Evidence for a serpentinized plate interface favouring continental subduction

Abstract

The dynamics of continental subduction is largely controlled by the rheological properties of rocks involved along the subduction channel. Serpentinites have low viscosity at geological strain rates. However, compelling geophysical evidence of a serpentinite channel during continental subduction is still lacking. Here we show that anomalously low shear-wave seismic velocities are found beneath the Western Alps, along the plate interface between the European slab and the overlying Adriatic mantle. We propose that these seismic velocities indicate the stacked remnants of a weak fossilised serpentinite channel, which includes both slivers of abyssal serpentinite formed at the ocean floor and mantle-wedge serpentinite formed by fluid release from the subducting slab. Our results suggest that this serpentinized plate interface may have favoured the subduction of continental crust into the upper mantle and the formation/exhumation of ultra-high pressure metamorphic rocks, providing new constraints to develop the conceptual and quantitative understanding of continental-subduction dynamics.

Fig. 1. Geologic setting and depth slices of Vs.

a Tectonic sketch map of the Adria–Europe plate boundary zone. b Geological sketch map of the western Alpine region (modified from Malusà et al.). The magenta line indicates the location of the cross section of Fig. 2a. Numbers 1–6 indicate the sites of the depth-velocity profiles illustrated in Fig. 2b. The purple line indicates the gravity anomaly of the Ivrea body (0 mGal isoline). Acronyms: Br, Briançonnais; DM, Dora-Maira; FPF, Frontal Pennine Fault; GP, Gran Paradiso; IF, Insubric Fault; La, Lanzo; MR, Monte Rosa; Se, Sesia-Lanzo; SL, Schistes lustrés; Vi, Viso. c, d Vs structure at different depths (40 km and 60 km) after TransD inversion. The green star marks the deep–low-velocity body (yellow to wheat area).

Fig. 2. Shear-wave velocity model across the Western Alps.

a Cross-section showing the absolute Vs in the 0–90-km depth range (see location in Fig. 1b). The white lines indicate the 3.7 and 3.8 km s⁻¹ contours. The green star marks the deep–low-velocity body. Earthquake foci (depth > 30 km) in a 40-km-wide swath parallel to the profile are plotted as purple circles. Letters a–c indicate regions of the model discussed in the text. The main crustal features (black lines) after Zhao et al. and Malusà et al. (acronyms as in Fig. 1b). b Depth–velocity profiles (±1σ) beneath six typical sites along the transect (see location in Fig. 1b and panel a). The orange vertical line is the 3.8 km s⁻¹ isovelocity.

Fig. 3. Measured Vs (km s⁻¹) for different rocks (green = ultramafic; blue = mafic; purple = granitic; brown = pelitic) at ambient conditions.

The size of the circles is proportional to Vs, while circle location depends on the pressure–temperature range at equilibrium for each rock type^,–,. The Vs range of 4.0–4.3 km s⁻¹ in quartzofeldspathic gneisses is based on calculations for modeled rocks at 700 °C and 2 GPa. The lower bound for HT serpentinite (3.4 km s⁻¹) is calculated from single-crystal elastic data and referred to Reuss approximation and 2 GPa. Lizardite (Lz) and antigorite (Atg) stability fields^,. The dashed gray lines indicate the metamorphic facies boundaries (Am amphibolite, Bs blueschist, Ec eclogite, Gr granulite, Gs greenschist). Note the progressive increase in Vs with depth and metamorphic grade in pelitic, granitic, and mafic rocks, and the sharp change in Vs across the Atg-out curve in ultramafic rocks. The green star marks the rock types consistent with Vs and depth of the analyzed low-velocity body.

Fig. 4. Environments of serpentinization during subduction and geologic interpretation of the deep–low-velocity body.

a Oceanic subduction. b Continental subduction. c Upper-plate divergent motion and exhumation of UHP rocks (crustal features color coded as in Fig. 1b). Lithospheric structure after Zhao et al.^,, Lyu et al., and Solarino et al.. Environments of serpentinization and fluid circulation after Deschamps et al.. Evolution of lizardite (Lz) and antigorite (Atg) destabilization isotherms after Liao et al.. Percentage of serpentinite in the exhumed mantle wedge after Solarino et al.. The deep–low-velocity body imaged in the subduction channel consists of abyssal and mantle-wedge serpentinites that lubricate the plate interface and facilitate continental subduction.