NEW TECTONIC IMPLICATIONS DERIVED FROM SOUTHERN OREGON PLATEAU BASALTIC VOLCANISM: DEFINING THE OWYHEE BLOCK

Located at the confluence of both the Yellowstone-Snake River Plain (YSRP) and High Lava Plains trends of age progressive continental silicic volcanism lies the southeastern-most Oregon Plateau, or the Owyhee Plateau.  Based primarily on chemical, isotopic, chronologic, and field evidence from mid-Miocene to recent basalts and on regional structural patterns, we hypothesize that the Owyhee Plateau has remained, since prior to the initiation of Miocene volcanism, a discrete lithospheric entity (“Owyhee Block”).

1) Commencing at ~17 Ma, Steens Basalt (SB) eruptions were focused along the margins of the Owyhee Plateau and at other localities throughout the Oregon Plateau.  Although SB dikes in the Northern Nevada rift may be older than SB lavas further north, no apparent temporal eruptive pattern is present along Owyhee Plateau margins, and no eruptive loci have been documented from within the Owyhee Plateau.

2) The ~11 Ma and younger basaltic volcanism of the Owyhee Plateau is focused along linear fractures and associated with numerous shield volcanoes and cinder cones concentrated primarily within the plateau confines.  The oldest manifestation of this activity are small shield volcanoes that formed and erupted at ~10 Ma, along NW trending fault zones.  Subsequently, basaltic volcanism ceased along a northwestward migrating front, punctuated by brief hiatuses at 6 and 3 Ma.  This pattern is consistent with upper mantle corner flow driven by the subducting Juan de Fuca plate.  Additionally, the basalt geochemical and isotopic data support the model of Humphreys et al. (GSA Today, 2000) that suggests the presence of basaltic melt depleted residuum beneath the Owyhee Plateau.

3) Physiographically, the plateau today is characterized by a relative lack of large-displacement normal faults, dissimilar to surrounding regions where extension was occurring prior to and concurrent with flood basalt volcanism.  Furthermore, there is no geologic evidence present within the Owyhee Plateau to suggest the presence of large, Mid-Miocene and younger extensional structures like those that exist adjacent to the plateau.  While this obvious lack of large scale faulting on the Owyhee Plateau may be accommodated partially by dike injection concurrent with extension, rift related sedimentation and volcanism both appear to be absent.