Showing papers by "Karl Fuchs published in 1991"

Large-scale variation in lithospheric structure along and across the Kenya rift

Abstract: THE Kenya rift is one of the classic examples of a continental rift zone: models for its evolution range from extension of the lithosphere by pure shear1, through extension by simple shear2, to diapiric upwelling of an asthenolith3. Following a pilot study in 19854, the present work involved the shooting of three seismic refraction and wide-angle reflection profiles along the axis, across the margins, and on the northeastern flank of the rift (Fig. 1). These lines were intended to reconcile the different crustal thickness estimates for the northern and southern parts of the rift4–6 and to reveal the structure across the rift, including that beneath the flanks. The data, presented here, reveal significant lateral variations in structure both along and across the rift. The crust thins along the rift axis from 35 km in the south to 20 km in the north; there are abrupt changes in Moho depth and uppermost-mantle seismic velocity across the rift margins, and crustal thickening across the boundary between the Archaean craton and Pan-African orogenic belt immediately west of the rift. These results suggest that thickened crust may have controlled the rift's location, that there is a decrease in extension from north to south, and that the upper mantle immediately beneath the rift may contain reservoirs of magma generated at greater depth.

Stress evaluation in offshore regions of Norway

Abstract: A review has been undertaken of all available information on crustal stresses in Norwegian continental shelf and North Sea. The data have principally come from the results of borehole breakouts, supplemented by focal mechanisms. A simple model of a soft linear inclusion was developed to explain the rapid changes in stress orientation in the Central Graben area. This example indicates that the stress field is affected locally by the geological or structural environment.

A Stress Measurement Profile to Mid-Crustal Depth in KTB Scientific Drilling Project, Southeastern Germany: Scientific Questions and Technological Challenges [and Discussion]

Abstract: Although many estimates of crustal stress levels at seismogenic depth have been made on the basis of laboratory data and simple faulting theory, the Kontinentales Tiefbohr (KTB) deep drilling project offers the first opportunity to directly measure at mid-crustal depth the absolute magnitudes of crustal stresses, the uniformity of stress changes with depth, the influence of pore pressure on stress magnitude and the magnitude and orientation of tectonic stresses in the vicinity of the brittle-ductile transition. The planned 10-12 km depth of the KTB Hauptbohrung (main borehole) will penetrate those depths where the shear stresses in the crust are highest and where the greatest discrepancy will exist between observations and predictions if either ‘Byerlee’s law' is not applicable to the mid-crust or near-lithostatic pore pressure exists which substantially weakens the crust.