Seismological constraints on the density, thickness and temperature of the lithospheric mantle in southwestern Tibet

Harry Matchette-Downes (Corresponding Author), Robert D. van der Hilst, Amy Gilligan, Keith Priestley

Research output: Contribution to journalArticle

Abstract

We constrain the lithospheric mantle in southwest Tibet to be cold, thick, and
dense by considering seismological observations, isostasy, and gravity anomalies. First, virtual deep seismic sounding (VDSS) indicates that the thickness of the crust increases from 50 ± 4 km beneath the Himalaya to 70 ± 4 km in the Lhasa terrane. This implies a ‘residual topography’ (difference between
isostatic elevation of crust and true elevation) of −2.4 ± 1.5 km. Taking into account deviations from isostasy, the lithospheric mantle must be dense enough to depress the surface by 0.9 to 4.5 km. Our joint inversion of fundamental-mode Rayleigh wave dispersion and receiver functions suggests that the vertically-polarised shear-wave speed (Vsv) is 4.6 ± 0.1 km s−1 at depths of 120 to 300 km. From the shear-wave speed profile, we estimate the geotherm, which is on average 200 ◦C below the 1350 ◦C adiabat, and suggest that the base of the lithosphere is at a depth of 290 ± 30 km. To match the negative buoyancy, the lithospheric must be denser, on average, than ‘normal’ fertile adiabatic mantle, which rules out a depleted (harzburgite) composition. The density excess can be explained solely by thermal contraction, but we cannot rule out additional density increases due to composition. Our observations are not consistent with a depleted Indian slab underthrusting Tibet in this region, which would result in a lower average density and lower temperatures.
Original languageEnglish
Article number115719
JournalEarth and Planetary Science Letters
Volume524
Early online date8 Aug 2019
DOIs
Publication statusPublished - 15 Oct 2019

Fingerprint

Tibet
Shear waves
isostasy
Earth mantle
mantle
Rayleigh waves
S waves
S-wave
crusts
Buoyancy
Chemical analysis
Topography
anomalies
crust
deep seismic sounding
Gravitation
wave dispersion
harzburgite
gravity anomalies
temperature

Keywords

  • Tibet
  • isostasy
  • lithosphere
  • density
  • mantle
  • seismology
  • SUBDUCTION
  • SENSITIVITY
  • CRUSTAL
  • PHASE
  • PLATEAU
  • THERMAL STRUCTURE
  • ATTENUATION
  • DYNAMICS
  • BENEATH
  • SHEAR-WAVE VELOCITY

Cite this

Seismological constraints on the density, thickness and temperature of the lithospheric mantle in southwestern Tibet. / Matchette-Downes, Harry (Corresponding Author); van der Hilst, Robert D.; Gilligan, Amy; Priestley, Keith.

In: Earth and Planetary Science Letters, Vol. 524, 115719, 15.10.2019.

Research output: Contribution to journalArticle

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note = "H. M.-D. thanks H. Fang, D. Forsyth, R. G. Green, A. Holt, V. Levin, C. Yu and especially L. Royden for helpful discussions, and to Schlumberger for a scholarship. He is also grateful to V. Levin and S. Roecker for making all seismo-grams from the Y2 network freely available through the IRIS Data Management Centre. We also thank three anonymous reviewers for thorough, constructive reviews. Data were processed using ObsPy. All figures were prepared using Matplotlib, alongside the Cartopy library for mapping.",
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N1 - H. M.-D. thanks H. Fang, D. Forsyth, R. G. Green, A. Holt, V. Levin, C. Yu and especially L. Royden for helpful discussions, and to Schlumberger for a scholarship. He is also grateful to V. Levin and S. Roecker for making all seismo-grams from the Y2 network freely available through the IRIS Data Management Centre. We also thank three anonymous reviewers for thorough, constructive reviews. Data were processed using ObsPy. All figures were prepared using Matplotlib, alongside the Cartopy library for mapping.

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AB - We constrain the lithospheric mantle in southwest Tibet to be cold, thick, anddense by considering seismological observations, isostasy, and gravity anomalies. First, virtual deep seismic sounding (VDSS) indicates that the thickness of the crust increases from 50 ± 4 km beneath the Himalaya to 70 ± 4 km in the Lhasa terrane. This implies a ‘residual topography’ (difference betweenisostatic elevation of crust and true elevation) of −2.4 ± 1.5 km. Taking into account deviations from isostasy, the lithospheric mantle must be dense enough to depress the surface by 0.9 to 4.5 km. Our joint inversion of fundamental-mode Rayleigh wave dispersion and receiver functions suggests that the vertically-polarised shear-wave speed (Vsv) is 4.6 ± 0.1 km s−1 at depths of 120 to 300 km. From the shear-wave speed profile, we estimate the geotherm, which is on average 200 ◦C below the 1350 ◦C adiabat, and suggest that the base of the lithosphere is at a depth of 290 ± 30 km. To match the negative buoyancy, the lithospheric must be denser, on average, than ‘normal’ fertile adiabatic mantle, which rules out a depleted (harzburgite) composition. The density excess can be explained solely by thermal contraction, but we cannot rule out additional density increases due to composition. Our observations are not consistent with a depleted Indian slab underthrusting Tibet in this region, which would result in a lower average density and lower temperatures.

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KW - THERMAL STRUCTURE

KW - ATTENUATION

KW - DYNAMICS

KW - BENEATH

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UR - http://www.mendeley.com/research/seismological-constraints-density-thickness-temperature-lithospheric-mantle-southwestern-tibet

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JO - Earth and Planetary Science Letters

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