligurian oceanic crust are also preserved in the Penninic ‘nappes’ of the Alps and in the Tuscan ‘nappes’ of the Apennines.
Within the Ligurian ‘nappes’ are some classic examples of continental margin ophiolites. These ophiolites consist of exhumed, subcontinental lithospheric mantle iherzolite directly overlain by basaltic pillow lavas. The ophiolites along the Ligurian coast along with those in the Central Alps and Apennines were instrumental in shaping the early understanding of obducted oceanic crust.
Sub-Ligurian Units (Paleocene – Miocene)
The composition of the Sub-Ligurian unit primarily consists of clastic sediments deposited in a shallow marine environment. In the Cinque Terre area, the primary Sub-Ligurian rocks exposed are members of the Canetolo Unit, a highly deformed Paleocene calc-argillitic rock.
Generally, rocks in the Sub-Ligurian units present a transition from deep marine limestones in the older section to sandstones deposited in a shallow environment in the younger section. As the depostional setting for Sub-Ligurian units was an oceanic basin being slowly closed up as the European and Adria plates moved towards each other, the shallowing-upward sequence makes sense.
Tuscan Units (Triassic to Miocene)
Tuscan unit rocks were deposited in a passive margin marine environment that existed on the northern edge of the Adria Plate from the Triassic through to the Miocene. The sedimentary package is between 3 and 4 kilometers thick, though compressional deformation resulting from the continental collision during the Miocene has overthrusted portions of the Tuscan units over themselves, causing a doubling of the section.
The most interesting stretch where to observe the exposed lithology of the Cinque Terre geology is by the villages of Riomaggiore, Manarola and Vernazza, right by the harbour; the Via dell'Amore is also quite impressive from the viewpoint of geology; unfortunately, however, at the moment it is not accessible to the public.