Time-space patterns of deformation
of the Brothers fault zone, central Oregon

The Brothers fault zone cuts obliquely the Oregon High Lava Plains, a province of age-progressive volcanism.  The age-progression is west-younging, and therefore inconsistent with the motion of the North American plate over a fixed hotspot.  One model for the cause of age-progressive volcanism is that it reflects the propagation of the Brothers fault zone across the province.  Working with Soren Klingsporn, an undergraduate student at Whitman College, we digitzed faults in seven domains with volcanic rocks of known age to document the time-space evolution of faulting in the province.  We digitized 3,973 points on 321 faults, and performed a series of calculations to document the magnitude of extension and extension rate in each domain.  The results are shown in the upper left block below.  Comparing the data  to ideal models of propagating and non-propagating fault systems (represented by yellow planes in the blocks below) it is clear that the data do not fit a propagating fault zone model.  The best fit can be achieved by allowing a slowing of the extension rate at about 5 Ma.  However, this may reflect greater difficulty in detecting extension in the less extended younger domains because more faults will be below the minimum scarp height necessary to be recognized on topographic maps and DEMs.  CONCLUSION: the Brothers fault zone did not propagate in the time frame of age-progressive volcanism but, rather, has been active along its length since at least 7.5 Ma.