Research Paper THEMED ISSUE: Subduction Top to Bottom 2 GEOSPHERE Deformation along the roof of a fossil subduction interface in the transition zone below seismogenic coupling: The Austroalpine GEOSPHERE; v. 16, no. 2 case and new insights from the Malenco Massif (Central Alps) https://doi.org/10.1130/GES02149.1 Paraskevi Io Ioannidi1, Samuel Angiboust1,2, Onno Oncken1, Philippe Agard3, Johannes Glodny1, and Masafumi Sudo4 11 figures; 5 tables; 1 set of supplemental files 1GFZ German Research Centre for Geosciences, D-14473 Potsdam, Germany 2Université de Paris, Institut de Physique du Globe de Paris, Centre National de la Recherche Scientifique (CNRS), F-75005 Paris, France 3 CORRESPONDENCE: [email protected] Institut des Sciences de la Terre de Paris (ISTeP)–Unité Mixte de Recherche (UMR) 7193, Sorbonne Université, F-75005 Paris, France 4Institut für Geowissenschaften, Universität Potsdam, 14476 Potsdam, Germany CITATION: Ioannidi, P.I., Angiboust, S., Oncken, O., Agard, P., Glodny, J., and Sudo, M., 2020, Deforma- tion along the roof of a fossil subduction interface in ABSTRACT the frame of previous studies on other segments documented thanks to a wealth of well-exposed the transition zone below seismogenic coupling: The of the same Alpine paleosubduction interface, key localities (Kitamura et al., 2005; Vannucchi Austroalpine case and new insights from the Malenco Massif (Central Alps): Geosphere, v. 16, no. 2, p. 510– A network of fossil subduction plate inter- and we propose that this system of shear zones et al., 2008; Bachmann et al., 2009b; Rowe et al., 532, https://doi.org/10.1130/GES02149.1. faces preserved in the Central Alps (Val Malenco, represents deformation conditions along the sub- 2013), our understanding of deeper deformation N Italy) is herein used as a proxy to study defor- duction interface(s) in the transition zone below processes (20–40 km depth) is hampered by the Science Editor: Shanaka de Silva mation processes related to subduction and the seismogenic zone during active subduction. scarcity of direct observations on localities devoid Guest Associate Editor: Gray E. Bebout subsequent underplating of continental slices (in of exhumation-related tectonic imprint. Even rarer particular the Margna and Sella nappes) at depths is the natural record of deformation along the Received 8 April 2019 Revision received 20 September 2019 reported to in the former brittle-ductile transition. ■ INTRODUCTION hanging wall of a subduction interface. Accepted 23 December 2019 Field observations, microfabrics, and mapping The European Central Alps represent a remark- revealed a network of shear zones comprising Subduction zones commonly evidence strong able natural laboratory giving the opportunity to Published online 13 February 2020 mostly mylonites and schists but also rare foliated interplate coupling and large magnitude earth- (nearly) continuously document hanging-wall cataclasites. These shear zones are either located quakes in the seismogenic zone (e.g., Hyndman processes from shallow levels exposed in Arosa at the contacts of the two nappes or within the et al., 1997; Conrad et al., 2004; Heuret and and Engadine regions (Bachmann et al., 2009a, boundaries of the Sella unit. Microprobe results Lallemand, 2005). Investigating deformation pro- 2009b) down to deeper segments exposed in point to two different white mica generations, with cesses taking place along and in the vicinity of the Dent Blanche and Sesia complexes (Trümpy, higher-pressure (Si-rich) phengites rimming low- their interface can shed light on properties such 1975; Konrad-Schmolke et al., 2011; Angiboust et er-pressure (Si-poor) phengites. Garnet is locally as the distribution of seismicity or the effective al., 2015; Jaeckel et al., 2018; Fig. 1). The subduc- observed overgrowing resorbed pre-Alpine cores. rheology along the interface (e.g., Stöckhert, 2002; tion and accretion of continental slivers from the Pressure-temperature estimates based on pseudo- Herrendörfer et al., 2015). Exhumed suture zones stretched Apulian margin over several millions of section modeling point to peak burial deformation are important targets since they enable a direct years against the Apulian buttress gave rise to the conditions of ~0.9 GPa and 350–400 °C, at ~30 km insight on ancient subduction settings and provide Austroalpine domain (Compagnoni et al., 1977; depth. Rb/Sr geochronology on marbles deformed opportunities to access the long-term record of Dal Piaz et al., 2003). This composite nappe stack during the Alpine event yields an age of 48.9 ± 0.9 subduction zone deformation (e.g., Ernst and Dal comprises individual slices separated by localized Ma, whereas due to incomplete recrystallization, Piaz, 1978; Platt, 1986; Stöckhert, 2002; Agard et shear zones interpreted as transient slip interfaces a wide range of both Rb/Sr and 40Ar/39Ar apparent al., 2018). Metamorphic rocks from ancient sutures and where local brittle and semi-brittle deforma- ages is obtained from deformed orthogneisses and may yield information on how rock fabrics evolve tion patterns have been reported (Polino et al., micaschists embracing 87–44 Ma. with depth from shallow brittle seismogenic faults 1990; Babist et al., 2006; Angiboust et al., 2014, Based on our pressure-temperature, structural (Sibson, 2013; Yamaguchi et al., 2014; Saffer and 2015; Locatelli et al., 2018; Menant et al., 2018). and geochronological observations, the studied Wallace, 2015) to deeper tremorgenic regions and While the Arosa-Engadine as well as Dent Blanche shear zones last equilibrated at depths downdip beyond (Angiboust et al., 2015; Obara and Kato, regions have been extensively investigated in This paper is published under the terms of the of the seismogenic zone in an active subduction 2016; Webber et al., 2018). While the shallow seg- the frame of the subduction interface model, the CC-BY-NC license. zone setting. We integrate these new results in ments of the plate interface have been extensively southern end of the Austroalpine complex in the © 2020 The Authors GEOSPHERE | Volume 16 | Number 2 Ioannidi et al. | The Austroalpine case and new insights from the Malenco Massif Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/2/510/4968578/510.pdf 510 by GeoForschungsZentrums Potsdam user on 29 April 2020 Research Paper A E 08 10 12 N Zurich 47 Bern Tauern Window Arosa Engadine St Moritz Geneva Sesia N 46 zone Dent Malenco Blanche Late Jurassic N Forno Platta MalencoMargna Err Sella Bernina Tertiary dextral shearing Legend Zermatt Combin European units Southern Alpine units Ivrea Canavese Sesia Penninic units Cenozoic intrusions Dent Blanche 100 km 50 km Austroalpine units Gosau group Lanzo AUSTROALPINE 3000 3100 B E 09°55'0'' E 09°57'0'' 2800 N 46°21'0'' 3000 2 Α 500 #18D 2900 N 2 2400 700 Punta Marinelli 3181 600 #23B 2 Cima Val Fontana 3068 #8* 2400 Cime di 2300 Musella 3088 2500 N 46°20'0'' 3000 Bocchetta di Caspoggio #01* 2700 3136 2600 2 7 #11Β 00 2400 2 #02 600 Α´ 2300 #13 2600 00 Monte delle 2500 22 Forbici 00 2910 2500 2 7 7 3000 2 2500 2 00 2900 2400 2 600 2800 1 km 2400 500 2800 2300 Lago di Gera Quaternary deposits 22 Sasso Moro 00 Bernina Sella Margna and glacial covers 3108 Magmatic Late-to-Post-Variscan rocks Permo-Mesozoic Andesitic-basaltic Meta-rhyolites Andesitic-basaltic Meta-rhyolites Fedoz gabbro metasediments dikes dikes (Mg-flasergabbro) Ultramafic Granodiorites Calcitic marbles Leucogranites Granites (gneiss) lenses (gneiss) Dolomitic marbles Crystalline pre-Permian basement rocks Malenco ultramafics (Grano)diorites, syenites, granites, Phyllites and micaschists Paragneisses Serpentinite (antigorite) paragneisses and micaschists and micaschists Megabreccia “Breccia Calcitic marbles d‘ Ur“ intercalated with Ca-silicate fels Figure 1. (A) Geological map of the Western and Central Alps showing the location of the Austroalpine nappes with respect to surrounding tec- tonic units. The studied area is located in the black box in the Val Malenco region. Inset: Paleogeographic reconstruction along the NW margin of the Apulian plate. Dent Blanche, Sesia, Margna, and Sella nappes are interpreted as extensional allochthons derived from the Apulian margin (modified from Froitzheim and Manatschal, 1996). (B) Geological map of the Val Malenco, modified from Montrasio et al. (2005). Shades of ma- genta colors represent units belonging to Sella nappe, beige to Margna nappe; also shown are the Malenco ultramafics (greenish) and the Bernina nappe (light brown). Black circles denote the sampling localities (e.g., #01* includes samples #01A, #01B, and #01F). For exact location, see also Table 1. Line A–Aʹ shows the position of the cross section in Figure 2B. GEOSPHERE | Volume 16 | Number 2 Ioannidi et al. | The Austroalpine case and new insights from the Malenco Massif Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/2/510/4968578/510.pdf 511 by GeoForschungsZentrums Potsdam user on 29 April 2020 Research Paper Central Alps (Malenco region) remains unexplored Austroalpine Units and Their Permo- locally contain dolomite breccias (Hermann and despite its potential importance as the missing Mesozoic Sedimentary Covers Müntener, 1992; Trommsdorff et al., 2005). Their element linking the depths of the Arosa-Enga- formation is connected to the rifting phase of the dine (10–20 km) and Dent Blanche–Sesia massifs The continental Austroalpine units are repre- Austroalpine units during Mesozoic times. Both (40–60 km). We herein report field, pressure-tem- sented in the study area by the Margna and the sedimentary units exhibit Alpine metamorphism. perature (P-T), and geochronological data from Sella nappes. Stratigraphic similarities between The Dent Blanche and Sesia units are consid- the aforementioned part of the Central Alps in the Margna cover and sediments from the Lower ered equivalent to the Margna and Sella nappes in order to study the processes taking place during Austroalpine Err nappe (Liniger and Guntli, 1988) the Central Alps (Froitzheim and Manatschal, 1996; subduction and underplating of the Margna and suggest an Apulian affinity for the Margna nappe Froitzheim et al., 1996; Schmid et al., 2004).