Magnetotelluric Experiment probes deep physical state of southeastern United States

Wannamaker, Philip E.; Chave, Alan D.; Booker, J. R.; Jones, Alan G.; Filloux, J. H.; Ogawa, Yasuo; Unsworth, M. J.; Tarits, Pascal; Evans, R. L.

doi:10.1029/96eo00227

Cited by 14 publications

(13 citation statements)

References 10 publications

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“…Sénéchal et al (1996), for instance, through coupled shear wave splitting measurements and magnetotelluric soundings along a transect in the Canadian shield, retrieved similar directions of anisotropy from both datasets and, considering that the measured electrical anisotropy originates between 50 and 150 km, they concluded that the seismic anisotropy also originates from a well-defined fabric in the lithospheric upper mantle fabric. Similar preliminary results have been obtained in the Appalachians (Wannamaker et al, 1996) and the Pyrenees (Pous et al, 1995;M. Daignières, pers.…”

Section: Tectonic Fabric Of the Continental Lithospheric Mantlesupporting

confidence: 72%

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Rheological heterogeneity, mechanical anisotropy and deformation of the continental lithosphere

1998

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This paper aims to present an overview on the influence of rheological heterogeneity and mechanical anisotropy on the deformation of continents. After briefly recapping the concept of rheological stratification of the lithosphere, we discuss two specific issues: (1) as supported by a growing body of geophysical and geological observations, crust=mantle mechanical coupling is usually efficient, especially beneath major transcurrent faults which probably crosscut the lithosphere and root within the sublithospheric mantle; and (2) in most geodynamic environments, mechanical properties of the mantle govern the tectonic behaviour of the lithosphere. Lateral rheological heterogeneity of the continental lithosphere may result from various sources, with variations in geothermal gradient being the principal one. The oldest domains of continents, the cratonic nuclei, are characterized by a relatively cold, thick, and consequently stiff lithosphere. On the other hand, rifting may also modify the thermal structure of the lithosphere. Depending on the relative stretching of the crust and upper mantle, a stiff or a weak heterogeneity may develop. Observations from rift domains suggest that rifting usually results in a larger thinning of the lithospheric mantle than of the crust, and therefore tends to generate a weak heterogeneity. Numerical models show that during continental collision, the presence of both stiff and weak rheological heterogeneities significantly influences the large-scale deformation of the continental lithosphere. They especially favour the development of lithospheric-scale strike-slip faults, which allow strain to be transferred between the heterogeneities. An heterogeneous strain partition occurs: cratons largely escape deformation, and strain tends to localize within or at the boundary of the rift basins provided compressional deformation starts before the thermal heterogeneity induced by rifting are compensated. Seismic and electrical conductivity anisotropies consistently point towards the existence of a coherent fabric in the lithospheric mantle beneath continental domains. Analysis of naturally deformed peridotites, experimental deformations and numerical simulations suggest that this fabric is developed during orogenic events and subsequently frozen in the lithospheric mantle. Because the mechanical properties of single-crystal olivine are anisotropic, i.e. dependent on the orientation of the applied forces relative to the dominant slip systems, a pervasive fabric frozen in the mantle may induce a significant mechanical anisotropy of the whole lithospheric mantle. It is suggested that this mechanical anisotropy is the source of the so-called tectonic inheritance, i.e. the systematic reactivation of ancient tectonic directions; it may especially explain preferential rift propagation and continental break-up along pre-existing orogenic belts. Thus, the deformation of continents during orogenic events results from a trade-off between tectonic forces applied at plate boundaries, plate geometry,...

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Section: Tectonic Fabric Of the Continental Lithospheric Mantlesupporting

confidence: 72%

“…Electrical anisotropy measurements in the Canadian shield (Sénéchal et al, 1996) and the eastern US (Wannamaker et al, 1996) show a very good agreement between the directions of conductivity anisotropy in the upper mantle and the crustal tectonic fabric, suggesting that the mantle and the crustal fabrics are similar.…”

Section: Rheology Of the Lithospherementioning

confidence: 82%

Rheological heterogeneity, mechanical anisotropy and deformation of the continental lithosphere

1998

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“…and one in the early-middle Paleozoic (Taconic), with final collision and plutonism in the Late Paleozoic (Alleghanian) (Hatcher, 1989;van Schmus et al, 1993;Rankin, 1994;Hibbard et al, 2002). Tectonic setting and problems, MT data collection, and basic response trends were discussed by Wannamaker et al (1996), and a 2D isotropic inversion model using just the sparser long period soundings was derived by Ogawa et al (1996). Merged wideband and long period data were inverted by Wannamaker et al (2004) and Wannamaker (2005).…”

Section: Fossil Transpressional Regimesmentioning

confidence: 98%

“…It is thus a fossil analogue to the current situation under the South Island of New Zealand, Figure 16. Three inversion cross sections derived from MT transect across southern Appalachians compressional orogenic belt (site map and data in Wannamaker et al, 1996). Top section is TM mode only, middle section is joint TM-TE phase-vertical H-field model, and bottom is TE impedance phase and vertical magnetic field with model adherence to the TM mode section.…”

Section: Fossil Transpressional Regimesmentioning

confidence: 99%

Anisotropy Versus Heterogeneity in Continental Solid Earth Electromagnetic Studies: Fundamental Response Characteristics and Implications for Physicochemical State

Wannamaker

2005

Surv Geophys

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134

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Electrical anisotropy, the effect of current density in a medium being a function of the orientation of the electric field, is being recognized increasingly as an important effect in explaining Earth electromagnetic observations. A consideration of anisotropy, however, in most cases is an admission of spatial aliasing in earth structure, wherein the averaging volume of diffusive EM fields may be greater than the characteristic dimensions of a family of oriented structures, thus leading to a response which is equivalent to a bulk anisotropic medium. Even for two-dimensional geometries, there can be strong non-parallelism of principal axes of vertical magnetic field relative to the impedance over broad areas, as well as impedance phase variations which leave normal quadrants, if there are multiple directions of anisotropy or anisotropy strike distinct from bulk geometric (2D) strike. This paper concentrates on experience with regional field studies in continental settings where bulk anisotropy is apparent. Upper crustal anisotropy may result from preferred orientations of fracture porosity, or lithologic layering, or oriented heterogeneity. Lower crustal anisotropy may result from preferred orientations of fluidized/melt-bearing or graphitized shear zones, but does not necessarily reflect current state of stress per se. In the upper mantle, the prior causes all may act in pertinent domains, but added to these is the possibility of strong electrical anisotropy due to hydrous defects within shear-aligned olivine crystals (solid-state conduction). Several field examples from continental MT investigations will be discussed, which roughly fall into active transpressional, active transtensional, and fossil transpressional regimes. A general challenge in interpreting data with apparent anisotropic effects is to establish the tradeoff between heterogeneity and anisotropy in the inversion of EM responses.

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“…Anisotropic electrical conductivity in the lithospheric mantle with a fast conduction direction subparallel to the polarization of the fast split S-wave and to the tectonic trend has been proposed, for instance, beneath the Grenville front (Sénéchal et al, 1996), the Great Slave shear zone (Eaton et al, 2004), the Appalachians (Wannamaker et al, 1996), the Neoproterozoic belts of southeastern Brazil (Padilha et al, 2006), and the Variscan belt in central Germany (Gatzemeier and Moorkamp, 2005). In contrast, in the Kaapvaal lithospheric mantle, inferred fast conduction directions are oblique to the polarization of the fast split S-waves (Hamilton et al, 2006).…”

Section: Accepted Manuscriptmentioning

confidence: 99%

Heterogeneity and anisotropy in the lithospheric mantle

2015

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International audienceThe lithospheric mantle is intrinsically heterogeneous and anisotropic. These two properties govern the repartition of deformation, controlling intraplate strain localization and development of new plate boundaries. Geophysical and geological observations provide clues on the types, ranges, and characteristic length scales of heterogeneity and anisotropy in the lithospheric mantle. Seismic tomography points to variations in geothermal gradient and hence in rheological behavior at scales of hundreds of km. Seismic anisotropy data substantiate anisotropic physical properties consistent at scales of tens to hundreds of km. Receiver functions imply lateral and vertical heterogeneity at scales < 10 km, which might record gradients in composition or anisotropy. Observations on naturally deformed peridotites establish that compositional heterogeneity and Crystal Preferred Orientations (CPOs) are ubiquitous from the mm to the km scales. These data allow discussing the processes that produce/destroy heterogeneity and anisotropy and constraining the time scales over which they are active. This analysis highlights: (i) the role of deformation and reactive percolation of melts and fluids in producing compositional and structural heterogeneity and the feedbacks between these processes, (ii) the weak mechanical effect of mineralogical variations, and (iii) the low volumes of fine-grained microstructures and difficulty to preserve them. In contrast, olivine CPO and the resulting anisotropy of mechanical and thermal properties are only modified by deformation. Based on this analysis, we propose that strain localization at the plate scale is, at first order, controlled by large-scale variations in thermal structure and in CPO-induced anisotropy. In cold parts of the lithospheric mantle , grain size reduction may contribute to strain localization, but the low volume of fine-grained domains limits this effect

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Magnetotelluric Experiment probes deep physical state of southeastern United States

Cited by 14 publications

References 10 publications

Rheological heterogeneity, mechanical anisotropy and deformation of the continental lithosphere

Rheological heterogeneity, mechanical anisotropy and deformation of the continental lithosphere

Anisotropy Versus Heterogeneity in Continental Solid Earth Electromagnetic Studies: Fundamental Response Characteristics and Implications for Physicochemical State

Heterogeneity and anisotropy in the lithospheric mantle

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