The Oregon-Idaho graben evolved after
widespread tholeiitic flood basalt volcanism associated with the arrival of the plume
headof the Yellowstone hotspot beneath southeastern Oregon approximately 17 - 16 Ma. The north- to
north-northeast-trending, 50 km-wide graben evolved between 15.5 and 10.5 Ma
during east-west extension. The graben is part of a middle Miocene back-arc rift
(1100 km long) that includes the northern Nevada rift, the Baker and
LaGrande
grabens, and the Chief Joseph dike swarm of the Columbia
River Basalt Group. The graben ceased to be a volcanotectonic depression as the
cross-cutting western Snake River plain began to
evolve at approximately 10.5 Ma.
At about
14.3 Ma and continuing to approximately 12.6 Ma the Oregon-Idaho graben began to
break up into subbasins that were bounded by intragraben fault zones. The break
up was accompanied by a change to calc-alkaline volcanism that persisted within
the Oregon-Idaho graben until 8.1 Ma, approximately 2.5 million years after
subsidence within the graben ceased. Three intragraben fault zones have been
identified and a fourth is inferred. The easternmost, the Devils Gate fault
zone, coincides with the magmatic axis of the graben at approximately 14.3 Ma.
By approximately 13.5 Ma the magmatic axis had shifted to the west along the Dry
Creek Buttes and Wall Rock Ridge fault zones. A diffuse magmatic axis developed
at approximately 12 Ma farther to the west where the fourth intragraben fault
zone is inferred. In addition to serving as conduits for the ascent of basalt,
basaltic andesite, rhyodacite, and rhyolite magmas, the zones focused the
discharge of large geothermal systems.
The intragraben fault zones display a common
evolutionary pattern. The 2 to 3 km-wide zones initially control the
distribution of facies in volcaniclastic sediments and the location of basalt
hydrovolcanic vents. During this stage, the fault zones may have had no surface
expression other than a low elevation change on a relatively low-relief
landscape. As relief developed by continued deformation along the fault zones,
strike-length of individual faults increased and weakly consolidated
volcaniclastic sediments, silicified sediments and hot-spring sinter, and
palagonite were eroded from the uplifted blocks and deposited in neighboring
subbasins. It is during this stage that the large hot-spring precious-metals
bearing prospects evolved. Displacements of over 150 m on a few faults within
the zones and possible reversal of the sense of displacement along the zones
occurred in this final stage of development. Locally, the fault zones were
remobilised during the subsidence of the cross-cutting western
Snake
River plain. The intragraben fault zones within the
Oregon-Idaho graben illustrate the relations among syntectonic volcanism and
sedimentation and the evolution of geothermal systems within continental rift
systems.