Structural evolution of faults and fault rocks

Faults and fault zones result from the failure of parts of the earth`s crust due to tectonically aligned stresses, and are essential zones of weakness in the upper parts of the crust. Within this project selected fault zones were investigated with regard to both their structural and temporal evolution (basic research), their consequence for surface evolution (geomorphology), and their implications for the permeability evolution (hydrogeology). The main sites of investigation were located along the Lavanttal fault zone between the Kor- and Saualpe, and the Talhof fault (Semmering area). The alignments of two major railway tunnels (Koralm base tunnel, Semmering base tunnel) partly transect these fault zones. Detailed structural investigations show that fault zones develop out of sets of subparallel fractures initially oriented oblique to the strike of the fault zone. The internal parts of the fault zone are therefore initially characterized by subparallel slices. Contraction and contemporaneous shearing cause rotation of these slices and subsequent fracturing along well defined planes. Fault zones comprising fractured and mechanically abraded rock material (fault rocks, cataclasites) develop out of these planes in the core of the fault zone, bordered by a network of fractures along a damage zone. The latter represent conduits for groundwater and hydrothermal fluids. Fault core zones are either characterized by minor permeability mainly parallel to the strike of the fault zone, or form barriers. Circulation of hydrothermal fluids through fault zones is accompanied by solution in the disintegrated parts, and by precipitation within pores and voids. This results in the cementation of fault rocks and damage zones, and in a reduction of permeability. By anaylzing the isotope composition we demonstrated that the fluids are mostly of meteoric origin, penetrating into deeper parts of the crust via the damage zone, warming up and interacting with the adjacent rock mass. These hydrothermal fluids as well as heat generated by shearing result in a modification of the thermal structure of the fault zone. The thermal evolution of some fault zones in the Eastern Alps (Mölltal fault, Lavanttal fault) was reconstructed by geochronological methods sensitive for temperatures between 400 and 40C. It can be shown that the evolution of the Eastern Alps is characterized by a phase of intense fault activity between 12 and 6 million years before present. Social and economic aspects within this project resulted from the investigation of the internal structure of fault zones and the permeability structure, both being essential for the design and development of subsurface constructions, in particular for tunneling, including the consequences for the application of construction techniques.