Because plate tectonics relies intimately on the deformation behavior of the upper mantle and lower crust, a thorough understanding of the factors that control lithospheric deformation is critical to our knowledge of many primary geodynamic processes. Importantly, although plate tectonic processes operate over the 10–1000 km scale, the ductile flow of rocks in the Earth depends on the deformation of crystals at the microscopic and sub-microscopic scale.

Our research focuses on how microscale features control the mechanics of plate tectonics.

Dislocation density from high-resolution EBSD

Dislocations are defects in crystals that are fundamental to plastic deformation. Many constitutive models for the rheological behavior of rocks include dislocation density as a key parameter. Thus, we are motivated to understand the temporal evolution and spatial heterogeneity of dislocation density in deforming rocks to better predict rheological behaviour under geological conditions. Outstanding questions include,

How does the character of dislocation structures developed during deformation vary as a function of dominant deformation mechanism?

Are there different types of dislocation that are more important than others in controlling the microstructural evolution or mechanical behavior?

How can we quickly and accurately determine the dislocation content with high spatial resolution and simultaneously over large representative volumes?

We have been exploring these questions primarily through scanning-electron microscopy in collaboration with the Oxford Micromechanics group in the Department of Materials. The presence of dislocations in a crystal necessarily gives rise to curvature of the lattice, which is measurable by high-resolution electron-backscatter diffraction. The densities of different types of dislocations can be determined, and we are working to relate the spatial distribution of those densities to mechanical behavior across a wide range of deformation conditions.

B3 for website

An example of high-resolution EBSD in a deformed olivine single crystal.

For publications related to this topic, see:

David Wallis, Lars N. Hansen, T. Ben Britton, Angus J. Wilkinson (2016), Geometrically necessary dislocation densities in olivine obtained using high-angular resolution electron backscatter diffraction, Ultramicroscopy, 168, 34–45, doi:10.1016/j.ultramic.2016.06.002. (PDF)

Seismic anisotropy and textural evolution of olivine aggregates

PT0718_movie_small

Olivine orientations from an aggregate experimentally deformed in torsion. The movie is constructed by scrolling through a radial section mapped by EBSD. In torsion, the shear strain scales linearly with the radius. Each point represents the orientation of a single grain. The grayscale indicates multiples of a uniform distribution.

Crystallographic texture, the alignment of many grains in a rock according to their crystallographic axes, is one of the key pieces of microstructural evidence used to determine the mechanisms by which a rock deformed. Because strongly textured rocks become anisotropic in their material properties (e.g., elasticity), much of the observed seismic anisotropy in the upper mantle has been attributed to the alignment of olivine grains. Although much experimental, theoretical, and observational work has been done to understand the evolution of olivine textures. Some significant questions remain. 

What is the steady-state texture developed at high strain?

What is the time scale of textural evolution,and how do pre-existing textures affect subsequent textural development?

Are laboratory-derived textures necessarily the ones that will be developed in Earth?

How can laboratory observations be used to interpret patterns of seismic anisotropy observed in Earth?

We are addressing these questions by quantifying the subtleties of olivine textural evolution in laboratory experiments under known conditions and comparing those results to textures observed in naturally deformed peridotites. For publications related to this topic, see:

L. N. Hansen, C. Qi, J. M. Warren (2016), Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1608269113.

P. Skemer and L. N. Hansen (2016), Inferring mantle flow from seismic anisotropy: An experimental perspective, Tectonophysics, 668-669, 1-14, doi:10.1016/j.tecto.2015.003. (PDF)

L. N. Hansen, J. M. Warren, M. E. Zimmerman, D. L. Kohlstedt (2016), Viscous anisotropy of textured olivine aggregates, Part 1: Measurement of the magnitude and evolution of anisotropy, Earth Planet. Sci. Lett., 445, 92–103, doi:10.1016/j.epsl.2016.04.008. (PDF)

L. N. Hansen and J. M. Warren (2015), Quantifying the effect of pyroxene on deformation of peridotite in a natural shear zone, J. Geophys. Res., 120, B011584. (PDF)

L. N. Hansen, Y. -H. Zhao, M. E. Zimmerman, D. L. Kohlstedt (2014), Protracted fabric evolution in olivine: Implications for the relationship among strain, crystallographic fabric, and seismic anisotropy, Earth Planet. Sci. Lett., 387, 157-158. (PDF)

Microstructural and rheological heterogeneity in natural peridotites

Folded pyroxenite dike within plastically deformed harzburgite of the Josephine Peridotite. Umbrella for scale.

One of the biggest challenges in scaling mechanical properties from the laboratory to Earth is the increase in compositional complexity from synthetic rocks to natural rocks. For example, the manner in which pyroxene affects the strength of a peridotite (dominantly olivine) is still controversial. We are working to elucidate the role of pyroxene in peridotite deformation by examining natural rocks. Variations in microstructural characteristics between pyroxene-poor and pyroxene-rich rocks yield valuable information about differences in strength. Additionally, larger structures such as folding (see photo at left) or boudinage can be used to quantify contrasts in viscosity due to changes in composition. This research focuses on three primary questions.

Does pyroxene primarily affect peridotite viscosity by keeping the olivine grain size small or by introducing strength heterogeneites?

Is pyroxene or olivine the weakest phase at low temperatures (600–800°C)?

How does the addition of pyroxene affect the dominant deformation mechanism and the development of crystallographic fabrics?

For publications related to this topic, see:

L. N. Hansen and J. M. Warren (2015), Quantifying the effect of pyroxene on deformation of peridotite in a natural shear zone, J. Geophys. Res., 120, B011584. (PDF)

Viscous anisotropy of strongly textured rocks

Schematic illustration of deformation experiments to test the magnitude of viscous anisotropy in olivine aggregates. Samples are initially deformed in torsion and then subsequently deformed in tension.

Olivine is a significantly anisotropic mineral, which implies that olivine-rich rocks with strong crystallographic textures will be similarly anisotropic (see above). The elastic anisotropy of mantle rocks is well studied and is used to infer patterns mantle convection. The viscous anisotropy of mantle rocks, however, has been essentially unmeasured. Geodynamic simulations are beginning to reveal that large degrees of viscous anisotropy in the upper mantle can affect convection patters, glacial rebound, and melt production at subduction zones.

Photograph of olivine aggregate after initially being deformed in torsion and subsequently deformed in tension. Tick marks are 0.5 mm.

To help place important constraints on future geodynamic simulations, we have been working on a set of experiments to measure the degree of viscous anisotropy in olivine aggregates. This project aims to measure several components of the viscosity tensor and to describe the evolution of those components with increasing crystallographic fabric strength. For publications related to this topic, see:

L. N. Hansen, C. Conrad, Y. Boneh, P. Skemer, J. M. Warren, D. L. Kohlstedt (2016), Viscous anisotropy of textured olivine aggregates, Part 2: Micromechanical model, J. Geophys. Res., 121, doi:10.1002/2016JB013240.

L. N. Hansen, J. M. Warren, M. E. Zimmerman, D. L. Kohlstedt (2016), Viscous anisotropy of textured olivine aggregates, Part 1: Measurement of the magnitude and evolution of anisotropy, Earth Planet. Sci. Lett., 445, 92–103, doi:10.1016/j.epsl.2016.04.008. (PDF)

Kohlstedt, D. L. and L. N. Hansen (2015), Constitutive equations, rheological behavior, and viscosity of rocks, Treatise on Geophysics, 2nd edition, vol 2., pp. 441-472. (PDF)

Skemer, P., J. M. Warren, L. N. Hansen, G. Hirth, P. B. Kelemen (2013), The influence of water and LPO on the initiation and evolution of mantle shear zones, Earth Planet. Sci. Lett., 375, 222-233 (PDF)

Hansen, L. N., M. E. Zimmerman, and D. L. Kohlstedt (2012), Laboratory measurements of the viscous anisotropy of olivine aggregates, Nature, 492, 415-418. (doi:10.1038/nature11671)

Hansen, L. N., M. E. Zimmerman, and D. L. Kohlstedt (2012), The influence of microstructure on deformation of olivine in the grain-boundary sliding regime, J. Geophys. Res., 117, B09201. (PDF)


Grain-boundary sliding in olivine

Sample assemblies after deformation in the Paterson apparatus at the University of Minnesota. The sample on the left was deformed in torsion, and the sample on the right was deformed in compression. Sample assemblies are jacketed in Fe. Tick marks are 0.5 mm.

Grain-boundary sliding accommodated by the motion of dislocations (GBS) has recently received considerable attention as a deformation mechanism in olivine. However, early datasets suggest that GBS is only important in the highest-stress portions of the lithosphere. We have set out to build a robust dataset for olivine deforming in the GBS regime to further our understanding of a potentially very important deformation mechanism. This research has concentrated on addressing three primary questions.

Does GBS control the viscosity of the upper mantle and produce a distinct crystallographic fabric?

Do constant-stress boundary conditions help GBS to initiate strain localization?

How does the viscosity of a rock deforming by GBS change with evolving microstructure?

Orientation maps constructed from electron-backscatter diffraction data. Maps for an undeformed aggregate of olivine (left) and an aggregate of olivine deformed in torsion (right). Maps are colored according to Schmid factor for the (010)[100] slip system with warmer colors indicating higher values.

The Paterson apparatus at the University of Minnesota, capable of deforming olivine with both axial compression and torsion deformation modes, has provided an ideal tool for addressing these questions. In addition, quantifying microstructures using detailed electron-backscatter diffraction mapping has greatly enhanced the precision of our results. For publications related to this topic, see:

Kohlstedt, D. L. and L. N. Hansen (2015), Constitutive equations, rheological behavior, and viscosity of rocks, Treatise on Geophysics, 2nd edition, vol 2., pp. 441-472. (PDF)

Hansen, L. N., M. E. Zimmerman, and D. L. Kohlstedt (2012), The influence of microstructure on deformation of olivine in the grain-boundary sliding regime, J. Geophys. Res., 117, B09201. (PDF)

Hansen, L. N., A. M. Dillman, M. E. Zimmerman, and D. L. Kohlstedt (2012), Strain localization in olivine aggregates at high temperature: An experimental comparison of constant-strain-rate and constant-stress boundary conditions, Earth Planet. Sci. Lett., 333-334, 134-145.  (PDF)

Hansen, L. N., M. E. Zimmerman, and D. L. Kohlstedt (2011), Grain-boundary sliding in San Carlos olivine: Flow-law parameters and crystallographic preferred orientation, J. Geophys. Res., 116, B08201. (PDF)

Deformation associated with oceanic detachment faults

An oblique view of the Kane Oceanic Core Complex on the Mid-Atlantic Ridge.

An oblique view of the Kane Oceanic Core Complex on the Mid-Atlantic Ridge.

Over the past several decades, oceanic core complexes have been recognized as a major feature of oceanic lithosphere formed at slow- and ultra-slow spreading mid-ocean ridges. Thus, the rheological characteristics of oceanic core complexes and their associated detachment faults are critical to the dynamics of much of Earth’s ridge system. Using samples collected by dredging and remotely operated submersible from the Mid-Atlantic Ridge near the Kane Fracture Zone, we have been striving to answer three prominent questions about oceanic core complexes.

How deep do oceanic detachment faults root?

What are the primary deformation mechanisms operating during detachment faulting?

What is the cause for differences in deformation style at different oceanic core complexes?

Gabbro mylonite associated with detachment faulting at the Kane Oceanic Core Complex. Field of view is approximately 1 cm.

Gabbro mylonite associated with detachment faulting at the Kane Oceanic Core Complex. Field of view is approximately 1 cm.

We have been using crystallographic, stereologic, and thermometric analytical techniques combined with laboratory-derived constitutive equations to address these questions. For publications related to this topic, see:

L. N. Hansen, M. Cheadle, B. John, H. Dick, B. Tucholke, M. Tivey (2013), Mylonitic deformation at the Kane oceanic core complex: Implications for the rheological behavior of oceanic detachment faults, Geochem., Geophys., Geosys., (14) 8, 3085-3108. (PDF)