Swiss J Geosci DOI 10.1007/s00015-016-0237-0 Ivrea mantle wedge, arc of the Western Alps, and kinematic evolution of the Alps–Apennines orogenic system 1 1 1 2 Stefan M. Schmid • Eduard Kissling • Tobias Diehl • Douwe J. J. van Hinsbergen • Giancarlo Molli3 Received: 6 June 2016 / Accepted: 9 December 2016 Ó Swiss Geological Society 2017 Abstract The construction of five crustal-scale profiles related to the lateral indentation of the Ivrea mantle slice across the Western Alps and the Ivrea mantle wedge towards WNW by some 100–150 km. (4) The final stage of integrates up-to-date geological and geophysical informa- arc formation (25–0 Ma) is associated with orogeny in the tion and reveals important along strike changes in the Apennines leading to oroclinal bending in the southern- overall structure of the crust of the Western Alpine arc. most Western Alps in connection with the 50° counter- Tectonic analysis of the profiles, together with a review of clockwise rotation of the Corsica-Sardinia block and the the existing literature allows for proposing the following Ligurian Alps. Analysis of existing literature data on the multistage evolution of the arc of the Western Alps: (1) Alps–Apennines transition zone reveals that substantial exhumation of the mantle beneath the Ivrea Zone to shal- parts of the Northern Apennines formerly suffered Alpine- low crustal depths during Mesozoic is a prerequisite for the type shortening associated with an E-dipping Alpine sub- formation of a strong Ivrea mantle wedge whose strength duction zone and were backthrusted to the NE during exceeds that of surrounding mostly quartz-bearing units, Apenninic orogeny that commences in the Oligocene. and consequently allows for indentation of the Ivrea mantle wedge and eastward back-thrusting of the western Alps Keywords Western Alps Á Apennines Á Ivrea mantle Á during Alpine orogeny. (2) A first early stage (pre-35 Ma) Teleseismic tomography Á Arcuate mountain belts of the West-Alpine orogenic evolution is characterized by top-NNW thrusting in sinistral transpression causing at least some 260 km displacement of internal Western Alps 1 Introduction and E-W-striking Alps farther east, together with the Adria micro-plate, towards N to NNW with respect to The Western Alps represent a prime example of a very stable Europe. (3) The second stage (35–25 Ma), further arcuate mountain belt curving around from an E-W strike accentuating the arc, is associated with top-WNW thrusting with dominant top-N nappe stacking in the Eastern Alps to a in the external zones of the central portion of the arc and is N-S strike in the northern Western Alps and finally into an E-W strike with top-S nappe stacking in the Ligurian Alps straddling the Mediterranean coast (Fig. 1). The question Editorial handling: C. Sue and A. G. Milnes. after the mechanisms of arc formation is an old and much debated issue in many orogens (Marshak 1988) involving, in & Stefan M. Schmid the case of the Alps, elements of oroclinal bending of an [email protected] initially straight orogen (Carey 1955; van der Voo 2004), 1 Institute of Geophysics, ETH Zu¨rich, Sonneggstrasse 5, induced by rigid body rotation of the hinterland (Vialon 8092 Zurich, Switzerland et al. 1989) and/or by W-directed indentation of the Adria 2 Department of Earth Sciences, Utrecht University, microplate (Laubscher 1971; Schmid and Kissling 2000). Heidelberglaan 2, 3584 CS Utrecht, The Netherlands Transpressional movements associated with large top NNW 3 Dipartimento di Scienze della Terra, Universita` di Pisa, Pisa, displacements of portions of the inner zone of the Western Italy Alps that occur at a small angle to the present-day N-S strike S. M. Schmid et al. Cenozoic basins Tectonic map of the Western Alps Cenozoic in general 6° 7° 8° 9° 10° 11° Cenozoic of Piedmontese basin Basel Lower Plate Units of the Alps European continental margin Zürich St. Gallen Dijon Cenozoic flysch Besançon (Dauphinois and Helvetic Nappes) 4 NFP 20 West 5 T Innsbruck European Mesozoic undeformed 47° icino European Mesozoic deformed 47° Bern Allochthonous Mesozoic & Cenozoic (Helvetic Nappes) Chur European basement (External Massifs) European upper crust of the Subpenninic Units High-pressure Subpenninic Units 3 Ec Bolzano Valaisan partly oceanic domain ors CROP Sion RSL Valaisan Genève PL 46° Locarno Briançonnais micro-continent GB 46° Sub-Briançonnais PF Briançonnais Aosta High-pressure Briançonnais Pre-Piemontais Acceglio Moûtiers Ivrea Sesia-Dent Blanche Extensional Allochthons Milano Verona Sesia, Dent Blanche, Margna, Sella Units Grenoble Piedmont-Liguria Ocean 45° 2 Pelvoux Torino TH Lower Plate Piedmont-Liguria MH 45° VVL Upper Plate Units of the Alps Briançon AFSV Upper Plate Piedmont-Liguria and distal continental margin of Adria Adria Permo-Mesozoic Gap PF Bologna Adria upper crust Cuneo Argentera Genova Adria lower crust ACL LA Austroalpine Permo-Mesozoic 44° Digne Austroalpine basement 44° 020100 km Cenozoic plutons 1 Argentera Apennines Nice Apenninic Units in general 6° 7° 8° 9° 10° 11° Fig. 1 Tectonic map of the arc of the Western Alps after Schmid Front, ACL Argentera-Cuneo Line, AFSV front of the metamorphic et al. (2004) and Bousquet et al. (2012b) with indication of the traces core of the Alps and Sestri-Voltaggio line, VVL Villalvernia-Varzi of the profiles depicted in Fig. 2. The selected faults indicated in red line. Abbreviations in black are: LA Ligurian Alps, TH Torino hills, are: RSL Rhone-Simplon Line, PL Periadriatic Line, PF Penninic MH Monferrato hills, GB Giudicarie transpressive belt of the Western Alps were also invoked for being responsible substantial recent improvements in revealing the deep for explaining the curvature from the Eastern Alps into the structure of the arc of the Western Alps. The new geo- N-S striking Western Alps (Ricou and Siddans 1986). There physical data set of unprecedented quality calls for inte- have also been attempts to explain diverging kinematic gration with a wealth of geological data collected over many indicators along the arc of the Western Alps ranging from decades to elucidate the formation of the arc of the Western top-N Western Switzerland to top-SW in southern France Alps that also demands to analyze the Alps–Apennines without taking resort to a multi-stage evolution allowing for transition area located in the Ligurian Alps and the Northern strike-slip movements and/or large rotations (Platt et al. Apennines (Molli et al. 2010). 1989; their group 3 kinematic indicators formed in the This contribution integrates the new results of crustal 40–15 Ma time interval). However, as pointed out by Butler tomography and improved knowledge on Moho depth (1986; his Figure 2), strongly divergent thrust transport (Spada et al. 2013) with a geological interpretation of five directions over large distances inevitably result in excessive crustal-scale transects across the Western Alps. Thereby orogen-parallel stretching. we focus on the role of the Ivrea mantle wedge. Another In the Western Alps many previous workers (e.g. Laub- important aim of our geological-geophysical interpreta- scher 1971; Butler 1983) recognized the crucial role of the tions is to put new constraints on the question as to how Ivrea mantle wedge residing at relatively shallow crustal much continental crust was subducted in the Alps and what levels and locally reaching the surface. Novel geophysical the variations in pre-orogenic crustal thickness might have methods such as teleseismic tomography aiming at the been (see discussions by, e.g., Me´nard and Molnar 1991; structure of the earth’s mantle (Lippitsch et al. 2003)and Butler 2013; Mohn et al. 2014). crustal tomography based on local earthquake data (Diehl Two transects are revised versions of previously pub- et al. 2009), together with older information based on con- lished profiles (Schmid et al. 2004) while three of them trolled source seismology (Blundell et al. 1992), led to were newly constructed. Special emphasis is given to Ivrea mantle wedge, arc of the Western Alps, and kinematic evolution of the Alps–Apennines… improved mapping of the outlines of the Ivrea mantle When comparing the velocity contours with the geological wedge based on geophysical data. This is then followed by interpretation it has to be kept in mind, however, that these an analysis of the complex multi-stage evolution of the arc contours have a low spatial resolution since they are based of the Western Alps and an attempt to map the associated on assigning a given velocity to a sizable cell volume of 20 displacement history of geological units of the Western 9 20 km (horizontal) and 15 km (vertical). Secondly, we Alps–Northern Apennines system using plate reconstruc- complemented these tomographic data by geophysical data tion methodology (GPlates free software of Boyden et al. better constraining Moho-depth. Where available we took 2011) embedded in the global plate kinematic reconstruc- Moho depths from Spada et al. (2013; their Figure 11; see tion of Seton et al. 2012. Finally, we provide a review-type also Wagner et al. 2012 regarding the methodology) who discussion of the Alps–Apennines transition area and dis- combined results of controlled-source seismology with cuss controversial issues concerning questions of multi- local receiver functions Moho depth maps and local stage vs. single subduction systems in the Apennines. earthquake tomography-derived Moho maps. Comparison between Moho depth compiled by Spada et al. (2013), characterized by a P-wave velocity jump, and 2 Crustal-scale profiles across the Western Alps the velocity model of Diehl et al. (2009) that defines Moho and the Ivrea mantle wedge integrating depth in terms a high velocity gradient in the range of geological and geophysical information 6.5–8.0 km s-1, noticeable over large parts of the profiles, shows that on average the Moho as defined by Spada et al.