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Geology topics

O. T. Tobisch

Publications and source records attributed to O. T. Tobisch.

4 recordsLinked to original sources
USGS1968

Exploration possibilities in the Western Chagai District, West Pakistan

A northerly-trending group of quartz diorite stocks that cut Cretaceous, Eocene, 01igocene(?) and Pleistocene( ?) sedimentary rocks near Saindak, West Pakistan , contain oxidized disseminated copper minerals which may occur in commercial concentrations at depth. The stocks are enclosed by an aureole of albite-epidote hornfels about 4 miles wide that locally is cut by veins containing sparse lead and copper minerals. Within the contact aureole the few thin beds of limestone and lenticular limestone reefs have been metasomatically replaced by iron sulfide or iron oxide. The sulfide is largely oxidized near the surface. The areas of metamorphosed rocks to which the veins are mostly restricted can be recognized on panchromatic aerial photographs and from a considerable distance on the ground by their rugged topography, dark color, and by the local abundance of dikes, thus greatly reducing the area requiring field examination during a prospecting program. The red and yellow soils mantling the sulfide-rich rocks are easily recognized from short distances on the ground, but cannot be differentiated from the unmetamorphosed rocks on conventional aerial photographs. Color photography or limited-spectral photo techniques probably can be used to reveal these distinctive soils and thus define the areas of sulfide mineralization.

west Pakistan↗
USGS1968

Gneissic amphibolite at Las Palmas, Puerto Rico, and its significance in the early history of the greater antilles island arc

The basal complex of Puerto Rico consists principally of serpentinite, minor amounts of chert and spilite, and locally small blocks of amphibolite . A detailed structural and metamorphic study of a relatively large block of gneissic amphibolite at Las Palmas reveals that the rock has undergone repeated deformation and regional metamorphism prior to contact metamorphism by intrusive serpentinite. The first event recorded by the amphibolite is amphibolite facies regional metamorphism. Foliation and some hornblende lineation were formed at this time. An increase in the intensity of regional metamorphism then produced quartzo-feldspathic segregations from the host rock, and was accompanied by small-scale folding and development of several types of lineation oblique (o the early hornblende lineation. The metamorphic intensity then decreased slightly and tectonic movement radically decreased, as nonfoliated but metamorphosed mafic dikes cut the gneissic amphibolite . As the metamorphic grade decreased further, late-stage kink-bands were developed at high angles to the earlier small-scale folding. Later, the amphibolite was intruded by serpen-tinite, causing incipient low-grade contact metamorphism and some calcium metasomatism. H. H. Hess has maintained that Puerto Rico and other islands in the Greater Antilles rest directly on oceanic crust, and that the intrusive serpentinite is derived from the bottom layer of the crust. In the Las Palmas area, the amphibolite formed prior to intrusion of serpentinite, indicating a complex preserpentinite history . Three interpretations of the gneissic amphibolite are considered. It may represent: (1) metamorphosed gabbroic rocks genetically related to the serpentinite, (2) regionally metamorphosed mafic rocks that are part of the oceanic crust, or (3) older mafic rocks regionally metamorphosed as part of an extensive pre-Late Cretaceous orogeny. The evidence from Las Palmas is not conclusive, but when considered in the regional framework, either of the last two interpretations seems reasonable.

Puerto Rico↗
USGS1966

Large-scale basin-and-dome pattern resulting from the interference of major folds

The geometry of individual major folds from the Glen Cannich area in the Northern Highlands of Scotland is described. The major folds are isoclinal, and their axial planes and fold limbs strike north or northeast and dip steeply; the fold axes plunge steeply toward the south or southeast. If pairs of individual folds are joined along a common axial-plane trace they form basins, domes, or other unusual culmination structures which resemble intensely flattened cylinders. As a group, the basins and domes form a continuous pattern , which covers an area of approximately 27 square miles, with possible extension into surrounding areas. This pattern has resulted from the interference of two sets of major folds , whereby the movement direction of the second fold set lay close to the axial plane of the first fold set. The geometrical conditions and /or intensity of the second folds that obtained during the second folding have rendered the first fold set unmappable, because the basins and domes are elongated entirely in the direction of the second fold set. The probable orientation of the first- fold axial plane is suggested by joining the culminations or depressions of the basm- and - dome pattern . Subsequent deformations have substantially altered the basin - and - dome pattern from its probable original form.

Scotland↗