Correlation of pseudo relative velocity spectra with site intensity, local soil classification and depth of sediments

Empirical regression equations are presented for scaling Pseudo Relative Velocity Spectra, PSV(T), in terms of the Modified Mercalli Intensity M.M.I., local soil classification at the recording site (‘rock’, stiff or deep soil), and local geological condition (depth of sediments). It is shown that both the local soil classification and the local geological depth of sediments should be used simultaneously in the evaluation of site specific response spectra.     

Clinoform deposition across a boundary between orogenic front and foredeep – an example from the Lower Cretaceous in Arctic Alaska

The Lower Cretaceous Fortress Mountain Formation occupies a spatial and temporal niche between syntectonic deposits at the Brooks Range orogenic front and post‐tectonic strata in the Colville foreland basin. The formation includes basin‐floor fan, marine‐slope and fan‐delta facies that define a clinoform depositional profile. Texture and composition of clasts in the formation suggest progressive burial of a tectonic wedge‐front that included older turbidites and mélange. These new interpretations, based entirely on outcrop study, suggest that the Fortress Mountain Formation spans the boundary between orogenic wedge and foredeep, with proximal strata onlapping the tectonic wedge‐front and distal strata downlapping the floor of the foreland basin. Our reconstruction suggests that clinoform amplitude reflects the structural relief generated by tectonic wedge development and load‐induced flexural subsidence of the foreland basin.     

Ecological controls on biogeochemical lfuxes in the western Antarctic Peninsula studied with an inverse foodweb model

Sea ice in the western Antarctic Peninsula (WAP) region is both highly variable and rapidly changing. In the Palmer Station region, the ice season duration has decreased by 92 d since 1978. The sea-ice...     

Applications of computer modeling to the study of the genesis of stratiform sulfide deposits

Stratiform sulfide deposits are considered to have formed at or near the discharge vents of submarine hydrothermal systems. Such deposits constitute an important source of zinc, lead, copper, silver, and gold. They are often divided on the basis of the dominant host-rock lithology into volcanogenic and sediment-hosted types. In terms of their economic base metal contents, volcanogenic deposits contain either Cu and Zn or Zn, Pb, and Cu, whereas sediment-hosted deposits usually contain predominantly Zn and Pb. Mathematical models of the chemical characteristics of stratiform sulfide deposits are based on a program (B)which allows the calculation of the equilibrium distribution of aqueous species in a hydrothermal solution. Program B is front-ended by Program A, which defines the components of the chemical system, and back-ended by Program C, which controls the mode of output of the calculated results. Programs A and C are tailored to the specific problem under investigation. Results of chemical modeling of the two chemically active zones of the hydrothermal system (1. the hydrothermal reservoir in which the ore-forming solutions are generated and, 2. the site of ore deposition)suggest explanations of various characteristics of stratiform sulfide deposits, and include

1. The consistency of the metal associations and metal ratios suggest that ore-forming hydrothermal solutions are saturated with respect to the major metals in the reservoir zone. The chemistry of the solutions is determined mainly by temperature and chemical equilibrium between the hydrothermal solution and the rock-forming minerals of the reservoir.
2. Metal associations and metal ratios corroborate isotopic and other evidence that volcanogenic deposits were formed from hotter hydrothermal systems (> 200°Capprox.)than sediment-hosted (< 200°Capprox.)Metal ratios suggest that the hydrothermal reservoirs were contained in mafic rocks (ferromagnesian-feldspar mineral assemblages)for the Cu-Zn association; in felsic rocks and/or sediments (feldspar-mica; clay mineral assemblages)for the Zn-Pb-Cu association: and in sediments (dominated by clay mineral assemblages)for the Zn-Pb association.
3. Chemical zonation patterns and ore textures characteristics of volcanogenic deposits are explained by rapid cooling of the hydrothermal solution immediately above its discharge vent.
4. Chemical zonation patterns and the fine laminations of sediment-hosted deposits are explained by the comparatively slow degeneration of hydrothermal solutions that have collected as a brine pool on the ocean floor.