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Hydration of the crust and upper mantle of the Hikurangi Plateau as it subducts at the southern Hikurangi margin.

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Title: Hydration of the crust and upper mantle of the Hikurangi Plateau as it subducts at the southern Hikurangi margin.
Authors: Herath, Pasan1 (AUTHOR) pasan.herath@vuw.ac.nz, Stern, Tim A.1 (AUTHOR), Savage, Martha K.1 (AUTHOR), Bassett, Dan2 (AUTHOR), Henrys, Stuart2 (AUTHOR), Boulton, Carolyn1 (AUTHOR)
Source: Earth & Planetary Science Letters. Jul2020, Vol. 541, pN.PAG-N.PAG. 1p.
Subjects: Subduction zones, Oceanic plateaus, Plateaus, Earthquake zones, Hydration, Oceanic crust, Lithosphere
Geographic Terms: North Island (N.Z.)
Abstract: • Hikurangi Plateau crust at the southern Hikurangi margin is ≈12 km thick. • A fast-upper mantle resides at a depth of ≈50 km beneath Hikurangi trough. • Crustal and upper mantle P-wave-speeds reduced by ≈10% beneath the Hikurangi trough. • Hikurangi Plateau is hydrated to depths of ≈50 km beneath the Hikurangi trough. Controlled-source seismic studies at most subduction zones show that bending of the subducting plate results in reduced seismic wave-speeds in the crust and upper mantle near the trench. Similar studies also have found unusually high P-wave-speeds (V p) in the upper mantle under oceanic plateaus. Onshore-offshore seismic profiling at the southern Hikurangi margin, where the ≈120 Ma old oceanic Hikurangi Plateau is subducting, indicates that a fast (V p ≈8.7±0.2 km/s) upper mantle layer lies beneath a ≈25 km thick mantle layer with more regular wave-speeds (V p ≈8.0±0.2 km/s) under the Hikurangi trough. This is consistent with previous findings of upper mantle V p ≈8.7-9.0 km/s in the margin-parallel direction under the North Island (≈100 km northwest of the deformation front) at depths ≈8-10 km below the Moho. Our profiles are margin-perpendicular, thus we show that the upper mantle lid of the subducting Pacific Plate is characterized by unusually high P-wave-speeds along all azimuths. We find an area of lowered V p in the ≈12±1 km thick Hikurangi Plateau crust beneath the trough. This drop in V p is ≈10%, and a similar drop in V p is deduced to depths of 25±2 km into the upper mantle. We interpret that the increase in thickness of the regular mantle beneath the trough results from the formation of a low-velocity zone in the faster upper mantle layer; this zone formed from serpentinisation by hydration through bending-induced normal faults and/or due to crack porosity introduced by thermal cracking, further enhanced by bending-related faulting. Thus the "regular mantle" (V p ≈8 km/s) is not in fact regular, but rather the high-speed mantle has mechanically bent, fractured, and altered. The absolute depth of fast mantle V p under the Hikurangi trough is around 50 km. The onset of the lower band of seismicity of the double seismic zone and high upper mantle V p under the North Island is observed at similar depths. This is consistent with the hypothesis that the lower band of earthquakes in a double seismic zone is due to antigorite dehydration processes, a hydrous mineral formed in the low velocity zone in the upper mantle beneath the trough. Our study on the Hikurangi margin is different, as the subducting plate here contains a ≈120 Ma old oceanic plateau with a ≈12 km thick crust, but the results are similar to other subduction margins where regular oceanic crust is subducting. [ABSTRACT FROM AUTHOR]
Copyright of Earth & Planetary Science Letters is the property of Elsevier B.V. and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Hydration of the crust and upper mantle of the Hikurangi Plateau as it subducts at the southern Hikurangi margin.
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  Data: <searchLink fieldCode="AR" term="%22Herath%2C+Pasan%22">Herath, Pasan</searchLink><relatesTo>1</relatesTo> (AUTHOR)<i> pasan.herath@vuw.ac.nz</i><br /><searchLink fieldCode="AR" term="%22Stern%2C+Tim+A%2E%22">Stern, Tim A.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Savage%2C+Martha+K%2E%22">Savage, Martha K.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Bassett%2C+Dan%22">Bassett, Dan</searchLink><relatesTo>2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Henrys%2C+Stuart%22">Henrys, Stuart</searchLink><relatesTo>2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Boulton%2C+Carolyn%22">Boulton, Carolyn</searchLink><relatesTo>1</relatesTo> (AUTHOR)
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  Data: <searchLink fieldCode="JN" term="%22Earth+%26+Planetary+Science+Letters%22">Earth & Planetary Science Letters</searchLink>. Jul2020, Vol. 541, pN.PAG-N.PAG. 1p.
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  Data: <searchLink fieldCode="DE" term="%22Subduction+zones%22">Subduction zones</searchLink><br /><searchLink fieldCode="DE" term="%22Oceanic+plateaus%22">Oceanic plateaus</searchLink><br /><searchLink fieldCode="DE" term="%22Plateaus%22">Plateaus</searchLink><br /><searchLink fieldCode="DE" term="%22Earthquake+zones%22">Earthquake zones</searchLink><br /><searchLink fieldCode="DE" term="%22Hydration%22">Hydration</searchLink><br /><searchLink fieldCode="DE" term="%22Oceanic+crust%22">Oceanic crust</searchLink><br /><searchLink fieldCode="DE" term="%22Lithosphere%22">Lithosphere</searchLink>
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  Data: <searchLink fieldCode="DE" term="%22North+Island+%28N%2EZ%2E%29%22">North Island (N.Z.)</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: • Hikurangi Plateau crust at the southern Hikurangi margin is ≈12 km thick. • A fast-upper mantle resides at a depth of ≈50 km beneath Hikurangi trough. • Crustal and upper mantle P-wave-speeds reduced by ≈10% beneath the Hikurangi trough. • Hikurangi Plateau is hydrated to depths of ≈50 km beneath the Hikurangi trough. Controlled-source seismic studies at most subduction zones show that bending of the subducting plate results in reduced seismic wave-speeds in the crust and upper mantle near the trench. Similar studies also have found unusually high P-wave-speeds (V p) in the upper mantle under oceanic plateaus. Onshore-offshore seismic profiling at the southern Hikurangi margin, where the ≈120 Ma old oceanic Hikurangi Plateau is subducting, indicates that a fast (V p ≈8.7±0.2 km/s) upper mantle layer lies beneath a ≈25 km thick mantle layer with more regular wave-speeds (V p ≈8.0±0.2 km/s) under the Hikurangi trough. This is consistent with previous findings of upper mantle V p ≈8.7-9.0 km/s in the margin-parallel direction under the North Island (≈100 km northwest of the deformation front) at depths ≈8-10 km below the Moho. Our profiles are margin-perpendicular, thus we show that the upper mantle lid of the subducting Pacific Plate is characterized by unusually high P-wave-speeds along all azimuths. We find an area of lowered V p in the ≈12±1 km thick Hikurangi Plateau crust beneath the trough. This drop in V p is ≈10%, and a similar drop in V p is deduced to depths of 25±2 km into the upper mantle. We interpret that the increase in thickness of the regular mantle beneath the trough results from the formation of a low-velocity zone in the faster upper mantle layer; this zone formed from serpentinisation by hydration through bending-induced normal faults and/or due to crack porosity introduced by thermal cracking, further enhanced by bending-related faulting. Thus the "regular mantle" (V p ≈8 km/s) is not in fact regular, but rather the high-speed mantle has mechanically bent, fractured, and altered. The absolute depth of fast mantle V p under the Hikurangi trough is around 50 km. The onset of the lower band of seismicity of the double seismic zone and high upper mantle V p under the North Island is observed at similar depths. This is consistent with the hypothesis that the lower band of earthquakes in a double seismic zone is due to antigorite dehydration processes, a hydrous mineral formed in the low velocity zone in the upper mantle beneath the trough. Our study on the Hikurangi margin is different, as the subducting plate here contains a ≈120 Ma old oceanic plateau with a ≈12 km thick crust, but the results are similar to other subduction margins where regular oceanic crust is subducting. [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Earth & Planetary Science Letters is the property of Elsevier B.V. and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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RecordInfo BibRecord:
  BibEntity:
    Identifiers:
      – Type: doi
        Value: 10.1016/j.epsl.2020.116271
    Languages:
      – Code: eng
        Text: English
    PhysicalDescription:
      Pagination:
        PageCount: 1
        StartPage: N.PAG
    Subjects:
      – SubjectFull: Subduction zones
        Type: general
      – SubjectFull: Oceanic plateaus
        Type: general
      – SubjectFull: Plateaus
        Type: general
      – SubjectFull: Earthquake zones
        Type: general
      – SubjectFull: Hydration
        Type: general
      – SubjectFull: Oceanic crust
        Type: general
      – SubjectFull: Lithosphere
        Type: general
      – SubjectFull: North Island (N.Z.)
        Type: general
    Titles:
      – TitleFull: Hydration of the crust and upper mantle of the Hikurangi Plateau as it subducts at the southern Hikurangi margin.
        Type: main
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            NameFull: Herath, Pasan
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            NameFull: Stern, Tim A.
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            NameFull: Savage, Martha K.
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              Text: Jul2020
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              Y: 2020
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              Value: 541
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