In search of transient subduction interfaces in the Dent Blanche–Sesia Tectonic System (W. Alps)
Graphical abstract
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.
Section snippets
Overview of the regional architecture
The studied area belongs to the Western Alps ophiolitic belt and was assembled during the closure of the Liguro-Piemontese slow-spreading ocean (and associated rifted continental margins) between ca. 100 Ma and ca. 40 Ma (e.g. Agard et al., 2001, Bousquet et al., 2008, Lagabrielle and Lemoine, 1997, Oberhänsli et al., 2004, Rosenbaum and Lister, 2005, Schmid et al., 1996). This suture zone extends N–S to NE–SW along the Italian border to France and Switzerland for approximately 300 km (Fig. 1a),
Electron probe micro-analysis (EPMA)
EPM analyses were performed with a JEOL-JXA 8230 probe at the GFZ Potsdam under common analytical conditions (15 kV, 20 nA, wavelength-dispersive spectroscopy mode), using a 10 μm beam. Standards used for the calibration were the following: orthoclase (Al, Si, K), fluorite (F), rutile (Ti), Cr2O3 (Cr), wollastonite (Ca), tugtupite (Cl), albite (Na), MgO (Mg), Fe2O3 (Fe), rhodonite (Mn). In order to obtain a representative dataset and to check for inter-sample heterogeneity, we analyzed phengite,
Field observations
The tectonic architecture of the DBT is relatively homogeneous along-strike (Fig. 2, Fig. 3). Stage 1 top-to-NW deformation gave rise to the dominant fabric in the Arolla gneiss. Stage 2 deformation (shear bands, crenulation folds) is often seen reworking Stage 1 mylonites in the DBT vicinity and also in the hanging wall of the Arolla unit (Valpelline shear zone; see Fig. 4a and b for pictures representative of each fabric). The mylonites resulting from shearing at the base of the Arolla
Tsaté Complex metasediments
The mineralogy of the Tsaté Complex calcschists is fairly homogeneous. The main foliation is defined by phengite + chlorite + calcite + quartz ± epidote ± titanite (Table 1). Quartz and calcite both show evidence for intracrystalline plastic deformation such as undulose extinction, subgrain formation, twinning and dynamically recrystallized grains. When albite is present, the foliation is deflected around these large (between 200 μm and 2 mm-wide), strain-free porphyroblasts which often preserve a relict
RSCM
In order to characterize the thermal history of the metasedimentary samples, we performed a series of Raman spectroscopy analyses on organic material from a set of 20 samples from the Tsaté Complex, one sample from the Monts Dolins slice and one sample from the Roisan Zone (DBTS; Fig. 2). Detailed results are shown in Table 1 and located on the geological map (Fig. 2). Three main temperature groups have been defined from the obtained maximum-temperature dataset. The Low-Temperature (LT) group
Tsaté Complex
The Rb/Sr mineral data of four calcschist samples from the Tsaté Complex provided relatively well-constrained age information. Selected isochron plots are shown in Fig. 7. The others are presented in the Appendix D. The complete set of isotopic data is shown in Table 5. The youngest age has been obtained for sample #29a from a High-T slice (494 ± 21 °C) from the base of the Tsaté Complex which yielded a six-point regression line corresponding to an age of 37.5 ± 0.9 Ma (Fig. 7a). Samples #05 and #19a
P–T–t path and dynamics of the Tsaté Complex
Our petrological investigations revealed the presence of three distinct groups (High-T, Medium-T and Low-T) of maximum temperatures comprised between 360 °C and 490 °C. The presence of very homogeneous intra-slice temperatures (see on Fig. 2 results for the Lac de Moiry area: 437–441 °C and for the Ollomont area: 396–403 °C) demonstrates the existence of several independent tectonic slices within the Tsaté Complex (in agreement with observations of Negro et al., 2013). Even though the detailed
Conclusions
We present here the first reconstruction of the Alpine P–T–t path recorded by the Dent Blanche Thrust (DBT) and the underlying ophiolite. The Tsaté ophiolitic domain exhibits a complex internal structure where km-sized slices with variable P–T–t paths were tectonically juxtaposed during exhumation in late Eocene times. The gneisses from the DBT show progressive record of deformation from blueschist- (12 kbar, 450 °C) to greenschist-facies conditions (7 kbar, ~ 500 °C). Combined thermodynamic
Acknowledgments
The authors thank Philippe Yamato, Amaury Pourteau and Philippe Agard for insightful discussions in the field. Benoit Dubacq is warmly thanked for his help with the Matlab software “Kit Chl-Phg”. Paola Manzotti, Michel Marthaler, Yves Gouffon and the Swiss Geological Office are also acknowledged for technical assistance. Christian Schmidt is acknowledged for providing help with Raman characterization of serpentinite minerals. Uwe Ring, an anonymous reviewer and the journal editor are thanked
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2022, TectonophysicsCitation Excerpt :Peak pressures are estimated at 1.6-1.7 GPa for the By unit and 0.8-0.9 GPa for the Cornet unit (Manzotti et al., 2021). The D2 to D3 exhumation-related episodes of these units were dated between 41 and 37 Ma (Reddy et al., 2003; Angiboust et al., 2014). Potentially independent tectonic slices, rooted in the Briançonnais area and belonging to the complex Valsavarenche mega-fold (Bucher et al., 2003), crop out around the Valgrisenche, Val de Rhêmes and Valsavarenche valleys.
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