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.