The morphology and development of folds

Imperial Users onl

Address at the opening of the session, 1881–82

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Correlations and contrasts in structural history and style between an Archaean greenstone belt and adjacent gneiss belt, NE Minnesota

An analysis of the deformation along the boundary between the Vermilion Granitic Complex (VGC) and the Vermilion district indicates that the two terranes have seen a similar deformation history since the earliest stages of folding in the area. Despite this common history, variations in structural style occur between the two terranes, such as the relative development of D sub 1 fabrics and D sub 2 shear zones, and these can be attributed to differences in the crustal levels of the two terranes during the deformation. Similarly, the local development of F sub 3 folds in the VGC, but not in the Vermilion district, is interpreted to be a result of later-D sub 2 pluton emplacement which was not significant at the level of exposure of ther Vermilion district

Julian of Norwich and her children today: Editions, translations and versions of her revelations

The viability of such concepts as "authorial intention," "the original text," "critical edition" and, above all, "scholarly editorial objectivity" is not what it was, and a study of the textual progeny of the revelations of Julian of Norwich--editions, versions, translations and selections--does little to rehabilitate them. Rather it tends to support the view that a history of reading is indeed a history of misreading or, more positively, that texts can have an organic life of their own that allows them to reproduce and evolve quite independently of their author. Julian's texts have had a more robustly continuous life than those of any other Middle English mystic. Their history--in manuscript and print, in editions more or less approximating Middle English and in translations more or less approaching Modern English--is virtually unbroken since the fifteenth century. But on this perilous journey, many and strange are the clutches into which she and her textual progeny have fallen

Experimental deformation of synthetic magnetite-bearing calcite sandstones: effects on remanence, bulk magnetic properties, and magnetic anisotropy

NSF EAR 88-04820 and NSERC A686

Going against the flow: testing the hypothesis of pulsed axial glacier flow

Hypothesised lobe‐like flow of a temperate glacier in southeast Iceland, proposed from an analysis of ice surface crevassing patterns, is appraised from both empirical and theoretical perspectives. The hypothesis comprises the migration of individual lobes (or ‘pulses’) of ice through the glacier body, with central lobes migrating more rapidly along a narrow, central, ‘axial flow corridor’. Our alternative hypothesis is that crevasse patterns at this glacier instead reflect simple surface ice responses to stresses caused by flow over uneven bed topography. To substantiate our rejection of the lobe‐like, pulsed axial flow hypothesis, we provide: (a) evidence for a prominent transverse foliation that exhibits no evidence of shear of the required magnitude to support the hypothesis; and (b) an analysis of ice surface displacement, obtained by feature tracking, that shows a uniform flow field throughout the glacier tongue. We argue that caution needs to be exercised when interpreting glacier flow solely from crevasse patterns and observations of minor displacements along near‐surface fractures and other features

Pore Pressure Evolution and Fluid Flow During Visco-Elastic Single-Layer Buckle Folding

Pore pressure and fluid flow during the deformational history of geologic structures are directly influenced by tectonic deformation events. In this contribution, 2D plane strain finite element analysis is used to study the influence of different permeability distributions on the pore pressure field and associated flow regimes during the evolution of visco-elastic single-layer buckle folds. The buckling-induced fluid flow regimes indicate that flow directions and, to a lesser degree, their magnitudes vary significantly throughout the deformation and as a function of the stratigraphic permeability distribution. The modelling results suggest that the volumetric strain and the permeability distribution significantly affect the resulting flow regime at different stages of fold development. For homogeneous permeability models (k \u3e 10-21 m2), low strain results in a mostly pervasive fluid flow regime and is in agreement with previous studies. For larger strain conditions, fluid focusing occurs in the buckling layer towards the top of the fold hinge. For low permeabilities (\u3c10-21 m2), local focused flow regimes inside the buckling layer emerge throughout the deformation history. For models featuring a low-permeability layer embedded in a high-permeability matrix or sandwiched between high-permeability layers, focused flow regimes inside the folded layer result throughout the deformation history, but with significant differences in the flow vectors of the surrounding layers. Fluid flow vectors induced by the fold can result in different, even reversed, directions depending on the amount of strain. In summary, fluid flow regimes during single-layer buckling can change from pervasive to focused and fluid flow vectors can be opposite at different strain levels, that is the flow vectors change significantly through time. Thus, a complete understanding of fluid flow regimes associated with single-layer buckle folds requires consideration of the complete deformation history of the fold

Formation of ice eddies in subglacial mountain valleys

Radar data from both Greenland and Antarctica show folds and other disruptions to the stratigraphy of the deep ice. The mechanisms by which stratigraphy deforms are related to the interplay between ice flow and topography. Here we show that when ice flows across valleys or overdeepenings, viscous overturnings called Moffatt eddies can develop. At the base of a subglacial valley, the shear on the valley sidewalls is transferred through the ice, forcing the ice to overturn. To understand the formation of these eddies, we numerically solve the non-Newtonian Stokes equations with a Glen's law rheology to determine the critical valley angle for the eddies to form. When temperature is incorporated into the ice rheology, the warmer basal ice is less viscous and eddies form in larger valley angles (shallower slopes) than in isothermal ice. We also show that when ice flow is not perpendicular to the valley orientation, complex 3-D eddies transport ice down the valley. We apply our simulations to the Gamburtsev Subglacial Mountains and solve for the ice flow over radar-determined topography. These simulations show Moffatt eddies on the order of 100 m tall in the deep subglacial valleys

Near-surface hydraulic conductivity of Northern Hemisphere glaciers

The hydrology of near‐surface glacier ice remains a neglected aspect of glacier hydrology despite its role in modulating meltwater delivery to downstream environments. To elucidate the hydrological characteristics of this near‐surface glacial weathering crust, we describe the design and operation of a capacitance‐based piezometer that enables rapid, economical deployment across multiple sites and provides an accurate, high‐resolution record of near‐surface water‐level fluctuations. Piezometers were employed at 10 northern hemisphere glaciers, and through the application of standard bail–recharge techniques, we derive hydraulic conductivity (K) values from 0.003 to 3.519 m day−1, with a mean of 0.185 ± 0.019 m day−1. These results are comparable to those obtained in other discrete studies of glacier near‐surface ice, and for firn, and indicate that the weathering crust represents a hydrologically inefficient aquifer. Hydraulic conductivity correlated positively with water table height but negatively with altitude and cumulative short‐wave radiation since the last synoptic period of either negative air temperatures or turbulent energy flux dominance. The large range of K observed suggests complex interactions between meteorological influences and differences arising from variability in ice structure and crystallography. Our data demonstrate a greater complexity of near‐surface ice hydrology than hitherto appreciated and support the notion that the weathering crust can regulate the supraglacial discharge response to melt production. The conductivities reported here, coupled with typical supraglacial channel spacing, suggest that meltwater can be retained within the weathering crust for at least several days. Not only does this have implications for the accuracy of predictive meltwater run‐off models, but we also argue for biogeochemical processes and transfers that are strongly conditioned by water residence time and the efficacy of the cascade of sediments, impurities, microbes, and nutrients to downstream ecosystems. Because continued atmospheric warming will incur rising snowline elevations and glacier thinning, the supraglacial hydrological system may assume greater importance in many mountainous regions, and consequently, detailing weathering crust hydraulics represents a research priority because the flow path it represents remains poorly constrained