Handy Etal 10 ESR.Pdf

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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Earth-Science Reviews 102 (2010) 121–158 Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Reconciling plate-tectonic reconstructions of Alpine Tethys with the geological–geophysical record of spreading and subduction in the Alps Mark R. Handy a,⁎, Stefan M. Schmid a,c,1, Romain Bousquet b,2, Eduard Kissling c,3, Daniel Bernoulli d,4 a Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100, D-12249 Berlin, Germany b Institut für Geowissenschaften, Universität Potsdam, Postfach 601553, D-14415 Potsdam, Germany c Institut für Geophysik, ETH-Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzerland d Geologisch-Paläontologisches Institut, Universität Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland article info abstract Article history: A new reconstruction of Alpine Tethys combines plate-kinematic modelling with a wealth of geological data Received 23 November 2008 and seismic tomography to shed light on its evolution, from sea-floor spreading through subduction to Accepted 1 June 2010 collision in the Alps. Unlike previous models, which relate the fate of Alpine Tethys solely to relative motions Available online 12 June 2010 of Africa, Iberia and Europe during opening of the Atlantic, our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record. The motions of Keywords: these microplates (Adria, Iberia, Alcapia, Alkapecia, and Tiszia) relative to both Africa and Europe during Late Tethys Alps Cretaceous to Cenozoic time involved the subduction of remnant Tethyan basins during the following three Mediterranean stages that are characterized by contrasting plate motions and driving forces: (1) 131–84 Ma intra-oceanic plate motion subduction of the Ligurian part of Alpine Tethys attached to Iberia coincided with Eo-alpine orogenesis in the subduction Alcapia microplate, north of Africa. These events were triggered primarily by foundering of the older (170– 131 Ma) Neotethyan subduction slab along the NE margin of the composite African–Adriatic plate; subduction was linked by a sinistral transform system to E–W opening of the Valais part of Alpine Tethys; (2) 84–35 Ma subduction of primarily the Piemont and Valais parts of Alpine Tethys which were then attached to the European plate beneath the overriding African and later Adriatic plates. NW translation of Adria with respect to Africa was accommodated primarily by slow widening of the Ionian Sea; (3) 35 Ma–Recent rollback subduction of the Ligurian part of Alpine Tethys coincided with Western Alpine orogenesis and involved the formation of the Gibraltar and Calabrian arcs. Rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable, retreating Adriatic and African slabs during slow convergence of Africa and Europe. The upper European–Iberian plate stretched to accommodate this slab retreat in a very mobile fashion, while the continental core of the Adriatic microplate acted as a rigid indenter within the Alpine collisional zone. The subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the Alps and Apennines, as well as beneath parts of the Pannonian Basin, the Adriatic Sea, the Ligurian Sea, and the Western Mediterranean. The predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10–30%, indicating that parts of slabs may be superposed within the mantle transition zone and/or that some of this subducted lithosphere became seismically transparent. © 2010 Elsevier B.V. All rights reserved. Contents 1. The controversial fate of Alpine Tethys ................................................. 122 2. Nomenclature of oceans and tectonic units in the Alps and adjacent mountain belts ............................ 124 ⁎ Corresponding author. Tel.: +49 30 838 70311; fax: +49 30 838 70734. E-mail addresses: [email protected] (M.R. Handy), [email protected] (S.M. Schmid), [email protected] (R. Bousquet), [email protected] (E. Kissling), [email protected] (D. Bernoulli). 1 Tel.: +49 30 838 70288; fax: +49 30 838 70734. 2 Tel.: +49 331 977 5809; fax: +49 331 977 5060. 3 Tel.: +41 1 633 2623; fax: +41 1 633 1065. 4 Tel.: +41 61 267 3639. 0012-8252/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2010.06.002 Author's personal copy 122 M.R. Handy et al. / Earth-Science Reviews 102 (2010) 121–158 3. Reconstructing the plate tectonics of Alpine Tethys – Approach and limitations . ........................... 128 3.1. Timing of pre-collisional events in the Alps ............................................ 128 3.2. Plate motion paths—boundary conditions and assumptions ..................................... 128 3.3. Methods used and their limitations in plate-motion reconstructions ................................ 131 4. Motion history of microplates in Alpine Tethys .............................................. 132 4.1. Motion of Africa leading to opening of the Piemont–Liguria Ocean and contemporaneous subduction/obduction of the Vardar Ocean . 132 4.2. Cretaceous microplate motions and transform-dominated tectonics ................................. 139 4.2.1. Opening of the Valais Ocean ............................................... 139 4.2.2. Eo-alpine Orogeny.................................................... 141 4.2.3. Partial subduction of the Ligurian Ocean ......................................... 144 4.2.4. Widening of the Ionian Sea ............................................... 145 4.3. Late Cretaceous to Early Cenozoic northward motions of Adria and Africa and the subduction of Alpine Tethys ............ 145 4.3.1. Subduction erosion at the NW tip of the Adriatic promontory ............................... 145 4.3.2. Rotation of Adria and accelerated subduction of Alpine Tethys ............................... 145 4.4. Adria–Europe collision and Ligurian rollback subduction following a change in subduction polarity .................. 147 4.4.1. Collision in the Alps ................................................... 147 4.4.2. Rollback subduction of the remaining Ligurian Ocean and backarc extension in the Western Mediterranean ......... 148 5. How much of Alpine Tethys can we see at depth? ............................................ 148 6. What governed the subduction of Alpine Tethys?............................................. 151 7. Conclusions .............................................................. 152 Acknowledgements ............................................................. 153 References ................................................................. 153 1. The controversial fate of Alpine Tethys (Dewey et al., 1973). However, they lacked crucial information on the size of Alpine Tethys and the precise age of its demise. The gap in Ever since Steinmann (1905) tried to relate the oceanic affinity of knowledge between plate-motion studies and field-based tectonic Alpine ophiolites to Alpine folding, a major challenge of Mediterranean syntheses was large, primarily because modern structural petrology geology has been to understand the fate of ocean basins preserved in the and geochronology were in their infancy and also because geologists circum-Mediterranean mountain belts—how they formed, their size were preoccupied with understanding collisional structures, which in and, finally, how they were consumed. The ophiolitic sutures that mark most areas overprint structures related to the spreading and these oceanic remains are imbricated with thrust sheets derived from subsequent subduction of oceanic lithosphere. the Early Mesozoic continental margins of the European and African Far from abating, controversy on the fate of Alpine Tethys has been plates, and of various microplates (Fig. 1). The lithospheric substratum fuelled by recent geological and geophysical research. On the one of these oceanic relics has been subducted and appears today as positive hand, simplistic plate-motion reconstructions suggest the creation of P-wave anomalies, many at the base of the upper mantle (Figs. 2 and 3; 650–1100 km of oceanic lithosphere within Alpine Tethys in an E–W e.g., Spakman et al., 1993; Wortel and Spakman, 2000; Piromallo and direction, i.e., perpendicular to the former spreading axis, and some Morelli, 2003). Relating these anomalies at depth to the history of 1500 km in a N–S direction (models reviewed in Capitanio and Goes, spreading, subduction and collision at the surface is crucial to 2006, their Fig. 3a). These amounts deduced from the motion of Africa understanding how mantle flow is coupled to the motion of tectonic with respect to

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