ABSTRACT

: Permocarboniferous red-beds cover large areas of folded and thrusted Palaeozoic meta sediments and older crystalline basement rocks. Small to medium scale basement faults and joints do not proceed into the Permocarboniferous cover. Fracture patterns are related to rock strength, friction controlled multi-layer interaction and local stress fields. Because Permocarboniferous red-beds are coarse grained rocks with only minor occurrences of silt- and clay-stones, friction is one of the principle geomechanical properties. The prominent cement is calcite while massive quartzitic cementation is restricted to areas with strong volcanic activity. Depending on the regional type of cementation the cohesion varies significantly. Another parameter, important for rock strength, is the packing density, which also varies significantly in red-beds. The results of this study show that fractures produced by extensional horizontal strain are log-normally or exponentially distributed with spacing types and dimensions related to the mechanical properties of multi-layered sequences. Fractal spacing distributions are indicative for local horizontal compressional strain.

1 INTRODUCTION

Rock fractures occur on many scales and are intensively studied by structural geologists and engineering geologists. A huge amount of theories and mechanical models for rock fracturing have been developed. They are based on material properties, layer geometries, multi-layer interaction and stress conditions (e.g. Hobbs 1967, Ladeira and Price 1981, Narr & Suppe 1991, Turcotte 1997). It is apparent that the combination of the many parameters involved with rock fracturing is very complex, and does not allow a universal model. Further problems arise from scale effects, that mean, the mechanical or hydraulic properties of rock-masses change with inspection scale (Da Cunha 1993). Engineering geologists are concerned with the range of inspection scales relevant to the dimensions of buildings, tunnels, quarries and so on, which rarely exceed three or four orders of magnitude. The most common problem in practice is the transition from sample or laboratory testing scales to the scales of engineering projects. However the picture becomes more complicated if the inspection is extended to the dimensions of micro-cracks or tectonic faults. The most successful geomechanical models are those adapted to petrophysical characteristics and strain paths of certain rock types and tectonic domains. The fewer litho-types and scales are introduced into a model the more significant results can be expected.

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