Viscous and viscoelastic deformation strongly affects the mechanical behavior of the Earth. This style of deformation has consequences for a wide range of geodynamic processes from large scale processes like the formation and maintenance of plate boundaries, to smaller scale processes like postseismic deformation on and near faults. One of the key features of viscous and viscoelastic deformation in the Earth is that it is observed to be self localizing under some circumstances. This is in spite of the tendency for viscous deformation to be pervasive in a deforming system. Many processes are thought to contribute to strain localization in the Earth: (1) viscous dissipation or shear heating (e.g., Braeck and Podladchikov, 2007; Braeck et al., 2009; Kameyama et al., 1999; Kelemen and Hirth, 2007; Ogawa, 1987), (2) grain size reduction (e.g., Braun et al., 1999; Montési and Hirth, 2003; Précigout and Gueydan, 2009), (3) lattice preferred orientation development (LPO) (e.g., Poirier, 1980; Tomassi et al., 2009), mixing of phases (e.g., Skemer et al., 2010a; Toy et al., 2010; Warren and Hirth, 2006) and geometrical interconnection of weak phases and materials (e.g., Handy, 1994). Utilizing both natural samples from Oman (Chapter 2) and theoretical work based on numerical modeling (Chapters 3 and 4) each chapter of this thesis evaluates the effect of a different one of these processes on strain localization, and in the case of Chapter 4 evaluates the additional feedback between two of these processes. In Chapter 2 we examine strain localization in a natural system in which two very rheologically different materials, gabbronorite (predominantly plagioclase) and harzburgite (predominantly olivine), were juxtaposed due to volcanic intrusion and subsequently deformed. We utilized field relationships, pyroxene and amphibole/plagioclase thermometry, metamorphic phase equilibrium, grain size piezometry and electron backscatter diffraction (EBSD) in order to constrain the deformation conditions for the field area. The viscosity of gabbronorite was found to be: (1) consistent with the predicted viscosities based on the extrapolation of experimental flow laws and (2) at least two orders of magnitude lower than the harzburgite while deformation was occurring. This suggests both that a significant viscosity contrast exists at the crust-mantle boundary where the crustal lithology is dominated by plagioclase and the mantle by olivine, and wherever deformation is geometrically allowed to localize within plagioclase rich layers. In Chapter 3 we examine the theoretical effect of shear heating as well as the feedback between viscous dissipation and temperature dependant viscosity on strain localization in a one-dimensional model of a viscoelastic shear zone. This model builds on the work of Kelemen and Hirth (2007) by utilizing a complex dry olivine viscoelastic rheology that includes dislocation creep, diffusion creep, dislocation accommodated grain boundary sliding (disGBS) and low temperature plasticity (LTP). We have found that increasing either the applied strain rate or the grain size system behavior is modified in three significant ways: (1) it causes the maximum stress the system can archive to increase, (2) it results in more unstable system behavior and (3) it causes the system to accommodate more deformation in the background. One consequence of enhanced background deformation is that system exhibits distinct periods of accelerated stress relaxation accompanied by increased strain rates, that do not necessarily go unstable. Consequently, we have shown that shear heating may play an important roll both in viscous deformation in the Earth and potentially in the occurrence of intermediate depth earthquakes and slow slip events. In Chapter 4, we extend Chapter 3 and examine the feedbacks between grain size evolution, viscous dissipation and a complex temperature and grain size dependant viscosity in a one-dimensional model of a viscoelastic shear zone. We evaluated both the grain size evolution models of Austin and Evans (2007) and a modified version of Hall and Parmentier (2003). We find that Austin and Evans predicts unrealistically fine background grain sizes while the predictions based on Hall and Parmentier (2003) are more reasonable. We also find that, based on this model and the experimental work of Mei et al. (2010), LTP may not contribute to grain size reduction in viscously deforming materials. Based on this model grain size evolution does not appear to strongly affect the peak stress or stability of a system for fine initial grain sizes as grain size reduction does not significantly alter the initial viscosity structure. However, in systems with coarser initial grain sizes, grain size evolution does appear to contribute to system instability. Additionally, for both initially coarse and fine systems, grains size evolution results in the emergence of stress evolutions displaying two distinct episodes of stress reduction. Much like Chapter 3, our observations in Chapter 4 suggest that grain size evolution may play an important role in viscous deformation in the Earth and may potentially be a mechanism for some intermediate depth earthquakes and slow slip events. Taken together the chapters in this thesis explore several of the potentially important processes that affect strain localization in the Earth. Thus providing significant insight into this important phenomenon.

The Hornelen basin, a tectonically-controlled Devonian extensional basin formed during the late stages of the Caledonian orogeny in SW Norway, provides an opportunity to investigate the geologic and tectonic evolution of the Caledonides through the use of single grain detrital thermochronometers. The Caledonides are an extensively studied orogen, which today expose medium- to high-grade metamorphic rocks from the core of the Siluro-Devonian continent-continent collision between Baltica and Laurentia. Important questions remain unresolved concerning the structural style of orogenic collapse, the geodynamic mechanism for the exhumation of ultra-high pressure metamorphic terranes, and the latest stage evolution of the orogen as a persistent topographic welt 400 million years after the main collisional event. This thesis presents the novel approach of using multiple chronometers on a single, large set of detrital samples from the Hornelen to elicit layers of information about the evolution of the basin source regions (and thus the ancient, regional geology of the now-long-since-eroded surface of the Caledonides). Using an extensive sample set that spans the basin through both space and time, and a suite of thermochronometers that span the range of closure temperatures from 850ºC (zircon) down to 60ºC (apatite fission-track), I interpret the data in terms of changes in basin provenance that are directly controlled by the structural setting of the basin, thus providing insight into the large-scale structure of an orogenic collapse. The low temperature chronometers (apatite and zircon fission-track) provide information about the post-depositional thermal history of the basin, and insights into the structural controls on this latest uplift history. Overall, these data shed new light on both the late-stage evolution of the Caledonian orogeny as well as the post-Caledonian history of uplift that continues to shape the modern Norwegian landscape. In chapter 1, I employ detrital zircon U/Pb dating on a suite of twenty samples that complement the previous work of Johnston (2006) and Pedersen (2011), and document a spatial pattern that supports Cuthbert’s (2000) observations of distinct N-S asymmetry in basin provenance. Chapter 2 presents detrital mica ⁴⁰Ar/³⁹Ar dating on twenty four samples from the same suite combined with (unpublished) data from three additional samples obtained by C. Warren at Open University; these data reflect a different dimension of the changes in basin provenance than the zircon data, documenting an up-section younging trend in mica ages. I interpret the detrital mica data to reflect progressive exhumation of the metamorphic core of the Caledonian orogen during basin opening. Chapter 3 delves further into the detrital zircon data using a petrochronological approach, complementing the U/Pb age data from Chapter 1 with trace element analyses from forty three zircons, mainly the youngest Caledonian (Scandian) aged grains, but also including an enigmatic population of Archean and Paleoproterozoic zircons. Single grain trace element data from these zircons (which have no obvious source in the allochthons) points to a provenance in the WGR for both the youngest, and oldest, detrital zircon populations in the Hornelen. Chapter 4 takes a different approach to detrital thermochronology, focusing on low temperature thermochronometers from the Hornelen and surrounding basement rocks, and interpreting two data sets (apatite and zircon fission-track) in terms of the latest uplift history of SW Norway: a history which is poorly constrained by the lack of onshore sedimentary deposits since the Devonian, and for which low-temperature thermochronology represents one of the most fruitful modern approaches. Based on subtle cooling-age differences around the Hornelen, I conclude that a single low angle detachment fault underlying the basin is unlikely, and instead, a series of reactivated, N-S trending normal faults have determined the structural evolution of the Hornelen area since the Devonian.

<p> The West Salton Detachment Fault (WSDF) is a low-angle normal fault that bounds the western Salton Trough and "desert ranges" (upper plate) from the Peninsular Ranges (footwall) in Southern California. Footwall and hanging wall fault rocks have quartzo-feldspathic plutonic protoliths at the study sites. The study&rsquo;s focus is on footwall fault rocks that formed mainly in the upper seismogenic zone and were exhumed mainly by WSDF slip. Footwall fault rocks show little overprint due to transiting the shallower aseismic zone. Hanging wall fault rocks formed at &lt; 2&ndash;3 km paleodepth, lack a well-developed ultracataclasite layer, and show clay, zeolite and/or potassic alteration. Zeolite and/or potassic alteration are minimal, post tectonic and/or absent in the footwall. A previous study performed (U-Th)/He dating of apatite and zircon from the hanging wall and footwall of the WSDF record 2.3&ndash;4 to 8 km of footwall exhumation and > 8&ndash;10 km of top-east slip accumulated on the WSDF mainly after ~12 Ma. Syntectonic upper-plate sediments suggest that most WSDF slip accumulated after ~5 Ma, possibly beginning ~8 Ma. WSDF slip ended at ~1.1 Ma when the detachment was cross-cut and deactivated by dextral strike-slip faults of the southern San Andreas fault system. The study focuses on footwall fault rocks that were unroofed from the upper seismogenic zone by WSDF slip. Two study sites along the WSDF were selected to analyze differences in fault-rock microstructure due to different slip rates; creep versus seismic (Agua Caliente (AC) and Powder Dump (PD), respectively). The PD footwall displays a two-part fault core (ultracataclasite above cataclasite), and has pseudotachylite fault and injection veins. This and other observations nearby indicate that the WSDF at the PD site experienced frequent seismic slip events. In contrast, at AC the footwall fault-core rocks lack pseudotachylite, ultracataclasite is thin (a few cm), and subjacent cataclasites are macroscopically foliated and lineated with normal-sense S-C-C' fabrics. Active hot springs issue from the WSDF and/or the nearby Elsinore fault at AC and strong upper-plate alteration at AC suggests this has been the case over significant time. The observed foliation in otherwise brittle, low-temperature Agua Caliente footwall fault rock suggests that significant slip accumulated by creep, perhaps assisted by fluid-related processes. </p><p> We conclude that size-dependent grain-shape variations (elongation, concavity, circularity, and roundness) differ between the two sites. AC has more convex, more round, and more equant grains within 1 meter of the detachment, while PD has more concave, less round, and more elongate grains within 1 meter of the detachment. There is a quantifiable difference in size-dependent elongation measurements between AC and PD. AC has relatively similar elongation averages for grains above 25 microns, but below 25 microns the elongation average of measured grains increases. At PD, elongation decreases with grain size for plagioclase, while quartz does not seem to have a clear relationship between grain size and elongation. There are also differences in concavity between mineral phases between the two sites. At AC, feldspar is generally more concave than quartz, while the opposite is true at PD. We infer that all of these observations are due to different fracturing processes at AC versus PD: AC grains fragmented by chipping and rolling, while PD grains fragmented by tensile cracking. These different fragmentation processes affect the grain shapes of mineral phases differently depending on their fracture, cleavage, and hardness properties. These different fragmentation processes may relate to either the rate of slip, or differences in confining pressure. </p><p> The grain size distribution (GSD) was determined at both sites by characterizing the slope, D, of a log-log distribution plot. Grain size distributions at both WSDF slip sites have D-values of 2.8&ndash;3.0, which shows that the grains fragmented by constrained comminution and were overprinted by subsequent shear. Additionally, the grains at AC were later rounded and chipped, so the GSD at AC reflects initial tensile cracking which was overprinted during unconstrained comminution. </p><p> Finally, pulverized rock was identified only at PD, within the fractured damage zone, where paleo-seismic slip events occurred. This supports previous arguments for pulverized rock being an indicator of seismic slip.</p><p>

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Geology, 1928. / Vita. / by William Vickers Smitheringale. / Ph.D.

Shape analysis methods based on the concept of the fractal dimension are emerging as a new method of quantifying complex particle shapes. The concept of the fractal dimension stems from the approximately linear relationship between the logarithm of the particle perimeter and the logarithm of the step length or unit of measure. as step length is shortened, the resulting particle perimeter is lengthened. The fractal dimension (D) is defined as 1 {dollar}-{dollar} b, where b is the slope of the log/log plot of perimeter against step length. Using two dimensional projected particle outlines measured by a video image digitizing system, the fractal dimensions of at least 400 randomly chosen quartz particles are calculated and used to characterize a specific sediment sample. The shape population from which these samples have been drawn is characterized via a fractal dimension density histogram. These histograms are used to make statistical comparisons of particle shape information contained in different samples. The fractal shape method has several advantages over the more widely used Fourier shape method. The fractal method has greater precision and sensitivity with particle shape discriminative power approximately 4.5 times that of the Fourier method. The fractal and Fourier methods were used in two applied shape analysis studies in order to compare shape method performance and recognize potential interpretive differences arising from the use of one method over the other. The first application investigated contrasting sediment sources and their distribution in Twofold Bay, New South Wales, Australia. These results correspond well with the conclusions of other independent sedimentological investigations of Twofold Bay. The Fourier method did not distinguish clearly between the terrestrial and marine compartments. The second application tested the resolution of shape change which occurs during the process of abrasion. The fractal method proved to be highly sensitive to small scale changes in particle roughness that occur initially as the result of particle to particle collisions, whereas the Fourier method did not. The fractal method detected far greater changes in overall shape than the Fourier method which appeared insensitive to fine-scale changes in particle shape. (Abstract shortened with permission of author.)

Using Q-mode factor analysis, 87 surficial sediment samples collected from Duck and Whalehead beach, North Carolina, were analyzed using the weight percent of the gravel and sand fraction subdivided at 0.5 phi class interval as variables. An additional data set composed of 178 surficial sediment samples from Duck beach (bimodal) and Coquina beach (unimodal fine) representing three years of sampling at monthly intervals were analyzed by the same technique using only the sand fraction. The spatial and temporal patterns of sediment factor groups support three main inferences. (1) Bimodal beaches display a more distinct sediment zonation than unimodal beaches. (2) On a long term basis (yearly), cross-shore grain-size distributions represent depositional processes. Particularly on bimodal beaches, the association of sediment factor groups with specific zones of the beach profile delineates a textural differentiation produced by the type and amount of energy inherent in each zone. Combinations of Q-mode factor analysis and other environmental sensitive techniques (e.g. log-probability plots of grain-size distributions) proved to be useful for interpreting sedimentary processes at the depositional site. (3) The cross-shore patterns which represent an average of the sedimentary processes occurring under fair weather and storm conditions indicate that coarse sediments are concentrated on the backshore. In contrast, fine sediments are located landward or shoreward of this zone where they are exposed to energy conditions that result in their depletion in the subaerial beach. Using the Q-mode factor model, 350 new sediment samples from beaches located between Duck and Oregon Inlet were "mapped" in the factor space defined by the Duck-Coquina data set. The along-coast results support the cross-shore trends observed in the previous studies and indicates that there are several sources of coarse sediments between Duck and Oregon Inlet. Sedimentologic, stratigraphic and seismic data offshore and landward of the barrier substantiate these findings and demonstrate that differences in subaerial beach morphology in this part of Currituck Spit, is primarily due to the availability of coarse sediments from the paleodrainage of the Albemarle river.

A total of 68 vibra-cores and 14-box cores in conjunction with high-resolution seismic records are used to describe the late Quaternary development of a twin-barrier island complex. Based on the stratigraphy, radiocarbon dates, and microfossils, a transgressive outer Holocene and inner Pleistocene barrier island complex are recognized. The two subaerial sub-parallel barriers are a result of separate marine transgressions that occurred before and after late Wisconsin glaciation. Pollen assemblages and ten radiocarbon dates from the lagoonal sediments below the older island concur on a date of approximately 30,000 years B.P., hence a probable mid-Wisconsinan age for the overriding barrier island. The uncertainty surrounding a sea level near today's position 30,000 years ago is not unnoticed; neotectonics may be an important consideration in this apparent rise in sea-level. Holocene sediments deposited in the backbarrier environment show a general shallowing and fining upward sequence. The Holocene stratigraphic sequence indicates a narrowing of the backbarrier region, a decrease in the tidal prism, and an increase in marsh and tidal flat infilling associated with calmer water conditions. Most backbarrier sediments are introduced through tidal inlets. Despite Holocene, backbarrier deposits greater than 8 meters thick, only 2 meters may be preserved below 75-100 cm thick nearshore sands in some areas. Inlet fill deposits will not be preserved. However inner barrier sands and lower Holocene backbarrier sands and muds have a strong preservational potential. The stacking of transgressive barrier deposits, albeit those from different transgressions, may provide a stratigraphic oil trap.

The Portland and Tualatin basins are part of the Puget-Willamette Lowland in the Cascadia forearc of Oregon and Washington. The Coast Range to the west has undergone Paleogene transtension and Neogene transpression, which is reflected in basin stratigraphy. To better understand the tectonic evolution of the region, I modeled three key stratigraphic horizons and their associated depocenters (areas of maximum sediment accumulation) through space and time using well log, seismic, outcrop, aeromagnetic, and gravity data. Three isochore maps were created to constrain the location of Portland and Tualatin basin depocenters during 1) Pleistocene to mid-Miocene (0-15 Ma), 2) eruption of the Columbia River Basalt Group (CRBG, 15.5-16.5 Ma), and 3) Mid-Miocene to late Eocene time (~17-35 Ma). Results show that the two basins each have distinct mid-Miocene to Pleistocene depocenters. The depth to CRBG in the Portland basin reaches a maximum of ~1,640 ft, 160 ft deeper than the Tualatin basin. Although the Portland basin is separated from the Tualatin basin by the Portland Hills, inversion of gravity data suggests that the two were connected as one continuous basin prior to CRBG deposition. Local thickening of CRBG flows over a gravity low coincident with the Portland Hills suggests that Neogene transpression in the forearc reactivated the Sylvan-Oatfield and Portland Hills faults as high angle reverse faults. This structural inversion separated the once continuous Portland and Tualatin basins in the mid-late Miocene. A change in the stress regime at that time marks the transition from Paleogene forearc extension to deformation dominated by north-south shortening due to collision of the forearc against the Canadian Coast Mountains. An eastward shift of the forearc basin depocenter over the Neogene likely reflects uplift of the Coast Range to the west. A change in regional stress in the mid to late-Miocene, along with uplift of the Oregon Coast Range, caused a 10-fold decrease in sediment accumulation rates across the Portland and Tualatin basins. Transpressional oblique-slip faulting continues to deform the region as the forearc undergoes clockwise rotation and collides with the rigid Canadian Coast Mountains to the north.

No description available.

The trilobite family Pliomeridae Raymond, 1913, is composed of 44 genera and some 190 species, with a global distribution through the Ordovician. Despite the common presence of pliomerids in paleotropical faunas, its phylogenetic structure is virtually unknown. Several pliomerid subfamilies are highly autapomorphic and clearly monophyletic, but the phylogenetic status of others is unclear at best. Higher level relationships within the family are unclear and untested, as are its inclusivity (i.e., what constitutes its basal node) and its relationship to other cheiruroidean groups such as the family Cheiruridae Hawle and Corda, 1857. A particular problem is lack of knowledge of stratigraphically early, potentially basal taxa. These taxa are particularly important because their morphology may critically inform the higher level phylogenetic problems. Abundant, well preserved, stratigraphically early taxa are available in well known Lower Ordovician successions in the Great Basin of the western United States. Pliomerid species from these strata have been poorly described or entirely overlooked, even though they are some of the most completely known taxa ever discovered, due to their silicified preservation. The Pseudocybele-Group of trilobites is one such group of early, potentially well known pliomerids. It forms a plexus of 39 species belonging to the genera Hintzeia Harrington, 1957, Panisaspis n. gen., Protopliomerella Harrington, 1957, Lemureops McAdams and Adrain, 2009, and Pseudocybele Ross, 1951. Members of this clade occupied shallow subtidal habitats in western Laurentia (present-day Great Basin, western United States), and radiated through the Tulean and Blackhillsian stages of the Ibexian Series (Upper Tremadocian-Floian). The goals of this project are 1) to revise and describe of all members of the Pseudocybele-Group from the Pogonip Group in western Utah and eastern Nevada and from the Garden City Formation in southeastern Idaho and northern Utah; 2) to reconstruct their phylogenetic relationships through species-level morphological parsimony analysis; and 3) to gain some insight into broader (subfamily-level) relationships within Pliomeridae using new knowledge from these species and their phylogenetic relationships.