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The relative motion of lithospheric plates and underlying mantle produces localized deformation near the lithosphere-asthenosphere boundary (1). The transition from rheologically stronger lithosphere to weaker asthenosphere may result from a small amount of melt or water in the asthenosphere, reducing viscosity (1-3). Either possibility may explain the seismic and electrical anomalies that extend to a depth of about 200 kilometres (4,5). However, the effect of melt on the physical properties of deformed materials at upper-mantle conditions remains poorly constrained (6). Here we present electrical anisotropy measurements at high temperatures and quasi-hydrostatic pressures of about three gigapascals on previously deformed olivine aggregates and sheared partially molten rocks. For all samples, electrical conductivity is highest when parallel to the direction of prior deformation. The conductivity of highly sheared olivine samples is ten times greater in the shear direction than for undeformed samples. At temperatures above 900 degrees Celsius, a deformed solid matrix with nearly isotropic melt distribution has an electrical anisotropy factor less than five. To obtain higher electrical anisotropy (up to a factor of 100), we propose an experimentally based model in which layers of sheared olivine are alternated with layers of sheared olivine plus MORB or of pure melt. Conductivities are up to 100 times greater in the shear direction than when perpendicular to the shear direction and reproduce stress-driven alignment of the melt. Our experimental results and the model reproduce mantle conductivity-depth profiles for melt-bearing geological contexts. The field data are best fitted by an electrically anisotropic asthenosphere overlain by an isotropic, high-conductivity lowermost lithosphere. The high conductivity could arise from partial melting associated with localized deformation resulting from differential plate velocities relative to the mantle, with subsequent upward melt percolation from the asthenosphere., Electromagnetic profiles of the lithosphere-asthenosphere system reveal zones of high electrical conductivity and electrical anisotropy, which vary with depth (Fig. 1) (5,7,8). High conductivity can be attributed to several factors, [...]

The mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics, and thus the understanding of the processes at the lithosphere-asthenosphere boundary (LAB) is fundamental to understand how our planet works. It is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the LAB. Data from borehole broadband ocean bottom seismometers show that the LAB beneath the Pacific and Philippine Sea plates is sharp and age-dependent. The observed large shear wave velocity reduction at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in meltless mantle, which accounts for the large viscosity contrast at the LAB that facilitates horizontal plate motions.

The primary aim of the present paper is (1) to review the tectonomagmatic evolution of the North Atlantic, and (2) constrain evolutionary models with new lithosphere strength estimates and interpretation of potential field data north of Iceland. Our interpretations suggest that the breakup along the entire eastern Jan Mayen Ridge occurred at c. 55 Ma. Calculations of lithospheric yield strength indicate that the continental rifting in East Greenland, which led to oceanic crustal formation west of the Jan Mayen Ridge at c. 25 Ma, could have started at c. 42.5 Ma. Symmetrical V-shaped gravimetric ridges, which can be traced back to c. 48 Ma, document large-scale asthenospheric flow both north and south of Iceland. Such flow is predicted by geodynamic models of mantle plumes, but has yet to be predicted by other mechanisms. The results from the compartments north of Iceland, viewed in a regional context, strengthen the hypothesis attributing the anomalous magrnatism in the North Atlantic area from c. 70 Ma to the present to the Icelandic plume.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.01.015 Byline: Karen Luttrell, David Sandwell Keywords: Europa; Ganymede; Callisto; Ices; Tectonics Abstract: Several approaches have been used to estimate the ice shell thickness on Callisto, Ganymede, and Europa. Here we develop a method for placing a strict lower bound on the thickness of the strong part of the shell (lithosphere) using measurements of topography. The minimal assumptions are that the strength of faults in the brittle lithosphere is controlled by lithostatic pressure according to Byerlee's law and the shell has relatively uniform density and thickness. Under these conditions, the topography of the ice provides a direct measure of the bending moment in the lithosphere. This topographic bending moment must be less than the saturation bending moment of the yield strength envelope derived from Byerlee's law. The model predicts that the topographic amplitude spectrum decreases as the square of the topographic wavelength. This explains why Europa is rugged at shorter wavelengths ([approximately equal to]10 km) but extremely smooth, and perhaps conforming to an equipotential surface, at longer wavelengths (>100 km). Previously compiled data on impact crater depth and diameter [Schenk, P.M., 2002. Nature 417, 419-421] on Europa show good agreement with the spectral decrease predicted by the model and require a lithosphere thicker than 2.5 km. A more realistic model, including a ductile lower lithosphere, requires a thickness greater than 3.5 km. Future measurements of topography in the 10-100 km wavelength band will provide tight constraints on lithospheric strength. Author Affiliation: Scripps Institution of Oceanography, La Jolla, CA 92093-0225, USA Article History: Received 10 June 2005; Revised 19 January 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.11.001 Byline: Trudi Hoogenboom, Gregory A. Houseman Keywords: Venus Abstract: In this study we explore the idea that coronae have formed on Venus as a result of gravitational (Rayleigh-Taylor) instability of the lithosphere. The lithosphere is represented by a system of stratified homogeneous viscous layers (low-density crust over high density mantle, over lower density layer beneath the lithosphere). A small harmonic perturbation imposed on the base of the lithosphere is observed to result in gravitational instability under the constraint of assumed axisymmetry. Topography develops with time under the influence of dynamic stress associated with downwelling or upwelling, and spatially variable crustal thickening or thinning. Topography may therefore be elevated or depressed above a mantle downwelling, but the computed gravity anomaly is always negative above a mantle downwelling in a homogeneous asthenosphere. The ratio of peak gravity to topography anomaly depends primarily on the ratio of crust to lithospheric viscosity. Average observed ratios are well resolved for two groups of coronae ([approximately equal to]40 mgalakm.sup.-1), consistent with models in which the crust is perhaps 5 times stronger than the lithosphere. Group 3a (rim surrounding elevated central region) coronae are inferred to arise from a central upwelling model, whereas Group 8 (depression) coronae are inferred to arise from central downwelling. Observed average coronae radii are consistent with a lithospheric thickness of only 50 km. An upper low-density crustal layer is 10-20 km thick, as inferred from the amplitude of gravity and topography anomalies. Author Affiliation: School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, UK Article History: Received 4 October 2004; Revised 4 August 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.09.010 Byline: Javier Ruiz (a), Rosa Tejero (a), Patrick J. McGovern (b) Keywords: Mars; Geophysics; Tectonics Abstract: Two independent sets of heat flow estimates provide constraints on the Hesperian-era surface and mantle heat flows, and the thickness of the heat-producing elements (HPE)-enriched upper crust, in the Solis Planum region of Mars. The calculations, which use the concentration of uppermost crust heat sources deduced from orbital gamma ray spectroscopy and soils geochemistry, are based on the effective elastic thickness of the lithosphere and the minimum depth of faults underlying winkle ridges. We find that, for the majority of analyzed settings, the HPE-enriched crust is thinner than the whole crust thickness in this region ([approximately equal to]65 km). Thus, our results strongly support a differentiated martian crust. Author Affiliation: (a) Departamento de Geodinamica, Facultad de Ciencias Geologicas, Universidad Complutense de Madrid, 28040 Madrid, Spain (b) Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, USA Article History: Received 27 April 2005; Revised 20 September 2005

Chinese continental lithosphere comprises three tectonic domains: (l) eastern China, a region characterized by rifting, extensional basins, and voluminous basaltic volcanism; (2) central China, a plexus of cratonic terrains with low-heat flow (40-50 mw/[m.sup.2]), including the Tarim, Erdos, and Yangtze blocks welded by pre-Cenozoic orogenic belts; and (3) western China, a region comprising the Qinghai-Tibet-Himalaya orogen. The relatively thin crust(~ 35 km) and lithosphere (~ 70 km) of eastern China is believed to reflect mantle upwelling, while near-normal crust(~ 45 km) and thickened lithosphere (>200 km) in central China suggest a mantle lithosphere root resembling those inferred for the Kaapvaal and Siberian cratons. Thickened crust (~ 70 km) and lithosphere (>150 km) in western China reflect the advanced development of an orogen root. The lower density and relative buoyancy of central cratonic mantle roots appear to have contributed to the long-term tectonic stability of the central Chinese lithospheric blocks. In contrast, gravitational instability produced by thickening of the denser western Chinese lithosphere and the resulting subsidence and eventual delamination of orogenic roots are believed to have led to postorogenic extensional collapse. Although the geological relationships suggest compressional forces caused both mountain building and orogen root development, extensional stress resulting from gravitational collapse is believed to have induced lithospheric and crustal thinning with concomitant reduction of topography. The term 'continental roots--plume tectonics' has been adopted to describe the configuration and dynamic condition of subcontinental lithosphere and upper mantle beneath China. Accordingly, it is proposed that supracrustal tectonic forms represent the surface expressions of, and responses to, deep 'continental roots--plume tectonics'. While the prevailing view is that the western orogenic belt is not genetically related to eastern continental rifling, such a relationship is inherent to the continental roots--plume tectonics model. It is further proposed that the formation of orogenic lithosphere roots triggered eastward extrusion of the asthenosphere along the 400-km depth mantle interface, and, in response to subduction at the eastern margin, produces plume-like upwelling beneath eastern China. We suggest that processes involved in continental roots--plume tectonics are directly responsible for the formation and evolution of the Eurasian supercontinent. Keywords: China; Lithosphere; Continental roots--plume tectonics