Applicability of palaeomagnetism of pyrrhotite to ore-genetic studies

Pyrrhotite has never before been used in palaeomagnetic investigations, being highly anisotropic magnetically. The aim of this paper is to show that fine-grained pyrrhotite aggregates of hydrothermal origin can be used for palaeomagnetic purposes. This has been proved with polymetallic Turkak veins from Kutná Hora in Central Bohemia as an example. It can be assumed that other localities with occurrence of hydrothermal pyrrhotite aggregates which contain directionless, uniformly scattered pyrrhotite grains and therefore forming a magnetically isotropic medium may be suitable for palaeomagnetic studies.

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Zusammenfassung Pyrrhotin, mit einem hohen Grad magnetischer Anisotropie, ist bisher für die paläomagnetische Datierung unbeachtet geblieben. Diese Arbeit will zeigen, daß feinkörnige Pyrrhotinaggregate hydrothermalen Ursprungs für paläomagnetische Untersuchungen geeignet sind, was am Beispiel polymetallischer Gänge von Turkak in Kutná Hora in Zentralböhmen dargestellt wird. Es ist anzunehmen, daß auch andere Vorkommen hydrothermaler Pyrrhotinaggregate in denen regelmäßig verteilte Körner richtungslos orientiert vorliegen und so ein magnetisch isotropes Medium bilden, sich für paläomagnetische Untersuchungen eignen.

     

Recent movements of the earth's crust

Summary It has been shown that it is necessary to combine several methods to be able to describe and explain recent movements of the Earth's crust.Presented at the XV^th General Assembly of the IUGG, Moscow 1971.     

Peralkaline acid tendencies in Gran Canaria (Canary Islands)

The study of a volcanic series from the island of Gran Canaria (Canary Islands) in which alkaline and peralkaline, saturated and undersaturated rocks coexist, is reported here. Materials with high volatile content (ignimbritic trachytes) were first emitted and the series ended with the eruption of phonolitic lavas. The average peralkalinity index in these rocks is typically about 1.0 and, therefore, peralkaline rocks coexist with non-peralkaline ones. However, a maximum in peralkalinity is found in the ignimbritic rocks of the lower part of the series. In spite of the evident acid peralkaline tendencies of these rocks, it does not seem appropriate to classify them as pantellerites or comendites. Nor are they consistent with the genetic processes proposed for rocks of similar composition and oceanic environment.The crystallization of the feldspars controls the variation trends among the different magmas but the fractionation alone does not sufficiently explain the genesis of successive fluids. Various factors seem to point to the important role which a gas-transfer process causing a geochemical stratification inside the magmatic chamber may have played.The occurrence of peralkaline silicics at Gran Canaria, which is located for away from the active Mid-Atlantic ridge, is not related to transitional basalts. These rocks are a deviation from the main undersaturated alkalic trend which characterizes the volcanism of the Canary Islands, their genesis being related to the realization of favourable local volcanic conditions.     

Comment on the effect of a vorticity centre on a frontal boundary

Zusammenfassung Es wird der Einfluss des sogenannten Vorticity-Zentrums auf die Grenzfläche zweier Luftmassen der mittleren geographischen Breiten studiert. Zwecks Aufklärung der Rolle des Bewegungs- und Druckfeldes in diesem Prozesse wurde ein einfaches Modell konstruiert und numerisch gelöst. Die sich entwickelnde frontale Welle kann beziehungsweise als ein Anfangswert beim Lösen der mit der Stabilität der Wellen verbundenen Probleme mittels der Gleichungen, linearisiert durch die Methode der kleinen Störungen, dienen. Bei der Konstruktion des Modells wird ausser anderem von der Geometrie der Bewölkung ausgegangen, wie sie auf den Aufnahmen der meteorologischen Satelliten erfasst ist.     

Results of deep seismic soundings along international profile VII in Czechoslovakia and poland

Summary The paper deals with the results of DSS measurements along international profile VII, carried out by Czechoslovak and Polish geophysicists in 1970 – 71. The profile situation is shown in Fig. 1. By 1971 part of the profile in the region of the Bohemian Massif between points 1 and 3 and in Poland between points 5 and 7 had been surveyed (Fig. 2). The seismograms were used to construct the travel-time curves of the fundamental types of waves P^K, P^M, P_n (Fig. 4). The mean velocities were computed from the travel-time curves of the reflected waves (P^M and P^K) and the refracted waves (P_g). Isolines of the mean velocities could be constructed for the region of the Bohemian Massif (Fig. 6). The velocity data found were used for the depth interpretation of the travel-time curves of the principal types of waves and to construct a seismic section (Fig. 8). In the region of the Pre-Sudeten block the thickness of the crust was found to be 34–37 km, and in the Sudeten it increased to 40 km. Towards the south the thickness of the crust gradually reduces to 30 km in the system of the Luice faults. In the Bohemian Cretaceous the thickness of the crust is about 30 km. Further to the south, in the region of the Moldanubicum, the thickness of the Earth's crust increases rapidly, and at the southern border of the Central Bohemian pluton it reaches values of about 42 km.     

Model of earthquake occurrence in the European area

Summary The time distribution of earthquake occurrence in the European area is investigated by statistical laws. The original data of shallow-focus earthquakes are taken from the European catalogue 1901–1967. Evidence is given that the process with the negative binomial entries as a model describing the occurrence of shallow-focus earthquakes is better than the Poisson process. Further, the influence of magnitude classes and magnitude threshold value on the time distribution of earthquake occurrence is examined.Communication presented at the XIII General Assembly of the European Seismological Commission in Brasov in 1972.     

Signal and noise coherence determination for the Uppsala seismograph array station

Summary The Uppsala seismograph array station (UPSAS) has been tested with respect to mutual resemblance of body-wave signals recorded by the individual array sensors. Records ofP-, pP- andPKP-phases from ten teleseismic earthquakes and the HANDLEY explosion, in the magnitude range fromm _b =5.8 tom _b =6.9, are investigated in detail. Calculated coherence and cross-correlation functions confirm that the area selected for UPSAS provides extremely favourable conditions for the operation of an array station. High signal similarity across UPSAS shows no obvious dependence upon the location of the focus or the magnitude. Natural and man-made events provide comparable results.