A north-south section across the Queyras Schistes lustrés (Piedmont zone, Western Alps): Syn-collision refolding of a subduction wedge

Abstract In the Sesia-Lanzo Zone, Western Alps, the Rocca Canavese Thrust Sheets (RCT) subunit is characterized by a mixture of mantle- and crust-derived lithologies, such as metapelites, metagranitoids, metabasics, and serpentinized mantle slices with sizes ranging from meters to hundreds of meters. Structural and metamorphic history suggests that the RCT rocks experienced a complex evolution. In particular, two different peak conditions were obtained for the metabasics, representing different tectono-metamorphic units (TMUs), namely, D1a under eclogite facies conditions and D1b under lawsonite-blueschist-facies conditions. The two TMUs were coupled during the syn-D2 exhumation stage under epidote-blueschist-facies conditions. The different rocks and metamorphic evolutions and the abundance of serpentinites in the tectonic mixture suggest a possible subduction-related mélange origin for the RCT. To verify whether a subduction-related mélange can record tectono-metamorphic histories similar to that inferred for the RCT, we compare the pressure-temperature evolutions with the results of a 2-D numerical model of ocean-continent subduction with mantle wedge serpentinization. The predictions of the numerical model fully reproduce the two peak conditions (D1a and D1b) and the successive exhumation history of the two TMUs within the subduction wedge. The degree of mixing estimated from field data is consistent with that predicted by the numerical simulation. Finally, the present-day location of the RCT, which marks the boundary between the orogenic wedge (Penninic and Austroalpine domains) and the southern hinterland (Southalpine domain) of the Alpine chain, is reproduced by the model at the end of the exhumation in the subduction wedge. Therefore, the comparison between natural data and the model results confirms the interpretation of the RCT as a subduction-related mélange that occurred during exhumation within a serpentinized mantle wedge.

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Diachronous evolution of the alpine continental subduction wedge: Evidence from P–T estimates in the Briançonnais Zone houillère (France – Western Alps)

Journal of Geodynamics ◽

10.1016/j.jog.2011.09.006 ◽

2012 ◽

Vol 56-57 ◽

pp. 39-54 ◽

Cited By ~ 58

Author(s):

Pierre Lanari ◽

Stéphane Guillot ◽

Stéphane Schwartz ◽

Olivier Vidal ◽

Pierre Tricart ◽

...

Keyword(s):

Western Alps ◽

Continental Subduction ◽

Subduction Wedge

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HP tectono‐metamorphic evolution of the Internal Piedmont Zone in Susa Valley (Western Alps): New petrologic insight from garnet+chloritoid‐bearing micaschists and Fe–Ti metagabbro

Journal of Metamorphic Geology ◽

10.1111/jmg.12574 ◽

2020 ◽

Cited By ~ 1

Author(s):

Stefano Ghignone ◽

Alessandro Borghi ◽

Gianni Balestro ◽

Daniele Castelli ◽

Marco Gattiglio ◽

...

Keyword(s):

Western Alps ◽

Metamorphic Evolution ◽

Piedmont Zone

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Lubrication Dynamics for Exhumation of high-pressure Rocks in Subduction Zones

10.5194/egusphere-egu2020-314 ◽

2020 ◽

Author(s):

Giridas Maiti ◽

Joyjeet Sen ◽

Nibir Mandal

Keyword(s):

High Pressure ◽

Subduction Zones ◽

Earth Science ◽

Dynamic Pressure ◽

Threshold Value ◽

Western Alps ◽

Plate Motion ◽

Gravitational Potential Energy ◽

Crustal Rocks ◽

Subduction Wedge

Subduction zones witness exhumation of deep crustal rocks metamorphosed under high pressure (HP) and ultra-high pressure (UHP) conditions, following burial to depths of 100 km or more. The exhumation dynamics of HP and UHP rocks still remains a lively issue of research in the Earth science community. We develop a new tectonic model based on the lubrication dynamics to show the exhumation mechanism of such deep crustal rocks in convergent tectonic settings. Our model suggests subducting plate motion produces a dynamic pressure in the subduction wedge, which supports the excess gravitational potential energy of the thicker and relatively denser overriding plate partly lying over the buoyant subduction wedge. A drop in the dynamic pressure causes the overriding plate to undergo gravitational collapse and forces the wedge materials to return to the surface. Using lubrication theory we estimate the magnitude of dynamic pressure (P) in the wedge as a function of subduction velocity (us), convergence angle (&#945;) and wedge viscosity (&#181;). We also conduct thermo-mechanical numerical experiments to implement the lubrication model in subduction zones on a real scale. Our analysis suggests that drop in wedge dynamic pressure below a threshold value due to decease in us&#160; and &#181;, or by other processes, such as slab rollback and trench retreat facilitate exhumation of deep crustal rocks. Finally we discuss their implications for the exhumation of deep crustal rocks in different subduction setups such as the Himalayan continental subduction, the Mediterranean realm (Calabria&#8211;Apennine and Aegean belts) and Western Alps.

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Pre-Alpine Tectono-Stratigraphic Reconstruction of the Jurassic Tethys in the High-Pressure Internal Piedmont Zone (Stura di Viù Valley, Western Alps)

Minerals ◽

10.3390/min11040361 ◽

2021 ◽

Vol 11 (4) ◽

pp. 361

Author(s):

Marcello De Togni ◽

Marco Gattiglio ◽

Stefano Ghignone ◽

Andrea Festa

Keyword(s):

Western Alps ◽

Piedmont Zone ◽

Ocean Ridges ◽

Stratigraphic Architecture ◽

Basin Floor ◽

Slow Spreading ◽

Mantle Exhumation ◽

Depositional Setting ◽

Unconformable Contact ◽

Stratigraphic Succession

We present a detailed description of the tectono-stratigraphic architecture of the eclogite-facies Internal Piedmont Zone (IPZ) metaophiolite, exposed in the Lanzo Valleys (Western Alps), which represents the remnant of the Jurassic Alpine Tethys. Seafloor spreading and mantle exhumation processes related to the Alpine Tethys evolution strongly conditioned the intra-oceanic depositional setting, which resulted in an articulated physiography and a heterogeneous stratigraphic succession above the exhumed serpentinized mantle. “Complete” and “reduced” successions were recognized, reflecting deposition in morphological or structural lows and highs, respectively. The “complete” succession consists of quartzite, followed by marble and calcschist. The “reduced” succession differs for the unconformable contact of the calcschist directly above mantle rocks, lacking quartzite and gray marble. The serpentinite at the base of this succession is intruded by metagabbro and characterized at its top by ophicalcite horizons. Mafic metabreccia grading to metasandstone mark the transition between the “complete” and “reduced” successions. The character of the reconstructed succession and basin floor physiography of the IPZ metaophiolite is well comparable with the Middle Jurassic–Late Cretaceous succession of both the Queyras Complex (External Piedmont Zone) and the Internal Ligurian Units (Northern Apennines) and with modern slow-spreading mid-ocean ridges.

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Source tracing of detrital serpentinite in the Oligocene molasse deposits from the western Alps (Barrême basin): implications for relief formation in the internal zone

Geological Magazine ◽

10.1017/s0016756811001105 ◽

2012 ◽

Vol 149 (5) ◽

pp. 841-856 ◽

Cited By ~ 15

Author(s):

S. SCHWARTZ ◽

S. GUILLOT ◽

P. TRICART ◽

M. BERNET ◽

S. JOURDAN ◽

...

Keyword(s):

Source Area ◽

Structural Data ◽

Western Alps ◽

Early Miocene ◽

Early Oligocene ◽

Source Tracing ◽

Transport Direction ◽

Internal Zone ◽

Relief Formation ◽

Subduction Wedge

AbstractWe present the first contribution of tracing the source area of ophiolitic detritus in the Alpine molasse deposits by Raman spectroscopy. The lower Oligocene molasse deposits preserved in the Barrême basin, in the SW foreland of the western Alpine arc, are known for the sudden arrival of the first ‘exotic’ detritus coming from the internal Alpine zones. Among them, the pebbles of serpentinized peridotites have so far not been studied. We show that they only consist of antigorite serpentinite, implying that they originate from erosion of high temperature blueschists. In contrast, the upper Oligocene/lower Miocene molasse shows mixed clasts of serpentine including antigorite and lizardite without any evidence of chrysotile. This suggests that they were derived from a less metamorphosed unit such as the low temperature blueschist unit. Taking into account the sediment transport direction in the basin and the varied metamorphic characteristics of the other ocean-derived detritus, we constrain the lithologic nature of the source zones and the location of the relief zones, identified as the internal Alps, SE of the Pelvoux external crystalline massif. Available structural data andin situthermochronological data allow the reconstruction of the Oligocene to early Miocene collisional geometry of the Palaeogene subduction wedge. This phase corresponds to two major phases of uplift evolving from a single relief zone located above the Ivrea body during early Oligocene times and persisting up to early Miocene times; then during late Oligocene/early Miocene times a second relief zone developed above the Briançonnais zone. At that time, the internal western Alps acquired its double vergency.

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