Macrofabrics have long been used to infer deformational patterns of subglacial till in glacial fl... more Macrofabrics have long been used to infer deformational patterns of subglacial till in glacial flutes. Flute macrofabrics, derived from the aggregate orientation of elongate pebbles, are either parallel to glacier flow or form a “herringbone” pattern along the flute’s central axis. Fabrics based on orientation of magnetic particles using anisotropy of magnetic susceptibility (AMS) have not been applied to flutes. AMS fabrics provide more complete, three-dimensional data on deformation kinematics than particle fabrics, since all principal susceptibility vectors, maximum [k1], intermediate [k2] and minimum [k3], are measured. Laboratory experiments using a ring-shear device indicate that all principal susceptibility vectors develop distinct, strong fabrics at moderate strains (7-30) and that those fabrics remain strong to higher strains. k1 fabrics form in the direction of shear and plunge mildly (<30˚) up-glacier. Under the same conditions k2 fabrics form perpendicular to the dire...
We use glass compositions (matrix glass and melt inclusions) from seven eruptive deposits dated b... more We use glass compositions (matrix glass and melt inclusions) from seven eruptive deposits dated between ~320 and 0.7 ka from four distinct calderas in the central TVZ, and we discuss advantages and limitations of the rhyolite-MELTS geobarometer in comparison with other geobarometers applied to the same eruptive deposits. Overall, there is good agreement with other pressure estimates from the literature (amphibole geobarometry and H 2 O-CO 2 solubility models). One of the main advantages of this new geobarometer is that it can be applied to both matrix glass and melt inclusions-regardless of volatile saturation. The examples presented also emphasise the utility of this method to filter out spurious glass compositions. Pressure estimates obtained with the new rhyolite-MELTS geobarometer range between ~250 to ~50 MPa, with a large majority at ~100 MPa. These results confirm that the TVZ hosts some of the shallowest rhyolitic magma bodies on the planet, resulting from the extensional tectonic regime and thinning of the crust. Distinct populations with different equilibration pressures are also recognised, which is consistent with the idea that multiple batches of eruptible magma can be present in the crust at the same time and can be tapped simultaneously by large eruptive events.
Stratigraphic and morphologic data previously collected from the forefield of Múlajökull, Iceland... more Stratigraphic and morphologic data previously collected from the forefield of Múlajökull, Iceland, suggest that its recent surge cycles are responsible for the formation of drumlins there and that their relief reflects both deposition on drumlins and erosion between them. We have tested these ideas and aspects of leading models of drumlin formation by studying past patterns of bed deformation and effective stress in basal tills of the glacier's forefield. Patterns of till strain indicated by the anisotropy of magnetic susceptibility (AMS) of ~2300 intact till samples indicate that till was deposited during shear deformation, with shearing azimuths and planes that conform to the drumlin morphology. Thus, till deposition occurred as drumlins grew, in agreement with LiDAR data indicating that the degree of aggradation of the glacier forefield is largest in areas subjected to the most surges. Previously described unconformities on the drumlin flanks, however, indicate that drumlin relief at Múlajökull has resulted, in part, from erosion. Given that the last surge deposited a till layer both on and between drumlins, a reasonable hypothesis is that erosion between drumlins occurred during normal (quiescent) flow of the glacier between surges. Densities of till samples , analyzed in conjunction with laboratory consolidation tests, indicate that effective stresses on the bed during such periods were on the order of 100 kPa larger between drumlins than within them, an observation consistent with subglacial channels at low water pressure occupying interdrumlin areas. Transport of sediment by turbulent flow in these channels or high effective stress adjacent to them causing enhanced till entrainment in ice or increased depths of bed deformation would promote the sediment flux divergence necessary to erode areas between drumlins. The observation that effective stresses were higher between drumlins than within them is the opposite of that presumed in leading models of drumlin formation. Moreover, the lack of AMS-fabric evidence of longitudinal compression in drumlin tills does not support some models of drumlin formation that invoke negative till-flux gradients in a deforming bed.
Macrofabrics have long been used to infer deformational patterns of subglacial till in glacial fl... more Macrofabrics have long been used to infer deformational patterns of subglacial till in glacial flutes. Flute macrofabrics, derived from the aggregate orientation of elongate pebbles, are either parallel to glacier flow or form a “herringbone” pattern along the flute’s central axis. Fabrics based on orientation of magnetic particles using anisotropy of magnetic susceptibility (AMS) have not been applied to flutes. AMS fabrics provide more complete, three-dimensional data on deformation kinematics than particle fabrics, since all principal susceptibility vectors, maximum [k1], intermediate [k2] and minimum [k3], are measured. Laboratory experiments using a ring-shear device indicate that all principal susceptibility vectors develop distinct, strong fabrics at moderate strains (7-30) and that those fabrics remain strong to higher strains. k1 fabrics form in the direction of shear and plunge mildly (<30˚) up-glacier. Under the same conditions k2 fabrics form perpendicular to the dire...
We use glass compositions (matrix glass and melt inclusions) from seven eruptive deposits dated b... more We use glass compositions (matrix glass and melt inclusions) from seven eruptive deposits dated between ~320 and 0.7 ka from four distinct calderas in the central TVZ, and we discuss advantages and limitations of the rhyolite-MELTS geobarometer in comparison with other geobarometers applied to the same eruptive deposits. Overall, there is good agreement with other pressure estimates from the literature (amphibole geobarometry and H 2 O-CO 2 solubility models). One of the main advantages of this new geobarometer is that it can be applied to both matrix glass and melt inclusions-regardless of volatile saturation. The examples presented also emphasise the utility of this method to filter out spurious glass compositions. Pressure estimates obtained with the new rhyolite-MELTS geobarometer range between ~250 to ~50 MPa, with a large majority at ~100 MPa. These results confirm that the TVZ hosts some of the shallowest rhyolitic magma bodies on the planet, resulting from the extensional tectonic regime and thinning of the crust. Distinct populations with different equilibration pressures are also recognised, which is consistent with the idea that multiple batches of eruptible magma can be present in the crust at the same time and can be tapped simultaneously by large eruptive events.
Stratigraphic and morphologic data previously collected from the forefield of Múlajökull, Iceland... more Stratigraphic and morphologic data previously collected from the forefield of Múlajökull, Iceland, suggest that its recent surge cycles are responsible for the formation of drumlins there and that their relief reflects both deposition on drumlins and erosion between them. We have tested these ideas and aspects of leading models of drumlin formation by studying past patterns of bed deformation and effective stress in basal tills of the glacier's forefield. Patterns of till strain indicated by the anisotropy of magnetic susceptibility (AMS) of ~2300 intact till samples indicate that till was deposited during shear deformation, with shearing azimuths and planes that conform to the drumlin morphology. Thus, till deposition occurred as drumlins grew, in agreement with LiDAR data indicating that the degree of aggradation of the glacier forefield is largest in areas subjected to the most surges. Previously described unconformities on the drumlin flanks, however, indicate that drumlin relief at Múlajökull has resulted, in part, from erosion. Given that the last surge deposited a till layer both on and between drumlins, a reasonable hypothesis is that erosion between drumlins occurred during normal (quiescent) flow of the glacier between surges. Densities of till samples , analyzed in conjunction with laboratory consolidation tests, indicate that effective stresses on the bed during such periods were on the order of 100 kPa larger between drumlins than within them, an observation consistent with subglacial channels at low water pressure occupying interdrumlin areas. Transport of sediment by turbulent flow in these channels or high effective stress adjacent to them causing enhanced till entrainment in ice or increased depths of bed deformation would promote the sediment flux divergence necessary to erode areas between drumlins. The observation that effective stresses were higher between drumlins than within them is the opposite of that presumed in leading models of drumlin formation. Moreover, the lack of AMS-fabric evidence of longitudinal compression in drumlin tills does not support some models of drumlin formation that invoke negative till-flux gradients in a deforming bed.
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Papers by Reba McCracken