Normal faulting in central Tibet since at least 13.5 Myr ago.

Tectonic models for the evolution of the Tibetan plateau interpret observed east-west thinning of the upper crust to be the result of either increased potential energy of elevated crust or geodynamic processes that may be unrelated to plateau formation. A key piece of information needed to evaluate these models is the timing of deformation within the plateau. The onset of normal faulting has been estimated to have commenced in southern Tibet between about 14 Myr ago and about 8 Myr ago and, in central Tibet, about 4 Myr ago. Here, however, we report a minimum age of approximately 13.5 Myr for the onset of graben formation in central Tibet, based on mineralization ages determined with Rb-Sr and 40Ar-39Ar data that post-date a major graben-bounding normal fault. These data, along with evidence for prolonged activity of normal faulting in this and other Tibetan grabens, support models that relate normal faulting to processes occurring beneath the plateau. Thinning of the upper crust is most plausibly the result of potential-energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath Tibet. This is supported by recent geophysical and geological data, which indicate that spatial heterogeneity exists in both the Tibetan crust and lithospheric mantle.

Rapid emplacement of young oceanic lithosphere: argon geochronology of the oman ophiolite.

(40)Ar/(39)Ar dates of emplacement-related metamorphic rocks beneath the Samail ophiolite in Oman show that cooling to <525 degrees C occurred within approximately 1 million years of igneous crystallization of the ophiolite. This unexpectedly short time span and rapid cooling means that old, cold continental or oceanic lithosphere must have been adjacent to the ophiolite during spreading and then been thrust beneath the ophiolite almost immediately afterward.

Time and metamorphic petrology: calcite to aragonite experiments.

Although the equilibrium phase relations of many mineral systems are generally well established, the rates of transformations, particularly in polycrystalline rocks, are not. The results of experiments on the calcite to aragonite transformation in polycrystalline marble are different from those for earlier experiments on powdered and single-crystal calcite. The transformation in the polycrystalline samples occurs by different mechanisms, with a different temperature dependence, and at a markedly slower rate. This work demonstrates the importance of kinetic studies on fully dense polycrystalline aggregates for understanding mineralogic phase changes in nature. Extrapolation of these results to geological time scales suggests that transformation of calcite to aragonite does not occur in the absence of volatiles at temperatures below 200 degrees C. Kinetic hindrance is likely to extend to higher temperatures in more complex transformations.

Observational evidence for a possible new diffusion path.

Transmission electron microscopy of experimentally deformed amphibolite suggests that submicroscopic intracrystalline tubes formed around linear defects may be a previously unrecognized kind of diffusion pathway. Deformed and compositionally altered plagioclase and amphibole crystals include moderate densities of linear defects that morphologically resemble unit dislocations but display unusual contrast. During prolonged electron irradiation, the core regions of the defects expand to well-defined tubes that are approximately 20 nanometers in diameter. Both observations suggest that the regions about the defect cores are glassy and were filled with silicate-water fluid during the experiments. Intracrystalline transport along these tubes would likely be several orders of magnitude faster than traditionally conceived processes of solid-state volume diffusion, grain-boundary solvent transfer, and ordinary pipe diffusion along dislocation cores.