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TitleMafic high-pressure rocks are preferentially exhumed from warm subduction settings
Authorvan Keken, P E; Wada, I; Abers, G A; Hacker, B R; Wang, KORCID logo
SourceGeochemistry, Geophysics, Geosystems (G3) vol. 19, 2018., Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20180130
PublisherAmerican Geophysical Union (AGU)
Mediapaper; on-line; digital
File formatpdf; html
Subjectstectonics; igneous and metamorphic petrology; geophysics; crustal studies; modelling; models; oceanic crust; oceanic lithosphere; tectonic environments; tectonic history; subduction zones; downgoing slab; shear heating; mafic rocks; eclogites; pressure-temperature conditions; thermal analyses; heat flow; fluid dynamics; fluid flow; seismic waves; p waves
Illustrationsmodels; phase diagrams; tables; profiles; plots
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2018 07 31
AbstractThe oceanic crust that enters a subduction zone is generally recycled to great depth. In rare and punctuated episodes, however, blueschists and eclogites derived from subducted oceanic crust are exhumed. Compilations of the maximum pressure-temperature conditions in exhumed rocks indicate significantly warmer conditions than those predicted by thermal models. This could be due to preferential exhumation of rocks from hotter conditions that promote greater fluid productivity, mobility, and buoyancy. Alternatively, the models might underestimate the forearc temperatures by neglecting certain heat sources. We compare two sets of global subduction zone thermal models to the rock record. We find that the addition of reasonable amounts of shear heating leads to less than 50ºC heating of the oceanic crust compared to models that exclude this heat source. Models for young oceanic lithosphere tend to agree well with the rock record. We test the hypothesis that certain heat sources may be missing in the models by constructing a global set of models that have high arbitrary heat sources in the forearc. Models that satisfy the rock record in this manner, however, fail to satisfy independent geophysical and geochemical observations. These combined tests show that the average exhumed mafic rock record is systematically warmer than the average thermal structure of mature modern subduction zones. We infer that typical blueschists and eclogites were exhumed preferentially under relatively warm conditions that occurred due to the subduction of young oceanic lithosphere or during the warmer initial stages of subduction.
Summary(Plain Language Summary, not published)
Temperature and petrology control the generation of earthquakes within the subducted oceanic plate (the slab). The most direct way of studying the in situ thermo-petrologic condition is to examine rocks that belonged to the slab but later exhumed to the surface. A controversy recently arose because in situ temperatures inferred from exhumed slab rocks appeared to be systematically higher then those predicted by theoretical models. This paper resolves the controversy by explaining that rocks of young and warm slabs are preferentially exhumed, biasing the temperature inferences. The currently used theoretical models are shown to be compatible with a wide range of geophysical and geological observations.

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