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Sub-slab mantle anisotropy beneath south-central Chile

Sub-slab mantle anisotropy beneath south-central Chile
Sub-slab mantle anisotropy beneath south-central Chile
Knowledge of mantle flow in convergent margins is crucial to unravelling both the contemporary geodynamics and the past evolution of subduction zones. By analysing shear-wave splitting in both teleseismic and local arrivals, we can determine the relative contribution from different parts of the subduction zone to the total observed SKS splitting, providing us with a depth constraint on anisotropy. We use this methodology to determine the location, orientation and strength of seismic anisotropy in the south-central Chile subduction zone. Data come from the TIPTEQ network, deployed on the forearc during 2004–2005. We obtain 110 teleseismic SKS and 116 local good-quality shear-wave splitting measurements. SKS average delay times are 1.3 s and local S delay times are only 0.2 s. Weak shear-wave splitting from local phases is consistent with a shape preferred orientation (SPO) source in the upper crust. We infer that the bulk of shear-wave splitting is sourced either within or below the subducting Nazca slab. SKS splitting measurements exhibit an average north-easterly fast direction, with a strong degree of variation. Further investigation suggests a relationship between the measurement's fast direction and the incoming ray's back-azimuth. Finite-element geodynamic modelling is used to investigate the strain rate field and predicted LPO characteristics in the subduction zone. These models highlight a thick region of high strain rate and strong S-wave anisotropy, with plunging olivine a-axes, in the sub-slab asthenosphere. We forward model the sub-slab sourced splitting with a strongly anisotropic layer of thick asthenosphere, comprising an olivine a-axis oriented parallel to the direction of subduction. The subducting lithosphere is not thick enough to cause 1.2 s of splitting, therefore our results and subsequent models show that the Nazca slab is entraining the underlying asthenosphere; its flow causes it to be strongly anisotropic. Our observation has important implications for the controlling factors on sub-slab mantle flow and the movement of asthenospheric material within the Earth.
south-central Chile, subduction, shear-wave splitting, seismic anisotropy, mantle flow, asthenospheric entrainment
0012-821X
203-213
Hicks, Stephen P.
036d1b3b-bb7a-4a22-b2ce-71618a1723a3
Nippress, Stuart E.J.
e57c54a5-12bb-4563-b736-e023af162bd0
Rietbrock, Andreas
e4656f22-06be-40fe-8f98-7f88c351c434
Hicks, Stephen P.
036d1b3b-bb7a-4a22-b2ce-71618a1723a3
Nippress, Stuart E.J.
e57c54a5-12bb-4563-b736-e023af162bd0
Rietbrock, Andreas
e4656f22-06be-40fe-8f98-7f88c351c434

Hicks, Stephen P., Nippress, Stuart E.J. and Rietbrock, Andreas (2012) Sub-slab mantle anisotropy beneath south-central Chile. Earth and Planetary Science Letters, 357-358, 203-213. (doi:10.1016/j.epsl.2012.09.017).

Record type: Article

Abstract

Knowledge of mantle flow in convergent margins is crucial to unravelling both the contemporary geodynamics and the past evolution of subduction zones. By analysing shear-wave splitting in both teleseismic and local arrivals, we can determine the relative contribution from different parts of the subduction zone to the total observed SKS splitting, providing us with a depth constraint on anisotropy. We use this methodology to determine the location, orientation and strength of seismic anisotropy in the south-central Chile subduction zone. Data come from the TIPTEQ network, deployed on the forearc during 2004–2005. We obtain 110 teleseismic SKS and 116 local good-quality shear-wave splitting measurements. SKS average delay times are 1.3 s and local S delay times are only 0.2 s. Weak shear-wave splitting from local phases is consistent with a shape preferred orientation (SPO) source in the upper crust. We infer that the bulk of shear-wave splitting is sourced either within or below the subducting Nazca slab. SKS splitting measurements exhibit an average north-easterly fast direction, with a strong degree of variation. Further investigation suggests a relationship between the measurement's fast direction and the incoming ray's back-azimuth. Finite-element geodynamic modelling is used to investigate the strain rate field and predicted LPO characteristics in the subduction zone. These models highlight a thick region of high strain rate and strong S-wave anisotropy, with plunging olivine a-axes, in the sub-slab asthenosphere. We forward model the sub-slab sourced splitting with a strongly anisotropic layer of thick asthenosphere, comprising an olivine a-axis oriented parallel to the direction of subduction. The subducting lithosphere is not thick enough to cause 1.2 s of splitting, therefore our results and subsequent models show that the Nazca slab is entraining the underlying asthenosphere; its flow causes it to be strongly anisotropic. Our observation has important implications for the controlling factors on sub-slab mantle flow and the movement of asthenospheric material within the Earth.

Text
10.1016.j.epsl.2012.09.017.pdf - Author's Original
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More information

Accepted/In Press date: 15 September 2012
Published date: 1 December 2012
Keywords: south-central Chile, subduction, shear-wave splitting, seismic anisotropy, mantle flow, asthenospheric entrainment
Organisations: Ocean and Earth Science, Geology & Geophysics

Identifiers

Local EPrints ID: 405575
URI: http://eprints.soton.ac.uk/id/eprint/405575
ISSN: 0012-821X
PURE UUID: 2c4cba0b-85f9-42aa-8ca5-c097e8eb6df3
ORCID for Stephen P. Hicks: ORCID iD orcid.org/0000-0002-7476-3284

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Date deposited: 06 Feb 2017 14:44
Last modified: 08 Jan 2022 03:40

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Contributors

Author: Stephen P. Hicks ORCID iD
Author: Stuart E.J. Nippress
Author: Andreas Rietbrock

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