Lateral variations in the crustal structure of the Indo-Eurasian collision zone

Amy Gilligan, Keith Priestley

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Abstract

The processes involved in continental collisions remain contested, yet knowledge of these processes is crucial to improving our understanding of how some of the most dramatic features on Earth have formed. As the largest and highest orogenic plateau on Earth today, Tibet is an excellent natural laboratory for investigating collisional processes. To understand the development of the Tibetan Plateau we need to understand the crustal structure beneath both Tibet and the Indian Plate. Building on previous work, we measure new group velocity dispersion curves using data from regional earthquakes (4424 paths) and ambient noise data (5696 paths), and use these to obtain new fundamental mode Rayleigh Wave group velocity maps for periods from 5-70 s for a region including Tibet, Pakistan and India. The dense path coverage at the shortest periods, due to the inclusion of ambient noise measurements, allows features of up to 100 km scale to be resolved in some areas of the collision zone, providing one of the highest resolution models of the crust and uppermost mantle across this region. We invert the Rayleigh wave group velocity maps for shear wave velocity structure to 120 km depth and construct a 3D velocity model for the crust and uppermost mantle of the Indo-Eurasian collision zone. We use this 3D model to map the lateral variations in the crust and in the nature of the crust-mantle transition (Moho) across the Indo-Eurasian collision zone.The Moho occurs at lower shear velocities below north eastern Tibet than it does beneath western and southern Tibet and below India. The east–west difference across Tibet is particularly apparent in the elevated velocities observed west of 84E at depths exceeding 90 km. This suggests that Indian lithosphere underlies the whole of the Plateau in the west,but possibly not in the east. At depths of 20-40km our crustal model shows the existence of a pervasive mid-crustal low velocity layer (⇠10% decrease in velocity, Vs<3.4 km/s) throughout all of Tibet, as well as beneath the Pamirs, but not below India. The thickness of this layer, the lowest velocity in the layer and the degree of velocity reduction vary across the region. Combining our Rayleigh wave observations with previously published Love wave dispersion measurements (Acton et al., 2010), we find that the low velocity layer has a radial anisotropic signature with Vsh>Vsv. The characteristics of the low velocity layer are supportive of deformation occurring through ductile flow in the mid-crust.
Original languageEnglish
Pages (from-to)975-989
Number of pages15
JournalGeophysical Journal International
Volume214
Issue number2
Early online date31 May 2018
DOIs
Publication statusPublished - 1 Aug 2018

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Tibet
collision zone
crustal structure
crusts
collisions
group velocity
plateaus
Earth mantle
crust
Rayleigh waves
India
low speed
wave group
ambient noise
Rayleigh wave
plateau
mantle
Moho
Pakistan
noise measurement

Keywords

  • Continental tectonics
  • compressional
  • Surface waves and free oscillations
  • crustal imaging
  • tomography
  • Asia

Cite this

Lateral variations in the crustal structure of the Indo-Eurasian collision zone. / Gilligan, Amy; Priestley, Keith.

In: Geophysical Journal International, Vol. 214, No. 2, 01.08.2018, p. 975-989.

Research output: Contribution to journalArticle

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keywords = "Continental tectonics, compressional, Surface waves and free oscillations, crustal imaging, tomography, Asia",
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note = "We thank Michael Ritzwoller and two anonymous reviewers for constructive comments that have helped improve the manuscript. The majority of the seismic data used in this study were downloaded from IRIS DMC. Data for the NGRI stations in India were provided by S. S. Rai, and Zahid Rafi provided the PMD Pakistan data. Kajal Borah provided the ambient noise cross-correlations for the Uttaranchal network. Figures were prepared using Generic Mapping Tools (GMT) software (Wessel and Smith, 1998). We thank Robert Herrmann for making the Computer Programs in Seismology freely available.",
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N2 - The processes involved in continental collisions remain contested, yet knowledge of these processes is crucial to improving our understanding of how some of the most dramatic features on Earth have formed. As the largest and highest orogenic plateau on Earth today, Tibet is an excellent natural laboratory for investigating collisional processes. To understand the development of the Tibetan Plateau we need to understand the crustal structure beneath both Tibet and the Indian Plate. Building on previous work, we measure new group velocity dispersion curves using data from regional earthquakes (4424 paths) and ambient noise data (5696 paths), and use these to obtain new fundamental mode Rayleigh Wave group velocity maps for periods from 5-70 s for a region including Tibet, Pakistan and India. The dense path coverage at the shortest periods, due to the inclusion of ambient noise measurements, allows features of up to 100 km scale to be resolved in some areas of the collision zone, providing one of the highest resolution models of the crust and uppermost mantle across this region. We invert the Rayleigh wave group velocity maps for shear wave velocity structure to 120 km depth and construct a 3D velocity model for the crust and uppermost mantle of the Indo-Eurasian collision zone. We use this 3D model to map the lateral variations in the crust and in the nature of the crust-mantle transition (Moho) across the Indo-Eurasian collision zone.The Moho occurs at lower shear velocities below north eastern Tibet than it does beneath western and southern Tibet and below India. The east–west difference across Tibet is particularly apparent in the elevated velocities observed west of 84E at depths exceeding 90 km. This suggests that Indian lithosphere underlies the whole of the Plateau in the west,but possibly not in the east. At depths of 20-40km our crustal model shows the existence of a pervasive mid-crustal low velocity layer (⇠10% decrease in velocity, Vs<3.4 km/s) throughout all of Tibet, as well as beneath the Pamirs, but not below India. The thickness of this layer, the lowest velocity in the layer and the degree of velocity reduction vary across the region. Combining our Rayleigh wave observations with previously published Love wave dispersion measurements (Acton et al., 2010), we find that the low velocity layer has a radial anisotropic signature with Vsh>Vsv. The characteristics of the low velocity layer are supportive of deformation occurring through ductile flow in the mid-crust.

AB - The processes involved in continental collisions remain contested, yet knowledge of these processes is crucial to improving our understanding of how some of the most dramatic features on Earth have formed. As the largest and highest orogenic plateau on Earth today, Tibet is an excellent natural laboratory for investigating collisional processes. To understand the development of the Tibetan Plateau we need to understand the crustal structure beneath both Tibet and the Indian Plate. Building on previous work, we measure new group velocity dispersion curves using data from regional earthquakes (4424 paths) and ambient noise data (5696 paths), and use these to obtain new fundamental mode Rayleigh Wave group velocity maps for periods from 5-70 s for a region including Tibet, Pakistan and India. The dense path coverage at the shortest periods, due to the inclusion of ambient noise measurements, allows features of up to 100 km scale to be resolved in some areas of the collision zone, providing one of the highest resolution models of the crust and uppermost mantle across this region. We invert the Rayleigh wave group velocity maps for shear wave velocity structure to 120 km depth and construct a 3D velocity model for the crust and uppermost mantle of the Indo-Eurasian collision zone. We use this 3D model to map the lateral variations in the crust and in the nature of the crust-mantle transition (Moho) across the Indo-Eurasian collision zone.The Moho occurs at lower shear velocities below north eastern Tibet than it does beneath western and southern Tibet and below India. The east–west difference across Tibet is particularly apparent in the elevated velocities observed west of 84E at depths exceeding 90 km. This suggests that Indian lithosphere underlies the whole of the Plateau in the west,but possibly not in the east. At depths of 20-40km our crustal model shows the existence of a pervasive mid-crustal low velocity layer (⇠10% decrease in velocity, Vs<3.4 km/s) throughout all of Tibet, as well as beneath the Pamirs, but not below India. The thickness of this layer, the lowest velocity in the layer and the degree of velocity reduction vary across the region. Combining our Rayleigh wave observations with previously published Love wave dispersion measurements (Acton et al., 2010), we find that the low velocity layer has a radial anisotropic signature with Vsh>Vsv. The characteristics of the low velocity layer are supportive of deformation occurring through ductile flow in the mid-crust.

KW - Continental tectonics

KW - compressional

KW - Surface waves and free oscillations

KW - crustal imaging

KW - tomography

KW - Asia

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JO - Geophysical Journal International

JF - Geophysical Journal International

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