Models of aggradation versus progradation in the Himalayan Foreland

A frequent goal of decompaction analysis is to reconstruct histories of basin subsidence and tectonic loading. In marine environments, eustatic and paleobathymetric uncertainties limit the resolution of these reconstructions. Whereas in the terrestrial basins, these ambiguities are absent, it is still necessary to account for depositional slopes between localities in order to analyze three-dimensional patterns of subsidence. We define two end-members for depositional surfaces: aggradation and progradation. The relative importance of either end-member is a function of the interplay between the rate of net sediment accumulation and the rate of basin subsidence. The models predict the patterns of major drainages (transverse versus longitudinal) and the way in which provenance should be reflected within different portions of a basin. Consequently, paleocurrent and provenance data from the ancient stratigraphic record can be used to distinguish between these endmembers. The subhorizontal depositional surfaces that dominate during times of aggradation provide a well defined reference frame for regional analysis of decompacted stratigraphies and related subsidence. Depositional slopes during progradation can not be as precisely specified, and consequently yield greater uncertainties in reconstructions of subsidence. These models are applied to the Mio-Pliocene foreland basin of the northwestern Himalaya, where sequences of isochronous strata have been analyzed throughout the basin. These time-controlled data delineate a distinctive evolution from largely aggradational to largely progradational depositional geometries as deformation progressively encroaches on the foreland. Such a reconstruction of past depositional surfaces provides a well constrained reference frame for subsequent integration of subsidence histories from throughout the foreland.

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Zusammenfassung Ein häufiges Ziel der Dekompaktionsanalyse ist es die Beckenabsenkung und die tektonische Belastung zu rekonstruieren. In marinen Ablagerungsräumen limitieren eustatische und paläobathymetrische Unsicherheiten die Auflösung der Rekonstruktion. Bei terrestrischen Becken fehlen diese Zweideutigkeiten; es ist aber trotzdem notwendig, Rechenschaft über den Ablagerungshang zwischen verschiedenen Lokalitäten abzulegen, um dreidimensionale Subsidenzmuster zu analysieren. Wir definieren zwei Endglieder von Ablagerangsflächen: Aggradation und Progradation. Die relative Wichtigkeit des jeweiligen Endglieds ist eine Funktion des Zusammenspiels zwischen der Nettorate der Sedimentakkumulation und der Beckensubsidenz. Die Modelle sagen die Hauptentwässerungsmuster (quer- oder längsverlaufend) vorher, sowie den Weg in dem die Sedimentherkunft innerhalb verschiedener Bereiche des Beckens berücksichtigt werden sollte. Folglich können Paläoströmungs- und Herkunftsdaten alter stratigraphischer Überlieferungen benutzt werden, um zwischen den Endgliedern zu unterscheiden. Die subhorizontale Ablagerungsfläche welche zur Zeit der Aggradation dominant ist, liefert einen gut definierten Referenzrahmen für die regionale Analyse von dekomprimierten Formationen und der damit verknüpften Subsidenz. Ablagerangshänge während Progradation können nicht präzise spezifiziert werden und beinhalten daher größere Unsicherheiten bei der Rekonstruktion der Subsidenz. Diese Modelle wurden übertragen auf das miozäne bis pliozäne Vorgebirgsbecken des nordwestlichen Himalayas, wo Sequenzen von isochronen Schichten durch das gesamte Becken analysiert werden konnten. Diese zeitkontrollierten Daten schildern eine ganz bestimmte Entwicklung, die von einer hauptsächlich aggradierenden zu einer progradierenden Ablagerangsgeometrie verlief, während der die Deformation schrittweise in Richtung Vorland übergriff. Diese Rekonstruktion von ehemaligen Ablagerangsflächen liefert einen guten Referenzrahmen für die folgende Integration der Subsidenzgeschichte des gesamten Vorlands.

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Résumé L'analyse de décompaction a souvent pour but de reconstituer l'histoire de la subsidence d'un bassin et de la charge tectonique. Dans les milieux marins, de telles reconstitutions sont limitées par des incertitudes de caractère eustatique et paléobathymétrique. Par contre, ces ambiguïtés ne se présentent pas dans le cas des bassins continentaux, où il convient néanmoins de tenir compte de la pente de la surface de dépôt entre les divers points considérés pour établir un schéma tridimensionnel de la subsidence. Nous définissons deux situations extrêmes pour les surfaces de dépôt: l'aggradation et la progradation. L'importance relative de ces deux extrêmes est fonction de l'interaction entre le taux d'accumulation net des sédiments et le taux de subsidence du bassin. Les modèles prévoient la répartition des drainages principaux (transverse ou longitudinal) et la manière dont l'origine des sédiments peut se répercuter dans les diverses parties d'un bassin. Il en résulte que des informations fournies par les relevés stratigraphiques à propos des paléocourants et de la source des sédiments peuvent être utilisées pour faire la distinction entre les deux cas extrêmes. Les surfaces de dépôt subhorizontales, qui prédominent pendant les périodes d'aggradation, fournissent un bon cadre de référence pour les analyses régionales de formations décompactées et de la subsidence qui leur est associée. Les surfaces de dépôt inclinées qui se présentent au cours des progradations ne peuvent pas être définies de manière aussi précise et engendrent par conséquent plus d'incertitude dans la reconstitution de la subsidence. Les auteurs appliquent ces modèles au bassin mio-pliocène d'avant-pays de l'Himalaya nord-occidental, dans lequel des séquences de couches isochrones ont été suivies à travers tout le bassin. Ces données, chronologiquement définies, fournissent l'image d'une évolution nette, depuis des géométries typiques d'aggradation jusqu' à des géométries typiques de progradation, au fur et à mesure de l'emprise progressive de la déformation sur l'avant-pays. Une telle reconstitution des surfaces de dépôt anciennes fournit un bon cadre de référence en vue de l'intégration ultérieure de l'histoire de la subsidence dans l'ensemble de l'avant-pays.

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V792 Her=HD 155638: A totally eclipsing RS CVn binary

Photometry of HD 155638=V792 Her has been analyzed to determine the elements of this totally eclipsing RS CVn binary. The light variation outside eclipse was found to have a period of 27^d.07±0^d.07, which is slightly different from the 27^d.5384±0^d.0045 orbital period. Analysis of the eclipses was achieved by a modification of the Russell-Merrill technique. With the aid of radial velocity measures, absolute elements were obtained for the hot and cool stars, respectively;R _h=2.58R ,R _c=12.28R ,M _h=1.40M ,M _c=1.46M ,i=80^o.61 and velocity semi-amplitudesK _c=48.36 km s^–1±0.79 km s^–1, andK _h=50.50 km s^–1±0.33 km s^–1. The apparent magnitudes areV _h=9 ^m .73 andV _c=8 ^m .48. The distance to HD 155638 was estimated to be 310 parsecs.     

The role of magnetohydrodynamics in heliospheric space plasma physics research

Magnetohydrodynamics (MHD) is a fairly recent extension of the field of fluid mechanics. While much remains to be done, it has successfully been applied to the contemporary field of heliospheric space plasma research to evaluate the macroscopic picture of some vital topics via the use of conducting fluid equations and numerical modeling and simulations. Some representative examples from solar and interplanetary physics are described to demonstrate that the continuum approach to global problems (while keeping in mind the assumptions and limitations therein) can be very successful in providing insight and large scale interpretations of otherwise intractable problems in space physics.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

Development of a real-time algorithm for detection of solar wind discontinuities

A study is presented of an algorithm, based on the statistical analysis of interplanetary magnetic field data, for the real-time detection of discontinuities in the solar wind. The analysis is based on the application of a sliding or searching algorithm together with a minimum variance treatment. A demonstration is given for a Pioneer data set from 29 August, 1966. This particular data set was chosen because it has been extensively studied (in terms of the fast forward MHD shock wave) by previous groups who utilized the three-dimensional Rankine-Hugoniot shock equations after identifying the shock on a subjective basis. Our procedure differs in that it is perfectly objective because of its searching technique. For this particular data set we identify the original shock with excellent agreement with the earlier studies of its normal vector, as well as an additional shock and tangential discontinuity, neither of which had been identified previously.     

Archean high-Mg monzodiorite–syenite,epidote skarn,and biotite–sericite gold lodes in the Granny Smith–Wallaby district,Australia: U–Pb and Re–Os chronometry of two intrusion-related hydrothermal systems

The Granny Smith (37 t Au production) and Wallaby deposits (38 t out of a 180 t Au resource) are located northeast of Kalgoorlie, in 2.7 Ga greenstones of the Eastern Goldfields Province, the youngest orogenic belt of the Yilgarn craton, Western Australia. At Granny Smith, a zoned monzodiorite–granodiorite stock, dated by a concordant titanite–zircon U–Pb age of 2,665 ± 3 Ma, cuts across east-dipping thrust faults. The stock is fractured but not displaced and sets a minimum age for large-scale (1 km) thrust faulting (D2), regional folding (D1), and dynamothermal metamorphism in the mining district. The local gold–pyrite mineralization, controlled by fractured fault zones, is younger than 2,665 ± 3 Ma. In augite–hornblende monzodiorite, alteration progressed from a hematite-stained alkali feldspar–quartz–calcite assemblage and quartz–molybdenite–pyrite veins to a late reduced sericite–dolomite–albite assemblage. Gold-related monazite and xenotime define a U–Pb age of 2,660 ± 5 Ma, and molybdenite from veins a Re–Os isochron age of 2,661 ± 6 Ma, indicating that mineralization took place shortly after the emplacement of the main stock, perhaps coincident with the intrusion of late alkali granite dikes. At Wallaby, a NE-trending swarm of porphyry dikes comprising augite monzonite, monzodiorite, and minor kersantite intrudes folded and thrust-faulted molasse. The conglomerate and the dikes are overprinted by barren (<0.01 g/t Au) anhydrite-bearing epidote–actinolite–calcite skarn, forming a 600-m-wide and >1,600-m-long replacement pipe, which is intruded by a younger ring dike of syenite porphyry pervasively altered to muscovite + calcite + pyrite. Skarn and syenite are cut by pink biotite–calcite veins, containing magnetite + pyrite and subeconomic gold–silver mineralization (Au/Ag = 0.2). The veins are associated with red biotite–sericite–calcite–albite alteration in adjacent monzonite dikes. Structural relations and the concordant titanite U–Pb age of the skarn constrain intrusion-related mineralization to 2,662 ± 3 Ma. The main-stage gold–pyrite ore (Au/Ag >10) forms hematite-stained sericite–dolomite–albite lodes in stacked D2 reverse faults, which offset skarn, syenite, and the biotite–calcite veins by up to 25 m. The molybdenite Re–Os age (2,661 ± 10 Ma) of the ore suggests a genetic link to intrusive activity but is in apparent conflict with a monazite–xenotime U–Pb age (2,651 ± 6 Ma), which differs from that of the skarn at the 95% confidence level. The time relationships at both gold deposits are inconsistent with orogenic models invoking a principal role for metamorphic fluids released during the main phase of compression in the fold belt. Instead, mineralization is related in space and time to late-orogenic, magnetite-series, high-Mg monzodiorite–syenite intrusions of mantle origin, characterized by Mg/(Mg + Fe_TOTAL) = 0.31–0.57, high Cr (34–96 ppm), Ni (22–63 ppm), Ba (1,056–2,321 ppm), Sr (1,268–2,457 ppm), Th (15–36 ppm), and rare earth elements (total REE: 343–523 ppm). At Wallaby, shared Ca–K–CO₂ metasomatism and Th-REE enrichment (in allanite) link Au–Ag mineralization in biotite–calcite veins to the formation of the giant epidote skarn, implicating a Th + REE-rich syenite pluton at depth as the source of the oxidized hydrothermal fluid. At Granny Smith, lead isotope data and the Rb–Th–U signature of early hematite-bearing wall-rock alteration point to fluid released by the source pluton of the differentiated alkali granite dikes.     

The Mordor Alkaline Igneous Complex,Central Australia: PGE-enriched disseminated sulfide layers in cumulates from a lamprophyric magma

The Mordor Alkaline Igneous Complex (MAIC) is a composite intrusion comprising a body of syenite and a funnel-shaped layered mafic–ultramafic intrusion of lamprophyric parentage, the Mordor Mafic–Ultramafic Intrusion or MMUI. The MMUI is highly unusual among intrusions of lamprophyric or potassic parentage in containing primary magmatic platinum-group element (PGE)-enriched sulfides. The MMUI sequence consists largely of phlogopite-rich pyroxenitic cumulates, with an inward dipping conformable layer of olivine-bearing cumulates divisible into a number of cyclic units. Stratiform-disseminated sulfide accumulations are of two types: disseminated layers at the base of cyclic units, with relatively high PGE tenors; and patchy PGE-poor disseminations within magnetite-bearing upper parts of cyclic units. Sulfide-enriched layers at cycle bases contain anomalous platinum group element contents with grades up to 1.5 g/t Pt+Pd+Au over 1-m intervals, returning to background values of low parts per billion (ppb) on a meter scale. They correspond to reversals in normal fractionation trends and are interpreted as the result of new magma influxes into a continuously replenished magma chamber. Basal layers have decoupled Cu and PGE peaks reflecting increasing PGE tenors up-section, due to increasing R factors during the replenishment episode, or progressive mixing of between resident PGE-poor magma and more PGE-enriched replenishing magma. The presence of PGE enriched sulfides in cumulates from a lamprophyric magma implies that low-degree partial melts do not necessarily leave sulfides and PGEs in the mantle restite during partial melting. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Quantitative textural analysis of packings of elongate crystals

The spatial distribution of grains in a solidifying igneous rock controls the physical properties of the crystal mush, and in turn is controlled by the rate of crystal growth and accumulation. A predominant non-spherical habit for igneous minerals brings into question the use of spherical particles in reference packings used for quantification of spatial distribution. Furthermore, variations of crystal clustering/ordering with length scale require spatial statistics which take into account the distribution of particles beyond nearest neighbours. Using random close packings of spherocylinders, we demonstrate the importance of aspect ratio for the aggregation index (usually known as R) and show that packings of spherical particles have more structure than packings of rods. The spatial distribution functions demonstrate that the plagioclase grains in the colonnade from the Holyoke basalt are clustered on a length scale of 0.5 mm. Understanding the controls on grain spatial distribution in igneous rocks will depend on the application of these techniques to well-understood environments.     

Oxidation of pyrite during extraction of carbonate associated sulfate

The sulfur isotopic composition of carbonate associated sulfate (CAS) has been used to investigate the geochemistry of ancient seawater sulfate. However, few studies have quantified the reliability of δ³⁴S of CAS as a seawater sulfate proxy, especially with respect to later diagenetic overprinting. Pyrite, which typically has depleted δ³⁴S values due to authigenic fractionation associated with bacterial sulfate reduction, is a common constituent of marine sedimentary rocks. The oxidation of pyrite, whether during diagenesis or sample preparation, could thus adversely influence the sulfur isotopic composition of CAS. Here, we report the results of CAS extractions using HCl and acetic acid with samples spiked with varying amounts of pyrite. The results show a very strong linear relationship between the abundance of fine-grained pyrite added to the sample and the resultant abundance and δ³⁴S value of CAS. This data represents the first unequivocal evidence that pyrite is oxidized during the CAS extraction process. Our mixing models indicate that in samples with much less than 1 wt.% pyrite and relatively high δ³⁴S_pyrite values, the isotopic offset imparted by oxidation of pyrite should be much less than ? 4‰. A wealth of literature exists on the oxidation of pyrite by Fe³⁺ and we believe this mechanism drives the oxidation of pyrite during CAS extraction, during which the oxygen used to form sulfate is taken from H₂O, not O₂. Consequently, extracting CAS under anaerobic conditions would only slow, but not halt, the oxidation of pyrite. Future studies of CAS should attempt to quantify pyrite abundance and isotopic composition.     

The composition and crystallinity of the near-surface regions of weathered alkali feldspars

Our ability to identify thin non-stoichiometric and amorphous layers beneath mineral surfaces has been tested by undertaking X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) work on alkali feldspars from pH 1 dissolution experiments. The outcomes of this work were used to help interpret XPS and TEM results from alkali feldspars weathered for <10,000 years in soils overlying the Shap Granite (north-west England). The chemistry of effluent solutions indicates that silica-rich layers a few nanometers in thickness formed during the pH 1 experiments. These layers can be successfully identified by XPS and have lower Al/Si, Na/Si, K/Si and Ca/Si values than the outermost ∼9 nm of unweathered controls. Development of Al-Si non-stoichiometry is coupled with loss of crystal structure to produce amorphous layers that are identifiable by TEM where >∼2.5 nm thick, whereas the crystallinity of albite is retained despite leaching of Na to depths of tens to hundreds on nanometers. Integration of XPS data over the outermost 6-9 nm of naturally weathered Shap feldspars shows that they have stoichiometric Al/Si and K/Si ratios, which is consistent with findings of previous TEM work on the same material that they lack amorphous layers. There is some XPS evidence for loss of K from the outermost couple of nanometers of Shap orthoclase, and the possibility of leaching of Na from albite to greater depths cannot be excluded using the XPS or TEM results. This study demonstrates that the leached layer model, as formulated from laboratory experiments, is inapplicable to the weathering of alkali feldspars within acidic soils, which is an essentially stoichiometric reaction.     

Quadruple sulfur isotope analysis of ca. 3.5 Ga Dresser Formation: New evidence for microbial sulfate reduction in the early Archean

Multiple sulfur isotope system is a powerful new tracer for atmospheric, volcanic, and biological influences on sulfur cycles in the anoxic early Earth. Here, we report high-precision quadruple sulfur isotope analyses (³²S/³³S/³⁴S/³⁶S) of barite, pyrite in barite, and sulfides in related hydrothermal and igneous rocks occurring in the ca. 3.5 Ga Dresser Formation, Western Australia. Our results indicate that observed isotopic variations are mainly controlled by mixing of mass-dependently (MD) and non-mass-dependently fractionated (non-MD) sulfur reservoirs. Based on the quadruple sulfur isotope systematics (δ³⁴S-Δ³³S-Δ³⁶S) for these minerals, four end-member sulfur reservoirs have been recognized: (1) non-MD sulfate (δ³⁴S = −5 ± 2‰; Δ³³S = −3 ± 1‰); (2) MD sulfate (δ³⁴S = +10 ± 3‰); (3) non-MD sulfur (δ³⁴S > +6‰; Δ³³S > +4‰); and (4) igneous MD sulfur (δ³⁴S = Δ³³S = 0‰). The first and third components show a clear non-MD signatures, thus probably represent sulfate and sulfur aerosol inputs. The MD sulfate component (2) is enriched in ³⁴S (+10 ± 3‰) and may have originated from microbial and/or abiotic disproportionation of volcanic S or SO₂. Our results reconfirm that the Dresser barites contain small amounts of pyrite depleted in ³⁴S by 15-22‰ relative to the host barite. These barite-pyrite pairs exhibit a mass-dependent relationship of δ³³S/δ³⁴S with slope less than 0.512, which is consistent with that expected for microbial sulfate reduction and is significantly different from that of equilibrium fractionation (0.515). The barite-pyrite pairs also show up to 1‰ difference in Δ³⁶S values and steep Δ³⁶S/Δ³³S slopes, which deviate from the main Archean array (Δ³⁶S/Δ³³S = −0.9) and are comparable to isotope effects exhibited by sulfate reducing microbes (Δ³⁶S/Δ³³S = −5 to −11). These new lines of evidence support the existence of sulfate reducers at ca. 3.5 Ga, whereas microbial sulfur disproportionation may have been more limited than recently suggested.