Deciphering the coupling between tectonic and metamorphic processes in the Monte Rosa nappe (Western Alps)

2021 ◽  
Author(s):  
Joshua D Vaughan Hammon ◽  
Cindy Luisier ◽  
Lorenzo G Candioti ◽  
Stefan M Schmalholz ◽  
Lukas P Baumgartner
Keyword(s):  
Subduction Zones ◽  
Numerical Models ◽  
Western Alps ◽  
Gravitational Potential Energy ◽  
Lithostatic Pressure ◽  
Burial Depth ◽  
Differential Stress ◽  
Metamorphic History ◽  
Monte Rosa ◽  
Metamorphic Facies

<p>Our refined ability to estimate metamorphic conditions incurred by rocks has increased our understanding of the dynamic earth. Calculating pressure (P), temperature (T) and time (t) histories of these rocks is vital for reconstructing tectonic movements within subduction zones. However, large disparities in peak P within a structurally coherent tectonic unit poses difficulties when attempting to resolve a tectono-metamorphic history, if a depth dependant lithostatic P is assumed. However, what is clear is that pressure, or mean stress, in a rock cannot exactly be lithostatic during an orogeny due to differential stress, required to drive rock deformation or to balance lateral variations in gravitational potential energy. Deviations from lithostatic P is commonly termed tectonic pressure, and both its magnitude and impact on metamorphic reactions in disputed.</p><p>For the ‘Queen of the Alps’ (the Monte Rosa massif), estimates for the maximum P recorded during Alpine orogenesis remain enigmatic. Large disparities in published estimates for peak P exist, ranging between 1.2 and 2.7 GPa. Moreover, the highest P estimates (2.2 - 2.7 GPa) are for rocks that comprise only a small percentage (< 1%) of the total volume of the nappe (whiteschist bodies and eclogitic mafic boudins). We present newly discovered whiteschist lithologies that persistently exhibit higher P conditions (<em>c.</em> 2.2 GPa) compared to metagranitic and metapelitic lithologies (<em>c.</em> 1.4 - 1.6 GPa). Detailed mapping and structural analysis in these regions lack evidence for tectonic mixing. Therefore, we suggest that a ΔP 0.6 ± 0.2 GPa during peak Alpine metamorphism could potentially represent tectonic pressure. Furthermore, we outline possible mechanisms that facilitate ΔP, namely mechanically- and/or reaction-induced. We present data from numerical models that exhibit significant ΔP (<em>c.</em> 0.4 GPa) during a transient period of high differential stress prior to buckling and subsequent exhumation of viscous fold nappes, similar to exhumation mechanisms suggested for the Monte Rosa nappe. As well as this, we present new routines for calculating metamorphic facies distribution within numerical models of subduction zones that agree with natural distributions within orogens.</p><p>The maximum burial depth of the Monte Rosa unit was likely significantly less than 80 km (based on the lithostatic pressure assumption and minor volumes of whiteschist at <em>c.</em> 2.2 GPa). Rather, the maximum burial depth of the Monte Rosa unit was presumably equal to or less than <em>c.</em> 60 km, estimated from pressures of 1.4 - 1.6 GPa recorded frequently in metagranite and metapelitic lithologies. In order to understanding, more completely, a rocks metamorphic history, consideration of the interplay between tectonic and metamorphic processes should not be overlooked.</p>

scholarly journals Metamorphic pressure variation in a coherent Alpine nappe challenges lithostatic pressure paradigm

2019 ◽  
Vol 10 (1) ◽  
Author(s):  
Cindy Luisier ◽  
Lukas Baumgartner ◽  
Stefan M. Schmalholz ◽  
Guillaume Siron ◽  
Torsten Vennemann
Keyword(s):  
Western Alps ◽  
Lithostatic Pressure ◽  
Burial Depth ◽  
Pressure Gradients ◽  
Mountain Ranges ◽  
Heterogeneous Rock ◽  
Mineral Assemblages ◽  
Petrology And Geochemistry ◽  
Monte Rosa ◽  
Temperature Time

Abstract Pressure–temperature–time paths obtained from minerals in metamorphic rocks allow the reconstruction of the geodynamic evolution of mountain ranges under the assumption that rock pressure is lithostatic. This lithostatic pressure paradigm enables converting the metamorphic pressure directly into the rock’s burial depth and, hence, quantifying the rock’s burial and exhumation history. In the coherent Monte Rosa tectonic unit, Western Alps, considerably different metamorphic pressures are determined in adjacent rocks. Here we show with field and microstructural observations, phase petrology and geochemistry that these pressure differences cannot be explained by tectonic mixing, retrogression of high-pressure minerals, or lack of equilibration of mineral assemblages. We propose that the determined pressure difference of 0.8 ± 0.3 GPa is due to deviation from lithostatic pressure. We show with two analytical solutions for compression- and reaction-induced stress in mechanically heterogeneous rock that such pressure differences are mechanically feasible, supporting our interpretation of significant outcrop-scale pressure gradients.

Pressure variations in the Monte Rosa nappe: new results from staurolite bearing metapelites

2020 ◽  
Author(s):  
Joshua David Vaughan-Hammon ◽  
Cindy Luisier ◽  
Stefan Schmalholz ◽  
Lukas Baumgartner
Keyword(s):  
Hydrostatic Stress ◽  
Western Alps ◽  
Garnet Growth ◽  
Lithostatic Pressure ◽  
Burial Depth ◽  
Tectonic History ◽  
Orogenic Wedge ◽  
Solution Models ◽  
Exhumation Rates ◽  
Monte Rosa

<p>Pressure recorded in metamorphic rocks is typically assumed to represent a hydrostatic stress and thus depends linearly on depth. Recently, work in the Monte Rosa nappe in the western Alps has challenged this lithostatic assumption. Observable pressure differences of 0.8 ± 0.3 GPa between chloritoid, talc, and phengite-bearing lithologies (locally known as ‘whiteschists’) at ca. 2.2 - 2.5 GPa, and metagranite lithologies at 1.4 – 1.6 GPa have been recorded. These pressure variations, rather than being attributed to variable rock kinetics, partial retrogression, or tectonic mixing, have been interpreted to be mechanically induced. As part of this ongoing investigation, we will present work undertaken on newly discovered staurolite-chloritoid bearing metapelites belonging to the Monte Rosa basement, in order to constrain the peak pressure and temperature conditions during burial within the Alpine orogeny and the resulting tectono-metamorphic and geodynamic implications.</p><p>Metapelitic samples from the Monte Rosa basement show a rich polymetamorphic history from high-T Variscan garnet growth through to high-P Alpine equilibration and decompression. Extensive phase petrology calculations have been undertaken on staurolite + chloritoid + phengite + paragonite assemblages, as well as garnet + chlorite + phengite + paragonite assemblages, representing equilibration at peak Alpine conditions. Various mixing models were employed due to non-negligible amounts of ZnO recorded in staurolite (~5% wt% and ~1 a.p.f.u) and the lack of available solution models. These result in peak Alpine conditions of 1.6 ± 0.2 GPa and 580 ± 15 ºC. These findings confirm the presence of significant disparities in pressure of 0.6 ± 0.2 GPa within the coherent Monte Rosa nappe.</p><p>Vital for the reconstruction and tectonic history for the western Alps is the maximum burial depth of units involved. We argue that the maximum burial depth of the Monte Rosa unit was significantly less than 80 km (based on the lithostatic pressure assumption and minor volumes of whiteschist at > 2.2 GPa). Rather, the maximum burial depth of the Monte Rosa unit was presumably equal or less than ca. 60 km, estimated from pressures of 1.4 - 1.6 GPs recorded frequently in metagranite and metapelitic lithologies. This depth is compatible with burial and exhumation within an orogenic wedge, rather than a complex exhumation mechanism such as within a weak and long subduction channel. Equally, the relatively slower exhumation rates from shallower crustal depths fit more reasonable tectonic velocities.</p>

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

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

<p>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 (<em>P</em>) in the wedge as a function of subduction velocity (<em>u<sub>s</sub></em>), convergence angle (<em>α</em>) and wedge viscosity (<em>µ</em>). 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 <em>u</em><sub><em>s</em>  </sub>and <em>µ</em>, 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–Apennine and Aegean belts) and Western Alps.</p>

Magnetite (U-Th-Sm)/He dating: analytical challenges and application

2021 ◽  
Author(s):  
Marianna Corre ◽  
Martine Lanson ◽  
Arnaud Agranier ◽  
Stephane Schwartz ◽  
Fabrice Brunet ◽  
...  
Keyword(s):  
Laser Ablation ◽  
Subduction Zones ◽  
Major Elements ◽  
Western Alps ◽  
Planetary Science ◽  
Mantle Xenoliths ◽  
Transform Faults ◽  
Geological Processes ◽  
History Of ◽  
Geochronological Constraints

<p>Magnetite (U-Th-Sm)/He dating method has a strong geodynamic significance, since it provides geochronological constraints on serpentinization episodes, which are associated to important geological processes such as ophiolite obductions, subduction zones, transform faults and fluid circulations. Although helium content that range from 0.1 pmol/g to 20 pmol/g can routinely be measured, the application of this dating technique however is still limited due to major analytical obstacles. The dissolution of a single magnetite crystal and the measurement of the U, Th and Sm present at the ppb level in the corresponding solution, remains highly challenging, especially because of the absence of magnetite standard. In order to overcome these analytical issues, two strategies have been followed, and tested on magnetite from high-pressure rocks from the Western Alps (Schwartz et al., 2020). Firstly, we purified U, Th and Sm (removing Fe and other major elements) using ion exchange columns in order to analyze samples, using smaller dilution. Secondly, we performed in-situ analyzes by laser-ablation-ICPMS. Since no solid magnetite certified standard is yet available, we synthetized our own by precipitating magnetite nanocrystals. The first quantitative results obtained by LA-ICP-MS using this synthetic material along with international glass standards, are promising. The laser-ablation technique overcomes the analytical difficulties related to sample dissolution and purification. It thus opens the path to the dating of magnetite (and also spinels) in various ultramafic rocks such as mantle xenoliths or serpentinized peridotites in ophiolites.</p><p>Schwartz S., Gautheron C., Ketcham R.A., Brunet F., Corre M., Agranier A., Pinna-Jamme R., Haurine F., Monvoin G., Riel N., 2020, Unraveling the exhumation history of high-press ure ophiolites using magnetite (U-Th-Sm)/He thermochronometry. Earth and Planetary Science Letters 543 (2020) 116359.</p>

Synconvergent and coherent (ultra)high-pressure crustal rock exhumation

2021 ◽  
Author(s):  
Lorenzo G. Candioti ◽  
Joshua D. Vaughan-Hammon ◽  
Thibault Duretz ◽  
Stefan M. Schmalholz
Keyword(s):  
Numerical Models ◽  
A Priori ◽  
Western Alps ◽  
Ultrahigh Pressure ◽  
Plate Motion ◽  
Crustal Rock ◽  
Plate Boundaries ◽  
Crustal Rocks ◽  
Natural Data ◽  
The Alps

<p>Ultrahigh-pressure (UHP) continental crustal rocks were first discovered in the Western Alps in 1984 and have since then been observed at many convergent plate boundaries worldwide. Unveiling the processes leading to the formation and exhumation of (U)HP metamorphic crustal rocks is key to understand the geodynamic evolution of orogens such as the Alps.</p><p> </p><p>Previous numerical studies investigating (U)HP rock exhumation in the Alps predicted deep (>80 km) subduction of crustal rocks and rapid buoyancy-driven exhumation of mainly incoherent (U)HP units, involving significant tectonic mixing forming so-called mélanges. Furthermore, these predictions often rely on excessive erosion or periods of divergent plate motion as important exhumation mechanism. Inconsistent with field observations and natural data, application of these models to the Western Alps was recently criticised.</p><p> </p><p>Here, we present models with continuous plate convergence, which exhibit local tectonic-driven upper plate extension enabling compressive- and buoyancy-driven exhumation of coherent (U)HP units along the subduction interface, involving feasible erosion.</p><p> </p><p>The two-dimensional petrological-thermo-mechanical numerical models presented here predict both subduction initiation and serpentinite channel formation without any a priori prescription of these two features. The (U)HP units are exhumed coherently, without significant internal deformation. Modelled pressure and temperature trajectories and exhumation velocities of selected crustal units agree with estimates for the Western Alps. The presented models support previous hypotheses of synconvergent exhumation, but do not rely on excessive erosion or divergent plate motion. Thus, our predictions provide new insights into processes leading to the exhumation of coherent (U)HP crustal units consistent with observations and natural data from the Western Alps.</p>

Formation and evolution of a subduction-related mélange: The example of the Rocca Canavese Thrust Sheets (Western Alps)

2019 ◽  
Vol 132 (3-4) ◽  
pp. 884-896 ◽  
Author(s):  
Manuel Roda ◽  
Michele Zucali ◽  
Alessandro Regorda ◽  
Maria Iole Spalla
Keyword(s):  
Numerical Model ◽  
Mantle Wedge ◽  
Western Alps ◽  
Metamorphic History ◽  
Orogenic Wedge ◽  
Thrust Sheets ◽  
History Of ◽  
Natural Data ◽  
Formation And Evolution ◽  
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.

Seismic imaging evidence that forearc underplating built the accretionary rock record of coastal North and South America

2019 ◽  
Vol 158 (1) ◽  
pp. 104-117 ◽  
Author(s):  
David W. Scholl
Keyword(s):  
South America ◽  
Subduction Zones ◽  
Seismic Imaging ◽  
Southern Chile ◽  
Late Palaeozoic ◽  
Aleutian Arc ◽  
Seaward Edge ◽  
Metamorphic Facies ◽  
North And South America ◽  
North And South

AbstractThe submerged forearcs of Pacific subduction zones of North and South America are underlain by a coastally exposed basement of late Palaeozoic to early Tertiary age. Basement is either an igneous massif of an accreted intra-oceanic arc or oceanic plateau (e.g. Cascadia(?), Colombia), an in situ formed arc massif (e.g. Aleutian Arc) or an exhumed accretionary complex of low and high P/T metamorphic facies of late Palaeozoic (e.g. southern Chile, Patagonia) and Mesozoic age (e.g. Alaska). Seismic studies at Pacific forearcs image frontal prisms of trench sediment accreted to the seaward edge of forearc basement. Frontal prisms tend to be narrow (10–40 km), weakly consolidated and volumetrically small (∼35–40 km3/km of trench). In contrast, deep seismic imaging of submerged forearcs commonly reveals large volumes (∼2000 km3/km of trench) of underplated material accreted at subsurface depths of ∼10–30 km to the base of forearc basement. Underplates have been imaged below the southern Chile, Ecuador–Colombia, north Cascade, Alaska, and possibly the eastern Aleutian forearcs. Deep underplates have also been observed below the Japan and New Zealand forearcs. Seismic imaging of northern and eastern Pacific forearcs supports the conclusion drawn from field and laboratory studies that exposed low and high P/T accretionary complexes accumulated in the subsurface at depths of 10–30 km. It seems significant that imaged underplated bodies are characteristic of modern well-sedimented subduction zones. It also seems likely that large Pacific-rim underplates store a significant fraction of sediment subducted in Cenozoic time.

scholarly journals Crustal reworking and hydration: insights from element zoning and oxygen isotopes of garnet in high-pressure rocks (Sesia Zone, Western Alps)

2020 ◽  
Vol 175 (11) ◽  
Author(s):  
Vho Alice ◽  
Rubatto Daniela ◽  
Lanari Pierre ◽  
Giuntoli Francesco ◽  
Regis Daniele ◽  
...  
Keyword(s):  
Oxygen Isotopes ◽  
Subduction Zones ◽  
Sea Water ◽  
Fluid Flows ◽  
Western Alps ◽  
Garnet Growth ◽  
Trace Element Geochemistry ◽  
Element Geochemistry ◽  
Chemical Zoning ◽  
Dehydration Reactions

Abstract Subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere. A better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial. In this study, major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the Sesia Zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin. Garnet shows a variety of textures requiring dissolution–precipitation processes in presence of fluids. In polycyclic metasediments, garnet preserves a partly resorbed core, related to pre-Alpine high-temperature/low-pressure metamorphism, and one or multiple rim generations, associated with Alpine subduction metamorphism. In eclogites, garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim. In metasediments, pre-Alpine garnet relics show δ18O values up to 5.3 ‰ higher than the Alpine rims, while no significant variation is observed among different Alpine garnet generations within each sample. This suggests that an extensive re-equilibration with an externally-derived fluid of distinct lower δ18O occurred before, or in correspondence to, the first Alpine garnet growth, while subsequent influxes of fluid had δ18O close to equilibrium. The observed shift in garnet δ18O is attributed to a possible combination of (1) interaction with sea-water derived fluids during pre-Alpine crustal extension and (2) fluids from dehydration reactions occurring during subduction of previously hydrated rocks, such as the serpentinised lithospheric mantle or hydrated portions of the basement.

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