The northern Walker Lane refraction experiment: Pn arrivals and the northern Sierra Nevada root

In May 2002, we collected a new crustal refraction profile from Battle Mountain, Nevada across western Nevada, the Reno area, Lake Tahoe, and the northern Sierra Nevada Mountains to Auburn, CA. Mine blasts and earthquakes were recorded by 199 Texan instruments extending across this more than 450-km-long transect. The use of large mine blasts and the ultra-portable Texan recorders kept the field costs of this profile to less than US$10,000. The seismic sources at the eastern end were mining blasts at Barrick's GoldStrike mine. The GoldStrike mine produced several ripple-fired blasts using 8000–44,000 kg of ANFO each, a daily occurrence. First arrivals from the larger GoldStrike blasts are obvious to distances of 300 km in the raw records. First arrivals from a quarry blast west of the survey near Watsonville, CA, located by the Northern California Seismic Network with a magnitude of 2.2, can be picked across the recording array to distances of 600 km. The Watsonville blast provides a western source, nearly reversing the GoldStrike blasts. A small earthquake near Bridgeport, CA. also produced pickable P-wave arrivals across the transect, providing fan-shot data. Arrivals from M5 events in the Mariana and Kuril Islands also appear in the records. This refraction survey observes an unexpectedly deep crustal root under the northern Sierra Nevada range, over 50 km in thickness and possibly centered west of the topographic crest. Pn delays of 4–6 s support this interpretation. At Battle Mountain, Nevada, we observe anomalously thin crust over a limited region perhaps only 150 km wide, with a Moho depth of 19–23 km. Pn crossover distances of less than 80 km support this anomaly, which is surrounded by observations of more normal, 30-km-thick crust. A 10-km-thick and high-velocity lower-crustal “pillow” is an alternative hypothesis, but unlikely due to the lack of volcanics west of Battle Mountain. Large mine and quarry blasts prove very effective crustal refraction sources when recorded with a dense receiver array, even over distances exceeding 600 km. New elastic synthetic seismogram modeling suggests that Pn can be strong as a first arrival, easing the modeling and interpretation of crustal refraction data. Fast eikonal computations of first-arrival time can match pickable Pn arrival times.     

Origin of rapidly solidified metal-troilite grains in chondrites and iron meteorites

Inclusions of troilite and metallic Fe,Ni 0.2–4 mm in size with a dendritic or cellular texture were observed in 12 ordinary chondrites. Cooling rates in the interval 1400?950°C calculated from the spacing of secondary dendrite arms or cell widths and published experimental data range from 10^?7 to 10⁴°C/sec. In 8 of these chondrites, which are breccias containing some normal slow-cooled metal grains, the inclusions solidified before they were incorporated into the breccias. Their cooling rates of 1–300 °C/sec indicate cooling by radiation, or by conduction in contact with cold silicate or hot silicate volumes only 6–40 mm in size. This is quantitative evidence that these inclusions and their associated clasts were melted on the surface of a parent body (by impact), and were not formed at depth from an internally derived melt. In Ramsdorf, Rose City and Shaw, which show extensive reheating to ? 1000°C, Fe-FeS textures in melted areas are coarser and indicate cooling rates of 10^?1 to 10^?4°C/sec during solidification. This metal may have solidified inside hot silicate volumes that were 10–300 cm in size. As Shaw and Rose City are breccias of unmelted and melted material, their melted metal did not necessarily cool through 1000°C within a few m of the surface. Shock-melted, fine-grained, irregular intergrowths of metal and troilite formed in situ in many irons and some chondrites by rapid solidification at cooling rates of ? 10⁵°C/sec. Their kamacite and taenite compositions may result from annealing at ~250°C of metallic glass or exceedingly fine-grained quench products.     

Comparison of fish and macroinvertebrate use of<Emphasis Type="Italic">Typha angustifolia,Phragmites australis</Emphasis>, and treated<Emphasis Type="Italic">Phragmites</Emphasis> marshes along the lower Connecticut River

Since 1965 large areas of lower Connecticut River tidelands have been converted from high diversity brackish meadow andTypha angustifolia marsh to near monocultures ofPhragmites australis. This study addresses the impact ofPhragmites invasion on fish and crustacean use of oligohaline high marsh. During spring tides from early June through early September 2000, fishes and crustaceans leaving flooded marsh along 3 km of the Lieutenant River, a lower Connecticut River tributary, were captured with Breder traps at 90 sites, equally distributed amongPhragmites, Typha, and treated (herbicide and mowing)Phragmites areas. Pit traps, 18 per vegetation type in 2000 and 30 each inPhragmites andTypha in 2001, caught larvae and juveniles at distances of up to 30 m into the marsh interior. There were no significant differences in fish species compositions or abundances among the vegetation types. Size distributions, size specific biomasses, and diets ofFundulus heteroclitus, the numerically dominant fish, were also similar. The shrimpPalaemonetes pugio was more abundant inPhragmites than in other types of vegetation, whereas the fiddler crabUca minax was least numerous inPhragmites. Mean numbers ofF. heteroclitus andP. pugio caught per site event were positively correlated with increasing site hydroperiod. Significantly moreF. heteroclitus were captured along the upper reach of the river where marsh elevations were lower than farther downstream. MoreF. heteroclitus and fewerP. pugio andU. minax were captured during the day than at night. A relatively small number of larval and juvenileFundulus sp. were captured in pit traps, but consistently fewer inPhragmites than inTypha, suggesting thatTypha and brackish meadow marshes may provide better nursery habitat. Vegetation was sampled along a 30 m transect at each trap site in 2000. Plant species diversity was greatest in treatedPhragmites areas and lowest inPhragmites sites.     

Letšeng diamond mine,Lesotho: a variant of Kimberley-type pyroclastic kimberlite emplacement

The Letšeng Diamond Mine comprises two ~91 Ma kimberlite pipes. An update of the geology is presented based on the 2012–2017 detailed investigation of open pit exposures and all available drillcores which included mapping, logging and petrography. Each of the steep-sided volcanic pipes comprises a number of phases of kimberlite with contrasting diamond contents which were formed by the emplacement of at least four batches of mantle-derived magma. The resulting range of textures includes resedimented volcaniclastic kimberlite (RVK), Kimberley-type pyroclastic kimberlite (KPK), coherent kimberlite (CK) and minor amounts of hypabyssal kimberlite (HK). The pipes are compared with KPK occurrences from southern Africa and worldwide. Many features of the Letšeng pipes are similar to KPK infilled pipes particularly those of the widespread Cretaceous kimberlite province of southern Africa. The differences displayed at Letšeng compared to other large KPK pipe infills described from around the world are attributed to the marginal or melnoitic nature of the magma and the upper diatreme to crater setting of the Letšeng pipes, where processes become extrusive. It is concluded that the pipes comprise a variant of Kimberley-type pyroclastic kimberlite emplacement. The classification of many of the Letšeng rocks as KPK is important for developing the internal geology of the pipes as well as for predicting the distribution of diamonds within the bodies.

     

TOPOCLIMATIC CONTROLS IN AN ALPINE FELLFIELD AND THEIR ECOLOGICAL SIGNIFICANCE

The interaction of topography and ambient atmospheric conditions and their combined effect on surface climate are studied in the alpine fellfield on Niwot Ridge, Colorado Front Range. Insolation, net shortwave and all-wave radiation, surface and air temperatures, ground heat flux, wind speed, and evapotranspiration are analyzed in order to characterize moisture and energy exchanges. The spatial pattern of daily energy and moisture fluxes is influenced more by cloud cover than topographic position. The daily radiation load is usually highest on east-facing slopes because clear mornings are commonly followed by cumulus development during midday and early afternoon. In the dominant westerly wind regime, wind speeds on east-facing slopes are lower than elsewhere. Strong insolation produces high rates of evapotranspiration from the thin, sandy/gravelly fellfield soil when moisture is abundant. After a few consecutive precipitation-free days, large radiation loads produce high surface temperatures, especially on east-facing slopes where wind speed and thus the sensible heat flux are small. During the 1985 summer drought period, the impact of hot, dry weather on plant water stress was most pronounced on east-facing slopes. The well-defined distribution of Dryas octopetala on Niwot Ridge, near the southern extent of its circumpolar range, is an example of the control of topoclimate over plant distribution owing to water stress during periods of drought The absence of Dryas from south-facing slopes does not correspond well with present topoclimatic patterns. The present distribution of Dryas may be a consequence of topoclimatic patterns throughout the middle of the last millennium when the summer climate in the Colorado Front Range was hotter and drier than today. Summer droughts then were probably more prolonged, and a combination of clearer days, lower wind speeds, or an increased frequency of northerly wind would have created more intense water stress on south-facing than east-facing slopes. [Key words: Topoclimate, alpine tundra, Dryas octopetala, energy balance.]     

Heterogeneous structure around the Jemez volcanic field, New Mexico, USA, as inferred from the envelope inversion of active-experiment seismic data

We analyse active-experiment seismic data obtained by the 1993 Jemez Tomography Experiment (JTEX) programme to elucidate the heterogeneous structure of the Jemez volcanic field, which is located at the boundary between the Colorado Plateau and the Rio Grande Rift. Using a single isotropic scattering assumption, we first calculate the envelope Green's functions for the upper and lower crust and the uppermost mantle. By fitting the theoretical envelopes with the observed three-component data, we estimate depth-dependent features of the scattering coefficients around Valles Caldera. We estimate the ratios of scattering coefficients, rather than scattering coefficients themselves, because of the uncertainty of the seismic efficiency of the explosive sources and knowledge of absolute site-amplification factors. The strongest scattering coefficients are observed at a shallow depth beneath the Valles Caldera. This is considered to be related to the complex structure caused by two episodes of caldera formation and the ensuing resurgent uplift in the caldera, etc. The depth-dependent characters of the scattering coefficients for the Colorado Plateau and the Rio Grande Rift are similar to each other: a transparent upper crust and a heterogeneous lower crust (small and large scattering coefficients, respectively). However, the scattering coefficients beneath the Rio Grande Rift are several times larger than those beneath the Colorado Plateau. Depths of the lower crust and the Moho boundary beneath the Rio Grande Rift are shallower than those of the Colorado Plateau. From their geological settings and other geophysical results around the region, we infer that the larger scattering coefficients of the rift are associated with rift formation and volcanic activity, such as magma ascent from the upper mantle to the crust.     

Biot consolidation analysis with automatic time stepping and error control Part 1: theory and implementation

A new finite element algorithm for solving elastic and elastoplastic coupled consolidation problems is described. The procedure treats the governing consolidation relations as a system of first-order differential equations and is based on the backward Euler and Thomas and Gladwell schemes with automatic subincrementation of a prescribed series of time increments. The prescribed time increments, which are called coarse time steps, serve to start the procedure and are chosen by the user. The automatic consolidation algorithm attempts to select the time subincrements such that, for a given mesh, the time-stepping (or temporal discretisation) error in the displacements lies close to a specified tolerance. Unlike existing solution techniques, the new algorithm computes not only the displacements and pore pressures, but also their derivatives with respect to time. These extra variables permit a family of unconditionally stable integration algorithms to be constructed which automatically provide an estimate of the local truncation error for each time step. This error estimate is inexpensive to compute and may be used to develop a simple and efficient automatic time stepping mechanism. For the elastic case, the displacements and pore pressures at the end of each subincrement may be solved directly without the need for iteration. For elastoplastic behaviour, however, the governing relationships are non-linear and a system of non-linear equations must be solved to compute the updates. Copyright © 1999 John Wiley & Sons, Ltd.     

A SYNOPTIC CLIMATOLOGICAL ANALYSIS OF SUMMERTIME PRECIPITATION INTENSITY IN THE EASTERN UNITED STATES

The spatial patterns of precipitation frequency and intensity over the eastern United States for summer from 1961 to 1990 are analyzed using a recently developed continental-scale air mass-based synoptic classification. This procedure, the spatial synoptic classification (SSC), is based on “seed” day identification of synoptic events and discriminant analysis to group together days that are within the same air-mass type. Results show differences in the types of precipitation associated with different air masses. Two air masses in particular-Moist Tropical and Moist Temperate-appear to be highly correlated with a majority of the precipitation, particularly in the southeast. The synoptic characteristics, daily intensity of rainfall, and radiosonde soundings during prevalence of these two air masses suggest that convective rainfall is common during Moist Tropical, and stratiform-type rainfall occurs during Moist Temperate. A simple stratification scheme based upon a synoptic-based air-mass delineation may be useful for studies that need to divide days into stratiform or convective regimes.     

Science and the St Elias: an evolving framework for sustainability in North America's highest mountains

The past, present, and future contributions of science in the St Elias Mountains, and its relationship with regional development, resource management, and traditional ecological knowledge is examined. Science has evolved from an early foundation of exploration, through stages of resource inventories and surveys, to deductive scientific research and, more recently, a promising reconnection with traditional knowledge. Directly and indirectly, events such as the Klondike Gold Rush, construction of the Alaska Highway, creation of the Arctic Institute of North America's Kluane Lake Research Station, and establishment of protected areas have helped foster scientific activities in the region. In turn, this scientific perspective has influenced regional development by providing detailed information that has been utilized, to varying degrees, in resource use, planning, and decisionmaking. Over the past decade, management of the region has become less sectoral and more cooperative in nature, due partly to the implementation of co-management agreements, regional land use planning, and settlement of first nations' land claims. Incorporating both science and traditional knowledge into this process through collaborative endeavours such as long-term ecological monitoring, adaptive management, and information integration will contribute to ecosystem-based management of the St Elias and ensure that both perspectives play an integral role in sustainable development of the region.