Sub-slab mantle anisotropy beneath south-central Chile

Hicks, Stephen P, Nippress, Stuart EJ and Rietbrock, Andreas
(2012) Sub-slab mantle anisotropy beneath south-central Chile. Earth and Planetary Science Letters, 357-35. pp. 203-213.

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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 olivine a-axes 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.

Item Type: Article
Additional Information: Times Cited: 1## TULIP Type: Articles/Papers (Journal) ## official_url: <Go to ISI>://WOS:000312621600020
Uncontrolled Keywords: south-central Chile, subduction, shear-wave splitting, seismic anisotropy, mantle flow, asthenospheric entrainment
Subjects: ?? Q1 ??
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Divisions: Faculty of Science and Engineering > School of Environmental Sciences
Depositing User: Symplectic Admin
Date Deposited: 04 Mar 2014 16:17
Last Modified: 17 Dec 2022 01:28
DOI: 10.1016/j.epsl.2012.09.017
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