From several studies of near-fault deformation, differences are observed in the behavior of sediments based upon their degree of cementation. In this study we are comparing the behavior of moderately cemented sandstones to shales with sandy clays collected from a sag-pond. The anisotropy of magnetic susceptibility (AMS) and the magnetic direction through demagnetization of the samples were measured.
For the cemented rocks the AMS increases approximately exponentially as the fault is approached with percent anisotropy increasing with increasing offset on the fault. The distance from the fault of the onset of AMS increase also increases with increasing offset. The soft sediments show no increasing AMS toward the fault, but show a greater scatter in the AMS. The cemented sediments show varying amounts of change in orientation of the AMS ellipsoid with the strongly sheared sediments having their maximum susceptibility axis oriented parallel to slip on the fault and the intermediate and minimum susceptibility axes in a girdle perpendicular to the fault. The soft sediments tend to have the maximum and intermediate axes in a girdle oriented about 15 degrees west of the trend of the right-lateral fault.
The magnetic directions of the cemented rocks show a slight drag of the rocks adjacent to the faults. Stratigraphic control makes this more difficult to determine in the soft sediments but the suggestion of drag is seen. Within the fault zone many steeply oriented directions are seen, particularly in the samples with greater AMS.
We interpret these results to indicate that deformation of these cemented rocks was penetrative affecting the rocks at greater distances from the faults. The deformation of the soft sediments appears to be along narrow (few cm) wide zones of intense deformation with regions of relatively little deformation, or turbulent deformation, around them. These results may also reflect differences in fault behavior and behavior of the sediments affected by grain size.