Earth-Science Reviews 125 (2013) 43–68 Contents lists available at SciVerse ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Stratigraphic evolution in the Ligurian Alps between Variscan heritages and the Alpine Tethys opening: A review A. Decarlis a,⁎, G. Dallagiovanna b, A. Lualdi b, M. Maino b, S. Seno b a IPGS – EOST, Université de Strasbourg, CNRS UMR 7516; 1, rue Blessig F-67084, Strasbourg Cedex, France b Università degli Studi di Pavia, Dipartimento di Scienze della Terra e dell'Ambiente, Via Ferrata 1, 27100 Pavia, Italy article info abstract Article history: The Ligurian Alps are the southernmost segment of the Alpine orogenic belt. Their formation is the result of a Received 11 September 2012 multistage process that comprises two orogenic cycles: the Variscan and the Alpine one. Since the Late Accepted 4 July 2013 Paleozoic onwards, superposed sedimentary successions deposited in this area, as a response to the changing Available online 12 July 2013 tectonic setting in the frame of the Western Mediterranean evolution. The transition between the two orogenies was characterized by the formation of a net of sedimentary basins controlled by diffused extension, which is not Keywords: correlated to a clear geodynamic context yet. As the fault network is severely affected by Alpine reworking, the Ligurian Alps Prepiedmont multiple pre-Alpine tectonic stages experienced by the Ligurian sector can be unravelled only by the analysis of Briançonnais the well exposed stratigraphic succession, which offers very distinctive feature for each tectonic phase. Geodynamics From the present day situation of a nappe-pile orogen backward to the originary basinal setting, the aim of Central Mediterranean this work is to reconstruct the paleogeographic evolution of the Ligurian Alps between Permian and Jurassic, through a detailed stratigraphic analysis. Several sedimentary packages belonging to different tectonic units are analysed, grouped into specific domains and correlated. Five major steps in the evolution of the Ligurian Alps have been pointed out; they have been contextualized within the latest paleogeographic reconstructions of the Alpine sector, contributing to detail the role of the study area in the plate tectonic dynamics. The sedimentary record is referred to successive geodynamic stages, from the Pangea break-up and Variscan belt dismantling, through a Triassic diffused extension, to the Alpine Tethys rifting and finally to the spread- ing that generated the Piedmont–Ligurian oceanic branch. The stratigraphic reconstruction of the Ligurian sector also indicates the lacking of an ocean interposed between the European continent and the Alpine collisional wedge, thus representing the southward termination of the Valais basin. On the whole, the paleogeographic reconstructions provided in this work highlight that the Ligurian Alps were a domain in which for over a hundred million years orogenesis, rifting and oceanisation strongly affect- ed the integrity of the upper crust. The heterogeneity of the crust that suffered multiple mantle uplifts, partial melting and extensive faulting over a long period is testified by field evidences and several effusive events. The last and most important rifting and the following Alpine Tethys oceanisation developed around a narrow, elongated area of crustal weakness generated by multiple geodynamic events. © 2013 Elsevier B.V. All rights reserved. Contents 1. Introduction and geological setting .................................................... 44 2. Alpine structure and evolution ...................................................... 45 3. Paleogeographic reconstruction ..................................................... 48 4. Stratigraphy .............................................................. 48 4.1. Permian ............................................................. 48 4.2. Uppermost Permian–Lower Triassic ................................................ 52 4.3. Middle Triassic .......................................................... 52 4.3.1. Costa Losera Formation (CLO; ) .............................................. 52 4.3.2. San Pietro dei Monti Dolomite (SPMD; ) .......................................... 52 4.4. Upper Triassic .......................................................... 54 ⁎ Corresponding author. Tel.: +33 368850427; fax: +33 368850402. E-mail address: [email protected] (A. Decarlis). 0012-8252/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.earscirev.2013.07.001 44 A. Decarlis et al. / Earth-Science Reviews 125 (2013) 43–68 4.4.1. Capo Noli Formation (CNF; ) ...............................................54 4.4.2. Rocca Prione Formation (RPF; ) ..............................................54 4.4.3. Monte Arena Dolomite (MAD; ) ..............................................54 4.4.4. Veravo Limestone (VL; ) .................................................54 4.5. Jurassic ..............................................................54 4.5.1. Siderolitico (SID) .....................................................55 4.5.2. Rocca Livernà Limestone (RLL; ) ..............................................55 4.5.3. Monte Galero Breccia (MGB; ) ...............................................57 4.5.4. Montaldo calcschists (MCS; ) ...............................................57 4.5.5. Nasino sandstone (NS; ) .................................................57 4.5.6. Arnasco Radiolarite (AR; ) .................................................57 4.5.7. Rio di Nava Limestone (RNL; ) ...............................................57 4.5.8. Val Tanarello Limestone (VTL; ) ..............................................57 5. Syn-sedimentary tectonics ........................................................58 5.1. Permian .............................................................58 5.2. Triassic ..............................................................58 5.3. Jurassic ..............................................................59 5.4. Paleomargin orientation ......................................................59 6. Major events recorded in the tectonic evolution ..............................................59 6.1. Events during the Permian period .................................................59 6.2. Middle Triassic events .......................................................61 6.3. Upper Triassic–Lower Jurassic events ................................................61 6.3.1. First rifting phase .....................................................61 6.3.2. Second rifting phase ...................................................61 6.3.3. Third rifting phase and post-rift ..............................................63 6.3.4. Paleogeography remarks .................................................65 7. Conclusion ...............................................................65 References ..................................................................66 1. Introduction and geological setting In the southern western Alps, at least four different domains are preserved: The renewed interest that has arisen around passive rifted margins leads us to reconsider the stratigraphy and geodynamics of (1) The Eastern Provençe domain representing the more external this peculiar and misunderstood sector of the Italian Alpine belt. areas with respect to the chain, mainly outcropping in France. The Western Italian Alps comprise one of the most extensively stud- It has been interpreted to derive from the proximal European ied areas in geological sciences, but the Ligurian Alps, representing a Tethyan passive rifted margin. At present it is well-preserved large part of the western belt, have been left out from recent debates due to the scarce involvement in the later Alpine build up. and are rarely cited in papers on pre-collisional geodynamic recon- (2) The Briançonnais domain is a debated key-area for the Tethyan structions. In our opinion this is due to the scarcity of international paleogeography and rift dynamics. It is formed by a basement literature on this Alpine sector, and to the widespread metamorphism and by a volcanic and sedimentary cover whose details are and tectonic displacement affecting almost all of the sedimentary presented in this paper. It is characterized by a long-lasting covers, discouraging field research. Nevertheless, once the complex depositional gap in the sedimentary succession from the puzzle of tectonic units has been restored, the Ligurian Alps represent Late-Middle Triassic to the Early–Middle Jurassic that records one of the best places to observe a complete section of a Mesozoic a peculiar isostatic history, contrasting with that of the Europe- passive margin. Furthermore, the preserved geological record per- an passive margin (Dauphinois), with the Adriatic Margin tains to two distinct orogenic events: the late Variscan and the Alpine, (Austroalpine and Southalpine) and with oceanic Tethyan documenting a complete Wilson cycle. basin realms (Piedmont–Ligurian). The latter, in fact, preserve The European margin of the Alpine Tethys is now partly preserved a quasi-complete succession with less sedimentary gaps. The in the Western Alps as a nappe-pile accreted during the Eocene Briançonnais has classically been interpreted as an “anoma- convergence. The distribution of tectonic slices in the present-day lous” domain placed between the proximal and the distal belt reflects their original pre-collisional geographic pertinence; part of the European passive