Doctor of Philosophy
Earth’s lithosphere is heterogeneous and composed of rheologically distinct elements at various scales of observations. This causes the flow of rocks to vary with space and time, which may influence the formation of various kinds of geological rock records. This thesis provides quantitative solutions to some first-order problems in structural geology regarding this heterogeneous flow variation and thereby the development of various geological rock records at different scales of observations.
Pressure in a rheologically heterogeneous element may deviate from its ambient value and if significant, may influence the metamorphic assemblages. This might cause problems in the routine use of geothermobarometry-based pressure estimates from mineral assemblages as a proxy for depth in geodynamic models of geological processes. A micromechanics-based multiscale model called Multi Order Power Law Approach (MOPLA) is applied to simulate pressure deviation in and around a rheologically distinct rock element embedded in a rock medium of a non-linear viscous anisotropic rheology. The results show that the pressure deviations are in the same order as the deviatoric stress levels and hence limited by the strength of rocks.
Flow variation can influence c-axis fabrics from quartz aggregate within feldspar-mica matrix in the natural high strain zones. If such effect is not accounted for, crucial geological information such as deformation temperature, deformation history and shear sense obtained using c-axis fabrics can be seriously misinterpreted. A multiscale approach coupling MOPLA with Viscoplastic Self-consistent (VPSC) model is used to simulate c-axis fabrics under partitioned flow. The results show that the quartz c-axis fabric variation showing apparent opposite senses of shear within a single thin section, can be explained by partitioned flow within the quartz domains and reflect finite strain gradient rather than a reversal of vorticity sense as previously thought.
Flow variation can form flanking structures around a cross-cutting element like a vein or a dyke that may provide kinematic information such as ‘shear sense’ and ‘finite strain’. Their correct interpretations are critical for understanding regional tectonics. A micromechanics-based modeling approach is used to simulate 3-D flanking structures and demonstrate how the flanking structure may vary with the cutting element’s shape, orientation, and rheological contrast to the ambient medium. A reverse-dynamic modeling approach is applied to quantitatively estimate kinematic vorticity number, viscosity contrast of the cutting element to the ambient medium, and finite strain from natural flanking structures.
Summary for Lay Audience
Earth’s lithosphere is continually subjected to forces that deform rocks and form patterns or structures of characteristic lengths ranging from crystal-scale to the scale of a lithospheric plate. Studying these geological structures can provide information about underlying geodynamic processes. However, field observations of geological structures can only allow access to few scales ranging from rock outcrops to hand specimens, to microscopic thin sections. A scale gap exists between the field observations and the tectonic processes that occur at the orogenic or plate boundary scale. Since the lithosphere is heterogeneous and composed of rheologically distinct elements, the flow in the rocks can vary, a phenomenon called flow partitioning. The geological structures from field observations are, therefore, relevant to the partitioned flow at the respective scales and cannot be directly linked to the tectonic scale processes. This thesis applies micromechanics-based numerical models to simulate such multiscale rock deformation in Earth’s lithosphere.
This thesis provides a quantitative understanding of geological phenomena due to the heterogeneous deformation of Earth’s lithosphere. Pressure may deviate in and around any rheologically heterogeneous rock element and may influence metamorphic assemblages. This can cause problems in using pressure-depth relationships in geodynamic models. Pressure deviations in and around the heterogeneous rock element were simulated, and it was found that the pressure deviations are in the same order as the deviatoric stress levels in rocks and hence limited by their strength. Flow variation in quartz aggregates can influence c-axis fabrics and cause problems in their interpretation. Quartz c-axis fabrics were simulated under partitioned flow, and it was found that the c-axis fabric variation showing apparent opposite senses of shear within a single thin section reflects a finite strain gradient rather than a reversal of vorticity sense as previously thought. Flow variation around any cross-cutting rock element such as a dyke, can form flanking structures, which are useful tools to infer kinematic information such as finite strain and shear sense. 3-D flanking structures were simulated, and it was demonstrated how flanking structures may vary with cutting element’s viscosity, 3-D shape, and orientation, as well as finite strains. A computer program is developed to quantitatively estimate kinematic information such as cutting element’s viscosity, finite strain, and kinematic vorticity number from a photograph of a natural flanking structure.
Bhandari, Ankit, "An investigation on flow field partitioning related to the rheological heterogeneities and its application to geological examples" (2021). Electronic Thesis and Dissertation Repository. 7763.