Obliquity between seismic and electrical anisotropies as a potential indicator of movement sense for ductile shear zones in the upper mantle

Ji, Shaocheng; Rondenay, S.; Mareschal, Marianne; Sénéchal, G.

doi:10.1130/0091-7613(1996)024<1033:obsaea>2.3.co;2

Cited by 73 publications

(38 citation statements)

References 18 publications

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“…Measurements of electrical anisotropy sensitive to the depth range s 150 km in Central Australia are more closely aligned with NNR-APM than with HS-APM [41,42]. If the discrepancy is signi¢cant the obliquity between seismic and electrical anisotropies might have implications for the kinematics of the deformation mech-anism [61]. These are, however, beyond the scope of this paper.…”

Section: Discussionmentioning

confidence: 69%

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

Simons

Hilst

2003

Earth and Planetary Science Letters

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We interpret the three-dimensional seismic wave-speed structure of the Australian upper mantle by comparing its azimuthal anisotropy to estimates of past and present lithospheric deformation. We infer the fossil strain field from the orientation of gravity anomalies relative to topography, bypassing the need to extrapolate crustal measures, and derive the current direction of mantle deformation from present-day plate motion. Our observations provide the depth resolution necessary to distinguish fossil from contemporaneous deformation. The distribution of azimuthal seismic anisotropy is determined from multi-mode surface-wave propagation. Mechanical anisotropy, or the directional variation of isostatic compensation, is a proxy for the fossil strain field and is derived from a spectral coherence analysis of digital gravity and topography data in two wavelength bands. The joint interpretation of seismic and tectonic data resolves a rheological transition in the Australian upper mantle. At depths shallower than V150^200 km strong seismic anisotropy forms complex patterns. In this regime the seismic fast axes are at large angles to the directions of principal shortening, defining a mechanically coupled crust^mantle lid deformed by orogenic processes dominated by transpression. Here, seismic anisotropy may be considered 'frozen', which suggests that past deformation has left a coherent imprint on much of the lithospheric depth profile. The azimuthal seismic anisotropy below V200 km is weaker and preferentially aligned with the direction of the rapid motion of the Indo-Australian plate. The alignment of the fast axes with the direction of present-day absolute plate motion is indicative of deformation by simple shear of a dry olivine mantle. Motion expressed in the hot-spot reference frame matches the seismic observations better than the no-net-rotation reference frame. Thus, seismic anisotropy supports the notion that the hot-spot reference frame is the most physically reasonable. Independently from plate motion models, seismic anisotropy can be used to derive a best-fitting direction of overall mantle shear. ß

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Section: Discussionmentioning

confidence: 69%

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

Simons

Hilst

2003

Earth and Planetary Science Letters

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“…Xenolith samples from the Rapide des Quinze locality present a similar obliquity between the LPO of olivine, which is responsible for the seismic anisotropy, and its shape-preferred orientation (SPO), which can be associated with the electrical anisotropy; this obliquity is due to finite noncoaxial strain (Ji et al 1996). Since MT data and shear-wave splitting analysis provide a comparable characterization of horizontal strain-induced anisotropy within the upper mantle, Ji et al (1996) speculate that the obliquity between seismic and electrical anisotropy can be employed to indicate the movement of transcurrent shear zones in the mantle. Thus, the 20°separation between the two anisotropies beneath the Abitibi-Grenville may reflect a dextral shear sense in the mantle, which is in agreement with the last cycle of regional deformation inferred from the surface geology (Ji et al 1996, and references therein).…”

Section: Shear-wave Splitting Analysismentioning

confidence: 99%

“…2b. For the entire array, the average direction of fast polarization is N101°E ± 10°, and the average splitting is 1.46 ± 0.21 s. Using this time shift and an S-velocity anisotropy of 3.2% inferred from local xenoliths samples, the thickness of the anisotropic layer is estimated at approximately 200 km (Ji et al 1996). Over the study area, the direction of seismic anisotropy correlates well (within a consistent bias of +20°) with the calculated direction of electrical anisotropy (N80°E; see section titled Geophysical coverage).…”

Section: Shear-wave Splitting Analysismentioning

confidence: 99%

“…Finally, Late Jurassic to Early Cretaceous diamondbearing kimberlites have been found at two locations in the study area: the "Rapide des Quinze" kimberlites (maximum age of 126 Ma; Ji et al 1996), and the Kirkland Lake kimberlite field (age ranging between 147 and 158 Ma; Meyer et al 1994, and references therein).…”

Section: Tectonic Settingmentioning

confidence: 99%

“…2a, 2b), it was the basis of the interpretation presented in Ji et al (1996). Xenolith samples from the Rapide des Quinze locality present a similar obliquity between the LPO of olivine, which is responsible for the seismic anisotropy, and its shape-preferred orientation (SPO), which can be associated with the electrical anisotropy; this obliquity is due to finite noncoaxial strain (Ji et al 1996). Since MT data and shear-wave splitting analysis provide a comparable characterization of horizontal strain-induced anisotropy within the upper mantle, Ji et al (1996) speculate that the obliquity between seismic and electrical anisotropy can be employed to indicate the movement of transcurrent shear zones in the mantle.…”

Section: Shear-wave Splitting Analysismentioning

confidence: 99%

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Teleseismic studies of the lithosphere below the Abitibi-Grenville Lithoprobe transect

Rondenay¹,

Bostock²,

Hearn³

et al. 2000

Rev. Can. Sci. Terre

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Abstract:In the past decade, the Abitibi-Grenville Lithoprobe transect has been the site of numerous geological and geophysical surveys oriented towards understanding the lithospheric evolution of the southeastern Superior and adjoining Grenville provinces. Among the different geophysical methods that have been employed, earthquake seismology provides the widest range of information on the deep structures of the upper mantle. This paper presents a review of studies, both complete and ongoing, involving teleseismic datasets that were collected in 1994 and 1996 along the transect. A complete shear-wave splitting analysis has been performed on the 1994 dataset as part of a comparative study on electrical and seismic anisotropies. Results suggest a correlation between the two anisotropies (supported by xenolith data) and favour a lithospheric origin for the seismic anisotropy. The two anisotropies are believed to represent the fossilized remnants of Archean strain fields in the lithospheric roots of the Canadian Shield. Preliminary splitting results for the 1996 experiment suggest that the S-wave azimuthal anisotropy may be depth dependent and laterally varying. Ongoing receiver function analysis and traveltime inversion studies provide velocity models of the crust and upper mantle beneath the study area. Preliminary receiver function results reveal the presence of an S-velocity increase at -90-100 km depth which appears to be laterally continuous over 200 km. Traveltime inversion models indicate the presence of an elongate, low-velocity anomaly beneath the southern portion of the 1996 array which strikes obliquely to major geological structures at the surface (e.g., Grenville Front). Preliminary interpretation relates this anomaly to the same process (e.g., fixed mantle plume, continental rifting) responsible for the emplacement of the Monteregian Hills igneous province.Résumé : Au cours de la dernière décennie, de nombreuses campagnes géologiques et géophysiques ont été effectuées le long de la traverse Abitibi-Grenville du projet Lithoprobe, dans le but de comprendre l'évolution lithosphérique de la partie sud-est de la Province du Supérieur et de la Province de Grenville. Parmi les différentes méthodes géophysiques employées, la sismologie passive est sans doute celle qui procure le plus d'informations sur les structures profondes du manteau supérieur. Cet article passe en revue les études, complétées et en cours, s'appuyant sur les données télésismi-ques enregistrées en 1994 et en 1996, le long de la traverse. Une analyse complète de biréfringence des ondes S a été effectuée sur les données de 1994, dans le cadre d'une étude comparative entre l'anisotropie électrique et sismique. Les résultats obtenus suggèrent une corrélation entre les deux anisotropies (observation supportée par l'analyse de xénoli-thes), et favorisent une anisotropie sismique d'origine lithosphérique. Les deux anisotropies semblent associées à des champs de déformation archéens fossilisés dans les racines lithosphériques du Bouclier canadien...

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Reexamination of magnetotelluric responses and electrical anisotropy of the lithospheric mantle in the Grenville Province, Canada

2015

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Magnetotelluric (MT) responses at the Proterozoic Grenville Front in Canada have been interpreted as being caused by lithospheric electrical anisotropy, and the area is often noted as a classic example of lithospheric anisotropy. This study reevaluates evidence for the electrical anisotropy using 56 MT stations. The spatially uniform MT responses noted at the Grenville Front extend to~200 km southeast and for at least 400 km along strike and are associated with rocks at less than 150 km depth. Examination of induction arrows at longer periods shows arrows at high angle to the MT conductive direction consistent with the presence of macroscopic resistivity structures. New 2-D anisotropic inversions show that electrical anisotropy is not required to fit the MT data. The results indicate that in the resistive mantle lithosphere beneath the Grenville Front, and in conductive lithosphere in adjacent areas, the maximum horizontal resistivity anisotropy is <10%, much less than the factor of 15 determined in earlier 1-D studies. The results suggest that the upper lithospheric mantle in the area is devoid of significant electrical anisotropy and that the observed MT response directionality is due to large-scale resistivity structure. We interpret the spatially consistent MT responses observed at the Grenville Front as being associated with the resistive Archean lithosphere extending southeast beneath the Grenville Front. The obliquity between seismic and MT responses arises because the Superior fabric is oblique to the seismic fast direction. If dextral shearing occurred, it appears to have not caused any significant shape preferred electrical anisotropy.

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Obliquity between seismic and electrical anisotropies as a potential indicator of movement sense for ductile shear zones in the upper mantle

Cited by 73 publications

References 18 publications

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

Teleseismic studies of the lithosphere below the Abitibi-Grenville Lithoprobe transect

Reexamination of magnetotelluric responses and electrical anisotropy of the lithospheric mantle in the Grenville Province, Canada

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