Timing of exhumation of meta-ophiolite units in the Western Alps: New tectonic implications from 40Ar/39Ar white mica ages from Piedmont Zone (Susa Valley)

Abstract

A multidisciplinary approach to the study of collisional orogenic belts can improve our knowledge of their geodynamic evolution and may suggest new tectonic models, especially for (U)HP rocks inside the accretionary wedge. In the Western Alps, wherein nappes of different origin are stacked, having recorded different metamorphic peaks at different stages of the orogenic evolution. This study focuses on the External (EPZ) and Internal (IPZ) ophiolitic units of the Piedmont Zone (Susa Valley, Western Alps), which were deformed throughout four tectonometamorphic phases (D1 to D4), developing different foliations and cleavages (S1 to S4) at different metamorphic conditions. The IPZ and EPZ are separated by a shear zone (i.e. the Susa Shear Zone) during which a related mylonitic foliation (SM) developed. S1 developed at high pressure conditions (Epidote-eclogite vs. Lawsonite-blueschist facies conditions for IPZ and EPZ, respectively), as suggested by the composition of white mica (i.e. phengite), whereas S2 developed at low pressure conditions (Epidote-greenschist facies conditions in both IPZ and EPZ) and is defined by muscovite. White mica defining the SM mylonitic foliation (T1) is mostly defined by phengite, while the T2-related disjunctive cleavage is defined by fine-grained muscovite. The relative chronology inferred from meso-and micro-structural observations suggests that T1 was nearcoeval respect to the D2, while T2 developed during D4. A new set of radiometric ages of the main metamorphic foliations were obtained by in situ Ar/Ar dating on white mica. Different generations of white mica defining S1 and S2 foliations in both the IPZ and EPZ and SM in the SSZ, were dated and two main groups of ages were obtained. In both IPZ and EPZ, S1 foliation developed at ~46-41 Ma, while S2 foliation developed at ~40-36 Ma and was nearly coeval with the SM mylonitic foliation (~39-36 Ma). Comparison between structural, petrological and geochronological data allows to define time of coupling of the different units and consequently to infer new tectonic implications for the exhumation of meta-ophiolites of the Piedmont Zone within axial sector of the Western Alps.

40 Ar/ 39 Ar geochronology of crustal deformation

“…This is to say that in a polyphased context, the interpretation of ages strictly in terms of crossing a given temperature barrier in one direction or the other can be debated without a proper understanding of the variable parameters that control the closure and opening of the chronometers being used. This is a highlight made several years ago by Chopin and Maluski [1980] in their studies of some high-pressure rocks of the Western Alps and subsequently validated by many geochronological works [see Agard et al, 2002, Ghignone et al, 2021 for in situ 40 Ar/ 39 Ar studies in the same area, or McDonald et al, 2016 for the HP gneisses of Norway].…”

“…However, many studies have shown that reliable textural-chemical-age relationships can be already established with current 40 Ar/ 39 Ar spatial resolution [e.g. Maluski and Monié, 1988, Kramar et al, 2001, Agard et al, 2002, Mulch and Cosca, 2004, Mulch et al, 2002, Augier et al, 2005, Mulch et al, 2005, Di Vincenzo et al, 2007, Schneider et al, 2012, Angiboust et al, 2015, Bellanger et al, 2015, Di Vincenzo et al, 2016, Kellet et al, 2016, Scharf et al, 2016, Berger et al, 2017, Laurent et al, 2017, Tan et al, 2019, Beaudoin et al, 2020, Ghignone et al, 2021, Di Vincenzo et al, 2022.…”

Paleocene‐Eocene High‐Pressure Carbonation of Western Alps Serpentinites: Positive Feedback Between Deformation and CO2‐CH4 Fluid Ingression Responsible for Slab Slicing?

“…(2002), Angiboust and Glodny (2020), and Ghignone et al. (2021). Abbreviations: LPU: Liguro‐Piemont Upper units; LPM: Liguro‐Piemont Middle units; LPL: Liguro‐Piemont Lower units.…”

“…Agard et al (2009) estimated that approximately 30%-50% of the sedimentary deposits were sliced off from the downgoing plate and exhumed, as opposed to only ∼5% of the oceanic crust and serpentinized mantle. After a peak burial between ∼60 and ∼40 Ma (e.g., Agard et al, 2002;Dragovic et al, 2020;Garber et al, 2020;Ghignone et al, 2021;Rubatto & Hermann, 2003), these rocks were successively scraped off from the subducting plate as tectonic slices and were exhumed from the Late Eocene to the Miocene (Agard et al, 2002(Agard et al, , 2009Angiboust & Glodny, 2020;Ghignone et al, 2021;Schwartz et al, 2020). This complex stack of slices therefore formed as a deep accretionary complex, built through progressive underplating (Agard, 2021;Agard et al, 2009Agard et al, , 2018Herviou et al, 2022;Platt, 1986).…”

Insights into the Subduction of the Ligure-Piemontese Oceanic Basin: New Constraints from the Metamorphism in the Internal Ligurian Units (Northern Apennines, Italy)

“…The only geodynamic setting where the estimated cold geothermal gradient can occur is a subduction zone [95][96][97][98]. Moreover, the P conditions are not compatible with a D1 phase developed in a subduction setting by frontal accretion because this mechanism implies a deformation that occurs at a depth shallower than 8-10 km [99][100][101][102].…”