Reconstruction of the tectonic evolution of the western

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					Rosenbaum, G., Lister, G. S. and Duboz, C. 2002. Reconstruction of the tectonic evolution of the western Mediterranean since the Oligocene. In:
Rosenbaum, G. and Lister, G. S. 2002. Reconstruction of the evolution of the Alpine-Himalayan Orogen. Journal of the Virtual Explorer, 8, 107 - 126.

                   Reconstruction of the tectonic evolution of the
                    western Mediterranean since the Oligocene
                                    G. ROSENBAUM, G. S. LISTER AND C. DUBOZ

                   School of Geosciences Australian Crustal Research Centre, Monash University
                                      Melbourne, Victoria 3800, Australia

             Abstract: We present a tectonic synthesis and an animation of the tectonics of the western Mediterranean since
             the Oligocene. This work is based on data derived from different geological datasets, such as structural geology,
             the distribution of metamorphic rocks, magmatic activity, sedimentary patterns, palaeomagnetic data and
             geophysics. Reconstruction was performed using an interactive software package (PLATYPLUS), which enabled
             us to apply rotational motions to numerous microplates and continental terranes involved in the evolution of the
             western Mediterranean basins. Boundary conditions are provided by the relative motions of Africa and Iberia
             with respect to Europe, and the Adriatic plate is considered here as an African promontory.
             The reconstruction shows that during Alpine orogenesis, a very wide zone in the interface between Africa and
             Europe underwent extension. Extensional tectonics was governed by rollback of subduction zones triggered by
             gravitational instability of old and dense oceanic lithosphere. Back-arc extension occurred in the overriding
             plates as a result of slow convergence rates combined with rapid subduction rollback. This mechanism can
             account for the evolution of the majority of the post-Oligocene extensional systems in the western
             Mediterranean. Moreover, extension led to drifting and rotations of continental terranes towards the retreating
             slabs in excess of 100-800 km. These terranes - Corsica, Sardinia, the Balearic Islands, the Kabylies blocks,
             Calabria and the Rif-Betic - drifted as long as subduction rollback took place, and were eventually accreted to
             the adjacent continents. We conclude that large-scale horizontal motions associated with subduction rollback,
             back-arc extension and accretion of allochthonous terranes played a fundamental role during Alpine orogenesis.

  Introduction                                                             drifted great distances to their present locations (e.g.,
                                                                           Calabria). However, some issues are yet to be resolved and
   The geological evolution of the western Mediterranean                   have been the subject of considerable debate. Different
 exhibits complicated interactions between orogenic                        models have been proposed to explain the evolution of the
 processes and widespread extensional tectonics. The                       Alboran Sea, namely, as a back-arc basin associated with
 region is located in a convergent plate margin separating                 a retreating slab (Lonergan & White, 1997), or as the
 Africa and Europe, and consists of marine basins - the                    result of an extensional collapse of thickened lithosphere
 Alboran Sea, the Algerian-Provençal Basin, the Valencia                   (Platt & Vissers, 1989; Houseman, 1996). The evolution
 Trough, the Ligurian Sea and the Tyrrhenian Sea (Figure                   of the Tyrrhenian Sea is also controversial, with some
 1) - which formed as back-arc basins since the Oligocene.                 fundamental problems in the current explanations of the
 The evolution of these basins, simultaneously with                        evolution of this basin.
 ongoing convergence of Africa with respect to Europe, has                    In this work, we aim to develop a coherent visual
 been the subject of numerous studies (e.g., Stanley &                     reconstruction that will best explain the large-scale
 Wezel 1985, Durand et al. 1999). Widespread extension                     tectonics of the western Mediterranean region. We use a
 associated with the formation of these basins led to                      wealth of accumulated knowledge as published in the
 considerable thinning of the continental crust (i.e., in the              literature, as well as a new software package that provides
 Alboran Sea and the northern Tyrrhenian) or to the local                  the ability to perform an interactive reconstruction. The
 initiation of sea floor spreading (i.e., in the southern                  reconstruction is presented as an animation, which clearly
 Tyrrhenian and Provençal Basin). Furthermore,                             demonstrates some fundamental features seen in
 extensional tectonism in the western Mediterranean was                    convergent plate margins. It shows the complex
 coeval with orogenesis in the adjacent mountain chains of                 interactions between subduction processes, horizontal
 the Rif-Betic cordillera, the Maghrebides of northern                     extension, block rotations and accretion events, and it
 Africa and Sicily, the Apennines, the Alps and the                        emphasises the roles of subduction rollback and the
 Dinarides (Malinverno & Ryan, 1986; Crespo-Blanc et al.,                  episodic accretion of allochthonous terranes during
 1994; Tricart et al., 1994; Cello et al., 1996; Azañón et al.,            orogenesis.
 1997; Frizon de Lamotte et al., 2000; Faccenna et al.,
 2001)(Figure 1).                                                          Summary of previous works
   The simultaneous formation of extensional basins
 together with thrusting and folding in adjacent mountain                    Over the course of the last century, and particularly
 belts has led to several tectonic models that acknowledge                 since the emergence of the modern plate tectonics theory,
 the role of large-scale horizontal motions associated with                numerous studies have aimed to reconstruct the evolution
 the retreat of the subduction trench (hereafter termed                    of the Mediterranean basins in the context of the Alpine
 subduction rollback) (Malinverno & Ryan, 1986; Royden,                    orogeny. The main concepts used in these reconstructions
 1993a; Lonergan & White, 1997). These provide an                          (as well as in the present study) correspond to continental
 explanation for the origin of allochthonous terranes, which               drift, microplate rotations and migration of subduction


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