Younger Faults Research Articles

Result of the preliminary geological investigation of natural steam fields in Indonesia

To meet the rising demand of electricity, the Indonesian Government has been forced to look for different sources of energy beside the hitherto known « conventional ones å. Since the Indonesian Island Arc is extremely volcanic, development of geothermal powerplants might provide the solution for the electrification of the rural areas. In the broad context of the recently launched Indonesian Five Year Plan, the Institute of Power Research in Jakarta, with the cooperation of the Bandung Institute of Technology, has started with a systematic investigation and geological mapping of the known existing steam fields. In 1968, several spots in Java have been singled out for closer investigation: four of these have been considered potentially favorable. These are the Dieng volcanic mountains in central Java, Mt. Tampomas and surroundings, the solfataric field of Kawah Kamodjang and the Hot Spring area of Tjisolok in western Java. Except for the solfataric field of Kawah Kamodjang, geological conditions of the other fields, structural as well as stratigraphical, are found to be promising. In these areas, a thick sedimentary series of Tertiary age ranging from sandstones to shales and marls which can play the role of reservoir and cap rocks, were intruded by magmatic stocks during the last phases of volcanism in Java. Besides, these areas are dissected by young faults which might lead to the formation of commercial steam-traps.

Schwefel-Isotopen-Untersuchungen am Pyrit-Sphalerit-Baryt-Lager Meggen/Lenne (Deutschland) und an verschiedenen Devon-Evaporiten

Sulphur isotope-ratios are reported for 65 sulphide samples and 72 barytes of the pyrite-sphalerite-baryte-deposit Meggen/Lenne. It was previously known that the Meggen “Kies”-deposit was formed in a shallow region of the Devonian ocean formed by the flat, dish-shaped top of a submarine elevation. The metal content (mainly pyrite) is derived from hydrothermal activity, and baryte was precipitated in a broad seam around the sulphide ore body. The S-isotope-ratios of both main components—sulphide and sulphate—differ only slightly from each other and show a markedly small spread. This contradicts any model of deriving both sulphur components from one common source : the transport of sulphide-and sulphate-ions in the same hydrothermal solution would give a large isotope fractionation due to the low temperature of formation (<100°C). Bacterial reduction of sulphate as a source of sulphide would give less fractionation but a large spread of δ-values, and concerning the deposition of baryte within a well separated seam, we can hardly derive the baryte-sulphur from oxydized ore-sulphide or hydrogen sulphide. Thus we must assume an individual source for each sulphur component: the sulphide sulphur has been brought by the metaliferous hydrothermal solutions probably in the form of complex ions, and the sulphate was precipitated from sea-water by Ba-ions of hydrothermal origin. Since no fractionation occurs in the normal sulphate precipitation, the baryte must preserve the S-isotope ratio of the Devonian ocean. This is proved by a plot of 25 new S-isotope measurements of Devonian evaporites from North America, published in this paper. Special attention is drawn to the evaluation of such evaporite data. Apart from the ore body, small amounts of sulphides of probably biogenic origin occur in various parts of the baryte-seam and in horizons above and beneath the deposit. They are depleted in S 34 and show a large spread of δ-values. The formation of a young fault within the ore body (Markhahnstörung) took place hydrothermally with a distinct fractionation of sulphur isotopes. From this fractionation a maximum temperature of 240°C is calculated for the mineralization.

Notes on the Yanagase Fault, Japan

The Yanagase fault can be traced 55 kilometers as the northeastern limit of the Biwa Lake basin.1) There are fault-line valleys that follow the line of the fault. It has been inferred that the fault is of recent origin on the basis of geomorphological and geological features.2) The earthquake of Anegawa in 1909 has been suggested to be related to the fault. Releveling of bench marks across the southern extension of the fault indicated uplift on the northeast side of it (fig. 3; after A. Imamura, 1928).3) The fault may be a left lateral strike slip one on the basis of some evidences (figs. 4 and 6) including offsets of dykes (fig. 5).4) In this region some younger faults with trends nearly parallel to the Yanagase fault show evidence of left-hand displacement (fig. 1).5) The stress distribution inducing the earthquake faults is believed to be rather consistent and indigenous to the region, at least during the Quaternary period.

Structural studies in the pre-Cambrian of western Greenland. Part III. Southern Sukkertoppen District

This paper summarises several summers field work within the southern Sukkertoppen district. Since detailed mapping has only been carried out within smaller areas within the region, the remainder being covered by reconnaissance mapping along the coasts, the results should be considered as preliminary. The southern Sukkertoppen district can be divided into three tectonic units, the Nordland, the Finnefjeld, and the Alángua complexes, which, most probably, were formed during the Ketilidian cycle (E. Wegmann, 1938). The metamorphic complexes are traversed by postorogenic dykes and faults (Berthelsen and Bridgwater, 1960). The dykes and faults were seemingly formed before the Nagssugtôqidian revolution which affected the country further to the north (Ramberg, 1948). The northern Nordland complex is shown to have passed through a metamorphic and structural evolution very similar to that which recently has been described from a small area within the complex (see table 2). An original granulite facies rock assemblage has been exposed to two successive imprints of retrograde metamorphism: first an amphibolite facies metamorphism; next a postorogenic epidote-amphibolite to greenschist facies metamorphism in connection with the formation of the younger faults. Evidence is brought forward that the tectonic phases established from Tovqussap nunâ may also be traced within the remaining parts of the Nordland complex. In one case (see fig. 3) an analysis of the basement structures reveals that the post-orogenic faulting is of the wrench fault type. The Finnefjeld complex which is built up of homogeneous hornblende-biotite-bearing quartz-dioritic gneisses is believed to have been originally composed of granulite facies rocks. Subsequent strong penetrative movements accompanied by low grade amphibolite facies metamorphism were responsible for the formation of the present Finnefjeld gneisses. This idea is strongly supported by the facts that relic patches of hypersthene gneiss and transgressive, but deformed, more or less uralitised diorite bodies occur within the Finnefjeld gneiss. The Alangua complex comprises abundant pelitic and semipelitic schists, amphibolites, ultrabasics and skarn rocks in addition to gneisses which are considered to be of metasomatic origin. The ultrabasic rocks have been described by H. Sørensen (1952,1953, 1954, and 1955). The rocks of this complex can also be shown to have passed through two periods of metamorphism (see also H. Sørensen, 1952); an original medium to high grade amphibolite facies metamorphism was succeeded by a later low grade amphibolite facies metamorphism accompanied by granitisation, pegmatisation etc., indicating the presence of a volatile-rich dispersed phase. Although not studied in detail, the structures of the Alángua complex are sufficiently well-known to establish the kinematic evolution of this complex. The first amphibolite facies metamorphism seems to correspond to the Smalledal-Pâkitsoq phases of the Nordland complex, while the subsequent period of low grade amphibolite metamorphism can be matched with the posthumous phase. During this latter, the northern part of the Nordland complex, which locally was thrust over the Alángua rocks (thereby causing their refolding) was converted into the present Finnefjeld gneisses. This interpretation explains the present differences between the three com· plexes as being due to Stockwerk tectonics, fig. 16. An alternative theory which holds that the Alángua rocks are younger than those of the southern complexes does not seem to concur with the field relation known so far. No mineral deposits of economic interest were found during the survey, but traces of sulfides (see tables 1 and 3), magnetite, molybdenite, corundum, monazite, zircon, talc and soapstone have been met with at various localities.

Tushar Uranium Area, Marysvale, Utah

A thick sequence of late Tertiary volcanic tuff covers portions of the Tushar Plateau in south-central Utah. Rhyolite of similar age fills parts of the Sevier River valley to the east. Mines along the valley produce uranium ore from deposits genetically related to the tuff and rhyolite. The threefold relation of mountain tuff, valley rhyolite, and uranium mineralization has been investigated by field mapping and laboratory study of the Mount Belknap volcanic group. Field mapping defines changes in the volcanic stratigraphy and shows the interfingering of the regional tuff with the local valley rhyolites. Primary features of compaction, intergradation, and flow folding suggest unusual modes of air-fall aggregation and subsequent gravity flow for the emplacement of some of the volcanic units. Thin section and clay-mineral investigations yield information about the primary structure of the volcanic units and the chemical and mineralogical reaction to alteration. Zeolites, quartz, and feldspar represent primary alteration features. Variations in kaolinite, montmorillonite, and mixed-layer clay suites reflect differences in the intensity of secondary alteration along fault zones. Quartz monzonite intrusives and associated ore bodies lie along a major east-west zone of weakness which passes through Marysvale, Utah. Younger fault systems cut across this zone and define the north-south axis of the Tushar Range. Uranium mineralization and related alteration phenomena have formed fissure veins along these younger fault trends.

Geomorphological Problems in the Inter-mount Basin, in Nagano Pref

KISO and INA horse mountains are situated along the southern border of the Inner Zone of Japanese Are, on the W side of the Median Line which is one of the main geotectonic lines in the Japanese Islands. INA intermount basin is located between these two horsts, and is irrigated by Tenryuriver that flows through the basin from north to south. In this paper the writer discussed on some geomorphological problems of the basin. The main subjects are summarized as follows: 1. There are some outcrops of younger faults along the geomorphological faultlines at the base of the above named two horst mountains on the margin of Ina Basin. 2. In the INA Basin, Y. Ichinose studied on the terraces, and classified them into 6 horizons in Ryusei, or the western reach of Tenryu, while 5 horizons were distinguished in Ryuto, or the eastern side of the river. Thereafter T. Shikama and K. Kobayashi classified the terraces and erosion surfaces in the basin into 9 stages. Ichinose concluded that No. 2 Terrace in the Ryusei were correlated to No. I in the Ryuto. The writer's conclusion through his study of these terraces are that No. 4 terrace in the Ryusei is comparable to No. 2 and 3 in the Ryuto. 3. At the higherr positions above No. 4 Terrace, the Hara topographies cited firstly in this paper have remarkable development around the basin. The Hara is a conception newly introduced by the writer in this paper, which means a kind of compound topography that combines alluvial fan with alluvial cone. Namely in its upper part it is formed by some or less flat plain, like the slope of alluvial fan, while its side slope is steep like alluvial cone. The development of this topography seemes to have been promoted by repeated faulting along the higher back fault-scarp, because the younger Hera deposits are piled. overr the older. 4. During the formation of the Hara at the level above No. 4 Terrace, lower terraces were developing below the level. of No. 4 Terrace.