Protracted Subduction of the European Hyperextended Margin Revealed by Rutile U‐Pb Geochronology Across the Dora‐Maira Massif (Western Alps)

Bonnet, Guillaume; Chopin, Christian; Locatelli, Michele; Kylander‐Clark, Andrew; Hacker, Bradley R.

doi:10.1029/2021tc007170

Cited by 24 publications

(30 citation statements)

References 180 publications

(364 reference statements)

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“…The internal geometry of the southern Dora-Maira Massif is relatively well known (Fig. 1a): several tectonic units have been identified on the basis of lithological, structural and petrological data and they are separated by thrust sheets and slivers of Mesozoic meta-sediments and ophiolites (e.g., Chopin 1984;Chopin et al 1991;Kienast et al 1991;Compagnoni et al 1995;Michard et al 1995;Chopin and Schertl 1999;Compagnoni and Hirajima 2001;Rubatto and Hermann 2001;Compagnoni and Rolfo 2003 and refs therein;Compagnoni et al 2012;Groppo et al 2019;Bonnet et al 2022;Michard et al 2022). The Alpine metamorphism reached by these units varies from garnet blueschist (Dronero Unit), to quartz eclogite (Rocca Solei) to coesite eclogite facies (Brossasco-Isasca) conditions (Fig.…”

Section: The Dora-maira Massif In the Western Alpsmentioning

confidence: 99%

A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps)

Manzotti¹,

Schiavi

Nosenzo³

et al. 2022

Preprint

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The distribution of ultrahigh-pressure metamorphism (UHP) at the scale of a mountain belt is of prime importance for deciphering its past subduction history. In the Western Alps, coesite has been recognized in the southern Dora-Maira massif, in the lens-shaped Brossasco-Isasca Unit, but has not been found up to now in the other parts of the massif. We report the discovery of a new UHP unit in the northern Dora-Maira Massif (Western Alps), named Chasteiran Unit (Manzotti et al. 2022). It is only a few tens of metres thick and consists of garnet-chloritoid micaschists. Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P&#8211;T evolution. Coesite crystals, which are pristine or partially transformed to palisade quartz occur as inclusions in the garnet outer cores. According to thermodynamic modelling, garnet displays a continuous record of growth during the prograde increase in P and T (25&#8211;27 kbar 470&#8211;500 &#176;C) (stage 1), up to the coesite stability field (27&#8211;28 kbar 520&#8211;530 &#176;C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to 15 kbar 500&#8211;515 &#176;C) (stage 3). The main regional, composite, foliation, marked by chloritoid and rutile, began to develop during this stage, and was then overprinted by chlorite-ilmenite (stage 4). The Chasteiran Unit is discontinuously exposed in the immediate hangingwall of the Pinerolo Unit, and it is located far away from, and without physical links to the classic UHP Brossasco-Isasca Unit. Moreover, it records a different, much colder, P&#8211;T evolution, showing that different slices were detached from the downgoing subduction slab. The Chasteiran Unit is the fourth and the coldest Alpine UHP unit known so far in the entire Alpine belt. Its P&#8211;T conditions are comparable to the ones of the Tian Shan coesite-chloritoid-bearing rocks. &#160; Manzotti, P., Schiavi, F., Nosenzo, F., Pitra, P., Ball&#232;vre, M. (2022). A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps). Contributions to Mineralogy and Petrology, in press. &#160;

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Section: The Dora-maira Massif In the Western Alpsmentioning

confidence: 99%

A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps)

Manzotti¹,

Schiavi

Nosenzo³

et al. 2022

Preprint

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“…700°C–750°C) within the Dora Maira massif (Figure 2; e.g., Bonnet et al., 2022; Groppo et al., 2019; Rubatto & Hermann, 2001). In our models with a weak interface, there are subducted and exhumed units that have peak P ‐ T values similar to the ones of most units of the Dora Maira massif (2–2.4 GPa and 500°C–540°C: Bonnet et al., 2022; Groppo et al., 2019). However, the UHP and high‐ T markers similar to values for the BIU are not exhumed (Figure 12d), but the largest peak P ‐ T values of rocks exhumed in our model (3.5 GPa and 620°C) are quite close to peak values of the BIU.…”

Section: Discussionmentioning

confidence: 99%

“…Inherited structural complexity deriving from extensional tectonics within the former passive margin (Figure 1a) may also contribute to the structurally complex and minor volumes of exhumed coherent UHP units observed throughout the Western Alps, for example, Brossasco-Isasca unit (BIU; peak P is c. 3.5-4.0 GPa and peak T is c. 700°C-750°C) within the Dora Maira massif (Figure 2; e.g., Bonnet et al, 2022;Groppo et al, 2019;Rubatto & Hermann, 2001). In our models with a weak interface, there are subducted and exhumed units that have peak P-T values similar to the ones of most units of the Dora Maira massif (2-2.4 GPa and 500°C-540°C: Bonnet et al, 2022;Groppo et al, 2019). However, the UHP and high-T markers similar to values for the BIU are not exhumed (Figure 12d), but the largest peak P-T values of rocks exhumed in our model (3.5 GPa and 620°C) are quite close to peak values of the BIU.…”

Section: Application To the Western Alpsmentioning

confidence: 99%

“…The BIU is a very small tectonic slice (c. 1 km thickness) and our model cannot resolve such small units and shear zones around such small units due to the numerical resolution of 1 × 1 km. Also, the BIU might have been an individual extensional allochthon, structurally separated from the main distal passive margin (e.g., Bonnet et al, 2022). Such extensional allochthonous are also not included in our preconvergence model configuration (Figure 3b).…”

Section: Application To the Western Alpsmentioning

confidence: 99%

“…Here, we investigate the metamorphic facies evolution and distribution for syn-convergent exhumation to test whether syn-convergent exhumation can occur without significant erosion and can generate a metamorphic facies distribution, which is comparable to that of the Western Alps. Other disputed issues concerning the Alpine orogeny include, for example, (a) the heat budget during Barrovian metamorphism in the Lepontine dome (Berger et al, 2011;Burg & Gerya, 2005;Jamieson et al, 1998;Ryan & Dewey, 2019;Stüwe, 1998), (b) potential local deviations from lithostatic pressure (Luisier et al, 2019;Schenker et al, 2015;Vaughan-Hammon et al, 2021b), and (c) the mechanisms by which small volumes of ultrahigh-pressure (UHP) rocks are exhumed and embedded in lower grade high-pressure (HP) units, for example, the c. 1 km thick Brosasco-Isasca UHP Unit within the HP Dora Maira massif (e.g., Bonnet et al, 2022;Chopin, 1987;Groppo et al, 2019;Hacker & Gerya, 2013;Kurz & Froitzheim, 2002;Warren, 2013). With the aim of contributing to eventually resolving the issues mentioned above, we also (a) investigate the temporal relation between peak pressure and peak temperature for pressure-temperature-time (P-T-t) paths, and (b) study the relation between the metamorphic grade of exhumed rocks and their original paleogeographic position.…”

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Metamorphic Facies Distribution in the Western Alps Predicted by Petrological‐Thermomechanical Models of Syn‐Convergent Exhumation

Vaughan‐Hammon

Candioti

Duretz

et al. 2022

Geochem Geophys Geosyst

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The distribution of metamorphic rocks throughout the western European Alps indicates subduction‐related metamorphism. However, processes by which high‐grade metamorphic rocks exhume remain disputed. Here, we present two‐dimensional petrological‐thermomechanical numerical models to investigate the metamorphic facies evolution during orogenesis. We model closure of an oceanic basin with exhumed mantle bounded by passive margins. To ensure thermomechanical feasibility, we model also this basin configuration. Before convergence, we replace the uppermost portions of exhumed mantle with serpentinite. Location and orientation of subduction are not predefined and subduction initiates self‐consistently during convergence. A weak subduction interface develops if serpentinite is initially thick enough (here 6 km) and can distribute along the interface. Syn‐convergent exhumation of (ultra)high‐pressure rocks occurs with minor erosion, enabled by local upper‐plate extension and a crustal‐scale normal‐sense shear zone. We calculate metamorphic facies evolutions with peak P and T values of 10,000 markers. Results show (a) peak P and T values agreeing with estimates from natural rocks, (b) exhumed, structurally coherent regions with identical metamorphic facies, indicating absence of significant mixing (mélange), (c) facies distributions corresponding to that of the Western Alps, which is from eclogite to blueschist to greenschist facies when going from internal to external domains, and (d) exhumation velocities larger than burial velocities. Models with stronger subduction interface (3‐km serpentinite thickness) develop an orogenic wedge with vertical metamorphic gradient and minor exhumation. Syn‐convergent exhumation is feasible for the Western Alps and calculating metamorphic facies distribution is useful when testing the applicability of models to natural orogens.

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A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps)

Manzotti

Schiavi

Nosenzo

et al. 2022

Contrib Mineral Petrol

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The distribution of ultrahigh-pressure metamorphism (UHP) at the scale of a mountain belt is of prime importance for deciphering its past subduction history. In the Western Alps, coesite has been recognized in the southern Dora-Maira Massif, in the lens-shaped Brossasco-Isasca Unit, but has not been found up to now in the other parts of the massif. We report the discovery of a new UHP unit in the northern Dora-Maira Massif (Western Alps), named Chasteiran Unit. It is only a few tens of metres thick and consists of graphite-rich, garnet–chloritoid micaschists, whose protoliths may be black shales of Silurian age. Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P–T evolution. Coesite crystals, which are pristine or partially transformed to palisade quartz occur as inclusions in the garnet outer cores. According to thermodynamic modelling, garnet displays a continuous record of growth during the prograde increase in P and T (25–27 kbar 470–500 °C) (stage 1), up to the coesite stability field (27–28 kbar 510–530 °C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to 15 kbar 500–515 °C) (stage 3). The main regional, composite, foliation, marked by chloritoid and rutile, began to develop during this stage, and was then overprinted by chlorite–ilmenite (stage 4). The Chasteiran Unit is discontinuously exposed in the immediate hangingwall of the Pinerolo Unit, and it is located far away from, and without physical links to the classic UHP Brossasco-Isasca Unit. Moreover, it records a different, much colder, P–T evolution, showing that different slices were detached from the downgoing subduction slab. The Chasteiran Unit is the fourth and the coldest Alpine UHP unit known so far in the entire Alpine belt. Its P–T conditions are comparable to the ones of the Tian Shan coesite–chloritoid-bearing rocks.

show abstract

Protracted Subduction of the European Hyperextended Margin Revealed by Rutile U‐Pb Geochronology Across the Dora‐Maira Massif (Western Alps)

Cited by 24 publications

References 180 publications

A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps)

A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps)

Metamorphic Facies Distribution in the Western Alps Predicted by Petrological‐Thermomechanical Models of Syn‐Convergent Exhumation

A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps)

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