In search of transient subduction interfaces in the Dent Blanche–Sesia Tectonic System (W. Alps)

Highlights

• The Dent Blanche Complex: a stack of blueschist metamorphic continental slices.

• Younger ages towards lowermost structural levels indicate underplating processes.

• Insights onto 30–40 km depths subduction dynamics at early Cenozoic times.

Abstract

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 continental, 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 reconstructed 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 experienced variable maximum burial temperatures between 360 °C and 490 °C at depths of ca. 25–40 km. Associated 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 between 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 lowermost 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 between 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 provide a new perspective for the understanding of geophysical results imaging the plate-interface region in active subduction zones.

Introduction

The plate interface zone at seismogenic depth and deeper has recently 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 techniques 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 therefore remains largely unknown (Hilairet and Reynard, 2009). In particular, 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 and references therein). Field-based studies also provide important constraints on the processes operating on exhumed subduction interfaces at relatively shallow depths (from the surface to ~ 25 km depth; e.g. Bachmann et al., 2009a, Bachmann et al., 2009b, Glodny et al., 2005, Meneghini et al., 2010, Ring et al., 2001). Some studies in particular characterize the mechanical interactions between the two plates and demonstrate a strong link between fluid circulation, tremor generation and seismic coupling (e.g. Fagereng and Sibson, 2010). However, the detailed tectonic architecture of deeper portions of exhumed plate interfaces (25–40 km) remains poorly resolved because of their rarity on the Earth surface and because of massive reworking during exhumation. This depth range of the interface has been identified by Ruff and Tichelaar (1996) as being associated to the downdip end of seismic coupling where the transition zone between creep and episodic slip and tremor features occurs (Schwartz and Rokosky, 2007).

The Dent Blanche area in the Western Alps potentially constitutes an example of an exhumed ‘deep’ subduction system. In this region, continental material from the upper plate (Austro-Alpine affinity) is thrust over a metasedimentary complex accreted within the Tethyan subduction zone (e.g. Trümpy, 1975; Fig. 1a and b). Both units experienced blueschist-facies conditions during Alpine orogeny (25–40 km depth; Oberhänsli et al., 2004) and therefore could potentially constitute one of the best natural analogues to understand the present-day processes imaged in geophysical subduction-zone surveys near the base of the upper plate crust.

However, controversies and uncertainties exist on tectonic paths and juxtaposition conditions in this part of the Alpine orogen (e.g. Ballèvre and Kienast, 1987, Ballèvre and Merle, 1993, Platt, 1986, Pleuger et al., 2007, Reddy et al., 1999, Ring, 1995, Wust and Silverberg, 1989). Some authors recently pointed out the need of a better geochronological characterization of the Dent Blanche massif history (Beltrando et al., 2010), for which the P–T–t evolution is far less well constrained than for its lateral equivalent, the Sesia Zone (Fig. 1a). We herein investigate the structure of the lower part of the Dent Blanche Tectonic System (DBTS) and the underlying metasediments using Rb/Sr geochronology and state-of-the-art thermobarometric tools in order to derive individual P–T–t metamorphic paths and the tectonic juxtaposition history. These results, which permit reconstructing the Alpine deep evolution in Early Cenozoic times, are of critical importance for (i) improving our vision of the evolution of the plate interface structure through time and (ii) a better understanding of subduction interface dynamics between 20 and 40 km depths.