Alexis Lavine

Sediment delivery after a wildfire

We use a record of sedimentation in a small reservoir within the Cerro Grande burn area, New Mexico, to document postfire delivery of ash, other fine-grained sediment carried in suspension within floods, and coarse-grained sediment transported as bedload over a five-year period. Ash content of sediment layers is estimated using fallout 137 Cs as a tracer, and ash concentrations are shown to rapidly decrease through a series of moderate-intensity convective storms in the first rainy season after the fire. Over 90% of the ash was delivered to the reservoir in the first year, and ash concentrations in suspended sediment were negligible after the second year. Delivery of the remainder of the fine sediment also declined rapidly after the first year despite the occurrence of higher-intensity storms in the second year. Fine sediment loads after five years remained significantly above prefire averages. Deposition of coarse-grained sediment was irregular in time and was associated with transport by snowmelt runoff of sediment stored along the upstream channel during short-duration summer floods. Coarse sediment delivery in the first four years was strongly correlated with snowmelt volume, suggesting a transport-limited system with abundant available sediment. Transport rates of coarse sediment declined in the fifth year, consistent with a transition to a more stable channel as the accessible sediment supply was depleted and the channel bed coarsened. Maximum impacts from ash and other fine-grained sediment therefore occurred soon after the fire, whereas the downstream impacts from coarse-grained sediment were attenuated by the more gradual process of bedload sediment transport.

Fault interaction and along-strike variation in throw in the Pajarito fault system, Rio Grande rift, New Mexico

The seismically active Pajarito fault system (PFS) of northern New Mexico, United States, is a complex zone of deformation made up of many laterally discontinuous faults and associated folds and fractures that interact in ways that have important implications for seismic hazards. Mapping and drilling projects in the PFS provide new insights into the structural geometry and paleoseismic history of the fault system. A 1.25 Ma old datum (the Bandelier Tuff) and high-resolution digital elevation data allow construction of throw-length profiles along the entire length of the PFS, revealing primary geometric features previously unrecognized. The fault system as a whole consists of numerous closely spaced overlapping sections ~8–14 km long. Slip maxima in some cases occur near the centers of these sections, and in others they are shifted toward one end. Along-strike asymmetrical throw profiles and throw deficits indicate fault branching, merging, and strain transfer. This pattern results from processes of fault linkage and conservation of strain on diverse structures of a large fault system. New mapping reveals that the northern end of the Pajarito fault terminates in a wide zone of extensional monoclines and discontinuous, small-displacement faults, and interacts with nearby antithetic faults. New paleoseismic data from a normal fault splay, interpreted in light of previous paleoseismic work, argue for three Holocene surface-rupturing earthquakes; one ca. 1.4 thousand calendar years ago (1.4 cal ka) on the Pajarito fault, a second 6.5–5.2 ka ago on the Pajarito fault that is consistent with an event 6.5–4.2 ka ago on the Guaje Mountain fault, and a third ca. 9 ka ago on both the Pajarito and the Rendija Canyon faults. This paleoseismic event chronology demonstrates that the Pajarito fault often ruptures alone, but sometimes ruptures either with the Rendija Canyon or the Guaje Mountain fault. When this occurs, the resultant seismic moment and therefore the earthquake magnitude are larger than when the main Pajarito fault ruptures alone. Evidence for fault interaction, and the presence of prominent bends in the Pajarito fault system, imply structural control of paleoseismicity and neoseismicity and suggest the potential for stress concentrations and earthquake triggering in complex linking fault systems.

Comparison of SAR techniques for luminescence dating of sediments derived from volcanic tuff

Abstract In this investigation we evaluate several proposed optically stimulated luminescence single-aliquot regeneration (OSL SAR) procedures to determine which technique has the greatest potential to yield accurate ages for samples collected from tuff-derived alluvial sediments within the narrow, sharply incised canyon systems of the Pajarito Plateau of northern New Mexico. The SAR data collection methods evaluated are: infrared-stimulated luminescence (IRSL), post-IR blue-OSL, IRSL with TL annealing cycles on polymineral fine-grains, and blue-OSL on quartz fine sand. A single-grain laser luminescence (SGLL) procedure for quartz sand is also evaluated. Age estimates obtained from these methods are compared with radiocarbon, soil PDI (profile development index), and IRSL multi-aliquot additive dose (MAAD) age constraints. Our results indicate that the modal D e of quartz sand SGLL dose distributions yield ages that are consistent with radiocarbon and PDI age constraints for the tuff derived sediments in this investigation and appears to be the most promising method for studies in this area. Additionally, two fine-grained polymineral methods, IRSL SAR and traditional IRSL MAAD, produced ages that were generally in agreement with the SGLL ages and with available 14 C and PDI age constraints. At the present stage of research, we advocate using quartz sand SGLL in conjunction with IRSL SAR or even IRSL MAAD for polymineral fine-grains to provide the most robust and reliable luminescence age data sets for tuff-derived sediments.

Morphology of a crater-filling lava lake margin, The Peninsula tuff cone, Tule Lake National Wildlife Refuge, California: implications for formation of peperite textures

Abstract A crater-filling lava lake basalt at The Peninsula tuff cone in northeastern California has features on its base formed by interaction of the basalt with wet hydrovolcanic tuff. A spectrum of features, including elongated, streamlined, flute-shaped lobes, and irregular corrugations, are exposed on the base of the lava lake. Flute-shaped lobes range in amplitude from several centimeters to 0.5 m, plunge downdip with respect to the subjacent layered tuff toward the center of the crater, and are better defined at depths more than 3 m below the lava lake surface. These may have been formed by growth of Rayleigh–Taylor instability waves caused by the density contrast between the wet tuff and basalt, combined with directed stress from flow of the lava lake radially outward from the crater. Formation of an insulating vapor film at the wet tuff–basalt interface and downward pressure exerted by the lava lake allowed the edge of the lava lake to quench gradually, and prevented formation of peperite at greater depths in the lake. Corrugations exposed on the upper 1.5 m of the lava lake margin increase in size from several millimeters to 10 cm in amplitude toward the top of the lava lake and approximately parallel the surface. Corrugations may be the remnant of fluidal peperite preserved on the edge of the lava lake, where the peperite was eroded, or may be deformation features formed within an insulating vapor film due to oscillation of the vapor film. These were formed higher in the lava lake, where the lava lake basalt was thinner, wet tuff deposits confining the lava lake were thinner, and temperature differences between the wet tuff and lava were greater. If corrugations represent deformation of the lava edge within the vapor film, rather than peperite formation, they preserve an intermediate process that is involved in the formation of peperite. This study offers a unique opportunity to evaluate some of the initial mechanisms involved in the formation of peperite based on features preserved in the lava where the host rock, in this case hydrovolcanic tuff, has been eroded but the properties of the host rock are known from adjacent exposures.

Total station geologic mapping: an innovative approach to analyzing surface-faulting hazards

Abstract We have developed an innovative application of high-precision geologic mapping with an electronic total station to assess the potential for seismic surface rupture in areas of Los Alamos National Laboratory (LANL). Our method of total station mapping enables recognition of secondary faults, with as little as 30 cm of vertical displacement that are not exposed at the surface, have no topographic expression, and would otherwise likely go unnoticed. It has been applied to preclude the presence of faulting in large areas (several km2) of proposed and existing critical facilities at LANL. The method involves surveying of points on geologic features, and detailed computer-aided and field analyses of anomalies in the elevations of surveyed points. We examine vertical anomalies in elevations that are the result of dominantly normal and reverse faulting; however, the method could also be applied to strike-slip faulting. Surveying of geologic contacts allows for easy integration of geologic data into a Geographical Information System (GIS) and detailed 3D analysis of small-scale structures. Field data are analyzed in profiles, 3D surface diagrams, and maps that are constructed with a variety of commercially available software packages. We apply the method to delineate volcanic map-unit boundaries in the 1.2-million-year-old Tshirege Member of the Bandelier Tuff to characterize portions of the Pajarito fault system. The ability of this method to identify faults with very small displacements that otherwise might be unrecognizable allows for discrimination of varying styles of deformation, decreases in displacement along strike through splaying into many smaller faults, monoclinal flexures, and cross structures between faults.