저작근 근전도에 관한 정상교합자와 ii급 부정교합자의 비교 연구

Data Reduction And Error Analysis For The Physical Sciences

[1] A recent global compilation of the thermal structure of subduction zones is used to predict the metamorphic facies and H 2 O content of downgoing slabs. Our calculations indicate that mineralogically bound water can pass efficiently through old and fast subduction zones (e.g., in the western Pacific), whereas hot subduction zones such as Cascadia see nearly complete dehydration of the subducting slab. The top of the slab is sufficiently hot in all subduction zones that the upper crust, including sediments and volcanic rocks, is predicted to dehydrate significantly. The degree and depth of dehydration in the deeper crust and uppermost mantle are highly diverse and depend strongly on composition (gabbro versus peridotite) and local pressure and temperature conditions. The upper mantle dehydrates at intermediate depths in all but the coldest subduction zones. On average, about one third of the bound H 2 O subducted globally in slabs reaches 240 km depth, carried principally and roughly equally in the gabbro and peridotite sections. The predicted global flux of H 2 O to the deep mantle is smaller than previous estimates but still amounts to about one ocean mass over the age of the Earth. At this rate, the overall mantle H 2 O content increases by 0.037 wt % (370 ppm) over the age of the Earth. This is qualitatively consistent with inferred H 2 O concentrations in the Earth's mantle assuming that secular cooling of the Earth has increased the efficiency of volatile recycling over time.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2010JB007922

Subduction factory: 4. Depth-dependent flux of H2O from subducting slabs worldwide

International Tables for X-Ray Crystallography (Published for the International Union of Crystallography.) Vol. 1: Symmetry Groups. Edited by Norman F. M. Henry and Kathleen Lonsdale. Pp. xi + 558. (Birmingham: Kynoch Press, 1952.) 105s.

/pdf/international-tables-for-x-ray-crystallography-1t8sk98xrd.pdf

International Tables for X-Ray Crystallography

Missing link

Geophysical data reveal that at subduction zones oceanic plates could be pervasively hydrated for several kilometres below the crust–mantle boundary. Numerical experiments suggest that such deep hydration is facilitated by negative pressure gradients that lead to the downward pumping of water along bending-related normal faults. Bending of oceanic plates at subduction zones results in extension and widespread normal faulting1 in the upper, brittle part of the slab2,3. Detailed seismic surveys at trenches reveal that this part of the oceanic plate could be pervasively hydrated for several kilometres below the crust–mantle boundary4,5,6,7. Similarly, heat-flow surveys indicate active fluid circulation within the slab8. Yet, the mechanisms that enable fluids to percolate to such depths in spite of their natural buoyancy remain unclear. Here we use two-dimensional numerical experiments to show that stress changes induced by the bending oceanic plate produce subhydrostatic or even negative pressure gradients along normal faults, favouring downward pumping of fluids. The fluids then react with the crust and mantle surrounding the faults and are stored in the form of hydrous minerals. We suggest that this process is the dominant mechanism of deep slab hydration, although it may be locally aided by the enhancement in porosity due to prefailure dilatancy9, pre-existing cracks10 and migrating fluid-filled cracks11. Our results have implications for the transport of water into the deeper parts of the mantle12, and for further clarifying the seismic anisotropy of slabs13.

Deep slab hydration induced by bending-related variations in tectonic pressure

In subduction zones, the anisotropic fast shear-wave component is generally observed to be orientated parallel to the strike of the trench. The interpretation of this shear-wave splitting above subduction zones has, however, been controversial and none of the inferred models appears to be explain the whole range of anisotropic patterns observed worldwide. Manuele Faccenda and colleagues now show that the amount and geometry of seismic anisotropy measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals formed along steeply dipping faults and from the larger scale vertical layering consisting of dry and hydrated crust-mantle sections whose spacing is several times smaller than teleseismic wavelengths. Faults orientations and estimated delay times are consistent with the observed shear-wave splitting patterns in most subduction zones. It is shown that the amount and geometry of seismic anisotropy measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals formed along steeply dipping faults and from the larger-scale vertical layering consisting of dry and hydrated crust–mantle sections, the spacing of which is several times smaller than teleseismic wavelengths. The variation of elastic-wave velocities as a function of the direction of propagation through the Earth’s interior is a widely documented phenomenon called seismic anisotropy. The geometry and amount of seismic anisotropy is generally estimated by measuring shear-wave splitting, which consists of determining the polarization direction of the fast shear-wave component and the time delay between the fast and slow, orthogonally polarized, waves. In subduction zones, the teleseismic fast shear-wave component is oriented generally parallel to the strike of the trench1, although a few exceptions have been reported (Cascadia2 and restricted areas of South America3,4). The interpretation of shear-wave splitting above subduction zones has been controversial and none of the inferred models seems to be sufficiently complete to explain the entire range of anisotropic patterns registered worldwide1. Here we show that the amount and the geometry of seismic anisotropies measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals (serpentine and talc) formed along steeply dipping faults and from the larger-scale vertical layering consisting of dry and hydrated crust–mantle sections whose spacing is several times smaller than teleseismic wavelengths. Fault orientations and estimated delay times are consistent with the observed shear-wave splitting patterns in most subduction zones.

Fault-induced seismic anisotropy by hydration in subducting oceanic plates

Styles of post-subduction collisional orogeny: Influence of convergence velocity, crustal rheology and radiogenic heat production

Water in the slab: A trilogy

Subducting oceanic plates carry a considerable amount of water from the surface down to mantle depths and contribute significantly to the global water cycle. A part of these volatiles stored in the slab is expelled at intermediate depths (70-300 km) where dehydration reactions occur. However, despite the fact that water considerably affects many physical properties of rocks, not much is known about the fluid flow path and the interaction with the rocks through which volatiles flow in the slab interior during its dehydration. We performed thermomechanical models (coupled with a petrological database and with incompressible aqueous fluid flow) of a dynamically subducting and dehydrating oceanic plate. Results show that, during slab dehydration, unbending stresses drive part of the released fluids into the cold core of the plate toward a level of strong tectonic under-pressure and neutral (slab-normal) pressure gradients. Fluids progressively accumulate and percolate updip along such a layer forming, together with the upper hydrated layer near the top of the slab, a Double Hydrated Zone (DHZ) where intermediate-depth seismicity could be triggered. The location and predicted mechanics of the DHZ would be consistent with seismological observations regarding Double Seismic Zones (DSZs) found in most subduction zones and suggests that hydrofracturing could be the trigger mechanism for observed intermediate-depth seismicity. In the light of our results, the lower plane of the DSZ is more likely to reflect a layer of upward percolating fluid than a level of mantle dehydration. In our models, a 20-30 km thick DSZ forms in relatively old oceanic plates without requiring an extremely deep slab hydration prior to subduction. The redistribution of fluids into the slab interior during slab unbending also has important implications for slab weakening and the deep water cycle. We estimate that, over the whole of Earth's history, a volume of water equivalent to around one to two oceans can be stored in nominally anhydrous minerals of the oceanic lithosphere and transported to the transition zone by this mechanism, suggesting that mantle regassing could have been efficient even without invoking the formation of high pressure hydrous minerals. Copyright 2012 by the American Geophysical Union.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011GC003860

Manuele Faccenda

Papers

Deep slab hydration induced by bending-related variations in tectonic pressure

Fault-induced seismic anisotropy by hydration in subducting oceanic plates

Styles of post-subduction collisional orogeny: Influence of convergence velocity, crustal rheology and radiogenic heat production

Water in the slab: A trilogy

Fluid flow during slab unbending and dehydration: Implications for intermediate‐depth seismicity, slab weakening and deep water recycling