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.
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 language | English |
|---|---|
| Article number | 115719 |
| Journal | Earth and Planetary Science Letters |
| Volume | 524 |
| Early online date | 8 Aug 2019 |
| DOIs | |
| Publication status | Published - 15 Oct 2019 |
Bibliographical 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.Keywords
- Tibet
- isostasy
- lithosphere
- density
- mantle
- seismology
- SUBDUCTION
- SENSITIVITY
- CRUSTAL
- PHASE
- PLATEAU
- THERMAL STRUCTURE
- ATTENUATION
- DYNAMICS
- BENEATH
- SHEAR-WAVE VELOCITY
