DEGRADATION OF NORMAL FAULT SCARPS IN BASALT, CENTRAL OREGON AND NORTHERN CALIFORNIA

Normal faulting characterizes regions of active extensional tectonics at many places on Earth, such as Iceland, the Basin and Range Province of North America, and the west flank of Kilahuea volcano on Hawaii. The geomorphic characteristics of fault scarps can provide constraints on the slip event(s) that produced the landform. In particular, understanding the processes of scarp evolution, and the relationship between scarp profile and age of the last slip event can be a powerful paleoseismological tool. Previous studies have found that for Quaternary events in arid climates, fault scarp degradation in colluvial material can be used to estimate fault age and earthquake recurrence. Scarps in bedrock may yield similar information, but there has been much less work on degradation of bedrock scarps. The focus of our study is the degradation of scarps in jointed basalt and the correlation between scarp profile and age. The principal goal is to evaluate the processes that affect degradation of jointed basalt scarps and to determine an analytical model for degradation in jointed bedrock. Current work includes topographic profiles of Quaternary scarps in Oregon and Northern California. The scarps displace jointed basalt from the associated flows of Newberry Crater in central Oregon, from flows related to Medicine Lake Volcano in northern California, and in flows of Devil's Garden on the Modoc Plateau, northern California. Detailed data about joint spacing, joint aperture, vesicle density, joint height, and flow composition were also collected; these data, along with estimates on the effects from a changing climate and the recurrence of earthquakes, helps constrain the variables which affect scarp degradation through topple of basalt columns. These variables provide limits for an analytical model of scarp degradation in jointed basalt; the topographic profiles provide a means to validate the model. Preliminary analysis shows a tentative correlation between column spacing, height, and the degree of degradation; smaller columns topple more readily than large columns. Initial work also shows that earthquakes may play a role in the modification of scarps. Likewise, scarp shape and rate of degradation may provide a record of subsequent regional groundshaking and therefore provide additional paleoseimological information.