Sr, C and O isotope systematics in the Pucará basin, central Peru

A combined Sr, O and C isotope study has been carried out in the Pucará basin, central Peru, to compare local isotopic trends of the San Vicente and Shalipayco Zn-Pb Mississippi Valley-type (MVT) deposits with regional geochemical patterns of the sedimentary host basin. Gypsum, limestone and regional replacement dolomite yield ⁸⁷Sr/⁸⁶Sr ratios that fall within or slightly below the published range of seawater ⁸⁷Sr/⁸⁶Sr values for the Lower Jurassic and the Upper Triassic. Our data indicate that the Sr isotopic composition of seawater between the Hettangian and the Toarcian may extend to lower ⁸⁷Sr/⁸⁶Sr ratios than previously published values. An ⁸⁷Sr-enrichment is noted in (1) carbonate rocks from the lowermost part of the Pucará basin, and (2) different carbonate generations at the MVT deposits. This indicates that host rocks at MVT deposits and in the lower-most part of the carbonate sequence interacted with ⁸⁷Srenriched fluids. The fluids acquired their radiogenic nature by interaction with lithologies underlying the carbonate rocks of the Pucará basin. The San Ramón granite, similar Permo-Triassic intrusions and their clastic derivatives in the Mitu Group are likely sources of radiogenic ⁸⁷Sr. The Brazilian shield and its erosion products are an additional potential source of radiogenic ⁸⁷Sr. Volcanic rocks of the Mitu Group are not a significant source for radiogenic ⁸⁷Sr; however, molasse-type sedimentary rocks and volcaniclastic rocks cannot be ruled out as a possible source of radiogenic ⁸⁷Sr. The marked enrichment in ⁸⁷Sr of carbonates toward the lower part of the Pucará Group is accompanied by only a slight decrease in ¹⁸O values and essentially no change in ¹³C values, whereas replacement dolomite and sparry carbonates at the MVT deposits display a coherent trend of progressive ⁸⁷Sr-enrichment, and ¹⁸O- and ¹³C-depletion. The depletion in ¹⁸O in carbonates from the MVT deposits are likely related to a temperature increase, possibly coupled with a ¹⁸O-enrichment of the ore-forming fluids. Progressively lower ¹³C values throughout the paragenetic sequence at the MVT deposits are interpreted as a gradually more important contribution from organically derived carbon. Quantitative calculations show that a single fluid-rock interaction model satisfactorily reproduces the marked ⁸⁷Sr-enrichment and the slight decrease in ¹⁸O values in carbonate rocks from the lower part of the Pucará Group. By contrast, the isotopic covariation trends of the MVT deposits are better reproduced by a model combining fluid mixing and fluid-rock interaction. The modelled ore-bearing fluids have a range of compositions between a hot, saline, radiogenic brine that had interacted with lithologies underlying the Pucará sequence and cooler, dilute brines possibly representing local fluids within the Pucará sequence. The composition of the local fluids varies according to the nature of the lithologies present in the neighborhood of the different MVT deposits. The proportion of the radiogenic fluid in the modelled fluid mixtures interacting with the carbonate host rocks at the MVT deposits decreases as one moves up in the stratigraphic sequence of the Pucará Group.     

A new series solution of Molodensky's problem

A complete series solution of Molodensky's boundary-value problem is derived using, instead of an integral equation, analytical continuation by means of power series. This solution is shown to be equivalent, term by term, to the Molodensky-Brovar series, but is simpler and practically more convenient.

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Zusammenfassung Eine vollst?ndige Reihenl?sung des Problems von Molodensky wird hergeleitet, wobei anstatt einer Integralgleichung die analytische Fortsetzung mittels Potenzreihen zugrunde gelegt wird. Es zeigt sich, dass diese L?sung gliedweise ?quivalent zur Reihe von Molodensky-Brovar ist, aber sie ist einfacher und praktisch brauchbarer.

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Résumé On déduit une série qui donne une solution complète du problème de Molodensky, en utilisant, au lieu d'une équation intégrale, la continuation analytique par une série de puissances. Il s'ensuit que cette solution est équivalente à la série de Molodensky-Brovar, mais elle est plus simple et plus pratique.

     

Ellipsoidal density models and hydrostatic equilibrium: Interim report

From the point of view of consistency with the Geodetic Reference System 1967, it would be desirable that the boundary surface of a Standard Earth Model is an exact equipotential ellipsoid. This is incompatible with the requirement that it be a figure of hydrostatic equilibrium. The report investigates the relation between equipotential ellipsoids and equilibrium figures. The principal conclusion is that it is possible to find an ellipsoidal model that has the same distribution of density and flattening (more precisely, of the parameter f′ as defined in the paper) as a hydrostatic model, the deviations being only of second order in the flattening.     

Imaging and regional distribution of basalt flows in the Faeroe-Shetland Basin

We demonstrate that the use of long-offset seismic data allows wide-angle reflections and diving waves to be recorded, and that these can be used in conjunction with prestack depth migrations to constrain and to image the base of the basalt flows and the underlying structure in the Faeroe-Shetland Basin. Crustal velocity models are built first by inverting the traveltimes of the recorded reflections and diving waves using ray-tracing methods. Finer details of the velocity structure can then be refined by analysis of the amplitudes and waveforms of the arrivals. We show that prestack depth migration of selected wide-angle arrivals of known origin, such as the base-basalt reflection, using the crustal velocity model, allows us to build a composite image of the structure down to the pre-rift basement. This has the advantage that the wide-angle first-arriving energy must be primary, and not from one of the many multiples or mode-converted phases that plague near-offset seismic data. This allows us to ‘tag’ these primary arrivals with confidence and then to identify the same arrivals on higher-resolution prestack migrations that include data from all offsets. Examples are drawn from the Faeroe-Shetland Basin, with a series of regional maps of the entire area showing the basalt depths and the thickness of the basalt flows and underlying sediment down to the top of the pre-rift basement. The maps show how the basalts thin to the southeast away from their presumed source west of the present Faeroe Islands, and also show the extent to which the structure of the pre-rift basement controls the considerable variations in sediment thickness between the basement and the cap formed by the overlying basalt flows.     

Shear-wave velocity estimation using a combination of ambient noise from small aperture array and small-scale active seismic measurements: a case study in the area of the natural gas fields of Northern Germany

In recent years, numerous induced seismic events have occurred in the proximity of the natural gas field in Northern Germany. To monitor the seismicity and to asses the seismic hazard potential, a local monitoring network was installed in the area. Focusing on the seismicity hazard assessment, a major challenge is the characterisation of potential site effects due to local soil characteristics. This is quantitatively performed by estimating the shear-wave velocity (V _s) variation with subsurface layer thickness. Such local effects can only be covered with a coarse spatial resolution due to the limited number of monitoring stations. Profiles were determined at three test sites (Langwedel, Walle and Bomlitz) by using a combined approach of small aperture 2D array ambient noise and small-scale active 1D measurements. The high-resolution frequency-wavenumber (HRFK), spatial autocorrelation (SPAC) and multichannel analysis of surface waves (MASW) methods were applied to the recorded ambient noise and active seismic data using various array sizes supplemented by the active measurements. This jointly allowed obtaining phase velocity dispersion curves covering a wide frequency range from 2 up to 32 Hz. The inversion of the obtained dispersion curves results in average S-wave velocity profiles down to depths of 70 m, identifying thin near-surface layers of a few meters as well as thicker layers of tens of meters in greater depth. A comparison with available borehole data shows a good correlation with the layering. Additionally, to asses the impact of a seismic event at the test sites, PGV estimations for various seismic events were performed. The final results of the test surveys demonstrate that the combined approach represents a suitable tool for near-surface characterisation, which can be used to improve the seismic hazard assessment in the area of the natural gas fields in Northern Germany.