Exhumation of the UHP Tso Morari eclogite as a diapir rising through the mantle wedge
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continental collision
This chapter presents a series of two-dimensional models of subduction are presented to investigate parameters controlling slab-induced mantle weakening and increased mantle flow rates in a simplified tectonic setting. Models using the composite viscosity formulation produce a zone of subduction-induced mantle weakening that results in reduced viscous support of the slab. In all cases, the magnitude of induced mantle flow is larger in the models using the composite viscosity formulation. The models suggest that the slab-driven mantle weakening and localized increased mantle flow phenomenon occurs in most subduction zones, and is primarily modulated by the mantle deformation mechanism, slab strength, and slab buoyancy, as suggested in 3D regional and global models. The results are consistent with three-dimensional models of subduction with a composite rheology and show that the rapid mantle flow predicted by these models is not due to the proximity of the model sidewalls.
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Reconstructing tectonic histories involving continental collision, subduction, and exhumation at plate-tectonic rates of ∼1 cm/year, requires precise U–Pb zircon geochronology. The Western Gneiss Region has exceptional exposures of high-pressure (HP) and ultra-high-pressure (UHP) rocks. The strategy adopted here involved sampling eclogite and associated late unstrained pegmatites to acquire the time of eclogite crystallization and subsequent exhumation, respectively. The oldest eclogite sampled is 415 ± 1 Ma from layered, probably UHP eclogite at Tevik, Averøya, also with a garnet–hornblende assemblage at 410 ± 1 Ma. The Flem Gabbro eclogite margin, with implied UHP conditions, is 410 ± 2 Ma. Hornblende eclogite at Seth, Lepsøya, never at UHP, is 412 ± 2 Ma. These compare to Devonian ages of 401 ± 1 Ma for overgrowths on Proterozoic baddeleyite in Selnes Gabbro, 402 ± 2 Ma for coesite eclogite at Hareidlandet, 405–400 Ma for coesite eclogite at Flatraket, and 405 ± 2 Ma for near-UHP eclogite at Hjelmelandsdalen. The 415 Ma eclogite at Tevik compares to granitic pegmatite in the same outcrop at 395.2 ± 1.3 Ma and to pegmatite in eclogite at Aspøya at 395.3 ± 2 Ma. The 410 Ma age at Flem compares to nearby pegmatite in eclogite at 396 ± 4 Ma. Collectively, these results imply 14–20 million years between deep eclogite crystallization at ∼130 km and return to amphibolite-facies conditions at ∼30 km, with crystallization of locally derived granitoid melts. Nearby garnet-pyroxenite records older ages (∼430) and greater depths (∼200 km), but on similar exhumation paths at ∼0.4–0.7 cm/year.
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In this study, we calculate timescales for the growth of gravitational instabilities forming in the sediment layer on the downgoing slab at subduction zones. Subducted metasediments are buoyant with respect to the overlying mantle and may form diapirs that detach from the slab and rise upwards into the mantle wedge. We use a particle-in-cell, finite-difference method to calculate growth rates for instabilities forming within a buoyant, wet-quartz metasediment layer underlying a dense mantle half-space composed of wet olivine. These growth rates are used to determine where sediment diapirs initiate and detach from the slab over a range of subduction zone thermal structures. We find that, given a sufficient layer thickness (200–800 m, depending on slab-surface and mantle-wedge temperatures), sediment diapirs begin to grow rapidly at depths of ∼80 km and detach from the slab within 1–3 Myr at temperatures ≤900 °C and at depths roughly corresponding to the location of the slab beneath the arc. Diapir growth is most sensitive to absolute slab temperature, however it is also affected by the viscosity ratio between the sediment layer and the mantle wedge and the length-scale over which viscosity decays above the slab. These secondary affects are most pronounced in colder subduction systems with old slabs and faster subduction rates. For a broad range of subduction zone thermal conditions, we find that diapirs can efficiently transport sediments into the mantle wedge, where they would melt and be incorporated into arc magmas. Thus, we conclude that sediment diapirism is a common feature of many subduction zones, providing a potential explanation for the 'sediment signature' in the chemistry of arc magmas.
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Eclogite facies rocks from the Dabie Mountains are mainly composed of garnet peridotite, eclogite, garnet phengite schist, eclogitic gneiss, garnet jadeite quartzite and eclogitic marble, which often occurs within granitic gneisses. The paragenetic sequence in eclogite indicates that eclogite facies rocks experienced progressive metamorphism from epidote amphibolite facies to coesite eclogite facies and retrograde metamorphism from amphibole eclogite facies through epidote amphibolite facies to greenschist facies. Granitic gneisses and metamorphosed volcanic sedimentary rocks were not subjected to ultrahigh pressure metamorphism, but they together with eclogite facies rocks underwent the same epidote amphibolite facies metamorphic event. This may imply that the two rock units were juxtaposed at crustal levels during the late continental collision in the Dabie Mountains.
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<p>Continental collision succeeds long term subduction of oceanic lithosphere into the earth's mantle whereby the negative buoyancy of the downgoing oceanic lithosphere (slab) provides the principal driving force for plate motions. Previous studies have shown that subduction-induced mantle flow could drive overriding plate shortening and orogenesis, and the arrival of the positively buoyant lithosphere at the trench affects the dynamics of the overriding plate and plate motions. The subsequent slab detachment at the subducted continent-ocean margin removes the driving force in the system and eventuates in cessation of subduction (Cloos, 1993)&#160; and plate convergence. The India-Eurasia subduction-collision system has multiple inferred slab break-off episodes (Replumaz et al., 2010), yet convergence is still ongoing. Here, we present 2D-cartesian buoyancy-driven numerical models of continental collision after subduction of a long oceanic plate (~6000 km) in a whole mantle reservoir (2880km), investigating the dynamics of such systems in the presence of detached slabs. These models&#8217; wide aspect ratio (6:1) allows for exploring deep subduction of oceanic slabs and detached slab(s), approximately at the centre of the domain, thereby minimising the effect of free slip sidewalls on obtained slab morphology in the mantle and associated mantle flow. Our results indicate that poloidal mantle flow induced by the sinking of the detached slab sustain long term convergence in collisional settings. Although 2D models lack the 3D components of mantle flow, these models can be used to understand the dynamics of the centre of >4000km wide subductions zones and facilitate interpretation in light of tomographic and plate reconstruction studies.</p><p>&#160;</p><p>References:</p><p>Cloos, M. (1993). Lithospheric buoyancy and collisional orogenesis: Subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts. <em>Geological Society of America Bulletin</em>, <em>105</em>(6), 715-737.</p><p>Replumaz, A., Negredo, A. M., Guillot, S., & Villase&#241;or, A. (2010). Multiple episodes of continental subduction during India/Asia convergence: Insight from seismic tomography and tectonic reconstruction. <em>Tectonophysics</em>, <em>483</em>(1-2), 125-134.</p>
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Abstract The Flatraket Complex, a granulite facies low strain enclave within the Western Gneiss Region, provides an excellent example of metastability of plagioclase‐bearing assemblages under eclogite facies conditions. Coesite eclogites are found <200 m structurally above and <1 km below the Flatraket Complex, and are separated from it by amphibolite facies gneisses related to pervasive late‐orogenic deformation and overprinting. Granulites within the Flatraket Complex equilibrated at 9–11 kbar, 700–800°C. These predate eclogite facies metamorphism and were preserved metastably in dry undeformed zones under eclogite facies conditions. Approximately 5% of the complex was transformed to eclogite in zones of fluid infiltration and deformation, which were focused along lithological contacts in the margin of the complex. Eclogitisation proceeded by domainal re‐equilibration and disequilibrium breakdown of plagioclase by predominantly hydration reactions. Both hydration and anhydrous plagioclase breakdown reactions were kinetically linked to input of fluid. More pervasive hydration of the complex occurred during exhumation, with fluid infiltration linked to dehydration of external gneisses. Eclogite facies shear zones within the complex equilibrated at 20–23 kbar, 650–800°C, consistent with the lack of coesite and with the equilibration conditions of external HP eclogites. If the complex experienced pressures equivalent to those of nearby coesite eclogites (> 28 kbar), unprecedented metastability of plagioclase and quartz is implied. Alternatively, a tectonic break exists between the Flatraket Complex and UHP eclogites, supporting the concept of a tectonic boundary to the UHP zone of the Western Gneiss Region. The distribution of eclogite and amphibolite facies metamorphic overprints demonstrates that the reactivity of the crust during deep burial and exhumation is strongly controlled by fluid availability, and is a function of the protolith.
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