Dennis L. Harry

Using ground penetrating radar to 'unearth' buried beaver dams

Beavers, once abundant and widespread in the Northern Hemisphere, are now substantially reduced. Although beaver dams trap sediment, the relative importance of this sediment in Quaternary valley aggradation remains uncertain. We use ground penetrating radar (GPR) and near-surface seismic refraction to quantify the magnitude of beaver-induced Holocene sedimentation in Beaver Meadows, Rocky Mountain National Park, Colorado (United States). GPR was used to identify radar packages of genetically related strata of glacial and non-glacial origins. We demonstrate that GPR is a useful tool for identifying buried beaver-induced sedimentation with little to no surficial expression. Seismic refraction was used to determine the total volume of sediment above bedrock. Beaver-induced sedimentation constitutes 30%–50% of surficial post-glacial sediments, and post-glacial sediments constitute ∼13% of the total valley fill. Beaver damming in montane valleys was thus an important process trapping sediments within the Holocene at this site. If geoscientists ignore the contribution of beaver-ponded sediments to Quaternary stratigraphy in a wide variety of riverine environments, they neglect a potentially important biotic driver of valley sedimentation.

Wilson cycles, tectonic inheritance, and rifting of the North American Gulf of Mexico continental margin

The tectonic evolution of the North American Gulf of Mexico continental margin is characterized by two Wilson cycles, i.e., repeated episodes of opening and closing of ocean basins along the same structural trend. This evolution includes (1) the Precambrian Grenville orogeny; (2) formation of a rift-transform margin during late Precambrian opening of the Iapetus Ocean; (3) the late Paleozoic Ouachita orogeny during assembly of Pangea; and (4) Mesozoic rifting during opening of the Gulf of Mexico. Unlike the Atlantic margins, where Wilson cycles were first recognized, breakup in the Gulf of Mexico did not initially focus within the orogen, but was instead accommodated within a diffuse region adjacent to the orogen. This variation in location of rifting is a consequence of variations in the prerift architecture of the orogens. The Appalachian-Caledonian orogeny involved substantial crustal shortening and formation of a thick crustal root. In contrast, the Ouachita orogeny resulted in minimal crustal shortening and thickening. In addition, rather than a crustal root, the Ouachita orogen was underlain by the lower plate of a relatively pristine Paleozoic subduction system that is characterized by a shallow mantle. A finite element model simulating extension on the margin demonstrates that this preexisting structure exerted fundamental controls on the style of Mesozoic rifting. The shallow mantle created a strong lithosphere beneath the orogen, causing extension to initiate adjacent to, rather than within, the orogen. On the Atlantic margins, the thick crustal root resulted in a weak lithosphere and initiation of extension within the interior of the orogen. Major features of the modern Gulf of Mexico margin, including the Interior Salt Basin, outboard unextended Wiggins arch, and an unusually broad region of extension beneath the coastal plain and continental shelf, are direct consequences of the prerift structure of the margin.

A matched interface and boundary method for solving multi-flow Navier-Stokes equations with applications to geodynamics

We have developed a second-order numerical method, based on the matched interface and boundary (MIB) approach, to solve the Navier-Stokes equations with discontinuous viscosity and density on non-staggered Cartesian grids. We have derived for the first time the interface conditions for the intermediate velocity field and the pressure potential function that are introduced in the projection method. Differentiation of the velocity components on stencils across the interface is aided by the coupled fictitious velocity values, whose representations are solved by using the coupled velocity interface conditions. These fictitious values and the non-staggered grid allow a convenient and accurate approximation of the pressure and potential jump conditions. A compact finite difference method was adopted to explicitly compute the pressure derivatives at regular nodes to avoid the pressure-velocity decoupling. Numerical experiments verified the desired accuracy of the numerical method. Applications to geophysical problems demonstrated that the sharp pressure jumps on the clast-Newtonian matrix are accurately captured for various shear conditions, moderate viscosity contrasts and a wide range of density contrasts. We showed that large transfer errors will be introduced to the jumps of the pressure and the potential function in case of a large absolute difference of the viscosity across the interface; these errors will cause simulations to become unstable.

Geodynamic models of continental extension and the formation of non-volcanic rifted continental margins

Abstract Finite-element models of continental rifting show that formation of non-volcanic rifted margins may be the result of extension of a rheologically homogeneous crust. In such circumstances lithosphere necking does not become well developed until late in the rift history, delaying the onset of decompression melting in the asthenosphere until the last 10% of the rifting episode. This result is robust over a broad range of mantle temperatures, margin geometries, and extension rates. A cool mantle is not required, so the models are able to account for the production of oceanic crust at the end of amagmatic rifting episodes. The duration of the syn-rift melting episode is most sensitive to changes in extension rate, with higher extension rates leading to shorter periods of melt production. The duration of the rifting episode is controlled by extension rate and initial crustal thickness, and the geometry of the margin after continental break-up is controlled by initial crustal thickness and the distribution of pre-existing rheological heterogeneity in the crust. The model results are generally compatible with the dimensions and extension rates of rifted continental margins across the globe, and provide a particularly good fit to the evolution of the Iberia Abyssal Plain margin.

Multichannel Analysis of Surface Waves Generated During High-resolution Seismic Reflection Profiling of a Fluvial Aquifer

Shear wave velocity profiles are estimated from surface wave dispersion analysis of data collected during a high-resolution P-wave reflection survey of a fluvial aquifer located in Columbus, Mississippi. The results demonstrate that useful velocity profiles of the upper 4m of a sedimentary sequence can be imaged, even when survey design parameters and noise conditions are not optimal for surface wave surveys. The data were collected with 100Hz geophones, a 10oz. hammer source, and a 1m geophone spacing with near and far offsets of 1 and 12m, respectively. In spite of these less-than-ideal survey parameters for surface wave analysis and the presence of substantial cultural noise arising from nearby quarrying operations and runway activity, the shear velocity profiles accurately locate the boundaries between a shallow soil layer and a meandering fluvial facies in the upper part of the aquifer, and the boundary between the meandering fluvial facies and a braided fluvial facies in the lower part of the aquife...