| Abstract: | Stress estimates as a function of depth are obtained for peridotite xenoliths from the upper mantle of three types of tectonic environments by applying revised recrystallizedgrain-size paleopiezometry and pyroxene thermobarometry. The general increase in grain size with depth and hence decrease in deviatoric stress, observed previously, is confirmed but reversals in these trends are now established and remain enigmatic. Stresses and temperatures obtained are combined with a representative creep-flow law to calculate strainrate and viscosity profiles that appear to be physically reasonable. Profiles for the highthermal-gradient rift/ridge environments show a complexity that is interpreted as.a rheological discontinuity resulting from the emplacement of asthenospheric diapirs during late stages of continental rifting. Profiles for broad continental extension zones (C.E.Z.), believed to be most representative of oceanic upper mantle, fluctuate between 50 and 80 km, with a general small increase in strain rate and decrease in viscosity with depth; deepest samples apparently come from the base of the lithosphere. Profiles for the infracratonic mantle of southern Africa show nearly a uniform increase in strain rate to values greater than 10−14/sec, and a decrease in viscosity to lower than 1021 poise, at a depth of 230 km. These profiles may transect the mechanically defined lithosphere—asthenosphere transition at about 200 km and, if so, there is no evidence for a mechanical discontinuity at the boundary. This observation, coupled with evidence that the sense of shear is homogeneous for all mantle profiles constructed, clearly favors a model whereby lithospheric plates are dragged by thermal convection of the asthenosphere below. Sea-floor spreading rates and relative plate-velocity estimates are consistent with this interpretation but do not independently permit a definitive choice between the two favored models advanced to explain the driving force for plate motions. |
