EFFECTS OF COMPOSITIONAL HETEROGENEITY on LOCALIZATION IN UPPER MANTLE DEFORMATION

While the lithologic complexity of the Earth’s crust is recognized as a contributing factor to the crust’s structural heterogeneity, the upper mantle is commonly assumed to be compositionally and structurally homogeneous, exhibiting homogeneous large-scale flow. Although the mantle is mineralogically less complex than the crust, studies of naturally deformed mantle rocks demonstrate that 1) mantle rocks also deform along mm- to km-scale localized zones, and 2) the compositional variability present in the upper mantle is responsible for much of this localized deformation.

Compositional variability leads to heterogeneous mantle deformation by causing rheological heterogeneities. Stress concentrations resulting from these heterogeneities result in increased strain energy, providing activation energy to promote reactions in polyphase rocks. Reactions, in turn, lead to grain size reduction and strain localization changing the dominant deformation mechanism from dislocation to diffusion creep.

Examples of rheological heterogeneity resulting in strain localization occur in the Red Hills ultramafic massif, New Zealand, where solid-state deformation is localized within narrow dunite bands in a harzburgite host. Phase variability, in the form of melt, also resulted in the juxtaposition of km-scale structural domains along a zone marked by increased volume of clinopyroxene dikes and a penetrative plagioclase foliation. In the Twin Sisters ultramafic massif, WA, pyroxenite dikes in a dunite host served as rheological heterogeneities that localized deformation within a cm-scale shear zone. Pyroxene in the sheared dunite promoted a reaction that produced fine-grained reaction products, changing the dominant deformation mechanism from dislocation to diffusion creep. Similarly, a continuous reaction associated with the spinel to plagioclase lherzolite transition promoted localization in the Turon de Técouère mylonites in the North Pyrenees.

In crustal rocks, fluids serve as reactants or products leading to strain localization. In the mantle, fluids may not be available. However, as polyphase mantle rocks move in P-T space, high T combined with strain energy may provide sufficient activation energy in highly strained zones for reactions to lead to grain size reduction and strain localization.