DRAINAGE SYSTEMS AND THE DEVELOPMENT OF NORMAL FAULTS - AN EXAMPLE FROM PLEASANT VALLEY, NEVADA

Drainage systems in regions of active extension contain information about fault zone structure and development, and particularly about the lateral growth of individual fault segments. that is difficult to obtain by more conventional means such as the dating of sedimentary material. We investigate drainage-fault interaction in Pleasant Valley, Nevada, where a large normal-faulting earthquake occurred in 1915. We find geomorphological evidence for a propagation (increase in length) of several km at both ends of one of the 1915 fault segments. At one end we estimate the propagation rate to be about 50 m per earthquake. This estimate is uncertain. but is within the range predicted by theoretical models of fault growth. Drainage along the axes of half graben in this region may be influenced by the spacing of tilted fault blocks: the highest valleys are also the narrowest. Whether the axial streams succeed in reaching lower neighbouring sinks depends on whether sedimentation rates are able to keep basins filled to the levels of the basement highs that separate half graben of opposite polarity. A knowledge of modern drainage-fault interactions may help predict drainage systems and the position of potential syn-rift reservoir sediments in older extensional terrains where fault configurations (but not facies distributions) are known from seismic reflection surveys.