Structural Evolution of the Apuane Alps: An Example of Continental Margin Deformation in the Northern Apennines, Italy

Abstract

The Apuane Alps region of the Northern Apennines provides exceptionally clear exposures of continental margin rocks deformed during collision of the Corsica-Sardinia microplate with Italy. Detailed structural mapping reveals a large scale crustal shear zone in which major isoclinal folds were rotated as much as 90°, bringing their axes into parallelism with the direction of nappe transport. A large tectonic window through the allochthonous cover sequences exposes, in ascending structural order, the following sequences: (1) the Apuane metamorphic sequences, which have been repeatedly deformed and metamorphosed to greenschist fades. They consist of continental margin, sedimentary rocks of Permian to Oligocene age deposited on Paleozoic continental basement. (2) The Tuscan nappe, which consists of an essentially unmetamorphosed, and only slightly deformed sequence of similar lithologies and ages. The Tuscan nappe has been thrust over the Apuane metamorphic sequences along an evaporite layer. (3) The Liguride sequences, which consist of deep water pelagic sediments and ophiolites. Structures formed during several phases of compressive deformation followed by a late stage of extension. During the first phase ($$D_{1}$$), the Tuscan nappe, together with the overlying Liguride sequences, was emplaced over the metamorphic sequences, which were ductiley deformed into tight, recumbent folds with flat axial surfaces. A second phase ($$D_{2}$$), and third phase ($$D_{3}$$) refolded all pre-existing structures and formed crenulation cleavages and conjugate schistosities. Both $$D_{2}$$ and $$D_{3}$$ phases are post-nappe emplacement. The late stage uplift of the region is related to regional extension in Tuscany. During the $$D_{1}$$ deformation of the metamorphic sequences, simple shear strain was dominant. This is demonstrated by the presence of schistosities oriented at a low angle to shear zone boundaries, strongly developed mineral elongation lineations parallel to the likely transport direction of fold nappes, and numerous strain discontinuities with geometries typical of simple shear zone boundaries. A progressive 90° change in the orientation of $$A_{1}$$ fold axes from directions parallel to the strike of the mountain belt into directions parallel to the mineral extension lineation can be seen within the metamorphic sequences. This is interpreted as an example of passive fold rotation during progressive simple shear. The above features suggest that the Apuane Alps region was deformed and metamorphosed within a large scale low-angle crustal shear zone with an overthrust sense of movement. The regional, greenschist facies metamorphism is confined to the rocks within the proposed shear zone and it is likely that frictional (shear) heating was significant in its evolution. The development of the shear zone within continental crust, the penetrative deformation and metamorphism, and the emplacement of the allochthonous sequences are the effects of compressive deformation of the Northern Apennine continental margin from the Oligocene to the Miocene. This is attributed to rotation of the Corsica-Sardinia microplate and its subsequent collision with the Italian continental crust.