Upper mantle tectonics: three-dimensional deformation, olivine crystallographic fabrics and seismic properties

Tommasi, Andréa; Tikoff, Basil; Vauchez, Alain

doi:10.1016/s0012-821x(99)00046-1

Cited by 217 publications

(244 citation statements)

References 49 publications

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“…We have resolved a change both in the character of seismic anisotropy and in its relation to strain in the depth interval between 150 and 200 km in the Australian subcontinental lithospheric mantle. The angles between the fast axes of seismic anisotropy and the principal shortening directions above 1502 00 km argue for a deformation mechanism that produced moderate strain due to horizontal pure shear [4,14]. Hence, the top 150^200 km of the Australian lithosphere primarily records the coherent signature of past deformation episodes.…”

Section: Discussionmentioning

confidence: 99%

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: 99%

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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“…Several processes can be invoked to explain this mantle LVZ beneath the NTD (Fig. 16): (i) seismic anisotropy, due to either mantle flow (Bagley & Nyblade 2013), lattice-preferred orientation (Tommasi et al 1999) or oriented melting cracks (Holtzman & Kendall 2010) can produce a local change in velocity. Our RF analysis can hardly be conclusive for the presence of azimuthal anisotropy within the lithosphere, and particularly in this LVZ.…”

Section: Upper Mantle Structurementioning

confidence: 99%

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

Plasman¹,

Tiberi

Ebinger

et al. 2017

Geophysical Journal International

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“…9d). The axial-[100] CPO indicates activation of the {0kl} [100] slip systems and may be associated with constrictional strain (Tommasi et al, 1999;Chatzaras et al, 2016) and high stress (Carter and Avé Lallemant, 1970;Jung and Karato, 2001).…”

Section: Olivinementioning

confidence: 99%

Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

et al. 2017

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Abstract. The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene–Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750–890 °C and pressures of 400–560 MPa, corresponding to 15–22 km depth. A hot crustal geotherm of 40 ° C km−1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12–33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12–17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1–1.3×1020 Pa ⋅ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016–1019 Pa ⋅ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 Pa ⋅ s, which indicates the lack of an upper-mantle lid beneath northern Baja California. Our data show that during post-seismic transients, the upper mantle and the lower crust in the Pacific–Baja California plate boundary are characterized by similar and low differential stress. Transient viscosity of the lower crust is similar to the viscosity of the upper mantle.

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Upper mantle tectonics: three-dimensional deformation, olivine crystallographic fabrics and seismic properties

Cited by 217 publications

References 49 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

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

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