GIS ANALYSIS OF EVOLUTION OF THE BROTHERS FAULT ZONE, HIGH LAVA PLAINS, OREGON

The Brothers Fault Zone (BFZ) occurs in the High Lava Plains province (HLP) of southeastern Oregon. The HLP is noted for age progressive volcanism which youngs from 10 Ma in the east, near Steens Mountain, to less than 1 Ma in the Newberry volcano area. This trend originates in the axis of middle Miocene flood basalt volcanism of the Steens and Columbia River basalts. The Yellowstone-Snake River Plain (YSRP) trend also originates in this area and progresses to the NE consistent with plate motion over a hotspot. The HLP trend complicates this interpretation of the YSRP as it is not simply explained by a hotspot. One alternative hypothesis for both trends is that they reflect propagation of shearing. We performed a GIS anaylsis of the Brothers Fault Zone to assess whether or not this system propagated in the timeframe of the HLP trend.

The BFZ comprises many normal faults of low apparent offset (generally 10s of m, occassionally over 100 m). These faults are comonly en echelon, trending N45°W, with individual scarps being continuous for 1-10 km. The area is arid and underlain by porous volcanic rocks, so little erosional degradation of fault scarps has occurred. Our analysis utilized a domain approach, with seven domains of varying age across the HLP. Coherent areas underlain by basalt of about the same age were selected as domains for GIS study. The scarps were digitized from USGS topographic maps utilizing ArcView 3.2a. Scarp height was recorded for each point entered along the scarp. For each domain an estimate was made of the sediment thickness against the scarps based on field observations and projection of tilted surfaces. Data analysis yielded percent extension and extension rate for each domain. The results indicate generally consistent rates of extension in most domains of about 0.2-0.5 %/Ma, with a higher rate in the oldest wentern domain (1.5 %/Ma). These results indicate that the faulting has not propagated across the HLP, but rather occured at the same rate across the entire HLP since at least 7.5 Ma. Greater extension in the west is attributed to the link between this area and the Cascade Graben. Fault propagation can clearly not explain migrating volcanism of the HLP. We prefer a model in which the head of the hypothesized Yellowstone plume not only drives middle Miocene flood basalt volcanism, but migrating volcanism of the HLP.