In search of transient subduction interfaces in the Dent Blanche–Sesia Tectonic System (W. Alps) Samuel Angiboust a, ⁎, Johannes Glodny a , Onno Oncken a , Christian Chopin b a GFZ German Research Center for Geosciences, D-14473 Potsdam, Germany b Laboratoire de Géologie, Ecole Normale Supérieure–CNRS, 24 rue Lhomond F-75005, Paris, France abstract article info Article history: Received 26 March 2014 Accepted 3 July 2014 Available online 11 July 2014 Keywords: Alps Subduction Thermobarometry Geochronology Dent Blanche Blueschists In this paper we study the Alpine metamorphic history of a major tectonic zone which formed during Alpine orogeny, the Dent Blanche Thrust (DBT). This contact, located in the Northern Western Alps, juxtaposes some ophiolitic metasediment-rich remnants of the Liguro-Piemontese ocean (Tsaté Complex) with a composite con- tinental, km-sized complex (Dent Blanche Tectonic System, DBTS) of Adriatic affinity thrusted over the ophiolite. In order to better understand the geodynamic meaning of the DBT region and adjacent units, we have recon- structed the pressure–temperature–time–deformation (P–T–t–d) history of these two units using modern thermobarometric tools, Rb/Sr geochronology, and field relationships. We show that the Tsaté Complex is formed by a stack of km-thick calcschists-bearing tectonic slices having ex- perienced variable maximum burial temperatures between 360 °C and 490 °C at depths of ca. 25–40 km. Asso- ciated deformation ages span a range between 37 Ma and 41 Ma. The Arolla gneissic mylonites at the base of the DBTS experienced high-pressure (12–14 kbar), top-to-NW deformation at ca. 450 °C between 43 and 48 Ma. A first age of ca. 58 Ma has been obtained for high-pressure ductile deformation in the Valpelline shear zone, atop Arolla gneisses. Some of the primary, peak metamorphic fabrics have been reworked and later backfolded during exhumation and collisional overprint (ca. 20 km depth, 37–40 Ma) leading to the regional greenschist-facies retrogression which is particularly prominent within Tsaté metasediments. We interpret the Dent Blanche Thrust, at the base of the Arolla unit, as a fossilized subduction interface active be- tween 43 and 48 Ma. Our geochronological results on the shear zone lining the top of the Arolla unit, together with previous P–T–t estimates on equivalent blueschist-facies shear zones cutting the Sesia unit, indicate an older tectonic activity between 58 and 65 Ma. We demonstrate here that observed younger ages towards lower- most structural levels are witness of the transient, downwards migration of the Alpine early Cenozoic blueschist- facies subduction interface. This down-stepping is interpreted to reflect the progressive underplating acting be- tween 30 and 40 km depth in the Alpine subduction zone between late Cretaceous and late Eocene. Underplating involved first continental material derived from the stretched Adriatic margin followed by underplating of ocean- derived rocks in the Eocene. These results shed light on subduction-zone accretion processes and therefore pro- vide a new perspective for the understanding of geophysical results imaging the plate-interface region in active subduction zones. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The plate interface zone at seismogenic depth and deeper has re- cently received increased attention due to detection of either transient slip processes or megathrust earthquakes (e.g. Sumatra, 2004; Chile, 2010; Japan, 2011). Recent developments in geophysical imaging tech- niques improved our vision on the location of this part of the subduction interface where a 5 ± 3 km wide “subduction channel” is believed to occur (e.g. Abers et al., 2006). This channel is possibly filled by accreted sediments in its upper portion (0–35 km) and possibly becomes serpentinite-rich deeper along the plate interface (35–90 km), both of which would essentially control the mechanical behavior (e.g. Guillot et al., 2009; Shreve and Cloos, 1986). However, limitations in resolution at Moho depths (typically 30–40 km) and deeper prevent resolving by geophysical means the internal structure of this interface which there- fore remains largely unknown (Hilairet and Reynard, 2009). In particu- lar, the degree of tectonic mixing occurring at the plate interface is still largely debated (e.g. Angiboust et al., 2012; Gerya et al., 2002; Rubatto et al., 2011). Mapping exhumed suture zones and deciphering rock-forming pressure–temperature–time (P–T–t) conditions enable unravelling burial histories of formerly subducted terranes (e.g. Agard et al., 2009 Lithos 205 (2014) 298–321 ⁎ Corresponding author at: GFZ, Lithosphere Dynamics Department, Telegrafenberg, D-14473 Potsdam, Germany. Tel.: +49 331 288 1363; fax: +49 331 288 1370. E-mail address: samuel@gfz-potsdam.de (S. Angiboust). http://dx.doi.org/10.1016/j.lithos.2014.07.001 0024-4937/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos