Subduction and obduction processes: the fate of oceanic lithosphere revealed by blueschists, eclogites and ophiolites

Agard, Philippe; Soret, Mathieu; Bonnet, Guillaume; Ninkabou, Dia; Plunder, Alexis; Prigent, Christophe; Yamato, Philippe

doi:10.1002/essoar.10510507.1

Cited by 4 publications

(4 citation statements)

References 67 publications

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“…Magmatic rocks with SSZ affinity (expected in ophiolites formed during subduction initiation; Agard et al., 2023; Stern et al., 2012), exposed in the Southern Coast Ranges (McLaughlin et al., 1988) only occur as blocks within the TCSM and have middle Jurassic ages (Shervais, Murchey, et al., 2005). The SSZ affinities of high‐grade blocks in the Franciscan (this study; Ghatak et al., 2012; Saha et al., 2005; Wakabayashi et al., 2010) needs however to be explained, as SSZ rocks are initially formed above a subduction zone, then metamorphosed at high‐grade in a subduction zone.…”

Section: Discussionmentioning

confidence: 99%

Seamount Subduction Dynamics and Long‐Term Evolution of the Franciscan Active Margin

Bonnet,

Apen,

Soret

et al. 2024

Tectonics

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The Snow Mountain Volcanic Complex (SMVC; northern California, USA) is a well‐preserved example of a coherently‐exhumed subducted seamount. This study reappraises the genesis and evolution of this complex and surrounding units through detailed field, petro‐structural and geochronological analyses. This work demonstrates that the SMVC (a) erupted at ∼166 Ma as a hotspot volcano on the Farallon Plate, (b) entered the Franciscan subduction trench at ∼118 Ma, and (c) was subsequently subducted to a depth of ∼20 km (within the seismogenic zone), as shown by local blueschist‐facies assemblages formed at 0.6 GPa, 240°C. Transient subduction interfaces are preserved above, within, and below the SMVC, making it an exceptional target to study seamount subduction dynamics. Like other seamounts, the subduction‐related deformation was mainly accommodated along kilometer‐scale internal thrust zones lubricated by serpentinite/metasediments, and within centimeter‐thick crack‐seal veins recording pulsed fluid flow near peak metamorphism. No unequivocal proof of seismic activity was found. The integration of other seamounts (some potentially belonging to a former seamount chain) in the Franciscan Complex suggests that exhumed seamounts are more abundant than previously thought. Moreover, pressure‐temperature‐time estimates of subduction metamorphism for the surrounding units, combined with previous work constrain the thermal maturation of the subduction zone through time and the in‐sequence emplacement of the SMVC. Rapid changes in age of the subducted oceanic plate when subducted additionally hint to the subduction of large‐offset transform faults on the former Farallon plate. Such a process might have been linked to changes in accretion dynamics and magmatic flare‐ups in the arc.

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Section: Discussionmentioning

confidence: 99%

Seamount Subduction Dynamics and Long‐Term Evolution of the Franciscan Active Margin

Bonnet,

Apen,

Soret

et al. 2024

Tectonics

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“…Rocks metamorphosed in subduction zones are thought to commonly undergo a rapid increase of pressure, accompanied by an increase of temperature, albeit only limited (typically in the order of 8°C·km −1 , e.g., Agard et al, 2022; Ernst, 2001; Miyashiro, 1972; Penniston‐Dorland et al, 2015). However, the apparent absence of a temperature increase during compression for our sample seems incompatible with this classic view and may question the tectonic significance of (U)HP metamorphism.…”

Section: Discussionmentioning

confidence: 99%

Isothermal compression of an eclogite from the Western Gneiss Region (Norway)

Simón

Pitra

Yamato

et al. 2022

Journal Metamorphic Geology

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In the Western Gneiss Region in Norway, mafic eclogites form lenses within granitoid orthogneiss and contain the best record of the pressure and temperature evolution of this ultrahigh‐pressure (UHP) terrane. Their exhumation from the UHP conditions has been extensively studied, but their prograde evolution has been rarely quantified although it represents a key constraint for the tectonic history of this area. This study focused on a well‐preserved phengite‐bearing eclogite sample from the Nordfjord region. The sample was investigated using phase‐equilibrium modelling, trace‐element analyses of garnet, trace‐ and major‐element thermobarometry and quartz‐in‐garnet barometry by Raman spectroscopy. Inclusions in garnet core point to crystallization conditions in the amphibolite facies at 510–600°C and 11–16 kbar, whereas chemical zoning in garnet suggests growth during isothermal compression up to the peak pressure of 28 kbar at 600°C, followed by near‐isobaric heating to 660–680°C. Near‐isothermal decompression to 10–14 kbar is recorded in fine‐grained clinopyroxene–amphibole–plagioclase symplectites. The absence of a temperature increase during compression seems incompatible with the classic view of crystallization along a geothermal gradient in a subduction zone and may question the tectonic significance of eclogite facies metamorphism. Two end‐member tectonic scenarios are proposed to explain such an isothermal compression: Either (1) the mafic rocks were originally at depth within the lower crust and were consecutively buried along the isothermal portion of the subducting slab or (2) the mafic rocks recorded up to 14 kbar of tectonic overpressure at constant depth and temperature during the collisional stage of the orogeny.

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“…By contrast with these continental eclogites, the oceanic eclogites and related rocks appear to have been returned from maximum pressures of less than ∼ 2.8 GPa (Agard et al, 2018). Processes related to the subduction and obduction of ocean lithosphere during the Phanerozoic were reviewed earlier this year by Agard et al (2023), and the processes by which subduction zones formed during the Cenozoic, from initiation to self-sustained subduction, were reviewed recently by Lallemand and Arcay (2021); consequently, these processes are not considered in detail herein. Lastly, I include only brief comments about xenolithic eclogites, where essential to the discussion, because they also are the subject of a recently published review (Aulbach and Smart, 2023).…”

Section: Introductionmentioning

confidence: 99%

Some thoughts about eclogites and related rocks

Brown

2023

Eur. J. Mineral.

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Abstract. The past 40 years have been a golden age for eclogite studies, supported by an ever wider range of instrumentation and enhanced computational capabilities, linked with ongoing developments in thermobarometry and geochronology. During this time, we have made robust estimates of pressure–temperature (P–T) conditions; determined ages related to the prograde, metamorphic peak and retrograde stages; and calculated time-integrated rates of cooling and exhumation for eclogites and related rocks, including blueschists, from orogenic belts worldwide. Improvements to single mineral thermometers and new developments in elastic barometry using inclusions of one mineral in another (e.g. quartz and/or zircon in garnet), coupled with ongoing innovations in petrochronology and diffusion modelling, presage a new age for eclogite studies in which detailed quantification of metamorphic conditions and timescales will be linked to an improved understanding of processes at all scales. Since the turn of the century, numerical modelling of subduction zone and rock exhumation processes has become increasingly important. As a result, subduction and exhumation are quite well understood, but the volume of continental crust subducted to and returned from mantle conditions and the amount lost to the mantle are largely unknown. We have generated sufficient data to investigate the spatiotemporal distribution of metamorphism and secular change but not without controversy in relation to the rare occurrence of orogenic eclogites and the absence of blueschists prior to the late Neoproterozoic and the emergence of plate tectonics on Earth. Since the turn of the century, the assumption that metamorphic pressure is lithostatic has come under increasing scrutiny. Whether local variations in stress extrapolate to the crustal scale and, if so, whether the magnitude of the calculated deviations from lithostatic pressure can be generated and sustained in mechanically heterogeneous rock units remains contentious. Could the paradigm of subduction of continental lithosphere to mantle depths be simply an artefact of the lithostatic assumption? Fluid cycling in subduction zones and understanding the role of fluids in the generation of intermediate-depth earthquakes remain important topics of current research. Dry (H2O-absent) conditions are unlikely around the peak of ultrahigh-pressure (UHP) metamorphism or during exhumation, due to dehydroxylation of nominally anhydrous minerals and breakdown of hydrous minerals at P–T conditions in the realm of supercritical fluid and hydrous melt. Indeed, the presence of melt may be necessary to facilitate the exhumation of HP and UHP tectonometamorphic rock units. Finally, our ability to interrogate inclusions in superdeep diamonds should lead to a better understanding of how the deep interior and surface are linked in the context of Earth as a fully coupled system.

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Subduction and obduction processes: the fate of oceanic lithosphere revealed by blueschists, eclogites and ophiolites

Cited by 4 publications

References 67 publications

Seamount Subduction Dynamics and Long‐Term Evolution of the Franciscan Active Margin

Seamount Subduction Dynamics and Long‐Term Evolution of the Franciscan Active Margin

Isothermal compression of an eclogite from the Western Gneiss Region (Norway)

Some thoughts about eclogites and related rocks

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