LENGTH/DISPLACEMENT SCALING OF SMALL FAULTS IN THE BROTHERS FAULT ZONE, A YOUNG CONTINENTAL TRANSFORM FAULT IN THE OREGON BASIN AND RANGE

Models of fault growth build from field relationships of fault length relative to displacement. Fault continuity and organization within a fault zone increase as shear zone growth accommodates increasing displacement. Utilizing the concepts that length/ displacement (L/D) relationships are a measure of fault maturity we are attempting to understand growth of the Brothers Fault Zone (BFZ) in the Oregon Basin and Range province. The BFZ is defined by a series of small, northwest trending normal faults extending across southeastern Oregon. Numerous small faults (1 – 10 km in length, less than 100 m in dip-slip separation) define the BFZ, which implies that the BFZ is a relatively young fault zone. Using the computer program ArcGIS, a digital elevation model of the northern BFZ was created and fault traces defined. Faults were defined from the DEM data on the basis of linear topographic breaks with slopes between 5 and 20 degrees. Lineaments along the base of the sloping regions were mapped as fault traces. From these traces, fault length (L) and displacement (D), inferred from topographic relief was measured. Measurement of L/D for faults primarily focused on faults that cut units of the same age. Faults that cut units of more than one age were avoided. Data were normalized and show an array of scaling of L/D values from .001 to .05 for faults of varying lengths within the BFZ. Most faults across the fault zone show a similar parabolic shape to their L/D measurements. Several faults have non-parabolic shapes indicating one of two possibilities. One potential cause for a on-parabolic L/D relation is that some part of the fault may have been covered by a younger lava flow, which would have the effect of causing an apparent variation in displacement at the surface along the length of the fault. Alternatively, discrepancies in longer faults may be due a discontinuity in the L/D relationship where two faults have linked but have not smoothed out the discontinuity in slip across the overlap region. These possibilities will be evaluated during the field session of this project to take place in the spring of 2008. With this new analysis, we hope that growth and maturity of the BFZ can be characterized using a relatively simple and consistent length/displacement characteristics.