Thesis advisor: Seth C. Kruckenberg / The Burlington Mylonite Zone (BMZ) is a northeast-trending, greenschist- to amphibolite-facies shear zone located entirely within the Boston Avalon terrane in Eastern Massachusetts along the tectonic boundary with the Nashoba terrane (the trailing marginal terrane of Ganderia). The juxtaposition of these terranes, and the development of the BMZ, is hypothesized to represent the amalgamation of Avalon and Laurentia during the late Silurian-early Devonian Acadian orogeny, but the timing of its formation and its structural evolution remain largely unconstrained. Field observations and microstructural analysis using electron backscatter diffraction (EBSD) of 24 samples from 16 field sites throughout the BMZ provide new constraints on the kinematics and conditions of deformation that facilitated the development of this large-scale crustal shear zone. The BMZ samples comprise a heterogeneous mix of quartzofeldspathic +/- hornblende-bearing gneisses and quartzites with varying microstructures. Nearly all samples contain abundant mixed, but predominantly sinistral, kinematic indicators (e.g., asymmetric porphyroclasts, tiled feldspars) and a strong crystallographic preferred orientation (CPO). Quartz – the dominant mineral by mode in all of the samples analyzed – is known from experimental deformation studies to develop distinct patterns of CPO which vary as a function of deformation kinematics, temperature, and strain geometry. Patterns of CPO in quartz are used to determine the dominant intracrystalline deformation mechanisms that accommodated the formation of the BMZ. Quartz CPO patterns in the BMZ samples are characterized by variably developed c- and a-axis distributions, broadly consistent with patterns expected for mixed<a> to prism<a> slip at intermediate temperatures of deformation. Corresponding intragranular misorientation axis plots are more diagnostic and indicate dominant prism<a> slip in all of the shear zone samples analyzed, consistent with microstructures observed in thin section (e.g., undulose extinction, subgrain development, grain boundary migration, dynamic recrystallization) and metamorphic conditions inferred from shear zone mineral parageneses. Application of the quartz recrystallized grain size piezometer places additional constraints on deformation conditions, indicating that the BMZ rocks record differential stresses ranging from ~44 to 92 MPa. Field and microstructural observations of shear sense indicators are combined with two analytical methods for determining aspects of kinematic vorticity and deformation geometry in the BMZ. This study applies a new analytical method - crystallographic vorticity axis (CVA) analysis - that leverages rotational statistics on crystallographic orientations within the interiors of grains to constrain the dominant axis of material rotation in deformed samples. This dominant axis provides a uniquely objective proxy for the vorticity normal reference frame required for further quantitative kinematic vorticity analyses. The rotational axis of kinematic vorticity, and its relationship to structural fabrics (i.e. foliation and lineation), provides an important constraint on the geometry of the deforming zone (e.g., monoclinic versus triclinic shear zones). The results of the CVA analysis are invariable across the entire length of the BMZ; the kinematic vorticity axis lies within the plane of mylonitic foliation perpendicular to lineation – the pattern expected for monoclinic deformation geometries. The mean kinematic vorticity number (Wm: a measure of the relative contribution of pure and simple shear) is calculated using Rigid Grain Net (RGN) analysis for the BMZ mylonites and ranges from 0.4-0.5, indicating general shear. Combined field, microstructural, and vorticity analyses are interpreted to suggest that crustal strain localization along the Avalon-Nashoba boundary, as recorded in the BMZ mylonites, involved the combined effects of pure and simple shear in a predominantly sinistral, monoclinic transpressional shear zone. Rock microstructures, patterns of crystallographic preferred orientation, and paleostress estimates suggest that mylonitization occurred at or near the brittle-ductile transition under relatively high stress conditions. This study demonstrates the power of new microstructural methods, such as CVA analysis of electron backscatter diffraction data, to augment traditional field-based methods of kinematics and deformation analysis in enigmatic, large-scale crustal shear zones.

Thesis (M.S.)--Washington State University, December 2006. / Includes bibliographical references (p. 88-93).

No description available.

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

The Archean North Caribou greenstone belt (NCGB) possesses abundant transpressive deformation zones on its northern margins, which appear to have formed under amphibolite facies conditions. Protracted deformation and regional metamorphism are coeval with widespread magmatism and accretion events in the Superior Province, yet the importance of these shear zones in the tectonic evolution of the NCGB is equivocal. Structural analyses support a transpressive system that strongly implicates horizontal tectonism. This is partly in contrast with some currently proposed models for Archean greenstone belts suggesting synchronous vertical and horizontal movements. Geochemical and microstructural analyses from shear zones indicate heterogeneous deformation/fluids conditions on a km-scale. Monazite and mica geochronological ages indicate metamorphism and deformation occurring during the amalgamation of the Superior craton from ca. 2.75 to 2.4 Ga. The high metamorphic grade background may obscure tectonic signatures, yet some structural and geochemical characteristics remain consistent with other greenstone belts where vertical and horizontal displacement are recorded.

I present physical shear-box experiments investigating the relationship between geometrical properties of shear zones and mechanical properties of deformed rocks. Experimental methodology is also examined critically and new materials for analogue modelling of shear localization are presented. First, I tested experimentally whether meaningful rheological information can be deduced from finite geometrical shear zone data. The results predict characteristic geometrical responses for certain end-member materials. However, it will be difficult to constrain such responses in the field. In the second part physical controls on deformation in the shear box are analysed for Newtonian and power-law fluids and an elastoviscoplastic strain-softening material. Since models always represent simplifications of the natural problem, it is essential to understand fully the physics of a given simulation. I show that displacement boundary conditions, model geometry, and rheology control shear zone geometry. Practical applications of the shear box for modelling natural shear localization and limitations of isothermal physical models with displacement boundary conditions in general are discussed. In the third part, new data on the rheology of highly-filled silicone polymers are introduced. Since dynamic similarity must be satisfied in analogue models to permit scaled, quantitative simulations of deformation processes, the choice of suitable rock analogues is critical for physical experiments. In particular, we address the problem of designing power-law fluids to model rocks deforming by dislocation creep. We found that highly-filled polymers have complex rheologies. Hence, such materials must be used with care in analogue modelling and only for certain experimental stress-strain rate conditions. Finally, I investigated whether fault network geometry and topography of brittle strike-slip faults are influenced by the degree of compaction of the host rock. Analogue shear experiments with loose and dense sand imply that the degree of sediment compaction may be a governing factor in the evolution of fault network structure and topography along strike-slip faults in sedimentary basins. Therefore, models of strike-slip faults should consider potential volume changes of deformed rocks.

In this thesis, I present field, microstructural, and Electron Backscatter Diffraction (EBSD) analyses of rock fabrics from high strain zones in exposures of lower crustal Cretaceous plutons at Breaksea Entrance, Fiordland, New Zealand. The interplay between deformation and metamorphism occurs across multiple scales at the root of a continental arc. I show a series of steps in which retrogressive metamorphism is linked to the accommodation of deformation. I define three main phases of deformation and metamorphism at Breaksea Entrance. The first phase (D1) involved emplacement of dioritic to gabbroic plutons at depths up to 60 km. The second phase (D2) is characterized by deformation and metamorphism at the granulite and eclogite facies that produced high strain zones with linear fabrics, isoclinal folding of igneous layering, and asymmetric pressure shadows around mafic aggregates. New structural analyses from Hāwea Island in Breaksea Entrance reveal the development of doubly plunging folds that define subdomes within larger, kilometer-scale gneiss domes. The development and intensification of S2 foliations within the domes was facilitated by the recrystallization of plagioclase and clinopyroxene at the micro-scale (subgrain rotation and grain boundary migration recrystallization), consistent with metamorphism at the granulite and eclogite facies and climb-accommodated dislocation creep. EBSD data show a strong crystallographic preferred orientation in plagioclase during D2 deformation. The third phase (D3) is characterized by deformation and metamorphism at the upper amphibolite facies that produced sets of discrete, narrow shear zones that wrap and encase lozenges of older fabrics. Structural analyses reveal a truncation and/or transposition relationship between the older S2 and the younger S3 foliations developed during D3. Progressive localization of deformation during cooling, hydration, and retrogression, resulted in the breakdown of garnet and pyroxene to form hornblende, biotite, fine plagioclase and quartz. EBSD data show a strong crystallographic preferred orientation in hornblende. During D3, hornblende and biotite accommodated most of the strain through fluid-assisted diffusion creep. The last two events (D2 and D3) reflect a transition in deformation and metamorphism during exhumation, as well as a focusing of strain and evolving strain localization mechanisms at the root of a continental arc. An examination of structures at multiple scales of observation reveals that fabrics seen in the field are a composite of multiple generations of deformation and metamorphism.

Décollements are important tectonic elements in thin-skinned fold-thrust belts. However, few studies have addressed the internal structure of décollements because most are deeply buried and internal features typically cannot be resolved in seismic reflection images. Upper Jurassic evaporite exposures in the Potosí uplift of northeastern Mexico provide a unique tectonic window into the décollement of the Laramide-age Sierra Madre Oriental fold belt. In order to constrain the three-dimensional geometry of décollement structures, I mapped a ~20 km2 portion of the décollement at a scale of 1:10,000. I created a new stratigraphy for the décollement interval during mapping, and made detailed structural observations at targeted sites. The ~900 m thick décollement interval consists of gypsum with five carbonate members (up to 120 m thick) and numerous thin (<5 m) carbonate interbeds. These carbonate units delineate map-scale structural patterns and define two structural domains. The middle and upper parts of the décollement in the western domain contain map-scale folds with local map-scale boudinage and thrust faults. The eastern domain exposes the lower part of the décollement, and contains thrust repetitions of carbonate members and a regionally-persistent basal shear zone. These map relationships indicate a stratigraphic variation in structural style. Western domain folds and eastern domain thrust sheets both appear to be related kinematically to overburden folding. In contrast, the basal shear zone accommodated décollement-parallel shear strain in response to overburden translation. Folding and faulting of carbonate members and intervening gypsum units drove localization of simple shear into the basal shear zone, because only the lowermost gypsum interval maintained a favorable orientation sub-parallel to the regional transport direction throughout deformation. This investigation demonstrates that décollements have complex internal structural patterns that are below typical seismic resolution and lateral variations in structural style that cannot be reconstructed from single well cores or small outcrops. Décollement stratigraphy controls variations in strain magnitude within the décollement interval, so that previous models that invoke homogeneous strain within the décollement are incorrect. Complex, laterally-variable structural style and stratigraphic control of strain distribution could be general characteristics of décollements where the décollement interval contains significant contrasts in bed rheology. / text

I present physical shear-box experiments investigating the relationship between geometrical properties of shear zones and mechanical properties of deformed rocks. Experimental methodology is also examined critically and new materials for analogue modelling of shear localization are presented. First, I tested experimentally whether meaningful rheological information can be deduced from finite geometrical shear zone data. The results predict characteristic geometrical responses for certain end-member materials. However, it will be difficult to constrain such responses in the field. In the second part physical controls on deformation in the shear box are analysed for Newtonian and power-law fluids and an elastoviscoplastic strain-softening material. Since models always represent simplifications of the natural problem, it is essential to understand fully the physics of a given simulation. I show that displacement boundary conditions, model geometry, and rheology control shear zone geometry. Practical applications of the shear box for modelling natural shear localization and limitations of isothermal physical models with displacement boundary conditions in general are discussed. In the third part, new data on the rheology of highly-filled silicone polymers are introduced. Since dynamic similarity must be satisfied in analogue models to permit scaled, quantitative simulations of deformation processes, the choice of suitable rock analogues is critical for physical experiments. In particular, we address the problem of designing power-law fluids to model rocks deforming by dislocation creep. We found that highly-filled polymers have complex rheologies. Hence, such materials must be used with care in analogue modelling and only for certain experimental stress-strain rate conditions. Finally, I investigated whether fault network geometry and topography of brittle strike-slip faults are influenced by the degree of compaction of the host rock. Analogue shear experiments with loose and dense sand imply that the degree of sediment compaction may be a governing factor in the evolution of fault network structure and topography along strike-slip faults in sedimentary basins. Therefore, models of strike-slip faults should consider potential volume changes of deformed rocks.

Thesis (M.S.)--University of Texas at El Paso, 2008. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.