Formation and exhumation of ultra-high-pressure rocks during continental collision: Role of detachment in the subduction channel

Abstract Sandy trench-fill sediments at accretionary margins are commonly scraped off at the frontal wedge and rarely subducted to the depth of high-pressure (HP) metamorphism. However, some ancient exhumed accretionary complexes are associated with high-pressure–low-temperature (HP-LT) metamorphic rocks, such as psammitic schists, which are derived from sandy trench-fill sediments. This study used sandbox analogue experiments to investigate the role of seafloor topography in the transport of trench-fill sediments to depth during subduction. We conducted two different types of experiments, with or without a rigid topographic high (representing a seamount). We used an undeformable backstop that was unfixed to the side wall of the apparatus to allow a seamount to be subducted beneath the overriding plate. In experiments without a seamount, progressive thickening of the accretionary wedge pushed the backstop down, leading to a stepping down of the décollement, narrowing of the subduction channel, and underplating of the wedge with subducting sediment. In contrast, in experiments with a topographic high, the subduction of the topographic high raised the backstop, leading to a stepping up of the décollement and widening of the subduction channel. These results suggest that the subduction of stiff topographic relief beneath an inflexible overriding plate might enable trench-fill sediments to be deeply subducted and to become the protoliths of HP-LT metamorphic rocks.

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Modelling tectonic styles and ultra-high pressure (UHP) rock exhumation during the transition from oceanic subduction to continental collision

Earth and Planetary Science Letters ◽

10.1016/j.epsl.2007.11.025 ◽

2008 ◽

Vol 267 (1-2) ◽

pp. 129-145 ◽

Cited By ~ 159

Author(s):

C.J. Warren ◽

C. Beaumont ◽

R.A. Jamieson

Keyword(s):

High Pressure ◽

Continental Collision ◽

Ultra High Pressure

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The role of buoyancy in the fate of ultra-high-pressure eclogite

Scientific Reports ◽

10.1038/s41598-019-56475-y ◽

2019 ◽

Vol 9 (1) ◽

Cited By ~ 6

Author(s):

Timothy Chapman ◽

Geoffrey L. Clarke ◽

Nathan R. Daczko

Keyword(s):

High Pressure ◽

Continental Crust ◽

Upper Mantle ◽

Crustal Rocks ◽

Ultra High Pressure ◽

Rock Types ◽

Mineral Assemblages ◽

Facies Metamorphism ◽

Point Of No Return

AbstractEclogite facies metamorphism of the lithosphere forms dense mineral assemblages at high- (1.6–2.4 GPa) to ultra-high-pressure (>2.4–12 GPa: UHP) conditions that drive slab-pull forces during its subduction to lower mantle conditions. The relative densities of mantle and lithospheric components places theoretical limits for the re-exposure, and peak conditions expected, of subducted lithosphere. Exposed eclogite terranes dominated by rock denser than the upper mantle are problematic, as are interpretations of UHP conditions in buoyant rock types. Their subduction and exposure require processes that overcame predicted buoyancy forces. Phase equilibria modelling indicates that depths of 50–60 km (P = 1.4–1.8 GPa) and 85–160 km (P = 2.6–5 GPa) present thresholds for pull force in end-member oceanic and continental lithosphere, respectively. The point of no-return for subducted silicic crustal rocks is between 160 and 260 km (P = 5.5–9 GPa), limiting the likelihood of stishovite–wadeite–K-hollandite-bearing assemblages being preserved in equilibrated assemblages. The subduction of buoyant continental crust requires its anchoring to denser mafic and ultramafic lithosphere in ratios below 1:3 for the continental crust to reach depths of UHP conditions (85–160 km), and above 2:3 for it to reach extreme depths (>160 km). The buoyant escape of continental crust following its detachment from an anchored situation could carry minor proportions of other rocks that are denser than the upper mantle. However, instances of rocks returned from well-beyond these limits require exceptional exhumation dynamics, plausibly coupled with the effects of incomplete metamorphism to retain less dense low-P phases.

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U–Pb zircon geochronology of eclogites from the Scandian Orogen, northern Western Gneiss Region, Norway: 14–20 million years between eclogite crystallization and return to amphibolite-facies conditionsThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

Canadian Journal of Earth Sciences ◽

10.1139/e10-076 ◽

2011 ◽

Vol 48 (2) ◽

pp. 441-472 ◽

Cited By ~ 45

Author(s):

Thomas E. Krogh ◽

Sandra L. Kamo ◽

Peter Robinson ◽

Michael P. Terry ◽

Kim Kwok

Keyword(s):

High Pressure ◽

Continental Collision ◽

Amphibolite Facies ◽

Granitic Pegmatite ◽

Zircon Geochronology ◽

Plate Tectonic ◽

Special Issue ◽

Western Gneiss Region ◽

Garnet Pyroxenite ◽

Ultra High Pressure

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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Late Orogenic Heating of (Ultra)High Pressure Rocks: Slab Rollback vs. Slab Breakoff

Geosciences ◽

10.3390/geosciences9120499 ◽

2019 ◽

Vol 9 (12) ◽

pp. 499 ◽

Cited By ~ 5

Author(s):

Elena Sizova ◽

Christoph Hauzenberger ◽

Harald Fritz ◽

Shah Wali Faryad ◽

Taras Gerya

Keyword(s):

High Pressure ◽

Continental Collision ◽

Crustal Material ◽

Ultra High Pressure ◽

Slab Rollback ◽

Slab Breakoff ◽

T Path ◽

Slab Bending ◽

Lower Crustal ◽

Collisional Orogens

Some (ultra)high-pressure metamorphic rocks that formed during continental collision preserve relict minerals, indicating a two-stage evolution: first, subduction to mantle depths and exhumation to the lower-crustal level (with simultaneous cooling), followed by intensive heating that can be characterized by a β-shaped pressure–temperature–time (P–T–t) path. Based on a two-dimensional (2D) coupled petrological–thermomechanical tectono-magmatic numerical model, we propose a possible sequence of tectonic stages that could lead to these overprinting metamorphic events along an orogenic β-shaped P–T–t path: the subduction and exhumation of continental crust, followed by slab retreat that leads to extension and subsequent asthenospheric upwelling. During the last stage, the exhumed crustal material at the crust–mantle boundary undergoes heating from the underlying hot asthenospheric mantle. This slab rollback scenario is further compared numerically with the classical continental collision scenario associated with slab breakoff, which is often used to explain the late heating impulse in the collisional orogens. The mantle upwelling occurring in the experiments with slab breakoff, which is responsible for the heating of the exhumed crustal material, is not related to the slab breakoff but can be caused either by slab bending before slab breakoff or by post-breakoff exhumation of the subducted crust. Our numerical modeling predictions align well with a variety of orogenic P–T–t paths that have been reported from many Phanerozoic collisional orogens, such as the Variscan Bohemian Massif, the Triassic Dabie Shan, the Cenozoic Northwest Himalaya, and some metamorphic complexes in the Alps.

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