Quantifying Syntectonic Weakening in Deep Orogenic Crust

The primary intellectual impact of this project will be in improving our understanding of the mechanics that shape the Earth\u27s crust. In recent years, earth scientists have used the increasing body of geodetic data towards that end, but the mechanical properties of the middle and lower crust remain only loosely constrained. This project focuses on the magnitude of strain weakening in shear zone networks. In detail, the research will explore the grain-scale and outcrop-scale deformation mechanisms in minerals that lead to this weakening, followed by modeling of the results to understand the weakening process on the larger scale. These conceptual and numerical models will allow better prediction of where and how fast the continental crust will deform and in turn this will benefit society. In the future, locations that have recently been deglaciated, and those in tectonically active areas where earthquakes are likely, may be interesting targets for further application of this research. This project also serves as a vehicle to continue and enhance ongoing educational and outreach initiatives at the K-16 and graduate levels. Support for students enrolled in the University of Maine\u27s Master of Science in Teaching program will allow pre-service K-12 teachers to become involved in an active research project and to participate in creating an environment where science is more accessible. The PI and colleagues will develop new content based around supercomputer, visualization, and field video library projects, the combined results of which will enhance professional development, graduate and undergraduate courses, and outreach to K-12 students in Maine\u27s rural areas.Throughout the lithosphere, strain localization plays a fundamental role in tectonic processes affecting, for example, seismicity, exhumation, fluid migration and mineralization, magma transport, and topographic and plate boundary evolution. Previous research involving field observations and numerical modeling has produced many constraints on the causes and consequences of strain localization, but researchers lack a thorough understanding of the magnitude of strength variation in the deep crust in particular. Due to exceptional exposure and deep exhumation this research group will use the Parry Sound domain of the Grenville Province, southern Ontario, as a natural laboratory. Along one margin of the domain, granulite facies mineral assemblages have been transformed under upper amphibolite facies conditions along meter-scale shear zones. After field-based mapping, the PI will quantify the change in strength associated with the development of the interconnected meter-scale shear zones through model calculations using the natural geometric framework. In addition, to develop better tools to predict weakening of this magnitude elsewhere, he will determine the mechanisms by which the shear zones developed and the weakening occurred. The PI has hypothesized that one such mechanism involved pegmatite-derived fluids that infiltrated adjacent zones, allowing mineralogical changes along which shear zones could nucleate. Establishing the crustal-scale significance of strength changes requires determining if meter-scale processes produced, for example, the regional-scale, domain-bounding Twelve Mile Bay shear zone. Preliminary observations indicate that fractures with no lateral offset assisted full transposition of the fabric as they evolved into interconnected meter-scale shear zones during progressive deformation. This research group will evaluate whether these preliminary interpretations are valid through the mapping of thermobarometric and geochronologic patterns along the inferred length of the Twelve Mile Bay shear zone

Early Paleozoic Orogenesis in the Maine-Quebec Appalachians

Accretionary orogens, such as the Appalachian orogen, form by episodic docking of oceanic and continental fragments. Two factors that exert significant control on the development of an accretionary orogen are: (1) the nature and source of the accreting fragments, and (2) the thermal and deformational structure of the crust. This study addresses aspects of both of these controls. In the Northern Appalachians, a long-lived but untested hypothesis suggested that Early Paleozoic accretion in western Maine, which marked the initiation of Appalachian development, involved the docking of an island arc. My goal was to test this hypothesis for the Maine-Qukbec segment of the orogen, where the Boundary Mountains terrane had been identified as a possible collider. Combining the techniques of mapping, structural analysis, petrography, U-Pb zircon and monazite geochronology, geochemistry, and geochemical modeling, I present the following interpretations related to the geologic history of the region. (1) the Chain Lakes massif, which cores the Boundary Mountains, was an Ordovician arc-marginal basin receiving sediments eroded from a Laurentian source. (2) Anatexis of the Chain Lakes massif disrupted the original sedimentaryvolcanic sequence. (3) The Boil Mountain Complex and Jim Pond Formation, which lie along the southern margin of the Chain Lakes massif, do not represent an ophiolite, as previously thought. (4) The Boundary Mountains represent a Laurentian-derived microcontinent that served as the nucleus for part of a regional arc system that collided with Laurentia in the Ordovician. The thermal and deformational processes described herein relate, respectively, to anatexis and pluton emplacement. Review and numerical modeling of the causes of lowpressure anatexis, which affected the Chain Lakes massif, indicate that appropriate pressure-temperature conditions are possible in regions of crustal-scale detachment faulting, percolative magma flow, or where thin lithosphere is accompanied by plutonic activity. Analytical kinematic modeling of the consequences of dike-fed pluton emplacement suggests that if published physical properties of granitic magmas are correct, host rocks surrounding an in-situ expanding pluton must deform at rates several orders of magnitude faster than typical tectonic strain rates. Such strain rates almost certainly must be accommodated by processes other than dislocation creep

Softening the Lower Crust: Modes of Syn-Transport Transposition Around and Adjacent to a Deep Crustal Granulite Nappe, Parry Sound Domain, Grenville Province, Ontario, Canada

The Parry Sound domain is a granulite nappe-stack transported cratonward during reactivation of the ductile lower and middle crust in the late convergence of the Mesoproterozoic Grenville orogeny. Field observations suggest the following with respect to the ductile sheath: (1) Formation of a carapace of transposed amphibolite facies gneiss derived from and enveloping the western extremity of the Parry Sound domain and separating it from high-strain gneiss of adjacent allochthons. This ductile sheath formed dynamically around the moving granulite nappe through the development of systems of progressively linked shear zones. (2) Transposition initiated by hydration (amphibolization) of granulite facies gneiss by introduction of fluid along cracks accompanying pegmatite emplacement. Shear zones nucleated along pegmatite margins and subsequently linked and rotated. The source of the pegmatites was most likely subjacent migmatitic and pegmatite-rich units or units over which Parry Sound domain was transported. Comparison of gneisses of the ductile sheath with high-strain layered gneiss of adjacent allochthons show the mode of transposition of penetratively layered gneiss depended on whether or not the gneiss protoliths were amphibolite or granulite facies tectonites before initiation of transposition, resulting in, e.g., folding before shearing, no folding before shearing, respectively. Meter-scale truncation along high-strain gradients at the margins of both types of transposition-related shear zones observed within and marginal to Parry Sound domain mimic features at kilometer scales, implying that apparent truncation by transposition originating in a manner similar to the ductile sheath may be a common feature of deep crustal ductile reworking. Citation: Culshaw, N., C. Gerbi, and J. Marsh (2010), Softening the lower crust: Modes of syn-transport transposition around and adjacent to a deep crustal granulite nappe, Parry Sound domain, Grenville Province, Ontario, Canada, Tectonics, 29, TC5013, doi:10.1029/2009TC002537

MRI: Acquisition of an SEM-EDS-EBSD-CL Microanalytical System for Solid Earth and Climate Change Research

Funding from the Major Research Instrumentation (MRI) Program grant will support acquisition of an Scanning Electron Microscope with secondary and backscattered electron detectors, electron backscatter diffraction capability, and live-color cathodluminescence capability for the Department of Earth Sciences at the University of Maine. The instrument will be used to support faculty and student research in geodynamics and crustal studies and studies of global climate change. The instrument will be the primary research tool of an early career researcher, but will be utilized by several faculty within the department. The scanning electron microscope facility is unique within the state of Maine and will thus operate as a regional facility for research collaboration with scientists from other universities, state government agencies, such as the Maine Geological Survey, and private industry. The facility and its personnel will also participate in outreach activities for K-12 education and the Penobscot Indian Nation

Integrated Analytical-Computational Analysis of Microstructural Influences on Seismic Anisotropy

The magnitudes, orientations and spatial distributions of elastic anisotropy in Earth\u27s crust and mantle carry valuable information about gradients in thermal, mechanical and kinematic parameters arising from mantle convection, mantle-crust coupling and tectonic plate interactions. Relating seismic signals to deformation regimes requires knowledge of the elastic signatures (bulk stiffnesses) of different microstructures that characterize specific deformation environments, but the influence of microstructural heterogeneity on bulk stiffness has not been comprehensively evaluated. The objectives of this project are to: (1) scale up a preliminary method to determine the bulk stiffness of rocks using integrated analytical (electron backscatter diffraction) and computational (asymptotic expansion homogenization) approaches that fully account for the grain-scale elastic interactions among the different minerals in the sample; (2) apply this integrated framework to investigate the effect on elastic anisotropy of several common crustal microstructures; (3) integrate time-dependent microstructure modeling with bulk stiffness calculations to investigate the effects of strain- and process-dependent microstructure evolution on elastic anisotropy in mantle rocks; and (4) disseminate open-source software for the calculation of bulk stiffnesses from electron backscatter diffraction data and creation of synthetic (computer generated) microstructures that can be used in sensitivity analyses among other applications. Because commonly used methods, such as the Voigt, Reuss and Voigt-Reuss-Hill averages, for calculating bulk rock stiffnesses do not account for elastic interactions among the constituent minerals, they exhibit marked, non-systematic differences from stiffnesses obtained using asymptotic expansion homogenization. These objectives are important because the results would substantially improve understanding of the nature of seismic anisotropy in the Earth\u27s crust, which is composed of rocks dominated by low symmetry minerals with complex structures. Traditional methods for performing these calculations do not easily incorporate these effects. This project will develop an elegant, easily-implemented alternative method for anisotropic materials. The scientific results and computational tools that result from this project will have global application across a number of solid Earth and engineering disciplines. Open-source codes developed in this project will made available through existing open-source ELLE platform. Classroom exercises developed for Earth Science and Mechanical Engineering courses that employ this software will be make available to the community, probably through the Science Education Resource Center website at Carleton College

The impact of temperature and crystal orientation fabric on the dynamics of mountain glaciers and ice streams

Streaming ice accounts for a major fraction of global ice flux, yet we cannot yet fully explain the dominant controls on its kinematics. In this contribution, we use an anisotropic full-Stokes thermomechanical flow solver to characterize how mechanical anisotropy and temperature distribution affect ice flux. For the ice stream and glacier geometries we explored, we found that the ice flux increases 1–3% per °C temperature increase in the margin. Glaciers and ice streams with crystallographic fabric oriented approximately normal to the shear plane increase by comparable amounts: an otherwise isotropic ice stream containing a concentrated transverse single maximum fabric in the margin flows 15% faster than the reference case. Fabric and temperature variations independently impact ice flux, with slightly nonlinear interactions. We find that realistic variations in temperature and crystallographic fabric both affect ice flux to similar degrees, with the exact effect a function of the local fabric and temperature distributions. Given this sensitivity, direct field-based measurements and models incorporating additional factors, such as water content and temporal evolution, are essential for explaining and predicting streaming ice dynamics