Damage zone and slip-surface evolution over mu m to km scales in high-porosity Navajo sandstone, Utah

Detailed mapping of throw variations and deformation along two kin-scale no rmal faults in the high-porosity Navajo sandstone, Utah, has been used to i nvestigate fault growth in this lithology. The faults consist of one or mor e through-going, striated, slip-surfaces, accommodating the greater part of the offset surrounded by a damage zone consisting of deformation band clus ters and short, unconnected slip-surfaces. In contrast to previous models f or deformation in this lithology, we find that the nucleation of slip-surfa ces begins where measurable throw is negligible and deformation bands are f orming and increasing in number. The microstructure and porosity of deforma tion bands and slip surfaces are distinct and independent of the amount of offset that they accommodate, i.e. they represent different and yet contemp oraneous deformation mechanisms. The point where measurable throw begins to accumulate (the fault tip) is marked by the first through-going connected slip-surface. Increase in throw towards the centre of the fault results in a three-dimensional strain field, producing orthorhombic structural geometr ies within the damage zone. We find that the total width of the damage zone increases as offset is accumulated. For these faults, the damage zone widt h is approximately 2.5 times the total fault throw. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.