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| Titel |
Flat Slab Subduction, Trench Suction, and Craton Destruction: Comparison of the North China, Wyoming, and Brazilian Cratons |
| VerfasserIn |
Timothy Kusky, Brian Windley  , Lu Wang, Zhensheng Wang, Xiaoyong Li, Peimin Zhu |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104084
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| Publikation (Nr.) |
 EGU/EGU2015-3507.pdf |
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| Zusammenfassung |
| We define and test a unifying plate tectonic driving mechanism that explains the
characteristics of the destruction of cratonic lithospheric roots. We document and model the
relationships between flat slab subduction, trench suction, and craton destruction, using
examples from the North China and Wyoming cratons, each of which locally lost
approximately 100 km of their lithospheric roots in the Cretaceous and which show
spatio-temporal relationships with episodes of flat slab subduction in the mantle transition
zone associated with deep mantle hydration, coupled with slab rollback and concomitant
influx of fertile mantle material to accommodate the space created by slab rollback. A similar
process has more recently operated along the western side of the Brazilian craton where it is
thrust beneath the thickened crust of the Andes in an area of trench rollback. The importance
of the mutual interaction between these processes for destruction of cratonic roots may be
greater than currently perceived. Together with the other processes of subduction
erosion and arc subduction, larger amounts of continental lithosphere may have been
subducted or otherwise returned to the sub-lithospheric mantle than previously
appreciated.
When oceanic lithosphere subducts, it hydrates the upper mantle beneath an arc from
well-known dehydration reactions. However, some hydrous phases (e.g., Phase A, Phase E,
and –γt and β-phase olivine) are stable to much greater depths and dehydrate even when a
slab is in the mantle transition zone. It is estimated that 40% of the water subducted
in hydrated oceanic crust, mantle, sediments, and subducted continental material
reaches the mantle transition zone between 410 and 660 km. For instance lawsonite
may contain up to 11% water, and is stable up to 11 GPa or about 300 km and
serpentinites can contain up to 13% water and are stable up to 7 GPa, and after
conversion to denser hydrous phases such as β-phase olivine they can be stable up to
50 GPa, well past the mantle transition zone. With increasing temperature (i.e.,
more time in the transition zone for deep flat slabs) these phases decompose to less
hydrous wadsleyite and ringwoodite with 2.2-3.3 wt % water, releasing water to
the deep mantle, which rises and hydrates the overlying mantle. During flat slab
subduction dehydration reactions therefore add water to the overlying mantle wedge.
As the subducting slabs roll back, they suck in mantle material to infill the void
space created by the slab roll back, and this fertile mantle becomes hydrated. The
roll-back causes concomitant lithospheric thinning of the overlying craton so the
flux of newly hydrated mantle material inevitably rises causing adiabatic melting,
generating new magmas that gradually destroy the roots of the overlying craton through
melt-peridotite reactions. Calculated fluxes of new mantle material beneath cratons that
have lost their roots range from 2.7 trillion to 70 million cubic kilometers, which is
sufficient to generate enough melt to completely replace the affected parts of the
destroyed cratons. Cratonic lithosphere may be destroyed in massive quantities through
this mechanism, warranting a re-evaluation of continental growth rates with time. |
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