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E-An Zen

Publications and source records attributed to E-An Zen.

11 recordsLinked to original sources
USGS2011

Thermal modelling of stepwise anatexis in a thrust-thickened sialic crust

One-dimensional modelling of the thermal history of a sialic crust thickened by multiple overstack thrusting of upper crustal material shows that anatexis is likely. both the uplift rate and the length of the incubation period between end of tectonism and start of uplift are important controls on the amount and temperature of the melt. Heat of fusion does not significantly affect the long-term thermal structure of the crust if the melt is not extracted because only a small fraction of conductive heat is converted to latent heat, though short-term thermal effects of latent heat can be locally important. Model results show that commonly <15% of mantle heat flux is converted to latent heat; even during peak melting in the most productive models, less than half of incremental mantle flux is converted. The results have obvious implications on the acceptability of proposed heat sources for crustal anatexis. Fusion could retard crustal temperature rise by nearly 100??C, but the system would recover except for situations of very rapid uplift. Understanding of the thermal evolution of a burial-uplift system requires knowledge not only of the timing of anatexis but of the pooling and movement of the magma, as well as the duration and nature of the incubation period; we are poorly equipped to measure these events. The model predicts that the characteristic time for anatexis is a thickened sialic crust is several tens of millions of years, comparable to the time lapse between orogenies; in making geological interpretations of magmatism, this time lag must be considered. -Author

Transactions of the Royal Society of Edinburgh, Ea↗
USGS2009

Land-use planning: One geologist's viewpoint

Planning for the best use of land and its resources should take fully into consideration the long-term consequences of each type of use in order to stretch out most beneficially the well-being of society in the future, and to protect the integrity of the land and its biota. Three kinds of land-use can be distinguished for planning purposes. Reversible land-use leaves the land, after use, essentially as it was before; little or no man-induced modification remains. An example of reversible use in the United States is the designation of certain public lands as Wilderness. Terminal land-use commits the land to a chosen particular use, and any attempt at reversal requires either time-scales that are long compared with the expected lifespan of the social and political institution, or a commitment of resources that is too high for society to consider worth bearing. Examples of terminal land-use are location of metropolises and sites of toxic and/or radioactive waste disposals; by its nature the list grows monotonically. A current source of some social tension arises from the fact that Wilderness designation appears to assign a terminal-use status by legislative fiat, whereas in fact the land is being used reversibly.

Environmental Conservation↗
USGS2004

Rapid late pleistocene incision of Atlantic passive-margin river gorges

The direct and secondary effects of rapidly changing climate caused large rivers draining the Atlantic passive margin to incise quickly into bedrock beginning about 35,000 years ago. Measured in samples from bedrock fluvial terraces, 10-beryllium shows that both the Susquehanna and Potomac Rivers incised 10- to 20-meter-deep gorges along steep, convex lower reaches during the last glacial cycle. This short-lived pulse of unusually rapid downcutting ended by 13,000 to 14,000 years ago. The timing and rate of downcutting are similar on the glaciated Susquehanna and unglaciated Potomac Rivers, indicating that regional changes, not simply glacial melt-water, initiated incision.

Science↗
USGS1984

Lithotectonic assemblages as portrayed on the new bedrock geologic map of Massachusetts.

Scale of 1:250,000. The map units are grouped into eight lithotectonic packages. Five "zones" of older rocks that cover the whole state are, from west to east, the Taconic-Berkshire, Rowe-Hawley, Bronson Hill, Nashoba, and Milford-Dedham. In central and western Massachusetts, these zones are overlain by the Connecticut Valley and Merrimack "belts" of Silurian-Devonian, and local Carboniferous, strata; the two belts are distinguished from each other on the basis of the lithofacies of the Silurian strata. In eastern Massachusetts, the Milford-Dedham Zone also includes Silurian-Devonian, Carboniferous, Mesozoic, and Tertiary strata. A Mesozoic "basin" is separately identified in the Connecticut Valley region

Massachusetts↗
USGS1973

Thermochemical parameters of minerals from oxygen-buffered hydrothermal equilibrium data: Method, application to annite and almandine

Reversed univariant hydrothermal phase-equilibrium reactions, in which a redox reaction occurs and is controlled by oxygen buffers, can be used to extract thermochemical data on minerals. The dominant gaseous species present, even for relatively oxidizing buffers such as the QFM buffer, are H 2 O and H 2 ; the main problem is to calculate the chemical potentials of these components in a binary mixture. The mixing of these two species in the gas phase was assumed by Eugster and Wones (1962) to be ideal; this assumption allows calculation of the chemical potentials of the two components in a binary gas mixture, using data in the literature. A simple-mixture model of nonideal mixing, such as that proposed by Shaw (1967), can also be combined with the equations of state for oxygen buffers to permit derivation of the chemical potentials of the two components. The two mixing models yield closely comparable results for the more oxidizing buffers such as the QFM buffer. For reducing buffers such as IQF, the nonideal-mixing correction can be significant and the Shaw model is better. The procedure of calculation of mineralogical thermochemical data, in reactions where hydrogen and H 2 O simultaneously appear, is applied to the experimental data on annite, given by Wones et al. (1971), and on almandine, given by Hsu (1968). For annite the results are: Standard entropy of formation from the elements, S 0 f (298, 1)=−283.35±2.2 gb/gf, S 0 (298, 1) =+92.5 gb/gf. G 0 f (298, 1)=−1148.2±6 kcal, and H 0 f (298, 1)=−1232.7±7 kcal. For almandine, the calculation takes into account the mutual solution of FeAl 2 O 4 (Hc) in magnetite and of Fe 3 O 4 (Mt) in hercynite and the temperature dependence of this solid solution, as given by Turnock and Eugster (1962); the calculations assume a regular-solution model for this binary spinel system. The standard entropy of formation of almandine, S 0 f,A (298, 1) is −272.33±3 gb/gf. The third law entropy, S 0 (298, 1) is +68.3±3 gb/gf, a value much less than the oxide-sum estimate but the deviation is nearly the same as that of grossularite, referring to a comparable set of oxide standard states. The Gibbs free energy G 0 f,A (298, 1) is −1192.36±4 kcal, and the enthalpy H 0 f,A (298, 1) is −1273.56±5 kcal.

Contributions to Mineralogy and Petrology↗
USGS1965

Solubility measurements in the system CaSO4-NaCL-H2O at 35°, 50°, and 70°cand one atmosphere pressure

The solubilities in the system CaSO 4 - NaCl -H 2 O, at 1 atm pressure and 35° , 50° , and 70°C, have been determined by approaching the solubility curves from both the undersaturation and supersaturation sides. The experimental runs are of long duration, as much as 3 months; these rates should be commensurate with those of the appropriate geological processes, and so the results should be directly applicable to the interpretation of evaporite deposits.

Journal of Petrology↗
USGS1965

Osmotic equilibrium and overthrust faulting

The two principal suggested modes of facilitating overthrust faulting are (1) lubrication at the sole by evaporite beds or micaceous shales and (2) flotation due to anomalously high (> hydrostatic) pore-water pressures. Past rapid sedimentation and tectonic compression have been suggested as important causes of anomalously high water pressure (Hubbert and Rubey, 1959). We suggest osmosis as another important possibility. Field data on shale beds and experimental studies on compacted clays show that such material can act as semipermeable membranes that greatly retard passage of dissolved electrolytes relative to H 2 O. Equilibrium osmotic pressure, π, across an ideal membrane is given by (a, activity; V H2O 0 1 , molar volume of distilled water). At 80°C, π is 470 bars between saturated halite solution and distilled water; it is 360 bars between saturated and 10 per cent solutions. At 25°C the values are 20 per cent lower. Other dissolved components, if present in similar proportions, will enhance the effect. Anomalous water pressures of at least 400 bars above hydrostatic have been measured in oil wells; many of these wells penetrate evaporites and/or shales which separate formation waters of differing salt concentrations. These pressures are explicable by assuming osmotic equilibrium across a membrane which separates saturated halite solution from solutions up to 10 weight per cent NaCl. Thus, osmotic equilibrium may be an important mechanism for floating thrust sheets. Lubrication of thrust sheets by shales or evaporites and flotation by anomalously high pressures may be simply different manifestations of the same geologic milieu. © 1965, The Geological Society of America, Inc.

Geological Society of America Bulletin↗
USGS1962

Problem of the thermodynamic status of the mixed-layer minerals

Minerals that show mixed layering, particularly with the component layers in random sequence, pose problems because they may behave thermodynamically as single phases or as polyphase aggregates. Two operational criteria are proposed for their distinction. The first scheme requires two samples of mixed-layer material which differ only in the proportions of the layers. If each of these two samples are allowed to equilibrate with the same suitably chosen monitoring solution, then the intensive parameters of the solution will be invariant if the mixed-layer sample is a polyphase aggregate, but not otherwise. The second scheme makes use of the fact that portions of many titration curves of clay minerals show constancy of the chemical activities of the components in the equilibrating solutions, suggesting phase separation. If such phase separation occurs for a mixed-layer material, then, knowing the number of independent components in the system, it should be possible to decide on the number of phases the mixed-layer material represents. Knowledge of the phase status of mixed-layer material is essential to the study of the equilibrium relations of mineral assemblages involving such material, because a given mixed-layer mineral will be plotted and treated differently on a phase diagram, depending on whether it is a single phase or a polyphase aggregate. Extension of the titration technique to minerals other than the mixed-layer type is possible. In particular, this method may be used to determine if cryptoperthites and peristerites are polyphase aggregates. In general, for any high-order phase separation, the method may be used to decide just at what point in this continuous process the system must be regarded operationally as a polyphase aggregate.

Geochimica et Cosmochimica Acta↗