Constraining slip rates and spacings for active normal faults

Numerous observations of extensional provinces indicate that neighbouring faults commonly slip at different rates and, moreover, may be active over different time intervals. These published observations include variations in slip rate measured along-strike of a fault array or fault zone, as well as significant across-strike differences in the timing and rates of movement on faults that have a similar orientation with respect to the regional stress field. Here we review published examples from the western USA, the North Sea, and central Greece, and present new data from the Italian Apennines that support the idea that such variations are systematic and thus to some extent predictable. The basis for the prediction is that: (1) the way in which a fault grows is fundamentally controlled by the ratio of maximum displacement to length, and (2) the regional strain rate must remain approximately constant through time. We show how data on fault lengths and displacements can be used to model the observed patterns of long-term slip rate where measured values are sparse. Specifically, we estimate the magnitude of spatial variation in slip rate along-strike and relate it to the across-strike spacing between active faults.     

Impedance spectroscopy of single and polycrystalline olivine: Evidence for grain boundary transport

The impedance spectra of single and polycrystalline olivine display two and three impedance arcs, respectively. Impedance spectra of single crystal olivine, polycrystalline olivine compacts, and natural dunite are compared to deduce the causes of the different impedance arcs. Variation of sample dimensions and use of three- and four-electrode configurations aid in the interpretation. The resistance of the two highest frequency mechanisms varies directly with the length to area ratio (l/A) of the sample. Experiments using the four-electrode configuration confirm that the lowest frequency impedance arc is caused by processes at the sample-electrode interface. In both single and polycrystalline samples the highest frequency mechanism is interpreted as bulk (grain interior) conduction, and the lowest frequency mechanism is attributed to sample-electrode interface effects. In the polycrystalline samples, the intermediate frequency mechanism is interpreted as the grain boundary conduction mechanism. The resistances of the grain interior and grain boundary mechanisms add in a series manner.     

Geochemistry and volcanic setting of the Ordovician Forbordfjell and Jonsvatn greenstones,Trondheim Region,central Norwegian Caledonides

Geochemical patterns in two, correlative, Ordovician, basaltic greenstone formations from the central Norwegian Caledonides corroborate field indications of a tripartite stratigraphic subdivision of the units. Lower and upper members show similar chemical traits and are transitional to LREE-enriched, high-magnesia to low-magnesia basalts. The thicker, more massive, middle member carries the geochemical signature of LREE-depleted, normal mid-oceanic ridge tholeiites. Stratigraphic-upward petrochemical variation discerned within the individual members points to processes of either closed or open-system low-P fractional crystallization. The geochemical trends can be ascribed to a mechanism of spreading accompanied by lithospheric thinning in a marginal ocean basin situation, followed by renewed crustal thickening. Geochemical and lithovolcanic polarity indicators denote that the basin was located to the east of an Arenig-Llanvirn magmatic arc sited within the Caledonian Iapetus Ocean.     

Measurement of marine radionuclide distribution using a towed sea-bed spectrometer

The application of in situ gamma-ray spectrometry to the measurement of radionuclide concentration in sea water and sea-bed sediment is discussed with special reference to a towed sea-bed gamma-ray spectrometer survey of radio-effluent distribution in the Irish Sea. The sensitivity and accuracy of the method are established by comparing the spectrometer measurements with laboratory analyses of sediment samples. Contour maps produced from the spectrometer profiles are matched with the distribution of bottom sediment types and conclusions are drawn concerning the modes of transport and deposition of ¹³⁷Cs in the Irish Sea.     

Trace element chemistry of pyrite: A useful guide to the occurrence of sulfide base metal mineralization

Trace element contents for pyrite from a range of sulfide mineral occurrences in the Kangiara region, eastern Australia, illustrate two main groups of pyrite. The first group, with higher Ag, Cu, Pb and Mo contents, corresponds to samples from sulfide base metal deposits and the second group, with higher Mn, Ti and Ni contents, contains samples from skarn mineralization, volcanic rocks and quartz veins. The model proposed for the development of pyrite in the Kangiara region is that the first group was formed from base metal-bearing solutions, while the second group reflects diagenetic pyrite and metamorphic pyrite. Thus, the pyrite trace element chemistry may provide a means of distinguishing types of mineral occurrences, in particular, those containing significant base metal mineralization.     

Acute toxicity of kepone to selected freshwater fishes

The acute toxicity of Kepone in freshwater was determined with three fish species,Ictalurus punctatus (channel catfish),Lepomis macrochirus (bluegills), andAnguilla rostrata (American eel). Elvers ofA. rostrata were most sensitive with a 96 h lethal concentration for 50% of the animals tested (LC50) of 35 μg per. 1. Bluegills were slightly less sensitive with a 96 h LC50 of 50 μg per 1. Catfish were most tolerant with a 96 h LC50 of 514 μg per l. Bluegills and catfish exposed to comparable concentrations of Kepone accumulated equivalent amounts in 96 h. This observation in conjunction with the markedly different 96 h LC50's for these species suggest a difference in the ways these fish cope with Kepone.     

Variations in storm response along a microtidal transgressive barrier-island arc

Storm response along the transgressive Chandeleur barrier-island arc southeast of the Mississippi delta plain is variable because of local differences in sediment supply, shoreline orientation and barrier morphology. A study of the morphological impact of Hurricane Frederic (1979) affirmed that tropical storms are the primary agents causing erosion and migration of this barrier arc.Frederic's greatest impact was in the duneless southern Chandeleurs, where sheet-flow overwash caused flattening of the barrier profile, destruction of a strip of marsh 50–100 m wide, and shoreline retreat of approximately 30 m. In contrast, overwash in the northern Chandeleurs was confined between dunes in channels established by previous storms. This channelized overwash breached the northern Chandeleur barriers in nineteen places. As Frederic passed, return flow through these channels transported overwashed sediment back to the nearshore zone. These ebb deposits were a source for longshore drift sediments, which quickly sealed storm channels, reestablishing a coherent northern Chandeleur barrier arc.These storm response patterns may help explain long-term changes in barrier morphology. During an 84-yr period (1885–1969) the southern Chandeleurs decreased 41 % in area, with an average retreat rate of 9.1 m yr^?1, compared to a 15% increase in area and an average shoreline retreat rate of 7.2 m yr^?1 for the northern Chandeleurs.