Collaborative Research: Determining Mantle Rheology from Field and Microstructural Observations of Naturally-deformed Peridotites

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A crucial and unsolved question in global earth science is the relative strength of the different lithospheric layers that make up the tectonic plates. Rheology, the term used to describe material behavior, quantifies the relation between the observed deformation (strain) and the applied forces (stress). The principal investigators of this grant combine quantitative, field-based geologic studies with data from experimental rock deformation studies to constrain the rheology of naturally-deformed mantle rocks. The fieldwork provides analysis of deformation at geologic strain-rates, spatial scales, and levels of compositional heterogeneity, while laboratory studies provide data from controlled deformation experiments at known stresses and strain-rates for monophase materials. The lithospheric (uppermost) mantle is the focus of this study for three reasons. First, mantle rocks have a relatively simple mineralogy (typically only three minerals, volumetrically dominated by the mineral olivine). Second, a large amount of experimental data has been conducted on olivine, allowing calibration of the field studies. Third, because of the high strength of olivine in experimental studies, many current models assume that the lithospheric mantle dominates the material behavior of the tectonic plates. Two extremely well-exposed sections of upper mantle rocks, the Dun Mountain ophiolite body (Red Hills section) in New Zealand and the Twin Sisters dunite in Washington State, USA are investigated in this study. The two field areas provide different, but complimentary, constraints on mantle rheology. The Twin Sisters dunite is an ideal field locality to: 1) Quantify deformation in a mantle section, by using geological markers known as dikes; 2) Observe spatial variations in mantle deformation; and 3) Determine the effect of the 2nd mineral phase (orthopyroxene) on the bulk rheology of natural peridotites. The Red Hills section of the Dun Mountain ophiolite, New Zealand, allows the PIs to investigate two additional aspects of mantle deformation: 1) The rheological implications of large-scale structures (e.g., folds) present in naturally deformed mantle bodies; and 2) Why localization, as inferred from ductilely sheared markers, takes place in the mantle. From these data sets, the bulk rheology of lithospheric mantle will be constrained. The grant supports a graduate student and multiple undergraduate students at two universities. The funding forms the basis for international cooperation with a New Zealand university, allowing students to visit both universities.