The effect of manganese on olivine-quartz-orthopyroxene stability

The effect of manganese on the stability of ferrosilite relative to fayalite + quartz has been experimentally determined to assess its importance to orthopyroxene barometry. Reaction reversals in a piston-cylinder apparatus were obtained to within 0.1-kbar intervals indicating instability of Fs₉₅Rh₅ below 10.3, 10.9, 11.4, 12.2, 12.9, 13.7 kbar and Fs₉₀Rh₁₀ below 9.8, 10.4, 10.9, 11.6, 12.4 and 13.2 kbar at 750, 800, 850, 900, 950 and 1000°C, respectively. Each mole % MnSiO₃ extends the pyroxene stability by approximately 0.12 kbar relative to FeSiO₃. Electron microprobe analyses of run products indicate a small preference of Mn for pyroxene over olivine withK_DMn-Fe^opx-oliv = 1.2?1.5, similar to values observed for natural pairs. Mössbauer spectra are consistent with a random distribution of Mn between the M1 and M2 sites in the orthopyroxene. These experimental data allow downward revision of pressure estimates based on the orthopyroxene barometer in areas where Mn is a significant component in orthopyroxene.     

Zur Frage der Beziehungen zwischen Tektonik und Landformung

Zusammenfassung Das Problem der Beziehungen zwischen Tektonik und Landformung beinhaltet die zeitliche und die genetische Verknüpfung der tektonischen Zyklen in der Erdgeschichte mit dem Ablauf des geomorphologischen Geschehens, Fragestellungen, welche besonders in Europa von verschiedenen Forschungsrichtungen (Geologie, physische Geographie) behandelt werden und welche einen Vergleich der erzielten Ergebnisse sehr nahelegen. Die Großformen der Erdrinde werden in erster Linie durch die endogenen, tektonischen Kräfte geschaffen. Die Landformung erscheint aber außerdem durch das Wirken der jeweiligen klimatischen Faktoren (Büdel, J., 1948) und durch den Stand des Meeresspiegels, des letzteren als der maßgeblichen, zeitlich wechselnden Erosionsbasis, beeinflußt; Umstände, auf welche, der Zielsetzung entsprechend, in diesem Aufsatz nicht näher eingegangen werden soll. Die morphologischen und die tektonischen Entwicklungen spiegeln sich in den Sedimentationen der jeweiligen Zeiträume der jüngeren geologischen Vergangenheit, unter Berücksichtigung der durch eine Filterwirkung bedingten Unterdrückung der Abbildung sekundärer Vorgänge (E.Wegmann, 1948), in großen Zügen wider, so daß auch diese zur Klärung der Erscheinungen heranzuziehen sind.Im folgenden sollen die einschlägigen Fragestellungen, hauptsächlich an Hand der jüngsttertiären Entwicklungsgeschichte der Alpen und ihrer weiteren Umrahmung, einschließlich der östlichen Randbecken, geprüft werden, wobei insbesondere eine Stellungnahme zu strittigen tektonischen Problemstellungen erfolgen muß, welch letztere für den Ablauf der Landformung von grundsätzlicher und maßgeblichster Bedeutung erscheinen.     

Subduction, back-arc spreading and global mantle flow

The shapes and orientations of Benioff zones beneath island arcs, interpreted as marking the location of subducted lithosphere, provide the best presently available constraints on the global convective flow pattern associated with plate motions. This global flow influences the dynamics of subduction. Subduction zone phenomena therefore provide powerful tests for models of mantle flow. We compute global flow models which, while simple, include those features which are best constrained, namely the observed plate velocities, applied as boundary conditions, and the density contrasts given by thermal models of the lithosphere and subducted slabs. Two viscosity structures are used; for one, flow is confined to the upper mantle, while for the other, flow extends throughout the mantle.Instantaneous flow velocity vectors match observed Benioff zone dips and shapes for the model which allows mantle-wide flow but not for the upper mantle model, which has a highly contorted flow pattern. The effect of trench migration on particle trajectories is calculated; it is not important if subduction velocities are greater than migration rates. Two-dimensional finite element models show that including a coherent high viscosity slab does not change these conclusions. A coherent high viscosity slab extending deep into the upper mantle would significantly slow subduction if flow were confined to the upper mantle. The maximum earthquake magnitude, M_w, for island arcs correlates well with the age of the subducted slab and pressure gradient between the trench and back-arc region for the whole mantle, but not the upper mantle, flow model. The correlations with orientations of Benioff zones and seismic coupling strongly suggest that the global return flow associated with plate motions extends below 700 km. For both models, regions of back-arc spreading have asthenospheric shear pulling the back-arc toward the trench; regions without back-arc spreading have the opposite sense of shear, suggesting global flow strongly influences back-arc spreading.     

Deformed graptolites, finite strain and volume loss during cleavage formation in rocks of the taconic slate belt, New York and Vermont, U.S.A.

An analysis of graptolites in the Taconic slate belt of eastern New York and western Vermont (U.S.A.) shows that they are nearly ideal strain markers. For the three species used in this study, Orthograptus whitfieldii, Orthograptus calcaratus and Climacograptus bicornis, the spacing of thecae is constant except for the first five or so thecae in the proximal part of the fossil rhabdosome. Further, the thecal apertures are perpendicular to the long axis of the stipe. Observations of the thecal spacing in deformed rocks leads to a determination of extension (e=(l_f−l₀)/l₀) and a measure of the angle between thecal aperture and stipe axis yields a direct determination of angular shear strain. In practice, we find it is most straightforward to use length changes to determine the magnitude of principal strains. In Taconic slates, e₁ ranges from 1.0 to 0.24, e₂ ranges from 0.23 to −0.43 and e₃ ranges from −0.56 to −0.74. Thus, we find that the absolute finite strain in these slates is constrictional at three sites, plane strain at another and a true flattening at only one site. An examination of volume changes based on strain results in determinations of between 81% volume loss and 7% volume gain, with volume losses between 28% and 81% in 9 out of 10 calculations. These conclusions are in accord with previous determinations of volume loss based on reduction spot analyses and are consistent with the observation that pressure dissolution was a common grain scale deformation process in cleavage formation but that these slates lack abundant veins, fibrous overgrowths or other identifiable sites of reprecipitation.     

1996年3月19日新疆阿图什6.9级地震烈度与发震构造

１９９６年３月１９日新疆阿图什、伽师县东北部发生６．９级主震－余震型地震，震中烈度达ＩＸ度。极震区内边坡裂缝、山体塌陷、山石崩塌。伽师县卧里托乎拉克乡、古尔鲁克乡、西克尔镇和阿图什市的格大良乡、农三师红旗农场等地许多房屋倒塌，大多数房屋出现不同程度的破坏。水坝和公路出现塌陷、裂缝。在地下水位较高、土质松散的地区出现大面积喷砂冒水现象。地震造成直接经济损失３．８７亿元。此次地震是托特供拜孜－阿尔帕勒克断裂最新活动的表现。     

Engineering implications of the 1985 nahanni earthquakes

A series of strong ground motions was recorded during a set of earthquakes which occurred in the North Nahanni River area of the Northwest Territories of Canada in late 1985. The strongest of these motions were recorded within 10 km of the epicentre of a magnitude 6.9 event; peak accelerations were well above 1g and peak velocities exceeded 0.4 m/s. This paper describes a study of the engineering characteristics of the Nahanni strong motions, particularly as they affect the response of building structures. The response characteristics are analysed in terms of the response spectra and the unit velocity base shear coefficients for multi-degree-of-freedom elastic frame structures. An analysis of the strong motion durations of the records and several measures of intensity is also included. The characteristics of the Nahanni records are compared with those of typical strong seismic ground motions and an ensemble of epicentral region seismic motions recorded under similar conditions. Comparisons are also made with the equivalent parameters in the 1985 edition of the National Building Code of Canada. It is concluded that the Nahanni records are very strong seismic ground motion records with relatively unusual response characteristics, even when compared with those of other records obtained within the epicentral region. The impact of these characteristics on engineering design is most significant for low period structures in epicentral regions. The specifications of typical design codes do not adequately describe the response to such records for very low periods.     

The Climax-Alma granite batholith of oligocene age and the prophyry molybdenum deposits of Climax, Colorado, U.S.A.

The porphyry molybdenum deposits of Climax, Colorado, are stockworks of quartz-molybdenite veinlets. They are apical to intrusive cupolas of leucocratic rhyolite-granite porphyry of Oligocene age (33 to 24 Ma). The upper parts of the orebodies are overprinted by overlying zones of phyllically altered rocks, and by stockworks of greisen-like veinlets, containing quartz, pyrite, topaz and huebnerite. The phyllic and/or greisen-like zones are surrounded by a propylitic zone, characterized by quartz-chlorite-hematite veinlets and chlorite after biotite. Late veins, radial and peripheral to the intrusive center, are common but widely scattered and discontinuous. Such veins typically contain quartz, fluorite, rhodochrosite, pyrite, dark sphalerite, galena, tetrahedrite, and traces of huebnerite, molybdenite, argentite, native gold and/or electrum.

The Alma district, 4 to 10 km southeast of Climax, contains widely scattered clusters of veins and replacement deposits. The veins cut chloritized to sericitized silicate host rocks, and the replacement deposits are in carbonate host rocks. The ore- and gangue-mineral assemblages of the veins of the Alma district resemble those of veins peripheral to the Climax molybdenum deposits. Veins of the Alma district cut dikes of Late white rhyolite (35 Ma), which resemble the rhyolite porphyries of Climax. A swarm of such dikes is parallel to the long axis of an elliptical drainage pattern, which bounds a structural dome, here called the outer Alma dome. The outer Alma dome (8 × 12 km) surrounds an inner Alma dome (6 × 8 km). Veins of the Alma district generally are axial and/or marginal to the outer Alma dome, and/or radial to the inner Alma dome. Veins and alteration minerals associated with the outer Alma dome formed between 35 and 27 Ma ago. Those associated with the inner Alma dome formed about 27 Ma ago, as indicated by fission-track age determinations on thermally annealed zircon grains from hydrothermally altered rocks, within and around the inner Alma dome.

Negative Bouguer gravity anomaly patterns indicate: (1) a regional, northeast-trending gravity low, associated with the composite monzonite-granodiorite-granite B batholith of the Colorado mineral belt (average density about 2.62 g/cm³); (2) a semi-regional gravity trough, within the gravity low of the Colorado mineral belt, extending northeastward across the Mosquito Range; (3) an oval pattern of gravity lows, coincident with the Alma domes; and (4) a closed gravity low associated with the Climax stock (density about 2.56 g/cm³) (Tweto and Case, 1972; Behrent and Bajwa, 1974; Corry, 1981).

The semi-regional gravity trough is interpreted as the gravimetric expression of a granite batholith, here called the Climax-Alma batholith. The gravity lows of the Climax and Alma districts are interpreted as expressions of cupolas on this granite batholith.

The highly differentiated rhyolite-granite intrusions of Climax plunge toward the Alma domes. The Climax intrusions are interpreted as products of apical differentiation of the inferred Climax-Alma granite batholith. Magmatic-hydrothermal fluids, associated with highly differentiated apical magmas, produced the Climax molybdenum orebodies. Most of the veins and replacement deposits of the Alma district are associated with lower, broader cupolas of the inferred granite batholith. The shape of the composite Climax-Alma magmatic-hydrothermal system resembles that of a teapot, with Climax above the spout and the Alma district above the lower, broadly domed lid.     

Sulfur dioxide in the North American Arctic

Determinations of atmospheric sulfur dioxide were made across the North American Aretic using gas chromatography with a detection limit of 25 parts per trillion by volume and a precision of 25% or better. The vertical distribution of sulfur dioxide in the Arctic atmosphere in April, 1986 was highly variable, with concentrations ranging from the detection limit to 15 parts-per-billion by volume (ppbv). While SO₂ exceeded 10 ppbv in an exceptional haze layer in the Alaskan Arctic, sulfur dioxide was sometimes in the 1 – 5 ppbv range when the haze was absent. This was particularly true for the Canadian Arctic in the vicinity of Alert. In the lower stratosphere over Ellesmere Island, sulfur dioxide was 0.85 ppbv.