Studies of free oscillations and body wave travel times have established that the one-way vertical S wave travel times through the upper mantle average about 5 s greater for oceans than for stable continental regions. This datum and the pure path dispersion data make it necessary to invoke significant differences between continental and oceanic shear velocity structures to depths exceeding 400 km. An analysis of the constraints on thermal profiles, including the constraints recently established by empirical geothermometry, indicates that temperature differences between the ancient shields and the ocean basins also extend deep into the mantle, with shields characterized by thermal gradients that are superadiabatic above 400 km. To at least 400-km depth beneath shields the dominant mechanism of heat transport is conduction, not advection. It is concluded that this entire region, termed here the continental tectosphere, translates coherently during lateral plate motions. It is proposed that the superadiabatic vertical temperature gradients beneath shields and the horizontal temperature gradients within the transition from shield to oceanic structures are stabilized by compositional gradients, the subcontinental mantle consisting of intrinsically less dense material. The existence of deep continental structure is apparently consistent with the dynamics of plate motions. It suggests that convection is not confined to the earth's upper mantle.

The continental tectosphere

An approximate theoretical relation is derived which relates stress during steady state creep to both subgrain size and dynamically recrystallized grain size. The relation results from equating the dislocation strain energy in the grain boundary to that in the enclosed volume. Available data on metals and silicates are in excellent agreement with the theory. For paleopiezometry, the recrystallized grain size must be preserved by quenching, by cooling under stress, or by inhibition of grain growth by intimate mixture of two or more phases. In general, stress may be underestimated using rocks in which grain size has been reduced by dynamic recrystallization, especially if the grain size is very small. Stress may be overestimated using coarse grained rocks in which the grain size has increased toward the steady state value. Quantitative limits remain to be established. The theoretical relation can in principle be applied to any metal or mineral if only the effective isotropic elastic moduli and the Burgers vector are known. When used as a paleopiezometer, the technique indicates that high stresses on the order of 100 MPa are not infrequently associated with mantle diapirism and with large scale thrust faulting. Consideration of the Mt. Albert ultramafic body suggests that texturally inferred stresses from peridotite massifs and from ultramafic xenoliths in alkali olivine basalts might reflect either horizontal variations in stress across a rising diapir or else a vertical variation in stress as defined by the pyroxene geobarometer (Mercier et al. 1977). In either case the stresses are probably characteristic of local diapirism. Stresses characteristic of global upper mantle flow might be inferred from xenoliths originating from above kimberlite-producing diapirs.

Theory and Applicability of a Recrystallized Grain Size Paleopiezometer

http://www.gm.univ-montp2.fr/PERSO/mainprice/W_data/My_Publications_PDF/Mainprice_Silver1993PEPI_SKS_SubContiental_Lithosphere.pdf

Interpretation of SKS-waves using samples from the subcontinental lithosphere

A data set of 1033 three-component, P wave seismograms from five broadband stations at the Yellowknife Array is assembled to investigate mantle structure below the southern Slave province in Canada's Northwest Territories. Following wave field decomposition, seismograms are source-normalized through simultaneous deconvolution to estimate the near-receiver impulse response as a function of epicentral distance and back azimuth. Images of impulse response reveal a well-developed mantle stratigraphy, anisotropic in part, extending from the Mohorovicic discontinuity to the transition zone, A layer of depth-localized anisotropy (±5%), termed H, is situated between ∼70 and 80 km depth with an average shear velocity comparable to that of the ambient mantle and a sharp upper boundary less than 100 m in transition width. The absence of free surface crustal reverberations on the transverse component affords a window into the upper mantle between 100 and 200 km depth. A sequence of at least two layers between 120–150 km depth, collectively termed X, is most clearly evident to the north and is underlain by a second structure L which dips from 170 km in the west to 230 km into the center of the Slave province. The deepest interface above the transition zone W marks a shear velocity inversion near 350 km depth whose signature is restricted to the SV component signalling a dominantly isotropic response. Consideration of these observations in light of data acquired in a recent LITHOPROBE seismic reflection traverse and in petrological studies of kimberlite xenoliths prompts speculation into the role of subduction in craton stabilization. It is suggested that the proto-Slave craton was assembled through processes of shallow subduction resulting in a near-horizontal mantle stratigraphy (i.e., H, X) both compositional and rheological in nature. Interpretation of L as the continuation of dipping reflectors on the seismic reflection profile argues for a final phase of craton assembly involving oblique underplating of subducted lithosphere in the Proterozoic. Subsequent modification of the lithosphere, as manifest by Phanerozoic kimberlite volcanism, may be related to W if an interpretation as the top of a layer containing a dense silicate melt fraction is invoked.

Mantle stratigraphy and evolution of the Slave province

Experimentally determined steady state flow properties and processes of important rock-forming materials are reviewed in reference to those of metals and ceramics and to physical conditions in the earth's crust and upper mantle. Dislocation motion controls the creep rate over a wide range of steady state conditions in the experiments, and the observation that the same processes have operated during natural deformations permits extrapolations of the mechanical data. Under these conditions, strain rate is related to stress raised to the power 2–9, depending on the material and conditions, and the resulting flow stresses and equivalent viscosities are compared at a representative geological strain rate of 10−14/s. The results are applied in brief discussions of diapirism, growth of folds, and flow in the upper mantle.

Steady state flow of rocks

A diapiric mechanism is presented to explain geochemical data on the thermal structure of the upper mantle. It is emphasised that any model to fit the data cannot be a steady state model, and it is concluded that the data are inconsistent with the popular interpretation of the seismic low velocity zone as a partially melted region.

Y. Gueguen

Papers

Origin of kimberlite pipes by diapiric upwelling in the upper mantle