Zagros blueschists: Episodic underplating and long-lived cooling of a subduction zone

Agard, Philippe; Glodny, Johannes; Omrani, Jafar; Oncken, Onno

doi:10.1016/j.epsl.2016.03.017

Cited by 70 publications

(118 citation statements)

References 52 publications

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“…A slightly younger episode of deeper recovery from depths of 50-55 km (at 70-80 Ma; Seghin unit: Agard et al, 2006;Angiboust et al, 2016;Sapi-Shergol: Groppo et al, 2016) is recorded from the same subduction system. Interestingly, in the nearby Sistan subduction zone, a pulse of rock recovery along~150 km is documented at 85 Ma (Eastern Iran; Broecker et al, 2013;Angiboust et al, 2013;Bonnet et al, 2018): two types of mafic rocks returned from depths of 30-40 and~80 km are found as m-to 10 m-scale blocks in a serpentinite mélange.…”

Section: Evidence That Transient Changes In Mechanical Coupling Contrmentioning

confidence: 84%

“…The punctuated exhumation record (Agard et al, 2009;Angiboust et al, 2016;Guillot et al, 2009) shows that rock recovery is intrinsically episodic: subducted rocks are returned over relatively short-lived episodes representing only a fraction of subduction lifetime (b20%, typically during a few Myr; Fig. 3a), and each time across only a small part of the subduction zone length (e.g.,b5% for the Neotethys or S. America).…”

Section: Episodic Rock Recovery From Specific Subduction Depthsmentioning

confidence: 99%

“…Some of the mélanges, for example, correspond to strongly deformed tectonic slices, as suggested by the progressive dismantling observed in sedimentary series or SB mixtures in shallow environments (e.g., Shimanto, SWJapan; Chrystalls Beach, N. Zealand). This is the case in SW Zagros, where metabasalt ridges (100 m thick by 500 m long) metamorphosed at~55 km depth are now 'floating' in a serpentinite matrix (Seghin unit; Agard et al, 2006;Angiboust et al, 2016).…”

Section: Data Selection and Rock Typesmentioning

confidence: 99%

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The subduction plate interface: rock record and mechanical coupling (from long to short timescales)

Agard

Plunder

Angiboust

et al. 2018

Lithos

206

294

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Short-and long-term processes at or close to the subduction plate interface (e.g.,mineral transformations, fluid release, seismicity and more generally deformation) might be more closely related than previously thought. Increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle. This contribution uses rocks recovered (episodically) from subduction zones, together with insights from thermomechanical modelling, to provide a new dynamic vision of the nature, structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones (e.g.,coupling inferred from geophysical monitoring) and fossil ones (e.g., coupling required to detach and recover subducted slab fragments). Based on critical observations and an exhaustive compilation of worldwide subducted oceanic units (for which the presence near the plate interface, rock types, pressure, temperature, T/P gradients, thickness and timing of detachment can be assessed), the present study demonstrates how long-term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery. Critical assessment of rock T/P characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to~2.8 GPa. Rock clusters are identified at depths of 30, 55-60 and 80 km, with some differences between rock types. Data also reveal a first-order evolution with subduction cooling (in the first~5 Myr), which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling, after which subduction is lubricated and mostly inhibits rock recovery. This contribution places bounds on the plate interface constitution, regular thickness (b300 m; i.e. where/when there is no detachment), changing geometry and effective viscosity. The concept of 'coupled thickness' is used here to capture subduction interface dynamics, notably during episodes of strong mechanical coupling, and to link long-and short-term deformation. Mechanical coupling depends on mantle wedge rheology, viscosity contrasts and initial structures (e.g.,heterogeneous lithosphere, existence of décollement horizons, extent of hydration, asperities) but also on boundary conditions (convergence rates, kinematics), and therefore differs for warm and cold subduction settings. Although most present-day subduction zone segments (both along strike and downdip) are likely below the detachment threshold, we propose that the most favorable location for detachment corresponds to the spatial transition between coupled and decoupled areas. Effective strain localization involves dissolution-precipitation and dislocation creep but also possibly brittle fractures and earthquakes, even at intermediate depths.

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Section: Evidence That Transient Changes In Mechanical Coupling Contrmentioning

confidence: 84%

Section: Episodic Rock Recovery From Specific Subduction Depthsmentioning

confidence: 99%

Section: Data Selection and Rock Typesmentioning

confidence: 99%

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The subduction plate interface: rock record and mechanical coupling (from long to short timescales)

Agard

Plunder

Angiboust

et al. 2018

Lithos

206

294

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“…Therefore, this type of correlation suggests either a prolonged deformation‐ or fluid‐related recrystallization process (i.e. apparently older white mica grains crystallized during the early metamorphic stages) or partial isotopic inheritance from the precursor rock (Angiboust, Agard, Glodny, Omrani, & Oncken, ; Angiboust, Glodny, Oncken, & Chopen, ). In consequence, the smallest mica crystals in a given rock will commonly be the most readily affected by dynamic recrystallisation processes or late, fluid‐limited recrystallization, and show isotopic signatures most closely associated to the last event of ductile deformation or fluid activity.…”

Section: Methodsmentioning

confidence: 99%

Preservation of high‐P rocks coupled to rock composition and the absence of metamorphic fluids

Skelton

Peillod

Glodny

et al. 2019

Journal Metamorphic Geology

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Eclogites, blueschists and greenschists are found in close proximity to one another along a 1-km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2-1.9 GPa and 410-530°C followed, at 43-38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5-2.1 GPa and 520-580°C. We explain co-existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10-100-m and veins on a scale of 1-10-cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11-16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high-P nappe stack consisting of belts in which high-P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension. K E Y W O R D Sbulk composition, Cyclades Blueschist Unit, HP-LT metamorphism, metamorphic fluids, Syros

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“…Here, we report the discovery of a unique, fully exposed subducted seamount, namely, the Siah Kuh (SK) massif, and we present its structure and evolution, which we detailed through extensive field and petrological data. This massif crops out within ophiolite fragments of the Neotethys Ocean subducted beneath Eurasia (Agard et al, 2011) in the easternmost portion of the Zagros Mountains, next to and below oceanic blueschists metamorphosed during the Late Cretaceous (Angiboust et al, 2016). This massif rises from Quaternary sediment infill as an 18 × 12-km-wide and ≥1.5-km-high feature (Figs.…”

Section: An Exceptionally Preserved Fossil Seamountmentioning

confidence: 95%

No large earthquakes in fully exposed subducted seamount

Bonnet

Agard

Angiboust

et al. 2019

Geology

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Bathymetric highs on the ocean floor ultimately sink into highly seismic subduction zones, raising vigorous debates on their potential to trigger or arrest large earthquakes (M w > 7.5). Many geophysical and seismological studies addressing this problem meet penetration and/or resolution issues and deal with only the most recent earthquakes. We herein present the missing piece of the puzzle with the time-integrated field and petrographic record of a unique, almost intact subducted seamount cropping out along a fossil subduction interface. We document seamount buildup and subduction down to ~30 km, and we show that this seamount did not behave as a large earthquake asperity and may have acted as a barrier.

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Zagros blueschists: Episodic underplating and long-lived cooling of a subduction zone

Cited by 70 publications

References 52 publications

The subduction plate interface: rock record and mechanical coupling (from long to short timescales)

The subduction plate interface: rock record and mechanical coupling (from long to short timescales)

Preservation of high‐P rocks coupled to rock composition and the absence of metamorphic fluids

No large earthquakes in fully exposed subducted seamount

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