Doctor of Philosophy
Earth’s lithosphere may be regarded as a composite material made of rheologically heterogeneous elements. The presence of these heterogeneous elements causes flow partitioning, making the deformation of Earth’s lithosphere heterogeneous on all observation scales. Understanding the multiscale heterogeneous deformation and the overall rheology of the lithosphere is very important in structural geology and tectonics. The overall rheology of Earth’s lithosphere on a given observation scale must be obtained from the properties of all constituents and may evolve during the deformation due to the fabric development. Both the problem of flow partitioning and characterization of the overall rheology are closely related and require a fully mechanical multiscale approach.
This thesis refines a micromechanics-based multiscale modeling approach called the self-consistent MultiOrder Power Law Approach (MOPLA). MOPLA treats the heterogeneous rock mass as a continuum of rheologically distinct elements. The rheological properties and the mechanical fields of the constituent elements and those of the composite material are computed by solving partitioning and homogenization equations self-consistently. The algorithm of MOPLA has been refined and implemented in MATLAB for high-performance computing. The micromechanical approach is used to investigate the deformation of ductile high-strain zones, advancing previous work on this subject to a full mechanical level.
This thesis considers a ductile high-strain zone as a flat heterogeneous inclusion embedded in the ductile lithosphere subjected to a tectonic deformation due to remote plate motion. The kinematic and the mechanical fields inside and outside the high-strain zone, including the finite strain accumulation in there, are solved by partitioning equations. The overall rheology of the high-strain zone is obtained by means of a self-consistent homogenization scheme.
Understanding the continental rheology requires an accurate quartz dislocation creep flow law. Despite decades of experimental studies, there are considerable discrepancies in quartz flow law parameters. This thesis proposes that the discrepancies could be explained by considering both the pressure effect on the activation enthalpy and the slip system dependence of the stress exponent. Two distinct dislocation creep flow laws corresponding to two dominant slip systems are determined based on the current dataset of the creep experiments on quartz samples.
Summary for Lay Audience
The Earth is a dynamic planet, and various parts of Earth interact. In response to the application of the deforming forces, Earth’s lithosphere deforms by frictional slip along preexisting faults near the surface; at greater depth, it deforms predominantly by crystalline plasticity, leaving abundant geological records, like fabrics and structures during the geological history. Structural geology deals with the fabrics and structures from a regional to a submicroscopic scale to reconstruct the lithospheric deformation process and understand the mechanical properties (rheology) of the rocks in the lithosphere.
Rock masses in Earth’s lithosphere are composed of many constituent elements, having distinct rheological properties. When the lithosphere is subjected to tectonic deformation, the mechanical and kinematic fields vary across the rheologically distinct elements because of the variations in rheology, leaving various fabrics and structures. The small-scale fabrics and structures can only relate to the relevant scale fields but not to the tectonic scale deformation process. In order to relate the small-scale features to the tectonic deformation, a multiscale approach is required.
On the other hand, as Earth’s lithosphere is composed of many rheologically distinct elements, the overall rheology of the lithosphere must be obtained from the properties of all constituents. This process requires the knowledge of the rheological properties of all constituents, which are mainly based on high-temperature and high-pressure creep experiments on natural rocks or synthetic mineral aggregates, and a fully mechanical multiscale approach to help us obtain the overall rheology.
In fact, the multiscale deformation in Earth’s lithosphere and the variation and evolution of the lithosphere rheology are closely related. Both require a fully mechanical multiscale approach combined with the observations from natural rocks and experiments. Recently a micromechanics-based self-consistent MultiOrder Power Law Approach (MOPLA) has been proposed to address the multiscale deformation in Earth’s lithosphere and simulate the mechanical behavior of the lithosphere. This thesis applied this fully mechanical multiscale approach together with the high-quality data of creep experiments on wet quartzites and the geological records in natural rocks to investigate the multiscale deformation in Earth’s lithosphere and the continental rheology.
Lu, Xi, "A Micromechanics-based Multiscale Approach toward Continental Deformation, with Application to Ductile High-Strain Zones and Quartz Flow Laws" (2020). Electronic Thesis and Dissertation Repository. 6965.